## Discussion
### Catalysis in Batch and in Flow
In order to improve this outcome, the influence of the reaction temperature was briefly investigated in a series of experiments carried out at 60 \({}^{\circ}\)C and with \(\oplus\) = 3 mL h\({}^{-1}\) (Table 2, entries 11-15). As expected, the combined increase of the temperature and reduction of the flow-rate succeeded in improving the conversion degree. At the same time, however, these conditions led to some decrease of catalytic activity towards the latest runs, which discouraged further pursuit of the approach. In this regard, it is worth pointing out that a cursory comparison of the entries 5 and 10 in Table 2 might give the feeling that the MCFR device (38% conversion and space-time yield of \(STY\) = 52 g L\({}^{-1}\) h\({}^{-1}\)) was provided with a lower catalytic activity than the PCFR one (57% conversion and \(STY\) = 70 g L\({}^{-1}\) h\({}^{-1}\), under otherwise identical conditions). However, when the relative copper contents and estimated residence times in the two reactors are taken into account (approx. 64 \(\mu\)mol vs. 170 \(\mu\)mol and \(\tau\) = 12 min vs. \(\tau\) = 19 min, respectively), it is possible to conclude that, similar to other studies [47], the monolithic device was actually endowed with higher specific activity than the packed-bed one. The last part of this investigation involved the PCFRs **R1**-**R5**, assembled by dry-packing **P7**-CuSO\({}_{4}\) (80-85 mg, approx. 14 \(\mu\)mol of copper) into PTFE tubing sections. Thanks to the translucent properties of PTFE, in this case, the reduction of the Cu(II) sites could be followed visually to confirm an essentially instantaneous process when the flush of sodium l-ascorbate in aqueous MeOH reached successive portions of the resin beads. Initial experiments were designed to benchmark the properties of reduced **P7**-CuSO\({}_{4}\) in flow. With this aim, two PCFRs (**R1** and **R2**; Table 3, entries 1 and 3) were fit to a dual-channel syringe pump and the whole apparatus was placed in a glove-bag under N\({}_{2}\) to prevent the oxidation of Cu(I) sites by atmospheric O\({}_{2}\). While the run in **R1** involved the MeOH/THF (4:1) reaction medium, already employed in batch (see Table 1, entries 9-12), for the reaction in **R2**, the use of a fully protic MeOH/H\({}_{2}\)O (4:1) solvent mixture was explored. With the adopted set-up, the influence of the flow-rate could be investigated simultaneously in the two PCFRs (Figure 4). Complete conversion of the azide at the outlet of both devices was observed for about 4.5 h at \(\oplus\) = 1.2 mL h\({}^{-1}\) (around 17 column volumes of feed stream). After that time, the flow-rate was increased to \(\oplus\) = 1.8 mL h\({}^{-1}\), with the applied change, which led to a progressive decrease of the conversions in the following 3.5 h. Besides the anticipated effect of the shorter residence time at higher \(\oplus\), the observed trend was likely to involve some catalyst deactivation phenomena. This conclusion is substantiated by the observation that switching back to the \(\oplus\) = 1.2 mL h\({}^{-1}\) setting caused a significant improvement of the reaction completeness, but did not restore the initial full conversions. Moreover, in the following 6 h at \(\oplus\) = 1.2 mL h\({}^{-1}\), a further loss of catalytic activity was recorded, which is a problem that could be partially compensated for by a reduction of the flow-rate to \(\oplus\) = 0.6 mL h\({}^{-1}\) in the final part of the run. Eventually, the trends in the instantaneous output of the reactors led to an 88% and 68% average degree of azide conversion in the cumulatively collected effluents from **R1** and **R2**, respectively. In this respect, it is important to note that, while some deactivation is evident for both PCFRs, the devices retained appreciable catalytic activity up to a time on stream (\(TOS\)) of almost 20 h. Moreover, due to the large S/C involved in the experiments (around 350 for each device), turnover numbers as high as \(TON\) = 304 and \(TON\) = 238 could be attained for **R1** and **R2**, respectively.