3.4 Applications of Ultrasound and Neoteric Solvents
47
3.4 Applications of Ultrasound and Neoteric Solvents
Ultrasound remains a very attractive technology in reason of its simplicity and easiness to be implemented together with milder conditions as compared to classical silent
conditions, i.e. absence of ultrasonic irradiation. Regardless of the incident working ultrasonic frequency, effects arising from cavitation phenomenon are intimately
dependent on the contents of the cavitation bubbles. In the world of sonochemistry,
low vapour pressure solvents are generally preferred to exalt consecutive direct and
indirect effects of both physical and chemical origin arising from optimized collapsing conditions. ILs and DESs possessing virtually none vapour pressure might appear
as ideal liquid media for ultrasound. Similarly, an SCF phase is reached under moderate to high pressure (and temperature) according to the fluid, making more difficult
the occurrence of cavitation but when the cavitation threshold is reached, more pronounced subsequent effects do occur. However, the optimized operating conditions
rely on a judicious balance between physicochemical properties of the NS, operating conditions of the ultrasonic system (frequency, output power, design of reactor)
and experimental conditions (temperature, pressure, surrounding gas, etc.). If such a
compromise is not found, lack of obvious effect or even decomposition of the considered medium may occur as for ILs (Oxley et al. 2003). Regardless of the class
of NS, biomass processing (cf. Chap. 4), extraction/separation processes, materials
preparation (not discussed in this monograph) and synthetic organic synthesis (cf.
Chap. 2) are nowadays the four main domains where coupling methodologies with
US are prominent.
3.4.1 Synthesis in Neoteric Solvents under Ultrasound
It is nowadays recognized that the substitution of environmentally unfriendly molecular solvents can reduce the negative impact of environment (Adams et al. 2004).
Among the aforementioned NS, ILs remain nowadays the most investigated in synthetic chemistry owing to their remarkable properties, seniority with respect to DES
and liquid phase at atmospheric pressure with respect to SCF. Numerous well-known
named, unnamed and derivatives organic reactions among Sonogashira, Knoevenagel, Suzuki, Michael and Baylis–Hillman additions, Friedel–Craft, Diels–Alder,
Mannich, Morita–Baylis–Hillman, etc. were thoroughly investigated in ILs/US systems. Invariably, improvements in terms of time, yield, selectivity, amount of catalyst
and other possible green advantages such as recycling, decreased energetic consumption and of waste production were reported as compared to silent conditions (Chatel
and MacFarlane 2014). Noteworthy, the general synthesis of ILs themselves has been
greatly improved by using ultrasound in a neat manner as early as 2002 (Leveque
et al. 2002; Namboodiri and Varma 2002). Although all these striking examples are
as many ideal boosters advocating for scale-up, pilot or industrial-scale applications
combining both US and ILs technologies are not of actuality and may even never
47
3.4 Applications of Ultrasound and Neoteric Solvents
Ultrasound remains a very attractive technology in reason of its simplicity and easiness to be implemented together with milder conditions as compared to classical silent
conditions, i.e. absence of ultrasonic irradiation. Regardless of the incident working ultrasonic frequency, effects arising from cavitation phenomenon are intimately
dependent on the contents of the cavitation bubbles. In the world of sonochemistry,
low vapour pressure solvents are generally preferred to exalt consecutive direct and
indirect effects of both physical and chemical origin arising from optimized collapsing conditions. ILs and DESs possessing virtually none vapour pressure might appear
as ideal liquid media for ultrasound. Similarly, an SCF phase is reached under moderate to high pressure (and temperature) according to the fluid, making more difficult
the occurrence of cavitation but when the cavitation threshold is reached, more pronounced subsequent effects do occur. However, the optimized operating conditions
rely on a judicious balance between physicochemical properties of the NS, operating conditions of the ultrasonic system (frequency, output power, design of reactor)
and experimental conditions (temperature, pressure, surrounding gas, etc.). If such a
compromise is not found, lack of obvious effect or even decomposition of the considered medium may occur as for ILs (Oxley et al. 2003). Regardless of the class
of NS, biomass processing (cf. Chap. 4), extraction/separation processes, materials
preparation (not discussed in this monograph) and synthetic organic synthesis (cf.
Chap. 2) are nowadays the four main domains where coupling methodologies with
US are prominent.
3.4.1 Synthesis in Neoteric Solvents under Ultrasound
It is nowadays recognized that the substitution of environmentally unfriendly molecular solvents can reduce the negative impact of environment (Adams et al. 2004).
Among the aforementioned NS, ILs remain nowadays the most investigated in synthetic chemistry owing to their remarkable properties, seniority with respect to DES
and liquid phase at atmospheric pressure with respect to SCF. Numerous well-known
named, unnamed and derivatives organic reactions among Sonogashira, Knoevenagel, Suzuki, Michael and Baylis–Hillman additions, Friedel–Craft, Diels–Alder,
Mannich, Morita–Baylis–Hillman, etc. were thoroughly investigated in ILs/US systems. Invariably, improvements in terms of time, yield, selectivity, amount of catalyst
and other possible green advantages such as recycling, decreased energetic consumption and of waste production were reported as compared to silent conditions (Chatel
and MacFarlane 2014). Noteworthy, the general synthesis of ILs themselves has been
greatly improved by using ultrasound in a neat manner as early as 2002 (Leveque
et al. 2002; Namboodiri and Varma 2002). Although all these striking examples are
as many ideal boosters advocating for scale-up, pilot or industrial-scale applications
combining both US and ILs technologies are not of actuality and may even never