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The 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, VWR, GeeJay Chemicals Ltd., Lawrence Industries, MTE Group, …) 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] 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, 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, December 5, 1961.

  2. Read, P. L. Sorption Vacuum Pump. US3264803A, 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.

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  • $\begingroup$ The "pore size" depends on what the analysis filled into the pores. Perhaps the standard gas used was changed at some time in the past? Nitrogen, argon, CO2, ... $\endgroup$
    – Karl
    Commented Jul 11, 2022 at 10:30

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