Catalytic CO2 hydrogenation to CO and methanol

Date
2019
Journal Title
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Publisher
University of Delaware
Abstract
The catalytic CO2 hydrogenation for carbon monoxide or methanol production has attracted significant attention recently as a strategy of introducing green H2 into the fuel economy and recycling carbon dioxide as a chemical feedstock. In the past decades, research into catalysts for this reaction has primarily centered on Cu-based materials supported on Al2O3, specially for the hydrogenation of synthesis gas to methanol. However, Cu-based materials show limited selectivity to methanol and limited stability. The aim of the research conducted in this thesis is to increase the energy efficiency of carbon-based technology by developing stable, selective, and efficient catalyst for catalytic CO2 hydrogenation process. ☐ The primary goal of the research described in first part of this dissertation (Chapters 3) has been develop an efficient, inexpensive and selective catalyst with good stability for the conversion of CO2 and H2 to CO, the so-called reverse water-gas shift (RWGS) reaction. The RWGS reaction is, fundamentally and practically, an essential reaction for sustainability because of the versatility of CO as a chemical intermediate and the simplest product for CO2 utilization. The catalytic properties of unsupported iron oxides, specifically magnetite (Fe3O4), were investigated at temperatures between 723 K and 773 K and atmospheric pressure. This catalyst exhibited fast catalytic CO formation rate (35.1 mmol h-1 gcat.-1), high turnover frequency (0.180 s-1), high CO selectivity (>99%), and high stability (753K, 45000 cm3h-1gcat.-1) under 1:1 H2 to CO2 ratio. Reaction rates over Fe3O4 catalyst displayed a strong dependence on H2 partial pressure (reaction order of ~0.8) and a weaker dependence on CO2 partial pressure (reaction order of 0.33) under equimolar flow of both reactants. X-ray powder diffraction patterns and XPS spectra reveal that the bulk composition and structure of the post-reaction catalyst was formed mostly of metallic Fe and Fe3C while the surface contained Fe2+, Fe3+, metallic Fe, and Fe3C. Catalytic tests using pure Fe3C (iron carbide) suggest that Fe3C is not an effective catalyst for this reaction at the conditions investigated. Gas-switching experiments (CO2 or H2) indicated that a redox mechanism is the predominant reaction pathway. ☐ The second part of this dissertation (Chapters 4-6) investigates the catalytic CO2 hydrogenation to methanol and the impact of a highly selective catalysts on the methanol synthesis process. Supported indium oxide catalysts (Chapter 4) are investigated for the CO2 hydrogenation to methanol at a total pressure of 40 bar (528-573K) using a laboratory flow reactor. Surface reducibility, optical spectral characteristics, and catalytic rates and selectivity were correlated to catalyst composition. Promoted catalysts, especially Yttrium or Lanthanum-promoted indium oxide, require higher temperatures (H2-TPR) for surface reduction and display higher CO2 desorption temperatures (CO2-TPD). The promoted samples also have higher methanol selectivity (about 20%) compared to the non-promoted catalyst, while methanol formation rates (0.330-0.420 gMeOH gcat.-1 h-1 at 573 K) remain close to the non-promoted catalyst. From 528 K to 558 K, methanol selectivity was over 80 %, over all the promoted catalysts, and nearly 100% selectivity was observed at the low temperature range (~528 K) investigated. The reaction kinetics of Y-promoted catalyst and the results of CO co-feeding experiments suggest that the formate pathway is the likely reaction mechanism for methanol formation. ☐ In Chapter 5, a novel supported bimetallic oxide, Co-In catalyst supported on ZrO2 (Co-In/ZrO2), was identified as an excellent catalyst for direct conversion of CO2 to methanol with high selectivity (>99%) under industrially relevant conditions (528 K-543 K, 40 bar). The evaluation of Co-In/ZrO2 catalyst over 40 h on stream showed outstanding stability without deactivation. The characterization investigations and catalytic behavior of Co-In/ZrO2 show that this hybrid oxide system is a promising alternative for Cu-based materials. ☐ Chapter 6 demonstrates the importance of implementing highly selective catalysts into the process of methanol synthesis by simulations using of Aspen Plus. The first section compares the difference of highly selective reaction of CO2 hydrogenation (methanol and water are the only two products) with the common CO2 hydrogenation (CO, methanol, and water are formed along with the competing reverse water-gas shift reaction) under equilibrium condition at60 bar, 543 K. The second scenario compares three different catalysts such as conventional Cu/Zn/Al2O3, La-In/ZrO2 (Ch4), and Co-In/ZrO2 (Ch5) with various conversion and selectivity. The lower net energy consumptions and higher productivity of methanol shown using the Aspen Plus process simulation software over our catalysts demonstrate the value and potential impact of using selective catalysts in the methanol synthesis process.
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