IR SPECTROSCOPY OF NEUTRAL AND IONIC HYDROGEN-BONDED COMPLEXES FORMEDUPON INTERACTION OF CH3OH, C2H5OH, (CH3)(2)O, (C2H5)(2)O AND C4H8O WITH H-Y, H-ZSM-5 AND H-MORDENITE - COMPARISON WITH ANALOGOUS ADDUCTS FORMED ON THE H-NAFION SUPERACIDIC MEMBRANE

A complete assignment of the IR spectra of the ionic and neutral hydro gen-bonded species formed by interaction of a series of bases of incre asing proton affinity [methanol, ethanol, dimethyl ether, diethyl ethe r and tetrahydrofuran (THF)] with H-Y, H-ZSM-5, H-mordenite (H-Mord) a nd H-Nafion is given. Special attention is devoted to the discussion o f spectral manifestations associated with structures varying with cont inuity from the true neutral AH ... B form (low B proton affinity) to the true ionic A(-)... HB+ forms (high B proton affinity) passing thro ugh intermediate situations characterized by proton potentials with a single broad minimum or two minima separated by a low barrier (hesitat ing proton) and substantial ionic character. The assignment is based m ore on the comparison of the whole series of spectra than on the minut e analysis of the specific properties of each spectrum. The assignment is also based on the comparison of the spectra of the adducts in zeol ites with the spectra of analogous species formed in homogeneous condi tions between acids and bases of comparable base and acid strengths (u sed as models). The main conclusions are: (i) adducts formed in H-Y ha ve neutral AH ... B character (only at the highest filling condition a re signs of formation of ionic dimeric B ... HB+ species observed for methanol and ethanol); (ii) the species formed upon adsorption of meth anol 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 f ormation of (B)(n) ... HB+ oligomers: 1, 2,..., n]; (iii) owing to the larger basicity of the ethers and THF, monomeric hydrogen-bonded addu cts characterized by nearly symmetric proton potentials and substantia l ionic character are directly formed on H-ZSM-5 and H-Mord without th e 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 sup eracidic H-Nafion membrane (used as a model of a superacidic solid) al l substrates directly form very strong and nearly symmetric hydrogen b onds without the assistance of other basic molecules: consequently the formed species always have prevalently ionic character even at the lo west dosages.