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

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508 12 Vanadium Phosphate Catalysts

This has been attributed to the use of ammonia as a probe molecule, since this

cannot distinguish between Lewis and Br ø nsted acidity. Cornaglia and coworkers

[79] measured the acid sites using pyridine and acetonitrile. However, the pyridine

results showed no correlation between the activity and selectivity to maleic anhy-

dride and either the Lewis - to - Br ø nsted acid sites ratio (L/B acidity ratio) or the

Lewis acid site concentration.

Adsorption of acetonitrile enabled the strength of the Lewis sites to be measured

and a greater number of strong Lewis sites were found to be present in organically

made catalysts than in those prepared in an aqueous medium. Thus, the greater

the concentration of strong Lewis acid sites in the catalyst, the higher the maleic

anhydride yield. Cornaglia and coworkers suggest that the strong Lewis sites are

responsible for butane dehydrogenation.

12.3

Preparation of VPP Precursors

Vanadium phosphate catalysts are obtained by activating the catalyst precursor in

the reaction feedstock. After pre - treatment, the catalyst is equilibrated and catalytic

activity remains consistent throughout the lifetime of the catalyst. The activated

catalysts are formed topotactically from the precursors [86] . For this reason, a great

deal of research is based around the preparation of catalyst precursors with well

deﬁ ned, favorable morphologies.

VOHPO

· ½H

O (Figure 12.4 ) is the catalytic precursor for (VO)

. A number

of preparation methods are commonly used to prepare VOHPO

· ½H

O. These

usually involve reacting V

and H

in the presence of a reducing agent.

Figure 12.4 Schematic diagram showing the layered structure

of hemihydrate precursors in the (a) (100) and (b) (010)

directions (both orthogonal to the (001) layer normal

direction) [87] .

Originally, catalyst precursors were prepared in aqueous media, most commonly

using hydrochloric acid as the reducing agent [80, 88 – 91] . This is commonly

referred to as the VPA route:

2. H

1. ∆, 2h

+ HCl VOHPO

·½H

(12.2)

Alternative aqueous routes have been used by a number of groups to prepare

VOHPO

· ½H

O. Oxalic acid [88, 92] , lactic acid [93] , phosphorous acid [93] and

OH.HCl [75] have all been investigated as reducing agents in place of hydro-

chloric acid.

There has also been investigation into alternative vanadium sources. Poli and

coworkers [88, 92] used NH

as the vanadium source in conjunction with

and oxalic acid, and Harouch Batis and coworkers [91] used a VCl

mixture instead of vanadium pentoxide. Mizuno and coworkers [95] reported a

Figure 12.5 (a) XRD pattern, (b) bright - ﬁ eld image and

precursor material [94] . (Reproduced with permission).

12.3 Preparation of VPP Precursors 509

510 12 Vanadium Phosphate Catalysts

preparative route using vanadium metal to reduce vanadium pentoxide. They

heated a mixture of phosphoric acid, cetyltrimethylammonium chloride, vana-

dium and vanadium pentoxide in an autoclave, at 200 ° C for 48 h. Schimoda and

coworkers [75] have also reported the direct reaction of V

and H

In the 1970s, catalyst precursors prepared in organic media became increasingly

popular. The most common route (the VPO route, Figure 12.5 ), is a one - pot

method using alcohol as both the solvent and the reducing agent:

∆, 16h

+ alcohol VOHPO

·½H

(12.3)

A number of alcohols have been used in this preparation, isobutanol being the

most common [96] . Another common organic route uses a mixture of isobutanol

and benzyl alcohol. V

is reﬂ uxed in the alcohol for an hour before H

added, and the mixture reﬂ uxed for a further hour [96, 97] .

Johnson and coworkers [86] described a method for the preparation of

VOHPO

· ½H

O by reduction of VOPO

· 2H

O with alcohol. This is known as the

VPD route. This route was investigated more fully by Horowitz and coworkers [98]

for short - chain alcohols, and Ellison and coworkers [99, 100] for longer chain

alcohols. VOPO

· 2H

O is prepared by heating an aqueous solution of V

and

under reﬂ ux conditions for 16 h. This is then reduced with alcohol to yield

VOHPO

· ½H

∆, 16h∆, 16h

alcohol

VOHPO

·½H

+ H

VOPO

·2H

(12.4)

Alternative vanadium sources have been investigated using organic solvents.

Doi and Miyake [101, 102] used V

as the vanadium source. V

was initially

reduced to V

by isobutanol. The V

was then reacted with H

with a range

of alcohols as the solvent. As with their aqueous preparations Harouch Batis and

coworkers [91] used a VCl

mixture instead of vanadium pentoxide.

Guilhaume and coworkers [103] investigated the effect of the reactant V

morphology on the morphology of the ﬁ nal catalysts. They found a correlation

between the size of the V

grains and the preferentially exposed planes of

(VO)

. This study was only carried out on a few V

samples, and further

research on this aspect of the preparation is needed.

Investigations comparing organically and aqueously prepared VOHPO

· ½H

found that the ﬁ nal catalysts had very similar speciﬁ c activities [89, 91] However,

the organic preparation routes tend to give precursors with a higher surface area

than those prepared in aqueous solution. As the morphology of the precursor is

retained in the active catalyst, the organically prepared catalysts show the better

catalytic performance (Figure 12.6 ).

Hutchings and coworkers [89] investigated VPA, VPO and VPD catalysts, pre-

pared with a range of alcohols (Figure 12.7 ). VPA catalysts were found to have a

cubic morphology and a low surface area and VPO catalysts were found to have a

platelet morphology. However, VPD catalysts prepared with primary alcohols gave

a high surface area catalyst with a rosette structure, characterized by an X - ray dif-

fraction pattern with only one peak corresponding to the (220) reﬂ ection. VPD

catalysts prepared with secondary alcohols had a similar morphology and surface

area to VPO catalysts and have a characteristic X - ray diffraction pattern containing

many peaks, with the (001) reﬂ ection as the dominant feature. The catalysts with

rosette morphology were found to have a considerably higher activity than the

platelets. This is probably due to the increased surface area, as all the VOHPO

· ½H

catalysts are reported to have similar speciﬁ c activities (the activity to maleic anhy-

dride per unit area).

Horowitz and coworkers [98] investigated a range of preparations in organic

solvents. They reported the rosette structure for catalysts prepared by a VPD

method with isobutanol and straight - chain alcohols, as well as a VPO type prepara-

tion using a 1 : 10 mixture of benzyl alcohol and isobutanol or 1 - butanol. This study

found that the platelet catalysts were more selective than the rosettes. They suggest

this is due to the rosette structure obscuring the active plane. The greater selectivity

of thick platelets has been conﬁ rmed by other researchers [26] .

Okuhara and coworkers [104 – 110] have modiﬁ ed the preparations using inter-

calated and exfoliated VOPO

· 2H

O. Using this technique, intercalating com-

pounds such as amines, amides, alcohols or carboxylic acids can replace the water

between the vanadium phosphate layers. These materials can then be delaminated

in a polar organic solvent and the exfoliated VOPO

reduced to give V

vanadium

phosphates with unusual morphologies. It is thought that this process can occur

in an alcohol, which leads to the formation of the rosette structures found by

reduction of VOPO

· 2H

O with a primary alcohol [106] .

The crystallization of VOHPO

· ½H

O from V

and H

has been studied

in detail by O ’ Mahony and coworkers [111, 112] using time - resolved X - ray diffrac-

tion. They found that an intermediate phase was formed initially but this then

disappeared as VOHPO

· ½H

O was detected. Concurrent focused ion beam

Figure 12.6 Relationship between catalyst activity and surface

area for standard vanadium phosphate catalysts for the

oxidation of n - butane [89] . (Reproduced with permission).

12.3 Preparation of VPP Precursors 511

512 12 Vanadium Phosphate Catalysts

microscopy showed rosette structures forming from delaminated plates as the

reaction proceeded (Figure 12.8 ).

A number of groups have also studied VO(H

)

as a catalyst precursor

[113 – 118] . Mount and Rafﬂ eson [119] prepared VO(H

)

by heating V

and H

with H

in an autoclave at 150 ° C. They found this material decom-

posed at 360 ° C to yield VO(PO

)

. Hannour and coworkers [120 – 122] prepared

VO(H

)

by heating an aqueous solution of V

, H

and oxalic acid. This

was calcined in air to give β - VO(PO

)

. They also prepared α - VO(PO

)

by heating

in a large excess of H

. Both of these catalysts were poorly selective to

maleic anhydride, with carbon oxides being the major products.

Figure 12.7 (a) An SEM micrograph, (b) an XRD pattern,

(c)

P NMR spin echo mapping spectrum from: activated

VPA, activated VPO, and activated VPD [5] . (Reproduced with

permission).

This is consistent with previous studies that have shown VO(PO

)

is not as

catalytically active as (VO)

. Hutchings and Higgins [123] found beneﬁ cial

results by removing VO(H

)

by solvent extraction, from the VOHPO

· ½H

This yielded a higher surface area precursor and a more active catalyst after activa-

tion. Most VOHPO

· ½H

O preparations include boiling in water as a ﬁ nal step

to remove water soluble impurities.

12.3.1

The Preparation of Novel Vanadium Phosphates

Bordes and Courtine [124] prepared a number of precursors that required

calcination in air, oxygen or nitrogen to give the ﬁ nal catalyst. NH

(VO

)

(NH

)

[(VO

)

(HPO

)

] · 5H

O and NH

HVPO

gave ﬁ nal catalysts mainly

comprising (VO)

, VO(PO

)

or V(PO

)

, depending on the activation

conditions.

The synthesis of new vanadium phosphate precursors as reported by Benziger

and coworkers [125] consisted of intercalated n - alkyl amine pillars inserted between

the layers of VOHPO

· ½H

O. The (VO)

catalysts derived from these precur-

sors show an increase in selectivity which is attributed to stacking faults created

by the pillars. Vanadyl phosphonates with the formula VOC

2 n +1

· x H

O, ( n = 0

to 4, x = 1 or 5) were also synthesized. These could be converted into (VO)

at considerably lower temperatures than VOHPO

· ½H

O and produced catalysts

with higher surface areas and increased yields of maleic anhydride.

The gas - phase synthesis of VOPO

· 2H

O has been reported [126] . A gas stream

of VOCl

, POCl

and H

O with N

as carrier, was passed through a furnace where

the powdered VOPO

· 2H

O was collected. This was converted to α - and β - VOPO

by calcination in nitrogen, before in situ activation gave the (VO)

catalyst.

Figure 12.8 A focused ion beam image of the samples

recovered 2 min after reaction of V

and H

in alcohol.

Rosette - shaped hemihydrate particles appear to grow out

from the basal plane of the platelets [112] . (Reproduced with

permission).

12.3 Preparation of VPP Precursors 513

514 12 Vanadium Phosphate Catalysts

Michalakos and coworkers [127] prepared vanadium phosphate catalysts using

an aerosol process. The aerosol was created with aqueous solutions of NH

and H

(with air as the carrier) and sprayed into a furnace. The solid was

collected at the reactor exit on a cooled ﬁ lter. The compound was found to be

VOPO

· n H

O which was converted to α

- VOPO

with a small amount of

VO(H

)

by calcining. The catalyst was found to be more active than VPA cata-

lysts, despite having a lower surface area.

A number of groups have prepared vanadium phosphate catalysts using hydro-

thermal synthesis [92, 93, 128 – 130] . Using standard reaction mixtures, Dong and

coworkers [128] showed that at elevated temperatures and pressures different

materials are synthesized from those obtained under reﬂ ux conditions. Pressure

did not seem to affect the product formed, but as the temperature increased to

> 200 ° C further reductions occurred and V

products formed. However, these

materials were not found to have enhanced catalytic activity compared to tradition-

ally prepared materials. At lower temperatures, hydrothermal syntheses have

produced catalysts with comparable activity to those prepared under standard

conditions [92, 93, 129, 130] . Tauﬁ q - Yap and coworkers [129] found an enhance-

ment in activity for hydrothermally prepared catalysts and suggested this was due

to a modiﬁ cation in the redox behavior of the catalysts evidenced by TPO/TPR

experiments.

Hydrothermal syntheses have also been used to prepare new porous materials.

Doi and Miyake [131] obtained a mesoporous vanadium phosphate compound by

intercalating a surfactant ( n - tetradecyltrimethyl ammonium chloride) between the

layers of VOHPO

· ½H

O. Furthermore, Bu and coworkers [132] have also reported

a new mesoporous vanadium phosphate compound synthesized with an organic

template molecule. NaVO

, V, H

, H

O and the template piperazine were

mixed in a molar ratio of 1.0 : 0.5 : 5.17 : 476 : 0.71. A dark blue gel was formed after

15 minutes stirring, and the mixture was then heated at 170 ° C for seven days in

an autoclave. Light blue, needle - like crystals were observed and then recovered by

ﬁ ltration. The catalytic activity of these mesoporous vanadium phosphates have

not been reported.

12.4

Activation of the Catalyst Precursors

Research into the activation of precursors falls into two categories: the study of

structural and morphological changes during the activation period, and the effect

of different activation methods on the ﬁ nal catalytic behavior.

12.4.1

Activation Procedures

The catalyst precursor VOHPO

· ½H

O must be activated to the (VO)

cata-

lyst. This is usually done in situ with the reaction feedstock of 1.5% butane in air.

A number of pre - treatments have been claimed to speed up the activation process.

These can involve heating the catalyst in an inert atmosphere, a reducing environ-

ment or an oxidizing environment (Table 12.1 ).

The effects of standard activation procedures (adopted by industry) and fast

activation procedures (often reported in the literature) have been investigated by

Lombardo and coworkers [80] . In the standard activation procedure, the catalyst

was heated in air up to reaction temperature, followed by introduction of the

butane in three steps, up to 1.5%. The gas hourly space velocity ( GHSV ) was

increased in four steps to 2500 h

− 1

. This procedure could take up to 380 hours.

In the fast activation procedure the catalyst was heated under 1.0% butane in

air up to reaction temperature, at a GHSV of 900 h

− 1

. This was held for 3 h before

the butane concentration was increased to 1.5% and the GHSV to 2500 h

− 1

. Lom-

bardo and coworkers found that the standard activation procedure gave ﬁ nal cata-

lysts that were more crystalline, had less V

phases present and were far more

active than the fast activated catalysts.

Albonetti and coworkers [133] have compared equilibrated and non - equilibrated

catalysts. Non - equilibrated catalysts, which had only been on - line for 100 hours,

were found to be poorly crystalline, with an oxidation state of +4.36 and a surface

area of 11 m

− 1

. The equilibrated catalysts that had been on - line for 1000 hours

were more crystalline, had an increased surface area (23 m

− 1

), and an oxidation

state of +4.00. The equilibrated catalyst was found to be considerably more active

and selective than the non - equilibrated catalyst.

The effect of oxidation pre - treatments on the catalyst have been investigated by

A ï t - Lachgar and coworkers [134] . Pure VPP catalysts were obtained by heating

VOHPO

· ½H

O in a ﬂ ow of N

at 750 ° C for 72 h. This was then oxidized in a

ﬂ ow of O

for between 0.5 and 24 h. The catalyst oxidized for 1 h showed the

highest selectivity to maleic anhydride, although all the pre - treated catalysts were

more selective than pure (VO)

. This is thought to be due to the introduction

of V

phases. Previous studies have found V

to be detrimental to catalysts, many

researchers suggesting they may be responsible for the total oxidation of butane

[22 – 24, 80, 133] .

Cheng and Wang [135] have studied the effect of calcining catalysts in air,

and CO

. VOHPO

· ½H

O could be converted into an amorphous V

phase,

Table 12.1 I n ﬂ uence of activation conditions on unpromoted V

–

O catalysts [135] .

Entry Activation

atmosphere

Temp ( ° C) Time (h) MA

selc

@400 ° C

(%)

Conv @400 ° C

(%)

Ref.

1 1% Bu/Air 380 100 50 20 [133]

2 1% Bu/Air 380 1000 80 20 [133]

3 O

500 1 84 ca. 12 [134]

4 30% O

in N

400 3 62.3 98.2 [135]

a Sample heated in N2 (750 ° C, 72 h) then heated in O

12.4 Activation of the Catalyst Precursors 515

516 12 Vanadium Phosphate Catalysts

crystalline VOPO

or (VO)

with varying crystallinity, depending on the calcin-

ing temperature and atmosphere. The calcining environment that resulted in the

best catalyst was different for promoted and unpromoted catalysts, depending on

how easily they were oxidized. It was found that for unpromoted VOHPO

· ½H

the best environment was 30% O

in N

. This gave a catalyst that was composed

of dispersed V

species on a (VO)

matrix.

The addition of water to the reaction feed was investigated [136] . This led to two

signiﬁ cant effects being noted. The selectivity to maleic anhydride increased (with

increased yields of acetic and acrylic acids). There was also an increase in the

surface area of the catalysts activated with the n - butane/water/air feed compared

to the dry activated catalyst. Arnold and Sundaresan excluded the possibility of the

water vapor acting as a diluent by performing experiments with an n - butane/N

/air

feed. Instead they proposed that water is adsorbed onto the surface, blocking sites

that are responsible for over - oxidation of the products.

Contractor and coworkers [137] conﬁ rmed the effects of water vapor in the

gas feed, using a riser reactor. They proposed that the water and oxygen are in

direct competition for catalytic sites. Adsorbed water decreases the amount

of activated oxygen available, which decreases the activity, but prevents over -

oxidation, which in turn increases the selectivity. It is suggested that lattice

oxygen is responsible for the selective oxygen, while adsorbed or gaseous oxygen

(which is inhibited by the steam) forms the total oxidation products. Further-

more, Vedrine and coworkers [138] proposed that water plays an important role

in maintaining a hydrated catalyst surface, which allows a redox mechanism to

occur easily.

Centi and Perathoner [139] have discussed the beneﬁ ts of small amounts of

sulfur dioxide in the reaction feed. The SO

is thought to adsorb on the surface to

form stable VOSO

, blocking the redox activity of surface V

sites (considered

to be responsible for over - oxidation). This results in an increase in selectivity to

maleic anhydride, particularly at high conversions.

Recently a very detailed study of activation conditions was carried out by

Patience and coworkers [140] . A number of parameters were investigated includ-

ing temperature, pressure, time and gas - phase composition with respect to O

and H

O. They found that standard conditions (390 ° C and atmospheric pressure)

gave the best performance although the activation time could be shortened by

increasing the pressure. Water had a deleterious effect on the catalyst even at

low concentrations and this was proposed to be due to an increase in the V

observed.

Mechanochemistry has been proposed as an activation method [141 – 146] . This

involves milling the catalyst precursor in a solvent, usually ethanol, prior to its

conversion to the active catalyst. Experiments have shown that this procedure

promotes the exposure of the (100) plane in the catalyst, which is considered to

be the active crystal face [141 – 143, 146] , and can reduce the particle size thus

increasing the surface area [141, 144 – 146] . Experiments with promoters show that

this procedure gives more active and selective catalysts than those prepared by

either chemical means or mechanical mixing [93, 141, 146] .

12.4.2

Structural Transformations during Activation

The importance of precursor morphology in determining catalyst performance

was reported by Johnson and coworkers [86] . This is due to the topotactic trans-

formation (Figure 12.3 ), so the (VO)

catalyst retains the morphology of the

VOHPO

· ½H

O precursor. Insight into how the transformation occurs can help

in the design of pre - treatments and activation procedures that will produce cata-

lysts with improved properties.

Kiely and coworkers [94] investigated the differences in activated catalysts which

were obtained from VOHPO

· ½H

O precursors prepared via VPA, VPO and VPD

routes. The VPA catalyst XRD pattern shows a mixture of VOPO

phases, with

(VO)

present as a minority phase, and there is a weak V

peak seen in the

P NMR spin echo mapping spectrum. TEM revealed α

- VOPO

, δ - VOPO

(VO)

and some amorphous material (Figure 12.9 ). The (VO)

widely

considered to be the active phase constituted only 10% of the catalyst.

Figure 12.9 Bright - ﬁ eld image showing a typical platelet from

the VPO - 0.1 sample. Corresponding dark - ﬁ eld micrographs

taken in (b) the g

pyro

= (024) reﬂ ection of (VO)

and (c)

the g

delta

= (022) reﬂ ection of δ - VOPO

( g = diffraction vector)

[94] . (Reproduced with permission).

12.4 Activation of the Catalyst Precursors 517