Pore Surface Tailored SOD‐Type Metal‐Organic Zeolites

Abstract

Natural and synthetic zeolites are important microporous materials. [ 1 ] The highly ordered structures and tunable compositions of aluminosilicate frameworks are responsible for their extraordinary performances. After the discovery of zeolitic metal imidazolate frameworks, ranging from zeolite-like cobalt imidazolates [ 2 ] to the SOD-, ANA-, and RHO-type (three-letter codes of zeolite topologies) zinc benziimidazolate [ 3 ] and 2-alkylimidazolates, [ 4 ] these novel porous coordination polymers (PCPs) have blossomed in the past few years. [ 5 ] The similarity of the bended exo-bidentate coordination mode of imidazolate with that of the O atom in aluminosilicates has been considered as a determinative construction principle ( Figure 1 ). The rich structure-directing role of imidazolate side groups provides an additional variable for enumeration of new zeolitic structures. [3][4][5] Nevertheless, there seems to be some inherent difference between the inorganic and metal-organic counterparts. For example, inorganic zeolites are negatively charged aluminosilicate frameworks and their pore surface property can be routinely tailored by adjusting the Al/Si ratio and/or cation exchange. In contrast, PCPs (including metal-organic zeolites) are well-known to be completely ordered materials.Among the large family of known metal-organic zeolites, SOD-[Zn(mim) 2 ] (Hmim = 2-methylimidazole [ 4 ] ) is noteworthy for its high porosity and exceptional stability. The synthesis, porous property, sorption mechanism, and application of MAF-4 (the metal azolate framework 4) have attracted much attention. [ 6 ] However, MAF-4 only adsorbs many important gases, such as H 2 , CO 2 , and C 2 H 2 weakly, due to the lack of a strong adsorption site on its pore surface. While new functionalities of MAF-4 are still emerging, efforts have been devoted to the structural modifi cation of this prototype framework. Because imidazolates are almost the shortest linkers in PCPs, variation of the metal ion and/or substituent groups on the imidazolate linker can largely alter porosity (size or volume), change the framework, [ 5e-h ] and even alter the whole network topology. Metal ion substitution can effectively change the pore surface properties of PCPs functionalized with coordinatively unsaturated metal centers. [ 7 ] However, the tetrahedral metal centers are shielded by the surrounding imidazolates in zeolitic frameworks and hence not effective for sorption. So far, tuning the sorption property of PCP by simple pore surface engineering instead of other porous parameters has rarely been reported. [ 7,8 ] Here, we report a straightforward approach for pore surface tailoring of the MAF-4 structure, which leads to not only drastic enhancement but also fi ne-tuning of the sorption performance for practical adsorptive applications. In principle, replacing a C-H moiety in MAF-4 with an isoelectronic N atom can provide a Lewis base active site for guest binding. [ 9 ] While the N atoms prefer coordination to metal ions during self-assembly, we show t...

Enhanced Framework Rigidity of a Zeolitic Metal-Azolate via Ligand Substitution

“…MAF-7 is isomorphous to ZIF-8 and crystallizes in the same cubic space group I-43m as reported by Zhang et al [16]. In the structure, the Zn 2+ ion is tetrahedrally coordinated with the triazolate ligand to form Zn(mtz)2 open framework with zeolitic sodalite (SOD) architecture.…”

“…In the structure, the Zn 2+ ion is tetrahedrally coordinated with the triazolate ligand to form Zn(mtz)2 open framework with zeolitic sodalite (SOD) architecture. The mtz − ligand coordinates with the zinc ion via its N atoms on the 2-and 4-positions, and the N on the 1-position unbonded [16,17]. It should be emphasized the lone pair electrons of the unbonded nitrogen can donate electron to the adjacent N at the 2-position, and this effect pushes the lone pair electrons of the coordinated N to bonded with the zinc center more tightly (Figure 1).…”

“…The electron-donating effect is indicated by arrows. The yellow surfaces indicate the solvent accessible voids of ZIF-8 and MAF-7 which are 49.8% and 51.7%, respectively [16]. Color codes used: zinc, red; carbon, grey; nitrogen, blue; hydrogen, white.…”

“…Single crystals of Zn(mtz)2 were prepared by slow evaporation from a clear aqueous ammonia solutions according to the method of Chen et al [16] In the procedure, 0.084 g Hmtz (Aladdin) and 0.198 g Zn(NO3)2·6H2O (Alfa Aesar) were dissolved in 10 mL aqueous ammonia solutions (25%) (Sinopham Chemical Reagent Co., Ltd., Shanghai, China). The colorless crystal have precipitated from liquid after a few days.…”

“…In this work, we present a systematic study on the elastic properties of a zeolitic triazolate, Zn(mtz)2 (MAF-7 [16], mtz = 3-methyl-1,2,4-triazolate), which is isomorphous to ZIF-8. To examine the electronic difference in elastic properties between 2-methylimidazolate and 3-methyl-1,2,4-triazolate, we performed firstprinciples calculations to extract the full elastic tensors of the cubic MAF-7.…”