Synthesis of chromanols using the eco-friendly catalyst AlPMo12O40/Carbon: The role of support

Abstract

To increase the yield of chromanol, the AlPMo12O40 salt catalyst should be supported on mesoporous materials because it has low specific surface area. In the present work, we prepared an organic carbon support via a sol-gel method using resorcinol and formaldehyde, and used it to immobilize the aluminum salt of H3PMo12O40 by using two impregnation techniques: pore-filling (ilp) and in situ (is). The Fourier-transform infrared spectra and X-ray diffraction patterns showed that, in the catalysts obtained by ilp, the salts were better dispersed on the support than in those prepared in situ. This was confirmed by scanning electron microscopy and a different morphology of the catalyst surface was observed. The textural properties of the catalysts were determined by N2 adsorption. The catalysts were tested in the synthesis of 2,5,7,8-tetramethyl-2-(4methylpent-3-enyl)-chroman-6-ol using trimethylhydroquinone (TMHQ) and geraniol. The reactions produced chromanol and one open-chain compound. The reaction using unsupported AlPMo12O40 was homogeneous when the solvent was chloroform and heterogeneous when the solvent was hexane. Heating by microwave radiation reduced the reaction time and increased the selectivity to chromanol. The AlPMo12O40 supported on carbon by ilp showed higher catalytic activity than the unsupported salt. The results using AlPMo12O40/Carbon as catalyst showed that the conversion of TMHQ was higher with the catalyst prepared by ilp than with that obtained by the in situ technique. Also, chromanol was obtained at highest yields using ilp catalysts. The catalyst with 30% supported salt prepared by ilp achieved 94% and 99% conversion in 5 h and 7 h reaction times, respectively and no by-products were formed. The heterogeneous catalyst (30-AlPMo/C3-800)ilp prepared has the characteristics of Lewis acid and has several advantages with respect to conventional catalysts, which makes it economically and environmentally attractive. Also, this catalyst could be reused.