Effect of maf-6 crystal size on its physicochemical and catalytic properties in the cycloaddition of co₂ to propylene oxide

Zeolitic imidazolate frameworks MAF-5 and MAF-6 based on Zn²⁺ and 2-ethylimidazole were demonstrated to be efficient heterogeneous catalysts in solvent-free coupling of CO₂ and propylene oxide (PO) to produce propylene carbonate (PC) at 0.8 MPa of CO₂ and 80^◦C. Activity of MAF-5 was lower in comparison with MAF-6 due to the difference in their structural and textural characteristics. MAF-6 samples with particle size of 190 ± 20, 360 ± 30, and 810 ± 30 nm were prepared at room temperature from [Zn(NH₃ )₄ ](OH)₂ and 2-ethylimidazole. Control of particle size was achieved by variation of type of alcohol in alcohol/cyclohexane media for the preparation of MAF-6. According to this comprehensive study, the yield of PC was found to decrease with increasing crystal size of the MAF-6 material, which was related to the change in textural properties and the number and localization of active sites. The combination of MAF-6 with particle size of with particle size of 190 ± 20 nm and tetrabutylammonium bromide ([n-Bu₄N]Br) as co-catalyst led to an approximately 4-fold enhancement in the yield of PC (80.5%). Compared with reported ZIFs catalysts, the efficiencies of MAF-5/[n-Bu₄N]Br and MAF-6/[n-Bu₄N]Br binary systems were comparable and higher under similar reaction conditions.