Production of Biodiesel Through Esterification Reaction Using Choline Exchanging Polytungstoboronic Acids as Temperature-Responsive Catalysts

High proton content polytungstoboronic acid H7BW11TiO40 (abbreviated as H7BW11Ti) had been prepared through titanium mono-substituted of H5BW12O40, which was evaluated in esterification of palmitic acid with methanol. H7BW11Ti could promote esterification reaction with almost 100% conversion of 417 mol/mol h TOF within very short time of 1 h at methanol/acid ratio of 25:1 under reaction temperature of 65 degrees C, which was attributed to the higher Bronsted acidity and cooperation of Lewis acidic site generating from Ti substituent. Limitation by its homogeneous performance, solidification of H7BW11Ti was designed via partial exchanging protons with choline cation (abbreviated as Ch(+)) to prepare a series of [(CH3)(3)NCH2CH2OH](n)H7-nBW11TiO40 (abbreviated as Ch(n)H(7-n)BW(11)Ti, n = 0 similar to 7). Among all solid catalysts, the highest efficiency had been achieved by with 99.0% conversion and 429 mol/mol h TOF within 50 min at 65 degrees C at methanol/acid ratio of 15: 1. The comparable activity was attributed to the cooperation of Ch(+)-self-assembling to form nanoreactor with amphiphilic surrounding to concentrate substrates and to resist to water-poison, temperature-responsive property to control form changing between heterogeneous to homogenous. This temperature-responsive HPA catalyst was available for production of biodiesel through esterification reaction. In addition, Separation of such heteropolyacid catalysts was easy by the lowering the reaction temperature to room temperature without appreciable loss of its high performance, which were reused for more than six times.