Pore architectures with

Based upon their well-defined, crystalline pore architecture with channel sizes at molecular dimensions, zeolites are becoming increasingly attractive as hosts for a... 

Designing Pore Architectures with Structure-Directing Agents... 

Figure 7. Conformational flexibility of the brick host architecture leads to three distinct sub-architectures with respect to the rotation of the pillars and the dimensionality of the pore structure in the galleries.

Figure 3.6 Some synthetic possibilities of self-assembly with surfactants (a) different micelle geometries can be obtained and used to produce many pore architectures,...

Both piperidine and pyridine serve as structure-directing agents in the commercial production of Ferrierite zeolite. More recently, use of DMAP has allowed preparation of novel metallo-aluminophosphate molecular sieves with both small- <2006W02006037437> and large-pore architecture <2006USA074267>. 

The inherent limitations of the use of zeolites as catalysts, i.e. their small pore sizes and long diffusion paths, have been addressed extensively. Corma reviewed the area of mesopore-containing microporous oxides,[67] with emphasis on extra-large pore zeolites and pillared-layered clay-type structures. Here we present a brief overview of different approaches to overcoming the limitations regarding the accessibility of catalytic sites in microporous oxide catalysts. In the first part, structures with hierarchical pore architectures, i.e. containing both microporous and mesoporous domains, are discussed. This is followed by a section on the modification of mesoporous host materials with nanometre-sized catalytically active metal oxide particles. 

It is clear that the Wacker cycle in a CuPdY zeolite incorporates the traditional features of the homogeneous catalysis combined with typical effects of a zeolite (303, 310). It also follows that whereas other cation exchangers in principle will show Wacker activity after cation exchange with Cu/Pd ions, the cage and pore architecture will probably be less suitable for Wacker chemistry than those of the faujasite structure. This is the case for fluoro-tetrasilicic mica, a synthetic layer silicate that swells under reaction conditions and allows access to the interlayer space (311). 

Separation of molecules with different sizes can be achieved by a proper choice of zeolites (nature of the zeolite and adjustment of the pore architecture, especially the pore size). The simplest forms of shape selectivity come from the impossibility of certain molecules in a reactant mixture entering the zeolite pores (reactant shape selectivity) or of certain product molecules (formed inside the pore network) exiting from these pores (product shape selectivity). In practice, reactant and product shape selectivities are observed not only when the size of molecules is larger than the size of the pore apertures (size exclusion) but also when their diffusion rate is significantly lower than that of the other molecules. Differences of diffiisivities by 2 orders of magnitude are required to produce significant selectivities between reactant species (35). 

Another effect of zeolite pore architecture on esterification is found in the lactonization of 15-hydroxypentadecanoic acid. With dissolved acid catalysts or with amorphous Si02-Al203, dimerization or polymerization are the dominant reactions, but when the reactant is adsorbed within the pores of a dealuminated HY zeolite, only the pentadecanolide is obtained (8) ... 

Clear-cut examples of effects of zeolite pore architecture on the selectivity of Diels Alder reactions are not easily found. For instance, 4-vinylcyclohexene is formed with high selectivity from butadiene over a Cu -Y zeolite however, the selectivity is intrinsically due to the properties of Cu1, which can be stabilized by the zeolite, and not to the framework as such (30-31). A simple NaY has been used in the cycloaddition of cyclopentadiene and non-activated dienophiles such as stilbene. With such large primary reactants, formation of secondary products can be impeded by transition state shape selectivity. An exemplary reaction is the condensation of cyclopentadiene and cis-cyclooctene (32) ... 

Since their discovery, microporous materials such as zeolites found major application fields in processes like separation, ion exchange and catalysis. Their uniform pore size and pore architecture are at the basis of separation processes whereas the chemical composition of these materials makes them unbeatable candidates to be used as a catalyst or an ion exchanger. Regardless of which process is used, the molecules engaged are adsorbed on the surface according to their molecular structure and properties. The bulkiness of the molecule compared to the pore size of the microporous material decides if or not the molecule can be trapped in the depth of the porous framework, thus there exists cases where molecules with larger diameters than the pore size are not able to enter the pores. This makes the microporous materials acting as a sieve in molecular level and they are hence referred to as molecular sieves. 

Amorphous Sn-, Si-, and Al-containing mixed oxides with homogeneous elemental distribution, elemental domains, and well-characterized pore architecture, including micropores and mesopores, can be prepared under controlled conditions by use of two different sol-gel processes. Sn-Si mixed oxides with low Sn content are very active and selective mild acid catalysts which are useful for esterification and etherification reactions [121]. These materials have large surface areas, and their catalytic activity and selectivity are excellent. In the esterification reaction of pentaerythritol and stearic acid catalytic activity can be correlated with surface area and decreasing tin content. The trend of decreasing tin content points to the potential importance of isolated Sn centers as active sites. 

Solid-state NMR spectroscopy is nowadays a well established technique for characterization of zeolites and other porous materials with respect to structure elucidation, pore architecture, catalytic behaviour and mobility properties (like diffusion). The objective of this paper is to highlight recent solid-state NMR results of zeolitic materials, based on new techniques, methods and pulse sequences. The intention is not to review recent NMR results, since a large number of such papers is easily available and one of the latest was presented during the 10th IZC Summer School on Zeolites in Wildbad Kreuth, Germany, two years ago (1). 

One of the main advantages of application of zeolitic or other porous materials is the shape-selectivity of this type of material, which arises due to differential diffusion of molecules with different sizes and shapes in the zeolitic or other porous materials. Iherefore, it is very instructive to monitor the pore architecture directly, with a molecule that "observes" the zeolitic type of structure. 

Advancements in the preparation of new PLS s nearly parallels that of the zeolite and zeolite-like phases. Initially the pillared smectite clays were investigated but the quest for new materials with new properties led to e qiloring the pillaring of other layered phases. These include, most notably, the layered zirconium phosphates, double hydroxides (hydrotalcites), sihcas and metal oxides. The parallel paths of discovery in new material compositions for the layered phases and the microporous (zeoUte) phases are summarized in Table 1. A conq>arison between the pore architectures of the zeohtes and the two dimensional PLS is shown in Table 2. 

Recently a strong synthetic effort has been devoted to developing amorphous silica-aluminas with controlled porosity, simultaneously new models for description of pore architecture have been studied. 

However, the shape of both the curves is different- This is caused through differences in pore architecture leading to a sharper autocatalytic and a more moderate retardative effect with the wide indow and wide-cavity HLfSY zeolite. 

Microporous materials with regular pore architectures comprise wonderfully complex structures and compositions.[1,2] Their fascinating properties, such as ion-exchange, separation, catalysis, and their roles as hosts in nanocomposite materials, are essentially determined by their unique structural characters, such as the size of the pore window, the accessible void space, the dimensionality of the channel system, and the numbers and sites of cations, etc. 

J.F. Wang, C.-K. Tsung, W.B. Hong, Y.Y. Wu, J. Tang, and G.D. Stucky, Synthesis of Microporous Silica Nanofibers with Controlled Pore Architectures. Chem. Mater, 2004,16, 5169-5181. 

Microporous materials with regular pore architectures comprise wonderfully complex structures and compositions. Their fascinating properties, such as ion-exchange, separation, and catalysis, and their roles as hosts in nanocomposite materials, are essentially determined by their unique structural characters, such as the size of the pore window, the accessible void space, the dimensionality of the channel system, and the numbers and sites of cations, etc. Traditionally, the term zeolite refers to a crystalline aluminosilicate or silica polymorph based on comer-sharing TO4 (T = Si and Al) tetrahedra forming a three-dimensional four-connected framework with uniformly sized pores of molecular dimensions. Nowadays, a diverse range of zeolite-related microporous materials with novel open-framework stmctures have been discovered. The framework atoms of microporous materials have expanded to cover most of the elements in the periodic table. For the structural chemistry aspect of our discussions, the second key component of the book, we have a chapter (Chapter 2) to introduce the structural characteristics of zeolites and related microporous materials. 

Transport phenomena in porous solids have been the subject of many studies [1-6,10]. Quantitative solutions are obtained however only in a number of limiting cases of generally formulated problems or in relatively simple cases. Such a case is, e.g., the permeation of a single gas in a membrane system with a relatively simple pore architecture and under conditions when a single mechanism is predominantly operating. 

By integrating optimized acid sites with superior mass transport characteristics and a pore architecture that reduces pore-mouth plugging, a catalyst with enhanced performance can be created. Figure 5 demonstrates that both the catalyst selectivity and lifetime are significantly improved. As shown in figure 6, which compares the performance of Exelus solid-acid catalyst with other commercially available systems, the new catalyst system is easily able to achieve a step-change in performance over other solid-acid catalysts. 

First-of-its kind results [14,15] with electron tomography for the study of zeolites have been obtained with Ag/Na-Y. The location of the Ag particles of about 10 nm could be unequivocally established with respect to the surface and the interior of the crystals. The moderate resolution of 5 nm of the reconstructed images in this study related to the rather large zeolite crystals (500 nm) involved. Hereafter, we first discuss in more detail the use of electron tomography to study mesopores in zeolite crystals and, secondly, case studies involving metal particles and pore architecture of ordered mesoporous materials. 

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