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1. Mechanism of selective benzene hydroxylation catalyzed by iron-containing zeolites

   https://doi.org/10.1073/pnas.1813849115  

   Snyder, Benjamin E.; Bols, Max L.; Rhoda, Hannah M.; Vanelderen, Pieter; Böttger, Lars H.; Braun, Augustin; Yan, James J.; Hadt, Ryan G.; Babicz, Jeffrey T.; Hu, Michael Y.; et al (November 2018, Proceedings of the National Academy of Sciences)

   A direct, catalytic conversion of benzene to phenol would have wide-reaching economic impacts. Fe zeolites exhibit a remarkable combination of high activity and selectivity in this conversion, leading to their past implementation at the pilot plant level. There were, however, issues related to catalyst deactivation for this process. Mechanistic insight could resolve these issues, and also provide a blueprint for achieving high performance in selective oxidation catalysis. Recently, we demonstrated that the active site of selective hydrocarbon oxidation in Fe zeolites, named α-O, is an unusually reactive Fe(IV)=O species. Here, we apply advanced spectroscopic techniques to determine that the reaction of this Fe(IV)=O intermediate with benzene in fact regenerates the reduced Fe(II) active site, enabling catalytic turnover. At the same time, a small fraction of Fe(III)-phenolate poisoned active sites form, defining a mechanism for catalyst deactivation. Density-functional theory calculations provide further insight into the experimentally defined mechanism. The extreme reactivity of α-O significantly tunes down (eliminates) the rate-limiting barrier for aromatic hydroxylation, leading to a diffusion-limited reaction coordinate. This favors hydroxylation of the rapidly diffusing benzene substrate over the slowly diffusing (but more reactive) oxygenated product, thereby enhancing selectivity. This defines a mechanism to simultaneously attain high activity (conversion) and selectivity, enabling the efficient oxidative upgrading of inert hydrocarbon substrates. 

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2. Structural characterization of a non-heme iron active site in zeolites that hydroxylates methane

   https://doi.org/10.1073/pnas.1721717115  

   Snyder, Benjamin E.; Böttger, Lars H.; Bols, Max L.; Yan, James J.; Rhoda, Hannah M.; Jacobs, Ariel B.; Hu, Michael Y.; Zhao, Jiyong; Alp, E. Ercan; Hedman, Britt; et al (March 2018, Proceedings of the National Academy of Sciences)

   Iron-containing zeolites exhibit unprecedented reactivity in the low-temperature hydroxylation of methane to form methanol. Reactivity occurs at a mononuclear ferrous active site, α-Fe(II), that is activated by N2O to form the reactive intermediate α-O. This has been defined as an Fe(IV)=O species. Using nuclear resonance vibrational spectroscopy coupled to X-ray absorption spectroscopy, we probe the bonding interaction between the iron center, its zeolite lattice-derived ligands, and the reactive oxygen. α-O is found to contain an unusually strong Fe(IV)=O bond resulting from a constrained coordination geometry enforced by the zeolite lattice. Density functional theory calculations clarify how the experimentally determined geometric structure of the active site leads to an electronic structure that is highly activated to perform H-atom abstraction. 

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3. Cage effects control the mechanism of methane hydroxylation in zeolites

   https://doi.org/10.1126/science.abd5803  

   Snyder, Benjamin E.; Bols, Max L.; Rhoda, Hannah M.; Plessers, Dieter; Schoonheydt, Robert A.; Sels, Bert F.; Solomon, Edward I. (July 2021, Science)

   null (Ed.)

   Catalytic conversion of methane to methanol remains an economically tantalizing but fundamentally challenging goal. Current technologies based on zeolites deactivate too rapidly for practical application. We found that similar active sites hosted in different zeolite lattices can exhibit markedly different reactivity with methane, depending on the size of the zeolite pore apertures. Whereas zeolite with large pore apertures deactivates completely after a single turnover, 40% of active sites in zeolite with small pore apertures are regenerated, enabling a catalytic cycle. Detailed spectroscopic characterization of reaction intermediates and density functional theory calculations show that hindered diffusion through small pore apertures disfavors premature release of CH 3 radicals from the active site after C-H activation, thereby promoting radical recombination to form methanol rather than deactivated Fe-OCH 3 centers elsewhere in the lattice. 

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4. Cooperative Fe sites on transition metal (oxy)hydroxides drive high oxygen evolution activity in base

   https://doi.org/10.1038/s41467-023-43305-z  

   Ou, Yingqing; Twight, Liam P; Samanta, Bipasa; Liu, Lu; Biswas, Santu; Fehrs, Jessica L; Sagui, Nicole A; Villalobos, Javier; Morales-Santelices, Joaquín; Antipin, Denis; et al (December 2023, Nature Communications)

   Fe-containing transition-metal (oxy)hydroxides are highly active oxygen-evolution reaction (OER) electrocatalysts in alkaline media and ubiquitously form across many materials systems. The complexity and dynamics of the Fe sites within the (oxy)hydroxide have slowed understanding of how and where the Fe-based active sites form—information critical for designing catalysts and electrolytes with higher activity and stability. We show that where/how Fe species in the electrolyte incorporate into host Ni or Co (oxy)hydroxides depends on the electrochemical history and structural properties of the host material. Substantially less Fe is incorporated from Fe-spiked electrolyte into Ni (oxy)hydroxide at anodic potentials, past the nominally Ni(2+/3+) redox wave, compared to during potential cycling. The Fe adsorbed under constant anodic potentials leads to impressively high per-Fe OER turn-over frequency (TOF(Fe)) of ~40 s−1 at 350 mV overpotential which we attribute to under-coordinated “surface” Fe. By systematically controlling the concentration of surface Fe, we find TOF(Fe) increases linearly with the Fe concentration. This suggests a changing OER mechanism with increased Fe concentration, consistent with a mechanism involving cooperative Fe sites in FeOx clusters. 

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5. Two-Dimensional Zeolite Materials: Structural and Acidity Properties

   https://doi.org/10.3390/ma13081822  

   Schulman, Emily; Wu, Wei; Liu, Dongxia (April 2020, Materials)

   Zeolites are generally defined as three-dimensional (3D) crystalline microporous aluminosilicates in which silicon (Si4+) and aluminum (Al3+) are coordinated tetrahedrally with oxygen to form large negative lattices and consequent Brønsted acidity. Two-dimensional (2D) zeolite nanosheets with single-unit-cell or near single-unit-cell thickness (~2–3 nm) represent an emerging type of zeolite material. The extremely thin slices of crystals in 2D zeolites produce high external surface areas (up to 50% of total surface area compared to ~2% in micron-sized 3D zeolite) and expose most of their active sites on external surfaces, enabling beneficial effects for the adsorption and reaction performance for processing bulky molecules. This review summarizes the structural properties of 2D layered precursors and 2D zeolite derivatives, as well as the acidity properties of 2D zeolite derivative structures, especially in connection to their 3D conventional zeolite analogues’ structural and compositional properties. The timeline of the synthesis and recognition of 2D zeolites, as well as the structure and composition properties of each 2D zeolite, are discussed initially. The qualitative and quantitative measurements on the acid site type, strength, and accessibility of 2D zeolites are then presented. Future research and development directions to advance understanding of 2D zeolite materials are also discussed. 

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