Conference-IWAMN 2009-Ti-MCM-41 with Various Ti Contents : Synthesis , Characterization and Catalytic Properties in Oxidation of α-Pinene

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 oxidation 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, consequently the conversion of the reaction and the pinandiol product yield increased. [DOI: 10.1380/ejssnt.2011.539]


I. INTRODUCTION
Nowadays, due to the increasingly important sustainability for the chemical industry, there are a lot of research interests to produce the new chemicals from renewable 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 chemicals and its epoxide is isomerized to produce campholenic aldehyde, which is an intermediate for the pharmaceuticals, agrochemicals and products for perfumery [1][2][3][4][5].α-Pinene is a valuable starting material to obtain its important oxidation products pinene oxide, verbenone and campholenic aldehyde.We have studied air oxidation of α-pinene in order to improve the preparation of campholenic aldehyde in one-pot starting from α-pinene.
Titanosilicates containing tetrahedral Ti species in the framework, being capable of activating hydrogen peroxide under liquid-phase conditions, prove to be promising catalysts for the selective oxidation of a variety of organic compounds [1][2][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 industrialized processes such as the hydroxylation of phenol to hydroquinone and catecol, and the ammoxination of cyclohexanone 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-veloped thereafter by hydrothermal synthesis or postsynthesis 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 oxidation of cyclic and branched alkenes and alkanes [3].Ti-containing mesoporous materials Ti-MCM-41 and Tisubstituted hexagonal mesoporous silica Ti-HMS have also been synthesized [3][4][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 coordination 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.

A. Synthesis and characterization
The titanium-containing mesoporous materials (Ti-MCM-41) were prepared by hydrothermal synthesis using cetyltrimethylammonium bromide (CTMABr) as template, 25 wt% aqueous solution of tetramethylammonium hydroxide (TMAOH, K + Na < 5 ppm).Tetraethoxysilane (TEOS) and tetrabutyl orthotitanate (TBOT) were used as the Si and Ti sources, respectively.An alcoholic 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 : H 2 O= 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 overnight.To remove the template, the samples were calcined at 550 • C for 6 h.The catalysts were characterized by XRD,DR-UV-Vis (V-650-spectro photometer Japan), STEM Japan), N 2 adsorption (Bell-Belsorp mini-Japan).

B. Catalytic experiment
The oxidation of α-pinene with H 2 O 2 were performed at 70 • C in a glass flask reflux under vigorous stirring.Typically, the reaction mixture consisted of 50 mg catalyst, 10 mmol of α-pinene, 2.5 mmol of H 2 O 2 and 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, column HP-5: 5% methylethylsiloxan 30 × 0.5 nm × 0.25 µm film thickness) at Petroleum Chemistry Center, Hanoi University of Science, Vietnam National University.

A. Physicochemical characterization of catalysts
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 confirmed 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 intensity 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 samples 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 indicates that most of Ti ions are isolated and in tetrahedral (Td) coordination.A shoulder at 250-270nm becomes significant in indicating the presence of higher coordinated Ti species (in penta-or octahedral coordination) in the samples with relatively high Ti content.This higher coordination 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 surface 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 unequivocally excluded.On the other hand, compared to the bulk anatase TiO 2 , the lack of an absorption band characteristic 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 titanium 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.
The samples exhibit type IV isotherms with a sharp infection at a relative pressure around P/P o = 0.34-0.45and a corresponding narrow and strong band in the pore size distribution curve, which is characteristic of well ordered 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 m 2 /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 surface area.On the other hand, the pore diameters increase slightly when increasing Ti loadings.
The TEM images of the Ti-MCM-41 materials with various 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 monodispersity 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 difference 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 performed by EDX confirmed that titanium oxide is located within the mesoporous pore.

B. Oxidation of α-Pinene
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 Table 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 established through many ways such as isomerization of αpinene, the direct epoxidation of C=C double bond forming α-pinene epoxide, oxidation follow radical mechanisms in C-C allyl bond, and isomerization of the epoxide regarded as a reactive intermediate.
Over Ti-MCM-41 catalyst, the oxidation was carried out in the mild condition and selectivity to α-pinene epoxide as a reactive intermediate.The selectivity to epoxide over Ti-MCM-41 was due to Ti 4+ atom substituted isomorphic Si atom in the framework [4,6].
The isomerization of α-pinene oxide regarded as a reactive intermediate over two catalysts gave the same campholenic aldehyde product (Fig. 6).    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 spectroscopy.
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 H 2 O 2 addition only at the beginning of reaction) practically show changes on activity 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 easily 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 rearrangement of α-pinene oxide to campholenic aldehyde.

IV. CONCLUSION
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 volume 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 coordination 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 TiO 2 anatase phase was not observed for any sample.The materials synthesized here showed a good activity for the epoxidation of α-pinene using H 2 O 2 as oxidant.The α-pinene conversion level and the nature of oxidation 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 products.The epoxide yield reached a maximum value at an Ti content in the catalyst of approximately 1 wt.% .

TABLE I :
Surface properties and chemical composition of Ti-MCM-41 with different Ti contents and pure silica MCM-41.