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Adsorption: Journal of the International Adsorption Society (v.17, #1)

Simulating the effect of carbon nanotube curvature on adsorption of polycyclic aromatic hydrocarbons by Radosław P. Wesołowski; Sylwester Furmaniak; Artur P. Terzyk; Piotr A. Gauden (pp. 1-4).

The results of Molecular Dynamics simulation of polycyclic aromatic hydrocarbons adsorption on single-walled (13,9) carbon nanotube are reported. We discuss the angular orientation and plausible adsorbed states of molecules. It is shown, that suggested by Gotovac et al. orientation of adsorbed molecules is correct.

Keywords: Adsorption PAH; Molecular dynamics simulation; Carbon nanotubes

MOF/graphite oxide hybrid materials: exploring the new concept of adsorbents and catalysts by Teresa J. Bandosz; Camille Petit (pp. 5-16).

Two types of metal-organic framework (MOF)/graphite oxide hybrid materials were prepared. One is based on a zinc-containing, MOF-5 and the other on a copper-containing HKUST-1. The materials are characterized by X-ray diffraction, sorption of nitrogen, thermal analyses, Fourier Transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). Their features are compared to the ones of the parent materials. The water stability and ammonia adsorption capacity of the hybrid materials were also evaluated. It was found that the latter compounds exhibit features similar to the ones of the parent MOF. In most cases, their porosity increased compared to the one calculated considering the physical mixture of MOF and GO. This new porosity likely located between the two components of the hybrid materials is responsible for the enhanced ammonia adsorption capacity of the compounds. However, for both the zinc-based and the copper-based materials (MOFs and hybrid materials), a collapse of the framework was observed as a result of ammonia adsorption. This collapse is caused by the interactions of ammonia with the metallic centers of MOFs either by hydrogen bonding (zinc-based materials) or coordination and subsequent complexation (copper-based materials). Whereas the MOF-5 based compounds collapse in presence of humidity, the copper-based materials are stable.

Keywords: Metal-organic frameworks; Graphite oxide; Hybrid material; Ammonia; Adsorption

Adsorptive desulfurization of propylene derived from bio-ethanol by T. Yamamoto; J. Chaichanawong; N. Thongprachan; T. Ohmori; A. Endo (pp. 17-20).

We studied adsorption characteristics of a series of LTA zeolite as an adsorbent for desulfurization of propylene, that was produced from bioethanol by ethanol-to-olefin (ETO) conversion. A breakthrough curve (BTC) for adsorption of methanethiol, as one of the sulfur impurities of propylene produced from bioethanol, in the presence of propylene was measured using a fixed-bed column packed with the LTA zeolite. The BTC revealed that the effect of the competitive adsorption of propylene on the LTA zeolite strongly depended on a cation species exchanged in the micropores of the zeolite. Among the cation species examined in this study, bivalent cation of zinc (Zn²⁺) was proved to be the most effective one to increase the amount of methanethiol adsorbed on the LTA zeolite under the presence of propylene. The specific interaction of methanethiol with the LTA zeolite exchanged with Zn²⁺ was confirmed by the measurement of a temperature-programmed desorption (TPD) spectrum of methanethiol.

Keywords: Adsorptive desulfurization; Breakthrough curve; Fixed-bed column; LTA zeolite; Temperature-programmed desorption

Methane adsorption in PIM-1 by Gregory S. Larsen; Ping Lin; Flor R. Siperstein; Coray M. Colina (pp. 21-26).

We report the results of Grand Canonical Monte Carlo (GCMC) simulations of methane adsorption in a prototypical polymer of intrinsic microporosity, PIM-1. Polymer chains were represented with a united-atom model, with Lennard-Jones parameters obtained from the TraPPE potential. Additionally, partial charges were calculated from ab initio methods using Gaussian (HF/6-31G^* basis set). Samples of PIM-1 were built at low density conditions, followed by a Molecular Dynamics compression protocol until densities of 1.2 g cm⁻³ were achieved. This protocol proved to be suitable for the realistic modeling of the amorphous structure of PIM-1. Surface areas and pore size distributions were measured and compared to available experimental data. The simulated pore size distribution present a peak at 4.3 Å, consistent with experimental results. GCMC simulations of methane adsorption were performed, and found to qualitatively reproduce the shape of the available experimental isotherm.

Keywords: Polymers of intrinsic microporosity; PIMs; Simulation; Adsorption; GCMC; MD

Single component and binary diffusion of n-heptane and toluene in SBA-15 materials by Qinglin Huang; Ramil Abu Qamar; Mladen Eić (pp. 27-38).

In this work, the diffusion properties of single component n-heptane and toluene as well as their binary mixtures in two SBA-15 samples with different structural characteristics were studied by the standard Zero Length Column (ZLC) technique under three different concentration levels. A theoretical ZLC desorption model considering the Generalized Maxwell-Stefan (GMS) formulation was developed. Using the independently measured single component equilibrium and kinetic parameters, the model was able to reasonably predict experimental binary ZLC desorption curve for countercurrent diffusion of toluene in the presence of n-heptane. However, there was a significant deviation between model prediction results and experimental data for countercurrent desorption of n-heptane in the presence of toluene. The diffusion of n-heptane is reduced by the presence of toluene, regardless of the relative content of micropores in the intrawall pores, while that of toluene is virtually unaffected by the counter-diffusion of n-heptane. The observed phenomena cannot be addressed by the simple model considering the cross term diffusion effect. The structural property of material and the molecular characteristics of probe molecules were used to account for the difference in the behavior between n-heptane and toluene.

Keywords: SBA-15; Binary diffusion; n-Heptane; Toluene; ZLC technique

Variable desorbent strength: influence on SMB operating conditions and performance by Catherine Laroche; Damien Leinekugel-le-Cocq; Philibert Leflaive (pp. 39-48).

The aim of this work was to evaluate the role of desorbent selectivity variation on the tuning and the performance of a SMB process. For this purpose a separation regions study in the case of a four-zones non-linear transfer limited SMB was carried out. The SMB unit was modelled considering the equivalent True Moving Bed (TMB). A set of 11 desorbents with different strength was considered: 3 desorbents with constant strength and 8 desorbents with variable strength. For each system, the optimal operating conditions are determined using the separation regions approach, with plots in both (Q₂, Q_F) and (Q₄, Q_D) planes. Depending on the separation considered, a variable selectivity desorbent can lead to an improvement or can be detrimental to process performances. A careful choice of desorbent selectivity (either constant or variable) is then crucial to achieve maximal process performance.

Keywords: Simulated Moving Bed (SMB); Desorbent strength; Robust operating conditions; Separation regions study

Studies on the growth of ice crystal templates during the synthesis of a monolithic silica microhoneycomb using the ice templating method by S. R. Mukai; K. Onodera; I. Yamada (pp. 49-54).

Porous materials with a wide variety of functions can be obtained through sol-gel synthesis. Recently, we found that sol-gel based materials can be molded into a monolithic microhoneycomb structure by simply freezing their parent hydrogels unidirectionally. The main feature of the monoliths obtained through this method, which we named the Ice Templating Method, is that they have straight and aligned macropores, the sizes of which are in the micrometer range. As these macropores are the traces of the ice crystals which are formed during freezing and which practically act as the template, the sizes as well as the shape of them depend on how the template ice crystals are formed and how they grow. Therefore in this work, the growth behavior of the ice crystals formed during the unidirectional freezing of a silica hydrogel was examined and the influences of this growth behavior on the properties of the resulting monoliths were verified.

Keywords: Ice templating; Freezing; Sol-gel process; Silica gel; Monolith; Microhoneycomb

A novel and consistent method (TriPOD) to characterize an arbitrary porous solid for its accessible volume, accessible geometrical surface area and accessible pore size by L. F. Herrera; Chunyan Fan; D. D. Do; D. Nicholson (pp. 55-68).

We present an improved Monte Carlo integration method to calculate the accessible pore size distribution of a porous solid having known configuration of solid atoms. The pore size distribution obtained with the present method is consistent with the accessible volume and the accessible geometric surface area presented in previous publications (Do and Do, in J. Colloid Interface Sci. 316(2):317–330, 2007; Do et al. in Adsorpt. J., 2010). The accessible volume, accessible geometrical surface area and the pore size distribution method construct an unambiguous and robust single framework to characterize porous solids. This framework is based on the derivation of the space accessible to the center of mass of a probe molecule. The accessible pore size presented is an absolute quantity in the sense that a zero value is possible. We present the entire framework of this characterization method and compare the improved method with the one presented previously for a set of porous solids such as graphitic slit pores, defective slit pores, bundle of carbon nanotubes, zeolite and some metal organic frameworks.

Keywords: Characterization; Adsorption; Monte Carlo integration; Pore size distribution; Accessibility

Diffusion and phase equilibria of binary fluids in mesopores by Philipp Zeigermann; Muslim Dvoyashkin; Rustem Valiullin (pp. 69-74).

The formation of adsorption hysteresis in mesoporous material with random pore structure may be interrelated with different distributions of the fluid density attained along different paths of the system preparation. To access microscopic details of these distributions, in addition to the main sorptive liquid, distribution of which along the pore space of Vycor porous glass was of interest, a small amount of a probe liquid with a substantially lower vapor pressure has been added. Molecular diffusivities of both liquids then have been traced using pulsed field gradient NMR. Due to different vapor pressures, the two molecular species explore different spaces occupied by the capillary-condensed (accessible for both species) and gaseous (accessible only for the molecules of the main sorptive) phases. Comparative analysis of the diffusion properties obtained at different states along the adsorption isotherm revealed further insight into peculiarities of the fluid distribution and mass transfer of binary fluids in pores.

Keywords: Adsorption; Diffusion; Binary mixture; Mesoporous materials; Nuclear magnetic resonance

Investigation of gas transport through porous membranes based on nonlinear frequency response analysis by M. Petkovska; A. Markovic; M. Lazar; A. Seidel-Morgenstern (pp. 75-91).

Theoretical development of a new experimental method for investigation of mass transport in porous membranes, based on the principle of the modified Wicke-Kallenbach diffusion cell and the nonlinear frequency response analysis is presented. The method is developed to analyze the transport of a binary gas mixture in a porous membrane. The mixture is assumed to consist of one adsorbable and one inert component. Complex mass transfer mechanism in the membrane, where bulk or transition diffusion in the pore volume and surface diffusion take place in parallel, is assumed. Starting from the basic mathematical model equations and following a rather standardized procedure, the frequency response functions (FRFs) up to the second order are derived. Based on the derived FRFs, correlations between some characteristic features of these functions on one side, and the whole set of equilibrium and transport parameters of the system, on the other, are established. As the FRFs can be estimated directly from different harmonics of the measured outputs, these correlations give a complete theoretical basis for the proposed experimental method. The method is illustrated by quantifying the transport of helium (inert gas) and C₃H₈ and CO₂ (adsorbable gases) through a porous Vycor glass membrane.

Keywords: Porous membranes; Mass transfer mechanisms; Frequency response functions; Transition diffusion; Surface diffusion; Knudsen diffusion

Diffusion of cyclohexane in native and surface-modified mesoporous glasses by M. Dvoyashkin; E. E. Romanova; W.-D. Einicke; R. Gläser; J. Kärger; R. Valiullin (pp. 93-99).

Diffusion of cyclohexane in mesoporous silica materials with different degrees of surface silanization has been probed by means of pulsed field gradient nuclear magnetic resonance. The self-diffusion coefficients have been measured at various pore fillings from about 10% of one monolayer coverage to complete pore saturation by the capillary-condensed phase. It is found that the surface modification, namely grafting of dimethyloctadecylmethoxysilane molecules to the silica surface, reduces diffusivities of guest molecules as compared to the native sample. The contribution of the Knudsen molecular diffusion to the measured diffusivity has been assessed using the model of fast molecular exchange between the adsorbed phase on the pore walls and the molecules in the gaseous phase in the pore interior. The diffusivity data were correlated with the degree of the surface modification, with the latter being probed by measuring ¹H and ¹³C spectra using magic angle spinning (MAS) solid state NMR, nitrogen adsorption and thermogravimetry.

Keywords: Adsorption; Surface modification; Diffusion; Mesopores; NMR

Concurrent dyes adsorption and photo-degradation on fly ash based substrates by M. Visa; L. Andronic; D. Lucaci; A. Duta (pp. 101-108).

Most of the dyes are organic compounds, with different degree of polarization and different groups with various steric effects, making their complete biodegradation slow or even impossible. Adsorption on fly ash and fly ash based substrates represents a possible alternative for simultaneous removal of dyes and heavy metals form wastewaters resulted in the textile industry. Adsorption (under visible light) and adsorption and photodegradation (under UV irradiation) studies were done on Methylene blue solutions and on their mixtures with heavy metals (copper and cadmium), in systems using fly ash as single substrate, or mixed with a wide band gap semiconductor (TiO₂). The titanium oxides and hematite content in fly ash proved to be responsible for photodegradation processes even in the absence of the TiO₂ powder, confirming that modified fly ash is a viable candidate in developing up scalable processes for advanced wastewater treatment. The kinetic and thermodynamic studies allow to calculate the parameters and to describe the complex mechanisms, involving competitive adsorption.

Keywords: Fly ash; TiO₂ ; Adsorption; Wastewater treatment

Effect of dead volume on performance of simulated moving bed process by Young-Il Lim; Suresh K. Bhatia (pp. 109-120).

The volume of surrounding equipments (pipe transfer lines and valves) in the simulated moving bed (SMB) unit, which is called the dead volume, is modeled as bed-head, bed-tail and bed-line. Since the dead volume can be significant especially in industrial-scale SMB units, the consideration of dead volume has been required for high performance operation. In this study, a simple and unified approach based on the method of characteristics (MOC), called the extended node model, is established to solve fluid concentration dynamics within dead volumes. The computational efficiency of the approach is evaluated for three case studies of a standard four-zone SMB process with a linear adsorption equilibrium model. Insertion of one zone to flush the fluid trapped in extract bed-line into the standard four-zone SMB improves substantially purity, while recovery is kept constant.

Keywords: Simulated moving bed (SMB); Dead volume; Mathematical modeling; Method of characteristics (MOC); Flushing; Purity improvement

Process intensification in PSA processes for upgrading synthetic landfill and lean natural gases by G. Spoorthi; R. S. Thakur; Nitin Kaistha; D. P. Rao (pp. 121-133).

Process intensification aims at reducing the size of equipment by orders of magnitude and is actively perused in separation processes. Its feasibility in Pressure Swing Adsorption (PSA) processes has been explored. A 4-bed PSA and a 3-bed PSA, which emulate the moving bed processes, and duplex PSA and a modified duplex PSA have been selected for the exploratory studies. Simulation studies on the separation of a mixture of CH₄–CO₂ over 5A zeolite were carried out to compare the performance of these processes. An index has been proposed to quantify the process intensification. The 3-bed PSA and the modified duplex PSA exhibited superior performance compared to the other two for a purity of 99.9 mol% of both the products. However, the performances of the processes other than duplex were comparable when purities were set at 95 mol%. In 3-bed PSA a modest process intensification of four times reduction in size and two times reduction in energy requirement appears to be feasible if benchmarked against the PSA based on the variant of the Skarstrom cycle.

Keywords: Process intensification; PSA; Duplex PSA; Synthetic landfill gas; Natural gas

Adsorption equilibrium and kinetics of copper ions and phenol onto modified adsorbents by Tae Young Kim; Sung Young Cho; Seung Jai Kim (pp. 135-143).

The adsorption equilibrium and kinetics of single and binary component copper ions and phenol onto powdered activated carbon (PAC), alginate beads and alginate-activated carbon beads (AAC) were studied. Adsorption equilibrium data for single component copper ions and phenol onto the adsorbents could be represented by the Langmuir equation. Multicomponent equilibrium data were correlated by the extended Langmuir and ideal adsorbed solution theory (IAST). The IAST gave the best fit to our data. The amount of copper ions adsorbed onto the AAC beads in the binary component was greater than that of phenol. The internal diffusion coefficients were determined by comparing the experimental concentration curves with those predicted from surface diffusion and pore diffusion model.

Keywords: Activated carbon; Alginate; Copper ions; Equilibrium; Kinetic; Phenol

Simulated moving bed technology to improve the yield of the biotechnological production of lactobionic acid and sorbitol by Eduardo A. Borges da Silva; Israel Pedruzzi; Alírio E. Rodrigues (pp. 145-158).

This work presents an analysis on the suitability of the Simulated Moving Bed (and pseudo-SMB) technology on separating the mixture obtained from the lactose oxidation catalyzed by glucose-fructose oxidoreductase and glucono-α-lactonase enzymes. These enzymes from the Zymomonas mobilis bacteria are able to oxidize lactose in presence of fructose to its respective organic acid—lactobionic acid—and sorbitol. Some alternative arrays of chromatographic systems, as fixed bed column, SMB unit, 4 section pseudo-SMB, have been explored to separate the multi-component mixture, in such way to make possible the product recovery and the recycle of substrates to the enzymatic reactor. This study involved the definition of appropriate operating conditions and the prediction of the performance of the separation units, or arrangement of units, through modeling and simulations tools. To define the proper operating conditions, inequalities from equilibrium theory and the concept of the separation volume analysis have been considered. In this analysis, equilibrium and kinetic parameters for the compounds adsorbing on DOWEX 50W-X4 resin, in K⁺ and Ca⁺² ion-loadings, have been obtained from chromatographic methods (pulse and adsorption-desorption techniques). The enzymatic kinetic of production of lactobionic acid and sorbitol using permeabilized cells of Z. mobilis is shown. The strategy of keeping the highest value of reaction rate by the integration of a chromatographic system proved to be viable when it was found the feasibility to apply the SMB system in cascade.

Keywords: Simulated moving bed; Lactobionic acid; GFOR enzyme; Pseudo-SMB configuration

Optimizing the separation of gaseous enantiomers by simulated moving bed and pressure swing adsorption by Jason Bentley; Qinglin Huang; Yoshiaki Kawajiri; Mladen Eic; Andres Seidel-Morgenstern (pp. 159-170).

The resolution of racemic gas mixtures by simulated moving bed (SMB) and pressure swing adsorption (PSA) is investigated by dynamic simulation and optimization. Enantiomer separation of inhalation anesthetics is important because there is evidence that the purified enantiomers may have different pharmacological properties than the racemate. The model parameters reported in an experimental investigation performed elsewhere are used to study the feasibility of this separation using SMB and PSA configurations. Both processes were modeled in gPROMS^® as systems of differential algebraic equations. Operating conditions are optimized such that the feed throughput and product recovery for each process were maximized subject to equal constraints on the pressures and superficial gas velocities. SMB was found to be capable of resolving racemic feed mixtures with purity and recovery exceeding 99%. On the other hand, PSA was also able to provide a single purified enantiomer with low recovery of about 30% which may limit its application to enantiomer separation. Nevertheless, PSA consumes less desorbent, and achieves higher throughput at the sacrifice of lower recovery.

Keywords: Optimization; SMB; PSA; Adsorption; Enantiomer separation; Enflurane

Boron adsorption mechanism on polyvinyl alcohol by Atsuhiro Harada; Toshiyuki Takagi; Sho Kataoka; Takuji Yamamoto; Akira Endo (pp. 171-178).

Recently, an increase in the use of boron compounds has led to an increase in boron emissions, and concern has grown regarding its detrimental effects on the human body. An adsorbent that adsorbs boron selectively has been developed as a countermeasure. Although certain commercially available boron selective adsorbents can be used to remove boron from aqueous solutions by utilizing the strong affinity between boron and hydroxyl groups, the adsorption capacity appears to be insufficient. So, we adopted polyvinyl alcohol (PVA), which contains many hydroxyl groups, as a model adsorbent. We investigated the boron adsorption characteristics of PVA, and then studied the relationship between the number of adsorption sites and actual adsorption amounts. We assessed the adsorption result by using adsorption site availability (ASA) as an indicator of the ratio of effectively functioning hydroxyl groups from the many hydroxyl groups in PVA. ASA was expressed as a percentage of the experimental equilibrium adsorbed amount in relation to the theoretical equilibrium adsorbed amount. We also compared the adsorption isotherms and ASA obtained with PVA, commercially available N-methylglucamine-type resin (CRB03 and CRB05) and the adsorbent we synthesized from polyallylamine (PAA) and glucose (PAA-Glu). Although PVA has many hydroxyl groups in a molecule, ASA analysis revealed that only 6% of the hydroxyl groups in PVA was used for boron adsorption. On the other hand, CRBs and PAA-Glu exhibited higher ASA values (about 15% and 35% respectively) and adsorption amounts, suggesting that the sterically congested adsorbent structure had a great influence on boron adsorption and ASA.

Keywords: Boron; Removal; Adsorption; Polyvinyl alcohol (PVA); Polyallylamine (PAA); Saccharide

Capture of CO₂ from flue gas by vacuum pressure swing adsorption using activated carbon beads by Chunzhi Shen; Jianguo Yu; Ping Li; Carlos A. Grande; Alirio E. Rodrigues (pp. 179-188).

Vacuum pressure swing adsorption (VPSA) for CO₂ capture has attracted much research effort with the development of the novel CO₂ adsorbent materials. In this work, a new adsorbent, that is, pitch-based activated carbon bead (AC bead), was used to capture CO₂ by VPSA process from flue gas. Adsorption equilibrium and kinetics data had been reported in a previous work. Fixed-bed breakthrough experiments were carried out in order to evaluate the effect of feed flowrate, composition as well as the operating pressure and temperature in the adsorption process. A four-step Skarstrom-type cycle, including co-current pressurization with feed stream, feed, counter-current blowdown, and counter-current purge with N₂ was employed for CO₂ capture to evaluate the performance of AC beads for CO₂ capture with the feed compositions from 15–50% CO₂ balanced with N₂. Various operating conditions such as total feed flowrate, feed composition, feed pressure, temperature and vacuum pressure were studied experimentally. The simulation of the VPSA unit taking into account mass balance, Ergun relation for pressure drop and energy balance was performed in the gPROMS using a bi-LDF approximation for mass transfer and Virial equation for equilibrium. The simulation and experimental results were in good agreement. Furthermore, two-stage VPSA process was adopted and high CO₂ purity and recovery were obtained for post-combustion CO₂ capture using AC beads.

Keywords: Vacuum pressure swing adsorption; Activated carbon beads; CO₂ adsorption; Simulation; Breakthrough curve

Adsorption and diffusion of argon in disordered nanoporous carbons by Jeremy C. Palmer; Joshua D. Moore; John K. Brennan; Keith E. Gubbins (pp. 189-199).

Application-specific optimization of disordered nanoporous carbons remains a formidable challenge due to the difficulty in accurately characterizing their microstructures with current empirical methods. Using molecular simulation techniques, we investigated the adsorptive and diffusive behavior of argon in three models of disordered nanoporous carbons. We found that the structural and morphological differences between these models gave rise to distinct phenomenological properties. The adsorptive behavior of argon in both the low and high pressure regimes was enhanced dramatically in the models with more crystalline microstructures. As for dynamic properties, we found that the adsorbent’s structure and energetic topology significantly alters the rates of diffusion as well as the characteristics of the underlying diffusion mechanisms.

Keywords: Anomalous diffusion; Single-file diffusion; Nanoporous carbons; Simulation; Adsorption; Molecular dynamics; Activated carbons

Adsorption equilibrium of the mixture of EtOH and TCE on FAU type high silica zeolite by K. Chihara; Y. Suzuki; S. Tomita; J. Kabe (pp. 201-203).

In this study, the adsorption equilibrium of the FAU type high silica zeolite was obtained by a gravimetric adsorption experiment device and a molecular simulation. The adsorbate used was ethanol and trichloroethylene, which are the system of azeotropic mixture.In the course of this study, it became necessary to check whether equilibration was disturbed or not by changing the dosing sequence, that is, whether first adsorbate in the crystal cage could exchange or not by second adsorbate because of each mutual displacement disturbance. Experimental confirmation was done by changing dosing sequence to get the obtained results coincident. Also, molecular simulations by MM and MD were tried to check the possibility of exchange movement of adsorbate in the pore.

Keywords: Azeotropic adsorption; Displacement adsorption; Gravimetric method

Preparation of macroporous carbon foam using emulsion templating method by N. Thongprachan; T. Yamamoto; J. Chaichanawong; T. Ohmori; A. Endo (pp. 205-210).

A macroporous carbon foam (MCF) possessing three-dimensionally interconnected porous structure, that was composed of macropores, mesopores and micropores, could be synthesized by the oil-in-water (o/w) emulsion templating method using ultrasound. For the preparation of the o/w emulsion as a template of the macropores formed in the MCF, a resorcinol-formaldehyde (RF) solution and cyclohexane were used as an aqueous phase and an oil phase, respectively. We examined the effects of the viscosity of the RF solution, the mass ratio of cyclohexane with the RF solution as well as the concentration of a hydrophilic surfactant (Tween80) contained in the RF solution on the size distribution of the macropores. Consequently, the suitable viscosity of the RF solution to obtain a MCF with a narrow size distribution of the macropores was determined. It was revealed that the size of the macropores increased with the increase in the mass ratio of cyclohexane with the RF solution or with the decrease in the concentration of Tween80. It was possible to increase the porosity of the prepared MCF larger than 90% using a concentrated o/w emulsion as the template of the macropores.

Keywords: Adsorbent; Emulsion templating method; Macroporous carbon foam; Pore size distribution; Ultrasound

Investigation on specific adsorption of hydrogen on lithium-doped mesoporous silica by Masaru Kubo; Hiroshi Ushiyama; Atsushi Shimojima; Tatsuya Okubo (pp. 211-218).

This paper reports the synthesis, structure, and hydrogen adsorption property of Li-doped mesoporous silica (MPS) with a 2D hexagonal structure. The Li-doping is achieved by impregnation of the cylindrical mesopores with an ethanol solution of lithium chloride followed by heat treatment. Detailed characterization by solid-state NMR, TG-MS, and FT-IR suggests that, during the heat treatment, lithium chloride reacts with surface ethoxy groups (≡Si-OEt) to form ≡SiOLi groups, while ethyl chloride is released into the gas phase. The hydrogen uptake at 77 K and 1 atm increases from 0.68 wt% for the undoped MPS to 0.81 wt% for Li-doped MPS (Li-MPS). The isosteric heat of adsorption is 4.8 kJ mol⁻¹, which is consistent with the quantum chemistry calculation result (5.12 kJ mol⁻¹). The specific hydrogen adsorption on Li-MPS would be explained by the frontier orbital interaction between HOMO of hydrogen molecules and LUMO of ≡SiOLi. These findings provide an important insight into the development of hydrogen storage materials with specific adsorption sites.

Keywords: Hydrogen adsorption; Li doping; Mesoporous silica; Quantum chemistry calculation

Liquid-phase adsorption on metal-organic frameworks by Antje Henschel; Irena Senkovska; Stefan Kaskel (pp. 219-226).

This work is focusing on the potential application of metal-organic frameworks as porous materials in heterogeneous catalysis where the substrate is in solution. The understanding of such a liquid-phase heterogeneous catalytic process requires adsorption equilibrium data in solution. For this purpose several metal-organic frameworks were synthesized as reference materials and tested as adsorbents for the adsorption of substrate molecules such as styrene or ethylcinnamate from the liquid phase. The adsorption capacity strongly depends on the polarity of the substrate with respect to the solvent. In several instances solvent and polarity effects are heavily superimposed on the pore size effects. Adsorption isotherms, rates and hydrogenation of the substrates are reported and discussed.

Keywords: Metal-organic framework; Liquid-phase adsorption; Palladium supported catalyst; Heterogeneous catalysis; Hydrogenation

A TPD-MS study of the adsorption of ethanol/cyclohexane mixture on activated carbons by Camelia Matei Ghimbeu; Roger Gadiou; Joseph Dentzer; Loïc Vidal; Cathie Vix-Guterl (pp. 227-233).

The adsorption of ethanol/cyclohexane binary mixtures on different types of activated carbons was studied in this work by temperature programmed desorption coupled with mass spectroscopy (TPD-MS). The texture, morphology and surface chemistry of the carbons were evaluated by N₂ adsorption, scanning electron microscopy (SEM) and TPD-MS techniques. The ethanol and cyclohexane TPD-MS desorption profiles showed that specific interactions between the carbon material and the adsorbate are involved during the adsorption. Most of the activated carbons adsorb strongly ethanol on the surface, leading to desorption temperatures above 100 °C. Only one carbon exhibits an affinity for cyclohexane. These observations were correlated to the different surface chemistry of the materials.

Keywords: Activated carbon; Temperature programmed desorption; Ethanol; Cyclohexane

Adsorption of CO₂ on nitrogen-enriched activated carbon and zeolite 13X by Diôgo P. Bezerra; Ronan S. Oliveira; Rodrigo S. Vieira; Célio L. Cavalcante Jr.; Diana C. S. Azevedo (pp. 235-246).

Adsorption may be a potentially attractive alternative to capturing CO₂ from stationary sources in the context of Carbon Capture and Sequestration (CCS) technologies. Activated carbon and zeolites are state-of-art adsorbents which may be used for CO₂ adsorption, however physisorption alone tends to be insignificant at high temperatures. In the present work, commercial adsorbents have been impregnated with monoethanolamine (MEA) and triethanolamine (TEA) in order to investigate the effect of the modified surface chemistry on CO₂ adsorption, especially above room temperature. Adsorption isotherms for CO₂, N₂ and CH₄ were measured in a gravimetrically system in the pressure range of UHV to 10 bar, at 298 and 348 K for activated carbon and zeolite 13X supports. The adsorbed concentration of CO₂ was significantly higher than those of CH₄ and N₂ for both adsorbents in the whole pressure range studied, zeolite 13X showing a remarkable affinity for CO₂ at very low pressures. However, at 348 K, the adsorbed concentration of CO₂ decreases significantly. The supports impregnated with concentrated amine solutions and dried in air suffered a detrimental effect on the textural properties, although CO₂ uptake became much less susceptible to temperature increase. Impregnations carried out with dilute solution followed by drying in inert atmosphere yielded materials with very similar textural characteristics as compared to the parent support. CO₂ isotherms in such materials showed a significant change with similar capacities at 348 K as compared to the original support at 298 K in the case of activated carbons. The impregnated zeolite showed a decrease in adsorbed phase concentration in low pressures for a given temperature, but the adsorbed amount also seemed to be less affected by temperature. These results are promising and indicate that CO₂ adsorption may be enhanced despite high process temperatures (e.g. 348 K), if convenient impregnation and drying methods are applied.

Keywords: CO₂ adsorption; Zeolite 13X; Activated carbon; Amine impregnation

Understanding phenol adsorption mechanisms on activated carbons by Leticia F. Velasco; Conchi O. Ania (pp. 247-254).

The interactions between phenol molecules and activated carbons were investigated in order to understand the adsorption mechanism of this aromatic compound. A series of activated carbons with varied chemical composition but similar porous features were synthesized and submitted to phenol exposure from aqueous phase, followed by thermogravimetric analysis and identification of the desorbed species by temperature programmed desorption coupled with mass spectrometry. Based on these experiments, both physi- and chemisorption sites for phenol were identified on the activated carbons. Our results demonstrate that physisorption of phenol depends strictly on the porosity of the activated carbons, whereas chemisorption depends on the availability of the basal planes in the activated carbons. Thus, oxidation of the carbon can suppress the fraction of chemisorbed phenol since the surface functionalities incorporate to the edges of the basal planes; notwithstanding, hydrophilic carbons may present a small but not negligible contribution of chemisorbed phenol depending on the extent of the functionalization. Moreover, these adsorption sites (chemi-) are recovered by simply removal of the surface functionalities after thermal annealing.

Keywords: Activated carbon; Phenol; Physisorption; Chemisorption

Modeling gas separation in metal-organic frameworks by Brad A. Wells; Alan L. Chaffee (pp. 255-264).

The gas adsorption and CO₂ separation properties of 9 different metal-organic frameworks (MOFs) have been modelled with grand canonical Monte Carlo (GCMC) adsorption simulations. Adsorption of both pure gases and gas mixtures has been studied. MOFs are shown to have high selectivity for polar gases such as CO₂ over non-polar gases such as N₂. Selectivity of one polar gas from another can be altered by changing the polarity of the framework, pore geometry and also temperature. Often features that lead to good selectivity of CO₂ from N₂ also lead to poor selectivity of CO₂ from H₂O.

Keywords: Metal-organic framework; Carbon dioxide; Separation; Simulation

Monte Carlo simulation of polymer adsorption by Christopher J. Rasmussen; Aleksey Vishnyakov; Alexander V. Neimark (pp. 265-271).

We developed and employed the incremental gauge cell method to calculate the chemical potential (and thus free energies) of long, flexible homopolymer chains of Lennard-Jones beads with harmonic bonds. The free energy of these chains was calculated with respect to three external conditions: in the zero-density bulk limit, confined in a spherical pore with hard walls, and confined in a spherical pore with attractive pores, the latter case being an analog of adsorption. Using the incremental gauge cell method, we calculated the incremental chemical potential of free polymer chains before and after the globual-random coil transitions. We also found that chains confined in attractive pores exhibit behaviors typical of low temperature physisorption isotherms, such as layering followed by capillary condensation.

Keywords: Monte Carlo; Chemical potential; Flexible chains; Lennard-Jones; Gauge cell; Polymers

Novel adsorbent hollow fibres for oxygen concentration by J. M. Nevell; S. P. Perera (pp. 273-283).

The research examined the development of adsorbent hollow fibres as a low pressure drop structure for the production of oxygen-enriched air. The potential benefits of using a low pressure drop flexible adsorbent structure with molecular sieving properties over a bed packed with pellets include a low attrition resistance which could extend the life of the adsorbent structure. Highly macroporous, highly adsorbent loaded (up to 90 wt%) fibres were produced. By increasing adsorbent density, the separative performance and nitrogen loading were improved. The separative performance of the adsorbent fibre was found to be slightly inferior to that of the bed of smaller 0.4–0.8 mm beads, as the diffusion path length was longer in the fibres and caused increased mass transfer resistances within the macroporous structure. The pressure drop through the fibre was found to be 40 to 70 times lower than that through an equivalent packed bed of 0.4–0.8 mm beads. This experimental feasibility study has demonstrated that the novel zeolite fibre configuration shows good potential for the production of oxygen-enriched air in a low energy, short cycle time, pressure swing process. The challenges of improving the performance of the adsorbent fibres and their operating parameters are described.

Keywords: Low pressure drop structure; Adsorbent hollow fibre; Pressure swing adsorption; Oxygen concentration

Erratum to: Simple model of surface-induced electrolytic dissociation of weak acids in organic solvents by Marek Kosmulski (pp. 285-286).

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Adsorption: Journal of the International Adsorption Society (v.17, #1)

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Adsorption: Journal of the International Adsorption Society (v.17, #1)