A density functional theory study of the mechanism of direct glucose dehydration to 5-hydroxymethylfurfural on anatase titania.

\u3cp\u3eA new mechanism for glucose dehydration to HMF without the intermediate isomerization to fructose is discussed for surface models of anatase TiO\u3csub\u3e2\u3c/sub\u3e using periodic density functional theory calculations. Activation of the glucose at glucose's C3-OH position by titania starts the reaction resulting in adsorbed 3-deoxyglucosone. The mechanistic study reveals two possible pathways for the acyclic form of glucose. Comparison of different surface models shows that the presence of tetrahedral Ti\u3csup\u3e4+\u3c/sup\u3e species on a defective TiO\u3csub\u3e2\u3c/sub\u3e(101) anatase surface is essential for explaining the activity. Synergy between a strong Lewis acidic Ti site and a vicinal basic oxygen site of a TiOH group is essential to establish the direct conversion of glucose to HMF. IR spectroscopy of adsorbed chloroform in the presence of water confirms the presence of water tolerant Lewis acid sites in close proximity to basic sites. In conventional glucose dehydration, Lewis acid sites and proton donors act as catalytic sites for efficient sequential isomerization-dehydration to HMF, while direct dehydration to HMF requires the cooperation of Lewis acid-base pairs.\u3c/p\u3

Unraveling the Nature of Extraframework Catalytic Ensembles in Zeolites : Flexibility and Dynamics of the Copper-Oxo Trimers in Mordenite

Extraframework cations define the chemical versatility of zeolite catalysts. Addressing their structural complexity and dynamic behavior represents one of the main fundamental challenges in the field. Herein, we present a computational approach for the identification and analysis of the accessible pool of intrazeolite extraframework complexes with a Cu/MOR catalyst as an industrially important model system. We employ ab initio molecular dynamics for capturing the ensemble of reactive isomers with the [Cu3O3]2+ stoichiometry confined in the mordenite channels. The high structural diversity of the generated isomers was ensured by concentrating the kinetic energy along the low-curvature directions of the potential energy surface (PES). Geometrically distinct [Cu3O3]2+ complexes were identified via a series of clustering procedures ensuring that one structure of each local minima is retained. The proposed procedure has resulted in a set of previously unknown peroxo-complexes, which are >50 kJ/mol more stable than the recently hypothesized chair-shaped structure. Our analysis demonstrates that the most stable peroxo-containing clusters can be formed under operando conditions from molecular oxygen and the Cu3O unit, similar to that in methane monooxygenase (MMO) enzymes

Unraveling the Nature of Extraframework Catalytic Ensembles in Zeolites: Flexibility and Dynamics of the Copper-Oxo Trimers in Mordenite

Extraframework cations define the chemical versatility of zeolite catalysts. Addressing their structural complexity and dynamic behavior represents one of the main fundamental challenges in the field. Herein, we present a computational approach for the identification and analysis of the accessible pool of intrazeolite extraframework complexes with a Cu/MOR catalyst as an industrially important model system. We employ ab initio molecular dynamics for capturing the ensemble of reactive isomers with the [Cu3O3]2+ stoichiometry confined in the mordenite channels. The high structural diversity of the generated isomers was ensured by concentrating the kinetic energy along the low-curvature directions of the potential energy surface (PES). Geometrically distinct [Cu3O3]2+ complexes were identified via a series of clustering procedures ensuring that one structure of each local minima is retained. The proposed procedure has resulted in a set of previously unknown peroxo-complexes, which are >50 kJ/mol more stable than the recently hypothesized chair-shaped structure. Our analysis demonstrates that the most stable peroxo-containing clusters can be formed under operando conditions from molecular oxygen and the Cu3O unit, similar to that in methane monooxygenase (MMO) enzymes. ChemE/Inorganic Systems EngineeringChemE/Algemee

Nature and catalytic role of extraframework aluminum in faujasite zeolite : a theoretical perspective

A comprehensive periodic DFT study complemented by ab initio thermodynamic analysis was carried out to determine the speciation of extraframework aluminium (EFAl) in faujasite zeolite. The structure and stability of a wide range of mono- bi-, tri- and tetranuclear EFAl complexes stabilized at different locations in faujasite were investigated. The thermodynamic cycles connecting these complexes were constructed involving such elementary steps as hydration/dehydration, proton transfer, and condensation reactions. Using ab initio thermodynamics analysis it was predicted that during high-temperature zeolite activation, the EFAl species self-organize into cationic clusters with more than one Al center. The resulting tri- and tetranuclear clusters are preferentially stabilized inside the small sodalite cages of faujasite that provide a favorable coordination and charge-compensation environment for the large multiply charged cationic clusters. The presence of such cationic EFAl clusters inside the inaccessible sodalite cages strongly enhances the protolytic propane cracking activity of vicinal supercage Brønsted acid sites

Relationship between acidity and catalytic reactivity of Faujasite Zeolite:a periodic DFT study

The fundamental aspects of Brønsted acidity and catalytic reactivity of faujasite-type zeolites were investigated by periodic DFT calculations. The adsorption energies of ammonia and pyridine on the Brønsted acid site (BAS) were used to determine the acidity. It is demonstrated that the acid strength of zeolite materials increases with rising Si/Al ratio (low-silica faujasite), and then levels off at high Si/Al ratio (high-silica faujasite). The presence of multinuclear extra framework Al (EFAl) in the sodalite cages substantially enhances the Brønsted acidity. The catalytic reactivity of faujasite toward protolytic propane cracking correlates well with the characterized acidity by base adsorption. However, for H/D exchange reaction of benzene the presence of EFAl species can induce deviations between the measured acidity and the reactivity of faujasite catalysts, indicating that acidity and reactivity are not always directly correlated

Electronic Structure of the [Cu \u3csub\u3e3\u3c/sub\u3e(μ-O) \u3csub\u3e3\u3c/sub\u3e] \u3csup\u3e2+\u3c/sup\u3e Cluster in Mordenite Zeolite and Its Effects on the Methane to Methanol Oxidation

\u3cp\u3eIdentifying 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 \u3csub\u3e3\u3c/sub\u3e(μ-O) \u3csub\u3e3\u3c/sub\u3e] \u3csup\u3e2+\u3c/sup\u3e 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 \u3csub\u3e3\u3c/sub\u3e(μ-O) \u3csub\u3e3\u3c/sub\u3e] \u3csup\u3e2+\u3c/sup\u3e 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 \u3csub\u3e3\u3c/sub\u3e(μ-O) \u3csub\u3e3\u3c/sub\u3e] \u3csup\u3e2+\u3c/sup\u3e. 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. \u3c/p\u3

Lewis acid catalysis by Zeolites

\u3cp\u3eThis chapter discusses recent developments in theory of Lewis acid zeolite catalysis. Special focus is laid on the role of active site cooperativity and synergistic effects as well as molecular recognition and confinement effects inside zeolite micropores on the reaction mechanisms and catalytic performance. With a selection of recent representative examples from our group, we illustrate the utility of modern computational techniques to reveal fundamental aspects of catalytic phenomena in zeolite micropores, which are not directly accessible to experimental techniques. The role of quantum chemical calculations in revealing the nature of the intrazeolite catalytic active sites is highlighted. Emphasis is placed on the necessity of utilizing realistic zeolite models accounting for the complexity of the intrazeolite reactive site environment.\u3c/p\u3

Synthesis of Sn-BEA with exclusive and high framework Sn content

Sn-Beta zeolite was prepared by acid dealumination of Beta zeolite, followed by dehydration and impregnation with anhydrous SnCl4. The formation of extraframework Sn (EFSn) species was prevented by the removal of unreacted SnCl4 in a methanol washing step prior to calcination. The resulting Sn-Beta zeolites were characterized by X-ray diffraction, Ar physisorption, NMR, UV/Vis, and FTIR spectroscopy. These well-defined Lewis acid zeolites exhibit good catalytic activity and selectivity in the conversion of 1,3-dihydroxyacetone to methyl lactate. Their performance is similar to a reference Sn-Beta zeolite prepared by hydrothermal synthesis. Sn-BEA zeolites that contain EFSn species exhibit lower catalytic activity; the EFSn species also catalyze formation of byproducts. DFT calculations show that partially hydrolyzed framework Sn-OH species (open sites), rather than the tetrahedral framework Sn sites (closed sites), are the most likely candidate active sites for methyl lactate formation

Dehydration of glucose to 5-Hydroxymethylfurfural using Nb-doped Tungstite

Dehydration of glucose to 5-hydroxymethylfurfural (HMF) remains a significant problem in the context of the valorization of lignocellulosic biomass. Hydrolysis of WCl6 and NbCl5 leads to precipitation of Nb-containing tungstite (WO3⋅H2O) at low Nb content and mixtures of tungstite and niobic acid at higher Nb content. Tungstite is a promising catalyst for the dehydration of glucose to HMF. Compared with Nb2O5, fewer by-products are formed because of the low Brønsted acidity of the (mixed) oxides. In water, an optimum yield of HMF was obtained for Nb–W oxides with low Nb content owing to balanced Lewis and Brønsted acidity. In THF/water, the strong Lewis acidity and weak Brønsted acidity caused the reaction to proceed through isomerization to fructose and dehydration of fructose to a partially dehydrated intermediate, which was identified by LC-ESI-MS. The addition of HCl to the reaction mixture resulted in rapid dehydration of this intermediate to HMF. The HMF yield obtained in this way was approximately 56 % for all tungstite catalysts. Density functional theory calculations show that the Lewis acid centers on the tungstite surface can isomerize glucose into fructose. Substitution of W by Nb lowers the overall activation barrier for glucose isomerization by stabilizing the deprotonated glucose adsorbate.\u3cbr/\u3