Selective Hydrogenation Of Crotonaldehyde Over Pt Catalysts

Unsaturated alcohols are widely applied in flavor and fragrance industries and in pharmaceutical manufacturing. Selective hydrogenation of a, p-unsaturated aldehydes is a main route to synthesize unsaturated alcohols. However, the hydrogenation of the C=C bond is much favored compared to that of C=O bond due to the thermodynamic factor, thus, promotion of selectivity to unsaturated alcohol has been the main focus in this research field. In this work, a series of supported platinum catalysts are prepared and tested for gas phase selective hydrogenation of crotonaldehyde. The catalysts were characterized using H2-TPR (H2 temperature-programmed reduction), XRD (X-ray power diffraction analysis), FTIR (Fourier transform infrared spectroscopy), TEM (Transmission electron microscope), NH3-TPD (NH3 temperature-programmed desorption), Raman (Raman spectroscopy) and CO-chemisorption to clarify the the reaction mechanism and active phase. The detailed results are as follows:1. Selective hydrogenation of crotonaldehyde over Pt/ZrO2 catalystsPt catalysts supported on high-surface area ZrO2 were prepared by an impregnation method and tested for gas phase selective hydrogenation of crotonaldehyde. The effects of Pt loading and reduction temperatures on the catalytic properties of Pt/ZrO2 catalysts were investigated. It was found that a 3Pt/ZrO2 catalyst with 3wt.% Pt loading and reduced at 500℃exhibited fairly good catalytic performance, with a selectivity of 55% to ctotyl alcohol and a 27% conversion of crotonaldehyde. Characterization results suggested that the catalytic performance depended on the strong Lewis acidic sites on the catalyst surface and the proper particle size of Pt (8 nm), which was favorable to the selective hydrogenation of crotonaldehyde to crotyl alcohol.2. Effect of Cr-modification on Pt/ZrO2 catalysts for selective hydrogenation of crotonaldehydeCr-modified Pt/ZrO2 catalysts were prepared and tested for vapor phase selective hydrogenation of crotonaldehyde. It was found that the reactivities of the catalysts declined with Cr contents, while the selectivity to crotyl alcohol increased with Cr contents. The decreasing activity of the Cr-modified catalyst is due to the decreasing Pt particle size compared to the un-modified catalyst and accumulation of absorbed organic compounds on the catalyst surface. Also, Lewis acidity and electron transfer between Cr and Pt were responsible for the enhancement of the selectivity to crotyl alcohol for the Cr-modified catalysts.3. A comparative study on deactivation and selectivity in crotonaldehyde hydrogenation over Pt/CeO2 and Pt/NaCl/ZrO2 catalystsVapor-phase hydrogenation of crotonaldehyde was carried out over Pt/CeO2 and Pt/NaCl/ZrO2 catalysts. It was found that both catalysts suffered deactivation due to the formation of C8 compounds by condensation reaction, as evidenced by the FTIR results. With respect to selectivity, these two catalysts behaved very differently in the reaction. For the Pt/NaCl/ZrO2 catalyst, selectivity to crotyl alcohol kept constant during the reaction, while for the Pt/CeO2 catalyst, the selectivity to crotyl alcohol decreased dramatically. Characterization of catalysts revealed that the loss of selectivity over the Pt/CeO2 catalyst could be attributed to the oxidation of Ce (III) cations to stable Ce (IV) and carbon deposit formed on the catalyst surface derived from CO reacting with Ce (III), as evidenced by in situ Raman spectroscopy of CO adsorption. In contrast, little carbon deposits are formed on the surface of Pt/NaCl/ZrO2 catalyst, which was responsible for the stable selectivity during the reaction process.