Mechanism investigation of propane dehydrogenation over metal cation exchanged zeolite catalysts
Date
2022
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Publisher
University of Delaware
Abstract
Propane dehydrogenation (PDH) has recently received extensive attention by the scientific community due to the growing gap between propylene demand and supply and the large availability of propane derived from shale gas. Pt- and Cr-based catalysts have been commercialized in the Oleflex process and Catofin process, respectively, as two on-purpose propylene synthesis processes. The high price of Pt and the environmental concerns of Cr have led to the search for other candidates for propane dehydrogenation. Metal cations exchanged inside zeolite have been shown to be efficient catalysts for propane dehydrogenation. However, identifying the active site in the zeolite remains an unresolved and important issue for new catalyst development. ☐ In this dissertation, we systematically investigated the metal speciation (Ga, Zn, In) in zeolite (MFI, CHA, AEI, [Fe]zeolites) catalysts and the corresponding reaction mechanisms for propane dehydrogenation using in situ transmission Fourier Transform infrared (FTIR) spectroscopy, pulse titrations, and reaction kinetics methods. Ga speciation in Ga/H-ZSM-5 with different Si/Al and Ga/Al ratios and PDH mechanisms are discussed in Chapters 3-5. We demonstrate in Chapter 3 that one Ga atom replaces one Brønsted acid site (BAS) at Ga/BAS ratio up to 0.7. Two types of exchanged Ga species are present on reduced Ga/H-ZSM-5, that is, Ga species exchanged with paired framework Al atoms and Ga exchanged with isolated Al atoms, which could be distinguished by employing H2 and H2O as probe molecules. Then, in Chapter 4, we determine the intrinsic reactivity and activation energies of the two different exchanged Ga sites, finding that Ga exchanged with paired framework Al atoms is more reactive to PDH than Ga exchanged with isolated Al atoms by an order of magnitude. Moreover, although both Brønsted acid and Ga sites interact with propane, FTIR results provide convincing evidence showing that the alkyl mechanism is more likely in the PDH on Ga/H-ZSM-5. In Chapter 5, we combined pulse titrations and in situ FTIR spectroscopy to identify the structure of the highly active Ga species as Ga2O22+, and the less active Ga species as Ga+. The PDH activity of Ga2O22+ is shown to be at least a factor of 18 higher than that of isolated Ga+, suggesting that increasing the density of the former could be an effective strategy for enhancing the catalytic performance. ☐ In Chapter 6, Zn speciation in Zn/H-ZSM-5 with varying Si/Al and Zn/Al ratios has been investigated. In contrast to Ga/H-ZSM-5, the exchange of Zn with BAS occurs at the impregnation and calcination stage, and ZnO in the external surface cannot be reduced and displace BAS. The relationship of BAS consumption vs. Zn/Al ratio demonstrated that: on Si/Al ratio of 15 with a fraction of paired Al pair sites, isolated Zn2+, [ZnOH]+, ZnOx cluster in the zeolite and crystalline ZnO species are introduced in sequence as the Zn/Al ratios rise. On Si/Al ratio of 39, the same sequence of Zn species is introduced without forming [Zn]2+. The [Zn-OH]+ site shows higher PDH rates than isolated Zn2+; however, the former species deactivates over time due to elemental Zn formation and subsequent sublimation. The condensation of two adjacent [Zn-OH]+ sites to form [Zn-O-Zn]2+ is stable after high-temperature reduction. We point out that fabrication of H-ZSM-5 with rich paired framework Al atoms is a strategy to prepare stable and reactive Zn/H-ZSM-5 catalysts for propane dehydrogenation. ☐ In addition to the investigation of H-ZSM-5 (a medium pore zeolite) supported gallium and zinc catalyst, the small pore zeolite chabazite (CHA) containing indium and gallium cations has been investigated for propane dehydrogenation (Chapters 7-8). In Chapter 7, we demonstrate that indium speciation depends on the support type: extra-framework In+ species are formed on In-CHA catalysts, while In2O3 was reduced to In(0) when using Al2O3 and SiO2 as support. In+ site in CHA shows better stability and C3H6 selectivity (~85%) than In2O3, 10In/SiO2 and 10In/Al2O3, consistent with a low C3H8 dehydrogenation activation energy (94.3 kJ/mol) and high C3H8 cracking activation energy (206 kJ/mol) in In-CHA catalyst. On the other hand, in Chapter 8 we show that extra-framework Ga+ sites are formed upon the reduction of Ga-CHA catalysts. This isolated Ga+ site reacts reversibly with H2 to form GaHx at 150 °C with an enthalpy of formation of approximately -51.2 kJ·mol-1, a result also supported by Density functional theory (DFT) calculations. The initial C3H6 dehydrogenation rates decrease rapidly during the first 100 min by approximately 40% and decline slowly in the latter part of the test, while the C3H6 selectivity is stable at ~ 96%. The catalyst recovers the PDH rate completely after regeneration. The higher reaction rate on Ga+ than In+ in CHA zeolites is the result of differences in propane activation and the stabilization of the transition state. ☐ In Chapter 9, we develop a novel catalyst preparation strategy, consisting of synthesizing Fe-containing zeolite such as [Fe]ZSM-5, [Fe]Beta, and [Fe]CHA, and then preparing Ga species exchanged with BAS in [Fe]zeolites through the reductive solid-state ion-exchange method. Ga/[Fe]ZSM-5, Ga/[Fe]Beta, and Ga/[Fe]CHA can achieve propylene selectivity of 96%. Although the catalytic rate is lower than that of the conventional Ga/[Al]ZSM-5 catalyst, the extraordinary propylene selectivity to Ga/[Fe] zeolite is explained by the stronger interaction between Ga+ and the Fe-containing zeolite framework. The investigation of Ga/[Fe]zeolites contributes to the application of Ga/zeolites catalyst for propane dehydrogenation and provides the guidance for further development of Ga-based catalyst. In the last Chapter, we summarize a toolbox to investigate the metal speciation and PDH mechanisms on metal cation exchanged zeolite catalysts, and point out some directions for future work.
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Keywords
Ga-CHA, Ga/H-ZSM-5, In-CHA, Metal zeolites, Propane dehydrogenation, Zn/H-ZSM-5