Heterogeneous CaO-ZrO2 acid-base bifunctional catalysts for vapor-phase selective dehydration of 1,4-butanediol to 3-buten-1-ol

Article abstract of DOI:10.1016/j.apcata.2013.06.020

A series of acid-base bifunctional CaO-ZrO₂ catalysts was prepared simply by the impregnation method and evaluated for the vapour-phase dehydration of 1,4-butanediol (BDO). The effects of CaO content and calcination temperature on the catalytic properties of the CaO-ZrO₂ catalysts were investigated. It was found that the catalyst with 12.5 wt% CaO and at a calcination temperature of 650°C exhibited favorable catalytic performance and good stability for the selective dehydration of BDO to 3-buten-1-ol (BTO). The maximum BTO selectivity and BDO conversion reached 68.9% and 94.6%, respectively. The formation of by-product, THF, was markedly suppressed. These catalysts were characterized by N₂ physisorption, XRD, FT-IR spectra, NH₃-TPD and CO₂-TPD. The results indicated that the CaO-ZrO₂ catalysts showed higher basicity density and similar acidity density compared to unmodified ZrO₂ due to the formation of Ca-O-Zr Hetero-linkage by CaO introduction. The catalytic performance depended on the coexistence of acidic and basic sites on the surface of catalysts.

Full text of DOI:10.1016/j.apcata.2013.06.020

Applied Catalysis A: General 466 (2013) 233–239
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Heterogeneous CaO-ZrO2 acid–base bifunctional catalysts for
vapor-phase selective dehydration of 1,4-butanediol to 3-buten-1-ol
Qian Zhang, Yin Zhang, Haitao Li, Chunguang Gao, Yongxiang Zhao∗
School of Chemistry and Chemical Engineering, Engineering Research Center for Fine Chemicals of Ministry of Education, Shanxi University, Taiyuan,
030006, Shanxi, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 March 2013
Received in revised form 16 June 2013
Accepted 19 June 2013
Available online 28 June 2013
A series of acid–base bifunctional CaO-ZrO2 catalysts was prepared simply by the impregnation method
and evaluated for the vapour-phase dehydration of 1,4-butanediol (BDO). The effects of CaO content and
calcination temperature on the catalytic properties of the CaO-ZrO2 catalysts were investigated. It was
found that the catalyst with 12.5 wt% CaO and at a calcination temperature of 650 ◦C exhibited favorable
catalytic performance and good stability for the selective dehydration of BDO to 3-buten-1-ol (BTO). The
maximum BTO selectivity and BDO conversion reached 68.9% and 94.6%, respectively. The formation
of by-product, THF, was markedly suppressed. These catalysts were characterized by N2 physisorption,
XRD, FT-IR spectra, NH3-TPD and CO2-TPD. The results indicated that the CaO-ZrO2 catalysts showed
higher basicity density and similar acidity density compared to unmodified ZrO2 due to the formation of
Ca–O–Zr Hetero-linkage by CaO introduction. The catalytic performance depended on the coexistence of
acidic and basic sites on the surface of catalysts.
Keywords:
1,4-Butanediol
3-Buten-1-ol
Dehydration
CaO-ZrO2 acid–base bifunctional catalyst
Ca–O–Zr Hetero-linkage
© 2013 Published by Elsevier B.V.
1. Introduction
generate rather than BTO. Thus, it is great significance to obtain a
catalyst that favors BTO production and suppresses THF production.
3-Buten-1-ol (BTO) is a valuable intermediate for the synthe-
sis of heterocyclic drugs, such as multidrug resistant compounds,
antiproliferative, antiviral and antitumor agents [1–4]. Currently,
there are three strategies for the synthesis of BTO: (1) formaldehyde
addition to propylene in aqueous solution [5,6], (2) 3,4-epoxy-1-
butene reduction by formic acid with a homogeneous catalyst in
tetrahydrofuran (THF) solvent [7,8], and (3) vapor-phase selective
dehydration of 1,4-butanediol (BDO) over metal oxide catalysts
[9–21]. The routes (1) and (2) encounter various disadvantages: the
high-temperature and high-pressure reaction conditions, the low
BTO yield, the difficulty in separation, utilization of organic solvent
and so on. In contrast, the route (3) offers mild reaction conditions,
simple experimental procedure, and more importantly, it is envi-
ronmental friendly. The dehydration of BDO is thermodynamically
feasible in two ways (Scheme 1) [10]. One is a cyclodehydra-
tion to THF by eliminating a hydroxyl group to form a carbenium
cation followed by nucleophilic substitution. The other is dehydra-
tion between terminal hydroxyl and adjacent ␤-H of BDO to form
BTO. And the competitive production, THF, is more preferential to
The catalysts used for BDO selective dehydration can be divided
into two types according to the reaction mechanism. One type is
redox oxides such as CeO2 [9–12] and In2O3 [13]. The metal ions
which have redox properties can interact with terminal hydroxyl
or ␤-H of BDO and produce corresponding radical intermediates.
When hydroxyl radical and ␤-H radical are simultaneously elimi-
nated from one BDO molecule, a BTO molecule is formed. During
above process, these intermediates can further generate a series
of by-products including 2-butene-1-ol, 1,3-butadiene, butanol,
butyraldehyde, and so on.
Another type is solid acid and base catalysts such as Al2O3, MgO
[14], ZrO2 [15,16], rare earth oxides [16–20] etc. During reaction
process, the acid sites interact with the terminal hydroxyl group
of BDO, while the basic sites interact with the ␤-H [9,20]. The sub-
sequent elimination of water leads to the formation of BTO. This
kind of catalyst has more advantages of less side-reaction, higher
activity than that of the redox catalysts. Stao and coworkers studied
the catalytic performance of ZrO2 and alkali metal (Li, Na, K) mod-
ified ZrO2 catalyst [15,16]. Among these catalysts, the pure ZrO2
showed highest BDO conversion, however, it is selectivity for BTO
is low. The introduction of alkali metal can improve BTO selectivity
greatly, while the conversion of BDO and the yield of BTO decrease
sharply. Further studies revealed that the reason for BDO conver-
sion decline resided in the quick reduction of surface acidic sites
∗
Corresponding author. Tel.: +86 351 7011587; fax: +86 351 7011688.
E-mail address: yxzhao@sxu.edu.cn (Y. Zhao).
0926-860X/$ – see front matter © 2013 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.apcata.2013.06.020

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Q. Zhang et al. / Applied Catalysis A: General 466 (2013) 233–239
The catalytic performance was evaluated using average of the initial
2–9 h.
2.3. Characterization
N2 physisorption measurement was performed on
a
Micromeritics ASAP 2020 instrument at −196 ◦C. Before the
measurement, the samples were degassed at 100 ◦C for 3 h. The
specific surface area was calculated by the BET method.
X-ray powder diffraction (XRD) analysis was performed using a
Bruker D8 Advance X-ray diffractometer with a Cu K␣ radiation at
40 kV and 40 mA. Scanning was carried out over a 2ꢀ ranging from
20◦ to 70◦ in increments of 0.05◦.
Scheme 1. Reaction scheme of 1,4-butanediol dehydration.
caused by the introduction of alkali metal. The key point for obtain-
ing a high active and selective catalyst for BDO dehydration to BTO
is how to prepare an acid–base bifunctional catalyst.
The FT-IR spectra were recorded on a Bruker Tensor 27 FT-IR
spectrometer with a resolution of 4 cm−1, using the KBr diluting
technique for the analysis of the samples. The spectra (100 scans)
were recorded in the 4000–400 cm−1 wavenumber range.
Temperature-programmed desorption (TPD) profiles of NH3 and
CO2 were measured to estimate the acidity and the basicity of the
catalysts, respectively. The samples (100 mg, 40–60 mesh) were
pretreated at 500 ◦C in a flow of helium (99.999%) at a rate of
60 mL min−1 for 60 min and saturated with a pure NH3 or CO2
flow after cooling to 100 ◦C. These pretreated samples were purged
under helium atmosphere at 100 ◦C to remove physisorbed NH3 or
CO2 until the baseline was stable, then these samples were heated
from 50 ◦C to 600 ◦C at a rate of 10 ◦C min−1 in the helium flow. The
amount of NH3 or CO2 evolved from the sample was determined
by thermal conductivity detector (TCD). The quantitative analysis
of the acid/basic amount was based on NH3/CO2-TPD profiles and
the acid/base density were expressed as the number of NH3/CO2
molecules per area of catalyst (NH3 nm−2, CO2 nm−2).
Acid–base bifunctional catalyst is a popular concept quoted
from enzyme catalysis reactions that contain two types of active
centers of acid and base in catalytic systems [22–27]. These
catalysts may act in a cooperative way to provide superior reactiv-
ity and selectivity than monofunctional materials. The acid–base
bifunctional catalysts can be prepared by the grafting method,
the co-condensation method and the formation of an organosil-
ica material [28]. However, there are several problems with these
routes due to the use of highly toxic raw materials, high process
cost, and complicated synthetic procedures to be disposed of.
Many references suggested that the formation of M1–O–M2
hetero-linkages in binary complex oxides can lead to the uneven
distribution of charges and subsequently generate new acid sites.
Our earlier study has also reported that the formation of Zr–O–Si
hetero-linkages generated a large number of acidic sites over
ZrO2–SiO2 mixed oxide surface [29,30]. Compared with alkali metal
promoters, the alkaline–earth metal ions especially for Ca2+ can
enter into the ZrO2 crystal lattice (Zr4+ ions are substituted by
Ca2+ ions) to form Ca–O–Zr hetero-linkages easily that is very
likely to generate new acidic sites. Herein, we developed a series
of CaO-ZrO2 catalyst and investigated their performance in BDO
dehydration. On basis of characterizations, the probable adsorption
model of BDO and active centers over the acid–base bifunctional
catalysts were also discussed in details.
3. Results
3.1. Dehydration of BDO over CaO-ZrO2 catalysts
3.1.1. Effects of CaO content
Distribution of products in the dehydration of BDO over XCZ-500
catalysts is listed in Table 1. It is found that the main prod-
ucts are BTO, THF and ␥-butyrolactone (GBL). Besides, dozens
of by-products are detected by gas chromatography, such as 2-
buten-1-ol, 1-butanol, and butanal, however, the selectivity of each
by-product is lower than 1%. Thus the main products of BTO, THF
and GBL are discussed in this dehydration reaction as references.
Over 0CZ-500 (unmodified ZrO2), the conversion of BDO is nearly
to 100%, but THF is dominantly produced (the selectivity is 79.6%).
With CaO content lower than 12.5 wt%, the BDO conversion is over
90% and the BTO selectivity increases from 18.1% of unmodified
ZrO2 to 63.8% of ZrO2 modified with 12.5 wt% CaO. When CaO
content further increases, the BDO conversion decreases and the
BTO selectivity remains at a high level. For the by-products of THF
and GBL, the selectivity is monotonously decreasing and gradually
increasing separately with increasing the CaO content.
2. Experimental
2.1. Catalyst preparation
ZrO2 aerogel support was prepared via heating alcohol–aqueous
solutions followed by supercritical fluid drying method that we
had reported previously [31]. The support was calcined at 400 ◦C
for 80 min before impregnation. An appropriate amount of cal-
cium nitrate was impregnated to the above-mentioned support,
and then the samples were dried at 120 ◦C over night and calcined
at an appropriate temperature for 3 h to obtain the CaO-ZrO2 cat-
alysts. The catalysts are designated as XCZ-T, in which X stands for
CaO content calculated by XCaO = mCaO/(mCaO + mZrO2 ) and T is the
calcination temperature.
3.1.2. Effects of calcination temperature
2.2. Catalytic reaction
The catalyst with 12.5 wt% CaO was chosen as the typical sample
to investigate the effect of calcination temperature on the catalytic
performance. The corresponding results are presented in Table 1.
From Table 1, the conversion of BDO exceeds 90% over the cata-
lysts calcined at 500–650 ◦C, and then decreases with a further rise
in temperature. Similarly, the selectivity to BTO initially increases
slowly, until a maximum (68.9%) is reached at 650 ◦C, and then a
decrease follows. And the maximum yield of BTO is 65.2% when cal-
cination temperature is 650 ◦C. For this dehydration reaction, the
group of S. Sato has systematically done fundamental researches.
The obtained maximum yield of BTO is 64.8% over Sc1.5Yb0.5O3 at
The dehydration of BDO was carried out in a fixed-bed flow reac-
tor. Before the reaction, 2.0 g catalyst (20–40 mesh) was preheated
in the N2 flow at 350 ◦C for 1 h. At this temperature, the BDO gas
was fed into the reactor at a flow rate of 2.0 mL h−1 together with
a N2 (carrier gas) flow of 30 mL min−1. In the poisoning experi-
ment, CO2 or 10%NH3–N2 were used as carrier gas. A flow rate was
also 30 mL min−1. Liquid effluent was collected periodically and
analyzed by gas chromatography (Agilent-7890A) using a 30 m cap-
illary column (AT.OV-1701) and a flame ionization detector (FID).

Q. Zhang et al. / Applied Catalysis A: General 466 (2013) 233–239
235
Table 1
Specific surface area, acid–base properties, and catalytic performance of CaO-ZrO2 catalysts with different CaO content and calcined at different temperature.
Catalyst (XCZ-T)a
SBET (m2 g−1
)
Acidityb (␮mol g−1
)
Basicityb (␮mol g−1
)
Conversionc (%)
Selectivityc (%)
BTO
THF
GBL
Othersd
0CZ-500
2CZ-500
5CZ-500
115
137
130
117
92
82
70
59
85
261
273
268
245
248
203
138
106
235
251
138
85
263
348
404
448
455
371
306
255
480
486
235
155
99.1
92.6
91.8
91.5
90.5
82.7
70.9
58.8
92.3
94.6
75.6
61.3
18.1
23.6
36.1
57.3
63.8
58.6
55.3
51.2
67.6
68.9
49.1
43.6
79.6
72.1
55.3
25.8
13.6
15.2
16.3
18.9
11.1
9.8
1.1
1.8
3.6
4.7
5.9
6.9
8.1
10.3
6.1
6.3
3.2
3.0
1.2
2.5
5.0
12.2
16.7
19.3
20.3
19.6
15.2
15
10CZ-500
12.5CZ-500
15CZ-500
20CZ-500
30CZ-500
12.5CZ-600
12.5CZ-650
12.5CZ-700
12.5CZ-800
82
76
73
15.6
21.7
32.1
31.7
a
X stands for the CaO content and T is the calcination temperature. Catalyst 2.0 g, 350 ◦C.
Acid and basic values are derived from NH3-TPD and CO2-TPD analysis, which are expressed as the mol number of NH3 or CO2 molecules per gram of catalyst.
Average conversion and selection are for initial 2–9 h.
b
c
d
Others include 2-buten-1-ol, 1-butanol, butanal, etc.
350 ◦C [19], suggesting that the CaO-ZrO2 catalysts have certain
practical value for further research. In addition, the selectivity to
the competitive products of THF and GBL changes within the range
of 9.8% to 21.7% and decreases from about 6% to 3.0% with the
calcination temperature.
inhibit the rapid decrease of surface area caused by the sintering of
ZrO2 from surface diffusion [32–34]. The reason of the fall in the SBET
is probable that the excessive CaO begins to aggregate on the ZrO2
surface. Also in Table 1, the SBET of 12.5CZ-T catalysts decreases from
92 m2 g−1 to 73 m2 g−1 with the calcination temperature increasing
from 500 ◦C to 800 ◦C.
3.1.3. Stability of 12.5CZ-650 catalyst to BDO dehydration
In view of optimum activity of 12.5CZ-650 catalyst, we evaluate
its stability in a long-time test (300 h). The dehydration activity
and BTO selectivity are shown in Fig. 1. Observing from Fig. 1, the
12.5CZ-650 catalyst exhibits high performance and good stability
within 300 h. The BDO conversion and BTO selectivity maintain a
high level for a long time. Only 200 h later, the selectivity to BTO
decreases slightly. Meanwhile, the selectivity to the by-products of
THF and GBL is limited to low levels (the average selectivity is 9.28%
and 5.87%, respectively).
From the XRD patterns of XCZ-500 catalysts shown in Fig. 2a,
it is clear that monoclinic and tetragonal phase coexist in ZrO2,
3.2. Characterization of CaO-ZrO2 catalysts
The BET surface areas (SBET) of the CaO-ZrO2 catalysts are shown
in Table 1. From Table 1, with the introduction of CaO, the SBET of
ZrO2 catalysts modified by low CaO content (10 wt%) is higher than
that of the unmodified ZrO2, while the SBET decreases obviously
with the elevated CaO content (from 137 m2 g−1 to 59 m2 g−1) for
all the CaO–modified ZrO2 catalysts. It is probable that the introduc-
tion of CaO can improve the anti-sintering ability of the catalyst and
Fig. 1. The long-time test of BDO dehydration over 12.5CZ-650 catalyst (catalyst
Fig. 2. XRD patterns of CaO-ZrO2 catalysts with different CaO content (a) and cal-
cined at different temperature (b).
2.0 g, 350 ◦C).

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Q. Zhang et al. / Applied Catalysis A: General 466 (2013) 233–239
Fig. 4. NH3-TPD profiles of CaO-ZrO2 catalysts with different CaO content and at
different calcination temperature. (a) NH3-TPD profiles of XCZ-500 (b) NH3-TPD of
12.5CZ-T.
Fig. 3. FT-IR spectra of CaO-ZrO2 catalysts with different CaO content (a) and cal-
cined at different temperature (b).
of these broad peaks (509 cm−1 and 750 cm−1) are speculated to
be caused by two factors. One is that Zr4+ ions are substituted by
Ca2+ ions partly to formed Ca–O–Zr hetero-linkages which weaken
the covalency of Zr O bond. The other is attributed to the phase
transition from monoclinic ZrO2 to tetragonal [36]. The absorp-
tion bands at 3643 cm−1, 1640 cm−1 and 870 cm−1, assigned to
OH stretching vibrations, OH bending vibration and vibration
of water absorbed on the surface respectively, appear in the high
CaO content samples, which is similar with the pure CaO phase.
This suggests that free CaO begins to aggregate and form crystal
phase [37]. This is consistent with the XRD results obtained above.
Fig. 3b shows the FT-IR spectra of 12.5CZ-T catalysts. As the calci-
nation temperature rising, the bands of Zr O (750 cm−1) and OH
(870 cm−1and 1640 cm−1) vibration absorption decrease and then
while the monoclinic peaks are less intense and the tetragonal
peaks become more intense with the introduction of CaO. This is
explained that more Ca2+ ions enter into the ZrO2 crystal lattice
(Zr4+ ions are substituted by Ca2+ ions partly and form Ca–O–Zr
hetero-linkages). Moreover, the presence of Ca2+ contributes to
the stabilization of the tetragonal phase. It can also be seen that
the characteristic diffraction peaks of CaO appear and the intensity
increase when CaO content is higher than 12.5 wt%. This sug-
gests that the excessive CaO begins to aggregate on the ZrO2
surface. For 30CZ-500, a characteristic diffraction peak of CaZrO3
is identified. The formation of this new phase indicates that CaO
reacted with ZrO2 and form a new stable Ca species [35]. The
XRD patterns of 12.5CZ-T catalysts are shown in Fig. 2b. With the
increase of calcination temperature, the monoclinic ZrO2 trans-
forms to tetragonal phase gradually and disappears at 12.5CZ-650.
And the diffraction peaks of CaZrO3 are observed as the calcina-
tion temperature exceeds 700 ◦C. This suggests that more Ca–O–Zr
hetero-linkages formation can explain the change of ZrO2 crys-
tal phase and the stable specie of CaZrO3 appears at the higher
temperature.
disappear, while the absorption band at around 526 cm−1 (Zr
O
vibration absorption) shifts toward a high wavenumber gradu-
ally. These results indicate that the high calcination temperature
make more Zr4+ ions be substituted by Ca2+ ions and form Ca–O–Zr
hetero-linkages or stable species CaZrO3.
NH3-TPD and CO2-TPD measurements of the CaO-ZrO2 catalysts
were carried out in order to evaluate their acidity and basicity,
respectively. Figs. 4 and 5 show the TPD profiles of samples and the
quantitative analysis of NH3 or CO2 desorbed is listed in Table 1.
From Table 1, the total amount of acidic sites is almost same when
CaO content ≤12.5% or T ≤ 650 ◦C. That is to say, the amount of
acidic sites changes little with the introduction of CaO. With the fur-
ther increase in CaO content or calcination temperature, the total
amount of acidic sites decreases significantly. Also in Table 1, the
The FT-IR spectra of XCZ-500 catalysts are gathered in Fig. 3a.
The broad absorption bands at around 509 cm−1 and 750 cm−1 are
attributed to the Zr O vibration absorption [36,37]. With CaO con-
tent increasing, the band at 509 cm−1 shifts gradually to a high
wavenumber (from 509 cm−1 to 530 cm−1). Correspondingly, the
intensity of the band at 750 cm−1 decreases by degrees. The changes

Q. Zhang et al. / Applied Catalysis A: General 466 (2013) 233–239
237
Fig. 6. BDO conversion and selectivity of BDO dehydration over 12.5CZ-650 catalyst
in different carrier gases (catalyst 2.0 g, 350 ◦C).
As an acid–base bifunctional catalyst, introduction of appropri-
ate CaO on the ZrO2 surface can increase the basic sites, while the
excess of CaO or generation of CaZrO3 may decline the amount of
basic sites. The amount of basic sites increases gradually as the
calcination temperature is lower than 650 ◦C, and then decreases
rapidly at higher temperatures. The results indicate that generation
of CaZrO3 may decline the amount of basic sites. In summary, For
CaO-ZrO2 catalyst, Ca2+ incorporates into the ZrO2 crystal lattice to
form Ca–O–Zr hetero-linkages, and these offer CaO-ZrO2 catalysts
not only new acidic sites but also higher basicity.
4.2. Relationship between acid–base properties and catalytic
activity
Fig. 5. CO2-TPD profiles of CaO-ZrO2 catalysts with different CaO content and at
different calcination temperature. CO2-TPD profiles of XCZ-500 (b) CO2-TPD profiles
of 12.5CZ-T.
To understand the relationship between acid–base properties
and catalytic activity, we have demonstrated in the poisoning
experiments of 12.5CZ-650 catalyst and the unmodified ZrO2. The
variations of BDO conversion and selectivity to BTO or THF are
entirely consistent, so we discuss the results taking the former cata-
lyst for example. Fig. 6 summarizes conversion and selectivity in the
dehydration of BDO over 12.5CZ-650 catalyst with different carrier
gases. Either 10%NH3-N2 or CO2 instead of N2 was used as carrier
gases. In acidic CO2 flow instead of N2, the formation rate of BTO
was markedly suppressed and the selectivity of THF, as competi-
tive product, increased correspondingly. The results of comparison
experiments on THF selectivity in CO2 gas without catalyst showed
that the BDO conversion was very low and no product was detected.
The acidic CO2 molecules react with the basic sites on the catalyst
surface. This clearly indicates that the basic sites play an important
role in the formation of BTO. Comparing the poisoning experiments
of Er2O3 and Yb2O3 [9,20,21,38], the conversion of BDO reaches
nearly 100% due to the rich acidic sites on unmodified ZrO2 surface.
Thus, the conversion of BDO decreases in basic 10%NH3–N2 flow,
while the selectivity to BTO has hardly changed due to the constant
basic sites. The poisoning experiments indicate that both basic and
acidic sites participate in the formation of BTO via the acid–base
tridentate coordination mechanism. The results strongly suggest
that the acidic and basic sites assure the conversion of BDO and the
selectivity to BTO, separately. In addition, a marked competition
exists between the products of BTO and THF.
total amount of basic sites increases initially and then a decrease
follows with elevated CaO content. Similarly, the amount of basic
sites increases gradually as the calcination temperature is lower
than 650 ◦C, and then decreases rapidly at higher temperatures. The
maximum amount of acidic sites and basic sites are 273 ␮mol g−1
and 486 ␮mol g−1
.
4. Discussion
4.1. Correlations between structure and acid–base properties
The formation of M1 M2 hetero-linkages in binary complex
O
oxides generates the uneven distribution of charges and subse-
quentlygeneratesnew acid sites. Our earlierstudyhas also reported
that the formation of Zr–O–Si hetero-linkages generated a large
number of acidic sites over ZrO2–SiO2 mixed oxide surface [29,30].
From XRD and FT-IR results shown above, when being introduced
to ZrO2 catalyst, Ca2+ ions enter into the ZrO2 crystal lattice to form
Ca–O–Zr hetero-linkages. This structure leads to the uneven distri-
bution of charges and makes the catalyst generate new acidic sites,
that is the reason why the total amount of acidic sites have little
change with CaO content lower than 12.5 wt% or calcination tem-
perature lower than 650 ◦C. However, the amount of acidic sites
reduces obviously due to the aggregation of CaO and generation
of CaZrO3 at the high CaO content or calcination temperature. This
can be interpreted as the aggregation of CaO and the generation of
CaZrO3, which is ascertained by XRD and FT-IR results.
In this study, through activity evaluation and characterization
of the catalysts, it is found that the acid–base properties of the
CaO-ZrO2 catalysts have significant influence in the dehydration of
BDO. Fig. 7 shows the relationship between acid–base density and
catalytic performance. From Fig. 7, the surface acid–base property

238
Q. Zhang et al. / Applied Catalysis A: General 466 (2013) 233–239
Fig. 7. Relationship between acid–base properties and catalytic performance.
exerted great influence on the catalytic performance and BTO
selectivity. The surface acid densities of unmodified ZrO2, 12.5CZ-
500, 12.5CZ-600 and 12.5CZ-650 are 1.37 NH3/nm2, 1.62 NH3/nm2,
1.66 NH3/nm2, and 1.84 NH3/nm2, respectively. They exhibited
high BDO conversion due to the rich acidic sites on their sur-
face. With the decrease of acid density, BDO conversion decreases
accordingly (from 99.1% to 58.8%), thus it is clear that BDO con-
version is closely related to acid density of catalysts. Given that
acidic sites act as an anchoring site for the OH group to activate
BDO [9,20], it follows that more acidic sites can activate more BDO
molecules to give higher BDO conversion. Fig. 7 also shows the rela-
tionship of BTO selectivity and surface base density of the catalysts.
It shows clearly that greater base density led to increase BTO selec-
tivity. The basic sites, probably composed of surface O2− anions
[9,20], are thought to interact with the ␤-H of BDO and thereby
contribute to the formation of BTO. 10CZ-500, 12.5CZ-600 and
12.5CZ-650 showed high BTO selectivity, which is attributed to the
abundant basic site. The maximum base density was 3.57 CO2/nm2
for 12.5CZ-650, and the BTO selectivity reached 68.9%.
5. Conclusions
In summary, we developed a simple preparation process to
synthesize acid–base bifunctional catalyst for the vapor-phase
dehydration of BDO to BTO. The catalysts exhibited good stability,
high BDO conversion and BTO selectivity. The surface acid–base
properties of the catalysts were improved by the introduction of
appropriate CaO into ZrO2. Increased basic sites were accompa-
nied by the generation of new acidic sites due to the formation of
Ca–O–Zr hetero-linkages. BTO was formed on acid–base concerted
sites of CaO-ZrO2 catalysts in activating the terminal hydroxyl
group and adjacent ␤-H of BDO. Our present research provides a
novel potential way toward adjusting the amount of acidic and
basic sites on the modified ZrO2 catalysts.
Acknowledgement
The authors gratefully thank the financial supports of
International
S
T
Cooperation Program of China (ISTCP)
In the vapor-phase selective dehydration of BDO, the terminal
hydroxyl group and the adjacent ␤-H of BDO interact with acidic
sites and basic sites, respectively, before the terminal hydroxyl
group and ␤-H are eliminated to form BTO [9,20]. This probable
adsorption structure was provided by S. Sato group, which is ver-
ified in our systems. In detail, every active center forming BTO is
composed of two acidic sites and one basic site, while the active
center includes acidic sites only for the competitive product of THF.
Thus the rich content of acidic and basic sites ensures the excel-
lent performance in BDO dehydration to BTO. In present study,
with the addition of CaO to ZrO2, the amount of basic sites initially
increases obviously and then a decrease follows (Table 1). Corre-
spondingly, the same tendency of BTO selectivity and the reverse
tendency of THF selectivity are also observed. Analogously, with
the increment of the calcination temperature (T > 650 ◦C), the BDO
conversion and the BTO selectivity decrease obviously due to the
decrease of acid–base active center. It is reasonable that acid–base
concerted site is favorable to the dehydration of BDO to BTO and
acidic sites can cyclize BDO to THF.
(No. 2013DFA40460) and the National Natural Science Funda-
tion of China (No. 21073114) The authors gratefully acknowledge
Dr. Zheng Jiang, Department of Chemistry, Inorganic Chemistry
Laboratory, Jesus College, University of Oxford for thorough
analysis and useful discussions.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.apcata.2013.06.020.
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