Reactions In Supercritical Water Research Articles

Effect of Supercritical Water Treatment on Porous Structure, Liquid-Phase Adsorption and Regeneration Characteristics of Activated Anthracite

Microporous activated anthracite was produced from waste anthracite powders by the conventional steam activation. The activated anthracite was also treated by supercritical water reaction (SWR) by using distilled water and hydrogen peroxide solution as a liquid medium for SWR treatment. It was found that SWR treatment can improve the mesoporosity of the activated anthracite though the micropore volume was reduced by the treatment. In liquid-phase adsorption and supercritical water regeneration studies, phenol and organic dye RED 31 were selected as the representative adsorbates. The adsorption and regeneration characteristics of activated anthracite were compared with those of a commercial activated carbon. The results indicate that the activated anthracite prepared showed comparable phenol adsorption capacity but significantly lower dye adsorption capacity than the commercial one. However, supercritical water regeneration efficiency was remarkably high. The first/second regeneration efficiencies of commercial activated carbon and activated anthracite exhausted with phenol were 55/55 and 65/65%, respectively, and in the case of RED 31, 78/79 and 338/317%, respectively, with losses of activated carbon less than 4% per regeneration. Because of little loss of activated carbon during successive regenerations, this SWR regeneration method was suitable for regenerating spent activated carbon or anthracite.

Organic reactions in high-temperature and supercritical water

This paper describes various examples of organic reactions carried out in high-temperature water (HTW) that have been performed at the University of Nottingham using either batch-type reactors or continuous-flow systems. Homogeneously and heterogeneously catalyzed partial oxidations of aromatic compounds, in situ hydrogenations, heterocyclic reactions, hydrolysis (HYD), saponification (SAP), and deuteration reactions are presented.

Supercritical water oxidation of o-dichlorobenzene: degradation studies and simulation insights

Supercritical water oxidation (SCWO) of an aromatic chlorinated species, o-dichlorobenzene ( o-DCB), is studied using a tubular flow reactor. o-DCB is often found in chemical waste streams and is similar to polychlorinated biphenyls (PCBs), an acute environmental hazard, in its physical–chemical properties. The experiments focus chiefly on the effect of temperature on the degradation efficiency of the SCWO process. Experiments are also performed for subcritical conditions. Molecular Dynamics simulations of OH radical and phenol in water are carried out in order to estimate the diffusion-controlled limit of the reaction between aromatic species and hydroxyl radical in aqueous solution. It is speculated that high diffusivity of the reacting species is a major factor responsible for high degradation efficiency of oxidation reactions in supercritical water.

Recovery of radioactivity as solids from nonflammable organic low-level radioactive wastes using supercritical water mixed with RuO 2

We investigated the distribution behavior of iron, cobalt, cesium, iodine and strontium attached to nonflammable organic materials, in solid, liquid and gas phases during the decomposition of these materials in supercritical water mixed with RuO 2. The distributions of these elements under various conditions (initial amounts, with/without precipitation reagent) were determined by using their radioisotopes as simulated low-level radioactive wastes in order to ease the detection of trace amounts of elements even in solid and gas phases. Iron and cobalt were found only in the solid phase when iron hydroxide was added as a precipitation reagent before the supercritical water reaction. Cesium, iodine and strontium were found in the liquid phase after the reaction. Therefore, by adding precipitation reagents such as sodium tetraphenylborate, and Na 2CO 3 (or NaHCO 3) and AgNO 3 aqueous solutions to each resultant liquid phase containing cesium, strontium and iodine, respectively, these elements can be successfully recovered only in the solid phase. The gases produced during the decomposition of the organic material contain no radioactivity under all conditions in this study. These results indicate that all of the elements investigated in this study (iron, cobalt, cesium, iodine and strontium) can be recovered successfully by this supercritical water process using RuO 2. Consequently, this process is suggested as a predominant candidate for the treatment of nonflammable organic materials in low-level radioactive wastes (LLW).

Mechanisms and Kinetics of Noncatalytic Ether Reaction in Supercritical Water. 1. Proton-Transferred Fragmentation of Diethyl Ether to Acetaldehyde in Competition with Hydrolysis

Noncatalytic reaction pathways and rates of diethyl ether in supercritical water are determined in a quartz capillary by observing the liquid- and gas-phase 1H and 13C NMR spectra. The reaction is investigated at two concentrations (0.1 and 0.5 M) in supercritical water at 400 degrees C and over a water-density range of 0.2-0.6 g/cm3, and in subcritical water at 300 and 350 degrees C. The neat reaction (in the absence of solvent) is also studied for comparison at 0.1 M and 400 degrees C. The ether is found to decompose through (i) the proton-transferred fragmentation to ethane and acetaldehyde and (ii) the hydrolysis to ethanol. Acetaldehyde from reaction (i) is consecutively subjected to the unimolecular and bimolecular redox reactions: (iii) the unimolecular proton-transferred decarbonylation forming methane and carbon monoxide, (iv) the bimolecular self-disproportionation producing ethanol and acetic acid, and (v) the bimolecular cross-disproportionation yielding ethanol and carbonic acid. Reactions (ii), (iv), and (v) proceed only in the presence of hot water. Ethanol is produced through the two types of disproportionations and the hydrolysis. The proton-transferred fragmentation is the characteristic reaction at high temperatures and is much more important than the hydrolysis at densities below 0.5 g/cm3. The proton-transferred fragmentation of ether and the decarbonylation of aldehyde are slightly suppressed by the presence of water. The hydrolysis is markedly accelerated by increasing the water density: the rate constant at 400 degrees C is 2.5 x 10(-7) s(-1) at 0.2 g/cm3 and 1.7 x 10(-5) s(-1) at 0.6 g/cm3. The hydrolysis becomes more important in the ether reaction than the proton-transferred fragmentation at 0.6 g/cm3. In subcritical water, the hydrolysis path is dominant at 300 degrees C (0.71 g/cm3), whereas it becomes less important at 350 degrees C (0.57 g/cm3). Acetic acid generated by the self-disproportionation autocatalyzes the hydrolysis at a higher concentration. Thus, the pathway preference can be controlled by the water density, reaction temperature, and initial concentration of diethyl ether.

Mechanisms and Kinetics of Noncatalytic Ether Reaction in Supercritical Water. 2. Proton-Transferred Fragmentation of Dimethyl Ether to Formaldehyde in Competition with Hydrolysis

Noncatalytic reaction pathways and rates of dimethyl ether (DME) in supercritical water are determined in a tube reactor made of quartz according to liquid- and gas-phase 1H and 13C NMR observations. The reaction is studied at two concentrations (0.1 and 0.5 M) in supercritical water at 400 degrees C and over a water-density range of 0.1-0.6 g/cm3. The supercritical water reaction is compared with the neat one (in the absence of solvent) at 0.1 M and 400 degrees C. DME is found to decompose through (i) the proton-transferred fragmentation to methane and formaldehyde and (ii) the hydrolysis to methanol. Formaldehyde from reaction (i) is consecutively subjected to four types of redox reactions. Two of them proceed even without solvent: (iii) the unimolecular proton-transferred decarbonylation forming hydrogen and carbon monoxide and (iv) the bimolecular self-disproportionation generating methanol and carbon monoxide. When the solvent water is present, two additional paths are open: (v) the bimolecular self-disproportionation of formaldehyde with reactant water, producing methanol and formic acid, and (vi) the bimolecular cross-disproportionation between formaldehyde and formic acid, yielding methanol and carbonic acid. Methanol is produced through the three types of disproportionations (iv)-(vi) as well as the hydrolysis (ii). The presence of solvent water decelerates the proton-transferred fragmentation of DME; the rate constant is reduced by 40% at 0.5 g/cm3. This is caused by the suppression of low-frequency concerted motion corresponding to the reaction coordinate for the simultaneous C-O bond scission and proton transfer from one methyl carbon to the other. In contrast to the proton-transferred fragmentation, the hydrolysis of DME is markedly accelerated by increasing the water density. The latter becomes more important than the former in supercritical water at densities greater than 0.5 g/cm3.

An extreame disposition method for low-level radioactive wastes using supercritical water (3)

Disposition of low-level radioactive wastes has been performed in supercritical water with RuO 2 as a catalyst without the addition of any oxidizing materials. These wastes arose from nuclear power plants etc., constituting of nonflammable organic materials, such as fire resistant sheeting and rubber gloves. We investigated the distribution behavior of iron and cobalt attached to nonflammable organic materials, in solid, liquid and gas phases during the decomposition of this method. The distributions of these elements under various conditions (initial amounts) were determined by using their radioisotopes as simulated low-level radioactive wastes in order to ease the detection of trace amounts of elements even in solid and gas phases. Iron and cobalt were found only in the solid phase when non-radioactive iron hydroxide was added as a precipitation reagent before the supercritical-water reaction.

Mechanisms and Kinetics of Acetaldehyde Reaction in Supercritical Water: Noncatalytic Disproportionation, Condensation, and Decarbonylation

Noncatalytic reaction pathways and their rates of acetaldehyde were determined in the neat system and in supercritical water at 400 °C and the density of 0.1−0.6 g/cm3. In supercritical water, acetaldehyde undergoes five types of reactions: (i) decarbonylation into methane and carbon monoxide, (ii) self-disproportionation producing ethanol and acetic acid, (iii) cross-disproportionation generating ethanol and carbonic acid, (iv) condensation forming crotonaldehyde, and (v) the subsequent polymerization of crotonaldehyde or its decarbonylated monomer. Reactions i and iv proceed irrespective of the presence of water, while water suppresses reaction i. Reactions ii and iii are characteristic of aldehyde under hydrothermal conditions. Although reaction ii produces the same products as the classical Cannizzaro reaction, it does not require any added catalysts. Reaction iii manifests the role of formic acid as a reducing aldehyde. Actually, it is shown that reaction iii involves a larger weight than reaction ii and leads to the excess production of ethanol. The rates of these reactions are sensitive to the water density, and path weight control is thus possible through variation of the thermodynamic conditions. New reaction mechanisms are proposed for the present set of high-temperature processes.

Kinetics and thermodynamics of 2-propanol dehydration in supercritical water

Kinetics and thermodynamics of 2-propanol dehydration in supercritical water (SCW) and the effect of density on this reaction have been investigated in a batch reactor. The studies were performed at water densities between 0.24 and 0.58 g/cm 3 and temperatures between 654 and 686 K. The main reaction products of 2-propanol dehydration were propene and water. Rate of the reaction was found to depend essentially on the water density. Two models are used to predict the effect of SCW density on the rate constant of 2-propanol dehydration reaction: a thermodynamic model (the model of activated complex) and a model of acid-catalyzed reaction. Redlich–Kwong–Soave equation of state was used for calculation of the partial molar volumes of the reaction participants in SCW including a transition state and activation volume of the reaction. Application of the activated complex model demonstrated that the calculated values of the activation volume depend strongly on pressure at fixed temperature, which contradicts the experimental data where the activation volume is independent of pressure. At the same time, the mechanism of 2-propanol dehydration reaction in SCW was shown to correlate well with the acid-catalyzed mechanism in solutions. Rate of the reaction is directly proportional to the concentration of 2-propanol and H 3O + ions. An increase in the H 3O + concentration in SCW with density correlates unequivocally with an increase in the reaction rate.

Heck Coupling Reaction of Iodobenzene and Styrene Using Supercritical Water in the Absence of a Catalyst

Heck coupling reaction of iodobenzene and styrene proceeds rapidly and selectively in supercritical water even without any catalyst in the presence of base. Both the choice of base and the reaction conditions had a significant effect on the conversion and the selectivity of the coupling products. The addition of a relatively mild base such as potassium acetate facilitated the cross-coupling reaction, while the hydrolysis of phenyl halide was favored in the presence of a strong base. The conversion and the yields of coupling products increased with increasing temperature, reaching a maximum at 650 K near the critical temperature of water, and then decreased as the temperature was further increased. Water density had a significant influence on the reaction rate, showing nearly 30% augmentation with a slight increase in density from 0.45 to 0.56 g cm(-3), but had less effect on the product selectivity. Two possibilities of the role of water responsible for the noncatalytic Heck coupling reaction in supercritical water, that is, ion and water-catalyzed mechanisms have been considered.

Control of reversible reactions in supercritical water: I. alkylations

AbstractThe reaction rates of alkylation of phenol and that of dealkylation of alkylphenol were evaluated in sub‐ and supercritical water. Dehydration of 2‐propanol gave propene, and then alkylation with phenol, which produced 2‐isopropylphenol. Then 2‐propylphenol was formed by rearrangement of 2‐isopropylphenol. The alkylation rate was higher than the dealkylation rate, as water density increased and temperature decreased at 613–713 K and 0.3–0.6 g/cm3 of water density. At around 673 K, the order of magnitude of the alkylation and dealkylation rates reversed and become sensitive to the system temperature and density. These results demonstrate some of the specific features of supercritical water: the reaction rate of both forward and reverse reactions, such as alkylation and dealkylations, can be controlled through manipulation of temperature and density (pressure). © 2004 American Institute of Chemical Engineers AIChE J, 50: 665–672, 2004

Development of a high-temperature and high-pressure nuclear magnetic resonance probe for studies of chemical reactions in supercritical water

A high-temperature and high-pressure nuclear magnetic resonance (NMR) probe has been developed to investigate chemical reactions in supercritical water up to 50 MPa and 500 °C. The NMR probe is of a continuous flow type with a high-pressure sample tube made of hot-pressed Si3N4 and a specially designed heat-resistant high-pressure seal. A mixing method was utilized to quickly realize the supercritical condition. The NMR probe can be used for in situ investigation of rapid reactions with several hundreds of ms half lives in continuously flowing supercritical water as well as slow equilibria under a stopped-flow condition.

Biodegradability Improvement and Structural Conversion of Polyvinyl Alcohol (PVA) by Sub- and Supercritical Water Reaction

The feasibility of improving the biodegradability of refractory pollutants in sub- and supercritical water was investigated. Poly vinyl alcohol (PVA) was chosen as a test material to represent high-molecular-weight refractory pollutants. At a temperature of 350°C, BOD increased from 100 mg/L to 3,350 mg/L within 17.4 min, while TOC reduction did not exceed 18%. BOD improvement was calculated based on the change of molecular weight distribution to the range lower than 380 and the decrease of UV absorbance to the range lower than 2.0 × 104 μV in the same molecular weight range. The structural conversion from PVA to biodegradable substances was mainly obtained from hydrolysis and thermal decomposition under sub- and supercritical water conditions. The suitable values of biodegradability (BOD/TOC) for conventional biological treatment methods, 0.78 and 0.79, were obtained in 17.4 min at 350°C and in 7.4 min at 400°C, respectively. The production of biodegradable substances from refractory pollutants by sub- and supercritical water reactions was the most important factor on the viewpoint of the pretreatment of refractory pollutants.

Kinetics of Water-Gas Shift Reaction in Supercritical Water

The kinetics of the water gas shift reaction was investigated in supercritical water using the Raman spectroscopic method for in situ quantitative measurement of reactants and relevant products in a stopped flow type reactor. The reaction proceeded according to the stoichiometric reaction as CO + H2O → CO2 + H2, whose rate was first order on the concentration of CO. The dependence on the reactant water was 1.5 ± 0.1 in the water density range from 4 to 25 mol dm–3 at a temperature of 380°C. The steady state concentration of formic acid was in the order of 10–4 against the initial concentration of CO and dependent on a 1.2 ± 0.3 order on the water density. These findings evidence that the rate determining step of the water gas shift reaction is the formation process of formic acid, which subsequently dissociates to CO2 + H2 at a much larger reaction rate. The slightly larger dependence than unity on the water density in the reaction rate as well as in the steady state concentration of the intermediate formic acid suggests that the transition state structure of the formic acid formation process resembles a certain kind of complex formed between a CO molecule and several water molecules such as suggested by Melius et al. (1990) by ab initio calculations.

Kinetics of cellulose conversion at 25 MPa in sub‐ and supercritical water

AbstractExperiments of microcrystalline cellulose conversion in subcritical and supercritical water were conducted at temperatures between 290 and 400°C, a pressure of 25 MPa, and residence times of 0.02–13.1 s using a continuous‐flow‐type microreactor. First, the reaction mechanism of microcrystalline cellulose in subcritical and supercritical water was proposed on the basis of detailed product analyses. Next, the kinetic description of this reaction in subcritical and supercritical water using a grain model was carried out to verify the proposed reaction mechanism and consequently found that the reaction‐rate models were able to express the reaction of microcrystalline cellulose at identical conditions. © 2004 American Institute of Chemical Engineers AIChE J, 50: 192–202, 2004

Effects of sulfates on the decomposition of cellobiose in supercritical water

The decomposition of cellobiose in supercritical water (SCW) with and without sulfates has been performed using a continuous reaction apparatus. For the reactions in pure supercritical water the conversions of cellobiose increase from 36.7 to 59.3% while the selectivities of glucose decrease from 19.9 to 12.8% by changing residence time between 0.05 and 0.15 s at 400 °C and 30 MPa. When we add extremely small amount of sulfuric acid, i.e. 6.4×10 −5 and 3.2×10 −4 mol/l, to supercritical water, the conversions of cellobiose reach high values between 54.2 and 93.9% and the selectivities of glucose show 35.4–73.5%. In order to investigate the effects of metal sulfate on the decomposition of cellobiose several metal sulfates, i.e. CuSO 4, FeSO 4 and MgSO 4 have been tested. Introducing CuSO 4 into supercritical water and keeping the concentrations between 6.4×10 −5 and 3.2×10 −4 mol/l, the conversions of cellobiose increase from 49.1 to 96.7% and the selectivities of glucose vary between 25.3 and 62.5% with residence time. The glucose selectivities of CuSO 4 are three or four times greater than those of pure supercritical water and similar to those of sulfuric acid. For the case of FeSO 4 the conversions are slightly lower than those of CuSO 4 while the selectivities of glucose are comparable to CuSO 4. When MgSO 4 is added to supercritical water, the conversion, yield and selectivity of glucose show relatively lower values than those of CuSO 4 and FeSO 4. Although the catalytic activities of metal sulfates are somewhat different, it is evident that high glucose yields may be obtained in a very short reaction time, e.g. 0.05–0.15 s, by employing extremely small amount of metal sulfates. From the product distribution it has been found that the hydrolysis path through the glucose/fructose gives larger effects than the pyrolysis paths through intermediate products, i.e. glycosyl-erythrose/glycolaldehyde and glycosyl-glycolaldehyde/erythrose, for the decomposition of cellobiose in supercritical water when metal sulfates are used as catalysts.

Decomposition reactions of plastic model compounds in sub- and supercritical water

Plastic model compounds having ether linkage were reacted in sub- and supercritical water to obtain information on the optimum conditions of plastic decomposition reaction into monomeric compounds. Conversion of dinaphthylether to naphthol was attained by the reaction in 3 ml of water including basic additives at 703 K. In the reaction in supercritical water, no production of naphthalene was confirmed suggesting chemical participation of water. Diphenylether was also decomposed into phenol in the reaction in supercritical water including Na 2CO 3. Decomposition reactions of 1,4-diphenoxybenzene and p-phenoxyphenol were also confirmed.

Hybrid quantum chemical studies for the methanol formation reaction assisted by the proton transfer mechanism in supercritical water: CH3Cl+nH2O→CH3OH+HCl+(n−1)H2O

The proton transfer along the chain of hydrogen bonds is involved in many chemical reactions in aqueous solution and known to play a decisive role. We have performed the hybrid quantum chemical simulations for the methanol formation reaction catalyzed by the proton transfer mechanism [CH3Cl+nH2O→CH3OH+HCl+(n−1)H2O, n=3] in supercritical water (SCW) to investigate the role of water solvent on the reaction. In the simulation, the electronic state of the chemically active solutes (CH3Cl+3H2O) has been determined quantum mechanically, while the static water solvent has been represented by a classical model. The activation free energy for the water–catalytic reaction in SCW has been found to be 9.6 kcal/mol, which is much lower than that in the gas phase (29.2 kcal/mol). The fractional charge analysis has revealed that the notable charge separation in the solute complex takes place at the transition state (TS) and the resulting huge dipole gives rise to the considerable stabilization of the TS as compared to the reactant. It has been shown that the reaction assisted by the proton transfer mechanism is energetically much favored than the ionic SN2 reaction (CH3Cl+OH−→CH3OH+Cl−, 18.8 kcal/mol). The present calculations suggest that the proton migrations through the chain of hydrogen bonds can be regarded as a probable candidate responsible for the anomalous reactivities observed in SCW.

Water density dependence of formaldehyde reaction in supercritical water

The water density dependence of formaldehyde (HCHO) reaction in supercritical water (SCW) was studied with batch experiments. Major products from the reaction were methanol (CH 3OH), formic acid (HCOOH), hydrogen (H 2), carbon monoxide (CO) and carbon dioxide (CO 2). It was found that the Cannizzaro reaction mechanism was the preferred reaction pathway for HCHO reaction in SCW. At higher water densities, CH 3OH yields increased confirming the predominance of the Cannizzaro reaction mechanism. At low water densities, CO yields increased and CH 3OH yields decreased, which indicated that monomolecular decomposition became the main reaction pathway. Addition of base to the reacting mixtures was found to promote the Cannizzaro reaction path whereas addition of acid promoted monomolecular decomposition.

Specific behavior of acid–base and neutralization reactions in supercritical water

Acid–base and its neutralization behaviors were analyzed with recently reported dissociation constant data determined by electrical conductivity, UV–Vis and pH methods at 400 °C and densities ranging from 0.3 to 1.0 g/cm 3. The systems are HCl, HNO 3, H 2SO 4, phenol, β-naphthol, β-naphthoic acid, NH 4OH, alkali metal hydroxides and alkali metal chlorides. Among these systems, HCl and KOH are strongest acid and base, respectively. Then neutralization behaviors of HCl+NaOH, dissociation behavior of NaCl and titration behavior of HCl+LiCl and LiOH+LiCl were studied with dissociation constant data and activity coefficient formulation. The results gave informations that (1) the equivalence point for neutralization was placed at a little smaller NaOH concentration than initial HCl concentration, (2) NaCl solution behaved as a base, (3) H + and OH − molalities in HCl and LiOH solutions, respectively, were decreased with increasing LiCl concentration.