Acid-redox bifunctional Fe/Al-AMS catalyst: Simultaneously oriented introducing Fe₂O₃ in the channels and Al in the framework of AMS and its enhanced catalytic performance

For the silica-supported catalysts, the acid-free property and weak interaction between metal active centers and silica cause great limitations in their catalytic applications. In this work, a modified metal-assisted templating method was designed for simultaneously introducing different metal sites into two different locations to obtain a novel bifunctional catalyst, in which the acid sites are produced by incorporating Al species in the framework and the redox sites are brought by encapsulating Fe₂O₃ in the channels of anionic surfactant-templated mesoporous silica (AMS), respectively. The functionalized Fe-APTES micelles establish a critical effect between the surfactant head groups and silica species by counterion-mediated interaction, and the adjustable pH value makes the co-hydrolysis of Al species and silica resource. The results show that Fe₂O₃ are highly dispersed in the channels with a strong interaction, and the framework Al species produce large amount of weak Lewis acid sites. The reaction of phenol hydroxylation was proceeded under the condition of phenol/H₂O₂ = 1:2 (molar ratio), reaction temperature 30 °C, reaction time 4 h. For this reaction, the redox active sites and acid sites in Fe/Al-AMS catalysts are both effective. In specific, the well-dispersed Fe₂O₃ in the confined channels will provide more exposed active sites, and more importantly, by replacement of toxic mineral acids, a large amount of surface acid groups stimulates and adsorbs more hydroxyl radicals, leading to the improvement of catalytic activity, also for the selectivity of hydroquinone. Compared with the reference catalysts, by the synergic effect of the metal active centers and surface acid groups, 3Fe/Al-AMS catalyst displays the best catalytic performance (Conv. 46.6%, CAT Select. 49.9%, HQ Select. 33.3%) and stability in the phenol hydroxylation reaction. This oriented introduction pathway of multiple metals in different locations of silica reveals new vision in the development of multi-functional catalysts for other catalytic applications.