Abstract: Nowadays, a new subclass of metal-organic frameworks (MOFs), such as zeolitic imidazolate frameworks (ZIFs), have attracted considerable attention as materials for catalysis due to combination of structure od zeolites and physicochemical properties of MOFs in one material. Here we demonstrated investigation of catalytic potential of zinc zeolitic imidazolate frameworks based on 2-methyl- (ZIF-8) and 2-ethylimidazolate (MAF-5 or ZIF-14) linkers in synthesis of propylene glycol methyl ether (1-methoxy-2-propanol, PGME) from methanol and propylene. PGME is widely used in chemical, cosmetic and pharmaceutical industry as pollution-free solvent. In the presence of ZIF-8 and MAF-5 materials reaction was found to proceed at 110-120oC with 92.1-93.8% selectivity towards PGME. Reaction rate in the presence of MAF-5 was higher in compared with ZIF-8 that was related to the higher in the basicity and in the accessibility of active sites. The lower basicity of ZIF-8 (858 kJ/mol) than MAF-5 (884 kJ/mol) due to the difference in pKa of conjugate acids of 2-methylimidazole (7.85) and 2-ethylimidazole (8.0). It was demonstrated that catalytic properties of ZIF-8 strongly depended on the crystal size. The conversion of PO and yield of PGME decreased rapidly with increasing crystallite size. Such behavior can be related with (a) the mass transfer resistance changes with the crystal size and morphology, and (b) varying localization of active sites. Moreover, the reaction selectivity is also determined by the crystal size of ZIF-8 materials. The lower the crystal size of the ZIF-8 sample, the higher the selectivity toward PGME that was explained by (a) different density of active sites on the external surface, and (b) electronic structure effects of the surface. The reaction mechanism in the presence of ZIF-8 catalyst was investigated by IR spectroscopy. According to the IR study, reaction between MeOH and PO in the presence of ZIF-8 systems proceeds on "Lewis acid site - basic site" pairs, where Lewis acid sites are preferably formed by Zn2+ ions without three or two organic linkers, whereas basic sites are formed by Zn-OH species or N atom of 2-methylimidazole. MAF-5 and ZIF-8 materials showed good reusability for 5 recycles in catalysis that can point to their high potential for catalytic application. Compared with reported catalysts, studied ZIFs, especially MAF-5, showed not only the highest conversion of PO but also the highest selectivity for 1-methoxy-2-propanol under the similar reaction conditions.

Cite: Timofeeva M.N. , Panchenko V.N. , Lukoyanov I.A. , Shefer K.I. , Shvydko A.V. , Ventlyand E.P. , Gerasimov E.Y.
Composition Systems Based on ZIF-8 and Alumina Nanofibers as Materials with Tunable Properties for Catalysis
The 5th EuChemS Inorganic Chemistry Conference 24-28 Jun 2019