Solvent-free systems

It is a misconception that most chemicals are manufactured in organic solvents. Most high-volume bulk chemicals are actually produced in solvent-free processes, or at least ones in which one of the reactants also acts as a solvent. Typical examples of such large-scale processes include the manufacture of benzene, methanol, MTBE, phenol and polypropylene. In addition, some heterogeneous gas-phase catalytic reactions, a class of solvent-free processes, are discussed in Chapter 4. 

Despite environmental concerns (Chapter 3), since 1980 MTBE has made a significant contribution to the lowering of VOC emissions from car exhausts. This is due to its clean bum properties, (producing fewer hydrocarbon by-products). MTBE is commonly produced in a fixed-bed reactor by passing a mixture of 2-methylpropene and excess methanol over 

The potential for solvent-free synthesis is relatively large, with examples of many well-known reaction types proceeding quite well under this type of regime these include transesterification, condensation and rearrangement reactions. Many workers have moved away from conventional thermal sources for providing the energy needed for these reactions - 

---

Owing to relatively low viscosity, these resins offer advantages for 100% soHds (solvent-free) systems. Higher filler levels are possible because of the low viscosity. Faster bubble release is also achieved. Higher epoxy content and functionaHty of bisphenol F epoxy resins can provide improved chemical resistance compared to conventional epoxies. 

Electron beam-initiated modification of polymers is a relatively new technique with certain advantages over conventional processes. Absence of catalyst residue, complete control of the temperature, a solvent-free system, and a source of an enormous amount of radicals and ions are some of the reasons why this technique has gained commercial importance in recent years. The modification of polyethylene (PE) for heat-shrinkable products using this technique has been recently reported [30,31]. Such modification is expected to alter the surface properties of PE and lead to improved adhesion and dyeability. 

The focus has largely been on polyether-based solvent-free systems for lith-... 

Kragl and coworkers investigated using organic-solvent-free systems to overcome the thermodynamic limitations in the synthesis of optically active ketone cyanohydrins. With organic-solvent-free systems under optimized reaction conditions, conversions up to 78% with > 99.0 enantiomeric excess (ee) (S) were obtained. Finally, 5 mL of (S)-acetophenone cyanohydrin with an ee of 98.5% was synthesized using MeHNL [52]. 

Von Langermann, J., Mell, A., Paetzold, E. et al. (2007) Hydroxynitrile lyase in organic solvent-free systems to over come thermodynamic limitations. Advanced Synthesis and Catalysis, 349, 1418-1424. 

The polymerization of dicarboxylic acids and glycols using lipase CA catalyst proceeded in a solvent-free system, despite the initial heterogeneous mixture of... 

In addition to their work with solvent free systems, Wang and co-workers reported a water mediated Friedlander quinoline synthesis using hydrochloric acid and conventional heating to synthesize a variety of substituted quinolines in high yields <06TL1059>. 

In chemical syntheses under the action of microwave irradiation the most successful applications are necessarily found to be the use of solvent-free systems [6], In these systems, microwaves interact directly with the reagents and can, therefore, drive chemical reactions more efficiently. The possible acceleration of such reactions might be optimum, because they are not moderated or impeded by solvents. Reactions on solid mineral supports and, in turn, the interaction of microwaves with the reagents on the solid phase boundary, which can substantially increase the rate of the reactions, are of particular interest [7]. 

The classical Pechmann approach for the synthesis of coumarins via the micro-wave-promoted reaction [68] has been extended to solvent-free system wherein salicy-... 

Chiral cyanohydrins are versatile intermediates in the synthesis of a-hydroxy acids, /3-amino alcohols, amino nitriles, a-hydroxy ketones and aziridines. For the synthesis of enantiopure cyanohydrins, the use of hydroxynitrile lyases is currently the most effective approach.Application of an organic-solvent-free system allows thermodynamically hindered substrates to be converted with moderate to excellent yields. With the use of the highly selective hydroxynitrile lyase from Manihot esculenta, the syntheses of several acetophenone cyanohydrins with excellent enantioselectivities were developed (Figure 8.2). (5)-Acetophenone cyanohydrin was synthesized on a preparative scale. ... 

The use of organic-solvent-free systems can be applied to the cyanohydrin synthesis of a wide range of acetophenone derivatives (Table 8.2) electronegative substituents (e.g. fluorine) facilitate high conversions and enantiomeric excess of the product, whereas electropositive substituents (e.g. methoxy-) result in low to no conversion into the corresponding cyanohydrins. 

The topochemical polymerization of 1,3-diene monomers based on polymer crystal engineering can be used not only for tacticity but also for the other chain structures such as molecular weight [ 102], ladder [84] or sheet [ 103] structures, and also polymer layer structures using intercalation reactions [ 104-107]. Some mechanical and structural properties have already been revealed with well-defined and highly or partly crystalline polymers [ 108-111 ]. A totally solvent-free system for the synthesis of layered polymer crystals was also reported [112]. 

In this equation. Act is taken as the maximum possible surface tension lowering. Hence for a solute-free continuous phase, Aa is the difference between the interfacial tension for the solvent-free system and the equilibrium interfacial tension corresponding to the solute concentration in the dispersed phase. Equation (10-6) indicates a strong effect of the viscosity ratio k on the mass transfer coefficient as found experimentally (LI 1). For the few systems in which measurements are reported (Bll, Lll, 04), estimates from Eq. (10-6) have an average error of about 30% for the first 5-10 seconds of transfer when interfacial turbulence is strongest. 

The scope and limitations of biocatalysis in non-conventional media are described. First, different kinds of non-conventional reaction media, such as organic solvents, supercritical fluids, gaseous media and solvent-free systems, are treated. Second, enzyme preparations suitable for use in these media are described. In several cases the enzyme is present as a solid phase but there are methods to solubilise enzymes in non-conventional media, as well. Third, important reaction parameters for biocatalysis in non-conventional media are discussed. The water content is of large importance in all non-conventional systems. The effects of the reaction medinm on enzyme activity, stabihty and on reaction yield are described. Finally, a few applications are briefly presented. 

Organic solvents Supercritical fluids Solvent-free systems... 

From the industrial point of view, it is advantageous to work with minimal amounts of solvents. This minimises the reactor size and thereby reactor cost. The extreme case is to omit the solvent completely. The use of solvent-free systems is attractive also because solvents can cause many problems (for example fire hazards, environmental problems and high costs). It has been proven possible in many cases to carry out bioconversions in solvent-free mixtures of substrates. Since most substrates are organic compounds, these mixtures behave like systems containing organic solvents and the advantages mentioned... 

The methods described below have been used for enzyme solubilisation in organic media but they should be applicable to supercritical media and solvent-free systems, as well. 

Esters are common components in cosmetics and skin-care products. They can be synthesized from fatty acids and alcohols using either chemical or enzymatic reactions. The chemical reactions are normally catalysed by acid catalysts. Enzymatic synthesis is carried out under milder conditions and therefore it provides products of very high purity. A range of esters such as isopropyl palmitate and isopropyl myristate are now produced industrially using enzymatic synthesis. The reactions are carried out in solvent-free systems using an immobilised lipase as catalyst. In order to get high yields in the reactions, water is removed continuously. 

Esterification between oleic acid and oleyl alcohol, catalyzed by the Mucor miehei immobihzed hpase in a batch-stirred tank reactor with supercritical carbon dioxide as solvent produced higher reaction rates at supercritical conditions than in the solvent-free system (Knez et al., 1995). 

A process based on the use of an immobilized C. antarctica lipase (Novozym 435) has been developed in order to perform direct esterification of natural alcohols with carboxylic acids in a solvent-free system. Based on the system represented in Figure 11.2, the process had to overcome the problem of recycling the nitrogen, once the reaction was over. 

Catalysis in reaction systems with undissolved substrates and products is not restricted to biocatalysis. High yields in sobd-state synthesis, sohd-to-sohd reactions, and solvent-free systems have also been reported for aldol condensation, Baeyer-Villiger oxidation, oxidative coupling of naphthols, and condensation of amines and aldehydes [1, 2]. 

With a finite value of necessarily some intramolecular hydrodynamic interaction or shielding must occur. The importance of eq. (3.53) lies at the present time, in the fact that it can be adapted for concentrated, solvent free systems like polymer melts. As Bueche (13) pointed out, in these systems every chain molecule is surrounded by chain molecules of the same sort. As all these molecules are necessarily equivalent, one cannot speak of a hydrodynamic shielding effect. This would imply that certain chains are permanently immobilized within the coils of other chains. The contrary is expected, viz. that the centre of gravity of each chain wiH independently foHow, in the average, the affine deformation of the medium as a continuum. From this reasoning Bueche deduces that the free-draining case should be applicable to polymer melts. Eq. (3.53) is then used (after omission of rj0) for the evaluation of an apparent friction factor . After introduction of this apparent friction factor into eq. (3.50), the set of relaxation times reads ... 

However, for some specific reactions, solvent-free systems are preferred because of their higher yields. The main area of development should be related to the hydrolysis of glycerides, transesterification, esterification and inter-esterification reactions. As lipases have high and stable activity in SC CO2 (even at the high temperature) an intensive development is expected. 

Advantages of high pressure crystallization are simple control of process parameters, and a solvent-free system. 

---