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

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of synthesis and activation and the modality of doping. These catalysts are pre-

pared by hydrothermal synthesis, which results in nucleation and growth of M1

and M2 phases with well deﬁ ned crystal morphologies.

Several articles on synthesis and characterization of Mo/V/M/O (M = Te, Sb,

Al) catalysts have been published recently [28] . The effects of metal oxide precursor

sources, synthesis conditions and post - synthesis treatments are subjects of current

studies. Asahi [29] has modiﬁ ed the composition of the Mitsubishi catalyst, by

incorporation of Sb in place of Te in the M1 phase; this catalyst is more stable

than the original one, and hence provides longer catalyst lifetime.

Standard Oil (later BP America, now Ineos) developed V

–

Sb - based catalysts

which show a high selectivity to acrylonitrile especially when using high propane

concentration [30, 31] . Single and dual catalyst compositions were patented, with

the second catalyst having the function of converting the propene intermediate to

acrylonitrile [30] . Claimed catalyst compositions are mixed oxides such as (i)

, where M is Mg, Ti, Sb, Fe, V, W, Cu, La, P, Ce or Nb; (ii) VSb

where M is one or more of Sn, Ti, Fe, Mn or Ga; (iii) Bi

, where A is

one or more alkali or alkaline metals, boron, W, Sn or La and B is one or more of

the elements Cr, Sb, Pb, P, Cu, Ni, Co, Mn or Mg; and (iv) Bi

FeMo

where D is one or more of the alkali metals; E is one or more of Mn, Cr, Cu, Zn,

Cd or La; F is one or more of P, As, Sb, F, Te, W, B, Sn, Pb or Se; and G is one

or more of Co, Ni or an alkaline earth metal. The gas feed composition was usually

propane/ammonia/oxygen/water in a 5/1/2/11 molar ratio, with excess alkane and

water used as a diluent.

A third catalytic system, based on vanadium aluminum oxynitrides ( VALON ),

has also been proposed [32] . Maximum acrylonitrile yield was about 30%, but with

an acrylonitrile productivity four times higher than V - Sb - W - Al - O catalysts and one

order of magnitude higher than Mo/V/Nb/Te/O catalysts [33] .

Other companies have studied and developed proprietary formulations, but in

general catalytic systems belong either to the antimonate family (Standard Oil,

Rhodia, BASF, Nitto, Monsanto) [34 – 38] or to the molybdate family (Mitsubishi,

Asahi).

All catalysts claimed are ‘ multi - functional ’ systems. Indeed, the formation of

acrylonitrile from propane occurs mainly via the intermediate formation of

propene, which is then transformed to acrylonitrile via the allylic intermediate. It

follows that the catalyst possesses different kinds of active site: one site that is able

to activate the parafﬁ n and oxidehydrogenates it to the oleﬁ n, and one site that

(amm)oxidizes the adsorbed oleﬁ nic intermediate. This second step must be very

rapid to limit, as much as possible, the desorption of the oleﬁ n. In order to develop

an effective cooperation between the two sites, it is necessary to have systems in

which they are in close proximity. The multi - functionality is achieved either

through the combination of two different compounds (phase - cooperation), or

through the presence of different elements inside a single crystalline structure. In

antimonate - based systems, the cooperation between the metal antimonate (having

the rutile crystalline structure), responsible for propane oxidative dehydrogenation

to propene and propene activation, and antimony oxide, active in allylic ammoxida-

tion, is made more efﬁ cient through the dispersion of the latter compound over

20.3 Propane Ammoxidation to Acrylonitrile 781

782 20 Catalytic Ammoxidation of Hydrocarbons on Mixed Oxides

the former one; in this case, an optimal ratio between the two compounds exists,

which makes the cooperation more efﬁ cient. In metal molybdates (the Mitsubishi

catalyst), one single crystalline structure contains the elements required for the

activation and oxidative dehydrogenation of the hydrocarbon (vanadium), and

those active for the transformation of the oleﬁ n and the allylic insertion of the

2 −

species (molybdenum and either tellurium or antimony). Niobium has the

role of improving the structural stability of the compound.

The selectivity to acrylonitrile versus propane conversion is plotted in Figure

20.7 for the best performances reported in patents issued by different companies,

and the temperature of reaction is also reported. In general, best per - pass yields

obtained with propane - rich conditions are between 20 and 25%. The yield to acry-

lonitrile reported is close to 60% at 87 – 89% propane conversion, with a selectivity

of 60 – 64% at 410 ° C, under propane - lean conditions, reported in Mitsubishi

patents with a catalyst made of mixed molybdate.

20.3.1

Mo/V/Te/Sb/(Nb)/O Catalysts

The elements constituting this catalyst are the basis for several crystalline struc-

tures, as illustrated in Figure 20.8 , which summarizes the main mixed oxides that

can be obtained by combination of V, Mo, Nb and Te(Sb). The ﬁ gure shows the

various bi - component systems in a triangular diagram of composition, as well as

the areas which include tri - and multi - component systems, exhibiting superior

performance in ethane oxidation, and in propane oxidation and ammoxidation.

Many of the reported structures are related; for example orthorhombic

5 − x

(V,Nb)

solid solutions are isostructural with θ - Mo

and with several

bi - component Sb/Mo, Nb/Mo and Te/Mo mixed oxides. The M1 phase is

Figure 20.7 A summary of best performances in propane

ammoxidation claimed by different companies.

Figure 20.8 Top: Triangular diagram of composition for the

Mo/V/Nb(+Te or Sb)/O system, indicating the stoichiometry

of compounds which form, and the area of existence for

systems active and selective in alkanes oxidation and

ammoxidation. Bottom: detail of the top Figure.

20.3 Propane Ammoxidation to Acrylonitrile 783

784 20 Catalytic Ammoxidation of Hydrocarbons on Mixed Oxides

structurally derived from the latter oxides, and the M2 phase is related to other

Mo - leaner Te/Mo and Sb/Mo mixed oxides.

Two main preparation methods have been used to synthesize Mo/V/Te/(Nb)/O

catalysts active in propane ammoxidation: (a) the dry - up and (b) the hydrothermal

synthesis. The dry - up method involves mixing aqueous slurries of metal oxide

precursors followed by a gradual evaporation of the combined aqueous slurry.

Solvent evaporation leads to nucleation and growth of precursor metal oxide

phases, which require further heat treatment to obtain active catalysts.

For the synthesis of Mo/V/Te/Nb/O catalysts, ammonium paramolybdate,

vanadyl sulfate, ammonium niobium oxalate and tellurium oxide or telluric acid

are mixed as aqueous solutions or slurries and dried at about 150 ° C. These mixed

slurries are then calcined at 500 – 650 ° C in a N

stream [39] . In addition to the

dominant M1 and M2 phases, impurity phases, for example MoO

, Mo

and

, are also observed [28c] . It should be noted that Mo - V - Te - O catalysts

cannot be obtained by the dry - up method.

The preferable method for catalyst preparation is the application of hydrother-

mal conditions [28a, 40] , because they allow the syntheses of single - phase complex

metal oxides, such as binary Mo/V/O, ternary Mo/V/Te/O and quaternary Mo/V/

Te/Nb/O, which have various crystal structures [40g, 40h] and performance [41] .

1.0

0.44

0.1

has an orthorhombic structure, Mo

1.0

0.81

0.64

a hexagonal

structure and Mo

1.0

0.25

a tetragonal structure. All these basically have the same

layered structure, in which networks of corner - shared MO

(M = Mo, V) octahedra

form slabs and the octahedra between the slabs also share corner oxygen forming

linear inﬁ nite chains of octahedra along the c - direction [41] . The structures differ

in the arrangement of octahedra within the slabs. In the orthorhombic structure,

the MO

(M = Mo, V) network in the slab is constructed with pentagonal and hex-

agonal rings of octahedra. The pentagonal bipyramidal sites may be occupied by

Mo and V. Te, as the third constituent element, is exclusively located in hexagonal

channels, whereas the heptagonal channels remain empty. The other structures

do not possess the heptagonal rings and showed much worse performance, indi-

cating that catalysts with the particular arrangement of MO

(M = Mo, V) octahedra

forming slabs with pentagonal, hexagonal and heptagonal rings in the (001) plane

of an orthorhombic structure is the active and selective phase in propane and

ethane conversion [41] . Mo and V are indispensable elements for the structure

formation. Te, located in the central position of the hexagonal ring, promoted the

conversion of intermediate propene. Introduced Nb occupied the same structural

position as V and the resulting catalyst clearly showed improved selectively.

Mo/V/Te/(Nb)/O catalysts owe their activity and selectivity towards propane

conversion to the essential presence of so - called M1 (orthorhombic) and M2

(pseudo - hexagonal) phases [42] . The M1 phase alone is capable of selective trans-

formation of propane [6b] , and the presence of the M2 phase is claimed to improve

the selectivity under more demanding conditions, such as high conversion [43] .

Signiﬁ cantly improved acrylonitrile yields from propane above those obtained with

pure M1 phase were observed using as catalyst a physical mixture with a composi-

tion of 50 wt% M1/50 wt% M2 and a surface area ratio of about 4/1. The two phases

must be thoroughly ground and brought into intimate contact with each other on

a micro - /nano - scale for synergy to occur.

The orthorhombic and hexagonal structures of the M1 and M2 phases was ﬁ rst

recognized by workers at CNRS and Elf Atochem [44] using electron microscopy.

They were even able to propose schematic structures for these materials, but these

models do not indicate which cation is found at a particular site and thus provide

no knowledge of cation co - ordination or valence state.

The M1 and M2 compounds are not simple phases. They have large unit cells

and do not form single crystals. Using combinatorial synthesis, it was possible to

ﬁ nd ways to prepare the M1 and M2 phases separately [45] and subsequently

perform structural analysis combining neutron and X - ray powder diffraction data.

The result for the M1 phase shows that the material has a layer structure with two

sets of channels, one approximately hexagonal, the other roughly heptagonal. The

channels allow the accommodation of large metal cations. This is where the large

cation sits. The large channel provides access to the cation ’ s lone pair electrons,

which is likely quite important in the ammoxidation reaction (Figure 20.9 ).

The MoVNbTeO

system for propane ammoxidation comprises three crystalline

phases: orthorhombic Mo

7.8

1.2

NbTe

0.94

28.9

( M1 ) ( Pba 2: a = 21.1337 Å ; b = 26.6440 Å ;

c = 4.01415 Å ; z = 4), pseudo - hexagonal Mo

4.67

1.33

1.82

19.82

( M2 ) ( Pmm 2:

a = 12.6294 Å ; b = 7.29156 Å ; c = 4.02010 Å ; z = 4) and a trace of monoclinic

TeMo

( P 21/ C : a = 10.0349 Å ; b = 14.430 Å ; c = 8.1599 Å ; β = 90.781 ° ; z = 1)

[45c] . The catalytically active and selective centers reside on the surface of the basal

plane of the M1 phase and are composed of an assembly of ﬁ ve metal oxide

octahedra (

032

068

VMo

062

038

VMo

Mo Mo

) and two tellurium – oxygen

sites 2

094

()

, which are stabilized and structurally isolated from each other (site

isolation) by four Nb

centers, each surrounded by ﬁ ve molybdenum – oxygen

octahedra (Figure 20.8 b). The V

surface sites, distinguished through their (V

O ↔ V

–

O) resonance structure, are the parafﬁ n activating sites capable of methy-

lene - H abstraction; the Te

sites (lone pair of electrons) are required for the α - H

abstraction of the chemisorbed propene molecule, once formed; and the adjacent

sites for NH insertion into the π - allylic surface intermediate. Within this

structure are contained all key catalytic elements needed to transform propane

directly to acrylonitrile, strategically arranged and within bonding distance of each

other, generating the active center of the M1 phase [45c] .

Under mild operating conditions, the M1 phase alone is enough to effectively

convert propane directly to acrylonitrile. Under demanding conditions symbiosis

between the M1 and M2 phases occurs, with the latter serving as a co - catalyst or

mop - up phase to the former, transforming unconverted, desorbed propene to

acrylonitrile. The M2 phase is incapable of propane activation, lacking V

sites,

but is a good propene ammoxidation catalyst. A maximum acrylonitrile yield from

propane of 61.8% (86% conversion, 72% selectivity at 420 ° C) was achieved with a

nominal catalyst composition of Mo

0.6

0.187

0.14

0.085

, identiﬁ ed by a combina-

torial methodology, which is composed of 60% M1, 40% M2 and a trace of

TeMo

[45c] . The Te environment is consistent with that observed by Millet and

coworkers using EXAFS in mixed - phase samples [46] .

20.3 Propane Ammoxidation to Acrylonitrile 785

786 20 Catalytic Ammoxidation of Hydrocarbons on Mixed Oxides

20.3.2

Rutile - Type Antimonate Catalysts

Several papers have been published dealing with the study of metal antimonates

having the rutile structure. In particular, V/Sb/O and Fe/Sb/O systems have been

the subject of many investigations, aimed at understanding the nature of these

mixed oxides, and at the identiﬁ cation of the active species.

Indeed, the preparation of a truly stoichiometric metal antimonate, MSbO

(M = metal), is a difﬁ cult task. The method of preparation employed affects the

nature of the catalysts prepared, but in general non - stoichiometry is a particular

feature of these systems [47 – 50] . The most striking case is the V/Sb/O system, for

which the composition V

0.92

( quasi - VSbO

) has been reported for the cata-

lyst with the V/Sb atomic ratio equal to 1/1 [49] . This cation - deﬁ cient structure,

which has 0.04 cationic positions unoccupied per O

2 −

anion, contains Sb

, while

Figure 20.9 (a) Unit cell of the M1(orthorhombic) phase

along the c axis with the regions considered to be the active

centers in propane ammoxidation shown in rectangular

boxes. (b) Enlarged view of the proposed active center in M1

phase (Mo

7.5

1.5

NbTeO

) in [001] projection with indication

(numbers) of the different atoms summarized below in the

scheme of the active site. (c) Elaborated from [6a, 45] .

vanadium is present both as V

and as V

. Electroneutrality is preserved for the

composition VVSbO

028

064

092

4.. .

++ +

[51] .

However, the ratio between V

and V

can vary depending on the method of

preparation; of particular importance are the atmosphere and temperature of

thermal treatment. The preparation procedure will affect the number of cationic

vacancies (for V

tot

/Sb ratio equal to 1.0) within the V

1 − y

series, which is

theoretically comprised between the two limiting compounds VVSbO

++ +

(the

stoichiometric compound, with no cationic vacancies and only V

) and

VV Sb O

089

089 4

(with only V

, and the maximum of 0.22 vacant cation positions),

with y varying between 0 and 0.11 [47, 48, 52] . This series of catalysts is preferen-

tially formed by treatment in air. Therefore, the presence of cation vacancies is a

consequence of V having an oxidation state higher than 3+ which explains why

V/Sb/O systems give such large deviations from stoichiometry, at least under

conditions that are not the most stable from the thermodynamic point of view.

Another possibility is to have a ratio between Sb

and V different from 1.0.

The V

0.9+ y

0.9

non - stoichiometric series has been described, with y theoretically

ranging from 0 to 0.2, and correspondingly with a degree of vacant cation posi-

tions ranging from 0.2 (in V

0.9

, with VV

, where the ratio V

tot

/Sb is 1.0)

to 0 (V

1.1

0.9

, with full occupancy of the cation sites, and with vanadium as

) [53] . In practice, vacant positions in V

0.9

(0.2 per unit formula) are

progressively occupied by V ions, with a corresponding increase of the V/Sb ratio

and of the V

ratio. The latter is mainly affected by the atmosphere of thermal

treatment, and therefore this parameter ﬁ nally affects the extent of cation site

occupancy in the rutile structure.

Berry and coworkers [47] described a compound of composition VSb

1 − y

4 − 2 y

where y ≈ 0.1, which does not formally contain cation vacancies, and which is

obtained when the thermal treatment of the catalyst is carried out with oxygen -

impure nitrogen. In practice, it is assumed that the ratio between V

/Sb

is equal

to 1.0, and the solid solution is enriched with V

ions (and obviously with addi-

tional O

2 −

ions); which represents a solid solution between VSbO

and VO

(both

rutile compounds). Only in the presence of oxygen - free nitrogen is it possible to

obtain monophasic compounds belonging to the series VSb

1 − y

4 − 1.5 y

. For y = 0.1

the compound V

1.039

0.935

is formed, with a small degree of cation site unoccu-

pancy (0.026 vacancies per unit formula), a V/Sb ratio higher than 1.0 and again

the presence of V

In most cases described in the literature, the rutile V/Sb/O system has been

reported to possess either equimolar amounts of V and Sb, or an excess of V.

Moreover, Sb is also present exclusively as Sb

. Excess Sb, if preparations are

carried out with an Sb/V atomic ratio higher than 2 and calcination is performed

in air, forms either α - Sb

or β - Sb

(the latter at calcination temperatures

higher than 800 ° C). Incorporation of small amounts of V in these Sb oxides is

also likely, lowering the temperature of the α β transformation. Antimony oxide

is also present in the form of amorphous oxide dispersed over the rutile, as sug-

gested by IR spectroscopy, for Sb/V ratios between 1 and 2 [54] .

20.3 Propane Ammoxidation to Acrylonitrile 787

788 20 Catalytic Ammoxidation of Hydrocarbons on Mixed Oxides

The case of FeSbO

is different. It is known that the structure can host an excess

of Sb. Berry and coworkers have demonstrated that on increasing the Sb/Fe ratio,

an increase in cell volume of the tetragonal structure occurs [55] . Moreover, instead

of a random distribution of iron and antimony in the cation sites, cation ordering

has been found, with development of a tri - rutile - like structure. The change from

the mono - to the tri - rutile - like ordering, from the Fe

SbO

stoichiometric com-

pound to the Fe

(with an Sb/Fe atomic ratio equal to 2) limiting compound

occurs with the reduction of Fe

to Fe

. Therefore, it is possible that the forma-

tion of Sb - enriched solid solutions preferentially occurs with metal ions which can

exist in stable 3+/2+ oxidation states. Indeed, the formation of very small amounts

of crystalline (VO)

has sometimes been observed [54a] . Fe/Sb/O systems

also have been studied as catalysts for propane and propene ammoxidation [34b,

36, 56, 57] .

The degree of structural defects can be checked by means of IR spectroscopy.

Speciﬁ cally, IR bands at 880 and 1015 cm

− 1

are attributed to the Sb

–

Sb and

–

stretching vibrations involving co - ordinatively unsaturated O

2 −

ions

adjacent to cationic vacancies [58] . Cationic vacancies play an important role in the

catalytic performance of rutile - type mixed oxides [59 – 61] . For instance, it was

found that an increased concentration of cationic vacancies and isolated V

species

in V/Sb/(Fe)/O systems, due to the introduction of increasing amounts of Fe in

the lattice, led to a proportionally higher activity [60c] . The formation of V

Cr/V/Sb mixed oxides had the similar effect of increasing propane conversion

considerably [62] .

The IR band at around 820 – 840 cm

− 1

, also observed in all antimonates, is not

typical of M

–

O stretching vibrations in rutile single oxides, and is also not observed

in any antimony oxide [63] . This band has been attributed to the stretching vibra-

tion of Sb

–

[50, 64] , present on the surface of the rutile, and possibly

constituting an active site for the reaction. Indeed, this vibration could be due to

the presence of SbO

- rich domains in the outer zones of non - stoichiometric rutile

crystallites. Non - stoichiometric FeSbO

prepared by thermal treatment at 800 ° C

with excess antimony in the structure, is characterized by a tetragonal cell larger

than that of stoichiometric FeSbO

and by the presence of the IR band at 820 cm

− 1

Treatment of this compound at 1000 ° C leads to (i) a decrease in the cell volume,

which becomes similar to that of stoichiometric rutile and (ii) a strong decrease

in the intensity of the mentioned IR absorption band [65] . Therefore, it is possible

that the development to a well - crystallized, ordered rutile structure occurs as a

consequence of a cation redistribution in the lattice at high temperature, with the

disappearance of features related to Sb - enrichment.

The best performances among the variety of compositions which have been

claimed for V/Sb/O - based systems are summarized in Table 20.2 [66] . It can be

seen that Standard Oil (then BP and now Ineos) has claimed the same catalyst

type for both propane - lean conditions, with high conversion of the parafﬁ n, and

propane - rich conditions, with low propane conversion.

In this system, the main catalyst component is the quasi - VSbO

rutile, which

activates the parafﬁ n and transforms it into the oleﬁ n - like adsorbed intermediate.

The intermediate may either desorb to yield propene, or be transformed to acry-

lonitrile over the SbO

‘ overlayers ’ , the amount of which is a function of the excess

Sb with respect to the rutile formula [22b] .

In systems developed by Rhodia [34, 37] the main component is SnO

(cassiter-

ite), which is inactive in propane ammoxidation, while it acts as the carrier for the

V/Sb/O and SbO

active components. Tin oxide is also able to disperse these

components, through the dissolution of V and Sb ions, thus yielding a multi - func-

tional catalyst where the two active compounds can effectively co - operate in the

reaction.

Other antimonates have been studied as catalysts for this reaction [23, 67 – 70] .

In the case of the Ga/Sb/O system [23, 67] , a decrease in the Ga/Sb ratio (from

to Ga

124

) leads to a progressive decrease in activity and increase

in selectivity to acrylonitrile. The best yield has been obtained at 550 ° C with a cata-

lyst having composition of Ga

124

, with 28.3% propane conversion and 35.3%

selectivity to acrylonitrile. The performance was improved by adding Ni, P and W

as dopants. Recently, V

- doped Al

0.5

was found to be active in propane

ammoxidation, although its performance was not outstanding [71] .

Rutile - type Cr antimonate, CrSbO

, is fairly active and selective in propane

ammoxidation [72] and adding V considerably improves the catalytic activity. The

(Cr + V) / Sb atomic ratio is the main compositional parameter affecting the nature

of these crystalline compounds [59a, 62, 73] . When almost equimolar ratios were

used (e.g. in Cr/V/Sb 1/1/1), the corresponding XRD pattern showed the presence

of a single, well - crystallized, rutile - type three - component CrVSbO

compound,

which in practice corresponds to an equimolar solid solution between CrSbO

and

(both characterized by the rutile - type structure). In Cr/V/Sb/O samples with

atomic ratios of Cr/V/Sb 1/ x /1, the general formula Cr

4+2 x

was extrapo-

lated, with compositions ranging from CrSbO

to CrVSbO

. In these catalysts, V

was present mainly as V

, and samples were extremely active in propane ammo-

xidation but poorly selective to acrylonitrile, leading to the predominant formation

of propene and carbon oxides. When samples were instead prepared with an

atomic ratio (Cr + V) / Sb < 1, V was present mainly in the rutile mixed oxide as a

species. The main peculiarity of these Cr - based antimonates is their ability to

Table 20.2 Performance of some V / S b / O - based catalysts described in the literature.

Catalyst formula T ( ° C) C

/NH

O/inert

(mol%)

conversion

(%)

Selectivity

to AN (%)

Ref.

VSb

0.5

–

SiO

500 6.5/13/12.9/19.4/48.4 68.8 56.7 [31a]

VSb

1.4

0.2

460 51/10.2/28.6/10.2/0 14.5 61.9 [29b]

VSb

1.4

0.2

0.1

480 6.4/7.7/18.6/0/67.3 40.3 47.5 [31h]

VSb

0.5

–

440 7.5/15/15/20/42.5 39 77 [34d]

VSb

450 8/8/20/0/64 30 49 [37b]

20.3 Propane Ammoxidation to Acrylonitrile

789

790 20 Catalytic Ammoxidation of Hydrocarbons on Mixed Oxides

host excess Sb in the structure, thereby developing non - stoichiometric compounds,

especially in samples with lower V content. The formation of cationic vacancies

was evidenced by an intense Raman band at 880 – 920 cm

− 1

, especially in samples

having very low (Cr + V) / Sb ratio (i.e. a large excess of Sb) and higher relative

amounts of V (i.e. a Cr/V ratio close to 1).

Also with these systems, an excess of Sb with respect to the stoichiometric

requirement for the rutile formation is necessary, in order to reach good selectivity

to acrylonitrile and low selectivity to carbon oxides.

An important dopant for rutile - based mixed oxides is Nb oxide [59a] . The

Nb/Sb/O system is active in the ammoxidation of ethane to acetonitrile [74] , and

the combination of Bi/Mo/O and alumina - supported Nb

gives good perfor-

mance in the ammoxidation of isobutane to methacrylonitrile [75] . Nb is one

component of the V/Nb/Sb/O catalyst for propane ammoxidation developed by

Nitto Chemical Industries [35] . When used as the support for V/Sb mixed oxide,

formed new phases by reaction with V and Sb under catalytic reaction

conditions [76] . These phases, of unclear nature, affected catalytic performance

in propane ammoxidation. When Nb was instead added as a promoter for the

alumina - supported V/Sb/O system [77] , the interaction between the active com-

ponents V, Sb and Nb led to an improvement of catalytic performance with

respect to undoped V/Sb/O. However, the Nb

–

Sb interaction may also lead to

the development of SbNbO

, which is not active for propane ammoxidation, and,

in addition, removes Sb and Nb sites that would have been co - ordinated with V

to form efﬁ cient V – Sb or V – Nb species. Nb also develops mixed (Cr)V/Nb/Sb

oxides with the rutile structure, and hence modiﬁ es the properties of Sb cations

in allylic ammoxidation [59a] .

The incorporation of other elements, either trivalent or tetravalent, in the rutile

lattice may substantially improve the performance of V antimonate [21] . This is

the case for Al

, Ti

and W

. In the Al - Sb - V

–

O system, the active phase was

identiﬁ ed as having the composition Al

1 − x

SbV

with 0 < x < 0.5 [78] . (Al,V)SbO

can be described as a solid solution between rutile - type AlSbO

and quasi - VSbO

A yield to acrylonitrile of almost 40% has been reported for an Al - Sb - V - W oxide

catalyst, using feed ratios of propane, oxygen and ammonia that are stoichiometric

with respect to the formation of acrylonitrile [26b] . A comprehensive study of the

Al - Sb - V - W - O system has shown that the active phase in this system is Sb

0.9

0.9 − x

which is a solid solution between V

0.92

and rutile WO

. Titanium substitu-

tion in rutile V/Sb/O produced an improved selectivity to acrylonitrile [79] ; Ti

replaced for both V

and V

/Sb

pairs, forming solid solutions of continuous

composition.

It was demonstrated that the better catalytic properties of (Al,V)SbO

and of

quasi - Sb(V,W)O

compared with the pure rutile V/Sb/O phase could be rational-

ized in terms of the ‘ site - isolation ’ theory. According to this theory, which was

originally formulated by Callahan and Grasselli [6c, 80] , a catalyst that is selective

for partial oxidation can be developed by creating structural isolation of the active

site. Thus, the improvement of the selectivity to acrylonitrile by substitution of

some of the V atoms with Al or W can be explained by the fact that substitution