Semiconductor-photocatalyzed sulfoxidation of alkanes

Article abstract of DOI:10.1002/anie.200800326

(Equation Presented) The C-H activation of alkanes with visible light is possible with a semiconductor as the photocatalyst. Upon irradiation, a mixture of titanium dioxide, sulfur dioxide, and oxygen can be used to convert alkanes into the corresponding sulfonic acids (see scheme).

Full text of DOI:10.1002/anie.200800326

Angewandte
Chemie
DOI: 10.1002/anie.200800326
Activation of Alkanes
Semiconductor-Photocatalyzed Sulfoxidation of Alkanes**
Francesco Parrino, Ayyappan Ramakrishnan, and Horst Kisch*
ꢀ
C H bond activation and functionalization is one of the major
challenges in chemistry.[1] A rare example of an industrially
applied process is the sulfoxidation of liquid alkanes by sulfur
dioxide and oxygen upon UV irradiation [Eq. (1)].[2]
like radical dimerization and radical–radical recombination
dominate and the reaction requires permanent irradiation.
However, addition of radical initiators or promoters like
acetic anhydride again induces a chain reaction.[4,5] In general
regioisomeric alkyl radicals are formed in the hydrogen-
abstraction step; one exception is the photosulfoxidiation of
adamantane in the presence of hydrogen peroxide which
affords regioselectively 1-adamantanesulfonic acid.[7] A rare
example of a sensitized process is the mercury-photosensi-
tized sulfination of alkanes with SO2 producing initially
sulfinic acids (RSOOH) and sulfinic esters, which can be
further oxidized to sulfonic acids with hydrogen peroxide.[8]
All the reactions mentioned above occur only upon
excitation of sulfur dioxide or mercury with UV light. We
report herein on the first catalytic photosulfoxidation of
alkanes with visible light. This reaction does not require UV
lamps or toxic sensitizers but only a nontoxic semiconductor
powder.
1= O2 þ hn ! RSO3H
ð1Þ
RꢀH þ SO2
þ
2
In the case of the linear C16–20 alkanes the resulting
alkanesulfonic acids are used as biodegradable surfactants.
The primary reaction steps of this rare alkane functionaliza-
[3]
tion consist of UV excitation of SO2 followed by hydrogen
abstraction from the alkane to produce an alkyl radical
(Scheme 1). Subsequent addition reactions with SO2 and O2
generate an alkylpersulfonyl radical, which in turn produces
another alkyl starter radical and the persulfonic acid.
Fragmentation and hydrogen abstraction [Eqs. (2) and (3)]
afford the alkanesulfonic acid.[4–6]
When a suspension of titania in n-heptane was irradiated
with visible light (l ꢁ 400 nm) under an atmosphere of SO2/O2
(1:1, v/v), the formation of n-heptanesulfonic acid (1) was
observed (Table 1, Figures S1 and S2 in the Supporting
Information). Only traces of the sulfonic acid were observable
in the absence of titania. Initial rates of product formation
were 3.5 mmolLꢀ1 hꢀ1 and 5.0 mmolLꢀ1 hꢀ1 for the anatase
materials Titanhydrat and Hombikat, respectively, whereas
for rutile and the mixed-phase powder P25 (75% anatase/
25% rutile) values of 6.0 mmolLꢀ1 hꢀ1 and 7.5 mmolLꢀ1 hꢀ1
were observed. Of the modified titania powders (entries 5–7,
Table 1), which are all good photocatalysts for the complete
oxidation of 4-chlorophenol with visible-light irradiation,[9–12]
only the titanium dioxide/chlororhodate complex and carbon-
or nitrogen-modified titania exhibited moderate rates of
3.5 mmolLꢀ1 hꢀ1.
Scheme 1. Mechanism of sulfoxidation of alkanes upon UVirradiation.
C
C
RSO2ꢀOꢀOꢀH ! RSO2ꢀO þ OH
ð2Þ
ð3Þ
C
C
RSO2ꢀO þ RꢀH ! RSO3H þ R
Under the given experimental conditions, formation of 1
stopped after 6 h of irradiation time (Figure S3 in the
Supporting Information). However, separating the catalyst
powder and washing with methanol restored the activity.
According to this reaction scheme, photosulfoxidation is a
photoinduced radical chain reaction and therefore proceeds
without further irradiation when the substrates are short-
chain alkanes (< C10) devoid of impurities. In the case of long
unbranched alkanes of insufficient purity, termination steps
Table 1: Initial rate ri for the formation of heptanesulfonic acid (1) in the
presence of various titania photocatalysts (see the Experimental
Section).
[*] F. Parrino, Dr. A. Ramakrishnan, Prof. Dr. H. Kisch
Department Chemie und Pharmazie
Entry
Photocatalyst[a]
ri [mmolLꢀ1 hꢀ1
]
Interdisciplinary Center of Molecular Materials (ICMM)
Universität Erlangen-Nürnberg
Egerlandstrasse, 1 91058 Erlangen (Germany)
Fax: (+49)9131-852-7363
1
2
3
4
5
6
7
Titanhydrat(A)
TiO2 (Hombikat, A)
TiO2 (B)
TiO2 (P25, A+B)
[TiO2]OPtCl4 (A)
[TiO2]ORhCl3 (A)
TiO2-C, TiO2-N (A)
3.5
5.0
6.0
7.5
0.0
3.5
3.5
E-mail: kisch@chemie.uni-erlangen.de
[**] This work was supported by Deutsche Forschungsgemeinschaft
(SFB 583).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200800326.
[a] A and B designate anatase and rutile modifications, respectively.
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Even after this procedure had been repeated three times, the
photocatalyst still retained its original activity (Figure 1). This
observation suggested that the reaction is inhibited by strong
product adsorption and that washing desorbs the sulfonic
acid. Accordingly, no product formed when 1 was added to
the suspension prior to irradiation. A similar deactivation and
Scheme 2. Proposed mechanism for alkyl radical generation.
photosulfoxidation summarized in Scheme 1 and Equa-
tions (2) and (3).
The proposed mechanism is supported by the observation
that the reaction is inhibited in the presence of only 10 vol%
2-propanol; this compound should be oxidized much more
readily than the alkane, and it is also an efficient OH radical
scavenger.[19]
Figure 1. Sequential photosulfoxidation of n-heptane. lirr ꢁ400 nm.
Reg=regeneration of catalyst.
ꢀ
The general applicability of the presented C H activation
is demonstrated by the successful photosulfoxidation of
cyclohexane and adamantane. Since adamantate is a solid,
the reaction was conducted in glacial acetic acid (see
Figures S5 and S6 in the Supporting Information). Of the
anatase powders investigated, Titanhydrat led to the highest
yield of 1-adamantanesulfonic acid (2) (Figure 2).
activation was observed during photooxidation of sulfur
dioxide in the presence of gaseous n-heptane at UV-irradiated
titania.[13] Product formation was also inhibited when small
amounts of water (0.3 vol%) were present in the suspension.
This may be because the reactive surface centers for heptane
oxidation are blocked by preferential adsorption.
When the reaction was stopped after 2 h of irradiation—
this corresponds to the formation of 1 in 15 mm concentra-
tion—and the reaction mixture was left for three days in the
darkat room temperature, product formation continued
affording 50 mm of 1. However, when the radical scavenger
hydroquinone[14] was present during the darkphase, the
production of sulfonic acid did not continue.
These findings suggest that this novel photosulfoxidation
is also a radical chain reaction. However, in this case the alkyl
starter radical is generated not by UV excitation of sulfur
dioxide but through absorption of visible light by the TiO2/n-
heptane/SO2/O2 system. Since only the modified titania
powders (entries 5—7, Table 1) are able to absorb visible
light, it seems likely that the unmodified materials (entries 1–
4, Table 1) form a charge-transfer complex with sulfur
dioxide. In fact, exposure of P25 to sulfur dioxide resulted
in a yellowish coloration of the powder, which corresponds to
a broad absorption maximum in the diffuse reflectance
spectrum at 410–420 nm (see Figure S4 in the Supporting
Information).
Figure 2. Yield of adamantanesulfonic acid (2) in the presence of the
photocatalysts Titanhydrat (a), P25 (b), TiO2-C (c), and TiO2-N (d).
ꢀ
In summary, this novel visible-light-induced C H activa-
tion can be classified as a semiconductor-photocatalysis
type B reaction, and the the previously known two-compo-
nent addition[20] is extended to a three-component system.
Accordingly, we propose the mechanism for alkyl radical
generation depicted in Scheme 2. Visible light excitation of
the charge-transfer complex affords a conduction band
electron [TiO2(eꢀ)] and an adsorbed sulfur dioxide radical
cation. Oxygen reduction by [TiO2(eꢀ)] produces superoxide,
and the adsorbed sulfur radical cation may oxidize the alkane
to the alkyl radical and a proton.[15] Superoxide may also
generate an alkyl radical through protonation by adsorbed
water or surface OH groups to the hydroperoxyl radical[18]
and subsequent hydrogen abstraction from the alkane. The
alkyl radical thus produced is expected to initiate a radical
chain reaction as formulated for the stoichiometric UV
Experimental Section
Titanhydrat (Kerr-McGee Pigments, 300 m2 gꢀ1) and P25 (Degussa,
50 m2 gꢀ1) were used as received. We are grateful to Prof. T. Egerton
for a sample of high-surface-area rutile (140 m2 gꢀ1). [TiO2]OPtCl4,
[9]
TiO2-C,[10] TiO2-N,[11] and [TiO2]ORhCl3,[12] were prepared according
to literature procedures and have surface areas of 260, 160, 170, and
230 m2 gꢀ1
, respectively. Adamantane (Acros) and n-heptane
(Fischer) were used as received.
Titania powder (30 mg) was suspended in n-heptane or cyclo-
hexane (15 mL) in a Solidex glass cuvette and sonicated for 15 min.
Thereafter, a gaseous mixture of O2 and SO2 (60 mL, 1:1 v/v) was
added by a syringe. Irradiation was performed with an Osram XBO
150 W xenon arc lamp (Io (400–520 nm) = 2 ” 10ꢀ6 Einsteinsꢀ1 cmꢀ2
)
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installed in a light condensing lamp housing (PTI A1010S) on an
optical train. A cutoff filter of l ꢁ 400 nm was placed in front of the
cuvette. The suspension was stirred magnetically. After 5 h of
irradiation, the photocatalyst was removed with a micropore filter
(Whatman 0.45 mm) and the filtrate was concentrated in vacuo. The
slightly yellow, oily residue was dissolved in methanol (3 mL) and
analyzed by HPLC[21,22] (see the Supporting Information). Adaman-
tane (136.2 mg, 1 mmol) was photosulfoxidized analogously in glacial
acetic acid (15 mL; see the Supporting Information). The high
photocatalyst concentration of 2 gLꢀ1 ensures complete light absorp-
tion in each experiment, and therefore the initial rates (calculated
from the product concentration at 5 h irradiation time) are compa-
rable.
[4] H. Ramloch, G. Täuber, Chem. Unserer Zeit 1979, 13, 157.
[5] M. Braun, M. T. Maurette, E. Oliveros, PhotochemicalTechnol-
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[6] R. Graf, Justus Liebigs Ann. Chem. 1952, 578, 50.
[7] G. W. Smith, H. D. Williams, J. Org. Chem. 1961, 26, 2207.
[8] R. R. Ferguson, R. H. Crabtree, J. Org. Chem. 1991, 56, 5503.
[9] W. Macyk, G. Burgeth, H. Kisch, Photochem. Photobiol. Sci.
2003, 2, 322.
[10] S. Sakthivel, H. Kisch, Angew. Chem. 2003, 115, 5057; Angew.
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[11] S. Sakthivel, M. Janczarek, H. Kisch, J. Phys. Chem. B 2004, 108,
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[12] Z. Dai, G. Burgeth, H. Kisch, unpublished results.
[13] J. Shang, Y. Zhu, Y. Du, Z. Xu, J. Solid State Chem. 2002, 166,
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[14] Y. Ishii, K. Matsunaka, S. Sakaguki, J. Am. Chem. Soc. 2000, 122,
7390.
Received: January 22, 2008
Revised: March 14, 2008
Published online: August 5, 2008
[15] When one applies the bond dissociation energy of adamantane
of about 4.2 eV[16] and E0(H+/H) = ꢀ2.4 V (H2O),[17] a potential
of 1.8 V is estimated for oxidation of RH to RC + H+.
[16] F. Recupero, A. Bravo, H. Bjorsvik, F. Fontana, F. Minisci, M.
Piredda, J. Chem. Soc. Perkin Trans. 2, 1997, 11, 2399.
[17] V. D. Parker, J. Am. Chem. Soc. 1992, 114, 7458.
[18] O. Carp, C. L. Huisman, A. Reller, Prog. Solid State Chem. 2004,
32, 33.
[19] K. S. Docherty, P. J. Ziemann, AerosolSci. Techno.l 2003, 37,
877.
[20] W. Schindler, H. Kisch, J. Photochem. Photobiol. A 1997, 103,
257.
ꢀ
Keywords: alkanes · C H activation · photocatalysis ·
sulfoxidation · sulfur dioxide
.
[1] a) R. G. Bergman, Nature 2007, 446, 391; b) T. Graening, Nachr.
Chem. 2007, 836.
[2] In industries processes high-pressure mercury vapor lamps (10—
40 kW) are used UV sources. See: Ullmannꢀs Encyclopedia of
IndustrialChemistry , 5E, Vol. A25, Wiley-VCH, Weinheim,
pp. 772–775.
[3] In n-hexane or isooctane SO2 shows a maximum absorption at
290 nm (e = 250mꢀ1 cmꢀ1); Y. Ogata, Y. Izawa, T. Tsuda,
Tetrahedron 1965, 21, 1349.
[21] J. R. Larson, J. Chromatogr. 1986, 356, 379.
[22] H. Small, T. E. Miller, Jr., Anal. Chem. 1982, 54, 462.
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