| a, p-unsaturated alcohols are important raw materials and widely applied in medicine and fine chemicals, which could be catalytically synthesized by selective hydrogenation of α, β-unsaturated aldehyde because it is a environmental-friendly reaction. In this work, Pt-based and Ir-based catalysts were prepared and tested for gas phase selective hydrogenation of crotonaldehyde as a model reaction. It was found that good catalytic activity and high selectivity to desired crotyl alcohol could be obtained over these catalysts. The catalysts were characterized using X-ray powder diffraction analysis (XRD), X-ray fluorescence diffuse reflectance (XRF), CO chemisorption characterization, NH3temperature-programmed (NH3-TPD), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) techniques and the catalytic performance was correlated to the catalyst properties. In addition, the machenism of catalyst deactivation was investigated. The detailed contents are as follows:1. Effects of Ir content on selective hydrogenation of crotonaldehyde over Ir/ZrO2catalystsA series of Ir/ZrO2catalysts with different Ir contents were tested for gas phase selective hydrogenation of crotonaldehyde. It was found that the reactivity of the catalyst increased with increasing Ir content, but rapid deactivation was observed on all the catalysts during the reaction, which might be due to CO poisoning generated by decarbonylation reaction. The highest selectivity (82.2%) to crotyl alcohol was obtained over a catalyst with3%Ir content, which was attributed to the proper Ir particle size and the highest amount of surface acid sites in the catalyst 2. Effects of reduction temperature on Ir/ZrO2catalyst for selective hydrogenation of crotonaldehydeThe Ir catalyst supported on ZrO2was prepared by impregnation, and the effect of reduction temperature on its catalytic performance for hydrogenation of crotonaldehyde in gas phase was tested. With increasing reduction temperature, the crotonaldehyde conversion and selectivity for crotyl alcohol over the3Ir/ZrO2catalyst first increase and then decrease. The catalyst reduced at400℃exhibits a highest crotonaldehyde conversion, reaching a value as high as32.2%, and the selectivity for ctotyl alcohol is74.3%. The results of X-ray photoelectron spectroscopy indicate that both Ir0and Ir3+species coexist on the catalyst surface coexisted after reduction of the catalyst at400℃. With increasing reduction temperature, the ratio of Ir0/Ir3+increases, and the surface Ir exists mainly as Ir0species when the reduction temperature is600℃. Additionally, NH3temperature-programmed desorption indicates that the amount and intensity of Lewis acid sites show a downward trend with increasing reduction temperature, which is attributed to the decline of Cl element in the catalyst. Therefore, it is concluded that the coexistence of Ir0and Ir3+and moderate-intensity surface Lewis acid help improve the yield and selectivity of crotyl alcohol.3. A comparative study on Pt/CeO2and Pt/ZrO2catalysts for crotonaldehyde hydrogenationVapor-phase hydrogenation of crotonaldehyde was carried out over Pt/CeO2and Pt/ZrO2catalysts. It was found that both catalysts suffered deactivation, due to the formation of organic compounds on the catalyst surface as revealed by temperature programmed oxidation technique on the spent catalysts, and the poisoning effect of strong CO chemisorptions on Pt atoms via decarbonylation reaction. For the Pt/ZrO2catalyst, selectivity to crotyl alcohol kept stable during the reaction, while for the Pt/CeO2catalyst, the selectivity to crotyl alcohol decreased dramatically. The decrease of selectivity for the Pt/CeO2catalyst was attributed to carbon deposit formed on the catalyst surface by the reaction between CO and reduced Ce3+ion in CeO2. |
PDF Full Text Request