Calculations of Site-Specific CO Stretching Frequencies for Copper Carbonyls with the “Near Spectroscopic Accuracy”: CO Interaction with Cu+/MFI

Zeolites at the Molecular Level: What Can Be Learned from Molecular Modeling

This review puts the development of molecular modeling methods in the context of their applications to zeolitic active sites. We attempt to highlight the utmost necessity of close cooperation between theory and experiment, resulting both in advances in computational methods and in progress in experimental techniques.

The effect of the zeolite pore size on the Lewis acid strength of extra-framework cations

The catalytic activity and the adsorption properties of zeolites depend on their topology and composition. For a better understanding of the structure-activity relationship it is advantageous to focus just on one of these parameters. Zeolites synthesized recently by the ADOR protocol offer a new possibility to investigate the effect of the channel diameter on the adsorption and catalytic properties of zeolites: UTL, OKO, and PCR zeolites consist of the same dense 2D layers (IPC-1P) that are connected with different linkers (D4R, S4R, O-atom, respectively) resulting in the channel systems of different sizes (14R × 12R, 12R × 10R, 10R × 8R, respectively). Consequently, extra-framework cation sites compensating charge of framework Al located in these dense 2D layers (channel-wall sites) are the same in all three zeolites. Therefore, the effect of the zeolite channel size on the Lewis properties of the cationic sites can be investigated independent of other factors determining the quality of Lewis sites. UTL, OKO, and PCR and pillared 2D IPC-1PI materials were prepared in Li-form and their properties were studied by a combination of experimental and theoretical methods. Qualitatively different conclusions are drawn for Li(+) located at the channel-wall sites and at the intersection sites (Li(+) located at the intersection of two zeolite channels): the Lewis acid strength of Li(+) at intersection sites is larger than that at channel-wall sites. The Lewis acid strength of Li(+) at channel-wall sites increases with decreasing channel size. When intersecting channels are small (10R × 8R in PCR) the intersection Li(+) sites are no longer stable and Li(+) is preferentially located at the channel-wall sites. Last but not least, the increase in adsorption heats with the decreasing channel size (due to enlarged dispersion contribution) is clearly demonstrated.

Advances in theory and their application within the field of zeolite chemistry

Zeolites are versatile and fascinating materials which are vital for a wide range of industries, due to their unique structural and chemical properties, which are the basis of applications in gas separation, ion exchange and catalysis. Given their economic impact, there is a powerful incentive for smart design of new materials with enhanced functionalities to obtain the best material for a given application. Over the last decades, theoretical modeling has matured to a level that model guided design has become within reach. Major hurdles have been overcome to reach this point and almost all contemporary methods in computational materials chemistry are actively used in the field of modeling zeolite chemistry and applications. Integration of complementary modeling approaches is necessary to obtain reliable predictions and rationalizations from theory. A close synergy between experimentalists and theoreticians has led to a deep understanding of the complexity of the system at hand, but also allowed the identification of shortcomings in current theoretical approaches. Inspired by the importance of zeolite characterization which can now be performed at the single atom and single molecule level from experiment, computational spectroscopy has grown in importance in the last decade. In this review most of the currently available modeling tools are introduced and illustrated on the most challenging problems in zeolite science. Directions for future model developments will be given.

Further evidence for the existence of a dual-Cu+ site in MFI working as the efficient site for C2H6 adsorption at room temperature

We have recently clarified the following point: a dual-type site, which is composed of a pair of monovalent copper ions (Cu(+)) formed in a copper-ion-exchanged MFI-type zeolite (CuMFI), functions as the active center for strong ethane (C2H6) adsorption even at room temperature rather than a single-type site composed of a Cu(+) ion. However, the character of the dual-Cu(+) site in a CuMFI is not yet fully understood. In this study, we have elucidated the nature of the active sites for C2H6 based on infrared (IR) and calorimetric data. On the basis of the results obtained, we came to the conclusion that the dual-Cu(+) site composed of Cu(+) ions giving the adsorption energy of 100 kJ mol(-1) and the absorption band at 2151 cm(-1) for carbon monoxide (used as a probe molecule) at room temperature functions as an adsorption site for C2H6. We also evaluated, for the first time, the interaction between the dual-Cu(+) site and C2H6 energetically, by the direct measurement of heat of adsorption. The value of 67 kJ mol(-1) that we recorded was higher than that for the single-Cu(+) site in this sample and also for other samples, such as NaMFI and HMFI.

Cu-ZSM-5: A biomimetic inorganic model for methane oxidation

The present work highlights recent advances in elucidating the methane oxidation mechanism of inorganic Cu-ZSM-5 biomimic and in identifying the reactive intermediates that are involved. Such molecular understanding is important in view of upgrading abundantly available methane, but also to comprehend the working mechanism of genuine Cu-containing oxidation enzymes.

Vibrational dynamics of adsorbed CO2: Separability of the CO2 asymmetric stretching mode

Separability of the CO2 asymmetric stretching mode is probed theoretically by performing highly accurate vibrational calculations on the CO2 and K+CO2 model systems. The proposed approach is applied to a model case of the vibrational dynamics of the CO2 molecule adsorbed in K-FER zeolite. The CCSD(T) level is fully adequate for quantitative decription of the CO2 vibrational dynamics, and all important effects on the vibrational dynamics of CO2 adsorption complexes can be estimated rather accurately (within 5 cm–1) at the DFT level of theory.

Probing the surfaces of heterogeneous catalysts by in situ IR spectroscopy

This critical review describes the reactivity of heterogeneous catalysts from the point of view of four simple, but essential for Chemistry, molecules (namely dihydrogen, carbon monoxide, nitrogen monoxide and ethylene) that are considered as probes or as reactants in combination with "in situ" controlled temperature and pressure Infrared spectroscopy. The fundamental properties of H(2), CO, NO and C(2)H(4) are shortly described in order to justify their different behaviour in respect of isolated sites in different environments, extended surfaces, clusters, crystalline or amorphous materials. The description is given by considering some "key studies" and trying to evidence similarities and differences among surfaces and probes (572 references).

The Variety of Carbon-Metal Bonds inside Cu-ZSM-5 Zeolites: A Density Functional Theory Study

Large-scale density functional theory calculations (DFT) found various types of binding of an unsaturated hydrocarbon (C₂H₂ and C₂H₄) to a ZSM-5 zeolite extraframework copper cation. We employed the DFT calculations based on the B3LYP functional to obtain local minima of an unsaturated hydrocarbon adsorbed on one or two copper cations embedded inside ZSM-5, and then compared their stabilization energies. The DFT results show that the stabilization energies are strongly dependent on the copper coordination environment as well as configurations of two copper cations. Consequently, the inner copper-carbon bonds are influenced substantially by a nanometer-scale cavity of ZSM-5.

Electronic view on ethene adsorption in Cu(I) exchanged zeolites

Ethene adsorption on isolated Cu(i) sites in two types of zeolites (faujasite and MFI) is investigated by means of the embedded cluster method. Structures, energetic stabilities and C[double bond, length as m-dash]C stretching vibrations in adsorption complexes are discussed. Furthermore, for interpretative purposes, the interaction energies are decomposed, using novel approaches based on so called natural orbitals for chemical valence. Ethene is always symmetrically bound to Cu(i) ion by both C atoms. In some cases two local minima of similar stability on the potential energy surface, differing by Cu(i) site relaxation can be found that may be simultaneously populated in equilibrium. Binding energies usually decrease with the degree of reconstruction of Cu(i) site after adsorption, however, in particular cases, a more distorted structure can be slightly more stable if favorable pi* back donation overwhelms the distortion effects. Calculated values of binding energies for Cu(i)-Y zeolite (about 80 kJ mol(-1)) agree well with microcalorimetric data. We predict that ethene binding in MFI is over two times stronger (to the best of our knowledge no experimental data are available). The C[double bond, length as m-dash]C stretching frequency is not site specific, but depends only on the type of copper connectivity to oxygen nodes. The appearance of two C[double bond, length as m-dash]C bands in IR spectra of Cu(i)-faujasite can be explained as the effect of coexistence of two types of adsorption complexes, with Cu(i) coordinated to one or two framework tetrahedrons, respectively. In Cu(i)-MFI, only one type of adsorption complex with Cu(i) ion coordinated to a single tetrahedron exists, as only a single C[double bond, length as m-dash]C band is present in IR spectra.

Adsorption of dioxygen to copper in CuHY zeolite

The adsorption of dioxygen to copper in CuHY zeolites has been studied by means of FTIR spectroscopy and model calculations at the quantum mechanical/molecular mechanics (QM/MM) level. Different Si/Al ratios, substitution patterns and adsorption sites within the cavities of the zeolite lead to a large number of different isomers to be studied. In addition, these parameters control the end-on vs. side-on adsorption of dioxygen. High-level multireference benchmark calculations for the singlet and triplet states of such adsorption complexes corroborate the use of density functional theory for the investigation of these systems. Comparison of the experimental and computed data allows for the identification of a preferred adsorption site and a small number of isomers which appear to be most relevant for the adsorption process. Redshifts of >250 cm(-1) are obtained for the vibrational frequencies of adsorbed O(2).

Computational and FTIR spectroscopic studies on carbon monoxide and dinitrogen adsorption on a high-silica H-FER zeolite

Adsorption (at a low temperature) of carbon monoxide and dinitrogen on a high-silica ferrierite-type zeolite (H-FER, Si : Al = 27.5 : 1) was investigated by means of variable temperature infrared spectroscopy and theoretical calculations at the periodic DFT level. This combined experimental and computational approach led to detailed characterization of several types of hydrogen-bonded OHCO and OHN(2) complexes, formed by interaction between the adsorbed molecules and the Brønsted acid OH groups of the zeolite. CO or N(2), forming linear complexes with OH groups pointing towards a sufficiently ample void space, show the largest adsorption enthalpy which was found to be in the (approximate) range of -25 to -29 kJ mol(-1) for CO and -15 to -19 kJ mol(-1) for N(2). Less stable OHCO or OHN(2) complexes can be formed when either the Brønsted acid OH group is involved in intra-zeolite hydrogen bonding or when the free space available is too small to allow formation of linear complexes without previous re-location of the proton of the OH group involved. The details of experimental IR spectra in the O-H, C-O, and N-N stretching regions could be interpreted on the basis of good agreement between experimental and calculated results.

Identification of two types of exchangeable sites for monovalent copper ions exchanged in MFI-type zeolite

Three different approaches have been used to characterize the state of exchanged copper ions in copper-ion-exchanged MFI (CuMFI) samples. (1) Two types of an ion-exchangeable site with different adsorption properties for N(2) or CO molecules were identified depending on the pre-treatment temperature (723 or 873 K) of a sample prepared by using an aqueous solution of CuCl(2). (2) The state of the active sites formed by the evacuation of a sample at 873 K that had been prepared using a mixture solution of aqueous NH(4)CH(3)COO and Cu(CH(3)COO)(2) was analysed utilizing both (13)C(18)O and (12)C(16)O to identify the two types of active adsorption sites for CO molecules. (3) CuMFI samples prepared by the ion-exchange method employing anhydrous CuCH(3)COO showed a surprising adsorption feature characterized by a single IR band occurring at 2159 cm(-1) due to the adsorbed CO molecules, but there was no corresponding IR band due to adsorbed N(2) molecules. A successful preparation of CuMFI, in which the monovalent copper ions exclusively occupied another one of the two types of ion-exchangeable sites, was also carried out utilizing the solid-ion exchange method using Cu(CH(3)COO)(2).H(2)O. This site exhibits an IR band occurring at 2151 cm(-1) for CO molecules and also acts as an active site for N(2) molecules. These experimental data correlate, and clearly indicate that there are at least two types of exchangeable sites for copper ions in MFI-type zeolites.

Theoretical studies of Cu(I) sites in faujasite and their interaction with carbon monoxide

Sitting, coordination, and properties of Cu(I) cations in zeolite faujasite are investigated using a combined quantum mechanics-interatomic potential function method. The coordination of Cu(I) ions depends on their location within the zeolite lattice. Cu(I) located inside the hexagonal prisms (site I') and in the plane of six-membered aluminosilicate rings on the walls of sodalite units (site II) is threefold coordinated, whereas Cu(I) located in the supercages (site III) is twofold coordinated. In agreement with available experimental data Cu(I) appears to be more strongly bound in sites I' and II than in site III. The binding energy of site II Cu(I) ions increases with the number of Al atoms, but only closest Al atoms have a substantial influence. The CO molecule binds more strongly onto sites with weaker bound cations and lower coordination. We assign the two CO stretching IR bands observed for Cu(I)-Y zeolites to sites II with one Al (2157-2161 cm(-1)) and two Al atoms (2140-2148 cm(-1)) in the six-membered aluminosilicate ring. For Cu(I)-X we tentatively assign the high frequency band to site III (2156-2168 cm(-1)) and the low-frequency band to site II with three Al atoms in the six-membered ring (2136-2138 cm(-1)).

Single and dual cation sites in zeolites: theoretical calculations and FTIR spectroscopic studies on CO adsorption on K-FER

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.

Thermodynamics of reversible gas adsorption on alkali-metal exchanged zeolites—the interplay of infrared spectroscopy and theoretical calculations

Detailed understanding of weak solid-gas interactions giving rise to reversible gas adsorption on zeolites and related materials is relevant to both, fundamental studies on gas adsorption and potential improvement on a number of (adsorption based) technological processes. Combination of variable-temperature infrared spectroscopy with theoretical calculations constitutes a fruitful approach towards both of these aims. Such an approach is demonstrated here (mainly) by reviewing recent studies on hydrogen and carbon monoxide adsorption (at a low temperature) on alkali-metal exchanged ferrierite. However, the methodology discussed, which involves the interplay of experimental measurements and theoretical calculations at the periodic DFT level, should be equally valid for many other gas-solid systems. Specific aspects considered are the identification of gas adsorption complexes and thermodynamic studies related to standard adsorption enthalpy and entropy.

A computational study on N2adsorption in Cu-ZSM-5

The present computational study investigates the adsorption of N(2) by Cu-ZSM-5, with particular regard to the interaction with pairs of Cu(+) ions, employing simple cluster models in the calculations. It shows that several interaction patterns between N(2) and couples of Cu(+) sites are possible within the Cu-ZSM-5 structure. In particular, when pairs of Cu(+) ions are located at opposite sides of ten-membered rings, in the region where linear and sinusoidal channels intersect each other, a quasi-linear Cu-N-N-Cu adsorption occurs. Although lattice restraints cause small deviations from linearity, such interaction turned out to be more favourable than other adsorption patterns within the Cu-ZSM-5 structure. The linearity of the Cu-N-N-Cu fragment and the relatively low concentration of the related sites cause a low extinction coefficient for the N-N IR stretching mode, which is usually detected with very low intensity or not detected at all. The results of the present calculations may explain the experimental evidence for a nearly IR-silent fraction of nitrogen strongly adsorbed in the Cu-ZSM-5 catalyst which, as shown in a previous work, is linearly related to the number of active sites for NO decomposition.

Carbon monoxide adsorption on low-silica zeolites—from single to dual and to multiple cation sites

Infrared spectra of CO adsorbed on the Al-rich Na-A zeolite were analysed by using a combined theoretical and experimental approach, showing that such spectra cannot be interpreted by assigning each IR band to CO interacting with a specific type of single cation site. This concept, which usually works well for high-silica zeolites, should not be uncritically extended to Al-rich zeolites that are crowded with cations in configurations which lead to preferential formation of CO adsorption complexes involving more than one cation site.

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)).

A Density Functional Theory Study of Molecular and Dissociative Adsorption of H2 on Active Sites in Mordenite

Adsorption and chemisorption of H2 in mordenite is studied using ab initio density functional theory (DFT) calculations. The geometries of the adsorption complex, the adsorption energies, stretching frequencies, and the capacity to dissociate the adsorbed molecule are compared for different active sites. The active centers include a Brønsted acid site, a three-coordinated surface Al site, and Lewis sites formed by extraframework cations: Na+, Cu+, Ag+, Zn2+, Cu2+, Ga3+, and Al3+. Adsorption properties of cations are compared for a location of the cation in the five-membered ring. This location differs from the location in the six-membered ring observed for hydrated cations. The five-membered ring, however, represents a stable location of the bare cation. In this position any cation exhibits higher reactivity compared with the location in the six-membered ring and is well accessible by molecules adsorbed in the main channel of the zeolite. Calculated adsorption energies range from 4 to 87 kJ/mol, depending on electronegativity and ionic radius of the cation and the stability of the cation-zeolite complex. The largest adsorption energy is observed for Cu+ and the lowest for Al3+ integrated into the interstitial site of the zeolite framework. A linear dependence is observed between the stretching frequency and the bond length of the adsorbed H2 molecule. The capacity of the metal-exchanged zeolite to dissociate the H2 molecule does not correlate with the adsorption energy. Dissociation is not possible on single Cu+ cation. The best performance is observed for the Ga3+, Zn2+, and Al3+ extraframework cations, in good agreement with experimental data.

Theoretical Investigation of CO Interaction with Copper Sites in Zeolites: Periodic DFT and Hybrid Quantum Mechanical/Interatomic Potential Function Study

Periodic DFT and combined quantum mechanics/interatomic potential function (QM-pot) models were used to describe the interaction of CO with the Cu+ sites in FER. The CO stretching frequencies were calculated using omega(CO)(CCSD(T))/r(CO)(DFT) scaling method relating frequencies determined using a high-level quantum-chemical (coupled clusters) method for simple model carbonyls to CO bond lengths calculated using periodic DFT and QM-pot methods for the Cu+-zeolite system. Both periodic DFT and QM-pot models together with omega(CO)/r(CO) scaling describe the CO stretching dynamics with the "near spectroscopic accuracy", giving nu(CO) = 2156 cm(-1) in excellent agreement with experimental data. Calculations for various Cu+ sites in FER show that both types of Cu+ sites in FER (channel-wall sites and intersection sites) have the same CO stretching frequencies. Thus, the CO stretching frequencies are not site-specific in the CO/Cu+/FER system. The convergence of the results with respect to the model size was analyzed. When the same exchange-correlation functional is used the adsorption energies from periodic DFT and QM-pot are in good agreement (about 2 kcal/mol difference) but substantially larger than those of the experiment. The adsorption energy calculated with the B3LYP functional agrees with available experimental data. The overestimation of the adsorption energy in DFT calculations (periodic or QM-pot) is related to a red-shift of the CO stretching mode, both result from an underestimation of the HOMO(5sigma)-LUMO(2pi) gap of CO and the consequent overestimation of the Cu(+)(d)-CO(2pi*) back-donation. For the adsorption energy, this can be overcome by the use of hybrid B3LYP exchange-correlation functional. For the frequency calculations, the DFT problem can be overcome by the use of the omega(CO)(CCSD(T))/r(CO)(DFT) correlation.