Heterogeneity characterization of ordered mesoporous carbon adsorbent CMK-1 for methane and hydrogen storage: GCMC simulation and comparison with experiment

Grand-canonical Monte Carlo (GCMC) simulations were performed to investigate the adsorption behavior of methane and hydrogen on a highly ordered carbon molecular sieve CMK-1 material. The rod-aligned slitlike pore (RSP) model was used to emphasize the grooved structure of the material, and the pore size distribution (PSD) was introduced to characterize the geometrical heterogeneity of the materials quantitatively. The PSD determined from adsorption isotherms of N-2 at 77 K indicates that the CMK-1 adsorbent is a mesoporous material. By combining the GCMC and PSD techniques, adsorption isotherms of CH4 at 303 K and H-2 at 303 and 77 K in the CMK-I materials were obtained. The simulated isotherms are in an excellent agreement with experimental data, suggesting that it is necessary and efficient to use the PSD to characterize the materials. The GCMC predictions demonstrate that gravimetric uptakes of CH4 and H-2 in the CMK-1 material at 30 MPa and 303 K are 31.23 and 1.19 wt %, respectively. Although a greater loading of 4.58 wt % for H-2 is favored at 77 K and the same pressure, it does not reach the U.S. Department of Energy target of 6.5 wt %. By analyzing isosteric heats, we found that the adsorptions of CH4 at 303 K and H-2 at 77 K exhibit an evidently energetic heterogeneous behavior in CMK-1 materials, with a broad range of isosteric heats of 10-27 kJ/mol for CH4 and 2.73-9.9 kJ/mol for H-2. However, the adsorption behavior tends to be energetically homogeneous for H-2 at 303 K, because the isosteric heat mainly centers on the range 4.82-6.65 kJ/mol. In addition, by exploring the relationship between the pore width and the surface excess, we found that, for CH4 at 303 K, the optimal operating conditions corresponding to the maximum surface excess are w = 1.2422 nm and P = 6 MPa, whereas for H-2 at 77 K, they are w = 1.0647 nm and P = 3 MPa.