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Adsorption: Journal of the International Adsorption Society (v.16, #1-2)
Foreword
by Arvind Rajendran; Mohammad Aman Ullah; Wang Kean; Zhiping Lai; Shamsuzzaman Farooq; Kus Hidajat; George X. S. Zhao; Chi Bun Ching (pp. 1-2).
The role of accessibility in the characterization of porous solids and their adsorption properties by D. D. Do; L. Herrera; Chunyan Fan; A. Wongkoblap; D. Nicholson (pp. 3-15).
This paper addresses the role of accessibility for adsorption in porous solids on the adsorption properties including Henry constant, adsorption isotherms and isosteric heat of adsorption. The relevant parameters are the accessible volume, the accessible geometrical surface area and the accessible pore size and its associated volume. This concept will be demonstrated to be important and calls for the need to consider adsorption characteristics in the most coherent and consistent manner. It is particularly reinforced by the limitations inherent in the conventional ways in determining the void volume, surface area and pore size. We provide a number of examples to support this; the challenge that faces us is the development of consistent experimental procedures to determine these accessible quantities. We define the accessible pore size as the size of the largest sphere that rests on three closest solid atoms in such a manner that any probe particle residing in that sphere would have a non-positive solid-fluid potential energy. For each accessible pore size there is an associated accessible pore volume, giving rise to a new accessible pore size distribution (APSD). This is distinct from the classical pore size distribution commonly used in the literature, and in our definition of accessible pore size, a zero pore size is possible. It is also emphasized that the accessible quantities that we introduce here are dependent on the choice of molecular probe, which is entirely consistent with the concept of molecular sieving.
Keywords: Characterization; Accessibility; Porous solids; Gas adsorption; Pore volume; Pore size distribution
Diffusion in zeolites: is surface resistance a critical parameter? by Laurent Gueudré; Elsa Jolimaîte; Nicolas Bats; Weï Dong (pp. 17-27).
Gravimetric uptake measurements were performed with cyclohexane for different Silicalite-1 crystals sizes. It was observed that the apparent diffusion coefficients vary with crystal size, confirming the existence of a surface resistance. Considering that surface and the intracrystalline characteristic diffusion times are additives, it was possible to dissociate the two resistances.Surface mass transfer coefficient was found to be in the same order of magnitude for the different samples and activated with temperature. The contribution of surface resistance to mass transfer limitation is lower at high temperatures and for the bigger crystals. Surface resistance is far from being negligible for the smaller crystals: for crystals of 2 μm, surface resistance represents more than 60% of the total mass transfer resistance at 398 K. And crystals of that size (in the order of 2 μm) are usually used industrially, in order to minimize mass transfer resistance.The surface of one of our sample was purified by etching with a solution of hydrogen fluoride, without any enhancement of the adsorption kinetic. Surface resistance may not be located at the extreme surface of the crystals but in a layer of non negligible thickness of distorted crystal structure around the crystals.
Keywords: Diffusion; Surface resistance; Cyclohexane; Silicalite
Diffusion of linear paraffins in silicalite studied by the ZLC method in the presence of CO2 by Artemis P. Guimarães; Andreas Möller; Reiner Staudt; Diana C. S. de Azevedo; Sebastião M. P. Lucena; Célio L. Cavalcante Jr. (pp. 29-36).
The diffusion behavior of C4–C10 n-alkanes in silicalite-1 has been investigated by using the Zero Length Column method. The diffusivities derived from measurements at different purge rates with different purge gases confirming intracrystalline diffusion control. Data are compared with results reported in the literature for MFI zeolites. The diffusivities were found to be consistent and agree well with data previous obtained by ZLC. However, these data showed a remarkable disagreement with other reported techniques (PFG-NMR, QENS and Permeation). The eventual influence of carbon dioxide (CO2) adsorption on diffusion properties of n-alkanes in silicalite was also investigated. For this purpose, a series of experiments was performed involving hydrocarbons mixed with CO2. Data were obtained at 303 K and flow rates between 20 and 80 mL/min. The presence of CO2 does not seem to influence the intracrystalline transport rate of the investigated light hydrocarbons (n-C4 and n-C6). On the other hand, the situation for n-C8 and n-C10 is more complex. The diffusivity values are higher compared to the previously reported values.
Keywords: Diffusion; Paraffins; Carbon dioxide; Silicalite; ZLC
Prediction of competitive adsorption on coal by a lattice DFT model by Ronny Pini; Stefan Ottiger; Giuseppe Storti; Marco Mazzotti (pp. 37-46).
Adsorption is one of the main mechanisms involved in the ECBM process, a technology where CO2 (or flue gas, i.e. a CO2/N2 mixture) is injected into a deep coal bed, with the aim of storing CO2 by simultaneously recovering CH4. A detailed understanding of the microscopic adsorption process is therefore needed, as the latter controls the displacement process. A lattice DFT model, previously extended to mixtures, has been applied to predict the competitive adsorption behavior of CO2, CH4 and N2 and of their mixtures in slit-shaped pores of 1.2 and 8 nm width. In particular, the effect of temperature, bulk composition and density on the resulting lattice pore profiles and on the lattice excess adsorption isotherms has been investigated. Important insights could be obtained; when approaching near critical conditions in the mesopores, a characteristic peak in the excess adsorption isotherm of CO2 appears. The same effect could be observed neither for the other gases nor in the micropores. Moreover, in the case of mixtures, a depletion of the less adsorbed species close to the adsorbent surface is observed, which eventually results in negative lattice excess adsorption at high bulk densities.
Keywords: Lattice DFT; ECBM; Competitive adsorption; Coal
Synthesis, characterization, and adsorption kinetics of titania nanotubes for basic dye wastewater treatment by Kuen-Song Lin; Hao-Wei Cheng; Wen-Ru Chen; Chian-Fu Wu (pp. 47-56).
Titanium dioxide has been recognized as an excellent photocatalyst material applied on many fields especially for environmental science or engineering. However, the effect of acid washing treatment on the morphology or phase and pore structures of titania nanotubes (TNs) has not still been clearly investigated. The variation of morphology, formation mechanism, phase structure, and pore structure of TN were thus characterized with FE-SEM, TEM, XRD, and N2 BET isotherms, respectively in the present work. Titania nanotube synthesized via a simple hydrothermal chemical process formed a crystalline structure with open-ended and multiwall morphologies. The XRD patterns and N2 BET isotherms implied that the acid washing techniques could improve the TN surface area and the pore size distribution up to 292 m2 g−1 and 40–60 nm, respectively. According to EXAFS/XANES spectra, the structure of the TN is closer to the anatase specimen and much more correlated with octahedral structure. Effects of the pore structure variation on basic dye (Basic Green 5 (BG5)) adsorption by TN were discussed in the present work. Moreover, the adsorption ability, mechanisms, and kinetics of BG5 dye onto TN were also examined with the aid of model analyses of the adsorption equilibrium and kinetic data. Therefore, the potential of TN for the removal of BG5 dye contaminant from wastewaters implied that further development would be warranted.
Keywords: Titania nanotube; BG5 basic dye; Hydrothermal method; Adsorption kinetics; XANES/EXAFS
Synthesis of carbon nanofibers from poly(ethylene glycol) with controlled structure by Yusuke Takahashi; Hirotaka Fujita; Wan-Hua Lin; Yuan-Yao Li; Takao Fujii; Akiyoshi Sakoda (pp. 57-68).
Through fine tuning of synthesis conditions, we successfully synthesized three types of carbon nanofiber (CNF) (herring-bone carbon nanofiber, platelet carbon nanofiber, and cup-stacked carbon nanofiber) by the thermal decomposition of a mixture of poly(ethylene glycol) (PEG) and nickel chloride (NiCl2). A series of experimental results demonstrated that the key factors for the selective synthesis of these CNFs were the (1) NiCl2/PEG ratio, (2) drying time of the polymeric mixture, (3) state of PEG (liquid or solid) before temperature rising, and (4) temperature profile during the thermal decomposition. Changes in these conditions contributed to the formation of Ni catalyst particles from the catalyst NiCl2 with different morphology, thereby resulting in the growth of different types of CNF or amorphous carbon products according to the catalyst particle’s shape. Also, we found that the mechanism of CNF growth in this synthesis method was fundamentally the same as that in chemical vapor deposition (CVD).
Keywords: Carbon nanofiber; Poly(ethylene glycol); Thermal decomposition
Flexible and transparent moisture getter film containing zeolite by Chien-Sheng Wu; Jung-Yu Liao; Shun-Yi Fang; Anthony S. T. Chiang (pp. 69-74).
Organic light-emitting devices (OLED) are extremely sensitive to moisture and oxygen. Without high-performance hermetic seals, the life of these devices will be limited. A large amount of desiccant has been used in the packaging of OLED to ensure a dry environment. However, for top-emitter OLEDs, particularly for active matrix displays where the OLEDs are directly integrated on top of a thin film transistor (TFT) layer, the light must pass through a transparent encapsulating layer above the OLED, leaving little room to house the desiccant. Thus, a transparent layer with embedded moisture getter, such as zeolite, would be desirable. Here we report an effort to prepare such a transparent composite film containing zeolite nanoparticles, and show that such architecture is indeed possible.
Keywords: Moisture getter; OLED; Nano-composite; Zeolite; Transparent
Chemically activated Ipomoea carnea as an adsorbent for the copper sorption from synthetic solutions by Michael Angelo Miranda; P. Dhandapani; M. Helen Kalavathy; Lima Rose Miranda (pp. 75-84).
An indigenously prepared zinc chloride activated Ipomoea carnea (morning glory), a low-cost and abundant adsorbent, was used for removal of Cu(II) ions from aqueous solutions in a batch adsorption system. The chemical activating agent ZnCl2 was dissolved in deionised water and then added to the adsorbent in two different ratios 1:1 and 1:0.5 adsorbent to activating agent ratio by weight. Studies were conducted as a function of contact time, initial metal concentration, dose of adsorbent, and pH. Activated Ipomoea carnea (AIC) were characterised using scanning electron microscopy (SEM), iodine number and methylene blue number. High iodine numbers indicates development of micro pores with zinc chloride activation. Maximum adsorption was noted within pH range 6.0(±0.05). Adsorption process is fast initially and reaches equilibrium after about 4 hours. The kinetic data were analysed using pseudo-first-order and pseudo-second-order models. The pseudo-second-order kinetic model was found to agree well with the experimental data. Adsorption equilibrium data were analyzed using Langmuir and Freundlich isotherm models. The Langmuir model represented the sorption process better than the Freundlich model. Based on the Langmuir isotherm, the monolayer adsorption capacity of Cu(II) ions was 7.855 mg g−1 for AIC (1:1) and 6.934 mg g−1 for AIC (1:0.5).
Keywords: Adsorption of copper; Ipomoea carnea; Zinc chloride activation; Kinetics; Micro- and meso-porous carbon
The separation of GMP from milk whey using the modified chitosan beads by Cunben Li; Xianghua Song; San Hein; Kean Wang (pp. 85-91).
The cationic property of chitosan was used to adsorb glycomacropeptide (GMP) molecules from milk whey proteins. β-cyclodextrin was immobilized to native chitosan beads by cross-linking with 1,6-hexamethylene diisocyanate (HMDI). The resultant modified beads (CS-HMDI-BCD) presented superior adsorption affinity and capacity towards GMP. At pH 3.0, 90.23% of GMP was adsorbed with a maximum adsorption capacity corresponding to 12.87 mg of sialic acid/g-adsorbent. Desorption experiments showed that the modified beads could be regenerated and used in many cycles without significant decreases in the capacity and selectivity.
Keywords: Chitosan; β-cyclodextrin; Glycomacropeptide; Adsorption; Milk whey