Spatial hydration structures and dynamics of phenol in sub- and supercritical water

The hydration structures and dynamics of phenol in aqueous solution at infinite dilution are investigated using molecular-dynamics simulation technique. The simulations are performed at several temperatures along the coexistence curve of water up to the critical point, and above the critical point with density fixed at 0.3 g/cm3. The hydration structures of phenol are characterized using the radial, cylindrical, and spatial distribution functions. In particular, full spatial maps of local atomic (solvent) density around a solute molecule are presented. It is demonstrated that in addition to normal H bonds with hydroxyl group of phenol, water forms pi-type complexes with the center of the benzene ring, in which H2O molecules act as H-bond donor. At ambient conditions phenol is solvated by 38 water molecules, which make up a large hydrophobic cavity, and forms on average 2.39 H bonds (1.55 of which are due to the hydroxyl group-water interactions and 0.84 are due to the pi complex) with its hydration shell. As temperature increases, the hydration structure of phenol undergoes significant changes. The disappearance of the pi-type H bonding is observed near the critical point. Self-diffusion coefficients of water and phenol are also calculated. Dramatic increase in the diffusivity of phenol in aqueous solution is observed near the critical point of simple point-charge-extended water and is related to the changes in water structure at these conditions.     

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.     

Dispersion stability of colloids in sub- and supercritical water

Dispersion stability of colloids has been investigated in sub- and supercritical water by measuring the hydrodynamic diffusion coefficients of the particles by means of dynamic light scattering. It is interestingly found that coagulation of the colloids in sub- and supercritical water is a universal phenomenon irrespective of the material of the colloids. Highly charged colloids were found to be more stable in water against high temperature. Numerical analysis reveals that the stability of the colloids at elevated temperature and pressure is primarily governed by the temperature dependence of the dielectric constant of the medium. The effect of the temperature dependence of the ion product of water (pKw) was found to be very little. Surface charge density and Stern potential may change with respect to temperature due to the readjustment of the ion concentration in the diffuse layer through the enhanced ion product and reduced dielectric constant of water. These are the secondary causes of the particle coagulations in sub- and supercritical water.     

Iridium-catalyzed H/D exchange into organic compounds in water

The air-stable complex Cp(PMe(3))IrCl(2) efficiently catalyzes the exchange of deuterium from D(2)O into both activated and unactivated C-H bonds of organic molecules without added acid or stabilizers. Selectivity is observed in many cases, with activation of primary C-H bonds occurring preferentially. A number of new stoichiometric transformations involving the iridiym catalyst precursor are also presented, including an oxidation-decarbonylation reaction with primary alcohols.     

Uncatalyzed partial oxidation of p-xylene in sub- and supercritical water

In sub- and supercritical water, partial oxidation of p-xylene was performed in a batch reactor without a catalyst at 240-500^oC, 220-300 bar. The loaded amount of hydrogen peroxide was set to 0-100% of the theoretically required oxygen amount. Conversion of p-xylene was reached over 99% within 15-20 min. In sub- and supercritical water, we propose two parallel pathways and major products that consist of p-tolualdehyde, p-toluic acid, terephthalic acid, toluene and benzaldehyde. Yields of major products in subcritical conditions were higher than in a supercritical conditions.     

The structure of H-bonded clusters in sub- and supercritical water

The structure of H-bonded complexes in sub- and supercritical water in regions close to and remote from the saturation curve was studied. The Car-Parrinello method was used to calculate water dipole moment distributions in 11 thermodynamic states. The dependence of the mean dipole moment of water molecules on the size of clusters and number of H-bonds was obtained.     

Complete replacement of H2 by D2 via Pd/C-catalyzed H/D exchange reaction

[reaction: see text] A general and in situ D2 gas generation method using 10% Pd/C-catalyzed H2-D2 exchange reaction in a H2-D2O system has been developed. H2 gas sealed in a reaction flask was efficiently converted into nearly pure D2 gas, which can be used for the reductive deuteration of substrates possessing reducible functionalities within the molecule.     

Schiff碱配合物催化过氧化氢氧化苯酚的动力学研究

按照文献方法合成了钴(Ⅱ)和铁(Ⅱ)Schiff碱配合物,研究了它们作为模拟过氧化物酶在缓冲溶液中以及在三种不同的表面活性剂(CTAB,Brij35,LSS)胶束中催化过氧化氢氧化苯酚的反应,结果表明：过氧化氢／配合物物质的量比和胶束微环境对金属配合物催化过氧化氢氧化苯酚的反应有明显的影响;本文所提出的Schiff碱金属配合物催化苯酚氧化反应的机理和动力学数学模型是合理的。     

H/D isotopic exchange between water molecules at ice surfaces

H/D isotopic exchange between H(2)O and D(2)O molecules was studied at the surface of ice films at 90-140 K by the technique of Cs(+) reactive ion scattering. Ice films were deposited on a Ru(0001) substrate in different compositions of H(2)O and D(2)O and in various structures to study the kinetics of isotopic exchange. H/D exchange was very slow on an ice film at 95-100 K, even when H(2)O and D(2)O were uniformly mixed in the film. At 140 K, H/D exchange occurred in a time scale of several minutes on the uniform mixture film. Kinetic measurement gave the rate coefficient for the exchange reaction, k(140 K)=1.6(+/-0.3) x 10(-19) cm(2) molecule(-1) s(-1) and k(100 K)< or =5.7(+/-0.5) x 10(-21) cm(2) molecule(-1) s(-1) and the Arrhenius activation energy, E(a)> or =9.8 kJ mol(-1). Addition of HCl on the film to provide excess protons greatly accelerated the isotopic exchange reaction such that it went to completion very quickly at the surface. The rapid reaction, however, was confined within the first bilayer (BL) of the surface and did not readily propagate to the underlying sublayer. The isotopic exchange in the vertical direction was almost completely blocked at 95 K, and it slowly occurred only to a depth of 3 BLs from the surface at 140 K. Thus, the proton transfer was highly directional. The lateral proton transfer at the surface was attributed to the increased mobility of protonic defects at the molecularly disordered and activated surface. The slow, vertical proton transfer was probably assisted by self-diffusion of water molecules.     

Acetone and water on TiO2(110): H/D exchange

Isotopic H/D exchange between coadsorbed acetone and water on the TiO2(110) surface was examined using temperature programmed desorption (TPD) as a function of coverage and two surface pretreatments (O2 oxidation and mild vacuum reduction). Coadsorbed acetone and water interact repulsively on reduced TiO2(110) on the basis of results from the companion paper to this study, with water exerting a greater influence in destabilizing acetone and acetone having only a nominal influence on water. Despite the repulsive interaction between these coadsorbates, about 0.02 monolayers (ML) of a 1 ML d6-acetone on the reduced surface (vacuum annealed at 850 K to a surface oxygen vacancy population of 7%) exhibits H/D exchange with coadsorbed water, with the exchange occurring exclusively in the high-temperature region of the d6-acetone TPD spectrum at approximately 340 K. The effect was confirmed with combinations of d0-acetone and D2O. The extent of exchange decreased on the reduced surface for water coverages above approximately 0.3 ML due to the ability of water to displace coadsorbed acetone from first layer sites to the multilayer. In contrast, the extent of exchange increased by a factor of 3 when surface oxygen vacancies were pre-oxidized with O2 prior to coadsorption. In this case, there was no evidence for the negative influence of high water coverages on the extent of H/D exchange. Comparison of the TPD spectra from the exchange products (either d1- or d5-acetone depending on the coadsorption pairing) suggests that, in addition to the 340 K exchange process seen on the reduced surface, a second exchange process was observed on the oxidized surface at approximately 390 K. In both cases (oxidized and reduced), desorption of the H/D exchange products appeared to be reaction limited and to involve the influence of OH/OD groups (or water formed during recombinative desorption of OH/OD groups) instead of molecularly adsorbed water. The 340 K exchange process is assigned to reaction at step sites, and the 390 K exchange process is attributed to the influence of oxygen adatoms deposited during surface oxidation. The H/D exchange mechanism likely involves an enolate or propenol surface intermediate formed transiently during the desorption of oxygen-stabilized acetone molecules.     

Intermediates and kinetics for phenol gasification in supercritical water

We processed phenol with supercritical water in a series of experiments, which systematically varied the temperature, water density, reactant concentration, and reaction time. Both the gas and liquid phases were analyzed post-reaction using gas chromatographic techniques, which identified and quantified the reaction intermediates and products, including H(2), CO, CH(4), and CO(2) in the gas phase and twenty different compounds--mainly polycyclic aromatic hydrocarbons--in the liquid phase. Many of these liquid phase compounds were identified for the first time and could pose environmental risks. Higher temperatures promoted gasification and resulted in a product gas rich in H(2) and CH(4) (33% and 29%, respectively, at 700 °C), but char yields increased as well. We implicated dibenzofuran and other identified phenolic dimers as precursor molecules for char formation pathways, which can be driven by free radical polymerization at high temperatures. Examination of the trends in conversion as a function of initial water and phenol concentrations revealed competing effects, and these informed the kinetic modeling of phenol disappearance. Two different reaction pathways emerged from the kinetic modeling: one in which rate ∝ [phenol](1.73)[water](-16.60) and the other in which rate ∝ [phenol](0.92)[water](1.39). These pathways may correspond to pyrolysis, which dominates when there is abundant phenol and little water, and hydrothermal reactions, which dominate in excess water. This result confirms that supercritical water gasification of phenol does not simply follow first-order kinetics, as previous efforts to model phenol disappearance had assumed.     

Trihydridoruthenium(IV) complexes: preparation and photo-induced H/D exchange reaction

The trihydrides (η⁵-C₅Me₅RuH₃(PR₃ = PMe₃, FEt₃, Pipr₃, PCy₃, PPh₂Me, and PPh₃) (2) are formed in the reaction of paramagnetic (η⁵C₅Me₅)RuCl₂(PR₃) (1) with NaBH₄ in ethanol. The reaction of 1 with NaBH₄, in THF yields intermediary tetrahydroborate complexes (η⁵-C₅Me₅)Ru(PR₃)(BH₄) (3), which are converted to the trihydrides 2 by treatment with ethanol. Irradiation of 2c and 2f in C₆D₆ solution with UV light causes H/D exchange reaction among the solvent, hydride ligands, and the coordinated phosphine.     

Fast thermal desorption spectroscopy study of H/D isotopic exchange reaction in polycrystalline ice near its melting point

Using fast thermal desorption spectroscopy, a novel technique developed in our laboratory, we investigated the kinetics of HD isotopic exchange in 3 microm thick polycrystalline H2O ice films containing D2O layers at thicknesses ranging from 10 to 300 nm at a temperature of -2.0+/-1.5 degrees C. According to our results over the duration of a typical fast thermal desorption experiment (3-4 ms), the isotopic exchange is confined to a 50+/-10 nm wide reaction zone located at the boundary between polycrystalline H2O and D2O ice. Combining these data with a theoretical analysis of the diffusion in polycrystalline medium, we establish the range of possible values for water self-diffusion coefficients and the grain boundary widths characteristic of our ice samples. Our analysis shows that for the grain boundary width on the order of a few nanometers, the diffusivity of D2O along the grain boundaries must be at least two orders of magnitude lower than that in bulk water at the same temperature. Based on these results, we argue that, in the limit of low concentrations of impurities, polycrystalline ice does not undergo grain boundary premelting at temperatures up to -2 degrees C.     

Focus on H/D exchange of proteins in solution

Editorial

Focus on H/D exchange of proteins in solution     

Molecular dynamic simulation of sub- and supercritical water with new interaction potential

Molecular dynamic simulation of water under sub- and supercritical conditions was performed with a new form of O···H pairwise potential with an additional non-electrostatic term corresponding hydrogen bond formation. Some structural properties of water and hydrogen bonds characteristics were calculated in a wide temperature range and pressure 50 MPa. Dependences of these values on temperature were analyzed.     

The renaissance of H/D exchange

The increasing demand for stable isotopically labeled compounds has led to an increased interest in H/D-exchange reactions at carbon centers. Today deuterium-labeled compounds are used as internal standards in mass spectrometry or to help elucidate mechanistic theories. Access to these deuterated compounds takes place significantly more efficiently and more cost effectively by exchange of hydrogen by deuterium in the target molecule than by classical synthesis. This Review will concentrate on the preparative application of the H/D-exchange reaction in the preparation of deuterium-labeled compounds. Advances over the last ten years are brought together and critically evaluated.     

钠离子取代对气相分子氢氘交换反应的影响

用质谱学方法测定分子的氢氘交换反应过程和结果是研究蛋白质分子等生物体系的分子构象,如蛋白质折叠状态的常用方法之一．本文利用线型离子阱．飞行时间质谱技术,在气相中测定了由不同个数钠离子所取代的简单多肽分子——五联丙氨酸（5Ala-nH＋nNa）．H^＋,,n=2,3,4,5的氢氘交换反应情况,得到了由不同个数钠离子取代的分子离子的氢氘交换反应速度以及交换的氢氘数量等,并研究了钠离子个数对分子氢氘交换反应的影响．结果表明,在本工作的实验条件下,钠离子的取代将影响分子离子的氢氚交换反应．当钠离子数为2时,（5Ala-2H＋2Na）．H^＋分子离子很难发生氢氘交换反应;当钠离子数增加时,对于（5Ala-3H＋3Na）．H^＋,（5Ala-4H＋4Na）．H^＋,和（5Ala-5H＋5Na）．H^＋等,分子的氢氘交换反应速度会加快．这可能是由于钠离子的取代导致了分子离子的构型以及分子的质子亲和势发生了显著的改变,因此影响它们的氢氘交换反应．此外,我们还研究了线型四极场的q值对离子氢氘交换反应的影响,发现对于某种离子,当其对应的四极场q值低于0．8时,对各种离子的交换反应速度影响不大,但当q值接近第一稳定区图的边界点,即当q接近0．908时,对某些离子的交换反应速度影响很明显．q值对离子的交换反应速度影响可以用离子的运动速度加快而导致碰撞反应能量增加来解释．     

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.     

Noncatalytic mono-N-methylation of aniline in supercritical methanol: the kinetics and acid/base effect

Aniline is easily N-methylated in supercritical methanol without catalyst at 350 degrees C and 0.237 g cm-3 to give mono-N-methylaniline with high selectivity, and the reaction rate is increased by a small amount of base (LiOH, NaOH, KOH, and CH3ONa), indicating a difference in the reaction mechanism from the ordinary acid-catalyzed one.