²⁹Si- and²⁷Al-MAS/NMR study of the thermal transformations of kaolinite

Abstract

Thermal transformations of kaolinite were studied by zgsi-and ZVA1-MAS/NMR and ESR techniques. In metakaolin, Si is still dominantly in the Q3 state (three Si atoms bonded to an Si-O4 tetrahedron) and A1 detectable by NMR is in both 4-coordination and 6-coordination. Coordination polyhedra around A1 or Fe replacing A1 are much distorted. Metakaolin crystallizes into y-alumina and mullite exothermically at ~ 980 ~ this crystallization being preceded by a faint endothermic reaction. The latter is due to segregation of SiO2 and A1203, which results in an increase of Si in the Q4 state and an increase of 6-coordinated A1203 .Kaolin minerals dehydroxylate at ~600~ this process corresponding to an intense endothermic peak on the DTA curve. The resultant anhydrous phase, called metakaolin or metakaolinite, is amorphous to X-rays. y-alumina plus mullite develop from the amorphous phase at ~ 980~ and this reaction is represented by a combination of a faint endotherm and a sharp and intense exotherm. The mechanism of this series of reactions has been studied from various standpoints, as reviewed by Brindley & Nakahira (1959), Tsuzuki (1971) andBrindley (1976). X-ray diffraction, electron microscopy, and X-ray fluorescence, IR absorption and M6ssbauer spectroscopy have all been used to study these reactions. Recent developments in high-resolution magic-angle-spinning MAS/NMR spectroscopy have allowed a new approach to this problem, Komarneni et al. (1985), MacKenzie et al. (1985b) and Brown et al. (1985) being forerunners in the application of this method to metakaolin and high-temperature phases of kaolinite. This paper gives the results of examination of a kaolinite heated to various temperatures, using 29Si-and 27A1-MAS/NMR and ESR techniques. Some new data are presented on the thermal transformation of kaolinite.

Kinetics of the chrysotile and brucite dehydroxylation reaction: a combined non-isothermal/isothermal thermogravimetric analysis and high-temperature X-ray powder diffraction study

“…vertical to the TO layers; 3 'circular' diffusion along the interlayer (b axis or a mixture between a and b axis) some of the liberated OH groups are trapped in nominally anhydrous forsterite (Trittschack unpublished work). Similar observations were made during dehydroxylation experiments of kaolinite where more than 10 % persist within the structure (MacKenzie et al 1985;Watanabe et al 1987). Those OH remnants are difficult to remove within the temperature range of the primary dehydroxylation reaction and may also contribute to a significant increase in E a .…”

Quantification of Crystalline and Noncrystalline Material in Ground Kaolinite by X-Ray Powder Diffraction, Infrared, Solid-State Nuclear Magnetic Resonance, and Chemical-Dissolution Analyses

“…Nevertheless, the NMR data appear to support the estimates by the chemical extraction methods. 27A1 and 298i MAS-NMR spectra of the ground kaolinite samples presented in Figures 6-8 bear a remarkable similarity to those reported for metakaolin (MacKenzie et al, 1985;Watanabe et al, 1987). The structural concepts proposed for metakaolin (Brindley and Nakahira, 1959;Freund, 1973) may therefore help explain the structural damage introduced by dry-grinding.…”

“…As seen in Figure 8, the AI(VI) signals were shifted from -4.1 to 2.0 ppm, and the AI(IV) signals shifted from 56 to 60 ppm as grinding time increased. A similar tendency for 27A1 NMR spectra to shift to the low field has been observed for kaolinite samples heated at various temperatures (Watanabe et al, 1987).…”

“…The signal intensity ratio, however, is not necessarily equivalent to the abundance ratio of Al(IV) and AI(VI) nuclei in the structure. Possibly some of the A1 atoms were in sites that are much distorted from spherical symmetry for NMR detection (Watanabe et al, 1987). This phenomenon could have been due to electric field gradients resulting from crystallographic distortions and/or defects that had been induced by grinding.…”

Solid-state NMR characterisation of the thermal transformation of a Hungarian white illite

“…There have been various multinuclear NMR reports (especially 29 Si and 27 Al) of the thermal decomposition of clay minerals. The most comprehensive NMR data set from clays has been for 1:1 clay minerals, such as kaolinite [14][15][16][17][18][19][20][21][22][23][24] and related phases such as halloysite [25]. For 2:1 type clay minerals pyrophyllite is a cation-free clay that has no iron and it shows different thermal breakdown characteristics from the cation-free 1:1 clay minerals, with much more AlO 5 in the intermediate dehydroxylate state [26][27][28].…”