Oxford Researchers Developing Method for Homogeneous Conversion of CO2 to Methanol Under Relatively Mild Conditions and Without Metal Catalysts
|CO2 was converted into methanol by adding it to frustrated Lewis Pairs in toluene under H2, heating the mixture, and vacuum distillation. Credit: Angewandte Chemie. Click to enlarge.|
Researchers at the University of Oxford are developing a method for the homogeneous conversion of CO2 to methanol (i.e., methanol is the only resulting C1 product) for use as a fuel using a “frustrated Lewis Pair” (FLP)-based nonmetal-mediated procedure at pressures of 1–2 atm and 160 °C. A paper on their work was published in the 21 December 2009 issue of the journal Angewandte Chemie International Edition.
Current approaches to hydrogenating CO2 to CH3OH (methanol)—a reaction that is themodynamically favorable, “but...not the most favorable transformation of CO2 with H2”, the authors note—rely on solid oxide catalysts.
However, these systems tend to give mixtures of C1 products: CO, CH3OH, and CH4. Furthermore, we are not aware of the homogeneous conversion of CO2 into CH3OH with nonmetal complexes...Herein we describe the heterolytic activation of hydrogen and subsequent insertion of CO2 into a B-H bond in the first homogeneous process for the conversion of CO2 into methanol.—Ashley et al.
A frustrated Lewis pair (FLP) is a Lewis acid - base pair in which steric hindrance (which occurs when the size of groups within a molecule prevent chemical reactions that occur in related smaller molecules) precludes the formation of a regular Lewis acid-base adduct (a molecule formed by the direct addition of two or more distinct molecules) formation—as in ammonia borane. Kept from the “normal” reaction, these “frustrated” groups turn to other molecules.
Other research groups have shown that FLPs can activate H2 heterolytically (cleaving a chemical bond). These systems can be used in metal-free catalytic hydrogenation and addition to olefins and other organic substrates.
Andrew E. Ashley, Amber L. Thompson, and Dermot O’Hare used a FLP consisting of the base tetramethylpiperidine (TMP) and the acid B(C6F5)3, which had been shown in other work to cleave hydrogen (H2) to form the salt [TMPH] [HB(C6F5)3].
...upon the addition of CO2 (1 equiv) to a 1:1 mixture of TMP/B(C6F5)3 (4 equiv) in C7D8 under an H2 atmosphere, quantitative conversion into CH3OB(C6F5)2 via 2 was observed after 6 days at 160 °C. Remarkably, vacuum distillation of the solvent (100 °C) led to the isolation of CH3OH (17–25% yield based on integration of the 1H NMR spectrum against internal Cp2Fe and GC analysis) as the sole C1 product, alongside C6F5H and TMP by-products. We expect that the formation of methanol results from the reaction of CH3OB(C6F5)2 with TMP or its conjugate acid.
...Current investigations are focused on increasing the stability of the system towards hydroxylic agents with the hope of thereby rendering the system catalytic.—Ashley et al.
Andrew E. Ashley, Amber L. Thompson, Dermot O’Hare (2010) Non-Metal-Mediated Homogeneous Hydrogenation of CO2 to CH3OH. Angewandte Chemie International Edition Volume 48 Issue 52, Pages 9839 - 9843 doi: 10.1002/anie.200905466