Characterization of the catalytic activity of a thermally activated hydrotalcite-like compound in propylene oligomerization.

Materials Chemistry and Physics, 26 (1990) 1- 12

CHARACTERIZATION LIKE COMPOUND

OF THE CATALYTIC

IN PROPYLENE

1

ACTIVITY

OF A THERMALLY

ACTIVATED

HYDROTALCITE-

OLIGOMERIZATION.

R. SPINICCI Dipartimento

di Energetica,

Universith

di Firenze,

Via S. Marta

3, 501%

Florence

(Italy) A. ULIBARRI Departamento

de Quimica

Cordoba,

14004 Cordoba

Received

November

Inorganica,

Facultad

de Ciencias,

Universidad

de

(Spain)

28, 1989; accepted

January

25, 1990

ABSTRACT A hydrotalcite-like

compound,

Ni4A12(OH)12C03'nH20,

silica in order to obtain a precursor oligomerization, decomposition temperature pretreatment

after thermal

temperature programmed

confirmed

treatment

reduction,

The significance

both by the minor role played

acid sites and by the experiments

revealed

that, once the reduced

activity

became

propylene between

were also carried

propylene

with adsorbed

by the strongest carbon monoxide

metal ion sites are blocked,

. Temperature

programmed

yelded Ni(1)

of these Ni(1) sites was

in the reaction

Bronsted

of the

the optimum

by ESR the chosen treatment

for the reaction.

on

in propylene

was studied and the

made in order to establish

As revealed

very low

was precipitated

used as a catalyst

up to 773 K. The effect

of Ni4A12(OH)l2C03'nH20

conditions.

sites, responsible

frequently

desorption

out in order to characterize

, which

the catalytic runs with adsorbed

the interactions

and the catalyst.

INTRODUCTION Some synthetic

anionic

carefully

controlled

catalysed

reactions

lactone catalyst

clay of formula

0254XI584/90/$3.50

[1,2], olefin

[3], etc. In this work the possibility

in the oligomerization Ni4A12(0H)12C03'nH20

subjected

have been shown to promote

such as aldol condensation

polymerizations active

clays such as MgbA13(OH)l6C03'nH20

heat activation

of lower olefins

to

a variety

of

isomerization of obtaining

121, a

from a similar anionic

is investigated

0 ElsevierSequoia/PrintedinThe Netherlands

The structure

of these materials

In this, the metal

Mg(OH)2. resulting

octahedra

divalent

metal

resembles

is octahedrally

share edges to form infinite

is isomorphously

replaced

virtually

interstitially. sheet

cation

groups:

the

(e.g. A13+),

charge per aluminum

of any type, but most frequently

In addition,

that of brucite,

by hydroxyl

sheets. When some of the

with a higher

M2+_A13+ -OH layer gains one unit of positive anions,

essentially

surrounded

some water molecules

carbonate,

the

ion. A host of

can be accomodated

lie between

each metal ion

[4,5].

By thermal active

treatment,

catalysts.

By heating

the decomposition converted

products

consisting

of nickel

may be converted

at over 573 K, the interstitial

were prepared

silica and by heating

materials

of the anion are released

to metal oxides and water

The catalysts

promote

these synthetic

oxide,

water molecules

and hydroxyl

and

groups are

[6,7].

by precipitation

this precursor

of Ni4A12(OH)l2C03'nH20

to yield NiO/A1203-Si02.

silica and alumina

the oligomerization

to highly

of lower olefins

on

Indeed catalysts

have been found to effectively

[8-lo].

EXPERIMENTAL The Ni/A1203-Si02 nickel-aluminum a surface

catalysts

were prepared

hydroxycarbonate

on silica supplied

Ni4Al2(OH)12C03'nH20

was prepared

as reported

up to pH = 8-8.5. The mixture

reaction

and the gel thus obtained

to remove

the excess

stirring.

Finally,

TWO precursors

activities

the solid obtained were prepared

second type of catalyst,

Rigaku-Thermoflex

method,

implemented

were mainly

6% wt Ni0/2%A1203-Si02

investigated

and

runs, the

on samples

of the did not

in detail.

of the samples

calcined

at

found on silica samples had not changed. on a Mettler

TA 2000 C analyser

X-ray powder diffraction

by the B.E.T.

patterns

by determining

apparatus.

The

the amount of

by the continuous

on a Perkin Elmer Mod. 2120 Sorptometer. Endor-ESR

and a

(XRD) were

by use of Co Ka radiation.

technique

on the sample, which was obtained

on a Brucker

at 353 K under

after some preliminary

PW 1130 with a Fe-filter

surface area was measured

to dryness

on those of the first type the activity

were recorded

instrument.

on a Philips

adsorbed

However,

to be characterized

that the values

the

was dried at 353 K for 24 h.

surface area measurements

DTA and TG diagrams

were performed

because

interesting

After preparation, 773 K confirmed

properties

of a NaOH

stirred during

to stand for 24 h and then washed

in order to obtain

respectively.

and the related

sufficiently

was thoroughly

was allowed

[ll], by

by slow addition

salt. Then it was slowly heated

9% wt Ni0/3%A1203-Si02,

nitrogen

by Akzo Chem, F-6 type, with

elsewhere

from Ni2+ and A13+ salt solutions

solution

recorded

of a double

area of 522 m2g-'.

coprecipitation

appear

by precipitation

flow

ESR experiments

Activities catalyst

were measured

amount

and propylene analysis

in a continuous

.

of 0.5 g

The reactor

(ratio 2:l) and the effluent

conditions

the catalyst

Temperature

programmed

desorption

(TPR) runs were performed adsorbate

hydrogen

the catalyst

reducing

packed with

PAW. Prior to

in situ under reducing

at 573 K for 2h.

described

programmed

elsewhere

reduction

[12], where

were sensed by a hot wire detector.

samples were pretreated

air at 773 K for 2 h, while

with columns,

(TPD) and temperature

in an apparatus

or, respectively,

experiments

and hydrogen

a

containing

of helium

on Chromosorb

sample was pretreated

with a flow of helium

reactor,

was checked by periodical

equipped

supported

and ethylhexilsebacate,

each experiment,

in situ by heating

in TPD runs the samples were pretreated

In TPR

in a flow of in a

flow at 573 K for 2 h: all samples were cooled down to room

jzemperature after heating. nitrogen, allowed:

preferably

TPR runs were carried

at an heating

to room temperature

readsorption

In TPD runs adsorption

flow of propylene

and starting

plus helium,

of catalyst

was

before

was kept to a minimum,

or avoid mass and heat transfer

in TPD runs was minimum

in

or 2) at room temperature,

desorption,

The amount

50-80 mg, in order to minimize

out by use of 10% hydrogen

rate of 8 K/min.

1) at 523 K in a continuous

under the same flow conditions.

though

composition

on a C.Erba 4200 gas chromatograph,

didecylphtalate

cooling

flow tubular

was fed by a mixture

restraints;

thanks to the small amount of sample,

it was taken into account.

RESULTS

AND DISCUSSION

The XRD patterns of the catalyst

of Ni,Al-hydroxycarbonate

of composition

with that corresponding The thermal resulted

S%Ni0/3%Al203-Si02,

to the unsupported

decomposition

in products

[6,13,14]

included

precipitated

are consistent compound

compound

and show an endothermic

and Si02. The weight

group and CO2. seems to continue with the temperatures

The XRD pattern with an amorphous

of the supported solid giving

compound

in unsupported

small amount

of NiO.

compound

I373 K shows the typical

crystalline

temperatures,

compound

even if

heated up to 773 K is consistent

to the maximum

of the supported NiO reflection

peak due to amorphous

intensities

[6], seem to be absent

The XRD pattern

to

and diffuse.

only a broad reflection

:silica. The broad lines corresponding detected

peak at low

up to higher

of the unsupported

at 773 K is much broader

elsewhere

loss, which corresponds

II20 from the hydroxyl

peak centerd

on Si02

to the loss of molecular

which

the expected

[6].

compound.

with those reported

(below 573 K), which can be attributed

can be compared

for comparison

to those given by the unsupported

for the unsupported

water from the laminar

on Si02, the precursor

are shown in Fig. 1, together

compound

of Ni,Al-hydroxycarbonate,

similar

Indeed the DTA-TG diagrams

temperatures

supported

probably

of NiO, because

sample heated

of the

up to

lines, but no lines

110

Ni2A1-CO3

Ni,Al-CO,/SiC,

773

K

50 Fig. 1. X-ray powder diffraction patterns for (a) Ni2Al-hydroxycarbonate, Ni Al-hydroxycarbonate supported on SiO2, (c) calcined at 773 K and (d) cafcined at 1373 K (0 SiO2 ; * NiO).

corresponding compound),

to the spine1 NiA1204

probably

because

(present

NiA1204

(h)

in the lines of the unsupported

is present

in small amount

(the NiO/NiAl204

ratio is 3/l, as shown in [6] ). The marked prompted

structural

differences

us to check for potential

between

samples heated at 773 K and 1373 K

differences

in their catalytic

though the activity

of the catalysts

strongly

by the very low surface area

influenced

On the basis of the results silica-alumina towards

catalysts

oligomerization

reducibility reduction

[15,16], which

of the precursors

heated

nickel

these temperatures. 773 K allows

of monovalent

reduction

steps correspond

maxima

centered

conducted

nickel

under UV irradiation

between

activity

ions, we tested the

up to 1373 K, by means of TPR runs. The by decomposition obtained

up to 773 K occurred

by the decomposition

silica nor alumina were reduced of two reduction

proceeds

process

by hydrogen

on two different

in two different

and subsequently

up to

at

steps in samples heated up to

of metallic

to two peaks, overlapped

respectively

for nickel oxide on

a relationship

of Ni(I1)

that nickel oxide is attached

that the reduction

formation

heated up to 1373 K was

step, as shown in Fig. 2. The reduction

The presence

one to assume

or more likely

indicate

that of the catalysts

oxide as neither

even

(l-2 m2/g).

in the literature

samples obtained

1373 K took place in a single involved

reported

and the reducibility

of the catalyst

in two steps, while

from the precursor

activity,

nickel.

The two

to a certain extent,

at 592 K and at 740 K. The reduction from the liquid nitrogen

sites

steps: first by the

with

was also

temperature

up to about

573

673

773

873

I

T

(K)

*

7

473

67?

Fig. 2. T.P.R. diagrams 1373 K (b).

of a catalyst

873

T

(K)

sample heated up to 773 K (a), and

, 123 Fig. 3. U.V. stimulated

T.P.R.

223

323

diagram

of a catalyst

T(K)

sample heated up to 773 K.

6

323 K. It was found to occur in a single step between 133 K and 323 K,as shown in Fig. 3. The presence of monovalent nickel on the catalysts surface was checked in reduced samples hu means of ERR experiments. In Fig. 4 is shown the ESR spectrum of a sampIe reduced at 573 K for 2 h
reduction can be ascribed to the process: Ni2+ f e* + Ni+. It is noteworthy

that the catalysts obtained by decomposition up to 1373 K do not show, after the single step reduction, the presence of:monovalent nickel: this is a further

proof of

the

struftural

differences already evidencedzdAccording to th0

literature \15,16,20] the presence of MI/I) on the catalyst surface plays

a

decisive role in rendering the catalyst appreciably active in the olefin oligomerization.Then, the samples decamposed up to 773 K were pretreated under reducing conditions {fixed in a flow of 33% hydrogen in helium) at a temperature sufficiently high

t5

ensure

-t&e

presence

of

Ni
species

free

from metal

nickel, We carried out also a reducing pretreatment in the same

flaw

conditions

Fig. 4 RSR spectrum of a catalyst Sample pretreated at 573X far 3 h in helium plus hydrogen (T-77 K; Scan Range-500 C; Mid Range=3JOQ G)

under UV irradiation, at a temperature ensuring the presence of Ni(1): in this case we opted for a temperature programmed pretreatment at a low heating rate (2 K/min) from the liquid nitrogen temperature to room temperature. The activity of the catalysts was initially tested in the temperature range 523-673 K, but we decided to investigate the catalytic activity at 573 K, the temperature resulting in the best ratio of activity to selectivity towards the dimerization. The activity of the catalysts obtained by decomposition of the precursors at 773 K was rather significant: conversions of about 35% were achieved by using either ethylene, propylene or isobutene, even if slightly decreasing in the order here reported of the hydrocarbons used: small amounts of cracking and metathesis products and a significant amount of the dimerization products were obtained. Thus the reaction of propylene on 9% Ni0/3% A1203-Si02 yielded 20.2% of cracking plus Imetathesisproducts (ethylene, 1-butene, 2-e

and

Z-trans butenes) and 79.8% of dimerization products (hexene and methylpentenes). The catalysts reduced by employing UV radiation at low temperatures also showed activities of this order (see Table I). We should note that the amount of cracking products present in ;okcreaction effluents, though small, can be accounted for on the basis of the silica alumina content in the catalyst.

Table 1. Propylene conversion on a hydrotalcite-likecompound containing (9XNiO + 3% A1203)/Si02.

Type

of

catalyst Activity (%)

Thermally reduced sample

uv

reduced sample

Selectivity (%) to Selectivity (%) to methylpentenes+hexenes metathesis+craclingpro.

34.5

78.9

20.2

33.2

81.0

19.0

(amount of catalyst: .5 g; flow rates: He = 15 ml.fmin. ; reaction temperature = = 573 K)

It must be underlined here that we also tested the catalytic activity towards the propylene catalyst result

oligomerization

completely

reduced

of urn-educed 9% Ni0/3%A1203-Si02 but these experiments

did not show any measurable

in terms of activity.

On the other hand, 1373 K was completely decisive

the catalyst inactive:

for this decrease

obtained

obviously

of catalytic

be due also to the lack of monovalent programmed evidence

reduction

of Ni(1)

occurs

with adsorbed

the precursor

as evidenced

at

area is

but we suppose

propylene

were performed

of these catalysts

that this can

by the fact that its shows no

adsorption

process.

and rather

somewhat

low to minimize

Indeed,

in order to

(particularly

up to 773 K) with propylene.

unfavoured

those obtained

The adsorption

to weakly

the propylene

reaction

in the first case, chemisorption

is characterized

at low temperatures

adsorbed

propylene,

was realized

during

(maximum

the

was kinetically consist

centered

while that obtained

of two

at about at higher

by a small area due to the small extent of

7

473 Fig. 5. T.P.D. diagrams 513 K (b), on thermally

673 pf propylene adsorbed reduced samples.

by

lower than that of the reaction,

slow: as shown in Fig. 5a, the TPD diagrams

main peaks. The peak appearing

temperatures

its very small surface

activity,

nickel,

or at a temperature

sufficiently

353 K) corresponds

of the precursor

in a single step and that its ESR analysis

the interactions

at room temperature but however

by decomposition

species.

TPD experiments characterize heating

and of the same

873

1073

at room temperature

T(K)

(a), and at

adsorption,

desorbed

at high temperatures

was conducted

This peak is somewhat

reactions,

than that obtained

The propylene

reinforces similar

desorption

TPD diagrams

as

Further

and aluminum propylene

related

propylene

treatment

obtained

for the catalyst

except

to be active

have the and this

and UV irradiation

lead to

TPD experiments

products

TPD experiments,

in these oligomerization

activity

runs indicate

indeed only a low conversion

involving

ammonia

adsorption

on the catalyst

The desorptograms

under TPD conditions)

activity

measured

in a helium

of 22.5% and a selectivity

indicates

that at the reaction affects

on catalyst

lower catalytic

towards

temperatures,

the catalytic

out. Carbon monoxide

broadened poisoned

one at by

temperature

activity,

namely

(i.e. a

of 72.3%. This

In other words

affect

of Bronsted

[21], only the weakest

based on the adsorption

with silica

the small amount of residual

activity.

of the participation

in literature

samples

C6 products

acid sites does not decisively

in the hypothesis

(Fig. 6) showed

performed

stream up to the reaction

showed a somewhat

conversion

were also carried

of

as catalyst

samples were

obtained

one, small, at about 565 K, and a flattened,

and then washed

TPD experiments,

K.

that no

of interest.

913-923 K. The catalytic

moderately

of nickel

was found, when using silica-alumina

out at room temperature.

another

with

out for this purpose

that the presence

a large peak at 343-353 K (also found in the experiments

of the strongest

the role played by

small peak in the range 923-1023

occurs on silica-alumina:

to cracking

as reported

and cracking

show no peak due to any form of chemisorbed

with the fact that catalytic

at the temperatures

and,

on the

or at 513 K)

catalysts

runs were carried

confirm

for a broad, flattened

This is consistent

process.

activity

propylene,

ammonia

at room

adsorption

catalysts

reduced

in order to clarify

ions in the catalytic

indeed the TPD diagrams

ammonia

found when the

to dimerization

to the photoreduced

were performed

reactions:

alumina),

is

at those temperatures.

that both thermal

The results

oxide is required

then carried

was obviously

at room temperature

to the thermally

and catalytic

on silica-alumina.

propylene

which

structures.

experiments

oligomerization

at about

propylene,

by adsorption

peak is associated

those related

the hypothesis

surface

adsorbed

(adsorption

the latter being unavoidable,

same features

centered

rather lower than that of reaction.

faster and more extensive

(Fig. 5b). In both cases

the high temperature

nickel

larger

chemisorbed

to high temperatures

at temperatures

indicating

temperature,

to strongly

with a reaction.

Only the peak corresponding adsorption

surface

it has a maximum

and, though broad and diffuse,

903 K. This second peak corresponds

the poisoning

the catalytic

activity,

sites to the reaction,

ones play an important

of carbon monoxide

role.

at room temperature,

is a strong Lewis base and has

I

423 Fig. 6. T.P.D.

diagram

473

1

623 of ammonia

673

-

823 adsorbed

1023

T (K)

at room temperature.

873

Fig. 7. T.P.D. diagrams of CO adsorbed at room temperature reduced sample (a). and on a photoreduced sample (b):

1073

w T (K)

on a thermally

interesting bonding properties to nickel: indeed, the presence of [Ni(CO)n]+ complexes was observed on photoreduction af silica supported nickel catalysts by carbon monoxide 122,231, testifying the occurrence and the involvement of metal sites or reduced

metal ion sites in the reaction.

The carbon monoxide desorptograms obtained upon thermal and UV stimulated reduction are shown in Fig. 7, where it is possible to see that strongly chemisorbod CO is still present on the catalyst surface at:573 K (the oligomerization temperature). As expected, catalytic activity measurements on catalysts poisoned with carbon monofid.@at room temperatuxe, and subsequently heated up to the reaction temperature h

a helium stream, showed very low values

of activity (conversion of 2%). This seems to support the hypothesis that dimerization is controlled by reduced nickel species, which occur to agreater or lesser extent in the active phase structure.

CONCLUSIONS On the basis of all our experimental,results it seems logical_to think that the thermal decomposition af the precursor, performed up to 773 K, and the subsequent reducing pretreatment can activate its laminar structure, which shares many common properties with the unsupported compound. In fact it is significant that this active phase can accomodate nickel icn sites which csn be partially reduced and can play a decisive role in the reaction mechanism, while the strongest Bronsted sires of silica a&

alumina seem to be less influential.

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