Structure, acidity and redox properties of MCM-22 ferrisilicate

• Published in: Microporous and mesoporous materials 2008-04, Vol.110 (1), p.51-63
• Main Authors: MIHALYI, R. M, LAZAR, K, KOLLAR, M, LONYI, F, PAL-BORBELY, G, SZEGEDI, A
• Format: Article
• Language: English
• Subjects: [Fe]MCM-22 zeolite; Chemistry; Colloidal state and disperse state; DRIFT spectroscopy; Exact sciences and technology; Ferrisilicate; General and physical chemistry; In situ Mössbauer spectroscopy; Intrinsic acid strength; Ion-exchange; Iron species; Porous materials; Surface physical chemistry; Zeolites: preparations and properties
• ISSN: 1387-1811; 1873-3093
• DOI: 10.1016/j.micromeso.2007.09.001

Aluminum-free MCM-22 type ferrisilicate, [Fe]MCM-22 (Si/Fe = 19.2) and, for comparison, MCM-22 aluminosilicate, [Al]MCM-22 (Si/Al = 13.1), were synthesized. The materials and their precursors were characterized by X-ray diffractometry (XRD), thermogravimetry (TG), transmission and diffuse reflectance Fourier-transform infrared spectroscopy (FT-IR and DRIFTS), temperature-programmed ammonia evolution (TPAE), in situ Mössbauer spectroscopy and temperature-programmed reduction with hydrogen (H 2-TPR). The presence of Fe 3+ T atoms (Fe FW) were indicated by the appearance of a ν OH IR-band at 3634 cm −1. The red shift of the ν OH band, induced by the H-bonding of bridged hydroxyls and N 2, was smaller for the [Fe]MCM-22 (64 cm −1) than for the [Al]MCM-22 (86 cm −1). The smaller shift evidences sites of weaker intrinsic acid strength. The rate and temperature of the TG-release of hexamethyleneimine template showed relation with the acidity of the sample. The amount of Fe 3+ species, present in highly distorted tetrahedral coordination state, was in good correspondence with the concentration of the Brønsted acid sites. After reduction with H 2 or CO, Fe 2+ appeared in a novel coordination state. Results suggest that these Fe 2+ species are bound to the framework via oxygen atoms. Reducibility of the framework iron is related to the high concentration of hydroxyl nests generated in both the wide (12-MR) and the narrow (10-MR) channels of [Fe]MCM-22 during the synthesis and/or template removal.