Among a number of terpenes, α-pinene is an important substance in the manufacture of a variety of synthetic aroma chemi-cals and its epoxide is isomerized to produce campholenic aldehyde
Trang 1e-J Surf Sci Nanotech Vol 9 (2011) 539-543 Conference IWAMN2009
-Ti-MCM-41 with Various Ti Contents: Synthesis, Characterization and
Tran Thi Nhu Mai, Nguyen Van Quyen, Nguyen Thanh Binh, and Le Thanh Son
Faculty of Chemistry, Hanoi University of Science, Vietnam National University, No.334-Nguyen Trai Str., Hanoi, Vietnam
Giang Thi Phuong Ly and Nguyen Thi Ha†
Faculty of Technology Chemistry, Hanoi University of Technology, No.1-Dai Co Viet Str., Hanoi, Vietnam
(Received 24 November 2009; Accepted 24 May 2010; Published 27 December 2011)
Ti-MCM-41 with various ratios Ti/Si was synthesized successfully and characterized by some measurements as XRD, DR-UV-Vis, TEM, EDAX and N2-Adsorption The peak in bands 230 nm in DR-UV-Vis spectroscopy showed that Ti was in tetrahedral coordination The TEM, EDAX images indicated that the obtained materials were of ordered mesoporous structures in the samples with low ratio of Ti/Si Moreover, increasing the thickness
of material wall calculated by BJH method was due to the incorporation of titanium in the framework The
oxida-tion of α-pinene over Ti-MCM-41 with various ratios Ti/Si indicated that the possibility of Ti in the tetrahedral coordination gave a priority to the selectivity of α-pinene oxide The Ti content of materials system increased,
con-sequently the conversion of the reaction and the pinandiol product yield increased [DOI: 10.1380/ejssnt.2011.539]
Keywords: Ti-MCM-41; EDAX; N 2-Absorption; Epoxidation; α-pinene; H2 O 2
Nowadays, due to the increasingly important
sustain-ability for the chemical industry, there are a lot of
search interests to produce the new chemicals from
re-newable resources Monoterpenes are cheap, abundant
and often fundamental raw chemicals for pharmaceutical,
fragrance and flavor industry Oxidation of monoterpenes
could provide interesting intermediates for the synthesis of
new chemicals for the chemical industry such as epoxides,
terpenic aldehydes, alcohols and polihydroxyl Among a
number of terpenes, α-pinene is an important substance
in the manufacture of a variety of synthetic aroma
chemi-cals and its epoxide is isomerized to produce campholenic
aldehyde, which is an intermediate for the
pharmaceu-ticals, agrochemicals and products for perfumery [1–5]
α-Pinene is a valuable starting material to obtain its
im-portant oxidation products pinene oxide, verbenone and
campholenic aldehyde We have studied air oxidation of
α-pinene in order to improve the preparation of
camp-holenic aldehyde in one-pot starting from α-pinene.
Titanosilicates containing tetrahedral Ti species in the
framework, being capable of activating hydrogen
perox-ide under liquid-phase conditions, prove to be promising
catalysts for the selective oxidation of a variety of organic
compounds [1–3] The representative titanosilicate has
been TS-1 of the MFI structure, which was reported two
decades ago The discovery of TS-1 has led to
industri-alized processes such as the hydroxylation of phenol to
hydroquinone and catecol, and the ammoxination of
cy-clohexanone to oxime [1, 3] To solve the problems that
medium pores TS-1 encounters in bulky reactions, many
others titanosilicates with larger pore have also been
de-∗This paper was presented at the International Workshop on
Ad-vanced Materials and Nanotechnology 2009 (IWAMN2009), Hanoi
University of Science, VNU, Hanoi, Vietnam, 24-25 November, 2009.
†Corresponding author: nguyenha1984@yahoo.com,hant-fct@mail.
hut.edu.vn
veloped thereafter by hydrothermal synthesis or
postsyn-thesis methods, for example Ti-β, Ti-ZSM-12, Ti-MOR,
Ti-MCM-48 [3] Particularly, Ti-Beta with 12-membered ring channels is a very attractive catalysts for the ox-idation of cyclic and branched alkenes and alkanes [3] containing mesoporous materials MCM-41 and Ti-substituted hexagonal mesoporous silica Ti-HMS have also been synthesized [3–5] Both materials pioneered the potential to oxidize bulky molecular which cannot enter the micropores of zeolites such as TS-1, TS-2 with the
MEL structure, and Ti-β.
In this paper, we report on the synthesis of
Ti-MCM-41 materials, together with a thorough characterization
of the catalysts by means of XRD, DR-UV-Vis, TEM, EDAX in order to give some insight into the coordina-tion state of Ti in Ti-MCM-41 samples, which is crucial for proper understanding of their structure and catalytic behavior Finally, the effect of Ti content on catalyst for
activity and product distribution in α-pinene oxidation
was studied
The titanium-containing mesoporous materials (Ti-MCM-41) were prepared by hydrothermal synthesis us-ing cetyltrimethylammonium bromide (CTMABr) as tem-plate, 25 wt% aqueous solution of tetramethylammonium
hydroxide (TMAOH, K + Na < 5 ppm)
Tetraethoxysi-lane (TEOS) and tetrabutyl orthotitanate (TBOT) were used as the Si and Ti sources, respectively An alco-holic solution of CTMABr was added to a mixture of TEOS and TBOT, following the methodology proposed
by Koyano and Tatsumi Molar ratio of gel Si : Ti : CTMABr : TMAOH : H2O= 1 : x : 0.15 : 1.3 : 150 (where x = 0.01 ÷ 0.03) This gel was transferred into
teflon-lined stainless-steel autoclave and kept at 100◦C
for 24 h The solid product was recovered by filtration, washed with doubly distilled water and dried at 60◦C
Trang 2Į
P
free and
Ti-vis spectra of synthesized samples
FIG 1: XRD patterns of Ti-free and Ti-MCM-41 Ti/Si =
0.01; 0.02; 0.03.
Į
Į
P
vis spectra of synthesized samples
FIG 2: Diffuse reflectance UV-vis spectra of synthesized
sam-ples
overnight To remove the template, the samples were
cal-cined at 550◦C for 6 h The catalysts were characterized
by XRD,DR-UV-Vis (V-650-spectro photometer Japan),
STEM Japan), N2 adsorption (Bell-Belsorp mini-Japan)
The oxidation of α-pinene with H2O2 were performed
at 70◦C in a glass flask reflux under vigorous stirring.
Typically, the reaction mixture consisted of 50 mg
cata-lyst, 10 mmol of α-pinene, 2.5 mmol of H2O2and 17.5 ml
of acetonitrile solvent (MeCN), time reaction 60 minutes
In all cases, the oxidant to substrate molar ratio was 1:4 in
order to minimize the possible Ti leaching The products
were separated and identified by the gas chromatography
mass spectrometry GC-MS (Detector MS-HP 5689,
col-umn HP-5: 5% methylethylsiloxan 30× 0.5 nm × 0.25
µm film thickness) at Petroleum Chemistry Center, Hanoi
The synthesis of Ti-MCM-41 has been carried out in the absence of alkali cations since they usually promote the formation of poor crystalline titanosilicates during the synthesis of TS-1 Titanium loading of the catalysts was varied by changing the amount of the Ti source in the synthesis gel In this way, four catalysts as MCM-41, Ti-MCM-41(0.01), Ti-MCM-41(0.02) and Ti-MCM-41(0.03) were prepared
The hexagonal arrangement of these catalysts is con-firmed by the XRD pattern shown in Fig 1 The sharp peak at 2.2 degrees is due to the diffraction plane 100 which indicates hexagonal symmetry Two additional high order-peaks were obtained in the case of MCM-41 and Ti-MCM-41(001) relating to the diffraction planes
110 and 200 In addition, there is a decrease in the in-tensity of this first peak, and an evident broadening for all peaks when increasing amounts of Ti, maybe due to
a reduction in the long-range order of the structure No diffraction peaks in the region of higher angles (10-50) could indicate the presence of bulk anatase in the sam-ples and suggest that Ti-MCM-41 sample is a pure phase DR-UV-Vis spectroscopy is a very sensitive method for characterization of the coordination site of Ti in zeolite framework The DR-UV-Vis spectra of the Ti-MCM-41 samples prepared with different Ti contents are shown
in Fig 2 The intense of ligand-to-metal charge transfer band at 230nm, which is present in all samples, clearly in-dicates that most of Ti ions are isolated and in tetrahedral (Td) coordination A shoulder at 250-270nm becomes sig-nificant in indicating the presence of higher coordinated
Ti species (in penta- or octahedral coordination) in the samples with relatively high Ti content This higher coor-dination environment of Ti could appear upon hydration
by insertion of water molecules as extraligands to the Ti (Td) species during preparation
Both the highly hydrophilic surface and the large sur-face area of these materials yield a high water adsorption capacity which would lead to a high hydration of Ti ions surface The possibility of some Ti–O–Ti clustering in the framework due to an incipient oligomerization of Ti species containing Ti–O–Ti bonds cannot be unequivo-cally excluded On the other hand, compared to the bulk anatase TiO2, the lack of an absorption band characteris-tic of octahedral extra-framework titanium at about
300-330 nm in the Ti-MCM-41 samples with Ti content of
2 mol.% and 3 mol.% suggests that no separated tita-nium phase is formed during the synthesis process In contrast with other results, the intensity of the 230 nm band is slightly increased, moreover this band does not shift towards higher wavelengths when increasing amount
of titanium [6, 10] From these results, it is possible to conclude that the tetrahedral component of Ti (V) almost remains even in the samples having high Ti content Nitrogen adsorption-desorption isotherms along with the corresponding BJH pore size distribution are shown
Trang 3FIG 3: Nitrogen adsorption-desorption isotherm and pore size distribution.
FIG 4: TEM images of Ti-MCM-41 samples with various Ti
contents.Ratio of Ti/Si from EDAX spectroscopy
FIG 5: The products of the oxidation of α-pinene.
in Fig 3
The samples exhibit type IV isotherms with a sharp
infection at a relative pressure around P/P o = 0.34-0.45
and a corresponding narrow and strong band in the pore
size distribution curve, which is characteristic of well
or-dered mesoporous materials with a narrow and uniform
pore size distribution [11]
All the samples showed great surface areas which
ranged from approximately 1000 to 1150 m2/g as seen in
Table I The decreasing specific surface area with the Ti
content may be correlated to the decrease in the structural
order, as observed in the XRD patterns Consequently, an
increasing amount of transition metal could obstruct the
structure-directing action of template and result in the
formation of partially broken pores as well as a lower
sur-face area On the other hand, the pore diameters increase
slightly when increasing Ti loadings
The TEM images of the Ti-MCM-41 materials with
var-ious Ti contents are shown in Fig 4 The TEM images
showed that the obtained materials with a low Ti content
were high ordered mesoporous structure and
monodisper-sity of Ti of Ti-MCM-41 The diameter of the pore was
estimated to be 2.5 nm The thickness of the mesoporous shell and the average particle diameter of Ti-MCM-41 are
in good agreement with the respective parameters of the Ti- mesoporous materials
Besides, EDAX spectra taken from different regions are shown in Fig 4 The root spectrum corresponds to the particle center and gives the value of mol % of Ti in the sample The EDAX results on surface of the samples showed that mounts of Ti varied according to the differ-ence positions This proves the possibility of Ti formation
is out of the framework mesoporous In the Ti-MCM-41 with a high Ti content, the local elemental analysis per-formed by EDX confirmed that titanium oxide is located within the mesoporous pore
The properties and product components of the α-pinene
oxidation were based on the nature of the catalyst The
reaction products of the α-pinene oxidation over
Ti-MCM-41 analyzed on GC-MS system were shown in Ta-ble II and Fig 5 According to GC-MS, the mixture contains the species which were formed by oxidation of both double bond and allylic C–H The formation of the
product I is attributed to the oxidation of π bond, the
compound V is produced by the rearrangement of III and the VI are formed by hydrolysis and opening of oxirane
ring of α-pinene oxide Products I and II are generated
by oxidation of allylic C–H bond
As above results, the oxidation of α-pinene was estab-lished through many ways such as isomerization of
α-pinene, the direct epoxidation of C=C double bond
form-ing α-pinene epoxide, oxidation follow radical mechanisms
in C-C allyl bond, and isomerization of the epoxide re-garded as a reactive intermediate
Over Ti-MCM-41 catalyst, the oxidation was carried
out in the mild condition and selectivity to α-pinene
epox-ide as a reactive intermediate.The selectivity to epoxepox-ide over Ti-MCM-41 was due to Ti4+ atom substituted iso-morphic Si atom in the framework [4, 6]
The isomerization of α-pinene oxide regarded as a
reac-tive intermediate over two catalysts gave the same camp-holenic aldehyde product (Fig 6) Those are intermediate chemicals, which have many applications in the synthesis
Trang 4Samples Ti content S BET Pore diamete Pore Volume d(100) a0 Wall thickness
TABLE II: Oxidation of α-pinene with H2O2 and Ti-MCM-41 (x)
Reaction conditions: acetonitrile as solvent; time, 1.0 h; temperature, 60◦C and H2O2as oxidant
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FIG 6: The isomerization of the intermediate α-pinene
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FIG.7: The ion fragments of campholenic aldehyde from
FIG 7: The ion fragments of campholenic aldehyde from
GC-MS result
of a fine chemical and pharmaceuticals In the GC-MS
result Fig 7, it indicated that the typical m/z fragment
ion of 108, 93, 67, 41, which are the typical fragment ions
of the fragment processing of campholenic aldehyde with
an efficient index of 90% to standard GC-MS mass
spec-troscopy
Results of α-pinene oxidations over Ti modified
MCM-41 catalysts are shown in Table II As it can be seen,
conventional reaction tests (with H2O2 addition only at
the beginning of reaction) practically show changes on
ac-tivity when rising amounts of Ti Based on the studied
reaction conditions, the change of Ti content on catalysts
has an effect on product distribution The smooth growth
of II and VI species during reaction shows that epoxide
is not stable at reaction conditions; furthermore it is eas-ily hydrolyzed and rearranged by acid sites of catalyst The nature of acid sites was studied for these catalysts
by adsorption/desorption of pyridine followed by FTIR Results of acidity characterization showed that Lewis and Bronsted acidity increased proportionally to Ti content The acidity also can explain the formation of species II,
as it is well known, Lewis acid sites induce the
rearrange-ment of α-pinene oxide to campholenic aldehyde.
Ti-MCM-41 molecular sieves with various compositions have been successfully prepared by direct synthesis In all cases, solids with high specific surface area, high pore vol-ume and a narrow pore size distribution were obtained
Ti was incorporated into the silica framework mainly in the tetrahedral isolated sites At high Ti content, the broadening of the main DR-UV-Vis band with a shoulder
at about 260-270 nm can be assigned to a higher coor-dination of Ti probably due to water molecules adsorbed
on the catalyst as well as to the formation of some
Ti-O-Ti clustering in the framework However, a segregated TiO2 anatase phase was not observed for any sample The materials synthesized here showed a good activity
for the epoxidation of α-pinene using H2O2 as oxidant
The α-pinene conversion level and the nature of
oxida-tion products were strongly influenced by the structure of the catalyst, the degree of metal loading and the chemical environment around the active sites The main oxidation
product was the α-pinene oxide, being the by-products
the corresponding hydrolysis and allylic oxidation prod-ucts The epoxide yield reached a maximum value at an
Ti content in the catalyst of approximately 1 wt.%
Acknowledgments
This study is supported by Grants-in-Aid from Viet-namese Government Fund
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