Alkali Hydrothermal Process Research Articles

Stepwise extraction and utilization of silica and alumina from coal fly ash by mild hydrothermal process

Coal fly ash (CFA) is a potential secondary resource. In this study, silica and alumina from CFA were leached and utilized through a gentle alkali hydrothermal process. The results of CFA desilication with a single NaOH solution showed that amorphous silica was dissolved at a relatively low temperatures. Under optimized conditions, the leaching rate of SiO2 reached 33.81 %, while that of Al2O3 only was 1.37 %. Amorphous honeycomb calcium silicate hydrate was then well prepared by using silicon-rich liquid with CaO. The new aluminosilicate structures (sodalite) were detected in desilicated coal fly ash (DSCFA), which hinder the silica dissolution. After the optimal hydrothermal leaching in Ca(OH)2+NaOH system, the leaching rate of Al2O3 in DSCFA reached 39.84 %. The dealuminated CFA were mainly composed of friendly nano-flake tobermorite. With the increase of temperature, time, alkali concentration and Ca/Si, the tobermorite phase gradually changed to hibschite. The obtained alumina lixivium was well transformed into valuable porous boehmite via a carbonation process. Finally, the pilot tests excellent results, leaching about 41 % of SiO2 and 42 % of Al2O3, further demonstrating the feasibility of the whole hydrothermal process of desilication with a single alkali followed by aluminum extraction with mixed alkali.

Hybridization of g-C3N4 quantum dots with 1D branched TiO2 fiber for efficient visible light-driven photocatalytic hydrogen generation

The hybrid 1D branched TiO2 loaded with g-C3N4 QDs was successfully fabricated that plays a significant role in photocatalysis. The 1D branched TiO2 prepared by electrospinning followed by alkali-hydrothermal process, and g-C3N4 QDs were grafted over it by a chemical vapor deposition method. The composite display enhancement in photocatalytic hydrogen evolution is about 10.57 mmol. g−1.h−1 in comparison to the g-C3N4 sample that only produces 0.32 mmol. g−1.h−1 while the HBTiO2 sample evolved a negligible amount of hydrogen under visible light. The composite sample shows quantum efficiency for HER at 420 nm light is 18.6% that is much higher than the other two samples. The specific surface area of the composite sample is 92.39 m2g-1 that is about 13 times more than bulk g-C3N4. The bandgap of HBTiO2/g-C3N4 QDs, g-C3N4, and HBTiO2 samples calculated as 2.71 eV, 2.67eV, and 3.24eV, respectively. The TRPL spectra imply that the duration of the lifetime of composite becomes longer which effectually overwhelm the electron-hole recombination. The 1D branched TiO2 fiber reduces the charge recombination by fast transfer of electron while g-C3N4 QDs facilitate the visible light absorption by improving the optical properties. The formation of the type II heterostructure system remarkably promotes the separation and transfer of electron holes and facilitates the photo-reduction reaction.

In situ decoration of g-C3N4 quantum dots on 1D branched TiO2 loaded with plasmonic Au nanoparticles and improved the photocatalytic hydrogen evolution activity

The hybrid 1D branched TiO2 loaded with surface plasmonic Au and decorated with g-C3N4 QDs were successfully fabricated which played a significant role in photocatalysis. The 1D branched TiO2 prepared by electrospinning followed by an alkali-hydrothermal process. The Au particles decorated by photo deposition and after that g-C3N4 QDs were grafted over it by a chemical vapor deposition method. The composite shows great improvement in photocatalytic hydrogen evolution and evolved about 2.22 mmol g−1 h−1 hydrogen. The composite sample has a quantum efficiency of 19.5% for hydrogen production at 420 nm light that depicts the performance behavior of the composite. The 1D branched TiO2 fiber facilitates the fast transfer of electrons and Au reduces the charge recombination due to its surface plasmonic effect while g-C3N4 QDs facilitate the visible light absorption by improving the optical properties and reduces the bandgap. The formation of the type II heterostructure system remarkably promotes the separation and transfer of electron holes and facilitates the photo-reduction reaction.

Energy Storage: In Situ Fabrication of Hierarchically Branched TiO2 Nanostructures: Enhanced Performance in Photocatalytic H2 Evolution and Li–Ion Batteries (Small 47/2017)

In article 1702357, Shengjie Peng, Wei Yan, and co-workers present direct in situ growth of hierarchically branched TiO2 nanofibers by a combination of an electrospinning technique and an alkali-hydrothermal process. The branched nanofibers have a unique morphology and can be used in both photocatalytic H2 generation from waste splitting, and as an anode in Li–ion batteries, where they exhibit electrochemical properties such as a high specific capacity.

In Situ Fabrication of Hierarchically Branched TiO2 Nanostructures: Enhanced Performance in Photocatalytic H2 Evolution and Li-Ion Batteries.

1D branched TiO2 nanomaterials play a significant role in efficient photocatalysis and high-performance lithium ion batteries. In contrast to the typical methods which generally have to employ epitaxial growth, the direct in situ growth of hierarchically branched TiO2 nanofibers by a combination of the electrospinning technique and the alkali-hydrothermal process is presented in this work. Such the branched nanofibers exhibit improvement in terms of photocatalytic hydrogen evolution (0.41 mmol g-1 h-1 ), in comparison to the conventional TiO2 nanofibers (0.11 mmol g-1 h-1 ) and P25 (0.082 mmol g-1 h-1 ). Furthermore, these nanofibers also deliver higher lithium specific capacity at different current densities, and the specific capacity at the rate of 2 C is as high as 201. 0 mAh g-1 , roughly two times higher than that of the pristine TiO2 nanofibers.

Highly efficient fluoride adsorption from aqueous solution by nepheline prepared from kaolinite through alkali-hydrothermal process

A direct alkali-hydrothermal induced transformation process was adopted to prepare nepheline from raw kaolinite (shortened form RK in this paper) and NaOH solution in this paper. Structure and morphology characterizations of the synthetic product showed that the nepheline possessed high degree of crystallinity and uniform surface morphology. Specific surface area of nepheline is 18 m2/g, with a point of zero charge at around pH 5.0–5.5. The fluoride (F− ions) adsorption by the synthetic nepheline (shortened form SN in this paper) from aqueous solution was also investigated under different experimental conditions. The adsorption process well matched the Langmuir isotherm model with an amazing maximum adsorption capacity of 183 mg/g at 323 K. The thermodynamic parameters (ΔG0, ΔH0, and ΔS0) for adsorption on SN were also determined from the temperature dependence. The adsorption capacities of fluoride on SN increased with increasing of temperature and initial concentration. Initial pH value also had influence on adsorption process. Adsorption of fluoride was rapidly increased in 5–60 min and thereafter increased slowly to reach the equilibrium in about 90–180 min under all conditions. The adsorption followed a pseudo-second order rate law.

Synthesis of crystalline perovskite-structured SrTiO3 nanoparticles using an alkali hydrothermal process

We report an experimental route for synthesizing perovskite-structured strontium titanate (SrTiO3) nanocubes using an alkali hydrothermal process at low temperatures without further heating. Furthermore, we studied the influence of heating time (at 180°C) on the crystallinity, morphology, and perovskite phase formation of SrTiO3. The SrTiO3 powder, which is formed via nanocube agglomeration, transforms into cubic particles with a particle size of 120–150 nm after 6 h of hydrothermal sintering. The crystallinity and percentage of the perovskite phase in the product increased with heating time. The cubic particles contained 31.24at% anatase TiO2 that originated from the precursor. By varying the weight ratio of anatase TiO2 used to react with the strontium salt precursor, we reduced the anatase-TiO2 content to 18.8at%. However, the average particle size increased when the anatase-TiO2 content decreased.

Hydrothermal extraction of potassium from potassic quartz syenite and preparation of aluminum hydroxide

Potassic quartz syenite, composed of microcline and quartz, is a promising source for the recovery of potassium, alumina, and silica values. Hydrothermal extraction of potassium by dissolution of the potassic quartz syenite powder in NaOH solution and utilization of alumina and silica in the potassic quartz syenite powder for preparation of acicular wollastonite and aluminum hydroxide were explored in this research. It mainly contains alkali-hydrothermal process, causticization process, and low-lime sintering process. The samples were characterized by using wet chemical analysis, X-ray diffraction, and scanning electron microscopy. The results indicated that the optimal extractions of K2O and SiO2 from the potassic quartz syenite powder were 93.2% and 62.5%, respectively. The purities of as-prepared acicular wollastonite, potassium carbonate, and aluminum hydroxide were 93.8%, 96%, 99%, respectively. The approach presented in this work is a promising process of comprehensive utilization of potassic silicate minerals such as K-feldspar.

Reaction behaviour of Al2O3 and SiO2 in high alumina coal fly ash during alkali hydrothermal process

The reaction behaviours of Al2O3 and SiO2 in high alumina coal fly ash under various alkali hydrothermal conditions were studied. The means of XRD, XRF, FTIR and SEM were used to measure the mineral phase and morphology of the solid samples obtained by different alkali hydrothermal treatments as well as the leaching ratio of SiO2 to Al2O3 in alkali solution. The results showed that with the increase of the hydrothermal treating temperature from 75 to 160 °C, phillipsite-Na, zeolite A, zeolite P, and hydroxysodalite were produced sequentially while the mullite and corundum phase still remained. Zeolite P was massively formed at low-alkali concentration and the hydroxysodalite was predominantly obtained at high-alkali concentration. By the dissolution of aluminosilicate glass and the formation of zeolites together, the leaching efficiency of SiO2 can reach 42.13% with the mass ratio of Al2O3/SiO2 up to 2.19:1.

Synthesis of kalsilite from microcline powder by an alkali-hydrothermal process

The properties of aluminosilicate kalsilite have attracted the interest of researchers in chemical synthesis, ceramic industry, biofuels, etc. In this study, kalsilite was hydrothermally synthesized from microcline powder in a KOH solution. The microcline powder, rich in potassium, aluminum, and silicon, was collected from Mountain Changling in Northwestern China. The effects of temperature, time, and KOH concentration on the decomposition of microcline were investigated. The kalsilite and intermediate products were characterized by means of wet chemistry analysis, X-ray Diffraction (XRD), infrared spectrometry (IR), 29Si magic angle spinning nuclear magnetic resonance (29Si MAS NMR), 27Al MAS NMR, and scanning electron microscope (SEM). With increasing temperature, the microcline powder transforms into a metastable KAlSiO4 polymorph before transforming further into pure kalsilite. A mixture of both kalsilite and metastable KAlSiO4 polymorph is obtained when the hydrothermal reaction is carried out within 2 h; but after 2 h, kalsilite is the predominant product. The concentration of KOH, which needs to be larger than 4.3 M, is an important parameter influencing the synthesis of kalsilite.

High surface area electrospun prickle-like hierarchical anatase TiO2 nanofibers for dye-sensitized solar cell photoanodes

In this study, a prickle-like hierarchical anatase TiO2 nanofiber (HTF) was synthesized via an electrospinning method followed by a template-free alkali hydrothermal process and calcination. It was found that dense nanotube prickle-like branches were grown on the HTF with a surface area of up to 154 m2 g−1 and this was ∼4.3 times larger than that of the pure electrospun TiO2 nanofiber (TNF). When applied in a dye sensitized solar cell (DSSC), the highest power conversion efficiency (7.86%) was achieved using a HTF-based bi-layer device. This result was ∼26% higher than that achieved using the TNF-based bi-layer device (6.24%). The significant improvement in DSSC efficiency was investigated using intensity-modulated photocurrent spectroscopy (IMPS), intensity-modulated photovoltage spectroscopy (IMVS) and electrochemical impedance spectroscopy (EIS) measurements. Examining the results of these measurements, we attribute the improvement to the hierarchical structure of HTF, as it results in greater dye loading and better light scattering, leading to higher light harvesting, as well as faster electron transport and less charge recombination, resulting in a higher charge collection efficiency.

Influence of the Preparation Processes on the Structure and Catalytic Properties of W-MCM-41 Mesoporous Materials

W-MCM-41 mesoporous material was prepared with the open system process, acid-hydrothermal process, and alkali-hydrothermal process, respectively. The samples were characterized by using various techniques. The influence of the different preparation methods on the structure and catalytic properties in the oxidation of cyclohexene was comparatively investigated in details. The results showed that all the samples exhibited good catalytic properties and the one prepared with alkali-hydrothermal method displayed the best mesoporous structure.

Preparation and Characterizations of Na<sub>2</sub>Ti<sub>3</sub>O<sub>7</sub>, H<sub>2</sub>Ti<sub>3</sub>O<sub>7</sub> and TiO<sub>2</sub> Nanobelts

In this paper, the Ti-O-Compound nanobelts from commercial TiO2 (annatase phase) were synthesized via the alkali-hydrothermal process. The as-synthesized nanobelts are sodium titanate, hydrogen titanate and anatase with general formula Na2Ti3O7, H2Ti3O7and TiO2, respectively. The nanobelts are characterized by Thermogravimetric/Differential Thermal Analysis (TG/DTA), X-ray Diffraction (XRD), Infrared Spectra (IR) and Scanning Electron Microscope (SEM) apparatuses. The characterization indicates that the nanobelts with typical widths of 50 to 200 nm, thicknesses of 20 to 50 nm, and up to a few millimeters in length. The conversion mechanisms between the layer titanate and anatase of nanobelts have been discussed in this study.

Structural resistance of chemically modified 1-D nanostructured titanates in inorganic acid environment

Sodium containing one-dimensional nanostructured layered titanates (1-D NSLT) were produced both from commercial anatase powder and Brazilian natural rutile mineral sands by alkali hydrothermal process. The 1-D NSLT were chemically modified with proton, cobalt or iron via ionic exchange and all products were additionally submitted to intensive inorganic acid aging (pH = 0.5) for 28 days. The morphology and crystal structure transformations of chemically modified 1-D NSLT were followed by transmission electron microscopy, powder X-ray diffraction, selected area electron diffraction and energy dispersive spectroscopy. It was found that the original sodium rich 1-D NSLT and cobalt substituted 1-D NSLT were completely converted to rutile nanoparticles, while the protonated form was transformed in a 70%–30% (by weight) anatase–rutile nanoparticles mixture, very similar to that of the well-known TiO 2-photocatalyst P25 (Degussa). The iron substituted 1-D NSLT presented better acid resistance as 13% of the original structure and morphology remained, the rest being converted in rutile. A significant amount of remaining 1-D NSLT was also observed after the acid treatment of the product obtained from rutile sand. The results showed that phase transformation of NSLT into titanium dioxide polymorph in inorganic acid conditions were controllable by varying the exchanged cations. Finally, the possibility to transform, through acid aging, 1-D NSLT obtained from Brazilian natural rutile sand into TiO 2-polymorphs was demonstrated for the first time to the best of authors' knowledge, opening path for producing TiO 2-nanoproducts with different morphologies through a simple process and from a low cost precursor.

Extraction of Rare Earth Elements by Alkali Hydrothermal Process

Extraction of Rare Earth Elements by Alkali Hydrothermal Process