The mechanism of the base-free Ru(PTA)4Cl2 catalyzed hydrogenation of CO2 to formic acid

Title : The mechanism of the base-free Ru(PTA)4Cl2 catalyzed hydrogenation of CO2 to formic acid

Abstract:

Catalytic hydrogenation of CO2 to fuels is an ideal strategy that offers solutions to two major concerns of the 21st century: global warming due to anthropogenic CO2 emissions and the rapid depletion of nonrenewable fossil fuel reserves. Of the many possible CO2 products, formic acid (HCOOH) has been attained recent attraction as a low-toxic liquid hydrogen storage material with a volumetric hydrogen density of 53 g of H2 per litre. Despite the progress in the development of homogeneous catalysts for the hydrogenation of CO2 to formic acid, none of these reactions could achieve industrial level applications due to the complications arises from the base additives. Even though base-free hydrogenations could evade the issues in the separation of formic acid, little efforts have been taken in this research direction. Recently, Laurenczy and co-workers have reported a robust ruthenium catalyst Ru(PTA)4Cl2 (PTA=1,3,5-triaza-7-phosphaadamantane) for the base-free hydrogenation of CO2 to HCOOH in acidic media without the need of any bases, additives or buffers [Nature. Commun. 2014, 5, 4017). In water, the catalyst affords to form 0.2 M formic acid with a turnover number of 74 whilst the same catalyst produces  1.9M formic acid in DMSO, an almost 10 times increase in the HCOOH formation. Employing density functional theory (DFT) calculations, we studied in detail the various mechanistic pathways for the hydrogenation of CO2 to formic acid for this novel catalysis. We found that the solvent itself act as a base and assist in the heterolytic cleavage of H2 to form the Ru-monohydride species Ru(PTA)4(Cl)(H) from the Laurenczy’s catalyst Ru(PTA)4Cl2. The computed enthalpic changes (ΔH) of 20.9 kcal/mol and 19.8 kcal/mol are in excellent agreement with the experimental activation enthalpies of 22.9 kcal/mol and 17.5 kcal/mol in water and DMSO, respectively. We have also explored the reaction pathways for the hydrogenation of CO2 to formic acid by the key species Ru(PTA)4(Cl)(H). The explicit role solvent in the reaction pathways were also investigated. We have found that the formation of the Ru-monohydride species from the Laurenczy’s catalyst is the rate-determinng step of the reaction. Unraveling the thermodynamic and kinetic aspects of the reaction pathways for the hydrogenation of CO2 for this novel catalysis can aid the further development of novel robust catalysts for the hydrogenation of CO2 in base-free reaction conditions.

Biography:

Prof. Brothers did his PhD at Penn State, a postdoc at Rice University, and has been at Texas A&M University at Qatar as a professor since 2008, where he also served as the Program Chair of Science for five years.  His research focuses on understanding the reaction mechanisms of metal bearing homogeneous catalysts.