IR spectroscopy of neutral and ionic hydrogen-bonded complexes formed upon interaction of CH3OH, C2H5OH, (CH3)2O, (C2H5)2O and C4H8O with H-Y, H-ZSM-5 and H-mordenite: comparison with analogous adducts formed on the H-Nafion superacidic membrane

Abstract

A complete assignment of the IR spectra of the ionic and neutral hydrogen-bonded species formed by interaction of a series of bases of increasing proton affinity [methanol, ethanol, dimethyl ether, diethyl ether and tetrahydrofuran (THF)] with H-Y, H-ZSM-5, H-mordenite (H-Mord) and H-Nafion is given. Special attention is devoted to the discussion of spectral manifestations associated with structures varying with continuity from the true neutral AH⋯B form (low B proton affinity) to the true ionic A^–⋯HB⁺ forms (high B proton affinity) passing through intermediate situations characterized by proton potentials with a single broad minimum or two minima separated by a low barrier (hesitating proton) and substantial ionic character. The assignment is based more on the comparison of the whole series of spectra than on the minute analysis of the specific properties of each spectrum. The assignment is also based on the comparison of the spectra of the adducts in zeolites with the spectra of analogous species formed in homogeneous conditions between acids and bases of comparable base and acid strengths (used as models).

The main conclusions are: (i) adducts formed in H-Y have neutral AH⋯B character (only at the highest filling condition are signs of formation of ionic dimeric B⋯HB⁺ species observed for methanol and ethanol); (ii) the species formed upon adsorption of methanol and ethanol on H-ZSM-5 and H-Mord are neutral at low coverage and prevalently ionic at the highest filling conditions [because of the formation of (B)_n⋯HB⁺ oligomers: 1, 2, …, n]; (iii) owing to the larger basicity of the ethers and THF, monomeric hydrogen-bonded adducts characterized by nearly symmetric proton potentials and substantial ionic character are directly formed on H-ZSM-5 and H-Mord without the intervention of other adsorbed molecules; (iv) THF and diethyl ether do not enter the H-Mord channels because the strongly adsorbed forms formed at the pore entrances block further penetration; (v) on the superacidic H-Nafion membrane (used as a model of a superacidic solid) all substrates directly form very strong and nearly symmetric hydrogen bonds without the assistance of other basic molecules: consequently the formed species always have prevalently ionic character even at the lowest dosages.