Lloyd L. Handbook of Industrial Catalysts

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288 Chapter 7

have been commercialized to date is the non-catalytic electrodimerization pro-

cess, as first used by Monsanto, and now by BASF:

2 CH

=CHCN + 2 H

+ 2 e

–

→ NC(CH

)

CN (7.33)

The reaction probably proceeds via the formation of a dianion by electron trans-

fer from an acrylonitrile molecule followed by combination with a second mole-

cule and proton addition from a water molecule.

Two rather different approaches to the catalytic dimerisation of acrylonitrile

have also been explored. In the first approach, a ruthenium catalyst was used in

the presence of hydrogen, at temperatures in the range 200–350°C and pressures

of 1–3 bar. Adiponitrile

was produced directly, but unfortunately, was accom-

panied by an equimolar amount of propionitrile, formed by direct hydrogenation

of the acrylonitrile:

2 CH

=CH.CN + H

→ NC(CH

)

CN (7.34)

=CH.CN + H

→ CH

CN (7.35)

It was not possible to eliminate the formation of the propionitrile and the cost of

converting the propionitrile to useful products was such that work on this route

was discontinued.

In the second approach, trivalent organophosphorus derivatives were used.

The main reaction product was α-methyleneglutaronitrile, which was not a use-

ful product, and which could not easily be converted to linear from. In subse-

quent work at ICI,

a range of arylphosphonite and diarylphosphinite catalysts

were developed, which under rigorously anhydrous conditions, and in isopropa-

nol solution, gave essentially linear dimer

at a good rate at temperatures as low

as 60°C. This process was developed successfully through the pilot plant stages,

but work was eventually terminated when ICI withdrew from the nylon business

for strategic commercial reasons.

Adiponitrile can be hydrogenated in the liquid phase, at about 99% selectiv-

ity, to produce hexamethylene diamine at pressures as low as 30 atm using nick-

el catalysts containing iron or cobalt at about 75°C. New routes to HMD are

being developed by major producers.

Adiponitrile was easily hydrogenated to

HMD, at pressures up to 600 atm, with a cobalt oxide catalyst containing small

amounts of silica and titania. Hydrogenation at 300–350 atm was also possible

with iron catalysts.

7.3.1.4. Nylon Polymer

Adipic acid and hexamethylene diamine are mixed in either a water or methanol

solution, where they form the salt, known as nylon salt, which crystallizes as

Petrochemical Catalysts 289

hexamethylene diammonium adipamide. The nylon salt is then heated to tem-

peratures of 250°–270°C to eliminate water and hence form the polymer. Care is

needed to control the dehydration conditions, to produce the optimum chain

length and molecular weight. Acetic acid can be added to control chain termina-

tion.

7.3.2. Nylon 6

Nylon was such a success that other companies already working on polymeriza-

tion began to search for competitive polyamides not covered by the DuPont pa-

tents. Paul Schleck of I.G. Farben quickly developed a type of nylon based on

ε−caprolactam.

The pure dry ε-caprolactam was difficult to polymerize, but the

problem was solved by the addition of catalytic amounts of an acid and water.

Production details were soon developed. Production of polycaprolactam (trade-

mark Perlon L) began in 1940. Nylons based on ε-caprolactam were soon being

produced by many other companies because after the war, details of the process

were released as intelligence reports and were free of patent restrictions.

The DuPont polymer then became known as nylon 66 because it was made

from two C

monomers, while the I.G. Farben polymer, from a single C

mono-

mer, was nylon 6.

7.3.2.1. Caprolactam

The preparation of ε-caprolactam via the Beckman rearrangement of cyclohexa-

none oxime was first described by Wallach in 1900.

In the I.G. Farben process,

phenol was hydrogenated to cyclohexanol, at 140°–160°C and 15 bar, with a

nickel oxide/silica catalyst. The cyclohexanol was dehydrogenated to cyclohex-

anone using a zinc/iron catalyst at 400°C, and the cyclohexanone oxime was

formed by reaction with hydroxylamine monosulfonate. Conversion to cyclo-

hexanone oxime was maximized at pH 7 by neutralizing the solution with am-

monia. The organic layer of crude oxime was crystallized and then isomerized to

ε-caprolactam with 20% oleum at 120°C. ε-Caprolactam was purified and the

ammonium sulfate recovered.

The basic industrial process is still essentially the same except that cyclo-

hexane is now used to provide more than 60% of the ε-caprolactam produced,

with phenol used for the balance. BASF switched to cyclohexane in feedstock

1963. A big disadvantage of the process has been the need to sell about 2.3 lb of

by-product ammonium sulfate for every pound of ε−caprolactam produced, that

is, about 5 kg of ammonium sulfate for each kilogram of ε- caprolactam. How-

ever, there is a ready market for ammonium sulphate in the fertilizer business.

290 Chapter 7

7.3.2.2. Cyclohexanone

Cyclohexanone was recovered from ketone/alcohol oil by distillation and then

dehydrogenating the remaining cyclohexanol using copper oxide/zinc oxide

catalysts at 400°–450°C, with about 90% conversion and 95% selectivity.

The original preparation of cyclohexanone from phenol by hydrogenation to

cyclohexanol followed by dehydrogenation has since been improved. Selective

hydrogenation of phenol to cyclohexanone is possible using a palladium catalyst

at 140°–170°C and 1–2 atm. An Allied/Vickers–Zimmer catalyst contained 0.5–

5% palladium, supported on a low-surface-area calcium aluminate which con-

tained about 8–9% calcium oxide.

7.3.2.3. Cyclohexanone Oxime

Cyclohexanone reacts with hydroxylamine sulfate in the presence of ammonia,

to produce the oxime with an equivalent of by-product ammonium sulfate. The

number of steps required to form hydroxylamine sulfate complicates the proce-

dure, and also gave rise to some further by-product ammonium sulphate.

In the DSM HPO process, nitrate ions are reduced to hydroxylamine using a

palladium/carbon catalyst in an aqueous phosphate buffer solution. It was possi-

ble to prepare cyclohexanone oxime by a process involving the extraction of

cyclohexanone from solution in toluene, into an immiscible aqueous solution of

hydroxylamine in a two phase liquid reactor. The spent solution was then recy-

cled with added nitric acid to the reduction step and sulfate production was elim-

inated:

+ H

+ 3H

→ NH

OH + 2H

O (7.36)

O + NH

OH → C

(OH)NHOH (7.37)

Further efforts to decrease by-product sulfate led to investigation of oxime

production from cyclohexanone by one step ammoxidation reactions. Ammoxi-

dation catalysts are usually unstable (phosphotungstates) or relatively inactive

(silica), but a more efficient catalyst was introduced by Montedipe (now Eni-

chem) in 1986.

This was based on a titanium silicalite catalyst with a sili-

ca/titania molar ratio greater than 30:1. Reaction took place with a hydrogen

peroxide oxidant, in tertiary butanol as solvent at 70°–90°C. Although the con-

version was almost 100% giving selectivity greater than 98%, ammoxidation has

not been used commercially. The photochemical reaction of benzene with nitro-

syl chloride, NOCl, has also been investigated in Japan, as an alternative route to

cyclohexane oxime.

Petrochemical Catalysts 291

7.3.2.4. Snia-Viscosa Process

Snia-Viscosa developed an ε−caprolactam process in 1958 based on the use of

toluene as the raw material,

which was licensed in 1962. The oxidation of tolu-

ene has been studied since at least 1892,

when the first products obtained were

benzaldehyde and benzoic acid. Catalysts based on platinum,

vanadium pent-

oxide, and various other oxides

have been described:

.CH

+ 3/2 O

→ C

.COOH + H

O (7.38)

.COOH + 3 H

→ C

.COOH (7.39)

Weiss and Downs

described the use of vanadium pentoxide for the cata-

lytic oxidation of toluene and naphthalene. Subsequently, Craver

suggested a

mixed oxide catalyst derived from uranium oxide and molybdenum trioxide in

molar ratios ranging from 3-13:1. Copper oxide was also suggested as a possible

promoter. When using the vanadium pentoxide catalyst, benzoic acid was the

main product at temperatures below 400°C, with some benzaldehyde formed at

higher temperatures. Selectivity was only about 50% at 5% conversion.

The Snia-Viscosa process now uses benzoic acid as an intermediate in ca-

prolactam production. The oxidation of toluene in the liquid phase is still used in

modern processes, with selectivity greater than 90%, at about 30% conversion.

Typical operating conditions are at temperatures around 140°C, and pressures in

the range 8–10 bar, using a cobalt acetate catalyst.

The benzoic acid is now hydrogenated to cyclohexane carboxylic acid with

a palladium/carbon catalyst at 170°C and 10–19 bar, whereas originally a nickel

catalyst was used. Reaction with nitrosyl sulfuric acid in oleum solution at 80°C

then gives ε-caprolactam. In the original Snia-Viscosa process, about 25% more

ammonium sulfate was formed than in the conventional process. However, sol-

vent extraction of the caprolactam from the acid solution using alkyl phenols

does avoid the production of the ammonium salt.

7.3.2.5. Conversion of Cyclohexanone Oxime to Caprolactam

Conversion of the oxime to caprolactam was achieved by mixing the oxime with

an equal weight of 20% oleum at a temperature of 120°C. After 15 minutes, the

yield of ε-caprolactam was about 90%. The solution was neutralized with am-

monia and the crude lactam separated recovered and purified by distillation.

In the DSM process, ammonium sulfate formation is reduced from about 5

kg for each kilogram of caprolactam to 1.8 kg. Boric acid catalysts can be used

for the rearrangement of the oxime to the lactam at 330°C in fixed or fluidized

beds, but have not been used commercially. Strong acid exchange resins have

292 Chapter 7

also been tested in a modified Beckman rearrangement using dimethylsulfoxide

solutions at 100°C.

7.3.2.6. Caprolactam from Butadiene

The DuPont direct hydrocyanation of butadiene process for the production of

hexamethylene diamine can also be adapted for the production of caprolactam.

Partial hydrogenation of the adiponitrile intermediate in which only one cyanide

group is converted to the amine, followed by hydrolysis of the remaining cya-

nide group and ring closure can produce ε-caprolactam.

The process has not

been developed commercially. It is also possible that a two-stage butadiene car-

bonylation process to produce caprolactam, developed by DSM and Shell, may

be further developed.

Caprolactam is polymerized to produce nylon 6 by opening the ring with

high temperature water at 250°–280°C to give ω-amino acid

[NH

(CH

)

COOH]. The amino acid can later react with either a second acid

molecule or a caprolactam molecule. Temperature control and the water content

of the mixture are important parameters during the polymerization process.

7.3.3. Polyesters

As a result of the DuPont patents for nylon, in 1937 Whinfield and Dickson, of

the Calico Printers’ Association in England, became interested in polyesters

based on aromatic dicarboxylic acids and ethylene glycol. Carothers had, of

course, abandoned his own early work following the disappointing results with

aliphatic dicarboxylic acids. Whinfield and Dickson quickly produced a long-

chain polyester and decided to patent the discovery, which was eventually li-

censed to ICI. The patent was actually granted in 1941 and ICI built the first

commercial production unit in late 1954.

By 1946 the ICI polyester technology had already been licensed to DuPont

and interest in the new material was on the rise. However, there were severe

practical difficulties in obtaining the almost insoluble terephthalic acid of suffi-

cient purity from the oxidation of p-xylene, without first preparing and convert-

ing the p-toluic acid to the methyl ester. Direct oxidation of p-xylene with air

was later developed, although complete oxidation remained a problem until Sci-

entific Design and Amoco introduced the use of bromine as a cocatalyst in con-

junction with manganese or cobalt salts. Ethylene glycol has remained the most

widely used alcohol.

Polyethylene terephthalate, or polyester, has become the most widely used

synthetic fiber, particularly when combined with cotton. It is also a very popular

thermoplastic resin for bottle and film production and new polyesters are now

being developed. Thermosetting unsaturated polyesters are based on maleic an-

hydride with a range of glycols. Polyethylene terephthalate is produced from

Petrochemical Catalysts 293

ethylene glycol, obtained by the direct oxidation of ethylene, and terephthalic

acid is obtained by oxidizing paraxylene. By the year 2000 almost 20 million

tonnes were being produced annually throughout the world.

7.3.3.1. Paraxylene

The main source of mixed xylenes is either the product from catalytic reformers

or pyrolysis gasoline. The proportion of ortho-, meta-, and paraxylenes is in the

ratio 1:2:1 in both reformate and pyrolysis gasoline. However, reformate aro-

matics contain less than 20% ethylbenzene, whereas pyrolysis gasoline aromat-

ics contains up to 50%. Ethylbenzene and o-xylene can be separated from the

mixture by fractional distillation, while p-xylene is separated by a zeolite ad-

sorption process. The remaining fraction, which is largely the unwanted m-

xylene, is usually isomerized with a catalyst to produce an equilibrium mixture

of the three isomers. This corresponds, approximately, to the 1:2:1, o-, m-, p

ratio in the original aromatic mixtures, depending on the isomerization tempera-

ture. In Table 7.7, typical catalysts and operating conditions for processes that

have been used commercially are shown.

The early silica/alumina catalysts for the isomerization of xylene suffered

from deactivation due to the deposition of carbon and the needed frequent re-

generation. The process was improved by both the use of catalysts impregnated

with platinum and the addition of hydrogen to the reactants, and thus led to a

reduction in the need for frequent regeneration. These catalysts also converted

ethyl benzene to xylenes. High-silica zeolites are now used to produce most of

the p-xylene obtained by isomerization, because high selectivity can be achieved

of equilibrium conversion. Mobil ZSM-5 is particularly useful because the pore

size promotes paraselectivity and controls the unwanted disproportionation reac-

TABLE 7.7. Xylene Isomerization Catalysts.

Catalyst Operation Comments

Silica/alumina 400

–500

C, atmospheric pressure Carbon deposition

No ethylbenzene conversion

Platinum/silica alumina 430

–450

C, 10–25 atm Hydrisomerization (+ H

)

No carbon—long life

Ethylbenzene conversion

1% Platinum/Al

+ mordenite

425

C, 12 atm Ammonia injection to control

zeolite acidity/activity

Platinum exchanged

ZSM-5

420

–460

C, 14–18 atm Hydrisomerization (+H

)

Equilibrium shifted to increase

conversion by 4%

294 Chapter 7

tions that produce trimethyl benzenes. Control of p-xylene isomerization within

the zeolite pores can lead to higher than equilibrium yields of the p-xylene. Zeo-

lites are more resistant to poisons, and do not require frequent regeneration.

7.3.3.2. Terephthalic Acid

The range of catalysts and manner in which the air oxidation of p-xylene to ter-

ephthalic acid has developed since the first plant started operation during the

1950s are shown in Table 7.8. The original oxidation of p-xylene using nitric

acid as oxidant, which led to nitrogen impurities in the product, soon became

obsolete. Direct oxidation with air was then developed using homogeneous cata-

lysts in the liquid phase. The oxidation of p-xylene produced p-toluic acid but

was not able to produce terephthalic acid directly:

p-C

(CH

)

+ 3O

(HNO

) → p-C

(COOH)

+ 2H

O (7.40)

p-C

(CH

)

+ 3/2O

→ p-CH

.COOH + H

O (7.41)

p-CH

.COOH → p-C

.COOH → p-OOCH

.COOH (7.42)

p-OOCH

.COOH → p-C

(COOH)

(7.43)

At first, the p-toluic acid was esterified with methanol in a separate step before

final oxidation to monomethyl terephthalic acid. Alternatively, if methanol was

used as a solvent for the p-xylene, both methyl groups were esterified in a single

step, if the residence time was longer than 20 h. The reaction mechanism in-

volved the conversion of the methyl group to a benzyl radical by reaction with

trivalent cobalt ions. The benzyl radical then reacts with oxygen and forms a

peroxy species that decomposes to the oxidation product. Trivalent cobalt ions

are regenerated as aldehydes from the peroxy or hydroperoxy species.

The purification of terephthalic acid is complicated because it does not melt

and, as it was not soluble in either water or other solvents, it could not be crys-

tallized. On the other hand, the dimethyl ester of terephthalic acid could be easi-

ly crystallized from methanol or xylene. When the Mid Century Process was

introduced by Scientific Design and Amoco in 1956, it became possible to pro-

duce and purify terephthalic acid directly. This process used air oxidation condi-

tions similar to those for previous processes, with a mixed trivalent cobalt and

manganese acetate catalyst in glacial acetic acid, but introduced an ammonium

bromide cocatalyst in conjunction with tetrabromomethane. Cobalt or molyb-

denum bromides or hydrobromic acid have also been used, and following reac-

tion with the trivalent cobalt, provided a source of bromine free radicals. The

free radicals activated the methyl groups of the p-xylene and led to the for-

Petrochemical Catalysts 295

TABLE 7.8. Development of p-Xylene Oxidation Processes.

Process Catalyst Operating conditions

(a) p-xylene to p-toluic acid.

(b) p-toluic methyl ester to

monomethyl terephthalic

acid (Witton 1951)

Cobalt or manganese acetates

promoted by p-toluene sulfonic

acid.

140

–170

C, 4–8 atm

250

–280

C, 20–25 atm

(p-xylene selectivity 85%)

p-xylene in methanol solvent

to dimethyl terephthalate

Cobalt salts 100

–200

C, 5–20 atm

(p-xylene selectivity 90%)

p-xylene to pure terephthalic

acid (Amoco/Scientific De-

sign, 1958)

Reaction: cobalt and manganese

acetates in glacial acetic acid/

cocatalyst NH

BR plus tetra-

bromomethane

Purification: palladium supported

on carbon

Reaction: 190

–205

C, 15–

30 atm in stirred auto-

claves (95% conversion;

p-xylene selectivity 90%)

Purification: 225

–275

C to

hydrogenate 4-carboxy

benzaldehyde to p-toluic

acid in water solution.

p-xylene oxidized (with sub-

stance that provides hydrop-

eroxides, e.g., paraldehyde)

to pure terephthalic acid

Cobalt acetate in acetic acid

Catalysts used represent about

0.1% of the TPA produced

100

–140

C, 30 atm

(selectivity >97% but by-

product acetic acid forms)

mation of aromatic aldehydes.

These were converted to the acids via the per-

oxy intermediates. A further novelty of the process was the purification of prod-

uct by dissolving it at high pressure in water at 225°–275°C, and then hydrogen-

ating any 4-carboxybenzaldehyde by-product to p-toluic acid with a palladi-

um/carbon catalyst before allowing the terephthalic acid to crystallize. The same

process was used to oxidize toluene to benzoic acid in the Snia-Viscosa capro-

lactam process. The Amoco process has been widely licensed and now produces

the bulk of the terephthalic acid used in polyester manufacture. Terephthalic

acid is preferable to dimethylterephthalate in polyester production because the

reaction rate is faster and a better yield is obtained. The use of methanol is also

avoided.

The polyester polymer is produced in two steps to avoid side reactions. The

acid is esterified in an excess of ethylene glycol at 100°–150°C and 10–70 bar

using a cobalt or zinc acetate catalyst. The monomer then condenses at 150°–

270°C as a liquid melt under vacuum using an antimony oxide catalyst. Ethylene

oxide is removed by distillation as the polymerization proceeds. The use of an-

timony oxide catalyst always led to a slightly grey-colored product, as a result of

reduction of some of the antimony to the metallic state. Pure white polymer

could be obtained by the use of germanium dioxide catalyst, but the problem

was the very high cost of germanium relative to antimony. This problem was

solved in ICI by W Hewertson with the addition of phosphine oxide ligands to

the catalyst, which stabilized the antimony towards reduction. However, as is

often the case in production, the more stable catalyst could be operated at a

296 Chapter 7

higher temperature than the conventional catalyst. This led to shorter polymeri-

zation times, and hence an increase in plant capacity, but at the expense of re-

turning to the slightly grey polymer! The operator then had a choice between

pure white polymer or an increase in total production.

A relatively low-molecular-weight polymer is produced and generally used

to make fibers and other products. The higher-molecular-weight polymer needed

for bottles or packaging is formed by further polymerization in the solid state.

7.3.3.3. Alternative Routes for Terephthalic Acid Production

Other syntheses for terephthalic acid have been developed, although none has

yet been used extensively. As with most chemical processes, the capital cost of

new equipment usually means that modifications to existing processes are more

easily adopted than completely different routes to the same product.

In the Henkel process, dipotassium phthalate can be isomerized to dipotas-

sium terephthalate at about 430°C and 5–20 bar using carbon dioxide with a

soluble organic zinc or cadmium salt. An alternative approach is the dispropor-

tionation of potassium benzoate to give dipotassium terephthalate and benzene

using zinc or cadmium benzoates at up to 50 bar pressure of carbon dioxide. In

both operations the potassium is recycled by reaction with either phthalic anhy-

dride or benzoic acid.

In a process developed by Lummus, p-xylene and ammonia are converted to

terephthalonitrile in a fluidized bed at 450°C. A supported vanadium pentoxide

catalyst is used and gives about 50% conversion. The vanadium pentoxide is

reduced during the reaction and can be reoxidized with air in a separate reactor.

The dinitrile is then hydrolyzed to terephthalic acid. The process has not been

used in polyester production, although a similar process used in Japan produces

diamines from terephthalonitrile or isophthalonitriles.

7.3.3.4. Use of Polyesters

Polyesters are widely used as fibers, although the market share of fibers fell

from more than 70% before 1995 to about 60% by 2000, as demand for polyes-

ter (PET) bottles increased. To cope with the increased demand for bottles since

1995 more isophthalic acid is being used as a copolymer with ethylene glycol.

At the same time, naphthalene dicarboxylate (NDC) has been used to produce a

new polyester known as polyethylene terenaphthalate (PEN).

The new polyes-

ter bottles, although originally very expensive, have lowered gas permeability

and are stronger and more heat resistant. Nevertheless, as prices fall, the demand

for all types of polyester will continue to rise.

Petrochemical Catalysts 297

7.4. HYDROFORMYLATION AND CARBONYLATION

Otto Roelen began studies on the OXO reaction in 1938, when he was working

on the Fischer–Tropsch reaction with Ruhrchemie AG in Germany.

He discov-

ered the presence of oxygenated organic compounds in the Fischer–Tropsch

paraffin waxes and concluded that they were the result of carbonylation and hy-

drogenation of the unsaturated hydrocarbons present in the feed gases:

This idea was confirmed by studies on the reaction of ethylene with synthe-

sis gas, (H

+ CO), at 150°–200°C and 150 bar using the standard Fischer–

Tropsch cobalt catalyst. Reaction products included propanaldehyde and isopro-

panaldehyde. In 1945 Ruhrchemie opened a plant with a capacity of 10,000

tonnes/year at Holten, to produce C

–C

alcohols from Fischer–Tropsch ole-

fins.

These were subsequently converted to detergent sulfates.

Other companies, including ICI in the United Kingdom, Shell in Holland,

and the Enjay Chemical Company in the United States investigated the process

from 1947 onwards and full-scale manufacture soon followed, often in redun-

dant high-pressure coal hydrogenation equipment.

By 1963 this had led to a

worldwide production of more than 500,000 tonnes/year, mainly as butanols and

–C

alcohols. The process, also known as hydroformylation, gave a mixture

of normal and isoalcohols, after hydrogenation of the initial carbonylation alde-

hydes. Some alcohols were formed during hydroformylation together with small

amounts of paraffins and heavy oils. In the early plant operated by Roelen, the

ratio of normal to isopropionaldehyde was about 2:1.

7.4.1. Cobalt Carbonyl Catalysts

During early experiments, Roelen used the standard Fischer–Tropsch catalyst

(30CoO/66SiO

/2ThO

/2MgO) in both stages of alcohol production. He soon

realized, however, that in the presence of carbon monoxide, cobalt octacarbonyl

was formed and reacted reversibly with hydrogen to give a homogeneous cata-

lyst. The reaction then proceeded in solution. For this reason, all further devel-

opments used a convenient and cheap soluble cobalt salt, such as cobalt naph-

thenate. Cobalt naphthenate was soluble in the organic reaction medium and was

available commercially.

The partial pressures of hydrogen and carbon monoxide are critical in main-

taining the concentration of the active species, cobalt hydridotetracarbonyl and

RCH=CH

+ H

+ CO RCH

(CH

)

CHO (7.44)

RCH(CHO)CH