Effects of water and ion-exchanged counterion on the FTIR spectra of ZSM-5. I. NaH-ZSM-5:: H-bonding with OH groups of zeolite and formation of H+(H2O)n Original Research Article

A dried (773 K, Ar, 1 h) NaH-ZSM-5 sample (Si/Al = 20, Na/Al = 0.5) showed two sharp O–H vibrations at 3745 cm−1 (SiOH, terminal OH) and at 3612 cm−1 [Si(OH)Al, bridged OH], a weak, wide band at 3300–3200 cm−1 due to H-bonded OH and three characteristic (T–O–T) vibrations at 1268, 975 and 834 cm−1 due to external asymmetric (νasext), internal asymmetric (νasint) and internal symmetric (νsymint) stretching lattice vibrations, respectively. The value of 975 cm−1 obtained here for the charge-sensitive νasint of the ZSM-5, was in good accordance with earlier results gained for bare Cu+, Cu+–CO or Cu2+ containing ZSM-5. The broadening of the 975 cm−1 band to 1010–990 cm−1 indicated an incomplete ion-exchange of H+ by Na+ obtained during the preparation of NaH-ZSM-5 from H-ZSM-5. Depending on the amount, water markedly influenced almost the whole FTIR spectra of NaH-ZSM-5, increasing more or less the intensities of the νasext, νasint and νsymint bands. At high H2O concentration, the H2O ligands in the aquacomplex completely screened the positive charge of Na+ ions. As a result, the perturbation effect on νasint disappeared, since νasint blueshifted to 1010 cm−1. Strongly H-bonded interactions with Fermi resonance and the formation of H+(H2O)n was highly suggested by the FTIR spectra at medium and high concentration of H2O, respectively. Even at higher temperatures, a part of silanol OH groups revealed unusually strong interactions with H2O molecules retained probably by the Na+ cations. The 880 cm−1 band attributed to out-of-plane OH bending mode and the νasint and νsymint bands varied in relation to each other, in three different stages.