Supercritical Synthesis Research Articles

Reactivity of Formaldehyde during 4-Hydroxy-2-butanone Synthesis in Supercritical State.

4-Hydroxy-2-butanone, an important intermediate for vitamin A and fragrances, is usually produced by aldol condensation of acetone and formaldehyde. Noncatalytic synthesis of 4-hydroxy-2-butanone in supercritical state, which was fast and had high production yield, was widely applied. Previous research on 4-hydroxy-2-butanone synthesis in the supercritical state focused on the formation and dehydration of 4-hydroxy-2-butanone while ignoring side reactions involving formaldehyde, which were studied in this paper. A reaction pathway of 4-hydroxy-2-butanone supercritical synthesis containing formaldehyde side reactions was proposed. The cross-disproportionation of formaldehyde and formic acid was found to be the main consumption of formic acid. The effects of initial formaldehyde and formic acid mass fractions in the feed on side reactions were studied. Based on the experiments conducted from 523.15 K to 563.15 K and 17 MPa, a kinetic model was suggested. The relative deviations between experimental and simulated data were less than 10%.

Carbon Formation during Methane Dry Reforming over Ni-Containing Ceria-Zirconia Catalysts.

Two series of Ni/Ce(Ti/Nb)ZrO2 catalysts were prepared using citrate route and original solvothermal continuous flow synthesis in supercritical isopropanol and studied in dry reforming of methane (DRM). TEM, XPS and FTIRS of adsorbed CO confirm influence of support composition and preparation method on the catalysts’ morphology and surface features. The oxygen mobility was studied by isotope heteroexchange with C18O2. After testing in DRM, carbon deposits after catalysts’ testing in DRM were investigated by temperature-programmed oxidation with thermo-gravimetric analysis. The lowest amounts of carbon deposits were obtained for unmodified Ni-CeZr and Ni-CeNbZr compositions. Ti addition lead to an increased amount of carbon, which was removed at higher temperatures. The use of supercritical supports also resulted in the formation of a higher amount of coke. Catalysts prepared by the supercritical synthesis were tested in DRM for 25 h. The highest activity drop was observed in the first three hours. For all compositions, close values of carbon deposits were revealed.

Photocatalytic Reduction of CO2 with N-Doped TiO2-Based Photocatalysts Obtained in One-Pot Supercritical Synthesis.

The objective of this work was to analyze the effect of carbon support on the activity and selectivity of N-doped TiO2 nanoparticles. Thus, N-doped TiO2 and two types of composites, N-doped TiO2/CNT and N-doped TiO2/rGO, were prepared by a new environmentally friendly one-pot method. CNT and rGO were used as supports, triethylamine and urea as N doping agents, and titanium (IV) tetraisopropoxide and ethanol as Ti precursor and hydrolysis agent, respectively. The as-prepared photocatalysts exhibited enhanced photocatalytic performance compared to TiO2 P25 commercial catalyst during the photoreduction of CO2 with water vapor. It was imputed to the synergistic effect of N doping (reduction of semiconductor band gap energy) and carbon support (enlarging e−-h+ recombination time). The activity and selectivity of catalysts varied depending on the investigated material. Thus, whereas N-doped TiO2 nanoparticles led to a gaseous mixture, where CH4 formed the majority compared to CO, N-doped TiO2/CNT and N-doped TiO2/rGO composites almost exclusively generated CO. Regarding the activity of the catalysts, the highest production rates of CO (8 µmol/gTiO2/h) and CH4 (4 µmol/gTiO2/h) were achieved with composite N1/TiO2/rGO and N1/TiO2 nanoparticles, respectively, where superscript represents the ratio mg N/g TiO2. These rates are four times and almost forty times higher than the CO and CH4 production rates observed with commercial TiO2 P25.

Supercritical synthesis and studies on non-isothermal kinetics and thermodynamics of solid state decomposition of nanoferrogels

Poly-N-isopropyl acrylamide (PNIPAM) and related nanoferrogels (NFG) were synthesized in supercritical carbon dioxide and investigated for their non-isothermal kinetic and thermodynamics of solidstate decomposition in oxidative media. Ferrite nanoparticles (FNPs, 10.5 nm) were prepared for NFG through chemical method. Thermal stability, along with kinetic and thermodynamic evaluationsof NFG has been shownthrough simultaneous thermogravimetric (TG) - differential thermal analysis (DTA) - differential thermogravimetry (DTG) in air.Thermal, non-isothermal kinetic and thermodynamic data of NFG was compared with FNP and PNIPAM. NFG have shown two step decompositions with reduction in their TG onset by 10 °C over FNPs and PNIPAM. This was associated with simultaneous increase in heat of fusion (mJ/mg) of NFG (2.88) over PNIPAM (2.55). Horowitz- Metzger (HM) and Coats-Redfern (CR) at reaction orders (n) ranging 0 to 3 reveals consistency in order of solid-state decomposition for FNPs (n = 1), PNIPAM and NFG (n = 3) with negative values of changes in entropy and enthalpy. HM methods reveals greater values of energy of activation (Ea) with respect to CR methods. Applied methods provide large differences in the values of Ea of solidstate degradations with marginal changes in thermodynamic parameters of NFG over PNIPAM. Kinetic and thermodynamic calculations reveal enhanced thermal stability of NFG over PNIPAM followed by FNPs. High thermal stability of NFG makes them possible for agricultural and biomedical applications.

Supercritical Fluid Synthesis of Highly Porous Polylactide Matrices: Fundamental Features and Technology of Formation, Development and Stabilization of Polymer Foams

Supercritical Fluid Synthesis of Highly Porous Polylactide Matrices: Fundamental Features and Technology of Formation, Development and Stabilization of Polymer Foams

A Review of Biodiesel Production from Renewable Resources: Chemical Reactions

Biodiesel has become an attractive renewable fuel due to carbon footprint reduction. Greenhouse gas footprint is minimised when the locally available renewable feedstock is utilised for cleaner biodiesel production. This biodiesel has similar characteristics of diesel oil which allows its use in compression motors without extensive engine modifications. One of the greenest choices to address the ever growing demand for cleaner energy is renewable biomass power. Contemporary development and work have been aimed at transforming these bio-based resources into practical gas and liquid fuels that comply with the current petro-based chemical production infrastructure. Biodiesel is a prospective alternative to petroleum fuels due to its clean, renewable and ecologically beneficial characteristics. The transesterification cycle is applied to the processing of cleaner biodiesel manufacturing. Comprehensive research is underway to develop more effective biodiesel processing processes. The different new technologies are both solid catalysts and supercritical processes that are not catalytic. A review of the various methods used for the successful synthesis of biodiesel includes the homogeneous, heterogeneous and enzyme catalyst, microwave and ultrasonic synthesis followed by the supercritical synthesis of biodiesel. The chemical reactions of biodiesel, i.e. transesterification, esterification and interesterification have been presented by this paper and the merits and limitations of these chemical reactions for biodiesel discussed.

Synthesis and Characterization of Nano Cerium Oxide Using Hydrothermal Technique

Cerium oxide rice grain like nanoparticles have been synthesized using wet chemical hydrothermal synthesis method. Hydrothermal synthesis method provides nanoparticles with unique dimension and shape due to supercritical synthesis conditions than conventional precipitation techniques. The hydrothermal synthesis of Cerium salt in the presence of urea produces Cerium carbonate hydroxide nanoparticles with rice-grain morphology, which are reduced into Cerium oxide by heat-treating at 600°C, for 4 hours, without changing its morphology. The formation of Cerium carbonate hydroxide with orthorhombic crystal structure and Cerium oxide with cubic crystal structure were confirmed by X-Ray Diffraction [XRD] spectroscopy studies. Thermo-gravimetric analysis [TGA] in inert nitrogen atmosphere and ambient air provides the reduction temperature of hydroxide and carbonate is about 600°C. Scanning electron microscope [SEM] studies show the uniform rice grain like morphology of as-prepared Cerium carbonate hydroxide and Cerium oxide, due to the reduction at elevated temperature and crystalline structural deformation Cerium oxide shows cracks and porous on its surfaces.

Supercritical fluid synthesis and possible properties of "cubic graphite"

Supercritical fluid synthesis and possible properties of "cubic graphite"

Facile one-pot supercritical synthesis of MoS2/pristine graphene nanohybrid as a highly active advanced electrocatalyst for hydrogen evolution reaction

Direct decoration of transition metallic dichalcogenides such as molybdenum disulfide (MoS2) on pristine graphene is extremely difficult owing to its surface inertness. Under assistance of supercritical carbon dioxide, we develop a unique synthesis of a high-quality hybrid nanostructure with ultrafine MoS2 nanosheets supported on pristine graphene that exhibits excellent properties as an advanced catalyst for electrocatalytic hydrogen evolution reaction (HER). These include a Tafel slope of ~42 mV·decade−1, onset potential as low as ~58 mV and small potential of 81 mV for 10 mA·cm−2 current density, much superior than those obtained from other carbon-based counterparts such as carbon black, carbon nanotubes, and reduced graphene oxide. Both these potentials are the best available values ever achieved so far among all graphene or carbon nanotube-supported molybdenum sulfide. The exceptional HER activity is found to be closely associated with high density of exposed edge sites of ultrafine MoS2 nanosheets being firmly and uniformly anchored on inherently conductive graphene sheets as a result of high wettability between the supercritical fluid and inert carbon surfaces. The facile yet effective supercritical method can also be applicable to a wide range of electrochemically active nanospecies in the development of highly efficient graphene-based nano-catalysts for broad energy applications.

Nickel-Containing Ceria-Zirconia Doped with Ti and Nb. Effect of Support Composition and Preparation Method on Catalytic Activity in Methane Dry Reforming.

Nickel-containing mixed ceria-zirconia oxides also doped by Nb and Ti have been prepared by a citrate route and by original solvothermal continuous flow synthesis in supercritical alcohols. Nickel was subsequently deposited by conventional insipient wetness impregnation. The oxides are comprised of ceria-zirconia solid solution with cubic fluorite phase. Negligible amounts of impurities of zirconia are observed for samples prepared by citrate route and doped by Ti. Supports prepared by supercritical synthesis are single-phased. XRD data, Raman, and UV-Vis DR (diffuse reflectance) spectroscopy suggest increasing lattice parameter and amount of oxygen vacancies in fluorite structure after Nb and Ti incorporation despite of the preparation method. These structural changes correlate with the catalytic activity in a methane dry reforming reaction. Catalysts synthesized under supercritical conditions are more active than the catalysts of the same composition prepared by the citrate route. The catalytic activity of samples doped with Ti and Nb is two times higher in terms of TOF (turnover frequency) and increased stability of these catalysts is attributed with the highest oxygen mobility being crucial for gasification of coke precursors.

Why Does Bi2WO6 Visible-Light Photocatalyst Always Form as Nanoplatelets?

Bi2WO6 nanocrystals exhibit excellent photocatalytic properties in the visible range of the solar spectrum, and intense efforts are directed at designing effective synthesis processes with control of size, morphology, and hierarchical structure. All known hydrothermal syntheses produce either nanoplatelet morphology or hierarchical structures based on such primary entities. Here we investigate the nucleation and growth of Bi2WO6 nanocrystals under hydrothermal conditions using in situ X-ray total scattering (TS) and powder X-ray diffraction (PXRD) measurements. It is shown that the preferential growth of Bi2WO6 nanoplates is due to the presence of disordered layers of Bi2O22+ molecular complexes in the precursor solution with an approximate length of 13 Å. These layers interact with tetrahedral WO42- molecular units and eventually form the disordered cubic (Bi0.933W0.067)O1.6) crystalline phase. When enough tungsten units are intertwined between Bi2O22+ layers formation of Bi2WO6 pristine nanoplates takes place by necessary sideways addition of units in the ac plane. The experimentally observed formation mechanism suggests that the Bi/W atomic ratio must play a central role in the nucleation (assembly of initial crystal layers). Indeed, it is observed in separate continuous flow supercritical synthesis that for a stoichiometric (Bi/W = 2:1) precursor, a (Bi0.933W0.067)O1.6) impurity phase is always observed together with the main Bi2WO6 product. Excess tungsten is required in the precursor to form phase-pure Bi2WO6 material. Thus, the present study also reports a fast, scalable, and green method for production of this highly attractive photocatalyst.

Formation mechanism of epitaxial palladium–platinum core–shell nanocatalysts in a one-step supercritical synthesis

Formation mechanism of epitaxial palladium–platinum core–shell nanocatalysts in a one-step supercritical synthesis

Formation Mechanism of Epitaxial Palladium–Platinum Core–Shell Nanocatalysts in a One‐Step Supercritical Synthesis

AbstractThe scarcity of platinum group metals provides a strong incentive to optimize the catalytic activity and stability, e.g., through nanoalloys or core–shell nanoparticles. Here, time‐resolved X‐ray total scattering and transmission electron microscopy characterization are used to study the formation of palladium–platinum core–shell nanoparticles under solvothermal conditions. It is shown that Pd rapidly forms small (5–10 nm), disordered primary particles, which agglomerate and crystallize when reaching 20–25 nm. The primary Pd particles provide nucleation sites for Pt, and, with extended reaction time, the Pd cores become fully covered with Pt shells. The observed core–shell material is surprising when considering the Pt–Pd phase diagram and relative surface energies, but it can be rationalized through the kinetics of precursor conversion. To bridge the gap between scientific studies and industrial demand for large‐scale production, the synthesis process is successfully transferred to a continuous flow supercritical reactor providing a simple scalable and green process for production of bimetallic nanocatalysts.

Supercritical flow synthesis of PtPdFe alloyed nanoparticles with enhanced low-temperature activity and thermal stability for propene oxidation under lean exhaust gas conditions

Supercritical flow technology was used for the one step production of PtPd and PtPdFe nanoparticles supported on high surface area γ-Al2O3.

Flow supercritical synthesis of brucite and magnesian T-O, T-O-T phyllosilicates: an opportunity to tune the structure with the solvent composition

ABSTRACTThis work presents the synthesis of minerals with a layered structure in supercritical water/ethanol mixtures to decrease the critical coordinates of the solvent regarding water. Depending on the water/ethanol ratio of the solvent, we obtained three different minerals adopting an octahedral brucitic sheet: (1) without a Si-tetrahedral sheet (O); (2) associated with one Si-tetrahedral sheet (T-O); or (3) intercalated between two Si-tetrahedral sheets (T-O-T). We have thus shown that ethanol in a supercritical water/ethanol mixture changes the solubility of silicon with a direct consequence on the formation of the tetrahedral silicon sheets and thus makes it possible to control the structure of the synthesized layered material.

Moderate supercritical synthesis as a facile route to mixed-valent uranium(iv,v) and (v,vi) silicates.

Mixed-valent uranium(iv,v) and (v,vi) phases represent a unique subset of known uranium compounds. Efforts to develop our current understanding of these materials have pointed to hydrothermal methods as effective preparative techniques. Herein we report the successful use of moderate supercritical conditions for the synthesis of five new U(v) containing phases.

Supercritical synthesis of platinum-modified titanium dioxide for solar fuel production from carbon dioxide

This paper investigates the properties of TiO2-based photocatalysts synthesised under supercritical conditions. Specifically, the characteristics of Pt dispersed on TiO2 catalysts obtained in supercritical CO2 are discussed and compared with those of commercial TiO2. The photocatalytic activity of the synthesised catalysts in the CO2 photoreduction reaction to produce solar fuel is tested. The main conclusion of the study is that photocatalysts with better or similar features, including high surface area, crystallisation degree, hydroxyl surface concentration, pore volume, absorbance in the visible range and methane production rate, to those of commercial TiO2 may be produced for the reduction of CO2 to fuel by synthesis in supercritical media.

Electrochemical CO2 Reduction to Fuels Using Pt/CNT Catalysts Synthesized in Supercritical Medium

The electrochemical reduction of CO2 in the gas phase has been carried out in a solid polymer electrolyte type cell (25 cm2 geometric area) in continuous operation mode using carbon nanotube-supported platinum catalysts (Pt/CNT). The main novelty of this work relies on the use of supercritical media (supercritical CO2) for Pt deposition on CNT. Supercritical synthesis has allowed obtaining small Pt nanoparticles divided into two modal distributions (for 3–4 nm and 8–9 nm, respectively) with a high deposition efficiency (about 80%). The main reaction products of the electrocatalytic conversion of CO2 have been formic acid (59–89%), methane (2–33%), CO (3–11%), methanol (0–1.9%), and small amounts of acetone, isopropanol, and methyl acetate. The CO2 conversion rate multiplies almost by four when increasing current density, although selectivity barely changes. Lower temperature promotes further reduction of CO2 to methane (33% of selectivity) to the detriment of formic acid and CO. However, increases of temp...

Fatty acid methyl esters synthesis from non-edible vegetable oils using supercritical methanol and methyl tert-butyl ether

Fatty acid methyl esters (FAMEs) are useful as biodiesel and have environmental benefits compared to conventional diesel. In this study, these esters were synthesized non-catalytically from non-edible vegetable oils: neem oil and mahua oil with two different methylating agents: methanol and methyl tert-butyl ether (MTBE). The effects of temperature, pressure, time and molar ratio on the conversion of triglycerides were studied. The temperature was varied in the range of 523–723K with molar ratios upto 50:1 and a reaction time of upto 150min. Conversion of neem and mahua oil to FAMEs with supercritical methanol was found to be 83% in 15min and 99% in 10min, respectively at 698K. Further, a conversion of 46% of mahua oil and 59% of neem oil was obtained in 15min at 723K using supercritical MTBE. The rate constants evaluated using pseudo first order reaction kinetics were in the range of 4.7×10−6 to 1.0×10−3s−1 for the investigated range of temperatures. The activation energies obtained were in the range of 62–113kJ/mol for the reaction systems investigated. The supercritical synthesis was found to be superior to the catalytic synthesis of the corresponding FAMEs.

Continuous supercritical synthesis of unsupported and high specific surface area catalyst precursors for deep-hydrodesulfurization

Unsupported and high specific surface area (SBET) catalyst precursors for deep-hydrodesulfurization (deep-HDS) are obtained using an environmental friendly, continuous and fast synthesis process in supercritical water/alcohol mixtures. This approach offers an access to high production rate and upscaling. Two mixed oxides were synthesized: NiMoO4, which is the preferred material when deep-HDS is investigated, and CoMoO4 preferred for simple HDS. The role of the water/alcohol mixture on the resulting specific surface area has been studied and allows a controlled adjustment of the resulting specific surface area. The obtained NiMoO4 material consists of the highly active hydrate NiMoO4.0.75H2O phase with a controlled composition and specific surface area up to 200m2g−1. Resulting materials have shown great performances when tested in deep-HDS catalytic tests.