Zeolitization characteristics

Adequate choice and adjustment of the zeolite characteristics, small path of diffusion (small crystallites, mesopores), good balance between hydrophobi-city and acidity. 

This chapter addresses the fundamentals of zeolite separation, starting with (i) impacts of adsorptive separation, a description of liquid phase adsorption, (ii) tools for adsorption development such as isotherms, pulse and breakthrough tests and (iii) requirements for appropriate zeolite characteristics in adsorption. Finally, speculative adsorption mechanisms are discussed. It is the author s intention that this chapter functions as a bridge to connect the readers to Chapters 7 and 8, Liquid Industrial Aromatics Adsorptive Separation and Liquid Industrial Non-Aromatics Adsorptive Separation, respectively. The industrial mode of operation, the UOP Sorbex technology, is described in Chapters 7 and 8. 

One of the most signiflcant variables affecting zeolite adsorption properties is the framework structure. Each framework type (e.g., FAU, LTA, MOR) has its own unique topology, cage type (alpha, beta), channel system (one-, two-, three-dimensional), free apertures, preferred cation locations, preferred water adsorption sites and kinetic pore diameter. Some zeolite characteristics are shown in Table 6.4. More detailed information on zeolite framework structures can be found in Breck s book entitled Zeolite Molecular Sieves [21] and in Chapter 2. 

Two categories of mesoporous solids are of special interest M41S type materials and pillared or delaminated derivatives of layered zeolite precursors (pillared zeolites in short). The M41S family, first reported in early 1990 s [1], has been extensively studied [2,3]. These materials exhibit broad structural and compositional diversity coupled with relative ease of preparation, which provides new opportunities for applications as catalysts, sorption and support media. The second class owes its existence to the discovery that some zeolite crystallizations can produce a lamellar intermediate phase, structurally resembling zeolites but lacking complete 3-dimensional connectivity in the as-synthesized form [4]. The complete zeolite framework is obtained from such layered zeolite precursor as the layers become fused, e.g. upon calcination. The layers posses zeolitic characteristics such as strong acidity and microporosity. Consequently, mesoporous solids derived from layered zeolite precursors have potentially attractive characteristics different from M41S and the zeolite species... 

Zhang and co-workers reported partial conversion of a mesoporous starting material (SBA-15) into a mesoporous aluminosilicate with zeolitic characteristics in a so-called vapour phase transport method.[82] In this process, Al is firstly introduced onto the mesoporous surface, followed by a filling of the mesopores with a carbonaceous species, and finally a partial recrystallization of aluminosilicate in the vapour of the SDA is conducted. The advantage of this method, compared with the hydrothermal recrystallization method of Kloetstra et al., lies in the fact that the mesopore structure collapses to a lesser extent as the crystallization is limited to the surface of the mesoporous precursor. 

Zhang, Y. W., Okubo, T. and Ogura, M. Synthesis of mesoporous aluminosilicate with zeolitic characteristics using vapor phase transport, Chem. Commun. 2005, 2719-2720. 

This short analysis of water adsorption over zeolites indicates that before pretreatment all the zeolite samples contain a certain amount of water depending on the storage and handling conditions as well as on the zeolite characteristics. Moreover, water adsorption at room temperature is so fast and occurs from so very low partial pressures that even exposure of the dry zeolite to the atmosphere for a short time results in the uptake of moisture. 

This simplified description of dealumination by steaming shows that the increase of the framework Si/Al ratio is accompanied by other changes in the zeolite characteristics. The partial collapse of the zeolite leads to the creation of secondary pores (supermicropores and mesopores) at the expanse of the initial micropores. The size and number of secondary pores increase with the degree of framework dealumination38. These secondary pores were shown to have a pronounced positive effect in liquid phase reactions which are often limited by product desorption. For these reactions (e.g. in alkylation of toluene with 1-heptene)32, the maximum in activity is significantly displaced to high values of the framework Si/Al ratio. These secondary pores were also... 

In the past 15 years, a number of research groups have investigated, often in batch reactors, the use of zeolites to catalyse liquid phase aromatic acylation. Good activities and selectivities have been achieved with activated aromatic substrates. Experiments in flow reactors show that deactivation is the main problem to be solved, which can be made by an adequate adjustment of the zeolite characteristics and especially of the operating conditions. 

One way to qualitatively check and/or discover some possible relations between the considered parameters (reaction conditions, zeolite characteristics) and the reaction efficiency (conversion of reactants, selectivity and yield of products), is to plot these parameters from selected experiments and to compute correlation coefficients. 

Easy chemical manipulation of all catalytic sites is an important and widely recognized zeolite characteristic. The near uniformity of the intracrystalline surface (in the absence of protic sites) provides an excellent opportunity to treat active sites uniformly. With protic zeolites, the presence of extra-framework alumina and silica-alumina phases and the need to optimize interaction between protic and Lewis acid sites make chemical manipulation complex, particularly with aluminum-rich zeolites. 

When considering as yet unexploited zeolite characteristics, we may need to consider recent electronic technology, particularly the formulation of computer chip sur ce structures, which approaches the atomic scale. Although zeolites do not possess electronic properties, their surfaces have a great variety of repeated pores that can be doped with metals or oxides. Such treatments may also introduce desired electronic characteristics. 

Another potentially interesting zeolite characteristic is the nature of gas diffiision in the intracrystalline pores. It has been suggested in the literature that certain adsorbed gas molecules close in size to the zeolite pores float within non polar zeolite crystals, instead of the standard adsorption-desorption mechanism. This concept opens the possibility that under certain circumstances, the emission of desorbed gas molecules may be directionally coherent as it emerges fi om each zeolite crystal face. Such a coherent gas emission - "a molecular laser" - may find applications in catalytic combustion or in other applications benefitting from "non thermalized" gas emissions. 

Based on these data and on the characterization of the acidity of zeolites by NH3-TPD cited previously, but not reported for brevity, it is possible to draw the following conclusions on the relationships between nature of vanadium species, zeolite characteristics and catalytic behavior ... 

Zeolites (characteristic length L = Volume/outer surface)... 

For an overview of the results, consider Table 1 which shows how the products of photolysis of DBK may be varied in a "catalytic" manner by varying zeolite characteristics. First consider the samples in the absence of any added guest molecules. The major product of photolysis of DBK adsorbed on MY is DPE (>90%) and is completely independent of M for the samples in the absence of added guests. In contrast, the major product of photolysis of DBK adsorbed on MX zeolites is strongly dependent on M for the samples in the absence of added guests (Table 1), with DPE the major product for LiX, p-DBK the major product for NaX, and o-DBK the major product for KX. 

Silanation of zeolites through chemical vapour deposition of tetraethyl orthosilicate has been proved to be a very convenient and useful laboratory technique to modily zeolite characteristics in order to make it highly selective for para-dialkyl benzenes specially with... 

The active surface of a zeolite is internal and intrinsic to the crystal structure. Diffraction techniques can therefore yield direct data on those structural features that control catalytic or sorptive performance. However, zeolite characteristics hamper the effective application of diffraction methods. Zeolite constituents have, generally, low atomic numbers and the normalized scattering power of a zeolite unit cell is relatively small. Zeolites have open firework structures supporting accessible void volumes which can be as much as 50% of the total crystal volume [1-3]. The void spaces are either empty (and hence contributing no scattered intensity to the measured diffraction pattern) or filled with species that are have positional or dynamic disorder and hence contribute to the diffraction peaks almost exclusively at low scattering angles. 

Metal-catalyzed reduction of methylviologen metal clusters internal or external location of metal cluster in zeolite Characteristic blue color of the reduced viologen radicals makes it ea to locate dusters. 

Here we focus on the outline of the phenomenon of NMR and basic approaches of both the use of solid-state NMR to derive the information on zeolite characteristics, dynamics of adsorbates, and reaction occurrence, and the performance of the solid-state NMR experiment itself to derive any particular useful for researcher information. 

Incidentally, these zeolites have been identified from the XRD diffractograms of the samples, as presented in Table 5.38. The mineralogical phase transition in the activated residues also reveals the higher zeolitization characteristics of the OHA for synthesizing a class of zeolites with a slightly higher SAR value (2.64 > SAR > 1.60), which is in close proximity to common fly ash zeolites, Faujasite and Analcime [47]. 

Incidentally, a decrease in G of the residues may be attributed to (i) enhanced formation of porous compounds and (ii) polymerization of Si and A1 tetrahedra, which also corresponds to increased volume of a zeolitic framework stmcture, present in the residues. Based on variations of G and SSA it can be inferred that Steps-2 and 3 (the R2), are indicative of better zeolitic characteristics of the residues (i.e., purified form of residues of Step-1). 

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