Microwave-assisted Hydrothermal Treatment Research Articles

Microwave radiation influence on the thermal and spectroscopic properties of Na-birnessite-type material

Abstract The aim of this work was to study the effect of the microwave radiation on the thermal and spectroscopic features, as well as about arrangement (order-disorder) and morphological properties, of the layered manganese oxide with birnessite-type structure. The route employed to obtain Na-birnessite matrix was redox precipitation. The products were characterized by X-ray diffraction, thermal analysis (TG-DTG-DSC), infrared (FTIR) and Raman spectroscopy, scanning electron microscopy (SEM) and nitrogen adsorption-desorption technique. The results showed that microwave radiation influenced in a short time (5 min) the octahedral ordering of birnessite, as well as in increasing the crystallite size. Thermal analysis showed that the thermal behavior of the lamellar matrix was different from that of birnessite under microwave radiation. After microwave-assisted hydrothermal treatment, FTIR and Raman spectroscopy investigations were used to differentiate ordered and disordered birnessites. Otherwise, there were no changes in SEM morphology of the lamellar-type phases, but the particle size changed.

Influence of microwave-assisted hydrothermal treatment time on the crystallinity, morphology and optical properties of ZnWO4 nanoparticles: Photocatalytic activity

In this work, zinc tungstate (ZnWO4) nanoparticles were obtained by microwave-assisted hydrothermal method at 140 °C and 2 atm. The hydrothermal treatment time was varied at 0, 5, 10, 30, 60 and 120 min. The nanoparticles were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscope (TEM), high-resolution transmission electron microscope (HRTEM), X-ray fluorescence (FRX), nitrogen adsorption by Brunauer-Emmett-Teller method (BET), UV–Visible spectroscopy (UV–Vis) and Photoluminescence (PL) properties. The adsorptive and photocatalytic properties were estimated against methylene blue (MB - cationic) and methyl orange (MO - anionic) dyes. The diffractograms show that the larger hydrothermal treatment time, anhigher the powders crystallinity. The BET and SEM analysis indicate that the increase in hydrothermal time does not interfere significantly in nanoparticles size but reduces their porosity. The change in the surface area is consistent with the adsorption tests, where the increase of the hydrothermal treatment time reduces the adsorption against the MB and MO dyes. However, increasing the hydrothermal treatment time provides an increase in photocatalytic activity.

GC/MS-screening analyses of valuable products in the aqueous phase from microwave-assisted hydrothermal processing of Lemna minor

Microwave-assisted hydrothermal treatment of Lemna minor plant material was studied under subcritical conditions. The water phases of this process were extracted with polar and nonpolar solvents and the extracts were investigated by GC/MS screening analyses for value added products. The polar extracts were dominated by compounds known as flavouring agents and partly as platform chemicals, e.g. hydroxyacetone, 1-hydroxybutan-2-one, furfural, cyclotene and pyrrolidone. Pyridinol, an important educt for heterocyclic chemistry, was another interesting product. Benzaldehyde and oleamide were detected as main products in nonpolar n-hexane extracts. An essential aspect is that a large proportion of compounds like aldehyds and ketones were obtained when mild hydrothermal processing was applied.

Insights into formation of anatase TiO2 nanoparticles from peroxo titanium complex degradation under microwave-assisted hydrothermal treatment

Several methods for the synthesis of anatase TiO2 have been reported in the literature. In particular, the microwave-assisted hydrothermal crystallization of Ti-peroxocomplexes has enabled the phase and morphological control of TiO2 nanoparticles by adjusting the synthesis conditions, such as pH, time and temperature, with significant increase in the reaction rate compared to the conventional sol-gel techniques. In this paper, we uncover the mechanism involved in the formation of anatase TiO2 nanoparticles from the Ti-peroxo complex precursor route. Structural characterizations (XRD, Raman, TEM and XANES) revealed that the Ti-peroxo complex precursor exhibits a titanate-like structure, comprised of bulk NH4+ ions and surface peroxo groups. The NH4+ ions release during hydrothermal treatment is determining on the anatase TiO2 nanoparticle formation, which occurs concomitantly with the decomposition of peroxogroups. UV-induced dye photodegradation tests involving the Ti-peroxo complex precursor and TiO2 nanoparticles showed that the precursor is not able to form hydroxyl radicals that are the main chemical species responsible for the dye photodegradation, despite the presence of peroxo groups at its surface. The results herein highlight the mechanism involved in the formation of anatase TiO2 nanoparticles, which could assist in the development of new synthesis approaches based on Ti-peroxocomplex precursors.

Spray-dried nanoporous NiO/PANI:PSS composite microspheres for high-performance asymmetric supercapacitors

In this study, we used a spray-drying method to synthesize a hierarchically structured binary nanocomposite comprising highly porous NiO nanosheets and polyaniline:poly(sodium 4-styrenesulfonate) (PANI:PSS) materials. In this system, the porous NiO nanosheets—obtained through thermal decomposition of β-Ni(OH)2 derived from microwave-assisted hydrothermal treatment—dispersed well in the PANI:PSS solution. The homogeneous suspension was then processed through spray-drying and crosslinking of PANI:PSS and GOPS to form NiO/PANI:PSS microspheres. The PANI polymer served as a good conductor and thin spacer that prevented the NiO nanosheets from undergoing layer-to-layer stacking. Moreover, the crosslinked PANI:PSS acted as a binder that held all of the active materials together, leading to high mechanical integrity and excellent water-resistance. Through a synergistic effect in this unique nanoarchitecture, the NiO/PANI:PSS nanocomposite displayed a higher capacitance (834 F/g) at 1 A/g compared with that of NiO (380 F/g) and PANI:PSS (264.5 F/g) individually. Furthermore, the NiO/PANI:PSS composite displayed remarkable rate performance and outstanding cycling life—88.9% retention of specific capacitance after 3000 repeated charge/discharge tests—as a result of its low resistance and high (SSA). We fabricated asymmetric supercapacitors (ASCs) incorporating NiO/PANI:PSS and active carbon (AC) to further explore the capacitive performance of the composite. The as-fabricated devices also delivered remarkable performance, with an energy density of 32.84 Wh/kg, a power density of 375 W/kg, and excellent cycle life.

Microwave-Assisted Hydrothermal Synthesis of Bi6(NO3)2O7(OH)2 and Its Photocatalytic Properties

Anhydrous basic bismuth nitrate Bi6(NO3)2O7(OH)2 was shown to be produced by reacting bismuth nitrate with aqueous ammonia under microwave-assisted hydrothermal treatment as a result of hydrolysis. The composition of the basic nitrate was verified by titration, CHNS analyses, DTG, and IR spectroscopy. Photocatalytic tests showed that Bi6(NO3)2O7(OH)2 has a photocatalytic activity, but its properties are interior to those of α-Bi2O3.

Unveiling the efficiency of microwave-assisted hydrothermal treatment for the preparation of SrTiO3 mesocrystals.

Material processing has become essential for the proper control, tuning and consequent application of the properties of micro/nanoparticles. In this case, we report herein the capability of the microwave-assisted hydrothermal (MAH) method to prepare the SrTiO3 compound, as a case study of inorganic compounds. Analyses conducted by X-ray diffraction, X-ray photoelectron and X-ray absorption spectroscopies confirmed that the MAH route enables the formation of pristine SrTiO3. The results indicated that the combination of thermal and non-thermal effects during the MAH treatment provides ideal conditions for an efficient and rapid synthesis of pristine SrTiO3 mesocrystals. Scanning electron microscopy images revealed a cube-like morphology (of ca. 1 μm) formed via a self-assembly process, influenced by the MAH time. Additionally, photoluminescence measurements revealed a broad blue emission related to intrinsic defects, which decreased with the MAH synthesis time.

The self-assembly of DyF3 nanoparticles synthesized by chloride-based route

The series of DyF3 nanoparticles was synthesized for the first time by a chloride-based route, using the water-soluble dysprosium chloride hexahydrate as a precursor. The synthesized nanoparticles have sizes of 3 to 7 nm, which is the smallest reported size for DyF3 nanoparticles. The influence of precursor concentration in an aqueous solution prior to the chemical reaction on the size of nanoparticles was studied. The influence of microwave-assisted hydrothermal treatment was also studied. The analysis of transmission electron microscopy (TEM) data revealed no correlation between the size of synthesized nanoparticles and the concentration of the aqueous solution. The AC/DC magnetic susceptibility of DyF3 nanoparticles was measured. The dipolar ferromagnetic transition for this sample was not observed down to 1.8 K that can be possibly explained by onset of superparamagnetism. The self-assembly of DyF3 nanoparticles was observed for the first time by analysis of XRD and TEM data and the evidence of superlattice formation along (020) and (210) lattice planes over the size of 20–30 nm (4–5 nanoparticles) was obtained.

Aerosol-based synthesis of silsesquioxane-graphene oxide and graphene-manganese oxide nanocomposites for high-performance asymmetric supercapacitors

A facile aerosol-based synthetic approach is demonstrated for the fabrication of silsesquioxane-graphene oxide (rGO-SQ) and reduced graphene oxide-manganese oxide (MnOx-rGO) nanocomposites, as the materials for negative and positive electrodes in an asymmetric supercapacitor (ASC), respectively. Microwave-assisted hydrothermal treatment is employed to form reduced graphene oxide (AS-rGO). Fourier-transformed infrared spectroscopy, thermogravimetric analysis, scanning electron microscopy and x-ray photoelectron spectrometry are used to provide complementary material characterizations for the synthesized nanocomposites. The results show that the composition and the morphology of the synthesized materials are tunable by the adjustment of precursor concentration and annealing temperature. From the shapes of cyclic voltammetric and galvanostatic charge-discharge curves, the conductivity and the subsequent capacitive performance of MnOx are enhanced effectively by the hybridization with rGO. The highest double-layer capacitance of AS-rGO is 118 F g−1, and the highest specific capacitance of MnOx-rGO reaches 183 F g−1 under a scan rate of 5 mV s−1. The ASC assembled with AS-rGO and MnOx-rGO possessed high charge-discharge reversibility at a cell voltage of 2.0 V. A high operation stability of ASC can be achieved, as evidenced by the high retention (98% of the retention) in the 10,000-cycle charge-discharge test at a current density of 2 A g−1. The maximum specific energy and specific power of the ASC respectively reach 16.6 Wh kg−1 and 1.052 kW kg−1 at a current density of 1 A g−1. This study demonstrates a prototype approach for the fabrication of nanocomposite electrode materials by design with the ability of scalable mass production.

Microwave-assisted low-temperature hydrothermal treatment of red seaweed (Gracilaria lemaneiformis) for production of levulinic acid and algae hydrochar

In this study, red seaweed (Gracilaria lemaneiformis) food waste with high carbohydrate content was valorized into levulinic acid (LA) and algae hydrochar through microwave-assisted low-temperature hydrothermal treatment in dilute acid solution. Various parameters including treatment temperature (160–200 °C), reaction time (1–40 min), acid concentration (0–0.6 M), and biomass-to-liquid ratio (1%–10%, w/v) were examined. The energy efficiency and carbon recovery of the proposed process were investigated. Under the experimental conditions of 5% (w/v) biomass loading, 0.2 M H2SO4, 180 °C, and 20 min, the highest levulinic acid yield of 16.3 wt% was produced. The resulting hydrochar showed approximately 45–55% energy yield and higher heating values of 19–25 MJ kg−1. The energy efficiency of the present study (1.31 × 10−6 g LA/J) was comparable to those of the conventional hydrothermal treatment of lignocellulosic biomass, while the reaction time (20 min) was much shorter with a high carbon recovery (73.3%).

Facile Synthesis by Peroxide Method and Microwave-Assisted Hydrothermal Treatment of TiO2 with High Photocatalytic Efficiency for Dye Degradation and Hydrogen Production

Facile Synthesis by Peroxide Method and Microwave-Assisted Hydrothermal Treatment of TiO₂ with High Photocatalytic Efficiency for Dye Degradation and Hydrogen Production

Degradation of PCDD/Fs in MSWI fly ash using a microwave-assisted hydrothermal process

Abstract In this work, microwave treatment was introduced to a hydrothermal treatment process to degrade PCDD/Fs (Polychlorinated dibenzo-p-dioxins and dibenzofurans) in municipal solid waste incineration (MSWI) fly ash. Three process additives (NaOH, Na2HPO4, H2O), temperatures (150 °C, 185 °C, 220 °C) and reaction times (1 h, 2 h, 3 h) were investigated to identify their effect on the disposal of fly ash samples through orthogonal experiments. High-resolution gas chromatography–mass spectrometry (HRGC/MS) was applied to determine the PCDD/F concentrations in MSWI fly ash. The experimental results revealed that 83.7% of total PCDD/Fs was degraded. Reaction temperature was the most important factor for the degradation of the total PCDD/Fs. Both direct destruction and chlorination reactions (the chlorination degree of PCDFs increased) took part in the degradation of PCDD/Fs in fly ash, which was a new discovery. Several PCDD/F indexes determined by the concentration of indicative congeners were found to quantitatively characterize the dioxin toxicity of the fly ash. Furthermore, heavy metals in the fly ash sample were solidified using microwave-assisted hydrothermal treatment, which provided an experimental basis for the simultaneous disposal of dioxins and heavy metals. Thus, the microwave-assisted hydrothermal process should be considered for the future disposal of MSWI fly ash.

Microwave assisted hydrothermal preparation of rice straw hydrochars for adsorption of organics and heavy metals

A series of rice straw hydrochars were produced through a microwave-assisted hydrothermal treatment method, characterized and used for the adsorption of three organics and two heavy metals from aqueous solutions. The hydrochars have carbon contents from 37.44% to 43.31%, are rich in oxygen containing functional groups, and the equilibrium of hydrothermal carbonization reactions could be reached rapidly in microwave environment. The hydrochars can effectively adsorb the model pollutants, the maximum adsorption capacities of Congo red, berberine hydrochloride and 2-naphthol at 298 K and initial concentration of 0.5 mg/mL were 222.1, 174.0 and 48.7 mg/g, respectively, and those of Zn2+ and Cu2+ were 112.8 and 144.9 mg/g, respectively. Adsorption thermodynamic parameters were calculated. These results suggest that microwave-assisted hydrothermal treatment is an effective method for the rapid production of hydrochars, and rice straw hydrochars are promising adsorbents for the removal of water pollutants such as organics and heavy metals.

Evolution of Heavy Metal Speciation in MSWI Fly Ash after Microwave-assisted Hydrothermal Treatment

In this work, microwave irradiation was introduced into a hydrothermal process to solidify heavy metals in MSWI fly ash. The reagent, Na2HPO4, showed a significant effect on the synthesis of zeolit...

Adsorption of heavy metal ions using zeolite materials of municipal solid waste incineration fly ash modified by microwave-assisted hydrothermal treatment

In this work, microwave-assisted hydrothermal process was applied to modify the municipal solid waste incineration (MSWI) fly ash and get zeolitic production. The condition of hydrothermal process was fixed at 200 °C and 30 min. The 1 mol/L Na2HPO4 was selected as the additive, and the liquid to solid ratio was 3 ml/g. X-ray diffraction results revealed that analogous zeolite crystals formed by the MSWI fly ash during the hydrothermal process. The cation exchange capacity (CEC) was 0.498 meq/g of the modified fly ash, which increased about 22 times compared to the raw MSWI fly ash. To obtain the adsorption mechanism of the modified fly ash, the experiments of adsorption isotherm and kinetics were researched. The adsorption isotherms of heavy metal cations in mixed solution could be described by Langmuir isotherm equations, with a high correlation coefficient value. The experimental products were better fitted by the pseudo second-order kinetics rather than the pseudo first-order kinetics. Although the adsorption capacity of the modified fly ash needs to be promoted, it is a potential utilization for MSWI fly ash to be the original materials as adsorbents.

Simultaneous production of lignin and polysaccharide rich aqueous solutions by microwave-assisted hydrothermal treatment of rapeseed meal

This work addresses a novel and green process for the co-production of lignin and oligosaccharides from rapeseed meal, examining the effects of the temperature (150–210 °C), reaction time (0–60 min) and catalyst amount (1–4 mol/L, CH3COOH) on the process. The yields to gas, liquid and solid varied by 0–18%, 22–64% and 34–74%, respectively. The solid consisted of high purity lignin (26–88 wt.%) together with unreacted cellulose (0–28 wt.%), hemicellulose (0–28 wt.%) and proteins (11–28 wt.%). Increasing the temperature and/or reaction time produced an increase in the liquid yield and a decrease in the solid yield due to the solubilisation of the cellulosic and hemicellulosic contents of the feedstock. Acetic acid exerted a positive catalytic effect, promoting the solubilisation of cellulose and hemicellulose and preventing humins formation. The relative amounts (wt.%) of C, H, O and N in the solid fraction shifted between 46–63, 5.8–6.4, 28–42 and 2–6, respectively. Py-GC/MS analysis revealed that the solid decomposed into phenols (1–19%), sugars (0–15%), N-compounds (0–31%), carboxylic acids (37–75%), hydrocarbons (4–20%) and furans (1–8%). The liquid phase comprised oligo- and mono/di-saccharides (33–51 C-wt.%, 0–3 C-wt.% and 0–6 C-wt.%) and carboxylic acids (40–62 C-wt.%). The progressive solubilisation of cellulose and hemicellulose produced an increase in the proportion of C together with a decrease in the amounts of H and O in the solid product, which also accounted for the increase and decrease observed in the proportions of phenols and sugars, respectively. An optimum was found at 186 °C using an acid concentration of 1 mol/L and a total reaction time of 2 min. These conditions maximise the solubilisation of cellulose and hemicellulose without altering the lignin content of the solid; thus allowing the selective and simultaneous production of high purity (85 wt.%) lignin together with a rich oligossacharide (51 C-wt.%) solution. The acid can be recovered from the sugar mixture, which not only improves the efficiency of the process but also allows the production of a pure saccharide (92 C-wt.%) product.

Production of fermentable species by microwave-assisted hydrothermal treatment of biomass carbohydrates: reactivity and fermentability assessments

Microwave-assisted hydrothermal depolymerisation: an efficient process for the production of fermentable liquids from biomass carbohydrates.

Bottom-up preparation of nitrogen doped carbon quantum dots with green emission under microwave-assisted hydrothermal treatment and their biological imaging

Carbon quantum dots (CDs) have standout as one of the most important carbon nanomaterials owing to their small size, optical properties, biocompatibility and low toxicity. These CDs have been extensively explored for various applications especially in the biomedical fields. Although great efforts have been devoted to the fabrication of CDs through both top-down and bottom-up methods, most of these CDs exhibited blue emission under UV light irradiation. In this work, we reported a novel, green, economic approach for preparation of nitrogen doped CDs by directly microwave-assisted heating small organic molecules at 120°C for 5min using Glu and amino group of MPD as the precursors and ethylene glycol as solvent. Thus prepared CDs emit green fluorescence when they were irradiated under 365nm UV light. Compared with other synthetic methods, this microwave-assisted heating method does not require strict reaction conditions and is easy for operation. The CDs display unique luminescence properties, high water dispersibility and good biocompatibility. These remarkable features make CDs great potential for various biomedical applications. Taken together, a microwave-assisted heating method has been developed for bottom-up preparation of green emission CDs in a rather mild route. Owing to the facile experimental procedure and outstanding features of CDs, this work will open up a new avenue for fabrication and biomedical applications of CDs.

Catalytic Performances of Various Solid Catalysts and Metal Halides for Microwave-Assisted Hydrothermal Conversion of Xylose, Xylan, and Straw to Furfural

Furfural is a multifunctional green chemical and a precursor for many furan-based chemicals. The drawbacks of the current furfural production on industrial scale are the processes with low yields, and the corrosive and non-environmental catalysts are being used in the processes. Here, the one pot catalytic conversion of xylose, xylan, and straw biomass (i.e., wheat straw, triticale straw and flax shives) to furfural was investigated in the only single-phase aqueous system using various solid catalysts and metal halides under microwave irradiation. Four different solid catalysts (i.e., Amberlyst®-15 dry, Amberlyst®-36, Nafion®-NR50, and DOWEX 50WX8-200) were tested at selected ranges of temperatures (120–180 °C). Catalytic performance of 14 metal halides (i.e., FeCl3, LaCl3, CrCl3, AlCl3, NiCl2, MnCl2, MgCl2, CaCl2, LiCl, NaCl, KCl, LiBr, LiI, and NaBr) was evaluated at 180 °C. A substrate:liquid ratio of 1:15, a catalyst loading of 10% (w/w), a residence time of 20 min were employed as the fixed microwave process conditions. The Amberlyst-36 wet was the most efficient heterogeneous catalyst for the conversion of xylose at 180 °C, which resulted in a furfural yield of 25.34%. However, for the conversion of xylan under same reaction conditions, only ~ 1% furfural yield was observed. Among metal halides, FeCl3 was the best catalyst for the microwave-assisted hydrothermal conversion of wheat straw, triticale straw and flax shives to furfural with yields of 35.3, 41.6, and 65.3%, respectively. Microwave-assisted hydrothermal treatment in the presence of FeCl3 can be applied for both the saccharification of straw biomass and production of furfural from straw biomass.

Particulate, Structural, and Optical Properties of D-Glucose-Derived Carbon Dots Synthesized by Microwave-Assisted Hydrothermal Treatment

Environmentally friendly and visible fluorescent carbon dots (CDs) have received attention as alternatives to compound semiconductor quantum dots containing toxic elements. Hydrothermal reaction of water-soluble carbohydrates can provide an eco-friendly and facile synthesis approach to CDs. In this study, we have attempted to efficiently produce uniform CDs using a microwave heating system that can maintain the reaction solution at a constant temperature. We synthesized CDs from D-glucose through hydrothermal treatment at 200°C for 5–60 min. From transmission electron microscopy, the particle size of the CDs increased in proportion to the hydrothermal duration. Furthermore, the absorbance and maximum photoluminescence intensity of the CDs aqueous dispersions (10 mg L−1) increased with an increase in hydrothermal duration. The maximum photoluminescence intensity of the dispersions decreased at pH < 4 and pH > 10; the absorbance at the excitation wavelength remained unchanged. These results indicate that the number of emission sites and absorption sites increased with an increase in the particle size and that emission sites were located near the surface of the obtained CDs.