Metal-free bifunctional silica for conversion of waste glycerol from biodiesel: Sustainable production of formic acid

This work describes for the first time the conversion of the waste glycerol from biodiesel production using a metal-free bifunctional silica with high porosity as catalyst. The mesoporous silica (synthetic SiO2) with high specific surface area of 1489 m(2)g(-1) was used in the conversion of the residual glycerol (supplied by Petrobras, without any purification) using continuous flow (PBR) reactor. Surprisingly, this metal-free silica showed high selectivity for the oxidative cleavage product (formic acid), reaching 83% in continuous flow. This indicates that there is no need for the use of supported metals as in the most of the works in the literature, which can reduce the cost of the catalyst. The surface defects in the synthetic SiO2 generated a high amount of Lewis acid sites (identified by pyridine adsorption experiments). Thus, the acid sites reacted in situ with the H2O2 (used as oxidizing agent during reaction for glycerol conversion), generating very active oxygen species on the catalyst surface. Therefore, the data presented in this work suggest that the prepared SiO2 is a bifunctional catalyst that presents Lewis acid sites besides reactive oxygen species.