Framework disorder and its effect on selective hysteretic sorption of a T-shaped azole-based metal-organic framework.

Metal-organic frameworks with highly ordered porosity have been studied extensively. In this paper, the effect of framework (pore) disorder on the gas sorption of azole-based isoreticular Cu(II) MOFs with rtl topology and characteristic 1D tubular pore channels is investigated for the first time. In contrast to other isoreticular rtl metal-organic frameworks, the Cu(II) metal-organic framework based on 5-(1H-imidazol-1-yl)isophthalate acid has a crystallographically identifiable disordered framework without open N-donor sites. The framework provides a unique example for investigating the effect of pore disorder on gas sorption that can be systematically evaluated. It exhibits remarkable temperature-dependent hysteretic CO2 sorption up to room temperature, and shows selectivity of CO2 over H2, CH4 and N2 at ambient temperature. The unique property of the framework is its disordered structure featuring distorted 1D tubular channels and DMF-guest-remediated defects. The results imply that structural disorder (defects) may play an important role in the modification of the performance of the material.

Introduction

Porous materials such as zeolites, molecular cages, activated n class="Chemical">carbons, covalent organic frameworks and metal–organic frameworks (MOFs) have been utilized as adsorbents for gas capture (e.g. CO2). Among these materials, MOFs with high surface area, tunable pore size and modifiable pore surfaces are receiving great interest because they show unique flexibility and dynamic behaviors (e.g. phase change) and also offer significant improvements in separation performance (Horike et al., 2009 ▸; Schneemann et al., 2014 ▸; Alhamami et al., 2014 ▸; Zhang et al., 2017 ▸). For example, their corresponding hysteretic gas adsorption/desorption behaviors are able to decrease the pressure of gas storage.

According to the isoreticular MOF concept introduced by Yaghi and others (Eddaoudi et al., 2002 ▸; Wilmer et al., 2012 ▸; Dn class="Gene">eng et al., 2012 ▸; Ma et al., 2010 ▸), a MOF structure is determined by the connectivity of the rigid bridging ligand, the secondary building unit (SBU) and the framework topology. By fixing a specific topology, the isostructural frameworks can be readily fine-tuned via organic ligand manipulation and metal ion selection (Yuan et al., 2010 ▸; Caskey et al., 2008 ▸). Analogous ligands with different functionalities can be especially employed to produce desired frameworks (Colombo et al., 2012 ▸; Sumida et al., 2013 ▸). Along these lines, the effect of some subtle factors on the pore properties can be investigated and analyzed by designing isoreticular MOFs (McDonald et al., 2012 ▸; Li, Zhang et al., 2012 ▸; Das et al., 2012 ▸; Zhang et al., 2012 ▸; Bae et al., 2012 ▸). However, much of the research emphasis remains on developing and characterizing highly ordered framework materials with regularly repeating crystal structures where all the pores have exactly the same size, shape and functionality. This ignores disorder and/or defects which are often concomitant with the growth of a long-range periodic framework. The introduction of defects may alter the regular porous interior and behavior of MOFs. Defects may play a crucial role in enhanced accessibility of the porous network and higher basicity of the metal centers. Defects in regular MOFs have been shown to impart unusual and useful physical properties to these framework materials (Cairns & Goodwin, 2013 ▸; Tucker et al., 2005 ▸; Goodwin et al., 2009 ▸; Cheetham et al., 2016 ▸; Fang et al., 2015 ▸; Tahier & Oliver, 2017 ▸; Cliffe et al., 2014 ▸; Li et al., 2013 ▸). Fundamental correlations between defects and properties of the resulting defective materials have been shown in some examples. Partial interpenetration of MOF NOTT-202a was observed by Schröder and co-workers to provide defect sites for gas recognition and storage, and to show selective hysteretic sorption of gas molecules (Yang et al., 2012 ▸, 2013 ▸). The mixed linker approach was reported by Zhou and co-workers to introduce functionalized disordered mesopores into MOFs (Park et al., 2012 ▸; Yuan et al., 2016 ▸). Missing-linker defects were shown to be extensively present [e.g. in the UiO-66 series (Trickett et al., 2015 ▸)], increasing the gas adsorption capacities (Rodríguez-Albelo et al., 2017 ▸) and enhancing the proton mobility (Taylor, Dekura et al., 2015 ▸; Taylor, Komatsu et al., 2015 ▸). Herein, we report a unique example to demonstrate how structural disorder can lead to unusual changes in the properties of MOFs (e.g. the sorption property) based on a series of isoreticular azole-based MOFs with rtl (rutile) topology (denoted rtl-MOFs).

Isoreticular rtl-MOFs are based on the square paddle-wheel SBU, one of the most common SBUs formed by metals and carboxyl­ates (Tranchemontagne et al., 2009 ▸). Such square SBUs have long been used to build various porous structures, for example, pillared layer structures with mixed ligands of N-containing heterocycles (e.g. pyridine, imidazole, pyrazole, tetrazole, 1,2,4-triazole) and carboxyl­ate compounds (Seki & Mori, 2002 ▸; Dybtsev et al., 2004 ▸; Pichon et al., 2007 ▸; Chen, Fronczek, Courtney et al., 2006 ▸; Chen, Ma, Zapata et al., 2006 ▸, 2007 ▸). We and others have implemented a new pillaring strategy (Suh et al., 2012 ▸) by amalgamation of one heterocycle and two carboxylate groups into a T-shaped heterofunctional ligand, resulting in the pillaring of 2D-edge transitive nets by 3-connected nodes (Xiang et al., 2011 ▸; Eubank et al., 2011 ▸; Wen et al., 2012 ▸; Zhang et al., 2010 ▸; Chen et al., 2011 ▸, 2015 ▸; Jia et al., 2011 ▸; Du et al., 2013 ▸; Kobalz et al., 2016 ▸; Cheng et al., 2017 ▸; Wei et al., 2014 ▸). Therefore, based on the square-grid sql layer (Zou et al., 2007 ▸; Zhong et al., 2011 ▸; Xue et al., 2007 ▸; Bourne et al., 2001 ▸; Gao et al., 2003 ▸) formed from the isophthalate unit, a series of isoreticular MOFs have been developed (Xiang et al., 2011 ▸; Eubank et al., 2011 ▸; Wen et al., 2012 ▸; Zhang et al., 2010 ▸; Chen et al., 2011 ▸, 2015 ▸; Jia et al., 2011 ▸; Du et al., 2013 ▸; Kobalz et al., 2016 ▸; Cheng et al., 2017 ▸). These microporous MOFs feature in (3,6)-connected 3D frameworks displaying rtl topology and point (Schläfli) symbol (4·62)2(42·610·83). The T-shaped ligand serves as a 3-connected node while the paddle-wheel cluster acts as a 6-connected node. A variety of heterocycles [pyridine (Xiang et al., 2011 ▸; Eubank et al., 2011 ▸; Chen et al., 2015 ▸), pyrimidine (Du et al., 2013 ▸), 1,2,4-triazole (Eubank et al., 2011 ▸; Wen et al., 2012 ▸; Kobalz et al., 2016 ▸) and tetrazole (Zhang et al., 2010 ▸)] have been incorporated into the T-shaped ligands and various metal ions [Cu (Xiang et al., 2011 ▸; Eubank et al., 2011 ▸; Wen et al., 2012 ▸; Zhang et al., 2010 ▸; Du et al., 2013 ▸; Kobalz et al., 2016 ▸; Chen et al., 2011 ▸), Zn (Chen et al., 2011 ▸) and Co (Jia et al., 2011 ▸)] have been utilized to form square paddle-wheel SBUs. One unique characteristic of the rtl topology is that such frameworks are forbidden from interpenetration, so that 1D tubular channels can be formed and isolated by the walls of the parallel pillars containing the aromatic rings. On one hand, the pore aperture is largely defined by the sql grid size or the distance between two carboxylate groups of the isophthalate unit. Thus, the pore sizes are similar for these isoreticular MOFs. On the other hand, the pore surface can be modified by changing or functionalizing the heterocyclic rings [e.g. with uncoordinated N atoms (Eubank et al., 2011 ▸) or alkyl groups (Kobalz et al., 2016 ▸; Cheng et al., 2017 ▸)], which significantly enhance the gas selectivity. In this paper, we add a unique member with framework (pore) disorder into an isoreticular rtl-MOF by incorporating an imidazole ring into the T-shaped ligand (Fig. 1 ▸). The remarkable effect of framework (pore) disorder on the sorption property, which induces a significant hysteretic sorption for CO2 at room temperature, is investigated.

Figure 1

T-shaped bridging ligands containing one azole five-membered-ring heterocycle (imidazole, 1,2,4-triazole and tetrazole) and two carboxylate groups, along with the X-ray crystal structure comparisons of the azole-based rtl-MOFs (Timi-Cu, Ttriaz-Cu and Ttetraz-Cu) showing variation of N sites along the Cu8 L 4 squares on the inner pore surface.

Results and discussion

Syntheses and crystal structures

The T-shaped ligand 5-(1H-imidazol-1-yl)isophthalate acid (denoted as n class="Chemical">Timi) used to introduce hetereocyclic imidazole was synthesized by acid-catalyzed ester hydrolysis of dimethyl 5-(1H-imidazol-1-yl)isophthalate, which was obtained via cyclization of formaldehyde with the diazabutadiene intermediate formed from a reaction of molar equivalents of di­methyl 5-amino­isophthalate, ammonium chloride and glyoxal. A mild solvothermal reaction of Timi with CuCl2·2H2O at 353 K in a mixture of DMF/EtOH (v:v 3:1) led to the new rtl-MOF member, herein denoted as Timi-Cu.

The structure of Timi-Cu was determined and checked by single-crystal X-ray diffraction at 150, 195 and 273 K with several randomly selected crystals, all displaying n class="Disease">disordered character. In general, Timi-Cu crystallizes in the monoclinic space group P21/c, which is isostructural to the previously reported Ttriaz-Cu (Eubank et al., 2011 ▸) and Ttetraz-Cu (Zhang et al., 2010 ▸) (Fig. 1 ▸, Table S1 of the supporting information). In contrast, the reaction of Timi with CuCl2·2H2O at 353 K in acidified DMF yields a different topological structure (Zhu et al., 2015 ▸). The reaction of Timi with CuBr2 at 353 K in acidified DMF–H2O yields the same topological structure, but no framework disorder is located (this structure is not stable during a sorption study) (Cheng et al., 2017 ▸). Therefore, regardless of the disorder in Timi-Cu, these azole-based rtl-MOFs have the same overall unit cell, building block geometry and lattice porosity. As shown in Fig. 2 ▸, the basic structural units are the dicopper paddle-wheel SBUs bonded together by four T-shaped ligands via carboxylate groups. The axial sites are occupied by the N donors of imdazole. Each Cu2 SBU joins six ligands and each ligand bridges three different Cu2 SUBs, thus generating the (3,6)-connected 3D framework of rtl topology. The tubular channels run along the a axis and have an opening of 11.9 × 14.5 Å (b × c, b ⊥ c) along the diagonals of the quadrangle cross section. The solvated DMF and H2O molecules are disordered and reside in the channels. Based on calculations using the program PLATON, the total potential solvent-accessible void volume is about 885.1 Å3 per unit cell with a pore volume ratio of 50.3%.

Figure 2

X-ray crystal structures of Timi-Cu: (a) Cu2 paddle-wheel SBU, (b) Cu8 L 4 square in sql layer formed via the isophthalate moiety in a 1,2-alternate fashion (up–up down–down), (c) side view of the 1D channel formed via pillared Cu8 L 4 square-grid sql layers, (d) solvent-accessible voids in the tubular channels perpendicular to the bc plane. Framework disorder, solvated molecules and hydrogen atoms have been omitted for clarity.

The detailed structural analysis of Timi-Cu reveals that the coordination framework is actually disordered in a specific fashion (Figs. 3 ▸ and S1). In a statistical sense, each Cu2 paddle-wheel SBU is distributed over two fractional positions in an approximate 3:1 ratio, and some local areas can be imagined to superimpose two partial rtl-MOFs in an offset way. In reality, each Cu2 SBU has a definite orientation in the individual asymmetric unit, and is closely related to the neighboring SBUs owing to the rigidity of the isophthalate moiety and consequently fixed Cu8 L 4 square-grid conformation. This is due to the fact that, in the rtl topology, sql sheets are formed via the isophthalate moiety in a predetermined 1,2-alternating fashion (up–up down–down). Thus, local primary 2D Cu8 L 4 square-grid layers are inherently inert to dynamic disorder. In the contrast, local interlayer gliding is relatively easy (Fig. 4 ▸) because: (i) the crystal structure is stacked with the parallel 2D sql square grids via pillars of the T-shaped Timi ligands, (ii) the Cu—N binding between imidazole N donors and axial positions of Cu2 SBUs is relatively labile, and (iii) the imidazole ring is freely rotatable along the N—C bond to the isophthalate moiety so as to adapt to the layer motions. Therefore, the disorder in Timi-Cu may be considered to originate from local random interpolated movement of the 2D Cu8 L 4 square-grid layers. This might also account for the observation of two isomerized Ttriaz-Cu structures (Eubank et al., 2011 ▸; Wen et al., 2012 ▸) which differ only in the arrangement of the 2D Cu8 L 4 square-grid layers in one direction. The crystal packing in the other two directions and the framework porosity are similar. However, as for Tteraz-Cu, noticeable disorder was not detected in Ttriaz-Cu, meaning that the chemical nature of heterocycles in this azole-based isoreticular rtl-MOF has a subtle influence on crystallization habit and framework isomerization (Makal et al., 2011 ▸; Lü et al., 2006 ▸). In the reaction of Timi with CuBr2 at 353 K in acidified DMF–H2O, the product has no noticeable disorder as well, showing that guest solvent molecules or ions may have a subtle effect on the disorder.

Figure 3

Comparison of the ideally ordered framework and one possible local disordered framework of Timi-Cu in the a (upper) and b (lower) directions: (a) ordered overlapping of the Cu8 L 4 square-grid sql layers, (b) one possible disordered offsetting model of the Cu8 L 4 square-grid sql layers, (c) top view of 1D straight channel in the ordered framework formed via overlapping of Cu8 L 4 square grids and (d) top view of the 1D distorted channel in the disordered framework formed via offsetting of the Cu8 L 4 square grid.

Figure 4

Disordered framework versus ordered framework for Timi-Cu.

To further probe the structure of Timi-Cu, X-ray photoelectron spectroscopy (XPS, Fig. S2) and X-ray absorption fine-structure (n class="Chemical">XAFS, Fig. S3 and Table S2) measurements were investigated. XPS confirms the presence of copper(II) in Timi-Cu. Cu 2p core-level photoelectron spectra for imi-Cu displayed doublets i.e. Cu 2p 3/2 and Cu 2p 1/2 at 935.8 and 955.0 eV, respectively. The Cu 2p 3/2 and Cu 2p 1/2 main doublets were separated by 20 eV and their satellite peaks present at binding energies of 943.0 and 964.0 eV are characteristic of the unfilled orbitals. The study of the Cu LMM signal also supports the presence of Cu(II), and the Cu LMM signal with a kinetic energy of 916.0 eV is assigned to Cu(II). Although being the most common technique for structure determination/identification and measurement of long-range order, powder X-ray diffraction (PXRD) generally provides little information on defects. XAFS was expected to provide some local order information. XAFS data show that Cu(II) is present and the average Cu—O/N bonds are calculated to be 1.97 ± 0.01 Å. However, the data of Timi-Cu are comparable with the data of Cu-Ttriaz and Cu-Ttetraz, which have no disorder. It can be concluded that Timi-Cu and the other two MOF materials have similar short-range coordination spheres.

The above results show that a unique structural feature of Timi-Cu is that the disorder does not cause absence of crystallinity. In other words, the disorder ocn class="Chemical">curs in a crystallographically ‘regular’ way to a certain extent. Various comparisons of the disordered structures of Timi-Cu with those of notionally ordered counterparts are depicted in Figs. 3 ▸, 4 and S1. It is speculated that partial Cu8 L 4 square-grid layers shift exactly half a unit cell along the c direction, leading to offset stacking of these 2D sql sheets along the a direction. In this way, all Cu2 SBUs need not change conformation except for a 180° rotation of the imdazole rings to adapt to the new axial coordination (Fig. 4 ▸). Since orientation of the Cu2 SBUs in every layer is fixed, gliding of the 2D sheet along any other direction will cause a severe geometry and connectivity mismatch. This is evident from observations of the nearly identical packing modes of the ordered and disordered frameworks in both the b and c directions (Fig. S1). Such a disorder phenomenon is distinct from the topological disorder in well known amorphous silica glass (α-SiO2) with a continuous random network (Cairns & Goodwin, 2013 ▸; Tucker et al., 2005 ▸). It is also distinct from the 2D layered square grids Ni(CN)2, which lack long-range order in the perpendicular direction (Cairns & Goodwin, 2013 ▸; Goodwin et al., 2009 ▸), although in a short-range or local environment they might be comparable. It is worth noting that some framework defects like coordination mismatch or connectivity distortion should also be present.

Therefore, the disordered n class="Chemical">Timi-Cu is a new member of the azole-based isoreticular rtl-MOFs containing different five-membered-ring heterocycles (imidazole, 1,2,4-triazole and tetrazole). A study of Timi-Cu may offer the following advantages: (i) Timi-Cu has a comparable pore volume regardless of chemical or structural variation. For the azole-based rtl-MOFs, the heterocycles protrude into the channels and slightly reduce the pore sizes in comparison with the pyridyl rtl-MOF (Xiang et al., 2011 ▸). However, the similar Cu8 L 4 square grids and pillaring nature of three five-membered azole rings principally decide the total comparable pore volume. (ii) The inner pore surfaces are modified by different heterocyclic azole rings. Two C—H moieties point into the channel in Timi-Cu, one C—H and one N donor in Ttriaz-Cu, while there are two N donors in Ttetraz-Cu (Fig. 1 ▸). (iii) Framework disorder in Timi-Cu produces different pore permeability from that in Ttriaz-Cu and Ttetraz-Cu. As seen from Figs. 3 ▸, 4 and S1, partial gliding of sql sheets does not cause significant pore changes along the b and c axes but does along the a axis. The main pore channels along the a axis in Timi-Cu become distorted in contrast to the straight channels with long-range order in Ttriaz-Cu and Ttetraz-Cu. (iv) The propensity for disorder and framework defects in Timi-Cu probably results in sorption dynamics compared with the more static frameworks in Ttriaz-Cu and Ttetraz-Cu.

Thermal stability

As-synthesized bulk samples of Timi-Cu display sharp PXRD patterns that closely resemble those simulated from the single-crystal data (Fig. S4), indicative of phase purity and air stability. Thermogravimetric analysis (TGA) of n class="Chemical">Timi-Cu reveals similar thermal stability to that reported for Ttetraz-Cu (Wen et al., 2012 ▸; Zhang et al., 2010 ▸). The TGA plot of as-synthesized Timi-Cu shows a weight loss of 26.2% from room temperature to ca. 513 K, corresponding to the release of solvent molecules (1.5 DMF and 0.5 H2O per formula unit; calculated weight loss 28.8%) residing in the pore channels (Fig. S5). Rapid decomposition occurs upon further heating above 543 K. The thermostability of the desolvated material is further revealed by PXRD experiments at various temperatures (Fig. S6). PXRD results reveal that the framework structure is unchanged up to 373 K and transforms into a new structure with a different framework topology between 383 and 443 K (the new structure will be reported in due course). This further indicates that the Cu2+-Timi system is rather sensitive to synthetic parameters including solvent, counteranions and temperature.

Permanent porosity

To evaluate the permanent porosity, nitrogen physisorption measurements were performed at 77 K. Prior to analysis, pore activation was performed by evacuating Timi-Cu by thermal activation under vacuum at 358 K following surface cleaning by EtOH. This gives rise to a partially desolvated sample, in which the DMF molecules occluded within the channels were not removed completely (Seo et al., 2010 ▸; Hijikata et al., 2013 ▸; Wang et al., 2013 ▸, 2015 ▸). FT–IR spectra confirm the presence of residual DMF (Fig. S7). An N2 adsorption isotherm of Timi-Cu reveals a steep uptake in the low-pressure region and the profile displays a type-I curve that is typical of microporous materials (Fig. 5 ▸). The Langmuir and BET surface areas are calculated to be 1145 and 771 m2 g−1, respectively, and the total pore volume is 0.31 cm3 g−1 (Table S3, Figs. S8–S10). Surprisingly Timi-Cu shows a double-peak pore size distribution centered around 5.1 and 6.8 Å according to the Horvath–Kawazoe method. The main pore size at 5.1 Å is consistent with other rtl-MOFs (Zhang et al., 2010 ▸), while 6.8 Å is unprecedented (see the discussion below). For comparison, Ttriaz-Cu has Langmuir and BET surface areas of 893 and 768 m2 g−1, while those of Ttetraz-Cu are 1055 and 766 m2 g−1, consistent with the reported data (Zhang et al., 2010 ▸). The total pore volumes are 0.29 and 0.30 cm3 g−1, respectively, and the pore sizes are comparable at around 4.9 Å.

Figure 5

(a) N2 adsorption–desorption isotherms for Timi-Cu measured at 77 K and (b) Horvath–Kawazoe micropore size distribution.

The pore enlargement of Timi-Cu is unexpected and can reasonably be related to the framework disorder. On one hand, the disorder in n class="Chemical">Timi-Cu causes the pore channels to become distorted; on the other hand, appearance of structural disorder in the crystal structure always implies concomitance of local defects due to coordination mismatch or topological distortion caused by interlayer gliding. Hence, enlargement of partial pores in Timi-Cu is understandable. The defects (probably containing uncoordinated metal centers) may be readily occupied by solvated DMF molecules. Moreover, the N2 sorption isotherms show an obvious H2-type hysteresis loop which is usually considered to be a characteristic of mesoporosity (Li et al., 2013 ▸; Fang et al., 2010 ▸; Zhao et al., 2011 ▸; Qiu et al., 2008 ▸), but we believe that this may also be attributed to the structural disorder. The clustering of numerous local defects may result in larger-scale mesoporosity, so the existence of some mesopores in the disordered framework is to be expected.

CO2 capture and framework dynamics

Low-pressure and high-pressure CO2 sorption has been studied. As depicted in Fig. 6 ▸, n class="Chemical">Timi-Cu adsorbs significant amounts of CO2 at various temperatures (195, 263, 273, 283 and 298 K). At 195 K, the isotherms are type I, which is typical for microporous materials. Timi-Cu displays an uptake capacity of 7.3 mmol g−1 (32.0 wt%) for CO2 at 1 bar. A striking feature is that Timi-Cu also shows high CO2 uptake at 273 K. Timi-Cu has CO2 storage capacity of 4.2 mmol g−1 (18.7 wt%) at 273 K, 1 bar. The adsorption isotherm of CO2 up to 30 bar at 298 K indicates that Timi-Cu shows a CO2 uptake capacity of 3.2 mmol g−1 (14.1 wt%) at 30 bar. The high CO2 uptake capacity of Timi-Cu, which has no open metal/N-donor sites on the inner pore surface revealed by the X-ray structure, hints at other contributions, namely framework disorder (see the discussion below).

Figure 6

CO2 adsorption–desorption isotherms for Timi-Cu measured at (a) 195 K (insert shows an enlargement of the low-pressure section); (b) 263, 273, 283 and 298 K; and (c) CO2, N2, H2 and CH4 adsorption–desorption isotherms for Timi-Cu measured at 273 K, and CO2, N2 and CH4 adsorption–desorption isotherms for (d) Timi-Cu, (e) Ttriaz-Cu and (f) Ttetraz-Cu measured under various pressures at 298 K.

Another noteworthy feature is that Timi-Cu shows remarkable hysteretic sorption behavior toward n class="Chemical">CO2, while the isostructural Ttriaz-Cu and Ttetraz-Cu do not show obvious hysteresis loops (Fig. 6 ▸). The stepwise sorption usually indicates filling of different types of pore sites, originating from the gate effect or dynamic nature (Kitaura et al., 2002 ▸; Thallapally et al., 2008 ▸; Chen, Ma, Hurtado et al., 2007 ▸; Nouar et al., 2012 ▸; Suzuki et al., 2016 ▸; Carrington et al., 2017 ▸; Taylor et al., 2016 ▸; Choi & Suh, 2009 ▸; Llewellyn et al., 2006 ▸) and framework defects (Yang et al., 2012 ▸). Careful examination reveals that Timi-Cu exhibits stepwise adsorption under low pressure at 195 K (Fig. 6 ▸ a). Surprisingly, such adsorption/desorption hysteresis becomes more prominent at elevated temperatures up to 273 K (Fig. 6 ▸ b). Note that a higher pressure is needed to initiate the hysteresis as temperature increases. The inducing pressure of P/P 0 = 0.01 at 195 K increases to 0.37 at 263 K and 0.54 at 273 K.

The adsorption isotherms of CO2 up to 30 bar at 298 K further reveal the temperature-gating pressure relationship (Fig. 6 ▸). n class="Chemical">Timi-Cu exhibits a distinct stepwise adsorption isotherm, while the desorption branch does not trace the adsorption branch, forming a remarkable hysteresis loop. At low pressure, only a small amount of CO2 (0.5 mmol g−1) is adsorbed. A sudden rise in the isotherm occurs at an inducing pressure of 125 kPa, which is higher than that at 273 K. This confirms the low-pressure observations that a higher pressure is needed to initiate the hysteresis as temperature increases.

Considering the above temperature-dependent variations of CO2 sorption capacity and hysteresis, and comparing the structural n class="Gene">nature of these isoreticular rtl-MOFs, we believe that the framework disorder in Timi-Cu imparts an essential effect on the CO2 sorption behavior. As discussed above, the structural disorder causes the pore channels to become too distorted for gas molecules to permeate through, and renders some local defects which might be partly maintained by the solvated DMF molecules. If gas molecules have a tendency to interact with the pore surface (protruding heterocycles) and defect sites (such as uncoordinated metal centers), gas uptake may induce framework dynamics. For Timi, the blocking/shielding DMF molecules can facilitate and affect the structural transformation, thus making hysteresis pronounced. At low temperature, i.e. 195 K, the coordination framework and shielding DMF molecules are relatively static, and the hysteretic steps are therefore relatively inexplicit. As the temperature increases, the kinetically hindered pores in the disordered framework become easier to break through, thus displaying a larger hysteresis loop. However, once the temperature rises above 283 K, hysteresis turns indistinctive again (Fig. 6 ▸ b), implying facile and expeditious structural conversions above this temperature. Additionally, the inducing pressure of hysteresis increases with rise in temperature, which may be due to higher thermal vibration of the framework, hindering DMF and adsorbed CO2 molecules at elevated temperature and resulting in weaker adsorbent/adsorbate interactions, thus requiring a larger pressure to push the motion of the framework. This may also account for the observation that the CO2 uptake capacity of Timi-Cu decreases as temperature increases relative to that of Ttriaz-Cu and Ttetraz-Cu. The CH4 isotherm also exhibits a broad hysteresis loop with an inflection point at 298 K and a higher pressure of ca. 940 kPa, which may relate to its larger polarizability (25.93 × 10−25 cm3) (Li et al., 2009 ▸; Sircar, 2006 ▸). This offers a new strategy for hysteretic sorption of CH4 (Taylor et al., 2016 ▸; Mason et al., 2014 ▸).

According to the above discussion, the present n class="Chemical">CO2 uptake process may represent a distinct strategy to drive adsorption/desorption hysteresis that is inherently related to framework (pore) disorder. In contrast to the gate effect and interpenetrating dynamics (Kitaura et al., 2002 ▸; Thallapally et al., 2008 ▸; Chen, Ma, Hurtado et al., 2007 ▸; Nouar et al., 2012 ▸; Suzuki et al., 2016 ▸; Carrington et al., 2017 ▸; Taylor et al., 2016 ▸; Choi & Suh, 2009 ▸; Llewellyn et al., 2006 ▸), the present CO2 (and CH4) hysteresis is temperature dependent, originating from the propensity of structural disorder which can be affected by guest intrusion. The structure dynamics can be induced selectively by CO2 (and CH4 at higher pressure) but not by N2 or H2.

In order to have further insight into the structure dynamics, PXRD investigation was performed. PXRD patterns of Timi-Cu remain unchanged under a high-pressure n class="Chemical">CO2 atmosphere up to 3.0 MPa even if accompanied by pulverizing of the crystal sample (Fig. S11). Moreover, the XRD patterns of Timi-Cu show no change after tablet compression with a tablet press from 0 to 20 MPa (Fig. S12). The results reveal that the long-range order of Timi-Cu is generally maintained under high pressure. The XRD signals become weak above 30 MPa, showing partial loss of long-range order under high pressure. Therefore, either no phase transformation occurs or the structural change is tiny under high pressure.

Two possible factors may contribute to the CO2 hysteresis. (i) Penetration of n class="Chemical">CO2 and CH4 through the disordered pore channels offers a greater driving force than through the straight 1D channels, and interactions of CO2 with the protruding imidazole rings facilitate their rotation along N—C bonds. Simulation of the CO2 adsorption isotherms at 263 K shows that a slight rotation of the imidazole ring of 5.448° has a dramatic effect on the adsorption results (Fig. S13). (ii) DMF guest molecules in the framework perhaps block the pore entrance, which becomes dynamic when the temperature and/or pressure increases. Such framework dynamics may result in partial loss of long-range order, as shown by the broadened PXRD pattern after sorption. Therefore, the framework dynamics are responsive to CO2 uptake depending on feasible conditions created by proper temperature. This is analogous with the partially interpenetrated NOTT-202a which shows temperature-dependent adsorption/desorption hysteresis only below the triple point of CO2 (216.7 K), corresponding completely to the framework defects. In our case, the framework dynamics may be more related to the topological disorder, because the disordered structure model established by single-crystal analysis prefers topological distortion to defect formation. Nevertheless, interaction of CO2 with the defect sites should also contribute to the dynamics of the disordered framework.

The CO2 adsorption capacity around room temperature is essential for potential industrial usage, such as n class="Chemical">CO2 capture and separation in upgrading of natural gas (natural gas clean-up, CO2/CH4), post-combustion (flue gas, CO2/N2) and pre-combustion (shifted synthesis gas stream, CO2/H2) (Sumida et al., 2012 ▸; Li, Sculley & Zhou, 2012 ▸; Nugent et al., 2013 ▸; Bloch et al., 2013 ▸). To investigate the sorption selectivity of Timi-Cu, CO2, N2, H2 and CH4 low-pressure adsorption isotherms were measured at 273 K, and CO2, N2 and CH4 high-pressure adsorption isotherms at 298 K, and compared in Figs. 6 ▸(e) and 6(f). For Timi-Cu, considerably larger amounts of CO2 are adsorbed than N2, H2 and CH4, suggesting that the gas uptake capacity drops remarkably for N2, H2 and CH4 but remains significant for CO2 at elevated temperatures (see Figs. S8–S10 for low-temperature data).

The above results verify that Timi-Cu has a high and general gas sorption selectivity for n class="Chemical">CO2 over H2/CH4/N2; the question is then how such sorption behavior happens. Various effects have been reported in the literature that enforce strong interactions between the host framework and CO2 under ambient conditions, e.g. immobilization of open metal sites or polarized functional groups (Yuan et al., 2010 ▸; Sumida et al., 2012 ▸; Cui et al., 2012 ▸; Gu et al., 2010 ▸; Demessence et al., 2009 ▸; McDonald et al., 2011 ▸; Banerjee et al., 2009 ▸). In particular, aromatic ligands containing uncoordinated N donors (e.g. tetrazole-based ligands) were found to improve the selective adsorption behavior for CO2 (Zhang et al., 2010 ▸; Cui et al., 2012 ▸; Lin et al., 2010 ▸, 2012 ▸; Qin et al., 2012 ▸). In the present azole-based rtl-MOF, no purposely introduced open metal sites exist on the pore surface and Timi-Cu has no polarized functional groups (open N-donor sites). So, the good selectivity of Timi-Cu probably relies on the above-described structural disorder, as well as the tubular pore channels characteristic of rtl-MOFs containing pillaring T-shaped ligands.

First of all, the tubular channels of narrow size in the present rtl-MOF have proved important (Du et al., 2013 ▸; An & Rosi, 2010 ▸). It is argued that, for MOFs with bigger pore sizes (>6 Å), substitution of C—H moieties with n class="Chemical">N donors does not significantly affect the adsorption capacity for H2 and CO2 (Park et al., 2011 ▸). The pore sizes of Timi-Cu justify this assumption well. Second, the quadruple moment of CO2 renders a stronger interaction with the host framework, which contributes excess energy for CO2 to enter the pore channels. Since CO2 is well known to have stronger adsorbent/adsorbate interactions due to strong polarizability (29.11 × 10−25 cm3) and quadruple moment (4.30 × 10−26 esu cm2) (Li et al., 2009 ▸; Sircar, 2006 ▸), selective hysteretic sorption of CO2 over N2, H2 and CH4 is understandable at low pressure. Third, the structural disorder may cause defects in the coordination framework, thus creating open metal sites. Such defects may exist in a small portion of the framework (see above). On the other hand, the framework disorder enables distorted pore channels as well as straight channels. The distorted channels with narrow size are appropriate for holding gas molecules kinetically within the channels, which increases the van der Waals interactions between the host framework and gas molecules (Wen et al., 2012 ▸). Most importantly, as temperature rises, CO2 uptake towards such disordered pores amplifies the framework dynamics, resulting in selective sorption hysteresis. Such adsorption/desorption hysteretic behavior at room temperature is rare (Hijikata et al., 2013 ▸; Wang et al., 2013 ▸, 2015 ▸); it greatly improves the adsorption selectivity for CO2 at ambient temperature, similar to observations of selective CO2 capture by flexible or dynamic MOFs under different conditions (Choi & Suh, 2009 ▸; Llewellyn et al., 2006 ▸; Mohamed et al., 2012 ▸; Burd et al., 2012 ▸; Eguchi et al., 2012 ▸). In addition, the present framework disorder induces hysteretic sorption in a much broader range from 195 K to room temperature. This allows the capture of CO2 at high pressure, but leaves CO2 trapped in the pores at low pressure, thus facilitating the separation of CO2 from H2/CH4/N2 under more industrially applicable conditions.

Conclusions

A unique rtl-MOF (n class="Chemical">Timi-Cu) with framework disorder was prepared by incorporating an imidazole ring into a T-shaped ligand and the gas sorption properties were evaluated. In contrast to other azole-based rtl-MOFs with five-membered-ring heterocycles (triazole, Ttriaz-Cu; tetrazole, Ttetraz-Cu), Timi-Cu does not integrate open N-donor sites while containing only two C—H moieties in its characteristic tubular pores. Remarkably, Timi-Cu displays crystallographically identifiable disorder of the framework. Considering the interesting effects induced by structural disorder on framework materials (Cairns & Goodwin, 2013 ▸; Tucker et al., 2005 ▸; Goodwin et al., 2009 ▸; Cheetham et al., 2016 ▸; Fang et al., 2015 ▸; Tahier & Oliver, 2017 ▸; Cliffe et al., 2014 ▸; Li et al., 2013 ▸; Allan et al., 2012 ▸; Amirjalayer & Schmid, 2008 ▸), Timi-Cu provides a unique example to investigate the effect of framework pore disorder on sorption properties. As a defective derivative, Timi-Cu retains the long-range order and topology of the parent framework of rtl-MOFs, while it exhibits pore disorder and a relatively large percentage of defects in an otherwise highly crystalline material. Single-crystal analyses establish the disordered structural model in relation to porosity, featuring distorted 1D tubular channels and DMF-guest-remediated defects. These factors endow Timi-Cu with good gas sorption capacity. Importantly, temperature-dependent hysteretic CO2 (and CH4) sorption is shown up to 298 K, which dramatically enhances selective adsorption of CO2 (and CH4) at elevated temperatures. Therefore, the present azole-based rtl-MOF shows strong binding with CO2 and high selectivity for CO2 over H2/CH4/N2 at ambient temperature. Furthermore, the results imply the significance of structure disorder (defects) on the modification of the performance of framework materials, providing a viewpoint for expanding the properties of framework materials.

Experimental

Materials and methods

All starting materials and solvents were obtained from commercial sources and used without further purification unless otherwise indicated. Di­methyl-5-(1H-imidazol-1-yl)n class="Chemical">isophthalate was prepared according to the published procedure (Wang et al., 2013 ▸). PXRD data were recorded on a Bruker D8 Advance diffractometer at 40 kV and 40 mA with a Cu-target tube and a graphite monochromator. Infrared spectra were measured on a Nicolet/Nexus-670 FT–IR spectrometer with KBr pellets. Thermogravimetric analysis was performed under N2 at a heating rate of 10 K min−1 on a Netzsch Termo Microbalance TG 209 F3 Tarsus. The sorption isotherms were measured with a Quantachrome Autosorb-iQ or Autosorb-iQ2 analyzer.

Synthesis of 5-(1H-imidazol-1-yl)benzene-1,3-di­carboxylic acid (Timi)

Ester hydrolysis of dimethyl-5-(1H-imidazol-1-yl)isophthalate was performed via an acid-catalyzed ester hydrolysis in HCl solution. Di­methyl-5-(1H-imidazol-1-yl)isophthalate (160 mg, 0.6 mmol) was refluxed for 36 h in 20% HCl (8 ml). The solvent was evaporated to obtain the product (140 mg, >99%). The product was soluble in DMF and MeOH. 1H NMR (300 MHz, DMSO-d 6): δ 9.76 (s, 1H), 8.54 (s, 1H), 8.50 (s, 2H), 8.41 (s, 1H), 7.87 (s, 1H). IR (cm−1, KBr): 3163 (w), 3087 (w), 1711 (m), 1674 (m), 1600 (w), 1539 (w), 1399 (m), 1348 (m), 1232 (s), 1068 (s), 876 (w), 757 (m), 672 (m), 616 (w).

Synthesis of Timi-Cu

A solution of Timi (6.0 mg, 0.025 mmol) in n class="Chemical">DMF (3 ml) and a solution of CuCl2·2H2O (8.5 mg, 0.05 mmol) in EtOH (1 ml) were mixed. The resultant clear solution was heated in a closed vial at 353 K for 3 days. Green crystals were collected using filtration (7 mg, 70%). Microanalysis found (calculated) for C11H6O4N2Cu·1.5DMF·0.5H2O: C, 45.22 (45.15); H 4.28 (4.28); N 11.74 (11.89)%. FT–IR (cm−1, KBr): 3426 (b), 3101 (w), 2929 (w), 1634 (m), 1594 (m), 1503 (w), 1385 (s), 1249 (w), 1070 (m), 922 (w), 782 (w), 730 (m), 656 (w). For thermally activated product under vacuum at 358 K following CH2Cl2 solvent exchange, microanalysis found (calculated) for C11H6O4N2Cu·3H2O: C 37.34 (37.99), H 3.55 (3.48), N 7.82 (8.06)%.

X-ray structure determination

X-ray reflection data were collected at 150 (2) K on an Oxford Gemini S Ultra diffractometer equipped with a graphite-monochromated Enhance (n class="Chemical">Cu) X-ray source (λ = 1.54178 Å). An empirical absorption correction was applied to the intensity data using spherical harmonics implemented in the SCALE3 ABSPACK scaling algorithm (Agilent, 2012 ▸). The structure was solved by direct methods following difference Fourier syntheses and was refined by the full matrix least-squares method against F o 2 using SHELXTL software (Sheldrick, 2015 ▸). The whole framework is disordered over two positions with an occupancy ratio of 0.694:0.306. The unit-cell volume includes a large region of disordered solvent (1.5 DMF and 0.5 H2O molecules). One DMF molecule is disordered over two positions with an occupancy ratio of 0.679:0.321. There are 0.25 water and 0.25 DMF molecules located at the inversion center. Modeled refinements were applied to the disordered parts including the imidazole ring, solvated water and DMF molecules to make them geometrically reasonable, resulting in a total of 1074 restraints.

Crystallographic data for Timi-Cu: C15.5H17CuN3.5O5.75, FW = 407.87, monoclinic, P21/c, a = 10.8431 (6) Å, b = 11.8835 (6) Å, c = 14.4823 (9) Å, α = 90°, β = 109.361 (7)°, γ = 90°, V = 1760.57 (17) Å3, Z = 4, T = 150 (2) K, λ = 1.54178 Å, ρcalc = 1.539 mg m−3, μ = 2.097 mm−1, 4661 reflections were collected (2553 were unique) for 4.93 < θ < 59.98, R(int) = 0.0325, R 1 = 0.0840, wR 2 = 0.2296 [I > 2σ(I)], R 1 = 0.0984, wR 2 = 0.2447 (all data) for 258 parameters, GOF = 1.073, CCDC reference 945810.

Related literature

The following references are cited in the supporting information: Vitillo et al. (2008 ▸); Zhou et al. (2008 ▸).

Crystal structure: contains datablock(s) I. DOI: 10.1107/S2052252518015749/lq5014sup1.cif

Supporting information file. DOI: 10.1107/S2052252518015749/lq5014sup2.pdf

CCDC reference: 945810