Montserrat R. Delgado

Combined DFT/CC and IR spectroscopic studies on carbon dioxide adsorption on the zeolite H-FER

Adsorption of carbon dioxide on H-FER zeolite (Si:Al = 8:1) was investigated by means of a combined methodology involving variable-temperature infrared spectroscopy and DFT/CC calculations on periodic zeolite models. The experimentally found value of adsorption enthalpy was ΔH0 = −30 kJ mol−1. According to calculations, adsorption complexes on isolated Si(OH)Al Bronsted acid sites (single sites) involve an adsorption enthalpy in the range of −33 to −36 kJ mol−1, about half of which is due to weak intermolecular interactions between CO2 and the zeolite framework. Calculations show clearly the significant role played by weak intermolecular interactions; adsorption enthalpies calculated with standard GGA type exchange-correlation functionals are about 13 kJ mol−1 underestimated with respect to experimental results. Good agreement was also found between calculated and experimentally observed stretching frequencies for these complexes. Calculations revealed that CO2 adsorption complexes involving two neighbouring Bronsted acid sites (dual sites) can be formed, provided that the dual site has the required geometry. However, no clear evidence of CO2 adsorption complexes on dual sites was experimentally found.

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

Adsorption of CO(2) in K-FER zeolite is investigated by a combination of variable-temperature IR spectroscopy and periodic DFT calculations augmented for description of dispersion interactions. Calculated adsorption enthalpies for CO(2) adsorption complexes on single extra-framework K(+) sites and on dual-cation sites where CO(2) interacts simultaneously with two extra-framework K(+) cations (-40 and -44 kJ mol(-1), respectively) are in excellent agreement with experimental values. The analysis of effects on the frequency of the asymmetric CO(2) stretching mode ν(3) shows that polarization of CO(2) by the K(+) cation leads to an increase in ν(3), while the interaction of CO(2) with the zeolite framework leads to a decrease in ν(3). In the case of K-FER, the latter effect is slightly larger than the former, and thus a small redshift in ν(3) results (-3 cm(-1) with respect to free CO(2)). For adsorption complexes on dual K(+) sites, where CO(2) interacts with one K(+) cation on each end of the molecule, the polarization of CO(2) molecules on both sides results in a blueshift of ν(3). The origin of the redshift in ν(3) when CO(2) is adsorbed in purely siliceous FER is also investigated computationally. Calculations show that the dispersion interaction does not affect the vibrational frequency of adsorbed CO(2).

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

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.

Variable‐Temperature Infrared Spectroscopy Studies on the Thermodynamics of CO Adsorption on the Zeolite Ca–Y

Variable temperature FT-IR spectroscopy (in the range of 298-380 K) is used to study the thermodynamics of formation of Ca(2+)...CO carbonyl species upon CO adsorption on the faujasite-type zeolite Ca-Y, and also the (temperature-dependent) isomerization equilibrium between carbonyl and isocarbonyl (Ca(2+)...OC) species. The standard enthalpy and entropy changes involved in formation of the monocarbonyl species resulted to be DeltaH(0)=-50.3 (+/-0.5) kJ mol(-1) and DeltaS(0)=-186 (+/-5) J mol(-1) K(-1), respectively. Isomerization of the (C-bonded) Ca(2+)...CO carbonyl to yield the (O-bonded) Ca(2+)...OC isocarbonyl involves an enthalpy change DeltaH(iso)(0)=+11.4 (+/-1.0) kJ mol(-1). These results are compared with previously reported data for the CO/Sr-Y system; and also, a brief analysis of enthalpy-entropy correlation for CO adsorption on zeolites and metal oxides is given.

Reply to the ‘Comment on “The nature of cationic adsorption sites in alkaline zeolites—single, dual and multiple cation sites”' by O. Cairon, Phys. Chem. Chem. Phys., 2012, 14, DOI: 10.1039/c2cp40963a

The preceding comments by O. Cairon, Phys. Chem. Chem. Phys., 2012, 14, DOI: 10.1039/c2cp40963, question several aspects of our perspective article on the nature of cationic adsorption sites in alkaline zeolites (P. Nachtigall, M. R. Delgado, D. Nachtigallova and C. O. Arean, Phys. Chem. Chem. Phys., 2012, 14, 1552). Questioning spans from experimental details, through methodology and to the very concept of dual and multiple cation sites in zeolites. While acknowledging that questioning constitutes an excellent method to foster understanding, we hope that the answers given herein will help to clarify the relevant points under discussion.

Probing Gas Adsorption in Zeolites by Variable-Temperature IR Spectroscopy: An Overview of Current Research

The current state of the art in the application of variable-temperature IR (VTIR) spectroscopy to the study of (i) adsorption sites in zeolites, including dual cation sites; (ii) the structure of adsorption complexes and (iii) gas-solid interaction energy is reviewed. The main focus is placed on the potential use of zeolites for gas separation, purification and transport, but possible extension to the field of heterogeneous catalysis is also envisaged. A critical comparison with classical IR spectroscopy and adsorption calorimetry shows that the main merits of VTIR spectroscopy are (i) its ability to provide simultaneously the spectroscopic signature of the adsorption complex and the standard enthalpy change involved in the adsorption process; and (ii) the enhanced potential of VTIR to be site specific in favorable cases.