Reactions In Supercritical Water Research Articles

Carbon formation in supercritical water reaction medium

Carbon formation in supercritical water reaction medium

Reforming of bioethanol over Ni/Al 2O 3 and Ni/CeZrO 2/Al 2O 3 catalysts in supercritical water for hydrogen production

Bioethanol was reformed in supercritical water (SCW) at 500 °C and 25 MPa on Ni/Al 2O 3 and Ni/CeZrO 2/Al 2O 3 catalysts to produce high-pressure hydrogen. The results were compared with non-catalytic reactions. Under supercritical water and in a non-catalytic environment, ethanol was reformed to H 2, CO 2 and CH 4 with small amounts of CO and C2 gas and liquid products. The presence of either Ni/Al 2O 3 or Ni/CeZrO 2/Al 2O 3 promoted reactions of ethanol reforming, dehydrogenation and decomposition. Acetaldehyde produced from the decomposition of ethanol was completely decomposed into CH 4 and CO, which underwent a further water–gas shift reaction in SCW. This led to great increases in ethanol conversion and H 2 yield on the catalysts of more than 3–4 times than that of the non-catalytic condition. For the catalytic operation, adding small amounts of oxygen at oxygen to ethanol molar ratio of 0.06 into the feed improved ethanol conversion, at the expense of some H 2 oxidized to water, resulting in a slightly lower H 2 yield. The ceria–zirconia promoted catalyst was more active than the unpromoted catalyst. On the promoted catalyst, complete ethanol conversion was achieved and no coke formation was found. The ceria–zirconia promoter has important roles in improving the decomposition of acetaldehyde, the enhancement of the water–gas shift as well as the methanation reactions to give an extremely low CO yield and a tremendously high H 2/CO ratio. The SCW environment for ethanol reforming caused the transformation of gamma–alumina towards the corundum phase of the alumina support in the Ni/Al 2O 3 catalyst, but this transformation was slowed down by the presence of the ceria–zirconia promoter.

Effect of supercritical water on upgrading reaction of oil sand bitumen

The advantages of supercritical water (SCW) as a reaction medium for upgrading oil sand bitumen were investigated through a comprehensive analysis of the output product, which includes gaseous products, middle distillate, distillation residue, and coke. Canadian oil sand bitumen mined by the steam assisted gravity drainage method was treated in an autoclave at 420–450 °C and 20–30 MPa for up to 120 min with three kinds of reaction media: SCW, high-pressure nitrogen, and supercritical toluene. The yields of gaseous products indicated that a very small amount of water was involved in the upgrading reaction. The analytical results of the middle distillate fractions were almost the same using water and nitrogen at 450 °C. The distillation residues produced in SCW had lower molecular weight distributions, lower H/C atomic ratios, higher aromaticities, and consequently more condensed structures compared to those produced in nitrogen. The coke produced using SCW also had lower H/C values and higher aromaticities. Judging from all the analytical results, the upgrading of bitumen by SCW reaction was primarily considered to be physical in nature. As a result, it is possible to highly disperse the heavy fractions by SCW. This dispersion effect of SCW led to intramolecular dehydrogenation of the heavier component and prevention of recombination reactions, and consequently gave the highest conversion.

ChemInform Abstract: Continuous Reactions in Supercritical Water: A New Route to La2CuO4 with a High Surface Area and Enhanced Oxygen Mobility.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Hydrothermal synthesis of BaTiO3 nanoparticles using a supercritical continuous flow reaction system

Abstract Highly crystalline BaTiO 3 nanoparticle was synthesized rapidly by hydrothermal reaction in supercritical water using a continuous flow reactor. The reactants of TiO 2 sol (or TiCl 4 )/Ba(NO 3 ) 2 mixed solution and KOH solution were used as starting materials and that was heated quickly up to 400 °C under the pressure of 30 MPa for 8 ms as reaction time. The XRD results revealed that the crystal phase of the obtained particles was cubic BaTiO 3 , indicating that the hydrothermal reaction in supercritical water was successfully proceeded under present reaction conditions. Primarily particle size of the BaTiO 3 nanoparticle was determined by means of BET surface area, as small as less than 10 nm with decreasing the reaction pH. In contrast, dispersed particle size in solution measured by DLS (dynamic light scattering) technique decreased from 260 to 90 nm with increasing the reactants concentration. Aggregation of BaTiO 3 nanoparticles might be depressed in the presence of coexisting nitrate anions.

Selective chemical reactions in supercritical carbon dioxide, water, and ionic liquids

Alternative solvents such as supercritical carbon dioxide, water, and ionic liquids are receiving an increase of interest as better replacements for conventional solvents in chemical reactions. They have been called sustainable green solvents because they are highly promising reaction mediums for organic synthesis. This review presents an overview of some selected chemical reactions that have been developed in these green solvents with a particular emphasis on metal-catalyzed reactions.

Formation of α- and β-phase Mn-doped zinc silicate in supercritical water and its luminescence properties at Si/(Zn+Mn) ratios from 0.25 to 1.25

Mn-doped Zn 2SiO 4 (ZSM) was synthesized in supercritical water under batch conditions to study the effect of Si molar ratio on ZSM phase formation and its luminescence properties. Precursors were prepared with zinc oxalate dihydrate, manganese oxalate, and amorphous SiO 2, where Mn/(Zn+Mn) molar ratio was a constant of 0.02 and Si/(Zn+Mn) molar ratios were varied from 0.25 to 1.25. Reaction conditions were chosen to be 400 °C, a pressure of 29 MPa, and for a reaction time of 90 min. At a stoichiometric Si ratio of 0.5, green-emitting α-Zn 2SiO 4 was only phase formed. At Si ratios from 0.55 to 1.25, an α-Zn 2SiO 4 phase and yellow-emitting β-Zn 2SiO 4 phase were formed along with unreacted amorphous SiO 2. As Si ratio increased, the emission color of the materials changed from the green region to the yellow–green region. The main phases present in the system, ZnO–SiO 2–H 2O, at 400 °C, 29 MPa, and 90 min were α-Zn 2SiO 4 and β-Zn 2SiO 4, while other phases such as ZnSiO 3 and Zn 4Si 2O 7(OH) 2·H 2O did not appear after reaction in supercritical water under these experimental conditions.

Rapid synthesis of γ-Al 2O 3 nanoparticles in supercritical water by continuous hydrothermal flow reaction system

Highly crystalline γ-alumina (γ-Al 2O 3) nanoparticle was synthesized hydrothermally in supercritical water by using a continuous flow reaction system from the starting reagent of Al(NO 3) 3·9H 2O. The study was performed under supercritical conditions of water; temperature ranging from 400 to 500 °C, pressures from 25 to 35 MPa and the reaction time was as short as 63 ms to 3 s. Products were characterized by XRD pattern, BET surface areas, transmission electron microscopy (TEM), and dynamic light scattering (DLS). XRD results revealed that γ-Al 2O 3 particles were obtained at 410 °C or higher through the dehydroxylation reaction in supercritical water, while γ-AlOOH phase was predominant at 400 °C. Primary particle size of γ-Al 2O 3 was about 4 nm and did not depend on the reaction temperature and time. Furthermore, DLS results revealed that the secondary particle size of γ-Al 2O 3 dispersed in water increased with increasing reaction temperature and time probably due to particle aggregation. It is noteworthy that secondary particle sizes of γ-Al 2O 3 dispersed in aqueous solution decreased as the Al(NO 3) 3·9H 2O concentration was increased, since particle aggregation was depressed by high zeta potential with lowering the pH.

Formation mechanism and luminescence appearance of Mn-doped zinc silicate particles synthesized in supercritical water

Luminescence appearance of Mn-doped zinc silicate (Zn 2SiO 4:Mn 2+, ZSM) formed in supercritical water at 400 °C and 29 MPa at reaction times from 1 to 4320 min was studied in the relation to its phase formation mechanism. Appearance of luminescent ZSM from green emission by α-ZSM and yellow emission by β-ZSM occurred over the same time period during the onset of phase formation at a reaction time of 2 min. Luminescence appeared at a much lower temperature and at shorter reaction times than the conventional solid-state reaction. Needle-like-shaped α-ZSM was the most stable particle shape and phase in the supercritical water reaction environment and particles formed via two routes: a homogenous nucleation route and a heterogenous route that involves solid-state diffusion and recrystallization.

Rapid synthesis of BaTiO 3 nanoparticles in supercritical water by continuous hydrothermal flow reaction system

Highly crystalline BaTiO 3 nanoparticle was synthesized rapidly by hydrothermal reaction in supercritical water using a continuous flow reactor. The reactant solution of 0.05 M TiO 2 sol and 0.06 M Ba(OH) 2 was used as starting materials and that was heated quickly up to 400 °C under the pressure of 30 MPa with in as short as 2 s to 7 ms reaction time. The X-ray diffraction (XRD) results revealed that the crystal phases of the obtained particles were only BaTiO 3, indicating that the hydrothermal reaction in supercritical water was successfully proceeded under present reaction conditions. Particle size of the BaTiO 3 nanoparticle was determined by means of BET surface area, as small as less than10 nm with reducing the reaction time from 2 s to 7 ms. Furthermore, high-resolution transmission electron microscopy (TEM) image of BaTiO 3 nanoparticles showed highly crystalline nature of BaTiO 3, even if the time for crystallization of BaTiO 3 was extremely reduced. The formation of highly crystalline BaTiO 3 nanoparticles by the short reaction period is thought to be strongly attributed to the properties of supercritical water. Especially, lower dielectric constant property of supercritical water expected to accelerate the hydrothermal reaction of BaTiO 3, and allowed the synthesizing of BaTiO 3 nanoparticles only in 7 ms.

Performance Evaluation of a High Pressure Microtube as a High-Speed Heating Device for Supercritical State Generation

The heat transfer performance of a microtube was experimentally investigated to verify its suitability as a temperature control device for supercritical water (SCW) reactions under high-pressure and high-temperature conditions (pressure: 23–45 MPa, temperature: 673–773 K, flow rate: 0.6–4.6 kg/h). The microtube was made of Ni-base alloy 625 and had a microchannel with an internal diameter (ID) of 0.258 mm. Ohmic heating was employed for elevating the temperature of the microtube. Using purified water as the operating fluid for thermal performance evaluation, rapid and effective fluid heating from room temperature to 673 K within 0.05 s was achieved inside the heater and heat exchanger, respectively. The electrical power input was converted to fluid enthalpy by the microtube heater with a thermal efficiency of 75–95%, heating rate of 104–105 K/s, and an overall heat transfer coefficient of 4950–35400 W/(m2 K). Experimental results indicate that the microtube is useful as a temperature control device, and the rapid and effective turbulent heat and mass transfer in the microchannel is suitable for the high-speed temperature control required for SCW reactions.

Electron-Transfer Reactions in Supercritical Water

Free energies and dynamics of electron-transfer reactions for a diatomic donor-acceptor complex in ambient and supercritical water are studied via molecular dynamics computer simulations using a two-state electronic description. The free energy perturbation method is employed to examine diabatic electronic curves relevant to charge separation and recombination and electron self-exchange. It is found that the diabatic curves are anharmonic and vary with the charge distributions of the donor-acceptor complex, consonant with earlier studies under ambient conditions. Nonetheless, the extent of their anharmonicity and dependence on charge distributions grows with decreasing solvent density so that the Marcus free energy relationship breaks down in low-density supercritical water. The influence of solvent dynamics associated with activation, deactivation, and adiabatic barrier crossing on reaction kinetics is analyzed. Although the transition state theory generally provides a reasonable framework to describe electron-transfer kinetics in a wide range of thermodynamic conditions, the deviation of the reaction rate from the transition state theory predictions increases as the water density decreases.

Potential of hydrogen from oil palm biomass as a source of renewable energy worldwide

Various catastrophes related to extreme weather events such as floods, hurricanes, droughts and heat waves occurring on the Earth in the recent times are definitely a clear warning sign from nature questioning our ability to protect the environment and ultimately the Earth itself. Progressive release of greenhouse gases (GHG) such as CO 2 and CH 4 from development of various energy-intensive industries has ultimately caused human civilization to pay its debt. Realizing the urgency of reducing emissions and yet simultaneously catering to needs of industries, researches and scientists conclude that renewable energy is the perfect candidate to fulfill both parties requirement. Renewable energy provides an effective option for the provision of energy services from the technical point of view. In this context, biomass appears as one important renewable source of energy. Biomass has been a major source of energy in the world until before industrialization when fossil fuels become dominant and researches have proven from time to time its viability for large-scale production. Although there has been some successful industrial-scale production of renewable energy from biomass, generally this industry still faces a lot of challenges including the availability of economically viable technology, sophisticated and sustainable natural resources management, and proper market strategies under competitive energy markets. Amidst these challenges, the development and implementation of suitable policies by the local policy-makers is still the single and most important factor that can determine a successful utilization of renewable energy in a particular country. Ultimately, the race to the end line must begin with the proof of biomass ability to sustain in a long run as a sustainable and reliable source of renewable energy. Thus, the aim of this paper is to present the potential availability of oil palm biomass that can be converted to hydrogen (leading candidate positioned as the energy of the millennium) through gasification reaction in supercritical water, as a source of renewable energy to policy-makers. Oil palm topped the ranking as number 1 fruit crops in terms of production for the year 2007 with 36.90 million tonnes produced or 35.90% of the total edible oil in the world. Its potentiality is further enhanced by the fact that oil constitutes only about 10% of the palm production, while the rest 90% is biomass. With a world oil palm biomass production annually of about 184.6 million tons, the maximum theoretical yield of hydrogen potentially produced by oil palm biomass via this method is 2.16×10 10 kg H 2 year −1 with an energy content of 2.59 EJ year −1, meeting almost 50% of the current worldwide hydrogen demand.

The role of local structure on formic acid decomposition in supercritical water: A hybrid quantum mechanics/Monte Carlo study

We explored water-assisted decompositions of formic acid in supercritical water in terms of local structure near reactant. A hybrid quantum mechanics/molecular mechanics (QM/MM) simulation used in this paper includes QM part as first solvation shell members around the reactant. A present QM/MM approach can simulate supercritical water solution with a reasonable computational load while keeping the simulation preciseness because a density functional theory of B3LYP/6-31+G(d) level was iterated at every 1000 Monte Carlo solute moves. The formic acid converts mainly decarboxylation by water-assisted mechanism, and the coordinated water molecules play an important role for understanding supercritical water density dependence of the reaction. We analyzed a contour map based on the solute–solvent interaction energy along with the reaction pathway. Coordinated water molecule restricted the dehydration pathway by means of hydrogen bond with formic acid, however, the coordinated water promotes the decarboxylation pathway by means of stabilization of the transition state structure with one catalytic water molecule. The contour map of the pair interaction energy along the reaction path elucidates the role of local structure on reactions in supercritical water.

Characterization of a new magnesium hydrogen orthophosphate salt, Mg 3.5H 2(PO 4) 3, synthesized in supercritical water

Beige crystals of a new magnesium hydrogen orthophosphate salt, Mg 3.5H 2(PO 4) 3, as well as nanoparticles of an amorphous, non-Mg-containing phosphate material, Fe 1− y K y PO 4 (0 < y < 1), have been produced by hydrothermal reactions in supercritical water (SCW) of equivalent quantities of aqueous MgCl 2·6H 2O (2 M), and K 4P 2O 7 (1 M) in concentrated HCl in a stainless-steel batch reactor at 400–450 °C and 25–32 MPa. The new salt has been characterized by single-crystal X-ray diffraction and IR and Raman spectroscopies. It crystallizes in the triclinic space group Pī, Z = 2 with the following unit-cell parameters: a = 6.438(1), b = 7.856(1), c = 9.438(1) Å; α = 104.57(1), β = 108.61(1), γ = 101.28(1)°, V = 739.99 Å 3. The effects of the SCW conditions on the nature of the products and their yields and morphologies have been studied by IR and Raman spectroscopies, X-ray powder diffraction, X-ray energy dispersive analysis, scanning electron microscopy, transmission electron microscopy and inductively coupled plasma analysis.

Proton concentration of supercritical water and high-concentrated carbon dioxide mixture using UV–vis spectroscopy

High-temperature and high-pressure UV–vis spectroscopy was applied to evaluate the proton concentration in supercritical water (SCW)–CO 2 (3 and 7 mol%) mixture at 380 °C and a water density of 400 kg m −3 using 4-nitrophenol as a probe molecule. From the absorption spectra measured at various NaOH concentrations, the first dissociation constant of aqueous carbon dioxide (CO 2,aq + H 2O = H + + HCO 3 −) was determined to be 3.3 × 10 −13. The determined dissociation constant was one order of magnitude smaller than that estimated from the revised HKF equation of state (2.5 × 10 −12). Using the determined dissociation constant, proton concentration in SCW–CO 2 mixture was estimated at various CO 2 concentrations (1–7 mol%) and was shown to be 6–19 times higher than that in pure SCW. The compound, 4-nitrophenol, was found to be a reliable probe for estimating proton concentrations in supercritical aqueous solutions. The estimated proton concentration in SCW–CO 2 mixture can be used for detailed analysis of SCW reactions, especially for acid-catalyzed ones.

P214 超臨界水反応における新規金属触媒の開発(ポスター発表)

P214 超臨界水反応における新規金属触媒の開発(ポスター発表)

A New Micromixer with Needle Adjustment for Instant Mixing and Heating under High Pressure and High Temperature

We successfully developed a new type of micromixer for instant mixing and rapid heating under high pressure and high temperature. The mixer has several distinguished features: the mixing within ten milliseconds, good insulation of heat, and high throughput. We confirmed these features by the examination of mixing experiments. Finally, we tried to produce bohmite nanoparticles by the super critical water reaction at 400°C in 30 MPa, and the fine particles the average diameter of which is ca. 20 nm were obtained. It is expected that the developed micromixer will be applied widely to various reactions that require severe reaction conditions.

Thermal Field-Flow Fractionation and Gas Chromatography−Mass Spectrometry in Determination of Decomposition Products of Expandable Polystyrene after Reactions in Pressurized Hot Water and Supercritical Water

Nonbiodegradable polymers are an environmental concern, and various techniques have been developed to recycle and reuse them. Pressurized hot water, or supercritical water, is an interesting alternative as a reaction medium for depolymerization, since water is a readily available “green” solvent and its physicochemical properties can be widely adjusted in the vicinity of the critical point. In the present study, various reaction conditions (reaction time, temperature, and use of additives and catalysts, i.e., H2O2, Pd, NaOH, and HCl) were applied to obtain as high styrene monomer yields as possible in the decomposition of industrial expandable polystyrene (EPS) in a pressurized, high-temperature aqueous medium. Other main reaction products were of interest as well. Reactions were carried out in laboratory-constructed reactors made of Hastelloy. The highest styrene recoveries, ca. 57 wt % of the initial EPS load as measured by gas chromatography−mass spectrometry (GC−MS), were obtained after NaOH addition ...

Decomposition of a long chain saturated fatty acid with some additives in hot compressed water

Stearic acid (C 17 H 35 COOH: C 17 -acid) was treated using a batch reactor with supercritical water (SCW) of 673 K and 0.17 g/cm 3 for 30 min. In SCW, C 17 -acid was stable (2% conversion) and the main products were CO 2 and C 16 alkene. An addition of alkali hydroxide (NaOH and KOH) in the SCW reaction enhanced the decarboxylation of C 17 -acid with the main products being CO 2 and C 17 alkane. Metal oxides (CeO 2 , Y 2 O 3 and ZrO 2 ) enhanced the decarboxylation of C 17 -acid and the main products were CO 2 and C 16 alkene. Based on the results, the reaction mechanism was proposed.