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 HNO₃leaching. 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-HNO₃treatments 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 °C 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.