Nanosized ZSM5 zeolites with microcrystal dimensions in the 20–120 nm range have been characterized by means of IR spectroscopy and HRTEM microscopy. The vibrational spectrum of the OH groups on the external and internal surfaces of H-ZSM5 and Na-ZSM5 samples of different crystallite dimensions has been investigated. For the sake of comparison the spectra of silicalite samples containing different concentrations of sodium and aluminium are also shown. For this purpose high-purity silicalite samples were prepared following a novel synthesis route. Carbon monoxide (a very weak Lewis base) was used to probe the acidity present on the external and internal surfaces of the zeolites through formation of 1 : 1 adducts with silanols (both internal and external), Brønsted-acid groups (both framework and extraframework), Na⁺ ions, and Lewis Al³⁺ centres (in extraframework and framework positions). The IR-active CO stretching modes of the complexes are shifted to higher wavenumber with respect to the free molecule; the positive shift can be used to estimate the acid strength. CO that was physically adsorbed in the zeolite channels has also been investigated.

A systematic investigation of the IR and Raman spectra of pure and Al-, B-, Ti- and Fe-substituted silicalites in the SiO stretching region (1500–700 cm^–1) is presented. As well as the characteristic stretching modes of the skeleton, silicalites containing hydroxylated nests show also broad bands at ca. 960 cm^–1(IR) and at ca. 976 cm^–1(Raman), associated with O₃Si—OH group modes, with prevailing Si—OH stretching character. The replacement of Si with heavier elements (like Ti or Fe) causes the appearance of new IR- and Raman-active modes (i) at 960 cm^–1(IR and Raman) and at 1127 cm^–1(Raman) in Ti silicalite; (ii) at 1015 cm^–1(IR) and at 1020 cm^–1(Raman) in Fe silicalite. Neither the Raman nor the IR spectra of the skeletal modes are substantially modified by the introduction of Al (ZSM5). The presence of boron induces the appearance in the IR spectra of a complex absorption at 1380 cm^–1 and at 960–930 cm^–1, which corresponds to the absorptions at 1417 and 976 cm^–1 in the Raman spectra. The assignment of the absorptions associated with heteroatoms and hydroxylated nests is discussed in detail.