Catalytic vapour phase hydrodehalogenation of 1,8-dichlorooctane over Pd@MIL-101(Cr)-NH2: a step forward in MOF-based technologies

Catalytic hydrodehalogenation (HDH) represents a promising strategy to remove halogens (Cl and Br) inpyrolysis oils derived from halogenated plastic waste. Metal–organic frameworks (MOFs) offer a potentialalternative to conventional HDH catalysts due to their outstanding properties in terms of porosity andsurface area, thus allowing better dispersion of active phases. In this work, a Pd@MOF catalyst is studiedfor the HDH of 1,8-dichlorooctane, selected as a model chlorinated organic compound, using a continuousfixed-bed reactor under vapor phase conditions. A first screening for MOFs with different structuresand compositions evidenced a much better performance of those synthesized under microwave radiation(UiO-66-NH2 and MIL-101(Cr)-NH2) due to their smaller particle sizes that improved the surface-tovolumeratios and reduced diffusional restrictions. Among these two MOFs, MIL-101(Cr)-NH2 wasselected for the dispersion of Pd nanoparticles due to its superior catalytic stability over time on stream,as compared with UiO-66-NH2. Using as a reference commercial catalyst (5 wt% Pd dispersed on Al2O3),a superior activity was observed over the Pd@MOF composite (100% conversion and almost full selectivityto n-octane after 4 h), which was attributed to the confinement of well-dispersed Pd nanoparticles withinthe MOF matrix, effectively preventing their agglomeration during the reaction, as confirmed by transmissionelectron microscopy (TEM) analysis. Through the combination of different characterization techniquessuch as active oxidative decomposition (AOD) combined with ion chromatography (IC), gaschromatography coupled with mass spectrometry (GC-MS), and X-ray photoelectron spectroscopy (XPS)analysis of the catalyst, we concluded that the released chlorine, from 1,8-dichlorooctane HDH, wasmostly retained in the Pd@MOF catalyst. Interestingly, the composite was regenerated by in situ thermaldesorption and reused keeping the initial activity for 12 h.