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) |
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.
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.
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:
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