Carbon capture using adsorption processes can significantly mitigate global warming. Mg-MOF-74 is a distinct reticular material amongst other adsorbents owing to its distinguished carbon dioxide adsorption capacity and selectivity under low- pressure applications, while MIL-100(Fe) has lower C O adsorption capacity, but extraordinary thermal and hydrostability in comparison to many classes of MOFs. In this paper, we present CO adsorption characteristics of new compounds formed by the incorporation of multi-walled carbon nanotubes (MWCNTs) into Mg-MOF-74 and MIL-100(Fe). This was done to improve the thermal diffusion properties of the base MOFs to enhance their adsorption capacities. The new composites have been characterized for degree of crystallinity, and the CO and N equilibrium uptake. The real adsorption separation 2 2 has been investigated by dynamic breakthrough tests at 297 K and 101.325. The equilibrium isotherm results showed that Mg-MOF-74 and 0.25 wt% MWCNT/MIL-100(Fe) (MMC2) have the highest CO uptake in comparison to the other inves- tigated composites. However, the interesting results obtained from breakthrough tests demonstrate that good improvements in the CO adsorption uptake and breakthrough breakpoint over pristine Mg-MOF-74 have been accomplished by adding 1.5 wt% MWCNT to Mg-MOF-74. The improvements of C O adsorption capacity and breakpoint were about 7.35 and 8.03%, respectively. Similarly, the CO adsorption uptake and breakthrough breakpoint over pristine MIL-100(Fe) are obtained by 0.1 wt% MWCNT/MIL-100(Fe) (MMC1) with improvements of 12.02 and 9.21%, respectively. Keywords Adsorption · Mg-MOF-74 · MIL-100(Fe) · MWCNTs · Characterization · Breakthrough · Carbon capture Abbreviations K Breakthrough fitting constant MFC1 0.1 wt% MWCNT/Mg-MOF-74 K T oth adsorption constant (1/kPa) eq MFC2 0.25 wt% MWCNT/Mg-MOF-74 L Adsorbent bed length (m) MFC3 0.5 wt% MWCNT/Mg-MOF-74 n Toth adsorption constant MFC4 0.75 wt% MWCNT/Mg-MOF-74 q Adsorption uptake (mmol/g) 3 −1 MFC5 1.5 wt% MWCNT/Mg-MOF-74 Q Outlet volumetric flow rate (m /s ) 3 −1 MMC1 0.1 wt% MWCNT/MIL-100(Fe) Q F eed volumetric flow rate (m /s ) MMC2 0.25 wt% MWCNT/MIL-100(Fe) P Pressure (Pa) MMC3 0.5 wt% MWCNT/MIL-100(Fe) r Adsorbent particle radius (m) MWCNT Multi-walled carbon nanotubes R Universal gas constant (J/mol K) v Velocity (m/s) List of symbols t Time (s) C Concentration (mol/m ) D Diffusion coefficient (m /s) Subscripts ε P orosity CO, N Carbon dioxide, nitrogen gas 2 2 k A dsorption time constant (1/s) i Gas species index g Gas m Maximum, limited * Mohamed A. Habib 0, in Inle t firstname.lastname@example.org out Outlet Mechanical Engineering Department and KACST-TIC on CCS, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia Vol.:(0123456789) 1 3 170 International Journal of Energy and Environmental Engineering (2018) 9:169–185 was also investigated for CO capture and was meas- Introduction ured to exhibit a fairly high adsorption capacity . A nickel isonicotinate-based ultramicroporous MOF, 1 [Ni- The fossil fuel burning processes produce greenhouse (4PyC)2·DMF], showed lower parasitic energy than Mg- gases, including CO, N , and CH . Global warming 2 2 4 MOF-74 and good stability with humidity . caused by these gases leads apparently to shore floods, MOF-74 is the most currently available MOFs with high hot atmosphere, soil droughts, and damage of eco system. CO adsorption uptake as well as an excellent CO over N In this scenario, carbon dioxide holds the most signifi- 2 2 2 selectivity [16, 17]. More specifically, Mg-MOF-74 was cant portion of the flue gases released to the atmosphere quantitatively identified as the highest adsorbent adsorbs . Thus, extensive efforts have been made by scientists, CO at low-pressure conditions (350 mg/g at 298 K) institutions, countries, and environmental organizations to . It is also reported that Mg-MOF-74 showed higher reduce the CO emissions. The principal source of CO is 2 2 H O hydrophilicity (593 ml/g at 298 K) than zeolite . a combustion processes that used fossil fuel. However, the Despite the high CO uptake of Mg-MOF-74, the existence utilization of fossil fuel is still very for satisfying energy of H O reduced the C O capture capacity, unlike HKUST- demands. Hence, the feasible solution to continue using 2 2 1, MIL-101(Cr), and MIL-100 (Fe) types . The study fossil fuel with mitigation of climate change is Carbon , moreover, showed the reduction of C O adsorption Capture and Storage (CSS). at different conditions. For example, at 1 bar and 298 K, A massive number of researchers have already stud- the CO adsorption capacity of the dry Mg-MOF-74 was ied CO capture using both experimental and simulation about 8.4 mmol/g of CO while with hydration 6.5 and 13%, approaches as well as synthesizing novel adsorbents . 2, the CO adsorption capacity values were 6.7 mmol/g and Using adsorption for C O separation is advantageous by 5.4 mmol/g, respectively . MIL-100(Fe) is remarked the ease of regenerating the adsorbent by being exposed to as the hydro- and thermal-stable adsorbent. It showed that heat and/or vacuum . The most common known adsor- some increase in CO uptake recorded with increasing RH, bents are activated carbons and zeolites being exploited −1 (up to 105 mg g for CO at 40% RH), with a large decrease for CO separation and storage. Zeolites could adsorb a in adsorption heat . higher quantity of C O than does by activated carbon at Less than 2 decades, carbon nanotubes (CNTs) have con- low operating pressures (< 20 kPa) [4, 5], whereas car- firmed a capability towards CO separation [21–25]. Specifi- bon-based adsorbents are better for CO storage appli- cally, the functionalized CNTs by amine-groups showed a cations [5, 6]. Conversely, the obvious merits of carbon significant improvement in the CO uptake [26–29]. The adsorbents over zeolites are cost penalties, hydrostability, CO uptake was reported around 2.59 mmol/g at 293 K lower regeneration energy, and easiness of production on using APTES functionalized CNTs. Incorporating CNTs a commercial scale . Despite, zeolite-based adsorbents with adsorbents could considerably improve the thermal have the relatively higher C O adsorption capacity, espe- diffusion of the adsorbents, thereby enhancing the CO cially at lower adsorption pressures (10–30 kPa (abs.) at adsorption uptake [30–32]. For example, the compound of T = 30 °C), the CO uptake is greatly reduced in case of CNT@13X/CaCl exhibited higher thermal conductivity CO /H O mixture and requires significantly higher heat 2 2 and then more C O uptake than did by 13X/CaCl and pris- of regeneration [8, 9]. 2 2 tine 13X [30, 31]. Furthermore, the thermal stability along Since 20 years ago, a novel class of reticulate adsor- with the adsorption uptake was improved by adding CNT to bents has been discovered and called metal organic frame- MIL-101-68 (Al) . Incorporating CNT and lithium ions works (MOFs) [10, 11]. In this context, the highest adsorp- −1 with MOF Cu (btc) resulted in improving the C O capacity tion CO adsorption capacity (1470 mg g ) was reported 3 2 2 by about 305% compared with those of the base adsorbent for MOF-177 at 35 bar . Over time, a large number of (Cu (btc) ) . In addition, the adsorption capacity of CO MOFs have been developed by scientists for maximize the 3 2 2 at high pressure and room temperature (10 bars and 298 K) CO uptake and selectivity. was improved by adding MWCNT to MIL101 . A certain class of MOFs incorporating functionalized A considerable number of experimental and numerical and open metal sites has shown high separation efficiency research attempts based on C O capture and separation have at ambient conditions like HKUST-1, Mg-MOF-74, and been conducted so far in terms of breakthrough and pressure NH MIL-53 (Al) . With regard to maximum CO 2 2 and temperature swing adsorption [35–49]. Nevertheless, adsorption capacity uptake, a few MOFs have shown a the majority amount of research chemically concentrated reasonably high C O uptake, such as CPM-5, MIL-53 on development of novel adsorbent materials targeted for (Al), UMCM-150, and Ni-STA-12, while others have been acquiring high C O adsorption and selectivity. At this point, evaluated for comparatively lower uptakes like MOF-5 the poor thermal conductivity shown by a vast majority of and MOF-177 . A nickel-based MOF, Ni/DOBDC, these adsorbents has been experienced as a major barrier 1 3 International Journal of Energy and Environmental Engineering (2018) 9:169–185 171 in improving the C O uptake of these materials in real MIL-100(Fe) was washed with copious amounts of water applications. In addition, a very limited number of research and ethanol and finally with an aqueous NH F solution in attempts have conducted the enhancement of CO capture purpose removing any unreacted species. Specifically, the and storage via improving the thermal properties of the base dried solid was, first, immersed in deionized water (60 ml adsorbent. For this reason, this paper aims at investigating per 1 g of solid) and the resulting suspension was stirred the effects of incorporating multi-walled carbon nanotubes at 70 °C for 5 h. Again, the suspension was centrifuged (MWCNTs) with Mg-MOF-74 and MIL-100(Fe), with the and the wash process was repeated using ethanol (60 ml) aim of enhancing the thermal properties of the adsorbents, at 65 °C for 3 h. This two-step purification was repeated and investigating the influence of the MWCNT addition on until the decanted solvent following centrifugation became the CO adsorption capacity and breakpoint of the resulting completely colorless, after which the solid was immersed MWCNT/Mg-MOF-74 and MWCNT/MIL-100(Fe) compos- in a 700 ml aqueous NH F solution and stirred at 70 °C for ites. The synthesized and activated Mg-MOF-74 and MIL- 5 h. The suspension was again centrifuged and the solid was 100(Fe) have been physically incorporated with different washed 5 times DI water at 60 °C, and finally dried in air at percentages of MWCNTs. They are characterized for the 75 °C for 2 days followed by 95 °C for 2 days. degree of crystallinity, CO adsorption isotherms, and CO The MWCNT/Mg-MOF-74 sample designations were 2 2 adsorption breakthrough. Moreover, the dynamic separations based according to the weight percentage of powder form of breakthrough tests are carried out to address the actual of MWCNTs which was physically mixed with as-synthe- CO separation from C O /N (20% v/v. C O and 80% v/v. N sis powder form of Mg-MOF-74 as follows. Mg-MOF-74, 2 2 2 2 2 for MWCNT/Mg-MOF-74, and 15% v/v. CO and 85% v/v. 0.1 wt% MWCNT/Mg-MOF-74, 0.25 wt% MWCNT/ N for MWCNT/MIL100(Fe)) mixture and then computing Mg-MOF-74, 0.5 wt% MWCNT/Mg-MOF-74, 0.75 wt% the level of enhancements on CO uptake and separation. MWCNT/Mg-MOF-74, 1 wt% MWCNT/Mg-MOF-74, and 1.5 wt% MWCNT/Mg-MOF-74 and shortly named as Mg- MOF-74, MFC1, MFC2, MFC3, MFC4, MFC5, and MFC6, Experimental work methodology respectively. The incorporation of powder form of MWCNT with as-synthesis powder form of MIL-100(Fe) has pro- MWCNT/Mg‑MOF‑74 and MWCNT/MIL‑100(Fe) duced MIL-100(Fe), 0.1 wt% MWCNT/MIL-100(Fe), samples preparation 0.25 wt% MWCNT/MIL-100(Fe), and 0.5 wt% MWCNT/ MIL-100(Fe) which were named as MIL-100(Fe), MMC1, We have followed a successful procedure for synthesizing MMC2, and MMC3, respectively. The particle size distri- Mg-MOF-74 as described in . Briefly, 0.337 g 2,5-dihy - bution for selective composites has been conducted using droxyterephthalic acid and 1.4 g Mg(NO3)2·6H2O were Particles-Size Analyzer, Model S3500, Microtrac, USA. dissolved in a solution of 135 ml dimethylformamide, 9 ml ethanol, and 9 ml water with sonication for 10 min. The Powder X‑ray diffraction (PXRD) analysis resulting stock solution was decanted into twelve 50 ml bot- tles. The bottles were tightly capped and heated at 398 K To determine the crystallinity of composites, PXRD patterns for 26 h. The mother liquor was, then, decanted. Following of MWCNT/Mg-MOF-74 were collected using a Bruker this, the products were washed with methanol, and then, left D8-Advance (Cu K λ = 1.54056 Å). The operating power of immersed in methanol. The products were combined to one the PXRD system was 30 kV/30 mA and the step-counting bottle and exchanged into fresh methanol daily for 4 days. method (step = 0.02 , time = 3 s) was used to collect data The activation process was carried out by evacuating the at range 2θ = 3–45 and 298 K. For MIL-100(Fe); the dif- product to dryness and then heated under vacuum at 523 K fraction data were collected between 3 and 45° (2θ) with a for 6 h. total scan time of 3 h. The synthesis of MIL-100(Fe) was performed in accord- ance with a previously reported procedure . We first dissolved Fe(NO ) ·9H O (4.04 g, 0.01 mol) in de-ionized Scanning electron microscopy (SEM) 3 3 2 water (50.2 ml, 2.8 mol) and the mixture was completely put in a 125 ml Teflon-liner containing BTC (1.4097 g, Scanning electron microscopy (SEM) was carried out using 0.00671 mol). After that, the Teflon-liner was tightly sealed a TESCAN LYRA3 FEG microscope to test structure of inside a stainless steel autoclave and was kept at 383 K Mg-MOF-74, 1.5 wt% MWCNT/Mg-MOF-74, MIL-100(Fe) for 14 h. After heating, the autoclave was slowly cooled and 0.5 wt% MWCNT/MIL-100(Fe). SEM samples were to ambient temperature, after which the “as-synthesized” prepared by placing a powder form of samples on Al tapes dark orange solid was recovered using a centrifuge that was and sputter coated with gold. The images were obtained at operated at 8000 rpm for about 45 min. The as-synthesized voltage of 20 kV. 1 3 172 International Journal of Energy and Environmental Engineering (2018) 9:169–185 MWCNT/MIL-100(Fe) composite (about 0.74 g). The system Gas physisorption measurements includes CO and N cylinders, two MFC (calibrated for CO 2 2 2 and N flow rates), two check valves, and bypass tube (for cali- The first step in the physisorption measurements of CO and 2 2 N is the sample degassing to remove any guest molecules brating the gases concentration detected by mass spectrometer from the inlet gas compositions). It, moreover, includes two within the pores of each material. Typically, 50–200 mg of each sample was transferred to pre-weighed empty sam- bourdon absolute pressure (manufactured by Baumer, accu- racy ± 1.6%), mass spectrometer (to measure the outlet gases ple cell with a 9 mm diameter. Degassing was conducted at 150 °C under vacuum for about 17 h for MWCNT/ composition leaving from the bed), vacuum pump and electric heater jacket (for regeneration purpose), and interconnecting MIL-100(Fe) and 220 °C under vacuum during about 5 h for MWCNT/Mg-MOF-74 using an Autosorb degas- stainless steel fittings and tubes to regulate the flow of carrier gas within the system. ser (Quantachrome Instruments, Inc.). Nitrogen adsorp- tion isotherms at 77 K were first recorded to estimate the The first step in the operation of breakthrough setup has involved the degassing of the activated MWCNT/MIL-100(Fe) Brunauer–Emmett–Teller (BET)-specific surface area (S ), average pore radius, and total pore volume. The or MWCNT/Mg-MOF-74 composite sample at 423 K under BET vacuum for 1 day period to remove any guest gases inside the interesting equilibrium adsorption isotherms for C O at 273, 298, and 313 K, and for N at ambient temperature (298 K) pores of MWCNT/MIL-100(Fe) or MWCNT/Mg-MOF-74 frameworks. The breakthrough experiments were conducted were recorded. The CO heat of adsorption was evaluated using the adsorption isotherms measured at 273 and 313 K at 297 K and 101.3 kPa. The flowrate of the feed gas, a mix- ture of 20% C O and 80% N (v/v) for MWCNT/Mg-MOF-74 in accordance with the Clausius–Clapeyron equation. 2 2 samples and 15% C O and 85% N (v/v) for MWCNT/MIL- 2 2 100(Fe) samples, was kept constant at 20 and 10 sccm for Breakthrough experiments of binary gas mixture (CO + N ) MWCNT/Mg-MOF-74 and MIL-MWCNT/100(Fe) compos- 2 2 ites, respectively. The full breakthrough capacity of CO and A dynamic C O /N breakthrough setup was constructed to N was measured by evaluating the ratio of compositions of the 2 2 downstream gas and the feed gas. The C O adsorption capac- separate CO from a CO /N mixture (representing a flue gas), 2 2 2 2 Fig. 1. The home-made setup is composed of a bed column ity of the adsorbents is evaluated from the C O molar flowrate inters and leaves the bed using the expression : with specifications of Inner diameter = 4 mm, outer diam- eter = 6 mm, and length = 7 cm. The column was filled with the MWCNT/Mg-MOF-74 composite (about 0.26 g), or the ⎡ ⎤ � � ⎢ ⎥ q = Q C − C(t)Q(t) dt − VC , (1) co F 0 0 ⎢ ⎥ ⎣ ⎦ Fig. 1 Scheme of CO /N adsorption separation breakthrough setup 2 2 1 3 International Journal of Energy and Environmental Engineering (2018) 9:169–185 173 where q (mmo/g) represents the C O uptake, Q and Q (t) Results and discussion co 2 F (m /s) are the input and output volumetric flow rates, C and C(t) (mol/m ) are the influent and effluent CO concentra- Powder X‑ray diffraction (PXRD) analysis tions, t (s) is the time, ε is the bed porosity, and V (m ) is the bed volume. Figure 2 shows the PXRD patterns of MWCNT/Mg- The term ( VC ) in Eq. 1, which represents the C O MOF-74 compounds as well as MIL-100(Fe). It can be seen 0 2 amounts still in the bed void (without being adsorbed), (Fig. 2a) that the PXRD pattern of MWCNT/Mg-MOF-74 has very small values comparing to the other terms, so and Mg-MOF-74 samples is in good agreement with the that, it can be ignored. However, we have considered it simulated pattern. The incorporation of MWCNTs has not in our calculations. decreased the crystallinity of the framework, as all the inten- sity peaks locations have represented the Mg-MOF-74 struc- ture. Hence, it can be concluded that the incorporation of less than 1.5 wt% MWCNTs using physical mixing preserves Fig. 2 PXRD patterns for a MWCNT/Mg-MOF-74 and b MIL-100(Fe) 1 3 174 International Journal of Energy and Environmental Engineering (2018) 9:169–185 the characteristic lattice structure of the Mg-MOF-74 frame- Table 1 Pores characterization of the MWCNT/Mg-MOF-74 and MWCNT/MIL-100(Fe) composites for N at 77 K work. The same conclusion was drawn for MWCNT/MIL- 100(Fe) as reported in the recent work . Figure 2b is Characterizations S (m /g) Pore volume Average pore BET (cc/g) radius (Å) to exhibit that the synthesized MIL-100(Fe) was in good agreement with the simulated pattern. Mg-MOF-74 1518 0.63 8.31 MFC1 1545 0.66 8.55 Electron microscopy analysis MFC2 1525 0.65 8.51 MFC3 1579 0.67 8.51 The morphologies of Mg-MOF-74, MIL-100(Fe), 1.5% MFC4 1562 0.71 8.51 wt MWCNT/Mg-MOF-74, and 0.5 wt% MWCNT/MIL- MFC5 1586 0.69 8.73 100(Fe) composites observed by SEM are shown in Fig. 3. MFC6 1477 0.63 8.52 The physical incorporation of MWCNTs does not change MIL-100(Fe) 1083 0.55 10.07 the shape of the Mg-MOF-74 and MIL-100(Fe) crystals. MMC1 1248 0.61 9.74 The crystal size of Mg-MOF-74 is much bigger than that of MMC2 1464 0.69 9.52 MIL-100(Fe). MMC3 1060 0.58 10.94 Adsorption equilibrium isotherms of carbon dioxide and nitrogen 1590 m /g. In addition, the total pore volume measured at 95% relative pressure (P/P0) and the pore size measured The N equilibrium isotherms for the MWCNT/Mg-MOF-74 and MWCNT/MIL-100(Fe) composites have been meas- were determined to be almost the same for all the samples by around 0.63–0.71 cc/g and 19 Å, respectively. The Mg- ured at 77 K. Table 1 lists the important porosity-related parameters estimated from the N adsorption data MWCNT/ MOF-74 BET surface area and total pore volume values are in good agreement with those reported in the literature [17, Mg-MOF-74 and MWCNT/MIL-100(Fe) composites. The measured BET surface area was almost close to each other 50, 52]. It can be deduced from the data shown in Table 1 that the addition of MWCNTs does not result in substantial in MWCNT/Mg-MOF-74 compounds between 1470 and Fig. 3 SEM micrographs of a Mg-MOF-74, b MIL- 100(Fe), c 1.5 wt% MWCNT/ Mg-MOF-74, and d 0.5 wt% MWCNT/MIL-100(Fe) 1 3 International Journal of Energy and Environmental Engineering (2018) 9:169–185 175 differences concerning its influence on the porosity-related parameters evaluated for the MWCNT/Mg-MOF-74 com- pounds. Similarly, the incorporation of MWCNTs with MIL- 100(Fe) did not change the porosity-related parameters of pristine MIL-100(Fe). There was almost slight increase in 2 −1 the surface area from 1083 m g for base MIL-100(Fe) 2 −1 to 1464 m g for MMC2. The total pore volume at 0.95 relative pressure was around 0.61 and 0.69 cc/g for MMC1 and MMC2, respectively, in comparison to 0.55 cc/g for the pristine MIL-100(Fe). Regarding the pore size, it is clearly that the pore diameter for all the MIL-100(Fe) and compos- ite samples was around 20 Å. These porous-property val- ues are close to those reported for MWCNT/MIL-100(Fe) composites . However, the synthesized MIL-100(Fe) in the present work is not the best qualitative fashion of this adsorbent due to the method of synthesis and purification we have followed. BET and pore volume of MIL-100(Fe) could 2 −1 be varied between 1090 and 2050 m g (for BET) and from 0.65 to 1.15 cc/g (for average pore volume) . The CO adsorption isotherms for MWCNT/Mg-MOF-74 and MWCNT/MIL-100(Fe) composites, measured at 273, 298, and 313 K, are exhibited in Figs. 4 and 5. It is obvi- ous that the adsorption uptake increased sharply in the region below 15 kPa and increased gradually with increas- ing adsorption pressure greater than 20 kPa. This behavior gives a good advantage for CO capturing in low-pressure applications including the CO separation from the flue gas (P = 10–20 kPa). However, as expected, an increase in CO2 the measured temperature showed an adverse effect on the recorded uptakes for each material. As obvious from Fig. 4, the highest C O uptake has been measured for pristine Mg- MOF-74 followed by MFC1 at all the measured tempera- tures (273, 298, and 313 K). For MWCNT/MIL-100(Fe) compounds (Fig. 5), the adsorption uptake increased more or less linearly with increasing adsorption pressure. It is obvi- ous that MMC2 showed optimal adsorbed amounts, and MMC1 resulted in the second highest uptake even greater Fig. 4 CO adsorption isotherms of MWCNT/Mg-MOF-74 compos- ites at: a 273 K, b 298 K, and c 313 K than the pristine MIL-100(Fe) and MMC3 composites, as shown in Fig. 5a–c. It is worth mentioning here that the N uptake by MWCNT/MIL-100(Fe) compounds. It is clear CO uptake for MWCNT/Mg-MOF-74 composites was 2 2 much higher than that adsorbed by MWCNT/MIL-100(Fe) that MMC1 had higher adsorbed values in comparison to the other compounds. For both adsorbent composites [MWCNT/ compounds. Another point is that the best version of MIL- 100(Fe) could adsorb about 1 and 1.52 mmol/g at 0.6 bar for Mg-MOF-74 and MWCNT/MIL-100(Fe)], the maximum uptake measured for N was observed to be significantly outgassing pretreatments 150 and 250 °C, respectively . Consequently, the reduction of C O capacity of the present smaller than that measured earlier for CO . In other words, all the samples have been noticed to exhibit preferential MIL-100(Fe) (q = 0.6 mmol/g at 0.6 bar and 298 K) is due to both the followed synthesis method (using Fe(NO ) ·9H O) selectivity of CO over the N . 2 2 3 3 2 To represent isotherms in mathematical models, Toth fit- and the activation treatments (75–95 °C). The N adsorption isotherms for MWCNT/Mg-MOF-74 ting (Eq. 2) is satisfied. For instance, the CO and N values 2 2 obtained using Toth fitting of MFC6 and MMC1 show an composites, measured at 298 K, are displayed in Fig. 6a. It is evident that the pristine Mg-MOF-74 exhibited the excellent agreement with those of the experimental iso- therms as plotted in Fig. 7. The respective fitting parameters largest uptake amount, followed by MFC4, MFC1, MFC6, MFC2, MFC5, and MFC3, respectively. Figure 6b shows at 95% level of confidence are tabulated in Table 2: 1 3 176 International Journal of Energy and Environmental Engineering (2018) 9:169–185 Fig. 6 N adsorption isotherms at 298 K for a MWCNT/Mg-MOF-74 and b MWCNT/MIL-100(Fe) composites observed to exhibit a more or less curvilinear correlation with the instantaneous C O uptake, as shown in Fig. 8a. In general, Q _ values locate between 25 and 40 kJ/mol; the st CO2 high values were sequentially associated with Mg-MOF-74, MFC4, MFC2, and MFC5. In contrast, MFC5 and MCF3 showed lower values of CO adsorption heat. Mg-MOF-74 heat of adsorption profile is close to that reported by Sim- mons et a. .The MWCNT/MIL-100(Fe) composites have been measured to have C O adsorption heat values between Fig. 5 CO adsorption isotherms of MWCNT/MIL-100(Fe) compos- 18 and 28 kJ/mol [for q > 0.3 mmol/g, Fig. 8b]; the highest ites at: a 273 K, b 298 K, and c 313 K values were measured for MIL-100(Fe) about 27.3 kJ/mol (for q > 0.3 mmol/g) and the lowest obtained by MMC1 about 18 kJ/mol (for q > 0.3 mmol/g). The CO adsorption q K P m,i eq,i i heat of MIL-100(Fe) show a close values with those of the q = . 1∕ni (2) ni same adsorbent reported by Mei et al. . 1 + K P eq,i i Experimental adsorption breakthrough test for MWCNT/Mg‑MOF‑74 and MWCNT/MIL‑100(Fe) Here, q is the equilibrium adsorption amount (mmol/g) of composites species i. q , K , and n are the Toth fitting constants. m eq Figure 8a, b depicts the variation of heat of adsorption Breakthrough experiments have been performed for the for CO , Q , against the instantaneous C O uptake for 2 st 2 binary gas (C O /N ) to quantify the improvements in C O 2 2 2 MWCNT/Mg-MOF-74 and MWCNT/MIL-100(Fe) com- adsorption uptake and breakpoint as a result of the incorpo- posites. For MWCNT/Mg-MOF-74, the Q _ values were st CO2 ration MWCNTs inside Mg-MOF-74 and MIL-100(Fe). In 1 3 International Journal of Energy and Environmental Engineering (2018) 9:169–185 177 Fig. 7 Equilibrium isotherms and Toth fittings for MFC6 and MMC1 at 298 K. a MFC6 and b MMC1 a typical procedure, predetermined amounts of MWCNT/ 1 and 88 µm. It is also shown that percentage of large Mg-MOF-74 and MWCNT/MIL-100(Fe) composite sam- sizes have been decreased by adding 1.5 wt% MWCNT. ples were first transferred to a tube (length L = 7 cm, inner For MWCNT/MIL-100(Fe) particle size distribution, it is diameter ∅ = 4 mm). All breakthrough experiments have obvious from Fig. 9b that the both tested samples (MIL- been performed at ambient temperature of 297 K. 100(Fe) and MMC3) have similar distribution (between 1 Particle size used in breakthrough was measured for some and 1400 µm) with almost the same percentages. This is selective composites including pure materials and the maxi- attributed to the small percentages of MWCNT added to mum MWCNTs contents composites, as shown in Fig. 9. the adsorbents. Mg-MOF-74 and MFC6 have almost close particle size dis- For systematic tests, the pressure drop has been dimin- tribution where the particle size values allocated between ished to a round zero as monitored by two bourdon meters 1 3 178 International Journal of Energy and Environmental Engineering (2018) 9:169–185 Table 2 Toth fitting parameters from experimental CO and N iso- ratio of the bed outlet has been evaluated to be less than 2 2 therms for MFC6 and MMC1 at 95% level of confidence 5%, was measured to be about 8.16 min (28.4 min/g) for MFC6 against 7.5 min (27.67 min/g) for Mg-MOF-74. Parameter Estimate Lower limit Upper limit This was followed by the value measured for MFC4 of MFC6 about 8.1 min, and then by 7.96 min for MFC1 (Fig. 10a). CO In the same manner, the highest breakthrough breakpoint q (mmol/g) 12.952 9.843 16.062 obtained by MWCNT\MIL-100(Fe) was associated with K (1/kPa) 0.666 0.266 1.067 Equation MMC2 by about 3.21 min (4.33 min/g) (Fig. 10b). The n 0.342 0.262 0.423 next breakthrough point was obtained by MMC1 at about N 3.19 min (4.32 min/g), and, then, by pristine MIL-100(Fe) q (mmol/g) 2.430 0.979 3.881 at about 2.9 min (3.69 min/g). K (1/kPa) 0.0029 0.0012 0.0045 Equation Adsorption breakthrough and separation processes n 1.399 0.921 1.876 can be also investigated numerically as described in our MMC1 previous works [57–59]. In addition, adsorption break- CO through curves, can be analytically represented by fitting q (mmol/g) 21.628 6.878 36.379 the experimental curves using some approaches reported −4 −4 −4 K (1/kPa) 8.42 × 10 3.17 × 10 13.66 × 10 Equation in the literature; one of these approaches is expressed in n 0.529 0.425 0.634 the following equation [60, 61], N � � √ √ q (mmol/g) 0.157 − 0.040 0.354 √ √ C_out 1 −4 −4 −4 = erfc − − − , K (1/kPa) 69.1 × 10 − 17.24 × 10 155.56 × 10 (3) Equation C_in 2 8 8 n 6.320 − 9.136 21.776 where L 1 − at the inlet and outlet of the bed. The adsorbent bed was = kK , (4) packed with almost the same packing density by about 0.292 ± 0.005 and 0.842 ± 0.002 g/cc for MWCNT/ Mg-MOF-74 and MWCNT/MIL-100(Fe) compounds, = k t − . (5) respectively. The samples have been pre-treated by heat- ing process for 20 h at about 423 K under vacuum. The Here, k an K are fitting constant; k (1/s), called adsorp- experimentally measured CO and N adsorption break- 2 2 tion time constant, could be used to determine the diffu- 15D 2 through curves for MWCNT/Mg-MOF-74 and MWCNT/ sion coefficient ( k = , D (m /s) is the diffusion coef- MIL-100(Fe) composites are displayed in Fig. 10. As ficient and r (m) is the adsorbent particle radius), L (m) evident, the outlet concentration ratios calculated each of is the bed length, and v (m/s) is the flow velocity. The these two gases have been plotted against the measurement breakthrough time (t) is taken in seconds. time. In general, it was observed in all the tested sam- For example, Fig. 11 shows the analytical and exper- ples that the concentration ratio evaluated for CO at the imental adsorption breakthrough of carbon dioxide bed outlet remained constant at zero for some time (e.g., adsorbed by MWCNT/Mg-MOF-74 composites. The about 6–7 min for MWCNT/Mg-MOF-74 and 2–3 min for analytical results using Eq. (3) provide a good indica- MWCNT/MIL-100(Fe) compounds), whereas the concen- tion to the whole adsorption time and behavior that are tration ratio for N increased up to about 1.3 (almost molar as close as to those of the experimental curves. The time fraction = 1) owing to the absence of CO which was pre- 2 constant values (k) obtained from analytical curves are adsorbed into the Mg-MOF-74 or MIL-100(Fe) compos- 0.258, 0.248, 0.164, 0.392, 0.433, 0.368, and 0.255 1/s for ite adsorbent bed. Following the first adsorption minutes Mg-MOF-74, MFC1, MFC2, MFC3, MFC4, MFC5, and of measurement time, the CO concentration ratio was MFC6, respectively. observed to increase up to 1, whereas the concentration To evaluate the improvements of C O adsorption capac- ratio of N was evaluated to gradually drop to a value close ity and breakpoint by adding MWCNT to Mg-MOF-74 and to 1. For MWCNT/Mg-MOF-74 composites, the optimal to MIL-100(Fe), the adsorbed amounts of CO have been value of the breakpoint, a time at which the concentration calculated from the experimental breakthrough curves 1 3 International Journal of Energy and Environmental Engineering (2018) 9:169–185 179 Fig. 8 CO heat of adsorption (Q ) a MWCNT/Mg-MOF-74 st and b MWCNT/MIL-100(Fe) composites using Eq. 1. The maximum CO adsorption capacity for respectively. This pair of statistics was followed by MFC1, base Mg-MOF-74 and MIL-100(Fe) calculated from the MFC4, and MFC5 composites for which the correspond- respective breakthrough curve was estimated to be about ing improvements in adsorption capacity and breakpoint 5.46 and 0.37 mmol/g, respectively. The maximum CO values over pristine Mg-MOF-74 have been evaluated to uptakes along with the adsorption breakpoint ratios for be 4.43 and 5.71, 2.21 and 7.3%, and 1.49 and 4.98%, Mg-MOF-74 as well as each of the six MWCNT/Mg- respectively. It is worth mentioning here that each of the MOF-74 composites are displayed in Fig. 12a. As evi- MFC6, MFC1, and MFC4 composites has already been dent, each of the six composites, except MFC2 and MFC3, characterized for lower values of heat of adsorption for exhibited a good improvement over pristine Mg-MOF-74 CO in comparison with pristine Mg-MOF-74, as shown with regard to both the adsorption capacity and the adsorp- earlier in Fig. 8a. This, theoretically, implies that each tion breakpoint ratio values. More specifically, the most of these composites should not only exhibit higher CO optimum combination of adsorption capacity and break- uptake values than pristine Mg-MOF-74, but also require point ratio values have been evaluated for MFC6 which comparatively lower energy for regeneration process (recy- has shown an improvement of 7.35 and 8.03% over pristine cling recovery). Mg-MOF-74 for adsorption capacity and breakpoint ratio, 1 3 180 International Journal of Energy and Environmental Engineering (2018) 9:169–185 −1 −1 of CNT is significantly high (2000–5000 W m K ) , the MWCNT/Mg-MOF-74 and MWCNT/MIL-100(Fe) composites’ effective thermal conductivity values could accordingly be higher than those of pristine adsorbents −1 −1 (0.2–0.4 W m K ). Therefore, the heat diffusion across the bulk composites is enhanced during adsorption processes, which helps in cooling down the adsorbent and enhances CO adsorption uptake. Furthermore, the enhancement of effective thermal conductivity of adsorbents helps in quickly heating the adsorbent particles during and desorption pro- cess which in turns accelerating the C O evacuation from the adsorbent. This research confirms a comparative dynamic CO uptake compared to the published data shown in Table 3. The MIL-100(Fe) shows the lowest values of adsorp- tion in a comparison to AC, 13X and Mg-MOF-74, while Mg-MOf-74 and MFC6 have the highest CO uptake. Mg- MOF-74 dynamic CO uptake was about 5.46 mmol/g which is greater than 4.06 mmol/g reported in the litera- ture [17, 50], because it was, in this study, measured at 20% CO molar fraction. The cost of adding very low quantity of MWCNT (< 1.5 wt%) to the adsorbents is believed to be neglected in a comparison to the C O separation improvements. In the literature, chemists usually use the adsorption isotherm data to compare the C O capacities of different adsorbents. However, we found out by carrying both adsorp- tion isotherm measurements and adsorption breakthrough experiments that they can give different ratings of adsorption Fig. 9 Particle size distribution (microns) for a Mg-MOF-74 and MFC6, and b MIL-100(Fe) and MMC3 capacity. Keeping in mind that adsorption isotherm meas- urements are taken under constant temperatures, while the breakthrough measurements are not, as the breakthrough Figure 12b shows the improvement in both adsorption bed is allowed to vary its temperature due to the heat dis- capacity and breakpoint due to adding MWCNT to the pris- sipation from the adsorbent to the ambient or surrounding tine MIL-100(Fe). As evident, MMC1 exhibited an optimal environments. The improved thermal diffusion cools down improvement reaches 12.02 and 9.21% for CO adsorption the adsorbent quickly. Therefore, the cooler is adsorbent, capacity and breakpoint, respectively. This improvement the higher is CO uptake which is also confirmed in the was followed by MMC2 for measured adsorption uptake and isotherms. The most accurate adsorption capacity if we are breakpoint about 8.74 and 9.47%, respectively, comparing joining to use a PSA/VSA/TSA is that measured in a break- with the base adsorbent (MIL-100(Fe)). On the contrary, the through setup. evaluated adsorption uptake and breakpoint improvement values for MFC2, MFC3, and MMC3 showed lower perfor- mance than the base adsorbents. This attribute indicates that Conclusions there is no a uniform improvement can be obtained for the incorporation of CNT with MOFs. The detected improve- Two types of MOFs, Mg-MOF-74 and MIL-100(Fe), were ment in the C O adsorption capacity and breakpoint primar- synthesized and incorporated with MWCNTs. In total, seven ily refers to an improvement in the thermal properties of compounds of Mg-MOF-74 materials containing 0, 0.1, Mg-MOF-74 and MIL-100(Fe) frameworks upon the incor- 0.25, 0.5, 0.75, 1, and 1.5 wt% MWCNTs and four com- poration of MWCNTs [30–32]. As the thermal conductivity pounds of MIL-100(Fe) involving 0, 0.1, 0.25, and 0.5 wt% 1 3 International Journal of Energy and Environmental Engineering (2018) 9:169–185 181 Fig. 10 Breakthrough curves for a MWCNT/Mg-MOF-74 composites for CO /N (0.2/0.8 2 2 v/v) and b MWCNT/MIL- 100(Fe) composites for CO / N (0.15/0.85 v/v), measured at 297 K and 101.3 kPa Fig. 11 Analytical represen- tation of CO breakthrough curves of MWCNT/Mg- MOF-74 composites; solid lines are analytical results; symbols are experimental data 1 3 182 International Journal of Energy and Environmental Engineering (2018) 9:169–185 Fig. 12 CO uptake (cubic bars) and adsorption breakpoint (cylindrical bars) enhancements for a MWCNT/Mg-MOF-74 and b MWCNT/MIL-100(Fe) composites measured at 297 K Table 3 Dynamic CO uptake of adsorbents BET surface area, and pore volume and size, indicating that the crystal lattices of Mg-MOF-74 and MIL-100(Fe) were Adsorbent Tempera- CO vol% CO capacity References 2 2 unaffected by the incorporation of MWCNTs using the phys- ture (K) (mmol/g) ical mixing (up to 1.5 wt% MWCNT for Mg-MOF-74 and Mg-MOF-74 298 15 4.06 [17, 50] 0.5 wt% MWCNT for MIL-100(Fe). AC 301 20 0.734  Equilibrium adsorption isotherms of CO measured at MIL-101(Cr) 298 10 0.49  273, 298, and 313 K, and N adsorption isotherms meas- 13X 297 20 2.56  ured at 298 K confirm that the highest adsorption capacities CNT/13X 297 20 3.29  for each of these two gases are exhibited by Mg-MOF-74 Mg-MOF-74 297 20 5.46 This work and 0.25 wt% MWCNT/MIL-100(Fe) (MMC2). Overall, MFC6 297 20 5.86 This work the MWCNT/Mg-MOF-74 composites have much larger MIL-100(Fe) 297 15 0.37 This work adsorption uptake values than those of MWCNT/MIL- MMC1 297 15 0.41 This work 100(Fe) composites. The key performance evaluation of the MWCNT/Mg- MOF-74 and MWCNT/MIL-100(Fe) composites has been achieved through the measurement of actual time-variant MWCNT have been characterized for the degree of crystal- linity, intrinsic porosity, CO adsorption capacity and sepa- CO breakthrough curves, which have revealed a good improvement in CO adsorption capacity as well as adsorp- ration, and dynamic adsorption breakthrough tests. The pow- der X-ray diffraction patterns as well as the porosity-related tion breakpoint due to the incorporation of MWCNTs in the Mg-MOF-74 and MIL-100(Fe) frameworks. The most opti- parameters for each of the composites did not include any substantial variation in peak intensities and peak locations, mum combination of these characteristics has been observed 1 3 International Journal of Energy and Environmental Engineering (2018) 9:169–185 183 10. 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