Using alkali metals to control reactivity and porosity during physical activation of demineralised kraft lignin

Demineralised kraft lignin was impregnated with between 6.2% and 50% NaCl or KCl and physically activated in CO2 at 750 C. The results presented show that a considerable reduction in activation time even at a comparatively low activation temperature could be achieved, particularly when using KCl. Considering a fixed level of burn-off, the impregnation did not affect the pore volume and only increased the pore width by about 0.1–0.2 nm, depending on the concentration of NaCl or KCl used. By controlling the conditions it was possible to obtain predominantly ultramicroporous materials with mean pore widths over the range 0.53–0.77 nm. On the other hand, at high levels of burn-off there was evidence for micropore widening into the small mesopore range and also for the formation of a secondary mesopore structure. Under these conditions it was possible to obtain materials with pore volumes as high as 0.82 cm3 g 1

Controlling the micropore size of activated carbons for the treatment of fuels and combustion gases

Exclusively microporous activated carbons have been prepared from cork by physical and chemical activation under different conditions. The results show that it is possible to control the pore size of the activated carbons and to obtain materials with narrow micropore size ( 0.69 nm) and high micropore volume ( 0.64 cm3 g 1) equal to or better than the best activated carbon fibres. Higher micropore volumes are generally obtained by chemical activation at higher temperature using dry or potassium hydroxide impregnation. On the other hand, wet or carbonate impregnation, as well as high temperature, or physical activation with CO2 or H2O under appropriate conditions, favours low mean pore widths

Application of different equations to adsorption isotherms of phenolic compounds on activated carbons prepared from cork

Activated carbons were prepared from solid cork wastes by physical activation with carbon dioxide or steam, and chemical activation by impregnation with phosphoric acid. In this work we show the possibility of using these activated carbons for the adsorption of phenolic compounds from the aqueous phase. The materials present a different response to the adsorptives used (p-nitrophenol, p-chlorophenol, p-cresol and phenol), depending on the type of activation and the parameters (burn-off, absolute concentration) used in each case. All the samples were capable of retaining the contaminants, with the best result being reached by the sample with higher burnoff and the worst with the carbonised, while intermediate values were reached with the remaining samples. The experimental isotherms were analysed with two and three parameters equations (Freundlich, Langmuir, Dubinin–Radushkevich–Kaganer and Redlich–Peterson). The results obtained from the application of the equations are similar in some aspects, but the degree of confidence is quite different. The best fit was achieved with the Redlich–Peterson equation, which can be explained by the fact that this has three adjustable parameters. However, overall the Freundlich and DRK equations appear to be more useful and provide parameters which can be correlated with the structural characteristics of the solids obtained from N2 adsorption measurements

New carbon materials with high porosity in the 1–7 nm range obtained by chemical activation with phosphoric acid of resorcinol–formaldehyde aerogels

Chemical activation of resorcinol–formaldehyde aerogels with phosphoric acid results in materials containing both intra-particle microporosity (pore volume ˇ0.18 cm3g 1 and mean-pore-width ˇ1 nm) and inter-particle micro/mesoporosity. The latter forms as a result of partial collapse of the mesopore structure of the organic aerogel and can be controlled by varying the phosphoric acid/organic aerogel ratio. Increasing this ratio leads to higher pore volume and size and it was possible to obtain micro/mesopore volumes as high as 1.23 cm3g 1 with pore widths up to ˇ7 nm. Over 90% of these pores were accessible even after blocking all of the ultramicroporosity by pre-adsorption of n-nonane

Influence of Degassing Temperature on the Performance of Carbon Molecular Sieves for Separations Involving O2, N2, CO2, and CH4

arbon molecular sieve Takeda 3A was studied. The results indicate that pore mouth barrier controls nitrogen transport. For oxygen and carbon dioxide, however, two mechanisms are present. Pore mouth barrier control determines the transport at lower temperature degassing, and micropore diffusion is present with a high temperature degassing. When the degassing temperature is increased, the adsorption as a function of contact time is almost constant for O2 and CO2, is almost null for CH4, and increases significantly for N2 between 373 and 653 K. The rates of diffusion also increase with increasing degassing temperature, but more appreciably for N2, which results in a significant decrease in selectivity for O2/N2 and CO2/N2 separations. These separations are therefore more efficient for Takeda 3A if it is only subjected to temperatures lower than 373 K. On the other hand, for CO2/CH4, the separation is more efficient if the Takeda 3A sample is first submitted to a degassing temperature around 673 K

Pore size control in activated carbons obtained by pyrolysis under different conditions of chemically impregnated cork

Activated carbons were prepared by the pyrolysis of cork impregnated with potassium and sodium hydroxides and carbonates as well as phosphoric acid and the effect of five experimental parameters, namely method of impregnation, impregnant concentration, mass ratio, precursor particle size and pyrolysis temperature, were studied. It is shown that cork is a versatile precursor and allows us to prepare a wide variety of materials with quite different pore structural characteristics by precise control of the impregnation and pyrolysis conditions. Even under relatively mild conditions, it was possible to produce cork based carbons with high pore volumes, in the range 0.5–0.7 cm3 g 1, and to simultaneously control the mean pore width over a three-fold range from a value as low as 0.7 nm up to a value as high as 2.2 nm. The best materials produced present pore structural characteristics which are significantly different to the vast majority of commercial activated carbons. In particular, the possibility of obtaining such high pore volumes in essentially microporous materials, containing virtually no mesoporosity in most cases, is noteworthy. Furthermore, the fact that it was possible with some samples to combine high pore volume and very narrow micropore size is a particularly notable achievement

Low-Cost Adsorbents: Growing Approach to Wastewater Treatment—a Review

Industrial, agricultural, and domestic activities of humans have affected the environmental system, resulting in drastic problems such as global warming and the generation of wastewater containing high levels of pollutants. As water of good quality is a precious commodity and available in limited amounts, it has become highly imperative to treat wastewater for removal of pollutants. In addition, the rapid modernization of society has also led to the generation of huge amount of materials of little value that have no fruitful use. Such materials are generally considered as waste, and their disposal is a problem. Also, there are some materials that are available in nature that have little or no use. The utilization of all such materials as low-cost adsorbents for the treatment of wastewater may make them of some value. An effort has been made to give a brief idea of an approach to wastewater treatment, particularly discussing and highlighting in brief the low-cost alternative adsorbents with a view to utilizing these waste/low-cost materials

Core-shell polymer aerogels prepared by co-polymerisation of 2,4-dihydroxybenzoic acid, resorcinol and formaldehyde

DHBARF polymer aerogels were prepared from mixtures containing a fixed stoichiometric amount of formaldehyde (F) and varying proportions of resorcinol (R) and 2,4-dihydroxybenzoic acid (DHBA), with the objective of combining the advantages of high mesopore volume and solids content of RF aerogels with the ion exchange capacity of DHBAF aerogels. The results show that surface area, pore volume, pore size and ion exchange capacity vary in a systematic fashion as the composition of the synthesis mixture is altered. It was found that up to a DHBA mole fraction of about 0.5 preferential phase separation of RF occurred and lead to the formation of primary particles consisting of an RF core and a rough DHBAF shell. When DHBAF and DHBARF aerogels were prepared using the same %DHBA, it was found that the addition of R to the synthesis mixture resulted in increased values of surface area, mesopore volume and mean diameter while simultaneously maintaining the ion exchange capacity of the wet gel. The optimum compromise was found for a DHBARF aerogel with core-shell structure prepared using 5% DHBA and 5% R. Under these conditions the ion exchange capacity was at its maximum value of slightly higher than 2 meq g 1, the surface area was 850 m2 g 1, the mesopore volume was 2 cm3 g 1 and the mesopore mean diameter was 17 nm

Separating Surface and Solvent Effects and the Notion of Critical Adsorption Energy in the Adsorption of Phenolic Compounds by Activated Carbons

A modified form of the Freundlich equation in which the solute equilibrium concentration is normalized with respect to the solute solubility is analyzed and applied to adsorption isotherms of phenol, 4-nitrophenol, 4-chlorophenol, and 2-chlorophenol at different values of pH on commercial activated carbon before and after oxidation. The analysis confirms the importance of normalizing the solute equilibrium concentration when analyzing the adsorption isotherms, and it is suggested that a parameter, KF10, obtained by taking 10%solubility as the reference point when applying the Freundlich equation, is probably the best comparative estimate of the relative adsorption capacity of the carbon for different phenolic compounds. In combination with the Freundlich exponent, nF, estimates of the adsorption capacity at any other reference point can then be obtained. Analysis of the experimental results also indicates a need to distinguish between two regimes of adsorption, characterized by an adsorption energy, Eads, greater than or less than a critical value, Eca. When Eads>Eca, the shape of the adsorption isotherm is determined by solute-solid interactions. On the other hand, when Eads < Eca, solute-solution interactions become more important

Adsorption of Bovine Serum Albumin onto Mesocellular Silica Foams with Differently Sized Cells and Windows

As a model protein with quite large dimensions, Bovine Serum Albumin (BSA) has been used to evaluate the influence of the distinct pore structural characteristics of three mesocellular foam (MCF) materials prepared with or without the addition of ammonium fluoride and with varying 1,3,5-trimethylbenzene/Pluronic P123 (TMB/P123) ratios. SBA-15 was also studied for comparative purposes. Characterisation by X-ray diffraction and electron microscopy confirmed the characteristic spheroid cell structure of the MCF pores. Nitrogen adsorption/desorption isotherms at 77 K revealed the different pore structural parameters of the MCF, viz. pore volume (1.8–2.4 cm3/g), cell size (24.6–28.5 nm) and window size (11.3–17.3 nm), as obtained by the NLDFT method. The equilibrium adsorption isotherms and the kinetic adsorption data for BSA at 298 K and pH 5 were well fitted by the Langmuir model and pseudo-second-order kinetic model, respectively. The results showed that adsorption onto the material possessing a window size of 11.3 nm was mostly restricted to the external surface, while a considerable increase in the maximum adsorption capacity from 120 mg/g to 500–600 mg/g was observed when the window size was above the critical dimension of 13.9 nm. In the latter cases, the maximum adsorption capacities could be related to the pore volume rather than to the total surface area. The confined BSA molecules were strongly immobilised in the cells, since only a small proportion was desorbed from the material with windows of 17.3 nm dimensions on contact with a buffer solution of pH 7