Extra-large pore materials structure

Since the phosphate-based, extra-large pore materials do not reveal stability that is likely sufficient for many commercial applications, I will limit my discussions below to silica-based materials. As I have discussed elsewhere, the lack of stability in phosphate-based materials may not be due to the presence of extra-large rings, but rather to the nature of the structural units [9], With silica-based, extra-large pore materials, it appears that bulky structure directing agents will be required for their preparation. I will make this assumption in further discussions below. 

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. 

VPI-5 has received great attention during the last years mainly due to the unusual structure and properties of this material with extra-large pores consisting of 18-membered rings. Wu et al. showed that 27 ai DOR is capable of resolving discrete framework... 

Richard Barter and his collaborators were the first to publish (in 1969) on the idea of extra-large pore, crystalline materials [5]. Barter and Villiger presented a series of hypothetical structures related to zeolites L, cancrinite, offretite and gmelinite that had 24 MR pores with free diameters of approximately... 

For future studies on MOF-based slurry systems, there is basic selection of criteria that needs to be satisfied by both MOF and the liquid solution. The selection of the MOF possessing the appropriate pore size for the preparation of the slurry system is very important to guarantee that the size of the liquid is large enough and does not occupy the pores which leaves no space for C02 to adsorb. Moreover, the structural stability of the MOF in the aqueous solution is essential so that it does not lose its porous framework nor its surface area. The selection of the liquid candidate is crucial, as it should not provide any extra mass transfer resistance for C02 molecules. Further, experimental and computational investigations are still required to understand the separation mechanism in slurry mixtures and to have insight into the different types of interactions between the gas, liquid, and solid materials. 

Zeolites can be classified in many ways. Two convenient methods are on the basis of pore size and chemical composition, that is, the Si/Al ratio. The pore diameter is determined by the size of the free apertures in the structure, which is dependent on the number of T atoms (T = Si or Al) that form the aperture. Table 10.1 summarizes some examples of zeolites based on pore size classification. It should be noted that the values typically reported in the literature are determined by crystallographic studies. While these numbers are good guides, it is important to note that the actual pore size depends on many factors, including temperature, firamework composition, and the type of extra-framework cations present in the zeolite. These factors can lead to subtle changes in effective pore sizes and subsequently large changes in material properties (adsorption/reactivity). 

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