Abstract
Highly active oxygen capable to selectively oxidize methane to methanol at low temperature can be prepared in transition-metal cation exchanged zeolites. Here we show that the α-oxygen stabilized by the negative charges of two framework aluminum atoms can be prepared by the dissociation of nitrous oxide over distant binuclear cation structures (M(II)…M(II), M = cobalt, nickel, and iron) accommodated in two adjacent 6-rings forming cationic sites in the ferrierite zeolite. This α-oxygen species is analogous to that known only for iron exchanged zeolites. In contrast to divalent iron cations, only binuclear divalent cobalt cationic structures and not isolated divalent cobalt cations are active. Created methoxy moieties are easily protonated to yield methanol, formaldehyde, and formic acid which are desorbed to the gas phase without the aid of water vapor while previous studies showed that highly stable methoxy groups were formed on isolated iron cations in iron exchanged ZSM-5 zeolites.
Highlights
Active oxygen capable to selectively oxidize methane to methanol at low temperature can be prepared in transition-metal cation exchanged zeolites
Our study shows that the selective oxidation of methane over the binuclear sites does not terminate after the formation of methoxy groups strongly bound to the catalyst
The α-oxygen with notable oxidation properties can be prepared on a Fe(II)-zeolite and on a zeolite exchanged with two other divalent cations—Co(II) and Ni(II)
Summary
Active oxygen capable to selectively oxidize methane to methanol at low temperature can be prepared in transition-metal cation exchanged zeolites. We show that the αoxygen stabilized by the negative charges of two framework aluminum atoms can be prepared by the dissociation of nitrous oxide over distant binuclear cation structures (M(II)...M (II), M = cobalt, nickel, and iron) accommodated in two adjacent 6-rings forming cationic sites in the ferrierite zeolite. This α-oxygen species is analogous to that known only for iron exchanged zeolites. High activities of Fe(II) exchanged catalysts featuring the α-oxygen regarding the selective oxidation of methane were reported only for a limited number of topologies of iron-exchanged silicon rich zeolites (ZSM-51,6,7,9,10,14,15, ferrierite[2,3], SSZ-135, and BEA*4,16)
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