Electronic Structure of the [Cu ₃(μ-O) ₃] ²⁺ Cluster in Mordenite Zeolite and Its Effects on the Methane to Methanol Oxidation

Identifying Cu-exchanged zeolites able to activate C-H bonds and selectively convert methane to methanol is a challenge in the field of biomimetic heterogeneous catalysis. Recent experiments point to the importance of trinuclear [Cu 3(μ-O) 3] 2+ complexes inside the micropores of mordenite (MOR) zeolite for selective oxo-functionalization of methane. The electronic structures of these species, namely, the oxidation state of Cu ions and the reactive character of the oxygen centers, are not yet fully understood. In this study, we performed a detailed analysis of the electronic structure of the [Cu 3(μ-O) 3] 2+ site using multiconfigurational wave-function-based methods and density functional theory. The calculations reveal that all Cu sites in the cluster are predominantly present in the Cu(II) formal oxidation state with a minor contribution from Cu(III), whereas two out of three oxygen anions possess a radical character. These electronic properties, along with the high accessibility of the out-of-plane oxygen center, make this oxygen the preferred site for the homolytic C-H activation of methane by [Cu 3(μ-O) 3] 2+. These new insights aid in the construction of a theoretical framework for the design of novel catalysts for oxyfunctionalization of natural gas and suggest further spectroscopic examination. </p

Ground-state properties of the narrowest zigzag graphene nanoribbon from quantum Monte Carlo and comparison with density functional theory

International audienceBy means of quantum Monte Carlo (QMC) calculations from first-principles, we study the ground-state properties of the narrowest zigzag graphene nanoribbon with an infinite linear acene structure. We show that this quasi-one-dimensional system is correlated and its ground state is made of localized π electrons whose spins are antiferromagnetically ordered. The antiferromagnetic (AFM) stabilization energy [36(3) meV per carbon atom] and the absolute magnetization [1.13(0.11) μ B per unit cell] predicted by QMC are sizable, and they suggest the survival of antiferromagnetic correlations above room temperature. These values can be reproduced to some extent by density functional theory (DFT) within the DFT+U framework or by using hybrid functionals. Based on our QMC results, we then provide the strength of Hubbard repulsion in DFT+U suitable for this class of systems

Finding the "missing components" during the synthesis of TS-1 zeolite by UV resonance raman spectroscopy

The catalytic performance of TS-1 zeolite greatly depends on the types of titanium species and their concentrations in the zeolite. Coupled with UV/vis spectroscopy, we present a UV resonance Raman spectroscopic investigation on the evolution of the titanium species during the crystallization of TS-1 zeolite. It is found that a small portion of Ti species leaches from the solid phase into the liquid phase. The "missing" Ti species in addition to the framework Ti species during the assembly process of TS-1 is identified as isolated "TiO6" species, due to the resonance Raman effect excited at 266 nm. The formation mechanisms of framework Ti species in TS-1 and the relation with the isolated "TiO6" species during the synthesis process are clarified. A synthetic strategy was designed to increase the concentration of the framework titanium by a factor of 1.5 for the actual synthesis of TS-1 zeolite

Enhancement of the visible light absorption intensity of microporous vanadosilicate AM-6

AM-6 with high intensity visible light absorption at 400–800 nm was synthesizedviaa hydrothermal route in the presence of F-ions. Structure characterization and DFT calculations indicate that the unusual optical properties originate from the connection of octahedral VO6wires with distorted tetrahedral VO4units, which are stabilized in the framework of AM-6

Aromatic Amine-Functionalized Covalent Organic Frameworks (COFs) for CO2/N2 Separation

CO2 is a prominent example for an exhaust gas, and it is known for its high impact on global warming. Therefore, carbon capture from CO2 emissions of industrial processes is increasingly important to halt and prevent the disruptive consequences of global warming. Covalent organic frameworks (COFs) as porous nanomaterials have been shown to selectively adsorb CO2 in high quantities and with high CO2/N2 selectivity. Interactions with amines are recognized to selectively adsorb CO2 and help capture it from exhaust emissions. Herein, a novel COF (Me3TFB-(NH2)2BD), which was not accessible via a direct condensation reaction, was synthetized by dynamic linker exchange starting with Me3TFB-BD. Despite the linker exchange, the porosity of the COF was largely maintained, resulting in a high BET surface area of 1624 ± 89 m2/g. The CO2 and N2 adsorption isotherms at 273 and 295 K were studied to determine the performance in carbon capture at flue gas conditions. Me3TFB-(NH2)2BD adsorbs 1.12 ± 0.26 and 0.72 ± 0.07 mmol/g of CO2 at 1 bar and 273 and 295 K, respectively. The COF shows a high CO2/N2 IAST selectivity under flue gas conditions (273 K:83 ± 11, 295 K: 47 ± 11). The interaction of the aromatic amine groups with CO2 is based on physisorption, which is expected to make the regeneration of the material energy efficient

Synergy between Lewis acid sites and hydroxyl groups for the isomerization of glucose to fructose over Sn-containing zeolites : A theoretical perspective

The mechanism of glucose isomerization to fructose catalyzed by Lewis acidic Sn sites in the framework of MOR, BEA, MFI and MWW zeolites was investigated by periodic DFT calculations. The main focus was on the influence of the nature of the active site and the zeolite topology on the rate-controlling hydride shift step. A general finding is that the Sn-catalyzed isomerization of glucose is strongly promoted by proximate hydroxyl groups. These hydroxyl groups can derive from co-adsorbed water molecules or internal silanols. The cooperative action of such proton donors with the Lewis acidic Sn sites results in more effective compensation of the negative charge developing on the O1 atom of glucose during the rate-controlling hydride shift reaction step. The variation in the shape of the micropores with a zeolite topology affects the mode and strength of carbohydrate adsorption, which is dominated by van der Waals forces. Their influence on the intrinsic reactivity of intrazeolite Sn sites is small. We propose that higher glucose adsorption energy in the narrower micropores of 10-membered ring zeolites (e.g., Sn-MFI and Sn-MWW) adversely affects the intrachannel diffusion compared to that in the zeolites with larger pores. The high catalytic performance of Sn-MWW towards glucose transformation is due to the lower barrier for the hydride shift step resulting from the presence of a relatively strong acidic bridging silanol group next to the Lewis acidic Sn site

Mechanistic investigation of benzene esterification by K₂CO₃/TiO₂: The catalytic role of the multifunctional interface

Potassium carbonate dispersed over a defective TiO2support (K2CO3/TiO2) is an efficient catalyst for benzene esterification with CO2and CH3OH. Density functional theory calculations reveal that this unique catalytic reactivity originates from the cooperation of the Ti3+/K+surface sites. The K2CO3promotor steers the stabilization of surface intermediates thus preventing catalyst deactivation.ChemE/Inorganic Systems EngineeringChemE/Algemee

Hydrogen bonding in homochiral dimers of hydroxyesters studied by Raman optical activity spectroscopy

Spectroscopic analysis of homochiral dimerization is important for the understanding of the homochirality of life and enantioselective catalysis. In this paper, (S)-methyl lactate and related molecules were studied to provide detailed structural information on hydrogen bonding in homochiral dimers of chiral α-hydroxyesters through the experimental and theoretical study of Raman optical activity. Different homochiral dimers can be distinguished by comparing their simulated Raman optical activity spectra with the experimental results. Hydrogen bonding motions are decoded with the aid of vibrational motion analysis, which are apparently involved in vibrational motions below 800 cm-1. A common feature related to the chain-bending mode also indicates the absolute configuration of methyl lactate and related molecules. The differing behavior of electric dipole-electric quadrupole invariants (β(A)2) compared with the electric dipole-magnetic dipole invariant (β(G')2), suggests that the intermolecular hydrogen bonding motion behaves differently from the intramolecular one in the asymmetric molecular electric and magnetic fields. These results may help understand hydrogen-bonded self-recognition and other dynamical features in chiral recognition