γ-Al2O3 catalysts

Applied Cotalysia, l(l981) Eleevier Scientific

59-70 Publishing Company,

59 Amsterdem

-Printed

in Belgium

DISPERSION OF MOLYBDENA AND HYDRODESULPHURIZATIDNACTIVITY OF MO/~-Al2OS AND Co (OR Ni) MO/Y-Al2DS CATALYSTS A. LOPEZ AGUDO, F.J. GIL LLAMBIAS', P. REYES* and J.L. GARCIA FIERRO fnstituto de Catalisis y Petroleoquimica, C.S.I.C., Serrano, 119, Madrid-C, Spain. Departamento de Quimica, Facultad de Ciencias. Universidad Tecnica de1 Estado, Santiago, Chile. 'Departanmnto de Quimica, Facultad de Ciencias, Universidad de Conception, Chile. (Received 1 December 1980, Accepted in revised form 9 February 1981) ABSTRACT Oxygen chemisorption and catalytic activity for hydrodesulphurizationof gas-oil on a series of MO/~-AleO and Co (or Ni) Me/y-A1203 oxide catalysts were studied. A linear relationship between molybdena loading, up to about 15 wt % MOOS, and equivalent molybdena area suggests a high molybdenum dispersion on the alumina surface. Similar behaviour for hydrodesulphurizationactivity versus equivalent molybdena area was observed, except for the highest molybdena content where a slightly higher activity was found. The equivalent molybdena area in Co (or Ni) promoted molybdena catalysts was slightly smaller than that found in non-promoted molybdena catalysts. However, catalytic activity for hydrodesulphurizationwas much higher in Co (or Ni) promoted catalysts. These results indicate clearly that Co (or Ni) in Mo containing catalysts does not increase molybdenum dispersion. INTRODUCTION Cobalt or nickel promoted molybdena catalysts supported on active alumina have been widely used for hydrodesulphurizationand hydrotreating in the petroleum industry and, more recently, in coal liquefaction and synthoil upgrading. Their importance is clearly shown by tfe large number of studies published in the last five years. This interest may also be due to the availability of better techniques and newer methods for evaluating the physico-chemicalproperties and for studying their correlation to their catalytic properties. Moreover, the old classical methods of catalyst characterization can also contribute to bring experimental evidence on this topic. In this respect, Weller (1,2) has recently renewed a method based on oxygen selective OlSS-9334/81/0000-0000/$02.50

0 1981 Elsevier Scientific

Publishing Company

60 chemisorption catalysts.

at low temperature More recently

catalysts

has been determined

Gas selective known example

on Pt/alumina

to the determination

This method

can also be reasonably

catalysts.

However,

catalysts samples,

unless this reduction

in which

authors

as a monolayer clarified

the promotor

catalysts

to transition

is carried

the existing

metal oxide

(or sulfide)

below 200 K.

of Mo03/A1203

groups.

Different

A question,

catalysts,

explanations

recently

(4) but, up to the present,

ive answer

to this problem

has not been provided,

still remains.

One of the suggested

promotor

increases

presented In

molybdena

dispersion.

of a series of T-A1203 by infrared

a definit-

so that much of the existing explanations

However,

(11) is that the

no clear evidence

the catalytic

catalysts,

spectroscopy.

activity

evidence

and molybdena

as well as the molybdena-alumina

has been

in the molybdenum

dispersion

of the high dispersion

activity

on the support.

of molybdena

dispersion

interaction

The main aim of this work is to elucidate

effect of cobalt or nickel on the molybdenum

to an increase mental

effect,

and sunarized

for this.

this work we have studied

enhancing

still not

of this promoting

have been proposed

controversy

catalysts,

surface

is the role of

each based on a limited amount of results, review by Massoth

supported

on reduced

groups grow onto the support hydroxyl

metals.

in molybdena

description

hydrodesulphurization

(cobalt or nickel).

the best

of supported

out at temperatures

a quantitative

molybdena

in an excellent

on these

in MoO3/SiO2

is undoubtedly

problem which arises

that molybdate

by replacing

in promoted

dispersion

of the bulk by oxygen chemisorption

(3-9) provide

it was postulated

of molybdena

area of molybdena

of the dispersion

applied

the most important

is the ready oxidation

Several

surface

(3).

chemisorption

applied

for measurements

the specific

whether

the

can be ascribed

Also, we show experi-

obtained.

EXPERIMENTAL Catalysts A Girdler

T-126 alumina, with a BET surface area of 188 m2 g-' and a pore -1 was used. It was ground and sieved to a particle size of volume of 0.39 cm3 g 1.19-0.86

nmn.

Catalyst impregnation

samples with different method

(impregnation

ure by impregnating lybdate

(or nickel)

CO-MO and Ni-Mo catalysts. a rotary evaporator.

solution

containing

nitrate was used to prepare

The solvent was evaporated

The catalysts

for 2.5 h and then calcined

content were prepared

in excess of the pore volume).

y-A1203 with a water

and cobalt

The composition

molybdena

were heated

by the "wet" Similar

both ammonium

procedparamo-

samples of supported

at 350 K and 27 kN mm2 in

in a forced flow of air at 650 K

at 823 K for 4.5 h.

of the catalysts

so obtained,

determined

by atomic absorption,

61 and expressed

as wt % of MoO3, Co0 or NiO per 100 g A1203,

are given in Table 1.

Oxygen chemisorption The method molybdena

used here to determine

supported

proposed

catalysts,

by Parekh and Weller

by Brunauer

and Emmett

is obtained

electrobalance where

The reduction

(13).

the classical

basically

between

were obtained = 10 -6 9).

of the catalysts

gravimetrically.

reduction

conditions

(for removal

and

rate, which was and non-severe

process of Moo3 to

changes

of the samples.

A 0.4 g sample was previously

at 703 K in a circulating

temperature

were given else-

at low H2 pressure

of the reduction

used was as follows.

it

at low temperature,

technique

weight

of the

using a Cahn micro-

was performed

a good control

devised

reduced;

two isotherms

The use of the gravimetric

allowed

procedure

reduced overnight nitrogen

previously

of the apparatus

Moo2 and Co0 (or NiO) to Co (or Ni) by following

of that one

technique

in the determination

gravimetrically

Details

area" for reduced

in order to keep the lowest level of reduction

monitored

The standard

molybdena

is a modification

in a catalyst

as the difference

isotherms

(sensitivity

low temperature

It consists

as a monolayer

graphically

The adsorption

(1) who adapted

(12).

amount of O2 chemisorbed

the "equivalent

by 02 chemisorption,

forced flow through

a trap at liquid

in MOO reduction). The sample -d N mm2) and, imnediately, was then pumped out at 703 K for 4 h in high vacuum (10 cooled down at 77 K. and physical

of H20 produced

At this temperature

adsorption)

at 195 K up to constant

the first O2 isotherm

weight

(for removal

O2 physically

cooled down at 77 K and then the second O2 isotherm Finally

the second

isotherm,

the BET surface

taking a 16.2 R2 for N2 cross section. of catalyst),

was calculated

(chemisorption

The sample was subsequently

was determined.

adsorbed

evacuated

in multilayer),

at 77 K was determined.

area at 77 K, was then measured,

The O2 chemisorbed,

from the difference

between

~1 (mg of O2 per g

the first and second

isotherms. The conversion (EMA) was based chemisorption reduced

on unsupported

unsupported

supported

molybdena

temperatures: isotherms

following

area to 02 chemisorbed

procedure

in monolayer

decreased

factor of conversion

factor was determined

area measurements

This ratio remained

both parameters

to a sintering

The calibrating

the standard

area"

of O2

Moo2 is the same; (ii) the use of a

(RUM) as a calibrating

catalysts.

molybdena

(i) the behaviour

77, 142 and 195 K on an unsupported

three temperatures. although

to "equivalent

two assumptions:

and supported

molybdena

and three BET surface

determined surface

of the amount of O2 chemisorbed

in the following

Moo2 sample.

above.

was practically

constant

in parallel.

Three double

at these temperatures

described

were

The ratio of BET the same for the

for successive This decrease

effect of a small amount of pyrophoric

for the

at three

molybdenum

determinations

seems to be due (%2 wt X)

62

~.)

°

'

F--

*

~

*

.

~

.

.

'

~

.

.

~

! !

!

E

I o4 0 E

.

E

.

.

.

.

I

*

*

1

"

-

o

'

1

o

*

*

'

l

o

o

'

*

*

*

*

1

~D

u

~-) C~l ~D

~L9

*

*

*

*

o

*

*

-

4~ 0 CO

4-)

*

-

e4-)

0 ul S. q)

0 (J

0 0

0

0

0

0

0

0

0

(J

qJ ~

C

°~

m s..

¢_) °~.. *rs.

.~

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¢D e0

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o

s.

¢.)

?

®

0 0

10-

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L

Q.

¢.~

0 0 5,-

..-1

¢~

63 contained

in the unsupported

ion of the H2 [MoO3(C204) sintering

may be caused,

573 K and 02 pressure brating

MoC3_6

~201.

H20 complex

Catalytic

Activity

for desulphurization

The average

This

process

at

value for the cali-

02, which

measurements

is close to

(1,2).

This value

EMA values.

LHSV = 8.8 h-';

PH

The catalyst particle

(HDS) of commercial

in a stainless

598, 623 and 648 K during

= 30.4~10~

reactor

(sulphur

Content

of

at temperatures

of

standard

N .-2; ratio H2 (gas, STP)/gas-oil

2was diluted,

1 to 1 by volume, with Carborundum

size of 1 mm, and'presulfided

of 20.3~10~

gas-oil

steel fixed-bed

4 h of run using the following

mixed with 7 vol % of CS2. pressure

decomposit

activity

1.2 wt %) was measured

average

by thermal

by the outgassing

is 8.84 m*/mg

from volumetric

used to calculate

prepared

in a H2 forced flow (14).

lower than 2~10~~ N mm2.

factor of the three temperatures

was subsequently

sample,

at least on the surface,

that of 9.52 m2/mg O2 obtained

402.

(6 2 1)

The presulfiding

condictions: (liquid)

=

with an

"in situ" with the feedstock was effected

for 2.5 h at 623 K,

N mm2, LHSV of 2.2 h-' and a ratio H2 (gas, STP)/gas-oil

volume of 250.

Infrared

spectroscopy

Self supporting particle

wafers

were obtained

by pressing

size, at 3~10~ N rnq2 for 1 min.

less cell and then evacuated for 4 h.

under a dynamic

The i.r. spectra were scanned,

30 mg powder,

They were placed

of 40 urn

into a special

vacuum of 1.3~10~~

grease-

N mm2 at 773 K

with the sample at room temperature,

on

a Perkin-Elmer

125 double beam grating spectrophotometer the accuracy of which -1 f 2 cm . Due to the low transmittance of the samples, attenuation

was within

of the reference procedure

beam was used.

A more detailed

used has been given elsewhere

account

of the spectroscopic

(15).

RESULTS Oxygen chemisorption Using the reduction the weight 99.6%) Thus, higher

conditions

loss in the reduction

to the theoretical

given previously of Mo03/A1203

value calculated

it is clear that the possibility reduction

in the experimental

catalysts

for a quantitative

of MO valence

section,

was very close reduction

(96.0 to Mo02.

being lower than 4 due to a

(16) does not take place in the reduction

conditions

of this

work. However,

in the case of molybdena

that the weight

loss in the reduction

of Co304 or NiO to Co or Ni metal

catalysts

containing

did not correspond

if we assume

Co or Ni it was found to the total reduction

that all Ho species

present on

64 catalyst

are quantitatively

molybdena

reduced

to

Since a fraction

catalysts.

with Al203 as irreducible

CoA1204

MOO*, in the same way as for unpromoted of Co or Ni oxide

(17, 18) or NiA1204,

is

very likely combined

it was considered

that

only a part of Co 0 or NiO would have been reduced to metal and all MO species 34 to Mo02. Then, from the weight changes registered in the reduction of A-9 and A-13 catalysts, corresponded,

it was derived

respectively,

that the irreducible

part of Co or Ni oxide

to 1.25 or 0.82 wt % of Co0 or NiO loading

in

catalysts. The O2 chemisorption corresponding

BET areas are summarized

mined by multiplying (8.84 m*/mg

values obtained

0

Am

for the catalysts in Table 1.

by the calibrating

studied

and the

The EMA values were deter-

factor

previously

determined

2 ) * These values are plotted in Figure 1 as a function of the wt %

Moo3 in catalysts.

A linear relationship

the metal oxide dispersion, Moo3 on the supported

defined

catalyst

through

the origin

as an apparent

and determined

surface

as EMA/SBET

is shown.

coverage

Also,

(ASC) of

x 100, is given

in

Table 1.

I

I

loo-

FIGURE

1

different

Relationship catalysts.

NiO-Mo03/y-A1203.

0,

between

equivalent

Mo03/y-Al2

3 ;

molybdena

area and Moo3

q , COO-Mo03/y-A1203;

loading on and

A ,

66 Catalytic

activity

The activity

of the catalysts

accorging

to the pseudo second-order

1

k= -$ -1 LHSV [ where, k = rate constant, product

with respect

As we would

of molybdena

equation

removal

supported

previously

in feedstock,

space velocity

expect,

ably more active than Mo03/A1203 activity

kinetic

So = wt % sulphur

and LHSV = liquid hourly

given in Table 1.

to the sulphur

for gas-oil

has been expressed as an apparent kinetic constant calculated

hydrodesulphurization

catalysts

(m3/m3h).

The results

The dependence

and EMA values

(19):

S = wt % sulphur

Co (or Ni) promoted

catalysts.

obtained

catalyst

in

are also

were consider-

between

catalytic

is illustrated

in

Figure 2.

. 648K A 623 *’ 0 598 ‘*

0

25

Apparent

FIGURE 2

vs equivalent

Infrared

constant

for gas-oil

hydrodesulphurization

area in Mo03/y-A1203.

spectra

in the hydroxyl

stretching

region of y-A1203

and Co0 (or NiO)

> taken partially from reference (15) are shown in Figure 3. For -1 , characteristic

Mo03/A1203

(Figure 3a) four bands at 3790, 3748, 3735 and 3710 cm

of isolated broadened probably

kinetic

EMAl rnzg-’

I 75

spectroscopy

Infrared

alumina

second order

molybdena

50

hydroxyl

bands,

groups on y-alumina

at frequencies

due to different

around

OH groups

The bond energy of these OH groups

surface

(20, 21), were found.

3650 and 3550 cm-', respectively,

bonded to the surface is considerably

through

diminished,

Two other are

hydrogen

bonding.

the proton having

66 a high mobility Spectra

which generates

of molybdena/alumina

1 wt % Mo03/A1203

Bronsted catalysts

acidity. are given in Figures

3d and 3e.

(Figure 3d) bands at 3790 and 3710 cm-', and a shoulder

3740 cm -l, due to OH groups on the support surface were observed. Mo03/A12D3 mechanism

(Figure 3e) these bands are no longer present. proposed

the support through

Figures

to the

OH groups on

leading to their partial

removal

catalysts

(simultaneous

impregnation)

are shown in

By comparison

the central

remains

species

According neighbour

at

On 3 wt %

of water.

of COO-Mo03/A1203

3b and 3c.

catalysts cm

interact with the molybdenum

formation

Spectra

by Dufaux et al (lo), the two nearest

For

to the background of the support, in these -1 The band at 3710 bands at 3730-3750 cm were not found.

unaffected

in COO-Mo03/A1203

catalyst

and is slightly

The band at 3770 cm -' for the former,

37;: cm-: in NiO-Mo03/A1203.

shifted at and at

3778 cm-' for the latter, catalyst are probably due to a shift toward -1 band present on the support.

low fre-

quency of the 3790 cm

1 4000

3.600

3600 3400

320C

WavenumberIcm-1 FIGURE 3

IR spectra for different

b, Co0 (3.8)~Moo3 A1203;

(8.5)/A1203;

and e, Mo03(8.2)/A1203.

samples.

a.

c, NiO (4.3)~Moo3

Background

of the alumina;

(8.7)/A1203;

d, Moo3

(l.O)/

67

DISCUSSION Mo03/A1203catalysts It is generally

accepted

that in molybdena-alumina

lo-15 wt % Moo3 the molybdena as a monolayer Figure

correlated

obtained

from chemisorbed

to the wt % Moo3 deposited

that, at least up to about 15 wt % of molybdena present

as an "interrupted

proposed

by Dufaux

a extensive consistent

surface,

monolayer"

results

(1 wt % Mo03) some of the OH groups in 8 wt % catalyst

(spectrum

According much higher weaker

segretation respect,

Al2O3

Spectrum

still remain

by

This is

d shows that

"free" on the

e) most of them have disappeared

These infrared

than that found on silica MO-silica

the molybdena

results

agree with those

to MO-alumina

is

Moo3

In this

monolayer.

size analysis

as compared

value of 69 8,

on alumina

(24) which favours

of a molybdate

must point out that the particle

gave an average

dispersion

This is clearly due to the relatively

(3).

as compared

in the former or the forming

we

Mo02/Si02

content

surface.

(15, 21, 23).

to what we were expecting,

interaction

is essentially

In the latter case only a broad band

with molybdenum species. -1 is observed. below 3660 cm

previously

suggests

to the mechanism

with molybdena

shown in Figure 3.

by interaction

reported

molybdena

according

increases

This result

of MO with the OH groups on the alumina

with the infrared

whereas

extending

up to

probably

02 as a monolayer,

on A1203.

loading,

(2) which,

et al (22), progressively

interaction

at low loading

containing

(4, 10).

1 shows that EMA values,

are linearly

catalysts

is in a very high degree of dispersion,

for the 12.8 wt %

to 25 fi for 15 wt % Mo03/

(3).

On the other hand, it does not seem molybdena Several

could be present

attempts

at obtaining

XRD patterns

ion of bulk 1~00~ were unsuccessful exclude 'the possibility layer or bulk-like the limitations

Moo3 microcrystals,

of this technique

cal value for our alumina be approx.

(25).

(8).

is in an octahedral

ating previous

(8, 22, 28).

either

high, up to molybdena

in monolayer

loadings

studied.

for the detectdo not completely

of Moo3 being present

in multi-

by XRD due to

the theoreti-

should be about 60% (ASC equal to that a small Moo3 fraction The presence

More recently

environment

Nevertheless, or partially

of about 15 wt %.

might

be

of these MO multi-

Delmon et al (27), by

in a 14.D wt % Mo03/A1203

shown that the MO (VI) results

samples

however,

is 53.9 % (see Table 1). whereas

suggests

reported

catalysts

The fact that the ASC corresponding

or Moo3 microcrystals.

layers has been previously

amount of

which could not be detected

(SBET = 188 m* 9")

means of XPS and DRS measurements

surface,

of the oxidized These results,

(4,26).

catalyst

25 wt % Mo03).

forming MO multilayers

alumina

in the different

of a very small fraction

to the 14.3 wt % Mo03/A1203

1 would

likely that a significant

as Moo3 microcrystals

catalyst,

in multilayers,

the MO dispersion in multilayers,

have also corroboron the

seems to be very

This is supported

by the results

68 shown in Figure 2, where

it can be seen that catalytic

linearly with the EMA value of the catalysts, molybdena

not clear. molybdena

toward higher activity,

There is the possibility content,

molybdena

ion of this method Very recently,

reported

capacity,

by Tauster

of molybdena

determined

between

1 it is clearly

content.

a small fraction the dispersion contrary,

observed. Figure

which appears

molybdenum

to be more pronounced

species

anchorage

results of Figure

of molybdenum

ously impregnated,

different

to contribute

Experimental the activity

of a less a "cover or

(27) and (iii)

the

results given here do Neverthe-

that Co or Ni can influence surface.

spectra.

Some well defined

Thus, the observed

on A1203 surface

coverage

of MO by Co or Ni.

showing

either

the OH

decrease

MO-CO

of a

(or Ni) (18)

Some experiments

are in

of this point.

that the promotor

for HDS is not due to an increase

effect of the Co or Ni on

in MO dispersion

in

to an effect of

or to the formation

species due to the interaction

to the clarification

evidence

do not chemisorb

is the most likely to occur.

3 indicate

is

shown in

on the surface when MO and Co or Ni were simultane-

of molybdenum

molybdenum

On the

loading

the formation

The chemisorption

as shown by infrared

distribution

it occur.

with Co or Ni (18), (ii)

species on the alumina

rather than to a partial progress

catalysts

(Figure 1) seems to be more likely associated

less reducible

section,

form and, therefore,

should

(31) or "CO-MO bilayer"

(8, 27).

groups on the support remained

EHA values

in a previous

by Co or Ni in the form "cobalt molybdate"

which of these hypotheses

less, the infrared

of A1203

the EMA decrease

These are (i)

by interaction

configurations

of MO multilayers

not show clearly

impregnation

for high molybdena

to explain

that reduced Co/A1203

(1, 30).

species

of molybdenum

formation

disulfides.

in the EMA values for cobalt or nickel containing

into account

"nickel molybdate"

activity

has been also

the EMA value, even at high

could be observed

can be suggested

oxygen at low temperatures

effect"

molybdenum

of MO could in this case be in multilayer

Three reasons

reducible

hydrodesulphurization

to the results discussed

effect of the promotor

1, taking

activities

containing-catalysts.

shown that the simultaneous

According

a slight decrease

catalysts,

catalytic

on the useful contribut-

catalysts

with both Mo and Co (or Ni) did not increase molybdena

is

measurements.

by a pulse dynamic method,

et al (29) for unsupported

Co0 (or NiO)-MoOfi20, In Figure

contents, evidence

further

The reason

could occur at high

with additional

found between molybdena

a poorer linear relationship

almost

at high temperature

mechanism

areas" provides

to the characterization

and 02 chemisorption

of the activity.

particularly

that another

but it should be confirmed

The good relationships and "equivalent

increases

except for the one with higher

loading, where there is a slight increase

for this tendency

activity

on the alumina

69 surface

is presented

catalysts

in Table

are slightly

the activities

1.

smaller

for gas-oil

The ASC values of Co0 (or NiO)-Mo03/A1203

than those found for molybdena

HDS of cobalt or nickel promoted

are much higher than those of non-promoted

catalysts.

role of Co (or Ni) agrees with the findings Co has no effect on molybdena Martinez surface

dispersion

et al (ll), who claimed of the alumina

different

methods

Significant

support.

differences

by varying

The results

and/or

in the surface

the preparation presented

and disagrees

here,

in addition

catalysts

(35), seem to indicate

is more of an electronic

CO-MO interaction

favours

for the dissociative these adsorbed

to the findings

reduced

of hydrogen

being easily

of other studies, activities

by the promotor nature.

structure

species

(33,34).

(18) and catalytic

than of a structural

chemisorption

species

of the Co and MO oxidic

that the role played

a partially

could be due to the

of the,alumina.used.

have been found

techniques

that

of

to spread over the

contradiction

structure

e.g. using XPS and DRS (27), gravimetric measurements

with the results

characteristics

of the catalysts

about the

(32), who showed

that Co causes molybdate

of preparation

However, catalysts

This conclusion

of Ben-Yaacov

This apparent

samples.

molybdena

in these

Likewise,

of molybdena

the

suitable

with a low heat of adsorption,

transferrable

to basic groups

on the sulfide

molecule.

CONCLUSIONS Oxygen

chemisorption

containing molybdena

molybdena

at low temperature

areas and, consequently,

prepared

by a "wet" impregnation surface within

as monolayer,

for the determination

MO dispersion.

to a series of y-alumina-supported

the alumina

on reduced alumina-supported

proved to be a useful method

molybdena procedure

of this method

(6.9-14.3 wt % MoO3)

shows that MO is highly dispersed

the Moo3 loading

but a very small fraction

The application

catalysts

catalysts of specific

range studied.

could be present

on

Most of MO is present

as multilayers

or bulk-

like microcrystals. The dispersion

of MO is not increased

MO and Co (or Ni) are simultaneously for hydrodesulphurization fore, the promotor significant

by the presence

impregnated;

on the studied

catalysts

increases

effect of Co (or Ni) can not possibly

increase

of the MO dispersion

of Co (or Ni) when both

however,

on the support

the catalytic

activity

considerably.

be attributed

There-

to a

surface.

ACKNOHLEDGEMENT The authors Asesora

gratefully

acknowledge

para la Investigation

the financial

Cientifica

y Tecnica

support from the Comision (Spain).

70 REFERENCES

:

3

9 10 11

15

;: 20 21 22

32 33 34 35

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