Title : Modified Mahoney Robinson reactor using a static catalytic foam characterization and catalytic applications
A stirred Mahoney-Robinson reactor has been modified by replacing its basket (usually filled with catalytic beads or extrudates) by a coated solid foam of the same external dimensions (hollow cylinder). Foams were either made of polyurethane or of stainless steel and Pd/C and Pd/alumina catalysts were coated at their surface.
The new reactor was first characterized in terms of G/L and L/S mass transfer. The batch absorption of hydrogen and the measurement of alpha-methylstyrene hydrogenation rate on Pd/alumina were used to evaluate G/L and L/S mass transfer coefficients. The foam showed slightly better performances than the reference bead-basket. However the productivity of the foams was up to 5 times higher than the bead bed due to specific area differences.
Stainless steel foams coated with Pd/C were used to catalyze nitrobenzene hydrogenation. Different sources of carbon were used for the coating (carbon black, activated carbon, carbon nanotubes (MWCNT)). Pd/MWCNT outperformed other coated Pd/C catalysts once coated. The sintering during the thermal treatment of the coating was less severe for this support. During the nitrobenzene hydrogenation, a catalyst deactivation was observed. The in situ electrochemical regeneration of spent Pd/C/foam catalysts was investigated. Its efficiency was assessed by monitoring the catalyst activity versus cumulative turnover number (TON). The results showed that the catalyst activity was fully recovered after regeneration and that the catalyst had the same deactivation rate after several reaction/regeneration cycles. This result validates the potential use of such modified Mahoney-Robinson reactor as a continuous stirred-tank reactor.
Finally, cylindrical polyurethane foams cut to fit a Mahoney-Robinson reactor were used. Polyurethane could bring several advantages compared to stainless steel, such as an easy availability, a low price, an easy handling and a better uniformity than metallic foams which generally suffered from successive process operations. To allow the coating of polyurethane foams (PUF) by a Pd/alumina catalytic layer we report the use of polydopamine (PDA) as a primer. Cylindrical polyurethane foams were first coated with a PDA layer and then successfully coated with a Pd/alumina powder catalyst. The coated foam was used in successive batch hydrogenations of alpha-methylstyrene. 6 successive runs were possible, reaching a total turnover number of 36000 mol(cumene)/mol(Pd)). The activity of the catalyst was still high at the end of the 6th run (more than half the activity of the fresh catalyst). No palladium leaching was noticed. The use of coated polyurethane foams in catalytic reactions is thus possible, whereas limited to low temperature reactions due to the thermal stability of polyurethane. Moreover, as the coating is very thin, such a configuration is recommended for very fast, mass-transfer limited reactions.