React. Kinet. Catal. Lett . , Vol. 41, No. 2, 309-314 (1990)

HYDROISOMERIZATION AND HYDROCRACKING OF 2-METHYLHEXANE

ON PtUSHY CATALYSTS.EFFECT OF PLATINUM CONTENT

F. Alvarez*, F. Ram~a Ribeiro* and M. Guisnet** I , f

*Grupo de Estudos de Catallse Heterogenea, I.S.T.,

Av. Rovisco Pais, 1096 Lisboa Codex (Portugal)

**UA CNRS 350, Unlverslte de Poitiers, 40,

Av. Recteur Pineau 86022 Poitiers Cedex (France)

Received June 21, 1989 Accepted September 21, 1989

The stability and the ratio of the isomerization to

cracking rates increase very much with platinum intro-

duction and the isomer and cracking product distribu-

tions are significantly modified. This can be explained

by the change from an acid to a bifunctional mechanism.

However, platinum has practically no effect on the rate

of 2-methylhexane transformation. This could be attributed

to the fact that neither on USHY nor on PtUSHY is the

formation of the carbenium ions with a 2-methylhexane

skeleton the limiting step.

C T a 6 H ~ b H O C T b H COOTHO~eHHe C K O p 0 C T e ~ H 3 O M e p H 3 a ~ H H H

K p e K H H F a CH~bHO HOBNmaDTC~ C B B e ~ e H H e M H~aTHHM~ q T 0

HpHBO~HT K 9 H a q H T e ~ b H O M y M 0 ~ H ~ H ~ H p 0 B a H H D p a c n p e ~ e n e -

HM~ HpO~yKTOB H 3 o M e p H s a ~ H H H K p e K H H F a . g T O 06%SCH~mT

~ S M e H e H H e M M e x a H H S M a 0T KHCnOFO K 6 H ~ y H K ~ M O H a ~ b H O M y .

0 ~ H a K O , n n a T H H a H p a K T M q e c K H He B n H s e T Ha C K 0 p O C T b

n p e B p a ~ e H H s 2 - M e T H n r e K C a H a . 9TO H p H H H C ~ B a ~ T TOMy

~ a K T y ~ q T 0 o 6 p a 3 o B a H H e K a p 6 e H H e B o F o HOHa CO c K e n e -

TOM 2 - M e T H n r e E c a H a He S B n S e T c s n M M H T H p y m ~ e ~ c T y n e H b m

HM Ha USHY, HH Ha P t U S H Y .

Akad~miai KiadS, Budapest

ALVAREZ et a l . : HYDROISOMERIZATION

INTRODUCTION

It has been shown that for n-alkane transformation on

PtUSHY catalysts the catalytic properties depend on npt , the

number of accessible platin~ atoms [1,2]. As can be expected

with bifunctional catalysis, the activity of the catalysts in-

creases with npt till it reaches a plateau; the stability in-

creases with npt and the isomer and cracking product distribu-

tions are significantly modified [1,3]. For methylcyclohexane

transformation similar changes are observed for the stability

and selectivit~ but the activity of the catalysts does not

depend on npt [43. This particular behavior could be due to

the presence of a methyl substituent or to the possibility of

forming an aromatic by dehydrogenation on the platinum sites.

To choose between these hypotheses, we study here the trans-

formation of a monobranched alkane, 2-methylhexane, on USHY

and on two PtUSHY catalysts.

EXPERIMENTAL

USHY was obtained by calcination of an ultrastable NH4Y

zeolite (LZY82 from Union Carbide) at 500~ under a dry air

flow for i0 h. Two platinum zeolites: 0.i PtUSHY (0.i wt.% Pt)

and 0.6 PtUSH (0.6 wt.%) were prepared by ion exchange with

[Pt(NH3)4]CI 2 under the same conditions as those already re-

ported [3]. The samples were calcined under dry air flow at

300~ and reduced by hydrogen at 500~ The platinum dispersitl

estimated by electron microscopy was greater than 90%.

The transformation of 2-methylhexane was carried out in a

flow reactor, at 250~ 1 arm and PH /Palkane =9" The activitie 2

were measured at a conversion of about 10%. Different conver-

sions were obtained by modifying contact time (35-100 mg of

catalysts; 0.2-12 cm 3 h -I of 2-methylhexane).

RESULTS AND DISCUSSION

On the three catalysts, 2-methylhexane leads to isomers (I

and to cracking products (C). As was the case for methylcyclo-

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ALVAREZ et a l . : HYDROSIOMERIZATION

hexane transformation [4], the stability of the catalysts and

the isomerization to cracking rate ratio increase with npt ,

the number of accessible platinum atoms, and the rate of trans-

formation remains practically constant (Table I). Thus the

initial activity of 0.6 PtUSHY is only 1.6 times greater than

that of USHY while for n-heptane transformation it was 12 times

greater [3]. The particular behavior found for methylcyclo-

hexane transformation [4] is therefore due to the presence of

a methyl group rather than to the possibility of dehydrogena-

tion into an aromatic ring.

As was the case for methylcyclohexane transformation [4],

the product distribution changes from that expected from an

acid mechanism on USHY to that expected from a bifunctional

mechanism of PtUSHY:

- On USHY isomers and cracking products are formed direct-

ly, while on 0.6 PtUSHY isomers are the only primary products;

on this latter catalyst cracking products appear only for a

percentage of conversion of about 10%.

- The isomer distributions are different on USHY and on

PtUSHY catalysts. On USHY, the distribution is that expected

from an acid mechanism [5]. Only the isomers to which co~espond

tertiary carbenium ions (3-methylhexane, ethylpentane, 2,3 and

2,4-dimethylpentanes, 2,2,3-trimethylbutane) are primary pro-

ducts, n-Heptane, 2,2 and 3,3-dimethylpentanes, to which only

secondary carbenium ions correspond, are not observed at low

conversion (Table 2). This can be explained by the fact that

the desorption of the isomers plays a significant role in the

rate of their formation. This desorption occurs through hydride

transfer from an alkane to the corresponding carbenium ions

e.g. C-C-~-C-C-C + RH -~- C-C-C-C-C-C + R +

and the higher the stability of the carbenium ions involved

the greater the rate [53. On 0.6 PtUSHY, n-heptane and all

311

ALVAREZ et a lo : HYDROISOMERIZATION

Table 1

Transformation of 2-methlyhexane on PtUSHY catalysts, npt (1019

atoms g-l) : number of accessible platinum atoms. A (10 -3 mol o

h-i g-l) : activity extrapolated to reaction time equal to zero.

A7: activity at reaction time equal to 30 minutes. I/C: isome-

rization to cracking rate ratio for a conversion equal to 10%.

USHY 0.1 PtUSHY 0.6 PtUSHY

npt 0 0.19 1.67

A ii0 130 180 o

Af/A ~ 0.2 0.7 0.9

I/C 6 13 20

the monobranched and bibranced isomers are primary products.

Only the tribranched isomer (2,2,3-trimethylbutane) is not

directly formed (Table 2). This distribution can be explained

by the bifunctional mechanism in which the limiting step would

be the rearrangement of the intermediate carbenium ions. The

isomers (n-heptane, 2,2 and 3,3-dimethylpentanes), which de-

sorb very slowly from a secondary carbenium ion (through

the acid mechanism), are formed here via olefinic intermediates:

-H 2 +H 2 + e.g. C-C-C-C-C-C-C ~ C-C-C=C-C-C-C ~ C-C-C-C-C-C-C

Pt

3-Methylhexane, which can be formed via an alkyl shift (type

A rearrangement [7]) is favored, n-Heptane and dimethylpentanes

are formed only via type B rearrangements (through protonated

cyclopropanes) which are known to be much slower [8]. 2,2,3-

Trimethlybutane whose formation requ~es two type B rearrange-

ments is not directly formed.

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ALVAREZ et a l . : HYDROISOMERIZATION

Table 2

Distribution of the isomers (%) ~ their mixture, at very

low conversion

USHY 0.6 PtUSHY Equilibrium [6 ]

n-heptane 0 9.8

3-methylhexane 59.3 64.2

ethylpentane 2.5 3.5

2,2-dimethylpentane 0 1.0

2,3-dimethylpentane 20.6 12.0

2,4-dimethylpentane 16.8 9.1

3,3-dimethylpentane 0 0.4

2,2,3-trimethylbutane 0.8 0

12.4

21.6

2.6

12.2

31.1

7.4

9.8

2.9

- The cracking product distributions are different on USHY

and on PtUSHY catalysts. On 0.6 PtUSHY propane and isobutane

in quasi-equimolar amounts are the main products ( > 95%). This

is what was found in n-heptane transformation on the same cata-

lyst [2]. These cracking products are formed by the scission

of carbenium ions having a 2,2 or a 2,4-dimethylpentane skeleton

(type B scission [9]). On USHY, the more complex distribution

obtained: presence of olefins, large amounts of C 5 and C 6 hydro-

carbons (25-30%) can be explained by an acid mechanism.

We can thus conclude that the transformation of 2-methyl-

hexane on PtUSHY catalysts occurs through a bifunctional mechan-

ism as was the case for n-heptane transformation [3]. The very

low effect of platinum on the reaction rate is due to the fact

that tertiary carbenium ions (which can be rapidly formed or

transformed into the corresponding alkanes through an acid

mechanism) are involved in 2-methylhexane as well as in methyl-

cyclohexane transformations. Therefore, neither on USHY nor on

PtUSHY the formation of the carbenium ions is the limiting step

of these transformations. The supplementary mode of formation

313

ALVAREZ et a l . : HYDROISOMERIZATION

or desorption of the carbenium ions (through bifunctional

catalysis) affects only the formation of the hydrocarbons to

which correspond no tertiary carbenium ions.

REFERENCES

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4. F. Alvarez, A. Montes, G. Perot, M. Guisnet: Accepted for

publication in: Proc. of the 8th Int. Zeolite Conf.,

Amsterdam, 1989.

5. M. Guisnet, J.J. Garcia, F. Chevalier, R. Maurel: Bull.

Soc. Chim., 1657 (1967).

6. D.R. Stull, E.F. Westrum, G.C. Sinke: The Chemical Thermo-

dynamics of Organic Compounds. John Wiley and Sons,

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7. H.F. Schulz, J. Weitkamp: Ind. Eng. Chem., Prod. Res. Dev.,

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8. F. Chevalier, M. Guisnet, R. Maurel: in: Proc. 6th Int.

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123 (1983).

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