Adsorption in zeolites

The study of sorption of guests within zeolite hosts is complementary to the study of diffusion in zeolites. Having discussed the pathways and trajectories of molecules through micropores, we now consider the favored sorption locations, conformations of sorbates, and sorption energetics. Indeed, so close are the two subjects that they are frequently considered within the same paper. 

It is therefore unsurprising that the MD and TST methods used to characterize diffusion processes are also used to simulate sorption. In the theoretical methodologies section that follows, these methods are not mentioned further as they were summarized in the preceding section. Monte Carlo methods are discussed in detail, including a recently developed technique to simulate the location and adsorption of longer chain molecules than would normally be possible by using conventional methods. Furthermore, we present the methodology of a combined MD/Monte Carlo/EM tech- 

The method of importance sampling confines the exploration of configurational space to regions of significant probability. In general, a particle is selected and displaced in a particular direction. In the case of a molecule, there is the possibility of displacement and rotation of the molecule about a fixed axis. The direction and degree of movement are selected at random. The energy of the new trial state, tria, is accepted if it is more favorable than the previous, initiai, or if 

Monte Carlo simulations are performed within a statistical ensemble. In the canonical ensemble (with the number of molecules, volume, and temperature fixed), the average value of a thermodynamic quantity, (T(x)), as a function of the states of system, x, is given by 

The parameter determined from simulations that can be most readily compared to an experimentally observable quantity is the heat of sorption. The mean total internal energy determined by the simulations, (V), can be equated to the isosteric heat of adsorption, Qst, in the limit of low occupancy, as follows  

The study of zeolites as adsorbent materials began in 1938 when Professor Barrer published a series of papers on the adsorptive properties of zeolites [28], In the last 50 years, zeolites, natural and synthetic, have turned out to be one of the most significant materials in modem technology [27-37], Zeolites have been shown to be good adsorbents for H20, NH3, H2S, NO, N02, S02, C02, linear and branched hydrocarbons, aromatic hydrocarbons, alcohols, ketones, and other molecules [2,31,34], Adsorption is not only an industrial application of zeolites but also a powerful means of characterizing these materials [1-11], since the adsorption of a specific molecule gives information about the microporous volume, the mesoporous area and volume, the size of the pores, the energetics of adsorption, and molecular transport. 

The Physical Chemistry of Materials Energy and Environmental Applications 

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For adsorption in zeolites, the biased Monte Carlo method as developed by Smit is an excellent method to determine the free energies of molecules adsorbed on zeolites [9bj. This method can be used to compute the concentration of molecules adsorbed on zeolites, as we discuss below. 

The low-temperature physisorption (type I isotherm) of hydrogen in zeolites is in good agreement with the adsorption model mentioned above for nanostructured carbon. The desorption isotherm followed the same path as the adsorption, which indicates that no pore condensation occurred. The hydrogen adsorption in zeolites depends linearly on the specific surface areas of the materials and is in very good agreement with the results on carbon nanostructures [24]. 

NH3 is similar to H2O in that they both possess large dipole moments and are both small molecules. The presence of NH3 in a zeolite is chemically similar to the presence of H2O in a zeolite. Therefore, the hydrated cation distribution in zeolites is probably more typical of NH3 adsorption in zeolites than the dehydrated cation distribution. According to Breck (18), for hydrated zeolite X, cations are found in sites SI, SI, SII, and SIV. Of these sites, SI, SII, and SIV would all be adsorption lattice solution sites. The cationic and anionic lattice solution sites (in the supercavity of NaX) are illustrated in Figure 8. For NH3, the subscript J1 will refer to SII sites, the subscript J2 will refer to SI sites, and J3 will refer to SIV sites. The anionic sites are two and are (l) in the center U-membered ring of the connecting frame and (2) near the center of the 0(2)—0(1)—0(l) triad of oxygen atoms. For NH3, the subscript il will refer to the first anionic site the subscript i2 will refer to the second anionic site. 

Because of the conceptual and mathematical simplicity of these models, they have been used recently to describe adsorption in zeolite and clay structures,53,54,55,56 based on either a cylindrical pore model or a packed sphere model. 

Subsequently, the parameters, T0 and AT, describe the desorption rate since, these parameters also represent the Gaussian function. Therefore, with the obtained results (Figure 4.40 and Table 4.9), it was shown that the parameter T, is associated with the adsorption energy of the adsorbate in the zeolite. In addition, the parameter AT was linked with the transport of molecules inside the zeolites channels during the nonisothermal desorption process as well with the heterogeneous character of adsorption in zeolites. 

Equation 5.21a is known in literature [10] as the Sips or Bradleys isotherm equation. This isotherm equation describes fairly well the experimental data of adsorption in zeolites and other microporous materials [26], The linear form of the osmotic equation can be expressed as follows... 

Many other equilibrium relationships have been applied to model sorption. For example, the Langmuir (Eq.44) and the Polanyi-Dubinin (Eq.45) isotherms have been widely applied to adsorption in zeolites [9] ... 

Similarly as for carbon materials, the maximum hydrogen uptake in different zeolites shows a good correlation with the BET specific surface area measured by nitrogen adsorption [73]. Beyond the specific surface area the interaction of hydrogen molecules with zeolites is strongly influenced by the type and the concentration of cations present in the framework channels. This is well reflected by, for example, the increase in the isosteric heat of adsorption in zeolites with increasing aluminum... 

Weitkamp, J. Schwaik, M. Emest, S. Removal of Thiophene Impurities from Benzene by Selective Adsorption in Zeolite ZSM-5. J. Chem. Soc. Chem. Commun., 1991, 1133. 

Adsorption plays an important role in permeation through microporous membranes. First of all, steps 1 and 5 involve adsorption and desorption processes. Second, the concentration dependence of the diffusion coefficient is often described by the adsorption isotherm. Some data on adsorption in zeolites will be presented in Section III.D. 

Adsorption in zeolites depends highly on the characteristics (type of cation, Si/Al ratio) of the zeolite [2]. The adsorption isotherms in zeolites are mostly of type I, which means that monolayer adsorption takes place and a saturation concentration is observed. Extensive reviews on adsorption on microporous media are given in several textbooks [48,49]. 

Although there are many ways to describe a zeolite system, models are based either on classical mechanics, quantum mechanics, or a mixture of classical and quantum mechanics. Classical models employ parameterized interatomic potentials, so-called force fields, to describe the energies and forces acting in a system. Classical models have been shownto be able to describe accurately the structure and dynamics of zeolites, and they have also been employed to study aspects of adsorption in zeolites, including the interaction between adsorbates and the zeolite framework, adsorption sites, and diffusion of adsorbates. The forming and breaking of bonds, however, cannot be studied with classical models. In studies on zeolite-catalyzed chemical reactions, therefore, a quantum mechanical description is typically employed where the electronic structure of the atoms in the system is taken into account explicitly. 

R. Shah, J. D. Gale, and M. C. Payne, /. Phys. Chem., 100, 11688 (1996). Methanol Adsorption in Zeolites—A First-Principles Study. 

Figure 3.2 Theoretical (x-axis) vs. measured (y-axis) heats of adsorption in zeolite Y and silicalite for a range of alkanes. The theoretical heats are multiplies by the "volume" of a methyl group (a ).

N2 adsorption in zeolites NaX and NaY is similar but higher than in the silicalite (S-1), which has a more dense structure (Table 2). In the case of the exchanged zeolites type Y (Figure 2), the amount of N2 adsorbed by the CaY sample is larger than for the NaY zeolite. The first material has half the amount of cations in comparison with NaY, but with similar size. Additionally, the increase in the size of the cation (Ca", Sr", Ba ) causes a decrease in the amount of N2 adsorbed. 

Fig. 7. Characteristic curves obtained from CO2 adsorption in zeolite NaY at 273K and 298K.

In many cases single gas adsorption in zeolites can be adequately described by a Langmuir-t3qje adsorption isotherm as given in Section 9.2.23. ... 

It is well known that water adsorption in zeolites releases heat while water desorption absorbs heat. This phenomenon has been exploited for the design of a heat pump for cooling applications using waste heat or solar energy. Currently, zeolite pellets are used, and the heat and mass transfer are inefficient, leading to bulky pumps. Recently, there have been renewed interests in zeolite coating heat pumps. A new in situ... 

Again, we do not exhaustively discuss molecular theories of diffusion and adsorption in zeolites but refer to other studies [32-34]. However, we highlight some important results significant to the kinetic analysis we are presenting. 

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