Structure and activity of fluorinated alumina. 2. Nature of the active site for 2-methylpropene oligomerization

Structure and Activity of Fluorinated Alumina 2. Nature of the Active Site for 2-Methylpropene Oligomerization F R A N S P. J. M. K E R K H O F , 1 J A N C. O U D E J A N S , J A C O B A . M O U L I J N , 2 AND E M I L R. A. M A T U L E W I C Z Institute for Chemical Technology, University of Amsterdam, Plantage Muidergracht 30, 1018 TV Amsterdam, The Netherlands Received August 15, 1979; accepted November 27, 1979 The oligomerization of 2-methylpropene catalyzed by fluorine-containing aluminas is studied in pulse, flow, and static reactors. The number of active sites of two series of fluorinated aluminas was measured by poisoning using 2,6-dimethylpyridine and n-butylamine. This number is compared with (i) the number of hydroxyl groups, measured by quantitative reaction of hexamethyldisilazane and the surface hydroxyl groups, and (ii) the number of Br0nsted sites measured by infrared spectroscopy. It is concluded that the active sites are BrCnsted sites which are formed by part of the hydroxyl groups. The influence of preparation methods on the structure of the catalysts is given. It is shown that catalysts which were prepared by impregnating alumina with an aqueous solution of aluminum nitrate and ammonium fluoride have a relatively higher surface area than catalysts prepared by other methods. A surface model for fluorinated alumina is presented. The reaction products are analyzed and identified. Mainly dimerization to trimethylpentenes occurs. It is shown that the products which are formed can be explained by a reaction mechanism based on the formation of a classical carbonium ion. The role of irreversibly adsorbed surface polymers as active centers is discussed. INTRODUCTION I n a p r e v i o u s p a p e r (1) w e s h o w e d t h a t 2,6-dimethylpyridine (DMPy) can be used as a specific p o i s o n f o r t h e B r 0 n s t e d sites of fluorinated alumina. The number of these sites w a s c o u n t e d b y m e a s u r i n g t h e i n t e n s i t y of the infrared bands of adsorbed DMPy. I n t h e p r e s e n t s t u d y t h e r o l e o f t h e s e sites as a c t i v e c e n t e r s in t h e p o l y m e r i z a t i o n o f a l k e n e s o v e r F/AI203 c a t a l y s t s is d i s c u s s e d . T h e r o l e o f B r 0 n s t e d sites as a c t i v e c e n ters on fluorinated alumina has been establ i s h e d b y o t h e r s , e . g . , b y A n t i p i n a et al. (2) for the cracking of cumene and by Scokart et al. (3) for d o u b l e b o n d m i g r a t i o n a n d skeletal isomerization. The fact that the a c t i v e sites in t h e p o l y m e r i z a t i o n o f l o w e r 1 Current address: Unilever Research, Vlaardingen, The Netherlands. 2 To whom all correspondence should be addressed. 0021-9797/80/090120-11 $02.00/0 Copyright © 1980 by Academic Press, Inc. All rights of reproduction in any form reserved.

a l k e n e s o v e r a c i d c a t a l y s t s a r e p r o t o n i c is w i d e l y a c c e p t e d (see, e . g . , O b l a d et al. (4)) a n d w a s d e m o n s t r a t e d b y S a t o et al. (5) a n d H o l m et al. (6) f o r t h e p o l y m e r i z a t i o n catalyzed by silica-alumina. However, r e c e n t l y T a k e et al. (7) c o n c l u d e d t h a t s t r o n g L e w i s sites w e r e a c t i v e in t h e oligom e r i z a t i o n o f l o w e r a l k e n e s . A s D M P y is a specific p o i s o n t o w a r d B r 0 n s t e d sites it s h o u l d b e u s e f u l to d i s c r i m i n a t e b e t w e e n these two different opinions. We therefore measured the catalytic activity of a series of f l u o r i n a t e d a l u m i n a s a n d p e r f o r m e d in situ p o i s o n i n g e x p e r i m e n t s w i t h D M P y . T h e reaction mechanism of the polymerization was further studied by investigating the product d i s t r i b u t i o n . I n p r i n c i p l e f r o m this d i s t r i b u t i o n it c a n b e c o n c l u d e d w h e t h e r c a r b o n i u m i o n s a r e r e a c t i o n i n t e r m e d i a t e s . It h a s b e e n c o n c l u d e d b y C l a r k a n d F i n c h (8) t h a t o n silica-alumina the formation of a carbonium 120 Journal of Colloid and Interface Science, Vol, 77. No. 1, September 1980

2-METHYLPROPENE OLIGOMERIZATIONOVER F/A1203CATALYSTS ion does not take place via a surface OH group but via an irreversibly adsorbed species. A similar conclusion was suggested for F/AlzO3 catalysts (9). The present study aims to correlate the catalytic activity of fluorinated alumina with BrCnsted or Lewis acidity and to prove a possible role of a proton donating irreversibly adsorbed species. MATERIALS AND METHODS The series of catalysts is the same as used in previous studies (10, 11). It was prepared by impregnating alumina with aqueous solutions of aluminum nitrate and ammonium fluoride followed by drying and subsequent calcination. The static reactor used was similar to the one described by Johnson (12). It consisted of an all glass reactor (250 ml) in a temperature-controlled furnace. The pressure drop that resulted from the polymerization was measured by a mercury manometer and a Texas Instrument Precision Pressure Gage (model 145-0). The reactor with external circulation was similar to the one used by Misono and Yoneda (13). The circulating pump had a capacity of 650 cma/min which was nearly independent of the pressure in the system. The reactor volume was 2.8 liters. Usually 0.25 g catalyst diluted with 2.25 g glass beads of the same size (0.2 mm) was used. Pulse experiments were performed with 0.3 g catalyst in a Pyrex reactor with a diameter of 4 mm. The standard activation of the catalysts consisted of a vacuum treatment at 550°K for 2 hr. After this treatment the temperature was decreased to 360°K and pulses of butene were injected in the helium carrier gas. Six-micromole pulses were used and the helium flow was 60 cm3/min (NTP). The pressure in the pulse reactor was slightly below atmospheric. Flow experiments have been performed in the same reactor. In that case ca. 20 mg catalyst diluted with 180 mg glass beads was used. A mixture of helium (250 cm3/min) and isobutene (5-15 cm3/min) was passed over

121

the catalyst bed. In the flow experiments the total pressure was 0.16 MPa. The products were separated on a short silica column (80 mm, 400°K) and detected by a thermal conductivity cell. The products of the reaction in the system with external circulation were frozen out and investigated as follows. First a mass spectral analysis of the product mixture was made. Then the products were separated in a dimer and trimer fraction on a Hupe preparative gas chromatograph (APG 402) using a 20% Apiezon-L/chromosorb column at 393°K. The dimer fraction was further analyzed by chromatography and nuclear magnetic resonance. The dimers were separated on a 3-m UCC-wax column operated at 440°K, mounted in a Hewlett-Packard chromatograph (5750G). A Varian T-60 A (60 MHz) NMR spectrometer was used in recording the spectra of the dimer fraction solved in CDC13. RESULTS AND DISCUSSION Preparation o f the Catalysts According to Choudhary (14) fluorinated catalysts can be prepared by vapor phase and impregnation methods. The first method applies vapors of fluorine-containing compounds, e.g., CF4, CF3OH, BF3, SF6 and HF, which are contacted with alumina at temperatures from 400 to 800°K. In the second method alumina is impregnated with aqueous solutions of, e.g., NH4F and HF. Antipina et al. (15) used 5 M H F and prepared catalytic aluminum hydroxyfluorides in this way. H F impregnation was also used by Gerberich et al. (16). Impregnation of alumina with NH4F leads to aluminas with fluorine incorporated in the surface and in A1F~ crystallites. Examples of this last procedure are the catalysts used by Dreiling (17), Scokart et al. (3), Holm et al. (6), and catalysts prepared according to the V K F process (18, 19). We studied the XRD patterns of aluminas impregnated with aqueous NH4 F and obJournal of Colloid and Interface Science, Vol. 77, No. 1, September 1980

122

KERKHOF

lOO

% Surface

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area

ET AL.

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15

20

FIG. 1. R e l a t i v e s u r f a c e a r e a s o f s e v e r a l c a t a l y s t s as a f u n c t i o n o f t h e f l u o r i n e c o n t e n t .

served the presence of NH4A1F4 and (NH4)aA1F6 in the dried samples. After calcination A1F3 was present in samples with a higher fluorine content. Obviously, already during impregnation part of the alumina is consumed to form ammonium aluminum fluorides which decompose during calcination. This consumption could explain the relatively large decrease in surface area of the catalysts prepared in such a way. This decrease is shown in Fig. 1. For the series of catalysts used in this and the previous study the method of preparation is different and the decrease is relatively small. The decrease in this case can be described by a simple dilution concept viz. S = So(1 - x), where S is the surface area of the catalyst withx wt% fluorine and So is the surface area of the support. In addition to X-ray diffraction, X-ray photoelectron spectroscopy (XPS) was used to study the catalysts. Whereas XRD gives information on crystalline material, XPS can be used to identify and quantify the amount of surface fluorine (10, 19). The resuits of the structural investigations are presented in Table I. The table shows that at low fluorine content only surface fluorine is present whereas at higher contents Journal of Colloid and Interface Science, Vol. 77, No. 1, September 1980

aluminum hydroxyfluorides are formed. If the synthesis is performed without carrier the coprecipitation of aluminum nitrate and ammonium fluoride in the molar ratio 1:3 yields A1Fa crystallites. Therefore, it is remarkable that the crystallites which are formed at higher fluorine content are TABLE

I

Surface Area and Composition of the Catalysts a

wt%F

Surface area (m 2 g-~)

Crystaflites (detected by XRD)

0 0.8 1.3 2.3 4.7 6.5 10.6 12.0 13.7 23.7 47.0

307 310 ° 310 b 350 291 313 309 310 ° 271 167 10

A A A A A/B

Surface fluorine (detected by XPS)

+ + + + + + + C C C C

" A , A1F1.96(OH)l.04; B , / 3 - A I F 3 ; C , s t r o n g s i g n a l o f F(ls) electron of the hydroxyfluoride interferes with the F(ls) signal of surface fluorine. b Estimated value.

2-METHYLPROPENE OLIGOMERIZATIONOVER F/AI20~CATALYSTS %Weight increasele}

OH

123

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- - ( o )

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i

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20

%F FIG. 2. H y d r o x y l g r o u p d e n s i t y as a f u n c t i o n o f the fluorine c o n t e n t .

hydroxyl group containing aluminum fluorides. The fact that our catalysts contain hydroxyfluorides and nearly no aluminum fluoride is tentatively explained as follows. By exchange of surface hydroxyl groups and oxygen atoms fluorine is incorporated in the surface of the alumina. The resulting solution is deficient in fluorine and upon crystallization fluorine-deficient species are formed. This theory is confirmed by experiments in which aluminum nitrate and ammonium fluoride were coprecipitated in the ratio 1:1 and A1F1.96(OH)~.04 was detected after drying and calcination. At low fluorine content the majority of the fluorine atoms is incorporated in the alumina surface and a substitution of surface oxygen and hydroxyl groups is to be expected. We therefore measured the number of hydroxyl groups by monitoring the weight increase of a catalyst after reaction with hexamethyldisilazane (HMDS). Using infrared spectroscopy Van Roosmalen and Mol (20) proved that the reaction of hydroxyl groups and HMDS is quantitative. The weight increase and hydroxyl group content are presented in Fig. 2. The value of 1.40H/nm 2 for y-A1203 is in good agreement with results reported by Peri (21). The figure shows that after incorporation of fluorine the OH number decreases. However, even at higher fluorine content part of the hy-

droxyl groups are still present. This is in agreement with the observation of Finch and Clark (9) that a 6.9 wt% F catalyst could have more than one OH/nm 2 (9). The presence of the OH groups after fluorination is also shown by the infrared spectra in the OH region (see, e.g., Scokart et al. (3)). These remaining OH groups are probably the strong protonic centers previously observed by infrared spectroscopy (1). Poisoning Experiments The upper limit of the number of active sites of a catalyst can be determined by poisoning. The poison can be preadsorbed or fed with the reactants. It was suggested by Kn6zinger (22) that if the poison is fed with the reactants the adsorption might take place at sites which are not involved in the catalytic process and, therefore, a preadsorption technique should be preferred. However, preadsorption also has disadvantages. If it is assumed that on the catalyst surface very active sites are present which are also strong adsorption sites, then these sites might be inactive at steady-state conditions. A preadsorption technique would count these sites and would overestimate the number of sites under reaction conditions. We preferred a pulse technique which enables in situ poisoning. In these pulse Journal of Colloid and Interface Science, Vol. 77, No. 1, September 1980

124

K E R K H O F ET AL. 100 Peak a r e a %

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experiments isobutene pulses were injected conversion and due to an excess of active in a helium flow. After constant conversion sites the first pulses of the poison do not of subsequent pulses was obtained DMPy result in loss of activity. The intercept of the was injected. As the reactant, a weak base, curves with the x axis is equivalent to the will preferentially react with acidic sites amount of DMPy needed to poison the cataDMPy, which is a stronger base, is a suit- lyst completely. The figure clearly shows able poison. the inhomogeneity in site activity. For exA typical example of the butene response ample, the 2.3 and 12 wt% F catalysts have before injection of the poison is given in about the same site density but the activity Fig. 3. After six pulses a constant conver- of the 12% catalyst is much higher. sion is obtained. The first response peaks As we performed the poisoning experiare small. This might be due to either a ments in a Pyrex reactor visual inspection higher conversion of these pulses or to of the poisoning process was possible. At chemisorption of a part of the isobutene. constant conversion the catalysts did not The first moments of the response peaks can have the white color of the fresh samples be used to calculate the adsorption constant but were slightly colored. After the first inof isobutene on the catalyst (23). By tem- jection of DMPy the white color was reperature variation heat and entropy of ad- stored in a sharp zone in the upper part of sorption can be determined. For all catalysts the catalyst bed. The length of this zone inand for pure alumina a value of 28 M/mole creased with the number of injections and for the heat of adsorption and 60 J/mole K when the total catalyst bed was white, zero conversion was obtained. These observafor the entropy of adsorption was found. These values indicate a physical adsorp- tions suggest that color and activity are cortion process and, obviously, the response related. If the color of the catalysts is due peak is due to desorption of the physical to the presence of a surface polymer, obviadsorbed isobutene whereas the molecules ously, poisoning results in depolymerizachemisorbed do not contribute to the re- tion. However, further experiments in a difsponse signal. It is therefore concluded that ferent type of reactor will show that chemion the catalyst surface two butene species sorbed species and activity do not correlate are present, viz. physically and chemically and therefore the depolymerization is probadsorbed butene. The decrease in butene ably a side reaction. It might be caused by conversion after DMPy injection is shown in sterical hindrance of the surface polymer Fig. 4. Under the conditions of these experi- by adsorbed DMPy. From data obtained in ments the most active catalysts yield 100% experiments as shown in Fig. 4 the number Journal of Colloid and Interface Science, Vol. 77, No. 1, September 1980

2-METHYLPROPENE OLIGOMERIZATION OVER F/AI20~ CATALYSTS

100 i "L

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FIG. 4. Influence of DMPy injections on the steady-state conversion in the pulse experiments. of active sites can be calculated. From infrared spectroscopy we (1) obtained the number of Br0nsted sites and a comparison of the poisoning experiments and the infrared experiments can be made. If all Br0nsted sites are active centers a plot of the number of Br0nsted sites vs the number of active sites would yield a straight line with a slope of 45 °. Figure 5 shows that indeed this correlation reasonably fits. / 06

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Thus, it can be concluded that Br0nsted sites are the active sites for olefin polymerization. In this connection also interesting are the results on a second series of fluorinated aluminas which were prepared by impregnation of Gibbsite with an aqueous solution of ammonia and ammonium fluoride (18, 24). With these catalysts the conversion of pentene-1 into pentene-2 was studied in a continuous flow reactor at 420°K and a W H S V of 3.4. In that study n-butylamine was used to poison the catalysts. Because of its size n-butylamine is a suitable poison. However, butylamine is not specific because it reacts with Br0nsted sites as well as with Lewis sites. But, if analogous to DMPy the reaction with Br0nsted sites would be irreversible then it still would be useful as a specific poison. The results of the poisoning studies with the second series are shown in Fig. 6. A good correlation between the site density measured by butylamine poisoning and activity in the isomerization is obtained. The site density for this series is somewhat higher than for the first series. This might be explained in several ways: (i) The second series, due to a different preparation method, has more protonic sites. (ii) Butylamine may react with Br0nsted and Lewis sites and therefore the site density is the sum of both Journal of Colloid and Interface Science, Vol. 77, No. I, September 1980

126

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sites. Antipina et al. (2) used Hr indicators and found a protonic site density increasing from zero for alumina up to 1.8 sites/nm2 for alumina with 6 wt% F. This fact indicates that the protonic site density of fluorinated alumina lies in the range of 0.5-1.8 sites/nmz and depends on the pretreatment of the sampies. The value measured by butylamine lies in this range and we therefore suggest that at the actual reaction conditions butylamine is a specific poison for BrCnsted sites. However, it must be mentioned that the poisoning with butylamine was performed at steady-state conditions. If polymeric species would be formed on Lewis sites then the specificity ofbutylamine is merely due to the fact that only Br0nsted sites are present. The suggested Br0nsted specificity should be further investigated by in situ infrared spectroscopy. The poisoning experiments and the infrared study of the first series clearly prove that BrCnsted sites are active in polymerization. Reaction Mechanism and Product Identifica tion

The liquid collected during the reaction in the batch reactor with external circulation Journal of Colloid and Interface Science, V o l . 7 7 , N o . 1, S e p t e m b e r

1980

consisted of isobutene dimers (95%) and trimers (5%). The dimer fraction was further investigated and the results of the gas chromatographical and NMR analysis are presented in Table II. The products obtained during the oligomerization of isobutene over fluorinated alumina are explained by a reaction mechanism similar to the one presented by Oblad et al. (4) for this polymerization catalyzed by sulfuric acid. In the first step of the reaction a proton from the surface reacts with an adsorbed molecule and carbonium ion A is formed. In the second step A reacts with a second butene molecule and a new tertiary ion B is formed. This is illustrated in Fig. 7. The products listed in Table II are explained as follows. Products 1 and 2 result from the desorption of the surface carbonium ion B when the protons of the carbon atom, located a to the electrondeficient carbon atom, return to the surface. Products 3 and 4 are formed after skeletal isomerization of the surface dimer and subsequent desorption. The fact that only trimethylpentenes are formed is probably due to the fact that other products can be formed only from a secondary and primary carbonium ion which, due to their instability, are less likely to occur. The low trimer concen-

2-METHYLPROPENE OLIGOMERIZATION OVER F/AI2Oz CATALYSTS

127

TABLE II Structure and Relative Concentration of the Dimers Formed during Oligomerization No.

Name

2,4,4-Trimethylpentene- 1

Concentration (mol%)

Structure

c

c

P

I

C~ C - - C - - C - - C

40

C

2,4,4-Trimethylpentene-2

C

C

f

I

C--C~C--C--C

30

f

c c 2,3,4-Trimethylpentene-2

3,4,4-Trimethylpentene-2

c

c

C--C~C--C--C c

c

I

I

10

C--C~C--C--C

20

I

c

tration indicates that the formation of the surface trimer is slow c o m p a r e d to the desorption of the surface dimer. Thus the product distribution is in a g r e e m e n t with the conclusion that BrCnsted sites are active centers for the polymerization of alkenes on F/A1203. This is in a g r e e m e n t with the results of others (5, 6, 12) for the polymerization catalyzed by silica-alumina.

fact two processes take place. We attribute process 1 to adsorption and process 2 to the actual polymerization. Extrapolation gives an estimate of the pressure drop due to adsorption. F o r the catalyst in the figure this is 0.2 kPa, which corresponds to a surface c o v e r a g e of 2 molecules/nm 2. This esti-

50 '°''",.,,.

Polymerization and Adsorption of 2-Methylpropene The pressure drop in the batch reactor with external recirculation during a typical e x p e r i m e n t is shown in Fig. 8. A first-order rate plot at a s o m e w h a t lower pressure is p r e s e n t e d in Fig. 9. The figure shows that in A

C r+ C-C-C

10.6% F "'%

13 kPa

420 K 4% ".. o. °..o -.Q

25

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FIG. 7. The surface dimerization reaction.

t__ s

i

i

250

500

FIG. 8. Pressure vs time during a typical experiment in the batch reactor with external recirculation. Journal of Colloid and Interface Science, VoL 77, No. I, September 1980

128

KERKHOF ET AL.

mate is based on a very small difference in pressure and it is clear that a small deviation in the slope of the lines will result in a large error. Therefore we performed several experiments similar to the one shown in Fig. 9 and plotted the amount of adsorbed material as a function of the gas phase concentration of isobutene in Fig. 10. In spite of the scattering caused by the error in the estimates clearly two phenomena can be observed: (i) A more than linear increase of the number of adsorbed molecules with pressure. (ii) An increase of the amount of adsorbed species at higher temperatures. The first observation might be due to an increasing chain length of the adsorbed polymer as was also observed by Clark and Finch (8). The second observation considering the increase of adsorbed material is surprising and indicates that the chain length of the adsorbed species increases not only with pressure but also with temperature. A similar increase of the amount of adsorbed species on a commercial silica-alumina

9.55

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,nL~ I

"-, "t g,45 t S |

150

3O0

FIO. 9. First-order rate plot o f an e x p e r i m e n t in the batch reactor w i t h external recirculation. Journal of Colloid and Interface Science, Vol. ?7, No. 1, September 1980

tO.7 %F

nm- I

4

molecules adsorbed

~

, 1

2

3

/ e/

~-~ ~--~ rnol m-° 4

.

L 5

FIG. 10. N u m b e r of adsorbed molecules as a function of the 2-methylpropene gas p h a s e concentration.

cracking catalyst was measured by Hartmann (25). However, the reaction rate for isobutene polymerization showed a maximum at 420°K and, in spite of the increased amount of adsorbed species, the rate dropped. We therefore suggest that for F/AI203 the surface polymer is not involved in the catalytic process. Measurements which are based on the increase of these species may indicate the rate of surface polymer formation but may not give information on the gas phase polymerization rate. An example of such a study is the work reported by Take et al. (7). These authors observed an increase of the infrared bands of adsorbed species with the number of Lewis sites and concluded that Lewis sites are responsible for the catalytic oligomerization. The fact that they measured the rate of surface polymer formation and not the actual product formation might explain why their conclusion is in disagreement with the widely accepted role of protonic sites. Our suggestion that the surface polymer is not involved in the catalytic process is in contrast with the suggestion of Clark and Finch (8). Their conclusion is based on experiments where perdeuterated butene is

2-METHYLPROPENE OLIGOMERIZATION OVER F/AI203 CATALYSTS

0

0®0 0~0 0 0 0

O0

-~

23nm

O0

AI203

I0%F / AI203

O Oxygen otom ® : Hydroxyl group • : Fluorine olom

FIG. 11. Surface model for the incorporation of fluorine.

adsorbed on the surface of the catalyst. From the observation that butene was deuterated by the adsorbed species it was concluded that these species played a role as active centers. However, if deuterium exchange of the polymeric species with the carbonium ion takes place a similar result would have been obtained. Obviously, the role of adsorbed polymers is not fully understood and needs further investigation.

Surface Model for Fluorinated Alumina Summarizing our results it is possible to construct a model for fluorine-containing aluminas. The surface structure of the crystallographic planes of alumina has been discussed by Peri (21), Lippens and Steggerda (26), and Kn6zinger and Ratnasamy (27). Our model of fluorinated alumina is constructed for the 100 plane which has a maximum hydroxyl group content of 1.25/nm 2. We assume that this is the average site density for alumina. If the (111) or (110) planes are preferentially exposed the results are slightly different. From our XPS results (10) it can be concluded that up to a fluorine content of 10 wt% the majority of the fluorine atoms is incorporated as surface groups. This fact is indicated by the binding energy of the F(ls) and Al(2p) electrons as well by their relative intensities, e.g., the F(ls)/Al(2p) XPS intensity ratio for such a catalyst is 90% of the monolayer prediction (10). From the maximum site density of the 100 plane (12.5/nm 2) 10 sites are occupied by fluorine. The hydroxyl group content is

129

1/nm 2 and therefore the maximum oxygen density is 1-1.5 sites/nm 2. This model is visualized in Fig. 11. The model shows that nearly all the oxygen atoms exposed are substituted by fluorine atoms. Only a limited number of oxygen atoms and hydroxyl groups remain exposed after fluorination. The actual sites are formed by exposed hydroxyl groups surrounded by several fluorine atoms. The lower activity of the catalysts containing only a few fluorine atoms is explained by the fact that the weakening of the O - H bonding, as also mentioned by Chapman and Hair (28), is not yet sufficient. At higher fluorine content the bonding is weak and strong protonic centers are formed. SUMMARY

(1) Fluorination of alumina results in a nearly complete substitution of the surface oxygen layer. (2) Hydroxyl groups are present after incorporation of the fluorine atoms. (3) The Br0nsted sites of fluorinated alumina catalyze the oligomerization of 2-methylpropene. (4) Irreversibly adsorbed species are formed after contacting catalyst and monomer. These species are probably not involved in the actual catalytic process. (5) 2,4-Dimethylpyridine (DMPy) is a specific poison toward the BrOnsted sites of fluorinated alumina. ACKNOWLEDGMENTS This study was supported by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for the Advancement of Pure Research (ZWO). We thank Texaco Oil Company for a fellowship for J.C.O. We are indebted to Mr. J. Kuisten from the Laboratory for Analytical Chemistry for help in the product analysis and Mr. H. van Cranenburgh for the OH group determinations. REFERENCES 1. Matulewicz, E. R. A., Kerkhof, F. P. J. M., Moulijn, J. A., and Reitsma, H. J., J. Colloid Interface Sci. 77, 110 (1980). Journal of Colloid and Interface Science, Vol. 77, No. 1, September 1980

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