Highly Selective Continuous Flow Hydrogenation of Cinnamaldehyde to Cinnamyl Alcohol in a Pt/SiO2 Coated Tube Reactor

A novel continuous flow process for selective hydrogenation of α, β-unsaturated aldehyde (cinnamaldehyde, CAL) to the unsaturated alcohol (cinnamyl alcohol, COL) has been reported in a tube reactor coated with a Pt/SiO2 catalyst. A 90% selectivity towards the unsaturated alcohol was obtained at the aldehyde conversion of 98.8%. This is a six-fold improvement in the selectivity compared to a batch process where acetals were the main reaction products. The increased selectivity in the tube reactor was caused by the suppression of acid sites responsible for the acetal formation after a short period on stream in the continuous process. In a fixed bed reactor, it had a similar acetal suppression phenomenon but showed lower product selectivity of about 47–72% due to mass transfer limitations. A minor change in selectivity and conversion caused by product inhibition was observed during the 110 h on stream with a turnover number (TON) reaching 3000 and an alcohol production throughput of 0.36 kg gPt day−1 in the single tube reactor. The catalysts performance after eight reaction cycles was fully restored by calcination in air at 400 ◦C. The tube reactors provide an opportunity for process intensification by increasing the reaction rates by a factor of 2.5 at the reaction temperature of 150 ◦C compared to 90 ◦C with no detrimental effects on catalyst stability or product selectivity.