Why is reported effective pore size smaller that the one declared in the name of the molecular sieve type X?

The [molecular sieve](https://en.wikipedia.org/wiki/Molecular_sieve) type is often coded with `<number><letter>` combination, where `<number>` specifies the pore size in ångströms and `<letter>` refers to the Linde type of zeolite. For sieve types 3A, 4A and 5A the reported pore sizes ubiquitously match the code, i.e. being 3 Å, 4 Å and 5 Å, respectively, with virtually no discrepancy neither on the manufactures' websites nor in the literature.


However, the majority of the companies selling 13X type sieves across both EU and US ([ADCOA](https://www.adcoa.net/product/item-ms13x00-molec-sieves-type-13x-custom-packaged/), [VWR](https://uk.vwr.com/store/product/2338947/molecular-sieve-13x-1-0-nm-10-a-pellets-oe-3-2-mm-flukatm), [GeeJay Chemicals Ltd.](https://www.geejaychemicals.co.uk/molecular-sieves), [Lawrence Industries](https://www.l-i.co.uk/products/molecular-sieve-air-gas-drying-zeolite), [MTE Group](https://www.mte-process.com/mte-catalyst-support/molecular-sieves/), …) report nominal pore size 10 Å instead of 13 Å. Analogously, the spec sheets for the mol sieves type 10X report the pore diameter 8 Å instead of expected 10 Å.


Surprisingly, there wasn't any mismatch in older literature and patents [1, 2], and the 13X sieves were claimed to have 13 Å pores, which seems logical. An editorial remark of a similar nature has been made for the 2006 paper [[3](https://doi.org/10.1063/1.2272835)] pointing out the inconsistency between expected pore size for 13X zeolite and the pore size reported by Sigma-Aldrich which authors used in their work. Based on discrepancy of the data presented in the literature at that time, the authors suggested that



> 
> microscopic structure of commercially produced 13X may not be fully independent of the sample preparation procedure
> 
> 
> 


and from the rest I draw a conclusion that they infer that commercially available sieves of 10X and 13X types could be [faujasite](https://en.wikipedia.org/wiki/Faujasite), zeolite of Linde type X and synthetic zeolites of *similar* composition, which could deviate from one manufacturer to another. I am not finding this completely convincing since the reported pore size doesn't fluctuate around one number, it's consistently understated by 2 Å and 3 Å for 10X and 13X sieves, respectively.


What exactly is going on here ~~and where can I get a refund for my missing 3 ångströms~~? Were the initial structures of the X type zeolites and, accordingly, the pore dimensions, incorrectly determined? Or did the manufacturers changed their product and continued to ship it using the former name?


### References


1. Meyer, J. R. *Method for Producing Exceptionally Pure Hydrogen*. [US3011589A](https://patents.google.com/patent/US3011589A/en), December 5, **1961**.
2. Read, P. L. *Sorption Vacuum Pump*. [US3264803A](https://patents.google.com/patent/US3264803A/en), August 9, **1966**.
3. Swenson, J.; Jansson, H.; Howells, W. S.; Longeville, S. *Reply to “Comment on ‘Dynamics of Water in a Molecular Sieve by Quasielastic Neutron Scattering’ ” [J. Chem. Phys. 125, 077101 (2006)]*. *J. Chem. Phys.* **2006**, 125 (7), 077102. DOI: [10.1063/1.2272835](https://doi.org/10.1063/1.2272835).


