Selective CO₂ adsorption in a microporous metal–organic framework with suitable pore sizes and open metal sites

Abstract

Microporous metal–organic frameworks, [H₂N(Me)₂]₂[Zn₄(L)₂(H₂O)_1.5]·5DMF·H₂O (1·DMF) or [H₂N(Me)₂]₂[Zn₄(L)₂(H₂O)_1.5]·4DMA·6H₂O (1·DMA), have been synthesized under solvothermal conditions using an undeveloped asymmetrical pentacarboxylate ligand, 2,4-di(3′,5′-dicarboxylphenyl)benzoic acid (H₅L). Structural analysis demonstrates that 1 is a 3D anionic framework based on two kinds of dinuclear [Zn₂(COO)₅(H₂O)] secondary building units (SBUs), which presents a rare framework constructed from ternary SBUs and shows a new (5,5,5)-connected topology. The desolvated anionic structure of 1·DMF (1a) contains two shapes of 1D channel with suitable pore sizes and a highly polar pore system decorated with open metal sites, diethylammonium cations and carboxylic oxygen atoms, exhibiting multipoint interactions between CO₂ molecules and the framework, resulting in high CO₂ uptake and significant selectivity for CO₂ over CH₄ and N₂. In addition, 1·DMF and 1·DMA display solid-state photoluminescence stemming from the ligand-centered fluorescent emissions of H₅L.

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Introduction

The effective capture and removal of CO₂ from industrial flue gas is becoming an important environmental issue due to escalated global warming.¹ Porous coordination polymers (PCPs) or metal–organic frameworks (MOFs) are rapidly emerging as a new class of porous materials, which show promising application for CO₂ capture and separation, and have attracted intense attention owing to their well-defined chemical structures and facile pore modification.^2,3 To be an optimal CO₂ capture material, a MOF should have high thermal stability, permanent porosity and high adsorption capacity for CO₂. A numbers of MOF-based materials showing selective adsorption of CO₂ over CH₄ and N₂ have been reported in the pastfew years,^4–6 but highly efficient materials are still rare. So far, how to enhance the separation and storage abilities of these materials from a structural modification point of view remains a great challenge. In an effort to develop porous MOFs for such applications, some strategies, for example generation of open metal sites (OMSs), forming charged frameworks, and the incorporation of specific polar functional groups (–NH₂, –OH, –CF₃, etc.) within the porous frameworks, have been investigated to augment the CO₂ adsorptive affinity and selectivity.⁷ Moreover, the pore-size-exclusive effect plays an important role in selective gas adsorption and separation in MOFs.⁸ Tuning/limiting the pores of MOFs to a certain size for selective adsorption and separation of specific gas molecules is an effective method.⁹ However, MOFs with ideal pore size are less reported because it still remains difficult to pre-design and target such made-to-order porous materials at the molecular level.

In the construction of MOFs, the judicious selection of appropriate organic ligands is proven to be one of the most crucial aspects.¹⁰ Herein, an unexploited 2,4-di(3′5′-dicarboxylphenyl)benzoic acid (H₅L) ligand (Scheme 1) is employed to build appropriate MOFs containing optimized pore sizes and OMSs based on the following advantages: (i) the isophthalate-containing linkers easily form porous frameworks based on paddlewheel SBUs,¹¹ and OMSs are often generated from the removal of terminally coordinated solvent molecules to the metal centers; (ii) compared with symmetrical 1,4-benzenedicarboxylate or 1,3,5-benzenetricarboxylate, which have been proven to be excellent organic linkers for constructing MOFs of high porosity and open large pore size,^12,13 H₅L possesses five carboxylic groups with multiple coordination sites, which is more conducive to forming a multiple-pore system with small pores. Such pores may result in a relative increase in the charge density surrounding the adsorbed CO₂ molecules and favor faster CO₂ adsorption kinetics in comparison with CH₄ and N₂, and in turn enhance high CO₂ uptake and selectivity.¹⁴ In addition, a new synthesis strategy that introduces types of building ligands with reduced symmetry to construct MOFs has been recently proposed,¹⁵ which offers a largely unexploited direction towards MOF synthesis by increasing the topological complexity and driving a new mode of framework assembly. In this contribution, we use the asymmetrical H₅L ligand to construct microporous MOFs, [H₂N(Me)₂]₂[Zn₄(L)₂(H₂O)_1.5]·5DMF·H₂O (1·DMF) and [H₂N(Me)₂]₂[Zn₄(L)₂(H₂O)_1.5]·4DMA·6H₂O (1·DMA), which exhibit high adsorption selectivity and storage capacity for CO₂ over CH₄ and N₂ under ambient conditions.

Scheme 1 Schematic structure of the organic linker H₅L.

Experimental section

Materials and general methods

The chemicals diethyl 5-bromoisophthalate (I), ethyl 2,4-dibromobenzoate, bis(pinacolato)diborane and Pd(PPh₃)₄ were purchased and used without further purification. Infrared spectra were obtained using KBr discs on a Nicolet Avatar 360 FTIR spectrometer in the 400–4000 cm⁻¹ region. Elemental analyses of C, H, and N were performed with a Perkin Elmer 2400C Elemental Analyzer. Thermogravimetric analyses (TGAs) were carried out in a nitrogen stream using a Netzsch TG209F3 instrument at a heating rate of 5 °C min⁻¹. Powder X-ray diffraction (PXRD) data were recorded on a Bruker D8 ADVANCE X-ray powder diffractometer (Cu Kα, 1.5418 Å). The sorption isotherms were measured with an automatic volumetric adsorption apparatus (Micromeritics ASAP 2020M).

Synthesis of 4,4,5,5-tetramethyl-2-(diethyl-3,5-dicarboxylate phenyl)-1,3-dioxolane (II)

A mixture of I (0.1 mol, 30.0 g), bis(pinacolato)diborane (11.8 mmol, 3.0 g), potassium acetate (0.29 mmol, 28.0 g), Pd (dppf)₂Cl₂ (7.0 mmol, 5.0 g), and dried 1,4-dioxane (500 mL) was stirred at 100 °C overnight and subsequently extracted with ethyl acetate (200 mL × 3). The organic layer was decolored with activated carbon and dried with anhydrous Na₂SO₄. The crude product was obtained via concentration under a vacuum and was purified by column chromatography (silica gel, ethylacetate–petroleum ether, 6 v%). Yield 78%. Anal. (%) calcd for C₁₈H₂₅BO₆: C, 62.09; H, 7.24. Found: C, 61.92; H, 7.16.

Synthesis of pentaethyl-2,4-di(3′,5′-dicarboxylphenyl)benzoate (III)

A mixture of II (0.1 mol, 34.82 g), ethyl 3,5-bromobenzoate (0.05 mol, 15.4 g), and K₃PO₄ (0.2 mol, 42.4 g) were mixed in 1,4-dioxane (500 mL), and deaerated with N₂ for 10 min. Pd(PPh₃)₄ (0.9 mmol, 1.0 g) was added to the stirred reaction mixture and the mixture was heated to reflux for ca. one week under N₂. The crude product of III was obtained after 1,4-dioxane was removed under a vacuum. Recrystallization from methanol afforded the pure product (Scheme 2). Anal. (%) calcd for C₃₃H₃₄O₁₀: C, 67.11; H, 5.80. Found: C, 67.02; H, 5.65.

Scheme 2 The scheme for the synthesis of H₅L.

Synthesis of 2,4-di(3′,5′-dicarboxylphenyl)benzoic acid (IV)

A mixture of III (0.05 mol, 29.5 g) and 10 g NaOH in 500 mL H₂O was refluxed for 2 hours, and then cooled to room temperature. The solution was neutralized with concentrated HCl. White powder was obtained in a yield of 90%. EI-MS: m/z [M − H]⁻, 449.06 (calcd for C₂₃H₁₄O₁₀, 450.06). Anal. (%) calcd for C₂₃H₁₄O₁₀, C, 61.34; H, 3.13 Found: C, 61.02; H, 2.96.

Synthesis of [H₂N(Me)₂]₂[Zn₄(L)₂(H₂O)_1.5]·5DMF·H₂O (1·DMF)

A mixture of Zn(NO₃)₂·6H₂O (0.1 mmol, 0.030 g), H₅L (0.04 mmol, 0.018 g), DMF (1.5 mL) and water (0.5 mL) was placed in a screw-capped vial, then the vial was capped and placed in an oven at 105 °C for 72 h. The resulting block crystals were washed with DMF three times to give 1·DMF. The yield was ∼24.0 mg (72.6% based on H₅L). Anal. Calcd for C₆₅H₇₄N₇O_27.5Zn₄: C, 47.17; H, 4.51; N, 5.92. Found: C, 47.02; H, 4.78; N, 5.63. IR (cm⁻¹): 3424m, 2965w, 2932w, 2807w, 2492w, 2026w, 1660s, 1628s, 1579s, 1450w, 1435w, 1390s, 1255w, 1163w, 1104m, 1061w, 1020w, 920w, 892w, 853w, 781s, 724s, 665w, 579w, 477w.

Synthesis of [H₂N(Me)₂]₂[Zn₄(L)₂(H₂O)_1.5]·4DMA·6H₂O (1·DMA)

A mixture of Zn(NO₃)₂·6H₂O (0.1 mmol, 0.030 g), H₅L (0.04 mmol, 0.018 g), DMA (1.5 mL) and water (1.0 mL) was placed in a screw-capped vial, then the vial was capped and placed in an oven at 105 °C for 72 h. The resulting block crystals were washed with DMA three times to give 1·DMA. The yield was ∼20.6 mg (60.4% based on H₅L). Anal. Calcd for C₆₄H₈₅N₆O_31.5Zn₄: C, 45.11; H, 5.03; N, 4.93. Found: C, 45.95; H, 4.88; N, 5.14. IR (cm⁻¹): 3423m, 2964w, 2938w, 2807w, 2487w, 2026w, 1627s, 1506m, 1405s, 1361s, 1261m, 1190w, 1107w, 1020m, 964w, 921w, 891w, 855m, 781s, 724s, 678w, 596w, 478w.

Crystallography

Diffraction data were collected with Mo Kα radiation (λ = 0.71073 Å) on a Bruker-AXS SMART CCD area detector diffractometer. Absorption corrections were carried out utilizing SADABS routine.¹⁶ The structures were solved using direct methods and refined using full matrix least squares refinements based on F². All non-hydrogen atoms were refined anisotropically with the hydrogen atoms added to their geometrically ideal positions and refined isotropically. The contribution of the disordered solvent molecules in 1·DMA was subtracted from the reflection data using the SQUEEZE method as implemented in the PLATON¹⁷ program. The final formulas of 1·DMF and 1·DMA were determined by combining the single-crystal structures, elemental microanalyses and TGA data. Selected crystallographic data and structure refinement results are listed in Table 1.

Table 1 Crystal data and structure refinements for 1·DMF and 1·DMA

Complex no.	1·DMF	1·DMA
a R 1 = ∑||Fo| − |Fc|)/∑|Fo|; wR2 = [∑w(Fo^2 − Fc^2)^2/∑w(Fo^2)^2]^1/2.
Empirical formula	C65H74N7O27.5Zn4	C46H21O21.5Zn4
Temperature [K]	293(2)	296(2)
Crystal system	Monoclinic	Monoclinic
Space group	P21/n	P21/n
a [Å]	14.123(4)	14.1192(17)
b [Å]	33.380(9)	33.389(4)
c [Å]	16.833(3)	16.938(2)
β [°]	111.943(16)	111.664(2)
V [Å^3]	7361(3)	7421.0(16)
Z	2	2
D calcd [g cm^−3]	1.493	1.055
μ [mm^−1]	1.373	1.330
GOF on F^2	1.122	1.142
Reflns collected/unique	38 297/13 772	46 438/18 188
R int	0.0440	0.0903
Final R^a	R 1 = 0.0684	R 1 = 0.0706
Indices [I > 2σ(I)]	wR2 = 0.2208	wR2 = 0.1757

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Results and discussion

Compound 1·DMF was solvothermally synthesized by heating a solution of Zn(NO₃)₂·6H₂O and H₅L in DMF at 105 °C for three days. Single-crystal X-ray diffraction reveals that 1·DMF crystallizes in the monoclinic space group P2₁/n and shows a 3D microporous framework. The asymmetric unit consists of four independent Zn²⁺ ions, two fully-deprotonated L⁵⁻ ligands, and one and half coordinated H₂O molecules (Fig. S1†). Zn1 and Zn2 are bridged by three μ₂-carboxylate groups and the axial position is further coordinated by two monodentate carboxylate groups to from a [Zn₂(COO)₅(H₂O)] SBU, where one coordinated H₂O molecule is bound to the Zn1 center (Fig. 1a). Similarly, Zn3 and Zn4 are bridged by three μ₂-carboxylate groups, one monodentate carboxylate group and one chelating carboxylate group to give a distinct [Zn₂(COO)₅(H₂O)] SBU, in which one coordinated H₂O molecule is bound to the Zn4 center (Fig. 1b). All the L⁵⁻ ligands in 1 join to eight Zinc atoms but show two distinct kinds of coordination mode: one exhibits μ¹–η¹, syn–syn-μ¹–η¹:η¹, syn–syn-μ²–η¹:η¹, and syn–anti-μ²–η¹:η¹, while the other adopts μ¹–η¹, and syn–anti-μ²–η¹:η¹ (Fig. S2†). The two kinds of SBU and L⁵⁻ ligand are alternately connected with each other to generate a 3D framework. This arrangement leads to the formation of 1D pore channels with two different shapes along the a-axis, labeled A and B in Fig. 2, with effective aperture sizes of 6.0 × 6.4 Ų and 4.8 × 11.7 Ų, respectively. The void volume calculated by the PLATON program is 54.4% of the total crystal volume after excluding H₂N(Me)₂ cations, DMF and H₂O molecules.

Fig. 1 The [Zn₂(COO)₅(H₂O)] SBUs in 1·DMF (a) and (b), and the [Zn₂(COO)₅(H₂O)] SBUs in 1·DMA (c) and (d).

Fig. 2 The 3D microporous framework of 1; A and B present different shapes of channels.

Notably, when DMA was used to replace DMF in the reaction system, 1·DMA was obtained, which is almost the same as the structure of 1·DMF. In contrast to the structure of 1·DMF, the main difference is the coordination modes of the zinc atoms, as shown in Fig. S3,† which shows the two kinds of SBU to have the same component [Zn₂(COO)₅(H₂O)] but show different ways of bridging with the carboxylate groups (Fig. 1c and d). It should be pointed out that many examples of MOFs can be obtained using different solvents, in which the metal centres and organic ligands have the same coordination and bridging modes,¹⁸ excluding the guest solvents. To the best of our knowledge, 1·DMF and 1·DMA present a unique example of having the same framework based on different building units. Moreover, a large number of MOFs based on various kinds of Zn₂(COO)₄ SBU have been reported,¹⁹ and a few MOFs constructed from Zn₂(COO)₃ SBUs have also been obtained,²⁰ while the [Zn₂(COO)₅] SBUs are only found in MOAAF-2–MOAAF-6 according to the latest Cambridge Structural Database.²¹ The [Zn₂(COO)₅(H₂O)] SBU in 1 is obtained for the first time among the reported MOFs.

Topologically, in the structure of 1, all the [Zn₂(COO)₅H₂O] SBUs and L⁵⁻ ligands act as five-connected nodes, and thus 1 shows a rare (5,5,5)-connected topology (Fig. 3). Although binary SBUs (with one kind of inorganic SBU and one kind of organic SBU) are common in the construction of MOFs, such as MOF-5,^12a HKUST-1,^13a MIL-101^22a and UiO-66,^22b MOFs based on ternary SBUs are unique and extremely difficult to form. To date, there have been only a few MOFs built from ternary SBUs: UMCM-150, CPM-5, MCF-39, USF-3 and -4, asc-1, -2 and -3, [Cu₆O(TZI)₃(H₂O)₉(NO₃)]_n, and JLU-Liu5 and -Liu6.²³ In 1, the two kinds of [Zn₂(COO)₅(H₂O)] unit act as inorganic SBUs and the L⁵⁻ as an organic SBU, and herein 1 gives the first example of a (5,5,5)-connected framework based on ternary SBUs.

Fig. 3 The unique (5,5,5)-connected topology of 1 (the colored balls represent: L⁵⁻, red; SBU1, blue; and SBU2, green.).

The experimental PXRD patterns of 1·DMF and 1·DMA match the simulated ones from the respective single crystal structures (Fig. S4†), confirming the phase purity. Thermogravimetric analysis (TGA) of 1·DMF and 1·DMA was performed (Fig. S6†). The TGA curve of 1·DMF indicates a weight loss of 24.9% in the temperature range of 30–310 °C, which is consistent with the release of five DMF molecules, one lattice H₂O molecule, and one and half coordinated H₂O molecules per formula unit (calc. 24.8%). For 1·DMA, a weight loss of 28.1% between 30 and 305 °C was observed, corresponding to four DMA molecules, six lattice H₂O molecules, and one and half coordinated H₂O molecules per formula unit (calc. 27.6%). The desolvated sample (1a) was prepared by heating an as-synthesized sample of 1·DMF at 160 °C for 6 h and then 180 °C for 3 h under vacuum, which was verified by TGA and IR analysis (Fig. S7†). The PXRD of 1a exhibits a shift and weakening of some peaks, indicating a possible framework change due to the numerous solvent releases (Fig. S5†).

To confirm the permanent porosity of the framework, gas-adsorption measurements (N₂, H₂, CO₂, and CH₄) of 1a were performed. The reversible N₂ adsorption isotherm at 77 K shows the characteristics of a microporous material with a type-I adsorption isotherm behavior (Fig. 4a). The Brunauer–Emmett–Teller (BET) and Langmuir surface areas of 1a are evaluated to be 544 and 589 m² g⁻¹, respectively, and the total micropore volume was estimated using the Dubinin–Astakhov (DA) method to be 0.205 cm³ g⁻¹. The saturated loading of 160 cm³ g⁻¹ at 1 bar is close to the calculated value of 196 cm³ g⁻¹ from the crystal density (1.123 g cm⁻³), free void (34%) and liquid N₂ density (0.808 g cm⁻³). The experimental pore size distribution (PSD) curve based on the NLDFT model demonstrates two kinds of pore in the material, with pore sizes of about 4.6 and 5.4 Å, respectively (inset Fig. 4a), corresponding to the two shapes of channel. The H₂ adsorption experiment at 77 K exhibits an uptake of 118.1 cm³ g⁻¹ (1.1 wt%) at 1 bar (Fig. 4b), which is slightly lower than those of MOF-5 (127 cm³ g⁻¹) and MOF-177 (123 cm³ g⁻¹),²⁴ but is superior to that of the best zeolite ZSM-5 (0.7 wt%).²⁵ Notably, the H₂ adsorption isotherm is steep in the low pressure region, showing the strong interactions between the relatively small pores and H₂ molecules (2.8 Å).²⁶

Fig. 4 N₂ (a) and H₂ (b) adsorption and desorption isotherms at 77 K and pore size distribution (inset) of 1a.

The establishment of the permanent porosity of 1a encourages us to examine its potential application for CO₂/CH₄ and CO₂/N₂ separation. As shown in Fig. 5, the CO₂ and CH₄ adsorption properties on 1a at 195, 273, and 298 K were investigated. At low temperature (195 K), the CO₂ uptake reaches 159.7 cm³ g⁻¹ (31.4 wt%) at 1 bar and displays a type-I isotherm. The CH₄ uptake of 83.7 cm³ g⁻¹ (6.0 wt%) is much lower than the value of CO₂ uptake under the same conditions. Such adsorption selectivity is more significant at high temperatures. The CO₂ uptakes are 85.5 cm³ g⁻¹ (16.8 wt%) at 273 K and 60.1 cm³ g⁻¹ (11.8 wt%) at 298 K and 1 bar. The CO₂ uptake of 1a at 298 K and 1 bar is inferior to the recently reported zeolite-like zinc–tetrazole framework [Zn(btz)] (22.0 wt%),²⁷ being comparable to MAF-23, which possesses multiple strong adsorption sites (11.0 wt%),²⁸ but is superior to some Zn₄O-based frameworks with large pore dimensions (9.0–32.0 Å), such as UMCM-1 (3.8 wt%),²⁹ MOF-177 (3.5 wt%),^2a and SNU-70 (3.5 wt%),^8c implying that the small pore size may be more attractive for CO₂ under low pressure. Meanwhile, the CH₄ uptakes are 26.7 cm³ g⁻¹ (1.9 wt%) and 14.8 (1.1 wt%) at 273 and 298 K, respectively. In addition, the N₂ sorption isotherm at 298 K was also measured, which shows only a trace of uptake (2.3 cm³ g⁻¹) at 1 bar. To evaluate the CO₂/CH₄ and CO₂/N₂ selectivities of 1a under mixture gas conditions with single component isotherms at 298 K, the ideal adsorbed solution theory³⁰ (IAST) can be used to predict the multicomponent adsorption behaviors (Fig. S8†). Fig. 6 shows the predicated selectivities for CO₂/CH₄ and CO₂/N₂ as a function of pressure when the gas phase mole fractions are 50/50 and 15/85, which are typical feed compositions for landfill gas and flue gas, respectively. It is worth noting that very high selectivities for both the CO₂/CH₄ and CO₂/N₂ are obtained. The initial values of selectivities for CO₂/CH₄ and CO₂/N₂ are 11.4 and 95.2 and the selectivities are 12.0 and 107.0 at 1 bar, respectively. The values of the selectivities for CO₂/CH₄ outperform many reported MOFs and carbon materials under the same conditions,³¹ and are comparable to the high selectivities of MOFs functionalized with OMSs or polar groups.³² The highly selective adsorption of CO₂ rather than CH₄ and N₂ suggests the possible application of 1a in capturing CO₂ from upgrading natural gas or flue gas. In addition, both the CO₂/CH₄ and CO₂/N₂ selectivities show a small increasing trend under relatively high pressure, which is rare in MOFs. Taking into account the PXRD pattern of the samples after gas adsorption, the increased selectivity should be attributed to the slight flexibility of the framework; as the pressure increases and gas molecules are adsorbed, a distortion of the framework may occur, which results in enhanced sieve effects and a corresponding higher selectivity. The increased selectivity of 1a indicates that the enhanced sieve effect of the pores might be a key effect that could be of benefit for CO₂ capture under relatively high pressure.

Fig. 5 The adsorption isotherms of 1a: (a) CO₂ and CH₄ at 195 K; (b) CO₂ adsorption and desorption at 273 K (a) and 298 K (b), CH₄ adsorption and desorption at 273 K (c) and 298 K (d), N₂ adsorption and desorption at 298 K (e).

Fig. 6 IAST adsorption selectivities of 1a for equimolar mixtures of CO₂ and CH₄ (a) and a mixture of CO₂ (15%) and N₂ (85%) (b) at 298 K.

The higher selectivity for CO₂ may be attributed to the effect of the OMSs (the exposed Zn²⁺ sites, stemming from the release of coordinated H₂O), the cationic framework, the dimethylammonium cationic guests, and the numerous uncoordinated O donors (located within the wall of the porous surface), which provide an additional electric field and make the framework highly polar,^8c,23g and effectively improve the interaction between highly quadrupolar CO₂ molecules and the host framework. Moreover, the narrow pore sizes of 1a and the small kinetic diameter of CO₂ (3.30 Å) compared to CH₄ (3.80 Å) and N₂ (3.64 Å) enable CO₂ molecules to easily enter the pores. To further check the affinity of 1a for CO₂, the CO₂ adsorption enthalpies (Q_st) were calculated according to the virial equation from the sorption isotherms at 273 and 298 K (Fig. S9†). At initial coverage, Q_st exhibits a maximum of 30.1 kJ mol⁻¹ (Fig. 7); this value is similar to that of dia-7i-1-Co (30 kJ mol⁻¹)^8a and comparable to those of MOFs decorated with typical active sites such as Lewis basic sites (LBSs) and OMSs, for example Cu(bcppm) (29 kJ mol⁻¹),^33a Cu-btc (28 kJ mol⁻¹),^33b and PCN-88 (27 kJ mol⁻¹),^9a but lower than those of some high polarity-functionalized MOFs (45–90 kJ mol⁻¹).^3a,34 This intermediate adsorption enthalpy implies a low-energy consumption for adsorbent regeneration, a desired property for separation applications. For comparison, the Q_st values for CH₄ were also calculated from adsorption data collected at 273 and 298 K for 1a (Fig. S10†). The maximum value of CH₄ is 21.9 kJ mol⁻¹ and is much smaller than that of CO₂, showing a weaker CH₄ binding affinity than CO₂ throughout the adsorption process.

Fig. 7 The isosteric heats of CO₂ and CH₄ adsorption for 1a estimated using the virial equation from the adsorption isotherms at 273 and 298 K.

In addition, the solid-state luminescence properties of 1·DMF and 1·DMA, as well as the free ligand H₅L, are investigated at room temperature. As shown in Fig. 8 and S11,†1·DMF and 1·DMA display fluorescence emission bands at 441 nm and 443 nm upon excitation at 344 nm and 341 nm, respectively. These bands can probably be assigned to the π–π* intra-ligand luminescent emission, since similar emission is observed at 414 nm upon excitation at 343 nm for the free ligand. In comparison with the emission of H₅L, a slight red-shift of ∼18 nm occurs in the maximum emission peaks of the compounds, which can presumably be associated with the coordinative environment around the ligand.

Fig. 8 Fluorescence emission spectra of the ligand H₅L (black), 1·DMF (red) and 1·DMA (blue) in solid state at room temperature.

Conclusions

In summary, by utilizing an asymmetrical pentacarboxylate ligand, a novel microporous MOF based on uncommon ternary SBUs has been successfully assembled. Interestingly, all the SBUs act as five-connected nodes to form a rare (5,5,5)-connected topology. The desolvated framework shows permanent porosity with suitable pore spaces and high polarity porous surfaces decorated with OMSs and LBSs, which enable this material to exhibit higher uptake capacities for CO₂ and high selectivity for CO₂ over N₂ and CH₄ gases. The unique structure and excellent gas adsorption properties further reveal that unsymmetrical polycarboxylate linkers can be used as a good building block to construct functional MOF materials.

Acknowledgements

We are grateful for financial support from the NSF of China (grants 21371142, 91022004, 21201139) and the NSF of Shaanxi, China (grants 2013JQ2016, 2013KJXX-26).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Additional figures, TGA, IR, PXRD, excitation spectra and additional sorption patterns. CCDC 1033728 and 1033729. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5qi00025d

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