Continuous hydrothermal synthesis of ZnGa2O4:Mn2+ nanoparticles at temperatures of 300–500 °C and pressures of 25–35 MPa

Mn²⁺ doped ZnGa₂O₄ particles were continuously synthesized in subcritical and supercritical water using a flow hydrothermal reaction system. Zn, Ga and Mn nitrates were used as the starting materials. The syntheses were carried out at temperatures from 300 to 500 °C, at pressures from 25 to 35 MPa, at KOH concentration of 0.04 M, and for residence times from 0.13 to 1.13 s. X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric differential thermal analysis (TG-DTA) and photoluminescent spectroscopy were used to characterize the samples. Single phase of ZnGa₂O₄ was achieved at residence times of less than 1 s. The crystallite sizes were in the range from 12 to 21 nm which tend to increase with an increase in the reaction temperature and with a decrease in the reaction pressure. Green emission was observed for the annealed samples under reducing conditions whereas as-synthesized samples did not exhibit photoluminescence.     

Techniques,applications and future prospects of diamond anvil cells for studying supercritical water systems

In this review, diamond anvil type cells (DACs) are reviewed as a method for studying supercritical water systems. The hydrothermal DAC provides easy and safe experimental access to high pressure (30–3000 MPa) and high temperature (400–800 °C) regions and the device allows exploration of supercritical systems at high density (400–1200 kg/m³), which is usually difficult or costly with batch or flow systems. In the first part of this review, characteristics of DACs regarding anvil type, DAC type, anvil alignment, heating, analytical methods, pressure and temperature determination, gasket, loading, physical size are discussed with emphasis on DACs that can be used to generate conditions of interest for understanding supercritical water systems. In the second part of this review, applications and key findings of studies on supercritical water systems from geology, chemical, biomass, energy, environmental, polymer, and materials related fields are discussed. Some of the key findings determined with DAC are related to the dissolution or existence of phases at conditions of high temperature and high pressure, however, DAC has been used in many quantitative studies to determine fundamental properties such as speeds of sound, phase behavior, solubilities, partition coefficients and viscosities. Future prospects for DAC as a method for exploring supercritical water systems include combination of DAC with transmission electron microscopy (TEM) for studying nanostructures, use of high-speed streak cameras to study high-speed reactions, combustions, and energetic materials, use of time-dependent loads to study kinetics, precipitation and crystallization phenomena, the use of DAC with synchrotron radiation to follow reaction and material processes in situ, and the many modifications that can be made to DAC anvils and rapid heating methods such as lasers and masers used in conjunction with in situ techniques. The DAC is a highly versatile instrument and should find widespread use in studying supercritical water systems.     

Combined UHV and ambient pressure studies of 1,3-butadiene adsorption and reaction on Pd(1 1 1) by GC,IRAS and XPS

The hydrogenation of 1,3-butadiene on Pd(1 1 1) at 300 K was studied at atmospheric pressure by infrared reflection absorption spectroscopy (IRAS) and gas chromatography (GC). Kinetic measurements showed 1-butene, trans-2-butene and cis-2-butene as primary products. Once 1,3-butadiene had been completely consumed, 1-butene was re-adsorbed on the surface producing trans-/cis-2-butene through isomerization and n-butane through hydrogenation. These results were corroborated by in situ IRAS spectroscopy. Post-reaction analysis by X-ray photoelectron spectroscopy (XPS) in the C1s region revealed a band at 284.2 eV, corresponding to adsorbed butadiene and/or carbonaceous deposits. Quantification of this peak revealed a total carbon coverage of 0.3 ML. Nevertheless, deactivation due to carbon deposition was a minor effect under our reaction conditions, as indicated by the kinetics of the subsequent butene hydrogenation reaction. Temperature-dependent XPS experiments after butadiene adsorption at 100 K indicated a high stability of the diene molecule with hardly any desorption and/or decomposition up to 500 K. Above this temperature, butadiene decomposed to carbon species that eventually dissolved in the Pd bulk above 700 K.     

Simple three parameter equations for correlating liquid phase compositions in subcritical and supercritical systems

Two Chrastil type expressions have been developed to model the solubility of supercritical fluids/gases in liquids. The three parameter expressions proposed correlates the solubility as a function of temperature, pressure and density. The equation can also be used to check the self-consistency of the experimental data of liquid phase compositions for supercritical fluid–liquid equilibria. Fifty three different binary systems (carbon-dioxide + liquid) with around 2700 data points encompassing a wide range of compounds like esters, alcohols, carboxylic acids and ionic liquids were successfully modeled for a wide range of temperatures and pressures. Besides the test for self-consistency, based on the data at one temperature, the model can be used to predict the solubility of supercritical fluids in liquids at different temperatures.     

Solubility of cyproheptadine in supercritical carbon dioxide; experimental and modeling approaches

Solubility of solute in supercritical fluids at different pressures and temperatures is one of the most important parameters necessary for design of any supercritical fluid-based processes. Among different supercritical fluids, carbon dioxide is one of the most widely used solvents due to its useful and green characteristics. In this work, with the assist of supercritical carbon dioxide as the solvent, solubility of cyproheptadine in different temperatures (308–338 K) and pressures (160–400 bar) are measured using static method. The obtained results demonstrated that solubility of cyproheptadine ranged between 3.35 × 10⁻⁵ and 3.09 × 10⁻³ based on mole fraction. A closer examination of measured solubility data show that not only solubility of cyproheptadine increases by increasing pressure but also experiences a cross over pressure about 200 bar. At last, the measured solubility data are correlated using four widely used density based correlations namely Mendez Santiago–Teja (MST), Kumar and Johnston (KJ), Bartle et al., and Chrastil models. The obtained results demonstrated that the best correlative capability was observed for KJ model leads to the average absolute relative deviation percent (AARD %) of 6.3%.     

Tricalcium silicate (C3S) hydration under high pressure at ambient and high temperature (200 °C)

The hydration of a tricalcium silicate paste at ambient temperature and at 200 °C under high pressure (up to 1000 bar) has been studied. Two high pressure cells have been used, one allows in-situ electrical conductivity measurements during hydration under high pressure. The hydration products were characterized by thermal analysis, X-ray diffraction and ²⁹Si NMR measurements. The pressure has a large kinetic effect on the hydration of a C₃S paste at room temperature. The pressure was seen to affect drastically the hydration of a C₃S paste at 200 °C and this study evidences the competition between the different high temperature phases during the hydration.     

A PLS regression model using NIR spectroscopy for on-line monitoring of the biodiesel production reaction

In this work, a Partial Least Squares (PLS) regression model using Near-Infrared (NIR) spectroscopy was developed to monitor the progress of the catalyzed transesterification reactions of soybean oil that produce biodiesel. The NIR spectra were collected during the transesterification reaction with a lab made spectrophotometric flow cell. Proton Nuclear Magnetic Resonance (¹H NMR) spectroscopy was employed for determining the conversion percentage of glycerides to methyl esters during the transesterification reaction and used as reference to build the PLS calibration model employing NIR spectroscopy data. The model, constructed with selected spectral range has not been tried before and allows the monitoring of the transesterification reaction in terms of conversion ratio for different temperatures. The model was validated and the values of Root Mean Square Error of Prediction (RMSEP) found for two different temperatures were 0.74% and 1.27% (of conversion) for reactions carried out at 20 ± 0.2 °C and 55 ± 0.2 °C, respectively.     

One-pot synthesis of fluorescent mesoporous materials for detection of the presence of Be<Superscript>2+</Superscript> ion

A fluorescent mesoporous silica solid (F-MCM-41) has been successfully prepared by one-pot hydrothermal synthesis from TEOS and 2-((3-(triethoxysily)propylimino)methyl)phenol in the presence of CTMABr surfactant, followed by removal of CTMABr in F-MCM-41 from extraction of ethanol solution at 50 °C for 5 h. The fluorescent sample was characterized with X-ray diffraction, nitrogen isotherms, nuclear magnetic resonance (NMR), and fluorescence spectroscopy, and the obtained results show that the sample has ordered hexagonal mesoporous symmetry, uniform pore size, high BET surface area, large pore volume as well as good fluorescent properties. When this sample is used to detect Be²⁺ ions in water, the degree of fluorescence quenching is dependent on the concentration of Be²⁺ solutions ranged from 1 to 20 μg/L.     

The relationship between the silica wet-gels relative optical transmittance at 500 nm and structural features of silica aerogels

Aerogels are usually synthesized from supercritical drying of wet-gels. High quality aerogels are usually characterized with high specific surface area (measured by BET method here) and high degree of polymerization of silicate (characterized by ²⁹Si NMR here). The quality of the aerogels depends highly on the quality of the corresponding wet-gels because the skeletal structure in the wet-gels changes only slightly in the supercritical drying process. Here we present a simple method to predict the quality of the desired silica aerogels before the supercritical drying. We found that the relative optical transmittance of the silica wet-gels at 500 nm had good linear correlation with the specific surface area and the degree of silicate polymerization in the corresponding aerogels. We can assess the quality and screen out the low-quality wet-gels from supercritical drying operation, and greatly reduce the cost for aerogel production.     

Synthesis of biodiesel from edible and non-edible oils in supercritical alcohols and enzymatic synthesis in supercritical carbon dioxide

Biodiesel is an attractive alternative fuel because it is environmentally friendly and can be synthesized from edible and non-edible oils. The synthesis of biodiesel from edible oils like palm oil and groundnut oil and from crude non-edible oils like Pongamia pinnata and Jatropha curcas was investigated in supercritical methanol and ethanol without using any catalyst from 200 to 400 °C at 200 bar. The variables affecting the conversion during transesterification, such as molar ratio of alcohol to oil, temperature and time were investigated in supercritical methanol and ethanol. Biodiesel was also synthesized enzymatically with Novozym-435 lipase in presence of supercritical carbon dioxide. The effect of reaction variables such as temperature, molar ratio, enzyme loading and kinetics of the reaction was investigated for enzymatic synthesis in supercritical carbon dioxide. Very high conversions (>80%) were obtained within 10 min and nearly complete conversions were obtained at within 40 min for the synthesis of biodiesel in supercritical alcohols. However, conversions of only 60–70% were obtained in the enzymatic synthesis even after 8 h.     

Gasification of charcoal using supercritical CO2 at high pressures

A novel one-shell high temperature and high pressure semi-continuous reactor has been developed for the study of the Boudouard reaction at temperatures up to 820 °C and pressures up to 32.5 MPa. Semicontinuous gasification of charcoal using supercritical CO₂ has been achieved at conversions up to 90.8% (w/w) at LSHV between 20 and 30 h⁻¹ after 5–9 h. A gasification model is proposed and validated. Effective rates of gasification (1.32 ± 0.12) × 10⁻⁶ to (6.10 ± 2.03) × 10⁻⁵ s⁻¹ were obtained. The results indicated that this method is technically feasible for the on-line production of high pressures streams of CO/CO₂ in the lab for carrying out further chemistries, avoiding the use of CO high pressure bottles.     

Supercritical fluids as reaction media in the ethylbenzene disproportionation on ZSM-5

Ethylbenzene disproportionation over HZSM-5, was studied in a wide range of temperatures and pressures including supercritical conditions, aiming to determine the effect of variations in reaction media properties on reaction parameters, focusing on the potential advantages of supercritical conditions, previously discussed in literature.The effect of pressure on conversion has been explained in terms of changes in the reaction media produced by pressure variations, whereas at high pressure conditions temperature has effect not only on the kinetics but also on media density.Pressure dos not have effect on the selectivity to the main product, diethylbenzene; however temperature should be taken into account since above certain temperatures secondary reactions are favoured.Conversion close to the equilibrium one has been reached at 400 °C and 45 bar, demonstrating that supercritical fluids are a satisfactory reaction media for ethylbenzene disproportionation.     

Catalytic conversion of olefins on supported cubic platinum nanoparticles: Selectivity of (1 0 0) versus (1 1 1) surfaces

The surface chemistry and catalytic conversion of cis- and trans-2-butenes on platinum (1 0 0) facets were characterized via surface-science and catalytic experiments. Temperature-programed desorption studies on Pt(1 0 0) single crystals pointed to the higher hydrogenation probability of the trans isomer at the expense of a lower extent of CC double-bond isomerization. To test these trends under catalytic conditions, shape selective catalysts were prepared by dispersing cubic platinum colloidal nanoparticles (which expose only (1 0 0) facets) onto a high-surface-area silica xerogel support. Infrared absorption spectroscopy and transmission electron microscopy were used to determine the conditions needed to remove the organic surfactants without loosing the original narrow size distribution and cubic shape of the original metal nanoparticles. Catalytic kinetic measurements with these materials corroborated the surface-science predictions, and pointed to a switch in isomerization selectivity from preferential cis-to-trans conversion with Pt(1 0 0) surfaces to the reverse trans-to-cis reaction with Pt(1 1 1) facets.     

Composition and structure of C-S-H in white Portland cement-20% metakaolin pastes hydrated at 25 °C

The microstructure and composition of water- and alkali-activated hardened pastes of white Portland cement-20% metakaolin blends have been studied using solid-state NMR spectroscopy and analytical TEM. The results show that after hydration for 1 day nearly half the cement had reacted in the water-activated paste but very little, if any, of the metakaolin; by 28 days two-thirds of the cement had reacted and most of the metakaolin. In contrast, whilst alkali-activation again led to about half the cement reacting by 1 day, about a quarter of the metakaolin had also reacted; and whilst most of the metakaolin had again reacted by 28 days, there had been no further reaction of the cement. The high degree of reaction of the MK in both pastes at 28 days resulted in long-chain highly aluminous C-S-H, with most of the bridging sites occupied by Al³⁺ rather than Si⁴⁺. The data for the C-S-H in the water-activated paste are consistent with both the tobermorite/jennite (T/J) and tobermorite/calcium hydroxide (T/CH) models for the nanostructure of C-S-H - although very little J- or CH-like structure is needed to account for the observed compositions - whilst those for the alkali-activated paste can only be accounted for on the T/CH viewpoint.     

Carbon incorporation and deactivation of MgO(0 0 1) supported Pd nanoparticles during CO oxidation

We have investigated the structure and composition of the model catalyst system Pd/MgO(0 0 1) during oxidation, CO reduction and CO oxidation at near atmospheric pressures by a combination of in situ X-ray diffraction and ex situ transmission electron microscopy and spectroscopy techniques. From the in situ X-ray experiments, we find: (a) the Pd nanoparticles with 9 nm in diameter transform into epitaxial PdO above 10⁻¹ mbar O₂ pressure at 570 K, (b) the oxidation process can be reverted by CO exposure, recovering Pd nanoparticles in their initial orientation, and (c) during CO oxidation in a mixture of 50 mbar O₂ and 50 mbar CO a new phase is evolving with lattice constant close to the MgO substrate value, which we assign to expanded Pd nanoparticles forming upon carbon incorporation. Ex situ transmission electron microscopy and different spectroscopy techniques uncover the CO₂ induced growth of a disordered overlayer containing C, Mg and O, which forms during CO oxidation and leads to an overgrowth of Pd nanoparticles thereby deactivating the catalyst.     

绿色产业超临界技术最新研究进展-在化学反应工程中的应用

超临界流体技术具有许多传统技术所没有的快速、高效、低能耗、污染少等优点，而且超临界流体无毒，不燃，不污染环境，可以设计实现许多新型、高效的绿色化工过程。介绍了超临界流体的密度、粘度、扩散系数等物理化学性质及其对化学反应的影响。重点论述了超临界水的性质、超临界流体在化学反应工程研究中的应用和由超临界流体实现的绿色化工过程。     

High-pressure phase equilibria for the binary system carbon dioxide + dibenzofuran

The experimental solubility of dibenzofuran in near-critical and supercritical carbon dioxide and the solid–liquid–vapor (SLV) equilibrium line for the CO₂ + dibenzofuran system are reported. The built in-house static view cell apparatus used in these measurements is described. The solubility of naphthalene in supercritical CO₂ and the CO₂ + naphthalene SLV line are also determined in order to assess the reliability and accuracy of the measurement technique. The solubility of dibenzofuran in carbon dioxide is determined at 301.3, 309.0, 319.2, 328.7 and 338.2 K in the 6–30 MPa pressure range. Solubility data are correlated using the Chrastil model and the Peng–Robinson equation of state. This equation is also used to predict the CO₂ + dibenzofuran SLV line. Results show the feasibility of using supercritical CO₂ to extract dibenzofuran.     

Comparison of Supercritical Fluid and Hexane Extraction Methods in Extracting Kenaf (<Emphasis Type="Italic">Hibiscus cannabinus</Emphasis>) Seed Oil Lipids

The objective of this study was to investigate and compare fatty acids, tocopherols and sterols of kenaf seed oil extracted by supercritical carbon dioxide and traditional solvent methods. Fatty acids, tocopherols and sterols were determined in the extracted oils as functions of the pressure (400 bar, 600 bar), temperature (40 °C, 80 °C) and CO₂ flow rate (25 g/min) using a 1-L extraction vessel. Gas chromatography was used to characterize fatty acids and sterols of the obtained oils while tocopherols were quantified by HPLC. No differences were found in the fatty acid compositions of the various oil extracts and the main components were found to be linoleic (38%), oleic (35%), palmitic (20%) and stearic acid (3%). Extraction of tocopherols using high pressure (600 bar/40 °C, 600 bar/80 °C) gave higher total tocopherols (88.20 and 85.57 mg/100 g oil, respectively) when compared with hexane extraction which gave yield of 62.38 mg/100 g oil. Extraction of kenaf seed oil using supercritical fluid extraction at high temperature (80 °C) gave higher amounts of sterols when compared with hexane extraction.     

Fabrication of fine microcapsulated red phosphorus particles by SCF-RESS with a new structure nozzle

A novel process of preparing microencapsulated red phosphorus particles by rapid expansion of supercritical fluids (RESS) was tested, in which a new kind of nozzle and supercritical CO₂ as a solvent were used. The structure of the microencapsulated red phosphorus particles were characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). These results show that red phosphorus particles can be effectively encapsulated with paraffin by using the method. At the same time, how the moisture absorption ratio of microencapsulated red phosphorus particles was dependent on the experimental conditions was further investigated. The results suggest that paraffin-microencapsulated red phosphorus particles show a lower moisture absorption ratio within the experimental conditions of the extraction column and nozzle temperature of 120°C, extraction column pressure of more than 16 MPa, and a mass flow of core particles (0.5 g/min).     

Dehydration of d-glucose in high temperature water at pressures up to 80 MPa

Reaction of d-glucose in water to yield 5-hydroxymethylfurfural (5-HMF), 1,2,4-benzenetriol (BTO) and furfural was studied at high temperatures (up to 400 °C) and high pressures (up to 80 MPa) using a continuous flow reactor. Maximum temperature and pressure conditions gave maximum furfural yield. Increasing pressure from 40 to 70 and 80 MPa enhanced dehydration reactions to 5-HMF, but also enhanced hydrolysis of 5-HMF leading to the production of BTO and thus lead to lower yields of 5-HMF (below 10%). Remarkably, the dehydration reaction to 5-HMF and the hydrolysis of 5-HMF were both enhanced by the increase in water density at 400 °C.