(6bt) Direct and Non-Oxidative Conversion of Methane to Value Added Products

Global energy demand has been continuously increasing at a rapid rate and tremendous research is being done to develop new sources of energy supply. The low cost of natural gas (predominantly contains methane) has created an interest in its conversion to high value chemicals and fuels. This methane can be converted into valuable chemicals by both direct and indirect routes. The indirect conversion of methane by Fisher-Tropsch (FT) to methanol followed by methanol-to-gasoline (MTG) is currently a more commercially viable process than the direct routes. Direct routes have barriers to commercialization including limited conversions, poor selectivity to desired products and catalyst deactivation.

My research has provided me an excellent learning opportunity to understand and work in one of the most interesting and highly debated problems in the field of heterogenous catalysis - catalytic conversion of methane under non-oxidative conditions. My research is focused on addressing the challenges faced in two different processes - i) Methane dehydroaromatization (MDA), a direct route to produce aromatics, primarily benzene by non-oxidative conversion of methane and ii) Catalytic decomposition of methane (CDM), a technique that involves production of high purity (CO_x free) hydrogen directly from methane.

The mechanism of MDA is still in debate and this work is focused on understanding the fundamental nature of catalysts used in this reaction through decoupling of the Mo/HZSM-5 catalysts to better understand the contribution of each component. The reaction mechanism was therefore studied by activating methane over Mo/SiO₂ and Mo/HZSM-5. The fresh and the spent catalysts were analyzed using BET, SEM, XRD and TGA to understand surface properties before and after the MDA reaction. The effect of H₂ pretreatment on the nature of active site formation and on induction time for benzene formation was investigated in detail. Results showed that H₂ reduction at 750 ^oC helped in the formation of metallic Mo sites which turned into active Mo₂C sites during methane activation and formed ethylene and aromatics products without any induction time. The significance of zeolites (porous catalyst materials) in MDA is revealed by decoding the mechanism of MDA. Further, our research involved in improving the performance of Mo/HZSM-5 catalyst by addition of promoter metals. A second metal (Fe or K or Rh) was added to Mo/HZSM-5 and its influence on methane conversion and benzene selectivity was studied.

Another part of this research includes the effect of different catalyst materials and their lifespan on hydrogen (% yield) produced from the methane decomposition reaction. A rare mixed metal oxide supported Fe catalyst (Fe/CeZrO₂) was subjected to methane activation and its activity for hydrogen production was investigated. Influence of a second metal (Co & Mo) and the effect of aging of catalyst after several cycles of regeneration on hydrogen yield was also studied. Addition of Co or Mo to the Fe catalyst has increased the H₂ yield by ~35% compared to the unpromoted Fe catalyst.

Future research direction:

• Perform molecular level studies (density functional theory) to support the experimental claims on Mo and Fe based catalysts used for non-oxidative methane conversion.
• Synthesis of zeolite-based catalysts to study their catalytic properties to design novel catalysts for different reaction pathways.

Teaching Interests:

Right from the early days of my bachelor's program, teaching has always been my passion. During my Undergraduate studies, I have been a math tutor for high school students and fellow undergraduate students. This tutoring experience made me realize that teaching could be my profession. As a teacher, I believe it is my responsibility to train students to develop their critical thinking and problem-solving ability. Explaining the fundamental concepts in a simple way and having lectures in an interactive way help students to develop interest in the subject. During my PhD, I have mentored 3 undergraduate students who worked with me on my research project. In addition to providing training in technical aspects, I also guided them in developing their soft skills including creative problem solving and presentation skills.

I am interested in teaching Chemical Engineering core courses including reaction kinetics, heat and mass transfer, process calculations, fluid mechanics, crude oil refining and industrial catalysis.

Awards and achievements:

• Peer reviewed journals: 2 published (first author)
• Conference presentations: 8
• Champan scholarship award, The University of Tulsa (2017)
• Honorary mention, TU Research colloquium, The University of Tulsa (2017, 2019)
• Bellwether Fellowship award, The University of Tulsa (2018)
• Graduate student Research grant, The University of Tulsa (2018)

Peer reviewed publications:

• Ramasubramanian, V., Ramsurn, , & Price, G. L. (2018). Methane dehydroaromatization–A study on hydrogen use for catalyst reduction, role of molybdenum, the nature of catalyst support and significance of Bronsted acid sites. Journal of Energy Chemistry. (https://doi.org/10.1016/j.jechem.2018.09.018)
• Ramasubramanian, V., Lienhard, D.J., Ramsurn, H., & Price, G. L. (2019). Effect of addition of K, Rh and Fe over Mo/HZSM-5 on methane dehydroaromatization under non-oxidative conditions. Catalysis letters. (https://doi.org/10.1007/s10562-019-02697-8)

Selected presentations:

• The Role of Molybdenum during the Conversion of Methane to Liquids Under Non- Oxidative Conditions, Vaidheeshwar Ramasubramanian., et al., Annual AIChE meeting, Minneapolis MN, 2017
• Effect of Addition of Promoters to the Mo/HZSM-5 during the Non-Oxidative Conversion of Methane to Aromatics, Vaidheeshwar Ramasubramanian., et al., Annual AIChE meeting, Pittsburgh, PA, 2018