Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

5.3 Results
Figure 5‐7: Oxidation of benzyl alcohol at a molar flow of 0.118 mol/min; conditions: 80 °C, 1.25 g
0.5%Pd/Al2O3 in a fixed bed reactor; feed composition: 0.9 mol‐% benzyl alcohol, 0.45 mol‐% O2, rest CO2; (●)
conversion, (○) benzaldehyde selectivity, (□) toluene selectivity, (∆) selectivity to overoxidation products.
visual inspection of the feed mixture in the inter‐connectable view cell of the setup. Thus, for the
feed mixture consisting of 0.9 mol‐% benzyl alcohol (Figure 5‐6b), clearly two phases were
observable at 150 bar, while only one was existing at 170 bar. At an intermediate pressure of 160 bar
no clear conclusion on the number of phases could be drawn from simple phase monitoring under
flow conditions. Decreasing the overall molar flow showed that the pressure at which the decrease in
reaction rate occurred remained almost the same (Figure 5‐7) matching again well with the
experimental data and the CPA predictions. Compared to the previously described experiments, the
selectivity was always higher than 90 %. A significant increase in conversion was found between 140
and 150 bar which was also reported under similar conditions [7]. Again, at both pressures the
system exists as two phases by visual inspection. Thus, the observed increase in reaction rate is not
due to a phase transition. The increased toluene selectivity indicates a faster transport of
benzaldehyde to the catalyst surface compared to O2. When changing from two phase conditions to
single phase conditions, the time until a steady‐state was reached was very long and a continuous
decrease in catalytic activity occurred. Note that in this study a steady‐state was assumed when the
141