Pd/Fe₃O₄ Nanofibers for the Catalytic Conversion of Lignin-Derived Benzyl Phenyl Ether under Transfer Hydrogenolysis Conditions

Abstract

Novel magnetite-supported palladium catalysts, in the form of nanofiber materials, were prepared by using the electrospinning process. Two different synthetic techniques were used to add palladium to the nanofibers: (i) the wet impregnation of palladium on the Fe₃O₄ electrospun support forming the Pd/Fe₃O₄[wnf] catalyst or (ii) the direct co-electrospinning of a solution containing both metal precursor specimens leading to a Pd/Fe₃O₄[cnf] sample. The obtained Pd-based Fe₃O₄ nanofibers were tested in the transfer hydrogenolysis of benzyl phenyl ether (BPE), one of the simplest lignin-derived aromatic ethers, by using 2-propanol as H-donor/solvent, and their performances were compared with the analogous impregnated Pd/Fe₃O₄ catalyst and a commercial Pd/C. A morphological and structural characterization of the investigated catalysts was performed by means of SEM-EDX, TGA-DSC, XRD, TEM, H₂-TPR, and N₂ isotherm at 77 K analysis. Pd/Fe₃O₄[wnf] was found to be the best catalytic system allowing a complete BPE conversion after 360 min at 240 °C and a good reusability in up to six consecutive recycling tests.

1. Introduction

In the last several decades, thanks to all opportunities offered by nanotechnology and nanoscience, a huge advance in the preparation methods of heterogeneous supported catalysts (impregnation, precipitation/co-precipitation, chemical vapor deposition, grafting, etc.) has been observed [1,2,3]. A great deal of attention has been paid to the development of synthetic procedures that can opportunely tune the textural properties of materials, thus driving their catalytic performances [1,2,3].

In this context, one-dimensional (1D) nanocatalysts have recently gained consideration [4,5,6], highlighting how nanofibrous supports can improve the metal dispersion, thus allowing an increase of the surface area-to-volume ratio that enhances the catalytic performance [7,8]. Among several methods adopted for producing 1D nanomaterials, the electrospinning technique can be successfully used to produce submicron fibers in a simple and feasible process [9,10,11,12,13,14,15,16].

In the electrospinning process, a precursor polymer solution, eventually containing the precursor of inorganic phases, is pumped by a capillary pump and charged by a supplied high voltage. The charged composite blend polymer jet undergoes stretching before it reaches the collector and, after the evaporation of the solvent, solidifies in nanofibers form. Then, the obtained composite is calcined at a prefixed temperature to obtain pure inorganic phase nanofibers. Indeed, the electrospun fibers show excellent features such as nanosize, mesoporous nanostructure, large specific surface area, and controllable morphology. These features make the nanofibers a promising and attractive candidate for catalyst support because they can provide more active sites for the catalyst, thus improving the catalytic efficiency [5]. Moreover, the electrospinning technique offers a simple and versatile route to immobilize metal particles in submicron-sized fibers improving the operational stability of samples when used, for example, in plug-flow reactors.

Recently different nanofibers have been proposed for different catalytic applications [17]. For example, an electrospun nanofiber NiO catalyst was tested in the transfer hydrogenation of aromatic aldehydes and hydration of aromatic nitriles and was highly catalytically efficient, with a yield of above 90% [18]. A supported catalyst in the form of a Cu-doped cerium oxide nanofiber sample was prepared by electrospinning, obtaining a fiber-like nanostructure with higher surface area and higher Cu²⁺ dispersion compared with a particle-like catalyst [8].

In this study, two novel Pd/Fe₃O₄ nanofiber-based catalysts were prepared via electrospinning by using two different synthetic approaches (Scheme 1):

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The impregnation of calcined iron(III) oxide electrospun nanofibers with a solution of a Pd(II) precursor followed by H₂ reduction (Pd/Fe₃O₄[wnf] catalyst).

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The direct co-electrospinning of the two solutions containing the metal solutions of the corresponding inorganic Pd²⁺ and Fe³⁺ precursors followed by calcination and H₂ reduction (Pd/Fe₃O₄[cnf] catalyst).

This type of heterogeneous Pd/Fe₃O₄ catalyst has been successfully used in the cleavage of C–O and C–C bonds of lignocellulosic derived molecules under catalytic transfer hydrogenolysis (CTH) conditions [19,20,21]. In CTH reactions, the use of an indirect H source such as 2-propanol, ethanol, methanol, or formic acid allows the lysis of carbon–carbon or carbon–heteroatom bonds in lignocellulosic biomasses and in their derived model molecules, thus lowering their oxygen content [22,23,24]. At the same time, under CTH conditions, it is possible to reduce problems related to the direct use of molecular hydrogen (purchase, transport, security hazards, expensive infrastructures, etc.).

The catalysts were tested in the transfer hydrogenolysis of benzyl phenyl ether (BPE), which is one of the simplest lignin-derived aromatic ethers, generally used as model molecule to mimic the catalytic cleavage of α-O-4 lignin linkages (Scheme 2) [25,26,27]. Lignin is the only fraction of lignocellulosic biomasses that, via catalytic depolymerization, is suitable for the sustainable production of aromatic compounds. Thus, it has attracted a great deal of attention in recent years [28,29]. The catalyst activity is decisive both in the reductive cleavage of C–C and C–O bonds, as well as in the stabilization of the lignin fragments [30,31,32]. Among the most adopted catalytic systems, Pd-based catalysts are surely among the most investigated [33,34,35,36,37].

2. Results and Discussion

2.1. Catalyst Characterization

Having the aim of obtaining a fibrous morphology, a key parameter in the electrospinning process is generally the viscosity of the spinnable solution. For both Pd/Fe₃O₄[wnf] and Pd/Fe₃O₄[cnf] systems, the optimal conditions to achieve a suitable viscosity (around 1.30–1.40 Pa at 25 °C) were identified. Moreover, after a number of experiments, the formation of the “Taylor cone” was finally realized at the optimized flow rate of 1.41 mL/h, high voltage of 17 kV, and needle-to-collector distance of 12 cm.

The SEM images of the as spun composite nanofibers showed a smooth surface (Figure 1), with an average diameter of around 450 nm for the electrospun support Fe(OH)_x[nf] and for the co-electrospun PdO/Fe(OH)_x[cnf] sample, highlighting that the presence of palladium in the precursor solution did not affect the nanofiber diameters.

The electrospun nanofibers were composed of organic polymer and ceramic and/or metallic precursor compounds. In order to remove the polymer, the as-spun fibers needed to be calcined. The calcination temperature was determined according to the thermal gravimetric analysis (TGA) of the polymer polyacrylonitrile (PAN) used in the precursor solution for electrospinning (Figure 2). PAN degraded completely below 500 °C [38], as shown by the thermal analysis results carried out on both samples, PdO/Fe₂O₃[wnf] and PdO/Fe₂O₃[cnf] (Figure 2). Moreover, the different catalyst preparation methods did not affect the thermal profiles, since both catalysts exhibited the relevant weight loss in the range 200–450 °C, attributable to polymer degradation.

Similarly, a comparative evaluation of specific surface areas obtained using Brunauer–Emmett–Teller (BET) equation applied to the N₂ adsorption–desorption isotherms at 77 K, on both electrospun samples, confirmed that there were no changes as a result of the different synthesis methods. The calculated specific surface areas of the analyzed samples are shown in

Table 1. Main characteristics of the supported palladium catalysts investigated (S.A.: surface area; d_n: mean particle size from TEM).

Samples	Palladium Loading (wt%)	Catalyst Preparation	S.A. (m2/g)	dn (nm)
Pd/Fe3O4[wnf]	4.3	Impregnation of Fe2O3 nanofibers	234	10.5
Pd/Fe3O4[cnf]	3.7	Co-electrospinning	228	8.5
Pd/Fe3O4	5.2	Impregnation of commercial Fe3O4	60	8.8
Pd/C	5.0	Commercial	600	10.2

.

The TPR-H₂ profiles of calcined samples, PdO/Fe₂O₃[wnf] and PdO/Fe₂O₃[cnf], are reported in Figure 3. In both cases, the first peak belongs to the palladium specimen reduction, whereas the higher-temperature peaks can be assigned to the Fe₃O₄ support reduction, slightly shifted towards lower temperatures with respect to the peak of the corresponding pure oxides, suggesting a metal–support interaction [39,40,41,42].

In the XRD patterns of the calcined samples (Figure 4), sharp peaks corresponding to the Fe₂O₃ phase were noticed (JCPDS card no. 33-0664). After the reduction under hydrogen flow, both samples showed the peaks related to the Fe₃O₄ phase (JCPDS card no. 19-0629). The absence of the (111) diffraction line of metallic palladium is indicative of extremely small highly dispersed Pd particles [22,43].

This was also confirmed by TEM results (Figure 5). For both reduced samples, the smooth composite nanofibers turned into rough nanofibers. Furthermore, the average nanofiber diameter decreased since the diameter of single nanofibers shrank down by about four times in both samples. According to TEM measurements (Figure 5), the catalyst sample Pd/Fe₃O₄[wnf] was composed of iron oxide grains decorated with palladium nanoparticles, having a mean diameter of 10 nm. For the catalytic sample obtained by co-electrospinning of Pd/Fe₃O₄[cnf], the metallic palladium particles still exhibited nanometric dimensions (mean diameter dimension: 9 nm) but also appeared to be embedded within the oxide fibers, which in turn seemed less wrinkled.

The homogeneous distribution of palladium in the reduced samples, co-electrospun Pd/Fe₃O₄[cnf] and impregnated Pd/Fe₃O₄[wnf], was also confirmed by EDX color mapping (Figure 6). EDX spectra indicate a Pd/Fe₃O₄ ratio in close agreement with the 5:95 ratio considered in the electrospinning precursor solution.

A more detailed physicochemical characterization of both Pd/Fe₃O₄[wnf] and Pd/Fe₃O₄[cnf] catalytic systems, including ICP-MS, is currently in progress and will be object of future contributions.

2.2. Catalytic Tests

Table 2. Performance of investigated Pd-based catalysts in the transfer hydrogenolysis of benzyl phenyl ether (BPE) in the presence of 2-propanol as solvent/H-source (reaction conditions: 0.125 g of catalyst; 40 mL 0.1 M BPE solution; time, 90 min; N₂ pressure, 10 bar; stirring, 500 rpm).

Entry	Catalyst	Temperature (°C)	Conversion (%)	Aromatic Selectivity (%)
1	Pd/Fe3O4[wnf]	240	59	100
2	Pd/Fe3O4[cnf]	240	21	100
3	Pd/Fe3O4	240	38	100
4	Pd/C	240	8	100

reports the catalytic results of investigated catalysts in the CTH of BPE at 240 °C after 90 min of reaction time.

By using Pd/Fe₃O₄[wnf], a good BPE conversion (60%) was registered, with toluene (TOL) and phenol (PHE) being the only reaction products observed. On the other hand, Pd/Fe₃O₄[cnf], impregnated Pd/Fe₃O₄ and commercial Pd/C—used as benchmark catalysts—exhibited a lower activity.

On the other hand, in the past, coprecipitated Pd/Fe₃O₄ catalyst showed a complete BPE conversion under analogous CTH reaction conditions (0.25 g of catalyst; 60 mL of entry solution; 240 °C; 10 bar N₂; 90 min) [25]. However, with all the investigated catalysts, reaction products derived from aromatic ring hydrogenation were not detected.

Results of reactions carried out at 240 °C allow us to discriminate which is the most important factor in the CTH of BPE. The pure Fe₃O₄ support (in the form of either commercial powder or nanofibers) did not give any BPE conversion, suggesting that Pd sites are an essential prerequisite for the CTH process. At the same time, Pd/C, with a specific surface area of 600 m²/g, was found to be poorly active, clearly indicating that the performance of Pd-based catalysts cannot be attributed to any surface area effect. Reasonably, the higher performance of the Pd/Fe₃O₄[wnf] catalyst can be attributed to a strong metal support interaction, as revealed by H₂-TPR analysis and in agreement with a recent report on analogous heterogeneous Pd/Fe catalytic systems on the CTH of lignin-derived aromatic ethers [19]. On the other hand, the lower activity shown by Pd/Fe₃O₄[cnf] can be explained on the basis of TEM analysis results. Indeed, our experiments highlight that palladium particles were embedded inside the iron oxide nanofibers, thus decreasing the availability of Pd sites on the catalyst surface [42,44].

Having identified the Pd/Fe₃O₄[wnf] system as the best catalyst in the CTH of BPE, we decided to evaluate its performance by investigating the effect of (i) the reaction temperature, (ii) the reaction time, and (iii) its stability in consecutive recycling tests.

Indeed, at lower temperature, the CTH of BPE showed a decrease in the conversion (20% at 210 °C and 5% at 180 °C, respectively) while the products distribution pattern remained unchanged within the investigated temperature range (Figure 7).

The investigation of the CTH of BPE at 240 °C was also carried out at different reaction times (Figure 8). The BPE conversion was completed after 6 h, while a small presence of hydrogenated products (cyclohexanol) after 12h of reaction time was observed.

The stability and reusability of the Pd/Fe₃O₄[wnf] catalyst was also assessed at 240 °C (reaction time: 6 h). Pd/Fe₃O₄[wnf] maintained its high catalytic activity after six consecutive recycling runs and no changes in product selectivity were found (Figure 9). Furthermore, Pd/Fe₃O₄[wnf] could be magnetically recovered from reaction media and directly reused after being washed with 2-propanol.

3. Materials and Methods

3.1. Catalyst Preparation

All chemicals were purchased from Carlo Erba Reagents (Italy), and used without additional purification. Before use, Pd/C was dried for 1 day under vacuum at 120 °C and finally reduced at 200 °C for 2 h under a hydrogen flow (1 cc/min).

The electrospinning apparatus consists of a syringe, a grounded collector, and a high-voltage power supply. In the spinning process, the syringe is filled with a polymer solution and a high voltage is applied between the syringe nozzle and the collector. The interaction between the charged polymer solution and the applied electric field provides the extrusion force. Many parameters such as applied voltage, solution feed rate, spinning distance, temperature, humidity, solution conductivity, and viscosity must be carefully adjusted in order to control morphology and properties of the electro-spun materials [17,45].

Two different approaches were explored for the synthesis of Pd-based iron oxide nanofibers: Pd/Fe₃O₄[wnf] and Pd/Fe₃O₄[cnf]. In the first case, the active metal was impregnated [46] onto the nanofibrous support, whereas in the second one, the active metal was directly added in the spinnable solution, as depicted in Scheme 1. In detail, in the production of both the nanofibers, polyacrylonitrile (PAN), N,N-dimethylformamide (DMF), and iron(III) acetate were used as polymer, solvent, and iron source, respectively. At the same time, palladium(II) acetylacetonate was utilized as a palladium source. FeAc₃ (5.1 wt%) was mixed with DMF (90.6 wt%) and, once the solution was homogeneous, PAN (6.5 wt%) was dissolved and magnetically stirred for 4 h at room temperature. In the case of the sample Pd/Fe₃O₄[cnf], the palladium precursor (nominal palladium loading of 5%) was added to the spinnable solution together with the iron precursor (iron(III) acetate). Electrospinning of solutions was performed using the CH-01 Electro-spinner 2.0 (Linari Engineering s.r.l.) system, working at room temperature and at 40% of the relative humidity. A 10-cc glass syringe was equipped with a 20-mm-long stainless-steel needle with a diameter of 1 mm. Different working conditions were tested. Basically, the needle-to-collector distance was varied between 10 and 20 cm, the applied voltage ranged between 12 and 20 kV, and the feeding rate of the solution was tested between 0.95 and 2.02 mL/h. During the process, DMF evaporated very rapidly, and a membrane consisting of smooth nanofibers was finally obtained. Thereafter, in both cases, membranes were calcined in air at 600 °C for 2 h to eliminate the organic constituents of the fibers and to generate the iron oxide from its precursor, respectively named PdO/Fe₂O₃[wnf] and PdO/Fe₂O₃[cnf]. The so-called Pd/Fe₃O₄[wnf] and Pd/Fe₃O₄[cnf] catalysts were obtained after reduction in a standard apparatus at 200 °C for 2 h under flowing hydrogen (1 cc/min).

In the case of Pd/Fe₃O₄ catalyst, the wet impregnation of the commercial Fe₃O₄ powder with an acetone solution of palladium(II) acetylacetonate was performed. After impregnation, also in this case, the sample was dried for 1 day under vacuum at 120 °C and finally reduced at 250 °C for 2 h under a hydrogen flow (1 cc/min).

The main characteristics of the tested palladium-based catalysts are reported in Table 1.

3.2. Catalyst Characterization

SEM-EDX analyses were carried out on a Phenom Pro-X scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectrometer (EDX) in order to investigate the morphological features and elemental compositions of synthesized materials.

The mean diameter size of nanofibers was obtained by counting several hundred fibers visible on the micrographs of each sample. The EDX analysis was used to evaluate the metal loading and its dispersion over the support, acquiring, for all samples, at least 20 points for three different magnifications.

The phase composition of the fresh catalysts was analyzed by powder X-ray diffraction (XRD) using a Bruker D2 Phaser using Cu Kα radiation at 30 kV and 20 mA. The diffraction angles 2θ were varied between 10° and 80° in steps of 0.02° and a count time of 5 s per step. Peak attribution was performed on the basis of the JCPDS database of reference compounds.

Samples’ thermal stability was evaluated by temperature-programmed TGA/DSC experiments with a Netzsch instrument. The temperature-programmed experiments were carried out in air in the range 25–550 °C with a heating rate of 10 °C/min.

The transmission electron microscopy (TEM) analysis was carried out using a JEOL 1400 Plus instrument operating at 120 kV, able to achieve a 0.19 nm point-to-point resolution and a 0.14 nm line resolution. Particle size distributions were obtained by counting several hundred particles visible on the micrographs of each sample.

The temperature-programmed reduction (H₂-TPR) was carried out with a Chemisorb Micromeritics 2750 instrument, to monitor the reduction of the metal oxides under a flux of 50 cm³/min of H₂/Ar (10 vol.%) in the temperature range 25–1000 °C, at atmospheric pressure.

The Brunauer–Emmett–Teller (BET) surface areas of the prepared samples were determined from nitrogen adsorption–desorption isotherms at 77 K (ChemiSorb 2750 Micromeritics).

3.3. Catalytic Tests

Reactions were carried out in a 100-mL stainless-steel autoclave at a stirring speed of 500 rpm. The reactor containing the reduced catalyst (0.125 g) suspended in a 0.1 M solution of BPE in 2-propanol (40 mL), was purged several times with N₂ (99.99%) and successively pressurized at the desired N₂ pressure (10 bar), and finally heated at the necessary reaction temperature. At the end of every reaction, the system was cooled down and, when at room temperature, the pressure was carefully released, and the liquid phase was analyzed using an off-line gas chromatograph (Agilent 6890N equipped with CP-WAX 52CB, 60 m, i.d. 0.53 mm).

In all recycling tests, after any run, the catalyst was magnetically recovered, carefully washed with 2-propanol, and reused under the same reaction conditions.

The conversion, product selectivity, and yield in the liquid phase were calculated on the basis of the following equations:

$$\mathrm{Conversion}[\%]=\frac{\mathrm{mol}\mathrm{of}\mathrm{reacted}\mathrm{BPE}}{\mathrm{mol}\mathrm{of}\mathrm{BPE}\mathrm{feed}}\times 100$$

(1)

$$\mathrm{Liquid}\mathrm{phase}\mathrm{selectivity}[\%]=\frac{\mathrm{mol}\mathrm{of}\mathrm{specific}\mathrm{product}\mathrm{in}\mathrm{liquid}\mathrm{phase}}{\mathrm{sum}\mathrm{of}\mathrm{mol}\mathrm{of}\mathrm{all}\mathrm{products}\mathrm{in}\mathrm{liquid}\mathrm{phase}}\times 100$$

(2)

$$\mathrm{Aromatic}\mathrm{selectivity}[\%]=\frac{\mathrm{mol}\mathrm{of}\mathrm{aromatic}\mathrm{products}\mathrm{in}\mathrm{liquid}\mathrm{phase}}{\mathrm{mol}\mathrm{of}\mathrm{reaction}\mathrm{products}\mathrm{in}\mathrm{liquid}\mathrm{phase}}\times 100$$

(3)

$$\mathrm{Product}\mathrm{Yield}[\%]=\frac{\mathrm{mol}\mathrm{of}\mathrm{specific}\mathrm{product}}{\mathrm{mol}\mathrm{of}\mathrm{substrate}\mathrm{feed}}\times 100$$

(4)

4. Conclusions

The electrospinning technique was efficiently employed to prepare Pd-based catalysts on iron oxide to be used in the transfer hydrogenolysis reaction of benzyl phenyl ether.

Two synthetic procedures to obtain catalyst nanofibers were explored, highlighting the differences in the features of the produced nanofibers. The palladium co-electrospun with the iron support (Pd/Fe₃O₄[cnf]) was finely dispersed and embedded in the nanofibers of the support, whilst the Pd nanoparticles impregnated on nanofibers of the iron oxide support (Pd/Fe₃O₄[wnf]) were evenly dispersed and appeared firmly attached on the surface of the iron oxide nanofiber support. In both cases, palladium particles having nanometric dimension were detected.

The Pd/Fe₃O₄[wnf] catalyst was found to be efficient in the catalytic transfer hydrogenolysis of benzyl phenyl ether (α-O-4 lignin model molecule) producing phenol and toluene as the unique reaction products. Moreover, the Pd/Fe₃O₄[wnf] catalyst could be used up to six consecutive recycling runs without any significative loss in its catalytic performance, and was easily magnetically recoverable from the reaction medium. All these features make the Pd/Fe₃O₄[wnf] nanofibers a promising catalytic system in the reductive valorization of lignocellulosic biomasses.