Characterization of microporous non-supported membrane top-layers using physisorption techniques

Gas physisorption is a generally used technique for the characterization of porous solids. Microporous solids, however, need a different approach compared to mesoporous solids due to enhanced adsorption in pores of molecular dimensions. Therefore, conventional theories and models can not be used. The application of physisorption for microporous solids, and the interpretation of the isotherms is elucidated. Some models presented in literature for pore size assessment in the micropore range are discussed. Based on a number of criteria, the Horváth-Kawazoe model, modified for cylindrical pore geometry, is selected. N₂ adsorption measurments on zeolites were performed to calibrate the method and to show the applicability and physical justification. Typical data obtained from experiments on non-supported microporous silica membrane top-layers show that the pore size distribution of these materials is bimodal, with a strong maximum at an effective diameter of 0.5 nm, and a weaker maximum at 0.75 nm.     

利用标准等温线分析活性炭的完全孔分布

利用我们从标准等温线性质发展的孔分布计算两个新方法——完全无模型孔分布计算法和改进的微孔分析法对有代表性的九个活性炭样品进行了完全孔分布分析.用吸附仪测量样品的低温氮吸附等温线,用Pickett方程关联了这些吸附等温线,发现n值在1.1—1.4之间.因常数C值在100—300之间,选用标准等温线n_2作为完全孔分布分析的依据.获得了总表面积、微孔的表面积和体积、非微孔的表面积和体积以及平均微孔水力半径等主要孔结构参数,并获得了所有样品的完全孔分布.两个比值SR和VR在0.9—1.1之间,结果是令人满意的.分析结果指出,DX-09-1-1,7S及J三个样品基本上全是微孔,而其余的炭毡及BPL炭样品都有相当数量的非微孔.这些结果说明,利用标准等温线进行活性炭样品的完全孔分布分析是实用的.     

非均匀多孔介质表面变压过程的分形特征

多孔物质的表面结构可以用分形维数D来表征,D与微孔结构存在对应的关系,不同的表面分形维数对应不同的微孔结构.详细讨论了分形维数和微孔结构随压力的变化规律.随着压力的增大,多孔介质表面形成更小的吸附微孔,孔径分布广,表面变得更粗糙,分形维数增大,至到趋于稳定值.分形维数表达表面结构简洁明了.     

非均匀多孔介质表面变压过程的分形特征研究

多孔物质的表面结构可以用分形维数D来表征,D与微孔结构存在对应的关系,不同的表面分形维数对应不同的微孔结构．详细讨论了分形维数和微孔结构随压力的变化规律．随着压力的增大,多孔介质表面形成更小的吸附微孔,孔径分布广．表面变得更粗糙,分形维数增大,至到趋于稳定值．分形维数表达表面结构简洁明了．     

ADSORPTION IN POROUS SOLIDS HAVING BIMODAL PORE SIZE DISTRIBUTION

The effect of micropore diffusion, macropore diffusion and adsorption on the transient uptake of porous solids having bimodal pore size distribution (for example, zeolites) is theoretically investigated in this paper. Two models commonly used in the literature—the pore diffusion model and the surface diffusion model— are considered and their equivalence is discussed. The bimodal diffusion model equations are solved analytically by using a generalized integral transform method. The solutions for the micropore, macropore concentrations and the fractional uptake are presented. Since two different processes control the transient uptake, the asymptotic solutions for micropore diffusion control case and macropore diffusion control case are obtained by using perturbation methods, so that these asymptotic solutions can be compared with that when these two processes equally contribute to the transient uptake.     

A self-consistent method to determine accessible volume,area and pore size distribution (APSD) of BPL,Norit and AX-21 activated carbon

Accessible volume, geometrical area and accessible pore size distribution are the fundamental structural parameters in the characterization of porous solids. We provide a novel “inverse” procedure, which is based on the mass balance and an MC optimization scheme, to determine these parameters from the analysis of experimental adsorption isotherms for a number of commonly used activated carbons: BPL, AX-21 and Norit. Our results, based on a kernel of model pores generated from GCMC simulations, are compared with, and shown to be different from, the results obtained from the conventional method. We show that the discrepancies arise from an incorrect evaluation of both micropore and mesopore sizes.     

NONUNIFORM STEADY STATES IN SYSTEMS OF INTERACTING CATALYST PARTICLES. THE CASE OF NEGLIGIBLE INTERPARTICLE MASS TRANSFER COEFFICIENT

The effect of micropore diffusion, macropore diffusion and adsorption on the transient uptake of porous solids having bimodal pore size distribution (for example, zeolites) is theoretically investigated in this paper. Two models commonly used in the literature—the pore diffusion model and the surface diffusion model— are considered and their equivalence is discussed. The bimodal diffusion model equations are solved analytically by using a generalized integral transform method. The solutions for the micropore, macropore concentrations and the fractional uptake are presented. Since two different processes control the transient uptake, the asymptotic solutions for micropore diffusion control case and macropore diffusion control case are obtained by using perturbation methods, so that these asymptotic solutions can be compared with that when these two processes equally contribute to the transient uptake.     

New adsorption-condensation theory for adsorption of vapors on porous activated carbons

A new analytical model to describe equilibrium adsorption of condensable vapors on porous adsorbents is developed. It accounts for the heterogeneous pore structure of the adsorbent, adsorption in the micropores by a pore filling mechanism and adsorption and condensation in the macropores. A gamma-type pore size distribution function is used. Langmuir-type adsorption equations are used to describe both micropore filling and adsorption on the macropore walls. The vapor condensation in the pores is described by the Kelvin equation. The model is successfully tested using isotherm data for adsorption of various condensable vapors on different porous activated carbons and charcoals. All three types (I, IV and V) of adsorption isotherms by the Brunauer classification which are depicted by the porous adsorbents can be described by the model.     

Phenol adsorption on porous and non-porous carbons

Phenol physisorption on a series of porous and non-porous amorphous carbons was studied at 298 K. Grand Canonical Monte Carlo computer simulations were performed to simulate phenol adsorption from the gas phase. Phenol is adsorbed in a solid-like state within the pores and there is no well-developed multilayer regime. The ‘t’ plot method was adapted to phenol adsorption and the results obtained are in agreement with the model solids employed. The simulated adsorption isotherms were compared with experimental results obtained for adsorption from aqueous solutions of phenol. BET surface areas were calculated. Other characteristics of the adsorption system analyzed were: adsorption energy distribution functions, density profiles, distribution of molecules according to gas-solid energy, and local isotherms.     

A new analysis method for the determination of the pore size distribution of porous carbons from nitrogen adsorption measurements

A new analysis method has been developed for the determination of the pore size distribution of porous carbons from nitrogen adsorption measurements. The method is based on a molecular model for the adsorption of nitrogen in porous carbon. It allows, for the first time, the distribution of pore sizes to be determined over both the micropore and mesopore size ranges using a single analysis method. In addition to carbons, this method is also applicable to a range of adsorbents, such as silicas and aluminas.     

Supercritical adsorption of ‘permanent’ gases under corresponding states on various carbons

Adsorption isotherms at 298 K have been measured on four different carbons for six gases: argon, carbon monoxide, hydrogen, methane, nitrogen and oxygen at pressures up to 7 MPa. For all the carbons, methane, on a molar basis, was adsorbed to a greater extent than all the other gases and hydrogen the least. These same gases were then adsorbed using the same carbons at temperatures corresponding to a reduced temperature of 2.36 up to reduced pressures of about 1.0. Under those conditions, five gases presented very similar uptakes on any given carbon although the porous nature of these carbons was very different. The exceptional gas was hydrogen. For every carbon, hydrogen was adsorbed to a much greater extent than the other gases. In all cases, the isotherms were reversible suggesting that only physisorption was involved. It would appear that hydrogen can penetrate into pores within the structure of carbons that go undetected by conventional pore or surface characterisation techniques. The carbon with the highest 77 K N₂ D-R micropore volume, AX-21, still showed the greatest hydrogen uptake under these reduced conditions.     

Characterization of the textural properties of metal loaded ZSM-5 zeolites

The textural properties of metal ion over-exchanged zeolites, Co–, Cu–, Ni–ZSM-5, were compared with those of the parent ZSM-5 by nitrogen adsorption measurements and photoelectron spectroscopy. The treatment of the adsorption isotherms permitted a thorough characterization of the microstructure of the samples. A first estimation of the microporous volume was made by the Dubinin–Radushkevich method. The “t-plot” method was used to determine the external surface and to obtain another estimation of the microporous volume. Effective micropore size distribution was obtained with the Horvath–Kawazoe approach. The Dubinin–Radushkevich method overestimated the microporous volume of the zeolites, in particular when the solid had an important external surface. It is shown that Cu–ZSM-5 had micropore size and micropore size distribution very close to those of ZSM-5, whereas Co–ZSM-5 and Ni–ZSM-5 had lower microporous volumes and larger external surfaces. The external to internal (microporous) surface ratios, r_s, and the mesoporous to microporous volume ratios, r_v, are proposed as practical parameters to classify the solids with respect to their microstructure. ZSM-5, a pure microporous solid, has low values of both r_s and r_v parameters. Ni– and Co–ZSM-5 have high r_s ad r_v parameters, indicative of some degree of mesoporous character. Cu–ZSM-5 has intermediate characteristics, typical for micro-mesoporous solid. XPS measurements confirmed that copper ions penetrated to a greater extent in the channels of ZSM-5 zeolite than cobalt and nickel ions, which merely deposited on the external surface of ZSM-5 zeolite as oxidic aggregates.     

Physisorption of nitrogen by porous materials

Nitrogen adsorption at 77 K is widely used for the determination of the surface area and pore size distribution of various porous materials. The first stage in the interpretation of a nitrogen isotherm is the identification of the physisorption mechanism(s): that is monolayer-multilayer adsorption, capillary condensation or micropore filling. The Brunauer-Emmett-Teller (BET) method cannot be used to provide a reliable evaluation of the surface area if the solid contains pores of molecular dimensions (i.e. narrow micropores). Furthermore, with such materials, it is necessary to use different probe molecules to determine the pore size distribution. Capillary condensation is involved as a secondary process in the filling of mesopores (pore width in the range 2–50 nm). Recent work on MCM-41, a model mesoporous adsorbent, has confirmed that capillary condensation-evaporation can occur reversibly in open-ended pores of about 4 nm width.     

Preparation of microporous activated carbon from Aegle marmelos fruit shell by KOH activation

Activated carbon (AC) is well‐known for its unique properties; hence, the search for new precursors and the investigation of new methods for the preparation of AC is still drawing attention of many researchers. In the present work, microporous AC was prepared from Aegle marmelos fruit shell (AMFS) by potassium hydroxide (KOH) activation. The effect of process parameters such as impregnation ratio, carbonisation temperature and holding time on porous characteristics was investigated. The porous characteristics of prepared AC samples were analysed by N₂ adsorption–desorption isotherms, and it was found that the isotherms obtained resemble typical microporous solids (Type‐I). The Langmuir surface area and total pore volume of the sample prepared at optimum conditions were found to be 937 m²/g and 0.33 cm³/g, respectively. The contribution of micropores to the porous characteristics of the prepared AC is very much appreciable, and about 97% of the total surface area and pore volume is attained by micropores. Pore size distribution (PSD) by Dubinin–Astakhov (DA) and micro‐pore (MP) methods confirmed the presence of micropores to a great extent with insignificant mesoporosity. © 2013 Canadian Society for Chemical Engineering     

Comparison of pore structure in Kentucky coals by mercury penetration and carbon dioxide adsorption

For four Kentucky coals, a comparison was made between pore volume distribution evaluated from CO₂ adsorption isotherms calculated from the Cranston-Inkley method and the Medek method. These distributions agree in the micropore range from approximately 0.6–2.0 nm. Most of the pore structure is in the micropore range. For the mercury penetration studies a rectilinear relation is found between penetrated volume and applied pressure from approximately 0.1 MPa (1 atm) up to 345 MPa (3400 atm). Surface areas were calculated from adsorption data by the BET, Langmuir, and Dubinin-Polanyi methods.     

磷酸活化法制备半纤维素基颗粒活性炭

以半纤维素的主要模型物木聚糖为原料,在不添加其他粘结剂的条件下,采用磷酸活化法制备半纤维素基颗粒活性炭。讨论了浸渍比和炭活化工艺对活性炭吸附性能和孔隙结构的影响。研究结果表明:浸渍比的增加,有利于颗粒活性炭的比表面积、亚甲基蓝吸附值、强度、总孔容积和中孔容积的提高。随着炭活化温度的升高,颗粒活性炭的碘吸附值、亚甲基蓝吸附值、比表面积、总孔容积和微孔容积呈下降的趋势,强度呈上升趋势。N₂吸附-脱附等温线和孔径分析表明,颗粒活性炭具有发达的微孔结构,炭活化温度的升高不利于孔隙结构的发达。     

Microporosity of Porous Glasses: New Investigation Techniques

Two new methods are developed for investigating the microporosity of porous materials. The first method, namely, diffusion diagnostics, is based on a computer simulation of the kinetics of gas desorption from a porous material into high vacuum. In this method, the kinetics of gas desorption is measured using mass spectrometry. Another method is based on the analysis of the equilibrium isotherms of gas desorption at low pressures. The microporous and mesoporous substructures of the porous glasses prepared by leaching of two-phase alkali borosilicate glasses in a 3 M HCl solution at 100°C are investigated using both the new methods and the classical adsorption methods. A correlation between the porous structure of glasses and their synthesis and phase separation conditions is revealed. It is demonstrated that porous glasses have a through mesoporous substructure with mean pore diameters ranging from 4 to 15 nm and a polymodal microporous substructure with pore sizes ranging from 0.35 to 2 nm. This microporous substructure is formed by ultramicropores of molecular sizes and also medium- and large-sized micropores. It is found that porous glasses contain bottle-shaped pores with sizes corresponding to the micropore range. An increase in the temperature of heat treatment of glasses from 550 to 700°C and an increase in the boron oxide content in the silica phase lead to an increase in the volume of micropores in porous glasses.     

一种新的计算有效扩散系数的孔结构模型——双参数模型

建立的孔结构模型包含两个孔结构参数.确定模型参数时不涉及由常规方法测得的孔分布数据.该模型用于文献中以及作者的测试结果,表明能良好地反映多孔介质的孔结构特征.     

Evaluation of effective diffusivity of adsorbate in the absence of adsorption and surface diffusion

A method is suggested for the evaluation of effective diffusivity of adsorbate in the absence of adsorption and surface diffusion, the precise value of which is required for estimating the effective surface diffusivity and the extent of influence of adsorption on the overall mass flux in a porous solid. The method does not require the knowledge of tortuosity factor, the accurate values of average pore radius for micro, transitional and macro pores, or the porosities corresponding to micro and macro pores, which are required in case of diffusion in the transitional region and in the solids containing bimodal pore size distribution.     

钢渣基高反应性焦炭气孔结构的溶损演化行为

在工业配合煤中添加1%钢渣制备高反应性焦炭,采用低温氮气吸附法分析高反应性焦炭（HRC）和普通焦炭（BC）在1100℃下溶损不同比率碳素（5%~50%）后焦炭的气孔结构,并结合分形理论研究焦炭溶损反应过程中孔结构的演化特性。结果表明,随着碳素的溶损,HRC的吸脱附曲线的变化幅度比BC的大,吸附等温线由Ⅰ型向Ⅱ型的转变较晚;焦炭的比表面积和微孔孔容随碳素的溶损先增大后减小、总孔容逐渐增大,而HRC的比表面积增率（ΔS _BET /Δx）比BC的大,孔径分布也相对较宽;HRC的分形维数D ₁和D ₂随碳素溶损而变化趋势与BC的有较大差异。这说明高反应性焦炭中的钢渣通过增加了焦炭表面上活性点影响焦炭溶损过程中气孔结构的演化行为。