N2O decomposition over Fe/ZSM-5: reversible generation of highly active cationic Fe species

Thermodynamics of Water-Cationic Species-Framework Guest-Host Interactions within Transition Metal Ion-Exchanged Mordenite Relevant to Selective Anaerobic Oxidation of Methane to Methanol

Low-temperature anaerobic methane conversion to methanol (MTM) using copper ion-exchanged mordenite (Cu-MOR) as the catalyst and water as the sole source of oxygen is promising for sustainable utilization of methane. Integrating in situ calorimetric, spectroscopic, and structural methodologies, we report a systematic study on energetics of water-cationic species-framework guest-host interactions as a function of water loading for several mordenites relevant to low-temperature MTM. Notably, the near-zero coverage hydration enthalpy on Cu-MOR is -133.1 ± 6.0 kJ/mol water, which is related to Cu-MOR regeneration using water as oxidant. The copper oxo sites are thermally stable up to 915 °C and remain chemically intact as an oxygen source after complete hydration and dehydration. This study underscores the importance of manipulating the oxidation state and coordination chemistry of transition metal guest species in zeolites by fine-tuning the partial pressure of water as a strategy for rational design, synthesis, and modification of catalysts.

Investigating the Influence of Fe Speciation on N2O Decomposition Over Fe-ZSM-5 Catalysts

The influence of Fe speciation on the decomposition rates of N₂O over Fe-ZSM-5 catalysts prepared by Chemical Vapour Impregnation were investigated. Various weight loadings of Fe-ZSM-5 catalysts were prepared from the parent zeolite H-ZSM-5 with a Si:Al ratio of 23 or 30. The effect of Si:Al ratio and Fe weight loading was initially investigated before focussing on a single weight loading and the effects of acid washing on catalyst activity and iron speciation. UV/Vis spectroscopy, surface area analysis, XPS and ICP-OES of the acid washed catalysts indicated a reduction of ca. 60% of Fe loading when compared to the parent catalyst with a 0.4 wt% Fe loading. The TOF of N₂O decomposition at 600 °C improved to 3.99 × 10³ s^-1 over the acid washed catalyst which had a weight loading of 0.16%, in contrast, the parent catalyst had a TOF of 1.60 × 10³ s^-1. Propane was added to the gas stream to act as a reductant and remove any inhibiting oxygen species that remain on the surface of the catalyst. Comparison of catalysts with relatively high and low Fe loadings achieved comparable levels of N₂O decomposition when propane is present. When only N₂O is present, low metal loading Fe-ZSM-5 catalysts are not capable of achieving high conversions due to the low proximity of active framework Fe³⁺ ions and extra-framework ɑ-Fe species, which limits oxygen desorption. Acid washing extracts Fe from these active sites and deposits it on the surface of the catalyst as Fe_xO_y, leading to a drop in activity. The Fe species present in the catalyst were identified using UV/Vis spectroscopy and speculate on the active species. We consider high loadings of Fe do not lead to an active catalyst when propane is present due to the formation of Fe_xO_y nanoparticles and clusters during catalyst preparation. These are inactive species which lead to a decrease in overall efficiency of the Fe ions and consequentially a lower TOF.

One-pot synthesis of hierarchical FeZSM-5 zeolites from natural aluminosilicates for selective catalytic reduction of NO by NH3

Iron-modified ZSM-5 zeolites (FeZSM-5s) have been considered to be a promising catalyst system to reduce nitrogen oxide emissions, one of the most important global environmental issues, but their synthesis faces enormous economic and environmental challenges. Herein we report a cheap and green strategy to fabricate hierarchical FeZSM-5 zeolites from natural aluminosilicate minerals via a nanoscale depolymerization-reorganization method. Our strategy is featured by neither using any aluminum-, silicon-, or iron-containing inorganic chemical nor involving any mesoscale template and any post-synthetic modification. Compared with the conventional FeZSM-5 synthesized from inorganic chemicals with the similar Fe content, the resulting hierarchical FeZSM-5 with highly-dispersed iron species showed superior catalytic activity in the selective catalytic reduction of NO by NH3.

Anchoring Fe Ions to Amorphous and Crystalline Oxides: A Means To Tune the Degree of Fe Coordination

We report on an IR spectroscopic study on the room-temperature adsorption of NO on different iron(II)-containing siliceous matrices. Fe2+ hosted inside the channels of MFI-type zeolites (Fe-ZSM-5 and Al-free Fe-silicalite) exhibits pronounced coordinative unsaturation, as witnessed by the capability to form, at 300 K, [Fe2-(NO)], [Fe2+(NO)2] and [Fe2+(NO)3] complexes with increasing NO equilibrium pressure. Fe2+ hosted on amorphous supports (high surface area SiO2 and MCM-41) sinks more deeply into the surface of the siliceous support and thus exhibits less pronounced coordinative unsaturation: only [Fe2+(NO)2] complexes were observed, even at the highest investigated NO equilibrium pressures. Activation at higher temperature (1073 K) of the Al-free Fe-silicalite sample resulted in the appearance of Fe2+ species similar to those observed on SiO2 and MCM-41, and this suggests that local (since not detectable by X-ray diffraction) amorphisation of the environment around Fe2+ anchoring sites occurs. The fact that this behaviour is not observed on the Fe-ZSM-5 sample activated at the same temperature suggests that framework Al species (and their negatively charged oxygen environment) have an important role in anchoring extraframework Fe2+ species. Such an anchoring phenomenon will prevent a random migration of iron species, with subsequent aggregation and loss of coordinative unsaturation. These observations can thus explain the higher catalytic activity of the Fe-ZSM-5 system in one-step benzene to phenol conversion when compared with the parent, Al-free, Fe-silicalite system with similar Fe content. The nature of the support and the activation temperature can therefore be used as effective means to tune the degree of Fe coordination.