Terephthalate Linkers Research Articles

Metal-Cation-Independent Dynamics of Phenylene Ring in Microporous MOFs: A 2H Solid-State NMR Study

Mobility of the organic linkers in metal–organic frameworks (MOFs) is an established phenomenon. Knowledge of the details of linker motion in MOFs could provide a great deal of information about the linker structure and the way the guest molecules interact with the organic framework. However, the mobility of the organic linkers is poorly characterized. The extent of the influence of the metal cation or guest molecules on linker motion is still unknown for MOFs with identical topologies. In this work, we have analyzed the rotational dynamics of the phenylene ring fragments of terephthalate (1,4-benzenedicarboxylate, bdc) linkers in the series of MOFs [M2(bdc)2(dabco)]·G (M = Co2+, Ni2+, Cu2+, Zn2+; dabco =1,4-diazabicyclo[2.2.2]octane; G = none or dimethylformamide, DMF). We have established that the reorientational motion of the phenylene rings is performed by π-flipping of the plane of the ring about its C2 axis. The dynamics of the phenylene rings is insensitive to the variation of the metal cation, whe...

New interpenetrated mixed (Co/Ni) metal–organic framework for dye removal under mild conditions

A new three-dimensional (3D) bimetallic metal–organic framework (MOF), [CoNi(μ3-tp)2(μ2-pyz)2] (1) (tp=terephthalic acid and pyz=pyrazine), was hydrothermally synthesized using mixed O- and N-donor ligands. The structure of 1 has been determined by single-crystal X-ray diffraction analysis and characterized by FT-IR, powder X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). Crystal structure shows that the metal centers are equally occupied by Co2+ and Ni2+ ions that was also confirmed by ICP technique. Two different coordination modes of terephthalate linkers connect three metal centers (CoNi), forming a (4,4)-connected 2D layers, which are pillaring by pyrazine ligands to form an interlocked 3D structure. Topological studies show a uninodal twofold interpenetrated structure with 3(412·63) topological network. The prepared bimetallic MOF was applied as catalyst for the removal of Bismarck brown as a diazo dye under mild conditions. The results showed that compound 1 had good activity and stability during the dye removal process and could be recovered and reused at least for three cycles without considerable loss of activity.

Facile synthesis and gas adsorption behavior of new functionalized Al-MIL-101-X (X = –CH3, –NO2, –OCH3, –C6H4, –F2, –(CH3)2, –(OCH3)2) materials

The synthesis of seven new functionalized aluminum terephthalates [Al3OCl(DEF)2(BDC-X)3] (DEF = N,N-diethylformamide; BDC = 1,4-benzene-dicarboxylate; X = –CH3, 1–CH3; –NO2, 2–NO2; –OCH3, 3–OCH3; –C6H4, 4–C6H4; –F2, 5–F2; –(CH3)2, 6-(CH3)2, –(OCH3)2, 7–(OCH3)2) having MIL-101 framework topology was carried out under solvothermal conditions (130 °C, 3–12 h) with conventional electric (CE) heating by using stochiometric mixtures of Al(ClO4)3·9H2O and H2BDC-X linkers in DEF. The previously reported Al-MIL-101–NH2 compound was also prepared under similar solvothermal conditions. Moreover, four of the compounds, namely 1–CH3, 3–OCH3, 4–C6H4 and 7–(OCH3)2, was synthesized under microwave-assisted solvothermal conditions (170 °C, 10 min, 150 W) by using similar reaction mixtures as the syntheses with CE heating. The compounds were fully characterized by X-ray powder diffraction (XRPD), Fourier transform infrared (FT-IR) spectroscopy, thermogravimetric (TG) and elemental analysis. The thermally activated forms of the as-synthesized compounds were obtained by exchange of the guest molecules with methanol, followed by heating the methanol-exchanged solids under dynamic vacuum. TG analyses carried out with the thermally activated compounds suggest high thermal stability up to 260–430 °C in an air atmosphere. All of the thermally activated compounds exhibit significant porosity, as confirmed by N2 and CO2 sorption experiments. The specific BET surface areas of the compounds fall in the 1328–2398 m2 g−1 range. The CO2 uptake values of the compounds lie in the 1.6–3.0 mmol g−1 range at 0 °C and 1 bar. The N2 adsorption capacities of the compounds are dependent on the size of the attached functional groups. On the other hand, the CO2 uptake values are dependent both on the size and the nature of functional groups attached to the terephthalate linker molecules.

Thorium terephthalates coordination polymers synthesized in solvothermal DMF/H2O system.

A series of thorium-based terephthalates have been solvothermaly synthesized in N,N-dimethylformamide (DMF) with different amounts of water and various temperatures (100-150 °C). Without the addition of water, the Th-H2bdc-DMF system gives rise to the formation of two phases, Th(bdc)2(DMF)2 (1) and Th6O4(OH)4(H2O)6(bdc)6·6DMF·12H2O (3) (bdc = 1,4-benzenedicarboxylate or terephthalate). Their structures are built up of isolated thorium centers ThO8(DMF)2 for (1) and the hexanuclear core Th6O4(OH)4(H2O)6 for (3). The latter adopts the UiO-66 metal-organic framework topology and exhibits a very high porosity for an actinides-based porous material (BET surface up to 730(6) m(2)·g(-1)). The synthesis of (3) is also favored upon adding water. However, for pure aqueous solutions or for a very low amount of water, a third solid Th(bdc)2 (2) crystallizes and contains thorium monomers ThO8. The main similitude with the parent system dedicated to tetravalent uranium concerns the possibility to stabilize the An6O8(H2O)6 core by terephthalate linkers and to reproduce An(bdc)2(DMF)2 for both actinides U(4+) and Th(4+). The thermal treatment of the latter shows a structural transition into the crystalline Th(bdc)2 (2) solid.

Effective adsorptive removal of indole from model fuel using a metal-organic framework functionalized with amino groups

Nitrogen-containing compounds (NCCs) should be removed from fuels because of the negative effect of NCCs on the environment and catalyst stability. NCCs are composed of basic materials such as quinoline (QUI) and neutral materials such as indole (IND). The NCCs can be removed by various methods including adsorption. Compared with basic NCCs, neutral NCCs are more difficult to remove through adsorption due to their less affinity toward adsorbents. In this report, adsorption of IND (as one of the representative neutral NCCs) was studied over the metal-organic frameworks (MOFs), UiO-66 and UiO-66-NH2, which contain terephthalate and aminoterephthalate linkers, respectively. In spite of the reduced porosity of UiO-66-NH2, the adsorption capacity of IND was improved upto 46% when compared with pristine UiO-66. Therefore, the additional amino group in the MOF imparts extra adsorption capability on the MOF. For a detailed investigation, adsorption of other NCCs such as QUI, pyrrole, and methylpyrrole was studied. The improved adsorption of IND over amino-functionalized MOFs could be attributed to the improved interaction of IND with the MOF via H-bonding because of the NH2 group. In addition to this remarkable improvement in IND adsorption, UiO-66-NH2 could be regenerated several times for the adsorption of IND by simple solvent washing.

Formation Mechanism of the Secondary Building Unit in a Chromium Terephthalate Metal–Organic Framework

A detailed mechanism, based on density functional theory calculations and simulation, is presented outlining the formation of the secondary building unit (SBU) of MIL-101, a chromium terephthalate metal organic framework (MOF). Formation of the metal core and of the SBU is key to MOF nucleation, the rate-limiting step in the synthesis process of many MOFs. A series of reactions that lead to the formation of the SBU of MIL-101 is proposed in this work. The highest barrier (∼35 kcal/mol) involves the formation of a dimetal-linker intermediate and high to low spin transition as a third Cr-linker moiety joins to form a three metal-linker group joined by a central oxygen. The terephthalate linkers play an important, key mechanistic role with the carboxylates first joining chromium atoms prior to the formation of bridging oxygens. Subsequent to metal core formation, stepwise linker addition reactions generate different assembly pathways due to structural isomers that are limited by the removal of water molecule...

Catalytic Performance of Vanadium MIL‐47 and Linker‐Substituted Variants in the Oxidation of Cyclohexene: A Combined Theoretical and Experimental Approach

AbstractThe epoxidation of cyclohexene has been investigated on a metal–organic framework MIL‐47 containing saturated V+IV sites linked with functionalized terephthalate linkers (MIL‐47‐X, X=OH, F, Cl, Br, CH3, NH2). Experimental catalytic tests have been performed on the MIL‐47‐X materials to elucidate the effect of linker substitution on the conversion. Notwithstanding the fact that these substituted materials are prone to leaching in the performed catalytic tests, the initial catalytic activity of these materials correlates with the Hammett substituent constants. In general, substituents led to an increased activity relative to the parent MIL‐47. To rationalize the experimental findings, first‐principles kinetic calculations were performed on periodic models of MIL‐47 to determine the most important active sites by creating defect structures in the interior of the crystalline material. In a next step these defect structures were used to propose extended cluster models, which are able to reproduce in an adequate way the direct environment of the active metal site. An alkylperoxo species V+VO(OOtBu) was identified as the most abundant and therefore the most active epoxidation site. The structure of the most active site was a starting basis for the construction of extended cluster models including substituents. They were used for quantifying the effect of functionalization of the linkers on the catalytic performance of the heterogeneous catalyst MIL‐47‐X. Electron‐withdrawing as well as electron‐donating groups have been considered. The epoxidation activity of the functionalized models has been compared with the measured experimental conversion of cyclohexene. The agreement is fairly good. This combined experimental–theoretical study makes it possible to elucidate the structure of the most active site and to quantify the electronic modulating effects of linker substituents on the catalytic activity.

Computational exploration of a Zr-carboxylate based metal–organic framework as a membrane material for CO2capture

A porous Zr-carboxylate based MOF functionalized with two free carboxylic groups on the terephthalate linkers was computationally explored for its membrane-based CO2 capture performances. This material in a pure or in a composite membrane was predicted to outperform Robeson's upper bound for two strategic gas mixtures (CO2/CH4 and CO2/N2).

Porous coordination polymers based on functionalized Schiff base linkers: enhanced CO2uptake by pore surface modification

We report the synthesis, structural characterization and adsorption properties of three new porous coordination polymers {[Cu(Meazpy)0.5(glut)](H2O)}n (2), {[Zn(azpy)0.5(terep)](H2O)}n (3), and {[Zn(Meazpy)0.5(terep)]}n (4) [glut = glutarate, terep = terephthalate, azpy = N,N'-bis-(pyridin-4-ylmethylene)hydrazine and Meazpy = N,N'-bis-(1-pyridin-4-ylethylidene)hydrazine] composed of mixed linkers systems. Structure determination reveals that all three compounds have three-dimensional (3D) coordination frameworks bridged by dicarboxylates and Schiff base linkers. In all cases 2D dicarboxylate layers are supported by paddle-wheel M2(CO2)4 SBUs extended in three dimensions by designed Schiff base linkers. Compound 1, which has been reported in a paper earlier by our group, is a robust porous three-dimensional (3D) framework whose pore surface was found to be decorated with the -CH=N- groups of a linear Schiff base (azpy) and it showed reversible single-crystal-to-single-crystal transformation and selective CO2 uptake. By using another linear Schiff base linker Meazpy, we have synthesized compound 2 which is isostructural with 1, having an additional methyl group pointing towards the pore. Like 1 it also shows a reversible single-crystal-to-single-crystal transformation upon dehydration and rehydration. The dehydrated framework of 2 exhibits 50% enhanced CO2 uptake compared to 1. This has been achieved by the pore surface modification effected upon changing the pillar backbone from a -CH=N- to -CMe=N- group. It also adsorbs water vapour at 298 K. In the case of the two isostructural 3D MOFs 3and 4, the use of a rigid carboxylate (terephthalate) linker arrested porosity by three-fold interpenetration. We showed that the use of aliphatic dicarboxylate (glutarate) results in a non-interpenetrated framework rather than the common interpenetrated framework with aromatic dicarboxylates in mixed ligand systems.

Enhancing CO2 Separation Ability of a Metal–Organic Framework by Post‐Synthetic Ligand Exchange with Flexible Aliphatic Carboxylates

A series of porous metal-organic frameworks having flexible carboxylic acid pendants in their pores (UiO-66-ADn: n=4, 6, 8, and 10, where n denotes the number of carbons in a pendant) has been synthesized by post-synthetic ligand exchange of terephthalate in UiO-66 with a series of alkanedioic acids (HO2 C(CH2 )n-2 CO2 H). NMR, IR, PXRD, TEM, and mass spectral data have suggested that a terephthalate linker in UiO-66 was substituted by two alkanedioate moieties, resulting in free carboxyl pendants in the pores. When post-synthetically modified UiO-66 was partially digested by adjusting the amount of added HF/sample, NMR spectra indicated that the ratio of alkanedioic acid/terephthalic acid was increased with smaller amounts of acid, implying that the ligand substitution proceeded from the outer layer of the particles. Gas sorption studies indicated that the surface areas and the pore volumes of all UiO-66-ADns were decreased compared to those of UiO-66, and that the CO2 adsorption capacities of UiO-66-ADn (n=4, 8) were similar to that of UiO-66. In the case of UiO-66-AD6, the CO2 uptake capacity was 34 % higher at 298 K and 58 % higher at 323 K compared to those of UiO-66. It was elucidated by thermodynamic calculations that the introduction of flexible carboxyl pendants of appropriate length has two effects: 1) it increases the interaction enthalpy between the host framework and CO2 molecules, and 2) it mitigates the entropy loss upon CO2 adsorption due to the formation of multiple configurations for the interactions between carboxyl groups and CO2 molecules. The ideal adsorption solution theory (IAST) selectivity for CO2 adsorption over that of CH4 was enhanced for all of the UiO-66-ADns compared to that of UiO-66 at 298 K. In particular, UiO-66-AD6 showed the most strongly enhanced CO2 uptake capacity and significantly increased selectivity for CO2 adsorption over that of CH4 at ambient temperature, suggesting that it is a promising material for sequestering CO2 from landfill gas.

Aluminum-1,4-cyclohexanedicarboxylates: High-Throughput and Temperature-Dependent in Situ EDXRD Studies

The system AlCl3·6H2O/cis-H2CDC/trans-H2CDC/solvent was systematically investigated with high-throughput methods to study the influence of the two 1,4-cyclohexanedicarboxylate isomers (cis- and trans-H2CDC) as flexible aliphatic linker molecules on the formation of new crystalline compounds. Using the cis-isomer, the layered inorganic-organic hybrid compound [Al(OH)(cis-CDC)] (1) is formed. The use of trans-H2CDC leads to the microporous MOF [Al(OH)(trans-CDC)]·H2O (2) denoted CAU-13. Its framework is related to the well-known MIL-53, which was previously described for trivalent cations and rigid terephthalate linker molecules. The crystal structures of 1 and 2 were derived from powder X-ray diffraction data. Temperature-dependent in situ energy dispersive X-ray diffraction (EDXRD) experiments for the synthesis of 2 were carried out at HASYLAB, DESY, Hamburg. The kinetic analysis, applying the Gualtieri model to the experimental data, revealed Arrhenius activation energies of 76 kJ/mol for both the nucleation and the growth process. These values do not differ much from the activation energies reported for MOFs with aromatic rigid linker molecules.

Synthesis of a New Interpenetrated Mixed Ligand Ni(II) Metal–Organic Framework: Structural, Thermal and Fluorescence Studies and its Thermal Decomposition to NiO Nanoparticles

Reaction of terephthalic acid and pyrazine with nickel nitrate under hydrothermal conditions forms a 3D metal–organic framework with the composition of [Ni(μ3 − tp)(μ2 − pyz)]n (1), where H2tp = 1,4-benzenedicarboxylic acid (terephthalic acid) and pyz = pyrazine. X-ray single crystal analysis reveals that Ni(II) centers adopt a distorted octahedral geometry which are surrounded by four oxygen atoms from three tp groups and two nitrogen atoms of pyrazine molecules in equatorial and axial positions, respectively. Coordination of Ni(II) centers by terephthalate linkers in ac plane forms infinite (4, 4)-connected 2D layers, which are further joined together by pyrazine groups to form an interlocked 3D structure. Topological studies show a two-fold interpenetrated uninodal 3D network with point symbol of (412·63). The polymer shows a blue emission band at 467 nm in the solid state at room temperature. The X-ray power diffractions (PXRD) and thermal gravimetric analysis of 1 are also presented and discussed. NiO nanoparticles have been prepared via direct calcination of 1 at 600 °C, with sizes of about 35–45 nm. Nanoparticles were further characterized by IR, scanning electron microscopy equipped with energy dispersive X-ray analyses and PXRD technique.

Water Sorption Cycle Measurements on Functionalized MIL-101Cr for Heat Transformation Application

The water loading capacity and water cycle stability (40 adsorption/desorption cycles) of four nitro- or amino-functionalized MIL-101Cr materials (1–4) is assessed for heat transformation applications. Amino- or nitro-functionalized (1, 3) and partially amino- or nitro-functionalized MIL-101Cr (2, 4) have been synthesized through time-controlled postsynthetic modification of MIL-101Cr. The partially functionalized materials (2, 4) contain about 78 mol % amino- or nitro-functionalized terephthalate linker. Hydrophilic nitro or amino functionalities were introduced into MIL-101Cr in order to achieve water loading at lower p/p0 values for possible use in thermally driven adsorption chillers or heat pumps. Among the four materials studied, fully aminated MIL-101Cr-NH2, 1, and partially aminated MIL-101Cr-pNH2, 2, showed the best water loadings (about 1.0 gH2O/gMIL) as well as water stability over 40 adsorption–desorption cycles. After 40 cycles, the X-ray powder diffractogram and Brunauer–Emmett–Teller (BET) surface determination of amino-functionalized materials indicated structural integrity with ΔBET = −6.3% after 40 cycles, while the nitro-functionalized MIL-101Cr exhibited a decrease in their BET surface of ΔBET = −25% and −20% for 3 and 4, respectively.

Host–guest and guest–guest interactions between xylene isomers confined in the MIL-47(V) pore system

The porous MIL-47 material shows a selective adsorption behavior for para-, ortho-, and meta-isomers of xylenes, making the material a serious candidate for separation applications. The origin of the selectivity lies in the differences in interactions (energetic) and confining (entropic). This paper investigates the xylene–framework interactions and the xylene–xylene interactions with quantum mechanical calculations, using a dispersion-corrected density functional and periodic boundary conditions to describe the crystal. First, the strength and geometrical characteristics of the optimal xylene–xylene interactions are quantified by studying the pure and mixed pairs in gas phase. An extended set of initial structures is created and optimized to sample as many relative orientations and distances as possible. Next, the pairs are brought in the pores of MIL-47. The interaction with the terephthalic linkers and other xylenes increases the stacking energy in gas phase (−31.7 kJ/mol per pair) by roughly a factor four in the fully loaded state (−58.3 kJ/mol per xylene). Our decomposition of the adsorption energy shows various trends in the contributing xylene–xylene interactions. The absence of a significant difference in energetics between the isomers indicates that entropic effects must be mainly responsible for the separation behavior.

The coordinatively saturated vanadium MIL-47 as a low leaching heterogeneous catalyst in the oxidation of cyclohexene

A Metal Organic Framework, containing coordinatively saturated V+IV sites linked together by terephthalic linkers (V-MIL-47), is evaluated as a catalyst in the epoxidation of cyclohexene. Different solvents and conditions are tested and compared. If the oxidant TBHP is dissolved in water, a significant leaching of V-species into the solution is observed, and also radical pathways are prominently operative leading to the formation of an adduct between the peroxide and cyclohexene. If, however, the oxidant is dissolved in decane, leaching is negligible and the structural integrity of the V-MIL-47 is maintained during successive runs. The selectivity toward the epoxide is very high in these circumstances. Extensive computational modeling is performed to show that several reaction cycles are possible. EPR and NMR measurements confirm that at least two parallel catalytic cycles are co-existing: one with V+IV sites and one with pre-oxidized V+V sites, and this is in complete agreement with the theoretical predictions.

Structural Chemistry, Monoclinic-to-Orthorhombic Phase Transition, and CO2 Adsorption Behavior of the Small Pore Scandium Terephthalate, Sc2(O2CC6H4CO2)3, and Its Nitro- And Amino-Functionalized Derivatives

The crystal structure of the small pore scandium terephthalate Sc(2)(O(2)CC(6)H(4)CO(2))(3) (hereafter Sc(2)BDC(3), BDC = 1,4-benzenedicarboxylate) has been investigated as a function of temperature and of functionalization, and its performance as an adsorbent for CO(2) has been examined. The structure of Sc(2)BDC(3) has been followed in vacuo over the temperature range 140 to 523 K by high resolution synchrotron X-ray powder diffraction, revealing a phase change at 225 K from monoclinic C2/c (low temperature) to Fddd (high temperature). The orthorhombic form shows negative thermal expansivity of 2.4 × 10(-5) K(-1): Rietveld analysis shows that this results largely from a decrease in the c axis, which is caused by carboxylate group rotation. (2)H wide-line and MAS NMR of deuterated Sc(2)BDC(3) indicates reorientation of phenyl groups via π flips at temperatures above 298 K. The same framework solid has also been prepared using monofunctionalized terephthalate linkers containing -NH(2) and -NO(2) groups. The structure of Sc(2)(NH(2)-BDC)(3) has been determined by Rietveld analysis of synchrotron powder diffraction at 100 and 298 K and found to be orthorhombic at both temperatures, whereas the structure of Sc(2)(NO(2)-BDC)(3) has been determined by single crystal diffraction at 298 K and Rietveld analysis of synchrotron powder diffraction at 100, 298, 373, and 473 K and is found to be monoclinic at all temperatures. Partial ordering of functional groups is observed in each structure. CO(2) adsorption at 196 and 273 K indicates that whereas Sc(2)BDC(3) has the largest capacity, Sc(2)(NH(2)-BDC)(3) shows the highest uptake at low partial pressure because of strong -NH(2)···CO(2) interactions. Remarkably, Sc(2)(NO(2)-BDC)(3) adsorbs 2.6 mmol CO(2) g(-1) at 196 K (P/P(0) = 0.5), suggesting that the -NO(2) groups are able to rotate to allow CO(2) molecules to diffuse along the narrow channels.

Complete Series of Monohalogenated Isoreticular Metal–Organic Frameworks: Synthesis and the Importance of Activation Method

A series of four IRMOFs with -F, -Cl, -Br, and -I terephthalate linkers was synthesized and characterized with respect to crystal structure, surface area, and nitrogen gas sorption. The activation of these materials was systematically evaluated, using several solvent-exchange methods and supercritical CO2 drying. The results demonstrate a strong dependence of surface area on the activation method used. Materials with surface areas comparable to predicted values were achieved, considerably improving upon previously reported values for IRMOF-2 and its -I variant. Finally, ambient-temperature nitrogen adsorption isotherms were measured. These data clearly demonstrate that the adsorption of a weakly interacting gas can be improved by increasing linker polarizability, confirming previous speculation in the literature.

Sulfation of metal–organic frameworks: Opportunities for acid catalysis and proton conductivity

A new post-functionalization method for metal–organic frameworks (MOFs) has been developed to introduce acidity for catalysis. Upon treatment with a mixture of triflic anhydride and sulfuric acid, chemically stable MOF structures MIL-101(Cr) and MIL-53(Al) can be sulfated, resulting in a Brønsted sulfoxy acid group attached to up to 50% of the aromatic terephthalate linkers of the structure. The sulfated samples have been extensively characterized by solid-state NMR, XANES, and FTIR spectroscopy. The functionalized acidic frameworks show catalytic activity similar to that of acidic polymers like Nafion® display in the esterification of n-butanol with acetic acid ( TOF ∼ 1 min −1 @ 343 K). Water adsorbs strongly up to 4 molecules per sulfoxy acid group, and an additional 2 molecules are taken up at lower temperatures in the 1-D pore channels of S-MIL-53(Al). The high water content and Brønsted acidity provide the structure S-MIL-53(Al) a high proton conductivity up to moderate temperatures.

Direct covalent post-synthetic chemical modification of Cr-MIL-101 using nitrating acid

For the first time, functionality has been covalently introduced into the Cr-MIL-101 network by post-synthetic modification of the terephthalate linker molecule through nitration. The nitro group was reduced and the amino group was reacted with ethyl isocyanate to yield the corresponding urea derivative.

Single Crystal X-ray Diffraction Studies of Carbon Dioxide and Fuel-Related Gases Adsorbed on the Small Pore Scandium Terephthalate Metal Organic Framework, Sc2(O2CC6H4CO2)3

The adsorption behavior of the microporous scandium terephthalate Sc2(O2CC6H4CO2)3 for small fuel-related molecules has been measured. The structure shows an adsorption capacity for N2 and CO2 of 6.5 mmol g(-1) and is able to take up straight chain hydrocarbons. The mechanism of adsorption of CO2, CH4, and C2H6 has been determined by single crystal synchrotron X-ray diffraction at approximately 230 K. Adsorption of CO2 at 235 K and 1 bar pressure and H2 at 80 K and 0.25 bar results in each case in a symmetry change from orthorhombic Fddd to monoclinic C2/c through tilts in the terephthalate linkers. CO2 molecules take up different sites in the two symmetrically different channels that result from this symmetry change. The structure remains orthorhombic in 9 bar of CH4 and 5 bar of C2H6, and the adsorption sites are located. CH4 and C2H6 are observed to adopt sites within the channels, and C2H6 is also observed to adopt adsorption sites between phenyl groups in the channel walls, suggesting that the structure is sufficiently flexible to allow diffusion of small molecules between adjacent channels.