(683h) Insights into the in-Situ Bronsted Acid Generation and Their Strength in WOx/Pt Nanocatalyst for Selective Glycerol Hydrodeoxygenation

WO_x/Pt is a widely used catalyst for the selective hydrodeoxygenation (HDO) of glycerol to 1,3 propanediol (1,3-PDO). Its selective C-O bond cleavage ability was attributed to the in-situ generated strong Bronsted acid sites on the WO_x clusters. However, the exact role of Bronsted acid and the need for finely dispersed Pt for high selectivity to 1,3-PDO is unclear. This computational investigation focuses on addressing some of these aspects. DFT simulations were performed using two catalyst models with the O₂W-O-WO₃ cluster: 1) 3 layer, 5x5 supercell of Pt(111), and 2) Pt(201) nanoparticle. The protonation dehydration mechanism for the C-O bond cleavage involved the nearest proton from the WO_x cluster attacking the secondary hydroxyl to form water (ΔE_r=-79 kJ/mole) followed by hydrogenation from the Pt surface to form 1,3-PDO. An alternative pathway involved the formation of hydroxypropenol (ΔE_r =-20 kJ/mole), followed by formation of hydroxypropanal, which upon sequential hydrogenation forms 1,3-PDO. 1,3-PDO can also be formed from secondary C-O cleavage, with the O resulting from the cleavage oxidizing the WO_x cluster. By comparing the energetics of different mechanisms (Fig.1), the protonation dehydration mechanism is found to be energetically the most favourable. Activated H diffusion from Pt to the WO_x cluster had barriers of 55 to 80 kJ/mole and is energetically uphill (> 40 KJ/mole) making these the controlling steps in the Bronsted acid generation. Binding energy of NH₃ at Bronsted acid sites, a descriptor for acid strength, was between -62 and -96 kJ/mole at single coordinated OH sites while it was -114 to 120 kJ/mole in the double coordinated acid sites. Especially, the hydroxyls coordinated to undercoordinated Pt atoms led to the strong acid formation. These fundamental insights will help to design efficient catalysts and choose appropriate reaction conditions for optimum Bronsted acid sites for selective HDO.