Sorption kinetics of n-hexane on MgA zeolites of different crystal sizes. Study of the rate-limiting transport mechanism

Abstract

Starting from the necessity for more substantial evidence with regard to the occurrence of intracrystalline diffusion as the rate-limiting transport mechanism in sorption processes on zeolites, the variation of zeolite crystal size is regarded as a criterion and used in the case of the sorption system n-hexane/MgA zeolite.

The experimental study has been carried out by using a constant volume variable pressure system. The uptake curves have been analysed by solution of the appropriate diffusion equation on the assumption that the uptake rate obtained is limited by intracrystalline diffusion. An additional analysis was done considering the sorption process as being limited by surmounting a transport resistance in the surface region of zeolite crystals.

The experimental parameters covered the following ranges: temperature: 373–623 K, pressure: ≈ 4 × 10^–3–265 Pa, amount sorbed: ≈ 2 × 10^–4–3 × 10^–1 mmol g^–1, crystal size (mean edge length of cubic crystals): 0.7–43 µm.

Over the whole of the ranges of amount sorbed and crystal size the apparent diffusion coefficient is independent of amount sorbed at constant temperature. At each temperature an unambiguous dependence of the apparent diffusion coefficient on zeolite crystal size has been found.

Taking into account the analysis of sorption kinetics by a surface barrier model, the shape of uptake curves, the values of the apparent energy of activation and the peculiarities of crystal habit of several sample sizes the following conclusions concerning the rate-limiting transport mechanisms can be drawn: (i) In the crystal size region 0.7–13 µm a surface barrier is possible (for several crystal samples a surface barrier dependent on the crystal size is probable at lower temperatures). (ii) In the case of zeolite crystals with the sizes 33–43 µm the uptake process seems to be limited by intracrystalline diffusion.

The energy of activation assumes values of ≳ 50 kJ mol^–1 and ≈ 42 kJ mol^–1 for the cases (i) and (ii), respectively.