84
M. H. Mahyuddin et al.
Int-3 subsequently forms a methanol complex Cu–CH 3 OH (Int-4) via TS4, where
the CH 3 moiety is now uncoordinated from the Cu center (H 3 C ··· Cu = 2.223 Å)
to interact with the OH ligand (H 3 C ··· OH = 2.013 Å) in the closed-shell singlet
state. The activation energy for this reaction step is calculated to be 14.6 kcal/mol,
comparable to that for C–H activation. Int-4 is then stabilized to a product complex
where the water molecule is now coordinated to the two Cu centers while the formed
methanol is completely detached from the Cu center. The coordination of a water
molecule makes the methanol desorption energy (E des = 26.5 kcal/mol) lower than
the direct methanol desorption leaving only two reduced Cu
I ··· Cu
I centers behind
(E des = 60.4 kcal/mol) [43].
3 Conclusion
In this chapter, we have overviewed recent advances in the mechanistic understanding of methane’s C–H bond cleavage by the [Cu–O–Cu]
2+ and [Cu 3 (μ-O) 3 ]
2+
active species in zeolites. Being the most well-accepted mechanism, the homolytic
hydrogen atom transfer mechanism becomes our focus of discussion. We show how
the high spin state and low spin state of the two active species are highly competitive in energetics since the difference between the two spin states lies only on the
spin direction of the Cu
2+ ions. In addition, we also demonstrate how the ground
state along the cleavage process on the [Cu–O–Cu]
2+ active site (triplet or open-shell
singlet state) formally remains unchanged.
More importantly, here we have reviewed the mechanism of methanol formation
from the cleaved
· CH 3 radical and the formed OH moiety, which has been debated in
literatures. While most of theoretical works suggested the direct HO–CH 3 rebound
as the most probable mechanism, recent experimental works suggested that the
· CH 3
radical first forms a surface methoxy species (O F –CH 3 , where O F is an O atom from
the zeolite framework) as an intermediate state before the formation of methanol.
Here, we show that the formation of O F –CH 3 species is as energetically probable as
the direct formation of methanol. The addition of a water molecule on the methoxy
intermediate facilitates the formation of a methyl complex (Cu–CH 3 ) via two possible
pathways that require activation barriers of 21.7 and 30.2 kcal/mol, both of which are
higher than that for cleaving the C–H bond of methane (14.4 kcal/mol). This, however,
is reasonable when one considers the fact that at the same reaction temperature, the
methanol extraction procedure using water steam takes longer time than the methane
activation and is performed several times to obtain maximum yield of methanol
[16, 49, 50].
Acknowledgements M.H.M. and H.K.D. acknowledge a research fund from the Indonesian
Ministry of Research and Technology/National Agency for Research and Innovation, and Indonesian Ministry of Education and Culture under World Class University (WCU) Program managed by
Institut Teknologi Bandung. K.Y. acknowledges the KAKENHI with grant numbers of JP24109014,
JP15K13710, and JP17H03117 from Japan Society for the Promotion of Science (JSPS) and the
Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT), the MEXT
M. H. Mahyuddin et al.
Int-3 subsequently forms a methanol complex Cu–CH 3 OH (Int-4) via TS4, where
the CH 3 moiety is now uncoordinated from the Cu center (H 3 C ··· Cu = 2.223 Å)
to interact with the OH ligand (H 3 C ··· OH = 2.013 Å) in the closed-shell singlet
state. The activation energy for this reaction step is calculated to be 14.6 kcal/mol,
comparable to that for C–H activation. Int-4 is then stabilized to a product complex
where the water molecule is now coordinated to the two Cu centers while the formed
methanol is completely detached from the Cu center. The coordination of a water
molecule makes the methanol desorption energy (E des = 26.5 kcal/mol) lower than
the direct methanol desorption leaving only two reduced Cu
I ··· Cu
I centers behind
(E des = 60.4 kcal/mol) [43].
3 Conclusion
In this chapter, we have overviewed recent advances in the mechanistic understanding of methane’s C–H bond cleavage by the [Cu–O–Cu]
2+ and [Cu 3 (μ-O) 3 ]
2+
active species in zeolites. Being the most well-accepted mechanism, the homolytic
hydrogen atom transfer mechanism becomes our focus of discussion. We show how
the high spin state and low spin state of the two active species are highly competitive in energetics since the difference between the two spin states lies only on the
spin direction of the Cu
2+ ions. In addition, we also demonstrate how the ground
state along the cleavage process on the [Cu–O–Cu]
2+ active site (triplet or open-shell
singlet state) formally remains unchanged.
More importantly, here we have reviewed the mechanism of methanol formation
from the cleaved
· CH 3 radical and the formed OH moiety, which has been debated in
literatures. While most of theoretical works suggested the direct HO–CH 3 rebound
as the most probable mechanism, recent experimental works suggested that the
· CH 3
radical first forms a surface methoxy species (O F –CH 3 , where O F is an O atom from
the zeolite framework) as an intermediate state before the formation of methanol.
Here, we show that the formation of O F –CH 3 species is as energetically probable as
the direct formation of methanol. The addition of a water molecule on the methoxy
intermediate facilitates the formation of a methyl complex (Cu–CH 3 ) via two possible
pathways that require activation barriers of 21.7 and 30.2 kcal/mol, both of which are
higher than that for cleaving the C–H bond of methane (14.4 kcal/mol). This, however,
is reasonable when one considers the fact that at the same reaction temperature, the
methanol extraction procedure using water steam takes longer time than the methane
activation and is performed several times to obtain maximum yield of methanol
[16, 49, 50].
Acknowledgements M.H.M. and H.K.D. acknowledge a research fund from the Indonesian
Ministry of Research and Technology/National Agency for Research and Innovation, and Indonesian Ministry of Education and Culture under World Class University (WCU) Program managed by
Institut Teknologi Bandung. K.Y. acknowledges the KAKENHI with grant numbers of JP24109014,
JP15K13710, and JP17H03117 from Japan Society for the Promotion of Science (JSPS) and the
Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT), the MEXT