Sulphuric acid silica support

The silica carrier of a sulphuric acid catalyst, which has a relatively low surface area, serves as an inert support for the melt. It must be chemically resistant to the very corrosive pyrosulphate melt and the pore structure of the carrier should be designed for optimum melt distribution and minimum pore diffusion restriction. Diatomaceous earth or synthetic silica may be used as the silica raw material for carrier production. The diatomaceous earth, which is also referred to as diatomite or kieselguhr, is a siliceous, sedimentary rock consisting principally of the fossilised skeletal remains of the diatom, which is a unicellular aquatic plant related to the algae. The supports made from diatomaceous earth, which may be pretreated by calcination or flux-calcination, exhibit bimodal pore size distributions due to the microstructure of the skeletons, cf. Fig. 5. 

In 1734, G. E. Stahl propounded the hypothesis that like sulphurous acid, nitric acid is a compound of sulphuric acid and phlogiston, but the proportions of phlogiston in the two acids are different. In 1750, J. G. Pietsch wrote a thesis in support of this view but in 1786, P. Thouvenal showed that while nitric acid is produced by the putrefaction of organic matter, sulphuric acid is never so formed. G. A. Saluzzo considered nitric acid to be a compound of an empyreumatic acid, an alkaline salt, lime, and a little silica and T. Bergman regarded it as a dephlogisticated plant acid produced by fermentation. 

Yet another method for the preparation of hydroxycitronellal (35) has been developed it depends on the fact that the immonium salt (36) is hydrated by aqueous sulphuric acid, hydrolysis of the imine group taking place with sodium hydroxide. The effect of catalysts supported on silica gel on the well-known thermal conversion of citronellal to isopulegol has been studied. The abstract... 

For the selective absorption of Cs-Cg olefins a reactor (2.0 x0.4 cm) containing silica gel impregnated with concentrated sulphuric acid, was used. The adsorbent was prepared by mixing concentrated sulphuric acid and silica gel (fraction of 60-200 mesh) (3 2, w/w). Silica gel is a good filler for sulphuric acid, as it remains friable even on absorption of a larger amount of acid than its own weight. Other supports (cement, refractory brick) were found to be inefficient. The absorbent must be stored in hermetically sealed vessels as the olefins are no longer absorbed quantitatively if the content of water in the sulphuric acid is about 12% or above. 

The EDX spectrum (Fig. 11.8) shows the main surface scale impurity peaks of silica, aluminium, sodium, chloride and iron. If this EDX is compared to that of a new, clean membrane surface (Fig. 11.9), the clean surface shows sulphur, carbon and oxygen, which is typical of a porous polysulphone support. It was concluded that the scale is amorphous, composed of aluminosilicate and silicate. These compounds are normally found in trace amounts in brine solutions. Analysis showed that the surface could be cleaned with hydrochloric acid and analysis of the dissolved scale was similar to the EDX spectrum analysis. Review of the plant operation determined that the precipitation was the result of high pH in combination with high silica concentrations in the brine. 

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