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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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254 N. Yoneda et al. / Applied Catalysis A: General 221 (2001) 253265

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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N. Yoneda et al. / Applied Catalysis A: General 221 (2001) 253265 255

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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