polymers

Article Preparation of Poly( glycol)@Polyurea Microcapsules Using Oil/Oil Emulsions and Their Application as Microreactors

Ahmad Zarour, Suheir Omar and Raed Abu-Reziq *

Casali Center of Applied and The Center for Nanoscience and Nanotechnology, Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 9190401, Israel; [email protected] (A.Z.); [email protected] (S.O.) * Correspondence: [email protected]; Tel.: +972-2-6586097; Fax: +972-2-6585469

Abstract: The development process of catalytic core/shell microreactors, possessing a poly(ethylene glycol) (PEG) core and a polyurea (PU) shell, by implementing an emulsion-templated non-aqueous encapsulation method, is presented. The microreactors’ fabrication process begins with an emulsi- fication process utilizing an oil-in-oil (o/o) emulsion of PEG-in-heptane, stabilized by a polymeric surfactant. Next, a reaction between a poly(ethylene ) (PEI) and a toluene-2,4-diisocyanate (TDI) takes place at the boundary of the emulsion droplets, resulting in the creation of a PU shell through an interfacial polymerization (IFP) process. The microreactors were loaded with palladium nanoparticles (NPs) and were utilized for the hydrogenation of and . Importantly, it was found that PEG has a positive effect on the catalytic performance of the developed microreactors. Interestingly,   besides being an efficient green reaction medium, PEG plays two crucial roles: first, it reduces the palladium ions to palladium NPs; thus, it avoids the unnecessary use of additional reducing agents. Citation: Zarour, A.; Omar, S.; Second, it stabilizes the palladium NPs and prevents their aggregation, allowing the formation of Abu-Reziq, R. Preparation of highly reactive palladium NPs. Strikingly, in one sense, the suggested system affords highly reactive Poly(ethylene glycol)@Polyurea semi-homogeneous catalysis, whereas in another sense, it enables the facile, rapid, and inexpensive Microcapsules Using Oil/Oil recovery of the catalytic microreactor by simple centrifugation. The durable microreactors exhibit Emulsions and Their Application as Microreactors. Polymers 2021, 13, 2566. excellent activity and were recycled nine times without any loss in their reactivity. https://doi.org/10.3390/ polym13152566 Keywords: non-aqueous interfacial polymerization; polyurea microcapsules; poly(ethylene glycol); oil-in-oil emulsions; hydrogenation Academic Editor: Eduardo Guzmán

Received: 14 July 2021 Accepted: 29 July 2021 1. Introduction Published: 31 July 2021 The pursuit of a sustainable future has become the driving force for today’s science, where pioneering approaches toward greener processes are continually being developed. Publisher’s Note: MDPI stays neutral In this regard, the design of efficient and facile catalytic systems is highly desired [1,2]. with regard to jurisdictional claims in Although the homogeneous route of catalysis is endowed with high reactivity and selectiv- published maps and institutional affil- ity [3,4], most of the industrial catalytic processes rely on the heterogeneous counterpart iations. owing to significant recovery and cost concerns [5,6]. Many efforts have been devoted to bridging the gap between the two routes and to utilizing their advantages; in this regard, numerous methods have been developed. Among others, this includes the use of Pickering emulsion systems [7,8], anchored single-atom catalysts [9], mesoscale nanostructures [10], Copyright: © 2021 by the authors. and tunable solvents [11]. Licensee MDPI, Basel, Switzerland. The use of metal nanoparticles constitutes another powerful tool for combining the This article is an open access article advantages of both disciplines because of metal nanoparticles’ high surface area [12,13]. distributed under the terms and However, despite their great reactivity and selectivity, metal nanoparticles require ad- conditions of the Creative Commons ditional immobilization techniques to avoid their aggregation, and usually involve the Attribution (CC BY) license (https:// use of different supports to enable catalyst recovery. The uses of organic polymer sup- creativecommons.org/licenses/by/ 4.0/). ports [14–16], dendrimers [17–19], supported ionic liquid phase (SILP) catalysts [20–22],

Polymers 2021, 13, 2566. https://doi.org/10.3390/polym13152566 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 2566 2 of 13

magnetic nanoparticles (MNPs) [23–25], and inorganic supports [26–28] are just a few examples of many immobilization techniques available for metal nanoparticles. However, along with leaching problems, these methods generally involve multiple synthetic steps and pre-functionalization processes, which make the development of new catalysts difficult, expensive, and cumbersome. Moreover, generally, their reactivity and selectivity are less efficient in comparison with the pure homogeneous catalysts. Recently, catalysis within microenvironmental entities, called in brief microreactors, has been proven to be efficient at least as the pure homogeneous route. The remark- ably enhanced catalytic performance refers to the local high concentrations of reagents within the dispersed microreactors, a fact that dramatically increases the reaction rate constants [29–34]. Accordingly, microcapsules (MCs) might serve as a useful tool for induc- ing such microenvironmental catalytic conditions [35–38]. Microencapsulation involves the process of confining various types of ingredients for diverse purposes, such as the controlled release of pesticides in agriculture [39–41], the release of drugs in pharmaceutical applications [42–44], the masking of odors and tastes in the food industry [45–47], and the protection and recycling of catalysts [48–52]. Interfacial polymerization (IFP) is a well-known chemical method for preparing MCs [53–56]. This process is based on a polycondensation reaction between two highly reactive monomers; it takes place at the boundary of two immiscible phases, and each phase contains one of the two monomers. Polyurea (PU) is the most common polymer prepared via IFP for the synthesis of MCs; it has been thoroughly investigated and applied in different fields [57–62]. PUs are generated from the reaction between and monomers. These MCs are considered valuable owing to their unique features, such as low Tg, rubbery characteristics, as well as their chemical [61] and mechanical [60] stability and biocompatibility, [63] which make them an ideal material for holding liquid in their core even for in vitro applications. In addition to the encapsulation of ionic liquids, which was demonstrated by our group [64], phase change materials [65] and more common organic solvents, such as xylene [66], were found to serve as an efficient platform for implementing PU MCs. The encapsulation process in the latter examples was preceded by an oil-in- water (o/w) emulsification process. Furthermore, water-in-oil (w/o) emulsion-templated PU MCs have been reported as well [67]. However, these two emulsification strategies (o/w and w/o) cannot provide an adequate solution in the case of moisture-sensitive ingredients, including catalysts. Thus, the development of a non-aqueous process, such as the use of oil-in-oil (o/o) emulsions, is of the utmost importance [68–72]. Although PU MCs and o/o emulsions themselves are well-established topics, their combination has barely been studied [73–77]. In addition, palladium NPs have been successfully supported on different catalytic systems, including magnetic carbon-coated cobalt nanobeads [78], silicon nanowire arrays [79], and cotton and filter paper [80], and were applied in the hydrogenation of alkenes. Moreover, palladium NPs were successfully encapsulated within PU MCs based on oil-in-water [81–83] and water-in-oil [84] emulsions. Herein, we present the first palladium-based PU microreactor prepared via a non- aqueous process that utilizes an o/o emulsion. A low-molecular-weight poly (ethylene glycol) (PEG) was successfully encapsulated, for the first time, within a PU shell. PEG represents a green reaction medium owing to its low vapor pressure and it is considered as a GRAS (generally recognized as safe) material [85,86]. In addition to its acting as a green medium for organic transformations, PEG was also found to play two additional roles that have a direct positive impact on the catalytic performance. First, it acts as a good stabilizing agent for palladium NPs, which is in line with the results obtained in other works [87–90]. Thus, it prevents the aggregation of palladium NPs and increases their reactivity. Second, it acts as a reducing agent, avoiding the need for additional reducing agents to activate the palladium NPs. Recently, PEG-based microcapsules were developed and applied for energy storage [91,92] or drug formulation [93–95] and for creating unique multilayer polymeric shells with complex structures [96]. Polymers 2021, 13, 2566 3 of 13

2. Materials and Methods 2.1. Materials The organic solvents heptane, toluene, xylene, and cyclohexane were purchased from Romical, along with chemicals and laboratory equipment. Poly(1-vinylpyrrolidone)-graft- (1-hexadecene) co-polymer (containing 80% by weight of C16 α-olefin and 20% by weight of polymerized vinylpyrrolidone) (Agrimer AL-22) was purchased from International Specialty Products (ISP). Bis-poly(ethylene glycol)/poly(propylene glycol)-14/14 dime- thicone (ABIL EM 97) was purchased from Evonik. Poly(ethylene glycol) 200, sodium tetra- chloropalladate (Na2PdCl4), dioctyl sulfosuccinate sodium salt (AOT), sorbitan monooleate (span 80), poly(ethylene glycol) oleyl (brij 92v), branched poly(ethylene amine) 800, 2,20(ethylenedioxy)bis(ethylamine), hexamethylenediamine (HMDA), and diethylenetri- amine (DETA) were purchased from Sigma Aldrich-Merck. Polymethylene polyphenyl isocyanate (PAPI 27) was contributed by FMC Corporation. Toluene-2,4-diisocyanate (TDI) was purchased from Acros Organics. All substrates were purchased either from Alfa Aesar (Tewksbury, MA, USA) or Sigma Aldrich-Merck (Budapest, Hungary).

2.2. Instrumentation The o/o emulsion containing 0.01% Rhodamine B in PEG200 was analyzed by a fluorescence microscope (Carl Zeiss, Axio Vision, Mátészalka, Hungary). The catalytic microcapsules were initially examined using a high-resolution scanning electron micro- scope (HR SEM) Sirion from FEI equipped with a Shottky-type field emission source and a secondary electron (SE) detector at 5 kV. Focused ion beam (FIB-SEM), using a FEI Helios nanolab 460S1 instrument, was used to investigate the inner and outer morphology of the microcapsules and to evaluate whether core–shell or matrix structures were obtained. Moreover, it was utilized for the mapping and EDXS (energy dispersive X-ray spectroscopy) analyses. 1H-NMR measurements were performed using a Bruker DRX-400 spectrometer to determine conversion and selectivity. Emulsifications were performed using the Kine- matica Polytron homogenizer PT-6100 equipped with dispersing aggregate 3030/4EC. IR was recorded with KBr pellets at room temperature in transmission mode on a PerkinElmer 65 FTIR spectrometer to determine the chemical composition of the microcapsules. In addition, thermogravimetric analysis (TGA) was performed using a Mettler Toledo TG 50 ◦ ◦ −1 analyzer at a temperature range of 25–700 C and at a heating rate of 10 C min under N2 atmosphere. Scanning transmission electron microscopy (STEM) and electron diffraction spectroscopy (EDS) were performed with (S)TEM Tecnai F20 G2 (FEI Company, Hillsboro, OR, USA) operated at 200 kV.

2.3. The Procedure for Preparing the PdNPs/PEG200 Polar Phase

In a vial of 10 mL, 100 mg (0.287 mmol) of Na2PdCl4 was dissolved and sonicated in 4.66 g of PEG200 for one hour. At this stage, the mixture’s color changes from light red to dark red and then to black, indicating that the palladium was reduced. Then, 0.34 g of an amine was added and the mixture was allowed to sonicate for one more hour.

2.4. General Procedure for Preparing the PdNPs/PEG200@PU Microreactors In a 100 mL beaker, 2.00 g of surfactant was dissolved in 13.40 g of heptane. The solution was homogenized at 10,000 rpm for 30 s. Then, the PdNPs/PEG200 polar phase was added and the homogenization proceeded for another two minutes. Afterward, a mixture of 0.6 g of an isocyanate dissolved in 4 g of xylene was added dropwise. The emulsion was allowed to stir at 500 rpm on a stirring plate for 4 h at room temperature. The resulting catalytic microreactors were separated by centrifugation and washed three times with heptane. The PdNPs/PEG200@PU MCs were then dispersed in heptane until the weight of the whole dispersion was 20.00 g. Polymers 2021, 13, 2566 4 of 14

heptane. The PdNPs/PEG200@PU MCs were then dispersed in heptane until the weight of Polymers 2021, 13, 2566 4 of 13 the whole dispersion was 20.00 g.

2.5. General Procedure for the Hydrogenation Reaction 2.5. GeneralIn a 25 Procedure mL glass-lined for the Hydrogenation autoclave equipped Reaction with a magnetic stirring bar, 1 g of the catalyticIn a 25 dispersion mL glass-lined (0.047 autoclavemmol of Pd equipped per 1 g of with dispersion) a magnetic was stirring added bar,to 2 1mL g ofof thehep- catalytictane. Then, dispersion a suitable (0.047 amount mmol of of Pd substrate per 1 gof wa dispersion)s added relative was added to the to 2substrate/catalyst mL of heptane. Then,(S/C) a ratio. suitable The amountautoclave of was substrate purged was three added times relative with H to2 and the then substrate/catalyst pressurized to (S/C)100 psi ratio.of H The2. The autoclave reactions was were purged carried three out at times room with temperature H2 and then for 2.5 pressurized h. Finally, tothe 100 MCs psi were of Hseparated2. The reactions from the were reaction carried mixture out at room by cent temperaturerifugation, forwashed 2.5 h. two Finally, times the with MCs heptane, were separatedredispersed from in the3 mL reaction of heptane, mixture and by then centrifugation, used for the washed next catalytic two times cycle. with The heptane, reaction redispersedcontent was in examined 3 mL of heptane, using 1H-NMR and then spectroscopy used for the in next order catalytic to determine cycle. the The conversion reaction contentand selectivity. was examined using 1H-NMR spectroscopy in order to determine the conversion and selectivity. 3. Results and Discussion 3. Results and Discussion 3.1. Synthesis and Optimization of the PEG200@PU MCs 3.1. Synthesis and Optimization of the PEG200@PU MCs The synthesis of the polyurea microcapsules (PU MCs) is based on an emulsion-tem- The synthesis of the polyurea microcapsules (PU MCs) is based on an emulsion- plated non-aqueous encapsulation process. The preparation procedure begins with an templated non-aqueous encapsulation process. The preparation procedure begins with emulsification process consisting of two immiscible oil phases: (1) the polar dispersed an emulsification process consisting of two immiscible oil phases: (1) the polar dispersed phase consists of branched poly(ethylene imine)800 (PEI800) and Na2PdCl4 dissolved in phase consists of branched poly(ethylene imine)800 (PEI800) and Na2PdCl4 dissolved in poly(ethylene glycol)200 (PEG200), and the PEG200 rapidly reduces the palladium (II) ions to poly(ethylene glycol)200 (PEG200), and the PEG200 rapidly reduces the palladium (II) ions to palladiumpalladium (0) (0) nanoparticles nanoparticles (NPs); (NPs); and and (2) (2) a polara polar continuous continuous phase phase of of heptane heptane containing containing thethe polymeric polymeric surfactant surfactant ABIL ABIL EM EM 90. 90. Then,Then, aa toluene-2,4-diisocyanatetoluene-2,4-diisocyanate (TDI) (TDI) dissolved dissolved in inxylene xylene was was slowly slowly added added while while the the homogenization homogenization process process was was running. running. Finally, Finally, an an in- interfacialterfacial polymerization process atat thethe boundaryboundary ofof the the emulsion emulsion droplets droplets between between the the PEIPEI and and the the TDI TDI is is executed, executed, leading leading to to the the creation creation of of a PUa PU MC MC possessing possessing a core/shella core/shell structurestructure owing owing to to the the insolubility insolubility of of the the PU PU in in PEG PEG (Scheme (Scheme1). 1).

Scheme 1. Schematic illustration of the PdNPs/PEG200@PU preparation procedure.

The selection of these materials was determined after conducting a series of optimiza- tion experiments in which other continuous phases (cyclohexane and toluene), surfactants 0 (Agrimer AL-22, AOT, span 80, and brij 92v), (2,2 (ethylenedioxy)bis(ethylamine), Polymers 2021, 13, 2566 5 of 14

Scheme 1. Schematic illustration of the PdNPs/PEG200@PU preparation procedure.

The selection of these materials was determined after conducting a series of optimi- Polymers 2021, 13, 2566 zation experiments in which other continuous phases (cyclohexane and toluene), surfac-5 of 13 tants (Agrimer AL-22, AOT, span 80, and brij 92v), amines (2,2′(ethylenedioxy)bis(ethyla- mine), HMDA, and DETA) and isocyanate (PAPI 27) were examined. Briefly, excluding the polymeric surfactants, ABIL EM 90, and Agrimer AL-22 (Figure S1), none of the sur- HMDA, and DETA) and isocyanate (PAPI 27) were examined. Briefly, excluding the poly- factants were able to stabilize the o/o emulsion; the stabilization of such emulsions is not meric surfactants, ABIL EM 90, and Agrimer AL-22 (Figure S1), none of the surfactants obvious and usually requires the involvement of multi-armed polymeric surfactants ra- were able to stabilize the o/o emulsion; the stabilization of such emulsions is not obvious ther than small ones; the former settle irreversibly at the interface of the droplets, as their and usually requires the involvement of multi-armed polymeric surfactants rather than departure from the interface requires that all the surfactants’ arms leave the interface con- small ones; the former settle irreversibly at the interface of the droplets, as their departure fromcomitantly, the interface which requiresis statistically that all less the possible. surfactants’ Moreover, arms leave whereas the interface the interfacial concomitantly, polymer- whichization is statisticallyof the PEI/TDI less possible. and DETA/TDI Moreover, whereaspairs fabricates the interfacial MCs, polymerization the 2,2′(ethylenedi- of the PEI/TDIoxy)bis(ethylamine)/PAPI and DETA/TDI pairs 27 fabricatesand HMDA/PAPI MCs, the 2,2 270(ethylenedioxy)bis(ethylamine)/PAPI pairs result in the formation of 27nanocapsules and HMDA/PAPI (Table 27S1). pairs However, result in the formationlatter two ofsuffer nanocapsules from aggregation (Table S1). problems, However, thewhereas latter in two the suffer case of from the DETA/T aggregationDI pair, problems, the encapsulation whereas in of the PEG case200 is of not the attainable. DETA/TDI In the case of cyclohexane only, the DETA/TDI pair affords MCs when Agrimer AL-22 was pair, the encapsulation of PEG200 is not attainable. In the case of cyclohexane only, the DETA/TDIutilized as the pair emulsion affords MCs stabilizer when (Figure Agrimer S2 AL-22); however, was utilized as already as the mentioned, emulsion the stabilizer encap- sulation of PEG200 with this pair is not viable. For toluene, well-defined MCs were not (Figure S2); however, as already mentioned, the encapsulation of PEG200 with this pair is notobtained viable. with For either toluene, of well-definedthe pairs. MCs were not obtained with either of the pairs.

3.2.3.2. CharacterizationCharacterization ofof thethe PdPdNPsNPs/PEG/PEG200200@PU@PU MCs MCs First,First, thethe catalyticcatalytic systemsystem waswas investigatedinvestigated beforebefore thethe penetrationpenetration ofof thethe palladium.palladium. ScanningScanning electronelectron microscopymicroscopy (SEM)(SEM) waswas employedemployed forfor determiningdetermining thethe morphologicalmorphological structurestructure of our system system (Figure (Figure 1a,b).1a,b). Smoot Smoothedhed spherical spherical surfaces surfaces and and some some pressed pressed cap- capsules,sules, caused caused by the by thehigh high vacuum vacuum applied applied in this in analysis, this analysis, werewere obtained. obtained. As can As be can easily be easilynoted, noted, a polydispersed a polydispersed system system with with sizes sizes ranging ranging from from 200 200 nm nm to to 15 15 µmµm was formed, whichwhich isis aa typicaltypical featurefeature ofof macroemulsionmacroemulsion systems.systems. TheThe measurementmeasurement ofof thethe sizesize ofof thethe microcapsulesmicrocapsules dispersed dispersed in in isopropyl isopropyl alcohol by by laser laser diffraction diffraction size size analyzer analyzer indicated indicated an averagean average size size of 12.48 of 12.48µm µm (d0.5 (d=0.5 12.48 = 12.48 micron, micron, Figure Figure S3). S3). Moreover, Moreover, fluorescent fluorescent microscopy micros- imagescopy images indicate indicate the presence the presence of the liquidof the PEGliquid200 PEG, accompanied200, accompanied by 0.01 by wt.% 0.01 of wt.% Rhodamine of Rho- Bdamine dye, within B dye, the within PU MCs the PU (Figure MCs1 c,d).(Figure 1c,d).

FigureFigure 1.1. ((aa,,bb)) Scanning Scanning electron electron microscopy microscopy (SEM) (SEM) and and (c, (dc),d fluorescent) fluorescent microscopy microscopy images images of of

PEGPEG200200@PU microcapsules microcapsules (MCs).

Thermogravimetric analysis (TGA) indicates the existence of two weight loss stages (Figure2). The first weight loss is associated with the decomposition of the encapsulated PEG200. This weight loss occurs almost at the same range of temperatures at which pure ◦ PEG200 decomposes (200–300 C), and constitutes 85% of the sample’s weight, which is in agreement with the calculated wt.% of PEG200 from the total amount of MCs (87%). The Polymers 2021, 13, 2566 6 of 14

Thermogravimetric analysis (TGA) indicates the existence of two weight loss stages (Figure 2). The first weight loss is associated with the decomposition of the encapsulated Polymers 2021, 13, 2566 PEG200. This weight loss occurs almost at the same range of temperatures at which 6pure of 13 PEG200 decomposes (200–300 °C), and constitutes 85% of the sample’s weight, which is in agreement with the calculated wt.% of PEG200 from the total amount of MCs (87%). The second weight loss (15%), centered at 400 ℃, apparently is associated with the thermal second weight loss (15%), centered at 400 °C, apparently is associated with the thermal decomposition of of the PU shell, which, accordin accordingg to the theoretical calculations, stands at 13%. Moreover, Moreover, the MCs are thermally stable up to 200 ◦°C;C; this makes them applicable even when elevated temperatures are needed.

Figure 2. Thermogravimetric analysis (TGA) curves of (a) PEG200@PU MCs and (b) pure PEG200. Figure 2. Thermogravimetric analysis (TGA) curves of (a) PEG200@PU MCs and (b) pure PEG200. Fourier-transform IR (FT-IR) was further employed to confirm the chemical composi- tion inFourier-transform the system. The resultsIR (FT-IR) presented was further in Figure employed3 revealed to confirm that polyurea the chemical polymerization compo- sitionwas achieved. in the system. The absence The results peak presented band at ~2270 in Figure cm− 13 confirmsrevealed thethat total polyurea polymerization polymeriza- of tionthe isocyanatewas achieved. monomer The absence (TDI), peak which band apparently at ~2270 reactedcm−1 confirms completely the total with polymerization the branched −1 ofPEI the800 isocyanateto form the monomer polyurea (TDI), shell. which The apparently absorption reacted bands completely at 846 cm with(CH the2 branchedrocking), −1 −1 −1 −1 −1 PEI890800 cm to form(C-OH the polyurea bending), shell. 1100 The cm absorption(C-O stretching), bands at 846 and cm 2870–2960 (CH2 rocking), cm 890refer cm to −1 −11 (C-OHPEG200 ,bending), whereas the1100 wide cm absorption (C-O stretching), band from and 3000 2870–2960 to 3700 cm referis attributed to PEG200 to, whereas the N-H theand wide O-H absorption stretching vibrationsband fromof 3000 polyurea to 3700 and cm− PEG.1 is attributed to the N-H and O-H stretch- ing vibrationsAfter confirming of polyurea the feasibilityand PEG. for the formation of the MCs, they were loaded with palladiumAfter confirming NPs and characterized the feasibility by for different the formation methods. of Thethe MCs, focused they ion were beam loaded (FIB-SEM) with palladiumimages indicate NPs and that characterized the MCs maintain by different their methods. morphology The andfocused their ion spherical beam (FIB-SEM) structure imagesafter the indicate penetration that the of MCs the palladiummaintain their (Figure morphology4a,b). Moreover, and their a spherical cutting process structure con- af- terfirms the the penetration existence of athe core/shell palladium structure (Figurewith 4a,b). a shellMoreover, thickness a cutting of ~1 processµm (Figure confirms4c,d); thehowever, existence the of thickness a core/shell varied, structure depending with a on shell the thickness MC size. of The ~1 presenceµm (Figure of 4c,d); the carbon, how- ever,nitrogen, the thickness and oxygen varied, elements depending was further on the confirmed MC size. byThe conducting presence of mapping the carbon, and EDXSnitro- gen,(energy and dispersiveoxygen elements X-ray spectroscopy)was further con analyses,firmed by for conducting the cut MC mapping and the completeand EDXS MCs (en- ergy(Figures dispersive S4 and X-ray S5). spectroscopy) analyses, for the cut MC and the complete MCs (Fig- ures S4However, and S5). although the EDXS analysis confirms the existence of palladium, the map- ping measurement was not sensitive enough to detect the presence of palladium. In this regard, we carried out the mapping measurement in scanning transmission electron mi- croscopy (STEM) mode (Figure5a). Moreover, the figure reveals that the palladium NPs were successfully encapsulated within the MCs. The presence of palladium was also veri- fied by EDXS analysis (Figure5b). Besides the already known positive stabilization effect of the PEG [87–90], the very small palladium NPs are further stabilized by the nitrogen atoms of the branched PEI800 and by the microencapsulation process itself; the latter constructs a concrete barrier between the palladium NPs and constitutes a hurdle to the possibility of high constrictions of palladium NPs, which eventually will lead to aggregation processes and, subsequently, to a drop in the catalytic activity. PolymersPolymers 20212021,, 1313,, 25662566 77 of of 1314

Figure 3. Fourier-transform IR (FT-IR) spectra of (a)) purepure PEG200PEG200 andand ((bb)) PEGPEG200200@PU@PU MCs. MCs.

Figure 4. Focused ion beam (FIB)-SEM images of the PdNPs/PEG200@PU MCs (a–d).

However, although the EDXS analysis confirms the existence of palladium, the map- ping measurement was not sensitive enough to detect the presence of palladium. In this regard, we carried out the mapping measurement in scanning transmission electron mi- Figure 4. Focused ion beamcroscopy (FIB)-SEM (STEM) images mode of the (Figure PdNPs/PEG200@PU 5a). Moreover MCs, the before figure etching reveals process that the (a,b palladium) and after NPs etching (c,d). were successfully encapsulated within the MCs. The presence of palladium was also ver- ified by EDXS analysis (Figure 5b). Besides the already known positive stabilization effect of the PEG [87–90], the very small palladium NPs are further stabilized by the nitrogen

1

Polymers 2021, 13, 2566 8 of 14

atoms of the branched PEI800 and by the microencapsulation process itself; the latter con-

Polymers 2021, 13, 2566 structs a concrete barrier between the palladium NPs and constitutes a hurdle to the8 pos- of 13 sibility of high constrictions of palladium NPs, which eventually will lead to aggregation processes and, subsequently, to a drop in the catalytic activity.

Figure 5. STEM mode applied for the ( a) mapping and ( b) energy dispersive X-ray spectro spectroscopyscopy (EDXS) analyses of of the PdNPs/PEG200@PU MCs. PdNPs/PEG200@PU MCs. 3.3.3.3. Evaluation Evaluation of of the the Catalytic Catalytic Performance Performance

TheThe catalytic catalytic performance performance of of the the Pd PdNPsNPs/PEG/PEG200@PU200@PU microreactor microreactor was was tested tested in the in hy- the drogenationhydrogenation of ofvarious various alkenes alkenes and and alkynes. alkynes. As As shown shown in in Table Table 1,1, the catalystcatalyst exhibitsexhibits highlyhighly desirable activity inin thethe hydrogenationhydrogenation of of alkenes. alkenes. Fully Fully saturated saturated alkanes could could be begenerated generated smoothly smoothly under under mild mild conditions conditions (Table (Table1, entries 1, entries 1–6). 1–6). The catalyst’sThe catalyst’s activity activity was wasalso testedalso tested in the in hydrogenation the hydrogenation of alkynes; of alkynes; aromatic aromatic terminal terminal alkynes werealkynes fully were converted fully convertedto the corresponding to the corresponding fully hydrogenated fully hydrogenated products (Table products1, entries (Table 7–9). 1, entries Interestingly, 7–9). Inter-para- estingly,substrates, para such-substrates, as 4-vinylanisole such as 4-vinylanisole and 4-methylstyrene, and 4-methylstyrene, exhibit a slightly exhibit better a catalyticslightly betterperformance catalytic compared performance with comparedmeta-substrates, with meta such-substrates, as 3-methylstyrene such as 3-methylstyrene and 3-chlorostyrene. and 3-chlorostyrene.Nevertheless, such Nevertheless, a finding requires such a finding further requires kinetic experiments further kinetic and experiments an in-depth and inves- an in-depthtigative studyinvestigative of the microreactorstudy of the microreact structure’sor porosity. structure’s In addition,porosity. In diphenylacetylene, addition, diphe- nylacetylene,which was also which fully was converted, also fully exhibits converted, exceptional exhibits behavioral exceptional patterns behavioral with amoderate patterns withselectivity a moderate of 61% selectivity towards theof 61%cis-stilbene towards and the 39% cis-stilbene towards and the 39% fully towards hydrogenated the fully biben- hy- drogenatedzyl. This partial bibenzyl. selectivity This partial could selectivity be attributed could to be the attributed steric hindrance to the steric of the hindrancecis-stilbene of theformed cis-stilbene initially, whichformed can initially, slow its hydrogenationwhich can slow within its the hydrogenation Pd/PEG@polyurea within microre- the Pd/PEG@polyureaactor (Table1, entry microreact 10). An excellentor (Table reactivity 1, entry 10). of ourAn excellent catalyst was reactivity also achieved of our catalyst when wasa substrate/catalyst also achieved when ratio a ofsubstrate/catalyst 5000 was applied ratio in of the 5000 hydrogenation was appliedof in styrenethe hydrogena- (Table1, tionentry of 11). styrene (Table 1, entry 11). Furthermore, the catalytic performance of the developed microreactor was compared with a completely homogeneous system. In this regard, the hydrogenation of styrene was used as a model reaction. Strikingly, the PdNPs/PEG200@PU microreactor exhibits catalytic supremacy over the pure homogeneous catalyst under the same conditions; the homogeneous catalyst reached 28% in PEG200, whereas the microreactor reached 100% conversion. These results indicate that the PEG stabilization does not ensure high catalytic Polymers 2021, 13, 2566 9 of 14

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In In th th ofis ofis theregard, theregard, developed developed the the hydrogenation hydrogenation microreactor microreactor of wasof was styrene styrene compared compared9 9 of was ofwas 14 14 supremacy over the pure homogeneous catalyst under the same conditions; the homoge- usedusedwithwithFurthermore, as Furthermore,aas a completelya completelya model model reaction. reaction. the homogeneousthe homogeneous catalytic catalytic Strikingly, Strikingly, performance performance system. system. the the Pd PdIn In NPsof NPsth ofth /PEGtheis/PEG theis regard, regard,developed developed200200@PU@PU the the microreactor microreactor hydrogenation microreactorhydrogenation microreactor exhibits exhibits was wasof of styrene compared styrene compared catalytic catalytic was was supremacyneous catalyst over reached the pure 28% homogeneous in PEG200, whereas catalyst the under microreactor the same reached conditions; 100% the conversion. homoge- withsupremacywithusedused Furthermore,aFurthermore, aascompletely ascompletely a a model overmodel the reaction. homogeneous the reaction.thehomogeneous pure catalytic catalytic homogeneous Strikingly, Strikingly, performance performance system. system. the the catalystIn PdIn Pdth of thNPsofisNPs theis /PEGtheregard, /PEG underregard, developed developed200200@PU thethe@PU the samehydrogenation microreactor hydrogenation microreactormicroreactor microreactor conditions; 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The The wascatalyst wascatalyst recycled recycled was was easilynine easilynine Table 1. The catalyticlocallocal performance concentration concentration of the of of Pdinteractions interactionsNPs/PEG200@PU between between microreactor the the catalyst catalyst in the hydrogenationand and the the substrates, substrates, reaction. leading leading to to an an timesimprovedimprovedseparatedtimesseparatedFinally, Finally,without without catalytic bycatalytic bythe the showingcentrifugation, centrifugation,showing recyclability recyclability performance performance any any loss ofloss washed,of washed, our ofourand ofand its catalyst itscatalyst highly activity,highly andactivity, and reused was efficientreusedwas efficient as asexamined. examined.seen seenfor for processes. processes.thein thein Figure nextFigure nextThe The cycle. catalystcycle. 6. catalyst 6. The The catalyst wascatalyst was recycled recycled was was easily easilynine nine TableTable 1. 1. The The catalytic catalytic performance performance of of the the Pd PdNPsNPs/PEG/PEG200200@PU@PU microreactor microreactor in in the the hydrogenation hydrogenation reaction. reaction. Entry Substrate separatedtimestimesseparatedFinally, Finally,without withoutConversion by by thecentrifugation, the centrifugation,showing showing recyclability recyclability (%) anya any loss washed, lossof washed,Entryof our ofour of its catalyst its catalyst andactivity, activity,and reused reused was was as as examined. examined.forseen seenSubstratefor the thein in nextFigure Figurenext The The cycle. cycle. catalyst 6. catalyst6. The The catalyst catalystwasConversion was recycled recycled was was easily (%)easily nine nine a TableTable 1. 1. The Thereactivity; catalytic catalytic performance however,performance performing of of the the Pd PdNPsNPs the/PEG/PEG reaction200200@PU@PU withinmicroreactor microreactor a microenvironmental in in the the hydrogenation hydrogenation entity reaction. reaction. guarantees EntryEntry SubstrateSubstrate separatedtimesseparatedtimes withoutConversion Conversionwithout by by centrifugation, centrifugation,showing showing (%) (%) a aany any loss washed, Entrylosswashed,Entry of of its its and activity,and activity, reused reused as as Substrateseenfor Substrateforseen the the in in next Figure nextFigure cycle. cycle. 6. 6. The The Conversioncatalyst Conversioncatalyst was was (%) easily (%)easily a a TableTable 1. 1. The The acatalytic catalytic high local performance performance concentration of of the the Pd of PdNPs interactionsNPs/PEG/PEG200200@PU@PU between microreactor microreactor the catalystin in the the hydrogenation hydrogenation and the substrates, reaction. reaction. leading separatedseparated by by centrifugation, centrifugation,a a washed, washed, and and reused reused for for the the next next cycle. cycle. a a EntryEntry1 SubstrateSubstrateto an improved ConversionConversion catalytic100 performance(%) (%) EntryEntry7 and highly efficientSubstrateSubstrate processes. ConversionConversion100 (%) (%) Table 1. The catalytic performance of the PdNPsNPs/PEG200200@PU microreactor in the hydrogenation reaction. Table 1. The catalytic performance of the Pda a /PEG @PU microreactor in the hydrogenation reaction. a a EntryEntry11 SubstrateSubstrate ConversionConversion100100 (%) (%) EntryEntry77 SubstrateSubstrate ConversionConversion100100 (%) (%) TableTable 1. 1. The The catalytic catalytic performance performance of of the the Pd PdNPsNPs/PEG/PEG200200@PU@PU microreactor microreactor in in the the hydrogenation hydrogenation reaction. reaction. a a a a EntryEntry1Table1 1. TheSubstrate catalyticSubstrate performance ConversionConversion of the100 Pd100NPs (%) (%)/PEG 200@PUEntryEntry7 microreactor7 in theSubstrateSubstrate hydrogenation reaction.ConversionConversion100100 (%) (%) a a EntryEntry SubstrateSubstrate ConversionConversion (%) (%) a EntryEntry SubstrateSubstrate ConversionConversion (%) (%) a Entry11 Substrate Conversion100100 (%) a 7Entry 7 Substrate Conversion (%)100100 a 2 100 8 100

11 100100 77 100100 22 100100 88 100100 1 11 100100 77 7 100 100100 22 100100 88 100100

2 2 100100 8 8 100100 3 96 9 100 2 22 100100 88 8 100 100100 33 9696 99 100100 22 100100 88 100100 33 9696 99 100100

3 3 3 9696 9 9 9 100 100100 4 100 10 b 100 33 9696 99 100100 44 100100 1010 b b 100100 33 9696 99 100100 4 44 100100100 101010 b b 100 100100

4 4 Cl 100100 1010 b b 100100 c 5 ClCl 92 11b 100 44 100100 1010 b 100100 5 5 9292 11 11c c c 100 100 454 ClCl 10092100 101110 b b 100100 c c 55 ClCl 9292 1111 100100 c c 55 ClCl 9292 1111 100100 6 6 100100 c c 55 ClCl 9292 1111 100100 66 100100 c c 55 9292 1111 a 1 b 100100 Reaction conditions: 0.2 mol% of Pd (substrate/Pd ratio = 500), 2.5 h, r.t., 100 psi of H2. Determined by HNMR. 61% cis-stilbene and 66 100100 2 a 1 b 39% bibenzyl.Reactionc 0.02 conditions: mol% of Pd 0.2 (substrate/Pd mol% of Pd ratio (substrate/Pd = 5000). ratio = 500), 2.5 h, r.t., 100 psi of H . Determined by HNMR. 61% cis- c 6stilbene and 39% bibenzyl. 0.02 mol% of100 Pd (substrate/Pd ratio = 5000). a a 1 1 b b ReactionReaction6 conditions: conditions: 0.2 0.2 mol% mol% of of Pd Pd (substrate/Pd (substrate/Pd100 ratio ratio = = 500), 500), 2.5 2.5 h, h, r.t., r.t., 100 100 psi psi of of H H2.2 . Determined Determined by by HNMR.HNMR. 61% 61% cis cis- - c c stilbene6stilbene6 and and 39% 39% bibenzyl. bibenzyl. 0.02 0.02 mol% mol% of of 100Pd 100Pd (substrate/Pd (substrate/Pd ratio ratio = = 5000). 5000). a a 1 1 b b Polymers 2021, 13, Reaction2566Reaction conditions: conditions: 0.2 0.2 mol% mol%Finally, of of Pd Pd the (substrate/Pd (substrate/Pd recyclability ratio ratio of our= =500), 500), catalyst 2.5 2.5 h, h, r.t., was r.t., 100 100 examined. psi psi of of H H2.2 The. Determined Determined catalyst wasby by HNMR.HNMR. recycled 61% nine61% cis cis- - 10 of 14 c c a a 1 1 b b Reaction6stilbeneReaction6 stilbene conditions:and conditions:and 39% 39% bibenzyl. bibenzyl. 0.2 times0.2 mol% mol% without 0.02 of0.02 of Pd mol%Pd mol% (substrate/Pd (substrate/Pd showing of of100 100Pd Pd (substrate/Pd (substrate/Pd any ratio ratio loss = =500), of 500), ratio its ratio 2.5 2.5 activity, =h, =h,5000). r.t.,5000). r.t., 100 as100 psi seen psi of of inH H2 Figure. 2. Determined Determined 6. The catalystby by HNMR. HNMR. was 61% easily 61% cis cis- - c stilbene and 39% bibenzyl. 0.02c mol% of Pd (substrate/Pd ratio = 5000). a 1 b ReactionReactionstilbene conditions: andconditions: 39% bibenzyl. 0.2 separated0.2 mol% mol% 0.02 of of byPd Pdmol% (substrate/Pd centrifugation, (substrate/Pd of Pd (substrate/Pd ratio ratio washed, = = 500), 500), ratio 2.5 and2.5 =h, h, 5000). reusedr.t., r.t., 100 100 psi forpsi of theof H H2 next.2 . aDetermined Determined cycle. by by HNMR.1HNMR. b61% 61% cis cis-- c c a 1 b stilbeneReactionstilbeneReaction and andconditions: conditions: 39% 39% bibenzyl. bibenzyl. 0.2 0.2 mol% mol% 0.02 0.02 of of mol%Pd mol%Pd (substrate/Pd (substrate/Pd of of Pd Pd (substrate/Pd (substrate/Pd ratio ratio = = 500), 500), ratio ratio 2.5 2.5 = = h,5000). 5000).h, r.t., r.t., 100 100 psi psi of of H H2.2 a. Determined Determined by by 1 HNMR.HNMR. b 61% 61% cis cis- - stilbenestilbene and and 39% 39% bibenzyl. bibenzyl. c c0.02 0.02 mol% mol% of of Pd Pd (substrate/Pd (substrate/Pd ratio ratio = =5000). 5000).

Figure 6. Recyclability of the Pd /PEG @PU microreactor in the hydrogenation reaction of Figure 6. Recyclability of theNPs PdNPs/PEG200 200@PU microreactor in the hydrogenation reaction of sty- styrene (reaction conditions: 0.2 mol% of Pd (substrate/Pd ratio = 500), 2.5 h, r.t., 100 psi of H ). rene (reaction conditions: 0.2 mol% of Pd (substrate/Pd ratio = 500), 2.5 h, r.t., 2100 psi of H2).

4. Conclusions With our continuous intent to bridge the gap between the homogeneous and hetero- geneous routes, highly reactive PdNPs/PEG200@PU microreactors were successfully devel- oped. The fabrication process is based on implementing an o/o emulsion-templated non- aqueous microencapsulation, through the interfacial polymerization of PEI800 and TDI. These microreactors were well characterized and utilized in the hydrogenation of aro- matic alkenes and alkynes, exhibiting extraordinary reactivity. The catalytic supremacy results from the very small and efficient palladium NPs; such a stabilization is achievable owing to the presence of PEG, which, to the best of our knowledge, was not encapsulated previously within a PU shell, surely not by a non-aqueous route. Not less important is the existence of the microenvironmental entity, which allows high local concentrations of in- teractions between the catalyst and substrates. Finally, the robust PU shell maintains the spherical structure of the microreactor and enables its facile recovery and recyclability.

Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: SEM images of PEG@PU microcapsules prepared using different surfactants; Table S1: PEG200@PU microcapsules obtained using different amines and isocyanate monomers; Figure S2: SEM image of PEG200@PU microcapsules obtained using PEG200-in-cyclohexane emulsions; Figure S3: FIB-SEM im- ages of PEG200@PU microcapsules; Figure S3: FIB-SEM images of Pdnano/PEG200@PU microcapsules. Author Contributions: Conceptualization, R.A.-R.; methodology, R.A.-R., A.Z. and S.O.; investiga- tion, A.Z. and S.O.; writing—Original draft preparation, A.Z.; writing—review and editing, R.A.- R., A.Z. and S.O.; supervision, R.A.-R.; project administration, A.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: We are grateful to the Ministry of Science, Technology, and Space for the fel- lowship of Ahmad Zarour. Conflicts of Interest: The authors declare no conflict of interest.

Polymers 2021, 13, 2566 10 of 13

4. Conclusions With our continuous intent to bridge the gap between the homogeneous and het- erogeneous routes, highly reactive PdNPs/PEG200@PU microreactors were successfully developed. The fabrication process is based on implementing an o/o emulsion-templated non-aqueous microencapsulation, through the interfacial polymerization of PEI800 and TDI. These microreactors were well characterized and utilized in the hydrogenation of aromatic alkenes and alkynes, exhibiting extraordinary reactivity. The catalytic supremacy results from the very small and efficient palladium NPs; such a stabilization is achievable owing to the presence of PEG, which, to the best of our knowledge, was not encapsulated previously within a PU shell, surely not by a non-aqueous route. Not less important is the existence of the microenvironmental entity, which allows high local concentrations of interactions between the catalyst and substrates. Finally, the robust PU shell maintains the spherical structure of the microreactor and enables its facile recovery and recyclability.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/polym13152566/s1, Figure S1: SEM images of PEG@PU microcapsules prepared using different surfactants; Table S1: PEG200@PU microcapsules obtained using different amines and isocyanate monomers; Figure S2: SEM image of PEG200@PU microcapsules obtained using PEG200- in-cyclohexane emulsions; Figure S3: FIB-SEM images of Pdnano/PEG200@PU microcapsules. Author Contributions: Conceptualization, R.A.-R.; methodology, R.A.-R., A.Z. and S.O.; investiga- tion, A.Z. and S.O.; writing—Original draft preparation, A.Z.; writing—review and editing, R.A.-R., A.Z. and S.O.; supervision, R.A.-R.; project administration, A.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: We are grateful to the Ministry of Science, Technology, and Space for the fellow- ship of Ahmad Zarour. Conflicts of Interest: The authors declare no conflict of interest.

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