SMSI effect in the butadiene hydrogenation on Pd-Cu bimetallic catalysts

407

CatalysisToday, 16 (1993)407-415 Elsevier Science Publishers B.V., Amsterdam

SMSI effect in the butadiene bimetallic catalysts M.M.Pereira,

on Pd-Cu

hydrogenation

F. B. Noronha and M.Schmal*

COPPE/NUCAT,Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brasil, fax (55-2112906626.

C.P.

68502,

CEP 21945,

Abstract Niobia Pd-Cu supported bimetallic catalysts have been studied in the 1,3-butadiene hydrogenation. On catalysts reduced at 573K, the addition of copper to palladium decreased the hydrogen adsorption capacity and the These turnover frequency but increased the trans/cis 2-butene ratio. results are ascribed to a bimetallic formation. After reduction at 773K. the hydrogen chemisorption and turnover frequency are drastically reduced due to SMSI effect. The 1,3-butadiene hydrogenation seems to be a structure sensitive reaction.

1. INTRODUCTION Since

the term “strong

by Tauster

et

chemisorption (titania,

influenced influence structure behavior

[1,21 a

niobial,

has received catalytic

al on

metal-support

to

describe

group

after

VIII

high

much attention

activity by on

the

the

of

reactions of

the

was introduced

suppression on

reduction

In addition,

[3-81.

of

(HTR),

it

has been shown that

TiOz supported

metals

the

SMSI effect

but

only

14,121.

Haller

a group

oxide

the SMSI effect

reactions

adding

Hz and CO

reducible

a

In general,

sensitive

effect

strong supported

temperature

SMSI [9-111.

insensitive with

the

metal

and selectivity

structure

interaction”(SMSI1

et

a

al.

Ib metal

has

minor [41

to

be

a great

effect

compared

group

the

could

VIII

on this metal

catalysts. In reaction is

contrast [4,6],

to

controversial.

turnover

hydrocarbon

the effect

frequency

hydrogenolysis,

of particle al.

size [131

a

structure

observed

Boitiaux

et

(TOFl for

the 1,3-butadiene

a

marked

hydrogenation

sensitive

hydrogenation

on the butadiene

decrease

reaction

0920-5861/93/$6.00 0 1993Elsevier Science Publishers B.V. All rights reserved.

of when

408 the metallic dispersion of Pd/AlsOa catalyst was increased. On the other hand,

Borgna

et

al.

1141 did

not

verify

such correlation

between

the

particle size and the activity/selectivity for this reaction. The addition of

chromium

decreased

the

catalytic

activity

the

of

bimetallic

palladium-chromium catalyst. However, these results were interpreted by a modification of the metal-unsaturated hydrocarbon

interaction induced by

ligand-effect. The aim of this work was to study the behavior of palladium catalysts, modified both by the addition of copper and by the SMSI effect, on the 1,3-butadiene hydrogenation. Our results allow to elucidate the sensibility of this reaction.

2.EXPEHIMENTAL 2.1. Catalyst preparation The NbaOs support (BET area: 30 m2/gl was obtained by calcination of niobic acid (CBMM, AD 376) in air at 773K for

2 hours. The catalysts were

prepared by incipient wetness impregnation of the support with

an

aqueous

solution of palladium chloride and copper chloride 1151, followed by drying at 393K for 16h and calcination at 673K for 2h. The prepared catalysts and their metal contents measured by atomic absorption spectroscopy are given in table 1.

2.2. Hydrogen chemisorption The frontal technique has been used for the hydrogen chemisorption, following a methodology similar to that presented in 1151. After reduction, the

catalyst

was

flushed

with

an

argon

gas

temperature for 30 min. Then, the catalyst was

flow

at

the

reduction

cooled to 343K and the

irreversible amount of adsorbed hydrogen was measured by a frontal method 1161. This [171.

adsorption

temperature was

chosen

to avoid B-PdH

x

formation

409

TABLE 1 Catalysts metal contents (xPd-vCu:x/r represents the theorical atomic ratio between Pd and Cu), hydrogen chemisorption and ratio of hydrogen adsorption after reduction at 573 and 773K.

Catalyst

Composition

Hz Chemisorption

(wt.-%) Pd

RCb

(pmolsH /ingPdl

cu

573K"

773Ka

0

2.08

0.62

3.35

Pd/NbsOs

0.87

lPd-2WNbaOs

0.89

1

0.52

0.22

2.36

lPd-5Cu/NbaOs

0.83

2.8

0.43

0.24

1.79

a reduction temperatures b RC= ratio of hydrogen adsorption amounts, after reduction at 573 and 773K

2.3. Catalytic activity The hydrogenation of 1,3-butadiene was performed in a flow system at atmospheric

pressure and 343K. The

catalyst

(ca. 10 mg), mixed with a

silica diluent (catalyst/diluent ratio : l/30), were reduced "in situ" with a mixture of 1.5% hydrogen in nitrogen at 573 or 773K. The reaction mixture consisted of 1,3-butadiene/hydrogen/nitrogen (10:10:80) and the conditions were

established

in order

products were analysed by

to keep

conversions below

12%. The

reaction

on line gas chromatography (VARIAN 2400, with a

80/100 Carbopack C/ 0.19% picric acid column at 313Kl. It was verified that the reaction is not limited by a diffusion process. The activities expressed in turnover frequency (TOFU were calculated from

the hydrogen

hydrogenation

chemisorption

results. The

selectivities for partial

(Shpl, 1-butene (S1l and the trans/cis 2-butene ratio were

determined as described in [141. All selectivities were obtained at the same conversion and temperature (iso-conversion).

410

3.RESULTS

3.1. Hydrogen

chemisorption

The amount of Ha irreversibly adsorbed on the catalysts, reduced at 573 and 7733, are shown present

significant

in table 1. The supported copper catalyst did not

irreversible

chemisorption

of

hydrogen.

In

the

bimetallic catalyst, an increase in the copper content and in the reduction temperature caused a decrease in the amount of hydrogen chemisorbed.

3.2. Catalytic test The catalytic activity (TOF) and selectivities are presented in table 2.

The

turnover

frequencies

decrease with

both

copper

addition

(after

reduction at 573K) and reduction at high temperature (HTR). The supported copper catalyst was virtually inactive in the temperature studied. The

1-butene selectivity and trans/cis 2-butene ratio increase for

higher copper content reduction at high

(after reduction at 57331 but do not change with

temperature. No n-butane was found in all bimetallic

catalysts.

Table 2 Turnover frequency (TOF), selectivities for I-butene (S1l, partial hydrogenation(Shp) and the translcis

2-butene ratio for niobia supported catalysts

after reduction at 573 and 773K.

Catalyst

S1(%l

S&(%1

t/c

TOF(s-'1

573

773

573

773

Pd/NbsOs

55

55

96

88

5.8

4.7

5.97

0.23

lPd-2Cu/NbaOs

58

71

99

100

7.2

9.2

3.00

1.35

lPd-5Cu/NbaOs

66

67

100

100

8.4

8.1

1.32

0.08

573

773

573

773

411 4. Discussion

SMSI state The suppression of hydrogen adsorption on Pd/NbzOs catalyst (table 1) after

high

temperature

reduction

(773KI

is

the

main

feature

of

the

so-called SMSI effect [1,21 and is consistent with previous works [15,181. Hu et al [19] have also reported that the hydrogen chemisorption and the catalytic activity for ethane hydrogenolysis were strongly suppressed by HTR.

These

species

behavior

were

(NbOxI which

explained by

cover

the presence

the surface of

of

reduced

rhodium particle

niobia

(geometric

effect). According

to

the

decoration model

[4,11,19,201, the presence of

these species on the metal surface would physically block active sites, preferentially

affecting those reactions that require a large ensemble of

atoms as active sites. Hence, HTR leads to a strong activity suppression for structure sensitive reactions. The

literature

[13,211 presents contradictory interpretations about

the effect of particle size on the 1,3-butadiene hydrogenation. On alumina and silica supported catalysts, Boitiaux et al [131 found a great decrease in the turnover number for 1,3-butadiene hydrogenation, when the dispersion was increased. Tardy et al

1211 performed this reaction on Pd particles

evaporated on carbonaceous supports and observed that it appears to be very size sensitive. They also observed that the catalytic activity increases when the particle size increases. In

this

1,3-butadiene drastically

work,

we

observe

hydrogenation after

on

H'IR. Then,

that

the

Pd/NbzOs we

specific catalyst

postulate

activity (Table

that

the

(TOF)

21

for

decrease

1,3-butadiene

hydrogenation is a structure sensitive reaction. A geometric explanation of the SMSI effect is also reinforced by the selectivities results. After high temperature reduction, the trans/cis 2butene ratio and the 1-butene selectivity are approximately constant, which excludes

an

electronic

explanation. However,

influnce of localized electronic effects.

we

can

not

rule

out

the

412 Metal-metal interactions

For the bimetallic decreased

catalysts reduced at 573K, the Presence of copper

the amount of irreversibly chemisorbed hydrogen. These results

are a clear evidence of bimetallic particle formation 122-241 and confirms our previous between

work

group

[151. Many

VIII-group

authors

Ib metals

[22-251 explained

in bimetallic

the interaction

systems by

geometric

effects, i.e., dilution or blocking of a fraction of the palladium surface by copper. Recently,

Leon y Leon and Vannice [221 studied the adsorption

properties of Pd-Cu/SiOz systems and observed also that the addition of copper to Pd/SiOz decreased the hydrogen and carbon monoxide chemisorption. This behavior was ascribed to a palladium-copper interaction. The geometric dilution of the palladium surface by copper was also observed by Noronha et al monoxide

adsorption

[151, by the decrease of hydrogen and carbon as

capacities,

well

as

an

increase

in

the

ratio

linear/bridged adsorbed carbon monoxide species. The catalytic activity and the selectivities results also confirm the bimetallic formation. According to a

second

metal

decreases

the

Haller and Resasco [41, the addition of

catalytic

activity

(TOF)

of

structure

sensitive reactions. From table 2, the turnover frequencies decreased when the copper content increased, after reduction at 573K. These results lead us to postulate that 1,3-butadiene hydrogenation is a structure sensitive reaction and agree fairly well with the chemisorption results. On the other hand, a pure geometric effect cannot explain the increase of

the

selectivity

for

1-butene

and

the

trans/cis

ratio

with

copper

addition. A modification of the electronic structure of palladium is thus certainly involved. Examining

the

hydrogen

chemisorption

results

of

the

bimetallic

catalysts (after reduction at 573K), we observe that the dispersion of this catalysts is approximately constant. In spite of this, the distribution of butenes

is

strongly

different.

The

lPd-5Cu/Nb20s catalyst

presented

S1

selectivity and trans/cis ratio higher than in the lPd-2Cu/Nbz0s catalyst. Moreover, all the bimetallic catalysts were fully selective for the partial hydrogenation, whereas the Pd/Nbz05 catalyst was only 90% selective. Such a modification of selectivity induced by an electronic effect has been already reported by Borgna et al.[141. They investigated the effect of chromium on the reactivity of palladium in the 1,3-butadiene hydrogenation.

413 The

addition

of

hydrogenation

chromium

and

the

increased

trans/cis

both

the

2-butene

seletivity

ratio.

These

for

partial were

results

interpreted by a modification of the electronic structure of palladium. From EXAFS analysis they showed a transfer of charge to the Pd 4d band due to the presence of Cr atoms. This

increase of the electron density of

palladium would induce a change in seletivity, since the bond strength of the hydrocarbons are affected differently. Recently, Noronha et al.[lSI have also suggested a modification of the eletronic structure of palladium

in Pd-Cu/NbaOs catalysts. According to

this work, copper decreased the adsorption strength of carbon monoxide on palladium, probably induced by an electron enrichment of palladium in the presence of copper. These results could be

interpreted by an eletronic

modification of the active sites, that is, the electron densities of Pd metal are increased by the electron transfer from copper. The effect of copper addition on the bimetallic catalysts reduced at 773K can be well understood by the ratio of hydrogen adsorbed amounts after reduction at 573 and 773K. This ratio must be higher than one [151, as the suppressed.

hydrogen chemisorption after reduction at 773K is strongly

However, this ratio decreases when the copper content increases. Noronha et al.[lSI also observed similar behavior in Pd-Cu/NbaOs catalysts (2 wt.% of palladium) but this ratio decreased more drastically. They proposed that the addition possibility agreement

of of

with

copper

to Pd/NbaOs catalysts

SMSI

formation.

these

results.

The On

turnover

limits or frequencies

Pd/NbaOs catalyst,

suppresses are

the TOF

also

the in

decreases

drastically, after HTR, due to the SMSI effect. The presence of copper inhibits

the

possibility

of

SMSI

formation.

Therefore,

the

TOF

after

reduction at 573 and 773K were approximately the same for the lPd-2Cu/NbsOs catalyst. However, the strong decrease of TOF observed for lPd-5Cu/NbaOs catalyst suggests that addition of extra Cu could not only suppress the SMSI formation but also principaly block the active sites.

CONCLUSIONS

In

all

catalysts,

the

specific

activity

for

1,3

butadiene

hydrogenation decreased after HTR, which confirmed that this reaction is structure sensitive.

414 The the

addition

trans-cis

of

ratio.

copper

improved the

Moreover,

the

selectivity

bimetallic

for

catalysts

l-butene and were

fully

selective for the partial hydrogenation. SMSI effects were attributed to both eletronic and geometric effects. However, the presence of copper inhibits or supress the SMSI formation.

Ackowledgment

This work has been supported by PRONAC (Programa National de Catalisel and

FINEP

(Financiadora

de

Estudos

e

Projetosl.

MMP

thanks

FCC

for

financial support. We are grateful to CBMM for the niobia supply.

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