Jack D.
Humby‡
a,
Oguarabau
Benson‡
b,
Gemma L.
Smith
a,
Stephen P.
Argent
c,
Ivan
da Silva
d,
Yongqiang
Cheng
e,
Svemir
Rudić
d,
Pascal
Manuel
d,
Mark D.
Frogley
f,
Gianfelice
Cinque
f,
Lucy K.
Saunders
f,
Iñigo J.
Vitórica-Yrezábal
a,
George F. S.
Whitehead
a,
Timothy L.
Easun
g,
William
Lewis
b,
Alexander J.
Blake
b,
Anibal J.
Ramirez-Cuesta
d,
Sihai
Yang
*a and
Martin
Schröder
*a
aSchool of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, UK. E-mail: Sihai.Yang@manchester.ac.uk; M.Schroder@manchester.ac.uk
bSchool of Chemistry, University of Nottingham, Nottingham, NG7 2RD, UK
cDepartment of Chemistry, University of Warwick, CV4 7AL, UK
dISIS Facility, STFC Rutherford Appleton Laboratory, Oxfordshire OX11 0QX, UK
eOak Ridge National Laboratory, Oak Ridge, TN 37831, USA
fDiamond Light Source, Harwell Science and Innovation Campus, Oxfordshire, OX11 0DE, UK
gSchool of Chemistry, Cardiff University, Cardiff CF10 3XQ, UK
First published on 12th October 2018
In order to develop new porous materials for applications in gas separations such as natural gas upgrading, landfill gas processing and acetylene purification it is vital to gain understanding of host–substrate interactions at a molecular level. Herein we report a series of six isoreticular metal–organic frameworks (MOFs) for selective gas adsorption. These materials do not incorporate open metal sites and thus provide an excellent platform to investigate the effect of the incorporation of ligand functionality via amide and alkyne groups on substrate binding. By reducing the length of the linker in our previously reported MFM-136, we report much improved CO2/CH4 (50:
50) and CO2/N2 (15
:
85) selectivity values of 20.2 and 65.4, respectively (1 bar and 273 K), in the new amide-decorated MOF, MFM-126. The CO2 separation performance of MFM-126 has been confirmed by dynamic breakthrough experiments. In situ inelastic neutron scattering and synchrotron FT-IR microspectroscopy were employed to elucidate dynamic interactions of adsorbed CO2 molecules within MFM-126. Upon changing the functionality to an alkyne group in MFM-127, the CO2 uptake decreases but the C2H2 uptake increases by 68%, leading to excellent C2H2/CO2 and C2H2/CH4 selectivities of 3.7 and 21.2, respectively. Neutron powder diffraction enabled the direct observation of the preferred binding domains in MFM-126 and MFM-127, and, to the best of our knowledge, we report the first example of acetylene binding to an alkyne moiety in a porous material, with over 50% of the acetylene observed within MFM-127 displaying interactions (<4 Å) with the alkyne functionality of the framework.
Common strategies for enhancing host–guest interactions in MOFs include incorporation of open metal sites,20 polar functional groups (e.g., –NH2, –CONH–, –OH, –F)17,21–24 and narrowing pore channels by use of small ligands.25 For example, polar nitrogen-containing groups remain a favored approach to enhancing CO2 adsorption, as shown in a crystallographic study that visualized CO2 molecules directly interacting with the amine group in a Zn–MOF.26 However, we have previously reported neutron diffraction and scattering data revealing that the high CO2 uptake in amide-functionalized MFM-136 is not solely due to guest–host interactions at the amide group, but is a combination of geometry, pore size and functionality that lead to improved gas sorption properties.27 Whilst many MOFs have been reported for their gas sorption properties, it is often difficult to fully account for differences in performance owing to many variables such as surface area, porosity and pore geometry, functionality and presence of open metal sites. Thus, to aid design-based approaches for improved materials, thorough investigations of isoreticular series of MOFs such as the IRMOF,28 UiO-66 (ref. 29) and MOF-74 (ref. 30) series are important.
Herein we present a comprehensive investigation into the roles of functional groups, pore geometry and porosity in enhancing selective gas binding through a series of six isostructural MOFs (MFM-126–128 and MFM-136–138; Table 1). MFM-137 and MFM-138 were designed by adapting previously reported MFM-136,27 replacing the amide group with an ethynyl bond and phenyl ring, respectively. Further modification of MFM-136–138 was achieved by removal of the central phenyl unit of the linker to produce ‘shortened’ derivatives, MFM-126–128. This systematic approach allows us to isolate either the effect of varying pore size or functionality to rationalize the properties of these materials. Our approach has been to focus on the role of ligand sites for substrate binding and, significantly, we report herein the first example of binding of acetylene to the alkyne groups in a porous material at crystallographic resolution.
MFM-126 | MFM-127 | MFM-128 | MFM-136a | MFM-137 | MFM-138 | |
---|---|---|---|---|---|---|
a Previously reported.27 | ||||||
Structures of linkers |
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H2L1 | H2L2 | H2L3 | H2L4 | H2L5 | H2L6 | |
Coordination environment |
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Cage sizes |
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Long cage (A) length/Å | 15.4 | 16.4 | 20.2 | 24.9 | 26.1 | 29.9 |
Short cage (B) length/Å | 12.3 | 12.7 | 14.1 | 16.2 | 16.4 | 18.1 |
BET surface area/m2 g−1 | 1004 | 1557 | 1491 | 1634 | 1749 | 1590 |
Pore volume (N2 isotherm)/cm3 g−1 | 0.47 | 0.52 | 0.57 | 0.65 | 0.61 | 0.60 |
Pore volume (single crystal)/cm3 g−1 | 0.52 | 0.57 | 0.57 | 0.64 | 0.67 | 0.62 |
The MOFs in this series all incorporate two types of cages, the larger of which, cage A (Fig. 1a) is comprised of six ligands and six [Cu2(RCOO)4(NR)2] paddlewheel units forming a hexagonal bipyramid. The six {Cu2} units form the six equatorial vertices of this cage and the hexagonal window of the Kagomé lattice (Fig. 1d). Six pyrimidyl units form the apical vertices, whereby ligands form six of the twelve faces of the hexagonal bipyramid. The smaller cage B (Fig. 1b) is constructed from six ligands and six {Cu2} paddlewheels forming a ditrigonal scalenohedral cage, whereby two sets of three {Cu2} paddlewheels bridged by three linker isophthalate units form triangular windows of neighboring Kagomé lattices. The overall structure of these MOFs is comprised of discrete cages A and B, which are packed in an alternating manner (Fig. 1c), giving highly porous and robust 3D frameworks. Views of the structure along the principal crystallographic axes of MFM-126 are shown in Fig. S3.† The phase purity of all material samples was confirmed by powder X-ray diffraction (PXRD) data (Fig. S4†).
CO2 adsorption isotherms were measured to 20 bar at 273 and 298 K for all MOFs (Fig. S5†). Throughout the series, it was found that the CO2 uptake at 20 bar increased with increasing pore volume, indicating that the pore functionality had little effect on high pressure gas adsorption, where porosity is the dominant factor. For example, a 10% increase in CO2 uptake at 20 bar is observed in MFM-127 compared with MFM-126 (11.3 and 10.2 mmol g−1 at 298 K, respectively), corresponding well with the 11% pore volume increase from MFM-126 to MFM-127. However, at 1.0 bar and 298 K, comparing phenyl-functionalized MFM-138 and MFM-128 (2.89 and 3.19 mmol g−1, respectively), it was found that the CO2 uptake is greater in MFM-128, indicating a stronger interaction with the MFM-128 framework at ambient pressure. This suggests that pore geometry has an important role in low pressure gas uptake, with MFM-126–128 all having greater uptake at 1.0 bar CO2 than the comparative extended derivatives, MFM-136–138 (Fig. 2a). The influence of functionality on CO2 uptake can be assessed by comparing MFM-136 and MFM-137 which show similar porosity (Table 1). MFM-136 and MFM-137 show CO2 uptakes of 7.30 and 5.76 mmol g−1 (273 K), respectively, suggesting that amide-functionalized MFM-136 possesses stronger affinity for CO2 compared to the alkyne groups in MFM-137.
Crucially, however, MFM-126, with the smallest pore volume, has the greatest CO2 uptake at low pressure across the series, reaching 4.63 mmol g−1 at 1 bar and 298 K, presumably due to the greater overlap of attractive interactions between gas molecules and the host framework. This is exemplified further at 0.15 bar, the partial pressure of CO2 in flue gas streams, where MFM-126 has a 52% higher uptake of CO2 compared with the next best-performing MOF, MFM-136 (2.94 and 1.94 mmol g−1, respectively) in this series.
MOF | CO2 uptake/mmol g−1 (1 bar) | CH4 uptake/mmol g−1 (1 bar) |
S
CO2/CH4 (50![]() ![]() |
S
CO2/N2 (15![]() ![]() |
Q st/kJ mol−1 (virial method 1) | |||||
---|---|---|---|---|---|---|---|---|---|---|
273 K | 298 K | 273 K | 298 K | 273 K | 298 K | 273 K | 298 K | CO2 | CH4 | |
MFM-126 | 7.00 | 4.63 | 1.50 | 0.897 | 20.2 | 11.7 | 65.4 | 39.6 | 30.7 | 17.3 |
MFM-127 | 5.72 | 2.97 | 1.71 | 0.991 | 5.08 | 3.33 | 10.6 | 7.65 | 25.8 | 13.0 |
MFM-128 | 5.76 | 3.20 | 1.77 | 0.953 | 5.46 | 4.53 | 35.8 | 18.9 | 20.4 | 22.7 |
MFM-136 | 7.29 | 4.28 | 2.93 | 1.64 | 4.07 | 3.35 | 37.0 | 23.2 | 20.1 | 18.9 |
MFM-137 | 5.76 | 2.92 | 1.41 | 0.870 | 6.09 | 4.08 | 27.6 | 15.7 | 19.2 | 17.1 |
MFM-138 | 6.08 | 2.89 | 1.75 | 1.02 | 5.42 | 3.87 | 17.1 | 15.5 | 30.0 | 18.8 |
To evaluate the dynamic separation performance of MFM-126, breakthrough experiments were performed with CO2/N2 (15:
85) and equimolar CO2/CH4 mixtures (Fig. 2e and f). The gas mixtures were flowed over a fixed-bed packed with MFM-126 with a total flow of 10 mL min−1 at 298 K (1 bar). These breakthrough experiments confirmed the separation potential for both gas mixtures as predicted by IAST selectivity calculations (Fig. 2d).
Although the amide-functionalised MFM-126 exhibits higher CO2 uptake compared with MFM-127, the functional groups were also investigated for their effect on acetylene adsorption. Interestingly, alkyne-functionalised MFM-127 exhibits a 68% higher acetylene uptake than MFM-126 at 273 K and 1 bar (9.28 and 5.54 mmol g−1, respectively, Fig. 2c). IAST analysis for MFM-127 reveals selectivity values for equimolar mixtures of C2H2/CO2 and C2H2/CH4 to be 3.7 and 21.2, respectively (Fig. S20†). This result is comparable to that observed for [Cu2(EBTC)] (H4EBTC = 1,1′-ethynebenzene-3,3′,5,5′-tetracarboxylic acid),34 which shows enhanced acetylene uptake compared with the non-alkyne bridged analogue MOF-505/MFM-100.35,36 Whilst the presence of CC⋯C
C interactions between acetylene gas and an alkyne organic linker may be postulated, this has not yet been confirmed or observed experimentally.
MFM-126 displays four binding sites for CO2, 1–4 (in decreasing order of occupancy; Fig. 3). CO2-1 is situated in small cage B and exhibits co-operative binding between crystallographically equivalent CO2 molecules [CCO2⋯OCO2 = 3.30(3) Å] where the linear bodies of the CO2 molecules lie parallel with an interaction to an adjacent amide [OCO2⋯Namide = 3.86(5) Å, <CO⋯N = 111°]. In addition, there are short contacts between a pyrimidyl ring [C–Hpyrimidine⋯OCO2 = 2.32(5) Å, <C–H⋯O = 143°] and an isophthalate ring [OCO2⋯centroidisophthal. = 3.15(4) Å, <C
O⋯centroid = 111°]. CO2-2 is located in the triangular window of cage B, with a close contact to a C–H group of the isophthalate unit [OCO2⋯H–Cisophthal. = 1.71(10) Å, <O⋯H–C = 152°] and a side-on interaction with two adjacent pyrimidine rings [OCO2⋯H–Cpyrimidine = 2.17(5) Å, <O⋯H–C = 128°]. CO2-3 is situated in a pocket between cages A and B with two short contacts to amido N–H units [OCO2⋯H–Namide = 3.77(6) Å, <O⋯H–N = 124° and OCO2⋯H–Namide = 3.98(8) Å, <O⋯H–N = 138°]. Further to this, there are two other close contacts with a pyrimidyl ring [OCO2⋯Cpyrimidine = 2.44(5) Å, C
O⋯C = 115°] and a side-on interaction with an isophthalate ring [CCO2⋯H–Cisophthal. = 3.34(12) Å, <C⋯H–C = 172°]. CO2-4 is positioned at the periphery of larger cage A with a hydrogen bond to an amido N–H [OCO2⋯H–N = 4.14(9) Å, <O⋯H–N = 149°] as well as lying in a crevice between two isophthalate units [OCO2⋯Cisophthal. = 3.14(11) Å, C
O⋯C = 81° and OCO2⋯Cisophthal. = 3.25(10) Å, C
O⋯C = 96°].
Analysis of CD4-loaded MFM-126 reveals seven sites 1–7, the first two of which have significantly higher crystallographic occupancies of 0.630 and 0.590 than the rest (Fig. 3). CD4-1, resides in cage B in an equivalent site to that of CO2-2, also with close interactions to isophthalate and pyrimidyl rings [D4C⋯H–Cisophthal. = 2.96(11) Å, <C⋯H–C = 122° and D4C⋯H–Cpyrimidine = 3.28(4) Å, <C⋯H–C = 159°]. CD4-2 is found in an identical site to that of CO2-1 located in the center of cage B with two key interactions to an isophthalate ring and an amido carbonyl of the framework [D4C⋯H–Cisophthal. = 2.81(4) Å, <C⋯H–C = 146° and D4C⋯OCamide = 3.00(5) Å, <C⋯O
C = 140°].
Refinement of NPD data of C2D2-loaded MFM-127 unveiled five sites, 1–5 (Fig. 3). Interestingly, over 50% of C2D2 were found having short contacts (<4 Å) to the framework alkyne units. C2D2-1 is situated in a narrow window between cages A and B, with closest interactions to pyrimidyl and isophthalate rings [DC2–D⋯Npyrimidine = 2.54(5) Å, <C–D⋯Npyrimidine = 145° and η2-D2C2⋯H–Cisophthal. = 2.78(2) Å, <C–H⋯CCcentroid = 108°]. Cooperative binding is visualized between C2D2-1 and C2D2-2 [1-D2C
C⋯D–C2D-2 = 1.81(3) Å, <C1⋯D–C2 = 127°]. C2D2-2 also binds to the framework alkyne [DC2–D⋯η2-C
Cframework = 3.89(5) Å, <C–D⋯C
Ccentroid = 123°] suggesting the framework alkyne has an integral role in the uptake of acetylene in MFM-127. C2D2-3 is located in the center of cage B with H-bonds to a framework alkyne moiety [DC2–D⋯η2-C
Cframework = 2.68(5) Å, <C–D⋯C
Ccentroid = 120°] as well as close contacts to pyrimidyl and isophthalate rings [η2-D2C
C⋯H–Cpyrimidine = 3.05(8) Å, <C–H⋯C
Ccentroid = 130° and η2-C
CD2⋯Cisophthal. = 3.48(3) Å]. C2D2-4 occupies the hydrophobic triangular window of cage B with strong binding to an oxygen of the {Cu2} paddlewheel [DC2–D⋯Opaddlewheel = 2.25(5) Å, <C–D⋯O = 150°] as well as to adjacent isophthalate rings [D2C2⋯H–Cisophthal. = 2.45(2) Å, <C–H⋯C = 120° and η2-C
CD2⋯H–Cisophthal. = 2.89(1) Å, <C–H⋯C
Ccentroid = 124°].
Three CO2 binding domains, 1–3, were found in MFM-127 (Fig. 3). CO2-1 is situated in an equivalent site to C2D2-4 with a close contact to a carboxylate Cδ+ [OCO2⋯CCOO = 3.96(6) Å, <C–O⋯C = 100°] as well as side-on interactions with isophthalate rings [OCO2⋯H–Cisophthal. = 2.27(1) Å, <C–H⋯O = 135° and CCO2⋯H–Cisophthal. = 2.95(7) Å, <C–H⋯C = 114°]. CO2-2 occupies the corresponding site to CO2-1 in MFM-126. As for MFM-126, cooperative binding is exhibited between crystallographically equivalent CO2-2 molecules in cage B of MFM-127 [CCO2⋯OCO2 = 3.04(2) Å, <CO⋯C = 128°]. Crucially, these results reveal the key role of the amide groups on selective adsorption of CO2 in MFM-126 as well as confirming that alkyne moieties do not act as effective adsorption sites for CO2 in MFM-127.
NPD data for CD4-loaded MFM-127 exposed only two noteworthy sites of interaction. Both sites correspond directly to CD4 sites 1 and 2 observed in MFM-126, respectively. In MFM-127, CD4-1 resides in the hydrophobic triangular windows of cage B [D4C⋯H–Cisophthal. = 2.93(1) Å, <C–H⋯C = 120°], further showing that this pocket provides an optimal environment for CD4. CD4-2 is found in the center of cage B and interacts weakly to the framework alkyne [D4C⋯η2-CCframework = 4.39(1) Å] revealing that the alkyne moiety has little effect on CD4 adsorption.
Thus, significantly, the NPD study reveals that it is a combination of cooperative binding as well as the amide functionality that leads to enhanced interaction of CO2 with MFM-126. On the other hand, NPD data reveal that alkyne-functionalized MFM-127 exhibits much weaker interaction with CO2, but that the alkyne moieties play critical roles in acetylene binding, with over half of the adsorbed acetylene molecules exhibiting interactions (<4 Å) to alkyne moieties in the pore of MFM-127. This represents the first example of direct visualization of acetylene binding to an alkyne moiety in porous materials.
Footnotes |
† Electronic supplementary information (ESI) available: Correspondence and requests for materials should be addressed to S. Y. and M. S. CCDC 1857732–1857743. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8sc03622e |
‡ These authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2019 |