Janos Kristof

Thermal treatment of mechanochemically activated kaolinite

The mechanochemical activation of a high defect kaolinite has been studied using a combination of high-resolution thermogravimetry and DRIFT spectroscopy. The effect of grinding causes a decrease in the dehydroxylation temperature and an increase in the amount of adsorbed/coordinated water. The temperature of dehydration also increases with grinding time. It is proposed that this dehydroxylation occurs through a homogenous process involving proton transfer through point heating. The amount of adsorbed water decreases with the increase in temperature of the thermal treatment.

Kaolinite-urea complexes obtained by mechanochemical and aqueous suspension techniques--a comparative study.

Intercalation compounds of low- and high-defect kaolinites have been prepared by direct reaction with urea aqueous solution as well as by co-grinding with urea in the absence of water (mechanochemical intercalation). The complexes formed were studied by X-ray diffraction, thermal analysis, DRIFT spectroscopy, and scanning electron microscopy. In aqueous solution the degree of intercalation for the low- and high-defect kaolinites was found to be 77 and 65%, respectively. With mechanochemical intercalation, both kaolinites were almost fully expanded after 1 h of grinding. Based on the results of DRIFT spectroscopy, a structural model for the bonding of urea to the siloxane surface is proposed. The kaolinite-urea intercalation compounds produced by mechanochemical intercalation have crystallite sizes lower than those obtained by the aqueous solution method.

The structure of an intercalated ordered kaolinite; a Raman microscopy study

Abstract Changes in the molecular structure of a highly ordered kaolinite, intercalated with urea and potassium acetate, have been studied using Raman microscopy. A new Raman band, attributed to the inner surface hydroxyl groups strongly hydrogen bound to the acetate, is observed at 3605 cm-1 for the potassium acetate intercalate with the consequential loss of intensity in the bands at 3652, 3670,3684 and 3693 cm-1. Remarkable changes in intensity of the Raman spectral bands of the low-frequency region of the kaolinite occurred upon intercalation. In particular, the 144 and 935 cm-1 bands increased by an order of magnitude and were found to be polarized. These spectroscopic changes provide evidence for the inner surface hydroxyl group-acetate bond being at an angle approaching 90° to the 001 face. Decreases in intensity of the bands at 243, 271 and 336 cm-1 were observed. The urea intercalate shows additional Raman bands at 3387, 3408 and 3500 cm-1 which are attributed to N-H vibrations after formation of the urea-kaolinite complex. Changes in the spectra of the inserting molecules were also observed.

Hydrazine-hydrate intercalated halloysite under controlled-rate thermal analysis conditions

The thermal behaviour of halloysite fully expanded with hydrazine-hydrate has been investigated in nitrogen atmosphere under dynamic heating and at a constant, pre-set decomposition rate of 0.15 mg min-1. Under controlled-rate thermal analysis (CRTA) conditions it was possible to resolve the closely overlapping decomposition stages and to distinguish between adsorbed and bonded reagent. Three types of bonded reagent could be identified. The loosely bonded reagent amounting to 0.20 mol hydrazine-hydrate per mol inner surface hydroxyl is connected to the internal and external surfaces of the expanded mineral and is present as a space filler between the sheets of the delaminated mineral. The strongly bonded (intercalated) hydrazine-hydrate is connected to the kaolinite inner surface OH groups by the formation of hydrogen bonds. Based on the thermoanalytical results two different types of bonded reagent could be distinguished in the complex. Type 1 reagent (approx. 0.06 mol hydrazine-hydrate/mol inner surface OH) is liberated between 77 and 103°C. Type 2 reagent is lost between 103 and 227°C, corresponding to a quantity of 0.36 mol hydrazine/mol inner surface OH. When heating the complex to 77°C under CRTA conditions a new reflection appears in the XRD pattern with a d-value of 9.6 Å, in addition to the 10.2 Ĺ reflection. This new reflection disappears in contact with moist air and the complex re-expands to the original d-value of 10.2 Å in a few h. The appearance of the 9.6 Å reflection is interpreted as the expansion of kaolinite with hydrazine alone, while the 10.2 Å one is due to expansion with hydrazine-hydrate. FTIR (DRIFT) spectroscopic results showed that the treated mineral after intercalation/deintercalation and heat treatment to 300°C is slightly more ordered than the original (untreated) clay.

Raman spectroscopy of urea and urea-intercalated kaolinites at 77 K.

The Raman spectra of urea and urea-intercalated kaolinites have been recorded at 77 K using a Renishaw Raman microprobe equipped with liquid nitrogen cooled microscope stage. The NH2 stretching modes of urea were observed as four bands at 3250, 3321, 3355 and 3425 cm(-1) at 77 K. These four bands are attributed to a change in conformation upon cooling to liquid nitrogen temperature. Upon intercalation of urea into both low and high defect kaolinites, only two bands were observed near 3390 and 3410 cm(-1). This is explained by hydrogen bonding between the amine groups of urea and oxygen atoms of the siloxane layer of kaolinite with only one urea conformation. When the intercalated low defect kaolinite was cooled to 77 K, the bands near 3700 cm(-1) attributed to the stretching modes of the inner surface hydroxyls disappeared and a new band was observed at 3615 cm(-1). This is explained by the breaking of hydrogen bonds involving OH groups of the gibbsite-like layer and formation of new bonds to the C=O group of the intercalated urea. Thus it is suggested that at low temperatures two kinds of hydrogen bonds are formed by urea molecules in urea-intercalated kaolinite.

INTERCALATION OF HALLOYSITE: A RAMAN SPECTROSCOPIC STUDY

Intercalates from an ordered halloysite with urea and potassium acetate were studied using Raman microscopy. The urea intercalate showed new Raman bands at 3387, 3410, 3497 and 3598 cm−1 which were attributed to the formation of a urea-Si2O5 complex. New Raman bands were observed at 3585 and 3602 cm−1 for the potassium acetate intercalate with concomitant loss of intensity of the bands at 3635, 3655, 3675 and 3696 cm−1. These new bands were attributed to the hydrogen bonds formed between the acetate and the inner surface hydroxyl groups. Remarkable changes in intensity in the lattice region of the halloysite were observed, the foremost being the reduction of the intensity of the bands at 243, 271 and 336 cm−1. Pronounced changes in the bands at 913 and 143 cm−1 attributed to the Al-OH librations were also observed.It is proposed that 2 distinct types of intercalation were present, as exemplified by: 1) urea intercalate, where the intercalating molecule hydrogen bonds to the Si-O of the halloysite layers and 2) potassium acetate intercalate, where the molecule is hydrogen-bonded to the inner surface hydroxyls of the halloysite layer and interacts with the tetrahedral sheet of the next adjacent halloysite layer. The Raman spectra of the intercalated halloysite strongly resembled that of an intercalated kaolinite.

Slow transformation of mechanically dehydroxylated kaolinite to kaolinite—an aged mechanochemically activated formamide-intercalated kaolinite study

Formamide-intercalated high defect kaolinite which was mechanochemically activated for periods of time up to 6 h has been aged for up to 1 year. These modified materials were studied using a combination of X-ray diffraction, thermal analysis and DRIFT spectroscopy. Ageing of the formamide-intercalated mechanochemically activated kaolinite results in de-intercalation of the formamide and the de-intercalated kaolinite returns to its original d-spacing. Thermal analysis shows that the temperature of dehydration and dehydroxylation increase by up to 30 °C. The temperature of the dehydroxylation of the aged samples was identical to that of the untreated kaolinite. The DRIFT spectroscopy showed that the spectrum of the aged samples approached that of the untreated kaolinite. The kaolinite showed partial de-intercalation and the 6 h sample had reformed to a mineral resembling the untreated kaolinite. The process of ageing the mechanochemically activated kaolinite enabled the reformation of the kaolinite.

Thermal behavior and decomposition of intercalated kaolinite

Intercalation complexes of a Hungarian kaolinite were prepared with hydrazine and potassium acetate. The thermal behavior and decomposition of the kaolinite-potassium acetate complex was studied by simultaneous TA-EGA, XRD, and FTIR methods. The intercalation complex is stable up to 300°C, and decomposition takes place in two stages after melting of potassium acetate intercalated in the interlayer spaces. Dehydroxylation occurred, in the presence of a molten phase, at a lower temperature than for the pure kaolinite. FTIR studies revealed that there is a sequence of dehydroxylation for the various OH groups of intercalated kaolinite. The reaction mechanism was followed up to 1000°C via identification of the gaseous and solid decomposition products formed: H2O, CO2, CO, C3H6O, intercalated phases with basal spacings of 14.1 Å, 11.5 Å, and 8.5 Å as well as elemental carbon, K4H2(CO3)3 · 1.5H2O, K2CO3 · 1.5H2O, and KAlSiO4.

FT-Raman spectroscopy of the lattice region of kaolinite and its intercalates

Abstract The structure of a highly ordered kaolinite from Kiralyhegy, Hungary, and an ordered halloysite from Szeg, Hungary, together with their urea and potassium acetate intercalates, have been studied using FT-Raman spectroscopy. Remarkable changes in the Raman spectra after intercalation of the kaolinites with urea or potassium acetate, in the lattice region of the kaolinite, were observed over the spectral range 150 to 1000 cm−1. In particular, for the potassium acetate intercalates, the 144 (Szeg) and 915 cm−1 (Kiralyhegy) bands increased by an order of magnitude and were found to be polarised with a depolarisation ratios of ca. 0.47. Decreases in intensity of the bands at 245, 274 and 334 cm−1 were observed upon intercalation. Changes in the Raman spectra of both the urea and potassium acetate intercalating molecules upon intercalation, form the basis of molecular models for the intercalation process.

Vibrational Spectroscopy of Intercalated Kaolinites. Part I.

Abstract The industrial application of kaolinite is closely related to its reactivity and surface properties. The reactivity of kaolinite can be tested by intercalation; that is, via the insertion of low-molecular-weight organic compounds between the kaolinite layers resulting in the formation of a nano-layered organo-complex. Although intercalation of kaolinite is an old and ongoing research topic, there is limited knowledge available on the reactivity of different kaolinites and the mechanism of complex formation, as well as on the structure of the complexes formed. Grafting and incorporation of exfoliated kaolinite in polymer matrices and other potential applications can open new horizons in the study of kaolinite intercalation. This article attempts to summarize (without completion) the most recent achievements in the study of kaolinite organo-complexes obtained with the most common intercalating compounds such as urea, potassium acetate, dimethyl sulphoxide, formamide, and hydrazine using vibrational spectroscopy combined with X-ray powder diffraction and thermal analysis.