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

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 degrees C for 5 h with 1.75% H(2)O(2 )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 Br & oslash;nsted (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 degrees 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 (similar to 82%) and H2 yield (similar to 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 similar to 1 x 106 and similar to 6 x 104 s-1, respectively, marking a breakthrough in sustainable syngas production.