Pore architecture

Pore architecture High surface area High accessibility... 

MOFs can be considered as organic zeolite analogs, as their pore architectures are often reminiscent of those of zeolites a comparison of the physical properties of a series of MOFs and of zeolite NaY has been provided in Table 4.1. Although such coordinative bonds are obviously weaker than the strong covalent Si-O and Al-O bonds in zeolites, the stability of MOF lattices is remarkable, especially when their mainly organic composition is taken into account. Thermal decomposition generally does not start at temperatures below 300 °C [3, 21], and, in some cases. 

This category includes a large variety of silica, zirconia, and alumina mesoporous films. Although the inorganic scaffold of such layers does not transport electric current, the pore architecture, which can be also used as a host matrix for incorporation of functional molecules, can alter electron transport to and from the conducting surface, thus influencing electronic properties of the complete system. 

MCM-41 is most extensively studied member of the M41S family because of its hexagonal array of unidimensional pore architecture. In addition to catalysis, MCM-41 type mesoporous materials are increasingly being explored for a variety of different applications, such as support, as sensors / carriers, surface modification etc. 

Zhorov BS, Tikhonov DB. Potassium, sodium, calcium and glutamate-gated channels pore architecture and ligand action. J Neurochem. 2004 88 782-799. 

Figure 6 Selectivity benefit of improved pore architecture.

Figure 7 Simplified Pore Architecture of fresh catalysts.

Conversion of these coke precursors prior to the formation of the coke. It is to some extent debatable whether this can be done. Some improvements are possible using an improved catalyst pore architecture and acidity distribution. 

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>. 

Hierarchical pore architectures combining microporous and mesoporosity 13... 

HIERARCHICAL PORE ARCHITECTURES COMBINING MICROPOROUS AND MESOPOROSITY... 

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). 

Fouling can result in bigger differences in selectivity of various catalysts, because of changes in pore architecture [2, 10]. 

Figure 10 groups the parameters according to geometry, bulk defects, surface phenomena, and extrinsic modifications. The geometry of a catalyst particle is given by its size, its habitus (meaning the anisotropy or deviation from a spherical shape), and by its pore system. Only for micro-and mesoporous samples is XRD a sensitive tool to determine the pore architecture (Chen et al., 2005 Davidson, 2002 Li and Kim, 2005 Liu et al., 2002 Ohare et al., 1998). In many solids that are more compact than most catalysts, only secondary effects are related to the pores. 

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) ... 

If the charge balancing cation in a zeolite is then the material is a solid acid that can reveal shape selective properties due to the confinement of the acidic proton within the zeolite pore architecture. An example of shape selective acid catalysis is provided in Figure 5.3.7. In this case, normal butanol and isobutanol were dehydrated over CaX and CaA zeolites that contained protons in the pore structure. Both the primary and secondary alcohols were dehydrated on the X zeolite whereas only the primary one reacted on the A zeolite. Since the secondary alcohol is too large to diffuse through the pores of CaA, it cannot reach the active sites within the CaA crystals. 

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). 

Finally, 29xe is a very suitable and sensitive isotope for probing the pore architecture of zeolitic materials. The extended Xe electron cloud is easily deformable due to interactions between, e.g. the Xe atoms and the channel wall of a zeolitic ftamewoik, and deformation results in a large low-field shift of the Xe resonance. In addition, 129xe NMR can be used to study metal particles in zeolites, while reduction-oxidation reactions can be monitored (13). Table 2 summarizes the NMR properties of a number of nuclei which have been used in NMR investigations of zeolitic materials (11). 

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. 

Planar faults are common in zeolites and related crystalline microporous solids. These can influence the sorptive characteristics in any one of several ways (i) they can have little influence on the overall accessibility or capacity, but alter the pore architecture, accessibility or difiusional constraints (ii) they can reduce the limiting dimensions of pore windows while leaving the tot pore volume unaffected (iii) they can block channels. Pores or pore access can also be blocked by detrital material such as alumina extracted from the framework, coke or sintered metal catalyst particles, immobile organic molecules or non-framework cations in blocking positions. 

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