This document provides information about the acetoxylation of olefins, specifically the conversion of ethylene to vinyl acetate. It discusses two processes - a solution-based process and a gas-phase process. The solution-based process uses palladium chloride and copper chloride catalysts in acetic acid. The gas-phase process uses palladium metal catalysts with alkali salts like potassium acetate. Both processes form vinyl acetate as the main product, with acetaldehyde and carbon dioxide as byproducts. The document provides details on the industrial development and operation of these processes, and proposed reaction mechanisms.

1. Assignment: Inorganic Chemistry (Minor)
Topic: Acetoxylation of Olefins
Submitted by: Muhammad Faisal (23-Reg)
Mohsin Abbas (34 -SS)
Class: BS Chemistry 8th Regular
Session: 2012-2016
Submitted to: Dr. Abdul Karim
Dated: 11-05-2016
Department of Chemistry,
University of Sargodha,
Sargodha

2. Acetoxylation of olefins:
The introduction of AcO group in olefins is called acetoxylation.
There are two types of processes for acetoxylation which are; Solution based process and gas
phase process.
The Solution-phase Ethylene to Vinyl Acetate Process::
The mechanism in this process is common with that of Wacker process. The process for vinyl
acetate is based on discovery by Moiseev that ethylene is stoichiometrically oxidized to vinyl
acetate by PdCl2 and sodium acetate in acetic acid.
The zerovalent Pd formed can be promptly reoxidized by Cu(II) and the catalytic cycle closed in
the same way as for acetaldehyde synthesis.
Based on this catalytic principle, ICI and Celanese developed industrial liquid phase processes
which led to the construction of large scale plants .
Hoechst independently developed a semi-commercial liquid-phase process . The liquid phase
processes resembled Wacker-Hoechst ’s acetaldehyde process, i.e., acetic acid solutions of
PdCl2 and CuCl2 are used as catalysts. The water produced from the oxidation of Cu(I) to
Cu(II) forms acetaldehyde in a secondary reaction with ethylene. The ratio of acetaldehyde
to vinyl acetate can be regulated by changing the operating conditions. The reaction takes place
at 110–130 ᵒC and 30–40 bar. The vinyl acetate selectivity reaches 93% (based on acetic acid).
The net selectivity to acetaldehyde and vinyl acetate is about 83% (based on ethylene), the by-
products being CO2, formic acid, oxalic acid, butene and chlorinated compounds. The reaction
solution is very corrosive, so that titanium must be used for many plant components. After a few
years of operation, in 1969–1970 both ICI and Celanese shut down their plants due to corrosion
and economic problems.
The Gas-phase Ethylene to Vinyl Acetate Process:
Ethylene acetoxylation was also developed as a gas phase process following the liquid phase
process and has been in commercial use since 1968. There is a notable difference between the
two processes: in the liquid phase the presence of palladium salts and redox systems results in
the formation of both vinyl acetate and acetaldehyde, whereas in the gas phase process, using
palladium metal, vinyl acetate is formed almost exclusively. Additionally there are no corrosion
problem s leading to cost savings in the use of stain less steel as the construction material.

3. One version of the gas phase process was developed by National Distillers Products (now
Quantum Chemical ) in the USA and another independently in Germany by Bayer together with
Hoechst. In both versions, ethylene is reacted with acetic acid and oxygen on a palladium-
containing fixed-bed catalyst at 5–10 bar and 175–200 ᵒC to form vinyl acetate and water.
The explosion limit restricts the O2 content in the feed mixture so that the ethylene conversion is
relatively small (B 10%). The acetic acid conversion is 20–35% with selectivities to vinyl acetate
of up to 94% (based on C2H4) and about 98–99% (based on AcOH). The most important side
reaction of this process is the total oxidation of ethylene to carbon dioxide and water. Other by-
products are acetaldehyde, ethyl acetate and heavy ends. After a multistep distillation the vinyl
acetate purity is 99.9% with traces of methyl acetate and ethyl acetate that do not
affect the subsequent use in polymerization.
In addition to palladium, the catalysts used commercially always contain alkali salts, preferably
potassium acetate. Additional activators include gold, cadmium, platinum, rhodium, barium,
while supports such as silica, alumina, aluminosilicates or carbon are used. The catalysts
remain in operation for several years but undergo deactivation. The drop in activity is due to a
gradual sintering of the palladium particles which causes the catalytically active area to
decrease progressively. Under reaction conditions potassium acetate is slowly lost from the
catalyst and must continuously be replaced.
The role of gold in the Pd/ Au/K acetate catalysts is to stabilize the size of Pd
crystallites and avoid sintering. The role of potassium acetate is to maintain the catalyst activity
and decrease CO2 selectivity. Potassium acetate favours a strong adsorption of acetic acid on
palladium, lowering the barrier to vinyl acetate formation. Gold by itself is inactive in the
catalysis of vinyl acetate. Pd only catalysts produce vinyl acetate at much lower rates than the
Pd/Au/K catalyst system and their activity decays rapidly.
The liquid and gas phase catalyst systems for vinyl acetate are based on the same
components; no coincidence as the latter was developed after the discovery of the former. They
differ mainly in the reoxidation of Pd(0), which is carried out by Cu(II) in the liquid phase process
and is not necessary in the gas phase process. It therefore seems tempting to suggest that the
chemistry is similar in both cases, at least as far as the vinyl acetate formation is concerned.
The kinetics and the mechanism of the gas phase acetoxylation of ethylene on palladium
catalyst has been the subject of many studies. These studies are based on a Langmuir-
Hinshelwood type mechanism in which all reacting species are chemisorbed on the Pd surface
and reaction occurs between chemisorbed species. Although a complete description is still
pending a commonly accepted proposal is shown in Figure 30.
The gaseous reactants interact with the Pd surface giving different chemisorbed species: i)
ethylene, which chemisorbs dissociatively producing a hydride and a vinyl group; ii) acetic acid
which initially adsorbs non dissociatively; and iii) O2 which dissociates yielding highly reactive
surface atomic oxygen.
In the following step oxygen abstracts a proton from acetic acid, yielding surface hydroxo- and
acetato- groups. Hydrogen abstraction from acetic acid and weakening of the Pd–O bond of
adsorbed acetate are promoted by the added alkali metal ions (potassium). The rate
determining step is the coupling reaction between the surface acetate and vinyl groups with the

4. formation of adsorbed vinyl acetate. The latter is finally desorbed along with water formed
by coupling of the surface hydride and hydroxide.
The main source of carbon dioxide, the main by-product, is thought to be the total oxidation of
ethylene by the highly reactive surface oxygen.
The similarities with Figure 2 8 are, i) the formation of a Pd-olefin complex; ii) the coupling
between the surface acetate and the vinyl group (equivalent to the nucleophilic attack depicted
in Figure 29), and iii) the formation of a coordinated/adsorbed Pd-vinyl acetate species.
It can be concluded that the understanding of Wacker chemistry with soluble species was a
major driving force in developing the heterogeneous catalyst for vinyl acetate synthesis.
Unfortunately, a similar heterogeneous catalyst formulation cannot be used for the original
Wacker process for the synthesis of acetaldehyde, as that would significantly simplify the
engineering of the process, since in the absence of a strong competing nucleophile (such as
acetate), the presence of surface oxygen would lead to carbon dioxide formation.
Uses of Vinyl Acetate:
The world production capacity of PVA was 4.35 Mt/a in 2005, of which 2.1 Mt were converted
into polyvinyl alcohol.
1) Most vinyl acetate is converted into polyvinyl acetate (PVA) which is used in the
manufacture of dispersions for paints and binders and as a raw material for paints.
2) It is also copolymerized with vinyl chloride and ethylene and to a lesser
extent with acrylic esters.
3) A substantial proportion of vinyl acetate is converted into polyvinyl alcohol by
saponification or transesterification of polyvinyl acetate. The main applications for
polyvinyl alcohol are either as raw material for adhesives or for fibres. It is also employed
in textile finishing and paper glueing, and as a dispersion agent (protective colloid).

5. Refrences:
1) Organotransition metal chemistry, Fundamentals and Applications by Akio Yamamoto.
2) Metal-Catalysis in Industrial Organic Processes By Gian Paolo Chiusoli and Peter M.
Maitlis.