I hope the following article could help you.

Article Metal Organic Framework MIL-101(Cr): Spectroscopic Investiga...

Ruichen Zhang The raw data are concerning as there is an unwarranted drop in adsorbed amount at very low relative pressure in the adsorption branch of the isotherm (actually goes negative in sample b).

A lot of pore volume in (b) can be attributed to the upturn in the isotherm at high P/Po which is often associated with nothing more than interparticle voids in a powder sample.

The desorption branch exhibits "stubborn" hysteresis and does not match up with adsorption below P/Po 0.4 which we would normally expect. This "open" isotherm is most often due to poor equilibration (i.e. not waiting long enough for each datum point to be fully adsorbed/desorbed), but can be due to structural change in susceptible materials (not MIL-100).

I would expect a MIL-100 to show significantly more uptake than as seen in your isotherms. At this point, the experimental gas sorption data do not look to be of sufficient quality to make any quantitative judgement about pore volume in either sample.

This is a useful paper for comparison: https://s3.amazonaws.com/academia.edu.documents/42571871/Hierarchical_MOF-xerogel_monolith_compos20160211-11806-tjd1hs.pdf?response-content-disposition=inline%3B%20filename%3DHierarchical_MOF-xerogel_monolith_compos.pdf&X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAIWOWYYGZ2Y53UL3A%2F20191230%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Date=20191230T190620Z&X-Amz-Expires=3600&X-Amz-SignedHeaders=host&X-Amz-Signature=9d8653fe675a8c3d6b90953233bf4e5ee2f00a575ec8d6484363272b6214688e

Dear Ruichen Zhang,

The BET surface area you have found for MIL-100(Fe) seems lower than what it has to be. Moreover, it is very interesting that your total pore volume increases after you load your MOF sample with Pd particles. The open hysteresis is a unexpected situation. You should have had a closed hysteresis for MIL-100(Fe). First of all, have you checked your Degas conditions? How long have you degassed your MOF samples? And how did you define your Degas temperature? In addition what is the model you have used for pore size distribution analysis, and how did you decide on that specific model? I recommend you to do a TGA analysis to decide on the Degas conditions. I believe that the better you degassed your samples, you would have a closed hysteresis and maybe, your BET surface area may even increase.

Dear Aysu Yurdusen

Thank you for your kind suggestion! For the questions:

1.My degas condition is 150C,12h under vacuum. I determined the temperature based on the thermal stability of the MOFs and the boiling point of the solvent.

According to previous literature, MIL-100(Fe) is stable under 150C (My synthetic condition is 200C, aqueous solution). However, I didn't pretreat the samples myself due to limited access to vacuum drying oven. I just sent the samples to the test center where they would do the degas treatment instead before testing;

2. I think the result is based on BJH model, but I'm not quite sure whether it applies well to mesoporous materials such as MIL-100(Fe);

3. I've run a TGA analysis on my samples, and I did not see a significant weight loss until about 230C. So I'm pretty sure MIL-100(Fe) remains stable under my degas condition. However, I've read articles about MOF activation, such as "Activation of metal–organic framework materials" (DOI: 10.1039/c3ce41232f) ,which points out that the crystal structure of MOFs might collapse during the activation process, leading to a much lower BET surface area. So now I'm really confused whether the low SBET is caused by poor material preparation or insufficient activation.

Dear Ruichen Zhang,

I have observed during my BET surface area tests that 12 hours of Degassing at 150 °C lead to change in the color of the crystals. So I have changed the Degas conditions to 120 °C for 6 hours. If your material is purely mesoporous you can go with BJH, but I would recommend you to use NLDFT. By NLDFT method, you can obtain information from microporous pores in case you have some.

I would still draw everyone's attention to the dip in the isotherm (circled) which means the data are unreliable... there could be a whole lot of micropore volume unaccounted for in both cases.

Dear Martin A Thomas

I see your point.I've retested my samples days after the question was asked, with the result still pending, and I'll keep this question updated. Meanwhile, what do you think has caused the inaccuracy? Is it due to the test conditions (i.e. Degas pretreatment, the simulation model), or the sample itself (e.g. it is not 100% pure MIL-100,or MIL-100 with poor crystallinity)? Thank you!

(Btw, I couldn't access the link you shared. Could you plz send the paper title instead?)

First, the paper:

Microporous and Mesoporous Materials 215 (2015) 143-153

Hierarchical MOF-xerogel monolith composites from embedding

MIL-100(Fe,Cr) and MIL-101(Cr) in resorcinol-formaldehyde xerogels

for water adsorption applications

Martin Wickenheisser, , Annika Herbst, Rene Tannert, Barbara Milow, Christoph Janiak

Now the data problem: looks to be instrument related due to pressure range... maybe an unzeroed transducer (does the instrument automatically zero, or only intermittently every so many runs, or only upon request by the operator?). This can be exacerbated by a sample that is still offgassing due to insufficient preparation. And could also be exacerbated by a void volume problem (re-using warm and cold free space, but values that do not actually apply to the cell being used.