Pd nanoparticles immobilized on boehmite by using tannic acid as structure-directing agent and stabilizer: A high performance catalyst for hydrogenation of olefins

Article abstract of DOI:10.1007/s11164-012-0959-1

Boehmite-supported Pd nanoparticles (Pd-TA-boehmite) were successfully synthesized by a hydrothermal method using tannic acid as the structure-directing agent as well as stabilizer. The physicochemical properties of the Pd-TA-boehmite catalyst were well characterized by XPS, XRD, N ₂ adsorption/desorption, and TEM analyses. Catalytic hydrogenation of olefins was used as the probe reaction to evaluate the activity of the Pd-TA-boehmite catalyst. For comparison, the Pd-boehmite catalyst prepared without tannic acid was also employed for olefin hydrogenation. For all the investigated substrates, the Pd-TA-boehmite catalyst exhibited superior catalytic performance than the Pd-boehmite catalyst. For the example of hydrogenation of allyl alcohol, the initial hydrogenation rate and selectivity of the Pd-TA-boehmite catalyst were 23,520 mol/mol h and 99 %, respectively, while those of the Pd-boehmite catalyst were only 14,186 mol/mol h and 93 %, respectively. Additionally, the hydrogenation rate of the Pd-TA-boehmite catalyst could still reach 20,791 mol/mol h at the 7th cycle, which was much higher than that of the Pd-boehmite catalyst (5,250 mol/mol h) at the 4th cycle, thus showing an improved reusability.

Full text of DOI:10.1007/s11164-012-0959-1

Res Chem Intermed
DOI 10.1007/s11164-012-0959-1
Pd nanoparticles immobilized on boehmite by using
tannic acid as structure-directing agent and stabilizer:
a high performance catalyst for hydrogenation
of olefins
•
Jing Liu Xuepin Liao Bi Shi
•
Received: 6 October 2012 / Accepted: 27 November 2012
Ó Springer Science+Business Media Dordrecht 2012
Abstract Boehmite-supported Pd nanoparticles (Pd–TA–boehmite) were success-
fully synthesized by a hydrothermal method using tannic acid as the structure-directing
agent as well as stabilizer. The physicochemical properties of the Pd–TA–boehmite
catalyst were well characterized by XPS, XRD, N2 adsorption/desorption, and TEM
analyses. Catalytic hydrogenation of olefins was used as the probe reaction to evaluate
the activity of the Pd–TA–boehmite catalyst. For comparison, the Pd–boehmite cat-
alyst prepared without tannic acid was also employed for olefin hydrogenation. For all
the investigated substrates, the Pd–TA–boehmite catalyst exhibited superior catalytic
performance than the Pd–boehmite catalyst. For the example of hydrogenation of allyl
alcohol, the initial hydrogenation rate and selectivity of the Pd–TA–boehmite catalyst
were 23,520 mol/mol h and 99 %, respectively, while those of the Pd–boehmite cat-
alyst were only 14,186 mol/mol h and 93 %, respectively. Additionally, the hydro-
genation rate of the Pd–TA–boehmite catalyst could still reach 20,791 mol/mol h at
the 7th cycle, which was much higher than that of the Pd–boehmite catalyst
(5,250 mol/mol h) at the 4th cycle, thus showing an improved reusability.
Keywords Pd nanoparticle Á Boehmite Á Tannic acid Á Hydrogenation Á
Catalytic activity Á Reusability
J. Liu Á X. Liao (&)
Department of Biomass Chemistry and Engineering, Sichuan University, Chengdu 610065,
People’s Republic of China
e-mail: xpliao@scu.edu.cn
B. Shi
National Engineering Laboratory for Clean Technology of Leather Manufacture,
Sichuan University, Chengdu 610065, People’s Republic of China
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J. Liu et al.
Introduction
Metal nanoparticle catalysts show very high catalytic activity in a variety of
chemical reactions because of their high surface-to-volume ratio, high surface
energy and large surface curvature [1–3]. In general, metal nanoparticle catalysts are
prepared by immobilizing metal nanoparticles onto inorganic solid matrices [4, 5].
However, the as-prepared heterogeneous catalysts often suffer the problems of
aggregation and leaching-out of active species due to their high surface energy and
weak interaction with the matrices, which inevitably lead to reduced catalytic
activity and reusability [6, 7], and great efforts have been made to solve these
problems. One frequently used strategy is to encapsulate metal nanoparticles into
porous supports [8, 9], whereas their preparation processes often involve the
unfavorable high temperature calcination of inorganic supports. Another possible
method is to functionalize the supports with specific organic molecular ligands or
surfactant to stabilize metal nanoparticles [10, 11], but the relatively high cost and/
or complicated procedures make these approaches less attractive for industrial
applications. Thus, it remains highly desirable to find new approaches to prepare
heterogeneous metal nanoparticle catalysts with high activity and stability.
Recently, natural macromolecules as environment-friendly stabilizers have
attracted growing attention for metal nanoparticle synthesis due to their special
physicochemical properties. For instance, cellulose rich in ether oxygen and
hydroxyl groups can stabilize metal nanoparticles (Ag, Au, Pt, Pd) [12, 13]. Silk
fibroin with abundant thiol and amine groups is able to in situ reduce Au3? ions to
Au nanoparticles and further stabilize them in a highly monodispersed nature [14].
Among them, plant tannin, a kind of natural biomass widely distributed in plant leaf
and fruit, has been found to have the ability to chelate with many metal ions by the
formation of a five-membered chelated ring and further stabilize the corresponding
metal nanoparticles [15, 16]. Accordingly, it can be reasoned that plant tannins can
act as the stabilizer to immobilize metal nanoparticles on the supporting matrices.
Boehmite, a commonly used supporting matrix, is usually prepared by a
hydrothermal method [17]. Previous researches have shown that the addition of
specific polymers and/or surfactants can direct the crystal growth and morphology
of boehmite during the hydrothermal process [18, 19]. Considering that tannic acid
possesses some surfactant-like characteristics [20], it is possible to direct the crystal
growth of boehmite by using tannic acid as the additive. During the hydrothermal
process, tannic acid will chelate with aluminum in the boehmite lattice through its
dense ortho-phenolic hydroxyls, and influence the structural properties of boehmite,
such as specific surface area and pore size. Moreover, the residual adjacent phenolic
hydroxyls of tannic acid may also chelate with Pd2?, and further stabilize the
corresponding Pd nanoparticles generated after reduction. As a consequence, tannic
acid can act as bridge to link metal nanoparticles and boehmite with strong
interaction, which will ensure even distribution and high stability of the Pd
nanoparticles on boehmite. Based on the above strategy, a novel method for the
synthesis of boehmite immobilized Pd nanoparticles (Pd–TA–boehmite) was
developed in this work. The influence of introducing tannic acid on the distribution
of Pd nanoparticles and the crystal structure of the boehmite were investigated.
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High performance catalyst for hydrogenation of olefins
The catalytic activity and stability of the Pd–TA–boehmite catalyst were evaluated
by using hydrogenation of olefins as the probe reaction.
Experimental
Reagents
Palladium chloride (PdCl2), Al(NO3)3Á9H2O, tannic acid, allyl alcohol, a-metha-
crylic acid, styrene, methanol and other chemicals were all analytic reagents and
used without further purification.
Preparation of the Pd–TA–boehmite catalyst
30 mL of 0.8 mol/L Al(NO3)3Á9H2O solution was slowly dropped into 50 mL of
tannic acid solution (3.5 wt%) under stirring. The pH of the mixture was adjusted to
4.5 using ammonia aqueous solution (5 wt%) and kept stirring for 1 h. The resultant
suspension was sealed in a Teflon-lined stainless steel autoclave and maintained at
160 °C for 48 h under N2 atmosphere. After cooling naturally to room temperature,
the precipitate was centrifuged, washed with distilled water, and dried at 60 °C for
24 h. The obtained intermediate product was denoted as TA–boehmite.
Next, 1.0 g of the obtained TA–boehmite was mixed with 10 mL of 1.0 g/L Pd2?
solution under stirring for 24 h, and the Pd2? absorbed on TA–boehmite (Pd2?–TA–
boehmite) was further reduced by NaBH4 solution. The resultant Pd–TA–boehmite
was centrifuged, washed with distilled water, and dried under vacuum at 60 °C for
24 h. For comparison, Pd–boehmite was prepared using a similar method but
without the use of tannic acid.
Characterization
X-ray photoelectron spectroscopy (XPS, Kratos XSAM-800, UK) analysis was
conducted by employing Mg-Ka X radiation (hv = 1,253.6 eV) and a pass energy
was 31.5 eV. All of the binding energy peaks of XPS spectra were calibrated by
placing the principal C 1 s binding energy peak at 284.7 eV. The crystalline structures
were characterized by XRD (X’Pert Pro, Philips PW3040/60, Netherlands) with Cu-
Ka radiation. The specific surface area and pore structure of catalysts were analyzed
by N2 adsorption/desorption using surface area and porosity analyzer (BET,
Micromeritics Tristar 3000, USA). Transmission electron microscopy (TEM; Jeol
2010, Japan) performed at an acceleration voltage of 200 kV was used to observe the
morphology of the Pd nanoparticles in the catalysts. The compositions of the catalytic
reaction system were analyzed by gas chromatography (Shimadzu, CG-2010, China).
Catalytic hydrogenation of olefins
A specific amount of catalyst (containing 2.5 lmol Pd) and catalytic substrate
(10 mmol) were added to 20 mL of methanol in the autoclave. Then, the autoclave
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was heated to 30 °C under constant stirring (1,200 rpm) and 1 MPa H2 atmosphere.
When the reaction was completed, the products were analyzed by gas chromato-
graph and the catalyst was recovered for reuse. The activity of the catalyst was
evaluated by the initial hydrogenation rate (mol/Pd mol h) determined by the initial
slope of the curve of product yield versus time [21].
Results and discussion
Preparation mechanism of Pd–TA–boehmite
The XPS analyses supply the opportunity to understand the preparation mechanism
of the Pd–TA–boehmite catalyst. As shown in Fig. 1A, the O 1s peak of TA–
boehmite can be fitted into three contributions. Two major peaks are located at
531.56 and 532.60 eV, respectively, which are correspondingly correlated to
boehmite [22] and phenolic hydroxyl of tannic acid [23]. A small peak located at
533.15 eV is the up-shift of phenolic hydroxyl, and can be assigned to phenolic
oxygen chelated with metal ions [23, 24]. These results confirm that the chelation
reaction between Al3? ions and tannic acids took place during the hydrothermal
process, and therefore tannic acid was successfully in situ immobilized on boehmite.
Fig. 1 O1s XPS spectrum of TA–boehmite (A), Pd2?–TA–boehmite (B) and Pd–TA–boehmite (C); Pd
3d XPS spectrum of Pd–TA–boehmite (D)
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High performance catalyst for hydrogenation of olefins
After the adsorption of TA–boehmite to Pd2? ions, the peak intensity at
532.51 eV assigned to phenolic hydroxyl obviously decreases in comparison with
that of TA–boehmite (532.60 eV, Fig. 1A), while the peak intensity at 533.11 eV
related to chelated phenolic oxygen sharply increases compared with that of TA–
boehmite (533.15 eV; Fig. 1A). These observations indicate that Pd2? ions were
chelated by the phenolic hydroxyls of TA–boehmite and therefore more chelated
phenolic oxygen was generated in Pd2?–TA–boehmite.
In contrast, in the O 1s spectrum of the final product of Pd–TA–boehmite
(Fig. 1C), the peak intensity at 532.45 eV of phenolic hydroxyl becomes intense
along with the peak intensity at 533.20 eV of chelated phenolic oxygen being weak.
In addition, the Pd 3d spectrum of Pd–TA–boehmite (Fig. 1D) has two character-
istic peaks of element Pd(0), 340.85 and 335.74 eV [25]. These observations
suggest that, during the process of Pd2? reduction, a part of the chelated phenolic
hydroxyls was released and converted into phenolic hydroxyls via recombining with
protons from NaBH4. Meanwhile, the intensity ratio of chelated phenolic oxygen to
phenolic oxygen is higher in Pd–TA–boehmite than that in TA–boehmite, which
indicates that the generated Pd(0) nanoparticles were still anchored by the phenolic
hydroxyls of tannic acid. On the basis of the above analyses, the synthetic route of
Pd–TA–boehmite can be deduced as Fig. 2.
Fig. 2 Schematic illustration showing the general synthetic procedures of Pd–TA–boehmite
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J. Liu et al.
Effect of tannic acid
The introduction of tannic acid influences the crystallization of boehmite. As shown in
Fig. 3, the diffraction patterns of the as-prepared samples exhibit the features of
boehmite (JCPDS #21-1307). However, the diffraction peaks of Pd–TA–boehmite are
boarder and less intense, which indicates that Pd–TA–boehmite had lower crystal-
linity and/or smaller crystallites. Possibly, the chelation interaction between phenolic
hydroxyls of tannic acid and Al3? species increased the nucleus number of boehmite
crystallites, which inhibited the normal growth process of boehmite crystallites.
Further observation reveals that the XRD pattern of Pd–boehmite shows the (111)
refection of Pd(0) crystal at 2h = 40.28, while no peaks related to Pd species can be
observed in the XRD pattern of Pd–TA–boehmite because of weak intensity of total
XRD patterns of the Pd–TA–boehmite catalyst. These differences in XRD patterns
may mean that small Pd(0) nanoparticles with good dispersion were formed in the Pd–
TA–boehmite catalyst, which is confirmed by following TEM observations.
The influence of tannic acid on the textural properties of boehmite was
investigated by N2 adsorption/desorption method. As shown in Fig. 4, the specific
Fig. 3 XRD patterns of the as-
prepared Pd–boehmite (a) and
Pd–TA–boehmite (b)
Fig. 4 N2 adsorption–desorption isotherms (A) and pore size distribution (B) of Pd–TA–boehmite
(a) and Pd–boehmite (b)
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High performance catalyst for hydrogenation of olefins
surface area of the Pd–TA–boehmite catalyst is determined to be about
268.65 m2 g-1, much higher than that of Pd–boehmite (168.14 m2 g-1). It is
generally accepted that the catalysts with high surface area provide more active sites
and enable more chemicals to participate in reactions [26]. Compared with the Pd–
boehmite catalyst, the Pd–TA–boehmite catalyst shows much narrower pore
distribution. Thus, it is reasonable to believe that the obtained Pd–TA–boehmite
should have higher activity and selectivity in catalytic reactions. Considering that all
the other experimental parameters are similar, the changes in textural structure of
the catalyst should be caused by tannic acid, which directed the crystal growth of
boehmite via the interaction between tannic acid and the growing boehmite crystal.
Further work is underway to study the underlying mechanisms.
TEM observations show substantial differences in particle size and dispersion of
the Pd nanoparticles contained in the Pd–TA–boehmite catalyst and in the
Pd–boehmite catalyst. As shown in Fig. 5A, the Pd nanoparticles with the small size
Fig. 5 TEM images of Pd–TA–boehmite (A), Pd–boehmite (B), Pd–TA–boehmite after seven cycles in
hydrogenation reaction (C) and Pd–boehmite after four cycles in hydrogenation reaction (D)
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J. Liu et al.
of 3.6 nm are highly dispersed in the Pd–TA–boehmite catalyst. In contrast, the Pd
nanoparticles in the Pd–boehmite catalyst (Fig. 5B) show poor distribution, and
have the relatively larger size of 8.5 nm. Occasionally, some Pd nanoparticles are
larger than 15 nm, indicating that the Pd nanoparticles suffered serious aggregation
in the Pd–boehmite catalyst. These results are consistent with the XRD analysis. As
for the Pd–TA–boehmite catalyst, tannic acid could serve as an effective stabilizer
to suppress the excessive growth and aggregation of the Pd nanoparticles, which
ensured the small size and good dispersion of the Pd nanoparticles. However, in the
absence of tannic acid, Pd nanoparticles would be overgrowth and/or aggregations
on boehmite.
Catalytic activity evaluation
Table 1 shows the catalytic results of the as-prepared catalysts in the hydrogenation
of olefins with different structures. For all substrates, the Pd–TA–boehmite catalyst
was found to be more active than the Pd–boehmite catalyst. Especially in the
hydrogenation of ally alcohol, the initial hydrogenation rate and selectivity of the
Pd–TA–boehmite catalyst reached 23,520 mol/mol h and 99 %, respectively, much
higher than those of the Pd–boehmite catalyst (14,186 mol/mol h and 93 %). The
obvious improvements of catalytic activity and selectivity of the Pd–TA–boehmite
catalyst are the contribution of tannic acid to the distribution of Pd nanoparticles and
the textural properties of the boehmite. As confirmed by TEM and N2 absorption/
desorption determinations, compared with Pd–boehmite, Pd–TA–boehmite has a
higher specific surface area and well-distributed Pd nanoparticles of much smaller
size, which provides a larger fraction of exposed active Pd atoms available for more
substrates to participate in reactions, thus leading to higher catalytic activity. On the
other hand, the narrower pore distribution of the Pd–TA–boehmite catalyst is
beneficial for the molecular-shape selectivity of the reaction and efficiently inhibits
Table 1 Catalytic activity and selectivity of the catalysts for the hydrogenation of olefins
Catalyst
Substrate
Initial hydrogenation
rate (mol mol h-1
Selectivity
(%)
Conversion
(%)
)
Pd–TA–boehmite
Pd–TA–boehmite
Pd–TA–boehmite
Pd–boehmite
Allyl alcohol
a-Methacrylic acid
Styrene
23,520
17,600
13,920
14,186
12,750
11,230
[99a
100b
100c
93a
100b
100c
89.3
73.2
58.1
59.2
53.4
49.2
Allyl alcohol
a-Methacrylic acid
Styrene
Pd–boehmite
Pd–boehmite
Catalyst/substrate 2.5 lmol/10 mmol, solvent 20.0 mL methanol, temperature 30 °C, H2 pressure
1.0 MPa, Stirring rate 1,200 rpm, Reaction time 10 min
a
The selectivity to propanol
b
The selectivity to isobutyricacid
c
The selectivity to phenylethane
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High performance catalyst for hydrogenation of olefins
side reactions of isomerization. Meanwhile, it is noted that the catalytic activity
advantage of the Pd–TA–boehmite catalyst over the Pd–boehmite catalyst was
reduced but still evident when the substrates with side chain (a-methacrylic acid) or
large molecules (styrene) were used. As shown in Table 1, the initial hydrogenation
rate of allyl alcohol on the Pd–boehmite catalyst was 14,186 mol/mol h, and it was
increased to 23,520 mol/mol h on the Pd–TA–boehmite catalyst, an increase of
65.8 %. However, when a-methacrylic acid and styrene were used as the substrates,
the initial hydrogenation rates were increased by 38.0 and 24.0 %, respectively. The
reasons for these differences should mainly be ascribed to the steric hindrance of
substrates with side chain or large molecules making it difficult for the substrates to
access the Pd nanoparticles located in the porosity of the Pd–TA–boehmite catalyst.
Heterogeneous metal nanocatalysts often suffer the problems of leaching and
aggregation of active species during their recycling and eventually lose their
catalytic activity [6, 7]. In the present investigation, however, the Pd–TA–boehmite
catalyst exhibited much higher stability during its recycling use. As shown in Fig. 6,
the catalytic activity of the Pd–TA–boehmite catalyst was hardly reduced when
recycled in the hydrogenation of allyl alcohol, and its hydrogenation rate was still as
high as 20,791 mol/mol h at the 7th repeated application. According to the analysis
of ICP-AES, the loss of Pd amount was determined to be only 8.3 %, and no
obvious aggregation of the Pd nanoparticles was observed in the TEM image
(Fig. 5C). In contrast, the Pd–boehmite catalyst suffered a sharp decrease in
catalytic activity after four cycles, of which the hydrogenation rate reduced to
5,250 mol/mol h, only retaining 37 % of its initial activity. Based on the ICP-AES
analysis, the loss of Pd content in the Pd–boehmite catalyst was 56 % after recycled
four times, and serious aggregation of Pd nanoparticles was also observed (Fig. 5D).
These results confirm that tannic acid plays a crucial role in achieving high
reusability for the Pd–TA–boehmite catalyst, which ensures the stability and even
dispersion of Pd nanoparticles.
Fig. 6 Recycles of Pd–TA–boehmite and Pd–boehmite in the hydrogenation of allyl alcohol
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J. Liu et al.
Conclusions
In this work, we prepared a highly active and reusable Pd catalyst (Pd–TA–
boehmite) by a hydrothermal method using tannic acid as the structure-directing
agent as well as stabilizer. The use of tannic acid increased the specific surface area
of the boehmite, and narrowed the pore distribution, which was beneficial for the
high activity of the Pd–TA–boehmite catalyst. Moreover, tannic acid can act as a
stabilizer to link Pd nanoparticles and boehmite, which ensured the high stability of
the Pd–TA–boehmite catalyst during recycles. In addition, this strategy might be
applied to the synthesis of other high-performance metal nanoparticle catalysts
considering that tannic acid has the ability to chelate with many kinds of metal ions.
Acknowledgments We acknowledge the financial supports provided by the National Natural Science
Foundation of China (21,176,161) and National High Technology R&D Program (2011AA06A108). We
also give thanks to Test Center of Sichuan University for the help of TEM tests.
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