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N.E. Musko et al. / Applied Catalysis A: General 443–444 (2012) 67–75
with the adsorption of hydrogen and 2-butenal. In addition, both
Zhao et al. [42] and Burgener et al. [43] found reaction of CO2 and
hydrogen under single phase conditions on noble metal particles.
Further CPA calculations of the P–T regions of co-existing phases
(Fig. 6) showed that the system phase transition at a constant tem-
perature and elevated pressures decreases towards lower pressures
as the reaction proceeds. This means that if the initial reaction mix-
ture (at t = 0) was in a single phase it stayed in one phase even after
all the 2-butenal had reacted. However, depending on the compo-
sition, T and P, in some cases the reaction may begin in the two
phase region, but at a certain conversion the phase transition may
occur and the system will become single phase. These changes of
the phase behaviour as a result of concentration changes during
reaction have to be considered for proper interpretation of reac-
tion kinetics and the CPA calculations proved to be an elegant tool
for this task.
medium, however, some recent findings revealed that under cer-
tain conditions, i.e. in the presence of hydrogen and noble metals
already at relatively low temperatures, CO2 can react with hydro-
gen in the so-called reverse water–gas shift reaction and form
carbon monoxide and water [43,44]. The former acts as cata-
lyst poison especially for low coordinated metal sites [45], and
thereby sometimes changes the catalyst selectivity [46]. Despite
poisoning by CO might be the case in the present study, this is
hardly the main reason because of the too strong drop in con-
version when excessively large amounts of CO2 were used. As it
was pointed out by Burgener et al. [43], the reverse water-gas shift
reaction takes place in many hydrogenation reactions and under
hydrogenation conditions metal surfaces are partially covered with
CO, however, this effect is mostly restricted to low coordinated
Pt-sites.
Another type of intermolecular interactions can play an impor-
des, ketones, esters, etc. Such interactions and their pressure
dependency have been studied using high-pressure FT-infrared
spectroscopy [40,41]. It was found that CO2 is capable of activating
of carbonyl groups in organic aldehydes and this effect is different
for saturated and unsaturated ones [41]. Thus, in the hydrogena-
tion of benzaldehyde the conversion into benzyl alcohol is merely
decreasing with increasing CO2 pressure due to dilution of the
system, whereas for cinnamaldehyde conversion reaches a max-
imum. The authors attribute it to the activation of C O bond in
cinnamaldehyde by CO2 at low pressures, and at elevated pres-
sures this effect disappears and conversion decreases due to the
dilution of the system. Despite in the present study another type
of hydrogenation is relevant, i.e. C C bond saturation, the presence
of intermolecular forces between CO2 and aldehydes is indirectly
indicated by binary interaction parameters kij (Table 4), which
is attributed to the non-ideality of the binary systems. The sig-
nificance of such interactions is that they determine the phase
behaviour of the system, which in turn determines the catalyst
performance as it was discussed above.
experimental data from the literature. The binary mixture car-
bon dioxide–2-butenal was experimentally studied in order to
find the bubble point pressures at different temperatures. Based
on this experimental data the binary interaction parameter was
obtained.
The CPA model is shown to be a powerful tool allowing ther-
modynamic calculations with high precision and accuracy. Using
CPA the number of co-existing phases was predicted, and the con-
centrations of the reacting components in coexisting phases were
calculated. These data are very important and useful for further
kinetic studies where knowledge of concentrations in individual
phases is a key element.
Furthermore, calculations using CPA gave insight into the phase
behaviour during the reaction, showing that the pressures and tem-
peratures at which a one phase region exists are decreasing as the
reaction proceeds.
The catalytic studies showed that maximum conversion was
achieved when the reaction mixture changed from one-phase to the
two-phase regions, near the critical point of the system. The con-
centrations of the components in the reaction mixture, calculated
with CPA, were shown to cause such behaviour.
Acknowledgements
The authors are grateful for financial support from the Techni-
cal University of Denmark (DTU), The Danish Research Council for
Technology and Production Sciences, Institute of Catalysis Research
and Technology (IKFT) at Karlsruhe Institute of Technology (KIT),
where all the catalytic experiments were performed, and also ETH
Zurich, Institute for Chemical and Bioengineering, Department of
Chemistry and Applied Biosciences, where the phase behaviour
measurements were carried out. Centre for Electron Nanoscopy
(DTU-CEN) is acknowledged for providing TEM images. Further-
more, Dr. Wolfgang Kleist, Dr. Loubna Gharnati, and Dr. Matthias J.
Beier are acknowledged for their help and support in building-up
experimental setups and performing the experiments. Dr. Ioannis
Tsivintzelis is acknowledged for his help with the thermodynamic
calculations. Many calculations presented in this study were made
using SPECS software – a program for phase equilibrium calcula-
tions developed at the Center for Energy Resources Engineering
(CERE), Department of Chemical and Biochemical Engineering,
Technical University of Denmark.
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