3.2.2. Catalytic Tests

The screening of the catalytic performances demonstrated the inability of the bare electrospun NYL membrane to catalyze the reaction in the studied conditions (Table 3, entry 1). Indeed, in this blank experiment, in the absence of the metallic active phase, more than 90% of the fed HMF was converted into degradation products and no oxidation occurred. The addition to the membrane of TiO2-supported AuPd NPs made the membrane active in the formation of HMFCA and FFCA (Table 3, entry 2) but still low production of FDCA was evidenced.


**Table 3.** Catalytic performances of NYL-derived materials. Reaction conditions: 4 h, 70 ◦C, O<sup>2</sup> pressure 10 bar, 25 mL water, HMF (0.018 M), HMF:NaOH molar ratio 1:2 and HMF:metal molar ratio 100:1.

Contrarily to what was observed using PAN-based fibers, using NYL material, a strong effect of reaction temperature was observed on FDCA formation, with yield increasing from 6% up to 27% (Figure S9). Nylon, having lower glass transition temperature and higher hydrophilicity, seems to lead to lower diffusion issues. *Processes* **2020**, *8*, 45 10 of 15 (Figure S9). Nylon, having lower glass transition temperature and higher hydrophilicity, seems to lead to lower diffusion issues. To further increase the performance of the membrane, different materials were synthetized

To further increase the performance of the membrane, different materials were synthetized inserting pristine AuPd NPs colloids in the mixture to be electrospun. In particular, two different membranes have been prepared by (i) electrospinning of a suspension containing AuPd NPs colloids and nylon (NYL + AuPd sample) and (ii) electrospinning a suspension containing bare TiO2, unsupported AuPd NPs and nylon (NYL + TiO<sup>2</sup> + AuPd sample). For both samples, after 4 h reaction time, HMF conversion was complete but considerable differences were seen among catalysts in term of product selectivity. In particular, the selectivity to FDCA increased significantly for TiO2-containing sample. This feature could be ascribed to the effect of the increase of specific surface area obtained by introducing TiO<sup>2</sup> in the membrane (Table 3). inserting pristine AuPd NPs colloids in the mixture to be electrospun. In particular, two different membranes have been prepared by (i) electrospinning of a suspension containing AuPd NPs colloids and nylon (NYL + AuPd sample) and (ii) electrospinning a suspension containing bare TiO2, unsupported AuPd NPs and nylon (NYL + TiO2 + AuPd sample). For both samples, after 4 h reaction time, HMF conversion was complete but considerable differences were seen among catalysts in term of product selectivity. In particular, the selectivity to FDCA increased significantly for TiO2 containing sample. This feature could be ascribed to the effect of the increase of specific surface area obtained by introducing TiO2 in the membrane (Table 3). To further demonstrate the higher suitability of such synthetic protocol, two tests at higher

To further demonstrate the higher suitability of such synthetic protocol, two tests at higher reaction temperature (90 and 110 ◦C, Figure 4) were performed. These tests proved that temperature has a strong positive effect on FDCA yield, since it rose from 14% to 67% by increasing reaction temperature. reaction temperature (90 and 110 °C, Figure 4) were performed. These tests proved that temperature has a strong positive effect on FDCA yield, since it rose from 14% to 67% by increasing reaction temperature.

**Figure 4.** Reaction temperature effect on the catalytic performance of NYL + TiO2 + AuPd sample. Operative conditions: 4 h, O2 pressure 10 bar, 25 mL water, HMF concentration 0.018 M, HMF:NaOH molar ratio 1:2, HMF:(Au + Pd) molar ratio 100:1. Legend: ■ HMF Conversion, • HMFCA yield, ▲ FFCA yield, ▼ FDCA yield, ◄ C-LOSS. **Figure 4.** Reaction temperature effect on the catalytic performance of NYL + TiO<sup>2</sup> + AuPd sample. Operative conditions: 4 h, O<sup>2</sup> pressure 10 bar, 25 mL water, HMF concentration 0.018 M, HMF:NaOH molar ratio 1:2, HMF:(Au + Pd) molar ratio 100:1. Legend: HMF Conversion, • HMFCA yield, N FFCA yield, <sup>H</sup> FDCA yield, C-LOSS.

In order to evaluate the NYL + TiO2 + AuPd membrane stability, reusability tests have been performed at 90 °C (Figure 5). Unexpectedly, NYL + TiO2 + AuPd catalytic membrane showed a significant increase in activity after the first catalytic test, with FDCA yield rising from 19% to 34%. In order to evaluate the NYL + TiO<sup>2</sup> + AuPd membrane stability, reusability tests have been performed at 90 ◦C (Figure 5). Unexpectedly, NYL <sup>+</sup> TiO<sup>2</sup> <sup>+</sup> AuPd catalytic membrane showed a significant increase in activity after the first catalytic test, with FDCA yield rising from 19% to 34%.

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**Figure 5.** Reusability tests performed over NYL + TiO2 + AuPd composite membranes. Operative conditions: 4 h, temperature 90 °C, O2 pressure 10 bar, 25 mL water, HMF concentration 18 mM, HMF:NaOH molar ratio 1:2, HMF:(Au + Pd) molar ratio 100:1. Legend: ■ HMF Conversion, ■ HMFCA yield, ■ FFCA yield, ■ FDCA yield, ■ C-LOSS. **Figure 5.** Reusability tests performed over NYL + TiO<sup>2</sup> + AuPd composite membranes. Operative conditions: 4 h, temperature 90 ◦C, O<sup>2</sup> pressure 10 bar, 25 mL water, HMF concentration 18 mM, HMF:NaOH molar ratio 1:2, HMF:(Au + Pd) molar ratio 100:1. Legend: HMF Conversion, HMFCA yield, FFCA yield, FDCA yield, C-LOSS. conditions: 4 h, temperature 90 °C, O2 pressure 10 bar, 25 mL water, HMF concentration 18 mM, HMF:NaOH molar ratio 1:2, HMF:(Au + Pd) molar ratio 100:1. Legend: ■ HMF Conversion, ■ HMFCA yield, ■ FFCA yield, ■ FDCA yield, ■ C-LOSS.

**Figure 5.** Reusability tests performed over NYL + TiO2 + AuPd composite membranes. Operative

Since metal leaching during the reaction was excluded by chemical analysis (XRF analysis revealed that no Pd, Au of Ti species were dissolved in the reaction mixture), this initial activation effect could be attributed to a modification of the interaction between the active phase and the polymer during the reactivity experiment in water. Indeed, it is possible to hypothesize that catalytic active sites, which are hindered in the original membrane network, are made accessible by the use of the membrane in the reaction conditions, probably due to thermal induced movement of polymer chains during the reaction, which could occur because of the fact that membrane operates in temperature conditions (90 °C) higher than its glass transition temperature (50 °C). In this frame, SEM micrographs of fresh and materials used at 90 °C and 110 °C (Figure 6) strengthened this hypothesis. Indeed, while some fiber shrinking was observed in the pictures, no significant changes in fibers morphology were observed. Moreover, the inorganic content, as evaluated by TGA, stayed unaffected even after catalytic tests, confirming that no leaching of TiO2 or metal nanoparticles Since metal leaching during the reaction was excluded by chemical analysis (XRF analysis revealed that no Pd, Au of Ti species were dissolved in the reaction mixture), this initial activation effect could be attributed to a modification of the interaction between the active phase and the polymer during the reactivity experiment in water. Indeed, it is possible to hypothesize that catalytic active sites, which are hindered in the original membrane network, are made accessible by the use of the membrane in the reaction conditions, probably due to thermal induced movement of polymer chains during the reaction, which could occur because of the fact that membrane operates in temperature conditions (90 ◦C) higher than its glass transition temperature (50 ◦C). In this frame, SEM micrographs of fresh and materials used at 90 ◦C and 110 ◦C (Figure 6) strengthened this hypothesis. Indeed, while some fiber shrinking was observed in the pictures, no significant changes in fibers morphology were observed. Moreover, the inorganic content, as evaluated by TGA, stayed unaffected even after catalytic tests, confirming that no leaching of TiO<sup>2</sup> or metal nanoparticles occurred during the reaction. Since metal leaching during the reaction was excluded by chemical analysis (XRF analysis revealed that no Pd, Au of Ti species were dissolved in the reaction mixture), this initial activation effect could be attributed to a modification of the interaction between the active phase and the polymer during the reactivity experiment in water. Indeed, it is possible to hypothesize that catalytic active sites, which are hindered in the original membrane network, are made accessible by the use of the membrane in the reaction conditions, probably due to thermal induced movement of polymer chains during the reaction, which could occur because of the fact that membrane operates in temperature conditions (90 °C) higher than its glass transition temperature (50 °C). In this frame, SEM micrographs of fresh and materials used at 90 °C and 110 °C (Figure 6) strengthened this hypothesis. Indeed, while some fiber shrinking was observed in the pictures, no significant changes in fibers morphology were observed. Moreover, the inorganic content, as evaluated by TGA, stayed unaffected even after catalytic tests, confirming that no leaching of TiO2 or metal nanoparticles occurred during the reaction.

**Figure 6.** SEM micrographs of (**A**) NYL + AuPd + TiO2; (**B**) NYL + AuPd + TiO2 after reaction at 90 °C **Figure 6.** SEM micrographs of (**A**) NYL + AuPd + TiO2; (**B**) NYL + AuPd + TiO2 after reaction at 90 °C and (**C**) NYL + AuPd + TiO2 after reaction at 110 °C; scale bar: 5 μm. **Figure 6.** SEM micrographs of (**A**) NYL + AuPd + TiO<sup>2</sup> ; (**B**) NYL <sup>+</sup> AuPd <sup>+</sup> TiO<sup>2</sup> after reaction at 90 ◦<sup>C</sup> and (**C**) NYL <sup>+</sup> AuPd <sup>+</sup> TiO<sup>2</sup> after reaction at 110 ◦C; scale bar: 5 <sup>µ</sup>m.

and (**C**) NYL + AuPd + TiO2 after reaction at 110 °C; scale bar: 5 μm. Preliminary analysis using the Attenuated Total Reflection coupled with Infrared Spectroscopy (ATR-FTIR) on fresh and used materials (Figure 7) showed that no furanic-compounds were present Preliminary analysis using the Attenuated Total Reflection coupled with Infrared Spectroscopy (ATR-FTIR) on fresh and used materials (Figure 7) showed that no furanic-compounds were present within both membranes, and no change in its chemical composition occurred during the reaction. Preliminary analysis using the Attenuated Total Reflection coupled with Infrared Spectroscopy(ATR-FTIR) on fresh and used materials (Figure 7) showed that no furanic-compounds were presentwithin both membranes, and no change in its chemical composition occurred during the reaction.

within both membranes, and no change in its chemical composition occurred during the reaction.

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**Figure 7.** ATR analysis of NYL + TiO2 + AuPd samples: fresh (black, up) and used (red, bottom). **Figure 7.** ATR analysis of NYL + TiO<sup>2</sup> + AuPd samples: fresh (black, up) and used (red, bottom).
