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New technologies to reduce CO2 emissions are essential for the chemical industry to become “greener”. An alternative to carbon capture is the use of CO2 as a feedstock in the production of value-added products such as methanol. However, the thermodynamic stability and limited reactivity of CO2 make its sequestration or use in chemical processes challenging.
A research team led by Doug Stephan at the University of Toronto targets this advancement using frustrated Lewis pairs (FLPs), acid-base combinations which cannot readily share electrons due to their bulky substituents and which are therefore highly reactive. Stephan’s team demonstrated the activation of small molecules, including reversible binding of CO2 and its reduction to methanol, using FLPs composed of relatively common and inexpensive reagents in comparison to transition-metal catalysts.
Converting CO2 into methanol and water catalytically could ultimately be carbon neutral as every molecule of CO2 removed from the atmosphere generates one molecule of CO2 when methanol is consumed. Ideally, the reducing agent would be hydrogen supplied not from conventional petroleum-based processes but another developmental technology, photochemical splitting of water, which requires minimal energy input. This new energy generation system would represent a transformative breakthrough in CO2 and “green” chemistry.
$268,000 over 2 years, awarded in 2011
Canada can be a scientific world leader in addressing the challenge of commercializing brand-new approaches to CO2 emissions. An unprecedented means of converting CO2 to a useful form, such as CO or methanol, could lead to a carbon-neutral transformation of Canada’s fuel sector with worldwide ramifications. Producing a liquid fuel such as methanol could utilize the existing distribution network, saving on costly new infrastructure.
Industries with high CO2 emissions could utilize this novel capture and catalytic system to potentially reutilize the CO2. Reduction to CO could employ well-known processes such as Fischer-Tropsch or hydroformylation to produce useful products. Other chemical industries such as polymerization processes could also benefit from new FLP-based catalytic systems.
This research will evaluate the use of frustrated Lewis pairs (FLP) in the activation, capture and utilization of CO2 using a three-phase approach:
Having established that CO2 can be captured with FLPs derived from phosphines and boranes, initial efforts will probe the range of Lewis acids and bases that effect similar binding.
Numerous combinations are proposed, including nitrogen (amine), aluminum, and transition metal systems.
The most promising and reactive systems will be used to develop catalytic reactions such as reduction to methanol and conversion to CO and water or to CO and oxide products. Structural modifications to the Lewis acid and base will maximize catalyst activity. Additional exploitation of FLP chemistry with, for example, silanes could open new paths to homogeneous catalytic CO2 reduction and even the formation of novel polymers.
Critical needs in this new FLP study are the reaction mechanisms and kinetic data, which provide the basis for development of a catalytic system for CO2 reduction. Reactions between CO2 and FLPs are rapid and thus difficult to study by conventional methods. The research team has demonstrated that the progress of the reaction of amines and CO2 can be followed by monitoring the size of gas bubbles in a microfluidic channel, offering a new approach to the acquisition of kinetic data gas/liquid reactions with minimal use of reagents and solvents.
The project has progressed from a proven CO2 capture system and demonstration of stoichiometric conversion to the successful catalytic reduction of CO2 to CO. Of the FLP candidates tested so far, zinc-based compounds show high reactivity, however the formation of an undesirable byproduct requires further investigation. In parallel and utilizing one reaction system for consistency, capabilities of the microfluidic cell system have advanced to include:
R.Dobrovetsky, D.W. Stephan, Stoichiometric Metal-Free Reduction of CO using Syn-Gas J. Am. Chem. Soc. 2013, 135, 4974–4977. DOI: 10.1021/ja401492s
G. Ménard, D.W. Stephan, CO2 Reduction via Aluminum Complexes of Ammonia Boranes Dalton Trans. 2013, 42, 5447-5453 .DOI: 10.1039/C3DT00098B.
MJ.Sgro, D.W. Stephan Activation of CO2 by Phosphinoamide Hafnium Complexes Chem. Commun. 49, 2610-2612 DOI:10.1039/C3CC38286A (cover invited).
R.Dobrovetsky, D.W. Stephan, Catalytic Reduction of CO2 to CO Using Zn(II) and In situ Generated Carbodiphosphorane as a Frustrated Lewis Pairs Angew. Chem.Int. Ed. 2013, 52,:2516-2519. doi: 10.1002/anie.201208817
M.J. Sgro, J.Dömer, D.W. Stephan, Stoichiometric CO2 Reductions using a Bis-Borane-based Frustrated Lewis Pair, Chem. Commun. 48, 7253-7255.
L.J. Hounjet, C.B. Caputo, D.W. Stephan, Phosphorus as a Lewis Acid: CO2 Sequestration with Amidophosphoranes, Angew. Chem. Int. Ed. 51, 4714–4717.
R.C. Neu, G. Ménard, D.W. Stephan, Exchange Chemistry of tBu3P(CO2)B(C6F5)2Cl, Dalton Trans , 41, 9016- 9018
G. Ménard, D.W. Stephan, Stoichiometric Reduction of CO2 to CO by Al-Based Frustrated Lewis Pairs, Angew. Chem. Int. Ed 50, 8396–8399.
I.Peuser, R.C.Neu,X. Zhao, M.Ulrich, B.Schirmer, G.Kehr, R.Fröhlich, S.Grimme,G.Erker, D.W.Stephan, CO2 and Formate Complexes of Phosphine-Borane Frustrated Lewis Pairs, Chem. Eur. J. 17, 9640–9650.
X.Zhao, D.W. Stephan, Bis-Boranes in the Frustrated Lewis Pair Activation of Carbon Dioxide, Chem. Commun. 1833-1835.
R.C. Neu, E. Otten, A. Lough, D. W. Stephan, The Synthesis and Exchange Chemistry of Frustrated Lewis Pair-Nitrous Oxide Complexes, Chem. Sci. 2, 170-176.
M.A. Dureen, D.W. Stephan, Reactions of Boron Amidinates with CO2 and CO and Other Small Molecules, J. Am. Chem. Soc. 132, 13559–13568.
Abolhasani, M.; Singh, M.; Kumacheva, E.; G?nther, A. Cruise Control for Segmented Flow. Lab Chip 12, 4787-4795 (2012).
Abolhasani, M.; Singh, M.; Kumacheva, E.; G?nther, A. Automated Microfluidic Platform for Studies of Carbon Dioxide Dissolution and Solubility in Physical Solvents. Lab Chip 12, 1611-1618 (2012).
Li, W.; Liu, K.; Simms, R.; Greener, J.; Jagadeesan, D.; Pinto, S.; Guenther, A.; Kumacheva, E.* Microfluidic Study of Fast Gas-Liquid Reactions. J. Am. Chem. Soc. 134, 3127-3132 (2012).