Synthesis, crystallization mechanism, and catalytic properties of titanium-rich TS-1 free of extraframework titanium species

Article abstract of DOI:10.1021/ja7100399

A new route to the synthesis of TS-1 has been developed using (NH ₄)₂CO₃ as a crystallization-mediating agent. In this way, the framework Ti content can be significantly increased without forming extraframework Ti species. The prepared catalyst had a Si/Ti ratio as low as 34 in contrast to the ratio of 58 achieved with the methods A and B established by the Enichem group (Clerici, M. G.; Bellussi, G.; Romano, U. J. Catal. 1991, 129, 159) and Thangaraj and Sivasanker (Thangaraj, A.; Sivasanker, S. J. Chem. Soc., Chem. Commun. 1992, 123), respectively. The material contained less defect sites than the samples synthesized by the other two methods. As a result, it showed much higher activity for the oxidation of various organic substrates, such as linear alkanes/alkenes and alcohols, styrene, and benzene. The crystallization mechanism of TS-1 in the presence of (NH₄) ₂CO₃ was studied by following the whole crystallization process with X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), thermogravimetry/differential thermal analysis (TG/DTA), inductively coupled plasma atomic emission spectrometry (ICP), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), diffuse reflectance UV-vis spectroscopy, and ²⁹Si MAS (magic-angle spinning) NMR spectroscopy techniques. It was shown that the presence of (NH ₄)₂CO₃ not only drastically lowered down pH, slowing down the crystallization process and making the incorporation of Ti into the framework match well with nucleation and crystal growth, but also modified the crystallization mechanism. It seems that the solid-phase transformation mechanism predominated in the crystallization process initiated by dissociation, reorganization, and recoalescence of the solidified gel although a small amount of nongelatinated Ti shifted to the solid during the crystal growth period. In contrast, a typical homogeneous nucleation mechanism occurred in the method A system. Thus, although in the method A system most of Ti cations was inserted into the lattice after the crystallization was nearly completed, the inclusion of Ti started at the earlier nucleation period in the presence of (NH ₄)₂CO₃. This is favorable for the incorporation of Ti into the framework, resulting in a more homogeneous distribution of Ti in the framework. Oxidation of 1-hexene and 2-hexanol over the samples collected during the whole crystallization process indicated that condensation of Ti-OH and Si-OH proceeded even after the crystallization was completed. This resulted in an increase in hydrophobicity and an overall improvement in microscopic character of Ti species and consequently a great increase in the catalytic activity with further progress of crystallization.

Full text of DOI:10.1021/ja7100399

Published on Web 07/10/2008
Synthesis, Crystallization Mechanism, and Catalytic
Properties of Titanium-Rich TS-1 Free of Extraframework
Titanium Species
Weibin Fan,†,‡ Ren-Guan Duan,§ Toshiyuki Yokoi,† Peng Wu,| Yoshihiro Kubota,
and Takashi Tatsumi*,†
Catalytic Chemistry DiVision, Chemical Resources Laboratory, Tokyo Institute of Technology,
Nagatsuta 4259, Midori-ku, Yokohama 226-8503, Japan, State Key Laboratory of Coal
ConVersion, Institute of Coal Chemistry, Chinese Academy of Sciences, 27 South Taoyuan Road,
Taiyuan 030001, China, Mail Stop D469, EES-6, Los Alamos National Laboratory, Los Alamos,
New Mexico 87545, Shanghai Key Laboratory of Green Chemistry and Chemical Processes,
Department of Chemistry, East China Normal UniVersity, North Zhongshan Road 3663,
Shanghai 200062, China, and Catalysis Laboratory, DiVision of Materials Science and Chemical
Engineering, Faculty of Engineering, Yokohama National UniVersity, Tokiwadai 79-5,
Hodogaya-ku, Yokohama 240-8501, Japan
Received November 14, 2007; E-mail: ttatsumi@cat.res.titech.ac.jp
Abstract: A new route to the synthesis of TS-1 has been developed using (NH4)2CO3 as a crystallization-
mediating agent. In this way, the framework Ti content can be significantly increased without forming
extraframework Ti species. The prepared catalyst had a Si/Ti ratio as low as 34 in contrast to the ratio of
58 achieved with the methods A and B established by the Enichem group (Clerici, M. G.; Bellussi, G.;
Romano, U. J. Catal. 1991, 129, 159) and Thangaraj and Sivasanker (Thangaraj, A.; Sivasanker, S. J. Chem.
Soc., Chem. Commun. 1992, 123), respectively. The material contained less defect sites than the samples
synthesized by the other two methods. As a result, it showed much higher activity for the oxidation of
various organic substrates, such as linear alkanes/alkenes and alcohols, styrene, and benzene. The
crystallization mechanism of TS-1 in the presence of (NH4)2CO3 was studied by following the whole
crystallization process with X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM),
thermogravimetry/differential thermal analysis (TG/DTA), inductively coupled plasma atomic emission
spectrometry (ICP), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS),
diffuse reflectance UV-vis spectroscopy, and 29Si MAS (magic-angle spinning) NMR spectroscopy
techniques. It was shown that the presence of (NH4)2CO3 not only drastically lowered down pH, slowing
down the crystallization process and making the incorporation of Ti into the framework match well with
nucleation and crystal growth, but also modified the crystallization mechanism. It seems that the solid-
phase transformation mechanism predominated in the crystallization process initiated by dissociation,
reorganization, and recoalescence of the solidified gel although a small amount of nongelatinated Ti shifted
to the solid during the crystal growth period. In contrast, a typical homogeneous nucleation mechanism
occurred in the method A system. Thus, although in the method A system most of Ti cations was inserted
into the lattice after the crystallization was nearly completed, the inclusion of Ti started at the earlier nucleation
period in the presence of (NH4)2CO3. This is favorable for the incorporation of Ti into the framework, resulting
in a more homogeneous distribution of Ti in the framework. Oxidation of 1-hexene and 2-hexanol over the
samples collected during the whole crystallization process indicated that condensation of Ti-OH and Si-OH
proceeded even after the crystallization was completed. This resulted in an increase in hydrophobicity and
an overall improvement in microscopic character of Ti species and consequently a great increase in the
catalytic activity with further progress of crystallization.
1. Introduction
a number of industrially important organic oxidation reactions
in the presence of aqueous H2O2 solution. Typical reactions
include the conversion of alkanes to alcohols and ketones,
secondary alcohols to ketones, secondary amines and ammonia
to dialkylhydroxylamines and hydroxylamine, as well as alkene
epoxidation, phenol oxidation, ketone ammoximation, etc.1–7
The discovery of TS-1 is a milestone in zeolite and
heterogeneous catalysis research fields. This is because it is an
active, highly selective, and environmentally benign catalyst for
† Tokyo Institute of Technology.
‡ Institute of Coal Chemistry, Chinese Academy of Sciences.
§ Los Alamos National Laboratory.
| East China Normal University.
(1) Tatsumi, T.; Nakamura, M.; Negishi, S.; Tominaga, H. Chem.
Commun. 1990, 476.
Yokohama National University.
(2) Notari, B. AdV. Catal. 1996, 41, 253, and references therein.
9
10150 J. AM. CHEM. SOC. 2008, 130, 10150–10164
10.1021/ja7100399 CCC: $40.75
2008 American Chemical Society

Synthesis and Properties of Ti-Rich TS-1
A R T I C L E S
The catalytic property of TS-1 depends on the lattice Ti content,
which is, however, usually less than 2 wt %.8,9 The effective
way to increase the Ti content in the framework of TS-1 is still
a huge challenge. Thangaraj and Sivasanker reported that eight
Ti ions could be incorporated in the lattice sites per unit cell
by an improved method (method B) in which titanium tetra-n-
butoxide was first dissolved in isopropyl alcohol before addition
to the hydrolyzed tetraethyl orthosilicate aqueous solution for
the purpose of avoiding formation of TiO2 precipitate by
reducing the hydrolysis rate of the alkoxide,10 but Schuchardt
et al. could not reproduce it, and found that there was no
difference in the framework Ti content between the samples
synthesized by methods A and B.11 This is also proven by our
data shown in the following section.
To synthesize Ti-rich TS-1, it is necessary and helpful to make
its crystallization mechanism clear. However, very few reports
have been devoted to the study of this subject.12 This might be
due to the complexity of the zeolite crystallization mechanism.
Indeed, it involves both nucleation and crystal growth,12–27 and
both have numerous simultaneous and independent equilibria
and condensation steps while nanoaggregates might evolve into
zeolite crystals.17–27 In addition, the synthesis materials, i.e.,
silica source, solvent, and templating/structure-directing agent
as well as the crystallization promoter or inhibitor, also
significantly affect the crystallization mechanism. As a result,
with respect to nucleation, there are several models, including
homogeneous nucleation, heterogeneous nucleation, autocata-
lytic nucleation, and secondary nucleation, and different nucle-
ation processes have been tried to be accounted for by different
models.12,13,17–27 In the case of crystallization growth, a
transition period involving a slow growth of a crystalline phase
and a relatively rapid crystal growth period are included.
Crystallization in hydrothermal systems may occur through the
liquid-phase transformation mechanisms,13–27 whereas in non-
aqueous systems the crystallization mechanism via solid-phase
transformation seems to predominate.13,14 In contrast, in the case
of synthesis of ZSM-5 in the system free of solvent by using
TPABr and NH4F as cotemplates, a vapor phase transport
mechanism has been suggested.28 Therefore, up to date, although
more than 170 types of zeolitic materials with various uniform
topologies have been synthesized and some of them have been
widely used in petrochemical and fine chemical industries as
active and selective catalysts and/or effective adsorbents, their
crystallization mechanisms have not been clear yet. A full
understanding of the crystallization mechanisms is desirable for
the synthesis of conventional zeolites with significantly im-
proved performance and new zeolites.
The crystallization process of titanosilicates is much more
complex than that of aluminosilicates because Ti4+ has a weak
structure-directing role and is much more difficult to be
incorporated into the framework than Al3+. As we know,
isomorphous substitution of metal atoms for Si in zeolites is
not only related to zeolite structures/framework composition
flexibility and the chemical nature of metals29–33 but is also
strongly influenced by the crystallization mechanism. The
framework composition flexibility of zeolites is chemically
important. Isomorphous substitution of transition metal ions for
Si requires a flexibility of the framework to bring together a
cluster of oxygen atoms around metal cations.32 This could be
made easier by hosting many defect sites in the framework.9,34,35
Therefore, the more flexible the framework of zeolites is, the
higher the substitution degree is.33 Thus, more Ti species could
be incorporated into the framework of Ti-Beta and Ti-MWW
than into that of TS-1.36,37 Nevertheless, Ti K edge extended
X-ray absorption fine structure (EXAFS) studies have shown
that the Ti-O bond length of tetrahedral Ti(OSi)4 species is
about 1.80 Å in contrast to the 1.61 Å of the Si-O bond
length.35,38–43 The Ti-O bond is much longer than the Si-O
bond, probably making the local structure around Ti seriously
(3) Clerici, M. G.; Ingallina, P. J. Catal. 1993, 140, 71.
(4) Bellussi, G.; Rigutto, M. S. Stud. Surf. Sci. Catal. 1994, 85, 177.
(5) Ratnasamy, P.; Srinivas, D.; Kno¨zinger, H. AdV. Catal. 2004, 48, 1.
(6) Bhaumik, A.; Kumar, R. J. Chem. Soc., Chem. Commun. 1995, 349.
(7) Bhaumik, A.; Samanta, S.; Mal, N. M. Microporous Mesoporous
Mater. 2004, 68, 29.
(8) Bordiga, S.; Damin, A.; Bonino, F.; Ricchiardi, G.; Zecchina, A.;
Tagliapietra, R.; Lamberti, C. Phys. Chem. Chem. Phys. 2003, 5, 4390.
(9) Lamberti, C.; Bordiga, S.; Zecchina, A.; Artioli, G.; Marra, G.; Spano,
G. J. Am. Chem. Soc. 2001, 123, 2204.
(10) Thangaraj, A.; Sivasanker, S. J. Chem. Soc., Chem. Commun. 1992,
123.
(11) Schuchardt, U.; Pastore, H. O.; Spinace, E. V. Stud. Surf. Sci. Catal.
1994, 84, 1877.
(12) Serrano, D. P.; Uguina, M. A.; Ovejero, G.; van Grieken, R.; Camacho,
M. Microporous Mater. 1996, 7, 309.
(13) Fan, W.; Li, R.; Ma, J.; Fan, B.; Dou, T.; Cao, J. Microporous Mater.
1997, 8, 131.
(14) Xu, R.; Tu, K.; Pang, W. Zeolite Molecular SieVes Structure and
Synthesis; Jilin University Publisher, 1987.
(15) Burkett, S. L.; Davis, M. E. J. Phys. Chem. 1994, 98, 4647.
(16) Burkett, S. L.; Davis, M. E. Chem. Mater. 1995, 7, 920.
(17) Dokter, W. H.; van Garderen, H. F.; Beelen, T. P. M.; van Santen,
R. A.; Bras, W. Angew. Chem., Int. Ed. Engl. 1995, 34, 73.
(18) Mintova, S.; Olson, N. H.; Valtchev, V.; Bein, T. Science 1999, 283,
958.
(28) Althoff, R.; Unger, K.; Schu¨th, F. Microporous Mater. 1994, 2, 557.
(29) Fan, W.; Schoonheydt, R. A.; Weckhuysen, B. M. Chem. Commun.
2000, 2249.
(19) Mintova, S.; Olson, N. H.; Bein, T. Angew. Chem., Int. Ed. 1999, 38,
3201.
(30) Weckhuysen, B. M.; Rao, R. R.; Martens, J. A.; Schoonheydt, R. A.
Eur. J. Inorg. Chem. 1999, 565.
(20) Davis, T. M.; Drews, T. O.; Ramanan, H.; He, C.; Dong, J.;
Schnablegger, H.; Katsoulakis, M. A.; Kokkoli, E.; Mccormick, A. V.;
Penn, R. L.; Tsapatsis, M. Nat. Mater. 2006, 5, 400.
(21) Ravishankar, R.; Kirschhock, C. E. A.; Knops-Gerritis, P. P.; Feijen,
E. J. P.; Grobet, P. J.; Vanoppen, P.; De Schryver, F. C.; Miehe, G.;
Fuess, H.; Schoeman, B. J.; Jacobs, P. A.; Martens, J. A. J. Phys.
Chem. B 1999, 103, 4960.
(31) Fan, W.; Fan, B.; Song, M.; Chen, T.; Li, R.; Dou, T.; Tatsumi, T.;
Weckhuysen, B. M. Microporous Mesoporous Mater. 2006, 94, 368.
(32) Hammonds, K. D.; Heine, V.; Dove, M. T. J. Phys. Chem. B 1998,
102, 1759.
(33) Fan, W.; Li, R.; Dou, T.; Tatsumi, T.; Weckhuysen, B. M. Microporous
Mesoporous Mater. 2005, 84, 116.
(22) Kirschhock, C. E. A.; Ravishankar, R.; Verspeurt, F.; Grobet, P. J.;
Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 4965.
(23) Kirschhock, C. E. A.; Ravishankar, R.; van Looveren, L.; Jacobs, P. A.;
Martens, J. A. J. Phys. Chem. B 1999, 103, 4972.
(34) Artioli, G.; Lamberti, C.; Marra, G. L. Acta Crystallogr., Sect. B 2000,
56, 2.
(35) Bordiga, S.; Bonino, F.; Damin, A.; Lamberti, C. Phys. Chem. Chem.
Phys. 2007, 9, 4854.
(24) Kirschhock, C. E. A.; Buschmann, V.; Kremer, S.; Ravishankar, R.;
Houssin, C. J. Y.; Mojet, B. L.; van Santen, R. A.; Grobet, P. J.; Jacobs,
P. A.; Martens, J. A. Angew. Chem., Int. Ed. 2001, 40, 2637.
(25) Kirschhock, C. E. A.; Kremer, S. P. B.; Grobet, P. J.; Jacobs, P. A.;
Martens, J. A. J. Phys. Chem. B 2002, 106, 4899.
(36) Corma, A.; Camblor, M. A.; Esteve, P.; Martı´nez, A.; Pe´rez-Pariente,
J. J. Catal. 1994, 145, 151.
(37) Wu, P.; Tatsumi, T. Chem. Commun. 2002, 1026.
(38) Le Noc, L.; Trong On, D.; Solomykina, S.; Echchahed, B.; Be´land,
F.; Cartier dit Moulin, C.; Bonneviot, L. Stud. Surf. Sci. Catal. 1996,
101, 611.
(26) De Moor, P. E. A.; Beelen, T. P.; van Santen, R. A. J. Phys. Chem.
B 1999, 103, 1639.
(39) Lamberti, C.; Bordiga, S.; Arduino, D.; Zecchina, A.; Geobaldo, F.;
Spano`, G.; Genoni, F.; Petrini, G.; Carati, A.; Villain, F.; Vlaic, G. J.
Phys. Chem. B 1992, 102, 6382.
(27) Kirschhock, C. E. A.; Ravishankar, R.; Jacobs, P. A.; Martens, J. A.
J. Phys. Chem. B 1999, 103, 11021.
9
J. AM. CHEM. SOC. VOL. 130, NO. 31, 2008 10151

A R T I C L E S
Fan et al.
MFI-type materials proceeds very quickly at high temperature when
TPAOH is used as a templating molecule. Therefore, herein the
samples were synthesized at the temperatures of 30, 60, 80, 100,
110, 120, 140, or 170 °C with accuracy of (2 °C in order to control
the process. When the designed time was reached, products were
withdrawn from the autoclave and quenched in an ice-water
mixture. The solid samples were centrifuged to separate them from
the mother liquid. Then, the solid samples were fully washed with
deionized water and dried at 100 °C. The dried sample was finally
calcined at 550 °C for 10 h and washed with 0.5 M aqueous solution
of HCl (at a liquid-to-solid ratio of 50 mL g-1). Part of the mother
liquid was diluted with deionized water to 100 times in volume
for elemental analyses. The crystallization experiments on the
method A system were carried out similarly.
2.2. Characterization of Samples. The X-ray diffraction pat-
terns (XRD) were recorded by a MAC Science M3X 1030 X-ray
diffractometer with Cu KR radiation (40 kV, 20 mA) to identify
the crystalline phase and estimate the crystallinity of TS-1. The
diffuse reflectance (DR) UV-vis spectra were measured on a Jasco
V-550 UV-vis spectrophotometer with a BaSO4 plate as the
reference. Framework infrared (IR) spectra were acquired on a
Perkin-Elmer Spectrum One Fourier transform infrared (FTIR)
spectrometer. Before recording the spectra, the samples were
evacuated at 500 °C for 2 h under high-vacuum conditions. Field
emission scanning electron microscopy (FE-SEM) images were
obtained on a Hitachi S-5200 scanning electron microscope. 29Si
MAS (magic-angle spinning) NMR measurements were performed
on a JEOL JNM-ECA 400 nuclear magnetic resonance spectrometer
at ambient temperature. The chemical shift was referred to an
external standard of poly(dimethylsilane). The spin rate of the rotor
was set at 5.0 KHz. A pulse length of 7 µs with a repetition time
of 30 s was applied, and 2000-3000 scans were accumulated. The
spectra were deconvoluted with Gaussian-Lorentzian mixed equa-
tion. X-ray photoelectron spectra (XPS) were attained on a
Shimadzu ESCA 3200 X-ray photoelectron spectrometer with an
Mg KR target. Thermogravimetry/differential thermal analysis (TG/
DTA) data were collected by a ULVAC-Riko TGD 9600 thermal
analysis system. The chemical compositions of the samples were
determined by a Shimadzu ICPS-8000E inductively coupled plasma
atomic emission spectrometer (ICP).
distorted. This results in the slow inclusion of Ti into the
framework, compared to Si ions. If crystallization proceeded
too fast, Ti ions would not have enough time to be incorporated
into the lattice. However, too slow crystallization would possibly
cause the formation of transition metal oxides, preventing metal
cations from being incorporated into the framework. In addition,
the difficult crystallization may also result from the strong
competition between the interaction of soluble silicate ions and
mother liquor and the condensation of silicate ions. Third, a
mismatch among hydrolysis of Ti and Si alkoxides, polymer-
ization of Ti4+ and/or Si4+ ions, nucleation, and crystal growth
would lead to much difficulty in the inclusion of Ti in the
framework. Since the chemical nature of Ti and the rigidity of
the framework of TS-1 cannot be altered, to find an effective
crystallization-mediating agent would be the sole way to increase
the lattice Ti content in TS-1 by harmonizing the hydrolysis
rate of Ti alkoxide with that of silicate species as well as the
nucleation and crystal growth rates. In this respect, we have
found the potential of (NH4)2CO3; the presence of (NH4)2CO3
in the crystallization gel not only considerably increased the
framework Ti content without forming extraframework Ti
species but also improved the hydrophobicity of the prepared
TS-1 catalyst, consequently showing significantly improved
activity for the oxidation of a variety of organic substrates.
2. Experimental Section
2.1. Synthesis of Samples. TS-1 samples were synthesized
basically according to the procedures reported by the Enichem
group44 (method A) except that (NH4)2CO3 was added after
removing alcohol. Namely, a certain amount of titanium tetra-n-
butoxide (TBOT) was first dissolved in an aqueous solution of
tetrapropylammonium hydroxide (TPAOH), followed by addition
of tetraethyl orthosilicate (TEOS) and stirring for 0.5 h. A H2O2
aqueous solution (31%) with the mass amount being 3 times as
much as that of TBOT was added before the addition of TPAOH.
Then, the solution was heated in a water bath kept at 55-60 °C
for about 4 h. Occasionally, a small amount of water was added
during the heating process to compensate for the vaporized portion.
After totally evaporating the alcohol, the resulting clear solution
was cooled down, and ammonium carbonate was added. The
composition of the final gel was maintained in the range of
0.3(NH4)2CO3/SiO2/(0-0.2)TiO2/0.5TPAOH/35H2O. The crystal-
lization was carried out at 170 °C for 3-6 days. The solid product
was filtered, washed with distilled water, and dried at 100 °C
overnight. After drying, the sample was washed with a 1 mol L-1
2.3. Catalytic Measurements. The liquid-phase oxidation of a
variety of organic substrates with H2O2 (31% in aqueous solution)
was carried out under stirring conditions in a round-bottom flask
(20 mL) equipped with a condenser. The temperature was controlled
with a water bath. The typical reaction conditions for the oxidation
of different organics are given in the footnote of the table and
captions of figures. For the oxidation of 1-hexene, 2-hexanol,
styrene, and linear n-alkanes, the products were analyzed by a
Shimadzu GC-14A gas chromatograph equipped with a 50 m OV-1
capillary column and a flame ionization detector. The amount of
the unconverted H2O2 was determined with a 0.1 M aqueous
solution of Ce(SO4)2 by using a titration method. In the case of
benzene oxidation, the product was analyzed by a Shimadzu LC-
10AD high-performance liquid chromatograph (HPLC), and the
H2O2 content was measured by the iodine titration method.
aqueous solution of HCl (at a liquid-to-solid ratio of 50 mL g-1
)
for 24 h, followed by calcining at 550 °C for 10 h. The calcined
sample was further treated with HCl under the same conditions as
those set before calcination. Here, this synthesis system is denoted
by the YNU system in contrast to the method A system.
The crystallization mechanism was studied by following the
crystallization process with several autoclaves (150 mL) filled with
the gel prepared from 20.1 g of TEOS, 0.65 g of TBOT (monomer,
97%), 41.8 g of TPAOH (20-25% in aqueous solution), 2.13 g of
(NH4)2CO3, and 25.9 g of distilled water according to the procedures
described in the above paragraph. Usually, the crystallization of
3. Results and Discussion
3.1. Preparation and Characterization of TS-1 Catalysts. The
as-synthesized sample showed an XRD pattern typical of the
MFI structure, confirming its high purity and crystallinity. Table
1 gives the sample notation of m-n with m and n representing
the synthesis method and the Si/Ti ratio in the synthesis gel,
respectively. It was found that the coordination states of Ti
species in the samples were markedly different when different
synthesis methods were used; the DR UV-vis spectra showed
that there was a considerable amount of octahedral Ti species
in the as-synthesized (curve a) and calcined (curve b) A (the
(40) Blasco, T.; Camblor, M. A.; Corma, A.; Pr´ez-Pariente, J. J. Am. Chem.
Soc. 1993, 115, 11806.
(41) Bordiga, S.; Coluccia, S.; Lamberti, C.; Marchese, L.; Zecchina, A.;
Boscherini, F.; Buffa, F.; Genoni, F.; Leofanti, G.; Petrini, G.; Vlaic,
G. J. Phys. Chem. 1994, 98, 4125.
(42) Gleeson, D.; Sankar, G.; Catlow, C. R. A.; Thomas, J. M.; Spano´, G.;
Bordiga, S.; Zecchina, A.; Lamberti, C. Phys. Chem. Chem. Phys.
2000, 2, 4812.
(43) Thomas, J. M.; Sankar, G. Acc. Chem. Res. 2001, 34, 571.
(44) Clerici, M. G.; Bellussi, G.; Romano, U. J. Catal. 1991, 129, 159.
9
10152 J. AM. CHEM. SOC. VOL. 130, NO. 31, 2008

Synthesis and Properties of Ti-Rich TS-1
A R T I C L E S
Table 1. Catalytic Results of the Prepared TS-1 for the Oxidation
of n-Hexanea
selectivity (%)
H2O2
Si/Ti
ratio in (%) based on based
conversion
TON
2-
and
2-
and conversion selectivity
sample
catalyst
n-hexane
on H2O2 3-ol 3-one
(%)
(%)
A-80
88.5
58.3
67.6
58.3
97.5
70.3
67.4
60.0
18.4
30.6
25.5
27.0
14.7
22.5
29.2
31.3
23.6
36.4
41.0
46.4
50.8
46.5
38.3
119
140
136
125
108
127
161
166
151
172
161
167
160
173
168
79.2 20.8
70.9 29.1
72.1 27.9
70.8 29.2
80.6 19.4
72.9 27.1
65.8 34.2
62.9 37.1
72.8 27.2
65.4 34.6
58.2 41.8
52.9 47.1
52.5 47.6
53.1 46.9
53.7 46.3
13.2
23.3
19.6
20.2
12.1
19.4
24.3
27.9
17.7
28.0
37.1
41.9
42.2
41.8
34.7
83.3
84.2
83.0
85.8
74.4
76.5
80.4
81.0
85.5
86.0
81.4
81.2
89.1
82.2
82.0
A-50
A-33
A-20
B-80
B-33
B-20
B-10
YNU-80 82.3
YNU-50 57.2
YNU-33 43.0
YNU-25 39.4
YNU-20 34.2
YNU-15 40.6
Figure 2. (A) Relationship between the unit-cell volume and Ti content
of the TS-1 catalysts prepared from the YNU system (the unit-cell volume
was calculated with the Rietveld method by using DBWS software (ref
47)).
YNU-5
47.2
a Reaction conditions: 0.1 g of catalyst, 60 °C, 4 h, 10 mL of CH3OH
as solvent, 10 mmol of n-hexane, 20 mmol of H2O2 (31% in aqueous
solution).
calcined samples, a small amount of octahedral Ti species and
Ti oxides were still present in the A samples (curve c).
Nevertheless, if the A samples were also treated with acid before
calcination, no extraframework Ti species could be detected with
the DR UV-vis spectroscopy (curve d). In order to ensure that
TS-1 catalysts prepared in the presence of (NH4)2CO3 are free
of extraframework Ti species, the same acid-treating processes
as for the A samples were also applied to all the YNU samples.
Table 1 shows that the Si/Ti ratio in the prepared A catalysts
reached a minimum of 58.3. This corresponds to 1.62 Ti atoms
per unit cell. This limitation was further substantiated by
progressively increasing the Ti amount in the synthesis gel up
to a Si/Ti ratio of 5 (not shown here for brevity). The samples
synthesized by method B gave similar results. In contrast, if
(NH4)2CO3 ((NH4)2CO3/SiO2 molar ratio of 0.3) was introduced
into the synthesis gel, the Ti content in the catalyst increased
with increasing Ti amount in the gel when the Si/Ti ratio in the
gel was higher than 20. Attempts to further increase the Ti
content in TS-1 by adding more Ti in the synthesis gel as well
as by adjusting the amounts of TPAOH and (NH4)2CO3 and
changing the silica source have been unsuccessful. A decrease
in the TPAOH content in the synthesis gel to a TPAOH/Si ratio
of 0.42 and 0.35 led to the formation of a TS-1 catalyst with a
Si/Ti ratio of 39.1 and 39.6, respectively, when a synthesis gel
having a Si/Ti ratio of 20 was used to synthesize TS-1 samples.
The use of tetramethyl orthosilicate (TMOS), tetrapropyl
orthosilicate (TPOS), fumed silica, and colloidal silica sol,
instead of TEOS, as silica sources resulted in the formation of
TS-1 catalysts having a Si/Ti ratio in the range of 37-43 under
the same synthesis conditions as for the YNU-20 sample. An
increase in the (NH4)2CO3 content to a (NH4)2CO3/Si ratio of
0.5 generated a significant amount of octahedral Ti species,
which could not be completely washed away by acid treatment,
as confirmed by an intense band at 260 nm in its DR UV-vis
spectrum. This also held true for the change of crystallization
temperature to 150 or 190 °C.
Figure 1. DR UV-vis spectra of (A) TS-1 samples prepared with a
synthesis gel having a Si/Ti ratio of 50 by using method A (thin lines (a-d))
and the YNU method (thick lines (e-h)). (a and e) as-synthesized, (b and
f) calcined, (c and g) calcined and acid-treated, (d and h) acid-treated,
calcined, and further acid-washed. (B) DR UV-vis spectra of TS-1 catalysts
prepared by the YNU method with a synthesis gel having a Si/Ti ratio of
(a) 80, (b) 50, (c) 33, (d) 20, (e) 15, (f) 6.67, and (g) 5.
Figure 2 shows that the unit-cell volume of the TS-1 catalysts
prepared from the YNU system linearly increased with increas-
ing Ti content (V ) 4400.8x + 5176.9, R ) 0.9836), providing
evidence that more Ti cations in the samples have been
incorporated into the framework although a systematic decrease
in the unit-cell volume was observed compared to the reported
values, possibly due to the measurement of the samples under
different conditions, the different lattice sites for the inclusion
method A employed) samples synthesized from a gel with a
Si/Ti ratio of 50, as evidenced by the presence of an intense
band around 260 nm in both curves (Figure 1A). In contrast,
extraframework octahedral Ti species were negligibly formed
in the YNU samples (curves e and f). After acid washing of
9
J. AM. CHEM. SOC. VOL. 130, NO. 31, 2008 10153

A R T I C L E S
Fan et al.
Figure 4. Crystallization curves of TS-1 and silicalite-1 synthesized in
the method A and YNU systems. (A) 30 °C, 1 day; (B) 60 °C, 1 day; (C)
80 °C, 1 day; (D) 100 °C, 1 day; (E) 140 °C, 1 day; (F) 170 °C, 1 day; (G)
170 °C, 3 days.
Figure 3. Intensity ratio of the peak at 957 cm-1 to that around 453 cm-1
as a function of the Ti/(Si + Ti) molar ratio in the TS-1 catalysts prepared
by the YNU method.
incorporated into the framework. This indicates that incorpora-
tion of about 2.73 Ti atoms per unit cell could be achieved with
the YNU method for sample YNU-20 (Si/Ti ) 34.2). This value
is much higher than that achieved by methods A or B. Although
some papers reported that the Ti content in the TS-1 could be
increased up to a Si/Ti ratio of 40,46,51 or even of 33,9,52
substantial difficulty was encountered in the synthesis of well-
manufactured samples.35,46 It requires extremely pure reagents
and severe control of the synthesis conditions,35,46 resulting in
a poor reproducibility. This is unlike the YNU method which
can use various types of silica sources such as fumed silica and
colloidal silica sol without the necessity to strictly control the
synthesis conditions, being a convenient and reliable method
for synthesis of Ti-rich TS-1 free of extraframework Ti species.
3.2. Crystallization Mechanism of TS-1 in the YNU System.
3.2.1. Crystallization Curves of TS-1 in the YNU and the
Method A Systems. As mentioned above, in order to study the
crystallization mechanism of zeolites, both the solid and liquid
fractions in the system should be considered. Figure 4 shows
the crystallinity of the TS-1 and silicalite-1 samples synthesized
under different conditions in the YNU and method A systems
(the Si/Ti ratio in the synthesis gel was 50 for both series of
samples). It is clear that the presence of Ti in the gel not only
severely retarded the crystallization process by prolonging the
induction period but also reduced the crystallinity of most of
the samples. Nevertheless, higher crystallinity of TS-1 was
finally achieved in the YNU system than in the method A
system.
of Ti, the increase in the particle size, and the decrease in the
defect sites with increasing framework Ti content (to be
discussed in detail below), and/or the use of a different
refinement program.45–47 The inclusion of more Ti in the lattice
sites with increasing Ti content in the samples is further
confirmed by IR spectroscopy. The 960 cm-1 band, probably
attributed to a stretching vibration mode of [SiO4] perturbed
by an adjacent framework Ti species or of [TiO4],48–50 has been
widely accepted as a proof of the isomorphous substitution of
Ti in the zeolitic lattice. In order to avoid the influence of water
and adsorbed molecules on the frequency and intensity of this
band,48 the framework IR spectra of all samples were measured
after evacuation at 500 °C under high-vacuum conditions. Figure
3 shows that the absorbance band at about 960 cm-1 could not
be observed in the IR spectrum of silicalite-1, whereas the
intensity of this band in the IR spectra of TS-1 catalysts
proportionally increased with increasing Ti amount in the
samples (y ) 19.891x, R ) 0.9966). The R obtained from the
function of (I957/I453) versus (Ti/(Ti + Si)) was larger than that
obtained from the function of unit-cell volume versus (Ti/(Ti
+ Si)), which reveals the considerable effects of the crystal size
and the conditions for treating samples on the obtained unit-
cell volume. Here, it is worth pointing out that the intensity of
the 957 cm-1 band was normalized on the basis of the 453 cm-1
band, which is assigned to structure-insensitive T-O bending
mode of tetrahedral TO4 units.19
DR UV-vis spectroscopy shows only the band at 210 nm,
but no band above 250 nm was observed for all the YNU
samples synthesized with the gels having different Ti contents
(Figure 1B). Although this cannot exclude the possibility of the
presence of extraframework Ti species, it definitely verifies that
most of the Ti species present in the samples have been
In comparison to the sample crystallized in the method A
system, TS-1 synthesized in the YNU system was more stable
against hydrolysis under the crystallization conditions after the
crystallization was completed. This may be due to the incor-
poration of more Ti cations into the framework associated with
a reduction in defect sites, resulting in higher hydrophobicity.
Lamberti and co-workers found that the intensity of the OH
band due to internal defective Si-OH groups progressively
decreased with increasing Ti amount incorporated in the
framework.9,35 This shows that the insertion of Ti into the lattice
of TS-1 has a mineralizing effect, reducing the framework
(45) Millini, R.; Massara, E. P.; Perego, G.; Bellussi, G. J. Catal. 1992,
137, 497.
(46) Lamberti, C.; Bordiga, S.; Zecchina, A.; Carati, A.; Fitch, A. N.; Artioli,
G.; Petrini, G.; Salvalaggio, M.; Marra, G. L. J. Catal. 1999, 183,
222.
(47) Biecher, L.; Sasaki, J. M.; Santos, C. O. P. J. Appl. Crystallogr. 2000,
33, 1189.
(48) Ricchiardi, G.; Damin, A.; Bordiga, S.; Lamberti, C.; Spano`, G.;
Rivetti, F.; Zecchina, A. J. Am. Chem. Soc. 2001, 123, 11409.
(49) Boccuti, M. R.; Rao, K. M.; Zecchina, A.; Leofanti, G.; Petrini, G.
Stud. Surf. Sci. Catal. 1989, 48, 133.
(51) Henry, P. F.; Weller, M. T.; Wilson, C. C. J. Phys. Chem. B 2001,
105, 7452.
(52) Marra, G. L.; Artioli, G.; Fitch, A. N.; Milanesio, M.; Lamberti, C.
Microporous Mesoporous Mater. 2000, 40, 85.
(50) Smirnov, K. S.; van de Graaf, B. Microporous Mater. 1996, 7, 133.
9
10154 J. AM. CHEM. SOC. VOL. 130, NO. 31, 2008

Synthesis and Properties of Ti-Rich TS-1
A R T I C L E S
Figure 5. Dependence of the Si/Ti molar ratio of the solid fraction on the
crystallization conditions in the YNU and method A systems. (A) 30 °C, 1
day; (B) 60 °C, 1 day; (C) 80 °C, 1 day; (D) 100 °C, 1 day; (E) 140 °C, 1
day; (F) 170 °C, 1 day; (G) 170 °C, 2 days; (H) 170 °C, 3 days.
Figure 6. Evolution of the Ti/Si molar ratio in the liquid fraction during
the whole crystallization process in the YNU and method A systems. (A)
30 °C, 1 day; (B) 60 °C, 1 day; (C) 80 °C, 1 day; (D) 100 °C, 1 day; (E)
140 °C, 1 day; (F) 170 °C, 1 day; (G) 170 °C, 3 days.
defects and, thus, increasing the hydrophobicity. This is also
evidenced by the 1H and 29Si MAS NMR spectroscopic findings
that silanol protons of H-bonded siloxy oxygens at defect sites
proportionally decreased with the Ti atoms per unit cell.53 In
addition, the neutron diffraction study has revealed that among
the 12 crystallographically independent T sites of the orthor-
hombic MFI framework, T6, T7, T10, and T11 are the most
possible sites for Ti to isomorphously substitute for Si, whereas
the other sites could have low probability for Ti occupancy.9,35
This is in good agreement with the Si vacancies in silicalite-
1,33 supporting the fact that the incorporation of more Ti atoms
into the framework is accompanied by the parallel decrease in
the defect sites.
An increase in the hydrophobicity by inserting Ti into the
framework is further substantiated by the crystallization curves;
the crystallinity of silicalite-1 decreased with increasing crystal-
lization time after reaching the maximum. The presence of defect
sites would make the material hydrophilic, causing an easy
attack by once-adsorbed H2O and, thus, a collapse of the
framework (eq 1).
process. The Si/Ti ratio in the solid samples synthesized in the
YNU system was kept almost constant, being in the range of
48 and 50.5. This is similar to that found in the synthesis of
TS-1 from amorphous wetness impregnated SiO2-TiO2 xerogels
and the nonaqueous synthesis of zeolites with solid reaction
mixtures where a solid-phase transformation mechanism
predominated.12,13 In addition, the solid yields recovered after
calcination of all the YNU samples were always higher than
95% based on the added SiO2 and TiO2. This is also consistent
with the occurrence of a solid-phase transformation mechanism.
This would be favorable for the incorporation of Ti cations into
the framework since dissociation, coalescence, and reorganiza-
tion are the main processes during nucleation and crystal growth.
In contrast, the Si/Ti ratio in the samples synthesized by method
A drastically increased during the period of crystal growth; here
the solid samples in the induction period were directly obtained
by drying the liquid at 100 °C since no solid was formed during
this period. This suggests that during the period of rapid crystal
growth in the method A system, silicic and/or silicate species
(t Si-OH, t Si-O-) were polymerized with each other at a
much higher rate than the condensation of silicic or silicate
species and titanic or titanate (t Ti-OH, t Ti-O-) species to
form TS-1 crystals. The Ti content in the solid remarkably
increased after the sample reached the highest crystallinity,
indicating that most of Ti cations were incorporated into the
framework after the crystallization was nearly completed. It was
found that nuclei were formed from a clear solution in the
method A system; the typical XRD pattern of the MFI structure
was detected when the synthesis was carried out at 80 °C for 1
day, verifying that the nuclei have been formed and that the
induction period has been completed, but at this stage the gel
remained transparent. This shows that the crystallization in the
method A system occurred via a homogeneous nucleation
mechanism although precursor aggregates might be formed
before nucleation, as reported by some researchers.17–27 It can
be also seen from Figure 5 that in this stage the Ti content in
the as-synthesized YNU products was higher than in the A
products, suggesting the incorporation of more Ti cations into
the framework.
t Si - O - Si t +H2O f 2 t Si - OH
(1)
Figure 4 shows that the crystallization rate of TS-1 was much
slower in the YNU system than that in the method A system.
This is contrary to the result obtained by Kumar et al., who
3-
reported that the presence of oxyanions such as ClO4-, PO4
,
2-
AsO43-, and CO3
ions in the synthesis system could
significantly accelerate the crystallization of zeolites by speeding
up the condensation process probably as a result of their great
polarizing ability for the hydrophobic hydration balls, facilitating
the nucleation of zeolites from the liquor.54 This inconsistency
may be due to the different crystallization mechanisms working
in the Kumar’s and the YNU synthesis systems. The crystal-
lization in the former system occurred via a solution-mediated
mechanism, whereas a solid-phase transformation mechanism
seems to predominate in the latter synthesis system, which will
be demonstrated in the following sections.
3.2.2. Evolution of the Chemical Compositions of the Solid
Fraction and the Mother Liquid. Figure 5 shows the Si/Ti ratio
of the solid samples collected in the whole crystallization
The evolution of the Ti/Si ratio in the mother liquid during
the whole crystallization process (Figure 6) is in agreement with
that of the solid samples. In the method A system, the Ti/Si
ratio of the liquid phase sharply increased from the point after
(53) Parker, W. O.; Millini, R. J. Am. Chem. Soc. 2006, 128, 1450.
(54) Kumar, R.; Bhaumik, A.; Ahedi, R. K.; Ganapathy, S. Nature 1996,
381, 298.
9
J. AM. CHEM. SOC. VOL. 130, NO. 31, 2008 10155

A R T I C L E S
Fan et al.
the nuclei formed and reached the highest value when crystal-
lization was almost completed. This shows that the majority of
Ti species were still present in the liquid during the crystal
growth period, while a large amount of silicic and silicate species
and/or silicate polyanions have condensed with tiny amorphous
precursor aggregates to form more nuclei or result in the growth
of crystals. It seems that after the zeolitic architecture was almost
built Ti cations started to be inserted into the framework. This
suggests that Ti cations did not randomly substitute for Si but
selectively occupied specific positions or defect sites, in
accordance with the assumption made by other researchers and
supported by the several types of spectroscopic and crystal-
lographic data on both TS-1 and defective silicalite-1.9,34,35,38,39,55
Here, it should be mentioned that although partially calcined
nano-TS-1 (nanoscopic building units of TS-1) devoid of
physisorbed TPAOH exhibited catalytic activity and selectivity
in the oxidation of some organics,56 it gave a conversion of
Figure 7. Evolution of the pH value of the liquid fraction during the whole
crystallization process in the YNU and method A systems. (A) After addition
of (NH4)2CO3 for the YNU system; after removal of ethanol for the method
A system. (B) 30 °C, 1 day; (C) 60 °C, 1 day; (D) 80 °C, 1 day; (E) 100
°C, 1 day; (F) 140 °C, 1 day; (G) 170 °C, 1 day; (H) 170 °C, 3 days.
1
about /12 as low as that obtained over the conventional TS-1
in the competitive epoxidation of 1-hexene and cyclohexene
(Table 2 in ref 56). However, if all the Ti cations were truly
incorporated in the framework, calcined nano-TS-1 would show
a higher activity than the conventional TS-1, particularly for
the epoxidation of cyclohexene, due to the significant decrease
in the steric constraint on the diffusion of substrates/products.
The observed contrary results suggest that most of Ti cations
were not incorporated in the framework, as confirmed by the
absence of an IR band at 960 cm-1 after calcination of the
sample at 550 °C.56 The presence of a small number of
tetrahedral Ti species in the nano-TS-1 would be possible, since
the MFI nanostructure (double five-membered ring) has been
formed. Therefore, the results obtained on the nano-TS-1 are
not contradictory to our finding that most of Ti cations were
inserted into the lattice after the crystallization was nearly
completed.
In contrast, in the YNU system, the Ti/Si ratio in the liquid
was almost constant (about 0.02) during the induction period,
being the same as that in the solid fraction. After nuclei formed,
a small amount of titanic and/or titanate species (more than 95%
of silicon and titanium was kept in the solid fraction through
the crystallization process) in the mother liquid quickly trans-
ferred into the solid, and the transfer rate was consistent with
the crystal growth speed, as revealed by the ICP analyses (Figure
6) in combination with XRD results (Figure 4); the Ti content
in the mother liquid decreased to 0 when the crystallization
conditions were changed from 100 °C for 1 day (the formation
of the MFI structure was just detected by XRD) to 140 °C for
1 day (the crystallization was nearly completed). This shows
that the crystallization mechanism in the YNU system is
completely different from that working in the method A system.
Nevertheless, the decrease in the Ti/Si ratio in the liquid during
the crystal growth period is indicative of the occurrence of a
liquid-to-solid mass transfer of Ti as well as the solid-phase
transformation which probably dominated the crystallization
process. The complete depletion of Ti species in the liquid in
the YNU system is in contrast to part of Ti species left in the
liquid in the method A system, further evidencing that more Ti
cations can be incorporated in the framework by the YNU
method than by method A.
3.2.3. Changes in the pH Value of the Mother Liquor
during the Whole Crystallization Process. The relationship
between the pH value of the mother liquor and the crystallization
conditions is illustrated in Figure 7. After the addition of
(NH4)2CO3, the pH value of the liquid sample swiftly lowered
from 11.9 to 10.4 and was thereafter unchanged during the
induction period. In contrast, the pH value in the method A
system remained 11.9 in the induction period. The hydroxide
anion is an excellent mineralizer. As a ligand for Si4+ and Ti4+
ions, it allows the condensation of moderately stable silicic/
silicate and titanic/titanate species to occur, resulting in the
formation of zeolite species. The hydroxide ion concentration
not only can alter the crystallization time for a particular
structure but also could induce a change in the composition and
sometimes the structure of the crystallized samples.57
In the synthesis of TS-1 in the method A system, the
hydroxide ion of TPAOH plays a major role in accelerating
the hydrolysis of the silica source and Ti alkoxide and the
oligomerization of Si-OH and Ti-OH species (eq 2). These
processes would lower down the pH value. Therefore, a high
pH value could promote these processes and thus decrease the
time for reaching the critical concentration of soluble silica and
titanium species from which zeolites are ultimately precipitated.
However, a very high alkaline condition would make the
synthesis gel unreacted but dissolve the silicate/titanosilicate
species (eq 3, from right to left), resulting in a lower crystallinity
of the products crystallized in the method A system and the
presence of more defect sites (Figure 4). In contrast, for the
YNU system, the synthesis gel was quickly solidified after
the addition of (NH4)2CO3. Thus, most of TPA+ species were
embedded in the solid with the amount of free OH- species
drastically decreasing and with the pH value of the liquid
lowering down. The lower the pH value, the higher the yield
of crystalline material57 or the more well-crystallized the product
because the condensation reaction (eq 3, from left to right)
proceeds to a greater extent. This accounts for the higher
crystallinity of the TS-1 samples achieved by the YNU method
than by method A.
(55) Marra, G. L.; Tozzalo, G.; Leofanti, G.; Padovan, M.; Petrini, G.;
Genoni, F.; Venturelli, B.; Zecchina, A.; Bordiga, S.; Ricchiardi, G.
Stud. Surf. Sci. Catal. 1994, 84, 559.
(56) Ravishankar, R.; Kirschhock, C.; Schoeman, B. J.; De Vos, D.; Grobet,
P. J.; Jacobs, P. A.; Martens, J. A. In Proceedings of the 12th
International Zeolite Conference; Treacy, M. M. J., Marcus, B. K.,
Bisher, M. E., Higgins, J. B., Eds.; Materials Research Society, 1999;
p 1825.
(57) Szostak, R. Molecular SieVes, 2nd ed.; Blackie Academic & Profes-
sional, an imprint of Thomson Science, 1998; p 100.
9
10156 J. AM. CHEM. SOC. VOL. 130, NO. 31, 2008

Synthesis and Properties of Ti-Rich TS-1
A R T I C L E S
x(TPA+OH-) + T(OR)4+(4 - x)H2O h
((TPA+)x(-O)x-T)(OR)4-x) + xROH + (4 - x)H2O h
(TPA+)x(-O)x-T)(OH)4-x) + (4 - x)ROH
very small and somewhat irregular in shape, rendering much
difficulty in differentiating them from amorphous materials.
Figure 8E suggests that nuclei might have evolved from the
inner part of the particles, which is in agreement with the
solid-phase transformation mechanism working in the YNU
system.13,14 It was recently proposed that high supersaturation
within the gel particles possibly coupled with preorganization
at the interface of the amorphous network with occluded
solution could be the driving force for nucleation of zeolites,
e.g., of zeolite A;18 however, this is based on the assumption
that the gel particle size is unchanged through the crystal-
lization process. Obviously, this is not in line with the FE-
SEM observations of the TS-1 samples synthesized in the
YNU system. Thus, the occurrence of a solid-phase trans-
formation mechanism would be more favorable in the YNU
system. After nuclei formed, irregular crystals of sizes less
than 200 nm progressively grew into spherical and further
boat-shaped particles accompanied by depletion of the
amorphous gel and restructuring (Figure 8F-I).
The formation of the secondary gel was also proved by TG
measurement. The YNU samples collected in the induction
period had four weight-loss regions (<100, 140-267 (or
110-263), 267-400, and 400-530 °C) (not shown here). The
weight loss below 100 °C is due to the desorption of physically
adsorbed water, whereas the weight losses in the last three
regions are correlated with gelatinated water and templating
molecules. The YNU sample synthesized at 60 °C for 1 day
showed an increased weight loss between 110 and 263 °C,
compared with the sample synthesized at 30 °C for 1 day. This
is perhaps a result of the increase in hydrophilicity. This could
be attributed to the dissociation and rearrangement/restructuring
of the initial solid gel to form a secondary gel.13,17
It has been confirmed by 1H-29Si CP MAS NMR spectros-
copy that in the synthesis of silicalite-1 by the same method as
method A except that Ti was absent, short-range intermolecular
interactions exist within van der Waals contact distance prior
to the formation of nuclei, resulting in the formation of a
preorganized inorganic-organic composite.15,16 Kirschhock and
co-workers also suggested that the formation of silicate poly-
anions with a curved hydrophobic SiO2 surface by means of
TPA+ cations initiates the nucleation of silicalite-1.22,23 The
presence of a large quantity of organic template in the gel
aggregates, where nuclei are formed, has been also found in
the synthesis of zeolites A and Y.18,19 Since the YNU samples
were fully washed with distilled water before drying, and the
dried gel was heavier than the materials added as sources of
SiO2, TiO2, and TPAOH, it is believable that most of the added
TPA molecules were primarily embedded in the solid after the
addition of (NH4)2CO3. This is also substantiated by the strong
and broad exothermal peaks between 200 and 500 °C in the
corresponding DTA curves (Supporting Information Figure S1,
parts A-C). The formation of inorganic-organic composites
in the YNU system provides the possibility for the nucleation
in the solid fraction.
The three weight-loss regions between 140 and 530 °C
observed for the YNU samples collected during the nucleation
period indicate different local environments of templating
molecules, as a consequence, showing broad exothermal peaks
in the DTA curves. In the synthesis of MFI-type molecular
sieves in the presence of TPAOH, it has been suggested that
hydrophobic hydration balls are formed by H2O molecules
encircling TPA+ cations with H2O partially or completely
substituted by silicate or aluminosilicate species.15,16 This is
(T ) Si, Ti)
(2)
t Si - O- + t TOH h t Si - O - T t +OH-
(T ) Si, Ti) (3)
As expected, an increase in the pH value during the crystal
growth period was observed for both synthesis systems, but the
degree was much more appreciable for the method A system
than for the YNU system. The pH value increased from 11.9 to
12.8 in the method A system, whereas it increased only slightly
from 10.4 to 10.7 in the YNU system (Figure 7). The increase
in the pH value is attributed to the incorporation of silicate
species into the framework of TS-1, which releases free OH-
(eq 3). This increase continued till the completion of crystal
growth. The higher OH- concentration in the method A system
also accelerates the crystal growth. Such a high crystallization
rate is not beneficial for the incorporation of Ti into the
framework since this process would make the local structure
around Ti distorted. In addition, the high alkalinity of the liquid
is unfavorable to the condensation of Ti-OH and silicate
species. In contrast, the presence of (NH4)2CO3 appropriately
slows down the crystallization rate by significantly decreasing
the pH value, buffering the synthesis gel, and introducing NH4
+
,
which is a structure-breaking cation in water,58 and consequently
reducing the polymerization rate of silicic/silicate/titanic/titanate
species. This would provide enough time for Ti species to be
inserted into the lattice during the crystallization process, as
indicated by the much lower Si/Ti ratio in the solid samples
obtained during the crystal growth period in the YNU system
than in the method A system. An optimum OH- concentration
could not only promote the crystallization of TS-1 and favor
the incorporation of Ti into the framework but also inhibit the
subsequent dissolution of the framework and/or the transforma-
tion to a more stable structure. It is not clear at this moment if
there exists an interaction between TPA+ cations and CO3
2-
anions, but this could not be excluded since the gel quickly
solidified and the pH value significantly lowered down after
addition of (NH4)2CO3. Work on the effect of the counteranions
of ammonium ion on the synthesis of TS-1 is in progress and
will be reported in the near future.
3.2.4. Structure Formation Process and Ti Coordination
States. Figure 8 shows the crystallite sizes and morphology
of the samples synthesized in the YNU system under different
crystallization conditions. It is clear that the initially formed
particles had a flat and smooth surface with a size of about
5 µm (Figure 8A). With crystallization going on, the bulky
gel particles gradually dissociated into aggregates of small
pieces. This is the so-called secondary gel since no diffraction
peaks characteristic of the MFI structure could be detected
with XRD. This is in line with the findings, obtained by
Dokter et al. through time-resolved small- and wide-angle
X-ray scattering techniques, that an intermediate gel phase
is required to start nucleation and crystallization.17 When the
sample was crystallized at 100 °C for 1 day, XRD measure-
ment demonstrated the formation of nuclei although the
particle morphology was basically unchanged; the XRD
pattern typical of the MFI structure could be just discerned
under these conditions (Figure 4D). The nuclei of TS-1 were
(58) Gabelica, Z.; Blom, N.; Derouane, E. G. Appl. Catal. 1983, 5, 227.
9
J. AM. CHEM. SOC. VOL. 130, NO. 31, 2008 10157

A R T I C L E S
Fan et al.
Figure 8. FE-SEM images of the solid samples collected under different crystallization conditions in the YNU system: (A) 30 °C, 1 day; (B) 60 °C, 1 day;
(C) 80 °C, 1 day; (D) 100 °C, 1 day; (E) 110 °C, 1 day; (F) 120 °C, 1 day; (G) 140 °C, 1 day; (H) 170 °C, 1 day; (I) 170 °C, 3 days.
certainly applicable to the synthesis of TS-1 in the method A
system since there is no essential difference with the exception
that Ti species are present. Addition of (NH4)2CO3 to the
synthesis mixture led to an immediate solidification of the gel
and, thus, the probable formation of hydrophobic hydration balls
with different sizes and/or condensation degrees around TPA+
by gelatinating the Si(OH)4 and Ti(OH)4 species with H2O
molecules followed by replacement of H2O molecules with
silicate and titanosilicate species. In addition, unlike in the
method A system, in the YNU system the physical embedding
of some TPAOH molecules could also be possible during the
gelatination process. With the progress of crystallization, the
inorganic-organic composite gel was restructured into zeolite
nuclei, followed by the crystal growth at the expense of
amorphous nutrients.
We concluded from the TG results that the unit-cell
composition of the YNU sample crystallized at 170 °C for 3
days was 3.9TPA+/96TO2/2.9H2O. This is consistent with
that of the ideal MFI-type structure where 4 TPA+ cations
were contained per 96 SiO2 units. This verified that, in the
as-synthesized TS-1 products, there was only one type of
TPA+ cation tightly enclathrated in the channel intersections.
This is supported by the DTA measurement that showed only
a very sharp exothermal peak at 385 °C with a shoulder
around 430 °C for the well-crystallized product (Supporting
Information Figure S1, part H).
As for the MFI-type molecular sieves, in addition to a
structural band at 440-470 cm-1 (T-O bending mode), a
typical band near 550 cm-1 should be observed in their
9
10158 J. AM. CHEM. SOC. VOL. 130, NO. 31, 2008

Synthesis and Properties of Ti-Rich TS-1
A R T I C L E S
Nevertheless, both the A and YNU samples displayed a split of
the band around 550 cm-1 into 549 and 559, and 546 and 559
cm-1, respectively. This split was slightly less obvious for the A
samples than for the YNU samples (Supporting Information Figure
S2). This is in agreement with the findings for silicalite-1 nanophase
material,61 whereas there is a small difference in band positions.
Although the basic reason is not clear, this is closely related to the
sizes of particles containing five-membered rings, as reported by
Kirschhock, Serrano, Hsu, and co-workers.22,62–64 The precursor
(2-3 nm) and nanoslabs (1.3 × 4 × 4 nm3) of silicalite-1 exhibit
the IR adsorption bands at 590-570 cm-1 22,62,64
whereas colloidal
,
particles with a size in the range of 18-100 nm, formed by
extending the crystallization time, show a doublet at 555 and 570
cm-1 61
.
In contrast, micrometer-sized crystals display a band
around 550 cm-1 15,16,22,59,60,62
.
This frequency shift also holds true
for the TS-1 and the agglomeration of its nanoparticles by
calcination,56 suggesting that both the A and YNU samples might
be composed of similar primary particles since both materials have
the same IR adsorption band at 559 cm-1. These primary particles
are further aggregated into zeolite crystals, resulting in the shift
to 546 and 549 cm-1 for YNU-50 and A-50, respectively. The
slightly lower frequency of the former sample compared to the
latter one might be due to the aggregation into larger
particles.22,56,61,62,64 This implies that zeolite crystals are possibly
formed through a similar aggregation process irrespective of
whether crystallization occurs via a solution-mediated mecha-
nism or a solid-phase transformation mechanism.
Another important and distinctive feature of IR spectra of
TS-1 is the band at about 960 cm-1. Although the assignment
of this band is still in debate,35,48–50 the presence of this band
after dehydration of samples under high-vacuum conditions is
generally accepted as a solid proof for the incorporation of Ti
into the framework. In agreement with previous results,48 this
band could not be observed in the IR spectra of the samples
obtained in the induction period in the method A system (curves
a-d in Figure 9A). When the crystallinity reached about 66%
after further crystallization, this band was discernible. Then it
sharply increased in intensity with the progress of crystallization.
This is consistent with the marked increase in the Ti content in
the solid and liquid fractions, verifying that Ti species were
mainly inserted into the lattice in the method A system after
the crystallization was nearly completed; no apparent increase
in the intensity of the band at about 550 cm-1 was observed
with the progress of crystallization after the sample was
crystallized at 100 °C for 1 day.
In contrast, Figure 9B clearly shows a broad band around
955 cm-1 even just after the addition of (NH4)2CO3 (curve a),
and this band progressively increased in intensity with crystal-
lization going on. This indicates that the Si-O-Ti groups were
formed by gelatination from the beginning in the YNU system.
With the solid gel structure dissociated and reorganized, more
and more Si-OH and Ti-OH species were condensed to form
Si-O-Ti groups, which then produced the secondary building
units of TS-1 with precursor aggregates and/or with Si-O-Si
and Si-OH species in the presence of TPA+ cations, and the
inorganic-organic composites were further gradually organized
Figure 9. Framework IR spectra of the samples synthesized in the method
A (A) and YNU (B) systems under different crystallization conditions: (a)
after removal of ethanol (method A system) or after addition of (NH4)2CO3
(YNU system); (b) 30 °C, 1 day; (c) 60 °C, 1 day; (d) 80 °C, 1 day; (e)
100 °C, 1 day; (f) 140 °C, 1 day; (g) 170 °C, 1 day; (h) 170 °C, 3 days.
framework IR spectra.15,16,59,60 This is due to the stretching
vibration of structural double five-membered rings.15,16 The
appearance of this band gives firm evidence that the sample
possesses an atomic ordering of the MFI structure. In the method
A system, the sample synthesized at 80 °C for 1 day showed a
shoulder band around 550 cm-1 (curve d, Figure 9A). This is
in agreement with the XRD finding that the peaks characteristic
of the MFI structure already appeared (Figure 4) under these
conditions, although the crystallinity was very low, viz., 2.8%.
However, in the case of YNU samples, these XRD peaks could
be detected only after the synthesis was conducted at 100 °C
for 1 day and increased in intensity with increasing crystallinity
(Figure 9B). These findings indicate that double five-membered
rings or secondary structural units were formed from the clear
solution in the method A system when the crystallization was
carried out at 80 °C for 1 day; in contrast, they were formed in
the YNU system when crystallization was continued at 100 °C
for 1 day. This shows that the crystallization rate was slower in
the YNU system, where the solid-phase transformation mech-
anism was operative, than in the method A system, where a
homogeneous nucleation mechanism worked.
(61) Ravishankar, R.; Kirschhock, C.; Schoeman, B. J.; Vanoppen, P.;
Grobet, P. J.; Storck, S.; Maier, W. F.; Martens, J. A.; De Schryver,
F. C.; Jacobs, P. A. J. Phys. Chem. B 1998, 102, 2633.
(62) Kirschhock, C. E. A.; Ravishankar, R.; Verspeurt, F.; Grobet, P. J.;
Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 2002, 106, 3333.
(63) Serrano, D. P.; van Grieken, R. J. Mater. Chem. 2001, 11, 2391.
(64) Hsu, C. Y.; Chiang, A. S. T.; Selvin, R.; Thompson, R. W. J. Phys.
Chem. B 2005, 109, 18804.
(59) Jacobs, P. A.; Derouane, E. G.; Weitkamp, J. J. Chem. Soc., Chem.
Commun. 1981, 591.
(60) Coudrier, G.; Naccache, C.; Vedrine, J. C. J. Chem. Soc., Chem.
Commun. 1982, 1413.
9
J. AM. CHEM. SOC. VOL. 130, NO. 31, 2008 10159

A R T I C L E S
Fan et al.
Figure 10. DR UV-vis spectra of the as-synthesized solid samples
obtained under different conditions: (a) after removal of ethanol (method
A system) or after addition of (NH4)2CO3 (YNU system); (b) 30 °C, 1 day;
(c) 60 °C, 1 day; (d) 80 °C, 1 day; (e) 100 °C, 1 day; (f) 140 °C, 1 day; (g)
170 °C, 1 day; (h) 170 °C, 3 days in the method A (A) and YNU (B)
systems, respectively.
Figure 11. Catalytic properties of the A and YNU samples collected in
different crystallization conditions for (A) the oxidation of 1-hexene (a)
after removal of ethanol (method A system) or after addition of (NH4)2CO3
(YNU system); (b) 30 °C, 1 day; (c) 60 °C, 1 day; (d) 80 °C, 1 day; (e)
100 °C, 1 day; (f) 140 °C, 1 day; (g) 170 °C, 1 day; (h) 170 °C, 3 days; (I)
170 °C, 5 days and for (B) the oxidation of 2-hexanol (a) after removal of
ethanol (method A system) or after addition of (NH4)2CO3 (YNU system);
(b) 30 °C, 1 days; (c) 60 °C, 1 day; (d) 80 °C, 1 day; (e) 100 °C, 1 day; (f)
140 °C, 1 day; (g) 170 °C, 1 day; (h) 170 °C, 3 days. (Reaction conditions:
0.05 g of catalyst, 10 mL of methanol, 10 mmol of substrate, 10 mmol of
H2O2, 60 °C, 2 h.)
into the MFI architecture. This further confirms that the insertion
of Ti into the framework started from the early nucleation period
in the YNU system and coincided well with the crystallization
process of the MFI structure. As a result, the incorporation of
more Ti cations into the lattice was attained.
The Ti coordination state during the whole crystallization
process was further investigated with DR UV-vis spectroscopy.
Both the as-synthesized YNU and A samples collected in the
induction period contained three types of Ti species, i.e.,
tetrahedral, octahedral, and tiny Ti oxide particles (Figure 10).
Nevertheless, many more tetrahedral Ti species were present
in the YNU samples, whereas the A samples collected at the
early crystallization period contained many more Ti oxides, as
evidenced by the presence of an intense band around 330 nm
in their DR UV-vis spectra. This is indicative of the presence
of a large number of isolated Ti(OH)4 species in the crystalline
gel of the method A system during the nucleation period. Even
after calcination and the following acid treatment, more
octahedral Ti species were still present in the A samples than
in the YNU samples since a more intense band at about 260
nm was observed for the A samples despite the disappearance
of the tiny Ti oxide particles (Supporting Information Figure
S3). This implies that the condensation degree of Si-OH and
Ti-OH species in the method A system was much lower than
in the YNU system during the nucleation period; all the samples
were dried at 100 °C and washed with acid under the same
conditions. In addition, the coordination state of Ti species in
the A samples obtained in the induction period was basically
unchanged with the progress of crystallization, giving another
piece of evidence for the predominant incorporation of Ti into
the framework after zeolitic architecture was nearly built. In
contrast, more and more Ti cations were transformed from
octahedral coordination species into tetrahedral-to-octahedral and
tetrahedral ones in the YNU system, as revealed by the gradual
shift to the lower-wavelength positions and the decrease in the
intensity of the bands ascribed to octahedral Ti species and Ti
oxides with increasing crystallization degree. This further proves
that substitution of Ti for Si concomitantly occurred with
nucleation in the YNU system.
3.2.5. Catalytic Performance of the Solid Samples Col-
lected under Different Conditions. Ti-containing solid samples
synthesized under different conditions in the method A and YNU
systems were further examined for the oxidation of 1-hexene
and 2-hexanol (Figure 11). Herein, all the samples were calcined
and successively treated with hydrochloric acid (0.5 mol L-1
,
liquid/solid ) 50 mL g-1) at room temperature for 24 h. Clearly,
regardless of the synthesis systems, the amorphous samples
obtained in the induction period showed a very low activity for
the oxidation of both 1-hexene and 2-hexanol although some
Ti cations have a tetrahedral coordination. After the formation
of nuclei, the activity sharply increased with increasing crystal-
linity for both series of samples. TS-1 catalysts prepared from
9
10160 J. AM. CHEM. SOC. VOL. 130, NO. 31, 2008

Synthesis and Properties of Ti-Rich TS-1
A R T I C L E S
Figure 12. 29Si MAS NMR spectra of the silicalite-1 and TS-1 samples
synthesized in the method A system under different conditions: (A) silicalite-
1, 170 °C, 1 day; (B) silicalite-1, 170 °C, 3 days; (C) TS-1, 170 °C, 1 day;
(D) TS-1, 170 °C, 3 days.
Figure 14. Dependence of the isolated Si-OH absorbance at 3738 cm-1
for the samples synthesized in the YNU system on the crystallization
conditions: (A) after addition of (NH4)2CO3; (B) 30 °C, 1 day; (C) 60 °C,
1 day; (D) 80 °C, 1 day; (E) 100 °C, 1 day; (F) 140 °C, 1 day; (G) 170 °C,
1 day; (H) 170 °C, 3 days.
Figure 13. 29Si MAS NMR spectra of the silicalite-1 and TS-1 samples
synthesized in the YNU system under different conditions: (A) silicalite-1,
170 °C, 1 day; (B) silicalite-1, 170 °C, 3 days; (C) TS-1, 170 °C, 1 day;
(D) TS-1, 170 °C, 3 days.
Figure 15. Catalytic results of the oxidation of various organic substrates
over the YNU-20 and A-50 catalysts (H2O2 based conversion (%): the
percent used for the oxidation of substrates. Reaction conditions: for
1-hexene, 0.05 g of catalyst, 10 mL of methanol, 10 mmol of substrate, 10
mmol of H2O2, 60 °C, 2 h; for n-hexane, 0.1 g of catalyst, 10 mL of
methanol, 10 mmol of substrate, 20 mmol of H2O2, 60 °C, 4 h; for benzene,
0.1 g of catalyst, 10 mL of sulfolane, 10 mmol of substrate, 1 mmol of
H2O2, 100 °C, 2 h; for styrene, 0.1 g of catalyst, 10 mL of acetone, 5 mmol
of substrate, 2.5 mmol of H2O2, 60 °C, 4 h).
the YNU system showed significantly higher conversion than
the samples prepared from the method A system partly as a
result of the incorporation of more Ti species into the frame-
work. This also holds true for the oxidation of 2-hexanol. It is
worth noting that the highest conversion for the oxidation of
1-hexene and 2-hexanol over the A samples was achieved when
crystallization was carried out at 170 °C for 1 day. In contrast,
with respect to the YNU samples, the activity in the oxidation
of 2-hexanol significantly increased when the crystallization time
at 170 °C was elongated from 1 to 3 days although the two
samples had similar crystallinities.
Figure 6 shows that in the method A system, Ti cations
continued to transfer from the liquid into the solid after the
sample was crystallized at 170 °C for 1 day. However, 1-hexene
and 2-hexanol conversions were not increased. This may result
from the partial tethering and/or absorption of these Ti species
on the solid, which can be easily removed by acid washing, as
indicated by similar Ti contents in the prepared catalysts (Figure
5). This is probably because most of the Ti cations have already
been inserted into the preferable lattice sites at this stage, and
thus, the rest of the sites would be much more difficult to be
replaced with Ti. However, this is not true for the YNU system.
As shown in Figures 5, 6, and 9, all Ti species entered into the
solid to achieve a tetrahedral coordination state when the
crystallization was carried out at 140 °C for 1 day. The further
increase in the activity might be due to a slight improvement
in microscopic Ti character stemming from further condensation
of Ti species into Si-O-Ti groups and the resultant increase
in hydrophobicity and slight change of the bond angle and the
bond length of Si-O-Ti groups.
3.2.6. Location of Ti Cations in the Samples. It was reported
that in the method A system Ti was preferably inserted into the
defect sites.9,34,35,38,39,55 This is supported by our experimental
data showing that the isomorphous substitution of Ti for Si
occurred mainly when crystal growth was almost completed and
that TS-1 has a much smaller number of defect sites than
silicalite-1 synthesized under the same conditions, as proved
by the low intensity of the IR vibration band at 3738 cm-1 (not
shown here), which is characteristic of isolated Si-OH groups.
This is further confirmed by 29Si MAS NMR spectroscopy; the
signal at about 103 ppm of the TS-1 sample is considerably
weaker in intensity than that of the silicalite-1 sample (Figure
12), which is solely attributed to Q3 species ((SiO)3SiOH) since
the signal assigned to the ((SiO)3SiOTi) species would appear
at a higher field than Q4 species ((SiO)4Si) for the TS-1
9
J. AM. CHEM. SOC. VOL. 130, NO. 31, 2008 10161

A R T I C L E S
Fan et al.
Figure 16. FE-SEM images of the A-50 (A), A-20 (B), YNU-50 (C), and YNU-20 (D) samples.
formation of more Si-O-Ti and Si-O-Si groups in the former
samples. Otherwise, the much higher crystallization rate in the
silicalite-1 system (Figure 4D) would contrarily result in a higher
condensation degree and, thus, the presence of a smaller amount
of defect sites for the silicalite-1 sample synthesized at 100 °C
for 1 day; under these conditions the crystallinity of silicalite-1
reached about 80%, whereas that of the YNU sample was less
than 3%. This would lead to a more homogeneous distribution
of Ti in the YNU samples than in the A samples because this
creates the conditions for Si-O-Si and Si-O-Ti groups
simultaneously to form a nucleus around TPA+ cations and
further build into a zeolitic architecture. In particular, it is
noteworthy that all Ti species have been incorporated into the
solid when the crystallinity reached the maximum. In contrast,
in the method A system, almost all Ti cations were inserted
into the defect lattice sites after crystallization was nearly
completed. This is supported by the XPS data; the surface Si/
Ti ratio in the calcined and subsequently acid-treated A samples
gradually decreased with the progress of crystallization after
reaching the highest crystallinity, whereas for the YNU samples
it was basically unchanged (not shown here). Nevertheless, this
cannot exclude the possibility that Ti prefers to replace Si in
some certain lattice sites, e.g., defect sites in silicalite-1, for
the YNU samples since TS-1 synthesized in the YNU system
also shows a Q3 signal much lower in intensity than silicalite-1
synthesized under the same conditions. An increase in the
intensity of the isolated Si-OH vibration band before the
formation of nuclei (Figure 14, parts C and D) is in accordance
with the TG and FE-SEM findings that original bulky gel
particles are first dissociated into aggregates of small pieces,
viz., secondary gel, and then reorganized into zeolitic framework.
3.3. Catalytic Properties of Ti-Rich TS-1 Prepared from
the YNU System. Table 1 shows the catalytic results of TS-1
prepared by the methods A, B, and YNU for the oxidation of
n-hexane. For each series of catalysts, their catalytic properties
were highly dependent on the framework Ti amount. Obviously,
YNU samples gave much higher n-hexane conversion and
ketone selectivity than either A or B (synthesized by method
B) samples, and the difference was more apparent in the
Figure 17. 29Si MAS NMR spectra of the prepared TS-1 catalysts (the
Si/Ti ratios are 58.3 and 57.2 for the A-50 and YNU-50 samples,
respectively).
material.65 The Q4/Q3 ratio of the silicalite-1 sample crystallized
at 170 °C for 1 day was 3.1 in contrast to the ratio of 4.3 for
the TS-1 synthesized under the same conditions. This is in
agreement with the finding made by neutron powder diffraction
study that Ti is not randomly distributed in the framework but
preferably occupies some sites such as T6, T7, T10, and T11.9,35
The silicalite-1 sample crystallized at 170 °C for 3 days showed
a lower Q4/Q3 ratio than the sample crystallized for 1 day,
indicating the presence of more defect sites in the former sample.
This is consistent with the fact that the crystallinity of silicalite-1
decreased when the synthesis time was extended from 1 to 3
days (Figure 4, parts F and G), which is probably due to partial
dissolution of the formed crystals upon further crystallization.
The presence of a smaller number of defect sites in the well-
crystallized TS-1 than in the silicalite-1 also holds true for the
YNU samples (Figure 13). Furthermore, it is worth noting that
the Ti-containing solids obtained during the induction period
in the YNU system contained less defect sites than pure silica
solid analogues (Figure 14D). This is probably due to the
(65) Tuel, A.; Ben Taarit, Y. J. Chem. Soc., Chem. Commun. 1992, 1578.
9
10162 J. AM. CHEM. SOC. VOL. 130, NO. 31, 2008

Synthesis and Properties of Ti-Rich TS-1
A R T I C L E S
Figure 18. Schemes for the crystallization process: (A) the proposed mechanism for the formation of TS-1 and (B) the evolution of solid particle morphology
occurring in the YNU system.
oxidation of other linear alkanes. The conversions based on
H2O2 over the YNU-15 sample were 14.2%, 16.3%, and 10.7%,
whereas they were 4.9%, 7.6%, and 3.7% on the A-50 sample
in the oxidation of n-pentane, n-heptane, and n-octane, respec-
tively, under the same reaction conditions. For comparison,
Figure 15 shows the catalytic results of the oxidation of various
organic substrates over the two best samples of A-50 and YNU-
20 obtained by the methods A and YNU, respectively. It is clear
that YNU-20 exhibited significantly higher catalytic activity than
A-50 in the oxidation of not only alkanes but also other organic
substrates such as 1-hexene, styrene, and benzene. This is
attributable to both the higher Ti content in the YNU samples
and the higher oxidation ability of Ti sites. YNU-20 gave
turnover numbers (TONs) of 100, 160, 16, and 24 in contrast
to 119, 140, 11, and 18 given by A-50 in the oxidation of
1-hexene, n-hexane, benzene, and styrene, respectively.
The oxidation ability of titanosilicates is correlated to the size
of crystals, and the small crystals usually exhibit high reactivity.
However, addition of (NH4)2CO3 to the synthesis gel resulted
in the formation of large crystals. FE-SEM measurements
showed that the uniform crystal size of the YNU-50 sample
was about 0.7 µm, being about 3.5 times as large as that of the
A-50 sample (Figure 16). The morphology and crystal sizes of
the B samples (not shown here) were similar to those of the A
samples. The crystal sizes of the YNU samples further increased
to larger than 1 µm with increasing Ti content for the YNU-
9
J. AM. CHEM. SOC. VOL. 130, NO. 31, 2008 10163

A R T I C L E S
Fan et al.
20. This is in accordance with the results reported by Cundy et
al. that incorporation of more Ti into the framework resulted in
the formation of larger crystals.66 In contrast, no marked change
in the crystal size was observed for the A and B samples when
the Si/Ti ratio in the gel was increased. The fact that the crystal
size of YNU samples was larger than of A samples is also
consistent with the framework IR spectra, where the band
assigned to the stretching vibration of double five-membered
rings appeared at a lower frequency for YNU-50 than for A-50
(546 vs 549 cm-1) (Supporting Information Figure S2).
One might consider that the increase in crystallite size would
lead to low oxidation efficiency. However, in the case of YNU
samples, crystals are well crystalline, as evidenced by higher
Q4/Q3 ratio (Figure 17). In contrast, more defective sites were
present in the A samples. This is consistent with the finding
that the silanol groups or the defect sites decreased with
increasing Ti content in the framework,35,53 since Ti has a
mineralizing effect and more Ti cations were incorporated in
the YNU samples than in the A samples. Therefore, in
comparison with the YNU samples, the A samples are not only
relatively hydrophilic but might contain more ((SiO)3TiOH)
species. The higher hydrophilicity of the A samples than of the
YNU samples is further corroborated by the fact that the weight
loss below 150 °C due to the desorption of water was about
1.0% for as-synthesized A-50 in contrast to 0.3% for as-
synthesized YNU-50 (not shown here).
Although the catalytic properties of titanosilicates are prob-
ably related to their crystalline structures and/or characters, the
presence of more defect sites in titanosilicates would be
unfavorable for the oxidation of alkanes, styrene, and benzene,
as suggested by the following facts: (i) compared with TS-1,
Ti-MWW, and Ti-Beta with more defect sites showed very low
activity for the oxidation of n-hexane, styrene, and benzene;
(ii) Ti-MCM-41, with a low Q4/Q3 ratio, showed very low
activity in the oxidation of n-hexane and 1-hexene; however,
silylation resulted in a remarkable improvement in the oxidation
activity.67 Furthermore, it has been proven that ((SiO)3TiOH)
species are much less active than ((SiO)4Ti) species for the
liquid-phase oxidation of organics over TS-1.68,69 Therefore,
although YNU samples have larger crystals, they showed higher
catalytic efficiency. On the other hand, the finding that the TON
of YNU samples increased with increasing Ti content up to the
Si/Ti ratio of 57.2 can be accounted for by the decrease in the
defect sites (Table 1). The fact that a further increase in the Ti
content resulted in a slight decrease in the TON is probably
due to the nonrandom distribution of Ti in the frame-
work,9,34,35,38,39,55 which would lead to the inclusion of some
Ti cations in the lattice sites hardly accessible to substrate
molecules.
than the A sample in the oxidation of a variety of organic
substrates, e.g., linear alkenes and alkanes, alcohols, styrene,
as well as benzene. This not only is due to the higher Ti content
in the framework but also results from the increase in
hydrophobicity.
Addition of (NH4)2CO3 to the synthesis gel drastically lowered
down the pH value to result in a slow crystallization rate, thus
making the speed for the incorporation of Ti into the framework
and the crystallization rate match well. Its addition also considerably
modified the crystallization mechanism. In comparison to the
typical liquid-phase transformation mechanism working in the
method A system, solid-phase transformation mechanism seems
to predominate in the YNU system although the simultaneous
occurrence of a liquid-to-solid mass transfer after the formation of
nuclei cannot be excluded. An overview of the crystallization
process in the YNU system is illustrated in Figure 18 (scheme I
for the proposed formation mechanism and scheme II for the
evolution of particle morphology).
After the addition of (NH4)2CO3, the clear solution quickly
solidified to form an inorganic-organic gel composed of bulky
particles, which were further depolymerized and reorganized
into aggregates of tiny particles (secondary gel). Nucleation
seems to start from the inner portion of the aggregates, followed
by a crystal growth by depleting the nutrients in amorphous
materials. The secondary gel particles first restructured into
zeolite nuclei and then grew into irregular objects less than 200
nm in size. This was followed by the formation of a spherical
shape and finally a boat habit.
During the induction period, no evidence was found for the
mass transfer from liquid to solid or vice versa in the YNU
system. This suggests that nuclei formed through dissociation
and restructuring of the solid gel, and incorporation of Ti into
the framework started from the gelatination process with the
assistance of (NH4)2CO3 in contrast to the method A system
where most of Ti species were inserted into the lattice after the
crystallization was nearly completed. With increasing crystal-
linity, a small part of nongelatinated Ti-OH species (and hence
left in the liquid) quickly transferred into the solid. Although
the crystallinity reached the highest value when crystallization
was carried out at 140 °C for 1 day, the condensation of Ti-OH
and Si-OH groups was still going on in the YNU system.
Therefore, a further crystallization led to not only an increase
in the hydrophobicity but also an overall improvement in the
microscopic character of Ti species. As a result, the samples
showed increased activity with the progress of crystallization
after the zeolitic architecture was built.
Acknowledgment. The authors gratefully thank the Core Research
for Evolutional Science and Technology of the Japan Science and
Technology Agency (JST) for support of this work. W.F. also thanks
JST for a postdoctoral fellowship and the Chinese Academy of
Sciences (“Hundred Talents” Project) and the National Science
Foundation of China (No. 20773153) for support of this work.
4. Conclusions
A Ti-rich TS-1 sample has been synthesized by using
(NH4)2CO3 as a crystallization-mediating agent. The Si/Ti ratio
in the framework can be decreased to 34 in contrast to 58
achieved by methods A or B under the same crystallization
conditions. The prepared catalyst showed much higher activity
Supporting Information Available: DTA results of the YNU
samples crystallized under different conditions, framework IR
spectra of the A-50 and YNU-50 catalysts, DR UV-vis spectra
of the calcined and further acid-treated solid samples synthesized
in the method A and YNU systems under different conditions,
Figures S1-S3. This material is available free of charge via
the Internet at http://pubs.acs.org.
(66) Cundy, C. S.; Forrest, J. O.; Plaisted, R. J. Microporous Mesoporous
Mater. 2003, 66, 143.
(67) Tatsumi, T.; Koyano, K. A.; Igarashi, N. Chem. Commun. 1998, 325.
(68) Srinivas, D.; Manikandan, P.; Laha, S. C.; Kumar, R.; Ratnasamy, P.
J. Catal. 2003, 217, 160.
(69) Zhuang, J.; Ma, D.; Yan, Z.; Deng, F.; Liu, X.; Han, X.; Bao, X.;
Guo, X.; Wang, X. J. Catal. 2004, 221, 670.
JA7100399
9
10164 J. AM. CHEM. SOC. VOL. 130, NO. 31, 2008