Pore topology

Column sorbents for aqueous media show )ust average properties. This is due to the different copolymerization process, which does not allow easy formation of macroporous beads with proper pore topology. This fact also reflects many experiences of SEC users, who have to input much more effort to get good aqueous SEC work accomplished. 

In the case of n-butene isomerization it was demonstrated (Figure 2) that the ideal micro-pore topology led to retardation of the C8 dimer intermediate and that the catalyst based on the ferrierite structure was close to optimal in this respect [1). For selective isodewaxing a one-dimensional pore structure which constrained the skeletal isomerization transition state and thereby minimized multiple branching such as the SAPO-11 structure was found to meet these criteria. Clearly, these are ideal systems in which to apply computational chemistry where the reactant and product molecules are relatively simple and the micro-porous structures are ordered and known in detail. 

MCM-50 consists of stacks of silica and surfactant layers. Obviously, no pores are formed upon removal of the surfactant layers. The silica layers contact each other resulting in a nonporous silica. It is noteworthy to mention that materials of M41S type were probably already synthesized by Sylvania Electric Products in 1971 [32], However, at that time the high ordering of the materials was not realized [33], M41S-type materials are synthesized under basic reaction conditions. Scientists from the University of Santa Barbara developed an alternative synthesis procedure under acidic conditions. They also used alkyltrimethyl ammonium as the surfactant. The porous silica materials obtained (e.g., hexagonal SBA-3 Santa BArbara [SBA]) had thicker pore walls but smaller pore diameters. Furthermore, they developed materials with novel pore topologies, e g., the cubic SBA-1 with spherical pores. 

Corma, A., Chica, A., Gull, J.M., Uopis, F.J., Mabilon, G., Perigon-Melon, A., and Valencia, S. (2000) Determination of the pore topology of zeolite IM-5 by means of catalytic test reactions and hydrocarbon absorption measurements. f Catal, 189, 382-394. 

New instrumental studies and molecular modeling lent further support to earlier observations. In conclusion, pore topology, acid strength, acid site density, and the location of acid sites are the main factors controlling catalyst performance.324 325 341 343... 

Colomban, P, and Vendange, V., Sintering of alumina and mullite prepared by slow hydrolysis of alkoxides the role of the protonic species and of pore topology, J. Non-Crystl. Solids, 147, 245, 1992. 

The most commonly used model for pore topology is to represent the material as composed of independent, non-interconnected pores of some simple geometry usually these are of slit shape for activated carbons, and of cylindrical geometry for glasses, oxides, silicas, etc. Usually, the heterogeneity is approximated by a distribution of pore sizes, it being implicitly assumed that all pores are of the same geometry and surface chemistry. In this case the excess adsorption, f(P), at a pressure P can be represented by... 

Detailed control of the structural and properties such as pore topology, pore diameter, pore connectivity, controlled multiscale porosity, surface properties, reactivity, functionalization, morphology, and macroscopic shape are desirable to reach the ultimate goals of industrial and commercial applications. Make mesoporous materials able to compete with other current using materials (e.g., zeolites). 

There have been several improvements to the slit-pore model and the description based on the concept of a pore size distribution. These improved models are also constructed by making detailed observations of the experimental data (electron micrographs. X-ray diffraction, adsorption isotherms, etc.), extracting more relevant features of the pore topology and the structure of the pore walls, and including these features in the models. For example, a 2D distribution of pore size and pore-wall thickness may be used, instead of a pore size distribution [22]. Most of these improvements are described in detail in a recent review [1]. 

Dynamic Change in Pore Topology by Design of Host Flexibility [24] 333... 

Fig. I. Influence of zeolite pore topology on the isobutene selectivity obtained at 350°C and 1 atm (5% of 1-butene in N2) and at a conversion of 41-47% (reproduced from Ref. [82])...

P-ll - n-Heptane hydroconversion and methylcyclohexane cracking as model reactions to investigate the pore topology of NU-88 zeolite... 

NOx reduction by propene NOx reduction with hydrocarbons NOx sorption, metal zeolites NOx storage, metal zeolites Noble metal zeolite Noble metals/MCM-4I Nonasil, synthesis Non-ionic surfactant Nu-86 synthesis NU-88, cracking catalyst NU-88, hydroconversion catalyst NU-88, pore topology nuclear waste Nucleation agent, zeolitic Nucleation and growth Nucleation, FAU Nucleation, LTA Nucleation, zeolites Nucleophilic Substitution... 

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