132
are subsequently hydrogenated to dioxymethylene and ∗H 2 COOH, which is then
cleaved into formaldehyde, e.g., ∗H 2 CO, and hydroxyl groups, e.g., ∗OH. Finally,
formaldehyde is hydrogenated into methoxy, e.g., ∗H 3 CO, or methylenoxy, e.g.,
∗H 2 COH, before final hydrogenation into methanol (Dang et al. 2018). In both cases,
carbon dioxide is usually considered as the main carbon source for methanol
synthesis.
Kim and co-workers (2017) investigated the reaction mechanism of carbon dioxide
hydrogenation to methanol over a Cu/ZrO 2 catalyst. Catalytic tests were performed in
a fixed-bed flow reactor at 230 °C and 25 bars with the molar ratio of hydrogen to
carbon dioxide equal to 3 to 1. They concluded that carbon dioxide was initially transformed into carbonate or bicarbonate, formate, and methoxy after adsorption and
hydrogenation on the Cu/ZrO 2 catalyst. This mechanism highlights the formation of
formate as key intermediate for methanol synthesis, which was formed at a rate equal
to 15 μmol s
−1
g Cu
−1
when both copper and zirconia were present. When copper and
silica were used as a catalyst, methanol was formed at a much lower rate of about
5 μmol s
−1
g Cu
−1
. These results showed the important role of the copper–zirconia interface for methanol production. Similarly, Hus and co-workers (2017) reported that the
formate pathway predominates during methanol synthesis on copper-based catalysts,
accounting for the bulk methanol production. The key intermediates of the reaction
were HCOO, H 2 COO, H 2 COOH, H 2 CO, and H 3 CO with the formation of H 2 COO and
H 2 COOH being the rate-determining steps (Huš et al. 2017a, b).
As previously mentioned, copper-based catalysts are the most used catalysts for
carbon dioxide hydrogenation to methanol. However, the copper species that are
considered as the active phase is still the object of some controversy. Liu and
co- workers (2017) investigated the influence of Cu(111) and Cu 2 O(111) on methanol production from the hydrogenation of carbon monoxide and carbon dioxide
mixtures and reverse water-gas shift reaction. The hydrogenation of carbon monoxide and carbon dioxide mixtures was performed at 280 °C and 80 atm with the molar
ratio of carbon monoxide and carbon dioxide mixtures to hydrogen equaled 20 to
80. The results revealed that the main source of carbon for methanol synthesis can
be either carbon dioxide or carbon monoxide, depending on the copper species present on the catalyst. Carbon dioxide is indeed the main carbon source when metallic
copper is involved. However, when Cu
+
species are present, carbon monoxide is the
primary carbon source. On the Cu(111) surface, carbon monoxide hydrogenation
yields methanol through H 2 CO∗ and H 3 CO∗ intermediates, and carbon dioxide is
hydrogenated via HCOO∗, H 2 COOH∗, and H 2 CO∗. On the other hand, on the
Cu 2 O(111) surface, carbon dioxide hydrogenation yields methanol via HCOO∗,
H 2 COOH∗, and H 2 COH∗, while carbon monoxide hydrogenation proceeds via
H 2 CO∗ and H 2 COH∗. These results highlight the importance of the catalyst preparation, which has an influence on the surface metal species, to the methanol production from carbon dioxide hydrogenation.
Grabow and co-workers (2011) also compared the mechanism for methanol synthesis on commercial Cu–ZnO–Al 2 O 3 catalyst with Cu(111) surface through carbon
dioxide and carbon monoxide hydrogenation. In contrast to previous studies, both
carbon dioxide and carbon monoxide hydrogenation pathways were reported to be
D. P. Minh et al.
are subsequently hydrogenated to dioxymethylene and ∗H 2 COOH, which is then
cleaved into formaldehyde, e.g., ∗H 2 CO, and hydroxyl groups, e.g., ∗OH. Finally,
formaldehyde is hydrogenated into methoxy, e.g., ∗H 3 CO, or methylenoxy, e.g.,
∗H 2 COH, before final hydrogenation into methanol (Dang et al. 2018). In both cases,
carbon dioxide is usually considered as the main carbon source for methanol
synthesis.
Kim and co-workers (2017) investigated the reaction mechanism of carbon dioxide
hydrogenation to methanol over a Cu/ZrO 2 catalyst. Catalytic tests were performed in
a fixed-bed flow reactor at 230 °C and 25 bars with the molar ratio of hydrogen to
carbon dioxide equal to 3 to 1. They concluded that carbon dioxide was initially transformed into carbonate or bicarbonate, formate, and methoxy after adsorption and
hydrogenation on the Cu/ZrO 2 catalyst. This mechanism highlights the formation of
formate as key intermediate for methanol synthesis, which was formed at a rate equal
to 15 μmol s
−1
g Cu
−1
when both copper and zirconia were present. When copper and
silica were used as a catalyst, methanol was formed at a much lower rate of about
5 μmol s
−1
g Cu
−1
. These results showed the important role of the copper–zirconia interface for methanol production. Similarly, Hus and co-workers (2017) reported that the
formate pathway predominates during methanol synthesis on copper-based catalysts,
accounting for the bulk methanol production. The key intermediates of the reaction
were HCOO, H 2 COO, H 2 COOH, H 2 CO, and H 3 CO with the formation of H 2 COO and
H 2 COOH being the rate-determining steps (Huš et al. 2017a, b).
As previously mentioned, copper-based catalysts are the most used catalysts for
carbon dioxide hydrogenation to methanol. However, the copper species that are
considered as the active phase is still the object of some controversy. Liu and
co- workers (2017) investigated the influence of Cu(111) and Cu 2 O(111) on methanol production from the hydrogenation of carbon monoxide and carbon dioxide
mixtures and reverse water-gas shift reaction. The hydrogenation of carbon monoxide and carbon dioxide mixtures was performed at 280 °C and 80 atm with the molar
ratio of carbon monoxide and carbon dioxide mixtures to hydrogen equaled 20 to
80. The results revealed that the main source of carbon for methanol synthesis can
be either carbon dioxide or carbon monoxide, depending on the copper species present on the catalyst. Carbon dioxide is indeed the main carbon source when metallic
copper is involved. However, when Cu
+
species are present, carbon monoxide is the
primary carbon source. On the Cu(111) surface, carbon monoxide hydrogenation
yields methanol through H 2 CO∗ and H 3 CO∗ intermediates, and carbon dioxide is
hydrogenated via HCOO∗, H 2 COOH∗, and H 2 CO∗. On the other hand, on the
Cu 2 O(111) surface, carbon dioxide hydrogenation yields methanol via HCOO∗,
H 2 COOH∗, and H 2 COH∗, while carbon monoxide hydrogenation proceeds via
H 2 CO∗ and H 2 COH∗. These results highlight the importance of the catalyst preparation, which has an influence on the surface metal species, to the methanol production from carbon dioxide hydrogenation.
Grabow and co-workers (2011) also compared the mechanism for methanol synthesis on commercial Cu–ZnO–Al 2 O 3 catalyst with Cu(111) surface through carbon
dioxide and carbon monoxide hydrogenation. In contrast to previous studies, both
carbon dioxide and carbon monoxide hydrogenation pathways were reported to be
D. P. Minh et al.