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Advancing Non-Oxidative Alkane Dehydrogenation with Siliceous Zeolite-Supported Catalysts in Hydrogen -Separative Membrane Reactors
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Olefins such as ethylene and propylene are essential building blocks of the petrochemical industry. Their demand is expected to increase due to global population growth and rising living standards. Traditionally, olefin production relies on the high-temperature cracking of crude oil or natural gas, which is energy-intensive and contributes significantly to carbon dioxide (CO2) emissions. An alternative approach, on-purpose olefin production via non-oxidative alkane dehydrogenation, offers higher carbon efficiency and lower CO2 emissions. However, the endothermic nature and equilibrium limitations of non-oxidative dehydrogenation necessitate high operating temperatures to achieve commercially viable conversions. These elevated temperatures promote undesirable side reactions, such as cracking and coke formation, leading to catalyst deactivation via coking and/or sintering. Membrane reactors (MRs), which integrate membrane separation with chemical reactions, can overcome thermodynamic equilibrium constraints by continuously removing hydrogen as a byproduct during non-oxidative alkane dehydrogenation. This approach enhances alkane conversion at lower temperatures, reducing overall energy input while mitigating catalyst deactivation. However, the hydrogen-deficient environment within membrane reactors often accelerates catalyst coking, posing a significant challenge for the deployment of state-of-the-art industrial catalysts in these applications.
This dissertation focuses on the development of active, selective, and stable catalysts for non-oxidative alkane dehydrogenation under hydrogen-separative membrane reactor conditions. Specifically, it investigates siliceous zeolite-supported metal catalysts for propane and ethane dehydrogenation in carbon molecular sieve (CMS) membrane reactors. The siliceous zeolite support enhances catalyst stability by suppressing cracking and oligomerization reactions due to the absence of Brønsted acid sites. Furthermore, the encapsulation of small metal clusters within the zeolite micropores and/or framework prevents metal sintering and subsequent deactivation. By employing catalysts with low activation temperatures and leveraging CMS membranes for hydrogen removal to shift the thermodynamic equilibrium toward higher conversions, this study achieves high alkane conversion and catalyst stability, with record-high durability for propylene and ethylene production.
The dissertation begins with an investigation of a silicalite-1 encapsulated platinum-zinc catalyst (Pt-Zn/S1) for non-oxidative propane dehydrogenation. This catalyst was synthesized via
in situ hydrothermal crystallization and characterized using various techniques. Catalytic performance was evaluated in a fixed-bed reactor, where the catalyst exhibited a remarkably low activation temperature of 275 °C and achieved equilibrium conversion under optimized process conditions. The catalyst demonstrated excellent stability, reaching equilibrium conversion with no drop in 110 h time on stream. A kinetic model was developed to describe catalyst performance in both fixed-bed and membrane reactor settings. When tested in a CMS membrane reactor, the catalyst exhibited unprecedented stability, surpassing all previously reported systems. A power-law rate equation was derived for coking kinetics, which was validated under both fixed-bed and CMS membrane reactor conditions. Additionally, an in-situ catalyst regeneration protocol was developed for propane dehydrogenation applications.
For ethane dehydrogenation, dealuminated BEA (DeAl-BEA) zeolite-supported cobalt catalysts (Co/DeAl-BEA) were developed. The catalyst was synthesized via impregnation of a cobalt precursor onto a dealuminated BEA support, followed by calcination treatment. An atomic-level investigation was conducted to elucidate the coordination environment and oxidation state of cobalt sites in DeAl-BEA support, from catalyst preparation to post-reaction conditions. The catalyst features tetrahedral Co2+ mononuclear sites, di-coordinated to the zeolite framework, with two adjacent silanol groups. These sites form upon exposure to hydrogen during induction and remain stable throughout the reaction. Catalytic performance was evaluated in a fixed-bed reactor, and a microkinetic model was developed. The catalyst was subsequently tested in a CMS membrane reactor under both sweep gas and vacuum conditions, demonstrating significantly enhanced ethane conversion compared to conventional fixed-bed reactors. The membrane reactor exhibited high durability, record-low deactivation rates, and excellent regenerability while maintaining high ethylene selectivity. This dissertation provides critical insights into catalyst design and membrane reactor integration for non-oxidative alkane dehydrogenation, paving the way for more efficient and impactful olefin production.