Water Resistant, Catalytically Active Nb and Ta Isolated Lewis Acid Sites, Homogeneously Distributed by Direct Synthesis in a Beta Zeolite

Nb and Ta solid Lewis acids able to work in the presence of water have been prepared by incorporating these elements in the framework of zeolite Beta by a direct synthesis procedure. Characterization of the samples by extended X-ray absorption fine structure and X-ray absorption near edge structure indicate that the calcined samples contain tetracoordinated Nb5+ (or Ta5+) with three siloxane bridges and one NbO (or TaO) bonds. The presence of Lewis acidity has been characterized by IR of adsorbed pyridine and cyclohexanone. The Nb- and Ta-Beta samples are catalytically active for the Meerwein−Ponndorf−Verley reduction of cyclohexanone with 2-butanol and the etherification of 2-butanol with p-methoxybenzalcohol. Both reactions can be successfully combined in a domino reaction to obtain the ether starting with p-methoxybenzaldehyde. The catalytic activity of Nb- and Ta-Beta has been compared with that of the homogeneous counterparts (NbCl5 and TaCl5) reported in the literature for the intramolecular carbonyl-ene reaction of citronellal to isopulegol, being the turnover number (TON) and the diastereoselectivity of the solids much higher than those of the homogeneous catalysts.