Zeolite Oxygen Atoms Research Articles

The role of the spatial arrangement of single rhodium sites on ZSM-5 in the oxidative methane carbonylation to acetic acid

Single atom rhodium catalysts based on microporous ZSM-5 zeolite (SiO2/Al2O3 = 30) were studied in the reaction of oxidative methane carbonylation to acetic acid. For highly dispersed rhodium distribution on the zeolite surface, nitrogen-containing polymers, namely chitosan hydrochloride, polyethyleneimine, and polyvinylpyrrolidone, were used. Using XAS spectroscopy, all rhodium was found to be as single atom metal sites in the zeolite structure, which contributes to the acetic acid formation. The catalytic properties of rhodium zeolite catalysts in the oxidative methane carbonylation to acetic acid were revealed to depend not only on the single atom rhodium distribution and the catalyst acidity, but also on the spatial rhodium atom arrangement in the zeolite, which was established by the XPS. According to DFT calculations in combination with EXAFS modeling, rhodium anchored at the zeolite channel intersections was shown to be coordinated with four zeolite oxygen atoms and one hydroxyl group oxygen atom and be the dominant catalytic species in the acetic acid formation.

Combining Kinetics and Operando Spectroscopy to Interrogate the Mechanism and Active Site Requirements of NOx Selective Catalytic Reduction with NH3 on Cu-Zeolites

NOx selective catalytic reduction (SCR) with NH3 on Cu-zeolites is a commercial emissions control technology for diesel and lean-burn engines. Mitigating low-temperature emissions remains an outstanding challenge, motivating an improved understanding of the reaction mechanism, active site requirements, and rate-determining processes at low temperatures (<523 K). In this Perspective, we discuss how operando spectroscopy provides crucial information about how the structures, coordination environments, and oxidation states of Cu active sites depend on reaction conditions and sample composition; when combined with kinetic measurements, such operando data provide insights into the Cu site and spatial density requirements for reduction and oxidation steps relevant to the Cu(II)/Cu(I) SCR redox cycle. Isolated Cu ions coordinated to zeolite oxygen atoms ex situ become coordinated to NH3 in situ and dynamically interconvert between mononuclear and binuclear NH3-solvated Cu complexes to catalyze SCR turnovers. We conclude with future research directions that can benefit from combining quantitative kinetic measurements with operando spectroscopy.

ChemInform Abstract: Quantum-Chemical Modeling of the Hydrocarbon Transformations in Acid Zeolite Catalysts

Results of quantum-chemical modeling of a number of elementary steps involved in the acid zeolite-catalyzed conversion of hydrocarbons are collected together and compared. The elementary steps considered are protolytic cracking, protolytic dehydrogenation, hydride transfer, skeletal isomerization, and β-scission. The hydrocarbon parts of transition states (TS) for these steps represent carbocations specific for each reaction. Geometry parameters of the TS and activation energies depend on the relative stability of these carbocations. The reactions considered can proceed via several alternative routes dependent on the species involved and on the details of the interaction of the hydrocarbon portion of the activated complex with the zeolite oxygen atoms. Variation of the acid strength of zeolite cluster models can be employed for studies of the acid strength sensitivity of the activation energies and other quantities of interest as well as for extrapolation of these quantities computed on small clusters towards zeolitic values.

Water nanodroplets confined in zeolite pores

We provide a comprehensive depiction of the behaviour of a nanodroplet of approximately equal to 20 water molecules confined in the pores of a series of 3D-connected isostructural zeolites with varying acidity, by means of molecular simulations. Both grand canonical Monte Carlo simulations using classical interatomic forcefields and first-principles Car-Parrinello molecular dynamics were used in order to characterise the behaviour of confined water by computing a range of properties, from thermodynamic quantities to electronic properties such as dipole moment, including structural and dynamical information. From the thermodynamic point of view, we have identified the all-silica zeolite as hydrophobic, and the cationic zeolites as hydrophilic; the condensation transition in the first case was demonstrated to be of first order. Furthermore, in-depth analysis of the dynamical and electronic properties of water showed that water in the hydrophobic zeolite behaves as a nanodroplet trying to close its hydrogen-bond network onto itself, with a few short-lived dangling OH groups, while water in hydrophilic zeolites "opens up" to form weak hydrogen bonds with the zeolite oxygen atoms. Finally, the dipole moment of confined water is studied and the contributions of water self-polarisation and the zeolite electric field are discussed.

Cover Picture: Molecular Heterogeneous Catalysis: A Single-Site Zeolite-Supported Rhodium Complex for Acetylene Cyclotrimerization (Chem. Eur. J. 26/2007)

Essentially molecular surface catalysis was demonstrated by D. A. Dixon, B. C. Gates, J. F. Haw et al. and is reported in their Full Paper on page 7294 ff. [Rh(C2H4)2(acac)] reacted with a dealuminated faujasite zeolite to give anchored rhodium complexes that retained their ethylene ligands, with each Rh atom bonded to two zeolite oxygen atoms, as shown by EXAFS, IR, and 13C NMR spectroscopy. Temperature-dependent NMR spectra demonstrated the structural uniformity of the complexes; acetylene reacted with the complexes, replacing ethylene and initiating catalytic acetylene cyclotrimerization.

Influence of support ionicity on the hydrogen chemisorption of Pt particles dispersed in Y zeolite: consequences for Pt particle size determination using the H/M method

Zeolite Y-supported Pt particles were studied by high-resolution transmission electron microscopy (HRTEM), extended X-ray absorption fine structure (EXAFS), and H 2 chemisorption experiments. The ionicity (i.e., electron richness of the zeolite oxygen atoms) was altered by using different cations (NaY, MgY, and LaY) and by steaming and introduction of protons (H-USY). Highly dispersed Pt particles were introduced inside the zeolite pores using a very careful synthesis procedure. Computer analysis of HRTEM images exhibited similar Pt particle size distributions with calculated average Pt particle sizes ranging from 0.98 to 1.26 nm for the various zeolite Y-supported Pt particles. The EXAFS results revealed that the first-shell Pt–Pt coordination number was around 6.5, in good agreement with the observed HRTEM results. H 2 chemisorption experiments found a much higher H/Pt value for Pt/NaY (1.63) than for Pt/H-USY(0.88), implying no relationship with the Pt dispersion as determined by HRTEM. Using the values for the dispersion (Pt s/Pt) as calculated from the HRTEM results, one can obtain a value for the number of chemisorbed hydrogen atoms per surface platinum atom (H/Pt s ratio). This value demonstrates a clearly decreasing trend with decreasing electron richness of the support oxygen atom: 1.96 (Pt/NaY) > 1.74 (Pt/MgY) > 1.51 (Pt/LaY) > 1.11 (Pt/H-USY). These results indicate that the hydrogen coverage on supported Pt particles depends strongly on the support ionicity. This has significant consequences for the catalytic behavior of hydrogenolysis and hydrogenation reactions catalyzed by Pt. These results also indicate that one should be cautious in interpreting H 2 chemisorption results only in terms of Pt dispersion. This approach is valid only when comparing Pt particles dispersed on the same support. Serious deviations in the real Pt dispersions occur if Pt is supported on oxides with different ionicities.

An ONIOM study of amines adsorption in H-[Ga]MOR

The two-layered ONIOM method (B3LYP/6-31G(d,p):HF/3-21G) is used to study the interaction of amines (NH 3, MeNH 2, Me 2NH and Me 3N) with H-[Ga]MOR. The optimization of the local structure of H-[Ga]MOR cluster leads to two stable bridging hydroxyl sites (O 10H and O 2H) in the zeolite framework, being different from that of H-[Al]MOR. In the adsorption complexes, all amines are protonated by the acidic proton of H-[Ga]MOR, and the protonated amines (HNR 3 +) are stabilized by hydrogen bonds between the negatively charged zeolite oxygen atoms and the hydrogen atoms of the N H and C H bonds in the adsorbates. This interaction is confirmed by the structure of the adsorption complexes as well as the calculated IR stretching frequencies. The calculated adsorption energies of amines agree reasonably with the available experimental data. It is found that NH 3 prefers to adsorb at the O 2H Brønsted site, while Me 2NH and Me 3N prefer to adsorb at the O 10H site, and MeNH 2 can be in equilibrium between O 2H and O 10H. The relative order of the basicity of amines on the basis of the computed adsorption energies agrees well with the experiments, but differs from those in the gas phase (proton affinity) and in solvents (p K a).

ZnO Clusters Encapsulated inside Micropores of Zeolites Studied by UV Raman and Laser-Induced Luminescence Spectroscopies

Using microporous zeolites as host, sub-nanometric ZnO clusters were prepared in the micropores of the host by the incipient wetness impregnation method. A small amount of sub-nanometric ZnO clusters were introduced into the channels of HZSM-5 zeolite, whereas a large quantity of sub-nanometric ZnO clusters can be accommodated in the supercages of HY zeolite and no macrocrystalline ZnO exists on the extra surface of the HY material. The vibrations of the zeolite framework and ZnO were characterized by UV Raman spectroscopy. The optical properties of these ZnO clusters were studied by UV-visible absorption spectroscopy and laser-induced luminescence spectroscopy. It is found that there are strong host-guest interactions between the framework oxygen atoms of zeolite and ZnO clusters influencing the motions of the framework oxygen atoms. The interaction may be the reason why ZnO clusters are stabilized in the pores of zeolites. Different from bulk ZnO materials, these sub-nanometric ZnO clusters exhibit their absorption onset below 265 nm and show a purple luminescence band (centered at 410-445 nm) that possesses high quantum efficiency and quantum size effect. This purple luminescence band most likely originates from the coordinatively unsaturated Zn sites in sub-nanometric ZnO clusters. On the other hand, the differences in the pore structure between HZSM-5 and HY zeolites cause the absorption edge and the purple luminescence band of ZnO clusters in ZnO/HZSM-5 show a red shift in comparison with those of ZnO clusters in ZnO/HY.

Observation of Single-Crystal-Type EPR Spectra from Monolayers of Copper-Exchanged Zeolite Na-A Crystals Assembled on Glass Plates

Single-crystal-type EPR spectra were observed for microcrystals of copper(II)-exchanged zeolite Na-A assembled on glass plates when the magnetic field was perpendicular to the glass surface. Two copper species (H and D) were identified both in the powder sample and in the assembled crystals. The g z direction of species H is parallel to [001] of the cubic crystal, while that of species D is tilted 9° away from [001]. Previously, other authors assumed the g z directions of the two corresponding species to be parallel to [111]. On the basis of the g z directions, we suggest that Cu 2 + in species H is displaced 0.149 nm along [111] from the six-ring plane into the β cage and octahedrally coordinated to three zeolite oxygen atoms in the six-ring plane and three water molecules. The g z direction of Cu 2 + in species D is not compatible with site II in the six-ring plane. Species D is probably a trigonal complex with Cu 2 + displaced into the β cage and coordinated to three zeolite oxygen atoms.

The interaction of cis-2-butene over a 10-ring Brönsted acid site of zeolite: a theoretical study

We have investigated theoretically the interaction of cis-2-butene with a Brönsted acid site (BAS) embedded within a ring of 10 tetrahedral zeolite cluster (T10-OH). Calculations were performed at ab Initio SCF-MO level with the STO-3G basis set. Cs symmetry restriction was imposed for the optimization geometry of the T10-OH cluster, and no symmetry restriction was assumed for the cis-2-butene–HO-T10 interaction. This interaction first leads to the formation of a molecular complex, where the CC is weakly coordinated to the proton of the BAS. Then, a hexa-cyclic transition state involving the coordination of one C atom of the CC to one oxygen atom next neighbor of the OH group and the other C atom to the acid proton. Finally, an intermediate secondary alkoxy complex is formed. The alkoxide structure is a very stable complex, dominated by a covalent C–O bonding between the C atom and zeolite oxygen atoms.

Quantum-chemical modeling of the hydrocarbon transformations in acid zeolite catalysts

Results of quantum-chemical modeling of a number of elementary steps involved in the acid zeolite-catalyzed conversion of hydrocarbons are collected together and compared. The elementary steps considered are protolytic cracking, protolytic dehydrogenation, hydride transfer, skeletal isomerization, and β-scission. The hydrocarbon parts of transition states (TS) for these steps represent carbocations specific for each reaction. Geometry parameters of the TS and activation energies depend on the relative stability of these carbocations. The reactions considered can proceed via several alternative routes dependent on the species involved and on the details of the interaction of the hydrocarbon portion of the activated complex with the zeolite oxygen atoms. Variation of the acid strength of zeolite cluster models can be employed for studies of the acid strength sensitivity of the activation energies and other quantities of interest as well as for extrapolation of these quantities computed on small clusters towards zeolitic values.

Spectral and energetic aspects of water adsorption by Li-, Na-, K- and Cs-X zeolites

The infra-red spectra of water adsorbed by X-type zeolites containing different alkali-metal cations have been studied. With small amounts of adsorption in LiNaX, NaX and KNaX zeolites water molecules interact mainly with the zeolite cations. The spectra show that one hydroxyl group of an adsorbed water molecule is bound to the oxygen of the zeolite skeleton and the other is “free”. In the case of CsX zeolite, where adsorption is weaker, both hydroxyl groups of the adsorbed water molecules interact, presumably, with the oxygen atoms of the zeolite. The relative amounts of water molecules interacting with the exchange cations, zeolite oxygen atoms and water molecules forming mutual hydrogen bonds depend on the cation and the number of water molecules in the zeolite cages. As these are filled there is a change in the energy of hydrogen bond formation between water molecules and the zeolite oxygen skeleton and between one another. This change becomes particularly apparent in the case of KX zeolite and is in accordance with the wave-like change of the differential heat of water adsorbtion by KX zeolite and with the maxima and minima of the heat capacity of adsorbed water.