Bimetallic Pd- and Co-Containing Mesoporous Carbons as Efficient Reusable Nanocatalysts for Hydrogenations of Nitroarenes and Enones Under Mild and Green Conditions

Easy and rapid preparations of magnetic Co- and Pd-containing mesoporous carbons (IM1, IM2 and DM) from green phenolic resins, amphiphilic templates and metallic salts via two synthetic routes are reported. Catalysts IM1 and IM2 are prepared via an indirect method involving two steps, i.e., the preparation of Co-containing mesoporous carbons with different Co contents (2.5 and 12.5%) and the further introduction of Pd (2.3%) via impregnation using a solution of a Pd salt and a process of thermal reduction. The mesoporous carbon obtained contains two distinct crystalline metallic phases, i.e., Co particles of 5.0 nm (IM1) and Pd nanoparticles of ~1.3 nm (IM1), while the increase in Co content triggers higher Co particle sizes of 23 nm and Pd particle sizes of 1.3 and 6.8 nm (IM2). Differently, the catalyst DM is prepared via direct synthesis, in one step, including all precursors and both metal salts. This results in Pd50-Co50 nanoalloys of 6.5 nm uniformly dispersed in the carbon matrix. The reactivity and reusability of catalysts IM1, IM2 and DM were then ascertained in organic synthesis for hydrogenations of nitroarenes and enones. It turned out that no reactions were observed in the presence of the catalyst DM due to the presence of Co in Pd50-Co50, which deactivates the catalytic activity of Pd. Gratifyingly, catalysts IM1 and IM2 were very efficient for mild hydrogenations of both nitroarenes and enones using only 5 mequiv. of supported Pd in EtOH at room temperature. The smaller Pd particle sizes (1.3 nm) and the high surface-to-volume area are probably responsible for the high reactivity observed. Catalysts IM1 and IM2 can be recovered by application of an external magnetic field. However, a more efficient magnetic recovery of catalyst IM2 compared to IM1 was observed due to its higher Co content. Catalyst IM2 can be successfully reused at least seven times without a loss of efficiency. Finally, almost-Pd-free products can be obtained directly after reaction without any purification step, since the Pd leaching is very low (<0.1% of the initial amount), thus decreasing waste and increasing the reaction’s efficiency.