KOH@NaX酸碱性调控及对甲苯甲醇侧链烷基化性能的影响

Acid-Base Control of KOH@NaX and Its Catalytic Performance in Side-Chain Alkylation of Toluene With Methanol

通过固-液连续混加法制备了KOH改性的NaX系列催化剂,并采用X射线衍射(XRD)、N_(2)物理吸附-脱附、傅里叶红外光谱(FT-IR)、扫描电子显微镜(SEM)、X射线能谱(SEM-EDS-Mapping)、CO_(2)-TPD、NH_(3)-TPD等表征手段对其进行分析,考察KOH改性的NaX系列催化剂在甲苯甲醇侧链烷基化反应中的催化性能。结果表明:KOH的均匀负载有利于增加NaX分子筛上强碱性位,同时抑制了中强酸性位;显著提高了甲醇转化率和苯乙烯与乙苯的总选择性,同时降低了甲烷和二甲苯的总选择性。在常压、425℃、质量空速1.0 h^(-1)、甲苯/甲醇摩尔比为5的条件下,当KOH的负载质量分数为7%时,甲醇转化率、苯乙烯和乙苯的总收率分别为99.81%和97.82%。通过对甲苯甲醇侧链烷基化反应机理的研究发现,甲醇在强碱性位和中强酸性位上存在一对竞争反应:甲醇在强碱性位上脱氢生成甲醛,进而与被活化的甲苯上的甲基反应生成苯乙烯和乙苯;而在中强酸性位上甲醇既可以加氢生成甲烷,也可以与甲苯发生苯环烷基化反应生成二甲苯。

KOH@NaX catalysts with different mass fractions of KOH loading were prepared by continuous solid-liquid mixing method,and characterized by XRD,N_(2) physical adsorption and desorption,SEM,SEM-EDS-Mapping,CO_(2)-TPD,NH_(3)-TPD etc,and the corresponding catalytic performance was evaluated by side-chain alkylation of toluene with methanol.It is found that the uniform loading of KOH is benefit for increasing the amount of strong base sites and thus decreasing the amount of medium acid sites,by which methanol conversion and the total selectivity of styrene and ethylbenzene are significantly enhanced and while the total selectivity of methane and xylene are reduced.When the reaction temperature of 425℃,n(Toluene)/n(Methanol)of 5,MHSV of 1.0 h^(-1),KOH mass loading of 7%,the conversion of methanol and the total yield of styrene and ethylbenzene reach 99.81%and 97.82%in maximum,respectively.Based on reaction mechanism,it is concluded that methanol has two competitive reactions over the strong base sites and the medium strong acid sites,respectively.That is,methanol is dehydrogenated to form formaldehyde over the strong base sites,and then reacts with the methyl of activated toluene to form styrene and ethylbenzene.In contrast,methanol is hydrogenated to methane or alkylated with toluene to generate xylene over the medium strong acid sites.