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

Abstract

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 C2H2/C2H4 have not been realized to date. Here, we report the development of tunable microporous enantiopure mixed-metal-organic framework (M'MOF) materials for highly selective separation of C2H2 and C2H4. The high selectivities achieved suggest the potential application of microporous M'MOFs for practical adsorption-based separation of C2H2/C2H4.

ARTICLE

Received 29 Sep 2010 | Accepted 24 Jan 2011 | Published 22 Feb 2011 DOI: 10.1038/ncomms1206

Sheng-Chang Xiang1, Zhangjing Zhang1, Cong-Gui Zhao1, Kunlun Hong2, Xuebo Zhao3, De-Rong Ding1, Ming-Hua Xie4, Chuan-De Wu4, Madhab C. Das1, Rachel Gill3, K. Mark Thomas5 & Banglin Chen1

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 olens using an organic solvent; both routes are costly and energy consuming. Adsorption technologies may allow separation, but microporous materials exhibiting highly selective adsorption of C2H2/C2H4 have not been realized to date. Here, we report the development of tunable microporous enantiopure mixed-metal-organic framework (MMOF) materials for highly selective separation of C2H2 and C2H4.

The high selectivities achieved suggest the potential application of microporous MMOFs for practical adsorption-based separation of C2H2/C2H4.

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

1 Department of Chemistry, University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249-0698, USA. 2 Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA. 3 Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China. 4 Department of Chemistry, Zhejiang University, Hangzhou 310027, China. 5 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]).

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Precise control of pore sizes and pore surfaces within porous materials is very important for their highly selective recognition and thus separation of small molecules, but is very chal

lenging and difficult to achieve in traditional zeolite materials1. The situation has been changing since the emergence of a new type of porous material, so-called microporous metal-organic frameworks (MOFs) or porous coordination polymers. The pores within porous MOFs, particularly those within isoreticular MOFs whose structures are pre-determined by the coordination geometries of the secondary building blocks, can be systematically modied by changing the organic bridging linkers and controlling the framework interpenetration25. Furthermore, the pore surfaces of porous MOFs can be functionalized by the immobilization of dierent recognition sites, such as open metal sites, Lewis basic/acidic sites and chiral pockets, to direct the recognition of small molecules614. Systematically tuning micropores can achieve size-specic encapsulation of small gas molecules, and immobilization of functional sites enables varying substrate interactions: microporous MOF materials have emerged as promising microporous media for the recognition and separation of small molecules1532.

Kitagawa pioneered the research on construction of porous mixed-metal-organic frameworks (MMOFs) by making use of MSalen metalloligands in 2004 (refs 15, 16). Such a novel approach eventually led to few porous MMOFs for heterogeneous asymmetric catalysis and enantioselective separation4,17,18. Recently, we have

successfully developed this metalloligand or pre-constructed building block approach to construct porous MOFs, and realized the rst such mixed-metal-organic framework (MMOF) Zn3(BDC)3

[Cu(SalPyen)](G)x (MMOF-1; BDC = 1,4-benzenedicarboxylate; G = guest molecules) with permanent porosity as clearly established by both gas and vapour sorption32. This new MMOF approach has provided us with a new ability to tune and functionalize the micro-pores within this series of isoreticular MMOFs by: the incorporation of dierent secondary organic linkers; the immobilization of dierent metal sites other than Cu2 + ; the introduction of chiral pockets/environments through the usage of chiral diamines; and derivatives of the precursor by the usage of other organic groups such as t-butyl instead of methyl group. And thus to explore novel functional microporous MMOFs for their recognition and separation of small molecules. We initiated studies on such endeavour by making use of Cu-Salen pre-constructed building blocks because of the straightforward and easy synthesis of the Cu-Salen precursors and the resulting porous MMOFs.

To make use of chiral (R, R)-1,2-cyclohexanediamine to construct the chiral metalloligand Cu(SalPyCy), enantiopure MMOF Zn3(BDC)3[Cu(SalPycy)](G)x (MMOF-2), which is isostructural to the nonchiral Zn3(BDC)3[Cu(SalPyen)](G)x (MMOF-1), can be readily assembled by the solvothermal reaction of this chiral building block with Zn(NO3)2 and H2BDC, leading to the chiral cavities within MMOF-2 (Fig. 1). Furthermore, such chiral cavities can be straightforwardly tuned by the incorporation of dierent bicarboxy-late CDC (CDC = 1,4-cyclohexanedicarboxylate) for their enhanced recognition and separation of small molecules.

Herein, we report the synthesis, structures, sorption and chiral recognition studies on these two new MMOFs Zn3(BDC)3[Cu(SalPycy )](G)x (MMOF-2) and Zn3(CDC)3[Cu(SalPycy)](G)x (MMOF-3). MMOF-3 exhibits signicantly enhanced selective separation of C2H2/C2H4 and enatioselective recognition of 1-phenylethyl alcohol (PEA) than MMOF-2, highlighting the rst example of microporous materials for highly selective separation of C2H2/C2H4, a very important industrial separation.

Results

Syntheses and structural characterization of MMOFs. The new

Salen-type chiral Schi base of pyridine derivative H2SalPyCy was prepared by condensation of 5-methyl-4-oxo-1,4-dihydro-

pyridine-3-carbaldehyde with (1R,2R)-cyclohexanediamine. Reaction of Cu(NO3)22.5H2O with H2SalPyCy formed the pre-constructed building block Cu(H2SalPyCy)(NO3)2 that was easily incorporated into MMOF-2 and -3 by the solvothermal reactions with Zn(NO3)26H2O and H2BDC or H2CDC, respectively, in dimethylformamide (DMF) at 373 K as dark-blue thin plates. They were formulated as Zn3(BDC)3[Cu(SalPyCy)]5DMF4H2O

(MMOF-2) and Zn3(CDC)3[Cu(SalPyCy)]5DMF4H2O (MMOF-3) by elemental microanalysis and single-crystal X-ray diraction (XRD) studies, and the phase purity of the bulk material was independently conrmed by powder XRD. The desolvated MMOFs- 2a and -3a for the adsorption studies was prepared from the methanol-exchanged samples followed by the activation under ultra-high vacuum at room temperature. The XRD proles of desolvated MMOFs-2a and -3a indicate that they maintain the crystalline framework structures (Supplementary Figs S14S16).

X-ray single-crystal structures reveal that MMOF-2 and -3 are isostructural three-dimensional frameworks, in which Zn3(COO)6

secondary building blocks are bridged by BDC or CDC anions to form the 36 two-dimensional tessellated Zn3(BDC)3 or Zn3(CDC)3

sheets that are further pillared by the Cu(SalPyCy) (Fig. 2, Supplementary Figs S1 and S2). Topologically, MMOF-2 and -3 can be described as a hexagonal primitive networks (Schi symbol 36418536),

which are the same as its achiral analogue Zn3(BDC)3[Cu(SalPyen)] (ref. 33); however, the incorporation of chiral metalloligand Cu(SalPycy) leads to enantiopure MMOF-2 and -3, exhibiting two chiral pore cavities of about 6.4 in diameter (Fig. 2a,b). The chiral pore cavities are lled with the disordered solvent molecules in which MMOF-2 and -3 have the pore accessible volume of 51.7 and 48.1%, respectively, calculated using the PLATON program34.

Microporous nature of MMOFs. To establish the permanent porosity, the methanol-exchanged MMOFs-2 and -3 were activated under high vacuum at room temperature overnight to form the desolvated MMOF-2a and -3a. Nitrogen adsorption on the activated MMOFs-2a and -3a at 77.3 K was very slow because of the activated diusion eects. Therefore, CO2 adsorption at 195 K was used for their pore characterization. Surprisingly, they exhibit remarkably dierent sorption isotherms attributed to the dierent dicarboxylates. The uptake of MMOF-2a (158 cm3 g 1) is about twice than that of MMOF-3a (86 cm3 g 1) at P/P0 of 1 (Fig. 3). Both MMOF-2a and -3a show hysteretic sorption behaviours, indicating their

R R

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3(BDC)3[Cu(SalPycy)].(G) 3(CDC)3[Cu(SalPycy)].(G)M'MOF-2

1.6 Enhanced separation selectivity of C2H2 / C2H4 25.5

21 Enhanced chiral recognition of 1-phenylethyl alcohol (ee) 64

x

x

M'MOF-3

Figure 1 | Syntheses and separation capacities of MMOFs-2 and -3.

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a c

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Figure 2 | X-ray crystal structures of MMOF-3 and MMOF-3S-PEA. (a) The hexagonal primitive network topology (Schi symbol 36418536)

and (b) the three-dimensional (3D) pillared framework with chiral pore cavities for MMOF-3. (c) The hexagonal primitive network topology and (d) the 3D pillared framework exclusively encapsulating S-PEA molecules for MMOF-3S-PEA. (Zn, pink; Cu, cyan; O, red; C, grey; N, blue;

H, white).

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framework exibility and the existence of the meta-stable intermediate frameworks, which have been also observed in other exible porous MOFs5. The Langmuir (BET) surface areas calculated from the rst step adsorption isotherms are 598(388) and 237(110) m2 g 1

for MMOF-2a and -3a, respectively, within the pressure range of 0.05 < P/P0 < 0.3 (Supplementary Figs S3S5). Assuming that the second step isotherms still t into the monolayer coverage model, the overall Langmuir surface areas of MMOF-2a and -3a are 939 and 551 m2 g 1. Their total pore volumes from the highest P/P0

values and pore volumes corresponding to the intermediate isotherm step are 0.301 (0.189) and 0.164 (0.049) cm3 g 1 for MMOF-2a and MMOF -3a, respectively.

Separation of acetylene and ethylene within MMOFs. The unique CO2 sorption isotherms encouraged us to examine the capacities of MMOF-2a and -3a for their selective separation of C2H2/C2H4 at 195 K, given the fact that these two molecules have comparable molecular sizes with CO2. For MMOF-2a (Fig. 3b), the shapes of the isotherms are complex, which are apparently attributed to the framework exibility during adsorption. The total pore volumes were calculated from the highest P/P0 values (P/P0 ~0.99) using densities of 0.577, 0.726 and 1.032 g cm 3 for the densities of C2H4, C2H2 and CO2, respectively. The total pore volumes were 0.306, 0.309 and 0.301 cm3 g 1 for C2H4, C2H2 and CO2, respectively, which are basically the same, indicating that all three gas molecules can have the full access to the pores within MMOF-2a. MMOF-3a, however, exhibits signicantly dierent sorption behaviours with respect to C2H4 and C2H2. MMOF-3a can take up the acetylene up to 147 cm3 g 1 with a one-step hysteresis loop, whereas only small amount of ethylene (30.2 cm3 g 1) without the marked loop at 1 atm

and 195 K (Fig. 3c). Accordingly, the total pore volumes were different, of 0.066, 0.236 and 0.165 cm3 g 1 for C2H4, C2H2 and CO2, respectively, as calculated from their highest P/P0 values, indicating that the three gas molecules have dierential degree of access to the pores at 1 atm and 195 K when the pores within MMOF-3a become smaller. Such subtle pore control is very important for these kinds of porous materials to exhibit highly selective gas separation. In fact, MMOF-2a can only slightly dierentiate C2H2 from C2H4 with a low

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P (mmHg) P (mmHg)

Figure 3 | Gas sorption isotherms on the two activated MMOFs at different temperatures. (a) CO2 sorption on MMOFs-2a (blue square)

and -3a (red dot) at 195 K. (b) C2H2 (green square) and C2H4 (blue triangle) on MMOF-2a at 195 K. (c) C2H2 (green square) and C2H4 (blue triangle) on MMOF-3a at 195 K. (d) C2H2 (blue square), CO2 (red dot)

and C2H4 (green triangle) on MMOF-2a at 273 K. (e) C2H2 (blue square), CO2 (red dot) and C2H4 (green triangle) on MMOF-2a at 295 K. (f) C2H2

(blue square), CO2 (red dot) and C2H4 (green triangle) on MMOF-3a at 273 K. (g) C2H2 (blue square), CO2 (red dot) and C2H4 (green triangle) on

MMOF-3a at 295 K. Adsorption and desorption branches are shown with closed and open symbols, respectively.

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selectivity of 1.6, whereas MMOF-3a displays signicantly higher selectivity of 25.5 and thus can exclusively separate C2H2 from C2H4

(Table 1). In the diusion of molecules into spherical or rectangular pores both the cross-section dimensions are important, whereas for slit shaped pores only the smallest dimension is important in determining selectivity. Such signicantly enhanced separation capacity of MMOF-3a over MMOF-2a is attributed to the smaller micropores within MMOF-3a, which favours its higher size-specic separation eect on the C2H2/C2H4 separation. The narrower molecular size of C2H2 (3.32 3.34 5.70 3) compared with that of C2H4 (3.28 4.18 4.84 3) has enabled the full entrance of the C2H2 into the micropores in MMOF-3a, whereas C2H4 molecules are basically blocked or the kinetics are very slow.

The adsorption isotherms of C2H2, C2H4 and CO2 on MMOF-2a and -3a were further measured at 273 and 295 K (Fig. 3). They showed type I sorption isotherms with very little hysteresis. The selectivities towards C2H2/C2H4 on MMOF-2a at 273 and 295 K were 1.5 and 1.9, respectively. Again, MMOF-3a exhibited enhanced C2H2/C2H4 selectivities of 4.1 and 5.2 at 273 and 295 K, respectively; which are 2.5 times higher than the corresponding values for MMOF-2a (Table 1).

The unique and temperature-dependent gas separation capacities of MMOF-3a are attributed both to thermodynamically and kinetically controlled framework exibility5,26. The more exible

nature of CDC in MMOF-3 has enabled the framework MMOF-3 more exible, thus resulting in narrower pores in activated MMOF-3a than those in MMOF-2a, as shown in their powder XRD patterns (Supplementary Fig. S16). To open pore entrances for the C2H2 uptake, the gate pressure of 166 mmHg needs to be applied on the thermodynamically exible framework MMOF-3a at 195 K. At higher temperature of 273 and 295 K, the rotation/swing of the

organic linker and metalloligand within MMOF-3a has enlarged the pore apertures for the access of both C2H2 and C2H4 molecules.

Such kinetically controlled framework exibility has been also revealed in several other porous MOFs and used for their temperature-dependent gas separation20,26.

Interactions of gas molecules with MMOFs. The coverage-dependent adsorption enthalpies of the MMOFs to acetylene, ethylene and CO2 were calculated based on the virial method and the vant Ho isochore. The virial graphs for adsorption of C2H4, C2H2 and CO2 on MMOF-2a and -3a at 273 and 295 K are shown in Supplementary Figures S6S11. It is apparent that the virial graphs have very good linearity in the low-pressure region. The parameters and the enthalpies obtained from the virial equation are summarized in Table 2. For MMOF-2a, C2H4 adsorption had A1 values increasing from 1,770 to 1,551 g mol 1 from 273 to 295 K, which has a similar trend of A1 values on a carbon molecular sieve increasing from 2,480 to 1,821 g mol 1 from 303 to 343 K (ref. 35); C2H2 adsorption had A1 values increasing from 1,621 to 1,353 g mol 1 from 273 to 295 K, which also has a similar trend of A1 values on a carbon molecular sieve increasing from 1,444 to 1,302 g mol 1

from 303 to 343 K. It is apparent that the virial parameters for C2H4

and C2H2 adsorption have similar values and trends. CO2 adsorption on MMOF-2a had A1 values from 1,071 to 940 g mol 1 from 273 to 295 K without well-dened trend, which has been also observed in CO2 adsorption on a carbon molecular sieve with the A1

values ranging from 1,000 to 1,045 g mol 1 from 303 to 343 K. The trends in the A1 parameters for C2H4, C2H2 and CO2 adsorption on MMOF-2a are consistent with the adsorbateadsorbate interactions decreasing with increasing temperature. In comparison with those for C2H4 and C2H2 adsorption on MMOF-2a, the

Table 1 | The Henrys constants or the product of the Langmuir equation constants (qmb) and the equilibrium selectivity for gases on the two MMOFs.

Henrys constants K or qmb (cm3 g 1 torr 1)

MMOF-2a MMOF-3aTemperature (K) 195 273 295 195 273 295

C2H2 239.370.0057 55.760.0078 55.090.0044 179.990.0058 50.360.0071 48.300.0058 CO2 194.650.0064 49.570.0044 43.140.0030 98.640.0089 30.180.0025 27.750.0012

C2H4 178.880.0048 38.720.0073 39.710.0031 710.175.7510 5 21.900.0040 12.450.0043

Selectivity 12C2H2/CO2 1.10 2.00 1.89 1.18 4.74 8.41

CO2/C2H4 1.46 0.77 1.02 21.60 0.86 0.62 C2H2/C2H4 1.61 1.54 1.93 25.53 4.08 5.23

Table 2 | Summary of the parameters and the enthalpies of gas adsorption on MMOFs at 273 and 295 K obtained from the virial equation.

Compounds Adsorbate T (K) A0/ln (mol g 1 Pa 1) A1/g mol 1 R2 Qst,n=0/kJ mol 1

MMOF-2a C2H4 273 15.234 0.059 1770.328 6.856 0.99975 32.7

295 16.302 0.054 1551.378 7.704 0.99946C2H2 273 14.070 0.012 1621.285 11.438 0.99950 37.7

295 15.300 0.011 1353.201 11.133 0.99892CO2 273 15.591 0.013 1070.628 35.231 0.99247 32.5

295 16.652 0.007 939.937 15.613 0.99697MMOF-3a C2H4 273 17.020 0.003 2143.060 6.940 0.99983 27.3

295 17.906 0.018 2199.728 94.199 0.99452C2H2 273 14.203 0.026 2056.965 25.871 0.99811 27.1

295 15.087 0.042 1693.176 41.686 0.99158CO2 273 16.679 0.026 3117.335 137.777 0.99031 40.5

295 17.999 0.007 1902.594 33.455 0.99753

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virial parameters for C2H4 and C2H2 adsorption on MMOF-3a are more negative due to its smaller pores, but still have similar values and trends from 273 to 295 K. The fact that the A1 values for C2H4

adsorption on MMOF-3a are not obviously changed indicates that the adsorbateadsorbate interactions might be independent of temperature from 273 to 295 K (Supplementary Fig. S11c).

The Qst,n = 0 values were 32.7, 37.7 and 32.5 kJ mol 1 for C2H4, C2H2 and CO2 adsorption on MMOF-2a over the temperature range of 273295 K. The Qst,n = 0 values for C2H2 and CO2 were comparable to those obtained for their adsorption on a carbon molecular sieve (35.0 kJ mol 1(C2H2) and 28.2 kJ mol 1(CO2)), whereas the value of 32.7 kJ mol 1 for C2H4 adsorption on MMOF-2a was signicantly lower than the value (50.1 kJ mol 1) obtained for C2H4 adsorption on a carbon molecular sieve in the temperature range of 303343 K. The comparison of the results from the two methods, the linear extrapolation and the virial equation shows that there is a very good agreement (Fig. 4). In all cases the isosteric enthalpies of adsorption gradually decreased with the increasing surface coverage. As expected, the isosteric enthalpies of adsorption are signicantly higher than the enthalpies of vaporization of 17, 14 and 16.5 kJ mol 1

for C2H4, C2H2 and CO2, respectively36. The isosteric enthalpies of adsorption for C2H2, C2H4 and CO2 on MMOF-2a are characteristic of their interactions with the hydrophobic pore surfaces presented in carbon molecular sieves.

The Qst,n = 0 for C2H4, C2H2 and CO2 adsorption on MMOF-3a were 27.4, 27.1 and 40.5 kJ mol 1, respectively, over the temperature range from 273 to 295 K. The systematically lower Qst,n = 0 values for

C2H4 and C2H2 adsorption on MMOF-3a than those observed on MMOF-2a (32.7 kJ mol 1 for C2H4 and 37.7 kJ mol 1 for C2H2) may be attributed to the deciency of interactions between these

molecules and CDC moieties on surfaces of pores in MMOF-3a. The results derived from the linear extrapolation are in very good agreement with those obtained from the virial equation (Fig. 4). It needs to be mentioned that the Qst values for C2H4 and C2H2 adsorption on MMOF-3a are almost the same, indicating that the selective C2H2/C2H4 separation cannot be realized by their dierential interactions with the pore surfaces, thus, the unique gas separation characteristics for MMOF-3a are mainly attributed to size-exclusive eect.

Enantiopure selective separation of PEA. The enantiopure pore environments within MMOF-2 and -3 motivated us to explore their potential for chiral recognition and enantioselective separation. Unlike the achiral MMOF-1 Zn3(BDC)3[Cu(SalPyen)], which

encapsulates both R- and S-1-PEA to form Zn3(BDC)3[Cu(SalPyen)] R/S-PEA (Supplementary Fig. S12), the enantiopure MMOF-3 exclusively takes up S-PEA to form MMOF-3S-PEA (Zn3(CDC)3

[Cu(SalPyCy)]S-PEA). In fact, the solvothermal reaction of the corresponding reaction mixture of Zn(NO3)26H2O, H2CDC and

Cu(H2SalCy)(NO3)2 in the presence of certain amount of racemic PEA in DMF at 100 C readily formed the enantiopure MMOF-3, which exclusively encapsulates S-PEA (Fig. 2d). The incorporated S-PEA can be easily extracted by immersing the as-synthesized MMOF-3S-PEA in methanol.

The chiral recognition and enantioselective separation of MMOF-2 and -3 for the R/S-PEA racemic mixture were examined using the bulky as-synthesized materials. The as-synthesized MMOF-2 and -3 were exchanged with methanol and then immersed in the racemic mixture to selectively encapsulate the S-PEA. Once such PEA-included MMOF-2 and -3 were immersed in methanol, the encapsulated PEA within the enantiopure MMOF-2 and -3 can be readily released from the chiral pores, making their potential application for enantioselective separation of R/S-PEA. Chiral HPLC analysis of the desorbed PEA from the PEA-included MMOF-2 yielded an ee value of 21.1%, and the absolute S congu-ration for the excess was conrmed by comparing its optical rotation with that of the standard sample. It must be noted that the used MMOF-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. The second and third such regenerated MMOF-2 provide an ee value of 15.7 and 13.2%, respectively. The low enantioselectivity of the enantiopure MMOF-2 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. The smaller chiral pores within the enantiopure MMOF-3 have signicantly enhanced its enantioselectivity for the separation of R/S-PEA with the much higher ee value of 64%. The regenerated MMOF-3 can also be further used for the separation of R/SPEA with the slightly lower ee value of 55.3 and 50.6%, respectively. The chiral pores within MMOF-2 and MMOF-3 basically match 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 1-phenyl-2-propanol.

Discussion

Separation of acetylene and ethylene is a very important but challenging industrial separation task. Ethylene, the lightest olen and the largest volume organic chemical, is largely stocked in petro-chemical industry and is widely used to produce polymers and other chemicals37. The typical ethylene produced in steam crackers contains on the order of 1% of acetylene38, although a p.p.m. level of acetylene ( > 5 p.p.m.) in ethylene can poison Ziegler-Natta catalyst during ethylene polymerizations and can also lower the product quality of the resulting polymers39. Moreover, the acetylenic compounds are oen converted into solid, thus blocking the uid stream and even leading to explosion40. There are mainly two commercial

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Figure 4 | Comparison of the gas adsorption enthalpies on the two activated MMOFs. Ethylene (a, d), acetylene (b, e) and carbon dioxide (c, f)

on MMOF-2a (left) and MMOF-3a (right) from two methods: the linear extrapolation (blue solid diamond) and virial equation (red open square).

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approaches to remove acetylenes in ethylene: partial hydrogenation of acetylene into ethylene over a noble metal catalyst such as a supported Pd catalyst41,42 and solvent extraction of cracked olens using an organic solvent to obtain pure acetylene43. However, the former process suers from the catalyst price and the loss of olens due to the over hydrogenation to paraffins, although the latter is also disadvantageous in terms of technical and economical aspects, partially, because of the low selectivities of acetylene over olens and also to the signicant loss of solvent aer multiple operations. Apparently, there is a signicant need to develop novel alternative C2H2/C2H4 separation approaches. Some recent attempts, hydrogenation by non-precious metal alloy catalysts44, ionic liquid extraction45, and -complexation46, have been made to reduce the cost or to enhance the selectivities. The realization of our rst example of microporous MOF materials for such challenging separation highlights the very promise of the emerging microporous MOFs to resolve this very important industrial separation task in the future. MMOF-3a is feasible for C2H2/C2H4 separation at moderate pressures over 200 mmHg at 195 K. To realize high C2H2/C2H4 separation at low pressures at 195 K, the gate pressure for the entrance of C2H2 needs to be further reduced, which might be fullled by the combinatorial approach outlined in the Introduction. C2H2/C2H4 separation selectivity of 5.23 has featured MMOF-3a as a practical media for this important separation even at room temperature.

We have developed the pre-constructed building block approach to introduce chiral pores/pockets within the MMOFs by the usage of the chiral diamine (R, R)-1,2-cyclohexanediamine. Such chiral pore/pocket sizes are rationally tuned by the incorporation of dierent bicarboxylates BDC and CDC within their isostructural MMOFs Zn3(BDC)3[Cu(SalPycy)](G)x (MMOF-2) and Zn3(CDC)3

[Cu(SalPycy)](G)x (MMOF-3). The slightly smaller pores within MMOF-3 have enabled the activated MMOF-3a exhibits much higher separation selectivities with respect to both C2H2/C2H4 and enantioselective separation of S-1-PEA than MMOF-2a, highlighting the very promise of this MMOF strategy to tune the micropores and immobilize functional sites to direct their specic recognition and thus separation of small molecules. We are now targeting at some more robust porous MMOFs and immobilizing dierent metal sites, such as Ni2 + , Co2 + , Zn2 + , Pd2 + and Pt2 + , to rationalize the contribution of such open metal sites for their recognition of dierent small molecules by synthrontron and/or neutron diraction studies. Furthermore, we will systematically further tune and functionalize the micropores within such microporous MMOFs by making use of dierent dicarboxylates such as BDC and CDC derivatives and a variety of metalloligands, and thus to further enhance the C2H2/C2H4 separation and to target at some other challenging separation tasks, such as O2/N2, C2H4/C2H6 and C3H6/C3H8 separation, and to address some practical enantioselective separation of racemic substrates of industrial and pharmaceutical importance.

Methods

Materials. All reagents and solvents used in synthetic studies were commercially available and used as supplied without further purication. 5-Methyl-4-oxo-1,4-dihydro-pyridine-3-carbaldehyde was synthesized according to the literature procedure47.

Synthesis of Cu(H2SalPyCy)(NO3)2. A solution of (1R, 2R)-(-)-1,2-cyclohexanediamine (0.846 g, 7.41 mmol) in EtOH (20 ml) was added dropwise to a solution of 5-methyl-4-oxo-1,4-dihydro-pyridine-3-carbaldehyde (1.794 g, 14.82 mmol) in EtOH (130 ml), and the resulting mixture was reuxed for 2 h to form a clear light brown solution. To this solution, a solution of Cu(NO3)22.5H2O (1.798 g, 7.73 mmol) in EtOH (20 ml) was added, forming a blue precipitate of Cu(H2SalPyCy)(NO3)2 that was collected by ltration, washed with EtOH and air dried (2.035 g, 51%).

Synthesis of Zn3(BDC)3[Cu(SalPyCy)]5DMF4H2O. A mixture of Zn(NO3)26H2O (0.236 g, 0.79 mmol), H2BDC (0.131 g, 0.79 mmol) and

Cu(H2SalPyCy)(NO3)2 (0.143 g, 0.24 mmol) was dissolved in 100 ml DMF, and heated in a vial (400 ml) at 373 K for 24 h. The dark-blue thin plates formed were

collected and dried in the air (0.21 g, 57%). Elemental analysis (%): calcd for Zn3(BDC)3[Cu(SalPyCy)]5DMF4H2O (C59H77N9O23CuZn3): C, 46.02; H, 5.04;

N, 8.19; found: C, 45.97; H, 4.98; N, 8.24.

Synthesis of Zn3(CDC)3[Cu(SalPyCy)]5DMF4H2O. A mixture of Zn(NO3)26H2O (0.236 g, 0.79 mmol), H2CDC (0.136 g, 0.79 mmol) and

Cu(H2SalPyCy)(NO3)2 (0.143 g, 0.24 mmol) was dissolved in 100 ml DMF, and heated in a vial (400 ml) at 373 K for 24 h. The dark-blue thin plates formed were collected and dried in the air (0.23 g, 62%). Elemental analysis (%): calcd for Zn3(CDC)3[Cu(SalPyCy)]5DMF4H2O (C59H95N9O23CuZn3): C, 45.48; H, 6.15;

N, 8.09; found: C, 45.35; H, 6.23; N, 7.96.

Synthesis of Zn3(CDC)3[Cu(SalPyCy)]S-PEA5DMF. A mixture of Zn(NO3)26H2O (0.018 g, 0.06 mmol), H2CDC (0.01 g, 0.06 mmol) and

Cu(H2SalPyCy)(NO3)2 (0.016 g, 0.30 mmol) was dissolved in 3 ml DMF and 2 ml D,L-PEA, and heated in a vial (23 ml) at 373 K for 24 h. The purple platelet crystals were colleted and dried in the air (0.01 g, 31%). Elemental analysis (%): calcd for Zn3(CDC)3[Cu(SalPyCy)]S-PEA5DMF (C67H97CuN9O20Zn3): C, 50.04; H, 6.08;

N, 7.84; found: C, 50.12; H, 6.15; N, 7.96.

Synthesis of Zn3(BDC)3[Cu(SalPyen)]R/S-PEA5DMF. A mixture of Zn(NO3)26H2O (0.018 g, 0.06 mmol), H2BDC (0.01 g, 0.06 mmol) and

Cu(H2SalPyen) (NO3)2 (0.015 g, 0.03 mmol) was dissolved in 3 ml DMF and 2 ml D,L-PEA, and heated in a vial (23 ml) at 373 K for 24 h. The purple platelet crystals were colleted and dried in the air (0.01 g, 32%). Elemental analysis (%): calcd for Zn3(BDC)3[Cu(SalPyen)]R/S-PEA5DMF (C63H73CuN9O20Zn3): C, 49.26; H, 4.79;

N, 8.21; found: C, 49.55; H, 4.85; N, 8.30.

Adsorption studies. After the bulk of the solvent was decanted, the freshly prepared sample of MMOF-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 methanol soaking and decanting for ten times, the solvent-exchange samples were activated by vacuum at room right overnight till the pressure of 5 mHg. CO2, ethylene and acetylene adsorption isotherms were measured on ASAP 2020 for the activated MMOFs. As the centre-controlled air condition was set up at 22.0 C, a water bath of 22.0 C was used for adsorption isothermsat 295.0 K, whereas dry ice-acetone and ice-water bathes were used for the isotherms at 195 and 273 K, respectively.

References

1. Kuznicki, S. M. et al. A titanosilicate molecular sieve with adjustable pores for size-selective adsorption of molecules. Nature 412, 720724 (2001).

2. Deng, H. et al. Multiple functional groups of varying ratios in metal-organic frameworks. Science 327, 846850 (2010).

3. Chen, B., Xiang, S.- C. & Qian, G.- D. Metal-organic frameworks with functional pores for recognition of small molecules. Acc. Chem. Res. 43, 11151124 (2010).

4. Ma, L., Falkowski, J. M., Abney, C. & Lin, W. A series of isoreticular chiral metal-organic frameworks as a tunable platform for asymmetric catalysis. Nat. Chem. 2, 838846 (2010).

5. Horike, S., Shimomura, S. & Kitagawa, S. So porous crystals. Nat. Chem. 1,

695704 (2009).

6. Britt, D., Furukawa, H., Wang, B., Glover, T. G. & Yaghi, O. M. Highly efficient separation of carbon dioxide by a metal-organic framework replete with open metal sites. Proc. Natl Acad. Sci. USA 106, 2063720640 (2009).

7. Shimomura, S. et al. Selective sorption of oxygen and nitric oxide by an electron-donating exible porous coordination polymer. Nat. Chem. 2, 633637 (2010).

8. Rabone, J. et al. An adaptable peptide-based porous material. Science 329, 10531057 (2010).

9. Devic, T. et al. Functionalization in exible porous solids: eects on the pore opening and the host-guest interactions. J. Am. Chem. Soc. 132, 11271136 (2010).

10. Seo, J. S. et al. A homochiral metal-organic porous material for enantioselective separation and catalysis. Nature 404, 982986 (2000).

11. Morris, R. E. & Bu, X. Induction of chiral porous solids containing only achiral building blocks. Nat. Chem. 2, 353361 (2010).

12. Chen, S., Zhang, J., Wu, T., Feng, P. & Bu, X. Multiroute synthesis of porous anionic frameworks and size-tunable extra framework organic cation-controlled gas sorption properties. J. Am. Chem. Soc. 131, 1602716029 (2009).

13. Yang, S. et al. Cation-induced kinetic trapping and enhanced hydrogen adsorption in a modulated anionic metal-organic framework. Nat. Chem. 1, 487493 (2009).

14. Xie, Z., Ma, L., de Krafft, K. E., Jin, A. & Lin, W. Porous phosphorescent coordination polymers for oxygen sensing. J. Am. Chem. Soc. 132, 922923 (2010).

15. Kitaura, R., Onoyama, G., Sakamoto, H., Matsuda, R., Noro, S.- I. & Kitagawa, S. Crystal engineering: immobilization of a metallo Schi base into a microporous coordination polymer. Angew. Chem. Int. Ed. 43, 26842687 (2004).

NATURE COMMUNICATIONS | 2:204 | DOI: 10.1038/ncomms1206 | www.nature.com/naturecommunications

2011 Macmillan Publishers Limited. All rights reserved.

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16. Chen, B., Fronczek, F. R. & Maverick, A. W. Porous Cu-Cd mixed-metal-organic frameworks constructed from Cu(Pyac)2 {Bis[3-(4-pyridyl)pentane-2,4-dionato]copper(II)}. Inorg. Chem. 43, 82098211 (2004).

17. Cho, S.- H., Ma, B., Nguyen, S. T., Hupp, J. T. & Albrecht-Schmitt, T. E. A metal-organic framework material that functions as an enantioselective catalyst for olen epoxidation. Chem. Commun. 25632565 (2006).

18. Liu, Y., Xuan, W. & Cui, Y. Engineering homochiral metal-organic frameworks for heterogeneous asymmetric catalysis and enantioselective separation.

Adv. Mater 22, 41124135 (2010).

19. Murray, L. J. et al. Highly-aelective and reversible O2 binding in Cr3(1,3,5-benzenetricarboxylate)2. J. Am. Chem. Soc. 132, 78567857 (2010).

20. Ma, S., Sun, D., Yuan, D., Wang, X.- S. & Zhou, H.- C. Preparation and gas adsorption studies of three mesh-adjustable molecular sieves with a common structure. J. Am. Chem. Soc. 131, 64456451 (2009).

21. McKinlay, A. C., Xiao, B., Wragg, D. S., Wheatley, P. S., Megson, I. L. & Morris, R. E. Exceptional behavior over the whole adsorption-storage-delivery cycle for NO in porous metal organic frameworks. J. Am. Chem. Soc. 130, 1044010444 (2008).22. Dubbeldam, D., Galvin, C. J., Walton, K. S., Ellis, D. E. & Snurr, R. Q. Separation and molecular-level segregation of complex alkane mixtures in metal-organic-frameworks. J. Am. Chem. Soc. 130, 1088410885 (2008).

23. Chen, B. et al. Microporous metal-organic framework for gas chromatographic separation of alkanes. Angew. Chem. Int. Ed. 45, 13901393 (2006).

24. Finsy, V. et al. Pore-lling-dependent selectivity eects in the vapor-phase separation of xylene isomers on the metal-organic framework MIL-47. J. Am. Chem. Soc. 130, 71107118 (2008).

25. Bae, Y.- S., Spokoyny, A. M., Farha, O. K., Snurr, R. Q., Hupp, J. T. & Mirkin, C. A. Separation of gas mixtures using Co(II) carborane-based porous coordination polymers. Chem. Commun. 46, 34783480 (2010).26. Zhang, J.- P. & Chen, X.- M. Exceptional framework exibility and sorption behavior of a multifunctional porous cuprous triazolate framework. J. Am. Chem. Soc. 130, 60106017 (2008).

27. Dybtsev, D. N., Chun, H., Yoon, S. H., Kim, D. & Kim, K. Microporous manganese formate: a simple metal-organic porous material with high framework stability and highly selective gas sorption properties. J. Am. Chem. Soc. 126, 3233 (2004).

28. Li, K.- H. et al. Zeolitic imidazolate frameworks for kinetic separation of propane and propene. J. Am. Chem. Soc. 131, 1036810369 (2009).

29. Vaidhyanathan, R. et al. A family of nanoporous materials based on an amino acid backbone. Angew. Chem., Int. Ed. 45, 64956499 (2006).

30. Nuzhdin, A. L., Dybtsev, D. N., Bryliakov, K. P., Talsi, E. P. & Fedin, V. P. Enantioselective chromatographic resolution and one-pot synthesis of enantiomerically pure sulfoxides over a homochiral Zn-organic framework. J. Am. Chem. Soc. 129, 1295812959 (2007).31. Dybtsev, D. N. et al. A homochiral metal-organic material with permanent porosity, enantioselective sorption properties, and catalytic activity. Angew. Chem. Int. Ed. 45, 916920 (2006).

32. Chen, B. et al. Surface and quantum interactions for H2 conned in metal-organic framework pores. J. Am. Chem. Soc. 130, 64116423 (2008).33. OKeee, M., Peskov, M. A., Ramsden, S. J. & Yaghi, O. M. The reticular chemistry structure resource (RCSR) database of, and symbols for, crystal nets. Acc. Chem. Res. 41, 17821789 (2008).

34. Spek, A. L. PLATON, A Multipurpose Crystallographic Tool (Utrecht University, 2001).

35. Reid, C. R., Okoye, I. P. & Thomas, K. M. Adsorption of gases on carbon molecular sieves used for air separation: spherical adsorptives as probes for kinetic selectivity. Langmuir 14, 24152425 (1998).

36. Chickos, J. S. & Acree, W. E. Enthalpies of vaporization of organic and organometallic compounds, 18802002. J. Phys. Chem. Ref. Data 32, 519878 (2003).

37. Sundaram, K. M., Shreehan, M. M. & Olszewski, E. F. Kirk-Othmer Encyclopedia of Chemical Technology 4th edn, 877915 (Wiley, 1995).

38. Collins, B. M. Selective hydrogenation of highly unsaturated hydrocarbons in the presence of less unsaturated hydrobarbons. US Patent 4126645 (1978).

39. Huang, W., McCormick, J. R., Lobo, R. F. & Chen, J. G. Selective hydrogenation of acetylene in the presence of ethylene on zeolite-supported bimetallic catalysts. J. Catal. 246, 4051 (2007).

40. Molero, H., Bartlett, B. F. & Tysoe, W. T. The hydrogenation of acetylene catalyzed by palladium: hydrogen pressure dependence. J. Catal. 181, 4956 (1999).

41. Choudary, B. M. et al. Hydrogenation of acetylenics by Pd-exchanged mesoporous materials. Appl. Catal. A 181, 139144 (1999).

42. Khan, N. A., Shaikhutdinov, S. & Freund, H.- J. Acetylene and ethylene hydrogenation on alumina supported Pd-Ag model catalysts. Catal. Lett. 108, 159164 (2006).

43. Weissermel, K. & Arpe, H.- J. Industrial Organic Chemistry 4th edn, 9198 (Wiley-VCH, 2003).

44. Studt, F., Abild-Pedersen, F., Bligaard, T., Srensen, R. Z., Christensen, C. H. & Nrskov, J. K. Identication of non-precious metal alloy catalysts for selective hydrogenation of acetylene. Science 320, 13201322 (2008).

45. Palgunadi, J., Kim, H. S., Lee, J. M. & Jung, S. Ionic liquids for acetylene and ethylene separation: material selection and solubility investigation. Chem. Eng. Proc. 49, 192198 (2010).

46. Wang, K. & Stiefel, E. I. Toward separation and purication of olens using dithiolene complexes: an electrochemical approach. Science 291, 106109 (2001).

47. Arya, F., Bouquant, J. & Chuche, J. A convenient synthesis of 3-formyl-4(1H)-pyridones. Synthesis 946948 (1983).

Acknowledgments

This work was supported by the Award CHE 0718281 from the NSF and AX-1730 from Welch Foundation (B.C.), AX-1593 from Welch Foundation (C.-G.Z.) and Leverhulme trust (K.M.T.). This research was partially conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Division of Scientic User Facilities, US Department of Energy.

Author contributions

S.-C.X., K.M.T. and B.C. designed the study, analysed the data and wrote the paper. K.-L.H. constructed part work to synthesize the precusor. C.-G.Z., D.-R.D., M.-H.X. and C.-D.W. performed the enantiomeric excess measurement. All authors discussed the results and commented on the manuscript.

Additional information

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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 ethylene. Nat. Commun. 2:204 doi: 10.1038/ncomms1206 (2011).

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