Silica support choice

Silica is the support of choice for catalysts used in processes operated at relatively low temperatures (below about 300 °C), such as hydrogenations, polymerizations or some oxidations. Its properties, such as pore size, particle size and surface area are easy to adjust to meet the specific requirements of particular applications. Compared with alumina, silica possesses lower thermal stability, and its propensity to form volatile hydroxides in steam at elevated temperatures also limits its applicability as a support. Most silica supports are made by one of two different preparation routes sol-gel precipitation to produce silica xerogels and flame hydrolysis to give so-called fumed silica. 

A significant volume of literature relates to our work. Concerning choice of support, Montassier et al. have examined silica-supported catalysts with Pt, Co, Rh Ru and Ir catalysts.However, these systems are not stable to hydrothermal conditions. Carbon offers a stable support option. However, the prior art with respect to carbon-supported catalysts has generally focused on Ru and Pt as metals.Additionally, unsupported catalysts have also been reported effective including Raney metals (metal sponges).Although the bulk of the literature is based on mono-metallic systems, Maris et al. recently reported on bimetallic carbon-supported catalysts with Pt/Ru and Au/Ru. In contrast, our work focuses primarily on the development of a class of rhenium-based carbon supported catalysts that have demonstrated performance equal to or better than much of the prior art. A proposed reaction mechartism is shown in Figure 34.2 °l... 

The newest and most commercially successful process involves vapor phase oxidation of propylene to AA followed by esterification to the acrylate of your choice. Chemical grade propylene (90—95% purity) is premixed with steam and oxygen and then reacted at 650—700°F and 60—70 psi over a molybdate-cobait or nickel metal oxide catalyst on a silica support to give acrolein (CH2=CH-CHO), an intermediate oxidation product on the way to AA. Other catalysts based on cobalt-molybdenum vanadium oxides are sometimes used for the acrolein oxidation step. 

The amorphous silica-supported amine systems show promising selectivity and recyclability for the heterogeneous catalysis of the Knoevenagel reaction (scheme 1). However they also demonstrate distinct limitations on the choice of solvent for the reaction and moderate turnover numbers.3 Materials prepared via grafting of HMS or in-situ preparation of organo-functionalised HMS will hopefully overcome these limitations. 

Aromatics such as benzene and toluene have also been used as a solvent for highly chelated metal complexes, such as in the preparation of supported metal acetylacetonates. Aromatics are a good choice for metal complexes formed with aromatic ligands such as bipyridil. Tetrahydrofuran (THF) has also been used as a solvent to prepare silica-supported Pd that was generated by the decoration of the surface with Pd(acac)2. °... 

The course of the decomposition of the mixed anhydrides [Scheme 44] which leads to the formation of expected esters (path A) or to a mixture of symmetrical carbonates and anhydrides (path B) strongly depends on the structures of the chloroformate and the carboxylic acid but also on the choice of the catalyst.. Because selective production of esters if of great interest, we have studied the thermal instability of the mixed anhydrides and developed a new efficient and selective esterification reaction with chloroformates using a silica supported guanidinium catalyst (Ref. 39). This method will be discussed in vol. 2 section 4-4. 

The vapor phase synthesis of methacrylic acid from propionic acid and formaldehyde was studied [42]. In particular, the choice of alkali metal cation and loading were evaluated for their effect on the activity and selectivity of silica supported catalysts. Experiments were carried out in 0.5 in. (o.d.) quartz reactors equipped with 0.125 in. thermowells. Alkali metal cations supported on silica are effective base catalysts for the production of methacrylic acid. Silica surfaces exchanged with alkali metal cations are capable of chemisorbing propionic acid yielding surface-bound silyl propionate esters and metal propionate salts. The alkali metal cation influences the temperature at which desorption of the ester occurs (Cs < Na < Li < support). For silica catalysts of equimolar cation loading, activity and selectivity to methacrylic acid show the opposite trend, Cs > K. > Na > Li. Methacrylic acid selectivity reaches a maximum at intermediate cation loadings where interaction of adjacent silyl esters is minimized [42]. 

In much of the definitive IR work on the silica surface researchers have chosen to work with fumed silica. This choice was mainly for experimental reasons (the ease of preparing the self-supporting disk), but also because it minimizes another important issue — the nature of porous silica surface. A major advance in the past decade has been in the controlled synthesis of many sUica polymorphs with variable pore size. Accordingly, the past decade has seen a renewed enthusiasm for the study of porous silicas, their reaction with chemical probes, and H2-D2 exchange reactions. An increasing body of evidence indicates that the basic silica structure is similar in both cases, but that accessibihty and derivatization of the porous silicas can stericaUy alter the process and the kinetics of the reactions. 

A typical example of this type of experiments is shown in Fig. 6. Two broad sets of signals are observable which are attributed to the Q - and Tn- groups present at the surface of the silica material SBA-3. The presence of different T -groups confirms not only the successful surface modification but also characterizes the method of attachment of APTES to the silica surface. This example clearly demonstrates that Si CP-MAS is the tool of choice for a detailed characterization of the modified surface of silica support materials. 

High-performance size exclusion chromatography is used for the characterization of copolymers, as well as for biopolymers (3). The packings for analyses of water-soluble polymers mainly consist of 5- to 10-/Am particles derived from deactivated silica or hydrophilic polymeric supports. For the investigation of organosoluble polymers, cross-linked polystyrene beads are still the column packing of choice. 

Determination of the acidic sites through IR spectroscopy of adsorbed CO is a valuable tool for the choice of the support when selective or multifunctional processes are to be set up. This technique allowed to identify a particular kind of silica as the support of choice for the selective hydrogenation of citral to citronellal and sepiolite as a Lewis acid support able to promote the one-step transformation of citral into menthol. 

Both ascending and descending paper chromatographic techniques have been used and, when thin-layer supports are employed, the use of either silica gel or cellulose is applicable. As the number of carbohydrates present in the sample is often small, the careful choice of solvent will generally make it unnecessary to perform the two-dimensional separations that are often needed when large numbers of substances, such as amino acids, are present. Reference solutions of each carbohydrate can be made up in concentrations of approximately 2 g 1 1 dissolved in an isopropanol solvent (10% v/v in water) and samples of about 10 fx should give discernible spots after separation. 

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