# Tannin-coated polyurethane foams as a non-innocent structured support for Pd-catalysed alkyne semi-hydrogenation, nitroarene reduction and Suzuki coupling under air

Sara Viggiano, Han Peng, Thierry Romero, Vasiliki Papaefthimiou, Maria Laura Alfieri, Lucia Panzella, Alessandra Napolitano, Vincent Ritleng

► To cite this version:

Sara Viggiano, Han Peng, Thierry Romero, Vasiliki Papaefthimiou, Maria Laura Alfieri, et al. Tannin-coated polyurethane foams as a non-innocent structured support for Pd-catalysed alkyne semi-hydrogenation, nitroarene reduction and Suzuki coupling under air. Molecular Catalysis, 2026, 588, pp.115540. 10.1016/j.mcat.2025.115540. hal-05495243

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Research Paper

# Tannin-coated polyurethane foams as a non-innocent structured support for Pd-catalysed alkyne semi-hydrogenation, nitroarene reduction and Suzuki coupling under air

Sara Viggiano<sup>a</sup>, Han Peng<sup>b</sup>, Thierry Romero<sup>c</sup>, Vasiliki Papaefthimiou<sup>c, d</sup>,
Maria Laura Alfieri<sup>a, d</sup>, Lucia Panzella<sup>a, d</sup>, Alessandra Napolitano<sup>a, *, d</sup>, Vincent Ritleng<sup>b, *, d</sup>

## ARTICLE INFO

Keywords:

Structured supported catalyst

Open cell polyurethane foam

Tannin-based coating

Palladium-catalysed reduction reactions

Suzuki coupling

## ABSTRACT

Polyurethane foams (PUF) were coated with hydrolyzable (HT) or condensed (CT) tannins, derived from chesnut and quebracho woods, and Pd was anchored onto the HT or CT layer via simple immersion of HT@PUF and CT@PUF in a water-ethanol (1:5) solution of Pd(NH<sub>3</sub>)<sub>4</sub>Cl<sub>2</sub>-H<sub>2</sub>O at room temperature under air to afford Pd@HT@PUF and Pd@CT@PUF composites. XPS showed a partial reduction of the Pd(II) salt to Pd(0) in agreement with the reducing properties of the HT and CT coatings, and SEM and SEM-EDX suggested a coordination of evenly dispersed Pd(II) ions by the phenol moieties of the HT or CT layer together with the formation of Pd (nano)particles. The palladium loading, as determined by ICP-AES measurements, varied with the concentration of the aqueous tannin solution (0.1 or 0.5 g/L) used for the HT or CT coating procedure; the higher the concentration, the higher the Pd loading. A high loading of Pd, hence of Pd(0), on the as-synthesized macroscopic catalyst guaranteed a stable catalytic activity and selectivity from the beginning without prior activation procedure in alkyne-to-alkene hydrogenation. A lower Pd loading resulted in a gradual improvement of the catalytic activity overtime that coincidated with a reduction of the chelated Pd(II) ions to Pd(0) under the reaction conditions. Notably, both Pd@HT@PUF and Pd@CT@PUF proved reusable for at least 6 cycles without significant catalyst activity and selectivity decrease, in a convenient "dip-and-play" mode, in selective alkyne semi-hydrogenation and/or nitroarene reduction, as well as in fast Suzuki coupling under air.

## 1. Introduction

Open-cell foams offer high surface-to-volume ratios, efficient mass transfer and low pressure drop. Their open structure and macroscopic shape allow an intimate mixing of the reagents, as well as an easy access of the latter to the active sites, while avoiding a tedious separation of the products from the catalyst [1]. These properties are beneficial for continuous processes. Metallic and ceramic foams, in particular, are excellent supports for catalytically active metallic nanoparticles (NPs) [2–4]. However, their preparation involves multiple steps, including high-temperature processes to produce the macroscopic host structures, and energy-intensive processes to deposit and activate the catalytic phase on their surface.³ In addition, these metallic or ceramic foams have numerous drawbacks, such as: (i) rigid structures that make them

fragile, (ii) the presence of randomly distributed closed cells that can affect reactions' reproducibility, and (iii) the need to often perform numerous chemical treatments to recover the metallic catalyst before disposal [5].

Because of their ease of manufacture, affordability, and interesting mechanical properties [6], open-cell polyurethane foams (PUFs) are an attractive alternative to ceramic and metallic foams, provided that a chemical or physical mean can be found to deposit an active phase without altering their mechanical and transport properties, as they lack microporosities. Inspired by a biomimetic approach [7], some of us succeeded in coating PUFs with a thin adhesive layer of polydopamine (PDA) via a simple dip-coating procedure in a buffered solution (pH 8.5) of dopamine in water [8], and then applied the resulting PDA@PUF elastic structured support in various dye removal [9–11] or (photo)

<footnote><sup>a</sup> University of Naples "Federico II", Department of Chemical Sciences, Via Cintia 4, 80126 Naples, Italy</footnote>

<footnote><sup>b</sup> Université de Strasbourg, Ecole européenne de Chimie, Polymères et Matériaux, CNRS, LIMA, UMR 7042, 25 rue Becquerel, 67087 Strasbourg, France</footnote>

<footnote><sup>c</sup> Université de Strasbourg, Ecole européenne de Chimie, Polymères et Matériaux, CNRS, ICPEES, UMR 7515, 25 rue Becquerel, 67087 Strasbourg, France</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: alesnapo@unina.it (A. Napolitano), vritleng@unistra.fr (V. Ritleng).</footnote>
catalytic [12–17] processes after further functionalization with metal [10,13,15] or metal oxide [9,14] nanoparticles, molecular catalysts [12, 16,17], or hydrides [10,11]. The strong adhesive properties of the PDA layer, resulting from a set of interactions ranging from weak forces to covalent or coordination bonds, enabled mainly by its catechol moieties [18], ensured both a solid coating of PUFs and a robust anchoring of the active agents, whatever their nature, and thus minimal leaching into the liquid reaction medium. This property, combined with the “dip-and-play” nature of the PDA@PUF macroscopic support resulted in easy-to-reuse catalysts [8–17]. In particular, a readily prepared Pd-functionalized PDA@PUF acted as an efficient and highly reusable heterogeneous catalyst for the selective semi-hydrogenation of alkynes under ambient conditions, in batch [15] and flow [19] as well as for the Suzuki coupling of aryl bromides under air [15]. Interestingly, although the standard redox potential of PDA is around +0.3 V/NHE, whereas that of the Pd²⁺/Pd⁰ redox couple is +0.99 V/NHE [20] and some precedents, such as the immobilization of K₂PdCl₄ on a PDA-coated cotton microfiber [21], resulted in the partial reduction of Pd(II) salt to Pd(0) NPs, no initial reduction of the grafted Pd(II) salt was observed by XPS, but a progressive reduction under the reaction conditions together with a partial reduction of the PDA layer. This resulted in a gradual activation of the macroscopic Pd@PDA@PUF catalyst over several catalytic runs, as shown by the gradual enhancement of the reaction kinetics over these runs [15].

With the aim of developing ever more efficient catalysts for these important reactions [22,23], we wondered whether natural phenolic compounds such as tannins, which are well-known for their adhesive and antioxidant properties [24,25], could serve as advantageous sustainable surrogates to polydopamine, by concomitantly immobilizing Pd (II) salts on PUF and reducing them as active Pd(0) species.

To achieve that, our attention turned towards largely exploited wood derived tannins, in particular those derived from Castanea sativa (chestnut) and Shinopsis lorentzii (quebracho) hardwoods, which are traditionally classified as hydrolysable (HT) and condensed (CT) (i.e.: non-hydrolysable) tannins, as the former are mainly composed of hydrolysable ellagitannins such as castalagin and its isomer, vescalagin [26,27], while the latter mainly comprise condensed tannins, of which linear profisetinidins represent the major constituents [28–30]. Both, indeed, display remarkable antioxidant coating properties [31], deriving from both their polyphenolic nature and their ability to interact with macromolecules such as polyurethane by hydrogen bonding and hydrophobic interactions [32], making them ideal candidates for this study.

In this context, we report here our studies toward: (i) the coating of open-cell polyurethane foams with HT and CT, (ii) the subsequent functionalization of the resulting HT@PUF and CT@PUF structured supports with Pd by simple immersion in a hydro-alcoholic solution of Pd(NH₃)₄Cl₂·H₂O at room temperature under air, (iii) the characterization of as-prepared macroscopic catalysts, Pd@HT@PUF and Pd@CT@PUF, by inductively coupled plasma-atomic emission spectrometry (ICP-AES), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM-EDX), and (iv) their use in a very convenient “dip-and-play” mode under batch conditions in selective phenylacetylene-to-styrene hydrogenation, nitrobenzene-to-aniline reduction, and Suzuki cross-coupling under air.

## 2. Experimental section

### 2.1. Materials and methods

All reagents and solvents were used as provided by commercial suppliers without any further purification or treatment. Polyurethane open cells foams (20 pores per inch, TCL 40,100 soft white reticulated) were given by FoamPartner. Hydrolysable (from chestnut wood) and condensed (from quebracho wood) tannins were provided by Silvateam

S.pA (San Michele Mondovì, Cuneo, Italy). [PdCl₂(NH₃)₄]·H₂O (≥99.9 % - 323,438), nitrobenzene (99 % - N10950), phenylboronic acid (≥97 % - 78,181) and 4-bromotoluene (98 % - B82200) were purchased from Sigma-Aldrich. 1,2-Diphenylethyne (95 % - 464,554) was purchased from Fluorochem. Methyl 2-bromobenzoate (99 % - A12435) was purchased from Alfa Aesar. 18.2 MΩ deionized water (TOC < 1 ppb), supplied by a Q20 Millipore system, was used for the preparation of aqueous solutions and washing procedures with water. TraceMetal grade HCl 37 % (Fisher Chemical) and Rotipuran Supra HNO₃ 69 % (Roth) were used for ICP-AES analyses. Element standards were purchased from CPI international.

SEM measurements were performed using a Hitachi SU8010 FE-SEM operating at 3 kV at room temperature. No metallization of the samples was carried out, but their edges were covered with metallic tape to evacuate excess charge.

Elemental mapping by SEM-EDX was performed with a Zeiss Gemini SEM 500 FEG equipped with an EDAX Octane Elite EDX detector. X-rays emitted upon electron irradiation were collected in the energy range of 0–20 keV. Quantification was performed using the standardless ZAF correction method implemented in the TEAM EDS software package (EDAX).

XPS measurements were performed in an ultrahigh vacuum spectrometer equipped with a RESOLVE 120 MCD5 hemispherical electron analyser. The samples were mounted on the sample holders using conductive double-sided carbon adhesive tapes. No treatment was applied on the samples before or during measurements. The non-monochromatic AlKα line at 1486.6 eV was used as incident radiation for the measurements. The constant pass energy mode with pass energies of 100, 50 (for the Pd 3d and N 1 s peaks) and 20 eV was applied to record survey and high-resolution spectra. A survey scan was initially acquired to determine the elements present on each sample. This was followed by high-resolution scans of the major element’s lines. The Pd 3d peak was analysed in components corresponding to metallic and oxidised Pd species. The Casa XPS 2.3.23 software was used for the analysis of the data in all cases.

ICP-AES measurements were performed for Pd loading determination. Pd-functionalized polyurethane foam samples were mineralized with aqua regia (2 mL HCl, 1 mL HNO₃) at 185 °C for 50 min under pressure (Multiwave ECO, Anton Paar). A blank test was carried out in parallel under the same conditions. Pd quantification in the clear obtained solutions was realized by ICP-AES (Varian 720ES) at two wavelengths: 340.458 nm and 360.955 nm. Measurements were performed by the Plateforme Analytique des Inorganiques de l'Institut Pluri disciplinaire Hubert Curien (UMR CNRS 7178), Strasbourg, France.

Gas chromatographic (GC) analyses were done on a GC Agilent with FID detectors using Agilent HP1 column (30 m, 0.35 mm, 0.25 µm), with hydrogen as gas carrier and with tetradecane as the internal standard. The retention times and GC responses in terms of area versus concentration of the reagents and products were calibrated by using pure components diluted in an acetone solution at various concentrations. The conversion and product distribution were calculated from the GC results.

High-performance liquid chromatography (HPLC) analyses were carried out with an Agilent 1100 binary pump instrument equipped with an SPD-10AV VP UV-Visible detector set at 254 nm. Chromatographic separation was performed on a reversed-phase column (octadecylsilane-coated column, 250 × 4.6 mm, 5 µm particle size; Phenomenex Spher-elcone ODS) using a flow rate of 0.7 mL/min. A binary gradient elution was applied, consisting of 0.1 % formic acid in water (solvent A) and acetonitrile (solvent B) at 50 % B in the first 10 min and from 50 % to 80 % B, from 10 to 30 min.

### 2.2. Coating of open cell polyurethane foams (PUF) with hydrolysable (HT) and condensed (CT) tannins

Cubic samples of PUF (ca. 2 cm x 2 cm x 2 cm) (PUF) were immersed
<figure><img src="image_2.png" /><figcaption>Fig. 1. Polyurethane open cell foam (PUF) coating with hydrolysable (HT) or condensed (CT) tannin and functionalization with Pd by simple dipping procedures.</figcaption></figure>

in an aqueous HT or CT solution (0.5 or 0.1 mg/mL) for 2 h under stirring. The resulting tannin-coated foams, 0.5HT@PUF, 0.1HT@PUF and 0.1CT@PUF, were taken out of the solution, washed 3 times with EtOH (120 mL) under vigorous stirring (755 rpm), at room temperature for 1 h, and air-dried. 0.1HT@PUF and 0.1CT@PUF were characterized by SEM (Fig. S1 and S2) and XPS (Fig S3 and S4).

## 2.3. Functionalization of the tannin-coated PUFs with [PdCl₂(NH₃)₄]·H₂O

Pd functionalization was adapted from our previously published procedure with polydopamine-coated PUF (PDA@PUF) [15]. A cubic sample of 0.5HT@PUF, 0.1HT@PUF or 0.1CT@PUF (ca. 2 cm x 2 cm x 2 cm) was immersed in a vigorously stirred (750 rpm) solution of [Pd(NH₃)₄Cl₂]·H₂O (20 mg, 0.076 mmol) in H₂O/EtOH (1:5) (120 mL) at room temperature for 19 h. The resulting Pd-functionalized foam, Pd@0.5HT@PUF, Pd@0.1HT@PUF or Pd@0.1CT@PUF, was removed from the reaction medium, washed in vigorously stirred water (3 × 100 mL) and ethanol (2 × 100 mL), and then air-dried.

Pd@0.5HT@PUF was characterized by XPS (Fig. S6) and ICP-AES revealing a Pd content of 1840 ± 72 mg/kg (0.184 ± 0.007 wt %). Pd@0.1HT@PUF and Pd@0.1CT@PUF were characterized by XPS (Fig. 2 and S5), SEM (Fig. 3), SEM-EDX (Fig. 4 and S7), and ICP-AES revealing a mean Pd content of 958 ± 55 mg/kg (0.096 ± 0.006 wt %) and 1155 ± 64 mg/kg (0.116 ± 0.006 wt %), respectively, from the analysis of three samples in each case.

## 2.4. Phenylacetylene-to-styrene hydrogenations with Pd@0.5HT@PUF or Pd@0.1HT@PUF

A stirring bar and a cubic sample of Pd@0.5HT@PUF (ca. 8 cm³, 281 mg; Pd content: 4.9 µmol, i.e.: 0.16 mol %) or of Pd@0.1HT@PUF (ca. 8 cm³, 260 mg; Pd content: 2.3 µmol, i.e.: 0.08 mol %) was introduced in a Schlenk tube. Ethanol (20 mL) was then added to totally immerse the foam, and H₂ was bubbled through the solution at room temperature for 30 min under a constant flow rate (13 mL/min). Afterwards, phenylacetylene (3 mmol) was added to the stirred solution (625 rpm), and t₀ reaction time was set. During the reaction course, H₂ was fed under ambient conditions (room temperature and atmospheric pressure) at the same constant flow rate of 13 mL/min. The reaction was kinetically monitored by sampling regularly the reaction medium and analysing the removed aliquots (50 µL) by GC (3 mL) for GC analysis. After each run, the foam was washed by immersing it in EtOH (3 × 20 mL) for 10 min under stirring and dried under vacuum before reuse in another phenylacetylene-to-styrene hydrogenation or in a nitrobenzene reduction.

## 2.5. Nitrobenzene-to-aniline hydrogenations with Pd@0.1HT@PUF or Pd@0.1CT@PUF

A stirring bar and a cubic sample of Pd@0.1HT@PUF (ca. 8 cm³, 260 mg; Pd content: 2.3 µmol, i.e.: 0.08 mol %) or Pd@0.1CT@PUF (ca. 8 cm³, 323 mg; Pd content: 3.5 µmol, i.e.: 0.12 mol %) were introduced in a Schlenk tube. Ethanol (20 mL) was then added to totally immerse the foam, and H₂ was bubbled through the solution at room temperature for 30 min under a constant flow rate (13 mL/min). Afterwards, nitrobenzene (3 mmol) was added to the stirred solution (625 rpm), and t₀ reaction time was set. During the reaction course, H₂ was fed under ambient conditions (room temperature and atmospheric pressure) at the same constant flow rate of 13 mL/min, and the reaction was kinetically monitored as described for the phenylacetylene-to-styrene hydrogenations. Similarly, after each run, the foam was washed by immersing it in EtOH (3 × 20 mL) for 10 min under stirring and dried under vacuum before reuse in another nitrobenzene reduction or phenylacetylene-to-styrene hydrogenation.

## 2.6. Stop-and-go experiment for the nitrobenzene-to-aniline hydrogenation with Pd@0.1HT@PUF

In this control experiment conducted for highlighting the absence or presence of Pd leaching into the liquid medium, the above-described general procedure for the catalytic nitrobenzene-to-aniline hydrogenations was followed using Pd@0.1HT@PUF as catalyst, except that, after 2 h reaction, the foam was removed from the reaction medium for 2 h. Then, Pd@0.1HT@PUF was reintroduced in the reacting vessel for 1.5 h, and finally removed definitively. During the whole process, H₂ was constantly fed into the medium as to maintain its concentration constant. The course of the reaction was monitored by sampling the reaction medium every 1 h, and analysing the removed aliquots (50 µL) by GC after dilution in acetone (3 mL).

## 2.7. Suzuki cross-couplings of aryl bromides and phenylboronic acid with Pd@0.1HT@PUF or Pd@0.1CT@PUF under air

A stirring bar and a cubic sample of Pd@0.1HT@PUF or Pd@0.1CT@PUF of ca. 1 cm³, whose mass was adjusted in function of its Pd content established by ICP-AES to have a Pd loading of 0.4 µmol or 0.5 µmol, respectively, were introduced in a tube in a similar fashion to the hydrogenation experiments. Ethanol/water (1:1) (2.25 mL) was then added to immerse the foam. This was followed by the addition of phenylboronic acid (0.17 mmol), K₂CO₃ (0.21 mmol), and the proper aryl bromide (0.14 mmol) (Pd content: 0.29 or 0.36 mol % vs. aryl bromide). The reaction mixture was then heated under stirring (625 rpm) by putting the tube in an oil bath at 65 °C. The course of the reaction was kinetically monitored by sampling regularly the reaction medium and
<figure><img src="image_3.png" /><figcaption>Fig. 2. XPS survey spectrum (A) and high-resolution spectrum of the Pd 3d level core region (B) of as synthesized Pd@<sup>0.1</sup>HT@PUF.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 3. SEM images with different magnifications of as-synthesized Pd@<sup>0.1</sup>HT@PUF (A-C) and Pd@<sup>0.1</sup>CT@PUF (D-F).</figcaption></figure>

analysing the removed aliquots (20 µL) by HPLC as such or after 1:10 dilution with methanol. The identity of the product was confirmed by comparison with an authentic standard while its quantitation was

obtained from a calibration curve built with an authentic standard.
<figure><img src="image_5.png" /><figcaption>Fig. 4. Elemental mapping of Pd obtained by SEM-EDX on as-synthesized Pd@0.1HT@PUF (A,B) and EDX spectra of selected spots and areas (C,D and E,F).</figcaption></figure>

### 3. Results and discussion

#### 3.1. Preparation and characterization of Pd@HT@PUF and Pd@CT@PUF

Coating of open cell polyurethane foam (PUF) with hydrolysable (HT) or condensed (CT) tannin was implemented by simply dipping a cubic sample of PUF of ca. 8 cm³ into an aqueous solution of the tannin at 0.5 mg/mL (HT) or 0.1 mg/mL (HT or CT) (Fig. 1). After 2 h of immersion, the 0.5/0.1HT- or 0.1CT-coated PUF was washed three times with EtOH under vigorous stirring at room temperature for 1 h each time and air-dried. The coating procedure, which does not require use of an alkaline medium as it is the case for polydopamine coatings from dopamine [7,33], exploited the well-known ability of tannins to adhere to many kinds of surfaces, particularly those able to establish hydrogen bonding [31,32], as is the case for polyurethane. Characterization of 0.1HT@PUF and 0.1CT@PUF by SEM at high magnification revealed

rough surfaces with dispersed aggregates (Fig. S1 and S2) akin to those observed with PDA@PUF, and XPS survey spectra showed a significant increase of the relative O(1 s) content on the topmost surface from <19 % in pristine PUF⁹ to ca. 28 and 26 % in 0.1HT@PUF and 0.1CT@PUF, respectively (Fig. S3 and S4), thereby suggesting the presence of oxygen atoms from the phenolic groups of HT and CT on the surface after the coatings. Noteworthy, in addition to O, elements N, C and Si were also observed on both samples.

Functionalization with Pd was obtained by immersion of the resulting 0.5/0.1HT@PUF and 0.1CT@PUF materials into a well stirred EtOH/H₂O (5:1) solution of [Pd(NH₃)₄Cl₂]·H₂O (0.63 mM) for 19 h at room temperature and subsequent thorough washings with water and ethanol (Fig. 1), according to our previously developed procedure with PDA-PUF foams [15]. ICP-AES measurements on several samples of Pd@0.1HT@PUF and Pd@0.1CT@PUF revealed mean Pd contents of 958 ± 55 mg/kg (0.096 ± 0.006 wt %) and 1155 ± 64 mg/kg (0.116 ± 0.006 wt %) respectively, and XPS spectroscopy Pd(II)/Pd(0) ratios of
<table><caption>**Table 1**
Conversion, selectivity, turnover number and frequency measured over 5 successive runs of semi-hydrogenation of phenylacetylene with Pd@<sup>0.5</sup>HT@PUF.</caption><thead><tr><th>Run</th><th>Reaction time (h)</th><th>PA conversion (%)</th><th>ST selectivity (%)</th><th>TON</th><th>TOF (h<sup>-1</sup>)</th></tr></thead><tbody><tr><td>1</td><td>5.5</td><td>97</td><td>93</td><td>552</td><td>100</td></tr><tr><td>2</td><td>4.5</td><td>97</td><td>92</td><td>546</td><td>121</td></tr><tr><td>3</td><td>4.5</td><td>91</td><td>94</td><td>524</td><td>116</td></tr><tr><td>4</td><td>5.0</td><td>94</td><td>92</td><td>529</td><td>106</td></tr><tr><td>5</td><td>4.0</td><td>96</td><td>93</td><td>546</td><td>136</td></tr></tbody></table>

Reaction conditions: phenylacetylene (3 mmol), Pd@<sup>0.1</sup>HT@PUF (8 cm<sup>3</sup>, Pd: 0.16 mol %), EtOH (20 mL), H<sub>2</sub> flow rate (13 mL/min), 25 °C, 625 rpm. TON expressed as (the number of mol of PA converted to ST)/(mol<sub>Pd</sub>). TOF = TON/h.

<table><caption>**Table 2**
Conversion, selectivity, turnover number and frequency measured over 4 successive runs of semi-hydrogenation of phenylacetylene with Pd@<sup>0.1</sup>HT@PUF.</caption><thead><tr><th>Run</th><th>Reaction time (h)</th><th>PA conversion (%)</th><th>ST selectivity (%)</th><th>TON</th><th>TOF (h<sup>-1</sup>)</th></tr></thead><tbody><tr><td>1</td><td>2</td><td>18</td><td>97</td><td>225</td><td>113</td></tr><tr><td>2</td><td>2</td><td>30</td><td>96</td><td>373</td><td>187</td></tr><tr><td>3</td><td>2</td><td>83</td><td>94</td><td>1013</td><td>506</td></tr><tr><td>4</td><td>2</td><td>95</td><td>92</td><td>1144</td><td>572</td></tr></tbody></table>

Reaction conditions: phenylacetylene (3 mmol), Pd@<sup>0.1</sup>HT@PUF (8 cm<sup>3</sup>, Pd: 0.08 mol %), EtOH (20 mL), H<sub>2</sub> flow rate (13 mL/min), 25 °C, 625 rpm. TON expressed as (the number of mol of PA converted to ST)/(mol<sub>Pd</sub>). TOF = TON/h.

<table><caption>**Table 3**
Conversion, selectivity, turnover number and frequency measured over 4 successive runs of nitrobenzene hydrogenation with Pd@<sup>0.1</sup>HT@PUF.</caption><thead><tr><th>Run</th><th>Reaction time (h)</th><th>NB-to-NA conversion (%)</th><th>TON</th><th>TOF (h<sup>-1</sup>)</th></tr></thead><tbody><tr><td>1</td><td>2</td><td>9</td><td>117</td><td>59</td></tr><tr><td>2</td><td>2</td><td>19</td><td>248</td><td>124</td></tr><tr><td>3</td><td>2</td><td>29</td><td>378</td><td>189</td></tr><tr><td>4</td><td>2</td><td>29</td><td>1200</td><td>150</td></tr><tr><td>4</td><td>2</td><td>29</td><td>378</td><td>189</td></tr></tbody></table>

Reaction conditions: nitrobenzene (3 mmol), Pd@<sup>0.1</sup>HT@PUF (8 cm<sup>3</sup>, Pd: 0.08 mol %), EtOH (20 mL), H<sub>2</sub> flow rate (13 mL/min), 25 °C, 625 rpm. TON expressed as (the number of mol of NB converted to AN)/(mol<sub>Pd</sub>). TOF = TON/h.

58:42 and 67:33 (Fig. 2 and S5). These results provided evidence that HT and CT coatings could act not only as adhesive coatings, anchoring Pd (II) species to the surface of the foam to an appreciable extent – the Pd loading being of the same order than that reported for other catecholamine or polyphenol coatings such as polydopamine [15], eumelanin [34] or gallic acid [35] – but is also able to partially reduce Pd(II) species to Pd(0). In agreement with these results, while the Pd content of

<figure><img src="image_6.png" /><figcaption>**Fig. 5.** Kinetics of PA hydrogenation with Pd@<sup>0.1</sup>HT@PUF (8 cm<sup>3</sup>) after 4 runs of PA hydrogenation and 4 runs of nitrobenzene reduction; black squares: PA conversion; red circles, ST selectivity; blue triangles: EB selectivity. Reaction conditions: PA (3 mmol), Pd (0.08 mol %), EtOH (20 mL), H<sub>2</sub> flow rate (13 mL/min), room temperature, 625 rpm.</figcaption></figure>

Pd@<sup>0.5</sup>HT@PUF established by ICP-AES analysis proved to be notably higher with a value of 1840±72 mg/kg (0.184 ± 0.007 wt %), due most probably to a thicker adhesive layer, the high-resolution XPS spectrum of the Pd 3d core region revealed a similar Pd(II)/Pd(0) ratio of 65:35 on the surface of the latter (Fig. S6). This ability of HT and CT coatings on PUF to reduce Pd(II) to Pd(0) contrasts with what was observed with PDA@PUF [15].

The successful anchoring of Pd on the foam surfaces was further assessed by SEM and SEM-EDX. While low magnification SEM images of Pd@<sup>0.1</sup>HT@PUF and Pd@<sup>0.1</sup>CT@PUF confirmed that both HT (Fig. 3A) and CT (Fig. 3D) were coated on the whole surface of PUF as rough films at the macroscopic level, higher magnification images, which look strikingly similar for both composites (Fig. 3B-C and 3E-F), highlighted numerous scattered aggregates, including some of lighter colour that could suggest the presence of Pd nanoparticles or aggregations. This assumption was confirmed by SEM-EDX mapping of Pd on both materials, which, in addition to a dispersion over the entire surface, clearly identified Pd aggregation on certain spots (Fig. 4A-B and Fig. S7A-B), whose EDX spectra left no doubt as to the existence of Pd (nano)particles (Fig. 4C-F and Fig. S7C-F). No TEM characterization could however be carried out to gain more information on these particles, because the elastic nature of the PUF host substrate prevented it from being cut into thin slices for observation, even after a freezing step in liquid nitrogen was attempted to reinforce its rigidity. Nevertheless, these results altogether suggest the coexistence on the surface of coordinated Pd(II) species [36] and of Pd(0) (nano)particles, hence of molecular and nano-catalytic species.

## 3.2. Catalytic hydrogenation with Pd@HT@PUF foams

The semi-hydrogenation of phenylacetylene (PA) to styrene (ST) is a reaction of great importance as this aromatic alkyne is an undesirable component of the gaseous reactant streams employed in polystyrene production plants and its elimination (at a concentration < 10 ppm) is mandatory to avoid deactivation/poisoning of polymerization catalysts [37]. It was thus chosen as a model reaction to evaluate our new structured catalytic materials Pd@<sup>0.5</sup>HT@PUF and Pd@<sup>0.1</sup>HT@PUF.

Pd@<sup>0.5</sup>HT@PUF, in the form of a cubic sample of ca. 8 cm<sup>3</sup> (Pd content: 4.9 µmol), was investigated as the catalyst for the PA-to-ST reaction in ethanol at room temperature with an initial PA concentration of 0.15 M (Pd loading of 0.16 mol %) under a constant flow rate of H<sub>2</sub> (13 mL/min) into the solution to maintain its concentration constant
<figure><img src="image_7.png" /><figcaption>Fig. 6. Conversion and selectivity measured over the 9 successive runs of semi-hydrogenation of phenylacetylene (runs 1 to 4 and 9) and reduction of nitrobenzene (runs 5 to 8) catalysed by a single cubic sample of Pd@0.1HT@PUF (8 cm³, Pd: 2.3 µmol, i.e.: 0.08 mol %). Hydrogenation's conditions: phenylacetylene or nitrobenezene (3 mmol), EtOH (20 mL), H₂ flow rate (13 mL/min), 25 °C, 625 rpm.</figcaption></figure>

<table><caption>**Table 4**

Liquid-phase hydrogenation catalysts for the selective PA-to-ST reduction.</caption><thead><tr><th>Entry</th><th>Catalyst</th><th>Solvent</th><th>T (K)</th><th>P (bar)</th><th>t (min)</th><th>Conv. (%)<sup>a</sup></th><th>Select. (%)<sup>b</sup></th><th>TON<sup>b</sup></th><th>TOF (h<sup>-1</sup>)<sup>b</sup></th></tr></thead><tbody><tr><td>1</td><td>Pd@0.1HT@PUF<sup>c d</sup></td><td>EtOH</td><td>298</td><td>1</td><td>120</td><td>95</td><td>92</td><td>1144</td><td>572</td></tr><tr><td>2</td><td>Pd@0.1HT@PUF<sup>c d</sup></td><td>EtOH</td><td>298</td><td>1</td><td>60</td><td>83</td><td>95</td><td>1076</td><td>1076</td></tr><tr><td>3 [15]</td><td>Pd@PDA@PUF<sup>e</sup></td><td>EtOH</td><td>298</td><td>1</td><td>255</td><td>98</td><td>95</td><td>776</td><td>183</td></tr><tr><td>4 [39]</td><td>Lindlar cat. (Aldrich)<sup>f</sup></td><td>n-hexane</td><td>–</td><td>1</td><td>150</td><td>82</td><td>92</td><td>176</td><td>70</td></tr><tr><td>5 [40]</td><td>Lindlar cat. (NHU Co.)<sup>g</sup></td><td>EtOH</td><td>303</td><td>1</td><td>270</td><td>5</td><td>>99</td><td>309</td><td>69</td></tr><tr><td>6 [41]</td><td>Lindlar cat. (TCI)<sup>h</sup></td><td>CH<sub>3</sub>CN</td><td>303</td><td>1</td><td>60</td><td>86</td><td>99</td><td>327</td><td>327</td></tr><tr><td>7 [42]</td><td>0.9Pd/C<sup>i</sup></td><td>EtOH</td><td>298</td><td>1</td><td>55</td><td>98</td><td>96</td><td>165</td><td>180</td></tr><tr><td>8 [39]</td><td>SiO<sub>2</sub>@CuFe<sub>2</sub>O<sub>4</sub>-Pd<sup>j</sup></td><td>n-hexane</td><td>–</td><td>1</td><td>150</td><td>98</td><td>98</td><td>224</td><td>90</td></tr><tr><td>9 [43]</td><td>Pd-Pb NSS<sup>k</sup></td><td>EtOH</td><td>313</td><td>4</td><td>30</td><td>100</td><td>95.8</td><td>549</td><td>1098</td></tr><tr><td>10 [44]</td><td>Pd/Ag@CeO<sub>2</sub>-1.5<sup>l</sup></td><td>EtOH</td><td>313</td><td>15</td><td>720</td><td>97</td><td>99</td><td>9.6</td><td>0.8</td></tr><tr><td>11 [40]</td><td>Pd/mpg-C<sub>3</sub>N<sub>4</sub><sup>m</sup></td><td>EtOH</td><td>303</td><td>1</td><td>85</td><td>100</td><td>94</td><td>1037</td><td>732</td></tr><tr><td>12 [41]</td><td>MWCNTs-Fe<sub>3</sub>O<sub>4</sub>-Cu<sub>2</sub>O-Pd<sup>n</sup></td><td>CH<sub>3</sub>CN</td><td>303</td><td>1</td><td>90</td><td>100</td><td>98</td><td>377</td><td>251</td></tr><tr><td>13 [45]</td><td>PyC<sub>12</sub>S-Pd/VC<sup>o</sup></td><td>CH<sub>2</sub>Cl<sub>2</sub></td><td>303</td><td>1</td><td>300</td><td>98</td><td>94</td><td>980</td><td>196</td></tr><tr><td>14 [46]</td><td>Pd+PEI(L)@HSS<sup>p</sup></td><td>MeOH/1,4-dioxane</td><td>303</td><td>1</td><td>240</td><td>97</td><td>87</td><td>169</td><td>42</td></tr><tr><td>15 [47]</td><td>1.3Pd-3.5Cu/TiO<sub>2</sub><sup>q</sup></td><td>Toluene</td><td>303</td><td>1</td><td>90</td><td>100</td><td>97.6</td><td>370</td><td>247</td></tr><tr><td>16 [48]</td><td>Pd/IL/Cu(BTC)<sup>r</sup></td><td>CH<sub>3</sub>CN</td><td>303</td><td>1</td><td>40</td><td>>99</td><td>>99</td><td>1524</td><td>2286</td></tr><tr><td>17 [49]</td><td>Pd@Ce-bptc<sup>s</sup></td><td>THF</td><td>298</td><td>1</td><td>150</td><td>100</td><td>93</td><td>990</td><td>396</td></tr><tr><td>18 [50]</td><td>Pd<sub>0.18</sub>Cu<sub>15</sub>/Al<sub>2</sub>O<sub>3</sub><sup>t</sup></td><td>n-hexane</td><td>298</td><td>6.9</td><td>480</td><td>90</td><td>94</td><td>–</td><td>–</td></tr><tr><td>19 [51]</td><td>Pd<sup>u</sup>-GDY<sup>u</sup></td><td>EtOH</td><td>298</td><td>2</td><td>120</td><td>100</td><td>99.3</td><td>12,586</td><td>6293</td></tr><tr><td>20 [52]</td><td>Pd<sub>1</sub>/SBA-15@N-C<sup>v</sup></td><td>EtOH</td><td>323</td><td>1</td><td>10</td><td>96</td><td>93</td><td>893</td><td>5358</td></tr></tbody></table>

a PA conversion and ST selectivity determined by GC analysis.

b TON expressed as (mol of PA converted to ST)/(molPd) and TOF as TON/(h of reaction).

c Entry 1 corresponds to run 4 and entry 2 to run 9 of Figure 6.

d Pd loading in the catalytic material (and in the reaction medium): 0.096 ± 0.006 wt % (0.08 mol %).

e 0.244 wt % (0.12 mol %).

f 5wt % (0.43 mol %).

g 1 wt % (0.016 mol %).

h 5wt % (0.26 mol %).

i 0.924 wt % (0.57 mol %).

j 4.15wt % (0.43 mol %).

k j2.6 wt % (0.175 mol %).

l 8 wt % (10 mol %).

m 5.64 wt % (0.091 mol %).

n 1.67 wt % (0.26 mol %).

o 1 wt % (0.094 mol %).

p (0.5 mol %).

q 1.27wt % (0.26 mol %).

r 1.27 wt % (0.066 mol %).

s 2wt % (0.094 mol %).

t 0.18 at %.

u 0.42 wt % (0.0079 mol %).

v 0.12 wt % (0.1 mol %).
<figure><img src="image_8.png" /><figcaption>**Fig. 7.** Yields measured over the 9 successive runs of Suzuki coupling of *p*-bromotoluene with phenylboronic acid catalysed by a single cubic sample of Pd@<sup>0.1</sup>HT@PUF (1 cm<sup>3</sup>, Pd: 0.4 µmol). Reaction conditions: *p*-bromotoluene (0.14 mmol), phenylboronic acid (0.17 mmol), K<sub>2</sub>CO<sub>3</sub> (0.21 mmol), EtOH/H<sub>2</sub>O (1:1) (2.25 mL), 65 °C, 625 rpm.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 8.** Kinetics of Suzuki coupling of 2-bromobenzoate with phenylboronic acid catalysed by a cubic sample of Pd@<sup>0.1</sup>HT@PUF (1 cm<sup>3</sup>, Pd: 0.4 µmol); pink circles: 2-bromobenzoate consumption; blue circles: 2-phenylbenzoate production. Reaction conditions: 2-bromobenzoate (0.14 mmol), phenylboronic acid (0.17 mmol), K<sub>2</sub>CO<sub>3</sub> (0.21 mmol), EtOH/H<sub>2</sub>O (1:1) (2.25 mL), 65 °C, 625 rpm.</figcaption></figure>

during the entire process. Under these conditions, a selectivity to ST of 93 % at 97 % conversion was observed after 5.5 h of reaction for the first run, giving a TON of 552 (expressed as [the number of mol of PA converted to ST]/[molPd]) and a TOF of 100 h<sup>-1</sup> (Table 1, entry 1), and ethylbenzene (EB) only started to be produced appreciably near full conversion of PA to ST (Fig. S8). Such a selectivity is similar to what had been observed with Pd@PDA@PUF [15] and may be explained by the fact that PA displays high adsorption properties on Pd@<sup>0.5</sup>HT@PUF, which could prevent ST to be adsorbed before almost all PA is consumed.

To shed light on the recycling properties of Pd@<sup>0.5</sup>HT@PUF and check if a gradual activation was occurring as for Pd@PDA@PUF [15]

five consecutive runs were conducted with the same foam under the same conditions. Satisfyingly, during these five runs, the activity and selectivity stayed stable, with the reaction completion time remaining around 4.75 h ± 0.75 h and the ST selectivity, at 94 ± 3 % PA conversion, maintaining at 93 ± 1 % (Table 1). The presence of 35 % Pd(0) on an as-synthesized HT-coated PUF with 0.184 ± 0.007 wt % Pd hence seemed to guaranty a stable catalytic activity and selectivity from the first run without prior activation procedure. In other words, the reducing power of the HT coating seemed to result in an efficient catalytic material without the need to carry out activation runs as was seen for PDA-coated PUF with a higher Pd loading of 0.244 ± 0.032 wt %
<table><caption>**Table 5**
Conversion, selectivity, turnover number and frequency measured over 5 successive runs of nitrobenzene hydrogenation with Pd@<sup>0.1</sup>CT@PUF.</caption><thead><tr><th>Run</th><th>Reaction time (h)</th><th>NB-to-NA conversion (%)</th><th>TON</th><th>TOF (h<sup>-1</sup>)</th></tr></thead><tbody><tr><td rowspan="3">1</td><td>2</td><td>4</td><td>34</td><td>17</td></tr><tr><td>4</td><td>10</td><td>86</td><td>21</td></tr><tr><td>15</td><td>57</td><td>489</td><td>33</td></tr><tr><td>2</td><td>16</td><td>99</td><td>849</td><td>53</td></tr><tr><td rowspan="2">3</td><td>2</td><td>29</td><td>249</td><td>124</td></tr><tr><td>4</td><td>58</td><td>497</td><td>124</td></tr><tr><td rowspan="2">4</td><td>2</td><td>26</td><td>223</td><td>111</td></tr><tr><td>4</td><td>60</td><td>514</td><td>129</td></tr><tr><td rowspan="3">5</td><td>2</td><td>29</td><td>249</td><td>124</td></tr><tr><td>4</td><td>64</td><td>549</td><td>137</td></tr><tr><td>7</td><td>95</td><td>814</td><td>116</td></tr></tbody></table>

Reaction conditions: nitrobenzene (3 mmol), Pd@<sup>0.1</sup>CT@PUF (8 cm<sup>3</sup>, Pd: 0.12 mol %), EtOH (20 mL), H<sub>2</sub> flow rate (13 mL/min), 25 °C, 625 rpm. TON expressed as (the number of mol of NB converted to AN)/(mol<sub>Pd</sub>). TOF = TON/h.

[15]. ICP-AES analysis of the used Pd@<sup>0.5</sup>HT@PUF foam after these 5 runs however revealed a significant decrease of the palladium loading from 1840±72 mg/kg to 1098 ± 43 mg/kg, i.e.: of ca. 40 %.

To overcome this critical leaching issue that could a priori be attributed to a loss of weakly embedded Pd(0) particles and/or to a partial loss of the HT coating from the PUF, we then turned our attention to the Pd@<sup>0.1</sup>HT@PUF foam, for which a lower concentration of HT (0.1 mg/mL) was used for the coating, and for which the ethanol washings were analysed spectrophotometrically until no detectable absorption was observed.

As for Pd@<sup>0.5</sup>HT@PUF, the catalytic activity of Pd@<sup>0.1</sup>HT@PUF was studied in consecutive catalytic PA-to-ST hydrogenation reactions, following the selectivity to ST during the conversion of PA over time.

Under similar conditions, but with a Pd content of 0.096 ± 0.006 wt %, i.e. of 2.3 µmol in a 8 cm<sup>3</sup> foam (0.08 mol % vs. PA) and an initial Pd(II)/Pd(0) ratio of 58:42, a gradual activation was observed in this case, the conversion of PA in 2 h reaction raising from 18 % in the 1<sup>st</sup> run to 95 % in the 4<sup>th</sup> run (Table 2), reminding the behaviour of Pd@PDA@PUF [15]. Satisfyingly however, the ST selectivity remained stable over time and successive runs, and a value as high as 92 % at 95 % PA conversion could be observed after 2 h reaction in the 4<sup>th</sup> run, giving a TON of 1144 and a TOF of 572 h<sup>-1</sup> (Table 2).

As both reusability and versatility are crucial properties of heterogeneous catalytic systems, we next examined the catalytic activity of Pd@<sup>0.1</sup>HT@PUF for the reduction of nitrobenzene (NB) to aniline (AN) – which is an essential synthesis method for aniline compounds preparation [38] – for four consecutive runs. Using the same foam as for PA-to-ST hydrogenations under similar reaction conditions, i.e. with a Pd loading of 0.08 mol %, an initial substrate concentration of 0.15 M in ethanol (20 mL), and a H<sub>2</sub> flow rate of 13 mL/min, continuation of the gradual activation was observed. Specifically, the NB-to-AN conversion after 2 h of reaction increased from 9 % in the 1<sup>st</sup> run to 29 % in the 3<sup>rd</sup> run, before stabilization in the 4<sup>th</sup> run (Table 3). Satisfyingly, up to 92 % yield in 8 h could be reached once the activity was stabilized, hence achieving a maximum TON of 1200 (Table 3 and Fig. S9).

As a last development, a final run of phenylacetylene semi-hydrogenation was carried out. Gratifyingly, 82 % conversion was observed in only 1 h with 95 % ST selectivity (Fig. 5 and 6). So, in total, 9 consecutive catalytic runs were conducted with a single Pd@<sup>0.1</sup>HT@PUF foam, consisting in 4 runs of semi-hydrogenation of phenylacetylene, followed by 4 runs of nitrobenzene reduction, and then again 1 run of phenylacetylene semi-hydrogenation (Fig. 6). During the first 7 runs, the conversion in 2 h, of either PA or NB, gradually increased before stabilisation in the 8<sup>th</sup> run. This all resulted in a final run of PA-to-ST hydrogenation with a TOF of 1076 h<sup>-1</sup>. Furthermore, in this case as well, EB started to be produced appreciably only after the full conversion of PA into ST, and the rate of formation of EB was found to be almost similar to that of ST (Fig. 5), which tends to show that the two successive hydrogenation reactions are initiated on similar active sites, and further

<figure><img src="image_10.png" /><figcaption>**Fig. 9.** Yields measured over the 8 successive runs of Suzuki coupling of *p*-bromotoluene with phenylboronic acid catalysed by a single cubic sample of Pd@<sup>0.1</sup>CT@PUF (1 cm<sup>3</sup>, Pd: 0.5 µmol). Reaction conditions: *p*-bromotoluene (0.14 mmol), phenylboronic acid (0.17 mmol), K<sub>2</sub>CO<sub>3</sub> (0.21 mmol), EtOH/H<sub>2</sub>O (1:1) (2.25 mL), 65 °C, 625 rpm.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Fig. 10.** "Stop-&-go" experiment for the hydrogenation of nitrobenzene with Pd@<sup>0.1</sup>HT@PUF; yellow triangles: NB conversion; orange circles: AN formation. Reaction conditions: nitrobenzene (3 mmol), Pd@<sup>0.1</sup>HT@PUF (8 cm<sup>3</sup>, Pd: 0.08 mol %), EtOH (20 mL), H<sub>2</sub> flow rate (13 mL/min), 25 °C, 625 rpm; 2nd run of Table 3.</figcaption></figure>

sustains that ST cannot be substantially adsorbed on Pd@<sup>0.1/0.5</sup>HT@PUF before all PA has been consumed.

These results prompted us to compare the catalytic performance of our readily prepared structured supported catalyst with state-of-the-art Pd-systems [39–52]. As Table 4 shows, Pd@<sup>0.1</sup>HT@PUF (entries 1–2) shows superior performances compared to Pd@PDA@PUF (entry 3) [15], both in terms of TON and TOF. Moreover, it compares well in terms of reaction conditions and selectivity with many catalytic systems, which range from simple Lindlar catalysts to single-atom catalysts, and often surpasses them in terms of TON and/or TOF (entries 4–15, 17 and 18) [39–47,49,50], the only notable exceptions being a Pd/ionic-liquid/metal-organic framework composite (entry 16) [48] and the graphdiyne- and SBA-15@N-supported Pd single-atom catalysts Pd<sup>S</sup>-GDY and Pd<sub>1</sub>/SBA-15@N—C (entries 19 and 20) [51,52], which can achieve TOF as high as 2286, 6293 and 5358 h<sup>-1</sup> with excellent to exceptional ST selectivity. However, none of them present the practicality of our structured supported catalyst that can be use in a

convenient "dip-and-play" mode, and the high-performing single atom-based catalysts necessitate tedious synthetic procedures, which precludes their use on an industrial scale.

### 3.3. Suzuki cross-coupling with Pd@<sup>0.1</sup>HT@PUF under air

The superior performances of Pd@<sup>0.1</sup>HT@PUF over Pd@PDA@PUF in PA-to-ST hydrogenation prompted us to compare their activity for the Suzuki coupling of 4-bromotoluene with phenylboronic acid. To that end, the reactions were conducted under the best conditions established for Pd@PUF@PDA, i.e.: in EtOH/H<sub>2</sub>O (1:1) at 65 °C under air in the presence of 1.2 equiv. of phenylboronic acid and 1.5 equiv. of K<sub>2</sub>CO<sub>3</sub> as base [15]. Using a cubic sample of Pd@<sup>0.1</sup>HT@PUF (0.096 ± 0.006 wt %) of ca. 1–1.5 cm<sup>3</sup> as the catalytic material, whose mass was adjusted to have a Pd loading of 0.4 µmol, i.e.: of 0.29 mol % as 0.14 mmol of phenylboronic acid was used, full conversion of the latter to 4-phenyltoluene was observed within 10 min, giving a TOF of 2100 h<sup>-1</sup>, which is about 20 times faster than the TOF of 107 h<sup>-1</sup> observed with Pd@PDA@PUF (with bromobenzene) [15]. In this case, Pd@<sup>0.1</sup>HT@PUF thus proved to be highly efficient from the first run, and this, without any pre-activation procedure in an aqueous medium under aerobic conditions. Even more satisfyingly, this catalytic efficiency remained unchanged over 6 runs with 99 % conversion in 10 min in each case (Fig. 7). Starting from the 7th run, the catalyst efficiency started to decrease with reaction yields after 10 min reaction gradually diminishing from 94 to 78 % between run 7 and 9. A longer reaction time in run 9 nevertheless allowed to reach 98 % conversion after 4 h.

The scope of the reaction was then briefly tested using as starting material the sterically hindered *ortho*-ester-substituted aryl bromide, methyl 2-bromobenzoate, that afforded the lowest conversion yields in previous investigations using Pd@PDA@PUF: 64 % in 4 h with a Pd loading of 1.4 mol % [15]. A conversion higher than 70 % was already observed after 2 h reaction and 90 % were reached after 24 h (Fig. 8).

### 3.4. Catalytic studies with Pd@<sup>0.1</sup>CT@PUF

For comparative purposes, we then studied the catalytic activity of the condensed tannin-coated foam, Pd@<sup>0.1</sup>CT@PUF, in both nitrobenzene reduction and Suzuki coupling reactions. Under similar reaction conditions than with Pd@<sup>0.1</sup>HT@PUF, but with a Pd content of 0.116 ± 0.006 wt %, i.e. of 3.5 µmol in a 8 cm<sup>3</sup> foam (0.12 mol % vs. NB) and an initial Pd(II)/Pd(0) ratio of 67:33, very similar results were observed for the nitrobenzene hydrogenation catalysed by Pd@<sup>0.1</sup>CT@PUF, with a gradual increase of the NB-to-AN conversion in 2 h reaction from 4 % in the 1<sup>st</sup> run to 29 % in the 3<sup>rd</sup> run, before stabilization in the following

<figure><img src="image_12.png" /><figcaption>**Fig. 11.** XPS survey spectrum (A) and high-resolution spectrum of the Pd 3d level core region (B) of Pd@0.1HT@PUF after 9 runs of hydrogenation.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>Fig. 12. SEM images with different magnifications of used Pd@<sup>0.1</sup>HT@PUF (A-C) and Pd@<sup>0.1</sup>CT@PUF (D-F) after Suzuki couplings.</figcaption></figure>

runs (Table 5). Furthermore, up to 95 % yield in 7 h could be reached in the 5th run, hence achieving a TON of 814 and a TOF of 116 h<sup>-1</sup> (Table 5 and Fig. S10), which are values of the same order than those obtained with Pd@<sup>0.1</sup>HT@PUF, albeit slightly lower.

Strikingly, a very similar behaviour to that of Pd@<sup>0.1</sup>HT@PUF was also observed for the Suzuki coupling of 4-bromotoluene with phenylboronic acid, but again with slightly reduced performances. Thus, using a cubic sample of Pd@<sup>0.1</sup>CT@PUF of ca. 1–1.5 cm<sup>3</sup> as the catalytic material, whose mass was adjusted to have a Pd loading of 0.5 µmol, i.e.: of 0.36 mol % as 0.14 mmol of phenylboronic acid was used, complete conversion of the latter to 4-phenyltoluene was also observed in 10 min, but with a TOF of 1680 h<sup>-1</sup>, instead of 2100 h<sup>-1</sup> with Pd@<sup>0.1</sup>HT@PUF. Furthermore, the catalytic efficiency also remained unchanged for several runs before declining progressively, but the number of cycles with maximal efficiency decreased from 6 to 5 runs (Fig. 9). An extended reaction time in run 8, for which only 84 % conversion was observed after 10 min, nevertheless enabled 89% conversion in 1 h, 92 % in 2 h and 100 % in 24 h.

### 3.5. Pd leaching evaluation and characterization of the used catalysts

An important criterion in assessing the reusability of a heterogeneous catalyst operated in a liquid medium is to evaluate the leaching of the

active species in the latter and its impact on the observed activity. A simple way to do that with a macroscopic catalyst is to carry out a “stop-and-go” experiment where the catalyst is removed from the medium for a given time to check if the reaction stops and later re-immersed to check if the reaction restarts. Such an experiment was carried out for the reduction of nitrobenzene catalysed by Pd@<sup>0.1</sup>HT@PUF (run 2, Table 3). Thus, while H<sub>2</sub> was constantly fed into the reaction medium to maintain its concentration constant, after 2 h reaction, the foam was removed from the medium for 2 h, then re-immersed in the solution for 1.5 h and finally removed again for 1.5 h (Fig. 10). Satisfyingly, this resulted in the quasi-quenching of the reaction as soon as Pd@<sup>0.1</sup>HT@PUF was removed from the reaction medium, and the restart of the reaction when the foam was re-immersed, indicating a low level of leaching of the active species into the solution and thus a minor influence on the overall activity. In agreement with this experiment, ICP-AES analyses of (i) the Pd@<sup>0.1</sup>HT@PUF-catalyzed NB-to-AN reaction media revealed low amounts of Pd in solution after each run (Table S1) and (ii) the used Pd@<sup>0.1</sup>HT@PUF foam after the 9 catalytic runs of PA-to-ST and NB-to-AN hydrogenation a Pd content of 735 ± 35 mg/kg, which corresponds to a loss of Pd of only 23 % compared to the 40 % loss in 5 runs for Pd@<sup>0.5</sup>HT@PUF (vide supra). Similar results were obtained for Pd@<sup>0.1</sup>CT@PUF with ICP-AES analyses of the NB-to-AN reaction media revealing amounts of Pd ranging from 1.6 ± 0.2 mg/
<figure><img src="image_14.png" /><figcaption>**Fig. 13.** Elemental mapping of Pd obtained by SEM-EDX on used Pd@<sup>0.1</sup>HT@PUF after 9 runs of Suzuki coupling (A,B) and EDX spectra of selected spots and areas (C,D and E,F).</figcaption></figure>

kg in run 1 to < 0.4 mg in runs 3 and 4 (Table S2), and of the used foam after these 5 runs, a Pd content of 953 ± 52 mg/kg which corresponds to a loss of Pd of ca. 18 % in 5 cycles.

Noteworthily, the levels of Pd leaching into the reaction media seemed to be a little higher for the Suzuki coupling reactions (see Table S3 and S4), especially when one considers the much shorter reaction times. This resulted in used foams showing a loss of Pd of 35 and 29 % for Pd@<sup>0.1</sup>HT@PUF (678 ± 75 mg/kg) and Pd@<sup>0.1</sup>CT@PUF (825 ± 94 mg/kg) after 9 and 8 runs, respectively.

### 3.6. Characterization of the used foam-based catalysts

To get an insight into both the observed activation and deactivation phenomena, we finally characterized the used catalytic materials by XPS, SEM, and SEM-EDX.

High-resolution XPS spectra of the Pd 3d core level region of the used

Pd@<sup>0.1</sup>HT@PUF and Pd@<sup>0.1</sup>CT@PUF foams after the 9 and 5 runs of hydrogenation revealed Pd(II)/Pd(0) ratios of 0:100 in both cases (Fig. 11 and S11). When compared to the Pd(II)/Pd(0) ratios of 58:42 and 67:33 observed for the as-synthesized foams, this increase in Pd(0) content most probably explains the gradual improvement of the catalytic activity observed over the first runs of hydrogenation reactions with both foams. Remarkably, this zero-oxidation state could be observed even after the foams had been left to stand in air for weeks, revealing a stabilizing effect of the HT and CT layers through surface interaction with the anchored metal, and a similar increase of the Pd(0) content was observed for the used Pd@<sup>0.1</sup>HT@PUF and Pd@<sup>0.1</sup>CT@PUF foams after the 9 and 8 runs of Suzuki couplings (Fig. S12 and S13).

In addition to these results, XPS survey scan spectra of the used Pd@<sup>0.1</sup>HT@PUF and Pd@<sup>0.1</sup>CT@PUF foams after either hydrogenation or Suzuki coupling reactions all showed a significant loss of Pd on the topmost surface of the foams (Table S5). It should be noted that this loss
of Pd appeared to be relatively more important than that measured by ICP-AES analyses of the bulk material, which suggests that it is the Pd on surface that has leached and that the one, that it is more embedded in the tannin, has stayed but would not be accessible for catalysis. This would explain why the foams slowly deactivated (after 5–6 runs of Suzuki couplings), even though significant amounts of Pd(0) remained (vide supra).

With the exception of Pd@<sup>0.1</sup>HT@PUF after 9 runs of hydrogenation, whose images showed a much smother surface with a wavy appearance and rare polymeric aggregates (Fig. S14), SEM images of all other used Pd@<sup>0.1</sup>HT@PUF and Pd@<sup>0.1</sup>CT@PUF foams still revealed numerous scattered aggregates of various sizes and shapes, ranging from nanometre-sized irregular particles to micrometre-sized spherical particles (Fig. 12 and Fig. S15). However, no Pd particles could be identified. This was confirmed by SEM-EDX mapping of Pd on all used catalytic materials and EDX spectra of selected small areas and spots (Fig. 13 and Fig. S16-S18). This observation further tends to show that it is the Pd on the upper surface that has been leached out, in particular the Pd particles that were observed on the as-synthesized materials.

## 4. Conclusion

In summary, open cell polyurethane foams (PUF) were readily coated with tannins derived from chesnut (HT) and quebracho (CT) woods, and Pd anchored onto the tannin layers via simple immersion of the tannin-coated foams, HT@PUF and CT@PUF, in a water-ethanol (1:5) solution of Pd(NH<sub>3</sub>)<sub>4</sub>Cl<sub>2</sub>-H<sub>2</sub>O at room temperature under air. The resulting easy-to-make composites, Pd@HT@PUF and Pd@CT@PUF, acted as reusable catalysts in a convenient “dip-and-play” mode under batch conditions in selective alkyne semi-hydrogenation and/or nitroarene reduction, as well as in fast Suzuki coupling under air, without any pre-activation procedure. This “ready-to-use” ability was favoured by the partial reduction of the Pd(II) salt into Pd(0) particles, as shown by XPS, SEM and SEM-EDX. A progressive gain of activity was nevertheless observed in the first catalytic runs as the remaining Pd(II) was reduced to Pd(0) under the reaction conditions, and after 5–6 runs a progressive deactivation as the surface Pd(0) particles were leached, due most probably to an attrition phenomenon. Use of both more reductive and more adhesive natural tannins as coatings should limit these inconvenient and studies in this direction are currently under way in our laboratories.

### CRediT authorship contribution statement

**Sara Viggiano:** Investigation. **Han Peng:** Investigation. **Thierry Romero:** Formal analysis, Data curation. **Vasiliki Papaefthimiou:** Formal analysis, Data curation. **Maria Laura Alfieri:** Validation, Supervision, Data curation. **Lucia Panzella:** Validation, Supervision. **Alessandra Napolitano:** Writing – original draft, Validation, Supervision, Methodology. **Vincent Ritleng:** Writing – review & editing, Validation, Supervision, Methodology, Funding acquisition.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

This work was supported by the University of Strasbourg (IdEx Unistra 2019). H.P. thanks The International Doctoral Program of the Université de Strasbourg for his PhD fellowship. Dr. Anne Boos and Mrs. Pascale Ronot from the IPHC (UMR CNRS 7178) are gratefully acknowledged for carrying out the ICP-AES measurements.

## Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.mcat.2025.115540.

## Data availability

Data will be made available on request.

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