Transesterification of Dimethyl Carbonate with Ethanol Catalyzed by Guanidine: A Theoretical Analysis.

Density-functional theory (DFT) was performed to investigate the mechanistic features of different guanidine-based catalysts, namely, 1,1,3,3-tetramethyl guanidine (TMG) and 1,5,7-triaza-bicyclo-[4.4.0]dec-5-ene (TBD), for the transesterification reaction of dimethyl carbonate (DMC) with ethanol (EtOH). Different possible pathways were suggested in which these catalysts act as either nucleophile or base within a homogeneous system. The DFT results allowed not only the study of the thermochemistry aspects of all elementary reactions featured in the two different activation modes but also the accurate calculation of the free energy barriers for each case. Our findings showed that the catalyzed reaction proceeded through simultaneous activation of DMC and EtOH, facilitated by hydrogen bonding for both catalysts. This feature led to the formation of a stable intermediate with a relatively low free energy barrier. TBD exhibited a potentially more efficient mechanism, owing to its planar structure and dual-activation mode. The free energy barrier of the rate-limiting step, identified as the formation of a zwitterionic complex, then declined by approximately 50% when compared with the reaction without catalysts. Overall, the DFT approach provides good insight into the reactivity of both catalysts and helps to find possibilities for further enhancing the mechanistic features of both catalysts for this type of transesterification reaction.

The effect of biochar mild air oxidation on the optimization of lead(II) adsorption from wastewater.

In the present study, the effects of mild air oxidation of a biochar produced by the Pyrovac Inc. pyrolysis process, on the adsorption of lead(II) from synthetic wastewater under batch experimental conditions have been investigated. The adsorption experiments were performed under several conditions suggested by the response surface methodology, which allowed finding the optimal conditions, in order to maximize the adsorption capacity (Q(mgg-1)), as well as the extraction efficiency (E (%)). The optimal conditions of lead ions adsorption were as follows: pH = 5, agitation time = 300 min, adsorbent mass = 0.5 g (per 50 cm3 of solution), and lead initial concentration = 100gm-3, resulted in an adsorption capacity of 7.9 mg g-1. Equilibrium adsorption was then obtained by keeping pH and adsorbent mass at the optimal values and changing the lead initial concentration for a sufficient agitation time. Results showed that mild air oxidation increased the equilibrium adsorption capacity of biochar from 2.5 to 44 mg g-1. Oxidized biochar after equilibrium adsorption was submitted to SEM/EDX and XPS analysis. From SEM it was found that lead particles were distributed heterogeneously after adsorption. From XPS analysis, it was revealed that the external surface of oxidized biochar particles becomes saturated for the initial point of equilibrium diagram, obtained at lead initial concentration of 100gm-3, suggesting that for a higher concentration, the internal surfaces of particles participate in the cations adsorption. The participation of surface functional groups in the adsorption process showed that carbonyl, carboxylic, and aromatic rings of oxidized biochar were involved in the adsorption. This work suggests that the very simple process of mild air oxidation can be used instead of the usual costly chemical activation, in order to improve biochar cation exchange capacity.

Eugenol Hydrodeoxygenation Over Mixed Mo-W Carbides.

The modification of molybdenum carbide catalysts by another transition metal has raised an increasing research interest due to the significant improvement of catalyst activity in hydrodeoxygenation of lignin derivatives. At par with the commonly used Co and Ni that add a strong hydrogenation functionality, it was found that the addition of the more oxophilic W restricts ring hydrogenation while allowing the deoxygenation of oxygenated compounds and thus yielding higher selectivity toward the formation of non-oxygenated aromatic compounds. The coexistence of Mo2C with W2C along with metallic W altered the electronic properties of Mo2C which resulted in an increase of catalyst active site density and facilitated further total eugenol deoxygenation. Propyl-benzene selectivity of up to 83 % was reached at close to 100 % eugenol conversion. These findings will allow a better overview of the effect of different metal phases of mixed carbides on the catalyst performance and raise the prospect of optimizing catalyst design for a hydrodeoxygenation processing of lignin depolymerization products.

A general chelate-assisted co-assembly to metallic nanoparticles-incorporated ordered mesoporous carbon catalysts for Fischer-Tropsch synthesis.

The organization of different nano objects with tunable sizes, morphologies, and functions into integrated nanostructures is critical to the development of novel nanosystems that display high performances in sensing, catalysis, and so on. Herein, using acetylacetone as a chelating agent, phenolic resol as a carbon source, metal nitrates as metal sources, and amphiphilic copolymers as a template, we demonstrate a chelate-assisted multicomponent coassembly method to synthesize ordered mesoporous carbon with uniform metal-containing nanoparticles. The obtained nanocomposites have a 2-D hexagonally arranged pore structure, uniform pore size (~4.0 nm), high surface area (~500 m(2)/g), moderate pore volume (~0.30 cm(3)/g), uniform and highly dispersed Fe(2)O(3) nanoparticles, and constant Fe(2)O(3) contents around 10 wt %. By adjusting acetylacetone amount, the size of Fe(2)O(3) nanoparticles is readily tunable from 8.3 to 22.1 nm. More importantly, it is found that the metal-containing nanoparticles are partially embedded in the carbon framework with the remaining part exposed in the mesopore channels. This unique semiexposure structure not only provides an excellent confinement effect and exposed surface for catalysis but also helps to tightly trap the nanoparticles and prevent aggregating during catalysis. Fischer-Tropsch synthesis results show that as the size of iron nanoparticles decreases, the mesoporous Fe-carbon nanocomposites exhibit significantly improved catalytic performances with C(5+) selectivity up to 68%, much better than any reported promoter-free Fe-based catalysts due to the unique semiexposure morphology of metal-containing nanoparticles confined in the mesoporous carbon matrix.

Stepwise and Microemulsions Epoxidation of Limonene by Dimethyldioxirane: A Comparative Study.

Limonene dioxide is recognized as a green monomer for the synthesis of a wide variety of polymers such as polycarbonates, epoxy resins, and nonisocyanate polyurethanes (NIPU). The developed green technologies for its synthesis over heterogeneous catalysts present a challenge in that the selectivity of limonene dioxide is rather low. Homogeneous epoxidation in the presence of dimethyldioxirane for limonene dioxide synthesis is a promising technology. This study reports the epoxidation of limonene by dimethyldioxirane (DMDO) using two approaches. The isolated synthesis of DMDO solution in acetone was followed by epoxidation of limonene in another reactor in 100% organic phase (stepwise epoxidation). Following this procedure, limonene dioxide could be produced with almost 100% conversion and yield. A second approach allowed using in situ generated in aqueous-phase DMDO to epoxidize the limonene forming a microemulsion with a solubilized surfactant in the absence of any organic solvent. The surfactants tested were hydrosulfate (CTAHS), bromide (CTAB), and chloride (CTAC) cetyltrimethylammonium. All these surfactants showed good stability of microemulsions at aqueous surfactant concentrations above their critical micellar concentrations (CMC). Stability is obtained at the lowest concentration when using CTAHS because of its very low CMC compared to CTAB and CTAC. The major advantages of epoxidation in microemulsions compared to DMDO stepwise epoxidation are the absence of an organic solvent (favoring a low reaction volume) and the very high oxygen yield of 60 to 70% versus 5% in a stepwise approach. The epoxides formed are easily separated from the aqueous medium and the surfactant by liquid-liquid extraction. Therefore, the developed in situ epoxidation process is a green technology conducted under mild conditions and convenient for large-scale applications.

Multivariate metal-organic framework MTV-MIL-101 via post-synthetic cation exchange: is it truly achievable?

The post-synthetic exchange (PSE) method is a well-proven route to replace, modify, and add different functionalities to metal-organic frameworks (MOFs). Particularly, the solvent-assisted cation substitution (SACS) technique has been reported to prepare mixed-metal multivariate metal-organic frameworks (MTV-MOFs). However, such a technique does not apply to all types of MOFs. In 2013, Szilágyi et al. reported the achievement of the mixed-metal MTV-MIL-101 framework via PSE. Since then, a debate has been taking place about the validity of these findings. On the other hand, the attainment of the mixed-metal MIL-101 was reported to be obtainable through the direct synthesis, which is, to some, the only way to achieve it. Here, we settle this dispute by investigating Szilágyi's method not only as described, but also at extended conditions of time and different metal precursors: all attempts were vain. However, by reconsidering the refluxing solvent (dimethylformamide "DMF" instead of water) and the applied reaction conditions (110 °C-20 h), mixed-metal MIL-101(Cr/Fe) was achieved via a simple PSE method.

Facile solvothermal synthesis of a MIL-47(V) metal-organic framework for a high-performance Epoxy/MOF coating with improved anticorrosion properties.

The vanadium-based metal-organic framework MIL-47 distinguishes itself among other MOFs for its distinctive structure and unique properties (e.g., flexible structure, high thermal stability, and high surface area). The synthesis of MIL-47 has been reported from various metal precursors, including vanadium(iii) chloride (VCl3) as a rich source of metal ions. Attempts have been made to include other starting materials, a step forward towards large-scale production. Synthesis from various solid materials is encouraged, seeking an economic and greener approach. In this study, vanadium pentoxide (V2O5), a readily abundant low-cost and thermodynamically stable metal source, was used to synthesize the MIL-47(V) framework via a facile solvothermal route. This precursor provides a controllable rate of metal ion production depending on the applied reaction conditions. In our method, the synthesis took place at a low temperature and reaction time (180 °C for 20 h, instead of 220 °C for 72 h), yielding MIL-47 microrods. Moreover, among its unique properties, the metal centers of MIL-47 oxidize under the influence of thermal or chemical treatments, preserving the framework structure. This unusual character is not commonly witnessed in comparable MOF structures. This property can be leveraged in anti-corrosion applications, whereby a redox reaction would sacrifice the framework components, protecting the metal in contact. However, the chemical stability of MIL-47 is doubted against a corrosive medium. Thus, an epoxy coating with 10 wt% MOF loading was incorporated in our investigation to extend the aluminum alloy (AA2024) surface protection for prolonged exposure duration. The uniformity of distribution of the prepared MOF within the epoxy matrix was confirmed using SEM/EDX. Electrochemical impedance spectroscopy (EIS) was used to evaluate the corrosion performance of the coated samples. The results showed that the inclusion of V-MOF offers extended corrosion prevention, over 60 days, for the AA2024 alloy against artificial seawater. The neat epoxy coating could not prevent the corrosion of AA2024 over two weeks of immersion, whereby pitting corrosion was clearly observed. The V-MOF could induce a series of redox reactions leading to the precipitation of vanadium on the cathodic sites of metal surfaces.

Proton exchange membranes for application in fuel cells: grafted silica/SPEEK nanocomposite elaboration and characterization.

Hydrogen technologies and especially fuel cells are key components in the battle to find alternate sources of energy to the highly polluting and economically constraining fossil fuels in an aim to preserve the environment. The present paper shows the synthesis of surface functionalized silica nanoparticles, which are used to prepare grafted silica/SPEEK nanocomposite membranes. The nanoparticles are grafted either with hexadecylsilyl or aminopropyldimethylsilyl moieties or both. The synthesized particles are analyzed using XRD, NMR, TEM, and DLS to collect information on the nature of the particles and the functional groups, on the particle sizes, and on the hydrophilic/hydrophobic character. The composite membranes prepared using the synthesized particles and two SPEEK polymers with sulfonation degrees of 69.4% and 85.0% are characterized for their proton conductivity and water uptake properties. The corresponding curves are very similar for the composites prepared with both polymers and the nanoparticles bearing the two functional groups. The composites prepared with the nanoparticles bearing solely the aminopropyldimethylsilyl moiety exhibit lower conductivity and water uptake, possibly due to higher interaction of the polymer sulfonic acid sites with the amine groups. The composites prepared with the nanoparticles bearing solely the hexadecylsilyl moiety were not further investigated because of very high particles segregation. A study of the proton conductivity as a function of temperature was performed on selected membranes and showed that nanocomposites made with nanoparticles bearing both functional moieties have a higher conductivity at higher temperatures.

Plasmonic Photocatalysts for Sunlight-Driven Reduction of CO2 : Details, Developments, and Perspectives.

Plasmonic photocatalysis is among the most efficient processes for the photoreduction of CO2 into valuable fuels. The formation of localized surface plasmon resonance (LSPR), energy transfer, and surface reaction are the significant steps in this process. LSPR plays an essential role in the performance of plasmonic photocatalysts as it promotes an excellent, light absorption over a broad wavelength range while simultaneously facilitating an efficient energy transfer to semiconductors. The LSPR transfers energy to a semiconductor through various mechanisms, which have both advantages and disadvantages. This work points out four critical features for plasmonic photocatalyst design, that is, plasmonic materials, size, shape of plasmonic nanoparticles (PNPs), and the contact between PNPs and semiconductor. Various developed plasmonic photocatalysts, as well as their photocatalytic performance in CO2 photoreduction, are reviewed and discussed. Finally, perspectives of advanced architectures and structural engineering for plasmonic photocatalyst design are put forward with high expectations to achieve an efficient CO2 photoreduction shortly.

Synthesis of g-C3 N4 Nanosheets by Using a Highly Condensed Lamellar Crystalline Melamine-Cyanuric Acid Supramolecular Complex for Enhanced Solar Hydrogen Generation.

A highly condensed lamellar melamine-cyanuric acid supramolecular (MCS) complex was synthesized in an autoclave at high pressure as a precursor for preparing g-C3 N4 nanosheets. Given the distinctive properties of the prepared MCS complex, an efficient g-C3 N4 nanosheet photocatalyst can be obtained by heat treatment of this MCS complex under Ar followed by calcination in air at 400 °C. The resulting nanosheets with in-plane nanoholes showed an extremely high specific surface area (≈270 m2 g-1 ) and significantly enhanced light absorption in the visible region. This phenomenon is observed for the first time in carbon nitride nanosheets. The enhanced light absorption results from the sizeable conjugated system of tri-striazine units in the carbon nitride framework, coupled with the structural defects arising from the presence of oxygen-containing groups induced during the synthesis. Consequently, the obtained carbon nitride nanosheets exhibited excellent performance for hydrogen generation under sunlight and especially under visible light. Its quantum efficiency (QE) of 20.9 % at 420 nm is one of the highest reported values for carbon nitride materials. A QE of 3.5 % could be observed even at 590 nm. The integrated QE of this material in the visible region (420-600 nm) is approximately 1 %. To the best of our knowledge this is the highest value compared to all other the carbon nitride nanosheet materials reported previously.

Hydrogenolysis of C-O Chemical Bonds of Broad Scope Mediated by a New Spherical Sol-Gel Catalyst.

The new spherical sol-gel hybrid material SiliaCat Pd0 selectively mediates the hydrogenolysis of aromatic alcohols, aldehydes, and ketones by using an ultralow catalytic amount (0.1 mol % Pd) under mild reaction conditions. The broad reaction scope as well as the catalyst's superior activity and pronounced stability open the route to green and convenient reductive deoxygenation processes of primary synthetic relevance in chemical research as well as in the fine chemical and petrochemical industries.

Impedance Analysis of Polyaniline in Comparison with Some Conventional Solid Electrolytes.

Doped polyaniline (PANI) is well-known as an electronic (polaronic) conductor and mostly is used as semiconductor in various applications. However, in the literature there are examples of employment of the acid doped form of PANI as electrolytic filler in proton exchange membranes. In order to distinguish between two types of conduction, in the present study powdered samples of polyaniline, either in the form of emeraldine base (PANI-EB) or in the form doped with camphorsulfonic acid (PANI-CSA), were investigated using impedance spectroscopy both in the dry state and in contact with liquid water. The obtained spectra were compared with the spectra of such conventional solid electrolytes, as zeolites X and ZSM5 and a strong electrolyte boron orthophosphate, acquired in identical conditions. The most important dissimilarity between conventional electrolytes and PANI was that ion diffusion dominates in the impedance response of the formers, whereas the behavior of PANI is under control of electron/hole displacement and the diffusion part is quite inessential. This corroborates the results of analysis of temperature dependence of PANI conductivity, which revealed values of activation energy twice as large as typical solid electrolytes. Equivalent circuits, simulating the impedance responses of all materials, were built up and used to estimate a possible diffusion coefficient of cations in the comparable solids. It was found that the diffusion in a strong electrolyte such as BPO4 is ∼2 orders of magnitude faster than evaluated for zeolites and ∼4 orders higher than what was PANI estimation. A conclusion was made that the slow cation diffusion both in protonated and in base form of PANI makes them less efficient solid electrolytes than conventional materials.

Solvent-Free Chemoselective Hydrogenation of Squalene to Squalane.

Squalene is selectively and entirely converted into squalane over the spherical sol-gel-entrapped Pd catalyst SiliaCat Pd(0) under solvent-free and mild reaction conditions of 3 bar H2 and 70 °C. The catalyst was reused successfully in eight consecutive cycles, with palladium leaching values <2 ppm, opening the route to sustainable and less-expensive hydrogenation of phytosqualene with important sustainability consequences.

A new route to the shape-controlled synthesis of nano-sized γ-alumina and Ag/γ-alumina for selective catalytic reduction of NO in the presence of propene.

We report a new route for the direct synthesis of γ-alumina nanocrystals with size and shape control in the presence of oleylamine as the capping agent. Their morphology can be controlled from nanospheres to nanorods by simply tuning a proper amount of concentrated nitric acid (67%) in the synthetic mixture. The as-made nanoparticle products after calcination show γ-alumina nano-size with unique porosity and high specific surface area and retained morphology. The XRD patterns of these calcined samples exhibit broad diffraction lines which are characteristic of nanocrystal size of γ-alumina. This synthesis procedure has been extended to the one-pot synthesis of nano-alumina based Ag catalysts with spherical and rod-shaped nano-alumina morphologies. Selective catalytic reduction (SCR) of NO with C3H6 over these catalysts was investigated. The results were compared to those of the conventional Ag/γ-Al2O3 and γ-nanoalumina alone. These nano-alumina based Ag catalysts exhibit excellent NO reduction activity in the presence of C3H6. Even in the presence of large oxygen concentration (15%), N2 yields as high as ∼90% at quite low temperature (∼350°C) have been achieved. The significantly high catalytic activity of this new type of nanocatalysts can also be attributed to their high surface area and good dispersion of silver species in the alumina matrix as well as the synergism and new properties that arise at the silver-nanoalumina interface.

Intercalation/Exfoliation mechanism of hybrid formation in polypropylene/lamellar mesostructured silica nanocomposites.

Understanding the optimal processing conditions for the fabrication of polymer nanocomposites is of fundamental importance in designing materials with balance of properties. To understand these conditions in the case of maleic anhydride grafted polypropylene (PP-g-MA)/layered mesostructured silica (LMS) nanocomposites, the effect of temperature, shear rate, and residence time during processing on the structure of the nanocomposites were studied. The results showed that the combination of temperature, residence time, and mechanical shears have strong effect on the structure of the nanocomposites, rather than just interfacial interactions between the polymer matrix and silicate layers. However, interfacial interactions between the polymer matrix and silicate layers primarily play an important role to the intercalation of polymer chains into the silicate galleries. On the basis of our experimental results, a first explanation of the formulation mechanism of PP-g-MA/LMS nanocomposites is proposed. Finally, a general concept of processing conditions for manufacturing of polymer nanocomposites by melt-compounding process in a batch-mixer is described.

Sorption of Water/Methanol on Teflon and Hydrocarbon Proton Exchange Membranes.

The sorption of water and methanol droplets on Teflon films, as well as on various representative classes of hydrocarbon-based proton exchange membranes (PEMs) was investigated using contact angle measurement (drop shape method) during wetting under ambient open-air conditions. Teflon films exhibited constant hydrophobic surfaces when contacted with water, but a significant sorption of methanol. The PEMs showed slow sorption of water, and a significant sorption of methanol. The differences in sorption of water and methanol on Teflon and PEMs arose from the match/compatibility in the surface free energies as well as polarities between a liquid and a membrane. The significant discrepancies in surface free energies and polarities between water (72.0 mJ m(-2) and 70.1%, respectively) and Teflon film (14.0 mJ m(-2) and 4.9%, respectively) lead to a highly hydrophobic surface and no discernible sorption of water on Teflon films, while the relative similarity or minor discrepancy in surface free energies and polarities between methanol (22.5 mJ m(-2) and 17.0%, respectively) and Teflon film (14.0 mJ m(-2) and 4.9%, respectively) results in a significant sorption of methanol on Teflon. The surface free energies of PEMs were calculated using the harmonic-mean approach, based on contact angle measurements using both water and diiodomethane as probes. The results show that PEMs have initial surface free energies ranging from 44.1 to 54.0 mJ m(-2) along with polarities in the range of 20.8 to 29.1%, for a selection of typical sulfonated polymers. The surface free energies of ionomers were principally contributed to by the nonpolar component, but the presence of polar groups in the polymer increased the polar component, leading to an increase in surface free energy. Of the PEMs investigated, sulfonated poly(aryl ether ether nitrile) has a higher surface energy than those of other ionomers with similar sulfonate contents. The compatibility between water/methanol and PEMs was investigated on the aspect of surface free energies. The present study provides a plausible strategy to prescreen potential PEMs and optimize membrane electrode assembly (MEA) fabrication.

Tuning surface hydrophilicity/hydrophobicity of hydrocarbon proton exchange membranes (PEMs).

The effect of annealing on the surface hydrophilicity of various representative classes of hydrocarbon-based proton exchange membranes (PEMs) is investigated. In all cases, a more hydrophilic membrane surface develops after annealing at elevated temperatures. The annealing time also had some influence, but in different ways depending on the class of PEM. Longer annealing times resulted in more hydrophilic membrane surfaces for copolymerized sulfonated poly(ether ether ketone) (SPEEK-HQ), while the opposite behavior occurred in sulfonated poly(aryl ether ether ketone) (Ph-SPEEK), sulfonated poly(aryl ether ether ketone ketone) (Ph-m-SPEEKK) and sulfonated poly (aryl ether ether nitrile) (SPAEEN-B). Increased surface hydrophilicity upon annealing results from ionic cluster decomposition, according to the "Eisenberg-Hird-Moore model" (EHM). The increased surface hydrophilicity is supported by contact angle (CA) measurements, and the cluster decomposition is auxiliarily supported by probing the level of atomic sulfur (sulfonic acid) within different surface depths using angle-dependent XPS as well as ATR-FTIR. Membrane acidification leads to more hydrophilic surfaces by elimination of the hydrogen bonding that occurs between strongly-bound residual solvent (dimethylacetamide, DMAc) and PEM sulfonic acid groups. The study of physicochemical tuning of surface hydrophilicity/hydrophobicity of PEMs by annealing and acidification provides insights for improving membrane electrode assembly (MEA) fabrication in fuel cell (FC).

Zeolitic Core@Shell Adsorbents for the Selective Removal of Free Glycerol from Crude Biodiesel.

Selective adsorption of free glycerol from crude biodiesel was investigated by using mesoporous silica spheres coated with a thin shell of microporous silicalite-1. A polycrystalline silicalite-1 shell was formed upon first covering the external surfaces of various core templates with discrete silicalite-1 nanocrystals, and this was followed by short hydrothermal treatment to ensure shell uniformity. Batch glycerol adsorption experiments were conducted to evaluate the ability of the sorbents to remove free glycerol selectively from crude biodiesel mixtures at various temperatures, also in comparison to that of conventional sorbents, for example, bare mesoporous silica gel spheres and zeolites. The silicalite-1 shell provided a microporous membrane that hindered the diffusion of fatty acid methyl esters into the mesopores of the composite sorbent, whereas the large pore volume of the mesoporous core enabled multilayer glycerol adsorption; this ultimately substantially enhanced the performance in terms of purification yield and adsorption capacity.