Effects of cations introduced into 12-molybdophosphoric acid on the catalyst properties

245

Applied Chtalyysis, 4 (1982) 246-256 Elsevier Scientific Publishing Company,

EFFECTS

OF CATIONS

INTRODUCED

Amsterdam

- Printed in The Netherlands

INTO 12-MOLYBDOPHOSPHORIC

ACID ON THE CATALYST

PROPERTIES

Mamoru

AI

Research

Laboratory

of Resources

Nagatsuta,

Midori-ku,

(Received

15 December

Yokohama,

Tokyo

Utilization,

Institute

of Technology,

227, Japan.

1981, accepted

IO July 1982)

ABSTRACT The effects of the kind and amount of a cation introduced into 12-molybdophosphoric acid, HSPMo 12’4fy on its function as an acid-catalyst, its reducibility and reoxidizabllity, I s oxidation activity, and its thermal stability are examined. With a change in the amount of a cation, the catalytic activity for acidcatalyzed reaction varies in a manner different from the acidity measured by means of NH-J adsorption, although there exists a correlation between them as far as the neutral salts are concerned. The neutral salts ar divi ed into two groups depending on the reducibility: Group A, salts of Ag+, Pd?+ , Cu $+ , and Cr3+ and Group B, the other salts. The reoxidizability varies in the same direction as the reducibility. The oxidation activity for 1-butene and methacrolein is related to the electronegativity of the metal ion, but is independent of the reducibility. The thermal stability is markedly enhanced by the introduction of Cr3+, whereas the effects of Cs+, Rb+ and K+ are limited by their deleterious effect on the catalytic activity.

INTRODUCTION Catalysts in certain

based on 12-molybdophosphoric

mild oxidations,

of their technical oxidation,

notably

importance

acid, H3PMo,2040,

in "acid-formation-type"

in the synthesis

much care has recently

of methacrylic

been taken to improve

[l-43. It should be noted that their selectivity the well-known

bismuth molybdate

on the characteristics

catalysts.

perform

excellently

reactions.

Because

acid by direct

their catalytic

is quite different

action

from that of

In a preceding paper [5], we reported

of IE-molybdophosphoric

acid and its salts as catalysts

for mild oxidation.

In the present paper, we attempt to clarify in more detail the effects of different

kinds and amounts

stressing

its function

and its thermal

of cation

introduced

as an acid-catalyst,

stability

into 12-molybdophosphoric

its reducibility

as well as its oxidation

acid,

and reoxidizability,

activity.

EXPERIMENTAL Catalysts IP-Molybdophosphoric 0166-9834/82/000&0000/$02.75

acid, H3PMo,2040.

aH20, from Kanto Chemical

0 1982 Elsevier Scientific Publishing Company

Co. was used

246 without

further

Salts of 12-molybdophosphoric

purification.

OJo (where M represents

an n valent cation and x is the extent

ment, 0 5 x 6 3), were prepared [6]. They were supported catalyst

preparation

according

to the principle

on pumice originating

was identical

[5], except that the calcination

Reaction

acid, M!+,3H3_xPMo12

from volcanic

with that described

temperature

of proton

reported

replace-

by Tsigdinos

rock. The method

in the preceding

of

paper

was fixed at 360°C.

procedures

The vapor-phase of 2-propanol perimental

air oxidation

were carried

procedures

Reducibility

out in a continuous-flow

and the dehydration

system. The reactor

were the same as those emplyed

and reoxidizability

The reducibility

of I-butene and methacrolein

in earlier works

and ex-

[5,7,8].

measurements

and reoxidizability

of the catalyst

a closed system using H2 and O2 as reductant

were measured

and oxidant

by means of

respectively. A 2.0 g 3. in volume and 15 cm

portion of the catalyst was put in a glass tube (about 8 cm in length) and heat-treated

in an electric

furnace

at about 330°C for 2 h under

a stream of dried air injected from a fine steel pipe inserted Immediately connected

with a BET apparatus

the catalyst

had cooled

modified

into the system,

was raised at a constant

programmed

furnace.

to measure

after which the temperature

with the rise in the temperature. temperature

until

was calculated

of the

rate (100°C h-l) by means of a temperature-

The same procedures

had previously

N2 in place of Hp. as a blank test, because

certain

it was quickly

a low area and pumped

to 0°C. A fixed amount of H2 (about 26 cm3 STP and 66

kPa) was then introduced catalyst

into the bottom.

after the glass tube had been taken out of the furnace,

been followed,

the pressure

but with

of the system

increases

The amount of H2 reacted with a catalyst from the difference

run with H2 and that with N2. The temperature 0.3 mole per mole of 12-molybdophosphoric

in pressure

between

at which the H2 consumption

acid, TR, was employed

at a the reaches

as the index

of the reducibility. After the determination

of the TR value, the reduction

ued until the H2 consumption measure

the reoxidizability

the system was outgassed Then, the same procedures the reduced catalyst

reached

1.0 mole per mole of H3PMo,2040

for a fixed degree of initial reduction.

until the catalyst were repeated

was obtained

ion of 0.15 mole per mole of H3PMo,20Q0,

were continin order to Thereafter,

had been cooled to room temperature.

with 02. The amount of 02 reacted with

from the difference

run with O2 and that with N2. The temperature

reoxidizability.

procedures

in pressure

corresponding

TRO, was employed

between

the

to the 02 consumptas the index of the

241 RESULTS Function

as acid-catalyst

It was found that, when the kind of countercation is changed, a manner

the oxidation

distinctly

different

of NH3 [51. This prompted function

activity

for butadiene

from the acidity

us to examine

is fixed, while

and methacrolein

measured

the effects

the amount

varies

in

by means of the adsorption

of countercations

on the

as an acid-catalyst.

The dehydration reaction.

of E-propanol

The activity

the presence

decreases

to propylene

was chosen

with the duration

of an excess of air. Therefore,

as a model acid-catalysed

of the reaction

time even in

care was taken to measure

the activi-

ty at a fixed time (30 min) after the start of each run. The main product was propylene;

the amounts

of acetone

and ether were

less than 3 mol %. The results

obtained

under fixed conditions (T = 15O'C; 2-propanol = 1.65 mol % in air; total -1 flow rate = 1.0 1 min ; catalyst = 2 to 10 g) are plotted as a function of the

extent of substitution manner

FIGURE

different

1

by a cation,

from the acidity

Activity

x, in Figure measured

of M~+,nH3_xPMo,2040

1. The activity

varies

in a

by means of the NH3 adsorption

catalyst

for dehydration

[5].

of 2-propanol.

248 As is shown in Figure 2, a correlation the electronegativity concerned,

is observed

between

the activity

ion, Xi [9], as far as the neutral

in the case of acid salts,

it is hard to find any correlation

O40'

5

while

of the metal

such as M"+ ,,nH2PMo,2040

between

salts are

and M2+ 2/nHPMo12

them.

I

-_-_-_-H+--__

4-

and

I

b

_g$/@d 0

0

T2

4

I 6

6

aNi**

I 10

I 12

I 14

Xi

FIGURE 2

Relation

E-propanol

between

the activity

and the electronegativity

Reducibility

of M~~,,PMo,~O~~ for dehydration

of metal

of

ion.

and reoxidizability

The reducibilility

- that is, the oxidizing

power - was measured

for a series

of neutral

salts, M"+ according to the method described above. The 3/nPMo12040' results are plotted as a function of the electronegativity of the metal ion,

X i, in Figure 3. Pd was readily reduced beyond the fixed extent even 3/2PMo12040 at 0°C; consequently, it was hard to evaluate the reducibility, TR. As may be seen in Figure 3, the salts are divided 3+ , the reducibility

salts of Pd 2+, Ag+, Cu2+, and Cr

than that of the free acid, H~PMo,~O~~. bility

is lower (TR is higher)

electronegativity

of the metal

In group A,

(TR is lower)

In Group 6, the other salts, the reduci-

than that of the free acid and is related

to the

ion.

On the other hand, the reducibility, of formation

into two groups. is higher

TR, is plotted

of the metal oxide corresponding

as a function

to the cation

of the heat

in Figure 4. It is

249

3oc

o,,,j2+

-_-_-

200

OFe’+ OS?+

Ii+-_-_-.-

Y N

%+

e

l

cu=+

1oc : I 2

An’

I 4

I 8

-6

Pdtn \I

I 10

I 12

I 14

Xi FIGURE 3

evident

TR as a function

of the electronegativity

that the heat of formation

Group A salts is low compared The effect of amount Ag+. The results markedly

of oxides corresponding

ion.

to the metal

ion of

with that of the other oxides.

of added cations

was examined

for Ca+, NH;, Cu2+, and

are shown in Figure 5. It was found that the reducibility

affected,

even when the amount of cation

Then, the reoxidizability, are plotted

of metal

as a function

of the reducibility,

the Group A salts stay about case of the Group B salts,

TRO, was measured

lOO"C, regardless

the TRo value

is

is small.

for each neutral

salt. The results

TR, in Figure 6. The TRO values of of the TR value. However,

increases

with an increase

in the

in the TR

value.

Oxidation

activity

First, the oxidation salt catalysts

of methacrolein

was conducted

over a series of neutral

T = 350°C; methacrolein - steam = 1.6 -1 7.5 mol % in air; total flow rate = 1.0 1 min , and catalyst = 10 g. The con-

version

under fixed conditions:

to methacrylic

the metal

acid is plotted

ion in Figure 7.

as a function

of the electronegativity

of

250

Ei!O

v0

200

’ -

i0 8 -m-e_-_- of=+ Q3 i,OJ

H,PMo,~O,,

b0,

z

1oc :A,0

0

&

-a

-AH FIGURE 4

TR as a function

the cation,

I 120

I

I 80

I 60

I 40

20

100

1 140

I kcal IO gdOm

of the heat of formation

of oxide corresponding

Except for BiPMo,2040, the activity

is related,

all the salts are less active to a certain

is, the electronegativity.

It should,

extent, however,

than the free acid, and

to the nature of the cation,

metals and Ag, which are very low in the activity

catalyzed

(Figure 1) and the acidity

reaction

to the same extent as the salts consisting

for acid-

[5], can produce methacrylic

of a metal

ion of a moderate

such as Mn 2+, Zn2+, Co2+, and Ni2+. The selectivity

from 35 to 55 mol %, and it appeared

that

be noted that the salts of alkali

and alkaline-earth

negativity,

to

AH.

acid

electro-

was in the range

to vary in the same direction

as the

activity. Since the activity

for oxidation

I-butene was chosen as the olefinic fixed conditions:

T = 35O'C;

of butadiene reactant.

1-butene

flow rate = 1.0 1 min -', and Catalyst is plotted catalyzed

Thermal

in Figure 8. The activity reaction

has previously

The oxidation

been studied

was conducted

[5],

under

- steam = 1.0 - 7.5 mol % in air; total = 10 g. varies

The conversion

to maleic

in the same manner

anhydride

as that for acid-

shown in Figure 2.

stability

12-Molybdophosphoric

acid is known to decompose

at around

400°C

C61, and to

253

X FIGURE 5

TR as a function

lose its catalytic disadvantage thermal

of the extent

activity

in an oxidation

stability

at lower temperatures Actually,

catalyst.

has been claimed

related

to the activity

to ascertain

activity

for a simple model reaction

- for example,

2.0 g portion of a catalyst

was heat-treated

was carried 2-propanol

in

[I]. This prompted

for olefin

reaction.

and aldehyde

It is, therefore,

from the lowering the dehydration

is

possible

of the activity

of 2-propanol.

at a fixed temperature

in a stream of air for 2 h. The dehydration

A

in the of 2-propanol

under fixed conditions: T = 160°C; -1 = 1.65 mol % in air, and total flow rate = 1.0 1 min . The activity

was measured

out over the heat-treated

X.

This is a serious

an improvement

literature

activity

for acid-catalyzed

range from 390 to 480°C

by a cation,

on the stability.

the oxidation

the loss of oxidation

[1,5,10]. however,

in the patent

us to study the effect of countercations As has been shown above,

of proton replacement

exactly

catalyst

30 min after the start of each run. The results

obtained

only from the catalysts

example,

the neutral

possessing

a relatively

salts of alkali and alkaline-earth

metals were too inert for the effect

could be

high activity; metals

of the heat-treatment

for

and some other

to be measured.

The

results are shown in Figure 9. It was found that: thermal

stability,

Rb+ and Cs+ enhance

(i)

(ii)

Li+, Na+, Ba2+, Cu2+, Zr4+, and A13+ reduce the the effect of 8i3+ and Fe 3+ is small,

the stability

to a certain

extent,

and (iv)

(iii)

NH:, K+,

Cr3+ shows an

/ 0

cs+

200 -

ry

# y+ *’

/ .ti+

H+./

l

' 4

d

++

.2*

N’

----I

L

0

100

FIGURE 6

Relation

excellent presence

effect.

between TRO and

It is noticeable

TR' that the stability

of steam in the surrounding

is not affected

by the

gas of the heat-treatment.

DISCUSSION The results

shown in Figures

1 and 2 may be summarized

as follows,

though

there are some discrepancies: (i)

When the extent of replacement acid-catalyzed

reaction

by means of the NH3 adsorption with the addition (ii)

The activity addition

by a cation

does not decline

of a certain

is still retained

of an alkali metal

is low, x $ 1, the activity

for

as steeply as the acidity measured

[53. In contrast,

the activity

increases

cation. or is even increased,

with the further

ion up to x = 2 (except for the case of Li+).

such as (iii) In the case of cations of a relatively high electronegativity, 2+ Bi3+, Fe3+, and Cu , the effect is small, even if the amount of the cation is great. (iv)

The activity

decreases

steeply

upon the addition

of a large amount

(x z 2)

of cation of a lower electronegativity. (VI

The activity

of neutral

(vi)

As far as the neutral

salts of alkali and alkaline-earthmetals

and Ag is

very low. salts are concerned,

clear correlations

exist among

253

J

20-

Bi3’ I I ,

H+-__

-_e__

I

1’ /’

lo-

I 2

00

I 4

6

8

10

I 14

12

Xi FIGURE 7

Relation

between

and the electronegativity

the activity,

the conversion of metal

the acidity,

of methacrolein

to methacrylic

acid

ion.

and the nature of the cation,

notably,

electro-

negativity. From the finding manner different

that the activity

from the acidity

we are induced to consider acidity

obtained

as follows.

by a static method

Bulk acid may be extinguished the cation; as a result, of the cation.

for acid-catalyzed

measured

As has been mentioned

represents

gradually

the acidity

to be different

and more complex. reflect

in preference

activity,

when the extent of acidic

however,

It is likely that (i)

(ii)

sites are generated replacement

new acidic

it is difficult

the bulk acid to the catalytic [3,51.

action

[51, the

by

the situation

the acidic

seems

activity

sites located

to the catalytic

of coordinat-

of the cation, is so eminent

action

and (iii)

in number of

At the present

stage,

the action of the bulk from that of the

surface for each salt of 12-molybdophosphoric

catalyst

[51,

of the protons

sites consisting

by the addition

of low, the catalyst

to distinguish

earlier

the catalytic

contribute

sites that all the sites do not act effectively.

however,

in a

fashion with the amount

the amount of bulk acid, because

to the bulk acid;

varies

the amount of bulk acid.

in a regular

on the surface or in the bulk near the surface

ively unsaturated

mainly

by the replacement

falls

In the case of catalytic

does not always

reaction

by means of the NH3 adsorption

acid, though the contribution

has been pointed

out for the free acid

of

254

thPi!nz*~~ 00 /co* /. $i2+ 0

0

2

4

6

6

Cr*

? ea+ I 12

I 10

I 14

Xi FIGURE 8

Relation

between

the electronegativity

of metal

It can be postulated of countercation formation

the conversion

of I-butene to maleic anhydride

ion.

from the results

is not the same between

of the oxides corresponding

shown in Figures

3 and 4 that the action

Group A and Group B salts. The heat of

to the cation of Group A salts is lower

than that for Moo2 + 0.5 02 + Mo03, -AH = 38 kcal [II], whereas ation of the other oxides

species,

the oxygen

in the &se

species

role in the reduction

to oxidize

of the Group 6 salts, oxygen

ing to this view and the results

of the catalyst.

shown in Figure 3, it can also be deduced

the reactivity,

i.e. the oxidizing

in the electronegativity

different

On the other hand,

bonded with MO reacts or migrates

role in the reduction

with a decrease

formity with the results obtained

the former oxygen

in the bulk, play an

is bonded more strongly with the counter-

the oxygen

more easily and plays an important

compounds

or migrate

of the bulk of the catalyst.

cation than with MO; as a result,

organic

bonded loosely with the counter-

species bonded with MO or P. Possibly,

having a higher ability

important

the heat of form-

is higher than 38 kcal (Figure 4). In the case of the

Group A salts, there may exist oxygen cation besides

and

power, of oxygen

that

bonded with MO decreases

of the countercation;

this is in con-

by an ESR study of the salts reduced with various

in ionization

potential

[12].

At any rate, it is true that most of the salts of 12-molybdophosphoric at least in a high oxidation

Accord-

state, are more eminent

in reducibility

acid,

than metal

255

I 'C

‘Temperature FIGURE 9

Catalytic

temperature

oxides;

activity

for 2-propanol

dehydration

as a function

of the

of heat-treatment.

for example,

the TR value of V205 is about 320°C and that of Moo3 is

above 380°C. It is surprising as the reducibility

to find that the reoxidizability (Figure 6), suggesting

From the results,

osites. governed

the bulk of the catalyst, strength.

we are induced to consider

by the same factor,

possibly

which

in the same direction

connected

by the results

are not opp-

that both properties

by the ease of the migration

is closely

This view is also supported

varies

that the two properties

are

of oxygen

with the metal-oxygen obtained

by Akimoto

in

bond et al.

c131. The oxidation acid-catalyzed mainly

activity

reaction,

by the activation

for I-butene indicating on acidic

In the case of methacrolein the same. The salts of Cs+, Ba

can be well related with the activity

that the oxidation

for olefins, produce methacrylic 2+ of Mn 2+, Zn2+, Co , and NJ.2+ , suggesting the presence

ectivity

for methacrylic

property

but, possibly,

is controlled mentioned

[53.

oxidation, however, the situation is not exactly 2+ , and Ag+, all showing a very low oxidation

activity

not necessitate

of olefin

sites, as has been previously

for

acid to the same extent as the salts that the oxidation

of methacrolein

does

of strong acid sites, and also that the high sel-

acid formation

is not ascribable

to the mild oxidizing

power.

to the strong acidic

256

It should be noted that the oxidation lated to the electronegativity

is a Group A or Group B salt. Moreover, increases with the addition bility decreases

markedly

activity

of the cation,

the oxidation

with the addition

by the oxidizing

istics which are connected The thermal gravimetric,

stability

of view [6,14,15]. the catalytic

thermal,

or melting

activity

can only with difficulty

the cation. unsuitable temperature

provide system

be predicted

because

activity

an answer to our old question

stability

suggest

temperature

salts corresponding

to

salts of Cs+, Rb+ and K+ are though their decomposition

salts, for example,

M2HPMo,2040,

about 200°C lower than the decom-

neutral

salt. At present,

remains

undetermined,

of why the addition

the reason

but the results

of Cr 3+ to W03-P205

the acidity

that the addition

of 12-tungstophosphoric

of catalytic

from the decomposition

of neutral

at a temperature

of the corresponding 3+ action of Cr addition

that is, the results

far lower than the decom-

of their low activity,

(calcined at 500°C) increases

point

shown in Figure 9 that

effect of Cs+, Rb+, K+, and NH: is re-

temperature

temperature

for the excellent

from the structural

and that the stability

are very high, and that the acidic

lose their catalytic position

at temperatures

It should be noted that the neutral as catalysts

is not

in the bulk, character.

salts has been studied by means of thermo.

temperature

the enhancing

lated to the high decomposition

the reduci-

bond strength.

and X-ray analyses

is destroyed

temperature

salt, although

for these compounds

for these compounds

of the oxygen

it is clear from the results

position

of neutral

activity

with the metal-oxygen

However,

activity

activity

is re-

the catalyst

of a small amount of NH: or Cs+

power or mobility

of many neutral

differential

of whether

of NH; or Cs+ up to x = 2 [5], whereas

(Figure 5). It is clear that the oxidation controlled

for olefin and aldehyde

irrespective

in a dramatic fashion [16]; 3+ of Cr also enhances the thermal

acid.

REFERENCES

12 13 14

T. Ohara and A. Niina, Symp. Catal. Sot. Japan, Fukuoka, No.10 and 11 (1976). T. Ohara, Shokubai (Catalyst), 19 (1977) 157. M. Otake and T. Onoda, Shokubai (Catalyst), 18 (1976) 169. W.W. Swanson, Molybdenum Catalyst Bibliography, Suppl. No.5. Climax Molybdenum Co. Michigan, (1978) 61. M. Ai, J. Catal., 71 (1981) 88. G.A. Tsigdinos, Ind. Eng. Chem. Prod. Res. Dev., 13 (1974) 267. M. Ai, J. Catal., 67 (1981) 110. M. Ai, J. Catal., 52 (1978) 16. K. Tanaka and A. Ozaki, J. Catal., 8 (1967) 1. H. Niiyama, H. Tsuneki and E. Echigoya, Nippon Kagaku Kaishi, (1979) 996. Kagaku Binran (Handbook of Chemistry), the Chem. Sot. Japan Ed. Maruzen, (1975) 950. M. Akimoto, Y. Tsuchida and E. Echigoya, Chem. Lett., (1980) 1205. M. Akimoto, K. Shima and E. Echigoya, Shokubai (Catalyst), 23 (1981) 281. "Topics in Current Chem.," Vo1.76, Springer-Verlag, Fi;j5;sigdinos, New York,

15 6

K. Eguchi, N. Yamazoe and T. Seiyama, M. Ai, J. Catal., 49 (1977) 305.

1 2 3 4 5 6 7 8 9 IO 11

Nippon Kagaku Kaishi,

(1981) 336.