Production of a renewable 1,3-diene containing a functional group from a furfural-acetone adduct in a fixed-bed reactor

Article abstract of DOI:10.1039/c9gc01481k

The synthesis of functionalized 1,3-dienes has received increasing attention due to their importance in the practical development of high-performance elastomers. In the present work, furfural and acetone, acting as renewable feedstocks, are first employed to produce a functionalized 1,3-diene containing a furan group (1-(2-furyl)-1,3-butadiene, F-diene). After selective hydrogenation of the CO group, the aldol adduct (4-(2-furanyl)-3-buten-2-ol, FAH) shows a nearly complete conversion, with an excellent selectivity (as high as 92%) towards F-diene over ceria-based catalysts in a fixed bed. BET, TEM, XRD, XPS, Raman, TPD, TPR and TGA were conducted to identify the relationship between the catalytic performance and catalyst structure. A plausible reaction pathway for the dehydration of FAH over ceria-based catalysts was proposed using a radical mechanism, which suggested the importance of the Ce⁴⁺-Ce³⁺ redox cycle for the dehydration of FAH to F-diene. Furthermore, the ceria-based catalysts exhibited a notable carbon resistance.

Full text of DOI:10.1039/c9gc01481k

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DOI: 10.1039/C9GC01481K
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Production of a renewable 1,3–diene containing a functional
group from furfural–acetone adduct in a fixed–bed reactor
Yanlong Qi a, Shijun Liu a,b, Long Cui a, Quanquan Dai a, Jianyun He a, Wei Dong a, Chenxi Bai a,*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The synthesis of functionalized 1,3–dienes has received increasing attention due to their importance in the practical
development of high–performance elastomers. In the present work, furfural and acetone, acting as renewable feedstocks,
are first employed to produce a functionalized 1,3–diene containing a furan group (1–(2–furyl)–1,3–butadiene, F–Diene).
After selective hydrogenation of the C=O group, the aldol adduct (4–(2–furanyl)–3–butene–2–ol, FAH) shows a nearly
complete conversion, with an excellent selectivity (as high as 92%) towards F–Diene over the ceria–based catalysts in a
fixed bed. BET, TEM, XRD, XPS, Raman, TPD, TPR and TGA were conducted to identify the relationship between the
catalytic performance and catalyst structure. A plausible reaction pathway for the dehydration of FAH over ceria–based
catalysts was proposed using a radical mechanism, which suggested the importance of the Ce4+–Ce3+ redox cycle for the
dehydration of FAH to F–Diene. Furthermore, the ceria–based catalysts exhibited a notable carbon resistance.
Notably, FA is also a precursor of a functionalized 1,3–diene,
containing a furan group. Inspired by this, we are interested in
exploiting a novel application of furfural and acetone in the
production of functionalized 1,3–dienes. The production of
1. Introduction
Diminishing fossil resources and growing concern regarding
environmental sustainability have boosted the development of
research and technologies for biofuels production and valuable
chemicals from biomass, which are a promising and renewable
energy resource.1, 2 Among the platform chemicals obtained from
biomass, furfural is one of most valuable and readily available
compounds, which can be produced by the dehydration of
hemicellulose, cellulose, xylose and glucose.3-5 Thus, it is a practical
and effective strategy to produce value–added chemicals by furfural
utilization. Consequently, many efforts have been devoted to
exploiting the potential applications of furfural through a wide
variety of catalytic processes.6-10 To manufacture liquid
transportation fuels (e.g., jet and diesel fuels), furfural reacts with
acetone (a by–product in phenol production) via aldol
condensation, which affords furfural–acetone adducts containing
8+ carbon atoms; these adducts are then converted into liquid fuel
after hydrogenation and deoxygenation reactions (Scheme 1).11-15
During these procedures, the C8 adduct (4–(2–furanyl)–3–butene–
2–one, FA) is an important intermediate, which has received a great
deal of attention.16-23
functionalized 1,3–dienes has received wide attention for decades,
because of the ability to design the functional groups (polar and/or
rigid groups), which can effectively solve the interfacial and
compatible problems of the resulting materials.24-29 Therefore, 1,3–
dienes, which bear polar and/or rigid groups such as silicon–,30-34
37, 38
39-42
cyano–,35, 36 ester–
and amino–
groups, as well as other
groups, have been synthesized. 43-45
Recently, Long et al.46 successfully synthesized a functionalized 1,3–
diene containing a furan group (1,3–2–(2–methylidenebut–3–enyl)
furan) from 2-iodofuran via the Grignard reaction and coupling
reactions in the presence of Pd(PPh3)4, vinyl magnesium bromide,
suspension of magnesium in anhydrous ether, 2,3–dibromopropene
and anhydrous THF. The furan displays polar and rigid properties,
a. Key Laboratory of High-Performance Synthetic Rubber and Its Composite
Materials, Changchun Institute of Applied Chemistry, Chinese Academy of
Sciences, Renmin Road, Changchun 130022, China;
b. Applied Chemistry, University of Science and Technology of China, Jinzhai Road,
Hefei 230026, China.
* Corresponding author: baicx@ciac.ac.cn
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Scheme 1. The production of transportation fuel from
furfural and acetone.
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which are desired in a functional group. Watkins et al.47 reported (15Ce/SiO2). Similarly, 1.00 g, 1.25 g, 1.50 g, 1.75 g, VaienwdAr2ti.c0le0Ognlionef
DOI: 10.1039/C9GC01481K
the production of 1–(2–furyl)–1,3–butadiene (F–diene) through the Ce(NO3)3·6H2O were used to prepare the catalysts 20Ce/SiO2,
Wittig
Reaction
using
3–(2–furyl)–2–propenal, 25Ce/SiO2, 30Ce/SiO2, 35Ce/SiO2, and 40Ce/SiO2, respectively. In
methyltriphenylphosphonium bromide in dry THF and sodium addition, Ag/SiO2, HPW/SiO2, P/SiO2, MoP/SiO2, HPW/SiO2, P/SiO2,
hexamethyldisilazane as starting materials, which sparked interest and MoP/SiO2 were prepared.
in this methodology.48 Similarly, methyl phosphonium bromide and
NaOH and NaBH4 were employed as catalysts for the aldol
condensation and reduction to allow for a high yield of the target
products.
3–(2–furyl) acrolein were utilized for the synthesis of F–diene in the
presence of potassium tert–butoxide and dry THF.49 Yasukawa et
al.50 synthesized F–diene using 4–(2–Furyl)–3–buten–2–one,
a
suspension of methyltriphenylphosphonium bromide in THF and n–
BuLi. Unfortunately, the outlined methods to produce
2.3 Characterization
functionalized 1,3–diene (including F–diene) involve unusual Surface area was measured by N2 adsorption–desorption isotherms
feedstocks (e.g., 2–iodofuran, 3–(2–furyl)–2–propenal, 2,3– on an Autosorb–iQ (Quantachrome), and X–ray diffraction (XRD,
dibromopropene) and hazardous reagents (e.g., Grignard reagents, Bruker D8 Advance, λ =0.154 nm) was performed to examine the
organophosphorus chemicals). More importantly, all of these sample structure. The morphology of samples was studied by (high–
chemicals are non–renewable resources or could lead to resolution) transmission electron microscopy (TEM, Tecnai G2 F20
environmental problems. Furthermore, these procedures require S-TWIN). Catalyst composition was revealed by inductively coupled
several elaborate operations under harsh conditions (e.g., inert and plasma (ICP). X–ray photoelectron spectroscopy (XPS, ESCALAB 250,
dry atmosphere, purified solvents). Notably, Wang et al.51 Thermo) was performed to examine the fresh catalysts. Raman
successfully employed furfural to synthesize F–diene; however, the spectra were recorded on a LabRAM HR800 (Horiba Jobin Yvon)
method still requires Grignard reagent, organophosphorus with a laser source of 532 nm. TGA (SDT Q600) was employed to
compound and complicated operations. Therefore, the production determine the carbon deposition of spent catalysts, using a heat
of F–diene from renewable and accessible feedstocks by a facile rate of 10 °C /min to 700 °C under air flow. The acidity of sample
route remains a challenge.
(NH3–TPD) was determined on TP5080 (Tianjin Xianquan) from 100
to 700 °C with a ramp of 10 °C/min. Firstly, the sample was
pretreated at 400 °C for 30 min under a flow of He (30 ml/min),
followed by saturated with 5% of NH3/He at room temperature,
physical adsorbed NH3 was removed successively at room
temperature and 100 °C for 30 min under 30 mL/min of He flow,
respectively. Temperature-programmed reduction (H2–TPR) was
conducted on TP5080 (Tianjin Xianquan), from 50 to 800 °C (heating
rate: 10 °C/min) under 10% of H2/Ar flow (20 mL/min). Prior to
experimental, the sample was pretreated at 150 °C for 30 min
under He flow (20 ml/min).
Herein, the present work focuses on the production of F–diene (a
functionalized 1,3–diene containing a furan group) from furfural
and acetone through condensation, hydrogenation and dehydration
reactions. The condensation and hydrogenation first allow for an
excellent yield of the target product under mild conditions.
Subsequently, the furfural–acetone adduct was converted into F–
diene over ceria–based catalysts in a fixed–bed reactor. The
relationship between the catalytic performance and catalyst was
studied, and a likely reaction pathway was proposed. To our
knowledge, this is the first report of F–diene production from a
furfural–acetone adduct under a readily accessible condition.
2.4 Reaction studies
Production of the furfural-acetone adduct
2. Experimental
2.1 Materials
The furfural–acetone adduct was prepared according to the
reported method with some modification.12 Furfural (10 g) and
acetone (60.75 g) were charged into a 500 mL round–bottom flask,
followed by the addition of a water–methanol solution (200 g, 1:1
in m/m) containing 1.0 g of NaOH. After stirring at 30 °C for 2 h, the
mixture was neutralized by dilute HCl and extracted by CH2Cl,
followed by removal of the solvent to give FA. A THF–water (50mL,
9:1 in v/v) solution containing 2.0 g of NaBH4 and 100 mg of
Ba(OH)2 was then added dropwise to a solution of THF–water (40
mL, 9:1 in v/v) containing 7.6 g of FA in an ice bath, followed by
stirring at room temperature for 12 h. The reaction was stopped by
the addition of dilute H2SO4, and THF was then removed. After
extraction with ether, the resulting liquid was washed with brine.
Finally, 4–(2–furanyl)–3–butene–2–ol (FAH) was obtained as yellow
oil after removal of the ether (Supporting materials).
Furfural and acetone were purchased from Aladdin. Ce(NO3)3·6H2O,
AgNO3, NaBH4, and (NH4)6Mo7O24·4H2O were supplied by Energy
Chemical. SiO2 (100 ~ 200 mesh, 370 m2/g) was purchased from
Qingdao Hailang (China), and n–heptane and dodecane were from
TCI (Shanghai). Tetrahydrofuran, diethyl ether, methanol, H3PO4
and NaOH were supplied by Beijing Chemical Works (China), and
phosphotungstic acid was supplied by Sinopharm Chemical Reagent
Co., Ltd. All chemicals were used as received.
2.2 Preparation of catalysts
All catalysts were prepared by an initial wetness method. Prior to
using as a support, SiO2 was first calcined at 550 °C for 4 h. In a
typical synthesis procedure, 0.75 g of Ce(NO3)3·6H2O was dissolved
in 8 mL of water at room temperature, and 5 g of SiO2 was added to
form a slurry. The slurry was aged over night at room temperature,
dried at 70 °C for 3 h, and then at 120 °C for 5 h in an oven. Finally,
the sample was calcined at 550 °C for 4 h to afford the catalyst
Production of functionalized 1,3-diene in a fixed-bed reactor
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C9GC01481K
Scheme 2. Synthesis of functionalized 1,3–diene from
furfural and acetone.
1,3–Diene was produced in a conventional continuous flow fixed–
bed reactor (a stainless steel tube, with an inner diameter of 7 mm
and length of 500 mm) under atmospheric pressure. The catalyst
(400 mg) was loaded and heated to the desired temperature under
N2 (99.999%), with a flow rate of 50 mL/min. Prior to the reaction,
the catalyst was heated at the desired temperature for 3 h. The
reactant of FAH (2.0 wt% in THF) was fed by a syringe pump
(Harvard Apparatus) with a rate of 2.6 mL/h and co–fed with 50
mL/min of N2. The products were collected using a trap, which was
charged with 20 mL of THF–dichloromethane solution in an ice
bath. The products were analyzed by GC–MS (Agilent 5975 MSD
with Agilent SE–54) and GC (Agilent technologies 7890B), and both
n–heptane and dodecane were used as internal standards.
Fig. 1 N2 adsorption isotherms (a) and pore size
distribution plots (b) of the ceria–based catalysts.
(a)
(b)
50
40
30
20
10
0
40
30
20
10
0
50 nm
50 nm
1
2
3
4
5
6
2 3 4 5 6 7
Particle size (nm)
Particle size (nm)
(c)
(d)
3. Result and discussion
3.1 Textural properties of the catalysts
The specific surface area, pore diameter and pore volume of the
ceria-based catalysts and support are summarized in Table 1, with
the N2 adsorption isotherms and pore size distribution displayed in
Fig. 1. All catalysts exhibited a similar N2 adsorption isotherm, with
a BJH pore size distribution similar to the SiO2 support. An increase
in CeO2 loading decreased the surface area of the catalysts, and
when the CeO2 loading was relatively high (e.g., 30Ce/SiO2,
35Ce/SiO2, and 40Ce/SiO2), the surface area showed only a slight
reduction (337, 336, and 334 m2/g, respectively). According to the
BJH pore size distribution, the samples have pores of 5 ~ 20 nm, and
the SiO2 support has a pore diameter of 11.89 nm, which was
significantly larger than that of the ceria–based catalysts, especially
for 15Ce/SiO2, 20Ce/SiO2, 25Ce/SiO2 and 30Ce/SiO2, which had pore
diameters of 8.39, 8.38, 8.39 and 8.38 nm, respectively. An
apparent difference was observed when the CeO2 loading was
relatively higher, with 35Ce/SiO2 and 40Ce/SiO2 catalysts having
pore diameters of 10.26 and 11.82 nm, respectively, which were
much larger than the other catalysts but approximately the same as
40
40
30
20
10
0
30
20
10
0
50 nm
50 nm
6
>13
8 10 12 14
Particle size (nm)
6
12 18 24
>21
Particle size (nm)
Fig. 2 TEM images and particle distribution of (a)
15Ce/SiO2, (b) 25Ce/SiO2, (c) 35Ce/SiO2 and (d) 40Ce/SiO2
the SiO2 support. Therefore, this appears to correlate with the
content and morphology of CeO2 in the catalysts (presented as
fallows).
To elucidate the CeO2 content of the catalysts, the content was
first determined by ICP, which was found to be within agreement of
the theoretical value (Table 1). The catalysts’ morphologies were
revealed by TEM, and the images were provided in Fig. 2. Fig. 2(a)
shows that the CeO2 nanoparticles (black) with a diameter of 1 ~ 3
nm are well dispersed on the SiO2 support for 15Ce/SiO2.
Table 1. Catalysts characteristics
Sample
Surface area (m2/g) Pore size (nm) a Pore volume (cc/g) CeO2 content (wt%) b Acidity (μmol/g) c
SiO2
370
355
350
343
337
336
334
11.89
8.39
8.38
8.39
8.38
10.26
11.82
0.98
0.96
0.95
0.89
0.90
0.93
0.92
–
–
3.01
8.63
10.24
10.39
11.82
11.29
15Ce/SiO2
20Ce/SiO2
25Ce/SiO2
30Ce/SiO2
35Ce/SiO2
40Ce/SiO2
a: Diameter of pore from BET
b: ICP results (theoretical data)
c: from NH3–TPD
5.66 (5.16)
7.26 (7.34)
9.06 (9.01)
10.97 (10.63)
12.45 (12.18)
14.90 (13.68)
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around 2θ of 28.4°, 33.0°, 47.5°, 56.1°, 59.1°, 69.3°, 76.6°, 79.0°, and
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DOI: 10.1039/C9GC01481K
88.3°, assigned to the (111), (200), (220), (311), (222), (400), (331),
(420) and (422) reflections of the fluorite structure of CeO2
respectively, according to the standard PDF cards of CeO2 (JCPDS
PDF#65-2975).52 Obviously, the CeO2 (111) facet was predominant
in all the samples, and the wide–range peak located within 17 ~ 23°
was attributed to the presence of the amorphous SiO2 support.
Moreover, three distinct diffraction peaks of 28.4°, 33.0° and 47.5°,
corresponding to the (111), (200) and (220) reflections, were
consistent with the results of high resolution TEM analyses (Fig. 4).
These exhibited well crystalline and defective particles with an
interplanar spacing of approx. 0.319 (d111), 0.274 (d200) and 0.198
(d220) nm. Furthermore, the polyhedral shape of CeO2 particles was
also confirmed.
Fig. 3 XRD patterns of the ceria–based catalysts
XPS was conducted to further clarify the structure of ceria–
based catalysts to afford more interface insights (Fig. 5), with the
XPS spectra of CeO2 also illustrated for comparison. The full spectra
of the catalysts are displayed in Fig. S2. The O 1s spectra of CeO2
(Fig. 5(a)) was divided into three peaks and marked as OL (529.3 eV),
OV (530.5 eV) and OC (532.1 eV). The OL was related to the O2-
species in the lattice of CeO2, OV was associated with the O2- species
in the oxygen vacancies, and OC originated from the contribution of
the surface–chemisorbed oxygen species.53 Fig. 5(c) depicts the O
1s spectra of the ceria–based catalysts, with all of the samples
having one distinct peak at approx. 532.1 eV, compared to the O 1s
spectra of CeO2. This may result from the absorbed oxygen,
hydroxyl species and/or water species on the surface, as well as
large amounts of O species from the SiO2 support. In this respect, it
is difficult to detect and evaluate the oxygen vacancies in SiO2
supported catalysts. As displayed in Fig. 5(b), the deconvoluted
peaks of Ce 3d spectra are labeled as u''' (916.6 eV), u'' (907.5 eV),
Additionally, 25Ce/SiO2 exhibited CeO2 particles with a relatively
uniform size (3 ~ 6 nm) (Fig. 2(b)). Increasing of the CeO2 loading
resulted in the appearance of some large CeO2 clusters, which were
aggregated by smaller particles, such as in 35Ce/SiO2 (Fig. 2(c)). In
40Ce/SiO2, the CeO2 particles presented an agglomerate form and
non–uniform particle size in the range of 3 ~ 30 nm (Fig. 2(d)).
Therefore, it may be inferred that the loading of CeO2 particles
resulted in a decrease in the SiO2 support surface area, and the
sufficient dispersion and smaller size of CeO2 particles, acting as a
coating on the SiO2 surface, narrowed the pore size. Alternatively,
the agglomerate form of CeO2 clusters may embed in the pore,
which has little affluence on pore diameter.
The XRD patterns of the ceria–based catalysts, as presented in
Fig. 3, show the characteristic diffraction peaks of CeO2, which were
Fig. 4 High resolution TEM images of the ceria–based catalysts
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Fig. 7 TGA of spent catalysts
Fig. 5 XPS patterns: (a) O 1s of CeO2, (b) Ce 3d of CeO2, (c)
O 1s of catalysts, (d) Ce 3d of catalysts.
and the slight smaller band at approx. 465 cm-1 observed in
35Ce/SiO2, when compared with 40Ce/SiO2 (Fig. 6(b)). This may
result from more of the agglomerate form of CeO2 clusters in
40Ce/SiO2 (as TEM suggested). Unfortunately, these two bands
were minimally detected in samples with a relatively lower CeO2
loading (e.g., 15Ce/SiO2, 20Ce/SiO2, 25Ce/SiO2), which can be
ascribed to the presence of a large amount of SiO2. Based on the
results from XPS and Raman, it can be concluded that the Ce3+/Ce4+
species related to the oxygen vacancies exist in the ceria–based
catalysts with various concentrations.
u' (902.2 eV), u (900.5 eV), u0 (899.3), v''' (898.3 eV), v'' (889.0 eV),
v' (884.3 eV), v (882.5 eV), and v0 (880.2 eV), where the v and u
represent the spin–orbit coupling of Ce 3d5/2 and Ce 3d3/2
,
respectively.54, 55 Generally, the peaks labeled as v, v'', v''', u, u'' and
u''' are attributed to Ce4+ species, and peaks labeled as v0, v', u0 and
u' are assigned to Ce3+ species. Therefore, both Ce3+ and Ce4+
species were identified to coexist in CeO2, with Fig. 5(d) implying
that the two Ce species also exist in the ceria-based catalysts. The
presence of Ce3+ species was always are associated with oxygen
vacancies,56 and which was further manifested by the Raman
results as illustrated in Fig. 6.
3.2 Production of the functionalized 1,3−diene
The functionalized 1,3−diene was produced from furfural and
acetone using an aldol condensation with various acid or base
catalysts. In the present work, NaOH was employed as the catalyst
to obtain a complete conversion of furfural and high selectivity to
the adduct product FA, FA was then converted into FAH in the
presence of NaBH4, which can have an excellent yield of the
Raman spectra of the ceria–based catalysts are shown in Fig.
6(a). A characteristic peak at approx. 465 cm−1, which was derived
from the symmetrical stretching vibration of Ce−O−Ce (F2g mode),
was observed in 35Ce/SiO2 and 40Ce/SiO2. The appearance of a
peak at approx. 601 cm−1 confirmed the existence of the oxygen
defect in both of the samples, which was ascribed to the second–
order transverse acoustic (2 TA) mode and linked to oxygen
vacancies in the CeO2 lattice.57-59 In our system, it was difficult to
obtain a precise value for the ratio of peak areas at approx. 601 and
465 cm−1 (A601/A465), which can be employed to quantitatively
calculate the concentration of oxygen vacancies. Whereas, it can be
inferred that 35Ce/SiO2 may have more oxygen vacancies than
40Ce/SiO2 due to the relatively stronger band at approx. 601 cm-1
unsaturated
alcohol
produced
from
the
unsaturated
aldehydes/ketones under mild conditions. Finally, the FAH solution
was fed through a fixed–bed reactor filled with ceria−based
catalysts, and the results are shown in Table 2. Additionally, the
catalytic results with some acid catalysts are summarized in Table
S1.
Compared with acid catalysts (e.g., PW/SiO2, Ag/SiO2), all
ceria−based catalysts displayed a high conversion of FAH. The effect
Fig. 8 NH3−TPD curves of the ceria−based catalysts (a) and
water−treated catalysts (b).
Fig. 6 Raman spectra of the ceria catalysts (a), and
magnified spectra of the 15Ce/SiO2 and 35Ce/SiO2 (b).
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in which an increase in CeO2 loading could decrease the carbon
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content. This was in line with the proven reDsuOltI: t1h0a.1t03th9e/Cc9eGrCia01h4a8s1Ka
carbon–resistance property.61 Furthermore, it also implied that the
Table 2. The dehydration of FAH over the ceria−based catalysts
Catalyst
Temperatur
e
(°C)
Conversion Selectivity Yield main byproducts were converted into carbon deposition.
(%)
(%)
(%)
Additionally, this indicated the definite relationship between the
concentration of oxygen vacancies (Ce3+/Ce4+ species) and
selectivity toward desired product, namely, more oxygen vacancies
accounting for efficiently the dehydration of FAH to F–diene.
15Ce/SiO2 300
20Ce/SiO2 300
25Ce/SiO2 300
30Ce/SiO2 300
35Ce/SiO2 300
40Ce/SiO2 300
35Ce/SiO2 275
35Ce/SiO2 325
99
99
99
100
100
100
99
50
61
77
84
92
89
54
88
50
60
76
84
92
89
53
88
3.3 Study of the catalytic performance
The oxygen vacancies of ceria are associated with its acid/base and
redox properties. It has been suggested that the oxygen vacancies,
together with the exposed Ce4+−O2− species in ceria−based
catalysts, displayed Lewis acid/base properties for many
reactions,62-66 with some of these studies demonstrating that the
catalytic performance relied on the concentration of the acid−base
sites or oxygen in ceria-based catalysts. In this work, catalyst acidity
was examined by NH3−TPD, as presented in Fig. 8(a). A weak peak,
located in the wide temperature range of 100 ~ 400 °C, was
observed for all the samples, which corresponded to the desorbed
NH3 linked to the acidity of the catalysts. The intensity of these
peaks displayed a slight increase with increasing CeO2 loading, with
40Ce/SiO2 showing a smaller peak when compared to 35Ce/SiO2,
which can be attributed to its agglomerate form of CeO2 clusters.
Raman and TEM suggested that the agglomerate of CeO2 decreased
the oxygen vacancies. The amount of acid sites of the ceria−based
catalysts is summarized in Table 1, with the samples all showing a
much smaller amount of acid sites.
100
Catalyst weight: 0.40g; N2 flow rate: 50 mL/min; the result
data are averaged within 1 h.
of reaction temperature (275, 300, and 325 °C) on the yield of F–
diene was studied. The yield of F–diene at 275 °C is much lower
than that at 300 and 325 °C, which can ascribe to a lower reaction
rate at a relative lower temperature. Typically, the dehydration of
FA was accompanied by other endothermic processes (gasification
of FAH, F–diene and THF), especially for FAH and F–diene having
high boiling point (Supporting materials). However, high
temperature favors the transformation of diene into carbon
deposition.60 Therefore, the reaction temperature in present work
is optimized at 300 °C. It can be presumed that the pore size of the
catalysts (5 ~ 20 nm) has littler influence on the catalytic procedure
(e.g., diffusion) because of it pretty larger that the reactant and
product molecules, in this respect, the CeO2 loading and its nature
played a crucial role in the dehydration of FAH. Thus, according to
the Raman results, the catalysts can be divided into two groups:
one group (G1) with a relatively high CeO2 loading (e.g., 35Ce/SiO2,
40Ce/SiO2) which exhibited significant Raman bands, and the other
group (G2), which included 15Ce/SiO2, 20Ce/SiO2, 25Ce/SiO2 and
30Ce/SiO2, that have no distinct Raman peaks. The yield of the
desired product over G1 catalysts was significantly higher than that
over G2 catalysts, with 35Ce/SiO2 having a yield as high as 92%,
which coincided with the Raman results. Notably, the yield over
40Ce/SiO2 (89%) was slight lower than that over 35Ce/SiO2, which
can be ascribed to 35Ce/SiO2 may have more oxygen vacancies as
suggested by Raman. No distinct Raman peaks were observed for
G2 catalysts, which resulted from the large amounts of SiO2, the
oxygen vacancies and Ce3+/Ce4+ species associated with the CeO2
content. In addition, based on the XPS results, it may be inferred
that a decrease in the oxygen vacancies and Ce3+/Ce4+ species
occurred in G2 catalysts with reduced CeO2 loading (ICP result),
which presented the order of the desired product yield (300 °C):
30Ce/SiO2 > 25Ce/SiO2 > 20Ce/SiO2 > 15Ce/SiO2. Therefore, the
catalytic performance strongly depended on the oxygen vacancies
and Ce3+/Ce4+ species in the catalysts.
The dehydration features with the production of water and
the interaction between H2O and Ce species have attracted great
interest because of the importance of understanding the catalytic
performance. Herein, the catalysts were treated in a flow of water
gas at 300 °C for 30 min, followed by NH3−TPD analysis (Fig. 8(b)).
The water−treatment catalysts showed little change in acidity when
compared with the corresponding counterparts. Moreover, some of
the solid acid catalysts (e.g., HPW/SiO2, P/SiO2, and PMo/SiO2)
displayed a low selectivity towards the desired product (Table S1).
Interestingly, all ceria–based catalysts showed high activity
(nearly complete conversion), with only a small amount of liquid
byproducts detected (as shown in Fig. S3), while the yield of the
desired product varied. To elucidate this, the spent catalysts were
collected and examined by TGA (Fig. 7). The spent G1 catalysts had
a much smaller carbon deposition compared to spent G2 catalysts,
Fig. 9 H2−TPR profiles of 15Ce/SiO2, 25Ce/SiO2 and
35Ce/SiO2
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Scheme 3. A probable reaction pathway for dehydration of FAH over CeO2
Therefore, in our system, the dehydration of FAH may be related to
the redox property (C3+⇄ C4+) of the ceria−based catalysts.
4. Conclusions
A renewable functionalized 1,3–diene containing a furan group (F–
Diene) was successfully produced from furfural and acetone. After
the selective hydrogenation of the C=O group, the aldol adduct
(FAH) displayed a nearly complete conversion and had excellent
selectivity (as high as 92%) towards the target product over the
ceria–based catalysts in a fixed–bed reactor. In addition, the ceria–
based catalysts exhibited an adequate carbon resistance. XPS,
Raman and H2–TPR analysis demonstrated that the oxygen
vacancies on ceria were primarily associated with the redox
property of the Ce3+/Ce4+ species, which corresponded to the
To estimate the redox properties of the ceria catalysts, H2−TPR
analysis was carried out (Fig. 9). Typically, the peak in the
temperature range of 400 ~ 600 °C was related to the reduction of
surface ceria, where probably involved the removal of surface
capping oxygen (O2− or O− anions).61, 67, 68 In the present work, the
H2 consumption peak corresponding to the reduction of surface
oxygen species was observed at 518 °C for 15Ce/SiO2 (black line)
and 501 °C for 35Ce/SiO2 (red line), and a shoulder peak developed
at approx. 380 °C, which were all in agreement with previous
catalytic performance.
A plausible reaction pathway for the
67
reports.61,
Specifically, compared with 25Ce/SiO2 (blue line),
dehydration of FAH over ceria–based catalysts was proposed
involving a radical mechanism, which suggested the importance of
the redox cycle of Ce4+⇄Ce3+ for the dehydration of FAH to F–Diene.
35Ce/SiO2 exhibited an apparent and large peak in the temperature
range of 450 ~ 550 °C. Thus, these findings indicated that 35Ce/SiO2
displayed a higher activity and more active sites when compared
with 25Ce/SiO2 and 15Ce/SiO2, which was supported by the results
of Raman, XPS and catalytic performance. Therefore, the
dehydration of FAH was dependently correlated with the redox
properties of the ceria–based catalysts (C3+⇄ C4+), which was similar
to the dehydration of unsaturated alcohols and diols reported by
Sato’s group.69-73
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Sato et al. suggested that three Ce cations together with an
oxygen–defect site (oxygen vacancy) on the CeO2 (111) surface are
the active center for the dehydration of diols or unsaturated
alcohols, which undergo a radical mechanism. Based on these
We acknowledge National Natural Science Foundation of China
(No. 51873203), Key Projects of Jilin Province Science and
Technology Development Plan (No. 20180201087GX), Special
Projects of Technology Transfer and Cooperation between
China and Arab states (NO. 2017EZ07), Special Fund Supported
Project for High−tech Industrialization of Science and
Technology Cooperation between Jilin Province and the
Chinese Academy of Sciences (2019SYHZ0003).
findings and the dehydration of secondary alcohols to 1-alkenes
75
over ceria,74,
we proposed a probable pathway for the
dehydration of FAH over ceria–based catalysts, which is similar to
the dehydration of an unsaturated alcohol to 1,3–diene (Scheme 3).
Here, three Ce atoms formed a triangle with an oxygen vacancy on
CeO2 (111). Initially, one H atom from the terminal methyl group in
FAH adsorbed on an oxygen defect site and was eliminated to
produce the radical species. Next, the H radical was oxidized to the
proton (H+) after donating one electron to reduce Ce4+ to Ce3+. The
reduced ceria then enable the cleavage of the C–OH bond to form
radicals.76 Followed by the elimination of an OH radical, the two
radicals on the intermediate combined to afford the target product.
Meanwhile, the OH radical abstracted one electron from Ce3+ to
yield Ce4+ and the OH radical itself was reduced to OH–, which
combined with the H+ to produce H2O. Therefore, the redox cycle of
Ce4+–Ce3+ on the surface plays a key role in the dehydration of FAH.
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