Figure
4. (a) Molar formation rate (per total Fe) of CH3OH and
CH3OD formed over MIL-100(Fe) when exposed to
D2O at 473 K following reaction. (b) Effect of the molar
ratio of H2O and D2O fed during the
product extraction step on the relative amount of CH3OH
and CH3OD formed. (c) Molar formation rate (per total
Fe) of CH316OH and
CH318OH over MIL-100(Fe) when exposed
to H218O at 473 K following reaction.
(1.6 kPa N2O, 1.5 kPa CH4, 473 K, 2 h).
Figure 4b reproduced from ref. 35 Copyright © 2020
WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Reaction with water to form methanol from intermediates that are not
desorbed from the surface in the absence of water mirrors several
observations reported in prior literature. Ethane oxidation over
MOF-74(Mg,Fe) to produce ethanol, acetaldehyde, and diethyl ether
required extraction with CD3CN following reaction at 348
K.36 Propane/ethane oxidation over MIL-100(Fe) also
required oxygenated products to be extracted with D2O,
with only unsaturated ethene/propene products desorbing into the gas
phase in the absence of D2O.39 DFT
calculations reported by Vitillo et al. evaluating the radical
rebound mechanism for methanol formation suggest that the step involving
formation of the Fe(IV)=O intermediate over MIL-100(Fe) carries the
highest activation barrier (140.5 kJ
mol-1).40 The authors suggest that
alcohol desorption may not readily occur under reaction conditions due
to the high activation barrier for the desorption for methanol (91.5 kJ
mol-1) in comparison to the heat of adsorption of
N2O (30 kJ mol-1). A competing pathway
to radical rebound to form the surface bound methanol product is one in
which the radical dissociates from the active
center.55,56 The activation energy for radical
desorption from Fe nodes in MIL-100(Fe) MOFs was predicted to be only
slightly greater (~ 5 kJ mol-1greater) than the barrier for radical rebound.40 We
note in this context that experimental evidence for catalyticmethane hydroxylation over MIL-100(Fe) has not yet been reported in the
literature. Though methoxy intermediates are identified in our study as
the predominant species formed prior to exposure to water vapor, the
identity of elementary steps that form them remain unclear, and the
possibility of minor quantities of methanol being formed is challenging
to disprove given the plausibility of methanol reacting with open-metal
iron sites, as demonstrated in the discussion that follows.
To test for the plausibility of methoxy formation mediated by either
adsorbed or gas phase methanol, thermally-activated MIL-100(Fe) was
first exposed to CH3OH at 373 K, purged for 6 h under
inert flow at 473 K to remove excess CH3OH, and then
exposed to D2O. 0.23 mol CH3OD (mol
Fe)-1 were measured upon introduction of
D2O subsequent to exposure to methanol, a value
coinciding closely with that formed following reaction with methane and
N2O (0.27 mol CH3OD (mol
Fe)-1)- Figure 5- evidencing the plausibility of
methoxy formation through methanol dissociation over
Fe2+ sites. The methanol dissociation observed is
analogous to water reacting with open-metal sites to reform hydroxyl
anions that have to be eliminated during thermal activations steps in
MIL-100(Fe) (Figure S11, SI), as also reported previously over
Cr2+ sites in MIL-100(Cr).45 The
susceptibility of methanol towards dissociation over
Fe2+ sites suggests that the formation of methanol
intermediates in our experiments cannot be excluded. Regardless of the
identity of steps mediating methoxy formation, its stoichiometric
formation exclusively over Fe2+ sites appears to
precede methanol formation upon extraction with water vapor. A 1:1
correspondence between methoxy concentrations and Fe2+site densities across a range of thermal activation
conditions35 suggests that methoxy formation involves
the participation of only one active center, and contrasts with prior
reports for iron-exchanged zeolites that propose the involvement of two
active ’α-oxygen’ sites per methoxy formed (CH4 +
2(O)α → (OH)α +
(OCH3)α).57,58