Indian Journal of Chemistry Vol. 47A, August 2008, pp. 1181-1186

Synthesis, characterization and catalytic properties of microporous silicotitaniumphosphate by neutral templating route

Krishanu Sarkar, Mahasweta Nandi & Asim Bhaumik*

Department of Materials Science and Centre for Advanced Materials, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India

Email: msab@iacs.res.in

Received 15 May 2008; revised 25 June 2008

A new series of microporous silicotitaniumphosphate and its organic-inorganic hybrid analogue have been synthesized under hydrothermal condition with the assistance of a neutral structure-directing agent, 4,4′-trimethylenedipiperidine under almost neutral pH condition. PXRD, FE-SEM, TEM and N2 sorption studies have been used to characterize these materials. FTIR and UV-visible studies suggest the presence of Si-O-P and Ti-O-Si bonds, and tetrahedral coordination of Ti(IV) in the samples. The thermal stability of the materials has been analyzed by TGA/DTA. These samples show good catalytic activity towards the liquid phase partial epoxidation of cyclohexene and styrene using dilute H2O2 as oxidant.

IPC Code: Int. Cl.8 B01J21/00; B01J29/00; B01J37/00

Nanoporous silicate1,2 and phosphate3,4 based molecular sieves have received considerable attention over the last decade because of their potential applications as adsorbents, catalysts and ion exchanger materials5-7. These materials are also useful for the synthesis of ultrafine nanorod arrays8 or ordered mesoporous carbons9 needed for device fabrication. Different hetero-element containing nanoporous materials find many practical applications because of the physical and chemical nature of the individual elements associated with the framework. Microporous titanium silicates (TS-1, TS-2 etc.) have been used widely for the selective, ecofriendly and industrially important organic reactions. In the presence of H2O2 these materials have shown excellent catalytic activity in a wide range of oxidation reactions, epoxidations, hydroxylations, ammoximations, etc.10-16. The use of hazardous organic peracids, inorganic acids and other harmful oxidizing agents has been largely replaced by these catalytic reactions12,13 without compromising on the selectivity for the desired products. The presence of Ti(IV) in the tetrahedral geometry in the silica framework has resulted in materials with highly active catalytic center, which can act as good oxidation catalyst16 in the presence of dilute H2O2 as oxidant. On the other hand, titanium-containing nanoporous silicophosphates have been reported which find application as hosts for fast ionic conductors and optical fibers17 and as catalysts18. Nanoporous

titanium phosphate materials19,20 also have many potential utilities. But the stability of the materials is relatively poor due to their highly charged structure. This may be overcome through the incorporation of Si in titanium phosphate based materials similar to the incorporation of Si in AlPO4 (SAPO21 based molecular sieves) and SnPO4

22. Again, purely silicate based porous structures have very high surface areas23 and can act as good adsorbents and catalyst supports24. However, these materials show almost no catalytic activity on their own as they are devoid of any redox center necessary for oxidation reactions. Purely microporous titanium phosphate sample on the other hand do not show a very good surface area, but they show excellent catalytic properties19. We have attempted to prepare nanoporous silicotitanium-phosphate framework, which can introduce catalytic oxidative property in the former and increase the surface area in the latter. Thus a nanoporous material with Si, P and Ti moieties in the oxide framework gives rise to a good catalyst with an improved surface area. Herein, we report for the first time the synthesis of a pure (STP) and a hybrid silicotitaniumphosphate (HSTP) material with microporous structure with the assistance of a neutral structure directing agent, 4,4′-trimethylenedipiperidine (TMDP) under hydrothermal and almost neutral pH in aqueous medium. A comparative study regarding the change in the surface area, catalytic properties, etc. of the materials have been discussed. A series of catalytic reactions have

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been studied with each of these samples using styrene and cyclohexene as the substrates. In both the cases good catalytic property was observed.

Materials and Methods

All the reagents were obtained commercially and used without any further purification. In this study, titanium (IV) isopropoxide (TIPO, 97%, Sigma Aldrich) and orthophosphoric acid (88% aqueous, E-Merck) were used as titanium and phosphorous sources respectively and tetraethyl orthosilicate (TEOS, Sigma Aldrich) and 1,2-bis (triethoxysilyl) ethane (BTEE, 96%, Aldrich) as silica precursors. Neutral surfactant 4,4′-trimethylenedipiperidine (TMDP, 97%, Sigma Aldrich) was used as the structure-directing agent. For catalytic studies, styrene and cyclohexene (both from Sigma Aldrich) were used as substrates, hydrogen peroxide (H2O2, 50%, aqueous, E-Merck) as oxidant and acetonitrile (E-Merck) as solvent. For the syntheses of STP and HSTP, required amount of TMDP was made soluble in 30 ml of water followed by stirring for 1 h. Then the desired amount of H3PO4 was taken in water and added dropwise to the above solution. After the solution became homogeneous, TEOS (in case of the pure material) or BTEE (in case of the hybrid material) was added slowly with continuous stirring and allowed to hydrolyze for one and half hour. Finally, the TIPO solution taken in isopropyl alcohol was added dropwise to the hydrolyzed silica gel under vigorous stirring. The final pH of the synthesised gels was maintained ca. 5.5 and the gels were stirred for another two hours and autoclaved at 413 K for two days under autogenous pressure. The pure silicotitaniumphosphate sample has been designated as sample 1 and the hybrid analogue has been designated as sample 2. The molar ratios of the various constituents of the syntheses gels of samples 1 and 2 have been listed in Table 1. After the hydrothermal treatment the solid products were filtered, washed repeatedly with water and dried in vacuum at low temperature in a lypholyzer. The

powdered samples were characterized by low angle powder X-ray diffraction using a Seifert XRD 3000P diffractometer on which the small and wide-angle goniometers were mounted. The X-ray source was Cu-Kα radiation (α = 0.15406 nm) with voltage and current of 35 kV and 30 mA, respectively. For recording the TEM images a JEOL JEM 2010 transmission electron microscope operated at an accelerated voltage of 200 kV was used. N2 adsorption/desorption isotherm measurements of the template free samples were carried out using a Quantachrome Autosorb 1C instrument at 77 K. Prior to N2 sorption experiment, the samples were degassed at 393 K for 4 h. Thermogravimetric (TG) and differential thermal analysis (DTA) were carried on a TA Instruments thermal analyzer SDT Q-600. UV-Vis diffuse reflectance spectra were obtained in a Shimadzu UV 2401PC using BaSO4 pellet as background standard. Fourier-transformed infrared (FTIR) spectra of these samples were recorded on KBr pellets, using a Nicolet MAGNA-FT IR 750 spectrometer series II. JEOL JEM 6700F Field Emission Scanning Electron Microscope (FE-SEM) with an EDS attachment was used for the determination of morphology and surface chemical composition of the samples. Wet chemical analyses of the samples were performed using a AA-6300 double beam Shimadzu atomic absorption spectrophotometer (AAS). The liquid phase reactions were performed in a magnetically stirred round-bottomed flask fitted with a condenser and placed in a temperature controlled oil bath. Typically, 1 g of the substrate was dissolved in 10 ml acetonitrile and to this 0.2 g of catalyst (20 weight % with respect to the substrate) was added and then the mixture was preheated to 343 K. The reaction started immediately after the addition of hydrogen peroxide (H2O2) to the reaction mixture. Aliquots from the reaction mixtures were collected at regular intervals and after cooling the filtrates the progress of the reactions were analyzed by capillary gas chromatography (Agilent 4890D, FID). The products were identified by known standards.

Table 1 — Composition of the samples, their surface areas, average pore diameters and pore volumes Sample TMDP

(mol) TEOS (mol)

BTEE (mol)

Ti(OBu)4 (mol)

H3PO4 (mol)

BET surface area (m2/g)

Avg. pore dia. a (Å)

STP 0.005 0.01 - 0.01 0.01 61 4.8

HSTP 0.005 - 0.01 0.01 0.01 130 5.0

aCalculated using HK(Horvath-Kawazoe) method.

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Results and Discussion Powder XRD

The powder X-ray diffraction patterns of the as-synthesized and extracted samples are shown in Fig. 1 (a & c for STP; b & d for HSTP respectively). A single intense peak for all the samples was observed in the low angle region. Absence of any high angle peaks except the broad humps characteristic of amorphous materials, suggests the disordered nature of the samples. These results are also in agreement with the TEM images obtained. The major diffraction peak was obtained at 2θ values of 4.62 (d=1.91 nm) and 5.12 (d=1.72 nm) (for STP and its extracted form respectively), 4.55 (d=1.94 nm) and 5.99 (d=1.47 nm) (for HSTP and its extracted form respectively). Electron microscopy (HRTEM and FE-SEM)

High-resolution transmission electron microscopy (HRTEM) image of one of the representative samples, HSTP, has been given in Fig. 2. Disordered wormhole-like micropores are clear throughout the area of investigation. Likewise, the HRTEM image for STP sample was also taken (not shown here). High resolution images for both the samples suggested the existence of micropores having diameter ca. 0.5 nm. Thus, the XRD and HRTEM

results reveal that these samples have disordered array of micropores with diameters of about 0.5 nm. Particle morphologies of STP and HSTP were examined by field emission scanning electron microscopy (FE-SEM) analysis. The FE-SEM images of STP and HSTP are shown in Fig. 3. Uniform spherical particles of about 20-50 nm are clearly seen for these microporous silicotitaniumphosphate materials. As seen from Fig. 3, some of these tiny particles agglomerated to form larger particles. Chemical analysis of the hybrid material from energy dispersive X-ray spectrum (Fig. 4) obtained from SEM analyses at different point of the image suggests almost uniform distribution of the constituent elements, viz., Ti, Si, O and P in the sample. From wet chemical analysis of the STP and HSTP samples using AAS, the amount of titanium loading was found to be 11.2% and 10.1% respectively. N2 sorption

The surface porosity of these samples was obtained from N2 sorption measurement. N2 adsorption/ desorption isotherms of STP and HSTP are shown in Fig. 5a and 5b respectively. Typical microporous materials with other chemical compositions follow similar type I isotherm at low P/P0 regions25. However at high P/P0 values (0.5-0.8), it deviates a little from the conventional type I isotherm to a nearly horizontal plateau, characteristic of type II isotherm. The pore size distribution of the materials was estimated by using Horvath-Kawazoe (HK) method26.

Fig. 1 — High angle PXRD pattern of as-synthesized and extracted samples of STP (a & c) and HSTP (b & d).

Fig. 2 — TEM image of the sample 2.

d

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Fig. 3 — SEM images of the samples. [a, sample 1, and, b, sample 2].

Fig. 4 — EDS data of sample 2.

It is interesting to note that the pore size distribution as shown in the inset of Fig. 5a and 5b, suggests the existence of micropores with peak pore diameter in the range of 0.5 nm. The pore diameter estimated from the N2 sorption data agrees well with the result obtained by the TEM image (ca. 0.5 nm) and PXRD data. The Brunauer-Emmett-Teller (BET) surface areas and the pore diameters of the samples have been given in Table 1. The surface area of the hybrid material (HSTP) is much higher as compared to that of the pure silicotitaniumphosphate (STP) sample. The enhanced surface area of HSTP may be attributed to the greater void space arising due to interparticle crosslinking initiated by the larger organic bridging moieties. Spectral analysis

The UV-visible spectra of the STP and HSTP samples show a very strong absorption band in the 220-320 nm wavelength region27 for both the samples. This may be due to the charge transfer electronic transition from O2- 2p to Ti4+ 3d orbitals. A similar

Fig. 5 — N2 adsorption/desorption isotherms of sample 1 (a) and 2 (b). Adsorption points are marked with filled circles and that for desorption by empty circles. Inset: Pore size distribution employing HK method.

a

b

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high-energy absorption edge due to tetrahedral coordination of Ti has been observed for other titanium containing molecular sieves28,29. The high-energy absorption band indicates that the tetrahedral coordination of Ti is predominant in these materials. The FTIR spectra of the extracted samples of STP (b) and HSTP (d) show the C-H vibrations21 at 2890 and 2835 cm-1 along with the bending vibration at the fingerprint region between 1400 and 800 cm-1 for the hybrid sample, indicating the presence of the organic group (-CH2-CH2-), which remain intact after the removal of TMDP molecules. A broad band in the hydroxyl region with maximum at 3400 cm–1 is observed. This corresponds to the framework Si-O-H, P-O-H and Ti-O-H groups as well as Si(OH)Ti groups interacting with the defect sites and adsorbed water molecules. The framework Ti-O-P vibrations appear at 950-1100 cm–1 for both the extracted samples. Thermal analysis

Thermogravimetric and differential thermal analyses (TGA/DTA) for STP (a) and HSTP (b) samples show high thermal stability for both the samples. It is interesting to note that initially both the samples show a weight loss of ca. 11% only, up to 425 K in the TGA plot, which is mainly due to the water molecules present in the external and internal surfaces of the material. Sharp endotherms corresponding to this weight loss is clear from the DTA plots of both the samples. Beyond this region, the template molecule TMDP gradually starts burning and the process is completed in the temperature region 790-800 K for sample 1. For sample 2, the weight loss is almost 22% in the temperature region 425-800 K, whereas for sample 1 it is only 14%. In the case of the former, in addition to burning of the TMDP molecules, the organic moiety (-CH2-CH2-) present in the framework of HSTP also decomposes. This explains the presence of two exotherms in the

DTA plot of HSTP corresponding to the decomposition of both the template TMDP molecule and the framework organic moieties, whereas for STP only one exotherm is observed corresponding to the decomposition of TMDP molecule. Liquid phase catalysis

Both pure and hybrid silicotitaniumphosphate materials were used as catalysts for the liquid phase partial epoxidation study of cyclohexene and styrene using dilute aqueous H2O2 as oxidant. Epoxidation of the substrates have been done under ambient pressure and at 343 K. The conversion, selectivity and turnover frequency (TOF) of different oxidized products of cyclohexene and styrene are listed in Table 2. It is quite clear from the results that the catalytic activity increases as we go from STP to HSTP. The trend that is observed can be directly correlated with the surface area of these materials; more the surface area more is the catalytic activity. AAS chemical analysis shows that titanium is not leaching out during oxidation reactions, as no titanium was detected in the liquid phase of the reaction mixture after the completion of reaction. After the reaction, both were filtered, washed with water, dried and heated at 373 K for activation. Regenerated catalyst obtained by this method was used for the epoxidation of styrene (as a representative case) for two additional cycles and for both, the conversion and selectivity remained almost same as that for the fresh catalyst (Table 2). These data clearly suggest that this hybrid microporous silicotitaniumphosphate sample is an efficient catalyst for repeated use and that the liquid-phase epoxidation reaction over microporous silicotitaniumphosphate materials is purely catalytic in nature. The active Ti sites are present in both STP and HSTP material. Thus, the activity of the catalyst is due to the Ti active centers as observed earlier over other Ti-containing molecular sieves. In the presence of dilute aqueous

Table 2 — Epoxidation of cyclohexene by various samples after 24 hrs

Samples Substrate Conversion (%)

Selectivity of epoxide (%)

Other oxidized products (%)

Turn over frequency (TOF)

1 Cyclohexene 78.8 29.7 70.3 0.87

2 Cyclohexene 89.9 44.7 55.3 1.08

1 Styrene 91.5 35.5 64.5 0.79

2 Styrene 95.5 39.8 60.2 0.91

2* Styrene 94.2 39.0 61.0 0.89

Reaction temp.: 343 K; Solvent: Acetonitrile (12 ml); Substrate: 1 g; Catalyst: 20% of the substrate; H2O2: 1:1 with respect to the substrate (in moles). *Regenerated catalyst (HSTP after two cycles).

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H2O2, titanium hydroperoxide30 species was formed on the microporous STP and HSTP surfaces. This could effectively catalyze the liquid phase epoxidation of cyclohexene and styrene. Based on the above results, we propose a model for the Ti, Si, P, C and O connectivity in the framework of the microporous hybrid silicotitaniumphosphate (HSTP) material (Fig. 6). Ti, Si and P have regular alternating tetrahedral arrangements. Spectroscopic results suggested this framework bonding in STP/HSTP materials.

Conclusions Microporous silicotitaniumphosphate and its organic-inorganic hybrid analogue have been synthesized under hydrothermal condition with the assistance of TMDP as structure directing agent under neutral pH. PXRD, TEM and N2 adsorption/ desorption results reveal the disordered microporous structure having pore diameter of ca. 0.5 nm. The spectroscopic results suggest that most of the titanium sites present in these samples are tetrahedrally coordinated Ti (IV). These silicotitaniumphosphate materials show good catalytic activity and selectivity in the liquid phase partial oxidation reaction of the substrates cyclohexene and styrene to their respective epoxides.

Acknowledgement KS and MN thank CSIR, New Delhi, for senior research fellowships. AB wishes to thank DST, New Delhi, for a Ramanna Fellowship grant. This work was partly funded by the Nano Science and Technology Initiative of DST.

References 1 Kresge C T, Leonowicz Roth W J, Vartuli J C & Beck J S,

Nature, 359 (1992) 710. 2 Kimura T, Tamura H, Tezuka M, Mochizuki D, Shigeno T,

Ohsuna T & Kuroda K, J Am Chem Soc, 130 (2008) 201. 3 Yu D, Qian J, Xue N, Zhang D, Wang C, Guo X, Ding W &

Chen Y, Langmuir, 23 (2007) 382. 4 Bhaumik A & Inagaki S, J Am Chem Soc, 123 (2001) 691. 5 Zhou X, Qiao S, Hao N, Wang X, Yu C, Wang L, Zhao D &

Lu G Q, Chem Mater, 19 (2007) 1870. 6 Vasylyev M & Neumann R, Chem Mater, 18 (2006) 2781. 7 Xu X, Han Y, Zhao L, Yu Y, Li D, Ding H, Li N, Guo Y &

Xiao F-S, Chem Mater, 15 (2003) 74. 8 Lee M, Park M-H, Oh N-K, Zin W-C, Jung H-T & Yoon D

K, Angew Chem Int Ed, 43 (2004) 6466. 9 Ryoo R, Joo S H, Kruk M & Jaroniec M, Adv Mater, 13

(2001) 677. 10 Goa Y, Wu P & Tatsumi T, J Phys Chem B, 108 (2004)

8401. 11 Notari B, Catal Today, 18 (1993) 163. 12 Corma A, Chem Rev, 97 (1997) 2373. 13 Bhaumik A & Kumar R, J Chem Soc Chem Commun, (1995)

349. 14 Kholdeeva O A, Melgunov M S, Shmakov A N, Trukhan N

N, Kriventsov V V, Zaikovskii V I & Malyshev M E, Catal

Today, 91 (2004) 205. 15 Taramaso M, Perego G & Notari B, US Pat, 4410501 (1983). 16 Huybrechts D R C, DeBruycker L & Jacobs P A, Nature, 345

(1990) 240. 17 Szu S P, Klein L C & Greenblatt M, J Non-Cryst Solids, 143

(1992) 21. 18 Oganowski W, Polish Pat P L, 159643 (1992). 19 Bhaumik A & Inagaki S, US Pat U S, 6660686 (2001). 20 Pan C, Yuan S & Zhang W, Applied Catalysis A: General,

312 (2006) 186. 21 Sarkar K, Laha S C & Bhaumik A, J Mater Chem, 16 (2006)

2439. 22 Chandra D, Mal N K & Bhaumik A, J Mol Cat A: Chemical,

247 (2006) 216. 23 Corma A & Garcia H, Chem Rev, 103 (2003) 4307. 24 Holland B T, Isbester P K, Blanford C F, Munson E J &

Stein A, J Am Chem Soc, 119 (1997) 6796. 25 Gregg S J & Sing K S W, Adsorption, Surface Area and

Porosity, (Academic Press, London) 1982. 26 Maffei A V, Budd P M & McKeown N B, Langmuir, 22

(2006) 4225. 27 Bhaumik A, Proc Indian Acad Sci, 114 (2002) 451. 28 Schofield L J, Kerton O J, McMorn P, Bethell D, Ellwood S

& Hutchings G J, J Chem Soc Perkin Trans, 2 (2002) 2064. 29 Kapoor M P, Bhaumik A, Inagaki S, Kuraoka K & Yazawa

T, J Mater Chem, (2002) 3078. 30 Bhaumik A, Kumar P & Kumar R, Catal Lett, 40 (1996) 47.

Fig. 6 — Proposed framework structure of the hybrid silicotitaniumphosphate (Sample 2).