Kinetics in microporous solids

Fundamentals of Adsorption Equilibrium and Kinetics in Microporous Solids 3... 

The main focus of this volume is on imderstanding the transport of molecules in microporous solids such as zeolites and carbon molecular sieves, and the kinetics of adsorption/desorption. This subject is of both practical and theoretical interest, since the performance of zeohte-based catalysts and adsorbents is strongly influenced by resistances to mass transfer and intracrystalline diffusion. However, at an even more basic level, the performance of microporous catalysts and adsorbents depends on favorable adsorption equilibria for the relevant species, so a general imderstanding of the fundamentals of adsorption equilibrium is a necessary prerequisite for understanding kinetic behavior. This chapter is intended to provide a concise summary of the general principles of adsorption equiHbriiun and of the main features of sorption kinetics in microporous solids, which generally depend on a combination of both equilibriiun and kinetic properties. 

This book will address the various fundamental aspects of adsorption equilibria and dynamics in microporous solids such as activated carbon and zeolite. The treatment of equilibria and kinetics, when properly applied, can be used for solids other than microporous solid, such as alumina, silica gel, etc. Recognizing that practical solids are far from homogeneous, this book will also cover many recent results in dealing with heterogeneous media. 

In Chapter 2, we discussed the fundamentals of adsorption equilibria for pure component, and in Chapter 3 we presented various empirical equations, practical for the calculation of adsorption kinetics and adsorber design, the BET theory and its varieties for the description of multilayer adsorption used as the yardstick for the surface area determination, and the capillary condensation for the pore size distribution determination. Here, we present another important adsorption mechanism applicable for microporous solids only, called micropore filling. In this class of solids, micropore walls are in proximity to each other, providing an enhanced adsorption potential within the micropores. This strong potential is due to the dispersive forces. Theories based on this force include that of Polanyi and particularly that of Dubinin, who coined the term micropore filling. This Dubinin theory forms the basis for many equations which are currently used for the description of equilibria in microporous solids. 

Physical adsorption in microporous solids shows type I isotherms because the micropores limit the adsorption to a few molecular layers. Using a kinetic approach, Langmuir described the type I isotherm, considering that adsorption was limited to a monolayer (32). This approach assumes that the adsorption energy is constant and is independent of the fraction of the surface occupied by the adsorbed molecules. 

Porosity refers to the volume of pores in a solid. It contributes to the internal surface area of the sample and can influence the kinetics of adsorption. Diffusion into and out of pores is often considered responsible for slow adsorption and desorption processes. Pores vary in size and shape. They have been classified according to their average widths as micropores which are of the order of molecular dimensions (<2 nm), meso- or transitional pores which are between 2-50 nm and macropores which are larger than 50 nm (Sing et al., 1985). The sum of all the pores is called the pore volume (porosity). 

Over the years there has been a lot of debate concerning the applicability of the Dubinin-Radushkevich equation on the very low pressure region of isotherms of microporous solids. The experimental downward deviation of the DR-plot for very low pressures is generally attributed to kinetic barriers, especially in the case of nitrogen adsorption at 77K. This low pressure region of isotherms of various adsorbents can be fitted with the Langmuir equation. Hence it is shown that the downward deviation is not due to experimental factors but reflects a different adsorption mechanism. 

Electron microscope examination by Burlein and Mastel (61) has shown the diameter of the track of a fission fragment in uranium oxide to measure 150 A. This dimension corresponds nearly to that of the micrograins of the microporous solids which were used. Consequently, the temperature of the surface of a certain number of micrograins that are in direct contact with the gaseous reactants may be raised to a very high value (more than 1000°C.) at this temperature kinetic and thermodynamic considerations applicable at the over-all macroscopic temperature cease to be valid. 

It is convenient to classify an overall soil reaction as a slow or rapid reaction. A soil reaction is slow when the kinetics are associated with an energy of activation. Slow reactions are those in which processes taking place at the solid phase are rate determining whether transport processes (such as surface diffusion, diffusion in micropores, penetration into the bulk, etc.), or chemical interactions. Mechanisms for slow soil reactions are discussed in detail in the following sections. 

Abstract Theoretical, experimental principles and the applications of the frequency response (FR) method for determining the diffusivities in microporous and bidispersed porous solid materials have been reviewed. Diffusivities of hydrocarbons and some other sorbates in microporous crystals and related pellets measured using the FR technique are presented, and the FR data are analysed to demonstrate the identification of the FR spectra. These results display the ability of the FR method to discriminate multi-kinetic mechanisms, including a surface resistance or surface barrier occurring simultaneously in the systems, which are difficult to be determined using other microscopic or macroscopic methods. The FR measurements also showed that the diffusivity of a system depends significantly on the subtle differences in molecular shape and size of sorbates in various... 

It is not easy to predict which frameworks will remain intact upon template removal, and which will collapse, but it should be borne in mind that all open frameworks are less stable (when empty) than dense crystalline forms, and barriers to recrystallisation are kinetic rather than thermodynamic. High-temperature treatment of microporous solids eventually results in recrystallisation to dense ceramics rather than other microporous solids. For example, the magnesium form of zeolite P transforms to magnesian cordierite, and aluminophosphates transform to dense AIPO4 polymorphs. 

Diffusion of adsorbate molecules throughout the pore space of microporous solids is an essential step in many applications of microporous solids and determines their utility and selectivity in applications. Whereas the thermodynamics of the adsorption determines the equilibrium situation, the kinetics of an adsorptive or catalytic process is controlled by the diffusion rates. This is exemplified in their use in shape-selective catalysis, where molecules must reach and leave active sites distributed through the crystallites and therefore products that diffuse faster will be enriched in the molecular mix leaving the solid. 

Separation of gases is a very important process in several industries (e.g., chemical, petrochemical, and related industries). Although cryogenics and absorption remain the most widely used processes, the last two decades have seen a tremendous growth in research activities and commercial applications of adsorption-based gas separation. Separation by adsorption is based on the selective accumnlation of one or more components of a gas mixture on the surface of a microporous solid. The separation is achieved by one of three mechanisms steric, kinetic, or equilibrium. Most processes operate by virtue of equilibrium (or competitive) adsorption of gases from binary or multicomponent mixtures [143]. 

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