Reaction and Raman spectroscopic studies of alcohol oxidation on gold-palladium catalysts in microstructured reactors

Oxidation of benzyl alcohol in the absence of solvent on 1% (Au–Pd)/TiO2 catalyst with pure oxygen was performed in silicon-glass micropacked-bed reactors (MPBRs). The overall size of the microreactor chip was 23 mm × 23 mm with a reaction channel dimension of 0.6 mm (W) × 0.3 mm (H) × 190 mm (L). A pillar structure (small rectangular posts of 60 μm (W) × 1 mm (L) 40 μm apart) was incorporated near the outlet of the reaction channel to retain the catalyst. The reaction was studied in the temperature range of 80–120 °C and at inlet pressures up to 5 bar(a). Benzyl alcohol conversion and benzaldehyde selectivity at 80 and 120 °C obtained in MPBRs were very close to those from conventional glass stirred reactors (GSRs) apart from the selectivity at 120 °C. Toluene was formed in the absence of oxygen, and its production was enhanced in the presence of oxygen. Increasing pressure improved both conversion and benzaldehyde selectivity. Mass transfer resistance in MPBRs was evaluated experimentally. The external mass transfer resistance could be ignored at a volumetric flow ratio of gas (STP) to liquid above 100, at a given liquid flow rate (0.003 mL/min). The effect of catalyst particle size on the reaction was examined with two ranges of particle size: 53–63 μm and 90–125 μm. Lower conversion was obtained with particle sizes of 90–125 μm, indicating the presence of internal mass transfer resistances. In situ Raman measurements in MPBRs were performed using a specially designed microreactor stage with a different microreactor configuration. Raman spectra obtained from liquid pockets at different points along the reaction channel could be used to obtain the benzaldehyde concentration profile along the catalyst bed. Bands due to formation of highly disordered graphitic carbon were observed on the catalyst surface.