Controllable self-aldol condensation of cyclopentanone over MgO-ZrO2 mixed oxides: Origin of activity & selectivity

Article abstract of DOI:10.1016/j.catcom.2016.04.008

Mesoporous MgO-ZrO₂ mixed oxides with different Mg/Zr ratios have been synthesized by a coprecipitation method and employed to catalyze self-aldol condensation of cyclopentanone in a solvent-free condition under atmospheric pressure. Effects of temperature, reaction time, catalyst amount and Mg/Zr ratio on the reaction have been investigated. Remarkable raise in catalytic activity and selectivity has been found on MgO-ZrO₂ mixed oxides as compared with pure MgO or ZrO₂. Characterization including XRD, XPS, BET and CO₂- TPD has been carried out to figure out the relationship between catalyst properties and their catalytic activity.

Full text of DOI:10.1016/j.catcom.2016.04.008

Controllable self-aldol condensation of cyclopentanone over MgO-ZrO 2
mixed oxides: Origin of activity & selectivity
Decheng Liang, Guozhu Li, Yanhong Liu, Jinmeng Wu, Xiangwen Zhang
PII:
DOI:
Reference:
S1566-7367(16)30128-5
doi: 10.1016/j.catcom.2016.04.008
CATCOM 4631
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
5 January 2016
17 March 2016
11 April 2016
Please cite this article as: Decheng Liang, Guozhu Li, Yanhong Liu, Jinmeng Wu, Xi-
angwen Zhang, Controllable self-aldol condensation of cyclopentanone over MgO-ZrO2
mixed oxides: Origin of activity & selectivity, Catalysis Communications (2016), doi:
1
0.1016/j.catcom.2016.04.008
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ACCEPTED MANUSCRIPT
Controllable self-aldol condensation of cyclopentanone over
MgO-ZrO mixed oxides: Origin of activity & selectivity
2
Decheng Liang, Guozhu Li,*Yanhong Liu, Jinmeng Wu and Xiangwen Zhang
Key Laboratory for Green Chemical Technology of Ministry of Education, School of
Chemical Engineering and Technology, Collaborative Innovative Center of Chemical
Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, China
E-mail: gzli@tju.edu.cn
1

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Abstract
Mesoporous MgO-ZrO mixed oxides with different Mg/Zr ratios have been
2
synthesized by a coprecipitation method and employed to catalyze self-aldol
condensation of cyclopentanone in a solvent-free condition under atmospheric
pressure. Effects of temperature, reaction time, catalyst amount and Mg/Zr ratio on
the reaction have been investigated. Remarkable raise in catalytic activity and
selectivity has been found on MgO-ZrO mixed oxides as compared with pure MgO
2
or ZrO . Characterization including XRD, BET and CO -TPD has been carried out to
2
2
figure out the relationship between catalyst properties and their catalytic activity.
2

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1
. Introduction
Synthesizing jet fuel from biomass platform chemicals has attracted great attention
of scientists for a long time since petroleum resource is on the decline.[1, 2]
Cyclopentanone is an important platform compound from the selective hydrogenation
of furfural,[3, 4] which has been produced on an industrial scale by the
hydrolysis-dehydration of the hemicellulose in agriculture wastes and forest residues.
As derived from biomass, cyclopentanone is a promising raw material to attain
renewable jet fuel.[5-9] Aldol condensation is one of the key steps in the process of
synthesizing fuel molecules with high density starting from cyclopentanone.
The steric hindrance of cyclopentanone makes its self-aldol condensation difficult
to initiate. Currently, the reaction is catalyzed by homogeneous aqueous-base catalyst
such as KOH and NaOH, which are difficult to reuse in industrial procedure. Many
efforts have been made for the development of active heterogeneous catalysts for
efficient aldol condensation of cycloketones.[10-12] Mono-condensed product is
preferred for the production of high-density fuel with suitable viscosity. Because the
mono-condensed products can further react with cyclic pentanone to provide
di-condensed products. Therefore, the selectivity of self-aldol condensation should
also be concerned for catalyst development.
Herein, MgO-ZrO mixed oxides[13-21] have been employed to catalyze self-aldol
2
condensation of cyclopentanone in a solvent-free condition under atmospheric
pressure. To our knowledge, MgO-ZrO has not been used in this specific reaction.
2
3

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We focus on not only catalytic activity and selectivity of various MgO-ZrO mixed
2
oxides but also on the correlation between their physical-chemical properties and
catalytic performance.
2
. Experimental section
2
.1 Catalyst preparation
MgO-ZrO catalysts were prepared by a simple ultradilution coprecipitation
2
method using Mg(NO ) ·6H O and ZrOCl ·8H O. [17] More detailed experimental
3
2
2
2
2
procedures for the preparation of MgO-ZrO catalysts and cubic ZrO are described in
2
2
section 1.1 of supporting information.
.2 Catalyst characterization
XRD patterns of the catalysts were collected by X-ray diffractometer (Rigaku,
2
D/MAX-2500). Measurements were made over a range of 20° < 2θ <80° at a scanning
rate of 4 deg/min. The X-ray photoelectron spectroscopy (XPS) studies were carried
out on a PHI5000VersaProbe system, using an aluminum anode (Al Ka, 1486.6 eV).
The spot size for these acquisitions was ∼100 μm. The takeoff angle was 45°. The
pass energy was 188 eV for the survey scan and 47 eV for the region scan. The data
of nitrogen physisorption at 77 K were collected by Micromeritics ASAP 2020,
surface area and pore volume were calculated by BET and BJH approach, respectively.
CO -TPD measurements were conducted on a Chemisorption Physisorption Analyzer
2
(Altamira, AMI-300) using thermal conductivity detector. The sample was activated
at 873 K for 1 h in He flow prior to the adsorption of CO at 353 K. After purging the
2
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physically adsorbed CO at 353 K for 45 min, the sample was heated to 773 K with a
2
ramping rate of 10 K/min.
2
.3 Catalytic reactions
Self-aldol condensation of cyclopentanone was performed in a three-neck flask
using various catalysts. Detailed experimental procedures are described in section 1.2
of supporting information. Products at different time were analysed by GC-MS and
GC. Cyclopentanone conversion was calculated by its consumption using
2
-methyl-2-butanol as internal standard. 2-cyclopentylidene-cyclopentanone (C10)
and 2,5-dicyclopentylidene-cyclopentanone (C15) have been detected in the
condensation product. C10 selectivity was determined by peak area normalization
method. C10 yield was calculated by the following formula: C10 yield =
cyclopentanone conversion × C10 selectivity.
3
. Results and discussions
XRD patterns and XPS results of the as-synthesized catalysts are depicted in Fig.
S1 and Fig. S2, respectively. In XRD patterns, cubic phase of MgO and ZrO has been
2
observed in the MgO-ZrO mixed oxides. However, pure ZrO maintains monoclinic
2
2
phase at low temperature (Fig. S1). Moreover, the diffraction peak of MgO at 2θ =
2.92° is unobvious in MgZr11. Similarly, binding energies of Zr(3d) are almost the
4
2
+
4+
same for both ZrO and MgZr11 in Fig. S2(b). Mg and Zr have similar ionic
2
2
+
4+
radius, which makes it possible for Mg to replace Zr in the ZrO structure and
2
2
+
form Mg Zr O phase.[18] The dissolution of Mg in ZrO unit cell results in
x
1−x 2−x
2
5

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phase transformation (from monoclinic phase to cubic phase) of ZrO as well as the
2
"absence" of MgO in both XRD pattern and XPS spectrum of MgZr11. In the MgZr11
and MgZr41 samples, the Mg Zr O mixed oxide is observed (Fig. S2). In the
x
1−x 2−x
MgZr61 sample, the shape of Zr(3d) peaks is different to those observed for MgZr11
and MgZr41. Moreover, the peaks are shifted towards lower binding energy,
indicating that the enrichment of MgO which covered the Mg Zr O mixed oxide
x
1−x 2−x
2
+
particles. Because the capacity of ZrO to accommodate Mg ions into its structure is
2
limited, when MgO content continues to increase, free MgO phase is able to form and
can be detected by XRD and XPS. It can also be validated by the increase of peak
intensity of MgO with the increasing Mg/Zr ratio from 1:1 to 6:1. Therefore, it can be
concluded that MgO and ZrO are not simply mechanically blended in the obtained
2
mixed-oxide catalysts. The formation of Mg Zr O
mixed oxide may bring
x
1−x 2−x
changes in many aspects, such as surface area, pore size, and basicity.
Then, the obtained solid base oxides had been used to catalyze aldol condensation
of cyclopentanone. To optimize the reaction conditions, we studied the effects of
temperature and catalyst amount on the conversion of cyclopentanone and selectivity
of mono-condensed product (C10).
According to Arrhenius equation, reaction temperature has great impact on reaction
rate. Cyclopentanone conversion increases sharply from 8.7% to 88.2% within 4.5h
when the reaction temperature varies from 383 K to 403 K, as shown in Fig. 1(a). We
observed that the reaction system would boil at 403 K under atmospheric pressure.
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Moreover, the selectivity of C10 decreases slightly from 100% to 95.9% when the
temperature is increased from 383 K to 403 K. Therefore, 403 K is the optimized
temperature for self-aldol condensation of cyclopentanone.
Fig. 1. Effects of temperature (a) and catalyst amount (b) on the conversion of
cyclopentanone and selectivity of C10. Reaction conditions: (a) 0.1 g MgZr41/g
cyclopentanone, 4.5 h. (b) 403 K, 4.5 h.
The positive correlation between catalyst amount and conversion of
cyclopentanone is displayed in Fig. 1(b). But the effect of conversion promotion will
be weakened with the increasing of catalyst amount. It is noticed that yield of C10
oxygenates has reached 84.6% using 0.1 g catalyst/g cyclopentanone, which is already
very high compared with the result reported previously.[9] Therefore, 0.1 g catalyst/g
cyclopentanone has been considered as a suitable value for cyclopentanone
condensation.
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Fig. 2 shows that 100% of cyclopentanone has been converted within the first 1.5
hours in the presence of NaOH due to its high activity. But only 31.4% of
cyclopentanone has been selectively converted to C10 oxygenates. The yield of C10
drops to 20.2% with the prolongation of reaction time. It seems that homogeneous
base catalyst is not suitable for catalyzing cyclopentanone condensation.
Conversion of cyclopentanone reaches to 40% and 20% in 6 hours over MgO and
monoclinic ZrO , respectively, as displayed in Fig. 2(a), indicating their low activity.
2
Though their selectivity of C10 is high (98.9% for MgO, 100% for monoclinic ZrO2),
the yield of C10 is still very low due to low conversion. It's interesting that
cyclopentanone did not conduct any condensation in the presence of cubic ZrO2.
In comparison, the behaviors of various mixed-oxide catalysts are obviously
different and strongly depending on the Mg/Zr ratio. Similarly, conversion of
cyclopentanone becomes nearly unchangeable after 4.5 h. Yield of C10 is increased
with time at first, and then decreased with time growth due to further condensation of
C10. Reaction time should be limited to 4.5 h to attain maximum C10. Therefore, the
optimized reaction conditions are under 403 K for 4.5 h, and with 0.1 g catalyst/g
cyclopentanone.
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Fig. 2. Dependence of cyclopentanone conversion and C10 yield on time over various
catalysts. Reaction conditions: 403 K, 0.1 g catalyst/g cyclopentanone.
The conversion of cyclopentanone and selectivity of C10 over various catalysts
have been compared under the same optimized reaction conditions (Fig. 3). The most
commonly used homogeneous base catalyst, NaOH, was used as background catalyst.
1
00% conversion of cyclopentanone has been achieved over NaOH. However, its
selectivity of C10 is only 19.7%. Therefore, heterogeneous base catalyst with both
high activity and selectivity should be developed for controllable condensation of
cyclopentanone. Since basicity is one of the key factors affecting cyclopentanone
conversion and C10 yield, ZrO and MgO were firstly evaluated in this reaction. Their
2
basicity follows the order of ZrO < MgO, and corresponding cyclopentanone
2
conversions at 4.5 h are 19.1% and 39.7%. However, strong basicity makes it easier to
conduct further condensation of C10, yielding low content of C10. This is validated
by the experimental results. Herein, C10 selectivity of ZrO (100%) is higher than that
2
of MgO (98.9%). Therefore, to maximize the yield of C10, suitable heterogeneous
9

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catalyst with appropriate basicity should be developed.
Fig. 3. Cyclopentanone conversion and C10 selectivity over various catalysts.
Reaction conditions: 403 K, 4.5 h, atmospheric pressure, and 0.1 g catalyst/g
cyclopentanone.
MgO-ZrO mixed oxides were reported to be active, selective and reusable solid
2
base catalysts for aldol condensation, e. g., reaction between furfural and acetone.[13,
1
8-20, 22] Concerning the case of heterogeneously catalyzed aldol reactions，[23, 24]
it is reasonable to assume that a similar mechanism could take place over MgO-ZrO2
mixed oxide. Firstly, cyclopentanone adsorbs on the surface of MgO-ZrO mixed
2
oxide. Then, the formation of adsorbed enol or enolate species happens. Finally, this
1
0

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species is added to another cyclopentanone molecule (probably coadsorbed), with the
formation of 2-cyclopentylidene-cyclopentanone on the surface following dehydration.
Herein, compared with MgO and ZrO , catalysts of mixed MgO-ZrO series show
2
2
remarkable raise in both catalytic activity and selectivity. As seen in Fig. 3, their
activities follow the sequence of MgZr41 > MgZr21 > MgZr11 > MgZr61 > MgO >
ZrO . Moreover, all the MgO-ZrO catalysts show high selectivity of C10 (>90%). It
2
2
is in consistent with previous reports on similar reactions.[18, 20] The acidic-basic
properties of the final solid can be modified through tuning Mg/Zr ratio during
preparation procedure. For instance, Sádaba et al. [18] found that when Mg/Zr ratio≥
1
，the mixed oxides were consisted of Mg Zr O and MgO. Their results showed
x 1−x 2−x
that MgO-ZrO with nominal Mg/Zr ratio =1 had the best performance in the aldol
2
condensation of furfural with acetone.
CO adsorption and subsequent desorption can detect the sites with different basic
2
strengths: bicarbonates (weak base sites); bidentate carbonates (bridged and chelating,
medium-strength base sites) and monodentates (strong base sites).[25] Based on the
desorption temperature of the formed species, it was obtained that base centers
associated with bicarbonates correspond with 293-373 K, bridged bidentate carbonate
with 373-473 K, chelating bidentate carbonate with 473-573 K and unidentate
carbonate with temperatures over 573 K.[26] Fig. 4 displays the CO -TPD results of
2
various MgO-ZrO catalysts synthesized in this study. ZrO has a weak desorption
2
2
peak at 403 K. While MgO has a strong desorption peak at 463 K and a medium
11

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desorption peak at 643 K. New peaks at 431 K and 588 K for mixed oxides appear,
which is in consistent with Faba et al.'s results [22]. It indicates the presence of
bidentate carbonate (medium-strength base sites) and monodentates (strong base sites).
The sequence of the amount of medium-strength base sites at 431 K (bridged
bidentate carbonate) is MgZr41> MgZr21> MgZr11> MgZr61, which is identical to
that of their catalytic activities. It is reasonable to assume that medium-strength base
sites resulting from the presence of Mg Zr O phase, is the active site to conduct
x
1−x 2−x
aldol condensation. Base sites on ZrO or MgO are either weaker or stronger than that
2
of mixed oxides, resulting in relative low catalytic activity.
Fig. 4. CO -TPD of different MgO-ZrO mixed oxides.
2
2
1
2

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Pore size distribution of MgO-ZrO catalysts was measured as displayed in Fig. S3.
2
MgO-ZrO series with various Mg/Zr ratios have similar distribution of mesoporous
2
size (8.9-22.2 nm). Therefore, no correlation between the selectivity and pore size of
MgO-ZrO is found.
2
Similar volcano plots of surface area and yield of C10 oxygenates versus Mg/Zr
ratio are displayed in Fig. 5. The trend of surface area of the mixed oxide depending
on the Mg/Zr ratio is similar to that of C10 yield catalyzed by corresponding oxide.
Surface area of MgO-ZrO mixed oxides reaches its maximum when Mg/Zr=4/1,
2
where the highest yield of C10 has been achieved. When Mg/Zr ratio is higher than
1
:1, both phases of Mg Zr O and free MgO are formed.[18] During preparation,
x 1−x 2−x
the presence of MgO with suitable amount controls the crystaline growth of
Mg Zr O phase, which affects surface area of the obtained mixed oxide.
x
1−x 2−x
Consequently, when Mg/Zr ratio is 4:1, the surface area of the obtained mixed oxide
reaches a maximum, leading to the improvement of accessibility of the base sites.
Therefore, it can be concluded that medium-strength base sites of MgO-ZrO mixed
2
oxides with suitable basicity and good accessibility are responsible for the high yield
of C10 oxygenates.[18]
1
3

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Fig. 5. Surface area and C10 oxygenates yield of different MgO-ZrO catalysts.
2
Reusability tests of MgZr41 were carried out at standard reaction conditions (403 K,
.5 h, atmospheric pressure, and 0.1 g catalyst/g cyclopentanone), and results are
4
shown in Fig. S4. C10 yield dropped to below 30% when the spent catalyst was
simply collected, dried and used in the next cycle of reaction (cycle 2 and cycle 3).
However, the spent catalyst was reactivated after calcination. C10 yield reached to 80%
after calcination of the spent catalyst at 873K for 10h (cycle 4).
4
. Conclusions
Various active and selective MgO-ZrO mixed oxides have been synthesized for
2
self-aldol condensation of cyclopentanone using a simple coprecipitation method. The
results of CO -TPD and BET reveal that mesoporous MgO-ZrO catalysts possess
2
2
1
4

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high surface area and new medium-strength base sites. The amount of
medium-strength base sites and their accessibility are strongly depended on the ratio
of Mg/Zr, which makes MgO-ZrO mixed oxide a tunable catalyst for various
2
potential applications. MgZr41 has been prepared with suitable basicity and good
accessibility of active base sites. Cyclopentanone conversion and C10 yield reach
8
8.2% and 84.6%, respectively, at 403 K under atmospheric pressure in 4.5 h over
MgZr41.
1
5

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Graphical Abstract
Keywords
MgO-ZrO ; self-aldol condensation; alkalinity; surface area; cyclopentanone
2
1
9

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Highlights

Synthesis of MgO-ZrO mixed oxides with various Mg/Zr ratio for self-aldol
2
condensation of cyclopentanone;

Systematical investigation of the effects of temperature, reaction time and catalyst
amount on the reaction;


Exploring the origin of high activity and selectivity of MgO-ZrO mixed oxides;
2
Achievement of self-aldol condensation of cyclopentanone with high yield
(84.6%) under mild conditions.
2
0