PINACOL REARRANGEMENT OVER METAL-SUBSTITUTED ALUMINOPHOSPHATE MOLECULAR-SIEVES

Pinacol rearrangement was found to proceed at relatively mild temperat ure over metal-substituted aluminophosphate molecular sieves. Pinacolo ne 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 elemen ts substituted in the AFI crystal structure, late transition metals se em to give better catalytic activities than the early ones. Fe3+, Cu2 and Ni2+ in order are the three which give the highest pinacol conver sions and pinacolone selectivities. The catalytic activity was found t o have no direct correlation with the acid strength or amount of the a cid 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-sub stituted molecular sieves of different crystalline structures, the act ivity decreased in the order of APO-5 > APO-11 greater than or equal t o APO-8 > VPI-5 > silicalite-1. Since the catalytic activity was indep endent of pore diameter, the liquid-phase reaction was considered to p roceed mainly on the outer surfaces of the catalysts. The hydrophilici ty of the aluminophosphate surface was in favor of catalyzing the pina col reaction, which involves polar reactants and less polar products. Moreover, high pinacol conversion was achieved by using solvents of lo w polarity. (C) 1998 Elsevier Science B.V. All rights reserved.