The system is based on hydroxyl polyimide hollow fibres that, upon treatment at 425 °C in inert atmosphere, rearrange to a polybenzoxazole structure featuring a unique porosity and transport behaviour, besides being amenable to incorporation into membrane reactor units packed with conventional catalytic materials (pictured, a). The authors explored the impact of the obtained membrane setup on different catalytic transformations that benefit from water removal. For instance, by using a commercial CuZn catalyst, they showed how the TR-PBO membrane reactor can systematically afford conversion levels beyond thermodynamic equilibrium for the reverse water–gas shift reaction (pictured, b; ΔQCO indicates the excess amount of CO produced beyond equilibrium). Remarkably, the effect is retained also at elevated temperatures, as shown via time-on-stream experiments with thermal ramps. The principle proved also successful for the oxidation of methane with a traditional Pd/Al2O3 catalyst — a system with a strong tendency to deactivate via H2O-mediated poisoning of the active sites. Unlike a conventional reaction setup, which rapidly deactivates, the TR-PBO membrane reactor showed constant activity levels over 150 h confirming its ability to effectively remove water from the proximity of the active sites. X-ray photoelectron spectroscopy analysis of the spent catalyst, in fact, confirmed that the structure of PdO — the active form of Pd during the process — is unaltered. As a final example, the team showed the potential of the membrane reactor to suppress side reactions during Fischer–Tropsch synthesis as a mean to control selectivity.
At this stage, the scalability of the approach beyond lab-scale set-ups remains unclear. However, the study offers a refreshing perspective on the general ability to control catalytic reactions via reactor engineering.
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