Synthesis and characterization of MPA (M=K, Cs)/ OMS-2/ MCM-41 nanocomposites

It has been reported that substitution of large monovalent cations, such as NH4+, K+, Cs+, etc., for H+ in MxH3-xPW12O40 catalysts improved surface area and thermal stability compared to parent acids. A problem associated with the use of Heteropolyacid salts is that the salt particles disperse as a colloid in water and organic solvents, making itdifficult to separate the salt from the reaction products by simple filtration. Therefore they were supported on a carrier. The POMs of the Keggin structure have molecular diameter of around 1.2 nm, it is therefore feasible to insert into the MCM-41. MCM-41 is a promising support because of its large surface area (˷1000 m2/g), high thermal stability and large pore size (1.5–8 nm). However, MCM-41 lacks Brønsted acid sites and exhibits only weak hydrogen-bonded type of sites [1]. For MCM-41 to be used as a catalyst or catalyst support in acid catalyzed reactions Brønsted acid sites need to be created and the acid strength must be enhanced. The acidity of MCM-41 can be increased by surface modification, through the introduction of strong acid species such as sulfate ions, sulfated zirconia or heteropolyacids with Keggin-type structures [2], either on the surface or within the inner channels of MCM-41. This can result in creating Brønsted acid sites which can lead to a significant improvement in the acid strength of MCM-41. Here we wish to report the results of a study on the use of MCM-41-supported heteropolyacids salts in esterification reactions. Previously, bulk HPA and carbon-supported HPA have been studied [3] as catalysts in esterification