I did the N2 adsorption-desorption of my synthesised ZIF-8 at 77K using TriStar. The sample was degassed at 100 degree C for 12 hours under vacuum. The BET surface area that I got is around 8.3 m2/g, which is way lower than those reported in literature (around 1000 m2/g). Is there something wrong with the method that I prepared the samples? Could the degassing temperature be too low? Some literatures suggested 150 degree C for the degassing under vacuum.
Hi Vanessa,
One reason for such low surface area could be the quality of the synthesised ZIF-8. Did you check PXRD to confirm the phase purity and crystallinity?. If its crystalline and phase pure, the second reason could be the washing step. The synthesised ZIF-8 has to washed properly to remove the unreacted ligands in the pores and solvent exchanged with volatile solvents. TGA is a good technique to see unreacted ligands in the pore. If this is the case, you will have a thermal decomposition of unreacted ligands from the pores preferable after 100 degree Celsius.
The third probable reason could be the activation. May be the vacuum was insufficient to remove the pore solvents (check the minimum vacuum level reached in the activation station). Hope this helps.
Best,
Anita Justin
Anita Justin Thanks so much for your suggestions. I will definitely consider them!
Hi Vanessa,
Apart from the points raised by Anita, another possiblity is that using BET for microporous samples can sometimes be tricky. I would recommend to have a look at the section "5.2.2 Application of the BET method to microporous materials " in the attached IUPAC tehcnical report. There is a procedure to determine the range in which the BET method should be applied when determining the surface area of microporous solids
Also, if you could attach the N2 77K isotherm graph, it could help to check if the problem is the sample or the method.
Best regards
Sebastien
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Sébastien Vaesen Thanks for your constructive comments! Attached is my N2 77K Isotherm Plot and the quantity adsorbed is relatively low compared to the reported ZIF-8. Could the reason for this low adsorbed quantity due to blocked pores/ washing not done properly?
Ashraf Alattar
Thank you for your advice. I am going to characterise this sample using XRD next week. When you mentioned 'decreasing with ratio of weight', what does that mean? Thank you very much for your explanation!
Vanessa Geh Thank you for the data
If there is no problem with the measurement procedure, clearly the sample is not proper ZIF-8. Your N2 isotherm shows low microporosity and high macroporosity with a total pore volume more than ten times lower than the expected values.
As mentionned by Anita Justin , you should confirm the cristallinity of the samples using PXRD and use higher temperature for the activation. In the original paper, they used 300°C for ZIF-8 activation under vacuum.
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We measured ZIF-8 on AS-iQ, degassed at 150°. All three steps are clear. A BET = 1800 m2/g. But one should keep in mind, that A BET is senseless here.
You have to check XRD first,
then degas at (150C) higher temperatures under vacuum
*recently we have synthesized we are getting the SA values which are similar to the reported literature.
Thé sample contents sommes impurities , you must check DRx tout sée its minérale composition
Hi Vanessa,
The reason might not be simply answered by "impurities" and "amorphous" as the factor of low BET area.
Actually, you have to consider the accessibility of the pore void. Although the pore size of ZIF-8 is ~ 1.16 nm, this is only the diameter of the widest point in the pore. the pores are interconnected by "...Zn-MIm-Zn- MIm-Zn..." channels, which only have an aperture of ~ 340 pm (closest atoms in the opposite position, center-to-center distance, not even considering the Van der Waals volumes).
Considering the size of the N2 molecule (420 pm in long axis and 310 pm in latitude, VdW size), it's easy to conclude that the N2 molecule is hard to access inner pores.
I think the pore you are measuring is only the surface (accessible) pores, and all inner pores are still not accessed thus not being taken into consideration of calculating BET surface area.
The MIm linkers, on the other hand, are able to rotate, to open up the aperture, and let the N2 molecules in under higher pressure (imagine forcing the N2 into the rotation-induced-flexible pore).
Typical ZIF-8 isotherms are nominally Type I with a gating step at low p/p0. These can be see in this paper https://www.nature.com/articles/s41598-018-28015-7 which discusses some influences of synthesis conditions. As others have already written, check with XRD that you have actually synthesize something resembling ZIF-8. Degassing under vacuum at 150 for 3 hours should be sufficient (at least under turbo level vacuum).