Computational and Variable‐Temperature Infrared Spectroscopic Studies on Carbon Monoxide Adsorption on Zeolite Ca‐A

Abstract

Carbon monoxide adsorption on LTA (Linde type 5A) zeolite Ca-A is studied by using a combination of variable-temperature infrared spectroscopy and computational methods involving periodic density functional calculations and the correlation between stretching frequency and bond length of adsorbed CO species (nu(CO)/r(CO) correlation). Based on the agreement between calculated and experimental results, the main adsorption species can be identified as bridged Ca(2+)...CO...Ca(2+) complexes formed on dual-cation sites constituted by a pair of nearby Ca(2+) cations. Two types of such species can be formed: One of them has the two Ca(2+) ions located on six-membered rings of the zeolite framework and is characterized by a C-O stretching frequency in the range of 2174-2179 cm(-1) and an adsorption enthalpy of -31 to -33 kJ mol(-1), whereas the other bridged CO species is formed between a Ca(2+) ion located on an eight-membered ring and another one on a nearby six-membered ring and is characterized by nu(CO) in the range 2183-2188 cm(-1) and an adsorption enthalpy of -46 to -50 kJ mol(-1). Ca(2+)...CO monocarbonyl complexes are also identified, and at a relatively high CO equilibrium pressure, dicarbonyl species can also be formed.

Probing zeolites by vibrational spectroscopies

“…273 These two successive studies confirmed the electrostatic nature of the interaction. In this regard, the theoretical works of Larin et al 259,260 are also worth mentioning together with the subsequent and more refined works of Nachtigall et al [274][275][276][277][278][279][280][281] Other minor spectroscopic features are present in the spectra shown in Fig. 7.…”

“…to the average charge of the first-shell oxygen atoms. In this regard, the theoretical works of Larin et al 259,260 are also worth mentioning together with the subsequent and more refined works of Nachtigall et al [274][275][276][277][278][279][280][281] Other minor spectroscopic features are present in the spectra shown in Fig. 8 are an indirect measure of the basicity of the zeolitic framework (see Section 4).…”

“…17,154,[274][275][276][277][278][279][280][281][283][284][285]288, In brief, the technique is based on displacement of the equilibrium between molecules in the gas phase and those adsorbed on a given type of surface site, which is carried out by changing the temperature T of the system, as shown in Fig. 17,154,[274][275][276][277][278][279][280][281][283][284][285]288, In brief, the technique is based on displacement of the equilibrium between molecules in the gas phase and those adsorbed on a given type of surface site, which is carried out by changing the temperature T of the system, as shown in Fig.…”

“…In the last decade, infrared spectroscopy has been widely used to determine the thermodynamics of molecular adsorption on internal sites of zeolites, mainly by the groups of Tsyganenko (Saint Petersburg, Russian Federation), Otero Arean (Palma de Mallorca, Spain), Garrone (Turin Polytechnic, Italy) and Zecchina (Turin University, Italy) on the experimental side and by the group of Nachtigall (Prague, Czech Republic) on the computational side. 17,154,[274][275][276][277][278][279][280][281][283][284][285]288, In brief, the technique is based on displacement of the equilibrium between molecules in the gas phase and those adsorbed on a given type of surface site, which is carried out by changing the temperature T of the system, as shown in Fig. 28a for CO adsorbed on chromocene (Cp 2 Cr) hosted inside a polystyrene (PS) porous polymeric matrix.…”

Interactions and Diffusion of Methane and Hydrogen in Microporous Structures: Nuclear Magnetic Resonance (NMR) Studies

“…The S1 sites are at the centers of the six-membered rings adjacent to the eight-sided opening of the α-supercages of the zeolite structure, and the S2 sites are located close to the octagonal openings of the α-supercages. For a detailed summary of the positions of these sites relative to the supercages the reader is referred to Figure 6 of [29], Figure 2 of [31], and Figure 2 of [32]. These discrete energies could be due to: (i) small energy barriers between the potentials at the S1 and S2 sites that allow molecules to jump form one site to another; and (ii) a number of closely spaced energy levels inside the binding potentials.…”