Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

CHAPTER 5
experimentally observed in the batch experiments. This flexible approach can predict the phase
behavior for a broad range of conditions and thus offers an alternative to often laborious
experimental phase behavior determination requiring special equipment. The model predictions for
the phase transition furthermore fell in the pressure regime where a considerable change in catalytic
activity was observed and gave a good indication of the optimal system pressure. Thus, the CPA EoS
allows for a more effective reaction optimization. This can be especially useful where little is known
about the phase behavior. In the present case, the phase behavior modeling suggested that the
catalytic reaction should be studied at higher pressures than previously done in order to reach a
single phase. Under very similar conditions as in ref. [7] also an increase in reaction rate between 140
bar and 150 bar (0.9 % benzyl alcohol, 0.45 % O2, rest CO2, cf. Figure 5‐7) was found here. Only when
extending the studies to pressures >160 bar a decrease in reaction rate was concurring with the
system being in a single phase which is similar to results reported for the oxidation of cinnamyl
alcohol [8]. Both at 140 bar and at 150 bar the mixture was biphasic though the phase behavior in
the catalyst pores may be different as indicated by infrared spectroscopic studies [7].
Pd catalysts used in alcohol oxidation are sensitive to the availability of oxygen [41, 44]. A
low availability of oxygen causes blocking of surface sites by adsorbed hydrogen and strongly
adsorbing CO which are both removed by oxidation. High amounts of oxygen favor overoxidation of
Pd which is connected to a lower catalytic activity [41, 44, 45]. Hence, a general recommendation is
to operate the reactor in the oxygen mass‐transport limited regime [3]. The lower catalytic activity
observed under single phase conditions might be connected to the oxidative deactivation of the
palladium particles on the catalyst surface which underlines that the oxygen mass transport to the
catalyst surface is improved compared to the biphasic situation. This is further corroborated by the
significantly higher selectivity to overoxidation products. Note that oxidized palladium can be re‐
reduced by the alcohol [46] so no permanent (short‐term) deactivation is caused by oxygen
treatment. Under biphasic conditions, XAS measurements showed that Pd was mainly reduced [41,
42] but analogue measurements under single phase conditions are still missing. If the oxidation state
of the active metal is the primary reason for the pronounced response to the phase behavior, then
the results made at 100 °C where no sudden change in conversion was observed might indicate that
palladium is always reduced at that temperature.
The oxidation of palladium occurs rapidly [6, 41] and does not explain the long time required
for reaching a steady‐state when operating in a single phase. A side product in the oxidation of
primary alcohols is the corresponding carboxylic acid which is only poorly soluble in CO2. Hence,
adsorption of benzoic acid on alumina and/or Pd during single phase conditions could cause
deactivation and was previously suggested from IR investigations [7]. In the presence of two phases,
the benzyl alcohol rich layer around the catalyst particles has a higher eluting power than the
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