Microwave-hydrothermal Conditions Research Articles

Valorization of hexoses into 5-hydroxymethylfurfural and levulinic acid in acidic seawater under microwave hydrothermal conditions

ABSTRACT Typical value-added platform chemicals 5-hydroxymethylfurfural (HMF) and levulinic acid (LA) can be obtained from hexoses under microwave hydrothermal (MHT) conditions. This study explored the detailed transformation process regarding the MHT products in acidic seawater obtained using glucose and fructose as raw materials. The facile conversion of fructose compared with glucose was mainly ascribed to their different activation energies (56.721 and 88.594 kJ mol−1, respectively). The HMF and LA product yields were strongly affected by the MHT temperature and holding time in two types of hexose solution. Undesirable humins were found to inevitably form under each set of reaction conditions. The carbon balance results for reactants and products showed that up to 60% of fructose carbon was converted into value-added chemicals, while 47% of glucose carbon underwent the same conversion in acidic seawater under the optimal MHT conditions. This study provides further knowledge regarding the role of microwave heating combined with acidic seawater in green chemistry and is a useful reference for the biorefinery industry.

One-Pot Green Preparation of Fluorescent Cellulose Nanofibers.

Fluorescent cellulose nanofibers (FCNFs), with a high yield, were prepared via one-pot hydrolysis and the grafting reaction of cellulose with thiazolipyridine carboxylic acid (TPCA). The hydrolysis and Fischer esterification of cellulose were conducted under microwave-hydrothermal conditions; meanwhile, TPCA formation was induced by the dehydration reaction between L-cysteine and citric acid. The effects of the reaction temperature and reaction time on the yield and performance of FCNF were investigated. The morphology and size, surface chemical property, crystal structure, thermostability, and fluorescent performance of FCNF were characterized. The results revealed that the yield of FCNF reached 73.2% under a microwave power of 500 W, reaction temperature of 110 °C, and reaction time of 5 h. The FCNF obtained presents a short rod-like morphology. The crystallinity of the FCNFs is 80%, and their thermal stability did not decline significantly. Additionally, the fluorescent performance of the FCNFs is excellent, which results in them having good sensitivity to chloride ions. The good fluorescent performance and significant responsiveness to chloride ions of FCNFs lead to them having broad prospects in bio-labeling, biosensing, information storage, chloride ion detection, among others.

Hierarchical ultrathin NiFe-borate layered double hydroxide nanosheets encapsulated into biomass-derived nitrogen-doped carbon for efficient electrocatalytic oxygen evolution

Exploring highly efficient, cost-effective, and stable electrocatalysts toward oxygen evaluation reaction (OER) is of great significance for hydrogen generation. Herein, in situ growth of NiFe-borate layered double hydroxide (NiFe-BLDH) nanosheets on waste biomass-derived nitrogen-doped carbon (NC) with hierarchical architecture under microwave hydrothermal conditions is reported. The as-synthesized NiFe-borate layered double hydroxide/nitrogen-doped carbon (NiFe-BLDH/NC) exhibits outstanding OER activity, showing an extremely low overpotential of 243 mV at the current density of 10 mA cm−2 and a relatively small Tafel slope of 42.7 mV dec−1, which are superior to the commercial RuO2 and many previously reported NiFe-LDH-based OER electrocatalysts. Additionally, the catalyst also shows remarkable stability with unobvious decrease of current density over a period of 24 h. These excellent catalytic performances could be attributable to the synergistic effects between the conductive NC scaffold and NiFe-BLDH with excellent hydrophilicity caused by the intercalated borate anions, which provide many advantages, such as large surface area, abundant active sites, fast charge and mass transport pathways. This work provides new insights into the design and facile preparation green and efficient electrocatalysts

PEG-assisted microwave hydrothermal growth of spherical mesoporous Zn-based mixed metal oxide nanocrystalline: Ester production application

In the present research, various spherical mesoporous ZnO-based nanocrystalline(ZnAl2O4, CuO-ZnO, TiO2-ZnO) were synthesized via post-calcination treatment at 600 °C for 4 h over N2 flow gas, which initially went through a simple PEG-assisted microwave hydrothermal route by adapting the varieties of PEG (i.e. PEG 2000, PEG 4000, PEG 6000, PEG 10000, and PEG 20000) using incomplete carbonized glucose (ICG) as a template. The as-synthesized mesoporous ZnO-basednanocrystallineswere further characterized by mean of XRD, N2 adsorption and desorption, NH3-TPD, FESEM, and EDX. The experimental outcomes revealed that the extent of PEG 6000 in mixture played the vital role in the growth of spherical mesoporous ZnO-based nanocrystallines compared to other PEG molecular weights. The N2 adsorption and desorption data confirmed the mesopore structure of the samples, possessing the specific surface areas in the range of 450.35–592.55 m2g−1. By verifying the optimum PEG-assisted microwave hydrothermal conditions, one-pot polymerization of poly(sodium4-styrenesulfonate) was employed to activate surface areas and mesopore walls by -SO3H functional groups using for ester productionthrough transesterification of used cooking oil, resulted in methyl ester content of 97.4% and excellent reusability for ten sequential reactions withoutanyfurtherregeneration. Moreover, biodiesel quality evaluation analysis showed that the prepared methyl ester satisfied both EN (14214) and ASTM (D6751) specifications.

Morphological aspects of Bismuth ferrite powders prepared by microwave-assisted hydrothermal method

BiFeO3 particles were synthetized by a fast, reproducible and environmental friendly microwave-assisted hydrothermal method. The aim of the study was the investigation of increased pressure and mineralizer concentration on the reaction mechanism, morphology of particles and aggregates as well as magnetic behavior. Powders were studied when using KOH mineralizer concentration from 4 to 12 M at room temperature and under microwave-hydrothermal conditions at 40 bar and 150 or 200°C. The reaction mechanism depends on the temperature of the microwave-hydrothermal treatment and the concentration of KOH mineralizer. The precursor powder obtained after coprecipitation step consists of Bi2O3, Fe2O3 and Bi25FeO40 in different ratios. After the treatment at 40 bar and 150°C for 1h the reaction between precursors is not complete, therefore resulting secondary phases of Bi25FeO40 and Fe2O3. BiFeO3 particles were successfully synthesized at 40 bar under microwave-hydrothermal conditions at a temperature of 200°C and a concentration of KOH of at least 8M. At 6M, the composition consisted of 98.5% BiFeO3 and 1.5% Bi2O3 as evidenced by Rietveld refinement. The competition between dissolution and precipitation of Bi3+ and Fe3+ in KOH solutions of different concentrations was investigated by Raman spectroscopy and EDS analysis. Thus, at 200°C there was evidenced a Bi2O3 secondary phase for 6 M KOH concentration and poor control of Bi/Fe ratio in BiFeO3 at KOH concentrations of 10 and 12 M. Particle and aggregates morphology depicted by FE-SEM investigations showed a tendency to form irregular shaped particles at KOH concentration of 6 M, for which the reaction is not complete. When increasing the concentration, BiFeO3 particles exhibit regular shapes from cubic at 8 M to cvasi-spherical at 14 M. M-H curves characteristics illustrate that Bi/Fe ratio and preferential orientation of crystal growth influence the magnetic behavior of BiFeO3 crystalline powders.

Ordered mesoporous Zn-based supported sorbent synthesized by a new method for high-efficiency desulfurization of hot coal gas

A new approach (microwave-hydrothermal plus oxidation) was proposed for the preparation of the Zn-based sorbents supported on the ordered mesoporous Si-based material. The hydrothermal synthesis and oxidation conditions of the sorbent precursor (ZnS/MCM41 mesophase) with Zn(AC)2 and thioacetamide (TAA) as the Zn and S sources, respectively, were optimized. The sorbents were evaluated in a fixed bed using the simulate gas of 2000 ppm H2S, 39% H2, 27% CO, 12% CO2, and N2 as balance gas. The structure of the sorbents was characterized by means of XRD, N2 adsoprtion, SEM, EDX, and TEM analysis. The results indicate that the optimal microwave-hydrothermal conditions are 400 W, 2.5 h, and 1:3 (the mole ratio of the Zn(AC)2 to TAA). The oxidation of ZnS in the sorbent precursor to ZnO is invalid at 550 °C due to unfavorable reaction kinetics. Both the fresh and used sorbents show ordered hexagonal mesoporous structure. It is confirmed that Zn2SiO4 present in the fresh and regenerated sorbents could be consumed during the desulfurization process via the reaction (Zn2SiO4 + 2H2S → 2ZnS + SiO2 + 2H2O), giving a positive effect on the desulfurization of the sorbents. The optimized Zn-based mesoporous sorbent (actual Zn content: 20.3%) exhibits superior (high breakthrough sulfur capacity: 5.4–5.7%) and stable desulfurization ability during five sulfidation/regeneration cycles. In addition, sulifidation reaction only leads to low-degree plugging of the pore structure of the regenerated sorbents, and the regeneration reaction could restore (at least mostly) the aforementioned changes in the pore properties.

Microwave Hydrothermal Assessment for CSH Formation in Silica-Lime-Water Systems

Microwave energy as a fast technique was utilized for the preparation of calcium silicate hydrates using hydrothermal reaction of silica quartz and/or silica fume with active lime in a solution. The silica/lime ratio was selected to be 60/40 in a mix as compared with the composition of sand-lime-based construction industries. The CSH formation was assessed after treatment at 140 and 160 °C under saturated steam microwave hydrothermal conditions. Moreover, the CSH formation was studied at 160 °C for periods up to 180 min. The prepared samples were characterized through XRD and FTIR. Further, DSC and SEM tools were used to investigate selected samples. The results led to the conclusion that the complete reaction of lime with the available silica sand was pronounced for samples prepared under saturated steam at 160°C/2 h, forming cross network needle structure morphology as seen from SEM results. Moreover, the results confirm the conversion phenomenon of CSH compounds changing in reaction time or in reactive silica percentage into another form of CSH. An amorphous structure like tobermorite is formed in samples containing silica fume as source of silica and microwave hydrothermally cured at 160 °C/2 h.

Effects of a donor concentration on the structure of Nb-doped nano-sized BaTiO3 powders prepared by microwave-hydrothermal synthesis methods

BaTi1–xNbxO3 compounds (with x = 0.00, 0.01, 0.03, 0.06, and 0.09) are synthesized by the microwavehydrothermal (MH) method. The donor concentration effect on the structural properties is investigated. The new MH method is used instead of the previous solid state reaction for the BaTiO3+Nb2O3 system. In the MH process, as the niobium incorporation rate increases, the particle growth substantially slows down. The synthesis of Nb-doped BaTiO3 is investigated under the MH conditions subjected to 150°C for only 2 h using C16H36O4Ti (tetra-n-butyl orthotitanate), Ba(OH)2∙8H2O, and NbCl5 as Ba, Ti, and Nb sources respectively. For the phase evolution studies, X-ray diffraction patterns are analyzed and Raman spectroscopy is performed. The transmission electron microscope and the field emission scanning electron microscope images are taken for the detailed analysis of the grain size, surface, and morphology of the compound.

Preparation of calcium silicates with long-fiber (needle) particles

The results of our experimental studies of processes for the synthesis of calcium silicate and hydrosilicate powders with needle-shaped particles from phosphogypsum (technogenic calcium-containing raw material) and sodium silicate block (silicon-containing product of chemical industry) under hydrothermal and microwave-hydrothermal conditions are presented.

Microwave-hydrothermal synthesis of Nb-doped BaTiO3 nanoparticles under various conditions

Tetragonal BaTiO3 powders were synthesized by the microwave-hydrothermal (MH) method. The effects of the reaction time and temperature on the MH synthesis were investigated. Typical experiments were performed with a solid state reaction for the system BaTiO3 + Nb2O3 at a high temperature. In the MH method, at a shorter time and lower temperature, the rate of the formation of tetragonal BaTiO3 and of the growth of particles went down substantially. The microwave heating can be very fast and uniform through a self-heating process that results from the direct absorption of the microwave energy in the reaction mixture. The synthesis of Nb-doped BaTiO3 has been investigated under MH conditions in the temperature range of 120–180°C for 1–3 hours using C16H36O4Ti, BaH2O2 · 8H2O, and NbCl5 as Ba, Ti and Nb sources, respectively. In the phase evolution studies, the X-ray diffraction measurements and Raman spectroscopy were employed. The Transmission Electron Microscope and the Field Emission Scanning Electron Microscopy images were taken for the detailed analysis of the grain size, surface, and morphology. The MH method was applied for the synthesis of Nb-doped BaTiO3 via provided advantages of the fast crystallization and decreased crystallite size. The nucleation rate should have been high because of a high heating rate in microwave heating processes.

Deposition of WO3 on Al2O3 via a microwave hydrothermal method to prepare highly dispersed W/Al2O3 hydrodesulfurization catalyst

A microwave hydrothermal method was developed to deposit WO3 on Al2O3 for rapidly preparing W/Al2O3 hydrodesulfurization (HDS) catalysts with both high tungsten dispersion and weak tungsten–alumina interaction. The catalysts were characterized by XRD, XPS, N2 adsorption, H2-TPR, NH3-TPD and HRTEM; their activity was tested by HDS of dibenzothiophene. Compared to the previous hydrothermal deposition method, the microwave hydrothermal method can further increase the dispersion of tungsten and attain the similar weak tungsten–alumina interaction, leading to the shorter and similar stacked WS2 and hence higher HDS activity, whereas the hydrothermal period is dramatically reduced from 12h to 15min. Moreover, this method can highly disperse more tungsten on alumina than the hydrothermal deposition and impregnation methods. It is attributed to the rapid nucleation and inhibited growth of WO3, and the enhanced mobility induced by microwave hydrothermal conditions. The significance of this method lies in its simplicity and efficiency for preparing tungsten-based catalysts with superior HDS activity.

Microwave hydrothermal disassembly for evolution from CuO dendrites to nanosheets and their applications in catalysis and photo-catalysis

CuO nanosheets and dendrites have been synthesized by a microwave hydrothermal method. The synthesized CuO nanostructures were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and transition electron microscopy (TEM). The formation process was discussed, which revealed that the disassembly of sheet-built CuO dendrites contributed to the formation of discrete CuO nanosheets under microwave hydrothermal conditions. However, such a disassembly process was inhibited by an ultrasonic heat pretreatment before the microwave hydrothermal treatment. The synthesized CuO nanostructures were active toward the thermal decomposition of ammonium perchlorate (AP) and allowed the decomposition temperature of AP to decrease. Furthermore, the synthesized CuO nanostructures promoted the photodegradation of rhodamine B and methyl orange.

Microwave Hydrothermal Transformation of Amorphous Calcium Carbonate Nanospheres and Application in Protein Adsorption

Calcium carbonate and calcium phosphate are the main components of biominerals. Among all of the forms of biominerals, amorphous calcium carbonate (ACC) and amorphous calcium phosphate (ACP) are the most important forms because they play a pivotal role in the process of biomineralization and are the precursors to the crystalline polymorphs. In this work, we first synthesized ACC in vitro using adenosine 5'-triphosphate disodium salt (ATP) as the stabilizer and investigated the transformation of the ACC under microwave hydrothermal conditions, and ACC/ACP composite nanospheres and carbonated hydroxyapatite (CHA) nanospheres were successfully prepared. In this novel strategy, ATP has two main functions: it serves as the stabilizer for ACC and the phosphorus source for ACP and CHA. Most importantly, the morphology and the size of the ACC precursor can be well-preserved after microwave heating, so it provides a new method for the preparation of calcium phosphate nanostructured materials using phosphorus-containing biomolecule-stabilized ACC as the precursor. Furthermore, the as-prepared ACC/ACP composite nanospheres have excellent biocompatibility and high protein adsorption capacity, indicating that they are promising for applications in biomedical fields such as drug delivery and protein adsorption.

MICROWAVE-HYDROTHERMAL SYNTHESIS AND CHARACTERIZATION OF HIGH-PURITY NB DOPED BATIO3 NANOCRYSTALS

The synthesis of Nb doped BaTiO3 has been investigated under Microwave-Hydrothermal (MH) conditions in the temperature of 150°C for only 2 h using C16H36O4Ti, BaH2O2.8H2O and NbCl5 as Ba, Ti and Nb sources, respectively. Typical experiments performed on MH processing have not yet reported for Nb doped BaTiO3. In the MH process, the formation of high purity nano tetragonal NbBaTiO3 was strongly enhanced. New hydrothermal method was used instead of the previous solid state reaction for the BaTiO3±Nb2O3 system. The new method uses high pressure to create nano dimension particles in a lower time and temperature. In case of the phase evolution studies, the XRD pattern measurements and Raman spectroscopy were performed. TEM and FE-SEM images were taken for the detailed analysis of the particle size, surface and morphology. Synthesis of Nb doped BaTiO3 with the Microwave-hydrothermal provides an advantage of fast crystallization and reduced crystal size when compared to existing methods.

Effects of Microwave-Hydrothermal Conditions on the Purity and Electrochemical Performance of Orthorhombic LiMnO2

This paper reports the different reaction conditions to synthesize the orthorhombic LiMnO2 (hereafter referred as o-LiMnO2) and to find the economic usage of raw material to obtain the pure o-LiMnO2 with ideal electrochemical performance via effective and energy-saving microwave-hydrothermal (MH) routine. The product formation mechanism in different process is discussed in detail. X-ray diffraction patterns (XRD) indicate that the MH temperature and time are more important than the LiOH concentration of the precursors to influence the phase purity of the products. The pure phased o-LiMnO2 prepared under optimized MH condition exhibits the maximum discharge capacity of 228 mA h/g and reversible capacity of 160 mA h/g after 50 cycles at 0.1 C rate. When the current density increased to 0.2 and 0.5 C, the maximum discharge capacities of o-LiMnO2 maintain at 197 and 165 mA h/g, respectively. The o-LiMnO2 obtained in controlled MH condition shows the enhanced electrochemical performance compared with those synthesized by other method, indicating its potential application in the lithium-ion battery field.

Novel synthesis of nanophase anatase under conventional- and microwave-hydrothermal conditions: DeNOx properties

Abstract Nanophase anatase was synthesized by a novel simplified one-pot method using Ti alkoxides and hydrogen peroxide under conventional- and microwave-hydrothermal conditions. The specific surface areas (SSAs) of anatase ranged from 147 to 298 m2/g under different hydrothermal conditions but higher SSAs were obtained under microwave-hydrothermal conditions. The novel method developed here results in nanophase anatase with superior DeNOx properties.

Microwave–Hydrothermal Crystallization of Polymorphic MnO2 for Electrochemical Energy Storage

We report a coupled microwave–hydrothermal process to crystallize polymorphs of MnO2 such as α-, β-, and γ-phase samples with plate-, rod-, and wirelike shapes, by a controllable redox reaction in MnCl2–KMnO4 aqueous solution system. MnCl2–KMnO4 redox reaction system was for the first time applied to MnO2 samples under the coupled microwave–hydrothermal conditions, which shows clear advantages such as shorter reaction time, well-crystallized polymorphic MnO2, and good electrochemical performances as electrode materials for lithium ion batteries. For comparison, we also did separate reactions with hydrothermal only and microwave only in our designed MnCl2–KMnO4 aqueous system. The present results indicate that MnCl2–KMnO4 reaction system can selectively lead to α-, β-, and γ-phase MnO2, and the as-crystallized MnO2 samples can show interesting electrochemical performances for both lithium-ion batteries and supercapacitors. Electrochemical measurements show that the as-crystallized MnO2 supercapacitors have...

Hierarchical Hollow Hydroxyapatite Microspheres: Microwave‐Assisted Rapid Synthesis by Using Pyridoxal‐5′‐Phosphate as a Phosphorus Source and Application in Drug Delivery

Three-dimensional (3D) hydroxyapatite (HAP) hierarchical nanostructures, in particular hollow nanostructures, have attracted much attention owing to their potential applications in many biomedical fields. Herein, we report a rapid microwave-assisted hydrothermal synthesis of a variety of hydroxyapatite hierarchical nanostructures that are constructed by the self-assembly of nanorods or nanosheets as the building blocks, including HAP nanorod-assembled hierarchical hollow microspheres (HA-NRHMs), HAP nanorod-assembled hierarchical microspheres (HA-NRMs), and HAP nanosheet-assembled hierarchical microspheres (HA-NSMs) by using biocompatible biomolecule pyridoxal-5'-phosphate (PLP) as a new organic phosphorus source. The PLP molecules hydrolyze to produce phosphate ions under microwave-hydrothermal conditions, and the phosphate ions react with calcium ions to form HAP nanorods or nanosheets; then, these nanorods or nanosheets self-assemble to form 3D HAP hierarchical nanostructures. The preparation method reported herein is time-saving, with microwave heating times as short as 5 min. The HA-NRHMs consist of HAP nanorods as the building units, with an average diameter of about 50 nm. The effects of the experimental conditions on the morphology and crystal phase of the products are investigated. The hydrolysis of PLP under microwave-hydrothermal conditions and the important role of PLP in the formation of 3D HAP hierarchical nanostructures are investigated and a possible formation mechanism is proposed. The products are explored for potential applications in protein adsorption and drug delivery. Our experimental results indicate that the HA-NRHMs have high drug/protein-loading capacity and sustained drug-release behavior. Thus, the as-prepared HA-NRHMs are promising for applications in drug delivery and protein adsorption.

Microwave Hydrothermal Synthesis of Nano Co3O4 with Various Morphologies

The precursors with various morphologies including nanowires, nanosheets, and hexagonal nanoplatelets were synthesized under different conditions by the microwave hydrothermal method. The precursors are transformed into Co3O4 with various nanostructures through thermal decomposition. It is very important to synthesize the precursors controlling the microwave hydrothermal conditions, such as the mineralizer, the reaction temperature, and the reaction time. The microwave assisted hydrothermal approach presented here opens a potential avenue to explore the synthesis of various other metal oxides with different nanomorphologies.

Synthesis, characterization and catalytic activities of bimetallic modified MCM-41 for epoxidation of styrene

Mesoporous molecular sieves MCM-41 modified by bimetal (Zr and Ti) ions with ordered hexagonal arrangement were prepared by direct synthesis under microwave–hydrothermal conditions at 403 K. FT-IR, N2 adsorption–desorption, scanning electron microscope, high-resolution transmission electron microscope, element mapping, X-ray photoelectron spectroscopy, diffuse Reflectance UV–Visible Spectra etc. were used to characterize as-prepared materials. The results showed that Zr and Ti were introduced into MCM-41 and bonds of Zr–O–Si and Ti–O–Si were formed in framework of the MCM-41. Moreover, their structures with ordered arrangement were still retained. The microwave–hydrothermal method might shorten the synthesis time from days to hours and the as-prepared catalyst had a high catalytic activity and selectivity for the liquid-phase epoxidation of styrene.