Fe-Zr-O catalyzed base-free aerobic oxidation of 5-HMF to 2,5-FDCA as a bio-based polyester monomer

Article abstract of DOI:10.1039/c7cy01704a

An environment-friendly and economical route for 5-hydroxymethylfurfural (HMF) aerobic oxidation to 2,5-furandicarboxylic acid (FDCA) in an ionic liquid (IL)-promoted base-free reaction system was reported using Fe-Zr-O as a catalyst. A series of Fe_xZr_1-xO₂ catalysts were synthesized by a hydrothermal method and the catalytic performance was investigated. Among these catalysts, Fe_0.6Zr_0.4O₂ exhibited excellent catalytic activity in the HMF oxidation. A 60.6% FDCA yield and 99.9% HMF conversion could be obtained after 24 h under 2 MPa O₂ pressure and base-free conditions. The good performance could be attributed to the large amount of acidic and basic sites on the surface of the catalyst and its high reducibility and oxygen mobility. In addition, the formation of humins and the reaction pathways in the ILs were also investigated, which revealed parallel reactions between FDCA and humin formation. A plausible reaction mechanism was proposed based on the results of a series of designed experiments. Finally, the catalyst was used five times without obvious loss of activity. To the best of our knowledge, this is the best result for a non-noble metal catalyzed base-free oxidation of HMF to FDCA using molecular oxygen as an oxidant.

Full text of DOI:10.1039/c7cy01704a

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ISSN 2044-4753
PAPER
Qingzhu Zhang et al.
Catalytic mechanism of C–F bond cleavage: insights from QM/MM
analysis of fluoroacetate dehalogenase
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PAPER
Fe-Zr-O catalyzed base-free aerobic oxidation of 5-HMF to 2,5-FDCA
as a bio-based polyester monomer
Received 00th January 20xx,
Accepted 00th January 20xx
a,b,c
b
b
b,c
b,c
a
Dongxia Yan , Jiayu Xin , Qiu Zhao , Kai Gao , Xingmei Lu , Gongying Wang* , and Suojiang
Zhang*
b
DOI: 10.1039/x0xx00000x
www.rsc.org/
An environment-friendly and economical route for 5-hydroxymethylfurfural (HMF) aerobic oxidation to 2,5-
furandicarboxylic acid (FDCA) in ionic liquid (IL)-promoted base-free reaction system was reported using Fe-Zr-O as catalyst.
A series of Fe
Among these catalysts, Fe0.6Zr0.4
HMF conversion could be obtained after 24 h under 2 MPa O
x
Zr1-x
O
2
catalysts were synthesized by a hydrothermal method and the catalytic performance was investigated.
exhibited excellent catalytic activity in the HMF oxidation. A 60.6% FDCA yield and 99.9%
and base-free condition. The good performance could be
O
2
2
attributed to the large amount of acidic and basic sites on the surface of the catalyst and its high reducibility and oxygen
mobility. In addition, the formation of humins and reaction pathways in the ILs were also investigated, which revealed
parallel reactions between FDCA and humins formation. The plausible reaction mechanism was proposed based on the
results of a series of designed experiments. At last, the catalyst was used five times without obvious loss of activity. To the
best of our knowledge, this is the best result for non-noble metal catalyzed base-free oxidation of HMF to FDCA using
molecular oxygen as oxidant.
4
, 5
environment and resources. Bio-based 2,5-furandicarboxylic acid
(FDCA), contains two carboxyl groups at the para position of the
furan ring, which is very similar to that of fossil based TPA and can
be a green substitute to produce plastics. Meanwhile, as an
analogue of PET, a FDCA-based polyethylene furandicarboxylate
Introduction
Due to the desirable properties such as durability, low production
cost, optical clarity and barrier properties, polyethylene
terephthalate (PET) has been the dominant manmade polymer with
applications ranging from beverage bottles, packaging materials,
medical devices to textile industry for the past four decades, of
which about 56 million tons was produced worldwide in 2013
(
PEF) has been well prepared and investigated, which is a
degradable bio-based polymer with superior properties to PET, such
as improved thermal stability and gas barrier properties while low-
2
, 5
1
,
2
temperature formability and reduced chain mobility. Therefore,
alone.
However, the presence of a high ratio of aromatic
as an important monomer for PEF production, a 100% bio-based
components in its structure and the corresponding limited mobility
of the polymer chains made the PET resistant to microbial attack
and non-degradable under normal environmental conditions, which
resulting in the accumulation of PET in environment and thus
6
-9
FDCA, has attracted much attention recently.
Generally, FDCA could be prepared by oxidation of 5-
hydroxymethylfurfural (HMF), and strong stoichiometric oxidation
2
−
2−
1, 3
reagents, such as chromate (CrO
4
), dichromate (Cr
2 7
O ) and
creating environmental hazards. More importantly, the monomer
of PET, terephthalic acid (TPA) is obtained from fossil resources,
which is non-renewable. Therefore, much research was conducted
to find bio-based alternatives to PET to reduce the pressure on the
−
permanganate (MnO
4
) salts, were often employed to accomplish
1
0
the reaction. However, these oxidants are high cost and
environment-unfriendly. Thus, conversion of HMF to FDCA with
green molecular oxygen is more attractive. Recently, the aerobic
oxidation of HMF to FDCA has been accomplished using a
homogeneous catalysts (e.g. Co/Mn/Zr/Br) combined with acetic
a.
Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu,
6
Beijing Key Laboratory of Ionic Liquids Clean Process, Key Laboratory of Green
Process Engineering, State Key Laboratory of Multiphase Complex Systems,
Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, P.
R. China
11
acid
as
solvent,
or
a
homogeneous
catalysts
10041, P. R. China
b.
(Co(OAc)
2
12
/Zn(OAc) /NaBr) combined with trifluoroacetic acid as an
2
additive, which is difficult to reuse the homogeneous catalysts.
Therefore, the use of molecular oxygen and heterogeneous catalyst
is quite advantageous for HMF oxidation. Thus, a large number of
reports focused on supported heterogeneous noble metal catalysts,
c.
University of Chinese Academy of Sciences, Beijing 100049, P. R. China.
Tel./Fax: +86-10-8262-7080; E-mail address: gywang@cioc.ac.cn (Gongying Wang),
sjzhang@ipe.ac.cn (Suojiang. Zhang)
13
14
15
16
7
†
Electronic Supplementary Information (ESI) available: [details of any
such as Au, Pt, Pd, Ru and their alloys. High yield of FDCA
can be obtained with the combined use of additional base.
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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However, the product obtained in these catalytic systems is in the Factory. [Bmim]Cl was commercially available (purchased from
form of FDCA salt, which cannot be used directly for polymer Lanzhou Institute of Chemical Physics, CAS) and dried under
production and need to be separated by adding a large amount of vacuum at 60 °C for 24 h before use. The commercial chemicals
strong mineral acids, such as HCl or H SO Therefore, the excessive were of analytical grade and therefore were used as received.
2
4.
use of base and high cost of catalysts resulted in the less efficient Catalyst preparation
and sustainable process. series of Fe
A
x
Zr1-x
O
2
mixed oxides were prepared by a
In order to reduce the cost of the catalyst, the heterogeneous non- hydrothermal method, where x is the atomic ratio of Fe/(Fe + Zr)
noble metal catalyzed oxidation of HMF was investigated recently. and varied from 0-1 (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0). The required
1
7, 18
Zhang et al.
be obtained over the nano-Fe
supported Co(II)-meso-tetra(4-pyridyl)-porphyrin catalyst using t- total concentration of Fe and Zr cation was 0.25 mol/L. The solution
BuOOH as oxidant. Recently, another group used Mn Fe mixed of Fe(NO ·9H O, Zr(NO ·5H O and their mixed solution were
oxide as a catalyst for the production of FDCA, while the yield was gradually dripped in an ammonia solution (5 mol/L) while
reported that moderate to high FDCA yields could amounts of Fe(NO
3 3 2 3 4 2
) ·9H O and Zr(NO ) ·5H O were dissolved in
3
O
4
-CoO catalyst or Merrifield resin- deionized water and well mixed for 2h at room temperature. The
x
x
y
3
)
3
2
3
)
4
2
1
9
very low by direct conversion of HMF. MnO
MnO -CeO were also used for the oxidation of HMF, and showed mixture was vigorously stirred for 2 h and settled for 2 h.
excellent catalytic performance in the presence of high Thereafter, 65 ml of slurry was transferred into 100 ml Teflon-lined
2
and its mixed oxides maintaining the pH at about 10 with stirring. Then, the reaction
x
2
2
0, 21
concentration of NaHCO or KHCO .
Generally, even though the stainless steel autoclaves. The autoclaves were tightly sealed and
3
3
noble metal catalysts showed excellent performance for FDCA placed in an oven for hydrothermal treatment at 220 °C for 48 h.
production, their high costs and excessive use of base imposed After cooling, the samples were washed with distilled water and
restriction for the further use. The non-noble metal can reduce the ethanol until a pH of 7 before air-drying at 110 ◦C for 12 h, thus, the
x 2
cost of catalyst, however, the catalytic systems often employ high Fe Zr1-xO mixed oxides were obtained.
cost and environment-unfriendly t-BuOOH as the oxidant as well as Catalyst characterization
7
the use of excessive base. Thus, it is believed that the economic, The prepared catalysts were systematically investigated using
environment-friendly and simple direct conversion of HMF to FDCA different characterization techniques like X-ray diffraction (XRD),
over non-noble metal catalysts in base free condition is highly Raman, X-ray photoelectron spectroscopy (XPS), hydrogen
desired.
Herein, with the purpose of developing an efficient and economic temperature-programmed desorption (TPD) of ammonia and
catalytic system for the green conversion of HMF, a series of Fe Zr1- carbon dioxide (NH and CO -TPD), transmission electron
catalysts were synthesized and employed as heterogeneous microscope (TEM), scanning electron microscopy (SEM), inductively
temperature-programmed
reduction
(H -TPR)
2
analysis,
x
3
2
x 2
O
catalysts for the production of FDCA using 1-butyl-3- coupled plasma optical emission spectroscopy (ICP-OES) and
methylimidazolium chloride ([Bmim]Cl) ionic liquids (ILs) solvent specific surface area analysis (BET). The details for the catalyst
and molecular oxygen under base free conditions. The catalytic characterization can be found in Electronic Supplementary
activity and stability towards the oxidation of HMF were examined, Informaꢀon (ESI†).
which showed good stability and obtained the best result for the Humins characterization
non-noble metal catalyzed base-free aerobic oxidation of HMF to The humins formed during the aerobic oxidation of HMF were
FDCA to the best of our knowledge. Furthermore, a series of analyzed by Fourier transform infrared spectroscopy (FT-IR),
characterization methods were used to reveal the factors affecting scanning electron microscopy (SEM), thermogravimetric (TG)
the activity of catalyst. Additionally, the formation of humins and analysis and gel permeation chromatography (GPC). The details for
reaction pathways in the [Bmim]Cl ILs were also investigated, and the humins characterization can be found in ESI†.
the plausible mechanism was proposed based on the results of a Catalytic reaction and product analysis
series of designed experiments.
In a typical experiment, 1 g of [Bmim]Cl ILs was added to a 50 ml
batch-type Teflon-lined stainless-steel autoclave along with 0.1
mmol HMF and 0.01 g powdery catalyst. The autoclave was purged
Experimental
3
times with O2 and then pressurized to 2 MPa at room
Materials
temperature. Once the system reached the desired temperature
HMF (98%), 5-hydroxymethyl-2-furancarboxylic acid (HMFA) (95%), (160 °C), the reaction was started by vigorous stirring with a
,5-diformylfuran (DFF) (98%), 5-formyl-2-furancarboxylic acid magnetic stirrer for 24 h. Then, reaction mixtures were diluted with
FFCA) (98%) and FDCA (97%) were purchased from the J&K deionized H O and filtered using PTFE 0.2 μm filters, subsequently
O was supplied by analyzed using a Shimazu LC-20A HPLC instrument equipped with
2
(
2
3 4 2
Chemical Co. Ltd. (Beijing, China). Zr(NO ) ·5H
Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). refractive index (RI) and photo-diode array (PDA) detectors. A Bio-
Fe(NO ·9H O was purchased from Xilong Chemical Co., Ltd. Rad Aminex HPX-87H column (300 mm × 7.8 mm ) and a 5 mM
Guangdong, China). Sulfuric acid (guarantee reagent) and ammonia SO aqueous solution were employed to perform the separation
wt% 25％～28％) were purchased from Beijing Chemicals Co. at 35 °C (0.6 ml min ). The retention times and calibrations for
3
)
3
2
(
(
H
2
4
−1
Ltd. (Beijing, China). O
2
(99.9%) was supplied by Beijing Beiwen Gas observed products were determined by injecting known
2
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aꢀ1
aꢀ2
bꢀ1
bꢀ2
2
ZrO (011)
ZrO
2
Fe Zr
O
2
0
.2 0.8
ZrO2
Fe Zr
O
2
Fe0.2Zr0.8O2
0.4 0.6
Fe0.4Zr0.6O2
Fe0.6Zr0.4O2
Fe Zr
O
0.4 2
0.6
Fe Zr
O
2
0.8 0.2
Fe0.8Zr0.2O2
Fe O
2
3
Fe O
2
3
10
20
30
40
50
60
70
80 28 30 32
100 200 300 400 500 600 700 800 900 1000 225
300
ꢀ1
ꢀ1
2
Theta (degree)
2
Theta (degree)
Raman Shift
(cm
)
Raman Shift (cm )
2 x 2
Fig. 1 (a) XRD patterns (a-2: amplification of the (011) reflection for ZrO ) and (b) Raman spectra (b-2: amplification of the Raman spectra) of Fe Zr1-xO samples.
2
.
5
concentrations of standard reference compounds, and quantified monoclinic phase of ZrO
2
2 3
For the Raman spectrum of Fe O , the
using an external standard calibration curve method.
diffraction signals at approximately 223, 244, 291, 408, 492 and 610
-1
26
cm indicate the hematite structure of Fe
that for Fe mixed oxides (x = 0.2, 0.4, 0.6 and 0.8), the main
bands of Fe O , such as at approximately 224 and 291 cm , are
2 3
O . It should be noted
Zr1-xO
x 2
-1
Results and discussion
2
3
shifted to lower wavenumbers (red shift) relative to Fe
see Fig. 1b-2). This phenomenon can be attributed to changes in
the surface strain or defects due to the incorporation of Zr into the
Fe crystal structure, because the strains and defects in the
crystal structure or effects of particle size can lead to the shifts in
2 3
O sample
Characterization of catalyst
(
2 3
The XRD patterns of the catalysts are presented in Fig. 1a. Fe O
and ZrO2 samples show the characteristic reflections of
rhombohedral phase of hematite (a-Fe ) (JCPDF 33-0664) and the
tetragonal as well as monoclinic phases of ZrO (JCPDF 50-1089 and
2
O
3
2 3
O
2
2
7
Raman peaks, which was consistent with the XRD results that a
certain amount of solid solution formed in the mixed oxides.
3
7-1484). For the Fe Zr O mixed oxides (x = 0.2, 0.4, 0.6 and 0.8),
x
1-x 2
both ZrO
2
and Fe
2
O
3
can be detected clearly. However, compared
peaks toward higher 2θ values
The samples were further analyzed by XPS to determine the
distribution of the elements and oxidation degrees on the surface
of the compound oxide catalysts. As shown in Fig. 2a, the Fe 2p XPS
spectra for the samples that contained iron revealed the presence
with the ZrO
2
, a slight shift of ZrO
2
were observed over the four samples (see Figure 1a-2), revealing
that a part of solid solution presented in the mixed oxides with
22, 23
some Fe doped into the lattice of ZrO
2
.
of Fe
2 3
O on the surface of the catalysts due to the shape and
Raman spectra are displayed in Fig. 2b For single ZrO , five obvious
2
-1
position of the Fe 2p3/2 peak at binding energy of 711.2 eV and the
peaks at 146, 267, 309, 460 and 641 cm are indexed as the
2
8, 29
2
.
4
Fe 2p1/2 peak at binding energy of 724.7 eV, respectively.
tetragonal phase of ZrO
2
2
The Raman spectrum of ZrO also
Additionally, a shake-up satellite of the Fe 2p3/2 peak, approximately
7.7 eV below the main line, was also observed at approximately
indicates the characteristic diffraction peaks at approximately 178,
-1
1
89, 331, 345, 381, 479 and 623 cm , which correspond to
a
Fe2p1/2
24.7eV
Fe2p3/2
b
Zr3d5/2
184.8eV
Zr3d3/2
7
711.2eV
Fe O3
182.4eV
2
Satellite
18.9eV
7
Fe0.8Zr0.2O2
Fe0.6Zr0.4O2
ZrO2
Fe0.2Zr0.8O2
Fe0.4Zr0.6O2
Fe0.6Zr0.4O2
Fe0.4Zr0.6O2
Fe0.2Zr0.8O2
Fe0.8Zr0.2O2
745 740 735 730 725 720 715 710 705
194 192 190 188 186 184 182 180 178 176 174
Binding Energy (eV)
Binding Energy (eV)
x 2
Fig. 2 XPS spectra of Fe Zr1-xO samples: Survey spectra of (a) Fe 2p, (b) Zr 3d.
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of surface oxygen species in all mixed oxides except the Fe0.6Zr0.4
sample (see Table 1), which performed the best catalytic
performance (will be discussed in section 3.2).
The reducibility of Fe samples was investigated by H
shown in Fig. 4a, while the H consumption values are displayed in
Table 2. The H -TPR profiles of ZrO do not show any peaks
O
2
Fe Zr
O
a
Fe Zr O
0.6 0.4 2
b
0
.8 0.2
2
x
Zr1-xO
2
2
-TPR as
O
Ⅰ
O
Ⅰ
2
O
Ⅲ
O
Ⅲ
O
Ⅱ
O
Ⅱ
2
2
5
38 536 534 532 530 528 526 524 522
538 536 534 532 530 528 526 524 522
Binding Energy (eV)
attributed to H
below 930 °C in hydrogen. For single Fe
attributed to the stepwise reduction of Fe O to Fe O and the
2
consumption due to non-reductive property of it
Binding Energy (eV)
37
2
3
O two distinct peaks are
Fe Zr
O
c
Fe Zr
O
d
2
3
3 4
0
.4 0.6
2
0.2 0.8
2
3
8
3 4
further reduction of Fe O to Fe. However, the Fe-Zr mixed oxides
represents three reduction peaks labeled α, β and γ, respectively. It
is reported that because of the interaction between Fe species and
O
Ⅰ
O
Ⅰ
other metals, the high temperature reduction peak of pure Fe
2 3
O
O
Ⅱ
OⅡ
O
Ⅲ
O
Ⅲ
can be divided into two peaks due to the stepwise reduction of
5
38 536 534 532 530 528 526 524 522
538 536 534 532 530 528 526 524 522
Binding Energy (eV)
36
Binding Energy (eV)
Fe
concluded that there should be an interaction between Fe and Zr
species which affects the reduction process of Fe in the mixed
2 3
O to Fe through FeO as an intermediate. Therefore, it can be
x 2
Fig. 3 O 1s XPS spectra of Fe Zr1-xO samples.
2 3
O
3
+
30
oxides. Thus, the reduction of Fe-Zr mixed oxides may be separated
to three steps which attributed to the reduction of Fe O to Fe O
4
7
18.9 eV, which was also the characteristic of Fe in Fe
the samples that contained zirconium (see Fig. 2b), the Zr 3d XPS
spectra showed the exiting of ZrO on the surface of the catalysts
because of Zr 3d3/2 and Zr 3d5/2 peaks were observed at binding
2 3
O . As for
2
3
3
(
3 4
peak α), the reduction of Fe O to FeO (peak β) and the reduction
2
of FeO to Fe (peak γ). It is worth mentioning that hydrogen
consumption during the TPR measurements increases with the
increase of Fe proportion in Fe-Zr mixed oxides, and the
2
8, 31
energy of 184.3 and 181.9 eV, respectively.
Moreover, the types of surface oxygen species were also identified
by the O 1s XPS spectra as displayed in Fig. 3. It can be seen that the
O 1s curves of the Fe Zr O mixed oxides (x = 0.2, 0.4, 0.6 and 0.8)
quantification results of H
In addition, the higher temperature reduction peaks in mixed oxides
2 3
are shifted to a lower temperature compared with single Fe O ,
2
-TPR experiments are included in Table 2.
x
1-x 2
can be fitted into three peaks, which were denoted as O
III, and the relative fractions of which were listed in Table 1.
According to literature, the main peak O at 529.6-530.2 eV is
I
, OII and
indicating the chemical interaction between Fe and Zr species,
O
which could enhance the reducibility and oxygen mobility of Fe-Zr
I
39, 40
2
-
mixed oxides.
2
Importantly, the Fe0.6Zr0.4O sample has the
ascribed to lattice oxygen atoms (O
2
); a shoulder peak OII at 531.0-
lowest reduction peaks, which reveals the highest reducibility and
oxygen mobility of it. This finding is in accordance with the O 1s XPS
5
31.8 eV is assigned to oxygen vacancies, surface adsorbed oxygen
-
-
2
ions (O or O ) or OH groups; another shoulder peak OIII at 532.4-
2
result that the Fe0.6Zr0.4O sample has the largest amount OII
5
32.9 eV is likely to be associated with adsorbed molecular water,
3
2, 33
fraction, which has higher mobility and is easier be activated.
respectively.
Importantly, surface oxygen species are
The acid-base properties of Fe Zr1-xO2 samples were revealed by
x
considered to be more reactive than the others because they have
higher mobility, and are easier to be activated. At the same time, it
3 2
NH -TPD and CO -TPD analysis, and the results are shown in Fig. 4b
-
and 4c. The total acidity and basicity which has been estimated
from the area of the TPD profiles are summarized in Table 2. The
results showed that all the samples have both acidic and basic sites,
is also reported that the high amount of oxygen vacancies and HO
groups (denoted as OII) contribute to the high activity of the
3
4, 35
catalysts.
x 2
As for the Fe Zr1-xO mixed oxides, a certain amount
as well as the same acid and base types. The NH
3
-TPD curves (Fig.
of OII and OIII species were observed. In combination with XRD,
Raman and the followed TEM and BET analysis, the oxygen
vacancies could be ascribed to the formation of Fe-Zr solid solution,
and the adsorbed oxygen ions or OH groups as well as molecular
water should be associated with the small particle size and high
4
b) show that the total acidity amount for Fe Zr1-xO2 samples
x
initially increased and then slightly decreased with the increase of
Zr proportion in Fe-Zr mixed oxides. Moreover, the amount of total
acidic sites on the Fe Zr O samples ranged from 0.20 mmol (Fe O
3
x
1-x
2
2
3
6
sample) to 0.60 mmol (Fe0.6Zr0.4
2 3
O sample) of NH per gram of
specific surface area of samples (see Table 2 in the following part).
Additionally, there is no obvious difference for the relative fractions
catalyst (see Table 2). There is no significant difference in the NH
3
-
x 2
Table 1 The binding energies and relative fractions of surface oxygen species for Fe Zr1-xO samples
O
I
O
II
O
III
Catalyst
B.E. (eV)
Fractions (%)
B.E. (eV)
Fractions (%)
B.E. (eV)
Fractions (%)
Fe0.8Zr0.2
Fe0.6Zr0.4
Fe0.4Zr0.6
Fe0.2Zr0.8
O
2
2
2
2
530.2
529.6
530.0
530.0
78.8
65.4
73.5
73.7
531.8
531.0
531.4
531.5
13.8
26.2
18.2
17.0
532.9
532.4
532.7
532.7
7.4
8.4
8.3
9.3
O
O
O
4
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ZrO2
a
b
c
Fe0.2Zr0.8O2
Fe0.4Zr0.6O2
Fe0.6Zr0.4O2
γ
ZrO2
ZrO2
Fe0.8Zr0.2O2
Fe2O3
β
α
Fe0.2Zr0.8O2
Fe0.2Zr0.8O2
Fe0.4Zr0.6O2
Fe0.4Zr0.6O2
Fe0.6Zr0.4O2
Fe0.6Zr0.4
O
2
Fe0.8Zr0.2O2
Fe2O3
Fe0.8Zr0.2O2
Fe2O3
100 200 300 400 500 600 700 800
100 200 300 400 500 600 700 800 900 1000
100 200 300 400 500 600 700 800 900 1000
Temperature (℃)
Temperature
(
℃
)
Temperature (℃)
Fig. 4 (a) H
2
-TPR profiles, (b) NH
-TPD and (c) CO -TPD of Fe Zr1-xO
3 2 x 2
samples.
TPD curves except the relative acid strength (Table S1†), suggesting generated (about 47.3 nm, Fig. S1
the same acid types. (about 5.0 nm), which still much smaller than pure Fe O particles
The CO -TPD profiles of all oxides in Fig. 4c present the basic (about 74.0 nm). The SEM images of the catalysts (Fig. S2†) also
properties of the oxides. Fe presents mainly medium and strong showed the smaller particle size of Fe-Zr mixed oxides than pure
†) except the small particles
2
3
2
2 3
O
basic sites, while ZrO presents preponderant weak and medium oxides.
2
basic sites. Additionally, as revealed in Fig. 4c and Table 2, both the The composition of the prepared catalysts was determined by ICP
total amount of basic sites and the base strength distribution are analysis (Table 2), and the results showed that the tested value was
influenced by the Zr proportion in Fe-Zr mixed oxides. The total approximately consistent with the designed value. The specific
x 2 2 3
amount of basic sites of Fe Zr1-xO samples initially increased and surface areas of the catalysts are show in Table 2. The Fe O sample
then decreased with the increase of Zr content in the Fe-Zr mixed presented the lowest BET surface because of the largest crystallite
oxides. The surface base concentrations increased from 0.91 mmol size of 74.0 nm (as shown in TEM). Additionally, it is clear that the
−
1
−1
CO
2
g
in Fe0.8Zr0.2
O
2
to 1.46 mmol CO
2
1
g
in Fe0.6Zr0.4
O
2
,
addition of Zr can significantly change the surface area of the
−
subsequently decreased to 0.96 mmol CO
2
g
in Fe0.2Zr0.8
2
O . More catalysts, which is consistent with the previous report that the
importantly, as can be seen from CO -TPD profiles, sites of medium formation of mixed oxides could help to increase the surface area of
2
41
2
strengths were the most abundant in Fe0.6Zr0.4O mixed oxide, with the catalysts. With the increase of Zr content in Fe-Zr mixed
−
1
2
-1
the content was as high as 0.81 mmol CO
2
g
(Table S2†).
oxides, specific surface area of the catalyst increased from 62 m g
2
-1
TEM images and size distributions of the prepared catalysts are in Fe0.8Zr0.2O to 122 m g in Fe0.2Zr0.8O sample. It should be noted
2
2
presented in Fig. 5, while the mean diameters of each catalyst are that due to the generation of large Fe
listed in Table 2. The results showed that a good metal dispersion TEM), the Fe0.8Zr0.2 sample has relatively low specific surface area
degree and homogeneity was achieved for all samples except compared with other mixed oxides.
. The Fe and ZrO clearly have a much larger average Catalytic performance for aerobic oxidation of HMF
particle size (74.0 and 8.2 nm, respectively) than the Fe-Zr mixed The reaction conditions were selected on the basis of our previous
2 3
O particles (as shown in
O
2
Fe0.8Zr0.2
O
2
2
O
3
2
4
2
oxides, which revealed that the generation of mixed oxides can work regarding the base-free conversion of HMF to FDCA in ILs.
significantly reduce the particle size of the catalysts, especially for We have already shown that the formation of FDCA nearly cannot
the Fe sample. It is worth mentioning that Fe0.6Zr0.4
, in take place below 120 °C over non-noble metal catalysts, and 160 °C
2
O
3
O
2
comparison with other mixed oxides, has a smaller particle size (7.0 is the most suitable temperature for the reaction. Therefore, the
nm). Additionally, because of the relatively high iron content in the catalytic performance of the synthesized Fe-Zr-O catalyst and the
2 2 3
Fe0.8Zr0.2O catalyst, there were lots of large Fe O
particles determination of the reaction pathways for the production of FDCA
Table 2 Physicochemical properties of the catalysts
a
X value of
Surface area
(m g )
15
Consumption of H
2
Total acidity
Total basicity
−1 f
Catalyst
Fe
Mean diameter (nm)
74.0
b
2
-1
c
−1
d
−1
)
e
Fe Zr1-xO
x 2
(mmol g
)
(mmol NH
3
g
2
(mmol CO g )
2
O
3
—
13.92
10.40
5.66
4.09
1.44
0.05
0.20
0.34
0.60
0.58
0.59
0.43
0.42
0.91
1.46
0.97
0.96
0.71
g
Fe0.8Zr0.2
O
O
2
2
5.0 / 47.3
0.76
0.58
0.37
0.18
—
62
96
102
122
113
Fe0.6Zr0.4
Fe0.4Zr0.6
Fe0.2Zr0.8
7.0
7.1
7.2
8.2
O
2
O
2
ZrO
2
a
b
c
d
Determined by TEM. Based on ICP analysis. BET surface area calculated from N
2
adsorption isotherms at 77 K. Consumption of H
2
from hydrogen TPD
e
f
g
measurements. Acid properties from ammonia TPD measurements. Base properties from carbon dioxide TPD measurements. A mixture of two different
sizes of particles.
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were performed with batch experiments at 160 °C and 2 MPa of O
2
.
Compared with the blank experiment, the ZrO
2
did not exhibit any
catalyst showed
The choice of [Bmim]Cl ILs as solvent not only can avoid the catalytic activity for HMF oxidation, while Fe
2
O
3
excessive use of base due to the unique dissolving abilities of ILs, 31.6% FDCA yield (Table 3, entry 2 and entry 3). Additionally, a
but also can effectively promote the oxidation of HMF into FDCA strong synergetic effect was performed when the Fe-Zr mixed
with non-noble metal catalyst, a reaction which cannot take place oxides were generated by a hydrothermal method. For the
when other solvents are used instead of ILs such as water, toluene, bimetallic oxides, with the increase in Zr content, the FDCA yield
4
2
CH
Table 3 presents the catalytic results of mixed oxides with different (Table 3, entries 4-7). Although all catalysts show similar conversion
Fe/Zr ratios and pure Fe O and ZrO2 oxides for the aerobic HMF (
98%), the employment of the Fe0.6Zr0.4O catalyst results in
3
CN, 1,4-dioxane, methanol, GVL or CH
3
COOH.
first sharply increased to 60.6%, then decreased rapidly to 27.1%
2
3
＞
2
4
2
oxidation of HMF to FDCA in ILs. As reported,
because of the the highest FDCA yield of 60.6%. It is well known that acid-base
strong hydrogen bonds formation ability and the unique dissolving properties of catalyst have great effect on catalysis. According to
1
6
ability for FDCA, [Bmim]Cl played a favorable role as a co-catalyst recent reports , the acidic active sites on the surface of the
and solvent in HMF oxidation, and a 21.6% of FDCA yield is achieved catalysts played an important role to enhance the yield of FDCA.
at 160 °C without any catalysts (Table 3, entry 1). Importantly, Recent literature reports also emphasized that the surface basicity
4
3
although over 98% conversion of HMF is achieved with all the of bimetallic oxides improve the oxidant strength of the catalyst,
catalysts, the FDCA yield is strongly influenced by the Fe/Zr ratio. especially the medium strength basic sites favor the oxidation of
x 2 2 2
Fig. 5 TEM images and histograms showing the size distributions of Fe Zr1-xO samples for (a) ZrO , (b) Fe0.2Zr0.8O , (c)
Fe0.4Zr0.6O
2
, (d) Fe0.6Zr0.4
O
2
, (e) Fe0.8Zr0.2
O
2
2 3
and (f) Fe O .
6
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a
Table 3 Catalytic performance for aerobic oxidation of HMF to FDCA in [Bmim]Cl over Fe
x
Zr1-x
O
2
catalysts
Entry
Catalysts
Blank
FDCA yield (%)
FFCA yield (%)
HMFA yield (%)
HMF conversion (%)
1
2
3
4
5
6
7
8
21.6±0.9
16.7±0.8
31.6±1.7
53.5±2.1
60.6±2.4
35.6±1.6
27.1±1.3
36.7±1.4
2.4±0.2
11.4±0.8
5.3±0.4
2.4±0.3
1.3±0.2
2.8±0.4
3.6±0.4
4.2±0.3
0.5±0.1
0.6±0.1
1.7±0.2
3.5±0.3
0.2±0.0
3.2±0.3
1.8±0.2
0.1±0.0
99.1±0.5
99.1±0.8
98.1±0.9
98.5±1.2
99.7±0.2
98.7±0.9
99.2±0.5
98.2±1.0
ZrO
Fe
2
2
O
3
Fe0.8Zr0.2
Fe0.6Zr0.4
Fe0.4Zr0.6
Fe0.2Zr0.8
O
2
O
O
O
2
2
2
b
2 3 2
Fe O +ZrO
a
b
Reaction conditions: HMF (0.1 mmol), catalyst (0.01 g), [Bmim]Cl (1 g), O
2
(2 MPa), 160 °C, 24 h. Physical mixture of Fe
O
2 3
2
and ZrO with atom ratio of
2
Fe0.6Zr0.4O .
19
HMF to FDCA. For the Fe0.6Zr0.4O2 catalyst, as we mentioned As shown in Fig. 6, the FT-IR spectrum of humins shows the broad
−
1
above (Fig. 4b, 4c and Table 2), a higher total number of acidic sites peak at 3378 cm attributed to O-H stretching vibrations,
4
5
as well as basic sites generated compared with pure oxides or other suggesting the existence of hydroxyl groups. The peaks at 2959-
−
1
mixed oxides. Importantly, basic sites of medium strengths were 2872 cm were due to stretching vibrations of methyl and
−
1
the most abundant in Fe0.6Zr0.4
resulted in the best catalytic performance of it.
Noteworthy, in contrast, the physical mixture of Fe O and ZrO2 corresponding to furanic rings exist in the IR spectrum of the
2
O catalyst (Table S2†), which methylene groups, and the peak at 1712 cm was ascribed to C=O
4
6
stretching vibrations in aldehydes.
Importantly, peaks
2
3
−1
with corresponding moles fraction as Fe0.6Zr0.4
2
O were added to the humins. As reported in the literature, peaks at 1611 and 1577 cm
reaction (Table 3, entry 8), however, much lower FDCA (36.7%) correspond to C=C stretching in furan rings, and peaks at 1407,
−
1
yield obtained from it. The significant difference between the 1364, 779 and 740 cm were ascribed to C-O-C stretching and
hydrothermal and physically mixing method prepared Fe0.6Zr0.4 aromatic C-H out-of-plane bending vibrations in furan rings,
O
2
4
7
−1
catalyst indicated the importance of the solid solution structure. As respectively. Additionally, the peak at 1018 cm , which is
we mentioned above, the XRD and Raman measurements along attributed to primary alcohols, is also observed in the IR spectrum
with O 1s XPS revealed the formation of solid solutions and oxygen of the humins. In conclusion, we can conclude that the formation of
2
6, 36
vacancy. As reported previously,
more oxygen vacancies on the humins may through the polymerization and cross-polymerization
4
8, 49
material could be created with the formation of solid solution. of HMF and intermediates as reported in the literature.
Therefore, the redox properties and activity of catalyst, which are Understanding and controlling the formation of humins are still
beneficial to the reaction, could be improved as well. In the Fe-Zr challenging, some work are ongoing in our laboratory to reduce the
2
mixed oxides, the Fe0.6Zr0.4O sample has relatively low reduction formation of humins and improve the yield of FDCA.
peaks (see Fig. 3a), revealing the highest reducibility and oxygen Further evidence for the formation of humins was obtained from
mobility of it, which is in accordance with the O 1s XPS finding that SEM and TG analysis. Typical SEM images of the catalyst and humins
there was largest amount of reactive oxygen species in the after aerobic oxidation of HMF are given in Fig. 7. It is showed that
2
Fe0.6Zr0.4O catalyst. It is worth stressing that the by-products during the humins consisted of lamellar membrane structure, which was
the oxidation of HMF are a small amount of FFCA and HMFA, which very different from the spherical particles as reported by the
could be detected by HPLC, and a relatively large amount of literature due to the use of very different reaction solvent
5
0
humins, which could be observed clearly after the reaction (Fig. [Bmim]Cl. TG behavior of the solid residues (contained insoluble
S4†) and will be discussed in the following part.
Formation of humins
4
4
As reported previously, HMF was not stable and degraded into
humins significantly at high temperature. Therefore, the FDCA yield
was restrained (60.6% under the optimal conditions) by the parallel
formation of insoluble humins even though the HMF conversion
reached over 99%. In our work, some yellowish-brown humins were
observed together with the used catalyst after the oxidation of HMF
1
712 1577 1407
1018
779
C=O
C=C
1611
C-O-C
C-O-C
2933
CH2
959
C-H
1364
C-H
740
CH OH
2
2872
2
3
O-H
378
(
Fig. S4†). In order to give more insights into their formation
mechanisms, the solid residues (contained insoluble humins and
used catalyst) were washed with ethanol for three times, collected
by centrifugation and then oven-dried overnight at 70 °C for FT-IR,
SEM, TG and GPC analysis.
4
000 3800 3600 3400 3200 3000
1600 1400 1200 1000 800 600 400
ꢀ1
Wavenumber
(
cm
)
Fig. 6 FT-IR spectrum of humins after aerobic oxidation of HMF. Reaction
conditions: HMF (0.1 mmol), Fe0.6Zr0.4 catalyst (0.01 g), [Bmim]Cl (1 g), O
2 MPa), 160 °C, 24 h.
O
2
2
(
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Fig. 7 SEM images of (a) fresh Fe0.6Zr0.4
conditions: HMF (0.1 mmol), Fe0.6Zr0.4
O
O
2
2
catalyst, (b) and (c) mixtures of used catalyst and humins after aerobic oxidation of HMF. Reaction
catalyst (0.01 g), [Bmim]Cl (1 g), O (2 MPa), 160 °C, 24 h.
2
humins and catalyst) showed significant weight loss (Fig. S5†), reaction, FDCA was detected as a major product, with HMFA and
corresponding to the decomposition of humins, which is also a FFCA being the main intermediates, while no DFF was detected. As
evidence for the formation of humins.
the reaction progressed, the yield of FDCA increased, while the
To gain insights in the average molecular weight distributions of yield of FFCA firstly increased and then decreased, and the yield of
humins, the solid residues were dissolved in THF and then subjected HMFA was always detected with low content. When prolonging the
to GPC analysis (Fig. S6†). The GPC result demonstrates that the reaction time to 24 h, FDCA yield achieved 60.6% while FFCA and
humins comprise of a mixture of components with the approximate HMFA yields declined to 1.3% and 0.2%, respectively.
weight-average molecular weight (Mw) of 277, 513 and 1065, which It should be underlined that there was no DFF generated during the
was two range of molecular weights of HMF. It should be note that reaction, suggesting that the oxidation of aldehyde group to
part of humins is insoluble or partly dissolve in THF, which exceed carboxyl group was much easier than the oxidation of
the detectable limit of instrument and their molecular weights hydroxymethyl group to aldehyde group in [Bmim]Cl ILs with
cannot be detected, as such, the molecular weights of the humins Fe0.6Zr0.4O2 catalyst. In order to confirm this, further oxidation
cannot be established unequivocally. However, it is evident that experiments of commercial HMFA (which contains hydroxymethyl
polymerization or cross-polymerization occurs to a significant group), DFF and FFCA (which contains aldehyde group) in [Bmim]Cl
extent because GPC showed obvious peaks. Coupling of the TG over Fe0.6Zr0.4O catalyst were performed. As shown in Table 4, the
2
behavior and the SEM characterization, we concluded that the main hydroxymethyl group of HMFA was slowly converted into the
by product was humins. While coupling of the FT-IR and GPC analyst aldehyde group of FFCA, and further converted into carboxyl group
we concluded that the polymerization or cross-polymerization of FDCA, just resulting in a 90.8% conversion of HMFA with a 0.6
occurs with two range of molecular weights of HMF.
and 12.9% yields of FFCA and FDCA after 8 h, respectively. While
the aldehyde groups in DFF and FFCA, were quickly converted into
Reaction pathways for aerobic oxidation of HMF to FDCA
The reaction pathways for aerobic oxidation of HMF in [Bmim]Cl the carboxyl group of FFCA or FDCA. Especially for the DFF, almost
over Fe0.6Zr0.4O2 catalyst are investigated by recording the complete conversion and a 1.2 and 44.8% yields of FFCA and FDCA
distributions and yields of the products at different reaction times. were attained as short as 2 h. When prolonging the reaction time to
As shown in Fig. 8, with the increase of reaction time, HMF 6 h, FDCA yield achieved 65.3% while the FFCA conversion
progressively consumed, reaching a conversion of 88.8% at 4 h and increased to 98.8%.
almost completely converted after 8 h. During the course of the Accordingly, the reaction pathways for the aerobic oxidation of
HMF to FDCA in [Bmim]Cl ILs over Fe0.6Zr0.4O2 catalyst could be
obtained as shown in Scheme 1, which proceeded via the formation
of HMFA intermediate rather than the DFF intermediate, and was
1
3, 51, 52
consistent with the previous reports.
Importantly, the yield
1
00
of HMFA was relatively low when compared to the yield of FDCA
and FFCA, and the FFCA yield was much higher than HMFA yield.
This phenomenon revealed that the conversion of aldehyde group
8
6
4
2
0
0
0
0
0
FDCA yield
FFCA yield
HMFA yield
HMF conversion
in furan rings to carboxyl group over Fe0.6Zr0.4
2
O catalyst is still a
challenge under base-free conditions, although it is very fast in the
53
presence of base. Therefore, conversions of HMF to HMFA and
humins occur in parallel depending on the Fe0.6Zr0.4 catalyst
O
2
0
2
4
6
8
10 12 14 16 18 20 22 24
Time (h)
employed, resulting in the high HMF conversion but limited FDCA
yield, while humins as the main by-product.
Proposed reaction mechanism
According to the above experimental and catalyst characterization
Fig. 8 Time evolution of HMF conversion and yields of products over
catalyst. Reaction conditions: HMF (0.1 mmol), catalyst (0.01 g),
Bmim]Cl (1 g), O (2 MPa), 160 °C.
Fe0.6Zr0.4O
2
[
1
3, 54, 55
2
results, as well as the reported work in the literature,
the
8
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x 2
Table 4 Oxidation of commercial HMFA, DFF and FFCA in [Bmim]Cl over Fe Zr1-xO catalysts under base free conditions
Reaction
Time (h)
Entry
Reactant
Conversion (%)
FFCA yield (%)
3.2±0.2
4.0±0.3
1.3±0.1
0.6±0.1
3.4±0.4
25.6±1.3
8.5±0.4
1.2±0.1
—
FDCA yield (%)
1.2±0.1
1.3±0.1
12.2±0.7
12.9±0.6
0.8±0.1
1
2
3
4
5
6
7
8
9
HMFA
HMFA
HMFA
HMFA
DFF
DFF
DFF
DFF
0.5
1
4
27.1±1.2
32.2±1.8
70.1±2.3
90.8±0.9
65.1±2.0
97.6±1.1
100.0±0.0
100.0±0.0
60.0±2.2
86.4±1.0
98.2±0.8
8
0.2
0.5
1
2
0.5
1
8.8±0.5
36.0±1.7
44.8±2.0
9.5±0.5
39.9±1.8
58.9±2.0
FFCA
FFCA
FFCA
1
1
0
1
—
—
4
6
1
2
FFCA
—
65.3±2.3
98.8±0.7
Reaction conditions: HMFA/DFF/FFCA (0.1 mmol), Fe0.6Zr0.4
O
2
2
catalyst (0.01 g), [Bmim]Cl (1 g), O (2 MPa), 160 °C.
plausible reaction mechanism for the aerobic oxidation of HMF over in the oxidation of HMF. In addition, as reported, the polar
catalyst is proposed and shown in Scheme 2. In the molecules or those containing strong proton donor functionality
Fe0.6Zr0.4O
2
initial stage of the reaction, the aldehyde groups of HMF were (such as carboxylic acids, phenols and diols) can be dissolved in ILs,
oxidized to corresponding monocarboxylic acid (HMFA) through the especially, when the ILs have H-bond acceptor capability, such as
58
formation of the intermediate hemiacetal. In this stage, both the chloride-containing ILs. We also found that the solubility of FDCA
4
2
acidity and basicity of the catalyst could facilitate the formation of in [Bmim]Cl was as high as 47.93%, therefore, the obtained HMFA,
5
6, 57
hemiacetal.
The acidic sites can favor the adsorption of FFCA and FDCA molecule could be desorbed from Fe0.6Zr0.4O
2
aldehyde group on the surface of the catalyst, while the basic sites catalyst with the help of [Bmim]Cl, and went into the ILs solvent
can further transform it into hemiacetal. During the transformation easily, thus, the catalyst was recovered.
of hemiacetal into HMFA, a dehydrogenation reaction occurred and Catalyst recycling
two hydrogen atoms leaved on the surface of the catalyst, which To test the reusability of the catalyst, Fe0.6Zr0.4
O
2
was successive
or O ) on the reused for five times under the optimal conditions for the oxidation
2
-
-
were oxidized into water by active oxygen ions (O
surface of the catalyst, thus, the catalyst is recovered. In the second of HMF to FDCA. After the reaction, the catalyst was thoroughly
stage, the hydroxymethyl group in HMF firstly adsorbed onto the washed with ethanol for three times, dried in a vacuum oven at 70
surface of catalyst to form a metal-alkoxide intermediate with the °C overnight and then reused. As shown in Fig. 9, there was no
help of acidic sites, and then further converted to an aldehyde to significant change in the HMF conversion and FDCA yield,
form FFCA through β-hydride elimination with the aid of basic suggesting the stability of the catalyst. In the first run, HMF
5
5
sites, also leaving two hydrogen atoms on the catalyst surface. As conversion and FDCA yield were 99.7 and 60.6%, respectively, while
for the third step, the oxidation of aldehyde in FFCA undergoes the in the fifth run, they were still maintained at 98.1% and 50.1%,
same oxidation process to the first step and finally generates FDCA. indicating the catalyst was used five times without significant loss of
42
It is worth noting that the both the acidity and basicity as well as activity. It is noteworthy that as our previous study pointed out, in
-
-
the active oxygen ions (O2 or O ) on the surface of the catalyst, the recycle experiments, some insoluble humins also formed, and
which could supply by molecular oxygen (O
2
), play important roles mixed together with the catalyst. However, it could be dissolved
OH
O
HO
O
O
2
O
2
OH
O
HO
OH
O
O
HMFA
O
O
2
O
HO
O
O
O
O
2
O
O
2
5
ꢀHMF
FFCA
FDCA
O
O
DFF
Scheme 1 Reaction pathways for the aerobic oxidation of HMF to FDCA in [Bmim]Cl over Fe0.6Zr0.4
O
2
catalyst.
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1
00
HMF conversion
FDCA yield
FFCA yield
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
HMFA yield
1
2
3
4
5
Run No.
Fig. 9 Recycle experiments of the catalyst. Reaction conditions: HMF (0.1
mmol), Fe0.6Zr0.4 catalyst (0.01 g), [Bmim]Cl (1 g), O2 (2 MPa), 160 °C,
4 h.
O
2
2
2
mechanism over the Fe0.6Zr0.4O catalyst was proposed based on
the understanding of the acidity and basicity as well as the
reducibility of the catalyst. Furthermore, given the cost and
environmental advantage of non-noble metal catalysts used in
base-free reaction system, more research could be further
performed in the development of highly active catalyst for FDCA
production, which are ongoing in our laboratory.
Scheme 2 Plausible reaction mechanism of aerobic oxidation of HMF to
FDCA over Fe0.6Zr0.4 catalyst under base free conditions.
O
2
Acknowledgements
completely by [Bmim]Cl ILs due to its unique dissolving abilities for
polar compounds (Fig. S7†). Therefore, the adsorption of FDCA and This work was supported by the National Basic Research Program of
humins on the active sites of the catalyst could be avoid, thus the China (973 Program, 2015CB251401), the National Natural Science
activity of catalyst could be maintained.
Foundation of China (No. 21576269, 91434203, 21210006,
1506231).
2
Conclusions
Notes and references
x 2
In summary, a series of Fe Zr1-xO non-noble metal catalysts were
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HMF to FDCA under base-free conditions. The catalysts
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different acidity and basicity as well as reducibility and oxygen
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