Zeolitic OH Research Articles

Infrared Spectrum of an Acidic Zeolite OH with Adsorbed Acetonitrile

The effect of acetonitrile adsorption on the infrared spectrum of an acidic hydroxyl group of a zeolite was studied using quantum-chemical calculations. The hydroxyl and its surroundings in the zeolite were modeled by a cluster molecule. Potential energy and dipole surfaces of the model were computed with density functional theory applying the Becke3LYP functional. A potential energy surface has been constructed as a function of the stretch, in-plane bending, and out-of-plane bending coordinates of both the hydrogen and the oxygen atom of the hydroxyl group, as well as the center-of-mass stretch coordinate of acetonitrile. Taking into full account anharmonicities, we computed the vibrational wave functions and infrared absorption intensities using a variational approach. To facilitate their interpretation, the computed spectra were decomposed with respect to the different vibrational coordinates. It was found that the use of center of mass conserving coordinates for the hydroxyl group is insufficient to obtain accurate hydroxyl stretch frequencies, and that oxygen coordinates need to be included in the calculation. The inclusion of oxygen coordinates furthermore improves the computed Fermi resonance splitting. A new explanation for the width of the A,B spectra is proposed.

Non-oxidative reactions of propane on Zn/Na-ZSM5

Propene formation rates during propane conversion at 773 K on Zn/Na-ZSM5 are about ten times higher than on Zn/H-ZSM5 catalysts with similar Zn content. The total rate of propane conversion is also higher on Zn/Na-ZSM5 by a factor of four. Propane reactions lead to high propene selectivities (>50%) as protons are replaced by Na cations in Zn/H-ZSM5 catalysts. The titration of acid sites with Na+ cations decreases the rate of acid-catalyzed chain growth reactions and the selectivity to C6–C9 aromatics. X-ray absorption studies at the Zn-K edge showed that aqueous ion exchange of Na-ZSM5 with Zn cations leads to isolated (ZnOH)+ species located at cation exchange sites. Unlike Zn species in Zn/H-ZSM5 (<1.0 wt.%), high temperature condensation reactions of (ZnOH)+ species with neighboring zeolite OH groups are less likely to occur in Zn/Na-ZSM5 and most Zn species remain as (ZnOH)+. Temperature programmed reduction studies show that Zn species in Zn/Na-ZSM5 reduce at lower temperatures than the (O-–Zn2+–O-) species present in Zn/H-ZSM5. D2 exchange with surface OH groups showed that some protons are formed during ion exchange. Higher deuterium contents in products of C3H8–D2 mixtures on Zn/Na-ZSM5 suggest that (ZnOH)+ species in Zn/Na-ZSM5 catalyze rate-determining hydrogen desorption steps during propane conversion more effectively than (O-–Zn2+–O-) sites present in Zn/H-ZSM5. The presence of (ZnOH)+ species and a lower acid site density in Zn/Na-ZSM5 leads to much higher propane conversion rates than on Zn/H-ZSM5. As the acid site density decreases, propene aromatization rates decrease, which leads to less hydrogen to be disposed by a more efficient hydrogen recombinative desorption species (ZnOH)+.

Methyl tertiary-butyl ether synthesis on zeolite HBeta investigated by in situ MAS NMR spectroscopy under continuous-flow conditions

Abstract In situ MAS NMR spectroscopy was applied to investigate the synthesis of methyl tertiary-butyl ether (MTBE) over zeolite HBeta (nSi/nAl=15.8) under continuous-flow conditions. Preliminary catalytic tests with on-line gas chromatographic analysis of the reaction products revealed an activity of zeolite HBeta in the MTBE synthesis comparable with that of Amberlyst-15. By exposing the calcined zeolite, filled into an MAS NMR rotor, to a flow of nitrogen gas loaded with methanol and isobutene, the interaction of these reactant molecules with the zeolite OH groups and the dimerization and oligomerization of isobutene were studied. 13 C MAS NMR spectroscopy performed during the conversion of a methanol/isobutene mixture over zeolite HBeta yielded signals at 77 ppm to 90 ppm due to secondary and tertiary carbon atoms of alkoxy complexes formed at the zeolite framework.

FT-IR study of NO + O2 co-adsorption on H-ZSM-5: re-assignment of the 2133 cm-1 band to NO+ species

Whereas NO adsorption at room temperature on activated H-ZSM-5 (Si/Al = 29) caused only negligible changes in its IR spectrum, addition on O2 to NO led to the appearance of bands at 2133 and 977 cm-1. Concomitantly, the number of acidic zeolite OH groups decreased while H2O hydrogen-bonded to zeolite OH groups developed. Introduction of small amounts of 18O2 did not change the 2133 cm-1 band wavenumber, nor the use of a partly deuteroxylated D–H-ZSM-5 sample. In such a case, HOD formation was detected. The results obtained evidence that the 2133 cm-1 band, generally considered as characterizing NO+2 species, is, in fact, due to NO+ species occupying cationic positions in the zeolite. The 977 cm-1 band is attributed to the Olattice–NO+ vibration. A scheme of the NO+ formation, involving NO2 molecules as NO oxidizing agent, is proposed.

Broensted acidity in zeolites

Theoretical and experimental investigations of intrinsic and enhanced Broensted acidity i) show no evidence for a significant effect on intrinsic acidity arising from neutralisation of some of the acid sites, ii) suggest a suitable scale for protic acidity using CO as a probe, iii) provide a fundamental functional linearity for the correlation of shift in hydroxyl frequency and changes in band intensity, iv) allow deconstruction of complex infrared spectra of zeolite OH region, and v) demonstrate the basis for generating Broensted sites of enhanced acidity by interaction of Lewis sites with proximate Broensted sites in zeolites.

Interaction of Water with Brønsted Acidic Sites of Zeolite Catalysts. Ab Initio Study of 1:1 and 2:1 Surface Complexes

The adsorption of one and two water molecules on cluster models of Brønsted acid sites of zeolite catalysts has been investigated by ab initio quantum chemical methods at the Hartree−Fock self-consistent field (HF), at the second-order Møller−Plesset perturbation theory (MP2), and at the density functional theory (DFT) levels. Among the two possible structures of the 1:1 adsorption complex, the water H-bonded to the zeolitic OH group (neutral complex) and the hydroxonium ion attached to the negatively charged zeolite surface (ion pair), only the former is a minimum. The ion pair complex is a transition structure for the proton transfer from one lattice oxygen to a neighboring one via the adsorbed water. However, the energy difference between both structures is only a few kJ/mol. For the neutral 1:1 adsorption complex we predict an average shift of the three protons involved of 7−8 ppm; the observed shifts are 6−7 ppm for one water molecule per site. The vibrational frequencies calculated for the ion pair structure do not permit an interpretation of the observed infrared spectrum. For the neutral structure (MP2) we predict frequencies of 1317 and 1022 cm-1 for the zeolitic in-plane and out-of-plane modes, respectively, while the zeolitic OH stretching mode is strongly red-shifted down to 2740−2850 cm-1. These data support a recent interpretation of the IR spectrum which explains the observed triplet of bands as a result of Fermi resonance between the strongly perturbed zeolitic OH stretch and the OH bending overtones. The MP2 calculations for the neutral complex also provide a complete assignment of the peaks observed by inelastic neutron scattering for water on H-mordenite. Inclusion of electron correlation proves crucial, and comparison of MP2 and DFT (gradient corrected functionals) methods is made. While energy differences are very similar, the DFT approach yields by far too large frequency shifts for OH donor groups in H bonds. When a second water molecule is added (2:1 complex), both the neutral and the ion pair structure prove to be local minima on the potential energy surface. The adsorption energy is found to drop by 25%, and the ion pair structure becomes the more stable one. Predictions are made on how the vibrational spectra and the 1H NMR chemical shifts change.

Interaction of ammonia with a zeolitic proton: ab initio quantum-chemical cluster calculations

The interaction of NH3 and a zeolitic cluster as well as the protonation of NH3 by zeolitic protons are studied by quantum-chemical calculations on small clusters at different levels of approximation. The focus of the paper is on a comparison of results obtained by the different methods. The clusters are studied at the SCF level as well as at the correlated level. Electron correlation is included through second-order Moller-Plesset perturbation theory. The basis-set superposition error (BSSE) was avoided by using the counterpoise scheme. Monodentate singly bonded NH3, that is NH3 being attached to one oxygen atom, forms a strong hydrogen bond with the zeolitic OH group. This bond has a strength of 60 or 67 kJ/mol, depending on the geometry of the zeolitic cluster. This value is approximately half the experimentally found heat of desorption. For this case, the O⋯N distance is found to be very short (2.74 or 2.73 A) and the intermolecular O-H-N stretching frequency is calculated to be 185 or 193 cm-1. The latter values agree reasonably with experimental data. Upon complexation with NH3, the OH stretching frequency shows a red shift of 551 cm-1. Proton transfer from the zeolitic cluster to NH3 is calculated to be unfavorable by 52 kJ/mol, as long as NH4+ is considered to be monodentate coordinated. The description of the hydrogen-bonded form is only slightly dependent on the basis set used. However, the proton-transfer energy does strongly depend on the basis set used. Electron correlation makes the proton transfer more favorable. The BSSE has a large influence on the description of the structures, especially if electron correlation is included. Although electron correlation has a nonnegligible effect on the proton-transfer energy, some conclusions can be drawn from SCF calculations on doubly and triply coordinated NH4+. The computed energy of adsorption now is approximately twice that computed for the hydrogen-bonded and singly coordinated NH3 and close to experimentally observed values of ammonia adsorption. From these results, it follows that these adsorption modes are prefered over the singly bonded form. These forms are preferred because of the favorable electrostatic stabilization of NH4+ when bonded to the cluster by two or three hydrogen bonds.

Infrared study on the mechanism of olefins and alcohols conversions over H-ZSM-5 zeolite

The IR spectra of 1-hexene, 1-heptene, cyclohexene, n-propanol and tert.-butanol, adsorbed on H-ZSM-5 zeolite, evacuated at 675 K, revealed a band at 1520 cm −1 after contacting with the zeolite at 373 J. The band was absent in the case of adsorbed benzene and cyclohexane and of very low intensity by methanol and ethanol adsorption. Considerations are given in favour of very strong interaction either of the type of X-complex or of carbenium ion. The second assumption was confirmed by the subsequent disappearance of the zeolitic OH bands.

Preparation of molybdenum zeolites from molybdenum hexacarbonyl. 1. Infrared studies

Ir spectroscopy has been used to study the adsorption and decomposition of molybdenum hexacarbonly (Mo(CO)/sub 6/) in the hydrogen (H) and sodium (Na) forms of zeolite Y. Two forms of adsorbed Mo(CO)/sub 6/ are found in both H-Y and Na-Y: a weakly (physically) adsorbed complex and a more strongly perturbed species. On being heated in vacuo above room temperature, the adsorbed Mo(CO)/sub 6/ decomposes. In H-Y, three distinct subcarbonyl species are formed reversibly. At temperatures of 200/sup 0/C or higher, decarbonylation is irreversible, and oxidation of the molybdenum occurs, as indicated by loss of zeolite OH groups and appearance of oxomolybdenum species. Carbon monoxide (CO) chemisorbed on oxidized molybdenum sites gives a characteristic ir band at 2170 cm/sup -1/. Ir evidence is presented for interaction of ammonia with intra-zeolitic molybdenum cations. Surface area, X-ray diffraction, and low-frequency infrared measurements indicate that the zeolite crystallinity is retained on heating in vacuo up to 500/sup 0/C, but loss of structure occurs on heating in oxygen at high temperature. In Na-Y, a single subcarbonyl species is formed reversibly upon initial decomposition of adsorbed Mo(CO)/sub 6/. At higher temperatures metallic molybdenum appears to be formed which does not adsorb appreciable quantities of CO. The more » resistance of the MoNa-Y to oxidation by O/sub 2/ depends on the temperature of prior activation. « less

Amphiprotic properties of OH groups in synthetic NaHY zeolites

The ir spectra of basic (pyridine) and acidic molecules (formic, acetic and benzoic acids) adsorbed on dehydrated NaHY zeolites were studied. Adsorbed pyridine molecules were transformed into pyridinium ions by reaction with the zeolitic OH groups characterized by the absorption peak at 3660 cm −1 and also to some extent with those exhibiting the absorption peak at 3565 cm −1. In the presence of acid molecules the same OH groups reacted with the formation of carboxylic anions and water molecules. The zeolitic OH groups behave therefore as Brønsted acid centers in the presence of basic molecules and as Brønsted basic centers in the presence of acid molecules. Similar amphiprotic properties were shown by OH groups in an amorphous silica-alumina cracking catalyst.