Applied Catalyclis, 6 (1983) 41-51 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

41

HYDROGENATION OF HIGHLY UNSATURATED HYDROCARBONS OVER HIGHLY DISPERSED PALLADIUM

CATALYST. PART I: BEHAVIOUR OF SMALL METAL PARTICLES

J.P. BOITIAUX, J. COSYNS and S. VASUDEVAN

Institut Fransais du Petrole, Rueil-Malmaison, France.

(Received 12 October 1982, accepted 20 January 1983)

ABSTRACT

Hydrogenation of 1-butyne over palladium catalysts with dispersions between 9 and 100% reveals that the reaction is sensitive to metallic dispersion. A similar sensitivity is also exhibited by 1,3-butadiene and isoprene, whereas I-butene is insensitive to dispersion. This behaviour of the highly unsaturated hydrocarbons is explained by the formation of a strong complex of these hydro- carbons with very small particles at high dispersion.

INTRODUCTION

The selective hydrogenation of acetylenics (I-butyne and vinylacetylene) in the

1,3-butadiene rich cut of the steam cracking reaction is a commercially attractive

process for meeting the stringent specification of polymer grade butadiene.

Although the literature abounds with work on the selective hydrogenation of acet-

ylene in the ethylene cut [1,23, there are very few publications, excluding patents,

on the selective hydrogenation of C4 cuts [3,4]. Kinetic studies of I-butyne

hydrogenation have been reported by Mann et al. [5-73 over different metals of

Group VIII. This study was however performed under conditions far removed from

those of industrial operations. Furthermore, there has been no work reported on

the influence of metallic dispersion on the hydrogenation of highly unsaturated

hydrocarbons. In a recent publication, we have shown that the hydrogenation of

1,3-butadiene is strongly influenced by the particle size [8]. We observed a marked

decrease in the turnover number (by a factor of 12), when the metallic dispersion

was increased from 20 to 100%. In this article we report a study of the influence

of dispersion on the kinetics of hydrogenation of other unsaturated hydrocarbons,

notably I-butyne, in order to genaralize the special behaviour of the small

particles.

EXPERIMENTAL

Materials

The 1-butyne used in our study was procured from Air Liquide (>95% pure). This

was further purified by passage through a bed of 4A + 13X molecular sieves. The

1-butyne was diluted in n-heptane (>99.8% pure) in order to obtain a concentration

’ 0166~9834/83/$03.00 0 1983 Elsevier Science Publishers B.V.

TABLE

1

Sumnary

of the

different

catalysts

used.

wt%

Support

area

Reduction

in

Desorption

Dispersion

b Turnover no.

Pd

/m2

g-'

I-Butyne hydrog.

H2/=C

O,,

/nm

Butene hydrog.

under

N2/'C

I%

activity/m01

s -1

for

hydrog.

of

activity/m01

g Pd-'

I-butyne/s-'

s-1

-1

o Pd

1 0.37

A-9

700

2 0.33

A-69

250

3 0.33

A-69

500

4 0.76

A-104

700

5 0.33

A-69

250

6 0.37

A-9

350

7 0.76

A-104

300

8 0.33

A-69

250

9 0.33

A-69

300

10

0.20

A-94

300

11

0.33

A-69

350

12

1.06

s-210

400

13

1.06

s-210

700

14

1.06

S-210

700

700

600

500

700

500

350

300

300

300

350

400

700

700

9 11.23

0.0517

60.5

20

4.2

0.0906

60.4

27

2.83

0.0717

28.3

27

2.83

0.0916

36.2

37

1.91

0.0376

11.33

44

1.54

0.0759

18.33

60

1.04

0.0696

12.33

67

0.91

0.0394

6.17

81

0.72

0.0510

6.83

85

0.67

0.0489

6.17

100

0.40

0.0555

5.83

56

1.13

0.0288

5.50

37

1.91

0.0582

16.67

37

1.91

0.0872

25.33

0.074

0.108

0.159

0.159

0.211

0.169

0.271

0.448

aSupport A

= alumina, S

= silica.

b Particle diameter calculated by assuming that the

crystallite

has

cubic

form with on side in contact

with the

support.

'Specific

activity for

the

hydrogenation

of I-butyne determined

at 70%

conversion.

48

of approximately 10% by weight of I-butyne. The hydrogen was of ultra pure grade

(>99.999%) and was further purified in a deoxo unit followed by 4A molecular

sieves.

The two precursors used for the preparation of the catalysts were palladium

acetylacetonate [Pd (CSH702)2] for alumina impregnation and palladium tetramine

nitrate [Pd (NH3141 (NO312 for silica impregnation. These were obtained from

Johnson Matthey, Paris.

Two types of alumina, an a-alumina and a y+_-alumina with

surface areas between 9 and 100 m* g -1

were used as supports in this study. The

silica had a surface area of 210 m* g -1

and a pore volume of 0.76 ml g-l.

Catalysts

Palladium based catalysts have been used in this study. The method of preparation

and the conditions of treatment employed to obtain a high dispersion have been

described in detail elsewhere C81 and are merely summarized here as follows. The

catalyst was prepared by wet impregnation of the alumina carrier with a known

quantity of palladium acetylacetonate dissolved in benzene. It was oven dried at

llO"C, air calcined at 300°C and reduced with hydrogen at temperatures between

300 and 8OO"C, according to the metal dispersion required.

The palladium on silica catalyst was prepared using the palladium tetramine

complex as precursor, as described in the literature C91. This catalyst was

calcined at 400°C and then reduced at two different temperatures to give two

different dispersions.

Table 1 summarizes the different catalysts used in this study, their conditions

of treatment, the metallic dispersions obtained by CO chemisorption and the mean

crystallite sizes (calculated on the basis of a fee structure). Some of these

catalysts were examined in an electron microscope (CTEM Jeol-120 X) and the average

crystallite diameter obtained was in close agreement with the diameter calculated

from CO chemisorption values.

Kinetic tests

Hydrogenation reactions were carried out in the liquid phase, in batch mode

in a CSTR reactor at 2 MPa (20 bar) constant pressure. The hydrogenation rate was

followed by recording the pressure drop in a constant volume hydrogen ballast,

upstream of the pressure regulator, as a function of time. Cooling water was

circulated in the reactor jacket in order to maintain the temperature at 20°C.

The hydrocarbon samples were analysed in an Intersmat IGC 112F chromatograph

provided with a flame ionization detector, using a 6 m 66' ODPN supported on

chromosat PAW column at room temperature.

44

RESULTS

Hydrogenation of l-butyne

In Figure 1 is reported the hydrogen consumption as a function of time for the

hydrogenation of 1-butyne over two catalysts with 9 and 67% metallic dispersion,

respectively. In both cases the hydrogenation started inediately, without any

induction period, and the rate increased as the reaction progressed. As the

hydrogenation rate varies with conversion, the catalyst activities should be

compared at the same conversion. In our study the 1-butyne hydrogenation rates

were evaluated at 70% butyne conversion. Curves A and B of Figure 1 reveal a

negative effect of the dispersion: 1-butyne hydrogenated more quickly on low

dispersion than on high dispersion catalysts.

I !

10 20 30 Tii(mn1 Tie (mud

FIGURE 1 Consumption of hydrogen vs. time during the hydrogenation of I-butyne

and I-butene over two catalysts of 9 and 67% dispersion.

FIGURE 2 I-butyne consumption and evolution of different products over a catalyst

of 85% dispersion.

Figure 2 shows the change of the reaction mixture composition with time over

a catalyst of 85% dispersion. During the hydrogenation of 1-butyne, the only

product of reaction was I-butene (up to a conversion of roughly 80%) if we except

the very small amounts of butane formed right from the beginning. No parallel

isomerization to E-butyne occurred. Once the 1-butyne had almost completely

disappeared, the 1-butene was consumed with a higher rate than the I-butyne (cf.

Figure 1 also). This I-butene hydrogenated into butane and isomerized into 2-

butenes (cis and trans) in two parallel ways. Unlike 1 -butyne, the 1 -butene

reaction did not change with time, the apparent order of reaction being nearly

equal to zero. Lastly, the 2-butenes formed hydrogenated (cis and trans success-

45

ively), but with a very slow rate. Thus the overall reaction scheme could be

represented as:

I-butyne4 I-butene __+ 2-butenes . l .

l * J / butane

Influence of the dispersion on the activity

I-butyne hydrogenation. As is seen in Figure 1, a highly dispersed catalyst

(67%) had an activity which was considerably lower than a catalyst having 9%

dispersion. In order to verify this strange behaviour of the small particles, we

carried out a series of hydrogenation tests over the whole range of dispersion.

We report the activity for the hydrogenation of 1-butyne, expressed as turnover

number (moles of 1-butyne converted per atom of palladium exposed per second).

The turnover number was determined at 70% conversion of 1-butyne and is plotted

in Figure 3 as a function of dispersion. We observed from the curve that the

activity remained constant up to 20% dispersion and decreased sharply thereafter.

The residual activity for very high dispersions close to 100% was around 12 times

lower. The nature of this curve is similar to the variation of activity for the

1,3-butadiene hydrogenation which we have reported earlier C83. Furthermore, we

I I

20 40 60 60 100 DkpanionM

16utene~6lltane l 16UtElW

L: H I------ ~16utem?exl r I

, 0.6 l /

20 40 60 60 100 aspniolorl

FIGURE 3 Variation of the turnover number as a function of dispersion for l-

butyne hydrogenation.

FIGURE 4 Variation of the hydrogenation activity for 1-butene as a function of

dispersion.

verified the variation of turnover number of I-butyne hydrogenation at different

conversions and found that at iso conversion the nature of the curve remained

identical.

I-butene hydrogenation. Referring again to Figure 1, we observed that, contrary

to l-butyne, the rate of hydrogenation of 1-butene (formed from I-butyne) was

higher at higher dispersions. In Figure 4 we plot the rate of hydrogenation of

1-butene (in moles of I-butene hydrogenated per second per gram of palladium) as

a function of dispersion and we observed that the activity increased linearly with

dispersion (i.e. the turnover number remained constant with dispersion). In order

to compare this activity of I-butene (ex 1-butyne) with pure 1-butene, we have

also included the values of the activity of pure I-butene hydrogenation at differ-

ent dispersions in Figure 4 and we find that the values are similar and lie on

the same line. This confirms that the turnover number of I-butene was not influ-

enced by particle size nor by the previous hydrogenation of the acetylenic compound.

Influence of dispersion on the selectivity

1-butyne hydrogenation. As reported previously, some butane was formed from

the beginning of the reaction. The extent of this formation increased with dis-

persion as is seen in Table 2, where the formation of butane at 90 and 99%

conversion of I-butyne is indicated. This behaviour is in line with the observat-

ions that at higher dispersions the I-butene hydrogenation rate remained high,

whereas the 1-butyne hydrogenation rate decreased sharply.

TABLE 2

Dispersion/% 9 44 60 85 100

% butane at 90% I-butyne conversion 2.4 2.5 2.8 3.2

4: butane at 99% 1-butyne conversion 10.20 13.0 24 27

1-butene hydrogenation. Pure l-butene and l-butene formed during the hydrogen-

ation of 1-butyne disappeared with similar rates (Figure 4). Nevertheless, the

similarity of behaviour is no so apparent if we consider the selectivities. The

ratios butane/z Z-butenes and tr-2-butene/cis-2-butene are plotted in Figure 5

for both feedstocks. Two observations can be made: firstly, the selectivities of

the parallel reactions remain constant over the whole range of dispersion and,

secondly, there was a significant difference in the hydrogenation/isomerization

ratio between pure I-butene and I-butene ex 1-butyne (values being 0,95 and 2.0

respectively). This fact is perhaps due to some inhibition of the double bond shift

induced by trace amounts of 1-butyne remaining on the catalytic surface as it is

well known that double bond shift is strongly inhibited by the presence of diolefins

or acetylenes.

41

2.4. .

2.0 . . *

1.6. ‘. 2TrBut~ne/ZCi&utem<~ 2Trans.Butene/2Cia.Buten~

v 1.2. y

0.6. 1 ButendIZButenes

0.4. 2 l :; :;-. t . . , t

I I. * * c 3 ” e 1 I. I. 0.. c 6 1 10 20 30 40 50 60 70 60 90 100 10 20 30 40 50 60 70 60 90100

Dispersion Dispersion ("/.I

FIGURE 5 Selectivity as a function of dispersion during the hydrogenation of

1-butene (pure and ex 1-butyne).

FIGURE 6 Selectivity as a function of dispersion during the hydrogenation of

1,3-butadiene.

1,3-butadiene hydrogenation. The influence of dispersion on the rate of

butadiene hydrogenation and on the butyne hydrogenation rate has been shown to

be similar [S]. In Table 3 the formation of butane at 90 and 99% conversion

of butadiene is shown. The influence of the dispersion on the selectivity

for consecutive reactions is similar for butadiene and for butyne.

TABLE 3 ~~ ~

Dispersion/% 15 37 44 60 81

% butane at 90% 1,3-butadiene conversion 0.45 0.60 1.50 2.30 2.10

% butane at 99% 1,3-butadiene conversion 2.0 3.7 3.9 7.8 8.7

Contrasting with these results, the selectivities for parallel reactions

(tr-2-butene/cis-2-butene and I-butene/z P-butenes) do not change with the metallic

dispersion, as is clearly seen from Figure 6. The difference between the trans/cis

ratios of the P-butenes formed from 1-butene (%1.6) and butadiene (12.0) may be

attributed to the predominance of the anti form (95%) in the butadiene.

DISCUSSION

A brief summary of our results is as follows: the turnover number for the

hydrogenation of I-butene (pure or ex 1-butyne) is constant, irrespective of the

dispersion. However, the turnover number for the hydrogenation of higher unsatur-

ated hydrocarbons, such as 1,3-butadiene and l-butyne, is strongly influenced by

48

the average particle size. In addition to

hydrogenation of isoprene and again found

dependent on dispersion and the nature of

butadiene or 1-butyne.

SsgO 5!6 125

these hydrocarbons, we also studied the

that the turnover number was strongly

the curve was similar to that of 1,3-

412 1:8 110 017 Ihm m

0 20 40 60 80 100

Dispdon 0

FIGURE 7 Relative variation of turnover number with respect to dispersion for the

hydrogenation of 1-butene,m ;l-butyne,n ;1,3_butadiene,+ and isoprene, A .

In Figure 7 we have plotted the variation of the relative turnover number as

a function of dispersion. The relative turnover number is the value obtained by

dividing the turnover number by its maximum value for each hydrocarbon. Thus, for

each hydrocarbon the maximum turnover number is normalised to the value of unity.

This figure demonstrates that at higher dispersion there is a spectacular decrease

in the activity of the hydrogenation of highly unsaturated hydrocarbons by a factor

of around 12 to 15. It is worthwhile emphazing that the hydrogenation studies

were performed over three different types of support, a-A1203, yt-A1203 and Si02,

their surface areas varying between 9 and 200 m2 g-l. The metal loading was varied

between 0.17 and 1.06 wt% and three different metal precursors, Pd(N03)2,

Pd(C5H702j2 and [Pt(NH3j41 (N03J2 were used. In spite of all these variations, the

turnover number seems to be strongly influenced only by the metallic dispersion.

As far as the selectivities are concerned, the consecutive formation of butane

increases with dispersion, whereas the selectivity for all parallel formations of

unsaturated intermediates does not vary with dispersion.

Several hypotheses can be proposed to explain these effects. Some of these are

as follows:

Polymerization. The higher dispersions could promote the polymerization of the

highly unsaturated hydrocarbons that would partially plug the catalytic surface.

49

Such an hypothesis can be easily eliminated as consecutive hydrogenation runs over

the same catalyst have been made with good reproducibility. Moreover, such a

trivial modification would certainly affect the hydrogenation of 1-butene coming

from 1-butyne.

Reducibility. The sharp decrease of turnover number obtained for dispersions

higher than 20% can not be solely explained by the presence of unreduced palladium

at high dispersions. With this interpretation, it is very difficult to explain

such a large effect on diolefin and alkyne hydrogenation (turnover number divided

by 10) while I-butene is not affected. Moreover, all catalysts with high dispersion

were prereduced at a temperature equal to or higher than 25O'C and controlled by

CO chemisorption measurement at room temperature. The stoichiometry observed is

close to one CO per palladium atom [83. In addition. 02-H2 titrations [9] have

shown that 30, is adsorbed on one Pd atom and 3/2 H2 is consumed by one palladium

atom during titration of this oxygen. Such stoichiometries can be explained only

if the major part of the palladium is in the reduced form.

Geometric effects. Geometric effects due to multiple site adsorption of the

hydrocarbons [lo] could be invoked as an hypothesis to explain the decrease in

activity at high dispersion, as multiple site adsorption normally becomes more

difficult as dispersion increases. This

results, wherein we find that the three

I-butyne, which probably have different

same manner with respect to dispersion.

parallel reactions remain constant with

by this hypothesis.

effect can not satisfactorily explain our

hydrocarbons butadiene, isoprene and

geometries of adsorption, behave in the

Furthermore, the selectivities of all

dispersion, which is again hardly explained

Support effects. It has been demonstrated by certain authors [11-123 that the

small metal particles attain the bulk properties only when they contain 100-300

atoms (corresponding to a diameter of 20 A). This would correspond to a dispersion

of about 50%. Certain authors proposed Cl33 that the metal-support interaction

becomes strong when the metal crystallite hasveryfew atoms (less than 200). Our

results demonstrate that the most spectacular fall in activity occurs between 20

and 40% dispersion, which would correspond to a total number of 500-6000 atoms

in the crystallite. With such a large number of atoms present, one can assume that

the particles show bulk metallic properties. In such particles the proportion of

atoms directly influenced by the support must be very low. Hence, even if some

support effect exists, it alone cannot explain the spectacular decrease of turn-

over number observed at dispersions higher than 20%. Hence it is difficult to

attribute the low activity to these type of effects.

Intrinsic metallic properties of the metallic site

If geometric and support effects can be, to a first approximation, neglected,

it is logical to suppose that this variation in activity is due to changes in the

electronic properties of the metal as dispersion increases. At high metallic

50

dispersions the surface atoms of small metal particles present a high coordinative

unsaturation. Under these conditions they can coordinate more strongly with

different species. Our earlier work on the infrared study of CO adsorption showed

that there is a displacement of the bands towards lower frequencies as the

dispersion increases from 27 to 86%, thus confirming a reinforcement of the M-CO

bond at higher dispersion [S].

FIGURE 8 Infrared spectra of the hydrogenation of I-butyne over two different

catalysts. 1, introduction of I-butyne; 2, introduction of hydrogen; 3, after

5 min; 4, after 15 min; 4', after 12 h.

We also tried-to simulate the hydrogenation of I-butyne in the IR cell and the

result of this study is given in Figure 8. On introduction of I-butyne along with

a stream of nitrogen over a catalyst wafer, we observe a peak at 2970 cm -1 , characteristic of adsorbed I-butyne. Over a low dispersion catalyst this peak

disappears completely in about 15 min when hydrogen is introduced, whereas over

a highly dispersed catalyst this peak does not disappear even after 12 h, thus

indicating the formation of a strong complex over the highly dispersed catalyst.

This type of formation of a strong complex with acetylenics and diolefins is known

to occur with metal complexes in coordination chemistry [14]. Over such homogeneous

catalysts the activity for the hydrogenation of butadiene and I-butyne is known

to be very low [15,16]. Thus it is logical to consider the highly dispersed catalyst

as a supported homogeneous catalyst. This type of comparison has already been made

in the literature 1173. Hence we suppose that the low activity shown by the highly

dispersed metal might be due to its electron deficient character. The surface atoms

51

of small particles coordinate more strongly with species which are electron donors.

This has been shown previously over platinum [18]. In our case we can consider

that highly unsaturated hydrocarbons are electron donors, as assumed by several

authors [14], thus explaining the formation of their strong complexes.

If too strong complexation with highly unsaturated hydrocarbons is the reason

why turnover is low at high dispersion, it would be possible, as in homogeneous

catalysis, to increase the activity and selectivity by adding a convenient electron

donating additive (ligand). We will show in a later paper how a nitrogen compound,

such as piperidine, can produce such a ligand effect.

ACKNOWLEDGEMENT

One of the authors (S.V) wishes to thank Engineers India Ltd. (India), for

study leave and the French Government for the grant of a scholarship to carry out

this work.

REFERENCES

1 W.T. McGown, C. Kemball, D.A. Whan and M.S. Scurrell, JCS Faraday Trans. I, 73 (1977) 627.

2 Al. Ammar and G. Webb, JCS Faraday Trans. I, 74 (1978) 195. 3 B. Nowicki, J. Klimeck and A. Masiarz, Przemysl Chemiczny, 55 (1976) 587. 4 H. Blume, R. Batz, R. Schubert and D. Bernhart, Chem. Techn., 25 (1973) 604. 5 R.S. Mann and K.C. Khulbe, J. Catal., IO (1968) 401. 6 R.S. Mann and K.C. Khulbe. J. Catal.. 13 (1969) 25.

17 18

19

R.S. Mann and K.C. Khulbe; J. Catal.; 17 (1970) 46. J.P. Boitiaux, J. Cosyns and S. Vasudevan, Proc. 3rd Int. Symp. on the Scientific Bases for the Preparation of Heterogeneous Catalysts, Belgium, Elsevier, (1982). S. Fuentes and F. Figueras, J. Chem. Sot. Faraday Trans. I, 18 (1978) 74. G. Webbs, in Comprehensive Chemical Kinetics, Vol. 20, Chap. 1. N.R. Avery and J.V. Sanders, J. Catal., 18 (1970) 129. D.N. Ross, K. Kinositha and P. Stonehart, J. Catal., 32 (1974) 163. J.R. Anderson, in Structure of Metallic Catalysts, Academic Press, (1975) 275. R. Ugo, Catal. Rev. Sci. Eng., 11 (1975) 225. E.W. Stern and P.K. Maples, J. Catal., 27 (1972) 120. V.M. Frolov, O.P. Parenago, L.P. Shokina and G.M. Gherkasis, React. Kinet. Catal. Lett., 16 (1981) 115. C. Carturan and G. Strukul, J. Catal., 57 (1979) 516. P. Gallezot, J. Datka, M. Primet and B. Imelik, Proc. 4th Int Cong. Catal., London, The Chemical Society, (1976), p. 697. S. Vasudevan, Thesis E.N.S.P.M. Paris (1982), Technip Ed.

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