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Rationally tuned micropores within enantiopure metal-organic frameworks for highly selective separation of acetylene and ethylene

Rationally tuned micropores within enantiopure metal-organic frameworks for highly selective... ARTICLE DOI: 10.1038/ncomms1206 Received 29 sep 2010 | Accepted 24 Jan 2011 | Published 22 Feb 2011 Rationally tuned micropores within enantiopure metal-organic frameworks for highly selective separation of acetylene and ethylene 1 1 1 2 3 1 sheng-Chang Xiang , Zhangjing Zhang , Cong-Gui Zhao , Kunlun Hong , Xuebo Zhao , De-Rong Ding , 4 4 1 3 5 1 ming-Hua Xie , Chuan-De Wu , madhab C. Das , Rachel Gill , K. mark Thomas & Banglin Chen separation of acetylene and ethylene is an important industrial process because both compounds are essential reagents for a range of chemical products and materials. Current separation approaches include the partial hydrogenation of acetylene into ethylene over a supported Pd catalyst, and the extraction of cracked olefins using an organic solvent; both routes are costly and energy consuming. Adsorption technologies may allow separation, but microporous materials exhibiting highly selective adsorption of C H /C H have not been 2 2 2 4 realized to date. Here, we report the development of tunable microporous enantiopure mixed- metal-organic framework (m′moF) materials for highly selective separation of C H and C H . 2 2 2 4 The high selectivities achieved suggest the potential application of microporous m′moFs for practical adsorption-based separation of C H /C H . 2 2 2 4 1 2 Department of Chemistry, University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249-0698, USA. Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA. Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese 4 5 Academy of Sciences, Qingdao 266101, China. Department of Chemistry, Zhejiang University, Hangzhou 310027, China. Northern Carbon Research Laboratories, Sir Joseph Swan Institute and School of Chemical Engineering and Advanced Material, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, UK. Correspondence and requests for materials should be addressed to K.M.T. ([email protected]) or to B.C. ([email protected]). nATuRE C ommunICATIons | 2:204 | DoI: 10.1038/ncomms1206 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. ARTICLE n ATu RE Commun ICATIons | Do I: 10.1038/ncomms1206 recise control of pore sizes and pore surfaces within porous materials is very important for their highly selective recogni- R R Ption and thus separation of small molecules, but is very chal- N N lenging and difficult to achieve in traditional zeolite materials . e Th Cu situation has been changing since the emergence of a new type of N O O N porous material, so-called microporous metal-organic frameworks (MOFs) or porous coordination polymers. e Th pores within porous MOFs, particularly those within isoreticular MOFs whose structures are pre-determined by the coordination geometries of the second- OH Zn(NO ) 6H O 3 2 2 ary building blocks, can be systematically modified by changing the OH organic bridging linkers and controlling the framework interpen- 2–5 etration . Furthermore, the pore surfaces of porous MOFs can be functionalized by the immobilization of different recognition sites, HO such as open metal sites, Lewis basic/acidic sites and chiral pockets, 6–14 to direct the recognition of small molecules . Systematically tun- O HO ing micropores can achieve size-specific encapsulation of small gas . . Zn (BDC) [Cu(SalPycy)] (G) Zn (CDC) [Cu(SalPycy)] (G) 3 3 x 3 3 x molecules, and immobilization of functional sites enables varying M'MOF-2 M'MOF-3 substrate interactions: microporous MOF materials have emerged 1.6 Enhanced separation selectivity of C H / C H 25.5 as promising microporous media for the recognition and separation 2 2 2 4 15–32 of small molecules . 21 Enhanced chiral recognition of 1-phenylethyl alcohol (ee) 64 Kitagawa pioneered the research on construction of porous mixed-metal-organic frameworks (M′MOFs) by making use of M- Figure 1 | Syntheses and separation capacities of M′MOFs-2 and -3. Salen metalloligands in 2004 (refs 15, 16). Such a novel approach eventually led to few porous M′MOFs for heterogeneous asymmet- 4,17,18 ric catalysis and enantioselective separation . Recently, we have pyridine-3-carbaldehyde with (1R,2R)-cyclohexanediamine. successfully developed this metalloligand or pre-constructed build- Reaction of Cu(NO ) ·2.5H O with H SalPyCy formed the pre- 3 2 2 2 ing block approach to construct porous MOFs, and realized the constructed building block Cu(H SalPyCy)(NO ) that was easily 2 3 2 first such mixed-metal-organic framework (M ′MOF) Zn (BDC) incorporated into M′MOF-2 and -3 by the solvothermal reactions 3 3 [Cu(SalPyen)]·(G) (M′MOF-1; BDC = 1,4-benzenedicarboxylate; with Zn(NO ) ·6H O and H BDC or H CDC, respectively, in x 3 2 2 2 2 G = guest molecules) with permanent porosity as clearly established dimethylformamide (DMF) at 373 K as dark-blue thin plates. by both gas and vapour sorption . This new M ′MOF approach has e Th y were formulated as Zn (BDC) [Cu(SalPyCy)]·5DMF·4H O 3 3 2 provided us with a new ability to tune and functionalize the micro- (M′MOF-2) and Zn (CDC) [Cu(SalPyCy)]·5DMF·4H O (M′MOF-3) 3 3 2 pores within this series of isoreticular M′MOFs by: the incorpora- by elemental microanalysis and single-crystal X-ray diffraction tion of different secondary organic linkers; the immobilization of (XRD) studies, and the phase purity of the bulk material was 2 + different metal sites other than Cu ; the introduction of chiral independently conr fi med by powder XRD. e Th desolvated MMOFs- pockets/environments through the usage of chiral diamines; and 2a and -3a for the adsorption studies was prepared from the derivatives of the precursor by the usage of other organic groups methanol-exchanged samples followed by the activation under ultra- such as t-butyl instead of methyl group. And thus to explore novel high vacuum at room temperature. e Th XRD profiles of desolvated functional microporous M′MOFs for their recognition and separa- MMOFs-2a and -3a indicate that they maintain the crystalline tion of small molecules. We initiated studies on such endeavour by framework structures (Supplementary Figs S14–S16). making use of Cu-Salen pre-constructed building blocks because of X-ray single-crystal structures reveal that M′MOF-2 and -3 are the straightforward and easy synthesis of the Cu-Salen precursors isostructural three-dimensional frameworks, in which Zn (COO) 3 6 and the resulting porous M′MOFs. secondary building blocks are bridged by BDC or CDC anions to To make use of chiral (R, R)-1,2-cyclohexanediamine to con- form the 3 two-dimensional tessellated Zn (BDC) or Zn (CDC) 3 3 3 3 struct the chiral metalloligand Cu(SalPyCy), enantiopure M′MOF sheets that are further pillared by the Cu(SalPyCy) (Fig. 2, Supple- Zn (BDC) [Cu(SalPycy)]·(G) (M′MOF-2), which is isostructural mentary Figs S1 and S2). Topologically, M′MOF-2 and -3 can be des- 3 3 x 6 18 3 to the nonchiral Zn (BDC) [Cu(SalPyen)]·(G) (M′MOF-1), can be cribed as a hexagonal primitive networks (Schäi fl symbol 3 4 5 6), 3 3 x readily assembled by the solvothermal reaction of this chiral build- which are the same as its achiral analogue Zn (BDC) [Cu(SalPyen)] 3 3 ing block with Zn(NO ) and H BDC, leading to the chiral cavities (ref. 33); however, the incorporation of chiral metalloligand 3 2 2 within M′MOF-2 (Fig. 1). Furthermore, such chiral cavities can be Cu(SalPycy) leads to enantiopure M′MOF-2 and -3, exhibiting two straightforwardly tuned by the incorporation of different bicarboxy - chiral pore cavities of about 6.4 Å in diameter (Fig. 2a,b). e Th chi - late CDC (CDC = 1,4-cyclohexanedicarboxylate) for their enhanced ral pore cavities are filled with the disordered solvent molecules in recognition and separation of small molecules. which M′MOF-2 and -3 have the pore accessible volume of 51.7 and Herein, we report the synthesis, structures, sorption and chiral rec- 48.1%, respectively, calculated using the PLATON program . ognition studies on these two new M′MOFs Zn (BDC) [Cu(SalPycy 3 3 )]·(G) (M′MOF-2) and Zn (CDC) [Cu(SalPycy)]·(G) (M′MOF-3). Microporous nature of M′MOFs. To establish the permanent x 3 3 x M′MOF-3 exhibits significantly enhanced selective separation of porosity, the methanol-exchanged M′MOFs-2 and -3 were activated C H /C H and enatioselective recognition of 1-phenylethyl alcohol under high vacuum at room temperature overnight to form the des- 2 2 2 4 (PEA) than M′MOF-2, highlighting the first example of micropo - olvated M′MOF-2a and -3a. Nitrogen adsorption on the activated rous materials for highly selective separation of C H /C H , a very M′MOFs-2a and -3a at 77.3 K was very slow because of the acti- 2 2 2 4 important industrial separation. vated diffusion effects. er Th efore, CO adsorption at 195 K was used for their pore characterization. Surprisingly, they exhibit remark- Results ably different sorption isotherms attributed to the different dicarbo - 3 − 1 Syntheses and structural characterization of M′MOFs. e Th new xylates. e Th uptake of M ′MOF-2a (158 cm g ) is about twice than 3 − 1 Salen-type chiral Schiff base of pyridine derivative H SalPyCy that of M′MOF-3a (86 cm g ) at P/P of 1 (Fig. 3). Both M′MOF- 2 0 was prepared by condensation of 5-methyl-4-oxo-1,4-dihydro- 2a and -3a show hysteretic sorption behaviours, indicating their n ATu RE Commun ICATIons | 2:204 | Do I: 10.1038/ncomms1206 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. n ATu RE Commun ICATIons | Do I: 10.1038/ncomms1206 ARTICLE T = 195 K a c a 0.0 0.2 0.4 0.6 0.8 1.0 P /P T = 195 K c T = 195 K b d 0 100 200 300 400 500 600 700 800 0 100 200 300 400 500 600 700 800 P (mmHg) P (mmHg) e T = 295 K d T = 273 K 60 50 Figure 2 | X-ray crystal structures of M′MOF-3 and M′MOF-3S-PEA. 6 18 3 (a) The hexagonal primitive network topology (s chäfli symbol 3 4 5 6) and (b) the three-dimensional (3D) pillared framework with chiral pore cavities for m′mo F-3. (c) The hexagonal primitive network topology and 0 200 400 600 800 0 200 400 600 800 (d) the 3D pillared framework exclusively encapsulating s -PEA molecules P (mmHg) P (mmHg) for m′mo F-3s -PEA. (Zn, pink; Cu, cyan; o , red; C, grey; n , blue; H, white). f g T = 273 K T = 295 K 50 50 framework flexibility and the existence of the meta-stable interme - 40 diate frameworks, which have been also observed in other flexible 30 30 porous MOFs e Th Langmuir (BET) surface areas calculated from 2 − 1 20 20 the first step adsorption isotherms are 598(388) and 237(110) m g for M′MOF-2a and -3a, respectively, within the pressure range of 0.05 < P/P < 0.3 (Supplementary Figs S3–S5). Assuming that the 0 0 second step isotherms still t fi into the monolayer coverage model, 0 200 400 600 800 0 200 400 600 800 the overall Langmuir surface areas of M′MOF-2a and -3a are 939 P (mmHg) P (mmHg) 2 − 1 and 551 m g . eir Th total pore volumes from the highest P/P values and pore volumes corresponding to the intermediate iso- Figure 3 | Gas sorption isotherms on the two activated M′MOFs at 3 − 1 therm step are 0.301 (0.189) and 0.164 (0.049) cm g for M′MOF-2a different temperatures. (a) Co sorption on m′mo Fs-2a (blue square) and M′MOF -3a, respectively. and -3a (red dot) at 195 K. (b) C H (green square) and C H (blue 2 2 2 4 triangle) on m′mo F-2a at 195 K. (c) C H (green square) and C H (blue 2 2 2 4 Separation of acetylene and ethylene within M′MOFs. e Th unique triangle) on m′mo F-3a at 195 K. (d) C H (blue square), Co (red dot) 2 2 2 CO sorption isotherms encouraged us to examine the capacities and C H (green triangle) on m′mo F-2a at 273 K. (e) C H (blue square), 2 4 2 2 of M′MOF-2a and -3a for their selective separation of C H /C H 2 2 2 4 Co (red dot) and C H (green triangle) on m′mo F-2a at 295 K. (f) C H 2 2 4 2 2 at 195 K, given the fact that these two molecules have comparable (blue square), Co (red dot) and C H (green triangle) on m′mo F-3a at 2 2 4 molecular sizes with CO . For M′MOF-2a (Fig. 3b), the shapes of 273 K. (g) C H (blue square), Co (red dot) and C H (green triangle) on 2 2 2 2 4 the isotherms are complex, which are apparently attributed to the m′mo F-3a at 295 K. Adsorption and desorption branches are shown with framework flexibility during adsorption. e Th total pore volumes closed and open symbols, respectively. were calculated from the highest P/P values (P/P ~0.99) using 0 0 − 3 densities of 0.577, 0.726 and 1.032 g cm for the densities of C H , 2 4 C H and CO , respectively. e Th total pore volumes were 0.306, and 195 K (Fig. 3c). Accordingly, the total pore volumes were dif- 2 2 2 3 − 1 3 − 1 0.309 and 0.301 cm g for C H , C H and CO , respectively, which ferent, of 0.066, 0.236 and 0.165 cm g for C H , C H and CO , 2 4 2 2 2 2 4 2 2 2 are basically the same, indicating that all three gas molecules can respectively, as calculated from their highest P/P values, indicating have the full access to the pores within M′MOF-2a. M′MOF-3a, that the three gas molecules have differential degree of access to the however, exhibits significantly different sorption behaviours with pores at 1 atm and 195 K when the pores within M′MOF-3a become respect to C H and C H . M′MOF-3a can take up the acetylene up smaller. Such subtle pore control is very important for these kinds 2 4 2 2 3 − 1 to 147 cm g with a one-step hysteresis loop, whereas only small of porous materials to exhibit highly selective gas separation. In fact, 3 − 1 amount of ethylene (30.2 cm g ) without the marked loop at 1 atm M′MOF-2a can only slightly differentiate C H from C H with a low 2 2 2 4 n ATu RE Commun ICATIons | 2:204 | Do I: 10.1038/ncomms1206 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. 3 –1 3 –1 3 –1 Adsorbed amount (cm (STP) g ) Adsorbed amount (cm (STP) g ) Adsorbed amount (cm (STP) g ) 3 –1 Adsorbed amount (cm (STP) g ) 3 –1 3 –1 3 –1 Adsorbed amount (cm (STP) g ) Adsorbed amount (cm (STP) g ) Adsorbed amount (cm (STP) g ) ARTICLE nATuRE C ommunICATIons | DoI: 10.1038/ncomms1206 selectivity of 1.6, whereas M′MOF-3a displays significantly higher organic linker and metalloligand within M′MOF-3a has enlarged selectivity of 25.5 and thus can exclusively separate C H from C H the pore apertures for the access of both C H and C H molecules. 2 2 2 4 2 2 2 4 (Table 1). In the diffusion of molecules into spherical or rectangular Such kinetically controlled framework flexibility has been also pores both the cross-section dimensions are important, whereas for revealed in several other porous MOFs and used for their tempera- 20,26 slit shaped pores only the smallest dimension is important in deter- ture-dependent gas separation . mining selectivity. Such significantly enhanced separation capacity of M′MOF-3a over M′MOF-2a is attributed to the smaller micropo- Interactions of gas molecules with M′MOFs. e Th coverage- res within M′MOF-3a, which favours its higher size-specific sepa - dependent adsorption enthalpies of the M′MOFs to acetylene, eth- ration effect on the C H /C H separation. e Th narrower molecu - ylene and CO were calculated based on the virial method and the 2 2 2 4 2 lar size of C H (3.32 × 3.34 × 5.70 Å ) compared with that of C H van’t Hoff isochore. e Th virial graphs for adsorption of C H , C H 2 2 2 4 2 4 2 2 (3.28 × 4.18 × 4.84 Å ) has enabled the full entrance of the C H into and CO on M′MOF-2a and -3a at 273 and 295 K are shown in Sup- 2 2 2 the micropores in M′MOF-3a, whereas C H molecules are basically plementary Figures S6–S11. It is apparent that the virial graphs have 2 4 blocked or the kinetics are very slow. very good linearity in the low-pressure region. e Th parameters and e Th adsorption isotherms of C H , C H and CO on M′MOF- the enthalpies obtained from the virial equation are summarized 2 2 2 4 2 2a and -3a were further measured at 273 and 295 K (Fig. 3). in Table 2. For M′MOF-2a, C H adsorption had A values increas- 2 4 1 − 1 e Th y showed type I sorption isotherms with very little hysteresis. ing from − 1,770 to − 1,551 g mol from 273 to 295 K, which has a e Th selectivities towards C H /C H on M′MOF-2a at 273 and similar trend of A values on a carbon molecular sieve increasing 2 2 2 4 1 − 1 295 K were 1.5 and 1.9, respectively. Again, M′MOF-3a exhibited from − 2,480 to − 1,821 g mol from 303 to 343 K (ref. 35); C H 2 2 − 1 enhanced C H /C H selectivities of 4.1 and 5.2 at 273 and 295 K, adsorption had A values increasing from − 1,621 to − 1,353 g mol 2 2 2 4 1 respectively; which are 2.5 times higher than the corresponding from 273 to 295 K, which also has a similar trend of A values on a − 1 values for M′MOF-2a (Table 1). carbon molecular sieve increasing from − 1,444 to − 1,302 g mol e Th unique and temperature-dependent gas separation capaci - from 303 to 343 K. It is apparent that the virial parameters for C H 2 4 ties of M′MOF-3a are attributed both to thermodynamically and and C H adsorption have similar values and trends. CO adsorp- 2 2 2 5,26 − 1 kinetically controlled framework flexibility . e Th more flexible tion on M′MOF-2a had A values from − 1,071 to − 940 g mol nature of CDC in M′MOF-3 has enabled the framework M′MOF-3 from 273 to 295 K without well-defined trend, which has been also more flexible, thus resulting in narrower pores in activated M ′MOF- observed in CO adsorption on a carbon molecular sieve with the A 2 1 − 1 3a than those in M′MOF-2a, as shown in their powder XRD pat- values ranging from − 1,000 to − 1,045 g mol from 303 to 343 K. terns (Supplementary Fig. S16). To open pore entrances for the e Th trends in the A parameters for C H , C H and CO adsorp- 1 2 4 2 2 2 C H uptake, the gate pressure of 166 mmHg needs to be applied tion on M′MOF-2a are consistent with the adsorbate–adsorbate 2 2 on the thermodynamically flexible framework M ′MOF-3a at 195 K. interactions decreasing with increasing temperature. In compari- At higher temperature of 273 and 295 K, the rotation/swing of the son with those for C H and C H adsorption on M′MOF-2a, the 2 4 2 2 Table 1 | The Henry’s constants or the product of the Langmuir equation constants (q ×b) and the equilibrium selectivity for gases on the two M′MOFs. 3 − 1 − 1 Henry’s constants K or q ×b (cm g torr ) M′MOF-2a M′MOF-3a Temperature (K) 195 273 295 195 273 295 C H 239.37×0.0057 55.76×0.0078 55.09×0.0044 179.99×0.0058 50.36×0.0071 48.30×0.0058 2 2 Co 194.65×0.0064 49.57×0.0044 43.14×0.0030 98.64×0.0089 30.18×0.0025 27.75×0.0012 − 5 C H 178.88×0.0048 38.72×0.0073 39.71×0.0031 710.17×5.75×10 21.90×0.0040 12.45×0.0043 2 4 Selectivity α C H /Co 1.10 2.00 1.89 1.18 4.74 8.41 2 2 2 Co /C H 1.46 0.77 1.02 21.60 0.86 0.62 2 2 4 C H /C H 1.61 1.54 1.93 25.53 4.08 5.23 2 2 2 4 Table 2 | Summary of the parameters and the enthalpies of gas adsorption on M′MOFs at 273 and 295 K obtained from the virial equation. − 1 − 1 − 1 2 − 1 Compounds Adsorbate T (K) A /ln (mol g Pa ) A /g mol R Q /kJ mol 0 1 st,n=0 m′moF- 2a C H 273 − 15.234 ± 0.059 − 1770.328 ± 6.856 0.99975 32.7 2 4 295 − 16.302 ± 0.054 − 1551.378 ± 7.704 0.99946 C H 273 − 14.070 ± 0.012 − 1621.285 ± 11.438 0.99950 37.7 2 2 295 − 15.300 ± 0.011 − 1353.201 ± 11.133 0.99892 Co 273 − 15.591 ± 0.013 − 1070.628 ± 35.231 0.99247 32.5 295 − 16.652 ± 0.007 − 939.937 ± 15.613 0.99697 m′moF- 3a C H 273 − 17.020 ± 0.003 − 2143.060 ± 6.940 0.99983 27.3 2 4 295 − 17.906 ± 0.018 − 2199.728 ± 94.199 0.99452 C H 273 − 14.203 ± 0.026 − 2056.965 ± 25.871 0.99811 27.1 2 2 295 − 15.087 ± 0.042 − 1693.176 ± 41.686 0.99158 Co 273 − 16.679 ± 0.026 − 3117.335 ± 137.777 0.99031 40.5 295 − 17.999 ± 0.007 − 1902.594 ± 33.455 0.99753 nATuRE C ommunICATIons | 2:204 | DoI: 10.1038/ncomms1206 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. n ATu RE Commun ICATIons | Do I: 10.1038/ncomms1206 ARTICLE virial parameters for C H and C H adsorption on M′MOF-3a are molecules and CDC moieties on surfaces of pores in M′MOF-3a. 2 4 2 2 more negative due to its smaller pores, but still have similar values e Th results derived from the linear extrapolation are in very good and trends from 273 to 295 K. e Th fact that the A values for C H agreement with those obtained from the virial equation (Fig. 4). It 1 2 4 adsorption on M′MOF-3a are not obviously changed indicates that needs to be mentioned that the Q values for C H and C H adsorp- st 2 4 2 2 the adsorbate–adsorbate interactions might be independent of tem- tion on M′MOF-3a are almost the same, indicating that the selec- perature from 273 to 295 K (Supplementary Fig. S11c). tive C H /C H separation cannot be realized by their differential 2 2 2 4 − 1 e Th Q values were 32.7, 37.7 and 32.5 kJ mol for C H , C H interactions with the pore surfaces, thus, the unique gas separation st,n = 0 2 4 2 2 and CO adsorption on M′MOF-2a over the temperature range of characteristics for M′MOF-3a are mainly attributed to size-exclu- 273–295 K. e Th Q values for C H and CO were comparable sive effect. st,n = 0 2 2 2 to those obtained for their adsorption on a carbon molecular sieve − 1 − 1 (35.0 kJ mol (C H ) and 28.2 kJ mol (CO )), whereas the value of Enantiopure selective separation of PEA. e Th enantiopure pore 2 2 2 − 1 32.7 kJ mol for C H adsorption on M′MOF-2a was significantly environments within M′MOF-2 and -3 motivated us to explore 2 4 − 1 lower than the value (50.1 kJ mol ) obtained for C H adsorption their potential for chiral recognition and enantioselective separa- 2 4 on a carbon molecular sieve in the temperature range of 303–343 K. tion. Unlike the achiral M′MOF-1 Zn (BDC) [Cu(SalPyen)], which 3 3 e Th comparison of the results from the two methods, the linear encapsulates both R- and S-1-PEA to form Zn (BDC) [Cu(SalPyen)] 3 3 extrapolation and the virial equation shows that there is a very good R/S-PEA (Supplementary Fig. S12), the enantiopure M′MOF-3 agreement (Fig. 4). In all cases the isosteric enthalpies of adsorp- exclusively takes up S-PEA to form M′MOF-3S-PEA (Zn (CDC) 3 3 tion gradually decreased with the increasing surface coverage. As [Cu(SalPyCy)]·S-PEA). In fact, the solvothermal reaction of the expected, the isosteric enthalpies of adsorption are significantly corresponding reaction mixture of Zn(NO ) ·6H O, H CDC and 3 2 2 2 − 1 higher than the enthalpies of vaporization of 17, 14 and 16.5 kJ mol Cu(H SalCy)(NO ) in the presence of certain amount of racemic 2 3 2 for C H , C H and CO , respectively . e Th isosteric enthalpies of PEA in DMF at 100 °C readily formed the enantiopure M′MOF-3, 2 4 2 2 2 adsorption for C H , C H and CO on M′MOF-2a are characteristic which exclusively encapsulates S-PEA (Fig. 2d). e Th incorporated 2 2 2 4 2 of their interactions with the hydrophobic pore surfaces presented S-PEA can be easily extracted by immersing the as-synthesized in carbon molecular sieves. M′MOF-3S-PEA in methanol. e Th Q for C H , C H and CO adsorption on M′MOF-3a e Th chiral recognition and enantioselective separation of st,n = 0 2 4 2 2 2 − 1 were 27.4, 27.1 and 40.5 kJ mol , respectively, over the temperature M′MOF-2 and -3 for the R/S-PEA racemic mixture were exam- range from 273 to 295 K. e Th systematically lower Q values for ined using the bulky as-synthesized materials. e Th as-synthe - st,n = 0 C H and C H adsorption on M′MOF-3a than those observed on sized M′MOF-2 and -3 were exchanged with methanol and then 2 4 2 2 − 1 − 1 M′MOF-2a (32.7 kJ mol for C H and 37.7 kJ mol for C H ) may immersed in the racemic mixture to selectively encapsulate the 2 4 2 2 be attributed to the deficiency of π–π interactions between these S-PEA. Once such PEA-included M′MOF-2 and -3 were immersed in methanol, the encapsulated PEA within the enantiopure M′MOF-2 and -3 can be readily released from the chiral pores, making their a d C H on M’MOF-2a C H on M’MOF-3a 2 4 2 4 potential application for enantioselective separation of R/S-PEA. Chiral HPLC analysis of the desorbed PEA from the PEA-included M′MOF-2 yielded an ee value of 21.1%, and the absolute S configu - ration for the excess was confirmed by comparing its optical rota - 27 20 tion with that of the standard sample. It must be noted that the used 10 M′MOF-2 keeps high crystallinity and can be regenerated simply by the immersion into the excess amount of methanol, and thus for further resolution of racemic R/S-PEA. e Th second and third 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.0 0.1 0.2 0.3 –1 –1 such regenerated M′MOF-2 provide an ee value of 15.7 and 13.2%, n (mmol g ) n (mmol g ) respectively. e Th low enantioselectivity of the enantiopure M ′MOF-2 b e C H on M’MOF-2a C H on M’MOF-3a 40 2 2 2 2 28 for the separation of R/S-PEA might be attributed to its large chiral pore environments, which have limited its high recognition of S-PEA. e Th smaller chiral pores within the enantiopure M ′MOF-3 have significantly enhanced its enantioselectivity for the separation of R/S-PEA with the much higher ee value of 64%. e Th regener - ated M′MOF-3 can also be further used for the separation of R/S- PEA with the slightly lower ee value of 55.3 and 50.6%, respectively. 20 8 e Th chiral pores within M ′MOF-2 and M′MOF-3 basically match 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.0 0.3 0.6 0.9 1.2 –1 –1 n (mmol g ) n (mmol g ) the size of S-PEA, which are not capable to separate larger alcohol enantiomers, such as 1-(p-tolyl)-ethanol, 2-phenyl-1-propanol and c f CO on M’MOF-2a CO on M’MOF-3a 1-phenyl-2-propanol. 2 2 Discussion Separation of acetylene and ethylene is a very important but chal- lenging industrial separation task. Ethylene, the lightest olefin and the largest volume organic chemical, is largely stocked in petro- chemical industry and is widely used to produce polymers and other chemicals . e Th typical ethylene produced in steam crack - 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 –1 –1 ers contains on the order of 1% of acetylene , although a p.p.m. n (mmol g ) n (mmol g ) level of acetylene ( > 5 p.p.m.) in ethylene can poison Ziegler-Natta Figure 4 | Comparison of the gas adsorption enthalpies on the two catalyst during ethylene polymerizations and can also lower the prod- activated M′MOFs. Ethylene (a, d), acetylene (b, e) and carbon dioxide (c, f) uct quality of the resulting polymers . Moreover, the acetylenic com- on m′mo F-2a (left) and m′mo F-3a (right) from two methods: the linear pounds are oen ft converted into solid, thus blocking the uid fl stream extrapolation (blue solid diamond) and virial equation (red open square). and even leading to explosion . er Th e are mainly two commercial n ATu RE Commun ICATIons | 2:204 | Do I: 10.1038/ncomms1206 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. –1 –1 –1 Q Q Q (kJ mol ) (kJ mol ) (kJ mol ) st st st –1 –1 –1 Q Q Q (kJ mol ) (kJ mol ) (kJ mol ) st st st ARTICLE nATuRE C ommunICATIons | DoI: 10.1038/ncomms1206 collected and dried in the air (0.21 g, 57%). Elemental analysis (%): calcd for approaches to remove acetylenes in ethylene: partial hydrogenation Zn (BDC) [Cu(SalPyCy)]·5DMF·4H O (C H N O CuZn ): C, 46.02; H, 5.04; 3 3 2 59 77 9 23 3 of acetylene into ethylene over a noble metal catalyst such as a N, 8.19; found: C, 45.97; H, 4.98; N, 8.24. 41,42 supported Pd catalyst and solvent extraction of cracked olefins using an organic solvent to obtain pure acetylene . However, the Synthesis of Zn (CDC) [Cu(SalPyCy)]·5DMF·4H O. A mixture of 3 3 2 former process suffers from the catalyst price and the loss of ole - Zn(NO ) ·6H O (0.236 g, 0.79 mmol), H CDC (0.136 g, 0.79 mmol) and 3 2 2 2 Cu(H SalPyCy)(NO ) (0.143 g, 0.24 mmol) was dissolved in 100 ml DMF, and fins due to the over hydrogenation to paraffins, although the latter is 2 3 2 heated in a vial (400 ml) at 373 K for 24 h. The dark-blue thin plates formed were also disadvantageous in terms of technical and economical aspects, collected and dried in the air (0.23 g, 62%). Elemental analysis (%): calcd for partially, because of the low selectivities of acetylene over olefins Zn (CDC) [Cu(SalPyCy)]·5DMF·4H O (C H N O CuZn ): C, 45.48; H, 6.15; 3 3 2 59 95 9 23 3 and also to the significant loss of solvent aer ft multiple operations. N, 8.09; found: C, 45.35; H, 6.23; N, 7.96. Apparently, there is a significant need to develop novel alternative Synthesis of Zn (CDC) [Cu(SalPyCy)]·S-PEA·5DMF. A mixture of C H /C H separation approaches. Some recent attempts, hydro- 3 3 2 2 2 4 44 Zn(NO ) ·6H O (0.018 g, 0.06 mmol), H CDC (0.01 g, 0.06 mmol) and 3 2 2 2 genation by non-precious metal alloy catalysts , ionic liquid extrac- Cu(H SalPyCy)(NO ) (0.016 g, 0.30 mmol) was dissolved in 3 ml DMF and 2 ml 2 3 2 45 46 tion , and π-complexation , have been made to reduce the cost or D,L-PEA, and heated in a vial (23 ml) at 373 K for 24 h. The purple platelet crystals to enhance the selectivities. e Th realization of our first example of were colleted and dried in the air (0.01 g, 31%). Elemental analysis (%): calcd for Zn (CDC) [Cu(SalPyCy)]·S-PEA·5DMF (C H CuN O Zn ): C, 50.04; H, 6.08; microporous MOF materials for such challenging separation high- 3 3 67 97 9 20 3 N, 7.84; found: C, 50.12; H, 6.15; N, 7.96. lights the very promise of the emerging microporous MOFs to resolve this very important industrial separation task in the future. Synthesis of Zn (BDC) [Cu(SalPyen)]·R/S-PEA·5DMF. A mixture of 3 3 M′MOF-3a is feasible for C H /C H separation at moderate pres- 2 2 2 4 Zn(NO ) ·6H O (0.018 g, 0.06 mmol), H BDC (0.01 g, 0.06 mmol) and 3 2 2 2 sures over 200 mmHg at 195 K. To realize high C H /C H separa- Cu(H SalPyen) (NO ) (0.015 g, 0.03 mmol) was dissolved in 3 ml DMF and 2 ml 2 2 2 4 2 3 2 D,L-PEA, and heated in a vial (23 ml) at 373 K for 24 h. The purple platelet crystals tion at low pressures at 195 K, the gate pressure for the entrance of were colleted and dried in the air (0.01 g, 32%). Elemental analysis (%): calcd for C H needs to be further reduced, which might be fulfilled by the 2 2 Zn (BDC) [Cu(SalPyen)]·R/S-PEA·5DMF (C H CuN O Zn ): C, 49.26; H, 4.79; 3 3 63 73 9 20 3 combinatorial approach outlined in the Introduction. C H /C H 2 2 2 4 N, 8.21; found: C, 49.55; H, 4.85; N, 8.30. separation selectivity of 5.23 has featured M′MOF-3a as a practical media for this important separation even at room temperature. Adsorption studies. After the bulk of the solvent was decanted, the freshly We have developed the pre-constructed building block approach prepared sample of M′MOF-2 or -3 (~0.15 g) was soaked in ~10 ml methanol for 1 h, and then the solvent was decanted. Following the procedure of metha- to introduce chiral pores/pockets within the M′MOFs by the usage nol soaking and decanting for ten times, the solvent-exchange samples were of the chiral diamine (R, R)-1,2-cyclohexanediamine. Such chi- activated by vacuum at room right overnight till the pressure of 5 µmHg. ral pore/pocket sizes are rationally tuned by the incorporation of CO , ethylene and acetylene adsorption isotherms were measured on ASAP different bicarboxylates BDC and CDC within their isostructural 2020 for the activated M′MOFs. As the centre-controlled air condition was M′MOFs Zn (BDC) [Cu(SalPycy)]·(G) (M′MOF-2) and Zn (CDC) set up at 22.0 °C, a water bath of 22.0 °C was used for adsorption isotherms 3 3 x 3 3 at 295.0 K, whereas dry ice-acetone and ice-water bathes were used for the [Cu(SalPycy)]·(G) (M′MOF-3). e Th slightly smaller pores within isotherms at 195 and 273 K, respectively. 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Adsorption of gases on carbon How to cite this article: Xiang, S.-C. et al. Rationally tuned micropores within enantiopure metal-organic frameworks for highly selective separation of acetylene and molecular sieves used for air separation: spherical adsorptives as probes for ethylene. Nat. Commun. 2:204 doi: 10.1038/ncomms1206 (2011). kinetic selectivity. Langmuir 14, 2415–2425 (1998). nATuRE C ommunICATIons | 2:204 | DoI: 10.1038/ncomms1206 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Nature Communications Springer Journals

Rationally tuned micropores within enantiopure metal-organic frameworks for highly selective separation of acetylene and ethylene

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ARTICLE DOI: 10.1038/ncomms1206 Received 29 sep 2010 | Accepted 24 Jan 2011 | Published 22 Feb 2011 Rationally tuned micropores within enantiopure metal-organic frameworks for highly selective separation of acetylene and ethylene 1 1 1 2 3 1 sheng-Chang Xiang , Zhangjing Zhang , Cong-Gui Zhao , Kunlun Hong , Xuebo Zhao , De-Rong Ding , 4 4 1 3 5 1 ming-Hua Xie , Chuan-De Wu , madhab C. Das , Rachel Gill , K. mark Thomas & Banglin Chen separation of acetylene and ethylene is an important industrial process because both compounds are essential reagents for a range of chemical products and materials. Current separation approaches include the partial hydrogenation of acetylene into ethylene over a supported Pd catalyst, and the extraction of cracked olefins using an organic solvent; both routes are costly and energy consuming. Adsorption technologies may allow separation, but microporous materials exhibiting highly selective adsorption of C H /C H have not been 2 2 2 4 realized to date. Here, we report the development of tunable microporous enantiopure mixed- metal-organic framework (m′moF) materials for highly selective separation of C H and C H . 2 2 2 4 The high selectivities achieved suggest the potential application of microporous m′moFs for practical adsorption-based separation of C H /C H . 2 2 2 4 1 2 Department of Chemistry, University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249-0698, USA. Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA. Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese 4 5 Academy of Sciences, Qingdao 266101, China. Department of Chemistry, Zhejiang University, Hangzhou 310027, China. Northern Carbon Research Laboratories, Sir Joseph Swan Institute and School of Chemical Engineering and Advanced Material, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, UK. Correspondence and requests for materials should be addressed to K.M.T. ([email protected]) or to B.C. ([email protected]). nATuRE C ommunICATIons | 2:204 | DoI: 10.1038/ncomms1206 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. ARTICLE n ATu RE Commun ICATIons | Do I: 10.1038/ncomms1206 recise control of pore sizes and pore surfaces within porous materials is very important for their highly selective recogni- R R Ption and thus separation of small molecules, but is very chal- N N lenging and difficult to achieve in traditional zeolite materials . e Th Cu situation has been changing since the emergence of a new type of N O O N porous material, so-called microporous metal-organic frameworks (MOFs) or porous coordination polymers. e Th pores within porous MOFs, particularly those within isoreticular MOFs whose structures are pre-determined by the coordination geometries of the second- OH Zn(NO ) 6H O 3 2 2 ary building blocks, can be systematically modified by changing the OH organic bridging linkers and controlling the framework interpen- 2–5 etration . Furthermore, the pore surfaces of porous MOFs can be functionalized by the immobilization of different recognition sites, HO such as open metal sites, Lewis basic/acidic sites and chiral pockets, 6–14 to direct the recognition of small molecules . Systematically tun- O HO ing micropores can achieve size-specific encapsulation of small gas . . Zn (BDC) [Cu(SalPycy)] (G) Zn (CDC) [Cu(SalPycy)] (G) 3 3 x 3 3 x molecules, and immobilization of functional sites enables varying M'MOF-2 M'MOF-3 substrate interactions: microporous MOF materials have emerged 1.6 Enhanced separation selectivity of C H / C H 25.5 as promising microporous media for the recognition and separation 2 2 2 4 15–32 of small molecules . 21 Enhanced chiral recognition of 1-phenylethyl alcohol (ee) 64 Kitagawa pioneered the research on construction of porous mixed-metal-organic frameworks (M′MOFs) by making use of M- Figure 1 | Syntheses and separation capacities of M′MOFs-2 and -3. Salen metalloligands in 2004 (refs 15, 16). Such a novel approach eventually led to few porous M′MOFs for heterogeneous asymmet- 4,17,18 ric catalysis and enantioselective separation . Recently, we have pyridine-3-carbaldehyde with (1R,2R)-cyclohexanediamine. successfully developed this metalloligand or pre-constructed build- Reaction of Cu(NO ) ·2.5H O with H SalPyCy formed the pre- 3 2 2 2 ing block approach to construct porous MOFs, and realized the constructed building block Cu(H SalPyCy)(NO ) that was easily 2 3 2 first such mixed-metal-organic framework (M ′MOF) Zn (BDC) incorporated into M′MOF-2 and -3 by the solvothermal reactions 3 3 [Cu(SalPyen)]·(G) (M′MOF-1; BDC = 1,4-benzenedicarboxylate; with Zn(NO ) ·6H O and H BDC or H CDC, respectively, in x 3 2 2 2 2 G = guest molecules) with permanent porosity as clearly established dimethylformamide (DMF) at 373 K as dark-blue thin plates. by both gas and vapour sorption . This new M ′MOF approach has e Th y were formulated as Zn (BDC) [Cu(SalPyCy)]·5DMF·4H O 3 3 2 provided us with a new ability to tune and functionalize the micro- (M′MOF-2) and Zn (CDC) [Cu(SalPyCy)]·5DMF·4H O (M′MOF-3) 3 3 2 pores within this series of isoreticular M′MOFs by: the incorpora- by elemental microanalysis and single-crystal X-ray diffraction tion of different secondary organic linkers; the immobilization of (XRD) studies, and the phase purity of the bulk material was 2 + different metal sites other than Cu ; the introduction of chiral independently conr fi med by powder XRD. e Th desolvated MMOFs- pockets/environments through the usage of chiral diamines; and 2a and -3a for the adsorption studies was prepared from the derivatives of the precursor by the usage of other organic groups methanol-exchanged samples followed by the activation under ultra- such as t-butyl instead of methyl group. And thus to explore novel high vacuum at room temperature. e Th XRD profiles of desolvated functional microporous M′MOFs for their recognition and separa- MMOFs-2a and -3a indicate that they maintain the crystalline tion of small molecules. We initiated studies on such endeavour by framework structures (Supplementary Figs S14–S16). making use of Cu-Salen pre-constructed building blocks because of X-ray single-crystal structures reveal that M′MOF-2 and -3 are the straightforward and easy synthesis of the Cu-Salen precursors isostructural three-dimensional frameworks, in which Zn (COO) 3 6 and the resulting porous M′MOFs. secondary building blocks are bridged by BDC or CDC anions to To make use of chiral (R, R)-1,2-cyclohexanediamine to con- form the 3 two-dimensional tessellated Zn (BDC) or Zn (CDC) 3 3 3 3 struct the chiral metalloligand Cu(SalPyCy), enantiopure M′MOF sheets that are further pillared by the Cu(SalPyCy) (Fig. 2, Supple- Zn (BDC) [Cu(SalPycy)]·(G) (M′MOF-2), which is isostructural mentary Figs S1 and S2). Topologically, M′MOF-2 and -3 can be des- 3 3 x 6 18 3 to the nonchiral Zn (BDC) [Cu(SalPyen)]·(G) (M′MOF-1), can be cribed as a hexagonal primitive networks (Schäi fl symbol 3 4 5 6), 3 3 x readily assembled by the solvothermal reaction of this chiral build- which are the same as its achiral analogue Zn (BDC) [Cu(SalPyen)] 3 3 ing block with Zn(NO ) and H BDC, leading to the chiral cavities (ref. 33); however, the incorporation of chiral metalloligand 3 2 2 within M′MOF-2 (Fig. 1). Furthermore, such chiral cavities can be Cu(SalPycy) leads to enantiopure M′MOF-2 and -3, exhibiting two straightforwardly tuned by the incorporation of different bicarboxy - chiral pore cavities of about 6.4 Å in diameter (Fig. 2a,b). e Th chi - late CDC (CDC = 1,4-cyclohexanedicarboxylate) for their enhanced ral pore cavities are filled with the disordered solvent molecules in recognition and separation of small molecules. which M′MOF-2 and -3 have the pore accessible volume of 51.7 and Herein, we report the synthesis, structures, sorption and chiral rec- 48.1%, respectively, calculated using the PLATON program . ognition studies on these two new M′MOFs Zn (BDC) [Cu(SalPycy 3 3 )]·(G) (M′MOF-2) and Zn (CDC) [Cu(SalPycy)]·(G) (M′MOF-3). Microporous nature of M′MOFs. To establish the permanent x 3 3 x M′MOF-3 exhibits significantly enhanced selective separation of porosity, the methanol-exchanged M′MOFs-2 and -3 were activated C H /C H and enatioselective recognition of 1-phenylethyl alcohol under high vacuum at room temperature overnight to form the des- 2 2 2 4 (PEA) than M′MOF-2, highlighting the first example of micropo - olvated M′MOF-2a and -3a. Nitrogen adsorption on the activated rous materials for highly selective separation of C H /C H , a very M′MOFs-2a and -3a at 77.3 K was very slow because of the acti- 2 2 2 4 important industrial separation. vated diffusion effects. er Th efore, CO adsorption at 195 K was used for their pore characterization. Surprisingly, they exhibit remark- Results ably different sorption isotherms attributed to the different dicarbo - 3 − 1 Syntheses and structural characterization of M′MOFs. e Th new xylates. e Th uptake of M ′MOF-2a (158 cm g ) is about twice than 3 − 1 Salen-type chiral Schiff base of pyridine derivative H SalPyCy that of M′MOF-3a (86 cm g ) at P/P of 1 (Fig. 3). Both M′MOF- 2 0 was prepared by condensation of 5-methyl-4-oxo-1,4-dihydro- 2a and -3a show hysteretic sorption behaviours, indicating their n ATu RE Commun ICATIons | 2:204 | Do I: 10.1038/ncomms1206 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. n ATu RE Commun ICATIons | Do I: 10.1038/ncomms1206 ARTICLE T = 195 K a c a 0.0 0.2 0.4 0.6 0.8 1.0 P /P T = 195 K c T = 195 K b d 0 100 200 300 400 500 600 700 800 0 100 200 300 400 500 600 700 800 P (mmHg) P (mmHg) e T = 295 K d T = 273 K 60 50 Figure 2 | X-ray crystal structures of M′MOF-3 and M′MOF-3S-PEA. 6 18 3 (a) The hexagonal primitive network topology (s chäfli symbol 3 4 5 6) and (b) the three-dimensional (3D) pillared framework with chiral pore cavities for m′mo F-3. (c) The hexagonal primitive network topology and 0 200 400 600 800 0 200 400 600 800 (d) the 3D pillared framework exclusively encapsulating s -PEA molecules P (mmHg) P (mmHg) for m′mo F-3s -PEA. (Zn, pink; Cu, cyan; o , red; C, grey; n , blue; H, white). f g T = 273 K T = 295 K 50 50 framework flexibility and the existence of the meta-stable interme - 40 diate frameworks, which have been also observed in other flexible 30 30 porous MOFs e Th Langmuir (BET) surface areas calculated from 2 − 1 20 20 the first step adsorption isotherms are 598(388) and 237(110) m g for M′MOF-2a and -3a, respectively, within the pressure range of 0.05 < P/P < 0.3 (Supplementary Figs S3–S5). Assuming that the 0 0 second step isotherms still t fi into the monolayer coverage model, 0 200 400 600 800 0 200 400 600 800 the overall Langmuir surface areas of M′MOF-2a and -3a are 939 P (mmHg) P (mmHg) 2 − 1 and 551 m g . eir Th total pore volumes from the highest P/P values and pore volumes corresponding to the intermediate iso- Figure 3 | Gas sorption isotherms on the two activated M′MOFs at 3 − 1 therm step are 0.301 (0.189) and 0.164 (0.049) cm g for M′MOF-2a different temperatures. (a) Co sorption on m′mo Fs-2a (blue square) and M′MOF -3a, respectively. and -3a (red dot) at 195 K. (b) C H (green square) and C H (blue 2 2 2 4 triangle) on m′mo F-2a at 195 K. (c) C H (green square) and C H (blue 2 2 2 4 Separation of acetylene and ethylene within M′MOFs. e Th unique triangle) on m′mo F-3a at 195 K. (d) C H (blue square), Co (red dot) 2 2 2 CO sorption isotherms encouraged us to examine the capacities and C H (green triangle) on m′mo F-2a at 273 K. (e) C H (blue square), 2 4 2 2 of M′MOF-2a and -3a for their selective separation of C H /C H 2 2 2 4 Co (red dot) and C H (green triangle) on m′mo F-2a at 295 K. (f) C H 2 2 4 2 2 at 195 K, given the fact that these two molecules have comparable (blue square), Co (red dot) and C H (green triangle) on m′mo F-3a at 2 2 4 molecular sizes with CO . For M′MOF-2a (Fig. 3b), the shapes of 273 K. (g) C H (blue square), Co (red dot) and C H (green triangle) on 2 2 2 2 4 the isotherms are complex, which are apparently attributed to the m′mo F-3a at 295 K. Adsorption and desorption branches are shown with framework flexibility during adsorption. e Th total pore volumes closed and open symbols, respectively. were calculated from the highest P/P values (P/P ~0.99) using 0 0 − 3 densities of 0.577, 0.726 and 1.032 g cm for the densities of C H , 2 4 C H and CO , respectively. e Th total pore volumes were 0.306, and 195 K (Fig. 3c). Accordingly, the total pore volumes were dif- 2 2 2 3 − 1 3 − 1 0.309 and 0.301 cm g for C H , C H and CO , respectively, which ferent, of 0.066, 0.236 and 0.165 cm g for C H , C H and CO , 2 4 2 2 2 2 4 2 2 2 are basically the same, indicating that all three gas molecules can respectively, as calculated from their highest P/P values, indicating have the full access to the pores within M′MOF-2a. M′MOF-3a, that the three gas molecules have differential degree of access to the however, exhibits significantly different sorption behaviours with pores at 1 atm and 195 K when the pores within M′MOF-3a become respect to C H and C H . M′MOF-3a can take up the acetylene up smaller. Such subtle pore control is very important for these kinds 2 4 2 2 3 − 1 to 147 cm g with a one-step hysteresis loop, whereas only small of porous materials to exhibit highly selective gas separation. In fact, 3 − 1 amount of ethylene (30.2 cm g ) without the marked loop at 1 atm M′MOF-2a can only slightly differentiate C H from C H with a low 2 2 2 4 n ATu RE Commun ICATIons | 2:204 | Do I: 10.1038/ncomms1206 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. 3 –1 3 –1 3 –1 Adsorbed amount (cm (STP) g ) Adsorbed amount (cm (STP) g ) Adsorbed amount (cm (STP) g ) 3 –1 Adsorbed amount (cm (STP) g ) 3 –1 3 –1 3 –1 Adsorbed amount (cm (STP) g ) Adsorbed amount (cm (STP) g ) Adsorbed amount (cm (STP) g ) ARTICLE nATuRE C ommunICATIons | DoI: 10.1038/ncomms1206 selectivity of 1.6, whereas M′MOF-3a displays significantly higher organic linker and metalloligand within M′MOF-3a has enlarged selectivity of 25.5 and thus can exclusively separate C H from C H the pore apertures for the access of both C H and C H molecules. 2 2 2 4 2 2 2 4 (Table 1). In the diffusion of molecules into spherical or rectangular Such kinetically controlled framework flexibility has been also pores both the cross-section dimensions are important, whereas for revealed in several other porous MOFs and used for their tempera- 20,26 slit shaped pores only the smallest dimension is important in deter- ture-dependent gas separation . mining selectivity. Such significantly enhanced separation capacity of M′MOF-3a over M′MOF-2a is attributed to the smaller micropo- Interactions of gas molecules with M′MOFs. e Th coverage- res within M′MOF-3a, which favours its higher size-specific sepa - dependent adsorption enthalpies of the M′MOFs to acetylene, eth- ration effect on the C H /C H separation. e Th narrower molecu - ylene and CO were calculated based on the virial method and the 2 2 2 4 2 lar size of C H (3.32 × 3.34 × 5.70 Å ) compared with that of C H van’t Hoff isochore. e Th virial graphs for adsorption of C H , C H 2 2 2 4 2 4 2 2 (3.28 × 4.18 × 4.84 Å ) has enabled the full entrance of the C H into and CO on M′MOF-2a and -3a at 273 and 295 K are shown in Sup- 2 2 2 the micropores in M′MOF-3a, whereas C H molecules are basically plementary Figures S6–S11. It is apparent that the virial graphs have 2 4 blocked or the kinetics are very slow. very good linearity in the low-pressure region. e Th parameters and e Th adsorption isotherms of C H , C H and CO on M′MOF- the enthalpies obtained from the virial equation are summarized 2 2 2 4 2 2a and -3a were further measured at 273 and 295 K (Fig. 3). in Table 2. For M′MOF-2a, C H adsorption had A values increas- 2 4 1 − 1 e Th y showed type I sorption isotherms with very little hysteresis. ing from − 1,770 to − 1,551 g mol from 273 to 295 K, which has a e Th selectivities towards C H /C H on M′MOF-2a at 273 and similar trend of A values on a carbon molecular sieve increasing 2 2 2 4 1 − 1 295 K were 1.5 and 1.9, respectively. Again, M′MOF-3a exhibited from − 2,480 to − 1,821 g mol from 303 to 343 K (ref. 35); C H 2 2 − 1 enhanced C H /C H selectivities of 4.1 and 5.2 at 273 and 295 K, adsorption had A values increasing from − 1,621 to − 1,353 g mol 2 2 2 4 1 respectively; which are 2.5 times higher than the corresponding from 273 to 295 K, which also has a similar trend of A values on a − 1 values for M′MOF-2a (Table 1). carbon molecular sieve increasing from − 1,444 to − 1,302 g mol e Th unique and temperature-dependent gas separation capaci - from 303 to 343 K. It is apparent that the virial parameters for C H 2 4 ties of M′MOF-3a are attributed both to thermodynamically and and C H adsorption have similar values and trends. CO adsorp- 2 2 2 5,26 − 1 kinetically controlled framework flexibility . e Th more flexible tion on M′MOF-2a had A values from − 1,071 to − 940 g mol nature of CDC in M′MOF-3 has enabled the framework M′MOF-3 from 273 to 295 K without well-defined trend, which has been also more flexible, thus resulting in narrower pores in activated M ′MOF- observed in CO adsorption on a carbon molecular sieve with the A 2 1 − 1 3a than those in M′MOF-2a, as shown in their powder XRD pat- values ranging from − 1,000 to − 1,045 g mol from 303 to 343 K. terns (Supplementary Fig. S16). To open pore entrances for the e Th trends in the A parameters for C H , C H and CO adsorp- 1 2 4 2 2 2 C H uptake, the gate pressure of 166 mmHg needs to be applied tion on M′MOF-2a are consistent with the adsorbate–adsorbate 2 2 on the thermodynamically flexible framework M ′MOF-3a at 195 K. interactions decreasing with increasing temperature. In compari- At higher temperature of 273 and 295 K, the rotation/swing of the son with those for C H and C H adsorption on M′MOF-2a, the 2 4 2 2 Table 1 | The Henry’s constants or the product of the Langmuir equation constants (q ×b) and the equilibrium selectivity for gases on the two M′MOFs. 3 − 1 − 1 Henry’s constants K or q ×b (cm g torr ) M′MOF-2a M′MOF-3a Temperature (K) 195 273 295 195 273 295 C H 239.37×0.0057 55.76×0.0078 55.09×0.0044 179.99×0.0058 50.36×0.0071 48.30×0.0058 2 2 Co 194.65×0.0064 49.57×0.0044 43.14×0.0030 98.64×0.0089 30.18×0.0025 27.75×0.0012 − 5 C H 178.88×0.0048 38.72×0.0073 39.71×0.0031 710.17×5.75×10 21.90×0.0040 12.45×0.0043 2 4 Selectivity α C H /Co 1.10 2.00 1.89 1.18 4.74 8.41 2 2 2 Co /C H 1.46 0.77 1.02 21.60 0.86 0.62 2 2 4 C H /C H 1.61 1.54 1.93 25.53 4.08 5.23 2 2 2 4 Table 2 | Summary of the parameters and the enthalpies of gas adsorption on M′MOFs at 273 and 295 K obtained from the virial equation. − 1 − 1 − 1 2 − 1 Compounds Adsorbate T (K) A /ln (mol g Pa ) A /g mol R Q /kJ mol 0 1 st,n=0 m′moF- 2a C H 273 − 15.234 ± 0.059 − 1770.328 ± 6.856 0.99975 32.7 2 4 295 − 16.302 ± 0.054 − 1551.378 ± 7.704 0.99946 C H 273 − 14.070 ± 0.012 − 1621.285 ± 11.438 0.99950 37.7 2 2 295 − 15.300 ± 0.011 − 1353.201 ± 11.133 0.99892 Co 273 − 15.591 ± 0.013 − 1070.628 ± 35.231 0.99247 32.5 295 − 16.652 ± 0.007 − 939.937 ± 15.613 0.99697 m′moF- 3a C H 273 − 17.020 ± 0.003 − 2143.060 ± 6.940 0.99983 27.3 2 4 295 − 17.906 ± 0.018 − 2199.728 ± 94.199 0.99452 C H 273 − 14.203 ± 0.026 − 2056.965 ± 25.871 0.99811 27.1 2 2 295 − 15.087 ± 0.042 − 1693.176 ± 41.686 0.99158 Co 273 − 16.679 ± 0.026 − 3117.335 ± 137.777 0.99031 40.5 295 − 17.999 ± 0.007 − 1902.594 ± 33.455 0.99753 nATuRE C ommunICATIons | 2:204 | DoI: 10.1038/ncomms1206 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. n ATu RE Commun ICATIons | Do I: 10.1038/ncomms1206 ARTICLE virial parameters for C H and C H adsorption on M′MOF-3a are molecules and CDC moieties on surfaces of pores in M′MOF-3a. 2 4 2 2 more negative due to its smaller pores, but still have similar values e Th results derived from the linear extrapolation are in very good and trends from 273 to 295 K. e Th fact that the A values for C H agreement with those obtained from the virial equation (Fig. 4). It 1 2 4 adsorption on M′MOF-3a are not obviously changed indicates that needs to be mentioned that the Q values for C H and C H adsorp- st 2 4 2 2 the adsorbate–adsorbate interactions might be independent of tem- tion on M′MOF-3a are almost the same, indicating that the selec- perature from 273 to 295 K (Supplementary Fig. S11c). tive C H /C H separation cannot be realized by their differential 2 2 2 4 − 1 e Th Q values were 32.7, 37.7 and 32.5 kJ mol for C H , C H interactions with the pore surfaces, thus, the unique gas separation st,n = 0 2 4 2 2 and CO adsorption on M′MOF-2a over the temperature range of characteristics for M′MOF-3a are mainly attributed to size-exclu- 273–295 K. e Th Q values for C H and CO were comparable sive effect. st,n = 0 2 2 2 to those obtained for their adsorption on a carbon molecular sieve − 1 − 1 (35.0 kJ mol (C H ) and 28.2 kJ mol (CO )), whereas the value of Enantiopure selective separation of PEA. e Th enantiopure pore 2 2 2 − 1 32.7 kJ mol for C H adsorption on M′MOF-2a was significantly environments within M′MOF-2 and -3 motivated us to explore 2 4 − 1 lower than the value (50.1 kJ mol ) obtained for C H adsorption their potential for chiral recognition and enantioselective separa- 2 4 on a carbon molecular sieve in the temperature range of 303–343 K. tion. Unlike the achiral M′MOF-1 Zn (BDC) [Cu(SalPyen)], which 3 3 e Th comparison of the results from the two methods, the linear encapsulates both R- and S-1-PEA to form Zn (BDC) [Cu(SalPyen)] 3 3 extrapolation and the virial equation shows that there is a very good R/S-PEA (Supplementary Fig. S12), the enantiopure M′MOF-3 agreement (Fig. 4). In all cases the isosteric enthalpies of adsorp- exclusively takes up S-PEA to form M′MOF-3S-PEA (Zn (CDC) 3 3 tion gradually decreased with the increasing surface coverage. As [Cu(SalPyCy)]·S-PEA). In fact, the solvothermal reaction of the expected, the isosteric enthalpies of adsorption are significantly corresponding reaction mixture of Zn(NO ) ·6H O, H CDC and 3 2 2 2 − 1 higher than the enthalpies of vaporization of 17, 14 and 16.5 kJ mol Cu(H SalCy)(NO ) in the presence of certain amount of racemic 2 3 2 for C H , C H and CO , respectively . e Th isosteric enthalpies of PEA in DMF at 100 °C readily formed the enantiopure M′MOF-3, 2 4 2 2 2 adsorption for C H , C H and CO on M′MOF-2a are characteristic which exclusively encapsulates S-PEA (Fig. 2d). e Th incorporated 2 2 2 4 2 of their interactions with the hydrophobic pore surfaces presented S-PEA can be easily extracted by immersing the as-synthesized in carbon molecular sieves. M′MOF-3S-PEA in methanol. e Th Q for C H , C H and CO adsorption on M′MOF-3a e Th chiral recognition and enantioselective separation of st,n = 0 2 4 2 2 2 − 1 were 27.4, 27.1 and 40.5 kJ mol , respectively, over the temperature M′MOF-2 and -3 for the R/S-PEA racemic mixture were exam- range from 273 to 295 K. e Th systematically lower Q values for ined using the bulky as-synthesized materials. e Th as-synthe - st,n = 0 C H and C H adsorption on M′MOF-3a than those observed on sized M′MOF-2 and -3 were exchanged with methanol and then 2 4 2 2 − 1 − 1 M′MOF-2a (32.7 kJ mol for C H and 37.7 kJ mol for C H ) may immersed in the racemic mixture to selectively encapsulate the 2 4 2 2 be attributed to the deficiency of π–π interactions between these S-PEA. Once such PEA-included M′MOF-2 and -3 were immersed in methanol, the encapsulated PEA within the enantiopure M′MOF-2 and -3 can be readily released from the chiral pores, making their a d C H on M’MOF-2a C H on M’MOF-3a 2 4 2 4 potential application for enantioselective separation of R/S-PEA. Chiral HPLC analysis of the desorbed PEA from the PEA-included M′MOF-2 yielded an ee value of 21.1%, and the absolute S configu - ration for the excess was confirmed by comparing its optical rota - 27 20 tion with that of the standard sample. It must be noted that the used 10 M′MOF-2 keeps high crystallinity and can be regenerated simply by the immersion into the excess amount of methanol, and thus for further resolution of racemic R/S-PEA. e Th second and third 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.0 0.1 0.2 0.3 –1 –1 such regenerated M′MOF-2 provide an ee value of 15.7 and 13.2%, n (mmol g ) n (mmol g ) respectively. e Th low enantioselectivity of the enantiopure M ′MOF-2 b e C H on M’MOF-2a C H on M’MOF-3a 40 2 2 2 2 28 for the separation of R/S-PEA might be attributed to its large chiral pore environments, which have limited its high recognition of S-PEA. e Th smaller chiral pores within the enantiopure M ′MOF-3 have significantly enhanced its enantioselectivity for the separation of R/S-PEA with the much higher ee value of 64%. e Th regener - ated M′MOF-3 can also be further used for the separation of R/S- PEA with the slightly lower ee value of 55.3 and 50.6%, respectively. 20 8 e Th chiral pores within M ′MOF-2 and M′MOF-3 basically match 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.0 0.3 0.6 0.9 1.2 –1 –1 n (mmol g ) n (mmol g ) the size of S-PEA, which are not capable to separate larger alcohol enantiomers, such as 1-(p-tolyl)-ethanol, 2-phenyl-1-propanol and c f CO on M’MOF-2a CO on M’MOF-3a 1-phenyl-2-propanol. 2 2 Discussion Separation of acetylene and ethylene is a very important but chal- lenging industrial separation task. Ethylene, the lightest olefin and the largest volume organic chemical, is largely stocked in petro- chemical industry and is widely used to produce polymers and other chemicals . e Th typical ethylene produced in steam crack - 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 –1 –1 ers contains on the order of 1% of acetylene , although a p.p.m. n (mmol g ) n (mmol g ) level of acetylene ( > 5 p.p.m.) in ethylene can poison Ziegler-Natta Figure 4 | Comparison of the gas adsorption enthalpies on the two catalyst during ethylene polymerizations and can also lower the prod- activated M′MOFs. Ethylene (a, d), acetylene (b, e) and carbon dioxide (c, f) uct quality of the resulting polymers . Moreover, the acetylenic com- on m′mo F-2a (left) and m′mo F-3a (right) from two methods: the linear pounds are oen ft converted into solid, thus blocking the uid fl stream extrapolation (blue solid diamond) and virial equation (red open square). and even leading to explosion . er Th e are mainly two commercial n ATu RE Commun ICATIons | 2:204 | Do I: 10.1038/ncomms1206 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. –1 –1 –1 Q Q Q (kJ mol ) (kJ mol ) (kJ mol ) st st st –1 –1 –1 Q Q Q (kJ mol ) (kJ mol ) (kJ mol ) st st st ARTICLE nATuRE C ommunICATIons | DoI: 10.1038/ncomms1206 collected and dried in the air (0.21 g, 57%). Elemental analysis (%): calcd for approaches to remove acetylenes in ethylene: partial hydrogenation Zn (BDC) [Cu(SalPyCy)]·5DMF·4H O (C H N O CuZn ): C, 46.02; H, 5.04; 3 3 2 59 77 9 23 3 of acetylene into ethylene over a noble metal catalyst such as a N, 8.19; found: C, 45.97; H, 4.98; N, 8.24. 41,42 supported Pd catalyst and solvent extraction of cracked olefins using an organic solvent to obtain pure acetylene . However, the Synthesis of Zn (CDC) [Cu(SalPyCy)]·5DMF·4H O. A mixture of 3 3 2 former process suffers from the catalyst price and the loss of ole - Zn(NO ) ·6H O (0.236 g, 0.79 mmol), H CDC (0.136 g, 0.79 mmol) and 3 2 2 2 Cu(H SalPyCy)(NO ) (0.143 g, 0.24 mmol) was dissolved in 100 ml DMF, and fins due to the over hydrogenation to paraffins, although the latter is 2 3 2 heated in a vial (400 ml) at 373 K for 24 h. The dark-blue thin plates formed were also disadvantageous in terms of technical and economical aspects, collected and dried in the air (0.23 g, 62%). Elemental analysis (%): calcd for partially, because of the low selectivities of acetylene over olefins Zn (CDC) [Cu(SalPyCy)]·5DMF·4H O (C H N O CuZn ): C, 45.48; H, 6.15; 3 3 2 59 95 9 23 3 and also to the significant loss of solvent aer ft multiple operations. N, 8.09; found: C, 45.35; H, 6.23; N, 7.96. Apparently, there is a significant need to develop novel alternative Synthesis of Zn (CDC) [Cu(SalPyCy)]·S-PEA·5DMF. A mixture of C H /C H separation approaches. Some recent attempts, hydro- 3 3 2 2 2 4 44 Zn(NO ) ·6H O (0.018 g, 0.06 mmol), H CDC (0.01 g, 0.06 mmol) and 3 2 2 2 genation by non-precious metal alloy catalysts , ionic liquid extrac- Cu(H SalPyCy)(NO ) (0.016 g, 0.30 mmol) was dissolved in 3 ml DMF and 2 ml 2 3 2 45 46 tion , and π-complexation , have been made to reduce the cost or D,L-PEA, and heated in a vial (23 ml) at 373 K for 24 h. The purple platelet crystals to enhance the selectivities. e Th realization of our first example of were colleted and dried in the air (0.01 g, 31%). Elemental analysis (%): calcd for Zn (CDC) [Cu(SalPyCy)]·S-PEA·5DMF (C H CuN O Zn ): C, 50.04; H, 6.08; microporous MOF materials for such challenging separation high- 3 3 67 97 9 20 3 N, 7.84; found: C, 50.12; H, 6.15; N, 7.96. lights the very promise of the emerging microporous MOFs to resolve this very important industrial separation task in the future. Synthesis of Zn (BDC) [Cu(SalPyen)]·R/S-PEA·5DMF. A mixture of 3 3 M′MOF-3a is feasible for C H /C H separation at moderate pres- 2 2 2 4 Zn(NO ) ·6H O (0.018 g, 0.06 mmol), H BDC (0.01 g, 0.06 mmol) and 3 2 2 2 sures over 200 mmHg at 195 K. To realize high C H /C H separa- Cu(H SalPyen) (NO ) (0.015 g, 0.03 mmol) was dissolved in 3 ml DMF and 2 ml 2 2 2 4 2 3 2 D,L-PEA, and heated in a vial (23 ml) at 373 K for 24 h. The purple platelet crystals tion at low pressures at 195 K, the gate pressure for the entrance of were colleted and dried in the air (0.01 g, 32%). Elemental analysis (%): calcd for C H needs to be further reduced, which might be fulfilled by the 2 2 Zn (BDC) [Cu(SalPyen)]·R/S-PEA·5DMF (C H CuN O Zn ): C, 49.26; H, 4.79; 3 3 63 73 9 20 3 combinatorial approach outlined in the Introduction. C H /C H 2 2 2 4 N, 8.21; found: C, 49.55; H, 4.85; N, 8.30. separation selectivity of 5.23 has featured M′MOF-3a as a practical media for this important separation even at room temperature. Adsorption studies. After the bulk of the solvent was decanted, the freshly We have developed the pre-constructed building block approach prepared sample of M′MOF-2 or -3 (~0.15 g) was soaked in ~10 ml methanol for 1 h, and then the solvent was decanted. Following the procedure of metha- to introduce chiral pores/pockets within the M′MOFs by the usage nol soaking and decanting for ten times, the solvent-exchange samples were of the chiral diamine (R, R)-1,2-cyclohexanediamine. Such chi- activated by vacuum at room right overnight till the pressure of 5 µmHg. ral pore/pocket sizes are rationally tuned by the incorporation of CO , ethylene and acetylene adsorption isotherms were measured on ASAP different bicarboxylates BDC and CDC within their isostructural 2020 for the activated M′MOFs. As the centre-controlled air condition was M′MOFs Zn (BDC) [Cu(SalPycy)]·(G) (M′MOF-2) and Zn (CDC) set up at 22.0 °C, a water bath of 22.0 °C was used for adsorption isotherms 3 3 x 3 3 at 295.0 K, whereas dry ice-acetone and ice-water bathes were used for the [Cu(SalPycy)]·(G) (M′MOF-3). e Th slightly smaller pores within isotherms at 195 and 273 K, respectively. 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Adsorption of gases on carbon How to cite this article: Xiang, S.-C. et al. Rationally tuned micropores within enantiopure metal-organic frameworks for highly selective separation of acetylene and molecular sieves used for air separation: spherical adsorptives as probes for ethylene. Nat. Commun. 2:204 doi: 10.1038/ncomms1206 (2011). kinetic selectivity. Langmuir 14, 2415–2425 (1998). nATuRE C ommunICATIons | 2:204 | DoI: 10.1038/ncomms1206 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved.

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Published: Feb 22, 2011

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