PREPARAÇÃO E MODIFICAÇÃO DE FIBRAS DE CARBONO ACTIVADAS A PARTIR DE FIBRAS TÊXTEIS ACRÍLICAS

Abstract

Activated carbon fibres were produced using three textile acrylic fibres from FISIPE-Fibras Sintéticas de Portugal, with a kidney bean cross section which is maintained after the activation, as seen by scanning electron microscopy. The carbon materials were produced by stabilisation of the precursors at 300ºC during 2 hours followed by carbonisation at 800ºC during 1 hour, both under a constant nitrogen flow. The fibres were then activated with carbon dioxide at 800 and 900ºC and kept at the final temperature for the appropriate time in order to obtain burn-offs within the range 10-90%.

The structural properties were determined by nitrogen adsorption at 77K and the isotherms analysed by s, Brunauer-Emmett-Teller (BET) and Dubinin-Radushkevich (DR) methods. The analysis showed that the fibres produced are microporous with apparent BET areas between 330 and 1535m2g-1, total micropore volumes within the range 0.16-0.73cm3g-1 and pore widths between 0.735 and 1.723nm. As the burn-off increases the total micropore volumes and pore widths also increase. The results show that the properties of the activated carbon fibres produced in this work have more interesting porosity characteristics than those produced from non-textile poliacrylonitrile, kevlar and nomex.

The chemical properties of these materials were analysed by Fourier Transform infrared spectroscopy (FTIR), elemental composition and point of zero charge (pzc). All the fibres have basic properties with pzc between 8-10 and contain a significant amount of nitrogen (>4%). The surface functional groups identified were amines, amides, lactones, pyrones, nitro, carbonyls and hydroxyls (free OH and phenol).

Selected samples were modified by thermal treatment using a microwave device. The results show that the treatment affects the porosity of the activated carbon fibres, causing a reduction in micropore volume and micropore size. More important, the results also show that the microwave treatment is a very effective and easy method for modifying the surface chemistry of the materials with a rapid obtention of very basic samples, with pzc approximately equal to 11. After storage in closed flasks for five months, without any precautions to prevent reoxidation, the samples were analysed and showed pzc’s and elemental compositions analogous to the as prepared materials, indicating that these samples are stable and do not suffer any significant ageing process.

Carbon vapour deposition by benzene pyrolisis (CVDb) at 800 and 900ºC reduces the pore widths and the micropore volumes (Vs in 47 and 59% and V0 in 40 and 52% respectively for the deposition at 800 and 900ºC), showing that during this treatment the carbon is deposited mainly on the pore walls. The best results for the improvement of molecular sieving properties for gas separation were obtained for the deposition at 900ºC.

The final section of this work deals with studies of the kinetics of adsorption at 25ºC of various gases of commercial importance, namely O2, N2, CO2 and CH4. The microwave thermal treatment did not produce good results in improving the ability for molecular sieving because the maximum adsorption capacity decreases after the treatment. The only exception is the sample F1-0m15 which showed a selective adsorption behaviour for CO2. Better performances were achieved by CVDb, mainly for the separation CO2/CH4 and with the sample F1-53d912.