Silica impurities

At high temperatures, coke reduces silica impurities in iron ore to elemental silicon ... 

Bands at 3745 and 3690 cm 1 are also present. The 3745 cm-1 band has usually been attributed to silica impurities occluded within the zeolite framework or to Si—OH groups terminating the lattice. The 3690 cm 1 band is due to undissociated water since its intensity is dependent upon the degree of hydration. 

Support used The type of support, e.g., gamma-, eta-, boehmite-alumina, could greatly affect the disposition and/or degree of interaction with Mo and Co. Also, impurities in the support could change the acidic character of the support and consequent interactions. Some commercial aluminas contain sodium, chloride, sulfate, or silica impurities. 

Calcium oxide, and other basic promoters, react with silica impurities in the raw materials to form glassy silicates. The glassy silicates enhance the thermal stability of the reduced iron. The main benefit is to minimize any neutralization of the K2O promoter, which would reduce its effectiveness204. 

The source of the silica can be either the treating reagent (14) or the zeolite sample itself through rearrangement of silica from another part of the framework or from a silica impurity within the sample. (Note that in the chromic salt case, chromia rather than silica is claimed to be inserted in the structure.) Because of the dealumination and the silica insertion, the treated samples are usually found to have a higher framework silicon to aluminum ratio and a higher thermal stability than the untreated materials. Furthermore, hydrophobic surface properties usually result from substantial dealumination (7). 

Also shown in Figure 2.1 are two other components a precolumn and a guard column. The precolumn, located in the system between the solvent reservoir and the pump, acts to filter out any impurities in the mobile phase before they reach the pump heads and the analytical column. In the case of analytical columns made of silica, impurities in the solvents can result in leaching of the silica. If solvents are made daily and are filtered (see below), a precolumn may not be needed. 

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