Zeolites molecular species

Qads.(max) = 5.7 molecules by unit cell). Generally speaking, Qacis.(max) is closely related to the molecular size, as it is observed for the other molecular species. Secondly, as shown on Figure 5, sorption isotherm sub-step observation could be another signature of zeolite inner surface influence. Such isotherm sub-step reflects a phase transition existence between a fluid phase and a solid phase stabilized by the inner surface sorption sites. 

Similar neutron diffractogram modifications have been al-ready observed several time during our studies concerning the structural proper-ties of confined molecular species (D2, Ar, N2, Kr, CD4, C2D6) in Silicalite-I zeolite. But for such a MFI type of framework porosity, characterized by a two dimensional micropore network, the intensity of the diffraction peaks (101) and (020), observed at small wave vector Q (A1) values, vanishes completely when increasing the confined phase loading ( as shown on figure 6, for the Ar / Silicalite-I system Ld. = 68 % ). 

The Brunauer type I is the characteristic shape that arises from uniform micro-porous sorbents such as zeolite molecular sieves. It must be admitted though that there are indeed some deviations from pure Brunauer type I behavior in zeoHtes. From this we derive the concept of the favorable versus an unfavorable isotherm for adsorption. The computation of mass transfer coefficients can be accompHshed through the construction of a multiple mass transfer resistance model. Resistance modehng utilizes the analogy between electrical current flow and transport of molecular species. In electrical current flow voltage difference represents the driving force and current flow represents the transport In mass transport the driving force is typically concentration difference and the flux of the species into the sorbent is resisted by various mechanisms. 

Mixed-matrix membranes comprising small-pore zeolite or small-pore non-zeolitic molecular sieve materials will combine the solution-diffusion separation mechanism of the polymer material with the molecular sieving mechanism of the zeolites. The small-pore zeolite or non-zeolitic molecular sieve materials in the mixed-matrix membranes are capable of separating mixtures of molecular species... 

The second part of the book covers zeolite adsorptive separation, adsorption mechanisms, zeolite membranes and mixed matrix membranes in Chapters 5-11. Chapter 5 summarizes the literature and reports adsorptive separation work on specific separation applications organized around the types of molecular species being separated. A series of tables provide groupings for (i) aromatics and derivatives, (ii) non-aromatic hydrocarbons, (iii) carbohydrates and organic acids, (iv) fine chemical and pharmaceuticals, (v) trace impurities removed from bulk materials. Zeolite adsorptive separation mechanisms are theorized in Chapter 6. 

In Table 4.3, the basic characteristics of the most important zeolite molecular sieve species are presented. Zeolite micropore openings are of the same order of magnitude as... 

Basic characteristics of some important zeolite molecular sieve species (Sherman, 1978)... 

Since mesoporous materials contain pores from 2 nm upwards, these materials are not restricted to the catalysis of small molecules only, as is the case for zeolites. Therefore, mesoporous materials have great potential in catalytic/separation technology applications in the fine chemical and pharmaceutical industries. The first mesoporous materials were pure silicates and aluminosilicates. More recently, the addition of key metallic or molecular species into or onto the siliceous mesoporous framework, and the synthesis of various other mesoporous transition metal oxide materials, has extended their applications to very diverse areas of technology. Potential uses for mesoporous smart materials in sensors, solar cells, nanoelectrodes, optical devices, batteries, fuel cells and electrochromic devices, amongst other applications, have been suggested in the literature.11 51... 

In principle, the determination of molecular uptake may be based on any experimentally accessible quantity which is a function of the amount adsorbed. Being directly sensitive to a certain molecular species, in this respect the application of spectroscopic methods is particularly suitable. IR spectroscopy has been successfully applied to studying molecular uptake by beds of zeolite catalysts [26-28] as well as—in combination with IR microscopy [29, 30]—on individual crystallites. Similarly, NMR spectroscopy has also been used to monitor the time dependence of the sorbate concentration within porous media [31]. Moreover, recent progress in NMR imaging allows the observation of concentration profiles within porous media with spatial resolution below the mm region [32-34],... 

The molecular sieves are aluminosilicates of particular importance. They are crystalline materials that have open structures that contain pores and channels that have molecular dimensions. This family of materials includes the zeolites, which have numerous applications in heterogeneous catalysis, ion-exchange materials, absorption of molecular species, and gas separation. While some zeolites occur naturally, they are usually manufactured from silicon and aluminum oxides mixed with tetraaUcylammonium templates in a high-pressure autoclave (see Zeolites). 

The previous chapters and examples were meant to familiarize the reader with current conventional ESR methodology and remove any barriers to the application of this kind of spectroscopy in electron-transfer studies. Unavoidably, not all aspects of inorganic ESR could be treated properly for instance, both the ESR spectroscopy of bioinorganic systems (e.g. heme species or iron-sulfur clusters) or materials chemistry (e.g. defect centers, radicals in zeolites) are vast and entirely independent research areas. Nevertheless, the molecular species presented here might serve as introductory examples to illustrate the versatility of this particular kind of magnetic resonance. 

Zeolites are a subclass of microporous materials in which the crystalline inorganic framework is composed of four-coordinated species interconnected by two-coordinated species. Traditionally these materials are aluminosilicates however, many different compositions have been synthesized. The templates used in the synthesis of microporous materials are typically small ionic or neutral molecular species. The function of the template in the synthesis of microporous materials is little understood, and there are at least four different modes by which an additive can operate in a zeolite synthesis a) It may act as a space filler occupying the voids in the structure, thereby energetically stabilizing less dense inorganic framework b) the additive may control the equilibria in the synthesis mixture, such as solution pH or complexation equilibria c) it may preorganize the solution species to favor the nucleation of a specific structure d) it may act as a true template determining the size and the shape of the voids in the structure. 

Occlusion or adsorption of molecular species by zeolites and related... 

Certain positions in the inner walls of the micropores of the zeolites serve as active sites, where catalytic conversions can take place. The size of the micropores and the location of cavities can be so adjusted that only one type of molecular species can reach the active sites. The regioselectivity in zeolites is demonstrated in the oxidation of alkenes to a single hydroperoxide [150c]. Zeolites contain many acidltlc sites [154]. The number of different types of acidic sites in a zeolite depends on the method of activation of the zeolite. Using NMR, Cao et al. demonstrated that CaY zeolites activated by heating in air contain more Bronsted acid sites than those activated in vacuum and this explains the difference in reactivity of dlarylethylenes in zeolite CaY [150d]. 

The equations and plots presented in the foregoing sections largely pertain to the diffusion of a single component followed by reaction. There are several other situations of industrial importance on which considerable information is available. They include biomolecular reactions in which the diffusion-reaction problem must be extended to two molecular species, reactions in the liquid phase, reactions in zeolites, reactions in immobilized catalysts, and extension to complex reactions (see Aris, 1975 Doraiswamy, 2001). Several factors influence the effectiveness factor, such as pore shape and constriction, particle size distribution, micro-macro pore structure, flow regime (bulk or Knudsen), transverse diffusion, gross external surface area of catalyst (as distinct from the total pore area), and volume change upon reaction. Table 11.8 lists the major effects of all these situations and factors. 

Chemical sensors are small devices for the detection and quantification of gaseous or solvated species. This is an active research area based on the need to obtain increasing amounts of data in chemical and food process streams as well as environmental monitoring. Most sensors consist of an appropriate transduction principle such as the quartz-crystal-microbalancc (QCM) and a chemically sensitive layer that imparts the desired chemical response behaviour. Most often a chemically selective response is desirable. Zeolite molecular sieves offer size- and shape-selective adsorption behaviour that can be combined with appropriate transduction concepts in order to construct chemically selective sensor devices. 

When two or more molecular species involved in a separation are both adsorbed, selectivity effects beeome important beeause of interaction between lha zeolite and the ndsorbate molecules. These interaction energies include dispersion and short-range repulsion energies, polarization energy, and components attributed to electrostatic interactions. 

As an alternative to x-ray imaging [10], NMR spin mapping has been repeatedly applied to the determination of concentration profiles during the penetration of molecular species into both porous [19-21] and compact [22,23] media. It is just this method that is of particular relevance for determining molecular distributions within beds of catalysts or adsorbents. As an example of the procedure. Fig. 4 shows the distribution of butane molecules within a bed of crystallites of zeolite NaCaA as determined by H NMR spin mapping during adsorption [ 19. The zeolite crystallites were packed into an NMR sample tube to... 

Research on the immobilization of metal complexes using carbon materials is scarce compared with inorganic supports, such as zeolites, silicas, and clay-based materials [1-10]. Nevertheless, carbon materials are unique supports, as they can provide a variety of surface groups at the edges and/or defects of graphene sheets that can be tailored by adequate thermal or chemical treatments, besides the inherent chemical-physical reactivity associated with the graphene sheets themselves, which are hydrophobic, have low polarity, and have a rich n-electron density [13-15]. This can lead to a huge diversity of methods for immobilization of molecular species. 

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