Synergistic role of Brønsted and Lewis acidity in alkali metal-exchanged heteropolyacid catalysts for esterification of acetic acid at room temperature

Abstract

A series of Cs exchanged phosphotungstic acid (PTA) catalysts were synthesized by the ion exchanged method and were characterized by X-ray diffraction, FT-IR spectroscopy, pyridine adsorbed FT-IR spectroscopy, SEM, BF-TEM, ICP-OES and BET surface area analysis. For comparison purposes, K exchanged PTA, Cs and K exchanged phosphomolybdic acid (PMA) catalysts were also prepared. XRD diffractograms showed that the crystallites of the Keggin ion are maintained, while FT-IR spectra also revealed the characteristic bands of the Keggin ion at all metal loadings of all the catalysts. From pyridine adsorbed FT-IR spectroscopy, it was observed that the Bronsted and Lewis acidity were significantly maintained at lower metal loadings, whereas STEM analysis showed a uniform distribution of the elements which correlated well with the theoretical atomic values of the loaded metals for all the catalysts, which were verified by ICP results. The efficiency of various metal-exchanged heteropolyacid catalysts was assessed for the esterification reaction using various substrates, and the Cs exchanged phosphotungstic acid catalysts showed superior activity compared to the other catalysts. In particular, the Cs exchanged phosphotungstic acid with a 1 wt% loading showed the highest activity and was most tolerant to the presence of water that was produced in the reaction. The catalytic activity correlates well with the Bronsted and Lewis acidity, as well as Keggin ion density of the catalysts.