Immobilization of polyoxometalate-based sulfonated ionic liquids on UiO-66-2COOH metal-organic frameworks for biodiesel production via one-pot transesterification-esterification of acidic vegetable oils

In accordance with the need of green and cleaner production, the aim of the present study is to fabricate an organic-inorganic hybrid recyclable catalyst with both Bronsted and Lewis acid sites for the one-pot production of biodiesel from the low-cost acidic oil feedstocks. To achieve this, the UiO-66-2COOH metal-organic framework materials (MOFs) were firstly functionalized with Keggin-type polyoxometalate (POM) acids through the in-situ preparation strategy, and then the sulfonated acidic ionic liquids (AILs) were further incorporated into the prepared POM/UiO-66-2COOH composites by pairing polyoxometalate anions with functionalized IL cations. The as-prepared AILs/POM/UiO-66-2COOH solid catalyst features a high surface area and an increased acidity, thus combining the merits of Bronsted and Lewis acids together with the heterogeneous microenvironment of MOFs. The solid acid catalysts were fully characterized by several techniques, and the results demonstrated that the polyoxometalate-based sulfonated AILs were virtually immobilized on the UiO-66-2COOH MOFs, and furthermore the porous crystalline structure of the UiO-66-2COOH material was almost unaltered after the immobilization processes. The solid catalyst had excellent catalytic activities toward the simultaneous transesterification and esterification of simulated low-cost oils with long-term stability and good reusability, and moreover still remained considerable activities even with the existence of 9 wt% free fatty acid and 3 wt% water in the low-cost oils. Therefore, this solid catalyst shows potential applications for the one-step biodiesel production especially from the low-cost feedstocks which usually contain high amounts of free fatty acid and water.