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Adsorption: Journal of the International Adsorption Society (v.7, #4)
The Compatibility of Compositional and Structural Features of Aluminous Large Pore Zeolites with Sorption Characteristics by P.N. Joshi; U.D. Joshi; T.H. Kim; K.I. Kim; V.P. Shiralkar (pp. 279-289).
The parent NaX and KL samples were prepared by hydrothermal crystallization and their corresponding K- and Na-exchanged forms were obtained by post-synthesis modification by conventional ion-exchange technique. NaX, NaKX, KL and NaKL samples were characterized by powder XRD, IR, low temperature nitrogen adsorption and chemical analyses. Their sorption uptake behaviour has been investigated by gravimetric method. The kinetics of sorption using different sorbate probe molecules such as water, benzene and n-hexane was measured at 298 K, at P/P 0 = 0.5 for 90 minutes. All the results obtained were discussed in terms of the compositional and structural variations of the adsorbents. Lack of high dimensionality of the pore system, less openness in structure, low average framework oxygen charge and less number of accessible nonframework cations are some of the prominent factors which has resulted in reduced hydro- and organo-philicity in LTL type zeolites as compared to X-type. The size and concentration of the accessible nonframework cations were also found to influence the sorptive properties of these aluminous large pore zeolites. Among all the probe molecules, n-hexane was found to be as a promising probe for differentiating the structural peculiarities especially dimensionality of the pore system.
Keywords: sorption; zeolite X; zeolite LTL; exchangeable cations; structural features
Specific Sorption Sites for Nitrogen in Zeolites NaLSX and LiLSX by Vadim B. Kazansky; Martin Bülow; Ekaterina Tichomirova (pp. 291-299).
Specific sorption sites for nitrogen, N2, in NaLSX and LiLSX zeolites were investigated using a DRIFT spectroscopic method. Sorption of molecular hydrogen, H2, by NaLSX or LiLSX zeolite at 77 K with DRIFT control of perturbation of sorbed molecules allowed to discriminate two or three different types of specific sorption sites in the respective zeolites. Their H–H stretching frequencies are 4077 and 4081 cm−1 for NaLSX, and 4061, 4084 and 4129 cm−1 for LiLSX. With reference to an independent investigation by methods of both sorption thermodynamics and molecular modeling for N2 sorption on LiLSX, the first two of the corresponding bands were ascribed to H2 sorption on lithium cations, Li+, localized in supercages of the faujasite, FAU, zeolite framework at sites SIII and SIII′, while the latter band most likely belongs to H2 sorption on Li+ cations at sites SII, and on hydroxyl groups, OH. Sorption of N2 by Li+ cations at sites SIII and SIII′ is the strongest, resulting in a decrease of intensity of the corresponding DRIFT bands that stem from subsequent H2 sorption. Nitrogen sorption by Li+ cations at sites SII is much weaker. Sorption of N2 on Na+ cations at sites SIII in NaLSX zeolite is also stronger than by Na+ cations at sites SII.
Keywords: sorption; DRIFT spectroscopy; hydrogen; nitrogen; zeolites NaLSX; LiLSX; cation sites
Short Communication: Diffusion of Simple Molecules in 4A Zeolite by Douglas M. Ruthven (pp. 301-304).
Recently reported diffusivity data for N2, CH4 and Kr in 4A zeolite pelleted adsorbent are compared with earlier data for the same sorbates in several different 4A samples. It is shown that, although there are large differences in diffusivity between the different adsorbent samples the activation energies are essentially constant. The data can be reconciled if it is assumed that the samples contain different fractions of “open” windows as a result of non-ideal distribution of the Na+ cations.
Keywords: diffusion; 4A zeolite; N2 ; Kr; CH4
A Film-Pore-Surface Diffusion Model for the Adsorption of Acid Dyes on Activated Carbon by Keith K.H. Choy; John F. Porter; Gordon Mckay (pp. 305-317).
The sorption of acid dyes from aqueous effluents onto activated carbon has been studied. The effects of initial dye concentration and activated carbon mass on the rate of Acid Blue 80, Acid Red 114 and Acid Yellow 117 removal have been investigated. A three-resistance mass transport model based on film, pore and surface diffusion control has been applied to model the concentration decay curves. The model incorporates an effective diffusion coefficient D eff, which is dependant on the equilibrium solid phase concentration or fractional surface coverage. The results of the film-pore-surface diffusion model are compared with the data obtained from the basic film-pore diffusion model. It has been found that the film-pore-surface diffusion model provides a major improvement over the data correlated by the film-pore diffusion model. Also, the relationship between surface diffusion and fractional surface coverage has been investigated for the adsorption of acid dyes on activated carbon.
Keywords: film-pore-surface diffusion; batch sorption; surface coverage; acid dyes; activated carbon
Enhanced Adsorption Capacity and Photo-Catalytic Oxidative Activity of Dyes in Aqueous Medium by Hydrothermally Treated Titania Pillared Clay by S.V. Awate; K. Suzuki (pp. 319-326).
Titania pillared montmorillonite clay was prepared by two different routes viz. 1) conventional ion exchange method and 2) modified method wherein the post hydrothermal treatment after ion-exchange was employed. The influence of the post hydrothermal treatment on the textural properties of titania-pillared clays was studied. The calcined clay was characterized by different physico-chemical techniques such as XRD, EDX, low temperature (77 K) nitrogen adsorption and UV-Vis diffuse reflectance spectroscopy. The content of pillared titania remained unchanged irrespective of the method of preparation. The method of preparation and severity of the conditions employed for the hydrothermal treatment resulted in alteration of the crystallinity and crystallite size of the anatase. The changes in the average pore diameter was found to comensurate with the changes in crystallite size of anatase phase. The increase in total pore volume as a function of the severity of the post hydrothermal treatment resulted in the decrease in micropore volume. The severity of the post hydrothermal treatment governed the extent of the blue shift in UV-Vis DRS spectra.The behaviour of titania pillared clays in adsorptive capacity and in photo-catalytic oxidation of methylene blue and victoria pure blue in aqueous medium was studied as a function of their physico-chemical characteristics. The titania pillared clays prepared by post hydrothermal route has shown enhanced adsorption capacity and photo-catalytic oxidation.
Keywords: titania pillared clay; hydrothermal treatment; anatase; photo-oxidation; methylene blue; victoria pure blue
A Thermodynamic Analysis of Ion Adsorption in the Metal Oxide/Electrolyte Systems in which PZC and CIP do not Coincide by W. Rudziński; W. Piasecki; W. Janusz; G. Panas; R. Charmas (pp. 327-338).
A theoretical-numerical analysis of two adsorption systems composed of the same kind of oxide- TiO2, and of two different electrolytes, NaCl and CsCl is presented. For one kind of the electrolyte (NaCl), PZC and CIP coincide, whereas they are different for the other (CsCl) electrolyte. The analysis is carried out by applying the popular TLM model, and by drawing formal-mathematical consequences of CIP existence in both kinds of adsorption systems. The values of the adsorption parameters are found by fitting simultaneously the obtained theoretical expressions to both experimental titration isotherms, and to the individual isotherms of cation adsorption measured using radiometric methods. That theoretical-numerical analysis suggests, that the inequality PZC ≠ $$frac{1}{2}$$ (pK int a1 + pK int a2) may be a general feature of the oxide/electrolyte systems including the systems in which PZC and CIP coincide.
Keywords: ion adsorption; surface charge; oxide/electrolyte interface
Effects of Feed Composition of Coke Oven Gas on a Layered Bed H2 PSA Process by Hyungwoong Ahn; Jaeyoung Yang; Chang-Ha Lee (pp. 339-356).
The effects of feed composition on the adsorption dynamics and the optimal process design were studied from the experimental and simulated results in the H2 layered bed PSA with activated carbon and zeolite 5A. The breakthrough results using the “base composition” (56.4 vol% H2; 26.6 vol% CH4; 8.4 vol% CO; 5.5 vol% N2; and 3.1 vol% CO2) in various layered beds were compared with those using the “higher nitrogen composition” and the “no nitrogen composition”. In the breakthrough dynamics, the propagation velocity of wave front of each component was closely related to the slope of isotherm estimated at its concentration in the feed. Breakthrough behavior at each layered bed in the “higher nitrogen composition” showed similar trends as that in the “base composition”. However, the “no nitrogen composition” showed different breakthrough behavior from the other groups. In this feed composition, it was observed that the order of CO and CH4 breakthrough times was reversed with a change in the carbon-to-zeolite ratio. Based on the adsorption dynamics and breakthrough behavior of each feed composition group in various layered beds, the effect of feed composition on a seven-step two-bed PSA process for the H2 recovery from coke oven gas was investigated numerically to develop a well-designed H2 PSA process under various operating conditions. As expected from the breakthrough results, the trends of the PSA performance in the “higher nitrogen composition” were similar to those in the “base composition” except for the slight decrease in the optimal carbon-to-zeolite ratio. However, in case of the “no nitrogen composition”, high purity product was obtained from the activated carbon-rich layered bed PSA because the adsorption capacity of the activated carbon for impurities was superior to that of zeolite. As a result, the optimum carbon-to-zeolite ratio at each operating condition was slightly changed depending on the propagation velocity of each component on each layer.
Keywords: 7 step H2 PSA process; breakthrough; feed composition effect; layered bed
Removal of H2S from Exhaust Gas by Use of Alkaline Activated Carbon by Jiun-Horng Tsai; Fu-Teng Jeng; Hung-Lung Chiang (pp. 357-366).
The purpose of this research was to select an activated carbon and alkaline solution blend that generated the best H2S adsorption on alkaline-activated carbon. RB2 (activated carbon) impregnated with NaOH solution was shown to have the optimum H2S removal efficiency. The optimum NaOH concentration was 50 mg per gram of carbon. H2S adsorption via RB2-NaOH50 was five times that of a corresponding fresh-activated carbon. The adsorption equivalent of H2S is nearly 1 (mol-H2S/mol-AOH), therefore, H2S + AOH → AHS + H2O was the major reaction. The H2S adsorption isotherm corresponded to the Freundlich isotherm.
Keywords: hydrogen sulfide; adsorption capacity; alkaline activated carbon; length of unused bed; adsorption wave