Microporous solids, sorption

Coal is microporous, with certain partial molecular sieve properties. (A microporous solid herein refers to that which contains pores with diameters of a few tens of A. or less.) Micropores can be considered as entities capable of sorbing foreign molecules, and it is known that additivity of their sorption potential fields enhances the sorption owing to dispersion interactions. As the pores become progressively narrower, the vapor adsorption isotherm (Figure 1) in the initial region up to point B becomes progressively steeper (toward the... 

Our current views, with some elaboration, are summarized below. We do not consider it meaningful with coals, or with microporous solids in general, to deduce specific surface values from sorption data, nor even to apply the concept of specific surface to these materials. We believe that sorption uptake (moles per unit weight or volume of a given adsorbent) under defined conditions is the correct parameter that should be used to describe the sorptive properties of such materials. Thus, whenever the sorption uptake by an active carbon, for a particular sorbate, is required in a practical application such as solvent recovery, purification, or gas sorption, what should be determined in the laboratory is the uptake under the conditions for which the value is to be used it is not possible to predetermine unequivocally the value for one... 

Figure 7.42 Types of gas sorption isotherm - microporous solids are characterised by a type I isotherm. Type II corresponds to macroporous materials with point B being the point at which monolayer coverage is complete. Type III is similar to type II but with adsorbate-adsorbate interactions playing an important role. Type IV corresponds to mesoporous industrial materials with the hysteresis arising from capillary condensation. The limiting adsorption at high P/P0 is a characteristic feature. Type V is uncommon. It is related to type III with weak adsorbent-adsorbate interactions. Type VI represents multilayer adsorption onto a uniform, non-porous surface with each step size representing the layer capacity (reproduced by permission of IUPAC).

Porosity and pore structure are properties that control diffitsive transport, selective reaction, and sorption-based separations of gases in adsorbents and catalysts [1,2]. Sorption porosimetry may be used to characterize the porosity of both mesoporous and microporous solids. The pore size distribution F H) is obtained from the experimental isotherm /[/ )... 

The theory of transport in microporous solids is complex and involves many aspects and steps. Although many aspects has been treated separately (e.g., sorption, diffusion, simulation studies, mechanisms, etc.) there are no coherent descriptions of permeation and separation in microporous membrcmes covering all the important aspects. In this chapter an attempt is made to introduce such a description. It is useful to give a qualitative picture first (Section 9.4.2.1). 

The wide range of structural units exhibited by the natural and synthetic phases suggests that many new open frameworks with ion-exchange and sorption properties can be obtained. Exploratory synthesis of novel microporous solids is currently an active research area. 

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. 

In the world of nanoparticles and microporous materials, the interaction forces between nanosized particles and molecules from the surrounding medium, or the forces between particles themselves, may exceed the mechanical forces between bodies of the macroscopic world. This is caused by the high surface-to-volume ratio of nanoparticles and microporous materials. When familiar materials become mainly surface, they acquire new optical, magnetic, electrical, chemical, and transport properties. Thus, dispersions tend to agglomerate, fine particles show increased mechanical strength, and microporous solids develop tremendous sorption and molecular sieving properties. 

Do, D.D., Sorption rate of bimodal microporous solids with an irreversible isotherm. Chem. Eng. Sci.. 44(8). 1707-1714 (1989). 

Do, D.D., Effect of surface barrier on sorption of gases in microporous solids. AIChE J.. 35(4), 649-655 (1989). Do, D.D., Role of temperature and length scales on sorption of gases in microporous solids, Chem. Eng. Commun., 77, 229-246(1989). 

In order to relate the effect of formamide on the gelation process and the subsequent structure of the xerogels, nitrogen sorption was performed. The results are provided in table II. The addition of formamide to acid catalyzed silica and silica/PVAc gels result in mesoporous, rather than microporous solids. Increasing the substitution of formamide for ethanol increases the BET surface area and pore volume. On the other hand, the addition of PVAc resulted in decrease of the BET surface area and pore volume for ual levels of formamide. 

A first SIM investigation of sorption-thermodynamic functions for binary [18-20] and ternary [21] mixtures of gases on microporous solids was presented by the Billow group, in the nineteen eighties and 1994, respectively Billow also introduced this technique to the Rees group at the ICSTM London [22]. Since 1989, the latter group published a series of papers, particularly on sorption equilibria for binary mixtures [23-26]. Thermodynamic analyses of the isosteric... 

Billow M. and Shen D., Direct Measurement of Sorption Isosteres for Gases on Microporous Solids, in Proc. Topical Conf. Sep. Sci. Technol., Part II, AIChE 1997 Annual Meeting, ed. by W.S.W. Ho and R.G Luo (Los Angeles, November 16-21, 1997) pp. 1150-1155. 

Adnadevic and Vucelic [86] compared the sorption of polar molecules with the BET gas adsorption technique for the surface area of microporous solids. He examined activated charcoal, zeolites and silica clay using MeOH, EtOH, CO2 and CeUe-... 

Powder XRD on a heated sample of 25d showed no ehange in the observed pattern. This would infer a robust architecture, but given the proposed strueture, void space would necessarily have to be generated in this desolvated solid, 25e. To further confirm the proposed strueture of 25d and definitively illustrate the porosity of this system, CO2 and N2 sorption isotherms were performed on 25e (Fig. 35). Both yielded type 1 isotherms eharaeteristie of microporous solids. For CO2, surface areas of 326, 373, and 380 m /g for BET, Langmuir, and Dubinin-Radushkevich (DR) models, respeetively, were obtained. 

FIGURE 35. Gas sorption isotherms with CO2 (light trace) and N2 (dark trace) for 25e. Both show reversible type 1 isotherms characteristic of a microporous solid. 

This overview shows the rich variety of sorption behavior of microporous solids that can be combined with piezoelectric devices to create selective sensors. Future work will explore additional parameters such as acid base reactions and coordination chemistry, as well as issues including chemical interferences imder real world conditions. 

Apparent hysteresis occurs mainly when complete equilibrium is not reached. Diffusion into the solid matrix or into micropores of aggregates is considered a main cause of apparent hysteresis. In a transitory state, sorption occurs concurrently with desorption and the concentration of contaminant in the liquid phase is erroneously low because some fraction is associated with sorption. 

Solid sorbents are materials with a microporous structure, whose internal pores and outer surfaces are accessible for sorption. Typical sorbents used for the collection of air samples have nominal size of 20/40 mesh, with pore diameters less than 50 k9 giving rise to surface areas up to 1000 m2/g. 

The only question that might be raised is whether any of the observations, particularly those relating to the use of concentrated NaCl solution, are an artifact of the procedures employed. To address this, it can be noted that the effect of 3.4 M NaCl is dramatically different from that of H2O or even sub-molar NaCl solution, so that comparisons with effects seen in these media cannot be made. Furthermore, after undergoing all of the treatment chemistry described, the recovered solids still retain full crystallinity, as measured by X-ray powder diffraction and micropore sorption capacity. 

A number of methods are used for studying the sorption of basic probe molecules on zeolites to learn more about zeolite acidity. A common disadvantage of all the examinations is that adsorbed basic probe increases the electron density on the solid and, thereby, change the acidic properties of the sites examined. From this aspect it seems advantageous to probe the acid sites with a weak base, e. g., with a hydrocarbon. It was shown that adsorption of alkanes is localized to the strong Brdnsted acid sites of H-zeolites [1, 2]. However, recent results suggest that usually the diffusion in the micropores controls the rate of hydrocarbon transport [3-5]. Obviously, the probe suitable for the batch FR examination of the sites has to be non-reactive and the sorption dynamics must control the rate of mass transport. The present work shows that alkanes can not be used because, due to their weak interaction with the H-zeolites, the diffusion is the slowest step of their transport. In contrast, acetylene was found suitable to probe the zeolitic acid sites. The results are discussed in comparison with those obtained using ammonia as probe. Moreover, it is demonstrated that fundamental information can be obtained about the alkane diffusivity in H-zeolites... 

Parise JB, Chen J (1997) Studies of ciystalhne solids at high pressure and temperature using the DIA multianvil apparatus. Eur J Solid State Inorg Chem 34 809-821 Parise JB, Coibin DR, Abrams L (1995) Stractural changes upon sorption and desorption of Xe from Cd-exchanged zeolite rho A real-time synchrotron X-ray powder diffraction study. Microporous Mater 4 99-110... 

This chapter aims to give guidelines on how to use adsorption methods for the characterization of the surface area and pore size of heterogeneous catalysts. The information derived from these measurements can range from the total and available specific surface area to the pore sizes and the strength of sorption in micropores. Note that this spans information from a macroscopic description of the pore volume/specific surface area to a detailed microscopic assessment of the environment capable of sorbing molecules. In this chapter we will, however, be confined to the interaction between sorbed molecules and solid sorbents that are based on unspecific attractive and repulsive forces (van der Waals forces, London dispersion forces). 

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