Lewis Base Centers Research Articles

Alteration in electrocatalytic water splitting activity of reduced graphene oxide through simultaneous and individual doping of Lewis acid/base center

Substrate adsorption-desorption is an important aspect in hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalysis process. The B and N are two neighboring elements of C which has Lewis acid and base characteristics. However, any thorough investigation has not been probed till date to understand the effect of simultaneous and individual incorporation of these Lewis acid and base centers in graphene towards electrocatalytic activity. In this investigation it was found that the incorporation of B and N in reduced graphene oxide (rGO) facilitated substrate adsorption-desorption and also modulated lattice parameters and electronic structure of it. Although N doped rGO showed better substrate adsorption efficiency in HER however, the substrate diffusion was sluggish in this case. Despite of moderate substrate adsorption efficiency of B and N-doped rGO, it showed better HER catalytic activity due to faster substrate diffusion and achieved 550 mA cm−2 current density at 327 mV overpotential. The B-doped rGO showed better substrate adsorption efficiency in OER. However, due to better charge transfer efficiency of N-doped rGO, it showed superior OER catalytic activity as OER was governed by charge transfer kinetics. In addition to electrocatalytic efficiency, the mechanistic of the electrocatalysis process also altered due to the simultaneous and individual incorporation of Lewis acid and base centers.

On the SCS Approach to the CeO2/CuO Nanocomposite: Thermochemical Aspects and Catalytic Activity in n-Hexane Conversion

Сerium(IV) oxide, copper(II) oxide and their composite (CeO2/CuO) were successfully synthesized via a simple method—solution combustion synthesis (SCS). The obtained samples were tested as a catalyst for the conversion of n-hexane, which could become the first step to further the synthesis of new simple catalysts constituted from oxide or composite in order to replace traditional ones and maintain a steady level of activity, which is urgently needed in petroleum refineries. The characteristics of the synthesized catalysts were achieved by various methods based on the analysis of TG/DSC, XRD, SEM, N2 adsorption−desorption and determination of acid−base centers. Through analysis of thermochemical aspects revealed that the SCS process can be divided into 3 stages: evaporation of water to form a viscous mixture, combustion complex compounds and removal of residual substances. The ignition temperature touched 205, 226, and 233°C for the synthesis of CeO2/CuO, CeO2, and CuO catalysts, respectively. The combustion mode took place at different rates, therein the synthesis of composite combusted earlier and weaker than its oxides. Here, the relationship between particle size, morphology, surface area, and catalytic activity is also discussed. It was shown that among catalysts mentioned above, the CeO2 sample exhibited the highest activity in the isomerization reactions of n-hexane conversion (68.7%), which was explained by the highest specific surface area (85.8 m2/g), and concentration of Lewis base centers (276.1 µmol/g). These findings are worth considering because of some of its advantages compared to commercial catalysts.

Reductive Amination of Aryl Boronic Acids: Parallelism of the Catalytic Reactivity of Transition Metals and Main Group Elements in the C(sp2)–N Bond-Forming Reactions

The results of the DFT studies on the mechanism of the PIII/PV=O catalyzed reductive amination of nitrosoarenes using ArB(OH)2 yielding diaryl amines are reported. This allowed a comparison of the reaction paths and key intermediates of the Cu(I)- and P(III)-mediated reductive aminations of aryl boronic acids using alkylnitrites, nitroso- or nitroarenes, and revealed important similarities in the catalytic reactivity of transition-metal and main-group elements in C(sp2)–N bond-forming reactions. It is shown that both transformations occur via ambiphilic nitrenoid-type key intermediates, the reactivity of which towards the aryl boronic acid is attributed to the presence of both a Lewis acid center (Cu or P) and a Lewis base center (the N or O atoms of the ‘N=O’ component).

Chitosan-Based Bio-Composite Modified with Thiocarbamate Moiety for Decontamination of Cations from the Aqueous Media.

Herein, we report the development of chitosan (CH)-based bio-composite modified with acrylonitrile (AN) in the presence of carbon disulfide. The current work aimed to increase the Lewis basic centers on the polymeric backbone using single-step three-components (chitosan, carbon disulfide, and acrylonitrile) reaction. For a said purpose, the thiocarbamate moiety was attached to the pendant functional amine (NH2) of chitosan. Both the pristine CH and modified CH-AN bio-composites were first characterized using numerous analytical and imaging techniques, including 13C-NMR (solid-form), Fourier-transform infrared spectroscopy (FTIR), elemental investigation, thermogravimetric analysis, and scanning electron microscopy (SEM). Finally, the modified bio-composite (CH-AN) was deployed for the decontamination of cations from the aqueous media. The sorption ability of the CH-AN bio-composite was evaluated by applying it to lead and copper-containing aqueous solution. The chitosan-based CH-AN bio-composite exhibited greater sorption capacity for lead (2.54 mmol g−1) and copper (2.02 mmol g−1) than precursor chitosan from aqueous solution based on Langmuir sorption isotherm. The experimental findings fitted better to Langmuir model than Temkin and Freundlich isotherms using linear regression method. Different linearization of Langmuir model showed different error functions and isothermal parameters. The nonlinear regression analysis showed lower values of error functions as compared with linear regression analysis. The chitosan with thiocarbamate group is an outstanding material for the decontamination of toxic elements from the aqueous environment.

Engineering Functionalized Chitosan-Based Sorbent Material: Characterization and Sorption of Toxic Elements

The present study reports the engineering of functionalized chitosan (CH)-based biosorbent material. Herein, a two-step reaction was performed to chemically modify the CH using 1,4-bis(3-aminopropyl) piperazine to incorporate nitrogen basic centers for cations sorption from the aqueous environment. The resultant functionalized chitosan-based sorbent material was designated as CH-ANP and characterized using various analytical techniques, including elemental analysis, Fourier-transform infrared spectroscopy (FTIR), 13C NMR (in solid-state), X-ray diffraction, and thermal analysis. Then, the newly engineered CH-ANP was employed for the removal of copper, lead, and cadmium in the aqueous medium. Langmuir sorption isotherm analysis revealed that the highest sorption abilities achieved were 2.82, 1.96, and 1.60 mmol g−1 for copper, cadmium, and lead, respectively. Linear and nonlinear regression methods were deployed on the sorption data to study the behavior of the Langmuir, the Freundlich, and the Temkin sorption isotherms. Among the four different forms, the Langmuir isotherm type 1 fit well to the experimental data as compared to the other models. It also showed the lowest values of error, and a higher correlation coefficient than the Freundlich and Temkin models; thus it was the best fit with the experimental data compared to the latter two models. In conclusion, the findings suggest that chemically modified novel materials with enhanced Lewis basic centers are useful and promising candidates for the sorption of various toxic cations in aqueous solution.

A computational study on hydrogenation of CO2, catalyzed by a bridged B/N frustrated Lewis pair

A DFT-based computational study has been performed on the hydrogenation of CO2, catalyzed by a bridged FLP. Formic acid might be formed in two possible pathways as revealed by this study. In one way, the Lewis basic center of the FLP activates H2, and the Lewis acidic center activates CO2 simultaneously in the first step of the reaction. Alternatively, the Lewis basic center of the FLP activates CO2, and the Lewis acidic center activates H2. The simultaneous activation of CO2 and H2 through single TS is confirmed by NBO analysis. Free energy profiles are also generated for both the possible pathways in solvent phase. It appears from these profiles that the first step, i.e., simultaneous activation of CO2 and H2, is the rate determining for both the reaction pathways. A significant amount of barrier height is reduced in comparison to that in the corresponding uncatalyzed reaction as observed in these profiles. The nature of donor-acceptor interactions present in the transition state geometries is further analyzed by energy decomposition analysis (EDA) methods. The EDA analysis shows that the HOMO of the FLP donates electron density to the LUMO of H2, the HOMO of H2 donates electron density to the LUMO of CO2, and several occupied MOs of CO2 donate electron density to the LUMO of FLP at the TS geometry.

Effect of Formation Conditions on the Functional Composition of the Surface of High-Silica Porous Glass

The possibility of controlling the content and ratio of Bronsted acid to Lewis base centers on the surface of high-silica porous glass (PG) with various compositions based on two-phase alkali-borosilicate glass by varying the conditions of the preparation of PG during the acid work-up of the initial glass is shown. The results demonstrate the possibility to control the features of the surface of the PG, and in particular, the ratio of silanol to siloxane structures.

Water-Stable Nanoscale Zirconium-Based Metal-Organic Frameworks for the Effective Removal of Glyphosate from Aqueous Media.

Two water-stable zirconium-based metal–organic frameworks (MOFs) (NU-1000 and UiO-67) have been synthesized in various size scales (100–2000 nm) for the adsorptive removal of glyphosate from the aqueous media. Both NU-1000 and UiO-67 possess a three-dimensional structure; NU-1000 consists of triangular micropores and wide mesoporous channels (31 Å), whereas UiO-67 has cage-like pores [octahedral (16 Å) and tetrahedral (14 Å) cages]. NU-1000 comprises Zr6(μ3-O)4(μ3-OH)4(H2O)4(OH)4, and UiO-67 contains Zr6O4(OH)4 as secondary building units. These units act as Lewis acid nodes and can interact with the Lewis base phosphate group of the glyphosate. The time taken for reaching equilibrium is found to be reduced considerably as the size of the MOF decreases. The smaller the particle size, the lesser is the diffusion barrier for the analyte, which enhances the interaction between Lewis acidic metal nodes and the Lewis basic center of the glyphosate molecule. NU-1000 was found to be better compared to UiO-67, both in terms of efficiency and reusability. This might be due to the larger pore diameters of the NU-1000. Theoretical calculations revealed that the interaction energy of glyphosate with the nodes of NU-1000 is higher (−37.63 KJ mol–1) compared to UiO-67 (−17.37 KJ mol–1), which might be the possible reason for the higher efficiency of NU-1000.

Direct catalytic hydrogenation of CO2 to formate over a Schiff-base-mediated gold nanocatalyst

Catalytic transformation of CO2 to formate is generally realized through bicarbonate hydrogenation in an alkaline environment, while it suffers from a thermodynamic sink due to the considerable thermodynamic stability of the bicarbonate intermediate. Here, we devise a route for the direct catalytic conversion of CO2 over a Schiff-base-modified gold nanocatalyst that is comparable to the fastest known nanocatalysts, with a turnover number (TON) of up to 14,470 over 12 h at 90 °C. Theoretical calculations and spectral analysis results demonstrate that the activation of CO2 can be achieved through a weakly bonded carbamate zwitterion intermediate derived from a simple Lewis base adduct of CO2. However, this can only occur with a hydrogen lacking Lewis base center in a polar solvent. This finding offers a promising avenue for the direct activation of CO2 and is likely to have considerable implications in the fields of CO2 conversion and gold catalytic chemistry.

Water adsorption on α-V2O5 surface and absorption in V2O5∙nH2O xerogel: DFT study of electronic structure

Adsorption of water on (001)-terminated α-V2O5 surface has been investigated in the framework of density functional theory. According to our calculations, molecular adsorption is preferred, and two types of adsorption sites (Lewis acid center on V atom and Lewis base center on O atoms) have been found. Water adsorption on Lewis acid site does not change the electronic structure of V2O5 surface. Adsorption on Lewis base site is less favorable but reduces the band gap value by half (from 2.5 to 1.3 eV). The reason of this decreasing is the localization of the non-bonding 1b2 electron state of water molecule within the forbidden gap of V2O5 surface. The idealized structure of V2O5∙nH2O xerogel has been computationally studied at n = 0 and 1. It is proved that only Lewis base sites are suitable for water positions. At n = 1, the water absorption energy in xerogel is close to adsorption energy of water on the oxygen atoms of α-V2O5 surface. The presence of the non-bonding states of physically absorbed water in the forbidden gap is also found in the xerogel case. This results in decreasing of the band gap from 2.7 eV to 0.7 eV.

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.

Hydrogen Activation by Frustrated Lewis Pairs Revisited by Metadynamics Simulations

Frustrated Lewis pairs have great potential as metal-free catalysts, for example, for the activation of molecular hydrogen. However, rational design of improved catalysts is hampered because the catalytic reaction mechanisms still remain largely unclear. In this study, we present a density-functional-theory-based metadynamics study of the hydrogen activation by a typical frustrated Lewis pair, tBu3P/B(C6F5)3. The computed free-energy landscape reveals a different reaction path compared with the ones in the literature. Importantly, we found different roles of the Lewis acid and base centers in the hydrogen activation. The rate-determining step is the hydride transfer to the Lewis acid, and the overall reaction is found to be exothermic once the proton transfer to the Lewis base step is accomplished.

Cooperative Catalytic Activation of Si-H Bonds: CO2 -Based Synthesis of Formamides from Amines and Hydrosilanes under Mild Conditions.

A simple cooperative catalytic system was successfully developed for the solvent-free N-formylation of amines with CO2 and hydrosilanes under ambient conditions, which was composed of a Zn(salen) catalyst and quaternary ammonium salt. These commercially available binary components activated the Si-H bonds effectively, owing to the intermolecular synergistic effect between Lewis base and transition metal center (LB-TM), and subsequently facilitated the insertion of CO2 to form the active silyl formats, thereby leading to excellent catalytic performance at a low catalyst loading. Furthermore, the bifunctional Zn(salen) complexes, with two imidazolium-based ionic-liquid (IL) units at the 3,3'-position of salen ligand, acted as intramolecularly cooperative catalysts, and the solvent-regulated separation resulted in facile catalyst recycling and reuse.

Site and chirality selective chemical modifications of boron nitride nanotubes (BNNTs) via Lewis acid-base interactions.

The pristine BNNTs contain both Lewis acid (boron) and Lewis base (nitrogen) centers at their surface. Interactions of ammonia and borane molecules, representatives of Lewis base and acid as adsorbates respectively, with matching sites at the surface of BNNTs, have been explored in the present DFT study. Adsorption energies suggest stronger chemisorption (about 15-20 kcal mol(-1)) of borane than ammonia (about 5-10 kcal mol(-1)) in both armchair (4,4) and zigzag (8,0) variants of the tube. NH3 favors (8,0) over the (4,4) tube, whereas BH3 exhibits the opposite preference, indicating some chirality dependence on acid-base interactions. A new feature of bonding is found in BH3/AlH3-BNNTs (at the edge site) complexes, where one hydrogen of the guest molecule is involved in three-center two-electron bonding, in addition to dative covalent bond (N: → B). This interaction causes a reversal of electron flow from borane/alane to BNNT, making the tube an electron acceptor, suggesting tailoring of electronic properties could be possible by varying strength of incoming Lewis acids. On the contrary, BNNTs always behave as electron acceptor in ammonia complexes. IR, XPS and NMR spectra show some characteristic features of complexes and can help experimentalists to identify not only structures of such complexes but also the location of the guest molecules and design second functionalizations. Interaction with several other neutral BF3, BCl3, BH2CH3 and ionic CH3(+) acids as well as amino group (CH3NH2 and NH2COOH) were also studied. The strongest interaction (>100 kcal mol(-1)) is found in BNNT-CH3(+) complexes and H-bonds are the only source of stability of NH2COOH-BNNT complexes.

Environmentally benign modified biodegradable chitosan for cation removal

The biodegradable biopolymer chitosan had its linear structure chemically modified in a two-step reaction, first by exploring the amino reactivity with glycidylmethacrylate, whose intermediate product, containing an aldehyde group, was then reacted with 1,2-ethanedithiol. Chitosan and the synthesized biopolymers were characterized by elemental analyses, infrared spectroscopy, nuclear magnetic resonance of the carbon nucleus in the solid state, X-ray diffractometry, thermogravimetry and scanning electron microscopy, to give a degree of immobilization of 3.00 mmol g−1. The available basic nitrogen, sulfur and oxygen Lewis centers attached to the enlarged pendant chains enriched the ability of the biopolymer for copper, lead and cadmium sorption from aqueous solution, to give maximum capacities of 2.05 ± 0.01; 2.53 ± 0.02 and 1.88 ± 0.01 mmol g−1, when compared to chitosan with 1.54 ± 0.33; 1.22 ± 0.04 and 1.12 ± 0.08 mmol g−1, respectively, using the Langmuir sorption isotherms. Based on the present results, the highest amount of these cation sorptions, especially with lead that is associated with sulfur–cation soft interactions, is dependent directly on not only the presence of long pendant chain attached, but also the availability of favorable Lewis base centers. The experimental data adjusted to the Langmuir, the Freundlich and the Temkin sorption isotherms using linear and non-linear regression methods and are in agreement with the best fit for Langmuir model type I.

Toward molecular recognition: three-point halogen bonding in the solid state and in solution.

A well-defined three-point interaction based solely on halogen bonding is presented. X-ray structural analyses of tridentate halogen bond donors (halogen-based Lewis acids) with a carefully chosen triamine illustrate the ideal geometric fit of the Lewis acidic axes of the former with the Lewis basic centers of the latter. Titration experiments reveal that the corresponding binding constant is about 3 orders of magnitude higher than that with a comparable monodentate amine. Other, less perfectly fitting multidentate amines also bind markedly weaker. Multipoint interactions like the one presented herein are the basis of molecular recognition, and we expect this principle to further establish halogen bonding as a reliable tool for solution-phase applications.

Cycloaddition Reaction of Propylene Oxide and Carbon Dioxide Over NaX Zeolite Supported Metalloporphyrin Catalysts

Metalloporphyrin catalysts has attracted extensive attention as the result of their outstanding biomimetic catalytic properties. In this paper, different kinds of unsupported (Co(TPP), MnIII(TPP)Cl and CoIII(TPP)Cl) and NaX zeolite supported metalloporphyrin catalysts (Co(TPP)/NaX, MnIII(TPP)Cl/NaX, CoIII(TPP)Cl/NaX) were prepared and characterized. It has been found that these metalloporphyrin catalysts show high catalytic activity in cycloaddition reaction of propylene oxide and carbon dioxide under mild reaction conditions. The NaX supported metalloporphyrin catalysts are more stable and present better catalytic performance than the unsupported metalloporphyrins. CoIII(TPP)Cl/NaX was better than MnIII(TPP)Cl/NaX and CoTPP/NaX, the propylene oxide conversion is 94.5 % and the selectivity to propylene carbonate is 95.6 %. A new kind of supported catalyst Metalloporphyrin/NaX, combined with Phenyltrimethylammonium tribromide was investigated in the cycloaddition of carbon dioxide and propylene epoxide. The results indicate that both a suitable Lewis acid center and base center are necessary for the reaction. A possible mechanism for the reaction was proposed.

ChemInform Abstract: Solid‐State NMR as a Spectroscopic Tool for Characterizing Phosphane‐Borane Frustrated Lewis Pairs

AbstractReview: 65 refs.

Reconstructed La-, Y-, Ce-modified MgAl-hydrotalcite as a solid base catalyst for aldol condensation: Investigation of water tolerance

MgAl-HT modified with La, Y, and Ce were evaluated in liquid-phase acetone self-aldolization as a model reaction. Calcined and rehydrated HT were characterized by XRD, N2 adsorption, IR spectroscopy, and calorimetry of CO2 adsorption. The results show that the introduction of rare earth elements (REE), of larger ionic radius than Al3+, can affect the degree of disorder of HT structure. Thus, La- and Y-modified HT exhibit larger surface areas and smaller particles sizes, resulting in higher specific amounts of medium/strong basic sites (Q>100kJ.mol−1). This is also correlated with the lower electronegativities of the REE ions, which may induce slight modifications of the electronic densities of the oxygen atoms, the Lewis basic centers. Calorimetry of CO2 adsorption indicates that the rehydrated HT exhibit basic sites in equivalent amounts but with more homogeneous basic strength distributions. The rehydrated HT catalysts show significant higher rate constants expressed per basic site. This confirms that acetone condensation proceeds faster over Brønsted basic sites compared with the Lewis ones. Over rehydrated HT, the rate constants per Brønsted basic sites are less dependent on the presence or nature of the REE, suggesting that the electronic effect of REE does not affect strongly the activity of the charge compensation anions OH−. The water tolerance of the catalysts was studied in the presence of 1wt% and 5wt% H2O in acetone, and the results show that rehydrated MgAl-HT modified with Y and La present a higher water tolerance for aldol condensation, which can be tentatively explained by their higher OH− content preferentially located on the defects of the HT platelets, where the solvation by interlayer water molecules could be limited.

Halogen bond with the multivalent halogen acting as the Lewis acid center

The halogen bond in complexes of BrF3 and BrF5 is analyzed where the trivalent and pentavalent bromine centers interact as the Lewis acid with the nitrogen Lewis base centers of HCN and N2 molecules as well as with the F−, Cl− and Br− anions. The halogen bonds analyzed here possess characteristics typical for other interactions as for example for the hydrogen bond or the halogen bond with the monovalent halogen center. However there are differences; the whole sphere of the multivalent Br-center is characterized by the positive electrostatic potential thus it acts as the Lewis acid, while the halogen monovalent atoms are usually characterized by the dual character since they may act simultaneously as the Lewis acid and as the Lewis base. The Quantum Theory of Atoms in Molecules and the Natural Bond Orbital method are applied here.