Reactions In Supercritical CO2 Research Articles

Starch benzoylation in supercritical CO2. A novel sustainable route towards biodegradable hydrophobic polymeric materials

Starch is one of the best candidates to replace non-renewable plastics thanks to its plentiful availability in nature, and low cost. Food industry produces large amount of starch-containing waste, which are not utilizable as nutrients, therefore have potential for non-food applications. However, some of the physicochemical characteristics of starch, such as low thermal stability and high hydrophilicity are undesirable for many applications. Supercritical CO2 is an attractive alternative media for the chemical modification of starch due to its unique tuneable properties, non-toxicity and reduced cost. In this study we performed the benzoylation of starch using vinyl benzoate via a base-catalysed transesterification reaction in supercritical CO2 employing different reaction conditions (temperature, pressure, reaction time, reagents feed) obtaining a wide range of degrees of substitution (0.01-3.00), as determined via elemental analysis and NMR. The modified starches were fully characterized (FT-IR, TGA, DSC, SEM, contact angle, rheology), revealing an overall increased hydrophobicity and improved thermal stability while retaining most of their biodegradability potential. To the best of our knowledge, this is the first report of benzoylation of starch in supercritical CO2 (scCO2), paving the way for the industrial application of this novel solvent in the hydrophobization of starches.

Physical and chemical effects of H2O on mineral carbonation reactions in supercritical CO2

Mineral carbonation through reaction with supercritical CO2 (scCO2) is the ultimate pathway to permanent carbon storage for geological sequestration. Whether and how much H2O is required for the scCO2-mineral interaction to proceed readily, and the role of H2O in the reaction have been actively researched topics. We designed and built a novel in situ Raman reactor to study the carbonation product during brucite [Mg(OH)2]-scCO2 interaction, and investigated the role of free H2O by comparing the products in neat, H2O, and formamide (FM) conditions. We introduced FM to provide a physical polarity environment similar to that of H2O but without the chemical protonation effect. We further evaluated the role of structural H2O by conducting experiments with periclase (MgO). The in situ Raman analysis revealed the occurrence of brucite carbonation both in the presence of H2O and FM, demonstrating the effect of polarity in the reaction; in contrast, carbonation of periclase only occurred in H2O-saturated scCO2. The lack of carbonation of periclase with the presence of FM was attributed to the difficulty of magnesite mineralization, as CaCO3 was able to form in the CaO-FM-scCO2 system. Post-experimental X-ray diffractometry and Fourier-transform infrared spectrometry of the products showed that brucite carbonation yielded nesquehonite and hydromagnesite in H2O and FM, respectively; whereas periclase-scCO2 interaction in H2O produced a mixture of the two products. Rate calculations suggested that the carbonation reaction was much faster in H2O; nonetheless, 40–50% carbonation was still achieved in the Mg(OH)2-FM-scCO2 system after 330 h. Overall, our results showed that the brucite-scCO2 reaction could proceed without H2O under certain conditions – the polarity effect of H2O/FM was large enough to break the Mg–OH and MgO–H bonds and promote the carbonation process.

Catalyst-free synthesis of 5-hydroxymethylfurfural from fructose by extractive reaction in supercritical CO2 – subcritical H2O two-phase system

An extractive reaction configuration using supercritical carbon dioxide (scCO2) as the extracting solvent was tested for the production of 5-hydroxymethyl furfural (HMF) from a 5 wt% fructose aqueous feed. In this configuration, extraction of HMF by scCO2 prevents HMF degradation in the aqueous phase. Because of water co-extraction by scCO2, the volume of the reactional mixture was maintained by continuous injection of water. Reaction was operated in a 90 mL high pressure reactor, where an HMF maximum yield of 62.4 % was achieved at 160 °C and 25 MPa, with a CO2 flow rate of 20 g.min−1 for 420 min. This is the first time that HMF is reportedly produced with such a yield by a catalyst-and organic solvent-free process. Besides, the separation efficiency reached 97.3 % and the relative purity of HMF in the extract was 95.8 wt%. Therefore, this configuration avoids post reactional purification which is needed in conventional batch processes or in extractive reaction processes using organic solvents. Based on kinetic and thermodynamic studies, modeling of the extractive reaction process was developed to perform a sensitivity analysis for CO2 flow rate and extraction efficiency, upon the HMF yield. As an example, it was shown that for 800 min reaction duration, a CO2 flow rate of 100 g.min−1 or an extraction efficiency increase by a 10-fold factor could theoretically led to HMF yields of 73.0 % and 73.7 %, respectively.

Formation mechanism of surface modified iron oxide nanoparticles using controlled hydrolysis reaction in supercritical CO2

In the synthesis of surface modified nanoparticles (NPs), supercritical CO2 is one of appealing reaction fields to realize simple and green chemistry approach without using an organic solvent. In this work, surface modified iron oxide NPs was synthesized in supercritical CO2 with various amounts of water between 5.0 and 90.0 mmol to elucidate the formation mechanism of surface modified NPs with the hydrolysis reaction. The synthesis was performed at 30.0 ± 0.9 MPa of CO2, 18 h and 100 °C, where iron(Ⅲ) acetylacetonate, water and oleic acid were used as starting materials. As a result, the smaller amount of water was revealed to be key factor to synthesize more densely modified iron oxide NPs without the aggregation and iron oxyhydroxide formation while the larger amount of water caused the sever aggregation of less modified NPs and the formation of iron oxyhydroxide. Additionally, the detailed analysis of iron oleate complex and the phase state prediction suggested that the smaller amount of water allowed more dominated formation of iron oleate and subsequent its hydrolysis reaction in uniform supercritical CO2 phase, resulting in the formation of densely modified NPs without the aggregation.

Production of 5-Hydroxymethylfurfural from Fructose by Extractive Reaction in Supercritical CO2 – Subcritical H2O Two-Phase System

Production of 5-Hydroxymethylfurfural from Fructose by Extractive Reaction in Supercritical CO2 – Subcritical H2O Two-Phase System

Green synthesis of substituted olefins (Heck Cross-Coupling Reaction) with palladium(II) catalysts bearing heterodonor P/N ligands

To develop new hemilabile ligand systems for the green synthesis of substituted olefins via Heck cross-coupling reactions in supercritical CO2, we designed and synthesized three palladium(II) complexes bearing heterodonor phosphine-imine ligands which were effectively used in organic solvents in terms of various Pd-catalyzed cross-coupling reactions. These newly prepared palladium(II) complexes and previously reported analogs by us were modified with perfluorooctyl (-C8F17) groups to provide their solubilization in supercritical CO2 to be applied as homogenous catalyst. Both catalytic efficiency and selectivity were benchmarked in the Heck cross-coupling reactions, this representing the first example of the use of hemilabile phosphine-imines as ligands in supercritical CO2 solvent. This ligand system leads to lower reaction times, good conversions and selectivities than the previously reported well-known mono- or bisphosphine palladium precatalysts. Moreover, the phosphine-imine based palladium(II) precatalysts show tolerance of a wide variety of substrates on both olefin and aryl halides.

Silicone aerogels with tunable mechanical properties obtained via hydrosilylation reaction in supercritical CO2

In the present work various sets of vinyl- and hydride-containing precursors are explored for aerogels synthesis via hydrosilylation route directly in supercritical CO2. The means to control the target mechanical properties of aerogels by changing the precursors composition are proposed. Aerogels with Young’s modulus as low as 3.6 kPa can be obtained using hyperbranched silsesquioxane hydrides, while the use of tetraallylsilane with a linear hydride-containing oligomeric siloxane allows synthesis of aerogels with Young’s modulus in the range of 40–124 kPa.

Photocatalytic activity of mixed-phase titanium oxide synthesized by supercritical sol-gel reaction

Photocatalytic activity of mixed-phase titanium oxide synthesized by sol-gel reaction in supercritical CO2 was investigated with dye degradation of methylene blue. Titanium oxides were synthesized using titanium isopropoxide (TIP), titanium butoxide (TBO), titanium ethylhexoxide (TEO) and titanium diisopropoxide bis(acetylacetonate) (TDB) in supercritical CO2. The photocatalytic activity of titanium oxide synthesized using TEO was higher than that of the commercial titanium oxide P25. The high concentration of TEO precursor in supercritical sol-gel reaction can increase the photocatalytic degradation rate constant of methylene blue in aqueous solutions. Supercritical sol-gel reaction achieved high negative surface charge of the titanium oxide in aqueous solutions, and consequently becomes highly-dispersed. The high dispersion of titanium oxide in aqueous solution results in a large surface area that promotes dye adsorption and photocatalytic degradation.

Study of the Sol−Gel Reaction Mechanism in Supercritical CO2 for the Formation of SiO2 Nanocomposites

Direct sol-gel reactions in supercritical CO2 (scCO2) have attracted significant interest for synthesizing nanomaterials by reacting alkoxides with a carboxylic acid. In this study, the hydrolysis of silicon alkoxides (TEOS or TMOS) was carried out using scCO2 as the solvent to generate silica nanoparticles within the matrix of polyethylene for the synthesis of polymeric nanocomposites. This methodology provides advantages of combining the sol-gel reactions and drying into a one-step process for producing polymer nanocomposites. The synthesized polymer silica composites were characterized by SEM, FTIR, and XPS. When the TEOS loading was below 10 wt % Si content, nanometer-sized silica particles were formed that were well dispersed within the polyethylene matrix. The mechanism of the silicon alkoxides reacting with acetic acid in scCO2 was further studied using online GC-MS and offline 13C NMR. Oligomer structures with a bridging methoxyl group between the two silicon atoms and the acetate monodentate were observed. This study suggests a new sol-gel pathway in scCO2 that is different from the hydrolysis-condensation reactions using the conventional sol-gel process.

Kinetics and Mechanism for the Reaction of Hexafluoroacetylacetone with CuO in Supercritical Carbon Dioxide

A kinetic model and mechanism were developed for the heterogeneous chelation reaction of thin CuO films with hexafluoroacetylacetone (hfacH) in supercritical CO2. This reaction has relevance for processing nanoscale structures and, more importantly, serves as a model system to tune the reaction behavior of solids using supercritical fluids. Precise control over reaction conditions enabled accurate etching rates to be measured as a function of both temperature [(53.5-88.4) +/- 0.5 degrees C] and hfacH concentration (0.3-10.9 mM), yielding an apparent activation energy of 70.2 +/- 4.1 kJ/mol and an order of approximately 0.6 with respect to hfacH. X-ray photoelectron spectroscopy and scanning electron microscopy were used to characterize the CuO surface, and a maximum etching rate of 24.5 +/- 3.1 A/min was obtained. Solvation forces between hfacH and the dense CO2 permitted material removal at temperatures more than 100 degrees C lower than that of the analogous gas-phase process. In the low concentration regime, the etching reaction was modeled with a three-step Langmuir-Hinshelwood mechanism. Small amounts of excess water nearly doubled the reaction rate through the proposed formation of a hydrogen-bonded hfacH complex in solution. Further increases in the hfacH concentration up to 27.5 mM caused a shift to first-order kinetics and an adsorption-limited or Rideal-Eley mechanism. These results demonstrate that relatively modest increases in concentration can prompt a heterogeneous reaction in supercritical CO2 to switch from a mechanism most commonly associated with a low-flux gas to one emblematic of a high-flux liquid.

On-chip Raman analysis of heterogeneous catalytic reaction in supercritical CO2: phase behaviour monitoring and activity profiling

Raman spectroscopy on a chip based Si/glass microreactor allows fast and simultaneous analysis of concentration profiles and phase behaviour of a heterogeneously catalyzed reaction at high pressure.

A Study of the Heck Reaction in Non-Polar Hydrocarbon Solvents and in Supercritical Carbon Dioxide

The effects of electronic and steric properties of phosphorus ligands on Heck reactions in supercritical CO2 and non-polar hydrocarbon solvents were studied. In Heck reactions between iodobenzene and butyl acrylate, higher yields were obtained with less electron-rich phosphine ligands. This trend was also observed with the electron-poor triphenyl phosphite. A range of sterically demanding phosphites were then investigated. Biphenyl-containing phosphites 8 and 13 were found to be highly effective. In the Heck reaction between the less-reactive bromobenzene and butyl acrylate, the bulky, electron-rich tri-tert-butylphosphine ligand produced the best yield. These results emphasize the importance of both electronic and steric properties of phosphorus ligands in the Heck reaction in non-polar solvents.

Molecular Simulations of Outersphere Reorganization Energies in Polar and Quadrupolar Solvents. The Case of Intramolecular Electron and Hole Transfer

Outersphere reorganization energies (lambda) for intramolecular electron and hole transfer are studied in anion- and cation-radical forms of complex organic substrates (p-phenylphenyl-spacer-naphthyl) in polar (water, 1,2-dichloroethane, tetrahydrofuran) and quadrupolar (supercritical CO2) solvents. Structure and charge distributions of solute molecules are obtained at the HF/6-31G(d,p) level. Standard Lennard-Jones parameters for solutes and the nonpolarizable simple site-based models of solvents are used in molecular dynamics (MD) simulations. Calculation of lambda is done by means of the original procedure, which treats electrostatic polarization of a solvent in terms of a usual nonpolarizable MD scheme supplemented by scaling of reorganization energies at the final stage. This approach provides a physically relevant background for separating inertial and inertialless polarization responses by means of a single parameter epsilon(infinity), optical dielectric permittivity of the solvent. Absolute lambda values for hole transfer in 1,2-dichloroethane agree with results of previous computations in terms of the different technique (MD/FRCM, Leontyev, I. V.; et al. Chem. Phys. 2005, 319, 4). Computed lambda values for electron transfer in tetrahydrofuran are larger than the experimental values by ca. 2.5 kcal/mol; for the case of hole transfer in 1,2-dichloroethane the discrepancy is of similar magnitude provided the experimental data are properly corrected. The MD approach gives nonzero lambda values for charge-transfer reaction in supercritical CO2, being able to provide a uniform treatment of nonequilibrium solvation phenomena in both quadrupolar and polar solvents.

Enzymatic Reactions in Supercritical CO2: Carboxylation, Asymmetric Reduction and Esterification

AbstractFor Abstract see ChemInform Abstract in Full Text.

The automation of continuous reactions in supercritical CO2: the acid-catalysed etherification of short chain alcohols

We describe a new, automated apparatus for continuous catalytic reactions in supercritical carbon dioxide, scCO2. The reactor incorporates on-line sampling, enabling us to quantify gaseous products such as light alkenes, which would normally be lost in the decompression stage of more conventional supercritical reactors. The on-line sampling is coupled to a gas chromatograph running an isothermal program, which provides automated analysis of the reaction products. We illustrate the operation of the reactor by studying the acid-catalysed etherification of linear alcohols, C1 to C5, both individually and in pairs, over a range of temperatures and pressures equating to a total of 180 different experiments.

Continuous hydrogenation reactions in supercritical CO2“without gases”

Continuous fixed-bed hydrogenation reactions are one of the most promising reactions studied under supercritical conditions. A reactor and supporting equipment has been developed in a collaboration between the University of Nottingham and HEL Ltd. to provide the means for small-scale experimental research. The high pressure gases required to achieve the supercritical state are not supplied by bottled or liquefied gases, but by the in situ decomposition of formic acid, HCO2H, which can be selectively decomposed to produce CO2 and H2. These gases can be used directly as both the supercritical solvent and reagent gases. Further control of the H2 concentration can be achieved by the parallel decomposition of other liquid precursors, namely ethyl formate HCO2Et, which can produce C2H6 + CO2. We report the hydrogenation of several organic substrates to demonstrate this approach, its application in research and its potential as a development tool.

Selective oxidation of cyclohexane in compressed CO2 and in liquid solvents over MnAPO-5 molecular sieve

The selective oxidation reaction of cyclohexane with MnAPO-5 molecular sieve catalyst to produce cyclohexanol and cyclohexanone was carried out in compressed CO2 at 398.2 K using oxygen as an oxidant, and the reaction mixture was controlled to be in the two-phase region, very close to the critical point, and in the supercritical (sc) region of the reaction system. The effect of a small amount of butyric acid cosolvent on the reaction in scCO2 was also studied. The reaction was also conducted in liquid butyric acid, benzene and CCl4, and in the absence of any solvent. It has been observed that the conversion and selectivity of the reaction in compressed CO2 change considerably with the phase behavior or the apparent density of the reaction system. Addition of a small amount (0.2 mol%) of butyric acid cosolvent to scCO2 enhances the conversion significantly, and the selectivity also changes considerably. The by-products of the reaction in compressed CO2 with and without the cosolvent are much less than that of the reaction in liquid solvents or in the absence of solvents. It is advantageous to conduct the reaction in supercritical CO2.

Conversion of pyrrole to pyrrole-2-carboxylate by cells of Bacillus megaterium in supercritical CO2.

Pyrrole was converted to pyrrole-2-carboxylate in supercritical CO2 using cells of Bacillus megaterium PYR 2910, and the yield of the carboxylation reaction in supercritical CO2 was 12 times higher than that under atmospheric pressure.

Kinetics of the Enzymatic Alcoholysis of Palm Kernel Oil in Supercritical CO2

Recently, the use of supercritical CO2 (SCCO2) as a solvent in enzyme-catalyzed reactions has been a matter of considerable research because of its favorable transport properties that can accelerate mass-transfer-limited enzymatic reactions. Further, biocatalysts, especially lipases, have substrate specificity and catalyze the syntheses of esters from a variety of acids and alcohols in SCCO2. Fatty acid esters obtained from alcoholysis of vegetable oils have many applications, such as antifriction agents, food preservatives, and emulsifiers. However, despite the industrial importance of the products obtained from alcoholysis of vegetable oils, conversions of such reactions in SCCO2 have hardly been reported in the literature. Hence, the main objective of this work is to study the ethanolysis of palm kernel oil in SCCO2 as catalyzed by two commercial lipases. The experiments were performed in a reactor vessel in the temperature range of 40−70 °C and from 60 to 200 bar using a water concentration of 0−10 wt...

Catalytic Chain Transfer Polymerization of Methyl Methacrylate in Supercritical Carbon Dioxide: Evidence for a Diffusion-Controlled Transfer Process

The mechanism of catalytic chain transfer polymerization with methyl methacrylate was studied in a range of media. The chain transfer reaction in supercritical CO2 was found to be significantly enhanced compared with similar experiments in toluene or in bulk methyl methacrylate. The results are consistent with a diffusion-controlled rate-determining step in the transfer process with cobaloxime catalysts. The gaslike viscosities in the supercritical medium result in an approximate chain transfer coefficient (ktr) of 108 L mol-1 s-1: an order of magnitude higher than values obtained in conventional organic solvents.