Tandem Reaction for CO2 Hydrogenation to Methanol At Low Temperature

Carbon dioxide has attracted much attention as a promising abundant and sustainable feedstock for the synthesis of fuels and commodity chemicals. The hydrogenation of CO₂ has the potential to become a carbon-neutral process, if hydrogen is obtained from renewable energy resources (e.g. H₂O). Methanol is an interesting first product, because it can be utilized as a fuel or precursor for other chemicals. State-of-the-art catalysts for CO₂ hydrogenation usually requires high operating temperatures (> 200 ºC), limiting the single-pass yield to methanol, which is thermodynamically favored at low temperatures [1].  

An alternative route to produce methanol from CO₂ via tandem reactions has recently been reported, with the promise of improving the methanol yield at milder temperatures (100-150 ºC) [2,3]. Methanol is synthesized via three sub-steps in tandem: i) Hydrogenation of CO₂ to formic acid; ii) esterification of formic acid to alkyl formate; iii) hydrogenolysis of alkyl formate to methanol. (Reaction scheme is shown in Figure 1.) An all-homogeneous cascade catalyst, consisting of viable catalyst for each single step, has recently been developed and demonstrated for this tandem, one-pot reaction [2]. However, the overall turnover number (TON) appeared to be limited by hydrogenolysis catalyst, due to its incompatability with step ii catalyst. This tandem reaction was also achievable using a single heterogenous catalyst, with hydrogenolysis being the rate-determining step [3]. Therefore, identifying more viable catalyst for the final step is crucial to enhance the overall TON of this triple-tandem reaction.

This talk will demonstrate ethyl formate hydrogenolysis over a series of copper-based catalysts  in a slurry-phase batch reactor. Several process parameters, including H₂ partial pressure and catalyst pretreatment protocol, were studied to identify favorable conditions for methanol formation. Figure 2 shows the reactant consumption and products formation profiles over a barium promoted-copper chromite catalyst. A total yield of 45.3% methanol was achieved at an ethyl formate conversion of 87.4% (8 hr, 135 ºC, 30 bar H₂). We also evaluated this catalyst for CO₂ hydrogenation under similar conditions; with just the Cu-based heterogeneous catalyst, an overall yield of 35.8% was achieved at a conversion of 41.4% (20 hr, 135 ºC, 10 bar CO₂, 30 bar H₂). The rates are likely to be improved when the appropriate homogenous catalysts are combined with this heterogneneous catalyst.

Figure 1: Tandem reaction scheme for CO₂ hydrogenation to methanol

Figure 2: Reactant consumption and products formation profiles of ethyl formate hydrogenolysis over barium promoted-copper chromite. 135 ºC, 30 bar, 200 rpm, 200 mg catalyst, 37.5 ml p-Dioxane, 0.55 mmole formate, 4.66 mmole H₂ (based on solubility in p-Dioxane).

 

References

 

1.      S. Natesakhawat, J. Lekse, J. Baltrus, P. Ohodnicki, Jr. B. Howard, X. Deng, C. Matranga, ACS. Catal. 2 (2012) 1667.

2.      C. Huff, M. Sanford, J. Am. Chem. Soc. 133 (2011) 18122.

3.      L. Fan, Y. Sakaiya, K. Fujimoto, Appl. Catal. A: Gen. 180 (1999)  L11.