PROTON-TRANSFER BETWEEN H-ZEOLITE AND ADSORBED ACETONE OR ACETONITRILE - QUANTUM-CHEMICAL AND FTIR STUDY

The proton-transfer energetics, heats of adsorption, and vibrational spectra of acetone and acetonitrile complexes with the H3Si-OH-A1H(2)-OH zeolite model were studied by ab initio HF/3-21G, HF/6-31G* and MP2/6-31+G** methods. The results were compared with FTIR examination of acetone, acetone-d(6), and acetonitrile-d(3) adsorbed at room temperature on the skeletal hydroxyls of HZSM- 5, HY, and HX zeolites. The experimental results indicate that both acetonitrile and acetone preferably form the H-bonded ''neutral'' complex with bridging hydroxyls of the zeolites. Yet, for acetone/HZSM-5 complex, the formation of the ion-pair type of hydrogen bond cannot be entirely ruled out. These findings are supported by the trend in proton transfer energies calculated for these systems. The higher ability of less basic water to be protonated at the zeolite surface is attributed to two point adsorption that enables coupled movement of protons in two adjacent hydrogen bonds. For acetone (acetonitrile), the interaction of the methyl group hydrogens with the skeletal oxygens can ease the proton transfer between the bridging hydroxyl and the adsorbate carbonyl (nitrile) in a similar but less effective way.