Strong Lewis Acid Research Articles

Aerobic oxidative lactonization of diols at room temperature over defective titanium-based oxides in water

Lactones are applied in pharmaceutical or agriculture/forestry industry as anesthetics/sedatives, pesticide intermediates or as plant growth regulators. Selective conversion of diols into value-added lactones under mild reaction conditions is still challenging. Here, we report on an environmentally benign catalytic approach using oxygen vacancies-enriched titanium-based oxides and water as solvent at room temperature for the lactonization of various aromatic and aliphatic diols. Lactonization was found to proceed in two consecutive steps: 1) the dehydrogenation of diols to hydroxyaldehyde intermediates catalyzed by the Pt cocatalyst; 2) the subsequent oxidative lactonization of the intermediates to the lactone products over defective titanium-based oxides. Diffuse reflectance infrared Fourier transform spectroscopy analysis using pyridine as probe molecule combined with electron paramagnetic resonance results suggest that oxygen vacancies as strong Lewis acid sites are pivotal for the superior catalytic activities at room temperature, which are attributed to the enhanced reaction rate of step 2 due to stronger adsorption of the hydroxyaldehyde intermediates on the Lewis acid sites.

Copper Foam-Supported CuxO@Fe2O3 Core–Shell Nanotubes: An Efficient Ozonation Catalyst for Degradation of Organic Pollutants

Copper foam (CF)-supported CuxO@Fe2O3 core–shell nanotubes (CF/CuxO@Fe2O3 NTs) were synthesized as a hybrid catalyst for heterogeneous catalytic ozonation (HCO). The hybrid catalyst exhibits high activity and excellent stability for the HCO of organic pollutants. The total organic carbon removal rates of methyl orange, ibuprofen, and nitrobenzene via HCO were 89, 82, and 88%, respectively, which are over 2 times higher than the corresponding values obtained without the catalyst. The hybrid catalyst also displays wide pH operation range, low metal leaching, easy separation from treated water, and excellent recyclability. We prove that large amounts of Lewis acid sites with medium acidity are created at the CuxO/Fe2O3 interface due to the synergic effect of CuxO and Fe2O3. These medium acid sites are the main active centers for the production of free hydroxyl radicals (•OH) and superoxide radicals (O2•– or HO2•) in neutral solution. Moreover, the 3D porous framework of the catalyst enables easy dispersion in and separation from water. This work develops a new strategy to design HCO catalysts by combining strong and weak Lewis acid oxides and also opens a new avenue for developing HCO catalysts on a 3D porous framework.

Stability and Acidity of Sites at the External Surface and at Point Defects of Faujasite

AbstractFaujasite is one of the most industrially employed zeolite for its catalytic properties. The strong Brønsted bulk bridging Al−OH−Si sites are often thought to be the main origin of the Brønsted acidity of faujasite. However, many reactions also take place at the surface of the material and of its mesopores, where other types of acid sites exist. This study aims at unraveling the nature and strength of faujasite bulk and surface acid sites. Using Density Functional Theory (DFT), we rank the faujasite acid sites based on their stability, investigate their dehydration properties, and study their Brønsted and Lewis acidity via the adsorption of pyridine. Calculations are performed on cells with high (47 for bulk cells and about 300 for surface slabs) and low Si/Al ratio (about 3) and on cells containing defects under the form of silanol nests. These environments generate Brønsted acid sites of various strengths, and strong Lewis acid sites, even in the absence of extra‐framework species.

SiO2- and Al2O3-Supported Sulfated Zirconiа Catalysts for Hexane Isomerization. Effect of the Support Nature

The effect of the support nature on the performance of hexane isomerization reaction was studied for the sulfated zirconiа catalysts supported on SiO2 and Al2O3, which have different textural characteristics. It was shown that a higher conversion of hexane is reached over sulfated zirconiа catalysts supported on aluminum oxides. IR spectroscopy of adsorbed CO revealed that the concentration of Broensted acid sites (BAS), which are characterized by adsorbed CO with the absorption band (a.b.) at 2170 cm–1, and strong Lewis acid sites (LAS), which are characterized by adsorbed CO with a.b. at 2210 and 2224 cm–1, is higher in the Al2O3-supported catalysts compared to the SiO2-supported ones. In the Al2O3-supported catalysts with different textural characteristics, an increase in the contribution of LAS to the total acidity essentially increases the yield of high-octane 2,2-dimethylbutane and the depth of hexane isomerization.

Aldol condensation of furfural and methyl isobutyl ketone over Zr-MOF-808/silica hybrid catalysts

Mesoporous silica-supported Zr-MOF-808 catalysts have been synthesised and tested in the aldol condensation of (biomass-derived) furfural and methyl isobutyl ketone to bio-jet fuel precursors. Growth of Zr-MOF-808 nanocrystals over silica scaffolds results in well-dispersed Zr species which confer strong Lewis acidity as determined by FTIR of chemisorbed pyridine. Hybrid Zr-MOF-808/silica materials exhibit higher condensation activity than the unsupported crystalline Zr-MOF-808 (which is also prone to rapid deactivation). Textural properties of the silica support strongly influence the catalytic performance, with a high surface area and sufficiently large mesopore desirable. In the screening, the optimum Zr-MOF-808/MCM-41 catalyst delivered 68 % furfural conversion and 90 % selectivity to the C11 aldol adduct, 1-(furan-2-yl)-5-methylhex-1-en-3-one, at 130 °C and a furfural:Zr mass ratio of 150:1. Although more stable than the pure Zr-MOF-808, the Zr-MOF-808/MCM-41 also suffered significant deactivation over successive condensation reactions due to strongly adsorbed organic residues, however this was largely ameliorated by decreasing the furfural:Zr ratio to 75:1, which also led to an outstanding catalytic performance (100 % furfural conversion and adduct selectivity), likely because of the suppression of furfural polymerization.

Design of Nickel Supported Hierarchical ZSM-5/USY Zeolite Bifunctional Catalysts for One-Pot Menthol Synthesis via Liquid-Phase Citral Hydrogenation.

Nickel-supported hierarchical zeolite catalysts were prepared through a desilication reassembly process under optimized conditions and applied in one-pot menthol synthesis. In this work, the hierarchical zeolite-supported metal bifunctional catalysts were prepared with the help of desilication re-assembly and wetness impregnation techniques and applied in menthol synthesis via citral hydrogenation. The prepared catalysts were characterized using PXRD, BET, FE-TEM, NH3-TPD, H2-TPR, pyridine adsorption, and ICP-OES techniques. As a result, the physicochemical and acidic properties, such as mesopore surface area, metal dispersion, acidity, catalytic activity, and strong Lewis acid sites of pure microporous ZSM-5/USY zeolites, were significantly improved. Consequently, with the occurrence of superior physicochemical and acidic properties, the Ni/HZ-0.5 M catalyst exhibited outstanding catalytic activity (100% conversion, TOF 7.12 h-1) and menthol selectivity (83%, 4 h) with uniform stability at 100 °C, 1.0 MPa hydrogen. Similarly, the cracking rate decreased with the decrease in Bronsted acid sites.

Light-activated carbon dot nanozyme with scandium for a highly efficient and pH-universal bio-nanozyme cascade colorimetric assay.

Nanozyme-based colorimetric assays have attracted much attention due to their cost-effectiveness, high stability, and sensitivity. In particular, the catalytic cascade imparted by the biological enzyme is highly selective. However, developing an efficient, one-pot, and pH-universal bio-nanozyme cascade remains challenging. Considering the tunable activity of the photo-activated nanozyme, we herein demonstrated a pH-universal colorimetric assay based on the Sc3+-boosted photocatalytic oxidation of carbon dots (C-dots). As a strong Lewis acid, Sc3+ shows ultra-fast complexation with OH- over a broad pH range and dramatically decreases the pH of the buffer solutions. In addition to regulating the pH, Sc3+ also binds to the C-dots to produce a persistent and strongly oxidizing intermediate based on photo-induced electron transfer. The proposed Sc3+-boosted photocatalytic system was successfully used in a cascade colorimetric assay with biological enzymes for assessing their activity as well as the detection of enzyme inhibitors at neutral and alkaline pH. Instead of designing new nanozymes for catalytic cascades, this work suggests that introducing promoters can be a convenient strategy in practical applications.

Water harvesting properties of a zwitterionic metal–organic framework

A nanoporous zwitterionic metal–organic framework is accessed for its atmospheric water harvesting properties due to its strong Lewis acidity and water stability. Findings show high cycling output and unique hydrochromic behavior.

Structural verification and new reactivity for Stang's reagent, [PhI(CN)][OTf

The structure of Stang's reagent [PhI(CN)][OTf] is confirmed by X-ray crystallography and is determined to be best described as an ion-pair in organic solution. It is found to be a strong Lewis acid, but reaction with pyridine ligands gives [Pyr-CN][OTf] salts via oxidation of pyridine giving a new derivative of the CDAP reagent widely used as an activation agent for polysaccharides.

Simple and Efficient Method for One Pot Multicomponent Synthesis of 3,4-Dihydropyrimidin-2-(1H)-One Derivatives Catalyzed by Organocatalyst: Benzoic Acid

The Biginelli reaction is one the useful multicomponent reactions and a very appropriate reaction for the synthesis of 3,4-dihydropyrimidin-2(1H)-ones derivatives. These 3,4-dihydropyrimidin-2(1H)-ones have biological and pharmacological properties which make them a very important class of medicinal chemistry. Although they are an important class of medicinal chemistry, the syntheses of these compounds have been catalyzed by large number of strong Bronsted acids and Lewis acids under thermal conditions. Small organic molecules, organocatalysts, have been used as catalysts for the Biginelli reaction in a small number as compared to Bronsted acids and Lewis acid. Benzoic acid, which is a small organic molecule, although an acid, has never been tested for the synthesis of 3,4-dihydropyrimidin-2(1H)-ones. Benzoic acid is an inexpensive, non-toxic molecule, it has been successfully tested here as a catalyst for the one-pot three component synthesis of 3,4-dihydropyrimidin-2-(1H)-one derivatives via Biginelli reaction between β-keto ester, a variety of aromatic aldehydes and urea or thiourea under thermal conditions using 20 mol% benzoic acid in acetonitrile solvent refluxed for 12 h to give good to high yields. This synthetic method includes inexpensive, non-toxic, easily available benzoic acid as a catalyst and is carried out in a simple operational procedure.

Imine-stabilized silylium ions: synthesis, structure and application in catalysis.

Novel norbornene-based imine-stabilized silylium ions 2 have been synthesized via the simple reaction of sulfide-stabilized silylium ion 1 with carbonyl derivatives. Those silylium ions were fully characterized in solution and in the solid state by NMR spectroscopy and X-ray diffraction analysis as well as DFT calculations. Unlike the previously reported phosphine-stabilized silylium ion VI, behaving as a Lewis pair, calculations show that 2 have a strong Lewis acid character. Indeed, imine-stabilized silylium ions 2 are able to activate Si-H bonds and catalyzed the hydrosilylation of carbonyl derivatives under mild conditions.

Phosphorus-Involved Wagner-Meerwein Rearrangement of Phosphiranes: An Entry to Four-Membered Phosphacycles.

Phosphenium ions [R2P]+ are important and highly reactive dicoordinate phosphorus species. Herein, we report a rearrangement of the carbocation into the phosphenium cation driven by ring strain. This phosphorus-involved Wagner-Meerwein rearrangement pathway converted the 1-acylphosphirane complex into phosphetane and 1,2-dihydrophosphete derivatives depending on the reaction temperature. The generation of the intermediate phosphenium cation was identified by the intramolecular reaction with ether, which also disclosed its strong Lewis acidity. This work expands the boundary of the phosphorus-carbon analogy.

IR-spectroscopic study of complex formation of nitrogen oxides (NO, N2O) with strong Lewis acid sites and the reactivity of adsorbed species in CO and CH4 oxidation

The formation of complexes and transformations of nitrogen oxides (NO, N2O) on strong Lewis acid sites of HZSM-5, H[Ga]ZSM-5, ZnO/HZSM-5 zeolites and dealuminated mordenites was investigated by diffuse-reflectance IR spectroscopy. Adsorbed N2O that is formed by disproportionation of NO is capable of oxidizing CO and CH4 molecules to CO2. The behaviour of strong Lewis acid sites in zeolites and cationic forms of zeolites in the NO disproportionation and CO or CH4 oxidation was comparatively studied.

Controlling the Hydrophilicity of the Electrochemical Interface to Modulate the Oxygen-Atom Transfer in Electrocatalytic Epoxidation Reactions.

The electrocatalytic epoxidation of alkenes at heterogeneous catalysts using water as the sole oxygen source is a promising safe route toward the sustainable synthesis of epoxides, which are essential building blocks in organic chemistry. However, the physicochemical parameters governing the oxygen-atom transfer to the alkene and the impact of the electrolyte structure on the epoxidation reaction are yet to be understood. Here, we study the electrocatalytic epoxidation of cyclooctene at the surface of gold in hybrid organic/aqueous mixtures using acetonitrile (ACN) solvent. Gold was selected, as in ACN/water electrolytes gold oxide is formed by reactivity with water at potentials less anodic than the oxygen evolution reaction (OER). This unique property allows us to demonstrate that a sacrificial mechanism is responsible for cyclooctene epoxidation at metallic gold surfaces, proceeding through cyclooctene activation, while epoxidation at gold oxide shares similar reaction intermediates with the OER and proceeds via the activation of water. More importantly, we show that the hydrophilicity of the electrode/electrolyte interface can be tuned by changing the nature of the supporting salt cation, hence affecting the reaction selectivity. At low overpotential, hydrophilic interfaces formed using strong Lewis acid cations are found to favor gold passivation. Instead, hydrophobic interfaces created by the use of large organic cations favor the oxidation of cyclooctene and the formation of epoxide. Our study directly demonstrates how tuning the hydrophilicity of electrochemical interfaces can improve both the yield and selectivity of anodic reactions at the surface of heterogeneous catalysts.

Reactivity and Structure of a Bis-phenolate Niobium NHC Complex.

We report the facile synthesis of a rare niobium(V) imido NHC complex with a dianionic OCO-pincer benzimidazolylidene ligand (L 1 ) with the general formula [NbL 1 (N t Bu)PyCl] 1-Py. We achieved this by in situ deprotonation of the corresponding azolium salt [H 3 L 1 ][Cl] and subsequent reaction with [Nb(N t Bu)Py 2 Cl 3 ]. The pyridine ligand in 1-Py can be removed by the addition of B(C6F5)3 as a strong Lewis acid leading to the formation of the pyridine-free complex 1. In contrast to similar vanadium(V) complexes, complex 1-Py was found to be a good precursor for various salt metathesis reactions, yielding a series of chalcogenido and pnictogenido complexes with the general formula [ NbL 1 (N t Bu)Py(EMes)] (E = O (2), S (3), NH (4), and PH (5)). Furthermore, complex 1-Py can be converted to alkyl complex (6) with 1equiv of neosilyl lithium as a transmetallation agent. Addition of a second equivalent yields a new trianionic supporting ligand on the niobium center (7) in which the benzimidazolylidene ligand is alkylated at the former carbene carbon atom. The latter is an interesting chemically "noninnocent" feature of the benzimidazolylidene ligand potentially useful in catalysis and atom transfer reactions. Addition of mesityl lithium to 1-Py gives the pyridine-free aryl complex 8, which is stable toward "overarylation" by an additional equivalent of mesityl lithium. Electrochemical investigation revealed that complexes 1-Py and 1 are inert toward reduction in dichloromethane but show two irreversible reduction processes in tetrahydrofuran as a solvent. However, using standard reduction agents, e.g., KC8, K-mirror, and Na/Napht, no reduced products could be isolated. All complexes have been thoroughly studied by various techniques, including 1H-, 13C{1H}-, and 1H-15N HMBC NMR spectroscopy, IR spectroscopy, and X-ray diffraction analysis.

Comparison effect of thin microrod‐nanowhisker manganese oxide (MnO2) and spherical grain‐like cobalt oxide (Co3O4) as nanofiller for performance‐enhanced composite gel terpolymer electrolyte‐based dye‐sensitized solar cells

AbstractGel polymer electrolyte (GPE) are better replacement of liquid electrolyte for dye‐sensitized solar cell (DSSC) but they have low efficiency due to insufficient segmental motion within the electrolyte. By incorporating transition metal oxide nanofiller, it can improve the performance of DSSC due to the existence of cross‐linking center that provides interaction with polymer side chain and redox mediator. Two different transition metal oxides (namely, manganese oxide, MnO2 and cobalt oxide, Co3O4) were synthesized using sonochemical method and incorporated as nanofiller. Poly (vinyl butyral‐co‐vinyl alcohol‐co‐vinyl acetate) (P(VB‐co‐VA‐co‐VAc)) as a host terpolymer and sodium iodide (NaI) as a dopant salt were used to formulate the composite gel terpolymer electrolytes (CGPEs). The two different shapes of nanofiller have impacted the performance of CGPE proven by the structural analysis. It was found that both systems, the highest ionic conductivity achieved at 2.5 wt% of MnO2 and Co3O4 content with values of 6.40 and 5.2 mS cm−1, respectively. Surprisingly, the performance DSSC of Co3O4‐based CGPE was higher due to the strong Lewis's acidity of Co3O4, which absorb more iodide ions into the surface and facilitating the ions mobility. CGPE containing 2.5 wt% Co3O4 yield the highest efficiency, with value of 3.69%.

Fe0/Fe3C-assisted Fe3O4 redox sites as robust peroxidase mimics for colorimetric detection of H2O2

Fe-based nanozymes have been shown promising peroxidase-mimicking activity and are used as colorimetric H2O2 sensing platforms. However, the redox cycle of Fe2+/Fe3+ in the Fe-H2O2 system to generate •OH that is needed to oxidize chromogenic substrate (3, 3′, 5, 5′-tetramethylbenzidine, TMB) is restricted due to a slower reaction rate of Fe3+ with H2O2 which leads to insensitive colorimetric detection of H2O2. In this study, Fe3O4-Fe0/Fe3C nanozyme was processed with a peculiar peroxidase-mimicking activity from a carbothermal reduction of ZnFe-double-layer hydroxide and glucose-derived carbon under a nitrogen environment. The nanozyme Fe3O4-Fe0/Fe3C was engineered upon thermal treatment and its peroxidase-mimicking activity was studied in detail. The Fe3O4-Fe0/Fe3C nanozyme exhibits excellent peroxidase activity with regard to the activation of H2O2 and oxidation of TMB due to the synergy of Fe3O4 and Fe0/Fe3C interface. A Fe0/Fe3C improves Fe2+/Fe3+ redox chemistry, and electron transfer kinetics to generate sufficient •OH while strong Lewis acid property of Fe3+ enhances adsorption of TMB. This synergy leads Fe3O4-Fe0/Fe3C nanozyme for highly selective and supersensitive sensing of H2O2 with a low limit of detection (67.1 pM). Furthermore, the proposed colorimetric sensor was impressively used for the real-time detection of H2O2 in milk and HepG2 living cells with excellent biocompatibility.

Lewis Acid-Base Interaction Triggering Electron Delocalization to Enhance the Photodegradation of Extracellular Antibiotic Resistance Genes Adsorbed on Clay Minerals.

The transformation of extracellular antibiotic resistance genes (eARGs) is largely influenced by their inevitable photodegradation in environments where they tend to be adsorbed by ubiquitous clay minerals instead of being in a free form. However, the photodegradation behaviors and mechanisms of the adsorbed eARGs may be quite different from those of the free form and still remain unclear. Herein, we found that kaolinite, a common 1:1-type clay, markedly enhanced eARG photodegradation and made eARGs undergo direct photodegradation under UVA. The decrease in the transformation efficiency of eARGs caused by photodegradation was also promoted. Spectroscopy methods combined with density functional theory calculations revealed that the Lewis acid-base interaction between P-O in eARGs and Al-OH on kaolinite delocalized electrons of eARGs, thus resulting in increased photon absorption ability of eARGs. This ultimately led to enhanced photodegradation of kaolinite-adsorbed eARGs. Additionally, divalent Ca2+ could reduce the Lewis acid-base interaction-mediated adsorption of eARGs by kaolinite, thereby weakening the enhanced photodegradation of eARGs caused by electron delocalization. In contrast, the 2:1-type clay montmorillonite without strong Lewis acid sites was unable to delocalize the electrons to enhance the photodegradation of eARGs. This work allowed us to better evaluate eARGs' fate and risk in real aqueous environments.

A Dual‐Function Additive to Regulate Nucleation Behavior and Interfacial Chemistry for Ultra‐Stable Na Metal Anodes beyond One Year

AbstractSodium (Na) metal anodes suffer from dendrite formation and inferior reversibility, mainly induced by the inhomogeneous nucleation/growth and fragile solid electrolyte interphase (SEI), which hinders their commercial application. Optimizing nucleation behavior or SEI features can improve Na deposition/stripping process, as observed in most currently available approaches, but its long‐term cyclic stability remains a great challenge because these issues are not fully optimized/solved in an individual method. Herein, a dual‐role crown ether additive (CEA) is introduced into electrolytes to circumvent these challenges concurrently. As revealed by experiments and theoretical calculations, CEA possesses a strong affinity with Na+ and effectively regulates desolvation kinetics, leading to the uniform Na nucleation/growth. On the other hand, the resultant Na+/CEA complexes with a strong Lewis acid feature easily attract anions, which enables an anion‐abundant solvation sheath, resulting in a NaF‐rich SEI. Consequently, Na|Cu cells deliver a high average Coulombic efficiency of 99.95% beyond one year and stable cyclic stability over 3000 h even under a high depth of discharge (75%), surpassing most previous studies. Furthermore, this concept is readily extended to zinc metal batteries, verifying that simultaneous nucleation control and interfacial chemistry regulation are promising ways to realize stable metal anodes.

Lewis Superacidic Heavy Pnictaalkene Cations: Comparative Assessment of Carbodicarbene-Stibenium and Carbodicarbene-Bismuthenium Ions.

We report a comprehensive assessment of Lewis acidity for a series of carbone-stibenium and -bismuthenium ions using the Gutmann-Beckett (GB) method. These new antimony and bismuth cations have been synthesized by halide abstractions from (CDC)PnBr3 and [(pyCDC)PnBr2][Br] (CDC = carbodicarbene; Pn = Sb or Bi; py = pyridyl). The reaction of (CDC)SbBr3 (1) with one or two equivalents of AgNTf2 (NTf2 = bis(trifluoromethanesulfonyl)imide) or AgSbF6 gives stibaalkene mono- and dications of the form [(CDC)SbBr3-n][A]n (2-4; n = 1,2; A = NTf2 or SbF6). The stibaalkene trication [(CDC)2Sb][NTf2]3 (5) was also isolated and collectively these molecules fill the gap among the series of cationic pnictaalkenes. The Sb cations are compared to the related CDC-bismaalkene complexes 6-9. With the goal of preparing highly Lewis acidic compounds, a tridentate bis(pyridine)carbodicarbene (pyCDC) was used as a ligand to access [(pyCDC)PnBr2][Br] (10, 12) and trications [(pyCDC)Pn][NTf2]3 (Pn = Sb (11), Bi (13)), forgoing the need for a second CDC as used in the synthesis of 5. The bonding situation in these complexes is elucidated through electron density and energy decomposition analyses in combination with natural orbital for chemical valence theory. In each complex, there exists a CDC-Pn double bonding interaction, consisting of a strong σ-bond and a weaker π-bond, whereby the π-bond gradually strengthens with the increase in cationic charge in the complex. Notably, [(CDC)SbBr][NTf2]2 (4) has an acceptor number (AN) (84) that is comparable to quintessential Lewis acids such as BF3, and tricationic pnictaalkene complexes 11 and 13 exhibit strong Lewis acidity with ANs of 109 (Pn = Sb) and 84 (Pn = Bi), respectively, which are among the highest values reported for any antimony or bismuth cation. Moreover, the calculated fluoride ion affinities (FIAs) for 11 and 13 are 99.8 and 94.3 kcal/mol, respectively, which are larger than that of SbF5 (85.1 kcal/mol), which suggest that these cations are Lewis superacids.