Microporous silica-aluminas

Non microporous silica alumina, SiO/AljO, =5.7, modified with 1,3-dithiane-... 

Between microporous silica-aluminas ERS-8, synthesized in basic medium shows very interesting textural properties with respect to SA, prepared in acidic medium, (higher surface area and higher pore volume, narrower pore size distribution). 

Three catalysts have been studied. The 1-35 catalyst is a microporous silica-alumina (1). The I-3S pellet catalyst, which is prepared by pelleting the I 35 once it was finely ground, has a structure constituted by mesopcres. The MZ-7P catalyst is a cracking zeolite (Akzo Chemie). 

Supports such as silica, alumina and carbon usually contain pores that offer a high internal surface area. The pore system of a support is usually rather irregular in shape and contains macropores, due to the spaces between individual crystallites, with diameters of the order of 100 nm, and micropores with characteristic dimensions of 5-10 nm. A good support offers... 

Although cracking also occurs on chlorine-treated clays and amorphous silica-aluminas, the application of zeolites has resulted in a significant improvement in gasoline yield. The finite size of the zeolite micropores prohibits the formation of large condensed aromatic molecules. This beneficial shape-selectivity improves the carbon efficiency of the process and also the lifetime of the catalyst. 

While our discussion will mainly focus on sifica, other oxide materials can also be used, and they need to be characterized with the same rigorous approach. For example, in the case of meso- and microporous materials such as zeolites, SBA-15, or MCM materials, the pore size, pore distribution, surface composition, and the inner and outer surface areas need to be measured since they can affect the grafting step (and the chemistry thereafter) [5-7]. Some oxides such as alumina or silica-alumina contain Lewis acid centres/sites, which can also participate in the reactivity of the support and the grafted species. These sites need to be characterized and quantified this is typically carried out by using molecular probes (Lewis bases) such as pyridine [8,9],... 

Fig. 3.23 shows pore volume distributions of some commercially important porous materials. Note that zeolites and activated carbon consist predominantly of micropores, whereas alumina and silica have pores mainly in the me.sopore range. Zeolites and active carbons have a sharp peak in pore size distribution, but in the case of the activated carbon also larger pores are present. The wide-pore silica is prepared specially to facilitate internal mass-transfer. 

The only ceramic membranes of which results are published, are tubular microporous silica membranes provided by ECN (Petten, The Netherlands).[10] The membrane consists of several support layers of a- and y-alumina, and the selective top layer at the outer wall of the tube is made of amorphous silica (Figure 4.10).[24] The pore size lies between 0.5 and 0.8 nm. The membranes were used in homogeneous catalysis in supercritical carbon dioxide (see paragraph 4.6.1). No details about solvent and temperature influences are given but it is expected that these are less important than in the case of polymeric membranes. 

Uhlhom, R. J. R., M. H. B. J. Huis in t Veld, K. Keizer and A. J. Burggraaf. 1989, High permselectivities of microporous silica-modified x-alumina membranes. J. Mater. Science Lett. 8(10) 1135-39. 

The activity advantage of zeolite catalysts over amorphous silica-alumina has well been documented, Weisz and his associates [1] reported that faujasite Y zeolite showed 10 to 10 times greater activity for the cracking of n-hexane than silica-alumina. Wang and Lunsford et al. [2] also noted that acidic Y zeolites were active for the disproportionation of toluene while silica-alumina was inactive. The activity difference between zeolite and silica-alumina has been attributed to their acidic properties. It is, however, difficult to explain the superactivity of zeolite relative to silica-alumina on the basis of acidity, since the number of acid sites of Y-type zeolite is only about 10 times larger than that of silica-alumina. To account for it, Wang et al. [2] proposed that the microporous structure of zeolite enhanced the concentration of reactant molecules at the acid sites. The purpose of the present work is to show that such a microporous effect is valid for pillared clay catalysts. 

Separation of gas streams by adsorption is becoming increasingly popular as improved technology comes on the market. Some examples of commercially practiced adsorption processes are shown in Table 1. These processes take advantage of the selective adsorption properties of a number of microporous adsorbents, including activated carbon, silica, alumina, and various synthetic and natural zeolites. 

D Ru on mesoporous silica E Pd on microporous alumina F Macroporous silica/alumina G Pt on mesoporous polystyrene... 

In summary, the main goal of the present work is the development of a hydrothermally stable microporous silica membrane with prescribed transport properties. Preferably, these steam stable membranes should have very high permselectivities. Because the permselectivity of a molecular sieving silica membrane will drop to the Knudsen value of the y-alumina supporting membrane when the silica membrane deteriorates under steam reforming conditions, a selectivity of the silica layer higher than the Knudsen selectivity is sufficient. In this way the measurement of the permselectivity is a powerful tool to assess the hydrothermal stability of a supported microporous membrane. 

The above mentioned advantages make the supports very suitable for the preparation of flat microporous silica membranes for lab-scale tests. However, due to the almost perfect particle packing, the hydrogen permeance may be too low for application in process industry. For stability testing, on the other hand, the permeance of the membranes is of a far lower importance than the selectivity of the layer under investigation. More information about stability testing can be found in chapter 5 and 6 for the y-alumina and the silica layer respectively. 

Somewhat surprisingly, however, only a very limited amount of literature is available on hydrothermal stability of even the most commonly applied mesoporous membrane type, namely y-alumina membranes on OC-AI2O3 supports. These mesoporous y-alumina membranes are the common supports for the microporous silica membranes to be used in membrane steam reformers. In the investigations that finally led to the present study, delamination of the y-alumina membrane from the OC-AI2O3 supports in hot steam was found to be a major compli-... 

Amorphous microporous silica membranes as discussed here, consist of a macroporous a-alumina support (pore diameter -100 nm) with a mesoporous y-alumina intermediate layer (Kelvin radius of 2.5 nm) and a microporous silica top layer (pore diameter -4 A) [1,2],... 

A common and well-known method to prepare silica membranes with molecular sieving properties is sol-gel coating [3-5], With this technique, microporous silica layers with a pore-size of about 0.5 nm are dip-coated on top of supported y-alumina membranes. The supports are porous a-alumina disks with pore diameters in the range from 100-200 nm. On top of these macroporous supports a 3 pm thick mesoporous y-alumina layer is coated, with a pore size of 3 nm. 

In some cases, when the polymerization appears, the energy distribution of micropores is negligible in comparison with the energy of polymerization. That is possible when the temperature of the treatment of the primary material (if this one can be polymerized, e.g., silica, alumina) is low (less 300-350 °C). In such cases, traditional methods of nonequilibrium thermodynamics are not effective, and the micropore formation can be considered as the result of the polymerization process which is described by methods of polymer science. However, models of macromolecular systems do not always give enough information about micropores as the empty space between polymers. For such systems, the application of fractal methods can allow us to obtain additional information, while one has to take into account the fact that they cannot be applied to very narrow pores (ultramicropores which are found, for instance, in some silica gels). 

IV. POLYMER MODELS OF MICROPOROUS MATERIALS LIKE SILICA/ALUMINA GELS. STATISTICAL POLYMER METHOD... 

Table III shows XRD and porosimetry data for calcined USY and AFS zeolites. All samples show shrinkage of the unit cell to comparable values following calcination. As a result, calcined samples are compared at similar silica-alumina framework ratios. All calcined samples have well developed microporous structures and comparable total pore volumes. These porosimetry data confirm that the hydrothermally dealuminated materials contain a significant fraction of mesopores relative to chemically dealuminated materials. The extensive washing given to AFS-1 results in higher micropore surface area and volume compared to AFS-2 and suggest that AFS-2 contains occluded fluoroaluminate and fluorosilicate compounds within the microporous structure.

The Surface Hydroxyl Croups of Microporous and Mesoporous Silica-Aluminas (SAs)... 

Various catalysts used in the two processes have been described as follows zeolite, alumina, silica-alumina, FCC catalyst, reforming catalyst, and others. The most common catalysts used in the cracking of heavy hydrocarbons are acidic catalysts alumina and silica-alumina with mesopores, and also zeolite with micropores, etc. They are typically used in the commercial petroleum process. For the chemical properties of catalyst, the... 

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