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 H 2O and changes in skeletal T–O–T vibrations

Abstract

The effect of water on Cu +(CO) 2–ZSM-5 ( n Si/ n Al=20, n Cu/ n Al=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 H 2O 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+H 2O⇌Cu +(CO)(H 2O)+CO], since the relevant FT-IR spectra showed very similar features to those presented recently for the [(Cu +CO)–ZSM-5+H 2O] system [see, J. Sárkány, Topics in Catalysis 18(2002)271]. Subsequent to H 2O addition, only first one ν( 12 CO) band appeared (stage C), namely at 2111 cm −1 which continuously shifted to 2118 cm −1 with decreasing H 2O concentration, then a new band series was observed (stage B) with gradual shifts from 2132 to 2139 cm −1, while finally the original Cu +( 12 CO ) 2 twin complex (2177 and 2151 cm −1) [and, according to the equilibrium, Cu +( 12 CO ) mono-carbonyl (2157 cm −1)] came back (stage A), via an isosbestic point at 2144 cm −1. Similarly, Cu +( 13 CO )( 12 CO ) mixed di-carbonyls (caused by 1.11% 13 CO being present naturally in CO) showed also three types of interactions with H 2O. Two of them were analogous to [ Cu +( 13 CO)+ H 2 O ] 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 ν as(CO)] band of Cu +( 13 C O)( 12 C O). The pertinent high-frequency-band (HF-band) [ ν sym(CO)] at 2167–2170 cm −1 could not be observed because of high overlapping with (very) strong ν(CO) bands of Cu +( 12 CO ) 2 and Cu +( 12 CO ) carbonyls. Other frequency regions of the FT-IR spectra also resulted in some useful informations. H 2O reversibly interacted at 293 K with zeolitic OH groups and with the whole zeolite ZSM-5. Large amount of H 2O notably enhanced the intensities of skeletal vibrations.