W/CO2 microemulsion

Figure 1. The conceptual diagram for the catalyst recovery and recycle method based on W/CO2 microemulsions...

Our system selection for studies on the stability of the W/CO2 microemulsions in the presence of organometallic catalysts is based on the hydroformylation of higher olefins. This reaction involves the formation of branched or linear aldehydes by the addition of H2 and CO to a double bond according to Scheme 1. The linear aldehydes are the preferred products and the selectivity in such reactions is usually expressed in terms of n iso ratio, which is the ratio of the linear aldehyde to the branched aldehyde. When conducted in the aqueous phase, the reaction is catalyzed with complexes formed in-situ from Rh(CO)2acac and 3,3 ,3 -Phosphinidynetris (benzenesuifonic acid), trisodium salt (TPPTS) in the presence of synthesis gas. 

We were able to form stable W/CO2 microemulsions when Rh(CO)2 acac, TPPTS, synthesis gas, water, di-HCF4, olefin and CO2 were combined together at appropriate concentrations. The olefins were also hydroformylated. The detailed compositions of the microemulsion systems which were investigated are listed below Figure 4. The product aldehyde concentrations were determined... 

Additional data on hydroformylation of 1-pentene, 1-octene and ethyl acrylate are provided in Table 1. In all the runs, the solutions became clear and yellow after a period of 10 minutes, which indicated the formation of the microemulsion with the catalyst formed in situ inside the water droplets. The solutions were clear and homogeneous during the entire run, which definitely excludes reaction via a biphasic pathway. Because of equipment limitations, the highest reaction temperature we investigated was 87.1 C. The stability of the W/CO2 microemulsion system at such a high temperature is remarkable. At the conditions employed, conversions ranged from 6 to 75%. The increase of temperature and the addition of NaOH were found to increase the reaction rate. The initial reaction rate for 1-pentene is about two times higher than that of 1-octene. In studies on hydroformylation of different olefins in aqueous biphasic systems, Brady et al. [2/] found that there is a marked dependence of the reaction rate on the solubility of the terminal olefins in water. The data shown in... 

The data presented in Table 1 also show that pH has an effect on activity in the W/CO2 microemulsion system. When 0.25M sodium hydroxide solution was used instead of pure water, the reaction rates for the hydroformylation of 1-pentene increased even though the temperature was decreased from 87 °C to 66 °C. The pH of the dispersed phase of a W/CO2 microemulsion system has been determined as 5 at a NaOH concentration of 0.25M and as 2.8 without NaOH [20], The increase in the rate might be due to the higher pH value aiding the formation of active species, such as the complete dissociation of the precursor to the monomeric species [19],... 

In summary, we demonstrated that it is possible to carry out reactions in W/CO2 microemulsions catalyzed by water soluble organometallic complexes. The microemulsions were stable in the presence of several electrolytes and the cloud points were not affected even at high electrolyte concentrations. In the future, we would like to investigate the factors controlling activity and selectivity and determine the feasibility of a catalyst recovery and recycle system based on such a microemulsion system. 

There are, of course, cases where the concept of correspondence between pool size and particle size seems to have been realized. One example is the synthesis of cadmium sulfide particles in W/CO2 microemulsions [229]. This work shows that the average nanocrystal radius was comparable with the corresponding water pool size. 

The water-sc CO2 system was also used by Ohde etal. [227]. The water phase contained dissolved silver nitrate and the surfactants were NaAOT and a perfluoropolyether-phosphate. The w value with only AOT was 12. The W/CO2 microemulsion formed at 38 C and 200 atm in a high pressure cell. After a stirring period of 1 h, one of the following reducing agents was injected into the reactor at 250 atm ... 

Surfactants have been designed to lower y in C02-based systems. The first generation of research involving surfactants in SCFs addressed water-in-oil (W/O) microemulsions and polymer latexes in ethane and propane, as reviewed elsewhere. (43-45). This work provided a foundation for studies in CO2, which has weaker van der Waals forces (a/v) than ethane. Surfactants with both C02-philic and C02-phobic segments have been used to form microemulsions, emulsions, and organic polymer latexes in CO2. 

Figure 3 Van der Waals interaction strength for W/C microemulsions versus the density of CO2 divided by the density of CO2 at the cloud point for temperatures from 15 to 80°C in the one-phase region. The interaction strength at the phase boundary where the droplets flocculate is 21.2 for a hard-sphere fluid with van der Waals attraction. (From Ref. 49.)...

Stable W/C macroemulsions for either liquid or supercritical CO2 have been formed with the surfactants (PFPE 000 )2 Mn2+ (13), PFPE COO-NH4 " (672-7500 MW) (9), and block copolymer pluronic surfactants composed of poly(propylene oxide) (PPO) and poly(ethylene oxide) (PEO) (51), PDMS and poly(acrylic acid) (PAA), or poly(methacrylic acid) and PDMS and PEO (2). The emulsions may be formed by shear through a 130 gm capillary or commercial homogenizer. The ratio of water to CO2 has been varied from 9 1 to 1 9 with less than 1 wt % surfactant. W/C microemulsions, on the other hand, contain less than 5 wt % water for this level of surfactant owing to the much higher interfacial area. 

In addition to w/c microemulsions, o/c microemulsions may be formed for systems with strong surfactant adsorption. The area occupied by PFPE-C00 NH4 at the interface between 600 molecular weight polyethylene glycol (PEG) md CO2 is 440 per molecule based upon measurement of the interfacial tension versus surfactant concentration [21]. This surface coverage is sufficient for microemulsion formation as was verified with phase behavior measurements. Only 0.55 wt% of 600 molecular weight polyethylene glycol is soluble in CO2 at 45 °C and 300 bar. With the addition of 4wt% PFPE-C00 NH4 surfactant, up to 1.8 wt% is solubilized. The additional PEG resides in the core of the microemulsion droplets, consistent with the prediction from the adsorption measurement. 

Figure 2.4-11 Conversion of benzyl chloride to benzyl bromide at 65 °C and 276 bar in a w/c microemulsion formed with 1.4 wt% PFPE-C00 NH4 at various initial concentrations of KBr in water and benzyl chloride in CO2.

In 1993, perfluoropolyether (PFPE) carboxylates, with average molecular weights between 2,500 and 7,500, were reported to be soluble in liquid CO2 (19). However, these high MW polymers were not effective at stabilizing w/c microemulsions. Later, Johnson et al. formed w/c microemulsions with an ammonium carboxylate PFPE (PFPE-COO NH4 surfactant of only 740 MW (30). Success with these surfactants was attributed to the chemical structure itself. PFPE constitutes an extremely C02-philic tail group, accentuated by the presence of pendant fluoromethyl groups, which tend to increase the volume at the interface on the CO2 side and thus favor curvature around the water. 

Scattering techniques provide the most definite proof of micellar aggregation. Zielinski et aL (34) employed SANS to study the droplet structures in these systems. Conductivity measurements (35) and SANS (36) were also used to study droplet interactions at high volume fraction in w/c microemulsions formed with a PFPE-COO NH4 surfactant (MW = 672). Scattering data were successfully fitted by Schultz distribution of polydisperse spheres (see footnote 37). A range of PFPE-COO NH/ surfactants were also shown to form w/c emulsions consisting of equal amount of CO2 and brine (38-40). 

Efforts have been made to develop hydrocarbon systems for CO2, as they could present significant advantages over high-cost fluorocarbon or siloxane counterparts. Recent advances are covered in section 3 of this article. Solubility of hydrocarbon materials in CO2 may be achieved by the addition of a polar cosolvent to CO2 to improve solvent polarity. For instance, AOT was shown by Ihara et ah to be completely soluble in CO2 with edianol as a co-solvent (59). Along similar lines, Hutton et aL in 1999, formed w/c microemulsions with 0.03 M AOT and 15 mol % ethanol or 10 mol % pentanol (60,61). 

Another approach to use AOT as a surfactant for CO2 microemulsion consists in mixing AOT with a PFPE-based surfactant. This may seem quite disconcerting at first sight, as fluorocarbons and hydrocarbons are notoriously known to be immiscible. However Fulton et aL (67) reported microemulsion formation by mixing 15 mM of AOT and 30 mM of PFPE-PO4 up to w = 12. These systems could be successfully used as micro reactors to synthesize metallic silver (3, 4) and copper nanoparticles (5) and to carry out catalytic hydrogenations (5). Eastoe et al also showed that hydrocarbon surfactants analogous to AOT with branched tails were CO2 compatible (68). More detail is given in section 3 below. 

Based on the polarity difference between CO2 and the interior of the micelles, w/c microemulsions have found many applications as extraction media. Furthermore, by modifying pressure and temperature, solvent quality may be changed and it becomes, therefore, possible to exert a real control over the extraction process uptake of solutes inside micelles may be varied. This may be of use for separations/extractions involving bio-chemicals and proteins. In conventional solvents their separation from the reaction medium can be quite complicated, involving tedious processes such as fluid-fluid extraction, decantation, chromatography column, filtration, precipitation. Use of supercritical fluid technology with extraction in reverse micelles seems advantageous for proteins (e.g. 19, 76). This process was also used for the extraction of metals (77-79) and more recently of copper from a filter paper surface (1). 

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