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[Article] www.whxb.pku.edu.cn
物理化学学报(Wuli Huaxue Xuebao)
Acta Phys. -Chim. Sin. 2014, 30 (9), 1727-1735
Received: April 22, 2014; Revised: July 7, 2014; Published on Web: July 8, 2014.∗Corresponding author. Email: [email protected], Tel: +86-21-62232251.The project was supported by the National Natural Science Foundation of China (20890122) and National Key Technology R&D Program, China
Fig.4 SEM images of samples synthesized at 150 °C for 3 d
and 120 °C for 12 d(a) S25-0.30-150, (b) S25-0.30-120
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Acta Phys. -Chim. Sin. 2014 Vol.30
of the templates physisorbed on the external surface of the zeolite
crystallites and decomposition of the organic templates trapped in
the zeolite pore channels, respectively.37,38 It might be explained
that silicalite-2 microspheres composed of nano-sized particles,
possessing high external surface area, lost much adsorbed water
and organic templates on external surfaces during calcination
process.
The 29Si MAS NMR spectra of the as synthesized and the
calcined S25-0.30-175 are depicted in Fig.6. Two broad resonance
signals are observed with chemical shifts at - 113 and - 102,
which belong to Q4 [Si (4Si)] and Q3 [Si (3Si, 1OH)] respec-
tively.39-41 The high content of Q3 species indicates the presence of
the large amount of external surface silanols and internal defects
resulted from the imperfect condensation. Upon the calcination,
the content of Q3 species obviously decreases due to the further
condensation of silanol groups. These results indicated that
structural ordering of the nano-sized sample is lower than that of
ordinary bulky crystals.1H-13C CP/MAS NMR spectroscopy is a very powerful tool to
investigate zeolite structure via the study of the occluded organic
templating molecules. The 1H-13C CP/MAS NMR spectra of the
as synthesized sample S25-0.30-175 is illustrated in Fig.7. The
chemical shifts at 60.6, 23.8, 20.2, and 14.2 can be unambiguously
assigned to the C1, C2, C3, and C4 in N+―CH2―CH2―CH2―CH3
chain, respectively. A slight difference is observed compared with
the results reported by Nagy et al.42 where a doublet was observed
at 14.8 and 12.8 in 1H-13C CP/MAS NMR spectrum of the micro-
sized zeolite crystals. For comparison, the standard 13C NMR
spectroscopy of TBAOH in D2O is shown in Fig.7 (inset). A single
line without any doublet is observed at 13.6. This can be ex-
plained by the special morphology of the sample S25-0.30-175
with assembled nano-sized particles, TBA+ ions are arranged in
the shorter channels of the sample with a higher degree of dis-
order, as proved by 29Si MAS NMR spectra, leading to the dis-
appearance of the weak peak at 12.8.
To explore the catalytic properties of micro-sized zeolite MEL
aggregates, Ti was incorporated in the framework at nSi/nTi=40.
Synthesis in the presence of titanium did not affect the structure
of MEL zeolite or prevent assembling of nano-sized primary
particles (Fig.8A and 8B). Pure MEL structured micro-sized TS-
2 spheres in size of ~8 µm were obtained at 175 °C for 3 d when
molar ratio of TBAOH/SiO2 is 0.30. Nitrogen adsorption iso-
therms indicate the presence of both micro- and mesoporosity in
the TS-2 microspheres, which possess high BET surface area (517
m2∙g-1) and large porosity (0.58 cm3∙g-1). Furthermore, the co-
ordination states of Ti species were studied by UV-Vis spectros-
copy. TS-2 showed a main absorption around 210 nm, which is
assigned to the charge transfer from O2- to Ti4 + (Fig.8D). This
adsorption is usually observed for Ti-substituted zeolites and is
characteristic of tetrahedrally coordinated Ti highly dispersed in
the framework.43 This could be beneficial for redox reactions with
large molecules that require tetrahedrally coordinated titanium. On
the other hand, TS-2 showed a broad adsorption at 230-270 nm,
implying the coexistence of some penta- or hexacoordinated Ti
species as a result of water adsorption.44 Large amount of silanol
group due to the formation of nano-sized primary particles might
be the main reason which results in high hydrophility and surface
Fig.5 FTIR spectra of silicalite-2 samples synthesized
at 175 °C for 3 d(a) S25-0.12-175, (b) S25-0.25-175, (c) S 25-0.30-175
Fig.6 29Si MAS NMR spectra of the as synthesized (a) and
calcined (b) sample S25-0.30-175
Fig.7 1H-13C CP/MAS NMR spectra of the as synthesized
sample S25-0.30-175inset: bar chart 13C NMR spectra of TBAOH in D2O
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CHEN Hong-Li et al.: Synthesis of Micro-Sized MEL-Type Zeolite AggregatesNo.9
area thus much adsorption of water.
The hydroxylation of phenol into catechol and hydroquinone
with aqueous H2O2 solution was selected as catalytic test reaction.
Titanium silicalite-1 (TS-1) with MFI structure was proved to be
an active catalyst in the hydroxylation of phenol,45,46 which has
been commercialized by EniChem.47 As a member of pentasil
family of zeolites as similar to TS-1, TS-2 also shows consider-
able catalytic performance in the hydroxylation of phenol.48
Meanwhile, It was reported that the framework structure differ-
ences did not perturb the catalytic performances by comparing
catalytic behaviors of TS-1 and TS-2 with similar Si/Ti molar
ratios and crystal sizes.48 This is not really surprising considering
that TS- 2 and TS- 1 framework structures are very similar.
However, it was demonstrated that the catalytic activity of TS-1
samples was very sensitive to the crystal size.49 Beyond 1 µm, the
diffusion of products and reactants is difficult and the autode-
composition of H2O2 is the prevailing reaction. Therefore, not only
low activities but also very low H2O2 efficiencies are observed. In
this case, TS-1 in size of ~100 nm was synthesized as a reference.
Fig.8 Structure and morphology of TS-2 microspheresMolar ratio of Si/Ti (nSi/nTi) is 40. (A) XRD pattern of TS-2 compared to that of the pure silicalite-2 S25-0.30-175, (B) SEM images of the TS-2 showing
uniform micro-sized spheres in size of ~8 µm, (C) N2 adsorption/desorption isotherms of calcined TS-2, (D) UV-Vis spectra of TS-2 showing
that Ti is incorporated in the MEL zeolite framework.
Table 2 Some parameters for hydroxylation of phenol on
the TS-1 and TS-2
a Nano-sized TS-1 was synthesized using TPAOH as microporous template with
nSi/nTi=40 in gel composition. b molar ratio of Si/Ti of final calcined products
quantified by inductively coupled plasma (ICP). c turnover number in
mol∙mol-1. d turnover frequency in mol∙mol-1∙h-1
Sample
TS-1a
TS-2
dcrystal/nm
~100
8000
nSi/nTib
41.6
51.1
Phenol conversion/%
26.0
23.2
TONc
140.8
153.5
TOFd
23.5
25.6
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Acta Phys. -Chim. Sin. 2014 Vol.30
The phenol conversion over micro-sized TS-2 is comparable to
that over nano-sized TS-1 (Table 2). Moreover, micro-sized TS-2
shows larger TON and TOF, indicating that the more effective
catalytic activity of Ti incorporated into the framework of TS-2
spheres. Most importantly, the formation of micro-sized aggregate
not only well keeps the advantages of acidic activity and shape
selectivity of micropores, but also curtails the difficulties in
separation and recovery. As a contrast, centrifugation was nec-
essary to collect nano-sized TS-1, which is difficult for indus-
trialization process. Furthermore, the generated extensive mes-
opore would reduce the rate of catalyst deactivation by slowing
down the process of coke formation.
4 ConclusionsMicro-sized silicalite-2 aggregates composed of nano-sized
primary particles (<50 nm) have been hydrothermally synthesized
from the synthetic system TBAOH-TEOS-H2O. The micro-sized
silicalite-2 aggregates could be formed in a wide range of
TBAOH/SiO2 molar ratios and crystallization temperatures with
appropriate amount of H2O, and show high surface area and large
porosity. In addition, micro-sized TS-2 spheres show comparable
catalytic activity to that of nano-sized TS-1 in the hydroxylation
of phenol, but show no difficulties in the separation for nano-sized
TS-1. These results prove that the formation of micro-sized ag-
gregates not only well keeps the advantages of nano-sized zeolites
in diffusion and catalytic activity but also curtails the difficulties
in separation and recovery.
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