Gold supported on Ti incorporated MCM-36 as efficient catalysts in propylene epoxidation with H2 and O2

Article abstract of DOI:10.1039/c5ra08621c

Novel catalysts consisting of gold nanoparticles (NPs) supported on the titanosilicate pillared MCM-36 (Si/Ti-MCM-36) with 2 nm mesopores were prepared. The influences of the Ti coordination environment on the particle size of the anchored Au NPs and their catalytic performances in propylene epoxidation with mixtures of H₂ and O₂ were investigated. The catalyst materials were characterized with Electron Probe Micro Analysis, X-ray diffraction, nitrogen sorption, UV-visible diffuse reflectance spectroscopy, transmission electron microscopy, and X-ray absorption spectroscopy. The coordination environment of Ti(iv) in Si/Ti-MCM-36 was found to be a mixture of octahedral (O_h) and tetrahedral (T_d). The Au loading was proportional to the amount of Ti incorporated in the porous silica. The higher loading of Ti in Si/Ti-MCM-36 anchored more Au and resulted in the formation of larger Au particles. The deposition-precipitation (DP) with NH₃ as a DP agent seemed to give Au NPs larger than those loaded by NaOH. The most efficient Au NPs size was about 1 nm, and Au NPs with sizes larger than 2 nm were not efficient for the propylene epoxide (PO) formation. By adjusting the Au and Ti synergy, the optimal PO formation rate over Au loaded Si/Ti-MCM-36 was 28.9 g_PO h^-1 g_cat^-1.

Full text of DOI:10.1039/c5ra08621c

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PAPER
Gold supported on Ti incorporated MCM-36 as
efficient catalysts in propylene epoxidation with H2
and O2†
Cite this: RSC Adv., 2015, 5, 61710
Fang Jin,ab Tsung-Han Lin,a Chih-Cheng Chang,a Ben-Zu Wan,c Jyh-Fu Leed
a
*
and Soofin Cheng
Novel catalysts consisting of gold nanoparticles (NPs) supported on the titanosilicate pillared MCM-36 (Si/Ti-
MCM-36) with 2 nm mesopores were prepared. The influences of the Ti coordination environment on the
particle size of the anchored Au NPs and their catalytic performances in propylene epoxidation with
mixtures of H2 and O2 were investigated. The catalyst materials were characterized with Electron Probe
Micro Analysis, X-ray diffraction, nitrogen sorption, UV-visible diffuse reflectance spectroscopy,
transmission electron microscopy, and X-ray absorption spectroscopy. The coordination environment of
Ti(IV) in Si/Ti-MCM-36 was found to be a mixture of octahedral (Oh) and tetrahedral (Td). The Au loading
was proportional to the amount of Ti incorporated in the porous silica. The higher loading of Ti in Si/Ti-
MCM-36 anchored more Au and resulted in the formation of larger Au particles. The deposition–
precipitation (DP) with NH3 as a DP agent seemed to give Au NPs larger than those loaded by NaOH. The
most efficient Au NPs size was about 1 nm, and Au NPs with sizes larger than 2 nm were not efficient for
the propylene epoxide (PO) formation. By adjusting the Au and Ti synergy, the optimal PO formation rate
Received 9th May 2015
Accepted 8th July 2015
DOI: 10.1039/c5ra08621c
ꢀ1
ꢀ1
www.rsc.org/advances
over Au loaded Si/Ti-MCM-36 was 28.9 gPO
h
g
cat
.
reaction of PO with methanol. Moreover, all the current
processes require multiple reaction steps in the liquid phase. In
1998, Haruta and coworkers2 discovered for the rst time that
propylene epoxidation could be achieved with hydrogen and
oxygen on-site generating H2O2 over gold nanoparticles sup-
ported on TiO2. Since then, the direct epoxidation of propylene
to PO has gained considerable attention in the past sixteen years.
Various titanosilicates, including TiO2/SiO2,3 Ti-TUD,4 Ti-SBA-
15,5 Ti-MCM-41,6 Ti-MCM-48,7 Ti-MWW,8 Ti-b,9 TS-2, TS-1,10,11
and alkaline treated TS-1 (ref. 12) have been investigated for
propylene epoxidation. Sinha et al.13 claimed that the Ti incor-
porated oxide supports with high surface and mesopores were
required to have well dispersed Au nanoparticles and give high
yields of PO. Moreover, well-dispersed tetrahedral Ti sites in the
silica supports and the size of the gold nanoparticles were
considered to be the two important factors for an active catalyst.
Although many studies have addressed the issue, the assignment
of the gold active sites is still under debate. Gold nanoparticles
with sizes in 2–5 nm were originally assigned as the active sites
for the PO reaction in Au/TiO2 and other Au/Ti-based oxides by
Haruta's groups.2,14 Later, Haruta's groups proposed that the
dominant gold active sites in Au/TS-1 for the PO reaction might
be 55 gold atom clusters of ca. 1.4 nm diameter.15 Recently, Feng
et al.16 identied the size-dependent activity of Au nanoparticles
(2–5 nm) deposited on the exterior surface of TS-1 by employing
uncalcined TS-1. The catalyst with Au nanoparticles about
2.5 nm was reported to have the best catalyst performance.
1. Introduction
Propylene epoxide (PO) is a very important feed stock in chem-
ical industry and widely used to produce a variety of derivatives
such as polyurethane, propylene glycol, polyester resins and
surfactants. The industrial PO production routes, such as chlo-
rohydrins and organic peroxide processes, suffer from gener-
ating either environmentally unfriendly chlorinated compounds
or co-products with lower demand and/or market values.1 The
direct epoxidation of propene with H2O2 over TS-1 catalyst and
methanol as solvent is an alternative rout in terms of greenness.1
Nevertheless, the industrialization of hydrogen peroxide process
suffers from some drawbacks, such as the expensive cost of the
oxidant H2O2, difficulties in separating the azeotropic mixture of
PO and methanol solvent, formation of the solvolysis side-
products of glycol ethers as a result of the ring-opening
aDepartment of Chemistry, National Taiwan University, Taipei 10617, Taiwan. E-mail:
chem1031@ntu.edu.tw
bKey Laboratory for Green Chemical Process of Ministry of Education, School of
Chemical Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan
430073, China
cDepartment of Chemical Engineering, National Taiwan University, Taipei 10617,
Taiwan
dResearch Division, National Synchrotron Radiation Research Center, Hsinchu 300,
Taiwan
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra08621c
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MCM-22 zeolite (IZA code MWW) exhibits advantages of high as the base.27,28 Approximately, 0.1 g of HAuCl4–3H2O (Alfa
exibility in varying the pore structures by different post-treat- Aesar, 99.99%) was dissolved in 25 mL of D.I. water stirring at
ments.17 The lamellar MWW structure comprises two indepen- 313 K, and the pH of the gold solution was adjusted with the
dent 10-membered ring (MR) channels and 12-MR cups on the base to 6. The neutralized gold solution was further stirred for
crystal exterior.17 Several of the MCM-22 derivatives were 2 h at 313 K before 0.35 g of the support was added. The pH
reported using the lamellar precursor, including delaminated value of the solution was further adjusted to 7 with 0.1 M base in
MWW (ITQ-2), partially delaminated MWW (MCM-56) and the about 1 h, and the mixture was stirred for another 2 h at the
silica-pillared MWW (MCM-36).17 Recently, the relevant MWW- same temperature and pH value. The solid was ltered, washed
type zeolitic materials were comprehensively reviewed.17–19 Ti- with 500 mL of D. I. H2O, and dried under ambient condition
incorpoated MWW structures have attracted scientists for 12 h. The prepared gold catalysts were stored in sealed
because of their catalytic properties in partial oxidation reac- amber bottles at 277 K and used in 12 h for propene epoxidation
tions17 Wu et al.20 and Fan et al.21 synthesized the titanosilicate reaction. The sample was designated mAu/xSi/Ti-MCM-36,
zeolites, Ti-MWW and Ti-YNU-1, respectively, with 3D MWW where m represents the nominal number of wt% of gold in
micropore structures by using different reactive processes. Ti- the DP solution versus the support. The m values of 1, 1.5, 3 and
YNU-1 has the same basic building unit as Ti-MWW but an 4.5 correspond to the Au3+ precursor concentrations in the DP
expanded interlayer window from 10 to 12-MR and an enlarged solution to be ca. 0.59, 1.1, 2.2, and 3.3 mM. The samples
22,23
˚
2.59 A interlayer spacing.
Recently, a new approach for prepared with 0.1 M NH3 as the base for controlling pH values
preparing Ti incorporated MCM-36 material (Si/Ti-MCM-36) are denoted as mAuNH3/xSi/Ti-MCM-36, while others used
was developed in our laboratory by intercalating the MWW NaOH as the base.
lamellar precursor with titanosilicate pillars.24 Furthermore,
when ERB-1, which was a borosilicate and aluminum-free
analogue of MCM-22, was used instead of MCM-22 as the
2.2. Characterization
precursor, a more efficient pillared 2 nm mesopores and higher X-ray powder diffraction patterns were recorded using a PAN-
surface area were obtained. The material also exhibited better alytical X'Pert PRO diffractometer with a Cu-Ka radiation (l ¼
˚
catalytic performance in liquid phase epoxidation of cyclo- 1.5418 A) operated at 40 mA and 45 kV. Diffuse reectance (DR)
hexene with tert-butyl hydroperoxide. This success encouraged UV-vis spectra were recorded using a Hitachi U-3310 spec-
us to investigate the performance of Au catalysts supported on trometer equipped with an integrating sphere detector, and
the new type of Ti incorporated mesoporous MCM-36 in direct BaSO4 was the reference. The elemental analyses were deter-
epoxidation of propylene with H2 and O2.
mined using Electron Probe Micro Analysis (EPMA) in JEOL JXA-
8200 Electron Probe. Transmission electron microscopy (TEM)
photographs were obtained from a JEOL JEM-1200EX II Trans-
mission Electron Microscope and Hitachi H-7100 Transmission
Electron Microscope.
2. Experimental methods
2.1. Preparation of materials
The borosilicate of MWW structure, namely ERB-1 precursor or
Au LIII-edge X-ray absorption spectra (XAS) were collected
ERB-1(P), was prepared following the synthesis method repor- using the uorescence mode at the beam lines 17C of the
ted by Millini et al.25,26 with the reactant ratio of National Synchrotron Radiation Research Center (NSRRC)
Si : B : pyperidine (PI) : H2O ¼ 1 : 0.75 : 1.4 : 19. The synthesis facility in Hsinchu, Taiwan. Standard operating condition was
of Si/Ti-MCM-36 was started with a swelling step in a solution 1.5 GeV and 350 mA. The photon energy was guided using a
mixture of tetrapropylammonium hydroxide (TPAOH) and xed-exit double-crystal Si(111) monochromator and calibrated
cetyltrimethylammonium bromide (CTMABr) with a relative with a metallic Au foil (LIII-edge, 11 919 eV).
weight ratio of TPAOH : CTMABr : H2O ¼ 1 : 4 : 330. The
The X-ray absorption near-edge structure (XANES) and
mixture sealed in a ask at 373 K for 16 h. The resulting swollen extended X-ray absorption ne structure (EXAFS) data were
ERB-1 material was collected by ltration, washing with processed using the programs of IFEFFIT.29 The EXAFS spectra
distilled water, and drying under vacuum. The swollen ERB-1 with k3-weighted functions were fourier transformed (FT) over a
was then dispersed in a mixture of tetraethylorthosilicate photoelectron momentum (k) range of 2.5–12.1 Aꢀ1. Structural
˚
(TEOS) and tetrabutylorthotitanate (TBOT) with TEOS/TBOT parameters were extracted from the rst coordination shell,
molar ratio varied in 40 : 1, 100 : 1, and 160 : 1, and the which was isolated by inverse transforming over a nonphase-
˚
mixture was stirred at 353 K for 25 h. The material collected by corrected radial distance range (r-space) of 1.8–3.4 A. The
ltration was dried at ambient condition. The hydrolyzation of spectra were then tted to the EXAFS equation.30 The amplitude
2
TBOT and TEOS was carried out by suspending the dried solid reduction factor (S0 ) value of 0.86 were determined from the
in an aqueous solution at 313 K for 6 h at pH 9 adjusted with 1 reference standard (unirradiated Au foil) and kept constant for
M NH3. Finally, the sample was calcinated at 723 K for 3 h in tting all spectra. The interatomic distance R, the coordination
nitrogen and at 812 K for 6 h in air (heating rate of 2 K minꢀ1). number of rst nearest neighbor (1NN), the difference of the
The resultant materials were designated as xSi/Ti-MCM-36, Debye–Waller factor from the reference (Ds2), and the correc-
where x is the Ti/Si molar ratio in the pillaring gel.
tion of the threshold energy (DE) were treated as free parameters
Gold was deposited on the supports by a deposition– during the tting. Fitting analysis in both k1-and k3-weighted
precipitation (DP) method using sodium hydroxide or ammonia fourier transforms was applied in order to obtain a unique set of
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coordination number (CN) and Ds2 parameters. The photo- The EPMA analysis results in Table 1 show that all the pillared
electron scattering-path amplitudes and phases were calculated MCM-36 samples do not contain boron anymore. It has been
by ab initio using FEFF8 (ref. 31) for a face-centered-cubic (fcc) conrmed that the swelling treatment of 3D-MWW precursor
˚
structure of Au with a rst nearest neighbor distance of 2.885 A. with TPAOH and CTMABr at elevated temperature and high pH
The size dependent average CN of atoms in a 12-fold coordi- value can leach nearly all the boron and also partially dissolve
nated spherical particle can be estimated by CN ¼ 12 (1 ꢀ 3/2D) the framework silicon.24,37 The pillaring step with the mixture of
R1NN, where R1NN is the nearest neighbor distance and D is the TEOS and TBOT incorporated Ti in the MWW structure. Table 1
average diameter of nanoparticles (NP).32–35
shows that the amount of incorporated Ti in Si/Ti-MCM-36
increases with decreasing the TEOS/TBOT ratio in the pillar-
ing reagent, and the Si/Ti ratio decreases.
2.3. Catalytic reaction
Table 1 also summarizes the Au loadings of the supported Au
catalysts. For Au-loaded 100Si/Ti-MCM-36 samples, the actual
Au loading is proportional to the concentration of the Au
precursor used in DP solution. Au loading of 0.84 wt% was
obtained with 3.3 mM Au3+ solution (4.5Au/100Si/Ti-MCM-36)
and 0.12 wt% loading with 0.59 mM Au3+ solution (1Au/100Si/
Ti-MCM-36).
When using identical concentrations of Au precursor for DP,
the Au loading on xSi/Ti-MCM-36 was found to change with the
Ti content in the support. The Au loading decreases slightly
with the decrease of Ti content in Si/Ti-MCM-36. For example,
the actual Au weight percentage loaded in the catalysts 40, 100
and 160Si/Ti-MCM-36 prepared with the same original 1.1 mM
Au precursor concentrations decrease from 0.31, 0.19 to 0.14%.
This is consistent with the results reported by other groups that
higher Ti content leads to higher Au loading.8,38 The presence of
Ti has been reported to be helpful in capturing the Au precursor
during the DP process, which is originated from a certain
affinity interaction between Ti species and Au precursor.13
In comparison of the DP agent for loading Au, NH3 is more
efficient to load Au on the supports than NaOH. When the same
Au precursor concentration in the DP solution is used, 1.5Au/
100Si/Ti-MCM-36 contains only 0.19 wt% Au, while 1.5AuNH3/
100Si/Ti-MCM-36, contains 0.74 wt% Au.
The catalytic performances of prepared catalysts were examined
using a xed bed ow system. The catalyst in powder form was
packed in a quartz tubular microreactor (35 cm in length, inner
diameter 6 mm, wall thickness 1.5 mm) equipped with an axial
quartz thermocouple well (2 mm O.D.), which allowed moni-
toring of the temperature of catalyst bed. Typically, the epoxida-
tion of propylene was carried out under atmosphere pressure,
using 0.25 g of catalyst without dilution and a gas feed rate of
28 cm3 minꢀ1 in total, with a feed concentration of 10/10/10/70
vol% of He (99.9%), O2 (99.9%), H2 (99.9%), and C3H6 (99.8%).
ꢀ1
The corresponding space velocity was about 7000 cm3 hꢀ1 gcat
.
Before reaction, the catalyst was pretreated with a mixture of
10 vol% H2 in He (28 cm3 minꢀ1) for 6 h, followed by 10 vol% O2
in He (28 cm3 minꢀ1) at 523 K for 0.5 h, aer that the temperature
was set to 473 K under He ow and the reactants were introduced.
Reaction products were analyzed by an online gas chro-
matograph (Shimadzu GC-14B), containing a CP-wax 52CB and
a Carboxen-1000 columns in two separate channels, with a
ame ionization detector (FID) and a thermal conductivity
detector (TCD), respectively. The CP-wax 52CB capillary column
(0.53 mm ꢁ 50 m) was used to detect oxygenates (i.e., acetal-
dehyde (AA), PO, acetone (ACE), propionaldehyde (PA), acrolein
(ACR)), respectively, whereas the Carboxen-1000 column was
used to analyze H2, O2, and CO2 and propylene, respectively.
The propylene conversion, PO selectivity, H2 conversion, and
H2 selectivity were dened as follows:
Table 1 The physico-chemical properties of the MCM-36 samples
with different Si/Ti ratios
Propylene conversion (XC H ) ¼ mol of (oxygenates + CO2/3)/mol
3
6
of propylene in feed.
Aua loading (%)
Sample
Si/Ti molara Si/B molara [wt%]
Product selectivity (SN) ¼ mol of corresponding product/mol of
(oxygenates + CO2/3),
ERB-1
40Si/Ti-MCM-36
Na
15.3
30
57
N
N
N
N
N
N
N
N
N
N
—
—
—
—
0.84
0.36
0.19
0.12
0.14
0.31
1.1
where N is PO, or oxygenated side products, i.e., AA, ACE, PA 100Si/Ti-MCM-36
160Si/Ti-MCM-36
94
and ACR.
4.5Au/100Si/Ti-MCM-36b 30
3Au/100Si/Ti-MCM-36
30
H2 conversion (XH ) ¼ mol of H2 consumed/moles of H2 in feed.
2
1.5Au/100Si/Ti-MCM-36 30
1Au/100Si/Ti-MCM-36b
30
H2 selectivity (SH ) ¼ mol of PO formed/moles of H2 consumed.
1.5Au/160Si/Ti-MCM-36b 94
2
1.5Au/40Si/Ti-MCM-36b
1.5AuNH3/40Si/
Ti-MCM-36
15.3
15.3
1.5AuNH3/100Si/
Ti-MCM-36
1.5AuNH3/160Si/
Ti-MCM-36
30
N
N
0.74
0.38
3. Results
3.1 Elemental analysis
94
a
Elemental analysis from EPMA. b Spent catalyst.
The ERB-1 discovered by Bellussi et al. in 1988 (ref. 36) is
actually an borosilicate isostructural with the MWW structure.
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Basically, the relative Au amounts loaded in the nal prod-
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ucts can also be determined by the edge jump of an X-ray
absorption spectrum collected in the transmission mode (like
the XRF technique) in the linear range.39,40 However, the uo-
rescence mode can only give semiquantitive analysis results.
The results shown in Fig. S1 (ESI†) obey the rule that the higher
edge jumps of original XAS data correspond to higher Au
contents in the catalysts, which conrms the EPMA results for
the Au wt% contents.
3.2 X-ray diffraction
The small- and wide-angle XRD patterns of the samples are
shown in Fig. 1(A) and (B), respectively. The ERB-1 precursor
shows the diffraction (001) at about 3ꢂ which is attributed to the
stacking of layered MWW structure along the c-direction with a
Fig. 2 (A) Small angle and (B) wide angle XRD patterns of (a) 4.5Au/
100Si/Ti-MCM-36, (b) 3Au/100Si/Ti-MCM-36, (c) 1.5Au/100Si/Ti-
MCM-36, (d) 1Au/100Si/Ti-MCM-36, (e) 1.5Au/160Si/Ti-MCM-36, (f)
1.5Au/40Si/Ti-MCM-36, (g) 1.5AuNH3/40Si/Ti-MCM-36, (h) 1.5AuNH3/
100Si/Ti-MCM-36, and (i) 1.5AuNH3/160Si/Ti-MCM-36.
˚
distance of 27 A. The Si/Ti-pillared MCM-36 samples show the
(001) reection shis down to around 2ꢂ, corresponding to an
˚
increased d-spacing of 44 A, and that implies mesopores of about
˚
20 A are generated by expanding the inter-layer space. Mean-
while, the (002) diffraction at 6.5ꢂ of the precursors almost
disappears. These results are evidences for the successful
expansion of the ERB-1 with the TEOS and TBOT mixture as the
pillaring solvent. However, unlike the merging of (101) and (102)
diffraction peaks for MCM-36 prepared from MCM-22 precursor
reported in the literature,18 the Si/Ti-MCM-36 prepared from ERB-
1(P) still exhibits two well resolved (101) and (102) diffractions
peaks of ERB-1(P). That implies that a portion of the particles,
probably the inner portion, may not be efficiently pillared.
The incorporation of Au did not signicantly change the
structure of Si/Ti-MCM-36 framework as shown in Fig. 2.
However, for the samples with relatively larger Au loadings,
diffraction peaks of Au metal appear at 38.2ꢂ and 44.4ꢂin the
XRD patterns, especially for the samples prepared with NH3 as
the DP agent, such as 4.5AuNH3/40Si/Ti-MCM-36 and
4.5AuNH3/100Si/Ti-MCM-36.
coordination geometry of the Ti(IV) cations. The spectra of Si/Ti-
MCM-36 materials are shown in Fig. 3. All the samples have two
peaks at 220 and 260 nm. However, the intensity of ꢃ260 nm
peak obviously decreases with the decrease of Ti loading.
According to Chen et al.,41 the 220 nm band is assigned to
charge transfer from O2ꢀ to Ti4+ of the tetrahedral (Td) coordi-
nation, while the 260 nm band is attributed to charge transfer
from O2ꢀ to Ti4+ in octahedral (Oh) coordination. Therefore, the
incorporated Ti is more favorable to be present in Td coordi-
nation state than Oh state when Ti loading is low or the Si/Ti
ratio is high. In other words, Ti is highly dispersed in the
silica matrix when Ti loading is low. In contrast, titanium oxide
clusters would form when the Ti loading increases.
Aer supporting Au on Si/Ti-MCM-36 by the DP method, the
materials have the color of white. However, the used catalysts
aer PO synthesis reaction display different colors, such as blue,
pink and brown, and it is a result of the presence of Au nano-
particles (NPs) of different amounts and particles sizes. These
results indicate that Au was reduced to metallic state during the
3.3 UV-vis diffuse reectance spectroscopy
The Ti-incorporated supports with different Si/Ti ratios were
examined with DR UV-vis analysis in order to understand the
Fig. 1 (A) Small angle and (B) wide angle XRD patterns of (a) ERB-1(P), Fig. 3 DR UV-vis spectra of (a) 40Si/Ti-MCM-36, (b) 100Si/Ti-MCM-
(b) 40Si/Ti-MCM-36, (c) 100Si/Ti-MCM-36, (d) 160Si/Ti-MCM-36.
36 and (c) 160Si/Ti-MCM-36.
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catalytic reaction, although it was originally deposited on the
supports in the hydroxide or oxide form. Their UV-vis spectra
exhibit a characteristic absorption band in the visible region
around 500–600 nm as shown in Fig. 4. The samples with larger
amounts of the Au loadings, such as the 1.5Au/100Si/Ti-MCM-
36, 1.5AuNH3/40Si/Ti-MCM-36, 1.5AuNH3/100Si/Ti-MCM-36,
1.5AuNH3/160Si/Ti-MCM-36 and 1.5AuNH3/160Si/Ti-MCM-36
have an obvious absorption band. This absorption band was
due to surface plasmon vibrations of gold, and the wavenumbers
are dependent on the gold particle size.42 The peak has red shi
with the increase of the NPs size. Fig. 4 also shows that the
wavelength of this Au plasmon UV-vis peak for 4.5Au/100Si/Ti-
MCM-36 is slightly lower than those of 1.5AuNH3/40Si/Ti-
MCM-36 and 1.5AuNH3/100Si/Ti-MCM-36, although they have
similar Au loadings. On the other hand, the peak position is
almost the same as that of 1.5AuNH3/160Si/Ti-MCM-36, which
contains more than twice amount of Au. This is evidence that the
Au NPs loaded by using NH3 as the DP agent would give larger Au
NPs than those prepared with NaOH.
Fig. 5 EXAFS of the Au LIII-edge phase-uncorrected Fourier transform
spectra of the c(k) ꢁ k3 weighted in R space after applying a window
around the first shell from 1.6–3.6 A˚. (a) Gold foil, (b) 4.5Au/100Si/Ti-
MCM-36, (c) 3Au/100Si/Ti-MCM-36, (d) 1.5Au/100Si/Ti-MCM-36, (e)
1Au/100Si/Ti-MCM-36, (f) 1.5Au/160Si/Ti-MCM-36, (g) 1.5Au/40Si/Ti-
MCM-36, (h) 1.5AuNH3/40Si/Ti-MCM-36 (vertical value multiple ꢀ1), (i)
1.5AuNH3/100Si/Ti-MCM-36 (vertical value multiple ꢀ1), (j) 1.5AuNH3/
160Si/Ti-MCM-36 (vertical value multiple ꢀ1).
3.4 Au LIII-edge XAS spectroscropy
EXAFS analysis of the Au LIII-edge XAS spectra of spent catalysts
was carried out to determine the CNs of Au atoms and thus to
calculate the sizes of the Au NPs. The fourier transforms (FT) of
Au LIII-edge k3-weighted EXAFS spectra of the catalysts Au/xSi/
Ti-MCM-36 are shown in Fig. 5, and the k3-weighted EXAFS
spectra and the curve tting results are summarized in Fig. S2
and S3 (ESI†). These results are compared with the spectrum of
the Au foil. The rened tting parameters are given in Table 2.
From the obtained coordination number, a mean Au particle
size was calculated by assuming Au metal in spherical particles
and fcc structure.5,43 The resulting mean particle sizes of Au on
the samples are also listed in Table 2.
The spectra and calculated results of 100Si/Ti-MCM-36
loaded with different amounts of Au varied from 0.84 to 0.36
wt% show that smaller Au NPs are generated when less amounts
of Au are loaded on the support, and concomitantly the lower
average coordination numbers are obtained for Au atoms. It is
also noticed that the rst shell near-neighbor distance, R,
deceases slightly with lower Au loading, in consistent with the
literature report.5 Furthermore, the Au NPs size has little vari-
ation with the Au loadings lower than 0.36 wt%.
In comparison of the catalysts with different Ti-contents, i.e.,
1.5Au/xSi/Ti-MCM-36, the correlations between Au NPs sizes
and Au loadings are similar. The Au NPs sizes increase with
increasing the Ti content. As to the samples prepared using NH3
as DP agent, relatively larger gold particles are formed, in
agreement with the results from the UV-vis spectra.
3.5 Transmission electron microscopy
Fig. S4 (ESI†) shows the TEM images of two series of samples,
namely Au supported on Si/Ti-MCM-36 prepared by NaOH and
NH3 as the neutralization agents with different Ti contents.
The size distribution of the gold NPs analyzed by Digital
Micrograph Demo soware are shown in Fig. 6. Fig. 6(a)–(c)
compare the samples with gold loadings of nominal 1.5 wt%
with NaOH as neutralization agent over the xSi/Ti-MCM-36.
The average sizes of gold NPs are about 2.0 nm, 1.5 nm, and
1.2 nm for x ¼ 40, 100, 160, respectively. The effect of
neutralization agents in DP method on the gold NPs size
distributions is shown in Fig. 6(a)–(c) versus Fig. 6(d)–(f). The
Fig. 4 DR UV-vis spectra of (a) 4.5Au/100Si/Ti-MCM-36, (b) 3Au/
100Si/Ti-MCM-36, (c) 1.5Au/100Si/Ti-MCM-36, (d) 1Au/100Si/Ti-
MCM-36, (e) 1.5Au/160Si/Ti-MCM-36, (f) 1.5Au/40Si/Ti-MCM-36, (g)
1.5AuNH3/40Si/Ti-MCM-36, (h) 1.5AuNH3/100Si/Ti-MCM-36 and (i)
1.5AuNH3/160Si/Ti-MCM-36.
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Table 2 Refined fitting parameters from Au LIII-edge EXAFS analysis of the first coordination shell for different supported Au samples in
comparison with that of Au foil as the reference materiala
2
2
2
˚
R (A)
˚
Sample
Ave. CN
S0 (set)
Ds (A )
DE (eV)
Estimated Au NPs size (nm)
Gold foil
12.0
9.9
8.6
2.86
2.86
2.84
2.82
2.82
2.82
2.84
2.85
2.85
2.85
0.86
0.86
0.86
0.86
0.86
0.86
0.86
0.86
0.86
0.86
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
4.62
5.99
5.30
4.72
4.59
5.28
5.54
5.04
5.21
4.08
—
4.5Au/100Si/Ti-MCM-36
3Au/100Si/Ti-MCM-36
1.5Au/100Si/Ti-MCM-36
1Au/100Si/Ti-MCM-36
1.5Au/160Si/Ti-MCM-36
1.5Au/40Si/Ti-MCM-36
1.5AuNH3/40Si/Ti-MCM-36
1.5AuNH3/100Si/Ti-MCM-36
1.5AuNH3/160Si/Ti-MCM-36
2.43
1.49
1.25
1.23
0.99
1.67
6.34
4.24
2.34
7.9
7. 9
6. 9
8.9
11.2
10.8
9.8
a
CN, R, S02, s2 and DE are the coordination number, Au–Au bond distance, amplitude reduction factor, Debye–Waller factor and energy correction
number, respectively.
shown. One is the specic rate or yield per hour based on per
kg of catalyst and are called PO formation rate. The other PO
rate is turnover frequency (TOF) based on per gram of gold,
and that can demonstrate the Au efficiency.3
Over the 100Si/Ti-MCM-36 supported Au catalysts, the four
catalysts with similar Au NPs sizes but the decrease of Au
loadings from 2 wt% to 1 wt%, the XC H decreases and SPO
3
6
increases at the same time. The maximum Au efficiency and
SH reach 14.6 gPO hꢀ1 gAu and 9.7% over 1.5Au/100Si/Ti-
ꢀ1
2
MCM-36 catalyst, and the PO formation rate over this cata-
lyst (27.7 gPO hꢀ1 kgcatꢀ1) is close to that over 3Au/100Si/Ti-
MCM-36 (28.9 gPO hꢀ1 kgcatꢀ1). It also shows that the cata-
lysts with the lower Au loadings have lower stability during
the reaction, when comparing the catalytic activities at
beginning and aer 6 h reaction. Furthermore, when Au
loadings are higher than 3.8 wt%, the Au/100Si/Ti-MCM-36
catalyst exhibits the worst catalytic performance, namely
50% selectivity of CO2, a high H2 conversion (about 33%) with
mostly generating H2O and producing only 3.8% PO (Table
S1†). This phenomenon is explained by its much too large Au
NPs size than others.
Although both series of the catalysts 1.5Au/xSi/Ti-MCM-36
exhibit similar tendency that the actual Au content and the
corresponding NPs size increase with the Ti content, their
catalytic performance trends are different. The optimum cata-
lytic activity was found for 1.5Au/xSi/Ti-MCM-36 with nominal
Si/Ti ratio of 100 in terms of the PO formation rate, Au effi-
Fig. 6 The size distributions of gold nanoparticles of (a) 1.5Au/40Si/Ti-
MCM-36, (b) 1.5Au/100Si/Ti-MCM-36, (c) 1.5Au/160Si/Ti-MCM-36, (d)
1.5AuNH3/40Si/Ti-MCM-36, (e) 1.5AuNH3/100Si/Ti-MCM-36, (f)
1.5AuNH3/160Si/Ti-MCM-36.
gold NPs prepared with NH3 as the neutralization agent are
apparently larger and have wider distribution than those
prepared with NaOH agent.
3.6 Catalytic performance
The Au/xSi/Ti-MCM-36 catalysts were applied for the direct
propene epoxidation with O2 and H2 in a plug-ow system for
about 6 h. The catalytic activities at time on stream (TOS) of 10
min and the average in 6 h are summarized in Table 3 and the
completed data are shown in Table S1 (ESI†). The catalytic
activities are expressed in terms of propene conversion (XC H ),
ciency, SPO and SH
.
2
The phenomena were observed on 1.5AuNH3/xSi/Ti-MCM-36
series that by using NH3 instead of NaOH as the DP neutrali-
zation agent, the PO yields decrease signicantly while the
propanal (PA) selectivities increase dramatically.
3
6
In the reaction temperatures of 433–498 K, it is found that
the XC H and XH increase but SPO and SH decrease with
PO selectivity (SPO) and side products selectivities, PO yield,
hydrogen conversion (XH ), and hydrogen selectivity (SH ). SH
2
3
6
2
2
2
2
increasing the reaction temperature over almost all the cata-
lysts. Moreover, based on the PO formation rate the best reac-
tion temperature is 453 K over Si/Ti-MCM-36 supported Au
catalysts.
is dened by moles of PO/moles of H2 consumed, which is to
differentiate the efficiency of H2 consumed for propylene
epoxidation from that consumed in the side reactions, such as
direct reaction with O2 for H2O production or hydrogenation
of propylene to form propane. Two PO formation rates are
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a
Table 3 Catalytic performance of Si/Ti-MCM-36 supported Au catalysts in propene epoxidation with H2 and O2
Selectivityb (%)
Si/Ti atomic
ratio
Catalyst
Au wt%
0.84
T (K)
PO yield (%)
Propene conv. (%)
PO
PA
ACR
ACE
4.5Au/100Si/Ti-MCM-36
30
30
30
453
(0.9)
0.67
0.51
0.44
(1.88)
1.01
0.97
0.98
(1.8)
1.39
1.33
1.1
(0.93)
0.48
(1.31)
0.39
(0.97)
0.39
0.37
(0.18)
0.29
(0.43)
0.36
(0.04)
0.19
(0.88)
0.71
0.55
0.55
(2.98)
1.57
1.2
2.05
(2.19)
1.61
1.49
1.6
(1.08)
0.61
(2.12)
0.65
(2.22)
0.97
0.84
(2.31)
1.63
(2.32)
2.13
(0.49)
0.59
(84.93)
93.02
94.49
79.43
(62.99)
63.08
81.07
48.63
(82.24)
86.61
90.74
67.45
(86.02)
77.49
(61.83)
62.9
(43.58)
41.39
57.02
(7.99)
16.81
(18.7)
16.82
(8.5)
(0)
0
0
4.97
(1.68)
0
(4.55)
0.91
1.14
3.55
(4.7)
14.68
8.25
27.09
(0)
(0)
0
0
433
473
453
0
3Au/100Si/Ti-MCM-36
1.5Au/100Si/Ti-MCM-36
0.36
0.19
(0.34)
0
0
0
(0)
0
433
473
453
0
5.17
(1.37)
0.23
0.3
2.6
(0)
0
433
473
453
0
0
0.92
(0)
0
(2.83)
0.57
(0)
0
(0)
0
(0)
0
(0)
0
1Au/100Si/Ti-MCM-36
1.5Au/40Si/Ti-MCM-36
1.5Au/160Ti-MCM-36
30
0.12
0.31
0.14
0
15.3
94
453
453
(5.19)
9.18
(18.47)
5.69
4.62
(87.88)
68.48
(79.31)
80.86
(73.47)
56.48
0
433
453
0
0
1.5AuNH3/40Si/Ti-MCM-36
1.5AuNH3/100Si/Ti-MCM-36
1.5AuNH3/160Si/Ti-MCM-36
15.3
30
1.1
(0)
0
(0)
0
(0)
0
(0)
0
(0)
0
(0)
0
0.74
0.38
453
453
94
31.39
a
ꢀ1
b
Reaction condition: 0.25 g catalysts, 433–498 K, space velocity: 7000 mL minꢀ1 gcat
.
PO: propylene oxide, PA: propanal, ACR: acrolein, ACE:
acetone. The catalystic performance data in the bracket is collect aer 10 min reaction; the other data is from the average of 6 h reaction.
S1.† Moreover, PA was found to be the principal product
obtained besides propylene oxide over these catalysts. For the
catalysts with different Au loadings but same supports, the
4. Discussion
4.1 Effect of Au particle size on PO synthesis
overloaded Au on the supports 100Si/Ti-MCM-36 causes the
formation of larger Au NPs (more than 2 nm), which are not
efficient for the PO formation. This observation is in consis-
tence with the literature reports.13,47 Possible explanation is that
the formed hydrogen peroxide is more difficult to migrate to Ti
sites on the larger Au particles because of longer distance and
less interfacial area between gold particles and the supports.
Instead, hydrogen peroxide is decomposed to H2O or involved
in nonselective oxidation.
The Au particle sizes estimated from the 1NN coordination
numbers of Au LIII-edge EXAFS suggest that the average Au
particles are ranged from 1 to 6 nm, which are slightly lower
than the average sizes determined from TEM images. It has
been reported that the Au NPs with size smaller than 1 nm are
invisible in the TEM images.11,15,27 In comparison, EXAFS is a
more sensitive technique to determine the average size of poly-
dispersed Au particles, especially toward diameter smaller than
1 nm.4 Nevertheless, there are correlations between the TEM
and EXAFS results of Au particle sizes, despite of the higher
sensitivity of EXFAS technique towards smaller particles.
The Au NPs size is considered to be one of the critical factors
to inuence the catalytic performance in propylene oxidation.4
Gold is believed to rstly catalyze the reaction of O2/H2 to form
hydrogen peroxide, which then migrates to Ti(IV) sites on the
supports to form titanium hydroperoxide species, and the latter
react with propylene to form epoxide.2,3,44–46 The Au catalysts
prepared using NH3 as the neutralization agent have much
higher Au loadings and larger NPs sizes than the corresponding
samples with NaOH as neutralization agent over both Si/Ti-
MCM-36 supports, as depicted from the EPMA, XRD, UV-vis
and EXAFS analysis results. All the catalysts with Au NPs sizes
larger than 2 nm have very low PO yields, as depicted in Table
4.2 Effect of Ti coordination environment on PO synthesis
For 1.5Au/xSi/Ti-MCM-36 catalysts, the actual Au loading and
the corresponding Au NPs size increase with the Ti content in
the supports. It is widely accepted that the higher Ti contents in
Si/Ti-MCM-36 can anchor more Au and result in the formation
of larger particles. The most active catalyst 1.5Au/100Si/Ti-
MCM-36 has Au NPs size of about 1.25 nm, which is in
between 0.99 nm of 1.5Au/160Si/Ti-MCM-36 and 1.66 nm of
1.5Au/40Si/Ti-MCM-36. According to literature,48–50 it is inter-
esting to nd that the Au NPs are larger in size when Ti in the
supports coexisted in Oh and Td coordinations, most of which is
incorporated in the mesopores by graing method. In contrast,
the active Au NPs are smaller in the supports containing
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absolutely isolated Td Ti, which are almost prepared by the nanoparticles loaded on these supports by DP method were
hydrothermal synthesis. These results are in consistence with capable to catalyze gas-phase propylene epoxidation with H2
our observation that the Au NPs supported on Si/Ti-MCM-36 and O2.
with higher contents of Oh coordinated Ti is a little larger
The small and wide angle XRD studies conrmed the
than that of lower contents of Oh coordinated Ti on the support. expanded structure between the layers and the reservation of
The coordination environment of incorporated Ti in the the internal MWW zeolitic structure aer incorporation of Ti
supports has great inuence on the loading and particle size of and Au. Aer Au loading by DP method the mesopores of Si/Ti-
the anchored Au, as well as its catalytic performance. The MCM-36 were still preserved. The UV-vis spectra and EPMA
optimal reaction temperature of Au/xSi/Ti-MCM-36 catalysts is analyses indicated that the Ti loading and ratio of Oh/Td coor-
evidence for the inuence of Ti species in the supports on the dinated Ti in Si/Ti-MCM-36 could be controlled by adjusting the
catalytic performance. The change in catalytic temperature of ratio of TEOS/TBOT precursors in the pillaring solvent.
Au on different supports for PO formation was reported in the
The support plays important roles in affecting the Au loading
literature.1 The propylene epoxidation reaction should be and NPs sizes. The Au NPs loaded by NH3 as DP agent were
carried out at temperatures below 373 K over the Au/TiO2 larger than those loaded by NaOH. On the other hand, the
catalysts, because PO is favorable to further oxidization to ACR, loading and size of gold particles had direct correlation with Ti
CO2 and H2O at higher temperatures. However, when replacing contents on both series of Si/Ti-MCM-36 catalysts. Moreover,
TiO2 with highly dispersed TiO2 on SiO2 surfaces or TS-1, the less amounts of Au and smaller NPs were loaded on Si/Ti-MCM-
operating temperature could be raised to 423–473 K.1 Therefore, 36 with lower content of Ti.
the optimal reaction temperature 453 K of Au/xSi/Ti-MCM-36
The catalytic performances of Au loaded Si/Ti-MCM-36 in
indicates that the Ti species is well dispersed in the supports. propylene epoxidation proved that the Au NPs with particle size
The most active Au NPs size of about 1.25 nm for the catalysts larger than 2 nm were not efficient for the PO formation and PA
1.5Au/xSi/Ti-MCM-36 is in consistent with the active Au particle was the principal product. The most efficient catalysts con-
size range for the catalysts with Ti in the supports coexisted in tained Au NPs of about 1 nm. The synergy of Au and Ti was
Oh and Td coordinations.4,5,50
found important for PO formation. Overloading Au could result
in the redundant C3H6 combustion and reduction of the H2
selectivity to PO. Nevertheless, the insufficient Au loading could
not generate enough H2O2 for the PO formation and excessive Ti
would inhibit desorption of formed PO from the supports and
cause catalyst deactivation. The optimal reaction temperature
for Au/Si/Ti-MCM-36 is 453 K.
4.3 The synergy of Au and Ti for PO formation
It has been reported that the epoxidation occurs at or close to
the interface of Au–Ti sites.4 A good synergy between gold and
titanium could limit water formation which occurs on the gold
sites not adjacent to titanium.3,8 The Au loadings in xAu/100Si/
Ti-MCM-36 samples with xed Ti content but Au precursor
concentrations varied in 1, 1.5 and 3 wt% have similar sizes of
Au NPs (1.2–1.5 nm). The variation in catalytic activities of these
three catalysts may demonstrate the synergy effect. The highest
Acknowledgements
The nancial supports from Ministry of Science & Technology,
Taiwan (NSC101-2113-M-002-012-MY3, NSC102-2811-M-002-
136) is gratefully acknowledged. F. Jin also acknowledges the
supports of Natural Science Foundation, China (NSFC
21306143) and Scientic Research Foundation for the Returned
Overseas Chinese Scholars, Education Ministry, China.
Acknowledgement is also extended to Prof. H.C. Lin and Mr.
C.Y. Kao, Ms. Y.Y. Yang and C.-Y. Chien of Instrumentation
Center, National Taiwan University for the assistances in EPMA
and EM experiments.
Au efficiency and SH are observed on 1.5Au/100Si/Ti-MCM-36
2
(Table S1†), which should reach the best synergy between Au
and Ti. Doubling the actual Au loading on 3Au/100Si/Ti-MCM-
36 increases the XC H and XH . However, the insufficient Ti
3
6
2
content causes the insufficient hydroperoxy intermediates
formed at the Ti sites for the PO formation. Instead, the
redundant C3H6 combustion occurs on the Au surfaces,13 and
CO2 and H2O are formed.3 It is evidenced by the increased SCO
2
and decreased SH in comparison of 3Au/100Si/Ti-MCM-36 with
2
1.5Au/100Si/Ti-MCM-36. On the other hand, the decrease of Au
loading from 1.5 to 1 wt% causes faster deactivation of the
catalyst, which is attributed to that insufficient hydrogen
peroxide is formed on Au NPs, and excessive Ti may inhibit the
desorption of formed PO from the supports and deactivate the
catalyst.8
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