Adsorption by Powders and Porous Solids

Principles, Methodology and Applications

Jean Rouquerol, Françoise Rouquerol, Philip Llewellyn, Guillaume Maurin, Kenneth Sing, Kenneth S.W. Sing

• Publisher: Academic Press
• Year: 2013
• eBook ISBN: 9780080970363
• Edition: 2
• Topic: Technology & Engineering
• Subtopic: Physical & Theoretical Chemistry
• Index: Technology & Engineering

The declared objective of this book is to provide an introductory review of the various theoretical and practical aspects of adsorption by powders and porous solids with particular reference to materials of technological importance. The primary aim is to meet the needs of students and non-specialists who are new to surface science or who wish to use the advanced techniques now available for the determination of surface area, pore size and surface characterization. In addition, a critical account is given of recent work on the adsorptive properties of activated carbons, oxides, clays and zeolites. - Provides a comprehensive treatment of adsorption at both the gas/solid interface and the liquid/solid interface - Includes chapters dealing with experimental methodology and the interpretation of adsorption data obtained with porous oxides, carbons and zeolites - Techniques capture the importance of heterogeneous catalysis, chemical engineering and the production of pigments, cements, agrochemicals, and pharmaceuticals

1

Introduction

Françoise Rouquerol*, Jean Rouquerol*, Kenneth S.W. Sing*, Guillaume Maurin** and Philip Llewellyn*, ^*Aix Marseille University-CNRS, MADIREL Laboratory, Marseille, France, ^**University of Montpellier 2, Institute Charles Gerhardt, Montpellier, France

Abstract

In addition to providing a general introduction to the main chapters of the book, this chapter also includes a short historical account of the development of the theoretical principles and applications of adsorption and also a comprehensive survey of the terms and properties associated with adsorption, powders and porous solids. The main differences between physisorption and chemisorption are summarised along with a revised classification of physisorption isotherms. Other sections deal briefly with energetics and molecular modelling and with the diffusion of adsorbed molecules.

Key Words

adsorption history; adsorption definitions; types of physisorption isotherms; adsorption energetics; adsorption modelling; diffusion and adsorption

Chapter Contents

1.1 The Importance of Adsorption

1.2 Historical Aspects

1.3 General Definitions and Terminology

1.4 Physisorption and Chemisorption

1.5 Types of Adsorption Isotherms

1.5.1 Classification of Gas Physisorption Isotherms

1.5.2 Chemisorption of Gases

1.5.3 Adsorption from Solution

1.6 Energetics of Physisorption and Molecular Modelling

1.7 Diffusion of Adsorbate

1.1 The Importance of Adsorption

Adsorption occurs whenever a solid surface is exposed to a gas or liquid: it is defined as the enrichment of material or increase in the density of the fluid in the vicinity of an interface. Under certain conditions, there is an appreciable enhancement in the concentration of a particular component and the overall effect is then dependent on the extent of the interfacial area. For this reason, all industrial adsorbents have large specific surface areas (generally, well in excess of 100 m² g^{− 1}) and are therefore highly porous or composed of very fine particles.

Adsorption is of great technological importance, as often stressed (Dabrowski, 2001). Thus, some adsorbents are used on a large scale as desiccants, catalysts or catalyst supports; others are used for the separation or storage of gases, the purification of liquids, controlled drug delivery, pollution control or for respiratory protection. In addition, adsorption phenomena play a vital role in many solid state reactions and biological mechanisms.

Another reason for the widespread use of adsorption techniques is the importance now attached to the characterisation of the surface properties and texture of fine powders such as pigments, fillers and cements. Similarly, adsorption measurements are undertaken in many academic and industrial laboratories on porous materials such as clays, ceramics and membranes. In particular, gas adsorption has become one of the most widely used procedures for determining the surface area and pore size distribution of a diverse range of powders and porous materials.

1.2 Historical Aspects

Various phenomena which we now associate with adsorption were known in antiquity. The adsorbent properties of such materials as clay, sand and wood charcoal were utilised by the ancient Egyptians, Greeks and Romans (Robens, 1994). These applications were wide-ranging and included the desalination of water, the clarification of fat and oil and the treatment of many diseases.

It has long been known that certain forms of charcoal can take up large volumes of gas. The earliest quantitative studies appear to have been made by Scheele in 1773 and independently by Priestley in 1775 and the Abbé Fontana in 1777 (Deitz, 1944; Forrester and Giles, 1971). The decolourising properties of charcoal were first investigated by the Russian chemist Lowitz in 1785 . The exothermal nature of gas adsorption was noted by de Saussure in 1814. Mitscherlich (1843) suggested that the amount of gas adsorbed in a porous carbon was such that it was probably in the liquid state. This prompted Favre (1854, 1874) to study the ‘wetting of solids by gases’ and to use adsorption calorimetry to show that the heat of adsorption of various gases on charcoal was larger than the heat of liquefaction, which he explained as due to a higher density in the vicinity of the pore walls. However, it was not until 1879–1881 that the first attempts were made by Chappuis (1879; 1881a,b) and Kayser (1881a,b) to relate the amount of gas adsorbed to the pressure. It was then that Kayser (1881a,b) introduced the term adsorption and over the next few years the terms isotherm and isothermal-curve were applied to the results of adsorption measurements made at constant temperature (see Forrester and Giles, 1971).

It was observed by Leslie in 1802 that heat was produced when liquid was added to a powder. The heat evolved by the immersion of dry sand in water was described by Pouillet in 1822. This exothermic phenomenon became known in France as the ‘Pouillet effect’. Gore (1894) recognised that the amount of heat was related to the surface area of the powder, while Gurvich (1915) suggested that it was also dependent on the polarity of the liquid and the nature of the powder.

The first recorded isotherms of adsorption from solution were probably those reported by van Bemmelen in 1881 (Forrester and Giles, 1972). In his investigations of the ‘absorptive’ power of soils, van Bemmelen noted the importance of the colloidal structure and drew attention to the relevance of the final state (i.e. equilibrium concentration) of the solution in contact with the soil. A number of solute–solid isotherms were determined over the next 20 years including those for the uptake of iodine and various dyes by charcoal and other adsorbents, but many of the investigators still believed that the process involved penetration into the solid structure. Freundlich, in 1907, was one of the first to appreciate the role of the solid surface. He proposed a general mathematical relation for the isotherm, which we now refer to as the Freundlich adsorption equation.

In 1909, McBain reported that the uptake of hydrogen by carbon appeared to occur in two stages: a rapid process of adsorption appeared to be followed by a slow process of absorption into the interior of the solid. McBain coined the term sorption to cover both phenomena. In recent years, it has been found convenient to use ‘sorption’, when it is not possible to make a clear distinction between these two stages of uptake and also to use it to denote the penetration of molecules into very narrow pores (Barrer, 1978).

During the early years of the past century, various quantitative investigations of gas adsorption were undertaken. The most important advances in the theoretical interpretation of gas adsorption data were made by Zsigmondy, Polanyi and Langmuir: their ideas set the scene for much of the research undertaken during the first half of the twentieth century.

In 1911, Zsigmondy pointed out that the condensation of a vapour can occur in very narrow pores at pressures well below the normal vapour pressure of the bulk liquid. This explanation was given for the large uptake of water vapour by silica gel and was based on an extension of a concept originally put forward by Thomson (Lord Kelvin) in 1871 (see Sing and Williams, 2012). It is now generally accepted that capillary condensation does play an important role in the physisorption by porous solids, but that the original theory of Zsigmondy cannot be applied to pores of molecular dimensions.

The theory proposed by Polanyi in 1914 was developed from an older idea of long-range attractive forces emanating from the solid surface. The adsorbed layer was pictured as a thick compressed film of decreasing density with increase in distance from the surface. The original ‘potential theory’ did not give an equation for the adsorption isotherm, but instead provided a means of establishing a ‘characteristic curve’ – relating adsorption potential to amount adsorbed – for a given system. In spite of its initial appeal, it soon became apparent that the principles underlying the potential theory were not consistent with the emerging treatment of intermolecular forces. However, the concept of a characteristic curve was subsequently modified and adopted by Dubinin and his co-workers in their theory of micropore filling.

The year 1916 brought a radical change in the approach to surface science. In that year, the first of Langmuir's monumental papers appeared (1916, 1917, 1918). Lord Rayleigh's earlier conclusion that certain films of polar oils on water were one molecule thick had not received the attention it deserved and Langmuir's great contribution was to bring together all the available evidence to support the unifying concept of the monomolecular layer (the monolayer). He proposed that adsorption on both liquid and solid surfaces normally involved the formation of a monomolecular layer. In retrospect, it is not surprising that the advent of the Langmuir theory produced a renaissance in surface science.

Langmuir's work on gas adsorption and insoluble monolayers prepared the way for more progress to be made in the interpretation of adsorption from solution data. In the light of the Langmuir theory, it seemed logical to suppose that the plateau of a solute isotherm represented monolayer completion and that the monolayer capacity could be derived by application of the Langmuir equation.

Another important stage in the history of gas adsorption was the work of Brunauer and Emmett, which preceded the publication of the Brunauer–Emmett–Teller (BET) theory in 1938. In 1934, Emmett and Brunauer made their first attempt to use low-temperature adsorption of nitrogen to determine the surface area of an iron synthetic ammonia catalyst. They noted that the adsorption isotherms of a number of gase...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface to the First Edition
• Preface to the Second Edition
• List of Main Symbols
• 1. Introduction
• 2. Thermodynamics of Adsorption at the Gas/Solid Interface
• 3. Methodology of Gas Adsorption
• 4. Adsorption at the Liquid–Solid Interface: Thermodynamics and Methodology
• 5. Classical Interpretation of Physisorption Isotherms at the Gas–Solid Interface
• 6. Modelling of Physisorption in Porous Solids
• 7. Assessment of Surface Area by Gas Adsorption
• 8. Assessment of Mesoporosity
• 9. Assessment of Microporosity
• 10. Adsorption by Active Carbons
• 11. Adsorption by Metal Oxides
• 12. Adsorption by Clays, Pillared Clays, Zeolites and Aluminophosphates
• 13. Adsorption by Ordered Mesoporous Materials
• 14. Adsorption by Metal-Organic Frameworks
• Index