Selective hydrogenation reactions: A comparative study of monolith CDC, stirred tank and trickle bed reactors

Monolith reactors offer several advantages over traditional random fixed beds or slurry reactors, such as better mass transfer characteristics, higher volumetric productivity for a smaller amount of catalyst, elimination of filtration step and lower pressure drop. However, achieving an even gas–liquid distribution across the monolith is difficult. The cocurrent downflow contactor (CDC) reactor presents an elegant solution to this problem by establishing a stable two-phase dispersion. When comparing the CDC reactor with traditional stirred tank and trickle bed reactors for the hydrogenation of 2-butyne-1,4-diol, greater selectivity towards the alkene intermediate is observed in the CDC for a range of solvents. This is partly due to a high dispersion (67%) of Pd on the washcoat support of the monolith and also to the optimisation of the surface species concentrations as a result of greater mass transfer of hydrogen to the catalyst surface compared with alternative reactor designs. Furthermore, when considering the effect of scale-out on the monolith by comparing a single capillary with 1256 and 5026 channel monoliths, initial reaction rates and selectivity were maintained. This supports the use of the CDC as a suitable method for dispersing gas–liquid mixtures through a monolith.