Geometric effect of Ru/HSAG@mSiO2: A catalyst for selective hydrogenation of cinnamaldehyde

Article abstract of DOI:10.1039/c4ra04811c

A new strategy to synthesize a catalyst consisting of graphite-supported Ru nanoparticles encapsulated by mesoporous silica layers was developed via a facile and scalable wet-chemical process. The intended structures were confirmed with N₂ sorption, CO chemisorption, TEM and SEM. The geometric effect of the pores in the silica layer of Ru/HSAG@mSiO₂ was evaluated in the hydrogenation of cinnamaldehyde and showed 15% higher selectivity to unsaturated alcohol and three times higher turnover frequency (TOF) relative to unconfined Ru/HSAG catalyst with the equivalent size of Ru particles. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra04811c

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Geometric effect of Ru/HSAG@mSiO2: a catalyst for
selective hydrogenation of cinnamaldehyde
Cite this: RSC Adv., 2014, 4, 30180
*
*
Hangjia Shen, Haodong Tang, Haiyu Yan, Wenfeng Han, Ying Li and Jun Ni
A new strategy to synthesize a catalyst consisting of graphite-supported Ru nanoparticles encapsulated by
mesoporous silica layers was developed via a facile and scalable wet-chemical process. The intended
structures were confirmed with N2 sorption, CO chemisorption, TEM and SEM. The geometric effect of
the pores in the silica layer of Ru/HSAG@mSiO2 was evaluated in the hydrogenation of cinnamaldehyde
and showed 15% higher selectivity to unsaturated alcohol and three times higher turnover frequency
(TOF) relative to unconfined Ru/HSAG catalyst with the equivalent size of Ru particles.
Received 21st May 2014
Accepted 27th June 2014
DOI: 10.1039/c4ra04811c
www.rsc.org/advances
various metals, such as Pt,9 Cu,10 Ru.11ꢀ13 In comparison with Pt
catalysts which has high affinity towards C]C bond hydroge-
nation,14 Ru catalysts have showed good selectivity in cinna-
Introduction
With the development of mesoporous materials, they have
found their increasing applications in protein separation,
catalysis, environment protection, and photonic crystals, due to
their extremely high surface areas combined with large and
uniform pore sizes.1 In catalysis, mesoporous materials have
been used as novel functional materials for selective adsorption
and to conne guest molecules in reactions where selectivity is
highly required. When mesoporous materials are used as a
coating of nanoparticles, they are able to not only prevent these
nanoparticles from aggregating in reactions, but also improve
catalytic selectivity due to the geometric and electronic effects of
pores in mesoporous materials.2–5
maldehyde hydrogenation as well as high activity.15,16 A very
high selectivity to cinnamyl alcohol (92% selectivity up to 80%
conversion) was obtained on multilayer graphitic nanotube
supported Ru catalysts, whereas a 20–30% selectivity on Ru/
A12O3 catalyst with similar-sized Ru particles was observed
under the same conditions.17The enhancement of selectivity are
likely attributed to the geometric effect of the graphitic nano-
tube, while the improvement of activity can be ascribed to the
electronic effect of the graphite support that graphite can serve
as the electron donor to enrich the electron densities of metals
as a result leading to the high hydrogenation rates.6
Although the high surface area graphite appears to be a
desired support of metal-supported catalysts for cinnamalde-
hyde hydrogenation to unsaturated alcohol,8,18 there is still a
challenge to synthesize a catalyst combining the merits of both
geometric and electronic effects together. Herein, we report a
facile and scalable wet-chemical process to prepare graphite-
supported metal nanoparticles covered with a mesoporous
silica layer (Ru/HSAG@mSiO2). The Ru nanoparticles, sup-
ported on the graphite, are conned at the bottom of the
pores in mSiO2 layers. Due to the geometric effect that mSiO2
imposed on the catalyst, with the equivalent size of Ru particles,
Ru/HSAG@mSiO2 with respect to Ru/HSAG afforded a 15%
increase in unsaturated alcohol selectivity in cinnamaldehyde
hydrogenation.
It is known that the hydrogenation of cinnamaldehyde is a
typical model reaction to investigate the effect of geometric
structure of catalysts on selectivity.6 Richard reported that the
chemioselectivity in cinnamaldehyde hydrogenation could be
greatly improved by using Pt and Rh metal clusters encapsu-
lated in Y-type zeolites, because shape-selectivity effects in the
zeolite micropores impose an end-on adsorption of the mole-
cule via the C]O group on the encapsulated metal.7 However,
this conclusion was drawn without taking the effect of particle
morphology into account, as the same author demonstrated in
his later paper that high selectivity of unsaturated alcohol could
be achieved with increasing the particle size of Pt and Rh
metals. The effect of metal particle size is because a steric effect
of the phenyl group hampers the molecule to adsorb parallel to
the at metal surface thus favouring the adsorption and
hydrogenation of the C]O group with respect to the C]C
group on large facetted metal particles.8 Analogous effect of
metal particle size was also reported in several literature using
Experimental section
Catalyst preparation
Chemicals. All chemicals were analytical grade and used as
received without further purication. High surface area
graphite (HSAG 300 with D90 ¼ 32.1 mm) was obtained from
Timcal Ltd. (Switzerland). Tetraethyl orthosilicate (TEOS) and
Institute of Industrial Catalysis, Zhejiang University of Technology, Hangzhou 310014,
Zhejiang, P. R. China. E-mail: tanghd@zjut.edu.cn; Junni@zjut.edu.cn; Fax: +86-571-
88320930; Tel: +86-571-88320092
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hexadecyltrimethylammonium
bromide
(C16TAB)
was
SEM studies were carried out by using a Hitachi S-4700 at
purchased from GuibaoC.Ltd. (Hangzhou, China) and Shanghai 300 kV, the samples were pretreated by metal spraying.
Bioscience
&
Technology Co. Ltd. (Shanghai, China),
respectively.
Hydrogenation of cinnamaldehyde
Synthesis of Ru/HSAG. High surface area graphite was used
as the support to prepare catalyst by mechanical ball milling
method. Briey, mix the graphite with Ru3CO12 in a casting can
with 2 big iron balls (diameter 20 mm), 14 middle ones
(diameter 10 mm) and 24 small ones (diameter 6 mm) at 200
rounds per minute for 1 hour. The obtained product was then
reduced in H2 at 300 ꢁC for 60 min, denoted as Ru/HSAG.
Synthesis of Ru/HSAG@mSiO2. The composite was prepared
through a surfactant-directed sol–gel coating process by using
CTAB as a template. Typically, a suspension in water con-
taining Ru/HSAG particles (0.2 g) was dispersed in a mixture
solution composed of CTAB (0.14 g), water (40 g), ethanol
(24 g), and ammonia (2.5 mL) under ultrasonic treatment for
30 min. Then, TEOS (1.00 mL) was added drop wisely and
The reaction mixture composed of 0.05 g Ru/HSAG (0.12 g Ru/
HSAG@mSiO2), 1.0 mL cinnamaldehyde, and 50 mL iso-
propanol, was introduced into a 250 mL autoclave. Prior the
reaction, the autoclave was purged with hydrogen, and then the
ꢁ
reaction was performed at 100 C and 4.0 MPa hydrogen pres-
sure with vigorous stirring at 1000 rpm. The reaction progress
was monitored by taking approximate 0.50 mL samples at
different time intervals for quantitative analysis on an SHI-
MADZU 2014 GC equipped with a FID detector and a HP-5
capillary column. Only CALD, HCAL, HCOL and COL are
detected in the reaction products. The conversion and selec-
tivity were determined by area normalization method and
calculated with the formulas as below, where A is the peak area:
ꢀ
ꢁ
ꢁ
stirred for 4 h at 25 C. The as-made composite was collected
ACALD
ConversionCALD
¼
1 ꢀ
by ltration and washed with water and ethanol, respectively,
and then extracted by HCl–ethanol (2 mL/50 mL) solution to
remove the surfactant CTAB, washed with water and ethanol
again. Finally, the composite was dried at room temperature
under vacuum for 12 h, denoted as Ru/HSAG@mSiO2. The
sample of directly dried of as-made composites at room
temperature under vacuum for 12 h without any wash was
denoted as Ru/HSAG@mSiO2-w.
ACALD þ ACALC þ AHCAD þ AHALC
ꢂ 100%
ꢀ
ꢁ
ACALC
ACALC þ AHCAD þ AHALC
SelectivityCALC
¼
ꢂ 100%
Results
Structure of catalysts
Catalyst characterization
Nitrogen adsorption–desorption measurement was performed
at ꢀ196 ꢁC using an NOVA 1000e (Quantachrome Instruments)
micromeritics system. Before the ꢁmeasurement, the catalysts
were outgassed in vacuum at 200 C for 12 h. The Brumauer–
Emmett–Teller (BET) method was utilized to calculate the
specic surface areas. The pore size distribution derived from
the adsorption branches of the isotherms was calculated by the
DFT (Density Functional Theory) method, and the total pore
volumes (Vt) were estimated from the adsorbed amount at a
relative pressure P/P0 of 0.995. The micro-mesopore volume and
surface area were calculated by the t-plot method at relative
pressure P/P0 between 0.3 and 0.5.
Nitrogen sorption isotherms and pore size distribution of
catalysts are presented in Fig. 1. In addition to the typical type I
curves at low relative pressure (P/P0) and a H3 hysteresis loop
according to the IUPAC classication,19 Ru/HSAG@mSiO2 also
showed typical IV curves with a capillary condensation step at
P/P0 from 0.2 to 0.4. The detailed structural parameters of
catalysts are listed in Table 1. The t-plot method at relative
pressure P/P0 from 0.3 to 0.5 was used to analyze the micro-
mesopores. As a material of plate-like particle, the specic
surface area of HSAG (268 m2 gꢀ1) was comprised mainly of its
plate surface area (204 m2 gꢀ1), and the surface area of micro-
mesopores (64 m2 gꢀ1). The total pore volume (Vt) of Ru/HSAG
was calculated to be 0.76 cm3 gꢀ1, to which the contribution of
The dispersion of Ru was obtained by CO chemisorption
method, which was carried out at 40 ꢁC on a Quantachrome
Autosorb-1/C chemisorb apparatus. Prior to measuꢁ rements, the
catalysts were reduced in situ for 10 min at 200 C in H2. The
metal dispersion was estimated based on the assumptions of a
spherical geometry of the particles and a stoichiometric
adsorption of one CO molecule on one Ru surface atom.
Thermogravimetric (TG) analyses were conducted on a
NETZSCH STA449C from 40 C to 500 C (10 C minꢀ1) under
ꢁ
ꢁ
ꢁ
owing argon (99.99%, 20 mL minꢀ1).
Transmission electron microscopy (TEM) experiments were
carried out on a Tecnai G2 F30 S-Twin (Netherlands) operated at
200 kV. The samples for TEM measurements were dispersed in
ethanol and then supported onto a lacey carbon lm on a
copper grid.
Fig. 1 Nitrogen sorption isotherms (A) and pore size distribution (B) of
Ru/HSAG (a) and Ru/HSAG@mSiO2 (b).
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a
Table 1 Textural properties of Ru/HSAG and Ru/HSAG@mSiO2
SBET
Sm
Sp
Vt
Vm
Poresize DRu
Ru content CO monolayer
D
(%)
(m2 gꢀ1
)
(m2 gꢀ1
)
(m2 gꢀ1
)
(cm3 gꢀ1
)
(cm3 gꢀ1
)
(nm)
(nm) (wt%)
uptake (mmol gcat )
ꢀ1
Catalyst
Ru/HSAG
Ru/HSAG@mSiO2
Ru/HSAG@mSiO2-w
268
613
—
64
523
—
204
90
—
0.76
0.49
—
0.03
0.33
—
—
2.7(1.3)
—
2.6
2.6
—
4.0
1.7
1.5
170
7.5
2.5
43
4.4
—
a
SBET: specic BET surface area; Sm: micro-mesopore surface area; Sp: SBET ꢀ Sm; Vt: total volume; Vm: micro-mesopore volume; DRu: Ru
nanoparticle size calculated by TEM; Ru content is nominal loading content; D: Ru dispersion.
micro-mesopores volume was negligible. Aer coated with a
mesoporous silica layer, Ru/HSAG@mSiO2 presented a specic
surface area of 613 m2 gꢀ1, mainly resulted from micro-meso-
pores (523 m2 gꢀ1). This result is also conrmed by the images
of SEM (Fig. 2). The surface of Ru/HSAG was rugged (Fig. 2A) but
became smooth aer covered with a layer of mesoporous silica
as in Ru/HSAG@mSiO2 (Fig. 2B).
Fig. 3 shows the TEM images of Ru/HSAG and Ru/
HSAG@mSiO2.Ru/HSAG exhibited a sharp-edged structure of
HSAG (Fig. 3A and B), while the extremities of Ru/HSAG@mSiO2
were blunt as the result of being covered with a 30 nm thickness
of mesoporous silica layer (Fig. 3C and D). Ru nanoparticles
with average size of 2.6 nm were selectively located at the edges
of the graphite basal planes, either decorating the outer edge of
graphite sheets or the steps of graphite adlayers (Fig. 3A and B).
In addition, Ru nanoparticles were also well dispersed on Ru/
HSAG@mSiO2 without any aggregation (Fig. 3C and D). The
chemical composition and structure of Ru/HSAG@mSiO2 were
further investigated by energy-dispersive X-ray spectroscopy
(EDX) (the inset of Fig. 3C). Comparing the EDX spectra of point
Fig. 3 TEM images of catalysts: (A and B) Ru/HSAG; (C and D) Ru/
A1 and A2, it is clearly that the point A1 had higher concen-
tration of silicon while lower concentration of carbon than A2,
indicating the point A1 was located on the silica layer while A2
was on the overlapped area where graphite was covered by
silica. The structure of Ru/HSAG@mSiO2 that Ru/HSAG was
encapsulated in a layer of mSiO2 can thus be conrmed.
Ru nanoparticles in Ru/HSAG@mSiO2 could be either
embedded in the silica wall at the bottom of the pores, which
made the Ru nanoparticles inaccessible to reactant molecules
and led to a drꢀa1matically decline in CO monolayer uptake from
HSAG@mSiO2. The insets in B and D are the Ru particle size distribu-
tion; the inset in C is the EDX of the points on two sides of the
boundary of Ru/HSAG@mSiO2.
its pores blocked by residual CTAB (Table 1). Ruꢀ/H1 SAG@mSiO2
had the CO monolayer uptake of 7.5 mmol gcat whereas Ru/
HSAG@mSiO2-w showed only 2.5 mmol gcatꢀ1, when 10 wt%
CTAB residues were in the pores of Ru/HSAG@mSiO2-w.
ꢀ1
170 mmol gcat to 7.5 mmol gcat measured by CO chemi-
Cinnamaldehyde hydrogenation
sorption method (Table 1), or accessible to reactant molecules
by molecule diffusion through the pores in silica layer. The later
hypothesis is validated by comparison of the CO monolayer
uptake of Ru/HSAG@mSiO2 with Ru/HSAG@mSiO2-w that had
Fig. 4 shows the course of reaction during the catalytic hydro-
genation of CALD (cinnamaldehyde) over catalysts. The reaction
rate declined from 0.92 mol gRuꢀ1 hꢀ1 on Ru/HSAG to 0.31 mol
ꢀ1
gRu hꢀ1 on Ru/HSAG@mSiO2 (Table 2), which can be attrib-
uted to the fact that part of Ru nanoparticles were capsulated in
the silica wall20 and were inaccessible to reactant molecules.
The selectivity of unsaturated alcohol remained at 65% when
the conversion approached to 100% on Ru/HSAG@mSiO2 (see
Fig. 5), while 50% of selectivity was observed over Ru/HSAG. The
variation of the contents of CALD, HALD (hydro-
cinnamaldehyde) and HALC (hydrocinnamyl alcohol) in the
reaction demonstrated that the hydrogenation of CALD to
HALC via HALD as intermediate is a rst-order consecutive
reaction. On the contrary, the rst-order relation was only
Fig. 2 SEM images of Ru/HSAG (a) and Ru/HSAG@mSiO2 (b).
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molecule containing an isolated C]O group and then the cin-
namyl alcohol molecule containing an isolated C]C group.22,23
Kinetic modeling
Scheme 1 shows the possible pathways of cinnamaldehyde
hydrogenation. To investigate the detailed kinetics and improve
the understanding of the reaction selectivity, a simplied model
is presented in Scheme 2. Taking into account of the results
reported in previous section, a kinetic model suitable to
describe the CALD hydrogenation over catalysts has been
formulated and tested. The kinetic model was developed by
using rate expressions of the Langmuir–Hinshelwood type
under the following assumption:6,22
(i) The competitive adsorption and hydrogenation of the C]C
double bond and the C]O group occur on the same type of
Ru sites.
(ii) The adsorption of the substrate and reaction products on
the active sites is reversible and competitive.
Fig. 4 Hydrogenation of CALD over the catalysts, T ¼ 100 ꢁC, PH
¼
2
4.0 MPa. (A) Ru/HSAG; (B) Ru/HSAG@mSiO2; (>) CALD; (O) CALC;
(,) HALD; (B) HALC. The dashes are the fitting line for CALD and
CALC.
(iii) H2 is activated on different types of sites, and it does not
compete with organic species for adsorption. The concentration
of Hc is thus a constant.
Table 2 Catalytic properties of Ru/HSAG and Ru/HSAG@mSiO2 and
the parameters of kinetic eqn (1)a
(iv) The rate-determining step is the surface reaction
between the adsorbed organic species and chemisorbed
hydrogen.
R
TOF
(hꢀ1
S60
A0
(mol gRu hꢀ1
)
)
(%) (mol Lꢀ1
)
k2/k3
ꢀ1
Catalyst
Process (1) is a rst-order parallel reaction with a reversible
rst-step, expressed by the equilibrium approximation:24
Ru/HSAG
Ru/HSAG@mSiO2 0.31
0.92
215
716
51
66
0.153
0.153
1.05
1.88
a
R: average reaction rate before the conversion of CALD achieves 100%;
k1A ¼ kꢀ1A*
S60: selectivity of CALC when the conversion achieves 60%; TOF ¼ RMRu
/
D, MRu ¼ 101.07, D is Ru dispersion; A0: the initial concentration of
CALD; T ¼ 100 ꢁC, PH ¼ 4.0 MPa.
2
And the rate equation is:
2
2
dA dA*
¼
ꢃ
ꢃ
¼ k1A ꢀ kꢀ1A* ꢀ k2A*½Hc ꢀ k3A*½Hc
observed in the hydrogenation of CALD to CALC (cinnamyl
alcohol) until the conversion of CALD achieved 100%, aer
which CALC started to be further hydrogenated to HALC. These
reaction tends are in agreement with previous studies,12,21 and
probably imply that the cinnamaldehyde molecule containing
conjugated C]C and C]O groups had the strongest adsorp-
tion on Ru sites, followed by the hydrocinnamaldehyde
dt
dt
2
k1
ꢃ
A½Hc
¼ ꢀ ðk2 þ k3Þ
k
ꢀ1
In process (2), the existence of A restrained the adsorption of
B before the conversion of A achieved 100% (see in Fig. 4). And
Scheme 1 The pathway of annimaldehyde hydrogenation: (A) (CALD);
Fig.
5 The CALC selectivity of catalysts: (a) Ru/HSAG; (b) Ru/ (B) (CALC); (C) (HALD); (D) (HALC); inside the dashed frame is the
HSAG@mSiO2.
adsorption type of substrate and reaction products.
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selectivity of cinnamaldehyde hydrogenation. Compared to
other supports, the selectivity was much higher on graphite
because of a charge transfer from the support to the metal,
which increased the charge density on metal, thus decreasing
the probability for the C]C bond activation.18 When the type of
supports was chosen, as the case of our study using high surface
area graphite as support, the geometric structure of catalyst
played an important role in determining the selectivity in
reactions. One of the geometric effects that metals have is the
particle size, as we know there is a steric hindrance for the C]C
bond adsorption on a at surface,8,11,12,25 which however can be
ruled out in our case as the particle size of Ru nanoparticles
maintained at 2.6 nm (see TEM images in Fig. 3) before and
aer being covered with the mSiO2 layer. Another geometric
effect imposed on the selectivity of cinnamaldehyde hydroge-
nation originates from the connement of metals in pores. We
have demonstrated that Ru/HSAG@mSiO2, in which Ru nano-
particles were supported on graphite and conned at the
bottom of silica pores, was able to deliver 15% higher selectivity
than its unconned counterpart. The schematic illustration of
the inuence of connement of metals by mSiO2 layers on the
selective hydrogenation of cinnamaldehyde is presented in
Fig. 6. As a shape-selective layer, the pores in silica layer
determined the adsorption mode of cinnamaldehyde molecules
on Ru nanoparticles. To reach the Ru surface, CALD molecules
should rst diffuse through the pores of mSiO2 layer. With a
branching chain in CALD molecule, the length of CALD is
longer than its width. In pores with average pore size of 2.7 nm,
the possibility of at-on adsorption of CALD molecules on Ru
surface was reduced, as a result of which less C]C bonds were
hydrogenated, the hydrogenation of terminal C]O bond was
thus favored. In this way, the selectivity of CALC (unsaturated
alcohol) in cinnamaldehyde hydrogenation was improved. On
the other hand, the TOF value on Ru/HSAG@mSiO2 was also
signicantly enhanced (three times higher than that on Ru/
HSAG), which was due to the fact that the at-on CALD mole-
cules cover more active sites on Ru nanoparticles than the
molecules with end-on adsorption (Table 2). To the best of our
knowledge, this is the rst report investigating the geometric
effect of catalysts aer ruling out the inuences from other
factors, namely, electronic property of supports and particle size
of metals.
Scheme
2 The kinetic model of cinnamaldehyde hydrogenation
before the cinnamaldehyde conversion achieves 100%.
the rate-determining step is: A* / B*, using the steady state
approximation,24 the rate equation for B* is:
dB*
2
¼ ꢀk2A*½Hcꢃ þ k4B* ¼ 0
dt
As a competitive reaction, the equation for the selectivity of B
is:
k2
S ¼
k2 þ k3
And the rate equation for B is:
dB
dt
k1
2
2
¼ k4B* ¼ k2A*½Hcꢃ ¼ k2
A½Hcꢃ
k
ꢀ1
ꢀ
ꢁ
k1
B
S
2
¼ k2
¼ k2
A0 ꢀ
½Hꢃ
k
ꢀ1
k1
k1ðk2 þ k3Þ
2
2
A0½Hcꢃ ꢀ
B½Hcꢃ
k
k
ꢀ1
ꢀ1
where A0 is the initial concentration of CALD.
When t ¼ 0, B ¼ 0 mol Lꢀ1, it gives
Hc 2t
k2
k2
½ ꢃ
kꢀ1
B ¼ ꢀ
A0eꢀ k1ðk2þk3Þ
þ
A0
(1)
k2 þ k3
k2 þ k3
When the eqn (1) was used to t the concentration curve of
CALC in Fig. 4, the Adj. R-square (coefficient of determination)
was determined to be 0.96 and 0.99 for Ru/HSAG and Ru/
HSAG@mSiO2, respectively. It indicates that this kinetic model
and the assumptions are suitable to describe the process of
cinnamaldehyde hydrogenation on these catalysts. This kinetic
model also supported the fact that there was a competitive
adsorption between CALD and CALC on Ru surface and the
existence of CALD restrained the adsorption of CALC before the
conversion of CALD achieved 100%. Since the cinnamaldehyde
hydrogenation is a parallel reaction, the ratio of reaction rate
constant (k2/k3) indicates the selectivity of CALC. The calculated
values of k2/k3 are 1.05 and 1.88 on Ru/HSAG and Ru/
HSAG@mSiO2, respectively (see Table 2), which are consistent
with the values obtained experimentally from Fig. 5.
Discussion
The electronic property of supports and the geometric structure
Fig. 6 The geometric effect of Ru/HSAG@mSiO2 for CALD hydroge-
of metals are two key factors of catalysts inuencing the nation to unsaturated alcohol.
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A general strategy for the facile preparation of graphite-sup-
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The nancial support from the National Natural Science
Foundation of China (NSFC Grant no. 21303163), the Qianjiang
Talent Project in Zhejiang Province (QJD1302011) and the
Scientic Research Fund of Zhejiang Provincial Education
Department (Y201328681) are gratefully acknowledged.
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