Conventional Hydrothermal Synthesis Research Articles

Materials with Nanoscale Porosity: Energy and Environmental Applications.

In this personal account, several key inventions on designing novel microporous and mesoporous materials, and their applications in energy and environmental research are reviewed. Although, crystalline materials with sub-nanometer pore size regime like zeolites, AlPOs, MOFs, ZIFs etc. are known over the years, silicious and non-silicious mesoporous materials have revolutionized the research on the materials with nanoscale porosity in last two and half decades. A wide range of inorganic, organic-inorganic hybrid as well as purely organic mesoporous materials with either periodic or disordered mesopores are known. Apart from conventional hydrothermal syntheses involving soft templating route, hard templating, evaporation induced self-assembly (EISA), electrochemical or solvothermal (using hydrophilic solvents) synthetic routes are often employed in designing a large spectrum of mesoporous materials. Ease of synthesis using available cheap raw chemicals and versatility in the framework compositions together with the unique surface properties like exceptionally high surface area, pore volume and tunability in pore dimensions have made these materials very exciting to a wide range of researchers working on materials chemistry. Nanoscale porosity in the semiconductor nanomaterials is highly beneficial for the photocatalytic, optoelectronic and related light-harvesting applications. Their high chemical stability has been explored intensively in designing novel heterogeneous catalysts for the synthesis of biofuels from biomass or CO2 fixation to reactive organic molecules for the synthesis of fine chemicals and fuels, which has a large impact on energy and environmental research for the years to come. Diversity in mesoporous frameworks and their potential applications related to light harvesting, generation of renewable energy and synthesis of value added fine chemicals and fuels through environment friendly routes are mostly focused in this review.

Amending the Structure of Renewable Carbon from Biorefinery Waste-Streams for Energy Storage Applications

Biorefineries produce impure sugar waste streams that are being underutilized. By converting this waste to a profitable by-product, biorefineries could be safeguarded against low oil prices. We demonstrate controlled production of useful carbon materials from the waste concentrate via hydrothermal synthesis and carbonization. We devise a pathway to producing tunable, porous spherical carbon materials by modeling the gross structure formation and developing an understanding of the pore formation mechanism utilizing simple reaction principles. Compared to a simple hydrothermal synthesis from sugar concentrate, emulsion-based synthesis results in hollow spheres with abundant microporosity. In contrast, conventional hydrothermal synthesis produces solid beads with micro and mesoporosity. All the carbonaceous materials show promise in energy storage application. Using our reaction pathway, perfect hollow activated carbon spheres can be produced from waste sugar in liquid effluence of biomass steam pretreatment units. The renewable carbon product demonstrated a desirable surface area of 872 m2/g and capacitance of up to 109 F/g when made into an electric double layer supercapacitor. The capacitor exhibited nearly ideal capacitive behavior with 90.5% capacitance retention after 5000 cycles.

Stabilization Mechanism of the Tetragonal Structure in a Hydrothermally Synthesized BaTiO3 Nanocrystal.

Higher OH concentration is identified in tetragonal barium titanate (BaTiO3) nanorods synthesized by a hydrothermal method with a 10 vol % ethylene glycol solvent (Inada, M.; et al. Ceram. Int. 2015, 41, 5581-5587). This is apparently inconsistent with the known fact that higher OH concentration in the conventional hydrothermal synthesis makes pseudocubic BaTiO3 nanocrystals more stable than the tetragonal one. To understand where and how the introduced OH anions are located and behave in the nanocrystals, we applied ab initio analysis to several possible microscopic geometries of OH locations, confirming the relative stability of the tetragonal distortion over the pseudocubic one because of the preference of trans-type configurations of OH anions. We also performed Fourier transform infrared and X-ray diffraction analysis, all being consistent with the microscopic picture established by the ab initio geometrical optimizations.

Effect of Surface Trap States on Photocatalytic Activity of Semiconductor Quantum Dots

Semiconductor quantum dots (QDs) are promising photocatalysts for water splitting due to the large specific area, but the influence of surface trap states on the photocatalytic activity of QDs is still not fully understood yet. To answer this question, CdSe QDs with the same morphology, diameter, crystal structure, and energy level are prepared following a hydrazine hydrate (N2H4) promoted synthesis strategy and conventional hydrothermal synthesis method. Through various characterizations and analysis, it is found that the conventional hydrothermal synthesized CdSe QDs (H-CdSe QDs) have a high concentration of Cd-involved shallow electron trap states, which seriously hinder the charge separation and transfer between CdSe and cocatalysts. In contrast, the N2H4 promoted synthesis strategy provides an energy-saving, low-cost, and facile pathway to eliminate the surface shallow electron traps, ensuring an efficient charge separation and H2 production in CdSe QDs. As a result, the N2H4-promoted synthesized CdS...

Ultrafast synthesis of *BEA zeolite without the aid of aging pretreatment

The present study focuses on the ultrafast synthesis of *BEA zeolite, which constitutes one of the most industrially important zeolites. A pure *BEA phase was successfully synthesized within 10 min at 210 °C by carefully tuning the composition of the initial reactant mixture. To date, the ultrafast synthesis of several industrially important zeolites is achieved with the aid of aging pretreatment, followed by high temperature hydrothermal treatment. In this study, we first demonstrated the 10-min synthesis of *BEA zeolite employing organic structure-directing agents (OSDAs), without any aging process. The employment of sealed pipe reactor in conjunction with the optimization of chemical composition of starting materials are critical in obtaining pure *BEA phase in a short time. The fast-synthesized *BEA zeolite possesses properties that are similar to those synthesized by conventional hydrothermal synthesis methods in terms of crystallinity and micropore volume. A comprehensive investigation on the effects of various parameters on the seed-assisted crystallization of *BEA zeolite was also studied.

Enhanced synthetic efficiency of CHA zeolite crystallized at higher temperatures

High silica chabazite zeolite (CHA) is very important for selective catalytic reduction of NOx with ammonia (NH3-SCR), but its synthesis is time-consuming under conventional hydrothermal conditions (160 °C for 90–120 h). Herein, we report a novel strategy for highly efficient synthesis of CHA zeolite by means of fast crystallization at high temperatures in the absence of water solvent (240 °C for 1.5 h). X-ray diffraction, N2-adsorption isotherms, inductively coupled plasma, scanning electron microscopy, 29Si and 27Al MAS NMR, as well as thermogravimetry-differential thermal analysis suggest that the samples have high quality. This concept significantly enhances the space-time yields (STY) for CHA zeolite prepared from high-temperature synthesis in the absence of water solvent (almost two orders of magnitude higher than those of conventional hydrothermal synthesis). Moreover, compared with hydrothermal synthesis, the samples obtained from fast crystallization at higher temperatures (Si/Al = 12, 2.0% Cu loading) shows enhanced catalytic performance at 350–550 °C in NH3-SCR.

Rationalization of Hydrothermal Synthesis of NaNbO3 by Rapid in Situ Time-Resolved Synchrotron X-ray Diffraction

In situ monitoring of the formation of crystalline phases during conventional hydrothermal synthesis is experimentally challenging. Here, we report an in situ time-resolved synchrotron X-ray diffraction study during hydrothermal synthesis of NaNbO3 using a high-pressure custom-made capillary cell penetrable to X-rays. The high time resolution (0.1 s) revealed a sequence of transient intermediate phases, including several unknown phases, before the final perovskite NaNbO3 was formed. These new findings highlight the complexity of the hydrothermal synthesis of NaNbO3 and demonstrate the potential for obtaining in-depth knowledge of the reactions taking place by time-resolved in situ X-ray diffraction.

COMPARATIVE STUDY ON SYNTHESIS OF METAL ORGANIC FRAMEWORK-199 BY MICROWAVE AND SOLVOLTHERMAL PROCESS

<p>In the present paper, a comparative study on the synthesis of metal organic framework-199 by microwave and solvolthermal method was demonstrated.<strong> </strong>The obtained materials were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), nitrogen adsorption/desorption isotherms and thermal gravity (TG). The microwave synthesis of MOF-199 has been compared to its conventional hydrothermal synthesis. It is found that by using microwave synthesis MOF-199 can be obtained in a much shorter synthesis time with improved yield and textural properties. It is supposed that the microwave energy emitted fast and uniformly, creating nuclei through the solution which quickly grew to crystals and both the nucleation and crystallization steps were accelerated. These make the microwave synthesis advantages over solvolthermal synthesis.<strong></strong></p>

Microwave synthesis and voltammetric simultaneous determination of paracetamol and caffeine using an MOF-199-based electrode

In the present paper, the microwave synthesis of MOF-199 and its application as an electrode modifier for the simultaneous voltammetric determination of paracetamol (PAR) and caffeine (CAF) were demonstrated. The obtained materials were characterised by X-ray diffraction, a scanning electron microscope (SEM), nitrogen adsorption/desorption isotherms and thermal gravity. The microwave (MW) synthesis of MOF-199 has been compared to its conventional hydrothermal synthesis. It is found that by using the MW synthesis, MOF-199 can be obtained in a much shorter synthesis time with improved yield and textural properties. The electrode modified by MOF-199 was used in order to develop an electroanalytical method that can be used to simultaneously quantify PAR and CAF. The kinetic parameters of the electrode reaction process were also investigated. This proposed method was successfully employed for the simultaneous detection of PAR and CAF in pharmaceutical formulations using the standard addition method and the obtained results compared with the results determined by means of HPLC were found to be statistically similar.

Synthesis of hierarchical ZSM-5 zeolites by solid-state crystallization and their catalytic properties

Hierarchically porous structure is important to promote mass transport and active site utilization of zeolites for many petrochemical reactions involving bulky molecules. Their conventional hydrothermal synthesis with meso-template materials is often complicated and expensive, and produces a large amount of aqueous pollute. We demonstrate here successful synthesis of mesoporous zeolites of MFI structure by solid-state crystallization without meso-template. Aluminosilicate nanogels are produced with only precursors and the dried nanoparticles are then transformed into nanocrystals in solid state using only their intrinsic water storage. After orientating with each other at edges, these nanocrystals join into monolithic zeolites with connecting inter-lattice mesopores. The produced hierarchical ZSM-5 zeolites exhibit superior stability and excellent catalytic performance in reactions such as Friedel-Crafts benzylation, lignin ethanolysis, and naphthalene hydrogenation. Compared with conventional synthesis routes, this new solid crystallization approach simplifies zeolite production, lowers the cost, and avoids toxic liquid waste, which may greatly benefit many energy applications.

Effect of ultrasound pretreatment on the hydrothermal synthesis of SSZ-13 zeolite

SSZ-13 zeolite has been identified as effective material for catalysts and membranes for years because of its structure, but the long crystallization time is the main obstacle for the industry application. The sonochemical-assisted method was identified as one of the potential pre-treatment methods which could reduce the formation duration of zeolites as well as other microporous and mesoporous materials. In this work, zeolite SSZ-13 was prepared by ultrasound pretreatment prior to hydrothermal crystallization. For comparison, samples by the conventional pretreatment and hydrothermal method were also prepared. The synthesized powders were characterized by XRD, SEM, BET, FTIR, NMR and TGA. The results showed that the complete crystallization of SSZ-13 synthesized by the ultrasound-assisted hydrothermal method was achieved within 72h, and much faster than conventional hydrothermal synthesis (120h). Furthermore, the ultrasound pretreatment showed very little effect on the structure properties of the final product compared with the conventional aging method.

Solvent‐Free Synthesis of ITQ‐12, ITQ‐13, and ITQ‐17 Zeolites

AbstractDevelopment of the sustainable routes for synthesis of ITQ‐family zeolites is very important because of their unique structures and excellent catalytic and adsorptive properties. The burden of costly raw materials and low efficiency of synthesis put a strong challenge for their widespread commercial application. Here, we show an alternative and simple route for synthesis of ITQ‐12, ITQ‐13, and ITQ‐17 zeolites using commercially available organic templates by a facile grinding process of anhydrous starting raw solids, followed by heating at 140—180 °C. Compared with the conventional hydrothermal synthesis, this approach has obvious advantages such as employment of low‐cost organic templates with very high effectiveness, high yield of zeolite products, short crystallization time, and relatively simple procedures. This methodology might open a pathway to synthesize ITQ zeolites with more sustainable manner.

Synthesis and characterization of Al-magadiite and its catalytic behavior in 1,4-pentanediol dehydration

Two layered aluminosilicate samples having magadiite structure and different Al-contents were prepared by conventional hydrothermal synthesis method. The samples were characterized by XRD, N2 adsorption isotherms, temperature-programmed ammonia evolution (TPAE), and 27Al MAS NMR spectroscopy. The acidity of the samples was characterized by the FT-IR spectra of adsorbed pyridine (Py). The specific surface area (SSA) of the crystalline samples was found to depend on the Al-content of the synthesis gel. The sample of low and high Si to Al ratio (Si/Al = 19 and 35) had SSA of 61 and 260 m2/g, respectively. 27Al MAS NMR spectra of the samples indicated tetra-coordinated Al, incorporated into Q3 and/or Q4 framework sites of the aluminosilicate layer. Aluminum in Q4 sites generates bridged hydroxyls (Si–OH–Al). These hydroxyls are strong Bronsted acid sites. Bases, like Py and ammonia, are bound to these sites strongly in protonated form. There are three-coordinated Al atoms in the Q3 sites. In dehydrated samples, these sites have Lewis-acidity. These acid sites, however, are only partially accessible to Py. Acid sites were also characterized in the reaction of 1,4-pentanediol (1,4-PD) dehydration. The acidity and catalytic behavior of the magadiite samples were compared to the corresponding properties of zeolite H-ZSM-5 (Si/Al = 20) and commercial γ-Al2O3.

Rapid synthesis of zinc oxide nanoforest: use of microwave and forced seeding

A rapid synthesis technique has been developed for zinc oxide (ZnO) nanoforest using microwave assisted hydrothermal method. This microwave assisted process takes less than 4 h whereas conventional hydrothermal synthesis takes 25 h to fabricate nanoforest of comparable dimensions. The simple dipping technique for secondary seeding on the primary nanorods has been studied and it has been shown that such seeding would take at least an hour. A ‘forced seeding’ technique has been developed which reduces the seeding time drastically. The microwave assisted branch growth occurs at a speed of 10 nm min−1 which is 10 fold faster than that of hydrothermal method.

Synthesis of distinctly thiol-capped CdTe quantum dots under microwave heating: multivariate optimization and characterization

The present work reports a straightforward one-step synthetic route assisted by microwave irradiation for the aqueous preparation of highly luminescence CdTe quantum dots capped with different ligands, namely, 3-mercaptopropionic acid (MPA), l-glutathione reduced and 2-mercaptoethanesulfonate. Utilization of microwave heating instead of convective heating, prevailing in the conventional hydrothermal synthesis, allows for rapid, uniform and controlled nucleation and growth of nanoparticles. Moreover, the use of air-stable Na2TeO3 as Te precursor enables a more user-friendly synthetic process averting the need for handling reagents in vacuum or under inert atmosphere. The experimental synthesis conditions for each capping were optimized by using a response surface methodology. The variables used for the optimization were the molar ratios of reagents and pH, and the optimization output was the highest photoluminescence quantum yield (QY). The optimal settings were determined by using a central composite design comprising a complete 23-factorial design as cubic points, with six axial points at a distance of α = 1.682 from the design center and four center points. Additionally, the influence of the microwave irradiation time (2–90 min) and the reaction temperature (85–150 °C) on the nanoparticles growth kinetics were thoroughly investigated. Under the optimized synthetic conditions highly luminescent (QY values up to approximately 60%), crystalline and monodisperse CdTe quantum dots with suitable surface functionalities were successfully obtained. The surface passivation with the respective capping ligand was characterized by FT-IR while the size and the high crystallinity of the as-prepared nanocrystals were confirmed by HR-TEM.

Creating hierarchies promptly: Microwave-accelerated synthesis of ZSM-5 zeolites on macrocellular silicon carbide (SiC) foams

Hierarchical ZSM-5 coatings supported on macrocellular β-SiC foams were prepared by a microwave-accelerated secondary growth method. Under microwave irradiation, differential heating was enabled due to the high microwave-adsorbing ability of SiC leading to the fast assembly of ZSM-5 zeolites on the SiC foams. The effects of crystallisation temperature, time and agitation on the growth of ZSM-5 zeolites on SiC foams were also investigated under microwave heating. A specific surface area of ca. 52m2g−1 was obtained under the optimum condition of 150°C and 4h. Comparative studies of conventional hydrothermal and microwave-accelerated synthesis were conducted and the resulting materials were characterised extensively using XRD, SEM and N2 adsorption. The combination of microwave heating, high microwave absorption of SiC substrates and agitation allowed a much more focused heating across the surface of SiC foams than conventional heating, which suppressed the formation of ZSM-5 crystals in the liquid phase and the consequent sedimentation. Additionally, for the first time, mesoscopic features of ZSM-5 coatings on SiC foam composites (>10μm) were studied in 3D by X-ray micro computed tomography (μ-CT), providing quantitative information on the quality of the ZSM-5 coating such as the global thickness and distribution of ZSM-5 across the SiC foam supports. It was concluded that microwave-accelerated secondary growth synthesis offered an efficient route to give high quality coatings on structured foams with the good crystal quality and uniform thickness as well as lower energy consumption, especially for the substrate with a high loss tangent.

Elucidating the effects of high temperature mixing method under hydrothermal condition (HTMM) on grain refinements and assembling structures

Crystal size and microstructure are of great importance in determining the physical and chemical properties of functional materials, and refine powders, especially their assembled porous structures have potential application in ceramic fabrication, absorption, catalysts and drug delivery, due to their characters of high activity and large specific surface area. Herein, high temperature mixing method under hydrothermal condition (HTMM) was adapted to synthesize barium strontium titanate [(Ba, Sr)TiO3, BST] powders with various Ba/Sr ratios (x=0.5, 1.0, 3.0 and 4.0). In comparison with conventional hydrothermal synthesis (CHS), the effects of HTMM on grain refinement and porous structure formation were exclusively investigated. XRD and SEM results indicated that, in the given condition, BST powders prepared by HTMM were much smaller than that by CHS, especially at a lower Ba/Sr ratio. Additionally, assembled porous architectures were constructed by HTMM. It's believed that the high temperature mixing process and continuous rotation contributed to the grain refinement and assembled porous structure, respectively. The assumption was further confirmed through the synthesis of sodium niobate (NaNbO3) powders by HTMM. It's demonstrated that HTMM is advantageous in preparation of refine powders and porous assembled architectures.

A new polymorph of NH4V3O7: Synthesis, structure, magnetic and electrochemical properties

NH4V3O7 micro-sized crystals have been successfully synthesized via a conventional hydrothermal synthesis route. The products were characterized by means of X-ray and neutron powder diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, static magnetization measurements, and electrochemical cycling. The diffraction patterns of NH4V3O7 can be indexed in the monoclinic space group P21 with the cell parameters a = 12.247(5) Å, b = 3.4233(10) Å, c = 13.899(4) Å, β = 87.72(3)°, and V = 582.3(4) Å3. The crystal structure is built up of (V3O7)-layers with V4+-and V5+-ions, which occupy oxygen octahedra and tetrahedra, respectively. The (V3O7)-layers are bonded by (NH4)+ -ions. Analysis of the magnetization data confirms that 2/3 of the V-ions are 4 + associated with S = 1/2. Roughly half of them are strongly coupled to antiferromagnetic dimers (J = 720 K), the other half is only weakly (J of several 10 K) antiferromagnetically interacting. Electrochemical cycling shows reversible lithium de-/intercalation into the layered NH4V3O7 host structure with an initial specific discharge capacity of 310 mAh/g at 7 mA/g.

Microwave-assisted and conventional hydrothermal synthesis of potassium merlinoite from K-feldspar

Potassium merlinoite was successfully synthesized from K-feldspar powder by microwave-assisted as well as conventional hydrothermal methods. Effects of aging, H2O/Al2O3 molar ratio, crystallization temperature, crystallization time, and mixed heating strategy on potassium merlinoite formation were systemically studied. It was observed that aging was not a necessary factor to the formation of merlinoite, and merlinoite was the unique crystalline product at a wide range of H2O/Al2O3 molar ratio, crystallization temperature and time. The reaction rate of microwave-assisted hydrothermal synthesis was 2–3 times as much as conventional method. The acceleration effect of microwave was proved to work only at the initial period of irradiation, and a mixed heating method was considered as an appropriate strategy to prepare merlinoite. A simple and economical chemical process has been proposed to convert the natural K-feldspar powder into merlinoite-type eco-fertilizer in this paper.

Conventional versus alkali fusion synthesis of zeolite A from low grade kaolin

Synthesis of zeolite A using low grade Bombowha kaolin from Ethiopia has been carried out by two methods: the conventional hydrothermal synthesis and alkali fusion followed by hydrothermal treatment. In the synthesis process, the metakaolinization was achieved by calcination of the kaolin at 600°C for 3h for the conventional method and for the alkali fusion method, the activation of the raw kaolin was done by alkali fusion for only 1h. Zeolite A was the main product obtained using 3M NaOH concentration, 100°C reaction temperature and 3h crystallization time in both methods. The molar gel composition for the best synthesis conditions for both methods was Na2O: Al2O3: 2SiO2: 37H2O. The main advantage of the alkali fusion method is that it allows using a low grade virgin kaolin without purification, whereas the conventional method only yields good quality zeolite A when the raw kaolin is purified. The starting raw kaolin, metakaolin and synthetic products were characterized by X-ray diffraction (XRD), ICP-OES elemental analyses, Scanning Electron Microscopy (SEM), Thermogravimetric Analysis (TGA) and Cation Exchange Capacity (CEC). Cubic crystals with rounded edges of zeolite A having crystallinity of about 85% and Cation Exchange Capacity of 300mg CaCO3/g of anhydrous zeolite was achieved by the alkali fusion method of synthesis using raw, virgin Bombowha kaolin.