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

Abstract

Catalytic hydrodehalogenation (HDH) represents a promising strategy to remove halogens (Cl and Br) in pyrolysis oils derived from halogenated plastic waste. Metal–organic frameworks (MOFs) offer a potential alternative to conventional HDH catalysts due to their outstanding properties in terms of porosity and surface area, thus allowing better dispersion of active phases. In this work, a Pd@MOF catalyst is studied for the HDH of 1,8-dichlorooctane, selected as a model chlorinated organic compound, using a continuous fixed-bed reactor under vapor phase conditions. A first screening for MOFs with different structures and 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-to-volume ratios and reduced diffusional restrictions. Among these two MOFs, MIL-101(Cr)-NH₂ was selected 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 selectivity to n-octane after 4 h), which was attributed to the confinement of well-dispersed Pd nanoparticles within the MOF matrix, effectively preventing their agglomeration during the reaction, as confirmed by transmission electron microscopy (TEM) analysis. Through the combination of different characterization techniques such as active oxidative decomposition (AOD) combined with ion chromatography (IC), gas chromatography 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, was mostly retained in the Pd@MOF catalyst. Interestingly, the composite was regenerated by in situ thermal desorption and reused keeping the initial activity for 12 h.