Lewis Acid Properties Research Articles

An efficient ordered mesoporous molybdate-zirconium oxophosphate solid acid catalyst with homogeneously dispersed active sites: Synthesis, characterization and application

Ordered mesoporous molybdate-zirconium oxophosphate (M-ZrPMo) solid acid catalysts with controllable molybdenum contents (0–20%) are designed and synthesized through a one-pot evaporation-induced self-assembly strategy. Afterwards, ordered mesostructure and molybdenum species in the materials are systematically researched by a variety of means. The results show that M-ZrPMo has highly ordered mesoporous structure with large specific surface area (∼200 m2·g−1), big pore volume (∼0.30 cm3·g−1) and pore size (∼6.5 nm). Additionally, ordered mesoporous structure of M-ZrPMo can be efficiently preserved even treated at 700 °C, presenting an outstanding thermal stability. Meanwhile, the molybdenum species are introduced as designed and homogeneously dispersed in mesoporous framework even at molybdenum content up to 20%. More importantly, the Brønsted and Lewis acidic properties of these materials are successfully enhanced with the introduction of molybdenum species. Meantime, the M-ZrPMo is employed as a solid acid catalyst for alkylation of aromatic compounds and esterification of levulinic acid with 1-butanol. The effect of molybdenum contents and calcination temperature on catalytic performance is thoroughly discussed. The excellent activity and reusability suggested that M-ZrPMo is a promising solid acid catalyst.

Aluminium Oxide Supported on SBA-15 Molecular Sieves as Potential Lewis Acid Catalysts for Epoxide Ring Opening Using Aniline

A series of aluminium oxide (Al2O3)-supported SBA-15 molecular sieves were prepared using a one-step wet-impregnation method. Powder X-ray diffraction, nitrogen adsorption/desorption, infrared spectroscopy and ammonia TPD were used to investigate the structures and chemical natures of the surface-bound species. The FT-IR studies of metal-impregnated SBA-15 materials revealed strong covalent interaction of Al2O3 on SBA-15 materials with strong Lewis acidic properties, evident from ammonia-TPD studies. The metal oxide-supported SBA-15 catalysts are active for epoxide ring opening with aniline at room temperature, and showed remarkably high stability and selectivity towards mono-alkylated products (about 86%) viz., 1-(phenylamino)propan-2-ol and 2-(phenylamino)propan-1-ol. The catalytic activities remained intact after several recycles. The observed activities and selectivities were compared with other metal oxide-loaded SBA-15 catalysts obtained by similar preparation methods. Aluminium oxide supported SBA-15 molecular sieves were prepared using a one-step wet-impregnation method. The materials showed strong Lewis acidic sites and promising catalytic activity for epoxide ring opening with aniline at room temperature.

Selective N-acetylation with concurrent S-oxidation of o-amino thiol at ambient conditions over Ce doped ZnO composite nanocrystallites

The oxidative SS coupling of thiol to disulfide is an imperative chemical transformation in the domain of biological processes and also finds numerous chemical applications. The CeO2 and ZnO are significant catalysts for oxidation of thiol to disulfide and N-acetylation of amines respectively. Dithiobis(phenylene)bis(benzyldeneimine) moiety containing N-acetyl and disulfide functional groups is a potential antimicrobial agent with Leishmanicidal and antihyperlipidemic activities. Herein, we report a synchronized catalytic application of Ce doped ZnO (Ce-ZnO) and CeO2-Ce-ZnO composites for selective synthesis of Dithiobis(phenylene)bis(benzyldeneimine) from o-amino thiol. The Ce-ZnO samples were synthesized by simple co precipitation method by calcination of hydroxide precursors at 400 °C to get 0–10% Ce-ZnO nanocrystallites. The formation of CeO2-Ce-ZnO composite material was observed beyond 1.5% Ce concentration. The synthesized materials were well characterized by IR, XRD, DRS spectroscopy and SEM-EDS analysis. The application of Ce doped ZnO as an efficient catalyst towards the selective N-acetylation and concurrent S-oxidation of o-amino thiol to afford Dithiobis(phenylene)bis(benzyldeneimine) at ambient temperature in acetonitrile was deliberated. Among all screened catalysts, the maximum selectivity was found for 7.5% Ce-ZnO as CeO2-Ce-ZnO composite catalyst. Lewis acidic property of catalyst supported probable mechanism for achieved dual transformations. Also, the 7.5% Ce-ZnO catalyst has demonstrated a versatile SS coupling ability for variety of thiol substrates with excellent stability.

Evaluation of Aggregation and Fibril Formation Propensity of Peptides Involved in Amyloid Diseases.

Amyloidosis is defined as a generic term given to a series of proteins/ polypeptides in the form of amyloid fibrils that are deposited in the tissues and give rise to a set of clinical disorders. This work developed an approach to first examine chain association propensities of several amyloidogenic peptides: SNNFGAILSS from the islet amyloid polypeptide (coded IAPP), NAGDVAFV from the protein responsible for corneal amyloidosis (coded Lactoferrin), and (1-42) β-amyloid (coded Amyloid). Fmoc-synthesis protocol was applied for the synthesis of IAPP and Lactoferrin whereas Amyloid was synthesized through the Boc-chemistry as early detailed. The fluorescence and light scattering experiments results indicated that Amyloid revealed a surprising reduction in the aggregation process as a function of time (decrease of about 20-30% in 3 days) through both methods. In contrast, the aggregation intensity of IAPP increased around 35% after 3 days via a light scattering procedure. These findings are very relevant for interpretation of the aggregation phenomenon of amyloidogenic peptides. The final part of this work proposed rules for dissolution of aggregated structures based on the Lewis acid and Lewis base properties of solvents. Very low solubility values (6 to 15%) were measured for peptides in water but with increased to around 90% in strong nucleophilic or strong electrophilic organic solvents. However, care should be taken when strong nucleophilic solvents such as DMSO are mixed with the strong electrophilic such as water. Both solvent molecules tend to attract each other rather than to dissolve peptide chains thus lowering the capacity of this type of solution for fibril dissolution.

Zn 1,3,5-benzenetricarboxylate as an efficient catalyst for the synthesis of cyclic carbonates fromCO2.

Development of heterogeneous catalysts for the cycloaddition of CO2 with epoxides to produce cyclic carbonates is a hot issue in the field of chemical fixation of carbon dioxide. It is fairly promising as production of by-products is quite low. In this study, the [Zn3(BTC)2]/n-Bu4NBr catalytic system was investigated for the solventless cycloaddition of carbon dioxide with epoxides and had an excellent synergetic effect in promoting the reaction. The reaction parameters were moderate i.e. (130 °C and 13 bar CO2 pressure) and were selected by a study of the catalytic system. Under the optimal reaction conditions, the yield of cyclic carbonate reached 99%. A decrease in the yield of cyclic carbonate was not apparent after [Zn3(BTC)2] was reused three times, indicating that [Zn3(BTC)2] was stable. At the same time, the catalytic activity of the catalyst for other epoxides was also verified. The acidic and alkaline nature of the [Zn3(BTC)2] catalyst did not change obviously after recycling the catalyst three times. In this study it is also verified that the [Zn3(BTC)2] catalytic cycloaddition reaction was closely related to the Lewis acid/base distribution. In addition, a plausible mechanism for the synergistic effect of the catalyst (Lewis acid and base properties) and the co-catalyst was suggested.

Small molecule activation of nitriles coordinated to the [Re6Se8]2+ core: formation of oxazine, oxazoline and carboxamide complexes.

Novel oxazine, oxazoline and carboxamide cluster complexes were prepared when different nucleophilic oxygen species reacted with nitriles coordinated to the Lewis acidic [Re6Se8]2+ cluster core. Reaction of ICH2CH2O- (generated in situ) with [Re6Se8(PEt3)5(NCR)]A2 (1A2 (R = Me) and 2A2 (R = Ph) where A = BF4-), leads to the formation of [Re6Se8(PEt3)5(2-methyloxazoline)]2+ (32+) and [Re6Se8(PEt3)5(2-phenyloxazoline)]2+ (42+). Similarly, reaction of BrCH2CH2CH2O- with the same nitrile complexes, 1A2 and 2A2 (where A = BF4- or SbF6-) leads to the corresponding oxazine complexes, [Re6Se8(PEt3)5(2-methyloxazine)]2+ (52+) and [Re6Se8(PEt3)5(2-phenyloxazine)]2+ (62+). In addition, reaction of 2(BF4)2 with KOH leads to the formation of the carboxamide complex, [Re6Se8(PEt3)5(phenylcarboxamide)](BF4) (7(BF4)). The neutral oxazine and oxazoline ligands can be removed using either heat or UV irradiation; UV irradiation was found to be more efficient at ligand removal as indicated by the shorter reaction times. The relative coordination strength of the neutral N-donor ligands was determined by these reaction times. X-ray structure determinations of 5(BF4)2 and 7(BF4) are also reported.

MIL-101(Cr) as a synergistic catalyst for the reduction of imines with trichlorosilane

The development of catalyst based on porous crystalline materials (PCM) constructed from metal ions or clusters and multidentate organic ligands is a topic of great interest. In view of the Lewis acidic and basic properties of PCMs, we report for the first time that MIL-101(Cr) works as a synergistic catalyst for the reduction of imines with trichlorosilane as the hydrogen source. Both ketimines and aldimines were tolerated with this protocol, giving the corresponding amines in moderate to high yields. The operational simplicity as well as mild reaction conditions renders this protocol an attractive approach for the synthesis of amines. Furthermore, a chiral MOF, CMIL-101, was also realized by grafting chiral N-formyl proline derivatives to the open metal sites. Moreover, CMIL-101 exhibited a comparable catalytic performance with its homogeneous counterpart in terms of yields and enantioselectivities.

Structural Defects, Lewis Acidity, and Catalysis Properties of Mesostructured WO3/SBA-15 Nanocatalysts

For the first time, creation of oxygen defects in the crystalline structure of WO3/SBA-15 catalysts and its correlation with Lewis acidity and catalytic activity in the oxidation of 4,6-dimethyldibenzothiophene (4,6-DMDBT) in a model diesel were reported. All the WO3/SBA-15 catalysts predominantly contained Lewis acid sites and some oxygen defects in the crystalline structure of WO3. The oxygen defect concentration increased from 1.67% to 16.66% per lattice cell unit, and the number of Lewis acid sites varied from 92 to 458 μmol/g as the WO3 content increased from 5 to 25 wt %. The 4,6-DMDBT conversion was almost proportional to both the number of Lewis acid sites and oxygen defects. Formic acid addition led to formation of peroxyformic acid which coordinated with surface W6+ to generate reactive oxygen species like peroxometallic complex, improving oxidant stability and 4,6-DMDBT oxidation efficiency. More than 99% of 4,6-DMDBT was removed with the best 25 wt % WO3/SBA-15 catalyst within 15 min of reacti...

Triarylborane-Based Materials for OLED Applications.

Multidisciplinary research on organic fluorescent molecules has been attracting great interest owing to their potential applications in biomedical and material sciences. In recent years, electron deficient systems have been increasingly incorporated into fluorescent materials. Triarylboranes with the empty p orbital of their boron centres are electron deficient and can be used as strong electron acceptors in conjugated organic fluorescent materials. Moreover, their applications in optoelectronic devices, energy harvesting materials and anion sensing, due to their natural Lewis acidity and remarkable solid-state fluorescence properties, have also been investigated. Furthermore, fluorescent triarylborane-based materials have been commonly utilized as emitters and electron transporters in organic light emitting diode (OLED) applications. In this review, triarylborane-based small molecules and polymers will be surveyed, covering their structure-property relationships, intramolecular charge transfer properties and solid-state fluorescence quantum yields as functional emissive materials in OLEDs. Also, the importance of the boron atom in triarylborane compounds is emphasized to address the key issues of both fluorescent emitters and their host materials for the construction of high-performance OLEDs.

Two faces of triel bonds in boron trihalide complexes.

The N⋅⋅⋅B triel bonds in complexes of boron trihalides, BX3 (X = F, Cl, Br, and I), with species acting as Lewis bases through the nitrogen center, NH3 , N2 , and HCN, are analyzed theoretically (MP2/aug-cc-pVTZ calculations). It is confirmed that stronger Lewis acid properties of the boron center are observed for the BCl3 moiety than for the BF3 one in complexes with the strong Lewis base (NH3 ); while the opposite order is observed for complexes with the weak Lewis base (N2 ). The BX3 NCH complexes (for X = Cl, Br, and I) are characterized by two tautomeric forms and by two corresponding N⋅⋅⋅B distances, the shorter one possesses characteristics of the covalent bond. In a case of the BF3 NCH complex one energetic minimum is observed. Ab initio calculations are supported by an analysis of molecular electrostatic potentials (EPs) and electron density distributions. The quantum theory of 'atoms in molecules' and the decomposition of the energy of interaction are applied. The aforementioned acidity orders as well as the existence of two tautomers for some of complexes result partly from the electrostatic interactions' balance; the EP distribution is different for the BF3 species than for the other BX3 species where X = Cl, Br, and I. © 2017 Wiley Periodicals, Inc.

Highly efficient and environmentally benign method for the synthesis of tetrahydrobenzo[b]pyrans using Ca9.5Mg0.5(PO4)5.5(SiO4)0.5F1.5 as a new bio- and nanocatalyst with Brønsted base and Lewis acid properties

Nanosized Ca9.5Mg0.5(PO4)5.5(SiO4)0.5F1.5 was used as a biocatalyst for the synthesis of tetrahydrobenzo[b]pyrans derivatives via one-pot three-component reaction of various aldehydes, dimedone and malononitrile. The reactions were easily carried out at 60 °C under mild and heterogeneous conditions. The catalyst was easily synthesized by high-energy ball milling, and the effect of some parameters such as specific surface area, pore volume, particle size, pH and crystallinity degree of the catalyst on the yield and reaction time was studied. The catalyst showed both Bronsted base and Lewis acid properties. The acidic sites were determined by FT-IR spectroscopy of the pyridine adsorption technique at different temperatures. Using a biocatalyst and an environmentally benign procedure, good to high yields of the products and short reaction time were the advantages of the method.

Dehydration of D-xylose to furfural using acid-functionalized MWCNTs catalysts**Invited talk at 5th Thailand International Nanotechnology Conference (Nano Thailand-2016), 27–29 November 2016, Nakhon Ratchasima, Thailand.

Acid-functionalized multi-wall carbon nanotubes (MWCNTs) catalysts were prepared by a wet chemical sonication with various acid solutions, i.e. H2SO4, H3PO4, HNO3, and HCl. Sulfonic groups and carboxyl groups were detected on MWCNTs with H2SO4 treatment (s-MWCNTs), while only carboxyl groups were presented from other acid treatments. The catalytic dehydration of D-xylose into furfural was evaluated using a batch reactor at 170 °C for 3 h under N2 pressure of 15 bar. The highest furfural selectivity was achieved around 57% by s-MWCNTs catalyst, suggesting a positive role of the sulfonic functionalized groups. The effect of Co species was related to their Lewis acid property resulting in the enhancement of xylose conversion with low selectivity to furfural product.

Introduction of a Simple Experiment for the Undergraduate Organic Chemistry Laboratory Demonstrating the Lewis Acid and Shape-Selective Properties of Zeolite Na-Y

A simple, inexpensive, discovery-based experiment for undergraduate organic laboratories has been developed that demonstrates the Lewis acid and shape-selective properties of zeolites. Calcined zeolite Na-Y promotes the electrophilic aromatic bromination of toluene with a significantly higher para/ortho ratio than observed under conventional conditions. The experiment can be completed in one three-hour laboratory period. The zeolite can be recovered and regenerated for later reuse. A broad range of organic course topics from orientation in electrophilic aromatic substitution to photochemistry can be discussed.

The Lewis acidity of three- and two-dimensional zeolites: The effect of framework topology

The zeolite activity in processes driven by Lewis acid sites is determined by the distribution of the extra-framework cations in the zeolite and their coordination with the framework (the stronger is the cation coordination, the weaker is its Lewis acidity). This study aims to test how much the transformation from 3D zeolite to its corresponding 2D layered form changes the extra-framework cation distribution and coordination and thus how much it affects the zeolite Lewis acid strength. Zeolites with two distinct topologies, MWW and MFI, which form layers with considerably different silanol density (1.1 and 2.9 silanols/nm2, respectively) were considered. To probe the Lewis acidity of extra-framework Li+ sites the carbon monoxide stretching frequencies and adsorption enthalpies were calculated, employing the dispersion-corrected density functional theory and the results are in good agreement with available experimental data. No or negligible differences in the Lewis acidity of Li+ sites in 2D and 3D form of zeolites with MWW topology are found; both, the extra-framework cation distribution and the coordination with the framework remain the same. On the contrary, there are significant differences in Lewis acidic properties of 3D and 2D zeolites with MFI topology. The Li+ cation coordination in 2D-MFI is considerably improved compared to 3D-MFI due to the large concentration of surface silanols and due to an increased flexibility of the material. Consequently, Lewis acidity of 2D-MFI is lower compared to corresponding 3D-MFI material. This Lewis acidity decrease also reflects the fact that the strongest Lewis acid sites found in 3D zeolite − the channel-intersection sites (coordinated with two framework oxygens only) − are not present in 2D-MFI. The results reported herein suggest that the flexibility of the layer, which increases with silanol density and varies for each zeolite topology, significantly influences its Lewis acidity; the larger is the flexibility of the layer, the better is the cation coordination with the framework and thus smaller is its Lewis acidity.

Nature of active sites on UiO-66 and beneficial influence of water in the catalysis of Fischer esterification

Zirconium terephthalate UiO-66 type metal organic frameworks (MOFs) are known to be highly active, stable and reusable catalysts for the esterification of carboxylic acids with alcohols. Moreover, when defects are present in the structure of these MOFs, coordinatively unsaturated Zr ions with Lewis acid properties are created, which increase the catalytic activity of the resulting defective solids. In the present work, molecular modeling techniques combined with new experimental data on various defective hydrated and dehydrated materials allow to unravel the nature and role of defective active sites in the Fischer esterification and the role of coordinated water molecules to provide additional Brønsted sites. Periodic models of UiO-66 and UiO-66-NH2 catalysts have been used to unravel the reaction mechanism on hydrated and dehydrated materials. Various adsorption modes of water and methanol are investigated. The proposed mechanisms are in line with experimental observations that amino groups yield a reduction in the reaction barriers, although they have a passive role in modulating the electronic structure of the material. Water has a beneficial role on the reaction cycle by providing extra Brønsted sites and by providing stabilization for various intermediates through hydrogen bonds.

Synthesis of dihydropyrano[2,3-c]pyrazoles using Ca9.5Mg0.5(PO4)5.5(SiO4)0.5F1.5 as a new nano cooperative catalyst

Ca9.5Mg0.5(PO4)5.5(SiO4)0.5F1.5 was used as a nano cooperative catalyst for the synthesis of dihydropyrano[2,3-c]pyrazoles derivatives via a one-pot four component reaction of aldehyde, ethyl acetoacetate, malononitrile and hydrazine hydrate. The reactions were carried out at 70 °C under mild and heterogeneous conditions. The catalyst was easily synthesized by high energy ball milling and the effect of some parameters such as specific surface area, pore volume, particle size, pH and crystallinity degree of the catalyst was studied on the yield and reaction time. The used catalyst showed both Bronsted base and Lewis acid properties. The acidic sites were determined by FT-IR spectroscopy of pyridine adsorption technique at different temperature. Using a new cooperative catalyst and an environmentally benign procedure, good to high yield of the products and relatively short reaction time were the advantages of the present method.

Photophysical and Lewis acidic properties of triarylboranes with meta-substituted 2-R-o-carboranes

Triarylboranes incorporating meta-substituted 2-R-o-carboranes (R = H, Me, iBu, Ph, 4-CF3C6F4) were prepared to investigate the effects of the meta-2-R-o-carborane on Lewis acidity. X-ray crystal structures revealed that the carboranyl CC bond distances increased with increasing steric and electron-withdrawing effects of the 2-R group. UV/Vis absorption titrations with fluoride ions showed high binding constants (K > 107 M−1 in THF and K = 3.9–6.6 × 103 M−1 in THF/H2O (9:1 vol)). Fluoride ion affinity was greater for the 2-aryl (4-CF3C6F4)-substituted triarylborane than 2-H and -alkyl (Me)-substituted triarylboranes, which was supported by anodic shifts in the reduction potentials of 2-aryl-o-carborane-substituted triarylboranes. Comparison of fluoride binding constants and reduction potentials of meta- and para-substituted triarylboranes indicated slightly lower Lewis acidity of meta-substituted triarylboranes. In contrast to para-substituted triarylboranes, the Lewis acidity of meta-substituted triarylboranes is likely to be primarily enhanced by the inductive effect of the 2-R-o-carborane.

Revisiting Acidity of SnBEA Catalysts by Combined Application of FTIR Spectroscopy of Different Probe Molecules

The Lewis and Bronsted acid properties of SnBEA zeolites as well as SnO2 supported on silica BEA have been examined by means of IR spectroscopy of adsorbed pyridine, 2,6-ditertbutylpyridine, and deuterated acetonitrile. Three types of surface sites have been detected and assigned to (i) framework Sn centers possessing Lewis acid properties; (ii) weak Bronsted acid sites associated with framework tin atoms; and (iii) nonframework SnO2 particles which possess Lewis acidity. The total amount of Lewis sites can be determined using pyridine as a probe molecule, while the type of Lewis sites can be distinguished by FTIR of adsorbed acetonitrile. The band at 2316 cm–1 is attributed to strong framework sites, whereas the band at 2287 cm–1 is attributed to nonframework sites. Bronsted acid sites can be characterized using 2,6-ditertbutylpyridine (bands at 3363, 1613, and 1530 cm–1) and deuterated acetonitrile (band at 2308 cm–1). The relative amount of Lewis and Bronsted acid sites on SnBEA can be varied by treatm...

Sensitive Determination of Cannabinoids in Whole Blood by LC-MS-MS After Rapid Removal of Phospholipids by Filtration.

Direct analysis of Δ9-tetrahydrocannabinol (THC) and other cannabinoids in crude acetonitrile extracts of whole blood by liquid chromatography-tandem mass spectrometry using pneumatically assisted electrospray ionization (LC-ESI-MS-MS) was subjected to pronounced ion suppression from co-eluting phospholipids (PLs). The interferences were mainly caused by the lysophosphatidylcholine and lysophosphatidylethanolamine classes of PLs. The PLs were easily removed from crude extracts by filtration through a sorbent with Lewis acid properties, which typically increased the THC and cannabinol (CBN) signal intensities by a factor of 5. Based on this technique, a simple high-throughput LC-MS-MS method was developed for the determination of cannabinoids in 100 μL samples of whole blood. The lower limits of quantification were 0.2 μg/L for THC, CBN, cannabidiol (CBD) and Δ9-tetrahydrocannabinolic acid A (THCA-A) and 0.5 μg/L for 11-hydroxy-Δ9-tetrahydrocannabinol (THC-OH) and 11-nor-9-carboxy-Δ9-tetrahydrocannabinol (THC-COOH). The mean ion suppression levels after clean-up were 10% (THC), 9% (CBN), 17% (CBD), 0% (THC-OH), 2% (THC-COOH) and 9% (THCA-A) at blood concentration levels of 1-10 μg/L. The mean true extraction recoveries were 97% (THC), 101% (CBN), 101% (CBD), 98% (THC-OH), 95% (THC-COOH) and 90% (THCA-A) at the same concentration levels. The relative intra-laboratory reproducibility standard deviations were <9% at concentrations of 1 μg/L or higher. The trueness expressed as the relative bias of the test results was within ±4% at concentrations of 1 μg/L or higher.

Lewis Acid Properties of Tetrel Tetrafluorides—The Coincidence of the σ-Hole Concept with the QTAIM Approach

Tetrel bond is analysed for a series of ZF4 (Z = C, Si, Ge) complexes with one and two NH3 or AsH3 ligands. The MP2/aug-cc-pVTZ calculations were performed and supported by results of the Quantum Theory of “Atoms in Molecules” (QTAIM) and the Natural Bond Orbitals (NBO) approaches. The Z-tetrel atoms of complexes analysed interact through their σ-holes with nitrogen or arsenic Lewis base centres; these interactions correspond to the Z…N/As bond paths according to the QTAIM approach. The QTAIM and NBO results show that these interactions are relatively strong and they possess numerous characteristics of covalent bonds. The theoretical analysis is supported by the discussion on crystal structures which are characterized by the same type interactions.