Acid Properties Of A Bidimensional Zeolite

B. Imelik et al. (Editors), Catalysis by Acids and Bases

© 1985 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

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ACID PROPERTIES OF A BIDIMENSIONAL ZEOLITE D. PLEE 1, A. SCHUTZ 2, G. PONCELET 2 and J.J. FRIPIAT 1 l C.N. R. S. - C.R.S.O.C.I., rue de la Ferollerie, 45045 Orleans (France) 2Groupe de Physico-Chimie Minerale et de Catalyse, Place Croix du Sud 1, 1348 Louvain-la-Neuve (Belgium). ABSTRACT Montmorillonite and synthetic beidellite have been intercalated with hydroxyaluminum polymers and characterized by ~everal techniques. The basal 001 reflection of the pillared clays is about 18 A and calcination slightly decreases it. From MAS-NMR measurements, it turns out that the pillaring agent is the All~ polymer. Calcination of pillared montmorillonite (with octahedral substitutlons) does not bring about modifications in the tetrahedral layer, whereas it does in pillared beidellite (with tetrahedral substitutions), in which coupling between an OH apex of an inverted Al tetrahedron and an OH of an Al octahedron from the Al13 polymer occurs, resulting in the exposure of the negative charge of the Al tetrahedron in the open in the interlamellar space. The thermal activation of pillared beidellite provokes protonation of Si IV0-A1IV linkages (infrared data). These protonated sites are strongly acid. INTRODUCTION Brindley et al. (ref.l) and Lahav et al. (ref.2) were the first to show that montmorillonite and, more generally, dioctahedral phyllosilicates may be expanded in a thermally stable structure by pillaring the bidimensional lattice ° with aluminum hydroxypolymers. The basal 001 refle~tion is slightly above 18 A in the air-dried solid. It decreases of about 0.5 A after calcination between ° 300 and 500°C. The free interlamellar space has thus a thickness of about 7.5 A. The BET surface area of these pillared clays is in the range of 250 - 300 m2/g. As shown further the acidity measured by the combined pyridine adsorption and infrared technique belongs mainly to the Bronsted type and in pillared montmorillonite the dinsity of acid sites decreases quite rapidly above 200°C. Indeed, the catalytic acitivity in hydrocracking and hydroisomerization is far below that of the ultrastable Y (USY) zeolite. The acid sites in pillared montmorillonite may be of two kinds according to their localization either on the surface of the clay or on that of the pillar. The main source of acidity of the montmorillonite surface is the hydration water (ref.3) and of course it fades away as dehydration progresses. The nature of the acid sites of the pillar is an open question which could be answered only if the nature of this pillar was clearly established. Vaughan et al. (ref.4) and Pinnavaia (ref.5) have suggested that the pillaring cation

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is most likely an Al 13 polyhydroxypolymer related to the known cation the schematic structure of which is shown in Fig. 1. The characteristic feature of this Al 13 polymer is the tetrahedral aluminum in the heart of 3 layers of aluminum octahedra. This structure is composed from the four ° spacing and its layers of oxygen or hydroxyls requir;d to obtain the dOOl 18 A 2. surface area is of the order of 110 A This provides enough void space to account for a rather large surface area. Till recently, nothing was known about the nature of the transformation of the Al 13 species after thermal activation above 300 "C. The formation of a spinel-like structure as those obtained upon calcining bayerite or gibbsite could be speculated. In that case the pillar would contribute to the acidity mostly by Lewis sites. Pillaring beidellite, e.g. a dioctahedral smectite with Si by Al substitutions, may create another source of acidity. Indeed, the Si-0-A1 IV linkage in that smectite is easily attacked by protons. It has been shown, for instance (ref. 6),that by decomposing an ammonium beidellite, Si-OH infrared .0 stretching bands appear at 3500 cm -1 and DOH @Hp 3420 cm- l. These bands were assigned to silanol groups formed upon a deamination reaction,as suggested by Uytterhoeven et al. for X and Y zeolites (ref. 7). The Al 13 polymer Fig. 1. Exploded representation of the Al13 polymer. being an acid, it might be anticipated that the thermal activation of the pillared beidellite could create similar Bronsted acid sites and enhanced catalytic properties related to acidity. The aim of this paper is thus to describe the structure of the solid resulting from the thermal activation of the pillared beidellite and its acid properties. Pillared montmorillonite will be used for comparison.

Al1304(OH)~4'

STRUCTURE AND TEXTURE OF PILLARED SMECTITES ° pillared smectites has been The preparation procedure used for obtaining 18 A described by Schutz et al. (ref. 8). Fundamenta l ly ,a clay slurry of a 3% (weight by weight) of the <2~ fraction is carefully di3persed. Separately, a 0.4 M Al(N03)3.9H20 solution is neutralized by a 0.3 MNaOH solution in order to adjust the molar OH/Al ratio in the range 1.2 to 1.8. This solution is aged at 50 °c for one hour and added to the clay slurry such as to obtain a final concentration between 20 and 30 meq Al per gram of clay. After stirring vigorously

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the mixture for 1 hour, the suspension is dialyzed for 4 days in a cellulose membrane in contact with distilled water. The suspension is lyophilized and then calcined between 300 and 500 DC. Fig. 2.1. shows the X-ray diffractogram of the pillared montmorillonite after calcining at 500 DC. The average structural formula obtained from 6 different samples prepared following this procedure after calcination at 900 DC, is VI . IV Na O. 064' 1.65 Al 13, (5'8) ,(Alleo.36t190.64) ,0 24. 2 assuming that pillars are made from 13 Al atoms as shown in Fig. 1. Al 13 in this formula stands for one pillar aluminum. Beidellite is synthesized and pillared according to the procedure described by Plee et al. (ref. 9). Its average formula is, from 5 different samples calcined at 900 DC : Na O. 9' (5i 7. 1A1 0. 9)IV, Al~I 022 The abundance of natural beidellite is so small that a systematic study would not have been possible. When pillared with the same procedure as that described above, the structural formula is : Na O. 06' 1.Sl Al 13, (5i 7. 1A1 0. 9)IV, Al~I, 023. 8 again on the basis of the solid calcined at 900 DC. The X-ray diffractogram of the pillared beidellite is shown in Fig. 2.2. It is very similar to that of pillared Wyoming bentonite. Because of the small number of 001 reflections, no quantitative infomation can be obtained from the diffractograms except that both types of solids have a turbostratic structure poorly ordered along the c axis. The most intense reflections are undoubtedly due to the 001 reflections of layers with stacking defects and perhaps some mixed layers contribution. Thus,as far as the long range order is concerned, pillared montmorillonite can18'\ not be distinguished from pillared beidellite. The specific surface area, microporous and porous volumes, ® obtained from the BET plot,the Dubinin equation (ref. 10) and 3.95'\ the total porous volume observed for the adsorption at -190 DC are shown for these two pil528 15 10 20 25 35 30 lared smectites in Table 1. Fig. 2. X-ray diffractogramme of 1) pillared The calcined pillared montmomontmorillonite and 2) pillared beidellite. rillonite (CPM) exhibits a specific surface area 20 % lower than that of calcined pillared beidellite (CPB).

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However, the main difference in between the microporous volumes is about 40 % less for CPM. The microporous volume calculated by assuming an interlayer diso tance of 7.5 A and an hexagonal packing of the pillars is also shown in Table 1. The rather good agreement between the calculated and observed values for CPB suggests a more ordered distribution of the pillars in CPB, as compared to CPM. TABLE 1 Surface area of calcined pillared montmorillonite (CPM) and calcined pillared beidellite (CPB). Total porous volume, microporous volume, residual CEC and calculated microporous volume for these 2 samples dried at 220 °c before N2 adsorption (see ref. 9). Sample

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0.18 0.19

SHORT RANGE ORDERING IN PILLARED SMECTITES It is clear that the results which are sketched above do not bring any clear distinction between pillared montmorillonite and beidellite. For this reason the study of short range ordering has been undertaken using high-resolution solid state 27Al and 29Si nuclear magnetic resonance under the conditions of magic-angle spinning (MAS-NMR). The 29Si and 27Al spectra were recorded using two spectrometers operating at 8.45 and 11.7 Tesla and spinning frequencies of about 2.6 and 3.5 kHz, respectively. The results of this study are reported at large in a paper by Plee et al. (ref. 11). Because of the high Fe content in paramagnetic impurities (mainly Fe3+) in Wyoming bentonite, pillared hectorite and laponite (trioctahedral smectites without tetrahedral substitutions) were compared with pillared beidellite, before and after calcination. The conclusion of this study can be summarized as follows: 1) The Al 13 polymer is indeed the pillaring agent for all the investigated smectites. 2) The calcination of the pillared clays doesn't lead to the transformation of the pillar into a pseudo-spinel structure. 3) The calcination of pillared smectites without tetrahedral substitutions doesn't lead to a modification of the tetrahedral layer of the sheet silicate. 4) The calcination of pillared smectite with tetrahedral substitutions (beidellite) modifies the tetrahedral layer and leads to a structural modification of the pillar. The proposed structure is shown in Fig. 3. The reaction of the hydroxylated pillar with the tetrahedral layers results

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from coupling the OH apex of an inverted aluminum tetrahedron and an OH of an aluminum octahedron belonging to Al 13. This leads to a Si-0-A1 IV linkage in which the negative charge of the inverted tetrahedron is no longer buried into a continuous tetrahedral network but is in the open in the interlamellar space. Pillaring beidellite with Al 13 and thermal activation would thus result in seeding the growth of a tridimensional network grafted on the bidimensional network of the clay. The resulting high area solid could be considered as a bidimensional zeolite. ACIDITY Protons from different sources may be at the origin of the acidity of pillared clays.The water molecules belonging to the hydration shell of .0 charge balancing cations are submitted to a strong electrical polarizing field and therefore they have a degree of dissociation several orders of magnitude larger than liquid water (ref. 3). Water molecules hydrating the aluminum pillars Fig. 3. Schematic view of Al13 are from that point of view a potential source of in the interlamellar space. acidity whereas it is known that the hydroxyl groups of the clay octahedral layer do not contribute to acidity. In beidellite, protons can be captured by tetrahedral SiIV_0_A1 IV linkages, yielding SiIV-OH ... A1 IV groups similar to those found in Y zeolites (ref. 6). Except for the very small number of tetrahedral sites where Si by Al substitution occurs in montmorillonite, this possibility does not exist in montmorillonite. Indeed, as shown by spectra (1) and (2) in Fig. 4, upon calcining PB, a rather strong OH band appears at 3440 cm- 1 whereas a weak shoulder is observed in CPM. It is not possible, by IR spectroscopy ,to distinguish between SiIV_0_A1 IV groups used for linking pillars to clay tetrahedral sheets through inverted Al tetrahedra with those which are not used for that purpose. Pyridine is a very well documented probe for acidity measurements. Self-supporting wafers of pillared clays were outgassed in the IR cell and the IR spectra were recorded in the 3000-3800 cm- 1 and 1400-1700 cm- 1 regions before and after adsorption of pyridine and evacuation of excess reagent at increasing temperature. For CPM as well as CPB, more than 90% of the acid sites are Bronsted sites protonating pyridine into pyridinium as identified easily by a characteristic

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IR band at 1540 cm- 1.

1(1'1-11 1600

1500

1400

Fig. 4. IR spectra. PB calcined at 400°C (1); PB + pyridine (2); PM calcined at 400°C (3); PM + pyridi ne (4).

The amount of chemisorbed pyridinium retained by pillared clays depends upon the pretreatment temperature at which the solid is activated and on the outgassing tempera ture. As shown in Fig. 5 A for CPM, increasing the pretreatment temperature decreases to a large extent the surface density in chemisorbed pyridinium. Also increasing the outgassing temperature works along the same way. For CPB, however, pretreating the solid between 350 and 500 °C increases the surface density in pyridinium, and the effect of the outgassing temperature is to decrease it to a lesser extent than in CPM (Fig. 5 B). Before interpreting these observations, it is worth summarizing the thermal behavior of the two pillared clays. The maximum of the endothermic peak resulting from the loss of physically adsorbed water is observed at about 100°C and the peak extends up to about 250 °C.

It is followed in PB by two well-defined endothermic peaks at 330°C and 540 °C, assigned to the pillar dehydration (or partial dehydroxylation) and to the loss of the octahedral OH's of the clay lattice, respectively. In PM, a continuous endothermic effect extends from about 250°C to 700 °C with maxima at 330°C and 630 °C. The coincidence between the temperature at which pillars dehydrate and the temperature at which the number of acid sites is reinforced (Fig. 5 B) suggests strongly that some of the protons libe\ated above 350°C are responsible for the formation of SiIV-OH ... A1 IV groups. That these groups are indeed acid is shown by their disappearance when CPB has adsorbed pyridine, as shown in spectrum 2, Fig. 4. Thus the nature and the strength of the acid sites in CPB offer a striking similitude with those in tridimensional zeolites. This is not the case for CPM. Thus from this point of view also CPB deserves the appelation of bidimensiona1 zeo1He. It could be objected that the network of pores in pillared clays is not

349

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G.W. Brindley and R.E. Sempels, Clay Min., 12 (1977) 229-237. N. Lahav, U. Shani and J. Shabtai, Clays and Clay Min., 26 (1978) 107-115. M.M. Mortland and K.V. Raman, Clays and Clay Min., 16 (1968) 393-398. D.E.W. Vaughan and R.J. Lussier, Proc. 5th International Conference on Zeolites, Editor L.V. Rees, Heyden and Sons U.K., 1980, pp. 94-101. T.J. Pinnavaia, Science, 220 (19B3) 365-371. B. Chourabi and J.J. Fripiat, Clays and Clay Min., 29 (1981) 260-268. J.B. Uytterhoeven, L.G. Christner and W.K. Hall, J. Phys. Chem., 69 (1965) 2117-2126. A. Schutz, G. Poncelet and P. Jacobs, Fr. Patent 81.16 387 (1981). D. Plee, L. Gatineau and J.J. Fripiat,Submitted to Clays and Clay Min.

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10. M.M. Dubinin, J. Coll. Interf. Sci., 23 (1967) 489-498. 11. D. Plee, F. Borg, L. Gatineau and J.J. Fripiat, Submitted to J. Am. Chem.Soc. ACKNOWLEDGMENT The authors acknowledge the Compagnie support.

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