Silica-Based Catalysts

Synthesis of 1,2,3-Triazoles Using Silica Gel-Supported Cu Catalysts Silica-based supports are easily available and environmentally benign. They have been widely utilized for heterogeneous Cu catalysis, which opens up new opportunities for the synthesis of greener clicked compounds. Wang s group immobilized a... 

An example for a non-structure-sensitive reaction is provided by Davis et al. [102], who investigated the liquid-phase hydrogenation of glucose over carbon and silica based ruthenium catalysts with particle sizes between 1.1 and 2.4 run. Depending on catalyst loading which was between 0.56 wt.% and 5 wt.%, dispersion decreased from 91% to 43%. At the same time, TOFs varied only insignificantly in a range between 0.21 1/s and 0.32 1/s. 

As it was mentioned, the chemistry of the silica based sol-gel process comprises several steps. First, silicate precursor (e.g., tetraethylortosilicate TEOS or tetramethylosilicate TMOS) is mixed with water and catalyst and stirred for a few hours. This process leads to hydrolysis of the Si-O-R bonds. The hydrolysis reaction can be catalyzed by acids (HC1, HF, etc.) or bases (NH4OH, NaOH, etc.). The process is schematically described by equation ... 

DeDiox A process for destroying polychlorinated dioxins and furans in flue-gases by catalytic oxidation with hydrogen peroxide. The catalyst is based on silica and the process is operated at 80 to 100°C. Developed by Degussa from 1994. The business was offered for sale in 1998. 

The high-density polyethylene is linear and can be manufactured by (i) coordination polymerisation of monomer by triethyl aluminium and tritanium chloride, (ii) polymerisation with supported Metal Oxide Catalysts. Such as chromium or molybdenum oxides supported over alumina-silica bases. 

Molecular imprinting is not limited to organic polymer matrices, but can also be applied to silica-based materials and even proteins. Proteins freeze-dried in the presence of a transition state analogue as template have been used successfully as catalysts, e.g., for the dehydrofluorination of a fluorobutanone. For instance, lyophilized 3-lactoglobulin imprinted in this manner with N-isopropyl-N-ni-trobenzyl-amine could accelerate the dehydrofluorination by a factor of 3.27 compared to the non-imprinted protein see Table 5 [62]. In a similar procedure, BSA was imprinted with N-methyl-N-(4-nitrobenzyl)-S-aminovaleric acid and showed an enhancement of the catalytic effect by a factor of 3.3 compared to the control protein for the same reaction see Table 5 [113]. 

Another way to produce acetic acid is based on a carbonylation of methanol in the so called Monsanto process, which is the dominant technology for the production of acetic acid today [15]. Acetic acid then is converted to VAM by addition of ethylene to acetic acid in the gas phase using heterogeneous catalysts usually based on palladium, cadmium, gold and its alloys (vinylation reaction 3 in Fig. 2) [16] supported on silica structures. 

Boron phosphate is used as an acid catalyst for dehydration of alcohols to olefins isomemization of olefins nitration of aromatic hydrocarbons polymerization of aldehydes and other synthetic reactions. It also is used as a flux in silica-based porcelain and ceramics special glasses and acid cleaners. 

Silica is of particular importance because of its use as a stable catalyst support with low acidity and its relationship to zeolite catalysts, which will be discussed in chapter 4. Silicon is an abundant material in the earth s crust and occurs in various forms including silica. Silica is also polymorphous with the main forms being quartz, cristobalite and trydimite. The stable room temperature form is quartz (Si02). Recently, a new family of stable silica-based ceramics from chemically stabilized cristobalites has been described using electron microscopy (Gai et al 1993). We describe the synthesis and microstructures of these ceramic supports in chapters 3 and 5. 

Stable silica-based ceramic oxide supports for catalysts some recent developments... 

Figure 11 Probable reaction mechanism for the transesterification of esters catalyzed by a supported titanate catalyst on silica (based on Blandy et al.f ...

TBT = tetrabutyl titanate and C94 catalyst = titanium and silica-based catalyst. 

The nature of the support can have a very profound influence on the catalyst activity. Thus, phosphinated polyvinyl chloride supports are fairly inactive (75), and phosphinated polystyrene catalysts are considerably more active (57), but rather less active particularly when cyclic olefins are the substrates than phosphinated silica supports (76). Silica-supported catalysts may be more active because the rhodium(I) complexes are bound to the outside of the silica surface and are, therefore, more readily available to the reactants than in the polystyrene-based catalysts where the rhodium(I) complex may be deep inside the polymer beads. If this is so, the polystyrene-based catalysts should be more valuable when it is desired to hydrogenate selectively one olefin in a mixture of olefins, whereas the silica-based catalysts should be more valuable when a rapid hydrogenation of a pure substrate is required. 

Since the first synthesis of mesoporous materials MCM-41 at Mobile Coporation,1 most work carried out in this area has focused on the preparation, characterization and applications of silica-based compounds. Recently, the synthesis of metal oxide-based mesostructured materials has attracted research attention due to their catalytic, electric, magnetic and optical properties.2 5 Although metal sulfides have found widespread applications as semiconductors, electro-optical materials and catalysts, to just name a few, only a few attempts have been reported on the synthesis of metal sulfide-based mesostructured materials. Thus far, mesostructured tin sulfides have proven to be most synthetically accessible in aqueous solution at ambient temperatures.6-7 Physical property studies showed that such materials may have potential to be used as semiconducting liquid crystals in electro-optical displays and chemical sensing applications. In addition, mesostructured thiogermanates8-10 and zinc sulfide with textured mesoporosity after surfactant removal11 have been prepared under hydrothermal conditions. 

The development of new porous materials that could be used as adsorbents, catalysts, catalyst supports, molecular sieves, etc. [1], are very well discussed by several authors [2-9], describing interesting properties and characteristics of materials such as MCM-41, MCM-48, M41S, FSM16, lamellar phases, intercalation products, special CMS (carbon molecular sieves), fullerenes, carbon nanotubes, etc. being some of them silica based materials, and carbon based the others. 

Figure 9 shows more completely the relationship between activity and calcining temperature. Here activity is defined as the inverse of the time needed to make SOOOg of polymer per gram of catalyst. Activity increases with increasing calcining temperature up to a maximum at around 925°C, and then declines as sintering destroys the surface area and porosity of the silica base. Krauss has shown that the coordinative unsaturation of Cr(II)/ silica follows a similar trend (36). 

Fig. 13. Titania, cogelled into the silica base, serves to promote the polymerization activity of chromium. The members of this series of catalysts, calcined at 760°C, differ in the amount of titania added.

The reaction of adipic acid with ammonia in either liquid or vapor phase produces adipamide as an intermediate which is subsequently dehydrated to adiponitrile. The most widely used catalysts are based on phosphorus-containing compounds, but boron compounds and silica gel also have been patented for this use (52—56). Vapor-phase processes involve the use of fixed catalyst beds whereas, in liquid—gas processes, the catalyst is added to the feed. The reaction temperature of the liquid-phase processes is ca 300°C and most vapor-phase processes mn at 350—400°C. Both operate at atmospheric pressure. Yields of adipic acid to adiponitrile are as high as 95% (57). 

In the supported systems the catalyst can be coated on the walls of the reactor, supported on a solid substrate or deposited around the case of the light source. Many are the supported materials used in literature, such as glass beads, and tubes [69], silica-based materials [70], hollow beads, membranes [71], optical fibers, zeolites, activated carbon, organic fibers [72], and so on. 

Scheme 3.13 Preparation of catalyst materials based on Pd-nanoparticles on different silica supports. Pd cluster [Pd561phen60(OAc)180], (Cp)Pd(allyl) [Pd(i73-C3H5)(f/5-CsH5)], TEOS tetraethoxysilane (Reproduced from Ref. [68] by permission of The Royal Society of Chemistry)...

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