Efficient and selective aqueous photocatalytic mono-dehydration of sugar alcohols using functionalized yttrium oxide nanocatalysts

Article abstract of DOI:10.1039/d0gc00914h

The mono-dehydration of sugar alcohols such as d-sorbitol and d-mannitol generates 1,4-sorbitan and 1,4-mannitan, respectively, which are relevant platform molecules for the synthesis of detergents and pharmaceuticals. Most reported catalytic systems provided access to di-dehydrated products, while mono-dehydration required special efforts, particularly regarding selectivity and reaction temperature. A series of functionalized yttrium oxides were prepared via sol-gel synthesis in this work, which not only showed an interesting micropipe-like morphology, but also contained functional components. These materials were investigated as photocatalysts in the dehydration of d-sorbitol and d-mannitol, exhibiting high selectivity to mono-dehydration. The effects of solvent, temperature and catalyst were fully discussed. A catalytic mechanism was proposed based on the experimental results and calculations.

Full text of DOI:10.1039/d0gc00914h

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Page 1 of 28
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DOI: 10.1039/D0GC00914H
Efficient and selective aqueous photocatalytic mono-dehydration of sugar
alcohols using functionalized yttrium oxide nanocatalysts
Juncheng Yanga, Xiaoyong Lia, Yu Chengb, Chao Fana, Cheng Pana, Benhua Huanga, Xu
Mengc, Junjie Zhanga, Aqun Zhenga, Xiaomo Mad, Lina Guoa, Rafael Luquee, Weining
Zhang*f, Yang Sun*a
Abstract
The mono-dehydration of sugar alcohols such as D-sorbitol and D-mannitol generates
1,4-sorbitan and 1,4-mannitan respectively, relevant platform molecules for the synthesis of
detergents and pharmaceuticals. Most reported catalytic systems provided access to
di-dehydrated products, while mono-dehydration deserved special endeavors, particularly
regarding selectivity and reaction temperature. A series of functionalized yttrium oxides were
prepared via sol-gel synthesis in this work, which not only showed an interesting micropipe-
like morphology, but also contained functional components. Materials were investigated as
photocatalysts in the dehydration of D-sorbitol and D-mannitol, exhibiting high selectivity of
a
Department of Applied Chemistry, School of Science, Xi’an Jiaotong University, No. 28, Xianning West Road, Xi’an
710049, P.R. China. E-mail: q62alsor@uco.es (Rafael Luque); sunyang79@mail.xjtu.edu.cn (Yang Sun)
b Xi’an No.1 Hospital; Shaanxi Institute of Ophthalmology; Shaanxi Key Laboratory of Ophthalmology; Clinical Research
center for Ophthalmology Diseases of Shaanxi Province; First Affiliated Hospital of Northwestern University, No. 30, Xi’an
City South Street powder lane, Xi’an 710002, P.R. China
c School of Material Science and Engineering, Xi’an University of Science and Technology, No. 48, Qin Tang Avenue, Xi’an
710600, P.R. China
d College of Humanities and Social Sciences, Xi’an Jiaotong University, No. 28, Xianning West Road, Xi’an 710049, P.R.
China
e Departamento de Quimica Organica, Universidad de Cordoba, Campus de Rabanales, Edificio Marie Curie (C-3), Ctra
Nnal IV-A, Km 396, E14014, Cordoba, Spain. E-mail: q62alsor@uco.es (Rafael Luque)
f Ningxia Orient Tantalum Industry Co. Ltd., P.O. Box 105, Dawukou District, Shizuishan City, Ningxia 753000, P.R. China.
E-mail: zhwn0608@sina.com (Weining Zhang)
† Electronic supplementary information (ESI) available: XPS survey scan of synthetic samples; XPS measurement of Ti 2p
and C 1s regions for synthetic samples; XRD comparison of C1 with that of standard Y2O3; low-angle XRD patterns of
powdered samples; FT-IR spectra of synthetic samples; GC-MS examples for catalytic products.
1

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mono-dehydration. The effects of solvent, temperature and catalyst were fully discussed. A
catalytic mechanism was proposed based on experimental results and calculations.
Introduction
The depletion of fossil fuels such as coal and oil, along with a worldwide increasing number
of carbon dioxide emissions mainly generated from combustion of these fuels are relevant
challenges that the world is facing nowadays.1 In response to this challenge, carbohydrates,
as one of the most abundant and renewable resources, emerged as a promising source of
chemicals and fuels.2 On the other hand, progress in biorefineries has attracted worldwide
and continuous interests which would determine levels of living and production in the future.3
The utilization of sugar alcohols such as sorbitol and mannitol appeared to be a highly
promising approach for development of fuels and/or fine chemicals. First of all, these sugar
alcohols could be produced on a large scale by hydrogenation of glucose, cellulose, or other
renewable biomass feedstocks.4 Importantly, the dehydrated forms of sugar alcohols have
been widely employed in many applications (i.e. isosorbide and isomannide as raw materials
for production of engineered plastics5).
Mono-dehydrated products of sorbitol and mannitol have comparatively received less
attention in literature. 1,4-Sorbitan (1,4-anhydro-D-sorbitol) can be obtained via
mono-dehydration of D-sorbitol, which could be transformed to polysorbate through
etherification, being widely used as stabilizer, oil-in-water emulsifier (detergent) or
dispersant.6 Comparably, 1,4-mannitan (1,4-anhydro-D-mannitol) has been employed as
intermediate for the synthesis of certain pharmaceuticals.7 To date, there is still large room
for improvements in mono-dehydration processes.
2

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Mineral acids including hydrofluoric, sulfuric and hydrochloric acids have been used as
catalysts for the dehydration of sugar alcohols, with inherent issues related to exothermic
reactions and corrosion of equipment, giving di-dehydrated products.4 Phosphates of tin,
zirconium and titanium were explored as alternative catalysts under homogeneous reaction
conditions at high temperatures.8,9 Heterogeneous catalysts including niobium oxide10,
sulfonic acid-functionalized silica11 and superhydrophobic mesoporous acids12 have also
been employed, in which isosorbide yields could be promoted by increasing catalyst acidity.
However, most systems provided high di-dehydration selectivity as compared to
mono-dehydrated products.4,8,10-12 Most known catalytic reactions demanded high
temperatures to guarantee satisfactory conversions including metal phosphate-catalyzed di-
dehydration (200 oC or higher)8 and sodium hydrogen sulfate-facilitated mono-dehydration
o
(180 C)13. Interestingly, the use of solid bases has not been considered for the
mono-dehydration of sugar alcohols.
In the past several decades, yttrium oxide has been extensively utilized as sintering agent
for the fabrication of transparent ceramics due to its high melting point, phase stability, as
well as low thermal expansion.14 Owing to its basic nature, yttrium oxide was also employed
as catalyst for a variety of reactions including conversion of alcohols to alkenes15, or
reduction of nitrogen oxide.16 Yttrium oxide featuring micro-/nanosized structures or specific
morphologies17, 18 have been prepared using a number of methodologies such as sol-gel19,
hydrothermal20 and non-hydrolytic synthesis.21
On the other hand, preparation and application of metal oxides for photocatalysis
aroused renewed interests in recent years. For example, titanium dioxide was the most
extended n-type semiconductor with strong absorbance of ultraviolet light, leading to wide
applications in pigments, photovoltaic cells or photocatalysis.22 Titanium dioxide-doped
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metal oxide exhibited much improved catalytic activity as compared to the individual oxide23,
for which titanium doping into various oxide can endow metal oxide nanomaterials (e.g.
yttrium oxides) with relevant photoactivities for photocatalytic applications.
Based on these premises, this contribution was aimed to provide an efficient,
chemo-selective, benign and cost-effective photoactive system for the dehydration of sugar
alcohols, exemplified for D-sorbitol and D-mannitol. A series of yttrium oxides were prepared
via sol-gel synthesis making use of Pluronic P123, PVA, and titanium to achieve a highly
photoactive multifunctional system. Additionally, a catalytic mechanism was proposed based
on experimental results and calculations.
Experimental Section
Starting materials
Yttrium(III) nitrate hexahydrate (Y(NO3)3·6H2O, 99.99 %), D-sorbitol (98 %) and D-mannitol
(98 %) were purchased from Adamas-beta. Titanium(IV) n-butoxide (Ti(OBu)4, 98 %),
Pluronic P123 (EO20-PO70-EO20; EO, ethylene oxide; PO, propylene oxide; average Mn =
5800), poly(vinyl alcohol) (PVA, 86 %-89 % hydrolyzed, medium molecular weight) were
commercially available from Aladdin. Solvents and other reagents were all analytically pure,
and provided by local suppliers. The distilled water was prepared in our laboratory.
Synthesis of Catalysts
The synthesis of yttrium oxide nanostructures was shown in Scheme 1. In practice,
Y(NO3)3·6H2O (1.15 g, 3.0 mmol) was dissolved into distilled water (10 mL) under vigorous
stirring at room temperature. Ti(OBu)4 (materials were denoted as C1 and C2, no titanium
precursor added; C3, 0.75 mmol; C4, 1.5 mmol) was added. After stirring for 0.5 h, Pluronic
4

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P123 (0.3 g) and PVA (0.1 g) were added together. The pH of this mixture was adjusted to
13.0 via dropwise addition of a solution of diluted NaOH (3.0 mol L-1). Sample C1 was
calcined (500ºC, Scheme 1), while C2 was not calcined.
The mixture was then transferred into an autoclave (50 mL) and aged at 200 oC for 24 h.
The resulting solids were filtered under reduced pressure, washed carefully with distilled
water (3 × 5 mL) and absolute ethanol (3 × 5 mL), dried at 60 oC overnight. C2 was directly
collected as yellow powders (1.10 g) and the other three samples were further calcined at
540 oC for 5 h. C1 was obtained as white powders (0.75 g), C3 and C4 as white powders
(0.89 g and 0.95 g, respectively).
C1: Ti(OBu)4 blank, Calcination used
1) P123
2) PVA
1) Aging, 200 oC,
24 h in autoclave
C2: Ti(OBu)4 blank, Calcination blank
C3: Ti(OBu)4 0.75 mmol, Y / Ti = 4 / 1,
Calcination used
.
Products
+
Hydrosol
Ti(OBu)4
Y(NO3)3 6H2O
2) Drying, 60 o
C
3) Stirred in distilled
H2O (10 mL), pH
to 13 by NaOH
3) Calcination, 500 oC,
5 h (optional)
(Optional)
(3 mmol)
C4: Ti(OBu)4 1.5 mmol, Y / Ti = 2 / 1,
Calcination used
Scheme 1 Synthesis of yttrium oxide nanostructures.
Characterization
BET surface area, pore volume, pore radius and pore size distribution were detected on
Micromeritics ASAP 2020, using N2 adsorption isotherms at 77.35 K. Solid samples were
o
degassed at 150 C in vacuum before testing. Surface area was calculated using the multi-
point Brunauer-Emmett-Teller (BET) method based on adsorption data with relative pressure
P / P0 of 0.06-0.3. Pore volume (radius, 8.5-1500 Å) was obtained from N2 adsorbed at P /
P0 = 0.97. Pore volume and pore radius were calculated by using Barrett-Joyner-Halenda
(BJH) method.
Surface acid properties of synthesized catalysts were determined by titration of
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n-butylamine with coeruleum bromocresolis as indicator.25 In practice, the material (300 mg)
was combined with n-butylamine (1.25 mmol, 25.00 mL of 0.05 mol L-1 toluene solution)
into a conical flask (250 mL). After stirring for 5 min under cover, 2-propanol (100 mL) and
coeruleum bromocresolis (1 drop of diluted solution) were both introduced. Then, this
mixture was titrated using a HCl solution (0.025 mol L-1) and the end point arrived when
solution color changed from blue to yellow. The acid amount of solid sample was the
adsorbed amount of n-butylamine, obtained by subtraction of total n-butylamine with the
residues in solution that determined by HCl titration.
X-ray photoelectron spectroscopy (XPS) was measured on Kratos Axis Ultra DLD, using
monochromatic Al Kα x-ray (1486.6 eV) as irradiation source, and binding energy scale was
calibrated by using C 1s peak at 284.8 eV. The peaks were fitted by employing
Gaussian-Lorentz (G / L) product function with 30 % Lorentzian. Both low-angle (2θ = 0.5o-
10o) and wide-angle (2θ = 10o-80o) x-ray diffraction (XRD) experiments were carried out on
Philips X’Pert Pro diffractometer using Cu-Kα radiation (λ = 1.5418 Å), with interval of
0.05o second-1.
Scanning electron microscopy (SEM) was performed on JEOL JSM-6700F at 20.0 kV
without Au coating. Transmission electron microscopy (TEM) was tested on JEOL JEM-
200CX at 120 kV. FT-IR was measured in KBr pellets on Bruker Tensor 27, having wave
numbers of 400-4000 cm-1.
GC-MS analysis was performed on GCMS-QP2010 Plus, Shimadzu, equipped with a
Rxi-5ms capillary column having length of 30 m and internal diameter of 0.25 mm. For part
of GC, column temperature was 60 oC, injection port temperature was 250 oC, sampling mode
was split-flow, split-ratio was 26, carrier gas was helium. For MS, ion source temperature
was 200 oC, and interface temperature was 250 oC.
6

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Photocatalytic dehydration of sugar alcohols
D-Sorbitol (or D-mannitol, 2.0 mmol) and catalyst (2 mol% yttrium over substrate, according
to XPS data in Table 1) were mixed in water (10 mL) into a round-bottomed flask (100 mL)
in a dark place and then subjected to UV irradiation (UV-blank experiments remained in
dark). UV irradiation was accomplished by using an UV lamp (P = 300 W, λ = 265 nm),
Ultra Vitalux, Osram. The mixture was vigorously stirred at 20 oC for 6 h. The solid catalyst
was then filtered, and sometimes reloaded in another flask with fresh reagents to test its
reusability. The filtrate was concentrated under reduced pressure, and then analyzed by GC-
MS for identification and quantification.
Calculations
The calculations were carried out over Gaussian 09 package.26 The key front molecular
orbitals (MOs) were obtained by using theoretical calculations on the basis of their ground
states. The functional RB3LYP was selected to optimize the substances in this work, as
appropriate to discuss reaction profiles.27 Furthermore, all nonmetallic atoms were obtained
by using 6-31 + G* group in calculations, and yttrium by using Lanl2dz group.
Results and discussion
Synthesis of yttrium oxide nanostructures is depicted in Scheme 1. C1 was obtained via
sol-gel process followed by calcination, while preparation, characterization and application
of C2 would show effects of organic residues due to lack of calcination. C3 was introduced
in order to discuss effects of titanium content, while C4 may show effects of a different
amounts of titanium in the catalytic reaction.
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Textural properties of synthesized materials were investigated by using nitrogen
physisorption (Table 1, Fig. 1). C1 and C2 exhibited type II isotherms without hysteresis
loops, along with pore size distributions centered at 12 nm (a and b, Fig. 1).30 In association
with BET surface areas (Table 1), C1 and C2 were essentially non-porous materials featuring
interparticle macroporosity.30 C3 and C4 comparably exhibited similar type II isotherms (c
and d vs. a, Fig. 1), but already some porosity was present in these materials (Table 1), similar
to previously reported titanium-containing phase including rutile or Metal-O-Ti mixed
oxides.23
Table 1 Textural properties of synthesized nanostructures
Sample
SBET
(m2 g-1)a
19
PV
PVmicro
(cm3 g-1)c
-
PR
(Å)d
80
Ρ
dS
dXRD
(nm)g
5.7
Acid amount
(mmol g-1)h
0.25
(cm3 g-1)b
0.08
(g cm-3)e
1.3
(nm)f
243
271
137
90
C1
C2
C3
C4
17
0.09
-
97
1.3
9.5
0.4
29
0.15
0.001
0.004
106
127
1.5
n. d.i
4.5
0.06
51
0.31
1.3
0.05
a Surface area determined by BET method based on N2 adsorption.
b BJH adsorption cumulative volume of pores with radius of 8.5 Å-1500 Å.
c t-Plot micropore volume.
d Pore radius, BJH method on N2 adsorption.
e Bulk density.
f Crystallite size based on BET surface area: dS = 6 / (SBET·ρ), ρ bulk density.28
g Diameter of product particle measured by XRD according to Scherrer’s equation29: using 222 diffraction of
yttrium oxide phase (a-d, Fig. 4).
h Determined by n-butylamine titration (see experimental section).
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i Not detected.
Fig. 1 Nitrogen adsorption-desorption isotherms and pore size distributions of (a) C1, (b)
C2, (c) C3, and (d) C4.
The binding energies and atomic compositions of synthesized materials were
subsequently studied by XPS and summarized in Table 2 and Fig. S1 (Sect. 1, ESI). C2
exhibited higher contents of carbon and sodium as well as lower those of oxygen, nitrogen
and yttrium as compared to C1 (Table 2), clearly indicating calcination removed organic
templates and improved metal contents and nitrogen on the sample surface (C2 vs. C1,
Scheme 1). Titanium was found to be effectively incorporated in the corresponding samples
(C3 and C4, Table 2).
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Table 2 Binding energy and atomic composition of C, O, N, Y, Ti and Na on sample surface
Sample C (1s)
O (1s)
N (1s)
Y (3p)
Ti (2p)
Na (1s)
b
C1
C2
C3
C4
284.8 (31.37)a 531.8 (37.50) 394.8 (16.45) 299.8 (14.67)
-
-
285.8 (57.08) 531.8 (33.61) 394.8 (2.91)
284.8 (27.06) 529.8 (44.78) 393.8 (6.87)
300.8 (5.25)
299.8 (7.01)
-
1070.8 (1.14)
1070.8 (8.53)
-
457.8 (5.75)
284.8 (32.12) 529.8 (39.31) 393.8 (12.93) 300.8 (11.80) 458.8 (3.84)
a Binding energy (eV), along with atomic percentage (at%) in parentheses.
b Not detected.
XPS provided additional information on the chemical states of elements in the final
materials. Yttrium 3d regions of C1 and C2 showed typical binding energies of Y 3d5/2 and
3d3/2 photoelectrons at 156.7-157.1 and 158.8-159.1 eV respectively (a and b, Fig. 2),
characteristic of framework Y3+ species present in octahedral Y2O3 materials.31 Interestingly,
C3 exhibited a bit more complex XPS spectra which could be deconvoluted into two
contributions, the first one was characteristic of framework Y3+ from Y2O3 (Y 3d5/2 and 3d3/2,
156.1 and 158.4 eV, c of Fig. 2), while another contribution at 156.7 and 160.1 eV was
indicative of Y3+ species coming from Y-O-Ti species due to the Ti doping into Y2O3 that
increased binding energies of Y 3d5/2 and 3d3/2 according to previous reports (c, Fig. 2).32 C4
showed a very similar Y 3d region as C3 (d vs. c, Fig. 2), proposing a similar yttrium chemical
state.
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Fig. 2 XPS spectra of Y 3d region for (a) C1, (b) C2, (c) C3, and (d) C4.
With regards to titanium, C3 exhibited binding energies of Ti 2p3/2 and 2p1/2
photoelectrons at 458.0 and 463.2 eV respectively, characteristic of Ti4+ species in titanium
dioxide33, as well as 458.7 and 463.9 eV from Ti4+ present in Ti-O-Y species (a, Fig. S2,
Sect. 2 of ESI).23 C4 exhibited similar Ti 2p spectra (b, Fig. S2). C 1s XPS data provided
information about the organic species in synthesized materials. The band at 285.0 eV on C1
was representative of saturated hydrocarbons (sp3 carbon), while the following three
contributions present at 286.4, 289.1 and 290.3 eV could be assigned to carbons of carbonyl,
carboxyl and ester groups respectively (c, Fig. S2).34,35 There were no fundamental
differences in the C 1s regions between C1 and C2 to C4 materials (d, e, f vs. c, Fig. S2).
O 1s regions were subsequently examined, providing multiple insights into the
compositions of the materials (Fig. 3). C1 exhibited contributions at 529.4 eV (ascribed to
31
oxygen in Y2O3 ), 532.0 eV (assigned to oxygen of residual organic species) and 534.2 eV
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(attributed to oxygen of adsorbed water-OH species36) (a, Fig. 3), similar to those of C2 (b,
Fig. 3). Comparably, C3 instead exhibited contributions at 531.3 eV (oxygen of TiO2
species37) and at 531.7 eV (oxygen of Y-O-Ti species) (c, Fig. 3). C4 exhibited four
components on O 1s region, namely 529.8 eV (oxygen in Y2O3), 530.9 eV (oxygen in TiO2),
531.9 eV (oxygen in Y-O-Ti species), and 532.7 eV (oxygen from the organic residues after
calcination, C4, Scheme 1) (d, Fig. 3).
Fig. 3 XPS spectra of O 1s region for (a) C1, (b) C2, (c) C3, and (d) C4.
The influence of doped titanium on the acidity of samples was subsequently investigated
via titration of n-butylamine (see experimental section). C2 possessed a higher amount of
acid sites as compared to C1 (Table 1), probably due to calcination removal of acidic organic
components and metal hydroxyls (Scheme 1). Interestingly, titanium loading sharply
decreased acid sites in C3 and C4 materials (C3 and C4 vs. C1, Table 1), indicating that the
acidity of C1 could be ascribed to Brønsted acidity stemming from surface hydroxyls of
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yttrium oxide of C1 backbone,38 reduced after titanium incorporation (Scheme 1).
XRD patterns of synthesized materials were included in Fig. 4. All samples exhibited
typical diffraction lines of yttrium oxide (Y2O3; 2θ = 29.15o (222), 48.54o (440), 57.63o (622);
PDF, No. 76-0151; grey cubes in a, Fig. 4; Fig. S3, Sect. 3 of ESI). No other obvious
diffraction lines were present in wide-angle XRD of all samples (a-c, Fig. 4), particularly no
titanium dioxide diffractions (Fig. S4, Sect. 4 of ESI), indicating all titanium-containing
components were amorphous on the tested samples. Low-angle XRD showed diffractions at
2θ of 0.93o-1.13o, along with d value of 7.77-9.46 nm (a-c, Fig. S5, Sect. 5 of ESI),
corresponding to 100 diffraction lines of long-axis SBA-15,39 mainly due to the introduction
of Pluronic P123 as template in catalyst synthesis (Scheme 1).
Fig. 4 Wide-angle XRD patterns of powdered (a) C1, (b) C2, and (c) C4.
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SEM and TEM images were also recorded to ascertain the morphologies and structural
features of the materials (Fig. 5 and Fig. 6). C1 was composed of very small particles having
diameters of less than 50 nm, together with micropipes featuring hexagonal cross section and
length of 1-10 μm (a and b, Fig. 5). On the basis of TEM, the pipes of C1 were hollow, whose
thicknesses were ca. 80 nm (a, Fig. 6). In the absence of calcination step, C2 still contained
both amorphous particles and micropipes (Scheme 1; c, d, e, Fig. 5), but the surfaces of C2
seemed comparably more dense than those of C1 (e vs. b, Fig. 5), probably due to non-
calcined organic templates on C2 (C2 vs. C1, Scheme 1).
Upon titanium doping, the morphology of resulting C3 became a little different, which
contained both small fibers (20-100 nm in length) and micropipes with hexagonal or square
cross sections (3-15 μm, f and g, Fig. 5). C4 showed very similar morphologies as C3 (h and
i vs. f and g, Fig. 5). These findings seemed to point out a certain morphology change upon
titanium incorporation in the sol-gel synthesis.
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Fig. 5 SEM images of (a) C1 (magnification of 5,000×), (b) C1 (20,000×), (c) C2 (5,000×),
(d) C2 (15,000×), (e) C2 (20,000×), (f) C3 (3,000×), (g) C3 (10,000×), (h) C4 (10,000×),
and (i) C4 (4,300×).
Fig. 6 TEM images of (a) C1 (magnification of 50,000×), (b) C1 (80,000×), and (c) C3
(100,000×).
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FT-IR spectroscopy provided additional insights on the functional group present in
yttrium oxide samples. C2 shows a sharp peak at 3614 cm-1 (b, Fig. S6, Sect. 6 of ESI),
illustrating O-H stretching of hydroxyl groups on the surface of yttrium oxide17, but this band
was not present in the FT-IR spectra of C1 (a, Fig. S6) as calcination facilitates dehydration
of hydroxyl groups from yttrium oxide. Furthermore, the broad band centered at 3364 cm-1
on C2 was characteristic of O-H stretching of hydroxyl groups from PVA or other organic
templates (b, Fig. S6), with this band also absent on C1 (a, Fig. S6). Bands at 2969 and 2869
cm-1 on C2 were characteristic of anti-symmetric and symmetric stretching of C-H bonds of
methyl groups, respectively39, invisible on C1 due to calcination (a vs. b, Fig. S6).
C2 showed other bands at 1488, 1373 and 1102 cm-1, corresponding to carbon-carbon
asymmetric stretching40, carbon–oxygen stretching of carboxyl and stretching of epoxy
groups41, respectively. However, these vibrations were removed or largely decreased in
intensity after calcination, the residual two small peaks that appeared at 1516 and 1397 cm-1
on C1 could be assigned to stretching of benzene ring and carbon–oxygen stretching of
carboxyl group, respectively (a, Fig. S6)42. Additionally, the peaks at 721 and 471 cm-1 on
C2 could be ascribed to stretching of yttrium-oxygen bond having different symmetries (b,
Fig. S6), which were shifted to lower wave numbers on C1 (557 and 460 cm-1, a, Fig. S6),
probably due to calcination (C1, Scheme 1). C3 and C4 exhibited similar FT-IR spectra to
that of C1, with the additional vibrations of titanium-oxygen bonds overlapped in the region
530-400 cm-1 (c, Fig. S6).
TGA and DSC analysis were performed and results included in Figure 7 and Figure S7
(Sect. 7, ESI), respectively. C1 exhibited a weight loss of 2.0 % at 30-150 oC (black line, Fig.
o
7), corresponding to a small endothermic peak that appeared at 40-150 C (black line, Fig.
S7, Sect. 7, ESI), typical of the evaporation of physisorbed water and/or organic solvents. On
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the other hand, the sharp endothermic peaks that occurred at 30-40 oC for all samples could
be ascribed to instrumental noise (all lines, Fig. S7). C2 exhibited three weight losses at
30-450 oC (red line, Fig. 7). The first (4%) taking place at 50-150 oC could be ascribed to the
removal of volatiles such as water and organic solvents, propelled by an endothermic band
o
that formed at 75-175 C (red line, Fig. S7). C2 showed the second (4.8 % loss) at 230-
285 oC, along with two endothermic peaks that centered at 276 oC and 295 oC respectively
(red line, Fig. S7), indicating partial removal of the organic template. The last weight loss
that appeared at 285-430 oC (ca. 16.6 %), could be ascribed to the removal of other organic
species and fully template removal (C2, Scheme 1).
There was only one obvious weight loss of 3.3 % that appeared at 85-155 oC on C3 (blue
o
line, Fig. 7), accompanied by an endothermic peak centered at 112 C (blue line, Fig. S7),
clearly indicating evaporation of volatiles. C4 showed a small weight loss of 1 % at 100-150
oC (cyan line, Fig. 7), corresponding to an endothermic band at 110-160 oC (cyan line, Fig.
S7), indicating C4 contained less adsorbed water or other volatiles as compared to C3.
Fig. 7 TGA curves of the synthesized samples.
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Photocatalytic experiments
The photoactivity of the designed yttrium nanostructures were subsequently tested in the
mono-dehydration of D-sorbitol and D-mannitol, with results summarized in Tables 3-6.
Importantly, the reaction could not be promoted by UV light (265 nm) in the absence of
catalyst (Table 3, entry 1), indicating the presence of the catalyst played a key role in the
selected transformations.
C1, C2, C3 and C4 exhibited low D-sorbitol conversions and yields of 1,4-sorbitan under
UV irradiation (265 nm), much less than those from blank experiments-in the absence of UV
(Table 3, entries 3 vs. 4, 5 vs. 6, 7 vs. 8, 9 vs. 10). Herein, the electrons in valence band of
yttrium oxide were excited to conduction band under UV light, leading to electron-hole pairs
on yttrium oxide surface, which could act as photo-sensitive sites.43 Importantly, these
centers were not active enough to promote the conversion of sugar alcohols, particularly
regarding the new phases formed in C1, C2 and C3, despite the presence of titanium doping
in C3 and C4 (C1-C4, Scheme 1). Electron-hole pairs may even destroy the original effective
Brønsted acidity sitesof synthesized samples, leading to poor outputs.
D-Mannitol was also employed as substrate, where C1, C2 and C3 exhibited poorer
photoactivities under UV than those observed in the absence of UV (Table 4, entries 3 vs. 4,
5 vs. 6, 7 vs. 8), confirming the already discussed UV effects over synthesized samples found
in the transformation of D-sorbitol (Table 4 vs. Table 3). The only exception came from C4
(Table 4, entries 9 vs. 10).
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Table 3. Photocatalytic dehydration of D-sorbitol to fine chemicals under UV irradiation
OH
HO
H
OH
OH
O
O
Catalyst (2 mol% Y)
OH
HO
+
HO
UV irradiation (265 nm)
H2O, 20 oC, 6 h
O
OH
OH
HO
1,4-Sorbitan
OH
H
HO
D-Sorbitol
Isosorbide
Entrya Catalyst UV
irradiation
Yes
Conversion
Yield of 1,4-sorbitan Yield of isosorbide
(%)b
(%)c
(%)d
1
2
3
4
5
6
7
8
9
10
0
0
0
None
Blank
Yes
0
0
0
29
67
30
66
30
93
26
48
28
64
29
66
29
85
26
48
1
C1
Blank
Yes
3
1
C2
Blank
Yes
0
1
C3
Blank
Yes
8
0
C4
Blank
0
a Reaction conditions: D-sorbitol (2.0 mmol), catalyst (2 mol% yttrium over substrate, according to XPS
data in Table 1), water (10 mL); UV irradiation: UV lamp (P = 300 W, λ = 265 nm), Ultra Vitalux,
o
Osram; reaction temperature: 20 C; time: 6 h; filtrate concentrated under reduced pressure, analysed
by GC-MS for identification and quantification.
b
Conversion of D-sorbitol to both mono- and di-dehydrated products, determined by GC-MS (Sect. 8,
ESI).
c Yield of 1,4-sorbitan based on original D-sorbitol, determined by GC-MS (Sect. 8, ESI).
d Yield of isosorbide based on original D-sorbitol, determined by GC-MS (Sect. 8, ESI).
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Table 4 Photocatalytic dehydration of of D-mannitol to platform chemicals under UV
irradiation
OH
HO
H
OH
OH
O
O
Catalyst (2 mol% Y)
OH
HO
+
HO
UV irradiation (265 nm)
H2O, 20 oC, 6 h
O
OH
OH
HO
OH
H
HO
D-Mannitol
1,4-Mannitan
Isomannide
Entrya Catalyst UV
irradiation
Yes
Conversion
Yield
of
1,4- Yield of isomannide
(%)b
mannitan (%)c
(%)d
1
2
3
4
5
6
7
8
9
10
0
0
0
None
Blank
Yes
0
0
0
37
85
40
94
68
74
97
73
36
85
40
75
68
74
87
72
1
C1
Blank
Yes
0
0
C2
Blank
Yes
19
0
C3
Blank
Yes
0
10
1
C4
Blank
a Reaction conditions: D-mannitol (2.0 mmol), catalyst (2 mol% yttrium over substrate, according to XPS
data in Table 1), water (10 mL); UV irradiation: UV lamp (P = 300 W, λ = 265 nm), Ultra Vitalux,
Osram; reaction temperature: 20 oC; time: 6 h; filtrate concentrated under reduced pressure, analyzed
by GC-MS for identification and quantification.
b
Conversion of D-mannitol to both mono- and di-dehydrated products, determined by GC-MS (Sect. 9,
ESI).
c Yield of 1,4-mannitan based on original D-mannitol, determined by GC-MS (Sect. 9, ESI).
d Yield of isomannide based on original D-mannitol, determined by GC-MS (Sect. 9, ESI).
Herein, all synthesized samples (C1-C4) not only contained inherent acidity (Brønsted-
type acid sites, Table 1) but also UV-promoted acidity (Lewis-type acid sites). Some catalytic
tendencies were changed with a different substrate, indicating the two types of acidities on
the catalysts both promoted the reaction, but one of them may dominate the reaction in the
light of different substrates with different structures and/or activation energies (Scheme 2).
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Additionally, catalyst durability was tested in recycling experiments. C3 exhibited
increasing outputs during subsequent reuses (four times) in the dehydration of D-sorbitol
under UV (Table 5), and a similar tendency was also found for the dehydration of D-mannitol
(Table 6). C4 could also be reused four times with only a slight activity loss (Table 6).
Table 5. Reuses of C3 in the photocatalytic dehydration of D-sorbitol to fine chemicals under
UV irradiation
OH
HO
H
OH
OH
O
O
Catalyst (2 mol% Y)
OH
HO
+
HO
UV irradiation (265 nm)
H2O, 20 oC, 6 h
O
OH
OH
HO
1,4-Sorbitan
OH
H
HO
D-Sorbitol
Isosorbide
Entrya Catalyst Conversion (%)b
Yield of 1,4-sorbitan (%)c
Yield of isosorbide (%)d
1
30
79
89
88
29
74
88
87
1
5
1
1
1R1
1R2
1R3
C3
a Reaction conditions: D-sorbitol (2.0 mmol), catalyst (2 mol% yttrium over substrate, according to XPS
data in Table 1), water (10 mL); UV irradiation: UV lamp (P = 300 W, λ = 265 nm), Ultra Vitalux,
Osram; reaction temperature: 20 oC; time: 6 h; filtrate concentrated under reduced pressure, analyzed by
GC-MS for identification and quantification; solid catalyst filtered, reloaded in another flask with fresh
reagents for recycling. In this column, entry 1 is copy of entry 7, Table 3, the following are continuous
recycling results.
b Conversion of D-sorbitol to both mono- and di-dehydrated products, determined by GC-MS.
c Yield of 1,4-sorbitan based on original D-sorbitol, determined by GC-MS.
d Yield of isosorbide based on original D-sorbitol, determined by GC-MS.
C3 and C4 were clearly stable under the investigated reaction conditions, with preserved
activities after four uses. Herein, the original Lewis acid sites of the catalysts may be turned
into Brønsted-type acid sites in protic solvents like H2O under UV irradiation, facilitating
catalytic dehydration in a more efficient way after the first use.44 In view of current advances,
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the present catalysts are comparable to reported systems in terms of mono-dehydration
selectivity4,5,8-10 as well as catalyst recycling4,9,12, showing promising prospects for large-
scale production.
Table 6. Reuses of C3 and C4 in the photocatalytic dehydration of D-mannitol to platform
chemicals under UV irradiation
OH
HO
H
OH
OH
O
O
Catalyst (2 mol% Y)
OH
HO
+
HO
UV irradiation (265 nm)
H2O, 20 oC, 6 h
O
OH
OH
HO
OH
H
HO
D-Mannitol
1,4-Mannitan
Isomannide
Entrya Catalyst Conversion (%)b
Yield of 1,4-mannitan (%)c Yield of isomannide (%)d
1
68
70
90
84
97
69
88
84
68
67
83
80
87
66
82
80
0
1R1
1R2
1R3
2
3
C3
C4
7
4
10
3
2R1
2R2
2R3
6
4
a Reaction conditions: D-mannitol (2.0 mmol), catalyst (2 mol% yttrium over substrate, according to XPS
data in Table 1), water (10 mL); UV irradiation: UV lamp (P = 300 W, λ = 265 nm), Ultra Vitalux,
Osram; reaction temperature: 20 oC; time: 6 h; filtrate concentrated under reduced pressure, analyzed
by GC-MS for identification and quantification; solid catalyst filtered, reloaded in another flask with
fresh reagents for recycling. In this column, entry 1 is copy of entry 7, Table 4, the following are
continuous recycling results, entry 2 that of entry 9, Table 4, the following the recycling too.
b Conversion of D-mannitol to both mono- and di-dehydrated products, determined by GC-MS.
c Yield of 1,4-mannitan based on original D-mannitol, determined by GC-MS.
d Yield of isomannide based on original D-mannitol, determined by GC-MS.
Proposed catalytic mechanism
With the experimental results obtained so far, a catalytic mechanism was proposed in
association with calculations. 1,4-dehydration of D-sorbitol may occur on yttrium oxide
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(Path 1, Scheme 2), with a transition state (TS1, Scheme 2) having energy of -26939.003 eV,
highest occupied molecular orbital (HOMO) of -0.18407 eV, and lowest unoccupied
molecular orbital (LUMO) of -0.05660 eV (a and b, Fig. 8), while difference of HOMO with
LUMO was 0.12747 eV.
The following dehydration leading to isosorbide was summarized in TS2, with an
energy of -24858.809 eV (Path 1, Scheme 2), along with HOMO of -0.17711 eV and LUMO
of -0.05109 eV (c and d, Fig. 8), having difference of 0.12602 eV. TS1 had a lower energy
and a higher difference between HOMO and LUMO as compared to TS2, indicating a higher
stability of TS1 45, which supported the preferential formation of mono-dehydrated products
under the investigated reaction conditions.
Path 1: The 1,4-dehydration of D-Sorbitol
O
O
Y
Y
H
HO
HO
OH
OH
O
O
O
Y2O3
Y2O3
- H2O
O
HO
OH
H
HO
HO
1-
4-
HO
OH
OH
HO
1,4-Sorbitan
OH
OH
D-Sorbitol
HO
TS1
OH
H
H
O
O
- H2O
H
O
O
O
OH
Y
HO
H
O
O
O
Isosorbide
Y
TS2
Path 2: The 1,4-dehydration of D-Mannitol
Y
O
O
O
HO
Y
H
H
OH
OH
O
O
HO
Y2O3
Y2O3
- H2O
O
OH
HO
4-
1-
HO
OH
HO
OH
OH
OH
OH
OH
TS3
1,4-Mannitan
D-Mannitol
HO
O
OH
H
O
- H2O
H
O
O
O
OH
Y
H
HO
Isomannide
H
O
O
O
Y
TS4
Scheme 2 Proposed catalytic mechanism.
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Comparably, D-mannitol 1,4-dehydration may bring about two transition states
including TS3 and TS4 (Path 2, Scheme 2). The energy, HOMO and LUMO of TS3 were -
26938.766, -0.20329 and -0.06935 eV, respectively (TS3, e and f, Fig. 8), while difference
of HOMO and LUMO was 0.13394 eV. And meanwhile, those of TS4 were -24858.803, -
0.17378 and -0.05064 eV (TS4, g and h, Fig. 8), while difference of HOMO with LUMO
was 0.12314 eV. Obviously, TS3 was more stable than TS4, in good agreement with the
mono-dehydrated product observed as major compound during catalytic conversion of
D-mannitol.
Fig. 8 The calculated contour of HOMO and LUMO for transition states in the proposed
mechanism.
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Conclusions
A series of functionalized yttrium oxides were prepared through sol-gel in order to catalyze
mono-dehydrations of sugar alcohols. Characterizations revealed all synthetic samples have
micropipe-like morphologies, composed of metal oxide nanoparticles and organic
components. All samples showed promising mono-dehydration selectivity under UV
irradiation. Synthesized samples contained both Brønsted-type acidity and Lewis-type
acidity, whose combination seemed to strongly determine catalytic outputs under UV
irradiation. The proposed mechanism showed some insights into the origin of mono-
dehydration selectivity. This study put forward an interesting series of functionalized yttrium
oxides, which showed high selectivities for mono-dehydrations of sugar alcohols under UV
irradiation.
Acknowledgements
This work was supported by the Fundamental Research Funds for the Central Universities
(No. xjj2014005).
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