Heteropolyacid encapsulated in Cu3(BTC)2 nanocrystals: An effective esterification catalyst

An original synthesis approach to prepare Cu3(BTC)2 (BTC = benzene tricarboxylic acid) encapsulated Keggin phosphotungstic acid (HPW) [HPW/Cu3(BTC)2] involving mixing of reagents at room temperature, quenching in liquid nitrogen and freeze drying has resulted in the formation of nanocrystals. The catalytic properties of the as-synthesized nanocrystalline materials were assessed using a model esterification reaction of acetic acid with 1-propanol. HPW/Cu3(BTC)2 catalyst is partially dissolved in presence of acetic acid. In the esterification reaction the molar ratio of acetic acid to 1-propanol is critical. At high molar ratio of acetic acid to 1-propanol (1:2) leaching of Cu2+ and HPW was observed. However, at low molar ratio of acetic acid to 1-propanol (1:40) leaching of Cu2+ and HPW could be prevented and the catalyst was stable. Nanosized HPW/Cu3(BTC)2 showed higher catalytic activity compared to micron-size Cu3(BTC)2 (HKUST-1), ultrastable Y zeolite and micron-sized HPW/Cu3(BTC)2 catalysts. The stability of the nanosized MOF catalyst in acidic reaction medium after esterification reaction was investigated by powder X-ray diffraction (PXRD), scanning electron microscope (SEM) and N2 adsorption.