Bimodal skeleton

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. 

Monolithic columns are another approach to provide lower pressure drops and higher rates of mass transfer. These are continuous solid columns of porous silica stationary phase instead of packed particles. Like perfusion packings, they have a bimodal pore structure (Figure 21.7). Macropores, which act as flowthrough pores, are about 2 fim in diameter. The silica skeleton contains mesopores with diameters of about 13 nm (130 A). It can be surface modified with stationary phases like Cig. The rod is shrink-wrapped in a polyetheretherketone (PEEK) plastic holder to prevent waU effects of solution flowing along the walls. The surface area of the mesopores is about 300 mVg, and the total porosity is 80%, compared with 65% for packed particles. The colunm exhibits a van Deemter curve approximating... 

Silica monoliths are prepared by sol-gel technology starting from alkoxide precursors, such as tetraethoxy-silane or tetramethoxy-silane [23]. The structure of a silica monolith is characterized by an interconnected silica skeleton and bimodal distribution of 1-3-pm macropores (flow through pores) and 10-20 nm mesopores. 

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