Cu/Mg/Al/Zr non-noble metal catalysts for o-phenylphenol synthesis

Article abstract of DOI:10.1039/c5ra25340c

Cu/Mg/Al/Zr hydrotalcite-like precursors with Zr⁴⁺:(Al³⁺ + Zr⁴⁺) atomic ratios between 0 and 1 were prepared by co-precipitation methods. The precursors were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric (TG) analysis and Fourier transform infrared spectroscopy (FT-IR). The results confirmed that well-defined layered double hydroxides (LDH) can be synthesized when the added Zr content is less than 0.25 in terms of Zr⁴⁺/(Al³⁺ + Zr⁴⁺) atomic ratio. The catalysts of Cu/Zn/Al/Zr mixed oxides can be obtained via thermal decomposition of hydroxide precursors, and can be used in dehydrogenation of 2-(1-cyclohexenyl) cyclohexanone (CHCH) to ortho-phenylphenol (OPP). Copper particles inside the catalyst act as active sites for dehydrogenation. Transmission electron microscopy (TEM), XRD, N₂O chemisorption and N₂ adsorption-desorption were performed to investigate the effect of Zr content on determining the copper particle size. Based on the catalytic performance test, it was concluded that the conversion of CHCH depends on the copper particle size of these catalysts.

Full text of DOI:10.1039/c5ra25340c

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Cu/Mg/Al/Zr NonꢀNoble Metal Catalysts for OꢀPhenylphenol Synthesis
Yongping Zeng*,† Tianchi Zhang†, Yueyang Xu ║, Peili Hu †, Ting Ye†, Zhehua
Jia†, Shengui Ju‡
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†College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou,
225002, China
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║Guodian Science and Technology Research Institute, Nanjing, 210031, China
‡ the State Key Laboratory of MaterialsꢀOriented Chemical Engineering, Nanjing
Tech University, Nanjing, 210009, China
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(* is the corresponding author: ypzeng@yzu.edu.cn)
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Abstract
Cu/Mg/Al/Zr hydrotalciteꢀlike precursors with Zr4+:(Al3++Zr4+) atomic ratios between
0 and 1 were prepared by coꢀprecipitation methods. The precursors were analyzed
using Xꢀray diffraction (XRD), scanning electron microscope (SEM),
thermogravimetric (TG) and Fourier Transform infrared spectroscopy (FTꢀIR). The
results confirmed that wellꢀdefined layered double hydroxides (LDH) can be
synthesized when the content of added Zr content is less than 0.25 in terms of
Zr4+/(Al3++Zr4+) atomic ratio. The catalysts of Cu/Zn/Al/Zr mixed oxides can be
obtained via thermal decomposition of hydroxides precursors, and can be used in
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dehydrogenation
of
2ꢀ(1ꢀcyclohexenyl)
cyclohexanone
(CHCH)
to
orthoꢀphenylphenol (OPP). Copper particles inside the catalyst act as active sites for
dehydrogenation. Transmission electron microscope (TEM), XRD, N2O
chemisorption and N2 adsorptionꢀdesorption were performed to investigate the effect
of Zr content on determining the copper particle size. Based on the catalytic
performance test, it was concluded that the conversion of CHCH depends on the
copper particle size of these catalysts.
Keywords:
Dehydrogenation;
Cu/Mg/Al/Zr
catalysts;
Hydrotalcite;
Orthoꢀphenylphenol
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1. Introduction
Many investigations have been made on the dehydrogenation of alicyclic
compounds. For instance, the only important industrial synthesis of oꢀphenylphenol
(OPP) is still the cyclohexanone method, in which 2ꢀ(1ꢀcyclohexenyl)cyclohexanone
(CHCH) is obtained from cyclohexanone via aldol condensation and dehydration. At
present, the catalysts used for OPP dehydrogenation generally use noble metals as the
active ingredients such as Pt.1,2 Only a few studies of the nonꢀnoble metals catalysts in
the OPP synthesis process have been reported in literature.3 Thus, the synthesis of
nonꢀprecious metals dehydrogenation catalysts is very important to minimize both
capital and operative expenses. Cuꢀbased catalysts are applied in various industrial
hydrogenation processes, and most of the catalysts when reduced with care can afford
high dispersion of copper, showing good activity. However, the catalysts still suffered
from thermal sintering and poisoning due to the poor dispersion. It is shown that the
Cuꢀcontaining catalysts with high dispersion show good catalytic selectivity and
activity in some reactions such as hydrogenation.4ꢀ6 However, Cuꢀcontaining catalysts
are not widely applied for dehydrogenation reactions of the organic compounds.
Hydrotalcite (HT)ꢀlike compounds are a new type of interesting materials which
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have been applied as ion exchangers, catalyst supports, catalysts, and composites.7ꢀ14
Their empirical formula is [MII
M
x(OH)2]x+[An− ]x−·mH2O, where MII is a
III
1−x
x/n
divalent cation such as Cu2+, Zn2+, or Mg2+ , MIII is a trivalent cation such as Fe3+, Al3+,
or Ga3+. An− is an anion with a negative charge such as CO3 and NO3 , and m is the
2−
−
molar amount of water in the interlayers.15,16 The asꢀsynthesized metallic oxides
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exhibit highly homogeneously dispersed M3+ and M2+ at the atomic level, high
specific surface area, high sintering stability, and strongly basic properties.17ꢀ19
Thermal treatments leads to dehydroxylation, dehydration, and destruction of
interlayer anions, and form the basic metallic oxides with high surface area, high
dispersion of the active centers and better resistance to sintering.20 The homogeneous
distributions of metallic oxides in precursors can form the highly dispersed and much
stable active metallic centers on catalyst surface after calcination and reduction.21
Thus, well dispersed Cuꢀbased catalysts with high stability, large surface area, and
moderate or strong basic properties can be synthesized via the thermal decomposition
of the hydrotalciteꢀlike precursors.
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In addition, the dispersibility and size of copper particles might significantly affect
the stability of the catalyst. Some literatures reported the relevant deactivation
mechanism for commercial catalysts.22ꢀ24 The low catalytic stability is currently a
limiting factor in applying Cuꢀcontaining catalystsin the hydrogenation process. How
to solve this problem is still the key to applying the Cuꢀcontaining catalysts in the
hydrogenation process. The introduction of Zr has been reported to improve the
surface basicity and Cu dispersion, and hence to influence the activity of catalysts.25ꢀ28
Furthermore, the presence of Zr can provide high stability and resistance to redox
cycles.29 However, very few studies reported the effect of addition of zirconium on
tuning the particle size of Cu inside the catalyst. The detailed structure–activity
relationship of the ZrꢀCu material is hence lacking and is the subject of this work.
In this work, a series of Cu/Mg/Al/Zr catalysts obtained from Cu/Mg/Al/Zr layered
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double hydroxides were prepared with Zr4+/(Al3++Zr4+) atomic ratios between 0 and 1.
Catalyst were characterized using Xꢀray diffraction (XRD), scanning electron
microscope (SEM), N2 adsorptionꢀdesorption, transmission electron microscope
(TEM), highꢀmagnification transmission electron microscope (HRTEM), N2O
chemisorption, thermogravimetric (TG), H2 temperature programmed reduction
(H2ꢀTPR), Fourier transform infrared spectroscopy (FTꢀIR) and CO2 temperature
programmed desorption (CO2ꢀTPD).
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2. Experimental
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2.1. Preparation of Catalysts
The Cu/Mg/Al/Zr hydrotalciteꢀlike samples were prepared by coꢀprecipitation
method. The Cu2+/Mg2+/(Al3++Zr4+) atomic ratio was kept at 2.5:3.5:2 in the starting
solution, while the atomic ratio of Zr4+/Al3+ was changed from 0 to 1. Calculated
amount of Cu(NO3)2•3H2O, Mg(NO3)2•6H2O, Zr(NO3)4•5H2O and Al(NO3)3•9H2O
were added into deionized water to prepare a 200mL mixed salt solution A, where the
total concentration of (Cu2++Mg2++Al3++Zr4+) is 0.9 mol/L. Another solution (called
B) was an aqueous of Na2CO3 and NaOH, in which the concentration of Na2CO3 and
NaOH are both 1 mol/L. The A and B were slowly added to a fourꢀnecked flask with
vigorous stirring and heated to the reaction temperature of 60 ˚C with the pH
maintained at 9.5. The resulting precipitant were aged for 6 hour and recovered with
filtration, then washed with distilled water to remove the trace sodium. Drying was
performed in the air at 90 ˚C for 12 hours prior to the calcination conducted in air at
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450 ˚C for 4 hours. Then the obtained powders were ground and extruded to strip
catalysts. The catalysts were reduced using H2 at 300 ˚C for 2 hours, followed by
reacted at 330 ˚C. Synthesized Cu/Mg/Al/Zr catalysts were denoted as CMAZꢀN,
where N = 0, 1, 2, 3 and 4 denoting the Zr4+/(Al3++Zr4+) ratio of 0, 0.25, 0.5, 0.75 and
1, respectively.
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2.2. Characterization of Samples
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Xꢀray diffraction (XRD) patterns were measured by the Xꢀray diffraction
instrument (Bruker AXS, D8 Advance) with the wavelength of the Cu Kα radiation
source being 1.5406Å. The intensity data was collected over the 2θ range of 5ꢀ70°
with a step size of 0.05° and counting time of 1s each point under the condition of 40
kV and 40 mA. SEM images of hydrotalciteꢀlike precursors and catalysts were
obtained by a field emission scanning electron microscopy (Hitachi, Sꢀ4800II).
Thermal decomposition and stability for the catalyst precursors was studied by
thermogravimetry analysis (TGꢀDTG, PerkinElmer Pyris 1) under the N2 atmosphere
(30 mL/min) with the heating rate of 10 ˚C /min from 50 ˚C to 750 ˚C. The infrared
spectra of samples were recorded by a Fourier Transform spectroscopy (FTꢀIR, Bruker,
Tensor 27), and measurements were performed in the wavenumber range of 400ꢀ4000
cmꢀ1. Transmission Electron Microscope (TEM) images of lowꢀmagnification were
collected using a Tecnai 12 model machine for lowꢀmagnification observe samples.
Highꢀmagnification Transmission Electron Microscope (HRTEM) was performed
using Tecnai G2 F30 SꢀTWIN model machine for highꢀmagnification to observe the
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samples. The specific surface areas of the samples were computed from N2
adsorptionꢀdesorption isotherms obtained at the temperature of liquid N2 on a
BELSORPꢀMAX instrument. Firstly, the powders were outgassed at the temperature
of 200 ˚C for 6 h to ensure a clean surface before the measurement of the adsorption
isotherms. H2ꢀTPR was performed on the instrument Finesorbꢀ3010. The catalyst was
firstly treated in Ar (30 ml/min) with a 10 ˚C/min heating rate to 200 ˚C and kept it at
200 ˚C for 30 min. After the samples were cooled down to 100 ˚C, TPR were
conducted in the mixture of H2/Ar (molar ratio 1:9) flowing at 30 ml/min at a 10
˚C/min heating rate until 500 ˚C. Finally, the catalyst was cooled to room temperature
in the presence of Ar. The exposed surface area of copper (SCu) and the dispersion of
copper (DCu) were determined by adsorption of N2O and performed on Finesorbꢀ3010
instrument, using the method similar to that reported by Xia et al.30,31 Catalysts were
reduced (the procedure is described in TPR section) in H2/Ar (molar ratio 1:9)
mixture with a flow rate of 30 mL/min at a heating rate of 10 ˚C/min until 450 ˚C and
was kept at 330 ˚C for 30 min. The consumption amount of H2 in the TPR was
recorded as X. Then the reactor bed was purged in Ar to 50 ˚C. The mixture of
N2O/Ar (the molar ratio 1:4, 30 mL/min) was changed to oxidize copper atoms of
surface to Cu2O at the temperature of 50 ˚C for 1 hour. The reactor bed was then
flushed with pure Ar to get rid of the oxidant. Another TPR procedure was conducted
in the mixture of H2/Ar (molar ratio 1:9) with a flow rate of 30 mL/min at a 10 ˚C/min
heating rate up to 330 ˚C and was kept at 330 ˚C for 30 min. The consumption of H2
in this TPR procedure was recorded as Y. The exposed surface area of copper and the
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dispersion of copper were computed by the following equations:
SCu = (2Y × NA )/(1.4×1019 × X × MCu )(m2 ⋅ g−1) (1)
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2Y
DCu =
×100%
(2)
X
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where SCu is the exposed surface area of Cu per gram sample, DCu is the dispersion of
Cu, MCu the Cu atomic mass, NA is Avogadro constant, and 1.4×1019 is the number of
Cu atoms per m2.32 The basicity of catalysts was measured by CO2 temperature
programmed desorption (CO2ꢀTPD, Finesorbꢀ3010). The catalysts were firstly treated
with Ar (30 ml/min) by raising the temperature at the rate of 10 ˚C/min to 330 ˚C and
keep it for 30 min. Catalysts were saturated with carbon dioxide (30 ml/min) for 60
min after cooling to 100 ˚C, and then flushed in Ar (30 ml/min) to get rid of all
physical adsorbed molecules for 30 min. Finally, the TPD were performed in Ar (30
ml/min) flowing at a 10 ˚C/min heating rate until 600 ˚C. Finally, the catalyst was
cooled to room temperature in the presence of Ar.
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2.3 Catalytic Reactions
The dehydrogenation reactions of CHCH were performed in a homeꢀmade setꢀup
with a tubular quartz glass flow reactor (Φ19×560 mm). The reactor temperature was
measured by a thermocouple located in the middle of the catalyst bed. The
dehydrogenation reactions were carried out at 330 ºC and atmospheric pressure. All
catalysts were preꢀtreated by reduction under H2 (flow rate of 20 mL/min) at 300 ºC
for 2 hours. Adjusting the H2 flow rate at 10 mL/min, the reagent was added into the
reactor using a syringe pump. The reaction products were dissolved in alcohol and
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cooled to room temperature and analyzed using a gas chromatograph equipped with a
flame ionization detector (FID) detector.
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3. Results and Discussions
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3.1. Textural and Structural Properties of the Precursors
The XRD patterns of the precursors are shown in Figure 1. It is shown that the
structure crystallinity of the samples decreases with increase of Zr amount. The
reflection peaks for CMAZꢀ0 and CMAZꢀ1 show a wellꢀcrystallized hydrotalcite
structure (JCPDS 48ꢀ0601). The introduction of Zr in CMAZꢀ1 still exist the
formation of hydrotalcite structure, which was also observed by Jeong and Gao et
al.33,34 But, further increasing the amount of Zr, the characteristic peaks of
hydrotalcite structure disappeared and the amorphous structure occurred instead. The
amorphous structure occurred probably because of the introduction of the distortions
in the hydrotalcite sheets as a result of the replacement of Al3+ by Zr4+.35 Another
reason may be the formation of amorphous precipitates (i.e. Mg and Zr salts of
hydroxyl carbonates and hydroxides).36 In these systems, hydrotalciteꢀlike
microcrystalline phases consisting of small poorly ordered layers are more likely to be
present, leading to the loss of crystallinity for the samples. Therefore, we expect that
Zrꢀcontaining hydrotalciteꢀlike compounds are the main component in these
precursors, especially for the composition of Zr4+/ (Al3++Zr4+) less than 0.25.
SEM images of all precursors are shown in Figure 2. It can be observed that a
wellꢀdeveloped layered and plateꢀlike structure of hydrotalcite prepared by
coꢀprecipitation method without Zr added, could be easily identified in Figure 2a.37,38
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When the Zr4+/(Al3++Zr4+) ratio is up to 0.25 (i.e. CMAZꢀ1), the materials still
exhibits a plateꢀlike structure of hydrotalcite, though a destructed morphology is
observed (Figure 2b). Also, it can be found that the plateꢀlike structures are
denseꢀstacking and homogeneous. However, when the amount of Zr increases as
shown in Figures 2cꢀe, platelet particles tend to break into small pieces for another
three samples35 These samples consist of irregular particles that existed in diverse and
larger agglomerates with poor crystallinity, which is consistent with the XRD results.
We can conclude that the Zr/Al ratio has an obvious influence on the crystal
morphology.
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FTꢀIR (Supporting Information) was used to identify anionꢀcation and bond types
for the interlayers of the precursors, shown in Figure S1. The intense and broad band
at about 3500 cmꢀ1 is ascribed to the bond stretching mode of H2O molecules of
interlayers and the hydroxyl groups on the layers (νOHstr). The broad bands show that
the hydroxyl species are hydrogen bonded. A weak absorption band occurs at about
1600 cmꢀ1 because of the deformation bending modes of H2O molecules. The bands
between 1300 and 1550 cmꢀ1 are due to bonding stretching modes of the carbonate
species. CMAZꢀ0 and CMAZꢀ1 exhibit medium vibration at about 420 cmꢀ1 ascribed
to OꢀMetalꢀO bonds in the sheets. This vibration band gradually disappeared, which
can be ascribed to the increase of the Zr content.36 It also shows that introduction of
Zr has a significant effect on the crystallinity of hydrotalcite layer structure.
To clarify the thermal decomposition process during the calcination, TG and
differential thermal gravity (DTG) analysis techniques were performed (Figure S2 in
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the Supporting Information). The results show that there are three major weight losses
for all samples. The first peak of weight loss occurred at 100ꢀ200 ˚C, which can be
ascribed to the removal of physically adsorbed H2O molecules in the interlayers. This
suggests that the temperature around 150 ˚C was sufficient for the removal of
physisorbed water. The second weight loss occurred at 250ꢀ400 ˚C, which may be
ascribed to the decomposition of carbonate ions and laminates hydroxy dehydration.
There are some differences for the third weight loss in these materials. The peak of
weight loss peak for CMAZꢀ0 occurred at 600ꢀ650 ˚C. The weight loss peak for
CMAZꢀ1 drops to 500ꢀ550 ˚C compared to CMAZꢀ0, which is ascribed to the
introduction of Zr in the structure. This might reduce the electrostatic interaction
between the anions and layers, that can result in lower thermal stability.39,40 For the
three samples of CMAZꢀ2, ꢀ3 and ꢀ4, the third stage almost disappeared, which
implies that hydrotalcite laminate structure disappeared with the increase of Zr
content.
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3.2. Textural and Structural Properties of the Calcined Samples
Figure 3 shows the XRD patterns for the samples calcined after H2 reduction for 2
hours. It can be seen that there are no characteristic peaks of hydrotalciteꢀlike
compounds in the XRD patterns. This shows the lamellar framework collapsed and
the structure is decomposed. Therefore, crystalline compounds containing Mg and Al
were not detected. There are no characteristic peaks of Zr in XRD patterns of
CMAZꢀ0, CMAZꢀ1 and CMAZꢀ2, which indicate that Zr was also in the amorphous
state.26,41 Cu0 is the main crystal phase in the CMAZꢀ0, CMAZꢀ1 and CMAZꢀ2. But,
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with the increase of Zr content (CMAZꢀ3 and CMAZꢀ4), the monoclinic zirconia
(mꢀZrO2, 2θ = 31.5°) characteristic peak occurs.
To investigate the effect of Zr content on the size of copper particles, the
full width at half maxima (FWHM) in the XRD patterns and the size of the copper
particles for different samples are listed in Table 1. The FWHM of the Cu0 particles
was calculated by Gaussian equation and the d(Cu0)XRD was calculated by Scherrer
formula (Equation 3).
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K
λ
d(Cu0 ) =
(3)
FWHM (
π
/180)cosθ
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Scherrer formula to calculate d(Cu0) size of each catalyst (K=0.89; λ=0.154 nm).
The results show that the average size of Cu0 crystalline was 3.97ꢀ13.23 nm after
H2 reduction. As the Zr content changes, the size of cooper particles reaches the
minimum value (3.97 nm) in these samples. Further increase of the Zr content results
in the increase of the copper particles size. It is clear that the particle size of Cu is
related to the amount of Zr added. Through adding the amount of Zr in the catalyst,
the size of copper particles can be controlled. However, excessive introduction of Zr
may cause the growth of copper particle.
Figure 4 shows a representative set of TEM images for copper particles synthesized
at two different compositions (CMAZꢀ1 and CMAZꢀ2) with the statistics of size
distributions for copper particles in Figure 4. The mean diameter and min/max
diameter were calculated by counting 100 particles (CMAZꢀ1) and 50 particles
(CMAZꢀ2) with a loupe from the TEM image of 195000 magnifications. From Figure
3 we can observe that CMAZꢀ1 and CMAZꢀ2 only present characteristic peaks of
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copper. Thus, we can confirm that particles in these two samples are copper
nanoparticles, which is confirmed by Zhang et al.42 A copper particle size on surface
of CMAZꢀ1 and CMAZꢀ2 was obtained from the corresponding size histograms. For
CMAZꢀ1, the average particle size is 3.85 nm, which is smaller than 7.79 nm for
CMAZꢀ2. It can be observed that copper particles are well dispersed in the oxide
matrix. Additionally, the dispersion of copper particles in CMAZꢀ1 is much better
than that CMAZꢀ2. This means that the introduction of Zr can tune the size of the
copper particles and have a significant impact on the dispersion of the copper particles
in the catalyst.29 In order to fully understand the inner structure of Cu particles in
CMAZꢀ1, the highꢀmagnification TEM (HRTEM) are obtained shown in Figure 5. Cu
particles size in Figure 5(a) is corresponding to the CMAZꢀ1 sample. Figure 5(b)
depicts the fringe spacing of 0.21nm ensures the presence of Cu (111) plane. HRTEM
elemental mapping images is shown in Figure S3 (Supporting Information), each
element is well dispersed in the catalyst, which illustrated the high dispersion of
Cu/Mg/Al/Zr catalyst.
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The pore volume and surface area of the catalysts after reduction are shown in
Table 2. The adsorption and desorption isotherms of samples are shown in Figure S4
(Supporting Information).The maximum surface area is 134.07 m2·gꢀ1 possessed by
CMAZꢀ1. Although CMAZꢀ0 surface area compare with CMAZꢀ1 is almost the same
(133.11 m2·gꢀ1), but the pore volume is nearly twice lower than CMAZꢀ1, respectively
0.358 cm3·gꢀ1 and 0.789 cm3·gꢀ1. It is speculated that the introduction of Zr may result
in smaller particle size and larger surface area and pore volume. Conversely, surface
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area and pore volume drop significantly in CMAZꢀ2, ꢀ3 and ꢀ4. This influence can be
ascribed to distortion and destruction of hydrotalcite laminates. The distortion
increases with the amount of Zr in the catalysts. The SBET of catalysts decreases
because of the destruction and distortion of hydrotalcite. The adsorption and
desorption isotherms of samples are shown in Figure S4 (Supporting Information).
The exposed catalyst surface area and dispersion of Cu were measured by N2O
adsorption and are shown in Table 2. When Zr4+/(Al3++Zr4+) ratio is less than 0.25, the
exposed surface area of Cu increased with the increase of Zr content. The addition
of Zr also enhanced the Cu dispersion, which may be due to the effect of valence
compensation and ion doping, i.e. the dissolved Zr in MgO crystal resulted in the
formation of cation defects on the surface of CuꢀMgO forming more active centers.43
This is consistent with the results observed by Velu et al.,44,45 who demonstrated that
substitution of Zr could improve the dispersion of Cu. With further increase of the
amount of Zr, Zr4+ may form the mꢀZrO2 crystalline structure and cause the decrease
of the proportion of Zr placed in the layered structures.46 Therefore, the existence of
mꢀZrO2 with little Zr4+ in the layered structures caused the lower DCu for the CMAZꢀ2,
CMAZꢀ3 and CMAZꢀ4.
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FTꢀIR spectra of calcined samples are shown in Figure S5 (Supporting Information).
Compared with the spectra of the precursors, the stretching vibration and absorption
bands positions were retained meaning that the layered structure of hydrotalcite is not
completely destroyed at the calcination temperature of 450 ˚C.
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the TPR profiles for all samples with various amount of Zr show a broad band of H2
consumption at the range of 150ꢀ400 ˚C. To get more insights into TPR results, the
band of H2 consumption were deconvoluted into two Gaussian forms denoted as α and
β peaks, respectively. The peak positions and corresponding contributions are listed in
the Table 3. The two peaks are attributed to the reduction of two different types of
CuO phase. The β peak at high temperature is due to the reduction of dispersed CuO.
The α peak is attributed to the reduction of highly dispersed CuO appearing at low
temperature.47 As shown in Table 3, α peak proportion in CMAZꢀ0 only is 14.97 %,
the lowest Cu content of all the samples. The proportion suddenly increased to
85.32 % in CMAZꢀ1 and reached the maximum. With the increasing amount of the Zr,
this proportion decreases. The addition of Zr caused the broad α peak for the highly
dispersed CuO phase. It is because Zr can strengthen the interactions of the Cu+/Cu2+,
leading to the higher reduction temperature.35 Excessive Zr amount assists to form the
mꢀZrO2, as observed in the XRD profile (Figure 3), thus resulting in the decrease of α
peak proportion.
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The surface basicity of the catalysts was detected with CO2ꢀTPD and is shown in
Figure 7. The broad band of CO2 consumption was deconvoluted into three Gaussian
forms denoted as weakly (α peak), moderately (β peak) and strongly (γ peak) basic
sites. The weakly basic sites are ascribed to hydroxyl groups, moderately basic sites
were related to AlꢀO, ZrꢀO or MgꢀO pairs, and strongly basic sites can be related to
oxygen atoms with low coordinations.35,47 The peak positions and corresponding
contributions are also shown in Table 4. The strength of strongly basic sites is
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markedly enhanced for CMAZꢀ1 after the introduction of Zr. While, the strength of
strongly basic sites decreased for subsequent CMAZꢀ2, CMAZꢀ3 and CMAZꢀ4. More
specifically, in CMAZꢀ4, strongly basic sites disappeared. CMAZꢀ1 has the most
strongly basic sites among these samples. The number of strongly basic sites increases
because of the increase of Zr content in the layered structures. The higher electronic
density of the layers in the samples with higher Zr content prohibits some oxygen
atoms to coordinate with Zr atoms.48 On the other hand, the number of strongly basic
sites decreases with further increase of Zr amount in the catalysts due to the
transformation of amorphous zirconia (amꢀZrO2) to monoclinic zirconia (mꢀZrO2)
(observed in Figure 3). This transformation of structure may lead to less strongly
basic sites.49
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3.3. Catalytic Performance
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Catalytic activity and selectivity of catalysts were tested in dehydrogenation of
CHCH to OPP. Figure 8 shows the selectivity and conversions as a function of the
Zr4+/(Zr4++Al3+) atomic ratio. Clear differences can be observed between the nature of
the catalysts and corresponding catalytic performance. There is a significant change in
the activity observed with the increase of the amount of Zr on these samples. The
average conversion of CHCH reaches around 98 % for CMAZꢀ0 to CMAZꢀ3 and
decreased to 67.5 % for CMAZꢀ4. The selectivity to OPP is also shown in Figure 8.
There is a significant difference for these samples. The selectivity of OPP reached a
maximum for Zr4+/(Zr4++Al3+) atomic ratio 0.25 (CMAZꢀ1). Further increase in the
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amount of Zr on the calcined samples leads to drastically toward 4.2 % with
Zr4+/(Zr4++Al3+) atomic ratio of 1. Cu/Mg/Al/Zr catalysts with a better dispersion was
prepared with the introduction of an appropriate Zr amount. In addition, the
sintering of catalysts can be effectively inhibited because the catalysts can form with a
more suitable structural morphology, which was an important factor. The enhanced
crystallinity enables more surface are of the catalyst support of the active sites for
dehydrogenation. The dehydrogenation performance follows the similar trend as that
of dispersion of Cu. For CMAZꢀ1 where Zr4+/(Zr4++Al3+) atomic ratio is low, the
sample has a high pore volume as observed by BET measurement (Table 2). The
catalysts exist in the form of the agglomeration of active sites at high Zr4+/(Zr4++Al3+)
atomic ratio resulting in largeꢀsized particles. From the CO2ꢀTPD profiles in Figure 7,
the OPP selectivity is influenced by the distribution of basic sites on the surface of
catalysts. The catalytic selectivity is also related to the amount of reducible copper,
although the selectivity is not proportional to the atomic ratio of Zr4+/(Zr4++Al3+)
shown in Figure 8. The appropriate amount of Zr added in the sample is 0.25 for the
atomic ratio of Zr4+/(Zr4++Al3+). H2ꢀTPR results and reaction results confirmed this
observation. CMAZꢀ1 shows the highest proportion of low temperature peak (around
85.3 %) in H2ꢀTPR, and the better dispersed CuO in this sample. Guo and Wang et al.
reported that catalytic activity of Cuꢀbased dehydrogenation catalysts is related to
well dispersed copper species.47, 50 Therefore, the variation trends of catalytic activity
of catalysts would be easy to understand in view of the similar trends of the dispersion
of copper species, which was illustrated by XRD and TPR results. In addition, it is
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clear that the OPP selectivity is also affected by the Zr amount, which tuned the
dispersibility of copper species. The exposed surface area of copper measured by
chemisorption of N2O is an important parameter for Cuꢀcontaining catalysts. The
catalytic activity depends on the exposed surface area of Cu. The exposed surface area
of Cu increased obviously with the increase of Zr content. The maximum dispersion
(22.5 %) of Cu is obtained for CMAZꢀ1 and the selectivity of OPP also reached a
maximum. The number of strongly basic sites decreases with further increase of Zr
amount in the catalysts because of the transformation of amorphous zirconia
(amꢀZrO2) to monoclinic zirconia (mꢀZrO2). This structural transformation may lead
to less strongly basic sites. Zhou et al. studied on Cu/ZrO2 catalysts, which consisted
of tetragonal zirconia with Cu content lower than 10 wt%.51 They referred to as the
interaction between Cu species and the ZrO2 support which hindered the phase
transformation to mꢀZrO2 based on the vacancy model of oxygen.52
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‐H2O
O
O
OH
OH
+H2
‐H2
‐H2
BP
OCC
OCHP
OPP
CHCH
‐H2
O
DBF
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Scheme 1 Synthesis route of OPP from CHCH and its byproducts.
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The goal of this investigation is to shed light on the effect of different Zr content on
the catalytic properties in relation to byproducts. The dehydrogenation process
consists of a sequence of hydrogenation or dehydrogenation reactions, in which the
CHCH is successively transformed into oꢀcyclohexylcyclohexanone (OCC),
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oꢀcyclohexylphenol (OCHP) and finally OPP (Scheme 1).53 Besides, there are small
amount of biphenyl (BP) and dibenzofuran (DBF) among byproducts. It is because
that OPP can further dehydrogenated to DBF and dehydrated to BP. Catalytic
selectivity for primary products of all catalysts is shown in Figure 9. When OPP
selectivity increases, the byproducts OCC and OCHP naturally decrease. Moreover,
OCC is the main byproducts. According to the route shown in the Scheme, the OPP
selectivity is significantly affected by the amount of Zr. Mean Cu0 crystalline sizes is
the smallest in CMAZꢀ1 and OCC content is the lowest. This suggests that smaller
copper particles size of catalyst results in the better copper dispersion. Thus, the
conversion of CHCH to OPP is more effective. While mean Cu0 crystalline sizes
increases, copper distribution becomes worse. The conversion of CHCH mainly
occurs in the first step reaction and the main byproduct is OCC. In conclusion, copper
particle size has a significant effect on the OPP catalytic activity and selectivity. The
introduction of Zr can control the copper particle size. The small mean Cu0 crystalline
size is in favor of the conversion of CHCH to OPP.
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Figure 10 shows the catalytic selectivity to OPP with the reaction time. The
corresponding conversions can be found in Table 1S (Supporting information). During
the reactions, CMAZꢀ1 always shows the highest catalytic performance and the
selectivity is still above 65% after 7 hours reaction. With the increase of Zr content,
CMAZꢀ4 almost lost its catalytic activity. The copper particles size and high
dispersion of containing active copper centers on surface have a significant impact on
the catalytic stability. Chraska et al. studied the effect of the particle size on the
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stability of the catalysts and observed that the stability for small particles can be
illustrated by the lower surface energy of ZrO2.54 Purnama et al. observed a significant
stabilizing effect of Cu centers in ZrO2.55 This may explain the stabilization of the Cu
particles with a diameter of about 3 nm. It is clear that the CMAZꢀ1 catalyst prepared
via coꢀprecipitation method shows a stable catalytic performance during this reaction
time.
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4. Conclusion
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CuꢀMgꢀAlꢀZr hydrotalciteꢀlike compounds with different atomic ratio of
Zr4+/(Al3++Zr4+) were successfully prepared by coꢀprecipitation methods. The
introduction of Zr can improve the catalytic properties of CuꢀMgꢀAl catalyst prepared
via hydrotalciteꢀlike precursors. The catalytic activity and selectivity for OPP
synthesis are related to the size of Cu species and the amount of Zr in catalysts. The
optimum content of Zr is about Zr4+/(Al3++Zr4+) = 0.25 (CMAZꢀ1), showing the
highest catalytic stability and selectivity above 65% even after 7 hours of reaction.
The existence of Cu nanopatricles is confirmed by analysis using XRD, TEM and
HRTEM. Introducing Zr affects the size of Cu nanoparticles, smaller Cu nanoparticles
always shows the better catalytic performance ; and CO2ꢀTPD results show that Zr
can lower the content of Al on the catalyst, and generate the strongly basic sites. This
may weaken the interaction between Al2O3 and CuO so to avoid the agglomeration of
active sites. CMAZꢀ1 always shows the highest catalytic stability and the selectivity
keep above 65% even after 7 hours reaction.
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Acknowledgements
3
We thank Dr. Peyman Moghadam at Northwestern University and Prof. Gui Han at
Yangzhou University for reading this manuscript and giving the valuable suggestions.
This work was supported by the Natural Science Foundation of China (Grant No.
20806064 and 21106125), the Natural Science Foundation of Jiangsu Province, China
(BK20131227). Generous allocations of test were provided by Testing Center of
Yangzhou University and a Project funded by the Priority Academic Program
Development of Jiangsu Higher Education Institutions.
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Supporting Information Available: FTꢀIR, DGꢀDTG, N2 adsorptionꢀdesorption
isotherms and elemental mapping images of catalysts. This material is available free
of charge via the Internet at http://pubs.rsc.org.
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Main Figures and Tables
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Fig. 1 XRD patterns of the Cu/Mg/Al/Zr precursors with various Zr content.
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Fig. 2 SEM images of uncalcined samples: (a) CMAZꢀ0, (b) CMAZꢀ1, (c) CMAZꢀ2,
(d) CMAZꢀ3, and (e) CMAZꢀ4.
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Fig. 3 XRD patterns of the calcined Cu/Mg/Al/Zr catalysts after reduction at 300 ˚C.
0；（ ）
(●) Cu
▽
◆
■
CuO; ( ) Cu2O; ( ) mꢀZrO2
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