Abstract
Divalent iron sites in tri-iron oxo-centered metal nodes in metal–organic frameworks (MOFs) catalyze light alkane oxidation. The first two steps of the reaction sequence, which are also the most energetically demanding ones, are the formation of the active species, Fe(IV)═O, by N2O decomposition and subsequent C–H bond cleavage. We have employed Kohn–Sham density functional methods to explore how modification of the microenvironment around the Fe(II) center can modulate its catalytic activity, akin to what noted in metalloenzymes. We have varied the substituents on the organic linker of the MIL-101(Fe) MOF, as a way to modulate the energy barriers associated with the first two steps of the methane to methanol reaction. The calculations show that varying substituents has a minimal electronic effect on the iron center and its first coordination shell. However, their proximity to the active site can modify the barriers by 20%. Hydrogen bond donors can lower both barriers, such that the resulting Fe(IV)═O species are simultaneously more stable and more reactive than those of the parent MOF. The screening of a large set of systems allowed us to establish rules for the selection of second coordination shell elements to improve the reactivity of oxoferryl-based catalysts: (i) functionality with a low pKa or large positive electrostatic potential, (ii) a distance around 1.5 Å between the oxoferryl and any atom of the ring substituent, and (iii) low conformational flexibility of the added substituent.
Highlights
Despite decades of active research, the quest for an efficient catalyst for the direct conversion of methane to methanol (MTM) is still ongoing.1 Di-iron active sites in methane monoxygenases are able to convert methane to methanol selectively at room temperature and atmospheric pressure.2,3 Among synthetic systems, single Fe(II) sites (α-Fe(II) sites) in iron-based zeolites can hydroxylate methane at room temperature.2,4,5 iron centers are hosted as extra framework species in the zeolites pores: this makes them intrinsically disordered, with the copresence of several possible species besides α-Fe(II).6Metal organic frameworks (MOFs) are a class of hybrid organic−inorganic materials characterized by a modular architecture.7 It is theoretically possible to choose separately the organic and the inorganic components to tailor their structure for a specific application
metal−organic frameworks (MOFs) have been demonstrated to be active in C−H bond activation,8−18 exploiting metal centers hosted in their pores as extra- or intraframework species
Triiron oxo-based MOFs catalyze various reactions involving C−H bond activation.23−27 We have recently identified them as promising catalysts for the oxidation of methane to methanol and ethane to ethanol, based on Kohn−Sham density functional (KS-DFT) calculations using N2O as the oxygen source
Summary
Despite decades of active research, the quest for an efficient catalyst for the direct conversion of methane to methanol (MTM) is still ongoing. Di-iron active sites in methane monoxygenases are able to convert methane to methanol selectively at room temperature and atmospheric pressure. Among synthetic systems, single Fe(II) sites (α-Fe(II) sites) in iron-based zeolites can hydroxylate methane at room temperature. iron centers are hosted as extra framework species in the zeolites pores: this makes them intrinsically disordered, with the copresence of several possible species besides α-Fe(II).6Metal organic frameworks (MOFs) are a class of hybrid organic−inorganic materials characterized by a modular architecture. It is theoretically possible to choose separately the organic (linker) and the inorganic (metal node) components to tailor their structure for a specific application. Iron centers are hosted as extra framework species in the zeolites pores: this makes them intrinsically disordered, with the copresence of several possible species besides α-Fe(II).. Most MOFs are crystalline solids, exhibiting nearly homogeneous physical and chemical properties All these features place MOFs as catalysts at the boundary between homogeneous and heterogeneous catalysis. MOFs have been demonstrated to be active in C−H bond activation,− exploiting metal centers hosted in their pores as extra- or intraframework species. While the former present the same problems of heterogeneity of species observed in zeolites, catalytic centers as part of the MOFs framework offer the possibility to tailor more carefully the active sites. The α-Fe(II) sites in zeolites are single iron sites characterized by high-spin ground states and constrained square planar coordination geometries. It is possible to introduce similar species in a MOF.
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