Scientists study the use of hybrid catalysts to accelerate the hydrogenation of CO2; they also investigate how CO2, in a supercritical state, reacts with hydrogen in the presence of different catalysts
Obtain value-added products from CO2 by using catalyzers that imitate the catalytic power of enzymes: this is one of the objectives of Project No. 30 of the FAPESP Shell Research Centre for Gas Innovation (RCGI). Coordinated by researcher Liane Rossi, of USP’s Chemistry Institute, the project “Innovative processes for converting CO2 into chemical products with a high aggregate value and fuels, based on hybrid catalysts” was based on a prior experiment of the team for developing selective catalysts for hydrogenation reactions (a reaction for adding hydrogen to organic molecules, from hydrogen gas, H2).
“For years we have been studying catalysts that behave differently from the classic forms, based on metallic nanoparticles and on the cooperative action of ligands. In nature, enzymes are natural and extremely efficient catalysts. Normally, they have a metallic center, like many of the classic catalysts, but the process is aided by ligands (binders) that are in the protein structure of the enzyme,” Liane explains. She says the enzymes called hydrogenases are able to break up hydrogen molecules under ambient conditions, when hydrogenation in metallic catalysts (classic) need special pressure and temperature conditions – including high temperatures, depending on the process used.
“After a number of different studies, we developed a hybrid catalyst that has a metallic component and a layer with an organic nature.” The catalyst contains metallic gold and has a wrap of organic nitrogenous binders. This system, when treated thermally at 400oC in an inert atmosphere (no oxygen), transforms all the organic material that is mixed with the metallic material into a layer of carbon doped with nitrogen. “We discovered, while working on a project financed by FAPESP, that this nitrogenous ligand, which was ‘burned’ and became a solid, has the necessary characteristics for acting cooperatively with the metal, gold in this case, and breakdown the hydrogen molecule under mild conditions.” The activation or breakdown of the H2 molecule is the first step in a hydrogenation reaction that proceeds via the transfer of H to the reagent to be hydrogenated.
“The gold catalyst developed was very active in several hydrogenation reactions. An example is the preparation of alkenes (compounds with double carbon bonds) from alkynes (compounds with triple carbon bonds), which gained us a patent.”
But, in order for the team to obtain from CO2 its desired final products – basically superior alcohols, like ethanol, butanol, and isopropanol; olefins or hydrocarbons – one more phase is needed: the CO2 molecule must be activated, which is very chemically stable. “The hydrogenation reaction of CO2 with classic catalysts can form products such as CO; CH4, which is methane; CH3OH, which is methanol; and some form of undesirable carbon. All of these processes compete with each other and the chosen catalyst will determine which product will be formed in detriment to the others – this is what we call selectivity. Being able to hydrogenate CO2 directly from alcohols or olefins is an even greater challenge. In terms of catalysis, nickel is often used, as well as iron, rhodium, platinum, or palladium… but not gold.” “Very little is known regarding the reactivity of gold, and new discoveries have stimulated its study.”
In the RCGI study, the group wants to explore the interaction of CO2 with the gold catalyst covered with carbon that has already been studied, which activates hydrogen gas (H2), as well as iron and nickel, which are also similar promising systems. The final objective is to find out whether the CO2 will also be activated for form products. “One of the phases of the project is to study the capture and activation of CO2. Now, we will begin to do very specific studies: optimizing the preparation of the catalyst, submitting it to hydrogen gas and to CO2. With detailed analysis techniques we will see how CO2 interacts in this system: whether it is merely absorbed or whether some important intermediary product can be formed, in both the reaction for forming methanol and of CO, of methane, or other products.”
The group of researchers, which includes the contributions of scientist Pedro Vidinha, is also interested in the reactions with CO2 in a super critical state. “The idea is to make CO2 in a supercritical state react with hydrogen, in the presence of this family of catalysts, and to see what the influence is on the products that are formed.” CO2 in its supercritical state, which is obtained under certain pressure and temperature conditions, presents properties that are distinctly different from conventional solvents and those properties can be modulated.
The group is attempting to capture the essential characteristics of biocatalysis and nanocatalysis, in order to design innovative catalysts and processes, which operate under milder temperatures and result in products with a greater aggregate value. But, according to Liane, this continues to be a big challenge.