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The relation between the intrinsic mechanism of various N2O conversions over FeMFI catalysts and the nature of the active iron site(s) has been analyzed. To this end, direct N2O decomposition and N2O reduction with CO in the absence or presence of NO were investigated using a combination of transient pulse and steady-state techniques over steam-activated FeMFI zeolites with a similar iron content (0.6–0.7 wt% Fe) and different framework compositions (Si–Al, Si–Ga, Si–Ge, and Si). The forms of iron in the catalysts were characterized by UV/vis and HRTEM. The intrinsic reaction mechanism determines the optimal iron site distribution, which can be modulated by tuning the steaming temperature during activation. Oligonuclear iron oxo clusters in the zeolite channels are essential in direct N2O decomposition due to a faster desorption of O2 as compared to isolated ions. Such forms of active iron can be achieved at a lower steam-activation temperature over FeAlMFI and FeGaMFI (900 K) than over FeGeMFI and FeMFI (1150 K). Contrarily, zeolites with a more uniform distribution of isolated iron species lead to higher activities in N2O reduction with CO as compared to highly clustered catalysts. In this case, O-removal as CO2 is strongly accelerated vis-à-vis O2 desorption in direct N2O decomposition. The dual role of NO as a promotor in N2O decomposition and as an inhibitor in N2O reduction also supports the participation of different sites in both types of conversions. NO selectively inhibits N2O reduction over isolated iron ions, further evidencing the essential role of oligonuclear iron clusters in the NO-assisted N2O decomposition. [Copyright &y& Elsevier]

Steam-activated Fe-zeolites (MFI and BEA) are efficient catalysts for the N2O-mediated oxidative dehydrogenation of propane, with initial propylene yields up to 25% at 723 K, but deactivate as a result of coke formation. A tapered element oscillating microbalance (TEOM) coupled to GC analysis has been applied to correlate activity and deactivation by simultaneous measurements of reaction and coking. The presence of extraframework iron species, generated during steam treatment, is essential for producing active catalysts. The nature and distribution of extraframework iron species, the zeolite host, and the catalyst acidity have no significant effect on the initial propene yield, but strongly influence the deactivation behavior. FeZSM-5 shows a remarkable resistance against deactivation, while a very rapid and complete deactivation occurred in FeBEA. The initial propene yield over FeZSM-5 and FeBEA was remarkably higher than over commercial H-ZSM-5 and H-BEA (with 30–40 times lower iron content), although the residual activities after 400 min on stream were very similar. [Copyright &y& Elsevier]

An accessibility index (ACI) was derived from infrared spectroscopy of substituted alkylpyridines with different size (pyridine: 0.57nm, 2,6-lutidine: 0.67nm, and 2,4,6-collidine: 0.74nm) over hierarchical porous ZSM-5 crystals. The samples were prepared by selective silicon extraction of a commercial sample in NaOH (desilication), and they contained different degrees of intracrystalline mesoporosity. Our results demonstrate the enhanced accessibility of Brønsted acid sites in the hierarchical zeolites. For example, a relatively bulky molecule such as collidine, which probes practically no acid site of the parent medium-pore MFI structure, can access up to 40% of the acid sites in the mesoporous sample. The ACI is a powerful tool to standardize acid site accessibility in zeolites, and can be used to rank the effectiveness of synthetic strategies to prepare hierarchical zeolites. [Copyright &y& Elsevier]

A temporal analysis of products (TAP) reactor was used to study relationships between the mechanism of direct N2O decomposition over metal-loaded zeolites and their resulting activity. Rh-ZSM-5 (prepared by incipient wetness) and Fe-ZSM-5 (prepared by liquid-ion exchange) were chosen as prototypic catalysts displaying low (<550 K) and high (>650 K) temperature activity, respectively. Transient studies at the same contact time revealed the higher activity of Rh-ZSM-5 below 623 K and significantly stronger N2O adsorption over Rh species than over Fe species in the zeolites. Several microkinetic models were applied for simultaneous fitting the transient responses of N2O, N2, and O2. Classical reaction schemes failed to describe the experimental data. The preferred models of N2O decomposition over Rh-ZSM-5 and Fe-ZSM-5 differ in the reaction pathways of O2 formation. For both catalysts, free active metal sites (*) and those occupied by monoatomic oxygen species (*-O) from N2O participate in the decomposition of gas-phase N2O. Gas-phase O2 is formed directly on N2O interaction with *-O over Rh-ZSM-5, whereas the latter reaction over Fe-ZSM-5 leads to a surface bi-atomic oxygen species (O-*-O), followed by its transformation to *-O2. The latter species desorbs as molecular oxygen. Comparison of ion-exchanged and steam-activated Fe-ZSM-5 [J. Phys. Chem. B 110 (2006) 22586] revealed that the reaction mechanism is independent of the iron constitution induced by the preparation and activation routes, despite important differences in catalytic activity. Our quantitative microkinetic analysis demonstrated that both the stronger reversible N2O adsorption and, most importantly, the faster desorption of O2 are distinctive mechanistic features of Rh-ZSM-5, likely indicating its high de-N2O activity. [Copyright &y& Elsevier]