Baeyer-Villiger oxidation in supercritical CO2 with potassium peroxomonosulfate supported on acidic silica gel

Supercritical carbon dioxide (scCO2) is an efficient reaction medium to perform the Baeyer-Villiger oxidation with hydrated silica-supported potassium peroxomonosulfate (h-SiO2.KHSO5) under flow-through conditions. Hydration modulates the reactivity of the active surface by softening the acidity of the KHSO4 present in the supported reagent. The reaction in scCO2 is much more efficient than in n-hexane under similar conditions, which is attributed to better transport and solvating properties of the supercritical medium with regard to n-hexane.     

Infrared spectra of geminal and novel triple hydroxyl groups on silica surface

We investigated the O---H stretching frequencies of geminal and triple hydroxyl groups on silica surface by FT-IR spectroscopy. The geminal and triple hydroxyl groups were individually prepared by the chemical reaction of alkylchlorosilane with surface isolated hydroxyl groups which are separated enough and following hydroxylation of surface chlorine groups. The O---H stretching frequencies of these generated geminal and triple hydroxyl groups were observed at frequencies by 2 and 3 cm⁻¹ lower than that of isolated ones, respectively. By heat treatment the triple hydroxyl groups are firstly eliminated, and secondly the geminal hydroxyl groups disappear. Lastly only isolated hydroxyl groups remain on silica surface.     

Influence of additives including amine and hydroxyl groups on aqueous ammonia absorbent for CO2 capture

Aqueous ammonia absorbent (10 wt %) was modified with four kinds of additives (1 wt %) including amine and hydroxyl groups, i.e., 2-amino-2-methyl-1-propanol (AMP), 2-amino-2-methyl-1,3-propandiol (AMPD), 2-amino-2-ethyl-1,3-propandiol (AEPD), and tri(hydroxymethyl) aminomethane (THAM), for CO(2) capture. The loss of ammonia by vaporization was reduced by additives, whereas the removal efficiency of CO(2) was slightly improved. These results were attributed to the interactions between ammonia and additives or absorbents and CO(2) via hydrogen bonding, as verified by FT-IR spectra and computational calculation. Molecular structures as well as binding energies were obtained from the geometries of (ammonia + additives) and (ammonia + additives + CO(2)) at the optimized state. These experimental and theoretical findings demonstrate that additives including amine and hydroxyl group are suitable for modifying aqueous ammonia absorbent for CO(2) removal.     

Fluorinated surfactants in supercritical CO2

Liquid or supercritical carbon dioxide has important environmental and economic advantages over petrochemical solvents currently used for industrial processes. However, low solubility in CO₂, particularly of polar compounds, is a hurdle to its implementation as an acceptable alternative. These solubility problems have been overcome by employing specialised fluorinated surfactants to stabilise water nano-droplets as water-in-supercritical/liquid CO₂ microemulsions. Such novel microemulsions can now facilitate innovative ‘green-and-clean’ applications of carbon dioxide technology.     

Tuning stability of mesoporous silica films under biologically relevant conditions through processing with supercritical CO2

Mesoporous materials have been proposed for use in numerous biological environments such as substrates for cell culture and controlled release for drug delivery. Although mesoporous silica synthesis is facile, recent reports (Dunphy et al. Langmuir 2003, 19, 10403; Bass et al. Chem. Mater. 2007, 19, 4349) have demonstrated instability (dissolution) of pure mesoporous silica films under biologically relevant conditions. In this work, we demonstrate a simple processing handle (pressure) to control the dissolution of mesoporous silica films that are synthesized using preformed template films and supercritical CO 2. Spectroscopic ellipsometry is utilized to quantify changes in both the film thickness and porosity; these properties provide insight into the dissolution mechanism. The pore size increases as the films are exposed to phosphate-buffered saline (PBS) through preferential dissolution at the pore wall in comparison to the film surface; a mechanism reminiscent of bulk erosion of scaffolds for drug delivery. Thin mesoporous silica film lifetimes can be extended from several hours using traditional sol-gel approaches to days by using CO 2 processing for identical film thickness. Osteoblast attachment and viability on these films was found to correlate with their increased stability. This enhanced stability opens new possibilities for the utilization of mesoporous silica for biological applications, including drug delivery and tissue engineering.     

皮革的CO2超临界流体脱灰

浸灰和脱灰是皮革制造过程的重要工序。在浸灰工序中,通过高浓度石灰乳液对动物皮的长时间处理,使其纤维介质被溶解,胶原纤维得到分散。脱灰是其后续工序,目的是除去动物皮中吸附和沉积的Ｃａ2+;调节ｐＨ值至中性并使其肿胀状态得以消除;促进鞣铬剂的发渗而与胶原纤维有效结合。常规制革工艺中,铵盐被广泛用作脱灰剂,其缺点是中和作用不充分不能有效除去Ｃａ2+,Ｃａ2+与动物油脂反应会产生“钙斑”,并产生令人不愉快的氨污染环境。而硼酸、甲酸、乙酸、柠檬酸等以单独或组合方式与铵盐一道用于脱灰[1]价格昂贵,还易引起裸皮的酸肿影响皮…     

Synthesis of gradient copolymers with complexing groups by RAFT polymerization and their solubility in supercritical CO2

We report the synthesis of new gradient fluorinated copolymers with complexing groups and soluble in supercritical carbon dioxide (scCO₂). Poly(1,1,2,2‐tetrahydroperfluorodecyl acrylate‐co‐acetoacetoxyethyl methacrylate) (poly(FDA‐co‐AAEM)) and poly(1,1,2,2‐tetrahydroperfluorodecyl acrylate‐co‐vinylbenzylphosphonic acid diethylester) (poly(FDA‐co‐VBPDE)) gradient copolymers were synthesized by reversible addition fragmentation chain transfer polymerization in α,α,α‐trifluorotoluene. Poly(1,1,2,2‐tetrahydroperfluorodecyl acrylate‐co‐vinylbenzylphosphonic diacid) (poly(FDA‐co‐VBPDA)) gradient copolymer was efficiently obtained by cleavage of the phosphonic ester groups of poly(FDA‐co‐VBPDE). The cloud points of these gradient copolymers in dense CO₂ were measured in a variable volume view cell at temperatures between 25 and 65 °C. The gradient copolymers show very good solubility in compressed CO₂ with the decreasing order: poly(FDA‐co‐AAEM) ≈ poly(FDA‐co‐VBPDE) > poly(FDA‐co‐VBPDA). Following a green chemistry strategy, poly(FDA‐co‐AAEM) gradient copolymer was successfully synthesized in scCO₂ with a good control over number‐average molecular weight and composition. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5448–5460, 2009     

The influence of hydroxyl groups on the ultraviolet spectra of substituted aromatic molecules adsorbed on silica surfaces

Infrared and uv spectroscopy have been used to study the interactions of a series of mono and di-substituted benzene molecules on both porous and nonporous high surface area silicas. It is confirmed that the strength of adsorption depends upon the presence and type of surface hydroxyl group but shown that the uv spectral shifts are not necessarily related to bond strength. Thus, when the surface OH groups reduce the effect of the electron donating side groups, stronger hydrogen bonds produce larger blue shifts in π-π* transitions. When n-π* transitions are involved, however, it is a dipole-dipole interaction which determines the magnitude of the red shift and not the strength of the hydrogen bond.     

Use of kinetic method for determining the concentration of hydroxyl groups on a surface of silica

The reactions of surface hydroxyl groups of pyrogenic silica, Aerosil with chromium oxychloride and trichlorosilane are satisfactorily described at the initial sections by the first and second order equations. These reactions were used as an example and it was shown that the kinetic method is applicable for determining the concentration of OH groups on the SiO₂ surface.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 2, pp. 252–255, March–April, 1987.     

Role of surface hydroxyl groups in promoting room temperature CO sensing by Pd-modified nanocrystalline SnO2

SnO₂/Pd nanocomposites were synthesized via sol-gel method followed by variable processing procedures. The materials are sensitive to CO gas in the concentration range 2-100 ppm at room operating temperature. It was shown that modification of nanocrystalline tin dioxide by Pd changes the temperature dependence of sensor response, decreasing the temperature of maximal signal. To understand the mechanism of room temperature CO sensitivity, a number of SnO₂/Pd materials were characterized by XRD, TEM, BET, XPS and TPR techniques. From the results of FTIR, impedance and sensing measurements under variable ambient conditions it was concluded that improvement in CO sensitivity for Pd-modified SnO₂ is due to alteration of CO oxidation pathway. The reaction of CO with surface OH-groups at room temperature was proposed, the latter being more reactive than oxygen species due to the possible chain character of the reactions. It was proposed that Pd additive may initiate chain processes at room temperature.     

FTIR study on the formation of TiO2 nanostructures in supercritical CO2

TiO(2) nanospherical and fibered structures were obtained via a one-step sol-gel method in supercritical carbon dioxide (scCO(2)) involving polycondensation of the alkoxide monomers titanium isopropoxide (TIP) and titanium butoxide (TBO) with acetic acid (HAc). The resulting materials were characterized by means of electron microscopy (SEM and TEM), X-ray diffraction (XRD), thermal analysis (TGA), and attenuated total reflection Fourier transmission infrared (ATR-FTIR) analysis. Depending on the experimental conditions, TiO(2) anatase nanospheres with a diameter of 20 nm or TiO(2) anatase/rutile nanofibers with a diameter of 10-100 nm were obtained. Fiber formation was enhanced by a higher HAc/Ti ratio and the use of the titanium isopropoxide (TIP) monomer. The mechanism of the microstructure formation was studied using in situ FTIR analysis in scCO(2). The FTIR results indicated that the formation of nanofibers was favored by a titanium hexamer that leads to one-dimensional condensation, while nanospheres were favored by a hexamer that permits three-dimensional condensation.     

Photocatalytic oxidation of n-octanol in aerated supercritical CO2 on hydrophobic TiO2

Supercritical CO₂ (scCO₂) has been used as a reaction medium for the photocatalytic oxidative degradation of n-octanol on a partially desilanized hydrophobic suspension of TiO₂ as photocatalyst. Hydrophobic sites on the catalyst surface are necessary to maintain a sustained suspension, and hence surface-mediated interfacial electron exchange, in this non-polar medium. The reaction rates for photooxidative degradation, ultimately to complete mineralization, depend only weakly on temperature and pressure of the supercritical fluid near the critical point. Product distributions were monitored in situ by on-line gas chromatographic analysis, which provides a convenient and rapid method for comparisons and optimization of the reaction conditions.     

Catalysis in supercritical CO2 using dendrimer-encapsulated palladium nanoparticles

Dendrimer-encapsulated nanoparticles are shown to be versatile catalysts for both the hydrogenation of styrene and Heck heterocoupling of iodobenzene and methacrylate in supercritical CO2 (scCO2).     

AKD-Modification of bacterial cellulose aerogels in supercritical CO2

Different approaches towards hydrophobic modification of bacterial cellulose aerogels with the alkyl ketene dimer (AKD) reagent are presented. If AKD modification was performed in supercritical CO₂, an unexpectedly high degree of loading was observed. About 15 % of the AKD was bound covalently to the cellulose matrix, while the other part consisted of re-extractable AKD-carbonate oligomers, which are novel chemical structures described for the first time. These oligomers contain up to six AKD and CO₂ moieties linked by enolcarbonate structures. The humidity uptake from environments with different relative humidity by samples equipped with up to 30 % AKD is strongly reduced, as expected due to the hydrophobization effect. Samples above 30 % AKD, and especially at very high loading between 100 and 250 %, showed the peculiar effect of increased humidity uptake which even exceeded the value of unmodified bacterial cellulose aerogels.     

Soft fluctuating surfactant membranes in supercritical CO2-microemulsions

The bending rigidity of surfactant membranes in novel bicontinuous CO(2)-microemulsions of the type H(2)O/NaCl-scCO(2)-Zonyl FSH/Zonyl FSN 100 was determined using both high pressure small angle neutron scattering and neutron-spin echo spectroscopy. As an important result it was found, that the stiffness of the membrane increases solely by an increase of the pressure.     

19F MAS NMR quantification of accessible hydroxyl sites on fiberglass surfaces

Solid-state 19F nuclear magnetic resonance (NMR) spectroscopy is used for the quantitative investigation of accessible hydroxyl sites on low surface area glass fibers. Samples with surface areas as low as 0.2 m2/g are investigated through covalent binding of (3,3,3-trifluoropropyl)dimethylchlorosilane. 19F is an ideal nucleus for solid-state NMR, as it has a nuclear spin of 1/2 and a natural isotopic abundance of 100%. High-speed MAS techniques (with rotor spinning frequencies greater than 15 kHz) sufficiently average the CSA and any strong dipolar couplings to allow for superior resolution, especially from terminal -CF3 groups. Studies of two model silica gels with higher surface area, but different pore sizes, provide chemical shift and spin-lattice relaxation rate parameters for probe molecules bound within different environments: pores approaching the size of the probe molecule and pores much larger than the molecular size where intermolecular interactions are assumed to be at a minimum. Resonances assignable to both types of binding environments are found in the spectra of similarly functionalized low surface area fibers. Accessible hydroxyl coverages in the range of 0.8-1.3 OH/nm2 have been measured, and an initial discussion of fiber surface roughness and microporosity is advanced.     

Homogeneous photochemical oxidation via singlet O2 in supercritical CO2

This communication describes the reaction chemistry of singlet oxygen in supercritical carbon dioxide, demonstrating rapid and quantitative conversion of alpha-terpinene to ascaridole.     

Rapid detection of hydroxyl groups on solid-phase

A rapid method for the qualitative detection of hydroxyl groups on solid-phase has been developed. The method employs N-methylisatoic anhydride to derivatise resin-bound substrates possessing free hydroxyl functionality. The resultant fluorescent ester can be detected by visualisation under a standard laboratory UV lamp at 365 nm excitation.