Catalytic cracking of tetralin on HY zeolite

Applied Catalysis A: General, 90 (1992) 97-l 15 Elsevier Science Publishers B.V., Amsterdam

97

APCAT A2359

Catalytic cracking of tetralin on HY zeolite

A.T. Townsend and J. Abbot

Chemistry Department, University of Tasmania, Hobart, Tasmania (Australia)

(Received 21 February 1992, revised manuscript received 6 August 1992)

Abstract

Catalytic reactions of tetralin have been studied on HY xeolite at 400°C. Product distributions have been obtained over a range of conversions so that both initial reaction processes and secondary reactions can be identified. The initial reactions of tetralin include isomerixation to methylindans and a set of complex bimolecular processes involving both cracking and hydrogen transfer. Products derived from naphthalene and benzene are the major species resulting from these bimolecular processes. Detailed examination of initial product selectivities and a carbon balance show that a complete set of simple reaction processes cannot be easily established.

Keywords: cracking, isomerixation, hydrogen transfer, HY xeolite, tetralin, xeolites.

INTRODUCTION

Various types of pure hydrocarbons have been studied as model compounds during catalytic cracking processes. There have been many reported studies using alkanes [l-5], alkenes [6-81 and alkyl aromatics [9], with the behaviour of cycloalkanes also being investigated [ 10-121. In contrast, there have been relatively few detailed investigations of the reactions of naphtheno-aromatic compounds, such as tetralin [ 13-151. It has long been recognised that extensive hydrogen transfer processes occur during reactions of tetralin over acid catalysts [ 161, leading to complex product distributions [ 131. A recent study of the reactions of tetralin over a Y zeolite at 480°C showed that the product distribution could be divided into approximately forty individual components and groups of components based on detailed gas chromatography (GC) analysis [ 131. Based on this study, reaction networks have been proposed. However, product distributions were reported only at very high conversion levels, where extensive secondary reaction processes would be expected to occur. In the present study we have examined cracking and isomerization pro-

Correspondence to: Dr. J. Abbot, Chemistry Department, University of Tasmania, Hobart, Tas- mania, Australia. Tel. (+61-02)202178, fax (+61-02)234074.

0926-3373/92/$05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.

98 A.T. Townsend and J. Abbot/Appl. Catal. A 90 (1992) 97-115

cesses of tetralin on HY over a range of conversions from 5 to 80%, which allows a detailed examination of both initial and secondary reaction phenomena. Studies of cracking reactions of tetralin can yield valuable information, as closely related structures are likely to be found after partial hydrogenation of cracking feedstocks derived from coal or heavy gas-oils.

EXPERIMENTAL

The tetralin feedstock (98.23%) was obtained from Aldrich and used without further purification. The presence of impurities, as shown in Table 1, was taken into account when calculating initial product selectivities.

HY zeolite (CBV 400, Si/A1=3.0) was provided by PQ zeolites, the Neth- erlands. Catalysts were sieved to mesh size SO/l00 and were calcined at 500’ C overnight in a flowing stream of dry air prior to use. Using data supplied by the manufacturer, the degree of cation exchange was calculated to be approximately 75%, after repeated exchange of the sodium form with ammonium nitrate.

All experiments were performed by using an integral, fixed-bed gas-phase plug flow reactor with an independently controlled three zone heater. The experimental apparatus and procedures used were similar to those described in previous studies [5]. All reactions were carried out at 400°C and 1 atm (1 atm = 101.325 kPa) pressure, with 10 g of feed being injected over the catalyst at varying rates, controlled by a feed syringe pump. Catalysts were mixed with granules of acid-washed sand, serving as a support matrix within the reactor. Blank experiments were undertaken to determine the extent of thermal cracking.

Liquid products were analysed using a Hewlett Packard 589011 gas chromatograph with a BP-10 capillary column (25 m x 0.32 mm I.D.) and a flame ionisation detector. Gaseous products were also analysed using a Hewlett Packard gas chromatograph of the same type, with a Chrompak capillary column (25 mX 0.32 mm I.D.). Hydrogen analysis was further carried out using an identical gas chromatograph with a molecular sieve 5A packed column (1 m X 3 mm I.D.) with a thermal conductivity detector. Data handling was facil- itated using a DAPA software package. Identification of hydrocarbons was as- sisted by the use of Hewlett Packard 5890 gas chromatograph coupled to a 5970 mass selective detector.

RESULTS AND DISCUSSION

Feedstock impurities and thermal cracking

The impurities present in the tetralin feedstock are given in Table 1. Con- sideration of the presence of these components is important when evaluating

A.T. Townsend and J. Abbot/Appl. Catal. A 90 (1992) 97-115

TABLE 1

Feedstock composition as determined by GC analysis

99

Component Weight percent

Tetralin 98.23 Dimethylnaphthalenes 0.63 Naphthalene 0.29 Indene 0.24 Decalin 0.21 Methylindans 0.09 Ethylnaphthelene 0.08 Methyltetralins 0.07 Ethylbenzene 0.06

CJ-La 0.04 Toluene 0.03 Dimethyltetralins 0.02 Pyrene/fluorene 0.02

Total 100.0

“Structure of compound not determined.

initial selectivities for the catalytic reactions of tetralin. Naphthalene, methylindans, methyltetralins, toluene and ethylbenzene are all observed as initial reaction products from tetralin, and the impurity concentrations present were taken into account when constructing yield-conversion plots.

It is also important to take into account possible thermal processes which could occur in parallel with catalytic phenomena [ 171. Thermal processes can be identified by passing the feedstock through the reactor at 400’ C in the absence of catalyst. It would of course be preferable to study cracking phenomena due to the catalyst under conditions where thermal contributions are negligible. Where this cannot be achieved, the contribution due to thermal processes can be subtracted from combined effects. The extent of thermal cracking in- creases with temperature, and also depends on the particular feedstock under investigation. Table 2 shows that tetralin is more susceptible to thermal conversion than other hydrocarbon types of similar molecular weight under the same reaction conditions. Our results show that decalin is much less susceptible to thermal cracking then tetralin, in agreement with the results of Mostad et al. [ 131 who found negligible thermal cracking of decalin, even at 480” C.

Typical products from the thermal cracking of tetralin are shown in Table 3. Blank runs at 400°C with tetralin were carried out with several types of packing inside the reactor, including washed silica sand, granulated quartz and beryl saddles. In each case the extent of conversion and product distributions observed were very similar. This is evidence that the observed reactions are thermal rather than cracking due to catalytically active sites on the packing

100

TABLE 2

A.T. Townsend and J. Abbot/Appl. Catal. A 90 (1992) 97-115

Extent of thermal cracking of various compounde at 600 s time-on-stream at 400°C

Product Conversion ( % )

Tetralin 2.6 Toluene 2.1 Indan 1.1 Cyclohexane 0.6 Decalin <O.Ol 1-Methylnaphthalene <O.Ol n-Octane <O.Ol

TABLE 3

Major thermal cracking producta of tetralin at 600 s time-on-stream at 400°C (in order of increas- ing molecular weight)

Product Weight percent

Toluene 0.20 Ethylbenxene 0.06 Indan 0.07 n-Propylbenzene 0.16 Naphthalene 0.50 CJLoa 1.10 Methylindans 0.16 GO&S” 0.09 Decalin 0.36 CJ-La 0.30 C&U” 0.16 C,+Lsn 0.66 CJ-La 0.21

“Structure not determined.

material itself. Approximately half of the thermal products were also found from catalytic processes, namely toluene, ethylbenzene, indan, n-propylbenzene, naphthalene plus methylindans. The remaining products could not be positively identified by gas chromatography-mass spectrometry (GC-MS) analysis and are shown as molecular formulae. The major thermal product was an unidentified CloHlo compound which may be an intermediate in the dehydrogenation of tetralin to give naphthalene. The difference in product distributions from thermal and catalytic cracking described in detail below would indicate that different processes are involved in each type of cracking considered. It should also be noted that five of the components listed in Table 3 retain the C,, carbon skeleton of tetralin, and these account for the majority of ob-

A.T. Townsend and J. Abbot/Appl. Catal. A 90 (1992) 97-115 101

served products. This greater propensity towards hydrogen transfer accounts for the higher susceptibility of tetralin towards thermal reactions compared to other hydrocarbons listed in Table 2. A small increase in decalin was found over that present as a feed impurity. This would suggest that decalin may be produced as a thermal product to some extent.

Catalytic cracking of tetralin

Previous detailed studies on the cracking reactions of tetralin have reported results only at high levels of feedstock conversion. For example, Mostad et al. [ 13 ] reported a product distribution from the reaction of tetralin on a Y zeolite at 460-480” C at 79% conversion. At this level of conversion it is impossible to separate the initial reaction processes involving tetralin itself from secondary processes in which initial cracking products undergo further reactions to produce other hydrocarbon species. In the present study we have examined product distributions from the cracking of tetralin on HY at 400°C with conversion levels ranging from 5 to 80%. Fig. 1 shows the variation in overall conversion of tetralin with time-on-stream with various levels of catalyst to feed, as well as the extent of thermal conversion in the absence of introduced catalyst at 400” C. Times-on-stream of less than 400 s were not considered due to reactor temperature fluctuations arising from rapid feedstock introduction. Plots of yield against conversion for each product identified, taking into account the contribution from thermal processes and impurities initially present, enable us to examine both initial reaction networks and secondary processes. This helps us to build up an overall reaction scheme for the cracking of tetralin on

0 400 800 1200 1600

TIME ON STREAM (r)

Fig. 1. Tetralin conversion on HY at 400°C against time-on-stream for a range of catalyst to feed (C/F) ratios, including blank runs (thermal reactions). (m) Blank (thermal cracking), (0 ) C/F =0.0025, (0) C/F=0.005, (A) C/F=O.Ol, (Cl) C/F=0.02.


I .c ,es--+e.c. I

0 20 40 60 80 100

4.0

3.0

2.0

1.0

0

0 20 40 60 80 100

CONVERSION (wt%)

Fig. 2. Yield-conversion plots for products from the reaction of tetralin of HY at 400°C. (a) Indene, (b) indan.

8.0 4.0 2.0

6.0 3.0

40 2.0 I.0

2.0 I.0

0 0 0 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100

i 1 .oo 0.30 7.0

i 0.80 6.0

3 0.60

0.20 5.0

4.0

0.40 3.0 0.10

2.0 0.20

I.0

0 0 0

0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100

CONVERSION (wt%) Fig. 3. Yield-conversion plots for aromatic products from the reaction of tetralin on HY at 400°C. (a) Benzene, (b) toluene, (c) ethylbenzene, (d) propylbenzene, (e) isobutylbenzene, (f) n-butylbenzene.

HY. For example, indene was not found as an initial product from tetralin, and the concentration of this component declined with increased conversion of tetralin (Fig. 2a), with hydrogenation to give indan considered likely (Fig. 2b). Indan was indeed observed as an initial product from the reaction of tetralin, and the contribution from the hydrogenation of indene was taken into account when calculating initial selectivities. Some chromatographic overlap from cis- decalin was noted for indene, but was considered unlikely to alter significantly the trend observed. The observation of decalin initially present in the feedstock is also important. The level of decalin was found to remain almost con-

A.T. Townsend and J. Abbot/Appl. C&cd. A 90 (1992) 97-115 103

40

(a) 30

20

10

0 t/” 0 20 40 60 a0 100

2.0

Ib)

t.0

0 m

0 20 40 60 80 100

4.0

3.0

2.0

I.0

0 I

0

(cl

A 20 40 60 80 100

4.0

2.0

1.5

I.0

0.5

(e)

I.0

OIP olr o- 0 20 40 60 60 100 0 20 40 60 80 100 0 20 40 60 80 100

CONVERSION (wtW)

Fig. 4. Yield-conversion plots for aromatic and cyclic producta from the reaction of tetralin on HY at 400°C. (a) Naphthalene, (b) 1-methylnaphthalene, (c) 2_methylnaphthalene, (d) methylindans, (e ) 5- and 6-methyltetralins, (f) methylcyclopentane.

0 20 40 60 00 100 0 20 40 60 00 100

(a) 1.00 -

(b)

0.80 . 0

o’60

0 0

0.40

0

0

O

0.20

0 j-:-:- 0 0, “, @8” o

CONVERSION (wt%) Fig. 5. Yield-conversion plots for products from the reaction of tetralin on HY at 400°C. (a) Hydrogen, (b) coke.

stant at about 0.2% throughout the conversion range studied for tetralin (5- 80% ). It is apparent from Table 1 that this corresponds closely to the level of decalin present in the original feedstock, showing that direct hydrogenation of tetralin to produce decalin is relatively insignificant during catalytic reactions of tetralin on HY at 400’ C.

Over 100 individual compounds were identified by GC-MS at high conversion levels from the cracking of tetralin at 400°C. However, inspection of the yield-conversion plots over the complete range of conversion showed that the majority of these products were, in fact, only produced as a result of secondary processes. Figs. 3 to 5 show examples of primary reaction products, while Figs.


0 20 40 60 60 100 0 20 40 60 60 100 0 20 40 60 60 100

0.60 0.002 0 so

ii 0.60 (d) 3 ti Ooo 0.20

0.40 0 0 0.001

0

0.20 0.10

*

0

0 0 0 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 60 100

CONVERSION (wt%)

Fig. 6. Yield-conversion plots for alkanes produced from the reaction of tetralin on HY at 400°C. (a) Methane, (b) ethane, (c) propane, (d) n-butane, (e) n-pentane, (f) L-methylpentane.

0.40

(a) 0 0

0.30

0 0.20

m

0

0 0

0.10 0

0 0 20 40 60 60 100

0 20 40 60 60 100

0.006 0 002

6.004 0.001

0.002

0 0 0 20 40 60 60 100 0 20 40 60 60 100

CONVERSION CwtW) Fig. 7. Yield-conversion plots for olefinic products produced from the reaction of tetralin on HY at 400°C. (a) Propene, (b) iaobutene, (c) 2-methyl-I-butene, (d) 2-methyl-2-butene, (e) tram- 2-pentene, (f) ck-2-pentene.

6 to 10 show examples of products classified as secondary. In most cases there is a clear distinction as many secondary products do not appear in detectable amounts below ca. 20% feedstock conversion.

Initial cracking processes

Approximately twenty individual hydrocarbon species were identified as being initial reaction products. These are shown in Table 4 which also gives

A.T. Townsend and J. AbbotjAppl. Catal. A 90 (1992) 97-115 105

L

$ a 0 20 40 60 60 100

se k 0.20 > I 0.10

& w 0.06

3 0.06

0.04

0.02

0

0 20 40 60 60 100

CONVERSION

(b) 0

i

0 20 40 60 80 100

-I

0 20 40 60 80 100

(wt%)

Fig. 8. Yield-conversion plots for cycloalkanes formed from the reaction of tetralin on HY at 400” C. (a) CyclopenLne, (b i dimethylcyclopentanes, (c) ethylcyclopentane, (4 dimethylcyclohexanes.

L 0.30 , 0.60 >

0.50

0.20 0.40

0.30

0.10 0.20

0.10

0 0 -I 0 20 40 60 60 100 0 20 40 60 80 100

CONVERSION (wtW) Fig. 9. Yield-conversion plots for secondary aromatic products from the reaction of tetralin on HY at 400°C. (a) o-xylene, (b) m- andp-xylene.

their respective initial weight selectivities, taken from the initial slopes of the corresponding yield-conversion plots. The initial molar selectivity of each product is also given, which assists in proposing initial reaction networks for the reaction of tetralin on HY. Isomerization to methylindans (Fig. 4d) ac-


CONVERSION (wtW)

Fig. 10. Yield-conversion plots for secondary naphthaIene derivatives from the reaction of tetrahn on HY at 400°C. (a) Dimethylnaphthalenes, (b) ethylnaphthalene, (c) propyhmphthalene.

TABLE 4

Initial products formed from the catalytic reaction of tetrahn on HY at 400°C

Product Type Initial selectivitfl

Weight Molar

Hydrogen Benzene Methylcyclopentane Toluene Methylcyclohexane Ethylbenzene Indan Propylbenzene Naphthalene Methylindans Butylbenzenes GoHis Methylnaphthalenes Methyltetrahns Cz-Naphthalene

Dimethyltetrabns C$‘-Naphthalene Anthracenelphenanthrene Cz -Naphthalene Coke

(1+2)s 0.0001 0.007 1s 0.126 0.213 (1+2)s 0.005 0.008 (1+2)s 0.019 0.027 (l+z)s 0.003 0.004 (1+2)s 0.013 0.016 (1+2)s 0.025 0.028 1u 0.023 0.025 (1+2)s 0.244 0.252 1u 0.245 0.245 1U 0.122 0.120 1u 0.015 0.014 (1+2)s 0.021 0.020 1u 0.114 0.103 1u 0.005 0.004

1u 0.004 0.003 1u 0.017 0.013 (1+2)s 0.002 0.002 1u 0.007 0.005 (1+2)s 0.003 0.003

Total 1.013

“1, Primary; 2, secondary; S, stable; U, unstable. ?The initial weight selectivity is determined by the initial slope of the corresponding yield-conversion plot.


counted for 24% of the initial weight selectivity. At the highest conversion considered, l- and 2-methylindan were the major isomers present with only small amounts of 4-methylindan being found. The ratio of the relative amounts of the three isomers (l- : 2- : 4-) was 1.0: 0.61: 0.099. Monocyclic aromatics (Fig. 3) account for 40% of the initial weight selectivity, with benzene and butylbenzene providing the major contribution. Bicyclic aromatics constitute about 29%, with naphthalene the major component in this classification (Fig. 4). Methyl and dimethyl tetralins make up about 11% (Fig. 4). The remainder includes small amounts of saturated alicyclics, anthracene and phenanthrene, coke and molecular hydrogen (Fig. 5). The product types given in Table 4 are included in distributions of products previously reported at high conversion levels of tetralin [ 131. However, various classes of compound are not formed initially under our reaction conditions, including acyclic alkanes and alkenes, and these are classified as secondary.

Initial reaction processes can be postulated from the product distribution presented in Table 4, as has been reported for the cracking of other types of hydrocarbons such as alkanes [l-3], cycloalkanes [ 10-121, alkenes [6-81 and alkyl aromatics [ 91. When considering the reaction of tetralin, this is more difficult than for other compound classes previously considered, as extensive hydrogen transfer accompanies cracking and rearrangement of the original carbon skeleton. It is however possible to propose classification of the initial processes into monomolecular and bimolecular reactions.

Monomolecular processes These include processes in which there is no loss or gain in the number of

carbon atoms from the Cl0 feed molecule, and there is also no requirement for another feed molecule to be immediately available to participate in hydrogen transfer. Two such processes can be identified and are illustrated in Fig. 11. The first is isomerization of the feed to methylindans. This reaction parallels the isomerization of cyclohexane to methylcyclopentane [lo] also observed at 400” C on a HY zeolite (Fig. 12). Ring contraction of saturated alicyclics [ 11,181 to produce a mixture of 5- and 6-membered rings has also been reported for larger ring systems, such as during the reaction of cyclooctane [ 181. The car- bocation intermediate during such a process may either be a carbonium ion or a carbenium ion. The second monomolecular process involves the formation of naphthalene, with concurrent evolution of moleoular hydrogen. Molecular hydrogen has been previously reported as a product during cracking of tetralin on Y zeolite [ 131. Evolution of molecular hydrogen has been associated with processes which lead to aromatics and coke during the reaction of cyclopentane on HY [ 121. The extent of evolution of molecular hydrogen in our study is, however, small when compared to the total formation of naphthalene during the initial reaction of tetralin, as seen from the initial molar selectivities in Table 4. This shows that the monomolecular process illustrated can only ac-

108 A.T. Townsend and J. AbbotfAppl. Catal. A 90 (1992) 97-115

Methylindans (0.245)

Tetralin

2H2

(1)

(2)

Naphthalene (0.252) Hydrogen (0.007)

Fig. 11. Monomolecular processes from the reaction of tetralin on HY at 400°C. Numbers in brackets indicate initial molar selectivities.

/

Aromatics (0.023)

/ 0 -v

Methylcyclopentane

\ (0.860)

/ \ Paraffins

Cracking products

(0.113)

Kliizclics)

Fig. 12. Reaction processes from the reaction of cyclohexane on HY at 400” C. Numbers in brackets indicate initial weight selectivities.

count for about 1.3% of the total amount of naphthalene observed. Aromati- zation to naphthalene through transfer of hydrogen to other feed molecules must occur preferentially over direct evolution of molecular hydrogen.

Bimolecular processes Sets of possible initial bimolecular reaction processes are illustrated in Figs.

13 and 14. These have been divided into two types, those in which net hydrogen formation would occur, and those in which net hydrogen removal would occur. It does not appear possible to formulate a series of independent bimolecular processes each with a net zero hydrogen balance. Obviously the processes ac-


BU

r + + hl (3)

0.252 0.120

co I

El

+ + 2P+l (5)

+ - 0.004 0.016 :

Me Pr

+ + P-hi (4)

t

+ + 2Wzl (7)

0.005 0.213

0.002 0.008

Fig. 13. Bim~l~~~lar processes with net hydrogen formation from the reaction of tetralin on HY at 400°C. Numbers in brackets indicate initial molar selectivities. [H,] indicates hydrogen transferred to another reactant molecule, but not observed as molecular hydrogen.

0.252

0.025

0 Et

I (10)

0.016

03 I (11)

0.014

Fig. 14. Bimolecular processes with net hydrogen removal from the reaction of tetralin on HY at 400°C. Numbers in brackets indicate initial molar selectivities. [H,] indicates hydrogen from associated reaction processes.

tually occurring must be more complex than those illustrated here, as both sets of processes must be coupled through the transfer of hydrogen. Despite the importance of hydrogen transfer processes during cracking reactions, little is reported in the literature regarding mechanistic details [ 191. Both ionic and


free radical processes have been postulated [ 201. However the processes shown in Figs. 13 and 14 give a partial insight into the rearrangements of the tetralin carbon skeleton. It is interesting to note that the initial selectivity for the formation of decalin on HY from the reaction of tetralin is very low. This shows that the bimolecular reaction:

a+a- + co

which does not involve a net production or removal of hydrogen is relatively unimportant under the conditions studied here. In agreement with other studies [ 131, decalin was produced only in low concentration at all levels of conversion studied.

The processes illustrated in Figs. 13 and 14 show how pairs of products could be formed to account for the redistributions of carbon atoms during cracking processes. For example, dehydrogenation of tetralin leads to naphthalene with concurrent ring cleavage in the second feed molecule to yield butylbenzene, as is illustrated in Fig. 13 (reaction 3). The observed ratio of n-butylbenzene to isobutylbenzene was 20 : 1. This would indicate that the formation of isobutylbenzene via a free carbenium ion is unlikely. However, transalkylation reactions can also occur to yield substituted naphthalenes together with the com- plementary alkylbenzene. Transfer of 1,2,3 or 4 carbon atoms can occur leading to propylbenzene, ethylbenzene, toluene and benzene itself. The side chain transferred to the naphthalene nucleus can be either saturated or unsaturated. Transfer of one carbon atom yields methylnaphthalenes (reaction 4). Trans- fer of 2,3 or 4 carbon atoms occurs to produce species with one double bond in the chain as illustrated in Fig. 13 (reactions 5,6 and 7). Transfer of 4 carbon atoms can also lead to a subsequent further reaction process in which a third aromatic ring is formed to give anthracene or phenanthrene (reaction 8). Transalkylation process can also apparently occur without dehydrogenation to produce the naphthalene nucleus. This is illustrated in Fig. 14 which illustrates the formation of methyltetralins and dimethyltetralins (reactions 9 and 10).

Inspection of the initial molar selectivities in Table 4, as well as the reactions described in Figs. 13 and 14, reveals that in some cases there is a reasonable correspondence in amounts of products formed for a given pair (e.g. process 4)) but in most cases there is a significant excess of one product. For example only about half the naphthalene observed can be accounted for by process (3) while less than 2.3% of the benzene can be accounted for by process (7). This implies that there are some “missing” carbon atom chains. One possible ex- planation is that these carbon species have formed coke on the surface of the catalyst. Other studies, including the reaction of cumene and gas oils [9,21], have concluded that a major route to coke is through the reaction of small ( C3 and C,) fragments. However, the initial selectivity for coke formation for the

A.T. Townsend and J. AbbotjAppl. Catal. A 90 (1992) 97-115 111

reaction of tetralin on HY at 400 o C as seen in Table 4 and Fig. 5 is less than l%, similar to the level found by Mostad et al. [13] (2 wt.-%) for tetralin cracking at 460” C on Y zeolite at 79% conversion. Greensfelder et al. [ 151 similarly found coke deposits of l-2 wt.-% for the cracking of tetralin on silica- zirconia-alumina catalysts at 500’ C. The low levels of coke found lead to the conclusion that a proportion of the aliphatic side chains which are removed during cracking are themselves converted into new aromatic nuclei. This can be seen by an examination of a molar balance for carbon based on atoms gained or lost from tetralin during initial reaction processes. Inspection of Table 5 shows that there is apparently a significant excess of carbon “lost” through cracking of the Cl0 framework, compared to carbon appearing as side chains on compounds in the “gain” column.

The initial reaction processes for tetralin can be compared with those of related feed molecules on this catalyst under similar conditions. Reactions of cyclohexane on HY at 400” C have been studied [lo] and Fig. 12 illustrates the observed initial processes. Isomerization to methylcyclopentane is the domi- nant initial process accounting for 86% of the initial weight selectivity, com-

TABLE 5

Carbon balance for the reaction of tetralin on HY at 400 ’ C

Carbon

Gain Loss

Benzene Methylcyclopentane Toluene Methylcyclohexane Ethylbenzene Indan Propylbenxene Naphthalene Methylindans Butylbenzenes

Cld& Methylnaphthalenes Methyltetralins Cl -naphthalene Dimethyltetralins CL -naphthalene Anthracene/phenanthrene C;-naphthalene Coke

0.020 0.103 0.008 0.006 0.039 0.008 0.020

0.852 0.032 0.081 0.012 0.032 0.028 0.025

Total 0.184 1.06


pared to 24% for the isomerization of tetralin. There is a marked contrast in the propensity towards hydrogen transfer for reactions of tetralin and cyclohexane under similar conditions. Formation of aromatics accounts for only 2% of the initial selectivity for the reaction of cyclohexane. This may be due to the lack of suitable receptors for the hydrogen which would be necessary for com- plementary formation of aromatics. Cleavage of the saturated ring of an alicyclic molecule occurs with much greater difficulty than for tetralin which leads to potential sites for hydrogen transfer. Formation of aromatics by dehydrogenation with concurrent formation of molecular hydrogen does not appear to be a favoured process for either cyclohexane or tetralin.

The reaction of tetralin can also be compared with that of an alkyl aromatic such as cumene under similiar conditions [ 91. In both cases cleavage of a C-C bond connecting the aromatic nucleus to a side chain can occur. In the case of cumene this cleavage leads to benzene and propene as the major products [ 91. For the reaction of tetralin direct cleavage of the corresponding C-C bond would lead to a phenylalkene product. This type of species was not detected in our present study, although reported as a product in the investigation by Mos- tad et al. [ 131, at higher temperatures. Hydrogen transfer to this species type prior to desorption leads to alkylaromatics such as butylbenzene which were observed as initial products.

Secondary processes

It has already been pointed out that most of the products present at high conversion are, in fact, not observed as initial reaction products. Yield-conversion plots of secondary reaction products are illustrated in Figs. 6-10. Table 6 gives a list of all secondary products identified. The effect of secondary reaction processes is most easily understood by classifying all products, both initial and secondary, into a smaller number of structural types. For example, products can be classified as benzene types, which includes benzene and al- kylbenzenes. Naphthalene types include naphthalene and substituted naphthalenes, where the side chain may be saturated or unsaturated. Tetralin types include the methylindan isomers and substituted tetralins such as methyl and dimethyltetralins. The alicyclic group includes cyclopentane, cyclohexane and related species formed through alkyl substitution. Species in these groups were observed as both primary and secondary products. The alkane group includes acyclic saturated molecules from C2 to C,, while the alkene group includes molecules from CZ to C5. Products classified in the alkane and alkene groups were observed only as secondary products.

Fig. 15 shows the behaviour of these six product groups with total conversion of tetralin on HY at 400’ C. The naphthalene types are seen as stable primary products which are also formed as secondary products at high conversion. The benzene types are also primary products, but may show some instability at high


TABLE 6

Secondary products identified from the reaction of tetralin on HY at 400°C at 50% conversion

Product wt.-%

Alkanes Methane 0.0012 Ethane 0.0020 Propane 0.075 Isobutane 0.18 n-butane 0.30 Isopentane 0.28 n-per&me 0 2-methylpentane 0.13 3-methylpentane 0.075 2-methylhexane 0.045 3-methylhexane 0 C&H,,” 0.0025

AIkenes

Alicyclics

Ethene Propene Isobutene trans-2-butene cis-2-butene

GHs“ trans-2-pentene cis-2-pentene 2-methyl-1-butene 2-methyl-2-butene

Cyclopentane Cyclohexane Dimethylcyclopentanes Ethylcyclopentane Dimethylcyclohexanes

0.040 0.12 0.16 0.11 0.080 0.0010 0.0020 0.0005 0 0.0005

0.10 0 0.25 0.075 0.040

Aromatics Xylenes 0.10-0.15 GH~z.= 0.15 CN,HU” 0 C,,H,,” 0.80 Dimethylnaphthalenes 0.025 Ethylnaphthaiene 0.32 C&H,,” 0.075 C,3H10a 0 C1aH1da 0.040 Propylnaphthaiene 0.030 Pyrene/fluoranthene 0.050 f&H,,” 0

“Structure not determined.

114 A.T. Townsend and J. AbbotfAppl. Catal. A 90 (1992) 97-115

0.00 0 20 40 60 60 1

% CONVERSION

0

Fig. 15. Plot of major reaction species against t&din conversion on HY at 400°C. (A ) Alkanes, ( 0 ) alkenes, (0 ) alicyclics, ( + ) benzene types, ( A ) naphthalene types, ( 0 ) t&din types.

conversion levels. The tetralin types formed initially are seen to be unstable at high conversion. Methylindans formed by isomerization of the tetralin feed are a major component of this group.

CONCLUSION

Reactions of tetralin on HY zeolite at 400°C have been studied both at low conversion showing initial reaction processes, and at higher conversion levels revealing secondary reaction processes. In addition, effects due to feedstock impurities and thermal processes have been taken into account. The study shows that even though the initial reaction products can be reduced to a set of about twenty individual hydrocarbon species, the initial reaction network is still very complex. The only significant simple monomolecular process appears to be isomerization to produce methylindans. All other initial processes appear to be bimolecular involving cracking and extensive hydrogen transfer. Dehy- drogenation and transalkylation leads to naphthalene and substituted naphthalenes concurrently. Cracking and hydrogenation leads to benzene and alkylated derivatives of benzene. Alkylated tetralins and alicyclic products are also formed through cracking and hydrogen transfer processes. Dehydroge- nation to produce aromatic species with evolution of molecular hydrogen is not a favoured process.

A detailed analysis of initial product selectivities coupled with an elemental carbon balance demonstrates that a complete set of simple processes cannot be identified, even if bimolecular processes are considered. Loss of carbon from the tetralm feedstock by cracking is significantly in excess of carbon gain which


can be accounted for by the appearance of alkylated naphthalenes and tetralins and coke. This implies that new aromatic nuclei must be formed from the fragments, as they are not observed as simple alicyclic fragments (i.e. alkanes and alkenes) at low conversion. This study also shows that hydrogen transfer processes are also complex, and must involve redistribution of hydrogen between more than two reacting feedstock molecules.

ACKNOWLEDGEMENT

Financial support for this work was provided by the Australian Research Council and the University of Tasmania.

REFERENCES

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Catalytic cracking of tetralin on HY zeolite