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

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• 作者：Avelino Corma ; Francesc X. Llabrs i Xamena ; Carmelo Prestipino ; Michael Renz ; Susana Valencia
• 刊名：Journal of Physical Chemistry C
• 出版年：2009
• 出版时间：July 2, 2009
• 年：2009
• 卷：113
• 期：26
• 页码：11306-11315
• 全文大小：247K
• 年卷期：v.113,no.26(July 2, 2009)
• ISSN：1932-7455

文摘

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.