Reaction monophasic condition

C02 as adjustable-solvent In this case, the reaction is carried out in scCOz under truly monophasic conditions and the catalyst is selectively precipitated through a change in temperature and/or pressure. Again, product isolation is achieved by SCF extraction. 

Behr et al. [83] also tested the linear diols 1,3-propanediol and 1,4-butanediol under the aqueous biphasic conditions described above. Yields of up to 60% could be obtained for 1,3-propanediol, but conversion dropped considerably as a yield of 31% was recorded for 1,4-butanediol. The TOFs for ethylene glycol (339 h ) and 1,4-butanediol (134 h-1) obtained under identical conditions clearly reflect the drop in activity upon elongation of the carbon chain. Palkovits et al. observed a different trend when they tested various diols under monophasic conditions (Figs. 6 and 7). Reaction conditions were optimized for 1,3-propanediol (butadiene/substrate ratio of 2 1, 353 K, 0.01 mol% Pd, TOMPP/Pd 5 1) giving the mono-telomer in 61% yield and 81% selectivity at 76% conversion with a TOF of around 21,000 h. ... 

Although the biphasic properties of fluorous-organic systems are desirable for separations, monophasic conditions would favour enhanced reaction rates. Therefore, it is important to know the general miscibilities of fluorous solvents and the effect of temperature (Tables 7.2 and 7.3). In Table 7.2, the temperature given for the phase separation is a consulate or upper critical solution temperature. However, these temperatures should only be taken as a guide, as... 

The Mizoroki-Heck reaction was carried out in water/scCOz and ethylene glycol /scC02 using the typical sulfonated triphenylphosphine ligand TPPTS [56]. The reaction is claimed to occur under monophasic conditions although this seems unlikely under the C02 pressures and temperatures with the amounts of catalyst and co-solvent employed. Catalyst recycling was achieved by phase separation after... 

Considerable scientific argument has arisen around the question of whether supercritical hydrogenation reactions proceed faster and more efficiently in either a single or multiple phases indeed conflicting reports have been published [44— 47]. Much of this debate has revolved around the LHSV of a reaction, but this parameter only addresses part of the issue from an industrial perspective. Other important factors which have to be taken into account include catalyst lifetime, overall conversion, and product selectivity, as well as the solvent compression costs need. The situation has been at least partly resolved by a key paper by Nunes da Ponte and co-workers [10]. They have shown that biphasic reactions can sometimes be faster than monophasic ones, because the concentration of subshate (as opposed to H2) is lower under monophasic conditions. 

The measurement of these phase equilibria clearly reveals that, for mixtures with a composition in excess of around 5% isophorone, quite substantial pressures and temperatures are required to render the system monophasic, a condition that has been reported to be essential for efficient and rapid hydrogenation [46]. By contrast, it has been shown that this reaction can be carried out with excellent selectivity and conversion with as much as 50% isophorone in the reaction stream, conditions that are clearly not single-phase. Furthermore, when conditions that facilitate single phase reactions are employed, a loss of desired product selectivity is observed as the high temperatures that are required often lead to the formation of unwanted side products. 

Although fluorous reactions can be conducted under heterogeneous liquid/Uquid biphasic conditions, from a rate standpoint it will normally be advantageous to operate under homogeneous monophasic conditions, as in the top sequence in Figure 3.2. For this reason it is important to know at what temperatures various fluorous and non-fluorous solvents become miscible (Table 3.5). 

This section describes the horseradish peroxidase-catalyzed synthesis of both homo- and copolymers of aromatic polymers based on phenols, naphthols, aniline, and their derivatives. Syntheses of novel optically active polymers are studied by changing the environment in which the enzyme functions, along with the organization of the monomers in the reaction mixture. To this objective, enzyme-catalyzed polymer syntheses are carried out in bulk monophasic conditions in which the solvent is miscible with water, biphasic solvent systems in which the solvents used for the syntheses are not miscible with water, and oil-in-water system in the presence of a detergent called reverse micelles. These experimental approaches are shown schematically in Fig. 4. 

Lin and coworkers disclosed that, at room temperature, nonenzymatic chemical addition was still observed in a water-organic solvent biphasic reaction system, though the volume of aqueous phases was relative small. Lin developed a method of preparing an active enzyme meal that contained essential water to retain its power for catalysis and found a new catalytic reaction system by application of the prepared meal in a nonaqueous monophasic organic medium (Figure 5.7). There was no problem over a wide range of temperature (from 0-30 °C) when the reactions were carried out under micro-aqueous conditions [50]. 

Figure 8.4. Chemoselectivity as a function of system pressure during the hydroformylation of long chain olefins with Rh/PEt3 catalysts. The change in chemoselectivity has been correlated with the transition from biphasic to monophasic reaction conditions...

Another way to produce biphenyl derivates using flow was described by Leeke et al. [34] where they performed a Pd catalyzed Suzuki-Miyaura synthesis in the presence of a base. First experiments were carried out in toluene/methanol solvent. A reaction mixture was passed through the encapsulated Pd filled column bed length 14.5 cm (some cases 10 cm) x 25.4 mm id. 45 g of PdEnCat. Base concentration, temperature and flow rate were optimized and at optimum parameters (0.05 M base concentration, 100°C and 9.9 mL/min) the conversion was 74%. Then the reaction was performed under supercritical conditions using supercritical CO2 at high pressure and temperature. After optimizing the concentration of base, flow rate, pressure and temperature, the highest conversion rate (81%) was observed at 166 bar and 100°C where the reactant mixture was monophasic in the supercritical state. This system is able to produce 0.06 g/min of the desired product. 

Laser pyrolysis has been shown to produce a wide variety of crystalline transition metal nitride and carbide nanoparticles with diameters as small as 2 nm. The nanopowders are in many cases monophasic and single crystalline. By varying the reaction conditions it is possible to control the particle diameter, and in some cases, the crystalline phase produced. Improvements in the synthesis are needed to control surface oxidation. 

When, under identical conditions, ascorbic acid was used instead of mercaptoethanol, the reaction gave products with 3°/2° carbon reactivity of 0.28-0.42, suggestive of an autoxidation process (12). Furthermore, the kinetics of the reaction are biphasic for 2-mercaptoethanol and monophasic for ascorbic acid. These kinetics are consistent with the generation of a new catalytic system by the coordination of the thiol to the ferric center(s). For either reductant, bleaching of the complex was observed within minutes in the absence of substrate. 

Much effort has been devoted to find inexpensive formulations compatible with an economical process. To obtain an inverse monophasic microemulsion, special conditions have to be met. Their main parameters are as follows surfactant concentration, HLB of the surfactant(s), temperature, nature of the organic phase and composition of the aqueous phase. A preferred class of surfactants are those forming microemulsions without any added cosurfactant (alcohol). The presence of an alcohol is indeed liable to favor chain transfer reactions, limiting the range of attainable molecular weights (19,20) In addition, the dilution procedure of these microemulsions containing alcohols is not trivial since the continuous phase consists of a solvent mixture of unknown composition. 

Not surprisingly, fhe classical Os-catalyzed asymmetric dihydroxylation of olefins (cf. Section 2.2) continues to be of interest. The basic principles, such as type of catalyst, stay relatively fhe same and instead efforts are concentrated on making the reaction more suitable for operation under process-like conditions. A modification fhat could improve fhe operabihty is to replace the conventional t-BuOH/ H2O solvent mixture by ionic liquid containing mixtures, either as a monophasic... 

Monophasic Ln x MnOs (Ln = La, Nd) perovskites with high surface areas (8-27 m /g) were synthesised at mild conditions by the freeze-drying method, and were found to be active for the catalytic combustion of ethane at low temperatures (573 to 648 K). As a general troid, the substitution of the rare earth cation by potassium decreased the intrinsic activity, reduced the reaction order in oxygen and, for the more substituted samples (x>0.10), it increased the selectivity to ethene. It was found that the rare earth cation also influenced the catafytic activity of the substituted perovskites. These effects were analysed in terms of structural modifications induced by the introduction of potassium in the perovskites. 

Biocatalysis in supercritical fluids, fluorous solvents, and under solvent-free conditions was recently reviewed (80). In this book, de Geus et al (17), Villarroya (41) and Bruns et al (32) all provide important examples of how supercritical CO2 can be used for enzyme-catalyzed reactions. Furthermore, Srienc et al (38) used ionic liquid media for enzyme-catalyzed polymerizations of p-butyrolactone in order to prepare poly(hydroxyalkanoic acids), PHA. The role of ionic liquids was to both maintain enzyme activity and propagating chain solubility so that high molecular weight products could be obtained in monophasic media. 

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