Selective Oxidation of 5-Hydroxymethylfurfural to 2,5-Diformylfuran by Visible Light-Driven Photocatalysis over In Situ Substrate-Sensitized Titania

Article abstract of DOI:10.1002/cssc.202002687

Solar energy-driven processes for biomass valorization are priority for the growing industrialized society. To address this challenge, efficient visible light-active photocatalyst for the selective oxidation of biomass-derived platform chemical is highly desirable. Herein, selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) was achieved by visible light-driven photocatalysis over titania. Pristine titania is photocatalytically inactive under visible light, so an unconventional approach was employed for the visible light (λ=515 nm) sensitization of titania via a formation of a visible light-absorbing complex of HMF (substrate) on the titania surface. Surface-complexation of HMF on titania mediated ligand-to-metal charge transfer (LMCT) under visible light, which efficiently catalyzed the oxidation of HMF to DFF. A high DFF selectivity of 87 % was achieved with 59 % HMF conversion after 4 h of illumination. The apparent quantum yield obtained for DFF production was calculated to be 6.3 %. It was proposed that the dissociative interaction of hydroxyl groups of HMF and the titania surface is responsible for the surface-complex formation. When the hydroxyl groups of titania were modified via surface-fluorination or calcination the oxidation of HMF was inhibited under visible light, signifying that hydroxyl groups are decisive for photocatalytic activity.

Full text of DOI:10.1002/cssc.202002687

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Selective Oxidation of 5-Hydroxymethylfurfural to 2,5-
Diformylfuran by Visible Light-Driven Photocatalysis over
In Situ Substrate-Sensitized Titania
Ayesha Khan,*[a] Michael Goepel,[b] Adam Kubas,[a] Dariusz Łomot,[a] Wojciech Lisowski,[a]
Dmytro Lisovytskiy,[a] Ariadna Nowicka,[a] Juan Carlos Colmenares,*[a] and Roger Gläser*[b]
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Solar energy-driven processes for biomass valorization are
priority for the growing industrialized society. To address this
challenge, efficient visible light-active photocatalyst for the
selective oxidation of biomass-derived platform chemical is
highly desirable. Herein, selective oxidation of 5-hydroxymeth-
ylfurfural (HMF) to 2,5-diformylfuran (DFF) was achieved by
visible light-driven photocatalysis over titania. Pristine titania is
photocatalytically inactive under visible light, so an unconven-
tional approach was employed for the visible light (λ=515 nm)
sensitization of titania via a formation of a visible light-
absorbing complex of HMF (substrate) on the titania surface.
Surface-complexation of HMF on titania mediated ligand-to-
metal charge transfer (LMCT) under visible light, which
efficiently catalyzed the oxidation of HMF to DFF. A high DFF
selectivity of 87% was achieved with 59% HMF conversion after
4 h of illumination. The apparent quantum yield obtained for
DFF production was calculated to be 6.3%. It was proposed
that the dissociative interaction of hydroxyl groups of HMF and
the titania surface is responsible for the surface-complex
formation. When the hydroxyl groups of titania were modified
via surface-fluorination or calcination the oxidation of HMF was
inhibited under visible light, signifying that hydroxyl groups are
decisive for photocatalytic activity.
Introduction
macrocyclic ligands, bio-based polymeric materials, pharma-
ceutical products, or antifungal agents etc.[7] Consequently, the
selective oxidation of HMF to DFF has received considerable
attention to date.[8–10]
Heterogeneous photocatalysis has great potential for the
partial oxidation of HMF to DFF.[11] Specifically, visible light-
driven photocatalytic conversion of HMF to DFF is an environ-
mentally benign and economical approach compared to ultra-
violet (UV) light-driven photocatalysis, which often is unselec-
tive (as it generates highly active oxidative species that over-
oxidize the substrate)[12] and conventional thermocatalytic
processes performed over costly noble metal catalysts at
Exploitation of renewable resources is crucial to ensure future
energy demands and alleviate environmental problems. Bio-
mass is a naturally abundant renewable resource, which has
potential to partially replace fossil fuel-based feedstocks for
sustainable production of fine chemicals.[1,2] To achieve this
goal, the selective oxidation of biomass-derived platform
chemicals into organic building blocks is a promising and
desirable strategy.[3] 5-hydroxymethylfurfural (HMF) is a key bio-
based platform chemical obtained from C6 monosaccharides
(derived from the depolymerization of cellulosic biomass),[4]
which can be further converted into high value-added chemical
such as 2,5-diformylfuran (DFF).[5,6] DFF has a broad range of
potential applications as an intermediate for the synthesis of
[13]
°
elevated temperatures (80–150 C).
Until now, few studies
were devoted to the partial oxidation of HMF to DFF under
visible light using different semiconductor photocatalysts.[14] For
example, Nb2O5 exhibited high selectivity for DFF (�91%) at
approximately 19% HMF conversion in benzotrifluoride after
6 h of illumination (λ>400 nm).[15] In addition, photocatalytic
oxidation of HMF performed using graphitic carbon nitride (g-
C3N4) in aqueous medium achieved 40% HMF conversion and
50% DFF selectivity after 4 h of solar irradiation.[16] In another
study, g-C3N4 in acetonitrile medium showed improved DFF
selectivity (�87%) with the HMF conversion of 31% after 6 h of
irradiation (λ>400 nm) under O2 flow.[17] Although these semi-
conductors have demonstrated the practical applicability of
visible light-driven photocatalysis for the transformation of HMF
to DFF, the preparation of these photocatalysts (Nb2O5 and g-
[a] A. Khan, Prof. A. Kubas, Dr. D. Łomot, Dr. W. Lisowski, Dr. D. Lisovytskiy,
Dr. A. Nowicka, Prof. J. C. Colmenares
Institute of Physical Chemistry
Polish Academy of Sciences
Warsaw 01-224 (Poland)
E-mail: akhan@ichf.edu.pl
jcarloscolmenares@ichf.edu.pl
[b] Dr. M. Goepel, Prof. Dr. R. Gläser
Institute of Chemical Technology
Leipzig University
Leipzig 04103 (Germany)
E-mail: roger.glaeser@uni-leipzig.de
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cssc.202002687
°
© 2021 The Authors. ChemSusChem published by Wiley-VCH GmbH. This is
an open access article under the terms of the Creative Commons Attribution
Non-Commercial License, which permits use, distribution and reproduction
in any medium, provided the original work is properly cited and is not used
for commercial purposes.
C3N4) involves high-temperature calcination (800 and �550 C,
respectively), which is not economical (due to high energy
consumption).
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Henceforth, the quest for a cost-effective, chemically stable,
omatic chlorinated products under visible light.[31,32] Higashimo-
to et al.[33] reported that benzyl alcohol on titania is oxidized
into its corresponding aldehyde (benzaldehyde) with 99%
selectivity under visible light via LMCT complex formation.[33,34]
Formation of LMCT complex between the titania surface and
substrate presents a significant potential for the development
of solar-driven photocatalytic systems.
Herein, we investigate the LMCT-mediated photocatalytic
activity of titania, prepared through sol-gel and hydrothermal-
assisted method (SGH-TiO2) for the selective oxidation of HMF
to DFF under visible light (λ=515 nm). It has been found that
the adsorption of substrate (HMF) on SGH-TiO2 surface enabled
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and efficient visible light-responsive photocatalyst is actively
pursued for the selective oxidation of HMF to DFF. Titania is a
widely used, inexpensive, and efficient photocatalyst employed
in various environmental and industrial applications.[18,19] How-
ever, its application in organic transformations is limited by the
wide bandgap (�3.2 eV), which requires UV light (accounts for
<5% of solar radiation) for photoactivation.[19,20] Notably,
pristine titania is considered to be highly unselective for the
oxidation of HMF, which was attributed to the over-oxidation of
the target products.[15] The highest selectivity to DFF achieved
by pristine titania until now is only 22% at 50% HMF
conversion in an aqueous medium under UV light (λ=365 nm)
irradiation.[21] Therefore, modifying the optical response of
titania to the visible light (accounts for �45% of solar
radiation[19]) range will make the application of photocatalysis
more economical and environment friendly for the production
of industrially relevant chemicals like DFF.
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the formation of
a visible light-absorbing complex. The
resulting complex mediates LMCT under visible light and the
electrons transferred from the surface-complex to the CB of the
SGH-TiO2 activate molecular oxygen and catalyze the oxidation
of HMF to DFF. This work provides a better understanding of
visible light sensitization of titania via LMCT approach and also
demonstrates its promising potential for the selective oxidation
of biomass-derived platform chemicals.
Numerous strategies have been used to tune the absorption
of titania toward the visible light region including metal (Fe,[22]
V,[23] Ni[24]) or non-metal (N,[25] S,[26] C[27]) doping and coupling
titania with dyes (dye sensitization)[28] or narrow bandgap
semiconductors such as CdS and WO3.[29] Another strategy that Results and Discussion
has been much less investigated is the ligand-to-metal charge
transfer (LMCT) by complex formation on the titania surface. In
this approach, the adsorbates or substrates (which do not
absorb visible light themselves) form a surface-complex on
titania. The resulting complex serves as a visible light sensitizer
and induces charge transfer from the highest occupied
molecular orbital (HOMO) of the adsorbates to the conduction
band (CB) of titania upon visible light irradiation.[19]
Characterization
The comparison of XRD patterns of SGH-TiO2 and P25 is shown
in Figure S1a,b in the Supporting Information. The XRD reflexes
observed at 25.4 (101), 37.9 (004), 48.0 (200), 54.4 (105), 63.2
°
(204), 69.3 (116), and 75.7 (215) correspond to the anatase
phase of titania (JCPDS Card No. 21–1272)[35] in both SGH-TiO2
and P25 (Figure S1a,b, Supporting Information), whereas the
Various organic compounds with hydroxyl functional group
(glucose, benzyl alcohol, phenol, catechol, chlorophenols,
ascorbic acid, etc.) and carboxylic acid functional group (formic
acid, citric acid, lactic acid, etc.) can form LMCT complexes on
titania, accompanied by the absorption of visible light,[19] which
is not exhibited by either adsorbates and titania alone. Several
studies have been carried out to investigate the LMCT-mediated
photocatalytic activity of surface-complexed titania under
visible light. Catechol surface-complexed titania is a typical
example, which extends the optical absorption of titania into
the visible light region (up to 600 nm). As a photocatalyst,
catechol-on-titania efficiently and selectively oxidizes amines
(with 55% conversion) into imines (with 90% selectivity)
ascribed to visible light-induced interfacial charge transfer.[30] In
addition, 2,4,5-trichlorophenol can also form a charge-transfer
complex on titania (P25) that is activated by light wavelengths
as long as 520 nm. The resulting charge-transfer complex forms
trichlorophenoxyl radicals, which upon coupling form polyar-
°
XRD reflex appearing at 30.8 (121) in SGH-TiO2 was assigned to
the brookite phase of titania (JCPDS no. 29-1360).[36] On the
other hand, P25 showed (Figure S1b, Supporting Information)
distinct reflexes at 27.3 (110), 36.1 (101), 41.2 (111), 53.9 (211),
°
and 56.6 (220), which are characteristic of the rutile phase of
titania (JCPDS card no. 21-1276).[35] The obtained results for
phase composition and crystallite size are presented in Table 1.
Textural properties of SGH-TiO2 and P25 evaluated by N2
sorption isotherms (Figure S2a,b, Supporting Information)
showed that SGH-TiO2 exhibited a type IV isotherm with H2
hysteresis, which is characteristic of mesoporous materials. P25
exhibited a type II isotherm with H3 hysteresis indicative of
plate-like particles or assemblages of slit-shaped pores.[37] The
microstructure properties such as specific surface area, pore
volume, and pore width (Figure S2c,d, Supporting Information)
of SGH-TiO2 and P25 are summarized in Table 1. Morphology,
phase composition, and average particle size of the as-
Table 1. Textural [Brunauer-Emmett-Teller specific surface area (SSA); Barrett-Joyner-Halenda pore volume (BJH Vp) and pore width (BJH wp)] and
crystallographic (phase composition and crystal size) properties of SGH-TiO2.
Sample
Ratio of crystalline phases
(anatase:brookite:rutile) [%]
Crystal size [nm]
Anatase
SSA
BJH Vp
BJH wp
[nm]
Brookite
6
Rutile
24
[m2 gÀ 1
]
[cm3 gÀ 1
]
SGH-TiO2
P25
74:26:0
87:0:13
5
17
177
53
0.20
0.22
3
16
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synthesized SGH-TiO2 nanoparticles were further corroborated
by TEM analysis. TEM characterization of SGH-TiO2 showed that
the as-prepared SGH-TiO2 nanoparticles consist of anatase and
brookite phase (Figure S3a, Supporting Information), with small
crystal size less than 10 nm (Figure S3b, Supporting Informa-
tion). Furthermore, the lattice spacing (Figure S3c, Supporting
Information) of 0.347, 0.290, and 0.351 nm corresponds to the
(111), (121), and (120) crystal planes of brookite, respectively,[38]
whereas the lattice spacing of 0.352 nm is ascribed to the (101)
crystal plane of anatase.[39] The hydroxyl group density (OH
nmÀ 2) based on thermogravimetric analysis (TGA) weight loss
(Figure S4, Supporting Information) of SGH-TiO2 and P25 is
calculated to be 9 and 7 OHnmÀ 2, respectively. The results
obtained are comparable with the literature-reported OH group
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density of P25 (8 OHnmÀ 2,[40] 5.3 OHnmÀ 2[41]
) and sol-gel-
prepared titania (10.2 OHnmÀ 2,[42] 8.7 OHnmÀ 2,[43] and
11.9 OHnmÀ 2[44]).
Photocatalytic behavior of SGH-TiO2
The photocatalytic performance of SGH-TiO2 was investigated
for the selective oxidation of HMF to DFF in acetonitrile under
UV light (λ=375 nm). SGH-TiO2 exhibited a high photocatalytic
activity with 52% HMF conversion and 36% DFF selectivity after
30 min of illumination under UV light (Figure 1). Besides DFF,
no other intermediate compounds were detected. However,
very low DFF selectivity (36%) indicated that the photocatalytic
oxidation of HMF may proceed at two different active sites,
through two parallel pathways:[21] i) partial oxidation of HMF to
DFF that is desorbed from the catalyst surface and may
compete with HMF for further oxidation and lead to mineraliza-
tion; (ii) complete oxidation of HMF to CO2 and H2O through
intermediates that do not release from the catalyst surface to
liquid phase. This is in agreement with the study carried out by
Yurdakal et al.[21] who compared different crystalline phases of
titania for the selective oxidation of HMF to DFF under UV light
in aqueous medium.
To explore the influence of incident light wavelengths, the
photocatalytic reaction was also performed under visible light
(λ=515 nm). Interestingly, SGH-TiO2 exhibited a high photo-
catalytic activity also under visible light, with 59% HMF
conversion and 87% DFF selectivity after 4 h of illumination
(entry 2, Table 2). The selectivity of the DFF increases gradually
in the beginning (Figure 2a), which may have ascribed to the
slow desorption of DFF from the catalyst surface. Moreover, DFF
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Figure 1. Conversion of HMF and selectivity for DFF in the photocatalytic
oxidation of HMF to DFF in an acetonitrile medium over SGH-TiO2 under UV
light (λ=375 nm).
Table 2. Results of the photocatalytic oxidation of HMF under different conditions.[a]
Entry
Photocatalyst
Light
Additive
HMF Conv.
[%]
DFF Sel.
[%]
DFF Yield
[%]
Carbon balance
[%]
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2
3
4
5
6
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8
SGH-TiO2
SGH-TiO2
P25
UV[b]
none
none
none
none
none
none
none
methanol
BQ
52
59
18
0
0
0
35
56
0
0
0
52
38
0
0
0
0
0
0
36
87
69
0
0
0
75
88
0
0
0
96
87
0
0
0
0
0
0
19
52
12
0
0
0
26
48
0
0
0
50
33
0
0
0
0
0
0
66
92
94
100
100
100
91
visible
visible
visible
visible
visible
visible
visible
visible
visible
visible
visible
visible
dark
none
SGH-TiO2-cal-600
F-SGH-TiO2
P90
SGH-TiO2
SGH-TiO2
SGH-TiO2
SGH-TiO2
SGH-TiO2
SGH-TiO2
SGH-TiO2
SGH-TiO2
SGH-TiO2
anatase
brookite
93
9
100
100
100
98
10
11
12
13
14
15
16
17
18
19
Ag+
Ar[c]
air
Ar:air
none
none
none
none
none
none
97
100
100
100
100
100
100
[d]
visible
visible
visible
visible
visible
[e]
Ana:brook (74:26)
[a] Reaction conditions: HMF (1 mm, 0.02 mmol), photocatalyst (1 gLÀ 1, 20 mg), scavenger (1:1 molar ratio of substrate, 0.02 mmol), reaction time (4 h). [b]
Reaction time: 30 min; reaction medium: acetonitrile. [c] Reaction medium: degassed acetonitrile. [d] Reaction medium: water. [e] Reaction medium: 90:10
acetonitrile/water. HMF solution volume: 20 mL; air flow rate: 25 mLminÀ 1 21% O2; Ar flow rate: 25 mLminÀ 1
, , 99.9% Ar; Ar:air flow rate:
15 mLminÀ 1:10 mLminÀ 1, 60% Ar and 8.4% O2); incident light wavelength: UV: 375 nm, visible: 515 nm; incident light intensity: �54 WmÀ 2 (6×
�9 WmÀ 2); ana:brook: physical mixture of anatase and brookite.
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Figure 2. (Left) Conversion of HMF and selectivity for DFF in the photocatalytic oxidation of HMF to DFF over SGH-TiO2 under visible light (λ=515 nm). (Right)
Oxidation of HMF and production of DFF over SGH-TiO2 as a function of time over SGH-TiO2.
selectivity remains stable under visible light during the course
of reaction (Figure 2a), which suggests that the low-energy
visible light radiation inhibits the over-oxidation of DFF.
The oxidation of HMF over SGH-TiO2 as a function of time
(Figure 2b) showed that the HMF was oxidized rapidly in the
beginning; the initial reaction rate observed after 60 min of
illumination was 8.02×10À 5 mmol minÀ 1. The oxidation of HMF
slowed down with time, which may correspond to the
deactivation of the active sites of the catalyst.[10] However, no
titanium leaching was observed after 4 h of photocatalytic
reaction. Commercial TiO2 (P25) was also tested in our reaction
system, which showed very low HMF conversion (18%) with
69% DFF selectivity (entry 3, Table 2) after 4 h of illumination. It
is noteworthy that both photocatalysts (SGH-TiO2 and P25), in
principle active only under UV light, showed activity under
visible light (λ=515 nm). This suggests that they either follow a
special mechanism under visible light or possess unique
features that impart visible light sensitivity.
Figure 3. Cycling runs for the photocatalytic oxidation of HMF over SGH-TiO2
under visible light irradiation.
and 2.2% at a wavelength of 515 nm, respectively, which are
comparable to the reported efficiency of titania (4.3%) for the
selective oxidation of benzyl alcohol under visible light
(500 nm).[33]
Recycling test of SGH-TiO2 for HMF oxidation
The stability and recyclability of the photocatalyst is of great
importance from the industrial application point of view. To
investigate the reusability of the catalyst, the recycling experi-
ment was carried out for the photocatalytic oxidation of HMF
using SGH-TiO2. As shown in Figure 3, SGH-TiO2 can be used in
multiple cycles without substantial drop in the HMF conversion
and DFF selectivity.
Formation of LMCT complex on SGH-TiO2
A series of characterizations was carried out to explore the
underlying reason accounting for the photocatalytic activity of
SGH-TiO2 and P25 for the oxidation of HMF under visible light.
We first examined the effect of HMF adsorption on the
absorption spectrum of the SGH-TiO2 and P25. For this purpose,
diffuse reflectance (DR-)UV/Vis absorption spectra of SGH-TiO2,
P25, HMF-adsorbed SGH-TiO2 (HMF-Ads-SGH-TiO2), and HMF-
adsorbed P25 (HMF-Ads-P25) in acetonitrile (under the same
photocatalytic reaction conditions) were recorded. The DR-UV/
Vis absorption spectra (Figure 4) show that the adsorption of
HMF on P25 extended the absorption in the visible light region
with the slight change in color of P25 (Figure 4). The adsorption
of HMF on SGH-TiO2 induces a marked increase in the visible
Apparent quantum yield measurement
The photon flux absorbed by the photocatalytic system is
calculated to be 11.63×10À 9 einsteinssÀ 1. The apparent quan-
tum yields (AQY) exhibited by SGH-TiO2 and P25 for DFF
production after 4 h of illumination were calculated to be 6.3
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Figure 4. DR-UV/Visible absorption spectra of SGH-TiO2, P25, HMF-Ads-SGH-TiO2, and HMF-Ads-P25.
light absorption up to 650 nm, which is also accompanied by a
significant change in color of SGH-TiO2 (from white to yellow,
Figure 4). Interestingly, HMF itself does not absorb light in the
visible region like titania. It is therefore hypothesized that the
visible light absorption results from the surface interaction of
HMF and SGH-TiO2. To test this hypothesis, a control experi-
ment performed in the absence of SGH-TiO2 showed no activity,
confirming that the reaction was not occurring through direct
photoactivation of the HMF (entry 4, Table 2).
TiO2. As seen in Figure S6b in the Supporting Information, the
HMF-Ads-SGH-TiO2 spectrum contained characteristic C=C sym-
metric vibrations of the furan ring at 1521 cmÀ 1,[46] which
indicates the adsorption of HMF on SGH-TiO2 surface. Besides
that, the broad band corresponding to the OÀ H stretching
vibrations at 3200–3400 cmÀ 1 (Figure S6c, Supporting Informa-
tion) in SGH-TiO2 and HMF spectra completely disappeared in
HMF-Ads-SGH-TiO2, suggesting the formation of
a LMCT
complex via a dissociative adsorption of the CH2OH group of
HMF and OH group of SGH-TiO2. Additionally, the HMF-Ads-
SGH-TiO2 spectrum showed a weak band at 2941 cmÀ 1, which is
assigned to CÀ H stretching vibrations.[47] A broad doublet
appearing at 1637 and 1680 cmÀ 1 may correspond to C=O
stretching vibrations associated to HMF. Moreover, the HMF-
To further investigate that the phenomenon of extended
absorption in the visible region has occurred due to the
interaction of HMF with the titania surface, Fourier-transform
(FT)IR analysis was performed for HMF, SGH-TiO2, and HMF-Ads-
SGH-TiO2 samples. The distinguishing differences in the spec-
trum of HMF-Ads-SGH-TiO2 compared to individual components
can be seen in Figure S5 in the Supporting Information. The
bands appearing in HMF-Ads-SGH-TiO2 sample at 1662, 1537,
and 1420 cmÀ 1 are characteristic IR bands for HMF, assigned to
C=O stretching vibration, symmetric stretching of the C=C
bonds, and asymmetric stretching of the C=C bonds,
respectively.[45] This suggests that HMF may have adsorbed on
the surface of SGH-TiO2. A weak band at 1198 cmÀ 1 in the HMF-
Ads-SGH-TiO2 spectrum corresponds to CÀ CÀ C multiple rocking
vibrations.[15] Interestingly, the band corresponding to the OÀ H
stretching vibrations in HMF and SGH-TiO2 [hydroxyl species
present in the form of free or H-bonded water or metal (Ti)
hydroxyl groups] at 3200–3400 cmÀ 1 is absent in the HMF-Ads-
SGH-TiO2 spectrum. The lack of an observable OÀ H stretching
band in the HMF-Ads-SGH-TiO2 spectrum is an indication that
the adsorption of HMF on SGH-TiO2 occurred through the À OH
group on the metal surface forming a metal alkoxide complex.
Moreover, the characteristic band of titania owing to the
TiÀ OÀ Ti and TiÀ OÀ C stretching vibration in the range of 800–
400 cmÀ 1 has also been observed for HMF-Ads-SGH-TiO2. The
bands appearing at 2357 and 2115 cmÀ 1 in SGH-TiO2 and HMF-
Ads-SGH-TiO2 spectra are ascribed to physically adsorbed CO
and CO2, respectively.
Ads-SGH-TiO2 spectrum showed
a new weak band at
1048 cmÀ 1, which may have attributed to the CÀ O stretching of
the alcoholate species formed between SGH-TiO2 and HMF.
These results indicated that the HMF molecules interact with
the SGH-TiO2 and formed a surface complex. Apart from that,
Raman spectra of HMF-Ads-SGH-TiO2 also confirmed the
presence of three characteristic Raman active modes of the
anatase phase of titania (Figure S6a, Supporting Information) at
399, 513, and 635 cmÀ 1 that are assigned to B1g, A1g/B1g
(unresolved doublet), and Eg symmetries, respectively.[48]
A
strong band at 152 cmÀ 1 corresponds to the A1g mode of
brookite.[49]
X-ray photoelectron spectroscopy (XPS) measurements were
recorded to obtain information about the changes in the
chemical environment of the surface of SGH-TiO2 after the
interaction of HMF. The XPS spectrum of SGH-TiO2 shows
(Figure S7a, Supporting Information) that the Ti2p split well
into four peaks: Ti2p3/2 at 458.5 eV and Ti2p1/2 at 464.3 eV are
consistent with Ti4+ in TiO2 lattice.[15] Ti2p3/2 at 457.0 eV and
Ti2p1/2 at 462.8 eV correspond to Ti3+ in Ti2O3.[50] This indicates
the presence of both Ti4+and Ti3+ in SGH-TiO2. When compared
with HMF-Ads-SGH-TiO2, the high-resolution XPS spectrum of
Ti2p did not show a prominent shift in the binding energies
and variation in the peak area (Figure S7b, Supporting
Information). This suggests that the electronic state of Ti
element has not changed after HMF adsorption. Moreover, after
Raman spectra (Figure S6, Supporting Information) have
also been recorded for HMF, SGH-TiO2, and HMF-Ads-SGH-TiO2
to further corroborate the surface interaction of HMF and SGH-
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the adsorption of the HMF the peak area of Ti4+ decreased
negligibly (�1%) and that of Ti3+ increased by approximately
1%, which accounts for a stable titania lattice and indicates the
probable interaction of HMF via a terminal OH groups rather
than the substitution of Ti. The O1 s spectrum of SGH-TiO2
(Figure S7c, Supporting Information) is deconvoluted into four
peaks. The peak at 529.7eV is attributed to lattice oxygen,
whereas the peaks at 530.5 and 531.3 eV correspond to oxygen
bound to titanium in Ti2O3 and oxygen bound to carbon (C=O),
respectively. The peak at 532.8 eV corresponds to hydroxyl
groups bound to titanium or carbon (TiÀ OH or CÀ OH). When
compared with the O1 s spectrum of HMF-Ads-SGH-TiO2 (Fig-
ure S7d, Supporting Information), the peak previously corre-
sponding to surface-bound oxygen in the form of hydroxyl
groups slightly shifted to lower binding energy (532.5 eV). This
may have ascribed to slightly higher electron density[15] around
OH group-associated oxygen after the adsorption of HMF.
However, there is a negligible increase in the peak area of O1 s
associated to lattice and surface-bound oxygen after HMF
adsorption (Figure S7d, Supporting Information).
To gain further insight into the HMF adsorption process at
the titania surface, we performed state-of-the-art quantum
chemical calculations. Here, we used a neutral, hydrogen-
terminated [Ti20O62H44]0 cluster to represent the anatase (101)
surface (Figure S8, Supporting Information). It was recently
shown that clusters of this size provide adsorption energies
close to the periodic values.[51] However, finite cluster approach
allows to use high-level methods such as DLPNO-CCSD(T)[52,53]
extrapolated to complete basis set limit (CBS) to get highly
accurate results as shown for the adsorption of small molecules
on anatase[51] and rutile[54] surfaces. H-termination instead of
point charge embedding made the geometry optimization step
and zero-point energy (ZPE) correction straightforward at the
more economic DFT/PBE level. Thus, the energies reported are
single-point DLPNO-CCSD(T)/CBS energies at the DFT/PBE geo-
metries and include ZPE corrections. Our calculations show the
interaction of HMF with the model surface is overall downhill in
energy (Figure 5a). Chemisorption is exothermic (À 2.70 eV) due
to H-bond formation between À OH group of the HMF and O
center at the TiO2 terrace accompanied with C=O!Ti coordina-
tion. Additionally, the titania surface interacts with the π system
of the HMF molecule via dispersion forces (C state, Figure 5a;
detailed structure is shown in Figure S9, Supporting Informa-
tion). Subsequently, 0.57 eV is released upon OÀ H heterolytic
bond cleavage (D state, Figure 5b). This results in a strong
covalent bond formation between HMF’s oxygen and the
titanium atom (bond length of 1.82 Å). According to the
computed IR spectrum (Figure 5e), the presence of CÀ O···Ti
stretching vibrations should be visible just above 1000 cmÀ 1,
while the peak above 1500 cmÀ 1 is ascribed to C=C ring
vibrations and should be broadened by unequal interactions at
opposite sides of the ring. The former is experimentally
obscured by TiO2 lattice vibrations, but the latter is clearly
visible in the experimental spectrum (Figure S5, Supporting
Information). Regarding the Raman spectra (Figure 5f), our
calculations show that symmetric C=C stretch of the furan ring
will have significantly higher intensity as compared to the same
mode of vibration detectable in IR spectra. This trend is evident
also in the experimental spectrum (Figure S6, Supporting
Information). Moreover, distinct features between 1350–
1400 cmÀ 1 are associated with the symmetric ring vibrations
accompanied with vibration of the CH2 group in the À CH2À OÀ Ti
group (rocking and wagging).
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Analysis of the PBE0 frontier molecular orbitals (Figure 5c,d)
of state D reveals that the HOMO is located at the HMF moiety
while the lowest unoccupied orbital (LUMO) is delocalized
between HMF and a combination of Ti d orbitals (TiO2 d-band).
Thus, the basic HOMO!LUMO transition will have LMCT
character although the computed HOMO-LUMO gap here is still
significant and varies between 2.4 and 4.2 eV at PBE and PBE0
level, respectively. Such variation is not exceptional as the
bandgap width is known to be functional-dependent.[55] More-
Figure 5. Theoretical analysis of HMF adsorption on TiO2 anatase (101) model surface. (a) Energetics of the process with a breakdown into two consecutive
steps: chemisorption (blue, C, dative C=O!Ti bond formation) and OH dissociation (red, D, OÀ Ti covalent bond formation). The structure of the state D is
shown in panel (b) while the corresponding HOMO and LUMO are shown in panels (c) and (d), respectively (�0.03 a.u. isosurface). Simulated IR (e) and Raman
(f) spectra change upon D state formation in comparison to free HMF in the 1000–2000 cmÀ 1 region.
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over, the HOMO-LUMO gap of the cluster should only be
considered as zero-order approximation to the true solid-state
bandgap. Interestingly, we found about 0.5 eV singlet-triplet
gap decrease upon HMF dissociative adsorption: T1À S0 energy
difference at the PBE0 level is 3.03 and 2.53 eV for free HMF and
state D, respectively. We note that in the triplet D state the
unpaired electron density resides mainly at the HMF moiety
(1.92 e according to Löwdin population analysis) with only
minor delocalization onto TiO2 cluster. 2.5 eV corresponds to
approximately 500 nm excitation wavelength but the accessi-
bility of T1 state will strongly depend on the spin-orbit coupling
matrix element HSOC = <S0 jHSOC jT1 >. We used complete active
space self-consistent field (CASSCF) approach to estimate HSOC
and found it to be 0.07 and 0.94 cmÀ 1 for free HMF and state D,
Supporting Information). In addition, the lack of an observable
OÀ H stretching band in the FTIR (Figure S10, Supporting
Information) and Raman (Figure S11, Supporting Information)
spectra of F-SGH-TiO2 further corroborate the exchange of
terminal OH groups with FÀ ions. As expected, surface-
fluorination completely blocked the activity of SGH-TiO2 (en-
try 6, Table 2). This is again consistent with inhibiting the
adsorption of the substrate (HMF) on the titania surface and,
thus, the formation of the LMCT complex through hydroxyl
group interaction.
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Active sites and conversion per surface OH group
!
respectively. Therefore, as the probability of T1 S0 transition is
In the field of catalysis, the term “active site” is often employed
for the adsorption site effective for catalytic reactions. For a
photocatalytic reaction it is often used for the dispersed
chemical species such as metal complexes or adsorbed
chemical species on the support.[58] However, the photocatalytic
reaction can take place only upon the absorption of light by
these species, and non-irradiated species cannot act as an
active site.[58–60] Therefore, the term “active site” is sometimes
misleading in photocatalysis, and its correlation with photo-
catalytic activity is hard to predict.[58] Nevertheless, the
estimation of photocatalytic activity on the basis of number of
active sites cannot be completely ruled out. Here, the hydroxyl
groups of the titania surface are considered to be the potential
active sites. Therefore, the photocatalytic activity was also
evaluated using the number of molecules of HMF converted
after 1 h of illumination and the number of initially present OH
groups. Table 3 shows the conversion of HMF with respect to
surface OH groups and area-related parameters. The number of
OH groups on SGH-TiO2 surface was found to be four times
higher than P25 (Table 3). However, the area-normalized rate
(conversion of HMF per surface OH groups per unit time)
observed for P25 was 1.5 times higher than SGH-TiO2. P25
showed two times higher conversion of HMF per surface OH
groups than SGH TiO2. Nevertheless, there are a number of
factors that may affect the conversion of HMF per surface OH
groups. The lower conversion of HMF per surface OH groups by
SGH TiO2 may indicate that all the OH groups are not
necessarily accessible to HMF for adsorption to activate the
catalytic cycle. Additionally, the whole surface area is not
accessed by light, especially inside the pores and aggregated
particles.[61]
2
in proportion to the jHSOC
j
we expect at least an order of
magnitude increase of this transition probability upon HMF
adsorption at the titania surface. This may indicate a possible
role of triplet states in LMCT process.[56] However, further
investigation of this phenomenon is beyond the scope of the
current study.
Role of surface OH groups for photocatalytic activity of
SGH-TiO2
In order to further elucidate the contribution of the surface
hydroxyl group in LMCT complex formation, SGH-TiO2 was
°
calcined at 600 C for 3 h in air with the aim to remove surface
hydroxyl groups by condensation. FTIR (Figure S10, Supporting
Information) and Raman (Figure S11, Supporting Information)
spectra of the as-calcined SGH-TiO2 sample (SGH-TiO2-cal-600)
exhibited that the band corresponding to OÀ H stretching
(3200–3400 cmÀ 1) and bending vibrations (1333 cmÀ 1) disap-
peared upon calcination. However, the calcined titania (SGH-
TiO2-cal-600) was inactive as a catalyst for the oxidation of HMF
under visible light (entry 5, Table 2), signifying that hydroxyl
groups are decisive for photocatalytic activity. They are likely
involved in the visible light-active LMCT complex formation.
In a further approach to study the role of the OH-groups for
photocatalytic activity, SGH-TiO2 was chemically modified via a
surface-fluorination to replace the surface hydroxyl groups with
FÀ ions.[57] The surface coverage of FÀ ion is calculated to be
7 OHnmÀ 2, which corresponds to approximately 80% of the FÀ
ion surface exchange with the hydroxyl groups. The XPS
analysis of surface-fluorinated SGH-TiO2 (F-SGH-TiO2) also con-
firmed that the FÀ ions are adsorbed on the TiO2 surface and
the surface OH groups of TiO2 were modified to TiÀ F (Table S1,
Table 3. Area-normalized conversion, area-normalized rate, conversion per surface OH groups, and conversion per surface OH groups per unit time achieved
by SGH-TiO2 and P25.[a]
Catalyst
Area-normalized Conv. [mÀ 2
]
Area-normalized rate [mÀ 2 sÀ 1
]
DOH [nm]
Number of OH groups
Conv. OHÀ 1
Conv. OHÀ 1 sÀ 1
SGH-TiO2
P25
8.1×1017
22.7×1013
34.9×1013
9
7
32.5×1018
7.8×1018
0.08
0.17
24.7×10À 6
47.2×10À 6
12.5×1017
[a] Reaction conditions: HMF (1 mm, 0.02 mmol), photocatalyst (1 gLÀ 1, 20 mg), reaction medium (acetonitrile), HMF solution volume (20 mL), reaction time
(1 h), incident light wavelength (515 nm), and light intensity [�54 WmÀ 2 (6×�9 WmÀ 2)]
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Mechanistic studies of oxidation of HMF via LMCT-mediated
activation of SGH-TiO2
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9
In order to identify the active species involved in the photo-
catalytic oxidation of HMF, different scavengers [methanol, 1,4-
benzoquinone (BQ), and silver cations] were separately added
in the reaction system under the same conditions. The addition
of methanol as hole (h+) trap does not greatly affect the HMF
conversion (56%) and DFF selectivity (88%), meaning that h+ is
not the key active species for the photocatalytic oxidation of
HMF (entry 8, Table 2). When BQ was added in the reacting
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*
À
suspension as superoxide radical anion (O2 ) scavenger, the
conversion of HMF and formation of DFF are completely
Figure 6. Reaction pathway for the photocatalytic oxidation of HMF over
SGH-TiO2.
*
À
inhibited (entry 9, Table 2). This is an indication that O2 is the
leading active species for the photocatalytic oxidation of HMF
to DFF. In contrast, the addition of silver cations as electron
acceptors totally stopped the photocatalytic activity (entry 10,
Table 2). This demonstrates their potential involvement in the
formation of active intermediates of HMF oxidation. The above
discussion suggests that Ag+ as electron acceptor may hinder
tion of HMF, which inject electrons into the CB of SGH-TiO2 and
transformed HMF to the corresponding radical cation (Step 2,
Figure 6). Dissolved oxygen gains an electron from the CB of
*
*
À
À
the formation of O2 , possibly responsible for the oxidation of
HMF. Moreover, to evaluate the effect of oxygen, the photo-
catalytic oxidation of HMF was carried out in anoxic conditions
(Ar 25 mLminÀ 1 and degassed acetonitrile) as well as by
bubbling air (25 mLminÀ 1) into the system. The anoxic con-
ditions greatly influence the photocatalytic activity and impede
the conversion of HMF (entry 11, Table 2). In contrast, bubbling
air into the photocatalytic system improved the DFF selectivity
up to 96% (entry 12, Table 2), although HMF conversion was
slightly reduced (52%), which indicates that increased amount
of oxygen prevents side reactions possibly by neutralizing the
reactive oxygen species (hydroxyl radical) produced by the
potential side product (H2O2). Moreover, bubbling air may also
assist in the desorption of DFF from the catalyst surface, which
further improved the DFF selectivity.
SGH-TiO2 and is converted to a superoxide radical anion (O2 ).
The radical cation then reacts with the superoxide radical anion
and generates hydroperoxyl radical and alkenyl radical (Step 3,
Figure 6). In the final step, the hydroperoxyl radical abstracts a
hydrogen atom from the alkenyl radical and forms the target
product (DFF) and side product (H2O2; Step 4, Figure 6). The
H2O2 produced may adsorb on the SGH-TiO2 and possibly react
with electrons to form a hydroxyl radical, which may also
influence the reaction mechanism and DFF selectivity. The
radical cation formed in the second step may or may not
release into the reaction medium; our experiments cannot
distinguish whether the radical cation is released or remains on
the surface. If the radical cation remains on the surface, it may
react with another HMF molecule and form an alkenyl radical
by deprotonation. Subsequently, the superoxide radical anion
may attack the alkenyl radical and produce DFF and a hydro-
peroxide anion. The hydroperoxide anion may react with the
proton and form hydrogen peroxide. However, the likelihood of
a free radical cation attacked by the superoxide radical anion at
the initial step is greater compared to the alkenyl radical
attacked at later stages.
To gain further insight about the photocatalytic oxidation of
HMF, a number of exploratory experiments were carried out
with some commercial catalysts and by changing the reaction
conditions. Interestingly, SGH-TiO2 showed better performance
for the selective oxidation of HMF than commercial P25 (entry 3,
Table 2). There are a couple of distinct features exhibited by
SGH-TiO2 that may improve its photocatalytic performance. For
example, HMF-ads-SGH-TiO2 showed pronounced shift in the
absorption of light from 400 up to 650 nm compared to HMF- Conclusions
ads-P25 (Figure 4). This may have ascribed to higher specific
surface area (177 m2 gÀ 1) of SGH-TiO2 compared to P25
(53 m2 gÀ 1). The higher specific surface area of SGH-TiO2
provided a greater (4 times) number of potential active sites
(OH groups) for the adsorption of HMF, which in turn enhanced
the activity of SGH-TiO2 for the selective oxidation of HMF.
Based on the characterization and photocatalytic experi-
ments carried out in the presence of different scavengers and
additives, a reaction pathway for the photocatalytic selective
oxidation of HMF to DFF under visible light is proposed
(Figure 6). The adsorption of HMF on the surface of SGH-TiO2
underpins all the photocatalytic oxidation steps. Visible light
irradiation activates the LMCT-complex formed by the adsorp-
The formation of a visible light-active ligand-to-metal charge
transfer (LMCT) complex between HMF and titania is shown to
enable the photocatalytic selective oxidation of the biomass-
derived platform chemical 5-hydroxymethylfurfural (HMF) to
the industrially relevant chemical 2,5-diformylfuran (DFF) in the
liquid phase. The key step for formation of the LMCT-complex is
the dissociative interaction of hydroxyl groups of HMF and the
titania surface. However, LMCT occurring in the singlet or triplet
state is still elusive as the first triplet state was found
approximately 500 nm above the ground singlet state with
predicted significantly enhanced accessibility as compared to
free HMF. Upon irradiation the LMCT-complex transfers elec-
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°
Afterwards, the F-SGH-TiO2 was collected and dried at 110 C for
12 h.
trons into the conduction band of titania, which react with
molecular oxygen in the reaction medium and generate super-
oxide radical anions responsible for the oxidation of HMF.
Generally, the visible light activity of the complex depends on
the available surface area of the catalyst, even when the specific
surface area of the titania is different. Moreover, the photo-
catalysts are recyclable and remain highly active over multiple
re-uses. The present findings, therefore, show that heteroge-
neous photocatalysis utilizing widely available titania may have
high potential for the valorization of biomass-derived platform
chemicals by selective oxidation. This may hold particularly true
if the photocatalytic conversion can be accomplished in the
presence of visible light with the reactants acting as a sensitizer
for titania through formation of a surface complex.
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Characterization of titania nanoparticles (SGH-TiO2): Powder XRD
measurements were performed employing Bragg-Brentano config-
uration. This type of arrangement was provided using Empyrean
diffraction platform from Malvern PANalytical Co., powered at
40 kV×40 mA and equipped with a vertical goniometer, with
theta–theta geometry using Ni filtered CuKα radiation. Data were
°
°
collected in range of 2θ=9–100 , with step size of 0.008 and
counting time up to 60 s per step. The percentage phase
composition of P25 and SGH-TiO2 was determined through the
Rietveld refinements of the XRD patterns. The average crystallite
size was determined according to the Scherrer equation [Eq. (1)],
where D is the average crystallite size of the catalyst [nm], λ is the
wavelength of the CuKα X-ray radiation (λ=0.154056 nm), k is a
coefficient usually taken as 0.94, β is the full width at half maximum
(FWHM) intensity of the peak observed at 2θ (radian), and θ is the
diffraction angle.
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Experimental Section
D ¼ kl=bcosq
(1)
Chemicals: Titanium(IV) isopropoxide (97+%, Sigma Aldrich), 2-
propanol (99.7%, Alfa Aesar), nitric acid (65%, Alfa Aesar), 5-
hydroxymethylfurfural (98%, Acros Organics), 2,5-diformylfuran
(98%, abcr), titanium(IV) oxide P25 (99.5%, Evonik), acetonitrile
(99.9%, POCH), methanol (99.9%, POCH), 1,4-benzoquinone (98%,
Sigma Aldrich), silver nitrate (99.8%, STANLAB), anatase (98%, Acros
Organics), brookite (99.9%, Sigma Aldrich), sodium fluoride (99%,
Chempur), sodium acetate (99.0%, Chempur), potassium trioxalato-
ferrate(III) trihydrate (98%, abcr), 1,10-phenanthroline (99.5%,
Chempur). Water used was purified to 18 MΩcm resistivity by Milli-
Q water purification system.
The specific surface area and pore width distribution of the SGH-
TiO2 and P25 was determined through N2 physisorption isotherms
by applying the BET and BJH method, respectively. The measure-
ments were carried out at Micrometrics ASAP 2000 automated
system. The FTIR spectrum of SGH-TiO2 was recorded on Bruker ATR
spectrometer in the range of 4000–400 cmÀ 1 in transmittance mode
with 16 scans and a resolution of 4 cmÀ 1. UV/Vis absorption
spectroscopy was performed using a UV/VIS/NIR spectrophotome-
ter Jasco V-570 equipped with an integrating sphere. The baseline
was recorded using SpectralonTM [poly(tetrafluoroethylene)] as a
reference material. Bandgap values were calculated using Tauc plot
applying the Kubelka-Munk function (Figure S12, Supporting
Information). The non-polarized Raman spectra were recorded in
the back-scattering geometry using in Via Renishaw micro-Raman
system equipped with an integrated Leica microscope. As a source
of excitation light, the infrared solid-state laser, operating at
785 nm was used. The power of the excitation light was fixed at no
more than 20 mW. The laser beam was focused on the sample
through the 20×/0.4NA objective. The spatial resolution of the
Raman spectra was about 2 μm. The samples were scanned in three
spectral range: 100–1000 cmÀ 1 (I), 1000–2000 cmÀ 1 (II), and 2750–
3450 cmÀ 1 (III) with the spectral resolution equal to 1 cm-1. The
Rayleigh radiation was block by a holographic notch filter. The
backscattered Raman light was dispersed by an 1800 mmÀ 1 (λ=
785 nm) holographic grating on the Peltier cooled CCD. All
measurements were performed at a room temperature. Artefacts
from cosmic ray were removed and analyses of the spectra were
performed in OPUS (Bruker) software. The leaching of titanium ion
after photocatalytic reaction was determined using the energy
dispersive X-ray fluorescence analysis (EDXRF) was carried out using
MiniPal 4 equipment from PANalytical Co, with a Rh-tube and
silicon drift detector (resolution 145 eV) to gain information on the
elemental composition of samples. The spectrum was collected in
atmosphere, without using a filter, at a tube voltage of 30 kV in
order to evaluate the presence of Ti. The time of acquisition was set
to 600 s and the tube current up to 50 μA. XPS experiments were
performed in a PHl 5000 VersaProbeTM spectrometer (ULVAC-PHI,
Chigasaki Japan). The XPS spectra were recorded using monochro-
matic AlKα radiation (hν=1486.6 eV) from an X-ray source operat-
ing at 100 μm spot size, 25 W, and 15 kV. Both survey and high-
resolution XPS spectra were collected with the analyzer pass energy
of 117.4 and 23.5 eV and the energy step size of 0.4 and 0.1 eV,
respectively. Casa XPS software (v.2.3.19, Casa Software Ltd,
Wilmslow, United Kingdom) was used to evaluate the XPS data.
Synthesis of titania nanoparticles: Titania nanoparticles were
synthesized through sol-gel-assisted method adapted from the
literature.[62] A TiO2 sol was prepared by acid-catalyzed hydrolysis of
titanium(IV) isopropoxide. In a typical synthesis, specified volume of
titanium(IV) isopropoxide (30.5 mL) was dissolved in 2-propanol
(25 mL) and stirred for 2 h at room temperature. Subsequently, 1 m
HNO3 (1 mL) was added to the solution under continuous stirring
for 5 min until gelation took place. Then, 25 mL of water was slowly
added to the gel and stirred for another 3 h. Afterwards, the
prepared titania nanoparticles were filtered, washed several times
°
with water, and then dried in an oven at 80 C for 12 h. The dried
sample was ground to powder and transferred to a Teflon-lined
autoclave filled (�80%) with water (50 mL) for hydrothermal
°
treatment at 150 C for 8 h. Finally, the obtained titania nano-
°
particles named SGH-TiO2 were dried at 110 C in an oven for 12 h.
Synthesis of HMF-adsorbed titania nanoparticles (HMF-Ads-SGH-
TiO2): For the preparation of HMF-Ads-SGH-TiO2, 20 mg of SGH-TiO2
was suspended in 1 mm HMF solution (20 mL) in acetonitrile in the
glass photoreactor. The suspension was stirred for 1 h in dark at
°
400 rpm. Afterwards, the catalyst was collected, dried at 80 C for
3 h, and used for further characterization.
Thermal and chemical modification of titania nanoparticles (SGH-
TiO2): The prepared SGH-TiO2 nanoparticles were modified further
via a thermal and chemical treatment (surface-fluorination) to
remove surface-bound OH groups. Thermal treatment involves the
°
calcination of SGH-TiO2 at 600 C for 3 h under air with the heating
À 1 [34]
°
rate of 5 Cmin
.
Thermally treated SGH-TiO2 was named as
SGH-TiO2-cal-600. Surface-fluorinated titania (F-SGH-TiO2) was pre-
pared according to the method reported in the literature[57] with
some modifications. In brief, 0.05 m aqueous solution of NaF was
prepared and the pH of the NaF solution was adjusted to 3.5 using
HCl. Afterwards, the SGH-TiO2 (0.06 g) was suspended in 6 mL of
NaF solution and stirred for 5 h to fluorinate the SGH-TiO2 surface.
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Shirley background subtraction and peak fitting with Gaussian-
Lorentzian-shaped profiles was performed. Binding energy scale
was referenced to the C1 s peak with a binding energy of 284.8 eV.
For quantification, the PHI Multipak sensitivity factors and deter-
mined transmission function of the spectrometer were used. The
fluorine content of F-SGH-TiO2 sample was calculated using
elemental analyzer (UNICUBE).
[Eq. (3)], DFF selectivity [Eq. (4)], and DFF yield [Eq. (5)] were
calculated as follows:
1
2
3
4
5
6
7
8
9
M0HMF À MHMF
HMF conversion ¼
DFF selectivity ¼
� 100
(3)
(4)
(5)
M0HMF
MDFF
M0HMFÀ MHMF
� 100
OH group density measurement: The OH group density of SGH-
TiO2 and P25 was determined via a TGA weight loss (Figure S4,
Supporting Information), performed using a thermobalance (Mettler
Toledo TGA/DSC 3+). The method employed for TGA analysis[63,64]
involved two steps. In step 1, the sample wÀa1s heated under air
MDFF
DFF yield ¼
� 100
M0HMF
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(40 mLminÀ 1) from 25 to 120 C at 5 Cmin and held at this
temperature for 3 h to remove physically bound water from the
°
°
Where M0
corresponds to the initial amount of HMF [mmol],
HMF
whereas MHMF and MDFF refer to the mmol of HMF and mmol of DFF
after the photocatalytic reaction, respectively.
°
surface. In step 2, the temperature was increased to 750 C at
À 1
°
10 Cmin and held for 1 h. The mass loss during step 2 was used
to calculate the number of hydroxyl groups per surface area
Active sites and conversion per surface OH group: The number of
OH groups and conversion of HMF with respect to area and active
sites of SGH-TiO2 and P25 were calculated as follows [Eqs. (6)–(10)]:
according to the following formula [Eq. (2)]:[43]
NA
MH2O
1018 � SSA
2ðmT À mCÞ
OH nmÀ 2 ¼ a
(2)
area À normalized conversion ¼
number of molecules of HMFconv:
(6)
Where mT is the mass loss per mg of the sample between 120 and
SSA � mcat
°
750 C, mC is the mass loss per mg of the sample due to carbon
content (on the basis of evolved CO2), SSA is specific surface area
[m2 gÀ 1], MH2O is the molar mass of H2O [gmolÀ 1], α is the
calibration constant (0.625),[43,64] and NA is Avogadro’s constant
[molÀ 1].
area À normalized rate ¼
number of molecules of HMFconv:
SSA � mcat � t
(7)
(8)
(9)
The carbon content in SGH-TiO2 and P25 was determined through
temperature programmed oxÀid1 ation (TPO) experiments. The sam-
number of OH groups ¼ DOH � SSA � mcat
conv: per surface OH groups ¼
number of molecules of HMFconv:
number of OH groups
°
°
ples were heated at 5 Cmin from 25 to 120 C in an oxidative
environment (air; 25 mLminÀ 1), with the holdup time of 3 h. After
that, samples were cooled down to room temperature anÀd1 then
°
°
heated from 25 to 750 C with the ramp of 10 Cmin (air:
25 mLminÀ 1). The oxidation process was analyzed by quadrupole
mass spectrometer (QMS) Dycor Ametek. The amount of carbon
calculated from the CO2 signals (Figure S13, Supporting Informa-
tion) of the QMS, calibrated using CO2 (with the pulse of 1.01×
10À 5 mol).
conv: per surface OH groups per unit time ¼
number of molecules of HMFconv:
(10)
number of OH groups � t
where area-normalized conversion is conversion of HMF per m2 of
the catalyst [mÀ 2], area-normalized rate is conversion of HMF per m2
per unit time [mÀ 2 sÀ 1], SSA is the specific surface area [m2 gÀ 1], mcat
is the mass of the catalyst [g], and DOH is the density of OH groups
per m2.
Photocatalytic selective oxidation of HMF: The photocatalytic
reaction was carried out in a glass photoreactor (20 mL). In a typical
run, 20 mL of 1 mm (0.02 mmol) HMF solution prepared in
acetonitrile and 20 mg of SGH-TiO2 photocatalyst (1 gLÀ 1) was
charged into a glass photoreactor. Subsequently, the suspension
was stirred (400 rpm) for 60 min in the dark to attain the
equilibrium. The photocatalytic reaction was commenced with the
irradiation of green LED lamp (λ=515 nm). The photo-intensity of
the LEDs was measured to be approximately 54 WmÀ 2 (6×
�9 WmÀ 2), recorded by Delta OHM HD 2302.0 light meter with LP
471 RAD probe having a spectral range of 400–1050 nm. The
distance between the light source and the wall of the reactor is
approximately 2 mm. At given irradiation time intervals, 0.15 mL
aliquots were collected and then filtered through a nylon filter
(pore size 0.2 μm) to remove the catalyst.
Apparent quantum yield measurement: The AQY (Φ) for the DFF
production is defined as a ratio of the amount of DFF produced per
unit time to the number of moles of photon absorbed by the
system per unit time [Eq. (11)]:
F ¼
number of moles of DFF produced per unit time
(11)
number of moles of photons absorbed per unit time
�100
After each run the catalyst was recovered by decanting the solvent,
°
To measure the AQY, the photon flux to the photoreactor was
determined by potassium ferrioxalate actinometry.[65–68] The actino-
metry experiments were performed in a dark room using 515 nm
LED with an intensity of approximately 54 WmÀ 2 (6×�9 WmÀ 2).
Briefly, 20 mL of 0.15 m potassium ferrioxalate solution was fed into
a glass photoreactor used for photocatalytic experiments. Then the
actinometer solution was irradiated for 30 s while stirring at the
speed of 400 rpm. Simultaneously, another sample prepared with
washed several times with water, dried at 110 C for 48 h, and
reused for next cycle with a fresh HMF solution. This process was
repeated up to five times of application.
The quantitative analysis of reactants and products was performed
on a HPLC instrument (Waters 2487) equipped with a C18 Thermo
scientific (250×4.6 mm) column using an eluent consisting of 55%
acetonitrile and 45% Milli-Q water at a flow rate of 1 mLminÀ 1. The
°
column oven temperature was kept at 25 C. The HMF conversion
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the same procedure was left in the dark as a control. Upon
completion of irradiation an aliquot of 0.15 m potassium ferriox-
alate was taken in glass vials and 500 μL of the 0.1% buffered
phenanthroline solution was added to irradiated and non-irradiated
samples. The samples were then allowed to develop in the dark for
0.5 h. Afterwards, the absorption of each of the sample was
measured at 510 nm on Thermoscientific Evolution 220 UV/Vis
spectrophotometer. The amount of Fe2+ produced during the
Numerical partial Hessian[80] was evaluated with PBE functional at
PBE-optimized structures to obtain zero-point energy corrections
that were added to the final single-point energies. These
calculations were also used to approximate IR and Raman spectra
in the 1000–2000 cmÀ 1 spectral envelope that is free from frozen
OH vibrations. Plotted spectra were broadened using Lorentzian
1
2
3
4
5
6
7
8
9
function with half-width of 50 cmÀ 1
.
If not stated otherwise, the reported energies are DLPNO-CCSD(T)/
CBS+ZPE energies. Comparison of energetics obtained at the DFT
+ZPE and MP2+ZPE levels is provided in Table S2, Supporting
Information.
irradiation was determined using the optical difference (ΔA510nm
)
between the irradiated and control (dark) sample and the extinction
coefficient at 510 nm (ɛ=11100 mÀ 1 cmÀ 1).
10
11
12
13
14
15
16
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53
54
55
56
57
Knowing that the quantum yield for Fe2+ production is 1.0 and that
the samples absorb >99% of the incident light, the photon flux
absorbed by the sample per unit time was calculated using the
following formula [Eq. (12)]:[68]
Spin-orbit coupling (SOC) matrix elements were computed with
complete active space self-consistent field (CASSCF) method.[81]
Initial orbitals were based on a converged triplet PBE0 natural
orbitals. A minimal active space was selected with two singly
occupied molecular orbitals and two electrons. SOC matrix
elements were estimated using the quasi-degenerate perturbation
theory.[82]
Nhv
t
moles of Fe2þ
(12)
¼
F t F
Coulomb and exact exchange integrals were evaluated with RI[83]
and COSX[84] approximations, respectively. Def2 family „/J“ and „/C“
auxiliary basis sets were applied.[85] DLPNO calculations were
performed with TightPNO settings as suggested for weak
interactions.[86] All calculations were performed with ORCA 4.2.0
suite of programs.[87]
where t is the irradiation time in seconds and F is the fraction of
light absorbed, which is unity in this case (Figure S14, Supporting
Information). Whereas, Nhν refers to number of moles of photon
absorbed in einstein.
Quantum chemical calculations: A H-saturated cluster model
[Ti20O62H44]0 was used to represent the TiO2 anatase (101) surface.
Such quantum model was extracted from the experimental crystal
structure according to the procedure depicted in the Figure S8,
Supporting Information. Pure cluster (surface model) was partially
relaxed by fixing terminal OH groups Cartesian coordinates to
remove bias towards relaxation to thermodynamically more stable
rutile-like structure.[69] Various HMF adsorption modes were tested
with economic GFN2-xTB method of Grimme and co-workers;[70] in
these calculations the HMF was fully relaxed while the surface
model was only partially relaxed (see above). The most stable
isomers were subject to further geometry refinement at the DFT
level using PBE functional,[71,72] def2-SVP basis set,[73] and two
corrections: D3BJ[74,75] and gCP,[76] to account for missing dispersion
interactions and basis set deficiencies, respectively. Here, the same
Cartesian fixing procedure as for GFN2-xTB was applied. Subse-
quently, single-point calculations were performed with the hybrid
PBE0 functional.[77] As a reference, we also performed single-point
calculations with the state-of-the-art coupled cluster singles,
doubles, and perturbative triples method in the domain localized
pair natural orbital approximate formulation [DLPNO-CCSD(T)].[52,53]
We note that the method allows to obtain “chemically accurate”
(error of <1 kcalmolÀ 1) energetics only at moderately higher cost
than DFT. In this computations, the complete basis set limit (CBS)
was approximated in a three-step procedure. First, DLPNO-CCSD(T)
calculations were performed with the def2-SVP basis set [this yields
accurate correlation energy with def2-SVP basis E(corr)CCdef2-SVP]. In
the next step, MP2 calculations were performed with def2-SVP and
def2-TZVP basis sets in order to obtain approximate correlation
energies at these basis sets that can be used in two-point
Acknowledgements
This publication is part of a project that has received funding from
the European Union’s Horizon 2020 research and innovation
programme under the Marie Skłodowska-Curie grant agreement
No. 711859 and from the financial resources for science in the
years 2017–2021 awarded for the implementation of an interna-
tional co-financed project. Roger Gläser and Michael Goepel
gratefully acknowledges support from the Leipzig Graduate School
of Natural Sciences: Building with Molecules and Nano-objects as
well as from the Research Academy Leipzig. Adam Kubas acknowl-
edges support from the National Science Centre, Poland, grant
number 2018/30/E/ST4/00004. Access to high performance com-
puting resources was provided by the Interdisciplinary Centre for
Mathematical and Computational Modelling in Warsaw, Poland,
under grant GB79-5. Moreover, authors also acknowledge Ms.
Agnieszka Wiśniewska (Soft Matter Research group, IPC PAS) and
Mr. Kamil Sobczak (Faculty of Chemistry, Biological and Chemical
Research Centre, University of Warsaw) for TGA and TEM measure-
ment, respectively.
Conflict of Interest
extrapolation scheme[78] to get MP2 correlation energy at the CBS
def2-TZVP
limit [these values are denoted as E(corr)MP2def2-SVP, E(corr)MP2
,
The authors declare no conflict of interest.
and E(corr)MP2CBS, respectively]. Similarly, one obtains Hartree–Fock
correlation energy at the CBS limit (EHFCBS). Finally, DLPNO-CCSD(T)/
CBS energy is estimated as:[79]
Keywords: 5-hydroxymethylfurfural · biomass valorization ·
photocatalysis · titania · visible light
�
�
CSSDðTÞ
CBS
def2À SVP
E DLPNO
� E þ EðcorrÞCC
HF
CBS
(13)
CBS
def2À SVP
þ½EðcorrÞMP2 À EðcorrÞMP2
�
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Manuscript received: November 19, 2020
Revised manuscript received: December 30, 2020
Accepted manuscript online: January 16, 2021
Version of record online: January 21, 2021
ChemSusChem 2021, 14, 1351–1362
www.chemsuschem.org
1362
© 2021 The Authors. ChemSusChem published by Wiley-VCH GmbH