catalysts
Article
Modifications in the Composition of CuO/ZnO/Al
2
O
3
Catalyst
for the Synthesis of Methanol by CO
2
Hydrogenation
Bianca Trifan
1
, Javier Lasobras
2
, Jaime Soler
2,
* , Javier Herguido
2
and Miguel Menéndez
2
Citation: Trifan, B.; Lasobras, J.;
Soler, J.; Herguido, J.; Menéndez, M.
Modifications in the Composition of
CuO/ZnO/Al
2
O
3
Catalyst for the
Synthesis of Methanol by CO
2
Hydrogenation. Catalysts 2021, 11,
774. https://doi.org/10.3390/
catal11070774
Academic Editor: Javier
Ereña Loizaga
Received: 9 June 2021
Accepted: 23 June 2021
Published: 25 June 2021
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1
School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, UK;
TrifanM@cardiff.ac.uk
2
Catalysis, Molecular Separations and Reactor Engineering Group (CREG), Aragon Institute of Engineering
Research (I3A), University of Zaragoza, 50009 Zaragoza, Spain; jlasobra@unizar.es (J.L.);
jhergui@unizar.es (J.H.); miguel.menendez@unizar.es (M.M.)
* Correspondence: jsoler@unizar.es; Tel.: +34-876555481
Abstract: Renewable methanol, obtained from CO
2
and hydrogen provided from renewable en-
ergy, was proposed to close the CO
2
loop. In industry, methanol synthesis using the catalyst
CuO/ZnO/Al
2
O
3
occurs at a high pressure. We intend to make certain modification on the tradi-
tional catalyst to work at lower pressure, maintaining high selectivity. Therefore, three heterogeneous
catalysts were synthesized by coprecipitation to improve the activity and the selectivity to methanol
under mild conditions of temperature and pressure. Certain modifications on the traditional catalyst
Cu/Zn/Al
2
O
3
were employed such as the modification of the synthesis time and the addition of Pd
as a dopant agent. The most efficient catalyst among those tested was a palladium-doped catalyst,
5% Pd/Cu/Zn/Al
2
O
3
. This had a selectivity of 64% at 210
◦
C and 5 bar.
Keywords: CO
2
hydrogenation; methanol synthesis; Cu/ZnO catalyst; Pd catalyst; energy storage
1. Introduction
The scientific community widely considers global warming as a major challenge to
our society. The main cause of this critical issue is the increase in the CO
2
concentration in
the atmosphere due to the massive use of fossil fuels [1], which grew in recent years [2].
One of the ways to address this problem is to use the CO
2
as resource in the synthesis
of valuable products [3]. Renewable methanol, i.e., methanol obtained from CO
2
and
hydrogen provided from renewable energy (solar or wind power), was proposed by the
Nobel Price G. Olah to close the CO
2
loop [4,5].
Methanol is already one of the key basic chemicals, being the second most manu-
factured compound from synthesis gas, after ammonia [6]. It is very soluble in water
and easily biodegradable, making it a viable alternative for large scale efficient energy
storage [7]. It stores both carbon and hydrogen in liquid form and may be converted into
light olefins, gasoline, and hydrocarbons [8]. In industry, methanol is produced using
synthesis gas and the traditional catalyst CuO/ZnO/Al
2
O
3
[9–11]. To optimize the process
of methanol synthesis and obtain high selectivity values, it is essential to be aware of the
reactions that take place in this synthesis.
CO
2
+3H
2
⇄ CH
3
OH + H
2
O ΔH
298K
= −49.5 kJ/mol CO
2
(1)
CO
2
+H
2
⇄ CO + H
2
O ΔH
298K
= −41.2 kJ/mol CO
2
(2)
CO + 2 H
2
⇄ CH
3
OH ΔH
298K
= −90.7 kJ/mol CO (3)
Methanol formation is exothermic and therefore the thermodynamic equilibrium is
favoured at low temperatures, while kinetics is favoured by high temperature. Although
reaction (1) as well as reaction (3) both give methanol as a major product, it was confirmed
Catalysts 2021, 11, 774. https://doi.org/10.3390/catal11070774 https://www.mdpi.com/journal/catalysts