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: 10.1021/ja208760