Heterogeneous catalyst synthesis

As can be seen from the works analyzed, modification of hemin-containing heterogeneous catalyst synthesis has been developed gradually. At present, catalysts obtained by the methods designed are being applied to complex production processes on pilot units and are approaching industrial use. 

Starburst dendrimers have received considerable attention in the area of heterogeneous catalyst synthesis in the last decade. Although Tomalia et al. and Bosman et al. originally discovered these hyperbranched macromolecules and their host-guest properties in the mid-1980s. Crooks and coworkers were the first to demonstrate the ability of poly(amidoamine) (PAMAM) starburst dendrimers to act as metal nanoparticle stabilizers that could potentially aid in the synthesis of supported metal catalysts. The benchmark work of Crooks et al. and subsequent literature reports have underscored the advantages of successfully utilizing PAMAM dendrimer-nanocomposite precursors over conventional catalyst preparation methods. 

These texts cover different catalytic principles and disciplines along with their application to industrial practice. Collectively they span a wide range of material including basic concepts in heterogeneous catalyst synthesis, characterization, kinetics, reaction engineering and their application to industrial catalytic systems. 

Attenlion should be drawn to ihe use of tin oxide systems as heterogeneous catalysts. The oldest and mosi extensively patented systems are the mixed lin-vanadium oxide catalysis for the oxidation of aromatic compounds such as benzene, toluene, xylenes and naphthalene in the. synthesis of organic acids and acid anhydride.s. More recenily mixed lin-aniimony oxides have been applied lo the selective oxidaiion and ammoxidaiion of propylene to acrolein, acrylic acid and acrylonilrile. 

A low-pressure process has been developed by ICl operating at about 50 atm (700 psi) using a new active copper-based catalyst at 240°C. The synthesis reaction occurs over a bed of heterogeneous catalyst arranged in either sequential adiabatic beds or placed within heat transfer tubes. The reaction is limited by equilibrium, and methanol concentration at the converter s exit rarely exceeds 7%. The converter effluent is cooled to 40°C to condense product methanol, and the unreacted gases are recycled. Crude methanol from the separator contains water and low levels of by-products, which are removed using a two-column distillation system. Figure 5-5 shows the ICl methanol synthesis process. 

Methanol synthesis over the heterogeneous catalyst is thought to occur by a successive hydrogenation of chemisorbed carbon monoxide. 

Active heterogeneous catalysts have been obtained. Examples include titania-, vanadia-, silica-, and ceria-based catalysts. A survey of catalytic materials prepared in flames can be found in [20]. Recent advances include nanocrystalline Ti02 [24], one-step synthesis of noble metal Ti02 [25], Ru-doped cobalt-zirconia [26], vanadia-titania [27], Rh-Al203 for chemoselective hydrogenations [28], and alumina-supported noble metal particles via high-throughput experimentation [29]. 

Table 6.2 Rationalization of the oxidation chemistry applied in the synthesis of heterogeneous catalysts and related functional materials.

In the following we shall study a model system for the synthesis of hydrogen peroxide (H2O2) over a heterogeneous catalyst containing a hypothetical metal M. It is proposed to split the reaction into the following elementary steps, which are all assumed to be in quasi-equilibrium except for step 3, which is assumed to be the rate-limiting step ... 

Clay-supported heteropoly acids such as H3PW12O40 are more active and selective heterogeneous catalysts for the synthesis of MTBE from methanol and tert-butanol, etherification of phenethyl alcohols with alkanols, and alkylation of hydroquinone with MTBE and tert-butanoi (Yadav and Kirthivasan, 1995 Yadav and Bokade, 1996 Yadav and Doshi, 2000), and synthesis of bisphenol-A (Yadav and Kirthivasan, 1997). 

The synthesis and olefin metathesis activity in protic solvents of a phosphine-free ruthenium alkylidene bound to a hydrophilic solid support have been reported. This heterogeneous catalyst promotes relatively efficient ring-closing and cross-metathesis reactions in both methanol and water.200 The catalyst-catalyzed cross-metathesis of allyl alcohol in D20 gave 80% HOCH2CH=CHCH2OH. 

Cong, P., Doolen, R., Fan, Q. et al. (1999) High-throughput synthesis and screening of combinatorial heterogeneous catalyst libraries. Angew. Chem. Int. Ed., 38, 484. 

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