Active sites in HZSM-5 with low Fe content for the formation of surface oxygen by decomposing N2O: is every deposited oxygen active?

Abstract

Surface-active centers were detected in HZSM-5 with a low content of iron (< 1000 ppm) activated by steaming and high-temperature calcination in inert atmospheres (up to 1323 K). These centers lead to the formation of surface oxygen (O) ad species from N 2 O and were characterized quantitatively by the transient response method. The total amount of active centers was proportional to the content of iron in the zeolites. Only a part of (O) ad deposited by decomposing N 2 O was active in CO oxidation at 523 K and appeared as sharp O 2 peaks at 666 K during the TPD measurements. A binuclear Fe center is suggested featuring a "diamond core" structure, similar to that of the monooxygenase enzyme, as an active center. The active O atoms were assigned to the paired terminal oxygen atoms each bonded to one Fe site (ferryl groups) in the diferric [Fe 2 O 2 H] + cluster. Oxygen pretreatment at 823 K decreases somewhat the total amount of active sites but does not affect the ratio of active/inactive oxygen. Zeolite presaturated by water vapor at 473-523 K generates (O) ad species from N 2 O completely inactive in the CO oxidation. Part of it appears as a broad peak at 940 K in the TPD profile. The total amount of the deposited oxygen corresponds to half of the stoichiometric amount of the surface Fe atoms and suggests that water blocks a half of the binuclear [Fe 2 O 2 H] + center, the remaining acting as a single Fe site.

FTIR study of the relation, between extra-framework aluminum species and the adsorbed molecular water, and its effect on the acidity in ZSM-5 steamed zeolite

“…This contradicts the frequently accepted mechanism that underlies the increasing number of Lewis acid sites at the expense of the bridged Si-OH-Al sites [1][2][3][4][5] , associated with Brönsted acidity, after hydrothermal treatment. This behavior may be attributed to the decreasing accessibility of pyridine molecules to Lewis acid sites inside the zeolite channels.…”

“…Several authors have propused [1][2][3][4][5] that the interaction between two bridged Si-OH-Al sites, associated 6 with Brönsted acidity, generate an extra-framework Al(OH) 2+ (or AlO + ), which has been considered [7][8][9] as the true carrier of Lewis acidity. It is generally accepted that the extra-framework aluminum (EFAL) generated after hydrothermal treatment is deposited in the form of 4, 5 and 6 coordinate monomeric species 10,11 , where 5 and 6 are related with the Lewis acidity in zeolites.…”

Fenton Reaction Driven by Iron Ligands

“…Ferryl species have been detected in complexes that stabilize Fe(IV) (33,34) , in enzymatic reaction of peroxidases (35) and heterogeneous catalysis by zero valent iron (36,37) . Some authors (38)(39)(40)(41) , have developed techniques to determine selectively the presence of [FeO] 2+ or •OH based on the reactivity again a substrate ( Figure 5).…”

Self-repairing properties of OPC clinker/natural zeolite blend in water and alkali carbonate environments at 270°C

“…water molecules situated in their micropores and channels (Armbruster and Gunter, 1991;Curkovic et al, 1997). Substantial amounts of iron may be intercalated in the crystalline lattice of zeolite (Dubkov et al, 2002;Kiwi-Minsker et al, 2003;Perez-Ramirez et al, 2003). Under alkali attack of calcium hydroxide from the hydration of lime and OPC, the aluminosilicate framework of zeolites disintegrates to form calcium silicates, calcium aluminates and/or calcium aluminium silicates in reaction with calcium cations (Liguori et al, 2015;Özen et al, 2016;Perraki et al, 2005;Uzal and Turanli, 2012).…”