Homogeneous Catalyzed Vinyl Acetate Synthesis

The acetoxylation of ethylene to form the vinyl acetate monomer (VAM) can be catalyzed by homogeneous catalysts comprised of PdCl/CuCl salts and carried out in glacial acetic acidl . The reaction is catalyzed by the Pd + species which are reduced by the adsorption of ethylene and subsequently reoxidized by oxygen and copper chloride. The speculated mechanism is as follows 

C2H4 -t- PdXa + HX [C2H4PdX3]- -b AcOH [C2H4PdX2(OAc)]-[C2H4PdX(OAc)l 

In the presence of water, Wacker cheudstiy tends to predominate, whereby ethylene reacts with water rather than acetic acid and forms acetaldehyde as the primary product. Heniy and Pandeyl l showed that the addition of alkali metal acetates can help to shift the product spectrum in order to form vinyl acetate in higher yields. The reaction is thought to involve the nucleophillic attack of ethylene by acetate to form a C2H4-OAC-Pd complex which subsequently undergoes /3-C-H activation to form VAM. Acetic acid or HCl can desorb from the complex to form Pd ° which is reoxidized back to copper chloride to regenerate Pd2+. 

Zaidil l and Samanosl suggested that a similar path, which is shown below, may give rise to catalysis in the supported liquid layer for heterogeneous Pd catalysts  

Pd is first oxidized to form a homogeneous Pd + cluster. Elthylene can then react via an inner- or outer-sphere mechanism to form the acetoxyethyl intermediate (see next section). The acetoxyethyl intermediate then imdergoes a /3-hydride elimination to form VAM. DFT calculations were performed in order to imderstand the potential presence of the liquid layer on a Pd surface and the energetics for the proposed homogeneous pathways. 

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