Fast microwave-assisted synthesis of tailored mesoporous carbon xerogels

Resorcinol-formaldehyde carbon xerogels with several initial pH were synthesized using two different heating methods (conventional and microwave heating). The effect of the pH of the precursor solution and the method of synthesis employed on the textural and chemical properties of the final materials was evaluated. It was found that both methods produce tailored carbon xerogels depending on the initial pH and that the pores of the carbon xerogels become larger as the initial pH decreases. High pHs result in exclusively microporous carbon xerogels, while a decrease in the amount of NaOH added, i.e. lower pH, causes the materials to evolve firstly into micro-mesoporous samples and then into micro-macroporous carbon xerogels. The main difference between the two heating methods studied, apart from the duration of the synthesis (i.e. approximately 5 h for the microwave-assisted synthesis as opposed to several days by conventional methods) lies in the meso-macroporosity of the resulting materials, since microwave radiation produces mainly mesoporous carbon xerogels with a specific mesopore size over a wider range of pH than conventional synthesis. For example, the pH range for mesoporous MW samples is 4.5-6.5 while equivalent samples that are conventionally synthesized require an initial pH of between 5.8 and 6.5. This work also illustrates a simple and precise method for determining the gelation point (t(g)) of different pH resorcinol-formaldehyde mixtures, based on varying the energy consumed by the microwave device during the synthesis of organic gels, without the need for other more complicated techniques.     

A Critical Assessment of the Specific Role of Microwave Irradiation in the Synthesis of ZnO Micro‐ and Nanostructured Materials

A rapid, microwave‐assisted hydrothermal method has been developed to access ultrafine ZnO hexagonal microrods of about 3–4 μm in length and 200–300 nm in width by using a 1:5 zinc nitrate/urea precursor system. The size and morphology of these ZnO materials can be influenced by subtle changes in precursor concentration, solvent system, and reaction temperature. Optimized conditions involve the use of a 1:3 water/ethylene glycol solvent system and 10 min microwave heating at 150 °C in a dedicated single‐mode microwave reactor with internal temperature control. Carefully executed control experiments ensuring identical heating and cooling profiles, stirring rates, and reactor geometries have demonstrated that for these preparations of ZnO microrods no differences between conventional and microwave dielectric heating are observed. The resulting ZnO microrods exhibited the same crystal phase, primary crystallite size, shape, and size distribution regardless of the heating mode. Similar results were obtained for the ultrafast preparation of ZnO nanoparticles with diameters of approximately 20 nm, synthesized by means of a nonaqueous sol–gel process at 200 °C from a Zn(acac)₂ (acac=acetylacetonate) precursor in benzyl alcohol. The specific role of microwave irradiation in enhancing these nanomaterial syntheses can thus be attributed to a purely thermal effect as a result of higher reaction temperatures, more rapid heating, and a better control of process parameters.     

A nonisothermal study of the kinetics of the nanoporosity elimination in sonogels-derived silica xerogels

A nonisothermal study of the kinetics of the nanoporosity elimination in monolithic silica xerogels, prepared from acid and ultrasound catalyzed hydrolysis of tetraethylortosilicate (TEOS), has been carried out by means of in situ linear shrinkage measurements performed with different heating rates. The study could be applied up to almost alpha approximately 0.6 of the volume fraction alpha of eliminated pores. The activation energy was found increasing from about 3.2 x 10(2) kJ/mol for alpha approximately 0.06 up to about 4.4 x 10(2) kJ/mol for alpha approximately 0.44. The sintering process accompanying the nanopore elimination in this set of xerogels is in agreement with a viscous flux sintering process with the hydroxyl content diminishing with the volume fraction of eliminated pores. All the volume fraction of eliminated pores versus temperature (T) curves can be matched onto a unique curve with an appropriate rescaling of the T axis, independent of the heating rate. This scaling property suggests that the path of sintering seems the same, regardless of the heating rate; the difference is that the rate is faster at higher temperature.     

Influence of Sol-Gel Synthesis Parameters on the Microstructure of Particulate Silica Xerogels

The influence of key sol-gel synthesis parameters on the pore structure of microporous silica xerogels was investigated. The silica xerogels were prepared using an acid-catalyzed aqueous sol-gel process, with tetraethoxysilane (TEOS) as the silicon-containing precursor. At high H2O : TEOS ratios, sols synthesized at pH 2–3 yielded minimum values of mean micropore diameter and micropore volume. Analysis of the resulting Type I nitrogen adsorption isotherms and the equilibrium adsorption of N(C4F9)3 indicated micropore diameters for these xerogels of less than approximately 10 Å.Xerogel micropore volumes corresponding to sols prepared at pH 3 and an H2O : TEOS ratio of r = 83 were consistent with nearly close packing of silica spheres in the xerogel. Xerogel microstructure was only weakly dependent upon H2O : TEOS ratio during sol synthesis for r > 10. Xerogel micropore volume increased rapidly with sol aging time during an initial induction period of particle formation. However, the xerogel microstructure changed only slowly with time after this initial period, suggesting potential processing advantages for the particulate sol-gel route to porous silica materials.Surface adsorption properties of the silica xerogels were investigated at ambient temperature using N2, SF6, and CO2. CO2 adsorbed most strongly, SF6 also showed measurable adsorption, and N2 adsorption was nearly zero. These results were consistent with the surface transport of CO2, and to a lesser extent SF6, observed in gas permeation studies performed through thin membrane films cast from similarly prepared silica sols.     

Synthesis of novel Schiff base doped sol–gel silicas

Sulfonamide Schiff bases were doped uniformly in silica sol–gels prepared from liquid precursors by a fast and easy way at room temperature and processed to form xerogels. Schiff bases are efficient chelating agents, bioactive and catalytically active compounds. The structures of the newly synthesized Schiff base doped xerogels were elucidated by their physical (morphology, surface area, porosity), spectral (FTIR) and analytical (CHNSO/Si) data. The powder X-ray diffraction studies were carried out to confirm the formation of single phase. Characterization confirmed that Schiff base molecules are entrapped inside the pores as well as physically bound onto the silica surface. All Schiff base doped xerogels are stable mesoporous materials showing hydrophilic properties. Loadings of Schiff bases from 0.10 to 0.23 g/g of xerogel were obtained resulting amorphous materials. The doping of Schiff bases with xerogel caused change in surface area, pore volume and pore diameter of xerogel without damaging the main framework of siliceous skeleton. Morphology and colour of xerogel was also changed after doping. The entrapment of Schiff bases in xerogel caused increase in their decomposition temperatures. The final Schiff base doped xerogels show remarkable thermal stability.     

A critical assessment of the specific role of microwave irradiation in the synthesis of ZnO micro- and nanostructured materials

A rapid, microwave-assisted hydrothermal method has been developed to access ultrafine ZnO hexagonal microrods of about 3-4 μm in length and 200-300 nm in width by using a 1:5 zinc nitrate/urea precursor system. The size and morphology of these ZnO materials can be influenced by subtle changes in precursor concentration, solvent system, and reaction temperature. Optimized conditions involve the use of a 1:3 water/ethylene glycol solvent system and 10 min microwave heating at 150 °C in a dedicated single-mode microwave reactor with internal temperature control. Carefully executed control experiments ensuring identical heating and cooling profiles, stirring rates, and reactor geometries have demonstrated that for these preparations of ZnO microrods no differences between conventional and microwave dielectric heating are observed. The resulting ZnO microrods exhibited the same crystal phase, primary crystallite size, shape, and size distribution regardless of the heating mode. Similar results were obtained for the ultrafast preparation of ZnO nanoparticles with diameters of approximately 20 nm, synthesized by means of a nonaqueous sol-gel process at 200 °C from a Zn(acac)(2) (acac=acetylacetonate) precursor in benzyl alcohol. The specific role of microwave irradiation in enhancing these nanomaterial syntheses can thus be attributed to a purely thermal effect as a result of higher reaction temperatures, more rapid heating, and a better control of process parameters.     

Time growing comparison for ZnO nanorod by using microwave irradiation method

In this study, zinc oxide (ZnO) nanorod were successfully prepared at different growth times (15, 30 and 60 min) using the microwave irradiation method. The ZnO nanorods were simply synthesized at a low temperature (90 °C) with low power microwave assisted heating (about 100 W) and a subsequent ageing process. The synthesized nanorod were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDX) and Ultraviolet–Visible spectroscopy (UV–Vis). The FESEM images showed nanorods with diameter ranging between 50 and 150 nm, and length of 150–550 nm. The XRD results indicate that ZnO nanorods of different time of growth exhibits pure wurtzite structure with lattice parameters of 3.2568 and 5.2125 Å. UV–Vis characterization showed that energy gap decreases with increase in time. The result also shows that growth of ZnO at 60 min produces an energy band gap of 3.15 eV. In general, the results of the study confirm that the microwave irradiation method is a promising low temperature, cheap and fast method for the production of ZnO nanostructures.     

Microwave Synthesis of SAPO‐11 and AlPO‐11: Aspects of Reactor Engineering

AlPO‐11 and SAPO‐11 are synthesized using microwave heating. The effects of precursor volume, reaction temperature, reactor geometry, stirring, applicator type and frequency on the microwave synthesis of SAPO‐11 and AlPO‐11 are studied. The nucleation time and crystallization rate are determined from crystallization curves for SAPO‐11 (and/or AlPO‐11), for the various parameters investigated. Increasing volume of the reacting material decreases the reaction rate of SAPO‐11 at 160°C. In particular, the nucleation time increases with increase in the reaction volume. Increasing the reaction temperature increases the crystallization rate and decreases the nucleation time, however it decreases the particle size. Nucleation of SAPO‐11 and AlPO‐11 under microwave heating is strongly dependant on the reaction temperature. Using wider geometry vessel (33 mm compared to 11 mm diameter) enhances the reaction rate, producing larger crystals in the same reaction time, even though the crystallization rate is decreased. The crystallization rate is enhanced by applicator type in the following order CEM MARS‐5 oven>CEM Discover “focused” system>monomode waveguide. Stirring the reacting solution during heating affects primarily the nucleation time. The effect of microwave frequency on the nucleation and growth of SAPO‐11 shows a dependence on the applicator type more than the specific frequency, for the frequency range 2.45–10.5 GHz. The difference between the crystallization rate observed at higher frequencies and that at 2.45 GHz maybe due to the multimode nature of the waveguide at frequencies above 2.45 GHz. Sweeping the microwave frequency linearly between 8.7 and 10.5 GHz at rates of 10 min^?1 and 100 min^?1 shows an intermediate crystallization curve to that for fixed frequencies of 2.45 GHz and that for 5.8, 8.7 and 10.5 GHz.     

Microwave-assisted synthesis and antitumor activity of the supramolecular complexes of betulin diacetate with arabinogalactan

In this work, a water-soluble supramolecular complex was synthesized in an aqueous suspension of betulin diacetate (BDA) and arabinogalactan (AG) upon microwave heating. Microwave heating allows reducing the time required for the complex formation, compared with conventional heating in a water bath. The specific effect of microwave irradiation on the initial reagents and preparation of a supramolecular complex was studied. In contrast to conventional heating, under microwave heating AG macromolecules may break into roughly equal fragments when the temperature increases up to 100 °C. A change in the surface morphology of BDA crystals under microwave heating of the suspension suggests that microwave irradiation facilitates the dissolution of BDA in water. It has been shown that the use of dimethylsulfoxide as a reaction medium for microwave heating led to a decrease in BDA content in the product due to the inclusion of DMSO into AG macromolecules. The BDA–AG complex was isolated from the microwave-heated aqueous solution, after water evaporation, as a thin amorphous film, which exhibited antitumor activity against Ehrlich ascites carcinoma cells and can be a promising material for pharmacological applications.     

Synthesis of Fe‐loaded MFI zeolite using silatrane as precursor and its CO activity

Fe‐MFI zeolite was successfully synthesized using silatrane as precursor and tetrapropyl ammonium bromide as template via the sol–gel process and microwave heating technique. The effects of ageing time, heating temperature, heating time and iron concentration were investigated, and it was found that Fe‐MFI synthesis favors higher heating temperatures, but is limited by the degradation of the incorporation of a template molecule. Moreover, longer ageing and heating times promote the incorporation of higher amounts of iron atom in the MFI structure. However, too long an ageing time decreases the incorporation of iron. The lower the percentage Fe loading, the greater the percentage of Fe³⁺ ions incorporated into the MFI framework. The catalytic activity of Fe‐MFI catalyst for the oxidation of CO was studied and it was found that these synthesized catalysts catalyzed the oxidation of CO very well. Copyright © 2005 John Wiley & Sons, Ltd.     

Parametric study of factors affecting melamine-resorcinol-formaldehyde xerogels properties

Resorcinol-formaldehyde (RF) xerogels are organic materials have been widely studied due to their industrially relevant characteristics, through which, RF gels have significant potential to be tailored to specific applications. Xerogel properties have been tailored, within this study, by altering the synthesis procedure with a focus on monomer concentrations, catalyst to monomer ratio, and the introduction of a nitrogen-rich precursor, thereby incorporating nitrogen into the structure to additionally affect the chemical properties of the final gel. Melamine (M) is used as the source of nitrogen, partially replacing the resorcinol (R) typically used, and resulting in a melamine-resorcinol-formaldehyde (MRF) gel; synthesis was facilitated by a sodium carbonate catalyst (C), as often used in RF gel production. R/C and R/F molar ratios, and M concentration ([M]), were chosen as parameters to study in-depth, as they have previously been shown to markedly influence sol-gel formation. The MRF gels produced were subsequently characterized to determine porous structure and chemical functionality. The results indicate that, texturally, increasing [M] produces a similar effect as increasing R/C values: increasing pore size, while decreasing surface area. Pore volume tends to increase when R/C or M increase individually but pore volume and surface area decrease drastically when both variables increase concurrently. Microporosity also tends to increase as R/C decreases, and as the concentration of M is decreased. Altering the gel matrix, by replacing M for R, results in a weakening of the gel structure, as the bridges formed during curing are reduced in quantity, which indicates a maximum level of substitution that can occur within these materials. Combined, these results suggest that nitrogen can be successfully incorporated into organic gel structures but that the interplay between process variables is crucial in determining final gel characteristics for specific applications.     

In situ SAXS/WAXS of zeolite microwave synthesis: NaY, NaA, and beta zeolites.

A custom waveguide apparatus is constructed to study the microwave synthesis of zeolites by in situ small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS). The WR-284 waveguide is used to heat precursor solutions using microwaves at a frequency of 2.45 GHz. The reaction vessels are designed to include sections of thin-walled glass, which permit X-rays to pass through the precursor solutions with minimal attenuation. Slots were machined into the waveguide to provide windows for X-ray energy to enter and scatter from solutions during microwave heating. The synthesis of zeolites with conventional heating is also studied using X-ray scattering in the same reactor. SAXS studies show that the crystallization of beta zeolite and NaY zeolite is preceded by a reorganization of nanosized particles in their precursor solutions or gels. The evolution of these particles during the nucleation and crystallization stages of zeolite formation depends on the properties of the precursor solution. The synthesis of NaA and NaX zeolites and sodalite from a single zeolite precursor is studied by microwave and conventional heating. Microwave heating shifts the selectivity of this synthesis in favor of NaA and NaX over sodalite; conventional heating leads to the formation of sodalite for synthesis from the same precursor. The use of microwave heating also led to a more rapid onset of NaA zeolite product crystallization compared to conventional heating. Pulsed and continuous microwave heating are compared for zeolite synthesis. The resulting rates of formation of the zeolite products, and the relative amounts of the products determined from the WAXS spectra, are similar when either pulsed or continuous microwave heating is applied in the reactor while maintaining the same synthesis temperature. The consequences of these results in terms of zeolite synthesis are discussed.     

Morphological behavior of spherically granulated hydrated zirconium dioxide during its chemical and hydrothermal treatments

Hydrated zirconium dioxide samples were synthesized via original sol–gel route by two-step neutralization of zirconium oxychloride. Spherical granules of zirconia materials were prepared using a well-known “oil-drop” method. By the use of low temperature nitrogen adsorption, it is shown that surface fractal dimension of synthesized xerogels strongly depends on synthesis conditions, chemical and hydrothermal treatments. It is established that the change of synthesis conditions allows to obtain zirconium containing materials with developed porous structure: BET surface area—250–450 m²/g, pore diameter—2.0–4.2 nm, surface fractal dimension—2.28–2.63. It is shown that surface fractal dimension is an important parameter allowing to control the final properties of synthesized material and to determine the reasons of non-reproducibility of adsorptive, catalytic and other physical and chemical properties of zirconium containing oxides from synthesis to synthesis.     

Titanium coated with high-performance nanocrystalline ruthenium oxide synthesized by the microwave-assisted sol–gel procedure

Ruthenium oxide coating on titanium was prepared by the sol–gel procedure from well-defined colloidal oxide dispersions synthesized by the microwave (MW)-assisted hydrothermal route under defined temperature and pressure heating conditions. The dispersions were characterized by dynamic light scattering (DLS) measurements and scanning electron microscopy (SEM). The electrochemical properties were analyzed as capacitive performances gained by cyclic voltammetry and electrochemical impedance spectroscopy and as the electrocatalytic activity for oxygen evolution from acid solution. The obtained dispersions were polydisperse and contained regular particles and agglomerates of increasing surface energy and decreasing particle size as the MW-assisted heating conditions were intensified. Owing to these features of the precursor dispersions, the obtained coatings had considerably improved capacitive performances and good electrocatalytic activity for oxygen evolution at high overpotentials.     

The optimization of hydrothermal process of MoS_2 nanosheets and their good microwave absorption performances

In this study,flower-like MoS_2 constructed by nanosheets was synthesized by a simple hydrothermal method.The hydrothermal process was optimized and the effects of hydrothermal condition,including reaction temperature,reaction time and the ratio of Mo source to S source(Mo:S) in precursor,on microwave absorption performances and dielectric properties were investigated.Our results showed that when the reaction temperature was 180℃,the reaction time was 18 h,and the Mo:S was 1:3.5,the synthesized MoS_2 had the best performance:Its minimum reflection loss could reach-55.78 dB,and the corresponding matching thickness was 2.30 mm with a wide effective bandwidth of 5.17 GHz.Further researches on the microwave absorption mechanism revealed that in addition to the destructive interference of electromagnetic waves,various polarization phenomena such as defect dipole polarization were the main reasons for microwave loss.We believe that MoS_2 is a candidate for a practical microwave absorbent.     

Synthesis of dye functionalized xerogels via nucleophilic aromatic substitution of fluoro aromatic compounds with aminosilanes

The one-pot synthesis of inorganic–organic hybrid materials via combination of sol–gel process and nucleophilic aromatic substitution reaction of various fluoro aromatic compounds and 3-aminopropyltrimethoxysilane has been studied. Both, nucleophilic aromatic substitution reaction and sol–gel process can be accomplished in the same reaction vessel due to the sol–gel precursor tetraethoxysilane acting as solvent during the first reaction step. Hydrogen fluoride, which forms as a by-product of the substitution reaction, is trapped by both silane species present and subsequently serves as catalyst during the sol–gel process. The obtained materials can be classified as type II xerogels, because of the covalent linkage between organic chromophor and inorganic silicon network. Fluoro aromatic compounds with different reactivities for nucleophilic aromatic substitution reactions containing azo, azomethine, and diphenylamine groups were used in order to (1) demonstrate the synthetic concept and (2) fine-tune the optical properties of the resulting chromophoric xerogels. The final chromophor content within the xerogels was varied by modifying the ratio of organosilicon precursor and tetraethoxysilane. All obtained organic–inorganic hybrid materials were characterized in detail using solid state NMR- and UV/vis spectroscopy. Latter one gave experimental confirmation of the partial hydrolysis of azomethine dyes in the xerogels, while no decomposition of azo or diphenylamine dyes was observed.     

Microwave pyrolysis of straw bale and energy balance analysis

The compressed wheat and corn straw bale were pyrolyzed on a microwave heating device self-designed and built with respect to the time-resolved temperature distribution, mass loss and product properties. Considering scale up and technology promotion of microwave pyrolysis (MWP), the investigations on electricity consumption and energy balance of MWP were carried out emphatically. The results indicated that MWP had obvious advantages over conventional pyrolysis, such as heating rapid and more valuable products obtained. The distribution of pyrolysis products such as gas, liquid and char was close to 1:1:1 due to the medium pyrolysis temperature and the slow heating rate, which was not favorable for the formation of gas and/or liquid products. The content of H₂ attained the highest value of 35 vol.% and syngas (H₂ and CO) was greater than 50 vol.%. The electricity consumption of MWP was between 0.58 and 0.65 kW h (kg straw)⁻¹ and with the increase of microwave power, the electricity consumption required for pyrolysis of unit mass of straw increased. The minimum microwave power for MWP was about 0.371 kW (kg straw)⁻¹ and the proportion of heat loss and conversion loss of electricity to microwave energy occupied in the total input energy was 42%. Data and information obtained are useful for the design and operation of pyrolysis of large-sized biomass via microwave heating technology.     

Silver Metallic Nanoparticles with Surface Plasmon Resonance: Synthesis and Characterizations

Silver nanoparticles were synthesized by the reduction of the silver nitrate (AgNO₃) using the latex copolymer in ethanol solution under microwave (MW) heating. The reaction parameters such as silver precursor concentration (from 0.005 to 0.1 g/l) and MW power (200–800 W) significantly affect the formation rate, shape, size and distribution of the silver nanoparticles. A significant reduction of irradiation time was observed when the MW energy is compared to conventional thermal reduction processes. The prepared silver nanoparticles show uniform and stable sizes from 5 to 11 nm, which can be stored at room temperature for approximately 12 months without any visible change. These peculiarities indicate that the latex copolymer is a good stabilizer for the silver nanoparticles. The optical properties, morphology, and crystalline structure of the silver-latex copolymer nanocomposites were characterized by the Ultraviolet–Visible spectroscopy, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The study of the TEM images at high magnifications identified the silver nanoparticles as face-centered cubic (fcc) structure with spherical and hexagonal shapes.     

Ultrafast Preparation of Monodisperse Fe3O4 Nanoparticles by Microwave‐Assisted Thermal Decomposition

Thermal decomposition, as the main synthetic procedure for the synthesis of magnetic nanoparticles (NPs), is facing several problems, such as high reaction temperatures and time consumption. An improved a microwave‐assisted thermal decomposition procedure has been developed by which monodisperse Fe₃O₄ NPs could be rapidly produced at a low aging temperature with high yield (90.1 %). The as‐synthesized NPs show excellent inductive heating and MRI properties in vitro. In contrast, Fe₃O₄ NPs synthesized by classical thermal decomposition were obtained in very low yield (20.3 %) with an overall poor quality. It was found for the first time that, besides precursors and solvents, magnetic NPs themselves could be heated by microwave irradiation during the synthetic process. These findings were demonstrated by a series of microwave‐heating experiments, Raman spectroscopy and vector‐network analysis, indicating that the initially formed magnetic Fe₃O₄ particles were able to transform microwave energy into heat directly and, thus, contribute to the nanoparticle growth.     

Influence of feed characteristics on the microwave-assisted pyrolysis used to produce syngas from biomass wastes

A series of biomass wastes (sewage sludges, coffee hulls and glycerol) were subjected to pyrolysis experiments under conventional and microwave heating. The influence of the initial characteristics of the raw materials upon syngas production was studied. Glycerol yielded the highest concentration of syngas, but the lowest H₂/CO ratio, whereas sewage sludges produced the lowest syngas production with the highest H₂/CO molar ratio. Coffee hull displayed intermediate values for both parameters. Microwave heating produced greater gas yields with elevated syngas content than conventional pyrolysis. Moreover, microwave pyrolysis always achieved the desired effect with temperature increase upon the pyrolysis products, whatever biomass material was employed. This could be due to the hot spot phenomenon, which only occurs under microwave heating. In addition, a comparison of the energy consumption of the traditional and microwave-assisted pyrolysis is also presented. Results point at microwave system as less time and energy consuming in comparison to conventional system.