Challenges and Prospects in the Catalytic Conversion of Carbon Dioxide to Formaldehyde

Abstract

Formaldehyde (HCHO) is a crucial C₁ building block for daily-life commodities in a wide range of industrial processes. Industrial production of HCHO today is based on energy- and cost-intensive gas-phase catalytic oxidation of methanol, which calls for exploring other and more sustainable ways of carrying out this process. Utilization of carbon dioxide (CO₂ ) as precursor presents a promising strategy to simultaneously mitigate the carbon footprint and alleviate environmental issues. This Minireview summarizes recent progress in CO₂ -to-HCHO conversion using hydrogenation, hydroboration/hydrosilylation as well as photochemical, electrochemical, photoelectrochemical, and enzymatic approaches. The active species, reaction intermediates, and mechanistic pathways are discussed to deepen the understanding of HCHO selectivity issues. Finally, shortcomings and prospects of the various strategies for sustainable reduction of CO₂ to HCHO are discussed.

Keywords: Carbon Dioxide Hydrogenation; Formaldehyde Production; Hydroboration; Hydrosilylation; Photo/Electrochemistry.

Figure 1

Schematic representation of the scope of the Minireview.

Figure 2

Overview of homogeneous catalysts used for synthesis of DMM from CO₂; Structures are redrawn from the references listed (Tmm=trimethylene methane).

Figure 3

Proposed pathway for CO₂ hydroboration using metal‐based catalysts in which products at the HCOOH (boryl formate), HCHO [bis(boryl)acetal], and CH₃OH (methoxy borane) oxidation levels are obtainable. Note that methoxy borane formation results in production of an equivalent of bis(boryl)oxide; Figure is redrawn from ref. ; Copyright 2018, American Chemical Society.

Figure 4

Overview of catalysts employed for CO₂ hydroboration and hydrosilylation together with the specific hydroborane or hydrosilane reagents used in each case; Structures are redrawn from the references listed.

Figure 5

Proposed overall catalytic cycle for hydrosilylation of CO₂ to the HCHO level by a bis(phosphino)boryl Ni−H complex (Figure 4b5). Reproduced from ref. ; Copyright 2018, American Chemical Society.

Figure 6

Proposed mechanistic pathway (including important transition states) for hydroboration of CO₂ by 1‐Bcat‐2‐PPh₂C₆H₄. Reproduced from ref. ; Copyright 2018, American Chemical Society.

Figure 7

a) Schematic representation of photochemical (PC) reduction. b) Quantity of HCHO produced during CO₂ reduction reaction over CeO₂‐based photocatalysts determined by gas chromatography; Figure reproduced from ref. ; Copyright 2020, Elsevier B.V. c) Schematic representation of electrochemical (EC) reduction. d) FE of products generated in eCO₂RR using various electrodes in CH₃OH electrolyte; Figure reproduced from ref. ; Copyright 2014, John Wiley and Sons. e) Schematic representation of photoelectrochemical (PEC) reduction of CO₂. f) Normalized FE of HCHO plotted against potential in the 040‐BVO|NaCl|Cu system; Figure reproduced from ref. ; Copyright 2017, American Chemical Society.