Layer-by-Layer Grafting of Titanium Phosphate onto Mesoporous

Jul 14, 2009 - (1-5) Although many open-framework and layered metal phosphates with diverse compositions and .... The 29Si MAS NMR spectra were fitted...
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Layer-by-Layer Grafting of Titanium Phosphate onto Mesoporous Silica SBA-15 Surfaces: Synthesis, Characterization, and Applications Jianan Zhang,†,‡ Zhen Ma,† Jian Jiao,† Hongfeng Yin,† Wenfu Yan,‡ Edward W. Hagaman,† Jihong Yu,*,‡ and Sheng Dai*,† †

Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, and ‡State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, People’s Republic of China Received May 17, 2009. Revised Manuscript Received June 22, 2009

Metal phosphates have many applications in catalysis, separation, and proton conduction, but their small surface areas and/or constrained pore structures limit their utilization. Here, we report two new methods for the liquid-phase grafting of titanium phosphate onto mesoporous silica (SBA-15) surfaces: (1) alternate grafting of Ti(OPri)4 and then POCl3 and (2) one-pot grafting of titanium phosphate formed in situ by employing Ti(OPri)4 (a base) and POCl3 (an acid) as an appropriate “acid-base pair”. Both the size of mesopores and the content of titanium phosphate can be changed by increasing the number of modification cycles in a stepwise (or layer-by-layer) fashion. The obtained products were characterized by inductively coupled plasma optical emission spectroscopy, X-ray diffraction, N2 adsorptiondesorption, transmission electron microscopy, 31P and 29Si magic-angle spinning NMR, and NH3 temperatureprogrammed desorption, and their performance in acid catalysis and metal ion adsorption was investigated. This work provides new methodologies for the general synthesis of supported metal phosphates with large surface areas, ordered nanoporous structures, and acid properties.

1. Introduction Metal phosphates hold great potential for catalysis, ion exchange, and proton conduction.1-5 Although many open-framework and layered metal phosphates with diverse compositions and properties have been developed,6-9 their applications are constrained by the lack of large surface areas and the dominance of small pores or interlayer spaces. Two synthesis approaches have been developed to address these deficiencies. The first approach involves the direct synthesis of mesoporous metal phosphates via surfactant templating.10-19 Notably, Zhao and *To whom correspondence should be addressed. E-mail: jihong @jlu.edu.cn (J.Y.) and [email protected] (S.D.). (1) Katz, H. E.; Scheller, G.; Putvinski, T. M.; Schilling, M. L.; Wilson, W. L.; Chidsey, C. E. D. Science 1991, 254, 1485. (2) Guang, C.; Hong, H. G.; Mallouk, T. E. Acc. Chem. Res. 1992, 25, 420. (3) Freiman, G.; Barboux, P.; Perriere, J.; Giannakopoulos, K. Chem. Mater. 2007, 19, 5862. (4) Jing, D. W.; Guo, L. J. J. Phys. Chem. C 2007, 111, 13437. (5) Das, D. P.; Parida, K. M. Catal. Surv. Asia 2008, 12, 203. (6) Poojary, D. M.; Shpeizer, B.; Clearfield, A. J. Chem. Soc. Dalton Trans. 1995, 111. (7) Bruque, S.; Aranda, M. A. G.; Losilla, E. R.; Olivera-Pastor, P.; Maireles-Torres, P. Inorg. Chem. 1995, 34, 893. (8) Ekambaram, S.; Sevov, S. C. Angew. Chem., Int. Ed. 1999, 38, 372. (9) Yu, J. H.; Xu, R. R. Chem. Soc. Rev. 2006, 35, 593. (10) Jimenez-Jimenez, J.; Maireles-Torres, P.; Olivera-Pastor, P.; RodriguezCastellon, E.; Jimenez-Lopez, A.; Jones, D. J.; Roziere, J. Adv. Mater. 1998, 10, 812. (11) Jones, D. J.; Aptel, G.; Brandhorst, M.; Jacquin, M.; Jimenez-Jimenez, J.; Jimenez-Lopez, A.; Maireles-Torres, P.; Piwonski, I.; Rodriguez-Castellon, E.; Zajac, J.; Roziere, J. J. Mater. Chem. 2000, 10, 1957. (12) Bhaumik, A.; Inagaki, S. J. Am. Chem. Soc. 2001, 123, 691. (13) Guo, X. F.; Ding, W. P.; Wang, X. G.; Yan, Q. J. Chem. Commun. 2001, 709. (14) Tian, B. Z.; Liu, X. Y.; Tu, B.; Yu, C. Z.; Fan, J.; Wang, L. M.; Xie, S. H.; Stucky, G. D.; Zhao, D. Y. Nat. Mater. 2003, 2, 159. (15) Shen, S. D.; Tian, B. Z.; Yu, C. Z.; Xie, S. H.; Zhang, Z. D.; Tu, B.; Zhao, D. Y. Chem. Mater. 2003, 15, 4046. (16) El Haskouri, J.; Guillem, C.; Latorre, J.; Beltran, A.; Beltran, D.; Amoros, P. Chem. Mater. 2004, 16, 4359. (17) Kapoor, M. P.; Inagaki, S.; Yoshida, H. J. Phys. Chem. B 2005, 109, 9231. (18) Anabia, M.; Rofouel, M. K.; Husain, S. W. Chin. J. Chem. 2006, 24, 1026. (19) Yu, D. H.; Wu, C.; Kong, Y.; Xue, N. H.; Guo, X. F.; Ding, W. P. J. Phys. Chem. C 2007, 111, 14394.

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co-workers synthesized mesoporous metal phosphates using triblock polymer templates via an “acid-base pair” approach.14 This synthesis method is general, but because of the high densities of metal phosphate frameworks, their surface areas are relatively low. The second approach involves the dispersion of metal phosphate guests on mesoporous silica hosts with large surface areas and ordered nanopores.20-22 For example, Wang and coworkers prepared SBA-15-supported iron phosphate via wetness impregnation of Fe(NO3)3 and NH4H2PO4 followed by hightemperature annealing.20 However, it is difficult to finely control both compositions and structures via wetness impregnation. These deficiencies prompted us to develop alternative methodologies for the synthesis of metal phosphates with controlled nanostructures. Solution-based layer-by-layer (LBL) deposition approaches have been extensively employed to make ultrathin films. Notably, Kunitake and co-workers fabricated metal oxide films on planar substrates via a hydrolytic surface sol-gel process involving sequential condensation and hydrolysis reactions.23,24 Park et al. deposited ultrathin oxide films on planar substrates via successive ion layer adsorption and reaction.25 Freiman and co-workers prepared titanium and zirconium phosphate films on planar silicon by alternately dipping the substrate in diluted Ti or Zr alkoxide and diluted phosphoric acid solutions.3 Wang and coworkers made ultrathin titanium phosphate films by repetitive immersion of a substrate in aqueous Ti(SO4)2 and then aqueous Na2HPO4 or NaH2PO4.26 However, general protocols for the (20) Wang, Y.; Wang, X. X.; Su, Z.; Guo, Q.; Tang, Q. H.; Zhang, Q. H.; Wan, H. L. Catal. Today 2004, 93-95, 155. (21) Li, X. K.; Ji, W. H.; Zhao, J.; Zhang, Z.; Au, C. T. Appl. Catal., A 2006, 306, 8. (22) Li, X. K.; Ji, W. H.; Zhao, J.; Zhang, Z. B.; Au, C. T. J. Catal. 2006, 238, 232. (23) Ichinose, I.; Senzu, H.; Kunitake, T. Chem. Lett. 1996, 831. (24) Ichinose, I.; Senzu, H.; Kunitake, T. Chem. Mater. 1997, 9, 1296. (25) Park, S.; Clark, B. L.; Keszler, D. A.; Bender, J. P.; Wager, J. F.; Reynolds, T. A.; Herman, G. S. Science 2002, 297, 65. (26) Wang, Q. F.; Zhong, L.; Sun, J. Q.; Shen, J. C. Chem. Mater. 2005, 17, 3563.

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controlled modification of powder materials by metal phosphates are still lacking.27 Previously, our group modified both nonporous and mesoporous oxide powders by TiO2 and other metal oxides using the hydrolytic surface sol-gel method.28,29 We have also developed a nonhydrolytic surface sol-gel process for the LBL modification of SBA-15 by TiO2.30 The objective of our current investigation is to extend the LBL and surface sol-gel functionalization strategy to synthesize supported metal phosphates. The two approaches in this work involve (1) the alternate liquid-phase grafting with titanium isopropoxide [Ti(OPri)4] and phosphorus oxychloride (POCl3) on SBA-15 (Scheme 1) and (2) the one-pot surface-mediated grafting of titanium phosphate formed in situ (Scheme 2). The rationale behind our choice of precursors for surface modification is based on the fact that Ti(OPri)4 and POCl3 can be used as suitable “acid-base pair” precursors for the synthesis of titanium phosphate.14 The prepared materials were characterized by elemental analysis, X-ray diffraction (XRD), N2 adsorption-desorption, transmission electron microscopy (TEM), 31P and 29Si magic-angle spinning (MAS) NMR, and NH3 temperature-programmed desorption (TPD). Finally, the applications of the materials were demonstrated by acid catalysis (isopropanol dehydration and cumene cracking) and metal ion adsorption tests. To the best of our knowledge, systematic work on the functionalization of SBA-15 using these methods and precursors has not been reported.

2. Experimental Section 2.1. Materials. Ti(OPri)4 (98%) and tetraethyl orthosilicate (TEOS, 98%) were purchased from Gelest. POCl3 (99.0þ%), triblock copolymer Pluronic P123 (EO20-PO70-EO20), hydrochloric acid (35%), anhydrous toluene, and anhydrous ethanol were purchased from Aldrich. SBA-15 was prepared using P123 as a template and TEOS as a silica source.31

2.2. Alternating Modification of SBA-15 with Ti(OPri)4 and POCl3 (Scheme 1). The synthesis was conducted in a

standard Schlenk line under N2. Typically, 1 g of predried SBA15 was loaded into a two-neck flask sealed with a rubber septum, and 20 mL of anhydrous toluene and 3.4 mmol of Ti(OPri)4 were injected by a syringe at room temperature. The slurry was refluxed under stirring for 2 h and filtered. The product was washed three times with anhydrous toluene and three times with deionized water for hydrolysis. Each washing cycle was conducted by putting the product and 30 mL of anhydrous toluene and deionized water in a centrifuge tube, dispersing the solid in the solvent using a vortexer, centrifuging, and discarding the supernatant. The sample was dried in air at 80 °C overnight. The sample, denoted as SBA-15-Ti, was then reacted with POCl3 by repeating the same procedure but with 10.2 mmol of POCl3 in place of the Ti(OPri)4. The product of this reaction was denoted as SBA-15-Ti-P. SBA-15-Ti-P-Ti-P with higher contents of Ti and P was made by repeating the entire procedure using SBA-15-Ti-P as the starting material.

2.3. One-Pot Grafting of Titanium Phosphate onto SBA15 (Scheme 2). In this method, 20 mL of anhydrous toluene

and ethanol (VT:VE = 1), 3.4 mmol of Ti(OPri)4, and 10.2 mmol of POCl3 were injected into a flask containing 1.0 g of predried (27) Kovalchuk, T. V.; Sflhi, H.; Korchev, A. S.; Kovalenko, A. S.; Il’in, V. G.; Zaitsev, V. N.; Fraissard, J. J. Phys. Chem. B 2005, 109, 13948. (28) Yan, W. F.; Chen, B.; Mahurin, S. M.; Hagaman, E. W.; Dai, S.; Overbury, S. H. J. Phys. Chem. B 2004, 108, 2793. (29) Yan, W. F.; Mahurin, S. M.; Overbury, S. H.; Dai, S. Top. Catal. 2006, 39, 199. (30) Yan, W. F.; Mahurin, S. M.; Overbury, S. H.; Dai, S. Chem. Mater. 2005, 17, 1923. (31) Zhao, D. Y.; Huo, Q. S.; Feng, J. L.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024.

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Zhang et al. Scheme 1. Modification of SBA-15 Surfaces by Alternate Grafting with Ti(OPri)4 and POCl3 (Method 1)

Scheme 2. Modification of SBA-15 Surfaces by One-Pot Grafting of Titanium Phosphate Formed in Situ (Method 2)

SBA-15 under anhydrous conditions. The slurry was stirred for 0.5 h and then refluxed at 110 °C (oil bath temperature) for 2 h. The filtered solid was washed several times with anhydrous toluene and deionized water, followed by drying at 80 °C overnight. This sample was designated SBA-15-TiP. SBA-15-TiP-TiP, with higher Ti and P contents, was prepared by repeating the above procedure with SBA-15-TiP as the starting material. Amorphous titanium phosphate was prepared by adding 3.4 mmol of Ti(OPri)4, 10.2 mmol of POCl3, 10 mL of toluene, and 10 mL of ethanol into a flask followed by refluxing for 2 h. The solution was opened to air and allowed to evaporate at room temperature for days. The glasslike product was washed with deionized water and dried at 80 °C. 2.4. Characterization. Elemental analysis of Ti and P was performed using inductively coupled plasma optical emission spectroscopy (ICP-OES) on a Thermo IRIS Intrepid II spectrometer. Powder X-ray diffraction (XRD) patterns were recorded on a Siemens D5005 diffractometer with Cu KR radiation. N2 adsorption-desorption isotherms at 77 K were measured on a Micromeritics TriStar instrument. Surface areas were calculated according to the Brunauer-Emmett-Teller (BET) method, and the pore-size distributions were obtained based on the adsorption branch of the isotherm with the Barrett-Joyner-Halenda (BJH) method. TEM images were obtained on a Hitachi HD-2000 STEM instrument operated at 200 kV. Solid-state 29Si and 31P MAS NMR spectra were collected on a Bruker MSL 400 spectrometer operating at resonance frequencies of 79.49 and 161.98 MHz, respectively, using a 4 mm MAS rotor spinning at 10 kHz. Single pulse excitation was applied in both 29 Si and 31P NMR experiments with a π/2 pulse of 1.0 μs. The chemical shifts of 29Si and 31P were calibrated according to tetramethylsilane (TMS) and 1.0 M H3PO4 solution, respectively. The 29Si MAS NMR spectra were fitted using a software of dmfit.32 NH3-TPD experiments were conducted on a Micromeritics Auto Chem II 2920 Chemisorption analyzer. A sample (50 mg) (32) Massiot, D.; Fayon, F.; Capron, M.; King, I.; Le Calve, S.; Alonso, B.; Durand, J. O.; Bujoli, B.; Gan, Z. H.; Hoatson, G. Magn. Reson. Chem. 2002, 40, 70.

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Figure 1. Small-angle (A) and wide-angle (B) XRD patterns of (a) SBA-15, (b) SBA-15-Ti-P, (c) SBA-15-Ti-P-Ti-P, (d) SBA-15-TiP, (e) SBA-15-TiP-TiP, and (f) titanium phosphate. was preheated at 200 °C for 1 h (titanium phosphate was preheated at 700 °C for 1 h) under N2 flow and cooled down to 100 °C under an Ar flow. The sample was saturated in NH3 (flow rate, 20 mL/min) at 100 °C for 1 h, followed by flushing with Ar for 1 h to remove the physically adsorbed NH3. The temperature was ramped from 100 to 600 °C in N2 flow at 17 °C/min to desorb NH3, and the signal was monitored using a thermal conductivity detector. 2.5. Acid Catalytic Activities. Isopropanol dehydration was studied in a plug-flow microreactor (Altamira AMI-200). A catalyst (50 mg) was loaded into a U-shaped quartz tube supported by quartz wool, preheated in 8% O2-He flow (37 mL/min) at 350 °C for 1 h, and then allowed to cool down to a set point of ca. 100 °C. The system was flushed by He to remove O2, and pure He was then bubbled through a glass saturator filled with isopropanol at room temperature (vapor pressure 44 mmHg = 5.9 kPa) and continuously passed through the catalyst at a rate of 5 mL/min. The reaction temperature was ramped using a furnace. The conversion was measured as a function of reaction temperature. A portion of the product stream was periodically extracted with an automatic sampling valve and analyzed by GC. Cumene cracking was conducted on the same instrument. A catalyst (50 mg) was pretreated in 8% O2-He flow at 514 °C for 1 h. The system was flushed with He, and He was then bubbled through a glass saturator filled with cumene at room temperature (vapor pressure 4.4 mmHg = 0.59 kPa) and continuously passed through the catalyst at a rate of 5 mL/min. The actual reaction temperature was fixed at 514 °C, and the conversion was measured as a function of reaction time on stream. A portion of the product stream was periodically extracted with an automatic sampling valve and analyzed by GC. 2.6. Ion Exchange Properties. A specified solid sample (60 mg) was suspended in a 25 mL solution of 20 mM AgNO3 (CuCl2 or PbCl2 in parallel experiments). The mixture was continuously shaken for 6 h in a shaking bath, followed by centrifugation at 8000 rpm for 5 min. After it was washed by deionized water several times and dried at room temperature, the powder was analyzed by scanning electron microscopy (SEM)-EDX to monitor the uptake of the metal ion.

3. Results and Discussion 3.1. Alternate Modification of SBA-15 with Ti(OPri)4 and POCl3 (Method 1). We previously reported the functionalization Langmuir 2009, 25(21), 12541–12549

of SBA-15 by TiO2 based on the iterative condensation and hydrolysis reactions involving Ti(OR)4 and water.28,29 Zhao and co-workers synthesized mesoporous titanium phosphate using Ti(OPri)4 and POCl3 as “acid-base pair” precursors.14 Fraissard and co-workers modified Ti-MCM-41 surfaces via the reaction of surface titanol groups with POCl3.27 On the basis of these works, here, we designed a new method (method 1) for the modification of SBA-15 by titanium phosphate (Scheme 1). In the first step, Ti(OPri)4 reacts with surface Si-OH groups and is then hydrolyzed to generate surface Ti-OH groups.24,28 In the second step, surface Ti-OH groups condense with POCl3. The chemisorbed phosphorus species derived from the above step is hydrolyzed to generate P-OH groups. Ti(OPri)4 is grafted onto SBA-15 first and POCl3 second because POCl3 barely reacts with SBA-15 under our conditions. By repeating the above sequence, multiple Ti and P layers can be added (not shown in Scheme 1). The loading of titanium phosphate onto SBA-15 was confirmed by ICP-OES analysis. The measured Ti and P contents of SBA-15-Ti-P are 7.2 and 3.1 wt % (1.5 and 1.0 mmol/g), and those of SBA-15-Ti-P-Ti-P are 12.9 and 4.3 wt % (2.7 and 1.4 mmol/g), respectively. It can be concluded that both the Ti and the P contents increase with the number of modification cycles, and the mass gains resulting from the coating process are obvious. The small-angle XRD patterns of SBA-15, SBA-15-Ti-P, and SBA-15-Ti-P-Ti-P present three well-resolved peaks at 2θ = 0.52° (traces a, b, and c in Figure 1A), confirming the maintenance of a mesoporous hexagonal structure. The peak intensity decreases with the number of the modification cycles, consistent with the filling of the mesopores by titanium phosphate. There is a slight shift of the small-angle peak to higher angles, indicating the decrease in d-spacing. Auroux and co-workers also found a similar decrease in d-spacing upon the modification of SBA-15 with Al2O3.33 The wide-angle XRD pattern of SBA-15 in Figure 2B displays a broaden peak around 2θ = 22° due to amorphous silica. SBA15-Ti-P and SBA-15-Ti-P-Ti-P show only this broad peak, excluding the presence of crystalline titanium phosphate that (33) Dragoi, B.; Dumitriu, E.; Guimon, C.; Auroux, A. Microporous Mesoporous Mater. 2009, 121, 7.

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Figure 2. N2 adsorption-desorption isotherms (A and B) and pore size distributions (C and D) of unmodified and titanium phosphatemodified SBA-15.

would otherwise show sharp peaks. In fact, bulk titanium phosphate prepared in our control experiment presents a broader peak at the same position, corresponding to an amorphous phase previously reported by Zhao et al.34 Our N2 sorption data demonstrate the coating of SBA-15 pores as well. In Figure 2A, SBA-15 shows a type IV isotherm with sharp capillary condensation steps at relative pressure (p/p0) of 0.7-0.8 and an H1 type hysteresis loop due to uniform cylindrical pores, and the average BJH pore size is 8.3 nm. SBA-15-Ti-P shows a sharp capillary condensation step with a little wide p/p0 range of 0.5-0.7, and its average pore size decreases to 7.0 nm. The hysteresis loop of SBA-15-Ti-P-Ti-P is still of H1 type, and the average pore size further decreases to 6.0 nm. Table 1 shows that the BET surface area (742, 524, and 438 m2/g) decreases and the thickness of the pore wall (3.5, 4.5, and 5.5 nm) increases with the number of modification cycles (0, 1, and 2 cycles). In the literature, the grafting of SBA-15 by organic species or metal (34) Zhao, J.; Tian, B. Z.; Yue, Y. H.; Hua, W. M.; Zhao, D. Y.; Gao, Z. J. Mol. Catal. A: Chem. 2005, 242, 218.

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oxides also reduces the surface area and pore size.35-39 By subtracting the wall thickness of SBA-15 from those of the modified samples and dividing those numbers by two, we estimate that the average film thicknesses of SBA-15-Ti-P and SBA-15-TiP-Ti-P are roughly 0.5 and 1.0 nm, respectively. Further TEM studies support the relatively homogeneous coating of SBA-15 surfaces. The TEM images of SBA-15, SBA15-Ti-P, and SBA-15-Ti-P-Ti-P viewed along the [110] direction show ordered two-dimensional hexagonal structures (Figure 3a,c, d). The walls of modified mesopores are as smooth as those of SBA-15, and the wall thicknesses of SBA-15, SBA-15-Ti-P, and SBA-15-Ti-P-Ti-P are estimated as 3.4, 4.5, and 6.0 nm, (35) Yang, W.; Wang, X. X.; Guo, Q.; Zhang, Q. H.; Wang, Y. New J. Chem. 2003, 27, 1301. (36) Tsoncheva, T.; Rosenholm, J.; Linden, M.; Ivanova, L.; Minchev, C. Appl. Catal., A 2007, 318, 234. (37) Krishnan, C. K.; Hayashi, T.; Ogura, M. Adv. Mater. 2008, 20, 2131. (38) Ni, L. I.; Ni, J.; Lv, Y.; Yang, P.; Cao, Y. Chem. Commun. 2009, 2171. (39) Aguado, J.; Arsuaga, J. M.; Arencibia, A.; Lindo, M.; Gascon, V. J. Hazard. Mater. 2009, 163, 213.

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Article Table 1. Physicochemical Properties of Unmodified and Modified SBA-15 along with Titanium Phosphate content wt%

samples SBA-15 SBA-15-Ti SBA-15-Ti-P SBA-15-Ti-P-Ti-P SBA-15-TiP SBA-15-TiP-TiP titanium phosphate

unit cell (nm)

BET surface area (m2/g)

pore size (nm)

pore volume (cm3/g)

wall thickness (nm)

average coating thickness (nm)

11.8 11.5 11.5 11.5 11.5 11.5

742 637 527 438 462 265 7.3

8.3 7.4 7.0 6.0 7.4 6.6

1.03 0.81 0.66 0.43 0.52 0.24

3.5 4.1 4.5 5.5 4.1 4.9

0 0.3 0.5 1.0 0.3 0.7

mmol/g

Ti

P

Ti

P

8.6 7.2 12.9 6.7 16.7 20.0

3.1 4.3 6.2 11.7 14.0

1.8 1.5 2.7 1.4 3.5 4.2

1.0 1.4 2.0 3.8 4.5

Figure 3. TEM images of (a) SBA-15, (c) SBA-15-Ti-P, (d) SBA-15-Ti-P-Ti-P, (e) SBA-15-TiP, and (f) SBA-15-TiP-TiP. (b) SEM image of titanium phosphate.

respectively, consistent with the values calculated based on the N2 sorption and small-angle XRD data (Table 1). 3.2. One-Pot Grafting of Titanium Phosphate onto SBA15 (Method 2). In method 1, we modified SBA-15 alternately by treating with Ti(OPri)4 and POCl3 and tuned the number of modification cycles. One drawback of method 1 is that the sample needs to be completely washed and dried in each cycle. Therefore, we attempted to modify SBA-15 by another method, that is, onepot surface grafting of titanium phosphate formed in situ, inspired by the nonhydrolytic sol-gel method by Corriu and co-workers Langmuir 2009, 25(21), 12541–12549

for the synthesis of bulk solids.40 In the nonhydrolytic sol-gel process, the formation of oxide linkages is caused by the thermal condensation between halide and alkoxide groups, with the elimination of alkyl halide. Corriu and co-workers have extended the approach to the synthesis of layered metal phosphates using metal alkoxides and RPOCl2 at 110 °C.41 More recently, Zhao (40) Corriu, R.; Leclercq, D.; Lefevre, P.; Mutin, P. H.; Vioux, A. Chem. Mater. 1992, 4, 961. (41) Corriu, R. J. P.; Leclercq, D.; Mutin, P. H.; Sarlin, L.; Vioux, A. J. Mater. Chem. 1998, 8, 1827.

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and co-workers prepared mesoporous metal phosphates using Ti(OPri)4 and POCl3 as “acid-base pair” precursors, where both condensation and self-assembly are driven by the evaporation of solvents and volatile condensation products.14 Here, we use Ti(OPri)4 and POCl3 as precursors for the controlled surface modification (Scheme 2). The success of our in situ surface “acidbase pair” approach relies on (i) the negligible condensation reaction of the “acid-base pair” precursors in solution and (ii) the surface activation of this condensation reaction on silica surfaces. The loading of titanium phosphate onto SBA-15 was confirmed by ICP-OES analysis. The measured Ti and P contents of SBA-15-TiP are 6.7 and 6.2 wt % (1.4 and 2.0 mmol/g), and those of SBA-15-TiP-TiP are 16.7 and 11.7 wt % (3.5 and 3.8 mmol/g), respectively, increasing with the number of modification cycles. It can be concluded that there are more Ti and P on SBA-15 surfaces in method 2 versus method 1. Figure 1 shows the XRD results. The small-angle XRD patterns of SBA-15-TiP and SBA-15-TiP-TiP exhibit up to three diffraction peaks at 2θ = 0.5-2° (traces d and e in Figure 1A). The small-angle peak slightly shifts to higher angles with the modification cycles, indicating the decrease in d-spacing as noticed in section 3.1, and the two higher angle peaks become weaker or even missing due to the introduction of titanium phosphate. The wide-angle XRD patterns in Figure 1B show that the supported titanium phosphate species are still amorphous. The N2 sorption isotherms of SBA-15-TiP and SBA-15-TiPTiP are shown in Figure 2B. The hysteresis loop of SBA-15-TiP is still close to type H1, and the average pore size decreases to 7.4 nm, smaller than that of SBA-15 (8.3 nm). The hysteresis loop of SBA-15-TiP-TiP changes to type H2, indicating ink-bottle pores as explained by the pore blockage, and the average pore size further decreases to 6.6 nm. Table 1 shows that the BET surface area (742, 462, and 265 m2/g) decreases and the thickness of the pore wall (3.5, 4.1, and 4.9 nm) increases with the modification (0, 1, and 2 cycles). By subtracting the wall thickness of SBA-15 from those of the modified samples and dividing those numbers by two, we estimate that the average film thicknesses of SBA-15-TiP and SBA-15-TiP-TiP are roughly 0.3 and 0.7 nm, respectively. One may note that SBA-15-TiP-TiP prepared by method 2 has a significantly smaller surface area (265 m2/g) and pore volume (0.24 cm3/g) than SBA-15-Ti-P-Ti-P prepared by method 1 (438 m2/g, 0.43 cm3/g), presumably due to the plugging of nanosized pores by the loaded titanium phosphate using method 2. TEM experiments demonstrate this possibility. As shown in Figure 3e,f, the pore surfaces of SBA-15-TiP and SBA-15-TiP-TiP are rough, and some amorphous titanium phosphate phases pierce into and even plug the nanochannels. Although method 2 leads to stuffed pore channels, under no circumstance are bulk titanium phosphate phases separated from SBA-15 surfaces formed. To see what bulk titanium phosphate looks like for comparison purposes, we prepared a mixture of Ti(OPri)4 and POCl3 and subjected the mixture to extensive aging and evaporation of the solvent for days. A glasslike transparent powder was finally isolated, and SEM experiments indicate the large sizes of the material (Figure 3b). It should be mentioned that simple mixing of Ti(OPri)4 and POCl3 under conditions identical to that used in the synthesis of SBA-15-TiP leads only to the formation of a transparent solution without any precipitation. Therefore, the formation of amorphous titanium phosphate on the surface of SBA-15 without any assistance of solvent evaporation suggests a surface-facilitated “acid-base pair” reaction. Considering the low reactivity of POCl3 on SBA-15 (as demonstrated by our 12546 DOI: 10.1021/la9017486

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31 P MAS NMR spectra of (a) SBA-15-Ti-P, (b) SBA-15Ti-P-Ti-P, (c) SBA-15-TiP, (d) SBA-15-TiP-TiP, and (e) titanium phosphate.

Figure 4.

ICP-OES analysis), we postulate that the one-pot surface modification is triggered by deposition of Ti(OPri)4 species on silica surfaces followed by the nonhydrolytic reaction with POCl3. 3.3. NMR Characterization. Figure 4 shows the 31P MAS NMR data. SBA-15-Ti-P and SBA-15-Ti-P-Ti-P show a broad resonance at around -13 ppm (traces a and b in Figure 4), due to the presence of a mixture of (tTiO)PO(OH)2 (-8 ppm) and (tTiO)2PO(OH) (-14 ppm) species.27,42 SBA-15-TiP and SBA15-TiP-TiP show a broad feature composed of overlapping peaks at 0.1, -8, -15, -21, and -29 ppm (traces c and d in Figure 4), corresponding to physically adsorbed H3PO4,43,44 (tTiO)PO(OH)2, (tTiO)2PO(OH), (tTiO)3PO,27,42,43 and TiP2O7,42,43 respectively. Titanium phosphate (trace e in Figure 4) shows three evident peaks at -1, -7, and -13 ppm due to H3PO4, (tTiO)PO(OH)2, and (tTiO)2PO(OH), respectively. If the titanium phosphate species are bonded to the silica surfaces, the local environment of silicon atoms will change and be reflected in the 29Si MAS NMR data. SBA-15 shows three peaks at -92, -102, and -110 ppm (Figure 5a), assigned to Si atoms with two (Q2), three (Q3), and four (Q4) siloxy bonds, respectively. The relative intensities of three Si peaks (Q2:Q3:Q4) are 3:39:58. These values change to 4:29:67 and 3:26:71 in SBA15-Ti-P-Ti-P and SBA-15-TiP-TiP, respectively, showing a (42) Alfaya, A. A. S.; Gushikem, Y.; de Castro, S. C. Chem. Mater. 1998, 10, 909. (43) Bogatyrev, V. M.; Brei, V. V.; Chuiko, A. A. Teor. Eksp. Khim. 1988, 24, 629. (44) Livage, J.; Barboux, P.; Vandenborre, M. T.; Schmutz, C.; Taulelle, F. J. Non-Cryst. Solids 1992, 147-148, 18.

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Figure 6. NH3-TPD curves of (a) SBA-15, (b) titanium phosphate, (c) SBA-15-Ti, (d) SBA-15-Ti-P, (e) SBA-15-Ti-P-Ti-P, (f) SBA-15-TiP, and (g) SBA-15-TiP-TiP.

Figure 5.

29

Si MAS NMR spectra of (a) SBA-15, (b) SBA-15-TiP-Ti-P, and (c) SBA-15-TiP-TiP.

decrease in Q3 and increase in Q4 (Figure 5b,c). The Qx analysis treats an Si-O-Si linkage as equivalent to an Si-O-Ti linkage in shift value at the silicon atom. This observation confirms the formation of the Si-O-Ti bond during the grafting process. It should be mentioned that since no high power decoupling was used in all 29Si MAS NMR experiments, the Q3 and Q2 peaks in 29Si NMR spectrum of SBA-15 broaden, which may be due to the strong 1H-29Si coupling between adsorbed water and surface Si atoms. Future 1H-29Si and 31P-29Si HETCOR or REDOR NMR experiments will be helpful for a better understanding of the interface at the molecular level. 3.4. Acidic Property and Catalytic Applications. Considering that some metal phosphates are solid acids and have great potential for catalytic reactions,45 we demonstrate the acidic and catalytic property of titanium phosphate-modified SBA-15. As shown in Figure 6a, SBA-15 has no evident NH3-TPD peak, indicating an inferior acid property, whereas SBA-15-Ti (trace c) exhibits a weak peak at 190 °C. SBA-15-Ti-P and SBA-15-Ti-PTi-P have two desorption peaks in the range of T1 = 160-200 °C and T2 = 250-350 °C (traces d and e). SBA-15-TiP and SBA-15TiP-TiP have T1 and T2 peaks, together with a shoulder at T3 = 350-500 °C (traces f and g). The T1 peak corresponds to weak acid sites, and T2 and T3 peaks correspond to moderate and (45) Tanabe, K.; Misono, M.; Ono, Y.; Hattori, H. New Solid Acids and Bases; Elsevier: Amsterdam, 1989.

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Figure 7. Conversion of isopropanol as a function of reaction temperature on different catalysts.

medium-strong acid sites, respectively. In contrast, titanium phosphate with a low surface area (7.3 m2/g) shows virtually no NH3 desorption. The catalytic dehydration of isopropanol is a probe reaction commonly used to compare the weak acidity of materials.45,46 As shown in Figure 7, SBA-15 is weakly active for isopropanol dehydration below 400 °C, and titanium phosphate is weakly active below 250 °C, whereas our titanium phosphate-modified SBA-15 samples all show considerable activities below 200 °C. The selectivity to isopropene is nearly 100%, and virtually no acetone is formed. The activity of SBA-15-Ti-P-Ti-P is slightly higher than or comparable to that of SBA-15-Ti-P, whereas the activity of SBA-15-TiP-TiP is lower than that of SBA-15-TiP. SBA-15-Ti is also active for this reaction, but its activity is lower than that of the titanium phosphate-modified SBA-15. These data are qualitatively consistent with NH3-TPD data because there (46) Ma, Z.; Yue, Y. H.; Deng, X. Y.; Gao, Z. J. Mol. Catal. A: Chem. 2002, 178, 97.

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Figure 8. Conversion of cumene as a function of reaction time on different catalysts.

is no significant NH3 desorption from SBA-15 and titanium phosphate, whereas there is obvious desorption of NH3 from titanium phosphate-modified SBA-15 below 200 °C. The catalytic cracking of cumene is a probe reaction commonly used to compare the intermediate-strong acidity of catalysts.45,46 Because the deactivation during cumene cracking is faster than the deactivation during isopropanol dehydration, as commonly known,46 here, we measure the conversion at one temperature (514 °C) as a function of reaction time on stream. As shown in Figure 8, SBA-15 and titanium phosphate show near 0% conversions. Titanium phosphate-modified SBA-15 samples show conversions. The activity of SBA-15-Ti-P-Ti-P is comparable to that of SBA-15-Ti-P, whereas the activity of SBA-15-TiP-TiP is lower than that of SBA-15-TiP. Among all of these samples, SBA15-TiP shows the highest activity, consistent with the population of medium-strong acid sites on its NH3-TPD profile (Figure 6). At odds is the fact that SBA-15-TiP-TiP has even more mediumstrong acid sites, but its activity in cumene cracking is lower than SBA-15-TiP. This is reasonable, because the surface area, pore size, and pore volume of SBA-15-TiP-TiP (265 m2/g, 6.6 nm, 0.24 cm2/g) are much smaller than the corresponding values of SBA-15-TiP (462 m2/g, 7.4 nm, 0.52 cm2/g), thus compromising the accessibility of bulky toluene molecules and hence the catalytic activity. SBA-15-Ti is also active for this cracking reaction, due to the formation of new acid sites by dispersing TiO2 on SiO2.45 3.5. Metal Ion Adsorption Tests. Titanium phosphate and phosphated titania have been used as ion exchangers because of the negative charges provided by surface hydrophosphate groups.47-50 For instance, titanium phosphate films have been developed as a carrier for silver ions by ion exchange, leading to antibacterial materials.26 Here, titanium phosphate materials with large surface areas were tested for metal ion-exchange capacities. In Figure 9, the uptakes of Pb2þ on SBA-15-Ti-P, (47) Llavona, R.; Suarez, M.; Garcia, J. R.; Rodriguez, J. Inorg. Chem. 1989, 28, 2863. (48) Deoliveira, S. F.; Airoldi, C. Mikrochim. Acta 1993, 110, 95. (49) Roca, S.; Airoldi, C. Thermochim. Acta 1996, 284, 289. (50) Shin, Y. S.; Arey, B. W.; Wang, C. M.; Li, X. H. S.; Engelhard, M. H.; Fryxell, G. E. Inorg. Chem. Commun. 2007, 10, 642.

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Figure 9. Metal ion competitive adsorption data for SBA-15, modified SBA-15, and titanium phosphate.

SBA-15-Ti-P-Ti-P, SBA-15-TiP, and SBA-15-TiP-TiP are 0.56, 0.73, 1.3, and 2.7 mmol/g, respectively, much higher than those on SBA-15 (0.01 mmol/g) and titanium phosphate (0.45 mmol/g). SBA-15-TiP-TiP shows much higher ion-exchange capability because of the high content of titanium phosphate (i.e., more hydrophosphate groups). The uptakes of Agþ and Cu2þ on titanium phosphate-modified SBA-15 are also higher than those on SBA-15 and titanium phosphate. Therefore, titanium phosphate-modified SBA-15 is a potential candidate not only for the removal of heavy metal ions (e.g., Pb2þ) but also for the construction of Agþ-containing antibacterial materials.

4. Conclusions Two liquid-phase grafting methods were developed for the functionalization of SBA-15 surfaces by amorphous titanium phosphate. In the first method, Ti(OPri)4 and POCl3 were alternately grafted onto SBA-15. In the second method, titanium phosphate was grafted onto SBA-15 in a one-pot reaction using Ti(OPri)4, POCl3, and SBA-15. The in situ formation of titanium phosphate on silica surfaces can be attributed to a surfacemediated reaction with conformal properties. In both methods, such grafting procedures can be repeated in an LBL fashion to load more titanium phosphate on surfaces. The ordered Langmuir 2009, 25(21), 12541–12549

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mesoporous structure is maintained after surface modification, and the mesopore diameter decreases and the thickness of the pore wall increases with the number of modification cycles. These titanium phosphate-modified SBA-15 materials display higher catalytic activity in isopropanol dehydration and cumene cracking as well as better ability in metal ion adsorption than SBA-15 and titanium phosphate. The presence of dispersed titanium phosphate species on the large surfaces of SBA-15 plays an important role in the better performance of the functionalized materials. To the best of our knowledge, these liquid-phase grafting methods were not previously applied in the synthesis of titanium phosphate-modified SBA-15. These methods are interesting from a methodological point of view, because most efforts have been placed on the modification of planar substrates by oxide films, whereas virtually no attention has been paid to the controlled modification of solid powders by metal phosphates.27 Although here we have chosen the grafting of titanium phosphate as an example, our ongoing work confirmed that some other metal phosphates can also be grafted onto SBA-15 using our protocols. The data obtained from those following experiments will be reported in due course. Here, we have chosen isopropanol dehydration and cumene cracking as common probe reactions to demonstrate the acid properties; nevertheless, considering the

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versatile catalytic applications of metal phosphates,45,51-53 we believe that these mesoporous acid catalysts may also be useful in other reactions such as biomass dehydration.54 Acknowledgment. S.D. thanks the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC. J.Y. thanks the grants provided by National Natural Science Foundation of China, the State Basic Research Project (973 Project) of China. This research was also supported by the appointment for Z.M., J.J., and H.Y. to the ORNL Research Associates Program, administered by Oak Ridge Associated Universities. Supporting Information Available: Figure of the surface areas of unmodified and titanium phosphate-modified SBA15 as a function of the duration of hydrothermal treatment at 120 °C. This material is available free of charge via the Internet at http://pubs.acs.org. (51) Takita, Y.; Sano, K.; Muraya, T.; Nishiguchi, H.; Kawata, N.; Ito, M.; Akbay, T.; Ishihara, T. Appl. Catal., A 1998, 170, 23. (52) Ai, M. Catal. Today 2003, 85, 193. (53) Ma, T. Y.; Zhang, X. J.; Shao, G. S.; Cao, J. L.; Yuan, Z. Y. J. Phys. Chem. C 2008, 112, 3090. (54) Corma, A.; Iborra, S.; Velty, A. Chem. Rev. 2007, 107, 2411.

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