(370g) Engineering Fe-HZSM-5 for Methane Dehydroaromatization

The abundance of domestic natural gas reserves has resulted in increasing interests in generating higher value chemicals from methane, the main component of natural gas. While methane upgrading is currently achieved through an indirect route via a syngas intermediate, direct upgrading offers conceptually a less energy intensive and costly alternative. One promising direct upgrading route is the non-oxidative conversion of methane to aromatics via dehydroaromatization (DHA). The reaction is typically conducted over catalysts composed of metal nanoparticles dispersed in zeolites, and zeolite channel size, the concentration of Brønsted acid sites, types of metal, and preparation methods are all known to influence the activity and selectivity for methane DHA. However, at the very high temperatures required for this reaction to proceed, carbon deposition becomes inevitable, results in rapid deactivation of the catalyst, and thus renders the process uneconomic.

Previous work in our group shows that atomically dispersed Fe could suppress coke formation on the metal cites while increase product selectivity toward benzene. In order to further mitigate coking on the zeolite acid site, here we investigated the effect of diffusion within ZSM-5 channel on minimizing undesired secondary reaction by tuning zeolite particle size and mesoporosity. We find that zeolite particle size can be controlled via the template/silica ratio, which also gives rise to a change in solution pH due to the alkalinity of tetrapropylammonium hydroxide solution (TPAOH) used as structure directing agent. The increase in alkalinity leads to higher mesoporosity. Balancing the pH by adding sodium hydroxide in turn affects the amount of sodium present during synthesis, which is also found to influence the mesopore volume of the sample. Careful and independent control of alkalinity, sodium amount, and TPAOH-to-silica ratio is hence required to isolate the effect of particle size and/or mesoporosity on activity and selectivity of the catalyst. Increasing mesoporosity is found to result in a strong increase in benzene selectivity and yield, while a change in particle size alone does not have a comparable impact on reactivity. This study hence demonstrated the effects of particle size and mesoporosity on the catalytic performance of Fe-ZSM5 in methane DHA and the ability to tune those parameters by carefully controlling template to silica ratio, alkalinity, and sodium amount, providing further insight into catalyst design.