is more acidic that the other zeolite. The peaks at 3607 cm -1 and 3681 cm -1 are associated to bridging Si-(OH)-Al group of H-ZSM-5 and H-MCM-22 zeolite, respectively. This agrees also with the previously reported experimental IR data of H-ZSM-5, 3616 cm -1 and H-MCM-22, 3622 cm -1 . It is believed that the acidic strength of the zeolites depend (directly or indirectly) on the local structure of BAS. The acidic strength increases, viz. We have also analyzed at the convergence of the free energy estimates; (Figure ). The free energy barrier is converged within 1.0 kcal mol -1 after 22 ps/window of the TASS simulations. The relatively slow convergence in the estimate can be ascribed to the diffusion of the methane molecule within the pore of the zeolite in the R state. In comparison to H-ZSM-5, the spatial probability distribution of methane in H-MCM-22 is broader, indicating a higher entropic contribution in the R state for the later zeolite compared to the former, (Figure ). In the transition state, the diffusion is limited, therefore the difference in the entropic contributions is minimal. Thus, the difference in the entropic contribution due to the translational freedom could be another reason for different free energy barriers in these two zeolites. To understand this better, we computed free energy barriers at 300 K for the reactions in both the zeolites using transition state structural optimization and normal mode analysis (Table ). Notably, the potential energy barrier is the same in the case of two zeolites, and the difference in the free energy barriers is mostly arising due to entropy. Although computed free energy barriers show similar trends as TASS, their values differ. In the free energy calculations using normal modes, entropy is computed from harmonic vibrational and translational partition functions and it is likely to miss a part of the entropy due to the flexibility of the unbound gas molecule in the R state. On the other hand MD simulations could sample the conformations better, and account for these effects.