2-butene alkylation on zeolite beta: Influence of post-synthesis treatments

/ APPLE ID CATALYSIS A: GENERAL

ELSEVIER

Applied Catalysis A: General 142 (1996) 139-150

Isobutane/2-butene alkylation on zeolite beta: Influence of post-synthesis treatments A. Corma a,*, A. Martfnez

a

P.A. Arroyo a,b J.L.F. Monteiro

b

E.F. Sousa-Aguiar c a lnstituto de Tecnologla Qulmica, UPV-CSIC, Universidad Polit~cnica de Valencia, Av. de los Naranjos s / n , 46022 Valencia, Spain b COPPE/UFRJ-Programa de Engenharia Qufmica, CP 68502, llha do Fund~o, CT-Bloco G, 21945-970 Rio de Janeiro, Brazil CENPES / PETROBP~S-Divis~o de Catalisadores, llha do Fundglo, Quadra 7, 21949-900 Rio de Janeiro, Brazil

Received 10 November 1995; revised 17 December 1995; accepted 17 December 1995

Abstract

The influence of the framework and extraframework composition of zeolite beta on its activity, selectivity, and deactivation behavior during the alkylation of isobutane with 2-butene has been studied by submitting a H-beta sample to different post-synthesis treatments. The parent HI3 sample was prepared by calcination, conventional NH~- exchange, and further calcination of a commercial TEA-beta. These treatments were seen to produce some framework dealumination with formation of dispersed cationic extraframework aluminum (EFAL) species. The HI3 was then dealuminated by two chemical treatments (hydrochloric acid and ammonium hexafluorosilicate) and by steam calcination, the former procedures leading to almost EFAL-free catalysts. Furthermore, HI3 was also treated with the required amount of ammonium hexafluorosilicate (HFS) in order to remove EFAL species generated during the activation of the parent TEA-beta sample. The catalysts were characterized by XRD, N 2 adsorption-desorption isotherms, 27A1and 29Si MASNMR, XPS, and IR spectroscopy with adsorption-desorption of pyridine. In general, dealumination of H[3 leads to less active, selective, and stable catalysts for isobutane/butene alkylation. The chemically dealuminated EFAL-free catalysts were much more active than the steamed sample, which was attributed to the presence of both cationic EFAL compensating framework charge and condensed type EFAL blocking acid sites in the latter catalyst. Furthermore, elimination of EFAL in HI3 by HFS treatment also decreased the alkylation activity of the zeolite, suggesting a

* Corresponding author. 0926-860X/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved. PH S0926-860X(96)000 14-2

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synergistic effect of dispersed cationic EFAL species with framework hydroxyls of HI3 to form Br~nsted acid sites of enhanced acid strength. Keywords: Alkylation, isobutane; 2-Butene; Ammonium hexafluorosilicate; Dealumination; Extraframework

dealumination; Framework dealumination; Hydrochloric acid; Isobutane; Zeolite Beta

1. Introduction Gasoline from alkylation of isobutane with C3-C 5 olefins is an ideal blending component for reformulated gasoline, since alkylate has a high octane number with a low octane sensitivity (difference between RON and MON), and is mainly formed by multibranched paraffins. If replacement of the environmental hazardous sulfuric and hydrofluoric acids used as commercial alkylation catalysts by more friendly solid acid catalysts [1] becomes technically and economically feasible, this would push forward the increase in alkylation capacity. Among the different solid acids studied, large pore zeolites [2-9] and solid superacids of the sulfated zirconia type [10,11] were seen to give a high alkylation activity during the initial reaction stages, but then the activity rapidly declined and olefin dimerization prevailed over the true alkylation. Among the large pore zeolites studied, those of the faujasite type, either in the protonic form or exchanged with di- and trivalent cations have been, by far, the most widely studied [2-5,12]. It was seen that both the framework [5] and extraframework [13] composition of the Y zeolite affected the catalytic activity, selectivity, and stability of the zeolite during isobutane/2-butene alkylation. Recently, the high-silica large pore zeolite beta has been shown to possess interesting properties for isobutane/2-butene alkylation [7,9,10]. Its catalytic performance for this important reaction was seen to depend on the zeolite S i / A I ratio and crystallite size, which in turn depended on the synthesis conditions used [7]. In this work we have studied the influence of post-synthesis treatments, i.e. hydrothermal and chemical dealumination, and elimination of extraframework A1 (EFAL) species, on the physicochemical and catalytic properties of zeolite beta for isobutane/2-butene alkylation.

2. Experimental

2.1. Preparation of catalysts The acid form of zeolite beta (HI3) was obtained from a commercial TEA-beta (Valfor CP806B-25) by calcination at 773 K for three hours to remove the template, followed by ion exchange with a 2M aqueous solution of NH4C1 at 353 K for two hours, and a final calcination step at 773 K for three hours. After that, the HI3 zeolite was partially dealuminated by using three

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different procedures, i.e., steam calcination at 873 K during three hours (sample H[3ST), acid treatment with HC1 0.1M at reflux for two hours (sample HCI[3), and ammonium hexafluorosilicate (HFS) treatment (sample HFS[3), the latter two procedures leading to almost EFAL-free beta catalysts. The HFS treatment was carried out in a 5M ammonium acetate buffer solution at 348 K with slow addition of a 0.3M hexafluorosilicate solution (12 cm3/h). The amount of HFS added was stoichiometrically calculated to extract the desired amount of either framework (dealumination) or extraframework (elimination of EFAL) aluminum. Afterwards, the temperature was raised to 368 K and kept there during eight hours after the addition of HFS. The sample was filtered, washed with excess hot water and dried at 373 K overnight. Furthermore, the parent HI3 was submitted to an HFS treatment (sample H[3/HFS) in order to selectively remove the EFAL generated during the activation of TEA-beta without practically modifying the framework composition. Finally, the steamed sample H[3ST was treated with a 0.1M hydrochloric acid solution (sample H[3ST/HC1) at reflux for two hours in order to remove most of the EFAL formed during the steaming process.

2.2. Catalysts characterization The chemical composition of the samples was determined by atomic absorption spectrometry (AAS) using a Varian SpectrAA-10 plus spectrometer. The crystallinity was determined by X-ray powder diffraction (XRD) in a Phillips PW1830 spectrometer (CuKo~ radiation) considering the original TEA-beta sample as 100% crystalline. Textural properties were determined from the nitrogen adsorption-desorption isotherms at 77 K obtained in a Micromeritics ASAP 2400 apparatus. The acidity of the samples was measured by IR spectroscopy combined with pyridine adsorption/desorption experiments on a Nicolet 710 FTIR spectrometer equipped with a data station. For the adsorption experiments, 6.6 × 102 Pa of pyridine were introduced into the IR cell at room temperature. After reaching equilibrium, the samples were outgassed at temperatures in the range of 523-723 K under vacuum, and the spectra recorded at room temperature. The solid-state 29Si and 27A1 MAS-NMR spectra were collected in a Varian VRX-300 FT NMR spectrometer at 7.05 T, equipped with a Varian CP-MAS probe. The 29Si MAS-NMR spectra were obtained at 59.8 MHz using 8.2 ~xs (7r/2) pulses and 20 s delay, a total of 500 pulses being accumulated. The magic angle spinning speed was 3 kHz. The 27A1 MAS-NMR spectra were obtained at 78.4 MHz using 0.7 txs ( r r / 2 ) pulses and 0.2 s delay, a total of 3000 pulses being accumulated. The magic angle spinning speed was 7 kHz. The zeolites were impregnated with a 38% v / v ethanolic solution of acetylacetone in order to account for the so-called invisible NMR A1 species [14]. The surface zeolite composition was measured by X-ray photoelectron spectroscopy (XPS). The XPS spectra were recorded at room temperature using

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a VG-Scientific ESCALAB MK II spectrometer with a hemispherical analyzer operated at constant pass energy mode (CAE = 46, 55 eV). A Mg K oL X-ray source was used, and the intensity of the XPS bands was determined using linear background subtraction and integration of peak areas.

2.3. Alkylation experiments Liquid phase alkylation of isobutane ( > 99% purity) with trans-2-butene ( > 99% purity) was performed in an automated continuous fixed-bed reactor at 323 K reaction temperature, 2.5 MPa total pressure, and an isobutane/2-butene molar ratio of 15. In order to obtain different initial conversions, the 2-butene WHSV was varied in the range of 1-8 h - ~ by changing the amount of zeolite in the reactor. Before being introduced into the reactor, the zeolites were pelletized, crushed, and sieved to particle size of 0.59-0.84 mm, and diluted with inert silica ( 5 0 / 5 0 by weight). A more detailed description of the reaction system and experimental procedure can be found elsewhere [5]. A multiloop sampling system (up to 8 loops) connected to the reactor outlet allowed to take samples of the product at very short times on stream (1 min) and in this way the activity of the catalysts could be compared in the absence of deactivation. Reactants and products were then automatically analyzed and separated in a GC equipped with a 100 m capillary column (fused silica, Supelco) and an FID detector. 3. Results and discussion

3.1. Characterization results The chemical composition and physicochemical characteristics of the beta samples prepared in this work are presented in Table 1. In general, dealumination of HI3 resulted in a decrease of surface area and micropore volume of the

Table 1 Physicochemical characteristics of beta catalysts Sample

TEA-!3 HI3 H~3/HFS HCI[3 HFS[3 H~ST HI3ST/HC1

Si/A1 ratio

Area

Pore volume (cm3/g)

XRD

Chemical analysis

XPS

29Si MASNMR

BET (m2/g)

Micropores a

Mesopores b

crystallinity (%)

12.5 12.8 16.1 39.5 33.5 12.8 35.7

12.4 16.3 18.0 34.0 nd c 15.9 46.7

7.1 15.1 nd nd nd 36.4 13.9

344 663 530 561 415 523 542

0.080 0.205 0.164 0.168 0.128 0.159 0.168

0.480 0.584 0.588 0.481 0.450 0.590 0.588

100 83 77 83 60 75 70

a Determined by t-plot method. b Determined by BJH method. c n d = not determined.

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zeolite, which is especially relevant for the sample treated with the more concentrated ammonium hexafluorosilicate solution (HFS[3). This probably indicates that some collapse of the zeolite structure occurred during the more severe HFS treatment. Indeed, it is known [15] that dealumination of zeolite Y with HFS above a certain extent cannot be done without seriously damaging the zeolite structure. The two chemically (almost EFAL-free) dealuminated samples (HCI[3 and HFS[3) show a similar bulk Si/A1 ratio, and this is close to the framework Si/A1 ratio (calculated from 29Si NMR) of the steamed H[3ST sample, suggesting a comparable framework Si/AI ratio between the three dealuminated catalysts, and higher than that of the parent HI3 catalyst. The XPS results indicate that the EFAL present in the steamed H[3ST sample is mainly located on the zeolite surface, and the treatment of this sample with HC1 (H[3ST/HC1) results in the elimination of this surface EFAL, together with dealumination of the zeolite framework at the surface. With respect to the NMR results, the 27A1MAS-NMR of H [3 showed, besides the band at ca. 56 ppm of tetrahedral framework A1, a sharp signal at ca. - 1.4 ppm of dispersed octahedral extraframework aluminum (EFAL). Such a signal was not detected in the parent TEA-beta sample. Additionally, the Si/A1 ratio of the HI3 obtained from 29Si MAS-NMR is slightly higher than the bulk one. After the steam dealumination (H[3ST sample), the intensity of the band located at ca. - 1 . 4 ppm decreases, and the band becomes broader, indicating the presence of different species of octahedral EFAL with a different degree of polymerization, while a signal at ca. 30 ppm of pentacoordinated EFAL is also visible. After the acid leaching of the steamed sample (H[3ST/HC1) only the signal at ca. 55 ppm is observed, indicating that most of the EFAL species were removed. In agreement with this, the bulk Si/A1 ratio of H[3ST/HC1 is similar 29r to the framework S i/A1 ratio of the H[3ST sample measured by S i MAS-NMR. However, the Si/A1 ratio of H[3ST/HC1 calculated from 29Si MAS-NMR was lower than expected, which can be attributed to the contribution of the Si(1OH) and Si(2OH) signals when calculating the framework Si/A1 ratio from the spectrum. This contribution was further confirmed by performing 29Si C P / M A S - N M R experiments. The acidity results obtained from the IR-pyridine experiments are presented in Table 2. It can be seen that steam dealumination of HI3 produced the strongest reduction in the number of B65nsted acid sites and Bfi5nsted/Lewis ratio ( B / L ) . Lewis acid sites can be associated with the formation of EFAL in which some cationic forms of the AlOe- type can occupy cationic exchange positions, neutralizing some of the potential BriSnsted acid sites. Surprisingly, elimination of EFAL in this sample by acid treatment (HI3ST/HC1) did not change appreciably the number and strength distribution of the BrSnsted acid sites, but increased the number of Lewis acid sites. This may indicate a depolymerization and redispersion of cationic (Lewis type) EFAL upon the acid treatment. Moreover, dealumination by acid and HFS treatments decreased both

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A. Corma et al./Applied Catalysis A: General 142 (1996) 139-150

Table 2 Acidity of the modified beta samples as measured by IR spectroscopy with adsorption of pyridine and desorption at different temperatures Sample

Density of acid sites (txmol P y / g catalyst) a Br~insted

H[3 H[3/HFS HFSI3 HCI[3 HI3ST HI3ST/HCI

Lewis

523 K

623 K

673 K

523 K

623 K

673 K

70 88 53 23 10 8

45 59 26 14 2 2

14 36 14 6 1 1

52 29 13 15 11 21

30 28 12 14 10 15

29 23 12 13 8 12

a Calculated using the extinction coefficients given in Ref. [16].

the Br~insted and Lewis acidity of the parent HI3 sample, but the latter procedure resulted in a higher concentration of BriSnsted sites as compared to the acid dealumination. However, treatment of HI3 with HFS to eliminate EFAL (H[3/HFS) increased the number of total Brtinsted acid sites in the zeolite. This suggests that some of the EFAL in HI3 was in the form of cationic species neutralizing part of the framework charge.

3.2. Isobutane / 2-butene alkylation experiments 3.2.1. Initial activity and selectivity The 2-butene conversion obtained at 1 min time-on-stream (TOS), and hence in the absence of catalyst deactivation, as a function of the olefin space velocity, WHSV, is presented in Fig. 1 for the different beta samples. At low WHSV the conversion is very high on all the beta catalysts, but under these conditions a strong decrease of the activity is observed upon increasing WHSV. The un-

100

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,o

!

i"

7O

X

5G 0

I

i

I

5

10

15

20

Oldln WlISV (I/h)

Fig. 1. Initial 2-butene conversion as a function of olefin space velocity (WHSV) for the different beta catalysts: ( O ) HI3; ( + ) HI3/HFS; ( ' v ) HFS[3; (M) HCll3; (¢z) HI3ST; ( A ) HI3ST/HC1.

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treated HI3 sample shows the highest conversion in the full range of WHSV studied. Moreover, Fig. 1 also shows that the chemically dealuminated HCI[3 and HFS[3 samples are much more active than the steamed H[3ST catalyst, which is in agreement with the much lower Br~Snsted acidity of the latter catalyst (Table 2). However, HCI[3 is slightly more active than HFS[3, despite the fact that the density of BriSnsted acid sites of the latter sample is higher. The much lower surface area and micropore volume of the sample dealuminated with HFS (Table 1), as a result of a higher destruction of the zeolite framework as compared to the sample dealuminated with HC1, may account for the lower activity of HFS[3. Finally, it can be observed that acid leaching of the steamed catalyst only produces a slight increase of the zeolite activity. Indeed, the acidity of the catalyst did not increase after acid leaching, even though some EFAL was effectively removed, as indicated by the higher bulk Si/A1 ratio of the acid treated sample, and by the 29Si NMR results given in Table 1. This may suggest that some cationic EFAL species were formed by depolymerization of condensed type EFAL during the acid treatment, and that these species could be compensating some framework charge and thus decrease the Br~Snsted acidity. This assumption is supported by the increase of the number of Lewis acid sites experienced after the acid treatment (Table 2). Furthermore, elimination of EFAL in HI3 by HFS treatment (sample HI3/HFS) results in a decrease of the zeolite activity, although the density of BriSnsted sites of the sample increased, as shown in Table 2. These results may be interpreted assuming that dispersed cationic Lewis type EFAL present in the parent H[3 can be interacting with framework hydroxyls increasing the acid strength of some Br~Snsted acid sites, as represented in Scheme 1 [17]. Table 3 compares the initial (1 min TOS) product distribution in the alkylates produced on the different beta samples at similar (ca. 90 wt%) 2-butene conversion levels. It can be clearly seen that elimination of EFAL in H[3 results in a lower cracking (C5-C 7) selectivity, and a higher selectivity to heavier products, C9÷, as well as in a lower selectivity to the desired TMP in the C 8 fraction. This would indicate that species represented in Scheme 1 not only have a beneficial effect on the activity, but also on the alkylation selectivity of zeolite beta. On the other hand, Table 3 shows that the steamed sample is the less selective to TMP, giving a high concentration of DMH and octenes formed by dimerization of the butenes. Moreover, the selectivity to light products and TMP

--0

H

/d(O,OHf

0""

0--

/\

/\ --0

0

0

Scheme 1.

O--

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A. Corma et al. / Applied Catalysis A: General 142 (1996) 139-150

Table 3 Comparison of initial (1 min TOS) product selectivity at 90 wt% 2-butene conversion level Sample

HI3

Composition of C5 + (wt%) C5-C 7 29 C8 62 C9+ 9 Composition of C~ (wt%) TMP 73 DMH 18 Octenes 9 T M P / D M H ratio 4.1 Composition of TMP (wt%) 2,2,4-TMP 51 2,2,3-TMP 7 2,3,4-TMP 27 2,3,3-TMP 15

HI3/HFS

HFS[3

HCI[3

H[3ST

H[3ST/HCI

19 45 36

17 36 47

30 44 26

15 60 25

26 60 14

60 26 14

52 32 16

62 28 10

17 33 50

25 37 38

2.3 43 9 32 16

1.6 43 9 31 17

2.2 50 8 28 14

0.5 28 11 39 22

0.7 33 12 35 20

slightly increases after the acid leaching of the steamed sample. This treatment was seen to remove preferentially condensed type EFAL, and did not produce significant changes in the acid strength distribution of the zeolite. The relatively high concentration of olefins in the product obtained on these samples indicates a poor hydrogen transfer activity in the steamed catalysts. Furthermore, dealumination of HI3 with HC1 produced a catalyst with a relatively high initial cracking selectivity, comparable to that of the most active HI3, and with a selectivity to TMP higher than that of the sample dealuminated with HFS, despite the density of Brt~nsted acid sites determined by IR-pyridine was higher for the latter sample. The much lower micropore volume of HFS[3, as well as the relatively low crystallinity of this sample (Table 1), suggest that some amorphous material produced during the severe HFS treatment may be blocking micropores and thus preventing the formation a n d / o r diffusion of the alkylation products (TMP) in the zeolite channels, then favoring the formation of heavier products, either by consecutive alkylations a n d / o r olefin oligomerization on the unhindered external sites. The high selectivity to heavier products, C9+, obtained on this catalyst seems to support the above hypothesis.

3.2.2. Catalyst deactiuation and product distribution with TOS The decrease of 2-butene conversion as a function of time on stream (TOS) for the modified beta catalysts is given in Fig. 2. In order to better compare the decay behavior of the zeolites the data of Fig. 2 correspond to experiments in where initial 2-butene conversions above 90 wt% were obtained, i.e., WHSV = 1 h -1 for samples H[3ST and H[3ST/HC1, WHSV = 2 h -1 for HI3, H[3/HFS, and HFS[3, and WHSV = 4 h-~ for HCI[3. It can be seen that the untreated HI3 gives a very high olefin conversion during the first 10 min TOS, and then the

A. Corma et al./Applied Catalysis A: General 142 (1996) 139-150

147

IO0

! i

I

i

10

15

20

25

T.O.S. (rain) Fig. 2. 2-butene conversion as a function of time-on-stream (TOS) for the modified beta samples: (O) HI3; ( + ) H~3/HFS; ( v ) HFS[3; (11) HCI[3; (~) H[3ST; ( A ) H[3ST/HC1.

activity starts to decline steadily at higher TOS. Treatment of this sample with HFS to eliminate EFAL (sample H[3/HFS) increases the deactivation rate of the zeolite. Among the dealuminated beta catalysts, those treated with HFS (HFS[3) and HC1 (HCI[3) having a low-EFAL content decay at a faster rate than the steamed EFAL-containing HI3ST sample. Furthermore, leaching of this sample to remove EFAL slightly improves the stability of the catalyst. In order to better discuss the deactivation behavior of the catalysts, and considering the different reactions taking place during isobutane/butene alkylation, the change of selectivity to TMP, that is, to the true alkylation products, is presented in Fig. 3 as a function of TOS. It is clearly seen there that the parent H[3 is the most selective to alkylation in the full range of TOS studied. The loss of TMP selectivity occurs at much shorter TOS after elimination of EFAL from this sample. On the other hand, the chemically dealuminated samples, HFS[3 and HCI[3, start to lose their alkylation selectivity from the initial reaction stages and is practically lost after 10 min TOS. Both the steamed and steamed plus acid

|. 2O 0 0

S

10

IS

20

25

T.O.S. (nan) Fig. 3. Change of TMP selectivity within the C 8 fraction as a function of TOS: (O) H[3; ( + ) H~3/HFS; ( • ) HFSI3; ( B ) HCII3; (~) HIgST; (A) HI3ST/HCI.

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A. Corma et al. /Applied Catalysis A: General 142 (1996) 139-150

1,°t

20 I

O0

$

10

15

20

25

T.O.S. (rain)

Fig. 4. Formation of octenes with TOS during isobutane/2-butene alkylation on beta catalysts: (O) HI3; ( + ) HI3/HFS; ( v ) HFS[3; (11) HCI[3; (Or) HI3ST; (A) HJ3ST/HCI.

treated samples show a very poor alkylation selectivity from the initial reaction stages, where the olefin conversion is relatively high (Fig. 2). For all the catalysts the loss of alkylation selectivity is accompanied by an increase of unsaturated compounds, mainly octenes, in the reaction product, as shown in Fig. 4. This indicates a loss of hydrogen transfer activity and a change of reaction path from alkylation to olefin dimerization in the most active samples. The results obtained after eliminating EFAL from HI3 suggest that, besides the positive effect on initial activity and selectivity already discussed, the dispersed cationic EFAL species present in this sample also have a beneficial effect on the stability of the zeolite towards deactivation with TOS. As it was suggested, this may be ascribed to the creation of BriSnsted acid sites of enhanced acidity upon interaction of cationic EFAL with framework hydroxyls. These sites would have a higher hydrogen transfer activity, which is crucial to produce a faster desorption of the adsorbed TMP + cations by hydride transfer from isobutane, and thus to maintain a high level of t-butyl cations necessary to continue the chain process for a longer period of time. The lower stability of the chemically dealuminated catalysts can be attributed to their lower framework acid site density. Indeed, previous results obtained for USY [5] and beta [7] zeolites showed that the alkylation activity and catalyst stability increased when increasing the framework A1 density, that is, when decreasing the framework S i / A I ratio. However, results of Fig. 2 show that the steamed samples are quite stable catalysts in terms of 2-butene conversion, but in these cases dimerization of the olefin was the predominant reaction taking place on these catalysts, as shown in Figs. 3 and 4. According to the IR-pyridine results of Table 2, samples H[3ST and H[3ST/HC1 possess BriSnsted acid sites of weak-medium acidity, which are the active sites for olefin dimerization. These sites would deactivate at a lower rate than the strong acid sites responsible for alkylation and cracking reactions, and this would explain the lower rate of decrease of 2-butene conversion observed for the steamed samples as compared

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to the chemically treated catalysts. The fact that the acid treatment of the steamed H[3ST sample in order to remove EFAL does not significantly improve the activity and stability of the zeolite is in agreement with the small changes in zeolite acidity reported in Table 2 for these two catalysts. This may be explained considering that during this treatment, besides the preferential extraction of condensed EFAL species from the zeolite surface, as shown by the increase of Si/A1 ratio measured by XPS (Table 1), some depolymerization of these species also takes place, leaving cationic type EFAL inside the zeolite channels that may be compensating part of the framework charge, thus decreasing the number of potential BriSnsted sites while increasing the number of Lewis acid sites, as can be seen in Table 2. The much lower Br~insted acidity of the acid treated H[3ST/HC1 with respect to the chemically dealuminated samples, all of them having similar framework Si/A1 ratios, and the lower acidity of the sample dealuminated with HC1 as compared to that treated with HFS, seems to support the above hypothesis.

4. Conclusions It has been shown that post-synthesis treatments, i.e., hydrothermal and chemical dealumination, and EFAL extraction, have a strong influence on the activity, selectivity, and decay behavior of zeolite beta during isobutane/2butene alkylation. Thus, activation (calcination, ion exchange) of TEA-beta to form the acid zeolite produces some framework dealumination leaving dispersed cationic EFAL species in the zeolite channels. These species have a beneficial effect on zeolite activity, alkylation selectivity, and catalyst stability, which is attributed to the interaction of cationic Lewis type EFAL with framework hydroxyls to form Br~Snsted acid sites of enhanced acidity. Thus, elimination of this species by HFS treatment results in a lower catalytic performance of HI3 for isobutane alkylation. On the other hand, chemical dealumination of HI3 by HFS and HC1 treatments to produce dealuminated EFAL-free beta catalysts decreases the alkylation activity and increases the deactivation rate of the zeolite, as compared to the parent HI3 catalyst. Hydrothermal dealumination produces beta samples with very poor alkylation activity from the initial reaction stages. This is thought to be due to the formation of condensed type EFAL that migrates to the zeolite surface decreasing the accessibility of reactants to the acid sites.

Acknowledgements Financial support by the Comisi6n Interministerial de Ciencia y Tecnologla of Spain (Project MAT 91-1152) is gratefully acknowledged. One of the authors (PAA) thanks CNPq-Brasil for a scholarship.

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