Synthesis and characterization of AMO LDH-derived mixed oxides with various Mg/Al ratios as acid–basic catalysts for esterification of benzoic acid with 2-ethylhexanol

Abstract

Proven to possess distinguishable physical and acid-base properties superior to conventional LDHs, Aqueous Miscible Organic solvent-Layered Double Hydroxides (AMO-LDHs) were thus synthesized and used as precursors to prepare the Mg/Al mixed oxide catalysts in this work. The AMO-LDH based oxide catalysts with various ratios of Mg/Al were studied for the chemical and physical properties and the activity on esterification of benzoic acid with 2-ethylhexanol. The catalysts were characterized using BET, XRD, TGA, and XPS. Moreover, the acid-base properties were studied by using NH3-TPD, CO2-TPD, and X-ray Absorption Spectroscopy (XAS) techniques, both XANES and EXAFS. As a result, the Mg/Al mixed oxides after calcination at 500 °C still had the clay structure, and were found to possess both acid and base sites. As the Mg/Al ratio increased, the total density of acid and basic sites decreased. Moreover, the acid-basic strength depended on their phase compositions and coordination number. The activity of calcined LDHs catalysts was tested for the esterification of benzoic acid with 2-ethylhexanol, aimed at producing 2-ethylhexyl benzoate as the desired chemical. The products were analyzed using GC–MS/TOF. In summary, the conversion of benzoic acid was enhanced significantly using the MgAl mixed oxides as the catalysts, owing to the acid-base sites (both Mg2+O2− and Al3+O2− pairs) of the catalysts. The catalyst with the Mg/Al ratio of 4:1 can convert 66% benzoic acid to 2-ethylhexyl benzoate. Moreover, the other products were composed of 2-ethylhexanal, 3-heptanone, and 3-heptanol because of acid-base pairs.