PVP-crosslinked electrospun membranes with embedded Pd and Cu₂O nanoparticles as effective heterogeneous catalytic supports

Abstract

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.

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1. Introduction

Electrospinning is a simple, cost-effective technique used for the production of fibrous materials comprised of long, continuous fibers with diameters ranging from a few nanometers up to a few micrometers.^1–3 While this process dates back to 1900,⁴ the first scientific publications on electrospun nanofibers appeared only in the early 1990s with the term “electrospinning” being popularised by Reneker.^5–7 The process is versatile and enables not only the fabrication of pristine polymer fibers but also of fibrous nanocomposites via the combination of polymers with inorganic nanofillers. This versatility has led to an exponential increase in the number of related publications, reaching to date around 2000 per year.⁸ These numbers reflect the importance of the electrospinning process, which, when combined with the current state of art can enhance the development of advanced, polymer-based fibrous nanocomposites destined for use in various technological applications. Among such nanoadditives, metal and metal oxide nanoparticles (NPs) with their unique crystalline structures and large specific surface areas often exhibit excellent catalytic properties.^9–14 It is anticipated that the combination of the catalytic properties of NPs, with the unique characteristics of electrospun fibrous materials (e.g., high surface-to-volume ratio, high porosity, ease of chemical/physical post-modification processes) will lead to the development of new, multifunctional fibrous nanocomposites with outstanding catalytic behaviour. Moreover, the immobilization of inorganic catalytic centers within an insoluble polymer matrix prevents nanoparticle aggregation that can reduce catalytic efficiency, and enables the catalyst's facile recovery and reuse. In contrast, analogous NP-containing colloidal systems cannot easily be separated from reaction products, which limits their use in practical applications.¹⁵

Electrospun fibers comprised of solely inorganic materials^16–21 or carbon nanofiber/nanotube-based electrospun systems^22–24 have been fabricated and evaluated in catalytic processes. However, there are only a few reports on the fabrication of polymer-NP electrospun fibrous nanocomposites and their evaluation in heterogeneous catalysis: Pd(0) metal nanoparticles (MNPs) were incorporated in electrospun polymer fibrous matrices and evaluated as catalytic supports in Heck²⁴ and hydrogenation^15,26,27 reactions; and Au or Ag MNPs were incorporated into fibrous electrospun mats and the resulting fibrous nanocomposites evaluated in reduction,^28,29 and electrocatalytic oxidation processes.³⁰

Herein, we describe the fabrication of PVP–Pd and PVP–Cu₂O electrospun fibrous nanocomposite membranes and their evaluation as heterogeneous catalytic supports in Heck, Suzuki and click chemistry reactions.

While two recent reports describe the evaluation of PVP–Pd electrospun fibers in Heck reaction processes,^15,25 no other coupling reactions have appeared in the literature using this type of polymer-based catalytic support. In the present work the PVP–Pd electrospun fibrous nanocomposite membranes were investigated in both Heck and Suzuki coupling reactions to demonstrate their versatility. Furthermore, the membranes were recovered, reused and shown to be effective for three consecutive coupling reactions.

In addition, and to the best of our knowledge, there are no reports on the fabrication and characterization of Cu₂O NP immobilized on electrospun PVP fibrous matrices that have been evaluated as catalytic supports in click chemistry: the Cu(I)-catalysed version of the Huisgen 1,3-dipolar cycloaddition of azides and terminal alkynes, was popularised by Sharpless in 2001 (ref. 31 and 32) and has numerous applications in drug-design and diverse chemical-biology applications.³³ Copper^34–36 and copper oxide^37–39 nanoparticles may act as effective catalysts in click chemistry reactions; consequently, their immobilization on electrospun PVP could extend the usability and flexibility of the click reaction.

Moreover, the chemical compatibility of the PVP–Pd and PVP–Cu₂O systems i.e. that the same polymer (PVP) is employed as a stabilizing agent for the generation of NP colloidal solutions and that in both cases the polymer-NP solutions are prepared in methanol, allows for their intermixing. This potentially enables the future development of multicomponent electrospun catalytic supports with an embedded “cocktail” of nanoparticulate catalytic centers.

2. Experimental

2.1. Materials and methods

Polyvinylpyrrolidone (PVP, M̄_n = 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 from Scharlau. The above-mentioned reagents were used as provided by the manufacturer without further purification. Benzyl azide was prepared from benzyl bromide and sodium azide according to the literature.⁴⁰ DMF was dried by azeotropically removing water with benzene and then distilling under vacuum over dried molecular sieves 4 Å and then kept under an argon atmosphere in a desiccator. Anhydrous Na₂SO₄ was used for drying organic extracts and all volatiles were removed under reduced pressure. All reaction mixtures and column eluents were monitored by TLC using commercial glass backed thin layer chromatography (TLC) plates (Merck Kieselgel 60 F₂₅₄). The plates were observed under UV light at 254 and 365 nm. The technique of dry flash chromatography was used throughout for all non-TLC scale chromatographic separations using Merck Silica Gel 60 (less than 0.063 mm).⁴¹

2.2. Synthesis

2.2.1. Synthesis of PVP-stabilized Pd nanoparticles. The PVP–Pd nanohybrid colloidal systems (mols vinyl pyridine units per mols palladium salt = 20 : 1) were prepared as follows:⁴² in a round bottom flask equipped with a magnetic 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) was added to the polymer solution and the reaction mixture was heated at reflux (65 °C) for 2 h. During this period, the colour of the solution changed from yellow to dark brown indicating the formation of Pd nanoparticles. After the completion of the reaction, the brown-coloured solution was allowed to cool to ca. 20 °C and it was then stored in sealed glass vials. The solutions were stable and no precipitation was observed even after several months.

2.2.2. Synthesis of PVP-stabilized Cu nanoparticles. The PVP–Cu nanohybrid colloidal systems (mols vinyl pyridine units per mols copper salt = 3.6 : 1) were prepared as follows: in a vial equipped with a magnetic stirrer, PVP (1.0 g, 9 mmol of vinylpyrridine units) was dissolved in MeOH (7.5 mL). Subsequently, copper(II) acetate monohydrate (500 mg, 2.5 mmol) was added to the polymer solution and the mixture was left to stir overnight at ca. 20 °C. During this period, the colour of the solution changed from colourless to blue. Afterwards, hydrazine monohydrate (972 μL, 20.0 mmol) was added and the colour of the solution became dark red, indicating the formation of Cu nanoparticles. Air exposure of the colloidal solution led to a colour change from red to light blue, indicative of the oxidation of the Cu NPs to Cu₂O.⁴³ The final PVP-capped Cu₂O nanoparticles exhibited high stability in methanol and no agglomeration/destabilization phenomena were observed even after several months.

2.2.3. Membrane fabrication of PVP–Pd and PVP–Cu₂O electrospun membranes. The PVP–Pd and PVP–Cu₂O colloidal solutions were used for the fabrication of Pd- and Cu₂O-containing PVP fibrous membranes by means of the electrospinning technique. All electrospinning experiments were performed at ca. 20 °C. Equipment included a controlled-flow, four-channel volumetric microdialysis pump (KD Scientific, Model: 789252), syringes with specially connected spinneret needle electrodes, a high-voltage power source (10–50 kV) and a custom-designed, grounded target collector, inside an interlocked Faraday enclosure safety cabinet. Systematic parametric studies were carried out by varying the applied voltage, the needle-to-collector distance, the needle diameter and the flow rate so as to determine the optimum experimental conditions for obtaining fibrous materials. Due to their hydrophilic nature, these membranes have the tendency to adsorb water vapour when exposed to high humidity conditions that in turn alters their morphological characteristics. For avoiding such hydration phenomena, the membranes were stored in sealed glass vials.

2.2.4. Membrane crosslinking. Insoluble PVP–Pd and PVP–Cu₂O fibrous nanocomposites were obtained via thermal crosslinking of the as-prepared corresponding electrospun fibers in an oven heated at ca. 180 °C for 5 h.

2.2.5. Heck reaction. To a stirred solution of iodobenzene (22 μL, 0.20 mmol) in dry DMF (1 mL) at ca. 20 °C, was added n-butyl acrylate (43 μL, 0.30 mmol), Et₃N (75 μL, 0.54 mmol) and the PVP–Pd 20 : 1 electrospun membrane (11 mg, 2.4 mol%). The mixture was heated to 125 °C in a sealed tube until complete consumption of iodobenzene (TLC). The mixture was then cooled to ca. 20 °C and t-BuOMe (10 mL) was added. The polymer precipitate was filtered and washed with t-BuOMe (10 mL), then dried under vacuum. The membrane was reused in subsequent reactions without other treatment. The organic washings were washed with H₂O (2 × 10 mL), combined, dried (Na₂SO₄) and evaporated to give after column chromatography (n-hexane/DCM, 1 : 1) n-butyl cinnamate (1) (36 mg, 88%) as a yellow oil; R_f 0.72 (n-hexane/DCM, 1 : 1); v_max/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; δ_H(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); δ_C(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-iodoanisole (117 mg, 0.50 mmol) in THF (0.8 mL) and H₂O (0.4 mL) at ca. 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 was heated to ca. 80 °C in a sealed tube until complete 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 filtered and washed with t-BuOMe (10 mL), then dried under vacuum. The membrane was reused in subsequent reactions without further treatment. The organic washings were washed with H₂O (2 × 10 mL), combined, dried (Na₂SO₄) and evaporated to give 4-methoxy-biphenyl (2) (90 mg, 98%) as colourless needles, mp 81–83 °C (from EtOH/H₂O, lit.,⁴⁵ 80–82 °C); R_f 0.30 (n-hexane); v_max/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; δ_H(500 MHz; CDCl₃) 7.56 (2H, d, J 7.9, Ar H), 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 Ar H), 6.99 (2H, d, J 8.7, Ar H), 3.86 (3H, s, CH₃); δ_C(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 to an 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 phenylacetylene (60 μL, 0.55 mmol), followed by Et₃N (77 μL, 0.55 mmol) and the crosslinked PVP–Cu₂O 3.6 : 1 membrane (11.6 mg, 5.0 mol% Cu). The mixture was heated to ca. 60 °C in a sealed 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 filtered and washed with t-BuOMe (10 mL), then dried under vacuum. The membrane was reused in subsequent reactions without further treatment. The organic washings were evaporated to give 1-benzyl-4-phenyl-1H-1,2,3-triazole (3) (102 mg, 87%) as colourless needles, mp 121–123 °C (from c-hexane, lit.,⁴⁶ 123–125 °C); R_f 0.83 (t-BuOMe); v_max/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; δ_H(500 MHz; 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); δ_C(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 nanohybrids stabilized in MeOH were recorded on a Jasco V-630 UV-vis spectrophotometer operating at ca. 20 °C, after appropriate dilution of the as-prepared colloidal solutions.

High resolution transmission electron microscopy (HRTEM) investigations of the membranes were performed by using a TECNAI F30 G2 S-TWIN microscope operated at 300 kV equipped with energy dispersive X-ray spectrometer (EDX). Samples were placed into a double copper grid (oyster) to be visualized by TEM. The morphology of the nanocomposite membranes was also characterised by scanning electron microscopy (SEM) (Vega TS5136LS-Tescan). The samples were gold-sputtered (∼15 nm) (sputtering system K575X Turbo Sputter Coater – Emitech) prior to SEM inspection.

For the characterisation of the final products obtained from the catalytic reactions: Melting points were determined using a PolyTherm-A, Wagner & Munz, Koefler – Hotstage Microscope apparatus. IR spectra were recorded on a Shimadzu FTIR-NIR Prestige-21 spectrometer with Pike Miracle Ge ATR accessory and strong, medium and weak peaks are represented by s, m and w, respectively. ¹H and ¹³C NMR spectra were recorded on a Bruker Avance 500 machine (at 500 and 125 MHz, respectively). Deuterated solvents were used for homonuclear lock and the signals are referenced to the deuterated solvent peaks. CH assignments are made based on DEPT 135 spectroscopy. MALDI-TOF mass spectra were recorded on a Bruker Autoflex III Smartbeam instrument.

3. Results and discussion

3.1. PVP–Pd, PVP–Cu and PVP–Cu₂O colloidal solutions

The PVP-coated Pd and Cu MNPs were prepared in the form of stable colloidal solutions in MeOH according to the literature.^42,47 The reaction process involved the reduction of Pd(II) ions into metallic Pd(0) NPs in refluxing MeOH that functioned as both, solvent and reductant in the presence of PVP which acted as a steric stabilizer. In the case of the Cu(II) ions, hydrazine monohydrate was used as the reductant for generating metallic PVP-capped Cu(0) NPs (Fig. 1).

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

The PVP, PVP–Pd and PVP–Cu MeOH solutions were characterised by UV-vis spectroscopy (Fig. 2). The broad absorption tail appearing between 300–800 nm is typical for Pd MNP colloidal systems,^48,49 whereas in the case of the Cu-containing solutions the characteristic surface plasmon resonance (SPR) signal appears at around 580 nm, indicating the existence of Cu MNP in solution.⁵⁰ When the freshly prepared PVP–Cu colloidal solution was exposed to air, the colour of the solution gradually changed from red to light blue (Fig. 1), indicating the oxidation of the Cu NP to Cu₂O NP. Similar solution colour changes (i.e. from red to light blue) were previously reported for oleylamine-coated Cu and Cu₂O NPs.⁴³

Fig. 2 UV-vis spectra of the pristine PVP MeOH solution and the PVP–Pd and PVP–Cu hybrid colloidal solutions (recorded upon appropriate dilution of the as-prepared colloidal solutions).

3.2. Membrane fabrication

Fibrous membranes comprised of PVP and either Pd or Cu₂O nanoparticles were prepared by electrospinning (Fig. 3).

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

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

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

Solutions concentration
Entry	Sample code	Solution concentration (% w/v)	Moles of VP units to metal salt ratio
1	PVP	15	—
2	PVP–Pd	10	20 : 1
3	PVP–Cu2O	10	3.6 : 1

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Electrospinning conditions
Entry	Needle (G)	Voltage (KV)	Needle-to-collector distance (cm)	Flow rate (μL min^−1)
1	18	20	20	3.0
2	16	15	20	5.0
3	16	20	10	2.5

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The morphology of the membranes was studied using SEM and TEM. As seen in the SEM images (Fig. 4), in all cases continuous, bead-free cylindrical fibers were obtained under the optimum electrospinning conditions employed. The average diameters of the PVP, PVP–Pd and PVP–Cu₂O fibers were 9.36 ± 1.68, 8.64 ± 2.98 and 7.11 ± 1.04 μm, respectively; determined using digimizer image analysis software. Moreover, no significant changes were observed in the morphology of the fibers in the presence of the embedded Pd and Cu₂O nanoparticles.

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

The as-prepared Pd and Cu₂O containing fibrous nanocomposites were also visualized by TEM (Fig. 5). From the TEM bright field images (Fig. 5a, b, d–f) the inorganic nanoparticles embedded in the membranes appear spherical in shape with average diameters of ∼5 nm. Moreover, a distinct dispersion of the NP in the PVP fibrous matrix resulting in high homogeneity was observed in both cases. HRTEM analysis was only feasible for the PVP–Pd membranes since the PVP–Cu₂O system was unstable in the electron beam. The Pd nanoparticles embedded in the fibrous PVP matrix are nanocrystals and HRTEM imaging (Fig. 5c) discloses the crystalline planes (111) and (200) of Pd NP with characteristic interplanar distances of 2.27 and 1.97 Å, respectively. In addition, the EDX spectrum corresponding to the PVP–Pd system (Fig. 5g) shows the presence of Pd and O as the major elements in the sample. The presence of Cu is attributed to the Cu grid employed in TEM investigations. In the case of the PVP–Cu₂O, the EDX analysis could not provide useful information since a Cu grid was employed during the sample analysis and the expected oxygen exists both in the polymer matrix and in the Cu₂O nanoparticles.

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

Thermal crosslinking of the membranes was carried out to prevent membrane dissolution and NP leaching during the use of the aforementioned materials as heterogeneous catalytic supports in the organic reaction media. The morphology and compositional characteristics of the nanocomposite fibers after thermal treatment were determined by means of SEM and TEM/EDX analysis.

SEM images of the thermally-induced crosslinked PVP–Pd and PVP–Cu₂O electrospun nanocomposite membranes (Fig. 6a and b) indicated that the membranes maintained their cylindrical shape and that there were no significant morphology changes compared to the non-crosslinked systems. After thermal treatment, the membranes were immersed in an aqueous solution where no membrane dissolution was observed (as in the case of the non-crosslinked analogues that are water-soluble due to the hydrophilic character of PVP) – Fig. 6c – indicating that the crosslinking process was successful.

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

In Fig. 7 the corresponding TEM images accompanied by EDX analysis spectra are provided. Owing to the superior stability of the crosslinked vs. non-crosslinked materials, it was possible to perform HRTEM analysis on both systems and confirm the existence of Cu₂O NP in the PVP–Cu₂O membranes. As in the case of the non-crosslinked PVP–Pd electrospun membranes, spherical Pd nanocrystals embedded in the crosslinked PVP matrix can be visualized in the bright field TEM images (Fig. 7a and b) while HRTEM disclosed the characteristic crystalline planes of Pd NP (Fig. 7c). The presence of the Pd NP in the crosslinked membranes was also verified by EDX analysis (Fig. 7d). HRTEM confirmed the existence of Cu₂O NP in the PVP–Cu₂O crosslinked membranes (Fig. 7g) since the characteristic crystalline planes (110) and (111) of Cu₂O NP with respective 3.02 and 3.47 Å interplanar distances can be clearly seen.

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

3.3. Catalysis

3.3.1. Heck reactions. During the course of our current study, the use of electrospun PVP–Pd nanoparticles in a Heck coupling reaction was reported by Bai and co-workers.¹⁵ These authors performed the reaction of phenyl iodide with butyl acrylate in DMF with 7.4 mol% Pd and after 23 h reaction time obtained a 99% conversion of acrylate 1 (based of GC). Moreover, they could recover their catalyst and use it repeatedly without loss of activity after washing several times with EtOH and water.

Having in our hands both a crosslinked and a non-crosslinked membrane containing Pd nanoparticles, we performed the same Heck reaction, using 1.5 equiv. of the acrylate, 2.7 equiv. of Et₃N and 7.4 mol% Pd from both the crosslinked and non-crosslinked material. After 14 h at ca. 125 °C, regardless of which catalyst was used, the starting iodide was completely consumed and product 1 was formed in 88% isolated yield (Scheme 1). Under these conditions the non-crosslinked membrane dissolved to give, as expected, a dark brown coloured solution, while the crosslinked membrane was more tolerant to the reaction conditions (DMF, 125 °C, 14 h). Nevertheless, attempts to recover the polymer catalysts after the reactions had completed by filtration led to low recoveries of up to ∼60%.

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

Reducing the catalyst loading and improving its post reaction recovery would prove the usability of the method. Using a lower loading of Pd (2.4 mol%) and lowering the reaction temperature to 100 °C with both the crosslinked and the non-crosslinked polymer systems, successfully gave the desired product in good yields (Table 2). Furthermore, modifying the reaction work-up allowed the recovery and reuse of the colloidal PVP–Pd nanocatalyst: the reaction was performed in a sealed tube to avoid the loss of volatile reagents and after the end of the reaction t-BuOMe was added to precipitate the PVP–Pd nanocomposite. The remaining solution was then decanted, washed with water to remove the DMF and chromatographed to isolate the pure product. In this manner the precipitated polymer-based catalyst can be reused without further processing. This procedure was repeated up to three times to demonstrate the reusability of the catalysts and examine whether there were any notable differences between the two catalytic systems. Gratifyingly, both catalysts were active even during the 3^rd run and gave consistent high yields of product.

Table 2 Heck reaction with PVP-Pd electrospun membranes

Reaction run	Electrospun membrane	Yield of 1 (%)^a	TON^b
a Yield is the average of duplicate runs whose yields varied < 2%.b TON = turnover number.
1	Crosslinked	97	40.5
2		88	36.7
3		89	37.1
1	Non-Crosslinked	98	40.9
2		96	40.0
3		95	39.6

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The reactions involving the crosslinked material showed a marked drop in product yield (from 97% in run 1 to 88 and 89% in runs 2 and 3), however, a similar drop was not observed with the non-crosslinked membrane. This phenomenon may be attributed to partial degradation of the membrane morphology under the reaction conditions that reduces its active surface area and consequently decreased its catalytic efficacy. To verify this hypothesis SEM analysis was performed on the crosslinked PVP–Pd membrane recovered after three consecutive reaction runs. Indeed, as seen in the SEM images (Fig. 8b), cylindrical fibers with larger mean diameters compared to the as-prepared crosslinked analogue (Fig. 8a) are obtained after the third reaction run. The above-mentioned problem may be overcome by performing the Heck reaction in different organic solvents or under milder reaction conditions.

Fig. 8 SEM images of (a) the as-prepared PVP–Pd crosslinked membrane and (b) the crosslinked PVP–Pd membrane recovered after three reaction runs.

3.3.2. Suzuki reaction. To further evaluate the versatility of the crosslinked PVP–Pd electrospun fibers as coupling agents, a classical Suzuki reaction was attempted. The Suzuki reaction between phenylboronic acid and 4-iodoanisole was successful in the absence of a phosphine ligand, using THF and water as the reaction solvents.⁵¹ Furthermore, the reusability of the catalyst was briefly investigated and up to three consecutive coupling reaction runs were effective. Interestingly, after three reaction cycles a gradual increase of the reaction times was observed indicating a drop in the reactivity of the catalyst, despite this, the yields of 4-methoxybiphenyl (2) remained high (>92%) (Scheme 2).

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

3.3.3. Click chemistry. Click chemistry is used to afford inert triazole units from Huisgen [3 + 2] cycloadditions between organic azides and terminal alkynes and is catalysed by copper(I) salts.³² Some click reaction strategies involve the in situ generation of Cu(I) from Cu(II) salts in the presence of a reducing agent as well as from metallic copper oxidized in situ by air.⁵² Improved procedures include the use of solid supported copper nanoparticles that provide reusable catalysts and reduce both the amount of copper needed in the catalysis and also copper contamination of the products.³⁴ This first use of electrospun PVP–Cu₂O nanoparticles expands the usability and flexibility of this reaction.

The click reaction between benzyl azide and phenylacetylene in the presence of Et₃N and PVP–Cu₂O nanoparticles was completed 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 in three consecutive reaction runs, and a small drop in product yield indicated a gradual loss of catalyst reactivity. SEM characterisation of the crosslinked PVP–Cu₂O membranes recovered after three runs revealed that although the fibers retain their cylindrical shape they had begun to aggregate (Fig. 9), similar to the above results on the PVP–Pd crosslinked membrane employed in Heck reaction processes.

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

Fig. 9 SEM images of the PVP–Cu₂O crosslinked electrospun membrane upon completion of the click reactions (after 3 reaction cycles).

4. Conclusions

Catalytic Pd and Cu₂O NP have been immobilized in electrospun PVP crosslinked mats and the resulting fibrous nanocomposites evaluated as heterogeneous catalytic supports in Heck, Suzuki (PVP–Pd) and click chemistry (PVP–Cu₂O) reactions. By employing a new recovery protocol, the crosslinked membranes can be recycled and exhibit high catalytic efficiencies even after three consecutive reaction runs. The versatility of PVP to act as effective steric stabiliser for various nanoparticles in organic solvents (including Ag, Au, Cu, Pd, Cu₂O etc.) allowing for their intermixing within a single electrospun fibrous mat, creates new prospects for the future development of multifunctional electrospun catalytic supports.

Acknowledgements

The authors wish to thank the Cyprus Research Promotion Foundation (Grant no. NEAYPODOMH/NEKYP/0308/02 and NEAYPODOMH/STRATHII/0308/06) and the following organizations and companies in Cyprus for generous donations of chemicals and glassware: the State General Laboratory; the Agricultural Research Institute; the Ministry of Agriculture; MedoChemie Ltd; Medisell Ltd; and Biotronics Ltd. The University of Cyprus and the Program “New Researchers” supporting P. Papaphilippou is greatly acknowledged. Furthermore, we thank the A. G. Leventis Foundation for helping to establish the NMR facility at the University of Cyprus.

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