Hollow mesoporous silica supported PtIr bimetal catalysts for selective hydrogenation of phenol: Significant promotion effect of iridium

Article abstract of DOI:10.1039/c7ra05653b

Selective hydrogenation of phenol is of great significant for the production of Nylon 6. Herein, highly efficient and stable Ir promoted Pt catalysts for the hydrogenation of phenol are prepared by deposition of Pt-Ir nanoparticles onto hollow mesoporous

Full text of DOI:10.1039/c7ra05653b

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Hollow mesoporous silica supported PtIr bimetal
catalysts for selective hydrogenation of phenol:
significant promotion effect of iridium†
Cite this: RSC Adv., 2017, 7, 31582
b
c
Yuying Zheng,a Peishan He,a Yanxiong Fang,a Xu Yang
*
*
and Huagen Liang
Selective hydrogenation of phenol is of great significant for the production of Nylon 6. Herein, highly
efficient and stable Ir promoted Pt catalysts for the hydrogenation of phenol are prepared by deposition
of Pt–Ir nanoparticles onto hollow mesoporous silica spheres (HMSs) through a facile wet-impregnation
and H2 reduction method. The obtained PtIr/HMS catalyst with Ir/Pt molar ratio of 0.1 shows a highest
hydrogenation rate of 4255 mmol hÀ1 gPt with cyclohexanone selectivity of 85% at 50 ꢀC and 0.5 MPa
À1
of pH , which is 3 times and 8.6 times higher than that of the mono Pt/HMS catalyst and commercial Pt
2
Received 19th May 2017
Accepted 14th June 2017
catalyst, respectively. The trace addition of Ir promoter essentially contributes to the enhanced
hydrogenation activity by improving the metal dispersion of Pt nanoparticles and promoting the charge
transfer between the Pt and Ir. This work provides an effective protocol in developing high performance
catalysts for phenol hydrogenation under mild reaction conditions.
DOI: 10.1039/c7ra05653b
rsc.li/rsc-advances
core–shell composites,15,16 Pt monolayer catalysts,17,18 alloying with
other metals,19 adding promoter,20,21 etc.
Introduction
Of these protocols, adding promoter is both economically
and effectively method due to its simple procedure and conve-
nient controlling. Moreover, literatures elsewhere have envi-
sioned that trace promoter not only can remarkably enhance
the activity, but also tune the selectivity for various chemical
reactions. For example, Tomita and co-authors have reported
that addition of iron oxide can effectively improve the activity of
Pt/Al2O3 towards CO oxidation.22 Burch et al. have investigated
a series of Pt/Al2O3 catalysts promoted by metal oxides or noble
metals (Ag, Au, Pd, Rh) for the NO reduction, claiming that the
promoters have a signicant effect on the activity of Pt/Al2O3.21
Luo and co-authors have found that the Ir promoted Ru/ZnO
showed high selectivity towards crotyl alcohol applied in cro-
tonaldehyde hydrogenation.23 They ascribed the improved
selectivity to the modied electronic properties of Ru induced
by the Ir promoter. These reports demonstrate well that high
performance catalyst can be achieved as the promoters are
elegant selected.
Catalytic hydrogenation of phenol is of industrial and scientic
signicance because cyclohexanone is an important interme-
diate for the production of Nylon 6 and Nylon 66.1 Thus,
developing high performance catalysts for the selective hydro-
genation of phenol to produce cyclohexanone under mild
reaction conditions is an attractive topic in the eld of catalysis
and materials.2–4
A lot of research work on the development of the catalyst has
been reported in the last decade.5–10 Current prevailing catalysts
used for hydrogenation include Ni and Pd supported catalysts; the
former ones are oen operated under rigorous reaction conditions
(T > 200 ꢀC or pH > 1 MPa),11,12 the latter are confronted with severe
2
metal sintering and leaching that oen lead to deactivation
issues.2,5,6 Noticeably, the Pt based catalysts exhibit excellent
activity and stability under mild reaction conditions.9,13,14 However,
one major obstacle for practical application of Pt based catalysts is
their scarce reserves and high cost. Therefore, several strategies
have been tried to increase the Pt utilization efficiency, such as
In this work, we attempt to obtain a high-performance Ir
promoted Pt bimetal catalysts for selective hydrogenation of
phenol. To the best of our knowledge, the promotion effect of Ir
on the Pt towards phenol hydrogenation has seldom been re-
ported. Previously, we have used our lab-made hollow meso-
porous silica (HMS) as support of Pd catalyst for phenol
hydrogenation.24 With high surface area (ꢁ800 m2 gÀ1) and
narrowed mesopores, the HMS can effectively disperse the Pd
components. Moreover, the HMS can exert its inactive siliceous
properties as an ideal candidate to investigate the promoter
effect with less interruption by metal-support interaction. Thus,
aSchool of Chemical Engineering and Light Industry, Guangdong University of
Technology, Guangzhou 510006, China
bGuangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou
510640, China. E-mail: yangxu@ms.giec.ac.cn; Tel: +86-020-87057620
cLow Carbon Energy Institute, School of Chemical Engineering & Technology, China
University of Mining and Technology, Xuzhou, 221116, China. E-mail: lhg654@
gmail.com; Tel: +86-0516-83883501
† Electronic
supplementary
information
(ESI)
available:
Catalyst
characterizations, schemes for reaction pathway and mechanistic proposition of
phenol hydrogenation. See DOI: 10.1039/c7ra05653b
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we herein selected the HMS as support to prepare PtIr/HMS Japan). X-ray photoelectron spectroscopy (XPS) with an AXIS Ultra
catalysts with various molar ratio of Ir/Pt through impregna- DLD (Kratos, Britain) was used to examine the catalysts' electronic
tion and reduction route. The effect of promoter on the Pt based properties. The binding energies were calibrated using a C 1s
catalysts was investigated comprehensively as to explore the Pt– binding energy of 284.5 eV. H2-TPR was performed on an
Ir synergy effect on the performance towards hydrogenation of AutoSorb-iQ-c automated system (Quantachrom, USA): a sample of
phenol.
50 mg was heated at a rate of 10 ꢀC minÀ1 from room temperature
to 700 ꢀC in 5% H2/Ar (40 ml minÀ1, STP).
Experimental
Catalyst evaluation
Sample preparation
The hydrogenation of phenol was used for catalyst evaluation
model reaction. The reaction mixture composed of 50 mg
catalyst, 0.5–1.0 g phenol, and 10 ml acetate as solvent, was
introduced into a 50 ml autoclave under stirring at 1200 rpm,
a value chosen to eliminate the mass transfer limitation. Air in
the autoclave was purged by hydrogen for 20 min, and then the
reaction proceeded at the required temperature (30–100 ꢀC) and
0.5 MPa of 99.99% pure hydrogen. The products were analysed
Hollow mesoporous silica (HMS) were prepared according to
route reported in document elsewhere:24 0.5 g dodecyl amine as
mesoporous template was dissolved in 20 ml ethanol under
mild stirring; aer the addition of 50 ml deionized water, 5 g
TEOS was added drop-wise to obtain a white mixture; aer 5
hours of stirring, white precipitates were obtained and then
ltered, washed, dried, and calcinated at 600 ꢀC for 2 h to
remove the residual template.
using
a gas chromatograph GC2010 (Shimadzua, Japan)
Pt–Ir bimetallic catalysts with various molar ratio of Ir/Pt
(from 0 to 0.2) were prepared according to a wet-impregnation
and H2 reduction route. Firstly, 1.0 g HMS supports were
impregnated with 20 ml ethanol solution of chloroplatinic acid
(0.03 g) and chloroiridic acid (0–0.008 g) with given molar ratio,
and kept stirring at room temperature. The solvent was then
evaporated in a water bath at 50 ꢀC. The PtIr/HMS catalysts were
nally prepared by reducing the sample under hydrogen ow at
equipped with a FID detector.
Results and discussion
Structure characterization of catalyst
Fig. 1a shows the XRD patterns of the calcined HMS materials. A
strong broad peak centred at 2.07ꢀ of 2q with a weak scattering
ꢀ
300 C for 4 h in a tubular furnace with programmed temper-
ature control, at a rate of 2 ꢀC minÀ1. The Pt loading was xed at
1.5% for all catalysts, and the basic specications of catalysts
are listed in Table 1. A commercial Pt/C catalyst was purchased
from Aladdin agent Company for reference sample.
Catalyst characterization
X-ray diffraction (XRD) patterns were obtained with a X-ray diffract
meter (Rigaku, Japan). The N2 adsorption–desorption isotherms
were measured with a Tristar 3010 isothermal nitrogen sorption
analyser (Micromeritics, USA) using a continuous adsorption
procedure. High-resolution transmission electron microscopy
(HRTEM) was carried out with a JEOL JEM2010 microscope (JEOL,
Table 1 Specifications of various catalysts
Metal loadingb
a
a
c
SBET
DPore
(nm)
DPt
Catalyst
(m2 gÀ1
)
Pt wt%
Ir wt%
(nm)
HMS
Pt/C
Pt/HMS
PtIr0.05/HMS
PtIr0.1/HMS
PtIr0.2/HMS
837
56
821
806
812
809
3.7
—
3.3
3.5
3.5
3.4
—
5
1.50
1.52
1.48
1.50
—
—
—
0.07
0.16
0.31
—
4.2
3.8
3.4
3.2
3.0
a
Surface area (SBET) and pore size (DPore) were derived from the iso-
b
thermal nitrogen ads–desorption curves.
Metal loading was
c
determined by ICP measurement. Average particle size of Pt
crystallite (DPt) was calculated from the XRD plot according to the
Scherrer equation.
Fig. 1 Small angle XRD patterns (a) and isothermal N2 adsorption–
desorption plot (b) of the calcined HMS materials.
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domain (indicated by arrow) around 4–5ꢀ of 2q reveals the
wormhole-like mesoporous structure of the sample.25 The cor-
responding isothermal N2 adsorption–desorption plot, shown
in Fig. 1b, displays an obvious sorption step at p/po of 0.5,
assigning to the capillary condensation in the mesopores.26 The
inset pore size distribution has a well-dened peak centred at
3.7 nm. Aer loading of Pt nano-particles, all of the HMS sup-
ported catalysts reserved high surface area and narrowed mes-
opores size (seeing Table 1), indicating the good thermal
stability of HMS.
Fig. 2 shows the wide-angle XRD patterns to characterize the
metal crystallites of the catalysts. A broaden peak centred at 24ꢀ
of 2q is observed for all samples, which is ascribed to the
amorphous silica. The Pt/HMS exhibits three clear diffraction
peaks located at 39.4, 46.1 and 66.9ꢀ of 2q, indexing to the (111),
(200), and (220) planes of the face-cantered cubic Pt crystallite.27
With the addition of Ir promoter, such three peaks become
broadened and diminished, suggesting that the Ir promoter can
weaken the crystalline degree of Pt, or improve the metal
dispersion. From the enlarged regional XRD between 30 and
50ꢀ, the half width of (111) peaks increase upon the Ir addition
elevated. Table 1 presents the crystallite size calculated by the
Scherrer equation. The particle size decreases from 3.8 to
Fig. 3 TEM images of (a and b) Pt/HMS catalysts, (c and d) PtIr0.1/HMS
catalysts.
3.0 nm as the molar ratio of Ir/Pt increase from 0 to 0.2. It is
clear that the addition of Ir promoter plays a crucial role for
enhancing the dispersion and decreasing the particle size of Pt
components.
Fig. 3 presents the TEM images of the mono-Pt/HMS and
PtIr0.1/HMS catalysts for comparison. It veries the micro-
structure of hollow cavity and mesoporous shell for the HMS
support. In addition, the highly dispersion of active compo-
nents on the HMS is seen clearly as well. The mono-Pt/HMS
shows some poly-dispersed Pt nano-particles on the meso-
porous shell (Fig. 3b), while the PtIr0.1/HMS presents homo-
geneously dispersed Pt nano-particles (Fig. 3d), further
conrming the promotion effect of Ir on the metal dispersion.
To determine the crystallite size and lattice fringe of Pt nano-
particles precisely, the high-resolution TEM was conducted for
these samples. As shown in Fig. 4, the PtIr0.1/HMS has
a smaller particle size (ꢁ2 nm) than that of the Pt/HMS (ꢁ4 nm).
Moreover, in the corresponding diffraction patterns, the PtIr0.1/
HMS exhibits less of diffraction light spots than the Pt/HMS
does, implying its less crystalline degree or smaller particle
Fig. 2 Wide angle XRD pattern (a) and enlarged region between 30
and 50ꢀ of 2q (b) for PtIr/HMS catalysts with various Ir/Pt molar ratio.
Fig. 4 HRTEM images of Pt/HMS and PtIr0.1/HMS catalysts.
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size, consistent with the XRD analysis. The (111) inter-planar
distances derived from the diffraction pattern, indicate 0.218
and 0.228 nm for PtIr0.1/HMS and Pt/HMS, respectively, further
revealing the contraction of Pt unit cell upon the addition of Ir.
This can be associated with the formation of PtIr alloy.28,29 It
demonstrates well that the addition of Ir promoter can result in
the formation of PtIr alloy, and improve the metal dispersion.
H2-TPR proles of the as-prepared catalysts aer impregna-
tion are presented in Fig. 5. The Pt/HMS exhibits a strong sharp
ꢀ
peak located at approximately 117 C, attributable to the easy
reducible Pt2+ ions. The PtIr0.1/HMS and PtIr0.2/HMS show
a relative high temperature reduction peak centred at 177 and
ꢀ
191 C, respectively, which implies the hard reducible bimetal
species on the catalysts. This can be ascribed to the strength-
ened interactions between the Pt precursors and silica,9 or to
the formation of PtIr alloy. It demonstrates that the addition of
Ir can intensify the metal support interaction, which would
modulate the electronic properties of Pt phase.
Fig. 6 shows X-ray photoelectron spectroscopy (XPS) of
samples to assess the surface information of metal states and
compositions. The Pt 4f spectra can be tted by two doublets
attributed to the higher binding energy of Pt2+ (73.2–73.8 eV)
and lower energy value of metallic Pt0 (71.9–72.2 eV).30
Compared with the Pt/HMS catalysts, a shi of ꢁ0.3 eV towards
high energy occurred for the bimetal PtIr0.1/HMS catalysts,
indicating the electronic properties of Pt can be modulated by
the Ir promoter. In addition, the XPS derived Pt0/Pt2+ atomic
ratio indicates that the PtIr0.1/HMS held more of metallic Pt
phase than the Pt/HMS has, suggesting the Ir promoter can
resist the re-oxidation of metallic Pt species. It further veries
the modulation of electronic properties of Pt caused by the Ir
promoter. The Ir 4f spectrum (Fig. 6c) indicates the 4f7/2 and
4f5/2 peaks at binding energy of 61.0 and 64.1 eV, respectively,
assigning to the metallic state of Ir phase.31 The well-dened
peak centred at 103.8 eV is observable for both samples, cor-
responding to the 2p binding energy of silica.
Fig. 6 XPS spectra of various catalysts: (a and b) Pt 4f, (c) Ir 4f and (d) Si
2p in Pt/HMS and PtIr0.1/HMS.
Fig. 5 H2-TPR curves of PtIr/HMS catalysts with various Ir/Pt molar
ratios.
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is 0.1. The recycle test was conducted for the PtIr0.1/HMS, as
shown in Fig. 7b. Aer fourth run, it exhibits slight decline of
conversion (<2%), accompanied with constant selectivity of 84%
towards cyclohexanone. The TEM images of spend catalyst
(seeing Fig. S1 in ESI†) aer hydrogenation suggested negligible
metal sintering, demonstrating the good stability of for the
PtIr0.1/HMS catalyst.
Several limitations of the current catalytic system, such as
harsh reaction conditions, deactivation and etc., have driven
scientists to develop high-performance catalysts applied in mild
reaction.7,25,32 Compared with the methods in documents else-
where,5,6,10–14 the high activity and good stability of the PtIr/HMS
make it one of promising catalysts applied for phenol hydro-
genation in mild reactions.
The inuence of reaction temperature on the PtIr0.1/HMS
was investigated, as shown in Fig. 8a. As the temperature is
elevated from 30 ꢀC to 100 ꢀC, the phenol conversion soars from
15% to 100%. However, the selectivity of cyclohexanone
decreases upon the increase of temperature, implying high
reaction temperature facilitates deep hydrogenation of cyclo-
hexanone to cyclohexanol. It reveals that the PtIr0.1/HMS is
suitable for production of cyclohexanone through phenol
hydrogenation under mild reaction. Based on the activity upon
the reaction temperature, we plotted the Arrhenius curves for
the Pt/HMS and PtIr0.1/HMS, as depicted in Fig. 8b. The
apparent activation energy was 39.5 and 31.4 kJ molÀ1 for Pt/
Catalytic performance of catalysts: hydrogenation of phenol
Fig. 7a shows the catalytic performance towards hydrogenation
of phenol over various catalysts. Generally, partial phenol
hydrogenation yields cyclohexanone, and deep hydrogenation
leads to the product of cyclohexanol, as shown the reaction
pathway of Scheme S1 of (ESI†). As the reaction proceeds at the
similar conversion, all of Pt based catalysts show similar
selectivity (80–85%) towards cyclohexanone regardless of Ir/Pt
ratio, thus we focused our discussion on phenol conversion.
The HMS is inactive under our conditions (not shown here). For
the Pt/HMS, it converts 21% phenol, comparable with the
commercial Pt/C that with Pt loading of 5 wt%, indicating the
much higher Pt utilization efficiency than the commercial Pt/C.
This suggests that the high surface area of HMS can facilitate
the dispersion of Pt components, and improve the metal utili-
zation efficiency. As the Ir promoter is introduced, the phenol
conversion rises up to 38% over the PtIr0.05/HMS, and elevates
to the highest value of 60% for the PtIr0.1/HMS. A high mass
activity of 4255 mmol hÀ1 gPtÀ1 is achieved, which is 3 times and
8.6 times higher than the mono Pt/HMS catalyst and commer-
cial Pt catalyst do, respectively.
Further increases the Ir addition, the conversion of PtIr0.2/
HMS shows a slight decline at 55%. These results demon-
strate that the Ir promoter could remarkably enhance the
phenol hydrogenation activity; the optimum molar ratio of Ir/Pt
Fig. 8 (a) Phenol hydrogenation over PtIr0.1/HMS upon temperature
evolution; (b) corresponding Arrhenius plots of various catalysts. The
Fig. 7 (a) Phenol hydrogenation over various catalysts, (b) Recycle test reaction mixture composed of 50 mg catalyst, 1 g phenol and 10 ml
over PtIr0.1/HMS. The reaction mixture composed of 50 mg catalyst, acetate as solvent, and proceeded with 0.5 MPa of pH for 0.5 h; the
2
0.5 g phenol, and 10 ml acetate as solvent, and proceeded at 50 ꢀC and selectivity towards cyclohexanone was determined at the conversion
0.5 MPa of pH for 1 h.
of 10% for each run.
2
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´
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of Ir. The promotion effect on the activity can be ascribed to the
improved metal dispersion and modulated electronic state. As
justied by the XRD and TEM results, the PtIr alloy effectively
hinders the particle growth. Higher metal dispersion means
more of active sites, resulting in accelerated hydrogenation
reaction. With respect to the electronic modulation, as shown in
TPR and XPS analysis, it veries the intensied metal-support
interaction and charge transfer induced by the Ir promoter,
and both can inuence the catalytic behaviour of reactants and
intermediates (e.g. dissociative, ads–desorption, etc.) during
hydrogenation.
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Conclusions
A high-performance Ir promoted Pt catalyst for hydrogenation of
phenol is prepared using the hollow mesoporous silica sphere
(HMS) as support through a facile wet-impregnation and H2
reduction method. The obtained PtIr0.1/HMS catalyst (with Ir/Pt
molar ratio of 0.1) shows highest hydrogenation rate of
4255 mmol hÀ1 gPt (with cyclohexanone selectivity of 85%) at
À1
ꢀ
50 C and 0.5 MPa of pH , which is 3 times and 8.6 times higher
2
than that the mono Pt/HMS catalyst and commercial Pt/C catalyst,
respectively. The addition of Ir promoter essentially contributes to
the enhanced hydrogenation activity by improving the metal
dispersion of Pt nanoparticles and promoting the charge transfer
between the Pt and Ir. This work provides an effective protocol in
developing a high performance catalyst for phenol hydrogenation
under mild reaction conditions.
Acknowledgements
This work was supported by the National Scientic Foundation
of China (project No. 21303210), and Open Project Program of
the State Key Lab of CAD&CG (Grant No. A1701) Zhejiang
University.
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