The widespread occurrence of negative thermal expansion in zeolites

Powder X-ray and neutron diffraction studies of the calcined siliceous zeol ites ITQ-7, ITQ-9 and CIT-5, and the aluminophosphates AlPO4-31 and Mg-dope d AlPO4-17 have been carried out as a function of temperature in order to d etermine their thermal expansivity properties. Of these, ITQ-7, ITQ-9 and M APO-17 show negative thermal expansivity over a wide temperature range, in common with several other previously studied microporous materials. CIT-5 a nd AlPO4-31 are anomalous in showing an expansion on heating. A detailed st udy of CIT-5 highlights the key structural parameters which determine its p ositive expansivity. From expansivity data currently available on seventeen microporous materials, it is clear that negative thermal expansivity must be considered the norm rather than the exception. Positive expansivity is e ncouraged by the structure having a relatively high framework density and a one-dimensional channel system.