Sorption and Diffusion Microporous Solids.

The most technologically important microporous materials are zeolites which are large alumino-silicate framework sructures with charge compensating extra framework cations such as Na, K and Mg ensuring charge neutallity. Zeolites are extensively used in industry for a number of tasks including water softening and as molecular sieves but most importantly for catalysis. In this case the extra framework cations can include 'protons' giving rise to acid catalysed reactions. Another feature of zeolites which is useful in catalysis is that the pore structure (size and shape) limits diffusion of reactants and products giving rise to shape selectivity.

Another class of microporous solids are the ALPO's. Many of their strutures are very similiar to zeolites except that the frameworks are composed of Al, P and O.

Structure of Zeolites.

Simulation can be used to help resolve structures of these complex unit cells and can predict the effect defects such as the extra framework cation will have on local strcuture.


Pure Zeolite L
(190K)

Thermal expansion of microporous materials.

Free energy minimisation can be used to examine the thermal expansion of crystals by calculating the variation in lattice vectors as a function of temperature. If this approach is applied to zeolites a surprising result is obtained. Out of the five siliceous zeolites considered, cancrinte, sodalite, omega, zeolite L and zeolite X, the latter three give rise to negative thermal expansion coefficients. i.e. the cells are predicted to contract on heating. Additional simulations have shown that zeolite L and X which contain Al and charge compensation cations also show negative thermal expansion.

The contraction in zeolite L is not isotropic and the table below shoes that the a axis is 70% and 40% more compressible than the c axis for siliceous and non siliceous.

50 K

100 K 200 K 300 K 400 K 500 K

siliceous

a (A)

18.051 18.048 18.041 18.034 18.026 18.020

c (A)

7.547 7.545 7.540 7.534 7.529 7.524

non siliceous

a (A)

18.199 18.200 18.194 18.190 18.187 18.184

c (A)

7.607 7.607 7.605 7.604 7.602 7.601

related references:

  1. Tschaufeser P. and Parker S.C.
    'Thermal expansion behaviour of zeolites and ALPO(4)s'
    J. Phys. Chem. 99, 10609-10615 (1995)

  2. Couves J.W., Jones R.H., Parker S.C., Tschaufeser P. and Catlow C.R.A.
    'Experimental verification of a predicted negative thermal expansivity of crystalline zeolites'
    J. Phys: Condens. Matter 5, L 329-332 (1993)

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