On the Synthesis and Characterization of Bi-functional Catalysts for the Conversion of CO2 into Light Olefins

Abstract

Due to the negative impact of climate change, reduction of CO2 concentrations in the atmosphere became urgent. One approach to achieve this is to capture and convert the CO2 to low-carbon fuels as these products are in high demand and it will reduce energy and capital costs. There are three main paths towards this conversion, however, direct hydrogenation is the most attractive route. Tandem catalysts, prepared by loading a metal/metal oxide catalyst into a zeolite support, are capable of catalyzing this reaction. SAPO-34 is one of the best candidates as a zeolite support, however, it suffers from a low activation rate due to coke deposition within its channels. To overcome this challenge, in this work, nanosheet SAPO-34 is synthesized by reducing the crystal size which has shown a higher activity rate in literature. This zeolite is then loaded with Cu-ZnO metal catalyst, as it is one of the best catalysts for the conversion of CO2 into methanol. The metals are loaded via ion exchange method to ensure the metals are located within the framework of the zeolite support while maintaining the crystallinity of the zeolite nanosheets. XRD was used to confirm the successful synthesis of CHA framework with some AEI intergrowth. SEM and TEM were used to confirm the nanosheet morphology before and after metal loading. TEM confirmed the presence of nanoparticles within the zeolite, while EDX was used alongside SEM to determine the elemental compositions of the synthesized catalysts. Nitrogen physisorption confirmed Type I isotherms of the catalysts, indicating the presence of micropores. H2 TPR was used to reduce the catalysts and confirmed the presence of a wide range of Cu and Zn species that are reduced. CO2 TPD showed a low CO2 uptake at low temperatures, while NH3 TPD confirmed low acidity of the catalysts synthesized. The prepared catalysts have the potential to be used in the direct hydrogenation of CO2 towards light olefins for the following reasons. The nanosized crystals of the SAPO-34 support can allow for easier diffusion of gases through the zeolite channels. The good dispersion of metal nanoparticles within the zeolite can allow for tandem conversion. The reducibility of the metal species, the low acidity (which helps in minimizing the coking), and CO2 uptake are all properties that can potentially make these catalysts suitable for CO2 conversion to light olefins. The characterization techniques confirmed the presence of these needed properties; however, catalytic testing is necessary to determine if optimization of the low acidity and basicity is necessary to ensure optimal performance.

Date of Award	Apr 2023
Original language	American English
Supervisor	Maryam Khaleel (Supervisor)