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Adsorption: Journal of the International Adsorption Society (v.18, #5-6)
Sorption interactions of volatile organic compounds with organoclays under different humidities by using linear solvation energy relationships by Yu-Huei Peng; Shih-Min Chou; Yang-Hsin Shih (pp. 329-336).
Organoclays are usually used as sorbents to reduce the spread of organic compounds and to remove them at contaminated sites. The sorption equilibrium and the mechanisms of volatile organic compounds (VOCs) on organoclays under different humidities are helpful for developing efficient organoclays and for predicting the fate of VOCs in the environment. In this study, the organoclay was synthesized through exchanging inorganic cations by hexadecyltrimethyl ammonium (HDTMA) into montmorillonite, resulting in 12 % of organic content. The surface area of organoclay was smaller than the unmodified clay due to the incorporation of organic cations into the interlayer. Both adsorption on organoclay surface and partition into the incorporated HDTMA in organoclay played roles on the sorption process. Compared the sorption coefficients in montmorillonite and different modified clays, the incorporated organic cations overcame the inhibition effect of hydrophilic surface of clay on the sorption process of hydrophobic organic compounds from water.The sorption coefficients of VOC vapors on organoclay were further characterized using a linear solvation energy relationship (LSER). The fitted LSER equations were obtained by a multiple regression of the sorption coefficients of 22 probe chemicals against their solvation parameters. The coefficients of the five-parameter LSER equations showed that high HDTMA-content montmorillonite interacts with VOC molecules mainly through dispersion, partly through dipolarity/polarizability and hydrogen-bonds as well as with negative π-/n-electron pair interaction. The interaction analysis by LSERs suggests that the potential predominant factors governing the sorption of VOCs are dispersion interactions under all tested humidity conditions, similar with the lower level modified clay. The derived LSER equations successfully fit the sorption coefficients of VOCs on organoclay under different humidity conditions. It is helpful to design better toxic vapor removal strategy and evaluate the fate of organic contaminants in the environment.
Keywords: Organoclay; Volatile organic compounds; Sorption; Inverse gas chromatography; Linear solvation energy relationship
Functionalized ordered mesoporous carbon for the adsorption of reactive dyes by Chun He; Xijun Hu (pp. 337-348).
A novel ordered mesoporous carbon containing basic nitrogen functional groups was synthesized by ammonia-tailoring at a temperature of 1173 K and was applied for reactive dye adsorption. The basic nitrogen-containing functional groups incorporated into the carbon surface could enhance the dispersive interactions between the carbon and dye molecules due to the electron-donating effect as well as the electrostatic interactions between the carbon surface and the anions of the dyes. It was found that this novel functionalized ordered mesoporous carbon could increase the adsorption capacity of reactive red 2 at 298 K by around 40 % and 100 % as compared with the unmodified carbon and a commercial activated carbon, respectively. The Freundlich isotherm showed better correlation with the experimental adsorption data of ammonia-tailored samples than the Langmuir isotherm due to the increased surface heterogeneity induced by the nitrogen-containing functional groups. Adsorption of reactive red 2 was an endothermic process as the adsorption capacity increased with increasing temperature. Low desorption efficiency revealed that the adsorption of reactive red 2 on the modified CMK-3 was extremely favorable, tending to be weakly reversible.
Keywords: Ordered mesoporous carbon; Surface modification; Reactive dye; Adsorption; Desorption
Effects of pore structure and surface chemical characteristics on the adsorption of organic vapors on titanate nanotubes by Chung-Kung Lee; Sheng-Kuo Fen; Huan-Ping Chao; Shin-Shou Liu; Fu-Chuang Huang (pp. 349-357).
Effects of pore structure and surface chemical characteristics of titanate nanotubes (TNTs) on their adsorptive removal of organic vapors were investigated. TNTs were prepared via a hydrothermal treatment of TiO2 powders in a 10 M NaOH solution at 150 °C for 24 h, and subsequently washed with HCl aqueous solution of different concentrations. Effects of acid washing process (or the sodium content) on the microstructures and surface chemical characteristics of TNTs were characterized with nitrogen adsorption-desorption isotherms, FTIR, and water vapor adsorption isotherms. For the adsorption experiments, gravimetric techniques were employed to determine the adsorption capacities of TNTs for four organic vapors with similar heats of vaporization (i.e., comparable heats of adsorption) but varying dipole moments and structures, including n-hexane, cyclohexane, toluene, and methyl ethyl ketone (MEK), at isothermal conditions of 20 and 25 °C. The experimental data were correlated by well-known vapor phase models including BET and GAB models. Isosteric heats of adsorption were calculated and heat curves were established. Equilibrium isotherms of organic vapors on TNTs were type II, characterizing vapor condensation to form multilayers. The specific surface area (and pore volume) and hydrophilicity of TNTs were the dominating factors for the determination of their organic vapors adsorption capacity. The GAB isotherm equation fitted the experimental data more closely than the BET equation. The heats of adsorption showed that the adsorption of organic vapors on TNTs was primarily due to physical forces and adsorbates with larger polarity might induce a stronger interaction with TNTs.
Keywords: Titanate nanotubes; Adsorption; Organic vapors; Pore structure; Surface chemical characteristics
NMR studies of carbon dioxide and methane self-diffusion in ZIF-8 at elevated gas pressures by Anne-Kristin Pusch; Tobias Splith; Lutz Moschkowitz; Shilpi Karmakar; Rajesh Biniwale; Marco Sant; Giuseppe B. Suffritti; Pierfranco Demontis; Janosch Cravillon; Evangelia Pantatosaki; Frank Stallmach (pp. 359-366).
Self-diffusion measurements with methane and carbon dioxide adsorbed in the Zeolitic Imidazolate Framework-8 (ZIF-8) were performed by 1H and 13C pulsed field gradient nuclear magnetic resonance (PFG NMR). The experiments were conducted at 298 K and variable pressures of 7 to 15 bar in the gas phase above the ZIF-8 bed. Via known adsorption isotherms these pressures were converted to loadings of the adsorbed molecules. The self-diffusion coefficients of carbon dioxide measured by PFG NMR are found to be independent of loading. They are in good agreement with results from molecular dynamic (MD) simulations and resume the trend previously found by IR microscopy at lower loadings. Methane diffuses in ZIF-8 only slightly slower than carbon dioxide. Its experimentally obtained self-diffusion coefficients are about a factor of two smaller than the corresponding values determined by MD simulations using flexible frameworks.
Keywords: ZIF-8; Self-diffusion; Carbon dioxide; Methane; NMR; Molecular dynamic simulation
Adsorptive separations for the recovery and purification of biobutanol by J. Cousin Saint Remi; G. Baron; J. Denayer (pp. 367-373).
A conceptual adsorption process for the recovery and purification of biobutanol is proposed. Different porous materials are tested on their ability to perform the adsorptive separations relevant to the process. The metal-organic framework ZIF-8, silicalite zeolite and active carbon were compared with respect to their adsorption capacity of 1-butanol dissolved in water, as obtained in static and dynamic conditions by respectively batch and breakthrough measurements at room temperature. Batch experimentation showed that other compounds present in a real ABE fermentation have no significant effect on the adsorption of 1-butanol on ZIF-8. The breakthrough separation of 1-butanol from an aqueous ABE mixture was performed with a ZIF-8 packed column. The desorption of 1-butanol from a saturated ZIF-8 packed column by a stepwise increase of the temperature to 423 K in combination with a purge of a nitrogen gas (60 ml/min) shows that 1-butanol desorbs at low temperature from ZIF-8. Adsorption isotherms of ethanol, 1-butanol and water in liquid phase on the zeolite SAPO-34 were determined by batch adsorption at 298 K. Also the separation of an ethanol/1-butanol mixture and the removal of ethanol from 1-butanol could be achieved with a SAPO-34 packed column. From this experimental work, two materials—ZIF-8 and SAPO-34—thus emerged as suitable adsorbents for the recovery and purification of biobutanol by adsorption.
Keywords: Adsorption; Alcohols; Recovery; ABE; Purification
Storage of hydrogen, methane, carbon dioxide in electron-rich porous aromatic framework (JUC-Z2) by Cuiying Pei; Teng Ben; Yan Cui; Shilun Qiu (pp. 375-380).
A 2D microporous electron-rich porous aromatic framework JUC-Z2 with high physicochemical stability and large surface area was studied in detail for their low-pressure N2, Ar, H2, CO2, CH4 sorption. Its hydrogen, methane, and carbon dioxide storage capacities are 181 cm3 g−1 (77 K/760 mmHg), 25 cm3 g−1 (273 K/760 mmHg), and 71 cm3 g−1 (273 K/760 mmHg), respectively. Gas molecule recognition at 273 K was performed and results show only greenhouse gases such as carbon dioxide and methane could be adsorbed onto JUC-Z2.
Keywords: Porous aromatic framework; Gas storage; Microporous materials; Electron-rich frameworks; Polytriphenylamine
Modeling the extra-column volume in a small column setup for bulk gas adsorption by Lisa Joss; Marco Mazzotti (pp. 381-393).
This study aims at highlighting the importance of an accurate characterization of the extra-column volume (ECV) and presents an experimental and computational protocol based on the characterization of the extra-column volume in terms of step-response experiments performed under various flow rates and pressures of 1 bar, 5 bar and 10 bar. The experiments are interpreted by describing the extra-column volume with a compartment model that reflects the geometry of the physical setup and that involves a stagnant zone to account for the non-ideal flow behavior through the piping system. The use of a mathematical model combining the description of the adsorption column and of the ECV can successfully predict experimental CO2–H2 breakthrough profiles performed at different pressures on an activated carbon adsorbent. This work shows how the presence of non-negligible extra-column effects can be accounted for, for the determination of adsorption transport parameters.
Keywords: Breakthrough experiment; Extra-column effects; Heat- and mass transfer coefficients; Mathematical modeling
Adsorption of copper(II), cadmium(II), nickel(II) and lead(II) from aqueous solution using biosorbents by Huan-Ping Chao; Chung-Cheng Chang (pp. 395-401).
Three types of agricultural waste, citrus maxima peel (CM), passion fruit shell (PF) and sugarcane bagasse (SB), were used to produce biosorbents for removing the heavy metal ions of copper(II), cadmium(II), nickel(II) and lead(II) from a pH 5.0 solution. The properties of biosorbents were characterized using scanning electron microscopy (SEM), zeta potential analyzer, Fourier transform infrared (FTIR) spectroscopy, elemental analyzer and tests of cation exchange capacity (CEC). The result indicated that the selected biosorbents possess rich carboxyl (COOH) and hydroxyl (OH) groups to produce a complexation with the heavy metals. Moreover, the negative surface charge of the biosorbent might adsorb the metal ions through the ion exchange. All of the adsorption isotherms indicated that L-type characters represented complexation and ion exchanges that were the adsorption mechanisms of biosorbents toward heavy metals. Biosorbents with higher oxygen content might generate high adsorption capacities. The adsorption capacities of CM and PF, estimated from the fitting to the Langmuir isotherm, are similar to those reported by others regarding biosorbents.
Keywords: Heavy metal; Adsorption; Biosorbent; Agricultural waste; Functional group
Flux response technology applied in zero length column diffusivity measurements by Ayodeji Sasegbon; Klaus Hellgardt (pp. 403-415).
Flux Response Technology (FRT) has been successfully adapted as an in situ perturbation technique in dynamic gas sorption measurements to extract hydrocarbon diffusion coefficients in alumina/CeZrO x washcoats of cordierite monoliths. FRT works by measuring minuscule changes in flowrate between two gas streams for any gaseous process involving a change in volume (δV/δt). Ad- and desorption transients, which can be collected during the same experimental run have been analysed using the zero length column (ZLC) method to study propane diffusivity within an alumina/CeZrO x washcoat as a function of temperature. Extracted diffusivities and activation energies compare favourably with literature data.
Keywords: Flux response technology; In situ; Diffusion; ZLC; Perturbation
Adsorption of organics on MSC5A in supercritical CO2, chromatographic measurements & stop & go simulation by Kazuyuki Chihara; Shingo Ito; Hideaki Nagashima; Mai Hikita; Ryota Suzuki (pp. 417-422).
Chromatographic measurements were made for the adsorption of benzene, toluene and m-xylene on molecular sieving carbon (MSC) in supercritical fluid CO2 mixed with organics. Supercritical chromatograph packed with MSC was used to detect pulse responses of organics. Adsorption equilibria and adsorption dynamics parameters for organics were obtained by moment analysis of the response peaks. Dependences of adsorption equilibrium constants, K ∗, and micropore diffusivity, D, on the amount adsorbed were examined. The dependencies of adsorption equilibrium constants, K ∗, and micropore diffusivity, D, of benzene, toluene and m-xylene, on molarity of benzene with each parameters of temperature or pressure were obtained. It was found that the values of K ∗ and D for an organic substance depended on the amount adsorbed of other organics strongly. And stop & go method was used as simulation method of perturbation chromatography for investigating adsorption equilibrium and rate. Numerical solution for multicomponent chromatogram in time domain could be obtained by appropriate model equations with experimental conditions. This simulated chromatogram can be compared with experimental chromatogram to determine the adsorption equilibrium and rate parameters. In addition, molecular simulation of multicomponent adsorption equilibria was performed, and potential parameters were determined by comparing the simulation with experimental results. Simulation soft ware is Cerius2 (Version 4.2) made by MSI. The purpose of performing simulation is to elucidate an adsorption mechanism on the molecule level.
Keywords: Supercritical fluid; MSC; Adsorption; BTX; Chromatography; Moment analysis
Selective adsorption of CO2 on amino-functionalized silica spheres with centrosymmetric radial mesopores and high amino loading by Shiyou Hao; Jing Zhang; Yijun Zhong; Weidong Zhu (pp. 423-430).
Amino-functionalized silica spheres with centrosymmetric radial mesopores and high amino loading were synthesized using the anionic surfactant N-lauroylsarcosine sodium (Sar-Na) as template and 3-aminopropyltrimethoxysilane (APTMS) as co-structure directing agent (CSDA) by an orthogonal experiment optimization. The synthesized amino-functionalized mesoporous silica (AFMS) was used as adsorbent to the selective adsorption of CO2. The effects of water vapor in the adsorptive stream on the adsorption properties of CO2 were investigated in detail. The results show that the synthesized adsorbent possesses a high adsorption selectivity for CO2 over CH4 and N2 due to the specific interactions between CO2 and amino groups. The presence of water vapor in the adsorptive stream can dramatically enhance the adsorbed amount of CO2 because of the partial formation of bicarbonate in the presence of moisture. Furthermore, the adsorbent shows a good stability, confirmed by adsorption-regeneration cycles. Based on these excellent properties, the application of the developed AFMS adsorbent in the selective adsorption of CO2 is anticipated.
Keywords: Amino-functionalized mesoporous silica; CO2 adsorption; Separation; Water vapor effect; Natural gas purification
Tetraethylenepentamine-modified mesoporous adsorbents for CO2 capture: effects of preparation methods by Shou-Heng Liu; Wei-Che Hsiao; Wun-Hu Sie (pp. 431-437).
Tetraethylenepentamine (TEPA)-modified mesocellular silica foams (MSFs) were fabricated via physical impregnation (MSF-T-x) and chemical grafting (MSF-CT-y) methods. The CO2 adsorption on these TEPA-modified MSFs was measured by using microbalances at 348 K and their adsorption capacities were observed to be 26.4–193.6 mg CO2/g-sorbent under ambient pressure using dry 15 % CO2. It was found that the CO2 adsorption capacities of MSF-CT-y were smaller than those of MSF-T-x sorbents which may be attributed to their higher density of amine groups. On the contrary, MSF-CT-y exhibited enhanced stability during repeated adsorption-desorption cycles compared to MSF-T-x sorbents. This notable enhancement in the durability of CO2 adsorption-desorption process was probably attributed to the decreased leaching of TEPA which is chemically bonded to the surface of MSF.
Keywords: Tetraethylenepentamine; Surface modification; Mesoporous silica; CO2 capture; Durability
Magnetic mesoporous carbon for efficient removal of organic pollutants by Tongbao Wang; Lin Liang; Runwei Wang; Yanqiu Jiang; Kaifeng Lin; Jianmin Sun (pp. 439-444).
Carbon materials such as activated carbons have been used in the field of water and wastewater treatments. However, the lack of mesopore and, particularly, the difficulty in recovering the spent carbon limited their applications. In this work, magnetic mesoporous carbon microspheres were synthesized by impregnating iron oxide precursors in the mesoporous carbon followed by the in situ conversion of the precursors into magnetite nanoparticles. The as-synthesized carbon microspheres with a high surface area of 742 m2/g and large mesopores of ∼4.4 nm exhibited an excellent adsorption capacity for aqueous organic pollutants. The superparamagnetic microspheres with a saturation magnetization of 7.15 emu/g can be easily separated from the treated solution by external magnetic field.
Keywords: Magnetic; Mesoporous carbon; One-pot synthesis; Adsorption; Organic pollutant
CO2 capture from flue gas by two successive VPSA units using 13XAPG by Lu Wang; Zhen Liu; Ping Li; Jin Wang; Jianguo Yu (pp. 445-459).
With the development of novel adsorbent material and adsorption process, adsorption technology has become a potential tool for the CO2 removal from flue gases. The reduction of carbon dioxide emissions from flue gases with two successive vacuum pressure swing adsorption (VPSA) units, using 13XAPG as the adsorbent, was investigated both theoretically and experimentally. A 3-bed 5-step VPSA process was designed to capture CO2 from flue gases, which included feed pressurization, adsorption, rinse, blowdown and counter-current purge. It was found that was difficult to achieve both high CO2 purity and high CO2 recovery by one VPSA unit when capturing CO2 from flue gases at atmospheric pressure. After the verification of one-column VPSA experiment for further concentrating CO2 stream from one VPSA unit to above 95 % purity, two successive VPSA units were designed, composed of 3-bed 5-step cycle for the first unit and 2-bed 6-step cycle for the second unit, and the effects of operating parameters on the separation behaviors were investigated by simulation. With the proposed VPSA process, a CO2 purity of 96.54 % was obtained with recovery of 93.35 %. The total specific power consumption of the two successive VPSA units was $528.39mbox{~kJ/kg}_{mathrm{CO}_{2}}$ , while the unit productivity was $0.031mbox{~kg}_{mathrm{CO}_{2}}mbox{/kg,h}$ .
Keywords: CO2 capture; Vacuum pressure swing adsorption; Zeolite 13X; Simulation; Two successive VPSA units
Regeneration of cobalt-contaminated activated carbon by supercritical carbon dioxide extraction by Ming-Tsai Liang; Roo-Chien Liang; Chih-Hsiung Lin (pp. 461-467).
A mixture of Di-(2-ethylhexyl) phosphoric acid (D2EPHA) and n-hexane or methanol is used as the extractant solution for extracting cobalt from activated carbon using supercritical carbon dioxide extraction technology. In this work, a semi-continuous pilot unit of SFE is employed to conduct the extraction. In order to feed the viscous extractant by HPLC pump, n-hexane or methanol is added as a diluent to reduce the viscosity of the extractant. The amount of cobalt removed along the time course of the extraction is recorded and plotted as an extraction curve. A kinetic model is also established and fit to the extraction curve, and the obtained parameters of the model are used to explain the regeneration mechanism. The effects of temperature ranged from 40 to 80 °C and the effect of the concentration of diluents on the extraction are investigated and discussed based on the established model. It is also found that the removal of cobalt ions reaches a maximum; this varies with the operational conditions and is known as maximum removal efficiency. It is presumed that the maximum removal efficiency is affected by the adsorption kinetics of the extractant and the rate of ion exchange between the extractant and metal ions on the surface of the activated carbon. After increasing the extraction temperature from 60 to 80 °C it is observed that the maximum removal efficiency is greatly increased, presumably resulting from the diminishing competitive adsorption between the extractant and diluents. The established model can help to reveal the extraction mechanism and to promote maximum removal efficiency for regenerating activated carbon without secondary pollution.
Keywords: Supercritical carbon dioxide; Activated carbon; Metal extraction; Regeneration; Acid extractant
Optimal flushing flow rates in para-xylene simulated moving-bed considering geometric factor of dead volume by Young-Il Lim (pp. 469-482).
The complex geometry of dead volume is often modeled into a simplified geometry, but little attention has been paid to how the simplified geometry of dead volume influences one-dimensional (1D) SMB modeling. This study investigated effects of the geometric factor of dead volume on 7-zone para-xylene (PX) simulated moving-bed (SMB). This work demonstrated that a complex geometry of dead volume can be modeled into a simple geometry by using a geometric factor in the 1D SMB simulations. Optimal flushing flow rates of the PX SMB were found by the parametric study on recovery and purity, employing the geometric factor to a simplified geometry.
Keywords: Simulated moving-bed (SMB); Para-xylene (PX); Dead volume; Geometric factor; Axial dispersion; flushing
Synthesis, characterization, and hydrogen storage study by hydrogen spillover of MIL-101 metal organic frameworks by Kuen-Song Lin; Abhijit Krishna Adhikari; Yu-Hsien Su; Chia-Wei Shu; Ho-Yang Chan (pp. 483-491).
MIL-101 is a chromium-based metal organic framework known to adsorb large amount of gases such as H2, CO2 and CH4. The framework was synthesized through solvothermal route and the H2 adsorption capacity was measured using a standard gravimetric method. X-ray absorption spectroscopy was performed to understand the fine structure, neighbors, coordination number and bond distance. The BET specific surface area of MIL-101nf (treated with NH4F) was 2,868 m2/g with a type I hysteresis loop measured from N2 adsorption isotherm. The hydrogen storage capacity was 0.16 wt% measured at 32 bar and room temperature for MIL-101nf. This capacity was increased up to 0.45 wt% by doping metal-supported carbon catalyst (Pd/AC and Pt/AC) through a carbon bridge with MIL-101. XANES spectra indicated that the valency of MIL-101 MOFs was Cr(III). EXAFS data also revealed that MIL-101 has a first shell of Cr-O bonding with the bond distance of 1.97 Å and the coordination number of 5.4.
Keywords: MIL-101; Metal organic frameworks; Porous material; Hydrogen storage; XANES/EXAFS; Hydrogen spillover
Adsorption, diffusion and catalysis of mesostructured zeolite HZSM-5 by Zhiping Liu; Weiming Fan; Jinghong Ma; Ruifeng Li (pp. 493-501).
Adsorption and diffusion properties of n-octane in meso-structured HZSM-5 zeolites were studied by high precision intelligent gravimetric analysis (IGA) and ZLC technology between 293 K and 393 K. As expected, great increase in adsorption capacity and diffusion efficient of n-octane in the mesostructured HZSM-5 zeolites was observed compared with conventional HZSM-5. At the same time, the adsorption activation energy of n-octane in the mesostructured HZSM-5 zeolites was significantly decreased. The adsorption heats with low n-octane loading showed a clear decline with increase of mesoporosity in the zeolite samples. These results clearly indicate that introduction of mesopores into the zeolites offered a short diffusion path and high diffusion rate for reactants and products, which resulted in a high yield of fuel oil and an enhanced resistance against the catalyst deactivation in the reaction of methanol to gasoline.
Keywords: Mesostructured zeolite HZSM-5; Adsorption isotherm; Diffusion kinetics; Activation energy
CO2 adsorption on zeolite X/activated carbon composites by Jinghong Ma; Chengming Si; Yixiao Li; Ruifeng Li (pp. 503-510).
Two series of zeolite X/activated carbon composites with different ratios of zeolite and activated carbon were prepared through a combination process of CO2 activation of the mixtures of elutrilithe and pitch and subsequent hydrothermal crystallization in alkaline solution. An additional surface modification was achieved in diluted NH4Cl solution. CO2 and N2 uptakes on the composites before and after modification were determined for pressures up to 101 kPa at 273 and 298 K, respectively. Langmuir-Freundlich and Toth adsorption models were used to describe the adsorption isotherms of CO2 and the corresponding heats of adsorption were estimated with the Clausius-Clapeyron equation. Both before and after modification, all composites exhibited a remarkable preferential adsorption of CO2 compared to N2, with the modified composites showing a higher adsorption selectivity to CO2 over N2 than the unmodified composites. With an increasing ratio of zeolite in the composites, adsorption capacity and adsorption heat of CO2 on the composites increased simultaneously. Lower adsorption heat was observed both before and after modification especially at the low-loading region and when there was less energetic heterogeneity on the surface of the modified composites. The results may be attributed to the elimination of strong basic sites on the modified composites, which is favorable for desorption of CO2 on the adsorbents and application in pressure swing adsorption processes.
Keywords: Zeolite/activated carbon composite; Surface modification; Adsorption heat; Heterogeneity