Adsorption and diffusion in microporous

Application of Nuclear Shielding Surfaces to the Fundamental Understanding of Adsorption and Diffusion in Microporous Solids... 

Another outstanding feature of TEOM is that it enables investigations of adsorption and diffusion in microporous and mesoporous materials as a function of the coke content (37,38). 

Thus, Volume 7 of the series Molecular Sieves - Science and Technology presents descriptions, critical analyses, and illustrative examples of applications of the most important methods for investigations of sorption and sorption kinetics in zeolite systems and related materials. The editors hope that the volume will be helpful for researchers as well as technologists who are confronted with the important phenomena of adsorption and diffusion in microporous materials as they occur, for instance, in separation processes and catalysis. 

In a series of papers, Ma and his co-workers [1 ] systematically examined the interrelationship between adsorption, permeation and diffusion in microporous silica membranes. Both equilibrium and nonequilibrium properties of the microporous inorganic gas separation membranes were studied. Both high pressure and low pressure gravimetric units were used in their adsorption measurements. 

Adsorption and diffusion in the micropores plays an important role in the activity of a zeolite. The adsorption of a base molecule is not only influenced by the strength and number of acid sites present in the molecular sieve, but also by the geometry of the micropores [68]. As the reaction rate of a test molecule is dependent on its adsorption properties, a measured rate will inherently depend on acidity and pore geometry factors. 

Regarding adsorption and diffusion without reaction, Jordi and Do (49) simulated the expected results for the frequency response by completely numerical methods, with no need for linearization. In a later study, they used a linearized model coupled with analytic solutions for the diffusion inside the particles, which also took into account transport in both macropores and micropores (50). The mathematical details are clearly presented in these papers. 

The application of the Maxwell-Stefan theory for diffusion in microporous media to permeation through zeolitic membranes implies that transport is assumed to occur only via the adsorbed phase (surface diffusion). Upon combination of surface diffusion according to the Maxwell-Stefan model (Eq. 20) with activated-gas translational diffusion (Eq. 12) for a one-component system, the temperature dependence of the flux shows a maximum and a minimum for a given set of parameters (Fig. 15). At low temperatures, surface diffusion is the most important diffusion mechanism. This type of diffusion is highly dependent on the concentration of adsorbed species in the membrane, which is calculated from the adsorption isotherm. At high temperatures, activated-gas translational diffusion takes over, causing an increase in the flux until it levels off at still-higher temperatures. 

Configurational diffusion in microporous (molecular sieve) membranes wiU be treated separately. Here the driving force must be described in terms of a chemical potential gradient, which is coupled to partial pressure via adsorption isotherms. In cases where several mechanisms operate simultaneously, the problem of additivity arises and in real membrane systems simplifying assumptions have to made. 

Soil reactions are generally classified according to the nature of the main chemical process involved adsorption, ion exchange, dissolution, etc. However, in order to assess the kinetics one should consider the nature and the rate of the transport processes associated with the chemical reaction flow and diffusion in the soil solution, transport across the solid-liquid interface, diffusion in liquid-filled pores and micropores, and surface diffusion penetration into the solid. An expression for the kinetics of soil reactions can be devised by assigning rate equations to transport and chemical processes and combining these equations. The expression finally obtained has to be validated by comparison to experimental results. 

Silicalite is a microporous crystalline silica molecular sieve with remarkable hydrophobic properties ( 1) and has been considered to offer practical applications in the clean-up of water contaminated with hydrocarbons and the separation of ethanol from dilute fermentation aqueous solutions (2 ii> 2) Many studies have been reported on the properties of adsorption and diffusion of gases in silicalite (e.g., 6, 8, , HI) However, despite the many potential applica-... 

More recently, continuous flow or open FR systems have been developed to measure the adsorption and diffusion properties of sorbate molecules in microporous materials [13-15]. hi these systems, either the concentration of the sorbate feed [13] or the pressure within the reactor [14,15] is oscillated and the resulting changes at the exit stream are measured by using mass spectrometry or a mass flow meter. For a flow FR system, the effect of adsorption heat is reduced as the flowing gases attenuates the temperature change. 

Sorption into, release from and diffusion inside microporous and mesoporous materials are of paramount interest in view of separation processes and catalysis by zeolites and related structures. Thus, volume 7 of the handbook-like series Molecular Sieves - Science and Technology is exclusively devoted to the phenomena of adsorption into, desorption out of and diffusion in the pores of zeolite crystalhtes. 

In the first part of this chapter I outline the theory and practice of adsorption studies, before going on to examine the nature of adsorption of different molecules at specific types of sites on the internal surface and the diffusion of adsorbates through channels and cages in microporous solids. In the light of this, I discuss the role of adsorption in particular applications. 

Diffusion in microporous solids occurs by activated jumps along the pore channels or across cages within the structure. The rate of diffusion over short distances, of the order of the unit cell repeat, is determined by the frequency of re-orientation of molecules into configurations that permit motion, the strength of interaction with the framework, the distance between adsorption sites and the presence of other molecules in the pores. Structural defects, including... 

The ability of microporous solids to act as high-capacity molecular sieves has long been exploited in a wide range of applications in adsorption and separation. The electrostatic interactions of the traditional cationic forms of aluminosilicates are well suited for the uptake of polar molecules (such as H2O) and are also able to separate oxygen from air. The development of microporous solids with varied chemistry has enabled adsorption and diffusion properties to be finely tuned for particular technologies. Pure silica zeolite polymorphs such as silicalite have particular importance, because they enable separation on the basis of a different range of polarity and on molecular size the absence of aluminium in the framework also prevents the presence of unwanted acidity, so adsorbed hydrocarbons do not undergo any catalytic transformation. 

D. M. Ruthven, S. Brandani and M. Eic, Measurement of Diffusion in Microporous Solids by Macroscopic Methods , in Molecular Sieves, eds. H. G. Karge and J. Weitkamp, Springer GmbH, 2008, Vol. 7, Adsorption and Diffusion, p. 45. 

A relatively recent development in the determination of acid site strength and quantity is the application of Ar adsorption (39). This technique, which is related to Ar TPD but stated not to suffer limitations of diffusion in micropores since it involves the determination of Ar adsorption isotherms under static equilibrium, involves the determination of heats of adsorption extracted from Henry or Langmuir regimes. Listed advantages of this method are that it can be applied... 

In this section, we discuss the general aspects of chemical bonding in zeolites and the zeolite O H bond. Brpnsted and Lewis acid catalysis by zeolites is presented in Section 4.2. Section 4.3 covers redox catalysis by zeolites. The final three sections describe the catalytic cycle and the role of adsorption and diffusion on catalytic performance. An important question that arises in each of these sections is the relation between the micropore structure of the zeolite and its activity and selectivity. 

The mechanisms of diffusion in these two systems (gas and liquid) are different and unrelated diffusion in gases is the result of the collision process, whereas that in liquids is an activated process (Bird et al., 1960). Diffusion in microp-orous materials is neither gaseous nor liquid diffusion. The closest case for such diffusion is surface diffusion, where molecules hop within the surface force field (see review by Kapoor et al., 1989b). Pick s law is used for both application (in modeling of adsorption processes) and experimental measurement of diffusion. Extensive reviews are available on diffusion in microporous materials and zeolites (Karger and Ruthven, 1992 Do, 1998). A lucid discussion on the nonlinear, and in some cases peculiar, phenomena in zeolite diffusion was given... 

The TAP reactor was used to simultaneously determine the ad i>tion and diffusion parameters of n-butane over chemically activate carbons in a packed bed. Measurements were performed over activated carbons prepared by phosphoric add activation of kraft lignin with varying carbonization temperatures (400-650 C), weight ratios of phosphoric acid to lignin (P/L=1.0-1.8) and impregnation times (1 and 48h). The diffusion and adsorption in these carbons were adequately described with a simplified three-parameter model. The determined adsorption and diffusion parameters agree well with the values reported in literature. TAP pulse responses allow the study of diffusion in the microporous region. 

Liquid chromatography is a technique widely used for the determination of adsorption and diffusion parameters in microporous materials such as zeolites [6], in meso- and macroporous materiels such as aluminas [7], as well as in biporous pelletized materials [8]. This technique enables also the development of suitable adsorbent for a given separation, for example the separation of isomers of xylenes on X or Y zeolites [9,10]. 

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