H2-Generation from Alcohols by the MOF-Based Noble Metal-Free Photocatalyst Ni/CdS/TiO2@MIL-101

Article abstract of DOI:10.1071/CH19255

The synthesis of important classes of chemical compounds from alcohols helps to conserve Earth's fossil carbon resources, since alcohols can be obtained from indigestible and abundantly available biomass. The utilisation of visible light for the activation of alcohols permits alcohol-based C-N and C-C bond formation under mild conditions inaccessible with thermally operating hydrogen liberation catalysts. Herein, we report on a noble metal-free photocatalyst able to split alcohols into hydrogen and carbonyl compounds under inert gas atmosphere without the requirement of electron donors, additives, or aqueous reaction media. The reusable photocatalyst mediates C-N multiple bond formation using the oxidation of alcohols and subsequent coupling with amines. The photocatalyst consists of a CdS/TiO₂ heterojunction decorated with co-catalytic Ni nanoparticles and is prepared on size-optimised colloidal metal-organic framework (MOF) crystallites.

Full text of DOI:10.1071/CH19255

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem. 2019, 72, 842–847
Communication
https://doi.org/10.1071/CH19255
H2-Generation from Alcohols by the MOF-Based Noble
Metal-Free Photocatalyst Ni/CdS/TiO2@MIL-101*
A
A
B
Dominic Tilgner, Mara Klarner, Sebastian Hammon,
A
C D
,
C E
,
Martin Friedrich, Andreas Verch,
Niels de Jonge,
B
A F
,
Stephan Kummel, and Rhett Kempe
¨
A
Inorganic Chemistry II, Catalyst Design, University of Bayreuth, Bayreuth 95440,
Germany.
B
Theoretical Physics IV, University of Bayreuth, Bayreuth 95440, Germany.
C
INM – Leibniz Institute for New Materials, Saarbrucken 66123, Germany.
¨
D
Carl Zeiss AG, Oberkochen 73447, Germany.
E
Department of Physics, Saarland University, Saarbrucken 66123, Germany.
¨
F
Corresponding author. Email: kempe@uni-bayreuth.de
The synthesis of important classes of chemical compounds from alcohols helps to conserve Earth’s fossil carbon resources,
since alcohols can be obtained from indigestible and abundantly available biomass. The utilisation of visible light for the
activation of alcohols permits alcohol-based C–N and C–C bond formation under mild conditions inaccessible with
thermally operating hydrogen liberation catalysts. Herein, we report on a noble metal-free photocatalyst able to split
alcohols into hydrogen and carbonyl compounds under inert gas atmosphere without the requirement of electron donors,
additives, or aqueous reaction media. The reusable photocatalyst mediates C–N multiple bond formation using the
oxidation of alcohols and subsequent coupling with amines. The photocatalyst consists of a CdS/TiO2 heterojunction
decorated with co-catalytic Ni nanoparticles and is prepared on size-optimised colloidal metal–organic framework (MOF)
crystallites.
Manuscript received: 4 June 2019.
Manuscript accepted: 22 July 2019.
Published online: 15 August 2019.
The dehydrogenation of alcohols is a fundamental reaction in
sustainable chemistry.[1] Alcohols can be obtained via pyrolysis
and hydrogenation from lignocellulose,[2] an abundantly avail-
able, indigestible, and barely used biomass.[3] Thus, alcohols are
a promising renewable carbon feedstock and the development of
alcohol refunctionalisation reactions is the key to its use.[4]
Alcohols are rather unreactive and dehydrogenation is an ele-
gant way of activating them. The oxidised form, carbonyl
compounds, can undergo condensation reactions permitting the
formation of C–C or C–N multiple bonds. The synthesis of
imines from alcohols and amines introduced by Milstein and co-
workers is an example of broad interest for such a C–N bond
formation reactions.[5] Our group has developed the synthesis of
aromatic N-heterocyclic compounds from alcohols in which
C–N and C–C multiple bond formation takes place concert-
edly.[4,6–9] We expected that a photocatalyst able to mediate
C–N bond formation via dehydrogenative coupling under very
mild reaction conditions is highly desirable to significantly
extend the applicability of existing thermally operating noble
metal-free catalysts for such reactions.[5b,6b,8b-c,10]
multiple bond formation takes place via dehydrogenation of
alcohols under very mild conditions without sacrificial electron
donors (alcohol splitting). Our noble metal-free photocatalyst
consists of a CdS/TiO2 heterojunction prepared on a porous
support that is decorated with co-catalytic Ni nanoparticles. The
metal–organic framework (MOF) MIL-101 (Cr)[11] was used as
the porous support material stabilising the photocatalytic sys-
tem.[12] We demonstrate the acceptorless dehydrogenation of
alcohols by splitting into hydrogen and carbonyl compounds at
room temperature with a broad substrate scope to generate
aldehydes and dialkyl, diaryl, and aryl-alkyl ketones. Visible
light-driven alcohol dehydrogenation without electron donors is
rarely found in the literature[13] and often requires aqueous
media, including additives.[14] Aqueous media are disadvanta-
geous regarding condensation steps, which are needed in many
dehydrogenative coupling/condensation reactions.
We expected that the arrangement of CdS and TiO2 nano-
crystals on a suitable MOF would be an elegant way to create
heterojunction systems with efficient charge carrier separation
and high photocatalytic activity. The pores of the MOF can be
selectively loaded with precursor molecules to control the size
and amount of the different nanocrystals without applying
Here, we report on a photocatalyst that mediates visible light-
driven imine synthesis from alcohols and amines. This C–N
*This paper is dedicated to Richard Robson, an outstanding pioneer in the field of porous coordination polymers and metal-organic frameworks.
Journal compilation Ó CSIRO 2019
www.publish.csiro.au/journals/ajc

Photocatalytic Alcohol Splitting
843
RЈ
OH+H2N RЉ
(a)
(b)
(c)
H2
H2O
RЉ
+
N
Ni
RЈ
Visible
light
e–
CB
VB
e–
MIL-101
200 nm
20 nm
(f)
Cds
500 nm
(d)
h+
TiO2
(e)
(h)
(k)
Scheme 1. Illustration of the Ni/CdS/TiO2@MIL-101 material with a
core–shell morphology. The MOF-supported Ni/CdS/TiO2 heterojunction
system is an efficient photocatalyst for the dehydrogenative coupling of
alcohols and amines under visible light illumination.
Cd/Ti
zoom
Cr
Cd/Ti
(g)
(i)
surface blocking ligands. MIL-101 (Cr) was chosen because of
its stability during solvothermal modifications and its large pore
volume. It was prepared according to a procedure described
previously with an average crystallite size of 300 nm (Fig. S1,
Supplementary Material).[15] This crystallite size is optimal
regarding catalytic performance. The crystallites are large
enough to ensure efficient separation for reusability and small
enough to have a sufficiently large outer surface for modifica-
tion with light harvesting materials. We recently reported a
synthesis protocol of crystalline TiO2 (anatase) on the surface of
MIL-101 crystallites leading to a core–shell morphology.[12]
Titanium(IV) isopropoxide [Ti(OiPr)4)] was infiltrated into
the pores of MIL-101 by gas-phase loading. The subsequent
hydrolysis of the Ti precursor led to the formation of amorphous
TiO2 inside the cavities of MIL-101. The crystallisation of
TiO2 (anatase) on the surface of MOF crystallites was performed
under hydrothermal conditions. The MIL-101 core is crucial
since the material collapses by removing the support of
the photocatalytic system.[12] In this work, the resulting
TiO2@MIL-101 material was modified with CdS nanocrystals
under solvothermal conditions using cadmium acetate dihydrate
and dimethyl sulfoxide as precursors to yield CdS/TiO2@MIL-
101.[16] MIL-101 serves as structure-directing centre to form
a dense photoactive shell. Thereby, the MOF core offers a
platform for the growth of CdS and TiO2 nanocrystallites
covering the outer surface of single MIL-101 crystallites.
Finally, we decorated the core–shell material with
homogeneously distributed nickel nanoparticles using
bis(cyclopentadienyl)nickel(II) [Ni(Cp)2] as the volatile Ni
precursor. The Ni/CdS/TiO2@MIL-101 photocatalyst and the
process of charge carrier separation between the semiconduct-
ing materials is illustrated in Scheme 1. The TiO2 and CdS
particles form a type II heterojunction at their interface where
CdS serves as light absorbing component for the visible spec-
trum. The directed electron transfer from the conduction band
of CdS into that of TiO2 is a well understood process.[17]
We believe that the enhanced photocatalytic efficiency of a
MOF-supported CdS/TiO2 heterojunction is due to a reduced
charge recombination rate as described in the literature. Metallic
Ni nanoparticles promote the separation of charges, since the
electrical potential gradient at the Ni/semiconductor interface
enhances the electron transfer.[18] Consequently, the charge
carriers initiate spatially separated redox reactions, since they
are distributed over three different components. The nanoparti-
cles of the non-noble metal Ni act as electron reservoirs and,
thus, as a co-catalyst for H2 liberation.
Ni
Ti
Cd
(j)
(l)
0.352 nm
(101) TiO2
O
0.335 nm
(111) CdS
5 nm
S
Fig. 1. (a) TEM image of Ni/CdS/TiO2@MIL-101. (b–l) HAADF-STEM
analysis of Ni/CdS/TiO2@MIL-101 with representative energy-dispersed
X-ray element maps. (k) Characteristic lattice planes for crystalline cubic
CdS and anatase TiO2.
The initial specific surface area of MIL-101 decreased
perceptibly during the generation of TiO2 and CdS, as investi-
gated by nitrogen physisorption measurements (Fig. S2, Sup-
plementary Material). Only a minor reduction of the specific
surface area was observed after the modification with nickel.
Crystalline TiO2 (anatase) and CdS (cubic) were identified by
X-ray powder diffraction (XRD) analysis (Fig. S3, Supplemen-
tary Material). The characteristic reflexes between 28 and 208
(2y) indicate the preserved structure of MIL-101. X-Ray photo-
electron spectroscopy (XPS) analysis confirmed the presence of
TiO2 and CdS. Signals of Ni0 and NiII were observed in the Ni 2p
spectrum. The presence of NiII can be attributed to the formation
of nickel(II) hydroxide when exposed to air during sample
handling (Fig. S4, Supplementary Material). We determined
the elemental composition of the materials by inductively
coupled plasma–optical emission spectrometry (ICP-OES)
measurements (Table S1, Supplementary Material). The
Ni/CdS/TiO2@MIL-101 material comprised 3.6 wt-% Ni, 18.8
wt-% CdS, and 26.0 wt-% TiO2. Diffuse reflectance ultraviolet-
visible spectra in the range from 450 to 800 nm show an
increased light absorption for TiO2@MIL-101 and

844
D. Tilgner et al.
(a)
(b)
1.0 ns
CdS@MIL-101
CdS/TiO2@MIL-101
Ni/CdS/TiO2@MIL-101
5 µm
4 µm
4 µm
0 ns
CdS@MIL-101
0.333 ns
CdS/TiO2@MIL-101
0.299 ns
Ni/CdS/TiO2@MIL-101
0.236 ns
3
4
5
6
7
8
9
10
Time [ns]
Fig. 2. TRPL studies of MIL-101 supported catalysts confirmed the directed electron transfer from excited CdS to TiO2 particles and Ni nanoparticles.
(a) TCSPC traces of CdS@MIL-101, CdS/TiO2@MIL-101, and Ni/CdS/TiO2@MIL-101 in semi-log representation with fits to the linear range (black lines;
Table S2, Supplementary Material). (b) Microscope images of fluorescing CdS@MIL-101, CdS/TiO2@MIL-101, and Ni/CdS/TiO2@MIL-101 after optical
excitation. The colour encodes the photoluminescence lifetime at each spatial point as indicated by the side bar. The photoluminescence lifetime of excited CdS
is calculated from the slope of the linear fit function. The values of the photoluminescence lifetime have to be compared relatively, since the instrument
response function (IRF) has not been taken into account in the data analysis.
metal nanoparticles.[19] We confirmed the directed electron
transfer process from excited CdS across the heterojunction
interface to TiO2 and further to Ni nanoparticles by TRPL
studies on MIL-101 supported catalysts. The photolumines-
cence lifetime of bare CdS@MIL-101 corresponds to the
recombination rate of electrons and holes after optical excita-
tion. The presence of TiO2 and Ni opens up additional relaxation
channels of the excited CdS through the directed electron
transfer induced by the potential gradient at the CdS/TiO2
and Ni/semiconductor interface. Thus, the photoluminescence
lifetime of CdS, which can be calculated from the linear range
of time-correlated single photon counting (TCSPC) traces in
semi-log representation, is reduced (Fig. 2; Table S2, Supple-
mentary Material). The charge carrier separation across three
different catalyst components leads to the enhanced photocata-
lytic activity of Ni/CdS/TiO2@MIL-101. To extend the insight
into the electron transfer between CdS and metallic species
synchrotron-based X-ray absorption spectroscopy and photo-
electrochemical characterisation could be combined.[20]
(a)
(b)
Fig. 3. Lowest energy geometries obtained from Born-Oppenheimer DFT
molecular dynamics simulations in which a benzyl alcohol molecule binds to
a 13 atom Ni particle (a) and a 13 atom Pd particle (b). See the Supplemen-
tary Material for computational details.
CdS/TiO2@MIL-101 in comparison to MIL-101 (Fig. S5, Sup-
plementary Material). The modification with CdS resulted in a
distinctive edge for CdS/TiO2@MIL-101, which is comparable
to the absorption of cubic CdS. We investigated the size and
shape of the Ni/CdS/TiO2@MIL-101 crystallites by transmis-
sion electron microscopy (TEM) and high-angle annular dark-
field scanning TEM (HAADF-STEM) measurements (Fig. 1).
The typical octahedral shape and the homogeneous size distri-
bution of the MIL-101 crystallites was also observed for
Ni/CdS/TiO2@MIL-101. Energy dispersed X-ray maps demon-
strate the uniform arrangement of TiO2, CdS, and Ni around
single MIL-101 crystallites. The interface between the crystal-
line semiconductors CdS and TiO2 was verified by assigning
characteristic lattice planes of adjacent semiconductor particles.
Nickel was located on CdS and TiO2 and showed a minimal
tendency for agglomeration (Fig. S5, Supplementary Material).
We investigated the TiO2@MIL-101 and CdS@MIL-101 mate-
rials by TEM measurements to determine the size of the CdS and
TiO2 crystallites formed on the surface of MIL-101 (Fig. S7,
Supplementary Material). The average particle size was
,35 nm for TiO2 and 20 nm for CdS.
The photocatalytic liberation of hydrogen via the oxidation
of alcohols leading to carbonyl compounds was performed
under argon atmosphere at room temperature without additives
or acceptor molecules. A 50 W blue LED (470 nm) was used as a
visible light source (Fig. S9, Supplementary Material). The
dehydrogenation of benzyl alcohol was investigated as a model
reaction. Acetonitrile was identified as the most suitable solvent
and a loading of 4 wt-% Ni showed the highest activity
(Tables S3 and S4, Supplementary Material). We investigated
the photocatalytic activity of CdS/TiO2@MIL-101 modified
with the noble metals Pd, Pt, and Au as co-catalysts to ensure the
superior performance of our Ni/CdS/TiO2@MIL-101 catalyst.
The Ni-modified catalyst system showed the highest activity
in alcohol splitting (Fig. S10a, Supplementary Material). In
combination with CdS, the noble metal Pt is especially used as
co-catalyst for hydrogen evolution reactions.[21]
A detailed, microscopic understanding of the mechanism of
such dehydrogenation reactions is a formidable task that is
beyond the scope of this work. However, one straightforward
first step towards understanding differences between Ni and
noble metals as co-catalysts can be taken computationally. Of
the three experimentally tested metals (Pd, Pt, and Au), we
chose Pd as the paradigm noble metal catalyst for the
Time-resolved photoluminescence (TRPL) studies on photo-
catalytic systems allow a deeper understanding of photosyn-
thetic electron dynamics between semiconductor materials and

Photocatalytic Alcohol Splitting
845
comparison with Ni. Choosing Pd is motivated by the observa-
tion that similar metal particle geometries are stable for Ni and
Pd, so binding properties can be readily compared. With the
help of density functional theory (DFT) calculations, we
checked whether there is a difference in the binding of benzyl
alcohol to the metal particles using a molecular model consist-
ing of a 13-atom metal particle and one benzyl alcohol mole-
cule. We chose the 13-atom clusters, since they form stable
structures[22] and can build an icosahedral geometry. The latter
is one of the smallest possible faceted structures and, therefore,
can be interpreted as a small model system for the binding to a
faceted metal nanoparticle. We obtained the geometrical and
electronic structure using the TURBOMOLE[23] code. Fig. 3
depicts the lowest energy, i.e. strongest bound, conformation in
which the alcohol binds to the Ni13 and Pd13 cluster. Details of
the computational procedures are reported in the Supplemen-
tary Material (see Theoretical Procedures). We paid particular
attention to the fact that d-electron metals pose special chal-
lenges to many exchange-correlation approximations.[24] Reas-
suringly, our calculations with different density functionals
consistently show that benzyl alcohol binds more strongly to
Ni13 than to Pd13 by several hundred meV (Table S10, Supple-
mentary Material). We therefore consider this trend as reliable.
We see this calculation as one first, small step to corroborate
the plausibility that Ni shows a different performance as a
co-catalyst than Pd. Extensive further computational work,
which is beyond the scope of the present manuscript, would
be needed to reveal the detailed mechanism.
The beneficial contribution of the heterojunction between the
semiconductors was demonstrated by the higher activity of
CdS/TiO2@MIL-101 in comparison to CdS@MIL-101. We also
demonstrated the favourable effect resulting from the support of
Ni/CdS/TiO2 on MIL-101 as compared to pure Ni, CdS, TiO2,
and Ni/CdS/TiO2. All materials without cadmium sulfide as the
light-absorbing component showed no activity under visible light
illumination (TableS5, SupplementaryMaterial). The reusability
of Ni/CdS/TiO2@MIL-101 was confirmed by performing ten
successive runs without a remarkable loss of photocatalytic
activity (Fig. S10b, Supplementary Material). The material was
analysed by TEM and XRD measurements after the last run to
demonstrate the structural integrity (Fig. S11, Supplementary
Material). A light on/off experiment was performed to examine
whether the dehydrogenation of benzyl alcohol is indeed a visible
light-mediated reaction (Fig. S12, Supplementary Material). The
amount of H2 released during the reaction was only increased
under illumination. We further checked whether hydrogen is
released in equimolar amounts during the photocatalytic reaction
for several substrates using methane as an internal standard
(Table S7, Supplementary Material). Performing the splitting
of alcohols under argon atmosphere is crucial for high yields and
the generation of molecular hydrogen (Table S7, Supplementary
Material). Our Ni/CdS/TiO2@MIL-101 photocatalyst can split a
broad range of alcohols 1 forming the corresponding aldehydes,
aryl-alkyl, diaryl, and dialkyl ketones 2 (Table 1). A variety of
functional groups was well tolerated, including halogens, meth-
oxy, hydroxy, trifluoromethyl, and amino groups. In addition,
hydrogenation-sensitive functionalities, such as nitrile and
nitro groups, and C=C bonds can be tolerated selectively. We
converted a total of 38 alcohols into the respective carbonyl
compounds in good to excellent yields (Table 1; Table S6,
Supplementary Material).
Table 1. Photocatalytic dehydrogenation of aryl, aryl-alky, diaryl, and
dialkyl alcohol compoundsA
OH
O
Ni/CdS/TiO2@MIL-101
+
H2
RЈ
RЉ
RЈ
RЉ
CH3CN, Ar, hv (470 nm)
H
1
2
Entry
Product
R
YieldB
[%]
1
O
2a R1 ¼ H, R2 ¼ H
97
96
92
81
83
71
97
96
93
83
90
2
2b R1 ¼ OMe, R2 ¼ H
2c R1 ¼ Me, R2 ¼ H
2d R1 ¼ H, R2 ¼ Me
2e R1 ¼ Me, R2 ¼ Me
2f R1 ¼ F, R2 ¼ H
2g R1 ¼ Cl, R2 ¼ H
2h R1 ¼ Br, R2 ¼ H
2i R ¼ Et
3
R1
R2
4
5C
6D
7
8
9
O
10C
11C
2j R ¼ Bu
R
2k R ¼ CH2OH
12
13
14
O
2l R ¼ H
88
91
82
2m R ¼ Me
2n R ¼ OMe
R
cycloheptanone
15D
16
17D
18D
19D
20
2o
86
.99
93
2p R1 ¼ H, R2 ¼ H
2q R1 ¼ CN, R2 ¼ H
2r R1 ¼ NO2, R2 ¼ H
2s R1 ¼ H, R2 ¼ NO2
2t R1 ¼ H, R2 ¼ NH2
2u R1 ¼ CF3, R2 ¼ H
2v
O
R1
R2
88
71
96
21
22D
70
O
70
AReaction conditions: 0.1 mmol alcohol, 0.6 mg Ni/CdS/TiO2@MIL-101,
Ar, 0.3 mL CH3CN, 278C, 24 h, 470 nm blue LED 50 W.
BDetermined by GC using n-dodecane as internal standard.
C1.2 mg Ni/CdS/TiO2@MIL-101.
D1.2 mg Ni/CdS/TiO2@MIL-101, 48 h.
alcohol and aniline was chosen to optimise the reaction condi-
tions (Table S8, Supplementary Material). Imines 5 were
obtained in good yields under very mild conditions at room
temperature. A notable tolerance for functional group was again
observed without the use of additives or sacrificial electron
donors. Methyl, methoxy, halide, and hydroxy-substituted ben-
zyl alcohols were well tolerated. In addition, we varied the
amine component and found the tolerance for methyl, halogen,
and methoxy substituents. We analysed the gas atmosphere to
quantify the amount of hydrogen released for selected examples
(Table S9, Supplementary Material). Again, no molecular
hydrogen and a decreased yield of imine were observed when
conducting the C–N bond formation without argon atmosphere.
Amines formed via hydrogenation of corresponding imines by
the hydrogen liberated were not detected.
Conclusion
In conclusion, we introduced photocatalytic C–N multiple bond
formation under visible light illumination by coupling activated
carbonyl compounds with amines. The reusable, noble metal-
free photocatalyst Ni/CdS/TiO2@MIL-101 permits quantitative
hydrogen generation from alcohols (alcohol splitting) under
very mild conditions, inert gas atmosphere, and without the
requirement of sacrificial electron donors. The photocatalyst is
Finally, we were interested in photocatalytic C–N multiple
bond formation reactions (Table 2). The reaction of benzyl

846
D. Tilgner et al.
Table 2. Photocatalytic synthesis of imines from primary alcohols and
aminesA
Network, the help of Florian Puchtler (XRD), Dr Ju¨rgen Seidel (XPS), Prof.
Dr Lothar Kador (FLIM) and Eduard Arzt for his support through INM. The
photoluminescence lifetime measurements were performed in the keylab
Electron and Optical Microscopy of the Bayreuth Polymer Institute (BPI).
Ni/CdS/TiO2@MIL-101
RЉ
+
RЈ
H2N RЉ
RЈ
N
OH
CH3CN, Ar, hv (470 nm)
3
4
5
–H2, –H2O
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88
53
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5g R1 ¼ Br, R2 ¼ H
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platform for a variety of photocatalytic C–N or C–C bond for-
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and condensation reaction(s) as key steps.
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Acknowledgements
This work was supported by grants from the Deutsche For-
schungsgemeinschaft (DFG, SFB 840(B1)). Note that parts of this work
were included in the PhD thesis of Dominic Tilgner, Bayreuth 2017.
The authors also acknowledge the support of the DAAD, Colloid/Polymer
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