Mesoporous mixed oxide catalysts via non-hydrolytic sol-gel: A review

Despite the enormous amount of research dedicated to this topic in the last 20 years or so, there is still a need for a general, cost-effective methodology allowing the synthesis of mesoporous mixed oxide catalysts. This review deals with the synthesis and catalytic applications of mixed oxides prepared by the non-hydrolytic sal-gel (NHSG) process based on the reaction of chloride precursors with ether or alkoxide oxygen donors. This NHSG process offers simple, one-step syntheses of mixed oxides with well-controlled compositions and non-ordered mesoporous textures, avoiding the use of supercritical drying or templates. Over the last decade, this process has been used to prepare various mesoporous mixed oxide catalysts, which showed real potential in major reactions such as partial and total oxidation, reduction of NOR, alkene metathesis, or alkylation. The main reactions involved in this NHSG process and the characteristics of the resulting mixed oxides are described in the first part of this review, underlining the decisive advantages in terms of simplicity and of control (in terms of composition, homogeneity or texture) offered by this process. In a second part, the literature dealing with mixed oxide catalysts prepared by this NHSG method is exhaustively reviewed and the catalytic performance of NHSG catalysts is compared, whenever possible, to that of catalysts with similar compositions prepared by other methods. The excellent catalytic performances of NHSG-catalysts (notably Si-Ti, Ti-V and Si Al-Mo catalysts) compared to state-of-the art aerogels or ordered mesoporous materials evidences the potential of this sol-gel method, which should open the door to the synthesis of improved catalysts and to the discovery of new catalysts. (C) 2012 Elsevier B.V. All rights reserved.