Hi Donya

Unfortunately, you can't!

Commercial software is provided by some instrument manufacturers, with kernels included for certain systems (a particular adsorbate-adsorbent pair at a given temperature) but otherwise you would have to write your own, which is not a trivial task.

It involves producing kernels, which are sets of isotherms representing adsorption in pores of different sizes at a given temperature, and then using these to solve what's known as the 'general adsorption isotherm' equation. That requires special algorithms too. So implementing this method yourself would be quite a large and complicated job.

Best regards,

Darren

Hi Darren,

Thank you so much for your reply.

To be honest, i got so excited when you have replied my question.

So, as BET method is not really reliable for microporous materials, do you have any other suggestion or alternative to do so?

Cheers,

Donya

Darren above is totally correct.

I see one solution if you don't have the software: send your isotherm to someone who has it. Since it does not necessarily be measured in the same equipment.

At least it works with the software from Quantachrome (forgot the name... AsiQwin perhaps?). Here, you create a raw isotherm file and send it to the person who has the software, who will import it. Then you (or your friend) proceeds with the analysis even if the software is not in your computer and you didn't measure the isotherm in that instrument.

Hi Donya

My pleasure!

What Fernando says makes sense, if you just want to apply NL-DFT to determine pore size distributions.

Otherwise, I think a good answer to your question would probably depend on your aims (and your material).

The BET Method is not really reliable for microporous materials but, providing you apply the set of criteria outlined in the current IUPAC guidelines (see p. 13):

http://sol.rutgers.edu/~aneimark/PDFs/IUPAC_Report_PAC_2015.pdf

you should get fairly repeatable results.

Therefore, if you are just looking to check differences between samples of the same material, for example, the BET Method is still useful. It's just important not to over interpret the physical meaning of the result.

NL-DFT, on the other hand, is one of a number of pore size distribution methods, which just differ in the way the kernel is produced. Surface areas can be extracted from the results, but really their purpose is different to BET measurements.

There are also limitations to DFT methods, in that the kernel must match the system you are studying, and the model used should describe the adsorption process accurately.

On this last point, I really like - and would therefore recommend - an article by Claudia Weidenthaler, published in Nanoscale in 2011:

http://pubs.rsc.org/en/content/articlelanding/2011/nr/c0nr00561d/

Much of the article is about other techniques but there are some really useful comments on pore size distribution methods near the end...

I will quote the final paragraph of the relevant section:

"Thus, using the NLDFT method is highly recommended if appropriate model isotherms are available with respect to adsorbent, adsorbate, and pore geometry. If this is not the case, NLDFT is as wrong as any other method and one should then

revert to classical methods as long as no better alternatives exist."

Some of the classical methods are covered in the IUPAC guidelines so that is probably a very good starting point if you are looking for alternatives. To make specific suggestions, I think we'd need to know what you're trying to achieve and what your materials are (because pore geometry can become quite an important consideration).

Cheers,

Darren

For activated carbons 

http://www.nldft.com/ 

is free and very good. 

Thank you Fernando, I think i need to find someone to do that for me.

Thank you Darren for your thorough explanation,

I have read the IUPAC report before and it was of much help for understanding the whole porosity concept and characterization methods.

I found the Claudia Weidenthaler's paper so interesting too.

To give you some idea, i am working on PIMs (polymers of intrinsic microporosity). I synthesize and modify (in some cases) them and need to investigate the porosity properties such as surface area, pore volume and size to compare with previous works. I need them to be microporous to physically adsorb H2 gas (as H2 storage materials).

PIMs are microporous due to their molecular structures orientations. In other words, the pores would be generated due to the stiff molecular chain with sites of contortion. This makes the polymers inflexible and unable to move around. Therefore, the free volume created between the molecular chains would be places to trap H2. So, as a result i do not know the shape of pores. However, most of studies using NLDFT, consider the slit-shaped pores.

Cheers,

Donya

Hi Donya

Thanks for the further information.

If you are looking to compare the surface area, pore volume and size of your materials to previous work on the same materials, then it does make sense just to use the same methods as others. However, it is certainly worth considering whether the techniques applied previously really make physical sense.

The pore shape point is a good one, but surface chemistry also matters. If kernels for N2 adsorption in slit-shaped carbon (graphitic) pores have been used, I'm not sure these really apply.

That could be one of the situations that Claudia Weidenthaler describes, where perhaps, for example, the HK method would be just as useful for comparing materials.

With regard to surface area, I would also look at the details of the BET analysis performed in previous studies and only use values from those that have been carried out using the consistency criteria, or similar. Low relative pressure ranges must be used for microporous materials, not "standard" ones like 0.05-0.3.

There can, however, be additional problems for materials like PIMs. Although they are relatively stiff, they can still swell. Hysteresis in N2 adsorption/desorption isotherms has previously been attributed to this:

http://pubs.acs.org/doi/abs/10.1021/la1028806

Although it's been argued that kinetic restrictions are more likely the problem:

http://pubs.acs.org/doi/abs/10.1021/la402630s

which seems quite convincing to me. I recommend those papers too. I think they point towards the need to be very careful indeed when using adsorption to characterise the porous properties of microporous organic polymers, including PIMs.

Best regards,

Darren

Dear Darren,

Thank you for your prompt reply.

It is so interesting that I have already come across the papers you suggest. Those two papers are also very interesting and useful. However, mentioning them at this stage will be a huge help for me to read through once again and decide upon.

Thank you so much for your time and advice.

Hope to see you one day,

Cheers,

Donya