High pressure sorption isotherms via differential pressure measurements

A differential pressure adsorption unit (DPAU) has been constructed which is capable of accurately measuring isotherm data up to 2000 psia with as little as 100 mg of sample. This non-traditional adsorption/desorption method has been benchmarked by comparing hydrogen and methane isotherms measured with standard volumetric and gravimetric instruments on a NaA (4A) zeolite and an activated carbon at near ambient temperatures. The results from stability tests and well as the details of the mathematical analysis for this differential pressure method are also provided.     

Gravimetric,Volumetric And Calorimetric Studies of The Surface Structure of Portland Cement

Dry cement powder and hardened cement paste were characterised by means of laser granulometry and volumetric measurement of the nitrogen adsorption isotherm at 77 K. Water sorption isotherms at ambient temperature were measured stepwise by means of a gravimetric apparatus. The isotherms show a very large hysteresis loop, reaching down to zero relative pressure, which reflects swelling of the cement gel. Thermoporometric measurements reveal that swelling of that gel consists in water take up within the continuously growing structure; no stable pore structure could be observed. This revised version was published online in August 2006 with corrections to the Cover Date.     

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

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).     

Characterization of gels via solvent desorption measurements

The present paper shows how a standard volumetric adsorption device can be used to measure solvent desorption isotherms on gels. As gels are very soft materials, they shrink significantly during the measurement, and the data have to be analyzed in terms of the mechanical properties of the gel’s skeleton. Methanol desorption isotherms are measured on a series of silica gels, and the results are compared with independent characterizations, notably beam bending.     

Thermal properties of pitch-polymer compositions and derived activated carbons

The evaluation of the possibilities to use coal-tar pitch modified with waste poly(ethylene terephthalate) or phenol?Cformaldehyde resin for the preparation of activated carbons was carried out. The measurement of thermal analysis (DSC), softening point, coking value, content of components insoluble in toluene and quinoline of pitch-polymer compositions were carried out. Coal-tar pitch and pitch-polymer compositions were carbonized and activated with steam at 800?°C to 50?% burn-off. For the obtained activated carbons the determination of thermal analysis (DSC), BET surface area on the basis of volumetric low-temperature adsorption of nitrogen, mesopore, and micropore volume from benzene adsorption/desorption isotherms (gravimetric McBain-Bakr method) were carried out.     

氢在A、X及ZSM-5型沸石上的高压物理吸附

The hydrogen adsorption properties and uptake capacities of the A, X and ZSM-5 types of zeolites were investigated at temperatures of 77, 195 and 293 K and pressures up to 7MPa, using a conventional volumetric adsorption apparatus. All hydrogen adsorption isotherms were basically type I, but the maximum in isotherm,a unique feature of supercritical adsorption, was observed at high pressures of 2-5 MPa at 77 K. The isosteric heats of adsorption were determined from the isotherms and the factors that influence their variations were discussed. Different types of zeolites exhibited remarkably different hydrogen uptake, based on both the framework structure and the nature of the cations present. The highest gravimetric storage capacity of 2.55wt% was obtained for NaX-type zeolite at 4 MPa and 77 K. In CaA, NaX and ZSM-5 types of zeolites,hydrogen uptakes were proportional to the specific surface areas, which were associated with the available void volumes of the zeolites. A threshold in hydrogen adsorption observed in NaA and KA was attributed to a pore blocking effect by large cations in KA. A ratio of the kinetic diameter of adsorbate to the effective opening diameter of zeolite was used to judge the blocking effect for physisorption.     

Effects of surface area, free volume, and heat of adsorption on hydrogen uptake in metal-organic frameworks

Grand canonical Monte Carlo simulations were performed to predict adsorption isotherms for hydrogen in a series of 10 isoreticular metal-organic frameworks (IRMOFs). The results show acceptable agreement with the limited experimental results from the literature. The effects of surface area, free volume, and heat of adsorption on hydrogen uptake were investigated by performing simulations over a wide range of pressures on this set of materials, which all have the same framework topology and surface chemistry but varying pore sizes. The results reveal the existence of three adsorption regimes: at low pressure (loading), hydrogen uptake correlates with the heat of adsorption; at intermediate pressure, uptake correlates with the surface area; and at the highest pressures, uptake correlates with the free volume. The accessible surface area and free volume, calculated from the crystal structures, were also used to estimate the potential of these materials to meet gravimetric and volumetric targets for hydrogen storage in IRMOFs.     

Direct measurements of pore fluid density by vibrating tube densimetry

The densities of pore-confined fluids were measured for the first time by means of vibrating tube densimetry (VTD). A custom-built high-pressure, high-temperature vibrating tube densimeter was used to measure the densities of propane at subcritical and supercritical temperatures (between 35 and 97 °C) and carbon dioxide at supercritical temperatures (between 32 and 50 °C) saturating hydrophobic silica aerogel (0.2 g/cm(3), 90% porosity) synthesized inside Hastelloy U-tubes. Additionally, supercritical isotherms of excess adsorption for CO(2) and the same porous solid were measured gravimetrically using a precise magnetically coupled microbalance. Pore fluid densities and total adsorption isotherms increased monotonically with increasing density of the bulk fluid, in contrast to excess adsorption isotherms, which reached a maximum and then decreased toward zero or negative values above the critical density of the bulk fluid. The isotherms of confined fluid density and excess adsorption obtained by VTD contain additional information. For instance, the maxima of excess adsorption occur below the critical density of the bulk fluid at the beginning of the plateau region in the total adsorption, marking the end of the transition of pore fluid to a denser, liquidlike pore phase. Compression of the confined fluid significantly beyond the density of the bulk fluid at the same temperature was observed even at subcritical temperatures. The effect of pore confinement on the liquid-vapor critical temperature of propane was less than ~1.7 K. The results for propane and carbon dioxide showed similarity in the sense of the principle of corresponding states. Good quantitative agreement was obtained between excess adsorption isotherms determined from VTD total adsorption results and those measured gravimetrically at the same temperature, confirming the validity of the vibrating tube measurements. Thus, it is demonstrated that vibrating tube densimetry is a novel experimental approach capable of providing directly the average density of pore-confined fluids, and hence complementary to the conventional gravimetric or volumetric/piezometric adsorption techniques, which yield the excess adsorption (the Gibbsian surface excess).     

Adsorption micro calorimeter

In this work, it is described an innovative heat flux micro calorimeter Tian-Calvet type designed to measure adsorption heats and reactions as well as adsorption isotherms. It consists in an adsorption instrument for volumetric gases, which is coupled to the micro calorimeter. The changes in the pressure are monitored by means of high sensitivity and high precision pressure transducers. The micro calorimeter has thermo elements that work by a Seebeck effect, in a twin cells system. The cells are inside a box in which the temperature can be adjusted from 77 to 300 K. The sensitiveness of the calorimeter is established by applying a perfectly known electric work. The results corresponding to the electric calibration, the base line stability determination and the time constant in the equipment are shown.     

Adsorption phenomena at high pressures and temperatures.

Equilibrium isotherms of adsorption of methane on crystalline Rho zeolite were measured with the use of a precision volumetric gravimetric setup that was developed for determining the equilibrium and Kinetic parameters of adsorption in the pressure range of 0.1–160 MPa and temperature range of 300–600 K. The method of determining the accessible volume of an adsorption system (free volume + micropore volume) and the micropore volume of a sorbent was used. Two independent methods for calculation of micropore volume on the basis of the isotherms of excess adsorption were used. The discrepancy in the results of the calculation of the micropore volume by three independent methods is within the limits of the calculation accuracy. An evaluation of a change in the isosteric heats of the excess and absolute adsorption of methane on Rho zeolite was carried out in relation to filling and temperature. An evaluaton of the adsorption volume above the outer surface of the zeolite crystals was performed. The results of experimental investigations of methane adsorption on Rho zeolite can be used to solve the problem of encapsulation of gases by solid sorbents. kg]Key words kw]adsorption kw]micropore volume kw]surface kw]isostere kw]heat kw]zeoliteFor Part 1 see Ref. I.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 568–573, March, 1996.     

Thermodynamic investigation of n-hexane thin films adsorbed on magnesium oxide

The thermodynamic properties of n-hexane adsorption on MgO(100) were determined using high-resolution volumetric adsorption isotherms in the temperature range 195-255 K. Two distinct layering transitions are observed in the isotherms. The isotherms are used to calculate the two-dimensional compressibility, the differential enthalpy and entropy, the heat of adsorption, and the isosteric heat of adsorption. Neutron Diffraction is used to identify where melting of the n-hexane monolayer takes place.     

Adsorption of hydrogen on multiwalled carbon natotubes at 77 K

Adsorption of H₂ on multiwalled carbon nanotubes was measured at 77 K by a volumetric method. Adsorption and desorption isotherms are largely reversible. The adsorption capacity increased remarkably after receiving heat treatment at 400 °C and being pressed into pellets. The isotherms show typical feature of supercritical adsorption and were satisfactorily modeled by the model that applied for usual supercritical adsorption. The maximal adsorption capacity of hydrogen under the condition tested is less than 0.5 wt%.     

A note on sorption measuring instruments

Gas sorption phenomena can be used to characterise porous solids and dispersed materials. Usually isotherms of nitrogen and noble gases like He, Ar, Kr are measured at low temperatures (77–90 K). Other gases and vapours like water, CO₂ and benzene are used at near ambient conditions of pressure and temperature. From the amount of gas adsorbed on the (external or internal) surface characteristic quantities like the specific surface area, specific pore volume and pore spectrum of the material are derived by standardised methods. Experimental techniques most often used are the carrier gas, the volumetric/manometric and the gravimetric method. A comprehensible overview of today's available instruments, their advantages and drawbacks is given.This revised version was published online in November 2005 with corrections to the Cover Date.     

A simple method for measuring excess adsorption isotherms of organic eluent components on reversed‐phase packing materials

A simple frontal analysis method has been developed for the reliable measurement of excess adsorption isotherms of an organic component on reversed‐phase adsorbents in a series of programmed concentration steps. In the present method, a peak, which is produced by refractive index change in column eluate, is detected at 589 nm; it represents the elution volume of the boundary. The method is applied to the measurement of the excess adsorption isotherms of organic eluent components from water on commercially available reversed‐phase stationary phases. The results are in good agreement with the previously reported isotherms. We also measure the excess adsorption isotherms of organic eluent components from solutions containing electrolytes. There are not any interference peaks on the elution traces. The method is thus reliably applicable to the evaluation of the excess adsorption of organic eluent components in practical systems.     

A revisit to the Gibbs dividing surfaces and helium adsorption

This paper addresses the long-standing problem of the so-called Gibbs dividing surface and the use of helium as a “non-adsorbing” gas for the determination of the “helium”-void volume and thence the Gibbs excess. Using helium is subject to some uncertainty because helium does adsorb (to call it a non-adsorbing gas is misleading) and it is able to access pore spaces that other larger adsorbates cannot. On the other hand, even helium atoms can not physically probe all the space described by the helium-void volume. To avoid these difficulties, we suggest an alternative to the formulation of the Gibbs dividing surface and the definition of the excess amount. We illustrate this with the two common tools to study adsorption—the volumetric and gravimetric techniques, and justify our new analysis with a computer simulation of a number of model adsorption systems. Furthermore, we also show that by using the correct accessible volume and inaccessible volume the excess amount obtained from a volumetric experiment is exactly the same as that obtained from a gravimetric experiment.     

AlPO4-11分子筛的吸附热力学性质

AlPO_4-n系列分子筛是八十年代初由美国联合碳化物公司开发的一类新型分子筛,据报导到目前为止已经合成出20种不同型号结构的AlPO_4分子筛。经X-射线多晶和单晶衍射结构分析,部分AlPO_4分子筛的结构已搞清楚,AlPO_4-11结构也已经测定,它具有十元环一维孔道,孔口大小为0.67nm×0.40nm的分子筛。     

Formation of Solvates of Phosphotungstic and Silicotungstic Heteropoly Acids in the Course of Ethanol Vapor Absorption

Absorption of ethanol vapor with dehydrated solid phosphotungstic and silicotungstic heteropoly acids was studied by a gravimetric adsorption procedure at room temperature. The absorption isotherms are steplike and reflect stepwise formation of proton-alcohol associates in the acid lattices.     

Correlation of sorption behavior of nitrogen, oxygen, and argon with Ca2+ locations in zeolite a: a grand canonical Monte Carlo simulation study

The adsorption isotherms for nitrogen, oxygen, and argon in various NaCaA zeolite samples were calculated theoretically using the grand canonical Monte Carlo simulation method. The adsorption capacity, selectivity, and heat of adsorption of nitrogen increase with an increasing number of calcium cations in zeolite A. The heat of adsorption of nitrogen showed a sudden increase when the calcium ion exchange to zeolite A was around 60%. Adsorption isotherms, determined experimentally by the volumetric adsorption method, support theoretically predicted isotherms. These observations have been explained in terms of the interaction of the nitrogen molecule with Ca2+ ions and their locations in zeolite A.     

Hydrogen storage in porous structures of adamantane‐based nitrogen‐heterocyclic ring with diamond‐like structure

Using the density functional theory and molecular mechanics methods, we calculated the binding energy and parameters about the primitive cell designed by us with the adamantane and the nitrogen heterocyclic ring, the vibrational frequencies about the small complexes. Grand canonical Monte Carlo simulations were performed to predict the capacities for the hydrogen storage and adsorption isotherms. The results show the positive effects of bigger specific surface area and pore volume on hydrogen storage and isosteric heat. The gravimetric hydrogen uptake of adamantane‐based nitrogen‐heterocyclic ring of quaterpyridyl can reach 9.02 wt % at room temperature and 100 bar. But the volumetric H₂ capacities of the four materials are low at T = 298 K because of weak interaction between the materials and H₂ molecule. © 2014 Wiley Periodicals, Inc.