(197bo) A Combined Experimental and Theoretical Study on Tuning Selectivity in Furfural Acetalization Reaction on Pd Nanostructures

Furfural acetalization reaction on well-defined Pd nanostructures showed structure sensitivity with respect to the particle shape. The acetalization reaction was performed over Pd cubes (~ 22 nm), octahedras (~ 26 nm) (Figure 1(a) and (b)) and spheres (~ 25 nm) at 303 K and 5 bar H₂ pressure for 3 hours in different solvents (ethanol, methanol, butanol and propanol). Pd cubes (exposing only the (100) facet) exhibited high furfural conversion (80%) and acetal selectivity (93%) compared to Pd octahedras (exposing only the (111) facets) (35% and 77%) and Pd Spheres (exposing both (111) and (100) facets) (41% and 75%) in ethanol as the solvent. The furfural acetalization reaction on Pd cubes was further studied in presence of methanol, propanol and butanol. The reactivity trend for different solvents followed the order: methanol (95%) > ethanol (80%) > propanol (62%) > butanol (56%). ab-initio Density Functional Theory (DFT) calculations further provided a mechanistic insight into the reactivity of two different Pd facets (100) and (111) towards furfural acetalization reaction at high coverage of solvents. DFT calculations showed that the binding energy of furfural on Pd (100) (E_b = -172.7 kJ/mol) (Figure 1 (c)) was higher than Pd (111) surface (E_b = -114.8 kJ/mol) (Figure 1(d)). On both the surfaces furfural acetal product was observed to be formed in three steps: alcohol dissociation into alkoxy intermediate followed by hydrogenation of carbonyl oxygen of furfural to form alkyl furan and nucleophilic addition of alkoxy group on the carbonyl carbon of alkyl furan to form acetal product (Figure 1(e)). The ethanol dissociation, furfural hydrogenation and acetalization step exhibited least barriers on Pd (100) (51.6, 27.7 and 77.2 kJ/mol) compared to Pd (111) surface (78.6, 35.8 and 92.2 kJ/mol) in presence of full coverage of ethanol. Calculated values of activation barriers for the three steps were significantly reduced on the Pd cubes in presence of methanol as the solvent (47.8, 24.2 and 64.6 kJ/mol) which was consistent with the experimental results.