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PVP-crosslinked

aDepartment of Mechanical and Manufacturi

Box 20537, 1678 Nicosia, Cyprus. E-mail: kbDepartment of Chemistry, University of CyprcUniversity Politehnica of Bucharest, Facu

Science, Department of Oxide Materials a

Street, 011061 Bucharest, RomaniadUniversity Politehnica of Bucharest, Facult

Bioengineering and Biotechnology, No 133

Bucharest, Romania

Cite this: RSC Adv., 2014, 4, 44911

Received 26th June 2014Accepted 5th September 2014

DOI: 10.1039/c4ra06294a

www.rsc.org/advances

This journal is © The Royal Society of C

electrospun membranes withembedded Pd and Cu2O nanoparticles as effectiveheterogeneous catalytic supports

Ioanna Savva,a Andreas S. Kalogirou,b Andrea Chatzinicolaou,a Petri Papaphilippou,a

Athena Pantelidou,a Eugeniu Vasile,c Eugenia Vasile,d Panayiotis A. Koutentisb

and Theodora Krasia-Christoforou*a

Palladium(0) (Pd) and copper(I) oxide (Cu2O) nanoparticles (NPs) were successfully embedded in

electrospun polyvinylpyrrolidone (PVP) fibrous membranes. The fabrication process involved the

synthesis of stable, PVP-capped Pd and Cu2O colloidal hybrid solutions in methanol that on subsequent

electrospinning afforded PVP–Pd and PVP–Cu2O fibrous mats. The morphology of the as-prepared

nanocomposite fibers was characterised using scanning electron microscopy (SEM) and transmission

electron microscopy (TEM). SEM revealed the presence of bead-free, cylindrical fibers with diameters in

the submicrometer range while TEM revealed the presence of spherical Pd and Cu2O NPs with

diameters below 10 nm that were evenly distributed within the fibers. Thermal treatment of the PVP–Pd

and the PVP–Cu2O membranes afforded crosslinked fibrous mats as supported by SEM. Furthermore,

the presence of homogeneously distributed Pd and Cu2O NPs within the crosslinked polymer fibers was

confirmed by HRTEM/EDX analyses. The above-mentioned nanocomposite fibers demonstrated high

catalytic efficacy as heterogeneous catalytic supports in Heck, Suzuki (PVP–Pd) and click (PVP–Cu2O)

reactions. Finally, the reusability of the membranes was briefly investigated with up to three consecutive

runs being effective.

1. Introduction

Electrospinning is a simple, cost-effective technique used forthe production of brous materials comprised of long, contin-uous bers with diameters ranging from a few nanometers upto a few micrometers.1–3 While this process dates back to 1900,4

the rst scientic publications on electrospun nanobersappeared only in the early 1990s with the term “electro-spinning” being popularised by Reneker.5–7 The process isversatile and enables not only the fabrication of pristine poly-mer bers but also of brous nanocomposites via the combi-nation of polymers with inorganic nanollers. This versatilityhas led to an exponential increase in the number of relatedpublications, reaching to date around 2000 per year.8 Thesenumbers reect the importance of the electrospinning process,which, when combined with the current state of art can enhance

ng Engineering, University of Cyprus, P.O.

rasia@ucy.ac.cy

us, P.O. Box 20537, 1678 Nicosia, Cyprus

lty of Applied Chemistry and Material

nd Nanomaterials, No. 1-7 Gh. Polizu

y of Medical Engineering, Department of

Splaiul Independentei Street, 060042,

hemistry 2014

the development of advanced, polymer-based brous nano-composites destined for use in various technological applica-tions. Among such nanoadditives, metal and metal oxidenanoparticles (NPs) with their unique crystalline structures andlarge specic surface areas oen exhibit excellent catalyticproperties.9–14 It is anticipated that the combination of thecatalytic properties of NPs, with the unique characteristics ofelectrospun brous materials (e.g., high surface-to-volumeratio, high porosity, ease of chemical/physical post-modica-tion processes) will lead to the development of new, multi-functional brous nanocomposites with outstanding catalyticbehaviour. Moreover, the immobilization of inorganic catalyticcenters within an insoluble polymer matrix prevents nano-particle aggregation that can reduce catalytic efficiency, andenables the catalyst's facile recovery and reuse. In contrast,analogous NP-containing colloidal systems cannot easily beseparated from reaction products, which limits their use inpractical applications.15

Electrospun bers comprised of solely inorganic mate-rials16–21 or carbon nanober/nanotube-based electrospunsystems22–24 have been fabricated and evaluated in catalyticprocesses. However, there are only a few reports on the fabri-cation of polymer-NP electrospun brous nanocomposites andtheir evaluation in heterogeneous catalysis: Pd(0) metal nano-particles (MNPs) were incorporated in electrospun polymer

RSC Adv., 2014, 4, 44911–44921 | 44911

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brous matrices and evaluated as catalytic supports in Heck24

and hydrogenation15,26,27 reactions; and Au or Ag MNPs wereincorporated into brous electrospun mats and the resultingbrous nanocomposites evaluated in reduction,28,29 and elec-trocatalytic oxidation processes.30

Herein, we describe the fabrication of PVP–Pd and PVP–Cu2O electrospun brous nanocomposite membranes and theirevaluation as heterogeneous catalytic supports in Heck, Suzukiand click chemistry reactions.

While two recent reports describe the evaluation of PVP–Pdelectrospun bers in Heck reaction processes,15,25 no othercoupling reactions have appeared in the literature using thistype of polymer-based catalytic support. In the present work thePVP–Pd electrospun brous nanocomposite membranes wereinvestigated in both Heck and Suzuki coupling reactions todemonstrate their versatility. Furthermore, the membraneswere recovered, reused and shown to be effective for threeconsecutive coupling reactions.

In addition, and to the best of our knowledge, there are noreports on the fabrication and characterization of Cu2O NPimmobilized on electrospun PVP brous matrices that havebeen evaluated as catalytic supports in click chemistry: theCu(I)-catalysed version of the Huisgen 1,3-dipolar cycloadditionof azides and terminal alkynes, was popularised by Sharpless in2001 (ref. 31 and 32) and has numerous applications in drug-design and diverse chemical-biology applications.33 Copper34–36

and copper oxide37–39 nanoparticles may act as effective catalystsin click chemistry reactions; consequently, their immobiliza-tion on electrospun PVP could extend the usability and exi-bility of the click reaction.

Moreover, the chemical compatibility of the PVP–Pd andPVP–Cu2O systems i.e. that the same polymer (PVP) is employedas a stabilizing agent for the generation of NP colloidal solu-tions and that in both cases the polymer-NP solutions areprepared in methanol, allows for their intermixing. Thispotentially enables the future development of multicomponentelectrospun catalytic supports with an embedded “cocktail” ofnanoparticulate catalytic centers.

2. Experimental2.1. Materials and methods

Polyvinylpyrrolidone (PVP, �Mn ¼ 1 300 000), palladium(II) acetate(98%) and copper(II) acetate monohydrate (98% ACS reagent)were purchased from Sigma-Aldrich. Methanol (analytical grade,ACS reagent) was purchased fromScharlau. The above-mentionedreagents were used as provided by the manufacturer withoutfurther purication. Benzyl azide was prepared from benzylbromide and sodium azide according to the literature.40 DMFwasdried by azeotropically removing water with benzene and thendistilling under vacuum over dried molecular sieves 4 A and thenkept under an argon atmosphere in a desiccator. AnhydrousNa2SO4 was used for drying organic extracts and all volatiles wereremoved under reduced pressure. All reaction mixtures andcolumn eluents were monitored by TLC using commercial glassbacked thin layer chromatography (TLC) plates (Merck Kieselgel60 F254). The plates were observed under UV light at 254 and 365

44912 | RSC Adv., 2014, 4, 44911–44921

nm. The technique of dry ash chromatography was usedthroughout for all non-TLC scale chromatographic separationsusing Merck Silica Gel 60 (less than 0.063 mm).41

2.2. Synthesis

2.2.1. Synthesis of PVP-stabilized Pd nanoparticles. ThePVP–Pd nanohybrid colloidal systems (mols vinyl pyridine unitsper mols palladium salt¼ 20 : 1) were prepared as follows:42 in around bottom ask equipped with amagnetic stirrer, PVP (1.0 g,9 mmol of vinylpyrridine units) was dissolved in MeOH (10 mL).Subsequently, palladium(II) acetate (100 mg, 0.45 mmol) wasadded to the polymer solution and the reaction mixture washeated at reux (65 �C) for 2 h. During this period, the colour ofthe solution changed from yellow to dark brown indicating theformation of Pd nanoparticles. Aer the completion of thereaction, the brown-coloured solution was allowed to cool to ca.20 �C and it was then stored in sealed glass vials. The solutionswere stable and no precipitation was observed even aer severalmonths.

2.2.2. Synthesis of PVP-stabilized Cu nanoparticles. ThePVP–Cu nanohybrid colloidal systems (mols vinyl pyridine unitsper mols copper salt¼ 3.6 : 1) were prepared as follows: in a vialequipped with a magnetic stirrer, PVP (1.0 g, 9 mmol of vinyl-pyrridine units) was dissolved in MeOH (7.5 mL). Subsequently,copper(II) acetate monohydrate (500 mg, 2.5 mmol) was addedto the polymer solution and the mixture was le to stir over-night at ca. 20 �C. During this period, the colour of the solutionchanged from colourless to blue. Aerwards, hydrazine mono-hydrate (972 mL, 20.0 mmol) was added and the colour of thesolution became dark red, indicating the formation of Cunanoparticles. Air exposure of the colloidal solution led to acolour change from red to light blue, indicative of the oxidationof the Cu NPs to Cu2O.43 The nal PVP-capped Cu2O nano-particles exhibited high stability in methanol and no agglom-eration/destabilization phenomena were observed even aerseveral months.

2.2.3. Membrane fabrication of PVP–Pd and PVP–Cu2Oelectrospun membranes. The PVP–Pd and PVP–Cu2O colloidalsolutions were used for the fabrication of Pd- and Cu2O-con-taining PVP brous membranes by means of the electro-spinning technique. All electrospinning experiments wereperformed at ca. 20 �C. Equipment included a controlled-ow,four-channel volumetric microdialysis pump (KD Scientic,Model: 789252), syringes with specially connected spinneretneedle electrodes, a high-voltage power source (10–50 kV) and acustom-designed, grounded target collector, inside an inter-locked Faraday enclosure safety cabinet. Systematic parametricstudies were carried out by varying the applied voltage, theneedle-to-collector distance, the needle diameter and the owrate so as to determine the optimum experimental conditionsfor obtaining brous materials. Due to their hydrophilicnature, these membranes have the tendency to adsorb watervapour when exposed to high humidity conditions that in turnalters their morphological characteristics. For avoiding suchhydration phenomena, the membranes were stored in sealedglass vials.

This journal is © The Royal Society of Chemistry 2014

Fig. 2 UV-vis spectra of the pristine PVPMeOH solution and the PVP–Pd and PVP–Cu hybrid colloidal solutions (recorded upon appropriate

Fig. 1 Schematic presentation of the synthetic pathways followed for the generation of the PVP–Pd, PVP–Cu and PVP–Cu2O colloidal solutionsstabilized in MeOH and corresponding photographs.

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2.2.4. Membrane crosslinking. Insoluble PVP–Pd and PVP–Cu2O brous nanocomposites were obtained via thermalcrosslinking of the as-prepared corresponding electrospunbers in an oven heated at ca. 180 �C for 5 h.

2.2.5. Heck reaction. To a stirred solution of iodobenzene(22 mL, 0.20 mmol) in dry DMF (1 mL) at ca. 20 �C, was added n-butyl acrylate (43 mL, 0.30 mmol), Et3N (75 mL, 0.54 mmol) andthe PVP–Pd 20 : 1 electrospun membrane (11 mg, 2.4 mol%).The mixture was heated to 125 �C in a sealed tube untilcomplete consumption of iodobenzene (TLC). The mixture wasthen cooled to ca. 20 �C and t-BuOMe (10 mL) was added. Thepolymer precipitate was ltered and washed with t-BuOMe(10 mL), then dried under vacuum. The membrane was reusedin subsequent reactions without other treatment. The organicwashings were washed with H2O (2 � 10 mL), combined, dried(Na2SO4) and evaporated to give aer column chromatography(n-hexane/DCM, 1 : 1) n-butyl cinnamate (1) (36 mg, 88%) as ayellow oil; Rf 0.72 (n-hexane/DCM, 1 : 1); vmax/cm

�1 3063w,3030w, 2958w, 2931w and 2874w (C–H), 1712s (C]O), 1639m(C]C), 1578m, 1558m, 1497w, 1465w, 1451m, 1383w, 1327m,1309m, 1281m, 1256m, 1202m, 1171s, 1119w, 1065w,1026w, 978m, 864m, 768m; dH(500 MHz; CDCl3) 7.69 (1H, d,J 16.0, ]CH), 7.53–7.51 (2H, m, Ar H), 7.39–7.36 (3H, m, Ar H),6.45 (1H, d, J 16.0, ]CH), 4.21 (2H, t, J 6.7, OCH2), 1.72–1.66(2H, m, CH2), 1.48–1.40 (2H, m, CH2), 0.97 (3H, t, J 7.4, CH3);dC(125 MHz; CDCl3) 167.05 (s), 144.5 (d), 134.4 (s), 130.15 (d),128.8 (d), 128.0 (d), 118.2 (d), 64.2 (t), 30.7 (t), 19.15 (t), 13.7 (q);identical to an authentic sample.44

2.2.6. Suzuki reaction. To a stirred solution of 4-iodoani-sole (117 mg, 0.50 mmol) in THF (0.8 mL) and H2O (0.4 mL) atca. 20 �C was added phenyl boronic acid (91 mg, 0.75 mmol),powdered K2CO3 (138 mg, 1.00 mmol) and the crosslinked PVP–Pd 20 : 1 membrane (20 mg, 1.6 mol%) and the mixture washeated to ca. 80 �C in a sealed tube until complete consumptionof the starting material (TLC). The mixture was then cooled toca. 20 �C and t-BuOMe (10 mL) was added. The polymerprecipitate was ltered and washed with t-BuOMe (10 mL), thendried under vacuum. The membrane was reused in subsequent

This journal is © The Royal Society of Chemistry 2014

reactions without further treatment. The organic washings werewashed with H2O (2 � 10 mL), combined, dried (Na2SO4) andevaporated to give 4-methoxy-biphenyl (2) (90 mg, 98%) as col-ourless needles, mp 81–83 �C (from EtOH/H2O, lit.,45 80–82 �C);Rf 0.30 (n-hexane); vmax/cm

�1 3067w, 3034w and 3003w (C–H),1605m, 1582w, 1522m, 1485m, 1464w, 1451w, 1439m, 1287m,1269m, 1248s, 1200s, 1184m, 1119w, 1042m, 1038m, 1015w,1003w, 833s, 804, 760s; dH(500 MHz; CDCl3) 7.56 (2H, d, J 7.9, ArH), 7.54 (2H, d, J 8.7, Ar H), 7.42 (2H, dd, J 7.6, 7.6 Ar H), 7.38(1H, dd, J 7.4, 7.4 ArH), 6.99 (2H, d, J 8.7, ArH), 3.86 (3H, s, CH3);dC(125 MHz; CDCl3) 159.1 (s), 140.8 (s), 133.8 (s), 128.7 (d), 128.1(d), 126.7 (d), 126.6 (d), 114.2 (d), 55.3 (t); m/z (MALDI-TOF) 185(MH+, 100%), 170 (32), 152 (61), 141 (18), 130 (32); identical toan authentic sample.

2.2.7. Click chemistry. To a stirred solution of benzyl azide(67 mg, 0.50 mmol) in dioxane (1 mL) at ca. 20 �C was added

dilution of the as-prepared colloidal solutions).

RSC Adv., 2014, 4, 44911–44921 | 44913

Fig. 3 Schematic presentation of the electrospinning process (set-up) used for the fabrication of the PVP–Pd and the PVP–Cu2O nano-composite membranes starting from colloidal nanohybrid solutions. Photographs show the solutions and corresponding electrospun nano-composite membranes.

Table 1 Optimum experimental parameters employed for the fabri-cation of the pristine PVP, and the nanocomposite PVP–Pd and PVP–Cu2O electrospun fibrous membranes

Solutions concentration

Entry Sample codeSolution concentration(% w/v)

Moles of VP unitsto metal salt ratio

1 PVP 15 —2 PVP–Pd 10 20 : 13 PVP–Cu2O 10 3.6 : 1

Electrospinning conditions

EntryNeedle(G)

Voltage(KV)

Needle-to-collectordistance (cm)

Flow rate(mL min�1)

1 18 20 20 3.02 16 15 20 5.03 16 20 10 2.5

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phenylacetylene (60 mL, 0.55 mmol), followed by Et3N (77 mL,0.55 mmol) and the crosslinked PVP–Cu2O 3.6 : 1 membrane(11.6 mg, 5.0 mol% Cu). Themixture was heated to ca. 60 �C in asealed tube until compete consumption of the starting material(TLC). The mixture was then cooled to ca. 20 �C and t-BuOMe(10 mL) was added. The polymer precipitate was ltered andwashed with t-BuOMe (10 mL), then dried under vacuum. Themembrane was reused in subsequent reactions withoutfurther treatment. The organic washings were evaporated to

44914 | RSC Adv., 2014, 4, 44911–44921

give 1-benzyl-4-phenyl-1H-1,2,3-triazole (3) (102 mg, 87%) ascolourless needles, mp 121–123 �C (from c-hexane, lit.,46 123–125 �C); Rf 0.83 (t-BuOMe); vmax/cm

�1 3123w, 3098w, 3065w,3057w, 3036w, 2974w, 2967w, 2928w and 2855 (C–H), 1607w,1497m, 1483w, 1466m, 1454m, 1443m, 1427w, 1358w, 1225m,1206m, 1196w, 1182w, 1155w, 1140w, 1076m, 1049m, 1028m,1003w, 976m, 914m, 827m, 804w, 779m, 766s, 727s; dH(500MHz; CDCl3) 7.80 (2H, d, J 7.3, Ar H), 7.68 (1H, s, Ar H), 7.41–7.35 (5H, m, Ar H), 7.32–7.29 (3H, m, Ar H), 5.55 (2H, s, CH2);dC(125 MHz; CDCl3) 148.1 (s), 134.6 (s), 130.45 (s), 129.0 (d),128.71 (d), 128.66 (d), 128.1 (d), 128.0 (d), 125.6 (d), 119.5 (d),54.1 (t); m/z (MALDI-TOF) 236 (MH+, 100%), 192 (40), 181 (18),166 (37); identical to an authentic sample.

2.3. Characterisation methods

The UV-vis spectra of the PVP–Pd and the PVP–Cu nanohybridsstabilized in MeOH were recorded on a Jasco V-630 UV-visspectrophotometer operating at ca. 20 �C, aer appropriatedilution of the as-prepared colloidal solutions.

High resolution transmission electron microscopy (HRTEM)investigations of the membranes were performed by using aTECNAI F30 G2 S-TWIN microscope operated at 300 kV equip-ped with energy dispersive X-ray spectrometer (EDX). Sampleswere placed into a double copper grid (oyster) to be visualized byTEM. The morphology of the nanocomposite membranes wasalso characterised by scanning electron microscopy (SEM) (VegaTS5136LS-Tescan). The samples were gold-sputtered (�15 nm)(sputtering system K575X Turbo Sputter Coater – Emitech) priorto SEM inspection.

This journal is © The Royal Society of Chemistry 2014

Fig. 4 SEM images of (a) the pristine PVP, (b) the PVP–Pd and (c) the PVP–Cu2O nanocomposite electrospun membranes.

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For the characterisation of the nal products obtained fromthe catalytic reactions: Melting points were determined using aPolyTherm-A, Wagner & Munz, Koeer – Hotstage Microscopeapparatus. IR spectra were recorded on a Shimadzu FTIR-NIRPrestige-21 spectrometer with Pike Miracle Ge ATR accessoryand strong, medium and weak peaks are represented by s, mand w, respectively. 1H and 13C NMR spectra were recorded on aBruker Avance 500 machine (at 500 and 125 MHz, respectively).Deuterated solvents were used for homonuclear lock and thesignals are referenced to the deuterated solvent peaks. CH

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assignments are made based on DEPT 135 spectroscopy.MALDI-TOFmass spectra were recorded on a Bruker Autoex IIISmartbeam instrument.

3. Results and discussion3.1. PVP–Pd, PVP–Cu and PVP–Cu2O colloidal solutions

The PVP-coated Pd and Cu MNPs were prepared in the form ofstable colloidal solutions in MeOH according to the litera-ture.42,47 The reaction process involved the reduction of Pd(II)

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Fig. 5 TEM bright field (a) and (b) and HRTEM (c) images of the as-prepared PVP–Pd nanocomposite electrospun membranes. TEM bright fieldimages (d)–(f) of the as-prepared PVP–Cu2O membranes. EDX spectrum of the PVP–Pd electrospun membrane (g).

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ions into metallic Pd(0) NPs in reuxing MeOH that functionedas both, solvent and reductant in the presence of PVP whichacted as a steric stabilizer. In the case of the Cu(II) ions,hydrazine monohydrate was used as the reductant for gener-ating metallic PVP-capped Cu(0) NPs (Fig. 1).

The PVP, PVP–Pd and PVP–Cu MeOH solutions were char-acterised by UV-vis spectroscopy (Fig. 2). The broad absorptiontail appearing between 300–800 nm is typical for Pd MNPcolloidal systems,48,49 whereas in the case of the Cu-containingsolutions the characteristic surface plasmon resonance (SPR)signal appears at around 580 nm, indicating the existence of CuMNP in solution.50 When the freshly prepared PVP–Cu colloidalsolution was exposed to air, the colour of the solution graduallychanged from red to light blue (Fig. 1), indicating the oxidation

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of the Cu NP to Cu2O NP. Similar solution colour changes(i.e. from red to light blue) were previously reported foroleylamine-coated Cu and Cu2O NPs.43

3.2. Membrane fabrication

Fibrous membranes comprised of PVP and either Pd or Cu2Onanoparticles were prepared by electrospinning (Fig. 3).

By systematically varying the solution concentration, theapplied voltage, the delivery rate of the solution, the diameter ofthe needle and the needle-to-collector distance, the optimumexperimental parameters were determined for the production ofbrous PVP–Pd (Table 1, entry 1) and PVP–Cu2O (Table 1,entry 2) electrospun membranes.

This journal is © The Royal Society of Chemistry 2014

Fig. 6 SEM images of (a) the PVP–Pd and (b) PVP–Cu2O crosslinked nanocomposite membranes (before reactions). (c) Photographs of thePVP–Pd and the PVP–Cu2O membranes immersed in water after crosslinking.

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The morphology of the membranes was studied using SEMand TEM. As seen in the SEM images (Fig. 4), in all casescontinuous, bead-free cylindrical bers were obtained underthe optimum electrospinning conditions employed. Theaverage diameters of the PVP, PVP–Pd and PVP–Cu2O berswere 9.36 � 1.68, 8.64 � 2.98 and 7.11 � 1.04 mm, respectively;determined using digimizer image analysis soware. Moreover,no signicant changes were observed in the morphology of thebers in the presence of the embedded Pd and Cu2Onanoparticles.

The as-prepared Pd and Cu2O containing brous nano-composites were also visualized by TEM (Fig. 5). From the TEMbright eld images (Fig. 5a, b, d–f) the inorganic nanoparticlesembedded in the membranes appear spherical in shape withaverage diameters of �5 nm. Moreover, a distinct dispersion ofthe NP in the PVP brous matrix resulting in high homogeneitywas observed in both cases. HRTEM analysis was only feasiblefor the PVP–Pd membranes since the PVP–Cu2O system wasunstable in the electron beam. The Pd nanoparticles embeddedin the brous PVP matrix are nanocrystals and HRTEM imaging(Fig. 5c) discloses the crystalline planes (111) and (200) of Pd NPwith characteristic interplanar distances of 2.27 and 1.97 A,respectively. In addition, the EDX spectrum corresponding tothe PVP–Pd system (Fig. 5g) shows the presence of Pd and O asthe major elements in the sample. The presence of Cu isattributed to the Cu grid employed in TEM investigations. In thecase of the PVP–Cu2O, the EDX analysis could not provide

This journal is © The Royal Society of Chemistry 2014

useful information since a Cu grid was employed during thesample analysis and the expected oxygen exists both in thepolymer matrix and in the Cu2O nanoparticles.

Thermal crosslinking of the membranes was carried out toprevent membrane dissolution and NP leaching during the useof the aforementioned materials as heterogeneous catalyticsupports in the organic reaction media. The morphology andcompositional characteristics of the nanocomposite bers aerthermal treatment were determined by means of SEM and TEM/EDX analysis.

SEM images of the thermally-induced crosslinked PVP–Pdand PVP–Cu2O electrospun nanocomposite membranes (Fig. 6aand b) indicated that the membranes maintained their cylin-drical shape and that there were no signicant morphologychanges compared to the non-crosslinked systems. Aerthermal treatment, the membranes were immersed in anaqueous solution where no membrane dissolution wasobserved (as in the case of the non-crosslinked analogues thatare water-soluble due to the hydrophilic character of PVP) –

Fig. 6c – indicating that the crosslinking process was successful.In Fig. 7 the corresponding TEM images accompanied by

EDX analysis spectra are provided. Owing to the superiorstability of the crosslinked vs. non-crosslinked materials, it waspossible to perform HRTEM analysis on both systems andconrm the existence of Cu2O NP in the PVP–Cu2Omembranes.As in the case of the non-crosslinked PVP–Pd electrospunmembranes, spherical Pd nanocrystals embedded in the

RSC Adv., 2014, 4, 44911–44921 | 44917

Fig. 7 Bright field (a and b) and high resolution (c) TEM images and corresponding EDX spectrum (d) of the PVP–Pd crosslinked electrospunnanocomposite membrane. Bright field (e and f) and high resolution (g) TEM images of the PVP–Cu2O crosslinked electrospun nanocompositemembrane.

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crosslinked PVPmatrix can be visualized in the bright eld TEMimages (Fig. 7a and b) while HRTEM disclosed the characteristiccrystalline planes of Pd NP (Fig. 7c). The presence of the Pd NPin the crosslinked membranes was also veried by EDX analysis(Fig. 7d). HRTEM conrmed the existence of Cu2O NP in thePVP–Cu2O crosslinked membranes (Fig. 7g) since the charac-teristic crystalline planes (110) and (111) of Cu2O NP withrespective 3.02 and 3.47 A interplanar distances can be clearlyseen.

3.3. Catalysis

3.3.1. Heck reactions. During the course of our currentstudy, the use of electrospun PVP–Pd nanoparticles in a Heck

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coupling reaction was reported by Bai and co-workers.15 Theseauthors performed the reaction of phenyl iodide with butylacrylate in DMF with 7.4 mol% Pd and aer 23 h reaction timeobtained a 99% conversion of acrylate 1 (based of GC). More-over, they could recover their catalyst and use it repeatedlywithout loss of activity aer washing several times with EtOHand water.

Having in our hands both a crosslinked and a non-cross-linked membrane containing Pd nanoparticles, we performedthe same Heck reaction, using 1.5 equiv. of the acrylate, 2.7equiv. of Et3N and 7.4 mol% Pd from both the crosslinked andnon-crosslinked material. Aer 14 h at ca. 125 �C, regardless ofwhich catalyst was used, the starting iodide was completely

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Scheme 1 Heck reaction between iodobenzene and n-butyl acrylatewith a PVP–Pd electrospun membrane.

Table 2 Heck reaction with PVP-Pd electrospun membranes

Reaction run Electrospun membrane Yield of 1 (%)a TONb

1 Crosslinked 97 40.52 88 36.73 89 37.11 Non-Crosslinked 98 40.92 96 40.03 95 39.6

a Yield is the average of duplicate runs whose yields varied < 2%. b TON¼ turnover number.

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consumed and product 1 was formed in 88% isolated yield(Scheme 1). Under these conditions the non-crosslinkedmembrane dissolved to give, as expected, a dark brown colouredsolution, while the crosslinked membrane was more tolerant tothe reaction conditions (DMF, 125 �C, 14 h). Nevertheless,attempts to recover the polymer catalysts aer the reactions hadcompleted by ltration led to low recoveries of up to �60%.

Fig. 8 SEM images of (a) the as-prepared PVP–Pd crosslinked membrareaction runs.

This journal is © The Royal Society of Chemistry 2014

Reducing the catalyst loading and improving its post reac-tion recovery would prove the usability of the method. Using alower loading of Pd (2.4 mol%) and lowering the reactiontemperature to 100 �C with both the crosslinked and the non-crosslinked polymer systems, successfully gave the desiredproduct in good yields (Table 2). Furthermore, modifying thereaction work-up allowed the recovery and reuse of thecolloidal PVP–Pd nanocatalyst: the reaction was performed ina sealed tube to avoid the loss of volatile reagents and aer theend of the reaction t-BuOMe was added to precipitate the PVP–Pd nanocomposite. The remaining solution was then dec-anted, washed with water to remove the DMF and chromato-graphed to isolate the pure product. In this manner theprecipitated polymer-based catalyst can be reused withoutfurther processing. This procedure was repeated up to threetimes to demonstrate the reusability of the catalysts andexamine whether there were any notable differences betweenthe two catalytic systems. Gratifyingly, both catalysts wereactive even during the 3rd run and gave consistent high yieldsof product.

The reactions involving the crosslinked material showed amarked drop in product yield (from 97% in run 1 to 88 and89% in runs 2 and 3), however, a similar drop was notobserved with the non-crosslinked membrane. Thisphenomenon may be attributed to partial degradation of themembrane morphology under the reaction conditions thatreduces its active surface area and consequently decreased itscatalytic efficacy. To verify this hypothesis SEM analysis wasperformed on the crosslinked PVP–Pd membrane recoveredaer three consecutive reaction runs. Indeed, as seen in theSEM images (Fig. 8b), cylindrical bers with larger meandiameters compared to the as-prepared crosslinked analogue(Fig. 8a) are obtained aer the third reaction run. The above-mentioned problemmay be overcome by performing the Heckreaction in different organic solvents or under milder reactionconditions.

3.3.2. Suzuki reaction. To further evaluate the versatility ofthe crosslinked PVP–Pd electrospun bers as coupling agents, a

ne and (b) the crosslinked PVP–Pd membrane recovered after three

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Scheme 3 Click reaction between benzyl azide and phenylacetylenein the presence of the crosslinked PVP–Cu2O membrane. TON:turnover number.

Scheme 2 Suzuki reaction between 4-iodoanisole and phenylboronicacid in the presence of the crosslinked PVP–Pd membrane. TON:turnover number.

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classical Suzuki reaction was attempted. The Suzuki reactionbetween phenylboronic acid and 4-iodoanisole was successfulin the absence of a phosphine ligand, using THF and water asthe reaction solvents.51 Furthermore, the reusability of thecatalyst was briey investigated and up to three consecutivecoupling reaction runs were effective. Interestingly, aer threereaction cycles a gradual increase of the reaction times wasobserved indicating a drop in the reactivity of the catalyst,despite this, the yields of 4-methoxybiphenyl (2) remained high(>92%) (Scheme 2).

3.3.3. Click chemistry. Click chemistry is used to affordinert triazole units from Huisgen [3 + 2] cycloadditions betweenorganic azides and terminal alkynes and is catalysed by cop-per(I) salts.32 Some click reaction strategies involve the in situgeneration of Cu(I) from Cu(II) salts in the presence of areducing agent as well as from metallic copper oxidized in situ

Fig. 9 SEM images of the PVP–Cu2O crosslinked electrospun membran

44920 | RSC Adv., 2014, 4, 44911–44921

by air.52 Improved procedures include the use of solid supportedcopper nanoparticles that provide reusable catalysts and reduceboth the amount of copper needed in the catalysis and alsocopper contamination of the products.34 This rst use of elec-trospun PVP–Cu2O nanoparticles expands the usability andexibility of this reaction.

The click reaction between benzyl azide and phenylacetylenein the presence of Et3N and PVP–Cu2O nanoparticles wascompleted in 2 h and gave 1-benzyl-4-phenyl-1H-1,2,3-triazole(3) in 87% yield (Scheme 3). The polymer catalyst was reused inthree consecutive reaction runs, and a small drop in productyield indicated a gradual loss of catalyst reactivity. SEM char-acterisation of the crosslinked PVP–Cu2Omembranes recoveredaer three runs revealed that although the bers retain theircylindrical shape they had begun to aggregate (Fig. 9), similar tothe above results on the PVP–Pd crosslinked membraneemployed in Heck reaction processes.

4. Conclusions

Catalytic Pd and Cu2O NP have been immobilized in electro-spun PVP crosslinked mats and the resulting brous nano-composites evaluated as heterogeneous catalytic supports inHeck, Suzuki (PVP–Pd) and click chemistry (PVP–Cu2O) reac-tions. By employing a new recovery protocol, the crosslinkedmembranes can be recycled and exhibit high catalytic efficien-cies even aer three consecutive reaction runs. The versatility ofPVP to act as effective steric stabiliser for various nanoparticlesin organic solvents (including Ag, Au, Cu, Pd, Cu2O etc.) allow-ing for their intermixing within a single electrospun brousmat, creates new prospects for the future development ofmultifunctional electrospun catalytic supports.

Acknowledgements

The authors wish to thank the Cyprus Research PromotionFoundation (Grant no. NEAYPODOMH/NEKYP/0308/02 andNEAYPODOMH/STRATHII/0308/06) and the following organi-zations and companies in Cyprus for generous donations of

e upon completion of the click reactions (after 3 reaction cycles).

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chemicals and glassware: the State General Laboratory; theAgricultural Research Institute; the Ministry of Agriculture;MedoChemie Ltd; Medisell Ltd; and Biotronics Ltd. TheUniversity of Cyprus and the Program “New Researchers” sup-porting P. Papaphilippou is greatly acknowledged. Further-more, we thank the A. G. Leventis Foundation for helping toestablish the NMR facility at the University of Cyprus.

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