Metal-Catalyzed Dehydrocyclization of Alkylaromatics

ADVANCES IN CATALYSIS, VOLUME 28

Metal = Catalyzed Dehyd rocyc Iizat ion of Alkylaromatics SIGMUND M. CSICSERY Chevron Research Company Richmond, California

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . 111. The Dehydrocyclization of C,, and Higher Alkylbenzenes . . . . . . . A. Metal-Catalyzed Cyclization . . . . . . . . . . . . . . . . . . B. Cyclization over Dual-Function Catalysts . . . . . . . . . . . . . C. Reactions that Accompany Dehydrocyclization . . . . . . . . . . IV. Double Cyclization of Cs and Higher Paraffins . . . . . . . . . . . . 11. The Dehydrocyclization of C, Alkylbenzenes

293 295 296 299 306 309 312

V. The Dehydrocyclization of Alkylbenzenes Over Chromia-Alumina Catalysts . . . . . . . . . . . . . . . . . . . . 314 VI. The Dehydrocyclization of Alkylnaphthalenes . . . . . . . . . . . . 3 15 VII. Dehydrocyclization of Diphenylalkanes . . . . . . . . . . . . . . . 3 18 VIII. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 19 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 19

I.

Introduction

Catalytic cyclization is a very important reaction, both commercially and theoretically. Here we review metal-catalyzed cyclization of alkylaromatic hydrocarbons, and we present reaction mechanisms that govern these reactions. Monoalkylaromatics and ortho-substituted dialkylaromatics containing at least three carbon atoms in the side chains can easily undergo dehydrocyclization to form an additional ring. One can classify cyclizations of . alkylaromatics in many different ways.

1. The new ring can have five or six (or very rarely more or fewer) carbon atoms. 2. The new bond can be formed between two sp3 hybridized carbon atoms (the cyclization of ortho-substituted dialkylaromatics) or between an sp3 and an sp2 hybridized carbon atom (the cyclization of monoalkylaro293

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0 1979 by Academic Press, Inc.

All rights of reproduction in any form reserved. ISBN 0-12-007828-7

294

SIGMUND M. CSICSERY

matics). A new ring can also be formed between two sp2 hybridized carbon atoms (the formation of fluorene from diphenylmethane). 3. Formation of the new bond can involve two primary carbon atoms, or a primary and a secondary carbon atom, or two secondary carbon atoms, or even tertiary carbon atoms. 4. The new ring can be saturated (e.g., indan, tetralin) or aromatic (e.g., indene, naphthalene). 5. If the alkyl side chain has five or more carbon atoms, the second ring can form entirely on the side chain, not involving the aromatic ring; for example, phenylcyclopentane, phenylcyclopentene, and phenylcyclopentadiene can be obtained from n-pentylbenzene.

n-Pentylbenzene

Phenylcyclopentane

Phenylcyclopentene

Phenylcyclopentadiene

Cyclization can be catalyzed by acids and by metals. Whereas the mechansim of acid-catalyzed cyclization is well understood, many questions remain about the nature of metal-catalyzed cyclization. It is, however, generally observed that the presence of an aromatic ring enhances the rate of cyclization : alkylaromatics cyclize at a much higher rate than aliphatic hydrocarbons. The dehydrocyclization of alkylaromatics was first described more than four decades ago. In 1936 Moldavskii and Kamusher reported the formation of naphthalene from n-butylbenzene on chromia at 475°C ( I , 2). In 1945 Herington and Rideal reported the formation of indene from n-propylbenzene over chromia-alumina (3). Platinum-containing catalysts were first used for these reactions in 1956 by Kazanskii and co-workers (4-6).

DEHYDROCYCLIZATION OF ALKYLAROMATICS

295

II. The Dehydrocyclization of C, Alkylbenzenes

The chemistry and kinetics of the cyclization reactions of 1-methyl-2ethylbenzene and n-propylbenzene were studied in great detail by Shephard and Rooney (7) and by Liberman, Bragin, and co-workers (6,8,9).Shephard and Rooney studied the reactions of n-propylbenzene, o-methylstyrene, and 1-methyl-2-ethylbenzene over a 0.5% platinum-on-?-alumina catalyst between 337°C and 490°C, in the presence of excess hydrogen. They used the microreactor pulse technique. Hydrogenation and dehydrogenation are very rapid over this catalyst. Equilibrium is obtained between the olefins and the corresponding paraffins at all the temperatures investigated. Cyclization gives indan. Subsequent hydrogenolysis of the five-membered ring of the indan yields n-propylbenzene from 1-methyl-2-ethylbenzene, and 1methyl2-ethylbenzene from n-propylbenzene. This reaction shows that platinum can catalyze isomerization, via cyclic intermediates. The mechanism i s quite general: it could involve normal and branched paraffins, and alkylaromatics as well (20-22).Shephard and Rooney believe that multiple x-bonded transition states are involved in the C,-cyclization of alkylbenzenes, in ring opening and in isomerizations involving cyclic intermediates. Barron, Gault, and co-workers favor an a, p, y-triadsorbed intermediate (20).Pdl, Dobrovolszky, and TttCnyi propose a dual-site mechanism for C, -cyclizations and isomerizations involving five-membered cyclic intermediates (224. Bragin and co-workers found that over platinum-on-carbon catalysts, both paraffins and alkylaromatics follow zero-order kinetics. Activation energy for C,-dehydrocyclization in which the new bond is formed between two sp3 hybridized atoms is substantially less than the activation energy of cyclization in which the new bond is formed between one sp3hybridized atom and the sp2 hybridized carbon atom of the aromatic ring. Over one batch of platinum-on-carbon catalyst, B r a e and co-workers obtained 20 kcal/mol and 27.5 kcal/mol activation energies for the dehydrocyclization of paraffins and monoalkylbenzenes, respectively (6). Another batch of platinum on carbon (which differed only in some minor details of preparation from the first batch), gave 14 kcal/mol for the cyclization of 1-methyl-2-ethylbenzene and isooctane, and 29 kcal/mol for the cyclization of secondary butylbenzene (8) (Fig. 1). The results of Shephard and Rooney support these observations. The activation energy for n-propylbenzene over platinum-on-alumina catalyst is twice that of the cyclization of 1-methyl-2-ethylbenzene (1 1.6 versus 5.8 kcal/mol; Fig. 2). However, these last values are based on pulse-reactor results; therefore, they should be used only for qualitative comparisons.

296

SIGMUND M. CSICSEXY 1.3-

0.9 1.1

0) 0

0.5 0.1

I

I

1.5

1.6

0.3

Bragin el al. (8).

s

rn

-0 1

1.0 0.8

-

1.4

I

I

I

1.7

10001T"K

FIG. 2. C,-Dehydrocyclization of 1-methyl-2-ethylbenzene (A) and n-propylbenzene ( 0 ) over Pt-y-alumina catalyst (0.5% Pt) in the presence of hydrogen. Plotted from the data of Shephard and Rooney (7) by Bragin et al. (8).

111.

The Dehydrocyclization of C,, and Higher Alkylbenzenes

Commercial reforming catalysts have both metal and acid sites. Both could contribute to cyclization. If there are four or more carbon atoms in the side chain of a mono-alkylaromatic or ortho-substituted dialkylaromatic hydrocarbon, cyclization can yield either five- or six-membered rings. This multiplicity of reaction pathways helps to clarify the roles of the metal and acid components in dehydrocyclization and other reactions. The most important reactions of alkylbenzenes over dual-functional catalysts exhibiting acidic and dehydrogenation activities are hydrogenation, dehydrogenation, isomerization, cyclization, hydrogenolysis, and cracking. To elucidate the mechanisms of these reactions, Csicsery reacted n-butylbenzene (13), n-pentylbenzene (14, and 2-phenylpentane (14) over catalysts covering a wide range of dehydrogenation and acid activities (Tables 1-111).

297

DEHYDROCYCLIZATION OF ALKYLAROMATICS

Catalyst

Platinum Surface Area, Micromole CO/g

Dehydrogenation Activity

Acid Activity

High

None

High

Intermediate

25

Intermediate

Strong

66

None

Strong

None

2% platinum on silica gel 0.75% platinum on alumina 2% platinum on silica-alumina Silica-alumina

TABLE I Reactions of n-Butylbenzene over Different Catalystfib Catalyst Platinum (2%) on silica gel

Platinum (0.75%) on alumina

Platinum (2%) on silicaalumina

Silicaalumina

Reaction temperature (“C) Products (moles per 100 moles of feed)

427

371

427

482

~

Methane Ethane Propane, propylene Butanes and butenes 1-Phenylbutenes 1-Methylindan Methylindenes Naphthalene sec-Butylbenzene Isobutylbenzene Benzene Toluene Ethyl benzene n-Propylbenzene Other Total n-butylbenzene converted (mole%) Csicsery (13, 29).

0.91 1.57 1.07 0.58 1.99 8.04 2.89 11.87 0.36 0.31 0.61 1.12 1.37 1.02 0.34 29.93

0.46 0.57 0.87 1s o 0.40 2.12 0.21 1.29 0.10 0.12 1.57 0.53 0.44 0.41 0.06 1.24

0.42 0.39 0.82 0.37 1.85 7.28 2.38 0.91 0.73 0.54 0.63 0.28 0.26 0.17 2.64 17.67

0.53 -

0.41 7.54 -

0.77 0.06 0.02 0.08 0.03 7.94 0.15

0.05 0.01 0.46 9.57

’Reaction conditions: Atmospheric total pressure, an LHSV of 9, and an initial hydrogen-to-

n-butylbenzene mole ratio of 3.

298

SIGMUND M. CSICSERY

Dehydrocyclization of n-butylbenzene produces 1-methylindan, methylindenes, and naphthalene:

j-

Naphthalene

H*

1 -Methylindan

1 -Methylindene

3-Methylindene

The primary products of n-pentylbenzene cyclization are 1 -ethylindan, 1-ethylindenes, and 1 -methylnaphthalene.Secondary (consecutive) isomerization may produce dimethylindans, 2-ethylindan, the corresponding indenes, and 2-methylnaphthalene:

n-Pentylbenzene

1 -Methylnaphthalene

2-Methylnaphthalene

1 -Ethylindan

1 -Ethylindene

3-Ethylindene

2-Ethylindan

Dimethylindans

Dimethylindenes

2-Ethylindene

DEHYDROCYCLIZATION OF ALKYLAROMATICS

299

The primary products of the cyclization of 2-phenylpentane are cis- and truns-l,3-dimethylindans,1,3-dimethylindene,and 1-methylnaphthalene :

2-Phenylpentane

1-Methylnaphthalene

Cyclization to five-membered rings forms the alkylindans and indenes ; cyclization to six-memberedrings gives tetralins and naphthalenes. Tetralins and decalins, however, were not observed in any of the experiments, because of unfavorable equilibrium. For example, less than 0.02-0.1% tetralin and less than 0.001% decalins would be expected if they were in equilibrium with the naphthalene formed in the n-butylbenzene experiments. [Equilibrium conversions calculated from the data of Egan (IS),Allam and Vlugter ( I d ) , and Frye and Weitkamp (IQ.1 In the cases of n-butylbenzene and 2-phenylpentane, there is an additional difference (other than ring size) between cyclization to naphthalenes and alkylindans. The reaction forming naphthalenes involves the addition of a primary carbon atom to the aromatic ring, while formation of alkylindans (and alkylindenes) involves secondary carbon atoms. With n-pentylbenzene, however, secondary carbon atoms are involved in both five- and six-ring cyclizations.

A. METAL-CATALYZED CYCLIZATION Cyclization to both five- and six-membered rings occurs over platinum on silica gel. Cyclization to six-membered rings has higher activation energy than cyclization to five-membered ring products. The (alkylindan plus alkylindenes)/naphthalene ratios therefore decrease with increasing temperature (Fig. 3). The activation energy difference is about 8-9 kcal/mol, in good agreement with the results of Kazanskii and Liberman (18). The dehydrocyclization rate constants (k, for five-membered ring cyclization,

300

SIGMUND M. CSICSERY

TABLE I1 Reactions of n-Pentylbenzene over Different Catalysts"*b Catalyst Platinum (2%) on silica gel Product composition (moles per 100 moles of feed) Unreacted n-Pentylbenzene Benzene Toluene Ethyl benzene n-Propyl benzene Indan n-Butylbenzene 1-Methylindan and 1-methylindene Naphthalene Phenylpentanes Phenylpentenes I-Ethylindan and 1-ethylindene Dimethylindans and dimethylindenes 1-Methylnaphthalene 2-Methylnaphthalene Methane Ethane, ethylene Propane, propylene Butanes, butenes Pentanes, pentenes

Platinum (0.75%) on alumina

Platinum (2%) on silicaalumina

Silicaalumina

Reaction temperature ("C) 427

371

427

427

77.06 0.43 0.38 0.68 0.33 0.25 0.34 0.53 0.23 1.02 2.89 6.85 3.32 4.43 0.10 0.97 0.46 0.45 0.35 0.39

89.29 0.92 0.42 0.56 0.18 0.04 0.20 0.15 0.03 0.28 0.58 3.40 1.oo 2.70 0.12 0.38 0.17 0.50 0.36 0.61

35.34 1.07 0.25 0.35 0.10 0.14 0.08 0.26 0.32 0.55 0.60 1.50 6.95 29.42 21.54 0.70 0.24 0.33 0.25 0.62

94.15 4.00 0.06 0.10 0.01 -

0.006 0.02 -

0.20 0.03 0.33 0.8 1 0.05 0.09 0.01 0.09 0.10 0.22 3.80

Csicsery (14,29). Reaction conditions: Atmospheric total pressure, an LHSV of 6.8, an initial hydrogen-tofeed ratio of 3.0, and an initial hydrocarbon partial pressure of 0.25 atm.

k, for six-ring cyclization) of three model hydrocarbons are compared in Table IV, assuming first-order kinetics. A comparison of dehydrocyclization rates over platinum on silica and other nonacidic platinum catalysts suggests the following. 1. Platinum-catalyzed cyclization of alkylaromatics is faster than the cyclization of paraffins because the presence of the aromatic ring enhances the rate. The rate of dehydrocyclization further increases with the number of aromatic rings in the feed molecule. A comparative study of the de-

301

DEHYDROCYCLIZATION OF ALKYLAROMATICS TABLE I11 Reactions of 2-Phenylpentane over Different CatalystfPb Catalyst Platinum (2%) on silica gel

Silicaalumina

Reaction temperature (“C)

Product Composition (moles per 100 moles of feed) Unreacted 2-Phenylpentane Benzene Toluene Ethyl benzene n-Propylbenzene Cumene n-Butylbenzene Phenyl-Methylbutanes 3-Phenylpentane Dimethylindans and dimethylindenes 1-Methylnaphthalene 2-Methylnaphthalene Methane Ethane, ethylene Propane, propylene Butanes, butenes Pentanes Pentenes ~~

Platinum (2%) on silicaalumina

371

37 1

427

96.27 0.32 0.03 0.19 0.02 0.10 0.03 0.69 1.45 0.20 0.06 0.1 0.2

44.66 29.20 0.20 4.06 0.08 0.16 0.10 3.71 2.16 11.96 0.27 0.25 0.9 0.2 2.4 24.5 1.2

13.73 80.80 0.29 0.72 0.07 0.34 0.07 0.33 1.11 1.03 0.03 0.04 0.2 3.8 5.5 68.7

0.26 0.02

~

Csicsery (14,29). Reaction conditions: Atmospheric total pressure, an LHSV of 6.8, an initial hydrogen-tofeed ratio of 3.0, and an initial hydrocarbon partial pressure of 0.25 atm. a

’

hydrocyclization of n-octyl- and dodecylbenzenes, and of 2-n-butyl- and 2-n-octylnaphthalene, shows that alkylnaphthalenes cyclize faster than alkylbenzenes (19). 2. The rate of dehydrocyclization is probably often inhibited by (bicyclic) product desorption. Apparent first-order rate constants in the dehydrocyclization of n-butylbenzene over platinum-on-silica-gel decreased with increasing levels of bicyclic aromatic product (Fig. 4). Sinfelt and co-workers also found that product desorption is rate-controlling in methylcyclohexane aromatization (20). 3. Relative cyclization rates of paraffins and alkylbenzenes may be

302

SIGMUND

22

-

20

-

y

M. CSICSERY

18-

W -I

a

$ 16n

a z

>l4v)

W

z

s+

z c3 4

P

F' I

86-

I-

W

2 4-

1000/T°K I

300

350

I

400

I

450

I

500

'C

TEMPERATURE FIG. 3. Cyclization of n-butylbenzene. Methylindan/naphthalene ratios. According to Csicsery (I3,29).

influenced by electronic, factors. Davis and co-workers modified catalytic selectivity for the formation of bicyclic aromatics from n-nonane and n-decane by adding either tin or sulfur compounds to platinum on neutralized alumina (21). While total dehydrocyclization rates are not affected in the case of nonane, and are increased in the case of decane, both sulfur and tin decrease the formation of bicyclic aromatics by more than a factor of ten (Tables V and VI). This selectivity change parallels a change in the ethylbenzenelortho-xylene ratio in n-octane dehydrocyclization. The ethylbenzenelortho-xylene ratio changes from 1.0 over pure platinum to 0.5 over platinum-tin (1 :4) or thiophene-doped catalyst. Davis and co-workers suggest that tin or sulfur modify the electronic properties of platinum. As a

303

DEHYDROCYCLIZATION OF ALKYLAROMATICS

TABLE IV Comparison of the Cyclization of Alkylbenzenef n-Butylbenzene

First-order cyclization rate constants

n-Pentylbenzene

2-Phenylpentane

371

421

371

421

311

0.12 0.08 1.46

0.48 0.52 0.92

0.14 0.05 2.8

0.31 0.13 2.3

0.034 0.0047 7.3

0.18 0.008 22.6

0.39 0.04 10.5

0.14 1.17 0.12

0.36 2.08 0.17

Platinum on silica gel

k,

k6 kdk6 Platinum on silica-alumina

a

0.45 0.025 18

Csicsery (13, 14,29).

2:

0.20

v)

2-

-

0

N -I

0.16

> V

u

LL

0

0.12

t\

\

a 0.04

U W

c3

m

0 I-

u)

LLL z o

NAPHTHALENE + METHYLINDENES FORMED, MOLE %

FIG.4. Dehydrocyclization of n-butylbenzene over 2% platinum on silica gel catalyst. (-) Cyclization to methylindan and methylindenes and (---) cyclization to naphthalene. According to Csicsery (23).

304

SIGMUND M. CSICSERY TABLE V Efjkcis of Tin and Thiophene on Bicyclic Aromatic Formation over Platinum CatalysPpb n-Nonane

Feed hydrocarbon Reagent added to platinum catalyst Total aromatics formed (%) Total bicyclic aromatics (%) Bicyclics (% of total aromatics)

None Thiophene 42 21 50

45 2 4

n-Decane Tin 66 1.2 2

None Thiophene 23 15 40

45 4.3 10

Tin 12 4.8 7

Davis et al. (24. Reaction conditions: Temperature, 482°C; Onstream time, 50 minutes.

TABLE VI The Effect of Thiophene on the Aromatization of n-Nonane over Nonacidic Platinum on Alumina-K Catalyst in the Presence of HZEsb Thiophene in n-Nonane feed (mol %) Time onstream (minutes) Thiophene in product (mol %) Total conversion to aromatics ( m o l x ) C, aromatics produced (mol% of total aromatics produced) n-Propylbenzene 1-Methyl-2-ethylbenzene 1-Methyl-3-ethylbenzene 1-Methyl-4-ethylbenzene Indan Indene

0 37 0 40

25.6 47.1 6.1 0.6 15.2 5.5

15.1

39 1.2 45

30.3 65.5 Trace Trace 4.2 Trace

* Davis et al. (21). Reaction conditions: Temperature, 482°C; LHSV, 0.3.

result, the monocyclic aromatics produced from nonane or decane desorb before they can undergo a second cyclization. 4. C5- and C,- cyclizations are parallel reactions. Csicsery has shown that isomerization of tetralin to methylindan over platinum-alumina at 371°C is extremely slow (22). Davis and Venuto provided further evidence by showing that methylindan is also not converted to tetralin or naphthalene over platinum on silica-alumina (23). This behavior is similar to that observed in the cyclization of aliphatic hydrocarbons. Davis and Venuto also reported that the major aromatic products obtained from ten C8-C, paraffins and olefins at 482°C are only formed by direct six-membered ring

DEHYDROCYCLIZATION OF ALKYLAROMATICS

305

closure (24). This also proves that C, C, ring interconversion is very limited in the absence of an acidic catalyst. 5. Other things being equal (such as in the case of n-pentylbenzene, where secondary carbons are involved in both five- and six-ring cyclizations), platinum-catalyzed cyclization favors five-membered ring products over six- membered rings (14). However, the difference between the two reactions is small and may be inverted by changing process conditions (e.g., by increasing the reaction temperature or decreasing the partial pressure of H2). 6 . Over platinum-on-carbon catalyst at relatively low temperature (31OOC), C,-cyclization of alkylbenzenesprobably proceeds by direct closure of the ring between the carbon atoms of the side-chain and the benzene ring, bypassing dehydrogenation to olefins (25-27). However, at higher temperatures and on platinum-alumina or platinum-on-silica C,-dehydrocyclization could involve olefinic intermediates (7, 13, 28). 7. Platinum catalyzes at least two types of c6-dehydrocyclization, one of which involves olefinic intermediates (13, 28, 29). In the case of paraffins, this latter reaction involves the ring-closure of hexatrienes (30, 31). In the C,-dehydrocyclization of n-butylbenzene and n-pentylbenzene, phenylbutadiene and phenylpentadiene could correspond to these triene intermediates (13, 14). The second C,-dehydrocyclization mechanism is similar to C,-dehydrocyclization, and may not involve olefinic intermediates. 8. The methyl group in the y-position of the side-chain interferes with cyclization. The rate of C,-cyclization of n-butylbenzene at 371"C over platinum-on-silica gel is 3.5 times higher than that of 2-phenylpentane (Table IV) (14). The difference in C,-cyclization is even larger. Now, the side-chain carbon atoms involved in the cyclizations of n-butylbenzene and 2-phenylpentane have identical natures (i.e., secondary in five-membered ring closure and primary in six-membered ring closure). The difference between the two molecules (the extra methyl group) is far removed from the two carbon atoms involved in the formation of the new bond:

Adsorption geometry could cause the observed rate difference. Crawford and Kemball have found that deuterium exchange of the two methyl groups of the isopropyl side-chain in cumene over nickel films occurs in two steps (32). Both methyl groups cannot exchange at the same time. The cumene molecule probably must leave the surface and re-adsorb before the second

306

SIGMUND M. CSICSERY

methyl group can exchange. A similar situation might exist over platinum. The phenyl ring is probably held by n-bonding parallel to the surface of the metal (32).This necessarily limits the configurations in which the side-chain can adsorb to two: with either the methyl group or the propyl group pointing away from the surface. Cyclization is possible only in the first case. If adsorption could happen both ways, the rate of cyclization of 2-phenylpentane would be half that of n-butylbenzene. A slight preference for adsorption of the methyl group will further decrease the rate of cyclization. 9. Alkyl side-chains may be removed from the new ring of the bicyclic product during dehydrocyclization. The relative proportion of bicyclics dealkylated increases with rising temperature. For example, in the dehydrocyclization of n-pentylbenzene over platinum-on-silica at 482"C, 12% of the cyclization product is dealkylated (14). Hydrogenolysis is the probable mechanism that removes short (one- or two-carbon-long) side-chains. There is more such dealkylation over platinum-on-silica catalyst than over platinum on (the very acidic) silica-alumina. Isomerization of alkyl side-chains around the ring, however, is very slow over platinum-on-silica and other nonacidic platinum-containing catalysts. For example, in the dehydrocyclization product of n-pentylbenzene, the amount of 2-methylnaphthalene is only 2-3% of the total methylnaphthalenes formed. Of course, over platinum on acidic carriers, isomerization of the side-chain around the ring is very fast. 10. Over platinum-on-silica catalysts, different alkylindans are at equilibrium with the corresponding alkylindenes. Similarly, I -methyIindene is at equilibrium with 3-methylindene (13, 14). 11. Neither C,-nor C,-cyclization involve carbonium-ion intermediates over platinum metal. The rates of the n-propylbenzene + indan reaction (where the new bond is formed between a primary carbon atom and the aromatic ring) and the n-butylbenzene + 1-methylindan reaction (which involves a secondary carbon atom) are quite similar (13). Furthermore, comparison of the C,-cyclization rates of n-butylbenzene and n-pentylbenzene (forming naphthalene and methylnaphthalene, respectively) over platinum-on-silica catalyst shows that in this reaction a primary carbon has higher reactivity than a secondary carbon (Table IV) (29). Lester postulated that platinum acts as a weak Lewis acid for adsorbed cyclopentenes,creating electron-deficient species that can rearrange like carbonium ions (33). The relative cyclization rates discussed above strongly contradict Lester's cyclization mechanism for platinum metal. B. CYCLIZATION OVER DUAL-FUNCTION CATALYSTS Cyclization selectivities are very different over platinum on silica-alumina than over platinum on silica gel (Table IV). In the case of n-butylbenzene, for example, methylindan/naphthalene ratios differ by about an order of

DEHYDROCYCLIZATION OF ALKYLAROMATICS

307

magnitude. Over platinum-on-silica-alumina, rates of cyclization to sixmembered rings for n-pentylbenzene are 50 to 100 times higher than for n-butylbenzene. As a consequence alkylindan/methylnaphthalene ratios in the reaction product of n-butylbenzene are 60 to 190 times higher than in the product of n-pentylbenzene reactions. Cyclization rates and k,/k, ratios of 2-phenylpentane are similar to those of n-butylbenzene, but differ significantly from those of n-pentylbenzene (13, 14). These rate differences show that, over platinum on silica-alumina, two cyclization reactions occur simultaneously. One is catalyzed by the platinum metal; it is the mechanism observed over platinum on silica. Acid catalyzed self-alkylation is the second reaction. The following steps are involved. Dehydrogenation of alkylbenzene over the platinum component yields phenylalkenes. Protonation of the phenylalkene over the acid component forms a carbonium ion. (A phenylallyl cation may be produced by proton addition to phenylbutadiene or by hydride ion removal from a phenylalkene.) Attack of this carbonium ion on the aromatic ring closes either a five- or six-membered ring. Stabilization of the product occurs by proton elimination or hydride abstraction. This step may be followed by dehydrogenation to the thermodynamically most-stable species (e.g., to an alkylnaphthalene in the case of six-membered ring closure). (See p. 308.) The stability of the intermediate carbonium ion determines whether cyclization forms five- or six-membered rings. In the case of n-butylbenzene and 2-phenylpentane, acid-catalyzed six-ring cyclization would involve very unstable primary carbonium ions: C6H5-CH2-CH,-CH2-CH,+ or C,H5CH(CH3)-CH,-CH,-CH, +. Therefore, only five-membered rings could be formed in acid-catalyzed cyclization from these two hydrocarbons. On the other hand, the n-pentylbenzene + methylnaphthalene reaction proceeds through C,H,-CH2-CH,-CH2-C+H-CH3, a secondary carbonium ion. As a consequence, acid-catalyzed cyclization produces both five- and six-membered rings. The possible carbonium ion intermediates leading to five- or six-membered ring closure may have similar structures (e.g., both are secondary carbonium ions, as in the case of n-pentylbenzene). If so, acid-catalyzed cyclization favors six-membered ring products, as shown by the k5/k6 ratios (Table IV). About half of the 1-methylnaphthalene formed from n-pentylbenzene and 2-phenylpentane isomerizes to 2-methylnaphthalene over platinum on silica-alumina (while over platinum on silica less than 3% of the methylnaphthalene isomerizes to 2-methylnaphthalene). Alkylindan (and alkylindene) isomerization is also considerable over platinum on silica-alumina (13, 14). Platinum on alumina has properties between those of platinum on silica gel and platinum on silica-alumina. Only about one-fifth of the methylindan is produced by the acid-catalyzed route (13). Also, the isomerization of

DEHYDROCYCLIZATION OF ALKYLAROMATICS

309

1-methylnaphthalene is substantially slower over the alumina-supported catalyst than over platinum on silica-alumina (14). Cyclization of alkylbenzenesis much slower over silica-alumina than over the platinum-containing catalysts. To clarify the successive steps of cyclization, Csicsery performed a set of experiments with 4-phenyl-l-butene, a dehydrogenation product of n-butylbenzene (28). Cyclization of phenylbutenes over the platinum-on-silica gel catalyst almost exactly parallels that of n-butylbenzene. Since rapid hydrogenation of the phenylbutenes results in about the same n-butylbenzenelphenylbutene ratio as observed with the n-butylbenzene feed, this is hardly unexpected. Over silica-alumina, the rate of cyclization of 4-phenyl-1-butene is about 2000 times higher than that of n-butylbenzene. Methylindan is the product in both cases. This reconfirms the mechanism proposed above for the acidcatalyzed cyclization of alkylbenzenes: a self-alkylation process involving carbonium-ion intermediates. Thus, alkylbenzenes cyclize over silicaalumina by first dehydrogenating to phenylalkenes (by thermal dehydrogenation or by acid-catalyzed hydrogen transfer). Carbonium ions are formed when the olefinic bonds are protonated. Attack on the aromatic ring by the electron-deficientcarbon atom of the side-chain completes the cyclization reaction. In the absence of a dehydrogenation component (such as for pure silica-alumina), the slow formation of phenylalkenes is the rate-limiting step. This mechanistic interpretation is based on the assumption that, once formed, five- or six-membered products of dehydrocyclization do not undergo interconversion. As discussed above, isomerizations are extremely slow at 317°C for tetralin to methylindan and methylindan to tetralin over alumina, silica-alumina, platinum-on-alumina, and platinum-on-silicaalumina catalysts (22, 23). C. REACTIONS THAT ACCOMPANY DEHYDROCYCLIZATION The cylization of alkylaromatics over platinum catalysts is usually accompanied by isomerization, hydrogenation and dehydrogenation, fragmentation (i.e., hydrogenolysis and cracking), and other reactions. Isomerization over the neutral platinum-on-silica gel catalyst proceeds by two different mechanisms. 1. C,-cyclic intermediates are involved in the 1-methyl-2-ethylbenzeneP n-propylbenzene isomerization :

310

SIGMUND M. CSICSERY

Similarly, n-butylbenzene may be converted to sec-butylbenzene (13), and n-pentylbenzene to 2- and 3-phenylpentanes (14):

I

I

and

\ CNC

I

c

2-Phenylpentane

ac;c CI

c,C

C c\ 3-Phenylpentane

2. Noncarbonium-ion-type 1-2-methyl shifts have been described by Barron et al. ( I ] ) , and by Anderson and Avery (34). The reaction proceeds through a-y-diasorbed intermediates over platinum on neutral supports and does not involve carbonium-ion intermediates. According to this mechanism, the n-butylbenzene i$isobutylbenzene reaction involves the following steps (surface sites are represented by *) :

All side-chain isomers are formed in acid-catalyzed isomerization. Carbonium ions are the intermediates here. Over dual-function catalysts, such as platinum-on-alumina and platinum-on-silica-alumina, platinum increases the rate of isomerization by dehydrogenating alkanes to olefins. This facilitates the formation of carbonium ions.

DEHYDROCYCLIZATION OF ALKYLAROMATICS

DEHYDROGENATION

n-PROPYLBENZENE

ETHYLBENZENE

>0

TOLUENE

.r

I

+'4'10

OR '4%

BENZENE

/

SECONMRFEL~TYLEENZENE FIG.5. Catalytic reactions of n-butylbenzene.According to Csicsery (I3,29).

31 1

312

SIGMUND M. CSICSERY

There are two catalytic fragmentation procedures : hydrogenolysis and cracking. In the former a molecule of hydrogen is added, and no new unsaturated bond is formed. In cracking, one of the fragments is formed with an additional unsaturated bond. However, differentiating between the two reactions is not always simple. In the presence of hydrogen over a hydrogenating catalyst, the olefinic products of cracking could become saturated ; or vice versa, the paraffinic products of hydrogenolysis could be dehydrogenated. Hydrogenolysis of the different side-chain bonds of n-alkylbenzenes occurs at an approximately equal rate over platinum on neutral supports. On secondary alkylbenzenes, the bond next to the phenyl ring is cleaved preferentially. Fragmentation of alkylbenzenes over silica-alumina occurs exclusively by acid-catalyzed cracking. The reaction selectively cleaves the bond between the phenyl ring and the a-carbon of the side-chain. This occurs more than 100 times more often than the cracking of all the other bonds combined. Cracking rates of secondary alkylbenzenes are about an order of magnitude higher than those of n-alkylbenzenes. Acid-catalyzed cracking and platinum-catal yzed hydrogen01ysis proceed simultaneously over dual-function catalysts. The distribution of the scission products is determined by the relative strengths of the acidic and metal-type catalytic components. These parallel and consecutive reactions for n-butylbenzene are shown in Fig. 5. IV.

Double Cyclization of C, and Higher Paraffins

Paraffins with more than eight carbon atoms can dehydrocyclize to form bicyclic products. According to Shuikin and Bekauri, bicyclic products can be formed from paraffins by either successive dehydrocyclization or by simultaneous closure of several carbon-carbon bonds (35). The second possibility follows Balandin’s “sextet” model (36). A large number of hydrocarbons follow the consecutive mechanism (27).Thus far there is no evidence for simultaneous closure. Il’in and Usov have shown that over acidic platinum-alumina catalyst there are at least two consecutive pathways from n-nonane to indan (37): either through alkylaromatic intermediates, by first closing the six-membered ring; or through alkylcyclopentane intermediates, by first closing a fivemembered ring (Table VII and Fig. 6).

313

DEHYDROCYCLIZATION OF ALKYLAROMATICS

TABLE VII Some Cyclic Products Formed in the Aromatization of n-Nonant? ~

Concentration in Product (wt. %) Unreacted n-nonane 1 -Methyl-2-n-propylcyclopentane 1,2-DiethyIcyclopentane n-Bu tylcyclopentane n-Propylbenzene 1-Methyl-2-ethylbenzene Indan

~

~~

Temperature ("C)

340

400

460

490

73.4 2.7

41.7 2.5

17.7 0.9

0.7 0.8

1.1 3.0

25.4 1.3 0.4 1.1

2.3 5.0

5.6

0.4 9.8

11.8

9.3 17.4

0.5

4.9

12.6

Il'in and Usov (37).

FIG.6. The conversion of n-nonane to indan.

0.3 20.4 14.5

314

SIGMUND M. CSICSERY

A third possible pathway could yield indan through cyclononane intermediate. We know that cyclononane undergoes transannular dehydrocyclization over platinum-on-charcoal catalyst at 300°C, to perhydroindan and then to indan (38);but so far there is no evidence for the direct cyclization of n-nonane to cyclononane. Unfortunately, Il’in and Usov used an acidic catalyst and we cannot separate the contributions of acid and metal catalysis to the two mechanisms. Experiments over nonacidic platinum catalysts could show the relative importances of the platinum metal in the two cyclization pathways. Kazanskii and co-workers have described an interesting special case, double cyclization of n-octane at 310°C over platinum-on-charcoal catalyst at 0.2 liquid hourly space velocity. The reaction product contains about 0.25% cis-octahydropentalane and 2.2-4.5% alkylcyclopentanes (an approximately 1 : 1mixture of trans-1-methyl-2-ethylcyclopentaneand n-propylcyclopentane) (39, 40). Indirect evidence suggests that most of the octahydropentalane is formed from l-methyl-2-ethylcyclopentane,which cyclizes significantly faster than n-propylcyclopentane.

l-Methyl-2ethylcyclopentane

cis -0ctahydropentalane (3,3,0-bicyclooctane)

n-Octane

n-Propylcyclopentane

V.

The Dehydrocyclization of Alkylbenzenes Over Chromia-Alumina Catalysts

It is interesting to compare the dehydrocyclization activity of platinum with that of chromia-alumina. Pines and Goetschel reacted different butylbenzene isomers over acidic and nonacidic chromia-alumina catalysts between 480°C and 492°C (44.Dehydrocyclization is much slower over

315

DEHYDROCYCLIZATION OF ALKnAROMATICS

chromia-alumina than over platinum-containing catalysts. No methylindan is produced from n-butylbenzene over the nonacidic chromia-alumina ; the only bicyclic products formed are tetralin and naphthalene, showing that nonacidic chromia-alumina does catalyse C,-dehydrOCyCliZatiOn. A moderate amount of methylindan is formed from secondary- and isobutylbenzenes over this catalyst. Apparently, nonacidic chromia-alumina catalyzes C,-dehydrocyclization only when the new bond is formed between a primary carbon atom and the aromatic ring. We know that the isomerization of alkylbenzenes over nonacidic chromia-alumina involves free-radical intermediates and proceeds by phenyl or vinyl migration (41,42).One can speculate that dehydrocyclization also has a free-radical mechanism here. Methylindan and smaller amounts of tetralin and naphthalene are formed from all three butylbenzene isomers over the acidic chromia-alumina catalyst. These reactions proceed by a cationic mechanism. VI.

The Dehydrocyclizationof Alkylnaphthalenes

The rate of dehydrocyclization increase with the number of aromatic rings in the molecule (29). The dehydrocyclization of alkylnaphthalenes can follow the same pathways as the cyclization of alkylbenzenes: C,-dehydrocyclization gives benzindans and benzindenes, while C,-dehydrocyclization yields anthracenes and phenanthrenes. In addition to these two pathways, a-substituted alkylnaphthalenes can cyclize to acenaphthenes and acenaphthylenes :

&*Q&-&&!) CH3

CH2

1 -Ethylnaphthalene

1 -Vinylnaphthalene

Acenaphthylene

Acenaphthene

This reaction was first observed by Plate, Erivanskaya, and KhalimaMansur over platinum-on-carbon and platinum-on-alumina catalysts (4348). Platinum-on-carbon catalyzes this reaction between 310°C and 390°C (above which the catalyst is poisoned) (44). Over an acidic platinum-alumina catalyst containing 0.5 wt% platinum and 0.1 wt% sodium, 16.7% acenaphthenes and 1.5% acenaphthylene are obtained at 460°C and at 0.4 liquid hourly space velocity in hydrogen diluent. Conversions are considerably lower in helium.

316

SIGMUND M. CSICSERY

Kinetic studies at short residence times fist suggested the following reaction sequence: ethylnaphthalene dehydrogenates to vinylnaphthalene ; vinylnaphthalene dehydrocyclizes to acenaphthylene ; and finally acenaphthylene is hydrogenated to acenaphthene. However, further work by Isagulyants and co-workers, using ''C-labeled 1-vinylnaphthylene, shows that over platinum on alumina at 470"C, acenaphthene and acenaphthylene are formed from both 1-ethylnaphthalene and 1-vinylnaphthalene. Vinylnaphthalene dehydrocyclizes about three times faster than ethylnaphthalene. The vinylnaphthalene intermediate remains adsorbed on the catalyst surface during the reaction (48). 1-Propylnaphthalene can give two types of products by C5-dehydrocyclization: dehydrocyclization at the peri-carbon atom of naphthalene gives 1-methylacenaphthene and 1-methyl-acenaphthylene, while dehydrocyclization involving the b-carbon atom of naphthalene gives 4,Sbenzindan and 45benzindene :

b-b+d+& 7H3

CH2

Peri-dehydrocyclization

p-dehydrocyclization

Hydrogen has an important directing effect here. In the presence of hydrogen over 0.5 wt% platinum on y-alumina, Erivanskaya and co-workers found that the rate of peri-dehydrocyclization of 1-propenylnaphthalene is about four times faster than the rate of P-C5-dehydrocyclization (49). However, if the catalyst, after the usual reduction, is treated with helium for three hours and the experiment is also carried out in helium, the rate of peri-dehydrocyclization decreases by about a factor of seven. The rate of j-dehydrocyclization does not change significantly. The rates for both types of C,-dehydrocyclization increase with the acidity of the alumina (50,51).

No investigator has observed C,-dehydrocyclization involving the pericarbon atom of naphthalene. Phenalene and 2,3-dehydrophenalene have not been detected over any of the catalysts investigated. The C,-dehydrocyclization of 2-n-butylnaphthalene can give 4 5 or 5,6-methylbenzindans and benzindenes. Similarly, C,-dehydrocyclization can yield either phenanthrene or anthracene. The product distribution depends on reaction temperature and catalyst type. The C,-dehydrocyclization of 1-(2-naphthyl)-butenedepends on the acidity of the alumina, but

DEHYDROCYCLIZATION OF ALKYLAROMATICS

317

C,-dehydrocyclization does not (50). As a consequence, over nonacidic platinum catalysts above 400"C, C,-dehydrocyclization predominates over C,-dehydrocyclization (27). Furthermore, the phenanthrene/anthracene ratio is independent of catalyst acidity (52).The effect of reaction temperature is, however, very interesting. Over platinum-on-carbon catalyst between 350°C and 400"C, more anthracene is formed than phenanthrene. Above 450°C phenanthrene is the main product (53).Phenanthrene is also the main product over chromia-alumina between 360°C and 440°C; whereas, as seen above, anthracene is formed in this temperature range over platinum-oncarbon catalyst (54). We know that C,-cyclization of l-(naphthyl-2)-butene is possible without metal catalysts. The products are dihydrophenanthrene over quartz and 1,2,3,4-tetrahydrophenanthreneplus phenanthrene over alumina (50). The latter apparently catalyzes the redistribution of hydrogen in dihydrophenanthrene. Neither anthracene nor dihydro- or tetrahydroanthracene are formed over quartz or alumina from l-(naphthy1-2)-butene. Plate and Erivanskaya concluded from this that the 2-alkylnaphthalene + anthracene reaction does not involve naphthylbutene intermediate (27). The a-position in naphthalene (and other condensed polycyclic aromatics) is sterically hindered. Hodges and Garnett have shown, for example, that at 100°C and in the presence of Pt(I1)-salts the p-hydrogen exchanges with deuterium 28 times faster than the a-hydrogen (55). This could suggest that Pt-catalyzed direct C,-cyclization will also favor the P-position. Such steric effects decrease with increasing temperature. On the other hand, thermal ring closure of 2-alkenylnaphthalenes occurs exclusively with the a-carbon, giving phenanthrene. More 2-alkenylnaphthalene is formed at higher temperatures. This, combined with the change of steric effect, determines anthracene/phenanthrene ratios at different temperatures (27).

If this is true, the simultaneous formation of anthracene and phenanthrene from 2-n-butylnaphthalene gives us an extraordinary and fortunate opportunity to differentiate between two types of C,-dehydrocyclization (27). Anthracene might be the product of direct cyclization, a mechanism related

318

SIGMUND M. CSICSERY

to C,-dehydrocyclization. Phenanthrene is probably formed through dehydrogenated intermediates. This mechanism probably corresponds to the cyclization of hexatriene, as described by P d l and Tetenyi (30, 31). The significance of the dependence of the anthracene/phenanthrene ratio on temperature and catalyst type cannot be overemphasized; one wishes that there were more experimental data available on the effects of hydrogen partial pressure and other variables, to give more support to this hypothesis. Erivanskaya and co-workers also studied the dehydrocyclization of 2-nbutylnaphthalene over supported palladium, rhodium, and iridium catalysts (56-58). Palladium-alumina showed the lowest C,-dehydrocyclization activity, but was the most active for the C,-dehydrocyclization of 2-n-butylnaphthalene. A later study showed, however, that this enhanced activity was due to the high chlorine content of the palladium-alumina catalyst and not to some mysterious inherent catalytic activity of palladium (56). The dehydrocyclization activity of rhodium-alumina is lower than that of platinum-alumina. Hydrogenolysispredominates over all the other reactions with this catalyst (57). The effect of temperature on the anthracenelphenanthrene ratio in the product from 2-n-butylnaphthalene is the same over iridium-alumina catalyst as that observed over platinum-alumina : more phenanthrene and less anthracene are formed at high temperatures (58). VII.

Dehydrocyclization of Diphenylalkanes

Scola studied the dehydrocyclization of diphenylmethane, bibenzyl, trans-stilbene, 1,4-diphenylbutadiene, and 1,l'-binaphthyl, over 0.6% platinum-on-silica gel catalyst, at around 550"C, and in the presence of hydrogen (59). Stable, fused polycyclic aromatics were formed through the abstraction of ortho-hydrogens from one or two of the phenyl rings. Diphenylmethane was thus converted to fluorene, with excellent yield (45% at 550°C and 12 seconds contact time). 1,ZDiphenylethane (bibenzyl) was converted to phenanthrene with 92% yield. On the other hand, trans-stilbene gave only 33% phenanthrene at the same conditions. 1,4-Diphenylbutadiene gave a nonvolatile liquid fraction which contained 53% 2-phenylnaphthalene, 16% 1-phenylnaphthalene, 10% fluoranthene, and 21% unidentified other products. 1,l '-Binaphthyl gave a mixture containing 42% benzofluoranthene and 39% perylene. Scola believes that carbonium ion intermediates are involved in all of these dehydrocyclization reactions. This is unlikely, as platinum-silica usually does not have sufficient acidity to generate carbonium ions. Dehydrocyclization follows molecular rearrangement when 1,l-diphenylethane reacts over platinum alumina between 420°C and 540°C in the

DEHYDROCYCLIZATION OF ALKYLAROMATICS

319

presence of steam. The product contains 1,l dphenylethylene, trans-stilbene, and phenanthrene. Small amounts of 9-methylfluorene and fluorene are also formed (60). The phenanthrene is probably formed through stilbene intermediates. In the absence of steam, the catalyst is deactivated. VIII.

Conclusions

1. The presence of the aromatic ring enhances the rate of dehydrocyclization; alkylaromatics dehydrocyclize faster than paraffins. Furthermore, the rate of dehydrocyclization increases with the number of aromatic rings within the reacting molecule. 2. Platinum can catalyze C5- and C,-dehydrocyclizations. The two reactions are parallel. Interconversion is very limited between five- and sixmembered ring products over nonacidic platinum catalysts. Platinumcatalyzed dehydrocyclization does not involve carbonium ions. 3. There are at least two C,-dehydrocyclization mechanisms; one of these proceeds through arylalkene intermediates and corresponds to the hexati-iene-type C,-dehydrocyclization of paraffins. The other pathway is direct ring closure. It is probably related to C5-dehydrocyclization. 2Butylnaphthalene may differentiate between the two mechanisms; phenanthrene is probably formed by the first reaction, anthracene by the second. 4. There appear to be two C5-dehydrocyclizations over platinum-oncarbon catalyst. Activation energy differences suggest that the reaction involving an sp2 and an sp3 carbon atom (a cyclization in which the new bond is formed between an aliphatic and an aromatic carbon atom) is different from cyclizations involving two sp3 carbon atoms (in which the new bond is formed between aliphatic carbon atoms of two side-chains). REFEXENCB 1. Moldavskii, B. L., and Kamusher, G. D., Dokl. Akad. Nu& SSSR I, 355 (1936). 2. Moldavskii, B. L., Kamusher, G. D., and Kobylskaya, M. V., Zh. Obshch. Khim. 7, 169 (1937). 3. Herington, E. F. G., and Rideal, E. K., Proc. R. SOC.London, Ser. A 184,447 (1945). 4. Liberman, A. L., Bragin, 0. V., and Kazanskii, B. A., Dokl. Akad. Nauk SSSR 111,1039 (1956). 5 . Liberman, A. L., Bragin, 0. V., and Kazanskii, B. A., Izu. Akad. Nauk SSSR, Ser. Khim. p. 879 (1956). 6. Liberman, A. L., Bragin, 0. V., Ming-Nan, C., and Kazanskii, B. A., Dokl. Akad. Nauk SSSR 129,578 (1959). 7. Shephard, F. E., and Rooney, J. J., J . Catal. 3, 129 (1964). 8. Bragin, 0.V., Gur’yanova, G. K., and Liberman, A. L., Dokl. Akad. Nauk SSSR 160,823 (1 965).

320

SIGMUND M. CSICSERY

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