Study of phosphorus-modified aluminum pillared montmorillonite

Applied Catalysis, 67 (1990) 93-106 Elsevier Science Publishers B.V., Amsterdam

Study of phosphorus-modified montmorillonite

93

aluminum pillared

I. Effect of the nature of phosphorus compounds Yan-Fei Shen, An-Nan Ko and Paul Grange* Catalyse

et Chimie des Matiriaux

Sud, 1, 1348 Louvain-la-Neuve

Divisds, (Belgium),

Universit6

Catholique

tel. (+32-10)473648,

de Louvain,

Place Croix du

fax. (+32-IO)433649

(Received 20 March 1990, revised manuscript received 30 May 1990)

Abstract Phosphorus and phosphoric acids induce effects on the physiochemical and catalytic properties of Alpillared montmorillonite. H,PO, results in a slight increase in the thermal stability and a significant enhancement of total acidity defined as an improvement of Brensted acidity and a loss of Lewis acidity. H,PO, causes a strong reduction in the thermal stability, BET area and both Lewis and Brensted acid sites. This substantial difference is attributed to different pH and different ease of AlPO, formation: low pH and AlPO, formation produce deleterious effects. Two different structural models and new acid site formation are proposed. Conversion in the 1-butanol dehydration reaction shows that the conversion is correlated with Lewis acidity. The medium and strong acid sites created explain the ratios of lbutene (1B) to dibutyl ether (DBE) and of 2-butenes to (1B +DBE). Keywords: phosphorus, montmorillonite, terization (acidity), butenes.

pillared clays, acidity, butanol dehydration, catalyst charac-

INTRODUCTION

Pillared clays (PILC) are a new class of solid acid materials synthesised by intercalating polycations into expandable clay minerals. Depending on the polycations [l-5], clays [ 5-81 and preparation conditions (9-121, pillared clays display a wide range of different basal spacings, thermal and hydrothermal stability, acidity and catalytic properties. Reviews have been recently published [ 12,141. Aluminum-pillared montmorillonite (Al-PM), one of the most widely studied pillared clays, has been increasingly attracting interest, due to its controllable pore size and hence its potential for applications in the area of shapeselective catalysts. Some successful applications in shape-selective catalysis have been reported in recent years [ 15-161.

0166-9834/90/$03.50

0 1990 Elsevier Science Publishers

B.V.

94

Pure Al-PM has been found to be weakly acidic, however, being especially weak, Bronsted acidic in most cases [ 17,181. Moreover, its applications, especially to catalytic cracking, require good thermal and hydrothermal stability. Attempts must be made, therefore, to the acidity and stability. It has been found that the treatment of clays with inorganic acids, such as hydrochloric, hydrofluoric and sulfuric acids, etc. result in an improvement of the acidity and specific surface area [ 19,201. Moreover, aluminum phosphate zeolites have been established as being strongly acidic and thermally stable catalysts [ 211. In addition, phosphorus compounds have been widely used as promoters of zeolites [ 22-27 1, and oxides such as alumina [ 281 or titania [ 291 have been used for the modification of acidity. However, the nature of the effects of phosphorus compounds on the acidity is still a matter of controversy, and different hypotheses have been proposed for zeolites [22-271 and for oxides [29]. For these reasons, H3P03 and H,PO, acids, which show different structures and different acidity, have been chosen in the present study as promoters in order to modify the acidity of Al-PM. We also try to understand the reasons for the modification of acidity. The following characterization techniques have been used: XRD, BET, CEC, NH3 TPD, FT-IR of pyridine, XPS and elemental analysis. Dehydration of 1-butanol was used as a catalytic reaction. EXPERIMENTAL

Catalyst preparation

Nat-montmorillonite was prepared by twice saturating a natural Wyoming montmorillonite, supplied by EEC International Ltd, with 1 N NaCl solution, and subsequently washing by centrifugation and dialysis to remove the excess of chlorine ions. Only the 12 mm size fraction of this clay was used, with a solution density of about 9-10 g/l. Aluminum polycations were prepared by slow titration of 0.10 M AICY3solution with 0.5 M NaOH solution under vigorous stirring and subsequent ageing at 90°C for 3 h and then at room temperature for 2 h. The final OH/Al molar ratio of the titrated solution was 2.0. H3P03 and H,PO, solutions (0.10 M) were dropped into the aged aluminum polycation solution with vigorous stirring at room temperature. Pure Al-PM, H3P03 and H,PO,-modified Al-PM samples were obtained by dropping the respective polycation solutions into the above freshly-prepared Na+ -montmorillonite solution with vigorous stirring, ageing at room temperature for 2 days, ten times washing by centrifugation until the electrical conductivity was less than 6 &Siemens and freeze-drying. The three pillared clays are designated as Al-PM, Al-PI-PM and Al-P,-PM, respectively. The initial Al/clay ratio and P/Al molar ratio are 4 and 0.10 mM/g, respectively.

95

Each sample was then calcined for 3 h at 200,300,400 and 500°C. For comparison, the Al-PM sample calcined at 500°C for 3 h was impregnated with H,PO, and H,PO, solutions. These samples are referred to as Al-PM-P1 and Al-PM-P,, respectively. Catalyst characterization Measurement of cation exchange capacity The cation exchange capacity (CEC) was determined by saturation of a sample with 1 N ammonium acetate solution, followed by successive washing with methanol and centrifugation up to complete elimination of excess ammonium ions. The remaining ammonium ions were determined by the microKjeldahl method. Physicochemical analyses X-ray diffraction film or powder patterns were obtained using a Philips 1130 diffractometer with a Ni filtered CuKa radiation. Temperature-programmed desorption (TPD ) of ammonia was conducted, after the sample had been pretreated in flowing helium at 500” C for 3 h, adsorbed with ammonia at 100” C for 0.5 h and outgassed in flowing helium at 100” C for 1 h. Infrared (IR) spectra of the sample were taken with a Bricker IFS 88 FT-IR spectrometer, after the sample was outgassed at 400’ C and ca. 5 x 10W4mbar for 3 h, followed by pyridine adsorption (5 torr) at 150°C for 5 min and desorption at given temperatures. The BET area was measured with a Micromeretics 2200 Surface Area Analyzer, following outgassing at about 200 oC for 1 h. Atomic absorption measurement allowed the determination of the chemical composition of the samples, except phosphorus, which was determined by a calorimetric method, after the solids had been dissolved using 50% sulfuric and 40% hydrofluoric acid. Evaluation of catalytic activity Dehydration of 1-butanol was used as a test reaction to characterize the catalytic properties of the pillared clays. Prior to the reaction at 25O”C, the samples calcined at 500 “C for 3 h were further pretreated with helium at 300 oC for 3 h. Previous measurements [30] have demonstrated that, in our experimental conditions, no diffusional limitations are observed. RESULTS

AND DISCUSSION

Effect on thermal stability and specific area XRD patterns of the three samples calcined at different temperatures are presented in Fig. 1. Basal spacings against calcination temperatures are plotted in Fig. 2.

96

I

b

a

C

I

a

6

L

20

Fig. 1. XRD patterns of the Al-PM (a), AI-PI-PM (b), AI-P,-PM (c) film calcined for 3 h at 500°C (1); 400°C (2); 300°C (3); 2OO’C (4) and room temperature (5).

I1 75

,

\

0

200 Calonation

LOO temperature

( 500 i”C)

Fig. 2. Basal spacings as a function of calcination temperature, (c] ) Al-PM, ( l ) Al-PI-PM, (m) Al-P,-PM.

Fig. 1 shows that the d(OO1) peak intensity and peak sharpness are decreased in the following order: Al-PI-PM > Al-PM > Al-P,-PM, indicating that H,PO, slightly increase the homogeneous pillar distribution and pillar density, while H3P04 largely decreases both of them. Moreover, Fig. 2 demonstrates that, for all the samples, the basal spacings decrease with the calcination temperature. In addition, calcination pretreatments result in a greater decrease in the basal spacings, as compared with other reports. This may be due to the fact that the clay used here had not been aged for a long enough time. Let us recall that it has been shown [ 13,141 that ageing of the clay before pillaring, namely

97

placing the Na montmorillonite in water for days or weeks, improves the thermal stability of pillared solid. Elemental analysis results, shown in Table 1, confirm the XRD observation. The following sequence of the A1203 composition is: Al-PI-PM > Al-PM> AlP,-PM, suggesting an identical order of pillar density. The significant difference of the effect of the two phosphorus compounds may be due, to some extent, to different pH-levels during the pillaring process. Table 2 lists the pH of the three suspensions before washing. It reveals that the H,PO,-modified sample suspension has a much lower pH than the other two, which give almost identical pH. In addition, Table 2 shows that after 2day pillaring at room temperature, the pH of the suspensions increases, revealing that during the pillaring of aluminum polycations, H+ cations were involved in the competitive exchange with cations on the clay. The pH affects the distributions of aluminum cations and the charge of polycation species in the solution [ 14,31-331 j The following main equilibria have been proposed: TABLE I Bulk concentrations of the fresh samples measured by elemental analysis Sample

Al-PM Al-PI-PM Al-P,-PM

Concentration (% )

&O,

Si02*

p205

MgO

Ba,O

Fe&

20.43 20.90 19.67

57.37 50.65 51.40

0 6.12 6.89

2.58 2.46 2.30

0.10 0.06 0.04

0.72 1.91 0.90

*Calculated by subtracting water content from total concentrations. TABLE 2 pH of suspensions of the Al-PM, Al-PI-PM, Al-P,-PM samples; cation exchange capacity (CEC) of the samples before calcination; BET surface area and acidity of the solids calcined at 500” C for 3h Sample pH just after pillaring pH after pillaring for 2 d CEC (meq/lOO g) BET area (m*/g) Total acidity (mmol/g) Specific acidity (~mol/m’)

Al-PM 4.22 4.68 51.8 188

1.74 9.3

Al-PI-PM 4.22 4.78 57.5 168

1.94 11.5

Al-P,-PM 2.87 3.43 96.2 52

1.01 19.4

98

pH=6

pH=4-6

It can be seen that a low pH would result either in the decomposition of [ Al,,O,. - - ] 7+ species or in lower positive charge of the [ Al,,O,* - - ] lz+ species. The lower-charge [ Al,,O,- - - ] nf species have been found to increase pillar density and thermal stability [32] and low pH has been reported to result in the decomposition of (A1i304. - - ) 7+ [ 33 1. The different effect of the two acids on the pillar distribution and density can thus at least partly be attributed to the difference in the concentration and charge of the pillared species. The second effect of pH may be that H’ can compete in exchange with cations on the clay during the pillaring of aluminum polycations. In addition to the increase in pH (Table 2), this competitive exchange would also be reflected in the cation exchange capacity (CEC) of the pillared samples. The CEC of the fresh samples are presented in Table 2. It indicates that the CEC decreases in the following order: Al-P,-PM >> Al-PI-PM > Al-PM. The CEC of the Al-P2-PM sample is even larger than that of Na+-montmorillonite (86 meq/lOO g). This reveals that during the pillaring, H+ or A13+ monomer cations in the solutions can exchange cations not only on the clay but also on the pillar of the sample, giving a low pillar density and an unusual high CEC. Such a proton effect has been found to result in the degradation and dissolution of clay sheet [ 14,311 and in the decrease of thermal stability [ 341. Moreover, from our above results and the fact that subsequent cation exchange of pillared clays produces an improved thermal stability [ 311, we believe that the partial pillar destruction in the Al-P,-PM could also be due to the migration of protons from the pillar surface into the bulk, causing its collapse during thermal treatment. The third effect of pH on thermal stability concerns the reaction of protons with pillar oxides. This can be deduced from Fig. 3, which shows that, when H,PO, and H3P03 solutions are impregnated onto a 500’ C-calcined Al-PM sample, Al-PM-P2 (Figs. 3-5) and Al-PM-P, samples are collapsed at room temperature and 5OO”C, respectively. This may be due to different proton effects on pillars or, most probably, to easier formation of AlPO, compounds in the A-PM-P2. The effect of the two phosphorus compounds on the thermal stability can be also evidenced by the evolution of BET areas. The BET areas of the three samples calcined at 500 oC for 3 h are shown in Table 2. It reveals that the BET

99

A

12

10

8

6

L

2

26 Fig. 3. XRD pattern of the AI-PM-P1 powder calcined for 3 h at room temperature (1); 3OO’C (2); 4OO’C (3) and5OO”C (4) and the Al-PM-P* powder without calcination (5).

area is decreased slightly if H,PO, is introduced, but the introduction of H3P04 causes a large reduction in the BET area. One of the main reasons for the difference in the BET area could be due to the different pillar density and pillar distribution homogeneity, as mentioned above. This can explain why the area of the Al-PI-PM sample is much larger than that of the Al-P2-PM. Moreover, pore blockage caused by the AlPOd may also play an important role in the reduction of the area, especially in the case of H,PO,. Such pore blockage has been observed over phosphate-modified ZSM5 zeolites [ 251 and sulfated Zr montmorillonite [ 351.

Effect on acidity

The TPD profiles of NH, and the calculated acidity are shown in Fig. 4 and Table 2, respectively. It is clear that all three of the samples have very broad desorption spectra at temperatures ranging from about 80°C to above 500°C the high-temperature region of which has been found not to be due to the base line, but to be ascribed to the desorption of ammonia [36]. The TPD profiles clearly indicate that both strong acid sites and total acidity are greatly reduced when H3P04 is introduced (Fig. 4-c)) while, when H3P03 is doped, the total acidity or the middle-strong acid sites corresponding to the ammonia desorbed in the region of 180-230°C are remarkably enhanced (Fig. 4-b). If specific acidity (acid sites per unit area) is considered, the Al-P,-PM sample is most

a

100

200

300

LOO

500 T I”Ci

Fig. 4. TPD profiles of NH, adsorbed on the AI-PM (a), Al-P,PM (b) and Al-P,-PM

100

200 Desorphon

300

LOO

temperature

(c).

500 (“Cl

Fig. 5. Lewis and Brensted acid intensity of pyridine IR spectra as a function of desorption temperature. Symbols as in Fig. 2.

acidic (Table 2), indicating that the loss of its total acidity is mainly due to the large reduction of its specific area. In order to see which kind of acid site, Lewis or Brransted, is modified, FTIR measurements of pyridine adsorption were taken and the intensity of the Lewis and Brensted acid bands was calculated on the basis of the integration and wafer density (mg/cm* ) . The calculation intensities of Lewis and Brnrnsted acidity are plotted against desorption temperature, as shown in Fig. 5. Lewis acid sites are predominant for the three samples. Both Lewis acid sites and Brransted acid sites decrease markedly for the Al-P,-PM, while for the Al-P,PM sample Brensted acid sites are increased, but Lewis acid sites are slightly

101

reduced. Moreover, evacuation at 400 oC almost completely removes Bronsted acid sites and largely reduces the Lewis acid sites of all three samples. Here we try to explain this significant difference of the effect of the two different phosphorus compounds on the acidity. Over ZSM-5 zeolites, phosphorus compounds have been found to affect the acid sites at the outer surface [ 22,24,27] through converting the strong Bronsted acid site of the Si - OH Al-bridged hydroxyl group to the weak acid site of a P - OH group [ 251. In the case of Al-PM, however, the situation may be different, because its acidity is weak and no reduction in strong acid sites can be found from the TPD profiles of ammonia adsorption in the Al-PI-PM sample (Fig. 4). In our opinion, the modification of the acidity may mainly be ascribed either to the adsorption of the acids on Lewis acid sites or, if in solution, to the substitution of the acids for the terminal OH groups, as in the case of phosphatemodified titania [ 29,371. Because of this, the Bronsted acid sites increase at the expense of the Lewis acid sites for the Al-PI-PM sample (Fig. 5). This interpretation can be further substantiated by the fact that the total acidity and strong acid sites of the samples prepared by impregnating solutions of phosphorus compounds onto calcined (500’ C ) Al-PM powders are larger than those of the samples prepared by the above method [ 341. It is proposed that the thermal treatment produces more Lewis acid sites, so that a larger amount of phosphorus can be adsorbed on the sites and more Bronsted acid sites can be formed. Moreover, high-temperature calcination probably produces some strong Lewis acid sites which can attract electrons from the oxygen of the acid species and hence reduces the strength of H - 0 bonds or increase the charge density on the hydrogen atom, inducing stronger Bronsted P - OH acidity. The following mechanism has therefore been proposed to explain the acidity modification of the two acids (Schemes 1 and 2 ) . The structures of the phosphorus compounds adsorbed on the surface of the pillared clay are also proposed in the above schemes on the basis of IR spectra. It is reported that the P=O double bond and the corresponding overtones could have IR bands in the 1200-1300 and 24-2600 cm-l regions, while the band at 3675 cm-l is ascribed to P - 0 groups [ 381. A strong band at 2505 cm-’ was detected in the Al-PI-PM sample pretreated at 400 aC, but not in the fresh AlPi-PM sample; however, no bands are observable in these two regions in both

OH

Al+- CL A

H3PO3

+H3m3

-----mm---m> in solution

Scheme 1.

I

P-OH

O=P-OH

I 0

OH ,I\\ pillar-Al-O-

OH

I

1 0

I I _Al_O----~~~~mAl-OA

102

HO

-Al+-O-

0 \

OH

1 / /‘X 9.

O-Al-CL

+ H++

H2m4-

--------in solution

Jo

/p\

0

.O

\ I’ O-AI- 0 --

\-. #’

?

-----,A1_O----->A~4

A

A

Scheme 2.

the fresh and pretreated at 400’ C Al-P,-PM sample. In addition, it is difficult to observe the band at 3675 cm-l, because of a very broad band in the region of 3600-3750 cm-i, due to structural OH groups [39] and probably the P OH group in the case of the P modified samples. In any case, the IR results are strong enough to conclude that adsorbed H3P03 species on the Al-PI-PM sample is oxidized during the thermal treatment, probably to HO - P=O-like structures, in which the electrons are localized and Brernsted acid sites are formed (Scheme 1)) as in the case of phosphate-modified zeolites [ 401. However, the adsorbed H,PO, species in the Al-P2-PM sample is converted to an electrondelocalized structure (Scheme 2 ), which is similar to that of phosphate-modified titania [ 29],and probably reacts with the Al pillar, resulting in the formation of AlPO, phases. In addition, the specific intensity of the OH band is decreased in the order: Al-PM > Al-PI-PM >> Al-P,-PM, indicating that no direct correlation exists between structural OH groups and Brensted acid sites, which differs from the results reported [ 171~ The loss of the total acidity of the Al-P,-PM, measured by ammonia TPD (Fig. 4), could be ascribed to the remarkable decrease in its specific area, as mentioned above. However, if the IR band intensities of both Bronsted and Lewis acid sites are divided by the specific area, the specific acidity measured by IR is still smaller in the Al-P,-PM than in the Al-PI-PM sample. This difference may be due to different porosities in the two samples. As mentioned before, H,PO, blocks micropores, hindering the entrance of pyridine into some pores. Such a steric block effect of H,PO, has also been observed in zeolites by other researchers [ 22,251. In order to investigate the source of the acidity and of the acidity modification, ammonia TPD spectra were also taken in the Na+-montmorillonite and H,PO,-modified Na+-montmorillonite prepared by introducing 0.1 M H3P03 solution into the clay and then washing and freeze-drying (phosphorus content = 0.4 mM/g clay). The TPD curves are similar to those obtained with the previous sample. The two samples have the same total acidity and the same acidity distributions. In addition, comparison of the results with those in Table 2 suggests that the clay mineral contributes about half of the total acidity of

103 TABLE 3 Superficial compositions of the P-modified pillared clay and Na clay analyzed by XPS Sample

Al-PI-PM Al-P,-PM H,POB-Na-Mont

Relative concentrations C

0

Al

Si

P

Mg

0.084 0.120 0.003

0.504 0.493 0.547

0.118 0‘111 0.078

0.210 0.203 0.292

0.059 0.052 0.037

0.024 0.023 0.021

Clay

umt ioyer

Fig. 6. Schematic representation of the phosphorus-pillared montmorillonite.

the Al-PM sample, which is somewhat different from a literature report about an acidity contribution of 30% by the clay [ 141. Two hypotheses may explain the acidity of these two samples: no H,PO, is deposited on the non-pillared clay or only the A&O3 pillars or the interconnections of the pillars and the clay are modified. The relative superficial compositions analyzed by XPS (Table 3 ) , indicate that the H3P03 impregnated on the clay is as much as about 60-70s of that deposited on the pillared clays, so the first hypothesis should be ruled out. We may therefore conclude that the acidity modification occurs only on the acid sites carried on pillars or in the linkage of pillars and the clay, rather than on the clay surface. However, we have no direct evidence to support this assumption. In short, we have observed a considerable difference in the effects of the two phosphorus compounds on the thermal stability, specific areas and acidity. In order to visualise these hypotheses, two model structures are proposed for the Al-PI-PM and Al-P2-PM samples, as illustrated in Fig. 6. Effect on catalytic properties The dehydration of 1-butanol to butenes and dibutyl ether (DBE) has been used to characterize surface acidity of aluminum phosphates [ 411. The ratio of 1-butene (1B ) to DBE or 2-butenes (2B) to (1B +DBE) has been identified as a parameter for measuring acid strengths and acid concentrations [30].

104 TABLE

4

Activity

and selectivity

of I-butanol

dehydration

over calcined Al-PM

and P-compounds

modified

Al-PM* Catalyst

Conversion

Yield (mmol min

1B

’)

(%I

Al-PM Al-PI-PM Al-P,-PM

28.8 19.1 7.8

1B

DBE

BAL

US-2B

trans-2B

4.58 1.97 1.84

7.21 1.14 4.02

0.45 0.53 0.52

4.48 1.90 1.2‘2

4.18 2.01 1.22

2B

DBE

lB+DBE

0.64 1.73 0.46

0.74 1.09 0.42

*Catalysts were pretreated in air at 500” C for 3 h and then in flowing helium at 300°C Reaction was conducted at 25O’C and WHSV = 50 h- *.

for 3 h.

Moreover, double bond migration of butenes and cis-truns rotation of 2-butene have been reported to take place on the so-called L-inter site and L-intra sites of aluminum phosphates, respectively [ 42,431. A characterization of the surface acidity may therefore be expected on the basis of this reaction. However, it is necessary to emphasize that acidity measurements and catalytic reaction are based on completely different experimental conditions. During dehydration of butanol, water is produced and Lewis acid sites may be converted to Bronsted sites. The results of l-butanol dehydration over the catalysts are presented in Table 4. It reveals an order of conversion: Al-PM > Al-PI-PM> Al-P,-PM, which is correlated with that of Lewis acidity determined by pyridine adsorption (Fig. 5). Moreover, the ratios of 1-butene (1B) to dibutyl ether (DBE) or 2-butenes (2B) to (1B +DBE) decrease in the sequence: Al-PI-PM> Al-PM> Al-P2PM. Compared with the acidity in Figs. 4 and 5 there is an agreement found between the ratios and the total Brernsted acidity. The correlation between Lewis acidity and the conversion probably means that the reaction is limited not by the initial protonation step, but by the neucleophilic attack on a carbocation. According to Tada and Sakai et al. [42,44], it is also likely that the lack of weak basic sites is responsible for the loss of the activity of the Al-P,PM catalyst, while the substantial conversion loss of the Al-P,-PM catalyst could be due to the considerable reduction of its specific area. On the other hand, the sequence of the ratios of lB/DBE and 2B/ (1B + DBE ) is correlated with the variation of medium and strong acid sites, as can be seen from Fig. 4. The small amount of butyraldehyde (BAL ) produced could be due to some basic sites, which are not characterized in this study. In addition, one can also find from Table 4 that the Al-PI-PM catalyst shows a much higher conversion than the Al-P,-PM catalysts but almost the same total product yields. This may be ascribed to the formation of some other kinds of products which have not be detected in the columns used. The same finding

105

has been made about other Al-PM catalysts modified with phosphorus compounds with large amounts of total acidity [ 451. CONCLUSIONS

The phosphorus compounds, H3P03 and H,PO,, have a significant effect on the thermal stability, specific area, acidity and catalytic properties of Al-pillared clay. The following conclusions may be drawn. H,PO, largely decreases the thermal stability of the Al-PM while H,PO, slightly improves it. This phenomenon is due to the greater availability of hydrogen ion of the former acid and easier formation of AlPO, precipitates or solid compounds in the Al-P2-PM sample. Too much hydrogen ion can induce the following effects: (1) decomposing aluminum polycations such as (Al,,O,-++; (2) competitively exchanging with Na+ or other cations on clay sheet or/and even on pillars; (3) interacting with AlgPOB pillars in the solid sample. The formation of AlPO, also causes the decomposition of aluminum polycations and collapse of pillars. H,PO, can block micropores of Al-PM much more severely than H3P03. This, along with its deleterious effect on thermal stability, results in the pronounced loss of the BET area of the Al-P,-PM. H,PO, significantly improves Bronsted acid sites and total acidity, but slightly decreases Lewis acid sites of the Al-PM. The acidity improvement occurs on the acid sites carried on the pillar surface or in the linkage of pillars and clay, rather than on clay sheet. H,PO, largely reduces both Lewis and Bronsted acid sites, mainly due to the loss of the BET area. A correlation between Lewis acid sites and the conversion of 1-butanol dehydration is obtained while the total acidity of Bronsted acid sites can be correlated with the ratios of lB/DBE and 2B/ (1B + DBE). The loss of the conversion is probably due to the lack of weak basic sites for the Al-PI-PM catalyst and to the reduction of BET area for the Al-P,-PM. ACKNOWLEDGEMENTS

We are grateful to the European Economical Community for a grant (H.F.S. ) and Services de la Programmation de la Politique Scientifique (SPPS), Belgium, for financial support. We also thank the following persons for the technical help: Messrs. E. Ponthieu, E. Laurent, A. Vieira and M. Genet. REFERENCES

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