Al2O3 Hydrogenation Catalysts with Lead by Using controlled surface reactions

M. Guisnet et al. (Editors), Heterogeneous Cetelvsis and Fine Chemicals © 1988 Elsevier Science Publishers BV., Amsterdam - Printed in The Netherlands

MODIFICATION OF Ni/A1203 HYDROGENATION CATALYSTS WITH LEAD BY SURFACE REACTIONS

145

USING CONTROLLED

" J.L.MARGITFALVI, S.GoBOLOS, M.HEGEDOS and E.TALAS

Central Research Institute for Chemistry of the Hungarian Academy 1525 BudapBst, PoB 17, Hungary

of SciBnces,

ABSTRACT In the present work alumina supported nickel catalysts modified by lead were used for the selective hydrogenation of acrylonitrile to propionitrile. The unmodified nickel catalyst was not selective in this reaction. The modification of the Ni/A120 3 catalyst, i.e. the preparation of Pb-Ni/A1 20 3 catalysts was carried out by using Controlled Surface Reactions (CSRs). CSRs applied in this work are based on the reactivity of hydrogen adsorbed on the nickel. This hydrogBn reacts selectively with tetraethyl lead resulting in bimetallic surface entities. Experimental results obtained both in the surface reactions involved in the modification of the catalyst and in the study of selective hydrogenation of acrylonitrile strongly indicate that catalyst modification via CSRs is a powerful tool to design selective hydrogenation catalysts. INTRODUCTION Raney type and alumina supported nickel catalysts are widely used in hydrogenation and reductive amination reactions to produce different

types

of

fine

chemicals (1). However in some hydrogenation and amination reactions the selectivity of nickel catalysts is not satisfactory (2). The selectivity pattern

of

nickel based catalysts can be improved by addition of well known catalyst poisons (Bi, Sn, Pb, S, As, P, V e.t.c.) (3). Impregnation with the solution of the poisoning compound is considered as the most general way to prepare selectively poisoned nickel catalysts. However, upon using conventional imprBgnation techniques the optimal selective poisoning of the most active sites of nickel cannot be guaranteed and part of the poisoning compound will

adsorb on the support

de-

creasing the efficiency of poisoning. Our approach used to modify different types of transition metal catalysts (e.g. Pt, Pd, Ni) is based on an anchoring process, in which Controlled Surface Reactions (CSRs) are used to anchore the precursor of the second metal to the surface of the first one (4). This new anchoring process is completely different from those used in previous approaches (5). CSRs used in our approach are based on the reactivity of tin and lead alkyls with hydrogen adsorbed on

Group VIII

metals (4,6). The use of tin alkyls to modify silica supported rhodium catalysts has already been mentioned, however, without claiming CSR with involvement of adsorbed hydrogen (7,8),

146

In this paper an example will be given for controlled selective poisoning of a commercial Ni/Al catalyst used in the preparation of propionitrile (PN) Z03 from acrylonitrile (AN) (See Scheme 1.J. This work was aimed to suppress both hydrogenation of the nitrile the formation of n-propyliJl'line (NPA) that of the secondary and tertiary amines from NPA (see Scheme 1.J. +H~

CHZ=CH-CN ~

+H Z -

CH3-CHZ-CI~

group

and

metals

can

CH 3-CHZ-CH=NH--

Scheme 1. SURFACE CHEMISTRY Recently we have demonstrated that hydrogen preadsorbed provide the driving

force for the

on the

formation of supported bimetallic

entities

with direct metal-metal interaction (4, 5). By suppressing side reactions, in which the surface OH groups will react with the precursor of the second metal, exlusive formation of bimetallic surface entities can be guaranteed. The formation and stabilization of the supported bimetallic Pb-Ni species was carried out by using CSRs as follows: xNi-H +Pb(C2H5)4 -

Nix-Pb(C 2H5) (4-x) + x C2H5

(1)

I + 4-x H

Ni -Pb

(2)

2

2

I

_

x

+

(4-x) C 2H5

In reaction (1) hydrogen adsorbed on nickel reacts selectively with the lead organic compound resulting in a primary surface complex (I) with metal-metal interaction. The latter is decomposed and stabilized in hydrogen atmosphere in the temperature interval between 50-250

0C.

In reactions (1) and (2) the formation of

ethane is the indication for the control of surface reactions. Characterization of surface species formed in reactions (1) and (2) will be given elsewhere (9). EXPERIMENTAL Catalyst preparation and characterization In this work a commercial CO poisoned Ni/AI

catalyst containing 50 w% Ni 203 with a cylindrical shape (diameter: 3.5 mm, length: 10 mm) was used. Prior to reaction (1) the catalyst was treated in HZ at 150°C for 5 hours, followed by different cooling procedures with cooling rate: 10C/min, and purged with nitrogen at

room temperature for 15 minutes. All of the experiments and procedures

were carried out under oxygen free conditions. Reaction (1) using n-hexane solvent was studied in a batch reactor, and was monitored by gas volumetry

as and

GC analysis as described elsewhere (5). The decomposition of surface complex (I)

147

(reaction (2J) was studied by Temperature Prograrrmed Reaction (TPPJ technicue. Ciltalysts prepared via CSRs will be denoted as C type two series of catalysts (A and 8 typeJ were

In aedition,

catalys~s.

prepared by adsorotion

J

on Ni!A1 used without hydrogen pretreatment. The lead content of catalysts 203 was determined by Atomic Absorption Spectroscopy (AASJ. The amount of hydrogen

t i r.r, (TPC) technique. Profile analysis of catalysts was carried out by electron microscopy using an energy dispersive X-ray analyzer (EDAX). Jdsorbed on catalysts was determined by Temperature-Programned

_'_0'-;;-:

Hydrogenation of acrylonitrile A conventional trickle bed reactor charged with 10 g

catalyst

was used

to

study selective hydrogenation of AN in n-hexane solution in the temperature and 0

pressure range of 80-140 C and 1-25 bar, respectively. Reaction products

were

analysed by GC using a Carbowax type column (18 % Carbowax 2000 + 5 % KOH

on

Chromosorb P NAW supportJ. The

following

reaction

products

were

analysed:

propionitrile (PN), n-propylimine (NPI), n-propylamine (NPA), di-n-propylamine (DNPA) and tri-n-propylamine (TNPA). RESULTS AND DISCUSSION Surface reactions Prior to CSRs the catalyst was treated in flowing

0

hydrogen at 150 C to

re-

mowe preadsorbed CO and to introduce one of the reaction partners, i.e. adsorbed hydrogen. As shown in Fig.

the Ni!A1203 catalyst strongly depends on the duration of the hydrogen treatment. The nega1

the amount of hydrogen desorbed from

0

tive peaks around 290 C are attributed to the consumption of adsorbed hydrogen in surface reactions, in which chemisorbed CO and oxygen can be involved. The fact, that upon increasing the duration of the hydrogen treatment at 150 0 C the total amount of hydrogen chemisorbed has

strongly increased with parallel

de-

crease of the negative peak, is considered as an indirect evidence for the relatively labile nature of the surface species discussed. It also means that the presence of chemisorbed CO rather than oxygen is responsible for the negative 0

peak. According to TPD data the treatment of Ni!A1 catalyst in H2 ae 150 C 2D 3 for 6 hours is sufficient to introduce relatively large amount (0.12 mmol!gcat) of adsorbed hydrogen. The amount of hydrogen available for reaction (1) could also be changed

by

using different cooling procedures, i.e. cooling in hydrogen (I) or in nitrogen (II) atmosphere. Reaction (1) was studied in details after treating the catalyst in H2 at 15~ ethane,

for 6 hours and applying cooling procedures I and II. Kinetic curves of

ethylene and methane formation are shown in Fig 2 and Fig. 3. Product selectiv-

148

.

H2 des. a.u.

a)

ii ;i

i .

8

duration of treatment

·N I , 'I i , Ii ., I. !I ~ l• ;"I

6

',

4

•1

;I

i' .I

2

!I II

0

12 reaction products

Ni/Alz03

x

Hz

O.5h 1.0h 4.0h 60h

\

10-5 mol

10

8 6

\

\ ~

b) 76% Pb-Ni/AIZ03 ([3)

o

200

400 T, O(

40

60 t, min

Fig. 1. Temperature-Programmed Desoprtion of H2 from Ni!A120 3 CaJ and (b) catalysts after hydrogen treatment at 1500C.

Pb-Ni!A12~

Fig. 2. Formation of hydrocarbons under non selective condition of reaction (1). Amount of catalyst: 1 g; cooling procedure: II; reaction temperature: 2;oC; [Pb(C 2H sJ 4 JO ~ 0.28 mol.dm- 3 TABLE 1 Reaction of Pb(C2H5)4 with hydrogen adsorbed on Ni!A1 0 catalyst 2 3 NO

Cooling

a

procedure 1 2 3 4 5

I

II II II

I

°c

[Pb(C 2Hs h Jo mol. dm- 3

Gas evolution -5 mol. 10

50 50 27 27 27

0.28 0.26 0.26 0.015 0.015

29.5 64,4 24.1 5.6 3.9

T,

b

Selectivities CH

4

10 17 13

6 5

b,

%

C 2H6

C 2H4

36 29 40 60 89

54 54 47 14 6

aSee text; bobtained after 65 minutes of reaction time; amount of catalyst ~ 19;

Vn-hexane ~ 20 cm 3 .

ities obtained after 65 minutes and the total amount gases

evolved in the sur-

face reaction are given in Table 1. Data given in Table 1 and Fig. 2 and 3 dicate that both the reaction temperature and the initial concentration of ethyl lead strongly affect the selectivity of the surface reaction.

intetr~

The higher

reaction temperature and the higher the initial higher the total gas formation and

concentration of PbCC the 2H5)4 the lower is the selectivity towards ethane

formation. The formation of ethylene can be attributed to the catalytic decomposition of tetraethyl lead, while the appearance of methane nickel can hydrogenolyze the C-C bound in the Nix-Pb(C2H5J4_~oieties. temperature and at low initial concentration of tetraethyl

may

indicate

lead the

that

At room rates of

149

6 reaction products x10- 5 mol

5

Pb in catalysts, %

15

theoretical

4

10 3 2 o....-oCZH4 :/3::8::0-0 CH4 20 40 60 t, min

o

2 4 Lead introduced, g

6

Fig.3. Formation of hydrocarbons under selective condition of reaction (1).Conditions see Fig. 2; [PbCC 2H s ) 4 ] O = 0.015 mol.dm~3.

Fig.4. Introduction of lead to Ni/A1 20 3 catalyst by adsorption (0 A and 0 B) and CSRs ( C). Amount of catalyst: 20 g; volume of solution: 20 cm 3; temperatur~ 27°C; time of adsorption: 24 hours, time of CSR: 1 hour; cooling procedure: I.

C2H4 and methane formation were relatively low. The change in the cooling procedure resulted in only slight alteration in the selectivity of products formed. However, after applying cooling procedure II the total amount of gases evolved in surface reactions is significantly higher than that

after using co-

oling procedure I. The elucidation of these differences requires additional experiments.

In the system studied the formation of ethylene and methane

considered as used to prepare

indirect evidences for the loss of

control of surface

reaction

bimetallic surface species with direct metal4netal interaction.

Under condition of reaction (1) in addition to CSR lead on Ni/A1

can be

adsorption of tetraethyl

catalyst can also take place, especially at high initial con-

203 centration of Pb(C

Therefore, additional experiments were carried out to 2HS)4' study the adsorption of tetraethyl lead on Ni/A1 catalyst. Two sereies of 203 catalyst (A and B type) with different lead contents were prepared. Prior to the preparation of A type catalysts no pretreatment of Ni/A1 catalyst was used, 203 whereas prior to preparation of B type catalysts it was dried in nitrogen at 0C 3 150 for 6 hours. In these experiments 20 g catalyst in 20 cm n-hexane was

150

conc., a.u.

a

Pb-Ni/ Al203

catalysts

3

Pb

[1

2

o

Ni

o

b

100

200

T,

O[

Pb

o

Ni

o

Fig.5. Temperature-Programmed Reaction of surface species (I) (Sample C1) and adsorbed Pb(CzH S) 4 (Sample A5 and B6). Amount of catalyst: 1 g, heating rate = = 4°C/min. Fig. 6. Radial distribution of Pb and Ni in extrudates. Catalyst: Pb-Ni/Al z0 3 (C1). I and II are determined prior to and after TPR, respectively. TABLE 2

Hydrogenation of acrylonitrile on Ni!Al z0 3 and Pb-Ni/Al z0 3 catalysts a Selectivities b Catalysts Pb T, Conversion % w% °c % PN NPI ONPA TNPA NPA Ni/A1 0 B1 96.6 B3.9 5.9 B.B 1.4 0 203 104 99.9 0 53.7 10.2 23.9 9.9 135 100 1.3 0 51.2 49.9 6.1 Pb-Ni!A1 D A4 4.0 106 100 41.7 15.5 25.5 15.3 0.9 2 J A5 6.0 104 72.8 98.9 0 1 .1 0 0 B4 3.1 102 99.5 47.9 14.1 13.2 0.4 24.4 B5 4.3 105 98.1 84.7 2.7 0 7.2 5.7 B6 6.1 104 98.1 95.5 0.5 0 2.5 1.5 B7 9.9 102 81.0 100 0 0 0 0 C1 5.1 100 100 93.9 1.4 1.1 0 3.6 C3 7.6 99 100 0 93.2 1.2 1.8 3.8 C5 13.0 100 85.0 100 0 0 0 0 aTrickle-bed reactor: liquid flow rate 0.3 kg.kg-1.h- 1, 15 % acrylonitrile in hexane, gas flow-rate: 6.6 l.h- 1, P 15 bar. hydrogen bpN = propionitrile, NPI = n-propylimine, NPA = n-propylamine, ONPA di-n-propylamine, TNPA = tri-n-propylamine

151 treated with different quantities of Pb(C at ZSOC for hours. The differZHS)4 ences between the preparation methods are demonstrated in Fig.4 where the lead content of the catalysts determined after adsorption or CSRs is plotted against the amount of lead added to the n-hexane suspensions of Ni/AI catalysts. No Z03 difference in the lead content of A and 8 type catalysts can be observed. This indicates that drying in N (8 type catalysts) does not alter the adsorption of Z • The lead content of C Type catalysts is higher than those Pb(CZHS)4 on Ni/AI Z03 of A and 8 cetolystsindicating that CSR is a more efficient way to introduce lead than

the adsorption. Theoretical curve in Fig. 4 corresponds

to 100 % of

conversion of introduced Pb(C in surface reaction (1). ZHS)4 The decomposition of the primary formed surface complex I (reaction (Z)) and the Pb(CZHS)4 adsorbed on the Ni/AI was studied by TPR technique.TPR results, Z03 i.e. formation of ethane in reaction (Z), given in Fig. S indicate that primary formed surface complex (I) decomposes in HZ below 1S00C, while the decomposition of adsorbed Pb(CZHS)4 requires higher temperatures (see sample C1 versus AS and 86). The lower intensitiy of the TPR peaks of catalyst samples AS and 86 than that of the

sample C1 can be explained by the lower Pb(C content of samZHS)4 ples pepared via adsorption. In addition, in samples AS and 86 40-S0 % loss of lead was observed in the TPR experiment. This fact was attributed to desorption of adsorbed but unreacted Pb(CZHS)4' of Pb-Ni/A1 20 3 catalysts

Characterizatio~

Pb-Ni/AI

Z03

catalysts were characterized by hydrogen TPD after HZ pretreat-

ment at 1S00C for 6 hours. Hydrogen TPD obtained on catalyst C3 is shown in Fig. 1.b. TPD data indicate that introduction of lead in 7-8 w% significantly suppresses the amount of hydrogen adsorbed on nickel. This fact can be considered as an evidence for direct interaction between lead an nickel. Radial distribution of both nickel and

lead

in

the extrudates

of

S.1 %

Pb-Ni/AI Z03 (C1) catalyst is shown in Fig. S. Results in Fig.6 a indicate that there is a slight enrichment of lead in the shell of the extrudates in the Pb-Ni/AI Z03 catalyst prepared via CSRs. This, can, probably, be explained by certain pore diffusion limitation in the relatively fast surface reaction

Upon TPR Pb-Ni/AI

in

(1).

HZ (reaction (Z)) no change in the lead distribution of C type catalyst (see Fig. 6 b) was observed. This fact indicates the

Z03 thermal stability of the Pb-Ni!AI catalyst prepared via Controlled Z03 Reactions.

Surface

Hydrogenation of acrylonitrile Conversion and selectivity data obtained in hydrogenation of acrylonitrile in Ni/AI Z03 and different types of Pb-Ni!Al

Z03

catalysts are given in Table Z.

On

152

catalyst upon increasing the reaction temperature from 51 to 1

the

selectivity towards propionitrilB drops almost to zero with parallel increasc in the NPA and DNPA selectivities.

~esults

in Table 2 indicate that at least 5

lead is needed to achieve 90-95% selectivity towards propionitrils. However,

of it

can also be SBen that the increase of the lead content abovs a certain level sis nificantly decreases the hydrogenation activity. Data in Table 2 snow thae the optimum lead content, at which both conversion and propionitrile selectivity are close to 95-100%, strongly depends on the type of the Pb-Ni/A1 catalyst. The 203 best results, i.e. the highest selectivity for propionitrile and the highest hydrogenation activity, were obtained on C type catalysts prepared by CSRs. Our results confirmed that Pb-Ni/A1

catalyst prepared via CSRs are highly active 203 and selective in hydrogenation of acrylonitrile to propionitrile. CONCLUSIONS In this work it was demonstrated that Ni/A1 catalyst can be modified by 203 lead using a new approach, i.e. the application of Controlled Surface Reactions. This approach is based on the surface reaction between adsorbed

hydrogen

lead alkyls. In this way highly active and very selective bimetallic

and

catalysts

with metal-metal interaction can be prepared. Pb-Ni/A1

catalysts prepared in 2D3 this study were tailored for the selective hydrogenation of acrylonitrile to

propionitrile. ACKNOWLEDGEMENTS Partial financial support given by the Hungarian Academy of Sciences

under

Contract DTKA-1003 is greatly acknowledged. REFERENCES

2 3 4 5 6 7 8 9

J.Volf and J.Pasek, in L. Cerveny (Editor) Studies in Surface Sciences and Catalysis Vol. 27, Elsevier, Amsterdam, 1986, pp. 105-144. J. Pasek, N. Kostova and B. Dvorak, Collect. Czech. Chern. Commun., 46 (1981) 1011 L.L. Hegedus, in R.W. McCabe (Editor) Chemical Industries Series Vol. 17, Catalyst Poisoning, Dekker, New York, 1984. J. Margitfalvi, M. Hegedus, S. G6bolos, E. Kern-T~las, P. Szedlacsek S. Szab6 and F. Nagy, Proe. 8th International Congress on Catalysis, Vol. IV. Verlag Chemie, Weinheim, 1984, pp. 903-914. Yu.I. Yermakov, B.N.Kuznetsov, V.A. Zakharov, Studies in Surface Science and Catalysis, Vol. 8, Catalysis by Supported Complexes, Elsevier, Amsterdam, 1981. E. Kern-Talas, M. Hegedus, S. G6bolos, P. Szedlacsek and J. Margitfalvi, in B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet (Editors) Preparation of Catalysts IV. Elsevier, Amsterdam, 1987, pp. 689-700. Ch.Travers, J.P. Bournonville and G. Martino, Proc. 8th International Congress on Catalysis, Vol. IV. Verlag Chemie, Weinheim, 1984, pp. 891-902. J.P. Candy, O.A. Ferreti, J.P. Bournonville and G. Mabilon, J. Chem. Soc., Chern. Comm., (1985) 1197. J.L. Margitfalvi, E. Talas and I. Bertoti, in preparation.