Silica pore diameter

FIGURE 2.8 Hydrophobic retention and selectivity with RP columns. The stationary phases are ordered according to the increasing retention of toluene in methanol-water 50-50 v-v. Dashed line Stationary phases with a silica pore diameter below lOnm. Solid line Stationary phases with a silica pore diameter >12nm. ( ) Stationary phases with polar-embedded functional groups. (( ) Stationary phase based on a wide pore silica (30 nm)). 

The ordered mesoporous silica (pore diameter 6 nm) was kindly provided by Dr. Ulla Junges and was prepared as described previously by G.D. Stucky et al. [15]. A commercial triblock copolymer (Pluronic-123 ) was used in the synthesis process. This polymer contained ethylene oxide/ propylene oxide/ ethylene oxide blocks, which formed a hexagonal mesophase in the synthesis mixture during the hydrolysis of the silicon alkoxides. 

The non-covalent immobilization of Mn complexes (R = Me, Ph) into SBA-15-type mesoporous silica with pore sizes ranging from 25 to 100 A was investigated by Coradin et al. [59,60]. As expected, the pore size deeply affects the amount of embedded cluster. Sihca with 60-A pores was found to be the most efficient to incorporate the acetate derivative (36 wt %) as small aggregates which appear to fill the channels in TEM micrographs. Size selectivity of a silica host toward Mni2 complexes was reported by Coronado et al., who studied the incorporation of the R = Me, Et, Ph and CeFs derivatives into MCM-41 silica (pore diameter ca. 26 A) [61,62]. Only the smaller Me and Et derivatives could be inserted into the mesoporous material, and no immobilization at all was observed after reduction of the pore diameter by trimethylsilylation. However, structural integrity was demonstrated for the R = Et derivative only and under special preparative conditions. 

C.H. Lochmflller and W.B. Thill, Jr. Dependence of site-site interactions on silica pore diameter in amine-modified stationary phases. Anal. Chim. Acta. 157 65-71 (1984). 

For the industrial preparation of supported metallocene catalysts, selection of the inorganic support, generally a type of silica, is very important. There are many varieties of silica available in the market and each has different specifications with respect to the particle surface area, pore size (diameter, volume), bulk density, and mechanical properties [189-194]. The silica pore diameter and volume must be chosen bearing in mind that these pores will be partially filled with MAO. 

Anotlier important modification metliod is tire passivation of tire external crystallite surface, which may improve perfonnance in shape selective catalysis (see C2.12.7). Treatment of zeolites witli alkoxysilanes, SiCl or silane, and subsequent hydrolysis or poisoning witli bulky bases, organophosphoms compounds and arylsilanes have been used for tliis purjDose [39]. In some cases, tire improved perfonnance was, however, not related to tire masking of unselective active sites on tire outer surface but ratlier to a narrowing of tire pore diameters due to silica deposits. 

Usually they are employed as porous pellets in a packed bed. Some exceptions are platinum for the oxidation of ammonia, which is in the form of several layers of fine-mesh wire gauze, and catalysts deposited on membranes. Pore surfaces can be several hundred mVg and pore diameters of the order of 100 A. The entire structure may be or catalytic material (silica or alumina, for instance, sometimes exert catalytic properties) or an active ingredient may be deposited on a porous refractory carrier as a thin film. In such cases the mass of expensive catalytic material, such as Pt or Pd, may be only a fraction of 1 percent. 

Separation of C oand C70 can be achieved by HPLC on a dinitroanilinopropyl (DNAP) silica (5pm pore size, 3(X)A pore diameter) column with a gradient from H-hexane to 50% CH2CI2 using a diode array detector at wavelengths 330nm (for C q) and 384nm (for C70). [J Am Chem Soc 113, 2940, 1991.]... 

SynChropak GPC supports were introduced in 1978 as the first commercial columns for high-performance liquid chromatography of proteins. SynChropak GPC columns were based on research developed by Fred Regnier and coworkers in 1976 (1,2). The first columns were only available in 10-yu,m particles with a 100-A pore diameter, but as silica technology advanced, the range of available pore diameters increased and 5-yu,m particle diameters became available. SynChropak GPC and CATSEC occasionally were prepared on larger particles on a custom basis, but generally these products have been intended for analytical applications. 

SynChropak size exclusion supports are composed of spherical uniformly porous silica that has been derivatized with a suitable layer. SynChropak GPC supports are available in six pore diameters ranging from 50 to 4000 A and particle diameters from 5 to 10 /zm. SynChropak CATSEC supports are available in four pore diameters. Table 10.1 details the physical characteristics of the product lines. 

FIGURE 23.5 Comparison of separation by silica gels and separation by CPG that have a similar pore diameter. (Reprinted from Polymer, 39,891, Copyright 1998, with permission from Elsevier Science.)... 

Microporous insulation materials consist mainly of highly dispersed silica with a particle size of only 5-30 nm. The highly dispersed silica powder is pressed to plates, which receive heat treatment up to 800 °C, after which the plates are self-supporting and possess a micropore structure with pore diameter of 0.1pm. The addition of opacifiers to the highly dispersed silica starting material reduces the loss of heat by radiation. The dates for such insulation boards are shown in Table 18. 

In which the ratio m/n is close to 3. The silane was produced by free radical copolymerization of vinyltriethoxysilane with N-vinylpyrrolidone. Its number-average molecular weight evaluated by vapour-phase osmometry was 3500. Porous silica microballs with a mean pore diameter of 225 A, a specific surface area (Ssp) of 130 m2/g and a pore volume of 0.8 cm3/g were modified by the silane dissolved in dry toluene. After washings and drying, 0.55% by weight of nitrogen and 4.65% of carbon remained on the microballs. Chromatographic tests carried out with a series of proteins have proved the size-exclusion mechanism of their separation. 

Graph of Pore Volume per Gram of Silica Gel against Log Pore Diameter for Ten Different Silica Gels... 

The column used in the separation depicted in figure 1 was 25 cm long and 6.2 mm in diameter packed with silica gel having a mean pore diameter of 100 A and a particle diameter of 5 jum. Thus, the column would have an HETP of approximately 0.001 cm (twice the particle diameter). Consequently, a column 25 cm long would have an 25... 

The mechanical incorporation of active nanoparticles into the silica pore structure is very promising for the general synthesis of supported catalysts, although particles larger than the support s pore diameter cannot be incorporated into the mesopore structure. To overcome this limitation, pre-defined Pt particles were mixed with silica precursors, and the mesoporous silica structures were grown by a hydrothermal method. This process is referred to as nanoparticle encapsulation (NE) (Scheme 2) [16] because the resulting silica encapsulates metal nanoparticles inside the pore structure. 

Silicas used in this study were FSM-16 (BET surface area 950m g pore diameter 2.7 nm), ethylene bridged... 

Zeolites. In heterogeneous catalysis porosity is nearly always of essential importance. In most cases porous materials are synthesized using the above de.scribed sol-gel techniques resulting in so-called amorphous catalysts. Porosity is introduced in the agglomeration process in which the sol is transformed into a gel. From X-ray Diffraction patterns it is clear that the material shows only weak broad lines, characteristic of non-crystalline materials. Silica and alumina are typical examples. Zeolites are an exception they are crystalline materials but nevertheless exhibit high (micro) porosity. Zeolites belong to the class of molecular sieves, which are porous solids with pores of molecular dimensions, i.e., typically the pore diameter ranges from 0.3 to 10 nm. Examples of molecular sieves are carbons, oxides and zeolites. 

Aldol condensation reactions have also been conducted. A good example is provided by Climent et al. (1998) for making a-n-amyl cinnamaldehyde (Jasmin aldehyde) by condensing benzaldehyde with n-heptaldehyde, in the presence of mesoporous MCM-41 aluminosilicates. Mesoporous silica-aluminas with a narrow range of pore diameter such as MCM-41 also... 

Aluminum oxides, similar to silica gels, are available as bulk materials and as precoated plates, to be used not only for straight phase adsorption chromatography, but also for partition PLC (see Table 3.3 and Table 3.4). In particular, the aluminum oxide type 150 (i.e., mean pore diameter 150 A [15 tun]) is suitable for partition chromatographic purposes. 

The silica gel matrix for the RP-18 PLC precoated plates consists either of a type 60 or 150 (indicating the respective mean pore diameter). The manufacturers and the properties of the PLC plates RP-18 available at present are listed in Table 3.8. 

Solid palladium scavengers, PVPy, QTU were pmchased from commercial somces. The mesoporous silica material, S102-SH, was prepared via reaction of SBA-15 (110 A pore diameter) with 3-mercaptopropyltrimethoxysilane (16). Specifically, a toluene suspension of SBA-15 and 3-mercaptopropyltrimethoxysilane was heated at reflux for two days under Ar. Water was then added to promote the cross-linking and the mixture was heated at reflux for an additional day. The sohds were filtered and washed with copious amounts of toluene, hexanes, and methanol to remove unreacted silanes. The solids were finally Soxhlet extracted with dichloromethane at reflux temperature for 3 days, dried, and stored in a nitrogen dry box. The final solid contained 7.5 wt% sulfur (2.3 mmole S/g solid). 

---