用催化剂表面修饰以进行苯选择加氢制环己烯的研究

1 前言常规的气相苯催化加氢反应,苯环大π键一经打开就全部加氢到底,产物中只能获得环己烷而极难得到选择加氢产物环己烯。生成环己烷的反应从热力学上看远比生成环己烯的反应容易进行很多,并且环己烯也非常容易进一步加氢生成环己烷。但催化剂的表面经修饰剂作用后可根本改变其性能,从而改进催化活性及选择性,或实现常规方法不可能实现的反应,获得不易得到的产物。在经表面修饰的催化剂上进行苯加氢反应可获得选择加氢产物环

A STUDY ON THE SELECTIVE HYDROGENATION OF BENZENE TO CYCLOHEXENE BY CATALYTIC SURFACE MODIFICATION

The surface modification of Ru and Ru/Al_2O_3 catalysts through the pre-adsorbed modifiers have been studied with the model reaction of benzene selective hydrogenation to cyclohexene.The Ru catalyst was prepared by the direct reduction of RuCl_3 with hydrogen.The Ru/Al_2O_3 catalyst was prepared by the impregnation of A1l_O_3 support with RuCl_3 solution,in which the Ru content was 1%(weight).It has been found that only when the surface of the above two catalysts have been pre-adsorbed with some modifiers,such as water,alcohols and so on,the catalytic hydrogenation of benzene will show some selectivity to cyclohexene.However,with different modifiers,the selectivity to cyclohecene will be different.For example,if water is used as modifier,only 7%(mol)selectivity to cyclohexene could be obtained,while with ethylence glycol as the modifier 40%(mol)selectivity to cyclohexene will be given.Several modifiers such as water,C_1-C_5 primary alcohols,C_2-C_4 glycols and so on have been used in this investigation.It was found that glycols could give high selectivity to cyclohexene(about 40%),but lead to a serious loss of catalytic activity.C_3-C_5 primary alcohols can give not only high selectivity to cyclohexene but also good activity.After careful modification of the surface and under proper reaction eonditions, almost 20%(mol)yield of cyclohexene can be obtained by the gas phase hydrogenation of benzene on Ru catalyst.On the other hand,high selectivity on modified Ru/Al_2O_3 catalyst could only be obtained at low conversion of benzene,because the high surface area of Ru/Al_2O_3 and long inner pores lead to further hydrogenation of cyclohexene to cyclohexane.It was suggested that the addition of modifiers to the catalysts surface can not only decrease the rate of further hydrogenation of cyclohexene,but also increase the desorption rate of cyclohexene.