or O 9 . We found that O 10 oxygen atom is not accessible for the proton transfer because it is oriented to the other site of the Brønsted acid site. The converged free energy surface for the proton exchange reaction in H-ZSM-5 and H-MCM-22 zeolites are given in Figure and Figure . The free energy barrier for the process is 29.2 ± 0.9 kcal mol -1 and 31.3 ± 1.0 kcal mol -1 in H-ZSM-5 and H-MCM-22 zeolites, respectively. The energy barriers reported for such exchange reactions vary between 24 -38 kcal mol -1 30-32,45,86-89 in different type of zeolites at varying temperatures.