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Adsorption: Journal of the International Adsorption Society (v.11, #3-4)

Wolfgang Schirmer—In memoriam and Recognition of his Merits by Gerhard Öhlmann; Martin Bülow (pp. 201-203).

Ordered Mesoporous Silicas with 2,5-Dimercapto-1,3,4-Thiadiazole Ligand: High Capacity Adsorbents for Mercury Ions by Oksana Olkhovyk; Mietek Jaroniec (pp. 205-214).

This work reports two-step synthesis of novel ordered mesoporous silicas (OMS), which contain mercury-specific multifunctional ligand and have high surface area and well-developed porosity. One pot co-condensation synthesis was employed to introduce chloropropyl functionality on the mesopore walls of hexagonally ordered silica. In the next step, 2,5-dimercapto-1,3,4-thiadiazole was reacted with chloropropyl groups during template-displacement process, which resulted in high affinity adsorbent towards mercury ions. The maximum adsorption capacity of this adsorbent for mercury ions from aqueous solutions was as high as 1.7 g/g, which is about three times higher than the concentration of surface ligand. This study shows that the surface properties of OMS can be tailored by proper choice of chemical modification method, which affects the ligand bonding density and determines the adsorbent capacity and affinity towards heavy metal ions. Three methods, one-pot synthesis, template-displacement and post-synthesis modification, were used for the introduction of surface ligands into MCM41 and SBA15 mesostructures to prepare mercury-specific adsorbents. In addition, adsorption properties of these adsorbents as well as their effectiveness for mercury removal from aqueous solutions were comparatively studied.

Keywords: MCM-41; SBA-15; mercury ion adsorption; 2,5-Dimercapto-1,3,4-Thiadiazole ligand; co-condensation synthesis of organosilicas; template displacement synthesis; post-synthesis modification of mesoporous silicas

Zeolite Membranes: From the Laboratory Scale to Technical Applications by Jürgen Caro; Manfred Noack; Peter Kölsch (pp. 215-227).

Current trends and novel concepts in R&D of zeolite membranes like the seeding supported crystallization and the preparation of Al-containing zeolite membranes are discussed. The influences of adsorption and diffusion on the permeation properties of zeolite membranes are considered. Dehydration of ethanol by steam permeation and pervaporation as the first zeolite membrane based industrial separation is presented. Membrane supported dehydrogenation and esterification are discussed as possible applications of a catalytic membrane reactors.

Keywords: molecular sieve; zeolite membrane; permeation; pervaporation

Application of Excess Formalism to Multicomponent Liquid Adsorption Equilibria by Grit Kalies; Peter Bräuer (pp. 229-234).

Thermodynamic equations for the description of multicomponent liquid adsorption equilibria on solids are often formulated in terms of absolute quantities instead of excess quantities. Considering the excess nature of liquid adsorption phenomena, the utilization of excess quantities seems to be more suitable for the analysis and prediction of multicomponent liquid adsorption. In this paper, a brief summary of four different possible applications of adsorption excess formalism is given. Calculations and experimental data are used to illustrate theoretical relations and to demonstrate their applicability.

Keywords: Excess formalism; thermodynamic prediction; adsorption isotherm; binary and ternary liquid mixtures; liquid/solid interface; liquid/air interface; activity coefficient; surface tension; calorimetric quantity

Sorption Kinetics and Intracrystalline Diffusion of Methanol in Ferrierite: An Example of Disguised Kinetics by P. Kortunov; C. Chmelik; J. Kärger; R. A. Rakoczy; D. M. Ruthven; Y. Traa; S. Vasenkov; J. Weitkamp (pp. 235-244).

Sorption kinetics of methanol in large crystals of ferrierite have been studied in detail by interference microscopy (IFM) and infra-red microscopy (IRM). The IFM measurements yield the transient concentration profiles, thus providing a direct measurement of both the surface resistance to mass transfer and the internal diffusion resistance. It is shown that, for this system, the uptake rate is controlled by the combined effects of surface resistance and diffusion through the 8-ring channels (in the y-direction). Transport through the 10-ring channels (in the z-direction) appears to be blocked by surface resistance. Although the overall uptake curves conform well to the “root t law” the diffusivity values derived from the uptake curves vary widely depending on the assumed direction of diffusion. Even if the correct direction of diffusion is assumed, the diffusivity values derived from the uptake curves are seriously in error as a result of the intrusion of surface resistance. The existence of transport resistances at the crystal surface is clearly apparent from the transient concentration profiles but is not obvious from the uptake curves.

Keywords: diffusion; zeolite; concentration profiles; surface barrier; interference microscopy; IR microscopy

Self-Diffusion Coefficients for Pure and Mixed Adsorbate Fluids in Narrow Pores by Yuriy K. Tovbin (pp. 245-257).

The equilibrium distribution and the concentration dependence of the local and average self-diffusion coefficients for pure fluid and binary mixture components in narrow slitlike pores were analyzed. The coefficients were calculated using the lattice gas model in the quasi-chemical approximation on the assumption of a spherical shape and approximately equal sizes of the components. For the pure adsorbate, these calculations were compared with molecular dynamics simulations. Both methods gave similar concentration profile changes and dynamic characteristics of interlayer particle redistributions in strong nonuniform adsorption fields for dense fluids. A satisfactory agreement was obtained for the temperature dependences of the self-diffusion coefficients along the pore axis. The influence of the molecule–wall potential and of intermolecular interaction were considered. The self-diffusion coefficients of the adsorbate were shown to strongly depend on the density of the mixture and the distance from pore walls.

Keywords: isotherms; coefficient of self-diffusion; argon; krypton; mixture; carbon slitlike pore

Diffusion in Surface Modified ZSM-5 Studied Using the ZLC Method by W. L. Duncan; K. P. Möller (pp. 259-273).

Deposition of silane on a zeolite's external surface is a well established method of increasing its shape selective properties by modifying diffusion resistances. In this work the intracrystalline diffusivity of cyclohexane in both a parent and silanized ZSM-5 samples were measured using the zero length column technique. It was found that the apparent intracrystalline diffusivity did indeed decrease in the modified samples. Models based on a surface barrier approach to describe pore mouth narrowing and an increase in intracrystalline tortuosity as a result of pore blockage were used to interpret the experimental data. It is found that the data correspond most consistently with the model describing pore blockage.

Keywords: characterization of zeolite properties; mathematical diffusion models; experimental diffusion data; intraparticle diffusion; silanization

Expressiveness of Adsorption Measurements for Characterization of Zeolitic Materials—A Review by Jan Kornatowski (pp. 275-293).

This critical review concerns the author's results and experience in adsorption studies on molecular sieves comprising crystalline microporous aluminosilicates and aluminophosphates as well as amorphous mesoporous aluminosilicates. The discussion is mainly based on three distinctly different standard adsorbates: nitrogen, benzene, and water. The highlights or advantages and the shortcomings or limitations are considered from the points of view of the experimental procedures and expressiveness or concluding. The results are compared to several other zeolitic materials and adsorbates. Adsorption technique is a valuable tool for characterization of the molecular sieves. Since the measurements are very sensitive to modification of the materials, the investigations require sufficiently thorough procedures and the results a careful interpretation. A comparison between the results for larger series of materials yields valuable conclusions that are much more expressive than those from a single measurement or material.

Keywords: adsorption; isotherms; adsorbent heterogeneity; adsorption centres; sorption capacity; adsorption mechanism; adsorption theory; adsorption equation

Further Validation of the Quartic Concentration Profile Approximation for Describing Intraparticle Transport in Cyclic Adsorption Processes by Sarang A. Gadre; Armin D. Ebner; James A. Ritter (pp. 295-314).

Models developed previously by the authors that describe nonlinear adsorption, and simultaneous pore and surface diffusion in a single particle, that are based on intraparticle quartic and parabolic concentration profile approximations, and that utilize the summation of the gas and adsorbed phases approach in the material balance formulations, were further validated under more diverse, yet more realistic, cycling conditions. Periodic square, sinusoidal and triangular wave functions were used to more accurately represent the periodic boundary conditions that the external surface of an adsorbent particle may be exposed to during repeated adsorption and desorption cycles in a fixed bed adsorber. Analytical solutions that describe the periodic uptake and release of the adsorbate by the adsorbent were obtained for all three periodic wave functions, and for both the quartic and parabolic profile approximations. By comparing the predictions obtained from both models with the exact numerical solution, the superiority of the quartic model over the parabolic model was clearly demonstrated for all wave functions, and for a wide range of adsorbate-adsorbent systems and bulk concentrations. Excellent agreement between the quartic and exact models was obtained in most cases. In general, the predictions improved as the wave function changed more gradually with time (triangular more gradual than sinusoidal and sinusoidal more gradual than square), as the degree of mathematical linearity of the adsorbate-adsorbent system increased, and as the maximum external surface concentration decreased (an isotherm nonlinearity effect). Subtle differences in the predictive ability of the new approximate models, stemming from the use of the different wave functions, were exposed. Overall, these results exemplify the importance of comparing the predictive ability of new approximate models that describe intraparticle transport under more diverse cycling conditions than are typically utilized in the literature, which has been dominated by the square wave function.

Keywords: linear driving force; LDF; simultaneous pore and surface diffusion; Langmuir isotherm; quartic concentration profile; parabolic concentration profile; cyclic adsorption

Tuning of Pressure Swing Adsorption Systems Based on Differential Pressure Profile by Yaping Lü; Shain-Jer Doong; Martin Bülow (pp. 315-324).

A method for tuning a Pressure Swing Adsorption (PSA) system aimed to achieve symmetrical operating conditions based on pressure differential in the adsorption vessels is developed in this study. Simulation of an oxygen Pressure-Vacuum Swing Adsorption (PVSA) process indicates that the pressure drop inside the adsorption vessel is closely related to the nitrogen concentration and gas velocity. The technique is applied to the tuning of an oxygen PVSA process. Adsorbent vessels of the PSA system are monitored and tuned by making corrective adjustments in each of the steps in a PSA cycle in response to imbalances in the differential pressure profiles in each of the adsorbent vessels. The method developed in this study provides a faster, easier, and more effective way to bring a PSA plant to its symmetrical, optimal state than those based on other parameters such as concentration, temperature, and pressure profile.

Keywords: PSA; VSA; PVSA; oxygen; plant; tuning; control; and pressure differential

Benzene Adsorption in Microporous Materials by Lijuan Song; Zhao-Lin Sun; Hong-Yan Ban; Min Dai; Lovat V. C. Rees (pp. 325-339).

The adsorption behaviour of benzene in silicalite-1, AlPO₄-5 and EU-1 has been investigated using gravimetric techniques and molecular simulation methods. For the one-dimensional, 12-membered ring (MR) channels of AlPO₄-5 and the unidirectional, 10-MR channels with 12-MR side pockets of EU-1, the isotherms of benzene show simple type I behaviour. For the three dimensional 10-MR channels of silicalite-1, an anomalous behaviour of the benzene molecules sorbed has been observed. Two steps at loadings of ca. 4 and 6 molecules per unit cell [m.(u.c.)⁻¹], respectively, and an hysteresis loop between loadings from 6 to 8 m.(u.c.)⁻¹ can be found in the isotherms of this system. These stepped isotherms can be classified as showing type VI isotherm behaviour but in this system the reasons behind the steps are of a new and novel nature. These abnormal adsorption properties have been ascribed to the subtle interplay of increased sorbate-sorbate interactions and decreases in the entropy of sorption due to the energetically heterogeneous surfaces which are present in silicalite-1. The composition and structure of the silicalite-1 samples also play an important role on the adsorption properties of this system.

Adsorption and Mobility of Water and Benzene Molecules in Carbon and Polymer Adsorbents by E. V. Khozina; R. Sh. Vartapetyan (pp. 341-356).

The correlation between the adsorption and the mobility of adsorbed molecules was analyzed. The peculiarities of molecular mobility in microporous adsorbents with relatively rigid (active carbons) and non-rigid (polymer super-crosslinked and methacrylate sorbents) structure were studied using pulsed nuclear magnetic resonance (NMR) techniques. It was shown that the translational behavior of water and benzene in pores are connected with the specific adsorption mechanism. The NMR data allowed analyzing the changes of molecular state in the processes of adsorption. Diffusional properties of water molecules in the super-crosslinked polystyrenes and active carbons with relatively rigid framework are similar and differ from that in methacrylate polymer sorbents. Moreover, the results of NM relaxation measurements allowed analyzing the porous structure over the scale of pore sizes, while the scale of the self-diffusion measurements comprises the areas of different porosity. The NMR data agree well with the data of adsorption measurements and complement them.

Keywords: nuclear magnetic relaxation; self-diffusion; porous media; microporous adsorbents; adsorption mechanism; pore sizes; pore size distribution

Extension of the Theory of Volume Filling of Micropores to Adsorption in Supermicropores by N. S. Polyakov; G. A. Petukhova (pp. 357-362).

An enhancement in characteristic energy of adsorption in large micropores is analyzed. The effect is due to the overlapping of potential fields from opposite walls of pores and to reduction of the surface adsorption film on filling the micropore volume. The effects of both factors are comparable in magnitude and dependent on the micropore size.

Keywords: non-porous and supermicroporous adsorbents; characteristic energy of adsorption; adsorption potential; polymolecular adsorption

Mesoporous Molecular Sieves Modified with Carbonaceous Deposits by M. Rozwadowski; M. Lezanska; K. Erdmann (pp. 363-377).

There were investigated aluminosilicate MCM-41 samples in the as-prepared form and those modified by the deposition of carbonaceous compounds during conversion of cyclohexene for 12 h. The amount of the deposits decreased with the rising reaction temperature and increased with the Al content of the samples. The cyclohexene conversion followed mainly two mechanisms: cyclohexene skeletal isomerization and hydrogen transfer. The products with 6 carbon atoms in a molecule prevailed in all cases. The process of conversion, proceeding on the Brønsted acid sites, resulted in formation of both coke deposits and volatile products. The creation of coke caused a decrease in the effective concentration of both the Brønsted and the Lewis acid sites. Thermodesorption of pyridine showed that (i) the concentration of these sites before and after the conversion differed only slightly and (ii) the acidic strength of the Brønsted sites was practically independent of their concentration and the sample Si/Al ratio. The chemical composition of the deposits was insignificantly affected by the Al content of the materials and depended mostly on the temperature and duration of the reaction. At relatively low temperatures, both aliphatic and aromatic compounds were formed, being rather weakly bound to the surface of the material. After a longer (55 h) reaction period, some deposits appeared that were strongly bound to the surface. Isotherms of adsorption of water, benzene, and nitrogen were determined, from which a mechanism of this process was derived. It included most probably multilayer adsorption at lower relative pressures, followed by capillary condensation. The sorption capacities of the uncoked samples for benzene and nitrogen were relatively high and independent of the sample Al content. In the case of water, however, an observed reduction in the sorption capacity with the increasing Al content suggested that clusters of the adsorbed molecules formed around the Al centers and caused partial clogging of the material pores. The deposited coke strongly decreased both the surface area and the sorption capacity of the materials.

Keywords: mesoporous molecular sieves; acid sites; conversion of cyclohexene; carbonaceous deposits; adsorption of water; benzene and nitrogen

Adsorption of Pure Gases and Mixtures on Porous Solids up to High Pressures by Reiner Staudt; Alexander Herbst; Silvio Beutekamp; Peter Harting (pp. 379-384).

Physisorption equilibria of multicomponent gases on microporous solids like zeolites or activated carbons are considered. An overview about adsorption measurements of pure gases H₂, He, O₂, N₂, Ar, CO₂, CO, CH₄, C₂H₄ and C₂H₆ and some of their mixtures in the pressure range vacuum < p < 50 MPa at different temperatures 10^∘C–70^∘C were investigated. Also a thermodynamic formalism based on a modified van Ness method and on a new 3 parameter Isotherm equation (3-PIG) to describe the excess amount adsorbed was developed. Results are shown and discussed.

Keywords: excess adsorption; mixed gas adsorption; gravimetric measurement; van Ness method

Gas-Chromatographic Studies of the Interaction between Water and Methanol Molecules and the Surface of Carbon Materials by Yu. I. Tarasevich; S. V. Bondarenko; A. I. Zhukova (pp. 385-391).

The gas-chromatographic method is used to study the interaction of water and methanol molecules with active hydrophilic centres existing at the surface of thermally exfoliated graphite and graphitized thermal carbon black. The concentration of carboxyl and phenol hydroxyl groups at the surface of these sorbents is determined, and heats of adsorption of the studied molecules are shown to be $$ar Q_a= 40-39$$ and 28−25 kJ/mol, respectively. It is also shown that adsorption of water at the hydrophilic centres at lowest relative pressure values takes place with formation of clusters consisting of n = 2 water molecules.

Keywords: hydrophobic model sorbents; hydrophilic centres; carbon materials; water; methanol; adsorption isotherm; gas chromatography; interaction energy; cluster adsorption

Influence of Cycle Temperatures on the Thermochemical Heat Storage Densities in the Systems Water/Microporous and Water/Mesoporous Adsorbents by H. Stach; J. Mugele; J. Jänchen; E. Weiler (pp. 393-404).

The adsorption equilibrium of water on microporous adsorbents (zeolites of NaA-, NaY- and NaX-type as well as their ion exchanged forms) and on mesoporous adsorbents (different silica gels and composite material i.e. silica gel + salt hydrate) has been studied experimentally and theoretically. Using the Dubinin theory of pore filling the characteristic curves of the adsorption systems and other relevant dependences such as isotherms, isobars, isosteres and the curve of the differential heat of adsorption were calculated. For all systems investigated the adsorption were calculated. A_ads and the desorption potential A_des of the closed heat storage system were estimated. These values define the working range of the adsorption/desorption cycle and allow to calculate the specific heat storage density Δ h_sp. On the basis of Δ h_sp the different adsorbents were compared in order to select the optimal porous storage material for a given application.The presented experimental and theoretical investigations show that the adsorption systems water-zeolite and water-composites are promising working pairs for thermochemical heat storage processes for hot tap water supply and space heating of single family dwellings. The advantage of the water-composite system is the low desorption temperature (solar energy) the main shortcoming the low temperature lift. The advantage of the water zeolite system is the high temperature lift, the shortcoming are the relative high desorption temperatures.

Keywords: thermochemical heat storage; water adsorption equilibrium; thermo-dynamical data; heat storage density; zeolites; composite adsorbents

Investigation and Modeling of the Hydrothermal Stability of Technically Relevant Zeolites by W. Lutz; H. Toufar; R. Kurzhals; M. Suckow (pp. 405-413).

The dense-structured zeolites of types ZSM-5 (MFI) and mordenite (MOR) of different SiO₂/Al₂O₃ moduli are relatively stable under treatment by liquid water for 72 hours up to 513 K. The open-structured zeolites of types Y in dealuminated modification (FAU) and beta (BEA) undergo strong decomposition in the same range. For these two sample types a mathematical model for the decomposition of the zeolite framework is established that takes into account the influence of modulus as well as temperature. Here the kinetic of the solid phase reaction is a superposition of two different mechanisms described by the Monod equation.

Keywords: zeolite; hydrothermal treatment; long-term stability; modeling

Heats of Adsorption of Ammonia and Correlation of Activity and Acidity in Heterogeneous Catalysis by I. V. Mishin; T. R. Brueva; G. I. Kapustin (pp. 415-424).

Adsorption microcalorimetry was applied to determine heats of adsorption of ammonia on zeolites Y, mordenite, ZSM-5, heteropolyacid H₃PW₁₂O₄₀, as well as silica gel and amorphous aluminosilicates. The plots of differential heats against coverage served to construct the acidity spectra and, in this way, to determine the number of acid sites with different acidity strengths. The behavior of these materials in acid-catalyzed reactions, primarily, in the transformations of hydrocarbons is discussed. Evidence is presented that heats of adsorption of ammonia can be used to obtain correlation plots that describe relations between acidic and catalytic properties of zeolite catalysts.

Keywords: differential heat of adsorption; acid site; acid site distribution; zeolite; framework composition; extra-framework aluminum; catalytic activity; correlations

Adsorption of Gases, Vapors and Liquids by Microporous Adsorbents by A. A. Fomkin (pp. 425-436).

Adsorption of Xe, Kr, Ar, N₂, O₂, H₂ CH₄, CO₂, He, and freons by PAU-10 and ACC microporous carbon adsorbents as well as by A and X zeolites was investigated over a wide range of pressures (0.1 Pa – 20 MPa) and temperatures (77, 120–600 K). The amount of gases, vapors and liquids adsorbed by microporous adsorbents increases steadily with increasing pressure and does not change dramatically if phase transitions occur in the adsorptive. Isosteres of adsorption constructed as a curve of ln P against f(1/T)_a retain a linear form over a wide range of pressures and temperatures. The slope of isosteres does not vary on going through the critical temperature of the gaseous phase. At high pressures, due to non-ideality of the gaseous phase and non-inert behavior of the adsorbent the differential molar heat of adsorption is dependent on temperature. At high fillings of micropores the differential molar isosteric heat capacities of adsorption systems show maxima that indicate the occurrence of structural rearrangements in the adsorbate.

Keywords: adsorption; isosteres; gas; vapor; liquid; microporous adsorbents; heat of adsorption; adsorption deformation; heat capacities of adsorption systems

DRIFTS Study of Kinetics of Diffusion-Limited Adsorption of Ethane from Mixtures with Hydrogen by Cationic Forms of Zeolites by V. B. Kazansky; N. A. Sokolova (pp. 437-446).

The diffusion-limited adsorption of individual ethane or of ethane from mixtures with hydrogen by thin pellets of NaA, CaA and LiLSX zeolites or by thick layers of granulated zeolites was studied at room temperature. The rates of adsorption were monitored by development of the bands from the symmetry forbidden C–H stretching vibrations that were not observed for gaseous molecules. Diffusivity of individual ethane in the micropores of the thin NaA pellet obtained by this method is equal to 6 ⋅ 10⁻¹⁶ m²/s. This value agrees well with that one previously reported in literature. For adsorption of pure ethane in the larger primary micro pores of CaA or LiLSX only the lower limits of diffusivities were estimated. Diffusion-limited adsorption of ethane from mixtures with hydrogen by the thicker layers of granulated zeolites is much slower and is limited by counterdiffusion inside much larger channels between the zeolite granules. Estimation of diffusion coefficients of such counterdiffusion indicated that they are by two orders of magnitude lower than those for diffusion in gaseous mixtures of similar composition.

Keywords: DRIFTS; diffusion; ethane-hydrogen mixtures; zeolites

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Adsorption: Journal of the International Adsorption Society (v.11, #3-4)

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Adsorption: Journal of the International Adsorption Society (v.11, #3-4)