Selective hydrogenation of citral: A catalytic challenge

The hydrogenation of citral was investigated using various palladium- and rhodium-based catalytic systems. We explored both Pd and Rh nanoparticles stabilized by K-carrageenan in a biphasic water/THF system, as well as several heterogeneous catalysts with low precious metal content. In particular, the heterogenous catalysts employed were Pd/Al2O3 (0.3% Pd w/w), Rh/Al2O3 (0.18% Rh w/w), and HNTs@PDA/PdNPs (0.8% Pd w/w), obtained using a site-specific polydopamine coating on the external surface of halloysite as support. Across all experiments, palladium-based catalysts consistently demonstrated superior activity and selectivity compared to their rhodium counterparts. Notably, the catalytic systems exhibit a very different selectivity. Indeed, both Pd- and Rh-based water-soluble catalysts, as well as the heterogeneous Rh/Al2O3 catalytic system, favor the formation of citronellal, due to the hydrogenation of the carbon-carbon double bond conjugated to the carbonyl group. In contrast, the two heterogeneous palladium-based catalysts strongly promote the formation of menthol through the cyclization of citronellal, in some cases achieving nearly complete selectivity.