Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

5.2.2.2 Catalytic studies
CHAPTER 5
Prior to each run the catalytic setup was flushed with CO2 (AGA A/S; grade 3.0). The reactor was
heated to the desired temperature (usually 80 °C). Benzyl alcohol (Sigma‐Aldrich, anhydrous, 99.8 %)
was introduced to the system by means of the HPLC pump. The CO2 pressure of the compressor was
set to ca. 200‐250 bar and fine‐tuned to the desired pressure by the interconnected reducing valve.
O2 (AGA A/S; grade 3.5) was compressed to ca. 200 bar (or higher if necessary) and regulated to a
pressure 10‐20 bar higher than the system pressure by the respective reducing valve. The desired
oxygen molar flow FO2 was applied by setting a shifting time tS (between 2‐5 s pulses) for the 6‐port‐
dosing valve and calculated from the coil volume Vcoil and the difference between the system
pressure Psys and the oxygen feed pressure PO2 with Eq. 5‐1.
(Eq. 5‐1)
F
O 2
=
( 2 )
P −P ⋅V
l ⋅bar
24.4 ⋅t
mol
O sys coil
S
The total flow of gaseous compounds (i.e. CO2 at low O2 concentrations) was regulated at the
exit of the setup by means of the expansion unit: with the first (heatable) reduction valve the system
pressure was decreased to ca. 15 bar, the second reduction valve was set to ca. 5 bar. The flow was
fine‐tuned with the needle valve after the gas‐liquid separator and measured with the ball flowmeter
calibrated daily. Samples were taken where indicated in Figure 5‐1 collected in ca. 2‐4 mL of ethyl
acetate cooled to ‐18 °C every 20 min. GC analysis was carried out with an Agilent 6890 N gas
chromatograph equipped with a 7683 B series autosampler, a flame ionization detector, and a DB‐
FFAP column (Agilent, 30 m x 0.25 mm x 0.25 µm). Conversions and selectivities were calculated from
the relative amounts of benzyl alcohol, benzaldehyde, benzoic acid, toluene and benzyl benzoate.
The mass balance was checked by comparing inlet and outlet flow masses and found to be >96 %
assuming that one molecule of water is formed per benzaldehyde molecule. Using an internal
standard gave unsatisfactory results especially in the two phase region. Samples were taken until the
obtained conversion stabilized; the conversion was then calculated by averaging 5‐10 samples taken
at steady‐state. Uncertainties were calculated as twice the standard deviation. The catalyst bed was
exchanged from time to time in order to stabilize the catalytic activity. Batch reactions were
conducted in the view cell previously used for phase behavior investigation. 200 mg of 0.5 % Pd/Al2O3
(as pellets), benzyl alcohol (336 µL, 3.2 mmol) and ca. 10 µL of tert.‐butylbenzene were introduced to
the cell. Maintaining the cell at 10 °C, the cell was flushed several times with oxygen of which 40 NmL
(1.6 mmol) were added followed by 15.6 g of CO2. The cell was then heated up to 80 °C while stirring.
By changing the cell volume, the system was kept in a single phase during the heat‐up phase. After
the heating jacket reached the desired temperature, the mixture was allowed to react for 1 h at the
desired pressure and then cooled to 10 °C. After careful venting a sample for GC analysis was taken.
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