(143d) Mechanistic Details of Vinyl Acetate Synthesis Reaction on Monometallic Pd Catalysts and the Influence of Cu Co-Catalysts

Vinyl acetate (CH₃COOC₂H₃, VA) is a high-value chemical produced from CH₃COOH (AA), C₂H₄ and O₂ using palladium-based catalysts typically involving alkali promoted PdAu bimetallic catalysts [1]. These reactions occur under high AA coverages and have been proposed to involve either heterogeneous reactions at metallic Pd sites [1,2,3] or dynamic formation diacetate species that mimic homogeneous catalysts [1]. The role of alkali promoters involves formation of more reactive diacetate clusters such as Pd₂K₂(OAc)₆, while Au has been proposed to involve stabilization of active metallic forms of Pd ensembles. Significant mechanistic details such as the nature of O₂ activation and the identity of steps and intermediates that control selectivity remain unresolved. Moreover, few other catalysts have been explored in detail. Here, kinetic, isotopic and spectroscopic measurements and density functional theory calculations are used to assess mechanisms of VA formation reactions on monometallic Pd catalysts and the selectivity enhancements and mechanistic influences of the addition of Cu in PdCu catalysts.

Measured VA formation rates decreased with increasing C₂H₄ residence time more significantly than the depletion of reactants by conversion, suggesting that products inhibit the reaction by blocking active sites. Co-feeding of products indicated that small CO concentrations in products account for this inhibition. On monometallic Pd, VA rates decrease with increasing C₂H₄ pressure, first increase and then get saturated with AA pressure, and increase linearly with O₂ pressure. These details are consistent with elementary steps in which O-H and C-H activations in AA and acetoxy ethyl intermediates form acetate and VA, respectively, using molecularly adsorbed O₂ in irreversible steps, and the C-O coupling step is quasi-equilibrated. The kinetic relevance of C-H and OH activation steps is probed using kinetic isotope effects and DFT. The prevalence of O₂ as the H-abstracting species is consistent with high O_2­ dissociation barriers in surfaces covered with acetates. DFT calculations also suggest that facile O-H activation at low coverages leads to strongly bound inactive acetate species, with active acetates forming only at near-saturation coverages. The VA rates decrease with C₂H₄ pressure, in spite of direct involvement of C₂H₄ in C-O coupling, because pseudo-steady-state concentration of active acetates is inversely proportional to C₂H₄ pressure, and C₂H₄ and CO (produced in higher concentration at higher C₂H₄ pressures) occupy significant active sites. Selectivity trends are consistent acetate oxidation and C₂H₄ decomposition steps as the major contributors to minor undesired products at high and low O₂ pressures, respectively. The C₂H₄/C₂D₄ and CH₃COOH/CH₃COOD kinetic isotope effects measured at highly acetate covered and partially C₂H₄ covered conditions reveal changes in kinetic relevant of C-H and O-H activation steps with acetate coverages.

Bimetallic PdCu samples with Pd:Cu atomic ratios below 0.1 exhibit significantly different rate and selectivity trends than Pd. The VA selectivity on monometallic samples increases with O₂ pressures and then levels-off and exhibits a slight decrease. PdCu samples exhibit much higher selectivity than Pd at low O₂ pressure, but the selectivity decreases more strongly with increasing O₂ pressure. The low VA selectivity in monometallic Pd at low O₂ pressure is a consequence of acetic acid decomposition indicated by detection of CO in the reactor effluent. Such decompositions are likely to require Pd-atom pairs that are absent in PdCu samples with low Pd content. The isolated nature of Pd in such samples was confirmed by exclusive detection of linear CO-adsorption features in infrared, and by x-ray absorption spectroscopy measurements. VA selectivity in PdCu samples increases with decreasing Pd:Cu ratios, and, for ratios below 0.1, reaches well above the selectivity in monometallic Pd samples at low O₂ pressures. VA rates on PdCu samples are first-order in both C₂H₄ and AA pressure, which cannot be explained by elementary steps consistent with the data for monometallic Pd; these catalysts likely involve mechanism in which acetate preferentially formed on Cu, and isolated Pd atoms provide sites for C₂H₄ adsorption and activation without decreasing acetate coverage for reactions at bimetallic domains. High VA selectivity at low O₂ pressures is consistent with much lower C₂H₄ decomposition at isolated Pd atoms than in monometallic Pd.

The results described here suggest that PdCu samples with low Pd content can exhibit high VA selectivity at low O₂ pressures, which may lead to efficient catalysts with lower costs than PdAu catalysts used currently in industrial processes.

References

1. E. K. Hanrieder, A. Jentys, J. A. Lercher, ACS Catal. 2015, 5, 5776.
2. (a) Stacchiola, D.; Calaza, F.; Burkholder, L.; Schwabacher, A.W.; Neurock, M.; Tysoe, W.T. Angew. Chem. Int. Ed., 2005, 44, 4572-4574. (b) F. Calaza, D. Stacchiola, M. Neurock, W. T. Tysoe, J. Am. Chem. Soc. 2010, 132, 2202.
3. Han, Y. F.; Wang, J. H.; Kumar, D.; Yan, Z.; Goodman, D. W. J. Catal. 2005, 232, 467-475.