Chem. Mater. 2004, 16, 263-269
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Stability of Pillared Clays: Effect of Compaction on the Physicochemical Properties of Al-Pillared Clays Mark Pichowicz and Robert Mokaya* School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, U.K. Received October 9, 2003. Revised Manuscript Received November 18, 2003
The stability of alumina-pillared montmorillonite clays, prepared at various Al/clay ratios, was investigated by compaction at various pressures. The physical properties of the pillared clays, before and after compaction, were studied by powder XRD and nitrogen sorption. We found that the Al/clay ratio used to prepare the pillared clays (and therefore the resulting pillar density) is not a critical factor in determining the overall mechanical stability. However, pillared clays with a higher density were subject to greater decreases in textural properties (surface area and pore volume) after compaction. It was found that the pillared clays (regardless of pillar density) are generally stable up to ca. 5.7 tons cm-2. Mechanical pressure below 5.7 t cm-2 resulted only in small decreases in basal spacing, surface area, and pore volume. This was, however, accompanied by significant changes in the proportion of micropore surface area and pore volume. Compaction at a pressure of ca. 7.5 t cm-2 decreased the surface area and pore volume by up to 20%, whereas after compaction at 8.3 t cm-2 the structural ordering of the pillared clay is significantly disrupted and the surface area and pore volume reduce by 55%. The porosity of the pillared clays also changes significantly with the micropore surface area and pore volume decreasing by 35% and 70% after compaction at 7.5 and 8.3 t cm-2, respectively, i.e., the pillared clays become less microporous. The (Brønsted) acidity of the pillared clays is affected to a lesser extent by compaction because the acid sites are mainly located on the clay layers rather than on the pillars. Consequently the loss of pillars after compaction has a limited effect on acidity.
1. Introduction One of the most common modifications made to cationic clay minerals to improve their porosity and stability is pillaring, which leads to the formation of a class of material known as pillared intercalated clays (PILCs).1-3 PILCs have increased surface area, pore volume, thermal stability, and (depending on the pillar) improved catalytic activity compared to the parent clays, making them suitable catalysts, ion exchangers, and adsorbents.3 PILCs are prepared by the “propping” apart of layered clay minerals with a variety of nanosized pillars. The pillars may be introduced into the interlayer region of the host clay via ion exchange or generated in-situ.4 The most common pillars are metal oxides. Metal oxide pillaring is essentially a cation exchange reaction in which the original interlayer cations in the clay are exchanged with polymeric metal ions which, on calcination, yield metal oxide “pillars” fixed in the interlayer region.2-4 The calcination, at temperatures up to 500 °C, leads to irreversible dehydroxylation of the polymeric metal ions resulting in the * To whom correspondence should be addressed. Phone: +44-0-115846-6174. Fax: +44-0-115-951-3562. E-mail: r.mokaya@nottingham. ac.uk. (1) Gil, A.; Gandia, L. M.; Vicente, M. A. Catal. Rev. - Sci. Eng. 2000, 42, 145. (2) Ding, Z.; Kloprogge, J. T.; Frost, R. L.; Lu, G. Q.; Zhu, H. J. Porous Mater. 2001, 8, 273. (3) Pinnavaia, T. J. Science 1983, 220, 365. (4) Schoonheydt, R. A.; Pinnavaia, T. J.; Lagaly, G.; Gangas, N. Pure Appl. Chem. 1999, 71, 2367.
pillars cross-linking the interlayer region of the clay.5 The pillars therefore prop open the layered structure of the clay increasing the basal (interlayer) spacing, surface area, and pore volume. In addition, the metal oxide pillars themselves can introduce new functionalities (e.g., catalytically active sites) to the clay.5 For example, they may be acidic thus enhancing the acidity of the PILC compared to that of the parent clay, or they may contain transition metals which are active for redox catalysis.5-7 The pillars prop open the clay layers to give basal spacings in the region of 20 Å and form a porous structure with pore diameters generally less than 10 Å and surface areas in the region of 200-600 m2g-1. A very large number of different types of PILCs have been reported1-4 with variations in the parent clay (including the use of montmorillonite, saponite, rectorite, fluohectorite, vermiculite, etc.)8-10 and pillar type (Al, Fe, Zr, Ti, Cr, Ni, Ta, Ga, and Si).11-13 The most studied (5) Jones, W. Catal. Today 1988, 2, 357. Bartley, G. J. J. Catal. Today 1988, 2, 233. Tzou, M. S.; Pinnavaia, T. J. Catal. Today 1988, 2, 243. Yamanaka, S.; Hattori, M. Catal. Today 1988, 2, 261. Warburton, C. I. Catal. Today 1988, 2, 271. Ming-Yuan, H.; Zhonghui, L.; Enze, M. Catal. Today 1988, 2, 321. (6) Bahranowski, K.; Kielski, A.; Serwicka, E. M.; Wisla-Walsh, E.; Wodnicka, K. Microporous Mesoporous Mater. 2000, 41, 201. (7) Zhu, H. Y.; Zhu, Z. H.; Lu, G. Q. J. Phys. Chem. B 2000, 104, 5674. (8) Moreno, S.; Sun Kou R.; Poncelet, G. J. Phys. Chem. B 1997, 101, 1569. (9) Vicente, M. A.; Ba˜nares-Mun˜oz, M. A. Langmuir 1996, 12, 5143. (10) Ohtsuka, K. Chem. Mater. 1997, 9, 2039. (11) Maes, N.; Heylen, I.; Cool, P.; Vansant, E. F. Appl. Clay Sci. 1997, 12, 43.
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pillared clays are those pillared with alumina because of their acidity and good thermal stability. Aluminapillared clays are prepared using an Al137+ polycation,14 the so-called keggin ion ([Al13O4(OH)4]7+). When studying pillared clays or evaluating their catalytic activity they are often compressed (at relatively high pressure) into pellets for ease of handling. For catalysis, the pellets are crushed to give compact particles. This removes the problem of silting, which is encountered if the uncompacted clay (made up of small particles) is used especially for reactions involving liquid reactants or products. It is usually assumed that the compaction has no effect on the structural/physical or chemical properties of the clay. Mechanical stability is therefore an important property of pillared clays. Good mechanical stability is a critical requirement to conserve the textural properties (pore volume, surface area, and pore diameter) of the pillared clay after compaction. The conservation of textural properties is essential to ensure that an open and porous structure is maintained which gives easy access to the internal space of pillared clays where the active sites (e.g., for adsorption or catalysis) are located. The layered 2-dimensional structure of nonmodified clay minerals is known to be mechanically robust.15 The interlayer forces present in nonmodified clay minerals can withstand considerable mechanical distortions without loss of the layered structure.15 However, on pillaring, a pseudo 3-dimensional clay structure is formed with pillars cross-linking the clay layers.16 The stability of the pillared clays is therefore no longer dependent on interlayer forces but on the tensile strength of the pillars. To the best of our knowledge, there has not been any report on the systematic study of the effects of compaction on the physicochemical properties of pillared clays. The aim of this study was, therefore, to investigate the effect of applying (mechanical) pressure on the physicochemical properties of PILCs and to identify any changes in their porous structure and basal spacing which may affect other properties such as acidity and catalytic activity. We also investigated the effect of pillar density (within the interlayer region) on the mechanical stability and physicochemical properties of the pillared clays. 2. Experimental Section 2.1 Materials Used. A Peruvian montmorillonite clay, designated M, with a cation exchange capacity (CEC) of 91 meq/100 g was used as the starting clay. The raw clay had a basal (001) spacing of 15.4 Å and an anhydrous structural (layer) formula of [Si7.86Al0.14][Al2.84Fe0.30Mg0.86] O20(OH)4. A commercial aluminum chlorhydrate (ACH) solution supplied by Albright & Wilson was used as source of the aluminum pillaring species. The ACH solution had an aluminum concentration of 6.5 M and an OH/Al ratio of 2.5. All other reagents used were of standard laboratory grade. 2.2 Sample Preparation and Characterization. Clay M was pillared using aluminum chlorhydrate (ACH) solution at various aluminum-to-clay ratios as follows. The clay was added to an appropriate ACH solution to give a liquid-to-clay ratio (12) Guiu, G.; Gil, A.; Montes M.; Grange, P. J. Catal. 1997, 168, 450. (13) Ahenach, J.; Cool, P.; Impens, R. E. N.; Vansant E. F. J. Porous Mater. 2000, 7, 475. (14) Johansson, G. Acta Chem. Scand. 1960, 14, 771. (15) Gregg, S. J.; Langford, J. F. J. Chem. Soc., Faraday Trans. 1 1977, 73, 747.
Pichowicz and Mokaya (w/w) of 100:1 and the resulting suspension was stirred at room temperature for 1 h. The suspension was then filtered, and the clay was washed with deionized water until free of chloride ions as tested with silver nitrate. The obtained precursor pillared clay was then air-dried overnight and subsequently calcined in air at 500 °C for 4 h. The final pillared samples were designated as PM(x) where x is the Al/clay ratio (in units of mmol Al per gram of clay) used in the preparation of the pillared clays. To assess mechanical stability, a known amount of calcined pillared clay sample was compressed in a 13-mm die at various pressures for 20 min. The obtained disks were crushed to obtain powdered samples, which were used for further characterization. The pressed (compacted) samples were designated as PM(x)XT, where X is the nominal applied pressure in tons. The nominal pressure was varied between 2.5 and 11 t. Compaction at a nominal pressure of 10 t (i.e., 740 MPa) is equivalent to an external pressure of 7.54 t cm-2 (calculated from the applied force and die area). X-ray diffraction was carried out on powder samples using a Phillips PW1830 diffractometer with Cu KR radiation (40 KV, 40 mA) 0.02° step size, and 1 s step time. Nitrogen sorption isotherms and textural properties of the pillared clays were determined at -196 °C using nitrogen in a conventional volumetric technique by a Coulter SA3100 sorptometer. Samples of average weight 0.2 g were degassed for 8-12 h at 200 °C before the nitrogen sorption isotherm was recorded at liquid nitrogen temperatures. The surface area was calculated using the BET method based on adsorption data in the partial pressure (P/Po) range 0.05 to 0.2, and total pore volume was determined from the amount of the nitrogen adsorbed at P/Po ) ca. 0.99. Micropore surface area and micropore volume were obtained via t-plot analysis. Infrared spectra were recorded on a Nicolet 210 spectrometer at room temperature. Samples were prepared by grinding a small amount of the clay with potassium bromide (KBr), which was then compressed in a 13-mm die at a nominal pressure of ca. 2.0 t for 2 min (i.e., lower than any of the analysis compaction pressures). Samples were allowed to stand in the spectrometer for 1 h in the presence of silica gel to remove moisture before spectra were recorded. The acid content of the samples before and after compaction was measured using established procedures employing thermal desorption of cyclohexylamine.17,18 The clay samples were exposed to liquid cyclohexylamine at room temperature, after which they were kept overnight (at room temperature) and then in an oven at 80 °C for 2 h to allow the base to permeate the samples. TGA curves were obtained for the cyclohexylamine containing samples using a Perkin-Elmer Pyris 6 TG analyzer with a heating rate of 20 °C/min under nitrogen flow of 25 mL/min. The weight loss associated with desorption of the base from acid sites, between 300 and 450 °C, was used to calculate the acid content in mmol of cyclohexylamine per g of sample assuming that each acid (H+) site interacts with one base molecule.
3. Results and Discussion 3.1 Physicochemical Changes on Compaction of Raw Montmorillonite Clay. We first checked the mechanical stability of the parent raw clay. The powder XRD patterns of the parent montmorillonite (M) before and after compaction at a nominal pressure of 10 t, i.e., 7.54 t cm-2 (sample MC10) or a nominal pressure of 11 (16) Bergaoui, L.; Mrad, I.; Lambert J. F.; Ghorbel, A. J. Phys. Chem. B 1999, 103, 2897. (17) Breen, C. Clay Miner. 1991, 26, 487. (18) Mokaya, R.; Jones, W.; Moreno, S.; Poncelet, G. Catal. Lett. 1997, 49, 87. (19) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Analysis and Porosity, Academic Press: London, 1982. (20) Yang, R. T.; Baksh, M. S. A. AICHE J. 1991, 37, 679. (21) Mokaya, R.; Jones, W.; Davies M. E.; Whittle, M. E. J. Mater. Chem. 1993, 3, 381. Mokaya, R.; Jones, W. J. Porous Mater. 1995, 1, 97.
Effect of Compaction on Al-Pillared Clays
Figure 1. Powder XRD patterns of a raw montmorillonite clay before (M) and after compaction at 7.54 t cm-2 (MC10) or 8.30 t cm-2 (MC11).
t (i.e., 8.3 t cm-2, sample MC11) are given in Figure 1. The XRD pattern of MC10 is virtually identical to that of the noncompacted clay with an unchanged basal spacing of 15.4 Å. This suggests that compression at 7.54 t cm-2 does not affect the layer-by-layer structural ordering of the raw clay. However, there was evidence from porosity data that the packing of the clay platelets was changed. Although the surface area and pore volume remained much the same after compaction at 7.54 t cm-2 (i.e., surface area of 57 and 67 m2/g and pore volume of 0.15 and 0.14 cm3/g for M and MC10, respectively), we observed that the proportion of smaller pores (
