Calculation of BET surface area of microporous materials at different relative pressure

BET equation was used to calculate surface area of different powder solids,such as mesoporous Al_2O_3 and microporous zeolites,at different relative pressure(P/P_0).The results showed that when relative pressure is in the range of 0.01-0.10,surface area of microporous materials calculated is more reliable.For most microporous materials,BET surface area calculated at P/P_0=0.05-0.20 is less than that at P/P_0=0.01-0.10.For some hydrothermally-treated ZSM-5 samples,their isotherms have hysteresis loop at lower relative pressure;BET surface area calculated at P/P_0=0.05-0.20 is smaller that that calculated at P/P_0=0.01-0.10.The more microporous material in the catalysts,the larger the difference in BET surface area calculated at the two relative pressure ranges.For some pure zeolites,this difference can be up to 15%.