zeolites data analysis 31 October 2017

Zeolite pore size oddity

Structural vs functional classification

One of the ways to classify zeolites is usually by the size of their pore openings given in the peculiar unit of T atom count. T atom stands for tetrachedrally coordinated atoms, usually the term lumps together silicon and aluminum in case of zeolites or aluminum and phosphorus in case of aluminumphpsphates. The structural units characterising pores are usually rings from which the tube-like channels are formed (as shown below) and therefore the as described as n membered rings or nMR in short. Where n can be as small as 4 and as large as 20.

[picture of 8MR, 10MR and 12MR with a corresponding tube]

By inspecting the figure below showing the distribution of ring sizes among the known zeolite structures it is easy to see that 8, 10, and 12 MR structures dominate over the remaining topologies. Which justifies the why this parameter might serve as a good classification. It should also be taken into account that for most of the interesting applications topologies with MR < 8 are not intersting since molecules cannot enter the small channels and channels larger than 14 would have very similar properties.

IUPAC classification into nano, micro and mesoporous structures in a technical report.

It is customary to call 8, 10 and 12MR, small, medium and large pore zeolites suggesting that this classification provides distinct groups of similar characteristing being size. While intuitively and qualitatively correct, applying it directly can lead to incorrect insights since this MR measure provides limited structural information. A closer inspection of each class revelas that in each class there is a distribution of sizes as evaluated by different criteria actually associated with a more functional definition of pore size.

apply clustering based on size largst cavity diameter (LCD) and some other features related to functional size and compare them to the classification based on MR and see which are misclassified.