Mechanochemical Preparation of Pd (II) and Pt (II) Composites With

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Article Mechanochemical Preparation of Pd(II) and Pt(II) Composites with Carbonaceous Materials and Their Application in the Suzuki-Miyaura Reaction at Several Energy Inputs

Mohamed M.A. Soliman 1,2 , Andreia F. Peixoto 3 , Ana P.C. Ribeiro 1 , Maximilian N. Kopylovich 1,* , Elisabete C.B.A. Alegria 1,2,* and Armando J.L. Pombeiro 1

1 Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisboa, Portugal; [email protected] (M.M.A.S.); [email protected] (A.P.C.R.); [email protected] (A.J.L.P.) 2 Área Departamental de Engenharia Química, ISEL, Instituto Politécnico de Lisboa, 1959-007 Lisboa, Portugal 3 REQUIMTE/LAQV, Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre s/n, 4169-007 Porto, Portugal; [email protected] * Correspondence: [email protected] (M.N.K.); [email protected] (E.C.B.A.A.); Tel.: +351-218317163 (E.C.B.A.A.) Academic Editors: Raquel G. Soengas and Humberto Rodríguez-Solla Received: 28 May 2020; Accepted: 23 June 2020; Published: 26 June 2020

Abstract: Pd(II) and Pt(II) composites with activated carbon (AC), graphene oxide, and multiwalled carbon nanotubes were prepared by ball milling and used as catalysts for the Suzuki-Miyaura reaction, under several energy inputs (mechanical grinding, conventional heating, and microwave irradiation). The catalytic composites were characterized by ICP-MS, BET, XPS analyses, TEM, and SEM. The average particle size of the prepared composites was estimated to be in the range of 6–30 nm, while the loadings of Pd and Pt did not signiﬁcantly aﬀect the surface area of the AC support due to the tendency to agglomerate as observed by the TEM analysis. The Pd/AC composites exhibit high mechanochemical catalytic activity in cross-coupling of bromobenzene and phenylboronic 1 acid with molar yields up to 80% with TON and TOF of 222 and 444 h− , respectively, achieved with Pd(4.7 wt%)-AC catalyst under the liquid assisted grinding for 0.5 h at ambient conditions, using cyclohexene as an additive.

Keywords: mechanochemical; liquid-assisted grinding; Suzuki-Miyaura reaction; cross-coupling; Palladium; Platinum; composites

1. Introduction The Suzuki-Miyaura (SM) cross-coupling of aryl halides with aryl boronic acids to form biaryls has become an exceptionally powerful tool in organic synthesis [1–4]. The notable expansion of this reaction is based on its applicability to a wide range of substrates with various functional groups and availability of the boronic acid starting materials. Generally, this chemical transformation needs a transition metal catalyst to proceed at a synthetically useful rate, and catalysts based on Pd dominate the scene [1–4]. The homogeneous catalytic conditions were the first choice of chemists because of high activity and selectivity; however, difficulties in the catalyst recovery and product purification are obvious drawbacks even at the laboratory scale. Moreover, catalysts that are active but difficult to recycle/recover from reaction mixture are generally not favored in the chemical industry. Hence, the recovery and reusability of catalysts is critically important for a sustainable development of the SM cross-coupling processes. These issues can be overcome by e.g., the catalyst heterogenization on

Molecules 2020, 25, 2951; doi:10.3390/molecules25122951 www.mdpi.com/journal/molecules Molecules 2020, 25, 2951 2 of 14 solid supports. However, in many cases the heterogenization decreases the overall catalytic efficiency, being also expensive and difficult to achieve on industrial scale. Therefore, elaboration of simple, scalable, eco-friendly and fast technics to prepare highly active and reusable dispersed catalysts for the SM reaction constitutes a significant challenge [5–7], and mechanochemical methods can eventually provide an effective solution [8–12]. The mechanochemical preparative technics involve chemical and physico-chemical transformations of substances produced by the effect of mechanical energy. These include influencing reactivity of solids by the presence of solid-state defects, interphase interactions and relaxations, which can enable time-convenient one-step syntheses of new nanomaterials with the desired properties in a reproducible way with high yields and under simple and easy operating conditions [8–12]. Due to these and other phenomena (e.g., formation of “hot spots” or mechanically stimulated phase transformations) [8], many mechanochemical processes take place under environmentally friendly and essentially waste-free conditions. Another advantage of the mechanochemical preparative technics involves their easy scaling up with the industrial milling equipment already available [8]. As it was mentioned above, Pd(II) salts or complexes are usually employed as catalytic agents for cross-couplings and related reactions. Such compounds are then reduced in situ to Pd(0) species which enter the catalytic cycle. The catalytic cycle involves the rate-determining activation of the aryl halide by oxidative addition, transmetalation and reductive elimination [1–7]. In each of these steps an effective charge transport from the metal nanoparticles to the π system of the aryl halides or boronic acids is important, and hence physicochemical properties of the support are relevant for the successful metal heterogenization, dispersion, charge transport, and, as result, the overall catalytic activity. The delocalized system of π electrons of carbonaceous materials is beneficial for π-π interactions with aromatic groups of substrates and for constructing electron-enriched vacancy defects which facilitate redox processes [9,13–16]. Due to these reasons as well as their high chemical stability, mechanical resistance, large surface area, tunable electronic, and physicochemical features, the carbon-based materials are broadly used as catalysts and catalyst supports in many industrially important processes [17–21], while mechanochemical methods have been applied for their preparation and functionalization [9–12,22–24]. Moreover, the use of supported Pd may prevent its aggregation upon reduction with the respective drop in activity [1–7]. Among different carbon materials, activated carbon (AC) is the most used and studied support for Pd catalysts and in the C-C coupling chemistry due to its low price and abundance, unique chemical and mechanical properties [20,21,25–29]. Thus, carbon is a high tensile material, able to sustain the amount of energy produced in ball milling, with the advantage of not inhibiting the SM reaction. Accordingly, in this study we intend to develop this line further and to prepare new composites composed from palladium salts and activated carbon as a support and component of catalytic system. For comparative purposes, platinum salts will be also tried as the main catalytic species and graphene oxide (GO) and multiwalled carbon nanotubes (MWCNT) as supports. From another prospective, not only the preparation of catalysts can be achieved by mechanochemical procedures, but also the catalytic reactions themselves [10,12]. The application of mechanical energy to promote C-C coupling is a hot topic nowadays, and several prominent papers have been published highlighting different aspects of this field [30–34]. The advantages of the mechanochemical treatment include decreased reaction times, simplified work-up procedures, omission or minimization of solvents and better energy balance in comparison with the conventional procedures. According to this view, in this study we intend to prepare “(metal salt)-(carbon material)” composites (where metal is Pd or Pt and carbon material is AC, GO, or MWCNT) by simple mechanochemical procedure and apply the prepared composites as catalysts in the mechanically activated SM model reaction, namely coupling of phenylboronic acid and bromobenzene as the aryl halide partner (Scheme1). For comparative purposes, other energy inputs, such as the conventional heating in oil bath with magnetic stirring and microwave (MW) irradiation, were also studied. Molecules 2020, 25, x FOR PEER REVIEW 3 of 14

Moleculeshalide partner2020, 25, (Scheme 2951 1). For comparative purposes, other energy inputs, such as the conventional3 of 14 heating in oil bath with magnetic stirring and microwave (MW) irradiation, were also studied.

Scheme 1. 1. SuzukiSuzuki-Miyaura-Miyaura reaction reaction of of bromobenzene bromobenzene and and phenylboronic aciacidd (typical reaction conditions). 2. Results and Discussion 2. Results and Discussion 2.1. Preparation of Pd and Pt Catalysts 2.1. Preparation of Pd and Pt Catalysts Pd(II) and Pt(II) composites were prepared by ball milling using their respective Pd(CH3COO)2 Pd(II) and Pt(II) composites were prepared by ball milling using their respective Pd(CH3COO)2 and K2PtCl4 salts as Pd and Pt sources, respectively, and commercially available AC as support and K2PtCl4 salts as Pd and Pt sources, respectively, and commercially available AC as support (see (see Section 3.1., Table1 and Electronic Supporting Material for details). The approximate theoretical Section 3.1., Table 1 and Electronic Supporting Material for details). The approximate theoretical metal-to-carbon ratio was 4.7 and 8.5 wt%, while the actual metal content was determined by Inductively metal-to-carbon ratio was 4.7 and 8.5 wt%, while the actual metal content was determined by Coupled Plasma-Mass Spectroscopy (ICP-MS, Table2). The prepared catalysts will hereinafter be Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS, Table 2). The prepared catalysts will denoted as AC-Pd1 and AC-Pt1 for the theoretical metal content of ca. 4.7 wt%, and AC-Pd2 and hereinafter be denoted as AC-Pd1 and AC-Pt1 for the theoretical metal content of ca. 4.7 wt%, and AC-Pt2 for the respective contents of ca. 8.5 wt%. For comparative purposes, palladium composites AC-Pd2 and AC-Pt2 for the respective contents of ca. 8.5 wt%. For comparative purposes, palladium with GO and MWCNT were similarly prepared by grinding in ball mill. As it was indicated above, composites with GO and MWCNT were similarly prepared by grinding in ball mill. As it was the use of the carbon-based material aims to improve the catalyst activity and stability in the reaction indicated above, the use of the carbon-based material aims to improve the catalyst activity and medium, allowing its recovery and reuse [2–7,17–21,25–29]. stability in the reaction medium, allowing its recovery and reuse [2–7,17–21,25–29]. Table 1. Denotation of the prepared composites. Table 1. Denotation of the prepared composites. Metal Precursor Pd(OAc)2 K2PtCl4 Metal Precursor Pd(OAc)2 K2PtCl4 Support (100 mg) 11 mg 22 mg 11 mg 22 mg Support (100 mg) 11 mg 22 mg 11 mg 22 mg AC AC-Pd1 AC-Pd2 AC-Pt1 AC-Pt2 AC MWCNTsAC-Pd1 CNT-Pd1AC-Pd2 CNT-Pd2 AC- CNT-Pt1Pt1 CNT-Pt2AC-Pt2 MWCNTs GOCNT-Pd1 CNT-Pd2 GO-Pd2 CNT-Pt1 CNT-Pt2 GO GO-Pd2 Table 2. Composition of the Pd/AC and Pt/AC composites. Table 2. Composition of the Pd/AC and Pt/AC composites. Pd(wt%) Pt(wt%) Sample TheoreticalPd(wt%) Found (ICP-MS) TheoreticalPt(wt%) Found (ICP-MS) Sample AC-Pd1Theoretical 4.7 Found (ICP 4.13-MS) Theoretical - Found (ICP - -MS) ACAC-Pd2-Pd1 4.78.5 4.13 7.63 - -- - AC-Pd2 8.5 7.63 - - AC-Pt1 - - 4.6 4.47 AC-Pt1 - - 4.6 4.47 ACAC-Pt2-Pt2 - -- -8.5 8.5 7.28 7.28

2.2. Characterization of the Pd/AC Pd/AC and PtPt/AC/AC CompositesComposites TToo deﬁne define the the surface surface morphology morphology of the the prepared prepared composites, composites, a set of characterization characterization techniques [ICP-MS,[ICP-MS, Scanning Scanning Electron Electron Microscopy Microscopy (SEM) and (SEM Energy) and Dispersive Energy X-ray Dispersive Spectroscopy X-ray (EDX),Spectroscopy Transmission (EDX), Transmission Electron Microscopy Electron (TEM), Microscopy X-ray (TEM), Photoelectron X-ray Photoelectron Microscopy Microscopy (XPS) and Brunauer-Emmett-Teller(XPS) and Brunauer-Emmett Surface-Teller Area Surface Analysis Area (BET)] Analysis has been(BET) undertaken.] has been undertaken. ICPICP-MS-MS analysis of the supported Pd and Pt catalysts was carried out to evaluate the actual loading amountsamounts ofof Pd Pd and and Pt Pt on on AC. AC. As As can can be be seen see fromn from Table Table2, the 2, the actual actual loading loading amounts amounts of Pd of and Pd Ptand on Pt AC on areAC 4.13are 4. wt%13 wt (for% AC-Pd1),(for AC-Pd1 7.63%), 7.63 (for% AC-Pd2),(for AC-Pd2 4.47%), 4.47 (for% AC-Pt1),(for AC-Pt1 and), and 7.28% 7.28 (for% AC-Pt2),(for AC- respectively,Pt2), respectively giving, giving more more than 80%than of 80% the of full the deposition. full deposition. The performed TEM (Figure1) and SEM (Figure2) conﬁrmed the presence of Pd and Pt particles supported on the AC surface. The produced dispersed materials consist of non-uniform crystallites MoleculesMoleculesMolecules 20202020 2020,, 25,25 25,, ,xx x FORFOR FOR PEER PEERPEER REVIEWREVIEW 4 of 4144 ofof 1414

Molecules 2020, 25, 2951 4 of 14 TheTheThe pp perererformedformedformed TEM TEMTEM (Figure(Figure 1)1) and andand SEM SEMSEM (Figure (Figure(Figure 2) 2)2) confirm confirmconfirmededed the thethe presence presencepresence of Pdofof Pd Pdand andand Pt particles PtPt particlesparticles supportsupportsupportededed onon on the thethe AC ACAC surface.surface. TheThe producedproduced dispersed disperseddispersed materials materialsmaterials consist consistconsist of ofofnon nonnon-uniform--uniformuniform crystallites crystallitescrystallites withwithwith tendencytendency tendency to toto agglomerate agglomerate agglomerate (( (ovalsovalsovals ininin Figure FigureFigure 1 11););)) ;this; thisthis creates createscreates some somesome difficult didifficultdifficultﬃcultiesiesiesies to todefineto definedefine deﬁne the the theparticle particleparticle sizesizesize and and and distribution. distribution. distribution. Nevertheless, Nevertheless, thethethe evaluation evaluationevaluation of of ofof the the thethe particle particle particleparticle sizes sizes sizessizes and/or and and/orand/or/or agglomerates agglomerates agglomeratesagglomerates present present presentpresent in in the prepared materials can be performed by the digital microscopy Motic plus image software, ininthe thethe prepared preparedprepared materials materialsmaterials can cancan be performed bebe performedperformed by the byby digital thethe digitaldigital microscopy microscopymicroscopy Motic MoticMotic plus image plusplus imageimage software, softwaresoftware giving,, giving average particle size around 18 nm for the AC-Pd2 dispersed nanomaterial (Figure 3). givinggivingaverage averageaverage particle particleparticle size around sizesize aroundaround 18 nm 18 for18 nmnm the forfor AC-Pd2 thethe ACAC dispersed--Pd2Pd2 disperseddispersed nanomaterial nanomaterialnanomaterial (Figure (Figure3(Figure). 3).3).

Figure 1. TEM images of AC-Pd1 (a), AC-Pd2 (b) and AC-Pt2 (c). FigureFigure 1.1. TEMTEMTEM imagesimages images ofof of ACAC AC-Pd1--Pd1Pd1 (( aaa))),,, ACAC AC-Pd2--Pd2Pd2 (( (bb))) and and ACAC AC-Pt2--Pt2Pt2 (( (cc)))...

Figure 2. SEM images of AC-Pd1 (a,b), AC-Pd2 (c,d), 10 µm scale (a,c) and 2000× magnification (b,d).

FigureFigure 2.2. SEMSEMSEM imagesimages images ofof of ACAC AC-Pd1--Pd1Pd1 (( aa,,,bb),),), ACAC AC-Pd2--Pd2Pd2 (( cc,,dd),), 1010 µmµmµm scale scale ( (aa,,cc)) and and 2000× 20002000× magnificationmagnificationmagniﬁcation (( bb,,,dd)))... ×

14 1616 12 1414 10 1212

Count 1010 6

88 4 Count Count 66 2

44 0 10 15 20 25 30 22 Length( μm)

00 Figure10 3.10 Particle size1515 distribution2020 in the AC-Pd22525 sample.3030 Figure 3. Particle size distribution in the AC-Pd2 sample. LengthLength( ( μm)μm) The SEM images for the AC-Pd1 (Figure2a) and AC-Pd2 (Figure2c) composites demonstrate micro (below 1 µm) irregularities. In the back-scatter mode, the metal can be diﬀerentiated from FigureFigure 3.3. ParticleParticle sizesize distributiondistribution inin thethe ACAC--Pd2Pd2 samplesample.. support; elements of greater atomic mass (e.g., Pd) appear brighter in a back scattered electron

Molecules 2020, 25, 2951 5 of 14 Molecules 2020, 25, x FOR PEER REVIEW 5 of 14 image. BothThe SEM images images were for collected the AC-Pd1 at (Figure magnification 2a) and AC 2000-Pd2(Figure (Figure2 2b,dc) composites for AC-Pd1 demonstrate and AC-Pd2, × respectively)micro (below and, 1 as µm) expected, irregularities the visible. In the amount back-scatter of dispersed mode, the Pd metal is low, can what be differentiated is in accordance from with support; elements of greater atomic mass (e.g., Pd) appear brighter in a back scattered electron image. low Pd loadings. The EDX analysis was performed for AC-Pd1 (Figure S1a) and AC-Pd2 (Figure S1b) Both images were collected at magnification 2000× (Figure 2b,d for AC-Pd1 and AC-Pd2, composites to determine the elemental composition and to confirm that Pd is the only hetero element. respectively) and, as expected, the visible amount of dispersed Pd is low, what is in accordance with The EDXlow Pd analysis loadings allowed. The EDXto analysis conclude was performed that possible for AC contamination-Pd1 (Figure S1 duea) and to AC the-Pd2 multiple (Figure collisionsS1b) betweencomposites medium, to determine reactor, andthe elemental grinding composition balls, detected and to in confirm some mechanochemicalthat Pd is the only hetero reactions element. [35 –39], was notThe observed. EDX analysis allowed to conclude that possible contamination due to the multiple collisions betweenThe qualitative medium, andreactor quantitative, and grinding surface balls, characterizationdetected in some mechanochemical of the Pd/AC and reactions Pt/AC [35 composites–39], werewas also not performed observed. by XPS (Figure4, Figures S2 and S3 and Table S1). No significant changes were observedThe in thequalitative high-resolution and quantitative spectra surface of the characterization catalysts prepared of the with Pd/AC di ffanderent Pt/AC content composites of Ptor Pd were also performed by XPS (Figures 4, S2,3 and Table S1). No significant changes were observed in precursors. High resolution Pd 3d region spectra presents a doublet corresponding to Pd 3d5/2 and the high-resolution spectra of the catalysts prepared with different content of Pt or Pd precursors. Pd 3d3/2 at 337.8 and 343.1 eV suggesting the predominant presence of Pd(II) species in the surface High resolution Pd 3d region spectra presents a doublet corresponding to Pd 3d5/2 and Pd 3d3/2 at of activated337.8 and carbon 343.1 eV (> suggesting80%) [40]. the The predominant presence presence of Pd(0) of was Pd(II) not species detected in the on surface the surface, of activated although previouslycarbon being(>80%) identified [40]. The presence by other of authors Pd(0) was in not similar detected systems on the [41 surface,]. The although small peaks previously observed being at 340.0, 341.7identified and 345.2 by eV other could authors be related in similar to parentalsystems [41]. species The small from peaks the Pd observed precursor at 340.0, [42]. 341.7 The deconvolutionand 345.2 of high-resolutioneV could be related spectra to parental Pt 4f region species (Figure from 4 the) revealed Pd precursor two bands, [42]. The each deconvolution comprised of of high a pair- of doublets:resolution the Ptspectra 4f7/2 Ptband 4f region at low (Figure binding 4) revealed energy two 72.2 ban eVds, and each Pt comprised 4f5/2 band of ata pair 75.5 of eV doublets: assigned to Pt(II).the The Pt 4f other7/2 band two at peakslow binding at higher energy BEs 72.2 74.2 eV (Pt4fand Pt7/ 24f)5/2 and band 77.5 at 75.5 eV (Pt4feV assigned5/2) correspond to Pt(II). The to Pt(IV)other [43]. The presencetwo peaks ofat higher divalent BEs platinum 74.2 (Pt4f7/2 in) and this 77.5 catalyst eV (Pt4f could5/2) correspond be due to to decomposition Pt(IV) [43]. The ofpresence the platinum of precursordivalent during platinum ball in milling this catalyst [44]. XPScould survey be due analysis to decomposition also revealed of the the platinum presence precursor of chlorine during species ball milling [44]. XPS survey analysis also revealed the presence of chlorine species on the carbon on the carbon surface (Figure S2) showing an incomplete decomposition of the platinum precursor surface (Figure S2) showing an incomplete decomposition of the platinum precursor under the used underexperimental the used experimental conditions. conditions.

Figure 4. XPS proﬁles of the catalysts in the Pd 3d and Pt 4f regions. Figure 4. XPS profiles of the catalysts in the Pd 3d and Pt 4f regions. In the carbon spectra a small peak at 290 eV was observed revealing the presence of carboxyl In the carbon spectra a small peak at 290 eV was observed revealing the presence of carboxyl groups, in addition to other typical peaks including the main peak of carbon at 284.6 eV (Figure S3). groups, in addition to other typical peaks including the main peak of carbon at 284.6 eV (Figure S3). Additionally, the high resolution O1s region spectra of the composites (Figure S3) give additional Additionally, the high resolution O1s region spectra of the composites (Figure S3) give additional informationinformation about about surface surface oxygen oxygen groups, groups, includingincluding the the peaks peaks at at ~531.5 ~531.5 eV eVattribute attributedd to C to-O/C=O C-O/ C=O andand M-O M functional-O functional groups groups and and di differentﬀerent peakspeaks at at 534.2 534.2 eV eV for for Pd/AC Pd/AC and and at 533.3 at 533.3 eV for eV Pt/AC for Pt /AC compositescomposites attributed attributed to the to presence the presence of carboxylic of carboxylic acid and acid anhydride and anhydride functional functional groups, groups, respectively. The contributionrespectively. The of chemisorbed contribution waterof chemisorbed located at water 536 eV located was observedat 536 eV inwas the observed Pd/AC samples.in the Pd/AC The ratio of totalsamples. oxygen The to ratio total of carbon, total oxygen O/C to (Table total carbon, S1,) relates O/C (Table to the S1 surface,) relates oxidation to the surface [45]. oxidation An increase [45]. in the oxygen content for the Pd/AC composites was observed in comparison to the pristine AC, probably as result of the carbon bonds cleavage during ball milling with the respective surface oxidation or due to a decomposition of the metal salt [41]. The XPS contents of Pd and Pt (Table S1) are lower to those Molecules 2020, 25, 2951Molecules 2020, 25, x FOR PEER REVIEW 6 of 14 6 of 14

An increase in the oxygen content for the Pd/AC composites was observed in comparison to the obtained by ICPpristine (Table AC1, )probab possiblyly as result due of to the the carbon tendency bonds cleavage of the during Pd ball and milling Pt nanoparticles with the respective to agglomerate, surface oxidation or due to a decomposition of the metal salt [41]. The XPS contents of Pd and Pt as supported also(Table by S1) SEM are lower and to TEMthose obtained (see above). by ICP (Table 1) possibly due to the tendency of the Pd and Pt The nitrogennanoparticles adsorption-desorption to agglomerate, as supported isomers also by for SEM AC and (FigureTEM (see above) S7),. Pd/AC and Pt/AC composites The nitrogen adsorption-desorption isomers for AC (Figure S7), Pd/AC and Pt/AC composites (Figure5) were determined at 196 C. According to the IUPAC classiﬁcation [46–48], a shape type (Figure 5) were determined− at ◦−196 °C. According to the IUPAC classification [46–48], a shape type (IV) isotherms and(IV) isotherms type IV and (H4) type IV hysteresis (H4) hysteresis loops loops werewere observed observed in the inrelative the pressure relative range pressure from range from 0.4 to 1.0 indicating the presence of mesopores, the formation of asymmetric, slit-shaped mesopores, 0.4 to 1.0 indicatingattributable the to presence rapid gas evolution of mesopores, and open channels the formation [46–48]. of asymmetric, slit-shaped mesopores, attributable to rapid gas evolution and open channels [46–48].

500 a) 500 b)

400 400 /g STP)

3 300 300 (Cm (cm³/g (cm³/g STP) Quantity dsorbed Quantity 200

Adsorption Adsorbed Quantity 200 Adsorption Desorption 100 Desorption 0 0.2 0.4 0.6 0.8 1 100 0 0.2 0.4 0.6 0.8 1 Relative Pressure (P/P ) Relative Pressure (P/P0) o

500 d) 500 c)

400 400

300 300 (cm³/g (cm³/g STP) (cm³/g (cm³/g STP)

Quantity Adsorbed Quantity 200

Quantity Adsorbed Quantity 200 Adsorption Adsorption Desorption Desorption 100 100 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1

Relative Pressure (P/Po) Relative Pressure (P/Po)

Figure 5. N2 adsorption-desorptionFigure 5. N2 adsorption-desorption isotherms isotherms of of synthesized synthesized AC-Pd1 ( AC-Pd1a), AC-Pd2 ( (ba),), AC AC-Pd2-Pt1 (c), and ( b), AC-Pt1 (c), and AC-Pt2 (d)AC composites.-Pt2 (d) composites.

Table 3 compares the BET surface areas, pore volume and pore size of the commercial AC with Table3 comparesthose of the the Pd/AC BET and surface Pt/AC supported areas, catalysts. pore volume The surface and area of pore the commercial size of theAC was commercial found AC with those of the Pdto/ ACbe 775 and m2/g Ptwith/AC a pore supported volume of 0.39 catalysts. cm3/g and BET The surface surface areas for area the Pd/AC of the and commercial Pt/AC AC was 2 2 composites2 arrange in the following order: AC-Pd1 (7473 m /g) > AC-Pt1 (722 m /g) > AC-Pd2 (648 found to be 775m2 m/g) >/ ACg with-Pt2 (584 a m pore2/g). Th volumee surface area of together 0.39 cmwith the/g type and and BET percentage surface of supported areas formetal the Pd/AC and Pt/AC compositescorrelates arrange with the in catalytic the following results (see order:below). ItAC-Pd1 should also(747 be noted m2 that/g) the> AC-Pt1micropores(722 volume m 2/g) > AC-Pd2 2 of the studied materials2 are close to the commercial AC value of 0.4 cm3/g, except of the AC-Pt2 (648 m /g) > AC-Pt2composite (584 which m exhibits/g). The a lower surface value area of 0.35 together cm3/g. As can with be seenthe, type no drastic and changes percentage in the of supported metal correlates with the catalytic results (see below). It should also be noted that the micropores volume of the studied materials are close to the commercial AC value of 0.4 cm3/g, except of the AC-Pt2 composite which exhibits a lower value of 0.35 cm3/g. As can be seen, no drastic changes in the textural properties of the studied samples were observed, suggesting that the diﬀerences in the adsorption capacities are mainly due to their surface properties.

Table 3. Brunauer—Emmett—Teller Surface Area Analysis (BET) characterization of Pd/AC and Pt/AC composites.

2 3 Sample SBET (m /g) Pore Volume (cm /g) Pore Size (nm) AC 775 0.39 6.2 AC-Pd1 747 0.42 6.9 AC-Pd2 648 0.41 7.1 AC-Pt1 722 0.40 6.9 AC-Pt2 584 0.35 7.0 Molecules 2020, 25, 2951 7 of 14

2.3. Catalytic Studies In order to investigate the eﬀects of various parameters on the SM reaction catalyzed by the prepared composites, a model cross-coupling of bromobenzene and phenylboronic acid has been selected. Additionally, the catalytic activity of the AC-Pd2 composite was compared with Pd catalysts supported on MWCNT, GO, and CaCO3 (Table4). Several reaction parameters (solvent, base, reaction time) were also evaluated to maximize the yield of the coupling reaction. The commercial salts used for the preparation of composites were also tested with the same mass (2.5 mg) giving ca. 75% yield (TON 34) for Pd(OAc)2 catalyst vs. 49% (TON 136) for the AC-Pd2 material. As can be seen, 1.5 yield drop is well compensated by the saving in Pd content. The use of commercial K2PtCl4 resulted in only 3% yield, indicating the importance of the AC support in improving the catalytic activity.

Table 4. Mechanochemical Suzuki—Miyaura cross-coupling reactions of bromobenzene with phenylboronic acid a.

b c 1 d Entry Catalyst Solvent Base Time (min) Yield (%) TON TOF (h− )

1 AC EtOH K2CO3 30 0 0 0

2 EtOH K2CO3 10 27 139 835

3AC-Pd1 EtOH K2CO3 30 30 155 309

4 - K2CO3 30 36 186 372

5 EtOH K2CO3 10 41 114 683

6 EtOH K2CO3 30 49 136 272

7 EtOH K2CO3 180 69 192 64 e 8 EtOH Na2CO3 30 31 86 172 9 e EtOH NaF 30 2 6 12 10 e AC-Pd2 EtOH KF 30 34 94 189 e 11 EtOH Cs2CO3 30 35 97 194 f 12 DCM K2CO3 30 3 8 17 f 13 ACN K2CO3 30 5 14 28 f 14 DMSO K2CO3 30 4 11 22 f 15 H2OK2CO3 30 1.6 4 9 3 16 EtOH K2CO3 10 26 228 1.4 10 AC-Pt1 × 17 EtOH K2CO3 30 5 44 88

18 EtOH K2CO3 10 2.5 13 81 AC-Pt2 19 EtOH K2CO3 30 2.3 12 25

20 GO-Pd2 EtOH K2CO3 30 53 - -

21 CNT-Pd1 EtOH K2CO3 10 37 - -

22 CNT-Pd1 EtOH K2CO3 30 29 - -

23 CNT-Pd2 EtOH K2CO3 10 32 - -

24 CNT-Pd2 EtOH K2CO3 30 50 - -

25 CaCO3_5%Pd EtOH K2CO3 30 12 - -

26 Pd(OAc)2 EtOH K2CO3 30 75 34 68

27 K2PtCl4 EtOH K2CO3 30 3 2.5 5 a Reaction conditions: 2.5 mg catalyst, 0.6 mmol PhB(OH)2, 0.5 mmol PhBr, 1.0 mmol K2CO3, 2 mL EtOH; ball milling: 10 milling stainless still spheres of 10 mm diameter, 500 rpm with 5 min rotation interval. b NMR yield (%) based on the relative integration areas of the limiting reagent (PhBr) and the product (biphenyl). c TON = turnover number = number of moles of product per mol of metal content, found by ICP-MS, in the catalyst. An example of TON and TOF calculation is provided in ESM. d TOF = TON per hour. e 1.0 mmol base. f 2 mL solvent. Molecules 2020, 25, x FOR PEER REVIEW 8 of 14

a Reaction conditions: 2.5 mg catalyst, 0.6 mmol PhB(OH)2, 0.5 mmol PhBr, 1.0 mmol K2CO3, 2 mL Molecules 2020, 25EtOH;, 2951 ball milling: 10 milling stainless still spheres of 10 mm diameter, 500 rpm with 5 min rotation 8 of 14 interval. b NMR yield (%) based on the relative integration areas of the limiting reagent (PhBr) and the product (biphenyl). c TON = turnover number = number of moles of product per mol of metal content, found by ICP-MS, in the catalyst. An example of TON and TOF calculation is provided in ESM. d TOF Pt catalysts= TON are per not hour. as commone 1.0 mmol base. as thef 2 mL palladium solvent. ones for the SM reactions; however, application of the AC-Pt1 catalyst resulted in 26% yield (TON of 228 and TOF of 1.4 103 h 1) of biphenyl after 10 min × − (Table4, entryPt 16). catalysts However, are not as after common 30 min as the there palladium was ones a drastic for the SM reduction reactions; in however, yield (Tableapplication4, entry 17), 3 −1 probably dueof the to AC the-Pt1 deactivation catalyst resulted of in the 26% catalyst. yield (TON Among of 228 and the TOF palladium-based of 1.4 × 10 h ) of biphenyl materials, after the 10 use of the min (Table 4, entry 16). However, after 30 min there was a drastic reduction in yield (Table 4, entry AC-Pd2 composite17), probably as due a catalystto the deactivation resulted of in the the catalyst. highest Among yields the palladium up to 49%-based after materials, 30 min the and use 70% after 180 min (Tableof the4 AC, entries-Pd2 composite 6 and 7). as Thea catalyst catalyst resulted with in the higher highest metal yield contents up to 49% showed after 30 highermin and yield 70% (30% for AC-Pd1 relativelyafter 180 min to 49%(Table for 4, entries AC-Pd2 6 and after 7). T 30he catalyst min, Table with higher4, entry metal 3); content however, showed with higher some yield decrease in TONs (155(30% and for 136, AC for-Pd1 AC-Pd1 relatively and to 49% AC-Pd2, for AC-Pd2 respectively, after 30 min after, Table 30 4, min). entry 3) The; however nanocrystalline, with some structure decrease in TONs (155 and 136, for AC-Pd1 and AC-Pd2, respectively, after 30 min). The of the catalysts,nanocrystalline the absence structure of contaminationsof the catalysts, the andabsence the of large contamina surfacetionsarea and the are large possible surface reasons area for the considerableare possible conversion reasons exhibited for the considerable by this catalyst conversion (Figure exhibit6ed). by this catalyst (Figure 6).

Figure 6. Cross-couplingFigure 6. Cross-coupling Suzuki-Miyaura Suzuki-Miyaura reaction reaction of bromobenzene bromobenzene and phenylboronic, and phenylboronic, catalyzed catalyzedby by AC-Pd1, AC-Pd2,AC-Pd1, AC AC-Pt1,-Pd2, AC and-Pt1, AC-Pt2and AC-Pt2 composites. composites. Reaction Reaction conditions: conditions: 2.5 mg 2.5 catalyst, mg catalyst,0.6 mmol 0.6 mmol PhB(OH)2,PhB(OH) 0.5 mmol2, 0.5 PhBr, mmol 1.0 PhBr, mmol 1.0 K mmol2CO K32,CO 2 mL3, 2 EtOH, mL EtOH, Ball Ball mill: mill: 10 10 milling milling spheres, spheres, 500 500 rpm, rpm, rotation interval (5rotation min) during interval 10(5 min) or 30 during min. 10 or 30 min. Considering the above catalyst screening, the AC-Pd2 composite was selected for the more Considering the above catalyst screening, the AC-Pd2 composite was selected for the more detailed catalytic studies. Thus, several inorganic bases, namely K2CO3, NaF, Na2CO3, KF, and detailed catalyticCs2CO3, were studies. tested Thus, for this several composite inorganic, and application bases, of namely potassium K2 carbonateCO3, NaF, resulted Na2CO in the3, highest KF, and Cs2CO3, were testedbiphenyl for this yield composite, of 49% (Table and 4 application, entry 6, Figure of S potassium8). The use of carbonate Na2CO3, KF resulted and Cs2CO in3 the gave highest lower biphenyl and similar yields in the range of 31–35% (Table 4, entries 8–10), while NaF provided the lowest yield of 49% (Table4, entry 6, Figure S8). The use of Na 2CO3, KF and Cs2CO3 gave lower and similar product yield of 2%. Although water is considered a green solvent, its use is not beneficial as well as yields in theapplication range ofof 31–35%dichloromethane (Table 4(DCM),, entries acetonitrile 8–10), while(ACN), NaFand dimethyl provided sulfoxide the lowest (DMSO), product while yield of 2%. Althoughimplementation water is of considered “green” ethanol a green significantly solvent, improves its use the is yield notbeneficial (Table 4, entries as well 6 and as 12 application–15, of dichloromethaneFigure S9 (DCM),). acetonitrile (ACN), and dimethyl sulfoxide (DMSO), while implementation of “green” ethanolTo check significantly if replacement improves of the AC support the yield by the (Table related4, entries carbon materials 6 and 12–15, would Figureinfluence S9). the catalytic results, GO and MWCNTs were used as supports (Table 4, entries 20–24). As can be seen, To checkthe AC if- replacementPd2 catalyst (yield of theof 49%, AC Table support 4, entry by 6) the is slightly related less carbon active than materials the GO-Pd2 would (yield influence of the catalytic results,53%, Table GO 4, entry and 20) MWCNTs and CNT-Pd2 were (yield used of 50%, as Table supports 4, entry (Table 24) analogs,4, entries under 20–24).the same reaction As can be seen, the AC-Pd2 catalyst (yield of 49%, Table4, entry 6) is slightly less active than the GO-Pd2 (yield of

53%, Table4, entry 20) and CNT-Pd2 (yield of 50%, Table4, entry 24) analogs, under the same reaction conditions (Figure S10), while the inorganic CaCO3 support provided a low yield of 12%, possibly due to inferior surface area. The promoting eﬀect of oleﬁns [31,32], namely cyclohexene, cyclooctene and 1,5-cyclooctadiene (1,5-COD), was also explored for the so-named liquid-assisted grinding (LAG) process with the AC-Pd2 composite as a catalyst (Table5). The presence of cyclooctene improved the yield to 59% with ethanol as a solvent (Table5, entry 1), whereas the presence of cyclohexene and 1,5-COD dropped the yield to 19% and 21%, respectively (Table5, entries 4 and 5). In solventless conditions, the yield increased from 59 to 80% and from 21 to 39% for the cyclooctene and 1,5-COD additives, respectively (Table5, entries 1, 2, 5, and 6). Moreover, the addition of EtOH as an additive (0.4 µL/mg catalyst) resulted in 40% yield (Table5, entry 7), while NaCl additive provided yield of only 28% (Table5, entry 8). Molecules 2020, 25, 2951 9 of 14

Table 5. Eﬀect of diﬀerent additives on the Suzuki-Miyaura cross-coupling reaction of bromobenzene with phenylboronic acid by AC-Pd2 a.

b c 1 d Entry Catalyst Solvent Base Additive Time (min) Yield TON TOF (h− ) 1 EtOH Cyclooctene 30 59 164 328 2 - Cyclooctene 30 80 222 444 3 EtOH Cyclooctene 180 38 106 35 4 EtOH Cyclohexene 30 19 53 106 AC-Pd2 K2CO3 5 EtOH 1,5-COD 30 21 58 117 6 - 1,5-COD 30 39 108 217 7 - EtOH 30 40 111 222 8 e - NaCl 30 27 75 150 a Reaction conditions: 2.5 mg catalyst, 0.6 mmol PhB(OH)2, 0.5 mmol PhBr, 1.0 mmol K2CO3, 2 mL EtOH; ball milling: 10 milling stainless still spheres of 10 mm diameter, 500 rpm with 5 min rotation interval. b NMR yield (%) based on the relative integration area of the limiting reagent (PhBr) and the product. c TON = turnover number = number of moles of product per mol of metal content, found by ICP-MS, in the catalyst. d TOF = TON per hour. e 0.6 g NaCl.

The eﬀect of other than mechanical energy inputs on the catalytic output has been also studied (Table6, Figure7). As can be seen, in case of convectional heating (CH) for 1 h yield is higher at 35 ◦C in respect to 75 ◦C (Table6, entry 2 vs. 5), probably due to the to the formation of Pd black with its agglomeration at higher temperatures. The CH after 30 min resulted in the complete conversion to biphenyl product, either at 75 or 35 ◦C, while MW irradiation has also a promoting eﬀect since 60 min of reaction at 75 ◦C resulted in total conversion. However, the described above liquid assisted grinding is much more sustainable and environmentally friendly taking in consideration the energy necessary to produce and recover the used solvents.

Table 6. Eﬀect of the other energy inputs for the solid-state Suzuki-Miyaura cross-coupling reaction of bromobenzene with phenylboronic acid by AC-Pd2 a.

Temperature b c 1 d Entry Catalyst Solvent Base Time (min) Yield (%) TON TOF (h− ) (◦C) CH 1 30 100 278 556 275 60 64 178 178 3 360 65 181 30 AC-Pd2 EtOH K2CO3 4 30 100 278 556 535 60 100 278 278 6 360 50 139 23 MW 7 30 61 169 339 875 60 100 278 278 9 360 69 192 32 AC-Pd2 EtOH K2CO3 10 30 52 144 289 1135 60 56 156 156 12 360 47 131 22 a Reaction conditions: 2.5 mg catalyst, 0.6 mmol PhB(OH)2, 0.5 mmol PhBr, 1.0 mmol K2CO3, 2 mL EtOH; ball milling: 10 milling stainless still spheres of 10 mm diameter, 500 rpm with 5 min rotation interval. b NMR yield (%) based on the relative integration area of the limiting reagent (PhBr) and the product. c TON = turnover number = number of moles of product per mol of metal content, found by ICP-MS, in the catalyst. d TOF = TON per hour. CH = conventional heating with oil bath; MW = under microwave irradiation (5–10 atm). Molecules 2020, 25, x FOR PEER REVIEW 10 of 14

10 35 30 52 144 289 11 60 56 156 156 12 360 47 131 22

a Reaction conditions: 2.5 mg catalyst, 0.6 mmol PhB(OH)2, 0.5 mmol PhBr, 1.0 mmol K2CO3, 2 mL EtOH; ball milling: 10 milling stainless still spheres of 10 mm diameter, 500 rpm with 5 min rotation interval. b NMR yield (%) based on the relative integration area of the limiting reagent (PhBr) and the product. c TON = turnover number = number of moles of product per mol of metal content, found by ICP-MS, in the catalyst. d TOF = TON per hour. CH = conventional heating with oil bath; MW = under Molecules 2020, 25, 2951 10 of 14 microwave irradiation (5–10 atm).

Figure 7. Conventional and microwave (MW) irradiation heating methods vs. time on the Suzuki- MiyauraFigure (SM) 7. Conventional reaction. and microwave (MW) irradiation heating methods vs. time on the Suzuki- Miyaura (SM) reaction. 3. Materials and Methods 3. Materials and Methods The activated carbon (AC, DARCO®, 100 mesh particle size), palladium acetate Pd(OAc) 47.5% − 2 Pd, ethanol,The toluene,activated ACN,carbon DCM, (AC, DARCO cyclohexene,®, −100 mesh 1-cyclooctene, particle size and), palladium 1,5-cyclooctadiene acetate Pd(OAc) were2 47.5% purchased Pd, ethanol, toluene, ACN, DCM, cyclohexene, 1-cyclooctene, and 1,5-cyclooctadiene were purchased from Aldrich (Sigma-Aldrich, Munich, Germany). Potassium tetrachloroplatinate(II) K2PtCl4 was purchasedfrom Aldrich from Alfa (Sigma Aesar-Aldrich, (Kandel, Munich, Germany), Germany bromobenzene). Potassium (99%) tetrachloroplatinate(II) from Panreac (Panreac, K2PtCl Darmstadt,4 was purchased from Alfa Aesar (Kandel, Germany), bromobenzene (99%) from Panreac (Panreac, Germany) and phenylboronic acid (95%) from Fluka (Fluka, Buchs, Switzerland). None of the used Darmstadt, Germany) and phenylboronic acid (95%) from Fluka (Fluka, Buchs, Switzerland). None commercialof the used salts commercial is signiﬁcantly salts is sensitive significantly to air sensitive or moisture to air andor moisture all the samplesand all the were sample handleds were under ambienthandled conditions, under ambient without conditions, additional without puriﬁcation. additional purification.

3.1. Preparation3.1. Preparation of Catalysts of Catalysts TheThe Pd Pd and and Pt-based Pt-based composites composites were were synthesized synthesized as asfollows: follows: 100 mg 100 of mg support of support (AC, GO, (AC, or GO, or MWCNT)MWCNT) and and 11 11mg mg oror 2222 mg of commercially commercially available available palladium palladium acetate acetate Pd(OAc) Pd(OAc)2 or potassium2 or potassium tetrachloroplatinate(II)tetrachloroplatinate(II) K2 PtClK2PtCl4 were4 were added added to a 250 250 mL mL stainless stainless steel steel grinding grinding bowl bowl with with 10 stainless 10 stainless steelsteel balls balls of 10 of mm10 mm diameter diameter and and milled milled inin a Retsch PM PM100100 planetary planetary ball ball mill mill for 1 for h at 1 hrotational at rotational speedspeed 500 rpm,500 rpm, with with rotational rotational inversions inversions everyevery 5 min, min, at at room room temperature. temperature. All Allthe composites the composites were were prepared in the absence of any added solvent (dry milling). All the samples were handled under prepared in the absence of any added solvent (dry milling). All the samples were handled under ambient conditions, without additional purification of the commercial reagents. ambient conditions, without additional puriﬁcation of the commercial reagents. 3.2. Characterization of Catalysts 3.2. Characterization of Catalysts The microstructure analysis (SEM and EDX) of the catalysts was conducted with a scanning electronThe microstructure microscope JEOL analysis 7001F (SEM(JEOL, and Akishima, EDX) Tokyo, of the catalystsJapan ) with was Oxford conducted light elements with a EDX scanning electrondetector microscope and EBSD JEOL detector. 7001F The (JEOL,morphology Akishima, of the composites Tokyo, Japan) was analyzed with Oxford by TEM light measurements elements EDX detector and EBSD detector. The morphology of the composites was analyzed by TEM measurements and performed on a Hitachi 8100 microscope (Hitachi, Tokyo, Japan) with a ThermoNoran light elements EDX detector and digital image acquisition. ICP-MS was carried out by Microanalytical Service of the Instituto Superior Técnico. XPS was performed with a Kratos AXIS Ultra HAS (KRATOS, Manchester, UK), coupled with VISION software, at Centro de Materiais da Universidade do Porto (CEMUP), Porto, Portugal. The equipment used a monochromatic Al Kα X-ray source (1486.7 eV); operation conditions: 15 kV (90 W); Fixed Analyser Transmission (FAT) mode; 40 eV (regions ROI) and 80 eV (survey) of pass energy. C1s band at 284.6 eV was used as internal standard for binding energy (BE) calibration. CASAXPS software was selected for data analysis. The BET surface areas were obtained from the N2 adsorption isotherms collected on a Micromeritics ASAP 2060 gas sorption instrument (Hiden Isochema, Warrington, UK) at 77 K. Pre-outgassing of the samples at high vacuum at 130 ◦C for 24 h was undertaken to remove the physisorbed species from the surface of the examined samples. Molecules 2020, 25, 2951 11 of 14

3.3. Catalytic Studies Typically, phenylbromide (0.5 mmol) and phenylboronic acid (0.6 mmol) as substrates, 2.5 mg composite as catalyst, potassium carbonate (1 mmol) as a base, and ethanol (2 mL) were introduced in a 250 mL stainless steel grinding jar and 10 stainless steel balls of 10 mm diameter were added. The grinding bowl was placed in the Retsch PM100 ball mill and the system was left under rotational speed 500 rpm, with rotational inversions every 5 min. At the end, 20 µL of the reaction mixture were 1 taken and measured amount of DMSO-d6 was added and the sample was analyzed by HNMR (for more details see Supplementary Materials). The alkene additives (1,5-COD, 1-cycoctene, and cyclohexene) were taken in concentration 1 µL/2.5 mg.

4. Conclusions In this study we demonstrated a simple and sustainable way to produce Pd(II) and Pt(II) and activated carbon heterogeneous nanocomposites. The preparation involves a simple, scalable, eco-friendly and fast ball milling technique and leads to dispersed catalytic materials, highly active for the Suzuki-Miyaura mechanochemical cross-coupling of bromobenzene with phenylboronic acid. The eﬀect of very small amounts of oleﬁns on the reaction yield was explored and in our most promising catalytic system (in the presence of AC-Pd2 and using cyclooctene as additive), relevant yields were achieved (ca. 80%). Moreover, the reported results show the importance of ball milling as a promising tool, in comparison with other energy inputs, in both synthesis and catalysis, and the advantages to combine a homogeneous catalyst with a supporting material under eco-friendly conditions. The green mechanochemical method will be further explored, expanding its scope and scaling up.

Supplementary Materials: The following are available online at http://www.mdpi.com/1420-3049/25/12/2951/s1, Calculation of the theoretical content of metals in the composite materials, Figure S1: EDX analysis of the AC-Pd1 (a) and AC-Pd2 (b) composite materials; Figure S2: XPS survey spectra of the Pt/AC composites; Table S1. Surface atomic percentages determined by XPS for the pristine AC and within the studied Pd/AC and Pt/AC catalytic materials; Figure S3. XPS profiles of the studied Pd/AC and Pt/AC catalytic materials in C 1s and O 1 1s regions; Figures S4–S6: Selected HNMR showing yield calculation; Figure S7: N2 adsorption–desorption isotherms of the pristine AC; Figure S8: Effect of the different bases in the cross-coupling Suzuki–Miyaura reaction of bromobenzene and phenylboronic, catalyzed by AC-Pd2; Figure S9: Effect of the different solvents in the cross-coupling Suzuki–Miyaura reaction of bromobenzene and phenylboronic, catalyzed by AC-Pd2; Figure S10: Effect of the different supports. Author Contributions: Conceptualization, E.C.B.A.A. and M.N.K.; Methodology, E.C.B.A.A., A.P.C.R.; Investigation, M.M.A.S. and A.F.P.; Writing-original draft preparation, M.M.A.S., E.C.B.A.A. and A.F.P.; Writing-review and editing, E.C.B.A.A., M.N.K. and A.J.L.P.; Supervision, E.C.B.A.A. and A.P.C.R.; Project administration, E.C.B.A.A.; Funding acquisition, E.C.B.A.A.; All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by MULTIFUN-Straightforward Synthesis of Multifunctional Core-Shell Composites and Their Applications, project number PTDC/QUI-QIN/29778/2017. Acknowledgments: Financial support from the Fundação para a Ciência e a Tecnologia (FCT), Portugal (project UIDB/00100/2020 of the Centro de Química Estrutura, fellowship PD/BD/114400/2016 to M.M.A.S., contract IST-ID/219/2019 to M.N.K., UIDB/04565/2020 and PTDC/QUI-QIN/29778/2017 projects and the UID/MULTI/04046/2019 research grant), from Instituto Politécnico de Lisboa (IPL/2019/STREAM_ISEL Project) and from LAQV-REQUIMTE (UID/QUI/50006/2019). APCR thanks Instituto Superior Técnico for the contract IST-ID/119/2018 under Decree-Law 57/2017. A.F.P. thanks FCT/MCTES the work contract (in the scope of the framework contract foreseen in the numbers 4, 5, and 6 of the article 23, of the Decree-Law 57/2016, of 29 August, changed by Law 57/2017, of 19 July) supported by national funds (OE). Conflicts of Interest: The authors declare no conflict of interest.

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Sample Availability: Samples of the composites are not available from the authors.

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