Zeolite showed about 600m2/g at 650C and 850 m2/g after cacine to 800C.
There was small amount continuous weight loss noticed in TGA from 600 to 800C.
Please explain or share your understanding.
At that temperature, you may be decomposing and thus changing the porous structure of the zeolite.
Did you check literature about this? Which zeolite are you using? What is its decomposition temperature? All of this should be well known and already published.
I am not sure which zeolite you are using but if your zeolites made up of even tiny bit of organic materials (templates etc) than during calcination you will observe burning of them which creates additional surface area hence weight loss . If you are not using any templates than you may want to find out what is the composition of your zeolite and which element decomposes in the temperature range of 650-800 C. You may additionaly characterise them with EDAX at 650C sample and 850C sample to find out which element is missing. Hope this may help .
Can you show the isotherms to see where is the difference?
I am also expecting some template and -OH groups.
There is increase in micropores and little at macro.
Do you think any quick adsorption of moisture while we doing BET measurement, particularly for 600C calcined sample?
I may suppose the DESHYDROXYLATION of the -OH groups on the zeolite surface. Two -OH groups form a -O- bond and a H2O water molecule is realesed. This behaviour has been reported for example in zeolite Y (FAU) zeolite in temperatures over 600 ºC.
Template releasing is also possible...
starting decomposition of zeolit structure through removing the zeolitic water
According to my experience with activated carbons, which is different from zeolite, I think that the macro- and mesopores collapsed into micropore sizes and hence the surface area increased upon the increase in temperature from 600 to 800oC. This discussion may be shown in pore size distribution.
Microporous zeolites cannot possess such high surface areas. so i expect it to be mesopore modified zeolite or Hierarchy established one.
Most of the organic templates can be removed when calcined at 650C ( with no furnace errors). However if the time of calcination is not sufficient, there will be carbon remains in the microporous interconnectivites which will block the pores and decrease the SA (in ur case 600 m2/g). when the calcination temp. increased in same time span, these remians can be removed (known from TGA) leading to the increase in SA. this can be clealy known from microporous area in BET. this could be one reason.
second one could be.... If its a proton-exchanged zeolite. most of your bronsted acid sites will lose water and turn in to Lewis cites at that higher temperatures (the weight loss in TGA can be due to loss of inherent water molecules). Degassed weight In case of BET, is comparably less and thus higher surface areas.
i dont think stuctural integrity of the zeolite is lost at 800C, since they are known for their high thermal stabilities. However there can be aggregation of nanocrystallites (with whatever the morphology your zeolite possess) to form intracrystalline mesopores which further increases SA. However this reason cannot justify since there is a weight loss observed in TGA.
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