Gaseous Reactants Inside FeAlPO4 Molecular Sieve
J. Phys. Chem. A, Vol. 104, No. 3, 2000 495
electron transfer. H+-transfer may involve deprotonation of O
rather than C, resulting in the initial formation of alkoxy radical
(CH3)2CH-O. However, rapid rearrangement to the more stable
(CH3)2C-OH radical may occur.41 The final product most likely
emerges from direct abstraction of H from the hydroxypropyl
radical by HOO. It is also conceivable that the two radicals
first combine to form (CH3)2C(O2H)OH, followed by elimina-
tion of H2O2. In fact, the latter has been proposed as an
intermediate of thermal 2-propanol oxidation.37 However, direct
H abstraction is more likely because we were unable to trap
any intermediate when conducting 2-propanol photooxidation
at -90 °C. A closed-shell molecule such as (CH3)2C(O2H)OH
is expected to be stabilized at such a low temperature. In any
case, the proposed mechanism is consistent with the single
photon nature of the photochemistry and the fact that the primary
products of the photoreaction of 2-propanol-16O with 18O2 are
acetone-16O and H218O.
gap excited iron oxide materials is particularly interesting to
photoactivation of molecules that are difficult to reduce, such
as carbon oxides.
V. Conclusions
In situ monitoring of probe reactions by FT-IR spectroscopy
has been employed to explore the reactivity of LMCT-excited
framework Fe centers of FAPO-5 at the gas-micropore
interface. According to the mechanism inferred from the
observed products, O2 acts as one-electron acceptor, thereby
reoxidizing the photoreduced Fe+II. Concurrently, methanol or
2-propanol molecules act as donors and reduce the transient
hole on framework oxygen. Subsequent proton transfer and H
atom abstraction leads to formaldehyde (acetone) and H2O2,
resulting in an overall two-electron transfer process. Among
these products, acetone is stable in the sieve, while formaldehyde
undergoes fast Cannizzaro reaction and H2O2 disproportionates
to H2O and O2. The finding that O2 is reduced efficiently by
transient framework Fe+II suggests that its reduction potential
lies at least one-half of a volt more negative than that of the
conduction band of dense-phase Fe2O3 particles. This may open
up demanding photoreductions not accessible by photochemistry
at iron oxide semiconductor materials. Using this method of in
situ FT-IR monitoring of laser-induced chemistry, we will
explore reactions with probe molecules covering a range of
electron acceptor and donor strengths that will allow us to
bracket the redox potentials of excited framework metals at the
gas-micropore interface. This will furnish the basis needed for
selecting microporous materials with appropriate framework
metals and oxidation states for a desired photochemical trans-
formation.
Enthalpies of formation of (CH3)2CH-OH (-65.2 kcal
mol-1),42 HOO radical (5.0 kcal mol-1),43 and (CH3)2CHO
radical (-12.5 kcal mol-1 44
) give an endothermicity of 58 kcal
mol-1 for the conversion of 2-propanol and O2 to the radical
pair. Hence, the observation that the reaction can be initiated
with photons as low in energy as 420 nm (68 kcal mol-1) is
consistent with the proposed mechanism.
The proposed mechanism for CH3OH photooxidation by O2
is analogous to that of 2-propanol, leading to a CH2OH/HOO
radical pair. Hydroxymethyl radical is an established species.30
On the basis of enthalpies of formation (∆H°(CH3OH) ) -48
f
kcal mol-1; ∆H°(HOO) ) 5.0 kcal mol-1; ∆H°(CH3O•) ) 4.2
f
f
kcal mol-1 ),41,43 we estimate an endothermicity of 57 kcal mol-1
for the reaction step
CH3OH + O2 9
hν8 CH3OH+ + O2- f CH3O• + HOO
Acknowledgment. This work was supported by the Director,
Office of Science, Office of Basic Energy Sciences, Chemical
Sciences Division of the U.S. Department of Energy under
Contract DE-AC03-76SF00098.
which, most probably, is followed by isomerization to CH2OH
before H abstraction leads to CH2dO and H2O2.45 The finding
that loading of CH2dO into the molecular sieve results in
spontaneous Cannizzaro reaction explains why HCO2H, formate,
and HCO2CH3 are the observed final oxidation products, rather
than formaldehyde.
The rather efficient photoreaction of O2 and alcohol at the
gas-micropore interface of FAPO-5 implies that the transient
Fe+II centers of the molecular sieve reduce O2 in a thermoneutral
or exoergic step
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