Silica composite beads

MIPs can be synthesized in the pores and on the surface of pre-made porous particles. Porous silica particles have been applied for this purpose. To ensure that the imprinted polymer is attached firmly to the particle, the particles are often chemically modified by coupling of polymerizable groups or initiator molecules to the particle surface prior to the MIP polymerization [100-102]. The use of immobilized initiators is often referred to as the iniferter (initiator-transfer agent-terminator) approach [103]. The method has been applied to the imprinting of a range of templates [104—107]. 

The silica can be removed from a MIP-silica composite bead by dissolution with hydrofluoric acid, leaving a MIP bead that is a replica of the pore structure of the original silica bead [108, 109], MIP composites have also been made using, for example, chitosan as the support [110, 111], 

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A preformed chitosan-silica composite with 60% weight inorganic part [7] is used as the source of silica for the zeolite synthesis. An alkaline solution of sodium aluminate (Na 2.1 M, Al 1 M) was used in three methods of preparation (A) beads of the chitosan-silica composite were stirred overnight in the aluminate solution, extracted and submitted to a hydrothermal treatment at 80 °C during 48h (B) beads of the chitosan-silica composite were immersed in the aluminate solution and the system underwent a hydrothermal treatment at 80 °C for 48h (C) beads of the chitosan-silica composite were stirred overnight in the aluminate solution, extracted, dried at 80 °C and exposed to water vapour at 80°C during 48h. 

In the X-ray powder diffraction patterns of the composites, the disappearance of the broad band centered at 22 °20, typical of amorphous silica, indicates that the zeolitisation of the mineral fraction of the parent composite was complete. In no diffraction pattern any sign of crystallised chitosan could be found. The two methods in which the silica-polymer beads were extracted from the aluminate solution after impregnation (methods A and C) allowed the formation of the expected zeolite X, with traces of gismondine in the case of the method C. The method B, in which excess aluminate solution was present during the hydrothermal treatment, resulted in the formation of zeolite A. 

Fig. 8 Schematic representation of the so-called sacrificial silica approach to porous MIP beads and examples of SEM micrographs of the resulting beads (a) silica beads (b) composite beads (silica + MIP) (c) MIP beads after silica etching (d) ground bulk MIP employed as control. Reproduced with permission from [107]...

Weisz (2) carried experiments on silica- alumina beads (many times the size of an average FCC particle). Heobservedthattheintrinsic cokebuming rate was independent of the coke composition and the catalyst characteristics but dependent on initial coke level and the diffusivity. Weisz (3) inanotherstudy, found that the CO /CO ratio during intrinsic coke burning is only a function of temperature. He also observed that this ratio i s affected by the presence oftrace metals like iron and nickel etc. Even though this study was elaborate, it was limited to only silica-alumina catalysts in the form of beads. 

To overcome the disadvantage of the low surface area (< 0.02 m g ) of pure Nafion NR50 beads, researchers at DuPont developed Nafion-silica composites, in which small (20-60 nm) Nafion resin particles are embedded in a porous silica matrix [7]. The composites, available under the trade name SAC 13 (containing 13 % (w/w) Nafion) are prepared by a sol-gel technique. Because of the higher surface area and accessibility of the active sites the application of this material as a solid-acid catalyst has become attractive. 

Different alkylation reactions have been investigated with Nafion as a catalyst. An example is the propylation of benzene to cumene. Large rate enhancements were observed on use of Nafion-silica composites, compared with bulk polymeric catalysts such as pure Nafion NR50 and Amberlyst-15. The activity of the composite was ca 6-7 times higher than that of pure Nafion beads on the basis of the total amount of catalyst. If this correlation were made on the basis of the total number of acid sites on Nafion, the activity would be ca 50 times higher. Amberlyst-15 was about twice as active as the Nafion NR50 particles (on the basis of the total amount of catalyst) [7]. 

Figure 7 Molecular imprinting using porous silica beads as support. Scanning electron micrographs for (a) porous silica bead support (b) silica-MIP composite beads and (c) MIP beads after removal of the support.

Particulate composites are made by blending silica flour, glass beads, even sand into a polymer during processing. 

Figure 4. (a) Adsorption-desorption isotherms of N2 at -196°C of 80°C-outgassed (empty squares) chitosan, (filled trangles) zeolite X-chitosan composite from in-situ zeolite synthesis and (empty triangles) zeolite Y-chitosan composite from encapsulation of the zeolite in the gelling chitosan. (b) Scanning electron micrographs of a calcined zeolite-chitosan bead prepared by zeolitisation of a silica-chitosan composite. 

Other composite photocatalysts were prepared by mounting immobilized anatase particles on mesoporous silica and silica beads [189-191], The behavior of anatase-mounted activated carbons was also studied in detail [192-194], It was even suggested that carbon-coated anatase exhibits better performance in photocatalysis than anatase itself, demonstrating high adsorptivity, inhibition of interaction with organic binders, etc. [195,196],... 

The desire to have catalysts that were uniform in composition and catalytic performance led to the development of synthetic catalysts. The first synthetic cracking catalyst, consisting of 87% silica (Si02) and 13% alumina (AI2O3), was used in pellet form and used in fixed-bed units in 1940. Catalysts of this composition were ground and sized for use in fluid catalytic cracking units. In 1944, catalysts in the form of beads about 2.5 to 5.0 mm in diameter were introduced and comprised about 90% silica and 10% alumina and were extremely durable. One version of these catalysts contained a minor amount of chromia (Cr203) to act as an oxidation promoter. 

Suspension arrays are based on addressable nano- or micrometric beads of various chemical natures (polymeric, silica, gold) and architectures (uniform composition, core-shell particles) to which NAs can be immobilized by means of activating chemistries similar to those described for fiat supports [64,65] depending on the composition of the particles outmost layer. 

To balance some of the drawbacks produced by the rubber toughening of thermosets, inorganic fillers that increase modulus and yield stress can be added to generate hybrid composites. Inorganic fillers such as glass beads, alumina, or silica - with high values of modulus and strength - are frequently included in thermoset formulations. 

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