**1. Introduction**

Monomer and polymer production from renewable feedstocks has become a relevant research target with the aim of providing more environmental friendly solutions to the actual fossil-based market [1–6]. In this framework, 5-(hydroxymethyl)furfural (HMF) is recognized as an ideal platform molecule to develop different green products, since it can be obtained via acid-catalyzed dehydration of biomass-derived sugars [7–9], and, in turn, it can be converted into a wide range of different high added value chemicals [10–13]. Among the HMF products, 2,5-furandicarboxylic acid (FDCA, Scheme 1) has been identified as one of the most interesting [14], since it can be considered as the bioderived counterpart of terephthalic acid for the production of polyesters [15], such as polyethylene 2,5-furandicarboxylate (PEF), being the latter the potential candidate to replace polyethylene terephthalate (PET) in bottle production [16,17]. *Processes* **2020**, *8*, 45 2 of 15

$$\stackrel{\mathsf{\*}}{\mathcal{J}}\_{\mathsf{G}}\bigotimes\_{\mathsf{G}}^{\mathsf{G}\mathsf{T}}\bigotimes\_{\mathsf{G}}^{\mathsf{G}\mathsf{T}\mathsf{S}}\bigotimes\_{\mathsf{G}}^{\mathsf{G}}\bigotimes\_{\mathsf{G}\mathsf{T}\mathsf{S}}^{\mathsf{G}\mathsf{T}}\bigotimes\_{\mathsf{G}}^{\mathsf{G}\mathsf{T}\mathsf{S}}\bigotimes\_{\mathsf{G}}^{\mathsf{G}\mathsf{T}\mathsf{S}}\bigotimes\_{\mathsf{G}}^{\mathsf{G}\mathsf{T}\mathsf{S}}\bigotimes\_{\mathsf{G}}^{\mathsf{G}\mathsf{T}}\bigotimes\_{\mathsf{G}}^{\mathsf{G}}\bigotimes\_{\mathsf{G}}^{\mathsf{G}}$$

**Scheme 1.** 5-(hydroxymethyl)furfural selective oxidation to 2,5-furandicarboxylic acid. **Scheme 1.** 5-(hydroxymethyl)furfural selective oxidation to 2,5-furandicarboxylic acid.

First attempts to industrially convert HMF to FDCA relied on the technologies developed for terephthalic acid production [18]. However, the use of corrosive solvent, homogeneous catalysts and harsh operative conditions forced the investigation of other catalytic systems in order to overcome such constraints. In this regard, precious metal nanoparticles showed their high potential, Au-based systems being the most investigated [19–21]. In the definition of the properties of such catalytic systems, several parameters seem to be fundamental such as: nanoparticle composition and dimension, support/nanoparticle interaction and support surface textural properties. The fine tuning of such parameters can lead to very active systems. For instance, by alloying Au with another metal (such as Cu [22,23] or Pd [24,25]) it is possible to effectively enhance FDCA yield, or by choosing suitable supports catalytic activity can be positively affected [26] and/or base addition can be avoided [27–29]. Besides the textural properties that have been shown to play a prominent role, pore dimension and acid/base sites can be considered of crucial importance [30]. Thus, it is evident that First attempts to industrially convert HMF to FDCA relied on the technologies developed for terephthalic acid production [18]. However, the use of corrosive solvent, homogeneous catalysts and harsh operative conditions forced the investigation of other catalytic systems in order to overcome such constraints. In this regard, precious metal nanoparticles showed their high potential, Au-based systems being the most investigated [19–21]. In the definition of the properties of such catalytic systems, several parameters seem to be fundamental such as: nanoparticle composition and dimension, support/nanoparticle interaction and support surface textural properties. The fine tuning of such parameters can lead to very active systems. For instance, by alloying Au with another metal (such as Cu [22,23] or Pd [24,25]) it is possible to effectively enhance FDCA yield, or by choosing suitable supports catalytic activity can be positively affected [26] and/or base addition can be avoided [27–29]. Besides the textural properties that have been shown to play a prominent role, pore dimension and acid/base sites can be considered of crucial importance [30]. Thus, it is evident that the design and synthesis of materials that possess suitable features can lead to optimal catalytic activity.

the design and synthesis of materials that possess suitable features can lead to optimal catalytic activity. As far as the setups employed to carry out this reaction, batch ones are the most studied. However, the economical sustainability of this kind of approach is still a concern [31]. Thus, to address this issue, efforts must be devoted to process intensification, for instance by evaluating the possibility of developing inexpensive catalytic systems to perform this reaction continuously [32,33], and, in this frame, recent reports have highlighted the interest of both industrial [34,35] and academic research [36,37]. In this context, the use of catalytic membranes is known to provide several advantages, making them efficient tools for applications in several industrial fields [38]. In the case of HMF oxidation, considering that the reaction is carried out under mild operative conditions (70– 120 °C), it is possible that the use of composite polymeric membranes might be advantageous. Along As far as the setups employed to carry out this reaction, batch ones are the most studied. However, the economical sustainability of this kind of approach is still a concern [31]. Thus, to address this issue, efforts must be devoted to process intensification, for instance by evaluating the possibility of developing inexpensive catalytic systems to perform this reaction continuously [32,33], and, in this frame, recent reports have highlighted the interest of both industrial [34,35] and academic research [36,37]. In this context, the use of catalytic membranes is known to provide several advantages, making them efficient tools for applications in several industrial fields [38]. In the case of HMF oxidation, considering that the reaction is carried out under mild operative conditions (70–120 ◦C), it is possible that the use of composite polymeric membranes might be advantageous. Along with all the advantages related to the more traditional inorganic membranes, these materials are characterized by low production costs, ease of handling and tunability of their properties [39].

with all the advantages related to the more traditional inorganic membranes, these materials are characterized by low production costs, ease of handling and tunability of their properties [39]. Electrospinning provides a convenient approach for the preparation and scale-up of membranes made of continuous sub-micrometric fibers characterized by large surface area and porosity [40,41]. This could represent an interesting strategy for the production of catalytic membranes, which can be used in processes for biomass valorization. Briefly, this technology uses electrostatic forces to uniaxially stretch a viscoelastic jet derived from a polymer solution to produce fibers having Electrospinning provides a convenient approach for the preparation and scale-up of membranes made of continuous sub-micrometric fibers characterized by large surface area and porosity [40,41]. This could represent an interesting strategy for the production of catalytic membranes, which can be used in processes for biomass valorization. Briefly, this technology uses electrostatic forces to uniaxially stretch a viscoelastic jet derived from a polymer solution to produce fibers having diameters ranging from a few tenths of nanometers to a few micrometers, collected as nonwovens with mesh porosity typically higher than 80% and pore diameters that can vary from a few to tens of micrometers.

diameters ranging from a few tenths of nanometers to a few micrometers, collected as nonwovens with mesh porosity typically higher than 80% and pore diameters that can vary from a few to tens of micrometers. Electrospun membranes are currently being investigated as materials for the support of heterogeneous catalysts by following different technological approaches, the main one being the production of ceramic fibers that support metal nanoparticles [40]. In this case, a polymer solution containing ceramic precursors and metal salts is electrospun and subsequently heat treated under inert gas to eliminate the organic components and reduce the metal precursor to metal nanoparticles [42–45]. This approach permits the achievement of high catalytic performances by exploiting the high surface area of the fibers but suffers from the high fragility of the completely inorganic nonwoven. Conversely, by keeping unaltered the organic polymeric component, membrane flexibility and Electrospun membranes are currently being investigated as materials for the support of heterogeneous catalysts by following different technological approaches, the main one being the production of ceramic fibers that support metal nanoparticles [40]. In this case, a polymer solution containing ceramic precursors and metal salts is electrospun and subsequently heat treated under inert gas to eliminate the organic components and reduce the metal precursor to metal nanoparticles [42–45]. This approach permits the achievement of high catalytic performances by exploiting the high surface area of the fibers but suffers from the high fragility of the completely inorganic nonwoven. Conversely, by keeping unaltered the organic polymeric component, membrane flexibility and handling can be massively improved. Following this approach, polymeric electrospun nanofibers have been decorated at the surface with metal nanoparticles (NPs) [46,47] in an elegant and effective way that exploits polymer bulk properties and maximizes the catalytic effect. However, NP immobilization at a fiber

handling can be massively improved. Following this approach, polymeric electrospun nanofibers

surface requires a further step in the production process that is time consuming and hardly applicable at the industrial level. Moreover, leaching of metal NPs from a fiber surface cannot be excluded.

In this work, electrospinning is used in a simple and scalable single-step approach for the production of electrospun polymer-inorganic catalytic membranes, potentially suitable for batch and continuous processes. Membranes were manufactured by incorporating preformed Au and Au/Pd NPs (Au/Pd molar ratio 6, which was demonstrated to promote the highest catalytic activity in this reaction [24]) and TiO<sup>2</sup> in the starting polymeric solutions. To optimize catalytic activity and stability, two different polymers have been tested—i.e., polyacrylonitrile (PAN) and Nylon 6,6 (NYL)—loaded with Au and alloyed Au/Pd NPs, either directly supported on TiO<sup>2</sup> or simply combined with TiO<sup>2</sup> during electrospinning. The catalytic activity of catalysts contained in different electrospun membranes towards HMF oxidation to FDCA has been investigated with the goal of highlighting the effect of polymer/inorganic combination on membrane performance. The materials were evaluated in batch experiments to assess the viability for use of electrospun polymer-based catalytic membranes in the conversion of renewable molecules in water.
