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active for methanol synthesis when Cu(111) surface was used. However, under classical industrial conditions for methanol synthesis, carbon dioxide hydrogenation
was responsible for approximatively two-thirds of the methanol production. Carbon
dioxide hydrogenation yielded methanol through HCOO∗, HCOOH∗, CH 3 O 2 ∗,
CH 2 O∗, and CH 3 O∗. Carbon monoxide was also hydrogenated in considerable
amounts to CO∗, HCO∗, CH 2 O∗, CH 3 O∗, and CH 3 OH∗, however, the rate of
CH 3 O∗ hydrogenation was slow.
Further investigations are still required to elucidate the reaction mechanism of
carbon dioxide hydrogenation into methanol over different catalytic systems, which
will contribute to the development of more efficient catalysts and to the basic understanding of methanol synthesis processes.
5.4 Selective Hydrogenation of Carbon Dioxide to Methanol
in the Liquid Phase: Homogeneous Catalysis
5.4.1 Introduction
The hydrogenation of carbon dioxide mediated by heterogeneous catalysts has been
studied extensively in the past, particularly using copper-based catalysts (Lim et al.
2009; Liu et al. 2001; Słoczyński et al. 2003) and palladium-based (Ma et al. 2009;
Shen et al. 2001). However, high operating temperature requirements, e.g., above
200 °C, for heterogeneous systems theoretically limited the yield of entropically
disfavored product, thus resulting in low product selectivity and activity. Also,
understanding heterogeneous catalysis and rational tuning of catalytic activity and
product selectivity remains challenging (Li et al. 2014a, b). A liquid phase methanol
synthesis process is considered favorable for heat management due to better reactor
temperature control, compared to heterogeneous catalysts in a fixed-bed process
(Tominaga et al. 1993). With recent major focus on climate change, the option to
utilize carbon dioxide as a means for its large-scale disposal is emerging. In this
section, metal-based homogenous catalysts and other efficient metal-free organocatalysts including frustrated Lewis pair catalysts and N-heterocyclic carbene catalyst will be reviewed. Photocatalytic hydrogenation of carbon dioxide to methanol
in liquid phase is also included.
5.4.2 Transition Metal Catalysts
Transition metal complexes plays a dominant role in homogeneous hydrogenation,
and among them, ruthenium catalysts exhibited the highest activity for hydrogenation of gaseous carbon dioxide to methanol. Homogeneous hydrogenation of carbon
dioxide to methanol catalyzed by Ru 3 (CO) 12 in the presence of potassium iodide at
240 °C under 90–140 atm was first reported in 1993 by Tominaga and co-workers
5 Selective Hydrogenation of Carbon Dioxide into Methanol
active for methanol synthesis when Cu(111) surface was used. However, under classical industrial conditions for methanol synthesis, carbon dioxide hydrogenation
was responsible for approximatively two-thirds of the methanol production. Carbon
dioxide hydrogenation yielded methanol through HCOO∗, HCOOH∗, CH 3 O 2 ∗,
CH 2 O∗, and CH 3 O∗. Carbon monoxide was also hydrogenated in considerable
amounts to CO∗, HCO∗, CH 2 O∗, CH 3 O∗, and CH 3 OH∗, however, the rate of
CH 3 O∗ hydrogenation was slow.
Further investigations are still required to elucidate the reaction mechanism of
carbon dioxide hydrogenation into methanol over different catalytic systems, which
will contribute to the development of more efficient catalysts and to the basic understanding of methanol synthesis processes.
5.4 Selective Hydrogenation of Carbon Dioxide to Methanol
in the Liquid Phase: Homogeneous Catalysis
5.4.1 Introduction
The hydrogenation of carbon dioxide mediated by heterogeneous catalysts has been
studied extensively in the past, particularly using copper-based catalysts (Lim et al.
2009; Liu et al. 2001; Słoczyński et al. 2003) and palladium-based (Ma et al. 2009;
Shen et al. 2001). However, high operating temperature requirements, e.g., above
200 °C, for heterogeneous systems theoretically limited the yield of entropically
disfavored product, thus resulting in low product selectivity and activity. Also,
understanding heterogeneous catalysis and rational tuning of catalytic activity and
product selectivity remains challenging (Li et al. 2014a, b). A liquid phase methanol
synthesis process is considered favorable for heat management due to better reactor
temperature control, compared to heterogeneous catalysts in a fixed-bed process
(Tominaga et al. 1993). With recent major focus on climate change, the option to
utilize carbon dioxide as a means for its large-scale disposal is emerging. In this
section, metal-based homogenous catalysts and other efficient metal-free organocatalysts including frustrated Lewis pair catalysts and N-heterocyclic carbene catalyst will be reviewed. Photocatalytic hydrogenation of carbon dioxide to methanol
in liquid phase is also included.
5.4.2 Transition Metal Catalysts
Transition metal complexes plays a dominant role in homogeneous hydrogenation,
and among them, ruthenium catalysts exhibited the highest activity for hydrogenation of gaseous carbon dioxide to methanol. Homogeneous hydrogenation of carbon
dioxide to methanol catalyzed by Ru 3 (CO) 12 in the presence of potassium iodide at
240 °C under 90–140 atm was first reported in 1993 by Tominaga and co-workers
5 Selective Hydrogenation of Carbon Dioxide into Methanol