Porous precipitated silicas

Silica is one of the most abundant chemical substances on earth. It can be both crystalline or amorphous. The crystalline forms of silica are quartz, cristobalite, and tridymite [51,52]. The amorphous forms, which are normally porous [149] are precipitated silica, silica gel, colloidal silica sols, and pyrogenic silica [150-156], According to the definition of the International Union of Pure and Applied Chemistry (IUPAC), porous materials can be classified as follows microporous materials are those with pore diameters from 3 to 20 A mesoporous materials are those that have pore diameters between 20 and 500 A and macroporous materials are those with pores bigger than 500 A [149],... 

Although commercially important, the precipitated silicas have received much less attention in the scientific literature than either the Aerosils or silica gels. In certain respects they are similar to pyrogenic silicas indeed, at one time they were treated as alternative non-porous silicas. Thus, the reversible Type II isotherms of nitrogen and argon obtained by Basset et al. (1968) were assumed to represent uncomplicated monolayer-multilayer adsorption. More recent work (Carrott and Sing, 1984) has shown that the Type II character is here the result of adsorption both on the external surface and within some micropores. 

KjO, GeOj and AI2O3. Non-Inertness of silica is another problem. At high pH all of them dissolve to some extent, precipitated silica and glass more so than quartz. As the dissolved silicates also consume base, correction for this phenomenon Is mandatory to obtain hysteresis-free specific surface area. Several silicas are porous for protons, as judged by the absolute values of cf°, If based on the BET (Njl-area. The trend Is that these Increase virlth porosity, as illustrated by fig. 3.65, with for several samples a° exceeding full monolayer coverage. 

Some materials, among the most porous, show a large volume variation due to mechanical compaction when submitted to mercury porosimetry. High dispersive precipitated silica shows, as low density xerogels and carbon black previously experimented, two successive volume variation mechanisms, compaction and intrusion. The position of the transition point between the two mechanisms allows to compute the buckling constant used to determine the pore size distribution in the compaction part of the experiment. The mercury porosimetry data of a high dispersive precipitated silica sample wrapped in a tight membrane are compared with the data obtained with the same sample without memlM ane. Both experiments interpreted by equations appropriate to the mechanisms lead to the same pore size distribution. 

High dispersive precipitated silica submitted to an increasing pressure in a mercury porosimeter shows successively a collapse mechanism of porous texture followed by a mechanism of mercury intrusion in the part of pore network which has resisted to the collapse. Such a behavior has been previously observed on low density xerogels and on some carbon black. Both mechanisms can be clearly distinguished by a sharp variation of slope of cumulative pore volume curve versus pressure. 

Before the distinctive adsorptive properties of porous silica can be described, the different ranges of pore size that are of special importance to the mechanisms of physisorption must be identified. Micropores are the pores of the smallest width (d < 2 nm) mesopores are of intermediate size (id 2-50 nm) macropores are the widest pores (d > 50 nm) (5). Amorphous silica gels tend to be mesoporous or microporous, whereas the crystalline zeolitic silicas possess intracrystalline microporosity. The precipitated silicas are macroporous and also, to a small extent, microporous. These and other aspects of the microstructures will be discussed in the following sections. 

Adsorptive properties of porous silicas compacts of pyrogenic powders, 506, 507f precipitated silicas, 507-509 silica gels, 509, 510-512 zeolitic silicas, 512-514 Adsorptive properties of silicas, challenges for improvement, 505 Aerogel(s) definition, 7, 620 porosities, 379, 380 ... 

Precipitated silicas are typically macroporous materials where the mercury method is generally the best method available for the reliable determination of pore sizes above 30 nm. Washburn et al. [22] introduced the mercury intrusion method to measure the pore size of a porous silica. 

Four main types of porous silica adsorbents have been identified compacts of pyrogenic powders, precipitated silicas, silica gels, and zeolitic silicas. The importance of porosity relative to the adsorptive properties of each group is reviewed, with particular reference to the adsorption of nitrogen, argon, and water vapor. The differences in size and specificity of these adsorptive molecules may be exploited to explore the surface properties of each grade of silica. A notable feature cf Silicalite I, which is the best known of the zeolitic silicas, is its remarkable hydrophobic character. Furthermore, the uniform tubular pore structure of this microporous silica is responsible for other highly distinctive properties. 

Polyethylene sheets are widely used in automobile batteries to separate the individual electrochemical cells. The polyethylene is highly filled with porous silica particles to provide a path for the migration of conductive ions. This has become a major market for precipitated silicas because the filler loading can be as high as 70%. 

Figure 2.46 allows one to compare the effect of the layer thickness on the absolute DR and DT at the absorption peak of the CN groups for two different-morphology silicas treated up to surface saturation by DMP.CN. These are Cab-O-Sil (nonporous silica with a primary particle diameter of 1-1.5 nm, aggregated in 10-nm grains with a specific surface area of 191 m g and a density of 150 g L ) and LiChrosorb Si 100 (precipitated porous silica characterized by a particle diameter of 5-10 turn, specific surface area of 321 m g , and density of 350 g L ). For 1-3-mm porous LiChrosorb silica layers, DR does not vary and DT is absent. Therefore, these layers can be considered as pseudoinfinite with R = Roo = 0.38 0.02. For the Cab-O-Sil sample, which has 50% less effective surface area and a generally looser structure, the asymptotic value of the reflectance is much lower R = R = 0.044 0.002). Even though the... 

SiOj-based (silicates) Clays LDHs Zeolites MCM-41, MCM-48, FSM, PMOs Silica gel, precipitated silica, fumed silica Porous glasses (VPG, CPG), sintered glasses... 

In determining the rate of dissolution of amorphous silica powders, the possible existence of a porous, rapidly soluble layer must be considered. Yates and Healy (212a) have shown that BDH-precipitated silica, widely used as a standard for study, has a surface layer that is impermeable to nitrogen but permeable to alkali. This gel layer dissolves more rapidly than the remainder of the silica and must be taken into account when the rate of dissolution is being measured. 

The results of Bolt (184) (Figure 4.10) and Heston, Her, and Sears (185) on nonporous silica particles are compared with those of Yates and Healy on precipitated silica (BDH) which is porous until heated to high temperature. It is evident that the surface charge density on the sol particles is of the same order as that on the precipitated powder that has been heated. The sol particles haVe never been heated over lOO C yet are dense because they were made by the slow deposition of molecular silica. Unless particles are grown in this way, porosity will be present as shown by the higher charge density. However, it appears that even these sol particles... 

Filter aids should have low bulk density to minimize settling and aid good distribution on a filter-medium surface that may not be horizontal. They should also be porous and capable of forming a porous cake to minimize flow resistance, and they must be chemically inert to the filtrate. These characteristics are all found in the two most popular commercial filter aids diatomaceous silica (also called diatomite, or diatomaceous earth), which is an almost pure silica prepared from deposits of diatom skeletons and expanded perhte, particles of puffed lava that are principally aluminum alkali siheate. Cellulosic fibers (ground wood pulp) are sometimes used when siliceous materials cannot be used but are much more compressible. The use of other less effective aids (e.g., carbon and gypsum) may be justified in special cases. Sometimes a combination or carbon and diatomaceous silica permits adsorption in addition to filter-aid performance. Various other materials, such as salt, fine sand, starch, and precipitated calcium carbonate, are employed in specific industries where they represent either waste material or inexpensive alternatives to conventional filter aids. 

Sedimentary rocks (like sandstone) have a microstructure rather like that of a vitreous ceramic. Sandstone is made of particles of silica, bonded together either by more silica or by calcium carbonate (CaCOj). Like pottery, it is porous. The difference lies in the way the bonding phase formed it is precipitated from solution in ground water, rather than formed by melting. 

Zebiihr et al. (29) developed an automated system for determining PAHs, PCBs and PCDD/Fs by using an aminopropyl silica column coupled to a porous graphitic carbon column. This method gives five fractions, i.e. aliphatic and monoaromatic hydrocarbons, polycyclic aromatic hydrocarbons, PCBs with two or more ortho-chlorines, mono-ort/io PCBs, and non-ortho PCBs and PCDD/Fs. This method employed five switching valves and was successfully used with extracts of sediments, biological samples and electrostatic filter precipitates. 

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