Abstract
Molybdenum supported on zeolite HZSM-5 is the most studied catalyst for methane dehydroaromatization. However, the nature of its catalytic sites and their deactivation mechanisms remain unclear and controversial. Here, we report further insights into this system: first, the crystal size of HZSM-5 determines the form and location of the catalytic active MoCx species, and thus the catalyst performance; second, MoCx sites are preferentially deactivated over acid sites, when supported on nano-sized HZSM-5; third, MoCx particles that are traditionally considered detrimental to the reaction can serve as active sites, if they are properly protected from coke deposition. These findings lead us to develop an “encapsulation” strategy, which reconciles the deactivation rates of the MoCx and acid sites, enabling a full utilization of both sites, and consequently leading to a 10-fold increase in catalyst lifetime. Encapsulation also allows us to design experiments to confirm the catalytic role of the acid sites.
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The crystal size-dependent catalytic behaviors of Mo/HZSM-5 in MDA are explainedPreferential deactivation of MoCx sites over acid sites in Mo/HZSM-5 is observedEncapsulating MoCx particles in zeolite greatly increases the life of the catalystiDPC-STEM provides direct evidence for the migration and evolution of Mo species
Molybdenum supported on zeolite HZSM-5 (Mo/HZSM-5) is an effective bi-functional catalyst for methane dehydroaromatization (MDA) that allows direct conversion of methane into aromatics. In this work, Wang et al. determine the forms, locations, and deactivation sequence of the active sites in Mo/HZSM-5 and prepare more stable MDA catalysts accordingly.