Support precipitated silicas

A thin layer of a mix of natural rubber, sulfur, precipitated silica, water, and some additives, such as carbon black and vulcanizing agents, is extruded on a paper support belt, calendered, and vulcanized as a roll in an autoclave under elevated pressure and temperature ( 180 °C). A modi-... 

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. 

Liquid/liquid partition chromatography was explored by Willstatter from 1913. The process was extensively developed by Martin and Synge (ca. 1941-1948) who partitioned amino acid derivatives between chloroform and water using precipitated silica as support for the aqueous phase. The preparations of silica were again very variable and it was difficult to prevent adsorption which interfered with the expected behavior of the aminoacids. At first methyl orange was added to the water phase to visualize the amino acids the separation of the acids then caused a red band to move down the columns. The quantitative reaction with ninhydrin was introduced by Moore and Stein in 1948 for both the detection and estimation of the amino acids. Consid-... 

A simple procedure for the preparation of Ni/Si02 catalysts involves the precipitation of nickel dimethylglyoxime from a suspension of silica in an aqueous solution of a nickel salt.38,39 Calcination is not required since the resulting supported precipitate is easily reduced in a stream of hydrogen at 200°-250°C, giving very highly dispersed supported nickel catalysts. 

Differently loaded (2-7 wt%) Mo03/Si02 and (2-50 wt%) 205/8102 catalysts were prepared by incipient wetness impregnation of a "precipitated" silica (PS) support (Si 4-5P Grade, Akzo Product S.A.bet, 400 m -g ) according to the procedure described elsewhere [3]. The list of the catalysts along with their composition and BET surface area values are reported in Table 1. 

A slight inflection near 3660 cm-1 can just be discerned in Figure 1 A. This band is due to perturbed or inaccessible internal silanols (self-supporting disks is increased. They are largely inaccessible to many reactant gases, and this topic is discussed in more detail later in the chapter. The 3720-and 3660-cm-1 features are not resolved in the spectrum of the precipitated silica (Fig. 2A), which instead exhibits a broad feature near 3695 cm-1. 

A 1.5 % w/w Ni supported on silica (Cab-O-Sil 5M, 194 m g ) catalyst precursor was prepared by homogeneous precipitation/deposition as described in detail elsewhere (24). The hydrated sample, sieved in the 150-200 pm range, was reduced, by heating directly in a 100 cm min stream of dry H2 (99.9%) at 5 K min to 673 1 K which was maintained for 18 h. Under these activation conditions the catalyst supports 1.1 x 10 exposed Ni atoms/g catalyst with a surface weighted average Ni diameter = 1.4 nm these values are based on chemisorption measurements (12). 

Ni supported on silica-alumina using three different techniques homogeneous deposition precipitation, impregnation and coprecipitation... 

This work was carried out with (1) a fumed silica (Cab-O-SU HS5 designated A-x, where x is the temperature of activation for 1 h in vacuum) having a Brunauer— Emmett—Teller (BET) (N2) surface area of 325 + 5 m /g and (2) a high-purity (Na 60 ppm, A1 <100 ppm, Fe 20 ppm, and Ti <20 ppm) nonporous precipitated silica (designated P-x) from Rhone-Poulenc (France) having a surface area of 285 5 m /g. The powder (50 mg total, lOmg/cm ) was compacted at 10 Pa into thin self-supporting disks for IR transmission studies. The IR cell [4] was constructed of quartz, had a volume of about... 

The filled composites containing 5 phr of nano-ZOS and 10-15 phr of SCC exhibited comparable tensile properties, tear strength and hardness as that of the microcomposite containing 5 phr of micro-ZnO and 30 phr of precipitated silica. These observations also support the findings that stearic acid-coated nano-ZnO (ZOS), nano-BIAT and SCC are better dispersed due to the small size of the particles as well as the higher specific surface area as already discussed in this chapter. 

Molybdate-Based Catalysts. The first catalyst commercialized by SOHIO for the propylene ammoxidation process was bismuth phosphomolybdate, Bi9PMoi2052, supported on silica (9). The catalytically active and selective component of the catalyst is bismuth molybdate. In commercial fluid-bed operation, the bismuth molybdate catalyst is supported on silica to provide hardness and attrition resistance in the fluidizing environment. Bismuth molybdate catalysts can be prepared by a coprecipitation procedure using aqueous solutions of bismuth nitrate and ammonium molybdate (10). The catal3ret is produced by drying the precipitate and heat treating the dried particles to crystallize the bismuth molybdate phase. Heat treatment temperature for bismuth molybdate catalysts is generally arovmd 500°C. 

Thus far, in this chapter, organic polymers have been used to create molecular imprinted networks but alternative supports have also been employed. Indeed, the first reported example of molecular imprinting was achieved with silica. Dickey precipitated silica gel using a dye as a template, and the corresponding matrix exhibited an increased affinity... 

Palladium chloride supported on silica gel and "y-alumina are active for the dimerization of ethylene [252]. PdCl2(Me2SO)2 and K PdCl,(MeSO) on KSH-2 silica gel catalyze ethylene dimerization. The yield of dimer and selectivity are high with use of 0.1% dimethylsulfoxide complex precipitated from acetone at 90°C and with 0.3% ionic complex precipitated from chlorobenzene at 100°C [252-255]. 

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