甲苯甲基化反应体系热力学分析

对生成二甲苯的甲苯与甲醇和甲苯与碳酸二甲酯反应体系进行了热力学分析，确定了二甲苯3种异构体选择性与其反应平衡常数或标准生成吉布斯自由能变的关系式。结果表明，在400~1000 K范围内，甲醇自身脱水、甲苯与甲醇甲基化、碳酸二甲酯(DMC)分解和甲苯与DMC甲基化反应都是自发进行的放热反应。从热力学角度比较，DMC甲基化选择性高于甲醇。反应达到平衡态时，间二甲苯(m-X)选择性最高；随着温度升高，间二甲苯选择性降低，邻二甲苯(o-X)选择性有所升高，对二甲苯(p-X)选择性稳定在22%左右。二甲苯3种异构体的选择性与反应物种类及其他产物种类无关。二甲苯异构体选择性的热力学计算结果与文献实验结果基本一致。

Thermodynamic Analysis of Toluene Methylation Reaction Systems

Thermodynamic analysis was carried out on the reaction system of toluene with methanol and toluene with dimethyl carbonate to produce xylene. Relationship between selectivity of the three xylene isomers and their reaction equilibrium constants or changes of standard Gibbs free energy formation were investigated. Thermodynamic results show that, from the temperature of 400 K to 1000 K, methanol self-dehydration, methylation of toluene with methanol, decomposition of dimethyl carbonate (DMC) and methylation of toluene with DMC are all spontaneous exothermic reactions. Thermodynamically, methylation selectivity of DMC is higher than that of methanol. In equilibrium state, m-xylene selectivity is higher than other isomers. When reaction temperature increases, m-xylene selectivity will be lower but o-xylene selectivity will be higher, and p-xylene selectivity maintains at about 22%. Selectivity of the three xylene isomers is independent of reactants and products. Thermodynamic calculation results of selectivity of xylene isomers are consistent with experimental results in literature.