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Modification of the Hydroxyl Surface of Kaolinite through Mechanochemical Treatment Followed by Intercalation with Potassium Acetate Ray L. Frost,*,† Janos Kristof,‡ Eva Mako,§ and Wayde N. Martens† Centre for Instrumental and Developmental Chemistry, Queensland University of Technology, 2 George Street, G.P.O. Box 2434, Brisbane, Queensland 4001, Australia, Department of Analytical Chemistry, University of Veszprem, H8201 Veszprem, P.O. Box 158, Hungary, and Department of Silicate and Materials Engineering, University of Veszprem, H-8201 Veszprem, P.O. Box 158, Hungary Received February 11, 2002. In Final Form: May 23, 2002 Changes in the hydroxyl surfaces of kaolinite have been studied through mechanochemical activation followed by intercalation with potassium acetate using a combination of X-ray diffraction and diffuse reflectance Fourier transform infrared (DRIFT) spectroscopy. X-ray diffraction shows that in the absence of air the intercalated kaolinite is expanded to 11.9 Å with an additional expanded phase at 9.07 Å. The degree of expansion is related to the orientation of the acetate between the kaolinite layers. Upon exposure to moist air, the kaolinite re-expands to 14.2 and 10.02 Å. Upon mechanochemical activation of the kaolinite for 1 h and intercalation with potassium acetate, a phase with a d spacing of 8.80 Å is obtained which upon exposure to moist air re-expands to 13.50 Å. Changes in the molecular structure of the mechanochemically activated intercalated kaolinite were followed by DRIFT spectroscopy. Fundamentally the intensity of the kaolinite hydroxyl stretching bands decreased exponentially with grinding time, and concomitantly the intensity of the bands attributed to the OH stretching vibrations of water increased. It is proposed that the mechanochemical activation of the kaolinite caused the conversion of the hydroxyls to water which coordinates the kaolinite surface. Significant changes in the infrared bands assigned to the hydroxyl deformation and translation modes were observed. The intensity decrease of the bands was linearly related to the grinding time.
Introduction Kaolinite is an extremely important industrial mineral, and the effects of the mechanochemical treatment of minerals such as kaolinite have been studied over a long period of time.1-6 Indeed, mechanochemical grinding of kaolinite has been used to insert inorganic molecules between the kaolinite layers.7-14 Recently, studies of the mechanochemical treatment of kaolinite have further elucidated the structural changes which occur upon the grinding of kaolinite.15-18 The mechanochemical activation * To whom correspondence should be addressed. E-mail: r.frost@ qut.edu.au. † Centre for Instrumental and Developmental Chemistry, Queensland University of Technology. ‡ Department of Analytical Chemistry, University of Veszprem. § Department of Silicate and Materials Engineering, University of Veszprem. (1) Takahashi, H. Clays and Clay Minerals: Proceedings of the 6th National Conference on Clays and Clay Minerals, Berkeley, CA, 1959; pp 279-91. (2) Takahashi, H. Bull. Chem. Soc. Jpn. 1959, 32, 235-45. (3) Takahashi, H. Bull. Chem. Soc. Jpn. 1959, 32, 245-51. (4) Takahashi, H. Bull. Chem. Soc. Jpn. 1959, 32, 252-63. (5) Takahashi, H. Bull. Chem. Soc. Jpn. 1959, 32, 381-7. (6) Wiegmann, J.; Kranz, G. Silikat. Technol. 1957, 8, 520-3. (7) Thompson, J. G.; Gabbitas, N.; Coyle, K.; Uwins, P. J. R.; Mackinnon, I. D. R. Clays, Controlling the Environment: Proceedings of the 10th International Clay Conference; CSIRO Pub.: East Melbourne, Victoria, Australia, 1995; pp 260-5. (8) Thompson, J. G.; Gabbitas, N.; Uwins, P. J. R. Clays Clay Miner. 1993, 41, 73-86. (9) Thompson, J. G.; Uwins, P. J. R.; Whittaker, A. K.; Mackinnon, I. D. R. Clays Clay Miner. 1992, 40, 369-80. (10) Yariv, S. Clays Clay Miner. 1975, 23, 80-2. (11) Yariv, S. Powder Technol. 1975, 12, 131-8. (12) Yariv, S. J. Chem. Soc., Faraday Trans. 1 1975, 71, 674-84. (13) Yariv, S. Int. J. Trop. Agric. 1986, 4, 310-22. (14) Yariv, S.; Shoval, S. Clays Clay Miner. 1976, 24, 253-61.
through dry grinding caused destruction in the crystal structure of kaolinite by the rupture of the O-H, Al-OH, Al-O-Si, and Si-O bonds. Grinding experiments were carried out for 1, 2, 3, and 4 h in a planetary mill. The rate of destruction of the kaolinite structure was followed by X-ray diffraction, thermal analysis, and diffuse reflectance Fourier transform IR (DRIFT) spectroscopy. The distortion and rupture of the kaolinite structure induced by grinding was reflected in line broadening, increases in mean lattice strain, and reduction of peak areas (intensities). The increased quartz content resulted in acceleration of the mechanochemically induced amorphization of the kaolinite structure. The mechanochemical activation of the kaolinite results in increased surface areas. This increase results from changes in the surface structure of the kaolinite. One very useful method of studying kaolinite surfaces is through modification with intercalation with potassium acetate.19-22 This modification through insertion of the intercalating molecule resulted in novel expanded phases of kaolinite as determined by X-ray diffraction. Vibrational spectroscopy using both DRIFT and Raman techniques enabled (15) Frost, R. L.; Mako, E.; Kristof, J.; Horvath, E.; Kloprogge, J. T. J. Colloid Interface Sci. 2001, 239, 458-66. (16) Frost, R. L.; Mako, E.; Kristof, J.; Horvath, E.; Kloprogge, J. T. Langmuir 2001, 17, 4731-8. (17) Mako, E.; Frost, R. L.; Kristof, J.; Horvath, E. J. Colloid Interface Sci. 2001, 244, 359-64. (18) Mako, E.; Kristof, J.; Juhasz, A. Z. Epitoanyag 1997, 49, 2-6. (19) Frost, R. L.; Kristof, J.; Horvath, E.; Kloprogge, J. T. J. Colloid Interface Sci. 1999, 214, 109-17. (20) Frost, R. L.; Kristof, J.; Horvath, E.; Kloprogge, J. T. Langmuir 1999, 15, 8787-94. (21) Frost, R. L.; Kristof, J.; Mako, E.; Kloprogge, J. T. Langmuir 2000, 16, 7421-8. (22) Frost, R. L.; Kristof, J.; Mako, E.; Kloprogge, J. T. Am. Mineral. 2000, 85, 1735-43.
10.1021/la0201422 CCC: $22.00 © 2002 American Chemical Society Published on Web 07/27/2002
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Figure 1. X-ray diffraction of kaolinite (a) intercalated with potassium acetate and kept under dry nitrogen; (b) intercalated with potassium acetate and exposed to moist air; (c) mechanochemically activated for 1 h, intercalated with potassium acetate, and exposed to dry nitrogen; (d) mechanochemically activated for 1 h, intercalated with potassium acetate, and exposed to moist air; (e) mechanochemically activated for 2 h, intercalated with potassium acetate, and exposed to dry nitrogen; and (f) mechanochemically activated for 2 h, intercalated with potassium acetate, and exposed to moist air.
the changes in the surface structure of the kaolinite to be followed. In this research, the combination of mechanochemical treatment of the kaolinite followed by intercalation is used to study the changes in the surface structure of the kaolinite. Experimental Methods Materials. The kaolin used in the experiments was the highgrade natural kaolin from Sedlec (Zettlitz) in Slovakia. Its chemical composition in wt % as oxides is MgO, 0.26; CaO, 0.54; SiO2, 46.97; Fe2O3, 0.37; K2O, 1.21; Al2O3, 36.32; TiO2, 0.05; loss on ignition, 13.38. The major mineral constituent is low-defect kaolinite (92 wt %) with a Hinckley index of around 0.7. Some minor amounts of quartz (4 wt %) and illite (4 wt %) are also present. This kaolin was selected for this experiment because of its low quartz content. The specific surface area is 18.5 m2/g. The arithmetic mean diameter determined by a Fritsch LaserParticle-Sizer “analysette 22” is 4.4 µm. The kaolin contains 10.1% of particles less than 1 µm and 9.3% of particles greater than 10 µm in size. Milling Procedure. A Fritsch pulverisette 5/2-type laboratory planetary mill was used to grind the mixtures of kaolinite and quartz. Samples were ground for 0, 1, 2, 3, and 4 h. Each milling was carried out with a 10 g air-dried sample in an 80 cm3 capacity stainless steel (18% Cr + 8% Ni) pot using eight (31.6 g) stainless steel balls (10 mm diameter). The applied rotation speed was 374 rpm. The Potassium Acetate Intercalated Mechanochemically Activated Kaolinite. Three hundred milligrams of the mechanochemically treated kaolinite was treated with 30 cm3 of 7.2 M potassium acetate solution. The sample was shaken for 80 h in a constant-temperature bath at ambient temperature. The excess solution on the clay was removed by centrifugation. The potassium acetate intercalated kaolinite was allowed to dry in air and stored in a desiccator above anhydrous calcium chloride before X-ray diffraction and spectroscopic analysis. To explore the modification of kaolinite surfaces through mechanochemical activation followed by intercalation with potassium acetate, a number of experiments were undertaken: (a) the starting kaolinite intercalated with potassium acetate, dried over silica gel, and measured under flowing nitrogen; (b) the starting material intercalated with potassium acetate, dried over silica gel, and exposed to room air for 1 h; (c) the kaolinite mechanochemically activated for 1 h, intercalated with potassium acetate, dried over silica gel, and measured under flowing nitrogen; (d) the kaolinite mechanochemically activated for 1 h, intercalated with potassium acetate, dried over silica gel, and exposed to room air for 1 h; (e) the kaolinite mechanochemically
activated for 2 h, intercalated with potassium acetate, dried over silica gel, and measured under flowing nitrogen; (f) the kaolinite mechanochemically activated for 2 h, intercalated with potassium acetate, dried over silica gel, and exposed to room air for 1 hour. X-ray Powder Diffraction. The normal room temperature and temperature-controlled X-ray diffraction (XRD) analyses were carried out on a Philips wide-angle PW 3020/1820 vertical goniometer equipped with curved graphite-diffracted beam monochromators. The d spacing and intensity measurements were improved by application of a self-developed computer-aided divergence slit system enabling constant sampling area irradiation (20 mm long) at any angle of incidence. The goniometer radius was enlarged from 173 to 204 mm. The radiation applied was Cu KR from a long fine focus Cu tube, operating at 40 kV and 40 mA. The samples were measured in static air and in a flowing nitrogen atmosphere at 15 L/h in stepscan mode with steps of 0.025° 2θ and a counting time of 1 s. Measured data were corrected with the Lorentz polarization factor (for oriented specimens) and for their irradiated volume. DRIFT Spectroscopy. DRIFT spectroscopic analyses were undertaken using a Bio-Rad FTS 60A spectrometer. Scans (512) were obtained at a resolution of 2 cm-1 with a mirror velocity of 0.3 cm/s. Spectra were co-added to improve the signal-to-noise ratio. Approximately 3 wt % ground kaolinite was dispersed in 100 mg of oven-dried spectroscopic grade KBr with a refractive index of 1.559 and a particle size of 5-20 µm. Reflected radiation was collected at ∼50% efficiency. Background KBr spectra were obtained, and spectra were ratioed to the background. The diffuse reflectance accessory used was designed exclusively for Bio-Rad FTS spectrometers. It is of the so-called “praying monk” design and is mounted on a kinematic baseplate. It includes two fourposition sample slides and eight sample cups. The cup (3 mm deep, 6 mm in diameter) accommodates powdery samples mixed with KBr using an achate mortar and pestle in 1-3% concentration. The reflectance spectra expressed as Kubelka-Munk unit versus wavenumber curves are very similar to absorbance spectra and can be evaluated accordingly. Spectral manipulation such as baseline adjustment, smoothing, and normalization was performed using the Spectracalc software package GRAMS (Galactic Industries Corp., NH). Band component analysis was undertaken using the Jandel “Peakfit” software package, which enabled the type of fitting function to be selected and allows specific parameters to be fixed or varied accordingly. Band fitting was done using a Lorentz-Gauss crossproduct function with the minimum number of component bands used for the fitting process. The Gauss-Lorentz ratio was maintained at values greater than 0.7, and fitting was undertaken until reproducible results were obtained with squared correlations of r2 greater than 0.995.
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Table 1. Results of the Band Component Analysis of DRIFT Spectra of the Hydroxyl Stretching Region of the Kaolinite as a Function of Grinding Time Followed by Intercalation with Potassium Acetate kaolinite polytype
band parameters
ν1
ν2
ν3
ν5
ν6
ν7
ν8
ν9
kaolinite ground for 1.0 hand then intercalated with potassium acetate
band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1
3693 8.9 14.2 3693 7.0 20.7 3695 3.3 18.2 3695 2.8 20.4
3668 0.3 9.4 3668 2.2 22.9 3666 2.0 36.0 3667 1.9 40.85
3653 2.7 32.4 3650 3.6 31.5 3642 0.1 6.1 3631 3.3 62.1
3619 3.5 11.7 3619 3.8 14.1 3620 4.9 60.2 3620 0.6 11.5
3599 0.6 15.8 3599 1.0 21.8
3521 9.5 152.0 3564 10.6 158.0 3552 7.0 146.5 3548 8.2 146.3
3383 21.7 247.9 3384 35.8 316.3 3342 47.6 302.7 3434 16.5 228.5
3192 25.6 318.4
kaolinite ground for 2.0 h and then intercalated with potassium acetate kaolinite ground for 6.0 h and then intercalated with potassium acetate kaolinite ground for 10.0 h and then intercalated with potassium acetate
3602 3.7 98.5
3198 2.2 142.0 3240 41.8 346.3
X-ray Diffraction Upon intercalation of kaolinite with potassium acetate, significant changes in the structure of the kaolinite occur through expansion with the inserting molecule.23,24 The X-ray diffraction patterns for the six experiments listed above are shown in Figure 1. When the kaolinite is not ground and is intercalated with potassium acetate, dried over silica gel, and maintained dry by the flow of dry nitrogen, a peak for the expanded kaolinite is found at 11.9 Å (curve a). A second expanded phase at 9.07 Å is also found. If the kaolinite is intercalated with the potassium acetate, dried over silica gel, and exposed to laboratory air, then a peak for the expanded kaolinite is obtained at 14.2 Å. At the same time, a second peak is found at 10.02 Å (curve b). The explanation for these different expansions lies with the role of the water in the intercalation complex and with the orientation of the potassium acetate. The 11.90 Å peak results when the kaolinite is expanded with the acetate at right angles to the hydroxyl surface and in the absence of water. If the orientation of the potassium acetate is inclined at an angle to the 001 surface, then the 10.02 Å peak results. If the kaolinite is expanded with potassium acetate, dried, and exposed to moist air, then the 14.2 Å peak results. In this case, the intercalation unit is acetate and water. When the kaolinite is mechanochemically activated for 1 h, intercalated with potassium acetate, dried, and measured over flowing nitrogen, no peak at either 14.20 or 10.02 Å was obtained but rather a peak at 8.80 Å was found (curve c). Upon intercalation of the kaolinite, which had been mechanochemically activated for 1 h, dried, and exposed to moist air, a peak was obtained at 13.50 Å and the one at 8.80 Å was not observed (curve d). It is considered that the 13.5 Å peak is attributable to a partially reconstructed water-acetate intercalation complex. When the kaolinite is mechanochemically treated for 2 h, intercalated with potassium acetate, dried, and maintained dry, then the expanded phase is observed at 8.95 Å (curve e). This expansion to the 8.95 Å d spacing is due to the parallel orientation of the acetate ion.21 For the 2 h mechanochemically treated intercalated kaolinite dried and then exposed to moist air, the only expansion observed is at 13.95 Å (curve f). To check that the reflections being observed are not due to the potassium acetate salt simply coating the particles’ outer surfaces, an experiment in which quartz was mixed with potassium acetate solution and then dried over silica gel was undertaken. The sample was kept under dry flowing nitrogen, and a second sample was exposed to moist air. The XRD patterns show no resemblance to the patterns (23) Frost, R. L.; Kristof, J.; Horvath, E.; Kloprogge, J. T. J. Colloid Interface Sci. 2001, 239, 126-33. (24) Frost, R. L.; Kristof, J.; Horvath, E.; Kloprogge, J. T. J. Raman Spectrosc. 2001, 32, 271-7.
Figure 2. DRIFT spectra of the hydroxyl stretching region of kaolinite mechanochemically activated for (a) 1, (b) 2, (c) 6, and (d) 10 h and intercalated with potassium acetate.
in Figure 1. Thus all the differences observed in the XRD patterns belong to the kaolinite structure. In this way, peaks, which belong to surface acetate and intercalated acetate, can be separated. If a peak belonged to an acetate coating, then all other reflections will change simultaneously. If only one peak is increasing in intensity, it can be assigned to kaolinite expansion. DRIFT Spectroscopy The use of infrared spectroscopy for the determination of changes in the structure of kaolinite is wellknown.11,12,18,25 Figure 2 displays the changes in the DRIFT spectra of the mechanochemically treated kaolinite, which has been ground for 1, 2, 6, and 10 h and then intercalated with potassium acetate. The band component analysis of the hydroxyl stretching region is reported in Table 1. The set of spectra cover three sets of infrared bands: (a) the hydroxyl stretching region of kaolinite, (b) the hydroxyl stretching region of water, and (c) the CH stretching region of the methyl group of the acetate. Four infrared bands are observed for kaolinite: these are (a) the 3693 cm-1 (25) Mendelovici, E.; Villalaba, R.; Sagarzazu, A.; Carias, O. Clay Miner. 1995, 30, 307-13.
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Figure 3. Relationship between the intensity of the hydroxyl stretching vibrations of kaolinite and water as a function of grinding time. Table 2. Results of the Band Component Analysis of DRIFT Spectra of the Hydroxyl Deformation Regions of the Kaolinite as a Function of Grinding Time Followed by Intercalation with Potassium Acetate kaolinite polytype
band parameters
νOH deform 1
νOH deform 2
νOH deform 3
νOH deform 4
kaolinite ground for 1.0 h and then intercalated with potassium acetate
band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1
939 4.8 16.4 939 2.7 14.5 939 0.59 14.5 939 0.05 14.5
920 6.8 22.5 925 2.6 14.7 920 0.6 6.0
914 8.0 15.0 914 8.6 15.3 914 8.6 4.4 914 3.9 4.5
895 1.6 16.2 895 2.6 16.1 895 0.1 8.5
kaolinite ground for 2.0 h and then intercalated with potassium acetate kaolinite ground for 6.0 h and then intercalated with potassium acetate kaolinite ground for 10.0 h and then intercalated with potassium acetate
Figure 4. DRIFT spectra of the hydroxyl deformation region of kaolinite mechanochemically activated for (a) 1, (b) 2, (c) 6, and (d) 10 h and intercalated with potassium acetate.
band assigned to the hydroxyl stretching vibration of the inner surface hydroxyls, (b) the 3619 cm-1 band assigned to the inner hydroxyls of kaolinite, and (c) two bands at 3668 and 3653 cm-1, which are attributed to the out-ofphase vibrations of the inner surface hydroxyls. Broad bands are observed at around 3383 and 3550 cm-1 and are assigned to the hydroxyl stretching vibrations of water. An additional low-intensity band is observed at around 3599 cm-1. This band is attributed to the hydroxyl stretching vibration of the inner surface hydroxyls, which are hydrogen bonded to the acetate. Several conclusions are made from the spectra: (a) the intensity of the hydroxyl stretching vibrations of kaolinite decreases with grinding time, (b) the intensity of water vibrations increases with grinding time, and (c) the intensity of the CH stretching vibrations increases with increased water content. As the length of grinding time increases, the out-of-phase vibrations observed at 3668 and 3650 cm-1 are no longer observed. This means that the in-phase/out-of-phase behavior of the kaolinite hydroxyls no longer exists. This results from increased disorder in the kaolinite through the mechanochemical activation. The changes in intensity of the bands with grinding time are shown in Figure 3. The decrease in intensity of the hydroxyl stretching vibration of the inner hydroxyl of kaolinite with grinding time appears to follow an exponential decay. The increase in the area of the water hydroxyl stretching vibration is linear with grinding time. The band observed at 3599 cm-1, which is attributed to the hydroxyl stretching vibration of the inner surface hydroxyl hydrogen bonded to the acetate, increases
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Figure 5. Relationship between the intensity of the hydroxyl deformation vibrations of kaolinite as a function of grinding time. Table 3. Results of the Band Component Analysis of DRIFT Spectra of the SiO Stretching Region of the Kaolinite as a Function of Grinding Time Followed by Intercalation with Potassium Acetate kaolinite kaolinite ground for 1.0 h and then intercalated with potassium acetate kaolinite ground for 2.0 h and then intercalated with potassium acetate kaolinite ground for 6.0 h and thenintercalated with potassium acetate kaolinite ground for 10.0 h and then intercalated with potassium acetate
νsilica νsilica νsilica νsilica νkaolinite νkaolinite νkaolinite band parameters SiO stretch 1 SiO stretch 2 SiO stretch 3 SiO stretch 4 SiO stretch 5 SiO stretch 6 SiO stretch 7 band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1
1115 4.5 15.7 1115 3.3 14.7 1111 2.2 21.7 1166 2.9 36.5
Figure 6. DRIFT spectra of the SiO stretching vibrations of kaolinite mechanochemically activated for (a) 1, (b) 2, (c) 6, and (d) 10 h and intercalated with potassium acetate.
linearly with the length of time of the mechanochemical activation. The figure shows that the rates of increase of the 3383 and 3599 cm-1 bands are in parallel. Since the rate of intercalation decreases with grinding time, the
1101 11.8 30.5 1103 14.0 35.3
1073 1.9 17.6 1060 24.8 60.2 1085 34.6 86.2 1095 39.3 91.1
1043 9.3 24.5
1046 18.0 31.5 1041 25.0 46.8
1031 19.2 20.6 1030 22.6 33.8 1031 8.1 21.6
1007 32.1 26.2 1005 14.2 24.5 1007 23.9 27.0 1004 27.5 43.6
3599 cm-1 band becomes more and more of a water band as the grinding time increases. One of the difficulties of studying the hydroxyl stretching region of kaolinite and water is the overlap of the bands. The spectral profile is like a continuum. One means of overcoming this problem is to study the hydroxyl deformation region of kaolinite between 875 and 950 cm-1. First, the region is exclusive of the water vibrations, and second, the phenomena of cooperative vibrations of the hydroxyl stretching modes are avoided. Figure 4 displays the hydroxyl deformation region of the mechanochemically activated kaolinite followed by intercalation with potassium acetate. The results of the band component analysis of the hydroxyl deformation modes are reported in Table 2. The DRIFT spectra of kaolinite show two bands at 935 and 914 cm-1 which are attributed to the hydroxyl deformation modes of the inner surface hydroxyls and the inner hydroxyl, respectively. For the 1 h mechanochemically activated kaolinite, four bands are observed at 939, 920, 914, and 895 cm-1. The 939 and 914 cm-1 bands are ascribed as previously to the deformation modes of the inner surface hydroxyls and to the inner hydroxyl. The 920 cm-1 band is attributed to the hydroxyl deformation modes of weakly hydrogen-bonded inner surface hydroxyls. The band at 895 cm-1 is assigned to the hydroxyl deformation modes of non-hydrogen-bonded inner surface hydroxyls. The variation of the intensity of the hydroxyl deformation modes with grinding time is illustrated in Figure 5. The graph clearly demonstrates the loss of intensity with grinding time. Indeed, the rate of decrease is similar for all three bands at 939, 920, and 895 cm-1. However, the intensity of the inner hydroxyl band at 914 cm-1 shows a significant decrease only after 6 h. The decrease in
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Figure 7. Relationship between the intensity of the SiO stretching vibrations of kaolinite as a function of grinding time. Table 4. Results of the Band Component Analysis of DRIFT Spectra of the Out-of-Plane OH Deformation Region of Kaolinite as a Function of Grinding Time Followed by Intercalation with Potassium Acetate kaolinite kaolinite ground for 1.0 h and then intercalated with potassium acetate kaolinite ground for 2.0 h and then intercalated with potassium acetate kaolinite ground for 6.0 h and then intercalated with potassium acetate kaolinite ground for 10.0 h and then intercalated with potassium acetate
band parameters center/cm-1
band % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1
ν1
ν2
ν3
797 0.8 12.6 797 0.7 11.7 797 0.3 11.8 797 0.1 15.2
787 0.6 9.0 787 0.5 7.4 788 0.2 7.1 788 0.1 7.1
753 2.9 20.3 752 2.7 20.8 754 2.5 19.5 751 2.4 21.0
intensity of the hydroxyl deformation modes also parallels the decrease of the stretching bands. The following conclusions are drawn from these analyses. First, the inner surface hydroxyls are lost before the inner hydroxyls. Second, the mechanochemical activation appears to strip the hydroxyls from the kaolinite. These hydroxyls react to give water, which hydrates the kaolinite surface. The DRIFT spectroscopy of the hydroxyl surfaces suggests that the OH units are being removed with mechanochemical treatment. Figure 6 displays the SiO stretching region for the mechanochemically activated and intercalated kaolinite. The results of the band component analysis of these spectra are reported in Table 3. What is clearly apparent is that the mechanical activation of the kaolinite causes the bands to broaden with grinding time. Two sets of bands are identified: (a) bands attributable to silica and (b) bands attributable to the siloxane tetrahedral units of kaolinite. The variation of the band areas is represented in Figure 7. Two bands, namely, the 1115 and 1031 cm-1 bands, decrease with grinding time, whereas the 1043 and 1073 cm-1 bands increase with grinding time. The DRIFT spectra of the hydroxyl translation modes are shown in Figure 8. The results of the band component analysis of this part of the spectral region are reported in Table 4. Figure 8 clearly shows that the bands observed at 797, 787, 701, and 689 cm-1 are significantly affected by the mechanochemical treatment of the kaolinite. The bands observed at 644 and 622 cm-1 are not affected. The decrease in intensity of these bands, which are attributed to hydroxyl translation modes of the kaolinite sheets, is
ν4
743 0.3 11.6
ν5 701 3.9 16.5 701 2.5 15.6 704 0.6 9.3 697 0.1 17.4
ν6
695 2.4 14.3
ν7 689 7.3 22.7 690 5.2 22.2 685 1.1 10.1 685 0.23 20.4
ν8
ν9
ν10
659 0.1 1.3
644 5.8 8.1 644 4.3 8.6 644 5.3 8.6 644 14.1 8.3
622 1.4 6.3 621 1.0 6.7 622 6.6 8.0 622 5.5 7.4
Figure 8. DRIFT spectra of the hydroxyl translation modes of kaolinite mechanochemically activated for (a) 1, (b) 2, (c) 6, and (d) 10 h and intercalated with potassium acetate.
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Figure 9. Relationship between the intensity of the hydroxyl translation modes of kaolinite as a function of grinding time. Table 5. Results of the Band Component Analysis of DRIFT Spectra of the Low-Frequency Region of Kaolinite as a Function of Grinding Time Followed by Intercalation with Potassium Acetate kaolinite/quartz kaolinite ground for 1.0 h and then intercalated with potassium acetate kaolinite ground for 2.0 h and then intercalated with potassium acetate kaolinite ground for 6.0 h and then intercalated with potassium acetate kaolinite ground for 10.0 h and then ntercalated with potassium acetate
band parameters
ν1
ν2
578 9.1 39.6 579 1.6 18.9 578 3.5 22.0
558 25.8 39.6 560 0.5 4.0 562 10.4 24.0 556 11.8 23.0
center/cm-1
band % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1
ν3
547 31.9 41.7
ν4
ν5
ν6
ν7
ν8
ν9
ν10
ν11
538 23.5 23.1 535 8.6 23.26 539 25.6 27.5 538 15.1 21.5
507 0.4 6.9 506 0.3 2.74
472 21.6 18.2 480 6.6 12.4 478 7.1 17.3 476 13.8 17.6
466 1.1 6.3 469 12.6 14.4 469 11.3 16.2 468 2.8 7.2
457 1.0 5.9 455 1.3 7.1 457 2.5 5.4 458 13.0 14.7
432 4.6 13.5 434 3.6 11.8 433 3.1 16.3 435 11.2 17.6
419 2.5 15.9 421 2.4 11.7 414 4.0 14.7 415 0.3 4.6
408 0.1 2.6 409 2.2 3.9
523 0.8 13.2
408 0.5 2.9
approximately a linear function of the grinding time (Figure 9). The bands at 644 and 622 cm-1 are ascribed to the bending modes of AlOSi units. The OSiO bending modes of the kaolinite sheets are shown in Figure 10, and the band component analysis of this spectral region is reported in Table 5. What this figure and the data in Table 5 mean is that the spectra show increased complexity with grinding time. More bands are observed in the 10 h spectrum as compared with the 1 h spectrum. Bands in this region are attributed to OSiO and OAlO bending vibrations. This means that the mechanochemical treatment of the kaolinite has resulted in an increased number of OSiO units. These results suggest that the effect of the mechanochemical treatment is to break the SiOAl bonds. The gibbsite-like layer is held to the siloxane layer through this unit. Conclusions
Figure 10. DRIFT spectra of the OSiO bending vibrations of kaolinite mechanochemically activated for (a) 1, (b) 2, (c) 6, and (d) 10 h and intercalated with potassium acetate.
The surfaces of kaolinite have been modified through mechanochemical activation followed by intercalation. X-ray diffraction shows that in the absence of air the kaolinite is expanded to 11.9 Å with an additional expanded phase at 9.07 Å. The degree of expansion is related to the orientation of the acetate between the kaolinite layers. Upon exposure to moist air, the kaolinite re-expands to 14.2 and 10.02 Å. This expansion is controlled by the orientation of the acetate ion between the kaolinite layers. Upon mechanochemical activation of the kaolinite for 1 h and intercalation with potassium acetate, a phase with a d spacing of 8.80 Å is obtained which upon exposure to moist air re-expands to 13.50 Å.
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In this case, the orientation of the acetate is parallel to the 001 plane or at an angle to the 001 plane. Changes in the molecular structure of the mechanochemically activated intercalated kaolinite were followed by DRIFT spectroscopy. Fundamentally the intensity of the kaolinite hydroxyl stretching bands decreased exponentially with grinding time, and concomitantly the intensity of the bands attributed to the OH stretching vibrations of water increased. It is proposed that the mechanochemical activation of the kaolinite caused the conversion of the hydroxyls to water which coordinates the kaolinite surface.
Frost et al.
Concomitant changes in the hydroxyl deformation and translation modes are observed. Acknowledgment. The financial and infrastructural support of the Queensland University of Technology, Centre for Instrumental and Developmental Chemistry, is gratefully acknowledged. Financial support from the Hungarian Scientific Research Fund under Grant OTKA T34356 is acknowledged. LA0201422
