Roles of Water Molecules in Modulating the Reactivity of Dioxygen-Bound Cu-ZSM-5 toward Methane: A Theoretical Prediction

Abstract

We propose theoretically that the reactivity of O2-bound Cu-ZSM-5 toward methane is enhanced by the presence of one water molecule near a dinuclear copper site inside a 10-membered ring of the zeolite cavity. The current study employed density functional theory (DFT) calculations with the B3LYP functional to elucidate reaction intermediates during dioxygen activation by Cu-ZSM-5 in the presence of one water molecule attached to a dicopper site. The initial event is the formation of a hydroperoxo species bridged by the dicopper site via an H atom transfer from an attached water to the bound dioxygen. After the formation of the intermediate, the hydroperoxo O–O bond is completely cleaved to form radical oxygen containing intermediates, such as a Cu–O–Cu species bound by two OH groups (HO–Cu–O–Cu–OH), as well as a copper oxyl group containing intermediate (HO–Cu–OH–CuO). The radical oxygen containing intermediates can cleave a methane C–H bond in a homolytic fashion. Examining the barrier for the C–H bond activation obtained from DFT calculations, we found that the two types of intermediates have the power to more effectively cleave methane C–H bonds than the Cu–O–Cu intermediate that has been proposed to be formed in the absence of a water molecule. The current DFT findings propose that O2-bound Cu-ZSM-5 in the presence of one water molecule is a potential candidate for catalysts desired for methane to methanol conversion under mild conditions. Recently, techniques for controlling the number of water molecules near the active site of a ZSM-5 zeolite have been developed, and therefore the DFT findings should stimulate experimental efforts for constructing catalysts for direct methane hydroxylation.

Copper-Modified Zeolites and Silica for Conversion of Methane to Methanol

“…The in situ infrared measurements reveal the formation of surface bound methoxy species as a likely important reaction intermediate during methane oxidation as well as the consumption of these methoxy species during water extraction, which is considerably more pronounced for the zeolitic systems. Hence, the higher methanol formation from Cu-ZSM-5 and Cu-SSZ-13 compared with Cu/SiO 2 can be attributed to the isolated Cu ions in consistence with previous suggestions [5,10,11,[14][15][16]46]. Notably, here, the minor amount of formed methanol (3.5-6.4 µmol/g Cu) over the Cu/SiO 2 sample regardless of oxidant is an interesting observation.…”

Copper–Oxygen Complexes Revisited: Structures, Spectroscopy, and Reactivity

“…70–76 Other dicopper species have also been suggested (Figure 1d), in large part stimulated by the identification by X-ray crystallography and EXAFS of a dicopper site in the enzyme. 29,77–80 These species include triplet 75 or mixed-valent Cu(II)Cu(III) 72 variants of the bis( μ -oxo)dicopper core, a ( μ -oxo)dicopper(II) unit akin to what has been proposed in Cu-doped zeolite catalysts (see section 1.2), 81–83 and dicopper units that incorporate a copper(II)-oxyl moiety. 77 Alternative hypotheses of mono- 84 and tricopper catalytic sites in pMMO have also been advanced, and proposals of additional tricopper reactive intermediates such as that shown in Figure 1d have been made.…”

“…More recent theoretical work led to the proposal of a pathway invoking peroxo and terminal hydroxo and oxyl intermediates (Figure 2b). 83 An alternative [Cu 3 ( μ -O) 3 ] 2+ core has been proposed in mordenite (Figure 2c). 118 While formally a mixed valent species (Cu(III) 2 Cu(II)), the cluster was described as having all Cu(II) ions with radical character on the O atoms on the basis of DFT calculations.…”

Die direkte katalytische Oxidation von Methan zu Methanol – eine kritische Beurteilung

“…Der Reaktortyp beeinflusst den Umsatz und die Selektivität nicht signifikant. [175] Eine neuere theoretische Analyse von Cu/ZSM-5 ergab einen dreikernigen Kupferoxocluster in ZSM-5 als mçglichen Kandidaten fürd ie Fçrderung eines Niedertemperatur-DMTM-Prozesses,w as durch EXAFS-Analyse untermauert wurde. [168] Das Absenken des Si/Al-Verhältnisses erhçhte die Methanolausbeute,d ad ie erhçhte Ionenaustauschkapazitätd ie Anzahl der aktiven Eisenspezies fürd ie Methanaktivierung erhçhte.E ine starke çkonomische Einschränkung dieses Systems besteht in der Tatsache,dass Wasserstoffperoxid teurer ist als Methanol.…”

“…[174] Sie argumentierten, dass die Identifizierung des aktiven Zentrums durch UV/Vis-Spektroskopie zweideutig sei, und griffen daher auf resonanzverstärkte Raman-Spektroskopie zurück, mit deren Hilfe ein gebogener Mono(m-Oxo)dikupferkern gefunden wurde.D ieses Postulat wurde kürzlich auch durch DFT-Rechnungen gestützt. [175] Eine neuere theoretische Analyse von Cu/ZSM-5 ergab einen dreikernigen Kupferoxocluster in ZSM-5 als mçglichen Kandidaten fürd ie Fçrderung eines Niedertemperatur-DMTM-Prozesses,w as durch EXAFS-Analyse untermauert wurde. [176,177] Va nelderen et al identifizierten zwei verschiedene [Cu-O-Cu] 2+ -Stellen, die fürdie Methanoxidation in Cu-MOR aktiv sind.…”