Single Turnover Epoxidation of Propylene by α-Complexes (FeIII-O?)α on the Surface of FeZSM-5 Zeolite

Article abstract of DOI:10.1021/acscatal.6b00930

Single turnover epoxidation of propylene by α-complexes (Fe^III-O^?)_α on the surface of FeZSM-5 zeolite was studied in the temperature range from +25 °C to -60 °C. After extraction, the reaction products were identified by gas chromatography (GC), gas chromatography-mass spectrometry (GC-MS), and nuclear magnetic resonance (NMR). The reaction C₃H₆ + (Fe^III-O^?)_α was shown to proceed in a similar way as the oxidation of propylene by the enzyme methane mono-oxygenase sMMO, i.e., via the addition of active oxygen over C=C bonds to yield propylene oxide, not affecting a weakly bound allylic hydrogen. This result, together with the previous results on the hydroxylation of methane and other hydrocarbons, shows the (Fe^III-O^?)_α to be the unique functional model of compound Q, the key intermediate of sMMO. In another aspect, the results relate to the low selectivity problem of the silver catalyst in propylene epoxidation and raise doubts about the presently accepted mechanism explaining an adverse effect of allylic hydrogen.

Full text of DOI:10.1021/acscatal.6b00930

Letter
pubs.acs.org/acscatalysis
Single Turnover Epoxidation of Propylene by α‑Complexes (FeIII−O•)α
on the Surface of FeZSM‑5 Zeolite
Gennady I. Panov,* Eugeny V. Starokon, Mikhail V. Parfenov, and Larisa V. Pirutko
Department of Heterogeneous Catalysis, Boreskov Institute of Catalysis, Pr. Lavrentieva 5, Novosibirsk 630090, Russian Federation
S
* Supporting Information
ABSTRACT: Single turnover epoxidation of propylene by α-
complexes (FeIII−O•)α on the surface of FeZSM-5 zeolite was
studied in the temperature range from +25 °C to −60 °C. After
extraction, the reaction products were identified by gas
chromatography (GC), gas chromatography−mass spectrometry
(GC-MS), and nuclear magnetic resonance (NMR). The reaction
C3H6 + (FeIII−O•)α was shown to proceed in a similar way as the
oxidation of propylene by the enzyme methane mono-oxygenase
sMMO, i.e., via the addition of active oxygen over CC bonds to yield propylene oxide, not affecting a weakly bound allylic
hydrogen. This result, together with the previous results on the hydroxylation of methane and other hydrocarbons, shows the
(FeIII−O•)α to be the unique functional model of compound Q, the key intermediate of sMMO. In another aspect, the results
relate to the low selectivity problem of the silver catalyst in propylene epoxidation and raise doubts about the presently accepted
mechanism explaining an adverse effect of allylic hydrogen.
KEYWORDS: propylene epoxidation, propylene oxide, α-complex (FeIII−O•)α, α-oxygen, FeZSM-5 zeolite, product extraction,
biomimetic model of compound Q
mong numerous catalytic oxidations by dioxygen, three
Avery difficult reactions can be distinguished: the oxidation
of methane to methanol, the oxidation of benzene to phenol,
and the oxidation of propylene to propylene oxide (PO).
Sometimes, these reactions are called “dream reactions”.1 If a
search history of efficient catalysts for the first two reactions has
more than 100 years,2,3 the oxidation of propylene to PO is a
relatively new challenge, arousing great research interest in the
field.4−12 According to a generally accepted view, the difficulty
of this reaction relates to the presence of weakly bonded allylic
hydrogen, which is readily subjected to oxidation, thus opening
a pathway for the transformation of propylene to CO2 and
water. In this respect, quite illustrative is the performance of the
commercial supported silver catalyst.4,5,13 With this catalyst, the
epoxidation of ethylene having no allylic hydrogen proceeds
with a selectivity of 90%, presenting one of the most efficient
modern chemical technologies, while the epoxidation of
propylene proceeds with a much lower selectivity, having no
practical interest.
hydroxylation ability, does not cleave and hydroxylate the
weak allylic C−H bonds of propylene (86.6 kcal/mol).
In this connection, FeZSM-5 zeolites are of great interest. In
the micropore space of the zeolites, stable complexes of bivalent
iron (FeII)α (α-sites) can be created.3,17 By reaction with
nitrous oxide (eq 1), the α-sites can be converted to oxidized α-
complexes (FeIII−O•)α,
(FeII)α + N2O → (FeIII−O•)α + N2
(1)
the formation of which has been shown by the Mossbauer,18,19
̈
resonant inelastic X-ray scattering (RIXS),20,21 and electron
paramagnetic resonance (EPR) methods.22,23 Because of the
availability of a very reactive species of radical oxygen (α-
oxygen, Oα), α-complexes exhibit an extremely high oxidizing
ability. The single turnover oxidation of methane by (FeIII−
O•)α proceeds immeasurably fast at −30 °C, leading to the
formation of hydroxy and methoxy groups:
CH4 + 2(FeIII−O•)α → (FeIII−OH)α + (FeIII−OCH3)α
(2)
In nature, the iron-containing methane monooxygenase
sMMO selectively performs a variety of oxidations, including
the “dream reactions”, which inspires new and repeated
attempts of researchers to create chemical models that mimic
the wonderful activity of the enzyme. The key intermediate of
sMMO is the compound Q, which contains very active oxygen
species, which are able to hydroxylate very strong C−H bonds
of methane and benzene (104.9 and 113 kcal/mol,
respectively) as well as add to the CC bonds of propylene
to form PO.14−16 It is remarkable that, in distinction to the
silver catalyst, sMMO, regardless of its highly powerful
The hydrolysis of methoxy groups that occurs at the
extraction step yields methanol.24,25 The oxidation of other
alkanes and aromatic compounds by α-complexes proceeds in a
similar way, resulting, as in the case of sMMO, in the formation
of hydroxylated products.26,27 These results, along with the data
on kH/kD kinetic isotope effects,28 allow one to conclude that
Received: March 31, 2016
Revised: May 13, 2016
© XXXX American Chemical Society
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Letter
the chemistry of α-oxygen is similar to that of the active oxygen
surface reaction of propylene with PO (see the Supporting
Information (section S1)):
of sMMO, which was noted in many works.1,28−31 The
Mossbauer spectra of α-complexes in the hydrated state are
̈
C3H6O + C3H6 → C6H12O
(5)
close to the spectra of Fe centers in sMMO.18,32
Note that α-complex (FeIII−O•)α can serve not only as the
single turnover oxidant but also as the key intermediate in the
catalytic oxidations of benzene to phenol33,34 and methane to
methanol,35 taking place at elevated temperatures.
The extract also contains some amount (12 mol %) of
products, denoted as “others”. They are represented by several
low-intensity chromatographic peaks that are difficult to
identify.
In the previous work,36 we studied the single turnover
oxidation of ethylene by (FeIII−O•)α. The reaction was shown
to attack the CC double bonds yielding ethylene oxide (EO)
as the only one primary product:
For the reaction at 25 °C, the total yield of extracted
products, calculated in terms of the number of moles of product
per mole of (FeIII−O•)α complex, is 41 mol %, while the
remainder (59 mol %) consists of nonextractable components.
Further experiments (see section S2 in the Supporting
Information) showed that both nonextractable and “other”
products are primarily (if not exclusively) formed directly from
PO, rather than from its isomeric forms (AA and PA).
As seen in Table 1, a temperature decrease strongly affects
the reaction outcome. First, it is related to PO, the fraction of
which, in extracted products, increases up to 59−63 mol % at
−60 °C. This increase correlates with the decrease of secondary
PO transformations, in particular its isomerization to PA and
conversion to C6H12O. Indeed, as the temperature decreases,
the PA fraction decreases by more than an order of magnitude
(from 24 mol % to 0.6 mol %), and the fraction of C6H12O
decreases 2-fold (from 43 mol % to 22 mol %). The fraction of
“other” products changes only slightly, being in a range of 10−
13 mol %. A decrease in the temperature increases the total
yield of extracted products from 41 mol % to 52 mol %.
Of particular interest is AA. As seen in Table 1, its small
amount, which was observed at 25 °C (3.5 mol %), further
decreases as the reaction temperature decreases and, at −60 °C,
becomes as low as 0.4−0.6 mol %. This value is 2 orders of
magnitude smaller than the amount of PO. Nevertheless, this
fact does not eliminate the mechanistic importance of the AA
origin.
At the reaction temperature of −60 °C, the EO fraction in
extracted products was 85%. The present work is devoted to a
similar reaction of propylene, C3H6 + (FeIII−O•)α. The most
intriguing question of the study is the oxidative strategy of the
(FeIII−O•)α. It may be the strategy typical of the silver catalyst
(i.e., the abstraction of allylic hydrogen) or the strategy typical
of sMMO (i.e., the addition of oxygen to the CC bond). The
answer to this question is one of the main goals of the present
work.
The experiments were carried out in a vacuum static setup
with the same FeZSM-5 sample (2.0 wt % Fe, 0.2 wt % Na, α-
site concentration = 58 μmol/g) that was used in the single
turnover epoxidation of ethylene.36 Briefly, the procedure
included the following steps:
(1) Oxidation of α-sites by N2O (eq 1) at 230 °C to create
(FeIII−O•)α complexes;
(2) Surface reaction of propylene with (FeIII−O•)α; and
(3) Extraction of the reaction products by methanol (10
vol % H2O) and their identification by the GC, GC-MS,
and NMR techniques.
There are two possible routes of AA formation (see Scheme
1). Route I implies the abstraction of allylic hydrogen with the
A more detailed procedure can be found elsewhere.36
Table 1 lists the composition and total yield of extracted
products after the reaction at different temperatures. The first
experiment carried out at 25 °C showed the formation of a
considerable amount of PO (18 mol %):
Scheme 1. Two Possible Routes of Propylene Oxidation by
α-Complexes (FeIII−O•)α
In addition to PO, its two isomerspropanal (PA, 24 mol
%) and allyl alcohol (AA, 3.5 mol %)are present in the
extract. The main fraction (43 mol %) is represented by a set of
isomeric compounds C6H12O, which are produced by the
formation of the hydroxy and propenoxy groups residing on α-
sites. The hydrolysis of propenoxy groups (at the extraction
a
Table 1. Effect of Temperature on the Reaction Products Extracted from the FeZSM-5 Surface
Distribution of Extracted Products (mol %)
expt
no.
reaction temperature
propylene oxide,
PO
propanal,
PA
allyl alcohol,
AA
total yield of extracted products based on Oα
(mol %)
(°C)
C6H12O others
1
2
3
4
5
25
0
18
37
59
59
63
24
3.5
4.5
1.4
0.6
0.4
43
39
24
26
22
11.5
9.7
41
51
52
51
52
9.7
2.9
1.7
1.6
−25
−60
−60
12.7
11.7
13
a
Reaction conditions: 0.1 g of FeZSM-5 with 5.8 μmol (FeIII−O•)α; propylene pressure = 2.0 (80 μmol C3H6); reaction time = 3 min.
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Figure 1. IR spectra of FeZSM-5 after the (FeIII−O•)α reaction with (a) methane and (b) propylene: initial spectra before reaction (spectrum 1),
after reaction at 25 °C (spectrum 2), and after reaction at −60 °C (spectrum 3).
step) leads to the formation of AA. Route II implies the
addition of α-oxygen to the CC bond with the formation of
PO, which can further partially isomerize to AA, PA, and also to
acetone, whose barely perceptible traces were observed in some
experiments (<0.1 mol %, not shown in Table 1). The
isomerization of PO is well-known in the literature.7 Upon
extraction, all of the listed products pass into solution, where
they do not undergo any further transformations.
An important manifestation of the allylic route I is the
formation of (FeIII−OH)α hydroxy groups, which can reliably
be identified by infrared (IR) spectroscopy. The application of
this method made it possible to show that the oxidation of
ethylene by (FeIII−O•)α proceeds without the formation of
(FeIII−OH) groups,36 i.e., only via route II with the addition of
α-oxygen to CC bonds.
Therefore, the reaction does not involve hydrogen abstraction
and, similar to the case with ethylene, proceeds only via route
II, yielding PO as the primary product. All other detected
products are the result of PO secondary transformations.
The epoxidation of CC bonds is a typical reaction for Fe-
containing enzymes mono-oxygenases, as well as for their
chemical models, the key intermediates of which are ferryl oxo-
complexes FeIVO.15,38−40 As shown by theoretical calcu-
lations, ferryl complexes can transform to their electromeric
oxyl form FeIII−O•,30,31,41 which seems identical to α-
complexes. According to Ye and Neese,41 the ferryl enters in
the oxidation step just in the oxyl form, having a more reactive
radical species of oxygen.
However, the α-complex (FeIII−O•)α cannot be considered
to be a complete analogue of the ferryl, because of the
extremely high reactivity of α-oxygen noted earlier. In
distinction to ferryl oxygen, Oα can readily hydroxylate
methane, benzene, and other hydrocarbons, as does the active
oxygen of sMMO. The key intermediate of sMMO, compound
Q, is represented, unlike the ferryl, by a binuclear [FeIV−
(μO)2−FeIV] complex having the “diamond core” structure.
This particular structure was strongly supported recently by
Banerjee et al., using resonance Raman spectroscopy.42,43 The
kinetics of the single turnover hydroxylation of methane and
other hydrocarbons, as well as the epoxidation of propylene by
compound Q, was investigated by Valentine et al.44
One may think that the similarity in oxidation chemistry of
α-complexes and compound Q should assume the similarity in
their structures. As was noted above, an exact structure of α-
sites, including the question of their mononuclear or binuclear
nature, remains debatable as yet. The situation is complicated
by the fact that FeZSM-5 zeolites, along with α-sites, may
contain a variety of other iron species: from isolated Fe2+ and
Fe3+ cations to oxide-like agglomerates. In a recent thorough
study, Li and co-authors45 concluded that the existence of
mononuclear [FeO]+ species in the micropore space of ZSM-5
is unlikely. Such species would be unstable and have a strong
tendency to transform to binuclear complexes, the composition
of which in both the reduced, [FeII−(μO)−FeII], and the
oxidized state, [FeIII−(μO)2−FeIII], is similar to the composi-
tion of compound Q, although with a different oxidation state
of iron.
Table 1 shows that, in the case of propylene oxidation, the
reaction also proceeds via route II, which is clearly evidenced by
the presence of PO in extracted products. However, there is an
important mechanistic question: Is this the only reaction route
or could the oxidation proceed also via the allylic route I?
Figure 1 shows the spectra of FeZSM-5 collected (see section
S3 in the Supporting Information) in the region of OH
stretching vibrations after the reaction of methane (Figure 1a)
and propylene (Figure 1b) with α-complexes (FeIII−O•)α. Let
us first consider the reaction of methane, which is a classic
example of the hydrogen abstraction mechanism, given here
just for comparison purposes. The spectrum of the initial
sample (spectrum 1 in Figure 1a) has a distinct absorption
band at 3745 cm−1, which is typical of the Si−OH silanol
groups of the zeolite. The reaction of methane with α-
complexes at 25 °C (spectrum 2 in Figure 1a) produces
dramatic changes in the spectrum. An intense band at 3675
cm−1 with a shoulder at 3633 cm−1 appears. Two types of (Fe−
OH)α groups can be attributed to the two types of α-sites that,
according to Mossbauer data,18 have different coordination
̈
states of iron. The assignment of these bands to (Fe−OH)α
groups was safely supported by isotopic experiments using 18Oα
and CD4.24,28,32,37 A similar spectrum also is observed after the
methane reaction at −60 °C (spectrum 3 in Figure 1a).
The spectra collected after the reaction of C3H6 + (FeIII−
O•)α have a completely different shape. One can see that the
reaction does not give rise to new hydroxy groups, neither at 25
°C (spectrum 2 in Figure 1b) nor at −60 °C (spectrum 3 in
Figure 1b). This allows a conclusion that, within the sensitivity
of the method, there are no products from allylic route I.
The binuclear structure of active iron species in FeZSM-5
zeolite agrees well with the data by Dubkov et al.,18 who
managed to quantitatively correlate the Mossbauer spectra
̈
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.6b00930.
■
S
Surface reaction of propylene with propylene oxide; on
the origin of nonextractable products; procedure of
collecting IR spectra (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Tel.: (007) 383 330-94-52. Fax: (007) 383 330-80-56. E-mail:
panov@catalysis.ru.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank V. A. Utkin for GC-MS analysis, I. E. Soshnikov for
NMR analysis, and G. M. Zhidomirov and E. P. Talsi for useful
discussions. The work was supported by RFBR (Project No.
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