Synthesis, Photocatalytic Activity, and Heavy Metal Ion Adsorption

Feb 6, 2008 - inverse opal method using 1-hydroxy ethylidene-1,1-diphosphonic acid ... added derivative (TOPβ) with hydroxy ethylidene groups inside ...
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J. Phys. Chem. C 2008, 112, 3090-3096

Ordered Macroporous Titanium Phosphonate Materials: Synthesis, Photocatalytic Activity, and Heavy Metal Ion Adsorption Tian-Yi Ma, Xue-Jun Zhang, Gao-Song Shao, Jian-Liang Cao, and Zhong-Yong Yuan* Institute of New Catalytic Materials Science, Engineering Research Center of Energy Storage and ConVersion (Ministry of Education), College of Chemistry, Nankai UniVersity, Tianjin 300071, P.R. China ReceiVed: NoVember 6, 2007; In Final Form: December 8, 2007

A three-dimensionally ordered macroporous titanium phosphonate material (TOP) was synthesized by an inverse opal method using 1-hydroxy ethylidene-1,1-diphosphonic acid (HEDP). The β-cyclodextrin (β-CD)added derivative (TOPβ) with hydroxy ethylidene groups inside the frameworks was also performed, having morphology modification and catalytic and adsorptive ability improvement. The samples were characterized by SEM, TEM, XRD, TG-DTA, FT-IR, and UV-vis absorption spectroscopy techniques. The introduced macroporous structure and the presence of the phosphorus and organic groups within the titanium phosphonate framework are believed to contribute the improvement of photocatalytic activity and heavy metal ion adsorption ability, demonstrated by the photodecomposition of Rhodamine B and the adsorption test of Cu2+, Cd2+, and Pb2+, respectively. A comparable photocatalytic activity with commercial P-25 and a selective complexation affinity for Cu2+ were observed in the synthesized hybrid materials, suggesting the promising catalysts and adsorbents for wastewater cleanup.

Introduction Inorganic-organic hybrid materials have been a topical object of a lot of studies because of their combining properties of the inorganic and organic components.1 These kinds of materials have the advantage of possessing a uniform distribution of functional organic groups within their framework, which allows us to tailor their density, chemical reactivity, and thermal stability. A large number of hybrid porous materials with various chemical compositions and organic groups have been welldocumented, showing interesting properties,2,3 and their applications as catalysts and adsorbents are emerging. Besides periodic mesoporous organosilicas (PMOs), which contain organic groups in siloxane bridges of the silica framework, that were widely reported since 1999,4,5 the preparation of porous hybrid materials by utilizing the surfactant templated strategy has recently been extended to non-silica-based inorganicorganic hybrid mesoporous materials. Kimura synthesized ordered mesoporous aluminum organophosphonate by using alkylene diphosphonic acid in the presence of alkyltrimethylammonium surfactant,6,7 oligomeric surfactant, or triblock copolymer.8 Haskouri et al. synthesized mesoporous aluminum phosphonates and diphosphonates from aluminum atrane complexes and methylphosphonic and/or ethylenediphosphonic acids.9 Shi et al. prepared mesoporous aluminum organophosphates with chiral L-proline groups in the pores through the atrane route.10 However, the possible applications of these mesoporous aluminum organophosphonates were not reported yet. Metal phosphonates have been of increasing interest in the past decade due to their applications in the fields of catalysis, ion exchange, proton conductivity, intercalcation chemistry, photochemistry, and materials chemistry.11-13 Microporous zirconium phosphonates and aluminum phosphonates have been * To whom correspondence should be addressed. Fax: +86 22 23509610. Tel: +86 22 23509610. E-mail: [email protected].

constructed from inorganic-organic layers12,14-16 through the controlled reactions of diphosphonic acids, phosphoric acid, and metal species. Vasylyev et al. prepared titanium and vanadium phosphonate materials with irregular porosity by a nonhydrolytic condensation of metal alkoxide with a dendritic tetraphosphonate very recently, which were able to catalyze oxidation of benzylic alcohols under aerobic conditions.17,18 Except from the scarce success examples on the ordered mesoporous aluminum phosphonate materials,6-9 metal organophosphonate with a threedimensionally (3D) ordered macrostructure was never reported. In this work, titanium phosphonate was used as construction material to form the 3D-ordered macroporous structure. The preparation was adapted from the inverse opal method using polystyrene (PS) spheres as the template and successive impregnation of titanium phosphonate solution into the voids.19 Removal of latex spheres was accomplished by extraction with a THF/acetone mixture at a relatively low temperature for the protection of the organophosphonate framework. Also, a β-cyclodextrin (β-CD)-added derivative of macroporous titanium organophosphonate was prepared. The obtained macroporous titanium phosphonate materials exhibited high photocatalytic activity in photodecomposition of Rhodamine B (RhB) and large capacity of heavy metal ion adsorption. Experimental Section Materials. Tetrabutyl titanate, dicyclohexanoneoxalyldihydrazone (BCOD), diphenylthiocarbazone, 1-(2-pyridinylazo)2-naphthol (PAN), monomer styrene, and potassium persulphate were obtained from Tianjin Kermel Chemical Co. β-cyclodextrin (β-CD: (C6H10O5)7) was obtained from Tianjin Guangfu Chemical Co. 1-Hydroxy ethylidene-1,1-diphosphonic acid (HEDP: H2PO3-C(OH)(CH3)-PO3H2) was donated from Henan Qingyuan Chemical Co. All chemicals were used as received without further purification. Synthesis of PS Spheres. Deionized water (300 g) was kept in a reactor at 70 °C under constant stirring for about 20 min.

10.1021/jp710636x CCC: $40.75 © 2008 American Chemical Society Published on Web 02/06/2008

Macroporous Titanium Phosphonate Materials Styrene monomer, which had been washed to remove the inhibitor, was added. After stirring for about 1 h, 0.13 g of potassium persulphate was added into the solution. Polymerization was performed under a nitrogen atmosphere for 30 h. Finally, colloidal suspensions of PS beads with a mean diameter of 500 nm were obtained. Before use, the spheres were centrifuged at 1000 rpm for 20 h and then allowed to air-dry. Synthesis of Macroporous Titanium Phosphonates. In a typical synthesis procedure, 0.515 g of HEDP was added into a mixed solution of 15 mL of deionized water and 7.5 mL of ethanol with or without β-CD under stirring (the obtained products were denoted as TOPβ or TOP, respectively), followed by the dropwise addition of tetrabutyl titanate. A homogeneous solution was obtained after 12 h of stirring, in which 0.5 g of as-synthesized PS spheres was immersed (for the synthesis of β-CD-added derivative, TOPβ, a microwave treatment of the PS spheres was needed before the addition). The obtained mixture was sealed in one Teflon-lined autoclave and aged statically at 60 °C. The product was filtered, washed with water, and dried at 50 °C. Removal of latex spheres was accomplished by extraction with a mixed solution of THF/acetone (1:1 of v/v) for 48 h. Characterization. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were carried out on a Shimadzu SS-550 microscope at 15 keV and a Philips Tecnai G20 at 200 kV, respectively. Fourier transform infrared (FTIR) spectra were measured on a Bruker VECTOR 22 spectrometer with the KBr pellet technique, and the ranges of spectrograms were 4000 to 400 cm-1. Diffuse reflectance UV-vis absorption spectroscopy was employed on a JASCO V-570 UV-V-NIR spectrophotometer over the wavelength range of 300-900 nm, using BaSO4 as a reference. X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max-2500 diffractometer with Cu KR radiation operated at 40 kV and 100 mA. Thermogravimetry-differential thermal analysis (TG-DTA) of the samples was conducted on a Rigaku Standard Model thermal analyzer in air atmosphere with a heating rate of 10 °C/min. The chemical compositions of Ti and P were analyzed by inductively coupled plasma (ICP) emission spectroscopy on a Thermo Jarrell-Ash ICP-9000 (N + M) spectrometer, and C, N, and H were analyzed on a Vario-EL elemental analyzer. Solid-state 31P MAS NMR measurement was performed on a Varian Unity plus-400 spectrometer at spinning rate of 12 kHz and resonance frequency of 161.9 MHz with a pulse length of 1.5 µs and a recycle time of 5 s. N2 adsorption-desorption isotherms were recorded on a Quantachrome NOVA 2000e sorption analyzer at liquid nitrogen temperature (77 K). The samples were degassed at 80 °C overnight prior to the measurement. The surface areas were calculated by the multipoint Brunauer-Emmett-Teller (BET) method. Photocatalytic Activity Test. The photocatalytic activity experiments on the obtained macroporous titanium phosphonate materials were performed by the degradation of RhB dye under UV-light irradiation in the air at room temperature. 5.5 mg of the obtained product was placed into a tubular quartz reactor of 100 mL of RhB aqueous solution (30 mg/L). A 125 W UV lamp with maximum emission at 365 nm was located 10 cm higher than the solution surrounded by a circulating water tube. The reaction mixture was stirred under UV-light irradiation. The mixture sampled at different times was centrifuged for 5 min to discard any sediment. The absorbance of reaction solutions was measured by a SP-722 spectrometer at λmax ) 554 nm. The commercial Degussa P-25 was also investigated under the identical conditions for comparison.

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Figure 1. (a and b) SEM and (c) TEM images of TOP; (d and e) SEM and (f) TEM images of TOPβ.

Metal Ion Adsorption Test. The potential of TOP and TOPβ for Cu(II) adsorption was tested as follows: 0.01 g of the sample was added into 50 mL of Cu(NO3)2 solution with different concentrations (10, 20, 30, 40, 50 mg/L). The mixture was stirred for 3 h, followed by centrifugation at 6000 rpm for 15 min. Twenty milliliters of obtained clear solution, 12 mL of ethanol, and 30 mL of BCOD solution (0.4 g of BCOD dissolved in 50 mL of ethanol and then adjusted to 500 mL with water) were mixed to 100 mL by adding more water and allowed to adjust with ammonia to pH ) 8-9, which is the best pH value for chromogenic reaction. The capacity of Cu(II) adsorbed was monitored by measuring the UV absorption at λmax ) 600 nm of the initial and final solutions. The competitive experiment was carried out in a 50 mL of mixed ionic solution containing Cu(II) (10 mg/L), Cd(II) (10 mg/L), and Pb(II) (10 mg/L). PAN and diphenylthiocarbazone were used as chromogenic reagents for Cd(II) and Pb(II), respectively, with λmax (Cd2+) ) 555 nm and λmax (Pb2+) ) 480 nm. Results and Discussion Material Synthesis and Characterization. The synthesis of macroporous titanium organophosphonates was performed by utilizing polymer colloidal crystals as templates. The synthetic flexibility of the colloidal crystal templating approach is beneficial for tailoring the reactivity of porous materials. Organic functionalization of porous materials provides a means of tuning the surface properties to control host/guest interactions and the hydrophobicity or hydrophilicity of the surface,20 as well as the mechanical and optical properties. The homogeneous titanium phosphonate solution filled into the voids between PS spheres, forming the macrostructured walls during autoclaving. Removal of latex spheres by extraction with THF/acetone mixture resulted in the formation of 3D-ordered macroporous titanium phosphonates (Figure 1), and the organic groups can be preserved during the extraction at a relatively low concentration of THF/acetone (1/1 of v/v).20 It is revealed from the SEM images in Figure 1a that the synthesized TOP sample presented a 3D-ordered arrangement of interconnected macropores with a mean pore diameter of 400 nm, which was ca. 10-15% smaller than the size range of original PS spheres templated, suggesting signifi-

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Figure 2. Formation mechanism of TOPβ scheme, where the step of filling with titanium phosphonate solution was not shown.

cant shrinkage during latex sphere extraction. The wall thickness is roughly estimated as 10-20 nm, which further demonstrated the complete filling of the interstitial voids during the sol-gel process. Each large cage in the surface layer contains four internal windows with the diameter range 50-100 nm, accessible to the second layers (Figure 1b), which are comparable to the previous 3D-ordered macroporous oxide materials.19,20 TEM images confirmed the reverse opal structure of the titanium phosphonate framework (Figure 1c), which is in agreement with the literature reports of colloidal crystal templated macroporous materials.21 Figure 1d-f shows the SEM and TEM images of TOPβ, indicating that the morphology has been modified a lot because of β-CD addition. The distribution and the size of macropores became less-ordered, and some of the pore walls were thickened obviously to about 200-400 nm. During synthesis, β-CD molecules can self-assemble into relatively ordered, channellike aggregates after stirring for a few hours22-24 and stick onto the surfaces of microwave predispersed PS spheres through electrostatic and hydrogen-bonding interaction, which affect the packing of PS spheres. As schemed in Figure 2, a derivative with extra wall thickness was then obtained by the following nanocasting with titanium phosphonate gel into the voids between the PS spheres, aging, and the template removal by extraction with THF/acetone solution. Because the β-CD aggregates in the pore walls of TOPβ could not be observed directly by TEM, the TG-DTA analysis was performed to find any evidence for their presence in the samples. Figure 3 shows the TG-DTA profiles of the synthesized TOP and TOPβ. The weight loss between 20 and 150 °C in the TG curves of both TOP and TOPβ, accompanied with an endothermic peak around 100 °C, could be attributed to the loss of surface adsorbed water. The weight loss between 150 and 500 °C on the TG curves, accompanied with a strong exothermic peak around 348 °C, is mainly due to the decomposition of organic species in the titanium phosphonate framework and the coke combustion. Compared with the DTA curve of TOP, a sharp endothermic peak appears on the DTA curve of TOPβ at 268 °C, which could be attributed to the breaks of potential hydrogen bonds, intermolecular hydrogen bonds of cyclodextrins,25 besides another additional peak at about 320 °C, superposed on the peak at 348 °C to form a much broader one. The exothermic peak at 320 °C might be relative to the decomposition of cyclodextrins. The total weigh loss of TOPβ

Figure 3. TG-DTA curves of TOP (a) and TOPβ (b).

(39.3%) is more than that of TOP (23.3%), suggesting the actual existence of the β-CD aggregations in the pore wall. All of the synthesized samples possess amorphous framework walls, as revealed by powder XRD patterns (Figure S1 in the Supporting Information). No crystalline TiPO4 or TiO2 phases appear. The ICP and conventional elemental analysis of the resultant solids revealed that TOP could be formulated as Ti(O3PC(CH3)(OH)PO3)0.98‚xH2O (experimental 8.79% Ti, 11.20% P, 4.34% C, and 6.79% H in mass), and for TOPβ was formulated as Ti(O3PC(CH3)(OH)PO3)0.93‚(β-CD)0.024‚xH2O (experimental 7.63% Ti, 9.23% P, 5.51% C, and 7.29% H in mass). The contents of P and Ti in TOPβ are lower than those in TOP because of the existence of β-CD in the pore wall of TOPβ. Figure 4 shows the FT-IR spectra of the synthesized samples. The strong broad band at 3400 cm-1 and the sharp band at 1638 cm-1 correspond to the surface-adsorbed water and hydroxyl groups.26 The band at 1040 cm-1 is due to P-O‚‚‚Ti stretching vibrations. The bands at 1380 and 1450 cm-1 could be attributed to phosphoryl (PdO) frequency and P-C stretching vibrations,27 respectively. In addition, two weak bands at 2930 cm-1 and 1155 cm-1, assigned to the C-H stretching mode and C-O stretching mode, were observed clearly in TOPβ, while TOP presented only a shoulder band of C-O around 1155 cm-1, which is due to the presence of β-CD in the TOPβ. The band at 927 cm-1 in the spectrum of HEDP, assigned to P-OH stretching vibrations, was not observed in either TOP or TOPβ; this implies the extensive condensation and coordination of the phosphoryl oxygen with the titanium atom, leading to mainly bidentate phosphonate units. The 31P MAS NMR spectrum of

Macroporous Titanium Phosphonate Materials

Figure 4. FT-IR spectra of β-CD, HEDP, TOP, and TOPβ.

Figure 5. Nitrogen adsorption-desorption isotherms of TOP and TOPβ.

the samples (see Figure S2 in the Supporting Information) shows the somewhat broadened resonance signal of the phosphorus nuclei around 16 ppm, which can be attributed to diphosphonate groups (≡P-C(OH)(CH3)-P≡).2,7 Figure 5 shows the nitrogen adsorption-desorption isotherms of TOP and TOPβ. The isotherms are close to type-II with a strong increase in nitrogen adsorbed volume at relative pressure higher than 0.85, in which the adsorption and desorption branches of the isotherm almost coincide. The similar isotherms have often been observed in the macroporous materials,28,29 indicative of an appreciable amount of macroporosity in the synthesized TOP and TOPβ samples. This is in agreement with the electron microscopic observation results (Figure 1). The BET surface areas of 23 and 25 m2/g were obtained for TOP and TOPβ, respectively. The low surface areas are due to the existence of organic groups in the pores and surfaces. Photocatalytic Activity. The UV-vis diffuse reflectance spectra of TOP and TOPβ materials were recorded (see Figure S3 in the Supporting Information). The band gap values of TOP and TOPβ are 3.23 and 3.19 eV, respectively, calculated by the formula Eg (eV) ) 1240/λg (nm), where λg stands for the wavelength value corresponding to the intersection point of the vertical and horizontal parts of the spectra.30 Their photocatalytic activities were evaluated by photodegradation of RhB under UV irradiation (Figure 6a) and compared with that of the commercial

J. Phys. Chem. C, Vol. 112, No. 8, 2008 3093 P-25 that has a band gap of 3.10 eV. A blank experiment (selfphotosensitized process) was also performed in the absence of any catalysts for comparison. As shown in Figure 6, TOP has a little lower photocatalytic ability than the commercial photocatalyst P-25, while TOPβ exhibits a better ability, though the band gap of TOPβ (3.19 eV) is higher than that of P-25 (3.10 eV). The photoabsorption efficiency, which is strongly influenced by the pore-wall structure of photocatalysts, is one of the main influence factors for the overall photocatalytic activity. In our macroporous titanium phosphonate material, the macroporous size is close to the light wavenumber, and the 3Dordered macropores with internal windows acted as light-transfer paths for the distribution of photon energy onto the inner surface of the macroporous titanium phosphonate, which allowed the light waves to penetrate deep inside the photocatalyst, making it a more efficient light harvester than nonporous P-25.31 Moreover, the doping of phosphorus into the framework of the synthesized materials contributed to the improvement of the photocatalytic ability.30,32 Yu et al.32-34 revealed that phosphated mesoporous TiO2 had a higher photocatalytic activity than P-25, due to the extended band gap energy. In the case of TOPβ, the adding of β-CD during the preparation offered a surface modification, resulting in possible carbon-doping in the framework of titanium phosphonates. The absorption edge shifting from 381 to 392 nm, compared with TOP, has thus been observed for TOPβ, leading to enhanced photocatalytic ability. Because the surface areas of the macroporous titanium phosphonates are close to that of P-25 (30 m2/g), the influence of the surface area on the photocatalytic ability can be ignored. The photocatalytic degradation reaction can be assumed to follow a pseudo-first-order expression: ln(C0/C) ) kt, where C0/C is the normalized organic compound concentration and k is the apparent reaction rate (min-1). The photocatalytic activity has been defined as the overall degradation rate constant of the catalysts. By plotting ln(C0/C) as a function of irradiation time through regression (Figure 6b), the k constant from the slopes of the simulated straight lines can be obtained. Because the degradation rate in the first 20 min was quite fast, and then from 20 to 80 min the degradation process tended to be slow (Figure 6a), the calculation of k constants was performed by two parts (first 20 min and second 60 min), and the results are listed in Table 1 to present the change of the reaction rates. The self-degradation rate constant is relatively low (3.03 × 10-3 min-1). The initial degradation rate constants in the first 20 min increase in the following order: TOP < P-25 < TOPβ, while this order is just reversed in the second 60 min. The k constants of TOP, TOPβ, and P-25 have a dramatic fall in the second 60 min compared with the first 20 min, which means that the degradation reaction rate descends along with the concentration of RhB. Heavy Metal Ion Adsorption. Heavy metal ions, especially mercury and lead, are highly toxic environmental pollutants. A series of appropriately functionalized porous materials, such as mesoporous organosilicas,35,36 have been developed recently for the removal of heavy metal ions from waste streams. The organic functionalities in these adsorbents typically serve to form complexes with heavy metal ions through acid-base reactions, and the solid support allows easy removal of the loaded adsorbent from liquid waste.20 Thiols, thiourea, and amines have been used as metal ion binding motifs for the efficient removal of toxic heavy metals like Hg(II), Cu(II), and Cd(II).37-39 However, metal organophosphonate materials for heavy metal ion adsorption was little investigated.40 In our work, the synthesized macoroporous titanium organophosphonate materi-

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Figure 6. (a) Photocatalytic activities of samples for RhB degradation under UV-light irritation; (b) Plots of ln(C0/C) versus the irradiation time, showing the fitting results using the pseudo-first-order reaction.

Figure 7. Percentage Cu2+ removed (a) and adsorption capacity curves (b) of TOP and TOPβ.

TABLE 1: Photocatalytic Activity and Metal Ion Competitive Adsorption Data for the Synthesized Macroporous Titanium Phosphonate Materials photodegradation rate constant (k, min-1) sample

first 20 min

second 60 min

adsorption capacity (mmol/g)

percentage ions removed (%)

Cu2+ Cd2+ Pb2+ Cu2+ Cd2+ Pb2+

TOP 5.2 × 10-2 3.4 × 10-2 0.157 0.064 0.045 19.7 10.1 9.9 TOPβ 8.8 × 10-2 2.3 × 10-2 0.166 0.070 0.046 20.8 10.7 10.0 P-25 7.3 × 10-2 2.5 × 10-2

als contain organic functional groups in the framework, which could demonstrate some interaction with the heavy metal ions, and thus their performances for heavy metal ion adsorption were studied. The competitive experiment, performed by treating mixed solutions containing Cu(II), Cd(II), and Pb(II) ions of the same concentration (10 mg/L) with the synthesized adsorbents, revealed a distinct preference of TOP and TOPβ adsorbants for the uptake of Cu2+ ions compared to those of Cd2+ and Pb2+ (Table 1), indicating that the synthesized macroporous titanium phosphonate materials have an innate specificity for the adsorption of Cu2+ over Cd2+ and Pb2+. Figure 7 shows the ability of TOP and TOPβ to remove Cu(II) from homoionic solution with different concentrations. When the concentration of Cu(II) was low (10-20 mg/L), most heavy metal ions in the solution could be removed (Figure 7a), leading to the stable percentage of Cu2+ being removed (TOP: 82.4%-81.1%; TOPβ: 90.1%-89.2%) but with rapidly rising

adsorption capacities (Figure 7b). When the Cu(II) concentration got higher (30-50 mg/L), the curves of the percentage of Cu2+ removed decreased dramatically, while the adsorption capacities increased slowly from 0.47 to 0.50 mmol/g for TOP and from 0.49 to 0.53 mmol/g for TOPβ. This phenomenon could be attributed to the maximum adsorption capacity (MAC) of the adsorbents. While treating the solution with a high ion concentration, the MAC value of synthesized adsorbents could be reached, that is, 0.50 mmol/g for TOP and 0.53 mmol/g for TOPβ. The distribution coefficient (Kd) was determined using the equation41,42 Kd ) (ci - cf)Vsoln/(cf mads), where ci is the initial metal ion concentration, cf is the ion concentration after adsorption, Vsoln is the volume of the solution (in mL), and mads is the amount of adsorbent (in g). The distribution coefficient profiles are shown in Figure S4 in the Surpporting Information. At low concentration of Cu(II) (10-20 mg/L), the Kd value of TOP (23000 mL/g) is smaller than that of TOPβ (45000 mL/ g). With the concentration of Cu(II) increasing to 50 mg/L, the Kd values decrease sharply to 2900 mL/g for TOP and 3300 mL/g for TOPβ. This suggests that the adsorption is closely related to the concentration of the ions, and the β-CD-added derivative TOPβ is a obviously better adsorbent than TOP. Moreover, the Kd value for TOP and TOPβ adsorbents are comparable to those of Cu(II) adsorbents made up of functionalized mesoporous silica,35,42,43 indicating that the TOP and

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J. Phys. Chem. C, Vol. 112, No. 8, 2008 3095 synthesis can be extended to other metal organophosphonates with different metal and organic groups. Acknowledgment. This work was supported by the National Natural Science Foundation of China (nos. 20473041 and 20673060), the National Basic Research Program of China (no. 2003CB615801), the Chinese-Bulgarian Scientific and Technological Cooperation Project, the MOE Supporting Program for New Century Excellent Talents (NCET-06-0215), and Nankai University. Supporting Information Available: XRD patterns, UVvis diffuse-reflectance spectra, 31P MAS NMR spectra, and the distribution coefficient (Kd) profiles of macroporous TOP and TOPβ, and the FT-IR spectra of TOP and Cu-TOP. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 8. UV-vis diffuse-reflectance spectra of TOP and Cu-TOP.

TOPβ adsorbents prove to be equally useful for removing metal ions such as Cu(II) from water. The observed high adsorption ability of inorganic-organic hybrid titanium phosphonates is caused mainly by the bridged phosphonates containing ligands for binding metal ions, including the OH connected to methylene and P-O of HEDP. These two different binding modes have represented some evidence on the FT-IR spectrum of Cu-TOP (TOP-loaded Cu2+) (Figure S5 in the Supporting Information). The spectrum revealed P-C stretching vibrations at 1454 cm-1 and a P-O‚‚‚Ti stretching mode at 1008 cm-1, which shows a slight shift as compared to the unloaded TOP: +4 cm-1 and -32 cm-1, respectively. Moreover, Cu-TOP was characterized by the UV-vis diffusereflectance spectroscopy (Figure 8). A sharp fall of absorbance between 300 and 350 nm can be seen in Cu-TOP, while a broad shoulder appears ranging from 700 to 900 nm. The phenomenon is also related to the Cu complex on the surface of the materials,35,44 and the titanium phosphonate framework is playing the most important role in providing coordination sites. As a result of the presence of β-CD aggregates in the pore wall of TOPβ, more coordination sites provided by OH of β-CD exist, leading to TOPβ possessing a higher adsorption ability than TOP. Conclusions 3D-ordered macroporous titanium organophosphonates have been synthesized by a simple “inverse opal method” using PS spheres as hard templates. The inorganic-organic hybrid titanium phosphonate frameworks possess an amorphous nature with intraframework organic functional groups that act as binding sites for heavy metal ion adsorption. The β-CD-added derivative was also synthesized, which shows obvious changes on both morphology and catalytic and adsorptive abilities. The X-ray diffraction, FT-IR and NMR spectroscopy, TG-DTA, and elemental analysis gave evidence of the presence of channels made up of β-CD aggregates in TOPβ. The introduced 3Dordered macroporous structure offers improved mass transport and light harvest, making the synthesized TOP and TOPβ materials potential adsorbents for heavy metal ion removal and photocatalysts for organic dye molecule degradation. TOPβ exhibited a higher efficiency than TOP in both photocatalysis and metal ion adsorption, due to the framework modification of β-CD. These materials may find any other properties and applications such as proton conduction and fuel cells, and the

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