Effect of the Nuclearity and Coordination of Cu and Fe Sites in β Zeolites on the Oxidation of Hydrocarbons

Abstract

Cu and Fe zeolites can activate oxygen for oxidative reactions of methane and stable hydrocarbon molecules. Here, we analyzed the activity of Cu and Fe sites with various nuclearities and coordinations in *BEA zeolites toward complete oxidation of methane, toluene, and ethanol by molecular oxygen. Spectral analysis of the interaction of methane with the individual Cu and Fe sites and a kinetic study show that the oxidation of methane is catalyzed by different active sites with different activation energies. In the low-temperature region (225–325 °C), the activity is primarily caused by reactive bridging oxygen atoms in dimeric Cu and Fe clusters. Isolated Fe3+-oxo ions in the octahedral coordination and Cu2+ partially charge-balanced by the framework with less rigid redox states contribute to the activity at higher temperatures of ≥350 °C. The isolated Fe3+-oxo ions tetrahedrally coordinated to the framework oxygen and the bare Cu2+ ions charge-balanced exclusively by the framework are strongly stabilized and cannot act as active sites. Oxidation of the hydrocarbons over both the dimeric and isolated Cu and Fe active sites is nonspecific with practically barrierless consecutive reactions of already activated molecules and therefore fully CO2-selective for all of the hydrocarbons in an excess of oxygen.

Low‐Temperature, Ambient Pressure Oxidation of Methane to Methanol Over Every Tri‐Iron Node in a Metal–Organic Framework Material

“…Synthetic heterogeneous catalyst design in this context has been heavily focused on Fe‐ and Cu‐exchanged zeolites, [4–8] in part inspired by existing proposals for active site structures prevalent in monooxygenase enzymes [9–11] . The study and development of iron‐modified zeolite catalysts, however, is significantly hampered by the highly heterogenous nature of iron speciation, the distribution and prevalence of which is contingent on many factors such as synthesis procedure, activation method, and total iron content (Table S1) [12–15] . For example, Fe−O, Fe−Fe, and Fe−Si/Al distances compiled from EXAFS data reported in the Fe‐ZSM‐5 literature evidence a range of first‐shell distances even at the lowest iron loadings evaluated (0.2 wt %, Figure S1).…”

“…[9][10][11] The study and development of iron-modified zeolite catalysts, however,i ss ignificantlyh ampered by the highly heterogenous natureo fi ron speciation, the distribution and prevalence of which is contingent on many factors such as synthesis procedure, activationm ethod, and total iron content (Table S1). [12][13][14][15] For example, FeÀO, FeÀFe, and FeÀSi/Al distances compiled from EXAFSd ata reported in the Fe-ZSM-5l iterature evidencearange of first-shell distances even at the lowest iron loadings evaluated (0.2 wt %, FigureS1). Such heterogeneity in site speciation not only leads to limitations in the decipherability of active site structure but also renders merely the differentiation of active sites from inactive ones extremely challenging!W ea ssert that despite decades of research,e vidence of ah eterogeneous iron-based catalystt hat is comprised exclusivelyo fi ron moieties that are active in the low-temperature conversion of methane to methanol has not been reported thus far in the literature.…”

Analysis of NH3‐TPD Profiles for CuSSZ‐13 SCR Catalyst of Controlled Al Distribution – Complexity Resolved by First Principles Thermodynamics of NH3Desorption, IR and EPR Insight into Cu Speciation**

“…[34] In turn, the cationic sites containing two Al atoms can stabilize bare Cu 2 + ions or Cu bridging species. [54] Due to the geometric constraints of the six-ring controlling the access to the Cu 2 + ions located in the ω, ω 3Si sites and pseudo octahedral coordination of Cu 2 + ions in these sites to the framework oxygens, the Cu 2 + ions in these sites can be supposed to adsorb guest molecules with difficulties.…”

Splitting dioxygen over distant binuclear transition metal cationic sites in zeolites. Effect of the transition metal cation

“…8000 fs) when the system fluctuated around the equilibrium and "snapshots" were collected and optimized. Similar time lengths were used for MD simulations of cationic sites in zeolites [4,15,[40][41][42]. The MD simulations serve to obtain the rearranged local structures (details are provided in our prior studies [14,24]).…”