Duration Of Hydrothermal Treatment Research Articles

Effect of organic-silane additives on textural–structural properties of mesoporous silicate materials

Reported are the influence of using various organic-silane additives: (3-aminopropyl)trimethoxysilane, (2-cyanoethyl)triethoxysilane and tris(trimethylsiloxy)silane and of hydrothermal treatment duration on textural–structural characteristics of materials. The role of organic-silane additives on formation of mesoporous silicate structures on the stage of template–silicate metaphase has been demonstrated. Introduction of said additives at equal basic components’ ratios and under same template synthesis regimes in the co-condensation process with the source of silicon:tetraethoxysilane at a ratio of 0.1:1 leads to formation of mesoporous materials having structures with various spatial organization, namely MCM-41 (using (2-cyanoethyl)triethoxysilane and (3-aminopropyl)trimethoxysilane) and MCM-48 (using tris(trimethylsiloxy)silane). The influence of the use of additives during synthesis of mesoporous materials on morphology of particles preserving their form even after decomposition of organic component of reaction mixture by baking at 650°C has also been demonstrated.

Catalytic hydrotreatment using NiMo/MAS catalysts synthesized from ZSM-5 nano-clusters

Mesostructured alumino-silcate (MAS) materials were synthesized from ZSM-5 nano-clusters and used as catalysts supports for the hydrotreatment of a model compound and real feed stock. Supports and catalysts were thoroughly characterized by using XRD, N2 adsorption analysis, FT-IR, 27Al MAS NMR, Raman, EXAFS and HRTEM techniques. The alumino-silicate materials exhibited different acid strength and textural properties depending on the duration of hydrothermal treatment of the zeolite seeds. Materials synthesized with low seeding time of 4h showed properties similar to Al-SBA-15 prepared using the direct synthesis route, whereas material with high seeding time (tseed=24h) resembled ZSM-5. The EXAFS study of the sulfided catalysts revealed that sulfidation was highest in the NiMo catalyst supported on mesoporous alumino-silicate synthesized from ZSM-5 seeds with 16h of hydrothermal treatment (MAS-16). Hydrotreating experiments were conducted using model compound (DBT) at 325°C and 600psi as well as real feed stock (Coker light gas oil) at industrial conditions. Catalytic activity in the hydrodesulfurization (HDS) of dibenzothiophene (DBT) was found to be maximum for this particular catalyst (MAS-16). The same catalyst also gave higher hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) conversion than NiMo/γ Al2O3 in the hydrotreating of Light Gas Oil.

Effects of cracking and coke formation on the selectivity of iron oxide catalysts in the dehydrogenation of methylbutenes to isoprene

The contributions of cracking and coke formation reactions to a decrease in the selectivity of domestic and imported catalysts for the dehydrogenation of methylbutenes (isopentanes and isoamylenes) into isoprene have been determined. It has been found that hydrothermal treatment in industrial reactors favorably affected an increase in the catalytic characteristics of a domestic iron oxide catalyst for dehydrogenation. The conditions for catalyst activation have been optimized with respect to the temperature and duration of hydrothermal treatment.

Hydrothermal Formation of Lath-Shaped Trioctahedral Smectite with High Crystallinity

Lath-shaped and highly crystalline trioctahedral smectites were synthesized under hydrothermal conditions. The quenched glasses with stoichiometrically dehydrated Na-smectite compositions were treated at (a) 500°C and 100 MPa for 1 and 11 days; and (b) 300°C and 100 MPa for 7 days. The crystallinity and particle size of products were dependent on the chemical composition of the starting glass, synthetic temperature and duration of hydrothermal treatment. The high structural ordering and large dimensions of the products were confirmed from the sharpness of XRD peaks; and hydration behavior under controlled relative humidity. Transmission electron microscopy was also performed for the characterization of the particle size of product. Particle sizes vary from ca. 10 nm to a few μm by changing the chemical compositions of the starting materials. The product with the highest structural ordering and largest dimension was obtained from Na0.33(Mg1.83Al0.67)Si4O11 glass treated at 500°C and 100 MPa for 1 day. The obtained results also confirmed the metastability and compositional dependency in the formation of highly crystalline trioctahedral smectite at hydrothermal conditions.

Synthesis of hierarchical nanocrystalline ZSM-5 with controlled particle size and mesoporosity

Development of new zeolite based materials is essential for a variety of applications in catalysis, separations, and medicine. Increasing the specific surface area and decreasing the micropore diffusion pathlength in zeolites are important factors for improving the performance of zeolites in catalytic applications and these factors can be optimized by decreasing the zeolite particle size. Creation of a hierarchical zeolite material that possesses both micro- and mesopores with very large surface areas and improved mass transport properties is an effective solution. In this study, a facile approach to a one step synthesis of nanocrystalline ZSM-5 zeolite from a single template system in 12–24 h at 140 °C and with high yield is presented. ZSM-5 zeolite crystals as small as 6 nm which form mesoporous aggregates of approximately 200 nm in diameter were synthesized using this method. The mesopore volume and size distribution showed a dependence on particle size such that smaller particles lead to higher mesopore volumes and narrower pore size distributions. The size of individual crystals, as well as mesopore surface area and pore volume can be controlled by adjusting the pH of the reaction mixture, as well as the hydrothermal treatment temperature and duration.

Tailored TiO2−SrTiO3 Heterostructure Nanotube Arrays for Improved Photoelectrochemical Performance

TiO(2) nanotube arrays formed on Ti substrate by electrochemical anodization have been converted into TiO(2)-SrTiO(3) heterostructures by controlled substitution of Sr under hydrothermal conditions. The growth of SrTiO(3) crystallites on the nanotube array electrode was probed by electron microscopy and X-ray diffraction. As the degree of Sr substitution increases with the duration of hydrothermal treatment, an increase in the size of SrTiO(3) crystallites was observed. Consequently, with increasing SrTiO(3) fraction in the TiO(2)-SrTiO(3) nanotube arrays, we observed a shift in the flat band potential to more negative potentials, thus confirming the influence of SrTiO(3) in the modification of the photoelectrochemical properties. The TiO(2)-SrTiO(3) composite heterostructures obtained with 1 h or less hydrothermal treatment exhibit the best photoelectrochemical performance with nearly 100% increase in external quantum efficiency at 360 nm. The results presented here provide a convenient way to tailor the photoelectrochemical properties of TiO(2)-SrTiO(3) nanotube array electrodes and employ them for dye- or quantum-dot-sensitized solar cells and/or photocatalytic hydrogen production.

Hydrothermal synthesis of Mn–mica

Hydrothermal synthesis of Mn–mica was achieved using metakaolin and manganese carbonate or silicic acid and aluminum nitrate as aluminosilicate sources along with manganese carbonate in aqueous solutions of KOH. The mixtures of starting materials were treated under hydrothermal conditions at 100–200 °C for 24–48 h. The use of metakaolin and manganese chloride also led to the synthesis of Mn–mica under hydrothermal conditions. From metakaolin and manganese carbonate, fine well-crystallized Mn–mica was hydrothermally synthesized at 200 °C using different KOH concentrations. However, from the mixtures of silicic acid, aluminum nitrate and manganese carbonate, poorly crystallized micas were synthesized at 200 °C using different Si/Al molar ratios of starting mixtures. Increasing the duration of hydrothermal treatment from 24 to 48 h yielded somewhat better crystallized Mn–mica as determined by X-ray diffraction (XRD). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed a particle size of about 1 µm and plate-like shape of the synthetic Mn–mica.

High-Temperature Formation of Titanate Nanotubes and the Transformation Mechanism of Nanotubes into Nanowires

Large-scale pure titanate nanotubes were synthesized through the hydrothermal reaction between TiO2 powders and concentrated NaOH under an unexpected high temperature of 240 °C, while it was generally claimed that it is impossible to form nanotubes at temperatures higher than 180 °C. The titanate nanotube was found to be an inevitable intermediate product, which finally transformed into a nanowire upon increasing the hydrothermal treatment duration. It was proven that the successive appearance of nanosheets, nanotubes, and nanowires are three unavoidable kinetic products of the reaction. Increasing the temperature could only accelerate the nanotube−nanowire transformation process but could not affect the sequence of the reaction events. The transformation kinetics from nanotubes to nanowires under different reaction temperatures was studied. Detailed studies indicate that this transformation process was accompanied by a coarsening process induced by both oriented attachment (OA) and Ostwald ripening (OR) ...

Novel hydrothermal preparation of pure brookite-type titanium(IV) oxide nanocrystal under strong acidic conditions

Titanium(IV) oxide (TiO2) nanocrystal with brookite phase was prepared by hydrothermal treatment of titanate nanotubes under strong acidic conditions (perchloric acid, hydrochloric acid and nitric acid). The relative fractions of crystal phases (anatase, rutile and brookite) were strongly dependent on hydrothermal treatment duration and the kind of strong acid used. A high brookite-type TiO2 content was obtained by hydrothermal treatment under perchloric acidic solution. Single-phase brookite TiO2 particles were extracted from as-prepared TiO2 by a centrifugal separation procedure, and they exhibited higher photocatalytic activity for acetaldehyde decomposition than TiO2 prepared without the separation procedure.

Hydrothermal Treatment Duration Effect on the Transformation of Titanate Nanotubes into Nanoribbons

Titanate nanotubes and nanoribbons have been synthesized hydrothermally using commercial anatase nanopowder (particle size ≈50−400 nm) and NaOH. The samples were characterized by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (TEM−SAED), and X-ray diffraction (XRD). It has been found that the hydrothermal temperature and the treatment duration have a strong effect on the morphology control of the resulting products. At a fixed temperature of 180 °C, the morphology of the products changes with the hydrothermal duration (5−72 h). Hollow nanotubes with an outer diameter of about 10 nm were formed at treatment durations between 5 and 20 h. Bundles of nanoribbons with widths ranging from 50 to 500 nm and lengths up to several tens of micrometers were produced at a treatment duration of 72 h. Experimental results on samples treated at different temperatures (135−195 °C) for different durations revealed the transformation of the nanotubes i...

Structural and morphological transformations of mesostructured titanium phosphate through hydrothermal treatment

The phase transformation of mesostructured titanium phosphate (TiPO) from hexagonal to lamellar structure was observed in a simply hydrothermal treatment, accompanied by drastically morphological changes in the micrometer-sized particles. XRD pattern revealed that different mesostructures were obtained by simply varying hydrothermal temperature or treatment duration. SEM and TEM observations showed the morphological evolution from individual particles to interconnected nanoplatelets. A significant blue shift in UV–vis spectra was observed for lamellar mesostructured material, which may be associated with the different coordinated Ti-sites in the hexagonal and lamellar mesostructures. FT-IR spectra and detailed 31P MAS NMR studies indicated that additional P OH groups were presented in the lamellar structure, which might play a key role in the structural and morphological transformations of mesostructures.

Hydrothermal synthesis of CeO2 nano-particles

Nano-crystalline particles of CeO2 have been synthesized via a low temperature hydrothermal synthesis process. Two types of precursors were studied—cerium hydroxide and ceria acetate. The precursors were adjusted to a basic (pH 10) and acidic (pH 4) medium before hydrothermal treatment at various durations of 6, 12, 18, and 24 h at 250 °C using a Teflon-lined hydrothermal bomb. The synthesized samples were characterized using DTA/TGA, XRD and TEM. Based on the characterization results, both precursor systems produced crystalline ceria nano-particles after 6 h of hydrothermal synthesis at 250 °C. The average crystallite sizes were 6 and 15 nm for the hydroxide and acetate system, respectively. The acetate precursor system appeared to produce better particles in terms of crystallinity and morphology. Based on the DTA/TGA analysis, hydrothermal synthesis had been effective in reducing the amount of intermediate products. With increasing hydrothermal treatment duration of up to 24 h, the samples did not exhibit a remarkable improvement in properties. The synthesized nano-particles were subsequently heat treated at 500 and 1000 °C for 2 h. After the heat treatment, enhanced crystallinity and growth in crystallite size was observed, but particles appeared more agglomerated.

Synthesis and Characterization of Mo‐Oxide Nanoribbons

Mixed valence Mo‐oxide nanoribbons have been synthesized using MoO3 as precursor and hexadecylamine (HDA) as surfactant under hydrothermal condition at 180°C for 1 to 10 d. The morphology of the product is found to depend strongly on the duration of hydrothermal treatment, temperature and HDA/Mo ratio. The powder diffraction patterns of pre‐and post‐hydrothermally treated materials exhibit the highly intense and sharp reflections at low scattering angles that are typical for layered structures. The IR spectrum of Mo‐HDA composite is quite similar to hydrothermal treated samples except shifting of few bands to higher energy side. The weight change in TG‐DTA curves indicates the presence of HDA as intercalates. The SEM images of samples reveal the layered and flat nanoribbons formation. Dimensions of these flat nanoribbons found from SEM images are about 160 nm in thickness, 430–620 nm in width and several micrometers in length. The high resolution TEM images of Mo‐oxide nanoribbons corroborate the XRD results and the fringe spacing paralleling to the nanolayers is estimated to be 3.52 nm, which is close to the (001) lattice spacing of lamellar Mo‐oxide nanoribbons.

Physicochemical features of the formation of siliceous porous mesophases. 4. Effect of duration and temperature of the hydrothermal treatment on the structural and textural properties

The results of investigation of mesoporous mesophase materials C12--SiO2--MMM and C16--SiO2--MMM prepared at the optimized composition of the reaction mixture and different durations of hydrothermal treatment (HTT) at 120 °C and 140 °C are presented. Hydrothermal treatment at 120 °C influences slightly the specific surface area and the volume of the mesopores but gives a more ordered structure. Prolonged HTT at 140 °C results in irreversible structure degradation. The samples obtained with the optimal HTT duration are characterized by the minimum width of X-ray reflections, the maximum surface and volume of mesopores, and the minimum external surface.

Hydrothermal modification of silica gels (xerogels) 2. Effect of the duration of treatment on their porous structure

Abstract The effect of the duration of hydrothermal treatment (in the range 1–20 h) on the parameters of the pore structure of silica xerogels was investigated. Silica gels with different initial porous structure parameters were used. The specific surface area (S) of the adsorbents changed from 830 to 38 m2 g−1. A simple mathematical equation describing the dependence of the specific surface area S on the autoclaving treatment time is proposed. The main changes in the geometrical structure of silica gel occur during the first 6 h of treatment. A long-term (over 6 h) hydrothermal treatment can be recommended to improve the homogeneity of the pore structure of silica gels.