Challenges in the Greener Production of Formates/Formic Acid, Methanol, and DME by Heterogeneously Catalyzed CO2 Hydrogenation Processes

Abstract

The recent advances in the development of heterogeneous catalysts and processes for the direct hydrogenation of CO₂ to formate/formic acid, methanol, and dimethyl ether are thoroughly reviewed, with special emphasis on thermodynamics and catalyst design considerations. After introducing the main motivation for the development of such processes, we first summarize the most important aspects of CO₂ capture and green routes to produce H₂. Once the scene in terms of feedstocks is introduced, we carefully summarize the state of the art in the development of heterogeneous catalysts for these important hydrogenation reactions. Finally, in an attempt to give an order of magnitude regarding CO₂ valorization, we critically assess economical aspects of the production of methanol and DME and outline future research and development directions.

Figure 1

Scope and aspects covered in this review.

Figure 2

Heterogeneous catalytic systems reported for the CO₂ hydrogenation to formic acid/formates.

Figure 3

Immobilization strategies of molecular complexes on grafted solid support for CO₂ hydrogenation. These “precatalyst” structures are created based on (a) refs (76) (n = 1,3), (75) (n = 2), and (77) (n = 3); (b) refs (−;) (c) ref (81); (d) refs (82) and (83); and (e) refs (, , and 85).

Figure 4

Immobilization of Ru/Ir molecular complexes on porous organic polymers. (a) Created after ref (89). (b) Adapted with permission from ref (90). Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA. (c) Adapted with permission from ref (91). Copyright 2016 Elsevier.

Figure 5

Immobilization of an Ir complex on CTF-based spheres. Reproduced with permission from ref (98). Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 6

Proposed mechanism for reversible CO₂ hydrogenation to formic acid using binuclear IrCp* catalyst with a thbpym ligand. Reproduced with permission from ref (65). Copyright 2012 Macmillan Publishers Ltd.

Figure 7

Mechanism of CO₂ hydrogenation over supported Au nanoparticles. Reproduced with permission from ref (58). Copyright 2016 Elsevier.

Figure 8

Mechanism of CO₂ hydrogenation over PdNi bimetallic surface. Reproduced with permission from ref (60). Copyright 2015 The Royal Society of Chemistry.

Figure 9

Chemical recycling of CO₂ to methanol and DME.

Figure 10

Equilibrium CO₂ conversion and methanol selectivity at different temperatures with initial H₂/CO₂ mixtures of 3 (left) and 10 (right), and at (a) 10 bar, (b) 30 bar, (c) 100 bar, (d) 200 bar, (e) 300 bar, (f) 400 bar, and (g) 500 bar. The calculation was performed with the same method as described in refs ( and 118).

Figure 11

Types of catalyst material reported for CO₂ hydrogenation to methanol. The percentages have been calculated based on ca. 200 papers selected from a Scopus search from 2006 to 2016 on the principal catalyst materials.

Figure 12

Microstructural features revealed by TEM of Cu–ZnO–Al₂O₃ catalyst. Adapted with permission from ref (132). Copyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 13

Synthesis methods of Cu–ZnO and Cu–ZnO–promoter catalysts. The percentage was calculated based on the publications over the past 10 years.

Figure 14

Simplified preparation procedure of Cu–ZnO or Cu–ZnO–Al₂O₃ via coprecipitation with widely used chemicals and resulting solid precursor phases.

Figure 15

Reported bed configurations of physically mixed catalysts in a fixed bed reactor.

Figure 16

Proposed reaction mechanisms for the CO₂ hydrogenation toward methanol.

Figure 17

Proposed reaction mechanism of methanol dehydration to DME. Adapted with permission from ref (247). Copyright 2016 American Chemical Society.

Figure 18

Conversion profiles (upper panel, bold lines) and equilibrium curve (upper panel, thin lines) for three types of conventional methanol synthesis reactors (lower panels): (a) tubular boiling water reactor; (b) series quench reactor; (c) series adiabatic reactor with interstage cooling. Reproduced with permission from ref (135). Copyright 1997 Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 19

Power requirements for water electrolysis (P_el), CO₂ capture (P_cap), and compression of hydrogen (P_compH2) and CO₂ (P_compCO2) at various reaction pressures in methanol synthesis by CO₂ hydrogenation. Reprinted with permission ref (184). Copyright 2016 Elsevier.