Copper‐exchanged zeolites are useful for stepwise conversion of methane to methanol at moderate temperatures. This process also generates some over‐oxidation products like CO and CO2. However, mechanistic pathways for methane over‐oxidation by copper‐oxo active sites in these zeolites have not been previously described. Adequate understanding of methane over‐oxidation is useful for developing systems with higher methanol yields and selectivities. Here, we use density functional theory (DFT) to examine methane over‐oxidation by [Cu3O3]2+active sites in zeolite mordenite MOR. The methyl group formed after activation of a methane C−H bond can be stabilized at a μ‐oxo atom of the active site. This μ‐(O−CH3) intermediate can undergo sequential hydrogen atom abstractions till eventual formation of a copper‐monocarbonyl species. Adsorbed formaldehyde, water and formates are also formed during this process. The overall mechanistic path is exothermic, and all intermediate steps are facile at 200 °C. Release of CO from the copper‐monocarbonyl costs only 3.4 kcal/mol. Thus, for high methanol selectivities, the methyl group from the first hydrogen atom abstraction step
Methane over‐oxidation by copper‐exchanged zeolites prevents realization of high‐yield catalytic conversion. However, there has been little description of the mechanism for methane over‐oxidation at the copper active sites of these zeolites. Using density functional theory (DFT) computations, we reported that tricopper [Cu3O3]2+active sites can over‐oxidize methane. However, the role of [Cu3O3]2+sites in methane‐to‐methanol conversion remains under debate. Here, we examine methane over‐oxidation by dicopper [Cu2O]2+and [Cu2O2]2+sites using DFT in zeolite mordenite (MOR). For [Cu2O2]2+, we considered the μ‐(η2:η2) peroxo‐, and bis(μ‐oxo) motifs. These sites were considered in the eight‐membered (8MR) ring of MOR. μ‐(η2:η2) peroxo sites are unstable relative to the bis(μ‐oxo) motif with a small interconversion barrier. Unlike [Cu2O]2+which is active for methane C−H activation, [Cu2O2]2+has a very large methane C−H activation barrier in the 8MR. Stabilization of methanol and methyl at unreacted dicopper sites however leads to over‐oxidation via sequential hydrogen atom abstraction steps. For methanol, these are initiated by abstraction of the CH3group, followed by OH and can proceed near 200 °C. Thus, for [Cu2O]2+and [Cu2O2]2+species, over‐oxidation is an inter‐site process. We discuss the implications of these findings for methanol selectivity, especially in comparison to the intra‐site process for [Cu3O3]2+sites and the role of Brønsted acid sites.
more » « less- Award ID(s):
- 1800387
- NSF-PAR ID:
- 10448187
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- ChemPhysChem
- Volume:
- 22
- Issue:
- 24
- ISSN:
- 1439-4235
- Page Range / eLocation ID:
- p. 2517-2525
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract must be stabilizedaway from copper‐oxo active sites. Indeed, it must be quickly trapped at an unreactive site (short diffusion lengths) while avoiding copper‐oxo species (large paths between active sites). This stabilization of the methyl group away from the active sites is central to the high methanol selectivities obtained with stepwise methane‐to‐methanol conversion. -
The formation and reactivities of [Cu–O–M] 2+ species (M = Ti–Cu, Zr–Mo and Ru–Ag) in metal-exchanged zeolites, as well as stabilities of these species towards autoreduction by O 2 elimination are investigated with density functional theory. These species were investigated in zeolite mordenite in search of insights into active site formation mechanisms, the relationship between stability and reactivity as well as discovery of heterometallic species useful for isothermal methane-to-methanol conversion (MMC). Several [Cu–O–M] 2+ species (M = Ti–Cr and Zr–Mo) are substantially more stable than [Cu 2 O] 2+ . Other [Cu–O–M] 2+ species, (M = Mn–Ni and Ru–Ag) have similar formation energies to [Cu 2 O] 2+ , to within ±10 kcal mol −1 . Interestingly, only [Cu–O–Ag] 2+ is more active for methane activation than [Cu 2 O] 2+ . [Cu–O–Ag] 2+ is however more susceptible to O 2 elimination. By considering the formation energies, autoreduction, cost and activity towards the methane C–H bond, we can only conclude that [Cu 2 O] 2+ is best suited for MMC. Formation of [Cu 2 O] 2+ is initiated by proton transfer from aquo ligands to the framework and proceeds mostly via dehydration steps. Its μ-oxo bridge is formed via water-assisted condensation of two hydroxo groups. To evaluate the relationship between [Cu 2 O] 2+ and other active sites, we also examined the formation energies of other species. The formation energies follow the trend: isolated [Cu–OH] + < paired [Cu–OH] + < [Cu 2 O] 2+ < [Cu 3 O 3 ] 2+ . Inclusion of Gibbs free-energy corrections indicates activation temperatures of 257, 307 and 327 and 331 °C for isolated [Cu–OH] + , paired [Cu–OH] + , [Cu 2 O] 2+ and [Cu 3 O 3 ] 2+ , respectively. The provocative nature of the lower-than-expected activation temperature for isolated [Cu–OH] + species is discussed.more » « less
-
more » « less
Abstract Catalysis of
O ‐atom transfer (OAT) reactions is a characteristic of both natural (enzymatic) and synthetic molybdenum‐oxo and ‐peroxo complexes. These reactions can employ a variety of terminal oxidants, e. g. DMSO,N ‐oxides, and peroxides, etc., but rarely molecular oxygen. Here we demonstrate the ability of a set of Schiff‐base‐MoO2complexes (cy‐salen)MoO2(cy‐salen=N,N’ ‐cyclohexyl‐1,2‐bis‐salicylimine) to catalyze the aerobic oxidation of PPh3. We also report the results of a DFT computational investigation of the catalytic pathway, including the identification of energetically accessible intermediates and transition states, for the aerobic oxidation of PMe3. Starting from the dioxo species, (cy‐salen)Mo(VI)O2(1 ), key reaction steps include: 1) associative addition of PMe3to an oxo‐O to give LMo(IV)(O)(OPMe3) (2 ); 2) OPMe3dissociation from2 to produce mono‐oxo complex (cy‐salen)Mo(IV)O (3 ); 3) stepwise O2association with3 via superoxo species (cy‐salen)Mo(V)(O)(η1‐O2) (4 ) to form the oxo‐peroxo intermediate (cy‐salen)Mo(VI)(O)(η2‐O2) (5 ); 4) theO ‐transfer reaction of PMe3with oxo‐peroxo species5 at the oxo‐group, rather than the peroxo unit leading, after OPMe3dissociation, to a monoperoxo species, (cy‐salen)Mo(IV)(η2‐O2) (7 ); and 5) regeneration of the dioxo complex (cy‐salen)Mo(VI)O2(1 ) from the monoperoxo triplet3 7 or singlet1 7 by a concerted, asynchronous electronic isomerization. An alternative pathway for recycling of the oxo‐peroxo species5 to the dioxo‐Mo1 via a bimetallic peroxo complex LMo(O)‐O−O‐Mo(O)L8 is determined to be energetically viable, but is unlikely to be competitive with the primary pathway for aerobic phosphine oxidation catalyzed by1 . -
Investigation of Cu–O 2 oxidation reactivity is important in biological and anthropogenic chemistry. Zeolites are one of the most promising Cu/O based oxidation catalysts for development of industrial-scale CH 4 to CH 3 OH conversion. Their oxidation mechanisms are not well understood, however, highlighting the importance of the investigation of molecular Cu( i )–O 2 reactivity with O-donor complexes. Herein, we give an overview of the synthesis, structural properties, and O 2 reactivity of three different series of O-donor fluorinated Cu( i ) alkoxides: K[Cu(OR) 2 ], [(Ph 3 P)Cu(μ-OR) 2 Cu(PPh 3 )], and K[(R 3 P)Cu(pin F )], in which OR = fluorinated monodentate alkoxide ligands and pin F = perfluoropinacolate. This breadth allowed for the exploration of the influence of the denticity of the ligand, coordination number, the presence of phosphine, and K⋯F/O interactions on their O 2 reactivity. K⋯F/O interactions were required to activate O 2 in the monodentate-ligand-only family, whereas these connections did not affect O 2 activation in the bidentate complexes, potentially due to the presence of phosphine. Both families formed trisanionic, trinuclear cores of the form {Cu 3 (μ 3 -O) 2 } 3− . Intramolecular and intermolecular substrate oxidation were also explored and found to be influenced by the fluorinated ligand. Namely, {Cu 3 (μ 3 -O) 2 } 3− from K[Cu(OR) 2 ] could perform both monooxygenase reactivity and oxidase catalysis, whereas those from K[(R 3 P)Cu(pin F )] could only perform oxidase catalysis.more » « less
-
Reactions of the bicompartmental bis(phenolato) compound 6,6′-methylenebis(2-((bis(pyridin-2-ylmethyl)amino)methyl)-4-chlorophenol)hemihydrate (H 2 L ½H 2 O) with 3d metal( ii ) ions afforded novel fully structurally characterized bridged acetato dinuclear complexes [Mn 2 (HL)(μ 1,2 -OAc) 2 ]PF 6 (1) [Zn 2 (HL)(μ 1,2 -OAc)(H 2 O) 0.75 (MeOH) 0.25 ](PF 6 ) 2 ·0.45(H 2 O) (5) and [Cd 2 (HL)(μ 1,1,2 -OAc)(OAc)(H 2 O)]PF 6 ·H 2 O (6) as well as the polymeric bridged-azido tetranuclear catena -[Cu 4 (HL) 2 (μ 1,1 -N 3 ) 2 (μ 1,3 -N 3 ) 2 ](NO 3 ) 2 ·5H 2 O (4). The complex [Cu 4 (HL) 2 (ClO 4 ) 3 (H 2 O) 5 ](ClO 4 ) 3 ·5H 2 O (2) was partially characterized. In addition, three more dinuclear complexes [Cu 2 (H 2 L)(NO 3 ) 2 (H 2 O) 2 ](NO 3 ) 2 (3), [Cu 2 (HL)(OAc)(CH 3 OH)](PF 6 ) 2 (7) and [Cu 2 (HL)(NCS) 2 ]NO 3 ·2H 2 O (8) were also isolated. All complexes were characterized by CHN elemental analysis, IR and UV-Vis spectroscopy, ESI-MS, conductivity measurements and X-ray single crystal crystallography for compounds 1, 4, 5 and 6, where the bis(phenolato) ligand displayed different deprotonation (H 2 L, HL − and L 2− ). The magnetic susceptibility measurements over the temperature range 2–300 K revealed very weak antiferromagnetic coupling in dimanganese( ii ) 1 ( J = −1.64(1) cm −1 ) and almost negligible magnetic interaction in dicopper( ii ) 2 ( J = 0(3) cm −1 ). In the azido catena -[Cu 4 (HL) 2 (μ 1,1 -N 3 ) 2 (μ 1,3 -N 3 ) 2 ](NO 3 ) 2 ·5H 2 O (4) complex, the J value of −133(3) cm −1 was obtained upon moderate-to-strong antiferromagnetic coupling through the di-μ 1,3 -N 3 -bridged dicopper( ii ) unit with no magnetic interaction between the two copper( ii ) ions in the di-μ 1,1 -N 3 -bridged unit.more » « less
