Recent developments in solid acid catalysts for biodiesel production

Biodiesel obtained via the transesterification process, a reaction frequently involving catalysis by homogeneous or heterogeneous catalysts, has shown considerable potential as a supplement to petroleum-based diesel; heterogeneous catalysts in the transesterification of vegetable oils are of special interest as they are relatively safer for the environment besides being endowed with recycling and regeneration attributes. The deployment of heterogeneous catalysts, however, necessitates a higher concentration of catalyst, a greater molar ratio of alcohol to oil, and an elevated temperature and pressure to produce biodiesel. A variety of solid-acid catalysts (SACs) have been utilized in the production of biodiesel, represented by Ti(SO4)O, SO4/Fe‒Al‒TiO2, sulfonated single-wall carbon nanohorns, Ti‒SBA‒15, La‒PW‒SiO2/single-wall carbon nanotubes, ionic liquid grafted NH2‒UiO‒66, ZrFe‒SA‒SO3H, magnetic SACs, sulfonic-reduced graphene oxide, CaO‒Al2O3‒SiO2‒CaSO4, and SiO2@Cs‒SO3H, which are deliberated here. These catalysts are highlighted and discussed in view of their exceptional catalytic activity and recyclability, in addition to comments on the existing challenges and potential future perspectives of the SACs.