3.1.1. Characterization

The different types of NPs supported onto TiO<sup>2</sup> were incorporated into PAN electrospun membranes, as described in detail in the Supporting Information, and the resulting morphology was observed by SEM (Figure 1) and TEM (Figure 2). 3.1.1. Characterization The different types of NPs supported onto TiO2 were incorporated into PAN electrospun membranes, as described in detail in the Supporting Information, and the resulting morphology was

**Figure 1.** SEM images acquired at different magnifications of: (**A**) PAN; (**B**) PAN+TiO2; (**C**) PAN + Au/TiO2 and (**D**) PAN + AuPd/TiO2. Scale bars: 10 µm (first column); 2 µm (second column) and 1 µm (third column). **Figure 1.** SEM images acquired at different magnifications of: (**A**) PAN; (**B**) PAN+TiO<sup>2</sup> ; (**C**) PAN + Au/TiO<sup>2</sup> and (**D**) PAN + AuPd/TiO<sup>2</sup> . Scale bars: 10 µm (first column); 2 µm (second column) and 1 µm (third column).

For all types of PAN-based membranes the continuous fibers have diameters in the range 200– 300 nm, similarly, to unloaded PAN membrane (Figure 1A). In PAN + TiO2 (Figure 1B) the TiO2 NPs are round-shaped, with a rough and corrugated surface and with a broad distribution of dimensions. The smallest particles (in the range of 0.5–1 µm) are clearly incorporated along the fibers while the different particle dimensions (Table S4).

distributed along fiber axis, mostly confined inside the PAN fibers. The particles are small (diameter 0.1–1 µm) though being bigger than fiber diameter, thus generating a "pearl necklace" morphology. The fact that large aggregates are entrapped in the pore of PAN + TiO2 while small particles are mostly incorporated in the fibers of PAN + Au/TiO2 and PAN + AuPd/TiO2 can be ascribable to the original

TEM inspection (Figure 2) indicates the successful incorporation of Au/TiO2 and AuPd/TiO2 in

TGA analysis (Figure S5) confirms the organic/inorganic composition of electrospun membranes. PAN membrane shows a constant weight up to 250 °C (apart from a modest weight loss of 1% at low temperature ascribable to residual DMF evaporation, see inset), followed by a sharp weight loss of about 35% and a slower second weight loss starting from about 500 °C, with a negligible weight residue at 700 °C, in line with previous results [48,49]. PAN membranes loaded with supported NPs show the same degradation pattern, with weight losses proportional to the PAN

PAN fibers and reveals that the particles observed in SEM images are aggregates of TiO2.

**Figure 2.** TEM analysis of PAN-Au/TiO2 and PAN-AuPd/TiO2 samples (**A**,**B**, respectively). Scale bars: 100 nm. **Figure 2.** TEM analysis of PAN-Au/TiO<sup>2</sup> and PAN-AuPd/TiO<sup>2</sup> samples (**A**,**B**, respectively). Scale bars: 100 nm.

3.1.2. Catalytic Tests Membranes were tested in the liquid phase oxidation of HMF. This molecule has two groups that can be oxidized: the alcoholic and the aldehydic group. The complete or partial oxidation of one or both groups may lead to the formation of different products (Scheme 1). The reaction on Au-based catalysts has been generally described in two steps: (i) the oxidation of aldehydic group to 5 hydroxymethyl-2-furancarboxylic acid (HMFCA) and (ii) the oxidation of alcoholic group—through the formation of 5-formyl-2-furancarboxylic acid (FFCA)—to 2,5-furandicarboxylic acid (FDCA). 2,5 diformylfuran (DFF) was not generally observed in the course of the reaction with gold-base catalysts. Our study (Table 1) indicated that the plain supporting phases (either PAN or PAN + TiO2) were inactive in the oxidation (entry 1 and 2), while forming very small amounts of HMFCA and by-For all types of PAN-based membranes the continuous fibers have diameters in the range 200–300 nm, similarly, to unloaded PAN membrane (Figure 1A). In PAN + TiO<sup>2</sup> (Figure 1B) the TiO<sup>2</sup> NPs are round-shaped, with a rough and corrugated surface and with a broad distribution of dimensions. The smallest particles (in the range of 0.5–1 µm) are clearly incorporated along the fibers while the biggest ones (up to 5 µm in diameter) are entrapped between the pores generated by the non-woven structure. In both PAN + Au/TiO<sup>2</sup> and PAN + AuPd/TiO<sup>2</sup> samples, the agglomerates are randomly distributed along fiber axis, mostly confined inside the PAN fibers. The particles are small (diameter 0.1–1 µm) though being bigger than fiber diameter, thus generating a "pearl necklace" morphology. The fact that large aggregates are entrapped in the pore of PAN + TiO<sup>2</sup> while small particles are mostly incorporated in the fibers of PAN + Au/TiO<sup>2</sup> and PAN + AuPd/TiO<sup>2</sup> can be ascribable to the original different particle dimensions (Table S4).

products derived from HMF degradation favored by the high pH, in agreement with previous studies [22]. On the other hand, when Au-decorated TiO2 was inserted in the membrane network, the TEM inspection (Figure 2) indicates the successful incorporation of Au/TiO<sup>2</sup> and AuPd/TiO<sup>2</sup> in PAN fibers and reveals that the particles observed in SEM images are aggregates of TiO2.

resulting materials display a certain activity (entry 3), which was far lower if compared to the powder catalyst (entry 5). This could be attributed to the fewer active sites exposed in the membrane respect to the overall active sites of the powder. The use of the bimetallic system induced a significant increase of the catalytic performance of the membrane: HMF conversion increased from 69% to 94%, and a small amount of FDCA (2%) was also detected with this catalyst (entry 4). The improved performances of the bimetallic system compared to the Au monometallic system is correlated to the cooperative effect of the two metals in the alloyed system, as demonstrated in previous papers [24,50]. TGA analysis (Figure S5) confirms the organic/inorganic composition of electrospun membranes. PAN membrane shows a constant weight up to 250 ◦C (apart from a modest weight loss of 1% at low temperature ascribable to residual DMF evaporation, see inset), followed by a sharp weight loss of about 35% and a slower second weight loss starting from about 500 ◦C, with a negligible weight residue at 700 ◦C, in line with previous results [48,49]. PAN membranes loaded with supported NPs show the same degradation pattern, with weight losses proportional to the PAN content, while the residual weight at 700 ◦C corresponds to the inorganic phase not subjected to thermal degradation and was in the range 61%–62% for all membranes, as a proof of process reproducibility.
