The initial bulk structure of pure siliceous ZSM-5 zeolite was obtained from the zeolite database, whereas the structure of MCM-22 was generated using the VESTA package. The bulk structures of both the zeolites were optimized using the MZHB MM potential developed in our group. We used GULP program for optimizations where the cell parameters were also allowed to relax together with all the atoms. MZHB potential is known to provide accurate description of structure and dynamics of siliceous materials and is suitable for QM/MM calculations. The periodic orthorhombic 2 × 2 × 2 supercells were constructed for both the zeolites to perform QM/MM calculations. In our QM/MM calculations, a 19T site of the zeolite, i.e., Si 19 O 51 in the zeolite framework, was treated within the QM potential for both the zeolites, whereas the rest of the zeolite framework was treated with MM potential; see also Figure . The ZSM-5 has three-dimensional pore channels and belongs to the MFI family, whereas the MCM-22 zeolite is from the MWW family which contains twodimensional pore channels. The preferred sites for the Al atom substitution replacing Si atom from the zeolite lattice are T12 and T3 for H-ZSM-5 and H-MCM-22 zeolites, respectively. We replaced the T12-Si atom in ZSM-5 and T3-Si atom in MCM-22 zeolites by Al atom. The negative charge of the zeolite framework is neutralized by the proton on one of the oxygen atoms of the shared SiO