Merging Metallic Catalysts and Sonication: a Periodic Table Overview

catalysts

Review Merging Metallic Catalysts and Sonication: A Periodic Table Overview

Claudia E. Domini 1,Mónica B. Álvarez 1, Gustavo F. Silbestri 1,*, Giancarlo Cravotto 2 and Pedro Cintas 3,*

1 Instituto de Química del Sur (INQUISUR), Departamento de Química, Universidad Nacional del Sur (UNS)-Consejo Nacional de Investigaciones Cientíﬁcas y Técnicas (CONICET), Avenida Alem 1253, Bahía Blanca 8000, Argentina; [email protected] (C.E.D.); [email protected] (M.B.Á.) 2 Dipartimento di Scienza e Tecnologia del Farmaco, University of Turin, Via P. Giuria 9, 10125 Turin, Italy; [email protected] 3 Departamento de Química Orgánica e Inorgánica, Facultad de Ciencias-Universidad de Extremadura, and Instituto del Agua, Cambio Climático y Sostenibilidad (IACYS)-Unidad de Química Verde y Desarrollo Sostenible, Avda. de Elvas s/n, 06006 Badajoz, Spain * Correspondence: [email protected] (G.F.S.); [email protected] (P.C.); Tel.: +54-291-459-5101 (G.F.S.); Tel.: +34-924-289-300 (P.C.)

Academic Editor: Keith Hohn Received: 8 March 2017; Accepted: 14 April 2017; Published: 19 April 2017

Abstract: This account summarizes and discusses recent examples in which the combination of ultrasonic waves and metal-based reagents, including metal nanoparticles, has proven to be a useful choice in synthetic planning. Not only does sonication often enhance the activity of the metal catalyst/reagent, but it also greatly enhances the synthetic transformation that can be conducted under milder conditions relative to conventional protocols. For the sake of clarity, we have adopted a structure according to the periodic-table elements or families, distinguishing between bulk metal reagents and nanoparticles, as well as the supported variations, thus illustrating the characteristics of the method under consideration in target synthesis. The coverage focuses essentially on the last decade, although the discussion also strikes a comparative balance between the more recent advancements and past literature.

Keywords: metal catalysts; nanoparticles; organic synthesis; sonication; supported metal reagent/catalyst; ultrasound

1. Introduction The interaction of ultrasound with metals dates back to early sonochemistry, from the 1950s onwards, when some scientists discovered, rather by serendipity, that organometallic reagents and their subsequent coupling with organic compounds could be greatly improved by sonication in ultrasonic baths. The method clearly rivaled traditional procedures employed for metal reagents, as the ultrasonic protocol worked well without inert conditions and undried solvents. These pluses were initially associated to cleaning effects that removed the passivating coat of the otherwise unreactive zero-valent metals. There has been considerable progress since, and the reader is referred to past and recent major reviews that document in detail the mechanisms involved in activation of metal surfaces, notably by creating reactive sites by atomic dislocation and corrosion effects, as well as facile metalations and further reactions with an organic compound [1–5]. Of major signiﬁcance in this context is the use of sonication in Barbier and Barbier-like reactions, where tedious and complex operations required to activate bulk metals are usually converted into simpler one-pot reactions [6,7].

Catalysts 2017, 7, 121; doi:10.3390/catal7040121 www.mdpi.com/journal/catalysts Catalysts 2017, 7, 121 2 of 29

The activation also applies to metal nanoparticles (NPs) [8–10], although their stability and reactivity may be substantially different from those of bulk metals, even though finely divided, and often require polymeric additives and/or deposition on inert supports, thus increasing the lifetimes of NPs in solution. Electron microscopy and X-ray diffraction reveal morphological surface changes induced by sonication on the crystal structure of metals such as Al, Cu, Zn, or Ag, which exhibit different hardness and natural roughness. As expected, cavitation plays a pivotal role, especially through mechanical effects generated by the collapse of microbubbles and the action of shock waves on a given surface. Both damage and deformation translate into structural reorganization of crystallites leading to small dimensional variations, weak grain cohesion, and atomic defects [11]. Chemical effects, especially surface oxidation induced by reactive species formed by substrate or solvent pyrolysis and released after bubble implosion, also contribute to structural changes of metal surfaces [12]. It is now well established that effects caused by cavitation can globally be rationalized by assuming a spherical model of bubbles, as microreactors, where three sonochemical reaction zones are identified [13]. The core or inner environment provides high local temperatures and pressures for volatile solutes capable of entering into the bubble and undergoing such intense conditions when it collapses. Given the extremely fast collapse and concomitant cooling rates (exceeding 1010 K per second), the structural reorganization and/or crystallization is highly hindered, thereby resulting in amorphous products. The interfacial region, between the core and the surrounding liquid, provides less intense conditions, yet high enough—ca. 1900 K—to bring about a chemical reaction. The interfacial region appears to be the prevalent zone where non-volatile substrates or those unable to enter into the bubble may react. Given the fast kinetics of collapse, the growth of the nuclei will also be considerable restricted and this may result in nanostructured or nanocrystalline materials depending on the temperature. The third zone comprising the bulk liquid at ambient conditions can also be the site for sonochemical reactions to occur, notably after releasing of reactive species generated by bubble collapse. However, the formation rate will also be lower [14]. The interplay between ultrasonic irradiation and metal activation can also be regarded within the green chemistry domain, even if some metal particles are inherently toxic materials and organic solvents cannot be completely avoided. However, these drawbacks are more than compensated for by the milder, faster, and more efficient processes run under sonication. Aqueous environments are often tolerated and new mechanistic pathways may afford functionalized derivatives not attained by thermal activation, thus switching conventional routes. In addition, it is worth pointing out that ultrasound along with other sustainable and enabling technologies provide sufficient activation to perform chemical reactions under solvent-free (i.e., neat reagents) and catalyst-free conditions [15]. This perspective is aimed at providing a good coverage of recent developments in the use of metal derivatives and metal NPs, generated and manipulated under ultrasound, hoping to be illustrative enough of the breadth and strength of the technique. The goals are focused on the capture of fundamental details that make ultrasound advantageous in a particular catalytic application. The preparation of micro- and nanomaterials with potential catalytic activity has grown almost exponentially. Since our analysis cannot be overlong, thus aiding readability and outlook of the applications presented, the article is largely restricted to synthetic strategies and/or specific applications, i.e., the sonochemical preparation of metal nanostructures or metal NPs without such detailed explorations cannot be treated, apart from a few exceptions in terms of discussion or comparative purposes. Accordingly, the authors hope to marry the properties of concision and representative cases, yet being informative and insightful.

2. Transition Metals and Supported Transition Metal Reagents The use of transition metals still represents the main domain of applications in synthesis and catalysis aided by ultrasound. Metal powders or metal alloys as well as other non-zero valent precursors, cheap metal oxides or metal halides for instance, are usually employed as precursors and Catalysts 2017, 7, 121 3 of 29 subjected to cavitation-induced treatments to generate active reagents and/or improve the synthetic transformation still further. To decrease metal loadings and improve the stability of metal particles, doping strategies on appropriate organic and inorganic supports are performed. Unfortunately, the nature of the resulting composite or soluble species is not always specified. The discussion Catalysts 2017, 7, 121 3 of 29 hereafterCatalysts identifies 2017, 7, 121 the material as stated in the original sources, while applications involving3 of NPs29 or nanostructurednature of the morphologies resulting composite will beor treatedsoluble species later. is not always specified. The discussion hereafter Late-transitionnatureidentifies of thethe resulting material metals, composite as copper stated or inin soluble particular,the origin speciesal provide issources, not always a while wide specified. rangeapplications ofThe synthetic discussion involving applications hereafter NPs or as Cu(I)identifiesnanostructured species arethe involvedmaterial morphologies as in stated efficient will bein andtreatedthe origin expeditious later.al sources, couplings. while applications Click reactions, involving generally NPs or based on copper-catalyzednanostructuredLate-transition morphologies [3+2]-cycloadditions metals, copper will be in treated particular, between later. provide azides a wide and range alkynes of synthetic (CuAAC) applications have met as many Late-transition metals, copper in particular, provide a wide range of synthetic applications as applicationsCu(I) species in materials are involved science, in efficient as well and asexpediti macromolecularous couplings. and Click biomedicinal reactions, generally chemistries based on [16 –18]. Cu(I) species are involved in efficient and expeditious couplings. Click reactions, generally based on Relativecopper-catalyzed to the conventional [3+2]-cycloadditions thermal cycloaddition, between azides which usuallyand alkynes yields (CuAAC) both 1,4- andhave 1,5-disubstituted met many copper-catalyzedapplications in materials [3+2]-cycloadditions science, as well between as macromolecular azides and andalkynes biomedicinal (CuAAC) chemistries have met [16–18]. many 1,2,3-triazoles, the Cu-catalyzed version is faster and highly regioselective leading to 1,4-regioisomers. applicationsRelative to inthe materials conventional science, thermal as well cycloaddi as macromoleculartion, which and usually biomedicinal yields chemistriesboth 1,4- and [16–18]. 1,5- The orthogonal character of this coupling in living organisms make it ideal for specific labeling of Relativedisubstituted to the 1,2,3-triazoles, conventional the thermal Cu-catalyzed cycloaddi versiontion, is which faster andusually highly yields regioselective both 1,4- leadingand 1,5- to biomolecules.disubstituted1,4-regioisomers. The 1,2,3-triazoles, active The orthogonal salt tothe generate Cu-catalyzed character the of correspondingversionthis coupling is faster in and copperliving highly organisms acetylides regioselective make as intermediates itleading ideal for to is Cu(I),1,4-regioisomers.specific although labeling the latterof The biomolecules. orthogonal can be formed Thecharacter active in situ ofsalt this from to generatecoupling a Cu(II) the in salt correspondingliving using organisms a reducing copper make agent,acetylides it ideal especially for as sodiumspecificintermediates ascorbate. labeling is Cu(I),of biomolecules. although the The latter active can salt be to fo generatermed in situthe correspondingfrom a Cu(II) salt copper using acetylides a reducing as Oneintermediatesagent, ofespecially the is first Cu(I),sodium sonochemical although ascorbate. the latter protocols can be was formed reported in situ from by Sreedhara Cu(II) salt and using Surendra a reducing Reddy in 2007agent, throughOne especially of the a first sodium three-component sonochemical ascorbate. protocols reaction was in reported aqueous by Sreedhar solution and at ambientSurendra Reddy temperature in 2007 [19]. One of the first sonochemical protocols was reported by Sreedhar and Surendra Reddy in 2007 The cycloadditionthrough a three-component was performed reaction in a commonin aqueou ultrasonics solution bathat ambient and hence, temperature its reproducibility [19]. The in through a three-component reaction in aqueous solution at ambient temperature [19]. The termscycloaddition of acoustic powerwas performed should in be a treated common with ultrasonic caution. bath However, and hence, sonication its reproducibility clearly enhancedin terms of mass cycloadditionacoustic power was should performed be treated in a withcommon caution. ultrasonic However, bath sonication and hence, clearly its reproducibility enhanced mass in termstransfer of transfer and yielded triazoles (3) in less than 30 min (Scheme1). The best results were obtained using acousticand yielded power triazoles should (3 be) in treated less than with 30 caution. min (Scheme However, 1). The sonication best results clearly were enhanced obtained mass using transfer Cu(I) Cu(I) iodide (92% yield), while other Cu(I) salts as well as the use of Cu(II) or Cu(0) species gave rise andiodide yielded (92% triazolesyield), while (3) in other less thanCu(I )30 salts min as (Scheme well as 1).the The use bestof Cu(II) results or wereCu(0) obtained species gaveusing rise Cu(I) to to loweriodidelower yields yields(92% and yield),and poor poor while selectivity.selectivity. other Cu(I ) salts as well as the use of Cu(II) or Cu(0) species gave rise to lower yields and poor selectivity.

SchemeScheme 1. Ultrasound-promoted 1. Ultrasound-promoted three-component three-component Cu-catalyzed Cu-catalyzed alkyne-azide alkyne-azide cycloaddition cycloaddition (CuAAC) Scheme(CuAAC) 1.in water.Ultrasound-promoted three-component Cu-catalyzed alkyne-azide cycloaddition in water.(CuAAC) in water. A further study by Cravotto and associates compared the efficiency of soluble and Aheterogeneous furtherA further study copperstudy by Cravotto catalysts,by Cravotto and as associates welland asassociates the compared influence compared the of efficiencyother the parameters efficiency of soluble likeof and solublesolvent heterogeneous and copperheterogeneoustemperature. catalysts, asMoreover, copper well as catalysts, theactivation influence as was well of carried otheras the parametersou influencet by either of like microwavesother solvent parameters and (MW) temperature. likeor simultaneoussolvent Moreover,and temperature. Moreover, activation was carried out by either microwaves (MW) or simultaneous activationmicrowave-ultrasound was carried out (MW/US) by either irradiation microwaves [20]. An (MW) analysis or simultaneous(Table 1) of data microwave-ultrasound gathered for the microwave-ultrasound (MW/US) irradiation [20]. An analysis (Table 1) of data gathered for the (MW/US)cycloaddition irradiation shown [20 ].in AnScheme analysis 2 indicates (Table1 )the of datapluses gathered of heterogeneous for the cycloaddition catalysis using shown an in cycloadditioninexpensive charcoal-supported shown in Scheme Cu(I) 2 indicates reagent preparthe plusesed from of Cu(I)heterogeneous and sonication, catalysis as well using as thean Scheme2 indicates the pluses of heterogeneous catalysis using an inexpensive charcoal-supported Cu(I) inexpensivebenefits of using charcoal-supported the MW/US combination. Cu(I) reagent prepared from Cu(I) and sonication, as well as the reagentbenefits prepared of using from the Cu(I) MW/US and combination. sonication, as well as the benefits of using the MW/US combination.

Scheme 2. CuAAC reaction activated by microwaves (MW) or simultaneous microwave-ultrasound SchemeScheme(MW/US) 2. CuAAC 2. irradiations. CuAAC reaction reaction activated activated by bymicrowaves microwaves (M (MW)W) or or simultaneous simultaneous microwave-ultrasound microwave-ultrasound (MW/US)(MW/US) irradiations. irradiations. An improved sonochemical protocol also developed by the Cravotto group employs commerciallyAn improved available sonochemical Cu turnings asprotocol catalyst also[21]. Ultrasounddeveloped smoothlyby the triggeredCravotto thegroup redox employs process commerciallybetween metallic available Cu and Cu Cu(II) turnings oxide as catalyston the metal [21]. Ultrasoundsurface leading smoothly to Cu(I) triggered species. the This redox protocol process is betweenmuch simpler metallic and Cu inexpensive and Cu(II) than oxide other on the catalyst metals surfaceemployed leading for CuAAC to Cu(I) reactions species. Thisand worksprotocol well is much simpler and inexpensive than other catalysts employed for CuAAC reactions and works well

Catalysts 2017, 7, 121 4 of 29

An improved sonochemical protocol also developed by the Cravotto group employs commercially available Cu turnings as catalyst [21]. Ultrasound smoothly triggered the redox process between metallic Cu and Cu(II) oxide on the metal surface leading to Cu(I) species. This protocol is much simplerCatalysts 2017 and, 7 inexpensive, 121 than other catalysts employed for CuAAC reactions and works well in water,4 of 29 where the latter itself serves as ligand and metallic wastes are easily removed by ﬁltration. As depicted in water, Figure 1where, both the sonication latter itself alone serves (with as aligand Ti horn) and and metallic simultaneous wastes are MW/US easily removed (pyrex horn) by filtration. can be successfullyAs depicted employedin Figure [1,21 both,22]. sonication alone (with a Ti horn) and simultaneous MW/US (pyrex horn) can be successfully employed [21,22]. Table 1. Cu-catalyzed alkyne-azide cycloaddition (CuAAC) reactions activated under different thermalTable 1. conditions. Cu-catalyzed alkyne-azide cycloaddition (CuAAC) reactions activated under different thermal conditions. Catalyst (10 mol %) Conditions (at 85 ◦C) Reaction Time Yield (%) Catalyst (10 mol %) Conditions (at 85 °C) Reaction Time Yield (%) (a) Cu(II)/C,Cu(II)/C,L L-ascorbic-ascorbic acidacid (a) oiloil bath bath 2 h 2 h 79 79 Cu(II)/C,Cu(II)/C, LL-ascorbic-ascorbic acidacid MW MW 10 10min min 83 83 (b) recycledrecycled Cu(II)/C Cu(II)/C (b) MWMW 30 30min min 84 84 Cu(II)/CCu(II)/C oil oil bath bath 3 h 3 h 30 30 Cu(II)/C MW 45 min 81 Cu(II)/C MW 45 min 81 Cu(I)/C oil bath 2 h 78 Cu(I)/C oil bath 2 h 78 Cu(I)/C MW 10 min 89 Cu(I)/CCu(I)/C MW/USMW 10 10min min 89 93 Cu(I)/C MW/US 10 min 93 (a) C = charcoal. (b) Catalyst recycled from the above reaction. (a) C = charcoal. (b) Catalyst recycled from the above reaction.

Figure 1.1. ClickClick cycloadditions with with metallic copper activated by ultrasound alone ( left) and the combined action of US andand MWMW irradiationsirradiations inin aa professionalprofessional MWMW reactorreactor ((rightright).). Set-ups from Cravotto’s group laboratory at the UniversityUniversity ofof Turin.Turin.

An advantageous application of the above procedure deals with the structural modification of An advantageous application of the above procedure deals with the structural modiﬁcation of cyclodextrins, which constitute versatile ligands and scaffolds in supramolecular chemistry, as well cyclodextrins, which constitute versatile ligands and scaffolds in supramolecular chemistry, as well as stationary phases in chiral chromatography. Cyclodextrins form stable sandwich-type inclusion as stationary phases in chiral chromatography. Cyclodextrins form stable sandwich-type inclusion compounds with copper ions, giving rise to a green-grayish coloration that require time-consuming compounds with copper ions, giving rise to a green-grayish coloration that require time-consuming purifications using chelate agents [23]. Metallic copper as catalyst in dimethylformamide (DMF) at puriﬁcations using chelate agents [23]. Metallic copper as catalyst in dimethylformamide (DMF) at high temperature affords cyclodextrin-triazole derivatives (9 and 10, Scheme 3) as white powders, high temperature affords cyclodextrin-triazole derivatives (9 and 10, Scheme3) as white powders, with only trace amounts of the metal [22]. with only trace amounts of the metal [22].

Catalysts 2017, 7, 121 4 of 29 in water, where the latter itself serves as ligand and metallic wastes are easily removed by filtration. As depicted in Figure 1, both sonication alone (with a Ti horn) and simultaneous MW/US (pyrex horn) can be successfully employed [21,22].

Table 1. Cu-catalyzed alkyne-azide cycloaddition (CuAAC) reactions activated under different thermal conditions.

Catalyst (10 mol %) Conditions (at 85 °C) Reaction Time Yield (%) Cu(II)/C, L-ascorbic acid (a) oil bath 2 h 79 Cu(II)/C, L-ascorbic acid MW 10 min 83 recycled Cu(II)/C (b) MW 30 min 84 Cu(II)/C oil bath 3 h 30 Cu(II)/C MW 45 min 81 Cu(I)/C oil bath 2 h 78 Cu(I)/C MW 10 min 89 Cu(I)/C MW/US 10 min 93 (a) C = charcoal. (b) Catalyst recycled from the above reaction.

Figure 1. Click cycloadditions with metallic copper activated by ultrasound alone (left) and the combined action of US and MW irradiations in a professional MW reactor (right). Set-ups from Cravotto’s group laboratory at the University of Turin.

An advantageous application of the above procedure deals with the structural modification of cyclodextrins, which constitute versatile ligands and scaffolds in supramolecular chemistry, as well as stationary phases in chiral chromatography. Cyclodextrins form stable sandwich-type inclusion compounds with copper ions, giving rise to a green-grayish coloration that require time-consuming purifications using chelate agents [23]. Metallic copper as catalyst in dimethylformamide (DMF) at Catalystshigh temperature2017, 7, 121 affords cyclodextrin-triazole derivatives (9 and 10, Scheme 3) as white powders,5 of 29 with only trace amounts of the metal [22].

Catalysts 2017, 7, 121 5 of 29 Catalysts 2017, 7, 121 5 of 29 Scheme 3. CuAAC reactions in the presence of Cu(0) with β-cyclodextrin derivatives as substrates. Scheme 3. CuAAC reactions in the presence of Cu(0) with β-cyclodextrin derivatives as substrates. Scheme 3. CuAAC reactions in the presence of Cu(0) with β-cyclodextrin derivatives as substrates. Further extension of the same conceptual basis to imidazolium-functionalized alkynes led to Further extension of the same conceptual basis to imidazolium-functionalized alkynes led to positively charged triazole-bridged cyclodextrins (Scheme (Scheme 44),), whichwhich exhibitexhibit hybridhybrid propertiesproperties ofof positively charged triazole-bridged cyclodextrins (Scheme 4), which exhibit hybrid properties of cyclodextrins as encapsulating hosts and ionic liquids [24]. [24]. This transformation can be conducted in cyclodextrins as encapsulating hosts and ionic liquids [24]. This transformation can be conducted in an aqueous environment and is more efﬁcientefficient underunder thethe simultaneoussimultaneous actionaction ofof US/MWUS/MW irradiation. irradiation. an aqueous environment and is more efficient under the simultaneous action of US/MW irradiation.

Scheme 4. CuAACCuAAC leading leading to hybrid cyclodextrin-triazole-based ionic liquids. Scheme 4. CuAAC leading to hybrid cyclodextrin-triazole-based ionic liquids. Jacob et al. developed porous glasses as a novel catalyst support for CuAAC reactions. The Jacob etet al.al. developed developed porous porous glasses glasses as aas novel a novel catalyst catalyst support support for CuAAC for CuAAC reactions. reactions. The catalyst The catalyst could easily be generated by sonication in 5–10 min from Cu(II) acetate in aqueous medium catalystcould easily could be easily generated be generated by sonication by sonication in 5–10 in min 5–10 from min Cu(II) from acetateCu(II) acetate in aqueous in aqueous medium medium using using an ultrasonic bath. The calcined catalyst was then employed in the click cycloaddition under usingan ultrasonic an ultrasonic bath. bath. The The calcined calcined catalyst catalyst was was then then employed employed in in the the clickclick cycloadditioncycloaddition under MW-assisted heating (Scheme 5) [25]. This resulted in excellent regioselectivity and yields within MW-assisted heatingheating (Scheme(Scheme5)[ 5)25 [25].]. This This resulted resulted in excellentin excellent regioselectivity regioselectivity and yieldsand yields within within short short reaction times. The experimental conditions were optimized using the cycloaddition of shortreaction reaction times. Thetimes. experimental The experimental conditions conditio were optimizedns were using optimized the cycloaddition using the ofcycloaddition phenylacetylene of phenylacetylene (5) and benzyl azide (12). Recyclability studies also showed that the catalyst could phenylacetylene(5) and benzyl azide (5) and (12 ).benzyl Recyclability azide (12 studies). Recyclability also showed studies that also the showed catalyst that could the be catalyst reused could up to be reused up to four times with comparable efficiency. befour reused times up with to four comparable times with efﬁciency. comparable efficiency.

Scheme 5. Click-reaction using a porous glass support. Scheme 5. Click-reactionClick-reaction using a porous glass support. With the advent and development of flow processes, which not only avoid hazards on With the advent and development of flow processes, which not only avoid hazards on manipulatingWith the toxic advent substances, and development but also miniaturization of flow processes, and whichintensification, not only an avoid interesting hazards click on manipulating toxic substances, but also miniaturization and intensification, an interesting click reactionmanipulating in continuous toxic substances, flow has but also also been miniaturization reported [26]. and The intensification, reactor in question an interesting consistsclick of a reactioncopper- reaction in continuous flow has also been reported [26]. The reactor in question consists of a copper- madein continuous cassette that flow integrates has also beena piezoelectric reported [transd26]. Theucer. reactor 1,4-Disubstituted in question consiststriazoles of were a copper-made obtained in made cassette that integrates a piezoelectric transducer. 1,4-Disubstituted triazoles were obtained in yields ranging from 15% to 55% and within 50–150 °C depending on the starting substrates in only yields ranging from 15% to 55% and within 50–150 °C depending on the starting substrates in only 5–10 min. 5–10 min. Couplings other than alkyne-azide cycloadditions mediated by Cu species and activated by Couplings other than alkyne-azide cycloadditions mediated by Cu species and activated by sonication have also proven to be useful in synthesis. Thus, ultrasonic irradiation was applied to the sonication have also proven to be useful in synthesis. Thus, ultrasonic irradiation was applied to the addition of metal acetylides to in situ generated imines leading to propargylamines (17), a protocol addition of metal acetylides to in situ generated imines leading to propargylamines (17), a protocol described more than one decade ago (Scheme 6). Even though the process was conducted in a described more than one decade ago (Scheme 6). Even though the process was conducted in a common ultrasonic bath, good yields were obtained at room temperature using CuI in aqueous common ultrasonic bath, good yields were obtained at room temperature using CuI in aqueous solution [27]. solution [27]. More recently, Cargnelutti et al. described a straightforward C-S coupling of aryl halides (18) More recently, Cargnelutti et al. described a straightforward C-S coupling of aryl halides (18) with thiols (19) in glycerol, catalyzed by CuI/bis(2-pyridyl)diselenoethers and using ultrasound (as with thiols (19) in glycerol, catalyzed by CuI/bis(2-pyridyl)diselenoethers and using ultrasound (as alternative energy source) to accelerate the process (Scheme 7) [28]. The efficiency of such C-S alternative energy source) to accelerate the process (Scheme 7) [28]. The efficiency of such C-S couplings in glycerol is analogous to other cross-coupling reactions in toxic organic solvents, which couplings in glycerol is analogous to other cross-coupling reactions in toxic organic solvents, which

Catalysts 2017, 7, 121 6 of 29 cassette that integrates a piezoelectric transducer. 1,4-Disubstituted triazoles were obtained in yields ranging from 15% to 55% and within 50–150 ◦C depending on the starting substrates in only 5–10 min. Couplings other than alkyne-azide cycloadditions mediated by Cu species and activated by sonication have also proven to be useful in synthesis. Thus, ultrasonic irradiation was applied to the addition of metal acetylides to in situ generated imines leading to propargylamines (17), a protocol described more than one decade ago (Scheme6). Even though the process was conducted in a common ultrasonic bath, good yields were obtained at room temperature using CuI in aqueous solution [27]. More recently, Cargnelutti et al. described a straightforward C-S coupling of aryl halides (18) with thiols (19) in glycerol, catalyzed by CuI/bis(2-pyridyl)diselenoethers and using ultrasound Catalysts(as alternative 2017, 7, 121 energy source) to accelerate the process (Scheme7)[ 28]. The efﬁciency of such6 of C-S 29 couplings in glycerol is analogous to other cross-coupling reactions in toxic organic solvents, which also alsoemploy employ more more expensive expensive catalysts catalysts based based on transition on transition metals. metals. The methodThe method allowed allowed the use the of use a wide of a widerange range of functionalized of functionalized reaction reaction partners, partners, giving giving the desired the desired products products in good in yields.good yields.

Scheme 6. Cu(I)-mediatedCu(I)-mediated synthesis synthesis of propargylamines.

Scheme 7. Synthesis of aryl sulﬁdes using bis(2-pyridyl)diseleno-ethers. Scheme 7. Synthesis of aryl sulfides using bis(2-pyridyl)diseleno-ethers.

Cheaper andand abundant abundant transition transition metals metals invariably invariably represent represent valuable valuable resources resources for metal-assisted for metal- assistedtransformations. transformations. Pirola et Pirola al. prepared et al. prepared supported supported iron catalysts iron catalysts for Fischer-Tropsch for Fischer-Tropsch (FT) reactions (FT) reactionsusing different using different methods, methods, both conventional both conventional and non-conventional and non-conventional [29]. Metal [29]. Metal deposition deposition on silica on silicawas conducted was conducted with with either either ultrasound ultrasound (US) (US) or or microwave microwave (MW), (MW), thereby thereby increasing increasing thethe catalyst activity. Data obtained for FT reactions showed th thatat the catalysts generated under sonication were the most most efficient, efﬁcient, especially especially under under an an inert inert (Ar) (Ar) atmosphere. atmosphere. MW-prepared MW-prepared catalysts catalysts afforded afforded results results comparable to those obtained with conventional protocols. Copper chromite chromite (with (with formula formula Cu Cu2Cr2Cr2O25O), 5sometimes), sometimes referred referred to as to the as Adkins the Adkins catalyst, catalyst, and TiOand2-supported TiO2-supported copper copper chromite chromite cataly catalystssts have havebeen beenprepared prepared by means by means of different of different methods, methods, and theirandtheir catalytic catalytic performance performance assessed assessed in the in the liquid-phase liquid-phase hydrogenation hydrogenation of of furfural furfural giving giving rise rise to furfuryl alcohol [30]. [30]. TiO 2-supported-supported catalysts catalysts obtained obtained under under ultrasonication ultrasonication exhibited exhibited the highest activity. Notably, Notably, the the catalytic catalytic activity activity was was depe dependentndent on on the the temperature temperature and and studies studies conducted conducted at ◦ 140,at 140, 170, 170, and and 200 200 °C showedC showed the thebest best results results at the at lowest the lowest temperature. temperature. The catalyst’s The catalyst’s efficiency efﬁciency was ascribedwas ascribed to the to amount the amount of reducible of reducible Cu(II) Cu(II) ions pres ionsent present in the incatalyst the catalyst and the and metallic the metallic surface. surface. Cu(0) was generated by reduction of the catalyst with H2. The activity was completely lost when carbon deposited on the surface or decreased the amount of metal species. Arena et al. also conducted a systematic research on the effects of Zn/Cu ratios on the structure and adsorption of Cu–ZnO/ZrO2 catalysts (Zn/Cu ratios = 0–3; ZrO2 = 42–44 wt %), synthesized via the reverse co-precipitation under ultrasound energy. Cu/ZrO2 and Cu–ZnO/ZrO2 systems ensured a superior activity in the hydrogenation reaction of CO2 to methanol with respect to conventional catalysts [31].

Catalysts 2017, 7, 121 7 of 29

Cu(0) was generated by reduction of the catalyst with H2. The activity was completely lost when carbon deposited on the surface or decreased the amount of metal species. Arena et al. also conducted a systematic research on the effects of Zn/Cu ratios on the structure and adsorption of Cu–ZnO/ZrO2 catalysts (Zn/Cu ratios = 0–3; ZrO2 = 42–44 wt %), synthesized via the reverse co-precipitation under ultrasound energy. Cu/ZrO2 and Cu–ZnO/ZrO2 systems ensured a superior activity in the hydrogenation reaction of CO2 to methanol with respect to conventional catalysts [31].

3. Main Group Metal Derivatives Unlike zero-valent transition metals, some exhibiting high hardness and being reluctant to undergo a direct metalation process [1,5], some main group elements, especially Group 13 and 14Catalysts metals, 2017, often7, 121 show characteristics typical of alkaline or alkaline-earth elements and can7 alsoof 29 Catalysts 2017, 7, 121 7 of 29 tolerate hydrophilic media and sensitive functional groups. Both metalation and couplings can conductedbe smoothly under conducted the action under theof ultrasound action of ultrasound waves. It waves. suffices, It suffices,for instance, for instance, to mention to mention that conducted under the action of ultrasound waves. It suffices, for instance, to mention that organostannanethat organostannane compounds compounds (alkyl, (alkyl, alkenyl, alkenyl, and andaryl arylderivatives), derivatives), which which are aretypically typically prepared prepared by organostannane compounds (alkyl, alkenyl, and aryl derivatives), which are typically prepared by transmetalationby transmetalation of the of corresponding the corresponding Grignard Grignard or organolithium or organolithium reagents reagents with bis(tributyltin) with bis(tributyltin) oxide, transmetalation of the corresponding Grignard or organolithium reagents with bis(tributyltin) oxide, canoxide, however can however be synthesized be synthesized via a via direct a direct Barbier-like Barbier-like reaction reaction unde underr sonication sonication [32,33]. [32,33]. A A similar similar can however be synthesized via a direct Barbier-like reaction under sonication [32,33]. A similar strategy employingemploying germaniumgermanium halides, halides, Mg Mg turnings, turnings, and and organohalides organohalides under under sonication sonication enables enables the strategy employing germanium halides, Mg turnings, and organohalides under sonication enables preparationthe preparation of the of correspondingthe corresponding organogermanium organogermanium compounds compounds [34]. [34]. the preparation of the corresponding organogermanium compounds [34]. Despite their their toxicity, toxicity, organostannates organostannates are are versat versatileile reagents reagents to accomplish to accomplish a variety a variety of metal- of Despite their toxicity, organostannates are versatile reagents to accomplish a variety of metal- mediatedmetal-mediated couplings, couplings, the Stille the Stillereaction reaction in particular. in particular. In 2009 In 2009 Lamandé-Langle Lamandé-Langle et etal. al.described described a mediated couplings, the Stille reaction in particular. In 2009 Lamandé-Langle et al. described a generala general procedure procedure that that allows allows the the efficient efficient synthe synthesissis of of various various organotin organotin compounds compounds in in following following a general procedure that allows the efficient synthesis of various organotin compounds in following a Barbier-likea Barbier-like strategy strategy as as well well (Schem (Schemee 8)8)[ [35].35]. The The process process offers offers a a number number of of advantages such as Barbier-like strategy as well (Scheme 8) [35]. The process offers a number of advantages such as simplicity and good yields, and reagent quality (both solvents and tin halides) does not constitute a simplicity and good yields, yields, and and re reagentagent quality quality (both (both solvents solvents and and ti tinn halides) halides) does does not not constitute constitute a critical issue. Moreover, careful purification before use and the presence of an additive are not acritical critical issue. issue. Moreover, Moreover, careful careful purification purification befo beforere use use and and the the presence presence of of an an additive additive are not required. Numerous organotin compounds could easily be synthesized in a simplified and improved required. Numerous organotin compounds couldcould easily be synthesized in a simplified simplified and improved one-step ultrasound-assisted Barbier reaction via cross-coupling of a variety of organic bromides (2525) one-step ultrasound-assisted Barbier reaction via cross-coupling of a variety of organic bromides ( 25)) with stannyl chlorides chlorides ( (2424: :Bu Bu3SnCl,3SnCl, Bu Bu2SnCl2SnCl2, BuSnCl2, BuSnCl3, SnCl3, SnCl4) promoted4) promoted by magnesium. by magnesium. The di- The and di- with stannyl chlorides (24: Bu3SnCl, Bu2SnCl2, BuSnCl3, SnCl4) promoted by magnesium. The di- and tri-functional derivatives (26) 26were further used in a Stille cross-coupling reaction, with different andtri-functional tri-functional derivatives derivatives (26)( were) were further further used used in ina aStille Stille cross-coupling cross-coupling reaction, reaction, with with different iodovinyl acids (or esters) to determine how many groups were transferred. iodovinyl acids (or(or esters)esters) toto determinedetermine how manymany groupsgroups werewere transferred.transferred.

Scheme 8. Sonochemical synthesis of organotin compounds. Scheme 8. Sonochemical synthesis of organotin compounds.

In 2012, Domini et al. reported an improved Stille synthesis of unsymmetrical biaryls (29) when In 2012, Domini et al. reported an improved Stille synthesis of unsymmetrical biaryls (29) when conducting the coupling under sonication (Scheme 9) [36]. Yields ranged from 57% to 97% in 30 min, conducting the coupling under sonication (Scheme9 9))[ [36].36]. Yields Yields ranged from 57% to 97% in 30 30 min, min, whereas the silent transformation took ca. 20 h to afford similar results. whereas thethe silentsilent transformationtransformation tooktook ca.ca. 2020 hh toto affordafford similarsimilar results.results.

Scheme 9. Synthesis of unsymmetrical biaryls. Scheme 9. Synthesis of unsymmetrical biaryls. Arylstannanes (30) have been recently employed as useful partners with alkanoyl chlorides (31) Arylstannanes (30) have been recently employed as useful partners with alkanoyl chlorides (31) in an indium-mediated regioselective synthesis of alkyl aryl ketones (32) (Scheme 10) [37]. The in an indium-mediated regioselective synthesis of alkyl aryl ketones (32) (Scheme 10) [37]. The protocol also works for aryl vinyl ketones. Yields were moderate to good (42%–84%) and, protocol also works for aryl vinyl ketones. Yields were moderate to good (42%–84%) and, remarkably, the transformation can be run under neat conditions, without requiring an organic remarkably, the transformation can be run under neat conditions, without requiring an organic solvent. Sonication dramatically reduced the reaction times compared to conventional conditions solvent. Sonication dramatically reduced the reaction times compared to conventional conditions (from 3–32 h to 10–70 min depending on the starting materials). Indium appears to be a radical (from 3–32 h to 10–70 min depending on the starting materials). Indium appears to be a radical initiator and the authors suggest a catalytic cycle involving a single-electron transfer (SET) initiator and the authors suggest a catalytic cycle involving a single-electron transfer (SET) mechanism. mechanism.

Catalysts 2017, 7, 121 8 of 29

Arylstannanes (30) have been recently employed as useful partners with alkanoyl chlorides (31) in an indium-mediated regioselective synthesis of alkyl aryl ketones (32) (Scheme 10)[37]. The protocol also works for aryl vinyl ketones. Yields were moderate to good (42–84%) and, remarkably, the transformation can be run under neat conditions, without requiring an organic solvent. Sonication dramatically reduced the reaction times compared to conventional conditions (from 3–32 h to 10–70 min depending on the starting materials). Indium appears to be a radical initiator and the authors suggest Catalysts 2017, 7, 121 8 of 29 Catalystsa catalytic 2017 cycle, 7, 121involving a single-electron transfer (SET) mechanism. 8 of 29

Scheme 10. Indium-mediated regioselective synthesis of alkyl aryl ketones. Scheme 10. Indium-mediatedIndium-mediated regioselective regioselective sy synthesisnthesis of alkyl aryl ketones. Certainly indium shows an ionization potential comparable to that of electropositive metals, and Certainly indium shows an ionization potential comparable to that of electropositive metals, and enablesCertainly a broad indium compatibility shows with an ionization numerous potential functional comparable groups and, to unlike that ofalkaline electropositive or alkaline-earth metals, enables a broad compatibility with numerous functional groups and, unlike alkaline or alkaline-earth andelements, enables has a broad the advantage compatibility of withperforming numerous reactions functional in groupsan aqueou and,s unlike environment alkaline or[38,39]. alkaline-earth In past elements, has the advantage of performing reactions in an aqueous environment [38,39]. In past elements,development has involving the advantage Barbier of allylation performing reactions, reactions the inallylindation an aqueous of environment1H-indole-3-carboxaldehyde [38,39]. In past development involving Barbier allylation reactions, the allylindation of 1H-indole-3-carboxaldehyde developmentwith azoles (pyrazole involving and Barbier imidazole allylation compounds) reactions, wa thes conducted allylindation in aqueous of 1H-indole-3-carboxaldehyde media under sonication. with azoles (pyrazole and imidazole compounds) was conducted in aqueous media under sonication. withMechanistically, azoles (pyrazole the one-step and imidazole protocol compounds) involves de washydration conducted and innucleophilic aqueous media addition under leading sonication. to a Mechanistically, the one-step protocol involves dehydration and nucleophilic addition leading to a Mechanistically,variety of indole the one-stepderivatives protocol (Scheme involves 11). dehydration Sonication and nucleophilicsignificantly addition accelerates leading the to aoverall variety variety of indole derivatives (Scheme 11). Sonication significantly accelerates the overall oftransformation indole derivatives without (Scheme affecting 11). Sonicationreaction yields significantly [40]. The accelerates work inspired the overall a multi-component transformation one-pot without transformation without affecting reaction yields [40]. The work inspired a multi-component one-pot affectingdomino synthesis reaction yieldsof convolutamydine [40]. The work A, inspired a natura al multi-component marine natural product one-pot with domino antinociceptive synthesis of domino synthesis of convolutamydine A, a natural marine natural product with antinociceptive convolutamydineeffects [41], as well A, as a further natural marineextension natural to the product preparation with antinociceptive of other functionalized effects [41 indolyl], as well derivatives as further effects [41], as well as further extension to the preparation of other functionalized indolyl derivatives extension[42]. to the preparation of other functionalized indolyl derivatives [42]. [42].

Scheme 11. IndoleIndole syntheses involving Barbier allylindation promoted by sonication. Scheme 11. Indole syntheses involving Barbier allylindation promoted by sonication. Soengas and associates have also paid attention to the effect of sonication on some indium- Soengas and and associates associates have have also also paid paid attention attention to the to effect the of effect sonication of sonication on some on indium- some promoted reactions, which can be conducted under milder conditions thus preserving high levels of promotedindium-promoted reactions, reactions, which can which be conducted can be under conducted milder under conditions milder thus conditions preserving thus high preserving levels of diastereoselection. The group applied the strategy to, for instance, the synthesis of β-lactam diastereoselection.high levels of diastereoselection. The group applied The groupthe strategy applied to, the for strategy instance, to, forthe instance,synthesis theof synthesisβ-lactam carbohydrates [43], β-amino acid and spirodiketopiperazines from sugar lactones [44], or allylation carbohydratesof β-lactam carbohydrates [43], β-amino [43 acid], β and-amino spirodiketopiperazines acid and spirodiketopiperazines from sugar lactones from sugar [44], lactonesor allylation [44], and Reformatsky reactions on oxime ethers [45]. An interesting variation of the classical Reformatsky- andor allylation Reformatsky and Reformatskyreactions on oxime reactions ethers on [45]. oxime An ethers interesting [45]. An variation interesting of the variation classical of Reformatsky- the classical Claisen rearrangement has been described by Ishihara et al. The reaction of substituted allyl α- ClaisenReformatsky-Claisen rearrangement rearrangement has been described has been by described Ishihara byet Ishiharaal. The etreaction al. The of reaction substituted of substituted allyl α- bromoacetates with indium and indium(III) chloride under ultrasound leads to functionalized bromoacetatesallyl α-bromoacetates with indium with indium and indium(III) and indium(III) chlori chloridede under under ultrasound ultrasound leads leads to to functionalized functionalized building blocks bearing quaternary centers [46]. The transformation is run in the presence of a building blocksblocks bearing bearing quaternary quaternary centers centers [46 ].[46]. The Th transformatione transformation is run is in run the presencein the presence of a silylating of a silylating reagent, and involves presumably the in-situ formation of In(I) species. The protocol offers silylating reagent, and involves presumably the in-situ formation of In(I) species. The protocol offers promising avenues in chiral transfer, as exemplified in the asymmetric reactions shown in Scheme 12 promising avenues in chiral transfer, as exemplified in the asymmetric reactions shown in Scheme 12 starting from diastereomerically different products. It should be noted that the preparation of starting from diastereomerically different products. It should be noted that the preparation of univalent indium salts can be successfully accomplished from indium metal employing sonication as univalent indium salts can be successfully accomplished from indium metal employing sonication as well [47]. well [47].

Catalysts 2017, 7, 121 9 of 29 reagent, and involves presumably the in-situ formation of In(I) species. The protocol offers promising avenues in chiral transfer, as exempliﬁed in the asymmetric reactions shown in Scheme 12 starting from diastereomerically different products. It should be noted that the preparation of univalent indium saltsCatalysts can 2017 be, successfully7, 121 accomplished from indium metal employing sonication as well [47]. 9 of 29 Catalysts 2017, 7, 121 9 of 29

Scheme 12. Sonochemical Reformatsky-Claisen rearrangement on chiral substrates. Scheme 12. Sonochemical Reformatsky-Claisen re rearrangementarrangement on chiral substrates. Like indium, lighter metallic gallium has attracted considerable interest in recent years. It is Like indium, lighter metallic gallium has attracted considerable interest in recent years.years. It is fairly stable under air and moisture and exhibits a low first ionization potential. Moreover, due to its fairly stable stable under under air air and and moisture moisture and and exhibits exhibits a alow low first first ionization ionization potential. potential. Moreover, Moreover, due due to its to low melting point (ca. 29 °C), it can easily be handled in the liquid phase [48]. Ga(0) has been exploited lowits low melting melting point point (ca. 29 (ca. °C), 29 it◦ canC), iteasily can easilybe handled be handled in the liquid in the phase liquid [48]. phase Ga(0) [48 has]. Ga(0) been exploited has been in a few stoichiometric Barbier reactions [49,50]. The ionic forms show distinctive properties; while inexploited a few stoichiometric in a few stoichiometric Barbier reactions Barbier reactions [49,50]. The [49, 50ionic]. The forms ionic show forms distinctive show distinctive properties; properties; while Ga(III) is a strong Lewis acid and has been employed in catalytic reactions, the chemistry of Ga(I) is whileGa(III) Ga(III) is a strong is a strongLewis acid Lewis and acid has and been has employed been employed in catalytic in catalytic reactions, reactions, the chemistry the chemistry of Ga(I) ofis largely underestimated as the low oxidation state tends to undergo disproportionation to Ga(III) and largelyGa(I) is underestimated largely underestimated as the low as oxidation the low oxidationstate tends state to undergo tends to disproportionation undergo disproportionation to Ga(III) and to Ga(0). In order to use Ga(0) in catalytic fashion, it should be converted into unstable Ga(I) species. In Ga(0).Ga(III) In and order Ga(0). to use In order Ga(0) to in use catalytic Ga(0) fashion, in catalytic it should fashion, be it converted should be into converted unstable into Ga(I) unstable species. Ga(I) In a recent study, Qin and Schneider described the first catalytic use of elemental gallium through in aspecies. recent study, In a recent Qin and study, Schneider Qin and described Schneider the described first catalytic the first use catalytic of elemental use of gallium elemental through gallium in situ oxidation by Ag(I) and ultrasonic activation [51]. Ga(I)-catalyzed carbon-carbon bond formations situthrough oxidation in situ by oxidation Ag(I) and by ultrasonic Ag(I) and activation ultrasonic [51]. activation Ga(I)-catalyzed [51]. Ga(I)-catalyzed carbon-carbon carbon-carbon bond formations bond involving allyl or allenyl boronic esters and acetals, ketals or aminals, proceeded with essentially involvingformations allyl involving or allenyl allyl boronic or allenyl esters boronic and acetals, esters and ketals acetals, or aminals, ketals orproceeded aminals, proceededwith essentially with complete chemo- and regioselectivity in good overall yields (Scheme 13). The reaction time was completeessentially chemo- complete and chemo- regioselectivity and regioselectivity in good inov gooderall overallyields (Scheme yields (Scheme 13). The 13 ).reaction The reaction time timewas significantly decreased by switching from conventional heating and stirring (40 °C in dioxane, 24 h) wassignificantly significantly decreased decreased by switching by switching from fromconventional conventional heating heating and stirring and stirring (40 °C (40 in ◦dioxane,C in dioxane, 24 h) to sonication (40–45 °C, 8 h). The Ga(0)/Ag(I) ratio and the virtual Ga(I) loading were decreased to to24 sonication h) to sonication (40–45 (40–45 °C, 8◦ h).C, 8The h). TheGa(0)/Ag(I) Ga(0)/Ag(I) ratio ratioand andthe virtual the virtual Ga(I) Ga(I) loading loading were were decreased decreased to 2:1 and 5 mol %, respectively, under such conditions without loss of activity. An external ligand [18], 2:1to 2:1 and and 5 mol 5 mol %, %,respectively, respectively, under under such such conditions conditions without without loss loss of of activity. activity. An An external external ligand ligand [18], [18], crown-6, employed to stabilize Ga(I) catalyst, proved to be critical for full conversion. Remarkably, crown-6, employed to stabilize Ga(I) catalyst, proved to be critical for full conversion. Remarkably, Remarkably, this reaction could be achieved on a gram scale at low catalyst loading (0.1 mol %). this reaction could be achieved on aa gramgram scalescale atat lowlow catalystcatalyst loadingloading (0.1(0.1 molmol %).%).

Scheme 13. Ga(I)-catalysis under sonication for C-C bond formation. Scheme 13. Ga(I)-catalysis under sonicati sonicationon for C-C bond formation. Finely dispersed gallium microspheres with concomitant encapsulation of organic dyes (which Finely dispersed gallium microspheres with concomitant encapsulation of organic dyes (which includeFinely Congo dispersed red, crystal gallium violet, microspheresphenanthroline, with and concomitantrhodamine 6G) encapsulation within them could of organic be achieved dyes include Congo red, crystal violet, phenanthroline, and rhodamine 6G) within them could be achieved (whichby means include of ultrasonication Congo red, crystal in water violet, for phenanthroline, a few minutes at and 55 rhodamine °C at a temperature 6G) within at them which could Ga beis by means of ultrasonication in water for a few minutes at 55 °C at a temperature at which Ga is achievedmolten. This by means application, of ultrasonication reported by in Gedanken water for aan fewd co-workers minutes at 55[52,53],◦C at even a temperature though disconnected at which Ga molten. This application, reported by Gedanken and co-workers [52,53], even though disconnected isfrom molten. synthesis, This application,provides potentiality reported by in Gedankenremoving organic and co-workers pollutants [52 by,53 ],such even microspheres though disconnected aided by from synthesis, provides potentiality in removing organic pollutants by such microspheres aided by fromthe mild synthesis, and relatively provides inexpensive potentiality use in of removing sonication. organic Scanning pollutants electron by micr suchoscopy microspheres (SEM) (Figure aided by2) the mild and relatively inexpensive use of sonication. Scanning electron microscopy (SEM) (Figure 2) and transmission electron microscopy (TEM) images indicated that entrapment of organic molecules and transmission electron microscopy (TEM) images indicated that entrapment of organic molecules takes place in the voids of hollow Ga particles, whereas alternative mechanisms based on adsorption takes place in the voids of hollow Ga particles, whereas alternative mechanisms based on adsorption onto the metal surface or interlattice inclusions were ruled out. onto the metal surface or interlattice inclusions were ruled out.

Catalysts 2017, 7, 121 10 of 29 the mild and relatively inexpensive use of sonication. Scanning electron microscopy (SEM) (Figure2) and transmission electron microscopy (TEM) images indicated that entrapment of organic molecules takes place in the voids of hollow Ga particles, whereas alternative mechanisms based on adsorption ontoCatalysts the 2017 metal, 7, 121 surface or interlattice inclusions were ruled out. 10 of 29

Figure 2. SEMSEM (scanning (scanning electron electron microscopy) microscopy) images images of of gallium gallium particles particles formed formed after after sonication sonication in in(A ()A distilled) distilled water water and and (B (B) )aqueous aqueous solution solution (9.1 (9.1 mM) mM) of 1,10-phenanthroline.1,10-phenanthroline. Reproduced withwith permission fromfrom Ref.Ref. [[53].53]. CopyrightCopyright 20142014 thethe RoyalRoyal SocietySociety ofof Chemistry.Chemistry.

4. Nanostructured Materials from Transition ElementsElements As mentioned above, the application of ultrasoundultrasound to the preparation of micro- andand nanosizednanosized materials from inorganic and organic materials representsrepresents a fertile field ﬁeld of mode modernrn research [8,10,54], [8,10,54], and cavitational implosion not only makes theirtheir synthesis easier but alsoalso inducesinduces profoundprofound morphological changes that affectaffect thethe reactivityreactivity againstagainst otherother molecules.molecules. AlthoughAlthough zero-valentzero-valent elements can be be employed, employed, sonochemical sonochemical procedures procedures often often harness harness the the facile facile formation formation of metal of metal ion ionspecies species in solution in solution to induce to induce surface surface modifications modiﬁcations and and formation formation of of supported supported catalysts catalysts with high activity. The following following examples examples include include cases cases involving involving early, early, middle, middle, and and late late transition transition metals, metals, as aswell well as asa few a few cases cases of ofso-called so-called inner inner tran transitionsition metals metals compri comprisingsing the the lanthanide lanthanide f-block.f -block.

4.1. First-Row Transition MetalMetal NPsNPs

Rahmani et al. synthesized, under the action of ultrasonic irradiation, Cr/clinoptilolite-ZrO2 Rahmani et al. synthesized, under the action of ultrasonic irradiation, Cr/clinoptilolite-ZrO2 nanocatalyst to investigate the CO2-enhanced dehydrogenation of ethane [55]. Sonication was carried nanocatalyst to investigate the CO2-enhanced dehydrogenation of ethane [55]. Sonication was carried out byby applyingapplying a a pulse pulse ratio ratio (on:off) (on:off) of of 0.3:0.1 0.3:0.1 s at s anat inputan input power power of 90 of W 90 for W 60 for min. 60 min. The samplesThe samples were thoroughlywere thoroughly characterized characterized by means by ofmeans various of physicochemicalvarious physicochemical techniques, techniques, and the authors and the determined authors thatdetermined sonication that generated sonication metal generated oxide-NPs metal with oxid sizese-NPs of aboutwith sizes 4–8 nm, of about promoting 4–8 nm, the promoting distribution the of metaldistribution particles of andmetal strengthening particles and the chromium-supportstrengthening the interaction.chromium-support The best performanceinteraction. atThe 700 best◦C forperformance 5 h was attained at 700 °C with for the5 h Cr/CLT-Z25(U)was attained with nanocatalyst the Cr/CLT-Z25(U) giving 38% nanocataly ethylenest yield. giving 38% ethylene yield.Iron-based nanomaterials are especially popular in terms of facile preparation from inexpensive precursors.Iron-based Both nanomaterials synthetic and degradation are especially applications popular in are terms frequently of facile reported preparation for such from NPs, inexpensive which can beprecursors. reused and/or Both synthetic recycled, thusand degradation contributing applications to sustainable are methodologies. frequently reported Thus, Dai for etsuch al. obtainedNPs, which an ozonationcan be reused catalyst and/or via chemicalrecycled, precipitation thus contributing and further to sustainable calcination, methodol assistedogies. by ultrasonication Thus, Dai et [ 56al.]. obtained an ozonation catalyst via chemical precipitation and further calcination, assisted by The authors prepared magnetic core/shell CeO2 nanoparticles with a Fe3O4 magnetic core, a silica ultrasonication [56]. The authors prepared magnetic core/shell CeO2 nanoparticles with a Fe3O4 mid-layer, and CeO2 outer layer (47: Fe3O4@SiO2@CeO2) for the degradation of aspirin (acetylsalicylic acid,magnetic ASA, core,46) a which silica couldmid-layer, also and enhance CeO2 the outer total layer organic (47: Fe carbon3O4@SiO removal2@CeO2 (Scheme) for the degradation14). The ASA of aspirin (acetylsalicylic acid, ASA, 46) which could also enhance the total organic carbon removal removal with Fe3O4@SiO2@CeO2 as catalyst conducted for 60 min reached 81.0%, while 67.3% with (Scheme 14). The ASA removal with Fe3O4@SiO2@CeO2 as catalyst conducted for 60 min reached 81.0%, while 67.3% with Fe3O4@SiO2, 66.1% with Fe3O4, and only 64.1% with O3 alone. This type of catalyst had magnetic recyclability and low metal leaching.

Catalysts 2017, 7, 121 10 of 29

Figure 2. SEM (scanning electron microscopy) images of gallium particles formed after sonication in (A) distilled water and (B) aqueous solution (9.1 mM) of 1,10-phenanthroline. Reproduced with permission from Ref. [53]. Copyright 2014 the Royal Society of Chemistry.

4. Nanostructured Materials from Transition Elements As mentioned above, the application of ultrasound to the preparation of micro- and nanosized materials from inorganic and organic materials represents a fertile field of modern research [8,10,54], and cavitational implosion not only makes their synthesis easier but also induces profound morphological changes that affect the reactivity against other molecules. Although zero-valent elements can be employed, sonochemical procedures often harness the facile formation of metal ion species in solution to induce surface modifications and formation of supported catalysts with high activity. The following examples include cases involving early, middle, and late transition metals, as well as a few cases of so-called inner transition metals comprising the lanthanide f-block.

4.1. First-Row Transition Metal NPs

Rahmani et al. synthesized, under the action of ultrasonic irradiation, Cr/clinoptilolite-ZrO2 nanocatalyst to investigate the CO2-enhanced dehydrogenation of ethane [55]. Sonication was carried out by applying a pulse ratio (on:off) of 0.3:0.1 s at an input power of 90 W for 60 min. The samples were thoroughly characterized by means of various physicochemical techniques, and the authors determined that sonication generated metal oxide-NPs with sizes of about 4–8 nm, promoting the distribution of metal particles and strengthening the chromium-support interaction. The best performance at 700 °C for 5 h was attained with the Cr/CLT-Z25(U) nanocatalyst giving 38% ethylene yield. Iron-based nanomaterials are especially popular in terms of facile preparation from inexpensive precursors. Both synthetic and degradation applications are frequently reported for such NPs, which can be reused and/or recycled, thus contributing to sustainable methodologies. Thus, Dai et al. obtained an ozonation catalyst via chemical precipitation and further calcination, assisted by ultrasonication [56]. The authors prepared magnetic core/shell CeO2 nanoparticles with a Fe3O4 magneticCatalysts 2017 core,, 7, 121 a silica mid-layer, and CeO2 outer layer (47: Fe3O4@SiO2@CeO2) for the degradation11 of of 29 aspirin (acetylsalicylic acid, ASA, 46) which could also enhance the total organic carbon removal (Scheme 14). The ASA removal with Fe3O4@SiO2@CeO2 as catalyst conducted for 60 min reached 81.0%,Fe3O4@SiO while2, 67.3% 66.1% with with Fe Fe33OO44@SiO, and2, only66.1% 64.1% with withFe3O4 O, 3andalone. only This 64.1% type with of catalystO3 alone. had This magnetic type of catalystrecyclability had magnetic and low metal recyclability leaching. and low metal leaching.

Catalysts 2017, 7, 121 11 of 29 Scheme 14. Degradation of a pharmaceutical compound using an iron-based ozonation catalyst in the Scheme 14. Degradation of a pharmaceutical compound using an iron-based ozonation catalyst in the presence of ultrasound. presence of ultrasound.

Gobouri et al.al. have have also also shown shown that that simultaneous irradiation of FeFe22O3-NPs-NPs (iron (iron oxide nanoparticles) withwith lightlight and and sonication sonication can can enhance enhance their their catalytic catalytic effects effects in degradation in degradation processes processes [57]. [57].The catalyticThe catalytic activity activity was evaluatedwas evaluated in the in degradation the degradation of Eosin of YEosin and RhodamineY and Rhodamine B under B certainunder certainexperimental experimental conditions conditions (pH, amount (pH, ofamount catalyst, of andcatalyst, organic and additives). organic additives). The catalytic The efficiencycatalytic efficiencyincreased significantly,increased significantly, from 53% tofrom 72.5% 53% and to 5.59%72.5% toand 20.79%, 5.59% respectively, to 20.79%, respectively, when the degradation when the wasdegradation conducted was under conducted simultaneous under simultaneous irradiation. irradiation. In line with the use of advancedadvanced oxidationoxidation processes (AOPs) as degradative strategies, a novel method of decomposing ibuprofen (IBP), a widely used anti-inflammatory anti-inflammatory drug, was developed by Ziylan and Ince [58]. [58]. Catalytic Catalytic ozonation using high-frequency ultrasound and Fe-based species was µ −1 applied. The optimum concentration of of IBP, O3 flowflow rate, and US powerpower werewere 5050 μM,M, 1212 mgmg minmin−1,, −1 and 0.23 W mLmL−1, ,respectively. respectively. The The most most critical critical parameter parameter was was found found to to be be pH, pH, as as the the latter determined the the mass mass transfer transfer and and decomposition decomposition of of O3 O as3 aswell well as the asthe diffusion diffusion of solutes of solutes from from the bulkthe bulk liquid liquid to the to theinterfaces interfaces (both (both gas–liquid gas–liquid and and solid–liquid). solid–liquid). The The maximum maximum degree degree of of oxidation oxidation could be obtained with zero-valent iron (ZVI) nanoparticlesnanoparticles at pH 3.0 (100%, 58%). The synergy of sonication and ZVI at acid pH was ascribed to a series of factors factors such as the existence existence of surface areas enriched with large reaction and nucleation sites, the role of active Fe, reactive oxygen species which promote Fenton-likeFenton-like reactions, reactions, and and finally finally the contribution the contribution of hydrodynamic of hydrodynamic shear forces shear to continuous forces to continuouscleaning of thecleaning catalytic of the surface. catalytic The surface. oxidation The and oxidation mineralization and mineralization of IBP was efficiently of IBP was accelerated efficiently in 1-haccelerated ozonation. in 1-h ozonation. The use of transitiontransition metalmetal nanocompositesnanocomposites as as catalysts catalysts under under sonication sonication has has also also been been applied applied to tothe the synthesis synthesis of mono- of mono- and disubstitutedand disubstituted dihydroquinazolinones dihydroquinazolinones (52) in ( good52) in yields good [ 59yields]. The [59]. reaction The reactiondepicted depicted in Scheme in 15 Scheme involves 15 a three-componentinvolves a three-component reaction of isatoic reaction anhydride of isatoic (49), anhydride primary amines (49), primaryor, alternatively amines or, ammonium alternatively acetate ammonium (50), with acetate aryl (50 aldehydes), with aryl (51 aldehydes) catalyzed (51) by catalyzed some transition by some transitionmetal multi-walled metal multi-walled carbon nanotubes carbon nanotubes (MWCNTs). (MWCNTs). The activities The activities of these speciesof these were species found were to founddecrease to decrease in the order: in the Co–MWCNTs order: Co–MWCNT > Pt–MWCNTss > Pt–MWCNTs > Cu–MWCNTs > Cu–MWCNTs > Ag–MWCNTs. > Ag–MWCNTs. The protocol The protocolshowed advantagesshowed advantages as mild reaction as mild conditions, reaction easyconditions, work-up, easy green work-up, character, green energy character, efficiency energy and efficiencyreusable catalysts. and reusable catalysts.

Scheme 15.15.One-pot One-pot multicomponent multicomponent condensation condensation catalyzed catalyzed by metal by multi-walled metal multi-walled carbon nanotubes carbon nanotubes(MWCNTs) (MWCNTs) under ultrasound under ultrasound irradiation. irradiation.

A related multicomponent strategy was reported by Safari and Zarnegar leading to an improved preparation of dihydropyrimidin-one/thione derivatives (56) starting from aromatic aldehydes (53), urea/thiourea (54), and β-dicarbonyl compounds (55), using Fe3O4–MWCNT as nanocatalyst under ultrasound irradiation [60] (Scheme 16). This Biginelli transformation tolerates numerous functional groups in the reaction partners and proceeds in short times and good yields, due to the high catalytic activity of the NPs. The catalyst was reused without significant losses in performance and was, in addition, easily recovered from the reaction mixture by means of an external magnet).

Catalysts 2017, 7, 121 12 of 29

A related multicomponent strategy was reported by Safari and Zarnegar leading to an improved preparation of dihydropyrimidin-one/thione derivatives (56) starting from aromatic aldehydes (53), urea/thiourea (54), and β-dicarbonyl compounds (55), using Fe3O4–MWCNT as nanocatalyst under ultrasound irradiation [60] (Scheme 16). This Biginelli transformation tolerates numerous functional groups in the reaction partners and proceeds in short times and good yields, due to the high catalytic activity of the NPs. The catalyst was reused without signiﬁcant losses in performance and was, in addition,Catalysts 2017 easily, 7, 121 recovered from the reaction mixture by means of an external magnet). 12 of 29

Scheme 16. Multicomponent synthesis of dihydropyrimidin-ones/thiones using Fe3O4-MWCNT Scheme 16. Multicomponent synthesis of dihydropyrimidin-ones/thiones using Fe3O4–MWCNT underunder ultrasonication. ultrasonication.

Nickel NPs, in the form of montmorillonite-supported nickel nanoparticles (Ni-MMT) are good Nickel NPs, in the form of montmorillonite-supported nickel nanoparticles (Ni-MMT) are good catalysts that can be easily generated through an ultrasound-assisted cation exchange impregnation catalysts that can be easily generated through an ultrasound-assisted cation exchange impregnation method within ca. 30 min [61]. Montmorillonites, which are phyllosilicates with a lamellar structure, method within ca. 30 min [61]. Montmorillonites, which are phyllosilicates with a lamellar structure, are ideal candidates for developing a low-cost, anti-coke, and anti-sintering support, thus preventing are ideal candidates for developing a low-cost, anti-coke, and anti-sintering support, thus preventing the agglomeration of NiNPs. The catalysts were evaluated in the production of H2 by the glycerol the agglomeration of NiNPs. The catalysts were evaluated in the production of H by the glycerol steam reforming reaction (GSR) with low CO concentration. The effect of Ni contents2 and calcination steam reforming reaction (GSR) with low CO concentration. The effect of Ni contents and calcination temperatures indicated that a mesoporous Ni-MMT catalyst with 20.9% Ni calcinated at 700 °C temperatures indicated that a mesoporous Ni-MMT catalyst with 20.9% Ni calcinated at 700 ◦C showed showed the highest activity and stability for GSR conducted at 600 °C. the highest activity and stability for GSR conducted at 600 ◦C. A nanostructured Co3O4 catalyst enables the selective oxidation of vanillyl alcohol to vanillin at A nanostructured Co3O4 catalyst enables the selective oxidation of vanillyl alcohol to vanillin at 75 °C and ambient pressure with H2O2 as the sole oxidant and water as solvent in conjuction with 75 ◦C and ambient pressure with H O as the sole oxidant and water as solvent in conjuction with ultrasound (at 19.95 kHz and volu2 me2 acoustic power = 0.25 W mL−1 determined by calorimety) [62]. ultrasound (at 19.95 kHz and volume acoustic power = 0.25 W mL−1 determined by calorimety) [62]. Although both yield and selectivity were modest, such figures were higher under ultrasound in only Although both yield and selectivity were modest, such figures were higher under ultrasound in 15 min (compared to 1 h under silent conditions). Moreover, a commercial Co3O4 nanopowder (with only 15 min (compared to 1 h under silent conditions). Moreover, a commercial Co O nanopowder a surface area do 32 m2 g−1) was also less efficient under sonication. The results3 4were ascribed to (with a surface area do 32 m2 g−1) was also less efficient under sonication. The results were ascribed enhanced physical (mass transfer) and chemical (production of OH radicals by sonicating H2O2) to enhanced physical (mass transfer) and chemical (production of OH radicals by sonicating H O ) effects promoted by ultrasound on the catalyst surface. 2 2 effects promoted by ultrasound on the catalyst surface. Noble and coinage transition metal NPs have been extensively used in recent years in view of Noble and coinage transition metal NPs have been extensively used in recent years in view of the numerous organic transformations accomplished with such metals. The use of NPs often provide the numerous organic transformations accomplished with such metals. The use of NPs often provide more efficient and environmentally benign approaches (by reducing metal loadings and wastes) than more efficient and environmentally benign approaches (by reducing metal loadings and wastes) the corresponding conventional catalysts employed under homogeneous or heterogeneous than the corresponding conventional catalysts employed under homogeneous or heterogeneous conditions. As a representative example, CuAAC reactions affording triazoles with high conditions. As a representative example, CuAAC reactions affording triazoles with high regioselectivity regioselectivity in good yields, have been performed with stable, water-soluble Cu-NPs synthesized in good yields, have been performed with stable, water-soluble Cu-NPs synthesized by means of by means of a double-hydrophilic block copolymer as template under 10-min sonication. Such Cu- a double-hydrophilic block copolymer as template under 10-min sonication. Such Cu-NPs could NPs could further be reused three times without any substantial loss of catalytic efficiency [63]. further be reused three times without any substantial loss of catalytic efficiency [63]. Surface modification of micro-sized TiO2 using high-intensity ultrasound was reported by Surface modification of micro-sized TiO using high-intensity ultrasound was reported by Stucchi and co-workers in order to overcome2 the lack of photocatalyst activity of the semiconductor Stucchi and co-workers in order to overcome the lack of photocatalyst activity of the semiconductor under visible light [64]. The authors studied the photodegradation of acetone and acetaldehyde under visible light [64]. The authors studied the photodegradation of acetone and acetaldehyde (hydrophilic and polar indoor pollutants) in the gas phase, using an LED lamp. The US-assisted (hydrophilic and polar indoor pollutants) in the gas phase, using an LED lamp. The US-assisted synthesis of metallic copper and copper oxides species at the TiO2 surface employed CuCl2 as synthesis of metallic copper and copper oxides species at the TiO2 surface employed CuCl as precursor (Cu amounts ranged from 1 to 75 wt %), showing diverse morphologies in accordance2 with copper content. Ultrasonic parameters (20-kHz frequency, 200 W, and a horn of 13-mm diameter) were important for the preparation of nanoparticles with a good distribution on the TiO2 support. The optimal ultrasonic operating conditions for the copper deposition (50 W cm−2, 2.5 h, 62 °C) accelerated the diffusion of solute in the reaction medium, generating available active sites on the TiO2 surface, thereby increasing the visible light absorption and improving the photocatalytic activity.

4.2. Pd and Pt NPs Versatile enough among metal catalysts, palladium has proven its efficiency and broad scope in promoting the quintessential point of organic synthesis, i.e., carbon-carbon bond-forming reactions.

Catalysts 2017, 7, 121 13 of 29 precursor (Cu amounts ranged from 1 to 75 wt %), showing diverse morphologies in accordance with copper content. Ultrasonic parameters (20-kHz frequency, 200 W, and a horn of 13-mm diameter) were important for the preparation of nanoparticles with a good distribution on the TiO2 support. The optimal ultrasonic operating conditions for the copper deposition (50 W cm−2, 2.5 h, 62 ◦C) accelerated the diffusion of solute in the reaction medium, generating available active sites on the TiO2 surface, thereby increasing the visible light absorption and improving the photocatalytic activity.

4.2. Pd and Pt NPs Versatile enough among metal catalysts, palladium has proven its efﬁciency and broad scope in promoting the quintessential point of organic synthesis, i.e., carbon-carbon bond-forming reactions.

Pd-NPsCatalysts serve 2017 as, 7, 121 precatalysts, which are produced in situ from Pd salts, usually Pd(II) precursors,13 of 29 thenCatalysts reduced 2017 to, 7 Pd(0),, 121 forming Pd-NPs [65]. 13 of 29 Pd-NPs serve as precatalysts, which are produced in situ from Pd salts, usually Pd(II) precursors, ZhangPd-NPs etserve al. as studiedprecatalysts, an aqueouswhich are Heckproduced reaction in situof from halogeno-arenes Pd salts, usually( 57Pd(II)) with precursors, oleﬁns (58) then reduced to Pd(0), forming Pd-NPs [65]. underthen ultrasonic reduced to irradiation Pd(0), forming at ambient Pd-NPs [65]. temperature affording the corresponding products (59) in Zhang et al. studied an aqueous Heck reaction of halogeno-arenes (57) with olefins (58) under moderateZhang to high et al. yields studied (Scheme an aqueous 17)[ 66Heck]. Pd-NPs reaction preparedof halogeno-arenes in-situ were (57) with also olefins a recyclable (58) under catalyst. ultrasonic irradiation at ambient temperature affording the corresponding products (59) in moderate Furthermore,ultrasonic the irradiation Heck reaction at ambient under temperat suchure benign affording conditions the corresponding offered high products regioselection (59) in moderate of para-over to high yields (Scheme 17) [66]. Pd-NPs prepared in-situ were also a recyclable catalyst. Furthermore, ortho-substitutionto high yields (Scheme in phenyl 17) [66]. iodides Pd-NPs (57 prepared), notably in-situ with were electron-donating also a recyclable groups.catalyst. Furthermore, Ultrasound was the Heck reaction under such benign conditions offered high regioselection of para-over ortho- the Heck reaction under such benign conditions offered high regioselection of para-over ortho- essentialsubstitution in the in formation phenyl iodides of Pd-NPs; (57), notably without with electron-donating it, slow generation groups. andUltrasound no aggregation was essential of the substitution in phenyl iodides (57), notably with electron-donating groups. Ultrasound was essential nanomaterialin the formation tookplace. of Pd-NPs; The without insertion it, slow of palladium generation into and theno aggregation C-I bond beneﬁts of the nanomaterial from the combined took in the formation of Pd-NPs; without it, slow generation and no aggregation of the nanomaterial took actionplace. between The insertion the surface of palladium energy of into Pd-NPs the C-I and bond the benefits energy from derived the combined from ultrasonic action between cavitation. the place. The insertion of palladium into the C-I bond benefits from the combined action between the Asurface related energy coupling, of Pd-NPs the and so-called the energy Sonogashira derived from reaction,ultrasonic cavitation. which enables the preparation of surface energy of Pd-NPs and the energy derived from ultrasonic cavitation. A related coupling, the so-called Sonogashira reaction, which enables the preparation of disubstitutedA related acetylenes coupling, (62 ;the Scheme so-called 18), Sonogashira has also been reaction, conducted which with enables nanoparticulated the preparation catalysts of disubstituted acetylenes (62; Scheme 18), has also been conducted with nanoparticulated catalysts and ultrasonicdisubstituted activation acetylenes by (62 Gholap; Scheme and 18), associates has also been [67 ].conducte The protocold with nanoparticulated could be performed catalysts in short and ultrasonic activation by Gholap and associates [67]. The protocol could be performed in short reactionand timesultrasonic with activation high chemoselectivity by Gholap and associates and good [67]. yields The atprotocol room temperature,could be performed under in sonication. short reaction times with high chemoselectivity and good yields at room temperature, under sonication. Comparativereaction times analyses with were high obtainedchemoselectivity from reactions and good conducted yields at room in acetone temperature, and in under an ionic sonication. liquid, in the Comparative analyses were obtained from reactions conducted in acetone and in an ionic liquid, in absenceComparative of a phosphine analyses ligand were andobtained co-catalysts. from reactions It was conducted determined in acetone that ultrasonic and in an irradiation ionic liquid, not in only the absence of a phosphine ligand and co-catalysts. It was determined that ultrasonic irradiation not the absence of a phosphine ligand and co-catalysts. It was determined that ultrasonic irradiation not generatedonly generated the Pd(0)-NPs the Pd(0)-NPs as the active as the catalyst, active catalyst, but also but enhanced also enhanced the catalytic the catalytic activity activity of this of species this in only generated the Pd(0)-NPs as the active catalyst, but also enhanced the catalytic activity of this the cross-couplingspecies in the cross-coupling reaction. reaction. species in the cross-coupling reaction.

Scheme 17. Sonochemical Heck reaction with PdNPs in aqueous medium. SchemeScheme 17. 17.Sonochemical Sonochemical Heck Heck reactionreaction with with PdNPs PdNPs in inaqueous aqueous medium. medium.

Scheme 18. Sonochemical Sonogashira coupling with in situ PdNPs. SchemeScheme 18. 18.Sonochemical Sonochemical SonogashiraSonogashira coupling coupling with with in insitu situ PdNPs. PdNPs. Palladium NPs merged with different supports have also found a variety of applications. The Palladium NPs merged with different supports have also found a variety of applications. The electrochemical performance of Pd-NPs deposited onto carbon-coated LiFePO4 (LiFePO4/C) surfaces electrochemical performance of Pd-NPs deposited onto carbon-coated LiFePO4 (LiFePO4/C) surfaces have been investigated recently [68]. Both Pd-NPs and deposition onto the support were obtained by have been investigated recently [68]. Both Pd-NPs and deposition onto the support were obtained by sonochemical procedure and stirring (at a frequency of 200 kHz and 8.4 W). The structural, sonochemical procedure and stirring (at a frequency of 200 kHz and 8.4 W). The structural, morphological and electrochemical performances were evaluated by means of TEM, X-ray diffraction morphological and electrochemical performances were evaluated by means of TEM, X-ray diffraction (XRD), and charge–discharge measurements. The size of NPs (homogenous size distributions) on the (XRD), and charge–discharge measurements. The size of NPs (homogenous size distributions) on the surface of LiFePO4/C were tested using various concentrations of Pd(II). It is worth pointing out that surface of LiFePO4/C were tested using various concentrations of Pd(II). It is worth pointing out that the technique can be used to ameliorate the electrochemical performance of LiFePO4 for battery the technique can be used to ameliorate the electrochemical performance of LiFePO4 for battery applications. applications. Heterometallic NPs can also be prepared by similar strategies of stabilization and deposition, Heterometallic NPs can also be prepared by similar strategies of stabilization and deposition, which are enhanced by ultrasonic irradiation, such as the sonochemical reduction of Pd and Au salts which are enhanced by ultrasonic irradiation, such as the sonochemical reduction of Pd and Au salts

Catalysts 2017, 7, 121 14 of 29

Palladium NPs merged with different supports have also found a variety of applications. The electrochemical performance of Pd-NPs deposited onto carbon-coated LiFePO4 (LiFePO4/C) surfaces have been investigated recently [68]. Both Pd-NPs and deposition onto the support were obtained by sonochemical procedure and stirring (at a frequency of 200 kHz and 8.4 W). The structural, morphological and electrochemical performances were evaluated by means of TEM, X-ray diffraction (XRD), and charge–discharge measurements. The size of NPs (homogenous size distributions) on the surface of LiFePO4/C were tested using various concentrations of Pd(II). It is worth pointing out that the technique can be used to ameliorate the electrochemical performance of LiFePO4 for battery applications. Heterometallic NPs can also be prepared by similar strategies of stabilization and deposition, which are enhanced by ultrasonic irradiation, such as the sonochemical reduction of Pd and Au salts to the zero-valent oxidation state followed by immobilization of the resulting nanoparticles on TiO2 surfaces [69]. The sonolytic oxidation of paracetamol in aqueous solution catalyzed by Pd–TiO2 and Pd/Au–TiO was especially effective due to their considerably smaller size (than that of Au–TiO ). Catalysts2 2017, 7, 121 14 of 29 2 Furthermore, the activity of the Pd–TiO2 nanocomposite was improved under ultrasonic and UV irradiationto at the 254 zero-valent nm. oxidation state followed by immobilization of the resulting nanoparticles on TiO2 Kim etsurfaces al. prepared [69]. The sonolytic Pd-Sn oxidation NPs under of parace ultrasonictamol in aqueous irradiation solution at catalyzed 20 kHz by with Pd–TiO an2 and input power Pd/Au–TiO2 was especially effective due to their considerably smaller size (than that of Au–TiO2). of 42 W/cm2 for 2 h [70]. The electrocatalytic activity for oxygen reduction was evaluated in Furthermore, the activity of the Pd–TiO2 nanocomposite was improved under ultrasonic and UV 0.5 M KOHirradiation and was at 254 found nm. to be greater than those of Pt or Pd NPs in alkaline media. Pd was present chieflyKim in et the al. metallicprepared Pd-Sn state NPs in theunder Pd-Sn ultrasonic NPs irradiation (with average at 20 kHz size with of an 3–5 input nm). power The of 42 factors that 2 affected theW/cm particle for 2 size h [70]. and The dispersion electrocatalytic of Pd-Sn activity NPs for and, oxygen therefore reduction controlled was evaluated the electroactivity in 0.5 M KOH and was found to be greater than those of Pt or Pd NPs in alkaline media. Pd was present for oxygenchiefly reduction in the metallic were state the molarin the Pd-Sn ratio NPs of (with Pd to average Sn ions size inof solution,3–5 nm). The their factors initial that affected concentrations, the volumethe of particle ethanol size (thereby and dispersion increasing of Pd-Sn hydrogen NPs and, therefore radicals controlled during the sonolysis), electroactivity as wellfor oxygen as the amount of citric acidreduction (acting were as the a molar stabilizer ratio ofand Pd to controllingSn ions in solution, the surfacetheir initial properties). concentrations, In the order volume to of assess the electrocatalyticethanol action,(thereby indium-dopedincreasing hydrogen tin radicals oxide during (ITO) sonolysis), electrodes as well were as the modified amount of withcitric acid Pd or Pd-Sn (acting as a stabilizer and controlling the surface properties). In order to assess the electrocatalytic NPs, whichaction, involve indium-doped the initial tin oxide oxidation (ITO) electrodes of the were electrode modified to with make Pd or it Pd-Sn OH-terminated, NPs, which involve followed by ◦ aminosilanizationthe initial withoxidation 3-aminopropyl-triethoxysilane of the electrode to make it OH-terminated, (APS) and followed subsequent by aminosilanization polymerization with at 100 C. The resulting3-aminopropyl-triethoxysilane APS-modified ITO electrode (APS) and was subsequent immersed polymerization in a citrate at solution 100 °C. The of Pd-Sn resulting NPs APS- (Scheme 19). modified ITO electrode was immersed in a citrate solution of Pd-Sn NPs (Scheme 19).

Scheme 19.SchemeIn-doped 19. In-doped tin oxide tin oxide (ITO) (ITO) electrodes electrodes modiﬁedmodified with with Pd–Sn Pd-Sn NPs, NPs,obtained obtained by sonochemical by sonochemical reduction.reduction. Reproduced Reproduced with permissionwith permission from from Ref. Ref. [[70].70]. Copyright Copyright 2009 2009 Elsevier Elsevier Ltd. Ltd.

A simple process to synthesize C60-Pd polymer spherical nanoparticles (20–200 nm in diameter) A simplewas reported process by to Brancewicz synthesize et al. C [71].60-Pd The polymerpolymer was spherical obtained nanoparticlesfrom a benzene solution (20–200 containing nm in diameter) was reportedprecursors by Brancewicz such as the well-known et al. [71]. complex The polymer of zero-valent was obtainedpalladium (Pd from2(dba) a benzene3·CHCl3) and solution fullerene containing (C60), and showed a tendency to precipitate in large cubic particles (20–80 μm) which are composed precursorsof such small as spherical the well-known nanoparticles complex (20–200 ofnm). zero-valent The procedure palladium takes advantage (Pd2(dba) of 3the·CHCl ultrasound3) and fullerene (C60), andenergy showed effect a tendencyto disintegrate to precipitatethe cubic particles in large in spherical cubic particlesnanoparticles. (20–80 Moreover,µm) whichthe size areof such composed of small sphericalC60-Pd nanoparticlesNPs can be fine-tuned (20–200 by the nm). concentration The procedure of the parent takes solution, advantage time of of polymerization, the ultrasound energy temperature, and stirring conditions. A solution containing a 3:1 ratio of [Pd] to [C60] led to the most homogeneous size of NPs. Dispersed C60-Pd NPs are stable in many organic protic and aprotic solvents. The thin solid films formed from chemically synthesized nanoparticles exhibited reversible electrochemical properties, e.g., high efficiency of electro-reduction and good capacitance. Highly homogeneous, spherical porous Pd nanostructures (SPPNs) with rough surfaces were obtained through a facile and rapid ultrasound-promoted reduction [72]. Sonication plays the dual role of speeding up the spontaneous assembly of Pd-NPs to avoid oriented attachment and maintaining a homogeneous dispersion of such nanostructures. The synthesis involves the sonication

Catalysts 2017, 7, 121 15 of 29

effect to disintegrate the cubic particles in spherical nanoparticles. Moreover, the size of such C60-Pd NPs can be fine-tuned by the concentration of the parent solution, time of polymerization, temperature, and stirring conditions. A solution containing a 3:1 ratio of [Pd] to [C60] led to the most homogeneous size of NPs. Dispersed C60-Pd NPs are stable in many organic protic and aprotic solvents. The thin solid films formed from chemically synthesized nanoparticles exhibited reversible electrochemical properties, e.g., high efficiency of electro-reduction and good capacitance. Highly homogeneous, spherical porous Pd nanostructures (SPPNs) with rough surfaces were obtained through a facile and rapid ultrasound-promoted reduction [72]. Sonication plays the dual role of speeding up the spontaneous assembly of Pd-NPs to avoid oriented attachment and maintaining a homogeneous dispersion of such nanostructures. The synthesis involves the sonication ◦ of an aqueous solution of K2PdCl4 plus ascorbic acid for 7 min at 40 C without any additives or templates. Isolated structures with a narrow size distribution and diameters ranging from 40 to 100 nm (controllable through the concentration of the Pd salt) were obtained. Typical products are formed by loosely packed grains of 2–3 nm with a diameter of 52 nm. The electro-oxidation of formic acid Catalysts 2017, 7, 121 15 of 29 was employed to assess the catalytic performance of the SPPNs. Long-term stability and recyclability makes thisof nanomaterialan aqueous solution a suitable of K2PdCl anode4 pluscatalyst ascorbic acid for formicfor 7 min acid-based at 40 °C without fuel cells.any additives or Likewise,templates. the Isolated ultrasonically structures assisted with a narrow synthesis size distribution of Pd-Cu and nanocatalysts diameters ranging with from various 40 to 100 Pd loadings nm (controllable through the concentration of the Pd salt) were obtained. Typical products are and their activities toward CO oxidation has also been reported [73]. The results of catalytic tests formed by loosely packed grains of 2–3 nm with a diameter of 52 nm. The electro-oxidation of formic indicatedacid that was NPs employed containing to assess a high the amountcatalytic perf of Pdormance showed of the a betterSPPNs. catalyticLong-term performance stability and at lower temperatures.recyclability A nanomaterial makes this nanomaterial of composition a suitable Pd(1.5%)-Cu(20%)/Al anode catalyst for formic acid-based2O3 exhibited fuel cells. the best activity after 95 min ofLikewise, ultrasonic the ultrasonically irradiation assisted exposure. synthesis of Pd-Cu nanocatalysts with various Pd loadings and their activities toward CO oxidation has also been reported [73]. The results of catalytic tests Colmenares et al. developed an outstanding photodeposition method combined with ultrasonic indicated that NPs containing a high amount of Pd showed a better catalytic performance at lower irradiation,temperatures. i.e., sonophotodeposition A nanomaterial of (SPD)composition method Pd(1.5%)-Cu(20%)/Al [74,75], for preparing2O3 exhibited bimetallic the best Pd-Auactivity alloy NPs supportedafter on titania95 min of (Pd-Au/TiO ultrasonic irradiation2 P90) [exposure.76]. The preparation was accomplished under mild conditions (ambient temperatureColmenares and et al. pressure),developed an in outstanding short times, photodeposition and without method requiring combined strong with ultrasonic reducing agents. irradiation, i.e., sonophotodeposition (SPD) method [74,75], for preparing bimetallic Pd-Au alloy NPs The bimetallic system exhibited remarkable catalytic activity (83%), selectivity (70%), and stability in supported on titania (Pd-Au/TiO2 P90) [76]. The preparation was accomplished under mild the oxidationconditions of methanol (ambient totemperature yield methyl and pressure), formate. in The short enhanced times, and catalytic without activityrequiring was strong rationalized in terms ofreducing synergistic agents. effectsThe bimetallic by combining system exhibited Au- and remarkable Pd-NPs, catalytic and theactivity strong (83%), interaction selectivity of Pd-Au with titania.(70%), and stability in the oxidation of methanol to yield methyl formate. The enhanced catalytic activity was rationalized in terms of synergistic effects by combining Au- and Pd-NPs, and the Pd NPs (with sizes between 10 and 17 nm) have also been prepared by a sustainable method that strong interaction of Pd-Au with titania. involves the usePd ofNPs a (with bioreductant, sizes between namely 10 and an 17 aqueousnm) have extractalso been of preparedPerilla frutescensby a sustainableleaf, method under sonication (ultrasonicthat bath involves for 2 the h at use 60 of◦ C)a bioreduc withouttant, any namely additional an aqueous surfactant extract of or Perilla capping frutescens agent leaf, [77 under]. It is thought that polyphenolssonication and (ultrasonic flavonoids bath for present 2 h at 60 in°C) the withou bio-organict any additional extract surfactant reduced or capping Pd(II) agent to Pd(0). [77]. The NPs It is thought that polyphenols and flavonoids present in the bio-organic extract reduced Pd(II) to were then applied to a high-yielding (81%–95%), three-component synthesis of aryl-substituted Pd(0). The NPs were then applied to a high-yielding (81%–95%), three-component synthesis of aryl- pyrazolylphosphonatessubstituted pyrazolylphosphonates from the corresponding from the pyrazolones,corresponding arylpyrazolones, aldehydes, aryl andaldehydes, triethylphosphite and in ethanol (Schemetriethylphosphite 20). in ethanol (Scheme 20).

Scheme 20.SchemeUltrasonic 20. Ultrasonic generation generation of of PdNPs PdNPs usingusing bioreduction bioreduction with withP. frutescensP. frutescens and subsequentand subsequent multicomponent synthesis of pyrazolone derivatives. Reproduced with permission from Ref. [77]. multicomponent synthesis of pyrazolone derivatives. Reproduced with permission from Ref. [77]. Copyright 2015 the Royal Society of Chemistry. Copyright 2015 the Royal Society of Chemistry. Together with Pd, the heavier congener Pt can be obtained as nanostructured material, stabilized by different ligands and additives under sonication. Pt-NPs have met a myriad of catalytic functions in basic and applied research, some largely related to photocatalysis and electrochemistry. Thus, from 2006 onwards, Colmenares et al. have been working with some titania-based catalysts doped with different transition metals, obtained by the sol-gel method using titanium tetraisopropoxide as precursor [78]. Magnetic stirring (MS) or ultrasound (US) were used to produce the aging of the gel. Sonication enabled the synthesis of pure anatase nanoparticles with increased surface areas, which influence the photocatalytic performance. Further pursuits included not only titanium tetraisopropoxide, but also titanium tetrachloride as titanium precursor, employing MS, US, microwave

Catalysts 2017, 7, 121 16 of 29

Together with Pd, the heavier congener Pt can be obtained as nanostructured material, stabilized by different ligands and additives under sonication. Pt-NPs have met a myriad of catalytic functions in basic and applied research, some largely related to photocatalysis and electrochemistry. Thus, from 2006 onwards, Colmenares et al. have been working with some titania-based catalysts doped with different transition metals, obtained by the sol-gel method using titanium tetraisopropoxide as precursor [78]. Magnetic stirring (MS) or ultrasound (US) were used to produce the aging of the gel. Sonication enabled the synthesis of pure anatase nanoparticles with increased surface areas, which influence the photocatalytic performance. Further pursuits included not only titanium tetra-isopropoxide, but also titanium tetrachloride as titanium precursor, employing MS, US, microwave (MW), and reflux methodologies for the sol-gel aging procedure [79]. The synthetic protocol was optimized and applied to the generation, by coprecipitation, of a Pt-doped TiO2 system that was evaluated against the selective photoxidation of propan-2-ol in the gas phase. The same research group also reported the complete degradation of 3-chloropyridine under the action of photocatalysts based on titania. This reaction was practically completed after 480 min of irradiation [80]. The synthesis of TiO2-based catalysts was performed using titanium tetra-isopropoxide and acetyl acetonates derived from different metals. The catalysts were obtained by aging the gel under US, MW and heating at reflux. The best solid in terms of material properties—100% anatase NPs, high crystallinity, and high surface area—was obtained under ultrasonic irradiation. Otherwise, the modification of TiO2 with metals (Fe, Pt, Pd) was unfavorable for photocatalysis. The team also used titanium isopropoxide or titanium tetrachloride as precursors of TiO2-based catalysts, for the selective photo-conversion of 2-butenol to 2-butenal in the liquid phase, as well as gas-phase photo-oxidation of 2-propanol to acetone [81]. Vinodgopal et al. employed an ultrasound dual frequency arrangement at 20 kHz (ultrasound horn-type transducer) and 211 kHz (hig- frequency transducer) operating in tandem to obtain bi- and single layered graphene with Pt-NPs dispersed on that 2D-material [82]. These hybrid composites showed good electrocatalytic activity in the oxidation of methanol. The dual frequency arrangement enabled the reduction of the graphene oxide sheets while ensuring a high level of exfoliation. The latter issue is critical as optimal properties of graphene largely depend on its thickness; ultrasound being a common energy source to achieve successfully liquid-phase exfoliation [83,84]. The enantioselective hydrogenation of 1-phenyl-1,2-propanedione (63) was performed by different Pt-modified catalysts: Pt/Al2O3, Pt/SiO2, Pt/SF (silica fiber), and Pt/C catalysts, also decorated with cinchonidine in a pressurized reactor subjected to ultrasonic irradiation (Scheme 21)[85]. The initial rate along with regio- and enantioselection trends were interrogated for different pretreatments, solvents, and US powers. Sonication improved significantly both enantioselection and activity of the Pt/SF catalyst. On using 5 wt % Pt/SF, pronounced enhancements of the initial rate, enantio- and regioselectivity could be observed under sonication relative to silent experiments. In solvents of high vapor pressure (e.g., methyl acetate) ultrasound had negligible effects on both reaction rate and product selection, while solvents exhibiting low vapor pressures (mesitylene in particular) resulted in the highest ultrasound-induced enhancements. From a chemical viewpoint, this asymmetric reduction is a complex process giving rise to four isomeric hydroxyketones (65 and 66), which can undergo further hydrogenation to the corresponding diols (64 as diastereomeric mixture). The authors only assessed the first hydrogenation step, measuring discrete enantiomeric excesses (47% ee at 50% conversion and up to 60% ee at 98% conversion). The effects caused by ultrasound were dependent on the catalyst and its pretreatment. The enhancement of ee under sonication was only noticeable with Pt/SF (34% ee versus 17% ee under silent conditions at 50% conversion). This was attributed to a substantial modification of metal particle size distribution (as viewed by SEM) under acoustic irradiation. Catalysts 2017, 7, 121 16 of 29

(MW), and reflux methodologies for the sol-gel aging procedure [79]. The synthetic protocol was optimized and applied to the generation, by coprecipitation, of a Pt-doped TiO2 system that was evaluated against the selective photoxidation of propan-2-ol in the gas phase. The same research group also reported the complete degradation of 3-chloropyridine under the action of photocatalysts based on titania. This reaction was practically completed after 480 min of irradiation [80]. The synthesis of TiO2-based catalysts was performed using titanium tetraisopropoxide and acetyl acetonates derived from different metals. The catalysts were obtained by aging the gel under US, MW and heating at reflux. The best solid in terms of material properties— 100% anatase NPs, high crystallinity, and high surface area—was obtained under ultrasonic irradiation. Otherwise, the modification of TiO2 with metals (Fe, Pt, Pd) was unfavorable for photocatalysis. The team also used titanium isopropoxide or titanium tetrachloride as precursors of TiO2-based catalysts, for the selective photo-conversion of 2-butenol to 2-butenal in the liquid phase, as well as gas-phase photo-oxidation of 2-propanol to acetone [81]. Vinodgopal et al. employed an ultrasound dual frequency arrangement at 20 kHz (ultrasound horn-type transducer) and 211 kHz (hig- frequency transducer) operating in tandem to obtain bi- and single layered graphene with Pt-NPs dispersed on that 2D-material [82]. These hybrid composites showed good electrocatalytic activity in the oxidation of methanol. The dual frequency arrangement enabled the reduction of the graphene oxide sheets while ensuring a high level of exfoliation. The latter issue is critical as optimal properties of graphene largely depend on its thickness; ultrasound being a common energy source to achieve successfully liquid-phase exfoliation [83,84]. The enantioselective hydrogenation of 1-phenyl-1,2-propanedione (63) was performed by different Pt-modified catalysts: Pt/Al2O3, Pt/SiO2, Pt/SF (silica fiber), and Pt/C catalysts, also decorated with cinchonidine in a pressurized reactor subjected to ultrasonic irradiation (Scheme 21) [85]. The initial rate along with regio- and enantioselection trends were interrogated for different pretreatments, solvents, and US powers. Sonication improved significantly both enantioselection and activity of the Pt/SF catalyst. On using 5 wt % Pt/SF, pronounced enhancements of the initial rate, enantio- and regioselectivity could be observed under sonication relative to silent experiments. In solvents of high vapor pressure (e.g., methyl acetate) ultrasound had negligible effects on both reaction rate and product selection, while solvents exhibiting low vapor pressures (mesitylene in particular) resulted in the highest ultrasound-induced enhancements. From a chemical viewpoint, this asymmetric reduction is a complex process giving rise to four isomeric hydroxyketones (65 and 66), which can undergo further hydrogenation to the corresponding diols (64 as diastereomeric mixture). The authors only assessed the first hydrogenation step, measuring discrete enantiomeric excesses (47% ee at 50% conversion and up to 60% ee at 98% conversion). The effects caused by ultrasound were dependent on the catalyst and its pretreatment. The enhancement of ee under sonication was only noticeable with Pt/SF (34% ee versus 17% ee under silent conditions at 50% conversion).Catalysts 2017, 7 ,This 121 was attributed to a substantial modification of metal particle size distribution17 of(as 29 viewed by SEM) under acoustic irradiation.

SchemeScheme 21. 21. StereoselectiveStereoselective hydrogenation hydrogenation of of 1-phenyl 1-phenyl-1,2-propanedione-1,2-propanedione improved improved by by sonication. sonication.

Highly dispersed Pt-NPs could also be incorporated into CeO2 nanopowders through a one-step Highly dispersed Pt-NPs could also be incorporated into CeO2 nanopowders through a one-step ultrasound-promoted reduction [86]. The efficiency of the resulting Pt/CeO2 catalysts was tested in ultrasound-promoted reduction [86]. The efficiency of the resulting Pt/CeO catalysts was tested in the the combustion reaction of ethyl acetate. Total conversion could be achieved2 at low temperature. It combustion reaction of ethyl acetate. Total conversion could be achieved at low temperature. It should be noted that the sonochemical preparation of the catalyst led to Pt-NPs with diameters of 2–4 nm and favored the homogeneous intercalation into CeO2 nanopowders. Selvaraj et al. reported the preparation of a new supporting material formed by polythiophene (PTh) and multiwalled carbon nanotubes (MWCNTs) (i.e., PTh-CNTs), which could be obtained through in situ polymerization of thiophene on carbon nanotubes using FeCl3 as oxidant under ultrasound [87]. Such polythiophene/CNT composites were further doped with Pt- and Pt-Ru NPs by reduction of the corresponding metal salts with formaldehyde as reducing agent at pH 11. The resulting Pt/PTh-CNT and Pt-Ru/PTh-CNT materials were tested in the electrocatalytic oxidation of ethylene glycol, finding that the bimetallic catalyst, Pt-Ru/PTh-CNT showed a prolonged stability and better storage characteristics than Pt/PTh-CNT. Nanoparticles made of Pt3Co with Pt-enriched shells on a carbonaceous support have been generated by a one-step ultrasonic polyol procedure from Pt(acac)2 (acac = acetyl acetonate) and Co(acac)2 as metal precursors [88]. Sonication promoted the transformation of Co(acac)2 and delayed the conversion of Pt(acac)2 into nanoparticles; an observation consistent with the different vapor pressures of Pt(II) and Co(II) acetyl acetonates. This sonochemical variation afforded Pt-NPs that were superior electrocatalysts for oxygen reduction than other commercially available catalysts like Pt/C. Likewise, Jiang et al. investigated the coverage of Pt atoms on PtCo-NPs and their catalytic effect on oxygen reduction [89]. PtCo-NPs having different coverages of Pt atoms were synthesized according to various methods, which included microemulsion synthesis and ultrasound-assisted microemulsion, the latter being especially efficient and yielding the most active catalyst (ascribed to an increase of active surface coatings of Pt atoms on NPs). The rate of oxygen reduction catalyzed by such ultrasound-generated PtCo-NPs was approximately four times higher than that of the PtCo catalyst obtained through a conventional method, and they could catalyze the oxygen reduction to water without detection of H2O2 as intermediate product. Nagao et al. evaluated the effect of sonication during the impregnation of bimetallic Pt-Ru-NPs on a carbonaceous support (ultrasonically dispersed) by the reduction of Pt and Ru ions with NaBH4 [90]. The catalysts prepared under sonication were single-nanometer-sized particles, while Pt-Ru particles with significant aggregation were instead detected in the conventional, non-irradiated sample. In addition, the former catalyst was much more efficient for methanol oxidation than Pt-Ru-NPs generated under silent conditions. The enhanced effect of ultrasound on the catalytic activity was also established at concentrations of Pt, Ru, and carbonaceous support as high as 3.0 mM, 1.2 mM, and 1065 mg L−1, respectively. In other words, ultrasonically-activated NPs remained unaffected in catalytic activity as the concentrations of metal ions and carbon increased. In a related study, series of Ag-, Pt-, and Pt/Ag-NPs supported on carbon substrates as electrocatalysts were generated under ultrasonic irradiation without consecutive thermal treatments [91]. Bimetallic-NPs were sphere-shaped agglomerates with a higher particle size than the corresponding single-metal species, which were less than 10 nm in diameter. The Pt-Ag/C system showed significant Catalysts 2017, 7, 121 18 of 29 enhancement of electrochemical activity (oxygen reduction), 1.5 times higher, with respect to a Pt/C catalyst from commercial suppliers.

4.3. Ag and Au NPs The preparations of silver and gold NPs have been widely documented in recent years and this trend will continue for years to come in view of their high versatility. Sonication has obviously added to the repertoire of methods employed in synthesis. Ag-NPs, currently obtained by different green routes [92], exhibit a remarkable antibacterial activity, whose mechanism of action has been investigated in detail as well [93]. Thus, Manjamadha and Muthukumar synthesized Ag-NPs in only 10 min using an aqueous extract of Lantana camara L. as bioreductant and capping reagent under ultrasound [94]. Both TEM (Figure3) and Fourier transform infrared spectroscopy (FTIR) studies conﬁrmed that the weed plant played a key role in the bioreduction and stabilization of NPs by electrostatic interaction

Catalystsof OH groups2017, 7, 121 present in the polyphenols and amide linkages of the protein. Finally, the antioxidant18 of 29 activity exerted by such Ag-NPs was checked using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) test. picrylhydrazylMoreover, the synthesized(DPPH) test. Ag-NPs Moreover, had the remarkable synthesized antibacterial Ag-NPs had activity remarkable versus antibacterial both Gram positiveactivity versusand Gram both negative Gram positive specimens. and Gram negative specimens.

Figure 3. 3. (a) (TEMa) TEM (transmission (transmission electron electron microscopy microscopy)) images images of Ag-NPs of Ag-NPs generated generated from Lantana from camaraLantana L. camara extractL. under extract ultrasound. under ultrasound. The box shows The box the shows resulting the particle resulting size particle distribution; size distribution; (b) SAED (selected(b) SAED area (selected electron area diffraction electron diffraction analysis) patte analysis)rn. Reproduced pattern. Reproduced with perm withission permission from Ref. from[94]. CopyrightRef. [94]. Copyright 2016 Springer. 2016 Springer.

Highly ordered TiO2 nanotube arrays loaded with Ag-NPs were synthesized at low AgNO3 Highly ordered TiO nanotube arrays loaded with Ag-NPs were synthesized at low AgNO concentrations using a common2 UV lamp through ultrasound-aided photochemistry [95]. It was3 concentrations using a common UV lamp through ultrasound-aided photochemistry [95]. It was shown that the concentration of Ag-NPs deposited on TiO2 nanotubes was sufficient to considerably shown that the concentration of Ag-NPs deposited on TiO2 nanotubes was sufﬁcient to considerably improve the photoelectrical and photocatalytic activities of the TiO2 nanotube array. Compared with improve the photoelectrical and photocatalytic activities of the TiO2 nanotube array. Compared with pure TiO2, Ag-doped TiO2 nanotubes generated from 0.006 M AgNO3 through the hybrid US-assisted pure TiO , Ag-doped TiO nanotubes generated from 0.006 M AgNO through the hybrid US-assisted photochemical2 methods 2 had higher photocatalytic degradation3 rate (3.7 times) and higher photochemical methods had higher photocatalytic degradation rate (3.7 times) and higher photocurrent photocurrent (1.2 times). The enhanced photocatalytic behavior of the Ag-TiO2 system was assessed (1.2 times). The enhanced photocatalytic behavior of the Ag-TiO system was assessed through the through the elimination of methylene blue (MB) dye in aqueous solution2 (at an initial concentration elimination of methylene blue (MB) dye in aqueous solution (at an initial concentration of 8 mg L−1). of 8 mg L−1). A 200-W high pressure Hg lamp with an emission wavelength of 365 nm was employed A 200-W high pressure Hg lamp with an emission wavelength of 365 nm was employed as UV-light as UV-light source, while the changes in MB concentration were obtained by measuring the source, while the changes in MB concentration were obtained by measuring the absorbance intensity absorbance intensity at 660 nm. Other authors have also synthesized Ag/TiO2 composite NPs in at 660 nm. Other authors have also synthesized Ag/TiO composite NPs in ethylene glycol in basic ethylene glycol in basic medium (NaOH) through a sonochemical2 process [96]. medium (NaOH) through a sonochemical process [96]. A new and stable nanostructured catalyst with high activity towards oxygen electro-reduction A new and stable nanostructured catalyst with high activity towards oxygen electro-reduction in acid electrolytes was introduced by Pech-Pech and co-workers [97,98]. The authors reported the in acid electrolytes was introduced by Pech-Pech and co-workers [97,98]. The authors reported the preparation of Ag-rich NPs within a PtPd-enriched area ([email protected]). The catalyst was preparation of Ag-rich NPs within a PtPd-enriched area ([email protected]). The catalyst was synthesized synthesized in a one-pot protocol by applying intense ultrasound, reducing Ag, and then Pt and Pd salts with NaBH4 in the presence of trisodium citrate, and finally adding XC-72 (a multi-purpose conductive carbon black). Structural characterization of the resulting catalysts revealed that the amount of Pt and Pd in the shell had a pronounced effect on electro-reduction. The optimal activity was obtained for a catalyst of composition [email protected]/C, which was a useful alternative to Pt/C. Nanostructured silver vanadates with nano-rod shape (diameter of 50–200 nm and length of 0.5– 2.0 μm) could be obtained by means of hydrothermal treatment assisted by US and MW techniques [99]. Silver vanadates showed a strong absorbance in the UV–vis zone at 470–500 nm and had much higher photocatalytic efficiency in the decomposition of MB under visible light exposure compared to common TiO2 NPs. Notably, such Ag vanadates also exhibited antibacterial activity, thus adding extra value to potential applications in environmental remediation. An easy and efficient technique for the synthesis of Ag-NP colloids with cherry- or raspberry- type morphologies was reported by Lei et al. [100]. The approach involves the self-assembly of a copolymer to micelles, which served as a compartment for the generation of NPs with the assistance of ultrasound. The above-mentioned morphologies depended on the amount of AgNO3 loading, the nature of the reducing agent and external conditions. Sonication favored the diffusion of AgNO3 and polyethyleneimine (PEI) affording nucleation sites that were well distributed through the micellar

Catalysts 2017, 7, 121 19 of 29 in a one-pot protocol by applying intense ultrasound, reducing Ag, and then Pt and Pd salts with NaBH4 in the presence of trisodium citrate, and finally adding XC-72 (a multi-purpose conductive carbon black). Structural characterization of the resulting catalysts revealed that the amount of Pt and Pd in the shell had a pronounced effect on electro-reduction. The optimal activity was obtained for a catalyst of composition [email protected]/C, which was a useful alternative to Pt/C. Nanostructured silver vanadates with nano-rod shape (diameter of 50–200 nm and length of 0.5–2.0 µm) could be obtained by means of hydrothermal treatment assisted by US and MW techniques [99]. Silver vanadates showed a strong absorbance in the UV–vis zone at 470–500 nm and had much higher photocatalytic efficiency in the decomposition of MB under visible light exposure compared to common TiO2 NPs. Notably, such Ag vanadates also exhibited antibacterial activity, thus adding extra value to potential applications in environmental remediation. An easy and efficient technique for the synthesis of Ag-NP colloids with cherry- or raspberry-type morphologies was reported by Lei et al. [100]. The approach involves the self-assembly of a copolymer to micelles, which served as a compartment for the generation of NPs with the assistance of ultrasound. The above-mentioned morphologies depended on the amount of AgNO3 loading, the nature of the reducing agent and external conditions. Sonication favored the diffusion of AgNO3 and polyethyleneimine (PEI) affording nucleation sites that were well distributed through the micellar cores and led to cherry-like Ag-NP colloids. Moreover, PEI is a nontoxic reducing agent with a slow reaction rate capable of controlling both nucleation and growth processes. Likewise, it serves as protective agent for the resulting Ag-NPs. Saha et al. described an ultrasound-assisted green procedure for the synthesis of carbohydrate polymer-inspired Ag-NPs using tyrosine and starch, both being naturally-occurring substances [101]. Starch-stabilized Ag-NPs were prepared in water using a small amount of tyrosine as a non-harmful reducing agent. The cytotoxicity of the resulting Ag-NPs on the peritoneal macrophages of Wistar rat was evaluated and confirmed a higher selectivity to filaricidal and larvicidal activity (Figure4) than that of commercially available ones. The sonogeneration and catalytic effect of Au-NPs is well documented in recent literature. Kuo et al. synthesized Au-NPs coated on TiO2 (ranging from 2 to 80 nm in diameter) by varying the pH values from 3 to 7, based on sonoelectrochemical reductions for 3 h [102]. Acetaldehyde solutions in ethanol were almost completely degraded (ca. 95%) based on Au-NPs having the smallest mean diameter of 2 nm. The catalytic activity for acetaldehyde decomposition was markedly reduced in the presence of Au-NPs of greater diameters. Moreover, the optimal sizes of Au-NPs on TiO2 to produce the strongest surface-enhanced Raman scattering (SERS) effects were ca. 60 nm, under a 785-nm irradiation for probe molecules of Rhodamine 6G and polypyrrole. In line with the above-mentioned applications, a 2011 review by Wen et al. detailed the use of supported noble metal NPs (Au/TiO2, Au/ZrO2, Ag/AgCl) as efficient catalysts for selective chemical syntheses and degradation of environmental pollutants [103]. Thus, the NPs could effectively catalyze solar-energy conversion into chemical energy (i.e., oxidation of alcohols, thiols, and benzene into carbonyls, disulfides, and phenol, respectively, or the reduction of nitro-aromatic compounds to the corresponding azo derivatives). Under ultrasound irradiation, the supported NPs could also catalyze the generation of hydrogen from water. Furthermore, a variety of contaminants (bearing different functional groups: aldehyde, alcohol, carboxylic acid, phenolic compounds, and organic dyes) were significantly decomposed, and completely degraded under the action of ultrasound. Two catalysts containing Au/TiO2/C were produced by means of US and MW irradiations by George et al. [104]. The deposition of Au colloids onto powdered TiO2/C, was performed by means of a solvated metal atom impregnation (SMAI) method, which provided highly-dispersed Au particles without requiring high-temperature calcination and reduction steps, which often lead to particle sintering. The catalytic performance of 1 wt % Au-TiO2/C-based catalysts, both sonochemically and microwave irradiated was evaluated for the oxidation of CO in the range of 0–300 ◦C and compared to that of a commercial catalyst containing 1 wt % Au-TiO2 (Degussa-P25). Figure5 shows the Catalysts 2017, 7, 121 20 of 29 different patterns of the above-mentioned catalysts viewed by transmission electron microscopy (TEM). An increase in CO conversion was observed for the ultrasonically-derived catalyst at low temperature. Furthermore, the reactivity observed for this oxidation followed the order Au/TiO2/C (US) > Au/TiO2 (P25)Catalysts > Au/TiO2017, 7, 1212/C (MW). 20 of 29

FigureFigure 4.4. EthidiumEthidium bromidebromide (EB)/acridine(EB)/acridine orange (AO) stainedstained oocyte,oocyte, microﬁlariamicrofilaria ofof S.S. cervicervi andand larvaelarvae of ofC. C. quinquefasciatus quinquefasciatus.(a.) Control,(a) Control, (b) parasites (b) parasites treated treated with 8.85 withµg/mL 8.85 ofμg/mL Ag-NP10), of Ag-NP10), (c) parasites (c) treatedparasites with treated 28.3 µwithg/mL 28.3 of μ Ag-NP10,g/mL of Ag-NP10, (d) control, (d) (control,e) larvae (e treated) larvae withtreated 21.95 withµg/mL 21.95 μ ofg/mL Ag-NP10, of Ag- andNP10, (f) and larvae (f) treatedlarvae treated with 117.1 withµ g/mL117.1 μ ofg/mL Ag-NP10. of Ag-NP10. Reproduced Reproduced with permission with permission from Ref.from [101 Ref.]. Copyright[101]. Copyright 2015 Elsevier 2015 Elsevier Ltd. Ltd.

Sonochemically prepared Au, Pt and Pd NPs could be successfully immobilized onto TiO2 after Sonochemically prepared Au, Pt and Pd NPs could be successfully immobilized onto TiO2 afterprolonged prolonged sonication sonication [105]. Sonochemically-generated [105]. Sonochemically-generated catalysts catalystsshowed higher showed activities higher than activities those thanprepared those by prepared conventional by conventional protocols protocolsas evaluate asd evaluatedin the production in the production of hydrogen of hydrogen from ethanol- from ethanol-containingcontaining aqueous aqueous solutions. solutions. The reactivity The reactivity was markedly was markedly dependent dependent on the on type the typeof metal of metal NPs NPs and andtheir their dimensions. dimensions. Small Small Pt-NPs Pt-NPs diddid effectively effectively restrict restrict side side reactions reactions arising arising from from recombination recombination of ofelectrons electrons and and holes holes and and released released hydrogen hydrogen at at a ahigher higher rate. rate. Core-shellCore-shell nanostructures consisting consisting of of Au-NPs Au-NPs within within shells shells of Ag of AgNPs-filled NPs-filled polymer polymer (Au- (Au-core/Ag-PVP-shell)core/Ag-PVP-shell) were were synthesized synthesized by byKan Kan and and co-workers co-workers in in a a two-steptwo-step sonication-assistedsonication-assisted reductionreduction procedureprocedure [[106].106]. Ag-NPs acted as reductivereductive agentagent forfor Au(III)Au(III) ionsions inin thethe presencepresence ofof poly(vinylpyrrolidone)poly(vinylpyrrolidone) (PVP).(PVP). AfterAfter sonication,sonication, solidificationsolidification andand spherulitespherulite growthgrowth ofof thethe polymerpolymer chainchain was was induced induced on on the as-formedthe as-formed Au-NPS, Au-NPS, such thatsuch tiny that Ag-NPs tiny Ag-NPs were embedded were embedded in the crystalline in the matrixcrystalline of polymer matrix spherulites.of polymer The spherulites. formation The of core-shell formation NPs of wascore-shell dependent NPs onwas the dependent redox potentials on the ofredox ions potentials in solution, of ions the coordinatingin solution, the ability coordinating of such ability ionicspecies of such toionic stabilize species PVP, to stabilize as well PVP, as the as crystallinewell as the and crystalline interlinkage and interlinkage degrees of suchdegrees a polymer of such during a polymer ultrasonic during irradiation. ultrasonic irradiation. An ultrasonic An cleaner,ultrasonic working cleaner, at working 40 kHz withat 40 ankHz output with an power outp ofut 360power W, of was 360 sufficient W, was sufficient to produce to theproduce Au/Ag the compositeAu/Ag composite in the absence in the of absence oxygen. of The oxygen. stabilizing The rolestabilizing of polymers role of and polymers surfactants and in surfactants nanoparticle in productionnanoparticle has production likewise been has highlighted likewise been by Park highligh et al.,ted who by synthesized Park et al., colloidal, who synthesized uniformly dispersedcolloidal, hybriduniformly nanocomposites dispersed hybrid constituted nanocomposites by polypyrrole consti (PPy)tuted and by Au-NPs polypyrrole (or Pt-NPs) (PPy) [ 107and]. Au-NPs The procedure, (or Pt- assistedNPs) [107]. by ultrasonic The procedure, irradiation, assisted involved by ultrasonic reduction irradiation, of Au(III) involved ions and reduction pyrrole monomer of Au(III) oxidation ions and inpyrrole an aqueous monomer solution oxidation containing in an sodiumaqueous dodecyl solution sulfate containing (SDS). sodium dodecyl sulfate (SDS).

Catalysts 2017, 7, 121 21 of 29 Catalysts 2017, 7, 121 21 of 29

2 FigureFigure 5. 5.TEM TEM images images of of ( a(a)) 1% 1% Au-TiO Au-TiO2 (anatase)/C(anatase)/C (under (under US), US), where where black black dots dots are are Au-NPs; Au-NPs; (b) (b)1% 1% Au/TiO Au/TiO2 2(anatase)/C(anatase)/C (under (under MW), MW), and and (c) 1%1% Au/TiOAu/TiO22-P-25, thethe lastlast support support coming coming from from Degussa.Degussa. Reproduced Reproduced with with permission permission from from Ref. Ref. [104 [104].]. Copyright Copyright 2007 2007 Elsevier Elsevier B.V. B.V.

The construction of a highly efficient anode catalyst for oxygen evolution reaction (OER) was The construction of a highly efficient anode catalyst for oxygen evolution reaction (OER) was reported in 2015 and consisted of a combination of multiwalled carbon nanotubes, metallic Au, and reported in 2015 and consisted of a combination of multiwalled carbon nanotubes, metallic Au, transition metal-oxide (i.e., CNTs-Au@Co3O4) generated under high-intensity ultrasound [108]. The and transition metal-oxide (i.e., CNTs-Au@Co3O4) generated under high-intensity ultrasound [108]. tubular architecture provides enough active centers for OER and enables fast mass and charge The tubular architecture provides enough active centers for OER and enables fast mass and charge transports. The Co3O4 layer protects Au-NPs against further detachment, and reciprocally Au transports. The Co3O4 layer protects Au-NPs against further detachment, and reciprocally Au facilitated the production of Co(IV) species for OER due to its greater electronegativity. facilitated the production of Co(IV) species for OER due to its greater electronegativity. It should be finally pointed out that Au-NPs can be efficiently generated by means of high- It should be finally pointed out that Au-NPs can be efficiently generated by means of high-intensity intensity focused ultrasound (HIFU), working at 463 kHz [109]. Formation of NPs proceeds through focused ultrasound (HIFU), working at 463 kHz [109]. Formation of NPs proceeds through the the reduction of Au(III) ions to Au(0) by radicals generated under ultrasonic cavitation. TEM images reduction of Au(III) ions to Au(0) by radicals generated under ultrasonic cavitation. TEM images revealed that NPs exhibit irregular shapes at low power (30 W), mainly icosahedral at 50 W, and revealed that NPs exhibit irregular shapes at low power (30 W), mainly icosahedral at 50 W, and finally finally nanorods at 70 W. The size decreased with a narrower size distribution as the acoustic power nanorods at 70 W. The size decreased with a narrower size distribution as the acoustic power increased. increased. 4.4. Lanthanide NPs 4.4. Lanthanide NPs Vilela and co-workers prepared highly crystalline lanthanide composite materials [Ln(Hpmd) Vilela and3+ co-workers3+ prepared3+ highly3+ crystalline lanthanide composite materials (H2O)], where Ln can be Eu , Gd , or Tb ions, and the organic linker is 1,4-phenylenebis [Ln(Hpmd)(H2O)], where Ln3+ can be Eu3+, Gd3+, or Tb3+ ions, and the organic linker is 1,4- (methylene)diphosphonic acid (H4pmd) as building block [110]. The authors used three different strategiesphenylenebis(methylene based on heating)diphosphonic procedures of acid the (H reaction4pmd) containers,as building includingblock [110]. classical The authors hydrothermal used three synthesisdifferent (180 strategies◦C for 3based days), on MW-assisted heating procedures heating (50-W of the power reaction at 40 ◦containers,C for 5 s), andincluding US-promoted classical hydrothermal synthesis (180 °C for 3 days), MW-assisted3+ heating (50-W power at 40 °C for 5 s), and synthesis which allowed, for highly diluted mixtures (Ln :H4pmd:H2O ratio = 1:1:7200), the generation 3+ ofUS-promoted isolated nano-crystals synthesis at room which temperature allowed, within for 5 minhighly of acousticdiluted irradiation. mixtures The (Ln Eu-containing:H4pmd:H2O compoundratio = 1:1:7200), was evaluated the generation in the methanolysisof isolated nano-cry of styrenestals oxideat room conducted temperature at 55 within◦C showing 5 min of diverse acoustic catalyticirradiation. activity The and Eu-containing selectivity. Using compound the ultrasonic was ev procedurealuated in it wasthe feasiblemethanolysis to significantly of styrene reduce oxide bothconducted the temperature at 55 °C showing (from 180 diverse◦C to catalytic room temperature) activity and andselectivity. reaction Using times the (from ultrasonic 3 days procedure to 5 s), withoutit was affectingfeasible to phase significantly purity and reduce crystallinity. both the temperature (from 180 °C to room temperature) and reaction times (from 3 days to 5 s), without affecting phase purity and crystallinity.

CatalystsCatalysts2017, 72017, 121, 7, 121 22 of 2922 of 29

In a recent and very detailed study on rare earth-doped nanopowders, ZrO2:Dy3+ NPs were 3+ Inobtained aCatalysts recent by 2017 and,sonication 7, 121 very detailed(ultrasonic study horn onat rare20 kHz, earth-doped 300 W at nanopowders,333 K) in the ZrOpresence22 of 229:Dy of NPs werecetyltrimethylammonium obtained by sonication bromide (ultrasonic (CTAB) horn as surfactant at 20 kHz, [111]. 300The re Wsulting at 333 NPs K) exhibited in the different presence of cetyltrimethylammoniumshapes andIn asizes recent depending and bromidevery detailedon the (CTAB) irradiationstudy ason surfactantrare time earth-dopeds and [the111 nanopowders,].quantity The resulting of CT ZrOAB2 NPs:Dy as 3+revealed exhibitedNPs were by SEM different images.obtained Flower-like by sonication morphologies (ultrasonic were horn obtained at 20 afte kHz,r prolonged 300 W at irradiation 333 K) in (up the to 6presence h) and increasedof shapes andcetyltrimethylammonium sizes depending on bromide the irradiation (CTAB) as surfactant times and [111]. the The quantity resulting ofNPs CTAB exhibited as different revealed by SEM with frequency (from 20 to 24 kHz). This suggests a complex evolution of self-assembly and growth images. Flower-likeshapes and sizes morphologies depending on were the irradiation obtained time afters and prolonged the quantity irradiation of CTAB as (uprevealed to 6 by h) SEM and increased of NPs, dictated to a significant extent by the surrounding surfactant (Figure 6). The nanopowders with frequencyimages. (fromFlower-like 20 to morphologies 24 kHz).This were obtained suggests afte ar complexprolonged evolutionirradiation (up of to self-assembly 6 h) and increased and growth had excellent luminescent properties that were utilized to reveal latent fingerprints (LFPs) on various of NPs, dictatedwith frequency to a signiﬁcant (from 20 to 24 extent kHz). This by thesuggests surrounding a complex evolution surfactant of self-assembly (Figure6). and The growth nanopowders surfacesof NPs, (Figure dictated 7). toIn a addition, significant th extente photocatalytic by the surrounding behavior surfactant of ZrO (Figure2:Dy3+ NPs6). The was nanopowders evaluated in the had excellentdegradationhad excellent luminescent of methylene luminescent properties blue properties dye. that that were were utilizedutilized toto reveal reveal latent latent fingerprints ﬁngerprints (LFPs) on (LFPs) variouson various 3+ surfaces (Figuresurfaces7 (Figure). In addition, 7). In addition, the photocatalyticthe photocatalytic behaviorbehavior of ofZrO ZrO2:Dy3+2:Dy NPs wasNPs evaluated was evaluated in the in the degradationdegradation of methylene of methylene blue blue dye. dye.

Figure 6. Schematic diagram depicting the formation of ZrO2:Dy3+-NPs (3 mol3+ %) with and without Figure 6. Schematic diagram depicting the formation of ZrO3+ 2:Dy -NPs (3 mol %) with and Figurecetyltrimethylammonium 6. Schematic diagram bromide depicting (CTAB) the formation and SEM of imagesZrO2:Dy of -NPsthe resulting (3 mol %) morphologies. with and without without cetyltrimethylammonium bromide (CTAB) and SEM images of the resulting morphologies. cetyltrimethylammoniumReproduced with permission bromide from Ref. (CTAB) [111]. Copyright and SEM 2017 images the American of the Chemical resulting Society. morphologies. ReproducedReproduced with with permission permission from from Ref. Ref. [111 [111].]. Copyright Copyright 2017 the the American American Chemical Chemical Society. Society.

Figure 7. Sequential illustration highlighting the development of latent fingerprint (LFP) detection 3+ using ZrO2:Dy (3 mol %): (i) impression of fingerprints on different substrates; (ii) applied onto the substrate to stain the fingerprint; (iii) UV light is then used to irradiate the fingerprint to emit white FigureFigure 7. Sequential 7. Sequential illustration illustration highlighting highlightingthe the developmentdevelopment of of latent latent fingerprint fingerprint (LFP) (LFP) detection detection using ZrO3+2:Dy3+ (3 mol %): (i) impression of fingerprints on different substrates; (ii) applied onto the using ZrO2:Dy (3 mol %): (i) impression of fingerprints on different substrates; (ii) applied onto the substrate to stain the fingerprint; (iii) UV light is then used to irradiate the fingerprint to emit white substrate to stain the fingerprint; (iii) UV light is then used to irradiate the fingerprint to emit white light, and (iv) revealing the fingerprint with high sensitivity and contrast. Reproduced with permission from Ref. [111]. Copyright 2017 the American Chemical Society. Catalysts 2017, 7, 121 23 of 29

4.5. Main-Group NPs

Aluminum NPs have been isolated and stabilized by reaction,reaction, underunder ultrasonication,ultrasonication, of SiClSiCl44 and LiAlH LiAlH44 inin the the presence presence of ofpoly(vinylpyrrolidone) poly(vinylpyrrolidone) (PVP) (PVP) at ambient at ambient temperature temperature [112]. [ 112The]. sonochemicalThe sonochemical procedure procedure (1 h under (1 h under nitrogen nitrogen stream) stream) was used was usedto produce to produce spherical spherical Al NPs Al with NPs withreasonably reasonably narrow narrow size sizedistribution distribution (2–15 (2–15 nm), nm), integrated integrated at atrandom random positions positions of of the the PVP PVP matrix. matrix. This transformation also releases H2 and volatile hydrosilanes as si sidede products. Thermal analysis of ◦ such composites unveiled their stability up to 422 °CC by gradual oxidation in air. In another interesting application, KumarKumar etet al.al. described described the the production production of of enantio-selective enantio-selective gallium particles by synthesizing them in aqueous solutions of D- or L-tryptophan, thereby inducing some imprinted chirality in their structure [[113].113]. Chiral Chiral imprinting could be achieved by ultrasonic ◦ irradiation (for 3 min at 55 °C)C) of of molten molten Ga Ga covered covered by by an an aqueous aqueous solution solution of of D- D- or or L-tryptophan, L-tryptophan, affording a grey suspension of particles, which were centrifuged at 6000 rpm for 10 min. The The resulting resulting products are micro- micro- or or nano-particles nano-particles of of Ga Ga encaps encapsulatingulating one one of of the the enantiomers. enantiomers. After After leaching leaching of theof the enantiomer enantiomer in inwater, water, molecular molecular scaffolds scaffolds are are left left on on the the surface surface of of the the Ga Ga particles, particles, which then enabled thethe entrapment entrapment of theof specificthe specific enantiomer enanti fromomer the racematefrom the (i.e., racemate DL-tryptophan). (i.e., DL-tryptophan). Unfortunately, Unfortunately,the enantiomeric the excesses enantiomeric were poor (fromexcesses 6% towere 12% eepoor) as determined(from 6% byto polarimetry,12% ee) as circular determined dichroism by polarimetry,and chiral HPLC. circular However, dichroism the method and chiral appears HPLC to. beHowever, promising the enough method en appears route to chiralto be imprintingpromising enoughmediated en by route metal to orchiral metal imprinting ion NPs generatedmediated underby metal sonication. or metal ion NPs generated under sonication. More recentlyrecently GedankenGedanken and and his his group, group, who who have ha signiﬁcantlyve significantly advanced advanced the subjectthe subject of Ga-based of Ga- basedmaterials materials sonofabrication, sonofabrication, reported reported a one-step a one-step procedure procedure that that yields yields Ga-doped Ga-doped carbon-dotscarbon-dots on Ga-NPs from polyethylene glycol (PEG 400) and molten gallium [114]. [114]. The resulting nanomaterial, Ga@C-dots@Ga-NPs, was was deposited deposited on on a a glass glass substrate substrate and and further further evaluated evaluated for for neural neural growth. growth. Cells grown on the aforementioned substrate show showeded a 97% increase in the number of branches resulting. It It is is noteworthy noteworthy that that the the sonochemical sonochemical protocol protocol (conducted (conducted with with a US a probe US probe at 20 atkHz, 20 kHz,70% ◦ amplitude,70% amplitude, 70 °C 70 forC 2.5 for h) 2.5 enabled h) enabled both both facile facile fabr fabricationication and and coating. coating. The The latter latter can can be attributed be attributed to ultrasonicto ultrasonic cleaning cleaning that that prevents prevents metal metal oxidatio oxidationn by by atmospheric atmospheric oxygen oxygen and and avoids avoids the the concomitant concomitant passivation on the C-dot surface. In a more-oriented synthetic application, Safari Safari et et al. al. reported reported a a convenient convenient synthesis synthesis of of 1,2,4,5- 1,2,4,5- tetrasubstituted imidazoles imidazoles ( (7070)) through a four-component one-step condensation of a substituted aromatic aldehyde ( 67), benzyl ( 68), phenyl amine ( 69), and ammonium acetate with nanocrystalline magnesium aluminate (MgAl2O4)) in in ethanol ethanol under under ultrasound ultrasound (Sch (Schemeeme 22)22)[ [115].115]. The The protocol protocol show show the typicaltypical advantages advantages of sonochemicallyof sonochemically accelerated accelerated reactions reactions in terms in ofterms high yields,of high mild yields, conditions, mild conditions,short reaction short times, reaction and easytimes, work-up. and easy work-up.

Scheme 22. Ultrasound-promoted synthesis of 1,2,4,5-tetrasubstituted imidazoles mediated by Mg/Al Scheme 22. Ultrasound-promoted synthesis of 1,2,4,5-tetrasubstituted imidazoles mediated by NPs.Mg/Al NPs.

5. Conclusions 5. Conclusions In this review we have provided some aspects and applications, mainly gathered in the last In this review we have provided some aspects and applications, mainly gathered in the last decade, of metals and metal ions, either massive or micro/nano-structured form, under the action of decade, of metals and metal ions, either massive or micro/nano-structured form, under the action of an ultrasonic field. The latter constitutes a well-known and established method that rivals and often an ultrasonic ﬁeld. The latter constitutes a well-known and established method that rivals and often surpasses the effects of conventional thermal protocols. Along with evident pluses of acceleration, surpasses the effects of conventional thermal protocols. Along with evident pluses of acceleration,

Catalysts 2017, 7, 121 24 of 29 selectivity, and good overall yields, sonication represents a low-cost strategy, affordable by most laboratories, which can be fine-tuned by simply adjusting empirical parameters such as frequency, input power, or solvents of choice as sound propagation and cavitational implosion are largely influenced by the fluid’s properties. In addition, ultrasonic devices can be combined with other irradiation and activation methods, and scaling up can also be easily accomplished. Clearly, one should look forward to developing and improving organic and organometallic reactions in the near future by placing metal derivatives in the vicinity of ultrasound beams.

Acknowledgments: The authors deeply acknowledge Universidad Nacional del Sur (UNS) and Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), the Junta de Extremadura-FEDER (Grant GR15022), and the University of Turin (Ricerca locale 2015) for financial support. Author Contributions: All authors contributed equally to the redaction of this review. All authors have given approval to the final version of the manuscript. Conflicts of Interest: The authors declare no conflict of interest. The founding institutions had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, as well as in the decision to publish the results.

References

1. Suslick, K.S.; Doktycz, S.J. Effects of ultrasound on surfaces and solids. In Advances in Sonochemistry; Mason, T.J., Ed.; JAI Press Ltd.: London, UK, 1990; Volume 1, pp. 197–230. 2. Pugin, B.; Turner, A.T. Inﬂuence of ultrasound on reactions with metals. In Advances in Sonochemistry; Mason, T.J., Ed.; JAI Press Ltd.: London, UK, 1990; Volume 1, pp. 81–118. 3. Tomlinson, W.J. Effect of ultrasonically induced cavitation on corrosion. In Advances in Sonochemistry; Mason, T.J., Ed.; JAI Press Ltd.: London, UK, 1990; Volume 1, pp. 173–195. 4. Luche, J.-L. Sonochemistry. From experiment to theoretical considerations. In Advances in Sonochemistry; Mason, T.J., Ed.; JAI Press Ltd.: London, UK, 1993; Volume 3, pp. 85–124. 5. Cintas, P.; Luche, J.-L. Organometallic sonochemistry. In Synthetic Organic Sonochemistry; Luche, J.-L., Ed.; Plenum Press: New York, NY, USA, 1998; Chapter 5; pp. 167–234. 6. Luche, J.-L. Ultrasonically promoted carbonyl addition reactions. In Advances in Sonochemistry; Mason, T.J., Ed.; JAI Press Ltd.: London, UK, 1990; Volume 1, pp. 119–171. 7. Cintas, P.; Palmisano, G.; Cravotto, G. Power ultrasound in metal-assisted synthesis: From classical Barbier-like reactions to click chemistry. Ultrason. Sonochem. 2011, 18, 836–841. [CrossRef][PubMed] 8. Manickam, S.; Kumar Rana, R. Production of nanomaterials using ultrasonic cavitation—A simple, energy efﬁcient and technological approach. In Ultrasound Technologies for Food and Bioprocessing; Feng, H., Barbosa-Cánovas, G.V., Weiss, J., Eds.; Springer: Heidelberg, Germany, 2011; Chapter 15; pp. 405–444. 9. Gedanken, A. Doping nanoparticles into polymers and ceramics using ultrasonic irradiation. Ultrason. Sonochem. 2007, 14, 418–430. [CrossRef][PubMed] 10. Xu, H.; Zeiger, B.W.; Suslick, K.S. Sonochemical synthesis of nanomaterials. Chem. Soc. Rev. 2013, 42, 2555–2567. [CrossRef] [PubMed] 11. Verdan, S.; Burato, G.; Comet, M.; Reinert, L.; Fuzellier, H. Structural changes of metallic surfaces induced by ultrasound. Ultrason. Sonochem. 2003, 10, 291–295. [CrossRef] 12. Skorb, E.V.; Möhwald, H.; Irrgang, T.; Fery, A.; Andreeva, D.V. Ultrasound-assisted design of metal nanocomposites. Chem. Commun. 2010, 46, 7897–7899. [CrossRef][PubMed] 13. Cravotto, G.; Cintas, P. Power ultrasound in organic synthesis: Moving cavitational chemistry from academia to innovative and large-scale applications. Chem. Soc. Rev. 2006, 35, 180–196. [CrossRef][PubMed] 14. Gedanken, A. Using sonochemistry for the fabrication of nanomaterials. Ultrason. Sonochem. 2004, 11, 47–55. [CrossRef][PubMed] 15. Gawande, M.B.; Bonifácio, V.D.B.; Luque, R.; Branco, P.S.; Varma, R.S. Solvent-free and catalysts-free chemistry: A benign pathway to sustainability. ChemSusChem 2014, 7, 24–44. [CrossRef][PubMed] 16. Thirumurugan, P.; Matosiuk, D.; Jozwiak, K. Click chemistry for drug development and diverse chemical-biology applications. Chem. Rev. 2013, 113, 4905–4979. [CrossRef][PubMed] 17. Witczak, Z.J.; Bielski, R. Click Chemistry in Glycoscience: New Developments and Strategies; Wiley-VCH: Weinheim, Germany, 2013. Catalysts 2017, 7, 121 25 of 29

18. Goldmann, A.S.; Glassner, M.; Inglis, A.J.; Barner-Kowollik, C. Post-functionalization of polymers via orthogonal ligation chemistry. Macromol. Rap. Commun. 2013, 34, 810–849. [CrossRef][PubMed] 19. Sreedhar, B.; Surendra Reddy, P. Sonochemical synthesis of 1,4-disubstituted 1,2.3-triazoles in aqueous medium. Synth. Commun. 2007, 37, 805–812. [CrossRef] 20. Cintas, P.; Martina, K.; Robaldo, B.; Garella, A.; Boffa, L.; Cravotto, G. Improved protocols for microwave- assisted Cu(I)-catalyzed Huisgen 1,3-dipolar cycloadditions. Collect. Czech. Chem. Commun. 2007, 72, 1014–1024. [CrossRef] 21. Cravotto, G.; Fokin, V.V.; Garella, D.; Binello, A.; Barge, A. Huisgen 1,3-dipolar cycloaddition catalyzed by metallic copper under ultrasound. J. Comb. Chem. 2010, 12, 13–15. [CrossRef][PubMed] 22. Cintas, P.; Barge, A.; Tagliapietra, T.; Boffa, L.; Cravotto, G. Alkyne-azide click reaction catalyzed by metallic copper under ultrasound. Nat. Protoc. 2010, 5, 607–616. [CrossRef][PubMed] 23. Klufers, P.; Piotrowski, H.; Uhlendorf, J. Homoleptic cuprates(II) with multiply deprotonated α-cyclodextrin ligands. Chem. Eur. J. 1997, 3, 601–608. [CrossRef] 24. Boffa, L.; Calcio Gaudino, E.; Martina, K.; Jicsinszky, L.; Cravotto, G. A new class of cationic cyclodextrins: Synthesis and chemico-physical properties. New J. Chem. 2010, 34, 2013–2019. [CrossRef] 25. Jacob, K.; Stolle, A.; Ondruschka, B.; Jandt, K.D.; Keller, T.F. Cu on porous glass: An easily recyclable catalyst for the microwave-assisted azide–alkyne cycloaddition in water. Appl. Catal. A Gen. 2013, 451, 94–100. [CrossRef] 26. Tu, N.P.; Hochlowski, J.E.; Djuric, S.W. Ultrasound-assisted click chemistry in continuous flow. Mol. Divers. 2012, 16, 53–58. [CrossRef][PubMed] 27. Sreedhar, B.; Surendra Reddy, P.; Veda Prakash, B.; Ravindra, A. Ultrasound-assisted rapid and efficient synthesis of propargylamines. Tetrahedron Lett. 2005, 46, 7019–7022. [CrossRef] 28. Cargnelutti, R.; Lang, E.S.; Schumacher, R.F. Bis(2-pyridyl)diselenoethers as versatile ligands for copper-catalyzed C-S bond formation in glycerol. Tetrahedron Lett. 2015, 56, 5218–5222. [CrossRef] 29. Pirola, C.; Bianchi, C.L.; Di Michele, A.; Diodati, P.; Boffito, D.; Ragaini, V. Ultrasound and microwave assisted synthesis of high loading Fe-supported Fischer-Tropsch catalysts. Ultrason. Sonochem. 2010, 17, 610–616. [CrossRef][PubMed] 30. Huang, W.; Li, H.; Zhu, B.; Feng, Y.; Wang, S.; Zhang, S. Selective hydrogenation of furfural to furfuryl alcohol over catalysts prepared via sonochemistry. Ultrason. Sonochem. 2007, 14, 67–74. [CrossRef][PubMed] 31. Arena, F.; Italiano, G.; Barbera, K.; Bonura, G.; Spadaro, L.; Frusteri, F. Basic evidences for methanol-synthesis catalyst design. Catal. Today 2009, 143, 80–85. [CrossRef] 32. Shih-Yuan, A.; Dai, W.-C. A simple and highly efficient preparation of arylstannane(s) via sonochemical Barbier reaction. Tetrahedron Lett. 1996, 37, 495–498. [CrossRef] 33. Lee, A.S.-Y.; Dai, W.-C. A facile and highly efficient sonochemical synthesis of organostannane via Barbier reaction. Tetrahedron 1997, 53, 859–868. [CrossRef] 34. David-Quillot, F.; Lunot, S.; Marsacq, D.; Duchêne, A. A novel access to organogermanium compounds. Tetrahedron Lett. 2000, 41, 4905–4907. [CrossRef] 35. Lamandé-Langle, S.; Abarbri, M.; Thibonnet, J.; Duchêne, A. A novel mode of access to polyfunctional organotin compounds and their reactivity in Stille cross-coupling reaction. J. Organomet. Chem. 2009, 694, 2368–2374. [CrossRef] 36. Domini, C.E.; Silbestri, G.F.; Fernández Band, B.; Chopa, A.B. Ultrasound-assisted synthesis of unsymmetrical biaryls by Stille cross-coupling reactions. Ultrason. Sonochem. 2012, 19, 410–414. [CrossRef][PubMed] 37. Lo Fiego, M.J.; Badajoz, M.A.; Domini, C.; Chopa, A.B.; Lockhart, M.T. Indium-mediated regioselective synthesis of ketones from arylstannanes under solvent free ultrasound irradiation. Ultrason. Sonochem. 2013, 20, 826–832. [CrossRef][PubMed] 38. Cintas, P. Synthetic organoindium chemistry: What makes indium so appealing? Synlett 1995, 1995, 1087–1096. [CrossRef] 39. Shen, Z.-L.; Wang, S.-Y.; Chok, Y.-K.; Xu, Y.-H.; Loh, T.-P. Organoindium reagents: The preparation and application in organic synthesis. Chem. Rev. 2013, 113, 271–401. [CrossRef][PubMed] 40. Cravotto, G.; Giovenzana, G.B.; Maspero, A.; Pilati, T.; Penoni, A.; Palmisano, G. Allylindation of 1H-indole- 3-carboxaldehyde in the presence of azoles–revisited. Tetrahedron Lett. 2006, 47, 6439–6443. [CrossRef] 41. Cravotto, G.; Giovenzana, G.B.; Palmisano, G.; Penoni, A.; Pilati, T.; Sisti, M.; Stazi, F. Convolutamydine A: The first authenticated absolute configuration and enantioselective synthesis. Tetrahedron Asymmetry 2006, 17, 3070–3074. [CrossRef] Catalysts 2017, 7, 121 26 of 29

42. Colombo, F.; Cravotto, G.; Palmisano, G.; Penoni, A.; Sisti, M. 3-Component indium-mediated domino allylation of 1H-indole-3-carboxaldehyde with electron-rich (het)arenes: Highly efﬁcient access to diversely functionalized indolylbutenes. Eur. J. Org. Chem. 2008, 2008, 2801–2807. [CrossRef] 43. Soengas, R.G.; Segade, Y.; Jiménez, C.; Rodríguez, J. Highly diastereoselective indium-mediated synthesis of β-lactam carbohydrates from imines. Tetrahedron 2011, 67, 2617–2622. [CrossRef] 44. Soengas, R.G. A straightforward route to novel α,α-disubstituted tetrahydrofuran β-amino acids and spirodiketopiperazines from sugar lactones. Synlett 2010, 2010, 2549–2552. [CrossRef] 45. Soengas, R.G.; Estévez, A.M. Indium-mediated allylation and Reformatsky reaction on glyoxylic oximes under ultrasound irradiation. Ultrason. Sonochem. 2012, 19, 916–920. [CrossRef][PubMed] 46. Ishihara, J.; Watanabe, Y.; Koyama, N.; Nishino, Y.; Takahashi, K.; Hatakeyama, S. Tetrahedron 2011, 67, 3659–3667. 47. Cooper, B.F.T.; MacDonald, C.L.B. Alternative syntheses of univalent indium salts including a direct route from indium metal. New J. Chem. 2010, 34, 1551–1555. [CrossRef] 48. Downs, A.J.; Himmel, H.-J. New light on the chemistry of the group 13 metals. In The Group 13 Metals Aluminium, Gallium, Indium and Thallium: Chemical Patterns and Peculiarities, 1st ed.; Aldridge, S., Downs, A.J., Eds.; John Wiley & Sons, Ltd.: Chichester, UK, 2011; p. 1. 49. Araki, S.; Ito, H.; Butsugan, Y. Gallium-induced Barbier coupling of carbonyl compounds with allyl iodide. Appl. Organomet. Chem. 1988, 2, 475–478. [CrossRef] 50. Goswami, D.; Chattopadhyay, A.; Sharma, A.; Chattopadhyay, S. [Bmim][Br] as a solvent and activator for the Ga-mediated Barbier allylation: Direct formation of an N-heterocyclic carbene from Ga metal. J. Org. Chem. 2012, 77, 11064–11070. [CrossRef][PubMed] 51. Qin, B.; Schneider, U. Catalytic use of elemental gallium for carbon-carbon bond formation. J. Am. Chem. Soc. 2016, 138, 13119–13122. [CrossRef][PubMed] 52. Kumar, V.B.; Gedanken, A.; Kimmer, G.; Porat, Z. Ultrasonic cavitation of molten gallium: Formation of micro- and nano-spheres. Ultrason. Sonochem. 2014, 21, 1166–1173. [CrossRef][PubMed] 53. Kumar, V.B.; Koltypin, Y.; Gedanken, A.; Porat, Z. Ultrasonic cavitation of molten gallium in water: Entrapment of organic molecules in gallium microspheres. J. Mater. Chem. A 2014, 2, 1309–1317. [CrossRef] 54. Shirazi, F.S.; Akhbari, K. Sonochemical procedures; the main synthetic methods for synthesis of coinage metal ion supramolecular polymer nanostructures. Ultrason. Sonochem. 2016, 31, 51–61. [CrossRef][PubMed]

55. Rahmani, F.; Haghighi, M.; Mahboob, S. CO2-enhanced dehydrogenation of ethane over sonochemically synthesized Cr/clinoptilolite-ZrO2 nanocatalyst: Effects of ultrasound irradiation and ZrO2 loading on catalytic activity and stability. Ultrason. Sonochem. 2016, 33, 150–163. [CrossRef][PubMed] 56. Dai, Q.; Wang, J.; Yu, J.; Chen, J.; Chen, J. Catalytic ozonation for the degradation of acetylsalicylic acid in

aqueous solution by magnetic CeO2 nanometer catalyst particles. Appl. Catal. B 2014, 144, 686–693. [CrossRef] 57. Gobouri, A.A. Ultrasound enhanced photocatalytic properties of α-Fe2O3 nanoparticles for degradation of dyes used by textile industry. Res. Chem. Intermed. 2016, 42, 5099–5113. [CrossRef] 58. Ziylan, A.; Ince, N.H. Catalytic ozonation of ibuprofen with ultrasound and Fe-based catalysts. Catal. Today 2014, 240, 5–8. [CrossRef] 59. Safari, J.; Gandomi-Ravandi, S. Application of the ultrasound in the mild synthesis of substituted 2,3- dihydroquinazolin-4(1H)-ones catalyzed by heterogeneous metal-MWCNTs nanocomposites. J. Mol. Struct. 2014, 1072, 173–178. [CrossRef]

60. Safari, J.; Zarnegar, Z. Biginelli reaction on Fe3O4–MWCNT nanocomposite: Excellent reactivity and facile recyclability of the catalyst combined with ultrasound irradiation. RSC Adv. 2013, 3, 17962–17967. [CrossRef] 61. Jiang, B.; Zhang, C.; Wang, K.; Dou, B.; Song, Y.; Chen, H.; Xu, Y. Highly dispersed Ni/montmorillonite catalyst for glycerol steam reforming: Effect of Ni loading and calcination temperature. Appl. Therm. Eng. 2016, 109, 99–108. [CrossRef] 62. Behling, R.; Chatel, G.; Valange, S. Sonochemical oxidation of vanillyl alcohol to vanillin in the presence of a cobalt oxide catalyst under mild conditions. Ultrason. Sonochem. 2017, 36, 27–35. [CrossRef][PubMed] 63. Kim, A.; Sharma, B.; Kim, B.-S.; Park, K.H. Double-hydrophilic block copolymer nanoreactor for the synthesis of copper nanoparticles and for application in click chemistry. J. Nanosci. Nanotechnol. 2011, 11, 6162–6166. [CrossRef][PubMed] Catalysts 2017, 7, 121 27 of 29

64. Stucchi, M.; Bianchi, C.L.; Pirola, C.; Cerrato, G.; Morandi, S.; Argirusis, C.; Sourkounie, G.; Naldonif, A.; Capucci, V. Copper NPs decorated titania: A novel synthesis by high energy US with a study of the photocatalytic activity under visible light. Ultrason. Sonochem. 2016, 31, 295–301. [CrossRef][PubMed] 65. Astruc, D. Palladium nanoparticles as efﬁcient green homogeneous and heterogeneous carbon-carbon coupling precatalysts: A unifying approach. Inorg. Chem. 2007, 46, 1884–1894. [CrossRef][PubMed] 66. Zhang, Z.; Zha, Z.; Gan, C.; Pan, C.; Zhou, Y.; Wang, Z.; Zhou, M.-M. Catalysis and regioselectivity of the aqueous Heck reaction by Pd(0) nanoparticles under ultrasonic irradiation. J. Org. Chem. 2006, 71, 4339–4342. [CrossRef][PubMed] 67. Gholap, A.R.; Venkatesan, K.; Pasricha, R.; Daniel, T.; Lahoti, R.J.; Srinivasan, K.V. Copper- and ligand-free Sonogashira reaction catalyzed by Pd(0) nanoparticles at ambient conditions under ultrasound irradiation. J. Org. Chem. 2005, 70, 4869–4872. [CrossRef][PubMed] 68. Saliman, M.A.; Okawa, H.; Takai, M.; Ono, Y.; Kato, T.; Sugawara, K.; Sato, M. Improved battery performance

using Pd nanoparticles synthesized on the surface of LiFePO4/C by ultrasound irradiation. Jpn. J. Appl. Phys. 2016, 55, 07KE05. [CrossRef]

69. Ziylan-Yavas, A.; Mizukoshi, Y.; Maeda, Y.; Ince, N.H. Supporting of pristine TiO2 with noble metals to enhance the oxidation and mineralization of paracetamol by sonolysis and sonophotolysis. Appl. Catal. B 2015, 172, 7–17. [CrossRef] 70. Kim, J.; Park, J.-E.; Momma, T.; Osaka, T. Synthesis of Pd-Sn nanoparticles by ultrasonic irradiation and their electrocatalytic activity for oxygen reduction. Electrochim. Acta 2009, 54, 3412–3418. [CrossRef] 71. Brancewicz, E.; Gradzka, E.; Basa, A.; Winkler, K. Chemical synthesis and characterization of the C60-Pd polymer spherical nanoparticles. Electrochim. Acta 2014, 128, 91–101. [CrossRef] 72. Tang, S.; Vongehr, S.; Zheng, Z.; Ren, H.; Meng, X. Facile and rapid synthesis of spherical porous palladium nanostructures with high catalytic activity for formic acid electro-oxidation. Nanotechnology 2012, 23, 255606. [CrossRef][PubMed] 73. Estifaee, P.; Haghighi, M.; Mohammadi, N.; Rahmani, F. CO oxidation over sonochemically synthesized

Pd–Cu/Al2O3 nanocatalyst used in hydrogen puriﬁcation: Effect of Pd loading and ultrasound irradiation time. Ultrason. Sonochem. 2014, 21, 1155–1165. [CrossRef][PubMed] 74. Colmenares, J.C.; Magdziarz, A.; Łomot, D.; Chernyayeva, O.; Lisovytskiy, D. A new photocatalytic tool in VOCs abatement: Effective synergetic combination of sonication and light for the synthesis of monometallic

palladium-containing TiO2. Appl. Catal. B 2014, 147, 624–632. [CrossRef] 75. Colmenares, J.C.; Magdziarz, A. Method of Depositing Metal Nanoparticles on the Surface of Semiconductor Materials and Surface Obtained by this Process. Patent No. P.401693, 20 November 2012. 76. Colmenares, J.C.; Lisowski, P.; Łomot, D.; Chernyayeva, O.; Lisovytskiy, D. Sonophotodeposition of bimetallic

photocatalysts Pd–Au/TiO2: Application to selective oxidation of methanol to methyl formate. ChemSusChem 2015, 8, 1676–1685. [CrossRef][PubMed] 77. Basavegowda, N.; Mishra, K.; Lee, Y.R. Ultrasonic-assisted green synthesis of palladium nanoparticles and their nanocatalytic application in multicomponent reaction. New J. Chem. 2015, 39, 972–977. [CrossRef] 78. Colmenares, J.C.; Aramendía, M.A.; Marinas, A.; Marinas, J.M.; Urbano, F.J. Synthesis, characterization and photocatalytic activity of different metal-doped titania systems. Appl. Catal. A Gen. 2006, 306, 120–127. [CrossRef] 79. Aramendía, M.A.; Colmenares, J.C.; Marinas, A.; Marinas, J.M.; Moreno, J.M.; Navío, J.A.; Urbano, F.J.

Effect of the redox treatment of Pt/TiO2 system on its photocatalytic behaviour in the gas phase selective photooxidation of propan-2-ol. Catal. Today 2007, 128, 235–244. [CrossRef] 80. Colmenares, J.C.; Aramendia, M.A.; Marinas, A.; Marinas, J.M.; Urbano, F.J. Titania nano-photocatalysts synthesized by ultrasound and microwave methodologies: Application in depuration of water from 3-chloropyridine. J. Mol. Catal. A Chem. 2010, 331, 58–63. [CrossRef] 81. López-Tenllado, F.J.; Marinas, A.; Urbano, F.J.; Colmenares, J.C.; Hidalgo, M.C.; Marinas, J.M.; Moreno, J.M. Selective photooxidation of alcohols as test reaction for photocatalytic activity. Appl. Catal. B 2012, 128, 150–158. [CrossRef] 82. Vinodgopal, K.; Neppolian, B.; Salleh, N.; Lightcap, I.V.; Grieser, F.; Ashokkumar, M.; Ding, T.T.; Kamat, P.V. Dual-frequency ultrasound for designing two dimensional catalyst surface: Reduced graphene oxide-Pt composite. Coll. Surf. A 2012, 409, 81–87. [CrossRef] Catalysts 2017, 7, 121 28 of 29

83. Cravotto, G.; Cintas, P. Sonication-assisted fabrication and post-synthetic modiﬁcations of graphene-like materials. Chem. Eur. J. 2010, 16, 5246–5259. [CrossRef][PubMed] 84. Ciesielski, A.; Samori, P. Graphene via sonication assisted liquid-phase exfoliation. Chem. Soc. Rev. 2014, 43, 381–398. [CrossRef][PubMed] 85. Toukoniitty, B.; Toukoniitty, E.; Mäki-Arvela, P.; Mikkola, J.-P.; Salmi, T.; Murzin, D.Y.; Kooyman, P.J. Effect of ultrasound in enantioselective hydrogenation of 1-phenyl-1,2-propanedione: Comparison of catalyst activation, solvents and supports. Ultrason. Sonochem. 2006, 13, 68–75. [CrossRef][PubMed]

86. Perkas, N.; Rotter, H.; Vradman, L.; Landau, M.V.; Gedanken, A. Sonochemically prepared Pt/CeO2 and its application as a catalyst in ethyl acetate combustion. Langmuir 2006, 22, 7072–7077. [CrossRef][PubMed] 87. Selvaraj, V.; Alagar, M. Ethylene glycol oxidation on Pt and Pt-Ru nanoparticle decorated polythiophene/multiwalled carbon nanotube composites for fuel cell applications. Nanotechnology 2008, 19, 045504. [CrossRef][PubMed] 88. Jang, J.-H.; Kim, J.; Lee, Y.-H.; Kim, I.Y.; Park, M.-H.; Yang, C.-W.; Hwang, S.-J.; Kwon, Y.-U. One-pot synthesis

of core-shell-like Pt3Co nanoparticle electrocatalyst with Pt-enriched surface for oxygen reduction reaction in fuel cells. Energy Environ. Sci. 2011, 4, 4947–4953. [CrossRef] 89. Jiang, R.; Rong, C.; Chu, D. Surface coverage of Pt atoms on PtCo nanoparticles and catalytic kinetics for oxygen reduction. Electrochim. Acta 2011, 56, 2532–2540. [CrossRef] 90. Nagao, D.; Shimazaki, Y.; Saeki, S.; Kobayashi, Y.; Konno, M. Effect of ultrasonic irradiation on carbon- supported Pt-Ru nanoparticles prepared at high metal concentration. Coll. Surf. A 2007, 302, 623–627. [CrossRef] 91. Ruiz-Camacho, B.; Álvarez, O.M.; Rodríguez-Santoyo, H.H.; López-Peréz, P.A.; Fuentes-Ramírez, R. Mono and bi-metallic electrocatalysts of Pt and Ag for oxygen reduction reaction synthesized by sonication. Electrochem. Commun. 2015, 61, 5–9. [CrossRef] 92. Cinelli, M.; Coles, S.R.; Nadagouda, M.N.; Blaszczyński,J.; Slowiński,R.; Varma, R.S.; Kirwan, K. A green chemistry-based classification model for the synthesis of silver nanoparticles. Green Chem. 2015, 17, 2825–2839. [CrossRef] 93. Chernousova, S.; Epple, M. Silver as antibacterial agent: Ion, nanoparticle, and metal. Angew. Chem. Int. Ed. 2013, 52, 1636–1653. [CrossRef][PubMed] 94. Manjamadha, V.P.; Muthukumar, K. Ultrasound assisted green synthesis of silver nanoparticles using weed plant. Bioprocess Biosyst. Eng. 2016, 39, 401–411. [CrossRef][PubMed] 95. Sun, L.; Li, J.; Wang, C.; Li, S.; Lai, Y.; Chen, H.; Lin, C. Ultrasound aided photochemical synthesis of Ag loaded TiO2 nanotube arrays to enhance photocatalytic activity. J. Hazard. Mater. 2009, 171, 1045–1050. [CrossRef][PubMed] 96. Jhuang, Y.Y.; Cheng, W.T. Fabrication and characterization of silver/titanium dioxide composite nanoparticles in ethylene glycol with alkaline solution through sonochemical process. Ultrason. Sonochem. 2016, 28, 327–333. [CrossRef][PubMed] 97. Pech-Pech, I.E.; Gervasio, D.F.; Godínez-Garcia, A.; Solorza-Feria, O.; Pérez-Robles, J.F. Nanoparticles of Ag with a Pt and Pd rich surface supported on carbon as a new catalyst for the oxygen electroreduction reaction (ORR) in acid electrolytes: Part 1. J. Power Sources 2015, 276, 365–373. [CrossRef] 98. Pech-Pech, I.E.; Gervasio, D.F.; Pérez-Robles, J.F. Nanoparticles of Ag with a Pt and Pd rich surface supported on carbon as a new catalyst for the oxygen electroreduction reaction (ORR) in acid electrolytes: Part 2. J. Power Sources 2015, 276, 374–381. [CrossRef] 99. Vu, T.A.; Dao, C.D.; Hoang, T.T.T.; Dang, P.T.; Tran, H.T.K.; Nguyen, K.T.; Le, G.H.; Nguyen, T.V.; Lee, G.D. Synthesis of novel silver vanadates with high photocatalytic and antibacterial activities. Mater. Lett. 2014, 123, 176–180. [CrossRef] 100. Lei, Z.; Zhang, L.; Wei, X. One-step synthesis of silver nanoparticles by sonication or heating using amphiphilic block copolymer as templates. J. Coll. Interface Sci. 2008, 324, 216–219. [CrossRef][PubMed] 101. Saha, S.K.; Roy, P.; Saini, P.; Mondal, M.K.; Chowdhury, P.; Babu, S.P.S. Carbohydrate polymer inspired silver nanoparticles for filaricidal and mosquitocidal activities: A comprehensive view. Carbohydr. Polym. 2016, 137, 390–401. [CrossRef][PubMed] 102. Kuo, T.-C.; Hsu, T.-C.; Liu, Y.-C.; Yang, K.-H. Size-controllable synthesis of surface-enhanced Raman

scattering-active gold nanoparticles coated on TiO2. Analyst 2012, 137, 3847–3853. [CrossRef][PubMed] Catalysts 2017, 7, 121 29 of 29

103. Wen, B.; Ma, J.; Chen, C.; Ma, W.; Zhu, H.; Zhao, J. Supported noble metal nanoparticles as photo/sono-catalysts for synthesis of chemicals and degradation of pollutants. Sci. China Chem. 2011, 54, 887–897. [CrossRef] 104. George, P.P.; Gedanken, A.; Perkas, N.; Zhong, Z. Selective oxidation of CO in the presence of air over

gold-based catalysts Au/TiO2/C (sonochemistry) and Au/TiO2/C (microwave). Ultrason. Sonochem. 2008, 15, 539–547. [CrossRef][PubMed] 105. Mizukoshi, Y.; Makise, Y.; Shuto, T.; Hu, J.; Tominaga, A.; Shironita, S.; Tanabe, S. Immobilization of noble

metal nanoparticles on the surface of TiO2 by the sonochemical method: Photocatalytic production of hydrogen from an aqueous solution of ethanol. Ultrason. Sonochem. 2007, 14, 387–392. [CrossRef][PubMed] 106. Kan, C.; Zhu, J.; Wang, C. Ag nanoparticle-ﬁlled polymer shell formed around Au nanoparticle core via ultrasound-assisted spherulite growth. J. Cryst. Growth 2009, 311, 1565–1570. [CrossRef] 107. Park, J.-E.; Atobe, M.; Fuchigami, T. Sonochemical synthesis of conducting polymer-metal nanoparticles nanocomposite. Electrochim. Acta 2005, 51, 849–854. [CrossRef] 108. Fang, Y.; Li, X.; Hu, Y.; Li, F.; Lin, X.; Tian, M.; Anc, X.; Fud, Y.; Jina, J.; Ma, J. Ultrasonication-assisted ultrafast

preparation of multiwalled carbon nanotubes/Au/Co3O4 tubular hybrids as superior anode materials for oxygen evolution reaction. J. Power Sources 2015, 300, 285–293. [CrossRef] 109. Yusof, N.S.M.; Ashokkumar, M. Sonochemical synthesis of gold nanoparticles by using high intensity focused ultrasound. ChemPhysChem 2015, 16, 775–781. [CrossRef][PubMed] 110. Vilela, S.M.F.; Ananias, D.; Fernandes, J.A.; Silva, P.; Gomes, A.C.; Silva, N.J.O.; Rodrigues, M.O.; Tomé, J.P.C.; Valente, A.A.; Ribeiro-Claro, P.; et al. Multifunctional micro- and nanosized metal-organic frameworks assembled from bisphosphonates and lanthanides. J. Mater. Chem. C 2014, 2, 3311–3327. [CrossRef] 111. Amith Yadav, H.J.; Eraiah, B.; Nagabhushana, H.; Darshan, G.P.; Daruka Prasad, B.; Sharma, S.C.; Premkumar, H.B.; Anantharaju, K.S.; Vijayakumar, G.R. Facile ultrasound route to prepare micro/nano superstructures for multifunctional applications. ACS Sust. Chem. Eng. 2017, 5, 2061–2074. [CrossRef] 112. Gottapu, S.; Padhi, S.K.; Krishna, M.G.; Muralidharan, K. Poly(vinylpyrrolidone) stabilized aluminum

nanoparticles obtained by the reaction of SiCl4 with LiAlH4. New J. Chem. 2015, 39, 5203–5207. [CrossRef] 113. Kumar, V.B.; Mastai, Y.; Porat, Z.; Gedanken, A. Chiral imprinting in molten gallium. New J. Chem. 2015, 39, 2690–2696. [CrossRef] 114. Nissan, I.; Kumar, V.B.; Porat, Z.; Makovec, D.; Sheﬁ, O.; Gedanken, A. Sonochemically-fabricated Ga@C-dots@Ga nanoparticle-aided neural growth. J. Mater. Chem. B 2017, 5, 1371–1379. [CrossRef] 115. Safari, J.; Gandomi-Ravandi, S.; Akbari, Z. Sonochemical synthesis of 1,2,4,5-tetrasubstituted imidazoles

using nanocrystalline MgAl2O4 as an effective catalyst. J. Adv. Res. 2013, 4, 509–514. [CrossRef][PubMed]

© 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).