Microporous molecular sieves

De Vos, D.E., Dams, M., Sels, B.F. and Jacobs, P.A. (2002) Ordered mesoporous and microporous molecular sieves functionalized with transition metal complexes as catalysts for selective... 

Characterization of mesoporous and microporous molecular sieves containing niobium and tantalum... 

Characterization of Mesoporous and Microporous Molecular Sieves Containing 201... 

FIGURE 4.2 XRD pattern of the opal sample 81C with an AlP04-5 microporous molecular sieve synthesized on the surface. 

Another example of the use of XRD in material characterization is shown in Figure 4.2. The XRD pattern of a sintered opal membrane covered with a molecular sieve is shown. The membrane was prepared with the opal sample 81C [47] afterward, the produced membrane was covered with an AlP04-5 microporous molecular sieve synthesized on the surface [41],... 

Diffusion, absorption, and mass transport properties, of course, also define microporous molecular sieve selectivity. No other physical proper-... 

Copper ions exchanged microporous molecular sieves, in particular Cu-ZSM-5, are active catalysts for the selective catalytic reduction of NO and N2O with hydrocarbons in the presence of O2 (HC-SCR). It has been reported that the catalytic activity may be controlled by intra-crystalline diffiisivity and by geometry-limited diffusion depending on the hydrocarbon molecular size and the zeolite pore size [1]. Therefore, it is of interest to prepare Cu-Al-MCM-41 mesoporous molecular sieves and to compare their activity with that of Cu-ZSM-5. 

To better understand why and how microporous molecular sieves (aluminosilicates and aluminophosphates) are highly suitable catalysis for the skeletal isomerization of n-butenes, it is important to discuss the mechanisms of the reactions that control the formation of the desired isobutylene and to evaluate the relative importance of secondary reactions that may... 

Characteristics of Microporous Molecular Sieves Used as Catalysts for the Conversion of n-Butenes... 

V. Skeletal Isomerization of n-Butenes Catalyzed by Medium-Pore Microporous Molecular Sieves... 

Microporous Molecular Sieves 1998. L Bonneviot, F Beland, C Danumah, S Giasson and S. Kaliaguine (Eds.), Studies in Surface Science and Catalysis Vol. 117, Elsevier Amsterdam 1998, pp. 343-350... 

It has been found that, except for a few silica or germanium oxide porous compounds, most of the microporous molecular sieves with a large aperture are metal phosphates with... 

In addition, in the field of dewaxing (gas oils, HDC residues, lubricating oil, etc.), synthesis of novel molecular sieves with better adsorption and separation abilities is highly desired. In the past 20 years, thanks to the discovery of many molecular sieves with new compositions and structural features [secondary building units (SBUs) and pores], there have appeared a number of new application fields for molecular sieves, such as basic catalysis, extra-large microporous molecular sieve catalysis, redox catalysis, asymmetric catalysis, and dual- and multi-functional catalysis.1-201 All of these will lay a further solid foundation for the development of molecular sieves in catalysis, adsorption, and separation. 

The application of solvent extraction to removal and recovery of templates or SDAs from zeolite channels was initiated by Whitehurst[7] in the 1990s for the extraction of surfactant from mesoporous M41S materials. This method or its improved analogs have become one of the most important techniques to recover surfactants from mesoporous molecular sieves. However, it is still difficult to use this technique to remove and to recover the SDAs from microporous molecular sieves because, first, the size of SDA molecules is similar to that of the channel openings and the molecules are not able to diffuse out from the channels, and, secondly, there are usually strong interactions between the microporous frameworks and the SDA molecules that prevent the SDA molecules from being extracted solely by solvents. Modification of the conventional solvent-extraction technique, such as addition of chemical agents which can adjust the... 

The history of mesoporous material synthesis is unintentionally or intentionally duplicating the development of zeolites and microporous molecular sieve. It starts from silicate and aluminosilicate, through heteroatom substitution, to other oxide compounds and sulfides. It is worth mentioning that many unavailable compositions for zeolite (e.g., certain transition metal oxides, even pure metals and carbon) can be made in mesoporous material form. 

The main difference between the synthesis of MCM-41 mesoporous material and traditional synthesis of zeolite or silica molecular sieve is the use of different templates. An individual organic molecule or metal cation is used for the traditional synthesis of silica microporous molecular sieve. For example, the typical template for ZSM-5 synthesis is tetrapropylammonium ion the crystal is formed through the condensation of silicate species around the template molecule, while for the formation of MCM-41, the typical template is the assembly of large molecules containing one hydrophobic chain with more than 10 carbons. 

The defects (faults) and intergrowth can be often found in zeolite or microporous molecular sieve crystals. Mesoporous material synthesis is controlled kinetically and there is no defect-free mesoporous material. Various defects can be found easily in mesoporous materials, even for so-called high-quality materials.[106]... 

In all sections macro-, meso-, and microporous (molecular sieving) systems will be treated separately. The focus will be on the most promising systems to obtain high selectivity (separation factors) in combination with reasonable permeation values. 

Configurational diffusion in microporous (molecular sieve) membranes wiU be treated separately. Here the driving force must be described in terms of a chemical potential gradient, which is coupled to partial pressure via adsorption isotherms. In cases where several mechanisms operate simultaneously, the problem of additivity arises and in real membrane systems simplifying assumptions have to made. 

The main characteristics of microporous molecular sieves is that they contain micropores with dimensions between 0.4 and 1.4 nm-similar to those of many reactant molecules of interest. Closer analysis of the characteristics of the materials reveals properties highly desired in most catalysts ... 

Silicoaluminophosphates (SAPO s) constitute a novel class of crystalline microporous molecular sieves, containing silicon, aluminum and phosphorus 1-3. SAPO-37 is isostructural to faujasite, and has large pores and an anionic framework 2-4. The negative charge arises from the presence of Si species, partially substituting P in a neutral... 

Silicoaluminophosphates (SAPOs) are a new generation of crystalline microporous molecular sieves. They have been discovered by incorporating Si into the fr unework of the aluminophosphates (AIPO4) molecular sieves. Several small-pore SAPO crystals have been synthesized. SAPO-17, SAPO-34 and SAPO-44 have pore openings of about 0.43 nm. SAPO-17 has an erionite-like structure, while SAPO-34 and SAPO-44 have a chabazite-like structure. 

New crystalline microporous molecular sieves have been synthesized by incorporating other elements into the AIPO4 freunework. Some of these elements are Co, Be, Mn, and Fe. ° They carry the generic naunes MAPO and MeAPO molecular sieves. The acidity of MAPO and MeAPO molecular sieves can vary widely (see Table 2). 

Figure 9.7 Simplified representation of the formation of microporous molecular sieves using a small single molecule organic stmcture-directing agent. (Adapted and reprinted with permission from T. J. Barton et al, Chem. Mater. 1999, 11, 2633. Copyright 1999 American Chemical Society.)...

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