Bromine Transformation during Catalytic Pyrolysis of Waste Electronic Circuit Boards (WECBs) in an Auger Reactor over the Dual-Catalyst HZSM-5/CaO

Effective bromine mitigation is a critical challenge in the sustainable recycling of electronic waste, where the uncontrolled release of brominated species compromises both environmental safety and product quality. This study unveils a novel synergistic transformation pathway of bromine (Br) during ex situ dual-catalyst pyrolysis of waste electronic circuit boards (WECBs). Experiments were conducted in a continuous auger reactor integrated with a fixed-bed catalytic unit employing a dual (HZSM-5/CaO) catalyst system. By tuning the weight hour space velocity WHSV from 0.6 to 1.0 h^-1, the catalytic process not only doubled the gas yield from 2.7 to 6.5 wt % but also selectively suppressed liquid formation from 18.0 to 12.5 wt %, while driving deeper deoxygenation and aromatic hydrocarbon enrichment. At lower WHSV, intensified secondary reactions promoted the generation of lighter aromatics and also accelerated coke deposition, highlighting the need for WHSV optimization. Mechanistic insights reveal that brominated phenols and aromatic hydrocarbons dominate the primary volatile fraction, where Br⁺ radicals undergo dual pathways: recombination with H⁺ and small fragments forming HBr/CH₃Br, or neutralization by CaO to yield stable CaBr₂. Importantly, 44 wt % of total bromine was retained in the solid residue as CaBr₂, drastically lowering bromine content in pyrolysis oils. The dual-catalyst strategy thus enables simultaneous Br-fixation, hydrocarbon upgrading, and catalyst regeneration, drastically reducing bromine in pyrolysis oils. These findings provide a scalable, mechanistically guided route for the valorization of cleaner electronic waste, coupling environmental protection with high-value fuel production.