Catalytic cracking of naphthenes and naphtheno-aromatics in fixed bed micro reactors

Applied Catalysis, 63 (1990) 345-364 Elsevier Science Publishers B.V., Amsterdam -

345 Printed in The Netherlands

Catalytic Cracking of Naphthenes and Naphthenoaromatics in Fixed Bed Micro Reactors H.B. MOSTAD*, T.U. RIIS and O.H. ELLESTAD Department of Hydrocarbon Process Chemistry, Center for Industrial Research, Box 124, Blindern, 0314 Oslo 3 (Norway), tel. (+47-2)452692, fax. (+47-2)452040 (Received 12 December 1969, revised manuscript received 13 April 1990)

ABSTRACT The product distributions from catalytic cracking of decalin and tetralin over an USY-zeolite catalyst (LZY82) in a fixed bed pulse and a standard micro activity test (MAT) reactor, have been analysed in detail by gas chromatography-mass spectrometry techniques. The product compositions from the pulse reactor and the MAT-reactor generally showed strong similarities. The main reaction routes have been outlined. For decalin, endocyclic bond cleavage was faster than dehydrogenation to tetralin and naphthalene. Several naphtheno-olefinic and mono-naphthenic compounds were observed, and the possible reaction routes for the formation and further reaction of some of these compounds have been discussed. For tetralin, alkylation of both the aromatic and the naphthenic ring has been established from the mass spectrometric identification of structural isomeric compounds. Keywords: zeolites, cracking catalysts, decalin cracking, tetralin cracking.

INTRODUCTION

The scope of our work has been to study the product distributions from catalytic cracking of decalin and tetralin over pure zeolites. These model compounds represent major classes of petroleum constituents in the feedstocks used for catalytic cracking. The reaction mechanisms of such di-naphthenic and naphtheno-aromatic compounds are crucial for the quality of the gasolineand diesel-fractions. Di-naphthenic compounds in the feed of the catalytic cracker may be converted to other hydrocarbon compounds by two mechanisms: they can undergo cracking to form paraffins and mono-naphthenes or be dehydrogenated to form aromatics. The content of aromatics from catalytic cracking of decalin and tetralin is strongly dependent upon the hydrogen transfer characteristics of the Y-zeolite [ 11. Di-aromatics boil in the light cycle oil range ( > 216°C) and reduce considerably the, nowadays, critical diesel cetane quality. However, the production of monoaromatics boiling in the gasoline range will improve gasoline octane quality [ 21. Thus, understanding and

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346

control of these mechanisms are important for improved fluid catalytic cracking (FCC) catalysts and processing. There are only a few publications within which decalin or tetralin have been considered as model compounds for catalytic cracking over zeolites. In the early work of Greensfelder and Voge, the C&-compounds were found to be the most prominent products in the liquid fraction (C,-C,,) from catalytic cracking of decalin over a silica-zirconia-alumina catalyst [ 31. Identification of specific compounds was not attempted, but strong indications of significant amounts of methylcyclopentane were reflected by the physical properties of the Csproducts. In 1970, Nate compared the cracking rate of mono-, di-, tri- and tetra-cyclic naphthenes over silica-alumina and a REHX-zeolite and found diffusional limitations for the three and four fused ring reactants [ 41. The product composition was analysed with low resolution gas-chromatography, and C,, compounds were not separated. The observation of abundant fivemembered ring products indicated, however, scission and recyclization reactants of the naphthenes. The product compositions from catalytic cracking of decalin and tetralin over a commercial catalyst (Mobil Durabead-8) in a fixed bed reactor at various temperatures and space velocities have been studied by Swaminathan [ 5 1. It was concluded that cracking of tetralin was accompanied by extensive hydrogen transfer reactions, while decalin was easily susceptible to cracking. Hernandez and co-workers cracked decalin over several zeolites in a flow reactor operated under differential conditions [ 61. The capillary gas-chromatographic system did not provide detailed product analysis. Offretite was the only catalyst observed to give appreciable cracking of decalin (conversion= 9 wt.-% at 550°C). Another purpose of this work has been to compare experiments performed in a MAT-reactor and a fixed bed pulse reactor. The MAT-reactor is the most accepted laboratory reactor for correlating bench scale data to fluid catalytic cracking processes. It would, however, be interesting if a pulse reactor could give relevant data, since it provides several experimental advantages in comparison with the MAT reactor. A fast test of the primary catalyst activity is possible, deactivation is less prominent, it permits the use of expensive model compounds and catalysts due to its small dimensions, and switching between different reactor feeds is easily performed. Even more advantageous is the possibility of connecting a gas chromatography-mass spectrometry (GC-MS) device for detailed identification of the rather complex product distributions. This also allows for investigations of compounds formed initially. The method employed has been described elsewhere [ 71. The investigation of the product composition from catalytic cracking of decalin over various zeolites, revealed shape selectivity of the cis- and trans-decalin isomers in the Y -zeolite. The selective conversion of cis-decalin has been presented separately [S]. In the present paper, the product analysis of the

347

decalin isomers is also discussed in detail and compared with the product composition from tetralin. EXPERIMENTAL

Catalyst Y-82 is a low sodium grade ultrastabilized molecular sieve, LZ-Y82 (65.6 wt.-% SiOz, 33.6 wt.-% A1203, 0.15 wt.-% NazO, 0.18 wt.-% Fe,O,, 0.03 wt.-% CaO) extrudates were obtained from Ventron GmbH. Fe203 and CaO originate probably from the binder, since these compounds were absent in the specified LZ-Y82 powder composition. The iron and calcium content were confirmed by inductively coupled plasma (ICP) analysis. The zeolite was crushed and sieved to a particle size of 70-325 mesh and hydrothermally treated in 100% steam (25 ml H,O/h) for 18 h at 730°C. The specific surface area was measured as 384 m2 g- ’ by nitrogen adsorption (BET). Hydrocarbons A mixture of cti/tram-decalin (decahydronaphthalene) , the pure decalin isomers and tetralin were used as model compounds for catalytic cracking. The cis/trans mixture was 60% : 40% (purity > 98%, EGA-Chemie). Pure cis- and trans-decalin were purchased from TCI-Tokyo Kasei (purity 99% and 98% respectively). Tetralin was obtained from Merck (purity z=-98% ) . Reactors and detection systems Fixed bed puke reactor The reactor was connected directly to a gas chromatograph-mass spectrometer. The model compounds were injected as pure liquids and carried through the reaction bed by the GC carrier gas (helium, 50 ml/min) . The analyses of the reactor effluent from catalytic cracking of decalin gave individual identification of compounds with carbon numbers from C3 to C,,. The details concerning the construction of the fixed bed pulse reactor and the quantification of the products from decalin have been described elsewhere [ 71. The reactor temperature was 460-480°C. The amount of catalyst and feed were 0.2 g and 10 pg respectively. Blank runs showed that thermal cracking was negligible ( ~0.5%) below reactor temperatures of 490°C. Micro activity test reactor (MAT) With the standard MAT reactor, the different reactor parameters could be more precisely controlled [ 91. The reactor was operated at standard ASTMconditions (T=482”C, amount of catalyst =4 g and feed= 1.3 g). The mass

348

balance of these runs accounted for more than 97% of the starting material. The carbonaceous deposit on the spent catalyst was measured on an element analyzer (Leco CHN-600). Only minor amounts of the C,-compounds were collected in the liquid phase from the MAT-reactor. Consequently, these compounds were not observed in the subsequent off-line GC-MS analyses of the liquid fraction, but instead analysed in the gas fraction by gas chromatography. The pulse reactor was, however, directly connected to the GC-MS equipment. The C4 compounds were therefore included in the analyses of the liquid fraction. Analyses The product distribution in the liquid fraction (C&i,) from the MATreactor was determined by mass spectrometry. The compounds were identified by matching their mass spectra with a NBS (National Bureau of Standards) spectrum library. The standard deviation was estimated to 1 area percent. The products were distributed in 37 compound groups and the amounts reported as weight percent. The compounds were also identified in a PIONA-library [lo] and classified as paraffins, iso-paraffins, olefins, naphthenes and aromatics. The variation in PIONA composition from parallel experiments in the MAT reactor was below 10% relative standard deviation. The gas chromatographs were a Finnigan 9610 and a Varian 3400. The mass spectrometers were Finnigan MAT 4000 and 8200. The GC column was a crosslinked Methyl Silicone Gum, 50 mx 0.2 mm I.D., 0.5 e film thickness (Hewlett Packard PONA-column). Injection of 0.08 fl model compound was done through a split injector and carrier gas was helium. The temperature program was -25°C (2 min), 2”C/min+‘?O”C, 2.5”C/min+150°C, 1O”C/min+25O”C and 250oC ( 10min). The mass-spectrometric conditions were electron impact at 70 eV and 250” C. The mass range, 35-200, was scanned at a speed of 0.6 s/ scan. RESULTS AND DISCUSSION

A di-naphthenic compound (decalin) and a naphtheno-aromatic compound (tetralin) were used as model compounds for catalytic cracking over an USYzeolite catalyst ( LZ-Y82 ) in a standard micro activity test (MAT) reactor. Tetralin produced more gasoline and diesel at the expense of gas compared to decalin (Table 1). The small amount of gas may be explained by less cracking and more isomerization/dehydrogenation of tetralin. The amount of coke was slightly higher from tetralin than decalin, probably due to faster aromatization of a compound with one initial aromatic ring. The two decalin isomers produced similar product compositions (Table 1) , as will be discussed below. In the PIONA-profile (paraffins, iso-paraffins, olefins, naphthenes and aromatics) of the liquid fraction from the MAT-experiments for tetralin, related

349

TABLE 1 Normalized composition of the converted material from catalytic cracking of decalin isomers and tetralin over an Y-82 catalyst in the MAT reactor. Feed

Tetralin cis-Decalin truns-Decalin c/t Decalin” “60% &DecaIin,

Average conversion ( % )

Gas (wt.-%)

< 216°C

> 216°C

Coke

(wt.-%)

(wt.-%)

(wt.-%)

79 97 87 89

5 18 17 16

82 74 76 77

11 7 6 6

2 1 1 1

40% truns-Decalin.

100

0

P

0

I Product

N

A

group

Fig. 1. PIONA-composition of the liquid fraction from catalytic cracking of tetralin, cis-decalin, truns-decalin and a cis/trans-decalin mixture (60 : 40) over an Y -82 catalyst in a MAT-reactor.

products were distinctly different from the ones from decalin (Fig. 1). More than 90% aromatics were produced from tetralin, while decalin produced below 40%. Note that these analyses included the methyl-naphthalenes (b.p.: 241245 oC ), but not the higher boiling compounds like the di- and tri-methyl naphthalenes and phenanthrene. However, only negligible amounts of these compounds were observed. The amount of unbranched paraffins (P) were, as expected from the cracking of cyclic compounds, very low both in the gas and gasoline fractions. Also, the amount of olefins (0) refers only to the small CdC7 alkenes (Fig. 1). The much more abundant olefinic naphthenes (about 8 wt.-% ) were included in the naphthenic groups (N). Close agreement of the normalized product compositions from the cis- and truns-decalin isomers was observed. (Tables 1 and 2 and Fig. 1) . This is expected since the tertiary carbocations produced from either cis- or trans-decalin by hydride abstraction are identical. Moreover, in a previous paper it has been shown that the cis-decalin isomer was selectively converted over the Y-

TABLE 2 Composition of the gas fraction from catalytic cracking of tetralin, cis-decalin, truns-decalin and c/t-decalin (60: 40) over an Y-82 catalyst in a MAT-reactor. Average values are given in wt.-%. Compound

Tetralin

HZ G G

0.1 0.1

C,= C, iC, C,= Sum

1.0 0.8 1.1 1.4 0.5 5.0

Ga

0.0

iCsn c 6+ a

0.8 3.7

cis-Decalin

tralzs-Decalin

c/t-Decalin

0.5 2.2 1.9 1.8 10.3 1.2 18.0

0.1 0.5 2.6 1.4 2.0 9.4 0.9 16.9

0.4 2.0 1.6 1.7 9.2 1.1 16.1

0.3 4.4 6.5

0.2 4.0 5.7

0.2 4.0 5.4

0.1

0.1

“The amounts of these compounds were added to the amount of gasoline ( < 216°C) in Table 1. Note that these C,- and C&-compounds do not contribute to the composition of the liquid fraction in Fig. 1 and Table 3.

zeolite due to pore discrimination effects [ 81. The shape selectivity was attributed to the slightly smaller molecular dimensions and the more flexible conformation of &-decalin. Thus, the cracking mechanism for trans-decalin will preferably be through isomerization to cis-decalin in a shape selective catalyst. The similar product distributions from cis- and tram-decalin permit either of them to be used in the evaluation of the product composition from decalin [ 11. It was necessary to investigate the normalized composition of the converted material from the isomers, due to the differences in conversion caused by the larger thermodynamic stability and pore discrimination of truns-decalin. To minimize conversion-dependent variations in the product profiles, experiments with similar conversion levels were selected for comparison (Table 1 and Fig. 1). Within the investigated conversion range, compositional variations were less than the calculated standard deviations of the MAT reactor. Average values of the product distribution in the liquid fraction (boiling point 5 245°C) from catalytic cracking of decalin and tetralin over an Y-82 catalyst in the MAT-reactor and the fixed bed pulse reactor are presented in Table 3. In general, a qualitative agreement between the product compositions from the model compound in the two reactor systems is observed. Since hydrogen was avoided, and because of the very detailed anlysis of all products from the MAT-reactor, it was possible to compare the stoichiometric composition in the feed and the products. The H/C ratio (mol-%) in the feed was 1.80, while the average value of the total amounts of products calculated from five MAT-experiments was 1.83 + 0.02 (Table 4). Thus, the difference in H/C

351 TABLE 3 Product composition (wt.-%) in the liquid fraction (C4-C&) from catalytic cracking of decalin and tetralin over an Y-82 catalyst in the MAT-reactor and the fixed bed pulse reactor No.

Product groups

Typee

MAT-reactor Tetralin

1 2

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 30 31 32 33 34 35 36 37

n-C,” n-C5 n-C-n-C,,, i-Caa i-C, i-C, i-C, i-Cs-i-C,, Cyclopentane Methyl-cyclopentane Di-Me/Et-cyclopentane Cyclohexane Methyl-cyclohexane C, mono-alkenes C, mono-alkenes Cyclopentene C&-C7alkenes C, mono-naphthener? Cs olefinic naphthenes’ Cg mono-naphthenesb Ca di-naphthenesd Cl0 mono-naphthene8 Cl0 olefinic naphthenes” truns-Decalin cis-Decalin Benzene Toluene Ethylbenzene Xylenes Cs benzenes C,, benzenes C,, benzenes Naphthalene C, naph./olef. benzenes Cl0 naph./olef. benzenes C,, naph./olef. benzenes Methylnaphthalenes Conversion

P P P P P P P P N N N N N 0 0 0 0 N N N N N N N N A A A A A A A A A A A A

Pulse-reactor Decalin

0.5

0.0

0.0 0.0 0.4

0.4 0.4 0.0 3.1

0.7 0.7 0.1 0.0 0.2 2.3 0.7 0.0 0.4 0.0 0.0 0.0 0.0

8.2 1.4 0.4 0.0 17.6 7.7

1.8 2.9 0.0 0.0

Tetralin 1.0 0.0 0.0 2.8 0.8 0.3 0.0 0.0 0.0 1.5 0.7 0.0 0.1 3.2 0.2 0.0 0.0

0.0

0.2 0.4 2.2

0.0 0.0 0.0

0.2 0.5 0.2

0.0 0.0 0.2 0.0

3.3 7.6 11.0 0.2

0.0 0.0 0.0 0.0 0.0

a.7 3.5

0.8 3.2

2.0 0.6

0.6

0.0

0.2 0.0 5.7

-

Decalin 1.5 0.3 0.0 10.1 4.8 4.5 0.0 0.0 0.4 8.9 3.5 0.4 2.1 5.4 0.0 0.7 0.0 0.3 0.0 0.0 0.0 0.9 4.8 11.1 0.7 1.4

2.5

2.0

1.0 0.4

0.3 1.6 2.1

1.2 8.2

3.4 2.4 3.6

0.2 31.3

0.1 2.5

0.7 5.6 0.0 34.6

2.6 30.5 0.7 4.4

1.0 10.3 0.9 1.3

2.0 30.9 0.7 4.9

16.9 0.8 1.2

79.4

88.8

83.0

88.2

6.2 0.0 6.5 1.0

“C, compounds from the MAT-reactor were mainly collected in the gas fraction, see Table 2. bIncludes small amounts of mono-alkenes. “Includes some di-naphthenes and di-alkenes. dIncludes some olefinic naphthenes and di-alkenes. ‘P = alkanes; N = naphthenes; 0 = alkenes; A = aromatics.

352 TABLE 4 Catalytic cracking of cis/trarts-decalin (exp. no. l-3), (exp. no. 5) over an Y-82 catalyst

cis-decalin (exp. no. 4) and trrans-decalin

Comparison of the H/C ratio in the feed and the products H/C ratio (mol-%) Experiment no.

1

2

3

4

5

Feed

1.80

1.80

1.80

1.80

1.80

Gas”’ < 216°C > 216°C’ Cokeb,c

2.47 1.79 1.01 0.5

2.45 1.82 0.92 0.5

2.45 1.77 0.95 0.5

2.46 1.78 0.95 0.5

2.49 1.75 0.94 0.5

Sum products

1.84

1.86

1.81

1.83

1.82

“Gas-analysis included H,, 02, CO, CO, and C,-C, compounds. bLimited accuracy, but with little influence on the stoichiometric calculations. “The composition of the product fractions were as in Table 1 for cis/truns-, cis- and trons-decalin respectively.

ratio of the feed and products was averagely 1.6%. This very satisfactory agreement in the stoichiometry made it possible to use the observed product composition of paraffins, olefins, naphthenes and aromatics to evaluate the main reaction routes. Tetralin

The best hydride donors form resonance-stabilized carbocations by losing hydride ions [ 111. Naphthenes and naphthene-aromatics are particularly active in releasing hydrogen by the aromatization reactions. Catalytic cracking of tetralin was observed to be characterized by two competing reactions: cleavage of the naphthenic ring, resulting in C&C,, mono-aromatics or dehydrogenation to naphthalene (Fig. 2). Although the main single product from tetralin was naphthalene, the total amount of di-aromatics was less than the total amount of mono-aromatics (Table 3), The carbocation is most likely produced by the protonation of the aromatic ring in tetralin (Fig. 2 ). However, further ring-opening of this carbocation by the p-scission mechanism will produce a primary carbocation unless simultaneous deprotonation or hydride abstraction occurs. Another possible reaction route to produce the observed amounts of n-butylbenzene from tetralin (Table 5)) is initial hydride abstraction at the j&position to the aromatic ring, yielding

353

-H+

-

)

(SEE

FIGURE

37

ISEE

F,GURE

4)

[:,:rnJ

Fig. 2. Proposed reaction scheme for catalytic cracking of tetralin.

a secondary carbocation (Fig. 2). Subsequent ring-opening may produce butylbenzene. Decreasing amounts of benzene, toluene and Cs-aromatics were found in the MAT-experiments (58: 24 : 18) and in the pulse-reactor experiments (59 : 26: 15).According to the proposed reaction scheme in Fig. 2, benzene was most likely produced from direct dealkylation of butylbenzene. The cracking of butylbenzene has been found to be very selective over an amorphous silicaalumina catalyst, with 97.5% benzene/C4 and 1.8% toluene/C, being observed at 482 aC [ 121.Appreciable more toluene compared to benzene was observed from cracking of tetralin over a pure Y-zeolite in the present work (see above). The n-butenylbenzene produced from initial ring-opening of tetralin, may isomerize into isobutenylbenzene. In fact, about 4 wt.-% of isobutenylbenzene (2-methyl-2-propenylbenzene) was observed among the products. Cracking of the tertiary carbocation of isobutenylbenzene will produce toluene and a secondary propene carbeniumion (Fig. 2 ) . Another possible reaction route to toluene, is hydride abstraction from the double-bonding carbon and subsequent cracking of n-butenylbenzene (Fig. 2 ).

354

TABLE 5 Relative distribution of aromatics produced from catalytic cracking of decalin and tetralin over an Y-82 catalyst in; A: the fixed bed pulse reactor at 457°C; B: the micro activity test (MAT) reactor at 482” C Product compound

Decalin A

Ce : Benzene C!, : Toluene Cs : Ethylbenzene Xylenes C, : Alkylbenzenes Naph./olef. benzenes C,,: Butylbenzenes 2-Me-propylbenzenes Polyalkylated benzenes Alkenylbenzenes Methylindanes Tetralin Naphthalene C,,: l-Me-4-(2-Me-propyljbz. Di-methylindanes Methyltetralins Methylnaphthalenes

3.5 5.0 0.8 4.0 5.3 2.5

-

1.8

13.8 5.9 34.3 2.1 16.3

Tetralin B

-

3.0

B

2.7 10.6 2.0 11.3 8.0 3.3

6.4 2.8 1.1 0.4 0.8 2.2

9.3 3.7 2.1 0.6 1.3 2.8

3.7

5.3 0.5

8.5 0.2

-

8.3 3.2 28.0 3.0 8.3

2.0

A

-

0.3 3.0 4.3

-

6.1 9.6 19.2 39.1

0.4 0.5 5.5

-

3.9 6.7 21.9 33.3 0.2 0.3 0.5 4.7

The further dominant reaction of toluene is disproportionation to benzene and xylenes [ 111. This adds to the amount of benzene from direct dealkylation. The relatively high amount of benzene from tetralin is in contrast to the results from decalin as will be discussed later. Tetralin produced comparable amounts of methylindanes and butylbenzene (Table 5). l-methylindane was the most abundant methylindane. This compound was probably produced by an isomerization mechanism based on a protonated cyclopropane-like intermediate formed from the initially tertiary carbocation of tetralin with a subsequent carbon bond scission and rearrangement (Fig. 2) [ 131. However, l-methylindane may also be produced from butylbenzene by addition of a secondary carbon atom to the aromatic ring as previously shown [ 141.n-Butylbenzene yielded only l-methylindane and not tetrahn with the carbocation type mechanism [ 141, reflecting the preference of secondary or tertiary carbocations in the side chain in the cyclization mechanism. Decalin, apparently formed by hydrogenation of tetralin, was produced only in very small amounts (Table 3). The observed C&-C, paraffins and mononaphthenes were most likely products from the subsequent cracking of decalin.

355

Significant amounts of alkylated products, boiling in the light cycle oil range, were identified in the product composition (Table 5). In addition to the methylnaphthalenes, very small amounts of 1-methyl-4- (2methyl-propyl) -benzene, small amounts of di-methylindanes and slightly higher amounts of the methyltetralins were detected. The observation of the methylated bicyclic compounds was somewhat surprising, since direct alkylation by a methyl-group is highly unlikely or even impossible. Thus, the observation of e.g. methylnaphthalene is an indication of bimolecular trans-alkylation reactions in the zeolites. This was further supported by the very small amounts of phenanthrene, since this compound may be produced from cyclization of butylnaphthalene. Six different structural isomeric methyltetralins and di-methyl indanes were identified by mass spectrometry, with 2-methyltetralin and 5-methyltetralin as the most abundant (ratio 1: 4). The reduced intensity of the [M - 15]+ ion in the mass spectrum of 2-methyltetralin, readily distinguish this isomer from the various di-methylindanes and l-methyltetralin. The latter compounds give mass spectra with a prominent [M - 15]+ ion, because of the formation of a carbocation stabilized by the aromatic ring. In the /?-position of 2-methyltetralin, loss of the methyl radical lacks this stabilizing effect. Consequently, the observed base peak in the mass spectrum of 2-methyltetralin (M+ = 146)) is not m/e 131, but m/e 104. The mass spectrum of 5-methyltetralin is similar to 6-methyltetralin, but is easily distinguised from both l- and 2-methyltetralin, di-methylindanes and isomeric alkenylbenzenes. Note that identification of these isomeric compounds based on GC retention times alone is difficult, due to the large number of possible structures and limited chromatographic resolution. A survey of the various observed alkylated products is given in Fig, 3, The most probable reaction leading to the observed 5-methyltetralin, is truns-alkylation from alkenylbenzene to the aromatic part of tetralin and subsequent cracking of the short living intermediate into 5-methyltetralin, one probable example being trans-alkylation by butenylbenzene produced from ring-opening of tetralin. Such bi-molecular reactions are promoted by the acidic zeolite cavities. Thus, producing the observed methylated bicyclic compounds and propane/propene. Appreciable amounts of C&-compounds were identified from cracking of tetralin (Table 2). Alkylation of the saturated part of a hydrocarbon is very unlikely to occur under these experimental conditions, because this reaction requires more acidic sites, lower temperature and longer residence time. The observation of significant amounts of 2-methyltetralin was therefore surprising. This suggests that 2-methyltetralin was produced by another reaction route than 5-methyltetralin. The small amounts of 2-methyltetralin may be produced from isomerization of the 5-methyltetralin or 5-butenyltetralin as suggested in Fig. 3. The most abundant di-methylindanes observed were 1,1-di-methylindane

= = = =

TA HT Fto

HYDROGENTRANSFER RING OPENING

TRANS.ALKYLATION ISOMERIZATION

Fig. 3. Survey of the observed methylated bicyclics from catalytic cracking of tetralin. Runs-alkylation of butane/butene example in the proposed reaction routes, since this compound may be transferred from tetralin after previous ring-opening.

TA

TA

-H+

has been used as an

357

and 1,6-di-methylindane in the ratio 1: 4. 1,6-di-methylindane may be produced from cracking of 1-methyl-6-alkylindane or from isomerization of 6methyltetralin. 1,1-di-methylindane may be produced by an isomerization mechanism based on a protonated cyclopropane-like intermediate formed from the tertiary carbocation of 2-methyltetralin with a subsequent carbon bond scission and rearrangement in accordance with the formation of l-methylindane from tetralin (Fig. 3 ). Decalin The proposed reaction scheme for catalytic cracking of decalin is presented in Fig. 4. The reaction mechanisms are based on the observed product composition at conversion levels of about 90% (Table 3). The change in catalyst selectivity as a function of conversion (60-100% ) does not change the product composition, but only the relative distribution [ 11. These experiments at rather high conversion levels, may therefore be used in the evaluation of the main reaction routes. Hydride abstraction from decalin will produce secondary and tertiary carbocations. However, the formation of tertiary carbocations is less favourable in bicyclic than in acyclic compounds, because planarity around the electron deficient carbon is not obtained. Moreover, since an initially formed tertiary carbocation only will produce primary carbocations by the /?-scission mechanism, the formation of secondary decalin carbocations was assumed to be more probable. The latter will subsequently yield secondary carbocations by the /?scission mechanism (Fig. 4). The hydrogen transfer and the endocyclic bond cleavage [15,16] are relatively slow reactions. The observed very small amounts of tetralin and naphthalene indicated that endocyclic ring opening proceeded at a higher reaction rate than dehydrogenation to aromatics. The high ratio of mono- to di-aromatics (9 : 1) further indicated that endocyclic ring opening to a large extent was preferred to dehydrogenation of one ring. Investigation of the product composition from tetralin, revealed that comparable amounts of mono- and diaromatics would have been expected if the endocyclic ring-opening of decalin occurred after dehydrogenation to tetralin. Although the small iron impurities on the catalyst surface may contribute to the amount of hydrogen transfer in addition to the acid site density, the dehydrogenation was less pronounced than ring-opening of decalin. These observations were in contrast to the data presented by Hernandez et al. [6]. They observed tetralin and naphthalene to be the only products from cracking of decalin over a cerium exchanged Y-zeolite (HCeY) in a flow reactor at 450-550’ C. However, our results were in satisfactory agreement with those from catalytic cracking of decalin over a commercial catalyst (Mobil Durabead-8) in a fixed bed reactor, rather similar to MAT conditions, at a

358

t

2+ t

359

conversion level of 94% (45O”C, 0.75 h-l), reported by Swaminathan [5]. Generally, less C&-C, paraffmic and naphthenic compounds and some more aromatics were observed in this work [ 51. Inspection of Tables 2 and 3 revealed that the most abundant products from decalin were iso-butane and methyl-cyclopentane. These compounds were probably produced from decalin by the type A/?-scission mechanism previously described by Weitkamp et al. [ 171. A characteristic feature of this mechanism (Fig. 5) is that it starts from and leads to a tertiary carbenium ion and selectively gives iso-butane and methyl-cyclopentane in the absence of spatial constraints. The initial product from decalin, the C,,-butenyl-cyclohexane, may undergo hydrogenation with concurrent rapid isomerization of the skeleton until a quarternary carbon atom is formed, which enables the so-called type A p-scission in the alkyl side chain (Fig. 5 ) . Exocyclic carbon-carbon bond cleavage will then occur rapidly, producing the observed high amount of iso-butane and methyl-cyclopentane. The hydrogen for this reaction was provided by concurrent aromatization reactions of decalin. The compound used as an example of a possible intermediate in type A /3-scission in Figs. 4 and 5, is selected because it is the most abundant precursor observed (in product group no. 22 in Table 3). However, since the product compositions in Table 3 were obtained at high

Fig. 5. Proposed reaction route for production of methyl-cyclopentane and iso-butane from decalin.

360

conversion levels, several others of these rather unstable precursors for type A /3-scission, probably exist among the more initial products. The observed higher ratio of iC,/nC, from decalin ( = 5.4 ) , in comparison with tetralin ( = 1.3), may be explained by the importance of the type A Pscission in the cracking of decalin. As indicated in Fig. 4, several other possible dealkylation reactions of the C,, mononaphthenes (product group no. 22 in table 3) may be possible, producing, among others, nC4. In these reactions, secondary carbocations are produced from tertiary or vice versa. Thus, they are probably less favourable than type A /?-scission, where tertiary carbocations are produced from tertiary ones. On the contrary, the C4 compounds from tetralin were probably produced from dealkylation subsequent to a ring-opening. Moreover, it is likely that the C-C cleavage near the aromatic ring in tetralin, producing n-butane, is faster than isomerization of the butyl-group into iso-butyl. The C,&&, mono-aromatics represent a major part of the products from decalin. Extensive hydrogen transfer reactions convert the initially produced C,, mono-naphthene to the corresponding butylbenzene. Several C,,polyalkylated aromatics were observed. The variety of these compounds were most likely produced by fast isomerization. The observed C9 aromatics were probably made from disproportionation reactions of either C,, aromatics (e.g. methylindanes in Fig. 4 produced di-methylindanes and indane) or Cs aromatics providing toluene and various C, alkylbenzenes. In contrast to the C&-C, aromatics from tetralin, C, and Cs aromatics were more important and benzene less important products from decalin (Table 5). The initially produced secondary carbocation of butenyl-cyclohexane may undergo fast isomerization to a tertiary carbocation. Exocyclic carbon-carbon bond cleavage of this ion by the /&scission mechanism and subsequent dehydrogenation will only produce toluene. The hydrogen provided by such aromatization reactions is essentially transferred to olefinic intermediates to form mono-naphthenes and iso-paraffins, which were among the major cracking products observed. 1-Methylindane was the main aromatic product from decalin (Table 5). In addition, appreciable amounts of 1-methylhydrindane was observed in product group no. 23 in Table 3. The latter may be produced by an isomerization mechanism based on a protonated cyclopropane-like intermediate formed from the initially tertiary carbocation of decalin with a subsequent carbon bond scission and rearrangement in accordance with the previously discussed isomerization of tetralin. The possibility of intermediate cyclopropane formation in the tertiary carbocation of decalin is doubled because of the two saturated rings. lmethylindane was probably produced from subsequent dehydrogenation of lmethylhydrindane. Another possible reaction route of 1-methylindane was isomerization of previously formed butenyl- or butylbenzene (Fig. 4). Neither methyldecalin nor methyltetralin were detected from decalin at the

361

investigated conversion levels. The production of di-methylindanes was, however, confirmed in addition to the methylnaphthalenes (Table 5). l-methylindane may undergo alkylation previous to and after transfer of the methyl group to the aromatic part of the molecule. 4,‘7-di-methylindane and 1,6-di-methylindane were the most abundant di-methylindanes. This indicates disproportionation or alkyl transfer to the aromatic ring, both before and after isomerization of 1-methylindane. Since the catalytic cracking experiments have been performed in a fixed bed pulse reactor or in a MAT reactor, it was possible to investigate the large group of initial cracking products, i.e. C,, olefinic mono-naphthenes, C,, di-naphthenes, C,, diolefins/decynes (product group no. 23 in Table 3) and several C,, mono-naphthenes (product group no. 22 in Table 3). The most abundant compounds in product group no. 23 (Table 3 ) were the olefinic mono-naphthenes (M+ = 138). The formation of of cis- and ~runscyclodecene from decalin has previously not been reported. A proposed reaction mechanism of these compounds from the cracking of the secondary carbeniumion of decalin is illustrated in Fig. 4. Jacobs et al. have on the other hand, observed internal ring alkylation to yield CL+ and trans-decalin from cracking of cyclodecane by bifunctional [ 181 and metal [ 191 catalysts. The various C,, alkenyl-cyclohexanes and cyclopentanes (Fig. 4 ) were most likely produced from the initial butenyl-cyclohexane by consecutive skeletal and double bond isomerization. Possible precursors to the production of methylcyclopentane and isobutane by the type A/%scission were observed among these compounds (e.g. l-methyl-l (2-methyl-2-propenyl) -cyclopentane) (Fig. 5). Several monocyclic compounds (i.e. product group no. 22 in Table 3 and Fig. 4 ) were detected (M+ = 140). Small amounts of Cl0 alkyl-cyclopropanes may arise from isomerization of the more abundant cyclopentanes and cyclohexanes (Fig. 5). Cyclization to various bicyclic compounds were also observed. The most abundant were 2,5-dimethyloctahydro-pentalene, Spiro [4S.]decane, 3,7,7-tri-methyl-bicycle [ 4.1.0.1 heptane, 2,6,6-tri-methyl-bicycle [ 3.1.1,] heptane and methylhydrindane (product group no. 23 in Table 3 and Fig. 4). Although dealkylation or cyclization to other bicyclic compounds was found to be more important, the observation of small amounts of Cl0 diolefins or decynes, indicated endocyclic ring-opening of both rings. Puke-reactor versus MAT-reactor It is virtually impossible to establish laboratory bench scale test systems giving equivalent conditions and reliable data to those obtained in a commercial FCC-process. The MAT-reactor is the most accepted method, giving a standard ASTM catalytic activity, which may be correlated to the process data. In more basic studies, the MAT-reactor will also give valuable information about product distributions. As previously mentioned, the pulse reactor pro-

362

vides several experimental advantages in comparison with the MAT-reactor, if potential disadvantages such as low experimental reproducibility and strong adsorption effects can be avoided. Although small differences were detected, the product compositions from both reactor systems were in qualitative agreement (Table 3). Investigation of Tables 3 and 5 revealed that the proposed main reaction routes from decalin and tetralin may be confirmed by both the MAT- and pulse-reactor experiments. A comparison of the amount of tetralins and naphthalenes produced from decalin in both reactor systems, revealed that only slightly more dehydrogenation took place in the pulse reactor than in the MAT-reactor (Table 6 ) . The same conclusion may be reached from the amount of naphthalene and methylnaphthalenes produced from tetralin in both reactor systems (Table 6). In addition, the observed dehydrogenation in the pulse reactor only increased slightly as the number of pulses over the catalyst bed was increased up to 10 [l]. The experimental conditions for the pulse reactor compared to the MAT reactor would in one way be expected to favour the more initial cracking products. However, this does not correspond with the above mentioned observations. Looking at the catalyst/feed ratio, this is about Z-10* in the pulse reactor, while only about 3 in the MAT-reactor. Thus, average residence time of the reacting molecules at the catalytic sites may become longer in the pulse reactor than in the MAT-system. The consecutive reactions will in this way be promoted. With the established successful experimental conditions for the pulse reacTABLE 6 Catalytic cracking of decalin and tetralin over an Y-82 catalyst in the MAT-reactor and the fixed bed pulse reactor; a comparison of the relative amount of the various reaction routes obtained from the model compounds

Decalin: dehydrogenation crackin< Tetralin: dehydrogenation hydrogenation cracking alkylation of tetrahn

MAT-reactor (wt.-%)

Pulse-reactor (wt.-%)

6 94

89

45 7 47 1

11

48 5 46 1

“Includes isomerization which proceeds through ring opening.

tor, full benefit was taken from the detailed analysis of the product composition. It is likely that under other experimental conditions, for example using lower temperatures, adsorption effects will give less favourable results. CONCLUSIONS

The two main reaction routes of tetralin were ring opening to butylbenzene, and dehydrogenation to naphthalene. The rather high amount of l-methylindane may be produced from scission of a protonated cyclopropane intermediate of the tertiary tetralin carbocation. Methylated tetralin compounds were most likely produced from cracking of short living alkylated intermediates. Endocyclic bond cleavage of decalin, producing mono-naphthalenes and mono-aromatics was faster than dehydrogenation to tetralin. The main nonaromatic products from decalin were methyl-cyclopentane and isobutane, while the main aromatic products were the methylindane isomers. The average difference in the H/C ratio between the feed and the total amounts of products from the MAT-experiments was below 2%. The strong similarity in product composition from the pulse-reactor and the MAT-reactor has demonstrated the applicability of the pulse reactor in catalytic cracking studies. ACKNOWLEDGEMENTS

The financial support of this work by the Norwegian Council for Scientific and Industrial Research (NTNF) is gratefully acknowledged.

REFERENCES

6 7

8 9 10

11

H.B. Mostad, T.U. Riis and O.H. Ellestad, Appl. Catal., 64 (1990) 119-141. H.U. Andreasson and L.L. Upson, Oil Gas J., Aug 5 (1984) 91. B.S. Greensfelder and H.H. Voge, Ind. Eng. Chem., 37 (1945) 1038. D.M. Nate, Ind. Eng. Chem. Prod. Res. Dev., 2 (1970) 203. S. Swaminathan, Diss. Abstr. Int. B, 41, (a), 3117, Univ. Microfilms Int.; Order No. 8103900, 1981. F. Hernandez, L. Moudafi, F. FajuIa and F. Figueras, in Proceedings of the 8th International Congress on Catalysis, Berlin, July 2-6,1984, Verlag Chemie Weinheim, 1984, Vol. II, p. 447. H.B. Mostad, T.U. Riis and O.H. Ellestad, in P. Longeviahe (Editor), Advances in Mass Spectrometry, Proc. of the 11th Int. Mass Spec. Conf., Bordeaux, Aug 29-Sept 2,1988, Heyden, London, llB, 1989, p. 1664. H.B. Mostad, T.U. Riis and O.H. Ellestad, Appl. Catal., 58 (1990) 105. ASTM D 3907-80, Fluid Cracking Catalysts by Micro Activity Test. K. Urdal and S. Sporstal, in P. Longeviahe (Editor), Advances in Mass Spectrometry, Proc. of the 11th Int. Mass Spec. Conf., Bordeaux, Aug 29-Sept 2, 1988, Heyden, London, llB, 1989, p. 1670. A. Corma andB.W. Wojciechowski, Catal. Rev.-Sci. Eng., 27 (1) (1985) 29-150.

364 12 13 14 15 16 17

18

19

SM. Csicsery, J. Catal., 9 (1967) 336. D.M. Brouwer and H. Hogeveen, in A. Streitwieser Jr. and R.W. Taft (Editors), Progress in Physical Organic Chemistry, Vol. 9, Wiley, New York, 1972, p. 179. S.M. Csicsery, Adv. Catal., 28 (1979) 293. D.M. Brouwer and H. Hogeveen, Rec. Trav. Chim. Pays-Bas, 89 (1970) 211. J. Weitkamp, S. Ernst and H.G. Karge, Erdijl Kohle, 37 (1984) 457. J. Weitkamp, S. Ernst and C.Y. Chen, in P.A. Jacobs and R.A. van Santen (Editors), Zeolites: Facts, Figures, Future, Proc., of the 8th Int. Zeolite Conf., Amsterdam, July lo-14,1989, (Studies in Surface Science and Catalysis, Vol. 49B), Elsevier, Amsterdam, 1989, p. 1115. P.A. Jacobs and J.A. Martens, in Y. Murakami, A. Iijima and J.W. Ward (Editors), New Developments in Zeolite Sience and Technology, Proc. of the 7th Int. Zeolite Conf., Tokyo, August 17-22,1986, (Studies in Surface Science and Catalysis, Vol. 28), Elsevier & Kodansha, Amsterdam, Tokyo, 1986, p. 23. P.A. Jacobs, M. Tielen and J. Martens, J. Mol. Catal., 27 (1984) 11.