Chemical conversion of cellulose as treated in supercritical methanol

The chemical conversion of cellulose as treated in supercritical methanol was studied using a batch-type reaction vessel at temperatures from 220 to 450°C and pressures from 14 to 72MPa. Supercritical methanol treatment at 350°C and 43MPa for 7min was sufficient to convert microcrystalline cellulose (avicel) to the methanol-soluble. To study the kinetics of the decomposition of cellulose, the decomposition rate constants were obtained, and rapid increase was observed at about 270°C which was about 30°C higher than the critical temperature of methanol. The main products from cellulose decomposition were methylated cellotriose, methylated cellobiose, methyl - and -D-glucosides, levoglucosan and 5-hydroxymethylfurfural. Monomeric compounds such as methyl - and -D-glucosides were stable in supercritical methanol, allowing high yields of monomeric products by supercritical methanol treatment. Based on these results, a pathway of cellulose decomposition treated in supercritical methanol was proposed. These findings suggest that the supercritical methanol treatment of various cellulosic materials may be suitable to obtain useful chemicals and liquid fuels without using fossil resources.     

Chemical conversion of various celluloses to glucose and its derivatives in supercritical water

The supercritical water biomass conversion system was designed and developed in our laboratory. The reaction vessel with cellulose sample was treated with this system at supercritical state of water for a designated period (3–105s) under the conditions of a tin bath temperature of 500°C and pressure of 35MPa. The recovered products of hydrolysates were then analyzed by high performance liquid chromatography. The obtained results indicated that a high amount of glucose and levoglucosan can be achieved from both celluloses I and II for 5–10s supercritical treatment, while that from starch for 3–5s treatment. Although this difference could be due to a difference in the molecular structure between cellulose and starch, a difference between celluloses I and II was not significant. Instead, an accessibility of the water towards cellulose molecules seemed to be significant for their chemical conversion. With the longer treatment, amounts of these compounds observed were decreased due to decomposition. Therefore, it may be concluded that, compared with acid hydrolysis or enzymatic saccharification, cellulose may be hydrolyzed to glucose and its derivatives more or less to the same degree as in corn starch under supercritical state. This finding suggests that the supercritical treatment can overcome the difficulties in hydrolyzing cellulose to glucose, found in the acid hydrolysis or enzymatic saccharification techniques.     

Kinetic study for liquefaction of tar in sub- and supercritical water

Liquefaction of tar from oil distillation was studied under sub- and supercritical water conditions using a batch reactor at 623 and 673 K and 25-40 MPa. The reaction scheme for tar liquefaction was determined as follows: the liquefaction process of tar occurs first and then intermediate chemical compounds are transformed into lighter molecular weight species. The effects of pressure and treatment time were combined into a single severity parameter that was used to monitor the conversion of tar. The main products from the liquefaction of tar were phenol (3.44 wt%), biphenyl (2.23 wt%), diphenylether (13.70 wt%) and diphenylmethane (1.30 wt%), respectively. Liquefaction of tar clearly increased with increasing water density at the same temperature reaction. It indicates that hydrolysis was important in the cleavage of the macromolecular structure of tar under sub- and supercritical conditions. Based on the results, this method could become an efficient method for tar liquefaction, producing high yields of valuable chemical intermediates.     

Pyrolysis of coal-tar asphaltene in supercritical water

Pyrolysis of coal-tar asphaltene, the main active component of coal tar in supercritical water (SCW), is investigated to further understand the upgrading mechanism of coal tar. It is found that coal-tar asphaltene convert to gas, maltene and char both in N₂ and in SCW, but the conversion of coal-tar asphaltene and the yield of maltene in SCW are significant higher than those in N₂. The effect of maltene and char in coal tar on the pyrolysis of coal-tar asphaltene is also studied. The results indicate that the presence of maltene could suppress the formation of char. And the addition of char could reduce the maltene yield. The analysis of pyrolysis product indicates the aromatic nucleus of asphaltene molecule is mainly composed of 2-4 rings aromatic hydrocarbons. Based on these results the pyrolysis mechanism of asphaltene in SCW was discussed.     

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.     

Decomposition kinetics of 2-propylphenol in supercritical water

The decomposition of 2-propylphenol (PP) at 673 K and a water density of 0–0.5 g cm⁻³ yielded 2-isopropylphenol (IPP), phenol and 2-cresol. Gas products were methane, carbon dioxide, ethylene and propene. The decomposition was found to occur through rearrangement and alkylation, that is, (1) rearrangement of the propyl functional group from PP to IPP, (2) dealkylation of PP to phenol, (3) dealkylation of PP to 2-cresol. The decomposition probably occurred by a free-radical mechanism. The reaction rate constants of each pathway were determined and it was found that these were invariant over all the water densities studied at the given temperature.     

Transformation of benzonitrile into benzyl alcohol and benzoate esters in supercritical alcohols

The reactions of benzonitrile in supercritical methanol, ethanol, and 2-propanol were investigated under non-catalytic conditions. In supercritical methanol, benzonitrile was converted to methyl benzoate in high yield. The esterification reaction also occurred in supercritical ethanol to afford ethyl benzoate in moderate yield. The esterification could occur via a route analogous to the Pinner reaction. On the other hand, benzonitrile in supercritical 2-propanol yielded no ester. Benzyl alcohol was the major product in supercritical 2-propanol. We investigated the reaction of the CN bond in supercritical 2-propanol. In supercritical 2-propanol, N-benzylideneaniline was transferred to the reduction product (N-benzylaniline) and hydrolysis products (benzyl alcohol and aniline). The hydrolysis reaction was restricted when the reaction was carried out in supercritical 2-propanol with a low water content. This indicates that the water in the 2-propanol acts as a reagent for the hydrolysis of the CN bond. These results suggested the following reaction process: C₆H₅CN→C₆H₅CHNH→C₆H₅CHO→C₆H₅CH₂OH.     

Solar energy conversion from water photolysis by biological and chemical systems

The production of chemicals and fuels, or energy-rich compounds, from water by sunlight is described as a particularly attractive means for the conversion of solar energy to a valuable renewable resource. The redox properties of photoexcited molecules and the operating mechanism of light-driven systems are first considered. The mechanism of water oxidation carried out by higher plants and green algae-which is actually one of the most important biochemical reactions—as well as that of artificial photosystems, up-to-now designed trying to simulate the natural process with higher efficiency and simplicity, are likewise discussed. A number of biological and chemical light-driven systems are presented as practical ways to solar energy conversion.     

A case against large cluster-induced nonequilibrium solvent effects in supercritical water

Using a partially compressible continuum solvation model, we have shown that solvent compression in just the first two solvation shells (or thereabouts) is all that is required to gain the bulk of the compression-induced enhancement to the solvation energy of ions in supercritical water. This result is found to hold even when the direct, equilibrium solvent-solute cluster involves well over a hundred solvent molecules. We argue that, for charge variation reactions in supercritical water, the observed short-range behavior of the compression-induced solvation free energy precludes the existence of any anomalously large nonequilibrium solvent effects which might be expected on the basis of the very large size of the equilibrium clusters. Received: 8 January 1997 / Accepted: 17 January 1997     

The catalytic role of transition metal salts on supercritical water oxidation of phenol and chlorophenols in a titanium reactor

The oxidation of phenol and some chlorophenols by molecular oxygen at a concentration of 1.5x10^-4 mol dm^-3 has been studied in a small static titanium tubular reactor at temperatures of 648, 657, 673 K and a pressures of 22 MPa. The effect of transition metal salts (CuSO₄, VSO₄, FeSO₄, MnSO₄, NiSO₄, CoSO₄), added in small environmentally acceptable amounts as homogeneous catalysts, was also studied, with copper showing a good catalytic effect. This revised version was published online in June 2006 with corrections to the Cover Date.     

CO在超临界水中的转化以及碱性钾盐的影响

构建了CO高压溶解的进气系统,在连续式反应系统中对超临界水条件下CO的转化规律进行了研究;针对生物质超临界水气化中钾盐的多样性,选择KHCO₃、K₂CO₃和KOH等三种钾盐成分,研究了它们在不同工艺条件（450-600℃、23-29 MPa、停留时间3-6 s）下对超临界水中水煤气转化过程的影响。结果表明,在无催化条件下,提高反应温度、延长停留时间均提高了CO的转化率,而压力对其影响在低压下（23-25 MPa）比较大,高压下（25-29 MPa）比较小,水煤气转化的反应动力学方程为k=10^3.75×exp（-0.66×10⁵/RT）（s^-1）。碱性钾盐均能显著提高CO转化率,其催化促进程度从大到小依次为：KHCO₃>K₂CO₃>KOH。添加碱性钾盐时,提高反应温度、延长停留时间均提高CO转化率,而压力的影响比较复杂。碱性盐对水煤气转化反应的催化是通过草酸盐（HC₂O₄^-）和甲酸盐（HCOO^-）作为中间产物进行的。     

CaO对褐煤在超临界水中制取富氢气体的影响

以褐煤在超临界水中制取富氢气体为目的,利用小型高压间歇反应装置,在Ca/C 摩尔比为0～0.60、温度450℃～680℃、压力23MPa～38MPa和停留1min～30min下,考察了小龙潭褐煤的反应特性。研究表明,CaO不仅可以固定气相中的CO2,提高H2的体积分数,而且可以提高碳转化率和气体产率。600℃、28MPa,Ca/C摩尔比为0.42时,气相产物中的CO2趋于完全固定,H2产率比无添加剂时提高2.5倍,H2体积分数为48％,其余为CH4和烃类气体。升高反应温度使CaO的催化作用更为显著, 碳转化率和气体产率（H2、CH4、烃类气体）随着反应温度的升高而逐渐增加,液相收率减少。增大反应压力可以促使煤转化率和气体产率升高,停留时间对反应的影响相对较小。以900℃热解焦为反应原料进行了气化实验,结果表明,在600℃和650℃反应5min后,碳转化率分别为8.6％和12.5％,CaO对气化反应和甲烷化反应起不同程度的催化作用。     

Relation between soil matrix potential changes and water conversion ratios during methane hydrate formation processes in loess

With a new apparatus designed and assembled by ourselves, the matrix potential of non-saturated loess was firstly measured and studied during methane hydrate formation processes. The experimental results showed that during two formation processes, the matrix potential changes of the loess all presented a good linear relationship with water conversion ratios. In addition, although it was well known that the secondary gas hydrate formation was easier than the initial, our experimental results showed that the initial hydrate formation efficiency in non-saturated loess was higher than that of the secondary.     

The study of model systems subjected to sub- and supercritical water hydrolysis for the production of fermentable sugars

Bioenergy obtained from lignocellulosic biomass is considered the most efficient way to achieve sustainable development in the future. However, there still are challenges in the cellulose conversion to hexoses, which could be used as raw material for the bioenergy production. Sub- and supercritical water hydrolysis have been researched as emergent technologies to obtain simple sugars from lignocellulosic biomass; however, the reaction pathways and kinetics of the hydrolysis of cellulose into oligomers and monomers, and their degradation under sub- and supercritical conditions, are not completely understood yet. Thus, this review provides an overview of the state-of-the-art on hydrolysis with sub- and supercritical water of model systems, cellulose and starch, in the context of elucidating the reaction pathways and kinetic behavior of the biomass hydrolysis to produce suitable fermentation substrates for the production of second generation bioethanol and other biofuels.     

Coupled mass transfer and reaction in water of an oil soluble chemical: A CT scan study

Chemical compounds of the family of alkoxysilanes, such as tetramethyl orthosilicate (TMOS), are soluble in oil but can be dissolved in water where they undergo a hydrolysis and a condensation reaction to form a stable silica gel. We have investigated in detail the coupled mass transfer–reaction process that takes place when oil with dissolved TMOS is brought in contact with water in a well-defined geometry. The main physical parameters (initial TMOS concentration in the oil phase, initial water to oil volume ratio, and water pH and salinity) were varied during the experiment. X-ray CT was used to visualize and quantify concentration changes in the oil and the water phases. We found that varying the initial concentration does not affect mass transfer rates out of the oil phase, but increasing initial concentrations do speed up gelation. The water to oil volume ratio has a larger impact on mass transfer, with a transfer rate that increases with increasing water to oil volume ratio. Low pH of the water phase promotes mass transfer, whereas high pH and addition of salt has no significant effect. Gelation is however more profoundly affected by both increasing pH and addition of salts.     

A kinetic study of the decross-linking of cross-linked polyethylene in supercritical methanol

The recycling of cross-linked polyethylene (XLPE) by a decross-linking reaction in supercritical methanol was studied using a batch reactor. XLPEs with initial gel contents of 45, 55 and 65% were employed and subjected to reaction temperatures between 320 and 360 °C. Complete decross-linking of XLPE was achieved in 10 min in supercritical methanol at 360 °C and 15 MPa. For the first time, chemical kinetics for the decross-linking reaction is proposed based on the gel concentration, and applicable to the reactor design. With respect to the gel concentration, the first-order reaction model agreed well with the experimental results. The evaluated kinetic constant was 0.0867 ± 0.0082 cm³/mg min at 350 °C, and the activation energy was 578 ± 25 kJ/mol.     

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

Luminescence appearance of Mn-doped zinc silicate (Zn₂SiO₄:Mn²⁺, 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.     

Effect of titanium dioxide solubility on the formation of BaTiO3 nanoparticles in supercritical water

Fine BaTiO₃ nanoparticles were prepared by hydrothermal synthesis under supercritical condition (400 °C and 30 MPa) from mixture of barium hydroxide and titanium dioxide as starting precursors. First, conditions for synthesizing BaTiO₃ were examined by using batch reactors. High pH condition, pH > 13, is necessary to obtain phase pure BaTiO₃. The reason was discussed based on the solubility of titanium dioxide, which that dissolution–recrystallization process is essential for the synthesis of BaTiO₃ nanoparticles. Rapid heating of the starting precursors by mixing with high temperature water in a flow reactor is effective to synthesize smaller size and narrower particle size distribution for the BaTiO₃ nanoparticles, compared with the case of slow heating with a batch reactor.     

Comparison of T-piece and concentric mixing systems for continuous flow synthesis of anatase nanoparticles in supercritical isopropanol/water

T-piece and concentric counter-flow mixing systems are compared in continuous flow supercritical solvothermal synthesis of TiO₂ at identical system parameters. The phase pure anatase nanoparticle products were characterized with powder X-ray diffraction (PXRD), transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS), and the particle size, size distribution and absolute crystallinity mapped as a function of temperature, precursor concentration, flow rate and pressure for the two different continuous flow reactors. The particles synthesized with the T-piece geometry are smaller with a narrower size distribution, possibly indicating a more effective mixing, than particles synthesized at the same conditions with concentric counter-flow geometry. In general, an increased synthesis temperature leads to an increase in absolute crystallinity. For the particles synthesized with the concentric reactor geometry crossing of the critical point of the solvent causes a decrease in the particle size and size distribution, and conditions just above the critical temperature are demonstrated to be optimal for continuous solvothermal synthesis of anatase.     

Comparison of supercritical fluid extraction and liquid-liquid extraction for isolation of selected pesticides stored in freeze-dried water samples

Summary The stability of freeze-dried water samples spiked with eight agrochemicals (atrazine, simazine, linuron, carbaryl, propanil, fenitrothion, parathion and fenamiphos) were examined to evaluate their suitability as candidate reference materials for their determination in water samples. In addition, two different extraction procedures, liquid-liquid and supercritical fluid extraction, were compared for the isolation and trace enrichment of target analytes from freeze-dried water samples. Final analytical determinations were by gas chromatography-nitrogen phosphorus detection and electronic impact mass spectrometry, and by liquid chromatography-diode array detection. The whole methodology developed in this paper permitted the determination of pesticides spiked in water at levels varying from 0.03 to 6.9 g L^–1.