Orbitals‐Driven Insights on the Reactivity of Boron Oxide with Dioxygen for Methane Oxidation on Singlet and Triplet Spin States

Abstract

Dedicated to Professor Amalendu Chandra (IIT Kanpur, India) on the occasion of his 60th birthday.Boron-based compounds such as frustrated Lewis pairs and borylenes that mimic transition-metal(TM)-like reactivity have attracted significant interest in recent years. This work examines the reactivity of boron oxide (B 2 O 3 ) towards dioxygen for methane activation. Density functional theory in combination with orbital analysis were utilized to derive mechanistic pathways for methane-to-formaldehyde conversion over B 2 O 3 in presence of closed-shell singlet and triplet O 2 . Multiple pathways were screened on the triplet spin state. Best route via the triplet spin state depicts the interaction of B 2 O 3 with dioxygen (O 1 =O 2 ) and CH 4 at a barrier of 49.8 kcal mol À 1 , to produce an intermediate having a BÀ O 1 À O 2 H unit and a free CH 3 radical, which later react together at a barrier of 57.6 kcal mol À 1 , to finally yield HCHO. In the singlet spin state, a free CH 3 radical does not form instead an intermediate with a BÀ O 1 À O 2 À CH 3 unit is found after crossing a barrier of 41.1 kcal mol À 1 , which further undergoes via a relatively lower barrier of 23.4 kcal mol À 1 to yield HCHO. Thus, the singlet pathway is energetically preferred over the triplet one. Orbitals further capture that at singlet and triplet states, methane activation follows a hydride and a hydrogen-atom-transfer mechanism, respectively.

Unidentical Twins: Geometrically Similar but Chemically Distinct Metal‐Free Sites in Boron Oxide for Methane Oxidation to HCHO, CO and CO2

“…In a more recent investigation, the same group showed the reaction of boron nitride with methane and dioxygen to accomplish methane oxidation [28] . In recent computational studies by our group, we showcased the mechanistic routes for the oxidation of methane over B 2 O 3 and h ‐BN [29,30] . In case of methane oxidation over B 2 O 3 , the routes leading to only HCHO were demonstrated [29] .…”

“…For the computational model of boron oxide, we utilized a primitive unit cell of B 2 O 3 and made a slab by cutting a 10$$\bar 1$$ plane. The choice of 10$$\bar 1$$ surface is associated with the ideal geometry of tri‐coordinated borons as in bulk [29,31–33] . Moreover, the 10$$\bar 1$$ surface has lowest surface energy in comparison to other low index surfaces [32] .…”

“…Therefore, it would be interesting to map the complete conversion of M2F and then formaldehyde to carbon monoxide and carbon dioxide over boron oxide (Figure 1(b)). The previous version of this work demonstrated the utilization of only one boron site [29] . However, the boron oxide system contains various probable boron active sites to accomplish the reaction (Figure 1(b)).…”

“…[28] In recent computational studies by our group, we showcased the mechanistic routes for the oxidation of methane over B 2 O 3 and h-BN. [29,30] In case of methane oxidation over B 2 O 3 , the routes leading to only HCHO were demonstrated. [29] However, further oxidation of HCHO to CO and CO 2 were unexplored.…”

“…[29,30] In case of methane oxidation over B 2 O 3 , the routes leading to only HCHO were demonstrated. [29] However, further oxidation of HCHO to CO and CO 2 were unexplored. Therefore, it would be interesting to map the complete conversion of M2F and then formaldehyde to carbon monoxide and carbon dioxide over boron oxide (Figure 1(b)).…”