82
3.4 Catalytic Hydrogenation of Carbon Dioxide
It is well known that carbon dioxide has many advantages, to be used for the production of methanol, such as its abundance, low cost, and nontoxicity (Huo et al.
2012). However, due to the high thermodynamic stability and low reactivity of the
carbon dioxide, the use of catalysts in presence of severe conditions like the high
pressure and temperature is essential to achieve the hydrogenation reaction
(Arakawa et al. 2001).
The products of the hydrogenation of carbon dioxide could be methanol or
methane based on the stoichiometry of the reactants, the type of catalyst, and the
reaction experimental conditions according to the following equations (Barbarossa
et al. 2014):
CO 4H
CH
H O
2
2
4
2
2
+
→
+
(3.1)
CO 3H
CH OH H O
2
2
3
2
+
→
+
(3.2)
Regarding the thermodynamic equilibrium, it was found that the sorption of the
produced water from the reaction medium had improved the production of the
methanol (Zachopoulos and Heracleous 2017).
Compared with the heterogeneous catalysts, the activity of the homogeneous
catalysts is very high (higher than 95% CO 2 conversion) at relatively low reaction
temperatures (lower than 145 °C). However, the homogenous reaction cannot be
accomplished without the assist of the high processing pressures (lower than 60 bar),
which are higher than the required pressure to achieve the heterogeneous catalytic
reaction (Ye and Johnson 2016).
3.4.1 Hydrogenation of Carbon Dioxide Using Transition
Metal Catalysts
Cu-based catalysts like Cu/ZnO/Al 2 O 3 (Deng et al. 1996), Cu/ZnO (Vesborg et al.
2009; Yang et al. 2010), Cu/ZnO/ZrO 2 (Guo et al. 2011), and Cu/ZnO/ZSM-5
(Ayodele et al. 2017) have been often utilized for methanol synthesis, whereas the
copper sites were acting as active sites while the oxide were acting as adsorbent for
the reactants.
In comparison with Cu/ZnO catalyst (Huo et al. 2012), the catalytic activity of
reduced Cu/ZnO/Al 2 O 3 (Dong et al. 2016) showed similar carbon dioxide conversion up to 14.4% with higher selectivity towards methanol production ~61% at
lower reaction temperature (230 °C). The mechanism of the carbon dioxide
hydrogenation using the reduced catalyst was found to be achieved through the
T. Zaki
3.4 Catalytic Hydrogenation of Carbon Dioxide
It is well known that carbon dioxide has many advantages, to be used for the production of methanol, such as its abundance, low cost, and nontoxicity (Huo et al.
2012). However, due to the high thermodynamic stability and low reactivity of the
carbon dioxide, the use of catalysts in presence of severe conditions like the high
pressure and temperature is essential to achieve the hydrogenation reaction
(Arakawa et al. 2001).
The products of the hydrogenation of carbon dioxide could be methanol or
methane based on the stoichiometry of the reactants, the type of catalyst, and the
reaction experimental conditions according to the following equations (Barbarossa
et al. 2014):
CO 4H
CH
H O
2
2
4
2
2
+
→
+
(3.1)
CO 3H
CH OH H O
2
2
3
2
+
→
+
(3.2)
Regarding the thermodynamic equilibrium, it was found that the sorption of the
produced water from the reaction medium had improved the production of the
methanol (Zachopoulos and Heracleous 2017).
Compared with the heterogeneous catalysts, the activity of the homogeneous
catalysts is very high (higher than 95% CO 2 conversion) at relatively low reaction
temperatures (lower than 145 °C). However, the homogenous reaction cannot be
accomplished without the assist of the high processing pressures (lower than 60 bar),
which are higher than the required pressure to achieve the heterogeneous catalytic
reaction (Ye and Johnson 2016).
3.4.1 Hydrogenation of Carbon Dioxide Using Transition
Metal Catalysts
Cu-based catalysts like Cu/ZnO/Al 2 O 3 (Deng et al. 1996), Cu/ZnO (Vesborg et al.
2009; Yang et al. 2010), Cu/ZnO/ZrO 2 (Guo et al. 2011), and Cu/ZnO/ZSM-5
(Ayodele et al. 2017) have been often utilized for methanol synthesis, whereas the
copper sites were acting as active sites while the oxide were acting as adsorbent for
the reactants.
In comparison with Cu/ZnO catalyst (Huo et al. 2012), the catalytic activity of
reduced Cu/ZnO/Al 2 O 3 (Dong et al. 2016) showed similar carbon dioxide conversion up to 14.4% with higher selectivity towards methanol production ~61% at
lower reaction temperature (230 °C). The mechanism of the carbon dioxide
hydrogenation using the reduced catalyst was found to be achieved through the
T. Zaki