The Jäntti approach using a two-layer model

Jäntti introduced a method to calculate the adsorption equilibrium by the measurement of the actual adsorbed amount at three times after a change of the gas pressure. He applied that method for gas/solid systems in which simple adsorption processes occur and for an infinite number of adsorption sites. In the present paper we discuss the case that the number of sites is decreasing with increasing coverage.This revised version was published online in November 2005 with corrections to the Cover Date.     

The relevance of Jäntti's method for the fast measurement of sorption of water vapour

The time necessary for the measurement of gas adsorption on a solid can be substantially reduced by using Jäntti's directions for data interpretation. This procedure already delivers results after measuring only a few values of the adsorbed mass. In earlier papers we introduced a second order adaptation to Jäntti's method to broaden its applicability. In the present paper we apply this second order treatment to the adsorption of water vapour from moistened air. Specially we pay attention to the diffusion delay caused by the boundary layer around the sample.This revised version was published online in November 2005 with corrections to the Cover Date.     

Application of jäntti's method to volumetric adsorption measurements

Jäntti introduced a method to calculate the adsorption equilibrium by measuring the actual adsorbed amount three times after a change to the gas pressure. By this method the experimental time for adsorption measurement can be considerably shortened. The procedure was developed for use in adsorption measurements where the adsorbed masses are directly measured with a balance. In the present paper we will demonstrate that the method is particularly useful in volumetric (manometric) measurements.     

Evaluation of the applicability of Jäntti’s method to volumetric sorption measurements

Jäntti published in 1970 a method to obtain values of the sorption parameters at an early stage of gravimetric sorption measurements. In the present paper we apply that method to volumetric sorption measurements to discuss its applicability. A method is presented to cope with inaccuracies when using Jäntti’s method. This will be of special importance for volumetric measurements where accuracy plays a major part. In the paper a second order regression method is used. We use a computer simulation as well as experimental data on desorption of nitrogen from a silica aerogel at 77 K.     

Measurement of Gas Adsorption with Jäntti's Method Using Continuously Increasing Pressure

Jäntti et al. published a method to reduce the time necessary for adsorption measurements. They proposed to extrapolate the equilibrium in the stepwise isobaric measurement of adsorption isotherms by measuring at each step three points of the kinetic curve. For that purpose they approximated the kinetic curve by an exponential function which they derived empirically from there measurements. In the present paper we discuss the applicability of the method for adsorption measurements under continuously varying gas pressure.     

Comparison of Sorptometric and Thermoporometric Measurements on Porous Glass and Some Others

The results of surface structure investigations involving the use of thermoporometry and gas-sorptometry on porous glass, silica and a zeolite were compared. The most frequent and the mean pore width were found to be in accordance, but major differences in specific surface were observed. These two measuring methods complement each other, because with gas adsorption a dry sample is investigated, while with thermoporometry a sample immersed in a liquid is investigated. The probe molecules (nitrogen or noble gas and water or benzene, respectively) differ in size. The features of the two methods are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Calorimetric and Gravimetric Measurements of Hydrocarbon Adsorption on a Granulated Active Carbon

Adsorption isotherms of n-butane on a granulated activated carbon were measured by two different but complementary experimental methods: calorimetry and gravimetry. Adsorption heats were determined in different ways. For the system studied, the experimental results prove that the adsorbent offers a homogeneous site distribution. Besides, there can be differences between the adsorption heat values which might come from the way they are obtained (by calculation or direct measurements). This revised version was published online in August 2006 with corrections to the Cover Date.     

Expressiveness of Adsorption Measurements for Characterization of Zeolitic Materials—A Review

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. Dedicated to the memory of Professor Wolfgang Schirmer.     

Measurement of Gas Mixture Adsorption Equilibria of Natural Gas Compounds on Microporous Sorbents

Physisorption equilibria of multicomponent gases on microporous solids like zeolites or activated carbons are considered. In view of lack of reliable and simple methods to calculate mixture adsorption isotherms from pure component data, experiments are still indispensable. An overview of classical and new methods to measure multicomponent gas adsorption equilibria is given. Some of the basic concepts like the Gibbs excess mass and the absolute mass adsorbed underlying these methods are discussed. Experimental data and a class of new adsorption isotherms for inhomogeneous microporous adsorbents of fractal dimension will be given in another subsequent paper (ADSO 635-98) by the same group of authors.     

Impedance Spectroscopic Measurements of Pure Gas Adsorption Equilibria on Zeolites

Physisorption equilibria of gases on inert porous solids like activated carbon or molecular sieves can be characterized by measuring the (frequency dependent) capacitance of a capacitor filled with a sample adsorbent. This quantity strongly depends not only on the physico-chemical structure of the empty adsorbent in vacuum, but also on the permanent or induced dipole moments of the molecules adsorbed and of the fluid phase. Consequently, it should be possible to determine the excess mass being adsorbed on the internal surface of a highly porous solid by measurements of the dielectric constant. The aim of this work is to show for various pure gases and adsorbents that the change of the capacitance of an adsorption system depends on the adsorbed mass. Therefore, this effect can be used to characterize porous solids and their adsorbates and, for example, to check the state or the quality of industrial adsorbents during a process on site if calibration measurements have been taken (Staudt et al., 1994, 1998).     

On the applicability of Jäntti’s method of shortening sorption measurements

Jäntti published in 1970 a method to obtain equilibrium values at an early stage of gravimetric sorption measurements. In former papers the authors criticised and extended that method. In the present work we discuss problems of its applicability on practical measurements.     

Criticism on Jäntti's Three Point Method on Curtailing Gas Adsorption Measurements

Jäntti introduced a method to reduce the time required for the stepwise measurement of adsorption isotherms (Jäntti et al., Progress in Vacuum Microbalance Techniques, Vol. 1, Heyden, London, pp. 345–353, 1972). After a pressure change he measured the adsorbed mass three times and calculated its equilibrium value for the new pressure. In the present paper we discuss the applicability of this method in a broader scope (without starting from a single combination of gas and solid adsorbent) and also discuss the influence of measurement inaccuracies. It is shown that the method can serve as an early warning system, which decides whether an improvement of the adsorption model is necessary.     

Adsorption of Cd(II) by Sol-Gel Silica Doped with N-(dipropylcarbamothioyl)thiophene-2-carboxamide

Sol-gel silica was doped with N,N-(dipropylcarbamothioyl) thiophene-2-carboxamide to investigate the sorption of cadmium (Cd) ions from aqueous media. In doped sol-gel silica, the large reagent molecules entrap into pores, whereas, small metal ions diffuse into pores where they make complex with doped reagent. This complexation can be accomplished by either ion exchange or chelation. Doped sol-gel sorbent was applied for removal of Cd(II) from aqueous solution in our study. Adsorption kinetics, adsorption isotherm, equilibration time, effect of initial concentration of adsorbate, and pH effect on the metal removal were studied to optimize the conditions. The prepared adsorbent shows rapid equilibrium and high stability toward high temperature and applied medium. In addition, desorption of metal ions was carried out by 1 M HCl and, thereafter, sol-gel silica adsorbent was regenerated and reused periodically.     

2D Covalent Organic Framework for Water Harvesting with Fast Kinetics and Low Regeneration Temperature

Atmospheric water harvesting represents a promising technique to address water stress. Advanced adsorbents have been rationally designed to achieve high water uptake, yet their water sorption kinetics and regeneration temperature greatly limit water production efficiency. Herein, we demonstrated that 2D covalent organic frameworks (COFs), featuring hydrophobic skeleton, proper hydrophilic site density, and 1D open channels significantly lowered the water diffusion and desorption energy barrier. DHTA-Pa COF showed a high water uptake of 0.48 g/g at 30 % R.H. with a remarkable adsorption rate of 0.72 L/Kg/h (298 K) and a desorption rate of 2.58 L/Kg/h (333 K). Moreover, more than 90 % adsorbed water could be released within 20 min at 313 K. This kinetic performance surpassed the reported porous materials and boosted the efficiency for multiple water extraction cycles. It may shed light on the material design strategy to achieve high daily water production with low-energy input.     

Chemistry of Soft Porous Crystals: Structural Dynamics and Gas Adsorption Properties

In this Minireview, we discuss the fundamental chemistry of soft porous crystals (SPCs) by characterizing their common structural features and the resulting structural softness and transitions. In particular, we focus on the recently emerging properties based on metastable transitions and those arising from local dynamics. By comparing the resulting adsorption properties to those of commonly applied rigid adsorbents, we highlight the potential of SPCs to revolutionize adsorption‐based technologies, considering our current understanding of the thermodynamic and kinetic aspects. We provide brief outlines for the experimental and computational characterization of such phenomena and offer an outlook toward next‐generation SPCs likely to be discovered in the next decade.     

Determination of Absolute Gas Adsorption Isotherms by Combined Calorimetric and Dielectric Measurements

A new method to determine absolute masses of gas adsorbed on the external and internal surfaces of a porous solid is proposed. It consists on a combination of calorimetric and dielectric measurements. These lead to the enthalpy and the dielectric polarization of the adsorbed phase from which by purely thermodynamic calculations the absolute mass adsorbed can be determined without using the so-called helium volume hypothesis nor any other equivalent assumption.As example adsorption of subcritical carbon dioxide (CO₂) on zeolite (Degussa DAY) at 298 K and pressures up to 0,4 MPa is considered. As expected data of absolute masses adsorbed are always somewhat larger than the corresponding Gibbs excess masses calculated from both volumetric and gravimetric measurements via the helium volume of the zeolite.     

Adsorption of Propene on Neutral Gold Clusters in the Gas Phase

The adsorption of propene on neutral gold clusters is investigated in a collision cell under a few collision conditions. The adsorption reaction is studied by pressure‐dependent kinetic measurements and delayed unimolecular dissociation of the excited Au_n?propene complexes. The cluster size (n=9–25) and temperature (T=90–300 K) dependence of the propene adsorption is analyzed. Strong size dependences of the absorption reaction are observed; a larger propene adsorption probability was found for gold clusters composed of an even number of atoms. Propene binding energies are estimated by comparison of the temperature‐dependent unimolecular dissociation rates with rates obtained by using statistical RRKM modeling. The Au_n–propene binding energies decrease non‐monotonously with cluster size and are in the range of 1.2–0.85 eV for n=9–25. Finally, the bonding of C₃H₆ on Au_n is qualitatively described and similarities with the absorption of CO molecules on gold clusters are discussed.     

Adsorption equilibria of CO/H2 with a magnetic suspension balance: Purely gravimetric measurement

Adsorption equilibria of pure gases and binary gas mixtures can be measured with a new magnetic suspension balance. For this measurement no additional concentration measurement is required, neither for the gas phase nor for the adsorbed phase. The new instrument allows gravimetric adsorption measurements in a wide range of pressure (UHV...50 MPa) and temperature (210 K...570 K) to be performed. The experimental procedure and the instrument set up is fairly easy and can be compared to pure gas adsorption experiments. The new instrument and experimental procedure have been tested by performing coadsorption measurements with CO/H₂ mixtures on a commercial 5A zeolite.This revised version was published online in November 2005 with corrections to the Cover Date.     

An Equilibrium Relation for Gas Adsorption Applications

A new multicomponent equilibrium relation is proposed for engineering applications. This relation is based on a lattice model for mixtures of unequally sized molecules. An approximation is introduced for loading-dependent enthalies of adsorption that simplifies the energy balance in fixed-bed models and allows efficient solution of the equation set in process simulations. Comparisons are made with nonideal binary data in the literature.