Microporosity and Acidity Properties of Alumina Pillared Titanates

Jan 29, 1999 - The micropore volume values were related to the amount of alumina and to the heat-treatment temperature. Most of the pillared titanates...
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Langmuir 1999, 15, 1090-1095

Microporosity and Acidity Properties of Alumina Pillared Titanates Fathi Kooli,* Takayoshi Sasaki, and Mamoru Watanabe National Institute for Research in Inorganic Materials, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

Cristina Martin and Vicente Rives Departamento de Quimica Inorganica, Universidad de Salamanca, 37008 Salamanca, Spain Received August 4, 1998. In Final Form: December 7, 1998 A titanate with lepidocrocite-like structure has been intercalated by hexylammonium (HA) and tetrabuthylammonium (TBA) organic cations, which were further exchanged with aluminum polyoxocations. The pillared structure of these materials was found to be stable up to 500 °C and exhibited a basal spacing of 1.4 nm. Nitrogen adsorption isotherms were measured to determine the surface areas and pore structure of these pillared titanates, and the effect of intercalation temperature and heat treatment on these surface characteristics is discussed. The micropore volume values were related to the amount of alumina and to the heat-treatment temperature. Most of the pillared titanates prepared from HA-Ti showed a sharp distribution of micropores with the average diameter of 2 nm, whereas the comparable materials obtained from TBA-Ti showed a broad distribution of pore diameter centered 4 nm in addition to micropores. Infrared spectroscopy monitoring the pyridine adsorption showed the presence of Lewis acid sites, and evolution of surface Bro¨nsted acid sites upon incorporation of the alumina pillars. There is a good correlation between the amount of alumina and the acidity in terms of proton concentration (Bronsted) as determined by adsorption/desorption of cyclohexylamine.

Introduction Pillared interlayered clays (PILCs) have caught much attention as a new type of microporous solid. These materials are usually prepared by ion exchange of interlayer cations in swollen clays with bulky polyoxocations. Upon calcination, the intercalated species are converted to metal oxide clusters which prevents the interlayer gallery from collapsing and creats microporosity associated with large surface area.1 The nature of host clays,1-3 the pillaring species,1,4-6 and calcination temperature7,8 can alter the microporous properties of PILCs. In addition, the acidity of PILCs has been improved as compared to their starting clays. The acidity is related to the type and the amount of interlayer pillars as well as the nature of the parent clays.1,3,9 Those metal oxide pillars in themselves are thought to be partially acidic.10,11 The acidity results from the protons produced by dehydroxylation of cationic pillars upon calcination,12 according to the following overall reaction, (1) Ohtsuka, K. Chem. Mater. 1997, 9, 2039, and references therein. (2) Sterte, J. Clays Clay Miner. 1991, 39, 167. (3) Kooli, F.; Bovey, J.; Jones, W. J. Mater. Chem. 1997, 7, 153. (4) Gil, A.; Montes, M. Langmuir 1994, 10, 291. (5) Yamanaka, S.; Inoue, Y.; Hattori, M.; Okumura, F.; Yoshikawa, M. Bull. Chem. Soc. Jpn. 1992, 65, 2494. (6) Sterte, J. Clays Clay Miner. 1986, 34, 658. (7) Tichit, D.; Fajula, F.; Figueras, F.; Ducoirant, B.; Mascherpa. G. Clays Clay Miner. 1988, 36, 369. (8) Brandt, K. B.; Kydd, R. A. Chem. Mater. 1997, 9, 567. (9) Bagshaw, S. A.; Cooney, R. P. Chem. Mater. 1993, 5, 1101. (10) Occelli, M. C.; Tindwa, R. M. Clays Clay Miner. 1982, 31, 22. (11) Figueras, F. Catal. Rev.-Sci. Eng. 1988, 30, 457. (12) Pinnavaia, T. J.; Landau, S. D.; Tzou, M. S.; Johnson, I. D. J. Am. Chem. Soc. 1985, 107, 7222.

2[AlO4Al12(OH)24(H2O)12]7+ f 13Al2O3 + 41H2O + 14H+ and is also associated with pillar-to-layer bonding sites on the clay sheets.11 It has been found that these PILCs have both Lewis and Bronsted acidity, and the ratio of Bronsted/Lewis acidity decreases as the calcination temperature increases. Above 400 °C, the acidity was mainly due to Lewis acid sites. A great deal of interest has been paid to pillaring of layered metal oxides with inorganic polycations in order to apply as catalyst supports, molecule sieves and etc.,13-16 such as silicates, metal phosphates, and double-layered hydroxides. Some of us have reported the incorporation of polyoxocation (Al13O4(OH)24-x(H2O)12+x)(7-x)+ (abbreviated as Al13) into a layered titanate with a lepidocrocitetype framework as well as the examination of the physicochemical properties of these new pillared titanates.17 Here we report detailed characterization of these pillared titanates in term of external surface area, micropore volume, and pore size distribution as well as their acidity associated to Bronsted and Lewis acid sites. Experimental Part Preparation. A titanate of Cs0.7Ti1.82500.175O4 was prepared by solid-state reaction of Cs2CO3 and TiO2 at 800 °C for (13) Landis, M. E.; Aufdembriuk, B. A.; Chu, P.; Johnson, I. D., Kirker, G. W.; Rubin, M. K. J. Am. Chem. Soc. 1991, 113, 1189. (14) Olivera-Pastor, P.; Maireles-Torres, P.; Rodgriguez-Castellon, E.; Jimenez-Lopez, A.; Cassagneau, T.; Jones, D. J.; Roziere, J. Chem. Mater. 1996, 8, 1758. (15) Nazar, L. F.; Liblong, S. W.; Yin, X. T. J. Am. Chem. Soc. 1991, 113, 5889. (16) Drezdzon, M. A. Inorg. Chem. 1988, 27, 4628. (17) Kooli, F.; Sasaki, T.; Watanabe, M. Microporous Mesoporous Mater., in press.

10.1021/la9809776 CCC: $18.00 © 1999 American Chemical Society Published on Web 01/29/1999

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Table 1. Aluminum Uptake and Surface Acidity of Pillared Titanates Calcined at 500 °C sample

mmol of Al g-1

mmol of H+ g-1

H-Ti HA-TiCl16rt HA-TiCl1680 HA-TiN1680 TBA-TiN1680 HA-TiN480 HA-TiN880 HA-TiN2480

0 3.06 4.75 4.42 5.42 3.58 4.11 4.91

0.10 0.38 0.35 0.44 0.17 0.25 0.62

20 h repeated twice.18 The protonated form of Cs-titanate (H0.7Ti1.82500.17O4‚H2O, H-Ti) has been obtained by leaching it in aqueous HCl solution (1 mol dm-3) at room temperature.19 The leaching was repeated by replacing the solution with a fresh one every 24 h for four times. Two host titanates were derived from this protonated titanate. One is hexylammonium titanate (abbreviated as HA-Ti) which was obtained by intercalation of hexylammoinium cations into H-Ti. The other host material is tetrabuthylammonium titanate (TBA-Ti) which was prepared via exfoliation of H-Ti in a TBAOH solution followed by subsequent restacking upon freeze-drying.19 These host titanates were further treated in a pillaring solution either at room temperature or at 80 °C with different ratios (R) of Al (mmol)/ titanate (g). The pillaring solution containing mainly Al13 cations was prepared by hydrolyzing aluminum chloride (Cl) or nitrate (N) solution with a tetramethylammonium hydroxide (TMAOH) solution adjusted at a OH/Al ratio of 2.5. Precursor materials intercalated with Al13 cations were collected by centrifugation, washed with distilled water, and then dried at room temperature; the detailed procedure has been reported elsewhere.17 Pillared titanates are labeled as M-TiXRT(C), where M-Ti represents starting materials (e.g. HA-Ti or TBA-Ti), X denotes the anion (Cl or N) of the starting aluminum solution, R is the ratio of Al (mmol)/titanate (g), T is the pillaring temperature of the Al13 cations, and C stands for the calcination temperature of the precursors. Different precursor materials were calcined at temperatures between 300 and 750 °C for 4 h in air. The term “pillared” is used for materials obtained after calcination. Characterization. Powder X-ray diffraction (PXRD) patterns were collected on a Rigaku Rint-2000 system with Cu KR radiation. Elemental analyses for Ti and Al were carried out by colorimetry and titration with EDTA, respectively, after dissolving the sample with a mixed acid solution of H2SO4 and HF. Specific surface areas were calculated according to the BET method from nitrogen adsorption isotherms measured with a Belsorp 28SA instrument. Samples were outgassed under vacuum for 8 h at 200 °C prior to analysis. Pore volume, t plots, and pore size distribution were determined using a computerized program developed by Rives.20 The concentration of the Bronsted acid sites (proton concentration) was estimated by the cyclohexylamine adsorption/ desorption method.21 The thermogravimetric analyses were performed on a Mac-ScienceTGA-DTA 2000S instrument under nitrogen gas flow (25 cm3 min-1). The weight loss between 240 and 420 °C was used to deduce the acidity in terms of millimoles of cyclohexylamine desorbed per gram of sample. It is assumed that each molecule of the base (cyclohexylamine) interacts with one protonic (Bronsted) acid site. 21,22 To distinguish between Bro¨nsted and Lewis acid sites,23 pyridine adsorption was monitored by the Fourier Transform Infrared (FT-IR) spectroscopy using a Perkin-Elmer 16PC FTIR spectrometer, equipped with a specially designed vacuum cell with CaF2 windows (IR radiation transparent above ca. 1000 (18) Grey, I. E.; Madsen, I. C.; Bursill, L. A. Acta Crystallogr. 1987, 66, 7. (19) Sasaki, T.; Nakano, S.; Yamauchi, S.; Watanabe, M. Chem. Mater. 1997, 9, 602. (20) Rives, V. Adsorpt. Sci. Technol. 1991, 8, 95. (21) Breen, C. Clay Miner. 1991, 26, 487. (22) Ballantine, J. A.; Purnell, J. H.; Thomas, J. M. Clay Miner. 1983, 18, 347. (23) Parry, E. R. J. Catal. 1963, 2, 371.

Figure 1. Nitrogen adsorption-desorption isotherms of samples: (a) H-Ti, (b)HA-TiCl16r, (c)HA-TiC1680, and (d)TBATiN1680. All of them were calcined at 500 °C. Open square and circle express adsorption and desorption branches, respectively. cm-1); 100 scans (nominal resolution 4 cm-1) were averaged to improve the signal-to-noise ratio.

Results and Discussion Pillared Titanates. The effect of synthetic conditions on the properties of pillared titanates has been reported in detail in our previous work.17 The pillaring temperature as well as the starting ratio (R) affects the amount of the intercalated aluminum in HA-Ti (Table 1). The crystallinity of the pillared materials is related to the nature of their starting host titanates. Pillared materials obtained from hexylammoniun titanate are more crystallized than those prepared from TBA-Ti. The thermal stability of pillared titanates is dependent on the amount of intercalated aluminum and on the calcination temperature. Adsorption Isotherms. Figure 1 shows the nitrogen adsorption-desorption isotherms for H-Ti and pillared titanates calcined at 500 °C in the air. The shape of the isotherm for the calcined H-Ti corresponds to type IV in IUPACs classification,24 being characteristic of a nonporous material. HA-TiCl1680(500) gives the characteristic shape of type I in a lower relative pressure range which probably arises from interlayer cavities produced by pillaring and the shape of type IV in a higher relative pressure range. The isotherm of the pillared HA-TiCl16rt(500) is very similar in shape to the H-Ti, except for a larger amount of adsorbed N2. No hysteresis loop was observed for the pillared materials from HA-Ti. However, the pillared materials from TBA-Ti showed a hysteresis loop of type H3 (Figure 1), indicating the presence of mesopores.25 The origin of this hysteresis is likely to be related to the morphology and the shape of the flaky TBATi particles compared to HA-Ti ones and/or to the platelets stacking. (24) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, T.; Siemienewska, T. Pure Appl. Chem. 1985, 57, 603. (25) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982.

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Table 2. Surface Areas of Pillared Titanates Calculated Using the BET Equationa sample

SBET (m2 g-1)

CBET

H-Ti(500) HA-TiCl16rt(500) HA-TiCl1680(500) HA-TiN1680(500) TBA-TiN1680(500) HA-TiN480(500) HA-TiN880(500) HA-TiN2480(500)

17 36 181 215 189 37 105 196

50 96 -204 -109 -448 72 -136 -96

correlation coefficient pore volumeb 0.999 91 0.999 92 0.999 37 0.999 15 0.999 57 0.999 9 0.999 25 0.998 94

0.031 0.060 0.150 0.160 0.202 0.051 0.107 0.142

a The corresponding constant (C BET) and correlation coefficients are also reported. b mL of liquid nitrogen g-1.

Table 3. Pore volume, Surface Area, and Related BET Parameters for HA-TiCl1680 Calcined at Different Temperatures (°C) sample

SBETa

CBET

correlation coefficient

pore volumeb

HA-TiCl1680(25) HA-TiCl1680(300) HA-TiCl1680(400) HA-TiCl1680(500) HA-TiCl1680(600) HA-TiCl1680(750)

176 210 252 181 52 35

-113 -86 -122 -204 67 49

0.998 48 0.998 55 0.990 8 0.999 37 0.999 94 0.999 91

0.148 0.150 0.180 0.150 0.079 0.065

a

Table 4. Variation of Correlation Coefficient in the BET Equation Constant (CBET) with the Subtracted Micropore Volume from the Experimental Isotherm to Estimate the Micropore Volume for HA-TiN1680(500)

m2 g-1. b mL of liquid nitrogen g-1.

The pillared materials of HA-TiCl1680 calcined below 500 °C have a very similar isotherm shape, but their adsorption amounts are different. The material heattreated above 600 °C gave an isotherm clearly different from the above and showed a character of nonporous materials. Surface Area and Microporosity. In Table 2 are collected total pore volumes and specific surface areas of the pillared titanates (calcined at 500 °C) as estimated on the basis of the BET equation. The corresponding BET constant (CBET) and the correlation coefficient of the BET plot are also listed. Specific surface areas and total pore volumes of the pillared titanates are larger than that H-Ti calcined at 500 °C. The variation in these values between various pillared titanates can be attributed to the different amounts of intercalated aluminum and to the type of the host titanates used. The largest specific surface area was obtained for the pillared titanate prepared from HA-Ti at 80 °C with R equal to 16. However, the largest pore volume was observed for the pillared titanate derived from TBA-Ti. As reported for a series of pillared titanates from HATiCl1680 in Table 3, their specific surface area and the pore volume reached the maximum at calcination temperatures of 400 °C and drastically decreased above 600 °C. This large reduction can be due to the collapse of the micropores resulting from the destruction of the pillars and/or to the collapse of the layered structure. The parent titanate (H-Ti) and different pillared titanates calcined between 500 and 750 °C, having low specific surface areas, give a good correlation coefficient and a positive CBET of the BET equation (Tables 2 and 3). However, other pillared titanates gave a low correlation coefficient and a negative CBET. According to the BET theory, CBET is related exponentially to the enthalpy (heat) of adsorption on the first adsorbed layer, and it gives an indication of the adsorbent-adsorbate interaction energy.25 Accordingly, the negative CBET has nonphysical meaning, and the low correlation coefficient indicates that the BET method is not suitable for the calculation of the specific surface area of these materials with primary

a

subtracted micropore volumea

correlation coefficient

CBET

0 0.046 0.062 0.065 0.069 0.077 0.085

0.999 15 0.999 67 0.999 92 0.999 95 0.999 98 0.999 90 0.996 56

-109 -119 124 89 57 25 6

mL (liquid nitrogen) g-1.

micropore filling.26 A similar situation has been reported in the case of La-Al montmorillonites,27 and Al, Ga, and Al-Ga pillared montmorillonites.28 In the case of microporous samples, external surface area and micropore volume provide very important information on their surface textures. The t-plot of de Boer et al.29 is commonly used to calculate micropore volume from vapor adsorption data. Nonporous samples show a straight line passing through the origin on the t-plot, and their specific surface area can be calculated from its slope. In contrast, the t-plot for microporous materials does not result in such a straight line and has a zero intercept according to the equation V ) Vm + 0.001Sextt. In such a case, the external surface area (Sext) is calculated from the slope of the straight part, while the micropore volume (Vm) is calculated from the zero intercept. However, exact estimation of these values should be influenced by the choice of the standard isotherm of nonporous material to estimate the statistical thickness of the adsorbed layer (t) and the range of the t values considered for the linear fitting.28 To avoid this ambiguity we have used another method to determine these parameters, which has been recently applied to PILCs.28 On the basis of this method, the BET equation can be derived from a modified isotherm despite the absence of primary micropore filling. This procedure corresponds to subtraction of the amount of N2 adsorbed on micropores (supposed to be constant in the range of 0.05 and 0.25) from the total experimental isotherm, which gives a good correlation coefficient and a positive CBET. Table 4 typically presents the variation of correlation coefficients with different micropore volumes subtracted from the experimental isotherm for HA-TiN1680(500). The correlation coefficient is improved as the subtracted micropore volume increased and then deteriorated with further subtractions. The expected micropore volume results in the best correlation coefficient. The external surface area is then determined according to the BET equation with the most appropriate correlation coefficient, and the related CBET becomes now positive. Table 5 reports the external surface areas, micropore volumes, and specific surface areas (equivalent to adsorption on micropores) of the pillared titanates determined by the method described above. It is concluded that the pillaring process drastically increases the micropore volume as compared to that of the protonated titanate. The micropore volume increases (26) Branton, P. J.; Sing, K. S. W.; White, J. W. J. Chem. Soc., Faraday Trans. 1 1997, 93, 2337. (27) Sterte. J. Clays Clay Miner. 1991, 39, 167. (28) Remy, M. J.; Vieira Coelho, A. C.; Poncelet, G. Microporous Mater. 1996, 7, 287. (29) de Boer, J. H.; Lippens, B. C.; Linsen, B. G.; Broekhoff, J. C. P.; van den Heuvel, A.; Osinga, Th. J. J. Colloid Interface Sci. 1966, 21, 405.

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Table 5. External and Micropore Surface Areas (m2g-1) of Pillared Titanates Determined Using the BET Equation after Subtraction of Micropore Volume sample

SBET externala

CBET

micropore volumeb

micropore surfacea

H-Ti(500) HA-TiCl16rt(500) HA-TiCl1680(500) HA-TiN1680(500) TBA-TiN1680(500) HA-TiN480(500) HA-TiN880(500) HA-TiN2480(500) HA-TiCl1680(25) HA-TiCl1680(300) HA-TiCl1680(400) HA-TiCl1680(600) HA-TiCl1680(750)

17 36 54 71 100 37 42 45 91 62 62 52 35

50 96 20 56 32 72 64 32 46 25 18 67 49

c c 0.062 0.067 0.046 c 0.031 0.076 0.043 0.077 0.093 c c

c c 174 196 130 c 87 213 122 218 261 c c

a

m2 g-1. b mL (liquid nitrogen) g-1. c Negligible.

in proportion with the density of pillars (see Tables 1 and 5), which has also been reported for pillared clay minerals.4,30-31 The pillared titanates from HA-Ti have an external surface area lower than those prepared from TBA-Ti. This fact would be attributed to smaller crystallite size of TBA-Ti. It has been reported that about 10-20 titanate sheets were restacked to form TBA-Ti upon freeze-drying of exfoliated titanate suspension.19 On the other hand, the microcrystallites of HA-Ti are composed of roughly several hundred of the titanate sheets. When the calcination temperature of HA-TiCl1680 increased, the micropore volume once increased and then sharply decreases. The increase in the first stage can be ascribed to the transformation of bulky Al13 cations to smaller oxide pillar clusters via dehydroxylation which may produce more free space between the pillars. At the temperatures above 600 °C, the estimated micropore volume was almost negligible due to the collapse of the micropores or degradation of the pillars. In fact the PXRD patterns showed that the intercalated structure was destroyed and a mixture of alumina and titania (anatase and rutile forms) crystallized.17 f-Plots. The f-plot, developed by Gregg,32 provides an easy method to compare a difference in porosity between two different samples or different states of the same sample. The f-plot is regarded as a plot of the ratio between the amounts adsorbed by a sample and a reference at the same relative pressures. The reference was usually a nonporous sample with a low specific surface area; if the sample is the same as the reference (in term of pore distribution), a straight horizontal line is obtained with a zero intercept on the f-axis of 1.0. An upward deviation means that the sample has an adsorption capacity larger than that of the reference. The f-plots of HA-TiCl16rt(500) and HA-TiCl1680(500) are shown in Figure 2, using H-Ti calcined at 500 °C as the reference. HA-TiCl16rt(500) gave an almost horizontal line with f slightly above 1.0. Both HA-TiCl16rt and H-Ti have similar textural properties (Figure 2a). However, HA-TiCl1680(500) shows a large upward deviation especially at low relative pressures in its f-plot (Figure 2b). This material includes a large amount of pores, compared to the reference (H-Ti). The effect of the starting host titanates on the f-plot is shown in Figure 3, where HA-TiN1680(500) is compared to TBA-TiN1680(500) as a reference. The upward devia(30) Vieira-Coelho, A. C.; Poncelet, G. Appl. Catal. 1991, 77, 303. (31) Kooli, F.; Jones, W. Chem. Mater. 1997, 9, 2913. (32) Gregg, S. G. J. Chem. Soc., Chem. Commun. 1975, 699.

Figure 2. f-Plots of (a) HA-TiCl16rt(500) and (b) HA-TiCl1680(500) pillared titanates (H-Ti calcined at 500 °C is taken as reference).

Figure 3. f-plot of HA-TiN1680(500) pillared titanate to TBATiN1680(500).

tion at P/P0 lower than 0.5 indicates that narrow pores are developed in HA-TiN1680 compared to TBATiN1680. However, large pores are detected in TBATiN1680 as shown by the downward deviation at P/P0 higher than 0.5. The f-plots of HA-TiCl1680 calcined at different temperatures are shown in Figure 4. The pillared titanate calcined at 750 °C, which had the lowest specific surface area and was regarded as nonporous, was used as reference. All the samples calcined below 500 °C showed a larger adsorption capacity at lower relative pressures indicating a large amount of pores with narrow diameters. The f-plot of HA-TiCl168(600) approximates the horizontal line with f equal to 1.0 (Figure 4). This dramatic change in the texture at 600 °C could be assigned to the collapse of the layered structure. Pore Size Distribution. The pore size distribution curves are given for typical samples in Figure 5. Only a small number of pores exist in H-Ti and in HA-TiCl16rt(500). On the other hand, the pillared titanate HATiCl1680(500) showed that a narrow distribution of micropores with a diameter of 1 to 2 nm had been produced. The increasing amount of intercalated aluminum enhances the formation of micropores in pillared titanates.

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Figure 4. f-Plots of (a) precursor HA-TiCl1680 and (b) calcined at 300 °C, (c) at 400, (d) at 500, and (e) at 600 °C. HA-TiCl1680(750) is taken as reference. Figure 6. FT-IR spectra recorded after adsorption of pyridine at room temperature on sample H-Ti (calcined at 500 °C) and outgassing at different temperatures.

Figure 5. Pore size distribution for pillared titanates calcined at 500 °C: (a) H-Ti; (b) HA-TiCl16rt; (c) HA-TiCl1680; (d) HATiN1680; (e) TBA-TiN1680.

The pore size distribution was also affected by the nature of the host materials. While pillared HA-TiN1680(500) was mainly microporous, TBA-TiN1680(500) had a large amount of mesopores with diameter around 4 nm in addition to micropores below 2 nm (Figure 5e). These characteristic micropores of HA-TiCl1680 were maintained up to 500 °C, but were not observed for materials calcined above 600 °C. These micropores are removed by the destruction of the pillars. Acidity Characterization. Bro¨ nsted Acidity. The acidity values have been quantitatively estimated from the thermal desorption studies of cyclohexylamine; the results are given in Table 1 in terms of millimoles of protons per gram of pillared titanate. The Bro¨nsted acid sites measured by this method are strong enough to interact with the base after being heated at 240 °C. The H-Ti calcined at 500 °C had a negligible amount of Bro¨nsted acid sites; however, after pillaring the number of acid sites increased. The acidity of these materials depended on the synthesis conditions and on the nature of the host titanates. The number of acid sites increased as the amount of intercalated aluminum increased for a

series of materials prepared from HA-Ti. Similar observations have been reported previously for alumina-pillared saponites 31 and montmorillonites clays.33 The acidity of the TBA-TiN1680 pillared titanate was larger than that of the material prepared from HA-Ti under the same conditions. This difference could be attributed to the amount of uptake aluminum. Bro¨ nsted and Lewis Acidity. FT-IR spectroscopy of pyridine was used to probe the nature of the acid sites of the pillared titanates. The FT-IR spectrum of pyridine adsorbed on H-Ti (calcined at 500 °C) was similar to that adsorbed on other titanium oxides.34,35 After outgassing at room temperature, adsorption bands were observed at 1605, 1575, 1490, and 1445 cm-1, as displayed in Figure 6, which are ascribed to modes 8a, 8b, 19a, and 19b of pyridine bound to surface Lewis acid sites, respectively. When the outgassing temperature increased up to 300 °C, the intensity of all the bands was markedly reduced but still observed. Figure 7 shows representative spectra recorded after adsorption of pyridine on pillared HA-TiCl16rt(500) (Figure 7A) and HA-TiCl1680(500) (Figure 7B) and after outgassing at different temperatures. New absorption bands at 1635 and 1545 cm-1 are due to modes 8a and 19b of the pyridinium ion, thus demonstrating the presence of Bro¨nsted acid sites. Those were absent in the H-Ti calcined at 500 °C (see Figure 6). In addition, other bands recorded at 1615, 1605, 1595 (shoulder), 1490, and 1450 cm-1 are originated to pyridine bound on Lewis acid sites. The band due to the mode 8a splits into three absorption bands at 1615, 1605, and 1595 cm-1 assigned to different types of Lewis acid sites associated with Al species. The similar split of this mode has been reported in the case (33) Bovey, J.; Jones. W. J. Mater. Chem. 1995, 5, 2027. (34) Miyata, H.; Nakagawa, Y.; Naguno, H. J. Chem. Soc., Faraday Trans. 1 1983, 79, 8343. (35) Martin, C.; Martin, I.; Rives, V.; Damyanova, S.; Spojakina, A. Spectrochim. Acta, Part A 1995, 51, 1837.

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and 1545 cm-1, or Bro¨nsted acid sites, were still present at 200 °C. The band at 1615 cm-1 should be assigned to pyridine bound to octahedral Al3+ sites, since it slightly shifted toward a higher wavenumber (e.g., 1619 cm-1).39 The band at 1605 cm-1 could be ascribed to pyridine absorbed on Ti4+ sites in the titanate sheets, as found in the H-Ti sample (Figure 6). At 400 °C, the Bronsted acid sites were completely absent and only Lewis acid sites remained. The intensity change of the band 8a was observed for pillared titanates HA-TiCl16rt(500) and HA-TiCl1680(500). For HA-TiCl1680(500) the band at 1615 cm-1 (pyridine bound to Al sites) was stronger than that at 1605 cm-1 (pyridine on Ti sites) compared to HA-TiCl16rt(500) (Figure 7B and Figure 7A). Qualitatively, the intensity of the 19b mode of the pyridinium ions was also stronger in HA-TiCl1680(500) than in HA-TiCl16rt(500) indicating a fewer number of Bro¨nsted acid sites in the latter. This difference would depend on the amount of intercalated aluminum. The creation of Bro¨nsted acid sites on pillared titanates with alumina could be associated to formation of Ti-OAl-OH sites on the analogy of the sites Si-O-Al-OH reported by other authors9,10,40 for alumina-pillared montmorillonites. However, it has been reported that incorporation of titania to alumina by the following different methods (e.g., impregnation, physical mixing) produces solids where only surface Lewis acidity is observed without any sort of chemical M-O-M′ (M ) Ti; M′ ) Al) interaction.35,41 Concluding Remarks

Figure 7. FT-IR spectra recorded after adsorption of pyridine at room temperature and after outgassing at different temperatures on pillared titanates (A) HA-TiCl16rt(500) and (B) HA-TiCl1680(500). Asterisk (*) corresponds to physisorbed or hydrogen-bound pyridine.

of pyridine adsorbed on R-alumina.36-38 Other bands at 1575-1590 and 1440 cm-1 were assigned to physisorbed or hydrogen-bound pyridine, because those bands rapidly vanished by heating the sample up to about 200 °C. In situ FT-IR observation under outgassing condition at higher temperatures revealed that the bands at 1635 (36) Morterra, C.; Chiorino, A.; Ghiotti, G.; Garrone, E. J. Chem. Soc., Faraday Trans. 1 1979, 75, 271. (37) Busca, G.; Lorenzelli, V.; Sanchez-Escribano, V.; Guidetti, R. J. Catal. 1991, 131, 167. (38) Knoinger, H.; Kietembrink, H.; Muller, H. D.; Schulz, W. Proc. 6th. Intern. Congr. Catal., Royal Society of Chemistry, London, 1976; p 183.

Different parameters have been examined to improve the micropore volume and the acidity of pillared titanates. The method proposed for PILCs with no primary micropore filling has been applied to calculate the external surface area and micropore volume of pillared titanates. The pillaring process induced microporous character in pillared materials prepared from a hexylammonium-intercalated host titanate, but both mesoporous and microporous character was evident in the pillared materials derived from TBA-Ti. The micropore volume was related to the pillar density in the interlayer space and/or to the nature of the starting host titanate. The acidity in term of proton concentration (Bro¨nsted acid sites) has been increased by the pillaring process. The number of Bro¨nsted acid sites is related to the accessibility of the probe molecule to the acid centers and to the amount of pillaring species between the titanate layers. FT-IR monitoring of pyridine adsorption has shown the presence of surface Lewis acid sites (not detected by cyclohexylamine adsorption/desorption) and has also confirmed the formation of Bro¨nsted acid sites (nonexisting in the Al-free host titanates) upon incorporation of the alumina pillars. LA9809776 (39) Connell, G.; Dumesic, J. A. J. Catal. 1986, 102, 216. (40) Bodoardo, S.; Figueras, F.; E. Garrone, E. J. Catal. 1994, 147, 223. (41) Nakabayashi, H. Bull. Chem. Soc. Jpn. 1992, 65, 914.