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-products derived from HMF degradation favored by the high pH, in agreement with previous studies [22]. On the other hand, when Au-decorated TiO<sup>2</sup> was inserted in the membrane network, the 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]. However, the catalytic activity of the AuPd-containing membrane was far lower than the one of the respective powder sample (entry 4 and 6, respectively), indicating that, also in this case, substrate access to the catalyst active sites is hindered by the polymer. It is indeed worth to point out that catalytic tests are carried out at 70 ◦C, a temperature significantly lower than the polymer glass transition (T<sup>g</sup> <sup>=</sup> <sup>108</sup> ◦C), and the polymeric phase might thus represent a diffusion barrier which is even harder to overcome in the glassy state. In order to overcome this problem, the PAN + AuPd/TiO<sup>2</sup> membrane was treated at a temperature higher than the PAN glass transition temperature. This has been done with the aim to allow polymer chain mobility, which could lead, in turn, to an improved exposure of the active sites. Alternatively, another batch of untreated membrane, was calcined at 300 ◦C, with the aim to understand if the thermal induced PAN cyclisation has a positive effect on the membrane catalytic performances. Nevertheless, these tests, also performed at higher temperatures (Figures S6 and S7) were not successful in recovering a higher catalytic activity.

**Table 1.** Catalytic performances of PAN-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.

