Optimisation of PET glycolysis by applying recyclable heterogeneous organocatalysts

Zsuzsanna Fehér; Johanna Kiss; Péter Kisszékelyi; János Molnár; Péter Huszthy; Levente Kárpáti; József Kupai

doi:10.1039/d2gc02860c

Optimisation of PET glycolysis by applying recyclable heterogeneous organocatalysts

Fehér, Zsuzsanna; Kiss, Johanna; Kisszékelyi, Péter; Molnár, János; Huszthy, Péter; Kárpáti, Levente; Kupai, József 2022-10-31 00:00:00 Green Chemistry View Article Online PAPER View Journal | View Issue Optimisation of PET glycolysis by applying recyclable heterogeneous organocatalysts† Cite this: Green Chem., 2022, 24, a a a b Zsuzsanna Fehér, Johanna Kiss, Péter Kisszékelyi, János Molnár, a b a Péter Huszthy, Levente Kárpáti and József Kupai * Chemical depolymerisation, or solvolysis, can be a sustainable plastic recycling method, as a circular economy can be achieved by recovering the pure monomers. Polyethylene terephthalate (PET) is a ubi- quitous plastic material with short-life application and slow biodegradation, so its waste management needs to be continuously improved. In this study, we tested three commercially available organocatalyst- modiﬁed silica gels in the glycolysis of PET and developed another, functionalized with triazabicyclode- cene (TBD), which was also tested. Organocatalysts are eﬃcient in PET glycolysis, but their recyclability needs to be improved for industrial application. The applied heterogeneous modiﬁed silica gels can be recovered easily by ﬁltration. Si-TEA catalyst was chosen for reaction optimisation because it has the highest thermal stability and good catalytic activity. The PET glycolysis process was optimised by fractional factorial experimental design and response surface methodology. Under optimal reaction conditions (PET (384 mg, 2 mmol), ethylene glycol (1.41 mL, 25.2 mmol), Si-TEA (15.5 mol%), 190 °C, 1.7 h), 88.5% non- isolated bis(2-hydroxyethyl) terephthalate (BHET) monomer yield was obtained. Si-TEA and Si-TBD cata- lysts were recycled in ﬁve reaction cycles, and with both catalysts, high cumulative BHET yields (89 and 88%, respectively) were achieved. Additionally, environmental energy impacts were calculated for the two Received 1st August 2022, catalysts and were compared favourably with other organocatalysts in the literature. A process scale-up Accepted 11th October 2022 was also implemented. Finally, it has been veriﬁed that modiﬁed silica gels have much higher catalytic DOI: 10.1039/d2gc02860c activities than native silica gel, as solvolytic reactions using the former catalysts took a signiﬁcantly shorter rsc.li/greenchem time. Plastic recycling methods fall into four categories: primary 1 Introduction (industrial scrap recycling), secondary (mechanical), tertiary 1 4 As global plastic production increases exponentially, there is a (chemical), and quaternary (incineration) recycling. growing demand for environmental awareness and sustainable Mechanical recycling is still considered the most favourable plastic recycling methods. Amongst the most prevalent plastic due to its lower energy requirement than chemical recycling. materials, polyethylene terephthalate (PET) is the one that In the case of PET bottles, the mechanical recycling technology becomes waste in the highest proportion of its produced quan- is indispensable, particularly when there is a deposit refund tity (97% of 33 million tons in 2015 ). This is because PET is system that can provide clean, homogeneous material for 2 5 primarily used as a short-life packaging material. Because of bottle-to-bottle recycling. However, if there is no such possi- its slow biodegradation and inadequate waste management, bility, chemical recycling can be adapted in the case of con- adaptive and well-designed recycling strategies are needed to taminated, heterogeneous waste streams. Chemical recycling prevent plastic’s harmful eﬀect on the environment and can be split into three categories: solvent purification, chemi- improve the sustainability of plastic use. cal depolymerisation (solvolysis or chemolysis), and thermal depolymerisation (thermal cracking or thermolysis). Among these technologies, solvolysis, which means the depolymerisa- a tion of plastic waste into its monomers and oligomers by a Department of Organic Chemistry and Technology, Faculty of Chemical Technology nucleophile reagent acting also as the solvent, has the most and Biotechnology, Budapest University of Technology and Economics, Műegyetem rkp. 3., H-1111 Budapest, Hungary. E-mail: [email protected] promise to complement mechanical recycling. This is because Laboratory of Plastics and Rubber Technology, Faculty of Chemical Technology and it can live up to the demands made for chemical recycling that Biotechnology, Budapest University of Technology and Economics, Műegyetem mechanical recycling sometimes struggles to achieve: infinite rkp. 3., H-1111 Budapest, Hungary virgin-grade recycling is technically feasible, food-grade pro- †Electronic supplementary information (ESI) available. See DOI: https://doi.org/ ducts can be produced, and the removal of contaminants is 10.1039/d2gc02860c This journal is © The Royal Society of Chemistry 2022 Green Chem., 2022, 24,8447–8459 | 8447 Open Access Article. Published on 12 October 2022. Downloaded on 10/16/2024 9:02:41 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Paper Green Chemistry possible. Sustainability requires the development of new analysis is that a process can be described explicitly at the methods to recover material of the same quality as the original least amount of time and expense. Factorial experimental one in an economical and environmentally friendly way, for design is an eﬃcient and straightforward method, and it is example, using recyclable catalysts in PET solvolysis. commonly used to determine the eﬀects of independent vari- Based on the applied nucleophile in solvolysis, PET is most ables that significantly influence the outcome of a process. often depolymerised by hydrolysis, methanolysis, glycolysis, and With the help of experimental design, the numerous methods aminolysis. Glycolysis is the simplest and earliest applied and catalysts used in PET glycolysis can be compared and eval- 7 3 method of PET depolymerisation. This transesterification reac- uated more eﬃciently. Chen and co-workers applied a 2 fac- tion proceeds with glycol excess at high temperatures torial experimental design to study the two- and three-factor (170–200 °C). It stands out with the advantage that its product, interaction eﬀects of reaction temperature, catalyst : PET ratio, bis(2-hydroxyethyl) terephthalate (BHET), can be used to and reaction time on the glycolysis of PET. They used cobalt 8,9 36,37 produce PET in only one polycondensation step. Moreover, on acetate or manganese acetate as catalysts. Aguado and co- average, it has the lowest environmental energy impact among workers also studied a fourth independent variable, the the solvolytic methods applied to rebuild PET (hydrolysis, reagent : PET ratio using Taguchi’s parameter design while 9 38 methanolysis, and glycolysis), and it is the most advanced in applying zinc acetate as a catalyst. A similar design was demonstrating commercial viability on a larger scale. applied by Zawadzki and co-workers with sodium metasilicate The glycolysis of PET proceeds through at least three stages: catalyst to optimise the synthesis of PET polyol from which oligomers, dimer, and monomer, as previously reported in the they prepared polyurethane adhesives. Van-Pham and co- literature. First, ethylene glycol diﬀuses into the polymer, workers applied a Box–Behnken design using sodium bicar- causing the polymer to swell up, thus increasing the diﬀusion bonate catalyst. There are multiple papers about applying rate. It was shown earlier that the depolymerisation of PET design of experiments in PET glycolysis for zinc acetate 41–43 with high polymerisation degree into PET with low polymeris- catalyst. Seeing the advantages of experimental design in ation degree, preferentially leading to the dimer, would these studies, e.g., finding the economically optimal reaction proceed in a relatively short time interval. So, the conversion conditions to obtain high yield and determining the eﬀects of of the dimer into BHET would be the rate-determining step. factors influencing the process, it is worth improving a cata- The transformation of dimer into BHET monomer is a revers- lytic recycling process by this method. ible process, and an equilibrium exists between the BHET In this work, multiple commercially available functiona- monomer and the remaining amount of the dimer. lized silica gels (modified with trialkylguanidine (Si-GUA), trialk- Catalysts are usually applied in the solvolysis of PET due to ylamine (Si-TEA), or dialkylthiourea (Si-THU)), and a newly syn- its high chemical stability and low solubility. The most thesised modified silica gel (modified with triazabicyclodecene common catalysts in PET glycolysis are metal catalysts (metal (Si-TBD)) catalyst were tested in PET solvolysis with ethylene 12–15 16–19 20–22 salts, metal oxides, and others ), basic glycol (EG). These heterogeneous catalysts are easily recoverable, 12,23–25 26–29 organocatalysts, ionic liquids, and deep eutectic which is necessary for being green and eﬃcient catalysts. To 30–33 solvents. Metal catalysts usually have high activity and identify the eﬀect of organocatalysts grafted to silica support, thermal stability if somewhat lower selectivity in some cases, the catalytic activities of the modified silica gels were compared but recovery from the reaction mixture is challenging. Ionic to unmodified silica gel (Si). To characterise the catalysts, liquids (ILs) oﬀer environmentally friendly alternatives to metal diﬀerent methods were used. The prepared Si-TBD catalyst was catalysts. However, their manufacturing is complex and costly. analysed by scanning electron microscopy with energy disper- The purification processes are cumbersome if the catalysts sive X-ray analysis (SEM-EDX). The thermal stability of all four contain metal, thus causing the procedures to be expensive, modified silica gels was determined by thermogravimetric ana- and sometimes, only low monomer yields can be obtained with lysis combined with diﬀerential scanning calorimetry (TG-DSC). them. Deep eutectic solvents emerge as new alternatives with The reaction conditions, such as reaction temperature, similar characteristics to ILs. However, as a novel method, only catalyst : PET ratio, EG : PET ratio, and reaction time, were opti- a few applications are described in the literature, in which some- mised using fractional factorial experimental design and times high catalyst loading and reagent excess are applied. response surface methodology. During the optimisation study, Organocatalysts are eﬃcient in PET glycolysis with usually good the yield of BHET was determined by high-performance liquid monomer yield; however, this area also needs improvements for chromatography (HPLC), which was compared with the isolated implementation in the industry: as the majority of them are yield in some cases. The recyclability of the two most eﬃcient relatively expensive, and organocatalyst loadings are often much catalysts was investigated in multiple cycles. higher than metal-based catalysts, their recyclability needs to be improved, for example, by grafting them to a solid support. These catalysts are prone to degradation; thus, their thermal 2 Experimental stability must be evaluated. 2.1 Materials There are numerous parameters that influence the solvoly- sis of PET, which can be examined systematically by experi- PET flakes from HUKE Ltd (Sárvár, Hungary) were used for gly- mental design. The advantage of experimental design and its colytic reactions. Post-consumer PET bottles were washed and 8448 | Green Chem., 2022, 24,8447–8459 This journal is © The Royal Society of Chemistry 2022 Open Access Article. Published on 12 October 2022. Downloaded on 10/16/2024 9:02:41 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Green Chemistry Paper 1 13 ground into small flakes (thickness 0.41 ± 0.08 mm; (9.9 ± The main product was identified by H, C nuclear mag- 2.9 mm) × (6.8 ± 2.2 mm)). Modified silica gels (Si-TEA, Si- netic resonance spectroscopy (NMR), high-resolution mass GUA, Si-THU, silica gel functionalized with glycidoxy groups spectrometry (HRMS), and Fourier-transform infrared spec- (Si-GLY)) were purchased from SiliCycle Inc, the unmodified troscopy (FTIR), and the BHET dimer side-product was identi- 1 13 silica gel (Si) and toluene from Merck. Ethylene glycol and fied by H, C NMR, and HRMS. TBD were purchased from Sigma-Aldrich. BHET. Melting point: 109 °C. Spectroscopic data (see Fig. S1–S4 in the ESI†) are fully consistent with those reported 2.2 Preparation of TBD-modified silica gel (Si-TBD) 23 1 in the literature. H NMR (500 MHz, DMSO-d ): δ 8.12 (s, 4H, Si-GLY (5 g, 5.55 mmol glycidyl group), toluene (45 mL), and CH), 4.98 (t, J = 5.7 Hz, 2H, OH), 4.32 (t, J = 4.9 Hz, 4H, O– TBD (927 mg, 6.66 mmol) were added to a two-necked round- CH ), 3.72 (q, J = 5.1 Hz, 4H, CH –OH) ppm. C NMR 2 2 bottomed flask. The reaction mixture was stirred under argon (126 MHz, DMSO-d ): δ 165.2, 133.8, 129.6, 67.1, 59.0 ppm. IR atmosphere and heated under reflux for 10 hours. The solid (KBr): ν 3447, 2964, 2946, 2932, 2880, 1716, 1689, 1505, max product was filtered on a sintered glass filter and washed with 1457, 1412, 1380, 1282, 1252, 1135, 1111, 1074, 1017, 910, 898, −1 + + a mixture of dichloromethane/methanol 5 : 1 (15 mL), followed 875 cm . HRMS (ESI ): m/z [M + Na] calcd for C H O Na: 12 14 6 by a mixture of dichloromethane/methanol/triethylamine 277.0688; found: 277.0689. 5 : 1 : 0.05 (2 × 15 mL), then again with dichloromethane/ BHET dimer. Melting point: 162–166 °C. To the best of our methanol 5 : 1 (2 × 15 mL). The product was dried in a drying knowledge, the spectroscopic data of the dimer has not been oven at 80 °C for 2 hours to give a mass of 5.36 g—the mole- reported so far (see Fig. S5–S6 in the ESI†). H NMR (500 MHz, −1 cular loading of Si-TBD was 0.58 ± 0.09 mmol g , determined DMSO-d ): δ 8.12 (dd, J = 13.5 Hz, and 8.5 Hz, 8H, CH), 4.95 (t, by SEM-EDX elemental analysis. J = 5.7 Hz, 2H, OH), 4.68 (s, 4H, O–CH –CH –O (middle)), 4.31 2 2 (t, J = 5.0 Hz, 4H, O–CH (next to CH –OH)), 3.71 (q, J = 5.1 Hz, 2 2 2.3 General procedure for PET glycolysis 13 4H, CH –OH) ppm. C NMR (126 MHz, DMSO-d ): δ 165.1, 2 6 The catalyst, PET flakes (384 mg, 2 mmol), and ethylene glycol 165.0, 133.9, 133.3, 129.6, 129.5, 67.0, 63.2, 59.0 ppm. HRMS + + were added to a 5 mL sealable vial. The reagent and catalyst (ESI ): m/z [M + Na] calcd for C H NaO : 469.1105; found: 22 22 10 ratios were calculated in relation to the amount of PET repeat- 469.1102. −1 ing unit (MW = 192.2 g mol ). The reactions were carried out BHET yield (non-isolated) was calculated based on HPLC under argon atmosphere, and magnetic stirring was used. A calibration method using an external standard, by fitting a sand bath was used for heating, and the internal temperature line to five data points (see Fig. S11 in the ESI†). The non-iso- (170–195 °C) was monitored by a thermometer placed in a sep- lated yield was also compared with the isolated yield in some arate 5 mL sealed vial filled with ethylene glycol. After the cases. The isolated yield was calculated as appropriate reaction time (1–2 h), the reaction mixture was BHET Yield½ % ¼ 100% ð1Þ allowed to cool to room temperature, and then HPLC samples BHET;0 were prepared to determine the non-isolated BHET yield. A where m and m refer to the actual and theoretical MeCN : H O = 5 : 1 mixture (3 × 2 mL) was added to the reac- 2 BHET BHET,0 mass of BHET, respectively. The theoretical weight of BHET tion mixture and filtered on a sintered glass filter to separate was calculated by the catalyst and any remaining solid oligomers from the desired product. The solid residue was washed with PET;0 m ¼ 254:2 ð2Þ BHET;0 MeCN : H O = 5 : 1 solvent mixture (3 × 3 mL). The filtrate was 192:2 then transferred to a 25 mL volumetric flask and filled with where m is the initial mass of PET; 192.2 and 254.2 are the PET,0 the above-mentioned solvent mixture to 25 mL. For the HPLC molecular weight of the PET repeating unit and BHET, sample, 50 μL of this solution was taken, and 950 μLof respectively. MeCN : H O = 5 : 1 solvent mixture was added. PET conversion was calculated as The BHET was isolated by analogy with the method of Zhang and co-workers. For determining the isolated BHET m m r c;0 Conversion½ % ¼ 1 100% ð3Þ yield, the acetonitrile was removed from the filtrate PET;0 (MeCN : H O = 5 : 1 mixture containing BHET, EG, and BHET where m and m refer to the mass of the solid residue and r c,0 dimer/trimer by-products) under reduced pressure, and the the initial mass of the catalyst, respectively. volume of the remaining aqueous solution was extended by water to 70 mL. Then, the water-insoluble oligomers (dimer 2.4 Catalyst recycling and, in some cases, trimer based on HPLC-MS) were removed by filtration using a filter paper, and the filtrate was evaporated For the catalyst recycling, 5 reaction cycles were carried out to approx. 4 mL under reduced pressure. The resulting solu- with Si-TEA and Si-TBD catalysts under optimal conditions. tion was stored for 12 hours in a refrigerator (4 °C) to give Each cycle was repeated once. To maintain the optimal ratio of BHET as white crystals. These crystals were filtered on a sin- the parameters, the mass of PET and the amount of ethylene tered glass filter and washed with the mother liquor, then with glycol were reduced in proportion to the mass of the recycled water. catalyst. If the reaction did not proceed with complete conver- This journal is © The Royal Society of Chemistry 2022 Green Chem., 2022, 24,8447–8459 | 8449 Open Access Article. Published on 12 October 2022. Downloaded on 10/16/2024 9:02:41 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Paper Green Chemistry sion, the solid residue remaining after filtration was trans- HPLC was performed using a Shimadzu LCMS-2020 device ferred to the next reaction cycle. equipped with a HALO C18 (2.7 µm; 4.6 × 150 mm) column. The samples were eluted with gradient elution, using eluent A 2.5 Scale-up of PET glycolysis process (0.1% HCOOH in H O) and eluent B (MeCN). The flow rate was −1 6-fold scale-up. The catalyst (15.5 mol% Si-TEA or 50 wt% set to 0.8 mL min . The initial condition was 5% eluent B, fol- Si), PET flakes (2.31 g, 12 mmol), and ethylene glycol (12.6 eq.) lowed by a linear gradient to 100% eluent B by 8 min; from 8 were added to a 50 mL round-bottomed flask. The reactions to 13 min, 100% eluent B was retained; and from 13 to were carried out under argon atmosphere, and magnetic stir- 14 min, it went back by a linear gradient to 5% eluent B, ring was used. An oil bath was used for heating. When the which was retained from 14 to 15 min. The column tempera- reaction was completed, the reaction mixture was allowed to ture was kept at 40 °C, and the injection volume was 1 µL. cool to room temperature. A MeCN : H O = 5 : 1 mixture 2.6 Experimental design (15 mL) was added to the reaction mixture and filtered on a sintered glass filter to separate the catalyst and any remaining The optimisation of PET glycolysis was conducted by response solid oligomers from the desired product. The solid residue surface methodology with gradient method. Statistica software was washed with the above-mentioned solvent mixture (3 × (TIBCO Software Inc.) was applied for data analysis at 5% sig- 5 mL). The acetonitrile was removed from the filtrate under nificance level (to calculate the regression model and perform 4-1 reduced pressure, and the volume of the remaining aqueous analysis of variance (ANOVA)). 2 fractional factorial design solution was extended by water to approx. 420 mL. The water- was applied with two centre point experiments, and all experi- insoluble oligomers (dimer and trimer) were removed by fil- ments were repeated once. The eﬀects of four independent tration using a filter paper, and the filtrate was evaporated to variables (i.e., reaction temperature, catalyst : PET ratio in approx. 25 mL under reduced pressure. The resulting solution mol%, EG : PET molar ratio, and reaction time) were investi- was stored for 12 hours in a refrigerator (4 °C) to give BHET as gated. Two dependent variables were chosen as responses, white crystals. These were filtered on a sintered glass filter and BHET yield (Y) and PET conversion. washed with the mother liquor, then with water. 2.7 Characterisation 18-fold scale-up. The catalyst (15.5 mol% Si-TEA), PET flakes (7.00 g, 36.4 mmol), and ethylene glycol (12.6 eq.) were added Infrared spectra were recorded on a Bruker Alpha-T FTIR to a 100 mL flat-bottomed flask. The reaction was carried out spectrometer. Silica gel 60 F (Merck) plates were used for under argon atmosphere, and mechanical stirring (800 rpm) thin-layer chromatography (TLC). Ratios of solvents are given −1 was used. An oil bath was used for heating. The reaction was in volumes (mL mL ). Melting points were measured in a completed after 5 hours, and the reaction mixture was allowed Boetius micro-melting point apparatus, and were uncorrected. to cool to room temperature, and then an HPLC sample was NMR spectra were recorded on a Bruker Avance 500 MHz 1 13 prepared to determine the non-isolated BHET yield. A spectrometer (500.13 MHz for H, 125.76 MHz for C) in MeCN : H O = 5 : 1 mixture (20 mL) was added to the reaction DMSO-d and were referenced to residual solvent proton 2 6 mixture and filtered on a sintered glass filter to separate the signals (δ = 2.50) and solvent carbon signals (δ = 39.51). All H C catalyst and any remaining solid oligomers from the desired chemical shifts are reported in parts per million (ppm). product. The solid residue was washed with the above-men- Coupling constants (J) are given in Hz. tioned solvent mixture (6 × 20 mL). The filtrate was then trans- HPLC-MS was performed using a Shimadzu LCMS-2020 ferred to a 500 mL volumetric flask and filled with the above- device, equipped with a Reprospher 100 C18 (5 µm; 100 × mentioned solvent mixture to 500 mL. For the HPLC sample, 3 mm) column and a positive/negative double ion source 60 μL of this solution was taken, and 940 μL of MeCN : H O= (DUIS±) with a quadrupole MS analyser in a range of 50–1000 5 : 1 solvent mixture was added. m/z. The samples were eluted with gradient elution, using For determining the isolated BHET yield, the acetonitrile eluent A (0.1% HCOOH in H O) and eluent B (0.1% HCOOH −1 was removed from the filtrate (MeCN : H O = 5 : 1 mixture con- in MeCN). The flow rate was set to 1.5 mL min . The initial taining BHET, EG, and by-products) under reduced pressure, condition was 5% eluent B, followed by a linear gradient to and the volume of the remaining aqueous solution was 100% eluent B by 1.5 min; from 1.5 to 4.0 min, 100% eluent B extended by water to 1.3 L. Then, the water-insoluble oligo- was retained; and from 4 to 4.5 min, it went back by a linear mers (dimer and trimer) and smaller-sized catalyst fraction gradient to 5% eluent B, which was retained from 4.5 to 5 min. (see Section 3.5) were removed by filtration using a filter paper The column temperature was kept at 30 °C, and the injection (the solid residue on the filter paper was washed with a volume was 1 µL. The purity of the compounds was assessed methanol : ethyl acetate = 1 : 1 mixture to recover the smaller- by HPLC with UV detection at 215 and 254 nm. High-resolu- sized catalyst fraction by dissolving the oligomers), and the fil- tion mass spectrometry was measured on a Q-TOF Premier trate was evaporated to approx. 53 mL under reduced pressure. mass spectrometer for BHET, and on a Bruker MicroTOF II The resulting solution was stored for 12 hours in a refrigerator instrument for the BHET dimer. The ionisation method was (4 °C) to give BHET as white crystals. These crystals were fil- electrospray ionisation (ESI) operated in positive ion mode. tered on a sintered glass filter and washed with the mother The stability of the catalysts was investigated by thermo- liquor, then with water. gravimetric analysis and diﬀerential scanning calorimetry 8450 | Green Chem., 2022, 24,8447–8459 This journal is © The Royal Society of Chemistry 2022 Open Access Article. Published on 12 October 2022. Downloaded on 10/16/2024 9:02:41 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Green Chemistry Paper (TG-DSC). A PerkinElmer STA 6000 instrument was used. In addition to the synthesised Si-TBD, three commercially During the measurement, the samples of about 30 mg were available functionalized silica gels, Si-GUA, Si-THU, or Si-TEA, −1 heated from 25 °C to 190 °C at a heating rate of 10 °C min were investigated (Fig. 1). The physical properties (particle and then held at the latter temperature for 360 min. size, specific surface area, molecular loading) of the commer- Si-TBD was analysed using a JEOL JSM-5500LV scanning cially available functionalized silica gels, which were given by electron microscope (SEM) at high vacuum with 10–20 kV the manufacturer, SiliCycle Inc, can be found in Table S1 in accelerating voltage. The elemental analysis of Si-TBD was the ESI.† To find the catalyst with the highest catalytic activity carried out with energy dispersive X-ray analysis (EDX with Si and recyclability while preserving the catalytic activity among (Li) detector) applying 10 kV accelerating voltage and sampling the modified silica gels tested, we investigated their thermal time of 40 s. stability by thermogravimetric analysis and diﬀerential scan- The physical properties of the commercially available func- ning calorimetry (TG-DSC), being held at 190 °C (the tempera- tionalized silica gels (particle size – measured by laser diﬀrac- ture which is most commonly used in PET glycolysis) for tion, specific surface area – measured by nitrogen adsorption/ 360 min. From the TG curves, which are similar for each tested desorption method at 77 K, molecular loading – measured by silica gel (see Fig. S7–S10 in the ESI†), the mass loss of each elemental analysis, or titration in the case of Si-GLY), which sample was determined. For each of the modified silica gels, a were given by the manufacturer, SiliCycle Inc, can be found in theoretical mass loss was also calculated based on the mole- Table S1 in the ESI.† cular loading, assuming that all the organic units are elimi- HS-GC-MS was performed using a Shimadzu GCMS-QP2010 nated from the modified silica gel. The theoretical and device equipped with an Rtx-5 (30 m × 0.32 mm × 0.25 µm) measured mass losses of the catalysts are summarised in column and an AOC-5000 auto-injector. The oven temperature Table 1. Since Si-TEA showed the lowest mass loss in the was initially held at 40 °C for 2 minutes, increased to 320 °C at TG-DSC measurement, it is assumed to have the highest −1 10 °C min with the final temperature held for 5 minutes. thermal stability. Si-TBD also showed a much lower mass loss The sample holder was thermostated for 45 minutes at 190 °C than the theoretical maximum. before the measurement. The carrier gas was argon, the linear During our experiments, the reactions are conducted in −1 flow rate on the column was set to 50 cm s , and the split inert atmosphere. Otherwise, the tertiary amines in the cata- ratio was 1 : 10. The injector was held at 300 °C. The interface lysts could react with oxygen to give N-oxides, and then the and the ion source temperature were set at 250 °C, the syringe Cope elimination of N-oxides would produce alkenes. There temperature was 150 °C, and the detector voltage was 1.3 kV. was no significant degradation of the catalysts observed. The Taking the molecular weights of the expected molecules into thermal stability of silica gel-supported organocatalysts was consideration, the mass scale was adjusted between mass to reported earlier. charge ratio (m/z)of 29–600. The morphology of the Si-TBD catalyst was investigated by SEM. Comparing its SEM image (Fig. 2(a)) with the starting 3 Results and discussion 3.1 Synthesis and characterisation of catalysts The Si-TBD was prepared from Si-GLY by reacting with TBD in toluene (Scheme 1). The Si-GLY used as starting material has a 2 −1 particle size of 40–63 μm, a specific surface area of 494 m g , and an active loading (the molecular loading measured by −1 titration by the manufacturer, SiliCycle Inc) of 1.11 mmol g . Then, leaching of TBD from the catalyst was investigated: the product was stirred in a solvent mixture (dichloromethane/ methanol 5 : 1) in which TBD was highly soluble, filtered, and washed. Analysis of the filtrate by HPLC-MS showed that less than 5% of the TBD on the catalyst surface (based on mole- Fig. 1 The applied, commercially available functionalized silica gels. cular loading) leached oﬀ from the catalyst. Scheme 1 Preparation of TBD-functionalized silica gel. This journal is © The Royal Society of Chemistry 2022 Green Chem., 2022, 24,8447–8459 | 8451 Open Access Article. Published on 12 October 2022. Downloaded on 10/16/2024 9:02:41 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Paper Green Chemistry Table 1 Theoretical and measured mass losses of the catalysts probably does not hinder the catalytic activity greatly because measured by TG-DSC (190 °C, 6 h) of the latter findings. Thus, we wanted to screen other silica gel-grafted nitrogen-based organocatalysts also, that can be Mass loss (%) easily accessed commercially, so we chose based on literature analogies: Si-TEA based on triethylamine, Si-THU based on Modified Theoretical Measured/theoretical silica gels maximum Measured maximum ratio (%) urea and another guanidine-based catalyst presumably similar to Si-TBD, Si-GUA. Si-GUA 16 12 75 Si-TEA 10 2 25 The catalytic activities of the commercially available modi- Si-THU 11 8 73 fied silica gels (Si-GUA, Si-TEA, Si-THU, with a particle size of Si-TBD 15 8 53 40–63 μm) and the newly synthesised Si-TBD were investigated in PET glycolysis (Scheme 2), and the results are shown in Fig. 3. In addition to the modified silica gels, unmodified silica gel (Si, also with a particle size of 40–63 μm) was also tested, and reactions without catalyst were also performed. The depolymerisation rate was low without catalyst (9% con- version), while it was slightly higher (34% conversion, 26% BHET yield) with Si. Thus, even the silica support has catalytic activity, but much lower than those of the modified silica gels: the three modified silica gels containing basic units (Si-TBD, Si-GUA, Si-TEA) showed excellent catalytic activities and selec- Fig. 2 SEM images of Si-TBD (a) and Si-GLY (b). tivities (96–100% PET conversions, 83–92% BHET yields) with relatively low standard deviation. Applying the Si-THU catalyst, high deviation was observed supposedly because of the cata- material, Si-GLY (Fig. 2(b)), both modified silica gels have irre- lyst’s instability at high temperature. As the aim was to recycle gular particle shapes with similar particle sizes. Therefore, it the catalyst in as many cycles as possible, Si-TEA was chosen can be assumed that the particle size distribution did not as the best catalyst for further reactions because of its relatively change during the modification of Si-GLY (40–63 µm, high catalytic activity and good thermal stability based on the measured by laser diﬀraction by SiliCycle Inc for Si-GLY). The TG-DSC measurements (see Table 1). molecular loading of Si-TBD was determined by SEM-EDX ana- To confirm the advantages of the Si-TEA modified silica gel lysis based on nitrogen content. Si-GLY was used as back- catalyst, triethylamine (TEA) was also applied as a catalyst. TEA ground for the calculation. should only be used in a closed reaction vessel; otherwise, it will evaporate from the reaction mixture. The latter catalyst 3.2 Catalyst screening also gave an excellent non-isolated yield (91%), but the remain- TBD is a bifunctional catalyst that can activate both an ester ing triethylamine traces in the reaction mixture probably hin- and alcohol through hydrogen bonding. Numerous nitrogen- dered the crystallization of BHET, as a poorer isolated yield based organocatalysts were screened for the depolymerisation (69%) was obtained than with the heterogeneous catalysts. To 25,47 of PET earlier in the literature. The catalytic activity was demonstrate the diﬀerence, in one case when Si-TEA was used, found to correlate with the basicity, but in the presence of a non-isolated yield of 90% was observed with an isolated yield short-chain diols such as ethylene glycol, which serve as a of 85%. cocatalyst for activating the ester carbonyl groups via hydrogen 3.3 Regression model based on experimental design bonding, the bifunctionality of TBD is less important, especially in the presence of excess ethylene glycol. The syn- The eﬀects of four independent variables (i.e., reaction temp- thesised catalyst, Si-TBD loses the bifunctionality because of erature, catalyst : PET ratio in mol%, EG : PET molar ratio, and the grafting onto silica support (and thus, its basicity reaction time) were investigated as quantitative factors most decreases – based on the reported pK of the conjugated acid frequently studied in the literature on glycolysis of PET. The of methyl-TBD ), but as we use excess ethylene glycol, this aim was to set up a simple model with relatively few experi- Scheme 2 PET glycolysis with EG using diﬀerent catalysts. 8452 | Green Chem., 2022, 24,8447–8459 This journal is © The Royal Society of Chemistry 2022 Open Access Article. Published on 12 October 2022. Downloaded on 10/16/2024 9:02:41 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Green Chemistry Paper cance of its eﬀect on the model response. Thus, all the main eﬀects and the B by C interaction eﬀect were found to be sig- nificant. The Pareto charts of standardised eﬀects in the case of BHET yield or PET conversion as the dependent variable can be found in Fig. S16 and S17 in the ESI.† After model reduction by removing the statistically insignif- icant factors, a new linear regression model was fitted to the experimental data, and it proved to be adequate based on the 2 2 curvature check with a high value of R (0.969) and adjusted R (0.955). Analysis of variance for the reduced linear regression model can be seen in Table S3 in the ESI.† Comparing the yields estimated from the model and the observed ones, it was Fig. 3 The catalytic activities of diﬀerent catalysts in PET glycolysis. found that the measured points are approximately along a line Reaction conditions: 190 °C, 10 mol% (or 40 wt% in the case of Si) cata- of slope 1 through the origin (Fig. 4), therefore the regression lyst (if used), EG : PET ratio of 16, 2 h. BHET yield was determined by model was adequate for predicting the BHET yield in the HPLC. design space, especially at high yields. The homogeneity of var- iances was checked comparing the predicted and residual values of BHET yield (see Fig. S18 in the ESI†), and using a 4-1 ments, so a 2 experimental design was developed to describe Cochran’s C test (see ESI†). Based on the latter, the assump- the variation of the dependent variable, non-isolated BHET tion that the random error variance is constant can be yield, as a function of the independent parameters using a accepted. Taking this into account, the reduced model can be linear regression model. Si-TEA catalyst was used during the used to determine the optimal conditions. experiments because of its highest thermal stability among the The reduced linear regression model representing the investigated modified silica gels and appropriate activity. The relationship between the BHET yield response Y and the coded lower and upper levels of the parameters were chosen based values of the four independent factors was obtained as on preliminary experiments by changing the parameters one by one (see Fig. S13–S15 in the ESI†) to obtain an approxi- Y ¼ 39:8 þ 30:6A þ 12:7B 11:9C þ 13:4D þ 4:2BC ð4Þ mately linear relationship between the dependent variable and Based on this model, the temperature (A) has the strongest the parameters in the chosen parameter ranges. Their coded eﬀect on the BHET yield, while the interaction of catalyst ratio and uncoded values are shown in Table 2. The experimental and EG ratio (BC) has low significance. design matrix and the observed BHET yields and PET conver- The interaction between catalyst : PET ratio and EG : PET sions from 20 experiments are presented in Table 3. ratio at a constant temperature of 190 °C and with a reaction In the fractional design, the triple interaction between time of 2 h was plotted in Fig. 5 on a response surface plot (a temperature, catalyst : PET ratio and EG : PET ratio is con- function of two factors with the others at constant values). At a founded with the fourth factor (reaction time). In this case, the fixed catalyst ratio, the yield decreases with increasing EG ratio main eﬀects are not mixed with each other or with two-factor in this investigating range. It should be noted, however, that in interactions, while the two-factor interactions are confounding our preliminary experiments, we found that the yield has a with each other (A by B with C by D; A by C with B by D; A by D maximum at a certain EG ratio value. For this, the explanation with B by C). Since out of the interactions, only A by D (and its could be that the transformation of dimer into monomer is a linear combination, B by C) were found to be significant, the B reversible process. Therefore, prolonging the reaction after by C interaction was chosen as the only significant interaction reaching equilibrium can cause a backward shift, increasing based on practical considerations (B by C interaction is the the amount of dimer at the expense of the BHET monomer. interaction of catalyst quantity and EG : PET ratio, which influ- In this case, the higher ratio of EG presumably shifted the ences the catalyst concentration in the reaction mixture). The equilibrium faster towards the dimer. In the design of experi- results of ANOVA for the regression model are shown in ments, however, the factor values were chosen so that there Table 4. A factor is considered significant if its p-value is less was an approximately linear relationship between yield and EG than 0.05, and the higher the F-value, the greater the signifi- ratio. On the surface plot, the interaction can be observed that at a lower catalyst ratio, the EG ratio has a greater negative Table 2 Coded and uncoded levels of independent variables eﬀect on yield than at a higher catalyst ratio, and similarly, at a higher EG ratio, the catalyst ratio has a greater positive eﬀect Coded level on yield than at a lower EG ratio. Independent variables Factor −10 +1 3.4 Optimisation of reaction conditions Temperature (°C) A 170 180 190 Catalyst : PET ratio (mol%) B 5 12.5 20 The optimisation was implemented using response surface EG : PET molar ratio (−) C 11 13.5 16 Reaction time (h) D 1 1.5 2 methodology with gradient method. Accordingly, with the step This journal is © The Royal Society of Chemistry 2022 Green Chem., 2022, 24,8447–8459 | 8453 Open Access Article. Published on 12 October 2022. Downloaded on 10/16/2024 9:02:41 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Paper Green Chemistry Table 3 Experimental design matrix and observed BHET yield and PET conversion Run Temperature (°C) Catalyst : PET ratio (mol%) EG : PET molar ratio (−) Reaction time (h) Conversion (%) BHET yield (%) 1 170 5.0 11.0 1.0 1 0 2 190 5.0 11.0 2.0 100 88 3 170 20.0 11.0 2.0 51 45 4 190 20.0 11.0 1.0 94 80 5 170 5.0 16.0 2.0 1 0 6 190 5.0 16.0 1.0 34 28 7 170 20.0 16.0 1.0 1 0 8 190 20.0 16.0 2.0 100 90 9 (C) 180 12.5 13.5 1.5 38 31 10 (C) 180 12.5 13.5 1.5 58 51 11 170 5.0 11.0 1.0 5 0 12 190 5.0 11.0 2.0 97 85 13 170 20.0 11.0 2.0 37 29 14 190 20.0 11.0 1.0 100 87 15 170 5.0 16.0 2.0 7 0 16 190 5.0 16.0 1.0 21 16 17 170 20.0 16.0 1.0 8 0 18 190 20.0 16.0 2.0 99 89 19 (C) 180 12.5 13.5 1.5 46 41 20 (C) 180 12.5 13.5 1.5 45 40 C: centre point. Table 4 Analysis of variance for linear regression model Factor Sum of squares DF Mean square F-Value p-Value Significant Curvature 2.81 1 2.81 0.07 0.7935 No (A) Temperature (°C) 14 945.06 1 14 945.06 382.09 <0.0001 Yes (B) Catalyst : PET ratio (mol%) 2575.56 1 2575.56 65.85 <0.0001 Yes (C) EG : PET ratio (−) 2280.06 1 2280.06 58.29 <0.0001 Yes (D) Reaction time (h) 2889.06 1 2889.06 73.86 <0.0001 Yes A by B 189.06 1 189.06 4.83 0.0502 No A by C 115.56 1 115.56 2.95 0.1136 No B by C 280.56 1 280.56 7.17 0.0215 Yes Error 430.25 11 39.11 Total sum of squares23 708.0019 DF: degree of freedom. Fig. 4 Comparison of observed and predicted BHET yields. intervals chosen adequately, steps are taken from the centre point of the design space towards the local maximum along the gradient of the reduced model because this is the direction of steepest ascent. The temperature step interval was chosen Fig. 5 Response surface plot for the interaction eﬀect between as the determining one (since this is the most diﬃcult to set catalyst : PET ratio and EG : PET ratio at a constant temperature of 190 °C to a certain value), which is 2.5 °C in all cases. The step inter- and with a reaction time of 2 h. 8454 | Green Chem., 2022, 24,8447–8459 This journal is © The Royal Society of Chemistry 2022 Open Access Article. Published on 12 October 2022. Downloaded on 10/16/2024 9:02:41 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Green Chemistry Paper vals for the other parameters can be calculated from this. The actual parameter value at a given point was obtained by adding the step interval to the parameter value in the previous step, the starting point being the centre point (experiment point 0), the value of which is known from the experimental design. The calculated step intervals and coded factor values at each point are shown in Table S4.† The parameter settings for the diﬀerent optimisation points are presented in Table 5, and the estimated and measured yields are shown in Fig. 6. Strictly speaking, the reduced model is only valid within the limits of the experimental design, while outside the design space, there may be a larger diﬀerence between the estimated and observed yields. For this reason, the optimisation experi- ments outside the design space (experiment points 5 and 6, Fig. 6 Observed and predicted yields in the optimisation experiments. Fig. 6) were conducted first. The BHET yield measured at point 5 is 9% lower than the estimated value (87.0% instead of 96.0%, Fig. 6), and at point 6, the yield is 8% lower (79.0%) straightforward, the use of these catalysts can be economical, than at point 5, so the yield decreases outside the design even if they have high prices. space. Returning to the limits of the experimental design, we After determining the optimal reaction conditions, we performed the reaction at point 4, during which we measured aimed to test the recyclability of Si-TEA, which has the highest high BHET yield (88.5%). This was accepted as the optimal thermal stability among the modified silica gels, and Si-TBD, setting because the model is valid inside the design space, so too, because this catalyst has the highest catalytic activity and experiments in points 1–3 are not needed to be conducted. a fairly good thermal stability. During the recycling experi- Thus, the most favourable reaction conditions were 190 °C, ments, catalyst mass decreased from cycle to cycle (see 15.5 mol% Si-TEA catalyst, EG : PET molar ratio of 12.6, and Table S5†), in the case of Si-TEA, by 6.5 ± 3.3%, and in the 1.7 h reaction time in point 4. case of Si-TBD, by 7.0 ± 3.2% per cycle compared to the orig- Comparing the results achieved under these optimal con- inal catalyst mass. Thus, to maintain the optimal ratio of the ditions with literature ones also attained by experimental parameters, the mass of PET and the amount of ethylene design (Table 6), excellent PET conversion (100%), BHET yield glycol were reduced in proportion to the mass of the recycled and selectivity (88.5%) were achieved with an easily recyclable, catalyst. The Si-TEA catalyst showed >89% BHET yield in the heterogeneous organocatalyst instead of inorganic salt cata- first four cycles of repeated PET glycolysis reactions, which lysts with costly recycling. This was achieved by performing slightly decreased (73%) in the fifth cycle (Fig. 7(a)). The Si- relatively few experiments but by testing four factors using a TBD catalyst resulted in only 78% BHET yield in the first cycle simple linear model. It can be observed that compared to in- but >91% in the next 3, while this catalyst also showed a organic metal salts, organocatalysts require a high catalyst slightly lower yield (79%) in the fifth cycle (Fig. 7(b)). The loading, but this can be acceptable by recycling the catalyst latter experience may be explained by stirring problems due to (see later in Section 3.5). the reduction in reaction size (because of the catalyst loss) during recycling. The slightly lower yield in the first cycle in 3.5 Catalyst recycling the case of Si-TBD could be attributed to the lack of end- The immobilisation of various organic compounds on silica capping of the catalyst, so for larger scales, it is worth consid- gels is well known in the literature, so the best-performing ering end-capping. Taking all five reaction cycles into account, catalyst could be produced on a larger scale more cheaply if the cumulative BHET yields are 89% (1.74 g) for Si-TEA and required. In any case, if the catalyst recycling is successful and 88% (1.70 g) for Si-TBD. Consequently, there was no signifi- Table 5 Parameter settings for the optimisation points * * * Experiment point Temperature (°C) Catalyst : PET ratio (mol%) EG : PET molar ratio (–) Reaction time (h) 0 180.0 12.50 12.50 1.50 1 182.5 13.28 12.26 1.55 2 185.0 14.03 12.02 1.61 3 187.5 14.76 11.80 1.66 4 190.0 15.47 11.58 1.72 5 192.5 16.15 11.37 1.77 6 195.0 16.81 11.17 1.83 Given to two decimal places to indicate monotonic growth of values. This journal is © The Royal Society of Chemistry 2022 Green Chem., 2022, 24,8447–8459 | 8455 Open Access Article. Published on 12 October 2022. Downloaded on 10/16/2024 9:02:41 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Paper Green Chemistry Table 6 Comparison of PET glycolysis methods using experimental design. The optimal reaction conditions are indicated, and the parameter ranges investigated are given in parentheses Temperature Reaction Catalyst : PET molar EG : PET molar Conversion Yield Ref. Catalyst Design (°C) time (h) ratio (%) ratio (−) (%) (%) Ref. 36 Co(OAc) 2 190 (130–190) 2.0 (0.5–2) 1.0 (0–1) 6.2 (fixed) 100 n.d. Ref. 37 Mn(OAc) 2 190 (130–190) 1.5 (0.5–2) 0.5 (0.05–0.5) 6.2 (fixed) 100 n.d. Ref. 38 Zn(OAc) Taguchi L 208 (195–220) 2.5 (2.5–3.5) 0.2 (0.2–1) 18.6 (6.2–18.6) n.d. 85 2 9 Ref. 41 Zn(OAc) Taguchi L 250 (MW) (fixed) 0.5 (fixed) 1.0 (1–4) 9.3 (3.1–15.5) n.d. 65 2 20 Ref. 40 NaHCO 4 factor Box–Behnken design 192 (192–200) 3.0 (3–5) 1.9 (1.4–2.3) 18.8 (7.7–23.2) n.d. 75.7 4-1 This work Si-TEA 2 190 (170–195) 1.7 (1–2) 15.5 (5–20) 12.6 (11–16) 100 88.5 Fig. 7 Catalyst recycling in PET glycolysis applying Si-TEA (a) and Si-TBD (b) catalysts. cant catalytic activity loss during the catalyst recycling ate from the irregular shaped silica gel at high temperature experiments. while stirring in the polar ethylene glycol. These particles go Stabilities of Si-TEA and Si-TBD catalysts were further inves- through the G3 porosity glass filter, thus causing the catalyst tigated by HS-GC-MS, and only a small amount of amines were mass loss. To examine this, we recovered some of the lost cata- detected as decomposition products only in the case of Si-TEA. lyst when a large amount of water was added to the filtrate (see After a 4-hour glycolysis reaction under optimised conditions, Section 2.3), and the precipitate was found to contain Si-TEA. we detected a very low amount of diﬀerent amines (triethyl- A catalyst recycling experiment was conducted with this recov- amine, diethylmethylamine, diethylamine) compared to a tri- ered smaller-sized Si-TEA, which remained active (64% non- ethylamine reference, which contained 5% of the TEA amount isolated BHET yield in the case of 5 mol% Si-TEA). It should grafted onto the silica gel (Fig. S19 and 20†). Their peak areas be noted, however, that the lost catalyst was diﬃcult to recover can be seen in Table S6† (those fragments of the detected com- and still contained BHET dimer after washing with a metha- pounds were depicted on the chromatogram, which had nol/ethyl acetate mixture. Consequently, future studies should approximately the same intensities considering the mass focus on examining the stability of spherical silica gels and spectra of the amines; thus, it gives a relative quantitative esti- other types of catalyst carriers. mation for the decomposition products). Besides amines, Based on the work of Thielemans and co-workers, the acetone and methyl 1,3-dioxolane (product of the reaction of environmental energy impact (ξ, eqn (5)) value equals the quo- ethylene glycol and acetaldehyde – the latter is a by-product of tient of the environmental factor (E ) and the energy factor PET degradation reactions ) were detected. In the case of Si- economy factor (ε). This green chemistry metric has allowed TBD, no decomposition products were detected by HS-GC-MS. for quantitative comparison between diﬀerent studies and for All reaction mixtures were also analysed by HPLC-MS, and no determining their relative feasibility. The lower the environ- decomposition products were detected in the case of Si-TEA. mental energy impact, the more favourable the technology is, However, in the case of Si-TBD, trace amounts of TBD were since the E is equal to the mass of waste divided by the factor detected, but only by the first application of the catalyst. After mass of product, and the energy economy factor is equal to catalyst recycling, it could not be detected. the yield divided by the temperature times the reaction time Thus, we assume that the reason for the slight catalyst (see Section 8 in the ESI†). Comparing Si-TEA and Si-TBD to mass loss (see Table S5†) is that smaller-sized particles separ- other organocatalysts (Table 7, and Table S7† in more detail), 8456 | Green Chem., 2022, 24,8447–8459 This journal is © The Royal Society of Chemistry 2022 Open Access Article. Published on 12 October 2022. Downloaded on 10/16/2024 9:02:41 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Green Chemistry Paper Table 7 Comparing the environmental energy impact, E and factor colysis of PET was further investigated. Silica gel was used as a energy economy factor of Si-TEA and Si-TBD to organocatalysts applied catalyst in a 6-fold scale-up reaction at 50 wt%. The reaction in PET glycolysis was carried out at 190 °C, using an EG : PET ratio of 12.6 in 10 h reaction time since the PET particles were broken down −1 −1 Ref. Catalyst ξ (°C min) E (−) ε (°C min ) factor in the reaction mixture during this time. Under these con- 9 −5 Ref. 25 TBD 24938 0.4875 1.955 × 10 ditions, complete conversion and 72% BHET yield were 9 −5 Ref. 23 TBD : MSA 12911 0.5440 4.213 × 10 −5 obtained, i.e., the silica gel could be successfully used as a This work Si-TEA 7585 0.3483 4.592 × 10 −5 Si-TBD 7758 0.3523 4.541 × 10 catalyst for the process. Nevertheless, the reaction took signifi- cantly more time, showing that silica gels functionalized with MSA: methanesulfonic acid. organic moieties have a much higher catalytic activity, and thus their application on an industrial scale may be more advantageous in terms of energy savings. their environmental energy impacts, environmental factors, To prove the feasibility of scaling-up by using mechanical and energy economy factors are slightly better, so these cata- stirring, an 18-fold scale-up experiment was performed with lysts can be feasible alternative catalysts in PET glycolysis. The mechanical stirring for 5 hours. The glycolysis reaction gave cumulative BHET yields (for five reaction cycles) were applied full conversion, 83% HPLC yield of BHET, and 72% isolated in the calculations, and the catalysts were not included in the BHET yield by crystallization. The isolated BHET yield could waste mass because of their recyclability (at a certain reaction be further increased by 7% (79% in sum) by column chromato- cycle number, the catalyst mass can be neglected) (see ESI†), graphy. Then, further side-products besides dimer of BHET as also reported in the work of Thielemans. were identified by HPLC-MS: BHET trimer, 2-hydroxyethyl ter- factor ephthalic acid (in a higher proportion than the rest), and 2-(2- ξ ¼ ð°C minÞð5Þ hydroxyethoxy)ethyl (2-hydroxyethyl) terephthalate (Fig. S12 and Table S2†). As only one experiment was performed, which While most heterogeneous catalysts applied in PET glycoly- was stopped based on the disappearance of PET flakes, the sis do not often maintain high BHET selectivity and require reaction time of the scaled-up reaction needs to be further higher temperatures than most other catalysts, in this work, optimised in the future. high selectivity was maintained in 5 reaction cycles at only These findings serve as preliminary studies for future scale- 190 °C. In the future, catalyst recycling in more cycles will be up optimisation. Glycolytic PET depolymerisation is already performed on a larger scale. applied on pilot and even commercial scales, however, these processes have limitations: using catalysts is not preferred 3.6 Preliminary scale-up of PET glycolysis process because of the issues associated with their often diﬃcult recov- To examine the achievable yield and how much longer it takes ery and high cost, and in the case of mixed PET waste pro- to depolymerise PET on a larger scale under the previously cesses, the removal of pigments and dyes is problematic. In optimised conditions at small scale, a 6-fold scale-up was our work, catalyst recovery is a crucial aspect, and by using applied using Si-TEA catalyst at 190 °C, and the catalyst : PET solid-supported organocatalysts, their reuse is made more ratio and the EG : PET ratio were adjusted to the optimal straightforward as they can be recycled by simple filtration. An values as previously determined (see detailed description of issue is the catalyst loss, which will be addressed in the future, the scale-up in Section 2.5). The reaction time was increased to and investigating the depolymerisation of mixed PET waste 2.5 h, as the reaction mixture still visibly contained PET par- streams is also planned. Later, the nature of the support can ticles after 1.7 h, which was the optimal time for the smaller also be optimised to address, e.g., issues of the catalyst recov- size. Full conversion was achieved, and the preparative BHET ery or the cost of the catalyst. The recovery of ethylene glycol 23,44 yield was 74% (meaning that the BHET dimer was still present was reported in earlier studies, this will be also investi- in the reaction mixture after the reaction time of 2.5 h). The gated in the future scale-up optimisation. slightly lower preparative BHET yield could probably be attrib- uted to the non-optimal reaction time because of the sub- optimal stirring on the larger scale, suggesting that further 4. Conclusions optimisation of the scale-up reaction time or stirring speed is required in order to obtain higher monomer selectivity and In this work, three commercially available functionalized silica yield. Although this reaction size is still far from the industrial gels and a newly synthesised one, namely solid-supported scale, the results suggest that the method may be suitable for TBD, were tested as heterogeneous organocatalysts in PET gly- the degradation of PET waste on larger scale. colysis. Si-TEA was found to have the highest thermal stability Using the unmodified silica gel (Si) as a catalyst, 34% PET based on TG-DSC and good catalytic activity, while Si-TBD had conversion and 26% non-isolated BHET yield were achieved in the highest activity. The eﬀects of four parameters were investi- 2 h reaction time during the catalyst screening experiments gated by experimental design, and a simple linear model was (see Fig. 3). Since silica gel is cheaper and thermally more set up to represent the relationship between the BHET yield stable than functionalized silica gels, its application in the gly- and the coded values of the four independent factors. The This journal is © The Royal Society of Chemistry 2022 Green Chem., 2022, 24, 8447–8459 | 8457 Open Access Article. Published on 12 October 2022. Downloaded on 10/16/2024 9:02:41 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Paper Green Chemistry reaction temperature had the highest eﬀect on BHET yield at Student Scholarship of Gedeon Richter’s Centenary 170–190 °C. The optimisation of PET glycolysis was Foundation (J. Kiss). The authors are grateful to Diana Daicu implemented using response surface methodology with gradi- from Budapest University of Technology and Economics ent method applying the most stable catalyst, Si-TEA. The (BUTE) for her assistance with the preliminary experiments optimal reaction conditions are 190 °C, 15.5 mol% Si-TEA cata- related to PET glycolysis, and to József Balla and Judit Mátyási lyst, EG : PET molar ratio of 12.6, and 1.7 h reaction time. This from BUTE for the HS-GC-MS measurements. method compared favourably with other methods applying experimental design in PET glycolysis optimisation. The best catalysts, Si-TEA and Si-TBD can be easily recycled by filtration while preserving high BHET yields in five reaction cycles. The References cumulative yields were similarly high for both catalysts (89% for Si-TEA and 88% for Si-TBD), and the methods applying 1 R. Geyer, J. R. Jambeck and K. L. Law, Sci. Adv., 2017, 3, these catalysts have low environmental energy impact, which e1700782. can be a revealing indicator of sustainability. Consequently, 2PlasticsEurope, Plastics—the facts 2019. An analysis of both catalysts can be good alternatives to organocatalysts or European plastics production, demand and waste data, https:// other heterogeneous catalysts to be applied in PET glycolysis plasticseurope.org/wp-content/uploads/2021/10/2019-Plastics- with high yield and selectivity. This work serves as a “proof of the-facts.pdf. concept” for using solid-supported organocatalysts in PET gly- 3 R. L. Smith, S. Takkellapati and R. C. Riegerix, ACS colysis. A 6-fold and an 18-fold scale-up (the latter by mechani- Sustainable Chem. Eng., 2022, 10, 2084–2096. cal stirring) were applied using Si-TEA catalyst, which showed 4 G. P. Karayannidis and D. S. Achilias, Macromol. 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Arias and W. Thielemans, Green Chem., tion, formal analysis, and writing – review & editing; Péter 2021, 23, 3765–3789. Huszthy: funding acquisition, and writing – review & editing; 10 R. López-Fonseca, I. Duque-Ingunza, B. de Rivas, L. Flores- Levente Kárpáti: conceptualisation, and writing – review & Giraldo and J. I. Gutiérrez-Ortiz, Chem. Eng. J., 2011, 168, editing; József Kupai: conceptualization, supervision, funding 312–320. acquisition, project administration, and writing – review & 11 M. Khoonkari, A. H. Haghighi, Y. Sefidbakht, K. Shekoohi editing. and A. Ghaderian, Int. J. Polym. Sci., 2015, 124524. 12 K. R. Delle Chiaie, F. R. McMahon, E. J. Williams, M. J. Price and A. P. Dove, Polym. Chem., 2020, 11, 1450– Conﬂicts of interest 13 R. Esquer and J. J. García, J. Organomet. Chem., 2019, 902, There are no conflicts to declare. 14 C. N. Hoang, T. T. N. Le and Q. D. Hoang, Polym. Bull., 2019, 76,23–34. Acknowledgements 15 L. Kárpáti, F. Fogarassy, D. Kovácsik and V. Vargha, This research was funded by the New National Excellence J. Polym. Environ., 2019, 27, 2167–2181. Program of the Ministry of Human Capacities, grant numbers 16 J. T. Du, Q. Sun, X. F. Zeng, D. Wang, J. X. Wang and ÚNKP-22-2-I-BME-146 (J. Kiss), ÚNKP-21-3-I-BME-311 (Z. F.), J. F. Chen, Chem. Eng. Sci., 2020, 220, 115642. and ÚNKP-20-5-BME-322 (J. Kupai), and the János Bolyai 17 M. R. Nabid, Y. Bide and M. Jafari, Polym. Degrad. Stab., Research Scholarship of the Hungarian Academy of Science (J. 2019, 169, 108962. Kupai). It was also supported by the National Research, 18 L. Bartolome, M. Imran, K. G. Lee, A. Sangalang, J. K. Ahn Development, and Innovation Oﬃce (grant numbers FK138037 and D. H. Kim, Green Chem., 2014, 16, 279–286. (J. Kupai) and K128473 (P. H.)), the Richter Gedeon Excellence 19 M. Imran, D. H. Kim, W. A. Al-Masry, A. Mahmood, PhD Scholarship of Gedeon Richter Talentum Foundation A. Hassan, S. Haider and S. M. Ramay, Polym. Degrad. (Gedeon Richter Plc.) (Z. 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Publisher: Royal Society of Chemistry
Copyright: © Royal Society of Chemistry 2024 Registered charity number: 207890
eISSN: 1463-9270
DOI: 10.1039/d2gc02860c
Publisher site: See Article on Publisher Site

Abstract

Green Chemistry View Article Online PAPER View Journal | View Issue Optimisation of PET glycolysis by applying recyclable heterogeneous organocatalysts† Cite this: Green Chem., 2022, 24, a a a b Zsuzsanna Fehér, Johanna Kiss, Péter Kisszékelyi, János Molnár, a b a Péter Huszthy, Levente Kárpáti and József Kupai * Chemical depolymerisation, or solvolysis, can be a sustainable plastic recycling method, as a circular economy can be achieved by recovering the pure monomers. Polyethylene terephthalate (PET) is a ubi- quitous plastic material with short-life application and slow biodegradation, so its waste management needs to be continuously improved. In this study, we tested three commercially available organocatalyst- modiﬁed silica gels in the glycolysis of PET and developed another, functionalized with triazabicyclode- cene (TBD), which was also tested. Organocatalysts are eﬃcient in PET glycolysis, but their recyclability needs to be improved for industrial application. The applied heterogeneous modiﬁed silica gels can be recovered easily by ﬁltration. Si-TEA catalyst was chosen for reaction optimisation because it has the highest thermal stability and good catalytic activity. The PET glycolysis process was optimised by fractional factorial experimental design and response surface methodology. Under optimal reaction conditions (PET (384 mg, 2 mmol), ethylene glycol (1.41 mL, 25.2 mmol), Si-TEA (15.5 mol%), 190 °C, 1.7 h), 88.5% non- isolated bis(2-hydroxyethyl) terephthalate (BHET) monomer yield was obtained. Si-TEA and Si-TBD cata- lysts were recycled in ﬁve reaction cycles, and with both catalysts, high cumulative BHET yields (89 and 88%, respectively) were achieved. Additionally, environmental energy impacts were calculated for the two Received 1st August 2022, catalysts and were compared favourably with other organocatalysts in the literature. A process scale-up Accepted 11th October 2022 was also implemented. Finally, it has been veriﬁed that modiﬁed silica gels have much higher catalytic DOI: 10.1039/d2gc02860c activities than native silica gel, as solvolytic reactions using the former catalysts took a signiﬁcantly shorter rsc.li/greenchem time. Plastic recycling methods fall into four categories: primary 1 Introduction (industrial scrap recycling), secondary (mechanical), tertiary 1 4 As global plastic production increases exponentially, there is a (chemical), and quaternary (incineration) recycling. growing demand for environmental awareness and sustainable Mechanical recycling is still considered the most favourable plastic recycling methods. Amongst the most prevalent plastic due to its lower energy requirement than chemical recycling. materials, polyethylene terephthalate (PET) is the one that In the case of PET bottles, the mechanical recycling technology becomes waste in the highest proportion of its produced quan- is indispensable, particularly when there is a deposit refund tity (97% of 33 million tons in 2015 ). This is because PET is system that can provide clean, homogeneous material for 2 5 primarily used as a short-life packaging material. Because of bottle-to-bottle recycling. However, if there is no such possi- its slow biodegradation and inadequate waste management, bility, chemical recycling can be adapted in the case of con- adaptive and well-designed recycling strategies are needed to taminated, heterogeneous waste streams. Chemical recycling prevent plastic’s harmful eﬀect on the environment and can be split into three categories: solvent purification, chemi- improve the sustainability of plastic use. cal depolymerisation (solvolysis or chemolysis), and thermal depolymerisation (thermal cracking or thermolysis). Among these technologies, solvolysis, which means the depolymerisa- a tion of plastic waste into its monomers and oligomers by a Department of Organic Chemistry and Technology, Faculty of Chemical Technology nucleophile reagent acting also as the solvent, has the most and Biotechnology, Budapest University of Technology and Economics, Műegyetem rkp. 3., H-1111 Budapest, Hungary. E-mail: [email protected] promise to complement mechanical recycling. This is because Laboratory of Plastics and Rubber Technology, Faculty of Chemical Technology and it can live up to the demands made for chemical recycling that Biotechnology, Budapest University of Technology and Economics, Műegyetem mechanical recycling sometimes struggles to achieve: infinite rkp. 3., H-1111 Budapest, Hungary virgin-grade recycling is technically feasible, food-grade pro- †Electronic supplementary information (ESI) available. See DOI: https://doi.org/ ducts can be produced, and the removal of contaminants is 10.1039/d2gc02860c This journal is © The Royal Society of Chemistry 2022 Green Chem., 2022, 24,8447–8459 | 8447 Open Access Article. Published on 12 October 2022. Downloaded on 10/16/2024 9:02:41 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Paper Green Chemistry possible. Sustainability requires the development of new analysis is that a process can be described explicitly at the methods to recover material of the same quality as the original least amount of time and expense. Factorial experimental one in an economical and environmentally friendly way, for design is an eﬃcient and straightforward method, and it is example, using recyclable catalysts in PET solvolysis. commonly used to determine the eﬀects of independent vari- Based on the applied nucleophile in solvolysis, PET is most ables that significantly influence the outcome of a process. often depolymerised by hydrolysis, methanolysis, glycolysis, and With the help of experimental design, the numerous methods aminolysis. Glycolysis is the simplest and earliest applied and catalysts used in PET glycolysis can be compared and eval- 7 3 method of PET depolymerisation. This transesterification reac- uated more eﬃciently. Chen and co-workers applied a 2 fac- tion proceeds with glycol excess at high temperatures torial experimental design to study the two- and three-factor (170–200 °C). It stands out with the advantage that its product, interaction eﬀects of reaction temperature, catalyst : PET ratio, bis(2-hydroxyethyl) terephthalate (BHET), can be used to and reaction time on the glycolysis of PET. They used cobalt 8,9 36,37 produce PET in only one polycondensation step. Moreover, on acetate or manganese acetate as catalysts. Aguado and co- average, it has the lowest environmental energy impact among workers also studied a fourth independent variable, the the solvolytic methods applied to rebuild PET (hydrolysis, reagent : PET ratio using Taguchi’s parameter design while 9 38 methanolysis, and glycolysis), and it is the most advanced in applying zinc acetate as a catalyst. A similar design was demonstrating commercial viability on a larger scale. applied by Zawadzki and co-workers with sodium metasilicate The glycolysis of PET proceeds through at least three stages: catalyst to optimise the synthesis of PET polyol from which oligomers, dimer, and monomer, as previously reported in the they prepared polyurethane adhesives. Van-Pham and co- literature. First, ethylene glycol diﬀuses into the polymer, workers applied a Box–Behnken design using sodium bicar- causing the polymer to swell up, thus increasing the diﬀusion bonate catalyst. There are multiple papers about applying rate. It was shown earlier that the depolymerisation of PET design of experiments in PET glycolysis for zinc acetate 41–43 with high polymerisation degree into PET with low polymeris- catalyst. Seeing the advantages of experimental design in ation degree, preferentially leading to the dimer, would these studies, e.g., finding the economically optimal reaction proceed in a relatively short time interval. So, the conversion conditions to obtain high yield and determining the eﬀects of of the dimer into BHET would be the rate-determining step. factors influencing the process, it is worth improving a cata- The transformation of dimer into BHET monomer is a revers- lytic recycling process by this method. ible process, and an equilibrium exists between the BHET In this work, multiple commercially available functiona- monomer and the remaining amount of the dimer. lized silica gels (modified with trialkylguanidine (Si-GUA), trialk- Catalysts are usually applied in the solvolysis of PET due to ylamine (Si-TEA), or dialkylthiourea (Si-THU)), and a newly syn- its high chemical stability and low solubility. The most thesised modified silica gel (modified with triazabicyclodecene common catalysts in PET glycolysis are metal catalysts (metal (Si-TBD)) catalyst were tested in PET solvolysis with ethylene 12–15 16–19 20–22 salts, metal oxides, and others ), basic glycol (EG). These heterogeneous catalysts are easily recoverable, 12,23–25 26–29 organocatalysts, ionic liquids, and deep eutectic which is necessary for being green and eﬃcient catalysts. To 30–33 solvents. Metal catalysts usually have high activity and identify the eﬀect of organocatalysts grafted to silica support, thermal stability if somewhat lower selectivity in some cases, the catalytic activities of the modified silica gels were compared but recovery from the reaction mixture is challenging. Ionic to unmodified silica gel (Si). To characterise the catalysts, liquids (ILs) oﬀer environmentally friendly alternatives to metal diﬀerent methods were used. The prepared Si-TBD catalyst was catalysts. However, their manufacturing is complex and costly. analysed by scanning electron microscopy with energy disper- The purification processes are cumbersome if the catalysts sive X-ray analysis (SEM-EDX). The thermal stability of all four contain metal, thus causing the procedures to be expensive, modified silica gels was determined by thermogravimetric ana- and sometimes, only low monomer yields can be obtained with lysis combined with diﬀerential scanning calorimetry (TG-DSC). them. Deep eutectic solvents emerge as new alternatives with The reaction conditions, such as reaction temperature, similar characteristics to ILs. However, as a novel method, only catalyst : PET ratio, EG : PET ratio, and reaction time, were opti- a few applications are described in the literature, in which some- mised using fractional factorial experimental design and times high catalyst loading and reagent excess are applied. response surface methodology. During the optimisation study, Organocatalysts are eﬃcient in PET glycolysis with usually good the yield of BHET was determined by high-performance liquid monomer yield; however, this area also needs improvements for chromatography (HPLC), which was compared with the isolated implementation in the industry: as the majority of them are yield in some cases. The recyclability of the two most eﬃcient relatively expensive, and organocatalyst loadings are often much catalysts was investigated in multiple cycles. higher than metal-based catalysts, their recyclability needs to be improved, for example, by grafting them to a solid support. These catalysts are prone to degradation; thus, their thermal 2 Experimental stability must be evaluated. 2.1 Materials There are numerous parameters that influence the solvoly- sis of PET, which can be examined systematically by experi- PET flakes from HUKE Ltd (Sárvár, Hungary) were used for gly- mental design. The advantage of experimental design and its colytic reactions. Post-consumer PET bottles were washed and 8448 | Green Chem., 2022, 24,8447–8459 This journal is © The Royal Society of Chemistry 2022 Open Access Article. Published on 12 October 2022. Downloaded on 10/16/2024 9:02:41 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Green Chemistry Paper 1 13 ground into small flakes (thickness 0.41 ± 0.08 mm; (9.9 ± The main product was identified by H, C nuclear mag- 2.9 mm) × (6.8 ± 2.2 mm)). Modified silica gels (Si-TEA, Si- netic resonance spectroscopy (NMR), high-resolution mass GUA, Si-THU, silica gel functionalized with glycidoxy groups spectrometry (HRMS), and Fourier-transform infrared spec- (Si-GLY)) were purchased from SiliCycle Inc, the unmodified troscopy (FTIR), and the BHET dimer side-product was identi- 1 13 silica gel (Si) and toluene from Merck. Ethylene glycol and fied by H, C NMR, and HRMS. TBD were purchased from Sigma-Aldrich. BHET. Melting point: 109 °C. Spectroscopic data (see Fig. S1–S4 in the ESI†) are fully consistent with those reported 2.2 Preparation of TBD-modified silica gel (Si-TBD) 23 1 in the literature. H NMR (500 MHz, DMSO-d ): δ 8.12 (s, 4H, Si-GLY (5 g, 5.55 mmol glycidyl group), toluene (45 mL), and CH), 4.98 (t, J = 5.7 Hz, 2H, OH), 4.32 (t, J = 4.9 Hz, 4H, O– TBD (927 mg, 6.66 mmol) were added to a two-necked round- CH ), 3.72 (q, J = 5.1 Hz, 4H, CH –OH) ppm. C NMR 2 2 bottomed flask. The reaction mixture was stirred under argon (126 MHz, DMSO-d ): δ 165.2, 133.8, 129.6, 67.1, 59.0 ppm. IR atmosphere and heated under reflux for 10 hours. The solid (KBr): ν 3447, 2964, 2946, 2932, 2880, 1716, 1689, 1505, max product was filtered on a sintered glass filter and washed with 1457, 1412, 1380, 1282, 1252, 1135, 1111, 1074, 1017, 910, 898, −1 + + a mixture of dichloromethane/methanol 5 : 1 (15 mL), followed 875 cm . HRMS (ESI ): m/z [M + Na] calcd for C H O Na: 12 14 6 by a mixture of dichloromethane/methanol/triethylamine 277.0688; found: 277.0689. 5 : 1 : 0.05 (2 × 15 mL), then again with dichloromethane/ BHET dimer. Melting point: 162–166 °C. To the best of our methanol 5 : 1 (2 × 15 mL). The product was dried in a drying knowledge, the spectroscopic data of the dimer has not been oven at 80 °C for 2 hours to give a mass of 5.36 g—the mole- reported so far (see Fig. S5–S6 in the ESI†). H NMR (500 MHz, −1 cular loading of Si-TBD was 0.58 ± 0.09 mmol g , determined DMSO-d ): δ 8.12 (dd, J = 13.5 Hz, and 8.5 Hz, 8H, CH), 4.95 (t, by SEM-EDX elemental analysis. J = 5.7 Hz, 2H, OH), 4.68 (s, 4H, O–CH –CH –O (middle)), 4.31 2 2 (t, J = 5.0 Hz, 4H, O–CH (next to CH –OH)), 3.71 (q, J = 5.1 Hz, 2 2 2.3 General procedure for PET glycolysis 13 4H, CH –OH) ppm. C NMR (126 MHz, DMSO-d ): δ 165.1, 2 6 The catalyst, PET flakes (384 mg, 2 mmol), and ethylene glycol 165.0, 133.9, 133.3, 129.6, 129.5, 67.0, 63.2, 59.0 ppm. HRMS + + were added to a 5 mL sealable vial. The reagent and catalyst (ESI ): m/z [M + Na] calcd for C H NaO : 469.1105; found: 22 22 10 ratios were calculated in relation to the amount of PET repeat- 469.1102. −1 ing unit (MW = 192.2 g mol ). The reactions were carried out BHET yield (non-isolated) was calculated based on HPLC under argon atmosphere, and magnetic stirring was used. A calibration method using an external standard, by fitting a sand bath was used for heating, and the internal temperature line to five data points (see Fig. S11 in the ESI†). The non-iso- (170–195 °C) was monitored by a thermometer placed in a sep- lated yield was also compared with the isolated yield in some arate 5 mL sealed vial filled with ethylene glycol. After the cases. The isolated yield was calculated as appropriate reaction time (1–2 h), the reaction mixture was BHET Yield½ % ¼ 100% ð1Þ allowed to cool to room temperature, and then HPLC samples BHET;0 were prepared to determine the non-isolated BHET yield. A where m and m refer to the actual and theoretical MeCN : H O = 5 : 1 mixture (3 × 2 mL) was added to the reac- 2 BHET BHET,0 mass of BHET, respectively. The theoretical weight of BHET tion mixture and filtered on a sintered glass filter to separate was calculated by the catalyst and any remaining solid oligomers from the desired product. The solid residue was washed with PET;0 m ¼ 254:2 ð2Þ BHET;0 MeCN : H O = 5 : 1 solvent mixture (3 × 3 mL). The filtrate was 192:2 then transferred to a 25 mL volumetric flask and filled with where m is the initial mass of PET; 192.2 and 254.2 are the PET,0 the above-mentioned solvent mixture to 25 mL. For the HPLC molecular weight of the PET repeating unit and BHET, sample, 50 μL of this solution was taken, and 950 μLof respectively. MeCN : H O = 5 : 1 solvent mixture was added. PET conversion was calculated as The BHET was isolated by analogy with the method of Zhang and co-workers. For determining the isolated BHET m m r c;0 Conversion½ % ¼ 1 100% ð3Þ yield, the acetonitrile was removed from the filtrate PET;0 (MeCN : H O = 5 : 1 mixture containing BHET, EG, and BHET where m and m refer to the mass of the solid residue and r c,0 dimer/trimer by-products) under reduced pressure, and the the initial mass of the catalyst, respectively. volume of the remaining aqueous solution was extended by water to 70 mL. Then, the water-insoluble oligomers (dimer 2.4 Catalyst recycling and, in some cases, trimer based on HPLC-MS) were removed by filtration using a filter paper, and the filtrate was evaporated For the catalyst recycling, 5 reaction cycles were carried out to approx. 4 mL under reduced pressure. The resulting solu- with Si-TEA and Si-TBD catalysts under optimal conditions. tion was stored for 12 hours in a refrigerator (4 °C) to give Each cycle was repeated once. To maintain the optimal ratio of BHET as white crystals. These crystals were filtered on a sin- the parameters, the mass of PET and the amount of ethylene tered glass filter and washed with the mother liquor, then with glycol were reduced in proportion to the mass of the recycled water. catalyst. If the reaction did not proceed with complete conver- This journal is © The Royal Society of Chemistry 2022 Green Chem., 2022, 24,8447–8459 | 8449 Open Access Article. Published on 12 October 2022. Downloaded on 10/16/2024 9:02:41 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Paper Green Chemistry sion, the solid residue remaining after filtration was trans- HPLC was performed using a Shimadzu LCMS-2020 device ferred to the next reaction cycle. equipped with a HALO C18 (2.7 µm; 4.6 × 150 mm) column. The samples were eluted with gradient elution, using eluent A 2.5 Scale-up of PET glycolysis process (0.1% HCOOH in H O) and eluent B (MeCN). The flow rate was −1 6-fold scale-up. The catalyst (15.5 mol% Si-TEA or 50 wt% set to 0.8 mL min . The initial condition was 5% eluent B, fol- Si), PET flakes (2.31 g, 12 mmol), and ethylene glycol (12.6 eq.) lowed by a linear gradient to 100% eluent B by 8 min; from 8 were added to a 50 mL round-bottomed flask. The reactions to 13 min, 100% eluent B was retained; and from 13 to were carried out under argon atmosphere, and magnetic stir- 14 min, it went back by a linear gradient to 5% eluent B, ring was used. An oil bath was used for heating. When the which was retained from 14 to 15 min. The column tempera- reaction was completed, the reaction mixture was allowed to ture was kept at 40 °C, and the injection volume was 1 µL. cool to room temperature. A MeCN : H O = 5 : 1 mixture 2.6 Experimental design (15 mL) was added to the reaction mixture and filtered on a sintered glass filter to separate the catalyst and any remaining The optimisation of PET glycolysis was conducted by response solid oligomers from the desired product. The solid residue surface methodology with gradient method. Statistica software was washed with the above-mentioned solvent mixture (3 × (TIBCO Software Inc.) was applied for data analysis at 5% sig- 5 mL). The acetonitrile was removed from the filtrate under nificance level (to calculate the regression model and perform 4-1 reduced pressure, and the volume of the remaining aqueous analysis of variance (ANOVA)). 2 fractional factorial design solution was extended by water to approx. 420 mL. The water- was applied with two centre point experiments, and all experi- insoluble oligomers (dimer and trimer) were removed by fil- ments were repeated once. The eﬀects of four independent tration using a filter paper, and the filtrate was evaporated to variables (i.e., reaction temperature, catalyst : PET ratio in approx. 25 mL under reduced pressure. The resulting solution mol%, EG : PET molar ratio, and reaction time) were investi- was stored for 12 hours in a refrigerator (4 °C) to give BHET as gated. Two dependent variables were chosen as responses, white crystals. These were filtered on a sintered glass filter and BHET yield (Y) and PET conversion. washed with the mother liquor, then with water. 2.7 Characterisation 18-fold scale-up. The catalyst (15.5 mol% Si-TEA), PET flakes (7.00 g, 36.4 mmol), and ethylene glycol (12.6 eq.) were added Infrared spectra were recorded on a Bruker Alpha-T FTIR to a 100 mL flat-bottomed flask. The reaction was carried out spectrometer. Silica gel 60 F (Merck) plates were used for under argon atmosphere, and mechanical stirring (800 rpm) thin-layer chromatography (TLC). Ratios of solvents are given −1 was used. An oil bath was used for heating. The reaction was in volumes (mL mL ). Melting points were measured in a completed after 5 hours, and the reaction mixture was allowed Boetius micro-melting point apparatus, and were uncorrected. to cool to room temperature, and then an HPLC sample was NMR spectra were recorded on a Bruker Avance 500 MHz 1 13 prepared to determine the non-isolated BHET yield. A spectrometer (500.13 MHz for H, 125.76 MHz for C) in MeCN : H O = 5 : 1 mixture (20 mL) was added to the reaction DMSO-d and were referenced to residual solvent proton 2 6 mixture and filtered on a sintered glass filter to separate the signals (δ = 2.50) and solvent carbon signals (δ = 39.51). All H C catalyst and any remaining solid oligomers from the desired chemical shifts are reported in parts per million (ppm). product. The solid residue was washed with the above-men- Coupling constants (J) are given in Hz. tioned solvent mixture (6 × 20 mL). The filtrate was then trans- HPLC-MS was performed using a Shimadzu LCMS-2020 ferred to a 500 mL volumetric flask and filled with the above- device, equipped with a Reprospher 100 C18 (5 µm; 100 × mentioned solvent mixture to 500 mL. For the HPLC sample, 3 mm) column and a positive/negative double ion source 60 μL of this solution was taken, and 940 μL of MeCN : H O= (DUIS±) with a quadrupole MS analyser in a range of 50–1000 5 : 1 solvent mixture was added. m/z. The samples were eluted with gradient elution, using For determining the isolated BHET yield, the acetonitrile eluent A (0.1% HCOOH in H O) and eluent B (0.1% HCOOH −1 was removed from the filtrate (MeCN : H O = 5 : 1 mixture con- in MeCN). The flow rate was set to 1.5 mL min . The initial taining BHET, EG, and by-products) under reduced pressure, condition was 5% eluent B, followed by a linear gradient to and the volume of the remaining aqueous solution was 100% eluent B by 1.5 min; from 1.5 to 4.0 min, 100% eluent B extended by water to 1.3 L. Then, the water-insoluble oligo- was retained; and from 4 to 4.5 min, it went back by a linear mers (dimer and trimer) and smaller-sized catalyst fraction gradient to 5% eluent B, which was retained from 4.5 to 5 min. (see Section 3.5) were removed by filtration using a filter paper The column temperature was kept at 30 °C, and the injection (the solid residue on the filter paper was washed with a volume was 1 µL. The purity of the compounds was assessed methanol : ethyl acetate = 1 : 1 mixture to recover the smaller- by HPLC with UV detection at 215 and 254 nm. High-resolu- sized catalyst fraction by dissolving the oligomers), and the fil- tion mass spectrometry was measured on a Q-TOF Premier trate was evaporated to approx. 53 mL under reduced pressure. mass spectrometer for BHET, and on a Bruker MicroTOF II The resulting solution was stored for 12 hours in a refrigerator instrument for the BHET dimer. The ionisation method was (4 °C) to give BHET as white crystals. These crystals were fil- electrospray ionisation (ESI) operated in positive ion mode. tered on a sintered glass filter and washed with the mother The stability of the catalysts was investigated by thermo- liquor, then with water. gravimetric analysis and diﬀerential scanning calorimetry 8450 | Green Chem., 2022, 24,8447–8459 This journal is © The Royal Society of Chemistry 2022 Open Access Article. Published on 12 October 2022. Downloaded on 10/16/2024 9:02:41 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Green Chemistry Paper (TG-DSC). A PerkinElmer STA 6000 instrument was used. In addition to the synthesised Si-TBD, three commercially During the measurement, the samples of about 30 mg were available functionalized silica gels, Si-GUA, Si-THU, or Si-TEA, −1 heated from 25 °C to 190 °C at a heating rate of 10 °C min were investigated (Fig. 1). The physical properties (particle and then held at the latter temperature for 360 min. size, specific surface area, molecular loading) of the commer- Si-TBD was analysed using a JEOL JSM-5500LV scanning cially available functionalized silica gels, which were given by electron microscope (SEM) at high vacuum with 10–20 kV the manufacturer, SiliCycle Inc, can be found in Table S1 in accelerating voltage. The elemental analysis of Si-TBD was the ESI.† To find the catalyst with the highest catalytic activity carried out with energy dispersive X-ray analysis (EDX with Si and recyclability while preserving the catalytic activity among (Li) detector) applying 10 kV accelerating voltage and sampling the modified silica gels tested, we investigated their thermal time of 40 s. stability by thermogravimetric analysis and diﬀerential scan- The physical properties of the commercially available func- ning calorimetry (TG-DSC), being held at 190 °C (the tempera- tionalized silica gels (particle size – measured by laser diﬀrac- ture which is most commonly used in PET glycolysis) for tion, specific surface area – measured by nitrogen adsorption/ 360 min. From the TG curves, which are similar for each tested desorption method at 77 K, molecular loading – measured by silica gel (see Fig. S7–S10 in the ESI†), the mass loss of each elemental analysis, or titration in the case of Si-GLY), which sample was determined. For each of the modified silica gels, a were given by the manufacturer, SiliCycle Inc, can be found in theoretical mass loss was also calculated based on the mole- Table S1 in the ESI.† cular loading, assuming that all the organic units are elimi- HS-GC-MS was performed using a Shimadzu GCMS-QP2010 nated from the modified silica gel. The theoretical and device equipped with an Rtx-5 (30 m × 0.32 mm × 0.25 µm) measured mass losses of the catalysts are summarised in column and an AOC-5000 auto-injector. The oven temperature Table 1. Since Si-TEA showed the lowest mass loss in the was initially held at 40 °C for 2 minutes, increased to 320 °C at TG-DSC measurement, it is assumed to have the highest −1 10 °C min with the final temperature held for 5 minutes. thermal stability. Si-TBD also showed a much lower mass loss The sample holder was thermostated for 45 minutes at 190 °C than the theoretical maximum. before the measurement. The carrier gas was argon, the linear During our experiments, the reactions are conducted in −1 flow rate on the column was set to 50 cm s , and the split inert atmosphere. Otherwise, the tertiary amines in the cata- ratio was 1 : 10. The injector was held at 300 °C. The interface lysts could react with oxygen to give N-oxides, and then the and the ion source temperature were set at 250 °C, the syringe Cope elimination of N-oxides would produce alkenes. There temperature was 150 °C, and the detector voltage was 1.3 kV. was no significant degradation of the catalysts observed. The Taking the molecular weights of the expected molecules into thermal stability of silica gel-supported organocatalysts was consideration, the mass scale was adjusted between mass to reported earlier. charge ratio (m/z)of 29–600. The morphology of the Si-TBD catalyst was investigated by SEM. Comparing its SEM image (Fig. 2(a)) with the starting 3 Results and discussion 3.1 Synthesis and characterisation of catalysts The Si-TBD was prepared from Si-GLY by reacting with TBD in toluene (Scheme 1). The Si-GLY used as starting material has a 2 −1 particle size of 40–63 μm, a specific surface area of 494 m g , and an active loading (the molecular loading measured by −1 titration by the manufacturer, SiliCycle Inc) of 1.11 mmol g . Then, leaching of TBD from the catalyst was investigated: the product was stirred in a solvent mixture (dichloromethane/ methanol 5 : 1) in which TBD was highly soluble, filtered, and washed. Analysis of the filtrate by HPLC-MS showed that less than 5% of the TBD on the catalyst surface (based on mole- Fig. 1 The applied, commercially available functionalized silica gels. cular loading) leached oﬀ from the catalyst. Scheme 1 Preparation of TBD-functionalized silica gel. This journal is © The Royal Society of Chemistry 2022 Green Chem., 2022, 24,8447–8459 | 8451 Open Access Article. Published on 12 October 2022. Downloaded on 10/16/2024 9:02:41 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Paper Green Chemistry Table 1 Theoretical and measured mass losses of the catalysts probably does not hinder the catalytic activity greatly because measured by TG-DSC (190 °C, 6 h) of the latter findings. Thus, we wanted to screen other silica gel-grafted nitrogen-based organocatalysts also, that can be Mass loss (%) easily accessed commercially, so we chose based on literature analogies: Si-TEA based on triethylamine, Si-THU based on Modified Theoretical Measured/theoretical silica gels maximum Measured maximum ratio (%) urea and another guanidine-based catalyst presumably similar to Si-TBD, Si-GUA. Si-GUA 16 12 75 Si-TEA 10 2 25 The catalytic activities of the commercially available modi- Si-THU 11 8 73 fied silica gels (Si-GUA, Si-TEA, Si-THU, with a particle size of Si-TBD 15 8 53 40–63 μm) and the newly synthesised Si-TBD were investigated in PET glycolysis (Scheme 2), and the results are shown in Fig. 3. In addition to the modified silica gels, unmodified silica gel (Si, also with a particle size of 40–63 μm) was also tested, and reactions without catalyst were also performed. The depolymerisation rate was low without catalyst (9% con- version), while it was slightly higher (34% conversion, 26% BHET yield) with Si. Thus, even the silica support has catalytic activity, but much lower than those of the modified silica gels: the three modified silica gels containing basic units (Si-TBD, Si-GUA, Si-TEA) showed excellent catalytic activities and selec- Fig. 2 SEM images of Si-TBD (a) and Si-GLY (b). tivities (96–100% PET conversions, 83–92% BHET yields) with relatively low standard deviation. Applying the Si-THU catalyst, high deviation was observed supposedly because of the cata- material, Si-GLY (Fig. 2(b)), both modified silica gels have irre- lyst’s instability at high temperature. As the aim was to recycle gular particle shapes with similar particle sizes. Therefore, it the catalyst in as many cycles as possible, Si-TEA was chosen can be assumed that the particle size distribution did not as the best catalyst for further reactions because of its relatively change during the modification of Si-GLY (40–63 µm, high catalytic activity and good thermal stability based on the measured by laser diﬀraction by SiliCycle Inc for Si-GLY). The TG-DSC measurements (see Table 1). molecular loading of Si-TBD was determined by SEM-EDX ana- To confirm the advantages of the Si-TEA modified silica gel lysis based on nitrogen content. Si-GLY was used as back- catalyst, triethylamine (TEA) was also applied as a catalyst. TEA ground for the calculation. should only be used in a closed reaction vessel; otherwise, it will evaporate from the reaction mixture. The latter catalyst 3.2 Catalyst screening also gave an excellent non-isolated yield (91%), but the remain- TBD is a bifunctional catalyst that can activate both an ester ing triethylamine traces in the reaction mixture probably hin- and alcohol through hydrogen bonding. Numerous nitrogen- dered the crystallization of BHET, as a poorer isolated yield based organocatalysts were screened for the depolymerisation (69%) was obtained than with the heterogeneous catalysts. To 25,47 of PET earlier in the literature. The catalytic activity was demonstrate the diﬀerence, in one case when Si-TEA was used, found to correlate with the basicity, but in the presence of a non-isolated yield of 90% was observed with an isolated yield short-chain diols such as ethylene glycol, which serve as a of 85%. cocatalyst for activating the ester carbonyl groups via hydrogen 3.3 Regression model based on experimental design bonding, the bifunctionality of TBD is less important, especially in the presence of excess ethylene glycol. The syn- The eﬀects of four independent variables (i.e., reaction temp- thesised catalyst, Si-TBD loses the bifunctionality because of erature, catalyst : PET ratio in mol%, EG : PET molar ratio, and the grafting onto silica support (and thus, its basicity reaction time) were investigated as quantitative factors most decreases – based on the reported pK of the conjugated acid frequently studied in the literature on glycolysis of PET. The of methyl-TBD ), but as we use excess ethylene glycol, this aim was to set up a simple model with relatively few experi- Scheme 2 PET glycolysis with EG using diﬀerent catalysts. 8452 | Green Chem., 2022, 24,8447–8459 This journal is © The Royal Society of Chemistry 2022 Open Access Article. Published on 12 October 2022. Downloaded on 10/16/2024 9:02:41 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Green Chemistry Paper cance of its eﬀect on the model response. Thus, all the main eﬀects and the B by C interaction eﬀect were found to be sig- nificant. The Pareto charts of standardised eﬀects in the case of BHET yield or PET conversion as the dependent variable can be found in Fig. S16 and S17 in the ESI.† After model reduction by removing the statistically insignif- icant factors, a new linear regression model was fitted to the experimental data, and it proved to be adequate based on the 2 2 curvature check with a high value of R (0.969) and adjusted R (0.955). Analysis of variance for the reduced linear regression model can be seen in Table S3 in the ESI.† Comparing the yields estimated from the model and the observed ones, it was Fig. 3 The catalytic activities of diﬀerent catalysts in PET glycolysis. found that the measured points are approximately along a line Reaction conditions: 190 °C, 10 mol% (or 40 wt% in the case of Si) cata- of slope 1 through the origin (Fig. 4), therefore the regression lyst (if used), EG : PET ratio of 16, 2 h. BHET yield was determined by model was adequate for predicting the BHET yield in the HPLC. design space, especially at high yields. The homogeneity of var- iances was checked comparing the predicted and residual values of BHET yield (see Fig. S18 in the ESI†), and using a 4-1 ments, so a 2 experimental design was developed to describe Cochran’s C test (see ESI†). Based on the latter, the assump- the variation of the dependent variable, non-isolated BHET tion that the random error variance is constant can be yield, as a function of the independent parameters using a accepted. Taking this into account, the reduced model can be linear regression model. Si-TEA catalyst was used during the used to determine the optimal conditions. experiments because of its highest thermal stability among the The reduced linear regression model representing the investigated modified silica gels and appropriate activity. The relationship between the BHET yield response Y and the coded lower and upper levels of the parameters were chosen based values of the four independent factors was obtained as on preliminary experiments by changing the parameters one by one (see Fig. S13–S15 in the ESI†) to obtain an approxi- Y ¼ 39:8 þ 30:6A þ 12:7B 11:9C þ 13:4D þ 4:2BC ð4Þ mately linear relationship between the dependent variable and Based on this model, the temperature (A) has the strongest the parameters in the chosen parameter ranges. Their coded eﬀect on the BHET yield, while the interaction of catalyst ratio and uncoded values are shown in Table 2. The experimental and EG ratio (BC) has low significance. design matrix and the observed BHET yields and PET conver- The interaction between catalyst : PET ratio and EG : PET sions from 20 experiments are presented in Table 3. ratio at a constant temperature of 190 °C and with a reaction In the fractional design, the triple interaction between time of 2 h was plotted in Fig. 5 on a response surface plot (a temperature, catalyst : PET ratio and EG : PET ratio is con- function of two factors with the others at constant values). At a founded with the fourth factor (reaction time). In this case, the fixed catalyst ratio, the yield decreases with increasing EG ratio main eﬀects are not mixed with each other or with two-factor in this investigating range. It should be noted, however, that in interactions, while the two-factor interactions are confounding our preliminary experiments, we found that the yield has a with each other (A by B with C by D; A by C with B by D; A by D maximum at a certain EG ratio value. For this, the explanation with B by C). Since out of the interactions, only A by D (and its could be that the transformation of dimer into monomer is a linear combination, B by C) were found to be significant, the B reversible process. Therefore, prolonging the reaction after by C interaction was chosen as the only significant interaction reaching equilibrium can cause a backward shift, increasing based on practical considerations (B by C interaction is the the amount of dimer at the expense of the BHET monomer. interaction of catalyst quantity and EG : PET ratio, which influ- In this case, the higher ratio of EG presumably shifted the ences the catalyst concentration in the reaction mixture). The equilibrium faster towards the dimer. In the design of experi- results of ANOVA for the regression model are shown in ments, however, the factor values were chosen so that there Table 4. A factor is considered significant if its p-value is less was an approximately linear relationship between yield and EG than 0.05, and the higher the F-value, the greater the signifi- ratio. On the surface plot, the interaction can be observed that at a lower catalyst ratio, the EG ratio has a greater negative Table 2 Coded and uncoded levels of independent variables eﬀect on yield than at a higher catalyst ratio, and similarly, at a higher EG ratio, the catalyst ratio has a greater positive eﬀect Coded level on yield than at a lower EG ratio. Independent variables Factor −10 +1 3.4 Optimisation of reaction conditions Temperature (°C) A 170 180 190 Catalyst : PET ratio (mol%) B 5 12.5 20 The optimisation was implemented using response surface EG : PET molar ratio (−) C 11 13.5 16 Reaction time (h) D 1 1.5 2 methodology with gradient method. Accordingly, with the step This journal is © The Royal Society of Chemistry 2022 Green Chem., 2022, 24,8447–8459 | 8453 Open Access Article. Published on 12 October 2022. Downloaded on 10/16/2024 9:02:41 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Paper Green Chemistry Table 3 Experimental design matrix and observed BHET yield and PET conversion Run Temperature (°C) Catalyst : PET ratio (mol%) EG : PET molar ratio (−) Reaction time (h) Conversion (%) BHET yield (%) 1 170 5.0 11.0 1.0 1 0 2 190 5.0 11.0 2.0 100 88 3 170 20.0 11.0 2.0 51 45 4 190 20.0 11.0 1.0 94 80 5 170 5.0 16.0 2.0 1 0 6 190 5.0 16.0 1.0 34 28 7 170 20.0 16.0 1.0 1 0 8 190 20.0 16.0 2.0 100 90 9 (C) 180 12.5 13.5 1.5 38 31 10 (C) 180 12.5 13.5 1.5 58 51 11 170 5.0 11.0 1.0 5 0 12 190 5.0 11.0 2.0 97 85 13 170 20.0 11.0 2.0 37 29 14 190 20.0 11.0 1.0 100 87 15 170 5.0 16.0 2.0 7 0 16 190 5.0 16.0 1.0 21 16 17 170 20.0 16.0 1.0 8 0 18 190 20.0 16.0 2.0 99 89 19 (C) 180 12.5 13.5 1.5 46 41 20 (C) 180 12.5 13.5 1.5 45 40 C: centre point. Table 4 Analysis of variance for linear regression model Factor Sum of squares DF Mean square F-Value p-Value Significant Curvature 2.81 1 2.81 0.07 0.7935 No (A) Temperature (°C) 14 945.06 1 14 945.06 382.09 <0.0001 Yes (B) Catalyst : PET ratio (mol%) 2575.56 1 2575.56 65.85 <0.0001 Yes (C) EG : PET ratio (−) 2280.06 1 2280.06 58.29 <0.0001 Yes (D) Reaction time (h) 2889.06 1 2889.06 73.86 <0.0001 Yes A by B 189.06 1 189.06 4.83 0.0502 No A by C 115.56 1 115.56 2.95 0.1136 No B by C 280.56 1 280.56 7.17 0.0215 Yes Error 430.25 11 39.11 Total sum of squares23 708.0019 DF: degree of freedom. Fig. 4 Comparison of observed and predicted BHET yields. intervals chosen adequately, steps are taken from the centre point of the design space towards the local maximum along the gradient of the reduced model because this is the direction of steepest ascent. The temperature step interval was chosen Fig. 5 Response surface plot for the interaction eﬀect between as the determining one (since this is the most diﬃcult to set catalyst : PET ratio and EG : PET ratio at a constant temperature of 190 °C to a certain value), which is 2.5 °C in all cases. The step inter- and with a reaction time of 2 h. 8454 | Green Chem., 2022, 24,8447–8459 This journal is © The Royal Society of Chemistry 2022 Open Access Article. Published on 12 October 2022. Downloaded on 10/16/2024 9:02:41 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Green Chemistry Paper vals for the other parameters can be calculated from this. The actual parameter value at a given point was obtained by adding the step interval to the parameter value in the previous step, the starting point being the centre point (experiment point 0), the value of which is known from the experimental design. The calculated step intervals and coded factor values at each point are shown in Table S4.† The parameter settings for the diﬀerent optimisation points are presented in Table 5, and the estimated and measured yields are shown in Fig. 6. Strictly speaking, the reduced model is only valid within the limits of the experimental design, while outside the design space, there may be a larger diﬀerence between the estimated and observed yields. For this reason, the optimisation experi- ments outside the design space (experiment points 5 and 6, Fig. 6 Observed and predicted yields in the optimisation experiments. Fig. 6) were conducted first. The BHET yield measured at point 5 is 9% lower than the estimated value (87.0% instead of 96.0%, Fig. 6), and at point 6, the yield is 8% lower (79.0%) straightforward, the use of these catalysts can be economical, than at point 5, so the yield decreases outside the design even if they have high prices. space. Returning to the limits of the experimental design, we After determining the optimal reaction conditions, we performed the reaction at point 4, during which we measured aimed to test the recyclability of Si-TEA, which has the highest high BHET yield (88.5%). This was accepted as the optimal thermal stability among the modified silica gels, and Si-TBD, setting because the model is valid inside the design space, so too, because this catalyst has the highest catalytic activity and experiments in points 1–3 are not needed to be conducted. a fairly good thermal stability. During the recycling experi- Thus, the most favourable reaction conditions were 190 °C, ments, catalyst mass decreased from cycle to cycle (see 15.5 mol% Si-TEA catalyst, EG : PET molar ratio of 12.6, and Table S5†), in the case of Si-TEA, by 6.5 ± 3.3%, and in the 1.7 h reaction time in point 4. case of Si-TBD, by 7.0 ± 3.2% per cycle compared to the orig- Comparing the results achieved under these optimal con- inal catalyst mass. Thus, to maintain the optimal ratio of the ditions with literature ones also attained by experimental parameters, the mass of PET and the amount of ethylene design (Table 6), excellent PET conversion (100%), BHET yield glycol were reduced in proportion to the mass of the recycled and selectivity (88.5%) were achieved with an easily recyclable, catalyst. The Si-TEA catalyst showed >89% BHET yield in the heterogeneous organocatalyst instead of inorganic salt cata- first four cycles of repeated PET glycolysis reactions, which lysts with costly recycling. This was achieved by performing slightly decreased (73%) in the fifth cycle (Fig. 7(a)). The Si- relatively few experiments but by testing four factors using a TBD catalyst resulted in only 78% BHET yield in the first cycle simple linear model. It can be observed that compared to in- but >91% in the next 3, while this catalyst also showed a organic metal salts, organocatalysts require a high catalyst slightly lower yield (79%) in the fifth cycle (Fig. 7(b)). The loading, but this can be acceptable by recycling the catalyst latter experience may be explained by stirring problems due to (see later in Section 3.5). the reduction in reaction size (because of the catalyst loss) during recycling. The slightly lower yield in the first cycle in 3.5 Catalyst recycling the case of Si-TBD could be attributed to the lack of end- The immobilisation of various organic compounds on silica capping of the catalyst, so for larger scales, it is worth consid- gels is well known in the literature, so the best-performing ering end-capping. Taking all five reaction cycles into account, catalyst could be produced on a larger scale more cheaply if the cumulative BHET yields are 89% (1.74 g) for Si-TEA and required. In any case, if the catalyst recycling is successful and 88% (1.70 g) for Si-TBD. Consequently, there was no signifi- Table 5 Parameter settings for the optimisation points * * * Experiment point Temperature (°C) Catalyst : PET ratio (mol%) EG : PET molar ratio (–) Reaction time (h) 0 180.0 12.50 12.50 1.50 1 182.5 13.28 12.26 1.55 2 185.0 14.03 12.02 1.61 3 187.5 14.76 11.80 1.66 4 190.0 15.47 11.58 1.72 5 192.5 16.15 11.37 1.77 6 195.0 16.81 11.17 1.83 Given to two decimal places to indicate monotonic growth of values. This journal is © The Royal Society of Chemistry 2022 Green Chem., 2022, 24,8447–8459 | 8455 Open Access Article. Published on 12 October 2022. Downloaded on 10/16/2024 9:02:41 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Paper Green Chemistry Table 6 Comparison of PET glycolysis methods using experimental design. The optimal reaction conditions are indicated, and the parameter ranges investigated are given in parentheses Temperature Reaction Catalyst : PET molar EG : PET molar Conversion Yield Ref. Catalyst Design (°C) time (h) ratio (%) ratio (−) (%) (%) Ref. 36 Co(OAc) 2 190 (130–190) 2.0 (0.5–2) 1.0 (0–1) 6.2 (fixed) 100 n.d. Ref. 37 Mn(OAc) 2 190 (130–190) 1.5 (0.5–2) 0.5 (0.05–0.5) 6.2 (fixed) 100 n.d. Ref. 38 Zn(OAc) Taguchi L 208 (195–220) 2.5 (2.5–3.5) 0.2 (0.2–1) 18.6 (6.2–18.6) n.d. 85 2 9 Ref. 41 Zn(OAc) Taguchi L 250 (MW) (fixed) 0.5 (fixed) 1.0 (1–4) 9.3 (3.1–15.5) n.d. 65 2 20 Ref. 40 NaHCO 4 factor Box–Behnken design 192 (192–200) 3.0 (3–5) 1.9 (1.4–2.3) 18.8 (7.7–23.2) n.d. 75.7 4-1 This work Si-TEA 2 190 (170–195) 1.7 (1–2) 15.5 (5–20) 12.6 (11–16) 100 88.5 Fig. 7 Catalyst recycling in PET glycolysis applying Si-TEA (a) and Si-TBD (b) catalysts. cant catalytic activity loss during the catalyst recycling ate from the irregular shaped silica gel at high temperature experiments. while stirring in the polar ethylene glycol. These particles go Stabilities of Si-TEA and Si-TBD catalysts were further inves- through the G3 porosity glass filter, thus causing the catalyst tigated by HS-GC-MS, and only a small amount of amines were mass loss. To examine this, we recovered some of the lost cata- detected as decomposition products only in the case of Si-TEA. lyst when a large amount of water was added to the filtrate (see After a 4-hour glycolysis reaction under optimised conditions, Section 2.3), and the precipitate was found to contain Si-TEA. we detected a very low amount of diﬀerent amines (triethyl- A catalyst recycling experiment was conducted with this recov- amine, diethylmethylamine, diethylamine) compared to a tri- ered smaller-sized Si-TEA, which remained active (64% non- ethylamine reference, which contained 5% of the TEA amount isolated BHET yield in the case of 5 mol% Si-TEA). It should grafted onto the silica gel (Fig. S19 and 20†). Their peak areas be noted, however, that the lost catalyst was diﬃcult to recover can be seen in Table S6† (those fragments of the detected com- and still contained BHET dimer after washing with a metha- pounds were depicted on the chromatogram, which had nol/ethyl acetate mixture. Consequently, future studies should approximately the same intensities considering the mass focus on examining the stability of spherical silica gels and spectra of the amines; thus, it gives a relative quantitative esti- other types of catalyst carriers. mation for the decomposition products). Besides amines, Based on the work of Thielemans and co-workers, the acetone and methyl 1,3-dioxolane (product of the reaction of environmental energy impact (ξ, eqn (5)) value equals the quo- ethylene glycol and acetaldehyde – the latter is a by-product of tient of the environmental factor (E ) and the energy factor PET degradation reactions ) were detected. In the case of Si- economy factor (ε). This green chemistry metric has allowed TBD, no decomposition products were detected by HS-GC-MS. for quantitative comparison between diﬀerent studies and for All reaction mixtures were also analysed by HPLC-MS, and no determining their relative feasibility. The lower the environ- decomposition products were detected in the case of Si-TEA. mental energy impact, the more favourable the technology is, However, in the case of Si-TBD, trace amounts of TBD were since the E is equal to the mass of waste divided by the factor detected, but only by the first application of the catalyst. After mass of product, and the energy economy factor is equal to catalyst recycling, it could not be detected. the yield divided by the temperature times the reaction time Thus, we assume that the reason for the slight catalyst (see Section 8 in the ESI†). Comparing Si-TEA and Si-TBD to mass loss (see Table S5†) is that smaller-sized particles separ- other organocatalysts (Table 7, and Table S7† in more detail), 8456 | Green Chem., 2022, 24,8447–8459 This journal is © The Royal Society of Chemistry 2022 Open Access Article. Published on 12 October 2022. Downloaded on 10/16/2024 9:02:41 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Green Chemistry Paper Table 7 Comparing the environmental energy impact, E and factor colysis of PET was further investigated. Silica gel was used as a energy economy factor of Si-TEA and Si-TBD to organocatalysts applied catalyst in a 6-fold scale-up reaction at 50 wt%. The reaction in PET glycolysis was carried out at 190 °C, using an EG : PET ratio of 12.6 in 10 h reaction time since the PET particles were broken down −1 −1 Ref. Catalyst ξ (°C min) E (−) ε (°C min ) factor in the reaction mixture during this time. Under these con- 9 −5 Ref. 25 TBD 24938 0.4875 1.955 × 10 ditions, complete conversion and 72% BHET yield were 9 −5 Ref. 23 TBD : MSA 12911 0.5440 4.213 × 10 −5 obtained, i.e., the silica gel could be successfully used as a This work Si-TEA 7585 0.3483 4.592 × 10 −5 Si-TBD 7758 0.3523 4.541 × 10 catalyst for the process. Nevertheless, the reaction took signifi- cantly more time, showing that silica gels functionalized with MSA: methanesulfonic acid. organic moieties have a much higher catalytic activity, and thus their application on an industrial scale may be more advantageous in terms of energy savings. their environmental energy impacts, environmental factors, To prove the feasibility of scaling-up by using mechanical and energy economy factors are slightly better, so these cata- stirring, an 18-fold scale-up experiment was performed with lysts can be feasible alternative catalysts in PET glycolysis. The mechanical stirring for 5 hours. The glycolysis reaction gave cumulative BHET yields (for five reaction cycles) were applied full conversion, 83% HPLC yield of BHET, and 72% isolated in the calculations, and the catalysts were not included in the BHET yield by crystallization. The isolated BHET yield could waste mass because of their recyclability (at a certain reaction be further increased by 7% (79% in sum) by column chromato- cycle number, the catalyst mass can be neglected) (see ESI†), graphy. Then, further side-products besides dimer of BHET as also reported in the work of Thielemans. were identified by HPLC-MS: BHET trimer, 2-hydroxyethyl ter- factor ephthalic acid (in a higher proportion than the rest), and 2-(2- ξ ¼ ð°C minÞð5Þ hydroxyethoxy)ethyl (2-hydroxyethyl) terephthalate (Fig. S12 and Table S2†). As only one experiment was performed, which While most heterogeneous catalysts applied in PET glycoly- was stopped based on the disappearance of PET flakes, the sis do not often maintain high BHET selectivity and require reaction time of the scaled-up reaction needs to be further higher temperatures than most other catalysts, in this work, optimised in the future. high selectivity was maintained in 5 reaction cycles at only These findings serve as preliminary studies for future scale- 190 °C. In the future, catalyst recycling in more cycles will be up optimisation. Glycolytic PET depolymerisation is already performed on a larger scale. applied on pilot and even commercial scales, however, these processes have limitations: using catalysts is not preferred 3.6 Preliminary scale-up of PET glycolysis process because of the issues associated with their often diﬃcult recov- To examine the achievable yield and how much longer it takes ery and high cost, and in the case of mixed PET waste pro- to depolymerise PET on a larger scale under the previously cesses, the removal of pigments and dyes is problematic. In optimised conditions at small scale, a 6-fold scale-up was our work, catalyst recovery is a crucial aspect, and by using applied using Si-TEA catalyst at 190 °C, and the catalyst : PET solid-supported organocatalysts, their reuse is made more ratio and the EG : PET ratio were adjusted to the optimal straightforward as they can be recycled by simple filtration. An values as previously determined (see detailed description of issue is the catalyst loss, which will be addressed in the future, the scale-up in Section 2.5). The reaction time was increased to and investigating the depolymerisation of mixed PET waste 2.5 h, as the reaction mixture still visibly contained PET par- streams is also planned. Later, the nature of the support can ticles after 1.7 h, which was the optimal time for the smaller also be optimised to address, e.g., issues of the catalyst recov- size. Full conversion was achieved, and the preparative BHET ery or the cost of the catalyst. The recovery of ethylene glycol 23,44 yield was 74% (meaning that the BHET dimer was still present was reported in earlier studies, this will be also investi- in the reaction mixture after the reaction time of 2.5 h). The gated in the future scale-up optimisation. slightly lower preparative BHET yield could probably be attrib- uted to the non-optimal reaction time because of the sub- optimal stirring on the larger scale, suggesting that further 4. Conclusions optimisation of the scale-up reaction time or stirring speed is required in order to obtain higher monomer selectivity and In this work, three commercially available functionalized silica yield. Although this reaction size is still far from the industrial gels and a newly synthesised one, namely solid-supported scale, the results suggest that the method may be suitable for TBD, were tested as heterogeneous organocatalysts in PET gly- the degradation of PET waste on larger scale. colysis. Si-TEA was found to have the highest thermal stability Using the unmodified silica gel (Si) as a catalyst, 34% PET based on TG-DSC and good catalytic activity, while Si-TBD had conversion and 26% non-isolated BHET yield were achieved in the highest activity. The eﬀects of four parameters were investi- 2 h reaction time during the catalyst screening experiments gated by experimental design, and a simple linear model was (see Fig. 3). Since silica gel is cheaper and thermally more set up to represent the relationship between the BHET yield stable than functionalized silica gels, its application in the gly- and the coded values of the four independent factors. The This journal is © The Royal Society of Chemistry 2022 Green Chem., 2022, 24, 8447–8459 | 8457 Open Access Article. Published on 12 October 2022. Downloaded on 10/16/2024 9:02:41 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Paper Green Chemistry reaction temperature had the highest eﬀect on BHET yield at Student Scholarship of Gedeon Richter’s Centenary 170–190 °C. The optimisation of PET glycolysis was Foundation (J. Kiss). The authors are grateful to Diana Daicu implemented using response surface methodology with gradi- from Budapest University of Technology and Economics ent method applying the most stable catalyst, Si-TEA. The (BUTE) for her assistance with the preliminary experiments optimal reaction conditions are 190 °C, 15.5 mol% Si-TEA cata- related to PET glycolysis, and to József Balla and Judit Mátyási lyst, EG : PET molar ratio of 12.6, and 1.7 h reaction time. This from BUTE for the HS-GC-MS measurements. method compared favourably with other methods applying experimental design in PET glycolysis optimisation. The best catalysts, Si-TEA and Si-TBD can be easily recycled by filtration while preserving high BHET yields in five reaction cycles. The References cumulative yields were similarly high for both catalysts (89% for Si-TEA and 88% for Si-TBD), and the methods applying 1 R. Geyer, J. R. Jambeck and K. L. Law, Sci. 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Optimisation of PET glycolysis by applying recyclable heterogeneous organocatalysts

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