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

Zeolite-Coated Mesostructured Cellular Silica Foams

ZSM-5- and NaY-coated MCF materials as new acid catalysts for conversion of bulky molecules were prepared by the coating procedure using diluted clear zeolite gel solutions. The resulting materials have high acidity and improved steam stability as compared to that of the corresponding MCF aluminosilica. This could be due to the zeolitic nature and relative hydrophobicity of the mesopore surface

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

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

Novel Polymer Nanocomposites from Templated Mesostructured Inorganic Materials

Novel Polymer Nanocomposites from Templated Mesostructured Inorganic Material

Nanocast LaNiO₃ Perovskites as Precursors for the Preparation of Coke-Resistant Dry Reforming Catalysts

Dry reforming of methane is gaining great interest owing to the fact that this process efficiently converts two greenhouse gases (CH4 and CO2) into synthesis gas (CO + H2), which can be further processed into liquid fuels and chemicals. Herein, a perovskite-derived nanostructured Ni/La2O3 material is reported as an efficient and stable catalyst for this reaction. High-surface-area LaNiO3 perovskite precursor is first synthesized by the method of nanocasting using ordered mesoporous silica SBA-15 as a hard template. The resulting nanostructured perovskite was found to possess high specific surface area as obtained from the BET method (150 m2 g–1). The reduction behavior of the nanocast perovskite was monitored by performing the temperature-programmed reduction of hydrogen (TPR-H2). It has been found that the complete destruction of perovskite structure occurs below 700 °C, leading to the formation of highly dispersed Ni0 in La2O3, as observed in the XRD pattern of the material after reduction. Similar behavior was observed for the LaNiO3 perovskite synthesized using the conventional citrate process. However, the specific surface area of the former material was found to be much higher than that of the latter (50 m2 g–1), which obviously resulted from the mesoporous architecture of the nanocast LaNiO3. It was found that the nanostructured Ni/La2O3 obtained from the reduction of the nanocast LaNiO3 exhibited high activity for the conversion of the reactant gases (CH4 and CO2) compared to the catalyst obtained from conventional perovskite, under the reaction conditions used in the present study. Particularly, no coke formation was observed for the mesoporous catalyst under the present conditions of operation, which in turn reflects the enhanced stability of the catalyst obtained from the nanocast LaNiO3. The improved performance of the nanostructured catalyst is attributed to the accessibility of the active sites resulting from the high specific surface area and the confinement effect leading to the stabilization of Ni nanoparticles

Amine Grafted Silica/SPEEK Nanocomposites as Proton Exchange Membranes

This work presents the elaboration of porous silica nanospheres, eventually amine functionalized, which are used as the inorganic filler in mixed matrix silica/SPEEK membranes. The surface of the silica nanoparticles is modified by grafting (3-aminopropyl)dimethylethoxysilane (APDMS). The two sets of nanocomposite membranes obtained at varying silica loadings are characterized for their proton conductivity and water uptake properties. At higher degrees of sulfonation, some cross-linking due to the interaction of the amine groups of the silica with the sulfonic acid groups of the SPEEK polymer is attested by the water uptake reduction between the composites made with amine grafted or pristine silica particles. However, even in these conditions the proton conductivity of the mixed matrix membrane is not essentially different in the two sets of nanocomposites. This indicates that the inorganic filler effect on proton conductivity is related to changes in the microstructure of the water channels in the polymer lattice

Selective Fragmentation through C–N Bond Cleavage of Carbon Nitride Framework for Enhanced Photocatalytic Hydrogen Production

A simple, practical approach for the structural modification of bulk g-C3N4 employing high-pressure NH3 and H2O formed by the polycondensation of urea is reported. The high-pressure processes the planarization of carbon nitride sheets that is disruptive because of structural distortion or defects, thus creating non-crystalline lines with highly reactive carbon species. The reaction of these carbon species with NH3 leads to highly selective and oriented fragmentation of the carbon nitride framework, which is entirely different from previous reports, producing nanofragments with a very small density of defects. The high pressure proceeds the sheet planarization and the structural condensation of nanofragments, resulting in very high crystallinity. The fragmentation also creating a high concentration of functional groups (−NH2 and −OH) on the edge of C3N4 sheets with a suitable proportion, constructing a large optimized hydrogen-bond network across intra- and interplanes that further enhance the crystallinity of the formed nanofragments. The high crystallinity, especially the strong planarization of carbon nitride sheets, significantly speeds up the charge separation and transfer, while the functional groups on the edge of sheets result in an excellent charge drive. Also, these groups simultaneously shift the conduction band to a higher level and improve proton adsorption and activation. As such, the as-prepared nanofragment photocatalyst exhibits a photocatalytic hydrogen production rate that is nearly five times increased, as compared to that of the bulk g-C3N4, with a high quantum efficiency of 12.3% at 420 nm

Role of Metal–Support Interactions, Particle Size, and Metal–Metal Synergy in CuNi Nanocatalysts for H₂ Generation

Efficient bimetallic nanocatalysts based on non-noble metals are highly desired for the development of new energy storage materials. In this work, we report a simple method for the synthesis of highly dispersed CuNi catalysts supported on mesoporous carbon or silica nanospheres using low-cost metal nitrate precursors. The mesoporous carbon-supported Cu_0.5Ni_0.5 nanocatalysts exhibit excellent catalytic performance for the hydrolysis of ammonia borane and decomposition of hydrous hydrazine with 100% hydrogen selectivity in aqueous alkaline solution at 60 °C. The chemical composition and size of the metal particles, which have a significant influence on the catalytic properties of the supported bimetallic CuNi materials, can readily be controlled by adjusting the metal loading and ratio of metal precursors. An exceedingly high turnover frequency of 3288 (mol_H₂ mol_metal^–1h^–1) and complete reaction within 1 min in dehydrogenation of ammonia-borane were achieved over a tailored-made catalyst obtained through precise monitoring of metal particle size, composition, and support properties

Development of Sinter-Resistant Core–Shell LaMn_<i>x</i>Fe_1–<i>x</i>O₃@mSiO₂ Oxygen Carriers for Chemical Looping Combustion

This work investigates the possibility of using LaMn_0.7Fe_0.3O_3.15@mSiO₂ as oxygen carriers for chemical looping combustion (CLC). CLC is a new combustion technique with inherent separation of CO₂ from atmospheric N₂. LaMn_0.7Fe_0.3O_3.15@mSiO₂ core–shell materials were prepared by coating a layer of mesostructured silica around the agglomerated perovskite particles. The oxygen carriers were characterized using different methods, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N₂ sorption, hydrogen temperature-programmed reduction (H₂-TPR), and temperature-programmed desorption of oxygen (TPD-O₂). The reactivity and stability of the carrier materials were tested in a special reactor, allowing for short contact time between the fluidized carrier and the reactive gas [Chemical Reactor Engineering Centre (CREC) fluidized riser simulator]. Multiple reduction–oxidation cycles were performed. TEM images of the carriers showed that a perfect mesoporous silica layer was formed around samples with 4, 32, and 55 nm in thickness. The oxygen carriers having a core–shell structure showed higher reactivity and stability during 10 repeated redox cycles compared to the LaMn_0.7Fe_0.3O_3.15 core. This could be due to a protective role of the silica shell against sintering of the particles during repeated cycles under CLC conditions. The agglomeration of the particles, which occurred at high temperatures during CLC cycles, is more controllable in the core–shell-structured carriers, as confirmed by SEM images. XRD patterns confirmed that the crystal structure of all perovskites remained unchanged after multiple redox cycles. Methane conversion and partial conversion to CO₂ were observed to increase with the contact time between methane and the carrier. Indeed, more oxygen from the carrier surface, grain boundaries, and even from the bulk lattice was released to react with methane. Upon rising the contact time, less CO was formed, which is desirable for CLC application. Increasing the reaction temperature and methane partial pressure lead to enhanced conversions of CH₄ under CLC conditions