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Hzsm-5 Zeolite
P.A. Jacobs et al. (Editors), Structure and Reactivity of Modified Zeolites
e 1984 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
279
SHAPE SELECTIVE ISOMERIZATION AND HYDROCRACKING OF NAPHTHENES OVER Pt/HZSM-5 ZEOLITE
J. WEITKAMp 1,2, P.A. JACOBS 3, and S. ERNST 1 1Engler-Bunte-Institute, Division of Gas, Oil, and Coal, University of Karlsruhe, D-7500 Karlsruhe 1 (Federal Republic of Germany) 2Author for correspondence 3Centrum voor Oppervlaktescheikunde en Colloidale Scheikunde, Katholieke Universiteit Leuven, B-3030 Leuven (Belgium)
ABSTRACT The pure naphthenes methylcyclopentane, methylcyclohexane, and ethylcyclohexane were converted over a bifunctional Pt/HZSM-5 zeolite catalyst under hydrogen pressure. For comparison the reactions of ethylcyclohexane were investigated over two large pore zeolites, viz. Pd/LaY and Pt/CaY. A variety of shape selectivity effects are encountered over Pt/HZSM-5, especially in the isomerization of methylcyclohexane and ethylcyclohexane. INTRODUCTI ON In the past, considerable efforts were undertaken to elucidate the mechanisms of catalytic hydrocarbon conversion via carbocations (ref. 1). Particular attention was devoted to large pore zeolite catalysts, .especially faujasites, due to their widespread use in the petroleum refining industry. It has been shown that model studies with alkanes of reasonably long carbon chains and suitably selected bifunctional zeolite catalysts furnish most valuable insight into the mechanisms of rearrangement (ref. 2,3) and S-scission (ref. 4,5) of alkylcarbenium ions. Much less work has been done with naphthenes even though they represent another major class of petroleum hydrocarbons. Voorhies et al. reported on the kinetics of cyclohexane (ref. 6,7) and decalin (ref. 8) hydroconversion over acidic mordenites and faujasites loaded with palladium. Schulz et al. (ref. 9) found that the isomerization of C6-naphthenes on Pt/CaY is accompanied by a disproportionation reaction leading to C7-/Ce-naphthenes and Cs-/C 4-alkanes. The isomerization of ethylcyclohexane and other naphthenes over non-zeolitic catalysts such as Ni/Si0 2-A1 20 3 was thoroughly studied by Pines et al. (ref. 10-12). More recently, zeolite catalysts of medium pore width were introduced on a commercial scale, e.g., in M-forming (ref. 13) and dewaxing (ref. 14). In these refinery processes catalysts of the ZSM-5 type are applied (ref. 13).
280
It is most likely that, inside the pores of such zeolites, the conversion of hydrocarbons is again determined by the chemistry of carbocations which, however, is modified significantly by shape selectivity effects~ It is common practice to classify the latter into (i) reactant shape selectivity, (ii) product shape selectivity, and (iii) restricted transition state shape selectivity, according to Csicsery's proposal (ref. 15,16). A variety of product and restricted transition state shape selectivity effects were recently shown to occur in the hydroconversion of long-chain n-alkanes over Pt/HZSM-5 zeolites (ref. 17,18). It is the intention of the present paper to extend these investigations to naphthenic model hydrocarbons with 6 to 8 carbon atoms. For comparison, some few data will be added which were obtained on large pore zeolite catalysts, viz. Pd/LaY and Pt/CaY. EXPERIMENTAL The pure naphthenes methylcyclopentane (M-CPn, purity 99.95 wt.-%, 0.05 % CHx), methylcyclohexane (M-CHx, no detectable impurities), and ethylcyclohexane (E-CHx, purity 99.06 wt.-%, 0.08 % heptanes, 0.21 %M-CHx, 0.48 %other Ca-naphthenes, 0.17 % unidentified impurities, presumably ethylcyclohexenes) were catalytically converted in the gas phase under hydrogen pressure in a fixed bed consisting of 0.47 g of Pt/HZSM-5. The flow type apparatus has been described in detail (ref. 19,20). The content of noble metal and the molar ratio Si/Al of the catalyst were 0.5 wt.-% and 60, respectively. Its preparation has been described previously (ref. 17,18). The 0.5 PtjCaY-86 zeolite was a commercial sample (Union Carbide, SK ZOO) which has been used extensively in prior studies by the Karlsruhe group (e.g., ref. 3). The 0.27 Pd/LaY-72 zeolite was prepared from NaY by ion exchange with an aqueous solution of La(N0 3 ) 3 at 80 DC followed by ion exchange with [Pd(NH 3)4]C1 2 • The Pd content, the molar ratio Si/Al, the total content of La and Na cations per Al, and the La ion exchange level amounted to 0.27 wt.-%, 2.46, 1.0 equiv.jmol, and 72 equiv.-%, respectively. All catalysts were used in a particle size between 0.2 and 0.3 mm and pretreated successively in a purge of oxygen at 300 DC, a purge of nitrogen at 350 DC, and a purge of hydrogen at 300 DC. High resolution capillary GLC with a flame ionization detector was employed for product analysis. During a run, at least two product samples were analyzed in the on-line mode, usually with polypropylene glycol (PPG) as stationary phase. In addition, liquid product samples were collected at ca. -190 DC which were later analyzed off-line using a different stationary GLC phase. In the experiments with ethylcyclohexane the combined application of a relatively polar (PPG) and a non-polar phase (e.g., OV-1) was a prerequisite for achieving satisfactory separation of feed isomers, especially when octanes were present. It was found that the variation of the temperature program in the
281
GLC oven is a valuable tool for separating unresolved peaks. The assignment of product peaks was based on the experience acquired in preceding studies on hydrocarbon conversion (e.g., ref. 3,9,19), on the boiling points of isomeric Cs-naphthenes, and ancillary GC/MS analyses (Hewlett-Packard 5987 A, ionization by 70 eV electron impact). All commercially available Cs-naphthenes were used as reference substances. Particular care was paid to the identification of methylcycloheptane. Unless otherwise stated the partial pressures of the feed hydrocarbon and hydrogen at the reactor inlet were 19.4 kPa and 2.0 MPa, respectively, and W/F was 135 g.h/mol. The reaction temperature was varied between 200 and 360 cc. Between two successive runs the catalyst was purged in pure hydrogen at 2.0 MPa and 300 cc. RESULTS AND DISCUSSION Conversion and types of reaction It was ascertained in preliminary experiments and by repeated control runs that there was no catalyst deactivation under the conditions applied in this study. In Figure 1 the conversions of the three naphthenes on Pt/HZSM-5 are plotted versus the reaction temperature. For comparison the conversion of ethylcyclohexane on the Pd/LaY zeolite is also given. As expected, the reactivity of the hydrocarbons increases with increasing carbon number. The Y type· 100
80 ~
0
Z
0
•
E-CHx 0 E-CHx 0 M-CHx A M-CPn
on on on on
II II
60
cJ)
a:
w >
Z
40
0
u
20
220
240
300
320
340
REACTION TEMPERATURE.·C Fig. 1. Conversion of naphthenes over bifunctional zeolite catalysts.
360
282
zeolite is slightly more active than Pt/HZSM-5 which is in all probability due to the higher ~umb~ of Br~nsted acid sites in Pd/LaY. Earlier TPD experiments with NH 3 revealed that HZSM-5 possesses very strong acidic sites (ref. 21) whereas it is a reasonable assumption that sites of intermediate acid strength predominate in Pd/LaY with its moderate exchange level of 72 %. The following types of reaction of the naphthenes will be distinguished: (i) Isomerization to other naphthenes, (ii) ring opening to alkanes with the same carbon number as the feed, (iii) hydrocracking to alkanes or cycloalkanes containing less carbon atoms than the feed, and (iv) formation of aromatics with the carbon number of the feed. It is evident from Figures 2 and 3 that under mild conditions isomerization is the sole reaction, regardless of the catalyst and the feed. Under more severe conditions on Pt/HZSM-5 methylcyclopentane undergoes mainly ring opening while hydrocracking predominates with methylcyclohexane and ethylcyclohexane. On Pd/LaY ca. 90 % of the ethylcyclohexane feed can be isomerized without substantial carbon-carbon bond rupture. It is noteworthy that on Pt/HZSM-5 methylcyclopentane does not disproportionate to any measurable extent as it does on, e.g., Pt/CaY (ref. 9). The potential products of such a disproportionation, i.e., C4 - and Cs-alkanes as well as Ca- and C7-naphthenes, could at least in part diffuse out of the intracrystalline framework since they are formed by hydrocracking and isomerization of M-CHx and E-CHx (vide infra). Therefore, it is concluded that the lack of disproportionation reactions in Pt/HZSM-5 is due to restricted transition state shape selectivity. Indeed, a bimolecular alkylation step is involved in the disproportionation reaction (ref. 9) which requires a bulky transition state. Evidence for some contribution of bimolecular reactions of ethylcyclohexane on Pd/LaY will be presented below. Isomerization Both on Pd/LaY and Pt/HZSM-5 isomerization proceeds in a stepwise manner. E.g., the primary products from ethylcyclohexane are the other monobranched Ca-naphthenes (Fig. 4), viz. mainly propylcyclopentane and little methylcycloheptane. Dibranched and tribranched isomers are formed in consecutive reactions. Presumably, the isomer distribution found on Pd/LaY around 300 DC is close to thermodynamic equilibrium. On Pt/HZSM-5 much higher selectivities of dibranched isomers are encountered. Probably, diffusion of most, if not all tribranched isomers (trimethylcyclopentanes) out of the ZSM-5 intracrystalline framework is severely hindered. It is reasonable to assume that, for the most part, these bulky isomers are. formed on the exten~al HZSM-5 surface or on platinum clusters located at the ext~t~al zeolite surface. The rapid formation of monobranched isomers from ethylcyclohexane is easily understood since this is a so-called type A isomerization (ref. 2,3,23): By definition, the number of ramifications remains constant in type A
283
100 IT---,--r--,---r----.-----,
FEED: M-CHx
FEED: M -CPn
ao ~ o
0
Cyclohexane
o
CI
Hexanes
o
Other C7 - Naphthenes Heptanes
to Cracked
to Cracked Products
60
'7
Products
Toluene
o
...J UJ
>
40
20
300
250
350 REACTION
250 300 TEMPERATURE,·C
350
Fig. 2. Yields of products from methylcyclopentane and methylcyclohexane over Pt/HZSM-5.
100
.0
eo Other Ca - NaPhthene~
80
-
66 ."7
0
0
60
0
...J
w >
Octanes Cracked Products Ca -Aromdtics
~ PtlHZSM-5 Pd I La Y
40
200
220
240
260 2aO 300 320 340 REACTION TEMPERATURE,·C
Fig. 3. Yields of products from ethylcyclohexane over Pd/LaY and Pt/HZSM-5.
360
284
TABLE Selectivity of isomerization of methylcyclohexane on Pt/HZSM-5 (expressed as moles of corresponding C7-naphthene formed per moles of M-CHx isomerized) and thermodynamic equilibrium distribution 1) of C 7-naphthenes. T,
°C % XM- CHx' YIsomers' %
300 50.4 40.3
220 250 1.0 11.2 1.0 11.0
360 90.5 14.4
227 327 Equil i bri um Distribution,
Selectivity, % M-CHx E-CPn 1,1-DM-CPn 1c2-DM-CPn lt2-DM-CPn 1c3-DM-CPn 1t3-DM-CPn
38.4 0.1 0 0.1 30.7 30.7
68 0 0 0 17 15
mol-%
21.8 1.8 0.2 0.5 37.8 37.9
57.9 4.8 7.2 2.0 12.3 10.0 5.8
17.5 9.0 4.3 15.9 26.6 26.7
38.8 9.2 9.0 3.6 16.5 14.0 8.9
1)Calculated from ref. 22 isomerizations whereas in type B isomerizations a new branching forms or an existing one disappears. It has been found previously with aliphatic substrates and a Pt/CaY zeolite (ref. 2) that type A isomerizations are somewhat faster
PtlHZSM - 5
Pdf La Y 100 .........---r---,.---r---r-,
dibranched
>-
I-
>
IU
monobranched
l1J
..J
l1J
CI'l
O~-'-"':::::;""""T""-.....r;=¥-J
200
250
300
REACTION
, "
200
250
300
350
TEMPERATURE • ·C
Fig. 4. Isomers formed from ethylcyclohexane (selectivity is defined as in Table 2).
285
TABLE 2 Selectivity of isomerization of ethylcyclohexane (expressed as moles of formed per moles of E-CHx isomerized in %). corresponding C 8-naphthene Catalyst T, °C PE-CHx' kPa PH2' MPa W/F, g'h/mol X, %
Y1somers , % P-CPn M-CHp
220 19.4 2.0 135 1.9 1.9 85.7 0
Pt/HZSM-5 290 240 19.4 19.4 2.0 2.0 135 135 78.1 9.0 8.9 60.4 48.5 0
5.3 0.1
330 19.4 2.0 135 97.6 19.5 4.1 0.1
Pt/CaY 220 25.1 3.9 250 3.9 3.9 59.9 6.1
Pd/LaY 200 280 19.4 19.4 2.0 2.0 280 135 4.9 87.6 4.9 85.7 58.8 4.0
4.3 0.1
-------------------------------------------------------------------------
(1-M-E)-CPn 0.5 2.8 0 0 2.1 0 0.8 0 0.3 1-E-1-M-CPn 1.4 1.1 1.7 1.3 0 1-E-c-2-M-CPn 0 1.9 0 0 1.1 2.8 0.7 13.8 1-E-t-2-M-CPn 0 0 1.5 14.7 4.7 0 1-E-c-3-M-CPn 17.5 17.0 2.2 2.1 6.0 15.4 4.2 1-E-t-3-M-CPn 6.6 15.3 17.4 3.3 3.7 12.5 8.8 1,1-DM-CHx 0 0.1 0 1c2-DM-CHx 1.8 0 0 0.4 1.6 0 1.2 5.4 1t2-DM-CHx 0 0 0.3 4.0 3.3 8.5 4.1 1c3-DM-CHx 0 0 0 5.4 30.0 32.4 1t4-DM-CHx 1.7 16.0 40.1 0 0 1t3-DM-CHx 0 0 0 5.8 0.9 1.4 9.7 1c4-DM-CHx 0 4.3 13.6 6.5 ------------------------------------------------------------------------1,1,2-TM-CPn 0 0 0 0 1.0 0 2.7 1,1,3-TM-CPn 0 0 0 1.7 3.3 0 6.8 1c2c3-TM-CPn 0 0 0 0 0 0 0.1 0 1c2t3-TM-CPn 0 0.1 1.0 0 1.9 0 0 1t2c3-TM-CPn 0 0.2 2.2 0 0 5.1 0 0 1c2c4-TM-CPn 0 0 0.1 0.6 1.5 0 1c2t4-TM-CPn 0 0 0 0 0.4 1t2c4-TM-CPn 0 0 3.0 0 7.6 than type B isomerizations. Mechanistically, type A rearrangements via carbocations are best accounted for by consecutive hydride and alkyl shifts while type B rearrangements are nowadays (ref. 3,23,24) believed to proceed via protonated cyclopropanes (PCPs). More detailed information on the selectivities of isomerization is presented in Tables 1 and 2. The type B isomerization of methylcyclohexane (Table 1) exhibits a pronounced shape selectivity: Up to at least 50 % conversion virtually no 1,1- and 1,2-dimethylcyclopentane are formed while both 1,3-isomers occur even at very low conversion. A clear discrimination between product and restricted transition state shape selectivity cannot be made as long as the
286
diffusion coefficients of the isomers are unknown. At this stage of the investigation, we believe that product shape selectivity is mainly responsible for the absence of 1,1-, 1cis2-, and 1trans2-dimethylcyclopentane. Even more pronounced shape selectivity effects are observed in the isomerization of ethylcyclohexane (Table 2). Whereas on Pt/CaY and Pd/LaY almost all possible isomers with two branchings occur at low conversion 1-ethyl-3-methylcyclopentane nnd 1,4-dimethylcyclohexane are the preferred products on Pt/HZSM5. A comparison of the low conversion data for the three zeolites clearly reveals that the selectivity is determined by the type of zeolite rather than by the nature of the noble metal. This rules out that isomerization on Pt/HZSM-5 occurs on external Pt clusters, except perhaps for the formation of trimethylpentanes. It is also interesting to notice that small amounts of methylcycloheptane are formed from ethylcyclohexane, especially on the large pore zeolites. Probable routes from ethylcyclohexyl cations to the products observed at low conversions are depicted in Figures 5 and 6. Based on earlier results obtained with aliphatic substrates (ref. 2,25) it was assumed that formation of PCPs (A,B,D in Fig. 5) starts from the set of secondary cations rather than from the tertiary cation. It is evident that the latter can only react to one particular PCP designated A in Fig. 5. However, such a pathway can be neglected at least for Pt/HZSM-5 since the predicted isomers formed via A are absent. The formation of dimethylcyclohexanes can be explained by a type A rearrangement starting from ethylmethylcyclopentyl cations with the positive charge in the side chain (Fig. 6). This way, the fast formation of 1,4-dimethylcyclohexane from ethylcyclohexane on Pt/HZSM-5 is readily accounted for. Alternatively, a pathway to dimethylcyclohexanes can be considered .in which the anticipated sequence of type B and type A rearrangements is changed: Type A rearrangements of ethylcyclohexyl cations lead to both propylcyclopentyl and methylcycloheptyl cations. Type B rearrangements of the latter give dimethylcyclohexyl cations with all possible carbon skeletons. It cannot be decided from the results of this study whether the interconversion of cycloalkanes and cycloalkylcarbenium ions proceeds via olefins or not. In principle, the noble metal opens the route via olefins. Another major function of the noble metal is to avoid catalyst deactivation by the formation of coke deposits. Ring opening Carbon-carbon bond rupture within the ring of cycloalkylcarbenium ions is a Slow reaction. This has been explained in terms of unfavorable orbital orientation during S-scission (ref. 26). Hence, the low rate of ring opening ~ven at high conversions of ethylcyclohexane on Pd/LaY is readily understood. The relatively high selectivities for trimethyl isomers (cf. Table 2 for Pd/LaY at 280°C) are in sharp contrast to isomerization of aliphatic hydrocarbons, e.g.,
287
IV
HV~ A _---~
--
.' ~ Il1
--~
CJ<"----,>
t-E-t -M-CPn
~~ l..!r ----> l-E-2-M-CPn <,
o
~CJ'
H--
---+
~ ---+t-E-3-M-CPn
Fig. 5. Type B rearrangements of secondary, ethylcyclohexyl cations. Full arrows lead to products which are strongly favored over Pt/HZSM-5.
- - - - - - - - - - - - - - ->
Ci' ----?
c;( -- ---"
cc
----~
t,2-DM-CHx
-----;
----~
t,3-DM-CHx
1,4-DM-CHx
Fig. 6. Formation of dimethylcyclohexanes by ring enlargement of ethylmethylCyclopentyl cations via tpye A rearrangements.
288
of n-octane over similar bifunctional catalysts (ref. 27,28). In particular, the occurrence of 1,1,3-trimethylcyclopentane is noteworthy since this isomer possesses an a,a,y-tribranched carbon skeleton and, hence, it is capable to undergo the energetically favored type A B-scission as defined in a recent paper (ref. 18). On Pt/HZSM-5 the maximum yield of isomers from ethylcyclohexane is considerably lower than on Pd/LaY (Fig. 3). Further work is needed to decide whether this is due to the higher strength of acidic sites in the ZSM-5 zeolite or to geometric constraints or to other reasons. It is interesting to note that on Pt/HZSM-5 the maximum yields of octanes from ethylcyclohexane (Fig. 3) and of heptanes from methylcyclohexane (Fig. 2) are relatively low. This means that there is no fast mechanism of desorption of the alkenyl cations formed by cleavage of the naphthenic rings. Rather, the cations formed by ring opening undergo consecutive S-scissions into smaller fragments. The alkanes formed by ring opening mainly consist of monomethyl isomers irrespective of the nature of the feed and the catalyst. Besides, the n-alkanes are present but there is no indication whatsoever that n-alkanes are primary products of ionic ring opening. Hence, there is no indication for a so-called direct ring opening via non-classical carbonium ions as proposed in the literature (ref. 29). With ethylcyclohexane as feed 2,4- and especially 2,5-dimethylhexane were always present in the products of ring opening. The latter isomer has the carbon skeleton which one would expect to result by a type As-scission of the tertiary 1,1,3-trimethylcyclopentyl cation. Hydrocracking Typical distributions of the cracked products from ethylcyclohexane are presented in Figure 7. Over Pt/HZSM-5 the carbon number distribution is essentially symmetrical with very low selectivities for CI and Cz and, correspondingly, C7 and CG. This curve is best interpreted in terms of an ionic mechanism of hydrocracking leading to C3 through C5 and a superimposed hydrogenolysis ~n Pt clusters. Relatively large amounts of C3 + Cs are formed on Pt/HZSM-5 compared to hydrocracking of Cs-hydrocarbons on large pore zeolites, e.g., ethylcyclohexane on Pd/LaY (Fig. 7, left side) or n-octane on Pt/CaY (ref. 27). On Pd/LaY neither methane nor ethane are formed at 300°C. Nevertheless, substantial amounts of C7 - and CG-hydrocarbons occur under the same conditions, and slightly more Cs than C3 is found. Most of the C7 - and C6-products are naphthenes. To account for all these findings some contribution of a bimolecular mechanism must be invoked: A Cs species is added to a second one which results in CI 6 unit. The latter undergoes rearrangements until a favorable structure for abstraction of C4 or Cs exists. This way, 2 Cs can be converted into, e.g., C4 + C5 + C7 • According to such a mechanism one could as well expect products with a carbon number above Cs, i.e., the occurrence of a disproportio-
289
Pd/LaY
-e • "0
QI oX
T =300 ·C
Pt/HZSM-5 T
=330
·C
80
v
(1
~
60
)(
r u I
40
UJ
C
-
c.
20
u
c
OL...i"--+--,.--+-~~"'T"""""".....J
1234567
1234567
CARBON NUMBER Cp OF CRACKED PRODUCTS Fig. 7. Hydrocracking of ethylcyclohexane. Distribution of the cracked products. nation type of reaction. However, no hydrocarbons with 9 or more carbon atoms were ever observed during this study. This indicates that the desorption of such intermediates from the acidic sites is slow compared to their cleavage. Hydrocracking of methyl cycl ohexane over Pt/HZSII-5 gi ves mainly propane and i-butane beside little n-butane and small amounts of hydrocarbons formed by hydrogenolysis. In contrast, the cracked products from methylcyclopentane, e.g., at 360°C contain large amounts of methane, ethane, butanes, and pentanes, i.e., the contribution of hydrogenolysis is high. This is another example for the fact that model hydrocarbons with six (or less) carbon atoms are often excluded from the favored pathways of ionic cracking and escape into different mechanisms (cf. ref. 27). The use of such small hydrocarbons for the investigation of ionic mechanisms is, therefore, discouraged. ACKNOWLEDGEMENTS The authors thank Mr. W. Stober and Mr. A. Dietl for valuable technical assistance and Mr. S. Maixner who carried out the mass spectroscopy work. Financial support by Deutsche Forschungsgemeinschaft and Fonds der Chemischen Industrie is gratefully acknowledged. REFERENCES 1 M.L. Poutsma, in J.A. Rabo (Ed.), Zeolite Chemistry and Catalysis, ACS Monograph 171, American Chemical Society, Washington, D.C., 1976, pp. 437-528. 2 J. Weitkamp and H. Farag, Acta Universitatis Szegediensis, Acta Physica et
290
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
Chemica, 24 (1978) 327-333. J. Weitkamp, Ind. Eng. Chern., Prod. Res. Dev., 21 (1982) 550-558. J. Weitkamp, Erddl, Kahle - Erdgas - Petrochem., 31 (1978) 13-22. M. Steijns, G. Froment, P. Jacobs, J. Uytterhoeven and J. Weitkamp, Ind. Eng. Chern., Prod. Res. Dev., 20 (1981) 654-660. A. Voorhies, Jr. and J.R. Hopper, in E.M. Flanigen and L.B. Sand (Eds.), Molecular Sieve Zeolites II, Adv. Chern. Ser. 102, American Chemical Society, Washington, D.C., 1971, pp. 410-416. , M.G. Luzarraga and A. Voorhies, Jr., Ind. Eng. Chern., Prod. Res. Dev., 12 (1973) 194-198. R. Beecher, A. Voorhies, Jr. and P. Eberly, Jr., Ind. Eng. Chern., Prod. Res. Dev., 7 (1968) 203-209. H. Schulz, J. Weitkamp and H. Eberth, Proc. 5th Intern. Congr. Catalysis, J.W. Hightower (Ed.), Vol. 2, North-Holland Publishing Co., Amsterdam, 1973, pp, 1229-1239. H. Pines and A.W. Shaw, J. Am. Chern. Soc., 79 (1957) 1474-1482. H. Pines and A.W. Shaw, Adv. Catal. 9 (1957) 569-574. H. Pines andN.E. Hoffman, in G.A. Olah (Ed.), Friedel-Crafts and Related Reactions, Vol. II, Part 2, Interscience Publishers, New York, London, Sydney, 1964, pp. 1211-1252. H. Heinemann, in J.R. Anderson and M. Boudart (Eds.), Catalysis - Science and Technology, Vol. 1, Springer-Verlag, Berlin, Heidelberg, New York, 1981, pp , 1-41. H.R. Ireland, C. Redini, A.S. Raff and L. Fava, Hydrocarbon Process., 58 (No.5, 1979) 119-122. S.M. Csicsery, in J.A. Rabo (Ed.), Zeolite Chemistry and Catalysis, ACS Monograph 171, American Chemical Society, Washington, D.C., 1976, pp. 680-713. S.M. Csicsery, Preprints, Div. Fuel Chern., Am. Chern. Soc., 28 (No.2, 1983) 116-126. P.A. Jacobs, J.A. Martens, J. Weitkamp and H.K. Beyer, Faraday Discuss. Chern. SOC. 72 (1982) 353-369. J. Weitkamp, P.A. Jacobs and J.A. Martens, Appl. Catal. 8 (1983) 123-141. H. Schulz and J. Weitkamp, Ind. Eng. Chern., Prod. Res. Dev., 11 (1972) 46-53. J. Weitkamp and K. Hedden, Chem.-Ing.-Tech., 47 (1975) 537. P.A. Jacobs, J.B. Uytterhoeven, M. Steijns, G. Froment and J. Weitkamp, in L.V.C. Rees (Ed.), Proc. 5th Intern. Conference Zeolites, Heyden, London, Philadelphia, Rheine, 1980, pp. 607-615. D.R. Stull, E.F. Westrum, Jr. and G.C. Sinke, The Chemical Thermodynamics of Organic Compounds, John Wiley &Sons, New York, London, Sydney, Toronto, 1969, pp. 348-350 and 357. D.M. Brouwer and H. Hogeveen, in R.W. Taft and A. Streitwieser, Jr. (E~s.), Progr. Phys. Org. Chern., Vol. 9, Interscience Publishers, New York, London, Sydney, Toronto, 1972, pp. 179-240. J. Weitkamp, in T. Seiyama and K. Tanabe (Eds.), Proc. 7th Intern. Congr. Catal., Elsevier Scientific Publishing Co., Amsterdam, Oxford, New York, 1981, pp , 1404-1405. J. Weitkamp and P.A. Jacobs, Preprints, Div. Petro Chern., Am. Chern. Soc. 26 (1981) 9-13. D.M. Brouwer and H. Hogeveen, Reel. Trav. Chim. Pays-Bas, 89 (1970) 211224. J. Weitkamp, in J.W. Ward and S.A. Qader (Eds.), Hydrocracking and Hydrotreating, Am. Chern. Soc. Symp. Ser. 20, American Chemical Society, Washington, D.C., 1979, pp.1-27. H. Vansina, M.A. Baltanas and G.F. Froment, Ind. Eng. Chern., Prod. Res. Dev., 22 (1983) 526-531. E.G. Christoffel and K.-H. Robschlager, Ind. Eng. Chern., Prod. Res. Dev., 17 (1978) 331-334.
e 1984 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
279
SHAPE SELECTIVE ISOMERIZATION AND HYDROCRACKING OF NAPHTHENES OVER Pt/HZSM-5 ZEOLITE
J. WEITKAMp 1,2, P.A. JACOBS 3, and S. ERNST 1 1Engler-Bunte-Institute, Division of Gas, Oil, and Coal, University of Karlsruhe, D-7500 Karlsruhe 1 (Federal Republic of Germany) 2Author for correspondence 3Centrum voor Oppervlaktescheikunde en Colloidale Scheikunde, Katholieke Universiteit Leuven, B-3030 Leuven (Belgium)
ABSTRACT The pure naphthenes methylcyclopentane, methylcyclohexane, and ethylcyclohexane were converted over a bifunctional Pt/HZSM-5 zeolite catalyst under hydrogen pressure. For comparison the reactions of ethylcyclohexane were investigated over two large pore zeolites, viz. Pd/LaY and Pt/CaY. A variety of shape selectivity effects are encountered over Pt/HZSM-5, especially in the isomerization of methylcyclohexane and ethylcyclohexane. INTRODUCTI ON In the past, considerable efforts were undertaken to elucidate the mechanisms of catalytic hydrocarbon conversion via carbocations (ref. 1). Particular attention was devoted to large pore zeolite catalysts, .especially faujasites, due to their widespread use in the petroleum refining industry. It has been shown that model studies with alkanes of reasonably long carbon chains and suitably selected bifunctional zeolite catalysts furnish most valuable insight into the mechanisms of rearrangement (ref. 2,3) and S-scission (ref. 4,5) of alkylcarbenium ions. Much less work has been done with naphthenes even though they represent another major class of petroleum hydrocarbons. Voorhies et al. reported on the kinetics of cyclohexane (ref. 6,7) and decalin (ref. 8) hydroconversion over acidic mordenites and faujasites loaded with palladium. Schulz et al. (ref. 9) found that the isomerization of C6-naphthenes on Pt/CaY is accompanied by a disproportionation reaction leading to C7-/Ce-naphthenes and Cs-/C 4-alkanes. The isomerization of ethylcyclohexane and other naphthenes over non-zeolitic catalysts such as Ni/Si0 2-A1 20 3 was thoroughly studied by Pines et al. (ref. 10-12). More recently, zeolite catalysts of medium pore width were introduced on a commercial scale, e.g., in M-forming (ref. 13) and dewaxing (ref. 14). In these refinery processes catalysts of the ZSM-5 type are applied (ref. 13).
280
It is most likely that, inside the pores of such zeolites, the conversion of hydrocarbons is again determined by the chemistry of carbocations which, however, is modified significantly by shape selectivity effects~ It is common practice to classify the latter into (i) reactant shape selectivity, (ii) product shape selectivity, and (iii) restricted transition state shape selectivity, according to Csicsery's proposal (ref. 15,16). A variety of product and restricted transition state shape selectivity effects were recently shown to occur in the hydroconversion of long-chain n-alkanes over Pt/HZSM-5 zeolites (ref. 17,18). It is the intention of the present paper to extend these investigations to naphthenic model hydrocarbons with 6 to 8 carbon atoms. For comparison, some few data will be added which were obtained on large pore zeolite catalysts, viz. Pd/LaY and Pt/CaY. EXPERIMENTAL The pure naphthenes methylcyclopentane (M-CPn, purity 99.95 wt.-%, 0.05 % CHx), methylcyclohexane (M-CHx, no detectable impurities), and ethylcyclohexane (E-CHx, purity 99.06 wt.-%, 0.08 % heptanes, 0.21 %M-CHx, 0.48 %other Ca-naphthenes, 0.17 % unidentified impurities, presumably ethylcyclohexenes) were catalytically converted in the gas phase under hydrogen pressure in a fixed bed consisting of 0.47 g of Pt/HZSM-5. The flow type apparatus has been described in detail (ref. 19,20). The content of noble metal and the molar ratio Si/Al of the catalyst were 0.5 wt.-% and 60, respectively. Its preparation has been described previously (ref. 17,18). The 0.5 PtjCaY-86 zeolite was a commercial sample (Union Carbide, SK ZOO) which has been used extensively in prior studies by the Karlsruhe group (e.g., ref. 3). The 0.27 Pd/LaY-72 zeolite was prepared from NaY by ion exchange with an aqueous solution of La(N0 3 ) 3 at 80 DC followed by ion exchange with [Pd(NH 3)4]C1 2 • The Pd content, the molar ratio Si/Al, the total content of La and Na cations per Al, and the La ion exchange level amounted to 0.27 wt.-%, 2.46, 1.0 equiv.jmol, and 72 equiv.-%, respectively. All catalysts were used in a particle size between 0.2 and 0.3 mm and pretreated successively in a purge of oxygen at 300 DC, a purge of nitrogen at 350 DC, and a purge of hydrogen at 300 DC. High resolution capillary GLC with a flame ionization detector was employed for product analysis. During a run, at least two product samples were analyzed in the on-line mode, usually with polypropylene glycol (PPG) as stationary phase. In addition, liquid product samples were collected at ca. -190 DC which were later analyzed off-line using a different stationary GLC phase. In the experiments with ethylcyclohexane the combined application of a relatively polar (PPG) and a non-polar phase (e.g., OV-1) was a prerequisite for achieving satisfactory separation of feed isomers, especially when octanes were present. It was found that the variation of the temperature program in the
281
GLC oven is a valuable tool for separating unresolved peaks. The assignment of product peaks was based on the experience acquired in preceding studies on hydrocarbon conversion (e.g., ref. 3,9,19), on the boiling points of isomeric Cs-naphthenes, and ancillary GC/MS analyses (Hewlett-Packard 5987 A, ionization by 70 eV electron impact). All commercially available Cs-naphthenes were used as reference substances. Particular care was paid to the identification of methylcycloheptane. Unless otherwise stated the partial pressures of the feed hydrocarbon and hydrogen at the reactor inlet were 19.4 kPa and 2.0 MPa, respectively, and W/F was 135 g.h/mol. The reaction temperature was varied between 200 and 360 cc. Between two successive runs the catalyst was purged in pure hydrogen at 2.0 MPa and 300 cc. RESULTS AND DISCUSSION Conversion and types of reaction It was ascertained in preliminary experiments and by repeated control runs that there was no catalyst deactivation under the conditions applied in this study. In Figure 1 the conversions of the three naphthenes on Pt/HZSM-5 are plotted versus the reaction temperature. For comparison the conversion of ethylcyclohexane on the Pd/LaY zeolite is also given. As expected, the reactivity of the hydrocarbons increases with increasing carbon number. The Y type· 100
80 ~
0
Z
0
•
E-CHx 0 E-CHx 0 M-CHx A M-CPn
on on on on
II II
60
cJ)
a:
w >
Z
40
0
u
20
220
240
300
320
340
REACTION TEMPERATURE.·C Fig. 1. Conversion of naphthenes over bifunctional zeolite catalysts.
360
282
zeolite is slightly more active than Pt/HZSM-5 which is in all probability due to the higher ~umb~ of Br~nsted acid sites in Pd/LaY. Earlier TPD experiments with NH 3 revealed that HZSM-5 possesses very strong acidic sites (ref. 21) whereas it is a reasonable assumption that sites of intermediate acid strength predominate in Pd/LaY with its moderate exchange level of 72 %. The following types of reaction of the naphthenes will be distinguished: (i) Isomerization to other naphthenes, (ii) ring opening to alkanes with the same carbon number as the feed, (iii) hydrocracking to alkanes or cycloalkanes containing less carbon atoms than the feed, and (iv) formation of aromatics with the carbon number of the feed. It is evident from Figures 2 and 3 that under mild conditions isomerization is the sole reaction, regardless of the catalyst and the feed. Under more severe conditions on Pt/HZSM-5 methylcyclopentane undergoes mainly ring opening while hydrocracking predominates with methylcyclohexane and ethylcyclohexane. On Pd/LaY ca. 90 % of the ethylcyclohexane feed can be isomerized without substantial carbon-carbon bond rupture. It is noteworthy that on Pt/HZSM-5 methylcyclopentane does not disproportionate to any measurable extent as it does on, e.g., Pt/CaY (ref. 9). The potential products of such a disproportionation, i.e., C4 - and Cs-alkanes as well as Ca- and C7-naphthenes, could at least in part diffuse out of the intracrystalline framework since they are formed by hydrocracking and isomerization of M-CHx and E-CHx (vide infra). Therefore, it is concluded that the lack of disproportionation reactions in Pt/HZSM-5 is due to restricted transition state shape selectivity. Indeed, a bimolecular alkylation step is involved in the disproportionation reaction (ref. 9) which requires a bulky transition state. Evidence for some contribution of bimolecular reactions of ethylcyclohexane on Pd/LaY will be presented below. Isomerization Both on Pd/LaY and Pt/HZSM-5 isomerization proceeds in a stepwise manner. E.g., the primary products from ethylcyclohexane are the other monobranched Ca-naphthenes (Fig. 4), viz. mainly propylcyclopentane and little methylcycloheptane. Dibranched and tribranched isomers are formed in consecutive reactions. Presumably, the isomer distribution found on Pd/LaY around 300 DC is close to thermodynamic equilibrium. On Pt/HZSM-5 much higher selectivities of dibranched isomers are encountered. Probably, diffusion of most, if not all tribranched isomers (trimethylcyclopentanes) out of the ZSM-5 intracrystalline framework is severely hindered. It is reasonable to assume that, for the most part, these bulky isomers are. formed on the exten~al HZSM-5 surface or on platinum clusters located at the ext~t~al zeolite surface. The rapid formation of monobranched isomers from ethylcyclohexane is easily understood since this is a so-called type A isomerization (ref. 2,3,23): By definition, the number of ramifications remains constant in type A
283
100 IT---,--r--,---r----.-----,
FEED: M-CHx
FEED: M -CPn
ao ~ o
0
Cyclohexane
o
CI
Hexanes
o
Other C7 - Naphthenes Heptanes
to Cracked
to Cracked Products
60
'7
Products
Toluene
o
...J UJ
>
40
20
300
250
350 REACTION
250 300 TEMPERATURE,·C
350
Fig. 2. Yields of products from methylcyclopentane and methylcyclohexane over Pt/HZSM-5.
100
.0
eo Other Ca - NaPhthene~
80
-
66 ."7
0
0
60
0
...J
w >
Octanes Cracked Products Ca -Aromdtics
~ PtlHZSM-5 Pd I La Y
40
200
220
240
260 2aO 300 320 340 REACTION TEMPERATURE,·C
Fig. 3. Yields of products from ethylcyclohexane over Pd/LaY and Pt/HZSM-5.
360
284
TABLE Selectivity of isomerization of methylcyclohexane on Pt/HZSM-5 (expressed as moles of corresponding C7-naphthene formed per moles of M-CHx isomerized) and thermodynamic equilibrium distribution 1) of C 7-naphthenes. T,
°C % XM- CHx' YIsomers' %
300 50.4 40.3
220 250 1.0 11.2 1.0 11.0
360 90.5 14.4
227 327 Equil i bri um Distribution,
Selectivity, % M-CHx E-CPn 1,1-DM-CPn 1c2-DM-CPn lt2-DM-CPn 1c3-DM-CPn 1t3-DM-CPn
38.4 0.1 0 0.1 30.7 30.7
68 0 0 0 17 15
mol-%
21.8 1.8 0.2 0.5 37.8 37.9
57.9 4.8 7.2 2.0 12.3 10.0 5.8
17.5 9.0 4.3 15.9 26.6 26.7
38.8 9.2 9.0 3.6 16.5 14.0 8.9
1)Calculated from ref. 22 isomerizations whereas in type B isomerizations a new branching forms or an existing one disappears. It has been found previously with aliphatic substrates and a Pt/CaY zeolite (ref. 2) that type A isomerizations are somewhat faster
PtlHZSM - 5
Pdf La Y 100 .........---r---,.---r---r-,
dibranched
>-
I-
>
IU
monobranched
l1J
..J
l1J
CI'l
O~-'-"':::::;""""T""-.....r;=¥-J
200
250
300
REACTION
, "
200
250
300
350
TEMPERATURE • ·C
Fig. 4. Isomers formed from ethylcyclohexane (selectivity is defined as in Table 2).
285
TABLE 2 Selectivity of isomerization of ethylcyclohexane (expressed as moles of formed per moles of E-CHx isomerized in %). corresponding C 8-naphthene Catalyst T, °C PE-CHx' kPa PH2' MPa W/F, g'h/mol X, %
Y1somers , % P-CPn M-CHp
220 19.4 2.0 135 1.9 1.9 85.7 0
Pt/HZSM-5 290 240 19.4 19.4 2.0 2.0 135 135 78.1 9.0 8.9 60.4 48.5 0
5.3 0.1
330 19.4 2.0 135 97.6 19.5 4.1 0.1
Pt/CaY 220 25.1 3.9 250 3.9 3.9 59.9 6.1
Pd/LaY 200 280 19.4 19.4 2.0 2.0 280 135 4.9 87.6 4.9 85.7 58.8 4.0
4.3 0.1
-------------------------------------------------------------------------
(1-M-E)-CPn 0.5 2.8 0 0 2.1 0 0.8 0 0.3 1-E-1-M-CPn 1.4 1.1 1.7 1.3 0 1-E-c-2-M-CPn 0 1.9 0 0 1.1 2.8 0.7 13.8 1-E-t-2-M-CPn 0 0 1.5 14.7 4.7 0 1-E-c-3-M-CPn 17.5 17.0 2.2 2.1 6.0 15.4 4.2 1-E-t-3-M-CPn 6.6 15.3 17.4 3.3 3.7 12.5 8.8 1,1-DM-CHx 0 0.1 0 1c2-DM-CHx 1.8 0 0 0.4 1.6 0 1.2 5.4 1t2-DM-CHx 0 0 0.3 4.0 3.3 8.5 4.1 1c3-DM-CHx 0 0 0 5.4 30.0 32.4 1t4-DM-CHx 1.7 16.0 40.1 0 0 1t3-DM-CHx 0 0 0 5.8 0.9 1.4 9.7 1c4-DM-CHx 0 4.3 13.6 6.5 ------------------------------------------------------------------------1,1,2-TM-CPn 0 0 0 0 1.0 0 2.7 1,1,3-TM-CPn 0 0 0 1.7 3.3 0 6.8 1c2c3-TM-CPn 0 0 0 0 0 0 0.1 0 1c2t3-TM-CPn 0 0.1 1.0 0 1.9 0 0 1t2c3-TM-CPn 0 0.2 2.2 0 0 5.1 0 0 1c2c4-TM-CPn 0 0 0.1 0.6 1.5 0 1c2t4-TM-CPn 0 0 0 0 0.4 1t2c4-TM-CPn 0 0 3.0 0 7.6 than type B isomerizations. Mechanistically, type A rearrangements via carbocations are best accounted for by consecutive hydride and alkyl shifts while type B rearrangements are nowadays (ref. 3,23,24) believed to proceed via protonated cyclopropanes (PCPs). More detailed information on the selectivities of isomerization is presented in Tables 1 and 2. The type B isomerization of methylcyclohexane (Table 1) exhibits a pronounced shape selectivity: Up to at least 50 % conversion virtually no 1,1- and 1,2-dimethylcyclopentane are formed while both 1,3-isomers occur even at very low conversion. A clear discrimination between product and restricted transition state shape selectivity cannot be made as long as the
286
diffusion coefficients of the isomers are unknown. At this stage of the investigation, we believe that product shape selectivity is mainly responsible for the absence of 1,1-, 1cis2-, and 1trans2-dimethylcyclopentane. Even more pronounced shape selectivity effects are observed in the isomerization of ethylcyclohexane (Table 2). Whereas on Pt/CaY and Pd/LaY almost all possible isomers with two branchings occur at low conversion 1-ethyl-3-methylcyclopentane nnd 1,4-dimethylcyclohexane are the preferred products on Pt/HZSM5. A comparison of the low conversion data for the three zeolites clearly reveals that the selectivity is determined by the type of zeolite rather than by the nature of the noble metal. This rules out that isomerization on Pt/HZSM-5 occurs on external Pt clusters, except perhaps for the formation of trimethylpentanes. It is also interesting to notice that small amounts of methylcycloheptane are formed from ethylcyclohexane, especially on the large pore zeolites. Probable routes from ethylcyclohexyl cations to the products observed at low conversions are depicted in Figures 5 and 6. Based on earlier results obtained with aliphatic substrates (ref. 2,25) it was assumed that formation of PCPs (A,B,D in Fig. 5) starts from the set of secondary cations rather than from the tertiary cation. It is evident that the latter can only react to one particular PCP designated A in Fig. 5. However, such a pathway can be neglected at least for Pt/HZSM-5 since the predicted isomers formed via A are absent. The formation of dimethylcyclohexanes can be explained by a type A rearrangement starting from ethylmethylcyclopentyl cations with the positive charge in the side chain (Fig. 6). This way, the fast formation of 1,4-dimethylcyclohexane from ethylcyclohexane on Pt/HZSM-5 is readily accounted for. Alternatively, a pathway to dimethylcyclohexanes can be considered .in which the anticipated sequence of type B and type A rearrangements is changed: Type A rearrangements of ethylcyclohexyl cations lead to both propylcyclopentyl and methylcycloheptyl cations. Type B rearrangements of the latter give dimethylcyclohexyl cations with all possible carbon skeletons. It cannot be decided from the results of this study whether the interconversion of cycloalkanes and cycloalkylcarbenium ions proceeds via olefins or not. In principle, the noble metal opens the route via olefins. Another major function of the noble metal is to avoid catalyst deactivation by the formation of coke deposits. Ring opening Carbon-carbon bond rupture within the ring of cycloalkylcarbenium ions is a Slow reaction. This has been explained in terms of unfavorable orbital orientation during S-scission (ref. 26). Hence, the low rate of ring opening ~ven at high conversions of ethylcyclohexane on Pd/LaY is readily understood. The relatively high selectivities for trimethyl isomers (cf. Table 2 for Pd/LaY at 280°C) are in sharp contrast to isomerization of aliphatic hydrocarbons, e.g.,
287
IV
HV~ A _---~
--
.' ~ Il1
--~
CJ<"----,>
t-E-t -M-CPn
~~ l..!r ----> l-E-2-M-CPn <,
o
~
H--
---+
~ ---+t-E-3-M-CPn
Fig. 5. Type B rearrangements of secondary, ethylcyclohexyl cations. Full arrows lead to products which are strongly favored over Pt/HZSM-5.
- - - - - - - - - - - - - - ->
Ci' ----?
c;( -- ---"
cc
----~
t,2-DM-CHx
-----;
----~
t,3-DM-CHx
1,4-DM-CHx
Fig. 6. Formation of dimethylcyclohexanes by ring enlargement of ethylmethylCyclopentyl cations via tpye A rearrangements.
288
of n-octane over similar bifunctional catalysts (ref. 27,28). In particular, the occurrence of 1,1,3-trimethylcyclopentane is noteworthy since this isomer possesses an a,a,y-tribranched carbon skeleton and, hence, it is capable to undergo the energetically favored type A B-scission as defined in a recent paper (ref. 18). On Pt/HZSM-5 the maximum yield of isomers from ethylcyclohexane is considerably lower than on Pd/LaY (Fig. 3). Further work is needed to decide whether this is due to the higher strength of acidic sites in the ZSM-5 zeolite or to geometric constraints or to other reasons. It is interesting to note that on Pt/HZSM-5 the maximum yields of octanes from ethylcyclohexane (Fig. 3) and of heptanes from methylcyclohexane (Fig. 2) are relatively low. This means that there is no fast mechanism of desorption of the alkenyl cations formed by cleavage of the naphthenic rings. Rather, the cations formed by ring opening undergo consecutive S-scissions into smaller fragments. The alkanes formed by ring opening mainly consist of monomethyl isomers irrespective of the nature of the feed and the catalyst. Besides, the n-alkanes are present but there is no indication whatsoever that n-alkanes are primary products of ionic ring opening. Hence, there is no indication for a so-called direct ring opening via non-classical carbonium ions as proposed in the literature (ref. 29). With ethylcyclohexane as feed 2,4- and especially 2,5-dimethylhexane were always present in the products of ring opening. The latter isomer has the carbon skeleton which one would expect to result by a type As-scission of the tertiary 1,1,3-trimethylcyclopentyl cation. Hydrocracking Typical distributions of the cracked products from ethylcyclohexane are presented in Figure 7. Over Pt/HZSM-5 the carbon number distribution is essentially symmetrical with very low selectivities for CI and Cz and, correspondingly, C7 and CG. This curve is best interpreted in terms of an ionic mechanism of hydrocracking leading to C3 through C5 and a superimposed hydrogenolysis ~n Pt clusters. Relatively large amounts of C3 + Cs are formed on Pt/HZSM-5 compared to hydrocracking of Cs-hydrocarbons on large pore zeolites, e.g., ethylcyclohexane on Pd/LaY (Fig. 7, left side) or n-octane on Pt/CaY (ref. 27). On Pd/LaY neither methane nor ethane are formed at 300°C. Nevertheless, substantial amounts of C7 - and CG-hydrocarbons occur under the same conditions, and slightly more Cs than C3 is found. Most of the C7 - and C6-products are naphthenes. To account for all these findings some contribution of a bimolecular mechanism must be invoked: A Cs species is added to a second one which results in CI 6 unit. The latter undergoes rearrangements until a favorable structure for abstraction of C4 or Cs exists. This way, 2 Cs can be converted into, e.g., C4 + C5 + C7 • According to such a mechanism one could as well expect products with a carbon number above Cs, i.e., the occurrence of a disproportio-
289
Pd/LaY
-e • "0
QI oX
T =300 ·C
Pt/HZSM-5 T
=330
·C
80
v
(1
~
60
)(
r u I
40
UJ
C
-
c.
20
u
c
OL...i"--+--,.--+-~~"'T"""""".....J
1234567
1234567
CARBON NUMBER Cp OF CRACKED PRODUCTS Fig. 7. Hydrocracking of ethylcyclohexane. Distribution of the cracked products. nation type of reaction. However, no hydrocarbons with 9 or more carbon atoms were ever observed during this study. This indicates that the desorption of such intermediates from the acidic sites is slow compared to their cleavage. Hydrocracking of methyl cycl ohexane over Pt/HZSII-5 gi ves mainly propane and i-butane beside little n-butane and small amounts of hydrocarbons formed by hydrogenolysis. In contrast, the cracked products from methylcyclopentane, e.g., at 360°C contain large amounts of methane, ethane, butanes, and pentanes, i.e., the contribution of hydrogenolysis is high. This is another example for the fact that model hydrocarbons with six (or less) carbon atoms are often excluded from the favored pathways of ionic cracking and escape into different mechanisms (cf. ref. 27). The use of such small hydrocarbons for the investigation of ionic mechanisms is, therefore, discouraged. ACKNOWLEDGEMENTS The authors thank Mr. W. Stober and Mr. A. Dietl for valuable technical assistance and Mr. S. Maixner who carried out the mass spectroscopy work. Financial support by Deutsche Forschungsgemeinschaft and Fonds der Chemischen Industrie is gratefully acknowledged. REFERENCES 1 M.L. Poutsma, in J.A. Rabo (Ed.), Zeolite Chemistry and Catalysis, ACS Monograph 171, American Chemical Society, Washington, D.C., 1976, pp. 437-528. 2 J. Weitkamp and H. Farag, Acta Universitatis Szegediensis, Acta Physica et
290
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
Chemica, 24 (1978) 327-333. J. Weitkamp, Ind. Eng. Chern., Prod. Res. Dev., 21 (1982) 550-558. J. Weitkamp, Erddl, Kahle - Erdgas - Petrochem., 31 (1978) 13-22. M. Steijns, G. Froment, P. Jacobs, J. Uytterhoeven and J. Weitkamp, Ind. Eng. Chern., Prod. Res. Dev., 20 (1981) 654-660. A. Voorhies, Jr. and J.R. Hopper, in E.M. Flanigen and L.B. Sand (Eds.), Molecular Sieve Zeolites II, Adv. Chern. Ser. 102, American Chemical Society, Washington, D.C., 1971, pp. 410-416. , M.G. Luzarraga and A. Voorhies, Jr., Ind. Eng. Chern., Prod. Res. Dev., 12 (1973) 194-198. R. Beecher, A. Voorhies, Jr. and P. Eberly, Jr., Ind. Eng. Chern., Prod. Res. Dev., 7 (1968) 203-209. H. Schulz, J. Weitkamp and H. Eberth, Proc. 5th Intern. Congr. Catalysis, J.W. Hightower (Ed.), Vol. 2, North-Holland Publishing Co., Amsterdam, 1973, pp, 1229-1239. H. Pines and A.W. Shaw, J. Am. Chern. Soc., 79 (1957) 1474-1482. H. Pines and A.W. Shaw, Adv. Catal. 9 (1957) 569-574. H. Pines andN.E. Hoffman, in G.A. Olah (Ed.), Friedel-Crafts and Related Reactions, Vol. II, Part 2, Interscience Publishers, New York, London, Sydney, 1964, pp. 1211-1252. H. Heinemann, in J.R. Anderson and M. Boudart (Eds.), Catalysis - Science and Technology, Vol. 1, Springer-Verlag, Berlin, Heidelberg, New York, 1981, pp , 1-41. H.R. Ireland, C. Redini, A.S. Raff and L. Fava, Hydrocarbon Process., 58 (No.5, 1979) 119-122. S.M. Csicsery, in J.A. Rabo (Ed.), Zeolite Chemistry and Catalysis, ACS Monograph 171, American Chemical Society, Washington, D.C., 1976, pp. 680-713. S.M. Csicsery, Preprints, Div. Fuel Chern., Am. Chern. Soc., 28 (No.2, 1983) 116-126. P.A. Jacobs, J.A. Martens, J. Weitkamp and H.K. Beyer, Faraday Discuss. Chern. SOC. 72 (1982) 353-369. J. Weitkamp, P.A. Jacobs and J.A. Martens, Appl. Catal. 8 (1983) 123-141. H. Schulz and J. Weitkamp, Ind. Eng. Chern., Prod. Res. Dev., 11 (1972) 46-53. J. Weitkamp and K. Hedden, Chem.-Ing.-Tech., 47 (1975) 537. P.A. Jacobs, J.B. Uytterhoeven, M. Steijns, G. Froment and J. Weitkamp, in L.V.C. Rees (Ed.), Proc. 5th Intern. Conference Zeolites, Heyden, London, Philadelphia, Rheine, 1980, pp. 607-615. D.R. Stull, E.F. Westrum, Jr. and G.C. Sinke, The Chemical Thermodynamics of Organic Compounds, John Wiley &Sons, New York, London, Sydney, Toronto, 1969, pp. 348-350 and 357. D.M. Brouwer and H. Hogeveen, in R.W. Taft and A. Streitwieser, Jr. (E~s.), Progr. Phys. Org. Chern., Vol. 9, Interscience Publishers, New York, London, Sydney, Toronto, 1972, pp. 179-240. J. Weitkamp, in T. Seiyama and K. Tanabe (Eds.), Proc. 7th Intern. Congr. Catal., Elsevier Scientific Publishing Co., Amsterdam, Oxford, New York, 1981, pp , 1404-1405. J. Weitkamp and P.A. Jacobs, Preprints, Div. Petro Chern., Am. Chern. Soc. 26 (1981) 9-13. D.M. Brouwer and H. Hogeveen, Reel. Trav. Chim. Pays-Bas, 89 (1970) 211224. J. Weitkamp, in J.W. Ward and S.A. Qader (Eds.), Hydrocracking and Hydrotreating, Am. Chern. Soc. Symp. Ser. 20, American Chemical Society, Washington, D.C., 1979, pp.1-27. H. Vansina, M.A. Baltanas and G.F. Froment, Ind. Eng. Chern., Prod. Res. Dev., 22 (1983) 526-531. E.G. Christoffel and K.-H. Robschlager, Ind. Eng. Chern., Prod. Res. Dev., 17 (1978) 331-334.