Conventional Hydrothermal Research Articles

Mechanistic insights of nitrate removal by MgFe/layered double hydroxides prepared by different synthesis pathways

In this study, a layered MgFe/double hydroxide (MgFe/LDH) adsorbent for nitrate removal from simulated systems was developed and investigated. Three different synthesis methods were used: coprecipitation at constant pH (CC), conventional hydrothermal (CH), and pre-ultrasonic followed by conventional hydrothermal (UCH). The XRD results indicated that all synthesized samples presented the characteristic structure of LDHs. The analysis of FT-IR spectra before and after nitrate adsorption allowed the analysis of NO3−MgFe/LDH interaction in different adsorbents. Adsorption kinetics showed that all adsorbents reached equilibrium around 120 min, with the adsorption capacity being 6.970, 6.690, and 5.610 mg g−1 for the UCH, CH, and CC, respectively. Pseudo-second order and Langmuir models were the best models for describing the kinetics and isotherm data. The highest maximum adsorption capacity was reached at 330 K, with a value of 21.18 mg g−1 for the UHC. Thermodynamic analysis indicated that the adsorption of nitrate was exothermic and spontaneous despite the system. The results of this study suggest that the MgFe/LDH adsorbent synthesized by the UCH method with hydrothermal treatment presents a promising and efficient way of removing nitrate from polluted water, with potential application in the purification of water for human and environmental use.

Synthesis of Pure Phase NaP2 Zeolite from the Gel of NaY by Conventional and Microwave-Assisted Hydrothermal Methods

The gel of zeolite NaY has potential as a precursor of other zeolites. The particular interest in this work is to convert the gel of NaY to NaP2. We found that the pure phase NaP2 can be produced simply by the conventional hydrothermal (CH) method at 150 °C for 24 h. This NaP2 sample, named CH150, has an average particle size of 10.3 µm and an Si/Al ratio of 1.82. In the case of single crystallization via microwave-assisted hydrothermal (MH) method, various parameters were studied, including the crystallization temperature (90, 150, 175 °C) and time (15, 30, 45, 60 min). The samples were analyzed by X-ray diffraction and scanning electron microscopy. However, mixed phases of P1 and P2 or ANA were obtained from all samples. Another attempt was made by a double crystallization via MH method as followed: at 90 °C for 1 h, quickly cooled down to room temperature in the microwave chamber and aged for 23 h, and finally at 150 °C for 1 h. The sample, named MH90A150, has an average crystal size of 16.45 µm and an Si/Al ratio of 1.85. The high Al content of NaP2 in both samples (CH150 and MH90A150) could lead to interesting applications.

Valorization of coal fly ash into nanozeolite by sonication-assisted hydrothermal method

The study aimed at the recycle and reuse coal fly ash (CFA) via a fast green method. Increasing dependence of coal in power sector results in increased production of CFA, the disposal of which is a major environmental issue. Sonication assisted hydrothermal (UAH) treatment has been employed to convert this CFA into versatile nanozeolite X (NZX) from CFA. The effect of CFA/NaOH, fusion temperature, fusion time and ultrasonication time were investigated. Initial precursor CFA and the synthesized NZX were characterized by XRF, XRD, FESEM, TEM, and FTIR analysis. UAH treatment produced NZX in shorter crystallization time than conventional hydrothermal (CH) method. The particle size of UAH-NZX were obtained from FESEM, BET, TEM analysis as 22–27 nm, 24.36 nm, and 2–5 nm respectively. Average crystal size of UAH-NZX was 21.58 nm as calculated using Scherrer's equation. The optimized condition for the UAH synthesis of NZX was found as CFA/NaOH ratio of 1.25, fusion temperature of 550 °C, fusion time of 1.5 h, and ultrasound time of 20 min. The characteristics of UAH-NZX zeolite was compared with CH-NZX and commercial zeolite X (CZX). The pure NZX was formed by UAH technique with 20 min ultrasound, followed by 6 h hydrothermal treatment at room temperature whereas CH technique took 96 h of hydrothermal digestion at 120 °C. The optimized CEC value of NZX, conventional hydrothermal zeolite X (CH-ZX), and CZX are 428 mmol/100 g, 242 mmol/100 g, and 293 mmol/100 g respectively. The UAH method produced NZX at shorter time with less consumption of energy than CH method with nanozeolite with higher CEC and surface area (797.53 m2/g) than both CH-NZX and CZX. The nanoscale zeolite X can be used efficiently as ion exchanger as well as adsorber.

Ultrafast microwave-hydrothermal synthesis of hexagonal plates of hematite

This paper reports on the synthesis of hexagonal plates of hematite, α-Fe2O3 crystals using microwave-hydrothermal (M-H) and conventional-hydrothermal (C-H) reactions. Platy α-Fe2O3 crystals which show dark red wine color were synthesized from 2M-FeCl3 and 8M-NaOH solution at both 140 and 180 °C and their morphology and crystal size were investigated. Average crystal size of hexagonal α-Fe2O3 was 0.9–1.6 μm at 140 °C and 0.8–2.1 μm at 180 °C under both M-H and C-H reactions. Compared to the formation of platy α-Fe2O3 crystals via C-H reaction, the M-H reaction led to increased rate of formation by 8 and 15 times at 140 and 180 °C, respectively. The M-H reaction yielded hexagonal plates of hematite in 2 min at 180 °C. The suspensions of platy α-Fe2O3 powders prepared at 140 and 180 °C by both C-H and M-H reactions showed dark red wine color due to similar crystal size and morphology, which have potential applications in cosmetic pigments, as anode materials in Li ion batteries.

Low-temperature hydrothermal pretreatment followed by dry anaerobic digestion: A sustainable strategy for manure waste management regarding energy recovery and nutrients availability

This study evaluated the feasibility of low-temperature hydrothermal (HT) pretreatment for improving dry anaerobic digestion (AD) of swine manure (SM) and nutrient elements reclamation, with specific goals to minimize the drawbacks of conventional HT process including high energy consumption, inhibitory compounds formation and unfavorable pH/alkalinity decrease. Pretreatment at 110–130°C for holding 30min increased the soluble organic carbon (SOC) concentration in SM by 13–26%. After being mixed with inocula, the pretreated SM was applied for dry AD tests successfully without initial pH adjustment, achieving a CH4 yield of 280.18–328.93ml/g-VSfed (14–34% increase compared to that from raw SM). Energy assessment indicated a positive net gain of 0.95kJ/g-VS by adopting HT pretreatment at 130°C. Except for increment in CH4 yield, low-temperature HT pretreatment also promoted organic-N mineralization, increasing N fractions in the digestate available for plants. After 70days’ dry AD, a high ammonia-N to total nitrogen (TN) ratio of 71% was obtained for the SM sample pretreated at 130°C, in sharp contrast to that of 38% in raw SM. P bioavailability in the final digestate was not greatly affected by the HT pretreatment since the labile organics were mostly degraded after AD, in which P existing forms were influenced by the multivalent metals content in SM. Overall, 23–27% of the total P was potentially bioavailable in all digestates.

TITANATE NANOTUBES PRODUCED FROM MICROWAVE-ASSISTED HYDROTHERMAL SYNTHESIS: CHARACTERIZATION, ADSORPTION AND PHOTOCATALYTIC ACTIVITY

Abstract - A microwave-assisted hydrothermal (MAH) method was employed to synthesize TiO 2 nanotubes (TNTs) and these were compared to the conventional hydrothermal (CH) method. The nanotubes were characterized for their crystallinity, morphology, surface area and photocatalytic activity. The TiO 2 P25 was used for comparison purposes and efficiently degraded both dyes. However, methylene blue was adsorbed on the nanotube surfaces, which can be attributed to the electrostatic interaction between the cationic methylene blue and the negatively charged TNT surface. On the other hand, all nanotubes had a photocatalytic performance with methyl orange (anionic dye). Additional degradation was observed for nanotubes synthesized via the MAH method at 150 °C. This can be directly related to their larger surface area. Adsorption mechanisms of TNTs were also discussed by applying the Langmuir and Freundlich isotherm. The results show an efficient dye wastewater treatmen t through the application of TNT photocatalyst.

Dry gel conversion as a suitable method for increasing the lifetime of SAPO-18 in MTO process

The crystalline and pure SAPO-18 was synthesized by dry-gel conversion (DGC) methods (including steam-assisted conversion (SAC) and vapor phase transport (VPT)) and by conventional hydrothermal (CHT) method. The physicochemical properties of the catalysts were analyzed by XRD, FESEM, EDXS, and TPD techniques. It was found that the DGC methods have no significant effect on the crystallization time of SAPO-18. The catalysts with smaller crystallite size and in a higher solid yield were achieved by DGC methods. The results also revealed that DGC method changes significantly the acidic properties of SAPO-18 by affecting the amount and mechanism of silicon incorporation into the framework. The catalytic behavior of the samples was investigated in methanol-to-olefins (MTO) reaction. Although no considerable change was observed in light olefin selectivity over the samples, SAC method showed an increase of about 20% in useful lifetime of SAPO-18. It can be due to the smaller crystallite size and desirable acidic characteristic. Finally, the results were compared with those of DGC-synthesized SAPO-34 reported in the literature. It suggests that SAPO-18 synthesized by SAC method can be an acceptable alternative to SAPO-34 by exhibiting a comparable activity and longer lifetime in MTO process.

Comparison of ZnO nanorod array coatings on wood and their UV prevention effects obtained by microwave-assisted hydrothermal and conventional hydrothermal synthesis

Abstract ZnO nanorod array coatings were successfully synthesized on wood surfaces using a conventional hydrothermal (CHT) and a microwave-assisted hydrothermal (MWHT) method. The reaction time of the MWHT method at high temperature was only 1/12th that of the CHT method. The MWHT-ZnO nanorods exhibited better crystallinity than those produced by the CHT method. The ZnO nanorod had a wurtzite crystal structure oriented along the c-axis. The MWHT nanorods with their higher (002)/(101) ratio were more conducive and improved the orientation degree of the ZnO nanorods. Field emission scanning electron microscopy (FE-SEM) images showed that the MWHT-ZnO nanorods had smaller diameters and narrower size distributions than those produced by the CHT method. However, both methods formed well-aligned array coatings that covered the wood surfaces, and the two coatings had similar UV protecting effects and inhibited discoloration.

LiMn2O4-based materials as anodes for lithium-ion battery

LiMn 2 O 4-based materials such as LiMn 2 O 4 and LiMn 1.53 Ni 0.47 O 3.67 were synthesized by both conventional-hydrothermal (CH), microwave-hydrothermal (MH) methods and calcination route. Both reaction temperature and reaction time during MH routes were lower than those of CH routes. LiMn 2 O 4 electrode materials were assembled into lithium-ion battery anodes and their electrochemical performances were studied. The results proved that these LiMn 2 O 4 electrodes can be served as conversion anode materials, and show electrochemical activity during the potential range of 0.01–3.0 V versus Li +/ Li . For LiMn 1.53 Ni 0.47 O 3.67 materials when used as lithium-ion battery anodes, we found that the introduction of Ni can change their electrochemical reaction and thus improve their electrochemical performances.

Photocatalysis of WO3 Nanoplates Synthesized by Conventional‐Hydrothermal and Microwave‐Hydrothermal Methods and of Commercial WO3 Nanorods

The degradation of methylene blue (MB) dye by tungsten oxide (WO3) photocatalyst synthesized by the 200°C conventional‐hydrothermal (C‐H) and 270 W microwave‐hydrothermal (M‐H) methods and commercial WO3 was studied under UV light irradiation for 360 min. The photocatalysts were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, Raman spectrophotometry, and UV visible spectroscopy to determine phase, morphology, vibration mode, and optical property. The BET analysis revealed the specific surface area of 29.74, 37.25, and 33.56 m2/g for the C‐H WO3 nanoplates, M‐H WO3 nanoplates, and commercial WO3 nanorods, respectively. In this research, the M‐H WO3 nanoplates have the highest photocatalytic efficiency of 90.07% within 360 min, comparing to the C‐H WO3 nanoplates and even commercial WO3 nanorods.

High-temperature-mixing hydrothermal synthesis of ZnO nanocrystals with wide growth window

High-quality and controllable growth of nanocrystals (NCs) have been attracting great attention. Here, a high-temperature-mixing hydrothermal (HTMH) method was designed to synthesize ZnO NCs with high crystallinity and narrow size distribution in a wide growth window. Compared with conventional hydrothermal (CH) growth, zinc source and alkali precursors were intentionally separated in temperature-rising stage and permitted to mix at the starting of heat preservation stage of HTMH growth. Highly crystalline ZnO NCs with uniform spherical morphology can be formed at alkali concentration and temperature windows as wide as 0.1–0.5 M and 160–200 °C, respectively. However, the products via CH method have much larger changes in not only morphology but also size. These results demonstrated that the high-temperature-mixing reaction greatly facilitates nucleation but depresses grain growth. Considering the simplicity and reproducibility, such HTMH method could have wide potentials for the fabrication of various functional nanocrystals.

Microwave-hydrothermal/solvothermal synthesis of kesterite, an emerging photovoltaic material

Abstract Rapid synthesis of kesterite (Cu2ZnSnS4), an emerging photovoltaic material was achieved by microwave-hydrothermal/solvothermal (M-H/S) process. Conventional-hydrothermal (C-H) process was also used for comparison with M-H/S process. The synthesized kesterite samples were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron diffraction and UV–vis spectroscopy. Nanophase kesterite, which is potentially suitable for coating was synthesized in ethylene glycol. Band gap values were calculated from UV–vis spectra of several synthetic kesterites and these values are close to the reported values for kesterite.

Structural refinement, growth mechanism, infrared/Raman spectroscopies and photoluminescence properties of PbMoO4 crystals

Lead molybdate (PbMoO4) crystals were synthesized by the co-precipitation method at room temperature and then processed in a conventional hydrothermal (CH) system at low temperature (70°C for different times). These crystals were structurally characterized by X-ray diffraction (XRD), Rietveld refinement, micro-Raman (MR) and Fourier transformed infrared (FT-IR) spectroscopies. Field emission scanning electron microscopy images were employed to observe the shape and monitor the crystal growth process. The optical properties were investigated by ultraviolet–visible (UV–Vis) absorption and photoluminescence (PL) measurements. XRD patterns and MR spectra indicate that these crystals have a scheelite-type tetragonal structure. Rietveld refinement data possibilities the evaluation of distortions in the tetrahedral [MoO4] clusters. MR and FT-IR spectra exhibited a high mode ν1(Ag) ascribed to symmetric stretching vibrations as well as a large absorption band with two modes ν3(Eu and Au) related to anti-symmetric stretching vibrations in [MoO4] clusters. Growth mechanisms were proposed to explain the stages involved for the formation of octahedron-like PbMoO4 crystals. UV–Vis absorption spectra indicate a reduction in optical band gap with an increase in the CH processing time. PL properties of PbMoO4 crystals have been elucidated using a model based on distortions of tetrahedral [MoO4] clusters due to medium-range intrinsic defects and intermediary energy levels (deep and shallow holes) within the band gap.

Microwave- and conventional-hydrothermal synthesis of CuS, SnS and ZnS: Optical properties

Abstract Sulfides of Cu, Sn and Zn were synthesized by conventional-hydrothermal (C-H) and microwave hydrothermal (M-H) methods using thioacetamide as a sulfide source. The synthesized CuS, SnS and ZnS were characterized by powder X-ray diffraction (XRD), scanning electron microscopy and UV–vis diffuse reflectance spectra. The results suggest that M-H process led to very rapid crystallization of all the sulfides compared to the C-H process. Band gaps of the CuS, SnS and ZnS were determined to be 1.8, 1.3 and 3.6, respectively matching the theoretical anticipated band gaps of 2.0, 1.2 and 3.54, respectively suggesting that pure phases were obtained. The three sulfides are potentially useful for the composite solid-state synthesis of kesterite, a photovoltaic material.

Cluster Coordination and Photoluminescence Properties of α-Ag2WO4 Microcrystals

In this paper, we report our initial research to obtain hexagonal rod-like elongated silver tungstate (α-Ag(2)WO(4)) microcrystals by different methods [sonochemistry (SC), coprecipitation (CP), and conventional hydrothermal (CH)] and to study their cluster coordination and optical properties. These microcrystals were structurally characterized by X-ray diffraction (XRD), Rietveld refinements, Fourier transform infrared (FT-IR), X-ray absorption near-edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopies. The shape and average size of these α-Ag(2)WO(4) microcrystals were observed by field-emission scanning electron microscopy (FE-SEM). The optical properties of these microcrystals were investigated by ultraviolet-visible (UV-vis) spectroscopy and photoluminescence (PL) measurements. XRD patterns and Rietveld refinement data confirmed that α-Ag(2)WO(4) microcrystals have an orthorhombic structure. FT-IR spectra exhibited four IR-active modes in a range from 250 to 1000 cm(-1). XANES spectra at the W L(3)-edge showed distorted octahedral [WO(6)] clusters in the lattice, while EXAFS analyses confirmed that W atoms are coordinated by six O atoms. FE-SEM images suggest that the α-Ag(2)WO(4) microcrystals grow by aggregation and the Ostwald ripening process. PL properties of α-Ag(2)WO(4) microcrystals decrease with an increase in the optical band-gap values (3.19-3.23 eV). Finally, we observed that large hexagonal rod-like α-Ag(2)WO(4) microcrystals prepared by the SC method exhibited a major PL emission intensity relative to α-Ag(2)WO(4) microcrystals prepared by the CP and CH methods.

Microwave-assisted hydrothermal synthesis of perovskite NaTaO3 nanocrystals and their photocatalytic properties

A fast and facile process for the preparation of perovskite NaTaO3 nanocrystals with Ta2O5 and NaOH as starting materials by a microwave-assisted hydrothermal (MHT) technique is reported. By pretreating the Ta2O5 powder by ball milling, pure-phase NaTaO3 can be synthesized under quite mild conditions through the formation of an intermediate pyrochlore Na2Ta2O6 phase, while much longer times are required in a conventional hydrothermal (CHT) process. After loading with NiO co-catalyst, the NaTaO3 nanocrystals prepared by the MHT method showed photocatalytic activity for overall water splitting more than two times greater than those prepared under CHT conditions.

Characterization and hydroisomerization performance of SAPO-11 molecular sieves synthesized by dry gel conversion

SAPO-11 molecular sieves were synthesized by two types of dry gel conversion (DGC) methods, namely, steam-assisted conversion (SAC) and vapor phase transport (VPT), and by conventional hydrothermal (CHT) method. The properties of these samples were characterized by XRD, XRF, SEM, N 2-adsorption, Pyridine-IR and 29Si MAS NMR methods. The results show that di- n-propylamine (DPA) is a suitable structure-directing agent (SDA) for the preparation of impurity-free SAPO-11. SAPO-11 molecular sieves synthesized by SAC and VPT exhibit higher crystallinity, higher Si content and more strongly acidic sites than that synthesized by CHT. 29Si MAS NMR results demonstrated that the two samples obtained by the DGC methods contain more Si(nAl)(0 < n < 4) species than that obtained by CHT. These facts suggest that the SAC and VPT methods contribute either to better Si incorporation into the AlPO 4 framework or to better Si dispersal, thus decreasing the size of Si islands and increasing the number and strength of acidic sites. The results from the hydroisomerization of n-dodecane indicate that the Pt/SAPO-11 synthesized by SAC possesses the highest hydroconversion activity and isomer yield among those three catalysts, due to the high acidity of the molecular sieve supporter.

Solvothermal/ Hydrothermal Synthesis of Metal Oxides and Metal Powders with and without Microwaves

Anatase and Ca, Sr and Ca0.5Sr0.5 hydroxyapatites were synthesized by conventional-hydrothermal (C-H) as well asmicrowave-hydrothermal (M-H)methods.Microwave-assisted reactions led to accelerated syntheses of anatase but no such acceleration of reactions could be detected with the syntheses of hydroxyapatites because the crystallization of the latter materials occurred at very low temperature. Cu and Au metal powders were produced by using glucose, fructose or sucrose as reducing agents under C-H conditions at 160 ℃, where fructose and sucrose were found to be stronger reducing agents than glucose. The crystallinity of all the powders was characterized by powder X-ray diffraction, and morphology and particle sizes were determined by scanning or transmission electron microscopy

Electrochemical hydrogen evolution over MoO 3 nanowires produced by microwave-assisted hydrothermal reaction

Molybdenum trioxide (MoO 3) nanowire with unprecedentedly high aspect ratios (>200) and good crystallinity was prepared via decomposition of (NH 4) 6Mo 7O 24·4H 2O under a microwave-assisted hydrothermal (MH) process. The nanowire was orthorhombic MoO 3 with 50 nm in diameter and 10–12 μm in length. The conventional hydrothermal (CH) reaction required higher temperature and longer reaction times to produce one-dimensional MoO 3, yet its quality was lower. In the electrochemical hydrogen evolution reaction in a H 2SO 4 solution, MoO 3 nanowire from MH process showed much higher electrocatalytic activity than MoO 3 prepared from CH method and commercial bulk MoO 3 particles. The facile vectorial electron transport along the nanowire axis was considered to be responsible for the excellent electrocatalytic activity of the MH–MoO 3 nanowire.

Conventional- vs microwave-hydrothermal synthesis of tin oxide, SnO 2 nanoparticles

Tin oxide nanophases or nanoparticles were synthesized under conventional- and microwave-hydrothermal conditions and characterized by powder X-ray diffraction, BET surface area determination as well as transmission electron microscopy. The use of microwave-hydrothermal (M-H) process led to higher yields in shorter time compared to the conventional-hydrothermal (C-H) process because of increased reaction rates in the former.