Liquid environment-friendly processes

This chapter reviewed recent advances in the preparation of hollow oxide nanoparticles by liquid-phase deposition methods. Hollow particles with various compositions, controlled shell structures, and controlled shapes and sizes have successfully been obtained by employing either template or template-free method, and many interesting properties arising from their unique structural features have been reported. Large-scale and low-cost production by environment-friendly processes promises practical applications in many fields. 

The three-step cumene process, including the liquid-phase reactions, is energyconsuming, environmentally unfavorable and disadvantageous for practical operation. The process also produces the unnecessary by-product acetone stoichio-metrically. Furthermore, the intermediate, cumene hydroperoxide, is explosive and cannot be concentrated in the final step, resulting in low phenol yield ( 5%, based on the amount of benzene initially used). Thus, direct phenol synthesis from benzene in a one-step reaction with high benzene conversion and high phenol selectivity is most desirable from the viewpoints of environment-friendly green process and economical efficiency. 

Non-harmful environmentally friendly processes are classified into two types. One of them is the closed system, as shown in Fig. 1. Generally, the substances and materials that have been used for conventional processes have had many merits and priorities from the viewpoint of performance. They have their own unique reasons why they have been used. The closed system makes the process completely closed. It often means zero emission to the outside environment. In some cases, the harmful gasses or liquids produced as byproducts or those that remain in the process might be reused for other purposes within the process. At any rate, no harmful materials would be discharged outside of the system (when using a closed system). The concrete examples are shown and explained in chapter Process Changes . 

One advantage of ILs is their negligible vapor pressure hence, they do not evaporate like liquids normally do. Over the last years the interest in ILs as environment-friendly green alternative to traditional highly volatile organic compounds (VOCs) has increased immensely. VOCs contribute to a great deal to atmospheric pollution by industrial processes and hence contribute to climate changes and affect human health. 

The choice of solvents for enzymatic reaction has been widened from organic solvents to various types of solvents such as supercritical fluids, ionic liquids, etc. The enzymatic reaction in organic solvent has been reported already in 1970s, the first biocatalysis in ionic liquids [4] was in 2000, and the first biocatalysis in supercritical fluids [5] was in 1985. Currently four kinds of liquid or fluid solvents, aqueous, organic solvents, ionic liquids, and supercritical fluids, are available for biocatalysis as shown in Figure 3.2. Moreover, biphasic or triphasic solvent systems consisting of two or more kinds of the solvents are also often employed for biocatalysis. Solid phase of immobilized enzymes and/or hydrophobic polymer to adsorb substrate and product may also exist. The performance of a biocatalyst depends significantly on the solvent system. The best medium should provide optimum reaction rates and simplify work-up procedure to make the process both economical and environment friendly. 

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