Lewis Acid Research Articles

Towards understanding the role of acidity on levulinic acid conversion using alumina-niobia mixed oxide-supported catalysts

The catalytic transformation of levulinic acid (LA) represents a pivotal advancement towards establishing a sustainable biorefinery industry. Hence, this study investigated the intricate role of the catalyst acidity, which remains relatively unexplored, in the catalytic transformation of LA. A range of alumina-niobia supported nickel-copper catalysts, with varying alumina (Al) to niobia (Nb) ratios, was synthesized and characterized. The results of characterization techniques revealed that the strength, quantity, and nature of the acidic sites of the catalysts were modified by the variations in the Al to Nb ratios. Further, a correlation between the acidic properties of the catalysts and the result of the reaction runs and poisoning experiments was established. Specifically, Lewis acid sites were responsible for the conversion of LA into angelica lactone (AL), which can be subsequently hydrogenated to gamma-valerolactone (GVL). Conversely, Brønsted acid sites are required for the ring-opening reactions of GVL which led to the formation of 2-methyltetrahydrofuran (MTHF) and valeric acid (VA). Notably, selectivities as high as 85.0% for MTHF and 67.0 % for VA were achieved with Ni-Cu/Al-Nb (6) and Ni-Cu/Nb2O5, respectively. To the best of our knowledge, this represents the highest MTHF selectivity in solvent-free catalytic conversion of LA. Additionally, based on our findings, we propose a comprehensive reaction pathway employing alumina-niobia-supported nickel-copper catalysts, shedding light on the intricate interplay of acidity in the LA conversion process. This study not only deepens our understanding of LA transformation but also holds great promise for optimizing catalytic processes in pursuit of sustainable and eco-friendly biorefinery technologies.

TiO2 modified limonite for selective catalytic reduction of NO from cement kiln flue gas with NH3

Limonite was used as a precursor and TiO2 as a modifier to synthesize TiO2/Limonite composites through the wet impregnation method. The influences of the TiO2 doping amount, the calcination temperature on the denitration efficiency and the structure and properties of composites were systematically studied. The results showed that the modified catalyst over the Ti/Fe mass ratio was 0.07 and the calcination temperature was 300°C exhibited excellent catalytic activity, good stability, and remarkable resistance to water and sulfur dioxide within reaction temperature of 100 ∼ 240°C. When NO initial concentration was 0.05 % and gas hourly space velocity (GHSV) was 36,000 h−1 and the reaction temperature was 130°C, the NO conversion efficiency reached 100 %, and the N2 selectivity was 99.6 %. When the catalyst was exposed to 10 %H2O at 150°C, it maintained a 100 % NO conversion efficiency. In the presence of 10 %H2O and 50 ppm SO2 at 200°C, the NO conversion efficiency remained at 100 %. The TiO2/Limonite catalyst possessed improved performance due to the introduction of the anatase TiO2, which led to more lattice defects, weak Lewis acid sites, and higher adsorbed oxygen mobility.

Carbon-free hydrogen production via plasma-catalytic ammonia decomposition over transition metal-based catalysts: In situ probing by DRIFTS and SVUV-PIMS

The lack of comprehensive understanding of the reaction mechanism hinders the development of plasma-catalytic ammonia (NH3) decomposition as a potential route for producing carbon-free hydrogen. Herein, we investigated the reaction process over transition metal-based catalysts in a dielectric barrier discharge (DBD) reactor. For the first time, a combination of an integrated in situ diffuse reflectance infrared Fourier transform spectroscopy-DBD (in situ DRIFTS-DBD) and Optical Emission Spectra (OES) were applied to reveal that as for Ni/Al2O3, less NH3* was observed in the gas phase and more NH3 adsorbed on the surface. Combined with online synchrotron vacuum ultraviolet-photoionization mass spectrometry (SVUV-PIMS) it was further illustrated that N2 desorbed more quickly from Ni/Al2O3 than other tested catalysts. Additionally, compared with thermal catalysis, plasma further enhanced the adsorption of NH3 on Lewis acid sites and facilitated the desorption of N2. This study provided a better understanding of the plasma-catalytic NH3 decomposition reaction.

Influence of Surface Groups on Electrochemical Properties of Molten Salt Synthesized Ti3C2Tx in Mild Aqueous Electrolytes

MXene, notable for its excellent electrical conductivity and tunable surface groups, has garnered widespread attention in the field of electrochemical energy storage. Here, Ti3C2Tx MXene was synthesized by a Lewis acid molten salt‐shielded synthesis (MS3). The surface groups (‐Cl, ‐O) were modified by washing Ti3C2Tx samples with various solutions (deionized water, 0.5 M hydrochloric acid (HCl), 0.5 M ammonium persulfate solution (APS)) and/or thermal treatments under an argon atmosphere at 300 °C, 500 °C, and 700 °C. It is shown that deionized water and HCl solution washing have minimal impact on the surface groups, while APS washing can increase the content of ‐O surface group. Conversely, thermal treatment may remove the ‐O. Electrochemical charge storage behavior of these Ti3C2Tx variants were further investigated in a 1 M acetate electrolyte buffered at pH=5.0. It is indicated that the ‐Cl surface group is electrochemically inert, whereas the ‐O may significantly improve the charge storage performance. Ti3C2Tx with high ‐O content delivered an impressive maximum capacity of 155 C g‐1. This research underscores the crucial role of surface groups on the electrochemical performance of Ti3C2Tx in mild aqueous electrolytes, offering valuable insights for future modifications and applications of Ti3C2Tx in energy storage technologies.

Promoted catalytic property of Cu/SSZ-13 by introducing a minority of Mn for NO removal from diesel engine exhaust

The Cu-exchanged SSZ-13 with the small-pore chabazite framework is considered as a highly efficient catalyst for selective catalytic reduction of NO with NH3 (NH3-SCR). In order to further improve the catalytic property, a series of Mn ion-assisted Cu/SSZ-13 powder catalysts were prepared by co-exchange method and stepwise exchange method. It is found that the NH3-SCR activity, N2 selectivity, hydrothermal stability and sulfur resistance of Cu/SSZ-13 are promoted by introducing a minority of Mn (0.15%–0.23%(mass)) through co-exchange method. Characterization results revel that the Cu,Mn co-exchange enables the higher amounts of Cu2+ active sites, the abundant medium strong and strong acid, the optimized ratio of lewis acid to Brønsted acid, etc., which are required for a good NH3-SCR catalytic property over broad temperature range and under harsh working environment. Moreover, a monolithic catalyst was prepared by impregnating a cordierite ceramic support into the coating slurry containing the optimized CuMn/SSZ-13 powder. The diesel engine bench tests show that Cu,Mn co-exchange gives the monolith catalyst a better catalytic property than commercial catalysts. This work provides an important guidance for the rational design of secondary-ion-assisted zeolites applied in NH3-SCR.

Regulation of Lewis acid sites on ZnO/SiW11Co for tunable photocatalytic activity toward water remediation

The function of surface acidity in catalytic reactions has been well studied in the field of conventional catalysis, nevertheless, regulating surface acidic sites and unveiling its roles during heterogeneous photocatalysis has been less frequently reported. To address this issue, we constructed a novel heterojunction photocatalyst of ZnO/SiW11Co with abundant Lewis acid sites for wastewater treatment. Under visible light irradiation, the optimal ZnO/SiW11Co was found to have the highest photocatalytic activity, achieving a degradation efficiency of 97.7% for Rhodamine B (RhB), 96.1% for tetracycline (TC), and 91.4% for methyl orange (MO). The RhB removal rate remained at 91.3% even after five cycles, indicating excellent stability and practicality of ZnO/SiW11Co. By compositing with SiW11Co, a typical polyoxometalates with unique super acid properties, ZnO/SiW11Co formed plentiful surface Lewis acid sites, as confirmed by the pyridine adsorption FT-IR and NH3-TPD. The as-formed Lewis acid sites could provide more active sites, making it easier for reactants to coordinate to the photocatalyst surface, which reduced the activation energy of photocatalytic reactions and accelerate RhB removal. Density functional theory (DFT) calculations unveiled the redistribution of electron-hole pairs, optimizing the light absorption and charge separation efficiency, which obeyed a Z-scheme mechanism. Combined active species capture experiments and electron spin resonance, •O2−, h+, and •OH were the main active substances for the photocatalytic degradation of RhB. Besides, the possible degradation pathways for RhB were analyzed using liquid chromatography-mass spectrometry (LC-MS). The work provided useful strategy for the design of high-performance photocatalysts via regulating surface Lewis acid sites toward water remediation.

On the synthesis of diphenhydramine: Steady state ki, solvation effects, and in-situ raman and benchtop NMR as PAT

Diphenhydramine is a first-generation antihistamine that is available over-the-counter to treat maladies such as allergic reactions and insomnia. Despite its ubiquity in society and high demand, the kinetics for either step of diphenhydramine synthesis have not been explored in detail. In this work, for the first step, a methodology has been devised to obtain kinetics from the heterogeneous halogenation of benzhydrol with hydrochloric acid in a batch reactor equipped with in-situ Raman probe and further verified with ex-situ low-field NMR spectroscopy. For the second step, the kinetics of etherification have been explored using a microfluidic flow reactor and at-line NMR spectroscopy in a variety of solvents. Arrhenius and Eyring parameters are determined for Step 2. We find that the reaction performance is well correlated with the acceptor number of the solvent indicating that the Lewis acidity of the solvent plays a significant role in the overall performance

Solvent Acidity and Basicity Scales: Analysis of Catalan’s SB and SA Scales and Gutmann’s Acceptor Number and Comparison with Kamlet and Taft’s β and α Solvent Scales, Gutmann’s Donor Number and Abraham’s B and A Solute Scales

AbstractThe use of experimental parameters to quantify solvent properties, for example in linear free energy relationships, is well established and several scales of solvent acidity, basicity and polarity/polarizability have been developed. The success of this approach raises questions of which molecular properties contribute to particular solvent parameters and whether these contributions are found in all parameters representing a particular solvent property. In the present study, Catalan’s hydrogen bond basicity and acidity parameters, SB and SA, and Gutmann’s acceptor number, AN, a measure of a solvent’s Lewis acidity, are correlated with molecular properties derived from computational chemistry. The results are compared with the results of similar correlations with Kamlet and Taft’s β and α Solvent Scales, Gutmann’s donor number DN) and Abraham’s B and A solute scales. The results show that measures of solvent basicity, SB, β and DN all correlate strongly with the partial charge on the most negative atom in the solvent molecule and the energy of the donor orbital and, in all cases, the parameter values for hydrogen-bonded solvents are anomalous. Abraham’s B, a measure of solute hydrogen basicity, depends only on the partial charge on the most negative atom and there is no anomaly in the values for solutes that, in the pure state, form hydrogen-bonded liquids. Similarly, all measures of solvent acidity, SA, α and AN, and Abraham’s A, a measure of solute hydrogen bond acidity, depend on the partial charge on the most positive hydrogen on the molecule.

IADOR yields diverse shape-selective solid Lewis acid catalysts

Lewis acid catalysts convert biomass compounds to green chemicals and fuels. Among these catalysts, stannosilicate zeolites display high thermal stability and reusability. However, only a few Sn-zeolites can be prepared using conventional synthesis methods, preventing us from developing shape-selective stannosilicate catalysts. Here, we propose a synthetic approach toward shape-selective Sn-zeolite catalysts, leveraging the Assembly, Disassembly, Organization, Reassembly (ADOR) strategy. By combining isomorphous Sn incorporation (i) into germanosilicate zeolites UOV, IWW, and IWR in the A step with structural modification in the DOR steps, we targeted new Sn-zeolite structures with either parallel or intersecting 12- and 8-ring pores predominantly containing Lewis acid centers with hydrolyzed Sn-O-Si bonds. The shape-selective performance of the designed Sn-zeolite catalysts was experimentally assessed and theoretically rationalized in transformations of citronellal into citronellol by Meerwein-Ponndorf-Verley reduction and into isopulegol isomers via intramolecular cyclization. Therefore, iADOR paves the way toward new zeolite catalysts with engineered Sn active sites for green chemistry.

Effect of sulfation on the catalytic performance of Mn-Co composite oxide catalyst for the selective catalytic reduction of NOx by NH3

The effect of sulfation on the catalytic performance of Mn-Co composite oxide catalyst for the selective catalytic reduction of NOx by NH3 (NH3-SCR) has been investigated. Different concentrations of SO2 have been used to sulfate Mn-Co composite oxide catalyst. The catalyst sulfated with 30 ppm SO2 showed the widest activity temperature window, over which 80% NOx conversion was obtained in the temperature range of 80–350 °C. The sulfation leads to the appropriate redox property of Mn-Co composite oxide, thus improving the N2 selectivity. Meanwhile, the sulfation contributes to the generation of more Brønsted acid sites, which is crucial for the enhanced activity. In situ diffuse reflectance infrared Fourier transform spectroscopy(DRIFTS) demonstrated that the adsorbed NH3 species on both Lewis acid and Brønsted acid sites, monodentate nitrite, bidentate nitrate and cis-nitrite species are the reactive intermediates in the NH3-SCR reaction. This study provides an idea for the regulating the activity temperature window of NH3-SCR catalyst to adapt to its practical application.

Engineering H2O2 Self-Supplying Platform for Xdynamic Therapies via Ru-Cu Peroxide Nanocarrier: Tumor Microenvironment-Mediated Synergistic Therapy.

Of the most common, hypoxia, overexpressed glutathione (GSH), and insufficient H2O2 concentration in the tumor microenvironment (TME) are the main barriers to the advancment of reactive oxygen species (ROS) mediated Xdynamic therapies (X = photo, chemodynamic, chemo). Maximizing Fenton catalytic efficiency is crucial in chemodynamic therapy (CDT), yet endogenous H2O2 levels are not sufficient to attain better anticancer efficacy. Specifically, there is a need to amplify Fenton reactivity within tumors, leveraging the unique attributes of the TME. Herein, for the first time, we design RuxCu1-xO2-Ce6/CPT (RCpCCPT) anticancer nanoagent for TME-mediated synergistic therapy based on heterogeneous Ru-Cu peroxide nanodots (RuxCu1-xO2 NDs) and chlorine e6 (Ce6), loaded with ROS-responsive thioketal (TK) linked-camptothecin (CPT). The Ru-Cu peroxide NDs (RCp NDs, x = 0.50) possess the highest oxygen vacancy (OV) density, which grants them the potential to form massive Lewis's acid sites for peroxide adsorption, while the dispersibility and targetability of the NDs were improved via surface modification using hyaluronic acid (HA). In TME, RCpCCPT degrades, releasing H2O2, Ru2+/3+, and Cu+/2+ ions, which cooperatively facilitate hydroxyl radical (•OH) formation and deactivate antioxidant GSH enzymes through a cocatalytic loop, resulting in excellent tumor therapeutic efficacy. Furthermore, when combined with laser treatment, RCpCCPT produces singlet oxygen (1O2) for PDT, which induces cell apoptosis at tumor sites. Following ROS generation, the TK linkage is disrupted, releasing up to 92% of the CPT within 48 h. In vitro investigations showed that laser-treated RCpCCPT caused 81.5% cell death from PDT/CDT and chemotherapy (CT). RCpCCPT in cancer cells produces red-blue emission in images of cells taking them in, which allows for fluorescence image-guided Xdynamic treatment. The overall results show that RCp NDs and RCpCCPT are more biocompatible and have excellent Xdynamic therapeutic effectiveness in vitro and in vivo.

Catalytic Hydroconversion of Lauric Acid Over Poly(N-vinyl-2-pyrrolidone)-Coated Pd Nanoparticles on ZIF-8

A subclass of Metal-Organic Frameworks, Zeolitic Imidazole Frameworks-8 (ZIF-8) is known as an emerging material that has the characteristic of a large surface area, good thermal stability as well as a high porosity. Instead of having extraordinary properties, ZIF-8 consists of Lewis acid and Lewis base site on its Zn metals and 2-methylimidazole which are the important components for the catalyst. In this study, Pd-Poly(N-vinyl-2-pyrrolidone) coated on ZIF-8 (Pd-PVP@ZIF-8) was synthesized by mixed Pd-PVP solution and ZIF-8 precursors at room temperature. The Pd-PVP solution was varied from 10 to 50 ml to differentiate the Pd concentration in ZIF-8. As-synthesized 50 ml of Pd-PVP on ZIF-8 (50Pd-PVP@ZIF-8) showed catalytic activity in the conversion of 98.6% lauric acid to produce 78.2% of 1-dodecanol at optimum condition 320 °C for 6 h. The synergy between Pd-PVP as metal and ZIF-8 as metal support as well as high dispersion of Pd particles could enhance performance in the conversion of lauric acid. Copyright © 2024 by Authors, Published by BCREC Publishing Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).

Oxygen-Vacancy-Mediated Large Binding Energy Exciton Dissociation in Nb3O7(OH) Nanorods with High Electron Mobility for CO2 Photoreduction.

Despite the excellent performance of Nb3O7(OH) in dye-sensitized solar cells and catalysis, its charge separation, transport, and structural properties remain poorly understood. Herein, the Nb3O7(OH) nanorods were prepared, and their structural characteristics, optoelectronic properties, and carrier mobility were also analyzed and investigated through a series of complex characterizations. Theoretical prediction suggested that the exciton binding energy of Nb3O7(OH) could be as high as 100.49 meV. The temperature-dependent photoluminescence (PL) of Nb3O7(OH) nanorods revealed two activation energies, and a higher proportion of long-lived components observed in the photoluminescence decay indicated effective electron trapping. That is, two energy states were present, hindering photogenerated charge recombination and promoting photocatalytic action. Current-voltage characteristics of the Nb3O7(OH) nanorod film were analyzed, revealing an ultrahigh carrier mobility of ∼310 cm2/V·s, ensuring fast and efficient electron transfer. Furthermore, Nb3O7(OH) nanorods were employed to reduce CO2, resulting in the effective production of CO and CH4. Overall, considering the presence of hydroxyl pairs on the surface of Nb3O7(OH), which facilitate the formation of the frustrated Lewis acid-base pairs and the activation of CO2, together with its effective electron trapping and charge transport, give Nb3O7(OH) nanorods a promising potential for CO2 reduction.

Comparative Selective Conversion of Biomass-Derived Mono- and Polysaccharides into Lactic Acid with Lanthanide Lewis Acid Catalysts

Comparative Selective Conversion of Biomass-Derived Mono- and Polysaccharides into Lactic Acid with Lanthanide Lewis Acid Catalysts

Bimetallic [Ge-Rh] complex catalyzed alkyne semihydrogenation: Theoretical investigations on the germanium promoted mechanism and selectivity

Transitional metal catalyzed alkynes functionalization is a very important method to obtain the organic intermediates. In this work, the mechanism of PhCCPh hydrogenation catalyzed by [Ge-Rh] bimetallic catalyst has been studied by means of density functional theory. The whole reaction consists of two cycles, cis-semihydrogenation of alkyne to give Z-alkene and trans-isomerization to get E-alkene. The rate-determining step is the hydrogen complexation in cis-semihydrogenation. Because the isomerization from Z-alkene to E-alkene is easy, while the reverse transformation is difficult, E-alkene is main product of alkyne semihydrogenation. The semihydrogenation would be facilitated by replacing the terphenyl ligands in the [Ge-Rh] bimetallic catalyst with the polar N-heterocyclic ligands. The addition of H2 is crucial in controlling the chemoselectivity of the reaction. The Ge acts as the Lewis acid to reduce the electron density on Rh, making Rh center becomes more electropositive than that in the monorhodium catalysts, thus increasing the selectivity of the [Ge-Rh] bimetallic catalyst. Our work elucidates the detailed mechanism of PhCCPh semihydrogenation catalyzed by the [Ge-Rh] catalyst and provides explanations for the experimental results.

Coordination Variations within Binuclear Copper Dioxygen-Derived (Hydro)Peroxo and Superoxo Species; Influences upon Thermodynamic and Electronic Properties.

Copper ion is a versatile and ubiquitous facilitator of redox chemical and biochemical processes. These include the binding of molecular oxygen to copper(I) complexes where it undergoes stepwise reduction-protonation. A detailed understanding of thermodynamic relationships between such reduced/protonated states is key to elucidate the fundamentals of the chemical/biochemical processes involved. The dicopper(I) complex [CuI2(BPMPO-)]1+ {BPMPOH = 2,6-bis{[(bis(2-pyridylmethyl)amino]methyl}-4-methylphenol)} undergoes cryogenic dioxygen addition; further manipulations in 2-methyltetrahydrofuran generate dicopper(II) peroxo [CuII2(BPMPO-)(O22-)]1+, hydroperoxo [CuII2(BPMPO-)(-OOH)]2+, and superoxo [CuII2(BPMPO-)(O2•-)]2+ species, characterized by UV-vis, resonance Raman and electron paramagnetic resonance (EPR) spectroscopies, and cold spray ionization mass spectrometry. An unexpected EPR spectrum for [CuII2(BPMPO-)(O2•-)]2+ is explained by the analysis of its exchange-coupled three-spin frustrated system and DFT calculations. A redox equilibrium, [CuII2(BPMPO-)(O22-)]1+ ⇄ [CuII2(BPMPO-)(O2•-)]2+, is established utilizing Me8Fc+/Cr(η6-C6H6)2, allowing for [CuII2(BPMPO-)(O2•-)]2+/[CuII2(BPMPO-)(O22-)]1+ reduction potential calculation, E°' = -0.44 ± 0.01 V vs Fc+/0, also confirmed by cryoelectrochemical measurements (E°' = -0.40 ± 0.01 V). 2,6-Lutidinium triflate addition to [CuII2(BPMPO-)(O22-)]1+ produces [CuII2(BPMPO-)(-OOH)]2+; using a phosphazene base, an acid-base equilibrium was achieved, pKa = 22.3 ± 0.7 for [CuII2(BPMPO-)(-OOH)]2+. The BDFEOO-H = 80.3 ± 1.2 kcal/mol, as calculated for [CuII2(BPMPO-)(-OOH)]2+; this is further substantiated by H atom abstraction from O-H substrates by [CuII2(BPMPO-)(O2•-)]2+ forming [CuII2(BPMPO-)(-OOH)]2+. In comparison to known analogues, the thermodynamic and spectroscopic properties of [CuII2(BPMPO-)] O2-derived adducts can be accounted for based on chelate ring size variations built into the BPMPO- framework and the resulting enhanced CuII-ion Lewis acidity.

Catalytic 1,1-diazidation of alkenes.

Compared to well-developed catalytic 1,2-diazidation of alkenes to produce vicinal diazides, the corresponding catalytic 1,1-diazidation of alkenes to yield geminal diazides has not been realized. Here we report an efficient approach for catalytic 1,1-diazidation of alkenes by redox-active selenium catalysis. Under mild conditions, electron-rich aryl alkenes with Z or E or Z/E mixed configuration can undergo migratory 1,1-diazidation to give a series of functionalized monoalkyl or dialkyl geminal diazides that are difficult to access by other methods. The method is also effective for the construction of polydiazides. The formed diazides are relatively safe by TGA-DSC analysis and impact sensitivity tests, and can be easily converted into various valuable molecules. In addition, interesting reactivity that geminal diazides give valuable molecules via the geminal diazidomethyl moiety as a formal leaving group in the presence of Lewis acid is disclosed. Mechanistic studies revealed that a selenenylation-deselenenylation followed by 1,2-aryl migration process is involved in the reactions, which provides a basis for the design of new reactions.

Surface Thermodynamic Properties of Poly Lactic Acid by Inverse Gas Chromatography

Poly lactic acid (PLA) is one of the most commonly used bio-derived thermoplastic polymers in 3D and 4D printing applications. The determination of PLA surface properties is of capital importance in 3D/4D printing technology. The surface thermodynamic properties of PLA polymers were determined using the inverse gas chromatography (IGC) technique at infinite dilution. The determination of the retention volume of polar and non-polar molecules adsorbed on the PLA particles filling the column allowed us to obtain the dispersive, polar, and Lewis’s acid–base surface properties at different temperatures from 40 °C to 100 °C. The applied surface method was based on our recent model that used the London dispersion equation, the new chromatographic parameter function of the deformation polarizability, and the harmonic mean of the ionization energies of the PLA polymer and organic molecules. The application of this new method led to the determination of the dispersive and polar free surface energy of the adsorption of molecules on the polymeric material, as well as the glass transition and the Lewis acid–base constants. Four interval temperatures were distinguished, showing four zones of variations in the surface properties of PLA as a function of the temperature before and after the glass transition. The acid–base parameters of PLA strongly depend on the temperature. The accurate determination of the dispersive and polar surface physicochemical properties of PLA led to the work of adhesion of the polar organic solvents adsorbed on PLA. These results can be very useful for achieving reliable and functional 3D and 4D printed components.

Electrografting of aryl amines on graphitic surfaces: An alternative to diazonium salts?

Aryl amines have recently been shown to be effectively grafted with the use of Lewis bases. Hence, they have been proposed as a potential alternative to aryl diazonium salts for the covalent modification of surfaces. In this work we compare grafting efficiencies of aryl amines, such as 4-bromoaniline (4-BA), with aryl diazonium salts, such as 4-bromobenzene diazonium (4-BBD), on carbon-based surfaces, namely highly oriented pyrolytic graphite (HOPG) and graphene. The Lewis base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) was employed to improve the grafting of 4-BA to the surfaces. The systems were studied with a range of techniques to confirm grafting of the organic molecules to the carbon surfaces, such as cyclic voltammetry, atomic force microscopy, scanning tunnelling microscopy and Raman spectroscopy. Density functional theory calculations demonstrate that the C-C bond is more favoured than the C-N bond. The grafting density of the aryldiazonium salts on graphite and graphene surfaces is much higher compared to aryl amines with or without the use of a Lewis base. Consequently, selection of the appropriate precursor for efficient covalent functionalization of a surface is required.

Hydroconversion of lignin-derived platform compound guaiacol to fuel additives and value-added chemicals over alumina-supported Ni catalysts

Hydroconversion of guaiacol (GUA) over γ-Al2O3 and phosphatized-γ-Al2O3 (γ-Al2O3(P)) supported Ni catalysts was initiated by Lewis-sites and active Ni sites. Conversion proceeded via transmethylation and hydrodemethylation/hydrodemethoxylation as major and minor pathways, respectively, resulting in mainly catechol and methylcatechols, and via series of consecutive hydrodehydroxylation (HDHY) and ring hydrogenation (HYD) reactions leading to partially and fully deoxygenated, saturated, and unsaturated products. High Ni-loading and high H2 pressure promoted the formation of cyclohexane and methyl-substituted cyclohexanes; however, above 300 °C the hydrogenation–dehydrogenation equilibrium favored the formation of benzene and methyl-substituted benzenes. Ni/γ-Al2O3(P) showed suppressed HYD/HDHY activity resulting in pronounced formation of catechol and/or phenol and their methyl-substituted derivatives. Surface phenolate species were substantiated as surface intermediates of hydrodeoxygenation. Phosphatizing reduced the concentration of both basic OH and Lewis acid (Al+) – Lewis base (O–) pair surface sites of the γ-Al2O3 support and, thereby, suppressed phenolate formation and hydrodeoxgenation.