128
efficient reactors and for improving methanol production (Takeshi et al. 2001). This
section will present a brief summary of the kinetic studies performed till this point
on carbon dioxide hydrogenation to methanol in gas phase using heterogeneous
catalysts, especially copper-based catalysts.
As presented in previous sections, carbon dioxide hydrogenation to methanol in
gas phase is achieved through Eqs. (1, 2 and 3) in Table 5.1. Even if the desired
reaction is the carbon dioxide hydrogenation, reverse water-gas shift and carbon
monoxide hydrogenation occur in parallel and must also be taken in consideration
for kinetic studies. Other side reactions (Table 5.1) could also impact the formation
rate of methanol.
The level of comprehension of the reaction over a certain catalyst surface will
define the scale of the kinetic model that can be applied (Maximilian et al. 2012).
Macrokinetic models, such as power laws, do not require hypothesis about the reaction mechanism and are generally used for reactor design and operation of chemical
reactors, whereas microkinetic models take elementary steps into consideration.
Macrokinetic models have been widely used over the years for investigating
methanol synthesis. The first macrokinetic models developed for methanol production from syngas mixtures containing both carbon dioxide and carbon monoxide
assumed that methanol was produced only from carbon monoxide, and thus pure
carbon dioxide streams could not be used. However, more recent kinetic models
along with
14
C-labeling experiments have proved that methanol is actually produced
mainly produced from carbon dioxide, which is primarily converted via the reverse
water-gas shift reaction (Kobl et al. 2016; Jadhav et al. 2014; Maximilian et al.
2012). Table 5.3 presents a summary of kinetic studies reported in the literature
highlighting the catalysts used, the reaction conditions, as well as the type of kinetic
model used.
Kobl and co-workers (2016) recently developed a power law kinetic model for
methanol production from carbon dioxide and hydrogen over classical Cu–ZnO–
Al 2 O 3 and Cu/ZnO/ZrO catalysts. Reactions 1, 2, and 3 were considered in the
kinetic model that was tested with temperatures ranging from 200 to 240 °C and
pressures ranging from 50 to 80 bars. The apparent activation energy of the reverse
water-gas shift reaction was much higher than that of methanol synthesis for both
catalysts, with Cu–ZnO–ZrO catalyst showing lower activation energy, suggesting
that this catalyst could have better catalytic performance than the commercial Cu–
ZnO–Al 2 O 3 catalyst. They also concluded that the influence of the hydrogen partial
pressure is much higher than carbon dioxide partial pressure, suggesting a strong
adsorption of carbon dioxide on the catalyst surface. However, since no mechanism
is taken into account, this power law model cannot explain the high dependency of
the reaction on hydrogen partial pressure.
Stanislaw and co-workers (2013) compared the performance of a commercial
Cu–ZnO–Al 2 O 3 catalyst with Cu–ZnO–Al 2 O 3 catalyst doped with magnesium and
zirconium in low-pressure carbon dioxide hydrogenation to methanol at temperatures ranging from 220 to 260 °C and pressures ranging from 23.7 to 30 bar. The
experimental results were then correlated to a simple power law model to better
compare the performance of both catalysts. The kinetic model showed that the reacD. P. Minh et al.
efficient reactors and for improving methanol production (Takeshi et al. 2001). This
section will present a brief summary of the kinetic studies performed till this point
on carbon dioxide hydrogenation to methanol in gas phase using heterogeneous
catalysts, especially copper-based catalysts.
As presented in previous sections, carbon dioxide hydrogenation to methanol in
gas phase is achieved through Eqs. (1, 2 and 3) in Table 5.1. Even if the desired
reaction is the carbon dioxide hydrogenation, reverse water-gas shift and carbon
monoxide hydrogenation occur in parallel and must also be taken in consideration
for kinetic studies. Other side reactions (Table 5.1) could also impact the formation
rate of methanol.
The level of comprehension of the reaction over a certain catalyst surface will
define the scale of the kinetic model that can be applied (Maximilian et al. 2012).
Macrokinetic models, such as power laws, do not require hypothesis about the reaction mechanism and are generally used for reactor design and operation of chemical
reactors, whereas microkinetic models take elementary steps into consideration.
Macrokinetic models have been widely used over the years for investigating
methanol synthesis. The first macrokinetic models developed for methanol production from syngas mixtures containing both carbon dioxide and carbon monoxide
assumed that methanol was produced only from carbon monoxide, and thus pure
carbon dioxide streams could not be used. However, more recent kinetic models
along with
14
C-labeling experiments have proved that methanol is actually produced
mainly produced from carbon dioxide, which is primarily converted via the reverse
water-gas shift reaction (Kobl et al. 2016; Jadhav et al. 2014; Maximilian et al.
2012). Table 5.3 presents a summary of kinetic studies reported in the literature
highlighting the catalysts used, the reaction conditions, as well as the type of kinetic
model used.
Kobl and co-workers (2016) recently developed a power law kinetic model for
methanol production from carbon dioxide and hydrogen over classical Cu–ZnO–
Al 2 O 3 and Cu/ZnO/ZrO catalysts. Reactions 1, 2, and 3 were considered in the
kinetic model that was tested with temperatures ranging from 200 to 240 °C and
pressures ranging from 50 to 80 bars. The apparent activation energy of the reverse
water-gas shift reaction was much higher than that of methanol synthesis for both
catalysts, with Cu–ZnO–ZrO catalyst showing lower activation energy, suggesting
that this catalyst could have better catalytic performance than the commercial Cu–
ZnO–Al 2 O 3 catalyst. They also concluded that the influence of the hydrogen partial
pressure is much higher than carbon dioxide partial pressure, suggesting a strong
adsorption of carbon dioxide on the catalyst surface. However, since no mechanism
is taken into account, this power law model cannot explain the high dependency of
the reaction on hydrogen partial pressure.
Stanislaw and co-workers (2013) compared the performance of a commercial
Cu–ZnO–Al 2 O 3 catalyst with Cu–ZnO–Al 2 O 3 catalyst doped with magnesium and
zirconium in low-pressure carbon dioxide hydrogenation to methanol at temperatures ranging from 220 to 260 °C and pressures ranging from 23.7 to 30 bar. The
experimental results were then correlated to a simple power law model to better
compare the performance of both catalysts. The kinetic model showed that the reacD. P. Minh et al.