AICI3

B. Imelik et al. (Editors), Catalysis by Acids and Bases 1985 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

213

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THE MECHANISM OF n-PENTANE TRANSFORMATION OVER SOLID SUPERACIDS - A1203/A1C13 r>1. MARCZEWSKI

Chemistry Dept., Warsaw Technical University, 00 662 Warsaw/ Poland!

ABSTRACT Superacid properties of A1203/A1C13 catalyst were studied. It was found that pentane low temperature isomerization occurs in presence of acceptor sites with activation energy of 10 Kcal/mol. In presence of this superacid catalyst pentane also decomposes to form isobutane. The mechanism of isobutane formation catalysed by surface attached carbocations has been discussed. RESUME Les proprietes superacides du catalyseur A1203/A1C13 ont ete etudiees. On montre que l'isomerisation du n-pentane a basse temperature est catalysee par les sites accepteurs avec une energie d'activation de IOKcal/mole. En presence de ce catalyseur superacide, le pentane se decompose aussi en donnant de 1'isobutene. On discute lemecanisme de formation de 1'isobutane par 1'intermediaire de carbocations lies a la surface. INTRODUCTION Aluminum oxide treated with A1C1 3 vapours is one of the most active acid catalysts and can be considered as solid superacid (ref. 1). The introduction of A1C1 3 onto alumina surfaces causes the formation of new stron~acceptor ~ites able to oxidize perylene into corresponding cation-radical without oxygen preadsorption (ref. 1). These centres are formed in the following way: +

9

~?A1C13

Al-o-Al-o -----.-Al-o-Al-o

A1C13 reacts with electron donating exposed oxygen ions 02- causing the electron shift towards A1C13 adsorbed molecule. As a result, surface aluminum cations with pronounced deficit of electrons are formed. In our previous paper we have proposed to relate catalytic activity of this catalyst with these sites (ref.l). Solid superacids are able to catalyse n-alkane reactions at low temperatures, even at 298K (ref.2). Products of these reactions are skeleton isomers and lower hydrocarbons. Pentane for example reacts to form isopentane and isobutane. The mechanisme of isobutane formation is still controversial. Tanabe et al. (ref 2) showed that isobutane is a secondary product of isopentane decomposition while Gates et al. (ref. 3) claim that it is formed from CIo intermediate.

214

The aim of this work was to verify the hypothesis that acceptor centres are responsible for superacidproperties of A1203/AIC13 system and to study the mechanism of pentane transformation initiated by this catalyst. METHODS Alumina. silica and silica-alumina (87 and 30 %of AI20~) were obtained by calcination at 823K aluminum and silicon hydroxides or their coprecipitated mixtures. The hydroxides were prepared by hydrolysis of aluminum isopropoxide or ethoxysilicon. Superacid catalysts were obtained by AlCl 3 sublimation (T = 573K. P = 1.3Nm-~) through the freshly calcined (T = 773K. P = 1.3 x 10-2Nm-2) support. IR investigation of NH 3 and pyridine adsorption were performed in a special IR cell (ref.l) using Specord IR 75 spectrophotometer. Oneelectron acceptor (o.e.a.) and one-electron donor (o.e.d.) properties were evaluated by perylene and tetracyanoethylene (TCNE) adsorption. The quantity of perylene and TCNE ion-radicals formed was measured using Jeol 3X ESR spectrometer. The number of surface hydroxyls of oxide carriers was estimated by sodium naphtalenide titration (ref.4). Catalytic activity measurements were carried out using a 150 cc batch reactor containing Ig of catalyst. RESULTS In order to evaluate the catalytic activity of acceptor sites. the catalysts which had been prepared from carriers possessing different quantity of o.e.d. centres were chosen. Presence of these sites is essential in acceptor centres formation. In Table 1 the properties of both carriers and superacid catalysts are presented. Obtained results indicate that for all oxides studied the mechanism of interaction between the surface and A1C1 3 vapours is similar. One can observe the disappearance of both surface hydroxyls and Bronsted acidity as well as substantial reduction of o.e.d. centres with simultaneous increase of o.e.a. sites. The properties of obtained superacids depend on the carrier composition. The number of acceptor sites increases with Si0 2 content in catalysts under study while Lewis acidity disappears for silica rich samples (30% AI203-Si02' Si0 2). One can expect that these two properties should change similarly because they are both connected with the presence of electron deficient aluminum cations (ref.5). The observed phenomenon may be easily explained if one assumes that NH3 can be coordinatively bonded only by Lewis sites from A1203 sublattice. X-Ray analysis confirmed that i(- A1203 phase was present only for two carriers studied i.e. Al 203 and 87% A1 203-Si0 2. Further confirmation of above assumption gives IR spectra of adsorbed Ml3 With the rise of Si0 2 content in the catalysts studied one can observe the rise of 1550 cm- 1 band intensity.

215

TABLE 1 Physico-chemical properties of catalysts studied Catalyst

Acidity B

L aua

A1 Z03 + A1Z 03 87-A1Z0 e3 87-A1Z03 + 30-Alz03 30-A1 Z03 + SiO Z ,SiOZ + I

A1C13 A1C13 A1C13 A1C1 3

0.18 0.03 0.18 O.OZ 0.05

0.10 0.Z9 0.05 0.17

One-electron prop. Acceptor Donor Total AlzO~ 10-15 spin/g

100 1Z6 190

100 100

4Z5 50 104 5Z tr.

OH

mmole/g

r d0 . -1 mln

0.6b 1.7 0.6 1.5 0.4 0.6

50 0.4

ZOO

0.3

a

Integrated intensity of adsorbed NH3 IR bands / 14Z0 cm- 1_ Bronsted acidity (B), 16Z0 cm- 1 - Lewis acidity (L)/,after desorption at 373K. bOne-electron acceptor sites of alumina phase. c Absence of IR absorption bands at 3700-3600 em-I. d Initial reaction rate of pentane isomerization (T = 473K, P = Ix 106Nm- Z). e 87-A1 203 means silica-alumina composed of 87% Al z03 and 13% SiO Z' This band was ascribed to the NH Z surface groups (ref.6). It seems that ammonia does not adsorb on surface sites: Si-0-A1Cl but reacts to form Si-0-A1NH Z species. Since Lewis acid sites can react in different ways with NH3 depending on their location (Al z03 or Si02 phase) the same phenomenon should be observed with o.e.a. centres. The number of o.e.a. sites of A1Z03 phase can be roughly estimated using the following formula: A1 Z0 3 o.e.a. sites = total o.e.a.-x.o.e.a. sites of SiOz/A1C13{x=:S7or.3) and is shown in Table 1. The comparison of the quantity of these centres with the number of o.e.d. sites of untreated supports confirms our hypothesis that at least one kind of acceptor centres is formed in the reaction between A1C13 molecules and o.e.d. sites of surface. Superacid properties of catalysts studied measured by initial reaction rate of n-pentane isomerization change in the same way as the concentration of o.e.a. centres of A1 Z0 3 phase.

216

The close relationship suggests that these sites can be responsible for catalytic activity. To check this hypothesis experiment with catalyst on which part of O.e.a. centres had been blocked with perylene was performed. Initial reaction rate of pentane isomerization diminished ca three times. On the basis of above findings one can beli~ve that o.e,a. centres of A1203 phase are responsible for pentane isomerization. Since Si02/A1C13 system possesses a small catalytic activity one cannot exclude that the sites: -0-A1C12 or (-0-)2A1Cl resulting from A1C13 and OH reaction have certain superacid properties. To study the mechanism of pentane interaction with the surface the nature of A1203/A1C13 catalyst under working conditions was examined~ The working conditions were simulated in IR experiments by adsorption of CSH12 at the reaction temperature. On such pretreated catalyst pyridine was adsorbed. The results are summarized in Fig. 1.

a

b

o~

c

z;

a

Vl Vl

L

Vl

:z -e ex

t-

2800

~--::;-:-:=----'---=t=-H--~=-=--"-----::~-=----:::;---

'3000

j

!

1600

Fig.l. absorption spectra of A1203/A1C13 (a), (a) + 2.7 x 10 4Nm- 2 C5H12 at 333K (b), (b) + 2.7 x 103Nm-2 Py after 1 hr evacuation at 333K (c). Pentane adsorption results in appearance of new absorption bands at 2970, 2930 and_2877 cm- 1 typi ca1 for stretchi ng vi brati ons of CH 3 and CH2 groups (ref. 7). Pyridine adsorbs on the surface forming both PyH+ (bands at 1533 em-I) and Py coordinatively bonded with Lewis centres (bands at 1460-1440 em-I). To study the mechanism of pentane isomerization the experiments with different initial substrate pressure has been performed. The results are presented on Fig. 2. The initial reaction rate of pentane isomerization does not depend on substrate concentration.

217 "'i

c E

...

"b

0

~ Z

lL

~6 -e

a: -J4

•

•

40

o

6

:Q20 L&.I >Z

s '=2 z

Q

o

u

200

PEN TA NE

400

&00

PRESSURE

Tr

~ig. 2. Dependence of initial reaction rate of pentane isomerization (T=333K) on substrate initial pressure (lTr = 133.3 Nm- )

800

20 40 60 TOTAL CONVERSION %

Fig. 3. Dependence of pentane to isopentane (a) and isobutane (b) converstions on total conversion (T = 333K, P = 2.7 x l04Nm- 2)

DISCUSSION Pentane reacts in presence of all catalysts studied. The following products are formed: isopentane, isobutane and small amounts of isohexanes-less than 1% (Fig.3). One can see that isopentane and isobutane are formed in parallel reactions. The isomerization reaction stops qUickly while the decomposition proceeds undisturbed. Hence, these two reactions may be considered as independent and catalysed by different active sites. Isopentane formaHon The linear correlation between initial rate of n-pentane isomerization and the number of o.e.a. centres of A1203 phase as well as the selective poisoning experiment indicate that superacid active sites possess a strong acceptor nature. The mechanism of pentane activation by these sites can be explained by an analogy with the action of liquid superacids. In superacid solution proton attacks C-H bond forming an unstable carbonium cation (I) (ref. 8), which decomposes with H2 evolution and carbocation (II) formation. One may suppose that in the case of solid superacids protons will be replaced by strong acceptor centres. Carbonium cation (III) resulting from an attack of acceptor site C-H bond in pentane decomposes to form adsorbed H- and CSH1I+ cation-isopentane precursor. The different decomposition of cation (III) is also possible. In such a case hydrocarbon chain (IV) remains on the surface exclusive of H-. (I II)

(I)

superacid solution

(II)+CH 2C4Hg+H-L (IV)L-CH2C4Hg+H+ solid super-acid

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The IR examination of pentane adsorption on A1 203/A1C1 3 catalyst confirmed the existence of such surface species (Fig. 2). It seems plausible that these species could be responsible for activity decay. Pentyl cation (II) formed in the adsorption step of the reaction can isomerize and desorb from the surface as isopentane. To confirm such a reaction pathway the Langmuir-Hinshelwool treatment has been applied. Since the initial reaction rate is independent on pentane pressure (Fig. 2) one can assume that either surface reaction-cation (II) isomerization, or isopentane desorption is the rate determining step. For these two steps the independence of initial reaction rate on substrate concentration is possible if the product of pentane initial concentration and pentane adsorption equilibrium constant is greater than 1 (ref. 9). Since on the basis of data presented in the work (ref. 10) one can assume that adsorption equilibruim constants of both pentane (KI)and isopentane (KIll) are very close and pentane to isopentane conversion (x) is for all catalysts studied less than 13% the following inequality: KI(I-x))K I I I X seems to be true. Taking this simplification two different rate expressions can be reduced to one: r = AS T (BC o)-1 where, A, B : constants, ST : number of active sites, Co : initial pentane concentration

Since ST has not a constant value but diminishes during the reaction causing activity decay, the reaction rate can be expressed in terms of Time On Stream Theory of catalyst deactivation in the following way (ref. 11) : r = Al (1 + A2 t)-N where, AI' A2' N :constant, t : reaction time. To solve this equation the least square method was applied. The experimental values of reaction rates were obtained by numerical differentiation of "spline" functions (ref. 12) which had been used to approximate the changes of pentane conversion vs. reaction time. The knowledge of Al allowed to calculate the rate constant and then the activation energy of isopentane formation. When desorPtton was rate limiting step Ea= 8kcal/mol, while for the other cases Ea = 10kcal/mol. Isobutane formation Isobutane, the main product of pentane transformation, is formed in presence of different active sites than those which catalyse pentane isomerization. Since isobutane formation needs certain induction period (fig. 3) one can suppose that the active centres in this reaction are formed on the catalyst surface upon substrate action. Pyridine adsorption on the A1203/A1C13 catalyst with

219

preadsorbed pentane proved the existence of acid centres of Lewis and Bronsted types (Fig.1). The time of pentane adsorption was as long as the time of reaction needed to total deactivation of isomerizing sites, so observed acid centres are different from those typical of fresh catalyst. The acid properties of pentane treated A1Z03/A1C13 catalyst may be connected with the presence of surface hydrocarbon-like species (IV). The unique explanation of acid properties of adsorbed hydrocarbon chains is an assumption that they are present in an ionized from i.e. as surface cations. Such cations may from as a result of interaction of adsorbed hydrocarbon (IV) with adjacent acceptor sites or with other cations present in reacting system: i-pentyl cations. To check the possibility of isobutane formation upon the action of surface bonded cations the reaction of pentane with ethyl chloride was performed. It is believed that in presence of superacids alkyl chlorides form corresponding alkyl cations (ref.3) CZHSCl influences isobutane formation rising its yield by four times. One can explain such a phenomenon as a result of following reactions L - R - CH Z,

',+

H3 C"

/>--CH ZC 3H7

+

CHZC 3H7 + L - R - CZHS

I:J

TABLE Z n-Pentane conversions into different products for the reactions of : pentane (I), (I) + CZHSC1/30% mol./ (II). Reaction temperature 473K.

Total conv.

Reaction

I

II

O.Z O.S

0.7 3.S

0.3 1.8

6.9

10.4

ZO.5

11.9

1.4 1.4

18.8 41.S

Surface cations (V) or CZH S+ attack the C-C bond of pentane molecule. The unstable ion (VI), product of this reaction, easily decomposes with butyl action and longer hydrocarbon-like species formation. Similar reaction were postulated by Olah et a1. (ref.l3) for alkyl chlorides with alkanes. Completing the reactions the isobutyl cation abstracts H- from surface bonded hydrocarbon and desorbs as isobutane to restore the active site.

220

REFERENCES 1. A. Krzywicki and M. Marczewski, J.C.S. Faraday I, 76 (1980) 1311-1322 2. O. Takahashi, T. Yamaguchi, T. Sakuhara, H. Hattori and K. Tanabe, Bull. Chern. Soc. Jpn. 53 (1980) 1807-1812. 3. G.A. Fuentes and B.C. Gates, J. Catal. 76 (1982) 440-449. 4. J. Kijenski and R. Hombek, J. Catal., 50 (1977) 186-189. 5. B.D. Flockhart, I.R. Leith and R.C. Pink, Trans. Faraday Soc., 62 (1966) 730-740. 6. J.B. Peri, J. Phys. Chern. 69 (1965) 231-239. 7. J. Datka, Zeolites, 1 (1981) 113-116. 8. G.A. Olah, G. Klopman and R.H. Schlosberg, J. Am. Chern. Soc., 91 (1969) 3261-3268. 9. Z.G. Szabo, D. Kallo (Eds.) Contact catalysis, Akademiai Kiado, Budapest 1976, pp. 480-537. 10. E. Baumgarten, F. Weinstrauch and H. Hoffkes, J. Chrom., 138 (1977) 347-354. 11. B.W. Wojciechowski, Cat. Rev. - Sci. Eng., 9 (1974) 79-113. 12. G. Dahlquist, A. Bjorck, Numerical Methods, PWN Warszawa 1983, pp. 128130. 13. G.A. Olah and J. Kaspi, J. Org. Chern., 42 (1977) 3046-3050.