Fabrication of a hierarchically structured HKUST-1 by a mixed-ligand approach

Abstract

A novel hierarchically structured HKUST-1 has been synthesized under hydrothermal synthesis conditions using a mixed ligand strategy, in which benzene-1,3,5-tricarboxylic acid was partially replaced by benzoic acid. The formation of the crystalline HKUST-1 material containing mesoporosity was characterized by a complementary combination of X-ray powder diffraction, Fourier transform infrared spectroscopy, N₂ adsorption–desorption isotherms, scanning electron microscopy, transmission electron microscopy and thermogravimetric analysis. These analyses indicated that incorporation of a certain amount of benzoic acid in the crystal lattice of HKUST-1 preserved its crystal structure. The benzoic acid ligand could be seen as “defect” sites that two carboxylate groups of one linker molecule are missing, leading to a distorted paddle-wheel unit, which induced the formation of an intra-crystalline mesoporous structure in HKUST-1. The hierarchically structured HKUST-1 combines the dual merits of two different pore structures, enabling it has maximum structural function in a limited space due to the enhanced diffusion through mesoporous channels. The presented method opens the door for the preparation of a variety of hierarchically structured MOFs.

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1. Introduction

Porous metal–organic frameworks (MOFs) are crystalline materials that possess various structure types with distinct pore dimensions, pore connectivity, and framework compositions,^1–3 and have received significant interest due to their potential in applications such as gas storage, molecular separation, catalysis, sensing and drug delivery,^4–6 owing to their higher versatility in tailoring the structure of frameworks using different metal-containing units and organic linkers.⁷ Porous MOFs are composed of an organic linker and a transition metal complex or cluster, forming a three dimensional open framework with a regular array of uniform micropores in a molecular dimension (pore dimensions are less than 2 nm).⁴ As a result of the small sizes of these inherent micropore channels or windows of pure MOFs, there is a significant obstacle to their application in processes involving bulky molecules. Reactants and products with sizes beyond the micropore dimensions are not able to diffuse into and out of the internal functional active sites of MOFs.^8,9 In such a diffusion-controlled regime, MOF applications are limited to small molecule separations and transformations. In addition, slow diffusion can cause the polymerization of by-products or reaction intermediates involved in the catalytic process,^10,11 and more severely, this can result in fast catalytic deactivation due to the frequent blocking of the diffusion paths in MOFs.¹²

To overcome the diffusion problem of guest molecules in MOFs, synthetic strategies to increase the size of MOF pores are highly desirable, including the synthesis of MOFs with extra-large micropores,^13–15 and the preparation of MOFs in the form of small nanoparticles.^16,17 The preparation of mesoporous MOFs through extended linkers and metal clusters can push pore metrics into the mesoporous regime. This type of novel mesoporous MOFs with extra-large pores was obtained by employing bulky molecules as the organic ligands, which induced the formation of large voids located in the interior of the MOF.¹³ However, only a limited number of successful examples have been reported for mesoporous MOFs,^18,19 which is attributed to the fact that MOFs with large pore sizes are easily plagued by collapses upon removal of the guest molecules and interpenetration between intra-framework space, resulting in a close packed structure.³ As a result, the development of a common strategy to fabricate robust mesoporous MOFs with tunable structures is still challenging; in other words, the synthesis of mesoporous MOFs with extra-large pores is limited. In the case of MOF nanoparticles, the synthesis process has to be optimized delicately so as to provide a degree of control over the resultant crystal size, leading to the formation of aggregates of MOF nanoparticles.^16,20 The synthesis should be tuned accurately in order to stop crystallization before growth to bulk crystals, which often results in a very low yield, and the final separation is cumbersome to filtrate. Therefore, from the viewpoint of material preparation, the synthesis of MOF nanoparticles is impractical.

Recently, novel hierarchically structured MOFs or mesoporous MOFs have received considerable attention, which possess mesopores in addition to the inherent micropores of MOFs.^8,9 These hierarchically structured MOFs can integrate the dual merits of mesoporosity and microporosity. The micropores can act as a sieve to separate the guest molecules based on size- or shape selectivity, while the mesopores provide a facile diffusion pathway to improve the mass transport ability of guest species through the interconnected micropore-to-mesopore networks. An alternative to produce the additional mesoporosity in MOFs is from specific defects or channels within MOFs, which can be induced through the chemical etching of crystals or space-filling effects from long-chain molecules during crystal growth.^4,21 The resultant hierarchical MOFs exhibited good performance in liquid phase separation processes, due to the enhancement of mass transfer for aromatic molecules.²² However, the chemical etching of crystals also induced an inevitable change in the MOF framework. The mesoporous structure can also be generated by the addition of a template, in which the organic template molecules are self-assembled into a supramolecular micelle that can function as a mesopore structure-directing agent (SDA). After crystallization, removal of the template from the resulting meso-MOFs induces the formation of MOFs within the intracrystal mesopores.⁴ The mesopore diameters can be tuned by control of the structure through molecular manipulation and can also be expanded by auxiliary swelling-agents such as 1,3,5-trimethylbenzene (TMB). In the following years, many new kinds of templates have been introduced to synthesize mesoporous MOFs. Among them, typical ones include amphiphilic molecules,^12,21,23,24 block copolymers,^25–27 non-ionic surfactants,^28,29 and termed structure-directing agents.³⁰ Besides, the defect-engineering of MOFs is also an effective emerging approach that could generate mesopores, which in turn help in overcoming diffusion limitations.³¹ Such structures commonly enable MOFs to have maximum structural function in a limited space and volume owing to the enhanced transformation ability and improved molecular accessibility.

Herein, we report a simple method to produce hierarchically porous HKUST-1 using a mixed-ligand strategy. This strategy, using modified and normal linker molecules, can prevent premature phase segregation, leading to new classes of hierarchically porous structures. The target of this contribution is to present the synthesis and properties of novel hierarchically structured HKUST-1 by a mixed-ligand strategy. The synthetic protocol described here is not limited to the preparation of hierarchical HKUST-1, but other hierarchical MOFs can also be synthesized using other mixed-ligands if they can self-assemble simultaneously without changing the crystal structure. This method opens the door for the synthesis of a variety of hierarchical micro- and mesoporous MOFs.

2. Experimental description

2.1. Synthesis of the hierarchically structured HKUST-1

In a typical synthesis, 1.068 g (4.59 mmol) of Cu(NO₃)₂·2.5H₂O was dissolved in 8 mL of distilled water. Then 0.461 g (2.19 mmol) of benzene-1,3,5-tricarboxylic acid and 0.134 g of benzoic acid (1.09 mmol) were dissolved in 16 mL of an absolute ethanol and dimethylformamide (DMF) mixture with v/v = 1 : 1. The mixture was stirred for 2 h at ambient temperature. After that, the two solutions were combined in a round-bottom flask and stirring was continued for another 2 h at room temperature. After stirring, the mixture was transferred into a 50 mL Teflon-lined autoclave, and heated to 383 K for 12 h. After crystallization, the collected precipitate was filtered, dried in air, and washed three times with ethanol. Finally, the product was centrifuged and dried at 373 K under vacuum overnight. The resulting hierarchically structured HKUST-1 sample is denoted as H-HKUST-1. For comparison, conventional HKUST-1 was prepared in this study following a literature report,²³ and is denoted as C-HKUST-1.

2.2. Characterization

Powder X-ray diffraction (XRD) patterns were obtained using a Bruker D8 ADVANCE diffractometer system equipped with Ni filtered Cu target K_α radiation (operation at 40 kV, 40 mA, wave length λ = 0.15418 nm). Infrared spectra of samples in the form of KBr pellets were recorded at room temperature on an FT-IR spectrometer (Bruker Vector 33) with a resolution of 1 cm⁻¹. Nitrogen adsorption and desorption isotherms were measured using an ASAP 2010 apparatus (Micromeritics) at 77 K. Before measurement, the samples were treated at 453 K for 10 hours under vacuum. The morphologies of the products were examined by Scanning Electron Microscopy (SEM) with Carl Zeiss, ZEISS Ultra 55 at a low landing energy (5.0 kV). Transmission Electron Microscopy (TEM) images were recorded with a JEOL JEM-2100 electron microscope operated at 200 kV and equipped with a Gatan 832 CCD camera. Thermogravimetric analyses were performed on a TA Instruments apparatus (Model TGA 2050) in a N₂ atmosphere with a heating rate of 10 °C min⁻¹.

3. Results and discussion

The powder XRD patterns of the hierarchically structured HKUST-1 and conventional HKUST-1 are shown in Fig. 1. The image clearly shows that both the H-HKUST-1 and C-HKUST-1 exhibit the typical topological structure of HKUST-1 reported in the literature,^32,33 and they are further compared with that of the simulated pattern provided by the Materials Studio package 5.0, as shown in Fig. 1 (bottom). The XRD patterns of synthesized H-HKUST-1 and C-HKUST-1 are well matched with the reflections of the simulated data. However, some differences in intensity between H-HKUST-1 and C-HKUST-1 are observed, which can be attributed to the fact that the mixed-ligands can influence the normal crystallization, leading to the formation of defect sites in the crystal lattice, contributing to the lower intensity of H-HKUST-1.

Fig. 1 XRD patterns of C-HKUST-1 and H-HKUST-1 materials; for structural comparison, a computer simulated XRD pattern of HKUST-1 is given at the bottom (black line).

H-HKUST-1 and C-HKUST-1 samples are further characterized by FTIR spectra (Fig. 2). A characteristic band at around 1640 cm⁻¹ (Fig. 2a) can be attributed to the antisymmetric stretch vibrations of the carboxylate group ν_as(OCO) in the benzene-1,3,5-tricarboxylic acid and benzoic acid ligands.³⁴ The signal at around 1560 cm⁻¹ (Fig. 2a) is assigned to the carboxylate group from the benzoic acid ligand, and a small shift to the left was observed due to the presence of a second linker compared to the pure HKUST-1 sample. A second characteristic signal is shown in Fig. 2b. The distinct absorption at 1108 cm⁻¹ is the C–H in-plane bending mode of the aromatic ring in the mixed ligands.³⁴ The differences between H-HKUST-1 and C-HKUST-1 are attributed to the fact that the mixed linkers will induce a different substitution pattern of the aromatic ring.

Fig. 2 IR spectra of C-HKUST-1 and H-HKUST-1 samples: (a) frequency range for ν_as(OCO) group; (b) frequency range of the C–H in plane bending mode of benzene-1,3,5-tricarboxylic acid and benzoic acid ligand, respectively.

N₂ adsorption–desorption isotherms and non-local density functional theory (NLDFT) pore size distributions are presented in Fig. 3. It can be seen that the C-HKUST-1 sample exhibits a typical type-I isotherm³⁵ with a sharp increase in the uptake (at low nitrogen relative pressure) followed by a plateau (Fig. 3a), which is typical of a microporous material.³³ In contrast, H-HKUST-1 shows a type IV isotherm as defined by IUPAC³⁵ (Fig. 3a). At a low relative P/P₀ pressure (0.0 < P/P₀ < 0.1), a steep rise in uptake, which corresponds to the filling of micropores with nitrogen^20,36 was observed. The hysteresis loop at a relatively high pressure (P/P₀ > 0.5) indicates capillary condensation of N₂ in mesopores.¹¹ These analyses confirm the co-existence of micropores and mesopores in the H-HKUST-1 sample, and similar conclusions were drawn by Fang et al.,³⁷ Kozachuk et al.³⁸ and Park et al.,³⁹ in which a series of defect-engineered MOFs were synthesized by a mixed-ligand approach, and these obtained MOFs exhibited pronounced hysteresis loop isotherms, revealing the formation of hierarchical porosity with both mesopores and micropores. It is obvious that the N₂ isotherm on C-HKUST-1 is higher that of H-HKUST-1 in the low relative pressure region of 0.0 < P/P₀ < 0.1 (Fig. 3a), indicating a possibly larger surface of C-HKUST-1 compared to H-HKUST-1. The (NLDFT) pore size distributions of these samples (Fig. 3b) determined from the adsorption branch of the N₂ isotherms using a cylindrical-type pore and silica absorbent model demonstrate that a narrow pore size with a mean value of about 1.0 nm is obtained for both C-HKUST-1 and H-HKUST-1, and the C-HKUST-1 sample shows a significantly higher microporosity than H-HKUST-1, suggesting a larger pore volume of C-HKUST-1 than H-HKUST-1. However, the H-HKUST-1 exhibits a broad pore size distribution of mesopore diameters centered at 14.8 nm compared with traditional C-HKUST-1, which is caused by the incorporation of mixed-ligands in the framework of HKUST-1, in which two of the bridging carboxylate groups located in the paddle-wheel unit are missing, resulting in the formation of defect sites in the crystal lattice. The BET surface area of H-HKUST-1 is 564 m² g⁻¹, and the micropore and mesopore volumes are 0.21 cm³ g⁻¹ and 0.25 cm³ g⁻¹, respectively. Comparing with conventional C-HKUST-1, the specific surface area and micropore volume of H-HKUST-1 are lower than for C-HKUST-1 (S_BET = 1242 m² g⁻¹, V_mic = 0.49 cm³ g⁻¹, V_meso = 0.11 cm³ g⁻¹), which can be attributed to the fact that the mixed linkers disturb the normal crystallization of HKUST-1, leading to a decrease in the degree of crystallinity. This is in good agreement with the aforementioned XRD data. Fang et al.³⁷ and Barin et al.⁴⁰ also reported that modifying MOFs by doping with defective linkers can induce a certain loss in specific surface area. However, H-HKUST-1 possesses a much larger mesopore volume than C-HKUST-1. These analyses validate the generation of mesoporosity in the H-HKUST-1.

Fig. 3 (a) Nitrogen adsorption/desorption isotherms of C-HKUST-1 and H-HKUST-1, (b) NLDFT pore size distributions of C-HKUST-1 and H-HKUST-1.

Fig. 4 shows scanning electron microscopy (SEM) images of the C-HKUST-1 and H-HKUST-1 samples. As shown in Fig. 4a, the conventional C-HKUST-1 sample exhibits a typical single-crystal morphology. The crystals are in octahedral shapes with circular edges. The H-HKUST-1 sample, prepared by the mixed-ligand strategy,still retains the overall morphology of the parent HKUST-1 (Fig. 4b). However, the detailed surfaces of H-HKUST-1 become rugged with various diameters of irregular needle-like pores. The surface roughness is likely to be due to the structural defects that have been introduced by the substitution of benzene-1,3,5-tricarboxylic acid by the benzoic acid ligand.

Fig. 4 SEM images of (a) C-HKUST-1 and (b) H-HKUST-1 samples.

Fig. 5 shows transmission electron microscopy (TEM) images of C-HKUST-1 and H-HKUST-1. The image of C-HKUST-1 shows that the entire particle is a single crystal with typical characteristics of the traditional HKUST-1 (ref. 41) (Fig. 5a). In contrast, the TEM image of H-HKUST-1 (Fig. 5b) exhibits a disordered mesostructured framework with three-dimensional connectivity, which is somewhat similar to the channel arrangement of KIT-1.⁴² These results are different from previously reported research work where porous coordination polymer crystals were fabricated by a coordination modulation method,^20,43,44 in which the size and morphology of HKUST-1 crystals could be tuned through adding different modulators into the reaction system, but all of the synthesized HKUST-1 crystals exhibited the characteristics of a microporous material⁴⁴ or hierarchical porosity²⁰ owing to interstitial grain voids. In the present work, the obtained H-HKUST-1 sample showed the characteristic of intracrystal mesopores, which can be attributed to the distortions caused by the linker substitution. In addition, the addition of a “terminal” linker likely truncates the crystal in a particular region, giving rise to this intra-crystalline mesoporosity, and such a hierarchical arrangement is important for mass transport.^12,27

Fig. 5 TEM images of (a) C-HKUST-1 and (b) H-HKUST-1 samples.

Thermogravimetric analysis of C-HKUST-1 and H-HKUST-1 in the presence of nitrogen is presented in Fig. 6a. The weight loss up to 393 K can be attributed to physically adsorbed water in the MOFs. The weight loss at higher temperature between 393 and 873 K is reasonably assigned to the decomposition of organic species in C-HKUST-1 and H-HKUST-1. It can be seen from Fig. 6a that the onset of decomposition of C-HKUST-1 is about 593 K, while H-HKUST-1 starts to decompose at around 543 K. Thus, C-HKUST-1 is more stable than H-HKUST-1. The differential thermogravimetric (DTG) curves of C-HKUST-1 and H-HKUST-1 (Fig. 6b) reveal that the thermal decomposition of both samples occurs in one step, centered at 615 K, which indicates that the substitution of benzene-1,3,5-tricarboxylic acid by benzoic acid does not significantly influence the crystal structure of HKUST-1.

Fig. 6 (a) Thermogravimetric and (b) differential thermogravimetric curves of C-HKUST-1 and H-HKUST-1 materials.

A proposed schematic strategy for the preparation of hierarchically structured HKUST-1 through the substitution of benzene-1,3,5-tricarboxylic acid by benzoic acid is given in Scheme 1. For C-HKUST-1, the benzene-1,3,5-tricarboxylic acid ligand can self-assemble with the Cu species to form a so-called paddle-wheel unit in which Cu₂-clusters are coordinated by four carboxylate groups that are arranged in a square.⁴⁵ By partially exchanging benzene-1,3,5-tricarboxylic acid molecules with benzoic acid, two carboxylate groups of one linker molecule are missing. Therefore, the paddle-wheel structure becomes distorted, resulting in the formation of a gap or void between the copper dimer and the linker molecules, which induces the generation of mesopores inside the HKUST-1 crystal framework. However, with the replacement of benzene-1,3,5-tricarboxylic acid by benzoic acid, two carboxylate groups are missing and two positive charges remain in that structure. Thus, residual nitrate ions or hydroxide species might compensate the additional charge.⁴⁶ With this strategy, a hierarchically structured HKUST-1 with intra-crystalline mesopores was successfully synthesized.

Scheme 1 Conceptual strategy for the preparation of hierarchically structured HKUST-1 with intra-crystalline mesopores through the substitution of benzene-1,3,5-tricarboxylic acid by benzoic acid.

4. Conclusions

In summary, we have developed a facile strategy to fabricate hierarchically structured HKUST-1 through a simple one-step mixed-ligand approach. The obtained hierarchical HKUST-1 material does not have a different crystal structure to HKUST-1, as confirmed by X-ray powder diffraction, but exhibited a mesoporous or hierarchical micro-/mesoporous system with an improved external surface area compared to conventional HKUST-1. The hierarchically structured HKUST-1 plays a significant role in heterogeneous catalysis, selective separation and drug delivery owing to the enhanced mass transport and overall storage capacity for bulk molecules. Despite the obvious potential of such materials, some issues still need to be addressed. Firstly, the molar ratio of benzene-1,3,5-tricarboxylic acid to benzoic acid could be varied to systematically control the porosity of the hierarchically structured HKUST-1. Next, a variety of hierarchical MOFs with different frameworks could also be obtained by choosing different building blocks, which can establish the feasibility of the design of hierarchically structured MOFs with controlled crystal structure and pore sizes. Further research is currently under way in our laboratory.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 20936001 and 21176084), the National High Technology Research and Development Program of China (No. 2013AA065005), SRFDP (No. 20130172110012) and Guangdong Natural Science Foundation (S2011 030001366), National Natural Science Foundation of China (No. 21276052) and Science and Technology Planning Project of Guangdong Province (No. 2012A090300006).

References

1. S. Kitagawa, R. Kitaura and S.-i. Noro, Angew. Chem., Int. Ed., 2004, 43, 2334–2375.
2. C. N. R. Rao, S. Natarajan and R. Vaidhyanathan, Angew. Chem., Int. Ed., 2004, 43, 1466–1496.
3. U. Mueller, M. Schubert, F. Teich, H. Puetter, K. Schierle-Arndt and J. Pastre, J. Mater. Chem., 2006, 16, 626–636.
4. D. Bradshaw, S. El-Hankari and L. Lupica-Spagnolo, Chem. Soc. Rev., 2014, 43, 5431–5443.
5. C.-D. Wu and W. Lin, Angew. Chem., 2007, 119, 1093–1096.
6. S. H. Jhung, J. H. Lee, J. W. Yoon, C. Serre, G. Férey and J. S. Chang, Adv. Mater., 2007, 19, 121–124.
7. H. Furukawa, K. E. Cordova, M. O’Keeffe and O. M. Yaghi, Science, 2013, 341.
8. Z. Wu and D. Zhao, Chem. Commun., 2011, 47, 3332–3338.
9. N. Linares, A. M. Silvestre-Albero, E. Serrano, J. Silvestre-Albero and J. Garcia-Martinez, Chem. Soc. Rev., 2014, 43, 7681–7717.
10. K. Na, M. Choi and R. Ryoo, Microporous Mesoporous Mater., 2013, 166, 3–19.
11. B. Liu, Y. Tan, Y. Ren, C. Li, H. Xi and Y. Qian, J. Mater. Chem., 2012, 22, 18631–18638.
12. L.-G. Qiu, T. Xu, Z.-Q. Li, W. Wang, Y. Wu, X. Jiang, X.-Y. Tian and L.-D. Zhang, Angew. Chem., Int. Ed., 2008, 47, 9487–9491.
13. X.-S. Wang, S. Ma, D. Sun, S. Parkin and H.-C. Zhou, J. Am. Chem. Soc., 2006, 128, 16474–16475.
14. G. Férey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour, S. Surblé and I. Margiolaki, Science, 2005, 309, 2040–2042.
15. G. Férey, C. Serre, C. Mellot-Draznieks, F. Millange, S. Surblé, J. Dutour and I. Margiolaki, Angew. Chem., Int. Ed., 2004, 43, 6296–6301.
16. J. Huo, M. Brightwell, S. El Hankari, A. Garai and D. Bradshaw, J. Mater. Chem. A, 2013, 1, 15220–15223.
17. Y. Pan, Y. Liu, G. Zeng, L. Zhao and Z. Lai, Chem. Commun., 2011, 47, 2071–2073.
18. Y. K. Park, S. B. Choi, H. Kim, K. Kim, B.-H. Won, K. Choi, J.-S. Choi, W.-S. Ahn, N. Won, S. Kim, D. H. Jung, S.-H. Choi, G.-H. Kim, S.-S. Cha, Y. H. Jhon, J. K. Yang and J. Kim, Angew. Chem., Int. Ed., 2007, 46, 8230–8233.
19. Q.-R. Fang, G.-S. Zhu, Z. Jin, Y.-Y. Ji, J.-W. Ye, M. Xue, H. Yang, Y. Wang and S.-L. Qiu, Angew. Chem., Int. Ed., 2007, 46, 6638–6642.
20. S. Diring, S. Furukawa, Y. Takashima, T. Tsuruoka and S. Kitagawa, Chem. Mater., 2010, 22, 4531–4538.
21. K. M. Choi, H. J. Jeon, J. K. Kang and O. M. Yaghi, J. Am. Chem. Soc., 2011, 133, 11920–11923.
22. A. Ahmed, N. Hodgson, M. Barrow, R. Clowes, C. M. Robertson, A. Steiner, P. McKeown, D. Bradshaw, P. Myers and H. Zhang, J. Mater. Chem. A, 2014, 2, 9085–9090.
23. S. S.-Y. Chui, S. M.-F. Lo, J. P. H. Charmant, A. G. Orpen and I. D. Williams, Science, 1999, 283, 1148–1150.
24. Y.-n. Wu, M. Zhou, B. Zhang, B. Wu, J. Li, J. Qiao, X. Guan and F. Li, Nanoscale, 2014, 6, 1105–1112.
25. T.-Y. Ma, H. Li, Q.-F. Deng, L. Liu, T.-Z. Ren and Z.-Y. Yuan, Chem. Mater., 2012, 24, 2253–2255.
26. Y. Wan, Y. Shi and D. Zhao, Chem. Commun., 2007, 897–926,  10.1039/b610570j.
27. Z. Xue, J. Zhang, L. Peng, B. Han, T. Mu, J. Li and G. Yang, ChemPhysChem, 2014, 15, 85–89.
28. Y. Zhao, J. Zhang, B. Han, J. Song, J. Li and Q. Wang, Angew. Chem., Int. Ed., 2011, 50, 636–639.
29. L. Peng, J. Zhang, J. Li, B. Han, Z. Xue and G. Yang, Chem. Commun., 2012, 48, 8688–8690.
30. J. Reboul, S. Furukawa, N. Horike, M. Tsotsalas, K. Hirai, H. Uehara, M. Kondo, N. Louvain, O. Sakata and S. Kitagawa, Nat. Mater., 2012, 11, 717–723.
31. Z. Fang, B. Bueken, D. E. De Vos and R. A. Fischer, Angew. Chem., Int. Ed., 2015, 54, 7234–7254.
32. Y. Li and R. T. Yang, AIChE J., 2008, 54, 269–279.
33. F. Xu, S. Xian, Q. Xia, Y. Li and Z. Li, Adsorpt. Sci. Technol., 2013, 31, 325–339.
34. N. Drenchev, E. Ivanova, M. Mihaylov and K. Hadjiivanov, Phys. Chem. Chem. Phys., 2010, 12, 6423–6427.
35. K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou, R. A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure Appl. Chem., 1985, 57, 603–619.
36. B. Liu, C. Li, Y. Ren, Y. Tan, H. Xi and Y. Qian, Chem. Eng. J., 2012, 210, 96–102.
37. Z. Fang, J. P. Dürholt, M. Kauer, W. Zhang, C. Lochenie, B. Jee, B. Albada, N. Metzler-Nolte, A. Pöppl, B. Weber, M. Muhler, Y. Wang, R. Schmid and R. A. Fischer, J. Am. Chem. Soc., 2014, 136, 9627–9636.
38. O. Kozachuk, I. Luz, F. X. Llabrés i Xamena, H. Noei, M. Kauer, H. B. Albada, E. D. Bloch, B. Marler, Y. Wang, M. Muhler and R. A. Fischer, Angew. Chem., Int. Ed., 2014, 53, 7058–7062.
39. J. Park, Z. U. Wang, L.-B. Sun, Y.-P. Chen and H.-C. Zhou, J. Am. Chem. Soc., 2012, 134, 20110–20116.
40. G. Barin, V. Krungleviciute, O. Gutov, J. T. Hupp, T. Yildirim and O. K. Farha, Inorg. Chem., 2014, 53, 6914–6919.
41. S. Li and F. Huo, Nanoscale, 2015, 7, 7482–7501.
42. R. Ryoo, J. M. Kim, C. H. Ko and C. H. Shin, J. Phys. Chem., 1996, 100, 17718–17721.
43. A. Umemura, S. Diring, S. Furukawa, H. Uehara, T. Tsuruoka and S. Kitagawa, J. Am. Chem. Soc., 2011, 133, 15506–15513.
44. H. R.-n. Na Li-yan, N. Gui-ling and O. Xiao-xia, Chem. Res. Chin. Univ., 2012, 28, 555–558.
45. K. Schlichte, T. Kratzke and S. Kaskel, Microporous Mesoporous Mater., 2004, 73, 81–88.
46. S. Marx, W. Kleist and A. Baiker, J. Catal., 2011, 281, 76–87.

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