Roman Bulánek

Reducibility and oxidation activity of Cu ions in zeolites: Effect of Cu ion coordination and zeolite framework composition

Reducibility and catalytic activity in propane oxidation was studied for the Cu ions exchanged in zeolites of structural types ZSM-5 (MFI), ferrierite (FER) and erionite (ERI) with different Cu/Al/Si compositions, and Na+ or H+ ions present as co-cations. The Cu ions were exchanged at conditions of various Cu concentrations in salt solutions, anion type and pH to receive samples with different Cu ion structures. Reducibility of the Cu ions in zeolites has been investigated by means of temperature-programmed reduction by hydrogen, and zeolite evacuation and treatment in CO atmosphere with subsequent detection of the Cu+–(CO)n complexes by IR spectroscopy. Redox properties of the Cu ions, represented by their reducibility and catalytic activity in propane oxidation to CO and CO2, dramatically differed depending on Cu loading in the zeolite, content of aluminum in the framework and presence of co-cations (Na+ versus H+ ions), as well as on the state of the Cu ions, i.e. if atomically dispersed as exchanged cations or present in CuO oxidic-like species. These results have been compared with our previous studies revealing several defined Cu ion sites characteristic for high-silica zeolites, differing in coordination-siting, positive charges, and population depending on Cu loading, and content of Al in the framework. It has been shown that, well-dispersed CuO species exhibit much higher oxidation activity and reducibility compared to the exchanged Cu ions. The redox properties of the exchanged Cu ions have been found to be highly controlled, besides the local geometry of the ligand field, by the total delocalized negative framework charge, given by the Al content in the framework, and the local negative charge adjacent to the cation, controlled by the Si–Al sequences. The Cu ions balanced by a single AlO2− entity, being most populated in zeolites with high Cu loading and in zeolites with low concentration of aluminum in the framework are suggested to be those easily reducible, and exhibiting high oxidation activity.

The vibrational dynamics of carbon monoxide in a confined space—CO in zeolites

Based on theoretical calculations, and a survey of infrared spectra of CO adsorbed on different cation exchanged zeolites, a model is proposed to explain the influence of the zeolite framework on the vibrational behaviour of CO confined into small void spaces (zeolite channels and cavities). The concepts developed should help to understand a number of details relevant to both, precise interpretation of IR spectra and a better understanding of the vibrational dynamics of small molecules in a confined space.

On the site-specificity of polycarbonyl complexes in Cu/zeolites: combined experimental and DFT study.

The preferred Cu(+) sites and formation of mono-, di-, and tricarbonyl complexes in the Cu-FER were investigated at the periodic density functional theory level and by means of FTIR spectroscopy. The site-specificity of adsorption enthalpies of CO on Cu-FER and of vibrational frequencies of polycarbonyl complexes were investigated for various Cu(+) sites in Cu-FER. Large changes in the Cu(+) interaction with the zeolite framework were observed upon the formation of carbonyl complexes. The dicarbonyl complexes formed on Cu(+) in the main channel or on the intersection of the main and perpendicular channels are stable and both, adsorption enthalpies and CO stretching frequencies are not site-specific. The fraction of Cu(+) ions in the FER cage, that cannot form dicarbonyl can be determined from IR spectra (about 7% for the Cu-FER with Si/Al = 27.5 investigated here). The tricarbonyl complexes can be formed at the Cu(+) ions located at the 8-member ring window at the intersection of main and perpendicular channel. The stability of tricarbonyl complexes is very low (DeltaH degrees (0 K)>or=-4 kJ mol(-1)).

Localization of Cu+ sites and framework Al positions in high-silica zeolites: Combined experimental and theoretical study

Three types of Cu+ sites are distinguished from the analysis of TPD spectra of CO in high-silica zeolites and the details about the structure and coordination of Cu+ at these sites are obtained from calculations.

Investigation of IR vibrational band of C–O bond of carbonyl species in Cu+-MFI zeolites

Interaction of CO molecules with Cu-MFI zeolites reduced in vacuum or CO atmosphere was investigated. The character of the absorption bands ascribed to intrazeolitic carbonyl species was explored in the dependence on the Cu loading, Si/Al ratio, degree of reduction and on the coverage of the Cu+ ions by CO molecules. The different character of individual Cu sites was revealed in the IR spectra of carbonyl species upon CO pressure. Some Cu ions are not able to form dicarbonyl species even at 7.5 kPa of CO. The band of residual monocarbonyl species strongly overlaps with the band of asymmetric vibration of dicarbonyl species, making the quantitative analysis of this band rather difficult. It was found that the position and the half-width of the monocarbonyl band, as well as the third and fourth statistical moment of the band are independent of the variable factors mentioned above. The main vibrational band of monocarbonyl species cannot be deconvoluted to individual bands corresponding to different Cu exchangeable sites. It means that IR spectroscopy of carbonyl species in the range of C–O stretching frequencies is not a suitable method for monitoring of the individual Cu+ sites in MFI zeolite framework. In addition, no correlation of –ΔHads and wavenumber of the fundamental vibration of C–O adsorbed on Cu+ ions in MFI zeolite matrix was found.

Characterization of Cu+ sites in FER: Combined computational and experimental TPD study

CO-TPD spectra of CO/Cu+/FER system were investigated and analyzed by a combination of experimental and theoretical techniques. Three Cu+ site types, characterized by adsorption energies -122, -76 and -59 kJ/mol were found using a combined quantum mechanical/interatomic potential function method. Three Cu+ site types with CO adsorption energies -121, -86, and -62 kJ/mol can be distinguished from the analysis of TPD spectra of CO/Cu+/FER. TPD spectra can be interpreted in terms of the computational results: CO desorbs first from the Cu+ sites in the perpendicular channel cavity. Sites in the main channel and on the intersection have the largest desorption energy. Information about the framework aluminum distribution in FER can be also obtained from the analysis of CO-TPD spectra.

Evidence of heterogeneous dual cation sites in zeolites by combined IR and DFT investigation

Abstract Interaction of the CO molecule with CuX-FER zeolites (X is an alkali-metal or proton as a co-cation) was investigated by IR spectroscopy and DFT calculations. An absorption band at 2138 cm −1 observed in IR spectra of CO on CuK- and CuCs-FER zeolites was assigned to a new type of CO adsorption complex on heterogeneous dual cation sites. CO molecule interacts simultaneously with Cu + and alkali metal cations (via C- and O-end, respectively) in this type of complex. Interaction of CO with the secondary (alkali metal) cation led to a slight destabilization of the carbonyl complex.

Combined theoretical and experimental study of the site-specificity of vibrational dynamics of CO adsorbed on monovalent metal cations in zeolites

The question of site-specificity of IR spectra of adsorbed CO molecule on metal ion exchanged zeolites was addressed. The vibrational dynamics of CO adsorbed on Cu + , Li + , and Na + sites in FER was investigated by the combination of experimental and theoretical methods allowing the calculations of CO frequencies with “near spectroscopic accuracy”. The details of experimental spectra in the CO stretching region are assigned based on the calculations. The effects contributing to the overall CO stretching are discussed. The general rules for site-specificity of vibrational dynamics of adsorbed probe molecules are suggested.

Combined oxidative and non-oxidative dehydrogenation of n-butane over VOX species supported on HMS

The combination of oxidative and non-oxidative dehydrogenation of n-butane as an attractive possibility for production of C4 olefins was studied over VOX based catalyst. Long-term activity and selectivity to desired products could be achieved over the catalysts with well dispersed monomeric vanadium oxide species supported on mesoporous silica support.

Oxidative dehydrogenation of propane by nitrous oxide and/or oxygen over Co beta zeolite

The oxidative dehydrogenation of propane has been studied with nitrous oxide (or mixture of nitrous oxide and oxygen) as oxidant. Nitrous oxide is a more selective but less active oxidant as compared with molecular oxygen. Upon increasing the concentration of N2O in the reaction mixture of propane and oxygen results in a substantial increase of propane conversion, while the selectivity to propene remains constant. The synergistic effect of O2 and N2O leads to a threefold higher yield of propene relative to than that of oxygen or nitrous alone.