Interaction of methanol with OH groups of zeolites: Comparison of theory and experiment

Summary

The nature of methanol interaction complexes with bridging hydroxyls of zeolites is studied experimentally and theoretically. Results of calculations using HF, DFT, and MP2 methods on a large H₂OH-Si-O-SiH₂-OH-AlH₂-O-SiH₂OH are compared with experimental studies, particularly 1H broad line and FTIR fragment spectroscopies on H-mordenite and H-ZSM-5. Both experimental and theoretical studies predict the hydrogen-bonded complex of methanol with zeolites. Two local minima of the hydrogen-bonded complex with a different orientation of methanol are found on the potential energy surface. In both structures, two active sites of zeolite are separated by one skeletal oxygen. The calculated adsorption enthalpies of both structures fall into the interval of measured adsorption heats. The calculated vibrational OH frequencies are in reasonable agreement with the experimentally measured frequencies. The calculations are also performed on a larger fragment which includes 4-oxygen ring and a part of the zeolite channel. The results obtained with this fragment are similar to those obtained with smaller fragments.