Reactions in the Photocatalytic Conversion of Tertiary Alcohols on Rutile TiO2(110)

Article abstract of DOI:10.1002/anie.201907917

According to textbooks, tertiary alcohols are inert towards oxidation. The photocatalysis of tertiary alcohols under highly defined vacuum conditions on a titania single crystal reveals unexpected and new reactions, which can be described as disproportionation into an alkane and the respective ketone. In contrast to primary and secondary alcohols, in tertiary alcohols the absence of an α-H leads to a C?C-bond cleavage instead of the common abstraction of hydrogen. Surprisingly, bonds to methyl groups are not cleaved when the alcohol exhibits longer alkyl chains in the α-position to the hydroxyl group. The presence of platinum loadings not only increases the reaction rate but also opens up a new reaction channel: the formation of molecular hydrogen and a long-chain alkane resulting from recombination of two alkyl moieties. This work demonstrates that new synthetic routes may become possible by introducing photocatalytic reaction steps in which the co-catalysts may also play a decisive role.

Full text of DOI:10.1002/anie.201907917

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Breaking Ground: Reactions in the Photocatalytic Conversion of
Tertiary Alcohols on Rutile TiO2(110)
Authors: Carla Courtois, Moritz Eder, Kordula Schnabl, Constantin
Alexander Walenta, Martin Tschurl, and Ulrich Heiz
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201907917
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Breaking Ground: Reactions in the Photocatalytic Conversion of
Tertiary Alcohols on Rutile TiO2(110)
Carla Courtois‡[a], Moritz Eder‡[a], Kordula Schnabl[a], Constantin A. Walenta[a], Martin Tschurl[a] and Ueli
Heiz*[a]
Abstract: According to textbooks, tertiary alcohols are inert towards
oxidation. The photocatalysis of tertiary alcohols under highly
defined vacuum conditions on a titania single crystal reveals
unexpected and new reactions, which can be described as
disproportionation into an alkane and the respective ketone. In
contrast to primary and secondary alcohols the absence of an α-H
leads to a C-C-bond cleavage instead of the common abstraction of
photocatalysis due to its reaction properties and availability.[8]
While it is commonly applied in a nanostructure form (e.g. as
P25), the materials’ structural complexity often deteriorates the
elucidation of exact reaction mechanisms.[9] Similar to thermal
catalysis, defined single crystal surfaces under highly defined
conditions in ultra-high vacuum (UHV) are more suitable for this
purpose.[10] In heterogeneous photocatalysis, rutile TiO2(110)
hydrogen. Surprisingly, bonds to methyl groups are not at all cleaved, surfaces have been comprehensively employed in alcohol
when the alcohol exhibits longer alkyl chains in the α-position to the
hydroxyl group. The presence of platinum loadings not only
increases the reaction rate but also opens up new reaction channels
which are the formation of molecular hydrogen and a long-chain
alkane resulting from recombination of two alkyl moieties. This work
demonstrates that new synthetic routes may become possible by
introducing photocatalytic reaction steps in which the co-catalysts
may also play a decisive role.
conversion thermally[11] or photochemically.[12] Thus, this material
represents the best-suited model system, even though other
titania systems (e.g. anatase) may exhibit better photoactivities.
For this report, we investigated the photochemical reaction
behavior of longer-chain tertiary alcohols (3-methyl-3-hexanol, 2-
methyl-2-pentanol, and 2-methyl-2-butanol) on bare and
platinum-loaded rutile TiO2(110) in an UHV-environment in the
absence of oxygen and water. We demonstrate that the alcohols
exhibit unexpected and new photocatalytic reactions, which
enable general mechanistic insights. Furthermore, we show that
the rich chemistry of tertiary alcohols makes them an interesting
model system for photocatalysis. For example, they enable the
elucidation of the behavior of alkyl radicals on surfaces,
important for the photo-Kolbe reaction[13] and the Fischer-
Tropsch process.[14] In the latter, TiO2 represents a common
support material.[15]
The selective oxidation of alcohols to aldehydes and ketones is
a fundamental topic in various fields of chemistry ranging from
heterogeneous catalysis to synthetic organic chemistry.[1] In
contrast to the facile oxidation of primary and secondary
alcohols, tertiary alcohols are usually not convertible in a similar
manner, due to the required cleavage of a C-C instead of a C-H
bond to establish carbonyl functionality.[2] As tertiary alcohol
oxidation is generally difficult, in particular in a selective manner,
publications on this subject are thus scarce and often a broad
product spectrum results.[1c, 2-3] Conventional synthetic methods
often rely on auxiliary compounds or quantitative amounts of
oxidants to enable the reaction in the first place.[4] Often the
conversion is conducted with the use of toxic metal oxides such
as chromium(VI) oxides.[5]
An alternative approach for alcohol reforming is photocatalysis
using semiconductors.[6] For example, Teichner and coworkers
successfully photooxidized 2-methyl-2-butanol by means of UV-
irradiation on a nonporous anatase catalyst in the presence of
oxygen. The proposed reaction pathways take place via olefin
intermediates, leading to the reaction products acetone, ethanal,
and 2-butanone.[7]
In general, titania is by far the most used material in
Scheme 1. Reaction scheme for the photo-reforming of 3-methyl-3-hexanol on
Ptx/r-TiO2(110) and on r-TiO2(110) under UV illumination. The reaction can
formally be seen as a hole-mediated disproportionation yielding an alkane and
a respective ketone, which however does not occur for the formation of
methane.
[a]
C. Courtois, M. M. Eder, K. Schnabl, Dr. C. A. Walenta, Dr. M.
Tschurl, Prof. Dr. U. Heiz
Chair of Physical Chemistry
Department of Chemistry & Catalysis Research Center
Technische Universität München
Lichtenbergstraße 4, 85748 Garching, Germany
Email: ulrich.heiz@mytum.de
The UV-illumination of the TiO2(110) crystal, decorated with a
defined coverage (0.1% monolayer (ML)) of platinum clusters in
a size range from Pt8 to about Pt25, leads to a photocatalytic
reaction of 3-methyl-3-hexanol, which results in a complex
fragmentation pattern in the mass spectrum. However, a
detailed analysis reveals (see Fig. S2) that only two parallel
reactions occur, both of which are an oxidation to a ketone and a
‡
These authors contributed equally to this work
Supporting information for this article is given via a link at the end of
the document.
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corresponding alkane (2-pentanone and ethane, 2-butanone and
propane, see Scheme 1). While the formation of higher alkanes
is observed, a reaction yielding methane is not. Monitoring mass
traces specific for a particular molecule (Fig. 1) demonstrates
that the reaction is truly catalytic under illumination and a
formation of unwanted surface species leading to catalyst
poisoning does not occur. Compared to the photoreforming of
other alcohols, the observed reaction pathways are unexpected.
The absence of an -H precludes the common C-H cleavage to
form H2 and the respective aldehyde or ketone, as detected for
primary and secondary alcohols.[2, 16] Therefore, the ejection of
radicals and concomitant stoichiometric H2 production is
expected in analogy to tert-butanol photoreforming.[17] Neither
any radical abstraction nor significant molecular hydrogen
formation is detected for prolonged reaction times. Instead, this
reaction, which to the best of our knowledge has never been
cleavage of a C-C bond. The role of alkoxies being the
photoactive species on TiO2(110) in alcohol photoreforming has
convincingly been demonstrated in the works of Henderson and
20]
others.[12,
Methyl radical ejection observed in tert-butanol
demonstrates that the photocatalytic oxidation reaction occurs
via a homolytic C-C bond scission. In contrast to methyl groups,
longer alkyl chains such as ethyl and propyl exhibit stronger
interactions with the TiO2 surface in their adsorption geometry
and thus remain on the surface. This is in perfect agreement
with their absence in the mass spectra.
described before, can be viewed as
a
photocatalytic
disproportionation yielding higher alkanes and the respective
ketones. Interestingly, reaction products originating from a bond
cleavage of the methyl group are not at all observed (Fig. S9). In
the same way, dehydration reactions, common in thermal
reactions, do also not occur (Fig. S9). The same reaction is
observed for tertiary alcohols with two methyl groups at the -C
(namely, 2-methyl-2-butanol and 2-methyl-2-pentanol), for which
only the long carbon chain is abstracted with 100% selectivity
(see Fig. S3 and S4). Consequently, the formation of acetone
and the respective alkane results exclusively. This demonstrates
the generality of our findings. The same products are even
observed under ambient conditions for 2-methy-2-butanol, but
the presence of oxygen and water leads to additional by-
products.[7, 18]
UV illumination
4x10-10
3-Methyl-3-Hexanol
3x10-10
Propane
2x10-10
2-Butanone
1x10-10
Ethane
2-Pentanone
0
0
200 400 600 800 1000 1200 1400 1600 1800
Time /s
Figure 1. Photocatalytic products of 3-methyl-3-hexanol photoreforming on
Ptx/r-TiO2(110) (0.1% monolayer (ML) cluster coverage). 3-methyl-3-hexanol
(m/z 73), propane (m/z 29), 2-butanone (m/z 72), ethane (m/z 30), and 2-
pentanone (m/z 86) signals are shown at 340 K under a 3-methyl-3-hexanol
background pressure of 1.7 × 10−7 mbar. The blue region highlights the period
of UV laser irradiation. The initial burst of the signal originates from higher
surface concentrations of the alcohol before the start of the illumination. Note
that the traces are offset for clarity. The traces demonstrate that two different
photocatalytic reactions occur in parallel yielding a ketone and the respective
alkane.
Figure 2. Photocatalytic conversion of 3-methyl-3-hexanol on Pt-decorated r-
TiO2(110). In a) the TOF of the ketones (sum of 2-butanone and 2-pentanone)
is shown for different Ptx cluster coverages. In b), the selectivities for 2-
pentanone and 2-butanone based on the TOFs are displayed for the bare r-
TiO2(110) and different Pt-loadings on r-TiO2(110). A monolayer ML is referred
to the surface atoms. 0% ML stands for the bare r-TiO2(110). While the
deposition of Pt clusters does not affect the reaction’s selectivity, it initially
increases the TOF. However, higher loadings do not have a similar effect.
These surface alkyl radicals undergo recombination in
a
consecutive thermal reaction step with hydrogen atoms
originating from the dissociative adsorption of the alcohol. This
reaction is also enabled on bare TiO2(110) (i.e., in the absence
of Pt) in contrast to the recombination of two hydrogen atoms.
Consequently, photoreforming of higher tertiary alcohols occurs
in a photocatalytic manner even without any co-catalyst (Fig. 2a)
This new reaction can comprehensively be explained with a
mechanism suggested by us for the photoreforming of alcohols
on TiO2 in the gas phase.[17a] The photoactive alkoxy species,
which are already formed upon surface adsorption in the dark[12,
19]
undergo a hole-mediated oxidation reaction, resulting in the
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and on a hydroxylated surface (as shown in Fig. S13 for 2-
methyl-2-pentanol photoreforming), in contrast to -H containing
alcohols. For the latter, surface hydroxylation results in
poisoning of the photocatalyst.[17a] The deposition of small
amounts of Pt clusters significantly increases the overall reaction
rate, with higher loadings leading only to a small rise in the
turnover frequency (TOF) (Fig. 2a). This trend is in good
agreement with findings from methanol photoreforming in
UHV[17a] and of colloidal systems.[21]
suggest the formation of methyl radicals to require significantly
more energy than the ethyl or propyl formation (see details in the
SI). In addition, longer chain alkyl moieties than methyl exhibit
stronger interactions with the surface. These radicals are
therefore not detected in the gas phase, contrary to methyl
radical ejection in tert-butanol photoreforming. For ethyl and
propyl formation, the difference in thermochemistry is less
pronounced compared to methyl. However, reactions yielding
ethyl are generally more endothermic than propyl formation. This
trend is reflected in the observed selectivity of the reaction;
pentanone and an ethyl radical are preferentially formed over
butanone and a propyl radical. Therefore, thermodynamic values
may be used as a rule-of-thumb to predict preferential bond
cleavage in similar photoreactions.
Scheme 2. Reaction scheme for the photo-reforming of 2-methyl-2-pentanol
on bare r-TiO2(110) and Ptx/r-TiO2(110) under UV illumination. While on the
bare titania only the hole-mediated disproportionation yielding acetone and
propane occurs, a second reaction pathway is enabled for the Pt-decorated
one above 2.0 × 10-7 mbar alcohol pressure. In the latter reaction, hydrogen
recombines on the Pt-clusters and two propyl radicals recombine forming
hexane.
Performing the reaction at different pressures (3 × 10-8 mbar to 5
× 10-6 mbar) for the Pt-loaded photocatalyst does not affect the
branching ratio for two reactions either (Fig. S8a). The overall
TOFs exhibit typical 1st order behavior when the reaction is
limited by reactant adsorption and 0th order in the case of
limitation by product desorption (see Fig. S5 and S6). Similarly,
the illumination-dependent TOFs (see Fig. S7) suggest a first-
order behavior at lower irradiation illumination intensities, which
transfer into a saturation regime (zeroth-order) at higher photon
fluxes, as in the photoreforming of other alcohols.[17a] However,
and more importantly, for platinum-decorated TiO2(110) an
additional side reaction becomes visible at higher pressures.
This is best illustrated for 2-methyl-2-pentanol photoreforming
(Fig. 3), for which all reaction products can clearly be quantified
and are not deteriorated by isobaric interference. As deposited
platinum clusters enable the efficient thermal recombination of
hydrogen atoms,[17a] the surface coverage of alkyl increases in
the steady state with rising pressure. Consequently, the
recombination product of two radicals (i.e., hexane)
accompanied by H2 formation is detected at 5 × 10-6 mbar of
alcohol pressure (Fig. 3a). As the formation of H2 is not
facilitated on bare titania, this side reaction is not observed in
the absence of a co-catalyst (Fig. 3b).
Consequently, this result also demonstrates that with the
addition of noble-metal clusters not only unwanted consecutive
reactions (as for example the hydrogenation of ketones recently
studied mechanistically in electrochemistry[22]), but also an
intrinsically different outcome of the photoreaction cycle must be
considered in applied systems.
To summarize, we discovered a new reaction for photoreforming
of tertiary alcohols on rutile, which can be described as hole-
mediated disproportionations yielding an alkane and the
respective ketone. Surprisingly, the abstraction of methyl groups
does not occur and only -C bonds to longer alkyl chains are
Figure 3. Photocatalytic products of 2-methyl-2-pentanol photo-reforming on
a) Ptx/r-TiO2(110) (0.1% ML cluster coverage) and b) r-TiO2(110). Propane
(m/z 29), acetone (m/z 58), hydrogen (m/z 2), and hexane (m/z 86) signals are
shown at 336 K under a 2-methyl-2-pentanol background pressure of 5.0 ×
10−6 mbar. The blue region highlights the period of UV laser irradiation. Note
that the traces are offset for clarity. Different to bare TiO2(110), the co-catalyst
loaded semiconductor enables another side reaction, which is alkyl radical
recombination and molecular hydrogen formation.
In good accordance with the interpretation of the photooxidation
on the semiconductor and a consecutive alkane formation, the
selectivity of the reaction of 3-methyl-3-hexanol remains
unaffected by the degree of Pt coverage (Fig. 2b). It also stays
constant with temperature (Fig. S8b) between 230 and 360 K,
further demonstrating the photocatalytic nature of the reaction.
In order to explain the observed selectivity of pentanone to
butanone of about 2:1 (i.e., the preferred cleavage of ethyl over
propyl and general absence of methyl), the thermochemistry of
the reactions may be used to obtain qualitative insights. All three
possible reactions displayed in Scheme 1 are endothermic by
about 20 to 30 kJ/mol (see the corresponding chapter in the SI).
Model reactions for the photocatalytic C-C bond cleavage
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Acknowledgements
The authors would like to thank Martin Stutzmann (TU Munich)
for helpful discussions and the DFG for project funding via grant
HE3435/22-1. CC acknowledges the support by the Luxembourg
National Research Fund (FNR), project code 12531916 and by
the TUM International Graduate School of Science and
Engineering (TUM-IGSSE).
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Keywords: photocatalysis • tertiary alcohols • titania • alcohol
reforming • reaction mechanisms
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Page No. – Page No.
Title
Photoreforming of tertiary alcohols on TiO2(110) reveals an unexpected reaction: a
selective disproportionation to an alkane and the respective ketone. Upon Pt-cluster
loading, the overall reaction rate is increased, accompanied by an additional
reaction pathway. In the latter, hydrogen evolution and alkane dimerization are
observed in addition to the respective ketone formation, a new reaction pathway in
photocatalysis under mild conditions.
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