Showing papers in "Adsorption-journal of The International Adsorption Society in 2014"

Enzyme immobilization by adsorption: a review

Abstract: Endowed with unparalleled high catalytic activity and selectivity, enzymes offer enormous potential as catalysts in practical applications. These applications, however, are seriously hampered by enzymes’ low thermal and chemical stabilities. One way to improve these stabilities is the enzyme immobilization. Among various tested methods of this process that make use of different enzyme-carrier interactions, immobilization by adsorption on solid carriers has appeared most common. According to these findings, in this review we present a comparative analysis of the literature reports on the recent trends in the immobilization of the enzymes by adsorption. This thorough study was prepared in order to provide a deeper understanding of the process. Both carriers, carrier modifiers and procedures developed for effective adsorption of the enzymes are discussed. The review may thus be helpful in choosing the right adsorption scheme for a given enzyme to achieve the improvement of its stability and activity for a specific application.

Physical adsorption characterization of nanoporous materials: progress and challenges

Abstract: Within the last two decades major progress has been achieved in understanding the adsorption and phase behavior of fluids in ordered nanoporous materials and in the development of advanced approaches based on statistical mechanics such as molecular simulation and density functional theory (DFT) of inhomogeneous fluids. This progress, coupled with the availability of high resolution experimental procedures for the adsorption of various subcritical fluids, has led to advances in the structural characterization by physical adsorption. It was demonstrated that the application of DFT based methods on high resolution experimental adsorption isotherms provides a much more accurate and comprehensive pore size analysis compared to classical, macroscopic methods. This article discusses important aspects of major underlying mechanisms associated with adsorption, pore condensation and hysteresis behavior in nanoporous solids. We discuss selected examples of state-of-the-art pore size characterization and also reflect briefly on the existing challenges in physical adsorption characterization.

Adsorption technology for CO2 separation and capture: a perspective

Abstract: The capture of CO2 from process and flue gas streams and subsequent sequestration was first proposed as a greenhouse gas mitigation option in the 1990s. This proposal spawned a series of laboratory and field tests in CO2 capture which has now grown into a major world-wide research effort encompassing a myriad of capture technologies and ingenious flow sheets integrating power production and carbon capture. Simultaneously, the explosive growth in materials science in the last two decades has produced a wealth of new materials and knowledge providing us with new avenues to explore to fine tune CO2 adsorption and selectivity. Laboratory and field studies over the last decade have explored the synergy of process and materials to produce numerous CO2 capture technologies and materials based on cyclic adsorption processes. In this brief perspective, we look at some of these developments and comment on the application and limitations of adsorption process to CO2 capture. We identify major engineering obstacles to overcome as well as potential breakthroughs necessary to achieve commercialization of adsorption processes for CO2 capture. Our perspective is primarily restricted to post-combustion flue gas capture and CO2 capture from natural gas.

Physical adsorption of gases: the case for absolute adsorption as the basis for thermodynamic analysis

Abstract: We discuss the thermodynamics of physical adsorption of gases in porous solids. The measurement of the amount of gas adsorbed in a solid requires specialized volumetric and gravimetric techniques based upon the concept of the surface excess. Excess adsorption isotherms provide thermodynamic information about the gas-solid system but are difficult to interpret at high pressure because of peculiarities such as intersecting isotherms. Quantities such as pore density and heats of adsorption are undefined for excess isotherms at high pressure. These difficulties vanish when excess isotherms are converted to absolute adsorption. Using the proper definitions, the special features of adsorption can be incorporated into a rigorous framework of solution thermodynamics. Practical applications including mixed-gas equilibria, equations for adsorption isotherms, and methods for calculating thermodynamic properties are covered. The primary limitations of the absolute adsorption formalism arise from the need to estimate pore volumes and in the application to systems with larger mesopores or macropores at high bulk pressures and temperatures where the thermodynamic properties may be dominated by contributions from the bulk fluid. Under these circumstances a rigorous treatment of the thermodynamics requires consideration of the adsorption cell and its contents (bulk gas, porous solid and confined fluid).

How do the HSDM and Boyd’s model compare for estimating intraparticle diffusion coefficients in adsorption processes

Abstract: Adsorption kinetics is a key-issue for a successful activated carbon selection and design of the treatment system. Crucial predictive aspects are the determination of the diffusion coefficients and the establishment of the controlling adsorption step. Several kinetic models have been developed and two of the most frequently used, the homogeneous surface diffusion model (HSDM) and the Boyd’s model, were applied to microcystins, and particularly MC-LR adsorption. Different initial MC-LR concentrations and carbons (particle diameter, porosity), yielding diverse Biot numbers (Bi), were tested. The model outcomes were compared, namely the Boyd’s effective intraparticle diffusion coefficient (Di) with the HSDM surface diffusion coefficient (Ds), as well as the Bi and Boyd’s criteria to establish the controlling adsorption step, which constitute a novel approach. Very good HSDM fittings were achieved with a constant diffusion model (Ds independent of MC-LR surface concentration). Di was similar to Ds whenever Boyd plots intercepted the origin. The Biot limit above which it may be considered that intraparticle diffusion is the rate limiting step depended on the carbon. A lower limit was observed for smaller carbons.

Adsorption of graphene for the removal of inorganic pollutants in water purification: a review

Abstract: Graphene has aroused widespread attention as a new type of adsorbents due to its outstanding ability for the removal of various pollutants from aqueous solutions. This review summarizes the application of graphene-based nanomaterials as an advanced adsorbent for the removal of inorganic pollutants including anionic and cationic type. The adsorption properties, mechanisms, isotherms, kinetics, thermodynamics and regeneration of adsorbents are all summarized, and the further research trends on graphene-based nanomaterials in the removal of pollutants are also given.

Adsorption isotherms of CO2, CO, N2, CH4, Ar and H2 on activated carbon and zeolite LiX up to 1.0 MPa

Abstract: The adsorption isotherms of CO2, CO, N2, CH4, Ar, and H2 on activated carbon and zeolite LiX were measured using a volumetric method. Equilibrium experiments were conducted at 293, 308, and 323 K and pressures up to 1.0 MPa. The adsorption isotherm and heat of adsorption were analyzed for two pressure regions of experimental data: pressures up to 0.1 MPa and up to 1.0 MPa. Each experimental isotherm was correlated by the Langmuir, Sips, Toth and temperature dependent Sips isotherm models, and the deviation of each model was evaluated. The Sips and Toth models showed smaller deviation from the experimental data of adsorbents than the Langmuir model. Isosteric heats of adsorption were calculated by the temperature dependent Sips model and are presented along with surface loading. From deviation analysis, it is recommended that the isotherm in the proper pressure range be used to appropriately design adsorptive processes.

Diffusion mechanism of CO2 in 13X zeolite beads

Abstract: A systematic study of the diffusion mechanism of CO2 in commercial 13X zeolite beads is presented. In order to gain a complete understanding of the diffusion process of CO2, kinetic measurements with a zero length column (ZLC) system and a volumetric apparatus have been carried out. The ZLC experiments were carried out on a single bead of zeolite 13X at 38 °C at a partial pressure of CO2 of 0.1 bar, conditions representative of post-combustion capture. Experiments with different carrier gases clearly show that the diffusion process is controlled by the transport inside the macropores. Volumetric measurements using a Quantachrome Autosorb system were carried out at different concentrations. These experiments are without a carrier gas and the low pressure measurements show clearly Knudsen diffusion control in both the uptake cell and the bead macropores. At increasing CO2 concentrations the transport mechanism shifts from Knudsen diffusion in the macropores to a completely heat limited process. Both sets of experiments are consistent with independent measurements of bead void fraction and tortuosity and confirm that under the range of conditions that are typical of a carbon capture process the system is controlled by macropore diffusion mechanisms.

Design of a H2 PSA for cogeneration of ultrapure hydrogen and power at an advanced integrated gasification combined cycle with pre-combustion capture

Abstract: A novel hydrogen pressure swing adsorption system has been studied that is applied to an advanced integrated gasification combined cycle plant for cogenerating power and ultrapure hydrogen (99.99+ mol%) with CO2 capture. In designing the H2 PSA, it is essential to increase the recovery of ultrapure hydrogen product to its maximum since the power consumption for compressing the H2 PSA tail gas up to the gas turbine operating pressure should be minimised to save the total auxiliary power consumption of the advanced IGCC plant. In this study, it is sought to increase the H2 recovery by increasing the complexity of the PSA step configuration that enables a PSA cycle to have a lower feed flow to one column for adsorption and more pressure equalisation steps. As a result the H2 recovery reaches a maximum around 93 % with a Polybed H2 PSA system having twelve columns and the step configuration contains simultaneous adsorption at three columns and four-stage pressure equalisation.

Tailoring microporosity and nitrogen content in carbons for achieving high uptake of CO2 at ambient conditions

Abstract: A series of nitrogen-containing carbon spheres (CS) was prepared using the modified Stober method. These CS were synthesized by using resorcinol and formaldehyde as carbon precursors, melamine as nitrogen precursor and ammonia as a polymerization reaction catalyst. Hydrothermal treatment followed by activation of these polymer spheres resulted in highly porous nitrogen-containing CS. Elemental analysis and N2 adsorption showed that the aforementioned CS exhibited high surface area (reaching 1,610 m2/g) with large fraction of fine micropores (volume of micropores smaller than 1 nm was estimated to be 0.40 cm3/g) and comparatively high nitrogen content (about 4.0 at.%). Interestingly, high CO2 adsorption capacities, 4.4 and 6.9 mmol/g, were obtained for these CS at 1 bar and two temperatures, 25 and 0 °C, respectively.

MFI zeolite as adsorbent for selective recovery of hydrocarbons from ABE fermentation broths

Abstract: 1-Butanol and butyric acid are two interesting compounds that may be produced by acetone, butanol, and ethanol fermentation using e.g. Clostridium acetobutylicum. The main drawback, restricting the commercialization potential of this process, is the toxicity of butanol for the cell culture resulting in low concentrations of this compound in the broth. To make this process economically viable, an efficient recovery process has to be developed. In this work, a hydrophobic MFI type zeolite with high silica to alumina ratio was evaluated as adsorbent for the recovery of butanol and butyric acid from model solutions. Dual component adsorption experiments revealed that both butanol and butyric acid showed a high affinity for the hydrophobic MFI zeolite when adsorbed from aqueous model solutions. Multicomponent adsorption experiments using model solutions, mimicking real fermentation broths, revealed that the adsorbent was very selective to the target compounds. Further, the adsorption of butyric and acetic acid was found to be pH dependent with high adsorption below, and low adsorption above, the respective pKa values of the acids. Thermal desorption of butanol from MFI type zeolite was also studied and a suitable desorption temperature was identified.

Branched versus linear alkane adsorption in carbonaceous slit pores

Abstract: The presence of carbonaceous deposits on the internal surfaces of a spark ignition engine has been linked in the literature to impaired vehicle performance, as manifested by increased knocking, higher fuel consumption, higher emissions and other adverse effects. One of the proposed mechanisms, in which the deposits affect the processes in the engine, is the adsorption and desorption of fuel components in the pores within the deposit. In this article we investigate this mechanism in more detail by considering single component adsorption of normal and branched alkanes in selected model slit pores representing the structure of the deposits. We further extend these studies to the binary mixture of normal heptane and isooctane, corresponding to a primary reference fuel blend. In particular, we show that in larger pores adsorption selectivity towards isooctane is about 1.2 on average throughout the pressure range. However, in the smaller 10 A pore selectivity towards isooctane can be in excess of three as a result of packing effects. These results are then placed in the context of engine performance issues.

Aqueous mercury adsorption in a fixed bed column of thiol functionalized mesoporous silica

Abstract: The aim of this work was to investigate the aqueous mercury adsorption in a fixed bed of mesostructured silica SBA-15 functionalized with propylthiol by co-condensation (SBA-15-SH). Powdered synthesized adsorbents were used to prepare pellets with sizes ranging from 0.5 to 1 mm. The physicochemical properties determined from N2 adsorption and chemical analysis were compared for powder and pellets. Batch static experiments were carried out to obtain the equilibrium mercury adsorption isotherms, resulting that although the maximum adsorption capacity was reduced from powder to pellets, the materials maintained high efficiency for mercury removal even at very low aqueous metal concentration. Dynamic experiments were carried out in a fixed bed column by modifying the volumetric flow rate, bed length, inlet concentration, and amount of propylthiol groups incorporated to the adsorbent, and analyzing the temporal scale and the mercury adsorption capacities. The elution of the fixed bed was carried out chemically by circulating an aqueous 2 M hydrobromic acid stream for 2 h so achieving a complete recovery of the mercury previously adsorbed. Simplified dynamic equations of Bohart–Adams and Wolborska were used for modeling the breakthrough curves.

Adsorption of carbon dioxide on graphene oxide supported layered double oxides

Abstract: Adsorption of CO2 on layered double oxides supported on graphene oxide has been studied under dry and wet conditions. In the first exposure to the adsorptive gas, the isotherms obtained for supported and unsupported materials are shown to fit to the Freundlich model indicating the existence of heterogeneous adsorption sites. After multiple temperature-swing cycles, the adsorption capacity decreased and the data is better described by the Langmuir model. The presence of graphene oxide is shown to reduce the loss of adsorption capacity, and helps to maintain the heterogeneity of the basic sites on the adsorbents. The use of wet gas mixtures was found to have a positive effect on the CO2 adsorption capacity of the graphene oxide hybrids. The presence of residual sodium on the materials resulted in a Freundlich isotherm with increased adsorption capacity.

Methane preconcentration by adsorption: a methodology for materials and conditions selection

Abstract: Methane (CH $$_4$$ ) adsorption has been widely studied, mainly in the context of natural gas purification. A much less prominent, but highly relevant application is the preconcentration of CH $$_4$$ from ambient air. In this study, we compare six different commercial adsorbent materials with respect to their effectiveness for methane preconcentration: a macroporous polymeric resin (HayeSep D), multi-walled carbon nanotubes, two microporous metal-organic frameworks (HKUST-1 and ZIF-8), and two zeolites (5A and 13X). The most relevant properties, such as isosteric enthalpy of adsorption, specific surface area and the selectivity for CH $$_4$$ adsorption over N $$_2$$ were characterized by analyzing adsorption/desorption isotherms. Using these parameters, we discuss the tested adsorbents with respect to the most important properties and identify the most promising candidates. Furthermore we identify the experimental conditions that are expected to give the best results with respect to practical applications.

Adsorption of carbon dioxide-methane mixtures in porous carbons: effect of surface chemistry

Abstract: A combined experimental and molecular simulation study of the coadsorption of CO2 and CH4 in porous carbons is reported. We address the effect of surface chemistry by considering a numerical model of disordered porous carbons which has been modified to include heterochemistry (with a chemical composition consistent with that of the experimental sample). We discuss how realistic the numerical sample is by comparing its pore size distribution (PSD), specific surface area, porous volume, and porosity with those for the experimental sample. We also discuss the different criteria used to estimate the latter properties from a geometrical analysis. We demonstrate the ability of the MP method to estimate PSD of porous carbons from nitrogen adsorption isotherms. Both the experimental and simulated coadsorption isotherms resemble those obtained for pure gases (type I in the IUPAC classification). On the other hand, only the porous carbon including the heterogroups allows simulating quantitatively the selectivity of the experimental adsorbent for different carbon dioxide/methane mixtures. This result shows that taking into account the heterochemistry present in porous carbons is crucial to represent correctly adsorption selectivities in such hydrophobic samples. We also show that the adsorbed solution theory describes quantitatively the simulated and experimental coadsorption isotherms without any parameter adjustment.

Removal of lead from aqueous solutions by adsorption with surface precipitation

Abstract: An activated carbon from Coconut (Cocos nucifera) shells was prepared by physical activation with carbon dioxide and water vapor. The activated carbon obtained has a surface area of 1058 m2 g−1 and such a high micropore volume of 0.49 cm3 g−1. This carbon was studied for the removal of lead from water. Sorption studies were performed at 30 °C, at different pH and adsorbent doses, in batch mode. Lead precipitation was observed on the surface of the activated carbon. Maximum adsorption occurred at pH 9 for an adsorbent dose of 2 g L−1. Kinetic studies, at the initial concentration of 150 mg L−1 of lead, pH 5 and an adsorbent dose of 1 g L−1, yielded an equilibrium time of 50 h for this activated carbon. The kinetic data were modeled with the pseudo first order, the pseudo second order and the Bangham models. The pseudo second order model fitted the data well. The sorption rate constant (7 × 10−4 mol−1 Kg s−1) and the maximum amount of lead adsorbed (0.23 mol kg−1) are quite good compared to the data found in literature. Sorption equilibrium studies were conducted in a concentration range of lead from 0 to 150 mg L−1. In an aqueous lead solution with an initial concentration of 30 mg L−1, at pH 5, adsorbent dose 1 g L−1, activated Coconut shell carbon removed at equilibrium 100 % of the heavy metal. The equilibrium data were modeled with the Langmuir and Freundlich equations, of which the former gave the best fit. The Langmuir constants Qmax eq (0.23 mol kg−1) and KL (487667 L mol−1) are in good agreement with literature. XPS studies identified adsorbed species as lead carbonates and/or lead oxalates and precipitates as lead oxide and/or lead hydroxide on the activated carbon surface. The Coconut shell activated carbon is a very efficient carbon due to its high surface area, to the presence of many micropores on its surface and to the presence surface groups like hydroxyls promoting adsorption in the porous system and lead crystal precipitation on the activated carbon surface.

Evaluating methane storage targets: from powder samples to onboard storage systems

Abstract: The development of a viable adsorbed natural gas onboard fuel system involves synthesizing materials that meet specific storage target requirements. We assess the impact on natural gas storage due to intermediate processes involved in taking a laboratory powder sample to an onboard packed or adsorbent bed module. We illustrate that reporting the V/V (volume of gas/volume of container) capacities based on powder adsorption data without accounting for losses due to pelletization and bed porosity, grossly overestimates the working storage capacity for a given material. Using data typically found for adsorbent materials that are carbon and MOF based materials, we show that in order to meet the Department of Energy targets of 180 V/V (equivalent STP) loading at 3.5 MPa and 298 K at the onboard packed bed level, the volumetric capacity of the pelletized sample should be at least 245 V/V and the corresponding gravimetric loading varies from 0.175 to 0.38 kg/kg for pellet densities ranging from 461.5 to 1,000 $$\hbox {kg m}^{-3}$$ . With recent revision of the DOE target to 263 V/V at the onboard packed bed level, the volumetric loadings for the pelletized sample should be about 373 V/V.

Modeling of high pressure adsorption isotherm using statistical physics approach: lateral interaction of gases adsorption onto metal–organic framework HKUST-1

Abstract: A novel theoretical approach for description of high pressure isotherm is developed in the present study. This approach is based on the grand canonical formalism in statistical physics by taking into account the lateral interaction between the adsorbate molecules. This leads to five parameters equation describing the high pressure adsorption equilibrium. This model is applied to experimental adsorption data of hydrogen, nitrogen, methane and carbon dioxide on metal–organic frameworks HKUST-1. There is a good correlation between experimental data and those calculated by the new model at pressure up to 50 MPa. It was found that hydrogen molecules can be considered as a ideal gas over a wide temperature range and even at high pressures. Lateral interactions were found to be necessary to describe the adsorption of N2, CH4 and CO2 at high pressures. The proposed model allows also prediction of some adsorption thermodynamic functions which govern the adsorption mechanism such as the entropy, the Gibbs free enthalpy and the internal energy.

Sorption equilibrium prediction of competitive adsorption of herbicides 2,4-D and MCPA from aqueous solution on activated carbon using ANN

Abstract: The simultaneous adsorption of two herbicides—2,4-dichlorophenoxyacetic acid (2,4-D) and 4-chloro-2-methylphenoxyacetic acid (MCPA)—from their aqueous binary mixtures onto granular activated carbon was studied. The quantities adsorbed were determined by HPLC with UV detection. The experimental data were analysed using the Freundlich adsorption isotherm. The high correlation coefficients indicated that the adsorption equilibrium fitted the Freundlich isotherm well. A multilayer perceptron (MLP) (an artificial neural network model—ANN) was applied to describe the adsorption equilibrium in multicomponent systems. This enabled sorption isotherms to be predicted for all possible combinations of the two herbicides. The experimental results and the calculated data obtained from MLP for the solutions of the individual components and their mixtures suggest that MCPA is better adsorbed onto activated carbon than 2,4-D.

Modeling water vapor adsorption/desorption cycles

Abstract: This modeling work deals with the adsorption of water vapor on different porous materials where it undergoes capillary condensation and its adsorption/desorption isotherms exhibit hysteresis. The focus is on the description of the so called scanning curves, i.e. the adsorption/desorption isotherms observed when such an adsorbent is repeatedly loaded and unloaded in a range of conditions where hysteresis is observed, and on the simulation of fixed bed adsorption/desorption cycles. We use an approach originally developed by Stěpanek et al. (Chem Eng Sci 55(2):431–440, 2000), and expand it so as to include more general isotherms (not only the Dubinin–Radushkevich and Dubinin–Astakhov model, but also the Guggenheim–Anderson–de Boer model and the Do and Do model) and to allow for less than infinitely fast heat transfer, so as to consider non-isothermal situations. From a modeling point of view the results are satisfactory and highlight the need for better experimental data on water vapor adsorption, which need to be measured in enhanced experimental set-ups, capable to tightly control the relative humidity of the gas phase.

Water vapor adsorption equilibria and mass transport in unmodified and modified cellulose fiber-based materials

Abstract: Water vapor adsorption isotherms of different unmodified and coated paper samples were studied to determine their suitability as water barrier packaging materials. The sorption behavior of these samples was compared with commercially available paper. The experimental data were analyzed using the Hailwood–Horrobin (H–H), Guggenheim–Anderson–De Boer (GAB) and BET models for extraction of isotherm parameters and determination of monolayer moisture contents. The H–H and GAB models were found to provide good fits to the experimental data. The monolayer moisture content of modified papers was less than 3.0 % (dry basis) as compared to unmodified paper samples (4.20 %), at saturation. It was also observed that the sorption behavior of modified paper samples differed with substrate type. Water vapor permeability (WVP) of unmodified and coated paper samples at the temperatures of 25 and 38 °C were also measured for a wide range of vapor partial pressure gradients. The permeabilities of the modified samples were found to be generally low compared to the unmodified (reference) paper sample. Among the investigated samples, PLA and PHBV coated paper samples showed higher mass transfer resistance to water vapor transport. Furthermore, the water vapor permeabilities of different samples were found to be relatively constant up to the modest relative humidity levels; however, at the higher humidity levels they showed increasing trend with the further increase in relative humidity. Results of this study confirmed that blocking of active surface sites by coating with PLA and PHBV is the most effective way to increase the water vapor barrier properties of modified papers, thus making them the appropriate candidates for green-based food packaging materials.

Microwave assisted vacuum regeneration for CO2 capture from wet flue gas

Abstract: Small scale processing of flue gas with the goal of enriching the stream in CO2 for sequestration or use is an interesting application area for adsorption technology. For example, boiler flue gas which may contain up to 10 % (v/v) CO2 in air can be readily enriched to a stream containing >70 % CO2 which may be ideal for use within a process such as acidification, precipitation, stripping, etc. The challenge in these applications is producing high purity CO2 without excessive energy use and handling high concentrations of water vapor without the added complication of a pre-drying stage. In this study we have examined the use of microwave assisted vacuum as a way of rapidly directing thermal energy to the adsorbent surface to liberate water and CO2. Preliminary “proof-of-concept” pump down experiments were conducted on a small transparent adsorption column of 13X zeolite pre-saturated with a 12 % CO2 in N2 gas mixture. Both wet and dry gas tests were conducted. The addition of microwave radiation improved the rapid desorption of CO2 and water and improved the integrated CO2 purity in the blowdown stream from 60 to 80 %. In the case of dry CO2 mixtures, the enhancement is due to microwave heating of the 13X zeolite facilitated by the high cation density in the faujasite structure. In the case of water and CO2 desorption, the temperature rise of the adsorbent upon microwave heating was much lower than that predicted by simple heating suggesting that the microwave radiation is absorbed primarily by the adsorbed water. A simplified energy analysis suggests that brief exposure of an adsorbent to microwave radiation will raise the required vacuum level for regeneration of high humidity flue gas streams and may lead to an overall lower energy penalty. The selective ability of microwave radiation to target different species provides scope for optimized, compact, flue gas treatment systems.

CO2 sorption in wet ordered mesoporous silica kit-6: effects of water content and mechanism on enhanced sorption capacity

Abstract: The sorption isotherms of CO2 in wet ordered mesoporous silica KIT-6 with different amounts of pre-adsorbed water were firstly collected experimentally using a volumetric method in the temperature range of 275–281 K. The isotherms show an inflection point indicating CO2 hydrates form in the pore spaces which is proofed by the enthalpy change calculated at the inflection pressure, and the quantity of water content shows considerable effect on the sorption capacity of CO2. The highest enhancement of sorption capacity in the presence of water is observed in wet KIT-6 sample with water loadings of 2.48, which is about 12.80 mmol/g and 1.86 times than that on dry sample. However, the saturation capacity is still far less than that what can be stored merely in the form of hydrates due to the low ratio of water utilization because of the large pore and the polar surface of KIT-6.

Effect on selective adsorption of ethane and ethylene of the polyoxometalates impregnation in the metal-organic framework MIL-101

Abstract: Impregnation of ionic nanostructured units in the pores of metal-organic frameworks (MOFs) is one approach to modify their host–guest interactions. Although, the effect of this approach is well investigated in catalysis, drug delivery, and bio imaging, still little is known about its impact on the selective adsorption properties of MOFs. Here we report the impregnation of two different polyoxometalate (POM) nanoclusters (PW11 and SiW11) into chromium terephthalate-based MOF, MIL-101(Cr), to investigate the post-impregnation changes in selective adsorption behavior, which are observed in terms of an important paraffin–olefin separation, using ethane and ethylene, at high pressure. The PW11 and SiW11 POMs bring π-accepting tendency and highly electronegative oxygen atoms on their surface to MIL-101 structure that selectively increases the affinity of material for ethylene, which is confirmed from isosteric heats of adsorption and selectivity calculation. Impregnated samples retain about 74–81 % of working adsorption capacity, after regeneration by decreasing the pressure. This study shows that anionic metal-oxide nanoclusters (POMs) may be used to change the selectivity of MOFs for olefin molecules.

Benzene, toluene and o -xylene (BTX) removal from aqueous solutions through adsorptive processes

Abstract: In this study, the monocomponent adsorption of benzene, toluene and o-xylene (BTX) compounds, as model contaminants present in the petrochemical wastewaters, was investigated using three types of adsorbents: activated carbon (Carbon CD 500), a polymeric resin (MN-202) and a modified clay (Claytone-40). Langmuir and Freundlich models were able to fit well the equilibrium experimental data. The BTX adsorption capacity increased in the following order: Claytone-40 < CD 500 < MN-202. The maximum uptake capacity of MN-202, given by the Langmuir fitting parameter, for benzene, toluene and o-xylene was 0.8 ± 0.1, 0.70 ± 0.08 and 0.63 ± 0.06 mmol/g at 26 °C. Desorption kinetics for polymeric resin with 50 % methanol solution were fast being able to reuse the resin in consecutive adsorption/desorption cycles without loss of sorption capacity. The adsorptive behaviour at batch system was modelled using a mass transfer kinetic model, considering that the sorption rate is controlled by a linear driving force model, which successfully predicts benzene, toluene and o-xylene concentration profiles, with homogeneous diffusivity coefficients, Dh, between 3.8 × 10−10 and 3.6 × 10−9 cm2/s. In general, benzene diffuses faster than toluene and o-xylene, which is in agreement with molecular diffusivity in water.

Effects of alumina phases on CO2 sorption and regeneration properties of potassium-based alumina sorbents

Abstract: A range of potassium-based alumina sorbents were fabricated by impregnation of alumina with K2CO3 to examine the effects of the structural and textural properties of alumina on the CO2 sorption and regeneration properties. Alumina materials, which were used as supports, were prepared by calcining alumina at various temperatures (300, 600, 950, and 1,200 °C). The CO2 sorption and regeneration properties of these sorbents were examined during multiple tests in a fixed-bed reactor in the presence of 1 vol% CO2 and 9 vol% H2O. The regeneration capacities of the potassium-based alumina sorbents increased with increasing calcination temperature of alumina. The formation of KHCO3 increased with increasing calcination temperature during CO2 sorption, whereas the formation of KAl(CO3)(OH)2, which is an inactive material, decreased. These results is due to the fact that the structure of alumina by the calcination temperature is related directly to the formation of the by-product [KAl(CO3)(OH)2]. The structure of alumina plays an important role in enhancing the regeneration capacity of the potassium-based alumina sorbent. Based on these results, a new potassium-based sorbent using δ-Al2O3 as a support was developed for post-combustion CO2 capture. This sorbent maintained a high CO2 capture capacity of 88 mg CO2/g sorbent after two cycles. In particular, it showed a faster sorption rate than the other potassium-based alumina sorbents examined.

Adsorption of carbonyl sulfide on modified activated carbon under low-oxygen content conditions

Abstract: Activated carbon sorbents impregnated with KOH, Fe(NO3)3, Cu(NO3)2, Zn(NO3)2 or Co(NO3)2 and their applications in catalytic oxidation reaction of COS were investigated. The results showed that the activated carbon modified with 10 % (mass percentage) KOH enhanced the adsorption ability significantly. And it was also found that the oxygen content and temperature were the two most important factors in the COS adsorption. Further investigation on the pore structures of the samples with X-ray photoelectron spectroscopy indicated that an adsorption/oxidation process happened in the KOH modified activated carbon in which the major existing forms of sulfur were SO4 2− and S species. The oxidation of COS suggested that KOH in the micropores may play a catalytic role during the adsorption. On the other hand, we found that the desorption activation energy from KOHW was higher than that from AC by the CO2-TPD spectra, which indicated the adsorption of CO2 on KOH impregnated activated carbon was stronger. The strong adsorption could be attributed to the basic groups on the activated carbon surface. In conclusion, the activated carbon impregnated with KOH promises a good candidate for COS adsorbent.

Chromatographic separation of prebiotic oligosaccharides. Case study: separation of galacto-oligosaccharides on a cation exchanger

Abstract: Chromatographic separation of prebiotic oligosaccharides, galacto-oligosaccharides from a typical post-reaction mixture containing unreacted substrate lactose and by-products glucose and galactose was investigated. A commercial cation-exchange resin Dowex 50WX4 was chosen as a prospective preparative-scale adsorbent and separation performance of its four ionic forms, H+, Na+, K+ and Ca2+, was tested. Since adsorption equilibrium isotherms were linear within the entire investigated range of concentrations, they were determined by pulse experiments for all saccharides present in a typical post reaction mixture including tri- and tetragalacto-oligosaccharides. From the four counter ions listed above, hydrogen offered the best selectivity and column performance. The selected H+-form of the cation exchanger was further investigated by means of frontal analysis where high ionic strength and elevated viscosity appear and can play a substantial role. Breakthrough curves were measured for monosaccharides, lactose, glucose–lactose mixture and a multicomponent commercial mixture of galacto-oligosaccharides, Vivinal® GOS. The breakthrough curves were successfully described by the dispersive plug flow model with linear driving force approximation.

Fluids in nanospaces: molecular simulation studies to find out key mechanisms for engineering

Abstract: We have analyzed various phenomena that occur in nanopores, focusing on elucidating their key mechanisms, to advance the effective engineering use of nanoporous materials. As ideal experimental systems, molecular simulations can effectively provide information at the molecular level that leads to mechanistic insight. In this short review, several of our recent results are presented. The first topic is the critical point depression of Lennard-Jones fluid in silica slit pores due to finite size effects, studied by our original Monte Carlo (MC) technique. We demonstrate that the first layers of adsorbed molecules in contact with the pore walls act as a “fluid wall” and impose extra finite size effects on the fluid confined in the central portion of the pore. We next present a new kernel for pore size distribution (PSD) analysis, based entirely on molecular simulation, which consists of local isotherms for nitrogen adsorption in carbon slit pores at 77 K. The kernel is obtained by combining grand canonical Monte Carlo (GCMC) method and open pore cell MC method that was developed in the previous study. We show that overall trends of the PSDs of activated carbons calculated with our new kernel and with conventional kernel from non-local density functional theory are nearly the same; however, apparent difference can be seen between them. As the third topic, we apply a free energy analysis method with the aid of GCMC simulations to investigate the gating behavior observed in a porous coordination polymer, and propose a mechanism for the adsorption-induced structural transition based on both the theory of equilibrium and kinetics. Finally, we construct an atomistic silica pore model that mimics MCM-41, which has atomic-level surface roughness, and perform molecular simulations to understand the mechanism of capillary condensation with hysteresis. We calculate the work required for the gas–liquid transition from the simulation data, and show that the adsorption branch with hysteresis for MCM-41 arise from spontaneous capillary condensation from a metastable state.