Applications of Ultrasound in Organic Synthesis - A Green Approach

Effects of Ultrasound and Microwaves on Selective Reduction: Catalyst Preparation and Reactions

“…The resultant inter-particle collisions are capable of inducing dramatic changes in surface morphology, composition and reactivity. [16] Therefore, US has been used as a successful tool for the synthesis of catalysts and catalytic materials in order to improve catalyst activity and enhance the dispersion or crystallization process. Sonication not only leads to substantial reductions in overall preparation time, [44][45][46][47] but also creates smaller particle sizes, higher metal dispersion and favours catalyst stability.…”

“…It offers the potential for shorter reaction cycles, cheaper reagents, and milder reaction conditions. [13,16,[89][90][91][92] The advantages of US in chemical reactions could be applied in industrial application in pharmaceutical or fine chemical industry. Sonochemistry can be used for fast reactions or in the synthesis of expensive products.…”

“…[40,97,98] Here the role of US should be more or less similar to that of high-speed stirrers. [16] The mechanical effect originates from the asymmetric collapse of cavitation bubbles near the liquid-solid phase boundary disrupting the interface and impelling jets of liquid into the catalyst surface (liquid jet impingement), [21,99] leading to the grinding of solid catalysts to become smaller particles, revealing new, previously unexposed surface and therefore, resulting in a dramatic increase in surface area, dispersion and BET surface area. [100] Increased adsorption/desorption rates of either liquid species at the catalyst surface or gas dissolution in the liquid phase may also enhance/alter reaction outcome.…”

“…[99] Thus, volatile reactants are brought into contact with clean surfaces, and solids, which may be unreactive due to high lattice energy, are induced to participate in chemical reactions. [16] Therefore, acoustic cavitation and microstreaming at the surface are both significant in heterogeneous solid-liquid systems. The former produces high speed microjets of liquid which impinge upon the surface and cause surface erosion and localized heating.…”

“…This innovation was devised to create more compact, safer, energy efficient and environmentally friendly processes. [15][16][17][18] Essentially, US has no direct action on chemical bonds, however, the significant enhancement in selectivity, rates and yields under mild reaction conditions in both homogeneous and heterogeneous systems has been achieved by means of sonication (the act of applying US energy), [17][18][19] which can even increase reactivity almost a million-fold. [20] The chemical and physical effects of US arise from cavitation phenomenon which produces localized extreme conditions (shortlived hot spots).…”