Applied Catalysis, 67 (1991) 307-324 Elsevier Science Publishers B.V.. Amsterdam

307

Methylcyclohexane and methylcyclohexene cracking over zeolite Y catalysts

Avelino Corma*, F. Mocholi and V. Orchilles Instituto de Catalisis y Petroleoquimica, C.S.I.C. Serrano 119,28006 Madrid (Spain)

and

Gerald S. Koermer* and Rostam J. Madon Engelhard Corporation, Menlo Park, CN 40, Edison, NJ 08818 (U.S.A.), (tel. (+ l-908)2055011, fax. (+ l-908)2055300

(Received 8 June 1990, revised manuscript received 24 August 1990)

Abstract

Naphthenes are an important class of molecules in fluid catalytic cracking. The cracking behavior of the model naphthenes, methylcyclohexane and methylcyclohexene was investigated over rare earth Y and USY zeolite catalysts. Initial products from methylcyclohexane are formed by a combination of protolytic and /?-scission cracking plus isomerization, H- transfer, H+ transfer and dehydrogenation reactions. Methylcyclohexane is a sensitive probe for characterizing the chemistry occurring on solid acid surfaces. Methylcyclohexene is the key intermediate in the formation of aromatics from methyl- cyclohexane. Methylcyclohexene cracks at a slower rate than methylcyclohexane but overall conversion is higher because hydride transfer reactions are fast.

Keywords: zeolites, cracking catalysts, methylcyclohexane cracking, methylcyclohexene cracking, na- phthene cracking.

INTRODUCTION

Naphthenes are important constituents of fluid catalytic cracking (FCC) feedstocks and products [ 11. Despite this, the cracking of alkanes, alkenes and short chain alkyl aromatics have been studied much more than naphthenes. Relatively little has been reported about the cracking chemistry of naphthenes over zeolite catalysts [ 2-51. Unsaturated naphthenes have been studied even less [6,7].

Currently, the aromatics content of FCC gasoline fractions is an important issue for both gasoline octane and reformulated gasolines. In this regard, naph- thenes are important in cracking chemistry as precursors for aromatics and coke [ 81 and as key agents in hydrogen transfer chemistry [ 2,3]. Thus a better understanding of naphthene cracking chemistry is important for controlling aromatics yields.

0166.9834/91/$03.50 0 1991 Elsevier Science Publishers B.V.

Nevertheless, the cracking chemistry of naphthenes is expected to be com- plex because a number of competing reactions such as cracking, hydride trans- fer, ring opening and isomerization can occur simultaneously. However this rich chemistry should make naphthenes a sensitive probe for catalyst proper- ties. In fact, there have been recent publications using cyclohexene as a test molecule for hydrogen transfer [ 9,101. However this work was done at tem- peratures significantly lower than commercial cracking temperatures.

We have investigated the cracking behavior of the model naphthenes, meth- ylcyclohexane (MCHA) and methylcyclohexene (MCHE), over zeolite Y cat- alysts. We report here (but see also refs. 11 and 12) the cracking chemistry of these compounds and the utility of MCHA as a probe molecule. The cracking chemistry provides insights into naphthene reactivity for both higher molec- ular weight and gasoline range naphthenes. Reaction product ratios allow us to assess subtle differences among zeolite catalysts.

EXPERIMENTAL

Materials

Methylcyclohexane (99% ) and 1-methyl-1-cyclohexene (97% ) were pur- chased from Aldrich and used without further purification.

Ultrastable Y (USY, Catalyst I) was prepared by standard hydrothermal techniques [ 131 from particles of approximately 80% zeolite Y in a low surface area, low activity y-alumina binder. After hydrothermal calcination and am- monium exchange, the zeolite unit cell size (UCS) was 24.53 A and the catalyst Na,O content was 0.69%.

Rare earth Y (Catalyst III) was prepared by ion exchanging mixed rare earth ions into approximately 80% zeolite Y bound with a low surface area alumina. The rare earth (RE,O,) content of the catalyst was 7.2 wt.-%, the Na,O con- tent was 1.41% and the UCS was 24.70 A.

Catalyst I was steamed in a fluidized bed at 732 C for 8 h at one bar of 100% steam to give Catalyst II which had a UCS of 24.31 A. Steaming Catalyst III in a fluidized bed at 732C for 8 h in 1 bar of 100% steam reduced the UCS to 24.45 A (Catalyst IV). Steaming reduced the absolute zeolite content of the unsteamed material by lo-15%.

Catalysis

Catalytic results were obtained using a fixed bed glass tubular reactor [ 141 with a 32-mm internal diameter. Reactions were done at atmospheric pressure and 500. Data was generated by two methods: (i) by holding time-on-stream constant and changing the amount of catalyst (i.e. changing the catalyst-to- oil ratio) to vary conversion and (ii) at constant cat/oil with a variable time-

309

on-stream. Standard optimum performance envelope techniques described previously [ 2,151 were used. Initial selectivities were calculated from the ini- tial slopes of plots of mol-% yield vs. percent conversion of feed.

Analysis

Product analysis was done by gas chromatography. Gaseous products were analyzed using a Poropak Q plus silica column. Liquid products were analyzed using a 60-m SE 39 capillary column. Product identification was done by re- tention time comparison to known samples and by gas chromatography-mass spectrometry (GC-MS) using a Shimadzu Model QP-1000 GC-MS equipped with a 100-m Supelco Petrocol DH capillary column.

REACTION CHEMISTRY

Methylcyclohexane (MCHA) cracking

MCHA has one primary, one tertiary and five secondary carbon atoms. In principle, MCHA cracking could occur by both protolytic [ 21 and p-scission mechanisms [ 2,3] via secondary and tertiary carbocation intermediates.

Protolytic cracking: C-C bond cleavage *

+ H- c

0

+ ii -0

+ CH,

(1)

(2)

The intermediate carbocations formed from protolytic cracking of ring C-C bonds can react in several ways:

- desorption as heptene by returning H+ to the catalyst (proton transfer from the carbenium ion).

- desorption as heptane after abstraction of H- from another hydrocarbon (hydride transfer).

- isomerization to branched carbenium ions, followed by desorption as de- scribed above.

- cracking before or after isomerization to give an alkene and an adsorbed carbenium ion. This carbenium ion can desorb by proton transfer or hydride transfer to give an alkene or alkane respectively.

Therefore, one expects heptanes, heptenes and products with less than seven carbon atoms as primary products from protolytic cracking of the MCHA ring. If however, the terminal methyl group is cleaved, methane and the products from the cyclohexyl carbenium ion result. These products will include cyclo-

hexene, cyclohexane, methylcyclopentane, methylpentanes, methylpentenes and products with less than six carbons.

Protolytic cracking: C-H bond cleavage The processes discussed above involve C-C bond cleavage. Alternatively

strong Bronsted sites can protonate a C-H bond to form hydrogen [ 161. This process is less favorable than C-C bond cleavage but is still feasible at cracking temperatures for molecules like MCHA that contain tertiary hydrogens [ 171. Protonation of the tertiary hydrogen in MCHA gives hydrogen and methyl- cyclohexyl carbocation (eq. 3 ). The methylcyclohexyl

(3)

carbocation can isomerize, lose a proton to give methylcyclohexene (MCHE ) or crack by the j?-scission mechanism to give the products described below.

Cracking by j%scission

(4)

Once a methylcyclohexyl carbocation is formed, it can crack by the conven- tional /3-scission process (eqn. 4 ) . This gives heptadiene, heptenes and prod- ucts with less than seven carbons, primarily Cqs and C,s.

Isomerization Methylcyclohexyl carbocation can also isomerize to give alkyl cyclopentyl

carbocations [5]. These species can either desorb to give alkylcyclopentenes and alkylcyclopentanes or crack giving mostly branched C, carbocations. Since the isomerization of cyclohexyl carbenium ions is fast, monoalkylcyclopen- tanes and monoalkylcyclopentenes should also occur as products.

Dehydrogenation As described above, protolytic cracking of a C-H bond can lead to dehydro-

genation of MCHA. Another dehydrogenation route involves hydrogen trans- fer. Here a surface carbenium ion abstracts a hydride from a donor such as MCHA, thereby generating a methylcyclohexyl carbocation. This species can lose H+ and desorb, thereby regenerating the protonic site and giving meth- ylcyclohexene (MCHE). MCHE can continue to lose hydrogens through H transfer [ 181 giving methylcyclohexadiene and finally toluene.

311

RESULTS AND DISCUSSION

Initial cracking products

Table 1 shows the observed initial products for MCHA cracking over cata- lysts I-IV at 500C. Fig. 1 contains plots of mol-% yield versus MCHA con- version for selected products from Catalyst I. In the following discussion, for brevity, unless otherwise noted, Catalyst I results are used.

Inspection of Table 1 indicates that the products can be rationalized by a combination of protolytic and j$scission cracking. In addition, both dehydro- genation mechanisms noted above seem to occur.

Additional information can be extracted from the data. For example, the amount of methane is slightly higher than the amount of cyclohexane. This can be explained by considering that cyclohexyl carbocation, before desorbing

TABLE 1

Initial molar selectivities for methylcyclohexane cracking over catalysts I-IV at 500C

Catalyst I II III IV

Methane 0.017 0.020 0.012 0.009 Ethane 0.005 0.004 0.005 0.003 Ethene 0.030 0.023 0.034 0.021 Propane 0.073 0.054 0.090 0.073 Propene 0.106 0.146 0.125 0.113 Butanes 0.186 0.131 0.205 0.194 Butenes 0.083 0.100 0.066 0.097 Pentanes 0.060 0.045 0.063 0.058 Pentenes 0.007 0.007 0.010 0.010 Cyclohexane 0.011 0.018 0.008 0.010 2-Methylpentane 0.027 0.012 0.034 0.020 3-Methylpentane 0.022 0.017 0.021 0.018 Hexane 0.005 0.004 0.004 0.004 Methylcyclopentane 0.002 0.001 0.001 0.001 Dimethylcyclopentanes 0.042 0.040 0.051 0.035 Dimethylpentane 0.004 0.003 0.005 0.005 2Methylhexane 0.012 0.016 0.014 0.017 3-Methylhexane 0.019 0.030 0.021 0.023 n-Heptane 0.152 0.156 0.146 0.167 Heptenes 0.219 0.258 0.206 0.250 Methylcyclohexenes 0.092 0.122 0.106 0.080 Methylcyclohexadienes 0.005 0.002 0.005 0.006 Toluene 0.058 0.035 0.052 0.040 Xylenes 0.033 0.018 0.028 0.022 Hydrogen 0.073 0.066 0.027 0.024 Coke 0.006 0.003 0.003 0.003

Mole % Yield

6 (A)

o- 0 10 20 30

Methylcyclohexane Mole % Yield

6i@)

4+

40 50

Conversion

_, 3-

2- / /

I- ,d

/

/

Ok

0 I

10 20 30 40 50

Methylcyclohexane Conversion

60

60

Fig. 1. Plots of mol-% yield vs. methylcyclohexane conversion over Catalyst I at 500C. (A) propene; (B) butenes; (C) heptane; (D) heptenes; (E) substituted cyclopentanes; (F) cyclo- hexane; (G) methylcyclohexenes; (H) toluene.

can react further to give methylcyclopentane, plus branched hexanes and hex- enes derived from ring opening. Indeed, a marked instability of cyclohexane is apparent in Fig. 1F even at conversion as low as 5%. If one adds the initial selectivities of all C!, products detected, the ratio of C,/C, is lower than one, indicating that C6 products could also be formed by reaction other than crack-

313

Mole % Yield

OS&- 0 10 20 30

% Methylcyclohexane Mole % Yield .^

40 50 60

Conversion

0 IO 20 30 40 50 60

% Methycylcohexane Conversion

ing of MCHA to methane and Cs products. One way to generate excess C6 products without methane could be transalkylation of methyl groups between two molecules of MCHA (eqn. 5 ) . Indeed, small amounts of dimethyl and even trimethylcyclohexanes were identified in the products by GC-MS spectral analysis. Moreover, it has been shown that when cracking heptane, the C,/C, ratio is also lower than one [ 141. Disproportionation reactions have been used to account for this observation [2]. Disproportionation reactions could also occur in the MCHA system and would produce some extra C, products. This may account for the low Cl/C6 ratio observed.

Mole % Yield

0 10 20 30 40 50

Methylcyclohexane Conversion Mole % Yield

0.16 ! 0.12 c

/

0.08 I 0.04

0 Y 0

_1

60

10 20 30 40 50 60

Methycyclohexane Conversion

Fig. 1.

20-b+ 0 (5) The open chain C7 products are also informative. Heptanes and heptenes

appear as primary products. Within experimental error, both heptane and hep- tenes are stable. Instability, if it occurs, is seen only at high MCHA conver- sions. This means that cracking products (

315

Mole % Yield

*I 0

Methylcyclohexane Mole % Yield

J

40 50 60

Conversion

7 (H) 6-

5

4

3 1

2-

l-

ok 0

A

10 20 30 40 50 60

% Methylcyclohexane Conversion

readsorption of heptanes or heptenes. If readsorption followed by cracking oc- curred extensively, heptane and heptenes would show marked curvature in the selectivity curves, Figs. 1C and D. This is not observed. In addition, it seems clear that little or no heptane is formed from saturation of heptenes by hydro- gen transfer reactions but mostly by hydride transfer from MCHA to C!, car- benium ions derived from ring opening of MCHA. Of course heptanes, hep- tenes and cracked products can also be formed from dimethylcyclopentanes. Our study does not address this or whether dimethylcyclopentanes crack more readily than MCHA. Regardless of the precise precursor for the ring opened

316

C, carbenium ion, the fate of this cation can be approximated by looking at the amount of cracking products and the amounts of heptenes and heptanes. Cracked products come from J?-scission. Heptanes will reflect the amount of H- transfer to the carbocation and heptenes reflect the amount of H+ transfer from the ion to the catalyst. We will discuss the implications of these results in the next section dealing with the use of MCHA as a probe molecule.

Aromatics formation is also an important process. The selectivity to hydro- gen is not sufficient to account for the yields of MCHE, methylcyclohexadiene and toluene by direct dehydrogenation. However, as a first approximation, hy- dride and proton transfer between saturated naphthenes, unsaturated na- phthenes, carbenium ions and the catalyst surface seem to be responsible for most of the unsaturated naphthenes and aromatics. We note that MCHE (Fig. lG), and methylcyclohexadiene are unstable primary products. Whereas tol- uene, (Fig. 1H) is a stable primary and secondary product. While MCHE is easily rationalized as a primary product, methylcyclohexadiene and toluene are much more difficult to explain. The formation of toluene by hydrogen transfer requires three consecutive bimolecular reactions with intermediate desorption and readsorption of MCHE and methylcyclohexadiene. Thus, as observed, MCHE and methylcyclohexadiene are unstable products. This multi- step bimolecular process for aromatics formation should have a very low fre- quency factor. The surprising appearance of toluene even at very low conver- sion could be due to the confined space in the zeolite cavities that produces high concentrations of reactants and relatively long residence times for the molecules in the pores. Also one must take into account the faster rate of de- hydrogenation of MCHE and methylcyclohexadiene compared with the initial dehydrogenation of MCHA to MCHE. This can be deduced from the magni- tude of Catalyst I initial selectivities to MCHE (0.092 ), methylcyclohexadiene (0.005) and toluene (0.058). In any case MCHE is clearly a key intermediate in the reaction chemistry of MCHA. The importance of MCHE as an inter- mediate will be discussed further in the section on MCHE reactions.

Methylcyclohexane as a catalyst probe molecule

Our results suggest that initial product ratios from MCHA cracking can be used as a sensitive probe to characterize and quantify the chemistry occurring on a catalyst surface. Based on the previous discussion, the ratio of cracking products to (MCHE + 2 (methylcyclohexadiene) + 3 (toluene ) + 3 (xylenes) + 6 (coke) -hydrogen) should reflect cracking relative to hydride transfer. In addition, the ratio of cracking products to heptenes should mirror cracking relative to H+ transfer to the catalyst. Finally, the ratio of heptenes to heptane should give the relative rate of proton transfer to hydride transfer for surface heptenium ions formed from either C5 or C, ring opening.

If the above ratios are indeed diagnostic for catalyst performance, these ra-

317

tios should vary in reasonable ways as the unit cell size (UCS) of Y zeolite in the catalyst is changed. We assessed this by determining the ratios for catalysts I through IV. The zeolite UCS varied from 24.70 to 24.31 A. The results are shown in Figs. 2-4.

Fig. 2 plots the ratio of cracking/H- transfer vs. UCS. The ratio increases as unit cell size decreases. This is consistent with previous work showing that

Cracking/Hydride Transfer

3-P

,i- ~ I A_ 24.3 24.4 24.5 24.6 24.7

Unit Cell Size (Angstroms)

Fig. 2. Cracking/ (MCHE+2(methylcyclohexadiene) + 3 (toluene) +6 (coke) - hydrogen) vs. zeolite unit cell size.

Cracking/Heptenes 3.5

2.5 -

2 1 I 24.3 24.4 24.5 24.6 24.7

Unit Cell Size (Angstroms) Fig. 3. Cracking/heptenes vs. zeolite unit cell size.

318

Heptenes/Heptane 1.3

, i_ 1 24.3 24.4 24.5 24.6

Unit Cell Size (Angstroms)

Fig. 4. Heptenes/heptane vs. zeolite unit cell size.

24.7

as zeolite UCS decreases cracking becomes more important and hydrogen transfer decreases [ 19,201.

Fig. 3 plots cracking/heptenes (which reflects cracking relative to H+ trans- fer) vs. UCS. This plot decreases with UCS, indicating that as UCS decreases, the rate of H+ transfer becomes steadily more important relative to cracking.

Finally, heptenes/heptane should increase as the H- transfer ability of the zeolite decreases with UCS [ 19,201. This indeed is observed (Fig. 4). So the ratio of heptane/heptenes is a unique measure of the partitioning of the C, ion between H- transfer to the ion and H+ transfer from the ion to the catalyst.

Methylcyclohexene (MCHE) reactions

As discussed above, MCHE is a key intermediate in understanding the cracking chemistry of MCHA and in forming aromatics from MCHA. MCHE cracking over Catalyst I at 500 C gives the initial molar selectivities shown in Table 2. Selected plots of mol-% yield vs. conversion are in Fig. 5. Comparison of the initial selectivities for MCHE and MCHA from the same catalyst (I ) at 500 C (Table 1) shows some striking differences. The major products from MCHE are MCHA and aromatics (toluene and xylenes ) . In contrast, MCHA conversion gives primarily cracking products (C&s, C4s) and ring opened prod- ucts (C,s). Clearly MCHE must have an alternate reaction pathway besides simple protonation of the double bond to give methylcyclohexyl carbocation.

The reason for the differences in product selectivities between MCHA and MCHE can be understood from Table 3 which gives the approximate initial rate constants for the appearance of cracking, ring opening, hydrogen transfer

319

TABLE 2

Initial molar selectivities for methylcyclohexene cracking over Catalyst I at 500C

Product I.S.

Methane 0.010 Ethane 0.003 Ethene 0.020 Propane 0.006 Propene 0.088 Butanes 0.040 Butenes 0.060 Pentanes 0.010 Pentenes 0.013 Hexanes 0.028 Hexenes 0.012 Methylcyclopentanes 0.003 Dimethylpentanes 0.001 Dimethylcyclopentanes 0.013 Cyclohexane 0.034 Heptane 0.050 Heptenes 0.087 Methylcyclohexane 0.290 Methylcyclohexadiene 0.020 Toluene 0.245 Xylenes 0.052 Hydrogen 0.004 Cg aromatics 0.002 Coke 0.029

and isomerization products from MCHA and MCHE. These rates were gen- erated from plots of time-on-stream vs. the sum of yields for products of each class [ 151.

Clearly the rate of cracking and ring opening for MCHE is less than the rate of hydrogen transfer. In addition, the rate of MCHE cracking is slower than that of MCHA. The relative cracking rates are surprising since alkenes gen- erally crack as much as two orders of magnitude faster than their saturated analogs [ 211.

However, the total rate of reaction of MCHE exceeds that for MCHA be- cause the transformation of MCHE to aromatics, methylcyclohexadiene and MCHA is very fast. Also noteworthy is the relatively high coke yield from MCHE and the lower hydrogen yield compared to MCHA.

The key to understanding MCHE cracking chemistry is the hydrogen trans- fer reaction. Rapid hydrogen transfer occurs for several reasons. First, MCHE is both an alkene and a naphthene, thus it can fulfill two roles. MCHE can act as both a hydrogen donor and acceptor. Protonation of MCHE leads to a high

320

Mole % Yield 0.5

(n)

0.4 I

% Methylcyclohexene Conversion

Mole % Yield

I2 (6)

10 -

8-

8-

0 10 20 30 40 50 60

% Methylcyclohexene Conversion

Fig. 5. Plots of mol-% yield vs. percent methylcyclohexene conversion over Catalyst I at 500C. (A) methylcyclohexadiene; (B ) toluene.

concentration of the hydride acceptor, methylcyclohexylcarbenium ion on the catalyst surface. Hydrogen transfer then occurs between this ion and MCHE which is present in high concentration since it is the feed. The net result is MCHA plus methylcyclohexenyl carbocation. Thus the bimolecular hydride transfer reaction is facilitated by a high concentration of both acceptor and donor.

321

TABLE 3

Initial rate constants (s-l) over Catalyst I at 500C

Methylcyclohexane Methylcyclohexene

Cracking 0.104 0.049 Isomerizationa 0.006 B

Hydrogen transfer 0.045 0.107 Ring opening 0.032 0.032

Formation of substituted cyclopentyl compounds. ?Not determined.

Another reason for rapid hydride transfer in this system is the formation of a relatively stable [ 221 allylic delocalized carbocation from MCHE.

Thirdly, the axial hydrogens adjacent to the double bond in MCHE are held by the cyclohexenyl ring in a conformation that makes them coplanar with a p orbital of the methylcyclohexene double bond. This stabilizes the developing positive charge in the C-H sp3-s bond as the hydride is being transferred. The net result is a relatively low activation energy for hydride transfer from an unsaturated naphthenic ring.

The resulting allylic carbenium ion loses a proton to give methylcyclohex- adiene which quickly again transfers a hydride and then eliminates a proton to give toluene. The high reactivity of methylcyclohexadiene can be seen from Fig. 5. Toluene formation is thermodynamically favorable and toluene is com- paratively very stable. Thus the overall process is fast and essentially irreversible.

A simplified reaction pathway for MCHE reaction is shown in Fig. 6. MCHA+ (I) is formed by protonation of MCHE. A hydride is then transferred to MCHA+ from MCHE to give the allylic carbenium ion II and MCHA. Ion II then loses a proton to the surface to give methylcyclohexadiene. Methylcy- clohexadiene gives up a hydride to another MCHA+ to form intermediate III, which quickly loses a proton to give toluene. Since /z3 >> k_,; k, >> k_,; and k6 >> k_6, the process is irreversible. Cracking, ring opened products and ring isomerization products arise from MCHA or I.

The role of allylic carbenium ions in strong acid zeolite catalysis has been invoked [ 181 but not well studied. However there is evidence for allylic car- benium ion formation from propene oligomers in mordenites and HNaY zeo- lites [ 231. In naphthene systems, allylic carbenium ions seem to play a more important role than in straight chain alkanes or alkenes.

It is interesting to compare the cracking chemistry of MCHE to its acyclic C, analogue, heptene. Heptene cracking chemistry has been reported for HY and ZSM-5 zeolites [ 24,251. These reports indicate that heptene cracking gives no aromatics or cyclic hydrocarbons as initial products. In contrast, aromatics

322

CRACKING 6 ISOMERIZATION

Fig. 6. Simplified reaction scheme for inital reaction of methylcyclohexene over zeolite catalysts.

and dehydrogenation products dominate MCHE reactivity. The lack of aro- matics from heptene suggests that the first and most difficult step in the aro- matization of acyclics is the cyclization of the carbenium ion [ 261. The absence of dialkenes as initial products from acyclic alkenes [ 261 suggests that allylic carbenium ions, while they may form, are of much less consequence in acyclic systems than in naphthenic systems.

Our data suggest hydride transfer cannot be the slow step that controls cracking at least in the MCHE system. Previous reports [27] have suggested that chain transfer via hydrogen transfer is the slow step in alkane cracking. In the MCHA/MCHE system, MCHE has a much higher hydrogen transfer rate than MCHA. Nevertheless, the cracking rate is lower for MCHE than for MCHA. This means that hydrogen transfer does not control cracking. As Fig. 6 illustrates, for MCHE feed, hydrogen transfer actually suppresses cracking by converting methylcyclohexyl carbocation to less reactive MCHA. MCHA cannot compete with MCHE for adsorption sites and thus is relatively stable at low conversions of MCHE.

CONCLUSIONS

Most of the cracking chemistry of methylcyclohexane can be explained by a combination of j&scission, and protolytic cracking plus isomerization, H-

323

transfer, H+ transfer and dehydrogenation reactions. MCHA can be used as a probe of acid catalyst behavior; specifically one can estimate the amount of ring opening, H+ transfer, H+ transfer vs. H- transfer and protolytic cracking occurring on the catalyst surface.

Mono-unsaturated naphthenes are key intermediates for aromatics forma- tion from saturated naphthenes. Once the mono-ene is formed, aromatics for- mation is relatively facile.

Methylcyclohexene is more highly reactive than methylcyclohexane but cracks more slowly than methylcyclohexane. This occurs because an alterna- tive reaction pathway, hydrogen transfer, competes effectively with cracking.

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2 3

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