Methane activation by gold-doped titanium oxide cluster anions with closed-shell electronic structures

Abstract

The complementary active sites of Au+ and O2– ions on gold-doped titanium oxide clusters activate methane under thermal collision conditions.

On the Origin of the Remarkably Variable Reactivities of [AlCeOx]+ (x=2–4) towards Methane as a Function of Oxygen Content

“…[2] These model studies,which often deal with methane activation, on the one hand mimic the active site and on the other hand may help to improve the catalytic conversion of methane in bulk systems. [14] Herein, we report that the closed-shell oxide [AlCeO 3 ] + abstracts ah ydrogen atom from methane under ambient conditions whereas the other two cations [AlCeO 2 ] + and [AlCeO 4 ] + are inert towards this substrate.M oreover,t he behavior of [AlCeO 3 ] + is in distinct contrast to that of other closed-shell species,s uch as [OTiH] + , [15] [AuCH 2 ] + , [16] [AuC] + , [17] [OSiOH] + , [18] [XHfO] + (X = F, Cl, Br), [19] [TaO 2 ] + , [20] [TaO 3 ] + , [21] [Al 2 TaO 5 ] + , [11] [AuTi 3 O x ] À (x = 7, 8), [22] and [AuV 2 O 6 ] + . [14] Herein, we report that the closed-shell oxide [AlCeO 3 ] + abstracts ah ydrogen atom from methane under ambient conditions whereas the other two cations [AlCeO 2 ] + and [AlCeO 4 ] + are inert towards this substrate.M oreover,t he behavior of [AlCeO 3 ] + is in distinct contrast to that of other closed-shell species,s uch as [OTiH] + , [15] [AuCH 2 ] + , [16] [AuC] + , [17] [OSiOH] + , [18] [XHfO] + (X = F, Cl, Br), [19] [TaO 2 ] + , [20] [TaO 3 ] + , [21] [Al 2 TaO 5 ] + , [11] [AuTi 3 O x ] À (x = 7, 8), [22] and [AuV 2 O 6 ] + .…”

“…[11,[18][19][20][21][22][23] Fort he latter systems,s ingle hydrogen-atom abstraction from methane is not possible,probably because generating two doublet species ([M x O y H]C +/À + CH 3 C)isenergetically not favored. [11,[18][19][20][21][22][23] Fort he latter systems,s ingle hydrogen-atom abstraction from methane is not possible,probably because generating two doublet species ([M x O y H]C +/À + CH 3 C)isenergetically not favored.…”

“…[11,[18][19][20][21][22][23] Whereas the unavailability of single hydrogen-atom abstraction by the latter species is due to the energetically unfavorable generation of the doublet products,t he thermal generation of [AlCeO 3 H]C + can be traced back to the higher stability of Ce III as compared to Ce IV . Ther easons for this distinctly different behavior have been elucidated by high-level quantum-chemical calculations.Furthermore,ithas also been found that the reactivity of PCETbased methane activation depends not only on the Lewis acidity/basicity of the methanide/proton acceptor,but also on how relaxed the structures of the associated transition states are.F urthermore,t he unprecedented single hydrogen-atom abstraction from methane by closed-shell [AlCeO 3 ] + is in distinct contrast to the performance of previously reported reactive closed-shell species.…”

Ursachen der unterschiedlichen Reaktivität von [AlCeOx]+ (x=2–4) gegenüber Methan in Abhängigkeit vom Sauerstoffgehalt

“…[1] Um Einblicke in diese Prozesse zu gewinnen, wurden zahlreiche Gasphasenuntersuchungen durchgeführt, mit denen die Reaktionsmechanismen auf strikt molekularer Ebene aufgeklärt werden konnten. Während eine Reihe von aluminiumhaltigen Clustern, wie [Al 2 O 2 ]C + , [3] [Al 2 O 3 ]C + , [4] [Al 2 O 7 ]C + , [5] [YAlO 3 ]C + , [6] [VAlO 4 ]C + , [7] [PtAl 2 O 4 ]C À , [8] [Al 2 Mg 2 O 5 ]C + , [9] [(Al 2 O 3 ) n ]C + (n = 3-5) [10] und [Al 2 TaO 5 ] + , [11] zur thermischen Aktivierung von Methan in der Lage ist, sind nur zwei Arten von cerhaltigen Clustern, nämlich [(CeO 2 ) n ]C + (n = 2-4) [12] und [(CeO 2 ) m (V 2 O 5 ) n ] + (m = 1, 2, n = 1-5; m = 3, n = 1-4), [13] [15] [AuCH 2 ] + , [16] [AuC] + , [17] [OSiOH] + , [18] [XHfO] + (X = F, Cl, Br), [19] [TaO 2 ] + , [20] [TaO 3 ] + , [21] [Al 2 TaO 5 ] + , [11] [AuTi 3 O x ] À (x = 7, 8) [22] und [AuV 2 O 6 ] + . Während eine Reihe von aluminiumhaltigen Clustern, wie [Al 2 O 2 ]C + , [3] [Al 2 O 3 ]C + , [4] [Al 2 O 7 ]C + , [5] [YAlO 3 ]C + , [6] [VAlO 4 ]C + , [7] [PtAl 2 O 4 ]C À , [8] [Al 2 Mg 2 O 5 ]C + , [9] [(Al 2 O 3 ) n ]C + (n = 3-5) [10] und [Al 2 TaO 5 ] + , [11] zur thermischen Aktivierung von Methan in der Lage ist, sind nur zwei Arten von cerhaltigen Clustern, nämlich [(CeO 2 ) n ]C + (n = 2-4) [12] und [(CeO 2 ) m (V 2 O 5 ) n ] + (m = 1, 2, n = 1-5; m = 3, n = 1-4), [13] [15] [AuCH 2 ] + , [16] [AuC] + , [17] [OSiOH] + , [18] [XHfO] + (X = F, Cl, Br), …”

“…Hinsichtlich der beiden unreaktiven Paare [AlCeO 2 ] + /CH 4 und [AlCeO 4 ] + /CH 4 handelt es sich bei den Reaktionsschritten 1 1! 1 9/ 1 10 von Bedeutung:Obwohl die Lewis-Basizitätv on O b in 1 [27] sind auch fürd as [YAlO 3 ]C + /CH 4 -System beschrieben worden, [6] durch die sowohl die Reaktionseffizienz (7 %f [11,[18][19][20][21][22][23] Während letztere wegen der energetisch ungünstigen Bildung der Dublettprodukte nicht zur einfachen Wasserstoffatomabstraktion in der Lage sind, kann die thermische Erzeugung von [AlCeO 3 H]C + auf die hçhere Stabilität von Ce III gegenüber der von Ce IV zurückgeführt werden. 1 TS1/3 b!…”