Use of Conventional Solvents

The carboxylation of epoxides (Equation 7.13) has long been known (IG Farben, 1943) [12, 117, 118], and today is available on stream from several production plants. 

The reaction of epoxides with C02 affords either CCs or polymers [119], and many reports have been made [120-125] and different active catalysts described [126-130] such as alkyl ammonium-, phosphonium-salts and alkali metal halides, in this respect. The main drawbacks here are the need for a high catalyst concentration, a high pressure (5 MPa of C02), and a temperature ranging from 370 to 400 K. The recovery of the catalysts for reuse is also a key issue, and in order to simplify the recovery process various hybrid systems have been developed, an example being that prepared by coupling 3-(triethoxysilyl)propyltriphenylphosphonium bromide with mesoporous silica [131]. In this case, the reaction was carried out in the absence of solvent, under very mild conditions (1 MPa, 263 K, 1 mol% loading of catalyst, 6h), such that the hybrid catalyst could be recovered and recycled several times. 

Very often, amides such as dimethylformamides (DMF) or dialkylacetamides (DAA) have been used as solvents in reactions where they may themselves promote the carboxylation of epoxides [139], if only to a limited extent. 

As noted above, the carboxylation of epoxides may afford polycarbonates, whether using Al-porphyrin complexes [144,145] or Zn-compounds [146], In fact, the Al-catalysts, which were the first to be described, are currently used in production plants (see Chapter 8). 

Functionalized CCs can also be obtained in good yields under mild conditions from epoxides and C02 by using an electrochemical procedure [149, 150]. For this, the CC formation is catalyzed by Ni(cyclam)Br and is carried out in singlecompartment cells fitted with a magnesium anode. The presence of functional groups such as chlorine, bromine, ether, ester or olefins is compatible with the reaction conditions. 

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Some insights can be obtained from the mechanisms of extraction in two-phase systems. As in conventional solvent extraction, the mechanism of transfer of the solute is either physical or chemical. In conventional solvent extraction, physical transfer is used for species that prefer the organic phase, i.e., their distribution coefficient D allows the use of conventional solvent extraction. In some cases of low solubility in the organic phase, microemulsions have proved to enhance extraction. An important example in this... 

However, the application of poly(aryl ether ketone)s in membrane fabrication has been limited owing to their intractability, which prevents the use of conventional solvent-based methods of membrane casting. Several routes are known for the preparation of poly(aryl ether ketone) membranes. 

The selected examples discussed above show that gold catalysis can be conducted in solvent-free conditions, water or ionic liquids. Although the vast majority of gold-catalysed protocols make use of conventional solvents, these examples should encourage researchers in the search for more sustainable protocols to explore gold-promoted reactions in neat or non-conventional solvents. 

The production of very high grade olive oils using dense CO2 is another possible commercial possibility. Once again, the potential of mild processing conditions without the use of conventional solvents would be of great advantage. 

This early work was followed intermittently by a flurry of activity that intensified diuing the 1970s and 1980s, partly as a result of environmental concerns over the use of conventional solvents. Major commercial processes currently in use are the SCE of caffeine from coffee and tea, the SCE of spice aromas, and the fractionation and purification of polymers. Processes under investigation include the treatment of wastewaters, activated carbon regeneration, and the SCE of edible oils and therapeutic agents from plant materials. 

In recent decades the development of preconcentration steps to be implemented prior to analytical determinations of trace level compounds has been explored in considerable depth. With a view to eliminating or at least minimising the use of organic solvents used in conventional liquid-liquid extraction, other methodologies have been developed, such as membrane extraction, solid-phase extraction, solid-phase microextraction, etc. 

The difficulties encountered in the early studies of anionic polymerization of methyl methacrylate arose from the unfortunate choice of experimental conditions the use of hydrocarbon solvents and of lithium alkyl initiators. The latter are strong bases. Even at —60 °C they not only initiate the conventional vinyl poly-addition, but attack also the ester group of the monomer yielding a vinyl ketone1, a very reactive monomer, and alkoxide 23). Such a process is described by the scheme. 

Very recently, we have developed one-pot synthesis of FePt nanoparticles larger than 5 nm with controlled composition by the polyol reduction of Pt(acac)2 and Fe(acac)3 in excess ligands without using the conventional solvents [23]. Figure 8 presents the TEM images... 

Applications References [99-102] have reported fractional extraction of cresolic and phenolic antioxidants from PE. Conventional analysis of PVC compounds requires the subsequent use of various solvents... 

As the vast majority of LC separations are carried out by means of gradient-elution RPLC, solvent-elimination RPLC-FUR interfaces suitable for the elimination of aqueous eluent contents are of considerable use. RPLC-FTTR systems based on TSP, PB and ultrasonic nebulisa-tion can handle relatively high flows of aqueous eluents (0.3-1 ml.min 1) and allow the use of conventional-size LC. However, due to diffuse spray characteristics and poor efficiency of analyte transfer to the substrate, their applicability is limited, with moderate (100 ng) to unfavourable (l-10pg) identification limits (mass injected). Better results (0.5-5 ng injected) are obtained with pneumatic and electrospray nebulisers, especially in combination with ZnSe substrates. Pneumatic LC-FI1R interfaces combine rapid solvent elimination with a relatively narrow spray. This allows deposition of analytes in narrow spots, so that FUR transmission microscopy achieves mass sensitivities in the low- or even sub-ng range. The flow-rates that can be handled directly by these systems are 2-50 pLmin-1, which means that micro- or narrow-bore LC (i.d. 0.2-1 mm) has to be applied. 

Whereas the use of conventional fast atom bombardment (FAB) in the analysis of polymer/additive extracts has been reported (see Section 6.2.4), the need for a glycerol (or other polar) matrix might render FAB-MS analysis of a dissolved polymer/additive system rather unattractive (high chemical background, high level of matrix-, solvent- and polymer-related ions, complicated spectra). Yet, in selected cases the method has proved quite successful. Lay and Miller [53] have developed an alternative method to the use of sample extraction, cleanup, followed by GC in the quantitative analysis of PVC/DEHP with plasticiser levels as typically found in consumer products (ca. 30 %). The method relied on addition of the internal standard didecylphthalate (DDP) to a THF solution of the PVC sample with FAB-MS quantitation based on the relative signal levels of the [MH]+ ions of DEHP and DDP obtained from full-scan spectra, and on the use of a calibration curve (intensity ratio m/z 391/447 vs. mg DEHP/mg DDP). No FAB-matrix was added. No ions associated with the bulk of the PVC polymer were observed. It was... 

During the past 30 years, there have been significant developments of parenteral disperse formulations. The use of parenteral emulsions can overcome the problems of low aqueous solubility and water hydrolysis of many drugs [184, 185]. Such formulations can avoid the use of conventional co-solvent systems and the undesirable effects caused by precipitation of drugs at the injection site. Recent developments of parenteral disperse formulations have the potential to provide sustained release and targeting of drugs [186-189],... 

From the studies covered in this chapter, it can be concluded that a completely green chemical process in the synthesis of this kind of material is still a challenge. Some protocols, despite using non-toxic precursors, are time- and/or energy-consuming processes or require the use of non-friendly and non-recyclable solvents. Reaction times in microwave-assisted reaction processes have shown to be shorter. On the other hand, the substitution of conventional solvents for chemical and thermally stable I Ls allowed the reutilization of the solvent and also provided control of the size and shape of NPs. 

Cyclic ketals (potential cosmetic ingredients) were obtained in excellent yields from a cineole ketone under the action of microwaves in solvent-free conditions or in toluene. The results reported compared very favorably with those obtained by use of conventional heating (Eq. (23) and Tab. 3.9) [76]. 

Colloidal catalysts in alkyne hydrogenation are widely used in conventional solvents, but their reactivity and high efficiency were very attractive for application in scC02. This method, which is based on colloidal catalyst dispersed in scC02, yields products of high purity at very high reactions rates. Bimetallic Pd/Au nanoparticles (Pd exclusively at the surface, while Au forms the cores) embedded in block copolymer micelles of polystyrene-block-poly-4-vinylpyridine... 

The use of alternative solvents in hydrogenation and hydroformylation reactions has developed at an incredible rate over the last few years. Many elegant systems have been designed which offer cleaner alternatives to those carried out in conventional organic solvents. Apart from the attractiveness of the separation process, catalyst lifetimes can be extended which represents another major advantage. In some cases, conventional organic solvents are completely removed from the system. 

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