Formation of iron active species on HZSM-5 catalysts by varying iron precursors for phenol hydroxylation

Abstract

Iron-containing HZSM-5 zeolite (Fen+-HZSM-5) are performed by ion exchange method with varying iron precursors of Fe2+, Fe3+ and equivalent molar of Fe2+/Fe3+. The Fen+-HZSM-5 materials are used as catalysts for phenol hydroxylation in the presence of H2O2. Bulk-iron oxides of α-Fe2O3 and α-FeOOH are synthesized via hydrothermal method for catalytic comparison to Fen+-HZSM-5. The iron structures on the Fen+-HZSM-5 catalysts are determined using UV–vis DRS and EXAFS techniques. The iron species found in the Fe2+-HZSM-5 and Fe3+-HZSM-5 catalysts is an isolated Fe3+ ion in tetrahedral geometry. Iron-oxo bonds with oxygen in zeolite framework (Fe-OF) are identified on both catalysts. On the other hand, Fe-O bond distances and coordination number on Fe2+/Fe3+-HZSM-5 exhibit the mixture of iron-oxo and iron-dioxo species (FO-Fe-OF) in octahedral geometry. The present iron species in the Fe2+/Fe3+-HZSM-5 catalyst show a significant improvement of the turnover number (TON) for phenol hydroxylation in comparison to the bulk α-Fe2O3 and α-FeOOH catalysts. Moreover, the existent iron-dioxo species on the Fe2+/Fe3+-HZSM-5 catalyst enhances catalytic selectivity to catechol product.