Title: Tandem Reactions of Carbon Dioxide Reduction and Hydrocarbon Transformation
Description: High atmospheric concentrations of CO2 contribute to adverse effects that impact human health and the climate. The need to reduce CO2 is evident, and climate stabilization will require a combination of mitigation, utilization, and even negative emission technologies. Thus, one key approach will be to transform abundant CO2 into a useful feedstock for processes that not only produce value-added products but also match the scale necessary to impact anthropogenic emissions. The tandem CO2 reduction and light alkane transformation reactions over specialized bifunctional catalysts have the potential to produce olefins or synthesis gas by efficiently utilizing the C2-C4 components in shale gas while reducing a greenhouse gas. The reactions of CO2 with light alkanes may occur through two distinct pathways, oxidative dehydrogenation (CO2 + CnH2n+2 → CnH2n + CO + H2O, CO2-ODH) and dry reforming (nCO2 + CnH2n+2 → 2nCO + (n+1)H2, DR). The two reactions can occur simultaneously at temperatures ≥823 K with considerable conversions.

Until recently, there has been little understanding regarding the identification of bimetallic catalytic systems that either selectively cleave the C-H bonds to produce olefins or effectively break all the C-C and C-H bonds to produce dry reforming products. In this work, we discuss a combined approach of flow reactor experiments, in situ characterization, and density functional theory (DFT) calculations to help create a design platform for catalysts that are inherently active and selective for the reactions of CO2 and light alkanes. Particularly, it was of interest to use propane as CO2 reduction feedstock due to its increasing abundance and highly marketable respective olefin. Through the combined approach, non-precious Fe3Ni1 and precious Ni3Pt1 supported on CeO2 were identified as promising catalysts for the CO2-ODH and DR of propane, respectively. In situ X-ray absorption spectroscopy measurements revealed the oxidation states of metals under reaction conditions and DFT calculations were utilized to identify the most favorable reaction pathways over the two types of catalysts.

While both the CO2-ODH and DR reactions of alkanes produce valuable molecules, the separation of gas phase products is challenging. Therefore, it was highly desirable to develop a tandem reaction scheme in which the reaction of CO2 and alkanes can produce liquid products. Another potential chemistry with increased similarity to the operating conditions of CO2-ODH, is the tandem reactions of CO2-assisted oxidative dehydrogenation and aromatization of light alkanes (CO2-ODA). In this process, alkanes are transformed directly into aromatics without the need for expensive naphtha while increasing the consumption of CO2 per mol of value-added product and facilitating downstream separation because of the production of liquid aromatics. One critical change upon the introduction of CO2 to the dehydrogenation/aromatization pathway is the formation of water. The presence of water under reaction conditions has been shown to be problematic for zeolites as it causes changes in the framework. Phosphorous modification at an optimal loading improved the hydrothermal stability of Ga/ZSM-5, reduced coke formation on the catalyst surface, and allowed for the formation of more liquid aromatics through the CO2-ODAE reaction pathway compared to the direct dehydrogenation and aromatization reaction.

With the aid of DFT calculations, the mechanisms for the production of aromatics from ethane were identified, providing insight on the effect of Ga modification on ethylene formation over ZSM-5 as well as the role of CO2 on the aromatization of ethylene. Future efforts should be geared toward enhancing aromatics yield through the design of hydrothermal stable zeolite-based materials with bimetallic active centers that are capable of activating CO2.