INTERPRETATION OF SOLID STATE 13C AND 295i NUCLEAR MAGNETIC RESONANCE SPECTRA OF KAOLINITE INTERCALATES

~3C and 295i nuclear magnetic resonance spectroscopy with magic-angle spinning bas been used to study the short-range ordering and bonding in the structures of intercalates of kaolinite with formamide, hydrazine, dimethyl sulfoxide (DMSO), and pyridine-N-oxide (PNO). The 29Si chemical shift indicated decreasing levels of bonding interaction between the silicate layer and the intercalate in the order: kaolinite: formamide (5 = 91.9, ppm relative to tetramethylsilane), kaolinite: hydrazine ( 92.0), kaolinite: DMSO (-93.1). The 29Si signal of the kaolinite: PNO intercalate (-92.1) was unexpectedly deshielded, possibly due to the aromatic nature of PNO. The degree of three-dimensional ordering of the structures was inferred from the 295i signal width, with the kaolinite: DMSO intercalate displaying the greatest ordering and kaolinite : hydrazine the least. ~3C resonances of intercalating organic molecules were shifted downfield by as much as 3 ppm in response to increased hydrogen bonding after intercalation, and in the kaolinite : DMSO intercalate the two methyl-carbon chemical environments were non-equivalent (6 = 43.7 and 42.5). Key Words--Dimethylsulfoxide, Formamide, Hydrazine, Intercalate, Nuclear magnetic resonance, Ordering, Pyridine-N-oxide. I N T R O D U C T I O N Solid-state 295i magic-angle spinning nuclear magnetic resonance (MAS/NMR) spectroscopy has proven to be sensitive to the short range ordering of minerals. In particular, the 29Si chemical shift in synthetic and natural silicates and aluminosilicates is sensitive to: (1) the chemical nature o f the a toms directly attached to oxygens of the silicate tetrahedra, also described as the second coordinat ion sphere of silicon (Lippmaa et al., 1980; M~igi et al., 1981); (2) interatomic distances (Smith and Blackwell, 1983; Higgins and Woessner, 1982) and angles (Smith and Blackwell, 1982); and (3) hydrogen bond strength (Thompson, 1984b). Solid state ~3C MAS/NMR, however, has not provided significantly more information than was available from solution studies, though a chemical shift non-equivalence not observable in solution has commonly been observed in the solid (Wasylishen and Fyfe, 1982). In this present work the above two techniques have been combined to investigate further the structures of and bonding in a selection o f well-documented kaolinite intercalates: kaol in i te : formamide (Adams et aL, 1976; Ledoux and White, 1966), kaolini te: dimethyl sulfoxide (DMSO) (Jacobs and Sterckx, 1970; Olejnik et aL, 1968; Sanchez and Gonzalez, 1970), kaolini te: pyridine-N-oxide (PNO) (Olejnik et aL, 1971; Weiss and Orth, 1973), and kaolinite: hydrazine (Weiss et aL, 1963; Ledoux and White, 1966). Solid-state 13C and 298i M A S / N M R spectroscopy should provide complementary information to the infrared spectroscopic data and to the oneand three-dimensional structural data Copyright 9 1985, The Clay Minerals Society already obtained. Conversely, the infrared and structural data should be indispensable in the interpretat ion of the N M R data. E X P E R I M E N T A L The material chosen for intercalation was kaolinite #4, Oneal pit, Macon, Georgia, supplied by Ward ' s Natural Science Establishment, Rochester, New York. It was chosen for its low iron content (< 0.5 % as Fe203), and narrow 29Si N M R resonance (signal width at hal f height (Av,/~) = 8 3 Hz). This kaolinite is relatively poorly crystallized, with a Hinckley crystallinity index o f 0.4 3 (Hinckley, 1963), and was found to intercalate more readily than well-crystallized kaolinites. The measured cation-exchange capacity (CEC) is 11.2 meq/100 g of kaolinite. The only contaminating phase present is about 5% smectite (d = 14.8 A). The intercalates with formamide (kaolinite: formamide), DMSO (kaolinite: DMSO), and hydrazine (kaolinite:hydrazine) were prepared by immersing the clay in the pure reagent (lab. grade) at 60~ for 2 weeks, 1 week, and 24 hr, respectively. The first two products were subsequently dried at 60~ for 48 hr and then left to stand for several weeks. The hydrazine intercalate, due to its instabil i ty in air, was prepared and dr ied immediately prior to analysis. The intercalate with PNO (kaolinite: PNO) was prepared by entra inment with hydrazine; i.e., kaol in i te :hydrazine was saturated in liquid PNO at 70~ for 24 hr, and dried at 100~ for 24 hr to evaporate the hydrazine. The excess PNO was removed by rapid washing with chloroform.