Competitive Adsorption and Modulated Product Binding Using Pyridines on Cobalt Phosphide Enhance Hydrogenation Catalysis.

The binding and interaction of substrates, products, and other reaction components at the surface are key to heterogeneous catalysis. Improved catalytic properties can be obtained from organic ligands that interact with the substrate at the surface. However, competitive binding phenomena of the ligands with not only substrates but also intermediates and products of a reaction can critically influence catalysis. The tuning of these interfacial phenomena to achieve a selective hydrogenation of α,β-unsaturated aldehydes is underexplored with heterogeneous catalysts, in particular, with transition metal phosphide-based catalysts. Herein, we systematically investigated the effect of seven pyridine-based additives with varied electronic, steric, and structural properties on the hydrogenation of trans-cinnamaldehyde by using a cobalt phosphide catalyst. Additives having electron-donating and relatively small substituents in para-position resulted in dramatically enhanced selectivity to the unsaturated alcohol by almost 2 orders of magnitude at long reaction times and enhanced overall productivity by factors of up to ∼2. This reaction outcome is dominated by the competitive adsorption of the pyridine-based additives with the produced unsaturated alcohol, which reduced product inhibition and undesired secondary reaction pathways. At early reaction times, substantial positive effects on catalytic selectivity were also observed, and these can mainly be attributed to dispersion effects and H-bonding interaction of the pyridines with the substrate. Our work highlights that judicious choice of organic additives and consideration of complex competitive adsorption phenomena and modulated binding of not only substrate but also products allow the development of more effective catalysts based on earth-abundant materials.