Slurry loop tubular membrane reactor for the catalysed aerobic oxidation of benzyl alcohol

A novel reactor that combines a catalyst slurry flowing inside a loop configuration, incorporating a tubular membrane (Teflon AF-2400) for controlled oxygen delivery was designed and employed for the aerobic oxidation of benzyl alcohol using a 1 wt% Au-Pd/TiO2 powdered catalyst. This reactor keeps the liquid phase saturated with oxygen, while avoiding the creation of bubbles in it, thus enhancing operation safety. Experimental results in batch mode compared with those of a conventional autoclave demonstrated that the slurry loop membrane reactor reached a similar oxidation turnover frequency (20,000–25,000 h−1) with comparable values of benzaldehyde selectivity (∼70%). Continuous operation was achieved by using a crossflow filter connected to the loop to keep the catalyst from exiting the reactor. Using a 60 cm long tubular membrane, with the slurry flow circulating at 10 mL/min, continuous reaction was performed at 100–120 °C, 0–5 bar oxygen pressure and 1.2–5.0 g/L catalyst loading. Selectivity to benzaldehyde increased by either decreasing the reaction temperature or increasing the external oxygen pressure. The oxygen consumption rate decreased linearly with the catalyst loading, suggesting negligible gas-liquid mass transfer resistance. This was further evidenced by doubling the length of the tubular membrane, which had no effect on the oxidation turnover frequency. The slurry loop membrane reactor showed significantly better performance than packed-bed membrane microchannel reactors, and similar performance as that of a trickle-bed capillary reactor. This reactor can be implemented for a wide range of applications, which rely on the use of powder catalyst, are limited by gaseous reactant availability and require safe operation