Olefin metathesis in supercritical carbon dioxide

Liquid or supercritical carbon dioxide (scCO(2)) is a versatile reaction medium for ring-opening metathesis polymerization (ROMP) and ring-closing olefin metathesis (RCM) reactions using well-defined metal catalysts. The molybdenum alkylidene complex 1 and ruthenium carbenes 2 and 3 bearing PCy(3) or N-heterocyclic carbene ligands, respectively, can be used and are found to exhibit efficiency similar to that in chlorinated organic solvents. While compound 1 is readily soluble in scCO(2), complexes 2 and 3 behave like heterogeneous catalysts in this reaction medium. Importantly, however, the unique properties of scCO(2) provide significant advantages beyond simple solvent replacement. This pertains to highly convenient workup procedures both for polymeric and low molecular weight products, to catalyst immobilization, to reaction tuning by density control (RCM versus acyclic diene metathesis polymerization), and to applications of scCO(2) as a protective medium for basic amine functions. The latter phenomenon is explained by the reversible formation of the corresponding carbamic acid as evidenced by (1)H NMR data obtained in compressed CO(2). Together with its environmentally and toxicologically benign character, these unique physicochemical features sum up to a very attractive solvent profile of carbon dioxide for sustainable synthesis and production.     

CO(2)-expanded solvents: unique and versatile media for performing homogeneous catalytic oxidations

The work summarized here demonstrates a new concept for exploiting dense phase CO(2), media considered to be "green" solvents, for homogeneous catalytic oxidation reactions. According to this concept, the conventional organic solvent medium used in catalytic chemical reactions is replaced substantially (up to 80 vol %) by CO(2), at moderate pressures (tens of bars), to create a continuum of CO(2)-expanded solvent media. A particular benefit is found for oxidation catalysis; the presence of CO(2) in the mixed medium increases the O(2) solubility by ca. 100 times compared to that in the neat organic solvent while the retained organic solvent serves an essential role by solubilizing the transition metal catalyst. We show that CO(2)-expanded solvents provide optimal properties for maximizing oxidation rates that are typically 1-2 orders of magnitude greater than those obtained with either the neat organic solvent or supercritical CO(2) as the reaction medium. These advantages are demonstrated with examples of homogeneous oxidations of a substituted phenol and of cyclohexene by molecular O(2) using transition metal catalysts, cobalt Schiff-base and iron porphyrin complexes, respectively, in CO(2)-expanded CH(3)CN.     

Lewis acid catalysis in supercritical carbon dioxide. Use of poly(ethylene glycol) derivatives and perfluoroalkylbenzenes as surfactant molecules which enable efficient catalysis in ScCO2

Lewis acid catalysis in supercritical carbon dioxide (CO(2)) was investigated. While solubility of most organic materials is low in scCO(2), poly(ethylene glycol) derivatives or perfluoroalkylbenzenes were found to work as surfactants to dissolve organic materials in scCO(2). In the presence of these molecules, Lewis acid catalyzed organic reactions such as aldol-, Mannich-, and Friedel-Crafts-type reactions proceeded smoothly in scCO(2). Formation of emulsions was observed in these reactions, and the systems were studied in detail.     

Supercritical carbon dioxide: an inert solvent for catalytic hydrogenation?

Various surface species originating from the reaction between CO2 and H2 over Al2O3-supported Pt, Pd, Rh, and Ru model catalysts were investigated by attenuated total reflection infrared (ATR-IR) spectroscopy under high-pressure conditions. Two different spectroscopic cells were used: a variable-volume view cell equipped with ATR-crystal and transmission IR windows (batch reactor) and a continuous-flow cell also equipped with a reflection element for ATR-IR spectroscopy. The study corroborated that CO formation from dense CO2 in the presence of hydrogen occurs over all Pt-group metals commonly used in heterogeneous catalytic hydrogenations in supercritical CO2 (scCO2). In the batch reactor cell, formation of CO was detected on all metals at 50 and 90 degrees C, with the highest rate on Pt. Additional surface species were observed on Pt/Al2O3 at 150 bar under static conditions. It seems that further reaction of CO with hydrogen is facilitated by the higher surface concentration at higher pressure. In the continuous-flow cell, CO coverage on Pt/Al2O3 was less prominent than that in the batch reactor cell. A transient experiment in the continuous-flow cell additionally revealed CO formation on Pt/Al2O3 at 120 bar after switching the feed from a H2-ethane to a H2-CO2 mixture. The in situ ATR-IR measurements indicate that CO formation in CO2-H2 mixtures is normally a minor side reaction during hydrogenation reactions on Pt-group metal catalysts, and dense ("supercritical") CO2 may be considered as a relatively "inert" solvent in many practical applications. However, blocking of specific sites on the metal surface by CO and consecutive products can affect structure sensitive hydrogenation reactions and may be at the origin of unexpected shifts in the product distribution.     

Proton transfer from 2-naphthol to aliphatic amines in supercritical CO2

The proton transfer from 2-naphthol to aliphatic amines was studied in supercritical CO(2) (scCO(2)) and in cyclohexane as reference solvent, by absorption and fluorescence spectroscopy and by time-resolved emission. Irradiation of 2-naphthol in scCO(2) in the presence of ethyldiisopropylamine shows dynamic fluorescence quenching of the acidic form of 2-naphthol and emission from the basic form. Fluorescence excitation spectra show that the emission of the basic form is originated upon excitation of the acidic form. The interaction between 2-naphthol and the amines is described by the formation of a complex with proton donor-acceptor character in the ground and excited states of 2-naphthol. The acidity increase of 2-naphthol upon electronic excitation to the first excited singlet in scCO(2) is as high as in water. Proton transfer quantum yields of 0.6 can be easily achieved in scCO(2). The results have implications for carrying out acid-base catalyzed reactions in scCO(2).     

超临界二氧化碳介质中过渡金融催化反应研究进展

超临界介质中的化学反应研究是目前热点研究领域之一，对超临界CO2介质中 过渡金属催化反应研究进行了总结和述评，同时讨论了有关提高过渡金属催化剂在 超临界CO2中溶液度的方法。     

Nucleophilic displacements in supercritical carbon dioxide under phase-transfer catalysis conditions. 2. Effect of pressure and kinetics

The nucleophilic displacement on n-octylmesylate (n-C(8)H(17)OSO(2)CH(3), 1) with four different anions (I(-), Br(-), N(3)(-), and SCN(-)) is investigated under liquid-supercritical phase-transfer catalysis (LSc-PTC) conditions, i.e. in a biphase system of supercritical carbon dioxide (scCO(2)) and water, in the presence of both silica supported and conventional onium salts. The CO(2) pressure greatly affects the concentration of 1 in the sc-phase and plays a major role on its conversion. For example, at 50 degrees C and with a supported PT-catalyst, the conversion of 1 into n-octyl iodide drops by a factor of 5 as the CO(2) pressure is increased from 80 to 150 bar, while in the same pressure range, the solubility of n-octylmesylate in scCO(2) shows a 6-fold increase, indicating that the reagent is desorbed from the catalyst. Under LSc-PTC conditions, pseudo-first-order kinetic rate constants, evaluated for the investigated reactions, show that the performance of scCO(2) as a PTC solvent and the relative nucleophilicity order of the anions (N(3)(-) > I(-) > or = Br(-) > SCN(-)) are comparable to those of toluene and n-heptane. The behavior of conventional phosphonium salts in the scCO(2)/H(2)O biphase system suggests that the reaction may take place either within small droplets of PT-catalyst containing water or in a separate third liquid phase of the PT-catalyst itself.     

Hydrothermal reactions of formaldehyde and formic acid: free-energy analysis of equilibrium

The chemical equilibria concerning formaldehyde and formic acid are computationally investigated in water over a wide range of thermodynamic conditions. The free energy is evaluated in the method of energy representation for the solvent effect on the decomposition processes of these two compounds. The solvation is found to suppress the production of nonpolar species from a polar. In the two competitive decomposition reactions of formic acid, the solvent strongly inhibits the decarboxylation (HCOOH-->CO2+H2) and its effect is relatively weak for the decarbonylation (HCOOH-->CO+H2O). The equilibrium weights for the two decomposition pathways of formic acid are determined by the equilibrium constant of the water-gas-shift reaction (CO+H2O-->CO2+H2), which is an essential and useful process in fuel technology. The reaction control by the solvent is then examined for the water-gas-shift reaction. Through the comparison of the equilibrium constants in the absence and presence of solvent, even the favorable side of the reaction is shown to be tuned by the solvent density and temperature. The reaction equilibrium is further treated for aldehyde disproportionation reactions involving formaldehyde and formic acid. The disproportionation reactions are found to be subject to relatively weak solvent effects and to be dominated by the electronic contribution.     

Selective hydrogenation of phenol in supercritical carbon dioxide

Liquid phase hydrogenation of phenol over Pt/C catalysts was investigated under conventional conditions and supercritical carbon dioxide (scCO2). The equivalent ration of hydrogen to phenol shows a significant effect on the product selectivity. Hydrogenation of phenol in different solvents was also studied, the experimental results show that polarity of solvents influences the yield of cyclohexanone remarkably, scCO2 has the highest one. Catalytic hydrogenation of phenol in scCO2 or sub-scCO2 was emphatically researched. The result is that near the critical point of CO2 phenol has higher reaction activity than that of normal organic solvents, cyclohexanone has 47% in yield and 87% in selectivity.     

Oxidation of alcohols to carbonyl compounds with CrO3.SiO2 in supercritical carbon dioxide

Supercritical carbon dioxide (scCO2) is an effective reaction medium to perform the oxidation of primary and secondary aliphatic alcohols to the corresponding carbonyl compounds with chromium trioxide supported on silica. These reactions were performed by flowing a solution of the alcohol in scCO2 through a column containing the supported reagent and recovering the product by depressurization. This method avoids the use of organic solvents and the contamination of the products with chromium species.     

Solvation structure and dynamics for passivated Au nanoparticle in supercritical CO2: a molecular dynamic simulation

All-atomic molecular dynamics simulations have been performed to study the interfacial structural and dynamical properties of passivated gold nanoparticles in supercritical carbon dioxide (scCO(2)). Simulations were conducted for a 55-atom gold nanocore with thiolated perfluoropolyether as the packing ligands. The effect of solvent density and surface coverage on the structural and dynamical properties of the self-assembly monolayer (SAM) has been discussed. The simulation results demonstrate that the interface between nanoparticle and scCO(2) solvent shows a depletion region due to the preclusion of SAM. The presence of scCO(2) solvent around the passivated Au nanoparticle can lead to an enhanced extension of the surface SAM. Under full coverage, the structure and conformation of SAM are insensitive to the density change of scCO(2) fluid. This simulation results clarify the microscopic solvation mechanism of passivated nanoparticles in supercritical fluid medium and is expected to be helpful in understanding the scCO(2)-based nanoparticle dispersion behavior.     

Interaction of cationic liposomes with cell membrane models

The colloidal dispersion stability of nano-sized graphene sheets in supercritical fluid (SCF) media is very important for developing SCF-based exfoliation and dispersion technologies for stabilization and solubilization of graphenes. We carried out molecular dynamics simulations to elucidate the stability mechanism of graphene in supercritical CO(2) (scCO(2)). The potential of mean force (PMF) between two graphene nanosheets in scCO(2) was simulated, and the effect of scCO(2) density and temperature on the PMF behavior has been investigated. The simulation results demonstrate that there exists a free energy barrier between graphenes in the scCO(2) fluid, possibly obstructing the aggregation of graphenes. The single-layer confined CO(2) molecules between the graphene sheets can induce a dominating repulsion interaction between graphene sheets. At higher scCO(2) fluid density, there are more confined CO(2) molecules within the interplate regions, resulting in a stronger repulsive free energy barrier. The effect of temperature on the PMF is relatively minor. The scCO(2) solvent structure shows layered confined arrangement in the interfacial region near the graphene nanosheets, which is correlated well with the PMF profile curve.     

Solubility of room-temperature ionic liquid in supercritical CO2 with and without organic compounds

Ionic liquid can dramatically dissolve in supercritical (sc) CO2 with polar organic compounds (ethanol, acetone) especially as the concentration of the compounds in scCO2 exceeds 10 mol%, while the effect of a nonpolar organic compound (n-hexane) in scCO2 on the solubility is very limited even when its concentration is as high as 30 mol%.     

Preparation of liposomes using an improved supercritical reverse phase evaporation method

Liposomes of various phospholipids were prepared using an improved supercritical reverse phase evaporation (ISCRPE) method that utilizes supercritical carbon dioxide (scCO(2)) as an alternative to organic solvents. Using this method, in the absence of any organic solvent including ethanol, the maximum trapping efficiency of glucose reached 36% for 20 mM l-alpha-dioleoylphosphatidylcholine (DOPC), compared to less than 10% using the Bangham method. Liposomes prepared by the ISCRPE method were highly stable for one month at room temperature. Freeze fractured TEM observations, osmotic shrinkage measurements, and DSC measurements revealed that the liposomes prepared by the ISCRPE method are unilamellar vesicles with loosely packed phospholipids. Comparison of nitrogen with scCO(2) revealed that the presence of CO(2) is necessary for the formation of liposomes.     

Sulfated mesoporous alumina: a highly effective solid strong base catalyst for the Tishchenko reaction in supercritical carbon dioxide

Heterogeneous strong base catalysis for the intramolecular Tishchenko reaction of aromatic 1,2-dicarbaldehydes to the corresponding phthalides in supercritical CO2CscCO2 has been realized with mesoporous alumina containing SO4(2-) ions in the alumina framework (mesoAl2O3/SO4(2-)). Infrared spectroscopy of pyrrole adsorbed on the alumina and strong poisoning by a weak Br?nsted acid of methanol revealed that the SO4(2-) ions in the framework slightly suppressed the average strength of base sites (O2-) on mesoAl2O3/SO4(2-), but there exists a small number of strong base sites that promote the Tishchenko reaction in scCO2. Although the intramolecular Tishchenko reaction of phthalaldehyde to phthalide in scCO2 was somewhat slower than those in organic solvents such as tetrahydrofuran (THF) and benzene, the addition of a small amount of THF as a cosolvent remarkably increased the reaction rate; the reaction in the scCO2-THF system proceeded 1.5-fold faster than those in pure benzene and THF solvents.     

Reduction of Aldehydes by Fe-H_2O-CO_2 System in Supercritical Carbon Dioxide

1 Introduction
Nowadays, green chemistry has received increased attention. The use of water and scCO2 as a solvent or reagent is an important field for organic reactions and green chemistry both in laboratory and industry.     

Phase behavior in model homopolymer/CO2 and surfactant/CO2 systems: discontinuous molecular dynamics simulations

Discontinuous molecular dynamics simulations are performed on surfactant (HmTn)/solvent systems modeled as a mixture of single-sphere solvent molecules and freely jointed surfactant chains composed of m slightly solvent-philic head spheres (H) and n solvent-philic tail spheres (T), all of the same size. We use a square-well potential to account for the head-head, head-solvent, tail-tail, and tail-solvent interactions and a hard-sphere potential for the head-tail and solvent-solvent interactions. We first simulate homopolymer/supercritical CO2 (scCO2) systems to establish the appropriate interaction parameters for a surfactant/scCO2 system. Next, we simulate surfactant/scCO2 systems and explore the effect of the surfactant volume fraction, packing fraction, and temperature on the phase behavior. The transition from the two-phase region to the one-phase region is located by monitoring the contrast structure factor of the equilibrated surfactant/scCO2 system, and the micelle to unimer transition is located by monitoring the aggregate size distribution of the equilibrated surfactant/scCO2 system. We find a two-phase region, a micelle phase, and a unimer phase with increasing packing fraction at fixed temperature or with increasing temperature at fixed packing fraction. The phase diagram for the surfactant/scCO2 system in the surfactant volume fraction-packing fraction plane and the density dependence of the critical micelle concentration are in qualitative agreement with experimental observations. The phase behavior of a surfactant/scCO2 system can be directly related to the solubilities of the corresponding homopolymers that serve as the head and tail blocks for the surfactant. The influence of surfactant structure (head and tail lengths) on the phase transitions is explored.     

DLVO-based estimates of adsorbed water film thicknesses in geologic CO2 reservoirs

When supercritical carbon dioxide (scCO(2)) is injected into deep subsurface reservoirs, much of the affected volume consists of pores containing both water and scCO(2), with water films remaining as the mineral-wetting phase. Although water films can affect multiphase flow and mediate reactions at mineral surfaces, little is known about how film thicknesses depend on system properties. Here, the thicknesses of water films were estimated on the basis of considerations of capillary pressure needed for the entry of CO(2) and disjoining pressures in films resulting from van der Waals and electric double-layer interactions. Depth-dependent CO(2) and water properties were used to estimate Hamaker constants for water films on silica and smectite surfaces under CO(2) confinement. Dispersion interactions were combined with approximate solutions to the electric double layer film thickness-pressure relationship in a Derjaguin-Landau-Verwey-Overbeek (DLVO) analysis, with CO(2) as the confining fluid. Under conditions of elevated pressure, temperature, and salinity commonly associated with CO(2) sequestration, adsorbed water films in reservoir rock surfaces are typically predicted to be less than 10 nm in thickness. Decreased surface charge of silica under the acidic pH of CO(2)-equilibrated water and elevated salinity is predicted to compress the electric double layer substantially, such that the dispersion contribution to the film thickness is dominant. Relative to silica, smectite surfaces are predicted to support thicker water films under CO(2) confinement because of greater electrostatic and dispersion stabilization.     

“嵌入模型”在γ-Al2O3负载单组分和双组分金属氧化物催化剂中的应用

探讨负载型金属氧化物催化剂的表面组分与载体之间的相互作用, 有助于理解相关催化剂的催化作用本质. 近年来, 我们对单组分CuO以及双组分CuO-Mn₂O₃, CuO-CoO等金属氧化物在γ-Al₂O₃载体表面的分散行为和存在状态, 及其物理化学性质和催化性能(CO+O₂和NO+CO模型反应)进行了研究. 结果表明, 这些金属氧化物在γ-Al₂O₃载体表面的分散行为和所得负载型催化剂样品的一些物理化学性质及其催化性能均可参照“嵌入模型”来解释. 在此基础上, 我们讨论了这些样品的“组成-结构-性质”间的关系, 并针对表面负载双组分金属氧化物样品提出了表面协同氧空位参与的NO+CO反应机理.     

Micelle-hosted palladium nanoparticles catalyze citral molecule hydrogenation in supercritical carbon dioxide

A new approach of employing metal particles in micelles for the hydrogenation of organic molecules in the presence of fluorinated surfactant and water in supercritical carbon dioxide has very recently been introduced. This is allegedly to deliver many advantages for carrying out catalysis including the use of supercritical carbon dioxide (scCO2) as a greener solvent. Following this preliminary account, the present work aims to provide direct visual evidence on the formation of metal microemulsions and to investigate whether metal located in the soft micellar assemblies could affect reaction selectivity. Synthesis of Pd nanoparticles in perfluorohydrocarboxylate anionic micelles in scCO2 is therefore carried out in a stainless steel batch reactor at 40 degrees C and in a 150 bar CO2/H2 mixture. Homogeneous dispersion of the microemulsion containing Pd nanoparticles in scCO2 is observed through a sapphire window reactor at W0 ratios (molar water-to-surfactant ratios) ranging from 2 to 30. It is also evidenced that the use of micelle assemblies as new metal catalyst nanocarriers could indeed exert a great influence on product selectivity. The hydrogenation of a citral molecule that contains three reducible groups (aldehyde, double bonds at the 2,3-position and the 6,7-position) is studied. An unusually high selectivity toward citronellal (a high regioselectivity toward the reduction of the 2,3-unsaturation) is observed in supercritical carbon dioxide. On the other hand, when the catalysis is carried out in the conventional liquid or vapor phase over the same reaction time, total hydrogenation of the two double bonds is achieved. It is thought that the high kinetic reluctance for double bond hydrogenation of the citral molecule at the hydrophobic end (the 6,7-position) is due to the unique micelle environment that is in close proximity to the metal surface in supercritical carbon dioxide that guides a head-on attack of the molecule toward the core metal particle.