More Like this

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. 

   more » « less
2. 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. 

   more » « less
3. Oxo dicopper anchored on carbon nitride for selective oxidation of methane

   https://doi.org/10.1038/s41467-022-28987-1  

   Xie, Pengfei; Ding, Jing; Yao, Zihao; Pu, Tiancheng; Zhang, Peng; Huang, Zhennan; Wang, Canhui; Zhang, Junlei; Zecher-Freeman, Noah; Zong, Han; et al (December 2022, Nature Communications)

   Abstract Selective conversion of methane (CH 4 ) into value-added chemicals represents a grand challenge for the efficient utilization of rising hydrocarbon sources. We report here dimeric copper centers supported on graphitic carbon nitride (denoted as Cu 2 @C 3 N 4 ) as advanced catalysts for CH 4 partial oxidation. The copper-dimer catalysts demonstrate high selectivity for partial oxidation of methane under both thermo- and photocatalytic reaction conditions, with hydrogen peroxide (H 2 O 2 ) and oxygen (O 2 ) being used as the oxidizer, respectively. In particular, the photocatalytic oxidation of CH 4 with O 2 achieves >10% conversion, and >98% selectivity toward methyl oxygenates and a mass-specific activity of 1399.3 mmol g Cu −1 h −1 . Mechanistic studies reveal that the high reactivity of Cu 2 @C 3 N 4 can be ascribed to symphonic mechanisms among the bridging oxygen, the two copper sites and the semiconducting C 3 N 4 substrate, which do not only facilitate the heterolytic scission of C-H bond, but also promotes H 2 O 2 and O 2 activation in thermo- and photocatalysis, respectively. 

   more » « less
4. 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. 

   more » « less
5. “Soft” oxidative coupling of methane to ethylene: Mechanistic insights from combined experiment and theory

   https://doi.org/10.1073/pnas.2012666118  

   Liu, Shanfu; Udyavara, Sagar; Zhang, Chi; Peter, Matthias; Lohr, Tracy L.; Dravid, Vinayak P.; Neurock, Matthew; Marks, Tobin J. (June 2021, Proceedings of the National Academy of Sciences)

   The oxidative coupling of methane to ethylene using gaseous disulfur (2CH 4 + S 2 → C 2 H 4 + 2H 2 S) as an oxidant (SOCM) proceeds with promising selectivity. Here, we report detailed experimental and theoretical studies that examine the mechanism for the conversion of CH 4 to C 2 H 4 over an Fe 3 O 4 -derived FeS 2 catalyst achieving a promising ethylene selectivity of 33%. We compare and contrast these results with those for the highly exothermic oxidative coupling of methane (OCM) using O 2 (2CH 4 + O 2 → C 2 H 4 + 2H 2 O). SOCM kinetic/mechanistic analysis, along with density functional theory results, indicate that ethylene is produced as a primary product of methane activation, proceeding predominantly via CH 2 coupling over dimeric S–S moieties that bridge Fe surface sites, and to a lesser degree, on heavily sulfided mononuclear sites. In contrast to and unlike OCM, the overoxidized CS 2 by-product forms predominantly via CH 4 oxidation, rather than from C 2 products, through a series of C–H activation and S-addition steps at adsorbed sulfur sites on the FeS 2 surface. The experimental rates for methane conversion are first order in both CH 4 and S 2 , consistent with the involvement of two S sites in the rate-determining methane C–H activation step, with a CD 4 /CH 4 kinetic isotope effect of 1.78. The experimental apparent activation energy for methane conversion is 66 ± 8 kJ/mol, significantly lower than for CH 4 oxidative coupling with O 2 . The computed methane activation barrier, rate orders, and kinetic isotope values are consistent with experiment. All evidence indicates that SOCM proceeds via a very different pathway than that of OCM. 

   more » « less