83
bifunctional route. The reaction was found to be related to the exposed copper
surface area, and the methanol selectivity is connected to the amount of basic sites
(Dong et al. 2016).
Regarding Cu-ZrO 2 catalysts, the high methanol turnover frequency (TOF) of
Cu/tetragonal-ZrO 2 with respect to Cu/amorphous-ZrO 2 and Cu/monoclinic-ZrO 2
was attributed to the enhanced spillover of atomic hydrogen from the copper surface
to the zirconia surface. This behavior caused an increase for the surface concentration of atomic hydrogen to carbon dioxide (Witoon et al. 2016).
The impregnation of the precious metals like palladium on ZnO (Bahruji et al.
2016), Cu-ZnO/MCM-41 (Siriworarat et al. 2017), or Cu-Zn/SiC (Díez-Ramírez
et al. 2017) enhanced the selectivity of the parent catalysts towards the methanol
production at a relatively lower reaction temperature (250 °C). Such activity was
attributed to the influence of palladium on the hydrogen spillover and the controlling of the size of the nanoparticles. The other precious metals such as gold and
silver enhanced the selectivity of the parent catalysts towards the hydrogenation of
the carbon dioxide into methanol (Sloczynski et al. 2004; Grabowski et al. 2011;
Asara et al. 2015; Vourros et al. 2017).
Also, the incorporation of a small amount of Ga
3+
on Cu/ZnO nanocatalyst facilitated the thermal deep reduction of ZnO support to Zn atoms in presence of the
hydrogen molecules, which created highly active catalytic sites towards the catalytic hydrogenation of carbon dioxide to methanol. Successively, selectivity towards
the formation of methanol significantly improved by increasing the Zn
0
content (Li
et al. 2016) and Cu
0
content (Liu et al. 2003; Behrens et al. 2012) in the catalyst.
Reduced perovskites such as LaCr 0.5 Cu 0.5 O 3 catalyst showed a medium catalytic
conversion (10.4%) and high selectivity (90.8%) at moderate reaction temperature
250 °C (Jia et al. 2009). This activity was attributed to the presence of two active
sites in the structure of the reduced perovskite:
1. Cu
α+
for hydrogen adsorption
2. Cu
2+
-O-Cr
4+
for carbon dioxide activation.
Also, this catalyst was distinguished by high selectivity towards the hydrogenation of carbon dioxide to methanol due to the presence of basic site created by lanthanum species, which facilitate the adsorption of reacted carbon dioxide having an
acidic nature. This attitude was noticed in another copper-based catalyst containing
CaO (Wisaijorn et al. 2017). Doped copper bromide catalyst by zirconium enhanced
the conversion of carbon dioxide to methanol to be achieved at low reaction temperature lower than 225 °C (Liaw and Chen 2001).
Generally, the drawbacks of the conventional catalysts can be summarized into
two major categories, which are the low yield of methanol and the necessary of high
reaction conditions (such as reaction temperature higher than 200 °C and reaction
pressure higher than 25 bar) (Ye and Johnson 2016).
3 Application of Metal Organic Frameworks in Carbon Dioxide Conversion to Methanol
bifunctional route. The reaction was found to be related to the exposed copper
surface area, and the methanol selectivity is connected to the amount of basic sites
(Dong et al. 2016).
Regarding Cu-ZrO 2 catalysts, the high methanol turnover frequency (TOF) of
Cu/tetragonal-ZrO 2 with respect to Cu/amorphous-ZrO 2 and Cu/monoclinic-ZrO 2
was attributed to the enhanced spillover of atomic hydrogen from the copper surface
to the zirconia surface. This behavior caused an increase for the surface concentration of atomic hydrogen to carbon dioxide (Witoon et al. 2016).
The impregnation of the precious metals like palladium on ZnO (Bahruji et al.
2016), Cu-ZnO/MCM-41 (Siriworarat et al. 2017), or Cu-Zn/SiC (Díez-Ramírez
et al. 2017) enhanced the selectivity of the parent catalysts towards the methanol
production at a relatively lower reaction temperature (250 °C). Such activity was
attributed to the influence of palladium on the hydrogen spillover and the controlling of the size of the nanoparticles. The other precious metals such as gold and
silver enhanced the selectivity of the parent catalysts towards the hydrogenation of
the carbon dioxide into methanol (Sloczynski et al. 2004; Grabowski et al. 2011;
Asara et al. 2015; Vourros et al. 2017).
Also, the incorporation of a small amount of Ga
3+
on Cu/ZnO nanocatalyst facilitated the thermal deep reduction of ZnO support to Zn atoms in presence of the
hydrogen molecules, which created highly active catalytic sites towards the catalytic hydrogenation of carbon dioxide to methanol. Successively, selectivity towards
the formation of methanol significantly improved by increasing the Zn
0
content (Li
et al. 2016) and Cu
0
content (Liu et al. 2003; Behrens et al. 2012) in the catalyst.
Reduced perovskites such as LaCr 0.5 Cu 0.5 O 3 catalyst showed a medium catalytic
conversion (10.4%) and high selectivity (90.8%) at moderate reaction temperature
250 °C (Jia et al. 2009). This activity was attributed to the presence of two active
sites in the structure of the reduced perovskite:
1. Cu
α+
for hydrogen adsorption
2. Cu
2+
-O-Cr
4+
for carbon dioxide activation.
Also, this catalyst was distinguished by high selectivity towards the hydrogenation of carbon dioxide to methanol due to the presence of basic site created by lanthanum species, which facilitate the adsorption of reacted carbon dioxide having an
acidic nature. This attitude was noticed in another copper-based catalyst containing
CaO (Wisaijorn et al. 2017). Doped copper bromide catalyst by zirconium enhanced
the conversion of carbon dioxide to methanol to be achieved at low reaction temperature lower than 225 °C (Liaw and Chen 2001).
Generally, the drawbacks of the conventional catalysts can be summarized into
two major categories, which are the low yield of methanol and the necessary of high
reaction conditions (such as reaction temperature higher than 200 °C and reaction
pressure higher than 25 bar) (Ye and Johnson 2016).
3 Application of Metal Organic Frameworks in Carbon Dioxide Conversion to Methanol