Cr, Zr-incorporated hydrotalcites and their application in the synthesis of isophorone

Article abstract of DOI:10.1016/j.catcom.2010.03.014

Cr³⁺ and Zr⁴⁺ cation-incorporated hydrotalcites (HTs) were prepared by coprecipitation method. Corresponding mixed oxide were obtained by the thermal decomposition of HTs at 773 K for 8 h and applied in the synthesis of isophorone (IP) from acetone. From the characteristic results, both Cr³⁺ and Zr⁴⁺ were introduced into the lattice of hydrotalcite producing the more disordered HT structures. Compared with Mg-Al mixed oxide, Cr and Zr modified mixed oxide demonstrated more amount of basic sites and stronger base strength, which were responsible for the improvement of catalytic activity. As a result, both of the modified mixed oxide exhibited IP selectivity of more than 70% under atmospheric pressure.

Full text of DOI:10.1016/j.catcom.2010.03.014

Catalysis Communications 11 (2010) 880–883
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Cr, Zr-incorporated hydrotalcites and their application in the synthesis of isophorone
a,
Yanxia Liu a,b, Kunpeng Sun a, Haowen Ma c, Xianlun Xu a, , Xiaolai Wang
⁎
⁎
a
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
Graduate University of Chinese Academy of Sciences, Beijing 100039, China
b
c
Lanzhou Petrochemical Research Center of Petrochina, Lanzhou 730000, China
a r t i c l e i n f o
a b s t r a c t
Cr3+ and Zr4+ cation-incorporated hydrotalcites (HTs) were prepared by coprecipitation method.
Corresponding mixed oxide were obtained by the thermal decomposition of HTs at 773 K for 8 h and
applied in the synthesis of isophorone (IP) from acetone. From the characteristic results, both Cr3+ and Zr4+
were introduced into the lattice of hydrotalcite producing the more disordered HT structures. Compared
with Mg–Al mixed oxide, Cr and Zr modified mixed oxide demonstrated more amount of basic sites and
stronger base strength, which were responsible for the improvement of catalytic activity. As a result, both of
the modified mixed oxide exhibited IP selectivity of more than 70% under atmospheric pressure.
© 2010 Elsevier B.V. All rights reserved.
Article history:
Received 6 January 2010
Received in revised form 16 March 2010
Accepted 26 March 2010
Available online 1 April 2010
Keywords:
Cation-incorporated hydrotalcite
Condensation
Isophorone
Solid base
1. Introduction
these reactions mentioned above can be catalyzed by solid base [9].
Mg–Al mixed oxide derived from hydrotalcite has been applied in the
Hydrotalcite (HT) or layered double hydroxide (LDH), e.g. Mg4Al2
(OH)12CO3·4H2O, has a structure similar to that of brucite, Mg(OH)2
[1]. In brucite, Mg2+ are octahedrally coordinated by hydroxyl ions,
which are edge-shared to form a sheet-like structure. In HT, some of
Mg2+ are replaced by Al3+, resulting in a net positive charge of the
brucite-like layers. Charge-balancing anions (typically CO23−) and
water molecules are situated in the interlayers between the stacked
brucite-like cation layers [2]. Upon calcinations, the dehydroxylation
of layers and the decomposition of carbonates take place, leading to
formation of a mixed oxide phase, Mg(Al)O. The mixed oxide holds
both strong Lewis basic and mild Lewis acid sites. The former are due
to the presence of the oxygen atoms of low coordination, and the
latter to Al3+ cations [3]. The acid-base properties of the mixed oxide
are related to its composition, preparation method, and treatment
conditions (calcination temperature) [4]. In HT, Mg2+ and Al3+ can be
partially substituted by other metal ions, which often introduced new
catalytic properties [5,6]. The cation-incorporation is also an efficient
method of modifying the acid-base property of the mixed oxide.
Isophorone (IP) is a kind of excellent high boiling solvent and
important chemical intermediate. It was typically synthesized from
acetone. As shown in Fig. 1, acetone firstly condensed and dehydrated
to produce mesityl oxide (MO) or isomesityl oxide (IMO), then the
MO condensed with another molecular of acetone and IP was formed
via 1,6-Meachel addition [7] or 1.6-Adol condensation reaction [8]. All
synthesis of IP in many previous studies [10–13]. A. A. Schutz
prepared hydrotalcite like material by suspending alumina gel and
MgO. The derived Mg–Al mixed oxide exhibited high selectivity of IP
(74.5%) under 623 K and 15 PSIG [10]. Because the complexity of the
preparing method, new preparing methods were continuously
reported [11–13]. Recently, Wang and coworkers [13] prepared HT
through coprecipitation-constant-temperature-crystal method and
the derived Mg–Al mixed oxide exhibited acetone conversion of 17.7%
and IP selectivity of 65.3% under 523 K. These reports mainly studies
on the method of synthesis hydrotalcite-like material, which was
precursor of the Mg–Al mixed oxide. But limited literatures is
available on the cation-doped Mg-Al mixed oxide for the synthesis
of IP. In this work, HTs incorporated by Cr and Zr were synthesized
through coprecipitation method and the Cr and Zr doped Mg–Al
mixed oxides were applied in the synthesis of IP. Results showed that
the doped mixed oxides exhibited excellent catalytic performance
and should be potentially used in the synthesis of IP.
2. Experimental
2.1. Catalyst preparation
The mixed nitrates (Mg:Cr/Zr:Al=39:1:20) with appropriate
molar ratio were dissolved in distilled water. Another aqueous
solution containing NaOH and Na2CO3 (molar ratio of NaOH and
Na2CO3 =4:1) was added to the above solution at 353 K under
stirring. The mixture was stirred at the same temperature for 5 h.
Then the precipitate was filtered, washed and dried at 393 K for 12 h.
⁎
Corresponding authors. Tel./fax: +86 931 4968217.
E-mail addresses: mibkcat@gmail.com (X. Xu), mibkcat@gmail.com (X. Wang).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.03.014

Y. Liu et al. / Catalysis Communications 11 (2010) 880–883
881
Fig. 1. General reaction mechanism for synthesis of isophorone.
The as-synthesized hydrotalcite was denoted as M/HTas in which M
represented the modified metal cations. In the same fashion, the
sample calcinated at 773 K for 8 h was denoted as M/HTox. For
comparison, the hydrotalcite HTas with molar ratio of Mg/Al=2:1
and the corresponding mixed oxide HTox were prepared via the same
method.
width at half maximum (FWHM) of (110) and (003) diffraction peaks,
using the Debye-Scherrer formula. The parameters a and c of Cr/HTas
and Zr/HTas were bigger than that of HTas. The enlargement of a of Zr/
HTas was related to the replacement of Mg2+ (0.65 Å) or Al3+
(0.50 Å) by cations with larger ionic radius Zr4+ (0.72 Å). As is known,
The radius of Zr4+ is larger than that of Cr3+ (0.62 Å). However, the
enlargement of a of Cr/HTas was larger than that of Zr/HTas. This may
be indicative of less introduced Zr4+ due to its lager radius. The
parameter c also showed the same tendency of a, which can be
attributed to the increase of the thickness of the brucite-like layer
upon incorporation of Cr3+ or Zr4+ [14]. The dimension of HTas in a-
direction was larger than that in c-direction, which was typical for the
plate-like hydrotalcite crystals [15]. Compared with HTas, particle size
of Cr/HTas and Zr/HTas was smaller. Moreover, their particle sizes in
a-direction was smaller than those in c-direction, indicating formation
of more irregular structures.
The XRD patterns of the as-prepared hydrotalcites were shown in
Fig. 2 A. All the three patterns showed the characteristic diffraction lines
typical for an HT compound, proving the formation of hydrotalcite-like
structure. There were no other diffraction peaks observed on patterns of
Cr/HTas and Zr/HTas, indicating that the modified cations were
incorporated into the lattice or highly dispersed on the surface.
Combined with the enlargement of lattice parameter a of the Cr/HTas
and Zr/HTas, the incorporations of the modified cations were confirmed.
After heat treatment of HTas at 773 K the layered compound was
destroyed and the mixed oxide (Mg(Al)O) was formed. From the XRD
patterns of the calcined mixed oxide (Fig. 2 B), Cr/HTox displayed
pattern similar to that of HTox. Besides diffraction peaks corresponding
to the Mg(Al)O, there wasanother diffractionpeak around 30° shown in
pattern of Zr/HTox. This peak could be attributed to ZrO2 (88-1007),
which suggested that the incorporation of large atom Zr was not stable
in the calcination process. Upon calcinations, color of Cr-modified
hydrotalcite changed from grey-blue of Cr/HTas to yellow of Cr//HTox.
The former was typically displayed by Cr (III) and the latter by Cr (VI).
These phenomena implied that both the coordination number and
valence of Cr changed in calcination process.
2.2. Catalyst characterizations
X-ray diffraction (XRD) patterns were obtained by Philips X Pert
Pro X diffractometer operated with a Ni-filtered Cu Kα radiation.
Physisorption of N2 was carried out at 77 K on a volumetric apparatus
(ASAP 2020). Temperature-programmed desorption of CO2 (CO2–
TPD) was performed on a homemade instrument. Typically, 50 mg of
sample was firstly pretreated in helium at 823 K for 30 min, then the
temperature was cooled to 323 K and the sample was saturated with
CO2. Desorption of CO2 was carried out from 323 K to 823 K under
flowing helium (30 mL/min) using a temperature rate of 10 K/min.
2.3. Catalytic activity test
The reactions were carried out in a fixed-bed microreactor (6 mm
i.d.) using 0.6 g of catalyst under atmospheric pressure. Acetone and
N2 were fed into the reactor from the top and mixed in preheating
section. The N2 (20 mL/min) was introduced with two aims. Firstly, N2
flow can avoid the interaction between catalyst and CO2 in air.
Secondly, it can carry away the product timely and prevent the
further-condensations. The experiments were carried out at 513 K for
2 h. The products were collected in a cold trap (held in an ethanol/dry
ice bath) and quantified by gas-chromatography (GC) equipped with
a flame ionization detector (FID).
3. Results and discussion
3.1. Characterizations
The lattice parameters and particle sizes of the hydrotalcites are
shown in Table 1. The lattice parameters a and c were calculated from
the expression a=2d(110) and c=3d(003), respectively. The average
particle sizes in the a- and c- directions were determined from full
The texture properties of the mixed oxides were shown in Table 2.
Compared with those of HTox, the surface area, pore volume and pore
size of Cr/HTox slightly decreased. These results could be indicative of
the similar structure of these two samples. Zr/HTox showed remarked
different texture properties, which implied significant modifications
stemming from the incorporation of Zr. Larger pore size of Zr/HTox
deduced the smaller surface area and pore volume. It was reported
that there was more interlayer waters in the Zr-modified hydrotalcite
[14]. This larger pores may be related to the departure of more water
molecules.
Table 1
Parameters from the XRD.
Sample
Cell parameter (Å)
Particle size (Å)
d(110)
a
c
d(003)
HTas
Cr/HTas
Zr/HTas
3.045
3.048
3.046
22.765
22.887
22.842
1421.5
409.3
314.6
557.0
424.6
466.3
CO2–TPD was usually employed to determine the density and
strength of basic sites in mixed oxides derived from the hydrotalcites.
The CO2 as probe molecule has enough acidity to probe all the basic

882
Y. Liu et al. / Catalysis Communications 11 (2010) 880–883
Fig. 3. CO2–TPD curves of calcined hydrotalcite: A. HTox, B. Cr/HTox and C. Zr/HTox.
3.2. Synthesis of IP
Acetone aldol condensation were tested at 513 K under atmo-
spheric pressure. The obtained results over different catalysts are
shown in Table 3. The main products detected were MO, IMO, IP, 2,6-
dimethylhepta-2,5-dien-4-one (also denoted as phorone), 4,4-
dimethylheptane-2,6-dione and minor quantities of further-conden-
sation products. From Table 3, the modified mixed oxide exhibited
improved catalytic performance compared with result of HTox. This
improvement of catalytic activity was consistent with the results of
CO2–TPD. From results of the CO2–TPD, Cr/HTox and Zr/HTox showed
stronger basic strength and larger amount of the basic site than HTox,
expressed by the corresponding temperature and area of peaks. The
Zr/HTox showed the biggest area of the desorption peaks. Accord-
ingly, Zr/HTox exhibited the highest acetone conversion of 36.8%.
Furthermore, more irregular structure of the modified hydrotalcite
were also beneficial for the increased catalytic activity. It was reported
that only basic sites near edges of hydrotalcite platelets took part in
aldol condensations [2]. More active basic sites were available in the
irregular modified mixed oxide leading to higher acetone conversions
over the modified mixed oxide. According to the reference [19], the
selectivity of IP depends on the strength of the basic site. The acetone
oligomerization can only be achieved over strong basic sites. The
presence of more strong basic sites on Zr/HTox and Cr/HTox was
helpful to the high selectivity of IP. MO or IP cannot desorb from the
stronger basic site timely which make further condensations take
place. As a result, there are more further condensed by-products
besides IP detected from the GC results of Cr/HTox and Zr/HTox.
The time-on-stream behavior for acetone condensation on Cr/HTox
was tested and shown in Fig. 4. A progressive deactivation process was
observed in 12 h. During the catalyst decay, formation of MO increases
at expensed of IP. Similar result has been observed by J.I. Di Cosimo et al.
on Mg–Al mixed oxide derived by HT synthesized by coprecipitation
method [20]. Because the main reason of the deactivation was coke
formation caused by Adol condensation synthesis of tetramers and
Fig. 2. XRD patterns of M/HTas (a) and M/HTox (b): A. HT, B. Cr/HT and C. Zr/HT.
sites [16]. CO2–TPD curves of the calcinated samples were shown in
Fig. 3. The desorption of CO2 from HTox started as low as 350 K
reaching the maximum at about 400 K. Moreover, a shoulder centered
at about 550 K was observed in the recorded desorption profiles. CO2–
TPD curves of Cr/HTox and Zr/HTox showed two desorption peaks,
indicating the existence of at least two types of basic sites. It is
reported that the low-temperature maximum of CO2 desorption
centered at about 400 K was attributed to carbon dioxide species
adsorbed on weakly basic OH groups [17] and the high-temperature
CO2 desorption peak observed at 550 K was assigned to CO2 adsorbed
on O2− anions [18]. The stronger basic strength of Zr/HTox may be
related to the higher net electronic charge of the layers in Zr/HTas,
which resulted in more low coordination oxygen atoms. The high-
temperature peak of Cr/HTox was found between 550 K and 800 K.
The shift of the desorption peak of Cr/HTox to higher temperature
may be caused by the higher value of electronegativity of Cr than that
of Mg and Al.
Table 2
Texture properties of the materials.
Table 3
a
Activity result of acetone condensation
.
Sample
SBET (m2/g)
Vpore (cm3/g)
Pore size (nm)
a
b
c
Catalyst
Conversion (%)
Selectivity of IP (%)
Selectivity of MO (%)
HTox
Cr/HTox
Zr/HTox
147.0
137.3
105.8
0.9137
0.7458
0.4449
21.5
19.4
24.1
HTox
Cr/HTox
Zr/HTox
15.8
25.2
36.8
53.0
73.9
72.5
15.5
11.9
8.9
a
BET surface area.
Single point total volume at P/P0=0.97.
BJH desorption average pore diameter.
b
c
a
: Experimental conditions: 0.6 g cat., acetone: 0.02 mL/min, 513 K and atmospheric
pressure.

Y. Liu et al. / Catalysis Communications 11 (2010) 880–883
883
synthesis of IP. Sincethe selectivity of IP reached 70% above, these doped
mixed oxides should be potentially used in the synthesis of IP. Further
improvement may be achieved by adjusting the Cr or Zr modification, as
well as method of preparing the HT precursor.
Acknowledgements
The authors thank Professor Yongxiang Zhao and Xu Liang of
Shanxi University for their helps on CO2–TPD experiments and helpful
discussions on the results.
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Cr and Zr were successfully introduced into the lattice of the
hydrotalcites via coprecipitation method and produced more irregular
hydrotalcite structures. From the result of CO2–TPD, the incorporation of
Cr, Zr lead to creation of new strong basic sites and increase of the basic
sites. The basicity of the mixed oxide was an important factor that
influence the catalytic activity in the synthesis of IP. Consequently, the
modified mixed oxide exhibited improved catalytic performances in the