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HYDROGENATIONF ETHYLENEND PROPYLENE

2543

Hydrogenation of Ethylene and Propylene over Palladium Hydride

by R. J. Rennard, Jr., and R. J. Kokes

Department

of

Che mis t r y , The J ohns Ho pk ins Un iv e r s i t y , B a l t imor e , Mar y la nd

(Received Februa ry

26, 1966

21218

T he ra te of ethy lene hydrogenation as a function of hydrogen c oncentra tion, tempera-

ture, ethylene pressure, and hydrogen pressure has been studied over palladium hydride

and palladium deuteride. Similar (but less extensive) studies have been carried ou t with

propylene. Th e rate of hydrogenation at

-78

is found to be nearly zero order in ethylene

and hydrogen pressure but first-order in th e hydride concentration. Although th e first-

order rate constant decreases with hydride concentration, th e activity increases. The re

is an inverse isotope effect with deu terium , and th e principal deuterated produc t is

CzH4Dz.

Analysis of t he da ta suggests tha t the slow ste p is the addition of ad sorbed hydrogen

atom s to adsorbed ethylene or adsorbed eth yl radicals.

Introduction

Num erous studies'-? have been made on catalytically

active alloy system s in which the com position is system-

atically varied in an attempt to correlate activity

changes to known changes in solid-state properties.

In such studies, the activities of a series of different

prepara tions are com pared (with6p6 or without' cor-

rections for differences in surface areas) on the assum p-

tion that the different preparative procedures, required

for different compositions, have only a trivial effect on

the activity. Similar studies on the palladium hy-

dride system offer the possibility of carrying o ut such

comparisons on a single palladium sample. Such a

stu dy of t he hydroge nation ac tivit y of palladium a s a

function of hy dride concentration is the subject of this

report .

Experimental Section

Palladium powder was prepared by a m ethod similar

to th at used by Gillespie and An aqueous solu-

tion of palladium chloride

(10%)

was treated with

amm onia and then with hydrochloric acid. Th e salt

formed was reprecipitated several times and then

reduced to metallic palladium by slowly heating t o

500

in a stream

of

hydrogen. Then, the sample was flushed

with helium, cooled, and washed with hot distilled

water and concentrated ammonia. After this, the re-

duction at

500

was repeated. Th is procedure yielded

a palladium sponge with a B E T surface area of a bout

0.4 m2/g.

Hydrogen, deuterium, and helium were purified by

passage through a charcoal trap at

-195 .

Ethylene

and propylene (CP grade) were fractionated prior to

use and checked for purity by gas chromatography.

Unless otherwise noted, before a ny of t he experi-

ments, the palladium sample was heated for

16

hr in

200 mm

of

hydrogen at

450 ,

degassed for

0.5

h r a t

this temperature, and cooled in helium to -78 .

At all times, the palladium was protected from mercury

vapor by a trap at

-78 .

Palladium hydride or deu-

teride was formed at

-78

by sorption from the gas

phase.

Kinetic studies

of

th e hydrogenation of ethylene

were carried out on 320 mg of palladium admixed with

1.5

g of powdered quart z, bot h 40-60 mesh. This

mixtur e was spread over t he bottom of a 30-cc conical

flask connected

vi

a capillary stopcock to a vacuuni

system. After pretre atm ent of the catalyst, prepara-

1)

(a) G . Reinacker and

E.

A. Bommer,

2

norg. A llgem.

Chem.

236,

263 (1939); (b) G. Reinacker,

E.

Muller, and

R

Burmann.

ibid. 251,

55 (1943).

(2) D . A . Dowden and P. W. Reynolds, Discussions Faraday

Soc.

8,

184 (1950).

(3) A. Couper and D. D. Eley, ib id . , 8, 172 (1950).

4)

M. Kowaka, N i p p o n K i n z o k u Gakkaishi 23, 625 (1959).

(5)

R .

J . Best and W . W . Russel, J . Am. Che m. Soc., 76 834 (1954).

(6) P.

H.

Emmett and W . K . Hall,

J .

P hy s . Che m., 6 2 , 817 (1958).

(7)

For a recent review see

G .

C. Bond, Catalysis by IIetills,

Academic Press Inc., Ne w York, N.

Y.,

1962,

pp

244-252.

(8) L. J.

Gillespie and

F.

P. Hall, J Am.

Chem. Soc . 48, 1207

(1926).

V o l u m e

70

N u m b e r 8

A u g u s t 1866

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2544

R.

J.

RENNARD,

R.,

N D R. J. KOKES

tion of th e hydride, and temp eratur e adjus tme nt, the

reac tant gas was adm itted. Fro m time to time a small

sample of the reactant gas (1.35%) was withdrawn

for chromatographic analysis on an alumina column.

Preliminary tests showed tha t th e sampling was repre-

sentative.

Results

Adsorption Studies . Isotherms were determined

from

0

to

400

at pressures up to

500

mm . Above 100

only the

o

phase was present and no hysteresis was

ob-

served; below 100 the

a-p

transition was present

an d hysteresis was evident. These results were con-

sistent with those reported by others.*-1° Ev en though

the hysteresis is pronounced at room temperature,

however, the hydrogen could be completely removed

by room tem per atu re evacuation. (For PdH0.28 com-

plete degassing required

1

h r; for PdHo.42 complete

degassing required

3

hr.) Further more , even when

hysteresis occurs, the pressure rapidly adjusts to a

ste ady quasi-equilibrium value.

Hyd ride formation with palladium was also detected

at -78,

-183,

and even -195 . At the lower tem-

peratures , sorption is slow. At -78 with inlets of

hydrogen corresponding to compositions up to PdHO.60,

the half-time for sorption is of th e order of 5 sec, the

residual pressure is negligible, and a 30-min evacuation

removes less than 0.05 of th e hydro gen (from PdH0.24).

Surface area measurements on a pure palladium

sample yield a value of 0.81 m2 , whereas those fo r a

PdH 0 .42ample prepared from th e same sam ple yielded

a value of 0.79 m2. Further studies revealed that

neither the standard pretreatment nor reformation of

the hydride was accompanied by a detectable change

in area.

Figure 1 shows the pressure fall accompanying the

sorption of hydrogen a t -78 by PdH o.24rom an equi-

molar mixture of hydrogen and ethylene or hydrogen

an d propylene. (Analysis of the gas phase shows th at

abo ut 10% of th e pressure fall could arise from a lkane

production.) I n th e absence of olefin a t this hydrogen

pressure (107 mm), the half-time for sorption would be

about

5

sec; hence, the presence of olefin decreases the

ra te by abo ut two orders of m agnitude.

Because of the rapid sorp-

tion of hydrogen compared to reaction it was only

possible to st ud y th e reaction of a hydrogen-olefin

mixture in which sorption was also occurring.

To

this end, the amount hydrogenation of a

50:50

hydro-

gen-olefin mixture over PdHo.24was compared to

that of a

50:50

helium-olefin m ixture. Th e results

are summarized in Table

I. It

appears from these

da ta th at the r ate of reaction decreases from 20 to

Reaction

with

Hydrogen.

The JOUTnaE gf Physical Chemistry

.4

0

4

8

12 16

Time (m i n . )

Figure 1.

-78O:

open circles,

50:50

CaH&:Hz,

P

( tota l )

= 214

mm ; closed circles, 50:50 C2H4:H2,

P

(tot'al) = 214 rnm.

Adsorption from an olefin-hydrogen mixture at

40% when the pressure of hydrogen decreases by four

or five orders of magn itude. In other words, the re-

action is essentially zero order w ith respect to hydrogen

for reaction over palladium hy dride.

Table I :

Hydrogenation over PdHo.24

Initial Final

pmoles of

Mixture P H ~ ,m P H ~ ,m paraffinb

107 3 0 3 . 6

CtHe-He . . . . . . 3 . 1

CaHsHz

107

34 24

CzHa-He

. . .

. . .

15

C zH r H z

a a

a

On the basis of th e residual pressure after sorption , this

Amount formed

In this t ime, about 200 pmoles of hydrogen was

value w ould be of th e orde r of 10-3mm of Hz.

after

18

min.

taken up by the catalyst .

T he effect of ethylene pressure on th e reaction with

two samples of palladium hydride is shown in Tabl e

11

These results show tha t th e reaction is nearly, b ut per-

haps n ot qu ite, zero order in ethylene pressure.

Th e kinetics of t he reaction of ethylen e with pal-

ladium hydride can be represented by the equation:

In C/Co

=

- k t where

Co

and

C

represent the hydrogen

content of the catalyst at

t = 0

and time t , respectively,

and

k

is a pseudo-first-order rate constant that depends

(9) D. M. Nace and J.

0

Aston,

J.A m . C hem. SOC . 9, 619

3623

3627 (1957).

(10) D. P.

Smith, Hydrogen in Meta ls, University

of

Chicago

Press, Chicago,

Ill.,

1948.

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2546

R.

J.

R E N N A R D ,

R.,

N D R.

J.

KOKES

O Y I

0

0.10 0.20

0.30 0.40

0.50

H 1

d

Figure

4.

Activity

us

hydride composition. Th e ordinate

represents th e amou nt

of

C2H6 formed per hour.

3 0m

2.0 -

Y

1.5 -

c

ct

1.0

-

0.5

-

01 I

4.5 5.0 5.5

x

to

Figure 5. Arrhenius plot

for

ethylen e hydrogenation: triangles,

PdHo.lt; open circles, PdH0.24; closed circles, PdHo+

If we compare the activity for a standard run to

that for these pretreatments, we find A reduces the

activity, B increases the activity, and C has no effect.

A comparable reduction in activity with pretrea tmen t

A is also fo un d fo r PdHo.12, PdH0.29,an d PdHo.89.

The rate constant was de-

termined for several hydride compositions over a

temperature range from -64 to -98 . These results

are summarized by the Arrhenius plots in Figure 5.

Values of the apparent activation energies were 8.6,

7.7, and

7.5

kcal for PdHo.11, PdH0.24, and PdHO.40,

respectively .

E ect of Temperature.

1.8

t

Y

1.6

Y

1.4

1.2

1.0

-

0 0,I

0.2

0.3 0.4 0.

11

I

, I

0 0,I

0.2

0.3 0.4 0.

H / Pd

Figure 6. Isotope effect

u s

hydride composition.

Reaction with Deuterium. The reaction of ethylene

or propylene with palladium deuteride followed the

first-order ra te law. I n general, the ra te of reaction

of t he deuteride with olefin was greater th an t ha t

for the hydride, but the effect was most pronounced

at lower temperatures and higher hydride concentra-

tions. A syste ma tic series of experiments with ethyl-

ene carried ou t alternately with th e deuteride and th e

hydride a t -78 yielded the value

of

kD/kH us hy-

dride composition. These results are summarized in

Figure 6. Th e ratio kD/kH is nearly u nity a t very low

hydride concentrations, increases abruptly near PdHo,l,

and increases more slowly at higher hydrogen concen-

trations to nearly 2. Near PdH oSl he reproducibility

was far worse than at higher or lower hydrogen con-

centrations; possibly this occurs because the surface

hydrogen concentration is often slightly above or below

the gross concentration with the results that near

PdHo.1 the surface can be in th e high or low kD/kH

region of Figure 6.

Th e rate con stant for reaction of propylene with

PdDo.24 was, as with the hydride, nearly an order of

magnitude less than t ha t for ethylene. I n this case

also, an inverse isotope effect was found; kD/kH for

propylen e was 1.9-2.0.

A series of a ltern ate deuterium a nd hydrogen runs

was made between 4 and 8 for th e single com-

position PdHo.zror PdDo.zr. These results are shown in

Figure 7 and indicate tha t kD/kH changes from 1.5 a t

4 o 2.3 at -98 . Th e activation energy for the deu-

terideis about 1kcal less than t hat for the hydride (7.9

kcal)

.

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HYDROGENATION

F

ETHYLENE

N D

PROPYLENE

2547

1.6 -

s

0

1.2

+

t

-

-

0 s

0 4

e

0

.5

5.0 5.5

T X x io3

Figure

7.

deuteride:

Arrhenius plot for reduction by hydride and

open circles, PdDa.,c; closed circles, PdHo.la.

For the reaction with deuterium, it was possible to

estimate the deuterium content of the product ethane.

The fragmentation pattern for deuterated ethanes was

assumed to be that of the nondeuterated ethane (at

25

v) with appropriate statistical corrections and a

relative proba bility of 1.2 an d

0.8

for cleavage of C-D

an d C-H bands, respectively.12 Th e results a t -78 ,

together with those reported by Bond and Wells13

for supported palladium at

- 3 6 O

are summarized by

the smoothed curves in Figure 8. Clearly there is

much less mixing of deuterium for etha ne production

by palladium deuteride.

Discussion

Palladium hydride is

a

two-phase system; .both

phases,

a

and

p

are cubic close packed with respect

to palladium atom^.'^^'^ At room temperature, the

CY phase alone is present for hydrogen concentrations

below Pd Ho,0 5nd the /3 phase alone is present for con-

centrations above PdHO.jj. At intermediate concen-

trations both phases, as judged by X-rays, coexist.

At the temperatures of interest to us, the hydrogen

atom s are inobilelj and occupy the octahedral holes in

th e close-packed lattice.16

Th e hydrogen-palladium system is not a typical

two-phase system. In the two-phase region the hy-

drogen fugacity increases with hydrogen content.

(Such behavior can be accounted for in part by analy-

sis

of

stress eff ects.Is) Fu rthe rm ore , physical proper-

ties such as magnetic susceptibility do not reflect the

phase transitions. Th e molar magnetic susceptibility

0 1 2 3 4 5 6

No. of D-otoms

/

Molecu le

Figure 8. Isotopic distribution in prod uct ethan e: open

circles, PdDo.2a lus ethylene a t -78 ; closed circles,

hydrogenation over supported palladium at -36O.13

decreases linearly as the hydrogen concentration is in-

creased and reaches zero near PdHo.6.19 This can be

rationalized by the assumption th at each hydrogen atom

donates one electron to th e existing holes in the d b and

of palladium. Similarly, the relative resistivity in-

creases linearly with hydrogen co ntent u p to PdH0.v6.

We are interested primarily in the relation of ac tivit y

to electronic structure, and the electronic structure,

as judged by susceptibility and resistivity, depends

on the hydrogen content, not on what phases are

present. Accordingly, we shall focus our atten tion on

the variation of activ ity with hydrogen content alone.

Th e very rapid up take of hydrogen at 8 suggests

a

rapid transfer of hydrogen between the surface and

bulk palladium. Zero-order dependence on hydrogen

pressure for olefin hydrogenation is in line with this;

for, then, we would expect the surface concentration of

hydrogen to be controlled by the bulk concentration

an d to be independent of the gas phase concentration.

Th e nearly zero-order dependence of hydrogenation

(12)

D. chissler, S.

0

Thompson, and J. Turkevich, Discuss ions

Faraday Soc .,

10 46 (1951).

(13)

G . C.Bond and P.

B.

Wells,

A dv an . Ca ta ly s i s ,

15,

91 (1964).

(14)

S.

D.

Axelrod and A. C. Makrides, J.

P hy s . Che m. ,

68

2154

(1964).

(15) R. .

Norberg, Phy s . Rev . ,

86,745 (1952).

(16)T.

. .Gibbs, P r o p . Inorg. Chem., 3,

422 (1962).

(17) D.

. verett and D. Norden, Proc . Roy .

8oc .

(London), A254,

341 (1960).

(18)

N.

A.

Scholtus and W.

K .

Hall,

J.

Che m. P hy s . , 39,

868 (1963).

(19)

C.Kitte l, Solid State Physics, John Wiley and Sons, Inc.,

New York, N.

Y. 958, 334.

V o l u m e 70 N u m b e r 8

A u o u s t

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R.

J.

R E N N A R D ,R., ND R .

J.

K O K E S

likely to b e one of th e following

C Z H

H CZH4

I_

C2H5

H CzHs zHs(g )

2CzH5

zH4 CZ&(g)

In the above sequence, C represents the

rate on olefin pressure suggests the surface is nearly

covered with olefin. T he reduc tion of hydrogen sorp-

tion ra te by a factor of to when olefin is

present s upp orts this view.

If the above be true, the rate-controlling step is

concentration, C2Hs(g) represents gaseous ethane, and

all other species are assumed to be a ttach ed to t he sur-

face. Reactions 2 an d 3 are usually assumed to occur

in olefin hydrogenation, and reaction 4 has been con-

sidered by BondeZ0I n any even t, if t he steady-

sta te approximation is applied to C2H6 ,we obtain

(I t is, of course, conceivable tha t th e ra te of hydro-

gena tion is controlled by diffusion of hyd rogen from t he

bulk to the surface. This can be ruled out on three

cou nts: (a) Th e kinetics are not consistent with dif-

fusion. (b) T he order of magnitu de difference in ra te

for ethylene and propylene is no t consistent with a r ate

controlled by diffusion. (c) T he inverse isotope effect

for the reaction is not consistent with the normal iso-

tope effect found for diffusion.)21

The marked difference in rate for ethylene and pro-

pylene rules out reaction 1 as th e rate-controlling step.

T he lack of isotopic mixing shows th at th e reverse of

reaction 2 c a n , in t he first ap proximation, be neglected.

If

we then make the assumption that the reverse re-

action (1) is much more rapid tha n (2), we can write

On integration, with CzH4 constant, this yields the ob-

served form

In C/C, =

-k t

where the constan t k is a composite qu ant ity given by

The observed kinetic isotope effect is qualitatively

consistent with the conclusion that the slow step is

the ra te of addition of a surface hydrogen atom to ad-

sorbed olefin. Figure 9 shows on the left an energy

Figure 9.

Relative energies of PdH and PdD (see text).

diagram for gaseous hydrogen, deuterium, palladium

hydride, and deuteride.

In order to construct this

diagram, it was assumed that differences in the heat

of form ationg of palladiu m d euterid e and palladium

hydride, 8.6 an d 9.6 kcal, respectively, stem primarily

from zero-point energy effects.

From these data and

th e zero-point energy of hydro gen vs. deuterium, we

find t ha t th e difference in zero-point energies for PdH

and PdD is about 0.8 kcal.

If, consistent with the

kinetic analysis, it is assumed that the slow step is

th e addition of a surface hydrogen to a carbon atom ,

the activated complex will have a nearly normal

carbon-hydrogen band. D at a for CC13D

vs

CCl3HZ 2

reveal the zero-point energy difference for this C-D

us C-H bond is 2.1 kcal.

Wi th th e same figure adopted

as the maximum zero-point energy difference for the

complex, we obtain the energy diagram on the right

of Figure 9.

If

we accept the foregoing qualitative

analyses as correct, the activation energy for reaction

with palladium deuteride is

ut

most 1.3 kea1 less than

that for palladium hydride. Thus, hydrogenation

would be expected t o show the observed inverse isotope;

the agreement with the observed difference in activa-

tion energies (1.0 kcal) is regarded as fortuitou s.

(We have no convincing explanation for the falloff

in isotope effect a t low conce ntrations of hyd rogen.

Data are not available for low hydride concentrations

which would permit con struction of a parallel t o Figure

9. We believe, however, th at th is change in isotope

effect may be indica tive of a change in mechan ism,

perhaps associated with th e pure phase.)

The first-order rate constant depends only on the

20) G. C. Bond, Trans. F a ra d a y SOC. 2 , 1235 (1956).

21)

W.

Jost, “Diffusion,” Academic Press Inc., New

York,

N .

T.

1952, p

308.

22) G. Herzberg, “hlolecular Spectra and Molecular Structure,

11,” D. Van Nostrand and Co., Inc., New

Tor k ,

N. Y. 945, 316.

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HYDROGENATIONF ETHYLENEN D PROPYLENE

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initial hydrogen concentration and remains constant

as the hydrogen content is reduced by reaction with

e th yle ne . M ic he l a n d G a l l i ~ o t ~ ~eported that although

sorption of h ydrogen red uces the magne tic suscepti-

bility, remov al of s orbed hydroge n by reaction a t low

temperature does not restore the initial magnetic

susceptibility. Implica tions of this observation have

been recently discussed by Cribbs.I6

If

this observa-

tion is correct, it would me an th at the electronic proper-

ties of the catalyst are governed wholly by the initial

hydrogen content. Thus, provided

kl/k- l

and (C2-

H4) do not depend on the initial hydrogen content,

k2 decreases as the holes in th e d b and are filled.

Regardless of the va lidity of the analysis in the pre-

ceding paragraph, however, the following conclusion

can be stated without equivocation. Th e activity

increases as the holes in the d band are filled; the first-

order rate constant decreases as the holes in the d

band are filled. This raises questions abo ut correla-

tions attempted solely on the basis of activity without

kinetic analyses.

The analysis of the d ata obtained with the sta ndard

pretreatment yielded a reasonable but admittedly

ten tativ e interp retatio n. Effects of varying this pre-

treatment are too complex for detailed interpretation.

It is, however, worth noting that cooling in hydrogen

poisoned the catalyst as has been observed for nickeP4

and also palladium.26 Perhaps these effects are due

to changes in surface structu re noted by Germ er.26

Acknowledgment. Acknowledgment is made to the

donors of the Petroleum Research Fund, administered

by t he Am erican Chemical Society, for supp ort of this

research.

(23)

A.

Michel and M. Gallisot, C m p t . Rend. 208, 434 (1939).

(24)W. .Hall and

P.

H . Emmett, J. hys . Chem.

63

1102 (1959).

(25) A. Couper and

D. D.

Eley, Discussions Faraday SOC. 8 172

(1950).

(26) L.

H.

ermer, Advan. Catalys is 13, 191 (1962).

Volume

70

Number

8

Auguet 1966