131
In summary, recent studies in carbon dioxide hydrogenation into methanol have
considered carbon dioxide as the main source of carbon for methanol synthesis.
Moreover, it is accepted that power law models are very useful when the reaction
mechanism is not yet defined and to investigate the influence of different reaction
parameters in the methanol production, which can be used for reactor design and
operation of chemical reactors. However, microkinetic models, such as those developed by Ovesen and co-workers (1996) and Van den Bussche and Froment (1996),
take into consideration the reaction mechanism and can be used to predict other
parameters, such as modification of catalyst morphology during the methanol reaction. Finally, most of the microkinetic models developed for classical Cu–ZnO–
Al 2 O 3 catalyst take into consideration that only copper active site participate in the
reaction. However, a few recent reports showed that other metals, such as zinc,
could also play a role in the reaction.
5.3.6 Mechanistic Study of the Hydrogenation of Carbon
Dioxide to Methanol in the Gas Phase
The reaction mechanism of carbon dioxide hydrogenation into methanol is still
under debate and is accepted to be very different depending on the catalytic system
used (Dang et al. 2018; Felix et al. 2015). This section will present an overview of
the main reaction mechanisms of carbon dioxide hydrogenation to methanol presented in the literature.
Copper-based catalysts are the most used catalytic system for the hydrogenation
of carbon monoxide and carbon dioxide mixtures to methanol, in which Cu–ZnO–
Al 2 O 3 is the catalyst currently used at commercial scale (Kobl et al. 2016). The
mechanism for carbon dioxide hydrogenation over copper-based catalysts has been
proposed by many researchers in the literature, and, despite some controversy, it is
generally accepted that metallic copper is the active phase for methanol synthesis
(Dang et al. 2018). Moreover, two major reaction mechanisms have been proposed
in the literature for carbon dioxide hydrogenation over copper-based catalysts.
The first major mechanism proposed is the redox mechanism, in which carbon
monoxide is formed via reverse water-gas shift reaction and syngas is then converted to methanol (Jadhav et al. 2014; Tursunov et al. 2017). Klier and co-workers
(1982) used this mechanism to explain the promoting/inhibiting effect of carbon
dioxide on methanol synthesis over copper-based catalysts. The authors reported
that the maximum rate of methanol production is determined by a balance between
the promoting effect of carbon dioxide that keep the catalyst in an active phase
through its oxidizing capacity and the inhibiting effect from its strong adsorption
when high carbon dioxide concentrations are used.
In the second mechanism, carbon dioxide reacts with atomic hydrogen to generate
formate, e.g., ∗HCOO, species as an intermediate. This step is reported to occur
either via Langmuir–Hinshelwood or Eley–Rideal mechanism. The ∗HCOO species
5 Selective Hydrogenation of Carbon Dioxide into Methanol
In summary, recent studies in carbon dioxide hydrogenation into methanol have
considered carbon dioxide as the main source of carbon for methanol synthesis.
Moreover, it is accepted that power law models are very useful when the reaction
mechanism is not yet defined and to investigate the influence of different reaction
parameters in the methanol production, which can be used for reactor design and
operation of chemical reactors. However, microkinetic models, such as those developed by Ovesen and co-workers (1996) and Van den Bussche and Froment (1996),
take into consideration the reaction mechanism and can be used to predict other
parameters, such as modification of catalyst morphology during the methanol reaction. Finally, most of the microkinetic models developed for classical Cu–ZnO–
Al 2 O 3 catalyst take into consideration that only copper active site participate in the
reaction. However, a few recent reports showed that other metals, such as zinc,
could also play a role in the reaction.
5.3.6 Mechanistic Study of the Hydrogenation of Carbon
Dioxide to Methanol in the Gas Phase
The reaction mechanism of carbon dioxide hydrogenation into methanol is still
under debate and is accepted to be very different depending on the catalytic system
used (Dang et al. 2018; Felix et al. 2015). This section will present an overview of
the main reaction mechanisms of carbon dioxide hydrogenation to methanol presented in the literature.
Copper-based catalysts are the most used catalytic system for the hydrogenation
of carbon monoxide and carbon dioxide mixtures to methanol, in which Cu–ZnO–
Al 2 O 3 is the catalyst currently used at commercial scale (Kobl et al. 2016). The
mechanism for carbon dioxide hydrogenation over copper-based catalysts has been
proposed by many researchers in the literature, and, despite some controversy, it is
generally accepted that metallic copper is the active phase for methanol synthesis
(Dang et al. 2018). Moreover, two major reaction mechanisms have been proposed
in the literature for carbon dioxide hydrogenation over copper-based catalysts.
The first major mechanism proposed is the redox mechanism, in which carbon
monoxide is formed via reverse water-gas shift reaction and syngas is then converted to methanol (Jadhav et al. 2014; Tursunov et al. 2017). Klier and co-workers
(1982) used this mechanism to explain the promoting/inhibiting effect of carbon
dioxide on methanol synthesis over copper-based catalysts. The authors reported
that the maximum rate of methanol production is determined by a balance between
the promoting effect of carbon dioxide that keep the catalyst in an active phase
through its oxidizing capacity and the inhibiting effect from its strong adsorption
when high carbon dioxide concentrations are used.
In the second mechanism, carbon dioxide reacts with atomic hydrogen to generate
formate, e.g., ∗HCOO, species as an intermediate. This step is reported to occur
either via Langmuir–Hinshelwood or Eley–Rideal mechanism. The ∗HCOO species
5 Selective Hydrogenation of Carbon Dioxide into Methanol