Can degassing temperature and time affect the BET surface area?
I changed the temperature and time from 80 to 120 degree and from 2h to 10 h respectively, and observed a significantly increased surface area for a well-known MOF. It's about 10 times more than expected one. Has anyone experienced this?
Dear Naime,
In our laboratory we have observed similar effects with temperature. As it happened to you, when increasing the temperature (by a small margin) there is a slight tendency in some materials to raise the specific area. This was also observed in some catalysts and in SiO2 nanoparticles when we changed the synthesis solvent (from water to water-methanol mixtures).
Best,
Fran
Metal-organic frameworks (MOFs) crystalline, porous materials that have been shown to exhibit very large BET surface areas.
# Temperature/ Route of synthesis maybe one of such case which affects BET surface area of MOF's.
Please do check the stability, even if you have a high surface, MOF will not be chemically stable.
Hi,
I don't have an answer, but it is not unusual for differences in pretreatment to affect the BET-SSA.
This is especially true for MOFs, because :
- They can hold very large amounts of moisture or solvents from synthesis. Incomplete removal makes the mass appear larger, and the SSA appears smaller.
- However, too much heating while solvent is present can cause damage or destruction to the MOF structure.
- Impurities can cause structural changes, and even cause opening/closing of some pores.
- Impurities can coordinate to sites of strong adsorption interaction, which are present in some MOFs. These sites can contribute to the apparent surface area.
However:
- An increase of 10 times seems very large - especially if the material is well characterised.
- Does the literature mention suitable pretreatment conditions?
- Is it possible that there is some contamination in the adsorption gas, or some problem with the measurement instrument ?
Please come back and tell us if you discover the answer !
C
Try to figure out what are the microporous materials. The answer to your question is: of course, yes, it cannot go other way, until you damage the MOF structure by heating.
Generally, degassing aims to desorb gases or physisorbed impurities on the surface of your material, so it will allow you to access the real specific surface area of your solid. The choice of the degassing temperature depends on several parameters in particular, the solvents used during the synthesis and the heat treatment (drying) of your materials. In my opinion, it is better to dry your MOF and go through the ATG to see its thermal behavior, and for the degassing time you set it at 10 or 12h and that is more than enough.
all the best,
Dear Naime,
I have faced this problem too in the experiments. The surface area of the MOF I synthesized was nearly 10 times lower than the reported one, which can be affected by many factors during materias synthesis, for example, the purity of reagents, the washing agents, and the drying ways.... Actually, this phenomenon has also been reported in other studies (ACS Appl. Mater. Interfaces 2018, 10, (22), 18619-18629; J. M. Chem. A 2016, 4, (28), 10816-10819). Speaking of this, I suggested that perhaps you can active the material in the vacuum drying oven at a moderate temperature for enough time (e.g., 12 h) to remove extra/blocked solvent molecules within the pore structures.
Best,
Kaixing
Increasing the degassing temperature as long as it does not change the structure of the MOF sample can increase in the surface area. I suggest dry your MOF sample and determine your material's thermal stability with TGA experiments.
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