| ?,?-Unsaturated alcohols are an important kind of fine chemical products.They are usually prepared from ?,?-unsaturated aldehydes/ketones by selectively hydrogenating C=O bonds to alcohols while retaining the C=C bond.Since the bond energy of the C=C bond is less than that of the C=O bond,the former is more easily hydrogenated in the thermodynamics.Therefore,it is difficult to achieve high selectivity for the catalytic hydrogenation of ?,?-unsaturated aldehydes to unsaturated alcohols.Acrolein and crotonaldehyde are the most difficult to produce unsaturated alcohols.The Ag-based catalyst has a high selectivity for the hydrogenation of the C=O bond of ?,?-unsaturated aldehydes,but also has the problem of low activity and easy deactivation.In this study,a series of monometallic silver catalysts supported with silica and modified silica were prepared by the impregnation method.The catalytic performance for the selective hydrogenation of crotonaldehyde under atmospheric conditions was also evaluated.The effects of factors including temperature,hydrogen-aldehyde ratio,airspeed and others were examined in details.The catalyst was characterized by XRD,TEM,TPR,XPS,NH3-TPD,O2 chemisorption and etc.The structure-activity relationship and inactivation mechanism of Ag-based catalysts have been investigated.The study found that:1.The silver-based catalyst had a significant carrier effect in the gas phase selective hydrogenation of crotonaldehyde.The silver particles attached to the surface of the carrier could exhibit the performance of selective hydrogenation to crotyl alcohol.The silica carrier was significantly superior to zirconia and alumina;2.The silica-supported silver catalyst exhibited non-structural sensitivity in the selective hydrogenation of crotonaldehyde.The size of silver nanoparticles had less effect on the selectivity of crotyl alcohol in the case of greater than 3 nm;3.The surface modification of silica with ZrO2 could significantly improve the catalytic performance of the Ag-based catalyst for the crotonaldehyde hydrogenation to crotyl alcohol.The Ag/Zr-SiO2 catalyst had better hydrogenation activity and crotyl alcohol selectivity than those of the unmodified Ag/SiO2 catalyst,and the stability was also improved.The selectivity of crotyl alcohol with the optimized catalyst 30Ag/5Zr-SiO2 at a higher conversion of crotonaldehyde(73.9%)could reach over 80%which was significantly higher than the level of Ag/SiO2 catalyst(<70%);4.After the modification of the surface of silica by zirconia,the number of acidic sites was obviously increased which was beneficial to the surface adsorption and dispersion of silver in the case of the catalyst was prepared by impregnation method.It also made the obtained silver catalyst had obviously improved metal dispersibility,thereby increasing the hydrogenation activity;5.The supported silver catalyst had an oxygen chemical adsorption capacity at 323 K,and the amount of oxygen adsorption had a positive correlation with the catalyst hydrogenation activity which indicated that these oxygen adsorption sites were closely related to the catalytic active sites;6.Based on the carrier effect and unique oxygen adsorption capacity exhibited by supported silver-based catalysts,it was assumed that the catalytic active sites may fall on the interface formed between the silver particles and the support.The silica was modified with the introduction of zirconia to form a special Ag-ZrO2-SiO2 ternary interface structure where Ag species on the interface tend to have a positive charge.This was favorable for the adsorption and hydrogenation of C=O bonds,thereby the production of target product crotyl alcohol was promoted;7.The supported silver catalyst was deactivated in the gas phase of crotonaldehyde hydrogenation.It is not obvious for the aggregation of silver particles during the reaction.The reason for the deactivation is mainly the surface coverage caused by the side reactions and the irreversible changes in the interface structure caused by the reaction atmosphere. |
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