Vapor Phase Selective Hydrogenation Of Crotonaldehyde On Ir Catalysts

α, β-unsaturated alcohols are widely applied in the synthesis of fine chemicals such as pharmaceutical, agrochemical, and fragrance compounds, which are mainly obtained by selective hydrogenation of α, β-unsaturated aldehydes. Crotonaldehyde is a typical representative of α, β-unsaturated aldehyde and the hydrogenation prefers to C=C bond compared with C=O bond to obtain saturated aldehyde. Thus it is challenging to improve the selectivity to unsaturated alcohols. Selective hydrogenation of crotonaldehyde to obtain crotyl alcohol on heterogeneous catalysts is an great synthesis of green chemistry. In this work, supportd Ir catalysts were prepared using an impregnation method to test their catalytic performance of selective hydrogenation of crotonaldehyde. The catalysts were characterized by H2temperature-programmed reduction (H2-TPR), CO chemisorption, Transmission electron microscope (HRTEM), NH3temperature-programmed desorption (NH3-TPD), Fourier transform infrared spectroscopy (FTIR), Temperature-programmed oxidation (TPO) and so on. It was found that the Ir-based catalysts obtained better catalytic activity and selectivity to crotyl alcohol. The detailed results are as follows:1. Selective hydrogenation of crotonaldehyde over supported Ir catalysts:Effect of surface acidity on catalyst deactivationIr catalysts supported on various oxides (TiO2, ZrO2, CeO2, SnO2, and La2O3) were prepared by an impregnation method and applied for selective hydrogenation of crotonaldehyde. It was found that the catalytic behavior strongly depended on the surface acidity of the catalysts. The highest activity and selectivity were obtained on the Ir/TiO2catalyst, with a crotonaldehyde conversion of43.2%and a selectivity to crotyl alcohol of80.9%at80℃. However, all the catalysts deactivated during the reaction due to polymeriation of crotonaldehyde molecules on the surface of the catalysts, which were evidenced by temperature-programmed oxidation and FTIR data of the used catalysts. It was also found that the deactivation rates were related to the amount of surface acid sites on the catalysts. The Ir/CeO2catalyst with the highest amount of surface acid sites deactivated more rapidly than the Ir/La2O3catalyst with the lowest amount of surface acid sites.2. Effect of Ir loadings on Ir/SiO2catalysts for selective hydrogenation of crotonaldehydeSelective hydrogenation of crotonaldehyde was conducted over Ir/SiO2catalysts prepared by an impregnation method to investigate the effects of Ir loading on the catalytic behaviors. It was found that the particle size of Ir in the catalyst increased with Ir loading, being2.1,3.9and6.7nm for the lIr/SiO2,3Ir/SiO2and5Ir/SiO2catalyst, respectively. The Ir species on the Ir/SiO2catalysts consisted of Ir0and Ird+, but the average valence of Ir species decreased with increasing Ir loading. Also, catalytic testing results revealed that the reactivity of the catalyst increased with Ir loading. It was found that the conversion of crotonaldehyede and the selectivity to crotyl alcohol gradually increasing during the reaction, and eventually reaching a steady state. The3Ir/SiO2catalyst got the highest selectivity (77.6%) to crotyl alcohol, and the conversion increased gradually to15.6%. The catalytic behavior of these stable catalysts could be attributed to the proper Ir particle size, the existence of Ir0and Ird+species on the surface, and surface acid sites.3. Effect of Fe modification over Ir/SiO2catalyst for selective hydrogenation of crotonaldehydeFe modified Ir/SiO2catalysts were prepared by an impregnation method, and then tested for selective hydrogenation of crotonaldehyde. In the reaction, the effect of Fe contents on the catalytic properties of Ir/SiO2catalysts was discussed. It was found that all the Fe/Ir/SiO2catalysts showed better reactivities and selectivity to crotyl alcohol than the Ir/SiO2catalyst. With increasing Fe/Ir ratio, the reactivities and selectivity to crotyl alcohol of Fe/Ir/SiO2catalysts first increased and then decreased. When the Fe/Ir molar ratio was0.1, the conversion of crotonaldehyde was36.9%, and the selectivity to crotyl alcohol was83%. However, the Fe modified Ir/SiO2catalysts suffered deactivation, and the deactivation rate increased with increasing Fe/Ir ratio. It was found that the catalyst deactivation could attribute to the deposition of organic compounds over the catalysts which could block the active sites and consequently suppress the activity. The catalytic behavior of these catalysts could be attributed to the Ir particle size, the existence of Ir0and Ird+species on the surface, and Lewis acid sites in these catalysts.