Search

Lignin, a renewable polymer derived from the lignocellulosic biomass represents more than 20% of the total mass of the Earth's biosphere. However, 98% of lignin is burned as a source of energy in the pulp and paper industry, essentially due to its complex structure. Today, the valorization of lignin into the production of value-added chemicals represents a real challenge in terms of both sustainability and environmental protection. The present contribution aims to provide a critical discussion on the crucial choice of the starting material to study lignin valorization. Next, a comparison between the different oxidation routes investigated by chemists over the past several years is presented, with emphasis on the major difficulties encountered. The main current challenges regarding the heterogeneous catalytic oxidation of lignin and its derivatives are also highlighted. Particular focus is given to innovative strategies favoring mild reaction conditions. Finally, we provide some recommendations and routes worthy of interest in this studied area of research in order to generate value-added chemicals from lignin oxidation through the use of heterogeneous catalysts. [ABSTRACT FROM AUTHOR]

Abstract: Nanometric Fe2O3 particles could be inserted inside the internal pore volume of SBA-15 mesoporous silica when Fe(III) chelates (EDTA, gluconate and citrate) were used as impregnating precursors. Fe(III) nitrate preferentially yields 8nm uniformly sized Fe2O3 clusters that selectively plug the SBA-15 channels through a geometric confinement effect. An oxidative degradation of Fe-chelate precursors yielded Fe(III) oxidic particles of various sizes and dispersion, depending on the nature and geometry of chelate anion. Fe(EDTA) precursors specifically generated Fe2O3 nanoparticles that selectively migrate towards two types of positions where the silica surface exhibits a high curvature. In samples involving low Fe loadings, Fe2O3 particles first creep towards the silica micropore mouths (nests), in which they are readily confined and stabilized. For higher Fe loadings, when most of the micropore nests were filled, oxidic particles eventually settle as a superficial film on the mesopore walls and undergo stabilization onto the surface roughness that can also favors their confinement, as theoretically predicted by Derouane and co-workers. Upon further reduction, as selectively followed by combined cyclic voltammetry and TPR, Fe2O3 readily yield FeO nanoslabs that remain even more efficiently confined within their respective nest positions through a further acid–base type stabilization. [Copyright &y& Elsevier]

Abstract: The selective synthesis of phytosterol esters from natural sterols and methyl esters was investigated on new grounds by a chemical process using basic solids that are reusable, active, and more selective than the currently used homogeneous catalysts (e.g., alkali hydroxides, carbonates). Various lanthanum oxides with different synthesis procedures and varying specific surface areas were tested. The catalytic performance of the new solids and, more specifically, the ability of La2O3 oxides in increasing phytosterol ester selectivity, was correlated with their acido-basic properties, which were fully characterized by infrared spectroscopy. All La2O3 oxides involved surface carbonate species with different structural states and strengths, as revealed by propyne adsorption. The selectivity to phytosterol ester varied from 90 to 96%, confirming that the side reaction of dehydration of β-sitosterol was strongly inhibited. The phytosterol ester yield could reach 89% and was related to the basic strength of the carbonate species. The lower the carbonate basicity, the higher the phytosterol ester yield. Moreover, IR spectra of catalysts diluted in KBr powder showed that the higher the intensity of unidentate carbonate bands, the higher the yield. It was concluded that the synthesis of phytosterol esters from fatty methyl esters and sitosterols in the presence of La2O3 catalysts requires ionic carbonate species of medium basic strength. [Copyright &y& Elsevier]

Review: 105 refs. [ABSTRACT FROM AUTHOR]

Abstract: Commercial relevance of glycerol as a biosourced safe organic building block for valuable commodity chemicals has increased considerably with the rapid development of oleochemistry. Among the various reaction pathways for catalytic conversion of glycerol, oligomerization to polyglycerols (PGs) is one of the viable and demanding processes due to the wide field of applications of PGs in cosmetics, polymers, food-additives, antifogging film industry, pharmaceuticals, biomedicals and drug delivery systems. This short review is dedicated to the selective synthesis of polyglycerols preferentially to low molecular-weight oligomers with emphasis on the pros/cons of reported homogeneous and heterogeneous acidic and basic catalytic processes. In addition, recent advances conducted in our laboratory will be given for a selective glycerol oligomerization in the presence of powdered and shaped alkaline earth-based mesoporous catalysts and of hydrothermally synthesized layered structured materials as newly developed basic catalysts. Some critical issues and challenges regarding the rational design of solid catalysts with emphasis on their stability in hot glycerol as well as alternative reaction engineering to overcome the drawbacks stemming from batch reactions will be addressed. Selectivity to the desired fraction of short-chain oligomers for food and cosmetic applications with respect to the European Union regulations will be also introduced. [Copyright &y& Elsevier]

Review: 37 refs. [ABSTRACT FROM AUTHOR]

Abstract: Hydrothermally synthesized Ni-containing 1:1 and 2:1 phyllosilicates (PS) were used as catalyst precursors for the CO2 reforming of CH4 to synthesis gas. The textural properties of the reduced and unreduced Ni-based PS were determined by N2 sorption analyses. In situ X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM) experiments were used to characterize the generated Ni0 nanoparticles after reduction of the Ni-containing PS under various conditions such as H2 concentration and temperature. In the case of thermally stable PS (2:1 type), the reduction of structural nickel led to the formation of nanometric Ni0 particles over the surface of the remaining unreduced PS, while for Ni 1:1 PS phases nickel particles were supported on silica only. All catalysts were also investigated in the reverse water gas shift (RWGS) reaction to elucidate the effect of RWGS on the reforming reaction. The catalytic results obtained over reduced Ni-containing 1:1 and 2:1 PS confirmed that CH4 conversion was favored by very small Ni0 particles size, whereas CO2 conversion was catalyst support dependent. Our study demonstrated that a number of side reactions can compete with CO2 reforming of CH4 over Ni-containing phyllosilicates. However, by choosing the suitable phyllosilicate structure as catalyst precursor, as well as appropriate reduction conditions and reaction temperature, it may be possible to suppress the parallel reactions to some extent, thereby increasing the selectivity towards the products of the reforming reaction (H2 and CO). Reduced Ni-containing 2:1 PS proved to be stable even at high temperature. Highly dispersed homogeneously sized supported nanometric Ni0 particles over the remaining unreduced 2:1 PS structure are the key factors for high catalytic activity in CO2 reforming of CH4. [ABSTRACT FROM AUTHOR]

Abstract: CO2 reforming of CH4 to synthesis gas was investigated on new grounds using Ni-containing phyllosilicates (PS) as catalyst precursors. Ni-containing 1:1 PS and 2:1 PS were synthesized hydrothermally from a gel containing sodium silicate and nickel chloride, then they were reduced under H2 flow before being used as catalysts. Several complementary techniques including XRD, TG-DTA, FTIR, and TPR were used to evaluate the structural, thermal, spectroscopic and redox properties of the samples, respectively. Ni 2:1 PS were found to be more thermally stable and catalytically active than Ni 1:1 PS. Well crystallized Ni 1:1 PS thermally treated at 700°C resulted in a transitional 2:1 PS phase. The reduction of structural nickel in thermally stable PS led to the formation of nanometric Ni0 particles over the surface of the remaining unreduced PS as well as on silica. The unreduced PS structure with surface OH groups appeared as one of the best supports for Ni0 nanoparticles, resulting in less coke formation, and thereby leading to a high catalytic stability. [ABSTRACT FROM AUTHOR]

A mesoporous SBA-15 doped iron oxide (Fe2O3/SBA-15) was synthesized by co-codensation, characterized and used as heterogeneous catalysts for the photo-Fenton decolorization of azo dye Orange II under UV irradiation. Response surface methodology (RSM) was used to investigate operating condition effects, such as hydrogen peroxide concentration, initial pH and catalyst loadings, on the decolorization rate. UV irradiation is found to enhance the activity of the catalyst in the process. RSM analysis evidenced the influence of the initial pH value and H2O2 concentration on the dye degradation rate. The coupled UV/Fe2O3/SBA-15/H2O2 process at room temperature is revealed as a promising friendly process for wastewater treatment. Indeed, the use of a heterogeneous catalyst allows an easy active phase recycling without multi-step recovering while the heterogeneous catalyst used here exhibits high catalytic activity for the reaction considered. [ABSTRACT FROM AUTHOR]

Abstract: Iron-based mesoporous silica materials were prepared according to different impregnation and co-condensation procedures. Several complementary techniques, including XRD, TEM/EDX and nitrogen sorption isotherms were used to evaluate the final structural and textural properties of the calcined Fe/SBA-15 materials. While Fe2O3 isolated particles of which the size is close to the silica pore diameter (∼7–8nm) were obtained using classical wet impregnation procedure, smaller iron oxide particles (∼2–4nm) homogeneously dispersed within the hexagonal pore structure of the SBA15 host support were generated by self-combustion of an impregnated iron-glycinic complex. By contrast, the various co-condensation routes used in this work were less efficient to generate iron oxide nanoparticles inside the silica mesopores. Catalytic performances of the materials were evaluated in the case of total phenol oxidation by H2O2 in aqueous solution at ambient conditions. Large differences in terms of catalytic activity and iron species stability were observed. While the impregnated solids proved to be the most active catalysts (highest Fe2O3 nanoparticles dispersion), iron leaching was observed in aqueous solution, accounting for a homogeneous catalytic contribution. In contrast, the co-condensed samples exhibiting larger iron oxide clusters stabilized over the silica surface proved more efficient as active sites in Fenton catalysis. [Copyright &y& Elsevier]

The purpose of the present study was to evaluate the ability of LaFeO3 perovskites prepared using the “self-combustion method” to behave as efficient and stable catalysts in phenol oxidation using H2O2 in ambient conditions. Depending on the synthesis conditions, strong differences in the structural, textural and reducibility characteristics of the perovskites, as well as in phenol conversion and TOC abatement level were observed. High TOC abatement (76%) and very low iron leaching (0.27 wt%) were obtained at 40 °C in the presence of LaFeO3 nanoparticles exhibiting the lowest crystal domain size. [ABSTRACT FROM AUTHOR]

Nanometric Fe2O3 particles could be inserted inside the internal pore volume of SBA-15 mesoporous silica and mesoporous alumina supports, when Fe(III) chelates (EDTA, gluconate or citrate) were used as impregnating precursors. The oxidative degradation of the chelating anions was followed by combined TG-DTA. Strong chelate-SiOH interactions (case of bulky EDTA), favored by the mesopore curvature, yield sub-nanometric extremely well dispersed Fe2O3 particles preferentially located at the micropore mouths (confinement effect). Fe2O3 even more strongly interacts with alumina walls, generating either (Fe,Al)2O3 mixed phases or Fe-aluminate micro domains. These iron-based mesoporous alumina composites proved very active catalysts in total oxidation of phenol at ambient conditions, with extremely low iron leaching (0.2%). [ABSTRACT FROM AUTHOR]

Abstract: Laser pyrolysis is a promising and versatile method allowing the synthesis of various nanoparticles, with well defined chemical composition, size and structure. This procedure is based on the interaction of a powerful IR laser beam with a mixture of gaseous or liquid precursors, with one of them absorbing the laser radiation. This paper presents the synthesis of nanoPt/TiO2 and nanoPd/TiO2 by laser pyrolysis and preliminary results about their efficiency for elimination of volatile organic compounds (deVOC). The nanoparticles have been synthesized by adding a metallic precursor in a liquid precursor (titanium isopropoxide). X-ray diffraction, transmission electron microscopy, thermogravimetric analysis and IR spectroscopy have been used to analyse their nanostructures and morphologies. The volatile organic compounds elimination (deVOC) tests performed on these nanomaterials demonstrate that Pd-catalysts are very efficient in the total elimination of chloro-compounds. These catalytic properties are unusual, especially because conventional Pd supported catalysts are generally not used for this application due to a production of a high amount of polychlorinated compounds. These results showed that the laser pyrolysis method allows the synthesis of well dispersed titanium-based catalysts not previously reported in the literature. [Copyright &y& Elsevier]

Starting from an as-synthesized Na-bearing MCM-41 sample with Si/Al=20, several Cs or K exchanged samples have been obtained. All samples, both washed after exchange to remove the excess alkali–metal salt and unwashed, were characterized by studying in the IR the adsorption of CO at a nominal temperature of 77 K, and of CO2 at room temperature.On unwashed samples, after thermal treatment at 500 °C, a basic alkali-oxide phase and a surface silicate were detected. In contrast, washed samples did not exhibit basic character and only Lewis acidic cationic centers were detected, the strength of which is close to that of alkali-cation exchanged ZSM-5 systems. Unwashed samples were inactive in the isomerization of 1-butene. Washed samples converted 1-butene to cis/trans-2-butene, with a ratio close to one (the presence of isobutene was negligible), as typical of mildly acidic Brønsted centers, which are probably present even after exchange: indeed, a sample of Al-MCM-41 in the protonic form, tested as reference, gave the same reaction with highest activity. [Copyright &y& Elsevier]