Single and Dual Cation Sites in Zeolites:  Theoretical Calculations and FTIR Spectroscopic Studies on CO Adsorption on K-FER

Abstract

Interaction of CO with K-FER zeolite was investigated by a combination of variable-temperature IR spectroscopy and computational study. Calculations were performed using omega(CO)/r(CO) correlation method in combination with a periodic density functional theory model. On the basis of agreement between experimental and calculated results, the following carbonyl complexes were identified: (i) mono- and dicarbonyl C-down complexes on single K(+) sites characterized by IR absorption bands at 2163 and 2161 cm(-1), respectively; (ii) complexes formed by CO bridging two K(+) ions separated by about 7-8 A (dual sites) characterized by a band at 2148 cm(-1); and (iii) isocarbonyl (O-down) complexes characterized by a band at 2116 cm(-1). The bridged carbonyl complexes on dual K(+) sites are about 5 kJ/mol more stable than monodentate (monocarbonyl) CO complexes. The C-O stretching frequency of monocarbonyl species in K-FER depends on K(+) location in the zeolite, and not on K(+) coordination to the framework. A combination of theoretical calculations using a periodic density functional model and experimental results showed formation of two types of monocarbonyls. The most abundant type appears at 2163 cm(-1), and the less abundant one at 2172 cm(-1). These experimentally determined wavenumber values coincide, within +/-2 cm(-1), with those derived from theoretical calculations.

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

“…[15,41] However, the calculations on CO/zeolite Ca-A reported here show that there is only a negligible difference between the stability of CO adsorption complexes on single-and dual-cation sites provided comparison is made between complexes involving the same type of primary cation site (i.e., the cation interacting with the C atom of the CO molecule). A reason for this could be that, in the case of zeolite Ca-A, the bridged CO complexes formed on dual-cation sites deviate largely from a linear arrangement, as shown in Fig-A C H T U N G T R E N N U N G ure 3 g for the S2-S1 A complex.…”

“…Di-and tri-carbonyls can also be formed in some cases when the CO equilibrium pressure is relatively high. [9,10,14] However, combined experimental and computational studies on CO adsorption on several alkali metal exchanged zeolites have recently shown that some gas adsorbing centres can actually involve two or more metal cations, [15,16] and such multiple-cation sites give rise to CO adsorption complexes that can differ from those formed on single-cation sites, not only in geometry, but also in CÀO stretching frequency and interaction energy (between CO and the cation site). While adsorption sites involving more than one cation are already well established for alkalimetal-exchanged zeolites, the case of zeolites containing divalent (extra-framework) cations is largely unexplored.…”

“…refs. [15][16][17][18][19][20]), but extension to complexes formed on zeolites containing alkaline earth metal cations has not been explored yet. On the experimental side, IR spectra were recorded over a temperature range with the aim not only of registering the spectroscopic features of each type of adsorption complex formed, but also of determining the corresponding value of the adsorption enthalpy.…”

Variable‐Temperature IR Spectroscopic and Theoretical Studies on CO2 Adsorbed in Zeolite K‐FER

“…Some calculations with double UC were also performed. The CO 2 adsorption complexes on the most stable K + sites in the vicinity of each of four distinguishable framework Al positions (determined previously [42] ) were investigated. The following K + sites were considered ( Figure 1): I2/T2 and I2/T4 in the 8-R window on the intersection of perpendicular (P) and main (M) channels, M7/T3 in the M channel, and P8/T1 in the P cage (for details on site notation see ref.…”

Intramolecular Hydroalkoxylation of Non‐Activated CC Bonds Catalysed by Zeolites: An Experimental and Theoretical Study

“…[45][46][47] In the case of CO adsorption on MFI, such a simultaneous adsorption of CO on dual cation sites was observed for samples with Si/Al = 14. [48] This threshold clearly depends on the size of the adsorbate and the zeolite topology.…”