Hydrogenation activity of Tungsten Carbides Preopared by CVD Method

本课题研究了CVD法制备碳化钨及其在加氢催化剂中的应用。六氟化钨与环烃在活性炭表面反应生成高比表面积的碳化钨。碳化物中的主要物质为WC和W_2C，前者对正丁烷加氢裂化有效。加氢裂化的碳化物的周转频率几乎与氧化铝负载的铂催化剂相同。但由于碳化物上的有效物种浓度较低，以单位催化剂质量计算的反应速率小于铂催化剂，碳化钨催化剂对乙烯加氢的催化活性与铂催化剂和镍催化剂基本相同。在铂和镍催化剂上的氢化反应速率随反应温度高于220”*“C而降低，但碳化物的催化活性在高达350”*“C时增加。这是碳化钨的非常独特的性质。以环己烯为催化剂，考察了制备条件对环己烯转化率和产物选择性的影响。反应的主要产物是由环己烯异构化产生的1-甲基-环戊烯。异构化反应表明该碳化物为固体酸催化剂。碳化物在700 ℃氢气氛中处理后，随着处理时间的延长，其加氢活性逐渐恢复。该结果表明，与碳化钨同时生成的氧化钨显示出固体酸性质。

The preparations of tungsten carbide by CVD method and the application of the carbides to hydrogenation catalyst have been studied in the present project. The tungsten hexafluoride reacted with cyclo-hydrocarbon on an activated carbon surface resulted in tungsten carbides with high surface area. Main substances in the carbides were WC and W_2C.The former was effective for hydrocracking of n-butane. The turnover frequency of the carbides for the hydrocracking was almost the same as that of a platinum catalyst supported on alumina. However, the reaction rate calculated from unit catalyst mass of the carbide catalyst was smaller than that of the platinum catalyst due to the low concentration of effective species on the carbides.The catalytic activity of the tungsten carbide for hydrogenation of ethylene was almost the same as those of platinum catalyst and nickel catalyst. The reaction rate of the hydrogenation over platinum and nickel catalysts decreased with reaction temperature higher than 220"*"C, but the catalytic activity of the carbides increased up to 350"*"C.This is very unique property of the tungsten carbides. The reaction of cyclohexene was carried out over the carbides to elucidate the effect of preparation conditions on the conversion of cyclohexene and the selectivity of the products. The main product for the reaction was 1-methyl-cyclopentene produced by isomerization of cyclohexene. The isomerization indicated that the carbide was solid acid catalyst. When the carbides was treated in hydrogen atmosphere at 700"*"C, it was found that the hydrogenation activity was gradually recovered with treating period. This result suggested that a tungsten oxide simultaneously produced with tungsten carbide showed the solid acid property.