Development of catalysts for direct non-oxidative methane aromatization

Abstract

Direct catalytic conversion of methane to benzene at non-oxidative condition is considered as one of key reactions for constitution of sustainable carbon-cycling processes, since either biomethane or CO₂-based synthetic methane can serve as its feed source. While this concern may motivate many researchers over the world to make their continuous effort to gain deep insight into the catalytic mechanism of this catalysis system and the essential cause of the catalyst deactivation, successful development of a catalyst with high performance, enhanced coking resistance and long-term operating stability will be the key to its industrial application. Here in this review paper, we demonstrate the high catalytic activity and stability of our two shaped Mo/HZSM-5 catalysts developed respectively for fixed-bed and fluidized-bed operations at severe reaction conditions. Thermodynamically, a possibly high aromatization temperature is required to attain a desired high benzene formation rate, but adopting such a temperature will certainly accelerate coke formation and catalyst deactivation. Therefore, the focus of the catalyst development was laid on finding various effective ways of suppressing coke accumulation and catalyst deactivation at practically required severe reaction conditions, and much effort was made to attain the purpose. As a result, a highly active and selective pelleted Mo/HZSM-5 catalyst has been successfully developed and was stably run in a fixed-bed reactor under cyclic regeneration operation mode over 1000 h. In parallel a binder-free, fluidizable Mo/HZSM-5 catalyst with certain mechanical strength has also been developed and successfully tested in a dual circulating fluidized-bed reactor system to provide a stable benzene yield of about 12% at 1073 K and 3000 ml/g/h space velocity.