Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

CHAPTER 5
conversion was stable within the experimental uncertainty during a few hours, so it cannot be
unambiguously excluded that further long‐term deactivation might occur.
5.3.5 Macroscopic mass transport in the two phase region
One observation made during the phase behavior measurements was that the alcohol‐rich phase is
only small in volume and viscous compared to the CO2 phase. This might hamper the transport of the
alcohol‐rich phase through the reactor. In a simple experiment under non‐reactive (i.e. oxygen‐free)
conditions the effluent mass flow was compared to the feed flow (Figure 5‐8) under definite single
(200 bar) and two phase (100 bar) conditions by measuring the weight increase of the accumulated
benzyl alcohol as a function of time at the system exit and comparison with the feed flow. Starting
with a system pressure of 200 bar, the effluent flow was constant and the catalyst particles only in
contact with the CO2‐dominated phase. Upon change to 100 bar (~20 min), the effluent benzyl
alcohol flow was decreasing to zero before it reached a constant level as high as at 200 bar again
(~70‐80 min). Thus, the organic substrate was deposited in the system including the reactor and the
catalyst particles. Increasing the pressure to 200 bar caused an intermediate massive increase in the
flow which reached again the value observed before (after roughly 20 min). Thus, under single phase
conditions previously deposited substrate is removed from the reactor (and the catalyst particles).
The deposition of substrate in the biphasic pressure region has two important consequences, i.e. a
higher concentration of the substrate in the reactor as compared to the feed and accordingly a
longer residence time of benzyl alcohol in the reactor. Thus, a direct comparison of continuous and
batch experiments is not possible offhand which is additionally complicated by a pressure‐dependent
induction phase observed under batch conditions [7]. Therefore, catalytic batch experiments were
performed in the view cell previously used for phase behavior determination. Also here, two phase
conditions resulted in a higher catalytic activity than single phase conditions. In contrast to the
continuous experiments, selectivities close to 100 % were observed. Due to conversion of benzyl
alcohol to the more readily soluble benzaldehyde, a phase transition from two phases to a single
phase was observed at low pressure (Figure 5‐9).
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