Teresa A. Centeno

Mesoporous carbons with graphitic structures fabricated by using porous silica materials as templates and iron-impregnated polypyrrole as precursor

Templated mesoporous carbons containing graphitic structures and a well-developed porosity were synthesised by using mesoporous silica materials (i.e. SBA-15 and silica xerogel) as templates and polypyrrole as carbon precursor. The polypyrrole was synthesised by in situ polymerization of pyrrole which was infiltrated into the silica pores containing FeCl3. The carbonization of polypyrrole in presence of iron allows carbon structures with some degree of graphitic order to be formed. The samples were characterised by XRD, nitrogen sorption, TEM, Raman and elemental analysis. The results show that the templated carbons have BET surface areas >1000 m2 g−1 and a porosity built up by two pore systems with sizes centred at ∼3–4 nm and at 10–14 nm. In addition, they have a good electrical conductivity due to the presence of graphitic structures in the carbon framework.

Carbon molecular sieve gas separation membranes based on poly(vinylidene chloride-co-vinyl chloride)

The preparation of a composite carbon membrane from Poly(vinylidene chloride-co-vinyl chloride) is described. The membrane is formed by a thin microporous carbon layer (Thickness, 0.8 μm) obtained by pyrolysis of a polymeric film supported over a macroporous carbon substrate (Pore size, 1 μm; Porosity, 30%). Single gas permeation experiments with pure gases of different molecular size (He, CO2, O2, N2 and CH4) were performed at different temperatures between 25°C and 150°C. It has been observed that the microporous carbon layer exhibits molecular sieving properties and it allows the separation of gases depending on their molecular size. The carbon membrane shows high selectivities for the separation of permanent gases like O2/N2 system (Selectivity≈14 at 25°C). Air preoxidation at 200°C for 6 h improves the permselectivity but a loss in gas permeance is observed.

On the characterization of microporous carbons by immersion calorimetry alone

Immersion calorimetry is a useful tool for the characterization of solid surfaces in general, but in the case of microporous solids it usually requires complementary information, obtained from an adsorption isotherm. This article demonstrates the possibilities and the limitations of this technique when used alone. It appears that the use of a standard value for the enthalpy immersion does not always provide a reliable assessment of the total surface area.

Carbon molecular sieve membranes derived from a phenolic resin supported on porous ceramic tubes

The preparation of a composite carbon membrane from a phenolic resin is described. The membrane is formed by a thin microporous carbon layer (thickness: 2 μm) obtained by pyrolysis (700°C, under vacuum) of a phenolic resin film supported on the inner face of a porous alumina tube. The separation characteristics of the resulting carbon membranes were analysed from permeation experiments with pure gases of different molecular size (He, CO2, O2, N2 and CH4) and separation of binary gas mixtures O2–N2 and CO2–CH4. An almost defect-free carbon membrane is obtained in only one casting step. The effective micropore size was estimated to be around 4.4 A. The prepared carbon membranes have demonstrated to be effective for separating gas mixtures such as O2–N2 (O2 permeance: 100 Barrer; O2–N2 separation factor: 12) and CO2–CH4 (CO2 permeance: 400 Barrer; CO2–CH4 separation factor: 150). The oxidation of the phenolic resin film with air at temperatures ranging from 150 to 300°C improves the gas permeance and originates a decrease in the permselectivity.

Water adsorption on activated carbons with different degrees of oxidation

A typical activated carbon, derived from olive stones, has been oxidized to different degrees with (NH 4 ) 2 S 2 O 8 and analysed by water vapour adsorption, immersion calorimetry, acid–base titration and temperature-programmed desorption of CO 2 and CO monitored by mass spectrometry. These techniques led to a coherent description of the surfaces and of their chemistry. The water adsorption isotherms, of type IV, were decomposed into type I and V contributions and analysed in terms of the Dubinin–Astakhov equation. The corresponding calculated enthalpies of immersion into water are in agreement with the experimental values. The number of carboxyl, lactone, phenol and basic groups identified by titration, can also be related to the parameters of the Dubinin–Astakhov equation and to the enthalpy of immersion into water. Finally, a good linear correlation is found between the amounts of CO 2 and CO desorbed from the surface, the enthalpies of immersion into water and the total number of sites identified on the surface.

Preparation of supported carbon molecular sieve membranes

Abstract Carbon molecular sieve membranes were prepared by carbonization of a polymeric film supported on a macroporous carbon substrate. Disk-shaped macroporous carbon supports (35 mm in diameter and 2.5 mm in thickness) were obtained by carbonization of fine particles of graphite blended with a phenolic resin. The polymeric precursor of the selective carbon film was a polyamic acid commercially available in solution that after imidization led to formation of BPDA-pPDA polyimide. The support coated with the polymeric solution was heated following the sequence: (i) drying at 150°C during 1 h in air (heating rate: 3°C/min); (ii) imidization under vacuum at 380°C (heating rate: 1°C/min); (iii) carbonization under vacuum at temperatures ranging between 550°C and 700°C (heating rate: 0.5°C/min). This procedure provided one-coat membranes. It was repeated to produce two- and three-coat carbon membranes. At 25°C the resulting carbon membranes showed selectivity O2/N2 of 14 and He/N2 of 136 and permeation rate of pure He and O2 of 27.2×10−10 and 2.8×10−10 mol/m2 s Pa, respectively.

Porous structure of polyarylamide-based activated carbon fibres

Abstract Activated carbons were prepared from two polyarylamide fibres (Kevlar and Nomex) by carbonization and subsequent steam activation to 25 and 50% burn-off. The porous structure of the carbons has been investigated by physical adsorption of CH2Cl2 and N2O at 293 K and immersion calorimetry into liquids of different molecular dimensions. The activated fibres have good sorptive capacities and relatively narrow micropores. The materials resulting from activation at low degrees of burn-off could therefore be of great interest in gas separation, owing to their molecular-sieve properties. Compared to industrial activated carbons, the fibres are hydrophilic and have small external surface areas.

Distillation of granulated scrap tires in a pilot plant

This paper reports the pyrolytic treatment of granulated scrap tires (GST) in a pilot distillation unit at moderate temperature (550°C) and atmospheric pressure, to produce oil, char and gas products. Tire-derived oil is a complex mixture of organic C(5)-C(24) compounds, including a very large proportion of aromatic compounds. This oil has a high gross calorific value (∼ 43 MJ kg(-1)) and N and S contents of 0.4% and 0.6%, respectively, falling within the specifications of certain heating fuels. The distillation gas is composed of hydrocarbons; methane and n-butane are the most abundant, investing the distillation gas with a very high gross calorific value (∼ 68 MJ Nm(-3)). This gas is transformed into electric power by a co-generation turbine. The distillation char is mostly made of carbon but with significant inorganic impurities (∼ 12 wt%). The quality of the solid residue of the process is comparable to that of some commercial chars. The quantity of residual solids, and the qualities of the gas, liquid and solid fractions, are similar to those obtained by conventional pyrolytic treatments of waste tires. However, the simplicity of the proposed technology and its low investment costs make it a very attractive alternative.

Water adsorption in carbons described by the dubinin–astakhov and dubinin–serpinski equations

It is shown that the water adsorption isotherm of type IV, observed for a number of carbons near room temperature, can be decomposed into two contributions of types I and V. The corresponding isotherms can be treated as Dubinin–Astakhov equations. The initial section suggests the presence of sites with characteristic energies, E, in the range of 5–8 kJ mol–1 and similar contributions to the molar enthalpy of immersion of the carbons into water. Similar values are obtained from the analysis of earlier data of Dubinin. The second part of the isotherm, of type V, is compatible with the earlier model of Dubinin and Serpinski. A satisfactory agreement is also found in most cases between the enthalpies of immersion calculated on the basis of the DA model and the experimental values.

Characterization of industrial activated carbons by adsorption and immersion techniques and by STM

Abstract The results obtained from a variety of techniques (vapour adsorption, selective adsorption of caffeine and immersion calorimetry) used to characterize activated carbons of industrial origin are compared. It is shown that gate effects due to constrictions, and often overlooked, can be revealed by simple experiments with larger molecular probles. Immersion calorimetry appears to be a simple and efficient technique, as illustrated by an activation series. Complementary information is also provided by scanning tunnelling microscopy (STM).