Ethanol/acetaldehyde conversion into butadiene over sol-gel ZrO2-SiO2 catalysts doped with ZnO

Article abstract of DOI:10.1039/c6ra25139k

ZnO promoted ZrO₂-SiO₂ catalysts synthesized by a sol-gel method were investigated in the two-step ethanol transforming to 1,3-butadiene process. The influence of promoters and the preparation method of the catalysts on the catalytic performance were studied in detail and the reaction conditions were optimized. The as-prepared catalysts were characterized by BET, X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Fourier Transform Infrared Resonance (FT-IR), X-ray photoelectron spectroscopy (XPS), FT-IR spectroscopy of adsorbed pyridine (Py-IR) and temperature-programmed desorption of NH₃ (NH₃-TPD). ZnO added ZrO₂-SiO₂ catalysts show the best catalytic activity and the catalysts prepared by the hybrid sol-gel method were superior to those prepared by the sol-gel coupled with impregnation method. The addition of promoters in the ZrO₂-SiO₂ system decreased the total acidity and lowered the selectivity to dehydration products. The best performance with ethanol/acetaldehyde conversion of 40.7% and 1,3-butadiene selectivity of 83.3% was reached at 310 °C, ethanol/acetaldehyde mole ratio of 3.5 and WHSV of 1.4 h^-1 using a 0.5 wt% ZnO doped ZrO₂-SiO₂ catalyst.

Full text of DOI:10.1039/c6ra25139k

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PAPER
Ethanol/acetaldehyde conversion into butadiene
over sol–gel ZrO2–SiO2 catalysts doped with ZnO
Cite this: RSC Adv., 2017, 7, 7140
ab
Yuchao Xu,ab Zongzhang Liu,ab Zheng Hanab and Minhua Zhang
*
ZnO promoted ZrO2–SiO2 catalysts synthesized by a sol–gel method were investigated in the two-step
ethanol transforming to 1,3-butadiene process. The influence of promoters and the preparation method
of the catalysts on the catalytic performance were studied in detail and the reaction conditions were
optimized. The as-prepared catalysts were characterized by BET, X-ray diffraction (XRD), Scanning
Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Fourier Transform Infrared
Resonance (FT-IR), X-ray photoelectron spectroscopy (XPS), FT-IR spectroscopy of adsorbed pyridine
(Py-IR) and temperature-programmed desorption of NH3 (NH3-TPD). ZnO added ZrO2–SiO2 catalysts
show the best catalytic activity and the catalysts prepared by the hybrid sol–gel method were superior to
those prepared by the sol–gel coupled with impregnation method. The addition of promoters in the
ZrO2–SiO2 system decreased the total acidity and lowered the selectivity to dehydration products. The
best performance with ethanol/acetaldehyde conversion of 40.7% and 1,3-butadiene selectivity of 83.3%
was reached at 310 ꢀC, ethanol/acetaldehyde mole ratio of 3.5 and WHSV of 1.4 hꢁ1 using a 0.5 wt%
ZnO doped ZrO2–SiO2 catalyst.
Received 12th October 2016
Accepted 5th December 2016
DOI: 10.1039/c6ra25139k
www.rsc.org/advances
Ever since the commercialization of ethanol into BD,
researchers have been studying the mechanism of this process
1. Introduction
for decades. Up to now, the mechanism is still in debate among
studies, and a generally accepted reaction network was used in
dozens of studies as reported in ref. 3–6. It's also believed that
the mechanisms for one-step process and two-step process
show no difference.7 The generally accepted mechanism
contains: (1) ethanol dehydrogenation into acetaldehyde; (2)
aldol condensation of acetaldehyde into acetaldol; (3) dehy-
dration of acetaldol into crotonaldehyde; (4) Meerwein–Ponn-
dorf–Verley reaction of crotonaldehyde by ethanol; and (5)
dehydration of crotyl alcohol into BD.
As reported in former literatures,8,9 MgO–SiO2 and ZrO2–SiO2
were regarded as the two most promising catalytic system in
ethanol to BD process, each for one-step and two-step, respec-
tively. Dozens of studies were performed on MgO–SiO2 catalytic
system.5,10–13 While, only a few studies were conducted using
ZrO2–SiO2 or ZrO2–SiO2 based catalysts in ethanol to BD process
before 2010. Aerwards, systematic studies have been con-
ducted by Vitaly L. Sushkevich et al. in order to choose the most
active component, accommodate acid–base properties, and
gure out the function of promoters. Many studies have been
conducted on ZrO2–SiO2 or metal oxides promoted ZrO2–SiO2
system in the past ve years, as reported in ref. 7, 14 and 15
Sushkevich et al.7 revealed the excellent catalytic capacity of Ag
promoted Zr-containing SiO2 molecular sieve (Zr-MCM-41 and
Zr-BEA) in the ethanol transforming to BD process. The best
performance was obtained with the ethanol conversion of 48%
and BD selectivity of 56% using 1 wt% Ag doped Zr-BEA catalyst.
1,3-Butadiene (BD) is one of the most important bulk chemicals
in the oil industry, and has been widely used in various elds of
synthesis of rubber elastomers and resins. Generally, the
ethanol to BD route is divided into two different parts, the one-
step process and the two-step process. The former was rst
introduced by Lebedev, a USSR chemist, in the 1920s. The latter
was a rened process by using an ethanol and acetaldehyde
mixture as the feed, and was rst commercialized by a US
company, the Carbide and Carbon Chemicals Corporation.
In recent years, the supply and the price of BD largely depend
on the supply of ethylene, one of bulk chemicals in oil industry.
With the improvement on the technology of exploiting syngas in
America and west Europe, there would be an shortage of the
supply of BD in the near future.1 Meanwhile, the technology of
bio-ethanol has been improved dramatically and used widely all
over the world. The bio-ethanol can be easily obtained not only
from crops such as corn and sugarcane but also from inedible
raw materials such as bre and lignin.2 In 2011, the output of
ethanol worldwide was over 100 billion litres, which showed
a fast growth. The production of high value-added products
from bio-ethanol has been attracting interest from all over the
world.
aKey Laboratory for Green Chemical Technology of Ministry of Education, R&D Center
for Petrochemical Technology, Tianjin University, Tianjin 300072, P. R. China. E-mail:
mhzhang@tju.edu.cn; Fax: +86-22-27401826; Tel: +86-22-27401826
bCollaborative Innovation Center of Chemical Science and Engineering, Tianjin, China
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Jian et al.15 reported a Zr/MCF catalyst of high performance, catalyst was dried at 383 K for 6 h and calcined at 923 K for 6 h
which showed the highest 1,3-butadiene yield and selectivity at with a heating rate of 5 ꢀC minꢁ1
the WHSV of 3.7 hꢁ1 and 1.5 hꢁ1, respectively. The former
.
studies conducted by other researchers showed a possibility of
industrial application of Zr–Si catalytic system in ethanol
conversion to BD process.
2.3 Catalyst characterization
N2 adsorption–desorption isotherms (BET) were measured at
ꢁ196 ꢀC using a Micromeritics Tristar 3000 surface area and
pore size analyser. Before measurement, all samples were pre-
treated at 300 ꢀC for 6 h. The Brunauer–Emmett–Teller (BET)
method was used here to calculate the surface areas.
In our previous work,16 we reported the application of sol–gel
ZrO2–SiO2 catalysts and gained a relatively high performance
compared with the results in other references. Meanwhile, the
selectivity to dehydration products remains high and the cata-
lysts show a relatively poor durability on the long-time stability
test. As reported,5,10,17–19 the addition of promoters into MgO–
SiO2 system would be benecial for elevating BD selectivity as
well as prolonging catalyst life.
Powder X-ray diffraction patterns (XRD) of the catalysts were
taken using a Rigaku D/Max 2500 type X-ray powder diffrac-
˚
tometer with Cu Ka radiation at a wavelength of 1.5456 A with
the scan range of 5–90ꢀ at the scan rate of 5ꢀ sꢁ1
.
Scanning Electron Microscope (SEM) images were taken
using a Hitachi S-4800 with the resolution of 1 nm. All the
samples were ground thoroughly and treated with conductive
coating before measurement, TEM images were obtained using
a Tacnai G2 F20 electron microscope operating at 200 kV. All the
samples were suspended in absolute ethanol with an ultrasonic
dispersion for 0.5 h. The suspension was doped on a copper grid
coated with amorphous carbon lm before measurement.
The infrared spectra were obtained using a Nicolet 6700
spectrometer with the scanning range from 4000 to 400 cmꢁ1
and at a resolution of 4 cmꢁ1. All the catalysts were prepared
with the addition of KBr at the weight proportion of 99%.
X-ray photoelectron spectroscopy (XPS) spectra were taken
on a PerkinElmer PHI-1600 spectrometer using an Mg Ka X-ray
radiation source at the pressure of 3.0 ꢂ 10ꢁ7 Mbar. The
collected binding-energy values were referenced to the C 1s line
at 284.6 eV.
Thus, in this study, metal or metal oxide promoted ZrO2–
SiO2 catalysts prepared via sol–gel method were characterized
and their catalytic performances for the formation of BD from
bioethanol and acetaldehyde were investigated. The effects of
addition of promoters and preparation method of catalysts on
the structure and acid properties were studied. The mass
content of the best promoter was also optimized. The experi-
mental conditions, i.e., the reaction temperature, the mole ratio
of ethanol to acetaldehyde and the WHSV were optimized. The
long-time durability study was also conducted as to gure out
the stability of the catalysts. Different characterization methods
were used to explore the relevance of the type of promoters and
acid properties to the catalytic performances.
2. Experimental
2.1 Chemicals and materials
The acidic properties were studied using FT-IR spectroscopy
of adsorbed pyridine. IR spectra were taken on a Nicolet 6700 FT-
IR spectrometer with the optical resolution of 4 cmꢁ1. Before
measurement, the samples were pretreated at 200 ꢀC under
a vacuum of 10ꢁ3 Pa for 2 h. Adsorption of pyridine was carried
out at 110 ꢀC, difference spectra were obtained by subtraction of
the blank spectra of the catalyst samples at 110 ꢀC from the
spectra of the samples with adsorbate at correlated temperatures.
The quantity and strength of acidity were determined by
NH3-TPD method using a Micromeritic Autochem II 2920. All
samples were pre-treated at 500 ꢀC for two hours under a helium
(99.999%) ow rate of 50 mL minꢁ1. Aer being cooled to 70 ꢀC,
the sample was saturated with ammonia (argon 99%, ammonia
1%) at a ow rate of 20 mL minꢁ1 for 50 min and subsequently
purged with He (30 mL minꢁ1) for 1 h to remove the physically
adsorbed NH3. Temperature programming process was then
conducted at the range frꢁo1m 70 ꢀC to 400 ꢀC with a temperature
ramp rate of 15 ꢀC min in helium at a ow rate of 50 mL
minꢁ1. The signal of desorbed NH3 was recorded by a Thermal
Conductivity Detector (TCD).
Zirconium oxynitrate (AR), acetaldehyde (97%), magnesium
nitrate hexahydrate (AR) were obtained from Aladdin Industrial
Corporation, Shanghai, China. Tetraethyl orthosilicate (AR),
zinc nitrate hexahydrate (AR), manganese nitrate (50% solution,
AR), silver nitrate (AR) were purchased from Tianjin Guangfu
Chemical Reagent Co., Ltd, Tianjin, China. Nitric acid (AR) was
purchased from Beijing Chemical Works. Dry ethanol (AR) and
pyridine were purchased from Tianjin Jiangtian Chemical
Technology Co., LTD.
2.2 Catalyst preparation
All ZrO2/SiO2 catalysts were prepared by using sol–gel method as
described in ref. 16. The ZrO2 mass loading for all samples was
2%. Promoter ZnO was added by using two different methods:
hybrid sol–gel method (HSG) and sol–gel, and impregnation
method (SGI). The catalysts prepared by the former method were
referred to as Zr–Si-A and the latter referred to as Zr–Si-B. In
general, the HSG catalysts were synthesized through the same
process as described in our previous work.16 In particular, the
precursor of the dopant, Zn(NO3)2$6(H2O), with a mass content
of 0.5%, and ZrO(NO3)2 were added simultaneously into the
2.4 Catalyst evaluation
solvent before gelation process. The SGI catalysts were synthe- The catalytic conversion of ethanol and acetaldehyde mixture
sized by incipient wetness impregnation of ZrO2/SiO2 catalysts into BD was performed on a xed bed reactor. Before experi-
with an aqueous solution of zinc nitrate hexahydrate to attain ment, the samples were ground and sieved into 20–40 mesh.
a ZnO mass content of 0.5%. Aer impregnation process, the The mole ratio range of ethanol to acetaldehyde was 2.5–4.5.
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The WHSV varied from 0.2 to 2.2 hꢁ1. The reaction temperature SiO2 or ZrO2 were found for three samples, only a halo at around
range was 290–340 ꢀC. The dry gas was analysed online by 2q ¼ 23.0ꢀ was obvious in all patterns, which can be explained
Agilent 7890A gas chromatograph using a 30 m HP-PLOT-Q as a well dispersed state of ZrO2 on SiO2 surface.21,22 As to ZnO
column. The products contain 1,3-butadiene (BD), ethylene promoted Zr–Si catalysts, a typical halo was also shown at 2q ¼
(EL), propylene (PL), butylene (BL), ethyl acetate (EA), and 23.0ꢀ. At lower ZnO loadings, no typical peaks of ZnO were
diethyl ether (DE).
recorded, which means a well dispersed state of ZnO species on
The conversion of ethanol and acetaldehyde mixture, and SiO2 surface.23,24 When ZnO loading increased up to 4.0 wt%,
selectivity towards the products were calculated as follows:20
some typical peaks at 2q ¼ 31.767ꢀ, 34.421ꢀ, 36.252ꢀ, 56.592ꢀ,
62.856ꢀ and 67.945ꢀ were observed, which were assigned to
(100), (002), (101), (110), (103), (112) crystal face of ZnO (JCPDS
36-1451), respectively.
Total conversion ¼
ðtotal C moles ꢁ ðCmole
ET þ Cmole
AAÞmolesÞ
unreacted
unreacted
total C moles
SEM and TEM characterizations were conducted to conrm
the information resulted from XRD patterns. Comparing the
SEM images in Fig. 2a and b, it's obvious that the particles size
in Fig. 2a was smaller and smoother than that in Fig. 2b, which
indicated a well-dispersed state of ZrO2 and ZnO on SiO2
through hybrid sol–gel method. TEM images in Fig. 2c and
d exhibited a better sight of the morphology of two types of
catalysts. In Fig. 2c, no observable ZrO2 or ZnO particles were
found at the scale of 200 nm, which elucidated a well dispersed
state of amorphous ZrO2 and ZnO on silica support. While in
Fig. 2d, the agglomeration of the particles can be seen at the
same scale of Fig. 2c. But no crystal ZrO2 or ZnO was found in
the map, implying that ZrO2 and ZnO were at amorphous state.
The results obtained from SEM and TEM images were in good
agreement with that of XRD patterns.
In order to verify the interaction between ZnO, ZrO2 and SiO2
phase, FT-IR studies of SiO2, ZrO2/SiO2, Zn1.0–Zr–Si, Zn1.0–Zr–Si-
B were performed. In Fig. 3, some typical wavenumbers were
shown at 400–2200 cmꢁ1. In particular, wavenumbers at 1082
cmꢁ1 and 1223 cmꢁ1 were assigned to nas(Si–O–Si),25,26 wave-
numbers at 804 cmꢁ1 and 463 cmꢁ1 were assigned to ns(Si–O–
Si)26,27 and wavenumber at 1631 cmꢁ1 was assigned to phys-
isorbed water on silica phase.28
It has been noted in ref. 29 that typical adsorption of Si–OH
at 960 cmꢁ1 can be observable on silica surface at low calcina-
tion temperatures, while at high temperatures, it disappears.
This can be explained as the dehydration of Si–OH groups
forming Si–O–Si moieties. But in the present work, a different
condition was observed, even the samples were calcinated at
a relatively high temperature (650 ꢀC), and a similar band at 964
cmꢁ1 remained distinct for SiO2 sample (line a in Fig. 3).
For SiO2 supported metal oxides, the vibration peaks at 962–
970 cmꢁ1 band is assigned to the metals incorporated into the
framework of the mesoporous silica materials.30 When metals
are incorporated into silica phase, the intensity of this band
increases, which is generally considered as a proof of the
incorporation of hetero-atoms into the silica framework. In the
present study, the wavenumber, at around 964 cmꢁ1, can be
described as the formation of Zr–O–Si structure with the tran-
sition of Zr atoms into silica framework.21,31
ꢂ100
Selectivity ¼
C moleproducts
total C moles ꢁ ðCmole unreacted ET þ Cmole
ꢂ100
AAÞmoles
unreacted
3. Results and discussion
3.1 Catalyst characterization
3.1.1 Catalyst structure and composition. The physical
properties of as-prepared catalysts were listed in Table 1. The
addition of promoters in Zr–Si decreased SBET, average pore size
and pore volume to some extent. This can be explained by
blockage of SiO2 pores when promoters were added through
sol–gel process. The addition method of ZnO promoter had
a great impact on the physical properties of ZnO promoted Zr–Si
catalysts. As can be seen (line 6 and line 9), the SBET, average
pore size and pore volume in Zn1.0–Zr–Si-A were much higher
than those of Zn1.0–Zr–Si-B, which proved to be an highly
dispersed ZnO species in Zn1.0–Zr–Si-A sample compared with
an relatively larger ZnO particle size which in turn caused the
blockage of pores in Zn1.0–Zr–Si-B sample. Different ZnO load-
ings showed no obvious changes on the physical properties of
the catalysts (line 7 to line 12). Only a slight decrease was
observed on these parameters.
XRD was performed in order to study the difference of the
structure of two methods, as shown in Fig. 1. No typical peaks of
Table 1 Pore structural characteristics of the catalysts
SBET (m2 gꢁ1
)
V (cm3 gꢁ1
)
˚
d (A)
Line
Sample
1
2
3
4
5
6
7
8
Zr–Si
601
544
614
448
561
514
607
594
570
580
568
548
26.1
24.3
23.9
24.1
25.0
23.1
25.6
25.5
25.2
25.4
25.1
24.5
0.39
0.33
0.37
0.27
0.34
0.30
0.38
0.36
0.36
0.37
0.36
0.36
Cu1.0–Zr–Si-A
Ag1.0–Zr–Si-A
Mn1.0–Zr–Si-A
Mg1.0–Zr–Si-A
Zn1.0–Zr–Si-B
Zn0.25–Zr–Si-A
Zn0.5–Zr–Si-A
Zn1.0–Zr–Si-A
Zn2.0–Zr–Si-A
Zn4.0–Zr–Si-A
Zn8.0–Zr–Si-A
XPS was conducted in order to study the chemical states of
ZnO and ZrO2 on silica surface and the forms it combined
between ZnO, ZrO2 and SiO2. In the present work, ZnO was
added as the promoter. Meanwhile, as depicted in the XRD
patterns, no characteristic peaks of ZnO were detected, which
may indicate a strong interaction between ZnO and SiO2.
9
10
11
12
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Fig. 1 XRD patterns for the catalysts (1) catalyst samples prepared by different method (a) Zr–Si; (b) Zn1.0–Zr–Si-A; (c) Zn1.0–Zr–Si-B; (2) catalyst
samples with different ZnO contents.
Fig. 2 SEM image (5 mm) of (a) Zn1.0–Zr–Si-A, (b) Zn1.0–Zr–Si-B. TEM image of (c) Zn1.0–Zr–Si-A (50 nm), (d) Zn1.0–Zr–Si-B (200 nm).
Considering this point of view, XPS proles of Zn 2p were shown and ZrO2, which may be presented by Zn–O–Zr bonds, rather
in Fig. 4A. As for Zn1.0–Zr–Si-B, the binding energy of Zn 2p was than the interaction between ZnO and SiO2. The results indi-
1022.7 eV, which means ZnO was more likely presented as bulk cated the existence of a Zn–Zr synergistic interaction in these
ZnO32 (binding energy varies from 1021.8 eV to 1022.5 eV) on catalysts.
SiO2 surface. On the contrary, the binding energy of Zn 2p for
Generally, synergistic effect of the promoter and loading
Zn1.0–Zr–Si-A sample was signicantly higher than that of bulk components may affect binding energy in XPS results. As shown
ZnO. The shi of the binding energy of Zn 2p was reported in in Fig. 4B, the binding energy of Zr 3d for three samples was
ZnO–SiO2 catalysts.23,33 Upon ZnO loading, the binding energy higher than that of bulk ZrO2. According to Sushkevich, Vitaly L.
of Zn 2p in ZnO–SiO2 catalysts shied to lower values compared et al.,7 this phenomenon can be explained by the formation of
with that of bulk ZnO, because the valence electron density of Zr–O–Si bonds upon ZrO2 loading through sol–gel process,
Zn in the Si–O–Zn bond is lower than that in the Zn–O–Zn which conrmed the result of FT-IR study. Interestingly, when
bond.23 Meanwhile, in the present study, this shi towards comparing the result of curve (a) and curve (b) or curve (b) and
higher values was due to the strong interaction between ZnO curve (c), one may nd that the binding energy of Zn1.0–Zr–Si-B
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As for the strength of acidity, temperatures vary from 119 ꢀC
ꢀ
to 136 C, Mg1.0–Zr–Si-A shows the highest strength of acidity
with a temperature of peak at 136 ꢀC. No change of Zn1.0–Zr–Si-A
was found compared with blank catalyst Zr–Si in terms of
temperature of peaks. The preparation method of catalyst oen
shows impact on the acid–base properties of the catalyst, i.e.,
quantity and strength of acidity. A comparison of quantity and
strength of acidity between Zn1.0–Zr–Si-A and Zn1.0–Zr–Si-B (line
9 and line 6, Table 2) was shown. No big difference was recorded
on the quantity of acidity, while the temperature of NH3
desorption peaks for Zn1.0–Zr–Si-B sample was higher than that
for Zn1.0–Zr–Si-A sample.
Upon increasing ZnO loadings, quantity of acidity decreased
lineally from 0.12 mmol gꢁ1 to 0.03 mmol gꢁ1 (line 7–12), which
means the majority of acidity was suppressed by ZnO. While as
to the strength of acidity, no big changes were found in different
ZnO loading samples. However, as we can see, the temperature
of NH3 desorption peaks were shied to higher values, implying
a relatively higher strength of acidity resulted from large
amount of ZnO (bulk crystal) in the ZrO2–SiO2 system, as
revealed in XRD patterns and TEM images.
In conclude, the importance of acid properties has been
revealed in former literatures,17,18 but few systematic study has
been conducted as to the relevance and weight between the
quantity and strength of acidity in ZrO2–SiO2 catalytic system.
Combining the results of the NH3-TPD with catalytic perfor-
mances of catalysts in this work, we predicted boldly that the
strength of acidity was more important than quantity of acidity
as to improving catalytic performance. A moderate strength of
acidity should be essential for high catalytic performance of the
catalyst.
In this work, an ordered study was performed by Py-IR to
analyze the acid type of acid site of as-prepared catalysts. As
reported,35 the adsorption at 1450 cmꢁ1 and 1608 cmꢁ1 were
typical bands of pyridine adsorbed on Lewis acid sites, 1490
cmꢁ1 was assigned to pyridine adsorbed on both Lewis and
Bronsted acid sites. For all promoter added catalysts, two bands
at ca. 1590 cmꢁ1 and 1445 cmꢁ1 were observed, which showed
a little gap from the typical bands of Lewis acid sites. Flanigen
et al.36 described this shis to lower wavenumbers as the change
Fig. 3 IR spectra of the samples: (a) SiO2; (b) Zr–Si; (c) Zn1.0–Zr–Si-B;
(d) Zn1.0–Zr–Si-A.
was similar to that of the blank sample Zr–Si. Meanwhile,
binding energy of Zn1.0–Zr–Si-A was much higher than that of
the counterparts, which also implied the interaction between
ZnO and ZrO2 in sol–gel process. As a result, binding energies of
both ZnO and ZrO2 were shied to higher values.
3.1.2 Acidic properties of the catalysts. Quantity of acid for
fresh samples was detected by thermal programmed desorption
(TPD) using NH3 as probe molecule. As shown in Fig. 5, NH3
desorption signal was recorded at the range from 100 ꢀC to
200 ꢀC. Only a broad peak was found for all samples, this peak
can be assigned to NH3 adsorbed on weak acid sites.34 The
relative quantity of acidity was calculated as shown in Table 2.
Interestingly, the blank catalyst Zr–Si showed the highest
quantity of acidity. With different promoters loaded on Zr–Si
catalyst, the quantity of acidity dropped to some extent, which
may result from the coverage of Lewis acid sites formed on the
inter-surface of ZrO2 and SiO2 by promoters through prepara-
tion process. Baylon et al.35 also reported a similar phenomenon
in Zn–Zr catalytic system used in ethanol conversion to BD
process.
Fig. 4 (1) XPS profiles of Zn 2p (a) Zn1.0–Zr–Si-B; (b) Zn1.0–Zr–Si-A; (2) XPS profiles of Zr 3d for: (a) Zr–Si; (b) Zn1.0–Zr–Si-B; (c) Zn1.0–Zr–Si-A.
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Fig. 5 NH3-TPD profiles of (1) Zn–Zr–Si catalysts prepared by two different methods: (a) Zn1.0–Zr–Si-A; (b) Zn1.0–Zr–Si-B; (2) different mental
oxide promoted Zr–Si catalysts; (3) Zn–Zr–Si-A catalyst samples with different ZnO contents.
Table 2 The acid properties of the catalysts
when ZnO loading was higher than 4 wt%, which means new
acid sites were formed on bulk crystal ZnO surface,37,38 as ana-
Quantity of
acidity
Temperature of
NH3 desorption
peaks (ꢀC)
lysed by XRD patterns. Interestingly, upon increasing ZnO
loadings the band of Lewis acid sites at ca. 1590 cmꢁ1 was
shied toward higher wavenumbers. This was in agreement
with the study performed by Baylon et al.35 They have
explained this phenomenon as an increase of Lewis acid
strength, which was in good agreement NH3-TPD results in the
present work.
Line
Sample
(mmol gꢁ1
)
1
2
3
4
5
6
7
8
Zr–Si
0.18
0.13
0.12
0.07
0.05
0.08
0.12
0.08
0.09
0.07
0.06
0.03
119
128
136
125
120
127
119
119
119
121
124
120
Mn1.0–Zr–Si-A
Mg1.0–Zr–Si-A
Cu1.0–Zr–Si-A
Ag1.0–Zr–Si-A
Zn1.0–Zr–Si-B
Zn0.25–Zr–Si-A
Zn0.5–Zr–Si-A
Zn1.0–Zr–Si-A
Zn2.0–Zr–Si-A
Zn4.0–Zr–Si-A
Zn8.0–Zr–Si-A
3.2 Catalyst evaluation in ethanol conversion into butadiene
3.2.1 Catalytic activity. Catalytic evaluation study was per-
formed under the condition of 320 ꢀC with an ethanol/
acetaldehyde mole ratio of 3.5, WHSV of 1.8 hꢁ1. As shown in
Table 3, the main product and by-product distribution
remained unchanged. Ethylene, butadiene, propylene,
butylene, diethyl ether, and butanol were detected, which were
simplied as EL, BD, PL, BL, DE, BU, and EA, respectively.
As shown in Table 3, among the testing results for different
metal oxide promoted catalysts (line 2–6), the highest BD
selectivity, 83.0%, was achieved using Zn1.0–Zr–Si-A catalyst.
Moreover, the conversion of Zn1.0–Zr–Si-A was also the best. The
addition of ZnO can boost the dehydrogenation ability in
ethanol conversion to acetaldehyde process as well as MPV
reaction of crotonaldehyde. Combining the analysis results
above, the addition of ZnO into Zr–Si catalytic system did
decrease the quantity of acidity to some extent, though, no
changes were observed as to the acid strength and acid type. It
can be inferred that moderate acid type and strength was crit-
ical to the process of ethanol conversion to BD. The addition of
9
10
11
12
of the diameter or positive charge of cations forming acid sites.
As can be seen in Fig. 6a, acid types for different promoter
added samples were mostly presented as Lewis acid sites. No
distinct Bronsted acid, which shown typical adsorption peak at
around 1540 cmꢁ1, was observable in the spectra of Zr–Si, Zn1.0
–
Zr–Si-A, Cu1.0–Zr–Si-A and Ag1.0–Zr–Si-A samples. Meanwhile,
the band at 1490 cmꢁ1, was distinguishable in the spectra of Mn
and Mg promoted samples. For ZnO promoted samples, as
shown in Fig. 6b, the bands at ca. 1445 cmꢁ1 and 1590 cmꢁ1
were evident. No obvious bands of Bronsted acid sites were
recorded at low ZnO loadings. Meanwhile, some observable
bands at ca. 1490 cmꢁ1, 1565 cmꢁ1 and 1580 cmꢁ1 were found
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Fig. 6 (1) Py-IR spectra of Py-IR spectra of different promoters doped ZrO2/SiO2 catalysts and (2) Zn–Zr–Si-A catalyst samples with different
ZnO contents.
Mn and Mg into ZrO2–SiO2 system didn't inhibit the formation
As known to all, the method of promoter addition also had
of dehydration products such as EL and DE effectively (line 4 a great impact on the conversion of ethanol and acetaldehyde
and 5). Combining the result of NH3-TPD and Py-IR with the and BD selectivity. Obviously, the conversion of ZnO added
catalytic performance of due catalysts, one can predict that the ZrO2–SiO2 fresh sample through SGI process (line 7) dropped
increase of the quantity of acidity and the formation of Bronsted dramatically compared with ZrO2–SiO2 counterpart (line 1).
acid sites for Mn and Mg added samples may cause the decrease This can be explained by the formation of bulk crystal of ZnO in
on the activity. Generally, Cu and Ag were regarded as dehy- SGI samples, which caused the blockage of pores, as analysed by
drogenation promoters, as reported in ref. 17, 19 and 39 which BET, XRD, TEM and XPS. Whereas highly dispersed ZnO species
in turn, boosted BD selectivity. However, in this study, Cu and obtained by HSG method on ZrO2–SiO2 surface can effectively
Ag showed inferior catalytic performances to that of other decrease the quantity of acidity of Zr–Si without much differ-
promoters, the BD selectivity was even lower than that of blank ence on the strength of acid sites, which hindered the dehy-
ZrO2–SiO2. The addition of Cu introduces redox-active sites dration property of Zr–Si catalyst and boosted BD selectivity by
which can boost dehydrogenation process of ethanol into the meantime (line 6).
acetaldehyde and inhibit the dehydration of ethanol. By the
It's also eye-catching that upon different promoters loading
meantime, CuO can also poison acid site of the catalyst, which on Zr–Si system, the conversion of promoted samples dropped
lowers the ethanol/acetaldehyde conversion and the selectivity to some extent compared with that of Zr–Si counterpart. This
to dehydration products (as shown in line 2).19 The dehydro- can be explained as the decrease of acidity of those promoted
genation ability of Ag was inferior to that of CuO, and more samples. A relatively low acidity of the catalyst means some
dehydration products (EL and DE) were formed at the expense amount of active sites of Zr–Si system was reduced or covered by
of lower BD selectivity. Therefore, as analysed above, ZnO was those metal oxides.
chosen as the best promoter in this study.
The catalytic data with different ZnO loading was shown in
Table 3 (line 2, 3–8). When ZnO loading increased from 0.25
Table 3 Activity evaluation results of the catalysts at 320 ꢀC, WHSV of 1.8 hꢁ1 and ethanol/acetaldehyde mole ratio of 3.5 for 10 h
Selectivity (C, mol%)
Line
Sample
Con (%)
EL
PL
BL
BD
DE
EA
BU
C6+a
1
2
3
4
5
6
7
8
Zr–Si
42.2
22.3
15.9
27.9
27.0
33.2
19.1
34.3
36.8
30.4
35.0
36.1
13.5
3.8
10.2
5.9
11.1
6.3
4.8
5.5
5.1
5.7
9.8
1.5
0
0.8
66.0
38.8
70.0
76.8
75.3
83.0
72.2
82.1
83.5
80.9
78.1
74.7
12.6
7.4
10.9
9.0
8.1
3.6
13.3
2.0
3.7
5.0
5.9
6.9
1.6
6.5
1.6
1.1
0.8
1.4
1.8
1.3
1.1
1.9
1.4
1.6
0.8
3.6
0
0.3
0
0.6
1.1
1.5
0.2
0.8
0.3
0.4
3.0
19.6
5.8
6.2
2.6
3.1
5.5
5.6
4.8
4.3
2.7
3.5
Cu1.0–Zr–Si-A
Ag1.0–Zr–Si-A
Mn1.0–Zr–Si-A
Mg1.0–Zr–Si-A
Zn1.0–Zr–Si-A
Zn1.0–Zr–Si-B
Zn0.25–Zr–Si-A
Zn0.5–Zr–Si-A
Zn2.0–Zr–Si-A
Zn4.0–Zr–Si-A
Zn8.0–Zr–Si-A
20.3
0.6
0.5
0.7
0.7
0.7
0.4
0.4
0.6
0.6
0.5
0.9
0.3
1.5
1.5
0.7
1.8
1.4
1.2
1.3
1.0
9
10
11
12
11.6
a
Unidentied heavier compounds in GC chromatography.
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wt% to 8.0 wt%, no big change on the ethanol and acetaldehyde selectivity increased at the range of 290 to 310 ꢀC and then
mixed conversion was observed, with a conversion range from dropped hereaer. The maximum of BD selectivity, 84.2%, was
30% to 37%. Meanwhile, selectivity to dehydration products (EL reached at 310 ꢀC, WHSV of 1.8 hꢁ1, ethanol/acetaldehyde mole
and DE) increased at the expense of lower BD selectivity. ratio of 3.5. Moreover, the selectivity to EL and DE increased
Notably, the maximum of BD selectivity, 83.5%, was reached at upon increasing reaction temperature. This reected that high
the ZnO loading of 0.5 wt%.
temperatures favour ethanol dehydration process.40,41 Notably,
3.2.2 Effect of reaction temperature. Different parameters, at higher temperatures, the selectivity to heavier compounds
ꢀ
i.e., reaction temperature, ethanol/acetaldehyde mole ratio and C6+ tend to decrease. Given all the analysis above, 310 C was
WHSV were studied as to optimize the reaction conditions and chosen as the optimum reaction temperature in ethanol and
Zn0.5–Zr–Si-A was chosen as a representative sample. The acetaldehyde conversion to BD process.
results were shown in Fig. 7. As shown in Fig. 7a, with
3.2.3 Effect of ethanol/acetaldehyde mole ratio. The
increasing temperature from 290 to 400 ꢀC, the ethanol/ optimum study on the ethanol/acetaldehyde ratio was con-
acetaldehyde conversion increased gradually. Whereas BD ducted based on the optimum reaction temperature as analysed
previously with a WHSV of 1.8 hꢁ1. The range of ethanol/
acetaldehyde mole ratio was from 2.5 to 4.5 and the results
were shown in Fig. 7b. It is obvious that the ethanol/
acetaldehyde conversion dropped with increasing ethanol/
acetaldehyde mole ratios. At high ethanol/acetaldehyde values,
the selectivity to dehydration products (EL and DE) were much
higher than that of low ethanol/acetaldehyde values. It's
generally accepted that ethanol dehydration process can
proceed on both Lewis acid sites than Bronsted acid sites.42
Lower amount of acetaldehyde in feed did not inhibit ethanol
dehydration ability on Zr–Si based catalysts. The best BD
selectivity was obtained at the ethanol/acetaldehyde mole ratio
of 3.5.
3.2.4 Effect of WHSV. The effect of WHSV was also studied
ꢀ
under the condition of 310 C and ethanol/acetaldehyde mole
ratio of 3.5. As can be seen from Fig. 7c, the ethanol/
acetaldehyde conversion boosted lineally with decreasing
WHSV values. The BD selectivity showed a maximum at the
WHSV of 1.8 hꢁ1. Strangely, the decrease of WHSV in the
present work didn't elevate BD selectively which was in contrast
with the results in ref. 15 and 43. Notably, the selectivity to EL
and BL shown a fast growing trend at lower WHSV values, this
could be explained by longer contact time at lower WHSV
values, which increased the dehydration possibility on the
catalyst.
3.2.5 Durability test of catalyst. Considering the relatively
inferior catalytic durability of the catalysts in ethanol and
acetaldehyde conversion into BD process as reported previously
in ref. 3, 10 and 17, a catalytic durability testing of Zn0.5–Zr–Si-A
sample was performed at 310 ꢀC with ethanol/acetaldehyde
mole ratio of 3.5, and WHSV of 1.8 hꢁ1. The catalytic result
was shown in Fig. 8. As clearly revealed, during the rst 20 h, the
catalytic index, i.e., the BD selectivity and the ethanol/
acetaldehyde conversion, were relatively high and stable. Aer
20 h testing, a slight decrease on both BD selectivity and
ethanol/acetaldehyde conversion was observed. This means
some amount of coke was doped on the active sites of the
catalyst, which in turn lowered the catalytic performance. Aer
45 h testing, both BD selectivity and ethanol/acetaldehyde
uctuated to some extent. Finally, aer 60 h, as we can see,
the catalytic performance dropped obviously with both BD
selectivity and ethanol/acetaldehyde conversion 10% lower than
Fig. 7 Effect of (a) temperature; (b) ethanol/acetaldehyde mole ratio
that of the fresh sample. The durability testing in this work
showed a super performance in terms of BD selectivity and
and (c) WHSV on the ethanol/acetaldehyde conversion and BD
selectivity.
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technical support and the large precision instrument platform
for XRD, XPS, SEM and HR-TEM measurements.
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