Modulation of Self-Separating Molecular Catalysts for Highly Efficient Biomass Transformations.

The energetically viable fabrication of stable and highly efficient solid acid catalysts is one of the key steps in the large-scale transformation processes of biomass resources. Herein, we report the covalent modification of the classical Dawson polyoxometalate (POMs) with sulphonic acids (-SO3H) by grafting the sulfonic acid groups on the POM's surface followed by oxidation of (3-Mercaptopropyl)trimethoxysilane. The acidity of TBA6-P2W17-SO3H (TBA = tetrabutyl ammonium) has been demonstrated using 31P.NMR, clearly indicating the presence of strong Bronsted acid sites. The presence of TBA counterions renders the solid acid catalyst as a promising candidate for phase transfer catalytic processes. The TBA6-P2W17-SO3H shows remarkable activity and selectivity, excellent stability and great substrate compatibility for the esterification of free fatty acids (FFA) with methanol and conversion into biodiesel at 70 degrees C with >98% conversion of oleic acid in 20 min. The excellent catalytic performance can be attributed to the formation of a catalytically active emulsion, which results in a uniform catalytic behavior during the reaction leading to efficient interaction between the substrate and the active sites of the catalyst. Most importantly, the catalyst can be easily recovered and reused without any loss of its catalytic activity due to its excellent phase transfer properties. This work offers an efficient and cost-effective strategy for large scale biomass conversion applications.