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Title: Mechanism of selective benzene hydroxylation catalyzed by iron-containing zeolites
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.
Bols, Max L.; Rhoda, Hannah M.; Snyder, Benjamin E.; Solomon, Edward I.; Pierloot, Kristine; Schoonheydt, Robert A.; Sels, Bert F.(
, 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.
Isostructural Cr and Fe nanoporous MIL-101, synthesized without mineralizing agents, are investigated for styrene oxidation utilizing aqueous hydrogen peroxide to yield valuable oxygenates for chemical synthesis applications. Styrene conversion rates and oxygenate product distributions both depend on metal identity, as MIL-101(Fe) is more reactive for total styrene oxidation and is more pathway selective, preferring aldehyde (benzaldehyde) formation at the α-carbon to the aromatic ring, where MIL-101(Cr) sustains epoxide (styrene oxide) production at the same α-carbon. These pathways often involve hydrogen peroxide derived radical intermediates (O, –HOO˙, –HO − ˙) and metallocycle transition states. We postulate that the higher reactivity of one of these surface intermediates, Fe( iv )O relative to Cr( iv )O, leads to higher styrene oxidation rates for MIL-101(Fe), while higher electrophilicity of Cr( iii )–OOH intermediates translates to the higher styrene oxide selectivity observed for MIL-101(Cr). Secondary styrene oxide and benzaldehyde conversions are observed over both analogs, but the former is more prevalent over MIL-101(Fe) due to higher Lewis/Brønsted acid site density and strength compared to MIL-101(Cr). Recyclability experiments combined with characterization via XRD, SEM/EDXS, and FT-IR and UV-vis spectroscopies show that the nature of MIL-101(Fe) sites does not change significantly with each cycle, whereas MIL-101(Cr) suffers from metal leaching, which impacts styrene conversion rates and product distribution. Both catalysts require active site regeneration, though MIL-101(Fe) sites are more susceptible to reactivation, even under mild conditions. Finally, examination of styrene conversion for three unique synthesized phases of MIL-101(Cr) rationalizes that nodal defects are largely responsible for observed reactivity and selectivity but predispose the framework to metal leaching as a predominant deactivation mechanism.
Ethylene oxidation by Ag catalysts has been extensively
investigated over the past few decades, but many key fundamental issues
about this important catalytic system are still unresolved. This overview of
the selective oxidation of ethylene to ethylene oxide by Ag catalysts critically
examines the experimental and theoretical literature of this complex
catalytic system: (i) the surface chemistry of silver catalysts (single crystal,
powder/foil, and supported Ag/α-Al2O3), (ii) the role of promoters, (iii)
the reaction kinetics, (iv) the reaction mechanism, (v) density functional
theory (DFT), and (vi) microkinetic modeling. Only in the past few years
have the modern catalysis research tools of in situ/operando spectroscopy
and DFT calculations been applied to begin establishing fundamental
structure−activity/selectivity relationships. This overview of the ethylene
oxidation reaction by Ag catalysts covers what is known and what issues still
need to be determined to advance the rational design of this important catalytic system.
Catalytic conversion of CH4 to transportable liquid hydrocarbons via partial oxidation is a promising avenue towards efficient utilization of natural gas. Single Fe atoms on N-functionalized graphene (FeN4/GN) have recently been shown to be active for partial CH4 oxidation with H2O2 at room temperature. Here, density functional theory (DFT) calculations combined with mean-field microkinetic modeling (MKM) have been applied to obtain kinetic understanding of partial CH4 oxidation with H2O2 to CH3OH and CH3OOH over FeN4/GN. CH3OH and CH3OOH are found to be minor and major reaction products, respectively, with a selectivity in agreement with reported experimental data. The kinetic modeling reveals two pathways for CH3OH production together with a dominant catalytic cycle for CH3OOH formation. The selectivity is found to be sensitive to the temperature and H2O2 concentration, with the CH3OH selectivity increasing with increasing temperature and decreasing H2O2 concentration. Turnover frequencies of both CH3OH and CH3OOH are found to decrease over time, due to a change in the Fe formal oxidation state from +6 to +4; Fe(+6) is more active, but less stable than Fe(+4). The present work unravels the detailed reaction mechanism for partial oxidation of methane by FeN4/GN, rationalizes experimental observations and provides guidance for efficient room-temperature methane conversion by single-atom Fe-catalysts.
The effect of olefin addition to a stream of dimethyl ether on the methanol homologation reaction is investigated using iron-substituted zeolites Fe-beta and Fe-ZSM-5. The reaction was investigated using plug-flow microreactors in the temperature range of 240-400 degrees C, at a total pressure of 0.239 MPa and a WHSV of 6.12 (g DME/ gcat-hr). For Fe-beta (Si/Fe= 9.2) catalysts, isobutene co-feeding almost doubles dimethyl ether (DME) consumption rate and shifts selectivity towards larger olefins with carbon numbers from 5 to 7. Addition of isobutene above 6.3%, however, resulted in a reduction of DME consumption rates, an effect assigned to the replacement of surface methoxy groups for adsorbed olefins in the zeolite pores. Below a temperature of 340 degrees C hydride-transfer rates are negligible; reaction rates are stable for over 5.5 h and the products consist almost exclusively of olefins and a small amount of methane. Above 360 degrees C the onset of catalytic hydride transfer processes is observed leading to fast catalyst deactivation rates and an increase in the concentration of aromatic species. Iron ZSM-5 (Si/Fe = 21.4) catalysts under similar reaction conditions consumes methanol faster than Febeta at approximately three times the TOF (on a per iron basis). The Fe-ZSM-5 catalyst was selective to a distribution of products (C5 to C8) as compared to Fe-beta which was selective to primarily C5 and C7. Co-feeding larger olefins (2-methyl-2-butene, 2,3-dimethyl-2-butene, 2,3,3-trimethyl-1-butene, and 2,4,4-trimethyl-2-pentene) at a 3.9% olefin concentration over Fe-beta changed selectivity towards cracking products (C4 compounds such as isobutene). As the size of the olefin increases, a reduction of DME consumption rate is also observed. These results show that co-feeding olefins with DME over Fe-zeolites is a promising route to increase methylation rates at relatively low temperatures producing larger branched olefins and that the product distribution is highly dependent on the zeolite pore size and structure of the olefin.
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., Zhao, Jiyong, Alp, E. Ercan, Hedman, Britt, Hodgson, Keith O., Schoonheydt, Robert A., Sels, Bert F., and Solomon, Edward I. Mechanism of selective benzene hydroxylation catalyzed by iron-containing zeolites. Proceedings of the National Academy of Sciences 115.48 Web. doi: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., Zhao, Jiyong, Alp, E. Ercan, Hedman, Britt, Hodgson, Keith O., Schoonheydt, Robert A., Sels, Bert F., & Solomon, Edward I. Mechanism of selective benzene hydroxylation catalyzed by iron-containing zeolites. Proceedings of the National Academy of Sciences, 115 (48). 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., Zhao, Jiyong, Alp, E. Ercan, Hedman, Britt, Hodgson, Keith O., Schoonheydt, Robert A., Sels, Bert F., and Solomon, Edward I.
"Mechanism of selective benzene hydroxylation catalyzed by iron-containing zeolites". Proceedings of the National Academy of Sciences 115 (48). Japan: Proceedings of the National Academy of Sciences. https://doi.org/10.1073/pnas.1813849115.https://par.nsf.gov/biblio/10079579.
@article{osti_10079579,
place = {Japan},
title = {Mechanism of selective benzene hydroxylation catalyzed by iron-containing zeolites},
url = {https://par.nsf.gov/biblio/10079579},
DOI = {10.1073/pnas.1813849115},
abstractNote = {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.},
journal = {Proceedings of the National Academy of Sciences},
volume = {115},
number = {48},
publisher = {Proceedings of the National Academy of Sciences},
author = {Snyder, Benjamin E. R. and Bols, Max L. and Rhoda, Hannah M. and Vanelderen, Pieter and Böttger, Lars H. and Braun, Augustin and Yan, James J. and Hadt, Ryan G. and Babicz, Jr., Jeffrey T. and Hu, Michael Y. and Zhao, Jiyong and Alp, E. Ercan and Hedman, Britt and Hodgson, Keith O. and Schoonheydt, Robert A. and Sels, Bert F. and Solomon, Edward I.},
}
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