Size-dependent effects in supported metal catalysts for liquid phase hydrogenation reactions
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abstract
Environmental awareness is pushing the development of more efficient and more selective heterogeneous catalysts for the production of fine chemicals and pharmaceuticals. The goal is to reduce the number of by-products and the consumption of hazardous solvents, making the processes efficient at molecular level. Heterogeneous catalysts can be tailored to specific needs, are easier to handle and lead to less amounts of waste chemicals in a given process. As preparation method, photochemical deposition of noble metals in different supports is gaining importance due to its simplicity and advantages. Its main advantage is the ability of spreading very effectively the metal throughout the support, leading to very high dispersions, resulting in higher molecular control, with a positive effect on both activity and selectivity.Hydrogenation of the carbonyl function in α,β-unsaturated olefins still attracts too much attention due to the importance of the unsaturated alcohol as intermediate in many synthetic routes [1]. The concepts of chemoselectivity and diastereoselectivity are extremely important issues in steroid hydrogenation.
Noble metal catalysts (Pt and Ir) supported on titania, with different loads (1 and 5%wt) were prepared by liquid phase photodeposition of the appropriate precursors. Calcination (under N2) and reduction (H2) at various temperatures provided materials with variable particle sizes at the nanometer scale. In order to explore the possibility of a metal-support interaction two types of titania were used: Degussa P25 (crystal mean diameter of 30 nm) and a sample of TiO2 prepared by a modified acid-catalyzed sol–gel method from alkoxide precursors (crystal mean diameter of 8 nm). These parameters will be discussed in terms of the performance of the catalytic materials in the hydrogenation of cinnamaldehyde. As a representative example, under laboratory conditions the catalyst Pt/TiO2 (5%) (N2 calcination at 773 K followed by H2 reduction at the same temperature) revealed a maximum selectivity to the unsaturated cinnamyl alcohol of 64% at 79% conversion, against 26% maximum selectivity at 29% conversion under similar conditions for the untreated catalyst. Preliminary tests in steroid selective hydrogenation were also extremely promising.
In addition to the above reported, similar results were found for Ir supported catalysts, leading to the conclusion that by very simple photodeposition method catalysts can be prepared, which are very selective for carbonyl hydrogenation.
