Effects of water and ion-exchanged counterion on the FT-IR spectra of ZSM-5: Part III. Cu+(CO)2–ZSM-5: interaction of Cu+(CO)2 complex with H2O and changes in skeletal T–O–T vibrations Original Research Article

The effect of water on Cu+(CO)2–ZSM-5 (nSi/nAl=20, nCu/nAl=0.75) has been studied at 293 K by time-resolved FT-IR technique in transmission mode, using self-supporting sample and flow system. After an H2O pulse addition into the CO/He line, (a) the IR bands of Cu+(CO)2 di-carbonyls at 2177 (sym) and 2151 cm−1 (as), (b) the skeletal T–O–T vibrations and (c) the OH vibrations considerably changed. In spite of the presence of 7.44 Torr (1 Torr=133.3 Pa) CO, water easily and reversibly displaced one carbonyl ligand [i.e. Cu+(CO)2+H2O⇌Cu+(CO)(H2O)+CO], since the relevant FT-IR spectra showed very similar features to those presented recently for the [(Cu+CO)–ZSM-5+H2O] system [see, J. Sárkány, Topics in Catalysis 18(2002)271]. Subsequent to H2O addition, only first one ν(12CO) band appeared (stage C), namely at 2111 cm−1 which continuously shifted to 2118 cm−1 with decreasing H2O concentration, then a new band series was observed (stage B) with gradual shifts from 2132 to 2139 cm−1, while finally the original Cu+(12CO)2 twin complex (2177 and 2151 cm−1) [and, according to the equilibrium, Cu+(12CO) mono-carbonyl (2157 cm−1)] came back (stage A), via an isosbestic point at 2144 cm−1. Similarly, Cu+(13CO)(12CO) mixed di-carbonyls (caused by 1.11% 13CO being present naturally in CO) showed also three types of interactions with H2O. Two of them were analogous to [Cu+(13CO)+H2O] with stages C (2063–2070 cm−1) and B (2083–2089 cm−1), while stage A revealed only one di-carbonyl-related characteristic band at 2111 cm−1 due to low-frequency (LF) [or νas(CO)] band of Cu+(13CO)(12CO). The pertinent high-frequency-band (HF-band) [νsym(CO)] at 2167–2170 cm−1 could not be observed because of high overlapping with (very) strong ν(CO) bands of Cu+(12CO)2 and Cu+(12CO) carbonyls. Other frequency regions of the FT-IR spectra also resulted in some useful informations. H2O reversibly interacted at 293 K with zeolitic OH groups and with the whole zeolite ZSM-5. Large amount of H2O notably enhanced the intensities of skeletal vibrations.