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Applied Catalysis A: General 221 (2001) 253265
Recent advances in processes and catalystsfor the production of acetic acid
Noriyuki Yoneda a, Satoru Kusano a,, Makoto Yasui b, Peter Pujado c, Steve Wilcher c
a Chiyoda Corporation, 3-13 Moriya-cho, Kanagawa-ku, Yokohama 221-0022, Japanb Chiyoda Corporation, 2-12-1 Tsurumichuo, Tsurumi-ku, Yokohama 230-8601, Japan
c UOP LLC, 25 East Algonquin Road, Des Plaines, IL 60017-5017, USA
Abstract
Novel acetic acid processes and catalysts have been introduced, commercialized, and improved continuously since the
1950s. The objective of the development of new acetic acid processes has been to reduce raw material consumption, energy
requirements, and investment costs. At present, industrial processes for the production of acetic acid are dominated by
methanol carbonylation and the oxidation of hydrocarbons such as acetaldehyde, ethylene, n-butane, and naphtha. This paper
discusses advances in acetic acid processes and catalysts according to the following routes: (1) methanol carbonylation; (2)
methyl formate isomerization; (3) synthesis gas to acetic acid; (4) vapor phase oxidation of ethylene, and (5) other novel
technologies. 2001 Published by Elsevier Science B.V.
Keywords: Acetic acid; Methanol carbonylation; Hydrocarbon oxidation; Reaction mechanisms
1. Introduction
Acetic acid is an important commodity chemical
used in a broad range of applications. As shown in
Fig. 1, acetic acid is used primarily as a raw ma-
terial for vinyl acetate monomer (VAM) and acetic
anhydride synthesis, and as a solvent for purified
terephthalic acid (PTA) production. The demand for
acetic acid has increased, especially in southeast Asia,
where several new PTA plants have been built. Withthe increased demand and installed capacity for PTA
in southeast Asia, the region has become a major
producer of polyester (PET) fiber, film, and resin.
Although the economic crisis in Asia momentarily
suppressed the demand for acetic acid to less than
expected levels, in the medium and long terms there
Corresponding author. Tel.: +81-45-441-9151;
fax: +81-45-441-1281.
is potentially a great demand for acetic acid in this
market.
The total world capacity of acetic acid has reached
approximately 7.8 million t in 1998 with BP-Amoco
and Celanese accounting for more than 50% of
the worlds capacity [1]. BP-Amoco and Celanese
have installed capacities of 1.5 million t (19%), and
2.0 million t (26%), respectively.
2. Processing routes to acetic acid
Originally, acetic acid was produced by aerobic fer-
mentation of ethanol, which is still the major process
for the production of vinegar. The first major com-
mercial process for the synthetic production of acetic
acid was based on the oxidation of acetaldehyde. In
an early process for the conversion of acetylene to ac-
etaldehyde introduced in 1916 in Germany and used
0926-860X/01/$ see front matter 2001 Published by Elsevier Science B.V.
P I I : S 0 9 2 6 - 8 6 0 X ( 0 1 ) 0 0 8 0 0 - 6
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Fig. 1. Use of acetic acid.
in China until recently, an organo-mercury compound
was used as the catalyst. The toxicity of the mercurycatalyst resulted in significant environmental pollu-
tion, and as a result, has essentially been phased out.
As the petrochemical industry developed in the 1950s,
the raw material for the production of acetaldehyde
shifted to ethylene. Other processes for the production
of acetic acid introduced in the 1950s and 1960s were
based on the oxidation of n-butane or naphtha. The
major producers of acetic acid via direct oxidation of
hydrocarbons were Celanese (via n-butane) and BP
(via naphtha). However, these reactions also produce
significant amounts of oxidation by-products, as sum-marized in Table 1, and their separation and recovery
can be very complex and expensive.
The homogeneous methanol carbonylation route to
acetic acid that used a homogeneous Ni catalyst was
first commercialized by BASF in 1955. An improved
process was later disclosed by BASF in 1960. The
process used an iodide-promoted CO catalyst and
operated at elevated temperature (230 C) and pres-
sure (600 atm). The product yields exhibited by this
Table 1
Acetic acid process
Catalyst Reaction condition
(C, atm)
Yield By-product
Methanol carbonylation Rhodium complex 180220, 3040 MeOH: 99%, CO: 85% None
Acetaldehyde oxidation Manganese acetate
or cobalt acetate
5060, atmospheric
pressure
CH3CHO: 95% None
Ethylene direct oxidation Palladium/heteropolyacid/
metal
150160, 80 Ethylene: 87% Acetaldehyde CO2
Hydrocarbon oxidation
(n-butane, naphtha)
Cobalt acetate or
manganese acetate
150230, 5060 n-Butane: 50%,
naphtha: 40%
Formic acid,
propionic acid, etc.
process were 90, and 70% based on methanol and CO
consumption, respectively, [2]. In 1970, Monsanto
commercialized an improved homogeneous methanol
carbonylation process using a methyl-iodide-promotedRh catalyst [36]. Compared to other acetic acid syn-
thesis routes (ethanol fermentation, and acetaldehyde,n-butane, or naphtha oxidation), homogeneous Rh
catalyzed methanol carbonylation is an efficient route
that exhibits high productivity and yields. The pro-
cess operated at much milder conditions (180220 C,
3040 atm) than the BASF process and exhibited su-
perior performance: acetic acid yields were 99 and
85% based on methanol and CO consumption, re-
spectively, [7]. Celanese and Daicel further improved
the Monsanto process during the 1980s by adding
a lithium or sodium iodide promoter to enable the
operation in a reduced water environment [815]. At
lower water concentrations, by-product formation via
the water gas shift reaction is reduced, thus improving
raw materials consumption and reducing downstream
separation costs.
Homogeneous metal catalysts less costly than Rh
(for example, Ni [16,17,75,76] and Ir [3,1824] with
other metal additives) have also been investigated.
The Ir-based process allows operation at reactor water
levels comparable to those of the improved Celanese
process and was commercialized by BP Chemicals in1996.
Until recently, virtually all new acetic acid ca-
pacity has made use of the homogeneous methanol
carbonylation technology developed by Monsanto
and practiced commercially by all major acetic acid
manufacturers, including BP-Amoco, Celanese, and
others. As a result, more than 60% of the world acetic
acid production employs the methanol carbonylation
methods, as shown in Fig. 2.
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Fig. 2. Acetic acid process routes.
Inherent to the homogeneous system, however, are
drawbacks relating to catalyst solubility limitations
and the loss of expensive Rh metal due to precipi-
tation in the separation sections. Accordingly, immo-
bilization of the Rh complex on a support has been
the subject of considerable investigation. Chiyoda and
UOP have jointly developed an improved methanol
carbonylation process for the production of acetic acid
based using a heterogeneous Rh catalyst system [25].
A direct oxidation process for the production of
acetic acid starting from ethylene was commercialized
by Denko in 1997. While the raw material, ethylene,
is more expensive than in the methanol carbonylationroute, the investment cost is reported to be lower and
competitive for small or medium-size capacity plants.
Wacker-Chemie plans to commercialize a new acetic
Fig. 3. Catalytic cycle for rhodium carbonylation.
acid process based on butylene feedstock. This process
also employs direct oxidation. Its key features are the
use of a relatively cheap raffinate-2 feedstock and com-
petitive economics in medium size plants. Recently,Poulenc and others have disclosed the direct produc-
tion of acetic acid from ethane; there are no indica-
tions of impending commercialization for this route.
Generally, the production cost of commodity chem-
icals such as acetic acid is dominated by the raw mate-
rial costs, and methanol carbonylation is still regarded
as the preferred route to produce acetic acid. Table 1
summarizes reaction conditions, catalysts and yields
for the major processes used to produce acetic acid. A
number of reviews on production of acetic acid have
been published and are referred [7,29,73,74].
3. Methanol carbonylation
3.1. Rhodium catalyzed methanol carbonylation
The methanol carbonylation process, Mon-
santo process, is operated under mild conditions
(180220 C, 3040 atm) and exhibits high selectivity
to acetic acid based on methanol (99%) and carbon
monoxide (85%) [7]. While the reaction, as shown
below, can be carried out in a variety of rhodium(I) or rhodium (III) complexes [6,18], under reaction
conditions they are almost invariably converted to the
active catalyst [RhI2(CO)2]1. As shown in Fig. 3,
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methyl iodide is provided by the reaction of feed
methanol with hydrogen iodide:
CH3OH+ CO
Rh complex
CH3COOHMethyl iodide is oxidatively added to the rhodium
dicarbonyldiiodide complex [RhI2(CO)2]1 (A)
to generate a rhodiummethyl complex (B). This
rhodiummethyl complex can rapidly undergo a
methyl migration to a neighboring carbonyl group in
the acetyl form (CH3CO) and react with CO (C) to
generate the rhodiumacetyl complex (D). Reductive
elimination of acetyl iodide (CH3COI) can then lib-
erate the original rhodium complex (A). Hydration of
acetyl iodide is very rapid in the presence of excess
water and will result in the formation of acetic acidand hydrogen iodide to complete the cycle.
The reaction rate is independent of methanol
concentration and carbon monoxide pressure. The
rate-determining step is believed to be the oxidative
addition of methyl iodide to the rhodium center of the
rhodium complex (A), and the reaction rate is essen-
tially of first order in both catalyst and methyl iodide
concentrations under normal reaction conditions:
reaction rate [catalyst][CH3I]
A substantial quantity of water (1415 wt.%) is re-quired to achieve high catalyst activity and also to
maintain good catalyst stability [8,9,1214]. However,
as rhodium also catalyzes the water gas shift reaction
(Fig. 4), the side reaction forming CO2 and H2 from
CO is significantly affected by water and hydrogen
iodide concentration in the reaction liquid [26,27].
Propionic acid is observed as the major liquid
by-product in this process. This is produced by the
carbonylation of ethanol that is often present as a
minor impurity in the methanol feed; however, other
Fig. 4. Mechanism for water gas shift.
routes are active since more propionic acid is observed
than can be accounted for by only this mechanism.
The rhodium catalyst system can generate acetalde-
hyde, and it is proposed that this acetaldehyde isreduced by hydrogen in the system to give ethanol
which subsequently yields propionic acid. One possi-
ble precursor for the generation of acetaldehyde is the
rhodiumacetyl species, as shown in the following
mechanism [28]:
[RhI3(CO)(COCH3)]+HI
[RhI4(CO)]+ CH3CHO
[RhI4(CO)] RhI3 + I
+ CO
Reaction of this species with hydrogen iodide would
yield acetaldehyde and [RhI4(CO)]1. The latter
species is well known in this system and is postulated
as the principal cause of catalyst loss by precipitation
of inactive rhodium tri-iodide [28].
Acetaldehyde undergoes self-condensation or aldol
condensation and yields butenal and higher aldehy-
des. These can undergo further reactions to alcohols
and carboxylic acids as summarized in the network of
Fig. 5 [28]. It would be expected that the homologa-
tion observed would result in unsaturates and iodides
having an even number of carbon atoms, and long
chain carboxylic acids with an odd number of carbonatoms. Particular problems are encountered with the
C6 species present. The boiling points of the unsatu-
rated compounds, including hexanal and some of its
isomers, are very similar to that of acetic acid. Further-
more, hexyl iodide is observed to form a constant boil-
ing azeotropic mixture with acetic acid. The presence
of the unsaturates, even at low parts per million con-
centrations can cause problems with product stability.
Separation of pure acetic acid product from the re-
action medium presents few problems. In this process,
however, the expensive Rh metal can be lost due to itsprecipitation and vaporization in the flash column. A
schematic of a conventional methanol carbonylation
plant configuration is shown in Fig. 6 [28]. Rhodium
catalyst is separated from the product acetic acid by
conducting a simple flash; the catalyst remains in the
liquor and can be recycled to the reactor. The sepa-
ration of light compounds, such as methyl iodide and
methyl acetate, may be carried out in the first distil-
lation column. This column is followed by a drying
column and then a column for the removal of heavy
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Fig. 5. Network of liquid by-products.
by-products. Energy usage in this fractionation train
can be high, depending on the concentration of waterand impurities such as propionic acid, heavy unsatu-
rates, and hexyl iodide present.
In the Monsanto process, because of the high water
concentration in the reactor (1415 wt.%), the separa-
tion of water from the acetic acid product is a major
energy consumer and can limit the unit capacity. In
addition, excess water causes carbon monoxide yield
loss due to the water gas shift reaction, and increases
the formation of by-products such as propionic acid,
thus lowering the acetic acid quality. Considerable sav-
ings in operating costs can be realized by operatingat low water concentration if a way can be found to
compensate for the consequent decrease in the reac-
tion rate and catalyst stability [29]. As a result, the
rhodium complex stability at low water concentrations
has been extensively investigated.
Fig. 6. Schematic of a acetic acid plant configuration.
Group I metal iodides, especially lithium iodide in
combination with methyl iodide, were identified earlyas a good agent for enhancing the stability of the
rhodium catalyst at low reactor water concentrations
(45 wt.%), and also for decreasing liquid by-product
formation [1214]. Further work in this area revealed
that the addition of a substantial quantity (1620 wt.%)
of group I metal iodides also enhanced the reactor
productivity even at quite low water concentrations
(2 wt.%) [811]. These features reportedly allow exist-
ing plants to expand their capacity for little incremen-
tal capital cost. The improved methanol carbonylation
process, low water process, effected by addinggroup I metal iodides to the Monsanto process was first
commercialized in the 1980s by Celanese and Daicel.
In this process, it is proposed that the addition of
a significant quantity of group I metal causes the Rh
complex to be more coordinated by CH3COO and
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Fig. 7. Reaction acceleration mechanism by iodide salt.
increases the rate of oxidative insertion of methyl io-
dide (the rate determining step), thus promoting theprimary carbonylation reaction (Fig. 7). As Figs. 7 and
8 shows the effect of the addition of lithium iodide
on the reaction rate, the overall carbonylation rate in-
crease is presumably due in part to the formation of
a strong nucleophilic five-coordinate dianionic inter-
mediate [Rh(CO2)2I2L]2 (L = I, OAc) which
is more active toward oxidative addition of methyl io-
dide [811,29].
The main advantages of the low water process rela-
tive to the conventional Monsanto process are reduced
raw materials consumption, increased productivity,
lower utility requirements, and lower capital costs
per unit of product. However, low water operation
Fig. 8. Effect of Li salt addition. Reaction condition:
[CH3] = 1.0 M, [MeOAc] = 0.3M, [H2O] = 1.0 M, temperature
= 190 C, total pressure = 400 psig.
with alkali-iodide promoters results in a higher iodide
environment, and higher residual iodide in the final
product. High iodide concentration in acetic acid leads
to catalyst poisoning problems in some downstreamapplications, such as in the manufacture of VAM. To
overcome the problems associated with high iodide
concentration in the final product, treatment by active
carbon [30], hydrogenation [31,32], and extra distilla-
tion [33,34] have been proposed. Celanese disclosed
the silver-guard process for the removal of very low
levels of iodide impurities from acetic acid in their
patent [35]. The use of silver metal on an ion exchange
resin such as Amberlyst-15 reduces the iodide level
to below 1 ppb, as opposed to 20 ppb more normally
achieved by conventional methods. One particular
advantage of this system is the ability to effectively
remove the halide impurity in a single step, thus avoid-
ing the need for additional distillation and recovery.
3.2. Nickel catalyzed methanol carbonylation
Recent studies have shown that nickel catalysts
can operate under mild conditions (190 C, 70 atm)
with the addition of methyl iodide as a co-promoter
[16]. The activity of nickel catalyst systems can be
increased and the volatility of nickel carbonyl com-
pounds lowered by the introduction of stabilizers suchas phosphines, alkali metals, tin, and molybdenum
[16,17,25,75,76]. The active catalysts are thought to
be Ni(0) complexes. For phosphine-promoted cata-
lyst, Ni(PR3)2 is considered an active form of catalyst
and, in addition, Ni(CO)4 was observed in all cases,
and its concentration was reduced by strongly coordi-
nating ligands and enhanced by weakly coordinating
ligands [76]. Recent work on nickel catalyst systems
shows that reaction rates and selectivities can ap-
proach those achieved in the rhodium catalyst system.
Although nickel catalysts have the advantage of beingmuch cheaper than rhodium, and are easy to stabi-
lize at low reactor water concentrations, [Ni(CO)4] is
known to be a very toxic and volatile compound. To
date, commercialization has not proceeded.
3.3. Iridium catalyzed methanol carbonylation
The potential use of iridium instead of rhodium was
identified as part of the early work done by Monsanto
[3,18,25], however, the reaction rate exhibited by the
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rhodium catalyst system was superior to that of irid-
ium. Recently, it was disclosed that an improved irid-
ium catalyst, in combination with a promoter metal
such as ruthenium, has higher activity and results inlower product impurity levels than reported in pre-
vious iridium systems [19]. The production of acetic
acid using the iridium catalyst system has been com-
mercialized by BP-Amoco in two world scale plants to
date, and has received wide publicity as the Cativa
process. Although much iridium is required to achieve
an activity comparable to the rhodium catalyst-based
processes, the catalyst system is able to operate at re-
duced water levels (less than 8 wt.% for the Cativa
process versus 1415 wt.% for the conventional Mon-
santo process). Thus, lower by-product formation and
improved carbon monoxide efficiency are achieved,
and steam consumption is decreased. Until the early
1990s, the difference in the prices of rhodium (US$
500/oz) and iridium (US$ 60/oz) was the driving force
for replacing rhodium with iridium. However, current
price increases for iridium (US$ 450/oz) negate the
advantage in catalyst price.
The unique differences between the rhodium cat-
alytic cycle and that of iridium in methanol car-
bonylation have been investigated [36]. The anionic
iridium cycle shown in Fig. 9, is similar to that shown
earlier for rhodium. Model studies have demonstrated
Fig. 9. Catalytic cycle for iridium carbonylation.
that the oxidative addition of methyl iodide to the
iridium center is of the order of 150 times faster
than the equivalent reaction with rhodium [36]. This
represents a possible improvement in the availablereaction rates, as methyl iodide addition is not the
rate determining step. The slowest step in this cycle is
the insertion of carbon monoxide to form the iridium
acetyl species, that involves the elimination of ionic
iodides and the coordination of an additional carbon
monoxide ligand. This would suggest the following
expression. The dependence on ionic iodide:
reaction rate [catalyst][CO]
[I]
suggests that high reaction rates should be achiev-
able by operating at low iodide concentrations. It
also suggests that the inclusion of species capable of
assisting in the abstraction of iodide should promote
the rate-limiting step. The patent would suggest that
ruthenium, or rhenium are the preferred promoters
[20,21]. In effect, a proprietary blend of promoters
has been found to increase reaction rate. The above
expression does not imply any effect from the water
present in the matrix, but water is found to have a
significant effect on rate [22].
In the improved iridium system, low water concen-
tration in the reactor results in the formation of fewer
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by-products such as propionic acid than in the original
Monsanto rhodium system, and no addition of lithium
iodide is required. Consequently, the iridium catalyst
system is also characterized by the formation of fewerhigher alkyl-iodide species than in the conventional
low water process.
3.4. Heterogeneous rhodium catalyzed
methanol carbonylation
In order to overcome the limitations of the ho-
mogeneous catalyst system (e.g. Rh precipitation
and catalyst solubility limitations), the immobiliza-
tion of the Rh complex on a support has been the
subject of considerable investigation. Active carbon
was investigated as a possible support and proposed
for vapor-phase operation [7,37,38]. However, the
reaction rate was 1/10001/10 that of Monsantos ho-
mogeneous process and selectivity was also poorer.
Inorganic oxides and zeolites were also investigated
for use in vapor-phase operation [39,40]. For exam-
ple, attaching the Rhphosphine ligand complex to
alumina by silylation was attempted [41,42]. The
resultant reaction rates for these catalysts were also
found to be poor relative to those observed for the
homogeneous system. To increase catalyst activity
for operation in the liquid phase, ion exchange resinsbased on cross-linked polystyrene and incorporating
pendant phosphines, or vinyl pyridine copolymers
have been evaluated [4345]. Although the activity of
these catalysts in the liquid phase was comparable to
Monsantos homogeneous catalyst, there were prob-
lems with rhodium metal leaching from the resins
and the decomposition of the resins during opera-
tion at elevated temperature. Vinyl pyridine resin was
known to be more robust and more tolerant of oper-
ation at elevated temperature relative to polystyrene
resins. It was disclosed that catalysts using pyridineresins exhibited high tolerance to operation at ele-
vated temperature and pressure, and higher reaction
rate than Monsantos rhodium system [46]. Further-
more, Chiyoda introduced novel pyridine resins and
catalysts that exhibited high activity, long catalyst
life, and no significant rhodium loss [4749]. Based
on this heterogeneous Rh catalyst, Chiyoda and
UOP have jointly developed an improved methanol
carbonylation process, called the acetica process,
for the production of acetic acid. Until the recent
development of a commercial heterogeneous Rh cata-
lyst system by Chiyoda, no successful demonstration
of such a catalyst had been known [7].
The heterogeneous catalyst commercialized for theacetica process consists of Rh complexed on a novel
poly-vinyl pyridine resin [50], which is tolerant of
elevated temperatures and pressures. Under reaction
conditions, the Rh is converted to its catalytically
active anion form [Rh(CO)2I2]1. Furthermore, the
nitrogen atoms of the resin pyridine groups become
positively charged after quaternization with methyl
iodide. Thus, the strong ionic association between the
pyridine nitrogen groups and the Rh complex causes
the immobilization (Fig. 10). The concentration of Rh
on the solid phase is determined by the ion exchange
equilibrium. Because equilibrium strongly favors the
solid phase, virtually all the Rh in the reaction mixture
is immobilized.
In the acetica process, the methanol carbonyla-
tion reaction is conducted at moderate temperature
(160200 C) and pressure (3060 atm) and at low
water concentration without any additives present.
Catalyst stability has been demonstrated in both once-
through and continuous-recycle pilot plant testing at
process conditions, low water content, and no Rh or
resin makeup. The catalyst exhibited no deactivation
after continuous operation for more than 7000 h [50].With homogeneous methanol carbonylation routes,
acetic acid productivity is directly proportional to
catalyst concentration in the reaction liquid, and as
a result, acetic acid production is restricted by the
solubility of the active metal. Limited success has
been achieved in improving catalyst solubility in
these systems by increasing the reaction-mixture wa-
ter concentration or by adding iodide salt stabilizers
[8,9,1214]. Both additives, however, result in in-
creased recycle and separation costs, higher corrosion
rates, and difficulty in product purification.With the heterogeneous catalyst system, catalyst
solubility limitations no longer govern reactor capac-
ity since catalyst concentrations several times greater
than those achievable in the homogeneous systems
are possible. Immobilization also significantly re-
duces the loss of expensive Rh metal because the
catalyst is confined to the reactor rather than circulat-
ing downstream, where reduced pressures may cause
precipitation of rhodium and vaporization losses of
metal carbonyl compounds. The lower water content
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Fig. 10. Rhodium immobilization.
of 37 wt.% typical of the acetica process results
in reduced production of CO2, and hydrogenated
by-products via the water gas shift reaction. Also,
because of the lower water content, less hydrogen
iodide is present in the system, and consequently the
process environment is less corrosive.
While the continuously stirred tank reactors (CSTR)
used in the conventional homogeneous processes
can be limited by gas solution rates to liquid and
are often prone to mechanical problems, the bubble
column, or gas lift reactor employed with the hetero-
geneous catalyst process does not suffer from such
problems and limitations. The acetica three-phase
gas lift reactor has no moving parts or mechanical
Fig. 11. Bubble column reactor and acetica process flow.
seals and was designed to maximize the performance
of its unique heterogeneous catalyst system without
any rotating equipment (Fig. 11). Methanol and CO
feeds are introduced at the reactor bottom, where the
compressed CO gas is distributed through a sparger.
Both of these feeds, along with the recycle liquid
and catalyst, flow up the reactor riser, where the
CO is consumed in the reaction. The process flow,
which is similar to that of a conventional homoge-
neous process is shown in Fig. 11. In cases where the
acetic acid product will be used for VAM production,
novel iodide removal technology is available to re-
duce the iodide in the acetic acid product to less than
3 ppb [51].
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4. Methyl formate isomerization
It has been proposed that acetic acid can be pro-
duced by isomerization of methyl formate in the pres-ence of a homogeneous rhodium catalyst together with
other metal additives [52,53]. Heterogeneous rhodium
catalysts supported on poly-vinyl pyridine resin have
also been proposed for this application [54]. This cata-
lyst has the same chemical morphology as a methanol
carbonylation catalyst. Methyl formate is produced by
dehydrogenation of methanol [55] or by methanol car-
bonylation under high pressure in the presence of:
HCOOCH3 CH3COOH
copper oxide and alkali catalyst. It is noted thatacetic acid production via methanol dehydrogena-
tion followed by methyl formate isomerization re-
quires only methanol and no carbon monoxide
plant:CH3OHHCHOHCOOCH3CH3COOH-
Acetic acid can be produced from only methanol us-
ing a Ru-Sn catalyst according to the following steps
[56,57]. Ru-Sn bimetallic complexes are proposed to
be the active species.
5. Synthesis gas route to acetic acid
A nearby synthesis gas plant to produce CO is nor-
mally required to provide feed to an acetic acid plant.
On the contrary, an efficient integrated synthesis
gas and methanol synthesis plant and acetic acid plant
are available by combination of current technology at
the natural gas source. This integrated process could
achieve a significant capital cost reduction relative to
the conventional flow scheme.
Applying this concept, Haldor Topsoe proposed
an integrated process that includes the synthesis ofmethanol and dimethyl ether (DME) in a first catalytic
reaction stage and the subsequent carbonylation of
methanol and DME into acetic acid [58,59]. Although
the reaction pressure required for methanol synthesis
is higher than the pressure used in acetic acid syn-
thesis, the combination of methanol synthesis with
dimethyl ether synthesis can reduce the pressure of
the first reaction step. The catalyst consists of a mix-
ture of the catalyst for methanol synthesis (Cu-Zn-Al
oxide, etc.) and a dehydration catalyst (H-ZSM-5,
etc.). The reaction is carried out at approximately
220 C and 40 atm:
CO+ 2H2 CH3OH
2CH3OH CH3OCH3 +H2O
H2O+ CO CO2 +H2
In the acetic acid synthesis step, carbonylation of
DME and methanol to acetic acid is carried out by
the rhodium carbonyl complex catalyst with carbon
monoxide being supplied from the synthesis gas
process unit:
CH3OH+ CO CH3COOH
CH3OCH3 + 2CO+H2O 2CH3COOH
Carbonylation reaction conditions of 170250 C and
2550 atm, can be used to obtain acceptable reaction
rates in the liquid phase.
6. Vapor phase oxidation of ethylene
The two-step oxidation process for the production
of acetic acid, starting from ethylene through acetalde-
hyde, was first commercialized in 1960:
CH2=CH2 +12 O2 CH3CHO
CH3CHO+12 O2 CH3COOH
This route involves the liquid phase oxidation of ac-
etaldehyde using air and typically a manganese ac-
etate catalyst operating at 5060 C. The reaction is
based on a free radical mechanism. Although this pro-
cess features high yield (approximately 90%) and a
relatively low capital investment cost, it suffers from
high acetaldehyde feedstock cost and a very corrosive
catalyst system. Many plants utilizing this technology
have been shut down over the last 20 years.
There is also an older process that entails liquid
phase free radical oxidation of n-butane or naphtha
in the C4C8 range. These reactions produce a wide
spectrum of oxidation by-products such as formic acid
and propionic acid:
CH3CH2CH2CH3+O2CH3COOH+ by-products
The direct production of acetic acid from ethylene via
an acetaldehyde intermediate is a desirable synthesis
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Scheme 1. Hydration route.
route that has yet to be developed. Much work has been
undertaken to develop a simpler, single stage process
for producing acetic acid directly from ethylene:
CH2=CH2 +O2catalyst CH3COOH
Various groups have carried out extensive research and
development in the area of direct vapor phase oxida-tion of ethylene to acetic acid. Catalyst systems con-
sisting of palladium chloride and V2O5 supported on
Al2O3 [60], and combinations of Pd (2%) and H3PO4(25%) on SiO2, Pd-V2O5-Sb2O3 on Al2O3 [61], or
Pd (1%) on V2O5 [62] have been proposed. These
catalysts have acetic acid selectivities in the range of
6090% based on ethylene. The routes have been pro-
posed according to the different catalyst systems in
Schemes 1 and 2.
Denko has developed a direct oxidation process for
the production of acetic acid based on the hydration
route [6365] and has commercialized this technol-
ogy in late 1997. The catalyst consists of either two
or three components. The first component is palla-
dium supported on a carrier, preferably in the range of
0.12% range. The second component is a heteropoly
acid and their salts, preferably phosphotungstic acid
salts of lithium, sodium, and copper. The third com-
ponent is copper, silver, tin, lead, antimony, bismuth,
selenium, or tellurium.
The reaction takes place in a fixed bed reactor at
operating temperatures and pressures of 150160 C,
and up to 8 atm, respectively. The gases fed to the re-actor are ethylene, oxygen, steam, and nitrogen that is
used as a diluent. The presence of steam is required to
Scheme 2. Partial oxidation route.
enhance the activity and selectivity of the process for
the production of acetic acid. The selectivity to acetic
acid is approximately 86%, since it appears that ac-
etaldehyde and carbon dioxide are necessarily formedin this type of process.
7. Other proposed technologies for the
production of acetic acid
7.1. Ethane oxidation
In the 1980s, an acetic acid route from ethane was
introduced. Two reaction mechanisms based on:
CH3CH3 +O2catalyst CH3COOH+ by-product
different catalyst systems were proposed: (1) partial
oxidation of the methyl group, and (2) ethane oxi-
dation to ethylene followed by ethylene hydration to
ethanol, or ethylene to acetaldehyde.
A patent refers to the production of acetic acid
by reacting ethane, ethylene, or mixtures of ethane
and ethylene with oxygen over a catalyst containing
molybdenum, vanadium, and one other metal (Z) in
the general formula MoxVyZz [66]. In one example,
the patent describes the gas phase oxidation of a 1/10mixture of ethane and ethylene at 255 C over a vana-
dium catalyst containing lesser amounts of molybde-
num, niobium, antimony, and calcium supported on
an LZ-105 molecular sieve to yield 63% selectivity
to acetic acid, and 14% selectivity to ethylene at 3%
ethane conversion. In the combined ethane/ethylene
feed case, the hydration catalyst further catalyzes the
hydration of ethylene to ethanol, which is then con-
verted to acetic acid (Scheme 1). The oxidation cat-
alyst catalyzes the reaction of ethylene to acetic acid
and other oxidation products that are converted toacetic acid (Scheme 2).
In another catalyst system, rhenium or a combi-
nation of rhenium and tungsten are introduced to
replace the molybdenum in the dehydrogenation cat-
alyst [67]. Tests showed that complete substitution
of molybdenum by rhenium (RexVyZz) is beneficial
in the reaction of ethane to ethylene, whereas partial
substitution can increase the selectivity to acetic acid.
Tests were not performed on ethylene feed, but tests
on ethane (21% ethane, 3.8% oxygen, and 75.2%
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nitrogen) resulted in acetic acid selectivity as high as
78% at an ethane conversion of 14.3%.
More recently in 1998, another oxidative process
and catalyst for the production of acetic acid fromethane, or ethylene was disclosed [68,69]. A new
molybdenum vanadate catalyst system promoted with
Nb, Sb, Ca, and Pd allows the gas phase oxidation of
ethane and/or ethylene to acetic acid, with high yield
and higher selectivity under milder operating condi-
tions than previously achieved. The patent discloses
the production of acetic acid with 86% selectivity and
11% ethane conversion per pass, at a temperature and
pressure of 250280 C and 15 atm, respectively.
In 1999, a catalyst for the co-production of ethylene
and acetic acid from ethane was disclosed [70]. It con-
sists of phosphorus-modified molybdenum-niobium
vanadate of formula Mo2.5V1.0Nb0.32Px in which the
optimum range for the phosphorus (x) is 0.010.06:
CH3CH3 +O2catalyst CH2==CH2 + CH3COOH+H2O
Ethane and air (15:85 (v/v)) at 260 C and 200 psig
(1100/h GHSV) reacted over the above catalyst system
(x = 0.042) to produce acetic acid and ethylene with
selectivities of 49.9, and 10.5%, respectively, at 53.3%
conversion. At phosphorus levels greater than 0.06%,
there is a marked increase in ethylene production with
a corresponding decline in acetic acid.
Recently, many attempts have been disclosed re-
garding the use of ethane as feedstock. Although
ethane is a relatively inexpensive and attractive raw
material for producing acetic acid, the oxidation pro-
cesses produce a variety of co-products, the disposi-
tion of which needs to be considered in any business
plan.
7.2. Methane carbonylation
Novel methods for producing acetic acid directly
from methane under relatively mild conditions have
been reported. It was first disclosed that acetic acid
can be produced from methane and carbon monox-
ide in the presence of: Pd(OCOCH3)2/Cu(OCOCH3)2/
K2S2O8/CF3COOH [71].
Secondly, it was reported that the mixture of
methane, carbon monoxide and oxygen formed acetic
acid in the presence of rhodium trichloride dissolved
in water [72]:
CH4 + CO+1
2 O2
RhCl3
CH3COOH
This reaction proceeds in an aqueous medium at a
temperature of approximately 100 C and gives a high
yield of acetic acid. The reaction rates are reported
to be too slow for an economically viable industrial
process, but this novel process route has the potential
to reduce the cost of acetic acid production.
8. Conclusions
Acetic acid represents a commodity chemicalgrowing at 3.54.5% per year from a significant
and large base capacity. Significant developments in
both process and catalyst technology have supported
the growth in this market since the 1950s when the
first commercial synthetic process was introduced.
Methanol carbonylation has emerged as the domi-
nant route to this product and currently over 60% of
the world acetic acid is produced using this route.
However, significant catalyst innovation has occurred
even within this production route resulting in greatly
improved yield, and selectivity at milder operatingconditions and lower cost of production. The lucra-
tive nature of this market and the need for the major
producers to continually protect their market position
and investments is expected to drive further inno-
vation within methanol carbonylation and the other
promising technology options looming on the horizon
that have been discussed in this paper.
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