Computational study on IM-5 zeolite: What is its preferential location of Al and proton siting?

The topological structure of IM-5 zeolite has remained a mystery for nearly 10 years. Stimulated by the recently structural solution of IM-5, we firstly report the computational study on the Al locations, acid sites and acid strength, which are important to understand the catalytic mechanism of IM-5. At the B3LYP/6-31+G(d,p) level, the 8T models were applied. The substitutions of Si by Al atom at 24 inequivalent tetrahedral crystallographic sites and the corresponding H proton localizations were examined by calculating the Al, H substitution energies, proton affinities, the atomic charges on proton and hydroxyl stretching vibrational frequencies. Based on the calculated results it was predicted that the most favorable sites for Al atom substitution in IM-5 were T4, T5, T14, T15 and T19 sites, whereas the least favorable sites were T1, T3, T8, T11, and T16 sites. There are about 40 preferable Al, H locations with relatively high acidity, including the nine strongest acid sites Al19H43 > Al14H18 > Al5H13 > Al4H8 > Al10H26, Al15H26, Al15H37, Al22H45 and Al24H47. The last five sites have equivalent proton affinity values. The numerous Al, H-sites with high acidity may be responsible for the high catalytic ability of IM-5. The calculated results should be helpful for understanding the chemistry of IM-5, the most complicated zeolite material known up to now.