Dehydrogenation of methylcyclohexene on a PtNaY catalyst. Study of kinetics and deactivation

103

Applied Catalysis, 26 (1986) 103-121 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

DEHYDROGENATION OF METHYLCYCLOHEXENE ON A PtNaY CATALYST. STUDY OF KINETICS AND DEACTIVATION Garcia de la Banda, A. Corma y F.V. Melo Instituto de Catalisis y Petroleoquimica, C.S.I.C. Serrano, 119.

~.F.

28006 Madrid.

(Received 27 October 1985, accerted 12 May 1986) ABSTRACT The kinetics of the dehydrogenation of methylcyclohexene to toluene on a PtNaY zeolite and the deactivation of the catalyst have been studied. Kinetic models for the dehydrogenation process and for catalyst decay have been established. By comparing the rate of dehydrogenation of methylcyclohexane and methylcyclohexene it has been found that, on this catalyst, the slowest step in the overall dehydrogenation process is the dehydrogenation of methylcyclohexane to methylcyclohexene, followed by the desorption of toluene from the metal. The deactivation of the catalyst is about six times higher when the reactant is methylcyclohexene instead of methylcyclohexane. INTRODUCTION The reactions of dehydrogenation of cyclohexane to benzene and of methylcyclohexane to toluene have been widely used for testing metal supported catalysts [1-4} However, there are two aspects of the reaction which remain controversial. Although it is generally agreed that the reaction takes place by successive dehydrogenation steps, some authors have shown that the controlling step is the desorption of the final product, i.e. toluene, from the metallic surface[l,

51.

while

according to others it would be the first dehydrogenation to give methylcycloh~xene[2, 6-~.

The second aspect of the reaction which is still not fUlly understood is the deactivation of the metal catalyst by coke deposition.

S~

Jossens and petersen[9]have studied the deactivation of platinum reforming catalysts during methylcyclohexane dehydrogenation using a deactivation model with two stages which are both time-on-stream dependent. In the first stage an equilibrium between the reactant and the adsorbed inactive intermediate products is established. the deactivation effect being due to the occupancy of active centers by these intermediate products. In the second stage, at longer reaction

times,

the deactivation is only partially reversible and depends on the over0166-9834/86/$03.50

© 1986 Elsevier Science Publishers B.V.

104

all rate of dehydrogenation of methylcyclohexane. Recently it has been shown [10]that toluene is not a precursor of coke formation, because it has no effect on the rate of deactivation of the catalyst. It was suggested that partially dehydrogenated products are responsible for the catalyst deactivation by coke deposition. In the present work the dehydrogenation of methylcyclohexene on a platinum on Y zeolite catalyst has been studied. The kinetics of the reaction and the influence of the variables of the process on catalyst decay has allowed us by comparison of the results with those obtained with methylcyclohexane, to establish the controlling step of the reaction and the influence of the partially dehydrogenated molecules (methylcyclohexene) on the catalyst decay.

EXPERIMENTAL Products Themethylcyclohexene used was synthesized following [11]with a purity higher than 99%. According to chromatography analysis its composition by weight was; 98.1% 1-methylcyclohexene; 1.6% 3-methylcyclohexene; 0.1% 4-methylcyclohexene and 0.2% dimethylcyclopentcnes. The gases (H 2 and N2) from Sociedad Espanola del Oxigeno had a given purity of 99.99%.

Catalyst preparation The catalyst used in this work was a PtNaY zeolite prepared as follows; A NaY zeolite (SK-40 Union Carbide with Si/bl

= 2.4) was put

in contact during 1h at 22QC with an aqueous solution of Pt(NH 3)4 Cl 2 under gentle stirring. The exchanged zeolite was filtered, washed with distilled water and dried at 110QC in air. The degree of exchange was determined analyzing the Na+ and the pt 2+ both in the liquids and in the solid by atomic absorption. The amount of pt 2+ exchanged was 0.48 wt % on the initial zeolite. The dried catalyst was heated in an air flow at 300QC during

3 hours, followed by 2 more hours at 450QC.

After this treatment, the sample was reduced in hydrogen according to the following thermal treatment; 150QC (0.5 hours), 250QC

(0.5 hours),

350QC (0.5 hours) and 450QC (3 hours). During the process of cooling down to room temperature the sample was kept in a hydrogen atmosphere. The calcination-reduction process used here has been recommended for obtaining high metal dispersions [121.

105

Analytical Techniques Reaction products were chromatographically analyzed using a chromosorb PNAW column (60-80 mesh) with di-isodecylphlate (20 wt %) for the liquid products, and a mixed column of silicagel and Porapak Q for the gaseous ones. 2/g. The BET surface area of the catalyst was 670 m The metallic surface was measured by hydrogen chemisorption, following the method given by Reyes [131 and by electron microscopy using a Phillips 300 and samples prepared with a microtome. The average sizes of Pt crystallites obtained were 30

Aby

hydrogen chemisorption and 35

A by

elec-

tron microscopy. In order to check the influence of the ionic exchange and the treatments used in the catalyst preparation on the crystallinity of the zeolite, measurements of the crystallinity of the initial zeolite and of the final catalyst were performed by X-ray diffraction using a Phillips PW 1051 diffractometer. No significant modification of the crystallinity was detected. The presence of surface hydroxylic groups in the catalyst was showed by

i.r.

spectroscopy using a Perkin Elmer spectrometer. Infrared

spectroscopic measurements of adsorbed pyridine were carried out in a conventional greaseless i.r. cell. The samples were pretreated overnight at 450QC and 10- 5 Torr of dynamic vacuum after which 5 Torr of p

pyridine were introduced into the cell at room temperature. After equilibration the samples were degassed at 150QC under vacuum, and the spectra recorded at room temperature.

Experimental method The experiments were run in a conventional fixed bed tubular reactor, at normal pressure and with electrical heating. The reactor had an internal diameter of 1.5 cm ,

and

20 cm in length. Methylcyclo-

hexene was fed using a positive displacement pump through a preheater. Liquid products were collected in sampling tubes after passing through a water cooled condenser. In a standard experiment, the methylcyclohexene was fed at a given constant flow rate and the reaction products were collected and analyzed at different reaction times. From these data yields to toluene and 3- and 4-methylcyclohexene was changed by

were obtained at each reaction time. Space velocity

changing the amount of catalyst.

The reaction products were toluene, 3- and 4-methylcyclohexene, methylcyclohexane,methylcyclohexadiene, dimethylcyclopentenes and C aln kanes with n ~5.

106

RESULTS In order to find the possible influence of homogeneous gas phase reactions an experiment was performed at 420QC with a methylcyclohexene flow of 2.8 mllh in the absence of catalyst. No dehydrogenation reaction took place and the double bond isomerization (formation of 3 + 4-methylcyclohexene) proceeded with a 4.8% conversion. To find the experimental conditions at which the process is not controlled by mass transfer, the initial rate of dehydrogenation of 1-methylcyclohexene was measured at the highest reaction temperature to be used in the kinetic study (390QC), and at different hydrocarbon flow rates, but keeping always constant the contact time. For 1-methylcyclohexene flows> 4.6 ml.h- 1 the reaction rate was independent of the hydrocarbon flow rate,i.e. the process was not controlled by external diffusion. The possible control by internal diffusion was checked by measuring the initial rate of dehydrogenation using PtNaY zeolite catalysts with different particle sizes. For particles

<

0.25 mm the rate

of dehydrogenation did not depend on particle size, i.e. the process was not controlled by internal diffusion. With the aim of finding the influence of the partial pressure of either 1-methylcyclohexene or hydrogen, experiments were done at 390QC changing the partial pressure of one of them and keeping the partial pressure of the other constant. Nitrogen was used as an inert gas in order to keep the total pressure equal to one atmosphere 5 (10 Pa). The flow of liquid 1-methylcyclohexene was kept constant and equal to 6.6 mllh, and the contact time was changed by modifying the catalyst weight used. Contact times were selected in order to have always conversions below 15%. Thus, initial rates were directly correlated with the initial partial pressures to obtain the kinetic data in absence of deactivation. Results are plotted in Figures 1a to 19. The initial conversion values (obtained by extrapolation to reaction time zero) and the contact time were fitted to a straight line and the slope gives the values of the initial rates of reaction (Table 1). In order to study the influence of the temperature over the catalyst activity, similar experiments were carried out at temperatures of 350 and 370QC. Results are shown in Figures 2a to 2g and 3a to 3g. The values of the initial rates have been included in Table 1. On a comparative bases the study of the 1-methylcyclohexene isomerization was also made on the original NaY zeolite and the results are given in Figure 4.

107

TABLE 1 Initial rates of dehydrogenation T,

.QC

PMCHe, atm.

p

atm. H 2,

r

o'

g MCHe/g.cat.min.

390

0.05

0.60

0.61

390

0.10

0.60

0.55

390

0.20

0.60

0.55

390

0.30

0.60

0.50

390

0.40

0.60

0.47

390

0.10

0.30

0.54

390

0.10

0.90

0.57 r

a

=0.54:!:0.1l(*)

0.05

0.60

0.32

370

0.10

0.60

0.36

370

0.20

0.60

0.40

370

0.30

0.60

0.31

370

0.40

0.60

0.39

370

0.10

0.30

0.35

370

0.10

0.90

0.37

370

r

a =0.36+0.08(*) -

350

0.05

0.60

0.27

350

0.10

0.60

0.24

350

0.20

0.60

0.28

350

0.30

0.60

0.29

350

0.40

0.60

0.30

350

0.10

0.30

0.29

350

0.10

0.90

0.26 r

a

=0.28:0.05(*)

(*) Average values and confidence intervals for 95% of the cases.

108

a

0.2

-0

b

c

0

~

>-

0.1

40

o

o

40 time on stream, min

20

Figure 1. Dehydrogenation and isomerization of I-methylcyclohexene at 3902C on PtNaY catalyst.---- yield to toluene, ------ yield to 3- and 4-methylcyclohexene. a) P

P 0.6 ; b) P 0.1, P MCHe=0.05, MCHe= H2= H2=0.6; c) P 0.2, P 0.6; d) P PH = 0.6; e) P 0.4, PH = MCHe= MCHe=0.3, MCHe= H2= 2 =0.6; f) PMCHe= 0.1, P 0.3; g) PMCHe= 6.1, PH =0.9 atm. Contact H2= times: . , 0.36 min; &, 0.24 min; . , 0.13 min~

0.2f ..... 0.1

..., .. a

b

...........

~

c

......

~0.1

s;

o,b-:~:::!::l~---=.;i;~~~~~ 40

o

20

40 time on stream, min

Figure 2.Dehydrogenation and isomerization of I-methylcyclohexene at 3702C on PtNaY catalyst. Contact time: 0,29 min. Other conditions as in Figure 1.

109

02

c

20

20

40

40

time on stream, min

Figure 3. Dehydrogenation and isomerization of I-methylcyclohexene at 350 2C on PtNay catalyst. Contact time: 0.36 min.

Other conditions as

in Figure 1.

03......-------------....,

01

o

. 20

40

time on stream, min Figure 4. Dehydrogenation and isomerization of 390 2C

~-methylcyclohexene at

on a NaY zeolite (treated with sodium acetate). P MCHe=0.4, PH =0.6 atm. Contact times . • ,0.62 min; .. ,0.26 min. - - , yield 2toluene; to ----, yield to 3- plus 4-methylcyclohexene.

110

DISCUSSION Reaction Scheme In figure 5 the yields of the different reaction products observed, i.e. toluene, 3- plus 4-methylcyclohexene, methylcyclohexane, methylcyclohexadienes, dimethylcyclopentenes and Cn (n < 5), were plotted versus total conversion. The shape of the curves show that all of these are primary products, which can be formed on the metal, on the zeolite, or on both. The active sites and the reactions producing those products will be discussed below.

0.2

-0

~0.1 >.

o

0.1 02 OJ toto I corwer sron

O.L.

Figure 5.Yields to products vs. total conversion at 3902C on PtNaY catalyst. P

MCHe cyclohexene;

=

0.4, PH

=

0.6 atm.

&, toluene;

. , metrtylcyclohexane; X,

. , 3 - plus 4-methyl-

dimethylcyclopentenes; 0,

cracking products. Reactions taking place in parallel to the dehydrogenation Double bond Isomerization. From a comparison of Figures Ie and 4 it can be seen that the amount

of 3- plus 4-methylcyclohexene produced

on the PtNaY is practically the same as on the NaY zeolite, indicating that a role of the Pt metal for the double bound isomerization does not need to be invoqued. Since it is known[14}that acid Bronsted sites are active for double bond isomerization, their amount was measured by pyridine adsorption on both PtNaY and NaY. After desorption at 1502C and 10- 4 torr, no band was clearly visible at 1545 em-I. This result, expected for the NaY zeolite, was not expected for the PtNaY zeolite, since in this case protons should be generated during the reduction of Pt

2+[15,

161, producing Bron s t e d acidity on the surface of the zeolite.

From these pyridine adsorption results we have to conclude that, in our case, the acid groups generated during the reduction of pt 2+, either do not retain pyridine at l50 2C and 10- 4 Torr, or they are loca-

111

ted in positions not accessible to I-methylcyclohexene. A third possibility can be that very few OH groups were formed and besides, part of them disappeared by dehydroxylation of the catalyst during heating in the i.r. cell, so that the pyridine adsorption procedure was not sensitive enough for their detection. To discuss these three possibilities an infrared study in the hydroxyl group region (3500-3750 ern

-1

) was

carried out after degassing the sample overnight at 450£C and 10-5 To r r. After this conventional treatment for zeolites, only a band at 3745 ern-I, which corresponds to non acidic silanol groups, was observed. Acidic OH groups located in accessible (3640 ern-I) or inaccessible (3560 em-I) places were not detected in the PtNaY, nor in the NaY zeolite sample. Therefore, it can be safely said that the OH groups created during reduction of Pt 2+ are removed during the heating in the i. r. cell. However, since it is known [14, 17Jthat weak acid sites can be active for double bond isomerization, and that under reaction conditions dehydroxylation should not be so dramaticasinthe i.r. cell, the remaining surface OH groups can catalyze the double bond isomerization. In any case, and in order to explain the activity of the NaY zeolite the intervention of other sites such as Lewis acids

[18] (tricoordinated

A1 3+ extraframework Aluminium and Na+ ions), which are able to chemisorb pyridine U~, cannot be neglected. Also there is the possibility that the double bond isomerization follows a radical mechanism, where the formed radicals are stabilized by the zeolites surface [20]. Hydrogenation of I-Methylcyclohexene to Methylcyclohexane. During the reaction of dehydrogenation of I-methylcyclohexene to toluene, methylcyclohexane is found as a primary product. In principle, the saturation of the double bond can take place, either by hydrogenation

[21J

or hydrogen transfer [22], on the surface of the zeolite via the following reactions:

ze~i~

ze~i~

112

Methylcyclohexane can also be formed by hydrogenation of l-Methylcyclohexene on the surface of the Platinum. Because of the experimental conditions used (high HZ/Hydrocarbon ratio and the value of the equiliMethylcyclohexane~Methylcyclohexene,Ke 3 9 0=0.015) and considering the low acidity of the zeolite carrier, we believe

brium constant for

that the methylcyclohexane should be mainly formed by hydrogenation on the surface of the metal. To check this point the initial

ra~e

of for-

mation of methylcyclohexane was obtained at different partial pressures of I-methylcyclohexene and constant partial pressure of HZ (0.60 atm). The results given in Figure 6 are consistent with a mechanism in which the controlling step is the surface reaction of a reactant molecule adsorbed on two sites (as was also found for the reverse reaction [11]) but not with a second order reaction as implied by the acid catalyzed reaction via hydrogen transfer.

c

'E ~ 0.15 u

--

01

<1l

I

~

0.10

01

~0.05 LJ ~

...

oL--__

-,L-:-_ _- : " -_ _---,L_ _- - ' ' - - - - - - '

Figure 6. Rate of formation of methylcyclohexane vs. l-methylcyclohexene pressure at 390 2C on PtNaY catalyst. PH = 0.6 atm. 2

Formation of dimethylcyclopentenes and Cn(n < 5) products. It has been seen above that the PtNaY zeolite has, if any, very few hydroxyl groups, and with a weak acid strength. Then, it is quite improbable that the chain isomerization involved in the formation of dimethylcyclopentene and the C-C breaking bond involved in the formation of C n (n <5) products, occur via a carbonium ion mechanism on the surface of the zeolite, since both reactions need the presence of medium and strong acid sites [14]. On the other hand, at high temperatures, as is this case, platinum is able to catalyze the chain isomerization and hydrogenolysis [Z3], [Z4],. Indeed, in our case C and C have been ob, . 1 2 as the main components of the cracked products.

ta~ned

113

Formation of methylcyclohexadienes and toluene. From the discussion on the formation of methylcyclohexane, we have to conclude that the traces of methylcyclohexadienes and toluene must be formed mainly by dehydrogenation on the platinum. Taking into account the behaviour of the reaction products discussed above, the global reaction scheme could be written as follows: (6 )

Adsorption on metal

I-MCRe

Adsorption on me t a 1

I-1CHde

H

(l)~ol

f I

I I

I

II Adsorption~~ __~~~

on zeolite

I

I I

He He

~~

Adsorption on ~ 4.1 zeolite

JI

- - ---_- ~_J DMC Pe, C

n

(n~

_

I J

5)

(1) Dehydrogenation on platinum (2) Double bond isomerization on zeolite

(3) Disproportionation on zeolite (4) Dehydrogenation on zeolite (5)

Hydrogenolysisand chain isomerization on platinum

(6) Hydrogenation on platinum Kinetic study of the dehydrogenation reaction As said before, traces of methylcyclohexadienes were observed in the products, which shows that, as in the case of methylcyclohexane

[11], the dehydrogenation on the Pt is taking place by a series of consecutive dehydrogenation reactions,which could be written as: MCHexene

MCHexadiene

U (MCHexene)ad

~

(MCHexadiene) ads

A reaction scheme of this process

Toluene

-

..

(Toluene)ads

type could be represented by a reaction

involving two active sites per dehydrogenation event, in the

following sequence, in where it is assumed that for MCHexenes and MCHexadienes the rate of dehydrogenation is the same for all the isomers:

A + 1 Al + 1 PI + 1

,.

• ..

. •

...

Al pI + 81 Rl + 81

(1) (2 ) (3 )

114

PI Rl Sl where: A,

,.

.

.. ... .. .

P + 1

(4 )

R + 1

(5 )

S + 1

(6 )

P, R, Sand 1 represent: Methylcyclohexene, methylcyclohexa-

diene, toluene, hydrogen, and active centres, respectively. Since MConly as traces, neither reaction (3) nor (4) can be

Hexadienes appear

the rate determining step, and therefore the following rate expressions can be written, depending on which one of the other steps is the slowest one. a) methylcyclohexene (A) r

adsorption control:

(7)

=

KpK S

1 + KRPR + KpP p + KSP S + ~

PpPS

b) Control by surface reaction of methylcyclohexene:

(8 )

r

c) Toluene (R) desorption control:

k

r

=

P K3K2 KA A L 2 p2 K S S

(9 )

K P 3K 2KA A 1 + KAPA + KpP p + 2 2 + KSP S K K S S where r = rate of reaction; k

rate constant of the rate determining

step; K2 and K = equilibrium constants of the surface reactions,res3 pectively; K , Kp' KR and K = adsorption equilibrium constants of meA S thylcyclohexene, methylcyclohexadiene, toluene and hydrogen; PA,P ,P

P

s =

partial pressures; and L

=

P

R

total number of active centres.

Taking into account that: a) dehydrogenation was carried out under differential conditions; b) the big excess of H2 in the reaction medium, so that PH ~ cte; and c) the partial pressures of methylcyclohexadienes wer8

alw~ys very low, the equations (7), (8) can be simplified

to:

k L a)

r

o

=(----

1 + KSP

S

o

(10)

115

(11)

c) In the case of equation (9), the above considerations would allow to write:

r

(12 )

=

It is possible to obtain

the numerical values of the equilibrium

constants for the reactions: methylcyclohexene

~

methylcyclohexadiene

+ H2, and methylcyclohexadiene~toluene+ H2. Knowing these values the relative contributions of the different terms in the denominator of equation (12), can be discussed. By using the group contribution method, the enthalpy

(~H298)

and en-

tropy

(~S298)

(~Cp)

for the reactant and products have been calculated the values

of formation and the specific heat at constant pressure

are given in Table 2. TABLE 2 Thermodynamic constants of formation of methylcyclohexene, methylcyclohexadiene , toluene and hydrogen.

Chemical compound

~HO

298

(Kcal) mol

SO

~C

(cal)

~ 298 mo1°K

a

p (cal/molOK) bxl0 3 cxl0 6

- 10.27

81. 78

-11.42 147.89 -53.90

Methylcyclohexadienes

18.12

80.59

-14.99 148.03 -52.82

Toluene

11.95

76.42

-

Hydrogen

0.00

31. 22

Methy1cyc1ohexenes

3.42 107.98 -42.04 6.62

0.81

0.00

Finally, with the data from Table 2 and using the thermodynamic expressions:

116

llHT + TllST o

llH29 8 + a T + llST

=

2b

T

2

+

3c

llS298 + a In T + bT +

T c

2

3

T

2

it has been found that at 390 2C the equilibrium constants for the dehydrogenation of methylcyclohexene to methylcyclohexadiene, and methylcyclohexadiene to toluene are 0.07 and 4810, respectively. Then, it is reasonable to assume that: K3K2KA K2 S

P A p2 S

» 1 + KAP A + ksps

If this is so, equation (12) can be reduced to: r

o

kL

(13)

It is obvious, that by measuring the initial rate of the reaction at different partial pressures of methylcyclohexene it is possible to discriminate among the mechanisms represented by equations (10),

(11)

and (13). Indeed, in equation (10) the initial rate changes linearly with the partial pressure, while in equation (11) it goes through a maximum, and in equation (13) the initial rate of the reaction does not depend on the partial pressure of methylcyclohexene. The data in Table 1 show that, within the experimental error, it is possible to say that the initial rate of methylcyclohexene

dehydrogena-

tion does not depend on either the partial pressure of methylcyclohexene or

hydrogen. Therefore, the kinetic model which best fits the ex-

perimental results is that corresponding to a mechanism where the desorption of toluene is the controlling step. With the values of kL at the three temperatures and the Arrhenius expression, an activation energy of 13.7 Kcal.mol- l for the dehydrogenation of methylcyclohexene on the PtNaY zeolite has been found. This value is much lower than

that of 25-30 Keal/mol obtained for the me-

thylcyclohexane dehydrogenation on the same catalyst [llJ. Moreover, if we compare the iryitial rates of dehydrogenation, that of dehydrogenation of methylcyelohexane is clearly lower than

that of dehydrogena-

tion of methylcyclohexene. As an example, at 390 (with hydrocarbon pressure = 0.1 At, and hydrogen pressure = 0.9 Atm) the initial rates 2C

117

of dehydrogenation of methylcyclohexane and methylcyclohexene are:0.21 . -1 respect1ve . 1 y. T h 1S . resu It 1S . and 0.57 g. hydrocar b on. g cat -1 .m1n in agreement with a mechanism where the rate determining step for the dehydrogenation of methylcyclohexane is the dehydrogenation of methylcyclohexane to methylcyclohexene. Kinetic study of the deactivation. Deactivation model Calling

0

the fraction of active sites which remain actives for the

dehydrogenation reaction, the rate of reaction under differential conditions will be: ~ ~

r

=

T

where x = toluene yield, apparent

T

k L

0

=

k

(14)

'0

= contact time and k'

kL

the initial

rate constant.

The concentration of active centres changes with the time of reaction. Assuming a power law type model the change of the active centre fraction with the time of reaction can be represented by: do dt where: t

P b S

= reaction time; k d

(15 )

rate of deactivation constant; m, a and

b are constants. Equation (15) can be transformed, at differential conditions (P constant and P s = constant) in: do dt

k ' dO m

A

(16)

where (17 )

Taking into account equation (14), the change in the rate of reaction with the time on stream will be: dr dt

k,do dt

(18)

From (16) and (18) it follows: dr dt where k"

k'

d

Ik' (m-l)

k ' k'

d

(kr,)m

(19)

118

Assuming different values of m, it has been found that the best correlation with the experimental results is obtained with m = 2. Then, equation (19) yields 1 r

Calculated

(20)

and experimental results from series of experiments per-

formed at 390QC are compared in Figure 7, while the values r o and k" obtained from equation (20) are shoNn in Table 3. Similar conditions are obtained at 370QC and 350QC. TABLE 3 Apparent constants of deactivation and initial rates of dehydrogenation at 390 2C. k

g cat/g MCRe

PH ' atm 2

r o' g MCHe/g cat.min

0.05

0.6

0.66

0.1

0.6

0.57

0.34

0.2

0.6

0.59

0.35

P MCHe, atm

U I

0.27

0.3

0.6

0.58

0.37

0.4

0.6

0.51

0.39

0.1

0.3

0.65

0.37

0.1

0.9

0.62

0.31

The calculated values of r o (Table 3) are very similar to those obtained by extrapolation (Table 1). Comparing the values of k" obtained from the series carried out either at constant pressure of methylcyclohexene or at constant pressure of hydrogen, the values of a and b in equation (17) can be calculated. Thus, at 3902C the values a

=

0.19

and b = -0.16 are obtained. From these values

it is concluded that the dependence of k" in

equation (19) on PA and P s could be of the type:

k"

(21)

Equation (21) represents a deactivation model in which, under rential conditions, the catalyst deactivation depends on the

diff~­

concentr~­

tion of adsorbed methylcyclohexene, and hydrogen and methylcyclohexene compete for the adsorption centres.

119 b

a

20

c

10 4i

I

u

~20 Ol

--

ou

40

--.... on 5 treom, min

Figure 7. Rates of methylcyclohexene dehydrogenation vs. reaction time Comparison between theoretical (lines) and experimental (points) results at 390QC on PtNaY catalyst. Other conditions as in Fig. 1. The best fit of equation (21) to experimental data was obtained with n = 1. The values of the constants obtained at 3902C were k"'= -1 -1 -1 -1 112 g catalyst. g MCHe . atm , K = 271 atm and K = 10 atm . S A A more realistic model would be to consider that the effect of hydrogen could be due to the hydrogenation of coke precursors. Then the deactivation equation would contain two terms, a positive term, dependent on the concentration of adsorbed methylcyclohexene (under differential conditions the concentration of the other adsorbed species would be very low) and a negative term, dependent on the hydrogen and methylcyclohexene concentrations, which would correspond to the rate of methylcyclohexene hydrogenation. A tentative equation, assuming a model of the Langmuir type, is:

kif

(22)

Our experimental results do not allow the proper calculation of parameters in (22). Nevertheless, qualitatively, the changes of k" as a function of PA and P s agree with such a kind of equation. Finally, if we compare the deactivation during the methylcyclohexene dehydrogenation (Figures 1 to 3) on the PtNaY catalyst with deacti-

120 vation during

the methylcyclohexane dehydrogenation [3] on the same

catalyst, it may be observed that in the last case the deactivation is much slower. Thus, for example, the average conversion of methylcyclohexene (at 390 2C, P = 0.1 atm. PH = 0.9 atm. and = W/F = 0.25min) MCHe changes from 0.14 at zero reaction ti~e to 0.02 at 30 minutes reaction time. Under tne same conditions, the average conversion of methylcyclohexane is 0.08 at zero reaction time and 0.07 at 35 minutes reaction time. This confirms the hypothesis given in the literature 19 , 10] concerning the importance of the unsaturated molecules, particularly olefines, as precursors of the coke formation, and therefore responsible for the catalyst deactivation. CONCLUSIONS In the range of partial pressures studied here, the initial rate of dehydrogenation of methylcyclohexene on a PtNaY zeolite catalyst does not depend on the partial pressure of the hydrocarbon nor on the partial pressure of hydrogen.

This indicates that the controlling step

of the process is the desorption of the toluene. By comparison of the initial rates of dehydrogenation ofmethylcyclohexane and methylcyclohexene it has been found that, on this catalyst, the dehydrogenation of methylcyclohexane to methylcyclohexene is slower than the desorption of toluene from the surface of the platinum. The self poisoning of the platinum during dehydrogenation of methylcyclohexene fits adequately either a potential decay function of a Langmuir type model. Finally,it has been found that the deactivation of the catalyst is about 6 times higher when the reactant is methylcyclohexene instead of methylcyclohexane. ACKNOWLEDGEMENT This work was partially supported by the Spanish CAICYT, under Project N2. 316/183. REFERENCES 1

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