Alkaline Hydrothermal Conditions Research Articles

Transfer hydrogenation of quinolines with glycerol over N-doped carbon encapsulated non-noble metal catalysts: Mott-Schottky effect on the catalytic activity

Catalytic transfer hydrogenation using glycerol (GLY) as a hydrogen donor presents a sustainable and secure approach for hydrogenation of quinoline and its derivatives. Conventional non-noble metal catalysts can effectively catalyze the transfer hydrogenation reaction with GLY. However, it is inevitable that metal leaching occurs under alkaline hydrothermal conditions. In this work, N-doped carbon encapsulated non-noble metal catalysts, such as Co@NC, Ni@NC, and CoNi@NC, were prepared by one-pot method combined with an acid leaching for this reaction. According to Mott-Schottky effect, encapsulating of Co, Ni, and CoNi nanoparticles with relatively smaller work function in NC shells constructed a rectifying contact and increased the electron density on NC shells. Density Functional Theory (DFT) calculations confirm that electrons transfer from metal cores to NC shell, with CoNi transferring the most, followed by Co and Ni. As a result, the catalytic activity followed the order of CoNi@NC＞Co@NC＞Ni@NC, where CoNi@NC exhibited highest catalytic activity, 92.2% GLY conversion and full quinoline conversion can be obtained over CoNi@NC at 180 °C with 89.7% and 99.3% of the selectivity to lactic acid and 1, 2, 3, 4-tetrahydroquinoline, respectively. In addition, CoNi@NC possessed excellent stability because NC shells physically isolate metal cores from the reactive environment. Such Mott-Schottky catalyst tuning could be an efficient way to design new non-noble catalysts.

Rapid one-step preparation of SrZrO3 using Zr4+ gel and SrSO4 ore under alkaline hydrothermal conditions

SrZrO3-structured perovskite particles were prepared under hydrothermal conditions in a KOH (5M) solution using Zr-gel and SrSO4 mineral precursors. The treatments were conducted between 150 and 240°C for different reaction intervals (1–96h), and the KOH solution volume varied between 7.5 and 30mL. To evaluate the effect of the Zr-gel precursor, the treatments were preliminarily conducted with a coprecipitated pasty Zr-gel (Zr(OH)4·9.64H2O) and subsequently with a lyophilized Zr-gel Zr(OH)4 powder. Generally, SrZrO3 particles were produced by a single-step reaction following the simultaneous bulk dissolution of the Zr4+ gel precursor and the SrSO4 powder. However, in the preliminary experiments, a dehydration reaction of the pasty Zr-gel preceded the ultimate single-step reaction, resulting in complete SrZrO3 particle formation taking place over a longer interval of 96h at 240°C. In contrast, when using the dried Zr-gel powder, complete feedstock dissolution occurred more rapidly, producing SrZrO3 particles at 200°C over 48h. The SrZrO3 particle sizes varied significantly depending on whether the pasty gel or dried powder Zr precursor was used. Particles prepared with the pasty gel exhibited a bimodal size distribution with mean particle sizes of 25 and 65μm with pseudocubic and star-shaped cuboidal morphologies, respectively. In contrast, particle growth resulting from the rapid dissolution of solid powder feedstock produced cubic-shaped particles, monomodally distributed with an average particle size of 10μm. Furthermore, byproduct (SrCO3) formation occurred predominantly under earlier stages together with SrZrO3 particle irrespective of the 5M KOH filling volume; however, at a volume of 15mL spontaneously achieved in situ the SrCO3 dissolution at intermediate stages of reaction. This reaction pathway did not proceed at small (7.5mL) and large (30mL) volumes of the alkaline fluid. A kinetic study indicated that the activation energy required to produce the SrZrO3 cubic-shaped particles was low in both cases, i.e., 15.05 and 22.27kJmol−1 between the powder and pasty Zr4+ precursors, respectively.

A Raman spectroscopic and ab initio investigation of aqueous boron speciation under alkaline hydrothermal conditions: evidence for the structure and thermodynamic stability of the diborate ion.

Raman spectra of aqueous sodium borate solutions, with and without excess NaOH, NaCl, and LiCl, have been obtained from perpendicular and parallel polarization measurements acquired using a custom-built sapphire flow cell over the temperature range 25 to 300 °C at 20 MPa. The solvent-corrected reduced isotropic spectra include a large well-defined band at 865 cm-1 which overlaps with the boric acid B(OH)3 band at 879 cm-1, and becomes increasingly intense at elevated temperatures. This band does not correspond to the spectrum of any other previously reported aqueous polyborate ions, all of which have symmetric stretching bands at frequencies below that of borate, [B(OH)4]-, at 745 cm-1. Based on the classic high-temperature potentiometric titration study by R. E. Mesmer, C. F. Baes and F. H. Sweeton, Acidity measurements at elevated temperatures. VI. Boric acid equilibriums, Inorg. Chem., 1972, 11, 537-543, the new band was postulated to arise from a diborate ion, [B2(OH)7]- or [B2O(OH)5]-. Ab initio density functional theory (DFT), together with chemical modelling studies, suggest that it is most likely [B2(OH)7]-. Thermodynamic formation quotients derived from the peak areas showed variations with ionic strength as well as charge-balance discrepancies, which suggest one or more unidentified minor equilibrium species may also be present. The most likely candidate is the divalent diborate species [B2O2(OH)4]2- which is also predicted to have a band near 865 cm-1 and is postulated to be present as a sodium ion pair. These are the first quantitative Raman spectra ever reported for borate-rich solutions under such conditions and provide the first spectroscopic evidence of a diborate species at PWR reactor coolant temperatures.

Hydrothermal humification of lignocellulosic components: Who is doing what?

The humification behavior of lignocellulosic components (i.e., cellulose, hemicellulose and lignin) under acid-alkali two-step hydrothermal treatment was investigated with particular interest of revealing the contribution of single components as well as their interaction to hydrothermal humic acid (HHA). Lignin with its rich aromatic structures, had the highest HHA contribution (51.8 ± 1.0 wt%), while cellulose and hemicellulose only contributed 8.8 ± 1.6 wt% and 20.1 ± 4.4 wt% HHA yields, respectively. The conversion pathway of these components toward humification indicated that the hydrochar (HC) rather than the hydrothermal solution (HS) had higher humification capability under alkaline hydrothermal conditions because the hydrochar-derived HHA yields of 40.8 ± 2.0 wt% (cellulose), 66.2 ± 6.4 wt% (hemicellulose) and 55.4 ± 6.1 wt% (lignin) were achieved. In addition, co-hydrothermal treatment of cellulose, hemicellulose and lignin under this process improved the HHA production ascribing to the increased HC yield (23.1–53.1 %). The chemical characteristics of HHA products from lignocellulosic components (cellulose, hemicellulose, lignin) and their combinations were similar to the acknowledged natural humic acids. The hydrothermal humification of biowaste mainly passes through a top-down pathway (i.e., HHA forms from the oxidation of large fragments of hydrochar) as well as two bottom-up pathways (i.e., the soluble chemicals conduct the polymerization reaction for HHA production and react with the fragments from HC to synthesize the HHA). This study filled the lack of knowledge in the field of biowaste humification.

From biomass to humic acid: Is there an accelerated way to go?

Acid hydrothermal treatment of biowaste is favorable for producing hydrochar and corresponding hydrothermal solution containing saccharides, furans and aromatic compounds, while the alkaline hydrothermal condition is beneficial for their humification to form hydrothermal humic acid (HHA). Herein, the humification potential of hydrochar, hydrothermal solution, and their mixture under alkaline hydrothermal conditions was compared to further reveal their contribution to HHA. The result showed the main source of HHA was hydrochar, occupying 70 wt% of total HHA yield from their mixture while hydrothermal solution only contributed 17 wt%. Moreover, the HHA that existed in hydrochar (denoted as HHAacid) was 32 wt% while the hydrochar-derived HHA obtained after alkaline hydrothermal treatment (denoted as HHAalk) increased to 65 wt%, close to that from commercial coal or lignite. The hydrochar was further separated into four components, i.e. hydrothermal fulvic acid (HFA), stable HHAacid (S-HHAacid) non-stable HHAacid (N-HHAacid) and remaining substance (RS), aiming to reveal the major pathway of the incremental HHA production under alkaline hydrothermal condition. Reactions of RS + HFA, RS + N-HHAacid and self-synthesizing of RS all contributed to the incremental HHAalk yield with the major contribution (75C%) from the first combination (RS + HFA). The pathways of alkaline hydrothermal humification of hydrochar were revealed after analyzing the molecular level structure of HHAalk by UPLC-MS. This in-depth analysis fills the knowledge gap in the mechanism of hydrothermal humification of biowaste.

Synthesis, characterization and study of the structural change of nanobelts of TiO2 (H2Ti3O7) to nanobelts with anatase, brookite and rutile phases

Titanium dioxide (TiO2) is well known for its wide variety of applications, especially for its physicochemical properties, which are affected by changes in the shape and crystal size, and crystalline structure. In this work, the morphological evolution and phase change of TiO2 nanobelts (H2Ti3O7) to nanobelts with anatase, brookite and rutile phases are reported. TiO2 nanobelts (H2Ti3O7) were synthesized from crystalline phases of TiO2 (anatase, brookite, rutile and the commercial material Degussa P25) under alkaline hydrothermal conditions using NaOH. From H2Ti3O7 nanobelts, phase changes to anatase, brookite and rutile were obtained by hydrothermal method using different hydrochloric acid solutions, in which a combination of the HCl concentration, synthesis time and temperature (set at 175 °C) determine the amount and type of each phase as well as its morphological change. The materials were analyzed using the X-ray powder diffraction technique, XRD, scanning electron microscopy, SEM and 2D micro-Raman and infrared FT-IR spectroscopies. The diffractograms and Raman spectra obtained are characteristic of each crystalline phase of TiO2 (anatase, brookite, rutile and their combinations); while SEM showed the morphology and average dimensions of the nanobelts obtained. Additionally, the analysis of the infrared spectra of each of the synthesized materials consistently correlates the changes that occur due to the morphology and the phases present.

Transformation Behavior of Iron Minerals in High-Iron Red Mud During High-Pressure Hydrothermal Reduction.

Nowadays, the comprehensive utilization of red mud is still a difficult problem. Using starch as the reducing agent, the "high-pressure hydrothermal reduction method" is used to separate and recover sodium, aluminum, and iron in high-iron red mud in one step in an alkaline hydrothermal system. This article focuses on the hydrothermal reduction behavior of the iron ore phase. The results showed that starch formed a strong reduction system through degradation under alkaline hydrothermal conditions, in which Fe2O3 was first partially dissolved and reduced to form Fe[OH]+, then Fe[OH]+ further reacted with Fe2O3 to form Fe3O4. At a temperature of 260°C, a C/S (calcium to silicon ratio) of 3.5, a Na2O concentration of 250g/L, Whigh-iron red mud/Wstarch = 10, the Fe reduction rate was 98.46%, and the transformation slag at a magnetic field strength of 1800 Gs resulted in a magnetic iron concentrate powder with a TFe grade of 65.75%.

Production of artificial humic acid from biomass residues by a non-catalytic hydrothermal process

Humic substance is vital for soil fertility and has wide applications such as soil remediation agents, organic fertilizer and adsorbents. The results of pH effect on artificial humic acids (AHAs) production demonstrated that acid hydrothermal conditions are favorable for biomass waste dissolution and formation of AHA precursors such as saccharides, furans, organic acids and phenols; whereas alkaline hydrothermal conditions are suitable for AHAs production from these precursors. Enhanced humification from biomass waste with a combined acid - alkaline hydrothermal process was proposed. The solid product known as hydrochar was reduced to only 5 wt% after this process indicating the significant improvement of dissolution and transformation. The AHAs concentration of 28.28 ± 0.98 wt% obtained from corn stalk was the highest yield so far, 10 times higher than ever reported. The carbon flow analysis indicated that the selectivity of AHA-C was 43.2% of the initial carbon in corn stalk. Our study provides a new way of mass production of AHAs in short period, which has great potential for industry application.

Phase transfer of Pyrophyllite to Riversideite, Sodic Zeolites and Hibschite Hydrogrossular under Alkaline Hydrothermal Conditions

Phase transfer of Pyrophyllite to Riversideite, Sodic Zeolites and Hibschite Hydrogrossular under Alkaline Hydrothermal Conditions

Synthesis of KAlSiO4 by Hydrothermal Processing on Biotite Syenite and Dissolution Reaction Kinetics

To make potassium from K-bearing rocks accessible to agriculture, processing on biotite syenite powder under mild alkaline hydrothermal conditions was carried out, in which two types of KAlSiO4 were obtained successfully. The dissolution-precipitation process of silicate rocks is a significant process in lithospheric evolution. Its effective utilization will be of importance for realizing the comprehensiveness of aluminosilicate minerals in nature. Two kinds of KAlSiO4 were precipitated in sequence during the dissolution process of biotite syenite. The crystal structures of two kinds of KAlSiO4 were compared by Rietveld structure refinements. The kinetics model derived from geochemical research was adopted to describe the dissolution behavior. The reaction order and apparent activation energy at the temperature range of 240–300 °C were 2.992 and 97.41 kJ/mol, respectively. The higher dissolution reaction rate of K-feldspar mainly relies on the alkaline solution, which gives rise to higher reaction order. During the dissolution-precipitation process of K-feldspar, two types of KAlSiO4 with different crystal structure were precipitated. This study provides novel green chemical routes for the comprehensive utilization of potassium-rich silicates.

Hydrothermal decomposition process of K‐feldspar in NaOH solution

AbstractThis work investigates the decomposition of K‐feldspar into analcime under alkaline hydrothermal conditions. Two types of raw materials, potassic syenite powder and microcline crystal lumps, were used for hydrothermal treatment. Research techniques of minerals alteration path simulation, powder decomposition experiments, and specific observations were conducted. Results show that K‐feldspar did not alter into analcime directly under hydrothermal conditions, and instead, hydroxycancrinite was an important intermediate. Formation route of analcime from potassic syenite powder was derived as follows: syenite dissolution → hydroxycancrinite precipitation → hydroxycancrinite assembling → reversed recrystallization → analcime, where the assembling of hydroxycancrinite into analcime‐shaped spheres was the key step for analcime formation. Above all, a novel and different to natural reaction process with feldspar decomposition and analcime formation process was proposed.

Hydrothermal alkaline conversion of actual acrylonitrile wastewater to organic acids

Acrylonitrile wastewater is a kind of highly toxic industrial wastewater, but it contains a lot of valuable chemical materials. In this study, by using the most promising hydrothermal technology, we investigated the best reaction conditions and possible reaction pathways for the conversion of the three nitriles (acrylonitrile, acetonitrile, and succinonitrile) to organic acids under alkaline hydrothermal conditions. Then, the reaction conditions for the conversion of actual acrylonitrile wastewater to organic acids were further optimized. The actual wastewater of acrylonitrile was converted into 1.33 × 104 mg/L acrylic acid, 1.98 × 104 mg/L formic acid and 9.40 × 103 mg/L acetic acid at optimal reaction conditions (reaction temperature 300 °C, reaction time 90 s, and initial NaOH concentration 1.0 mol/L). It is the theoretical basis of the application of the hydrothermal alkali-catalyzed method in the acrylonitrile wastewater resource engineering.

CaNa[Cr(OH)6] – A Layered Hydroxochromate(III) with Ordered Brucite Structure and its Thermal Decomposition

Green single‐crystals of the hydroxochromate(III) CaNa[Cr(OH)6] were grown under highly alkaline hydrothermal conditions at about 200 °C. The starting materials Ca(NO3)2·6H2O and Cr(NO3)3·9H2O were reacted in a mixture of water and sodium hydroxide with the molar ratio of 2.8:1. CaNa[Cr(OH)6] crystallizes in the non‐centrosymmetric trigonal space group R3 with the lattice parameters a = 583.86(2) pm and c = 1428.73(6) pm [T = 100(1) K]. Characteristically, the crystals are reverse‐obverse as well as inversion twins. The crystal structure is a stack of uncharged metal hydroxide layers, which can be regarded as a cation‐ordered rhombohedral variant of the Mg(OH)2 (brucite) structure type. The oxidation state of chromium(III) and its coordination by hydroxide groups was confirmed by UV/Vis and IR spectroscopy, respectively. Temperature‐dependent magnetic measurements revealed paramagnetic behavior with an effective moment of 3.82 μB per chromium atom. The thermal decomposition of CaNa[Cr(OH)6] takes place at about 225 °C, where the fast elimination of 1.5 equivalents of water is followed by the oxidation of chromium(III) to chromium(VI). Upon further heating to 1000 °C and 1200 °C, the intermediate decomposition products CaCrO4 and Na2CrO4 transform into the oxochromates(V) Ca5(CrO4)3O0.5 and Ca3(CrO4)2, respectively.

Promotion of protocell self-assembly from mixed amphiphiles at the origin of life.

Vesicles formed from single-chain amphiphiles (SCAs) such as fatty acids probably played an important role in the origin of life. A major criticism of the hypothesis that life arose in an early ocean hydrothermal environment is that hot temperatures, large pH gradients, high salinity and abundant divalent cations should preclude vesicle formation. However, these arguments are based on model vesicles using 1-3 SCAs, even though Fischer-Tropsch-type synthesis under hydrothermal conditions produces a wide array of fatty acids and 1-alkanols, including abundant C10-C15 compounds. Here, we show that mixtures of these C10-C15 SCAs form vesicles in aqueous solutions between pH ~6.5 and >12 at modern seawater concentrations of NaCl, Mg2+ and Ca2+. Adding C10 isoprenoids improves vesicle stability even further. Vesicles form most readily at temperatures of ~70 °C and require salinity and strongly alkaline conditions to self-assemble. Thus, alkaline hydrothermal conditions not only permit protocell formation at the origin of life but actively favour it.

Possible mechanisms of CO2 reduction by H2 via prebiotic vectorial electrochemistry.

Methanogens are putatively ancestral autotrophs that reduce CO2 with H2 to form biomass using a membrane-bound, proton-motive Fe(Ni)S protein called the energy-converting hydrogenase (Ech). At the origin of life, geologically sustained H+ gradients across inorganic barriers containing Fe(Ni)S minerals could theoretically have driven CO2 reduction by H2 through vectorial chemistry in a similar way to Ech. pH modulation of the redox potentials of H2, CO2 and Fe(Ni)S minerals could in principle enable an otherwise endergonic reaction. Here, we analyse whether vectorial electrochemistry can facilitate the reduction of CO2 by H2 under alkaline hydrothermal conditions using a microfluidic reactor. We present pilot data showing that steep pH gradients of approximately 5 pH units can be sustained over greater than 5 h across Fe(Ni)S barriers, with H+-flux across the barrier about two million-fold faster than OH–-flux. This high flux produces a calculated 3-pH unit-gradient (equating to 180 mV) across single approximately 25-nm Fe(Ni)S nanocrystals, which is close to that required to reduce CO2. However, the poor solubility of H2 at atmospheric pressure limits CO2 reduction by H2, explaining why organic synthesis has so far proved elusive in our reactor. Higher H2 concentration will be needed in future to facilitate CO2 reduction through prebiotic vectorial electrochemistry.

Hydrothermal processing on potassic syenite powder: Zeolite synthesis and potassium release kinetics

To make potassium accessible to agricultural application from K-silicates, processing on potassic syenite powder under mild alkaline hydrothermal conditions was conducted, as a result of which, potassium became accessible into solution, and to two zeolite powders (analcime and hydroxycancrinite) were successfully obtained. A scale-up operation in 25L-hydrothermal reactor supported the feasibility of further industrial implementation. The kinetic model derived from geochemical study was adopted to describe K+ release behavior, and two kinetic factors, reaction order and apparent activation energy, over temperature range of 477–531 K were 2.61 and 46.61 kJ/mol. The concentrated alkaline solution giving rise to higher reaction order and lower apparent activation energy was a dominated factor accelerating K-feldspar dissolution accomplished within hours. The hydrothermal approach proposed in this work is a simplified and green chemical way for the comprehensive utilization of K-silicates.

Porous PdRh nanobowls: facile synthesis and activity for alkaline ethanol oxidation.

Optimizing structure and composition with respect to electrocatalytic performance is critical to achieve outstanding Pd-based electrocatalysts. Herein, we have successfully developed a novel electrocatalyst of hollow and porous PdRh nanobowls (PdRh NBs) for the ethanol oxidation reaction (EOR) by using urea as a guiding surfactant. Under alkaline hydrothermal conditions, urea molecules can release bubbles (NH3 and CO2) that in turn guide the formation of PdRh nanobowls. The porous bowl-like structures of PdRh NBs expose abundant surface sites, which allows for increased collision frequency via confining reactants within open spaces. In regards to composition, the reason for introducing Rh is that not only is the redox potential of Rh approximate with that of Pd (beneficial to the formation of high PdRh alloy phase), but also it can effectively facilitate the breakage of C-C bond on the electrode surface (enhancing the total oxidation of ethanol to CO2). Benefiting from the compositional and structural advantages, the newly developed PdRh NBs exhibit significantly improved electrocatalytic activity for the EOR compared with those of the pure Pd NBs, PdRh nanoparticles (PdRh NPs) and commercial Pd black. These attributes might make them good anodic candidates for application in direct ethanol fuel cells.

Controllable interlayer space effects of layered potassium triniobate nanoflakes on enhanced pH dependent adsorption-photocatalysis behaviors

Despite the extensive study of two-dimensional layered KNb3O8 (KN), there still remains some vital problems need to be clarified for future applications in environmental purification. Here we demonstrated the successful preparation of interlayer-controlled KN nanoflakes using alkaline hydrothermal conditions by adjusting the amount of thiourea in the reaction. This process resulted in KN nanoflakes with a larger specific surface area than previously reported. Moreover, the initial pH of dye solution and discrepant preferential orientation of interlayer peak have been proved to significantly influence the adsorption and photocatalysis performances of KN. In addition, relevant photocatalysis mechanisms have been expounded, by combined the first-principles calculation. The present work could be helpful in revealing the intrinsic adsorption-photocatalysis features of KN and other similar niobates.

Carbonized polydopamine wrapping layered KNb3O8 nanoflakes based on alkaline hydrothermal for enhanced and discrepant lithium storage

Although the photochemical and ion exchange properties of layered KNb3O8 (KN) have been extensively studied, its potential lithium storage applications were ignored. Unlike typical acid hydrothermal method, this work demonstrate that interlayer-controlled KN nanoflakes can be prepared based on alkaline hydrothermal conditions. Pristine KN performs a high first-discharge capacity and superior cyclic stability. Moreover, polydopamine derived carbon as a conductive coating shell was firstly applied to modify KN for enhancing its lithium insertion ability. It performs outstanding rate character (310, 255, 110 mA h g−1 at the current density of 0.2, 1, 10 A g−1, respectively), as compared with pristine KN (96, 51, 13 mA h g−1). It also shows excellent long-term cycling feature (209 mA h g−1 after 3000 cycles, corresponds to 95% capacity retention). In addition, relevant energy storage mechanisms have been expounded. The present work could be helpful in developing potential multifunctional applications of KN-based and other similar niobates.

Hydrothermal synthesis of zeolites using sanitary ware waste as a raw material

Abstract Zeolites are extensively produced by hydrothermal treatment of different raw materials, such as kaolin. Sanitary ware (SW) is a kaolin-based ceramic ware usually found in sinks, urinals and bathtubs whose production is led by Spanish domestic clay product industry. SW production generates approximately 8% of solid waste in Spain, which increases landfills sizes and pose economic, social and technical problems. This study aims to evaluate the feasibility of solid waste of SW industry to produce zeolites once subjected to a conventional hydrothermal treatment during different times and temperatures. Ground SW was subjected to dissolution in a highly basic medium (NaOH 5 M) inside a Teflon-lined stainless steel reactors, at temperatures of 100, 150 and 200 °C for 1 to 30 days. X-ray powder diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) were used to characterize both the raw material and resultant mineral phases. Quartz and mullite present in SW transform into zeolites, as temperature and time increase. Mineral assemblage at 100 °C includes zeolite P, Na-faujasite and sodalite as major component and analcime, natrolite, and zeolite A as minor, which behave as metastable phases and their relative abundance depends on aging time. Analcime and cancrinite crystallization is favored by high temperature (150 °C to 200 °C) and increasing aging time produces analcime disappearance. Cancrinite is the dominant mineral after 30 days. Conventional SW waste is an optimal raw material for zeolites synthesis under high alkaline hydrothermal conditions. Combining time and temperature it is possible to drive crystallization towards a target phase.