Syngas production from phenolic pollutants via a series of hydroxylation, ring cleavage, and aqueous-phase reforming catalyzed by a hydrotalcite-supported Fe–Mn–Ni alloy

Abstract

A trifunctional catalyst facilitating a series of hydroxylation, oxidative ring opening, and aqueous-phase reforming reactions was developed to convert phenolic wastewater into syngas. The definitive screening design experiment at 250 °C for 5 h with 1.75% H₂O₂ and 2 wt% catalyst loading demonstrated the importance of Fe, Mn, and Ni among the first-row transition metals to be impregnated into hydrotalcite to acquire the trifunctional feature. The surface chemistry characterization revealed that they improved the amount of strong and weak Brønsted (SBrA and WBrA) and Lewis (SLA and WLA) acidic active sites. The mechanistic roles of these sites via semi-continuous kinetic investigation at 200–300 °C for 1–5 h with 1.75% H₂O₂ and 2 wt% catalyst loading were unraveled: (1) SBrA (surface metal oxyhydroxides) facilitated hydroxylation and homolytic cleavage producing hydroxyphenols; (2) WBrA (surface metal hydroxides) promoted ring opening of hydroxyphenols yielding oxo- and di-carboxylic acids; (3) WLA (mineral phase with a tetrahedral coordination) catalyzed reforming of acids into syngas; and (4) SLA (mineral phase with an octahedral coordination) improved the H₂ yield by promoting the water–gas shift reaction. The optimal content of Fe, Mn, and Ni was 49.4, 21.2, and 29.4 wt%, respectively, from 20 wt% of active metals on the support to achieve the maximal organic carbon removal (∼82%) and H₂ yield (∼80%) with a CO-to-H₂ ratio of 0.6, useful for chemical building block synthesis. The optimized catalyst demonstrated high activity and reusability, with a turnover number and frequency of ∼1 × 10⁶ and ∼6 × 10⁴ s⁻¹, respectively, marking a breakthrough in sustainable syngas production.