Active Sites in Fe/ZSM-5 Zeolite

Abstract

Fe/ZSM-5 zeolite has shown great potential in the selective oxidations of hydrocarbons such as methane and benzene. The various competing active sites of Fe/ ZSM-5 zeolite are reviewed, including the mono-iron, oxygen-bridged [Fe, X] (X = Fe, Al) and peroxide species; in addition, the influences of H 2 pretreatment are considered. For the mono-iron species, the H 2 molecules are chemisorbed on the Fe(III) sites via the g 2 -binding mode. Both high-and low-spin Fe(III) ions play an important role during the H 2 reduction process whereas the former predominates in the N 2 O decomposition process. As the calculated energy barriers indicate, the Fe(III) ions are facile to be reduced by H 2 pretreatment and therefore the active site of the mono-iron species should be in the FeO(OH) form. Instead, the oxygen-bridged [Fe, X] and peroxide species remain stable by H 2 pretreatment. The suitable oxygenbridged [Fe, X] structures are screened out by comparisons with the experimental data and energy considerations from computational aspects. The geometries are in good agree-

A DFT study on direct benzene hydroxylation catalyzed by framework Fe and Al sites in zeolites

“…3 indicates, the energy barrier for N 2 O decomposition (step 2) equals 49.9 kcal mol −1 and exceeds that of the extra-framework Fe sites. 5,[8][9][10]26 With the use of 5-T cluster models, an energy barrier of 44.6 kcal mol −1 has been predicted for the extra-framework mononuclear-Fe site, 5 and similar computational methodologies as in this work obtain a barrier of 33.7 kcal mol −1 . 26 This suggests that the α-oxygen site is more likely to produce extra-framework Fe sites, and then migrate to framework Fe Lewis acidic sites and catalyze the benzene hydroxylation reaction.…”

“…2,3,[5][6][7][8][9] The exact structure of these species is still under debate. 7,10 Notwithstanding, zeolites with framework Fe sites were also detected to show catalytic activity for the above mentioned reaction. [11][12][13][14][15] At a given Fe content, zeolites synthesized with framework Fe sites are significantly more catalytically active than those only with cation exchange (extra-framework Fe).…”

“…This is distinct from the extra-framework Fe sites where this step has a high-energy transition state. 5,[8][9][10]26 Then phenol is produced as a result of an intra-molecular proton transfer from C1 to O1 (step 5, 5 → 6), and the reaction is completed by desorption of phenol.…”

Nature of the active sites in CO oxidation on FeSiBEA zeolites

“…Zeolites containing iron are active in numerous reactions, including decomposition of nitrogen oxides, selective catalytic reduction of NO with hydrocarbons, benzene oxidation or CO oxidation [7][8][9][10]. Nevertheless, the nature of the active sites in zeolites containing iron is still a matter of discussion [11][12][13].…”

Highly dispersed iron species created on alkali-treated zeolite for ammonia SCR

“…species for the NH 3 -SCR reaction, and therefore controlling the formation of highly dispersed Fe species attracts a lot of interest [11,12]. However, to obtain such active species is a difficult process for the exchange of Fe in ZSM-5 because of the strong diffusional control in zeolite crystals and the tendency for highly dispersed Fe nþ species to form iron oxide particles [13].…”