SiO2-supported dodecatungstophosphoric acid and Nafion-H prepared by ball-milling for catalytic application

Dodecatungstophosphoric acid (HPW) and Nafion-H were supported on SiO2 using the ball-milling technique. Samples were characterized by physical (BET surface, SEM, DSC, XRD, DRIFT and Raman spectroscopies) and chemical (acid-base titration) methods. Catalytic properties were studied in a range of chemical transformations, which can be induced by electrophilic catalysts including Friedel-Crafts reactions (alkylation of toluene with benzyl alcohol, acylation of anisole with acetic anhydride) and the dimerization of α-methylstyrene. Milling conditions (stainless steel or polystyrene apparatus, milling time), loading of the active components, and the type of support were found to have profound effects on the physical characteristics and catalytic performance of the samples. Ball-milled HPW-SiO2 samples prepared under appropriately selected conditions exhibit catalytic properties similar to catalysts made by impregnation. HPW in ball-milled samples, in turn, proved to be more resistant to extraction with polar solvents (hot methanol) and these catalysts display higher stability in recycling studies. Supported Nafion-H samples prepared by ball-milling show high activities in test reactions. The best catalyst exceeds SAC-13 in the selective synthesis of 1,1,3-trimethyl-3-phenylindane in the dimerization of 2-phenylpropene. Catalysts with high catalytic performance require the use of optimal milling time, because particle coalescence and the resulting decrease in BET surface area hinder the accessibility of the active sites.