3A, 4A, 5A, 13X… What’s the Difference?

Type A

All molecular sieves are composed of sodium and aluminosilicate, which are grown to form a three-dimensional crystal structure. It’s this crystalline structure that allows molecular sieve to adsorb some molecules while rejecting other molecules that are too large to fit inside the crystal


When A-Type crystals are grown, they initially are all Type 4A, meaning the pore openings are four Angstroms in diameter with an A-Type crystal structure. These 4A crystals are capable of adsorbing molecules that are smaller than 4 angstroms in diameter, such as water, while allowing larger molecules, such as methane, to pass along to the desired product stream.

Smaller Pore Sizes

The pore size of Type 4A can be manipulated, which allows molecular sieve to be used to adsorb, or exclude, certain molecules during refining and purification processes.  If 4A crystals are exposed to potassium for instance, an ion exchange can occur, replacing a sodium ion with a larger potassium ion, and creating a smaller pore opening.  This ion exchange is how Type 3A molecular sieve crystals are made.  Exchanging one ions allows more selectivity for adsorbing water and is critical in processes such as Ethanol Dehydration.  Since ethanol molecules are roughly 3.6 Angstroms in diameter, 4A molecular sieve is able to adsorb ethanol, which means that the desired product would be co-adsorbed while trying to remove water from ethanol streams.  To offer higher efficiency in the dehydration process, Type 3A molecular sieve is commonly used due to its pore size, which enables 3A sieve to adsorb water while rejecting ethanol from entering the crystals.

Larger Pore Sizes

To offer a slightly larger pore opening, calcium is used in an ion exchange to produce Type 5A molecular sieve.  A sodium ion is exchanged with the slightly smaller calcium ion, which increases the pore size of the crystal.  Type 5A is able to adsorb certain mercaptans while still small enough to block larger hydrocarbons from entering the crystal.  These properties allows this molecular sieve to be utilized in LNG and LPG refining to remove sulfur compounds while allowing the desired hydrocarbons to pass through to the product stream.  Type 5A is also useful for producing relatively pure streams of hydrogen or oxygen since it is able to adsorb nitrogen, carbon monoxide, and other contaminants.

Type X

Type X

To create very pure streams of oxygen, a Type 13X crystal is more likely to be used.  Type X crystals are shaped differently from Type A crystals are tend to offer much larger pore sizes, about 9 Angstroms in diameter.  13X molecular sieve is often used in air separation process, such as in the production of high purity oxygen.  Aside from oxygen production, X type crystals are commonly used in cryogenic distillation processes to deeply dehydrate LNG and LPG streams.  It is critical to remove all water from these streams to prevent blockage and freezing in pipelines.  There are many variations of Type X crystals with most being Type 13X, although some can be produced to have pore openings about 8 Angstroms in diameter, sometimes referred to as Type 10X.

While all molecular sieves serve the purpose of selectively adsorbing smaller molecules from larger ones, it’s critical to select the right product for your application.  Ion exchanges are the key differences in Type A crystals, which offer different capabilities of adsorption, while Type X crystals have a different shape and offer much larger pore openings than Type A crystals.

Authored By:
Kolten Burkes
Hengye Inc.
Houston, Texas

40 thoughts on “3A, 4A, 5A, 13X… What’s the Difference?”

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  4. So 13X is used for air separation? meaning that water vapor can actually adsorb on the crystal?
    also, can 4A also adsorb water vapor?

    1. Hi Ali,

      Yes, 13X has many applications amongst which include air separation functions, but 5A is also commonly used in industrial air separation processes.

      Regarding adsorbing water vapor, all types of molecular sieve 3A, 4A, 5A, and 13X are capable of adsorbing water.

  5. Hi
    Can you use zeolite molecular sieve 3 Angstrom for nitrogen generation because of the relative sizes differences between nitrogen 3.1 and oxygen 2.9 why do i see Carbon molecular sieve being specifically sold for this purpose of nitrogen generation, if the pore throat diameter is the critical factor why is zeolite molecular sieve never marketed for this??
    Thank you for your time.

    1. Hi Alex,
      3A is generally only capable of adsorb water and sometimes acetylene. Carbon Molecular Sieve is a preferred adsorbent for nitrogen generation because of its affinity for adsorbing oxygen over nitrogen. Thus the CMS will entrap the oxygen and release a near pure stream of nitrogen and can offer this in rapid, mass production.

  6. Based on the information you provided in the article; would there be any benefits to a 2 stage process for the production of high purity oxygen from air? For instance, would pumping air through a chamber filled with 13x then outputting to another chamber filled with 5a or vice versa provide any benefits? Also, what would the process be for removing the captured elements and recharging the zeolite, much in the same way a water softener does using sodium or potassium chloride in it resin beads?
    Thank you in advance for your time

    1. Hi Aric,

      A single stage oxygen concentrator can generate oxygen streams with a purity of around 90-95%, with about 5% being noble gases. If you wanted to protect your investment in high-quality, comparably expensive molecular sieves such as LithiumX, you could use a 2 stage concentrator to pass the air through a less expensive 4A or 5A molecular sieve first to help dehydrate the air, thus preventing the second molecular sieve unit from becoming as saturated. I have not seen this done, personally, as the additional operation and equipment costs might offset any costs associated with replacing the molecular sieves in only one unit.

      For regenerating molecular sieve, most of these operations are pressure swing applications (PSA) and utilize vacuum pressure to release nitrogen and other contaminants, allowing the sieve to be used many times. Long term use of molecular sieve does have diminished returns in capacity and efficiency, over time, but oxygen generators can often run for 2 to 10 years before the sieve needs to be replaced.

      1. …. But aric never mentioned lithium X sieves; for O2 streams you and him both only mentioned the 13X sieves. Could I be missing something? Could it be that the 13X sieve and lithium X sieve are one and the same? And if they are not, can I still use the less expensive 13X sieve instead of the lithium X sieve for my OWN concentrator?

  7. Since the machines I bought were not supplying with Seive column ( zenolite catridges)…I want to buy zenolite from Alibaba but not sure what grade to buy..please guide me. I am suffering from COPD and have to use the POC machine 20 hours a day.

  8. Info. is really useful. Iam having a doubt regarding the nomenclature. Zeolite-A is given by names as 3A, 4A, 5A based on pore size. How the naming is given as 10X and 13X foe zeolite-X having a different pore sizes.

    1. Hello Mahima,

      When 13X molecular sieve was first synthesized in the late 1940s, the pore opening was measured as approximately 13 Angstroms. So as a Type X crystal, it was named Type 13X molecular sieve. Later on, as better microscopes were built, the pore opening was measured again and confirmed to be closer to 9 Angstroms in diameter; however, this happened years later, so the name remained as Type 13X molecular sieve. The term 10X also refers to the Type X crystalline structure, although it measures about 8 Angstroms in pore diameter; this molecular sieve is also referred to as CalciumX.

      Similar to the “X” in Type 13X, the “A” in Type A (3A, 4A, 5A) molecular sieve actually refers to its A-Type structure and not the word Ångstrom.

      1. Hi, about the structure of FAU zeolite, type X crystalline structure is different to type Y crystalline structure? don’t type X and Y have the same framework?

        1. Hello Alex,

          Type X and Type Y faujasite both have similar frameworks; the key difference is the silica:alumina ratios of the each. Having different Silica/Alumina ratios permits each faujasite to be used in different ways where Type Y has high thermal resistance and is commonly used for its catalytic advantages while Type X is used more as a traditional molecular sieve for separation processes.

  9. Hi there, thanks for your useful information about molecular sieves. I want to ask you that, which kind of these molecular sieve was favored and commercialized for separating methanol and water from ethylene and other olefin in the ethylene production from ethanol plant (olefin plant with ethane as feed stock).

  10. 3a, 4a, 5a, 13X zeolite sample request

    Hi, this is Keju An who is a postdoc at Arizona State University.

    I am working on CO2 adsorption research using advanced adsorbents.

    Would you mind send me a sample of 3a, 4a, 5a, 13X zeolites of your company? Or the quote of 3a, 4a, 5a, 13X?

    What size options of 3a, 4a, 5a, 13X zeolites?


    Keju An

  11. I wish to generate nitrogen from air.What is the appropriate carbon molecular sieve to use? How can I get a quantity of about 40 – 50kg during this period of COVID-19 lockdown in Nigeria. Can Amazon supply? The company I inquired from expects me to buy a minimum order of over 500kg.

  12. Hi, can any ofthese type trap isobutylene in a normal 1-butene purification system? given usual operation conditions, like 20C-40C ambient temp, and high pressure like 30 barg.

  13. Sir your article and all the answers that you provided are extremely helpful. My sincere thanks!

  14. Hi, thanks for your information, I want to ask one question though , we have PSA system and we are using 13X zeolite for oxygen concentrator, but purity we are getting is very low about 35 % , is there any different version of 13X which we should have used, internet says it should be lithium grade 13X, what is that actually ?

  15. Hi, do we need to use 5A and 13x molecular sieve in 50% ratio in the industrial scale PSA based oxygen generator to achieve high purity and high recovery

  16. You have explained it in a lucid and simple way ! We had been researching on producing Zeolite from green sources and this definitely brings lot of clarity to our thought process. Thank You !

  17. hi
    5A and 13X zeolite is used as an oxygen concentrator(OC) .both have different diameters 13x have 10 A and 5X have 5A. They are very useful for separate oxygen and nitrogen molecules in OC, MY QUESTION is oxygen and nitrogen have to size is 2.99 A and 3.05A respectively, both are small for 5A and 13x zeolite so how only oxygen is pass and nitrogen adsorb?

  18. if the 5a is better than 13x, why does the oxygen concentrator use type 13x instead of 5a!

  19. Should I use 4a, 5a or 13x for hydrogen drying and why? (Removing water vapour from hydrogen gas). Thanks

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