Wide-Pore Amorphous Silica

Entry Number Material Silica Gel (pm) Polymer (Mw) Density (g/mL) Silica Density Volume (mL/g)] 

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Figure 6.5. Absolute adsorption isotherms for water on different wide-pore amorphous silicas at room temperature (after the silicas were treated at 200 °C). The line shows the average of data from the literature (Kiselev, 1986 Zhuravlev, 1993).

In the production of linear alkylbenzenes zeolites with ten membered rings yield the desired product, the 2-phenyl isomer, vriiich is the smallest of the isomers, more selectively, but the activity of these zeoUtes is low [98], In a conparative study of several wide pore zeolites it was observed that the selective towards 2-phenyl dodecane in the reaction of dodecene with benzene at 498 K, 0.6 MPa and alkene to benzene ratio of 0.1 decreased in the order H-Mordenite > H-Beta > HY> amorphous silica-ahunina >RE-YwHF. Ahhou the production of the 2-phenyl isomer is desirable in the production of LAB, it is much more inportant to avoid the formation of diphenyl-isomers and branched phenyl isomers. The production of these conpounds can be suppressed by using HY as a catalyst [98,60],... 

Amorphous silica-alumina materials represent an important class of porous inorganic solids which have not long-range order and usually have a wide distribution of the pore size, in the micro and mesopore region. They show outstanding catalytic behaviours in several acid catalysed reactions (5, 6). 

Zeolites such as Y, Beta, and ZSM-5 are widely used commercial catalysts, but their applications are strongly limited by their small pore sizes. One solution is to use ordered mesoporous materials such as MCM-41 and SBA-15. These materials exhibit good catalytic properties for the catalytic conversion of bulky reactants. Although the mesoporous silica materials made from normal synthesis processes, such as MCM-41 and MCM-48, have high thermal stability, their hydrothermal stability is poor. Calcined samples can be destroyed with moisture or water, even at room temperature. Most calcined samples became amorphous in cold water within a few minutes. The main reason is the hydrolysis reaction of the amorphous silica wall (Si—O Si bonds broken). 

Zhuravlev, Gorelik, and co-workers (77, 80,120-122,143) carried out kinetic studies of water vapor adsorption and isotopic exchange (D2O + =Si-OH) for different types of amorphous silica. A relationship was established between the nature of the porosity and the shape of the kinetic curve. For example, for bidispersed silica gels containing both wide mesopores and very fine ultramicropores, the kinetic plot consists of two sections a very short period due to the mass transfer of water vapors through transport mesopores, and a very long period (tens of hours) due to the diffusion of water molecules inside very fine pores that have diameters comparable with that of the water molecule. Thus, such plots provide information on the type of pores present in the silica sample. 

In the 1950s, A. Kiselev, Zhdanov, and co-workers (12, 84, 155-159) showed that when the adsorption isotherms of water are expressed as absolute isotherms (referred to as the unit surface of the SiC>2 sample), widely different forms of amorphous silica having a completely hydroxyl-ated state adsorb the same amount of water at the same relative pressure (p/po <0.3). Thus the plots of absolute adsorption isotherms for different samples showed that the surfaces of these samples are of a similar nature. The adsorption properties of nonporous silica and silica having large pores (i.e., an absence of micropores) depend above all on the presence of OH groups and on the degree of hydroxylation of the surface. 

Alumina and amorphous silica-alumina are usually mesoporous materials with a wide distribution of pore sizes. The surface area, pore size and pore volume of alumina and amorphous silica-alumina depend greatly on the preparation method, hence their textural properties can be controlled to a certain extent by changing the synthesis conditions. These parameters are also highly relevant in determining the catalytic properties of these materials. 

Regarding their use as cracking and isomerization catalysts, bulk oxides such as clays and amorphous silica-aluminas have been widely displaced by molecular sieve compounds (e.g., zeolites, aluminophosphates), whose well-defined pore structures generally offer higher selectivity and flexibility. Nevertheless, bulk oxides continue to be used for various cracking and isomerization applications in the petroleum industry. 

Another worthwhile area of endeavor is new support materials, ones which are as single-sited as the catalysts they carry. Eor example, the zeolite carriers discussed above have more uniform pore sizes and volumes than the amorphous silicas hitherto widely used. The importance of single-site catalysts lies in the ability to characterize them comprehensively and alter them in a rational fashion in order to enhance desired polymer properties extending this degree of understanding and control to their interaction with supports and their performance in numerous polymerization processes could only be beneficial. 

Attempts to determine the solubility of amorphous silica in salt water solutions at near neutral pH and 0 to 5 C or 22 to 25 C have yielded a wide range of values9 which results in part from aging of the silica surface in contact with solution This makes determination of an initial solubility for silica difficult. Low temperature aging in salt water solutions or seawater causes a decrease in surface area, in specific pore volxime, and in solubility. Solubilities determined at pressures to 1000 atmospheres (lx 10 pascals) indicate that aging causes an increase in density of the surface silica this data also allows calculation of the partial molal voltime of dissolved silica. Identification of specific processes involved with aging of an amorphous surface are necessary for understanding silica solubility. 

Methods for preparation of silica samples exhibiting or not porosity are known since a long time [10]. Before the discovery of mesoporous silica [11] zeohtes were the most important materials with micropores forming a regular array of channels with uniform size [12-14]. Mesoporous materials were mainly represented by amorphous silica [1], pillared clays, sihcates [15,16] and certain forms of alumina. A common characteristic of those materials is the irregular spatial distribution of pores and a wide spectrum of pore sizes [17]. In 1992 Beck et al. announced the synthesis and characterization of new mesoporous silica-based materials [11,18]. These mesoporous molecular sieves received the general... 

Silica gel with narrow or wide pores. This adsorbent consists of amorphous Si02 (97.3 %) and is commercially available as a particle material. It is chemically neutral and resistant to acids with the exception of hydrofluoric acid. Silica gel with wide pores is sometimes used for the separation of tiny droplets... 

Silica gel structure is destroyed and the amorphous silica crystallizes at quite low temperature when 5 mole % of the alkali metal oxides is present. Lithium promotes crystallization to quartz, and sodium to cristobalite, at only 700 C. Potassium acts almost as rapidly at 700 C (354). The surface area of such alkali-containing gel drops almost to zero at 650-700 C (355). When the gel is taken at an intermediate stage as it shrinks, and is treated with acid, washed, and dried, it contains very wide pores, 300-1200 A diameter. Also there are micropores which are probably left after the sodium is extracted from the thin glassy layer on the.surface. Any surface area from 70 to 6 m g can be obtained by cooling the sample at the right point. 

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