Carbonates: A Promising Approach for Low-Temperature Carbon Dioxide Conversions
By Vanda Glezakou and Roger Rousseau
Almost two hundred years into the Industrial Revolution, we are faced with a mounting problem regarding fossil fuel use -- too much carbon dioxide in the air. Turning carbon dioxide into fuels would address two energy issues at once: reducing the gas's accumulation in the atmosphere and recycling the finite resource that is fossil fuels. While others are exploring ways to collect carbon dioxide from power plant exhaust and use it as a raw material in industrial processes, we think it may also be possible to recycle the carbon dioxide back into fuels via a novel chemistry involving the conversion of carbonates. Here's how.
Power plants produce carbon dioxide around the country. Pressurizing and transporting all that gas to a central location is prohibitively expensive and energy intensive, so capturing carbon dioxide and turning it into fuels or other forms of stored energy should be performed as close to the source as possible. But that raises a problem. Current well-established methods, dry reforming and Fischer-Tropsch synthesis, require extreme conditions that industrial-sized plants achieve cost-effectively. They are simply not amenable to decentralized conversion. Instead, we need to focus on low-temperature conversion systems that are less expensive.
To achieve low-temperature carbon dioxide conversion, we can take inspiration from two sources: biomass and enzymatic conversion of carbon dioxide. Both perform the tasks of breaking carbon-oxygen bonds and creating carbon-hydrogen bonds to store energy, all at the temperatures we're aiming for. Researchers in the fundamental and applied catalysis groups at PNNL have shown that efficient conversion of oxygenated species can be achieved by multifunctional catalysts that form C-H bonds at a metal site while cleaving C-O bonds at an acid site1.
Guided by the same principles discovered in biomass and enzymatic studies, we believe that we can create active and selective catalysts for C-H bond-making and C-O bond-breaking2. So far PNNL researchers have made substantial progress on the C-O bond-breaking part. They have shown that multifunctional catalysts based on isolated single atom sites such as palladium or ruthenium can facilitate carbon dioxide reduction at reasonable temperatures3,4.
However, low temperature C-H bond formation remains a challenge. Hydrogenation studies with homogeneous catalysts show the key thermodynamic parameter for controlling hydrogenation is the bond strength between the metal site and hydrogen. We can manipulate the bond strength experimentally, and therefore the thermodynamics, because the electron density at the metal site is ultimately determined by the ligand environment. We will use this approach to try and lower the temperature at which hydrogenation proceeds efficiently.
This is where the carbonates come in. If we include an acid/base function in carbon dioxide capture solvents, carbonates, bicarbonates or alkyl carbonates form, even at ambient temperatures. This process occurs regardless of whether the acid/base function is added on the catalyst or as part of the reaction media.
Those carbonates have surprising reactivity in the capture solvents we've been exploring, ionic liquids that reversibly binds carbon dioxide. Recently, PNNL researchers have shown that carbon dioxide—having taken the form of alkyl-carbonates in the ionic solvent system—converts to methyl-formate, which is a precursor to some types of plastic. This happens when the capture is coupled to a homogenous catalyst at the relatively low temperature of 393 K and atmospheric pressure5. Thus, we postulate that by manipulating the catalyst functionalities and the reaction media, we can achieve a robust carbon dioxide reduction under mild process conditions.
We've made as much progress as we have so far thanks to teams of experimentalists and theorists working closely together. In the first years of our work, we successfully designed carbon dioxide capture solvents through a combination of experiment, theory and simulation6. Such collaboration could be fruitful for the next step, successfully converting carbon dioxide in the capture solvent, as long as the models account for the complexity of a catalyst and its environment under operating conditions. The theoretical investigation of the controlled activation and reduction of carbon dioxide is a challenging task because it demands that we have the ability to obtain reliable structural, spectroscopic and energetic information for large systems. This information must have high chemical complexity and be available for tractable timescales. Therefore, the key for making substantive advances in relatively short times will be to embrace the complexity of these catalytic systems and intellectually rise to the challenge of understanding them at a quantitative level. PNNL is uniquely positioned both experimentally and theoretically to do this, and we are passionate for the opportunity.
References
- Lercher, J.A., Appel, A.M., Autrey, T., Bullock, R.M., Camaioni, D.M., Cho, H.M., Dixon, D.A., Dohnalek, Z., Gao, F., Glezakou, V.A. and Henderson, M.A., ...et al 2014. Multifunctional Catalysts to Synthesize and Utilize Energy Carriers (No. PNNL-SA-103068). Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL). [PDF]
- Raugei, S., DuBois, D.L., Rousseau, R., Chen, S., Ho, M.H., Bullock, R.M. and Dupuis, M., Toward Molecular Catalysts by Computer. Acc. Chem. Res., 2015, 48(2), pp.248-255. DOI: 10.1021/ar500342g
- Kwak, J. H.; Kovarik, L.; Szanyi, J., CO2 reduction on supported Ru/Al2O3 catalysts: cluster size dependence of product selectivity. ACS Catal. 2013, 3, 2449-2455. DOI: 10.1021/cs400381f
- Kwak, J. H.; Kovarik, L.; Szanyi, J., Heterogeneous catalysis on atomically dispersed supported metals: CO2 reduction on multifunctional Pd catalysts. ACS Catal. 2013, 3, 2094-2100 DOI: 10.1021/cs4001392
- Yadav, M.; Linehan, J. C.; Karkamkar, A. J.; van der Eide, E.; Heldebrant, D. J., Homogeneous Hydrogenation of CO2 to Methyl Formate Utilizing Switchable Ionic Liquids. Inorg. Chem. 2014, 53, 9849-9854. DOI: 10.1021/ic501378w
- Cantu, D.C., Lee, J., Lee, M.S., Heldebrant, D.J., Koech, P.K., Freeman, C.J., Rousseau, R. and Glezakou, V.A., Dynamic Acid/Base Equilibrium in Single Component Switchable Ionic Liquids and Consequences on Viscosity. J. Phys. Chem. Lett., 2016. 7(9), pp.1646-1652 DOI: 10.1021/acs.jpclett.6b00395
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