I am working on PEI fictionalized mesoporous silica for CO2 adsorption. In normal physical adsorption process, capacity is decreases with temperature. However, in case of PEI, adsorption capacity is increases with temperature (available many Paper) and saturated with CO2 in very low pressure. please suggest me, how to calculate the heat of adsorption as well as its value becomes -ve or +ve ?
Dear Rupak,
The easiest way for determining the enthalpy of adsorption of any system is to use a microcalorimeter in which adsorption occurs. However, you did not mention the temperature at which you obtained your adsorption isotherm, but if it is a low temperature (sublimation CO2 temperature, or nitrogen boiling tempreature for instance), it is rather difficult. You can look at papers published by Grillet et al in the 90's.
If adsorption occurs at a convenient temperature, a microcalorimeter is the best option.
If you do not have the possibility to measure the enthalpy of adsorption, the other option is to calculate it through the Clausius Clapeyron equation.
As suggested by Hongning, you need at least two adsorption isotherms performed at two different temperatures to calculate : DH = RT1T2/(T1-T2)* Ln(p2/p1). This is called the isosteric method.
In this equation, you simply choose an adsorbed amount on your adsorbed isotherms and you read the corresponding values of the absolute pressure. After calculation, you have the enthalpy of adsorption for this adsorbed amount.
By changing the adsorbed amount , you can determine the enthalpic profile (DH vs Qads).
This method is very well documented in many famous books such as those published by K.S. Sing et al.
I hope it helps.
Dear Philippe Trens, Thank you very much. I have learned a lot from your answer.
A question of calculating the enthalpy of n-hexane adsorption was raised to me, and I used the Clausius Clapeyron equation with the adsorption isotherm at 25 degree and 35 degree.
As to microcalorimeter, is this paper Journal of Thermal Analysis, Vol. 38 (1992) 603-609 of Grillet et al in the 90's that u have mentioned?
Thanks again.
Dear Hongning,
Yves Grillet published many papers in the field, as a specialist of adsorption at low temperature. As an example :
Methane and krypton adsorption on single-walled carbon nanotubes by Grillet et al , LANGMUIR, 16(17), pp 7019-7022, 2000
Microcalorimetr ic study of argon, nitrogen, and carbon monoxide adsorption on mesoporous vycor glass by Grillet et al, JOURNAL OF COLLOID AND INTERFACE SCIENCE,: 206(2), pp 527-531, 1998
And many others...
The one you indicated is also an excellent choice of what can doa microcalorimeter for understanding sorption interactions
I hope it helps
Dear Philippe Trens,
Thanks a lot!
I will read the papers you have recommended,. If I have further questions, pls give me help again.
Have a nice day!
Hongning Wang
The variation of CO2 sorption with temperature is described for PEI loaded SBA-15 in Knowles, Gregory P. et al "Silica/Polyeththyleneimine composite adsorbent S-PEI for CO2 capture by vacuum swing adsorption (VSA)" chapter 9 in M. Attalla (Ed) "Recent Advances in post combustion CO2 capture chemistry", ACS symposium series 1097, ACS, 2012. Heats of adsorption are reported, measured directly via combined Thermo-Gravimetric Analysis (TGA) and Differential Thermal Analysis (DTA). Measurement of heats of adsorption similarly reported in earlier papers, as calibrated against heat of fusion of various metals. For equilibrated data, heats of adsorption can be estimated for isotherm data sets collected at minimum two different temperatures via application of virial equations to the Clausius Clapeyron relationship. Refer method in Hawes, Chris S. et al "Co-ordination polymers from a highly flexible alkyldiamine-derived ligand:structure, magnetism, and gas adsorption studies", Dalton Transactions, 2015, DOI: 10.1039/C5DT02323H. Let me know if you would like further assistance.