Abstract:

MgO-modified ZSM-5 was prepared by impregnation of ZSM-5 with an aqueous solution of magnesium nitrate. Selective synthesis of p-xylene by alkylation of toluene with dimethyl carbonate（DMC）was carried out in a continuous flow fixed-bed rector over MgO-modified ZSM-5 zeolite. Physico-chemical properties of MgO/ZSM-5 catalysts were characterized by means of XRD，NH₃-TPD，2,4-dimethylquinoline-IR and pyridine-IR.The results of experiments indicated that alkylation of toluene with DMC occurred mainly on Brønsted acid sites. With increasing MgO content，there was a decrease in toluene conversion and increase in selectivity for p-xylene. With further increase in MgO content from 9% to 12%，no significant change in toluene conversion and selectivity for p-xylene was observed. The results of FT-IR characterization with 2,4- dimethylquinoline adsorption indicated that the MgO was only dispersed over the external surface of ZSM-5 zeolite. With increasing MgO content，the amount of Brønsted acid sites decreased. When the amount of MgO was >9%，Brønsted acid sites on the external surface almost disappeared.

摘要：

以乙酸镁为前体，采用等体积浸渍法制备不同负载量的MgO/ZSM-5催化剂，在气相连续流动固定床反应器上对甲苯与碳酸二甲酯（DMC）择形烷基化合成对二甲苯进行了研究。采用XRD、NH₃-TPD、吡啶吸附红外和2,4-二甲基喹啉吸附红外等手段对催化剂进行了表征。结果表明：B酸中心是甲苯与DMC烷基化反应的活性中心；随着MgO负载量的增加，甲苯转化率下降，对二甲苯的选择性上升；当MgO负载量大于9%时，甲苯转化率和对二甲苯选择性基本保持不变。2,4-二甲基喹啉吸附红外表征结果表明，乙酸镁在改性过程中未进入ZSM-5孔道内，焙烧分解生成的MgO主要分布在分子筛的外表面，分子筛孔内酸位并未受到影响。随着MgO负载量的增加，催化剂外表面的B酸位数量下降，当负载量大于9%时，外表面的B酸位基本消失。