Theoretical Study of the Direct Conversion of Methane …
41
The second crossing seam occurs in the vicinity of the hydroxo intermediate HO–
Fe
+ –CH 3 . The third crossing seam is located in the exit channel in which CH 3 OH is
eliminated from the product complex. We calculated the strength of the SOC along the
reaction pathway to estimate the probability of spin inversion. The strength of SOC
is 133.6 cm
−1 in the reactant complex and 21.4 cm
−1 in the hydroxo intermediate,
and the SOC value decreases along the reaction pathway, approaching zero in the
product complex. Although the strength of SOC decreases as we follow the reaction
coordinate, the SOC value in the second crossing seam is not negligible. However,
this spin inversion is not preferred for the production of CH 3 OH because TS2 in the
sextet state is relatively high. When the quartet state changes to the sextet state in the
vicinity of the second crossing seam, another reaction branch that leads to FeOH
+
+
• CH 3 will occur. Finally, we conclude that the spin inversion from the sextet state to
the quartet state should occur in the first crossing seam, whereas the spin state should
remain unchanged in both the second and third crossing seams. Our computations
suggest that the spin inversion can promote the reaction of FeO
+ with CH 4 and that
the spin inversion would not occur in the reverse reaction (Fe
+
+ CH 4 ) that starts in
the third crossing seam.
5 Summary
In this chapter, we reviewed mechanisms for the direct conversion of CH 4 to CH 3 OH
by bare transition-metal monoxide cations (MO
+ ), as deduced from DFT calculations. The calculated electronic structures of the MO
+ ions are important for the
reactivity and catalytic properties. DFT calculations indicated that MO
+ (M = Sc,
Ti, V, Cr, Mn, Fe, Co, Ni, Cu) reacted with CH 4 . Both the high- and low-spin state
potential-energy surfaces have been characterized at the B3LYP/6-311G** level of
theory. In conclusion, the CH 4 –CH 3 OH conversion by MO
+ proceeds in the following
way.
MO
+
+ CH 4 → OM
+
(CH 4 ) → TS1 → HO−M
+
−CH 3 → TS2
→ M
+
(CH 3 OH) → M
+
+ CH 3 OH
(M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu)
Potential energies measured from the dissociation limit indicate a substantial
activation barrier, consistent with experimental results. The behavior of the forward
and reverse reactions in the MO
+ complexes with CH 4 systems is consistent with
reactions that proceed via the hydroxo intermediates CH 3 –M
+ –OH. Ab initio calculations showed that the formation of CH 3 OH via CH 3 –M
+ –OH is energetically more
favorable than the byproduct pathway leading to methyl radical, except in the case
of ScO
+ and TiO
+ .
The potential-energy surface crossing points between the high- and low-spin states
play a substantial role, leading to a decrease in the barrier height of transition states.
41
The second crossing seam occurs in the vicinity of the hydroxo intermediate HO–
Fe
+ –CH 3 . The third crossing seam is located in the exit channel in which CH 3 OH is
eliminated from the product complex. We calculated the strength of the SOC along the
reaction pathway to estimate the probability of spin inversion. The strength of SOC
is 133.6 cm
−1 in the reactant complex and 21.4 cm
−1 in the hydroxo intermediate,
and the SOC value decreases along the reaction pathway, approaching zero in the
product complex. Although the strength of SOC decreases as we follow the reaction
coordinate, the SOC value in the second crossing seam is not negligible. However,
this spin inversion is not preferred for the production of CH 3 OH because TS2 in the
sextet state is relatively high. When the quartet state changes to the sextet state in the
vicinity of the second crossing seam, another reaction branch that leads to FeOH
+
+
• CH 3 will occur. Finally, we conclude that the spin inversion from the sextet state to
the quartet state should occur in the first crossing seam, whereas the spin state should
remain unchanged in both the second and third crossing seams. Our computations
suggest that the spin inversion can promote the reaction of FeO
+ with CH 4 and that
the spin inversion would not occur in the reverse reaction (Fe
+
+ CH 4 ) that starts in
the third crossing seam.
5 Summary
In this chapter, we reviewed mechanisms for the direct conversion of CH 4 to CH 3 OH
by bare transition-metal monoxide cations (MO
+ ), as deduced from DFT calculations. The calculated electronic structures of the MO
+ ions are important for the
reactivity and catalytic properties. DFT calculations indicated that MO
+ (M = Sc,
Ti, V, Cr, Mn, Fe, Co, Ni, Cu) reacted with CH 4 . Both the high- and low-spin state
potential-energy surfaces have been characterized at the B3LYP/6-311G** level of
theory. In conclusion, the CH 4 –CH 3 OH conversion by MO
+ proceeds in the following
way.
MO
+
+ CH 4 → OM
+
(CH 4 ) → TS1 → HO−M
+
−CH 3 → TS2
→ M
+
(CH 3 OH) → M
+
+ CH 3 OH
(M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu)
Potential energies measured from the dissociation limit indicate a substantial
activation barrier, consistent with experimental results. The behavior of the forward
and reverse reactions in the MO
+ complexes with CH 4 systems is consistent with
reactions that proceed via the hydroxo intermediates CH 3 –M
+ –OH. Ab initio calculations showed that the formation of CH 3 OH via CH 3 –M
+ –OH is energetically more
favorable than the byproduct pathway leading to methyl radical, except in the case
of ScO
+ and TiO
+ .
The potential-energy surface crossing points between the high- and low-spin states
play a substantial role, leading to a decrease in the barrier height of transition states.