The role of hydrotalcite-modified porous alumina spheres in bimetallic RuPd catalysts for selective hydrogenation

Article abstract of DOI:10.1016/j.catcom.2014.05.017

By utilizing alumina previously modified by in-situ growth of varying amounts of hydrotalcites, a series of heterogeneous bimetallic catalysts with constant low loading (0.3 wt.%) and identical Ru:Pd ratio (1:1) were prepared based on co-impregnation for selective hydrogenation of dimethyl terephthalate to dimethyl cyclohexane-1,4-dicarboxylate. Nanoparticles confined in the reticular structure were observed. The resulting candidates show the superior catalytic activity over that supported on the unmodified alumina, and reach the highest in the optimized sample RuPd/HT_C-Al₂O ₃-1. This promotion and optimization effect could be ascribed to the improved dispersion and more supply of hydrogen and acid sites especially with medium strength.

Full text of DOI:10.1016/j.catcom.2014.05.017

The Role of Hydrotalcite-modified Porous Alumina Spheres in Bimetallic
RuPd Catalysts for Selective Hydrogenation
Jiali Chen, Li Guo, Fazhi Zhang
PII:
DOI:
Reference:
S1566-7367(14)00198-8
doi: 10.1016/j.catcom.2014.05.017
CATCOM 3911
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Catalysis Communications
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Revised date:
Accepted date:
17 March 2014
6 May 2014
19 May 2014
Please cite this article as: Jiali Chen, Li Guo, Fazhi Zhang, The Role of Hydrotalcite-
modified Porous Alumina Spheres in Bimetallic RuPd Catalysts for Selective Hydrogena-
tion, Catalysis Communications (2014), doi: 10.1016/j.catcom.2014.05.017
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ACCEPTED MANUSCRIPT
The Role of Hydrotalcite-modified Porous Alumina Spheres in
Bimetallic RuPd Catalysts for Selective Hydrogenation
Jiali Chen, Li Guo, and Fazhi Zhang*
State Key Laboratory of Chemical Resource Engineering
Beijing University of Chemical Technology
Beijing 100029, China
Tel: +86 10 64425105
Fax: +86 10 64425385
E-mail: zhangfz@mail.buct.edu.cn
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Abstract
By utilizing alumina previously modified by in-situ growth of varying amounts of
hydrotalcites, a series of heterogeneous bimetallic catalysts with constant low loading
(0.3 wt%) and identical Ru:Pd ratio (1:1) were prepared based on co-impregnation for
selective
hydrogenation
of
dimethyl
terephthalate
to
dimethyl
cyclohexane-1,4-dicarboxylate. Nanoparticles confined in the reticular structure were
observed. The resulting candidates show the superior catalytic activity over that
supported on the unmodified alumina, and reach the highest in the optimized sample
RuPd/HTC-Al2O3-1. This promotion and optimization effect could be ascribed to the
improved dispersion and more supply of hydrogen and acid sites especially with
medium strength.
Keywords: supported bimetallic catalysts, layered double hydroxides, selective
hydrogenation, dimethyl terephthalte, dimethyl cyclohexane-1,4-dicarboxylate.
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1. Introduction
Dimethyl cyclohexane-1.4-dicarboxylate (DMCD) is one important polymer
modification and prerequisite intermediate in the process of dimethyl terephthalte
(DMT) to 1,4-cyclohexanedimethanol (CHDM), one preferred linker candidate for
highly valued polyester paints and resins, (Scheme 1) [1]. Industrially, DMCD
production is carried out from DMT selective hydrogenation by sole precious Pd
catalysts with high loadings under the H2 pressure range (30-48 MPa) and reaction
temperature region (160-180 ºC). Hence, taking into the more severe supply of scarce
sources, various alternatives have been proposed for cutting the cost and maximizing
the catalytic efficiency [2-4]. For example, Pd nanoparticles with low loading (0.3
wt%) and high dispersion were reported as one promising candidate [2]; also, the
low-cost metal Ru was held as one potential case [3-4].
Historically, bimetallic catalysts have attracted continuous attention potentially
for the fascinating synergistic effect [5-9]; however, heterogeneous ones are generally
prepared by co-impregnation of precursors containing metal elements followed by
prevalent calcinations and reductions. Thus, the resulting properties are normally not
satisfying for the poor reproducibility and fragile immobilization. The typical facts are
partial aggregation, sintering, unevenness and leaching of particles frequently
happening in preparation and applications especially for highly exothermic reactions.
Accordingly, kinds of additional organic stabilizers or capping agents were employed
for some essential protection of dispersed particles [10-13]; unfortunately, they are
very susceptible to thermal treatments, and not gratifying from the view of “green”
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synthesis [14]. Thus, it remains one challenge to build potential candidates with
desirable dispersion for practical applications.
Recently, layered double hydroxides (LDHs), generally known as one class of
anionic clays with brucite-like structure where cations in layer and anions in hydrated
interlayer gallery can be tuned, have been of much interest [15-21]. Their distinctive
flexibility offers one access to fabricate novel carriers, catalyst or relevant precursors
with regulated composition, tailored structure, controlled active sites and desired
properties [17-19]. For instance, by coordinating compounds with LDHs surface,
Kohki et al. [18] fabricated one robust RuMnMn system with uniform composition
and distribution. Recent studies reported the promotion effect of stable hydrotalcite
platelets on catalytic performances in heterogeneous fields [2,20-21]. Therefore, by
taking advantages of these merits, it would be one desirable strategy to immobilize
bimetallic cases with high dispersion.
Here, inspired by the phenomenon that species at the perimeter of nanoparticles
could strongly anchor or immobilize metal sites escaping from migration across the
surface, agglomerating or coalescencing for large particles [22], in-situ modification
by varying amounts of MgAl-LDHs was previously implemented on the surface and
in the pores of θ-Al2O3, leading to the tailored reticular morphologies constructed by
cross stacking and oriented growing of platelets; and consequently, RuPd samples
with high dispersions were obtained by co-impregnation. Also, one reference with
same loadings of Ru and Pd was prepared by utilizing the unmodified θ-Al2O3. And,
for insights into the support influence, DMT selective hydrogenation tests and several
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characterizations were adopted.
2. Experimental
2.1. Preparation of modified porous alumina spheres
The porous θ-Al2O3 spheres with purity of 99.5% and an average particle size of
20-40 mesh, previously synthesized in our laboratory, were modified by in-situ
growth [2,23-24]. According to Mg consumption amount (wt%), modified alumina
was denoted as HT-Al2O3-0.5, HT-Al2O3-1, and HT-Al2O3-2, respectively. After
calcinations (450 ºC; 6 h), the terminal supports (marked as HTC-Al2O3-0.5,
HTC-Al2O3-1, and HTC-Al2O3-2, respectively) were obtained.
2.2 Preparation of Supported Bimetallic RuPd Catalysts
Bimetallic RuPd catalysts were prepared by co-impregnation. In each case, the
total loading of two metals is 0.3 wt% and the Ru:Pd ratio is 1:1. In order for
understanding the effect of metals-loading on catalytic reactivity, two referenced
samples (Ref 1 and Ref 2) were synthesized with total metal amounts (0.2 wt% and
0.5 wt%, respectively) and the same Ru:Pd ratio 1:1 by using the pre-prepared support
HTC-Al2O3-1. More details including compositions and nomenclatures for
identification in the remainder of this paper are listed in Table 1. In addition, prior to
the liquid-phase reaction, each sample was pre-reduced by NaBH4. Specific
parameters are given in Supporting Information.
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2.3. Characterization
ICP-AES, XRD, SEM, BET, HRTEM, H2-TPR/TPD, NH3-TPD, and CO
chemisorptions were applied for comprehending composition, texture, structure,
morphology and surface properties. Detailed parameters are given in Supporting
Information.
2.4. Selective hydrogenation test
In each test, one starting material system consists of the reactant DMT (10 mmol),
the solvent ethyl acetate (80 mL) and catalyst (0.50 g), and the condition is 180 ºC
and 8.0 MPa for 6 h, which has been confirmed as the optimal for this reaction in our
previous experiments. The catalytic reactivity of each fresh case and its reusability is
investigated. Specific operations are described in Supporting Information.
The terminal products were quantified by GC-2014C (Shimadzu). Consequently,
the primary product is DMCD, and the main by-products are cyclohexanecarboxylate,
methyl benzecarboxylate, and trace 4-methyl-1-methyl benzecarboxylate. The specific
catalytic performances are expressed by following simplified equations:
R
DMT DMT
A
C = (1
)×100%
n
R
DMT
A
DMT  RDMCD
ADMCD

R
i i
A

i1
R
DMCD DMCD
A
S =
×100%
n
R
DMCD
A
DMCD

R
i i
A

i1
R
DMCD DMCD
A
Y =
×100%
n
R
DMT
A
DMT  RDMCD
ADMCD

R
i i
A

i1
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Where C represents catalytic conversion of DMT (%), S catalytic selectivity toward
DMCD (%), Y catalytic yield of DMCD (%), i a certain by-product, R the relative area
rectifying factor, and A denotes area percentage determined by GC.
3. Results and Discussion
3.1. DMT Selective Hydrogenation
Table 2 lists the test results of bimetallic RuPd catalysts for selective
hydrogenation of DMT to DMCD. Obviously, under the employed conditions, three
RuPd/HTC-Al2O3 catalysts with total metal loading 0.3 wt% and Ru:Pd ratio 1:1
exhibit the higher DMT conversion and DMCD yield than RuPd/Al2O3, while DMCD
selectivity in each case runs at a high value (~96.0%). And, for RuPd/HTC-Al2O3
catalysts, DMT conversion and DMCD yield follow this tendency:
RuPd/HTC-Al2O3-1 ＞ RuPd/HTC-Al2O3-0.5 > RuPd/HTC-Al2O3-2
Besides, as displayed in Fig. S1, bimetallic catalysts especially for
RuPd/HTC-Al2O3-1 behave the improved reusability, as compared with RuPd/Al2O3.
Also, two designed reference samples with different total metal loading amounts were
tested (Table 2). Under the employed reaction condition, Ref 1 with total metal
loading 0.2 wt% shows the lower DMT conversion than RuPd/HTC-Al2O3-1, while
Ref 2 with total metal loading 0.5 wt% exhibits a similar DMT conversion to
RuPd/HTC-Al2O3-1. DMCD selectivity is well preserved for each reference case. In
the following sections, RuPd/Al2O3, RuPd/HTC-Al2O3-1 and RuPd/HTC-Al2O3-2, as
the typical samples, were given into consideration for understanding the reaction
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results mentioned above.
Very recently, we have detailedly investigated the composition regulation of
bimetallic RuPd catalysts supported on porous alumina spheres for selective
hydrogenation of DMT to DMCD under various conditions; [25] and consequently,
the results suggest that DMT conversion shows one volcano-type tendency, and
reaches the highest value in the bimetallic sample with the Ru:Pd ratio 1:1, while
DMCD selectivity is well preserved in each bimetallic case despite somewhat
negative influence potentially derived from the progressive Ru introduction; and,
under each condition, bimetallic RuPd catalysts show higher reactivity over
monometallic Ru and Pd ones.
3.2. Compositional, Structural and Textural Features of Samples
ICP-AES (Table 1) gives the exact determinations of Ru, Pd and Mg loadings,
which are very similar to the feeding amounts, indicting the fine composition control.
As displayed in Fig. 1, the detected peaks (002), (111), (422) and (442) of θ-Al2O3 in
each case imply the well-preserved θ-Al2O3 after various treatments; characteristic
peaks (003) and (006) of LDHs, which appear in HT-Al2O3-1 and HT-Al2O3-2, and
disappear in HTC-Al2O3-1 and HTC-Al2O3-2, reveal the successful growth and
subsequent structural transformation of MgAl-LDHs in θ-Al2O3 [25-26]. Significantly,
the stronger typical peaks of MgAl-LDHs in HT-Al2O3-2 than in HT-Al2O3-1 could
indicate more formation of MgAl-LDHs in the former sample. And, no obvious peaks
of Ru and Pd are detected in catalysts, which might be determined by low loadings.
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The textural data (Table 3) show that the specific surface area and pore volume
are higher for RuPd/HTC-Al2O3 catalysts than for RuPd/Al2O3, and reach the highest
value in RuPd/HTC-Al2O3-1. These alterations could be rationalized by in-situ growth
of MgAl-LDHs, which leads to the formation of mixed oxides with higher surface
area after calcinations, and somewhat inevitable blocking, stacking or covering on the
surface or in the pores especially for RuPd/HTC-Al2O3-2 with more hydrotalcites
[2,20-21]. Thus, the catalyst textures are improved and optimized by regulated
supports, which might be beneficial for dispersed deposition of metal sites.
3.3. Morphology and Dispersion of Samples
SEM images (Fig. 2) show that, compared to the bare θ-Al2O3 and RuPd/Al2O3,
staggered platelets are easily observed in RuPd/HTC-Al2O3-1 and RuPd/HTC-Al2O3-2,
illustrating the more lush surface geography. HRTEM images (Fig. 3) show the
situations of sphere-like nanoparticles in each case. It is noteworthy that the reticular
grid where nanoparticles are located can be roughly distinguished, implying the
confined migration and mobility of nanoparticles on the surface or across the pores,
while this phenomenon is absence in RuPd/Al2O3. Relevant histograms show the
more concentrated distributions in RuPd/HTC-Al2O3-1 and RuPd/HTC-Al2O3-2
especially for the former than in RuPd/Al2O3; also, the detected largest is ~7 nm for
RuPd/HTC-Al2O3 cases, while it is ~9 nm for RuPd/Al2O3. The quantitative results
from CO chemisorptions (Table 1) show that the dispersion is higher for
RuPd/HTC-Al2O3 catalysts than for RuPd/Al2O3, and reaches the highest value in
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RuPd/HTC-Al2O3-1. These results suggest the improved dispersion especially in
RuPd/HTC-Al2O3-1, which would be helpful for high catalytic activity.
3.4. Hydrogen Reducibility, Desorption, and Surface Acid Properties of Samples
H2-TPR interprets the interaction between metals and that of metals with supports
from the standpoint of reducible behavior (Fig. S2). Compared to no obvious peaks in
θ-Al2O3, two overlapped peaks in each RuPd case could be attributed to the continual
reduction of binary sites with intimate interaction. The onset temperature moves
toward a lower position in RuPd/HTC-Al2O3-1 than in RuPd/Al2O3, suggesting the
facilitated reduction probably brought by the high dispersion; however, it increases
for RuPd/HTC-Al2O3-2, implying the delayed reduction potentially due to the
intensified interaction with supports. The hydrogen consumption amounts are 0.110,
0.099 and 0.081 mmol/g for RuPd/Al2O3, RuPd/HTC-Al2O3-1 and RuPd/HTC-Al2O3-2,
respectively, which indicate the occurrence of gradual inhibition for hydrogen
adsorption and migration on surface during reduction process. Utilizing the
hydrotalcite-modified support may lead to the formation of local confined space
where nanoparticles are positioned, which should be responsible for the limited
supply of accessible surface for hydrogen overflowing toward support surface.
Based on H2-TPD results (Table 4), RuPd/HTC-Al2O3 catalysts behave the higher
hydrogen desorption amounts than RuPd/Al2O3, and reach the highest one in
RuPd/HTC-Al2O3-1, which could be interpreted in terms of space supply for hydrogen
chemisorptions and storages [27-29]. The analysis results of NH3-TPD (Table 4)
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reveal that improved NH3 desorption behaviors including more acid sites especially
with medium intensity occur in RuPd/HTC-Al2O3 catalysts, and reach the optimal in
RuPd/HTC-Al2O3-1. With attention to the fact that more supply of hydrogen and acid
sites especially with medium intensity is conducive to the superior reactivity [30-32],
these results would offer some interpretations for the enhanced catalytic activity.
4. Conclusions
A series of bimetallic RuPd catalysts with constant loading of two metals (0.3
wt%) and identical Ru:Pd ratio (1:1) were prepared based on co-impregnation on
hydrotalcite-modified alumina by in-situ growth of MgAl-LDHs with varying
amounts. The obtained cases behaved the superior catalytic activity and improved
reusability over the conventional one by unmodified alumina, and reached the highest
in the optimized sample RuPd/HTC-Al2O3-1 for DMT selective hydrogenation.
Several measurements suggest that the contribution of designed samples especially for
RuPd/HTC-Al2O3-1 toward structural and morphological features, and surface
properties should be responsible for catalytic reactivity enhancement.
Acknowledgements
We would like to thank Professor Dianqing Li at Beijing University of Chemical
Technology, who offered us the pristine alumina spheres. This work was supported by
the National Natural Science Foundation of China, the 973 Program (No.
2011CBA00506), and the Program for Changjiang Scholars and Innovative Research
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Team in University (No. IRT1205).
Appendix A. Supplementary data
Supporting Information to this article is included.
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Figure Captions
Scheme 1. The industrial two-step hydrogenation process of DMT to CHDM.
Fig. 1. Powder XRD diffractions of θ-Al2O3 (a), HT-Al2O3-1 (b), HT-Al2O3-2 (c),
HTC-Al2O3-1 (d), HTC-Al2O3-2 (e), RuPd/HTC-Al2O3-1 (f), RuPd/HTC-Al2O3-2 (g)
and RuPd/Al2O3 (h).
Fig. 2. Representative SEM images of θ-Al2O3 (a), RuPd/Al2O3 (b),
RuPd/HTC-Al2O3-1 (c) and RuPd/HTC-Al2O3-2 (d).
Fig. 3. Representative HRTEM images of RuPd/Al2O3 (a), RuPd/HTC-Al2O3-1 (c) and
RuPd/HTC-Al2O3-2 (e) after NaBH4 pre-reduction. Corresponding distribution
histograms are also included.
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Fig. 1
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Figure 2
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Figure 3
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Scheme 1
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Table 1
Compositions, nomenclatures and metal dispersion of the RuPd catalysts.
Composition* (wt%)
Sample$
Dispersion# (%)
Mg
Ru
Pd
Total&
RuPd/Al2O3
RuPd/HTC-Al2O3-1
RuPd/HTC-Al2O3-2
Ref 1
-
0.15
0.14
0.15
0.11
0.24
0.15
0.15
0.16
0.10
0.26
0.30
0.29
0.31
0.21
0.50
56.7
75.8
62.1
--
0.96
2.05
0.98
1.02
Ref 2
--
$ Sample Ref 1 and Ref 2 were prepared using pre-prepared HTC-Al2O3-1 as support.
* determined by ICP.
& the sum of Ru and Pd.
# calculated based on CO chemisorptions.
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Table 2
Catalytic performances for DMT selective hydrogenation of the RuPd catalysts.
Sample
RuPd/Al2O3
DMT Conv / %
DMCD Sel / %
DMCD Yield / %
88.3
94.3
99.6
90.4
89.7
97.8
96.1
96.0
96.1
95.8
95.3
94.8
84.9
90.5
95.7
87.6
85.5
92.7
RuPd/HTC-Al2O3-0.5
RuPd/HTC-Al2O3-1
RuPd/HTC-Al2O3-2
Ref 1
Ref 2
* Reaction conditions: H2 pressure 8.0 MPa, temperature 180 ºC, reaction time 6 h,
and further details are listed in Experimental section.
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Table 3
Textural properties of samples.
Specific
Most
probable pore
size (nm)
32.0
Total
pore volume
(cc/g)
Sample
surface area
(m2/g)
83.6
θ-Al2O3
1.21
RuPd/Al2O3
85.2
30.9
0.99
RuPd/HTC-Al2O3-1
RuPd/HTC-Al2O3-2
94.8
30.0
1.12
91.6
29.4
1.06
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Table 4
Desorption behaviors of hydrogen and NH3 for concerned samples.
Total
Desorption
amount#
(mmol/g)
0.019
LTR*
MTR*
HTR*
Sample
amount*
(mmol/g)
2.093
/ %
/ %
/ %
RuPd/Al2O3
6.8
8.9
41.9
50.7
48.6
51.3
40.4
38.8
RuPd/HTC-Al2O3-1
RuPd/HTC-Al2O3-2
2.626
0.032
12.6
2.309
0.027
* calculated by NH3-TPD, LTR denotes the lower temperature region than 200 ºC,
MTR represents the moderate one between 200 ºC and 400 ºC, and HTR is the higher
one than 400 ºC.
# determined by H2-TPD.
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Highlights
New heterogeneous RuPd catalysts with high dispersion were synthesized.
Activity for DMT selective hydrogenation was promoted by modified supports.
RuPd/HTC-Al2O3-1 showed the highest catalytic performances.
Nanoparticles located in the reticular structure were detected.
Contributions of structure, morphology and surface properties were considered.
25