In-situ FTIR study on the mechanism of both steps of zeolite-catalysed hydroesterification reaction in the context of biodiesel manufacturing

While biodiesel manufacturing using transesterification is a mature process, a number of key issues need to be addressed for widespread application. These include amongst others feedstock availability and waste products. In this paper, we report on a different approach to synthesize biodiesel, namely a two-stage process which alleviates the need to utilise purified oils and only contains relatively pure glycerol as by-product. We investigated a route for waste cooking oil hydroesterification (hydrolysis/esterification) over zeolites. We tried to understand the mechanism of both steps of zeolite-catalysed hydroesterification and investigate the adsorption of probe molecules, along with the compounds formed by ester hydrolysis and fatty acid esterification reactions over the ZSM-5 zeolite surface by employing in-situ FTIR. The 40% conversion level of waste oil hydrolysis into fatty acid mixture and 63% conversion level of the synthesised fatty acids esterification to biodiesel was observed over ZSM-5 (Si/Al ratio = 15) after 4 h. The results from FTIR spectroscopy evident the protonation of the carbonyl groups of ester and fatty acid via hydrogen bonding with bridging OsbndH in SisbndOHsbndAl group (Brønsted acid sites) of the zeolite. The electrophilic reaction of protonated fatty acid, adsorbed on the surface of zeolite, with alcohol was observed from FTIR spectroscopy of esterification reaction.