Ordered microporous structures

More recently, various attempts have been made to develop cracking catalysts from pillared smectite clays, in which the layers are separated and held apart by the intercalation of large cations. Pillared clays (PILCs) have large surface areas within fairly well-ordered micropore structures (pore widths in die approximate range 0.6-1.2 nm). It is not surprising that these materials have attracted considerable interest with the prospect of an alternative type of catalytic shape selectivity (Thomas, 1994 Thomas etal., 1997 Fripiat, 1997). 

Figure 32. (a) Schematic representation of hierarchical self-organization of 30 into ordered microporous structure, (b) Fluorescence photomicrograph of solution-cast micellar film of 30 with m = 10 and n = 300. (Reprinted with permission from ref 109. Copyright 1999 American Association for the Advancement of Science). 

Microporous nanoparticles with ordered zeolitic structure such as Ti-Beta are used for incorporation into walls or deposition into pores of mesoporous materials to form the micro/mesoporous composite materials [1-3], Microporous particles need to be small enough to be successfully incorporated in the composite structure. This means that the zeolite synthesis has to be stopped as soon as the particles exhibit ordered zeolitic structure. To study the growth of Ti-Beta particles we used 29Si solid-state and liquid-state NMR spectroscopy combined with x-ray powder diffraction (XRPD) and high-resolution transmission electron microscopy (HRTEM). With these techniques we monitored zeolite formation from the initial precursor gel to the final Ti-Beta product. 

McCusker, L.B., Liebau, F., and Engelhardt, G. (2003) Nomenclature of structural and compositional characteristics of ordered microporous and mesoporous materials with inorganic hosts (lUPAC recommendations 2001). Micropor. Mesopor. Mat., 58 (11), 3-13. 

Diffusivities of xylene isomers in MCM-41 samples are of the same order of magnitude as in NaX zeolite indicating the existence of additional microporous structure in MCM-41 materials. On the other hand diffusion of bulkier 1-3-5 triisopropylbenzene (1,3,5 TIPB) is about one order of magnitude faster than in NaX indicating that diffusion of this sorbate, and of the bulkier perfluorotributylbenzene (PFTBA), occurs in the MCM-41 cylindrical mesopores. 

Relatively straightforward is the definition of nanoscopic voids. Nanopores and nanocavities are elongated voids or voids of any shape, and nanomaterials can incorporate especially nanopores in an ordered or disordered way. The former is of crucial importance for many of the hybrid materials discussed in the book (e.g., in Chapters 16 or 18). Nanochannel is also frequently used instead of nanopore, often in biological or biochemical contexts. Besides nanoporous, the term mesoporous is often found in hybrid materials research. Interestingly, the IUPAC has defined the terms mesoporous (pores with diameters between 2 and 50 nm), microporous (pores with diameters <2 nm) and macroporous (pores with diameters >50 nm), yet has not given a definition of nanoporous in the IUPAC Recommendations on the Nomenclature of Structural and Compositional Characteristics of Ordered Microporous and... 

This kind of carbon with such unique pore structure, extremely large surface area, and micropore volume has never been reported. Sections 3.3.3.1 and 3.3.3.2 will introduce the details of the production method of the ordered microporous carbons and compare the extraordinary pore structure of this carbon with commercial microporous carbons with a large surface area. In Section 3.3.3.3, it will be demonstrated that a similar ordered microporous carbon containing N atoms can be prepared by the template technique, and the N-doping influences the adsorption behavior of H20 molecules [136], In Section 3.3.3.4, the use of this unique carbon as an electrode for supercapacitor will be introduced [137],... 

All of these mesoporous materials are characterized by having narrow pore size distributions comparable to microporous materials, and extraordinary hydrocarbon sorption capacities. These materials are important mostly because of their exceptional ordered mesopore structure, which is not common for other adsorbent materials. As a result, mesoporous-ordered silica has been proposed as a reference material for the study of adsorption processes in mesopores [16]. 

The kinetics of physical adsorption were reviewed by Brunauer in 1943 (5). A 1 careful literature survey (6) reveals that the present status of the subject is substantially unchanged. Elementary considerations indicate that the process of physical adsorption should proceed very rapidly and be practically complete within the order of magnitude of one minute, and indeed, experiment usually confirms this. Deviations from this behavior have been accounted for by the time required to dissipate the heat of adsorption or by a very slow surface diffusion into a microporous structure. 

A striking feature of MCM-41 and other related ordered mesoporous materials is the magnitude of their specific surface areas. It is remarkable that it is possible to produce non-microporous structures with stable surface areas well in excess of 1000 mz g 1. Of course, the implication of the combination of high area and high pore volume is that the pore walls must be quite thin - in some cases the walls may be no more than two oxygen atoms thick ... 

The microporous structure of the PILCs is characterised by the distance between the clay layers and the distance between the intercalated units (pillars), the interlayer spacing and the interpillar spacing respectively [6]. Several phenomena can influence these distances such as the elimination of organic moieties from the polycations and the dehydration or sintering of the pillars. Regarding the MPSDs presented in Figures 2-4, more information about the distribution of the aluminium fixed and the evolution of the textural properties with the calcination temperature is required in order to achieve a better knowledge of the microporous structure of these solids. 

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