USP researchers are increasingly closer to transforming CO2 into fuels or plastics via catalysis

They were able to obtain CO, under high-pressure conditions, by developing a new catalyst. Results were highlighted in two foreign publications.

Under high-pressure conditions, a new catalyst based on nickel, zinc, and carbon was able to transform carbon dioxide (CO2), one of the main greenhouse gases (GHG), into carbon monoxide (CO), which is an important intermediary for generating added-value products. “The result of our research shows that we are getting closer and closer to producing such petroleum derivatives as plastics and fuels, via catalysis,” declares Liane Rossi, Professor in the Institute of Chemistry at the University of São Paulo (IQ-USP) and Coordinator of the study being performed within the scope of the FAPESP Shell Research Centre for Gas Innovation (RCGI).

This study was recently the lead story of two foreign publications. One of them is the European Journal of Inorganic Chemistry (Eur JIC), which featured the article on the front cover of the November 2021 issue, Zeolitic-Imidazolate Framework Derived Intermetallic Nickel Zinc Carbide Material as a Selective Catalyst for CO2 to CO Reduction at High Pressure, signed by the USP research team. With its focus on the general public, and not merely academics, the website ChemistryViews also reported on the study.

The published work is a byproduct of a previous study, coordinated by Rossi, where the researchers found that a nickel catalyst performed better after being subjected to an elevated temperature (800 degrees Celsius), in an atmosphere of CO2 and hydrogen (H2) or methane or propane. “This process provided an excellent catalyst for CO2 reduction, generating only CO, with no sign of the less desirable product, methane (CH4),” Rossi points out. Those results were published in the Journal of the American Chemical Society in March 2021.

However, the researchers unsuccessfully tested this same catalyst under high-pressure conditions (between 20 and 100 bar) in an attempt to adapt the reactions required for the subsequent transformation of CO into liquid products. Rossi comments that “when we forced the conditions to higher pressures, we found that in addition to CO, much methane was also produced. This is a problem because we wanted to obtain only CO, which is more reactive, because it is able to form liquids with long carbon and hydrogen chains and, thus, generate value-added products. Methane, on the other hand, is not as easily transformed into liquid products.”

The solution came about through a nickel, zinc, and carbon-based catalyst developed by Nágila Maluf, a doctoral student at IQ-USP and a member of the team of researchers coordinated by Rossi. “This combination changes the way the molecules interact on the surface of the catalyst, compared to pure nickel,” Rossi notes. The experiments were carried out by two research groups at IQ-USP: the Laboratory of Nanomaterials and Catalysis, coordinated by Rossi, and the Laboratory of Sustainable Carbon, coordinated by Professor Pedro Vidinha, co-author of the article. The USP Physics Institute, located in the city of São Carlos, in outstate São Paulo, and the Pacific Northwest National Laboratory (PNNL), in the city of Richland, Washington, in the United States also took part in the testing phase.

Rossi explains that catalysts are widely used in industry, but they are also used on a day-to-day basis to purify automotive exhaust fumes. “Catalysts are substances that promote chemical reactions between two or more molecules. For example, they can be enzymes or metallic surfaces (as is the case in this study). Catalysts generally accelerate the reaction between molecules that would not react naturally, or that would only react very slowly.”

Catalysts also function in the selection of a reaction path, that is, they direct the reaction to provide the desired product. Rossi further explains, “the molecules, when submitted to a catalyst at a given temperature, bind to it and undergo a process that involves breaking up and forming new chemical bonds, which allow new compounds to be formed and leave the catalyst. Reactions between gases can also benefit from increased pressure, such as in the experiment we are performing, because the processes already known to transform CO into liquids, such as alcohols, hydrocarbons, or olefins, take place in pressurized reactors.”

She emphasizes that success during the phase of transforming CO2 into CO at high pressure levels is important precisely for integration with subsequent steps, which will use this intermediate product (CO) with other catalysts to, then, generate the liquid products. “The more similar the operating conditions of the two stages are, the better the process will function, since we can evaluate the use of two catalysts in the same reactor. Therefore, for this process to be viable from a commercial standpoint, we need to use high pressure.”

The research team is now preparing to proceed with the study. “The next step is to use two different catalysts in the same reactor. One of them is based on nickel, zinc, and carbon, and the other, on iron or copper,” Rossi says. She completes her presentation by explaining that the second catalyst should favor the reaction between CO and H2 molecules, in order to produce alcohols or hydrocarbons, which are value-added products: “This will be possible through the Fischer-Tropsch synthesis, a process discovered in the 1920s that is capable of producing synthetic fuels, but which really never was accepted for industrial use, due to competition with cheaper products obtained directly from petroleum. Now, with global warming and worldwide interest in processes for mitigating CO2 emissions, the story may be different.”