CO2 to Methanol via Cascade Triple Catalysis

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U. Michigan researchers have published a paper in the ACS Journal demonstrating the production of methanol from carbon dioxide using three catalysts operating in a single vessel -- "cascade catalysis."

Huff and Sanford targeted a cascade catalysis sequence involving:

hydrogenation of CO2 to formic acid
esterification to generate a formate ester
hydrogenation of the ester to release methanol
They used three different homogeneous catalysts—(PMe3 )4Ru- (Cl)(OAc); Sc(OTf)3; and (PNN)Ru(CO)(H)—operating in sequence in different combinations and under different conditions.

They found that a combination of the three operating at 135 °C demonstrated the viability of cascade catalysis, producing 2.5 turnovers of methanol—i.e., a proof of principle. However, they noted, the methanol yield was significantly lower than expected.

They found that the major problem for cascade catalysis was the deactivation of one catalyst by another. As a “low-tech” solution, they physically separated the cross-reactive catalysts within the high-pressure vessel. Two catalysts were placed in a vial in the center of the vessel, while the third was placed in the outer well of the reactor. This resulted in 21 turnovers of CH3OH from CO2 under an initial temperature of 75 °C, with a ramp to 135°C.

This communication has demonstrated the viability of cascade catalysis for the reduction of CO2 with H2. This approach offers the distinct advantage that it provides opportunities for detailed analysis of the molecular basis of catalyst incompatibilities, the modes of catalyst decomposition, and the slow step of the sequence. As such, we anticipate that it will enable rational tuning of each of the individual catalysts (A–C) in order to improve the turnover numbers and turnover frequencies for this process. Efforts in all these areas are currently underway in our group and will be reported in due course.

—Huff and Sanford

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We have been talking about "The Methanol Economy" for several years. Research groups from Europe to North America to Japan have been successful in devising ways of converting CO2 to methanol by various methods. But none of them are particularly economical -- nor are they likely to be any time soon.

Converting methane to methanol is another story, and we are likely to see a lot more of that particular conversion quite soon. Particularly since there are economical ways of converting methanol to gasoline (via the Exxon Mobil MTG process) as well as to a number of high value chemicals such as ethylene. Methanol is also used in the production of biodiesel, can be blended with gasoline or petro-diesel as a fuel extender, and can be used as a fuel in its own right. Methanol fuel cells are likely to become widely used at scales from power supplies for small consumer electronics, up to household and industrial sizes.

The idea of converting CO2 into fuels is something of a romantic idea -- a chemical version of "poetic justice," but it isn't practical in the modern economic climate. If you want to do something constructive with CO2, feed it to algae farms and green house plants.

Otherwise, feed it to the atmosphere. The Earth's atmosphere has dealt with far higher levels of CO2 than humans can conceivably produce, for billions of years.