Jackson S.D., Hargreaves J.S.J. Metal Oxide Catalysis

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Catalytic Ammoxidation of Hydrocarbons on Mixed Oxides

Fabrizio Cavani , Gabriele Centi , and Philippe Marion

771

Metal Oxide Catalysis. Edited by S. David Jackson and Justin S. J. Hargreaves

ISBN: 978-3-527-31815-5

20.1

Introduction

Ammoxidation, sometimes also termed oxidative ammonolysis, describes the

process of catalytic oxidation of hydrocarbons (particularly alkenes, alkanes, alkyl -

aromatics and alkyl - pyridines) to organic nitriles in the presence of ammonia,

typically using mixed oxide catalysts:

RCH O NH RCN HO−−++→ +

323 2

15 3.

(20.1)

The most important example of this process is the synthesis of acrylonitrile from

propene. Acrylonitrile is a large - volume commodity chemical (within the top

twenty). The world ’ s production capacity for acrylonitrile was 6.14 million tons per

annum in 2005 and the output reached 5.24 million tons, an increase of 0.5% over

2004. The operating rate of production units was more than 85%, lower than the

operating rate of 90% in 2004. The drop in utilization of capacity has been higher

than the growth rate of the new capacity in recent years, and is essentially related

to the market situation and relatively old plant technology. There is a supply deﬁ cit

of acrylonitrile in the world today, but proﬁ tability in the acrylonitrile sector is still

low. The mean spread, that is the difference between the acrylonitrile price and the

raw material cost, between 2002 and 2006 was about $ 150/metric ton. Figure 20.1

reports the world acrylonitrile demand (by use) over the 1995 – 2006 period and

projection up to 2010 [1, 2] . Figure 20.1 also shows that acrylonitrile is a chemical

intermediate used mainly in acrylic ﬁ bres, ABS ( acrylonitrile – butadiene – styrene ),

SAN ( styrene – acrylonitrile ) and NBR ( nitrile – butadiene – rubber ).

There is a mean annual increase in world demand of about 3%, driven mainly

by ABS/SAN resin and other applications. Acrylonitrile is also used to produce

adiponitrile (for manufacture of hexamethylenediamine used in Nylon - 6,6 ﬁ bers

and resins) and acrylamide for water - treatment polymers. Approximately 52% of

the total EU production of acrylonitrile is used in production of ﬁ bres, 15% in

production of ABS and SAN resins, 15% in the production of acrylamide and

adiponitrile, and 18% for other uses [2] .

772 20 Catalytic Ammoxidation of Hydrocarbons on Mixed Oxides

The global market for acrylonitrile is currently quite balanced. After a period of

oversupply in 2001, brought about by the start - up of two large production units,

many closures in Western countries offset capacity growth and brought the market

into better balance. In general, a shift of capacity from Western to Asian countries

has occurred between 2004 and 2008. Figure 20.2 shows the world acrylonitrile

Figure 20.1 World acrylonitrile demand by derivative

(data estimated from 2006 to year 2010). Data from PCI

Acrylonitrile Ltd [1] .

Figure 20.2 World acrylonitrile trade ﬂ ow by year 2005 and major

manufacturers ’ capacity. Data from PCI Acrylonitrile Ltd [1] .

trade ﬂ ow in 2005 [1] with an indication also of the major manufacturers ’ capacity.

North America is still the largest acrylonitrile exporter and Asia is the major import

area. However, recent and future expansion of production in Asia will result in a

shrinking export market for North America. Costs of acrylonitrile production are

highly dependent upon propene and ammonia prices. Since 2006, prices for acry-

lonitrile have increased substantially (by more than one - third), mainly because of

rising feedstock costs. For this reason, and also because of weak demand, acrylo-

nitrile margins remain poor.

The cost of propene is mainly driven from competing areas of application (Table

20.1 ). Acrylonitrile accounts for only about 9% of propene demand and, because

of the expansion of the market for other applications, a further increase of propene

cost is expected. Global propene demand grew from 16.4 million tons in 1980 to

around 30 million tons in 1990 and about 52 million tons in 2000, with thus an

average annual growth of about 6% per year, which is much higher than the

average annual growth of demand for acrylonitrile. Expected demand for propene

by 2010 will be 81 million tons with demand growing faster than supply. Propene

supply/demand conditions and pricing are strongly dependent on reﬁ nery produc-

tion and the supply/demand balance, operating rates and feedstock in the ethane

industry. Globally, more than 25% of the new crackers currently planned are based

on ethane, and therefore will produce little propene. Propene production by dehy-

drogenation of propane, although increasing, is still an expensive route. Therefore,

higher prices for propene are forecast in the next decade.

Owing to the increasing cost of propene, interest in using alternative cheaper

raw materials, particularly propane, is expanding rapidly. Asahi Kasei Chemicals

Corporation ( AKC ) has begun commercial operation of the world ’ s ﬁ rst propane

process for acrylonitrile production, with the start - up and validation of a propane -

process line at the Ulsan Plant of its Korean subsidiary Tongsuh Petrochemical

Corporation. An existing 70 000 ton/year acrylonitrile line was modiﬁ ed to use the

propane process. Propene, however, will still remain the main raw material to

manufacture acrylonitrile for several years. The most used process is Innovene ’ s

(Ineos) acrylonitrile technology, known in the industry as the SOHIO acrylonitrile

Table 20.1 Propene demand by application (year 2005).

Application Use (thousand metric tons) %

Polypropene 39 289 60

Acrylonitrile 5 684 9

Propene oxide 4 780 7

Cumene 3 629 6

2 - Ethylhexanol 2 424 4

Butanols 2 261 4

Isopropanol 1 350 2

Oligomers 1 327 2

Others 3 566 6

20.1 Introduction 773

774 20 Catalytic Ammoxidation of Hydrocarbons on Mixed Oxides

process, which is used in the manufacture of over 90% of the world ’ s acrylonitrile.

In 1996, this process, which involves the oxidation of propene and ammonia to

acrylonitrile in a ﬂ uid - bed reactor, was designated the eleventh National Historic

Chemical Landmark by the American Chemical Society.

The SOHIO process involves the catalytic oxidative reaction of propene with

ammonia in the vapor phase [3] . Approximately stoichiometric amounts of propene

and ammonia, combined with air, are passed through the reactor in a single - pass

operation with a residence time of just a few seconds.

H C CHCH O NH H C CHCN H O

kJmol

23232 2

32 3

515

==++→ +

°=−

−

∆H

(20.2)

The reaction is highly exothermic and the heat of reaction is generally used to

make high - pressure steam, utilized downstream in separation and puriﬁ cation

operations. The main useful by - products from the process are HCN (about 0.1 kg

per kg of acrylonitrile), which is used primarily in the manufacture of methyl

methacrylate, and acetonitrile (about 0.03 kg per kg of acrylonitrile), a common

industrial solvent. Smaller quantities of carbon oxides and nitrogen (from ammonia

combustion) are also obtained. Unreacted ammonia in the reactor efﬂ uent is

neutralized with sulfuric acid. The resulting ammonium sulfate can be recovered

for use as a fertilizer.

Since the reactions generating by - products (carbon oxide formation and

ammonia combustion) are themselves highly exothermic, the total exothermicity

of the reaction is around 530 – 660 kJ mol

− 1

, making control of the reaction tempera-

ture critical.

Prior to 1960, acrylonitrile was produced commercially by processes based on

either ethylene oxide and hydrogen cyanide or acetylene and hydrogen cyanide. In

the late 1950s, Standard Oil (later Sohio and then BP) [4] and Distillers [5] devel-

oped a heterogeneous vapor - phase catalytic process for acrylonitrile by selective

oxidation of propene and ammonia using a catalyst based on bismuth, tin and

antimony salts of molybdic and phosphomolybdic acids and bismuth phospho-

tungstate. The Bi

PMo

/SiO

system was ﬁ rst reported in 1955 for propene

oxidation. The system works either in a cyclic oxidant process mode, or as a

genuine redox catalyst. The ﬁ rst attempt to use it in a cyclic oxidant mode was

abandoned after demonstration pilot operations, because about 200 kg of the solid

catalyst had to be circulated to produce 1 kg of acrolein [6] . However, the same

system operates successfully as a redox catalyst and was ﬁ rst commercialized for

the production of acrolein from propene in 1957 (Degussa licensee), and subse-

quently commercialized internally in 1959 for the production of acrylonitrile from

propene (licensed worldwide).

Around the same time, but slightly later, Edison (later Montedison) was also

developing a similar process, but based on different catalysts (tellurium -

cerium oxides) [7] . Although at that time the performance of these catalysts were

equal or slightly superior to bismuth molybdenum oxides, the process was never

commercialized.

As a consequence of the introduction of the new catalytic ammoxidation process,

a steep rise in the production of acrylonitrile occurred starting from the year 1960

[6] . Further step changes were observed as a consequence of the introduction of

each new generation of more effective catalysts (the acrylonitrile yield has been

increased over the past 40 years from 50 to over 80%). The substitution of the inef-

ﬁ cient and expensive process of IG Farben (HCN + acetylene) to produce acryloni-

trile by the highly efﬁ cient and environmentally friendly SOHIO processs

(propene + ammonia + air) can be considered as one of the ﬁ rst examples of ‘ green ’

sustainable chemistry. At the time of writing (2008), over 90% of the worldwide

production of acrylonitrile is made using the Sohio ammoxidation process [8] .

20.2

Propene Ammoxidation to Acrylonitrile

Commercial catalysts are made of multicomponent Bi - Fe - Ni - Co molybdates,

also containing several additives: Cr, Mg, Rb, K, Cs, P, B, Ce, Sb and Mn are

those more frequently mentioned in the several patents issued. The mixed

oxide is dispersed in silica (50 wt%) for ﬂ uid - bed reactor applications. Owing

to the high exothermicity of the reaction and by - product formation, most of

the plants use a ﬂ uidized - bed technology. A typical empirical composition is

(K,Cs)

0.1

(Ni,Mg,Mn)

7.5

(Fe,Cr)

2.3

0.5

/SiO

[9, 10] . The role of the various ele-

ments in the multi - component catalyst, in relation to the multi - step reaction

mechanism, are discussed in detail by Grasselli [10] . Production improvements

since 1980 have stemmed largely from the development of several generations of

increasingly more efﬁ cient catalysts. In addition to mixed metal oxide catalysts

based on multimetal molybdates ( MMM ), other types of commercially used cata-

lysts were based on iron antimony oxide, uranium antimony oxide (in the past),

and tellurium molybdenum oxide. MMM - based catalysts are the most commonly

used nowadays.

Even though the literature on this topic has been mainly focussed on the struc-

tural and chemical - physical properties of Bi molybdates, and on the reactivity of

its various polymorphs (the α , β and γ structures), the industrial catalyst consists

of several divalent and trivalent metal molybdates. Indeed, Bi is present in minor

amounts in catalyst formulations. The two classes of molybdate contribute differ-

ently to catalytic performance: (1) trivalent Bi/Fe/Cr molybdates, having the Schee-

lite - type structure, contain the catalytically active elements while (2) divalent

Ni/Co/Fe/Mg molybdates, having the Wolframite - type structure, mainly enhance

the catalyst re - oxidation rate.

The catalyst also contains excess Mo with respect to the stoichiometric require-

ment for the formation of the molybdates. The excess Mo is considered important

for catalyst performance because:

• it provides a molecular bridge between the co - operating molybdates if their

crystalline match is not perfect;

20.2 Propene Ammoxidation to Acrylonitrile 775

776 20 Catalytic Ammoxidation of Hydrocarbons on Mixed Oxides

• it provides Mo to those phases partly depleted of it because of the redox

cycle, especially under more reducing conditions (Mo is lost in the form of

volatile MoO(OH)

). For instance, owing to the reduction of Fe

to Fe

, part of

the Mo is excluded from the molybdate, and is ﬁ nally lost in volatile form.

Excess MoO

, when present in the catalyst composition from the beginning,

or when added during reaction, migrates along hydrated silica surfaces towards

Mo - lean catalytically active phases. Often, Mo - enriched make - up MMM catalyst

is preferentially added in place of MoO

during reactor operation. Fundamental

understanding of these complex catalysts and the surface - reaction mechanism

of propene ammoxidation has contributed substantially to the development of

new catalyst generations (currently at the fourth generation). Detailed mecha-

nisms for selective ammoxidation of propene over bismuth molybdate and anti-

monate catalysts have been proposed [11] . The rate - determining step is abstraction

of an α - hydrogen of propene by an oxygen in the catalyst to form a π - allyl

complex on the surface [11, 12] . Lattice oxygens from the catalyst participate in

further hydrogen abstraction, followed by oxygen insertion to produce acrolein

in the absence of ammonia, or nitrogen insertion to form acrylonitrile when

ammonia is present [13] . The oxygen removed from the catalyst in these steps

is replenished by gas - phase oxygen, which is incorporated into the catalyst struc-

ture at a surface site separate from the site of propene reaction. In the ammoxi-

dation reaction, ammonia is activated by an exchange with oxygen ions to form

isoelectronic NH

2 −

moieties, which are inserted into the allyl intermediate to

produce acrylonitrile.

The active site on the surface of a selective propene ammoxidation catalyst con-

tains three critical functionalities associated with the speciﬁ c metal components

of the catalyst [14] : an α - H abstraction component such as Bi

, Sb

or Te

; an

oleﬁ n chemisorption and oxygen or nitrogen insertion component such as Mo

or Sb

; and a redox couple, such as Fe

/Fe

or Ce

/Ce

, to enhance transfer of

lattice oxygen between the bulk and surface of the catalyst. Moreover, in general,

it may be considered that the large improvement in the selectivity of these catalysts

derives from the application of seven principles ( ‘ seven pillars ’ ) [6] : lattice oxygen,

metal – oxygen bond strength, host structure, redox activity, multi - functionality of

active sites, site isolation and phase co - operation.

The process schematic of propene ammoxidation is shown in Figure 20.3 [15,

16] . A single - pass conﬁ guration is possible, because over 95 wt% conversion can

occur with selectivities to acrylonitrile which nowadays are well above 80%. Air,

ammonia and propene are sent to a ﬂ uidized - bed reactor, which may contain up

to 70 – 80 tons of catalyst in the form of ﬁ ne spherical particles ( < 40 µ m in diameter)

highly resistant to mechanical attrition. The purity of reactants is very high ( > 90%

for propene and > 99.5% for ammonia). The ammonia to propene molar ratio is

in the range 1.05 to 1.2 and the O

/propene ratio in the range 1.9 – 2.1; typically,

oxygen - enriched air is used in industrial operation. Reaction temperature is in the

420 – 450 ° C interval, residence time is between 3 and 8 s, with a linear gas velocity

from 0.2 and 0.5 m s

− 1

and pressure between 1.5 and 3 atm. Since the rate of acry-

lonitrile synthesis is ﬁ rst order with respect to propene, with higher orders being

shown towards the by - products, an increase in pressure has a negative effect on

selectivity. However, positive pressure is necessary to maintain the correct ﬂ uidiza-

tion characteristics in the reactor.

The feed enters the reactor through separate inlets, in order to minimize homo-

geneous reactions and prevent local ﬂ ammable compositions developing, although

owing to the effect of the solid in inhibiting radical propagation within the

ﬂ uidized - bed reactor, it is possible to work inside the ﬂ ammability region in

the catalytic bed. The ﬂ uidized - bed reactor contains many steam coils to keep the

reaction temperature constant, avoid bubble coalescence and reduce gas back -

mixing. By proper design of these coils a virtually plug - ﬂ ow regime may be possi-

ble. An expansion area to homogenize the gas velocity proﬁ les, and thus minimize

the entrapment of solid particles, and cyclones, to recover the ﬁ nest particles, are

integrated into the ﬂ uidized - bed process.

The hot reactor efﬂ uent is sent to a water absorber where it is quenched counter -

currently, while unreacted ammonia is neutralized with sulfuric acid. The result-

ing ammonium sulfate can be recovered and used as a fertilizer. The off - gases

containing N

, carbon oxides and unreacted hydrocarbon are sent to incineration.

The solution of acetonitrile/acrylonitrile is a heteroazeotrope. After settling, an

aqueous and an organic phase are obtained. The ﬁ rst is reﬂ uxed, while the latter,

rich in acrylonitrile and HCN, is sent to the puriﬁ cation step. The aqueous aceto-

Figure 20.3 Schematic ﬂ ow sheet of the SOHIO process of

propene ammoxidation to acrylonitrile. Adapted from [11] .

20.2 Propene Ammoxidation to Acrylonitrile 777

778 20 Catalytic Ammoxidation of Hydrocarbons on Mixed Oxides

nitrile recovered at the bottom is further concentrated by azeotropic distillation.

The crude acrylonitrile is ﬁ rst puriﬁ ed in two in - series columns to separate HCN

and impurities (acetone, acetaldehyde, propionaldehyde, acrolein) and ﬁ nally

further puriﬁ ed under vacuum. Final polymer grade must have a purity higher

than 99.4%. Disposal of the process impurities includes deep - well disposal (not

sustainable), wet air oxidation, ammonium sulfate separation, biological treatment

and incineration [17] .

20.3

Propane Ammoxidation to Acrylonitrile

In the current process technology for the manufacture of acrylonitrile, the propene

feedstock cost represents about 67% of the full (or ﬁ xed) cost of production [18] .

The price differential between propene and propane depends on many factors, but

can be estimated to be on average $ 360 per ton during 2007 [19] , compared with

around $ 320 per ton in 2004 [18] . This price differential makes propane ammoxida-

tion competitive using the currently available catalysts. In fact some plants have

already started to be revamped. For example, at Asahi Kasei Corporation an exist-

ing 70 000 ton/year acrylonitrile line was modiﬁ ed to use the propane process.

Production began in January 2007 [20a] ).

The reaction conditions claimed by the various companies are substantially dif-

ferent. As shown in Figure 20.4 , sometimes propane - rich conditions have been

claimed, as in earlier patents from Standard Oil, while in other cases propane - lean

conditions are preferred. In the former case, the conversion of propane is low, and

therefore recycling of the unconverted parafﬁ n becomes necessary. Mitsubishi was

the ﬁ rst to claim the use of hydrocarbon - lean conditions, that is conditions in

which very high propane conversions can be reached. In more recent patents, BP

has claimed the use of analogous conditions, using oxygen - rich feed, and propane

as the limiting reactant. However, the lower activity of antimonates makes it neces-

sary to use temperatures which are approximately 50 ° C higher than those employed

with the Mitsubishi catalyst.

Figure 20.4 Feed composition in

propane ammoxidation claimed by

different companies.

A once - through process is the preferred option [18a] (Figure 20.5 ). The catalyst

and process conditions are tailored to achieve as near total feedstock conversion

as possible. Products are separated and fresh feedstock is introduced into the

reactor on a continuous basis. In addition to providing the lowest capital cost for

a new - build plant, this also provides the best possibility for retroﬁ tting existing

plants by simply replacing catalyst and feedstock. This option is preferable to the

combination of an alkane dehydrogenation unit with an existing propene - to -

acrylonitrile process, owing to the signiﬁ cant capital cost involved. The signiﬁ cant

improvement in selectivity at high conversion using V/Mo/Nb/Te oxide catalysts,

compared to the earlier generation of catalysts based on V - Sb oxide, also makes

the once - through option preferable to the recycle option (Figure 20.5 ) [18a] .

Although selectivity is higher when operating at lower propane conversion, addi-

tional equipment is required for recovery and recycling of the unreacted feedstock.

Nevertheless, if selectivities to acrylonitrile could exceed 80% at a propane conver-

sion lower than about 30% using new generation catalysts, the recycle process

could become an interesting option. The recycling of unconverted propane is also

an option when high alkane conversion is reached, because this allows not only

the complete recovery of propane but also propene recycling (one side - product of

the reaction, which is a precursor of acrylonitrile), along with carbon dioxide,

which may act as a ballast for the reaction. The Mitsubishi process makes use of

the BOC - PSA technology for the removal of N

(both present in the feed and gen-

erated in the reactor by ammonia combustion), while the purge stream is inciner-

ated [20b] . One Mitsubishi patent claims the staged feeding of ammonia along the

catalytic bed [20c] ; this is a necessary option if the catalyst is very active in ammonia

Figure 20.5 Once - through and recycle processes for propane to

acrylonitrile (ACH: acetone cyanohydrins). Adapted from [18a] .

20.3 Propane Ammoxidation to Acrylonitrile 779

780 20 Catalytic Ammoxidation of Hydrocarbons on Mixed Oxides

combustion to N

, in order to avoid a lack of ammonia in the ﬁ nal part of the

reactor, which would favor combustion of the hydrocarbons and the formation of

propene.

More complex process concepts have also been proposed which attempt to

optimize the processes. One option is the use of multiple reaction steps operating

at different reaction conditions [18a] . Three sequential process steps are used, each

operated with increasing conversion of propane. Product is separated after each

step and the unreacted propane is sent to the next reactor with fresh ammonia

and air/oxygen. An overall propane conversion of 95% with overall acrylonitrile

yield of 63% is achieved in three steps. Another approach is to increase the process

ﬂ exibility in synthesizing multiple products. By changing the ratio of propane/

ammonia in the feed to the reactor, acrylonitrile and acrylic acid could be simul-

taneously produced with variable yields, depending on NH

ratio (Figure

20.6 ). Even if the complexity of product puriﬁ cation and recovery sections increases

the process costs, this option provides the opportunity to optimize the operation

of the plant based on changes in feedstock and product price and demand.

A number of reviews have discussed the catalyst and catalytic reaction chemistry

in propane ammoxidation [21 – 25] . So far, two main catalytic systems have been

proposed in the literature. They are based on V - antimonates with rutile structure

or multi - component molybdates (Mo/V/Nb/Te/O). Among the antimonates are

compositions of the Al/Sb/V/W/O system which give the highest acrylonitrile

yields reported (about 39%) [26] . The most promising catalyst discovered thus far

is the Mo/V/Nb/Te/O system, which has been shown to be both active and selec-

tive, giving acrylonitrile yields up to 62% [6b, 27] . Mitsubishi Kasei [27] originally

developed this catalyst composition which gives the highest yield of acrylonitrile

to date, although the long - term stability is still unclear. The optimal catalyst com-

position is MoV

0.3

0.23

, which gives a yield of acrylonitrile of about 50%;

this can be further increased to nearly 60% by the addition of Sb, B or Ce. Key

characteristics of the catalyst are not only its composition, but also the procedure

Figure 20.6 Multiple product option with propane feedstock

(US Patent 6,166,241 (2000)). Adapted from [18a] .