Heteropoly acids immobilized into a silica matrix:: characterization and catalytic applications

Four dodecaheteropoly acids of the Keggin-type (H-n[XM12O40]; n = 3, 4; X = Si, P; M = Mo, W) were immobilized into a silica matrix by means of a sol-gel technique. It was shown by ammonia thermodesorption that W-containing samples possess higher acidity than their Mo-containing counterparts which is in agreement with previous results for the neat acids. These silica-included heteropoly acids proved to be effective catalysts in ester hydrolysis, Friedel-Crafts alkylations (adamantylation and transalkylation of toluene with 5-tert-butyl-1,2,3-trimethylbenzene), and in the synthesis of methyl tert-butyl ether (MTBE) from methanol and tert-butyl alcohol. Friedel-Crafts alkylations were found to be sensitive enough to detect minute differences in catalyst characteristics. When compared to silica-supported heteropoly acids the silica-entrapped samples exhibited higher specific activity and proved to be recyclable catalytic materials. (C) 1999 Elsevier Science B.V. All rights reserved.