M. Manuela L. Ribeiro Carrott

Adsorption of n-Pentane on Mesoporous Silica and Adsorbent Deformation

Development of quantitative theory of adsorption-induced deformation is important, e.g., for enhanced coalbed methane recovery by CO2 injection. It is also promising for the interpretation of experimental measurements of elastic properties of porous solids. We study deformation of mesoporous silica by n-pentane adsorption. The shape of experimental strain isotherms for this system differs from the shape predicted by thermodynamic theory of adsorption-induced deformation. We show that this difference can be attributed to the difference of disjoining pressure isotherm, responsible for the solid-fluid interactions. We suggest the disjoining pressure isotherm suitable for n-pentane adsorption on silica and derive the parameters for this isotherm from experimental data of n-pentane adsorption on nonporous silica. We use this isotherm in the formalism of macroscopic theory of adsorption-induced deformation of mesoporous materials, thus extending this theory for the case of weak solid-fluid interactions. We employ the extended theory to calculate solvation pressure and strain isotherms for SBA-15 and MCM-41 silica and compare it with experimental data obtained from small-angle X-ray scattering. Theoretical predictions for MCM-41 are in good agreement with the experiment, but for SBA-15 they are only qualitative. This deviation suggests that the elastic modulus of SBA-15 may change during pore filling.

Hydrocarbons adsorption on templated mesoporous materials: effect of the pore size, geometry and surface chemistry

Adsorption–desorption isotherms of aromatic (mesitylene and toluene) and aliphatic (methylcyclohexane, neopentane and n-pentane) hydrocarbons were measured on ordered mesoporous materials, including MCM-41, MCM-48, SBA-15, SBA-16 and MCF silicas, a periodic mesoporous organosilica and a CMK-3 carbon, in order to evaluate the effect of the adsorbent characteristics on the organic compounds adsorption behaviour. A clear separation between aliphatic and aromatic hydrocarbons is observed at low p/po for the materials having pores accessible by narrow openings. The presence of narrow pore openings causes an increase in the volume adsorbed of mesitylene, toluene, methylcyclohexane and n-pentane prior to capillary condensation that does not occur for neopentane. The increase of the hydrophobicity and change of the surface structure, resulting from the incorporation of chloromethyl groups on the silica walls, causes the p/po at which the aromatic hydrocarbons condense to increase while the introduction of aromatic rings into the pore walls has a less significant effect on the condensation pressures. However, at low p/po, all the hydrocarbons have higher affinity for the periodic mesoporous organosilica surface than for the pure silica or silica with chloromethyl groups surfaces, while these lower the affinity of the aromatic hydrocarbons. Good estimates of pore size (Dp) are obtained using the classical Kelvin equation from the mesitylene, toluene and methylcyclohexane adsorption data, for spheroidal pores of at least ∼20 nm. With n-pentane this occurs for pores of ∼30 nm while neopentane underestimates Dp values even for pores as large as these, although to a lesser extent than nitrogen.

Ex-hydroxide magnesium oxide as a model adsorbent for investigation of micropore filling mechanisms

The thermal decomposition of magnesium hydroxide to magnesium oxide has been studied under carefully controlled conditions. Analysis of nitrogen and neopentane adsorption isotherm measurements shows that the micropore structure of the decomposition products is very well defined and consists of slit-shaped pores of width 0.93 ± 0.03 nm from 40% to 90% decomposition. The mean pore width increases, up to ca. 1.8 nm, at higher levels of decomposition. For larger pore sizes two stages of secondary micropore filling by nitrogen (surface coverage followed by enhanced adsorption in the body of the pore) are clearly discernible. The results also show very clearly how the mechanism of micropore filling, and hence the shape of the isotherm, is dependent on the ratio of pore width to molecular diameter (d/σ), and not just on d.

Adsorption Properties of Activated Carbons Prepared from Recycled PET in the Removal of Organic Pollutants from Aqueous Solutions

Activated carbons (ACs) were prepared from recycled polyethylene terephthalate (PET) by chemical activation with KOH and used successfully for the removal of 4-chloro-2-methylphenoxyacetic acid (MCPA) and Methylene Blue (MB) from aqueous media. In order to improve the adsorption capacity towards pollutant removal, the AC was submitted to different post-treatments. After oxidation, the sample exhibited an increase in MB adsorption capacity but showed a decreased MCPA adsorption removal extent. High-temperature treatment was not favourable for increasing the MB or MCPA adsorption capacity. However, after treatment with sodium hydroxide or urea, the samples showed a significant increase in their MCPA adsorption capacities. Normalisation of the MB and MCPA adsorption data with respect to the micropore volume led to a clearer understanding of the influence of the textural and chemical characteristics of the AC on the removal of MB and MCPA. The MB and MCPA adsorption isotherms were analysed via the Langmuir and Freundlich models, with the calculated parameters accurately fitting the experimental data and exhibiting good agreement between them.

Adsorption of Aqueous Mercury(II) Species by Commercial Activated Carbon Fibres with and without Surface Modification

The adsorption of HgCl2, [HgCl4]2– and Hg2+ onto a series of activated carbon fibres was studied. These included the as-received commercial activated carbon fibre (K), that obtained after modification via by sulphuric acid oxidation (KAC) and that obtained after modification by reaction with pentaethylenehexamine (KBAS). The effects of concentration (10–1500 mg/l), solution pH (1–10) and temperature (25°C, 35°C and 45°C) were studied. The mercury(II) adsorption isotherms followed the Langmuir model with maximum adsorption capacities of 361.0, 142.2 and 300.3 mg/g for HgCl2, [HgCl4]2– and Hg2+, respectively. Fibre K proved to have the highest adsorption capacity towards HgCl2 but the best results for the adsorption of [HgCl4]2– and Hg2+ were obtained with the fibre KAC. The performance of fibre KBAS was always worse than those of the other two fibres tested. The negative values obtained for ΔH0 and ΔG0 indicate that the adsorption was an exothermic and spontaneous process and also demonstrated that the adsorption of Hg(II) is a feasible process.

Textural Development of Activated Carbon Prepared from Recycled PET with Different Chemical Activation Agents

Activated carbons (AC) were prepared from waste granulated Polyethyleneterephthalate (PET) by chemical activation with phosphoric acid, sodium hydroxide and potassium hydroxide. All AC were characterised by N2 adsorption at 77 K, and those prepared with H3PO4 had a narrow pore size around 0.8 nm, those prepared with NaOH had a larger pore size higher than 1.52 nm and those prepared with KOH presented a pore size varying between 0.66 and 1.58 nm. The results suggest that H3PO4 and NaOH are not the most suitable activating agents for preparing AC with a high pore volume from waste PET. The AC produced with KOH presented a very high porosity, which passed through a maximum of 0.75 cm3g-1, due to an enlargement of the small micropores with an increase of the carbonisation temperature.

Activated Carbons Prepared from Natural and Synthetic Raw Materials with Potential Applications in Gas Separations

A carbon molecular sieve for the purification of a gas mixtures containing O2, N2 and CO2, CH4 was produced from a waste granulated PET by means of a single carbonisation step at 973 K. Activated carbon materials presenting good adsorption capacity and some selectivity for O2/N2 and CO2/CH4 were prepared from granulated PET and cork oak with pore mouth narrowing using CVD from benzene. The diffusion coefficients of O2, N2, CO2 and CH4 in these materials were calculated and are comparable to published values determined on Takeda 3A and on a carbon molecular sieve prepared from PET textile fibres by means of carbonisation and subsequent CVD with benzene. However, the selectivities were not quite as good as those given by Takeda 3A. However, taking into account that this is a first attempt at producing CMS from PET, the results are encouraging, and it is to be expected that further development of the experimental procedure will result in new materials with improved performance.

Simulations of Phenol Adsorption onto Activated Carbon and Carbon Black

Grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations have been used to study the adsorption of phenol onto carbon materials. Activated carbon was modelled in terms of its pore-size distribution based on DFT methods, while carbon black was represented by a single carbon slab with varying percentages of surface atoms removed. GCMC results for adsorption from the corresponding gas phase were in reasonable agreement with experimental adsorption results. MD simulations, that studied the influence of the presence of water and surface roughness on the arrangement of the adsorbed phenol molecules, showed that the interaction between the adsorbed molecules was strongly influenced by the presence of water.

Volatile organic compound adsorption on a nonporous silica surface: how do different probe molecules sense the same surface?

In this work, we compare experimental results to molecular simulation results of volatile organic compound (VOC) adsorption on nonporous silica. We adopted an effective model for the rough solid surface, obtained by a temperature annealing scheme, plus an experimental/simulation nitrogen adsorption tuning process over the silica energetic oxygen parameter. The measurement/prediction of selected VOCs, specifically, n-pentane and methylcyclohexane, is presented in terms of adsorption isotherms, with an emphasis on the angle distribution analysis of the three studied probe molecules with respect to the same modeled surface.

Nitrogen adsorption studies on non-porous silica: the annealing effect over surface non-bridging oxygen atoms

This contribution compares experimental nitrogen adsorption isotherms with molecular simulation results on effective non-porous silica models. A Molecular Dynamic (MD) annealing temperature scheme was adopted for the formulation of three samples of the solid surface. By means of Grand Canonical Monte Carlo (GCMC) simulations, nitrogen adsorption isotherms were obtained for each sample, with charged and uncharged nitrogen molecules, and compared with relevant experimental data. Following comparison of the experimental and simulated nitrogen results, a tuning process over the energetic parameter of the oxygen atoms was performed for the final proposed model. Specifically, the results are presented in terms of adsorption isotherms with an emphasis on the annealing effect over relevant model parameters, such as the surface concentration and the distribution of non-bridging oxygen atoms.