119
5.3.1 Copper-Based Catalysts for Carbon Dioxide
Hydrogenation in the Gas Phase
The copper-based catalysts were initially developed for mixtures of carbon monoxide, carbon dioxide, and hydrogen. They were also studied for pure carbon dioxide
hydrogenation and modified to improve the reactivity of carbon dioxide. Whatever
the carbon source, carbon monoxide or carbon dioxide, copper remains the metal of
choice for methanol synthesis (Goeppert et al. 2014; Wang et al. 2011; Saito 1998),
even if other metals are also studied, as it will be discussed in the next sections.
For most copper-containing methanol catalysts, the catalytic performance is
based on the copper–zirconium oxide couple. The role of zirconium oxide has been
proven to improve the dispersion of copper (Arena et al. 2007, 2009), thus increasing the amount of active sites for the dissociation of hydrogen on the surface of the
catalyst. In addition to the role of dispersant, it is proposed that the presence of
zirconium oxide allows the formation of Cu
+
–O–Zn active species, partially dissolved into copper particles and leading to the formation of a CuZn alloy, which
strongly enhances the catalytic performance (Choi et al. 2001; Behrens et al. 2012).
Thus, zirconium oxide has two functions: (1) it works as a promoter for the dispersion of copper particles, and (2) it allows the formation of surface of copper–zinc
active species which affect the properties for carbon dioxide adsorption (Zander
et al. 2013).
The effect of the oxide support for the copper–zirconium oxide couple has been
extensively studied. The current industrial methanol catalysts are of Cu–ZnO–Al 2 O 3
type. Many works claim that alumina acts as a simple support, without any direct
catalytic activity, that allows improving methanol productivity by favoring active
phase dispersion (Xu et al. 1991; Saito et al. 1996; Li et al. 2014a, b) and inhibiting
sintering of copper particles (Baltes et al. 2008).
The effect of promoters, e.g., silica, titanium oxide, and silica–titanium oxide
mixtures, on the performance of Cu–ZnO–Al 2 O 3 for methanol synthesis from carbon dioxide was investigated by Zhang and co-workers (Zhang et al. 2012). The
results showed that all studied promoters enhanced the dispersion of copper and led
to both better carbon dioxide conversion and methanol selectivity.
Other oxides such as gallium oxide, chromium oxide, ceria, or zirconia were
investigated. The good behavior of Cu–ZnO–Ga 2 O 3 in carbon dioxide hydrogenation to methanol reaction was related to the small gallium(III) oxide, e.g., Ga 2 O 3 ,
particle size which was proposed to favor the formation of Cu
+
(Cai et al. 2015).
Through it led to good performance for the production of methanol, the presence of
Ga 2 O 3 as support also catalyzed the undesired side reactions, and some hydrocarbons, such as methane, ethane, and ethylene, was detected in the products (Toyir
et al. 2001). Chromium(III) oxide, e.g., Cr 2 O 3 , was shown, as gallium(III) oxide, to
promote activity by optimizing the Cu
+
/Cu
0
ratio at the surface of copper particles
(Saito et al. 1996; Toyir et al. 2001). The addition of magnesium oxide, e.g., MgO,
helps reducing copper sintering and, due to its basic properties, is beneficial for
5 Selective Hydrogenation of Carbon Dioxide into Methanol
5.3.1 Copper-Based Catalysts for Carbon Dioxide
Hydrogenation in the Gas Phase
The copper-based catalysts were initially developed for mixtures of carbon monoxide, carbon dioxide, and hydrogen. They were also studied for pure carbon dioxide
hydrogenation and modified to improve the reactivity of carbon dioxide. Whatever
the carbon source, carbon monoxide or carbon dioxide, copper remains the metal of
choice for methanol synthesis (Goeppert et al. 2014; Wang et al. 2011; Saito 1998),
even if other metals are also studied, as it will be discussed in the next sections.
For most copper-containing methanol catalysts, the catalytic performance is
based on the copper–zirconium oxide couple. The role of zirconium oxide has been
proven to improve the dispersion of copper (Arena et al. 2007, 2009), thus increasing the amount of active sites for the dissociation of hydrogen on the surface of the
catalyst. In addition to the role of dispersant, it is proposed that the presence of
zirconium oxide allows the formation of Cu
+
–O–Zn active species, partially dissolved into copper particles and leading to the formation of a CuZn alloy, which
strongly enhances the catalytic performance (Choi et al. 2001; Behrens et al. 2012).
Thus, zirconium oxide has two functions: (1) it works as a promoter for the dispersion of copper particles, and (2) it allows the formation of surface of copper–zinc
active species which affect the properties for carbon dioxide adsorption (Zander
et al. 2013).
The effect of the oxide support for the copper–zirconium oxide couple has been
extensively studied. The current industrial methanol catalysts are of Cu–ZnO–Al 2 O 3
type. Many works claim that alumina acts as a simple support, without any direct
catalytic activity, that allows improving methanol productivity by favoring active
phase dispersion (Xu et al. 1991; Saito et al. 1996; Li et al. 2014a, b) and inhibiting
sintering of copper particles (Baltes et al. 2008).
The effect of promoters, e.g., silica, titanium oxide, and silica–titanium oxide
mixtures, on the performance of Cu–ZnO–Al 2 O 3 for methanol synthesis from carbon dioxide was investigated by Zhang and co-workers (Zhang et al. 2012). The
results showed that all studied promoters enhanced the dispersion of copper and led
to both better carbon dioxide conversion and methanol selectivity.
Other oxides such as gallium oxide, chromium oxide, ceria, or zirconia were
investigated. The good behavior of Cu–ZnO–Ga 2 O 3 in carbon dioxide hydrogenation to methanol reaction was related to the small gallium(III) oxide, e.g., Ga 2 O 3 ,
particle size which was proposed to favor the formation of Cu
+
(Cai et al. 2015).
Through it led to good performance for the production of methanol, the presence of
Ga 2 O 3 as support also catalyzed the undesired side reactions, and some hydrocarbons, such as methane, ethane, and ethylene, was detected in the products (Toyir
et al. 2001). Chromium(III) oxide, e.g., Cr 2 O 3 , was shown, as gallium(III) oxide, to
promote activity by optimizing the Cu
+
/Cu
0
ratio at the surface of copper particles
(Saito et al. 1996; Toyir et al. 2001). The addition of magnesium oxide, e.g., MgO,
helps reducing copper sintering and, due to its basic properties, is beneficial for
5 Selective Hydrogenation of Carbon Dioxide into Methanol