Chapter 13 - Characterization of zeolites by sorption capacity measurements

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This chapter covers the sorption capacity measurement as it provides one of the simplest and most direct ways of characterizing a zeolites. However, the information derived from capacity measurement generally provides only an estimate of sample purity and/or evidence of consistency with a known structure, rather than a means of differentiation between different structures. The molecular volume of the adsorbed phase corresponds closely to that of the saturated liquid sorbate at the same temperature so the measured saturation capacity can be easily converted to the specific micropore volume or, knowing the crystal density, to the intracrystalline void fraction. Automated BET measurement systems such as Omnisorb can also be used to measure saturation capacity. In such devices a dilute stream of sorbate in an inert (He) carrier is passed through the sample and the capacity is found by integration of the measured breakthrough curve. Prior to a capacity measurement any organic template from the synthesis must be removed by oxidation and the sample must be thoroughly degassed. In principle almost any of the common small molecules can be used as the probe sorbate although, in practice, the choice is generally restricted to Ar, O2 or N2. Linear paraffins such as nhexane have sufficient flexibility to pack within the micropores almost as effectively as the smaller molecules so n-hexane capacities are also commonly used to measure specific micropore volume. Bulkier molecules such as i-butane pack less efficiently and thus yield erroneously low estimates for the micropore volume.

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