A study on the stability of MCM-41-supported heteropoly acids under liquid- and gas-phase esterification conditions

MCM-41-supported heteropoly acids (HPAs:H3PW12O40(PW12) and H4SiW12O40(SiW12)) are used as catalysts in the liquid-phase esterification of 1-propanol and hexanoic acid, and in the gas-phase esterification of acetic acid and 1-butanol. For both reactions MCM-41-supported HPAs proved to be active catalysts. However, X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies show that the initially high dispersion of HPA units is lost during reaction. Large clusters (similar to 10 nm) of HPA are formed on the outer surface of the MCM-41 support. Since this clustering also occurred when less polar substrates were applied, not only the polarity of the substrates is responsible for the cluster formation. It is expected that water, formed in esterification, plays a major role in the transport of the HPA from the MCM-41 pores to the outer surface, leading to formation of HPA clusters on the outer surface. (C) 1999 Elsevier Science B.V. All rights reserved.