Tunable Photocatalytic Activity of Palladium-Decorated TiO2: Non-Hydrogen-Mediated Hydrogenation or Isomerization of Benzyl-Substituted Alkenes

Article abstract of DOI:10.1021/acscatal.6b02832

Palladium-decorated TiO₂ is a moisture- and air-tolerant versatile catalyst. Its photocatalytic activity can be tuned in favor of hydrogenation or isomerization of benzyl-substituted alkenes simply by changing the irradiation wavelength. Benzyl-substituted alkenes are selectively isomerized to phenyl-substituted alkenes (E-isomer) with complete conversion over Pd@TiO₂ under H₂-free conditions. The reaction can be thermally induced under air or driven by visible-light irradiation at room temperature under Ar. UV irradiation in methanol solvent leads to efficient hydrogenation. The fine-tunability of the catalyst can also be used for selective deuterium incorporation using deuterated solvents; here H/D exchange is used as a mechanistic tool but with clear potential for isotope substitution applications.

Full text of DOI:10.1021/acscatal.6b02832

Subscriber access provided by UB + Fachbibliothek Chemie | (FU-Bibliothekssystem)
Article
Tuneable photocatalytic activity of Palladium-decorated TiO2: non-hydrogen
mediated hydrogenation or isomerization of benzyl-substituted alkenes
Ayda Elhage, Anabel E. Lanterna, and Juan C. Scaiano
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02832 • Publication Date (Web): 22 Nov 2016
Downloaded from http://pubs.acs.org on November 22, 2016
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street
N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 7
ACS Catalysis
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Tuneable photocatalytic activity of palladium-decorated TiO : non-
hydrogen mediated hydrogenation or isomerization of benzyl-
substituted alkenes
Ayda Elhage, Anabel E. Lanterna* and Juan C. Scaiano*
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Department of Chemistry and Biomolecular Sciences and Centre for Catalysis Research and Innovation (CCRI). University of
Ottawa, 10 Marie Curie, Ottawa, ON K1N 6N5, Canada
ABSTRACT: Palladium-decorated TiO is a moisture and air-tolerant versatile catalyst. Its photocatalytic activity can be tuned in
2
favour of hydrogenation or isomerization of benzyl-substituted alkenes simply by changing the irradiation wavelength. Benzyl-
substituted alkenes are selectively isomerized to phenyl-substituted alkenes (E-isomer) with complete conversion over Pd@TiO2
under H -free conditions. The reaction can be thermally induced under air or driven by visible light irradiation at room tempera-
2
ture under Ar. UV irradiation in methanol solvent leads to efficient hydrogenation. The fine tunability of the catalyst can also be
used for selective deuterium incorporation using deuterated solvents; here H/D exchange is used as a mechanistic tool but with
clear potential for isotope substitution applications.
Keywords: photocatalysis, titanium dioxide, palladium, isomerization, hydrogenation.
Alkene isomerisation reactions have often been encountered
as undesired side-reactions during asymmetric catalytic hy-
drogenation or metathesis reactions,2-3 that have the poten-
tial to compromise enantioselectivity and promote the for-
mation of by-products. However, the deliberate and con-
trolled isomerisation of a pre-existing alkene bond can afford
positional and geometric isomers that are difficult to prepare
using conventional methods. Established methods for carry-
ing out these transformations include: treatment of an easily
The allyl phenyl ether group is important in the perfumery
and food industries, where t-anethole and i-eugenol have
high industrial demand as intermediates for the synthesis of
various perfumery chemicals, pharmaceutical compounds,
and beverages. t-Anethole is a naturally occurring product,
extracted from anise and fennel oils, with variable proportion
of its cis isomer as an impurity. However the increasing indus-
trial demand has made it necessary to develop alternative
synthetic routes for t-anethole, the only valuable product due
to the toxicity of its cis isomer. From an academic point of
view, several methodologies have been reported to isomerize
1
4
5
6
prepared alkene substrate with radicals , boranes , halogens,
7
and strong acids . However, there are many disadvantages
associated with these methods due to functional group in-
compatibility with the harsh reaction conditions that are of-
ten required for isomerisation to occur. Photochemical meth-
ods have been applied to the (E)/(Z)-isomerisation of al-
the estragole to anethole either catalyzed by metal complex-
16-17
es of Pd, Ru and Rh,
or carried out over catalysts belong-
18
ing to the hydrotalcite family. Most of the selective bond
isomerization of estragole studies has been done using ho-
mogeneous catalysis under a nitrogen atmosphere to main-
tain the catalyst stability. Nevertheless they are not commer-
cially viable due to the high synthetic costs.
8
kenes, but the success of this approach is substrate depend-
ent, with mixtures of products controlled by the photosta-
9
tionary state often being produced. The use of transition
metal-based catalysts offers several advantages over these
more conventional methods, such as functional group toler-
ance, mild reaction conditions, product selectivity and relia-
The use of supported PdNPs has been explored in the last few
years because of their potential to replace Pd complexes as
19-21
catalysts.
Imamura et al. have developed a solvent-
1, 10-14
bility.
From an industrial perspective, catalytic processes
mediated hydrogenation of alkenes using the photocatalytic
are more desirable than those using stoichiometric reagents,
as product separation is greatly simplified, particularly if het-
erogeneous catalysts are used. This is especially advanta-
geous for the synthesis of pharmaceutical compounds in-
tended for human consumption, which must adhere to strict
ability of TiO combined with the well-known H-Pd surface
interaction. On the other hand, Zhu and Shon have recently
2
22
presented a mechanism for hydrogenation/isomerization
23-24
reactions in the presence of H on PdNP surface.
Here we
2
present a water and air tolerant PdNP-decorated TiO2
Pd@TiO ) catalyst that shows dual switchable catalytic activi-
15
purity requirements.
(
2
ties changing from hydrogenation to isomerization on-
ACS Paragon Plus Environment

ACS Catalysis
Page 2 of 7
demand, by minor variations of the reaction conditions. Thus, studied for the synthesis of t-anethole using estragol. Table 1
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
the ability of Pd@TiO to photo-assist the hydrogenation of
alkenes upon UV excitation can be switched to lead isomeri-
zation simply by using visible light or thermal heating, without
summarizes the conditions screened for the optimization of
the thermally induced isomerization of estragol under air. The
best conditions were found when estragol was mixed togeth-
2
addition of H . Like the hydrogenation reaction, the photo-
er with Pd@TiO catalyst in i-propanol and refluxed for 24 h.
2
2
induced isomerization is also performed under Argon, whilst
the thermally induced reaction is air tolerant. The photocata-
lyst can be reused several times with excellent performance
Changing the catalyst support for a known inert material such
as crystal nanodiamonds (CND), the yield of the reaction de-
creases dramatically (cft. entries i-iii and iv-vii in table 1). The-
(
vide infra). Both conditions selectively lead to the trans reac-
se results show that the role of TiO is much more than simply
2
tion product. The selective isomerization of estragol to t-
anethole has been studied using i-propanol and methanol as
solvents and reducing agents under both reflux and visible
light irradiation, respectively. Further, mechanistic studies
demonstrate the reaction is solvent-mediated allowing selec-
tive deuterium incorporation. The scope of the catalytic activ-
ity has been explored with a group of valuable industrial
compounds, including i-eugenol.
anchoring the PdNP. Additionally, the reaction is highly fa-
voured in alcohols, although small conversion and lower se-
lectivity has been found using non-protic solvents (table 1,
entries viii-ix). These two conditions might be explained be-
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
cause of the reduction of PdO over TiO by secondary alco-
2
27
hols, which is favoured at high temperature. Control reac-
tions in the presence of TiO or in the absence of catalyst
2
were also performed.
Table 1: Effect of solvents and catalyst supports on the
thermal isomerization of estragol
RESULTS AND DISCUSSION
2+
PdNPs were synthesized by reduction of Pd under UVA irra-
25
26
diation in the presence of TiO2 (band gap energy ~3.2 eV ),
Scheme 1. The major advantage of this synthetic route is that
it does not require an organic ligand/reductant that could
arrest particle growth; it also maximizes the available nano-
particle surface. Pd@TiO were characterized by inductively
Solvent
Catalyst
% Conv
20
% A Yield
18 (≥90)
32 (71)
ND
2
coupled plasma optical emission spectrometry (ICP-OES),
transmission electron microscopy (TEM) and X-ray photoelec-
tron spectroscopy (XPS) to determine the total Pd loading,
the particle size and the oxidation state of the Pd species,
respectively. The synthesis described in the SI leads to 2 % Pd
loading with average particle size of (1.6 ± 0.9) nm (Scheme 1
and Figure S1). The XPS analysis of the binding energies of Pd
was calculated on the basis of the binding energy (284.8 eV)
i
i-propanol
Toluene
Pd@CND
Pd@CND
Pd@CND
Pd@TiO2
Pd@TiO2
ii
35
iii
iv
v
Acetonitrile
i-propanol
ND
100
ND
70
>99 (>90)
ND
b
i-propanol
a
vi
vii
viii
ix
x
Methanol
Pd@TiO
69 (86)
6 (>90)
33 (60)
8 (53)
ND
2
27
b
of C 1s and using reference values from the literature. Some
differences are expected on the binding energy of an element
in the nanoscale and in bulk. A high-resolution scan over the
range 327-347 eV reveals several peaks associated to differ-
ent Pd species present in the sample (Figure S6). Two pairs of
doublet signals can be recognized after deconvolution: a
double of a Pd (3d5/2) peak at around 334.7 eV and a Pd
Methanol
Pd@TiO2
11
Toluene
Pd@TiO2
45
Acetonitrile
i-propanol
i-propanol
Pd@TiO2
15
--
ND
ND
xi
TiO2
ND
a
b
(
3d3/2) peak at around 339.9 eV which can be assigned to
Conversion yields drop to ca. 39% in deuterated methanol.
Reaction time: 6h. Solvent boiling point: i-propanol = 85 °C, Ace-
tonitrile = 85 °C, Toluene = 111 °C, Methanol = 65 °C. % Yields
and conversions were calculated by GC-FID and/or H NMR using
28
Pd(0); the second pair located in a higher region (337.2 and
3
42.3 eV) in comparison with the Pd(0) indicates the presence
1
29-30
of PdO.
t-butylbenzene and dimethyl sulfone as external standards, re-
spectively. Percent A Yield correspond to the total isomerization
products, values between brackets show the selectivity towards
the trans isomer. No hydrogenation products have been detect-
ed under these conditions. Similar yields were found under an
inert atmosphere.
Considering that upon UV light irradiation Pd@TiO can cata-
2
22
lyze the hydrogenation of alkenes (tested here for estragol
(table 2, entry vi)), and that it has an important absorption in
the visible region, probably due to the presence of Pd or PdO
NPs (See Figure S2); the same reaction was tested upon visi-
ble irradiation at room temperature. Unexpectedly, only
isomerization products are found after 4 h of irradiation, as
can be noticed in table 2, entry i. The reaction works well in
methanol and goes to completion in 24 h (with traces of hy-
Scheme 1. Proposed mechanism for the photochemical syn-
thesis of supported PdNPs
The direct, selective, and non-hydrogen mediated isomeriza-
tion of benzyl-substituted alkenes over Pd@TiO catalyst was
2
ACS Paragon Plus Environment

Page 3 of 7
ACS Catalysis
pected due to minor changes in the nature and light scatter-
drogenation product, probably due to some UV light contam-
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
ination on the LED emission, see Figure S3) under Ar atmos-
phere. The temperature of the mixture rises up to ca. 50 ºC
upon excitation at 465 nm. Notice that the total irradiance
used under visible light is about 3 times higher compared to
UV light. Neither the hydrogenation nor the isomerization
work under air, suggesting that a reducing environment is
required for both reactions. Different catalyst concentrations
were tested for both isomerization and hydrogenation (Table
S1). Overall, different products can be obtained under essen-
tially the same conditions, simply by switching the excitation
ing of the suspension, therefore in the QY calculation the
absorbed photon flux was replaced by the incident photon
flux. The latter was determined using Ru(bpy) Cl as the acti-
nometer. Considering 4 h of reaction, the AQY was found
~0.29 % and ~0.53 % when 10 mg and 15 mg of catalyst were
used, respectively.
3
2
33
Pd-based catalysts are also known to perform deuterium in-
34-36
corporation in the presence of H by H-D exchange.
Here
2
we evaluated the deuterium incorporation in order to gain
insight the mechanism of the thermally and light-induced
isomerization and to evaluate its potential synthetic value.
Yields about 40 % isomerization (no double bond reduction)
were found after refluxing the reaction mixture for 24 h in
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
wavelength. Further, when TiO is replaced by CND (with
2
same NPs size distribution, cft. Figure S1 c and d), the isomer-
ization products are also formed, albeit in lower yields. How-
ever, for Pd@CND, hydrogenation products could not be de-
tected, even upon 368 nm irradiation. Finally, comparing the
isomerization yields obtained after conventional heating (en-
try vii, table 1), microwave irradiation (entry x, table 2) and
light irradiation (entry i, table 2) there is clear evidence that
CD OD, in contrast to the ca. 70 % found using regular meth-
3
anol (cf. entry vi, table 1). This isotopic effect hints to an im-
portant role of the solvent during the rate-limiting step of the
1
reaction. H NMR spectra were recorded showing ~21% of
deuterium incorporation in position a (Scheme 2) exclusively
the catalytic activity of the Pd@TiO is dominated by a pho-
2
(D ). The same experiment was carried out upon 465 nm irra-
1
toinduced process even if a minor thermal contribution may
be involved in the isomerization process. This is reminiscent
of what has been recently described for Pt NPs (<10 nm) that
diation as per conditions in table 2, entry ii. Although a great
isotopic effect was found using methanol-d (vide infra), min-
4
imal deuterium incorporation was detected by integration of
1
can indeed absorb visible light when embed on SiO supports
2
the H NMR signals.
resulting from enhanced near field of resonant scattering
31
modes.
Table 2: Effect of irradiation light and atmosphere on the
photocatalytic isomerization of estragol at room tempera-
ture
Scheme 2. Deuterium incorporation for isomerization upon
reflux in CD OD.
3
UV irradiation (4 h) in CD OD led to full reduction of the dou-
3
1
2
ble bond. H and H NMR show deuteration in positions a, b
and c with 0.75, 0.72 and 0.56 D incorporation, respectively
Time hν (  )
Solvent
%
Conv
48
100
58
55
0
100
0
33
15
0
% DA
% A
(
h)
(
See Figure S4). Deuteration in position c demonstrates the
i
ii
iii
iv
v
vi
vii
viii
ix
x
4
465
465a
465
465b
465d
368
Methanol
Methanol
i-propanol
Methanol
Methanol
i-propanol
Methanol
Methanol
Methanol
Methanol
Toluene
ND
7
48 (75)
93 (86)
58 (81)
55 (80)
ND
photocatalytic hydrogenation process occurs concurrently
24
24
24
24
4
with isomerization. Thus, the deuterated products (D and D )
2
3
ND
ND
ND
100
ND
ND
ND
ND
ND
ND
could be obtained by isomerization of estragol to anethole as
proposed in Scheme 3, followed by deuterium incorporation
(
path a) or by direct reaction of estragol (path b). Although,
ND
ND
no generation of estragol was detected when anethole is
submitted to 368 nm irradiation for 4 h, additional experi-
ments in deuterated solvent show also deuteration in posi-
tions a, b and c. This reinforces the idea that isomerization
can also take place upon UV irradiation, but these products
cannot be detected before they inexorably goes to full hydro-
genation process in the time range studied here, see SI (Fig-
ure S4 C and Table S2).
24
24
4
4
24
24
368c
465d
368d
MWe
465
33 (73)
15 (53)
ND
22 (82)
21 (81)
xi
xii
25
26
465
Toluene-d
8
a
Conversion yields drops to ca. 56 % in deuterated methanol
b
c
d
e
Catalyst two months old. Under air. Pd@CND. 55 ºC and 20
bars. % Yields and conversions were calculated by GC-FID and/or
H NMR using t-butylbenzene and dimethyl sulfone as external
1
-2
standards, respectively. Irradiance 10,035 mWm using 465 nm
-2
LED and 4,865 mWm using 368 nm LED. Percent A Yield corre-
sponds to the total isomerization products, values between
brackets show the selectivity towards the trans isomer.
Although the determination of the quantum yield (QY) of
32
heterogeneous catalysts is well known to be complex we
decided to measure the ‘apparent’ QY (AQY) of the isomeriza-
tion reaction. Uneven absorption of the photon flux is ex-
ACS Paragon Plus Environment

ACS Catalysis
Scheme 3. Deuterium incorporation for hydrogenation un-
Page 4 of 7
Scheme S1 in the Supporting information presents each case
as a separate panel.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
der 368 nm irradiation in CD OD.
3
Zhu and Shon have recently suggested that the hydrogena-
tion/isomerization reactions in the presence of H on PdNP
2
H
2
surface must occur through a mono-σ bonded Pd-alkyl inter-
mediate followed by β-hydride elimination as they previously
23-24
proposed.
When there is no H present, a π-allyl Pd hy-
2
dride mechanism is suggested. Figure 1 shows the analysis of
the gas phase of the Pd@TiO catalyst suspended in methanol
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
under Ar atmosphere upon irradiation at 368 nm or 465 nm.
As expected, a large amount of H can be identified upon UV
O2
N
2
2
irradiation (Scheme S2) but no generation of H was detected
2
when the catalyst was exposed to blue light (465 nm). The
3
.6
3.8
4.0
4.2
4.4
4.6
4.8
5.0
5.2
5.4
5.6
5.8
6.0
Retention Time (min)
presence of H suggests the mechanism involved in the hy-
2
drogenation reaction, although run at almost the same condi-
tions as the isomerization, might be different. We show here
that either under heating or visible light irradiation, benzyl
Figure 1. Gas chromatogram using Thermal Conductivity Detec-
tion (TCD). Gas phase of the Pd@TiO in methanol upon 465 nm
irradiation (black) and 368 nm irradiation (blue).
2
alkenes can be isomerized without addition of H , but
2
through a solvent-dependant reaction. Thus, although small
deuterium incorporation was found under both conditions,
the reaction yields are reduced to a half in methanol-d , sug-
4
gesting a significant isotopic effect on the isomerization itself.
When the same experiments were performed in toluene-d8
no isotopic effect was found (Table 2, entries xi and xii). We
believe the solvent might be involved in the decomplexation
step of the reaction, when the π-bonded Pd-allyl intermediate
has to be released from the Pd surface to lead to products
(
Scheme 4 A). This process might be critical to obtain high
yields when the reactions are performed in alcohols. Also, it is
known that alcohols can decompose on Pd and PdO surfaces,
27
releasing CO, a good reducing agent for PdO, which would
account for the differences found between MeOH and i-
37
propanol. As the sample is heated or illuminated with visible
light, the decomposition of the alcohol can reduce the surface
2
7
of the PdNP and promote isomerization. The use of deuter-
ated alcohols can decrease the rate of this process and all the
concomitant surface processes (i.e. isomerization). The major
implication of this mechanism is that the formation of π-allyl
Pd-hydride does not require H gas to proceed. This is simply
Scheme 4. A. Isomerization of alkenes upon irradiation at
465 nm. The formation of the π-allyl Pd-hydride adducts on
2
explained because the Pd−H adduct is generated from the
oxidative addition of Pd to the C−H bond α to the alkene
group to produce a π-allyl metal-hydride intermediate
the Pd surface does not require the presence of H
alkyl Pd hydride adduct can be formed on the Pd surface
activated upon H generation by UV light excitation. Notice
gas. B. σ-
2
(
Scheme 4 A). On the other hand, as reported by Imamura et
2
al., 368 nm excitation of Pd@TiO composites in methanol
generates H2 by alcohol oxidation on the TiO2 surface
in this cartoon the size of the particles has been exaggerated
to show easily the process on the Pd surface, but they are
not on scale. The grey boxes magnify the surface phenome-
na.
2
(
Scheme 4B and S1D). The Pd-alkyl mechanism shown in
Scheme 4 B requires the presence of the Pd−H species for the
catalytic reaction to occur. Our results show that H can be
generated in-situ by the oxidation of methanol to methanal
2
The scope of the isomerization reaction studied with different
allyl benzyl derivates under thermally or light-induced cataly-
sis is summarized in tables 3 and 4, respectively. Table 3
shows that the photocatalytic isomerization of benzyl alkenes
generally works better than the thermally induced reaction.
There are however, some exceptions where only thermal
catalysis can drive to isomerization in less than 48 h (cf. en-
tries iv and vi in table 3 and table 4). The selectivity of the
reaction towards the trans product was excellent in both cas-
es. Linear alkenes and cyclic dienes were also tested for these
reactions but the volatility of both, the reagents and the
on the TiO surface. Thus, in the absence of TiO , i.e. when
2
2
CND is used as catalyst support, only isomerization products
can be found upon 368 nm irradiation, simply because no H2
is generated on the inert diamond surface. Scheme 4 summa-
rizes mechanistically the four situations encountered in this
work, that is, thermal isomerization under air, photochemical
isomerization with visible light under Ar, photochemical hy-
drogenation and hydrogen generation (a process that also
occurs in the absence of substrate) both under UV light and
Ar. In order to facilitate visualizing the various situations,
ACS Paragon Plus Environment

Page 5 of 7
ACS Catalysis
cycle and becomes negligible by the third use; the photocata-
products, made the analysis of the reaction progress very
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
difficult.
lytic activity of Pd@TiO remains quite reasonable after four
2
catalytic cycles (see SI for more details). We believe that un-
der these conditions (methanol and an argon atmosphere) Pd
is reduced in situ to lead to essentially the same behavior as
the fresh samples. Interestingly, even a sample aged for two
months under air still shows 50% of its ‘fresh’ activity (see
Table 2, entry iv).
Table 3. Thermal isomerization of different allyl benzyl de-
rivativees catalyzed by Pd@TiO2 after 24 h reflux in i-
propanol.
100
%
Selectivity
trans
*
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
% Conv
% I Yield
80
i
100 (100*)
66 (72)
55 (69)
45 (11)
30 (59)
15 (NR)
NR (NR)
100 (93)
66 (72)
55 (69)
45 (11)
30 (59)
13 (--)
--
93 (86)
73 (86)
76 (61)
77 (100)
80 (88)
60 (--)
--
6
4
0
0
ii
iii
iv
v
20
0
†
1
2
3
4
vi
vii
Catalytic cycle
Figure 2. Catalyst reusability test upon 465 nm LED irradiation
solid bar) or dark conditions at 85 ºC (lined bar), using the same
*7% yield of product H. NR: No reaction. ND: No detectable. %
yields and conversions were calculated by GC-FID and/or H NMR
using t-butylbenzene and dimethylsulfone as external standards,
respectively. Values in brackets correspond to the % yields ob-
tained under irradiation as per conditions in Table 4 after 24 h.
(
1
conditions as in table 3. †No conversion detected. *Aꢀer catalyst
recovery treatment with I-2959.
Storing the catalyst under air as well as its use might lead to
partial oxidation of PdNPs as was confirmed by XPS (Figure
S7). The shifting of the Pd 3d peaks with the aging of the cata-
lyst to higher binding energy indicates that Pd can be slowly
Table 4. Light induced isomerization of different allyl benzyl
derivatives catalyzed by Pd@TiO2
2+
oxidized to Pd . Significant loss of the catalytic activity occurs
after multiple uses, as shown in Figure 2 for the thermal
isomerization at 85 ºC. In order to reactivate the used cata-
lyst, three different reduction treatments detailed in the Sup-
porting Information were performed: 1) heating treatment in
% Yield
27
Time
% Conv
i-propanol; 2) H treatment; and 3) photochemical treat-
2
I
H
ment using Irgacure-2959 (I-2959), a photoinitiator well-
38
known for the generation of reducing ketyl radicals. Even
i
24 h
42 h
30 h
48 h
48 h
48 h
48 h
100
100
100
24
93 (86)
7
when the three recovery methods seem to generate more
reduced Pd species (Figure S7); surprisingly, only I-2959
treatment fully recovers the thermally induced catalytic activ-
ity of Pd@TiO2.
ii
90 (87)
100 (92)
24 (71)
100 (92)
traces
10
--
iii
iv
v
--
CONCLUSIONS
The tunability of the Pd@TiO catalyst was demonstrated for
100
ND
ND
--
2
the isomerization/hydrogenation of benzyl-substituted al-
kenes. The hybrid composite is known to drive hydrogenation
vi
vii
--
of alkenes under UV light irradiation by generation of H upon
2
traces
--
methanol oxidation. However, minor changes on the reaction
conditions can drive the reaction to totally different but con-
trollable products. It has been demonstrated that the isomer-
1
%
yields and conversions were calculated by GC-FID and/or H
NMR using t-butylbenzene and dimethylsulfone as external
standards, respectively. Values between brackets correspond to
the % selective yield towards trans isomer
ization of benzyl alkenes could be performed over Pd@TiO in
2
methanol by both, thermally and visible light induced cataly-
sis, the latter by working under an inert atmosphere.
Pd@TiO catalyst was tested for potential reuse in the direct,
2
The proposed isomerization mechanism suggests a π-allyl Pd-
hydride intermediate that can be formed in the absence of H2
and mediated by solvent participation. Multiple deuterium
incorporations can be achieved managing different labelling
position by slightly modifying the reaction conditions.
selective and non-hydrogen mediated isomerization of es-
tragol under thermally and photo-induced catalysis, by per-
forming the reaction at least three times (Figure 2). The reac-
tion was scaled up to facilitate catalyst recovery and reuse.
Whilst the thermal catalytic activity decreases after the first
ACS Paragon Plus Environment

ACS Catalysis
Page 6 of 7
The reusability of the catalyst is very good when used under
14.
Borszeky, K.; Burgi, T.; Zhaohui, Z.; Mallat, T.; Baiker, A., J.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Catal. 1999, 187, 160-166.
light exposure. Although its catalytic activity is reduced after
use in thermal catalysis, the catalyst can be easily reactivated
by UVA irradiation in presence of the benzoin I-2959.
15.
Davies, I. W.; Matty, L.; Hughes, D. L.; Reider, P. J., J. Am.
Chem. Soc. 2001, 123, 10139-10140.
1
6.
Catal. Commun. 2005, 6, 205-209.
7. Lastra-Barreira, B.; Crochet, P., Green Chem. 2010, 12,
311-1314.
8. Kishore, D.; Kannan, S., J. Mol. Catal. A: Chem. 2006, 244,
Sharma, S. K.; Srivastava, V. K.; Pandya, P. H.; Jasra, R. V.,
ASSOCIATED CONTENT
Supporting Information
Experimental details, instrumentation used and NMR spectra, as
well as characterization of materials are shown in the supple-
mentary information. The Supporting Information is available
free of charge on the ACS Publications website.
1
1
8
1
3-92.
19.
McKenna, F. M.; Wells, R. P. K.; Anderson, J. A., Chem.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Commun. 2011, 47, 2351-2353.
20. McKenna, F. M.; Anderson, J. A., J. Catal. 2011, 281, 231-
40.
1.
015, 7, 872-876.
22. Imamura, K.; Okubo, Y.; Ito, T.; Tanaka, A.; Hashimoto, K.;
Kominami, H., RSC Advances 2014, 4, 19883-19886.
3. Sadeghmoghaddam, E.; Gu, H.; Shon, Y.-S., ACS Catalysis
2012, 2, 1838-1845.
2
2
Seth, J.; Kona, C. N.; Das, S.; Prasad, B. L. V., Nanoscale
AUTHOR INFORMATION
Corresponding Author
2
*
*
E-mail: titoscaiano@mac.com
E-mail: anabel.lanterna@icloud.com
2
24.
Gavia, D. J.; Maung, M. S.; Shon, Y. S., ACS Appl. Mater.
ACKNOWLEDGMENT
We thank the Natural Sciences and Engineering Research Council
of Canada, the Canada Research Chairs program and the Canadi-
an Foundation for Innovation for generous support.
Inter. 2013, 5, 12432-12440.
25.
Yang, D. C.; Feng, W.; Wu, G. J.; Li, L. D.; Guan, N. J., Catal.
Today 2011, 175, 356-361.
26.
Linsebigler, A. L.; Lu, G. Q.; Yates, J. T., Chem. Rev. 1995,
9
5, 735-758.
2
7.
Jurgensen, A.; Heutz, N.; Raschke, H.; Merz, K.;
REFERENCES
Hergenroder, R., Anal. Chem. 2015, 87, 7848-7856.
1
.
Borszeky, K.; Mallat, T.; Baiker, A., Catal. Lett. 1999, 59, 95-
28.
Noack, K.; Zbinden, H.; Schlögl, R., Catal. Lett. 1990, 4,
9
7.
145-155.
2
.
Hong, S. H.; Sanders, D. P.; Lee, C. W.; Grubbs, R. H., J. Am.
2
9.
Casaletto, M. P.; Longo, A.; Martorana, A.; Prestianni, A.;
Chem. Soc. 2005, 127, 17160-1.
Higman, C. S.; Lanterna, A. E.; Marin, M. L.; Scaiano, J. C.;
Fogg, D. E., Chemcatchem 2016, 8, 2446-2449.
4. Ichinose, Y.; Nozaki, K.; Wakamatsu, K.; Oshima, K.;
Utimoto, K., Tetrahedron Lett. 1987, 28, 3709-3712.
Venezia, A. M., Surf. Interface Anal. 2006, 38, 215-218.
0. Venezia, A. M.; Pantaleo, G.; Longo, A.; Di Carlo, G.;
Casaletto, M. P.; Liotta, F. L.; Deganello, G., J. Phys. Chem. B 2005,
09, 2821-2827.
1. Zhang, N.; Han, C.; Xu, Y. J.; Foley, J. J.; Zhang, D. T.;
3
.
3
1
3
5
.
Brown, H. C.; Bhatt, M. V., J. Am. Chem. Soc. 1966, 88,
Codrington, J.; Gray, S. K.; Sun, Y. G., Nat. Photonics 2016, 10, 473-
483.
1
440-&.
6
.
Ling, T. T.; Poupon, E.; Rueden, E. J.; Kim, S. H.;
32.
3.
Kisch, H., Angew Chem Int Edit 2010, 49, 9588-9589.
Pitre, S. P.; McTiernan, C. D.; Vine, W.; DiPucchio, R.;
Theodorakis, E. A., J. Am. Chem. Soc. 2002, 124, 12261-12267.
3
7
8
.
.
Hubert, A. J.; Reimling.H, J Chem Soc C 1969, 944-&.
Hashimoto, K.; Masuda, Y.; Kominami, H., ACS Catalysis
Grenier, M.; Scaiano, J. C., Sci Rep 2015, 5, 16397-16407.
4. Modutlwa, N.; Maegawa, T.; Monguchi, Y.; Sajiki, H., J
Labelled Compd Rad 2010, 53, 686-692.
5. Kurita, T.; Hattori, K.; Seki, S.; Mizumoto, M.; Aoki, F.;
3
2013, 3, 1349-1355.
9.
Turro, N. J.; Ramamurthy, V.; Scaiano, J. C., Modern
3
molecular photochemistry of organic molecules. University Science
Books: United States of America, 2010; Vol. 1.
Yamada, Y.; Ikawa, K.; Maegawa, T.; Monguchi, Y.; Sajiki, H., Chem-
Eur J 2008, 14, 664-673.
1
0.
816-9826.
1. Sharma, S. K.; Srivastava, V. K.; Jasra, R. V., J. Mol. Catal. A:
Chem. 2006, 245, 200-209.
Larionov, E.; Li, H.; Mazet, C., Chem. Commun. 2014, 50,
36.
Synthesis-Stuttgart 2009, 2674-2678.
7. Al-Azri, Z. H. N.; Chen, W. T.; Chan, A.; Jovic, V.; Ina, T.;
Idriss, H.; Waterhouse, G. I. N., J. Catal. 2015, 329, 355-367.
8. McGilvray, K. L.; Decan, M. R.; Wang, D. S.; Scaiano, J. C., J.
Am. Chem. Soc. 2006, 128, 15980-15981.
Maegawa, T.; Ito, N.; Oono, K.; Monguchi, Y.; Sajiki, H.,
9
1
3
1
2.
16.
13.
Borszeky, K.; Mallat, T.; Baiker, A., J. Catal. 1999, 188, 413-
3
4
Borszeky, K.; Mallat, T.; Baiker, A., Tetrahedron:
Asymmetry 1999, 10, 4781-4789.
ACS Paragon Plus Environment

Page 7 of 7
ACS Catalysis
Insert Table of Contents artwork here
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
7
ACS Paragon Plus Environment