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

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518 12 Vanadium Phosphate Catalysts

No crystalline phases were detected in the VPO catalyst using XRD. The

NMR spin echo mapping spectrum showed two peaks that were assigned to dis-

organized and crystalline (VO)

. TEM showed large plates, proposed to be

amorphous material with small rectangular crystals of (VO)

, preferentially

exposing the (100), (021) and (012) planes. About 20% of the sample was composed

of δ - VOPO

plates containing cracks, which may be formed by the initial loss of

water from VOHPO

· ½H

The VPD catalyst appears more crystalline than the VPO catalyst. XRD and

NMR both show crystalline (VO)

, with a small amount of VOPO

and disor-

ganized (VO)

also visible in the spin echo mapping spectrum. The TEM study

shows the characteristic rosettes make up 95% of the catalyst, with a few ﬂ at

platelets. Diffraction patterns showed the rosettes to be made up of (VO)

(100) planes, while the ﬂ at plates can be indexed to α

- VOPO

It is clear that the method of preparation, as well as the activation procedure,

can have an effect on the structure of the active catalyst. This must be taken into

account when comparing activation procedures carried out on catalyst precursors

prepared by different means.

Research has been carried out to investigate the changes observed during the

activation process. This takes the form of heating a catalyst precursor up to the

reaction temperature in an n - butane/air feedstock, and leaving it on - line for

varying times (the standard times that have been adopted are 0.1 hour, 8 hours,

84 hours and 132 hours). The samples are then taken off - line and characterized

as fully as possible.

Abon and coworkers [32] report that during the activation period the catalyst

goes through a number of changes. Initially, VOHPO

· ½H

O is transformed into

poorly crystalline (VO)

and δ - VOPO

. The initial δ - VOPO

is transformed into

- VOPO

and (VO)

, which become more crystalline with time on - line. The

increase in crystallinity and surface area are proposed to be responsible for the

increase in activity observed. A decrease in V

phases (responsible for total oxida-

tion) is observed with activation time, which accounts for the increase in selectivity

to maleic anhydride.

Additionally, a similar study was conducted by Kiely and coworkers [94] on VPO

catalysts. Prior to activation, rhomboidal plates of the precursor VOHPO

· ½H

were observed by TEM. As the sample is heated in the reaction feedstock, cracks

are formed in the plates, due to the loss of water of crystallization (0.1 hour)

(Figure 12.10 a). Even after a short time on - line (VO)

and δ - VOPO

are seen

in the diffraction pattern. The δ - VOPO

and VOHPO

· ½H

O phases gradually

diminish with time, until they can no longer be observed in the 132 hour sample

(Figure 12.10 d).

As the activation proceeds, a crystalline rim starts to form around the edge of

the plate, while in the center of the plate a more disordered phase is formed, with

dispersed crystals of δ - VOPO

(8 hours) (Figure 12.10 b). After 84 hours the rim

(made up of small oblong crystallites of (VO)

) has thickened and large holes

start to appear in the center of the plates (Figure 12.10 c). After 132 hours the inside

of the platelet has also become more crystalline (VO)

(Figure 12.10 d).

12.5

Promoted Catalysts

Industrial catalysts for oxidation reactions rarely use a single bulk phase. A number

of promoter elements are added that can act purely as textural promoters, or

enhance the activity and selectivity of the bulk catalyst. The role of promoters on

vanadium phosphate catalysts has been addressed mainly in the patent literature

and Hutchings [147] has provided an extensive review of these patents.

A number of groups have tested a wide range of promoter elements and com-

pounds. Hutchings and Higgins [148] found that chromium, niobium, palladium,

antimony, ruthenium, thorium, zinc and zirconium had very little effect on the

speciﬁ c activity of (VO)

. A signiﬁ cant increase in surface area was observed

with zirconium, zinc and chromium, which could be of use as structural promot-

ers. Iron - , cesium - and silver - doped catalysts showed a decrease in the speciﬁ c

activity, while cobalt and molybdenum were the only promoters found to increase

the speciﬁ c activity.

The selectivity decreased for catalysts doped with cesium, palladium, ruthe-

nium, zinc and zirconium. This was thought to be due to these metals promoting

the over - oxidation of maleic anhydride to carbon oxides. However, molybdenum

Figure 12.10 Bright - ﬁ eld transmission electron micrographs of

platelet morphologies in (a) VPO - 0.1, (b) VPO - 8, (c) VPO - 84

and (d) VPO - 132 activated catalysts [94] . (Reproduced with

permission).

12.5 Promoted Catalysts 519

520 12 Vanadium Phosphate Catalysts

was found to poison the over - oxidation reaction. From this work Hutchings and

Higgins concluded that only cobalt and molybdenum act as promoters. Other ele-

ments reported as promoters are only responsible for an increase in surface area

of (VO)

Ye and coworkers [149 – 154] tested a large number of promoter elements and

found the activity to maleic anhydride changes in the order:

Zr Ce La Fe Co Cu Nb Ti Mo Ca Si W Ni Ge K>>>>>>>> >>>>>>

However, the activity reported by Ye and coworkers is not the speciﬁ c activity,

and the surface areas of the promoted catalysts show a large variation (26.3 to

50.8 m

− 1

). Hutchings [155] has plotted (Figure 12.11 ) the activity against the

surface area for a number of promoted catalysts and deduced that most of the

catalysts conform to a linear correlation. The only enhancement of the speciﬁ c

activity was given by the Ce - promoted catalyst. This shows that care must be taken

in the interpretation of results, particularly when catalysts prepared by different

methods are compared. A review of the promoter literature revealed that Ce, Co,

Cr, Cu, Fe, Hf, La, Mo, Nb, Ni, Ti and Zr are commonly reported to enhance the

activity [155] . These cations are suggested to form solid solutions, [(VO)

1 - x

]

(where M is a promoter cation). The inclusion of cations of different size or charge

in the VPP lattice is likely to cause defects, which could then function as active

sites for butane oxidation. In this section we shall discuss the common promoter

elements that have been shown to give an increase in activity compared to undoped

catalysts.

A number of other groups have also found that zirconium enhances the activity

of vanadium phosphate catalysts [11, 56, 146, 148, 150, 154, 156 – 163] . Zeyss and

coworkers [158] investigated catalysts doped with 5 to 15% zirconium. Unlike the

Figure 12.11 Plot of the activity against surface area for a number

of promoted catalysts [155] . (Reproduced with permission).

observations of Hutchings and Higgins [148] , the zirconium was not incorporated

into the (VO)

lattice, but was found in an amorphous phase, which is pro-

posed to be the catalytically active phase. This is probably the reason that zirco-

nium was not found to increase the surface area as Ye and coworkers [153]

observed. Zeyss suggested that the zirconium inﬂ uences the amount of VOPO

phases formed during the activation of the catalyst and that these are the cause of

the increased activity. The positive or negative effect of VOPO

in the active catalyst

is the subject of considerable discussion (see Section 12.2 ), so this explanation of

the promotional effect of zirconium is quite controversial. Sant and Varma [164]

also studied the role of zirconium as a promoter. They found that low concentra-

tions of zirconium lowered the temperature required to reach the maximum yield.

Various reasons for this observation have been put forward. The increase in

surface area and the increase in oxygen transport rates can be sufﬁ ciently altered

by the zirconium to result in high yields of maleic anhydride at lower tempera-

tures. The roles of zirconium, zinc and titanium have been studied as catalytic and

structural promoters [156] . The results showed that all these promoters had a sig-

niﬁ cant effect, if added in the correct proportion. Conﬁ rming previous ﬁ ndings,

1.5% zirconium had the most beneﬁ cial effect on the activity; good catalytic per-

formance could be achieved at lower temperatures.

It is proposed that zirconium and titanium both create acidic surface sites on

the vanadium phosphate surface. This prevents the desorption of reaction inter-

mediates (butene, butadiene and furan), while facilitating the desorption of the

acidic maleic anhydride. A large amount of zinc promoter resulted in a loss of

surface acidity, leading to over - oxidation of strongly adsorbed maleic anhydride.

However, a small addition of 0.6% zinc enhanced the catalyst performance by

creating basic sites, which increase the rate of butane activation. At low zinc con-

centrations the slight loss in surface acidity does not have a great effect. Takita

and coworkers [157, 165] studied the effects of zinc oxide on the catalyst. They

found an increase in catalytic performance that was ascribed to the increased rate

of re - oxidation of catalyst. Additionally, a range of transition metals and transition

metal oxides were tested to determine if they could act as promoters. It was found

that the conversion is signiﬁ cantly increased over catalysts containing manganese,

cobalt and zirconium, but decreased over the catalysts containing TiO

and MoO

Also the selectivity to maleic anhydride showed an increase of 8 to 10% for catalysts

containing TiO

, copper and zinc.

The conclusions of their study related the speciﬁ c activity to maleic anhydride

to the electronegativity of the promoter elements added. They found that the V

stretching mode has a larger wavenumber as the electronegativity of the promoter

increases, and that the larger the wavenumber of the V

O stretching mode, the

smaller the speciﬁ c activity becomes. The stronger the V

O bond is, the higher

the frequency of the V

O stretching mode will be, hence the larger the wavenum-

ber will be. So the more electronegative the promoter, the stronger the V

O bond,

and the lower the speciﬁ c activity.

Bej and Rao [166 – 170] conducted a detailed study of molybdenum - and cerium -

promoted vanadium phosphate catalysts. They found an increase in the selectivity

12.5 Promoted Catalysts 521

522 12 Vanadium Phosphate Catalysts

of these catalysts, compared to the unpromoted catalyst, albeit with a slight decrease

in activity. They attribute this ﬁ nding to the promoters preventing over - oxidation

of the maleic anhydride to carbon oxides. They also found that the promoted cata-

lyst could withstand more severe reaction conditions, which was again attributed

to less carbon oxides being formed, which can poison the catalyst.

In common with Hutchings and Higgins [148] , Bej and Rao suggest that the

molybdenum prevents the reduction of the V

ions to V

, a species that is con-

sidered to be responsible for the formation of total oxidation products. Cerium is

proposed to increase the conversion of butane.

The promotional effects of cobalt [71, 74, 150, 152, 154, 157, 162, 171 – 184] and

iron [71, 110, 148, 151, 152, 160 – 163, 173, 175, 176, 182, 185 – 189] have been widely

studied. Ben Abdelouahab and coworkers [173] looked at the effect of various pro-

moters on the structure of organically prepared catalysts. Both cobalt and iron

promoters were found to increase the selectivity to maleic anhydride, but butane

conversion was found to decrease with cobalt promoters and increase with iron

promoters.

As with the zirconium study by Zeyss and coworkers, cobalt and iron were found

to promote the formation of VOPO

phases during activation of the precursor to

the active catalyst. The difference in activity is considered to be due to the redox

potentials of the promoters. As the V

ratio decreases the butane conversion

is stabilized by iron (as the Fe

/Fe

redox potential is lower than the V

redox

potential). As the Co

/Co

redox potential is higher than the V

redox poten-

tial, the conversion of butane decreases when the V

ratio decreases.

A similar promotional effect was observed for catalysts prepared using an

aqueous route [174] . The iron - and cobalt - promoted catalysts are associated with

an increase in selectivity. The iron - doped catalyst showed an increase in activity

while the cobalt - doped catalyst activity decreased. The decrease in activity of the

cobalt - promoted catalyst is attributed to the formation of VOPO

· 2H

O in the ﬁ nal

catalyst. The VOPO

· 2H

O is formed by the oxidation of VOHPO

· ½H

O during

the introduction of the promoters using the incipient wetness technique.

The method of preparation of the catalyst was found to alter the effect of the

promoter [177] . With standard organically prepared VPO, the effect of cobalt and

iron was found to be the same as previously described [133, 173 – 176, 183] . The

increase in catalytic performance is proposed to be due to the stabilization of

–

dimers; the proposed active site. However, with catalysts prepared from

VOPO

· 2H

O in organic solvents, iron has no promotional effect. This is proposed

to be due to the loss of crystallinity and surface area of the rosette crystals formed

by this preparative route. Similarly the increase in activity due to cobalt is thought

to be a structural effect, inﬂ uencing the development of the (100) plane of

(VO)

Zazhigalov and coworkers [143] investigated cobalt - doped VPO catalysts pre-

pared by co - precipitation and impregnation methods. The performance of catalysts

prepared by both methods was increased, compared to the unpromoted catalyst.

The cobalt is thought to be present as cobalt phosphate, which is considered to

stabilize excess phosphorus at the surface, which has previously been found to be

an important feature of active catalysts (Section 12.2 ).

Oxygen donors (Sb

and BiPO

) were used by Ruiz and coworkers [48], and

it was found these increased the activity and selectivity when used to promote a

high P/V ratio catalyst. Tamaki and coworkers [190] tested the promotional affects

of magnesium, manganese, lanthanum and bismuth. They found bismuth to be

the most effective promoter, increasing the selectivity at conversions up to 90 mol%,

compared with unpromoted VPP. In line with studies on the active catalyst by

Morishige and coworkers [52] , there is speculation that the bismuth is incorpo-

rated into a phosphorus - rich amorphous surface species. Again, the additive is

thought to reduce the over - oxidation of products and the total combustion of

butane to carbon oxides.

The promotional effects of alkali and alkali earth metals, were investigated by

Zazhigalov and coworkers [143] . The promoters can easily donate electrons to the

(VO)

. This was reported to lead to an increased negative charge on the oxygen

atoms and an increase in the basic properties of the catalyst. The activation of

butane by dehydrogenation occurs more readily on a basic catalyst and so the rate

is increased. Acid sites are also proposed to be important to enable desorption of

products to prevent over - oxidation. It has been suggested that surface species must

be tuned by the promoters, to have a mix of acid and base sites in appropriate

amounts for butane activation to be enhanced while allowing the selectivity to

maleic anhydride to remain undiminished.

Centi and coworkers [84] have examined the effect of potassium doping, which

was found to inhibit the P

–

OH Br ø nsted acid sites. The lack of Br ø nsted sites was

thought to have two unfavorable effects on the catalyst. Firstly; the formation of

lactones and maleic anhydride from furan was inhibited and, secondly, carbon -

containing residues were strongly adsorbed onto the surface, deactivating the

catalyst.

An increased performance has been reported with catalysts doped with indium

and tetraethylorthosilicate ( TEOS ) [185] . The increase in catalytic performance is

only observed with both promoters in the catalyst. It is proposed that the promot-

ers work by facilitating the oxidation of the catalyst during activation, giving rise

to VOPO

phases, and drastically decreasing the thickness and size of the (VO)

crystallites leading to a higher surface area.

Harouch Batis and coworkers [91] investigated the effect of chromium, which

Hutchings and Higgins [148] observed had no effect on speciﬁ c activity or selectiv-

ity. In this study, a surface enrichment of chromium was found to give a decreased

maleic anhydride yield, while at high conversion, the catalyst was deactivated by

surface coking. It is thought that the active site is different on the doped catalyst

and the undoped vanadium phosphate. This leads to the formation of butene and

furan at low conversions, which cannot usually desorb from vanadium phosphate

catalysts, and at high conversions to over - oxidation to carbon oxides. Matsuura and

coworkers [191] report that using niobium phosphate as promoter leads to an

increase in activity of vanadium phosphate catalysts. It is thought that an increase

in Lewis acid sites is responsible for the enhanced performance.

Zazhigalov and coworkers [142, 143] have reported the incorporation of bismuth

compounds into vanadium phosphate catalysts using mechanochemistry.

This involves milling the catalyst precursor and the promoter in ethanol. The

12.5 Promoted Catalysts 523

524 12 Vanadium Phosphate Catalysts

mechanochemistry preparation yields catalysts with a higher activity and greater

selectivity to maleic anhydride than those prepared by chemical means, or mechani-

cal mixtures.

As illustrated by the work by Sananes - Schulz and coworkers [177] the prepara-

tion method of the catalyst can alter the effect of the promoter, as can the method

of doping. Hutchings and Higgins have warned against misinterpretation of pro-

motional effects for catalysts prepared by incipient wetness and co - precipitation

using acid solutions. Acidic solutions can cause the formation of VOPO

· 2H

which has a detrimental effect on the catalytic performance that can be mistakenly

attributed to the promoter. They recommend that these methods of introducing

promoters must be used with care, and in particular that the acidity of the impreg-

nation solution should be carefully monitored. Further confusion is caused by

different groups reporting contrasting results for the same promoters.

The effect of promoters on VPO performance has been summarized by Ballarini

and coworkers (Table 12.2 ) [192] .

12.6

Mechanism of n - Butane Partial Oxidation

The oxidation of n - butane to maleic anhydride is a 14 - electron oxidation. It involves

the abstraction of eight hydrogen atoms, the insertion of three oxygen atoms, and

a multi - step polyfunctional reaction mechanism that occurs entirely on the

adsorbed phase. No intermediates have been observed under standard continuous

ﬂ ow conditions, although mechanisms for this process have been proposed based

on a variety of experimental and theoretical ﬁ ndings. The description of the active

site is linked to the mechanism and is the subject of considerable debate in the

literature. The mechanisms are linked to the researchers ’ hypotheses of the active

site, which will be discussed in a separate section in this chapter. It is widely

accepted that the (100) plane of vanadyl pyrophosphate, (VO)

, (referred to as

the (020) plane by certain authors) plays an important role in the selective oxida-

tion of butane.

The structure has been determined by XRD and consists of edge - sharing VO

units linked by pyrophosphate tetrahedra (Figure 12.2 ). This is viewed as the active

surface for most of the proposed mechanisms. Here we will discuss several of the

mechanisms thought to account for the production of maleic anhydride, which

have been debated in the literature.

12.6.1

Consecutive Alkenyl Mechanism

A consecutive alkenyl mechanism has the widest support in the literature [58 – 64,

83, 200]. Once butane has adsorbed onto the vanadium phosphate surface, it is

transformed via adsorbed alkenyl intermediates into maleic anhydride. A summary

of the mechanism is shown in Scheme 12.2 .

Table 12.2 Summary of recent achievements on the effect of

promoters on VPO performance [192] .

Dopant, optimal

amount (%w.r.t. V)

Promotional effect

Reasons for promotion Ref.

Co, 0.77 C 15 – 25%, S 0 – 11%,

under hydrocarbon - rich

conditions

Control of the optimal V

surface ratio; stabilization of

an amorphous Co/V/P/O

compound

[71, 193,

194]

Co, 13% C 55 – 79%, S 43 – 35%,

at 653K

Optimal surface Lewis acidity [179, 183,

195, 196]

Ce + Fe C 44 – 60%, S63 – 66% in

the absence of O

Improvement of redox

properties

[182]

Fe, 8% Increase of catalytic

activity

Fe replaces V

in VPP. The

re - oxidation rate is increased

[105, 197]

Ga, 10% C 22 – 73%, S 55 – 51% Increase of surface

area + increase of intrinsic

activity (electronic effect)

[198]

Nb C 20 – 17%, S 35 – 53% Increase of surface acidity

promotes desorption of MA

[159]

Nb, 1% C 58 – 75%, S 70 – 70% Nb concentrates at the

surface, where defects are

generated.

Nb acts as an n - type dopant;

development of a more

oxidized surface

[199]

a C = conversion; S = selectivity for the undoped compared with the

doped catalyst, under ﬁ xed reaction conditions.

Scheme 12.2

12.6 Mechanism of n-Butane Partial Oxidation 525

526 12 Vanadium Phosphate Catalysts

The initial step is thought to be hydrogen abstraction from n - butane ( 1 ), giving

1 - butene ( 2 ), followed by a further hydrogen abstraction to form 1, 3 - butadiene

( 3 ). A 1, 4 insertion of an electrophilic surface oxygen atom occurs, producing

dihydrofuran ( 4 ). Dihydrofuran is then oxidized to the asymmetric lactone ( 5 ) from

which maleic anhydride ( 6 ) is formed by a ﬁ nal oxidation of the remaining CH

group.

There are different ways in which gaseous oxygen can adsorb onto the surface

of the catalyst. In their theoretical study, Schi ø tt and J ø rgensen [58, 59] suggest

that the gaseous oxygen is adsorbed in an η

- peroxo coordination mode (Scheme

12.3 ) as this leads to a favorable overlap of the φ

–

and φ

–

orbitals. Furan is

formed by oxygen insertion into adsorbed 1, 3 - butadiene ( 7 ). The φ

–

orbital

donates electron density into the φ

–

orbital, weakening the C

–

H bond and

forming an O

–

H bond ( 8 ). Then there is a favorable carbon – oxygen interaction to

give intermediate ( 9 ). The asymmetric lactone intermediate ( 10 ) is ﬁ nally formed

by the loss of water. This process is repeated on the reverse side of the lactone to

give maleic anhydride.

A consecutive reaction mechanism was also proposed by Gleaves and Centi [61] .

This was based on experimental work to back up the theoretical calculations of

Schi ø tt and J ø rgensen. Although the proposed intermediates are not detected

under reaction conditions they have been seen with fuel - rich gas feeds and under

temporal conditions. Using a TAP reactor, the products are detected in the order:

butane → butene → butadiene → furan. However, these conditions differ signiﬁ -

cantly from standard continuous ﬂ ow reaction conditions. Tauﬁ q - Yap and cowork-

ers [64] surmised the same mechanism from temperature programmed reaction

( TPR ) and temperature programmed desorption ( TPD ) experiments on n - butane,

1 - butene and 1, 3 - butadiene. Temperature programmed oxidation ( TPO ) experi-

ments suggest that the active oxygen species for selective oxidation is lattice

oxygen, and that the replenishment of the surface oxygen from the bulk is the rate

determining step.

The active oxygen species was investigated by Abon and coworkers [57] , using

isotopic labeling experiments. Initially, they found the products contained only

lattice

O. As the reaction proceeded, more

O atoms were incorporated into the

products. They also concluded that lattice oxygen was the active oxygen species,

Scheme 12.3

and that it was replenished by the gas - phase oxygen. This is widely accepted to be

the case and has been conﬁ rmed by numerous studies [31] .

Although Misono and coworkers [63] agreed with the consecutive mechanism,

they suggested a different rate determining step to Tauﬁ q - Yap and coworkers

[64] . By determining kinetic data for the reaction of n - butane, 1 - butene and 1, 3 -

butadiene over VPP, they concluded that the initial dehydrogenation of butane

was rate determining. The mechanism for this H abstraction has been studied in

more detail by Millet [201] .

Other research [65, 67, 202] has shown that butene oxidation can produce many

selective products (furan, acetaldehyde and methyl vinyl ketone) which are not

detected during butane oxidation. It cannot be assumed that the oxidation of

butane and the unsaturated reactants proceed along the same pathway. The kinetic

data must be viewed with this in mind, although butane activation is widely

accepted to be the rate determining step. The intermediates are capable of desorb-

ing from the surface, (as seen in the TAP studies) but do not do so, indicating that

the further reactions occur more readily than desorption.

12.6.2

Consecutive Alkoxide Mechanism

A consecutive mechanism was proposed by Zhang - Lin and coworkers [68, 69] .

The mechanism was based on kinetic data calculated for the oxidation of butane,

1 - butene, 1, 3 - butadiene and furan over (VO)

and VOPO

phases. Unlike TAP

studies, the kinetic data suggested that furan is not an intermediate for butane

oxidation, but is an intermediate for butadiene oxidation. The differences observed

in the oxidation of butane and the unsaturated hydrocarbons questions the validity

of applying butene and butadiene oxidation results to the butane system.

The consecutive alkenyl mechanism (Scheme 12.4 ) was put forward by Zhang -

Lin and coworkers [68, 69] as the route for oxidation of unsaturated reactants

such as 1 - butene. The weakly adsorbed intermediates are in equilibrium with

the gas phase, which enables furan to be seen as a product for butene oxidation

[65 – 67] .

Scheme 12.4

12.6 Mechanism of n-Butane Partial Oxidation 527