Pressurized Dehydrocondensation of Methane toward Benzene and Naphthalene on Mo/HZSM-5 Catalyst: Optimization of Reaction Parameters and Promotion by CO2 Addition

The rate of benzene formation with time on stream is remarkably stabilized under pressurized condition in the dehydrocondensation reaction of methane. Under a methane pressure of more than 0.2 MPa and a suitable CO2 concentration in methane feed, the high activity and stability were achieved at elevated reaction temperatures of more than 1023 K and methane space velocities of more than 5400 ml/(g·h) due to minimized coke formation on the Mo/HZSM-5 catalyst. A high and stable benzene formation rate of 4000 nmol/ (g-cat·s) was obtained on the 6% Mo/HZSM-5 catalyst under the optimized reaction conditions, e.g., 1073 K, 0.3 MPa, and 7200 ml/(g·h), and with 5% CO2 addition. A temperature-programmed oxidation study demonstrated that both addition of CO2 to the methane feed and increasing of the methane pressure greatly suppressed the deposition of coke on the Mo/HZSM-5 catalyst. Moreover, the H/C ratio of the coke on the catalyst increased due to CO2 addition and methane pressure increment, implying that the surface carbon had relatively high hydrogen content at the prevailing reaction conditions. It suggests that the added CO2 reacted with coke before its transformation to inactive graphitic carbon blocking the microporous channels of HZSM-5 zeolite, which is more detrimental for methane dehydrocondensation reaction.