Study of Ethylene Oligomerization on Bronsted and lewis Acidic Sites of Zeolites Using Diffuse Reflectance IR Spectroscopy

P.A. Jacobs et aJ. (Editors), Structure and Reactivity of Modified Zeolites © 1984 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

241

STUDY OF ETHYLENE OLIGOMERIZATION ON BRONS TED AND LEWIS ACIDIC SITES OF ZEOLITES USING DIFFUSE REFLECTANCE IR SPECTROSCOPY L.M. KUSTOV, V.YU. BOROVKOV and V.B. KAZANSKY N.D. Zelinsky Institute of Organic Chemistry, Academy of Sciences of the USSR, Moscow (USSR)

ABSTRACT Different types of Lewis acidic sites in zeolites (three-coordinated aluminium and silicon atoms of the lattice and extralattice aluminium) were detected by means of the diffuse reflectance IR-spectroscopy using low-temperature adsorption of molecular hydrogen. The lattice Lewis sites unlike the extralattice centres were found to initiate C?H oligomerization via a catio4 of branched products. The nic mechanism resulting in formarion reaction on the strongest Brdnsted acidic sites of the high silica containing zeolites leads to linear oligomers. Ethoxy groups were suggested to be intermediates in this reaction. INTRODUCTION Catalytic oligomerization of C2-C3 olefins is an important stage'of the fuel synthesis from non-oil raw materials (methanol, ethanol, light olefins etc.) (ref.i). It has been established that ethylene oligomerization proceeds on acidic OH-groups of ZSM type zeolites even at 300K (ref.2-4). The reaction mechanism is supposed to involve carbonium ion formation (ref.3). The less acidic HY zeolites are inactive in this reaction. On the other hand, Liengme and Hall (ref.5) and Kubelkov~ et al. (ref.6) have shown that dehydroxylation of HY zeolite at high temperatures resulted in the appearance of activity in ethylene oligomerization. On this basis they supposed that this reaction may be also catalyzed by Lewis acidic sites. However, the nature of these sites and the conditions of their formation in zeolites are still not elucidated enough. Therefore, the question of the participation of these centres in ethylene oligomerization on dehydroxylated samples remains unsolved. Recently we have developed a new method for the identification of aprotic acidic sites on the surface of adsorbents and catalysts based on IR spectroscopic study of low-temperature adsor-

242

ption of molecular hydrogen (ref.7). It allows us to distinguish experimentally different types of Lewis acidic sites arising during dehydroxylation. In the present work this technique together with spectroscopic study of ethylene adeorption was applied to elucidate the role of Lewis acidic sites in oligomerization of light olefins over highly dehydroxylated zeolites. In addition, we have also investigated C conversion over H-forms of zeolites 2H4 containing only Brdnsted acidic sites. EXPERIMENTAL HY zeolites (Si/Al=2.9) with a decationization degree of ~=99.5 and 75%, H-mordenite (Si/AI=5.0),~=96% and H-ZSM-5 (Si/Al=35), ~=99% were prepared from Na-forms by an ion exchange with a 0.2N NH 4Cl solution followed by a thermovacuum treatment at various temperatures. Dealuminated mordenite (Si/AI=12.5) was prepared by an acidic treatment of the Na-form with a iN HC1. IR-spectra of powdered samples in the 2000-4000cm- 1 range were measured at 300K using a Perkin-Elmer 580B spectrophotometer supplied with a home-made diffuse reflectance adapter (ref.8). The measurements of IR-spectra of adsorbed hydrogen (P=4kPa) were carried out at 77K using a Beckman Acta M-YII spectrophotometer according to the procedure described in (ref.7). Ethylene was purified from water traces by a prolonged storage over silica gel calcined at 970K in vacuum. The oligomerization reaction was studied at 300K spectroscopically directly in IR-cells under static conditions. RESULTS AND DISCUSSION Interaction of ethylene with Lewis acidic sites of zeolites. Fig.la represents the evolution of IR-spectra of HZSM-5 and HY zeolites, pretreated at 1270 and 970K, respectively, after C2H4 adsorption at 300K. With the increase of time the intensity of the bands at 2870, 2930 and 2970cm- 1 which are attributed to the vibrations of CH (2870 and 297Qcm-1 ) and CH (2930cm -i ), are 3 2 increasing. These groups definitely arise in the course of ethylene oligomerization leading to the formation of branched hydrocarbon chains. The absence of the bands in the region of 35503650cm- 1 (Fig.ia) indicates that the samples under study do not contain any acidic OH-groups. Dehydroxylation of zeolites is known to result in the formation of Lewis acidic sites. They can be detected IR- spectro-

243

2930 2870 1

1

.2970 40101 r4035 3740

.:». ~

3140 ~

40'!>5

2

2

a.)

b)

Fig.1. a) IR spectra of HZSM-5 (1) and HY (2) zeolites measured in 1, 8, 20, 100 and 200 min after adsorption of ethylene. The samples were pretreated at 1270 and 970K, respectively, in vacuum. b) IR spectra of H adsorbed at 77K on the pretreated 2 HZSM (1) and HY (2) zeolites (solid lines) and after ethylene adsorption at 300K (dotted lines). scopically by the adsorption of molecular hydrogen. In Fig.lb IR -spectra of hydrogen adsorbed at 77K on dehydroxylated HZSM and HY zeolites are shown. According to our previous data (ref.g,10)

the bands at 4010cm -1 and 4035cm -1 belong to H molecules adsor2 bed on the lattice three-coordinated aluminium atoms and silicon

ions, respectively. The lines in the regi0n of 4100-4160cm -1 correspond to the molecules interacting with residual Na+ catiOns or non-acidic SiOH-groups. The previous oligomerization of C on these zeolites at 2H4 300K does not affect the intensity of the bands of adsorbed hy-

. drogen at 4100-4160cm -1 • Consequent 1 y, t h e growlng oligomer chains do not block sodium cations and. SiOH groups inside the cavities and channels of these zeolites. Ethylene preadsorption leads, however, to a disappearance of the bands at 4010cm- 1 and 1 4035cm(Fig.1b, dotted lines), which are connected with the

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lattice Lewis acidic sites. These data show that active sites of ethylene oligomerization on dehydroxylated zeolites contain either three coordinated aluminium or silicon atoms in the lattice. Oligomerization of C branched oligomers also takes place 2H4 on the acidic Lewis sites of the lattice of decationized and dealuminated mordenites. The extent of the chain branching depends on the structure of a zeolite. The highest branching degree was observed for HY zeolites (ICH3/ICH2~3, where I are the intensities of the bands of CH 3 and CH 2 groups at 2970 and 2930cm -1 ,respectively). On the contrary, with HZSM and HM zeolites which hav& narrow channels the oligomers exhibit comparable amounts of CH 3 CH CH and CH 2 fragments (I 3/1 2=0.8-1.2). In addition to the lattice Lewis sites, dehydroxylation sometimes also results in the formation of extralattice aluminium atoms, which are formed because of the partial dealumination of the zeolitic framework. These sites are connected with the band at 4060cm -1 in the spectra of adsorbed hydrogen (see Fig.2). According to IR-spectra they do not, however, take part in ethylene oligomerization and form only weak complexes with ethylene molecu les which can be removed by evacua t ion at 3001< (see Fig. 2) • CH3 "

/CH

..... Si

/"

\

f)

,,/CH nC2~Si \

CH3

r

>;

I

CH

I

3

(CHI - CHf> n-l ~ •••

CH3

Fig.2. IR spectra of hydrogen adsorbed at 771< on H-forms of zeolites of Y (a) and mordenite (b) type pretreated at 970K under deep-bed conditions (1), after adsorption of ethylene (2) and after evacuation at 3001< for 1h (3). All these results about the formation of branched oligomers on the Lewis acidic sites of the lattice could be explained by the cationic mechanism given above. Interaction of ethylene with Brdnsted acidic sites. In Fig.3 IR-spectra of ethylene adsorbed at room temperature

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on HZSM-5 zeolite pretreated at 870K in vacuum are shown. The sample contains only Br~nsted acidic sites which are indicated by the IR band at 3610cm- 1. Ethylene adsorption results in hydrogen bonding between these hydroxyl groups and C2H4 molecules (the broad band at 3250cm- 1) as well as in the increase of the intensity of the lines at 2860 and 2930cm- 1 belonging to the vibration of CH groups in oligomerization products. This is an evi2 dence for the linear structure of the oligomer chains in agreement with the data presented in (ref.3). In the course of reaction the concentration of the hydrogen-bonded complexes of acidic OH-groups with C2H4 molecules decreases whereas the concentration of the complexes formed between these hydroxyl groups and saturated hydrocarbon chains of the growing oligomers increases, resulting in the appearance of the band at 3470cm- 1. A similar line was earlier observed in (ref.9) after the adsorption of C on HZSM-5 zeolite. During the reaction the release of some 6H14 part of acidic OH-groups also takes place. These transformations are connected with the existence of two isobestic points in the IR-spectra near 3400 and 3000cm- 1.

Fig.3. IR spectra of HZSM zeolite pretreated at 870K (1) and 1, 8, 15, 120 and 240 min after adsorption of 2.10 2 0 ethylene molecules per gramm of zeolite (2-6). Adsorption of ethylene on the deuterium form of ZSM-5 zeolite and of C 2D 4 on HZSM-5 shows that at 300K there is a fast proton exchange between hydroxyl groups and ethylene molecules occuring in hydrogen-bonded complexes. For example, in the case of

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C adsorption on HZSM, the appearance of the intensive bands at 2D4 2660cm -1 and 2400cm -1 attributed to f ree OD-groups and those perturbed by ethylene, respectively, was detected even one minute after ethylene adsorption. The rate of proton exchange considerably exceeds that of oligomerization. It can proceed via formation of either ethoxy structures (I) or carbonium ions (II), de-pending on the ionic character of the c-o bond:

~ 1\

CHdCH2 +

C2H 4;::

~

,0,

(j)CH

2-CH3

(II)

Q

,0\

As a result of such a fast exchange, C or C molecu2HD3 2H3D les are formed. which are transformed into oligomers containing CHD methylene fragments. They are displayed in the IR-spectra by the lines in the regions of C-H or C-D vibrations at 2920cm- 1 -1 and 2143cm ,respectively. Thus, there are two possible ways of C2H4 oligomerization with the participation of the strong Br~nsted acidic sites of zeolites: the carbonium ion mechanism (a) suggested in (ref.3) and t h e concerted mechanism (b) with the structures I or II as active intermediates, respectively:

(j)gH 2CH 3

,0,

~

(j)gH 2CH 2CH 2CH3 _

_

R

CH3o,. /CH~ GH 2 CH I

~O <, Al ~C\. 1\

.•• (a)

2

_

.. (b)

247 We believe that the mechanism (b) which should lead to the formation of the linear polymethylene chains is more probable. Besides that, the carbonium ions should isomerize to more energetically preferable secondary or tertiary ions resulting in the formation of the branched oligomers. This does not agree with the experimental data. As it follows from the experiment on ethylene oligomerization with the participation of Lewis acidic sites, the branching is not

restrict~d

by the size of the cavities and chan-

nels in all zeolites under study. In principal, the possibility of formation of the branched chains in channels of ZSM zeolites is also confirmed by the data on propene oligomerization (ref.3). The study of ethylene oligomerization over decationized and dealuminated mordenites which do not contain Lewis sites shows that this reaction proceeds only on the strongest acidic OH -groups which are characterized in the spectra by the band at 3610cm- 1• Thus, the data obtained in this work demonstrate that the active sites of ethylene oligomerization at 300K on different types of zeolites could involve both strong

Br~nsted

acidic sites

and lattice Lewis acidic centres. The structure of hydrocarbon chains which are formed in this reaction depends on the nature of these active sites

(Br~nsted

or Lewis). This could be used for

the identification of the nature of active centres. REFERENCES 1 J.H.C. van Hooff, in P.Prins and G.C.A. Schuit (Eds.), Chemistry and Chemical Engineering of Catalytic Processes, NATO Advanced Study Institute Series, Series E, Applied Sciences, 1980, No.39, p , 599. 2 V. Bolis, J.C. Vedrine, J.P. van den Berg, J.P. Wolthuizen and E.G. Derouane, J. Chem. Soc., Faraday Trans. I, 76(1980),1606. 3 J. Nov~kov~, L. Kubelkov~, Z. Dolej§ek and P. JirO, ColI. Czechosl. Chern. Commun., 44 (1979), 3341. 4 J.P. Wolthuizen, J.P. van den Berg and J.H.C. van Hooff, in B. Imelik et a L, (Eds.), Catalysis by Zeolites, Elsevier, Amsterdam, 1980, p.85. 5 B.V. Liengme and W.K. Hall, Trans. Faraday Soc., 62(1966),3229. 6 L. Kubelkov~, J. Nov~kov~, B. Wichterlov~ and P. JirO, ColI. Czechosl. Chem. Commun., 45 (1980), 2280. 7 L.M. Kustov, A.A. Alexeev, V. Yu.Borovkov and V.B. Kazansky, Doklady Akad. Nauk SSSR, 261 (1981), 1374. 8 L.M. Kustov, V. Yu. Borovkov and V.B. Kazansky, J. Catal., 72 (1981), 149. 9 V.B. Kazansky, L.M. Kustov and V.Yu. Borovkov, Zeolites, 3 (1983),77. 10 V.B. i