The BET model is often used, in its standard form to calculate the  surface area of zeolite but it  does actually not apply to microporous materials (if you check the C value reported with the BET surface area, it is likely to be negative, which has no physical sense). That said, it is common to report an "equivalent surface area" for a zeolite. Quite recently, Rouquerol et al. proposed to use the BET model in zeolite but on a very specific domain of P/Po (this domain beeing selected based on  a self consistency criterion) https://www.researchgate.net/publication/229182183_Is_the_BET_Equation_Applicable_to_Microporous_Adsorbents

Article Is the BET Equation Applicable to Microporous Adsorbents?

The BET model is often used, in its standard form to calculate the  surface area of zeolite but it  does actually not apply to microporous materials (if you check the C value reported with the BET surface area, it is likely to be negative, which has no physical sense). That said, it is common to report an "equivalent surface area" for a zeolite. Quite recently, Rouquerol et al. proposed to use the BET model in zeolite but on a very specific domain of P/Po (this domain beeing selected based on  a self consistency criterion) https://www.researchgate.net/publication/229182183_Is_the_BET_Equation_Applicable_to_Microporous_Adsorbents

Article Is the BET Equation Applicable to Microporous Adsorbents?

If, the BET surface area (SBET) of a porous material (either microporous or micro-mesoporous) is calculated taking into account all consideration recomended for the IUPAC (https://www.researchgate.net/publication/282624978_Physisorption_of_gases_with_special_reference_to_the_evaluation_of_surface_area_and_pore_size_distribution_IUPAC_Technical_Report) proposed by Rouquerol et al. (https://www.researchgate.net/publication/229182183_Is_the_BET_Equation_Applicable_to_Microporous_Adsorbents), this SBET value can be considered as the apparent surface area more suitable to report.

On the other hand, the DR equation can be applied successfully when the material is purely microporous, but when the material present mesoporosity, the micropore volume obtained by the DR equation is overestimated, and therefore also the micropore surface area is overestimated. This overestimation increases with the mesoporosity of the material, as was shown by Villarroel-Rocha et al. (https://www.researchgate.net/publication/275458174_Importance_of_the_a_s_-plot_Method_in_the_Characterization_of_Nanoporous_Materials?ev=prf_pub).

To calculate the micropore surface area, it's necessary to apply the αS-plot method to estimate the external surface area (Sext), and then the micropore surface area (Smi) can be determined from (SBET - Sext).

Article Is the BET Equation Applicable to Microporous Adsorbents?

Article Importance of the α s -plot Method in the Characterization o...

Article Physisorption of gases, with special reference to the evalua...

Dear Juliette Blanchard  and Jhonny Villarroel-Rocha,

Thank you very much for explanation!

I typically use the DFT model to calculate surface areas of microporous materials, ideally using Ar as sorptive at 87K.  The value you get should be closer to a "true" surface area than using the BET method.  There are numerous DFT models available (I use the ones built in to my Quantachrome sorption analyzer), covering a range of sorptives and surface properties.

The advice of Juliette and Jhonny is also good, if you want to stay with the BET approach.

Good luck! 

I think it is experimental error 

you can check this from the y plotting the Relative Pressure (P/Po) against 1/[Q(Po/P - 1)] and it should give a straight line with R2 (correlation coefficient), 1

Because for correct using BET for microporous systems demands to choice P/P0 range

see,.e.g Langmuir 2010 26(8) p5475

or Rouquerol Stud Surf Sci Catal 2007 p 160