Experimental investigation of the promotion effect of CO on catalytic behavior of Mo/HZSM-5 catalyst in CH4 dehydroaromatization at 1073 K

Addition of a small amount of CO into CH4 feed was previously observed to remarkably improve the activity, stability and lifetime of Mo/HZSM-5 catalyst for methane non-oxidative dehydroaromatization (Ohnishi et al., J. Catal., 182 (1999) 92). However, at an elevated reaction temperature of 1073 K the present study shows that CO addition has little improving effect on extending the catalyst lifetime whereas it indeed helps increase the catalytic activity and stability of a 5%Mo/HZSM-5 catalyst in the benzene selectivity-stable period. The experimental data obtained at different CH4 space velocities and different CO addition contents as well as DFT calculations based on CO dissociation on various sites all suggest that CO in CH4 feed only functions to protect MoCx sites from being deactivated by coke deposition and plays no role in suppressing coke accumulation on acidic sites and/or Si sites inside the zeolite channels. Thus, it becomes obvious that at 1073 K the gradual narrowing and blocking by coking of zeolite channels, not the deactivation of Mo sites, mainly controls the catalytic stability and lifetime of Mo/HZSM-5. This work clarifies a key issue of designing a highly coke-resistant Mo/HZSM-5 catalyst lying in suppressing the formation of internal coke in the zeolite.