Characterisation of Porous Solids V (eBook)

J.P. Baselt, G. Kreysa, KK Unger (Herausgeber)

eBook Download: EPUB

2000 | 1. Auflage
700 Seiten
Elsevier Science (Verlag)
978-0-08-052889-2 (ISBN)

Infos

The Fifth International Symposium on the Characterisation of Porous Solids (COPS-V) was held at Heidelberg, Germany, from May 30 to June 2, 1999. About 220 participants from 25 countries enjoyed a very successful meeting with 32 lectures and 155 poster presentations.
The Symposium started with a highly stimulating lecture by Sir John Meurig Thomas, Cambridge, highlighting the recent developments in engineering of new catalysts. The following two full sessions were devoted to theory, modelling and simulation which provide the basis for the interpretation of pore structural data of adsorbents and finely dispersed solids. Sessions 2 and 3 focused on the advances in the synthesis and characterisation of highly ordered inorganic adsorbents and carbons. Sessions 4 and 5 addressed important questions with respect to the characterisation of porous solids by sorption measurement and other related techniques.
The intensive three-day programme provided a stimulating forum for the exchange of novel research findings, concepts, techniques and materials which are collected in this volume.

Inhalt

Cover 1
Contents 6
Foreword 14
Scientific Committee 16
Financial Support 16
Part I: Theory, Modelling and Simulation 18
Chapter 1. Adsorption of Argon and Xenon in Silica Controlled Porous Glass: A Grand Canonical Monte-Carlo Study 18
Chapter 2. The role of isosteric enthalpy of adsorption in micropore characterisation: A simulation study 28
Chapter 3. Capillary Condensation and Hysteresis in Disordered Porous Materials 38
Chapter 4. Molecular Simulation Study on Freezing in Nano-pores 48
Chapter 5. Characterisation of porous materials using density functional theory and molecular simulation 58
Chapter 6. Density Functional Theory of Adsorption Hysteresis and Nanopore Characterization 68
Chapter 7. Characterization of controlled pore glasses: Molecular simulations of adsorption 78
Chapter 8. A new method for the accurate pore size analysis of MCM-41 and other silica based mesoporous materials 88
Chapter 9. Comparison of the experimental isosteric heat of adsorption of argon on mesoporous silica with density functional theory calculations 98
Chapter 10. A computational exploration of cation locations in high-silica Ca-Chabazite 106
Chapter 11. Density functional theory: Diatomic nitrogen molecules in graphite pores 116
Chapter 12. Modelling studies of the influence of macroscopic structural heterogeneities on nitrogen sorption hysteresis 128
Chapter 13. Condensation-evaporation processes in simulated heterogeneous three-dimensional porous networks 138
Chapter 14. Characterisation of porous solids for gas transport 148
Chapter 15. Experimental and simulation studies of melting and freezing in porous glasses 158
Chapter 16. A fast two-point method for gas adsorption measurements 168
Part II: Highly Ordered Porous Inorganic Systems 172
Chapter 17. Rational design, tailored synthesis and characterisation of ordered mesoporous silicas in the micron and submicron size range 172
Chapter 18. Relationship between intrinsic pore-wall corrugation and adsorption hysteresis of N2, O2, and Ar on regular mesopores 184
Chapter 19. Study of the morphology of porous silica materials 194
Chapter 20. Adsorption hysteresis and criticality in regular mesoporous materials 204
Chapter 21. Comprehensive structural characterisation of MCM-41: From mesopores to particles 214
Chapter 22. Characterisation of mesoporous MCM-41 adsorbents by various techniques 224
Chapter 23. Characterisation of mesoporous molecular sieves containing copper and zinc: An adsorption and TPR study 232
Chapter 24. On the applicability of the Horwath-Kawazoe method for pore size analysis of MCM-41 and related mesoporous materials 242
Chapter 25. Dynamic and structural properties of confined phases (hydrogen, methane and water) in MCM-41 samples (19Å, 25Å and 40Å) 252
Chapter 26. Estimating pore size distribution from the differential curves of comparison plots 260
Chapter 27. Rotational state change of acetonitrile vapor on MCM-41 upon capillary condensation with the aid of time-correlation function analysis of IR spectroscopy 268
Chapter 28. Systematic sorption studies on surface and pore size characteristics of different MCM-48 silica materials 276
Chapter 29. Synthesis and characterisation of ordered mesoporous MCM-41 materials 286
Chapter 30. Textural and spectroscopic characterisation of vanadium MCM-41 materials - Application to gas-phase catalysis 296
Chapter 31. On the ordering of simple gas phases adsorbed within model microporous adsorbents 306
Chapter 32. Textural and framework-confined porosity in S+I– mesoporous silica 314
Part III: Carbons 320
Chapter 33. Use of immersion calorimetry to evaluate the separation ability of carbon molecular sieves 320
Chapter 34. Molecular simulations and measurement of adsorption in porous carbon nanotubes 330
Chapter 35. Application of the alpha s method for analysing benzene, dichloromethane and methanol isotherms determined on molecular sieve and superactivated carbons 340
Chapter 36. Characterisation of porous carbonaceous sorbents using high pressure - high temperature adsorption data 350
Chapter 37. Influence of the porous structure of activated carbon on adsorption from binary liquid mixtures 364
Chapter 38. Adsorption mechanism of water on carbon micropore with in situ small angle x-ray scattering 372
Chapter 39. Ultra-thin microporous carbon films 378
Chapter 40. Electrochemical investigation of carbon aerogels and their activated derivatives 388
Chapter 41. Evolution of microporosity upon CO2-activation of carbon aerogels 398
Chapter 42. On the determination of the micropore size distribution of activated carbons from adsorption isotherms 408
Chapter 43. Role of pore size distribution in the binary adsorption kinetics of gases in activated carbon 418
Chapter 44. Confined state of alcohol in carbon micropores as revealed by in situ x-ray diffraction 428
Part IV: Interpretation of Data, Membranes 438
Chapter 45. Critical appraisal of the use of nitrogen adsorption for the characterisation of porous carbons 438
Chapter 46. Structural characterisation and applications of ceramic membranes for gas separations 446
Chapter 47. SANS charcterisation of mesoporous silicas having model structures 456
Chapter 48. Pore-scale complexity of a calcareous material by time-controlled mercury porosimetry 466
Chapter 49. SANS analysis of anisotropic pore structures in alumina membranes 476
Chapter 50. Zeolite membranes - charcterisation and applications in gas separations 484
Chapter 51. A modified Horvath-Kawazoe method for micropore size analysis 492
Part V: Miscellaneous techniques 502
Chapter 52. Further evidences of the usefullness of CO2 adsorption to characterise microporous solids 502
Chapter 53. Interaction between menisci in adjacent pores 512
Chapter 54. Studies on the formation and properties of some highly ordered mesoporous solids 522
Chapter 55. Pore structure of zeolites of type Y and pentasil as the function conditions of preparation and methods of modification 532
Chapter 56. Characterisation of activated carbon fibers by positron annihilation life- time spectroscopy (PALS) 540
Chapter 57. Investigation of the textural characteristics and their impact on in vitro dissolution of spray dried drug product size fractions 550
Chapter 58. The response function method as a novel technique to determine the dielectric permittivity of highly porous materials 562
Chapter 59. Mesopore characterisation by positron annihilation 574
Chapter 60. Characterisation of vanadia-doped silica aerogeis 582
Chapter 61. Shear strength of mineral filter cakes 590
Chapter 62. A frequency-response study of diffusion and adsorption of C1-C5 alkanes and acetylene in zeolites 604
Chapter 63. Novel Mn-based mesoporous mixed oxidic solids 610
Chapter 64. Mercury porosimetry applied to precipitated silica 620
Chapter 65. Synthesis and textural properties of amorphous silica-aluminas 630
Chapter 66. Porous texture modifications of a series of silica and silica-alumina hydrogels and xerogels: A thermoporometry study 640
Chapter 67. Comparison of specific surface areas of a micronised drug substance as determined by different techniques 650
Chapter 68. Investigations on the surface properties of pure and alkali or alkaline earth metal doped ceria 660
Chapter 69. Comparison of the porosity evaluation results based on immersion calorimetry and gravimetric sorption measurements for activated chars from a high volatile bituminous coal 670
Chapter 70. Measuring permeability and modulus of aerogels using dynamic pressurisation in an autoclave 680
Author Index 688
Other volumes in the series 692

Textauszug

Adsorption of Argon and Xenon in Silica Controlled Porous Glass: A Grand Canonical Monte-Carlo Study

R.J.-M. Pellenq,*; A. Delville; H. van Damme; P. Levitz    Centre de Recherche sur la Matière Divisée, CNRS et Université d’Orléans 1b rue de la Férollerie, 45071 Orléanscedex 02, France.

We have studied adsorption of argon (at 77 K) and xenon (at 195 K) in a mesoporous silica Controlled Porous Glass (CPG) by means of Grand Canonical Monte-Carlo (GCMC) simulation. Several numerical samples of the CPG adsorbent have been obtained by using an off-lattice reconstruction method recently introduced to reproduce topological and morphological properties of correlated disordered porous materials. The off-lattice functional of Vycor is applied to a simulation box containing silicon and oxygen atoms of cubic cristoballite with an homothetic reduction of factor 2.5 so to obtain 30 Å-CPG sample. It allows to cut out portion of the initial volume in order to create the porosity. A realistic surface chemistry is then obtained by saturating all oxygen dangling bonds with hydrogen. All numerical samples have similar textural and structural properties in terms of intrinsic porosity, density, specific surface and volume. The adsorbate (Ar,Xe)/adsorbent potential functions as used in GCMC simulations are derived from the PN model. Ar and Xe adsorption isotherms are calculated for each sample: they exhibit a capillary condensation transition but with a finite slope by contrast to that obtained in simple geometries such as slits and cylinders. The analysis of the adsorbed density reveals that the adsorption mechanism for argon (at 77 K) differs from that for xenon (at 195 K): Ar forms a thin layer which covers all the surface prior to condensation while Xe condensates in the higher surface curvature regions without forming a continuous film. This is interpreted on the basis of the Zisman law for wetting: it is based on a contrast of polarizability between the adsorbate and the atoms of the adsorbent. The difference of behavior upon adsorption has important implications for the characterization of porous material by means of physical adsorption especially as far as the specific surface measurement is concerned.

1 INTRODUCTION

Disordered porous solids play an important role in industrial processes such as separation, heterogeneous catalysis… The confinement and the geometrical disorder strongly influence the thermodynamics of fluid adsorbed inside the porous network. This raises the challenge of describing the morphology and the topology of these porous solids [1]. A structural analysis can be achieved by using optical and electron microscopy, molecular adsorption…

Vycor is a porous silica glass which is widely used as a model structure for the study of the properties of confined fluids in mesoporous materials. The pores in vycor have an average radius of about 30-35 Å (assuming a cylindrical geometry) and are spaced about 200 Å apart [2-3]. A literature survey indicates that there are two kinds of (Coming) vycor glasses: one type has a specific surface around 100 m2/g while the other is characterized by a specific surface around 200 m2/g (both values are obtained from N2 adsorption isotherms at 77 K).

The aim of this work is to provide an insight in the adsorption mechanism of different rare gases (argon and xenon) in a disordered connected mesoporous medium such as vycor at a microscopic level.

2 SIMULATION PROCEDURES

2.1 Generating vycor-like numerical samples

We have used on an off-lattice reconstruction algorithm in order to numerically generate a porous structure which has the main morphological and topological properties of real vycor in terms of pore shape: close inspection of molecular self-diffusion shows that the off-lattice reconstruction procedure gives a connectivity similar to experiment. One challenge was to define a realistic mesoporous environment within the smallest simulation box. In a previous study, it was shown that chord distribution analysis on large non-periodic reconstructed 3D structures of disordered materials allows to calculate small angle scattering spectra. In the particular case of vycor, the agreement with experiment is good: on a box of several thousands Å in size, the calculated curve exhibits the experimentally observed correlation peak at 0.026 Å− 1 [4]. The first criterium that our minimal reconstruction has to meet is to reproduce this correlation peak in the diffuse scattered intensity spectrum which corresponds to a minimal (pseudo) unit-cell size around 270 Å. In fact, such a simulation box size still remains too large to be correctly handled in an atomistic Monte-Carlo simulation of adsorption. This is the reason why we have applied an homothetic reduction with a factor of 2.54 so that the final numerical sample is contained in a box of about 107 Å in size (see below). This transformation preserves the pore morphology but reduces the average pore size from 70 Å to roughly 30 Å. Note that a reconstructed minimal numerical sample is still well within the mesoporous domain. The atomistic pseudo-vycor porous medium has been obtained by applying the off-lattice functional to a box containing the silicon and oxygen atoms of 153 unit cells of cubic cristoballite (a siliceous non-porous solid). This allows to cut out portions of the initial volume in order to create the vycor porosity. The off-lattice functional represents the gaussian field associated to the volume autocorrelation function of the studied porous structure [5]. This approach encompasses a statistical description: it allows to generate a set of morphologically and topologically equivalent numerical samples of pseudo-vycor. Periodic boundary conditions are applied in order to simplify the Grand Canonical Monte-Carlo (GCMC) adsorption procedure. In order to model the surface in a realistic way and to ensure electroneutrality, all oxygen dangling bonds are saturated with hydrogen atoms (all silicon atoms in an incomplete tetrahedral environment are also removed). The gradient of the local gaussian field allows to place each hydrogen atom in the pore void perpendicular to the interface at 1 Å from the closest unsaturated oxygen.

2.2 The Grand Ensemble Monte-Carlo simulation technique as applied to adsorption

In this work, we have used a PN-type potential function as reported for adsorption of rare gas in silicalite (a purely siliceous zeolite): it is based on the usual partition of the adsorption intermolecular energy which can written as the sum of a dispersion interaction term with the repulsive short range contribution and an induction term (no electrostatic term in the rare gas/surface potential function) [6]. The dispersion and induction parts in the Xe/H potential are obtained assuming that hydrogen atoms have a partial charge of 0.5e (qO = -1e and qSi = -2e respectively) and a polarizability of 0.58 Å3; the adsorbate/H repulsive contribution (Born-Mayer term) is adjusted on the experimental low coverage isosteric heat of adsorption (13.5 and 17 kJ/mol for argon [7,8] and xenon [9,10] respectively). The adsorbate-adsorbate potential energy was calculated on the basis of a Lennard-Jones function (ε = 120 K and σ = 3.405 Å for argon and ε = 211 K and σ = 3.869 Å for xenon). In the Grand Canonical Ensemble, the independent variables are the chemical potential, the temperature and the volume [11]. At equilibrium, the chemical potential of the adsorbed phase equals that of the bulk phase which constitutes an infinite reservoir of particles at constant temperature. The chemical potential of the bulk phase can be related to the temperature and the bulk pressure. Consequently, the independent variables in a GCMC simulation of adsorption in vycor are the temperature, the pressure of the bulk gas and the volume of the simulation cell containing the porous sample as defined above. The adsorption isotherm can be readily obtained from such a simulation technique by evaluating the ensemble average of the number of adsorbate molecules. Note that the bulk gas is assumed to obey the ideal gas law. Control charts in the form of plots of a number of adsorbed molecules and internal energy versus the number of Monte-Carlo steps were used to monitor the approach to equilibrium. Acceptance rates for creation or destruction were also followed and should be equal at equilibrium. After equilibrium has been reached, all averages were reset and calculated over several millions of configurations. In order to accelerate GCMC simulation runs, we have used a grid-interpolation procedure in which the simulation box volume is split into a collection of voxells [12]. The adsorbate/pseudo-vycor adsorption potential energy is calculated at each corner of each elementary cubes.

3 RESULTS AND DISCUSSION

3.1 Properties of pseudo-vycor numerical samples

We have generated a series of ten numerical samples. The porosity ranges from 0.291 to 0.378 % while the density ranges from 1.369 to 1.562 g/cm3. The average density and porosity values are 1.467 g/cm3 and 0.334 respectively (the values for real vycor are 1.50 g/cm3 and 0.30). Density and porosity exhibit fluctuations...

Produkt-Details

• Erscheint lt. Verlag: 11.4.2000
• Sprache: englisch
• Themenwelt: Naturwissenschaften ► Chemie ► Physikalische Chemie
• Themenwelt: Naturwissenschaften ► Chemie ► Technische Chemie
• Themenwelt: Naturwissenschaften ► Geowissenschaften ► Mineralogie / Paläontologie
• Themenwelt: Technik ► Maschinenbau
• ISBN-10: 0-08-052889-9 / 0080528899
• ISBN-13: 978-0-08-052889-2 / 9780080528892