Hydroconversion of n-nonane catalyzed by PdHY zeolites

Applied

Catalysis, 69 (1991)

Elsevier Science Publishers

15

15-31 B.V., Amsterdam

Hydroconversion

of n-nonane catalyzed by PdHY zeolites

I. Influence of catalyst pretreatment J. Leglise, A. Chambellan and D. Cornet* Catalyse et Spectrochimie, (France),

fax. (+33)

C!A.CNRS

04.0414, I.S.M.Ra,

University

of Caen, 14050 Caen CZdex

31452877.

(Received 25 January 1990, revised manuscript received 28 September 1990)

Abstract Pd/HY catalysts reduced at temperatures ranging from 573 to 873 K were compared in respect of the hydroconversion of n-nonane performed at 473 K and 0.1 MPa. The catalyst reduced at 573 K had a stable activity and a high selectivity for isomerization, but samples reduced at 773 K or above initially favoured cracking at the expense of isomeriaation. The evolution of the reaction mechanism with the temperature of reduction is related to a change in the dispersion of palladium revealed by TEM. Keywords: palladium/zeolite, zeolites, nonane hydroconversion, (TEM ), selectivity (isomerization), pretreatment.

dispersion, catalyst characterization

INTRODUCTION

HY zeolites containing palladium or platinum are effective catalysts for the hydroconversion of n-alkanes into cracked as well as isomerized products [ 1,2]. The activity levels of catalysts containing either platinum or palladium are not very different [ 3,4]. Fundamental studies generally consider catalysts reduced at a rather low temperature, and operating at 430-530 K and at a moderate pressure. Under such conditions the metal is highly dispersed and the extent of isomerization depends only upon the overall conversion, whatever the reaction temperature or pressure [5,6]. For a series of alkanes ranging from C, to C12, ideal hydrocracking was defined as producing a high yield of isomers, the cracked products arising from a single C-C bond splitting [ 71. But ideal hydrocracking requires a precise balance between the number of acidic and metallic sites, which is very sensitive to the conditions of catalyst preparation and pretreatment [8,9]. When these conditions deviate from the optimum, the hydrogenation function is generally weakened, and cracking is favoured [lo]. Furthermore, the activity declines with time-on-stream [ 111. In the present paper the influence of the pretreatment conditions on the properties of a Pd/HY catalyst is considered. Samples calcined and then re-

0166-9834/91/$03.50

0 1991 Elsevier Science Publishers

B.V.

16

duced at different temperatures were tested by studying the hydroconversion of n-nonane carried out at 473 K and at 0.1 MPa. The selectivity into isomeric or cracked products was specially examined. The behaviour of the catalyst is related to variations in metal dispersion or in the acidity of the support. EXPERIMENTAL

Catalysts

The carrier was the stabilized Linde LZY 82 zeolite (denoted as SY), containing 0.15% sodium and 2.8% ammonium by weight. From the overall silicon : aluminium ratio of 2.7 and the framework silicon : aluminium ratio of 4.5, it appeared that one third of the total aluminium was non-framework. Palladium was introduced by exchanging the SY zeolite with a 106” molar solution of Pd(NH,),Cl, at pH= 10. The Pd/SY-A zeolite contained 1.2% Pd, as determined by atomic absorption. Activity

measurements

The catalytic conversion of n-nonane was measured in a flow reactor containing about 0.08 g of dry catalyst, and operating at atmospheric pressure. Samples of the palladium-exchanged zeolite were slowly heated (1 K mn- ’ ) under flowing oxygen up to Tax= 723 or 773 K. No self reduction of the palladous complex occurred at this stage [ 111. The catalysts were then reduced with hydrogen for 3 h at T,=573, 673, or 773 K, etc. Catalysts were denoted as A (573 ) , A (673 ), etc., according to the temperature of reduction. The temperature was finally set at 473 K for the catalytic test. The reaction mixture ( H2/C9H2D= 228) was obtained by passing hydrogen through a thermostated saturator filled with liquid nonane (purity> 99.5%) held at 293 K. The total flow-rate could be varied between 17 and 130 mmol h-l, and the weight time, defined as r= W&/F,, was in the range 1.3.10W4 to 0.15*10-4 h kg 1-l. Gas samples were taken at the reactor exit and analyzed with a gas chromatograph equipped with an OV 101 capillary column. Fractional conversions f, into each product or pseudospecies i (C, isomers or cracking products) could be determined, as well as the selectivity ratio R =fi,,/fc,,. Catalyst characterization

X-ray patterns of the catalysts were recorded with a Philips diffractometer using the Cu K, radiation (Crystallography and Materials Science Laboratory, Caen) . Transmission electron micrographs were obtained on a Jeol12OX microscope at the Institut Francais du Petrole (Rueil). Infrared transmission spectra were recorded on a Nicolet MX-1 FTIR spectrometer.

0 1

0.5

0

Totalconversion Fig. 2. Measured

conversions

(f) and selectivity

to monobranched

isomers

of the total conversion

of n-C, over Pd/SY-A

at 473 K. Symbols:

( A ) cracked products;

( A ) total nonane isomers; (0 ) monobranched

tibranched nonanes, MTB; selectivity to monobranched (*) Pd/SY-A

0

(this work);

(0

) PtCa/Y

(7), Pt/HY

50

(L&e),

fractional

isomers, Sm=

nonanes, (MB)/

as a function

conversions MB;

f into

( H ) mul-

[ (MB) + (MTB)

1:

[ 121.

100

percent n-Cg converted

Fig. 3. Products of n-C, hydroisomerization isomer composition (9%)

over Pd/SY-A

catalyst

at 473 K: monobranched

branched C, isomers (MB ) , the C9 isomers with more than one branch (MTB ) , and the cracked products (C ). The fractional conversions fi into each of these groups, measured at 473 K and at several flow-rates, are shown on Fig. 2 as a function of overall conversion: the pattern of curves is very similar to that reported by Weitkamp [7] for the reaction of octane over a PtCa/Y catalyst under rather different conditions. Thus, in the conversion of nonane over the Pd/SY catalyst, hydroisomerization is clearly the primary process, going through a maximum at about 80% total conversion. The cracking then rapidly increases, The points reported in Fig. 2 refer to conversions f higher than 15%)

0 to 20 0 to 5

0.5

5

0.5

-

573 673

773

773

873

673

Cracking

16.2

63.6 64.3 95.8 68.9 90.8 65

15.0 15.3 21.5 12.2 16.9 17.5

17.2 23.2 68.5 12.7 63.0 3.9

temperature 439 K.

13.8

12.2

17.2 17.3

i-C,

c,

18

over Pd/SY-A

8.8 3.7 5.8

50.7 75.7 58.4

11

12.7 12.5

72.6 72.4

71

n-C,

i-Cs

2.9 0.6 1.9 0.5

8.3 10.4 17

0.5 0.6

17.4 16.3 15.3

n-C,

i-C,

(mol products/100

TK

201 200

-

193

205

199 199

mol C, cracked)

3.0

4.0

1.2

0.4 0.2

C,

N P.-B

catalysts reduced at various temperatures

mol/lOO mol C, cracked

n-C,

Molar distribution:

(wt.-%)

conversion

of the cracked products

zeolites were calcined at 773 K before reduction.

(h)

(K)

“Data taken from ref. 12. Reaction

“The Pd/SY-A

Pt/HYp

Pd/SY-A”

Aging time

r,,

of n-nonane at 473 K: molar distributions

Reaction

Catalyst

1

TABLE

20

and do not allow any decision to be made as to whether both MB and MTB isomers are formed initially. But when f exceeds 70%, the monobranched isomers decline, while the multibranched ones still increase and the selectivity into monobranched isomers SMB= MB/ (all isomers) decreases steadily from 0.75 to 0.60 (Fig. 2). This is an indication that the MB isomers appear first. Selectivities to MB isomers previously reported for the reaction of nonane over PtCa/Y at 548-573 K and 4 MPa [7], and over Pt/HY at 439 K and 0.1 MPa [ 121 are shown for comparison: S MB varies little with temperature or pressure, but the palladium zeolite gives slightly more multiple branching than the platinum catalysts at low conversions. In any case, the observed SlvrBexceeds the thermodynamic ratio, which is 0.34 at 473 K [ 131. By contrast, the internal distributions among isomers with the same degree of branching (monobranched or dibranched) were found to be nearly constant, at the conversions reported (f> 15% ) . The distribution of the monobranched isomers is shown in Fig. 3: the methyloctanes are rather close to mutual equilibrium, whereas the ethylheptanes, which amount to 10% of the group, are only slightly below their equilibrium value (dashed line on Fig. 3). The dibranched fraction consists mainly of dimethylheptanes, again in near-equilibrium, with about 4% ethylmethylhexanes. The tribranched isomers, among which 2,2,5_trimethylhexane was predominant, accounted for less than 0.5% of all isomers. These features are characteristic of the hydroisomerization of long-chain alkanes [ 12,14,15]. The distributions of the cracked products were found to be independent of overall conversion. An example is shown in the first line of Table 1: the molar distribution is nearly symmetrical, showing that primary cracking is largely predominant. The central bonds in nonane are preferentially split, as the ratio (C, + C, ) / ( C3 + C,) amounts to 5, but the cracked alkanes contain a high proportion of iso structures. A similar behaviour has been reported for a Pt/HY catalyst (bottom line in Table 1) . In fact, the molar ratios C,/C!, and C&/C, somewhat exceeded unity, being 1.3 and 1.07, respectively. Moreover, small amounts of heptanes were formed, while ethane and methane were absent from the products. Reaction mechanism The above results are consistent with the mechanism of ideal hydrocracking [ 151: the n-nonane is first dehydrogenated over a metal site, and it is then readily protonated, yielding a secondary nonyl carbenium ion. This ion rearranges via cyclopropyl intermediates, so that monobranched, dibranched and tribranched structures are successively formed, and cracking appears at high conversion levels. The preferential splitting of the tray-trisubstituted Cg carbenium ions (mechanism A) explains the high proportion of isoalkanes among the cracked products, since the splitting of ions with an iso structure (mechanism B ) would produce equal amounts of n- and isoalkanes [ 151.

21

n-nonane -H,lt n-nonene +H+ U n-nonyl+

dimethylheptane ?l dimethylheptene

methyloctane t4 methyloctene t1

G

trimethylhexane tl. trimetylhexene t1

t4

P

methyloctyl+

dimethylheptyl+ (B) 1

*

trimethylhexyl+ (A) 1 Cracking

In ideal hydrocracking, the dehydrogenation step is fast, and the concentration of nonene is high enough to ensure a rapid desorption of the branched carbenium ions into Cs isomers. Equilibrium between ions with different degrees of branching is not attained, as the monobranched Cs isomers are favoured. However, the carbenium ions with the same degree of branching are in near-equilibrium, since methyl shifts are very easy. Kinetic analysis

Since the above network contains a great number of reactions, the products are usually grouped into families with similar properties. Thus only a limited number of reaction steps is considered for any given catalyst. As the partial pressure of nonane is kept low, all rates are assumed to be first order in reactant, so that the rate of appearance of species i is: ri= - Clz;C,+ Ck,C, Several reaction networks were considered: - In Scheme 1, cracking is preceded by two isomerization steps, as suggested by Steijns et al. [ 51: k

k,z

k,l

n-nonane I (N)

monobranched Cg F k:l

multibranched C, 2 ktz

(MB)

(MTB)

cracking CC)

In Scheme 2, the monobranched and multibranched C, are grouped into a single species, as for the reaction of n-heptane over Pt/HY catalysts using the SY support [9]:

-

km,

kiso

n-nonane _ k:., (N)

isomeric Cg -

cracked products (Cl

(1)

In Scheme 3, three lumped species are again considered, but a step of nalkane cracking is introduced [ 9,101:

-

km,

isomeric C, (I )

n-nonane (N) B k’iso

\ k’cm

J

cracking (C)

k,,,

22

Following earlier studies on the conversion of C8, C,, and C,, alkanes, the rate constants for reversible isomerization steps were assumed to be bound by equilibrium conditions (Schemes la, 2a and 3a). But this is not necessarily the best procedure, especially when all branched isomers are grouped into a single lump, since the monobranched isomers exceed their equilibrium value. Therefore, apparent rate constants, not bound by any equilibrium condition, were also derived from the experimental distributions (case b). For any of the above schemes, and in plug flow conditions, the mole fraction fi of any species i varies with the weight time 7 according to the matrix equation:

$=(k).(f) A numerical integration yielded the rate constants k; by a regression based on a second-order development of Q(k), the weighted sum of square residuals:

Q,(k) =

C Cfq-fiJ) "/(fLj)

1

i = lumps

j = runs

where f, and jLJare experimental and calculated conversions. The rate constants determined for catalyst A (573) using data at four weight times are shown in Table 2 for the three simplified schemes. The standard deviations at 95% confidence level appear as % of the final k values. The coefficients R2 of multiple nonlinear regression were always found to be very close to one. The discrimination between models involving 3 or 4 species was based on the sum of TABLE

2

Kinetic

Results for the Hydroconversion

Model

No of parameters

C&x 103 k‘l

of n-Nonane

over Pd/SY-A

6’“iI

ki,

&

la

3

14.1

14829(3.4)

1501

16893(21.9)

lb

5

4.9

16015(4.5)

2756(30.2)

15627(11.3)

2a

2

8.3

14429(3.2)

2b

3

1.9

16120(3.3)

2376(21.7)

ho

&so

3a

3

7.7

14687(4.4)

3b

4

1.8

16192(3.9)

“Rate constants

expressed in 1 h-r kg-‘.

501

8619 37820(9.2)

k cm

16141(16.7) 28616(13.0)

4822 (5.9) 4652(3.3)

k

kbr*

CL-B

510

5162(12.2)

2343(25.6) Standard

at 473 K”

4789(7.9)

deviations

-270(165) -109(244)

(% ks) are given in parentheses.

23 100

80

60

40

20

0 1.0

0.5 Weight

time,

WC’N/F’N

: 10m4

kg

h/liter

Fig. 4. Conversion of n-C9: computed profiles of hydrocarbon lumped species as a function of weight time over Pd/SY-A at 473 K. Model 2b (non-equilibrated isomerization rate): (0 ) nnonane: ( A ) isomeric products; ( A ) cracked products. t 0.04

Model

0.04

Model

a

la

b

2b

0 -0.04

,

I

0

0.5 Total

conversion

1.0

-0.04

l----_-A 0.5

0 Total

1 .o

conversion

Fig. 5. Plot of the residuals on hydrocarbon groups. (a) Model la: (four species: isomerization rates equilibrated). (b ) Model 2b: (three species: isomerization rate non-equilibrated). Symbols: (0 ) n-nonane; (A ) isomeric nonanes; (0 ) monobranched nonanes; ( + ) multibranched nonanes; (A ) cracked products.

errors Qj3(for model 1, the residues on MB and MTB were added up ), and also on the deviations on the ks. Model 3 (a or b ) could be discarded at once, since the rate constant for direct n-alkane cracking was found to be insignificant, in agreement with earlier work on Cl0 [ 161 and C, alkanes [ 171. The other models tested were all acceptable on statistical criteria. For Scheme la, nearly identical rate constants were obtained when mini-

24

mizing an objective function @ involving 4 species ( D4) or 3 species (c&). Incidentally, the rate constants kil, ki2 and k,,, appearing in the first line of Table 2 fall exactly between the values determined for the hydroconversion of n-octane and n-decane over Pt/HY at the same temperature, but at much higher pressures [ 16,171. In Scheme 2a, with only three species, the rate of n-nonane disappearance is found to be very close to that of Scheme la. The ratio between the rates of cracking !zC,,(2a) /k,, (la) = 0.30 departs from the equilibrium value of [ MTB] / ( [ MTB] + [MB] ) = 0.66, but is close to the ratio of the same concentrations (0.35) observed at 50% conversion of n-nonane (Fig. 2). Thus, models la and 2a give consistent results. However, the deviations and the CDvalues were systematically lower with model 2a. Finally, the isomerization rate constants were considered free of any equilibrium condition, e.g. in case 2b, ki,,/ki,, was not fixed at 28.8 The values obtained for 0 and R* are more favorable, but this is to be expected since the number of parameters is higher. The standard deviations also appear to be slightly better. Fig. 4 shows the agreement between the observed product distributions and those calculated from the rate constants of model 2b. This scheme appears most appropriate since the residues are low and better distributed than in any other model (Fig. 5 ) . Thus, from a limited number of data, any catalyst may be characterized by the two rate constants in Scheme 2b, kis,, and k,,,, the significance of the third constant kf,, being limited. This will be useful in the evaluation of modified catalysts.

Catalysts pretreated

at high temperature

Kinetic aspects

In the previous section, the Pd/SY-A catalyst was calcined above 700 K, in order to achieve a good dispersion of the palladium ions [ 181 before reduction at 573 K. Such a treatment seems optimal, since sample A, calcined at either 723 or 773 K and then reduced at 573 K, showed the same stable activity and selectivity. The rate constants are nearly identical (Table 3, lines 1 and 2). Similarly, raising the temperature of reduction from 573 to 673 K did not modify the catalyst properties (Tables 1 and 3). However, when catalyst A was reduced at 773 K or above, cracking was dramatically enhanced. But deactivation was so fast that the rate constants could not be determined as above. Values of kiso and kcr, recorded after 1 h and 5 h on stream are reported in Table 3 (lines 4 and 5): compared to the results of A(573), ki, is found to be lower, and decreases with time, whereas k,,, is increased more than loo-fold at the start of the run. Such values of k,,, are unrealistic, and show that the consecutive Scheme 2 no longer applies.

25

TABLE 3 Rate constants lzisO and k,, (1 hh’ kg-‘) deduced from the consecutive network (2b) for Pd/SYA zeolites after different pretreatments Pretreatment temperature (K)

Catalyst age (h )

0,

H,

723 773 773

573 573 673

oto20 oto20 0 to 5

773 773

773 773

1 5

Rate constants k&u x1(-”

k xcra low

16.1 16.0 16.7

4.7 5.3 5.1

6.4 2.5

640 110

Selectivity in production formation Examination of the products confirms that the Pd/SY catalyst reduced at high temperature departs from ideal behaviour. The amount of isomerized C, was rather small. Nevertheless, the selectivity, SMB,to monobranched isomers could be measured at all times during a deactivation run, and is plotted in Fi.g. 6 against the overall conversion f:S MBincreases from 0.60 to 0.75 while the conversion decreases from 70 to 9%. These values are only slightly smaller than over catalyst A (573) operating at similar conversions, and still stand well above the thermodynamic ratio 0.34 (Fig. 6). The proportion of tribranched isomers over catalyst A( 773) was very small, about 0.2%. Thus the whole is,o-

0

0

0.5

1.0

Tolai c0Wersl0"

Fig. 6. Selectivity to monobranched isomers (&a) versus overall conversion of n-nonane over Pd/SY-A reduced at 573 K (0 ) or at 773 K (A ). Comparison with thermodynamic data (dotted line ) .

26

merization process is slower when the temperature of the catalyst reduction goes from 573 to 773 K and, according to SMB, the first step is slightly more affected than the second one. Fig. 7 shows that the distribution of the cracked products markedly changed with time-on-stream over catalyst A (773). During the first hours, the proportion of C, and C, decreased, while the C5 and C, fractions increased. Sizeable amounts of heptanes also appeared, but neither ethane nor methane were ever detected. Molar distributions observed after 0.5 and 5 h on-stream are reported in Table 1, and the individual amounts of CnH2n+2 are plotted in Fig. 8 as a function of the overall conversion. Compared with sample A(573) which is close to ideal behaviour, the fresh catalyst A( 773) gave much more C, and C4, and the total number of moles IV,,, formed upon cracking 100 moles of C9 exceeded the value 200 expected from a single splitting of C, carbenium ions. As catalyst A (773) became aged, the amounts of C, and C, got closer to C6 and C5, respectively (Fig. 8) but the number PI,,, fell below 200 (Table 1). Furthermore, the individual amounts of cracked products declined more rapidly than the overall conversion (Fig. 8), so that cracking appears as a secondary process. Meanwhile, the proportion of C7 among the reaction products increased. Since C, molecules can only arise from the splitting of carbenium ions with at least 10 carbons, it is likely that cracking over the aged A (773 ) involves the participation of a bimolecular mechanism. The proportion pn of branched alkane in every cracked fraction CnHSn+2 was also examined. As noted previously, the iso selectivities pn for catalyst

Fig. 7. Distribution crackedmoles

(*),C,

of cracked products (a),&

(A),&

versus catalyst age for Pd/SY-A (A),‘&

(0)

and&

(O)products.

reduced at 773 K. Total

27

i-C,

0.6

0

Fig. 8. Yield of individual sion for catalyst

Pd/SY-A

(0 1. (b) Isobutane

cracked products

0.4

0.2

(mol per mol C9 converted)

reduced at 7’73 K. (a) Propane

( A ) and n-butane

( A ); isopentane

(0);

.8

0.6

versus the overall conver-

isohexanes

(0 ) and n-pentane

(0)

and n-hexane

( q ).

A (573 ) did not depend upon conversion nor catalyst age, and stood well above the thermodynamic ratios. The distributions reported in Table 1 show that the selectivity to isobutane p4 was constant over A (773) also, but at a higher level than on A (573 ) : 85% against 79%. However, p5 and p6 increased with time on stream for the catalyst reduced at 773 K. Over the fresh catalyst, p5 starts at 85%, as on A(573), but reaches 95% after 5 h. On the other hand, ps starts at 70% and finally comes close to that of catalyst A(573). Thus the degree of branching among the cracked products is higher for the catalysts reduced at 773 or 873 K, except for the hexanes. Physical characterisation of the catalysts As the location and the dispersion of palladium are very sensitive toward the temperature of pretreatment [ 191, reduced samples of the Pd/SY-A catalyst were examined by electron microscopy. The pictures revealed the diffraction pattern of crystalline zeolite Y, but they also displayed some amorphous zones anchored at the surface of the microcrystals. This amorphous material is probably formed during the steaming treatment. Black spots on the micrographs correspond to palladium entities. On the catalyst reduced at 573 K these particles were rather small, and clearly located in the zeolite array. However, on catalyst A (773 ), the palladium particles were considerably larger, and most of them appeared at the surface of zeolite microcrystals, or at the borders between them. The palladium size distributions (Fig. 9 ) , the average diameters cl,, and metal dispersion D (Table 4 ) are completely modified when the reduction temperature is raised. The narrow distribution of small particles observed on A (573) is considerably broadened on A (773)) and the average diameter d, grows from

crystallite

diameter

Crystallite

: nm

Fig. 9. Distribution of palladium reduced at 573 K or at 773 K.

crystallites

diameter

: nm

versus particle diameter, d,, for the Pd/SY-A

zeolite

TABLE 4 Summary of spectroscopic data from various techniques for reduced Pd/SY -A catalyst Reduction

d:

temperature (K )

(nm )

TEM

XRD

Relative OH intensities Dispersion DB

573 773

3.0 10.7

FT-IR

0.44 0.12

Metal area

a

Crystallinity:’

(m’Pd/g

(nm)

(%)

2.452 2.450

78 78

cat)

1.73 0.52

“d, is the surface average diameter, d,= In,d:

/In,df

Qeducedfrom D=GV(Pd)/d,S Base 100% is NaY.

taken from ref. 20.

Pd); S(Pd)

IZaZ6

I 3745

1

1 1.07

0.70

3 to 10.7 nm. Similar values have already been reported for palladium in a NaHY zeolite [ 211. The SY zeolite is not otherwise modified upon reduction at 773 or 873 K as the crystallinity is essentially maintained (Table 4). In the IR spectrum, the band at 3626 cm-’ due to the framework hydroxyls (HF) is slightly less intense than on A (573 ) , while the band at 3745 cm- ‘, characteristic of terminal Si-OH species, is nearly unaffected (Table 4). Therefore the acidity of the zeolite does not greatly depend upon the temperature of reduction.

Reaction mechanism When the Pd/SY catalyst is reduced at 773 K or above, the conversion of nnonane after a short time on stream is about the same as for a catalyst reduced at 573 K. The acidity of the zeolite is still very high, but the hydrogenation function is much poorer, due to a loss in palladium dispersion. The concentration of nonene is then too small to allow the rapid desorption of carbenium ions required for ideal hydrocracking, and the amount of C9 isomers becomes

29

negligible. Since the rearrangements and the splitting of the carbenium ions are not affected, cracking appears as a primary process [ 91. Among the cracked products, the degree of branching is generally higher than in the ideal case, which means that most of the C-C& still arise through splitting of a tribranched nonyl carbenium ion (mechanism A), and there is no secondary isomerization of the cracked products. However, the quantities of C, and C, greatly exceed those of C, and C5, and this cannot be related to any secondary cracking, which would only convert the Cs into C,. Thus, in addition to the main monomolecular mechanisms (A and B ) , another process contributes to the cracking reaction. The importance of this second process may be roughly estimated using the data in Table 1. Considering the distribution over catalyst A (573) as characteristic for the main monomolecular process, the excess of C, and C, observed in the initial period over A (773) is about equivalent to the missing C, and Ce and corresponds to 15% of the cracked C,. This departure from ideal behaviour cannot be explained by protolytic cracking, i.e. by the splitting of carbonium ions. This process occurs at much higher temperatures, and produces C1 and C, as well. Since the alkenes arising from primary cracking are slowly hydrogenated, it is suggested that heavy carbenium ions are formed via bimolecular processes [22] such as: C,H,, +C,H& - [C,+,H&+,,]

dcracking

The presence of some heptanes among the reaction products may be explained in this way. The heavy carbenium ions may undergo more than three successive splitting since, on average, every C, is cracked into more than 2 molecules. The very large amount of isobutane suggests that these carbenium ions are highly branched. But they are also likely to be coke precursors, so that the reaction mechanism at short time-on-stream may be written as: iso-Cg n-C9

\

d-+

’

coke

(Scheme

3’)

I cracking/

The rapid deactivation of catalyst A (773) did not allow measurements at several flow rates, and the rate constants in the above scheme could not be determined. The activity level of the catalyst became quasi-constant after 6 h on stream at 473 K, while the coke deposit amounted to 3%. Incidentally, the number of Cg molecules that had been converted was about equal to the number of Brcansted acid sites on the zeolite. At this stage, the distribution of the cracked products gets near to the ideal hydrocracking pattern (Table 1) . The amount of isomeric C, is much higher

30

than over the fresh catalyst, and the cracked products appear after the C, isomers (Fig. 8). Thus, acid centers have been poisoned by the build-up of coke, and the balance between the acidic and the hydrogenation function becomes closer to the ideal. However, there are significant differences between the cracking distributions observed over the aged A (773 ) and the ideal A (573) catalysts, the former giving more heptanes and isopentane. The increase in C; suggests that the bimolecular mechanism remains important over the coked zeolites; this is even more pronounced for a catalyst reduced at 873 K (Table 1). However, fewer than two fragments are formed for every cracked Cg, meaning that the splitting of the heavy carbenium ions becomes less frequent upon ageing. The increase in branching of the C, and C6 observed during the course of catalyst deactivation is more difficult to explain. It has been previously stated that the degrees of branching p,, obtained in ideal hydrocracking were lower than with hydrocracking catalysts of reduced hydrogenation activity, and much lower than those found over cracking catalysts [ 71. In the present case, the increase in p5 and ps is associated with a drop in overall conversion. This may be the main factor, but an evolution of the bimolecular process with catalyst age may also be important. CONCLUSIONS

The conversion of n-nonane is very sensitive to the balance between the hydrogenation and the acidic function of the Pd/SY catalysts. The hydrogenation function is predominant over the catalyst reduced below 700 K, and the reaction may be analyzed using a simplified consecutive network. Upon reduction at 773 K or above, the palladium dispersion considerably decreases, and the acidic function becomes predominant. Cracking is the only reaction observed after a brief time on stream, and coke deposition takes place. The acidic centers are deactivated more rapidly than the dehydrogenation centers. ACKNOWLEDGEMENTS

We thank Prof. J. Weitkamp (Stuttgart), Drs. C. Marcilly and P. Dufresne (Rueil) for helpful discussions, and Dr. R. Scymanski (Rueil), who performed the TEM measurements.

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