Vapor-Phase Selective Hydrogenation Of Crotonaldehyde Over Noble Metal (Pt, Ir)Catalysts

The vapor-phase selective hydrogenation of crotonaldehyde, a classical a, β-unsaturated aldehyde, to crotyl alcohol over heterogeneous catalysts is an appealing green process of great importance from both industrial and academic points of view. In this work, a series of Pt-and Ir-based catalysts were prepared and tested for gas phase selective hydrogenation of crotonaldehyde. The catalysts were characterized using X-ray powder diffraction analysis (XRD), X-ray fluorescence diffuse reflectance (XRF), Transmission electron microscope (HRTEM), CO chemisorption, H2temperature-programmed reduction (H2-TPR), NH3temperature-programmed desorption (NH3-TPD), Temperature-programmed oxidation (TPO), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy (Raman), Diffuse reflectance infrared transform spectra of CO adsorption (CO-DRIFTS) and X-ray photoelectron spectroscopy (XPS) techniques, it was found that the Pt/Ceo.gLao.201.9and Ir/TiO2catalysts were highly active for this reaction, and the active sites and mechanism of the reaction were discussed. The detailed results are as follows:1. Pt/Ce0.8La0.201.9catalysts for selective hydrogenation of crotonaldehydeThe vapor-phase selective hydrogenation of crotonaldehyde was carried out over Pt/Ceo.8La0.2O1.9catalysts with different Pt contents prepared by an impregnation method. The catalysts were characterized by XRD, H2-TPR, NH3-TPD, and Raman spectroscopy. It was found that with increasing Pt loading in the Pt/Ce0.8La0.2O1.9, both the activity (TOF) and selectivity to crotyl alcohol first increased and then slightly decreased. The3Pt/Ce0.8La0.2O1.9],9catalyst showed the highest activity (TOF) and selectivity to crotyl alcohol in the hydrogenation of crotonaldehyde (4.01×10-3s-and53.6%, respectively). It was concluded that the catalytic performance of the catalysts depended on the surface acidity and the Pt particle size. Carbon deposition formed on the catalyst surface during the hydrogenaton process may account for the loss of catalytic activity and selectivity to crotyl alcohol.2. Effect of reduction temperature on selective hydrogenation of crotonaldehyde over Ir/TiO2catalystsIr catalysts supported on TiO2were prepared by an impregnation method and tested for gas phase selective hydrogenation of crotonaldehyde. The effects of reduction temperatures on the catalytic properties of Ir/TiO2catalysts were investigated. The catalyst reduced at100℃contained large amount of strong Lewis acid sites, which was detrimental to the reactivity because of the strong adsorption of crotonaldehyde molecules on the active sites. Also, the selectivity to crotyl alcohol was suppressed due to the interaction between C=C bond and the charge-deficient Ir species, which favored the formation of butanal. When the catalyst was reduced at high temperature, it was found that300℃was appropriate for high reactivity and selectivity, due to the moderate interaction between the C=O bond and the Ir species, as well as the weakened surface Lewis acid sites. However, higher reduction temperature (500℃) led to a strong bonding of carbonyl carbon and the charge-rich Ir species and suppresses the reactivity.3. Effect of Ir contents on selective hydrogenation of crotonaldehyde over Ir/TiO2catalystsVapor-phase selective hydrogenation of crotonaldehyde was carried out over Ir/TiO2catalysts with different Ir contents prepared by an impregnation method. The catalysts were characterized by XRD, H2-TPR, CO-DRIFTS, NH3-TPD, Raman spectroscopy and TPO. It was found that with increasing Ir content in Ir/TiO2, both the activity (TOF) and selectivity to crotyl alcohol first increased and then slightly decreased. The3Ir/TiO2catalyst showed the highest activity,(with a TOF of9.3×10-3s-1) and the highest selectivity to crotyl alcohol (74.6%) in the hydrogenation of crotonaldehyde. The results of CO-DRIFTS indicated that the reduced catalyst contains a mixture of Iro and Irδ+. It was concluded that the catalytic performance of the catalysts depended on the strength of surface acidity and the Ir particles size for the selective hydrogenation of crotonaldehyde to crotyl alcohol. All Ir/TiO2catalysts show severely deactivity process, which should be attributed to the decarbonylation and polymerization reactions.