Synthesis of monoesters and diesters using eco-friendly solid acid catalysts—Cerium(IV) and thorium(IV) phosphates

• Ce(IV) and Th(IV) phosphates have been used as solid acid catalysts for the first time.
• Materials have been synthesized under conventional and microwave irradiation.
• Catalytic activity has been explored and compared by studying esterification reaction.
• Monoesters and diesters have been synthesized by optimizing several parameters.

In the present endeavour, amorphous cerium phosphate (CP) and thorium phosphate (TP) have been synthesized by sol–gel method and also under microwave irradiation to yield CPM and TPM. CP, TP, CPM and TPM have been characterized for elemental analysis (ICP-AES), spectral analysis (FTIR), thermal analysis (TGA), X-ray diffraction studies, SEM, EDX, surface area (BET) and surface acidity (NH3-TPD). The potential use of these materials as solid acid catalysts has been explored by studying esterification as a model reaction. Monoesters such as ethyl acetate (EA), propyl acetate (PA), butyl acetate (BA), benzyl acetate (BzAc) and diesters such as diethyl malonate (DEM), diethyl succinate (DES), dibutyl phthalate (DBP), dioctyl phthalate (DOP) have been synthesized. Esterification conditions have been optimized by varying several parameters such as reaction time, catalyst amount and mole ratio of reagents. The catalytic activity has been compared and correlated with reference to surface acidity of the catalysts. It is found that catalytic activity of CPM > CP > TPM > TP. The regenerated catalysts could be reused upto two catalytic runs without significant loss in % yields of esters formed. The highlighting feature of the present work is the catalysts CPM and TPM that are synthesized in a much shorter reaction time with higher surface acidity giving good % yield of esters.

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