Guanidine post-functionalized crystalline ZIF-90 frameworks as a promising recyclable catalyst for the production of biodiesel via soybean oil transesterification

In the current research, a porous solid base catalyst (ZIF-90-Gua) was prepared through covalent post-functionalization of zeolitic imidazolate framework ZIF-90 with organic guanidine via an imine condensation reaction. Various techniques such as XRD, SEM, FT-IR, XPS, EDX, TPD, Hammett indicator titration and nitrogen porosimetry measurements were employed to characterize the as-prepared solid catalyst. It was shown that the organic guanidine had been bound on the ZIF-90 frameworks by covalent imine linkages, and the primary crystalline structure of the imidazolium-based ZIF-90 was essentially maintained after the guanidine incorporation. This ZIF-90-Gua catalyst possessed an enhanced basicity, and interconnectivity, leading to highly catalytic activities. As a robust catalyst, the catalytic performances were evaluated in the heterogeneous transesterification of soybean oil with methanol for biodiesel production, and the maximum oil conversion to biodiesel of 95.4% was attained at reaction temperature of 65 °C, with a methanol/oil molar ratio of 15:1, catalyst dosage of 1 wt% (based on the oil mass) within reaction duration of 6 h. The solid catalyst could be easily recovered by filtration and reused for five times without significant decay of the catalytic activity, showing that it has great potential to be used as an efficient and durable catalyst for the clean production of biodiesel.