Pinacol rearrangement over metal-substituted aluminophosphate molecular sieves

Pinacol rearrangement was found to proceed at relatively mild temperature over metal-substituted aluminophosphate molecular sieves. Pinacolone and 2,3-dimethyl-1,4-butadiene were the two main products obtained. The conversion of pinacol and the selectivities of these two products were found to be influenced by the metal species, the metal content, the crystalline structure of aluminophosphate and the solvent used in the liquid-phase reaction. Among the first row transition metal elements substituted in the AFI crystal structure, late transition metals seem to give better catalytic activities than the early ones. Fe3+, Cu2+ and Ni2+ in order are the three which give the highest pinacol conversions and pinacolone selectivities. The catalytic activity was found to have no direct correlation with the acid strength or amount of the acid sites on the catalysts. A mechanism involving the redox ability of Fe(III) and its stabilization of the carbenium ion intermediates was proposed. Besides, in comparison of the catalytic activities of Fe-substituted molecular sieves of different crystalline structures, the activity decreased in the order of APO-5 > APO-11 greater than or equal to APO-8 > VPI-5 > silicalite-1. Since the catalytic activity was independent of pore diameter, the liquid-phase reaction was considered to proceed mainly on the outer surfaces of the catalysts. The hydrophilicity of the aluminophosphate surface was in favor of catalyzing the pinacol reaction, which involves polar reactants and less polar products. Moreover, high pinacol conversion was achieved by using solvents of low polarity. (C) 1998 Elsevier Science B.V. All rights reserved.