Hydrothermal Crystallization Process Research Articles

SAPO-34分子筛晶化过程的研究

The structure and composition of SAPO-34 zeolite synthesized by conventional hydrothermal process and dry gel liquid phase crystallization process were investigated by XRD,SEM,ICP-AES and MAS NMR characterization methods.The results showed that SAPO-34 zeolite with impurity was obtained by conventional hydrothermal process.Impurity was appeared due to fewer active silicon species with high rate of silica source depolymerization and polycondensation.Pure SAPO-34 zeolite was synthesized by dry gel liquid-phase crystallization process.The rate of nucleation and growth was controlled by slower rate of silica source depolymerization.Samples of relative crystallinity increased gradually.

Metathesis of 2-butene to propylene over W-mesoporous molecular sieves: A comparative study between tungsten containing MCM-41 and SBA-15

Mesoporous silica (MCM-41 and SBA-15) having tungsten oxide in the framework was synthesized by hydrothermal crystallization process using CTAB and P123 as structure directing agents, respectively. Tungsten oxide was supported on the MCM-41 and SBA-15 by wet impregnation method for a comparative study. The synthesized materials were characterized by XRD, N2 adsorption–desorption, NH3-TPD, pyridine-FT-IR, UV–vis DRS and Raman spectroscopy techniques. The catalysts were used to perform metathesis reaction of 2-butene in a fixed-bed reactor at different temperatures and under atmospheric pressure. It was observed that catalysts having tungsten oxide in the framework of mesoporous silica exhibited higher activity as compared to tungsten oxide impregnated catalysts. The results revealed that WO3-MCM-41(30) catalyst has much higher activity at a low temperature (450°C) as compared to WO3-SBA-15(30). At 550°C, WO3-MCM-41(30) and WO3-SBA-15(30) showed comparable activity with highest propylene yield of 39mol% and 37mol%, respectively. The higher metathesis activity of WO3-MCM-41 catalysts can be related to the well-dispersed active tetrahedral tungsten oxide species as evidenced from XRD, UV–vis DRS and Raman spectroscopy results.

Utilization of bagasse fly ash for carbon–zeolite composite preparation

Bagasse fly ash (BFA), a solid waste from sugar cane industries, contains significant amount of carbon as well as silica. The coarse particles with high carbon content can be separated and further activated to produce BFA-based activated carbon, while silica content can be extracted from fine BFA particles to be used for zeolite crystallization. The zeolite crystal may be grown on a suitable solid surface to create a zeolitic composite. In this study, silicate extract from fine BFA particles were combined with pretreated carbon rich coarse BFA particles in a hydrothermal crystallization process to produce particular carbon–zeolite composites. The carbon rich particles could be subjected to any necessary activation or surface treatment before being used in the composite preparation. Meanwhile, a simple method based on thermogravimetry is proposed to evaluate the zeolite particles distribution on the carbon surface. Furthermore, the composite ability for treating mixed organic and inorganic pollutants in aqueous solution has been investigated.

A study of thermal treatment and hydrothermal crystallization stages in production of granulated NaA zeolite from mechanically activated metakaolin

NaA zeolite was synthesized from mechanically activated granulated mixtures of metakaolin, sodium hydroxide, and aluminum oxide. The thermal treatment and hydrothermal crystallization processes were studied. It is shown that the optimal temperature of thermal treatment of grains based on a mechanically activated metakaolin is 600°C at a NaA zeolite content of about 65 wt %. Raising the calcination temperature leads to a decrease in the content of the zeolite and to formation of nepheline. It was found that the optimal concentration of the crystallization solution of NaOH is 2–4 M. Under these conditions, the content of the zeolite exceeds 90 wt %. Raising the alkali concentration leads to a decrease in the content of the zeolite and to its recrystallization into sodalite.

Preparation of Mullite Nanocomposites Powders by the Hydrothermal Crystallization Method from Coal Gangue

Mullite nanocomposites powders have been successfully synthesized from pretreated coal gangue via hydrothermal crystallization process. The mullite nanocomposites powders are investigated and charactered by laser particle size analysis, X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM) analysis, selected area electron diffraction (SAED) analysis and energy-dispersive X-ray (EDX) analysis. Moreover, the reaction mechanism of this process is speculated according to MAS-NMR results. It is indicated that mullite nanocomposites powders can be prepared at 80-90 °C for 3 h with NaOH concentration of 2-4 mol/L and L/S ratio (liquid/solid ratio in mass) of 10 mL/g by using calcined coal gangue as precursor. The agglomerate powder size of D50 is about 3.83-4.71 mm. Furthermore, some imperfective puncheon-shaped crystals exist among a large number of whole puncheon-shaped crystals which are about 30-70 nm in length and about 10-20 nm in diameter. MAS-NMR analysis indicates that the nano-size mullite grow around the central atom-Al, Si of [AlSi]O4 tetrahedral and the growth unit Al[OH]4- exist in this process definitely.

Single-pass flow-through reaction cell for high-temperature and high-pressurein situneutron diffraction studies of hydrothermal crystallization processes

A large-volume single-pass flow-through cell forin situneutron diffraction investigation of hydrothermal crystallization processes is reported. The cell is much more versatile than previous designs owing to the ability to control independently and precisely temperature (up to 673 K), pressure (up to 46 MPa), flow rate (0.01–10 ml min−1) and reaction-fluid volume (≥65 ml). Such versatility is realized by an innovative design consisting of a room-temperature and ambient-pressure external fluid supply module, a high-pressure reaction module which includes a high-temperature sample compartment enclosed in a vacuum furnace, and a room-temperature and high-pressure backpressure regulation module for pressure control. The cell provides a new avenue for studying various parameters of hydrothermal crystallizations independently,in situand in real time at extreme hydrothermal conditions (e.g.supercritical). The cell was successfully commissioned on the high-intensity powder diffractometer beamline, Wombat, at the Australian Nuclear Science and Technology Organisation by investigating the effect of pressure on the hydrothermal pseudomorphic conversion from SrSO4(celestine) to SrCO3(strontianite) at a constant temperature of 473 K and flow rate of 5 ml min−1. The results show that the increase of pressure exerts a nonlinear effect on the conversion rate, which first increases with increasing pressure from 14 to 20 MPa, and then decreases when pressure further increases to 24 MPa.

A Comparative study on synthesis, characterization and photocatalytic activities of MgO and Fe/MgO nanoparticles

MgO and Fe/MgO particles in the nanometer-size regime have been successfully synthesized by a hydrothermal crystallization process using dodecyl benzenesulphonic acid as a surfactant. The prepared catalysts were characterized in details by multiform techniques: XRD, SEM, TEM, UV-DRS, TGA-DSC, BET surface area, and EPR spectroscopy. Comparative photocatalytic activity of the samples was evaluated through the photodegradation of Acid Blue 80 dye under sunlight. The results of the photocatalytic degradation of Acid Blue 80 dye in aqueous solution demonstrated that Fe ions doping greatly improved the photocatalytic efficiency of MgO nanoparticles. Most interestingly, this is the first time that it has been observed that the Fe/MgO system is quite an effective visible light active catalyst.

Phonon and magnetic properties of nanocrystalline MnWO 4 prepared by hydrothermal method

Well-crystallized MnWO 4 nanoparticles with 20–100 nm sizes were synthesized by a mild hydrothermal crystallization process. X-ray diffraction and TEM results show that particles have tendency to grow in a shape of nanorods elongated in the c direction. They also show that the particles synthesized from MnCl 2 have smaller concentration of defects than those synthesized from MnSO 4·H 2O. Raman spectra revealed weak changes with decreasing particle size due to phonon confinement effect and defects. In contrast to this behavior, IR spectra showed much more pronounced changes. Origin of this behavior is discussed. Raman and IR spectra showed also presence of additional bands at 923 and 914 cm −1, respectively. These bands have been attributed to presence of terminal W O bonds at the surface of MnWO 4 nanocrystallites. Specific heat and magnetization studies revealed broad anomalies at 11.0–11.4 K, which correspond to the two magnetic transitions found for bulk MnWO 4 at 12.5 and 13.5 K.

Removal of UO 22+ onto synthetic NaA zeolite. Characterization, equilibrium and kinetic studies

The removal of UO 2 2+ ions from aqueous solutions on zeolite NaA was investigated. The effects of initial uranium (VI) concentration, solution pH, solid/liquid ratio and temperature were studied in batch experiments. Zeolite NaA was synthesized according to a hydrothermal crystallization process. The Dubinin–Radushkevich, the Freundlich and the Langmuir models have been applied and the adsorption equilibrium has been found to follow the Langmuir model. Kinetic studies showed that the pseudo first-order sorption model was the most prevalent for the adsorption of UO 2 2+ ions. The rate constant of the exchanged ions appears to be controlled by chemical sorption process. Rate limiting investigations suggested that the UO 2 2+ ions diffusion was the determining step. The thermodynamic parameters were also determined and the negative values of adsorption entropies indicated that no significant change occurred in the framework of the NaA zeolite during the sorption of UO 2 2+. The obtained optimal parameters were applied to real effluents containing UO 2 2+ ions.

Synthesis of ordered cubic mesoporous ZrMCM-48 molecular sieve with aid of fluoride ions

Using cetyltrimethyl ammonium bromide (CTAB) as the template and tetraethyl orthosilicate (TEOS) as silica source, ordered cubic mesoporous ZrMCM-48 molecular sieves were synthesized by addition of sodium fluoride in hydrothermal crystallization process. The structure and property of the calcined sample were characterized by XRD, TEM, XPS, and N 2 physical adsorption, respectively. The effect of crystallization time, crystallization temperature and F −/Si molar ratio on synthesis process of sample was systematically investigated. Also, the different molar ratios of Si/Zr on textural properties of ZrMCM-48 samples were also evaluated. The results show that the optimum synthesis conditions of cubic ZrMCM-48 sample are the following: crystallization temperature is 120 °C, crystallization time is 24 h and F −/Si ratio is 0.1. A small amount substitution of Zr ions for Si 4+ in the framework of MCM-48 did not significantly change the textural properties while the higher Zr content incorporated leads to decrease of the surface area and deterioration of structural regularity.

Preparation of Mullite Nanocomposites Powders from Coal Fly Ash via Hydrothermal Crystallization

Mullite nanocomposites powders have been successfully synthesized from high-aluminium coal fly ash via hydrothermal crystallization process. The mullite nanocomposites powders are investigated and charactered by laser particle size analysis, BET surface area analysis, X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM) analysis, selected area electron diffraction (SAED) analysis and energy-dispersive X-ray (EDX) analysis. Moreover, the reaction mechanism is speculated according to MAS-NMR results. The experimental results show that mullite nanocomposites powders can be prepared at 80-90 °C for 3 h with NaOH concentration of 2-4 mol/L and L/S ratio (liquid/solid ratio in mass) of 10 mL/g. The powder size of D50is about 1.88-3.27 mm. Mullite nanocomposites are mainly rod-like, acicular and fibroid in shape with an range of 30-80 nm in length and an range of 4.5-30 nm in diameter. MAS-NMR analysis indicates that the nano-size mullite grow around the central atom-Al, Si of [AlSi]O4tetrahedral and the growth unit Al[OH]4-is existed in this process definitely.

Zeolite microspheres with hierarchical structures: formation, mechanism and catalytic performance

An improved polymerization-induced colloid aggregation (im-PICA) method was developed to prepare zeolite microspheres (ZMSs) with a hierarchical porous structure and a uniform size, which could easily be carried out within half an hour by simply adding urea and formaldehyde into an acidic pH-pre-controlled colloidal solution as-obtained from a hydrothermal crystallization process. This procedure successfully avoided the repeated ultrasonic washing and high-speed centrifugation of the nanozeolite colloid during the traditional PICA method, and thereby became energy and time-economical. After removing the polymeric component, solid and hollow ZMSs as well as the mono-dispersed nanozeolite can be obtained under different preparation conditions. The structural transformation could be attributed to the correlation of the polymerization velocity on the zeolite nanocrystal surface and in the bulk solution with variation of acidity. Due to the short diffusion routes and abundant exposed active sites of nanozeolites as well as their unique secondary mesopore architecture, ZMSs displayed a clear advantage on catalysis reactions compared to commercial zeolite catalysts, such as the dehydration of fructose into 5-hydromethyl furfural.

Room‐Temperature Solid‐State Reaction Behavior, Hydrothermal Crystallization and Physical Characterization of NaRE(MoO4)2, and Na5Lu(MoO4)4 Compounds

In the context, a novel composite synthesis technology is engaged in the preparation of NaRE(MoO4)2 (RE=Y, La, Nd, Eu, Gd, Tb, Er, and Yb) and Na5Lu(MoO4)4 compounds, which involves a room‐temperature solid‐state reaction and hydrothermal crystallization process. The synthesis mechanism is predicted, indicating that higher temperature and moisture can speed up the reaction process and especially the existence of crystalline water molecules in the precursor is necessary for the solid‐state reaction at room temperature. It is found that different rare‐earth nitrate precursors present different reactivity to sodium molybdate at room temperature. The crystallization degree of the products after the room‐temperature solid‐state reaction depends on the melting point of rare‐earth nitrate precursors. The hydrothermal treatment is beneficial for the good crystallization of NaRE(MoO4)2 (RE=Y, La, Nd, Eu, Gd, Tb, Er, and Yb) and Na5Lu(MoO4)4. Finally, the photoluminescent spectra for these NaRE(MoO4)2:Eu3+ (La, Gd, and Y) are studied, which depend on the species of rare‐earth ions.

Photocatalytic activities of Cd-doped ZnWO 4 nanorods prepared by a hydrothermal process

The ZnWO 4 nanorods doped with cadmium ions have been successfully synthesized by a hydrothermal crystallization process. The products were characterized in detail by multiform techniques: X-ray diffraction (XRD), energy dispersive X-ray analysis (EDS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results of the photocatalytic degradation of rhodamine B (RhB) in aqueous solution showed that cadmium ions doping greatly improved the photocatalytic efficiency of ZnWO 4 nanorods. The Cd-doped ZnWO 4 nanorods with atomic ratio of Cd to Zn being 0.06 had the best activity in photo-degradation of RhB in aqueous solution under UV light irradiation, when the nanorods have prepared at pH 8.

Synthesis, morphology and phase transition of the zinc molybdates ZnMoO 4·0.8H 2O/α-ZnMoO 4/ZnMoO 4 by hydrothermal method

ZnMoO 4 with a rhombus sheet or flower-like structure, α-ZnMoO 4 and needle-like ZnMoO 4·0.8 H 2O were successfully synthesized by simple hydrothermal crystallization processes with citric acid. ZnMoO 4·0.8 H 2O was easily synthesized in a shorter reaction time (2 h) at a higher reactant concentration. It gradually transformed into ZnMoO 4 with a monoclinic wolframite tungstate structure with an increased reaction time, and pure ZnMoO 4 was obtained with a longer reaction time (8 h). Citric acid (CA) played an important role in controlling the morphology of the as-obtained molybdates. The α-ZnMoO 4 and ZnMoO 4 were synthesized by heating ZnMoO 4·0.8 H 2O at 130 °C for 4 h and 8 h, respectively, under hydrothermal conditions. With transforming of ZnMoO 4·0.8 H 2O to α-ZnMoO 4 and further to ZnMoO 4, the needle-like crystals gradually disappeared and were transformed into crystals with rhombus sheet morphology and then further to pentacle or flower-like crystals that can be ascribed to continuous splitting and growing of the rhombus sheets.

Synthesis of Hollow Zeolite Composite Spheres by Using β-Zeolite Crystal as Template

Abstract Core–shell microspheres with BEA and FAU zeolite structures (denoted as BEA–FAU) were prepared through a two-step hydrothermal crystallization process in which a β-zeolite was used as silicon source for the synthesis of Y zeolite as well as a template for the formation of hollow sphere. Scaning electron microscopy and intrusive mercury experiments displayed the as-synthesized sample had extra-large pores of 2–3 µm; a possible formation mechanism of BEA–FAU hollow spheres was proposed and discussed.

Synthesis and phonon properties of nanosized Aurivillius phase of Bi2MoO6

AbstractNanosized crystallites Bi2MoO6 of a platelet shape were synthesized by a mild hydrothermal crystallization process. The effect of particle size on the structure and properties of Bi2MoO6 was studied by X‐ray diffraction, transmission electron microscopy, as well as Raman and infrared (IR) spectroscopies. Moreover, temperature‐dependent Raman spectra were collected for bulk and nanocrystalline Bi2MoO6. These studies showed that the thickness of the smallest crystallites is about 8–11 nm. Raman and IR studies have revealed that the damping and the intensity of Raman and IR bands are significantly modified with decreasing particle size and temperature. Moreover, some bands experience significant shifts both towards lower and higher wavenumbers. The obtained results indicate that, similar to the isostructural Bi2WO6, the orthorhombic distortion decreases with decreasing particle size. However, the structure of nanocrystalline Bi2MoO6 remains orthorhombic and noncentrosymmetric, and the structural changes are not the same as those observed in Bi2WO6. Copyright © 2010 John Wiley & Sons, Ltd.

Phonon properties of nanosized bismuth layered ferroelectric material—Bi2WO6

AbstractNanosized Bi2WO6 was synthesized by a mild hydrothermal crystallization process. This method allowed obtaining plate‐like crystallites of very small thickness (down to 3 nm). The effect of particle size on the structure and properties of Bi2WO6 was studied by X‐ray diffraction (XRD), transmission electron microscopy, and Raman and infrared spectroscopies. It has been shown that the orthorhombic distortion decreases with decreasing particle size, but the structure of the smallest crystallites is still orthorhombic. Raman studies have also revealed a very strong intensity decrease for those modes that appear mainly for incident and scattered light polarized perpendicular to the layers. This behavior has been attributed to a decrease in the orthorhombic distortion and a plate‐like shape of the nanocrystallites. Copyright © 2009 John Wiley & Sons, Ltd.

Dynamic crystal-growth process observed during hydrothermal coarsening of nanocrystalline hydroxyfluorapatite

The formation process of hydroxyfluorapatite under hydrothermal environment has been studied in detail. The fluoride ion has been substituted for the hydroxide ion in the OH lattice position, which has been characterized by X-ray diffraction (XRD) and FT–IR spectra. On the basis of the experimental results obtained from the transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), a detailed four-step growth mechanism has been proposed for the formation of the 3D nanostructure, which involves the initial nucleation (conventional hydrothermal crystallization process), the self-assembly of nanoparticles (oriented attachment), the self-assembly of thin nanorods (oriented attachment) and the grain growth (Ostwald ripening).

Synthesis and Catalytic Property of a Zeolite Composite for Preparation of Dimethyl Ether from Methanol Dehydration

A zeolite composite with Beta and Mordenite zeolite structures (denoted as BMZ) was synthesized with a two-step hydrothermal crystallization process, and characterized by X-ray diffraction, scanning electron microscopy, N2 adsorption, NH3-TPD and in situ IR spectra of pyridine. The results show that the composite has microprous and mesoporous structures and distinguished acid properties, the acid amount and concentration ratio of Lewis to Bronsted-acid sites in the composites can be controlled by changing the alkalinity in the second-step synthesis. Over the composite catalyst H-BMZ, the conversion of methanol in the mild condition reached 90% and the selectivity of dimethyl ether almost kept 100%.