Homogeneous Hydrogenation of CO2 and CO to Methanol: The Renaissance of Low-Temperature Catalysis in the Context of the Methanol Economy

Abstract

The traditional economy based on carbon-intensive fuels and materials has led to an exponential rise in anthropogenic CO₂ emissions. Outpacing the natural carbon cycle, atmospheric CO₂ levels increased by 50 % since the pre-industrial age and can be directly linked to global warming. Being at the core of the proposed methanol economy pioneered by the late George A. Olah, the chemical recycling of CO₂ to produce methanol, a green fuel and feedstock, is a prime channel to achieve carbon neutrality. In this direction, homogeneous catalytic systems have lately been a major focus for methanol synthesis from CO₂ , CO and their derivatives as potential low-temperature alternatives to the commercial processes. This Review provides an account of this rapidly growing field over the past decade, since its resurgence in 2011. Based on the critical assessment of the progress thus far, the present key challenges in this field have been highlighted and potential directions have been suggested for practically viable applications.

Keywords: CO2 recycling; carbon capture and utilization; homogeneous catalysis; hydrogenation; renewable methanol.

Figure 1

Sustainable production based on Power‐to‐Liquid (PtL) and Carbon Capture and Utilization (CCU).

Figure 2

a) Global methanol demand and production capacity and b) methanol usage by industrial sectors. Based on data from MMSA.[ ²⁹ , ⁴¹ ]

Figure 3

Selected examples of the first‐generation homogeneous catalysts instrumental in developing CO₂ hydrogenation to methanol.

Figure 4

CO₂ reduction to methanol: selected categories of H‐source, catalysis and process covered by this Review (framed).

Figure 5

Cascade catalysis of CO₂ to methanol. Based on ref.  and ref. .

Figure 6

Ruthenium–triphos catalysis for CO₂ to methanol. Based on ref. .

Figure 7

Biphasic system to separate methanol and catalyst. Adapted from ref.  with permission from the Royal Society of Chemistry.

Figure 8

Comparative performances of Ru‐4 and Ru‐6. Adapted from ref.  with permission from the American Chemical Society.

Figure 9

Comparison of Ru catalysts based on neutral C‐triphos, N‐triphos and cationic ^MeN‐triphos.

Figure 10

Amine‐assisted one‐pot CO₂‐to‐methanol system.

Figure 11

Amine‐assisted sequential methanol synthesis from CO₂ via N‐formylation.

Figure 12

Low‐pressure CO₂‐to‐methanol route via oxazolidinone. Based on ref. .

Figure 13

Amine‐assisted CO₂ hydrogenation to methanol using Ru‐Macho‐BH (Ru‐9). a) Recycling of reaction components with PEHA and CO₂:H₂ (1 : 9); b) Repeated pressure refill experiment with PEHA and CO₂:H₂ (1 : 3); c) Effect of amine molecular structures on methanol formation; and d) Effect of solvent volume on yields of methanol and intermediates with PEHA. Adapted from ref.  and ref.  with permission from the American Chemical Society.

Figure 14

Pyrrolizidine‐assisted CO₂ hydrogenation to methanol.

Figure 15

Ruthenium catalysts for amine‐assisted CO₂ to methanol.

Figure 16

Co‐production of glycol and methanol from epoxide and CO₂.

Figure 17

MOF‐encapsulated catalysts for one‐pot CO₂ to methanol. Adapted from ref.  and ref.  with permission from Elsevier Inc. and American Chemical Society.

Figure 18

CO₂ to methanol via formic acid disproportionation.

Figure 19

Multinuclear Ir complexes for gas–solid phase hydrogenation of CO₂ to methanol.

Figure 20

Transfer hydrogenation of CO₂ to methanol using ethanol as H‐source.

Figure 21

Solid supported amines for CO₂ hydrogenation to methanol. Reproduced from ref.  with permission from John Wiley and Sons.

Figure 22

Cobalt‐based catalysis with triphos‐derived ligands.

Figure 23

Sequential amine‐assisted CO₂ to methanol using base metal catalysts.

Figure 24

Mn‐catalyzed direct hydrogenation of CO₂ to methanol.

Figure 25

Fe–scorpionate catalysis for CO₂ to methanol.

Figure 26

Indirect methanol synthesis from various derivatives and capture products of CO₂.

Figure 27

CO₂ capture with amines and metal hydroxides.

Figure 28

Integrated carbon capture and conversion to methanol. Based on ref. .

Figure 29

Homogeneous catalytic systems for hydrogenation of CO to methanol.

Figure 30

Modes of bond activation by metal–PN(H)P complexes.

Figure 31

Proposed catalytic route with Ru–PN(H)P catalysts for amine‐assisted CO₂ hydrogenation to methanol. Adapted from ref.  with permission from the American Chemical Society.

Figure 32

Mechanistic insights into the role of catalyst molecular structure on methanol formation. A) Effect of modulating catalyst structure on methanol formation, B) Reactivation of biscarbonyl complexes under H₂, C) Single crystal X‐ray structure of deactivating complexes, D) Observation of carbonyl peaks in ¹³C NMR spectrum, E) Lability trend of axial carbonyl as seen in IR. Adapted from ref.  and ref.  with permission from the American Chemical Society.

Figure 33

Addition of heterogeneous Lewis acid (ZnO) to enhance CO₂ hydrogenation to methanol. Adapted from ref.  with permission from the American Chemical Society.

Figure 34

Modes of bond activation by aromatized Ru–PNP complexes.

Figure 35

Catalytic cycle for aromatized Ru–PNP/PNN complexes for hydrogenation of A) carbamic acid derivatives and B) CO₂. Adapted from ref.  and ref.  with permission from the Springer Nature and American Chemical Society.

Figure 36

Mechanistic insights and catalytic cycle of Ru–triphos‐based CO₂ to methanol. Adapted from ref.  with permission from the Royal Society of Chemistry.

Figure 37

Anthropogenic carbon cycle in the context of a circular methanol economy. Reproduced from ref.  with permission from Springer Nature.