Heterogeneous silica-supported catalysts

Finally, the concept was broadened by supporting these titanium silsesquiox-anes on silica remarkably, the heterogeneous silica-supported catalysts displayed an epoxidation activity per mole of titanium (94% TBHP conversion after 3 h) similar to that of the homogeneous titanium-silsesquioxane a2b4 complexes, although with a lower selectivity towards 1,2-epoxyoctane (92%). These heterogeneous catalysts did not leach active species and proved to be recyclable [45]. 

Silica-supported Lewis acids are useful catalysts with microwave irradiation for conjugate additions. The silica-supported catalysts are obtained by treatment of silica with ZnCh [Si(Zn)], Et2AlCl [Si(Al)] or TiCl4 [Si(Ti)] [ 150-152], The Michael addition of methyl a-acetamidoacrylate (196) with indole (2) under Si(M) heterogeneous catalysis assisted by microwave irradiation afforded the alanine derivative 197 within 15 min and/or bis-indolyl 198, depending on the reaction conditions (Scheme 45) [153]. While the bis-indolyl product 198 is only formed when Si(Zn) was used as catalyst, the alanine derivative 197, as a single product is formed under thermal heating in a yield of 12%. The best yields were observed with Si(Al) (Table 5). The product 198 was obtained by elimination of acetamide followed by a-Michael addition between intermediate 199 with a second mole of indole. 

The hydroperoxidation method was first developed by Halcon Corp and Atlantic Richfield Oil Corp (now Lyondell) in the 1970s, and was then also implemented commercially by Shell. The catalysts for this reaction are either homogeneous Mo complexes (Halcon/ARCO) or heterogeneous silica-supported Ti" (Shell) [2, 3]. 

Shell subsequently developed a heterogeneous, silica-supported titania catalyst [11,12] which forms the basis of the commercial process for the epoxidation of propylene with ethylbenzene hydroperoxide. The co-product alcohol is dehydrated, in a separate step, to styrene. Ti(IV)Si02 was the first truly heterogeneous epoxidation catalyst useful for continuous operation in the liquid phase. 

The heterogeneous catalyst was prepared by reacting 0.5 g of silica containing MAO within the silica pores with 25 mg of complex 24 and 10 ml of additional toluene and 5 ml of additional MAO/toluene solution. The contents of this slurry were stirred and ethylene was added (1 atm) to initiate a prepolymerization process that was carried out to increase the total solids to 2.06 grams which were isolated by solvent evaporation in vacuo. The silica-supported catalyst was evaluated in a 2-liter reactor containing one liter of isobutane at 80°C, 35 bar total pressure with TIBA as cocatalyst. Linear polyethylene was produced with an M of 1470 and M M of 2.13 with a catalyst activity of 1309 Kg PE/g Cr/hr. These results clearly demonstrate that this particular single-site catalyst could be operated in commercial polyethylene manufacturing operations. 

In a heterogeneous SPS polymerization at a typical reaction temperature (40-90 °C) with a silica-supported catalyst, the reaction mixture undergoes a series of physical changes as SPS particles precipitate out from the liquid phase. Initially, the total solid content (TSC) in the reaction mixture is very small and it is a clear liquid (Fig. 8.5a). 

Rajasekhar et al. also synthesized a new class of diethyl a-aryl/2-thienyl-a-[2-(phenylthio)phenylamino]methylphosphonates (54) via a three-component Kabachnik-Fields reaction of 2-aminodiphenylsulfide (53), substituted phenyl/heterocyclic aldehydes (44), and diethyl-phosphate (2) in the presence of heterogeneous nano-silica-supported nano-BFs SiOa under solvent-free conditions under microwave irradiation (Scheme 25). The procedure has several advantages such as short reaction time, low loading of catalyst, good yields, and reusability of the heterogeneous silica-supported nano-catalyst. The title compounds were found to exhibit considerable in vitro antibacterial and antifungal activity. 

The metal catalyzed production of polyolefins such as high density polyethylene (HDPE), linear low density polyethylene (LLDPE) and polypropylene (PP) has grown into an enormous industry. Heterogeneous transition metal catalysts are used for the vast majority of PE and all of the PP production. These catalysts fall generally within two broad classes. Most commercial PP is isotactic and is produced with a catalyst based on a combination of titanium chloride and alkylaluminum chlorides. HDPE and LLDPE are produced with either a titanium catalyst or one based on chromium supported on silica. Most commercial titanium-based PE catalysts are supported on MgCl2. 

Silica-supported heterogenous Cr systems, such as the Phillips247,248 and Union Carbide catalysts,249,250 are used in the commercial production of polyethylene. The active sites are widely agreed to contain low-valent Cr centers. The relatively ill-defined nature of these catalysts has led to considerable efforts to synthesize well-defined homogeneous Cr-based catalysts. 

Both heterogeneous and homogeneous CO reduction catalyst recipes often contain electrophilic components such as silica supports, metal oxides, and A1Cl3 [1,5,33,34,35,36]. 

The use of heterogeneous catalysts in this reaction has also been achieved palladium-montmorillonite clays [93] or palladium/activated carbon [94] in the presence of dppb transformed 2-allylphenols into lactones, the regiose-lectivity of the reaction being largely dependant on the nature of the support. Very recently, palladium complexes immobilized onto silica-supported (polyaminoamido)dendrimers were used as catalysts in the presence of dppb for the cyclocarbonylation of 2-allylphenols, 2-allylanilines, 2-vinylphenols, and 2-vinylanilines affording five-, six-, or seven-membered lactones and lactams. Good conversions are realized and the catalyst can be recycled 3-5 times [95]. 

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