Effect of pretreatment on textural characteristics of an alumina-supported nickel-tungsten catalyst

~ut~~als Chemistry and Physics, 24 (1990) 443-456

EFFECT AN

OF

PRETREATMENT

ON

443

TEXTURAL

CHARACTERISTICS

OF

ALUMINA-SUPPORTED NICKEL-TUNGSTEN CATALYST

Z.FOLTYNOWICZ and Z.SARBAK Faculty of Chemistry , Adam Mickiewicz University 60-780 Poznah (Poland) Received April 27,1989 : accepted June 1.1989

ABSTRACT The results of the investigation of the porous structure of oxide

,

reduced and sulfided

forms of alumina supported . Microdistribution

tungsten catalyst are presented was

determined ,

adsorption

on

the

intrusion porosimetry different

basis

of

the

while macrodistribution

forms

of

low

nickel

of the pore5

temperature

was determined

nitrogen

by mercury

. The change in pore size distribution the

NiWfAl,O,

catalyst

relation to the condition5 of its activation

is .

discussed

of in

It was revealed

that the process of reduction by hydrogen as well as simultaneous reduction-sulfiding

by a mixture of hydrogen and hydrogen sulfide

result in new surface species formation which affect the porous pattern of catalyst5 .

Catalysts for hydroprocesses cations: Cr

,

MO or W

cations as promotors often

used

catalysts

are obtained by a deposition of

as active

components and Fe

, onto a gamma alumina support are

prepared

components and promotors: CO-MO

,

from

a

mixture

Ni-Mo and Ni-W

, Co or Ni . The

of

most

active

, with nickel

considered the most preferable for economic reasons .

444 Classical support

preparation

impregnation

,

of the promotors

of

with

the

above

a nitrate

,

during

catalysts

solution

before

is

based

containing

or after

on

cations

impregnation with . Preparations

ammonium paramolybdate and ammonium metatungstate

obtained in this way are dried and calcined . As a result , oxide forms are produced . Oxide catalysts prior to use are activated by reduction with hydrogen

and

temperatures

sulfidation . As a

with ,

result

hydrogen

catalysts

sulfide active

at

higher

in hydrocarbon

hydrogenation and hydrogenolysis of such bonds as C-N , C-S , C-O or C-Metal are obtained . Depending

on

the

applied

conditions

in

the

preparation

procedure , catalysts with a specific structure of the surface are yielded

. The structure

type

catalytically

of

in

,

turn

active

determines

,

the

amount

and

. From the point of view

centres

of catalysis

it is the very importnnt to define these centres

Henceforth,

this is the mnin

.

workers

,

However

they

field of study of many research

often

tend

to

neglect

the

problems

concerning the porous structure As is well know , the type of pores and their sizes as well as the type and concentration

of active centres hnve

influence on

the activity of the catalysts . The question of the porosity is of considerable importance in the processes of molecule transportation to and from the catalyst active centres , particulary in the case when heavy fractions of petroleum or coal liquids are Large

organic

move freely active

centres

respective well

as

the

on which

reagent

,

taking with

catalyst

used

the

products

.

,

place

in

hydrogen

NiW/A150, in

reuson

access

would

be

and hydrogen is of

one

of

active

petrochemical

very

leave

centres

. to

system sulfide

as the

and as a result

important

the

formed

freely

deactivated

porous

to the

and mny undergo

blocked

to the

should

access

products

should

set

it is very

, which

processing

chemisorbed

molecules

the

to gain

conversion

would

have

the catalyst

mentioned

activation

the

, pores not

in order

may be

unreacted

consequence changes

they

the nbove products

UP

system

. Also

would

.

building

pore

. Otherwise

molecule5

above

the

conversion

catalysts other

molecules

inside

used

Due

to

the

examine

the

during of

oxide

active and

. In

therma 1 forms

of

catalysts

carbochemical

445

EXPERIMENTAL Catalyst preparation In the experiments described below a nickel-tungsten catalyst was used . It the

was made of 5.1 wt% of NiO and 22.0 wt% of WO, by

impregnation

technique

labelled as 0-NiW/Al,O,

. The

catalyst

in oxide

form was

.

The oxide

form was reduced with hydrogen at temperature of -1 for 2 hours. The reduced 400°C at the flow rate of 100 cm3 min .

form was labelled as H-NiW/Al,O, ,

On the other hand

the sulfided

form of the catalyst was

marked as S-NiW/Al,O, and was obtained under the same conditions as the previous one . However

,

instead of hydrogen

, a mixture . Details

of 10 ~01% of hydrogen sulfide in hydrogen was applied

of the preparation conditions were desccribed in paper [ll.

Determination of the pore size distribution in the pore radius range 5-150 A ,

The microdistribution temperature

(77 K) nitrogen adsorption measurments

out on a Sartorius microbalance the measurements

low

were carried

(Gravimat , type 4133) . Prior to

the samples were outgassed at 523 K in a vacuum

of 10-s Torr . The pore size microdistribution

in the pore radius range from

5-150 %,was calculated from the desorption branch of the isotherm by the method of Cranston and Inkley performed onan In

results

I21 .A11 calculations were

IBM computer using a specially elaborated routine.

,the data

on

cumulative

pore

volume

,

Vcum

the

mean pore radius r cumulative pore surface area S as well cum ’ av as plots of the pore volume and pore surface area distributions as functions of the pore radius were obtained . The

pore

size

75-75000

%, was

performed

on

pressure

macrodistribution obtained

a Carlo

range

macrodistributions

Erba

0-1000 ,

by AG

in

mercury 65a

atm.

the

pore

porosimeter The

radius

range

intrusion measurements in the

parameters

i.e. AV/Alg(rl,AS/Alg(rl

computer based on the Ritter and Drake method

of

applied pore

were calculated by I31 .

The t-plots representing the relationships between the amount of

nitrogen

adsorbed

V

layer t were calculatedaby

and

the thickness of the adsorption the Lippens-de Boer method [41 based

on the result of nitrogen adsorption measurements.

446

RESULTS AND DISCUSSION The pretreatment

of the oxide form of NiW/Al,O,

catalyst

in

hydrogen or hydrogen and hydrogen sulfide mixture results in new species formation on surface of the catalyst such

a

and

species

its

existance

in

[ll . Appearance of

different

geometrical

configurations should affect the surface area as well as Porous structure of different forms of NiW/Al,O, catalyst . Nitrogen adsorption as well as mercury instrusion measurments confirmed our assumptions . The total amount of adsorbed nitrogen in case of 0-NiWfAlsO, and S-NiW/Al,O, is comparable , whereas in the

case

of

H-NiW/Al,O,

nitrogen capacity catalysts

and

it

is

distiguished

by

much

larger

. Adsorption isotherms for nitrogen on reduced

their

precursor

are

given

in

Figs. 1-3.

Va (cm3 g'-*)

P/P, Fig. 1. Nitrogen adsorption - desorption isotherms 0-NiW/AlZ03 catalyst : 'A'-adsorption; 'o'-desorption.

for

an

447 Va

(cm3 g's)

I

0.0

0.1

0.2

0.3

0.3

0.F

0.5

0.7

0.8

0.Q

1.C

P/P, Fig.

2. Nitrogen

H-NiW/A1,03

Va

adsorption

:

catalyst

- desorption

'A'-adsorption;

isotherm

for

an

for

an

'o'-desorption.

(cm3 g-2)

1

53o/m

P/P, Fig.

3. Nitrogen

S-NiW/A1203

adsorption

catalyst

- desorption

: 'A '-adsorption;

isotherm

'IX'-desorption.

All isotherms are of type exibit

a

sharp

0-NiW/Al,O,

knee

IV in BDDT .

(point B)

classification

However

(Fig.1) approches the line P/P_=

whereas those on H-NiW/Al,O, a final upward turn

,

the

. They

isotherm

on

1 asymptotically

,

(Fig.2) and S-NiW/Al,O, (Fig.3) show

. The isotherm on 0-NiW/Al,O,

IFig.1) shows

hysteresis with a shoulder point at p/pm= 0.35 , indicating small pores

.

The

hydrogen-reduced

form

shows

a

hysteresis

loop

extending up to p/p*- 0.58 (Fig.2) . This indicates the origin of a

great

number

of

larger

pores

.

Simultaneous

sulfiding results in smaller pore formation on

the

S-NiW/Al,O,

isotherm

(Fig.3) occurs

reduction

and

. The shouder point at

lower relative

pressure (p/pa= 0.52) . The hysteresis loops are of a mixed shape and resemble those of E or B'type with admixture of A , according to de Boer

[51 . This

is equivalent

to the presence of tubular

capillaries with wide and narrower parts as well as so-called ink bottle capillaries , which is reflected on the plots of pore size distribution given in Fig. 4-6 .

As/Ar (ms g-l A-+)

AV/Ar 1BJ(crn3 g i h-%)

Pore radius (A) as functions of pore radius Fig. 4. pore size microdistributions for an 0-NiW/A1203 catalyst : a) pore volume - 'A'; b) pore surface area - '0'.

449

AV/Ar

103(cm3

Pore Fig.

5. Pore

As/Ar

g-l h-1)

radius

(&I

size microdistributions

for an H-NiW/A1,03

catalyst

10J(cm3

6. Pore

surface

g-+ A-1)

Pore Fig.

as functions

: a) pore volume b) pore

AV/Ar

catalyst

radius

area

- '0'.

(rn2 g-1 A-1)

(A)

size microdistributions

for an S-NiW/A1,03

of pore

- 'A';

As/Ar

radius

(ml g-i %I-l)

as functions

: a) pore volume b) pore

surface

of pore

- 'A'; area

- 'a'.

radius

450

The pore size distribution for 0- and H-NiW/Al,O, forms in the radius range 15-150 fi (so called micropores

I611 is practically amplitudes. In the

monodispersed with wide maxima of different precursor 0-NiW/Al,O, pores

less

than

.

(Fig.4a)

The

significant surface As

a

60

porous

change

of the pore

most A

wide

with

structure

during

volume

is contained

dominating of

hydrogen

radius

0-NiW/Al,O, ,

treatment

at

in

35

A

undergoes

a

which

causes

a

layer reduction as well as catalyst component migration

result

of

surface

layer restructurization

upon

hydrogen

treatment narrower pores are formed with dominating radius at 25 A (Fig.Sa) ; became

local maxima

levelled but

about 100 A for the 0-NiW/Al,O,

some knee

came

into view

sample .

at 60 A

This

family of pores developed completely during reduction -sulfiding in HZ-H,S

mixture

,

which

can

be

seen

in

Fig.6a

as

a

new

intensive peak exactly at 60 A . As a consequnce of sulfiding new pores are formed , which

is reflected on the plot of pore size

distribution as a maximum at 85 A . From Figs. 4b-6b we can see clearly that the main contribution to cumulative pore surface area comes exactly from the same pores as in the case of cumulative pore volume.

Table 1. Characterization of the pore

structure on the

basis of

nitrogen adsorption-desorption measurements. Sample

Pore volume Vc,,,,, Mean pore cm3g-l

Surface area

radius F av A

m2g-1 Cranston Inkley method S ClJUl

0-NiW/l,O, H-NiW/l,O, S-NiW/l,O,

0.37 0.48 0.34

388 342 353

182 231 165

BET method SET

238 305 210

The analysis of nitrogen adsorption data is shown in Table I . we can see , both processes of pretreatment effect the parameters of pore structures in opposite manner . Hydrogen

As

treatment

leads to the significant

increase of Vcum

, Scum as

well as SBST , while pretreatment with the mixture of hydrogen and hydrogen sulfide cauaea a decline of above mentioned

451

parameters H,S

. The diminishing of pore structure parameters due to

presence

,

formation sulfide

ions

0-NiW/Al,O, hydrogen phase and

can

pore

.

[7,81 surface

atmosphere

, while

to

(macropore)

on

On

the

NiO

phase

,

hand easily

[l]

. The

of

of

new

exchanged

be

explain

enlargement

basis

are

other

can

layer would

species

for

some

parts

converted presence

in of

the 28% increase

Vcum

larger

is a result

of the

such

a

of Sewn of

large

.

formation surface

the

oxygens

species

30%

The cumulative

area of pores

Scum,determined

by the CI

[21 , is much smaller than the specific surface area of the

method

samples from

explained

terminal

in the surface

SBET

tions

be

where

estimated

the nature

of the sample

of both methods.

indicate

the

relationships

existence

,

.

by the BET method surface

The values of

presented

These

as well

of mean

macropores in Fig.7

result

as from the assump-

statistical

. The

,

differences

pore

linearity

confirms

the

of

radius Va-

presence

t of

macropores.

‘a

(cm= s-1)

mo,

150 -

Fig.

7. Relationship

between

the amount

and

the

of the

adsorption

thickness

a) 0-NiW/A1203

- 'A'

b) 0-NiW/A1,03

of

nitrogen

layer

- '0'

c)

t for

adsorbed catalysts

0-NiW/A1203

V

a

:

- '0'

452

In order to describe the porous structure more completly , the macrodistribution

in

(the pores falling

the

radius

range

75 to

75000 A) has also been studied on the basis of the results of mercury

intrusion

measurements

. The

analysis

of

the

results

obtained is shown in Table II. Table II. Characterization of the pore structure on the basis

of

mercury porosimetry measurements. Pore volume Vcum Mean pore

Sample

0-NiW/Al,O, H-NiW/Al,O, S-NiW/Al,O,

is

m2g-l

6.24 7.66 5.52

837 1015 1416

0.40 0.36 0.19

contribution

to

insignificant

.

precursor

sample

reduced

sulfiding

caused

50% decline

at

least

the

two

processes

of

formation

the

results

of

.

assumption

. However

is

‘av

new

upon

treatment

The

connected

of

of

V

of

Vc_

walls

of

fiav

takes

of

pores

value place

shows

that

The

change

.

of

value

in

surf ace

:

formation data

.

we claim

macropores

-

, while Vvum

is

give

their layer

a large

the for

our

increase

of

S-NiW/Al,O,

smaller

,

than

for

species

on

. The enlargement

restructurization

macropore

the

sulfide

new bulky

volume

layer

On

confirms

of H-NiW/Al,O,

surface

reducing

.

hydrogen-hydrogen

or

reduced

the

that

enlargement

which

the

and i?av we may conclude

increases significantly , but ‘av 0-NlW/Al,O, . Reduction and sulfiding the

of of

into

adsorption

, in the case

with

Scum

Taking

species

Rav

area

, while

parameter

their

surface

hydrogen

change

surface

pretreatment

slightly this

CUllI

nitrogen

total

Hydrogen

affected

and

pore

the

Scum only

behavior

restructurization basis

area

A

only

consideration

Eav Surface

cm3g-l

The macropore sample

radius

structure

is

also

shown

in

detail in Figs. 8-10, which represent differential curves of the pore volume (Figs. 8a-10a) as well as pore surface distribution (Figs. 8b-lob) as a function of the pore radius. Generally area

comes

distinguishes pore

surface

some small is

, the

only contribution

from the by its

pores

own porosity

distribution local

less are

maxima , while

open to the mesopores

.

to the total pore surface

than

200

pattern. practically the

plot

fi

.

Each

Differential

sample

is

curves

of

monodispersive for

H-NiW/Al,O,

with (Fig.9b)

453

AV/Alsr

(cm3 s-2 &-+I

As/Algr

1

(cm2 g-% W-it

i

lo2

103

lo4

1Q5

Pore radius (%i) Fig.6. Differential curves of the pore volume(A) and the pore surface area (a)macrodistributions the pore

AV/Algr

Ae/Algr

(cma g-1 &-A)

I

(cm2 g-i A-1)

G‘B

~1,~ ,

120 I10 loo

as functions of

radius for an 0-NiW/Al,O, catalyst.

‘i

0.7

GIlI

Il.8

830

D.6

75 Eo

: xl ZI IO 0

0.6

i

i

:

0.4

‘n.3

cl.2 n.i

1

i

10

103

104

0.0

10=

Pore radius (h) Fig.9. Differential curves of the pore volume (A)and the Pore surface area(o) microdistributions radius for an H-NiW/Al.O, catalyst.

as functions of the pore

454

AV/Algr

(cm3 CT1 A-i)

As/Algr

I&or

(cm2 g-% A+)

, I.0

I

t

102

103

1%5

104

Pore radius (8) Fi!3.10. Differential curves of the pore volume (A)and the pore surface (n) macrodistributions

as functions of the

pore radius for an S-NiW/Al,O, catalyst.

On the contrary , pore volume distribution is polydispersive In the case of 0-NiW/Al,O, pore structure components smoothes

(Fig.SbI the sample polydispersity of

is a result

on

the

.

A1202

out polydispersity,

of

imposition

matrice specially

.

of Ni

Hydrogen

and W

oxides

pretreatment

in the region

of pores

with radius below 40% fi , with the formation of sharp maxim% at 180 %I (Fig.9b) . These pores became nearly completely reduced in sulfide atmosphere , but eimultaneous restructurization of the surface layer results in formation of a

hydrogen-hydrogen

new family of pores in the radius range 15% to 25% % (Fig.l%b) . Hydrogen

treatment

macropores

led to the development

, already present

of three branches

in the 0-NiW/Al,O, precursor

of

,i.e.

range 1000 - 75%00 A . This results in a Vcum increase for H-NiW/Al,O, sample , After hydrogen-hydrogen sulfide treatment the first (2000 - 4000 A) and third (more than 1000% 5) maxima decline about 40% , which is connected with a Vcu, in the radius

decrease . The changes described above in poroua structure of the oxide form

of

NiWfAl,O,

catalyst

*

occurins

after

the

thermal

are due to chemical

and hydrogen sulfide

activation in hydrogen

processes taking place on the pore surface . The species occuring on the surface of pores were X-Ray

,

Diffraction

,

,

UV-Vis , Raman

Thermal

, Magnetic Susceptibility

Spectroscopy

:

Infrared Spectroscopy

Differential

Diffuse Reflectance Spectroscopy measurments

identified

. The following should be pointed out

using numerous methods

Analysis

(Faraday method1

and EPR

[ll .

Upon thermal activation in the atmosphere of oxygen different species are formed on the surface of pores which determine

the

porous structure

and

polymeric

of the sample

species

containing

. The existence

octahedrally

octahedrals

is more

or

similar

to NiWO, , WO,

and

ions

A part of these

. Morever

less deformed

W '+

coordinated

coupled by oxygen bridges W 6+_O_W6+ was found structures

of dimeric

,

formation

of

heteropolytungstates

was indicated . Reduction

with

hydrogen

the

of

above

leads

sample

to

a

transformation of some of the species found on the pore surface into others . Namely , the presence of NiO ant+binuclear spegfes single

with

or

respectively

double

, was confirmed

bridges

.

partial reduction

. On the other hand

and

the

sulfiding

changes complete

of

of

porous

oxide

structure

substitution

of

form ,

1

1

and W, 0, W, 0, [ [ Heteropolytungstates undergo a

oxygen

,

simultaneous

leads to

which

terminal

even

is due

oxide

to

ions

reduction

more a

partial

(in the

existing on the surface of pores) for volumetrically

profound or

species

big sulfide

ions .

CONCLUSION The

qualitative

and

quantitative

changes

induced

in

the

formed species by applying thermal activation in the mixture of hydrogen and hydrogen sulfide affect the textural the examined catalyst are

produced

by

the

properties

of

. In our opinion the most profound changes change

of

terminal

oxide

ions

for

considerably biger sulfide ions . The described changes in the catalyst texture are confirmed by the catalytic

properties

.

It was

concluded

that the sulfide

form of the NiW/Al,O, catalyst is characterized by high activity in the

hydrodenitrogenation

derivatives

[1,9] .

of

carbazole

and

its hydrogenated

456

This

indicates

thermal

that

the

pore

in

the

mixture

activation

sulfide allows ,

carbazole

for a diffusion

tetrahydrocarbazole

system of

produced

in result

hydrogen

and

of relatively and

big molecules

hexahydrocarbazole

catalytically active centres and for a diffusion direction

of

hydrogen to

of the

in the opposite

.

REFERENCES Z.Sarbak

,

Catalysts

Structure (in Polish)

and

Activity

of

,Adam Mickiewicz

Hydrodenitrogenation Press University

Poznari , 1985. R.W.Cranston and F.A.Inkley ,Adv.Catal. ,9 (19571 143. H.L.Ritter and L.E.Drake ,Ind.Eng.Chem.,Anal.Ed.,l7

(1945) 782.

B.C.Lippens and J.H.de boer ,J.Catal. ,4 (1965) 319. J.H.de Boer ,The Structure and Properties of Porous Materials Butterworths ,London ,1985 ,p.68. ,Pure Appl.Chem. ,45 (1976) 71.

6

R.L.Burwell

7

Z.Sarbak ,Surface Technol. ,26 (1985) 331

8

Z.Foltynowicz

and

Z.Sarbak

,Surface

Coatings

Technol.

(1988) 29. 9

Z.Sarbak *React. Kinet.Catal.Lett.

,32 (1986) 435.

.35