碱金属改性对Pd﹣Au／Al 2 O3催化剂上异戊二烯选择性加氢性能的影响

摘要

采用等体积浸渍法制备了不同K2 O、Li2 O 含量的 Pd﹣Au∕Al2 O3催化剂，采用低温氮吸附法、吡啶吸附红外光谱、程序升温还原（H2﹣TPR）等方法进行了表征研究。吡啶吸附红外光谱表征结果表明，催化剂表面只有L酸，无B酸，K2 O改性比Li2 O改性催化剂的表面酸性弱。H2﹣TPR表征结果表明，催化剂表面形成了Pd﹣Au二元合金，加入碱金属改性后，催化剂的还原峰向高温方向移动。以异戊二烯的模型化合物为原料评价了自制的改性Pd﹣Au∕Al2 O3催化剂的选择加氢性能，结果表明：催化剂经过Li2 O 改性后，异戊二烯的转化率和单烯烃选择性提高，而用K2 O改性时，异戊二烯的转化率降低，单烯烃选择性提高；采用K2 O质量分数为1．2％的催化剂，在反应温度为70℃时异戊二烯的转化率为90．0％，单烯烃的选择性为86．0％。%A series of Pd﹣Au∕Al2 O3 catalysts modified by different amount of alkali metal K or Li were prepared by incipient wetness impregnation and characterized by low temperature nitrogen adsorp﹣tion,FT﹣IR and H2﹣TPR techniques. The isoprene was used to evaluate the selective hydrogenation performance of the modified catalysts. The FT﹣IR results show that only Lewis acid sites exist on the catalysts,no Bronsted acid is found. The acid strength of K﹣modified catalyst is weaker than Li﹣modi﹣fied catalyst. The H2﹣TPR results indicate that Pd﹣Au binary alloy is formed,the reduction peaks of catalysts shift to higher temperature after modified by alkali metals. The reaction results of isoprene se﹣lective hydrogenation on the modified Pd﹣Au∕Al2 O3 catalysts show that the isoprene conversion and iso﹣pentenes selectivity increase when addition of K2 O. The LiO modification reduces the conversion of iso﹣prene but isopentenes selectivity increases. At 70 ℃,isoprene conversion reaches 90. 0%,isopentenes selectivity is 86. 0% on the catalyst Pd﹣Au∕Al2 O3 with 1. 2% K2 O.