Chem. Mater. 1997, 9, 567-572
567
Characterization of Synthetic Microporous Pillared Beidellites of High Thermal Stability Kerstin B. Brandt and Ronald A. Kydd* Department of Chemistry, University of Calgary, 2500 University Dr. NW, Calgary, Alberta, Canada, T2N 1N4 Received July 29, 1996. Revised Manuscript Received November 22, 1996X
The clay mineral beidellite was synthesized and pillared, separately, with the three different tridecameric cations AlO4Al12(OH)24(H2O)127+, GaO4Al12(OH)24(H2O)127+, and GaO4Ga12(OH)24(H2O)127+. The basal spacings and surface areas of the resulting pillared clays were monitored as a function of temperature, and it was found that the beidellites were usually more thermally stable than the corresponding pillared montmorillonites. 27Al MAS NMR spectra were obtained in an effort to understand the structural changes taking place in the pillars and in the clay layers upon calcination; the signal from tetrahedral aluminum diminished considerably after calcination at 500 °C. The argon adsorption isotherms were measured, and the pillared clays were found to have a bimodal micropore size distribution, with maxima at 0.6 and 0.8 nm.
Introduction The quest for highly microporous and thermally stable cracking catalysts continues to be the focus for the preparation of pillar interlayered structures. Recent studies examining the thermal stability of PILCs include the comparison of the relative thermal stabilities of Al13-, Ga13-, and GaAl12-pillared montmorillonite.1,2 It was found that the GaAl12-polyoxocations afford extremely stable pillared structures. Similar reports of highly stable Ga-Al pillared montmorillonite were published shortly thereafter.3,4 Recently, the preparation and characterization of a highly stable GaAl12pillared rectorite5 and a GaAl12-pillared bentolite-H6 have been reported; these PILCs are thermally stable above 700 °C. Pillaring clay layers with bulky cations creates interlayer microporosity (see diagram) resulting in large
Abstract published in Advance ACS Abstracts, January 15, 1997. (1) Bradley, S. M.; Kydd, R. A. Catal. Lett. 1991, 8, 185-192. (2) Bradley, S. M. In Synthesis and Characterization of Heteropoly and Isopoly Oxometal Cations and their Use in the Formation of Pillared Clay Mineral Catalysts. Ph.D. Thesis, University of Calgary, 1991. (3) Gonza´lez, F.; Pesquera, C.; Benito, I.; Mendioroz, S. J. Chem. Soc., Chem. Commun. 1991, 587-588. (4) Gonza´lez, F.; Pesquera, C.; Blanco, C.; Benito, I.; Mendioroz, S. Inorg. Chem. 1992, 31, 727-731. (5) Bagshaw, S. A.; Cooney, R. P. Chem. Mater. 1995, 7, 13841389. (6) Tang, X.; Xu, W.-Q.; Shen, Y.-F.; Suib, S. L. Chem. Mater. 1995, 7, 102-110. X
S0897-4756(96)00403-6 CCC: $14.00
surface areas. It is important to know the dimensions of these micropores as the existence of very small pores influences the interaction of the catalysts with gases, since diffusion into narrow pores may be very slow or negligible. Therefore a quantitative evaluation of the micropore size distribution is important for the design of new catalytic materials. The micropores in the pillared clay minerals can be altered by varying the charge density of the clay layers, the charge of the pillars, and the size and the shape of the interlayer cations.7 The purpose of the work reported here is to investigate the thermal stability of beidellite pillared with the tridecameric polyoxocations AlO4Al12(OH)24(H2O)127+ (abbreviated Al13), its gallium analogue GaO4Ga12(OH)24(H2O)127+ (Ga13), and the mixed gallium/aluminum cation GaO4Al12(OH)24(H2O)127+ (GaAl12) and compare the results with published data for montmorillonite pillared with the same cations.1 Two types of beidellite were usedsan “unsubstituted” one, in which the framework metals are Al and Si, and a second sample in which 15% of the aluminum in the reaction mixture was replaced by gallium. Electron probe microanalyses of “as-synthesized” and “ion-exchanged” samples showed that the gallium was present in nonexchangeable positions in the framework, and ICP analysis showed that 90% of that present in the reaction mixture was retained;8 we were interested to find out how its larger size (compared to Al) would affect the properties of the pillared clays. Pore size distributions for the different combinations of layer type and pillar were calculated from adsorption data, using the Saito-Foley model.9 27Al MAS NMR spectra were also obtained in an attempt to understand the structural changes which occur while calcining pillared beidellite. (7) Barrer, R. M. Clays Clay Miner. 1989, 37, 385-395. (8) Brandt, K. B. In A Comparison of Natural and Synthetic Clay Minerals in the Formation of Pillar Interlayered Clay Mineral Catalysts. Ph.D. Thesis, University of Calgary, 1996. (9) Saito, A.; Foley, H. C. AIChE J. 1991, 37, 429-436.
© 1997 American Chemical Society
568
Chem. Mater., Vol. 9, No. 2, 1997
Brandt and Kydd
Experimental Section Mg-beidellite was synthesized under hydrothermal conditions in a 300 cm3 stainless steel autoclave using a modified form of the nitrate decomposition method described by Diddams.10 Initially a gel was prepared by dissolving 46.52 g Al(NO3)3‚9 H2O and 3.97 g Mg(NO3)2‚6 H2O in 1 L of deionized distilled water. Under stirring, this mixture was then heated at about 2 °C/ minute to 80 °C, followed by a dropwise addition over approximately 30 min of 37.2 g of Ludox AS 40 (colloidal silica, 40%). After stirring for 1.5 h, the gel that formed was heated to dryness and calcined in a stream of air at 200 °C (4 h), 500 °C (6 h), and 700 °C (8 h). For unsubstituted beidellite, the oxide precursor had a molar ratio of 8SiO2/2Al2O3/0.5MgO; to synthesize Mgbeidellite (0.15 Ga), 15% of the Al was replaced by Ga in the reaction mixture on a molar basis. The calcined precursors were redispersed in deionized distilled water and treated hydrothermally at 350 °C and autogenic water vapor pressure (2400 psi) for 7 days. The fine fraction of Mg-beidellite and Mg-beidellite (0.15 Ga) was obtained by centrifuging (6 min; g ) 1.56). Natural montmorillonite samples (STx-1) were obtained from the Source Clay Minerals Repository, University of Missouri. The
