Solids large pore

Adsorbents, and activated carbon in particular, are typically characterized by a highly porous structure. Adsorbents with the highest adsorption capacity for gasoline or fuel vapors have a large pore volume associated with pore diameters on the order of 50 Angstroms or less. When adsorption occurs in these pores, the process is comparable to condensation in which the pores become filled with hquid adsorbate. Fig. 5 depicts the adsorption process, including transfer of adsorbate molecules through the bulk gas phase to the surface of the solid, and diffusion onto internal surfaces of the adsorbent and into the pores. 

Another class of solid supports is based on rigid structures containing large pores and is often referred to as macroporous nonswelling beads. 

Within the scope of thermoelectric nanostructures, Sima et al. [161] prepared nanorod (fibril) and microtube (tubule) arrays of PbSei. , Tej by potentiostatic electrodeposition from nitric acid solutions of Pb(N03)2, H2Se03, and Te02, using a 30 fim thick polycarbonate track-etch membrane, with pores 100-2,000 nm in diameter, as template (Cu supported). After electrodeposition the polymer membrane was dissolved in CH2CI2. Solid rods were obtained in membranes with small pores, and hollow tubes in those with large pores. The formation of microtubes rather than nanorods in the larger pores was attributed to the higher deposition current. 

Zeolites have ordered micropores smaller than 2nm in diameter and are widely used as catalysts and supports in many practical reactions. Some zeolites have solid acidity and show shape-selectivity, which gives crucial effects in the processes of oil refining and petrochemistry. Metal nanoclusters and complexes can be synthesized in zeolites by the ship-in-a-bottle technique (Figure 1) [1,2], and the composite materials have also been applied to catalytic reactions. However, the decline of catalytic activity was often observed due to the diffusion-limitation of substrates or products in the micropores of zeolites. To overcome this drawback, newly developed mesoporous silicas such as FSM-16 [3,4], MCM-41 [5], and SBA-15 [6] have been used as catalyst supports, because they have large pores (2-10 nm) and high surface area (500-1000 m g ) [7,8]. The internal surface of the channels accounts for more than 90% of the surface area of mesoporous silicas. With the help of the new incredible materials, template synthesis of metal nanoclusters inside mesoporous channels is achieved and the nanoclusters give stupendous performances in various applications [9]. In this chapter, nanoclusters include nanoparticles and nanowires, and we focus on the synthesis and catalytic application of noble-metal nanoclusters in mesoporous silicas. 

For n-decane isomerization, when a good balance between the metal phase and the acidic phase of the catalysts is reached, the isomerization and cracking yield curves of the catalysts are a unique function of the conversion, meaning that these yields do not depends on the porosity nor the acidity of large pore materials. Formation of the most bulky isomers, such as 4-propylheptane and 3-ethyl-3-methylheptane was favored in mesoporous solids (figure 1). Criteria based on the formation of these particular isomers are linked with mesoporosity and could be useful to discriminate between zeolites catalysts with and without mesopores. 

Zeolites are crystalline porous solids with pore dimensions at the molecular level. Some zeolite types, such as the faujasites (zeolite X and Y or their hexagonal isomer EMT), possess large supercages with an internal diameter of approximately 1.2 nm, connected by pores with a diameter of approximately 0.75 nm. A metal complex will be confined in the supercage, when its size exceeds 0.8 nm. 

Figure 1.6 Top Low-temperature nitrogen adsorption ( ) and desorption (x) isotherms measured on a calcined SBA-15 mesoporous silica solid prepared using an EO20PO70EO20 block copolymer [54]. Bottom Pore size distribution derived from the adsorption isotherm reported at the top [54]. A high surface area (850 m2/g), a uniform distribution of cylindrical nanopores (diameter —90 A), and a large pore volume (1.17 cm3/g) were all estimated from these data. These properties make this material suitable for use as support in the preparation of high-surface-area solid catalysts. (Reproduced with permission from The American Chemical Society.)...

In the first instance (1,3) two types of nickel are used on the side exposed to the gas, large pores are produced in the metal and adjacent to this structure, a network of smaller pores are produced to hold back the electrolyte. The reacting gases diffuse rapidly in the large pores and come in intimate contact with the electrolyte present in the small pores. For the electrochemical reaction po occur, a three phases contact is needed since a gaseous reactant produces a solvated reactior oro uct nd in this process an electron is given or withdrawn from a solid conducting substrate. 

With this purpose, several different types of solid acid catalysts have been investigated for the acylation of aromatics, but the best performances have been obtained with medium-pore and large-pore zeolites (3-9). In general, however, the use of acylating agents other then halides, e.g., anhydrides or acids, is limited to the transformation of aromatic substrates highly activated towards electrophilic substitution. In a previous work (10), we investigated the benzoylation of resorcinol (1,3-dihydroxybenzene), catalyzed by acid clays. It was found that the reaction mechanism consists of the direct 0-benzoylation with formation of resorcinol monobenzoate, while no primary formation of the product of C-benzoylation (2,4-dihydroxybenzophenone) occurred. The latter product formed exclusively by... 

The assumption usually made is that the ratio Fu /Sbet has the same value at a given relative pressure independent of the solid. A plot therefore of t versus P/Pq should give the same curve for any non-porous solid (see Fig. 8.6). In fact, plots of the number of adsorbed layers versus P/Pq show some discrepancies which for the analysis of large pores is not significant. Therefore, the Halsey equation can be used for the statistical thickness in that application. However, for micropore analysis, a statistical thickness must be taken from a t versus P/Pq curve that has approximately the same BET C value as the test sample. The unavailability of t versus P/Pq plots on numerous surfaces with various C values would make the MP method of passing interest were it not for the fact that t can be calculated from equation (8.36). This implies that surface area can be accurately measured on microporous samples. Brunauer points out that in most instances the BET equation does correctly measure the micropore surface area. 

The shape of the front is given by the spreading of the front around the point S. The spreading depends mainly on toxic mass transfer kinetics between gas and solid phases. Thus numerous parameters can be considered air flow rate, size of the toxic molecule, pore sizes of the carbon, carbon grain diameter. With large pores and small grains, the mass transfer from gas to carbon is very fast the spreading... 

Concentrated sulfuric acid and hydrogen fluoride are still mainly used in commercial isoalkane-alkene alkylation processes.353 Because of the difficulties associated with these liquid acid catalysts (see Section 5.1.1), considerable research efforts are still devoted to find suitable solid acid catalysts for replacement.354-356 Various large-pore zeolites, mainly X and Y, and more recently zeolite Beta were studied in this reaction. Considering the reaction scheme [(Eqs (5.3)—(5.5) and Scheme 5.1)] it is obvious that the large-pore zeolitic structure is a prerequisite, since many of the reaction steps involve bimolecular bulky intermediates. In addition, the fast and easy desorption of highly branched bulky products, such as trimethylpentanes, also requires sufficient and adequate pore size. Experiments showed that even with large-pore zeolite Y, alkylation is severely diffusion limited under liquid-phase conditions.357... 

Aromatics. The application of solid acid catalysts provides excellent possibilities to carry out aromatic electrophilic substitutions in an environmentally friendly way. Various zeolites were found by Smith and coworkers to exhibit high activities and selectivities.250 Acetyl nitrate generated in situ from acetic anhydride and HNO3 transforms alkylbenzenes to the corresponding para-nitro derivatives in high yield (92-99%) and with excellent selectivity (79-92%) when applied in the presence of large-pore H-Beta zeolites.251 Lattice flexibility and the coordination of acetyl... 

For the automated solid-phase synthesis of oligonucleotides, however, silica was found to be the support of choice [192-197]. Silica with large pore size (25-300 nm), so-called controlled pore glass (CPG), is generally used for this purpose. The main advantages of CPG, as compared with silica gel, are its more regular particle size and shape, and greater mechanical stability. 

This would indicate that (3 has stronger acid sites than dealuminated HYD samples, something which agrees with IR data of both solids, where the wavenumber of the accesible acidic hydroxy groups is 3620-3630 cm-1 on dealuminated HY, and 3612 cm- in HJ3 zeolites (ref. 8), A similar conclusion was reached by measuring the desorption of pyridine on these two types of large pore zeolites. 

Ferey, G., Mellot-Draznieks, C., Serre, C., et al, A chromium terephthalate-based solid with unusually large pore volumes and surface area. Science 2005, 309, 2040-2042, correction 2005, 310, 1119. 

The hypothesis that the constituents of the mixture have a Lagrangian microstructure (in the sense of Capriz [3]) means that each material element of a single body reveals a microscopic geometric order at a closer look then it is there assigned a measure Vi(x) of the peculiar microstructure, read on a manifold Mi of finite dimension rnp e.g., the space of symmetric tensor in the theory of solids with large pores or the interval [0, v) of real number, with v immiscible mixture (see [5, 9]). We do not fix the rank of the tensor order parameter u%. 

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