Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

CHAPTER 5
This would additionally limit the comparability of (short‐term) batch experiments with continuous
experiments as described here. Interestingly, the catalyst recovered slightly during overnight
shutdown phases. The concentration of oxygen can both increase the reaction rate and deactivate
the catalyst due to oxidation of palladium [41, 42]. Studying the influence of O2 in higher than
stoichiometric amounts caused a decrease in conversion both under single and two phase conditions
(Figure 5‐10). In general, the selectivity to toluene decreased and the amount of benzoic acid and
benzyl benzoate increased with increasing oxygen concentrations though still under two phase
conditions a relatively high amount of toluene prevailed. Under single phase conditions, the
conversion decreased continuously over two subsequent days indicating catalyst deactivation. After
2 days, the conversion was stable over a few hours though a further decrease over a longer period
cannot be excluded. The deactivation was reversible; after 7 days under high oxygen concentrations
and single phase conditions (i.e. measuring Figure 5‐10a), a conversion of 89.7 % ± 1.4 % was found
when switching back to two phase conditions compared to 86.4 % ± 2.4 % obtained before (for
conditions see Figure 5‐7). Note that some long‐term deactivation was, however, observed.
Figure 5‐10: Benzyl alcohol oxidation with different oxygen concentrations under single phase (a) and two
phase conditions (b); reaction conditions: 80 °C, 1.25 g 0.5%Pd/Al2O3 in a fixed bed reactor; feed
composition: 0.5 mol‐% benzyl alcohol, O2 as indicated, rest CO2, total flow 0.177 mol/min, (a) 180 bar, (b)
140 bar; (●) conversion, (○) benzaldehyde selectivity, (□) toluene selectivity, (∆) selectivity to overoxidation
products.
5.3.6 Higher reaction temperature
(a) (b)
So far, the reaction was studied at a temperature of 80 °C and a clear influence of the phase behavior
was observed. Performing the reaction at 100 °C, a different reaction profile was found (Figure 5‐11).
While still a decrease in reaction rate was found at higher pressures there was no abrupt change in
conversion although CPA modeling suggests a phase transition to occur in the investigated pressure
range. The decrease in toluene selectivity observed between 130 and 140 bar might be due to a
change in the number of phases but no clear conclusions can be drawn here. In contrast to the
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