Tracer-exchange experiments with positron emission profiling: diffusion inzeolites

A new technique for tracer-exchange experiments with positron emission prof iling, was developed which permits the spatially resolved, in situ investig ation of kinetic processes in fixed-bed reactors under steady-state conditi ons. Modeling of the complete reactor proves to be a valuable tool for the design and interpretation of experiments. To validate the new method, the micropore diffusion of 2-methylpentane and n-hexane in silicalite has been investigated. For n-hexane, a diffusivity o f 7+/-1 x 10(-8) cm(2) s(-1) was measured at a temperature of 413 K with an activation energy of 19+/-3 kJ mol(-1). The diffusivity of 2-methylpentane at the same temperature was 1.0 +/- 0.2 x 10-8 cm(2) s(-1) with an activat ion energy of 29 +/- 3 kJ mol(-1), clearly showing the influence of the add itional steric hindrance due to branching. (C) 2000 Elsevier Science B.V. A ll rights reserved.