Intramolecular cyclization of (+)-citronellal using supported 12-tungstophosphoric acid on MCM-41

Supported heteropolyacids (HPAs) have been used as catalysts by fine-tuning their heterogenous and Bronsted acidic properties. In this work, supported H3PW12O40 (HPW) on MCM-41 was prepared (2-40 wt%) and applied in the intramolecular cyclization of (+)-citronellal. The synthesized materials were characterized by P-31 MAS NMR, XRD, FTIR, pyridine gaseous adsorption, low temperature nitrogen adsorption and thermal analysis. The impregnation of 20% HPW on the surface of MCM-41 decreased the characteristic crystallographic reflections of the support, suggesting that HPW modifies the long-range order. The results show that the HPW was deposited preferentially inside the mesopores of the support and affected the acid strength of the supported catalyst. FTIR studies of pyridine adsorption confirmed the presence of Bronsted and hydrogen bonded acid sites. All synthesized materials were active in the cyclization reaction, but the 20%HPW/MCM-41 sample was the most active. This catalyst showed about 96% conversion and 65% selectivity to the most important stereoisomer (-)-isopulegol, under 1 h reaction. This catalyst was reused four times with a fair degree of deactivation. The decreased activity was attributed to the formation of small agglomerates in the channels of MCM-41. (C) 2012 Elsevier B.V. All rights reserved.