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U Michigan team demonstrates viability of cascade catalysis for lower temperature conversion of CO2 to methanol


Cascade catalysis for the lower temperature conversion of CO2 to methanol. Credit: ACS, Huff and Sanford. Click to enlarge.

Researchers at the University of Michigan have demonstrated the viability of cascade catalysis—i.e., the use of a series of different homogeneous catalysts operating in a single vessel—for the reduction of CO2 with hydrogen to produce methanol. A paper on their work is published in the Journal of the American Chemical Society.

One potentially attractive approach under investigation for carbon-neutral alternatives to fossil feedstocks is the conversion of CO2 (captured from the atmosphere) with hydrogen (ideally derived from a renewable source) to produce methanol (CH3OH), note Chelsea Huff and Melanie Sanford in their paper. Methanol is both a potential gasoline replacement and a starting material for the synthesis of important platform chemicals, including ethylene and propylene.

Previous work in this area has focused on developing single catalysts that promote the multistep sequence of reduction reactions required to transform CO2 into CH3OH...However, these systems suffer from the significant disadvantage that they require high operating temperatures (200–250 °C), which limits the theoretical yield of the entropically disfavored reduction products. In addition, rational tuning of the reactivity and selectivity of heterogeneous catalysts remains challenging.

For these reasons, significant recent work has aimed at the development of homogeneous catalysts for the low(er) temperature conversion of CO2 to methanol. Several such systems that operate at room temperature and contain tunable supporting ligands have been reported. However, current catalysts generally remain limited by the requirement for impractical and expensive hydrogen sources such as boranes and hydrosilanes.

We sought to address these challenges by exploiting cascade catalysis for the homogeneous catalytic reduction of CO2 with H2 to produce CH3OH. This approach involves the use of a series of different homogeneous catalysts operating in a single vessel to promote the various steps of the CO2 reduction sequence. Importantly, our strategy precludes the requirement of isolating thermodynamically disfavored and/or chemically unstable intermediates (e.g., HCO2H or HCOH, respectively).

Furthermore, it offers the distinct advantage that the rate and selectivity of each step can potentially be tuned by simply substituting an alternative catalyst. The major challenge for this approach is to identify a series of catalysts that are compatible with one another, operate effectively under the same reaction conditions, and are not poisoned by catalytic intermediates and/or products.

—Huff and Sanford

Huff and Sanford targeted a cascade catalysis sequence involving:

  1. hydrogenation of CO2 to formic acid
  2. esterification to generate a formate ester
  3. 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

Resources

  • Chelsea A. Huff and Melanie S. Sanford (2011) Cascade Catalysis for the Homogeneous Hydrogenation of CO2 to Methanol. Journal of the American Chemical Society DOI:

Comments

HarveyD

Done on a very large scale in a few thousand places, it could eventually turn excess CO2 into usable liquid fuels....and start the cycle all over again?

Henry Gibson

There have been some stories about feeding hydrogen and CO2 or CO to ethanol fermenting units. It is a known step to make CO out of H2 and CO2 if necessary. In fact every ethanol producer that has natural gas should make some hydrogen and CO2 from methane to feed to the fermentation bath to save on corn expense. The hydrogen can be made from plant residues as well. The fermentation process produces some CO2 from sugar. ..HG..

mahonj

Presumably you would get the CO2 from a power station.
Where you get the H2 is another matter.
If you are generating it from electricity, why not put the electricity into the grid.
(Unless you have excess) (Night time or very windy).

In which case, why not generate H2 and O2 and react them in the same power station.
You might be able to improve the combustion efficiency of the coal or gas using the H2 and O2.

[ I am guessing here ]
Anyone qualified to comment or correct ?

Roger Pham

Significant degree of efficiency lost will incur when converting H2 into methanol. Getting CO2 out of the air requires significant amount of energy input. CO2 + H2 =methanol is another energy losing step. Methanol is only important as transportation fuel since it is easy to carry and dispense. Methanol is highly corrosive and poisonous, making it unsuitable for transportation on pipeline.

On a volume basis, methanol has 4 times the energy density as compressed H2 at 350 bar, however, methanol combustion engine has about half the peak efficiency of a PEM-H2 Fuel cell.
Given that, methanol only has twice the energy density as H2 in surface transportation. Direct Methanol fuel cell has low power density and is inefficient, therefore, is not viable for powering transportation.

Davemart

Roger,
Transporting methanol can be managed.
The main problem would appear to be the low power density of direct methanol fuel cells, but it is not clear that the difficulty is insurmountable and in view of the storage problems etc of hydrogen it would seem premature to write off hydrogen as a transport fuel, although it is certainly not viable with current technology.

Engineer-Poet

If you can find a way to make NADH either from NAD+ and hydrogen or by electrolysis, .

A D

I said many times to capture the co2 from big chimneys and recirculate it in doing methanol and then refeed it at the input for old natural gas and coal power plants. There is no shame in recirculating the fuel endlessly in any fuel consuming devises

HarveyD

Sharp/Japan will soon produce solar cells with 40% efficiency. Will all the unused very sunny desert and semi-desert land areas around, the world will not have to worry much about future clean energy. More effective E-energy transportation and lower cost storage require more research.

A. Rossi's E-cat reactors may be another alternative way to get unlimited clean energy. If it ever comes about at an affordable price, (which is somewhat doubtful) the steam turbines may make a very strong come back.

Account Deleted

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