Crisotilas brasileiras : caracterização dos sitios superficiais por cromatografia inversa, microscopia de força atõmica e espectroscopia no infravermelho

AUTOR(ES)
DATA DE PUBLICAÇÃO

1995

RESUMO

Chrysotile, a fibrous serpentine, is an asbestos mineral with potential applications in many chemical areas. Although extensively studied, some aspects of the nature and properts of its surface sites are obscure, specially for the brazilian chrysotiles. ln this work we owe the characterization of the surface sites of brasilian chrysotile coming from the mine of Canabrava, Uruaçú (Go). Three differents and complemenmry techniques, were used to reforce the knowledge about chrysotile overall properties. The samples studied were sieved and used both in the natural form and lixiviated with HCl attack on reflux. Images of these samples were obtained by atomic force microscopy. Adsorption of probe molecules was followed by inverse chromatography. Infrared adsorption spectra of adsorbed samples were also recorded. Atomic force images showed that both natural and lixiviated chrysotile are constituted of cillindric fibers, with mean diameters of about 14 nm and 183 nm respectivelly. In the case of natural chrysotilc the ends af the sheets were observed predominantly parallel to the fiber lenght, and this sheets are about 7 Å in thickness, in agreement with cristalographical data. Images show that lixiviated chrysotile is microporous. Liquid chromatography columns packed with these matherials were used to obtain adsorption isotherms with benzene, toluene and anisole, at 30, 35 and 40°C for each column. The isotherms obtained showed good linearity and effetive variations with the temperature. Adsorption enthalpy changes for the studied systems were calculated by means of the Clausius-Clapeyron equation. The values obtained are 57, 64 and 73 kJ/mol for benzene, toluene and anisole on the natural chrysotile, and 54, 126 and 116 kJ/mol on the lixiviated chrysotile. The colunms used were evaluated, and the efficiency obtained was about 2000 and 1000 theorical plates per metter for the natural and the lixiviated chrysitiles, respectivelly. They provided the complete separation of mixtures benzene/anisole and benzene/acetone, respectivelly. Infrared data analysis in the OH stretching region showed that the surface OH are the effective adsorption sites. The spectras showed a reduction in the free OH stretching absorption intensity with consequently increase in its intensity at the pereturbated frequency, when is increased adsorbed quantities. The sum of the frequencis shifts was about 140 and 66 cm for the natural chrysotile, and 280 and 85 cm for the lixiviated chrysotile, and is consistent with the adsorption enthalpy values obtained from chromatographic data for the same systems. The fact that the surface hydroxylcs are the most important groups in the adsorption process is consistent with the geometrical data of the natural chrysotile fibers in wich 95% of the surface is covered by hydroxyles. The adsorption enthalpy values were associated with the benzenic ring substituents power to donnoute or accept the eletrons of the p system. This fact points to an interaction of chemical nature, consistent with the observation of higher adsorption at higher temperature, for all systens studied.

ASSUNTO(S)

espectroscopia de infravermelho analise cromatografica crisotila

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