Author ORCID Identifier

https://orcid.org/0000-0001-5726-3344

Semester

Summer

Date of Graduation

2023

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Chemical and Biomedical Engineering

Committee Chair

Jianli Hu

Committee Member

Charter Stinespring

Committee Member

Debangsu Bhattacharyya

Committee Member

V'yacheslav Akkerman

Committee Member

Christina Wildfire

Abstract

The world’s demand for natural gas has been consistently growing over the past few decades. Natural gas is considered a potential alternative to oil and coal because of its ability to burn cleaner. Although not completely emission-free, it has dominated the energy industry by providing a continuous supply of cheap fuel. Currently, natural gas makes up 41% of North American energy production. The conversion of natural gas to value-added chemicals typically involves the indirect production of synthesis gas (syngas) as an intermediate before the production of the desired product. Steam reforming and partial oxidation are two of the most common indirect methods of natural gas conversion. Steam reforming is the production of syngas by reacting steam (water) with natural gas at high temperatures. Partial oxidation also produces a lower quantity of syngas than steam reforming by reacting oxygen with natural gas. However, both involve the intermediate synthesis of syngas, which causes these processes to be both energy and capital-intensive. Alternatively, the direct conversion of light alkanes, such as methane and ethane, to aromatics eliminates the expensive intermediate production of syngas and the greenhouse gas CO2 emissions from steam reforming. This direct conversion path has been heavily investigated over the past few decades due to its importance in the production of value-added chemicals and fuels. Typically, the direct conversion of natural gas is performed under either non-oxidative and oxidative conditions, both having their own set of advantages and challenges. The conversion of natural gas by the non-oxidative direct method shows promise as a cost-efficient approach. However, the commercialization of this process faces some obstacles. The main technical challenges of non-oxidative dehydroaromatization (DHA) are related to low selectivity, catalyst coking, deactivation, and regeneration. The accumulation of carbon on the catalyst’s surface can block the active sites, therefore resulting in a decrease in the activity or deactivation of the catalyst over time. Regeneration of the catalyst is typically carried out in an oxidative atmosphere, where an exothermic reaction burns the coke off the catalyst, thus producing carbon dioxide. The exothermic reaction can be hard to control the temperature distribution in the reactor. The addition of oxygen species, such as O2, CO, and/or CO2 to the inlet gas as a co-feed or pulse has been proposed to avoid this rapid deactivation of the catalyst. Oxidative dehydrogenation to ethylene (ODHE) and oxidative dehydroaromatization (ODHA) using oxygen as an oxidant. However, carbon dioxide can also be used as a soft oxidant for ODHE (ODHE-CO2) and ODHA. It comes with several advantages: reduced coke formation, prevention of over oxidation of metal catalyst, and enhanced equilibrium conversion obtained due to the removal of hydrogen via the water gas shift mechanism. Zeolites have been extensively researched and optimized for a wide variety of applications over many decades. They have been used in many industrial processes in oil refineries, biomass conversion, direct valorization of natural gas, and more. The zeolite’s framework provides a unique shape-selective channel that can be optimized for a specific application or reaction. The zeolite, ZSM-5, is an MFI-type zeolite, composed of pentasil units which are linked to form pentasil chains. Oxygen bridges connect these chains to form corrugated sheets, oxygen bridges then connect each sheet to the next to form a three-dimensional 10-ring channel system containing micropores of about 0.55 nm. The micropore size of ZSM-5 is optimal for the dynamic molecular sizes of the BTX aromatics (benzene, toluene, and xylene). The MFI-type zeolites pores are large enough for the BTX to diffuse out but small enough to hinder polyaromatic species growth within the pores. These unique features make the ZSM-5 an ideal support for the direct conversion of light alkanes to aromatics. Zeolites can be categorized by what is known as a SiO2/Al2O3 ratio (SAR), which can influence the zeolites acidity, catalytic activity, hydrophobic/hydrophilic behavior, and polarization towards reactants and products. The research described in this dissertation was aimed at catalyst synthesis and development towards improved catalytic activity, stability, and efficiency for natural gas conversion under non-oxidative and oxidative conditions. The catalysts performance, activity, and product selectivity can depend on the choice and composition of the transition-metal promoter(s). Over the course of this work Mo, Fe, Ga, and Pt are the active metal species in the direct transformation of ethane to aromatics on commercially purchased zeolites as well as synthesized zeolites. Gallium and platinum metal-loaded H/ZSM-5 catalysts were tested for the ethane DHA reaction after different hydrogen pretreatment reduction conditions in a thermal fixed bed reactor. Also, in this work, the effects of incorporating microwave heating on the ODHE reaction with carbon dioxide as a soft oxidant were investigated and compared to the thermal process. Further, the non-oxidative DHA and ODHE-CO2 reactions were performed using conventionally synthesized zeolite as well as innovative microwave synthesized H-ZSM-5 and a galloaluminosilicate zeolite, HGaAlMFI.

Embargo Reason

Publication Pending

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