New perfluorinated rhodium-BINAP catalysts and hydrogenation of styrene in supercritical CO2

New perfluorinated BINAP ligands and their cationic rhodium complexes have been synthesized and characterized. The catalysts have been tested in homogeneous hydrogenation of the styrene in both methanol and supercritical carbon dioxide (scCO₂). The selective hydrogenations of the styrene in scCO₂ were carried out under the same reaction conditions of 343.15 K temperature and 120 bar CO₂ pressure after 3 h (molar ratio of substrate to catalyst = 500). While all catalysts showed 100% conversion in methanol, same activity was not carried out in scCO₂. (R)-6,6′-diperflorooktil-2,2′-bis(diphenylphosphino)-1,1′-binaphtyl-[Rh(COD)]BArF was showed the highest conversion (96.4%) in scCO₂. Furthermore, methanol was used as co-solvent in the supercritical media in hydrogenation of styrene for the perfluorinated catalysts, which were inactive in scCO₂. Addition of methanol in scCO₂ has rather increased the conversion.     

Hydrogenation of Nitrobenzene with Supported Transition Metal Catalysts in Supercritical Carbon Dioxide

Transition metal catalysts such as Pd, Pt, Ru, and Rh supported on carbon, silica and alumina have been examined for the hydrogenation of nitrobenzene (NB) in supercritical carbon dioxide (scCO₂) and in ethanol. The order of hydrogenation activity is Pt>Pd>Ru, Rh in scCO₂ and in ethanol. The effectiveness of the support is C>Al₂O₃, SiO₂ for either Pt or Pd in scCO₂. For all the catalysts, higher selectivity to aniline has been obtained in scCO₂ compared with ethanol. Hydrogenation of nitrobenzene catalyzed with Pd/C and Pt/C catalysts was successfully conducted in scCO₂ with a 100% yield to aniline at a lower reaction temperature of 35 °C. The product aniline (organic phase) can be easily separated from the side‐product water (aqueous phase), solvent (scCO₂), and catalyst (solid) by a simple phase separation process. The hydrogenation of NB is a structure‐sensitive reaction in ethanol as well as in scCO₂ except for a few Pt/C catalysts in which the degree of metal dispersion is small (<0.08).     

Physically and chemically mixed TiO2-supported Pd and Au catalysts: unexpected synergistic effects on selective hydrogenation of citral in supercritical CO2

The selective hydrogenation of citral was studied with various TiO₂-supported monometallic and bimetallic Pd and Au catalysts and their physical mixtures in supercritical CO₂ (scCO₂). Significant synergistic effects appeared when active Pd species was chemically or physically mixed with less active Au species. The total rate of conversion was greatly enhanced and the selectivity to citronellal (CAL) was improved. The physical properties of those catalysts were characterized by TEM, HRTEM-EDS, XPS, and UV/Vis and their features of H₂ desorption were examined by TPD. The physical and chemical characterization results were used to discuss the reasons for the unexpected synergistic effects observed. The same selective hydrogenation was also conducted in a conventional non-polar organic solvent of n-hexane to examine the roles of scCO₂. The use of scCO₂ was effective for accelerating the hydrogenation of citral and improving the selectivity to CAL.     

Catalytic Oxidation of Oleic Acid in Supercritical Carbon Dioxide Media with Molecular Oxygen

Oxidation of oleic acid was performed over various ordered porous catalysts containing transition metal in supercritical carbon dioxide (scCO₂) media with molecular oxygen. Oleic acid was completely decomposed into mono- and dicarboxylic acids over porous catalysts, viz., mesoporous molecular sieves (CrMCM-41, MnMCM-41, CoMCM-41) and microporous molecular sieves (CrAPO-5, CoMFI, MnMFI) using scCO₂ at 353 K for 8 h. Among the different catalysts studied, microporous and mesoporous catalysts containing chromium, in presence of scCO₂ showed high distribution of azelaic and pelargonic acids as compared to their analogs containing cobalt or manganese. The presence of scCO₂ medium with the catalysts increased the distribution of azelaic and pelargonic acids. The effect of CO₂ pressure, reaction temperature and reaction time on oxidation of oleic acid over CrMCM-41 was also investigated. Additionally it is noticed that the catalyst can be recycled with negligible loss of catalytic activity.     

Asymmetric Epoxidation of Styrene on the Heterogenized Chiral Salen Complexes Prepared from Organo-Functionalized Mesoporous Materials

Organosilane-modified mesoporous materials have been prepared under mild and acidic conditions by a solvent evaporation method using C₁₆TMABr surfactant as a template. The mesoporous samples synthesized in ethanol solvent by using this evaporation method showed a fully disordered pore system, but those obtained under hydrothermal conditions had highly ordered pores. The chiral salen Mn(III) complexes were immobilized on these organosilane-functionalized mesoporous silicas by a grafting method. The catalysts used in the asymmetric epoxidation of styrene and cis-stilbene and the effect of different mesoporous structures on the reactivity was investigated. Similar enantioselectivities were observed by using these heterogenized salen complexes as compared with reaction under homogeneous conditions.     

Gold-Catalyzed Aerobic Oxidation of Benzyl Alcohol: Effect of Gold Particle Size on Activity and Selectivity in Different Solvents

The effect of the size of gold particles deposited on CeO₂ and TiO₂ supports on their catalytic behavior in the aerobic oxidation of benzyl alcohol in different solvents (mesitylene, toluene, and supercritical carbon dioxide) has been investigated. The size of supported gold particles deposited via a colloidal route was in the range 1.3–11.3 nm, as determined by means of EXAFS and HAADF-STEM measurements. The catalytic performance of the supported gold catalysts in the different solvents revealed a significant effect of the gold particle size. Optimal activity was observed for catalysts with medium particle size (ca. 6.9 nm) whereas smaller and bigger particles showed inferior activity. Identical trends for the activity–particle size relationship were found using Au/CeO₂ and Au/TiO₂ for the reaction at atmospheric pressure in conventional solvents (mesitylene, toluene) as well as under supercritical conditions (scCO₂). Selectivity to benzaldehyde was only weakly affected by the gold particle size and mainly depended on reaction conditions. In supercritical CO₂ (scCO₂) selectivity was higher than in the conventional solvents under atmospheric pressure. All catalysts tested with particle sizes ranging from 1.3 to 11.3 nm showed excellent selectivity of 99% or higher under supercritical conditions.     

Process design for the removal of residual monomer from latex products using supercritical carbon dioxide

One of the most important environmental incentives for the polymer industry is to reduce the residual monomer in polymer products. Although several techniques are available for the removal of residual monomer, most methods are rather energy intensive and time consuming. Since supercritical carbon dioxide (scCO₂) is known to have a plasticizing effect on many polymers and is a good solvent for most commonly used monomers, a scCO₂-based process forms an interesting alternative. This paper focuses on the development of a post-emulsion polymerization process, which decreases the amount of methylmethacrylate (MMA) in polyMMA lattices using scCO₂. Typically, the method comprises a counterflow process, in which part of the residual monomer is converted by the increased diffusion inside the polymer particles due to the swelling by scCO₂. In addition, the amount of residual monomer is further reduced by the extraction capacity of scCO₂. Experimental studies have shown that the scCO₂ extraction of MMA is the predominant effect as compared to the enhanced polymerization due to plasticization. From a mass transfer model, the largest resistance in the extraction process appears to be situated in the water phase near the scCO₂ interface due to a relatively small surface area as compared to the overall polymer-water interfacial area. A viability study, including equipment sizing and economic evaluation, has shown that the removal of residual monomer from latex products using scCO₂ in principle yields a process which is both technically and economically feasible, capable to meet the future requirements.     

Hydrogenation of olefins catalyzed by Pd(II) complexes containing a perfluoroalkylated S,O-chelating ligand in supercritical CO2 and organic solvents

Pd(II) complexes with the tridentate ligands heptadecafluorodecyl-thiophene-2-carboxylate (L₁ = SOSR_f1, R_f1 = (CH₂)₂(CF₂)₇CF₃) and heptadecafluorononil thiophene-2-carboxylate (L₂ = SOOR_f2, R_f2 = (CH₂)(CF₂)₇CF₃) have been synthesized, and their catalytic activities for the homogeneous hydrogenation of olefins have been studied with molecular hydrogen in scCO₂ and organic solvents. Both perfluoroalkylated-heterocycle ligands potentially have three donor atom sets (SOO/SOS). Depending on the soft-hard interaction between palladium and the donor atoms of the ligands, different activities for the Pd(II) complexes were observed. The nature of the solvent had an effect upon the hydrogenation of olefins. The effects of temperature and H₂ pressure on the hydrogenation of styrene were also investigated.     

Total replacement of solvent in polyurethane synthesis using carbon dioxide soluble 1,3‐dichlorodistannoxane catalysts

This work demonstrates catalytic synthesis of polyurethanes using 1,3‐dichlorodistannoxane catalysts ( 1 ) in carbon dioxide (CO₂) and carbon dioxide expanded liquids (CXL). Catalytic polyurethane synthesis was also performed in pure organic solvent (dimethylformamide) for comparison. In this study, mainly, 4, 4′‐methylene‐bis‐(phenyl isocyanate) (MDI) as the diisocyanate precursor and ethylene glycol (EG) as the diol precursor were used for polyurethane synthesis. In addition to MDI, hexamethylene diisocyanate (HDI), toluene diisocyanate (TDI), and p‐isocyanatobenzylisocyanate (PIBI) were also used for polyurethane synthesis with different diols or triol in CO₂. Polyurethanes with a molecular weight ranging from 3000 to 70,000 were synthesized depending upon the combination of diisocyanate and diol used. Comparable yields of polyurethanes were obtained using an all butyl group substituted ( 1a ) catalyst in CO₂ (55 bars, 50°C) and in DMF (50°C). Additionally, the yield and polydispersity index (PDI) of polymer formed in neat CO₂ was comparable with those synthesized in the largely used organic solvent DMF. Interestingly, catalyst 1a in CXL (55 bars, 50°C) gave higher yields, and polymers with lower PDI (1.19). Reactions carried out in scCO₂ at 145 bars using PIBI and EG were found to be about three times faster than the reaction carried out in DMF. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Tandem Staudinger-Aza-Wittig reaction in supercritical CO2: Synthesis of a pharmaceutical interest compound

The aim of this research was to investigate the capability of using supercritical CO₂ (scCO₂) as reagent and solvent in the synthesis of pharmaceutical cyclodextrinyl derivatives in smooth reaction conditions. In this way we have followed the kinetics of a synthesis which was previously realized in DMF, by using tandem Staudinger-Aza-Wittig (S.A.W.) reaction in scCO₂ at 200 bars pressure in a 100 mL reactor. The results show that the reaction in scCO₂ showed a second order kinetics with the yield of 92%.     

Host (nanopores of zeolite Y)-guest (oxovanadium(IV) tetradentate schiff-base complexes) nanocomposite materials: synthesis,characterization and liquid phase hydroxylation of phenol with hydrogen peroxide

Oxovanadium(IV) tetradentate Schiff-base complexes; [VO(X₂-haacac)] (X = H, Cl, CH₃ and NO₂), X₂-haacac = substituted bis(2-hydroxyanil)acetylacetone; and encapsulated in the nanopores of zeolite NaY; [VO(X₂-haacac)]-NaY; have been synthesized and characterized. The host-guest nanocomposite materials; [VO(X₂-haacac)]-NaY; was characterized by chemical analysis and spectroscopic methods (FT-IR, UV/VIS, XRD, BET and DRS). The analytical data indicated a composition corresponding to the mononuclear complex of Schiff-base ligand. The characterization data showed the absence of extraneous complexes, retention of zeolite crystalline structure and encapsulation in the nanopores. Substitution of the aromatic hydrogen atoms of the Schiff-base ligand by electron withdrawing groups like −Cl, and −NO₂ has two major effects: (1) retention and concentration of the oxovanadium(IV) complex in the zeolite cavities is enhanced (due to the larger size of the substituents) and (2) the electronic and spectral properties of the encapsulated complex are modified. Liquid-phase selective hydroxylation of phenol with H₂O₂ to a mixture of catechol and hydroquinone in CH₃CN has been reported using oxovanadium(IV) Schiff-base complexes encapsulated in zeolite-Y as catalysts. Reaction conditions have been optimized by considering the concentration of substrate and oxidant, amount of catalyst, effect of time, volume of solvent and temperature. Under the optimized reaction conditions, [VO((NO₂)₂-haacac)]-NaY has shown the highest conversion of 42.3% after 6 h. All these catalysts are more selective toward catechol formation. Encapsulated oxovanadium(IV) complex is catalytically very efficient as compared to other neat complexes for the hydroxylation of phenol and is stable to be recycled without much deterioration.     

Synthesis of core-shell polyurethane-polydimethylsiloxane particles in cyclohexane and in supercritical carbon dioxide used as dispersant media: a comparative investigation

Polyurethane-polydimethylsiloxane particles of tuneable diameter in the range 0.5-5 μm, have been synthesized by step-growth polymerization, both in cyclohexane and in supercritical carbon dioxide (scCO₂) used as dispersant media. The feasibility of the polyaddition between ethylene glycol and tolylene-2,4-diisocyanate, selected as monomers, was first demonstrated in cyclohexane, in the presence of hydroxy- or isocyanate-terminated polydimethylsiloxane used as a surfmer. The polymerizations were then carried out in scCO₂ at 60 °C and above 200 bar, after the solubility of each reactant has been determined in scCO₂, at various pressures and temperatures. Both in cyclohexane and in scCO₂, the reaction extent was determined by means of FTIR spectroscopy. The structure and the morphology of the resulting powdery polyurethane-polydimethylsiloxane ‘core-shell’ material were confirmed by NMR, SEC, TEM and SEM techniques.     

Synthesis of Zirconia with Nanoporous Structure by a Supercritical Carbon Dioxide Microemulsion Route

Supercritical carbon dioxide (scCO₂) fluid as a green solvent for processing can markedly reduce the use of organic solvents. Nanoporous clusters of zirconia (ZrO₂) were synthesized in a [Zr⁴⁺](aq)/scCO₂ microemulsion using hydrazine solution as the precipitating agent, and the resulting nanostructures (surface area, morphology) under different reaction temperatures and pressures were determined by BET and TEM. The inter droplet separation of microemulsions increases with the reaction pressure at a fixed temperature, giving different networked nanostructures. The closed pore diameters were in the range of 5–150 nm (mesopores and macropores) at 17.3 MPa and 80°C.     

Synthesis and characterization amino-salicylaldimine palladium-based complexes and their Suzuki coupling reaction study

A series of amino-salicylaldimine ligands, 5-R-3-morpholinomethyl-salicylaldimine (R = t-Bu, CH₃, Br) have been synthesized and structurally analyzed. These tridentate ligands add to palladium (II) to give high yields of air-stable complexes that are characterized by IR, ¹H NMR and elemental analysis. Crystal structure details of complex 1 are presented. These Pd (II) complexes with amino-salicylaldimine ligands are versatile and efficient Suzuki catalysts towards cross-coupling of a variety of aryl chlorides with phenylboronic acid using DMF as solvent.     

VULCAN-SUPPORTED Pt ELECTROCATALYSTS FOR PEMFCs PREPARED USING SUPERCRITICAL CARBON DIOXIDE DEPOSITION

In this study, supercritical carbon dioxide (scCO₂) deposition was used to prepare vulcan-supported Pt (Pt/Vulcan) electrocatalysts for proton exchange membrane fuel cells (PEMFCs), and the effects of process variables on the properties of the electrocatalysts were investigated. The two different methods used to reduce the organometallic precursor were thermal reduction in nitrogen at atmospheric pressure and thermal reduction in scCO₂. In the former method, the maximum Pt loading achieved was 9%, and this was governed by the adsorption isotherm of the Pt precursor between the scCO₂ phase and the Vulcan phase. By using the latter method, higher Pt loadings of 15% and 35% could be achieved. The Pt/Vulcan catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and cyclic voltammetry (CV). The average particle sizes for Pt/Vulcan 9%, 15%, and 35% catalysts were 1.2, 1.3, and 2 nm, respectively. Electrochemical surface areas obtained from CV data were found to vary with the Pt loading.     

Foaming of PCL/clay nanocomposites with supercritical CO2 mixtures: The effect of nanocomposite fabrication route on the clay dispersion and the final porous structure

Supercritical fluids have been established as alternative foaming agents in various polymers as well as nanocomposite systems. Most recently, supercritical carbon dioxide (scCO₂) has also been used in some studies as a medium of clay dispersion in the polymer matrix providing a solvent-free fabrication route for nanocomposites. In this work, this latter route was followed for the development of porous poly(ɿ-caprolactone) (PCL)/clay nanocomposites after pressure quench. Similarly, PCL/clay nanocomposites were also prepared using the solvent casting and melt blending methods and were then processed with scCO₂ with the batch foaming technique (isothermal pressure quench) to produce their porous counterparts. Poor clay dispersion and non-uniform porous structures were observed when pure CO₂ was used as a dispersion medium for nanocomposite preparation and as a blowing agent, respectively. On the contrary, polymer intercalation and more uniform cell structures were produced when CO₂⿿ethanol mixtures were used as blowing agents.     

Investigation of machinability in milling of Inconel 718 with solid Sialon ceramic tool using supercritical carbon dioxide (scCO2)-based cooling conditions

Milling of hard-to-machining materials is still a challenge since the high cutting temperature caused by the cooling lubrication problems and the property of materials. This paper proposes the use of supercritical carbon dioxide (scCO₂), supercritical carbon dioxide based minimum quantity lubrication cutting fluid (scCO₂-MQL), and supercritical carbon dioxide based minimum quantity lubrication with oil droplets cutting fluid (scCO₂-OoW) as the eco-friendly cooling-lubrication methods for milling of Inconel 718 superalloy. The cutting forces, cutting temperatures, surface roughness, surface topographies, subsurface characteristics and tool wear were performed to quantify the effect of various cooling methods. The results indicated that the application of scCO₂-based cooling conditions was an effective cooling and lubrication technology for the ceramic tool since it could reduce the cutting force and temperature and improve the surface finish with lower peaks and valleys dispersion compared with other cooling conditions. Compared with the scCO₂-MQL, only scCO₂ and dry milling conditions, the topographies of machined surface under the scCO₂-OoW condition have been significantly improved. Furthermore, the scCO₂-OoW cooling technique has facilitated the removal of debris adhering to the ceramic tool and improved lubrication of the cutting zone.     

Sorption of tryalkoxysilane in low-cost porous silicates using a supercritical CO2 method

The internal surface of micro and nanoporous substrates can be modified in terms of charge, functionality or even reactivity or stability by means of bifunctional organic molecules able to self-assembly. This work investigates the impregnation with trialkoxysilanes of porous systems using supercritical carbon dioxide (scCO₂) as a solvent, which combines several advantages such as liquid-like density and high solvating power with gas-like transport properties. Moreover, scCO₂ does not have specific interactions with the substrate and there is no competition for absorption between added reagents and solvent molecules. The work aims at describing a generic liquid-solventless impregnation method applicable to macro (perlite), meso (silica gels and agglomerated nanoparticles) and microporous (zeolite) silica-based substrates using scCO₂. The hydrophobic octyltriethoxysilane was used to impregnate the internal surface of the chosen substrates with the objective of obtaining high capacity oil adsorbents. FTIR, TGA and N₂ adsorption isotherms were used to determine the interactions between the substrate and the silane deposited monolayer. The acquired hydrophobic character was evaluated using the Karl Fischer method to measure the loss in water adsorption capacity after silane grafting.     

Supercritical carbonation of calcium aluminate cement

The microstructural changes occurring during supercritical carbonation (scCO₂) of calcium aluminate cement (CAC) and changes to its strength have been investigated. Cylindrical specimens of CAC cured at different temperatures were prepared and then subjected to scCO₂. It is shown that CAC carbonation in supercritical conditions is accelerated with a positive effect on the compressive strength. Due to the scCO₂ treatment, both conversion and alkaline hydrolysis are avoided. The best behaviour of the studied specimens was attained for samples cured at 25 °C. The residual compounds after the scCO₂ process, i.e. monocalcium aluminate, calcium carbonate and aluminium hydroxide are durable in normal ambient conditions. Complete carbonation of CAC is particularly important for the reinforcement of CAC with polymer fibres to improve its mechanical strength.     

Pincer Type Ditertiary Aminomethylphosphine–Pd(II) Complexes Supported on Multi-Walled Carbon Nanotube: Catalytic Properties in Heck C–C Coupling Reactions

Pincer type aminomethylphosphine–Pd(II) complexes supported on multi-walled carbon nanotube (MWCNT) have been synthesized and characterized using X-Ray diffraction spectrometry, scanning electron microscopy, thermal analysis, energy dispersive X-Ray, Fourier transform infrared spectrometry, transmission electron microscopy (TEM) and ultraviolet–visible spectrometry techniques. The novel complexes were tried as catalysts in Heck C–C coupling reactions. The crystallite size and lattice strain of the MWCNT based compounds were calculated by the Scherrer’s equation. The optical parameters of the MWCNT based structures were analyzed and the band gap enhanced from 4.42 to 4.98 eV. Different solvents (toluene, 1,4-diooxane, DMF and NMP) and bases (Et₃N, Na₂CO₃, NaOAc and K₂CO₃) were tried at different temperatures (80, 100 and 110 °C) in the cross-coupling of bromobenzene with styrene. The optimum yield was found in the presence of K₂CO₃, 110 °C in 1,4-dioxane solvent system.