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A neutral templating route to mesoporous titanium phosphate molecular sieves with enhanced thermal stability
Applied Catalysis A: General 312 (2006) 186–193 www.elsevier.com/locate/apcata
A neutral templating route to mesoporous titanium phosphate molecular sieves with enhanced thermal stability Chunliu Pan a,b,*, Shi Yuan a, Wenxiang Zhang c b
a Department of Physics, University of Miami, Florida 33146, USA Department of Chemistry and Biochemistry, Florida Atlantic University, Boca Raton 33431, USA c Institute of Chemistry, Jilin University, Changchun 130023, China
Received 15 June 2005; received in revised form 5 June 2006; accepted 28 June 2006 Available online 7 August 2006
Abstract For the first time the mesoporous titanium phosphates with high surface area and highly thermal stability were synthesized using long-chain nalkylamine as structure-directing agents under an ethanol system containing a small quantity of water. Powder X-ray diffraction (XRD) and transmission electron micrograph (TEM) show that the disorder mesopores exist in the mesostructured materials. The mesoporous titanium phosphates calcinated at 500 8C for 10 h still retain the refined pore structure and the high surface area. UV–vis absorption suggests most of titanium is tetrahedrally coordinated in the framework of calcinated mesoporous titanium phosphates. The high catalytic activity in the liquidphase partial oxidation of cyclohexene with H2O2 oxidant supports the tetrahedral coordination of titanium in these materials. # 2006 Elsevier B.V. All rights reserved. Keywords: Mesoporous; Titanium phosphates; Thermal stability; Cyclohexene
1. Introduction With the discovery of ordered mesostructured silica phases using supramolecular templating assembly in 1992 by Mobil Oil scientists [1,2], considerable synthesis effort has been devoted to this area during the past few years due to their large internal surface areas, narrow pore size distribution and potential application as catalysts, absorbents, and host materials [3]. Some mesostructured metal oxides such as zirconium oxide [4,5], niobium and tantalum mixed oxide [6], titania [7], tungsten oxide [8] and ceria [9] have been prepared in recent years using cationic, anionic or nonionic surfactants. Researches about nanoporous metal phosphates have led to the discovery of mesoporous aluminophosphates [10–13], nickel(II) phosphate [14], iron phosphate [15], tin phosphate [16], zirconium phosphates [17] and titanium phosphates [18,19]. Among them, mesoporous titanium phosphates have been found to show remarkable catalytic activity in selective oxidation in the presence of oxidant (H2O2) due to the presence of tetrahedrally coordinated titanium [19]. Zhao et al. reported the low * Corresponding author. E-mail address: [email protected] (C. Pan). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.06.047
temperature (at 80 8C) synthesis of mesoporous titaniumcontaining aluminophosphates with hexagonal, cubic and lamellar structure similar to MCM-41, MCM-48 and MCM-50 [20]. Kapoor et al. also obtained hexagonal mesoporous titanium aluminophosphate molecular sieves using fluoride route [21]. Although mesoporous titanium phosphates have been synthesized using cationic and anionic surfactants as templates, there are no reports about long-chain n-alkylamine surfactants as structure-directing agents to synthesize mesoporous titanium phosphate. Although mesoporous titanium phosphates using cationic surfactants reported by Jones et al. are calcinated at 540 8C for 4 h, a few surfactants still exist and can be seen via infrared spectra [18]. While the mesoporous titanium phosphate materials using both cationic and anionic surfactants reported by Bhaumik and Inagaki have high catalytic activity in the liquid-phase partial oxidation, their uses are limited by the thermal unstability (thermally stable only up to 300 8C) [19]. In this paper, we give a first report on the synthesis of thermally stable mesoporous titanium phosphate under basically nonaqueous conditions with low quantities of water present in the reaction mixture; it is also reported for the first time that longchain n-alkylamine as structure-directing agents leads to porous mesostructured titanium phosphate materials.
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2. Experimental
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vis diffuse reflectance spectra were obtained with a Shimadzu UV-365.
2.1. Materials 3. Results and discussion n-Alkylamine surfactants CH3(CH2)nNH2 with n = 11, 15 and 17 were provided by Boxing Grease Co. Titanium isopropoxide (Ti(OiC3H7)4, Ti(OiPr)4) was obtained from Shandong Chemical Co. Phosphoric acid (85% H3PO4) was obtained from Changchun Chemical Co. Ethanol (>99.7%) was purchased from Beijing Chemical Co. All chemicals were used as received. 2.2. Synthesis In a typical synthesis, 11.5 g titanium isopropoxide is added to a solution of 4.6 g phosphoric acid in 50 g ethanol and 1.4 g water. The mixture is stirred vigorously for 2 h at room temperature. A solution of CH3(CH2)nNH2 in 10 g ethanol is then added (corresponding to an approximate molar ratio in the synthetic mixture of Ti/P/surfactant/ethanol/water = 1/1/0.5/ 32/3). After being stirred at room temperature for another 2 h, the slurry is transferred into a Teflon-lined autoclave and kept at 90 8C for 48 h without agitation. The product is filtered off, washed with ethanol and dried at 90 8C for 12 h. Due to the unstability of as-synthesized mesostructured titanium phosphates, a postsynthetic treatment is employed to strengthen their stability as reported by Tiemann [10]. In a postsynthetic treatment, 6 g of the dry sample is stored in a closed container over 15 g water. The container is sealed and stored at 90 8C for 12 h without agitation. By this procedure the sample is exposed to water vapor, but is not in contact with liquid water. For extraction of the surfactant, 5 g of sample (after the postsynthetic treatment) is dispersed in a solution of 20 g ethanol and 5 g of aqueous hydrochloric acid (1 mol/l). The mixture is stirred at room temperature for 15 min, filtered off and washed with ethanol. The process is repeated for five times. After then, the samples are calcinated at a speed of 1 8C/min until to the temperature 500, 540 and 580 8C in air and kept at the temperature for 10 h, respectively.
Titanium phosphates with disordered mesostructure can be synthesized using long-chain primary alkylamines as structuredirecting agents under alcoholic conditions and a small quantity of water. Fig. 1(a) shows the typical XRD patterns of assynthesized samples prepared with C12-NH2, C16-NH2 and C18NH2, respectively. A relatively broad reflection at low diffraction angle can be seen in each pattern and there are no distinctive higher order peaks. Analogous single peak patterns corresponding to large d-spacings have been observed previously for disordered HMS [22] and MSU-X silicas [23], synthesized by a neutral assembly pathway using nonionic surfactant. This is also the case for a mesoporous alumina material, synthesized by reacting tri-sec-butoxyaluminum with nonionic polyethylene oxide surfactants [24], KIT-1, prepared via an electrostatic templating route in the presence of ethylenediaminetetraacetic acid tetrasodium salt (EDTANa4) [25], and also a mesostructured/mesoporous aluminophosphate using n-alkylamine
2.3. Characterization XRD patterns were recorded on a SHIMADZU transmission diffractometer with Cu Ka radiation. The scattering data were collected in a continuous scan mode from 1.48 to 108 (2u) at a scanning rate of 18/min. N2 sorption isotherms were recorded using a Micromeritics ASAP 2010 Sorptometer. The samples were pretreated at 250 8C for 2 h. Specific surface areas were calculated by the BET equation; pore size distributions were calculated by the BJH formula from the adsorption isotherm. The TEM image was taken on a Hitachi H-8100 electron microscope. FTIR spectra of as-synthesized and surfactant-free samples were recorded on a Nicolet Impact 410 FTIR spectrometer. Samples were prepared by the KBr disk technique. For 31P MAS NMR measurements, a Varian Infinity plus 400 spectrometer was used. The chemical shifts on 31P MAS NMR were referenced to H3PO4 at 0 ppm. UV–
Fig. 1. XRD patterns of mesostructured titanium phosphates prepared with C12NH2, C16-NH2 and C18-NH2, respectively: (a) as-synthesized; (b) after thermal treatment; (c) after extraction of the surfactant.
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surfactants as structure-directing agents [10]. The pore structure of these materials is believed to be a three-dimensional, disordered network of short, wormlike channels, while the channel widths are uniform. With the carbon chain length increasing from dodecyl-, hexadecyl-, to octadecylamine, the dspacings of the three products corresponding to XRD diffraction angles vary from 2.6, 2.9 to 3.1 nm. The postsynthetic thermal treatment is an effective and key means to improve the thermal stability of the samples as well as the degree of structural order, since mesopores collapse after direct calcinations or extraction by acidified ethanol. Fig. 1(b) shows the powder XRD patterns of the samples after thermal treatment. The intensity of all the reflecting peaks has increased, which is probably due to the farther framework condensation to improve the stability as well as the structural order. The location of the reflection is shifted to a lower diffraction angle corresponding to a larger d-spacing. The increase in the d-spacing may be interpreted by a growth of the inorganic walls due to the additional condensation of the inorganic framework, or the expanding of the surfactant, or the action of both of them. Solvent extraction is a good alternative to eliminate the organic part while preserving the mesoporosity of the materials [23]. After the postsynthetic thermal treatment, the samples are stable enough to retain their mesostructures and most of surfactants can be removed by the extraction of acidified ethanol. The powder XRD patterns are shown in Fig. 1(c). After extraction, the low-angle XRD reflections still exist, but the dspacings are higher than those both as-synthesized and thermal treatments, which is unusual. In addition, the samples using C16NH2 and C18-NH2 as structure-directing agents exhibit the characteristic, narrower and more highly intense low-angle XRD reflection; whereas the sample using C12-NH2 as templates shows a broader and slightly less intense one. We can assume that more regular mesopores are remained in the samples using the templates with longer carbon chain after extraction. Thus, the removal of the template by solvent extraction tends to preserve the crystallinity for the samples using C16-NH2 and C18-NH2 as templates. The partial mesophase in samples with C12-NH2 as template collapses after extraction by HCl/ethanol. Analogous changes that the larger intensities of the XRD reflection for the calcined forms than for the as-synthesized materials can also be observed for HMS silicas and MSU-x aluminas [24–26]. Recent modeling studies by Mahanti [27] and Tanev and Pinnavaia [28] have showed that the removal of the occluded organic template from hexagonal mesotructures enhances the Bragg scattering cross-section. The lower XRD reflection of the production from using the C12-NH2 surfactant is observed after extraction by acidic ethanol, indicating that the regularity of the wormlike pore decreases. After extracted by HCl/ethanol, there also remain a few surfactants or a fraction of surfactants (as can be seen in Fig. 5). So the samples were calcined at 500 8C for 10 h in air after extraction by acidified ethanol to eliminate the remaining structure-directing agents. The XRD patterns of the calcined samples prepared with different n-alkylamines are shown in Fig. 2. The single diffraction peaks are still preserved, which
indicates that the mesopore of materials is retained even after calcination temperature highly up to 500 8C. With the increase in carbon chain length of alkylamines, the narrower XRD diffraction can be observed, which indicates that samples using the surfactants with longer carbon chain as the structure-directing agents own more regular mesopore in nature. In addition, calcinations at 500 8C lead to lower Bragg degrees corresponding to higher d values, comparing with the products after extraction by acidic ethanol. For most mesostructured metal phosphates which have hexagonal or wormlike structures, calcinations to remove the template will reduce the order of mesopores and result in lower d-spacings in XRD diffractions [10–13,29]. Fig. 3 shows the XRD patterns of the mesoporous titanium phosphates which are prepared with C18-NH2 after extraction and calcinations for 10 h in air at three different temperatures. With the increase of calcination temperature, the d-spacings decrease from 5.2 to 4.05 nm at temperature up to 580 8C. Furthermore, the width of the diffraction peak becomes wider and the intensity decreases, suggesting a little reduction in the mesoporous order of the structure. The intense diffraction peak of sample calcinated at 580 8C still exists, indicating that most of mesoporous structures were preserved. Compared with those reported by Bhaumik and Inagaki [19] synthesized using cationic and anionic surfactants as templates (thermally stable only up to 300 8C) and reported by Jones et al. [18] with cationic surfactants (thermally stable up to 540 8C), the
Fig. 2. XRD patterns of mesoporous titanium phosphates after extraction and then calcinated at 500 8C for 10 h in air.
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Fig. 3. XRD patterns of mesoporous titanium phosphates prepared with C18-NH2 after extraction and then calcinated for 10 h in air, respectively: (a) 500 8C; (b) 540 8C; (c) 580 8C.
mesoporous titanium phosphates synthesized in this paper have higher thermal stabilities after the thermal treatment. The neutral templates such as alkylamines [10] and PEObased surfactants [30,31] are known to adopt spherical to long
Fig. 4. TEM image of mesoporous titanium phosphate using C18-NH2 as structure-directing agents after extraction and then calcinated at 500 8C for 10 h.
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‘‘wormlike’’ micellar structures. The pore structures of the titanium phosphate calcined at 500 8C using C18-NH2 as structure-directing agents reflect the wormlike motif of the micellar structure, as evidenced by the representative transmission electron micrograph (TEM), as can been seen in Fig. 4, which shows a large disorder in the mesopore shape. This result suggests that the wormlike mesopore is still holding, which is consistent with the XRD patterns (Fig. 2). Fig. 5 shows the IR spectra of the sample using C16-NH2 surfactant as templates, the same material after thermal treatment, HCl/ethanol extraction and calcinations. In Fig. 5(a) and (b), the sharp bands at 2930 and 2850 cm 1 correspond to asymmetric and symmetric C–H stretching, respectively. Bands at 1464 and 1375 cm 1 can be assigned to C–H deformation vibrations. The absorption at 720 cm 1 comes from skeletal vibrations in the hydrocarbon chains in the surfactant [10,32,33]. The absorption band at 1517 cm 1 is assigned to NH3+ deformation vibration. The broad band centered at 3100 cm 1 is the corporate action from both NH3+ and C–H stretching vibration [34,35]. From the results, it can be indicated that the neutral C16-NH2 exhibits as C16-NH3+ in the as-synthesized and thermal treatment samples. Broad bands in the hydroxyl region with maximum at 3400 cm 1 correspond to O–H stretching vibrations of the residual water, OH and defective OH [9]. The characteristic band of H2O at 1635 cm 1 also can be observed. Strong spectra in the region between 1000 and 1050 cm 1 centered at 1010 cm 1 are from Ti–O–P skeletal stretching vibrations [19]. The spectra range in 400–790 cm 1 are TO4 either bending or stretching vibrations. After extraction by HCl/ ethanol solution, a spot characteristic C–H vibration band at 2930 and 2850 cm 1 still can be observed, which indicates a few surfactants or fragments are still remained in the samples (Fig. 5(c)). The surfactant can be removed completely after calcinations, indicated by the entirely disappeared characteristic C–H vibrations (Fig. 5(d)). The similar IR absorptions also can
Fig. 5. IR spectra of mesostructured titanium phosphate prepared with C16NH2: (a) as-synthesized; (b) after thermal treatment; (c) after extraction of the surfactant; (d) calcinated at 500 8C for 10 h in air.
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be observed in the samples using C12-NH2 and C18-NH2 as templates. The porous properties of all the mesoporous titanium phosphates using n-alkylamines with different carbon chain as
templates were characterized based on their N2 adsorption behavior. Prior to the adsorption measurement, samples were heated at 250 8C for 2 h in a vacuum to remove adsorbed water. Fig. 6 shows typical N2-sorption isotherms of the calcined
Fig. 6. Nitrogen adsorption–desorption isotherms (a, c and e) and BJH pore size distribution (b, d and f) at 77 K of mesoporous titanium phosphate calcinated at 500 8C for 10 h using C12-NH2 (a, b), C16-NH2 (c, d) and C18-NH2 (e, f) as structure-directing agents.
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Table 1 Physical properties of various mesoporous titanium phosphates using C12-NH2, C16-NH2 and C18-NH2 as structure-directing agents calcinated at 500 8C for 10 h in air Sample name
Ti/Pa
SSA (m2/g)
Pore diameter (nm)
Pore volume (cm3/g)
1 2 3
1.18 1.19 1.19
359 430 497
1.7 1.9 3.3
0.26 0.28 0.63
1: C12-NH2 used as structure-directing agents; 2: C16-NH2 used as structuredirecting agents; 3: C18-NH2 used as structure-directing agents. a Ti/P molar ratio obtained from ICP measurements.
samples. The type IV isotherms clearly illustrate the mesoporous nature of the materials. The far narrower pore size distribution curves can be observed on the materials using C18-NH2 as template than the C12-NH2 and C16-NH2, and the micropore seems to exit in the latter two materials after calcinations at 500 8C for 10 h. This result indicates that extraction and then calcinations at 500 8C to remove the templates will result in collapse of the partial mesophase using C12-NH2 and C16-NH2, which is consistent with the XRD results. The textural properties of samples are summarized in Table 1. The specific surface areas of the mesoporous titanium phosphates calcined at 500 8C increase with the carbon chain length of the surfactants. Compared with the mesoporous titanium phosphate calcinated at 540 8C for 4 h using trimethylammonium surfactants as template reported by Jones et al. (surface area of only 260 m2/g) [19], the mesoporous titanium phosphates obtained by us have larger surface areas. With the increase in carbon number of the surfactants, pore diameters of samples calculated on the basis of BJH analysis from desorption branch of the isotherms have their maximum at range from 1.7, 1.9 to 3.3 nm, respectively, as shown in Fig. 6. Due to the disordered porous framework, the corresponding pore walls thickness of the samples were calculated simply about 2.65, 2.6 and 1.9 nm, respectively, a bit larger than or similar to those of siliceous MCM-41 materials and Ti-MCM41 (1.4–2.2 nm), but much smaller than Ti-SBA-15 (6.4 nm) [36–38]. The pore volumes of samples calculated from corresponding desorption curve of the isotherms also increase with the length of carbon chain of surfactants. The local environments of P atom of the mesoporous titanium phosphates are studied using the MAS NMR technique. Fig. 7 shows the 31P MAS NMR spectra of the typical sample using C18-NH2 as structure-directing agents after extraction and calcinations at 500 8C. There exists a single broad resonance between 5 and 40 ppm centered at ca. 25.1 ppm, as expected for phosphorus in a large range of environments. The signal seems to consist of several resonances which are badly resolved from each other. Studies of titanium phosphates of known crystal structure have shown a correlation between connectivity and chemical shift. As the connectivity increases, an up-field shift is observed from 5.3 to 10.6 ppm for H2PO4, to 18.1 ppm for HPO4 and finally to 19 to 32.5 ppm for PO4 [39–41]. A comparison of the position of the signals obtained (Fig. 7) with the above reported data
Fig. 7. 31P MAS NMR spectra of the mesoporous titanium phosphate calcinated at 500 8C for 10 h in air.
suggests that these resonances may be attributed to four-fold coordinated P with O–Ti and various amounts of H2O or OH groups. The UV–visible spectra of the mesoporous titanium phosphate materials after calcinations at 500 8C are shown in Fig. 8. The calcined materials using different carbon chain as templates show similar and very strong absorption bands in the 200–350 nm wavelength region, centered at 220–240 nm together with a shoulder band at 280 nm. The band at 220– 240 nm was attributed to ligand-to-metal charge transfer associated with isolated Ti(IV) framework sites in tetrahedral coordination [19,42]. The shoulder at 280 nm can be attributed to framework-linked Ti atoms with octahedral symmetry or to partially polymerized Ti species, which contain Ti–O–Ti bonds
Fig. 8. UV–vis spectra of mesoporous titanium phosphates using: (a) C12-NH2; (b) C16-NH2; (c) C18-NH2 as structure-directing agents after extraction and then calcinated at 500 8C for 10 h in air.
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Table 2 Oxidation of cyclohexene over various Ti-containing catalysts Samples
a
TiP TS-1c Ti-MMSd Ti-MCM-41e a b c d e
Cyclohexene conversion (%)
99 0.7 20.6 52.9
Selectivity (%) Epoxide
1-ol
1-one
Mono-ethers
Diols, diethers
0 36.4 2.3 0
20.2 0 5.8 3.6
0 0 15.3 14.3
0 0 38.3 23.0
79.8b 63.6 38.3 59.1
Mesoporous titanium phosphate using C18-NH2 as template; reaction conditions: 0.1 g catalyst, cyclohexene:H2O2 = 1:1, 10 h, solvent methanol, 333 K for 10 h. The selectivity includes 1-methyl-1,2-cyclohexanediol (66.4%) and 1,2-cyclohexanediol (13.4%). Reaction conditions: 0.05 g catalyst, cyclohexene:H2O2 = 2:1, solvent methanol, 333 K for 2 h. See reference [51]. Reaction conditions: 0.2 g catalyst, cyclohexene:H2O2 = 3.3, solvent methanol, 333 K for 3 h. See reference [52]. Reaction conditions: 0.1 g catalyst, cyclohexene:H2O2 = 1:1, solvent methanol, 313 K, 6 h. See reference [53].
[43–45]. That the Ti/P ratios of mesoporous titanium phosphate materials in Table 1 are a little bit larger than 1 also indicates that some Ti–O–Ti bonds exist. A similar high-energy absorption edge due to tetrahedral coordination has been observed for mesoporous titanosilicate (300–330 nm) [46], titanium-containing amorphous silica (325–350 nm) [47], and mesoporous titanium phosphates (220–330 nm) [19]. The absence of a band at 370–410 nm in the UV–vis spectra rules out the presence of free TiO2 [48,49]. The microporous titanium silicate materials, with tetrahedral titanium, show strong absorption bands at 200–220 nm. The high-energy absorption bands are attributable to the low content of titanium in TS materials, otherwise the mesoporous titanium phosphates reported here having a high content of titanium. An increase in content of titanium always leads to a higher wavelength [47]. In addition, the red shift also depends strongly on the nature of the ligands and hydration state [42]. The local environment of titanium attached with –O–P in mesoporous titanium phosphate materials instead of –O–Si in microporous titanium silicates may be also responsible for this shift of absorption band. Since Ti–OP and Ti–OH should have different electronic transitions as the oxygen electron density in a phosphor–oxygen bridge is lower than that of OH groups, the unavoidable presence of Ti– OH may also account for the red shift [50]. Mesoporous titanium phosphate (after extraction and calcinations using C18-NH2 as template) is chosen for the liquid-phase oxidation of cyclohexene with H2O2 (30% aqueous) oxidant under mild conditions (333 K). In a typical reaction (0.1 g catalyst, substrate:H2O2 = 1:1, 10 h reaction time, solvent methanol), 99 mol% cyclohexene was converted into 1-methyl-1,2-cyclohexanediol (66.4% selectivity), 1,2cyclohexanediol (13.4% selectivity) and 2-cyclohexene-1-ol (20.2% selectivity). The oxidation reactions of cyclohexene using different catalysts are summarized in Table 2. Compared with TS-1, Ti-MMS and Ti-MCM-41, mesoporous titanium phosphate shows the highest catalytic activity, due to its larger pore and more accessible titanium in tetrahedral coordination [53]. The tetrahedral framework titanium in mesoporous titanium phosphate is the effectively active site for such liquid-phase oxidation reactions in the presence of H2O2. In course of the oxidation reaction, the titanium hydroperoxo species can be generated between the tetrahedral titanium and H2O2 [54]. The formation of 1-methyl-1,2-cyclohexanediol,
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A neutral templating route to mesoporous titanium phosphate molecular sieves with enhanced thermal stability Chunliu Pan a,b,*, Shi Yuan a, Wenxiang Zhang c b
a Department of Physics, University of Miami, Florida 33146, USA Department of Chemistry and Biochemistry, Florida Atlantic University, Boca Raton 33431, USA c Institute of Chemistry, Jilin University, Changchun 130023, China
Received 15 June 2005; received in revised form 5 June 2006; accepted 28 June 2006 Available online 7 August 2006
Abstract For the first time the mesoporous titanium phosphates with high surface area and highly thermal stability were synthesized using long-chain nalkylamine as structure-directing agents under an ethanol system containing a small quantity of water. Powder X-ray diffraction (XRD) and transmission electron micrograph (TEM) show that the disorder mesopores exist in the mesostructured materials. The mesoporous titanium phosphates calcinated at 500 8C for 10 h still retain the refined pore structure and the high surface area. UV–vis absorption suggests most of titanium is tetrahedrally coordinated in the framework of calcinated mesoporous titanium phosphates. The high catalytic activity in the liquidphase partial oxidation of cyclohexene with H2O2 oxidant supports the tetrahedral coordination of titanium in these materials. # 2006 Elsevier B.V. All rights reserved. Keywords: Mesoporous; Titanium phosphates; Thermal stability; Cyclohexene
1. Introduction With the discovery of ordered mesostructured silica phases using supramolecular templating assembly in 1992 by Mobil Oil scientists [1,2], considerable synthesis effort has been devoted to this area during the past few years due to their large internal surface areas, narrow pore size distribution and potential application as catalysts, absorbents, and host materials [3]. Some mesostructured metal oxides such as zirconium oxide [4,5], niobium and tantalum mixed oxide [6], titania [7], tungsten oxide [8] and ceria [9] have been prepared in recent years using cationic, anionic or nonionic surfactants. Researches about nanoporous metal phosphates have led to the discovery of mesoporous aluminophosphates [10–13], nickel(II) phosphate [14], iron phosphate [15], tin phosphate [16], zirconium phosphates [17] and titanium phosphates [18,19]. Among them, mesoporous titanium phosphates have been found to show remarkable catalytic activity in selective oxidation in the presence of oxidant (H2O2) due to the presence of tetrahedrally coordinated titanium [19]. Zhao et al. reported the low * Corresponding author. E-mail address: [email protected] (C. Pan). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.06.047
temperature (at 80 8C) synthesis of mesoporous titaniumcontaining aluminophosphates with hexagonal, cubic and lamellar structure similar to MCM-41, MCM-48 and MCM-50 [20]. Kapoor et al. also obtained hexagonal mesoporous titanium aluminophosphate molecular sieves using fluoride route [21]. Although mesoporous titanium phosphates have been synthesized using cationic and anionic surfactants as templates, there are no reports about long-chain n-alkylamine surfactants as structure-directing agents to synthesize mesoporous titanium phosphate. Although mesoporous titanium phosphates using cationic surfactants reported by Jones et al. are calcinated at 540 8C for 4 h, a few surfactants still exist and can be seen via infrared spectra [18]. While the mesoporous titanium phosphate materials using both cationic and anionic surfactants reported by Bhaumik and Inagaki have high catalytic activity in the liquid-phase partial oxidation, their uses are limited by the thermal unstability (thermally stable only up to 300 8C) [19]. In this paper, we give a first report on the synthesis of thermally stable mesoporous titanium phosphate under basically nonaqueous conditions with low quantities of water present in the reaction mixture; it is also reported for the first time that longchain n-alkylamine as structure-directing agents leads to porous mesostructured titanium phosphate materials.
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2. Experimental
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vis diffuse reflectance spectra were obtained with a Shimadzu UV-365.
2.1. Materials 3. Results and discussion n-Alkylamine surfactants CH3(CH2)nNH2 with n = 11, 15 and 17 were provided by Boxing Grease Co. Titanium isopropoxide (Ti(OiC3H7)4, Ti(OiPr)4) was obtained from Shandong Chemical Co. Phosphoric acid (85% H3PO4) was obtained from Changchun Chemical Co. Ethanol (>99.7%) was purchased from Beijing Chemical Co. All chemicals were used as received. 2.2. Synthesis In a typical synthesis, 11.5 g titanium isopropoxide is added to a solution of 4.6 g phosphoric acid in 50 g ethanol and 1.4 g water. The mixture is stirred vigorously for 2 h at room temperature. A solution of CH3(CH2)nNH2 in 10 g ethanol is then added (corresponding to an approximate molar ratio in the synthetic mixture of Ti/P/surfactant/ethanol/water = 1/1/0.5/ 32/3). After being stirred at room temperature for another 2 h, the slurry is transferred into a Teflon-lined autoclave and kept at 90 8C for 48 h without agitation. The product is filtered off, washed with ethanol and dried at 90 8C for 12 h. Due to the unstability of as-synthesized mesostructured titanium phosphates, a postsynthetic treatment is employed to strengthen their stability as reported by Tiemann [10]. In a postsynthetic treatment, 6 g of the dry sample is stored in a closed container over 15 g water. The container is sealed and stored at 90 8C for 12 h without agitation. By this procedure the sample is exposed to water vapor, but is not in contact with liquid water. For extraction of the surfactant, 5 g of sample (after the postsynthetic treatment) is dispersed in a solution of 20 g ethanol and 5 g of aqueous hydrochloric acid (1 mol/l). The mixture is stirred at room temperature for 15 min, filtered off and washed with ethanol. The process is repeated for five times. After then, the samples are calcinated at a speed of 1 8C/min until to the temperature 500, 540 and 580 8C in air and kept at the temperature for 10 h, respectively.
Titanium phosphates with disordered mesostructure can be synthesized using long-chain primary alkylamines as structuredirecting agents under alcoholic conditions and a small quantity of water. Fig. 1(a) shows the typical XRD patterns of assynthesized samples prepared with C12-NH2, C16-NH2 and C18NH2, respectively. A relatively broad reflection at low diffraction angle can be seen in each pattern and there are no distinctive higher order peaks. Analogous single peak patterns corresponding to large d-spacings have been observed previously for disordered HMS [22] and MSU-X silicas [23], synthesized by a neutral assembly pathway using nonionic surfactant. This is also the case for a mesoporous alumina material, synthesized by reacting tri-sec-butoxyaluminum with nonionic polyethylene oxide surfactants [24], KIT-1, prepared via an electrostatic templating route in the presence of ethylenediaminetetraacetic acid tetrasodium salt (EDTANa4) [25], and also a mesostructured/mesoporous aluminophosphate using n-alkylamine
2.3. Characterization XRD patterns were recorded on a SHIMADZU transmission diffractometer with Cu Ka radiation. The scattering data were collected in a continuous scan mode from 1.48 to 108 (2u) at a scanning rate of 18/min. N2 sorption isotherms were recorded using a Micromeritics ASAP 2010 Sorptometer. The samples were pretreated at 250 8C for 2 h. Specific surface areas were calculated by the BET equation; pore size distributions were calculated by the BJH formula from the adsorption isotherm. The TEM image was taken on a Hitachi H-8100 electron microscope. FTIR spectra of as-synthesized and surfactant-free samples were recorded on a Nicolet Impact 410 FTIR spectrometer. Samples were prepared by the KBr disk technique. For 31P MAS NMR measurements, a Varian Infinity plus 400 spectrometer was used. The chemical shifts on 31P MAS NMR were referenced to H3PO4 at 0 ppm. UV–
Fig. 1. XRD patterns of mesostructured titanium phosphates prepared with C12NH2, C16-NH2 and C18-NH2, respectively: (a) as-synthesized; (b) after thermal treatment; (c) after extraction of the surfactant.
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surfactants as structure-directing agents [10]. The pore structure of these materials is believed to be a three-dimensional, disordered network of short, wormlike channels, while the channel widths are uniform. With the carbon chain length increasing from dodecyl-, hexadecyl-, to octadecylamine, the dspacings of the three products corresponding to XRD diffraction angles vary from 2.6, 2.9 to 3.1 nm. The postsynthetic thermal treatment is an effective and key means to improve the thermal stability of the samples as well as the degree of structural order, since mesopores collapse after direct calcinations or extraction by acidified ethanol. Fig. 1(b) shows the powder XRD patterns of the samples after thermal treatment. The intensity of all the reflecting peaks has increased, which is probably due to the farther framework condensation to improve the stability as well as the structural order. The location of the reflection is shifted to a lower diffraction angle corresponding to a larger d-spacing. The increase in the d-spacing may be interpreted by a growth of the inorganic walls due to the additional condensation of the inorganic framework, or the expanding of the surfactant, or the action of both of them. Solvent extraction is a good alternative to eliminate the organic part while preserving the mesoporosity of the materials [23]. After the postsynthetic thermal treatment, the samples are stable enough to retain their mesostructures and most of surfactants can be removed by the extraction of acidified ethanol. The powder XRD patterns are shown in Fig. 1(c). After extraction, the low-angle XRD reflections still exist, but the dspacings are higher than those both as-synthesized and thermal treatments, which is unusual. In addition, the samples using C16NH2 and C18-NH2 as structure-directing agents exhibit the characteristic, narrower and more highly intense low-angle XRD reflection; whereas the sample using C12-NH2 as templates shows a broader and slightly less intense one. We can assume that more regular mesopores are remained in the samples using the templates with longer carbon chain after extraction. Thus, the removal of the template by solvent extraction tends to preserve the crystallinity for the samples using C16-NH2 and C18-NH2 as templates. The partial mesophase in samples with C12-NH2 as template collapses after extraction by HCl/ethanol. Analogous changes that the larger intensities of the XRD reflection for the calcined forms than for the as-synthesized materials can also be observed for HMS silicas and MSU-x aluminas [24–26]. Recent modeling studies by Mahanti [27] and Tanev and Pinnavaia [28] have showed that the removal of the occluded organic template from hexagonal mesotructures enhances the Bragg scattering cross-section. The lower XRD reflection of the production from using the C12-NH2 surfactant is observed after extraction by acidic ethanol, indicating that the regularity of the wormlike pore decreases. After extracted by HCl/ethanol, there also remain a few surfactants or a fraction of surfactants (as can be seen in Fig. 5). So the samples were calcined at 500 8C for 10 h in air after extraction by acidified ethanol to eliminate the remaining structure-directing agents. The XRD patterns of the calcined samples prepared with different n-alkylamines are shown in Fig. 2. The single diffraction peaks are still preserved, which
indicates that the mesopore of materials is retained even after calcination temperature highly up to 500 8C. With the increase in carbon chain length of alkylamines, the narrower XRD diffraction can be observed, which indicates that samples using the surfactants with longer carbon chain as the structure-directing agents own more regular mesopore in nature. In addition, calcinations at 500 8C lead to lower Bragg degrees corresponding to higher d values, comparing with the products after extraction by acidic ethanol. For most mesostructured metal phosphates which have hexagonal or wormlike structures, calcinations to remove the template will reduce the order of mesopores and result in lower d-spacings in XRD diffractions [10–13,29]. Fig. 3 shows the XRD patterns of the mesoporous titanium phosphates which are prepared with C18-NH2 after extraction and calcinations for 10 h in air at three different temperatures. With the increase of calcination temperature, the d-spacings decrease from 5.2 to 4.05 nm at temperature up to 580 8C. Furthermore, the width of the diffraction peak becomes wider and the intensity decreases, suggesting a little reduction in the mesoporous order of the structure. The intense diffraction peak of sample calcinated at 580 8C still exists, indicating that most of mesoporous structures were preserved. Compared with those reported by Bhaumik and Inagaki [19] synthesized using cationic and anionic surfactants as templates (thermally stable only up to 300 8C) and reported by Jones et al. [18] with cationic surfactants (thermally stable up to 540 8C), the
Fig. 2. XRD patterns of mesoporous titanium phosphates after extraction and then calcinated at 500 8C for 10 h in air.
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Fig. 3. XRD patterns of mesoporous titanium phosphates prepared with C18-NH2 after extraction and then calcinated for 10 h in air, respectively: (a) 500 8C; (b) 540 8C; (c) 580 8C.
mesoporous titanium phosphates synthesized in this paper have higher thermal stabilities after the thermal treatment. The neutral templates such as alkylamines [10] and PEObased surfactants [30,31] are known to adopt spherical to long
Fig. 4. TEM image of mesoporous titanium phosphate using C18-NH2 as structure-directing agents after extraction and then calcinated at 500 8C for 10 h.
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‘‘wormlike’’ micellar structures. The pore structures of the titanium phosphate calcined at 500 8C using C18-NH2 as structure-directing agents reflect the wormlike motif of the micellar structure, as evidenced by the representative transmission electron micrograph (TEM), as can been seen in Fig. 4, which shows a large disorder in the mesopore shape. This result suggests that the wormlike mesopore is still holding, which is consistent with the XRD patterns (Fig. 2). Fig. 5 shows the IR spectra of the sample using C16-NH2 surfactant as templates, the same material after thermal treatment, HCl/ethanol extraction and calcinations. In Fig. 5(a) and (b), the sharp bands at 2930 and 2850 cm 1 correspond to asymmetric and symmetric C–H stretching, respectively. Bands at 1464 and 1375 cm 1 can be assigned to C–H deformation vibrations. The absorption at 720 cm 1 comes from skeletal vibrations in the hydrocarbon chains in the surfactant [10,32,33]. The absorption band at 1517 cm 1 is assigned to NH3+ deformation vibration. The broad band centered at 3100 cm 1 is the corporate action from both NH3+ and C–H stretching vibration [34,35]. From the results, it can be indicated that the neutral C16-NH2 exhibits as C16-NH3+ in the as-synthesized and thermal treatment samples. Broad bands in the hydroxyl region with maximum at 3400 cm 1 correspond to O–H stretching vibrations of the residual water, OH and defective OH [9]. The characteristic band of H2O at 1635 cm 1 also can be observed. Strong spectra in the region between 1000 and 1050 cm 1 centered at 1010 cm 1 are from Ti–O–P skeletal stretching vibrations [19]. The spectra range in 400–790 cm 1 are TO4 either bending or stretching vibrations. After extraction by HCl/ ethanol solution, a spot characteristic C–H vibration band at 2930 and 2850 cm 1 still can be observed, which indicates a few surfactants or fragments are still remained in the samples (Fig. 5(c)). The surfactant can be removed completely after calcinations, indicated by the entirely disappeared characteristic C–H vibrations (Fig. 5(d)). The similar IR absorptions also can
Fig. 5. IR spectra of mesostructured titanium phosphate prepared with C16NH2: (a) as-synthesized; (b) after thermal treatment; (c) after extraction of the surfactant; (d) calcinated at 500 8C for 10 h in air.
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be observed in the samples using C12-NH2 and C18-NH2 as templates. The porous properties of all the mesoporous titanium phosphates using n-alkylamines with different carbon chain as
templates were characterized based on their N2 adsorption behavior. Prior to the adsorption measurement, samples were heated at 250 8C for 2 h in a vacuum to remove adsorbed water. Fig. 6 shows typical N2-sorption isotherms of the calcined
Fig. 6. Nitrogen adsorption–desorption isotherms (a, c and e) and BJH pore size distribution (b, d and f) at 77 K of mesoporous titanium phosphate calcinated at 500 8C for 10 h using C12-NH2 (a, b), C16-NH2 (c, d) and C18-NH2 (e, f) as structure-directing agents.
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Table 1 Physical properties of various mesoporous titanium phosphates using C12-NH2, C16-NH2 and C18-NH2 as structure-directing agents calcinated at 500 8C for 10 h in air Sample name
Ti/Pa
SSA (m2/g)
Pore diameter (nm)
Pore volume (cm3/g)
1 2 3
1.18 1.19 1.19
359 430 497
1.7 1.9 3.3
0.26 0.28 0.63
1: C12-NH2 used as structure-directing agents; 2: C16-NH2 used as structuredirecting agents; 3: C18-NH2 used as structure-directing agents. a Ti/P molar ratio obtained from ICP measurements.
samples. The type IV isotherms clearly illustrate the mesoporous nature of the materials. The far narrower pore size distribution curves can be observed on the materials using C18-NH2 as template than the C12-NH2 and C16-NH2, and the micropore seems to exit in the latter two materials after calcinations at 500 8C for 10 h. This result indicates that extraction and then calcinations at 500 8C to remove the templates will result in collapse of the partial mesophase using C12-NH2 and C16-NH2, which is consistent with the XRD results. The textural properties of samples are summarized in Table 1. The specific surface areas of the mesoporous titanium phosphates calcined at 500 8C increase with the carbon chain length of the surfactants. Compared with the mesoporous titanium phosphate calcinated at 540 8C for 4 h using trimethylammonium surfactants as template reported by Jones et al. (surface area of only 260 m2/g) [19], the mesoporous titanium phosphates obtained by us have larger surface areas. With the increase in carbon number of the surfactants, pore diameters of samples calculated on the basis of BJH analysis from desorption branch of the isotherms have their maximum at range from 1.7, 1.9 to 3.3 nm, respectively, as shown in Fig. 6. Due to the disordered porous framework, the corresponding pore walls thickness of the samples were calculated simply about 2.65, 2.6 and 1.9 nm, respectively, a bit larger than or similar to those of siliceous MCM-41 materials and Ti-MCM41 (1.4–2.2 nm), but much smaller than Ti-SBA-15 (6.4 nm) [36–38]. The pore volumes of samples calculated from corresponding desorption curve of the isotherms also increase with the length of carbon chain of surfactants. The local environments of P atom of the mesoporous titanium phosphates are studied using the MAS NMR technique. Fig. 7 shows the 31P MAS NMR spectra of the typical sample using C18-NH2 as structure-directing agents after extraction and calcinations at 500 8C. There exists a single broad resonance between 5 and 40 ppm centered at ca. 25.1 ppm, as expected for phosphorus in a large range of environments. The signal seems to consist of several resonances which are badly resolved from each other. Studies of titanium phosphates of known crystal structure have shown a correlation between connectivity and chemical shift. As the connectivity increases, an up-field shift is observed from 5.3 to 10.6 ppm for H2PO4, to 18.1 ppm for HPO4 and finally to 19 to 32.5 ppm for PO4 [39–41]. A comparison of the position of the signals obtained (Fig. 7) with the above reported data
Fig. 7. 31P MAS NMR spectra of the mesoporous titanium phosphate calcinated at 500 8C for 10 h in air.
suggests that these resonances may be attributed to four-fold coordinated P with O–Ti and various amounts of H2O or OH groups. The UV–visible spectra of the mesoporous titanium phosphate materials after calcinations at 500 8C are shown in Fig. 8. The calcined materials using different carbon chain as templates show similar and very strong absorption bands in the 200–350 nm wavelength region, centered at 220–240 nm together with a shoulder band at 280 nm. The band at 220– 240 nm was attributed to ligand-to-metal charge transfer associated with isolated Ti(IV) framework sites in tetrahedral coordination [19,42]. The shoulder at 280 nm can be attributed to framework-linked Ti atoms with octahedral symmetry or to partially polymerized Ti species, which contain Ti–O–Ti bonds
Fig. 8. UV–vis spectra of mesoporous titanium phosphates using: (a) C12-NH2; (b) C16-NH2; (c) C18-NH2 as structure-directing agents after extraction and then calcinated at 500 8C for 10 h in air.
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Table 2 Oxidation of cyclohexene over various Ti-containing catalysts Samples
a
TiP TS-1c Ti-MMSd Ti-MCM-41e a b c d e
Cyclohexene conversion (%)
99 0.7 20.6 52.9
Selectivity (%) Epoxide
1-ol
1-one
Mono-ethers
Diols, diethers
0 36.4 2.3 0
20.2 0 5.8 3.6
0 0 15.3 14.3
0 0 38.3 23.0
79.8b 63.6 38.3 59.1
Mesoporous titanium phosphate using C18-NH2 as template; reaction conditions: 0.1 g catalyst, cyclohexene:H2O2 = 1:1, 10 h, solvent methanol, 333 K for 10 h. The selectivity includes 1-methyl-1,2-cyclohexanediol (66.4%) and 1,2-cyclohexanediol (13.4%). Reaction conditions: 0.05 g catalyst, cyclohexene:H2O2 = 2:1, solvent methanol, 333 K for 2 h. See reference [51]. Reaction conditions: 0.2 g catalyst, cyclohexene:H2O2 = 3.3, solvent methanol, 333 K for 3 h. See reference [52]. Reaction conditions: 0.1 g catalyst, cyclohexene:H2O2 = 1:1, solvent methanol, 313 K, 6 h. See reference [53].
[43–45]. That the Ti/P ratios of mesoporous titanium phosphate materials in Table 1 are a little bit larger than 1 also indicates that some Ti–O–Ti bonds exist. A similar high-energy absorption edge due to tetrahedral coordination has been observed for mesoporous titanosilicate (300–330 nm) [46], titanium-containing amorphous silica (325–350 nm) [47], and mesoporous titanium phosphates (220–330 nm) [19]. The absence of a band at 370–410 nm in the UV–vis spectra rules out the presence of free TiO2 [48,49]. The microporous titanium silicate materials, with tetrahedral titanium, show strong absorption bands at 200–220 nm. The high-energy absorption bands are attributable to the low content of titanium in TS materials, otherwise the mesoporous titanium phosphates reported here having a high content of titanium. An increase in content of titanium always leads to a higher wavelength [47]. In addition, the red shift also depends strongly on the nature of the ligands and hydration state [42]. The local environment of titanium attached with –O–P in mesoporous titanium phosphate materials instead of –O–Si in microporous titanium silicates may be also responsible for this shift of absorption band. Since Ti–OP and Ti–OH should have different electronic transitions as the oxygen electron density in a phosphor–oxygen bridge is lower than that of OH groups, the unavoidable presence of Ti– OH may also account for the red shift [50]. Mesoporous titanium phosphate (after extraction and calcinations using C18-NH2 as template) is chosen for the liquid-phase oxidation of cyclohexene with H2O2 (30% aqueous) oxidant under mild conditions (333 K). In a typical reaction (0.1 g catalyst, substrate:H2O2 = 1:1, 10 h reaction time, solvent methanol), 99 mol% cyclohexene was converted into 1-methyl-1,2-cyclohexanediol (66.4% selectivity), 1,2cyclohexanediol (13.4% selectivity) and 2-cyclohexene-1-ol (20.2% selectivity). The oxidation reactions of cyclohexene using different catalysts are summarized in Table 2. Compared with TS-1, Ti-MMS and Ti-MCM-41, mesoporous titanium phosphate shows the highest catalytic activity, due to its larger pore and more accessible titanium in tetrahedral coordination [53]. The tetrahedral framework titanium in mesoporous titanium phosphate is the effectively active site for such liquid-phase oxidation reactions in the presence of H2O2. In course of the oxidation reaction, the titanium hydroperoxo species can be generated between the tetrahedral titanium and H2O2 [54]. The formation of 1-methyl-1,2-cyclohexanediol,
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