THERMAL REGENERATION OF ACTIVATED CARBONS

The thermal regeneration of activated carbons loaded with p-nitrophenol (PNP) and other aromatic compounds was studied using a thermal balance. After pyrolysis of the adsorbates in nitrogen at 700°C, the residues were gasified with oxygen at 415 to 500°C or with steam at 840 to 920°C. Residues from PNP were several times more reactive to oxygen than the base carbons and also showed greater chemisorption of oxygen. For steam gasification, only small differences between spent and fresh carbons were found.     

Diffuse reflectance fourier transform infrared spectroscopy of phospholipid adsorption onto silicic acid

Diffuse reflectance Fourier transform infrared spectroscopy was used to study the mode of adsorption of phosphatidylcholine (PC) in hexane onto silicic acid (SA). PC adsorption was mainly through the charged phosphate group with minimal binding through the ester carbonyl. When the SA surface with adsorbed PC is washed with hexane, containing a small concentration of isopropanol, the desorbed PC is recovered without structural change, i.e., there is no evidence of PC hydrolysis in the adsorption process. Adsorbent misture probably promotes PC adsorption due to the increased availability of surface water hydroxyl groups for interaction with the PC phosphate groups. Isopropanol promoted PC binding by destabilizing PC reverse miscelles in solution, thus promoting its adsorption.     

Ethanol extraction of oil,gossypol and aflatoxin from cottonseed

Commercial processing of cottonseed requires hexane to extract and recover edible oil. Gossypol and aflatoxin are not removed from extracted meals. A bench-top extraction process with 95% (vol/vol) aqueous ethanol (EtOH) solvent has been developed that extracts all three of the above materials with a much less volatile solvent. In this process, cottonseed is pretreated and extracted with ambient 95% EtOH to remove gossypol and then extracted with hot 95% EtOH to extract oil and aflatoxin. Membranes and adsorption columns are used to purify the various extract streams, so that they can be recycled directly. A representative extracted meal contained a total gossypol content of 0.47% (a 70% reduction) and 3 ppb aflatoxin (a 95% reduction). Residual oil content was approximately 2%. Although the process is technically feasible, it is presently not economical unless a mill has a continual, serious aflatoxin contamination problem. However, if a plant cannot meet the hexane emission standards under the Clean Air Act of 1990, this process could provide a safer solvent that may expand the use and increase the value of cottonseed meal as a feed for nonruminants. Presented in part at the AOCS annual meeting, Toronto, Canada, May 1992.     

Surface chemistry and kinetics of the hydrolysis of isocyanic acid on anatase

In order to meet the stricter NO_x and particulate emission limits for commercial vehicles, the selective catalytic reduction (SCR) with urea is currently seen having the highest potential. The conversion of urea into ammonia and carbon dioxide consists of two consecutive reactions, in which isocyanic acid is an intermediate that is hydrolyzed over TiO₂. The intrinsic kinetics and the surface chemistry for this reaction are explored. Up to a temperature of 132 °C the reaction was in the intrinsic kinetic regime (E_A = 73 kJ/mol), while at higher temperatures the reaction was limited by pore and external diffusion constraints, respectively. In the presence of NO, NH₃ and NO₂, the catalytic activity was negatively influenced, increasing in severity in the sequence mentioned indicating that nitrates formed from NO₂ were most effective in blocking cations and anions of TiO₂. IR spectroscopy indicates that dissociative adsorption of HNCO on TiO₂ forms Ti–NCO and hydrogen bonded OH species. In the presence of water, isocyanic acid was so rapidly hydrolyzed that only adsorbed ammonia was observed on the catalyst surface. The presence of NO, NH₃ and NO₂ retards hydrolysis leading to the appearance of isocyanate species on the surface.     

Lipase PS-catalyzed transesterification of citronellyl butyrate and geranyl caproate: Effect of reaction parameters

Pseudomonas sp. lipase PS was immobilized by adsorption and tested for its ability to catalyze the synthesis of citronellyl butyrate and geranyl caproate by transesterification in n-hexane. The reaction parameters investigated were: enzyme load, effect of substrate concentration, added water, temperature, time course, organic solvent, pH memory, and enzyme reuse. Yields as high as 96 and 99% were obtained for citronellyl butyrate and geranyl caproate, respectively, with 300 units (approx. 15% w/w of reactants) of lipase PS. Increasing amounts of terpene alcohol inhibited lipase activity, while excess acyl donor (triacylglycerol) concentration enhanced ester production. Optimal yields were obtained at temperatures from 30–50°C after 24-h incubation time. Yields of 90 and 99% were obtained for citronellyl and geranyl esters, respectively, with 2% added water. Solvents with log P values ≥ 2.5 showed the highest conversion yields. pH 7 and 6–8 seemed to be ideal for citronellyl butyrate and geraniol caproate, respectively. The lipase remained active after reusing 12 times.     

Preparation of porous carbons from thermoplastic precursors and their performance for electric double layer capacitors

Porous carbons with high surface area were successfully prepared from thermoplastic precursors, such as poly(vinyl alcohol) (PVA), hydroxyl propyl cellulose and poly(ethylene terephthalate), by the carbonization of a mixture with MgO at 900 °C in an inert atmosphere. After carbonization the MgO was dissolved out using a diluted sulfuric acid and the carbons formed were isolated. The mixing of the PVA carbon precursor with the MgO precursors (reagent grade MgO, magnesium acetate or citrate) was done either in powder form or in an aqueous solution. The BET surface area of the carbons obtained via solution mixing could reach a very high value, such as 2000 m²/g, without any activation process. The pore structure of the resultant carbons was found to depend strongly on the mixing method; the carbons prepared via solution mixing were rich in mesopores, but those produced via powder mixing were rich in micropores. The size of mesopores was found to be almost the same as that of the MgO particles, suggesting a way of controlling the mesopore size in the resultant carbons. Measurement of capacitance was carried out in 1 mol/L H₂SO₄ electrolyte. The porous carbon with a BET surface area of 1900 m²/g prepared at 900 °C through solution mixing of Mg acetate with PVA showed a fairly high EDLC capacitance, about 250 F/g with a current density of 20 mA/g and 210 F/g with 1000 mA/g. The rate performance was closely related to the mesoporous surface area.     

Synthesis and characterization of porous media produced by a sol-gel method

Mesoporous silica materials were synthesized by sol-gel method using tetraethoxysilane (TEOS) as precursors and surfactants i.e., cetyltrimethyl ammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and polyoxyethylene cetyl ether (Brij 56) as templates. Surfactant templates were completely removed by calcination to form mesoporous structure. The effects of type and amount of surfactants on the characteristics of samples were studied. The textural characteristics such as surface area, pore volume, pore size, and pore size distribution were determined by nitrogen sorption isotherms. Fourier transform infrared (FTIR) spectroscopy was employed to qualitatively identify the chemical functionality and to confirm the removal of surfactant template. Scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to directly observe surface morphology and mesoporous structure, respectively. The adsorption capacity of the synthesized adsorbent for toluene vapor was examined. We found that the pore volume and pore size of mesoporous materials affected the adsorption capacity. The sample prepared with high content of CTAB under basic condition (pH ∼7) yielded large pore volumes and pore sizes and subsequently possessed the high adsorption capacity for toluene vapor.     

Intercalation of methylene blue in n-alkanethiolated self-assembled monolayers: Versatile electrochemical platforms for characterizing surfactant adsorption on hydrophobic surfaces

A versatile electrochemical platform for characterizing the adsorption of neutral and positively charged surfactants on hydrophobic surfaces was established using methylene blue (MB) as the probe. As a rigid, planar and electroactive species, MB can intercalate inside the regular self-assembled monolayers (SAMs) of n-hexanethiol and exhibit well-defined electrochemical responses. The adsorption of surfactants on the hydrophobic SAMs through the intercalation interaction between the hydrophobic tails of surfactants and the SAMs might change the density of the SAMs and influence the electrochemical behaviors of MB, providing a simple but effective approach for characterizing surfactant adsorption on hydrophobic surfaces. As an example, the adsorptive behaviors of cetyltrimethylammonium bromide (CTAB), a positively charged surfactant, and Triton X-100, a neutral surfactant, on hydrophobic surfaces were investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The results showed that these surfactants generally experienced three different adsorptive behaviors: the monomer adsorption at low concentrations, the loose monolayer adsorption at intermediate concentrations and the dense monolayer adsorption at high concentrations. In the case of CTAB, a new additional submonolayer adsorptive behavior between the monomer and the loose monolayer adsorption was observed for the first time, due to its rather long hydrophobic tail.     

A model for the adsorption of single metal ion solutes in aqueous solution onto activated carbon produced from pecan shells

Adsorption isotherms for activated carbon made from pecan shells have been obtained at 25 °C and an approximate pH of 3 for a number of metal ion solutes. It was found that the Slips and Freundlich equations were satisfactory for explaining the experimental data. The correlation of metal ion adsorption with the solute parameters of metal ion electronegativity and first stability constant of the metal hydroxide was investigated. In the case of most of the metal ions studied, higher electronegativities and stability constants corresponded to the higher adsorption levels of metal ions onto the activated carbon. A correlation was developed that predicts the constants of the Freundlich equation from the selected parameters of the metal ions, and thus can predict the adsorption isotherms at constant pH. The developed correlation gives results with acceptable deviations from experimental data. A procedure is proposed for obtaining similar correlations for different conditions (temperature, pH, carbon type and dosage). The ratio of equivalent metal ions adsorbed to protons released is calculated for the studied metal ions over a range of concentrations. In most cases, particularly at low concentrations, this ratio is close to one, confirming that ion exchange of one proton with one equivalent metal ion is the dominant reaction mechanism.     

Electrochemical reduction of benzyl halides at a silver electrode

The electrochemical reduction of benzyl halides PhCH₂X (X = Cl, Br and I) has been investigated at Ag and glassy carbon (GC) electrodes in CH₃CN + 0.1 M Et₄NClO₄. At both electrodes reduction of PhCH₂X involves irreversible electron transfer concerted with breaking of the carbon-halogen bond. All three halides exhibit a single 2e⁻ reduction peak at GC, whereas up to three peaks can be observed at the Ag electrode. Silver exhibits remarkable catalytic properties for the reduction process, which is positively shifted by 0.45-0.72 V with respect to GC. The mechanism of reduction of the organic halides at Ag involves adsorption of both the starting reagents and their reduction products. Adsorption of PhCH₂Cl and PhCH₂Br is weak and slow, whereas PhCH₂I is more rapidly and strongly adsorbed, so that two distinct peaks can be observed for the reduction of the dissolved and adsorbed molecules. Controlled-potential electrolyses at Ag have shown that the process may be directed to the production of bibenzyl or toluene, depending on the applied potential.