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1. Advances in the synthesis, characterisation, and mechanistic understanding of active sites in Fe-zeolites for redox catalysts

   https://doi.org/10.1039/D0DT01857K  

   Bols, Max L.; Rhoda, Hannah M.; Snyder, Benjamin E.; Solomon, Edward I.; Pierloot, Kristine; Schoonheydt, Robert A.; Sels, Bert F. (November 2020, Dalton Transactions)

   null (Ed.)

   The recent research developments on the active sites in Fe-zeolites for redox catalysis are discussed. Building on the characterisation of the α-Fe/α-O active sites in the beta and chabazite zeolites, we demonstrate a bottom-up approach to successfully understand and develop Fe-zeolite catalysts. We use the room temperature benzene to phenol reaction as a relevant example. We then suggest how the spectroscopic identification of other monomeric and dimeric iron sites could be tackled. The challenges in the characterisation of active sites and intermediates in NO X selective catalytic reduction catalysts and further development of catalysts for mild partial methane oxidation are briefly discussed. 

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

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

   Snyder, Benjamin E. R.; Bols, Max L.; Rhoda, Hannah M.; Vanelderen, Pieter; Böttger, Lars H.; Braun, Augustin; Yan, James J.; Hadt, Ryan G.; Babicz, Jr., 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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3. Mechanistic understanding of methane-to-methanol conversion on graphene-stabilized single-atom iron centers

   https://doi.org/10.1039/D1CY00826A  

   Hong, Sungil; Mpourmpakis, Giannis (January 2021, Catalysis Science & Technology)

   null (Ed.)

   The functionalization of methane to value-added liquid chemicals remains as one of the “grand challenges” in chemistry. In this work, we provide insights into the direct methane-to-methanol conversion mechanisms with H 2 O 2 as an oxidant on single Fe-atom centers stabilized on N-functionalized graphene, using first principles calculations. By investigating a series of different reaction paths on various active centers and calculating their turnover frequencies, we reveal that a H 2 O 2 -mediated radical mechanism and a Fenton-type mechanism are energetically the most plausible pathways taking place on di- and mono-oxo centers, respectively. Due to the thermodynamic preference of the mono-oxo center formation over the di-oxo under reaction conditions, the Fenton-type mechanism appears to determine the overall catalytic activity. On the other hand, the hydroxy(oxo) center, which is thermodynamically the most favorable center, is found to be catalytically inactive. Hence, the high activity is attributed to a fine balance of keeping the active centers as oxo-species during the reaction. Moreover, we reveal that the presence of solvent (water) can accelerate or slow down different pathways with the overall turnover of the dominant Fenton-type reaction being decreased. Importantly, this work reveals the nature of active sites and a gamut of reaction mechanisms for the direct conversion of methane to methanol rationalizing experimental observations and aiding the search for room temperature catalysts for methane conversion to liquid products. 

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4. Oxygenates production in a microfluidic dielectric barrier discharge device sustained in Ar/CH4/O2

   https://doi.org/10.1063/5.0239464  

   Meyer, Mackenzie; Hartman, Ryan; Kushner, Mark J (January 2025, Journal of Applied Physics)

   Reforming of methane (CH4) is a process to produce syngas (CO/H2) and other value-added chemicals including oxygenates such as methanol (CH3OH). Atmospheric pressure plasmas have the potential to be more energy efficient than traditional reforming methods as value-added chemicals can be synthesized directly in the plasma without requiring an additional step. In this paper, we discuss the results from a computational investigation of the formation of oxygenates by CH4 oxidation in the presence of Ar, including CH3OH and CH2O, in a nanosecond pulsed dielectric barrier discharge. The plasma is formed in a microfluidic channel whose small dimensions are ideal for plasma formation at atmospheric pressure. The production and consumption mechanisms of dominant radicals and long-lived species are discussed in detail for the base case conditions of Ar/CH4/O2 = 50/25/25. CH3OH is produced primarily by CH3O reacting with CH3O and CH3O2 reacting with OH, while CH2O formation relies on reactions involving CH3O and CH3. The most abundant oxygenate formed is CO (produced by H abstraction from CHO). However, the greenhouse gas CO2 is also formed as a by-product. The effects of gas mixture are examined to maximize the CH3OH and CH2O densities while decreasing the CO2 density. Increasing the Ar percentage from 0% to 95% decreased the CH3OH and CH2O densities. At low Ar percentages, this is due to an increase in consumption of CH3OH and CH2O, while at high Ar percentages (>40% Ar), the production of CH3OH and CH2O is decreased. However, both CO and CO2 reached peak densities at 70%–90% Ar. Changing the CH4/O2 ratio while keeping 50% Ar in the discharge led to increased CH3OH and CH2O production, reaching peak densities at 35%–40% CH4. The CO and CO2 densities decreased beyond 20% CH4, indicating that a CH4 rich discharge is ideal for forming the desired oxygenates. 

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5. Controlling Product Selectivity During Dioxygen Reduction with Mn Complexes Using Pendent Proton Donor Relays and Added Base

   https://doi.org/10.1039/D3SC02611F  

   Cook, Emma N.; Courter, Ian M.; Dickie, Diane A.; Machan, Charles W. (January 2024, Chemical Science)

   The catalytic reduction of dioxygen (O2) is important in biological energy conversion and alternative energy applications. In comparison to Fe- and Co-based systems, examples of catalytic O2 reduction by homogeneous Mn-based systems is relatively sparse. Motivated by this lack of knowledge, two Mn-based catalysts for the oxygen reduction reaction (ORR) containing a bipyridine-based non-porphyrinic ligand framework have been developed to evaluate how pendent proton donor relays alter activity and selectivity for the ORR, where Mn(p-tbudhbpy)Cl (1) was used as a control complex and Mn(nPrdhbpy)Cl (2) contains a pendent –OMe group in the secondary coordination sphere. Using an ammonium-based proton source, N,N′-diisopropylethylammonium hexafluorophosphate, we analyzed catalytic activity for the ORR: 1 was found to be 64% selective for H2O2 and 2 is quantitative for H2O2, with O2 binding to the reduced Mn(II) center being the rate-determining step. Upon addition of the conjugate base, N,N′-diisopropylethylamine, the observed catalytic selectivity of both 1 and 2 shifted to H2O as the primary product. Interestingly, while the shift in selectivity suggests a change in mechanism for both 1 and 2, the catalytic activity of 2 is substantially enhanced in the presence of base and the rate-determining step becomes the bimetallic cleavage of the O–O bond in a Mn-hydroperoxo species. These data suggest that the introduction of pendent relay moieties can improve selectivity for H2O2 at the expense of diminished reaction rates from strong hydrogen bonding interactions. Further, although catalytic rate enhancements are observed with a change in product selectivity when base is added to buffer proton activity, the pendent relays stabilize dimer intermediates, limiting the maximum rate. 

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