Lewis Acid Pair Research Articles

Selective Production of C3 Polyols from Cellulose over Hydrogen Spillover Promoted Pd–Mo/TiO2 Catalyst with Adjacent Lewis Acid Pairs

Selective Production of C₃ Polyols from Cellulose over Hydrogen Spillover Promoted Pd–Mo/TiO₂ Catalyst with Adjacent Lewis Acid Pairs

Selective Catalytic Reduction of NOx by Methanol on Metal-Free Zeolite with Brønsted and Lewis Acid Pair

Selective Catalytic Reduction of NO<i>_x</i> by Methanol on Metal-Free Zeolite with Brønsted and Lewis Acid Pair

Efficient ambient ammonia synthesis by Lewis acid pair over cobalt single atom catalyst with suppressed proton reduction

A Lewis acid pair over a single Co atom anchored on sponge-like nitrogen-doped mesoporous hollow carbon nanofibers was designed. The catalyst exhibits an NH3 production rate of 67.6 μg h−1 mg−1 and a maximum Faraday efficiency of 56.9% at a peak potential of −0.1 V vs. RHE.

Metal-free electrocatalyst for reducing nitrogen to ammonia using a Lewis acid pair

Metal-free Lewis acid catalyst is developed as a high-efficiency electrocatalyst to enable nitrogen reduction under ambient conditions. A new nitrogen activation mechanism via the pull–pull effect between Lewis acid sites and nitrogen is also proposed.

Ultrastabilization of Zeolite Y Transforms Brønsted-Brønsted Acid Pairs into Brønsted-Lewis Acid Pairs.

Two pairs of Brønsted acid sites have been identified in H,Na-Y zeolite, located in the supercage and in the sodalite cage, which upon ultrastabilization (dealumination) are transformed into pairs of Brønsted and Lewis acid sites. This mild postsynthetic modification step is an important process for converting this material into an active catalyst for large-scale commercial reactions. Pairing structures and their transformations have been investigated using 1 H double-quantum NMR spectroscopy experiments for dehydrated zeolite, H,Na-Y, and its ultrastabilized form, H,Na-USY. This approach enables the detection of pairs of Brønsted and Lewis acid sites with unprecedented 1 H resolution and distinguishing them from isolated acid sites. The dealumination is also detected by static 27 Al solid-state NMR experiments.

Structures and thermodynamic stability of cobalt molybdenum oxide (CoMoO4-II)

This contribution reports density functional theory (DFT) calculations on structural and electronic properties of bulk and surfaces of cobalt molybdenum oxide CoMoO4-II; i.e., a material that enjoys a wide array of chemical catalytic and optical applications. Estimated lattice constants and atomic charges for bulk CoMoO4-II reproduce limited analogous experimental measurements. Bader's charges confirm the ionic nature for metal-O bonds in bulk and surfaces of CoMoO4-II. Plotted partial density of states reveal a narrow band gap of 1.8 eV for bulk CoMoO4-II. We found that cleaving bulk of CoMoO4-II along the low-Miller indices afford twelve distinct surfaces. Upward displacement of oxygen atom becomes evident when contrasting bulk positioning of atoms with relaxed surfaces. The two mixed Mo/O- and Co/O–terminated surfaces dominate the thermodynamic stability diagram at 1 atm and 300 – 1400 K, and across a wide range of oxygen chemical potential. The presence of surface oxygen atoms in these stable surfaces is expected to facilitate the occurrence of oxygen reduction reactions as experimentally demonstrated. Likewise, the adjacent surface cations (Mo4+/Co2+) and anions (O2−) serve as Lewis-acid pairs; i.e., very potent active sites in prominent catalysis reactions.

Surface Analogues of Molecular Frustrated Lewis Pairs in Heterogeneous CO2 Hydrogenation Catalysis

The discovery of homogeneous, solution-based molecular frustrated Lewis pairs, denoted FLPs, comprising main-group elements that can activate H2 heralded a paradigm shift in chemistry and catalysis. In FLPs, unquenched Lewis base and Lewis acid sites (B···A) are able to polarize and dissociate H2 heterolytically to form adjacent proton and hydride sites (BH–···AH+), which can enable reactions such as CO2 reduction. In this paper, we draw attention to a relationship between these well-known molecular FLPs and the surface active sites comprised of proximal Lewis base and Lewis acid pairs, which have been reported multiple times in the literature to be responsible for driving various heterogeneous catalytic reactions. From our recent studies that described one such surface site in a nanostructured defect laden indium oxide, capable of activating H2 and enabling the hydrogenation of CO2, it was conjectured that these sites are surface FLPs. Significantly, the conversion rate for this hydrogenation reaction is...

Cooperative reaction chemistry derived from a borata-diene framework.

The bifunctional frustrated BH(-)/B hydridoborate nucleophile/borane Lewis acid pair 6 is prepared starting from the borata-diene 3 by a sequence of sequential protonation/hydride attachment, followed by hydroboration with Piers' borane [HB(C6F5)2]. The trans-1,2-bifunctional system 6 reduced carbon monoxide eventually to the aldehyde product 10.

The role of acid sites induced by defects in the etherification of HMF on Silicalite-1 catalysts

Defects in Silicalite-1 (structure type MFI) generate acid sites in the form of (i) hydroxyl nests present inside the channels or (ii) Brønsted–Lewis acid pairs, associated with terrace step kink sites present on the external surface of zeolite crystals. The amount of the sites has been changed by controlling the thermal treatment and pH value during preparation, or by post-synthesis treatments (ionic-exchange, silylation). These sites have been quantified by NMR, XRD and FT-IR pyridine adsorption/desorption at room temperature and 150°C, and correlated with the catalytic etherification of 5-hydroxymethyl-2-furfural (HMF) with ethanol, a relevant reaction to produce diesel additives. The results show that the surface acidity of Silicalite-1 materials presents an heterogeneous distribution of Lewis and Brønsted acid sites with different degrees of strength. The amount and distribution of such active centers determine the catalytic performance in HMF etherification. It is also shown that adsorption of water, produced in situ by ethanol condensation, has a promoter effect at lower concentration and high temperature, due to the dissociation of S2R siloxanes bonds, and formation of Lewis and Brønsted acid centers with medium-high strength, responsible for the activation pathways of HMF conversion to 5-(ethoxymethyl)furfan-2-carbaldehyde (EMF) and ethyl-4-oxopentanoate (EOP), respectively.

Role of Directed van der Waals Bonded Interactions in the Determination of the Structures of Molecular Arsenate Solids

Bond paths, local energy density properties, and Laplacian, L(r) = -wedge(2)rho(r), composite isosurfaces of the electron density distributions were calculated for the intramolecular and intermolecular bonded interactions for molecular solids of As(2)O(3) and AsO(2) composition, an As(2)O(5) crystal, a number of arsenate molecules, and the arsenic metalloid, arsenolamprite. The directed intermolecular van der Waals As-O, O-O, and As-As bonded interactions are believed to serve as mainstays between the individual molecules in each of the molecular solids. As-O bond paths between the bonded atoms connect Lewis base charge concentrations and Lewis acid charge depletion domains, whereas the O-O and As-As paths connect Lewis base pair and Lewis acid pair domains, respectively, giving rise to sets of intermolecular directed bond paths. The alignment of the directed bond paths results in the periodic structures adopted by the arsenates. The arrangements of the As atoms in the claudetite polymorphs of As(2)O(3) and the As atoms in arsenolamprite are similar. Like the As(2)O(3) polymorphs, arsenolamprite is a molecular solid connected by relatively weak As-As intermolecular directed van der Waals bond paths between the layers of stronger As-As intramolecular bonded interactions. The bond critical point and local energy density properties of the intermolecular As-As bonded interactions in arsenolamprite are comparable with the As-As interactions in claudetite I. As such, the structure of claudetite I can be viewed as a stuffed derivative of the arsenolamprite structure with O atoms between pairs of As atoms comprising the layers of the structure. The cubic structure adopted by the arsenolite polymorph can be understood in terms of sets of directed acid-base As-O and base-base O-O pair domains and bond paths that radiate from the tetrahedral faces of its constituent molecules, serving as face-to-face key-lock mainstays in forming a periodic tetrahedral array of molecules rather than one based on some variant of close packing. The relatively dense structure and the corrugation of the layers in claudetite I can also be understood in terms of directed van der Waals As-O bonded interactions. Our study not only provides a new basis for understanding the crystal chemistry and the structures of the arsenates, but it also calls for a reappraisal of the concept of van der Waals bonded interactions, how the structures of molecular solids are viewed, and how the molecules in these solids are bonded in a periodic structure.

One-Step MIBK Synthesis: A New Process from 2-Propanol

The one-step MIBK synthesis from 2-propanol was investigated as an alternative process to current conventional technology that produces MIBK from acetone. The reaction was studied at 473 K and atmospheric pressure using bifunctional Cu-base catalysts. Single MIOx, binary MIMIIOx, and Cu-containing CuMI(MIIOx mixed oxides, where MI and MII are metal cations such as Mg2+, Al3+, or Ce3+, were obtained by thermal decomposition of precipitated precursors. The density and strength of surface basic sites were obtained by CO2 chemisorption and by temperature-programmed desorption (TPD) of CO2, whereas the acid site densities were measured by TPD of NH3. The MIBK synthesis reaction network involves consecutively the initial 2-propanol dehydrogenation to acetone, the aldol condensation of acetone to mesityl oxide, and the final hydrogenation of this compound to MIBK. Cu/SiO2 promoted 2-propanol dehydrogenation to acetone but did not form any C6 condensation products. When copper was supported on protonic HY zeolite, 2-propanol was essentially dehydrated to propylene and the formation of acetone was negligible. Bifunctional Cu-base catalysts were active and selective for MIBK synthesis. The highest MIBK formation rates were obtained on Cu-base solids containing a high density of medium-strength Brønsted base-weak Lewis acid pair sites for promoting the acetone aldol condensation step. CuMg10Al7Ox was the best catalyst because it contained in intimate contact highly dispersed Cu0 crystallites with proper Al3+ Lewis sites and Mg+2–O2− basic pairs and efficiently combines the active sites required for consecutive reactions leading to MIBK. Catalysts containing strongly basic O2− sites, such as CuMg10Ox and CuMg10Ce2Ox, presented lower condensation rates because isolated O2− hindered stabilization of anionic intermediates for acetone condensation. The effect that the reacting atmosphere (N2 or H2) has on catalyst activity and selectivity was investigated on CuMg10Al7Ox. It was found that the catalyst activity is enhanced in N2, but for a given 2-propanol conversion the selectivity for C6 aldol condensation products is higher in hydrogen. MIBK yields as high as 25% were achieved in comparison to the 30% typically obtained in current commercial high-pressure processes from acetone.