Degree

Doctor of Philosophy (PhD)

Department

Chemical Engineering

Document Type

Dissertation

Abstract

One of the most challenging aspects of modern-day catalysis is the conversion of methane. Direct conversion of methane via dehydroaromatization (MDHA) is a well-known process which can produce valuable hydrocarbons. Mo oxide supported on ZSM-5/MCM-22 has been studied extensively in recent years for MDHA. Mo carbides are responsible for activating methane by forming CHx species. These are dimerized into C2Hy and oligomerized on ZSM-5/MCM-22 Brønsted acid sites to form aromatics. Sulfated zirconia (SZ) supported Mo catalyst contains the acid sites necessary to produce benzene in MDHA. Here, sulfated hafnia (SH), a homologous oxide like SZ, has been proposed to provide the necessary acid sites as a novel support for Mo in MDHA. Conversion increased with higher temperature and lower space velocity and gradually deactivated with time. This can be attribute to catalytic surface coking, confirmed with subsequent TPO analysis. Benzene product selectivity increased with higher Mo loading, lower temperature and lower space velocity, while gradually decreasing with time. A direct comparison of conventional Mo/HZSM-5 synthesized here and under identical reaction conditions showed lower activity compared to the Mo-SH catalyst. To address catalytic coking and improve aromatics selectivity, several extensions of this project were carried out in this work. Additional of promoters like Pt, Cr, Pd to Mo-SZ catalysts showed improved benzene selectivity and overall activity of the modified catalysts. MDHA studies using group VIB metals (Cr, Mo, W) supported on SZ were also carried out to understand the effect of these active metals on SZ, which showed the superiority of Mo in terms of catalytic activity and benzene selectivity. Direct conversion of methane to C2 hydrocarbons using W/SZ is another demonstration of the versatility of this catalytic process. Additionally, Mo/SH was used to directly activate ethane and propane and selectively produce important industrial feedstocks like ethylene and propylene. Few alternate routes were suggested for low temperature conversion of methane using CO2 via bimetallic catalytic approaches to produce high value oxygenates. Based on thermodynamic analysis, prospective catalytic reaction mechanisms are discussed to overcome the thermodynamic energy constraints for CO2 and methane activation at low temperature with selective production of target oxygenates.

Committee Chair

Spivey, James

DOI

10.31390/gradschool_dissertations.5630

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