Microporous solids adsorption

Bulow, M., and Micke, A., Determination of transport coefficients in microporous solids. Adsorption, 1(1), 29-48 (1995). 

In the simplest case, adsorption in a microporous solid leads to an isotherm of Type I consequently it is convenient to approach the subject by a discussion, from a classical standpoint, of Type I isotherms. 

Perhaps the most direct method of evaluating microporosity is to fill up the micropores with some suitable adsorbate whilst leaving the mesopores, macropores and external surface free. The use of n-nonane as a preadsorbate was proposed by Gregg and Langford on the basis of earlier work on the adsorption of n-alkanes C, to C, on ammonium phos-phomolybdate, a microporous solid. This work had shown that the rate at... 

A vast amount of research has been undertaken on adsorption phenomena and the nature of solid surfaces over the fifteen years since the first edition was published, but for the most part this work has resulted in the refinement of existing theoretical principles and experimental procedures rather than in the formulation of entirely new concepts. In spite of the acknowledged weakness of its theoretical foundations, the Brunauer-Emmett-Teller (BET) method still remains the most widely used procedure for the determination of surface area similarly, methods based on the Kelvin equation are still generally applied for the computation of mesopore size distribution from gas adsorption data. However, the more recent studies, especially those carried out on well defined surfaces, have led to a clearer understanding of the scope and limitations of these methods furthermore, the growing awareness of the importance of molecular sieve carbons and zeolites has generated considerable interest in the properties of microporous solids and the mechanism of micropore filling. 

Nitrogen adsorption/desorption isotherms of all the activated carbons are of Type I, i.e. characteristic of basically microporous solids. There is a lack of adsorption/desorption hysteresis. More careful analysis permits to notice significant differences in the porous texture parameters depending on precursor origin. 

The parent zeolites, MOR and BEA, show reversible type-I adsorption/desorption isotherm with a steep rise at pipe, < 0.01, typical for microporous solid while the recrystallized exhibit rather sharp steps at pipe, 0.35, corresponding to the existence of uniform mesopores (typical for MCM-41 phase). According to BJH calculation, the size of the mesopores was about 3.0 nm. The contribution of micro- and mesopores in recrystallized materials was adjusted by variation of alkalinity during recrystallization procedure [2] (Table 1). The formation of mesopores resulted in significant increase of pore volumes of the samples upon recrystallization. 

In order to obtain satisfactory absorption spectra of the substance under investigation it is essential that the beam pass through a large number of adsorbed monomolecular layers. In practice this is most satisfactorily achieved by adsorption on transparent microporous solids with a high surface area to mass ratio (200-600 m2/g). The solids found most suitable have been silica gel, silicic acid, and microporous glass. 

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... 

Fig. 1 shows the benzene adsorption isotherms of samples before and after the boronation. Compared with the parent sample, the isotherms of boronated samples are so slanting at medium and high relative pressure that they deviate greatly from Brunauer Type I curve characteristic of microporous solid. At the same relative pressure, the adsorption capacities of boronated samples are significantly larger than that of parent sample, suggesting that the void volumes in the boronated samples increase and more spaces inside pores are accessible to benzene molecules. N, adsorption isotherms in Fig.2 further show that the hysteresis... 

To achieve a significant adsorptive capacity an adsorbent must have a high specific area, which implies a highly porous structure with very small micropores. Such microporous solids can be produced in several different ways. Adsorbents such as silica gel and activated alumina are made by precipitation of colloidal particles, followed by dehydration. Carbon adsorbents are prepared by controlled burn-out of carbonaceous materials such as coal, lignite, and coconut shells. The crystalline adsorbents (zeolite and zeolite analogues are different in that the dimensions of the micropores are determined by the crystal structure and there is therefore virtually no distribution of micropore size. Although structurally very different from the crystalline adsorbents, carbon molecular sieves also have a very narrow distribution of pore size. The adsorptive properties depend on the pore size and the pore size distribution as well as on the nature of the solid surface. 

The search for a suitable adsorbent is generally the first step in the development of an adsorption process. A practical adsorbent has four primary requirements selectivity, capacity, mass transfer rate, and longterm stability. The lequiiement foi adequate adsoiptive capacity icstiicts the choice of adsorbents to microporous solids with pore diameters ranging from a few tenths to a few tens of nanometers. 

Adsorption. Although several types of microporous solids are useful as adsorbents for the separation of vapor or liquid mixtures, the distribution of pore diameters does not enable separations based on the molecular-sieve effect. The most important molecular-sieve effects are shown by crystalline zeolites. The sieve effect ntay be total or partial. 

Chemical Potential. Equilibrium calculations are based on the equality of individual chemical potentials (and fugacities) between phases in contact (10). In gas—solid adsorption, the equilibrium state can be defined in terms of an adsorption potential, which is an extension of the chemical potential concept to pore-filling (physisorption) onto microporous solids (11—16). 

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

Gas-adsorption processes Involve the selective concentration (adsorption) of one or more components (adsorbates) of a gas (or vapor) at the surface of a microporous solid (adsorbent) The attractive forces causing the adsorption are generally weaker than those of chemical bonds and are such that, by Increasing the temperature of the adsorbent or reducing an adsorbate s partial pressure, the adsorbate can be desorbed The desorption step Is quite Important in the overall process First, desorption allows recovery of adsorbates In those separations where they are valuable, and second, It permits reuse of the adsorbent for further cycles ... 

Type II isotherms (e.g. nitrogen on silica gel at 77 K) are frequently encountered, and represent multilayer physical adsorption on non-porous solids. They are often referred to as sigmoid isotherms. For such solids, point B represents the formation of an adsorbed monolayer. Physical adsorption on microporous solids can also result in type II isotherms. In this case, point B represents both monolayer adsorption on the surface as a whole and condensation in the fine pores. The remainder of the curve represents multilayer adsorption as for non-porous solids. 

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).

Now let us overview the theoretical adsorption models for characterization of the pore structures according to the pore size range. For physical adsorption of the gas molecules on such microporous solids as activated carbons and zeolites, Dubinin and Radushkevich95 developed an empirical equation, which describes the volume filling process in the micropores. Their theory incorporates earlier work by Polanyi96 in regard to the adsorption potential Ad defined as... 

On the other hand, it is impossible to apply the SP method to the correct description of gas adsorption in the micropores, since the adsorption in the micropores does not occur by multilayer adsorption but by micropore volume filling process. In this case, the pore fractal dimension gives a physical importance for the description of structural heterogeneity of the microporous solids. Terzyk et al.143"149 have intensively investigated the pore fractal characteristics of the microporous materials using gas adsorption isotherms theoretically simulated. 

Physical adsorption of gases is, undoubtedly, the most widely used technique [4], Due to the considerable sensitivity of nitrogen adsorption isotherms to the pore texture in both microporous and mesoporous ranges and to its relative experimental simplicity, measurements of subcritical nitrogen adsorption at 77 K are the most used. However, this technique has some limitations, and other complementary techniques are needed for the characterization of microporous solids. 

He adsorption at 4.2 K has been proposed [14-16] as a promising method for the accurate determination of microporosity. The He atom is the smallest one it has a spherical shape and interacts weakly with any solid surface [14], He adsorption requires lower equilibrium times, and the amount adsorbed is higher than in the case of N2 at 77 K. From this research, the authors concluded that the micropore analysis by N2 adsorption at 77 K is insufficient and may give misleading conclusions [14], In spite of the interesting results obtained with He, the experimental conditions used (adsorption at 4.2 K) make this technique unavailable for routine characterization of microporous solids. 

The adsorption on microporous solids is not so well understood as in nonporous or mesoporous solids. When the pore size is similar to the size of the adsorbate molecule and the adsorption temperature is below the critical temperature, the adsorption does not take place by a progressive completion of a monolayer followed by multilayer adsorption, but by the filling of the MPV with the adsorbate in a liquid-like condition. There are a number of problems associated with adsorption in micropores including the following important points [23] ... 

All the aforementioned points about peculiarities of adsorption in micropores show that special attention is needed when microporous solids (i.e., activated carbons, ACFs, nanotubes, CMSs, charcoals, etc.) are characterized by the physical adsorption methods. 

Setoyama N, Ruike M, Kasu T, Suzuki T, and Kaneko K. Surface characterization of microporous solids with He adsorption and small angle x-ray scattering. Langmuir, 1993 9(10) 2612-2617. 

Pinto ML, Pires J, Carvalho AP, and de Carvalho MB. On the difficulties of predicting the adsorption of volatile organic compounds at low pressures in microporous solid The example of ethyl benzene. J. Phys. Chem. B, 2006 110(1) 250-257. 

Adsorption on microporous solids can be described by Dubinin s equation ... 

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