Platinum/Carbon nanotube nanocomposite synthesized in supercritical fluid as electrocatalysts for low-temperature fuel cells

Carbon nanotube (CNT)-supported Pt nanoparticle catalysts have been synthesized in supercritical carbon dioxide (scCO(2)) using platinum(II) acetylacetonate as metal precursor. The structure of the catalysts has been characterized with transmission electron micrograph (TEM) and X-ray photoelectron spectroscopy (XPS). TEM images show that the platinum particles' size is in the range of 5-10 nm. XPS analysis indicates the presence of zero-valence platinum. The Pt-CNT exhibited high catalytic activity both for methanol oxidation and oxygen reduction reaction. The higher catalytic activity has been attributed to the large surface area of carbon nanotubes and the decrease in the overpotential for methanol oxidation and oxygen reduction reaction. Cyclic voltammetric measurements at different scan rates showed that the oxygen reduction reaction at the Pt-CNT electrode is a diffusion-controlled process. Analysis of the electrode kinetics using Tafel plot suggests that Pt-CNT from scCO(2) provides a strong electrocatalytic activity for oxygen reduction reaction. For the methanol oxidation reaction, a high ratio of forward anodic peak current to reverse anodic peak current was observed at room temperature, which implies good oxidation of methanol to carbon dioxide on the Pt-CNT electrode. This work demonstrates that Pt-CNT nanocomposites synthesized in supercritical carbon dioxide are effective electrocatalysts for low-temperature fuel cells.     

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.     

Effect of solvation on induce-fit molecular recognition in supercritical fluid to organic crystals immobilized on a quartz crystal microbalance.

The inclusion behavior of guest molecules to a solid apohost of an orthogonal anthracene-bis(resorcinol)tetraol (1) was investigated in supercritical carbon dioxide (scCO(2)) by using a 9 MHz quartz-crystal microbalance (QCM). Compound 1 forms crystals composed of molecular-sheet bound together by an extensive hydrogen-bonded network. The selective binding of gaseous ethyl acetate to the apohost-immobilized QCM in scCO(2) was observed, and the inclusion amount of ethyl acetate showed a drastic increase above a threshold concentration, [Guest](th) = 0.08 M, and the apparent Gibbs' free energy for the binding was DeltaG(app) = -1.3 kcal mol(-1). Similar selective bindings of ethyl acetate or ethanol had been observed in the gas phase and in water: [Guest](th) = 0.002 M with DeltaG(app) = -3.5 kcal mol(-1) and [Guest](th) = 0.5 M with DeltaG(app) = -0.41 kcal mol(-1), respectively. These values obtained in scCO(2) were intermediate between those in the gas and water phases. Since various physical properties (viscosity, density, polarity, diffusion constant, and solvation) of supercritical fluid are known to be intermediate between gas and liquid, these values clearly reflect the solvation behavior of guest molecules. Thus, the lower solvation of guest molecules indicates the lower threshold concentration and the larger binding energy in the following order: in air > in scCO(2) > in water.     

Dramatic pressure-dependent quenching effects in supercritical CO2 assessed by the fluorescence of 4'-dimethylamino-3-hydroxyflavone. Thermodynamic versus kinetics control of excited-state intramolecular proton transfer

Steady-state fluorescence of 4'-dimethylamino-3-hydroxyflavone (DMA3HF) was observed in supercritical carbon dioxide (scCO(2)). Excited-state intramolecular proton transfer (ESIPT) occurs resulting in two well-separated emission bands corresponding to the normal and tautomer forms. As the scCO(2) density exceeds 0.7 g/mL, the relative intensity of the two bands tends to a constant value, comparable to that observed for organic solvents with ET(30) = 33.0 +/- 0.5 kcal/mol, such as toluene and di-n-butyl ether. At lower densities, the substantial decrease of the total fluorescence intensity (a 600-fold decrease as the pressure decreases from 100 to 80 bar) is accompanied by an even more accentuated decrease of the tautomer fluorescence. This can be explained by a shift in the equilibrium between normal and tautomer forms, concomitant with a more efficient quenching of the less solvated fluorophore, that may change the thermodynamic control of the relative population of the two emissive species to a kinetic control.     

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.     

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.     

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.     

In situ infrared spectroscopic study of brucite carbonation in dry to water-saturated supercritical carbon dioxide

In geologic carbon sequestration, whereas part of the injected carbon dioxide will dissolve into host brine, some will remain as neat to water saturated supercritical CO(2) (scCO(2)) near the well bore and at the caprock, especially in the short term life cycle of the sequestration site. Little is known about the reactivity of minerals with scCO(2) containing variable concentrations of water. In this study, we used high-pressure infrared spectroscopy to examine the carbonation of brucite (Mg(OH)(2)) in situ over a 24 h reaction period with scCO(2) containing water concentrations between 0% and 100% saturation, at temperatures of 35, 50, and 70 °C, and at a pressure of 100 bar. Little or no detectable carbonation was observed when brucite was reacted with neat scCO(2). Higher water concentrations and higher temperatures led to greater brucite carbonation rates and larger extents of conversion to magnesium carbonate products. The only observed carbonation product at 35 °C was nesquehonite (MgCO(3)·3H(2)O). Mixtures of nesquehonite and magnesite (MgCO(3)) were detected at 50 °C, but magnesite was more prevalent with increasing water concentration. Both an amorphous hydrated magnesium carbonate solid and magnesite were detected at 70 °C, but magnesite predominated with increasing water concentration. The identity of the magnesium carbonate products appears strongly linked to magnesium water exchange kinetics through temperature and water availability effects.     

The role of carbon dioxide in chemoselective hydrogenation of halonitroaromatics over supported noble metal catalysts in supercritical carbon dioxide

Chemoselective hydrogenation of halogenated nitrobenzenes over Pt/C catalysts proceeds effectively in supercritical carbon dioxide (scCO2) to produce halogenated anilines with excellent selectivity; the rate of the hydrogenation of nitro groups is markedly enhanced in scCO2 compared to the neat reaction, and the dehalogenation reaction is significantly suppressed.     

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.     

Efficient batch and continuous flow Suzuki cross-coupling reactions under mild conditions, catalysed by polyurea-encapsulated palladium (II) acetate and tetra-n-butylammonium salts

Suzuki cross-coupling reactions are effected in both conventional organic solvents, under continuous flow conditions at 70 degree C, and in batch mode in supercritical carbon dioxide (scCO2), at temperatures as low as 40 degrees C in the presence of palladium(II) acetate microencapsulated in polyurea [PdEnCat] and tetra-n-butylammonium salts.     

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.     

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.     

Solvent effects on the 9-hydroxymethylanthracene + N-ethylmaleimide Diels-Alder reaction. A theoretical study

[structure: see text] The origin of the high reaction rates of the 9-hydroxymethylanthracene + N-ethylmaleimide Diels-Alder reaction in fluorous solvents and supercritical carbon dioxide is analyzed through a combination of regression analyses and theoretical calculations. Both approaches allow the solvent effects on the activation barrier (a decrease by solvophobic interactions, an increase by dipolarity-polarizability) to be attributed to the existence of a hydrogen bond between the two reactants in the transition state, refuting a previous hypothesis based on strong solvophobic interactions.     

Some Organic Reactions in Supercritical Carbon Dioxide

Organic reactions in supercritical carbon dioxide (scCO2) have facilitated great progress in recent years . ScCO2, as an environmentally friendly reaction medium, may be a substitute for 1 volatile and toxic organic solvents and show some special advantages. Firstly, CO2 is inexpensive, nonflammable, nontoxic and chemical inert under many conditions. Secondly, scCO2 possesses hybrid properties of both liquid and gas, to the advantage of some reactions involving gaseous reagents. Control o…     

Investigating the effect of the zwitterion/lactone equilibrium of rhodamine B on the cybotactic region of the acetonitrile/scCO2 cosolvent

We investigated the effect of adding acetonitrile to supercritical carbon dioxide (scCO(2)) in the presence of rhodamine B. This spectroscopic investigation of the scCO(2)/acetonitrile, rhodamine B/scCO(2), and rhodamine B/acetonitrile interactions revealed that rhodamine B, which possesses a temperature dependent equilibrium between a zwitterionic form and a neutral form, had a strong affect on the cybotactic region. To confirm that this effect was only dependent upon the rhodamine B/acetonitrile interactions and not merely due to the bulk-phase behavior of the scCO(2), we measured the compressibility of the scCO(2)/acetonitrile mixture and found it to be independent of the acetonitrile concentration to less than approximately 0.047 mol fraction. We fit the compressibility data using the Peng-Robinson equation of state because it is most appropriate for fluids in the region between 1.72 and 12.45 MPa and between 313 and 333 K.     

Deposition of small organic molecules by the displacement of two immiscible supercritical phases

A new coating process is described (deposition from two immiscible supercritical phases, or DISP) in which a solution of supercritical carbon dioxide (scCO2) with a desired solute is displaced by supercritical helium (scHe). After depressurization, the solute is deposited on substrates initially submerged in the coating solvent. Micron-sized particles and thin films of sucrose octaacetate (SOA) were formed on silicon wafer substrate coupons from DISP at relatively low temperatures and pressures (< or = 6500 psi and < or = 60 degrees C). The particle size, film thickness, and morphology of SOA were characterized as a function of coating conditions-solution concentrations, withdrawal velocities, and pressures. Particles in the range of 1-14 microm in diameter were deposited at low solute concentrations (< or = 0.2 wt % at 4500 psi), whereas films in the range of 0.1-0.5 microm in thickness were deposited at higher solute concentrations (> or = 1.5 wt % at 4500 psi). Particle sizes decreased with increasing displacement velocity and increasing pressure. Estimates of characteristic times for diffusion and nucleation indicate that DISP is a diffusion-limited process. Optical microscopy and atomic force microscopy (AFM) were used to characterize film morphology, including defect formations and film roughness. Highly uniform films with low root-mean-square (RMS) roughness (approximately 10 angstroms) were obtained at a low displacement velocity of 0.0035 cm/s, while ring-like defect structures were observed in films deposited at a higher displacement velocity of 0.035 cm/s. The film thickness and morphology of the films deposited from DISP were compared with films from normal dip coating with typical organic solvents (acetone and toluene). Films deposited from scCO2 by DISP were much thicker, more uniform, and exhibited much fewer drying defects and lower RMS roughness compared with films from the organic solvents.     

Supercritical fluid extraction of phenol compounds from olive leaves

A clean, highly selective supercritical fluid extraction (SFE) method for the isolation of phenols from olive leaf samples was examined. Total phenol extracts were determined using the Folin-Ciocalteu reagent. Dried, ground, sieved olive leaf samples (30 mg) are subjected to SFE, using carbon dioxide modified with 10% methanol at 334 bar, 100 degrees C (CO(2) density 0.70 g ml(-1)) at a liquid flow-rate of 2 ml min(-1) for 140 min. Diatomaceous earth is used to reduce the void volume of the extraction vessel. The influence of extraction variables such as modifier content, pressure, temperature, flow-rate, extraction time, and collection/elution variables, were studied. Supercritical fluid extracts were screened for acid compounds such as carboxylic acids and phenols using Electrospray-MS (in the negative ionization mode). SFE was found to produce higher phenol recoveries than sonication in liquid solvents such as n-hexane, diethyl ether and ethyl acetate. However, the extraction yield obtained was only 45%, using liquid methanol.     

Ab initio molecular-dynamics study of supercritical carbon dioxide

Car-Parrinello molecular-dynamics simulations of supercritical carbon dioxide (scCO(2)) have been performed at the temperature of 318.15 K and at the density of 0.703 g/cc in order to understand its microscopic structure and dynamics. Atomic pair correlation functions and structure factors have been obtained and good agreement has been found with experiments. In the supercritical state the CO(2) molecule is marginally nonlinear, and thus possesses a dipole moment. Analyses of angle distributions between near neighbor molecules reveal the existence of configurations with pairs of molecules in the distorted T-shaped geometry. The reorientational dynamics of carbon dioxide molecules, investigated through first- and second-order time correlation functions, exhibit time constants of 620 and 268 fs, respectively, in good agreement with nuclear magnetic resonance experiments. The intramolecular vibrations of CO(2) have been examined through an analysis of the velocity autocorrelation function of the atoms. These reveal a red shift in the frequency spectrum relative to that of an isolated molecule, consistent with experiments on scCO(2). The results have also been compared to classical molecular-dynamics calculations employing an empirical potential.     

Characterization of varietal differences in essential oil components of hops (Humulus lupulus) by SFC-FTIR spectroscopy

A supercritical fluid chromatographic method combined with Fourier-transform infrared spectroscopy detection (SFC-FTIR) was developed for determination of varietal differences in essential oil constituents in hops (Humulus lupulus). Infrared spectra (IR) of the major constituents of essential oil of hops were taken as films deposited on AgCl discs and compared with those obtained after chromatographic separation in the IR flow-cell with supercritical carbon dioxide (scCO2). Spectra from AgCl discs were comparable to those in scCO2, but in scCO2 most of the bands appeared approximately 8-10 cm-1 to higher wave numbers. Open-tubular SFC-FTIR analysis of the essential oil of 4 different hop varieties was performed. The SFC-FTIR chromatograms showed differences in the location and relative intensity of the peaks depending on the variety, which was further confirmed by consideration of their FTIR spectra.