水对改性HZSM-5催化联苯甲基化反应的影响

以纳米(n(SiO₂)/n(Al₂O₃)=26) 分子筛为母体, 采用水热处理、水热处理结合酸洗等方法对其进行改性, 并以联苯(BP)与甲醇的烷基化为探针反应, 在固定床反应器上考察了反应体系中生成的水及通入的水对改性HZSM-5催化性能的影响, 并通过吡啶吸附傅里叶变换红外(FTIR)光谱、热重(TG)分析对改性前后样品的酸性及反应后样品积炭量进行了表征. 结果表明, 水热处理及水热处理结合酸洗都提高了催化剂的稳定性,酸洗后效果更好. 在改性的HZSM-5 催化联苯甲基化反应体系中, 不通水的条件下, 联苯的转化率随时间呈抛物线变化; 通水后, 联苯转化率30 h 内稳定在8.6%左右, 4-甲基联苯(4-MBP)的选择性增加至约60%; 通水条件下升高反应温度至500℃, 联苯的转化率在反应30 h后达最高点, 选择性稳定在58%左右. 水的存在不仅改善了催化剂的活性及稳定性, 还能提高对位选择性.

Effect of Water on Biphenyl Methylation over Modified HZSM-5

Nanosized HZSM-5 (n(SiO₂)/n(Al₂O₃)=26) samples were hydrothermally treated with and without subsequent HNO₃ leaching. The performance of the samples for the alkylation of biphenyl (BP) with methanol to 4-methylbiphenyl (4-MBP) under fixed-bed down-flow conditions was investigated. Characterization was conducted by the adsorption of pyridine using Fourier transform infrared (FTIR) spectroscopy and thermogravimetric (TG) analysis. The effect of water on the catalytic performance of modified HZSM-5 was investigated. Both hydrothermal and combined hydrothermal-HNO₃ treatments improved catalytic stability, with the latter exhibiting better stability. Without the addition of water, BP conversion behavior resembled an open down parabolic vs reaction time over modified HZSM-5. However, this change disappeared upon the addition of water to the reaction system. Both catalytic stability and selectivity of 4-MBP were improved upon the addition of water. BP conversion after 30 h on stream was 8.6%, and the selectivity of 4-MBP was as high as about 60%. Elevating the reaction temperature to 500℃ in the presence of water led to increased BP conversion with time on stream up to 30 h, and the selectivity was stable at ~58%. The activity and stability of HZSM-5 were improved, and para selectivity was enhanced with the addition of water.