Dehydroxylation Of Montmorillonite Research Articles

Thermostability of Organobentonite Modified with Poly(acrylic acid).

A new type of organobentonite foundry binder composed of a composite of bentonite (SN) and poly(acrylic acid) (PAA) was analyzed using thermal analysis (TG-DTG-DSC) and pyrolysis gas chromatography mass spectrometry (Py-GC/MS). The temperature range in which the composite retains its binding properties was identified using thermal analysis of the composite and its components. Results showed that the thermal decomposition process is complex and involves physicochemical transformations that are mainly reversible at temperatures in the ranges of 20-100 °C (related to evaporation of solvent water) and 100-230 °C (related to intermolecular dehydration). The decomposition of PAA chains occurs between 230 and 300 °C, while complete decomposition of PAA and formation of organic decomposition products takes place at 300-500 °C. Dehydroxylation of montmorillonite (MMT) in bentonite begins at about 500 °C, which leads to a drastic structural transformation. An endothermic effect associated with the remodeling of the mineral structure was observed on the DSC curve in the range of 500-750 °C. The produced SN/PAA composite was found to be thermostable during degradation in both oxidative and inert atmosphere, similar to the starting bentonite, and even maintained over a relatively higher and wider temperature range compared to organic binding materials used. At the given temperatures of 300 °C and 800 °C, only CO2 emissions occur from all the examined SN/PAA samples. There is no emission of compounds from the BTEX group. This means that the proposed binding material in the form of the MMT-PAA composite will not pose a threat to the environment and the workplace.

Thermogravimetry and differential thermal analysis of montmorillonite treated with 1,4-diaminoanthraquinone

Neat organic dye 1,4-diaminoanthraquinone (DAAQ) was investigated by TG, DTG, and DTA in air and under nitrogen up to 900 °C. Total mass loss was 79.1 and 38.3 % in air and under nitrogen, respectively. The residues in both heating cells were dark black, suggesting that they were charcoals which required temperatures higher than 900 °C for oxidation or pyrolysis. Complexes of montmorillonite (MONT) with this organic dye were investigated by TG, DTG, and DTA in air and under nitrogen up to 900 °C. Aqueous solubility of DAAQ is very low (0.33 mg/1 l) and consequently the loading of the clay by the dye is very small (5–24 mmol DAAQ/100 g clay). Five saturated dye solutions were prepared for the synthesis of five complexes, (i) in distilled water and (ii–v) in 0.1, 0.5, 1.0, and 2.0 molar HCl. The DTA and DTG peaks of the thermal reactions of the organoclay complexes (oxidation in air and pyrolysis under nitrogen) were very weak and many were not detectable. On the other hand, inflections were detected in the TG curves and were considered as upper and lower borders of mass-loss steps. Conclusions on reaction steps which are responsible for mass loss were based on previous knowledge of the thermal reactions of organoclays. Six mass-loss steps were identified in air and seven steps under nitrogen. Mass-loss steps in air were attributed to evolution of interlayer water, H-bonded water, dehydroxylation of MONT, and to three steps of oxidation of intercalated DAAQ. Under nitrogen, steps were attributed to evolution of interlayer water, H-bonded water, dehydroxylation of MONT, and four steps of pyrolysis of DAAQ.

Studies on the solid acidity of heated and cation-exchanged montmorillonite using n-butylamine titration in non-aqueous system and diffuse reflectance Fourier transform infrared (DRIFT) spectroscopy

The effects of heating and cation exchange on the solid acidity of montmorillonite were investigated using n-butylamine titration in non-aqueous system and diffuse reflectance Fourier transform infrared spectroscopy. The number of total, Bronsted, and Lewis acid sites showed the same modulation tendency with increasing heating temperature, reaching a maximum at 120 °C and subsequently decreasing until it reaches a minimum at 600 °C. The Lewis acid sites result from unsaturated Al3+ cations, and their number increased with the heating temperature due to the dehydration and dehydroxylation of montmorillonite. The generation and evolution of Bronsted acidity were mainly related to interlayer-polarized water molecules. Water adsorbed on the unsaturated Al3+ ions also acted as a Bronsted acid. The acid strength of the Bronsted acid sites was dependent on the polarization ability of the exchangeable cation, the amount of interlayer water, and the degree of dissociation of the interlayer water coordinated to exchangeable cations. All cation-exchanged montmorillonites exhibited different numbers of acid sites and various distributions of acid strength. Bronsted acidity was predominant in Al3+-exchanged montmorillonite, whereas the Na+- and K+-exchanged montmorillonites showed predominantly Lewis acidity. Moreover, Mg2+- and Li+-exchanged montmorillonites exhibited approximately equal numbers of Bronsted and Lewis acid sites. The Bronsted acidity of cation-exchanged montmorillonite was positively correlated with the charge-to-radius ratios of the cations, whereas the Lewis acidity was highly dependent on the electronegativity of the cations. The acid strengths of Al3+- and Mg2+-exchanged montmorillonites were remarkably higher than those of monovalent cation-exchanged montmorillonites, showing the highest acid strength (H 0 ≤ −3.0). Li+- and Na+-exchanged montmorillonites exhibited an acid strength distribution of −3.0 < H 0 ≤ 4.8, with the acid strength ranging primarily from 1.5 to 3.3 in Li+-exchanged montmorillonite, whereas only weaker-strength acid sites (1.5 < H 0 ≤ 4.8) were present in K+-exchanged montmorillonite. The results of the catalysis experiments indicated that montmorillonite promoted the thermal decomposition of the model organic. The catalytic activity showed a positive correlation with the solid acidity of montmorillonite and was affected by cation exchange, which occurs naturally in geological processes.

Influence of heating on the solid acidity of montmorillonite: A combined study by DRIFT and Hammett indicators

The solid acidity of montmorillonite and its heated products was determined by combining Hammett indicators and diffuse reflectance Fourier transform infrared spectroscopy. The total amount of solid acid sites was 0.10 mmol/g montmorillonite and increased after heating. The amount of Lewis acid sites, unsaturated Al 3+ ions, increased due to the dehydration and dehydroxylation of montmorillonite. The Brønsted acid sites resulted from interlayer polarized water molecules, H 3O + ions adsorbed by the negatively charged AlO 4, and water adsorbed on the unsaturated Al 3+ ions. The Brønsted acid amount increased after heating and the largest value was found at 120 °C due to the enhanced dissociation of the interlayer water molecules. It decreased with further heating due to the gradual removal of the interlayer water. The determination of solid acid sites was affected by the small basal spacing of montmorillonite after calcination at 400 and 600 °C. High-strength acid sites, appearing after heating at 120, 200 and 600 °C were attributed to interlayer polarized water and rehydrated unsaturated Al 3+ ions.