Microwave Hydrothermal Treatment Research Articles

Synthesis of zeolite from coal fly ash by microwave hydrothermal treatment with pulverization process

Coal fly ash was hydrothermally treated with a NaOH aqueous solution at 373K using microwave heating with zirconia beads. We investigated the effect of the pulverization of coal fly ash on the generation rate and crystalline phase of synthesized zeolite. As a result, it was found that the pulverization process increased the generation rate of phillipsite in the early stage of the hydrothermal treatment because it enhances the dissolution of aluminate and silicate ions from coal fly ash and the generation rate of the aluminosilicate-gel precursor. On the other hand, prepulverization before the hydrothermal treatment and long-term pulverization during the hydrothermal treatment promote the formation of hydroxysodalite rather than phillipsite. In addition, we examined the effect of the addition of an aluminum source on the yield of synthesized phillipsite. This examination revealed that the excess addition of aluminate ions promotes the generation of hydroxysodalite and that timely and proper addition improves the yield of synthesized phillipsite. From these results, it was concluded that pulverization in the early stage of the hydrothermal treatment is effective for the generation of phillipsite from coal fly ash. Moreover, the timely and proper addition of an aluminum source improves the yield of synthesized phillipsite.

In situ impregnation−gelation−hydrothermal crystallization synthesis of hollow fiber zeolite NaA membrane

Chitosan-assisted in situ impregnation−gelation−hydrothermal (IGH) crystallization process has been developed for the preparation of hollow fiber zeolite NaA membranes. Firstly, chitosan-zeolite NaA composite hollow fibers were prepared successfully by assistance of a simple homemade tube-in-orifice spinneret. The composite hollow fibers were initially prepared by in situ impregnation–gelation–hydrothermal transformation of chitosan-silica hollow fibers in aluminate solution. Zeolite NaA membranes can be subsequently obtained on the outer surface of chitosan-zeolite NaA composite hollow fibers by in situ microwave hydrothermal treatment. The zeolite crystals in the composite hollow fibers serve as seeds for the growth of zeolite membrane. Moreover, the chitosan-silica hollow fibers were prepared by solidification of chitosan hollow fibers, which were formed in the tube-in-orifice spinneret from a chitosan-silica sol viscous aqueous solution, in the sodium hydroxide solution. Pervaporation for separation of 90 wt% ethanol aqueous solution was employed to examine the obtained membranes. The hollow fiber membranes showed high permeation flux and high stability.

Synthesis of zinc oxide by microwave hydrothermal method for application to transesterification of soybean oil (biodiesel)

ZnO nanostructures were synthesized by microwave hydrothermal treatment using two different mineralization agents (NaOH and NH4OH), and were evaluated as catalysts for biodiesel synthesis. The materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Brunauer-Emmett-Teller (BET) surface area analysis. The XRD patterns indicated the formation of the hexagonal wurtzite phase in both samples. SEM analysis showed completely different morphologies based on the mineralization agent employed. The ZnO nanostructures synthesized with NaOH (ZONa5 and ZONa5P) presented plate-like agglomerates, resulting in a quasi-spherical morphology, whereas the materials synthesized with NH4OH (ZONH5 and ZONH5P) presented a flower-like morphology. The ZONa5P sample showed an activity of 77.82% for the catalytic conversion of soybean oil into biodiesel by transesterification using methanol.

Impounding of ortho-Chlorophenol by Zeolitic Materials Adapted from Bagasse Fly Ash: Four Factor Three Level Box-Behnken Design Modelling and Optimization

The present study enlightens the impounding of ortho-chlorophenol (OCP) onto zeolitic composites derived from agricultural waste Bagasse Fly Ash (BFA). The OCP impounding was enhanced by modifying native BFA by electrolyte supported microwave hydrothermal treatment (MZBFA) and magnetic modification (MMZBFA). The synthesized sorbents were characterized by instrumental techniques (XRF, PXRD, SEM and FTIR). The adsorption process was optimized under four different process variables like: pH (4–7), agitation time (30–120 min), initial sorbate concentration (\({50-150\,{\rm mg}\,{\rm L}^{-1}}\)), sorbent dosage (\({1-4\,{\rm g}\,{\rm L}^{-1}}\)) based on Box-Behnken design with response surface methodology. The highest predicted adsorption capacities at pH 7 with \({1\,{\rm g}\,{\rm L}^{-1}}\) sorbent dose for MZBFA and MMZBFA were found to be 29.95 and \({31.94\,{\rm mg}\,{\rm g}^{-1}}\) for agitation time of 75 and 120 min and initial sorbate concentration 150 and \({100\,{\rm mg}\,{\rm g}^{-1}}\) , respectively, which were approximated with laboratory results. The isotherm data and kinetic data were best described by Langmuir model and pseudo-second-order model, respectively. The sorbents are stable after three cyclic runs when regenerated with 0.5M NaOH solution.

Lithium-doped hydroxyapatite nano-composites: Synthesis, characterization, gamma attenuation coefficient and dielectric properties

Lithium-hydroxyapatite (0, 1, 5, 10, 20, 30 and 40wt% Li-HAp) nano-composites were synthesized by sol-gel technique followed by microwave-hydrothermal treatment. The composites were characterized by X-ray diffraction (XRD), Field emission scanning electron microscope (FE-SEM), energy dispersive spectroscopy (EDS), Fourier transform infrared (FTIR) and Raman techniques. Gamma attenuation coefficient and the dielectric properties for all composites were investigated. The crystallinity degree of Li-doped HAp was higher than that of un-doped HAp. Gamma attenuation coefficient values increased from 0.562cm−1 for 0wt% Li-HAp to 2.190cm−1 for 40wt% Li-HAp. The alternating current conductivity increased with increasing frequency. The concentration of Li affect the values of dielectric constant where Li doped HAp of low dielectric constant can have an advantage for healing in bone fractures. The calcium to phosphorus ratio decreased from 1.43 to 1.37 with the addition of lithium indicating the Ca deficiency in the studied composites. Our findings lead to the conclusion that Li-HAp is a new nano-composite useful for medical applications and could be doped with gamma shield materials.

Preparation of nanoporous nickel phosphate VSB-5 nanorods carbon paste electrode as glucose non-enzymatic sensor

In this paper, nanoporous nickel phosphate VSB-5 nanorods was synthesized with microwave irradiation and hydrothermal treatment and characterized by X-ray diffraction, FT-IR spectroscopy, Filed emission scanning electron microscopy (FESEM) and EDS analysis. FESEM technique exhibited the presence of nickel phosphate nanorods morphology. The BET surface area, total pore volume and average pore diameter of synthesized nickel phosphate were obtained to be 102.5 m2 g−1, 0.14 cm3 g−1 and 3.6 nm, respectively. Carbon paste electrode (CPE) was modified by VSB-5 nanorods and then Ni2+ ions incorporated to this electrode to obtain Ni-VSB-5/CPE. The current intensity of glucose oxidation increases impressively on the surface of Ni-VSB-5/CPE and modified CPE can reduce the overvoltage of glucose oxidation in comparison with Ni-CPE. The values of electron transfer coefficient, diffusion coefficient and mean value of catalytic rate constant for glucose and redox sites of electrode were found to be 0.87, 6.18 × 10−4 cm2 s−1 and 3.06 × 105 cm3 mol−1 s−1, respectively. The good catalytic activity, high sensitivity, good selectivity and stability and ease of preparation rendered the Ni-VSB-5/CPE to be a capable electrode for glucose electro-oxidation.

Synthesis of Bi–Fe–Sb–O Pyrochlore Nanoparticles with Visible‐Light Photocatalytic Activity

AbstractA method has been developed for the preparation of Bi–Fe–Sb–O pyrochlore nanoparticles by microwave hydrothermal treatment. Experiments revealed that the preparation procedure for the precursors is an important factor that affects the phase composition of the product. The morphology of the pyrochlore particles was investigated by SEM, HRTEM, and selected‐area electron diffraction (SAED) methods. The samples consist of numerous spherical agglomerates from 100 to 200 nm formed from 20 nm mutually oriented nanocrystallites. A mechanism for the formation of the particles is proposed, and the magnetic and photocatalytic properties of the Bi–Fe–Sb–O pyrochlore nanocrystallites are reported.

Selective hydrothermal microwave synthesis of various manganese dioxide polymorphs

Hydrothermal microwave treatment of mixed solutions of potassium permanganate and hexamethylenetetramine within the pH range 0.5–6.9, resulted in various polymorphs of nanocrystalline manganese dioxide: α-MnO2 (cryptomelane), γ-MnO2 (nsutite), β-MnO2 (pyrolusite), and δ-MnO2 (birnessite). The pH values of the medium at which single-phase samples form were determined.

Testing nanocrystalline CdWO4 doped with Yb3+ as a possible down-conversion phosphor

The CdWO4:Yb3+ (0.1, 2, 3%) nanophosphors have been synthesized using microwave-hydrothermal treatment. The spectral and kinetic characteristics of their luminescence were studied with the aim to check the possibility of cooperative energy transfer from CdWO4 self-trapped excitons to Yb3+ pairs, which can provide down-conversion of UV photons absorbed by the CdWO4 host into two lower-energy (IR) photons emitted by two Yb3+ ions. The obtained results have shown the conversion of absorbed UV photons into Yb3+ IR fluorescence. However, the obtained parameters of decay kinetics of both intrinsic luminescence of CdWO4 and IR fluorescence of Yb3+ did not confirm the existence of cooperative energy transfer from STE to two Yb3+ ions in the studied CdWO4:Yb3+ nanophosphors. The reason is supposed to be rather low concentrations of Yb3+ obtained in the synthesized samples which prevents the creation of high enough concentration of Yb3+ pairs with short distances between the ions.

Cellulose biotemplates for layered double hydroxides networks

Synthesis of porous network structures of layered double hydroxides (LDH) has been studied using microcrystalline cellulose (MCC) as a hard template. LDH was synthesized using cellulose as biotemplate and post treated by microwave hydrothermal treatment (MWHT). The prepared LDHs were thoroughly characterized by Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy, X-ray Diffraction (XRD) studies, and Thermogravimetric (TGA) analysis, which proved the formation of LDH structure in the presence and absence of MCC as templates. The morphological structures of the composite materials were analyzed by Scanning Electron Microscopy (SEM), which showed that the LDH formed in the presence of cellulose had porous network structures, while LDH particles synthesized in the absence of the MCC exhibited a hexagonal platelet-like morphology. Energy Dispersive X-ray (EDX) measurements confirmed the presence of essential elements in the particles. TEM showed the fibrillar nature and size reduction of the particles. The surface area of the material was measured by Brunauer–Emmett–Teller (BET) analysis, exhibiting a twofold increase in surface area and increased specific pore volume in comparison with particles synthesized in absence of cellulose and thus showcasing the potential of MCC as templates for production of porous LDH with high surface area (152 m2/g). Further, the adsorption capacities of the materials were evaluated with orange II dye.

Rapid synthesis of tunable-structured short-pore SBA-15 and its application on CO2 capture

The well-ordered hexagonal short-pore SBA-15 (SSBA-15) has been successfully synthesized via an energy-efficient process under the microwave-hydrothermal condition, in which the formation duration [stirring duration (ripening step) and microwave-hydrothermal treatment duration (aging step)] has been greatly decreased from 48 h for conventional hydrothermal treatment to <2 h. The extension of the ripening duration led to the evidently improved uniformity and regularity of particles while the increase of aging duration resulted in less intrawall pores and more structural mesopores. The optimal condition to get highly ordered SSBA-15 in this work was the duration of 2 h for both steps, and the SSBA-15 obtained was composed of cuboid-like particles, whose pore length was ca. 450 ± 50 nm and pore size was around 10 nm. The SSBA-15 was modified with different amine agents to prepare solid amine adsorbent, the obtained adsorbents showed considerable CO2 adsorption ability, indicating that this fast-prepared SSBA-15 can be a promising support of CO2 adsorbents.

BiPO4@glucose-based C core–shell nanorod heterojunction photocatalyst with enhanced photocatalytic activity

BiPO4@glucose-based C (BiPO4@C) core–shell nanorod heterojunction photocatalyst was successfully prepared by the facile microwave hydrothermal method. [BiPO4(hexagonal phase)]H+ could be absorbed by the negatively charged glucose molecule to replace the hydroxyl of the glucose molecule. BiPO4@C was formed by the carbonization of glucose around the BiPO4 nanorods involving the intermolecular cross-linking and dehydration of the glucose molecules during the microwave hydrothermal treatment. The steric hindrance effect inhibited the crystal growth of BiPO4. BiPO4@C core–shell nanorod heterojunction photocatalyst showed an enhanced photocatalytic activity for the degradation of Rhodamine B (RhB) under UV light irradiation. The degree of photocatalytic activity enhancement strongly depended on the coverage of carbon on the surface of BiPO4. A carbon shell of about 10 nm thickness for BiPO4@21C exhibited the highest photocatalytic activity. Greater than 94% photodegradation of RhB under the exposure of UV light was achieved within 10 min, and its degradation rate was 4.1 times higher than that of commercial TiO2 (P25) and 2.3 times higher than that of pure BiPO4. This was attributed to the synergistic effect based on the intimate interfacial contacts between the carbon layers and BiPO4, which could prolong the lifetime of photogenerated electron–hole pairs. Moreover, the TOC removal efficiency could reach 60% for BiPO4@21C after 20 min of UV photocatalytic reaction, which exceeded that of BiPO4 by 2.3 times. BiPO4/21C displayed very low photocatalytic activity because of the destruction of BiPO4@C core–shell structure and the increase of bulk defects in the BPO4 lattice after calcination.

NIR fluorescence quenching by OH acceptors in the Nd3+ doped KY3F10 nanoparticles synthesized by microwave-hydrothermal treatment

We performed site-selective spectroscopy and analyzed static (Direct Energy Transfer, DET) Förster kinetics of impurity quenching N(t) in the water-dispersible 0.1% Nd3+: KY3F10 nanoparticles (NPs) synthesized by microwave-hydrothermal treatment (MWHT) to determine the acceptor space dimension D and estimate the concentration of –OH acceptors. As a result we found two types of optical sites for Nd3+ ions and revealed that a moderate amount of –OH quenching acceptors is distributed in the volume of the nanoparticles, rather than on their surface. The result is similar to the one obtained for the previously studied water-dispersible the Nd3+: KYF4 and Nd3+: YPO4 NPs, and thus may be extended to all nanoparticles synthesized by water-based techniques. Therefore, all of the previously obtained results concerning the fluorescence quenching of such NPs should be reviewed in the light of the new data on the actual source of the quenching. Also, we compared quantitatively the near IR fluorescence quenching in the Nd3+: KY3F10 NPs with that in the Nd3+ doped KYF4 and YPO4 nanoparticles with close sizes and size distribution (all the nanoparticles were synthesized by MWHT). We found minimum fluorescence quenching and accordingly maximum fluorescence quantum yield for the Nd3+: KY3F10 nanocrystals, which is in agreement with the relative amount of –OH molecular groups in the volume of the NPs estimated from Förster kinetics and with the results of TG/DTG-DTA data analysis. The analysis of the fluorescence kinetics of the 1% Nd3+: KY3F10 NPs revealed the realization of the late fluctuation stage. The formation of the fluctuation stage confirmed the high concentration of the –OH acceptors in the volume of the NPs compared to the optically active Nd3+ ions. Furthermore, the analysis of the fluctuation stage confirmed the reduced –OH concentration in the Nd3+ doped KY3F10 NPs in comparison with the KYF4 and YPO4 NPs. This makes the Nd3+: KY3F10 NPs especially promising material for near IR imaging.

Promotional effect of microwave hydrothermal treatment on the low-temperature NH3-SCR activity over iron-based catalyst

Microwave hydrothermal was introduced into the preparation of iron–cerium–titanium mixed oxide catalyst for NH3-SCR of NOx, and the catalysts after microwave hydrothermal treatment or not are characterized by X-ray diffraction (XRD), Infrared spectrum (IR), Transmission Electron Microscope (TEM), nitrogen adsorption–desorption and temperature programmed desorption (NH3/NO-TPD). Microwave hydrothermal treatment enhanced the low-temperature NH3-SCR activity of iron–cerium–titanium mixed oxide catalyst, and made its NH3-SCR reactive temperature window shift to the low-temperature region. Microwave hydrothermal treatment accelerated the crystallite rate of the precursors for iron–cerium–titanium mixed oxide catalysts, adjusted the microscopic pore structure of catalyst, and enlarged the pore diameter and the pore volume of catalyst. Microwave hydrothermal treatment increases Fe and Ce dispersion on the surface of catalyst, and the molar ratio of Ce3+/Ce4+ on the surface of mixed oxide catalyst could also be enhanced after microwave hydrothermal treatment, and improved the relative concentration of Ce3+, thereby enhanced the oxidation and reduction properties of catalyst. At the same time, microwave hydrothermal treatment enhanced the low-temperature adsorption intensity of NH3 over catalyst, thereby enhanced the low-temperature SCR activity of iron–cerium–titanium mixed oxide catalyst.

Synthesis of nanocrystalline birnessite and cryptomelane by microwave hydrothermal treatment

Effect of reaction medium acidity on the phase composition and morphology of manganese dioxide obtained by microwave hydrothermal treatment was studied. Conditions for the preparation of single-phase nanocrystalline polymorphs of manganese dioxide, birnessite and cryptomelane, were revealed. The obtained samples were characterized by X-ray diffraction, Raman spectroscopy, and scanning electron microscopy.

Preparation of a highly dispersed Ni2P/Al2O3 catalyst using Ni–Al–CO32 − layered double hydroxide as a nickel precursor

We now report a novel method for the synthesis of a Ni2P/Al2O3-LW catalyst using Ni–Al–CO32− layered double hydroxide (Ni–Al–CO32−-LDH) as a nickel precursor and ammonium dihydrogen phosphate as a phosphorous precursor under microwave–hydrothermal (MWH) treatment for 20min at 363K. The catalysts were characterized by XRD, TPR, BET, CO uptake and XPS. MWH treatment can promote the formation of smaller and highly dispersed Ni2P particles and a higher surface area of the catalyst. The Ni2P/Al2O3-LW shows hydrodesulfurization activity of 99.3%, which was much higher than that found for the Ni2P/Al2O3 catalyst obtained via an impregnation method.

Hydrothermal microwave valorization of eucalyptus using acidic ionic liquid as catalyst toward a green biorefinery scenario

The application of the acidic ionic liquid (IL), 1-butyl-3-methylimidazolium hydrogensulfate ([bmim]HSO4), as a catalyst in the hydrothermal microwave treatment (HMT) and green upgradation of eucalyptus biomass has been investigated. The process was carried out in a microwave reactor system at different temperatures (140–200°C) and evaluated for severities. The xylooligosaccharides (XOS, refers to a DP of 2–6) yield up to 5.04% (w/w) of the initial biomass and 26.72% (w/w) of xylan were achieved. Higher temperature resulted in lower molecular weight product, and enhanced the concentration of monosaccharides and byproducts. The morphology and structure of the solid residues were performed using an array of techniques, such as SEM, XRD, FTIR, BET surface area, and CP/MAS 13C NMR, by which the increase of crystallinity, the destruction of surface structure, and the changes in functional groups and compositions were studied after the pretreatment, thus significantly enhancing the enzymatic hydrolysis.

Nanocrystalline manganese dioxide synthesis by microwave-hydrothermal treatment

An efficient method is proposed for preparing manganese dioxide nanoparticles via microwave-hydrothermal (MW-HT) treatment of potassium permanganate and sodium nitrite solutions in the presence of sulfuric acid. The effect of preparation parameters on the phase composition and morphology of products is analyzed. A change in pH in the reaction mixture is shown to affect the phase composition and morphology of MnO2 nanoparticles.

Laser heating of the Y_1-xDy_xPO_4 nanocrystals

We propose a novel material prepared by microwave-hydrothermal treatment, the tetragonal xenotime-type yttrium orthophosphate YPO4 nanocrystals doped by different concentrations of Dy3+. It may be suitable for laser-induced local heating of cancer tumors for hyperthermia. We heated a powder consisted of the nanoparticles by focused quasi-CW laser irradiation at different wavelengths in the near IR spectral range fitting the transparency window of biological tissues. The local temperature on the surface of the powder in the place of irradiation increases linearly with increasing laser power and increasing the Dy3+ concentration. At the same time the efficiency of local heating Φ = ΔT / (P f) (ΔT is a local temperature increase, f is an oscillator strength of absorption transition, and P is the quantity of laser power) is proportional to the energy of the initially excited electronic level. The proposed method allows for high rates of heating and cooling. The laser power used for heating was rather low, tens of milliwatts that together with short heating time to required temperature may result in extremely low doses of laser irradiation for heating.

Phase composition and morphology of nanoparticles of yttrium orthophosphates synthesized by microwave-hydrothermal treatment: The influence of synthetic conditions

Herein we report the study of the influence of synthesis conditions during the microwave-hydrothermal crystallization of freshly precipitated gels on the phase composition and morphology of the rare-earth doped yttrium orthophosphates nanoparticles. We characterize the nanoparticles of YPO4 and YPO4⋅0.8H2O using X-ray diffraction analysis, TEM, and FT-IR spectroscopy. Furthermore, we argue that for the given phase the degree of crystallinity and thus the sample morphology depend strongly on the synthesis conditions. We establish that the hexagonal hydrate phase can be obtained by means of microwave-hydrothermal method if one uses phosphate anion excess or adjusts pH of the reaction mixture. Also we show that the metastable hydrate phase is most likely stabilized by hydroxyl groups at elevated temperatures.