CAREER: Tailoring the Synergy between Catalyst Design and Reaction Engineering for Direct Conversion of Methane to Aromatics

The vast abundance of methane in natural resources makes it an attractive chemical feedstock for conversion into higher hydrocarbon fuels and bulk chemicals of industrial interest. However much of the natural gas is found in stranded locations and is flared due to high transportation and processing costs. Direct transformation routes of methane into liquid chemicals can reduce processing costs but are currently not economically viable due to unanswered scientific challenges.

The proposal targets catalytic methane dehydroaromatization (MDA) for the direct conversion of methane to benzene and hydrogen, both mainstays in the chemicals industry. The proposed study seeks to integrate catalysis and reaction engineering to overcome technological challenges arising during MDA. The research results will be integrated with educational and outreach activities.

Two major challenges currently prevent the implementation of MDA at an industrial level: (1) thermodynamic limitations leading to low methane equilibrium conversion; (2) rapid catalyst deactivation. While processes involving removal of the hydrogen product from the reaction medium are successful in shifting the equilibrium to the right and increasing methane conversion to benzene, they also accelerate coking.

Thus, catalyst deactivation in MDA is a ubiquitous issue that must be addressed. Molybdenum (Mo) oxide supported on HZSM-5 zeolite is the most commonly studied catalyst for the MDA reaction. It is agreed that during an induction period Mo oxide species transform to Mo carbide species that are responsible for the conversion of methane to aromatics.

Adding oxidant co-reactants can help the process thermodynamics, however, only low oxidant concentrations can be used since the active Mo carbide phases will otherwise be oxidatively destroyed. At low oxidant concentrations benzene yields are improved but the oxidant distribution in a packed-bed reactor (PBR) is not even, thus kinetic measurements are not representative of the entire catalyst bed.

Distributed-feed membrane reactors (DFMR) can overcome the reactor heterogeneity problem and increase benzene yields, but catalyst coking is not fully prevented. The PI has discovered a method to increase catalyst resistance to coking by preparing Mo carbide phases ex situ in the presence of a second metal X (X= Fe, Co, or Ni), but the nature of the Mo-X interaction remains unknown.

The proposed project includes studies of stable Mo-X/HZSM-5 catalysts in a DFMR with an oxidant (oxygen or carbon dioxide) as co-reactant to determine the role of Mo-X-C interactions on the reaction and catalyst deactivation pathways. Model catalysts will be prepared by ensuring that the metals (Mo and X) are located inside the zeolite channels by blocking the anchoring sites on the external surface.

The structure, location and evolution of the Mo-X phases will be monitored by in situ and Operando experiments using advanced characterization techniques, including X-ray absorption and high-resolution powder diffraction. DFMR operating conditions will be tuned to ensure even axial distribution of the oxidant and maximum enhancement in benzene yield.

Kinetic testing will be performed with the DFMR and compared to a PBR as reference. The combination of the kinetic tests, in situ structural characterization and theoretical calculations will result in the determination of the reaction and deactivation pathways of MDA in the presence of oxidants and will provide the basis for the rational design of catalysts tailored for a DFMR.

The PI will develop two new graduate courses focused on an integrated interdisciplinary approach to catalysis and an interdisciplinary Science, Technology, Engineering, Art, Mathematics (STEAM) project with the School of Theatre and Dance at Texas Tech University to use storytelling to increase the awareness of the public on the importance of science and engineering and attract students into STEM careers. The stories will be propagated in various formats including theatre performances at local schools, digital stories and podcasts posted on a YouTube channel.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.