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

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 emulsification 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 imine) (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 alkenes and alkynes. 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. Second, it stabilizes the palladium NPs and prevents their aggregation, allowing the formation of highly reactive palladium NPs. Strikingly, in one sense, the suggested system affords highly reactive semi-homogeneous catalysis, whereas in another sense, it enables the facile, rapid, and inexpensive recovery of the catalytic microreactor by simple centrifugation. The durable microreactors exhibit excellent activity and were recycled nine times without any loss in their reactivity.


Introduction
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. In this regard, the design of efficient and facile catalytic systems is highly desired [1,2]. Although the homogeneous route of catalysis is endowed with high reactivity and selectivity [3,4], most of the industrial catalytic processes rely on the heterogeneous counterpart 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], and tunable solvents [11].

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 microscope (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 Kinematica 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 analyzer at a temperature range of 25-700 • C and at a heating rate of 10 • C min −1 under N 2 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.

The Procedure for Preparing the Pd NPs /PEG 200 Polar Phase
In a vial of 10 mL, 100 mg (0.287 mmol) of Na 2 PdCl 4 was dissolved and sonicated in 4.66 g of PEG 200 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.

General Procedure for Preparing the Pd NPs /PEG 200 @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 Pd NPs /PEG 200 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 Pd NPs /PEG 200 @PU MCs were then dispersed in heptane until the weight of the whole dispersion was 20.00 g.

General Procedure for the Hydrogenation Reaction
In a 25 mL glass-lined autoclave equipped with a magnetic stirring bar, 1 g of the catalytic dispersion (0.047 mmol of Pd per 1 g of dispersion) was added to 2 mL of heptane. Then, a suitable amount of substrate was added relative to the substrate/catalyst (S/C) ratio. The autoclave was purged three times with H 2 and then pressurized to 100 psi of H 2 . The reactions were carried out at room temperature for 2.5 h. Finally, the MCs were separated from the reaction mixture by centrifugation, washed two times with heptane, redispersed in 3 mL of heptane, and then used for the next catalytic cycle. The reaction content was examined using 1 H-NMR spectroscopy in order to determine the conversion and selectivity.

Synthesis and Optimization of the PEG 200 @PU MCs
The synthesis of the polyurea microcapsules (PU MCs) is based on an emulsiontemplated non-aqueous encapsulation process. The preparation procedure begins with an emulsification process consisting of two immiscible oil phases: (1) the polar dispersed phase consists of branched poly(ethylene imine) 800 (PEI 800 ) and Na 2 PdCl 4 dissolved in poly(ethylene glycol) 200 (PEG 200 ), and the PEG 200 rapidly reduces the palladium (II) ions to palladium (0) nanoparticles (NPs); and (2) a polar continuous phase of heptane containing the polymeric surfactant ABIL EM 90. Then, a toluene-2,4-diisocyanate (TDI) dissolved in xylene was slowly added while the homogenization process was running. Finally, an interfacial polymerization process at the boundary of the emulsion droplets between the PEI and the TDI is executed, leading to the creation of a PU MC possessing a core/shell structure owing to the insolubility of the PU in PEG (Scheme 1). heptane. The PdNPs/PEG200@PU MCs were then dispersed in heptane until the weight of the whole dispersion was 20.00 g.

General Procedure for the Hydrogenation Reaction
In a 25 mL glass-lined autoclave equipped with a magnetic stirring bar, 1 g of the catalytic dispersion (0.047 mmol of Pd per 1 g of dispersion) was added to 2 mL of heptane. Then, a suitable amount of substrate was added relative to the substrate/catalyst (S/C) ratio. The autoclave was purged three times with H2 and then pressurized to 100 psi of H2. The reactions were carried out at room temperature for 2.5 h. Finally, the MCs were separated from the reaction mixture by centrifugation, washed two times with heptane, redispersed in 3 mL of heptane, and then used for the next catalytic cycle. The reaction content was examined using 1 H-NMR spectroscopy in order to determine the conversion and selectivity.

Synthesis and Optimization of the PEG200@PU MCs
The synthesis of the polyurea microcapsules (PU MCs) is based on an emulsion-templated non-aqueous encapsulation process. The preparation procedure begins with 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 poly(ethylene glycol)200 (PEG200), and the PEG200 rapidly reduces the palladium (II) ions to palladium (0) nanoparticles (NPs); and (2) a polar continuous phase of heptane containing the polymeric surfactant ABIL EM 90. Then, a toluene-2,4-diisocyanate (TDI) dissolved in xylene was slowly added while the homogenization process was running. Finally, an interfacial polymerization process at the boundary of the emulsion droplets between the PEI and the TDI is executed, leading to the creation of a PU MC possessing a core/shell structure owing to the insolubility of the PU in PEG (Scheme 1). Scheme 1. Schematic illustration of the Pd NPs /PEG 200 @PU preparation procedure.
The selection of these materials was determined after conducting a series of optimization experiments in which other continuous phases (cyclohexane and toluene), surfactants (Agrimer AL-22, AOT, span 80, and brij 92v), amines (2,2 (ethylenedioxy)bis(ethylamine), 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 surfactants were able to stabilize the o/o emulsion; the stabilization of such emulsions is not obvious and usually requires the involvement of multi-armed polymeric surfactants rather than small ones; the former settle irreversibly at the interface of the droplets, as their departure from the interface requires that all the surfactants' arms leave the interface concomitantly, which is statistically less possible. Moreover, whereas the interfacial polymerization of the PEI/TDI and DETA/TDI pairs fabricates MCs, the 2,2 (ethylenedioxy)bis(ethylamine)/PAPI 27 and HMDA/PAPI 27 pairs result in the formation of nanocapsules (Table S1). However, the latter two suffer from aggregation problems, whereas in the case of the DETA/TDI pair, the encapsulation of PEG 200 is not attainable. In the case of cyclohexane only, the DETA/TDI pair affords MCs when Agrimer AL-22 was utilized as the emulsion stabilizer ( Figure S2); however, as already mentioned, the encapsulation of PEG 200 with this pair is not viable. For toluene, well-defined MCs were not obtained with either of the pairs.

Characterization of the Pd NPs /PEG 200 @PU MCs
First, the catalytic system was investigated before the penetration of the palladium. Scanning electron microscopy (SEM) was employed for determining the morphological structure of our system (Figure 1a,b). Smoothed spherical surfaces and some pressed capsules, caused by the high vacuum applied in this analysis, were obtained. As can be easily noted, a polydispersed system with sizes ranging from 200 nm to 15 µm was formed, which is a typical feature of macroemulsion systems. The measurement of the size of the microcapsules dispersed in isopropyl alcohol by laser diffraction size analyzer indicated an average size of 12.48 µm (d 0.5 = 12.48 micron, Figure S3 The selection of these materials was determined after conducting a series of optimization experiments in which other continuous phases (cyclohexane and toluene), surfactants (Agrimer AL-22, AOT, span 80, and brij 92v), amines (2,2′(ethylenedioxy)bis(ethylamine), 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 surfactants were able to stabilize the o/o emulsion; the stabilization of such emulsions is not obvious and usually requires the involvement of multi-armed polymeric surfactants rather than small ones; the former settle irreversibly at the interface of the droplets, as their departure from the interface requires that all the surfactants' arms leave the interface concomitantly, which is statistically less possible. Moreover, whereas the interfacial polymerization of the PEI/TDI and DETA/TDI pairs fabricates MCs, the 2,2′(ethylenedioxy)bis(ethylamine)/PAPI 27 and HMDA/PAPI 27 pairs result in the formation of nanocapsules (Table S1). However, the latter two suffer from aggregation problems, whereas in the case of the DETA/TDI pair, the encapsulation of PEG200 is not attainable. In the case of cyclohexane only, the DETA/TDI pair affords MCs when Agrimer AL-22 was utilized as the emulsion stabilizer ( Figure S2); however, as already mentioned, the encapsulation of PEG200 with this pair is not viable. For toluene, well-defined MCs were not obtained with either of the pairs.

Characterization of the PdNPs/PEG200@PU MCs
First, the catalytic system was investigated before the penetration of the palladium. Scanning electron microscopy (SEM) was employed for determining the morphological structure of our system (Figure 1a,b). Smoothed spherical surfaces and some pressed capsules, caused by the high vacuum applied in this analysis, were obtained. As can be easily noted, a polydispersed system with sizes ranging from 200 nm to 15 µm was formed, which is a typical feature of macroemulsion systems. The measurement of the size of the microcapsules dispersed in isopropyl alcohol by laser diffraction size analyzer indicated an average size of 12.48 µm (d0.5 = 12.48 micron, Figure S3). Moreover, fluorescent microscopy images indicate the presence of the liquid PEG200, accompanied by 0.01 wt.% of Rhodamine B dye, within the PU MCs (Figure 1c,d).  second weight loss (15%), centered at 400°C, apparently is associated with the thermal decomposition of the PU shell, which, according to the theoretical calculations, stands at 13%. Moreover, the MCs are thermally stable up to 200 • C; this makes them applicable even when elevated temperatures are needed.
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 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 second weight loss (15%), centered at 400 ℃, apparently is associated with the thermal decomposition of the PU shell, which, according to the theoretical calculations, stands at 13%. Moreover, the MCs are thermally stable up to 200 °C; this makes them applicable even when elevated temperatures are needed. Fourier-transform IR (FT-IR) was further employed to confirm the chemical composition in the system. The results presented in Figure 3 revealed that polyurea polymerization was achieved. The absence peak band at ~2270 cm −1 confirms the total polymerization of the isocyanate monomer (TDI), which apparently reacted completely with the branched PEI800 to form the polyurea shell. The absorption bands at 846 cm −1 (CH2 rocking), 890 cm −1 (C-OH bending), 1100 cm −1 (C-O stretching), and 2870-2960 cm −1 refer to PEG200, whereas the wide absorption band from 3000 to 3700 cm −1 is attributed to the N-H and O-H stretching vibrations of polyurea and PEG.
After confirming the feasibility for the formation of the MCs, they were loaded with palladium NPs and characterized by different methods. The focused ion beam (FIB-SEM) images indicate that the MCs maintain their morphology and their spherical structure after the penetration of the palladium (Figure 4a,b). Moreover, a cutting process confirms the existence of a core/shell structure with a shell thickness of ~1 µm (Figure 4c,d); however, the thickness varied, depending on the MC size. The presence of the carbon, nitrogen, and oxygen elements was further confirmed by conducting mapping and EDXS (energy dispersive X-ray spectroscopy) analyses, for the cut MC and the complete MCs (Figures S4 and S5). Fourier-transform IR (FT-IR) was further employed to confirm the chemical composition in the system. The results presented in Figure 3 revealed that polyurea polymerization was achieved. The absence peak band at~2270 cm −1 confirms the total polymerization of the isocyanate monomer (TDI), which apparently reacted completely with the branched PEI 800 to form the polyurea shell. The absorption bands at 846 cm −1 (CH 2 rocking), 890 cm −1 (C-OH bending), 1100 cm −1 (C-O stretching), and 2870-2960 cm −1 refer to PEG 200 , whereas the wide absorption band from 3000 to 3700 cm −1 is attributed to the N-H and O-H stretching vibrations of polyurea and PEG.
After confirming the feasibility for the formation of the MCs, they were loaded with palladium NPs and characterized by different methods. The focused ion beam (FIB-SEM) images indicate that the MCs maintain their morphology and their spherical structure after the penetration of the palladium (Figure 4a,b). Moreover, a cutting process confirms the existence of a core/shell structure with a shell thickness of~1 µm (Figure 4c,d); however, the thickness varied, depending on the MC size. The presence of the carbon, nitrogen, and oxygen elements was further confirmed by conducting mapping and EDXS (energy dispersive X-ray spectroscopy) analyses, for the cut MC and the complete MCs ( Figures S4 and S5).
However, although the EDXS analysis confirms the existence of palladium, the mapping measurement was not sensitive enough to detect the presence of palladium. In this regard, we carried out the mapping measurement in scanning transmission electron microscopy (STEM) mode (Figure 5a). Moreover, the figure reveals that the palladium NPs were successfully encapsulated within the MCs. The presence of palladium was also verified by EDXS analysis (Figure 5b). Besides the already known positive stabilization effect of the PEG [87][88][89][90], the very small palladium NPs are further stabilized by the nitrogen atoms of the branched PEI 800 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.  However, although the EDXS analysis confirms the existence of palladium, the mapping measurement was not sensitive enough to detect the presence of palladium. In this regard, we carried out the mapping measurement in scanning transmission electron microscopy (STEM) mode (Figure 5a). Moreover, the figure reveals that the palladium NPs were successfully encapsulated within the MCs. The presence of palladium was also verified by EDXS analysis (Figure 5b). Besides the already known positive stabilization effect of the PEG [87][88][89][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.

Evaluation of the Catalytic Performance
The catalytic performance of the PdNPs/PEG200@PU microreactor was tested in the hydrogenation of various alkenes and alkynes. As shown in Table 1, the catalyst exhibits highly desirable activity in the hydrogenation of alkenes. Fully saturated alkanes could be generated smoothly under mild conditions (Table 1, entries 1-6). The catalyst's activity was also tested in the hydrogenation of alkynes; aromatic terminal alkynes were fully converted to the corresponding fully hydrogenated products (Table 1, entries 7-9). Interestingly, para-substrates, such as 4-vinylanisole and 4-methylstyrene, exhibit a slightly better catalytic performance compared with meta-substrates, such as 3-methylstyrene and 3-chlorostyrene. Nevertheless, such a finding requires further kinetic experiments and an in-depth investigative study of the microreactor structure's porosity. In addition, diphenylacetylene, which was also fully converted, exhibits exceptional behavioral patterns with a moderate selectivity of 61% towards the cis-stilbene and 39% towards the fully hydrogenated bibenzyl. This partial selectivity could be attributed to the steric hindrance of the cis-stilbene formed initially, which can slow its hydrogenation within the Pd/PEG@polyurea microreactor (Table 1, entry 10). An excellent reactivity of our catalyst was also achieved when a substrate/catalyst ratio of 5000 was applied in the hydrogenation of styrene (Table 1, entry 11).

Evaluation of the Catalytic Performance
The catalytic performance of the Pd NPs /PEG 200 @PU microreactor was tested in the hydrogenation of various alkenes and alkynes. As shown in Table 1, the catalyst exhibits highly desirable activity in the hydrogenation of alkenes. Fully saturated alkanes could be generated smoothly under mild conditions (Table 1, entries 1-6). The catalyst's activity was also tested in the hydrogenation of alkynes; aromatic terminal alkynes were fully converted to the corresponding fully hydrogenated products (Table 1, entries 7-9). Interestingly, parasubstrates, such as 4-vinylanisole and 4-methylstyrene, exhibit a slightly better catalytic performance compared with meta-substrates, such as 3-methylstyrene and 3-chlorostyrene. Nevertheless, such a finding requires further kinetic experiments and an in-depth investigative study of the microreactor structure's porosity. In addition, diphenylacetylene, which was also fully converted, exhibits exceptional behavioral patterns with a moderate selectivity of 61% towards the cis-stilbene and 39% towards the fully hydrogenated bibenzyl. This partial selectivity could be attributed to the steric hindrance of the cis-stilbene formed initially, which can slow its hydrogenation within the Pd/PEG@polyurea microreactor (Table 1, entry 10). An excellent reactivity of our catalyst was also achieved when a substrate/catalyst ratio of 5000 was applied in the hydrogenation of 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 Pd NPs /PEG 200 @PU microreactor exhibits catalytic supremacy over the pure homogeneous catalyst under the same conditions; the homogeneous catalyst reached 28% in PEG 200 , whereas the microreactor reached 100% conversion. These results indicate that the PEG stabilization does not ensure high catalytic reactivity; however, performing the reaction within a microenvironmental entity guarantees a high local concentration of interactions between the catalyst and the substrates, leading to an improved catalytic performance and highly efficient processes. These results indicate that the PEG stabilization does not ensure high catalytic reactivity; however, performing the reaction within a microenvironmental entity guarantees a high local concentration of interactions between the catalyst and the substrates, leading to an improved catalytic performance and highly efficient processes. Finally, the recyclability of our catalyst was examined. The catalyst was recycled nine times without showing any loss of its activity, as seen in Figure 6. The catalyst was easily separated by centrifugation, washed, and reused for the next cycle. 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 reactivity; however, performing the reaction within a microenvironmental entity guarantees a high local concentration of interactions between the catalyst and the substrates, leading to an improved catalytic performance and highly efficient processes. Finally, the recyclability of our catalyst was examined. The catalyst was recycled nine times without showing any loss of its activity, as seen in Figure 6. The catalyst was easily separated by centrifugation, washed, and reused for the next cycle. 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 reactivity; however, performing the reaction within a microenvironmental entity guarantees a high local concentration of interactions between the catalyst and the substrates, leading to an improved catalytic performance and highly efficient processes. Finally, the recyclability of our catalyst was examined. The catalyst was recycled nine times without showing any loss of its activity, as seen in Figure 6. The catalyst was easily separated by centrifugation, washed, and reused for the next cycle. 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 reactivity; however, performing the reaction within a microenvironmental entity guarantees a high local concentration of interactions between the catalyst and the substrates, leading to an improved catalytic performance and highly efficient processes. Finally, the recyclability of our catalyst was examined. The catalyst was recycled nine times without showing any loss of its activity, as seen in Figure 6. The catalyst was easily separated by centrifugation, washed, and reused for the next cycle. neous catalyst reached 28% in PEG200, whereas the microreactor reached 100% conversion.
These results indicate that the PEG stabilization does not ensure high catalytic reactivity; however, performing the reaction within a microenvironmental entity guarantees a high local concentration of interactions between the catalyst and the substrates, leading to an improved catalytic performance and highly efficient processes. Finally, the recyclability of our catalyst was examined. The catalyst was recycled nine times without showing any loss of its activity, as seen in Figure 6. The catalyst was easily separated by centrifugation, washed, and reused for the next cycle. neous catalyst reached 28% in PEG200, whereas the microreactor reached 100% conversion.
These results indicate that the PEG stabilization does not ensure high catalytic reactivity; however, performing the reaction within a microenvironmental entity guarantees a high local concentration of interactions between the catalyst and the substrates, leading to an improved catalytic performance and highly efficient processes. Finally, the recyclability of our catalyst was examined. The catalyst was recycled nine times without showing any loss of its activity, as seen in Figure 6. The catalyst was easily separated by centrifugation, washed, and reused for the next cycle. These results indicate that the PEG stabilization does not ensure high catalytic reactivity; however, performing the reaction within a microenvironmental entity guarantees a high local concentration of interactions between the catalyst and the substrates, leading to an improved catalytic performance and highly efficient processes. Finally, the recyclability of our catalyst was examined. The catalyst was recycled nine times without showing any loss of its activity, as seen in Figure 6. The catalyst was easily separated by centrifugation, washed, and reused for the next cycle. These results indicate that the PEG stabilization does not ensure high catalytic reactivity; however, performing the reaction within a microenvironmental entity guarantees a high local concentration of interactions between the catalyst and the substrates, leading to an improved catalytic performance and highly efficient processes. Finally, the recyclability of our catalyst was examined. The catalyst was recycled nine times without showing any loss of its activity, as seen in Figure 6. The catalyst was easily separated by centrifugation, washed, and reused for the next cycle. however, performing the reaction within a microenvironmental entity guarantees a high local concentration of interactions between the catalyst and the substrates, leading to an improved catalytic performance and highly efficient processes. Finally, the recyclability of our catalyst was examined. The catalyst was recycled nine times without showing any loss of its activity, as seen in Figure 6. The catalyst was easily separated by centrifugation, washed, and reused for the next cycle. however, performing the reaction within a microenvironmental entity guarantees a high local concentration of interactions between the catalyst and the substrates, leading to an improved catalytic performance and highly efficient processes. Finally, the recyclability of our catalyst was examined. The catalyst was recycled nine times without showing any loss of its activity, as seen in Figure 6. The catalyst was easily separated by centrifugation, washed, and reused for the next cycle. local concentration of interactions between the catalyst and the substrates, leading to an improved catalytic performance and highly efficient processes. Finally, the recyclability of our catalyst was examined. The catalyst was recycled nine times without showing any loss of its activity, as seen in Figure 6. The catalyst was easily separated by centrifugation, washed, and reused for the next cycle. Finally, the recyclability of our catalyst was examined. The catalyst was recycled nine times without showing any loss of its activity, as seen in Figure 6. The catalyst was easily separated by centrifugation, washed, and reused for the next cycle.

Conclusions
With our continuous intent to bridge the gap between the homogeneous and geneous routes, highly reactive PdNPs/PEG200@PU microreactors were successfull

Conclusions
With our continuous intent to bridge the gap between the homogeneous and heterogeneous routes, highly reactive Pd NPs /PEG 200 @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 PEI 800 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.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.