Controlling iron-catalyzed oxidation reactions: From non-selective radical to selective non-radical reactions

Article abstract of DOI:10.1002/chem.200801432

A study was conducted to demonstrate the relationship between the absolute proton concentration (pH value), change of pH value, and catalytic performance in important iron-catalyzed selective oxidation reactions. As per this hypothesis, the high selectivi

Full text of DOI:10.1002/chem.200801432

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DOI: 10.1002/chem.200801432
Controlling Iron-Catalyzed Oxidation Reactions:
From Non-Selective Radical to Selective Non-Radical Reactions
[a, b]
Feng Shi,[a] Man Kin Tse,[a, b] Zuopeng Li,[c] and MatthiasBeller*
Among all elements widely distributed in nature, iron is
probably the most versatile and important redox center for
life and natural transformation processes.[1–2] With respect to
sustainable chemistry and following natureꢀs principles, iron
is the ideal metal for more environmentally benign catalyst
generations in future because of availability, low toxicity,
and price. Although iron plays wonderfully in nature, the
difficulty to prevent non-selective radical reactions makes
its usage on laboratory scale, but especially for industrial
production, more difficult—except being directly used as
Lewis acid.[3–5] In order to apply iron catalysts for a specific
reaction with sufficient selectivity and activity, so far the
general concept is based on the design of a suitable ligand
which is further on optimized. In recent years iron catalysis
has become a “hot topic” and notable progress has been ac-
complished applying this approach. For instance, novel iron
containing systems have been developed for hydroxyl-
ation,[6–8] sulfide oxidation,[9–11] cross-coupling reactions,[12–14]
systems.[24–28] Its high efficiency for oxidations of alkanes to
ketones, benzene to phenol, and for environmental pollutant
treatment has been extensively explored and has significant
industrial potential.[29–32] Notably, after more than a century
of the first report, detailed investigations of the Fenton reac-
tion still reveal two possible mechanisms,[24–28] that is, the
classical free radical mechanism and the oxygenated Fenton
chemistry. Until today there is no concrete evidence to ex-
clude each other completely, because both mechanisms have
their own supporting evidence.[24–28] In former reports, the
importance of acids in Fenton chemistry has been stud-
ied,[33–39] however a detailed study on the crucial function of
pH variation on the catalyst activity and selectivity has been
ignored.
Earlier on, we discovered the pH dependency of the se-
lectivity in osmium-catalyzed dihydroxylations of olefins ap-
plying oxygen as the terminal oxidant.[40] Based on our
recent experience in iron catalysis,[41–43] we supposed that
the proton concentration in the reaction system might be
also important for Fe-dependant oxidation reactions.
Indeed, studying the oxidation of benzyl alcohol to benzal-
dehyde as a typical model reaction, a remarkable influence
of the pH is observed (Table 1).
heterolytic RO OH bond cleavage,[15] hydroamination,[16] al-
À
lylic alkylation or amination,[17–19] epoxidation,[20–21] and alco-
hol oxidation.[22–23] However, the intrinsic problem of non-
selective radical side-reactions in the redox chemistry of
iron has not been generally solved. This is especially evident
for Fenton- or Gif-type reactions, that is, Fen+ with H2O2,
which constitute one of the most famous iron salt-dependent
Table 1. Fe-catalyzed selective oxidation of benzyl alcohol.[a]
[a] Dr. F. Shi, Dr. M. K. Tse, Prof. Dr. M. Beller
Leibniz-Institut für Katalyse e.V. an der Universität Rostock
Albert-Einstein-Str. 29a, 18059 Rostock (Germany)
Fax : (+49)381-1281-5000
Entry
KH2PO4 [mol%]
Conv. [%][b]
Sel. (CHO/CO2H) [%][c]
1
2
3
4
5
6
7
0
72
85
90
87
71
43
16
43 (81:19)
53 (77:23)
71 (75:25)
80 (75:25)
87 (70:30)
91 (56:44)
99 (50:49)
E-mail: matthias.beller@catalysis.de
0.5
1.0
2.0
3.0
4.0
6.0
[b] Dr. M. K. Tse, Prof. Dr. M. Beller
University of Rostock
Center for Life Science Automation (CELISCA)
Friedrich-Barnewitz-Str. 8, 18119 Rostock-Warnemünde (Germany)
[c] Z. Li
Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences
Lanzhou, 730000 (China)
[a] Benzyl alcohol/H2O2/Fe(NO3)3·9H2O/KH2PO4 1:1.5:0.02:0–0.06, sol-
A
vent free; H2O2 was added with a syringe pump in 12 h. [b] Benzyl alco-
hol conversion. [c] Total selectivity to benzaldehyde and benzoic acid
(ratio of benzaldehyde/benzoic acid).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801432.
Chem. Eur. J. 2008, 14, 8793 – 8797
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8793

Table 2. Iron-catalyzed oxidation of benzyl alcohol.[a]
Initially, the correlation of acidity of the reaction system,
catalyst activity and selectivity was investigated applying Fe-
Entry
Iron salts
Additive
[mol%]
Conv.
[%][b]
Sel. (CHO/CO2H)
[%][c]
(NO3)3·9H2O as the catalyst and hydrogen peroxide as the
oxidant in the presence of various concentration of KH2PO4
(Table 1). It is apparent that the selectivity of the reaction
increased dramatically with increased concentration of
1
2
3
4
5
6
FePO4·4H2O
FePO4·4H2O
FeCl3·6H2O
FeCl3·6H2O
0
2 H2SO4
0
3 KH2PO4
0
8
99 (99:<1)
90 (86:14)
54 (83:17)
84 (74:26)
69 (88:12)
87 (81:19)
80
78
74
84
85
KH2PO4 (Table 1, entries 1–7). Pure Fe(NO3)3·9H2O gave a
G
Fe
A
selectivity of only 43%. On the other hand, 99% selectivity
is achieved when 6% KH2PO4 was added (Table 1, entries 1
and 7). Interestingly, also the activity of the catalyst in-
creased adding a small amount of KH2PO4. Hence, the con-
version increased from 72 to 90% when 1% KH2PO4 was
employed (Table 1, entries 1–3). However, when the
KH2PO4 loading was further increased, the conversion de-
creased. Importantly, in the absence of KH2PO4 a significant
amount of black tar polymer was obtained after removal of
volatiles from the reaction mixture (Figure 1). It is of
common knowledge that such tar polymers are produced by
radical type reactions, for example, Fenton chemistry.[30]
Fe2
A
0.7 KHSO4
[a] Reaction conditions: See Table 1. [b] Benzyl alcohol conversion.
[c] Selectivity to benzaldehyde and benzoic acid (ratio of benzaldehyde/
benzoic acid).
while the selectivity is controlled by the change of pH value
(DpH) during the reaction. If this hypothesis is right, the
high selectivity should be independent of the pH value of
the buffer and the nature of the buffer system. Thus, the ac-
tivity of Fe(NO3)3·9H2O in the presence of various buffers
C
was tested (Table 3). In fact all catalytic reactions gave high
selectivity (83–99%) if the DpH
is ꢀ0.09, albeit the absolute pH
values. The conversion in-
creased with an increasing acid-
ity of the reaction system and
reached ~90% (Table 4, en-
tries 1–7). In the presence of
SO42À-based buffering systems,
similar results are obtained
(Table 4, entries 8 and 9). It is
clearly shown that the pH and
DpH but not the buffer compo-
sition is the key factors for the
high activity and selectivity. It
is important to note that all the
Figure 1. An illustration for the [Fe3+] and [Fe3+]/buffer catalyst system for selective oxidation of benzyl alco-
hol a) without pH control; b) before hydrogen peroxide addition, and c) precise pH control.
buffered reaction systems kept
homogeneous during the reac-
tion; although FePO4 precipita-
tion is observed under solvent
If the improved catalyst activity and selectivity originate
from the different proton concentration and not because of
the addition of KH2PO4, other iron salts should also catalyze
the reaction in similar manner. Hence, FeCl3·6H2O,
free conditions (Table 1, entries 2–7, Table 2, entries 2 and
4). Under iron-free conditions at pH value lower than ~1.20
Table 3. Catalytic activity of Fe
ACHTREUNG
FePO4·4H2O, and Fe2(SO4)3·5H2O were tested. KH2PO4,
A
Buffer pH[b]
H2SO4 and KHSO4 were used to tune the pH of the system.
Indeed, high selectivity to the aldehyde and acid was ach-
ieved and the catalyst activity is maintained (Table 2, en-
tries 1–6).
In order to understand the effect of the additives, we mea-
sured the pH of the reaction systems.[44] Surprisingly, the
precise pH value of all the reactions with high conversion
and selectivity is close to 1.00 (Table 1, entries 3 and 4,
Table 2, entries 2, 4 and 6). At the same time, the deviation
of the pH value (DpH) is close to 0.10. However, in reac-
tions with lower selectivity DpH is significantly larger
(>0.2).
Entry
1
2
3
4
5
6
7
0.38 (0.54/0.48)
0.61 (0.66/0.61)
0.78 (0.83/0.74)
1.00 (0.99/0.97)
1.19 (1.18/1.17)
1.39 (1.36/1.34)
2.00 (1.74/1.67)
1.00 (0.98/0.99)
0.98 (0.99/0.97)
87
87
86
85
82
38
27
87
89
86 (80:20)
85 (81:19)
83 (83:17)
87 (84:16)
85 (82:18)
84 (82:18)
89 (83:17)
88 (85:15)
89 (83:17)
0.06
0.05
0.09
0.02
0.01
0.02
0.07
0.01
0.02
8[e]
9[f]
[a] Reaction conditions: See Table 1. 5 mL buffer solution was used as
the solvent. [b] pH value of buffer solution (pH value of buffer solution
after the addition of reaction mixture before/after reaction).[45] [c] Benzyl
alcohol conversion. [d] Selectivity to benzaldehyde and benzoic acid
(ratio of benzaldehyde/benzoic acid). [e] Buffer solution based on
Na2SO4. [f] Buffer solution based on K2SO4.
Hence, we suppose that the catalyst activity is controlled
by the absolute proton concentration (actual pH value),
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Iron-Catalyzed Oxidation Reactions
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also some conversion to unknown products is observed (5–
20%). Here, no benzaldehyde or benzoic acid is formed
(see Supporting Information, Table S1).
of view, it is important to note that also the inhibition of tar
formation can be realized just through precise pH control.
Finally, we proved our theory in the benzene hydroxyl-
ation catalyzed by FeSO4·7H2O, which is a classical radical
initiated Fenton reaction. In the presence of FeSO4·7H2O as
catalyst, benzene conversion was ~25% and less than 1%
phenol is produced. A deep black reaction mixture formed
and the side products are tar polymers. Parallel investigation
of benzene oxidation under identical reaction conditions
except for the presence of buffer showed complete inhibi-
tion of any radical type reaction (Table 4, entries 3 and 4).
Based on well defined Fenton and Gif reactions, the pro-
posed mechanism of our reaction is shown in Scheme 1.[24–28]
As the production of tar polymers is significantly reduced in
our system, the non-radical process is apparently the main
pathway and the radical pathway is effectively inhibited by
maintaining the pH value.
Table 4. FeSO4·7H2O-catalyzed selective oxidation reactions.[a]
Entry
Substrate
Buffer ([mL])
Conv. [%][b]
Sel. [%][c]
40(83:17)
85 (81:19)
<1
1
2
3
4
–
73
77
25
<2
ACHTREUNG
benzyl alcohol
pH 0.99 (5)
–
pH 0.99 (5)
benzene
–
[a] Reaction conditions: See Table 1. [b] Benzyl alcohol or benzene con-
version. [c] Selectivity to benzaldehyde and benzoic acid (ratio of benzal-
dehyde/benzoic acid) or phenol.
This indicates that side reactions are possibly caused by
the acidic conditions but not from the iron catalyst itself. In
agreement with our hypothesis, the selectivity of the reac-
tion decreased at pH ~1.00 at lower concentration of the
buffer. mVariation of the concentration of buffer solutions
from 0 to 0.4m (based on PO43À) showed that buffer strength
3À
of ~0.025m of PO4 was sufficient to provide high conver-
sion with good selectivity (Table 5, entries 1–9).
Table 5. Effect of buffer concentration on the benzyl alcohol oxidation.[a]
3À
Entry Buffer pH[b]
c
(PO4
)
]
Conv. Sel.
DpH
[molLÀ1
[%][c]
A
1
2
3
4
5
6
7
8
9
6.69 (1.97/1.29)
0
97
96
97
97
94
92
87
87
83
48 (99:1)
55 (93:3)
70 (84:16)
72 (85:15)
82 (78:22)
87 (86:14)
89 (88:12)
89 (87:13)
88 (88:12)
0.68
0.21
0.14
0.07
0.04
0.03
0.01
0.02
0.02
1.01 (0.92/0.71) 0.003
1.02 (0.94/0.80) 0.006
1.02 (0.95/0.88) 0.013
0.98 (0.86/0.82) 0.025
0.98 (0.84/0.81) 0.050
1.02 (0.89/0.90) 0.100
1.02 (0.96/0.98) 0.200
0.99 (0.96/0.98) 0.400
Scheme 1. Proposed mechanism for the Fe-catalyzed alcohol oxidation.
[a] Reaction conditions: See Table 1. 5 mL buffer solution was used as
the solvent. [b] pH value of buffer solution (pH value of buffer solution
after the addition of reaction mixture before/after reaction).[45] [c] Benzyl
alcohol conversion. [d] Selectivity to benzaldehyde and benzoic acid
(ratio of benzaldehyde/benzoic acid).
Direct evidence for this hypothesis was attained by cyclic
voltammetry (CV) analysis of the reaction mixture
(Figure 2). The reduction potential of FeIII to FeII decreased
from 0.14 to À0.23 V, which indicated that the reduction of
FeIII to FeII is minimized. Additionally, no oxidation peak of
FeII to FeIII is observed up to 1.2 V after the addition of
KH2PO4. Thus, the oxidation of FeII to FeIII, which is respon-
sible for the production of hydroxyl radicals, is inhibited. In
other words, the catalytic cycle between FeII and FeIII is
blocked.
Kinetic studies of the oxidation of para-substituted benzyl
alcohols showed that the rate determining step involves gen-
eration of a carbocation from the respective benzyl alcohol.
The relative rate of oxidation of para-substituted benzyl al-
cohols to the corresponding benzaldehydes catalyzed by Fe-
It is worth to note that the highest yield is achieved using
a buffer composed with 0.05m PO43À. The pH value of the
whole system, KH2PO4, H2SO4, and Fe(NO3)3, was ~0.8 and
A
the DpH was only 0.03 (Table 5, entry 6). Again the result
from the buffer free system suggests that a pH change
during the reaction is favorable for radical generation reac-
tion, that is, high activity but low selectivity (Table 5,
entry 1).
Unexpectedly, omitting unwanted radical reactions by
precise pH value control can be also applied in other typical
radical systems, that is, FeSO4·7H2O with hydrogen perox-
ide. Here, only 40% selectivity is obtained at 73% conver-
sion (Table 4, entry 1). However, after the addition of 5 mL
buffer (pH 0.99),[45] the product yield is enhanced to 65 with
85% selectivity (Table 4, entry 2). From an industrial point
A
a large negative 1 (À2.5), which falls in the range of formal
hydride equivalent abstraction by an electrophilic metal–oxo
species and is significantly more negative than hydrogen
atom abstraction (Figure 3).[47–49]
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8795

M. Beller et al.
Scheme 2. Proposed mechanism for the Fe-catalyzed alcohol oxidation to
benzaldehyde.
Figure 2. Cyclic voltammetry (CV) analysis of Fe
A
+
PhCH2OH (red curve) and Fe(NO3)3·9H2O + KH2PO4 + PhCH2OH
AHCTREUNG
(blue curve). For the detailed information of the CV measurement,
please see Supporting Information.
higher catalyst activity and a small DpH during the reaction
is responsible for improved selectivity. Essentially, iron oxo
and radical pathways in iron-catalyzed redox reactions are
switchable using DpH value as an on–off. Although we still
cannot explain this phenomenon on a molecular level, we
believe it is of general importance in oxidation chemistry
and it should provide useful inspiration to rethink the mech-
anism of iron-catalyzed reactions, especially the century con-
troversial mechanism of Fenton chemistry.
Experimental Section
General procedure for the selective oxidation of benzyl alcohol to benz-
aldehyde: All reactions were carried out applying a multi-reactor (Carou-
sel 12 station, RADLEYS). Typically, 10 mmol benzyl alcohol (1.08 g),
Figure 3. Hammett plot of logkrel against substituent constants sp of
benzyl alcohols.[46]
0.2 mmol Fe(NO3)3·9H2O (80.8 mg), and 0–0.60 mmol KH2PO4 (0–
A
81.6 mg) were added sequentially to a glass vessel ~50 mL. Then, the re-
action mixture was vigorously stirred (500–750 rpm) at 758C. 15 mmol
H2O2 (30wt% in water, from Merck, 1.5 mL) was added continuously
with a syringe pump in 12 h. The mixture is cooled to room temperature
and 1,4-dioxane (1760 mg, 20 mmol) is added for qualitative analysis by
GC-FID. Depending on the pH the conversion was 16–90% with a selec-
tivity of 43–99% (aldehyde and acid).For the detailed information of CV
analysis, kinetic studies (logkrel vs sp) and isotope effect studies see Sup-
porting Information.
In
a competitive reaction of benzyl alcohol and
[D7]benzyl alcohol, a kinetic isotope effect (kH/kD) of 2.3
was observed, indicating the benzylic C H bond breaking as
À
the rate-determining step; this is consistent also with hy-
dride equivalent abstraction but far smaller than hydrogen-
atom abstraction by other metal–oxo complexes.[47–53] More-
over, in the presence of >30 equivalent of H218O to H216O2,
the ratio of 16O-benzaldehyde/18O-benzaldehyde was 70:30
as determined by GC-MS measurements. In the control re-
action, the ratio of 16O-benzaldehyde/18O-benzaldehyde was
46 (Æ2.7):54 (Æ2.7) and no 18O-exchange of benzyl alcohol
was observed. Hence, the isotope exchange mainly occurs
after the formation of benzaldehyde. Therefore, the direct
formation of PhCH16O from the carbocation should be the
main pathway. In agreement with studies of Fenton and Gif
reactions,[24–28] the kinetic studies here support the formation
of high valent iron oxo species and the non-radical pathway
is favorable (see Scheme 2).
Acknowledgements
This work has been supported by the State of Mecklenburg-Western
Pommerania, the Deutsche Forschungsgemeinschaft (SPP 1118 and Leib-
niz prize). Dr. Feng Shi thanks the Alexander-von-Humboldt-Stiftung for
an AvH Fellowship.
Keywords: Fenton chemistry · heterogeneous catalysis ·
iron · pH control · radicals
In conclusion, for the first time the relationship between
the absolute proton concentration (pH value), change of pH
value (DpH) and catalytic performance in important iron-
catalyzed selective oxidation reactions is demonstrated. It
clearly shows that an increased proton concentration led to
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tration of K2HPO4 was 0.40 molLÀ1. The pH value was then adjust-
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posed by Na2SO4 or K2SO4 and H2SO4: the concentration of Na2SO4
or K2SO4 was 0.60 molLÀ1. The pH value was then adjusted to lower
pH value with 98 wt%H2SO4.
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Received: July 16, 2008
Published online: September 2, 2008
Please note: Minor changes have been made to this publication in
Chemistry—A European Journal Early View. The Editor.
Chem. Eur. J. 2008, 14, 8793 – 8797
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