Conversion of CO2 to light olefins by cascade reactions over bifunctional nanocatalysts

The use of CO2 as carbon source for industrial production of commodity products may potentially lead to substantial benefit for society; with respect to lower carbon footprint for the production of value-added chemicals, reduced dependency on fossil fuels and reduced pollution. Light olefins (ethene, propene and butenes) are key building blocks in the petrochemical industry, with an annual production of more than 120 million metric tons. Capturing and using CO2 as raw material for the production of light olefins would be an important contributor to an anthropogenic carbon cycle, provided the energy input comes from renewable sources. In this context, CO2LO envisages to directly convert CO2 to light olefins through cascade reactions over a bifunctional catalyst. The proposed cascade process connects hydrogenation of CO2 to CO and water (Rx. 1), conversion of COx and hydrogen to methanol (Rx. 2), and conversion of methanol to olefins (Rx 3). These reactions are well-known industrial processes but requiring two reactors, individually operating at widely different temperatures (250 vs. 450 oC) and pressures (>50 vs 3-5 bar), due to thermodynamic restrictions of the first two reactions. The combined process represents a strong thermodynamic driving force, enabled by the third, energetically favored reaction. The CO2LO project aimed to develop a bifunctional catalyst by integrating an active metal, alloy or metal oxide catalyst for the CO2-to-methanol reaction with a zeolite/zeotype catalyst for selective methanol-to-olefins conversion. Catalyst optimization is targeted by gaining fundamental knowledge on property-performance relationships for the combined system and interfacing a positive feedback loop on catalyst synthesis, characterization, testing and mechanistic studies. In CO2LO, we have synthesized and tested previously published, as well as new, catalyst combinations for reactions 1-2 and/or 3. Studies of their joint performance by catalytic testing and in operando characterization revealed new insight into their combined properties. Promising results (>50 % selectivity to a single product, propane, at close to 40% CO2 conversion) is obtained for conversion of CO2/H2 to alkanes. Selective olefins production is more challenging, yet results obtained in this project give clear hints to how higher olefins yield may eventually be obtained in the future. Fundamental studies have been carried out for both catalytic functions. Studies of the CO2/H2-to-methanol catalyst builds on results from a previous RCN-funded project, CONFINE (250795). There, studies of model systems revealed the catalytic sites which yield selective methanol formation. In CO2LO, we have identified sites that yield unwanted byproducts, CO and methane. Hence, our toolbox for catalyst design has been enlarged. Studies of methanol-to-olefins catalysts build on results obtained in another, prior RCN-funded project, NanoReactor (239193). As part of CO2LO, we performed transient kinetics studies that showed how catalytic sites influence alkane and alkene diffusion in the micropores of zeolites and zeotypes, and furthermore, how the acid strength of the catalytic sites influences reaction selectivity. This insight is an important addition to the toolbox of future catalyst development and design.

CO2LO-prosjektet har gitt økt innsikt i potensialet for å omdanne CO2/H2 via metanol til lette olefiner og alkaner over såkalte tandem-katalysatorer, det vil si en blanding av to ulike katalysatorer i en reaktor. Grunnleggende studier av hver enkelt katalysator har gitt innsikt i hvilke material-parametre som bidrar til å danne ønsket produkt, og hvilke som bidrar til å danne uønskede biprodukter. Prosjektet har også bidratt til utdannelsen av to PhD-kandidater (hvorav en var ansatt i prosjektet, og en annen veiledet gjennom prosjektet) og en postdoc-kandidat.

CO2, the primary driver of climate change via the Greenhouse Effect, is also a sustainable carbon resource for the production of value-added chemicals. Nature uses 110 gigatons of CO2 every year as the primary carbon source for production of carbohydrates via photosynthesis. Mimicking nature by using CO2 as a feedstock would result in substantial benefit for society with respect to lower carbon footprint for the production of value-added chemicals, reduced dependency on fossil fuels and reduced pollution. Light olefins (ethene, propene and butenes) are key building blocks in the petrochemical industry, with an annual production of more than 120 million metric tons. Capturing and using CO2 as raw material for the production of light olefins would be an important contribution to an anthropogenic carbon cycle, provided the energy input comes from renewable sources. In this context, CO2LO envisages to directly convert CO2 to light olefins through cascade reactions over a bifunctional catalyst. The proposed cascade process connects hydrogenation of CO2 to CO and water, conversion of CO and hydrogen to methanol, and conversion of methanol to olefins, all well-known industrial processes but requiring two reactors, individually operating at widely different temperatures (250 vs. 400 ?C) and pressures (50 vs 1 bar), due to thermodynamic restrictions of the first two steps. The combined process represents a strong thermodynamic driving force, enabled by the third, energetically favored reaction. The project aims to develop a bifunctional catalyst by integrating, at the molecular scale, an active metal alloy or metal oxide for the CO2-to-methanol reaction onto a zeotype catalyst for selective methanol-to-olefins conversion. Catalyst optimization will be targeted by gaining fundamental knowledge on property-performance relationships for the combined system and interfacing a positive feedback loop on catalyst synthesis, characterization, testing and mechanistic studies.