Sonocatalysis – The Effects of Sound Generated Waves on Catalytic Chemistry
with the increase of the phase boundary the chemical reaction rate rises as the phase boundary is the active area, where the chemical reaction can take place.
Phase Transfer Catalysis Phase transfer catalysis (PTC) is a special form of heterogeneous catalysis and is known as a practical methodology for organic synthesis. By using a phase transfer catalyst, it becomes possible to solubilise ionic reactants, which are often soluble in an aqueous phase but insoluble in an organic phase. General advantages of phase transfer catalysis are short preparation, simple experimental procedures, mild reaction conditions and the use of inexpensive and environmentally benign reagents, such as quaternary ammonium salts and solvents and the possibility of conducting large-scale preparations.2
A variety of liquid–liquid and liquid–solid reactions have been intensified and made selective by using simple phase-transfer (PT) catalysts such as quats and polyethylene glycol-400, which allow ionic species to be ferried from the aqueous phase to the organic phase. Thus, the problems associated with extremely low solubility of the organic reactants in the aqueous phase can be overcome. In the pesticide and pharmaceutical industries, PTC is used extensively and has changed the fundamentals of business.3
By the application of ultrasonic irradiation, the reaction time for PTC can be substantially reduced, resulting in complete avoidance of phase transfer catalyst. In combination with ultrasound, PTC is an invaluable chemical methodology for organic synthesis from two and more immiscible reactants: PTC enables the more efficient and cost-effective use of raw materials in chemical processes. The enhancement of chemical reactions by PTC is an important tool for chemical production that can be improved dramatically by the use of ultrasound.
Examples for Ultrasonically Promoted Phase Transfer Catalysis Reactions • Synthesis of new N’-(4,6-disubstituted-pyrimidin-2-yl)-N-(5-aryl-2- furoyl)thiourea derivatives using PEG-400 under ultrasonication.4
• The ultrasonically assisted synthesis of mandelic acid by PTC in ionic liquid shows a significant enhancement in reaction yields under ambient conditions.5
• Kubo et al.6
report the ultrasonically assisted C-alkylation of phenylacetonitrile in a solvent-free environment. The effect of the ultrasound in promoting the reaction was attributed to the extremely large interfacial area between the two liquid phases. Ultrasonication results in a much faster reaction rate than mechanical mixing.
•
Sonication during the reaction of carbon tetrachloride with magnesium for the generation of dichlorocarbene results in a higher yield of gem-dichlorocyclopropane in the presence of olefins.7
•
Ultrasound provides the acceleration of the Cannizzaro reaction of p-chlorobenzaldehyde under phase transfer conditions. Of three phase transfer catalysts – benzyltriethylammonium chloride (TEBA), Aliquat and 18-crown-6 that have been tested by Polácková et al.8
– TEBA was found to be the most effective. Ferrocenecarbaldehyde and p-dimethylaminobenzaldehyde gave, HYDROCARBON WORLD – VOLUME 6 ISSUE 2
Figure 1: Ultrasonically Generated Cavitation on the Sonotrode of a 1,500 W Ultrasonic Processor
Image has been illuminated for better visibility. Figure 2: Impact of Particle/Droplet Size on the Surface Area
50,000 100,000 150,000 200,000 250,000 300,000 350,000
0 10 1.0 Particle size (mm)
Creating a fine emulsion or dispersion, the chemical reaction can be speeded up remarkably and will occur more completely as more surface area is available to react. This saves time and raw material.
under similar conditions, 1,5-diaryl-1,4-pentadien-3-ones as the main product.
• Lin-Xiao et al.9
have shown that the combination of ultrasonication and PTC promotes effectively the generation of dichlorocarbene from chloroform in a shorter time with better yield and less amount of catalyst.
21 0.1 0.01
Total particle surface (μm2)
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