Direct synthesis of V-containing all-silica beta-zeolite for efficient one-pot, one-step conversion of carbohydrates into 2,5-diformylfuran

Article abstract of DOI:10.1039/c7cy01834g

Vanadium (V)-containing all-silica beta-zeolites (VSi-Beta) were directly synthesized using a dry-gel-conversion route, in which the gel was prepared from the acid-catalysed co-hydrolysis of a silica precursor and ammonium metavanadate. Besides the framework isolated V species, VSi-Beta contained tetrahedral [V₂O₇]^4- and (VO₃)_n^n- species that were bonded to the framework with a high dispersion. These V species resisted the over-oxidation of fructose and efficiently catalyzed the oxidation of 5-hydroxymethylfurfural (HMF) into 2,5-diformylfuran (DFF). As a result, VSi-Beta demonstrated high activity in the one-pot and one-step conversion of fructose into DFF using atmospheric oxygen (O₂) in the presence of sulfuric acid, affording a high yield of 86.3% and a maximum turnover number (TON) of 270. The catalyst can be easily recovered by filtration and exhibits good reusability. Various other carbohydrates (glucose, sucrose, inulin, raffinose, maltose and starch) were also effectively converted into DFF using VSi-Beta.

Full text of DOI:10.1039/c7cy01834g

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Qingzhu Zhang et al.
Catalytic mechanism of C–F bond cleavage: insights from QM/MM
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Direct synthesis of V-containing all silica Beta zeolite for efficient
one-pot and one-step conversion of carbohydrates into 2,5-
diformylfuran
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Wei Zhang, Wei Hou, Tongsuo Meng, Wenxia Zhuang, Jingyan Xie, Yu Zhou* and Jun Wang*
www.rsc.org/
Vanadium (V) containing all silica Beta zeolites (VSi-Beta) were directly synthesized in a dry-gel-conversion route, in which
the gel was prepared from acidic cohydrolysis of silica precursor and ammonium metavanadate. Besides the framework
4
-
n-
2 7 3 n
isolated V species, VSi-Beta contained tetrahedral [V O ] and (VO ) species that were bonded to the framework with a
high dispersion. These V species resisted the over-oxidation of fructose and efficiently catalyzed the oxidation of 5-
hydroxymethylfurfural (HMF) into 2,5-diformylfuran (DFF). As a result, VSi-Beta demonstrated high activity in the one-pot
2
and one-step conversion of fructose into DFF by using atmospheric oxygen (O ) in the presence of sulfuric acid, affording a
high yield of 86.3% and a maximum turnover number (TON) of 270. The catalyst can be facilely recovered by filtration and
reused with well reusability. Various other carbohydrates (glucose, sucrose, inulin, raffinose, maltose and starch) were also
effectively converted into DFF by using VSi-Beta.
numbers (TON) and turnover frequency (TOF). For example,
TON values were normally below 40 even by using noble metal
Introduction
-1 7-10
catalyst, with the TOF values below 3 or even 1 h
. Further,
Biomass, the only carbon-containing sustainable resources, is a
1
promising alternative to the non-renewable fossil resources
-3
stability and reusability are still to be improved for many of
these catalysts. It remains a challenge to develop low cost,
efficient and stable heterogeneous catalysis systems with high
TON/TOF for the direct synthesis of DFF from carbohydrates.
Zeolites are crystalline aluminosilicates that have been
widely industrially applied in adsorption, separation and
catalysis, particularly in the petrochemical and fine chemical
.
Carbohydrates are the major component of biomass and great
efforts have been made in the conversion of carbohydrates
into chemicals and fuels. Acid-catalytic dehydration of
carbohydrates offers a valuable versatile platform molecule 5-
hydroxymethylfurfural (HMF), which acts as a significant
bridge that links biomass to numerous high value-added
1
4,15
4
,5
industry
. They have large surface areas, well-defined
products . 2,5-Diformylfuran (DFF) is one of the most
important chemicals derived from HMF and has many
application prospects in the synthesis of pharmaceuticals,
antifungal agents, heterocyclic ligands and organic conductors
uniform micropores, and high thermal/hydrothermal stability.
More importantly, their chemical composition, topology and
morphology can be facilely adjusted in a wide range through
controlling the synthetic strategies and parameters. In this
context, heteroatom zeolites are able to deliver highly
dispersed active sites, with the advantage of fascinating
atomic efficiency. Various heteroatom zeolites were developed
from direct synthesis and post-treatment. They served as
efficient heterogeneous catalysts in many reactions also
including biomass conversion. For example, Sn-Beta
6
,7
.
Direct synthesis of DFF from carbohydrates is highly
attractive due to the easy availability of raw materials and the
simplification of the reaction path. Various catalytic systems
were fabricated by using either homogeneous or
heterogeneous catalysts. The one-pot conversion of
carbohydrates into DFF involves a tandem reaction, thus,
binary catalysts composed of an acid and an oxidative catalyst
1
6
7
-
demonstrated high activity in the isomerization of glucose
.
or catalysts with multifunctional active sites were employed
1
1
7
18
3
TS-1 and VS-1 were able to efficiently catalyze the gas-
phase epoxidation and liquid-phase oxidation. Mg-containing
zeolites such as MgS-1 were active in Knoevenagel
.
Normally, excessive catalyst dosage or long reaction time
was involved in most cases, suffering from low turnover
19
condensation
, a typical base reaction. It is greatly
encouraging but still remains one blank to explore the
application of heteroatom zeolites in direct synthesis of DFF
from carbohydrates.
The topology and Si/Al molar ratio of the framework (SAR)
are two crucial parameters for the application of zeolites.
Therefore, many studies focused on the synthesis and
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application of zeolites with target topology and tunable SAR. Rigaku ZSX Primus II) with Rh end-window tube and “SQX”
The all silica zeolites are a large class of silicates without Al in calculation software. Nitrogen sorption isotherms were
the framework, featuring high hydrophobicity, thermal measured by using a BELSORP-MAX analyzer with the samples
stability and coke resistance. Incorporation of transition metals pre-degassed at 573 K for 3 h. The pore-size distribution curves
into the framework of all silica zeolites provides many were calculated by Horvath-Kawazoe (HK) method. Fourier
promising heterogeneous catalysts. One of the widely studied transform infrared spectroscopy (FT-IR) spectra were recorded
all silica zeolites is Silicalite-1 (S-1). Besides, there are also on an Agilent Cary 660 instrument (KBr disks) in the 4000-400
-1
various other all silica zeolites with different topologies. Beta cm region. Solid UV-vis spectra were measured on a
zeolites with BEA topology and three-dimensional 12- Shimadzu UV 2600 spectrometer and BaSO was used as an
4
membered ring (0.56×0.67 nm) have been industrially applied internal standard. Raman spectra were obtained on a Horiba
in alkylation, hydroisomerization, dewaxing, hydration of HR 800 spectrometer (excitation line: 514 nm). Electron spin
olefines and so on. All silica Beta zeolites can be synthesized in resonance (EPR) spectra were recorded on a Bruker EMX-
hydrothermal route or prepared from dealumination. 10/12 spectrometer at the X-band at ambient temperature. X-
Transition metals such as V were also incorporated into the ray photoelectron spectra (XPS) were conducted on a PHI 5000
Beta zeolites to deliver active sites for the oxidative reactions. Versa Probe X-ray photoelectron spectrometer equipped with
2
9
51
Zeolites with V species were active in a variety of reactions Al Kα radiation (1486.6 eV). Si and V magic-angle spinning
2
such as the direct oxidation of benzene to phenol , oxidative (MAS) nuclear magnetic resonance (NMR) spectra were
0
2
1
dehydrogenation of alkanes , cyclohexane oxidation and analyzed on a Bruker Avance III spectrometer. Hydrogen
2
2
epoxidation of 1-octene in the liquid phase , oxidation of temperature-programmed reduction (H
2
-TPR) was performed
. The synthetic on a BELCAT-B instrument equipped with a Gow-Mac thermal
strategy significantly affects the surface state of V species and conductivity detector (TCD) and a glass flow system (10%
2
3-25
HMF to DFF and pyrolysis of biomass
3
-1
their catalytic activity. Though various V-containing Beta
2
2
H /Ar, 30 cm min ).
6-28
zeolites were reported
, rare V-containing all silica Beta
zeolite has been directly synthesized to date.
Catalyst preparation
Herein, we reported the synthesis of V-containing all silica
Beta zeolite (VSi-Beta) in a dry-gel-conversion route and
Synthesis of V-containing all silica Beta zeolites (VSi-Beta)
applied them as the efficient heterogeneous catalysts for the was carried out in a dry-gel-conversion route by using
straightforward synthesis of DFF from carbohydrates. Gel tetraethylammonium hydroxide (TEAOH, 35 wt% aqueous
preparation involved the cohydrolysis/condensation of silica solution, Jintan Huadong, AR) as the structure directing agent.
and V precursors under mild acid condition, followed by aging In a typical run, 10.71 g of tetraethoxysilane (TEOS, Sinopharm)
at base condition. This strategy enabled the formation of was mixed with 7.2 g of deionized water. Concentrated
highly dispersed V species in or bonded to the framework. The hydrochloric acid (HCl) was slowly dropped to reach pH=1.0,
obtained VSi-Beta effectively catalyzed the conversion of followed by the addition of ammonium metavanadate
fructose into DFF in a one-pot and one-step reaction under (NH
VO , Aladdin). The resulting mixture was stirred at 20 °C
3
4
ambient O
2
pressure (balloon), delivering high yield and for 20 h. Subsequently, the mixture of TEAOH and sodium
TON/TOF. Systematic characterizations and comparison with hydroxide (NaOH, 99 wt%, Sinopharm Chem. Reagent Co., AR)
Al-containing and supported analogues emphasized the was added, and then the concentrated HCl was slowly
important role of the special V species in the all silica BEA dropwise added. The obtained slurry was aged at room
framework. After reaction, the catalyst can be facilely temperature under dynamic (stirring) conditions for 24 h. The
recovered and stably reused. The scope was studied by using final gel molar composition was 1 SiO
: 0.27 (TEA)
HF: 9.4 H O. The pH value of the gel during the aging process
2
: 1/n V: 0.54
2
2
various other carbohydrates.
was regulated to be 12.3 by controlling the amount of NaOH.
After aging, the gel was dried at 80 °C in an oven and then
ground into the powder-state dry gel. Finally, 0.5 g of dry gel
was placed on a raised Teflon holder and sealed in a 50 mL
Teflon-lined stainless steel autoclave involving 0.5 g of
deionized water at the bottom for static crystallization at
Experiment
Materials and methods
All chemicals were analytical grade and employed as
-1
received. X-ray diffraction (XRD) patterns (5-50°, 0.2° s ) were
collected on a SmartLab diffractmeter (Rigaku Corporation)
equipped with a rotating anode Cu source (9 kW, 45 kV and
1
5
80 °C for 3 d. The template was removed by calcination at
50 °C for 5 h in air. The obtained zeolites were termed VSi-
Beta(n), in which n denotes the Si/V molar ratio in the gel (In
other words, n was the molar ratio of Si to V in the starting raw
materials). All silica Beta zeolite (Si-Beta) was synthesized with
the similar procedure to VSi-Beta in the absence of NH VO . V
1
00 mA). The morphologies were recorded on a field emission
scanning electron microscope (SEM) instrument of Hitachi S-
800. Energy dispersive X-ray spectrometry (EDS) analysis and
4
4
3
elemental mapping images were performed with an
acceleration voltage of 20 kV. Transmission electron
microscope (TEM) images were measured on a JEM-2100 F.
The metal contents were detected by X-ray Fluorescence (XRF,
and Al containing Beta zeolite (VAlSi-Beta) and Si-Beta
supported V O (V O @Si-Beta) were synthesized, with the
2
5
2
5
details in the Supporting Information.
2
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Catalytic test
ARTICLE
control the V content, offering two products VSi-Beta(100, 50)
with high crystallinity. Further increasing the V content in the
gel (e.g. n=33) will hinder the formation of BEA topology
structure (Fig. S7). The sample VSi-Beta-basic synthesized from
the cohydrolysis/condensation of silica and vanadium
precursors under the conventional base condition exhibited no
diffraction peaks of BEA phase in the XRD pattern (Fig. S8),
suggesting that the initial cohydrolysis/condensation step
plays an essential role.
Syntheses of DFF from HMF, HMF from fructose and DFF
from carbohydrates (fructose, glucose, sucrose, inulin,
maltose, raffinose and starch) were performed on a glass tube
by using target catalyst, substrates and gas atmosphere. In a
typical run for the conversion of typical carbohydrate
(fructose) into DFF, fructose (0.55 mmol, 0.1 g), concentrated
sulfuric acid (H SO , 0.09 g), catalyst (0.02 g) and dimethyl
2
4
sulfoxide (DMSO, 5 mL) were stirred in a glass tube, which was
vacuumized and equipped with an oxygen balloon. The
reaction proceeded at 130 °C for 4 h. After reaction,
isopropanol (0.05 g) was added as an internal standard. The
products were analysed by gas chromatography (Agilent GC
7890B) equipped with a hydrogen flame ionization detector
and a capillary column (HP-5, 30m×0.25mm×0.25 μm). The
conversion of fructose was monitored by liquid
1
1
chromatography-tandem mass spectrometry (LC-MS/MS) . A
five-run recycling test was used to assess the reusability. After
each run, the catalyst was separated by filtration, washed,
dried, calcined (550 °C for 5 h) and then charged into the next
run.
Results and discussion
Synthesis of VSi-Beta
Scheme 1 depicts the synthesis of VSi-Beta(
n). Various
synthesis factors including the silica precursors, H
2
O/SiO
2
molecular ratio, pH values in the acid hydrolysis and
crystallization step plus crystallization temperature and time
were investigated with n=100 (Figs. S1-S6). Silica precursors
significantly affect the formation and growth of the primary
particles. Water provides an essential medium to dissolve and
Scheme 1 Preparation of VSi-Beta.
2
9
diffuse aluminosilicate . Therefore, silica precursors and
water amount exhibited to be two important factors for the
synthesis. The employment of TEOS as the silica precursor
Fig. 1A depicts the XRD patterns of Si-Beta and VSi-Beta(n)
reached the successful synthesis, while other silica precursors series (n=100 and 50) synthesized under the optimal
of silicic acid and sodium silicate gave weak crystallinity or conditions (gel composition: 1 SiO : 0.27 (TEA) : 1/n V: 0.54
impurity (Fig. S1). Moderate H O/SiO
ratio of 9.4 was HF: 9.4 H O; acid hydrolysis pH: 1.0; aging pH: 12.3;
2
2
2
2
2
favorable for the formation of pure BEA phase. By contrast, crystallization: 180 °C, 3 d). All of them display a set of
low water amount failed to generate Beta zeolite while high diffraction peaks assignable to BEA topology without other
water amount led to slight impurity of NaF (Fig. S2). The impurities, index of well crystallized zeolite structure. The unit
investigation of the pH values in the initial acid hydrolysis step cell parameters of VSi-Beta(n) series were calculated from the
indicated that (Fig. S3) pure BEA phase formed in the range of XRD patterns (Table 1). The larger unit cell volumes of VSi-Beta
0
.6 to 1.0, while lower (0.4) or higher (1.2) pH value caused the than that of Si-Beta imply the incorporation of the V species
amorphous products. The favorable pH value in the into the zeolite framework, because the ionic radius of V
5
+
4
+
30
crystallization process was 12.3 by exploring from 12.1 to 12.8 (0.054 nm) is larger than that of Si (0.04 nm)
Fig. S4). The impurity phase of quartz was observed at the low
sorption experiments were performed to provide
pH value of 12.1. Crystallization at the temperature from 160 porosity information (Fig. 1B). The corresponding textural
to 180 °C afforded the pure BEA phase. The low temperature properties are listed in Table 1. N sorption isotherms of these
.
(
N
2
2
of 140 °C caused an impurity phase of NaF while the high zeolites were type I+IV with a sharp uptake at low relative
temperature of 200 °C led to weak crystallinity (Fig. S5). The pressure and a slowly increasing one at higher relative
influence of crystallization time was studied by fixing the pressure. This phenomenon suggests that they are typical
crystallization temperature at 180 °C (Fig. S6). Partially faint micro/mesoporous materials possessing mainly zeolite
diffraction peaks corresponding to BEA phase appeared at 1 d microchannels and certain mesopores derived from the
and high crystallized BEA zeolite was obtained at 3 d. The aggregations of primary particles. As shown in Table 1, all the
concentration of V precursor in the initial gel was adjusted to
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samples presented the large surface areas and pore volumes, index of abundant open pores. The analysis of the chemical
Table 1 Textural properties
[b]
[
a]
2
-1
Pore volume
Unite cell parameters
Sample
V (wt%)
Surface area (m · g )
3
−1
3
(cm · g )
a (Å)
b (Å)
c (Å)
V (Å )
Si-Beta
VSi-Beta(100)
VSi-Beta(50)
0
0.71
1.48
618
620
516
0.34
0.30
0.23
12.4559
12.4612
12.4675
12.4559
12.4612
12.4675
26.3317
26.3323
26.3333
4085.35
4088.92
4093.21
[
a] V content. [b] Unit cell parameters are determined by using the MDI Jade software (Jade 7 XRD Pattern processing Software).
State of V species
The state of V species in these VSi-Beta zeolites was
systematically characterized. For comparison, four control
samples of VAlSi-Beta (V and Al containing Beta zeolite),
V
2
O
5
@Si-Beta (Si-Beta supported V
2 5 2 5 4 3
O ), V O and NH VO
were parallel measured (Figs. S9-S16 and Table S1).
Fig. 3A shows the UV-vis spectra. VSi-Beta(100) exhibited
two bands at 263 and 320 nm, attributable to the
5
+
31,32
mononuclear and polymerized tetrahedral V species
Fig. 1. (A) XRD patterns and (B) Nitrogen sorption isotherms respectively. The red shift of the band from 320 to 326 nm
,
(
the inset is the pore size distributions calculated by the HK with enhanced peak intensity for VSi-Beta(50) indicates that
5
+
32
method) of VSi-Beta(
n
) series. Isotherms were collected at these tetrahedral V species become more polymerized
.
−
7
high-accuracy mode (relative pressure p/p
0
of 10 Pa).
The band at 331 nm in VAlSi-Beta(100) and V
2
O
5
@Si-Beta is
tentatively assigned to the extra-framework V oligomer with
compositions (Table 1) indicated that the V content increased the octahedral structure (Fig. S11). The band at 417 nm for
5
+
with the initial V concentration in the gel, with slightly smaller
value of the former relative to the latter.
SEM images (Fig. 2) shown that these V-containing zeolites The band at 960 cm is assigned to a stretching vibration
2 5
V O @Si-Beta is attributable to the extra-framework
2
polymerized V species . Fig. 3B illustrates the FTIR spectra.
8
-1
were composed of identical squarish particles with the size of mode of a [SiO
4
] unit bonded to a V atom (Si–O–V), further
3
3,34
several micrometers, similar to Si-Beta. Magnified SEM image revealing the existence of framework V species
. The band
-1
35
demonstrated that these particles were composed of small at 1023 cm is assigned to the vibration of V=O . These bands
2 5
slice-like primary particles. The elemental mappings of the are absent in Si-Beta and V O @Si-Beta (Fig. S12), due to the
typical sample VSi-Beta(100) confirmed the relatively uniform absence of framework V species. EPR spectra were performed
dispersion of V species (Figs. 2D-F). TEM image and selected
area electron diffraction pattern of VSi-Beta(100) (Figs. 2H and
I) further revealed the well crystallized zeolite structure.
A
G
B
C
E
I
D
F
H
Fig. 2. SEM images of (A) Si-Beta, (B, C, D) VSi-Beta(100) with
EDS elemental mapping images of (E) Si and (F) V element, (G)
VSi-Beta(50). TEM images of (H, I) VSi-Beta(100).
2
Fig. 3. (A) UV-vis, (B) IR, (C) EPR spectra and (D) H -TPR curves.
4
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[a]
Table 2 Direct synthesis of DFF from fructose in a one-pot and one-step reaction
Fructose
amount (g)
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
Catalyst
dosage (g)
-
0.02
0.01
0.02
-
0.02
0.02
0.02
0.02
0.02
0.02
0.02
Con.
(%)
16
YHMF
YDFF
(%)
[e]
TOF
(h )
-
40.5
67.5
20.5
-
27.5
9.0
-
Entry
Catalyst
[b]
[c]
[d]
TON
-1 [f]
(%)
0
0/0
0
g
1
-
1.8
-
h
h
2
3
4
5
6
VSi-Beta(100)
VSi-Beta(100)
VSi-Beta(50)
-
VSi-Beta(100)
VSi-Beta(100)
Si-Beta
100
100
100
100
100
100
100
100
100
93
82/83.2
68.3
86.3
15.1
55.6
18.3
16.8
29.8
63.6
60.2
48.3
162
270
82
-
110
36
-
57
131
2.0
1.2
0
i
1.5
0.2
0
0
0
2.6
5.1
7.8
j
k
7
8
9
VAlSi-Beta
V O @Si-Beta
14.3
32.8
0.5
0.3
1
1
1
0
1
2
2
5
NH
4
VO
3
V
2
O
5
0.1
79
[
2 4 2
a] Reaction conditions: 0.09 g H SO , 5 mL DMSO, 130 °C, 4 h, O balloon. [b] Conversion of fructose. [c] Yield of HMF. [d] Yield of DFF. [e]
Turnover number (TON): mole of DFF produced over per molar V; TON=(mole of produced DFF)/(mole of V in the catalyst). [f] Turnover
frequency (TOF): mole of DFF produced over per molar V per hour; TOF=(mole of produced DFF)/[(mole of V in the catalyst)×(the whole
reaction time)]. [g] In the absence of H
balloon, 1 bar), and the second step is the further addition of VSi-Beta(100) to oxidize HMF into DFF (reaction time: 3 h; O
bar). [i] In the presence of H SO . [j] Air balloon. [k] N balloon.
2
SO
4
. [h] Two-step reaction: the first step is the conversion of fructose into HMF (reaction time: 1 h;
N
2
2
balloon, 1
2
4
2
(Figs. 3C and S13). Anisotropic hyperfine structure was
became stronger over VSi-Beta(50) and V O @Si-Beta,
2 5
observed on the EPR spectrum of VSi-Beta(100), indicating
+
suggesting enhanced polymerization of these oligomeric V
species. These observations of Raman, together with the UV-
vis and EPR spectra, imply that the V species are highly
4
that there exist isolated V species (Fig. 3C). EPR signals
became stronger over VSi-Beta(50) and VAlSi-Beta, reflecting
4
more V species existed than VSi-Beta(100, 50) (Fig. 3C and
+
dispersed over VSi-Beta(n).
S13). H -TPR curves are presented in Figs. 3D and S14. VSi-
2
Beta(100) displayed two reduction peaks at 608 and 683 °C,
attributable to the existence of highly dispersed tetrahedral
5
+
36
species . These reduction temperatures are close to the
V
one of NH VO , but lower than the ones of V O . Owing to
4
3
2 5
the existence of extra-framework V species, additional
reduction peaks at relatively low temperatures were
observed in H -TPR curves of VSi-Beta(50), VAlSi-Beta and
2
3
6,37
V
2
O
5
@Si-Beta
Raman spectra are illustrated in Figs. 4A and S15. These
Beta zeolites exhibited typical bands corresponding to the
.
3
8
-1
silicalites's framework . The peaks at 400 and 460 cm in
the spectra of VSi-Beta(100 and 50) are broader and weaker
than the ones of Si-Beta, reflecting the distortion of the
harmonic vibration in the BEA framework due to the
3
9
incorporation of V species . Apparent signals corresponding
to the V species were observed over these V-containing Beta
-
1
zeolites. The band at 780 cm for VSi-Beta(100 and 50) and
VAlSi-Beta comes from the framework V-O-Si, as this band
2
Fig. 4. (A) Raman, (B) V 2p XPS, (C) Si and (D) V MAS NMR
spectra.
9
51
-
1
was absent for V
2 5
O @Si-Beta. Bands at 1030 and 1060 cm
for VSi-Beta(100) are assigned to the V=O symmetric
4
0
stretching mode of the isolated V species in the framework
These bands were ambiguous over VSi-Beta(50), VAlSi-Beta
and V @Si-Beta, indicating that their isolated V species
were in low content. Besides, VSi-Beta(100) also
.
XPS analyses were performed over VSi-Beta(100 and 50),
VAlSi-Beta and V
2 5
O @Si-Beta (Figs. 4B and S16). All of them
2 5
O
presented a strong peak at high (517.3 eV) binding energy for
5
+
the V species and a weak one at low binding energy (516.2
-1
4+
43
demonstrated the bands at 953, 919, 637 and 508 cm
corresponding to the metavanadate chains, suggesting the
eV) for the V species . The proportion of these two kind
surface V species for different samples was calculated from
the normalization of the peak area of the V 2p3/2 signals
n
-
41,42
existence of tetrahedral (VO
3
)
n
species
. These bands
4
+
were observable over other V-containing Beta zeolites and
(Table S2). Content of V species for these samples is in the
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ARTICLE
Journal Name
order of VSi-Beta(100)<VSi-Beta(50)<V
2
O
5
@Si-Beta<VAlSi-
For comparison, Si-Beta gave the full conversion of
fructose but the DFF yield of 16.8% in the presence of H SO
4
Beta. This phenomenon suggests that low V content of VSi-
2
5
Beta benefits high proportion of V species, while post-
+
(entry 8), suggesting that the V species are the major active
centers in the oxidation step. VAlSi-Beta offered a low DFF
yield of 29.8% (entry 9), implying that the Al enriched zeolite
framework is harmful for this reaction. The DFF yield was
4
+
impregnation method causes more V species. VSi-Beta(100)
was further characterized by MAS NMR spectra (Fig. 4C and
2
D). Si MAS NMR spectrum presented three strong peaks at
9
4
4
(Q )
-
111, -112 and -115 ppm attributable to the Si(OSi)
4
4
2 5
63.6% for the V O @Si-Beta catalyzed conversion of fructose
4 51
.
structure, further revealing the high crystallinity
V MAS
(entry 10), suggesting that the framework V species are more
active than the extra-framework ones. This is further
NMR spectrum (Fig. 4D) exhibited a strong signal at -574 ppm
n-
45,46
that can be assigned to the (VO
peak at -563 ppm comes from the [V
weak peak was observed at -625 ppm, in line with the
3
)
n
species
. The shoulder
4 3 2 5
reflected by the facts that NH VO and V O exhibited inferior
4
]
-
46,47
2
O
7
species
. A
activity than the supported V species even by using the same
catalyst dosage, affording much low TON (entry 11 and 12).
5
+
n-
presence of isolated tetrahedral V species in the framework
4
NH
species displayed higher activity than V
these V species are more active than the octahedral ones of
. All of these indicate that the highly dispersed
4 3 3 n
VO containing highly polymerized tetrahedral (VO )
8
.
These signals are in agreement with results of UV-vis and
2 5
O , suggesting that
Raman spectra.
2 5
V O
Direct synthesis of DFF from fructose
tetrahedral V species in the all silica BEA network benefit the
high activity.
Table 2 summarizes the catalytic performance of VSi-
Beta(n) and various controls in the one-pot and one-step
conversion of fructose into DFF by using the atmospheric O
2
Insight into the reaction
as the oxidant. The conversion of fructose was only 16.0% in
the absence of catalyst, giving the DFF yield of 1.8% (entry 1).
VSi-Beta(100) gave the DFF yield of 82.0% with a high TON of
In order to gain insightful understanding, the kinetic curve
of VSi-Beta(100) in the one-pot and one-step conversion of
fructose into DFF was analyzed (Fig. 5A). Fructose was rapidly
converted into HMF within a short reaction time, with the
slow formation of DFF in the initial stage. The highest HMF
yield of 90.2% was obtained at 1 h, with the DFF yield of 4.8%.
After that, HMF was continuously converted into DFF with
relatively high reaction rate and afforded the DFF yield of 82%
at 4 h. These observations indicate that the direct synthesis
of DFF from fructose catalyzed by VSi-Beta(100) is a tandem
reaction involving the formation of HMF and successive
oxidation to DFF.
-1
2 4
162 (TOF: 40.5 h ) in the presence of H SO (entry 2). Activity
of VSi-Beta(100) under different reaction conditions was
systematically investigated by exploring the influence of
H
2
4
SO amount, fructose amount, catalyst amount, DMSO
amount, reaction temperature and solvents (Figs. S17-S22).
-1
The maximum TON of 270 (TOF: 67.5 h ) was obtained with
DFF yield of 68.3% (entry 3) by using low catalyst dosage (Fig.
S19). VSi-Beta(50) afforded a higher DFF yield of 86.3% with
-1
2 4
the TON of 82 and TOF of 20.5 h (entry 4). H SO itself can
catalyze the conversion of fructose into HMF, but gave a low
Activity of VSi-Beta(100) and V
respectively assessed in each step (Table S4 and S5) to
understand the role of different V species. H SO itself
efficiently converted fructose into HMF with high yield of
94.0% (entry 1, N balloon). Even under O atmosphere,
SO afforded the HMF yield of 91.0% (entry 2). In the
presence of H SO , VSi-Beta(100) gave the similar HMF yield
of 90.2% (entry 3, O balloon), which is higher than the one
by using V @Si-Beta (82.2%, entry 4). The DFF yield over
VSi-Beta(100) was 4.8%, delivering a high carbon balance of
95.0%. By contrast, the DFF yield over V @Si-Beta was only
2.3%, with a low carbon balance of 84.5% due to the over-
were superior or at least comparable to previous ones (39-84 % oxidation of fructose to form undetectable by-products like
2 5
O @Si-Beta was also
DFF yield of 15.1% (entry 5). This result reveals that an
efficient catalytic system is fabricated by using VSi-Beta(
n
)
2
4
and H SO . This system was even active under atmospheric
2
4
air condition, with a moderate DFF yield of 55.6% catalyzed
by VSi-Beta(100) (entry 6). It is the first efficient heteroatom
zeolite that was applied in the conversion of fructose into DFF.
This catalytic system has the feature of high atom efficiency
with remarkable TON and TOF. In previous works, TON values
were normally below 10 for the transition metal catalysts and
2
2
H
2
4
2
4
2
2 5
O
7
-13
below 40 for the noble metal Ru catalysts (Table S3)
.
2 5
O
Yields over VSi-Beta( ) catalyzed system for this reaction
n
4
9
for the catalytic systems including both the homogeneous
and heterogeneous catalysts in the one-pot and one-step
reaction). The maximum TON of 270 is greatly superior to
humins . This result indicates that VSi-Beta resisted the
over-oxidation of fructose in the presence of O to offer a
2
high HMF yield that benefits the formation of DFF in the
sequential oxidative step. Table S5 compares the activity of
(
more than 20 times) all the previous non-noble metal
catalytic systems in either one-step or two-step reactions
Table S3) and even much exceeds the best one (TON: 36.6)
VSi-Beta and V
(O balloon). The auto-oxidation of HMF bestowed the DFF
yield of 29.1% in the presence of H SO (entry 2). The DFF
yield by using VSi-Beta(100) was 92.9% (entry 3), higher than
the one over V @Si-Beta (80.3%, entry 4). The higher
activity of VSi-Beta(100) than V @Si-Beta in the oxidation
2 5
O @Si-Beta in the oxidation of HMF to DFF
(
2
of previous noble metal Ru catalyzed one-pot and two-step
9
conversion of fructose under high oxygen pressure . The
2
4
results allow to draw that the VSi-Beta(
n
) catalyzed
2 5
O
conversion of fructose into DFF endows high yield and
TON/TOF under mild conditions.
2 5
O
step also responds to the high activity of VSi-Beta(100) in the
one-pot and one-step conversion of fructose into DFF. Briefly,
6
| J. Name., 2012, 00, 1-3
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Cat Pa ll ey as si es dS oc ni eo nt ca ed j &u s Tt me ca hr gn i on ls ogy
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DOI: 10.1039/C7CY01834G
Journal Name
ARTICLE
V species of VSi-Beta promote the activity by resisting the
over-oxidation of fructose and accelerating the oxidation of
HMF to DFF.
under N2 balloon (1 bar), VSi-Beta(100) was added under
O2 balloon for 4 h (entries 1-5) or 7 h (entry 6).
Furthermore, the recovered VSi-Beta(100) without
calcination, VSi-Beta(100)-I, was characterized (Figs. S23-29).
Similar XRD pattern and UV-vis spectrum to the fresh one was
observed over this sample, suggesting that the crystal
structure remained well during the reaction. EPR spectrum
tetrahedral V species during the reaction. XRD pattern and
th
TEM image of the recovered sample after 5 run (VSi-
Beta(100)-II) indicated the well-preserved crystal structure
(Figs. S23 and S24). The V content of VSi-Beta(100)-II (the
th
recovered sample after 5 run) was 0.69 wt%, very close to
4
+
(
Fig. S28) exhibited the strong signal for V species,
the one of the fresh catalyst (0.71 wt%). This suggests that V
leaching of VSi-Beta(100) in this reaction was negligible. Two
factors contribute to such stability of VSi-Beta(100) in the
acid media. On the one hand, the all silica framework resists
the collapse of the skeleton. On the other hand, the
5
indicating that V species are the major active centers in the
+
4
oxidation step and part of them changed to be V species
+
-1
-1
after reaction. Disappeared bands at 920 cm and 956 cm in
the Raman spectrum (Fig. S29) suggests that the lattice
5
+
5+
oxygen of V =O species attend the reaction, which follows
the Mars-van Krevelen mechanism by referencing previous
framework related tetrahedral V species is stable in the acid
5
3
condition . In a word, the framework V species in the all
silica skeleton of VSi-Beta(100) provide sufficient resistance
to corrosion under the acid environment for good stability. It
should be pointed out that the acid concentration was only
50-52
studies
. The mechanism is further reflected by the fact
that a certain amount of DFF (18.3%, Table 2, entry 7) was
obtained over VSi-Beta(100) under N atmosphere, indicating
2
-1
that the lattice oxygen species start the reaction.
2 4
0.18 mmol mL and the ionization of H SO was also inhibited
+
in organic solution (DMSO), enabling a low H concentration.
Therefore, VSi-Beta(100) was stable in the reaction,
endowing good reusability.
The scope of VSi-Beta(100) was investigated in the
conversion of various other carbohydrates including glucose,
sucrose, inulin, raffinose, maltose and starch (Table 3). The
results indicated that VSi-Beta(100) was effective in the one-
pot and one-step reaction, affording moderate yields even for
those more difficult substrates. Still high TON values (7.6-41.6)
were obtained, revealing the well substrate compatibility and
high activity of this catalyst. In a two-step reaction, increased
yields were obtained. All of these reveal the good substrate
compatibility and high activity of VSi-Beta(100).
Fig. 5. (A) Kinetic curve and (B) reusability of VSi-Beta(100) in
the one-pot and one-step conversion of fructose into DFF.
Reaction conditions: 0.1 g fructose, 0.02 g catalyst, 0.09 g
H SO , 5 mL DMSO, 130 °C, O balloon.
2
4
2
Conclusions
V-containing all silica Beta (VSi-Beta) zeolites were directly
Reusability and scope
After reaction, VSi-Beta(100) can be facilely separated by
filtration. Reusability of VSi-Beta(100) was assessed by a five-
run recycling test (Fig. 5B). No apparent decline of yield was
observed, indicating stable activity during the reuse process.
The well reusability can be attributed to the preservation of
the zeolite framework and stable framework-related
synthesized in
demonstrated high activity in the one-pot and one-step
synthesis of DFF from carbohydrates with atmospheric O (1
a
dry-gel-conversion process and
2
bar) in the presence of sulfuric acid. The excellent
performance results from the highly dispersed tetrahedral V
species in or bonded to the framework of all silica BEA zeolite,
which resisted the over-oxidation of fructose and served as
effective active sites in the oxidative step. This work opens a
new possibility of transition metal contained zeolites for the
application in the synthesis of high value-added chemicals
from carbohydrates.
Table 3 VSi-Beta(100) catalyzed synthesis of DFF in a one-pot
[a]
and one-step reaction
TOF
-1
(h )
DMSO Time YHMF
Y
DFF
[e]
(%)
[f]
Entry Substrate
[b]
mL) (h)
[c]
[d]
TON
(
(%)
[g]
h
h
38.2/50.1 15.1
1
2
3
4
5
6
glucose
sucrose
inulin
3
6
6
6
6
6
9
0/0
2.1/1.3
2.5
6.9
2.6
4.7
1.3
1.2
h
h
h
32.1/39.5 41.6
3
5
5
5
5
h
h
39.3/51.6 15.5
Conflicts of interest
0/0
0/0.8
h
h
20.1/27.6 28.4
raffinose
maltose
starch
There are no conflicts to declare.
h
18.1/23.4 7.6
1.9/0.2
h
1.6/0
h
28.2/37.9 11.1
[
0
a] Reaction conditions: 0.1 g carbohydrate, 0.1 g catalyst,
Acknowledgements
.15 g H SO , 130 °C, O balloon. [b] Solvent amount. [c]
2
4
2
Reaction time. [d] Yield of HMF. [e] Yield of DFF. [f]
Turnover number (TON): mole of DFF produced over per
molar V. [g] Turnover frequency (TOF): mole of produced
DFF per molar V per hour. [h] Two-step reaction: after 2 h
This work was supported by the National Natural Science
Foundation of China (Nos. U1662107, 21476109, 21303084
and 21136005), the Jiangsu Provincial Science Foundation for
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Journal Name
Youths (Nos. BK20130921), the Specialized Research Fund for
the Doctoral Program of Higher Education (No.
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Table of contents
V-containing all silica Beta zeolite exhibited high atom-efficient in the direct synthesis of
,5-diformylfuran from carbohydrates.
2