Macroporous Aluminum Phosphonate Hybrid

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J. Phys. Chem. C 2009, 113, 12854–12862

Hierarchical Meso-/Macroporous Aluminum Phosphonate Hybrid Materials as Multifunctional Adsorbents Tian-Yi Ma, Xue-Jun Zhang, and Zhong-Yong Yuan* Institute of New Catalytic Materials Science, Key Laboratory of Energy-Material Chemistry (Tianjin) and Engineering Research Center of Energy Storage and ConVersion (Ministry of Education), College of Chemistry, Nankai UniVersity, Tianjin 300071, People’s Republic of China ReceiVed: April 14, 2009; ReVised Manuscript ReceiVed: May 24, 2009

Inorganic-organic hybrid aluminum phosphonate (AlPPh) materials with hierarchical meso-/macroporous structure were synthesized by using two different kinds of organophosphonic acids: amino tri(methylene phosphonic acid) and bis(hexamethylenetriamine)-penta(methylenephosphonic acid). The preparation was accomplished both with and without the assistance of surfactant F127. All the samples possess a uniform macroporous (500-2000 nm) structure of mesoporous (4-5 nm) framework, which were characterized by SEM, TEM, N2 sorption, XRD, TGA-DSC, elemental analysis, MAS NMR, and FT-IR spectroscopy techniques. The as-prepared AlPPh materials were used as multifunctional adsorbents for the efficient removal of heavy metal ions (e.g., Cu2+) and the adsorption of proteins (e.g., lysozyme). The heavy metal ion adsorption results show that the AlPPh materials have a large adsorption capacity, comparable to those of previous reported Cu(II)-adsorbents made up of functionalized mesoporous silica. The isotherms for lysozyme adsorption are of type L (Langmuir isotherm), and different monolayer capacities were calculated using Langmuir equation. The differences between the metal ion and the lysozyme adsorption were mainly caused by the nature of inorganic ions and proteins and the interactions between the adsorbents and adsorbates. The synthesized AlPPh hybrid materials were confirmed to be useful multifunctional adsorbents for both metal ions and proteins. 1. Introduction Adsorption and separation, as one of the most important applications of the nanoporous materials, has attracted tremendous researching interest and a large number of adsorbents have been put into practice.1-4 For the adsorption of inorganic ions like heavy metal ions, especially mercury and lead, which are highly toxic environmental pollutants, a series of silica-based mesoporous organic-inorganic hybrid materials have recently been developed for removal of them from waste streams,5,6 where 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 the liquid waste.7 Thiols, thiourea, and amines have been used as metal ion binding motifs in the mesoporous silicas for the efficient removal of toxic heavy metals like Hg(II), Cu(II), and Cd(II).8,9 Mesoporous titaniaphosphonate10 and macroporous titanium phosphonate11 hybrid materials were also synthesized for the selective adsorption of heavy metal ions. The adsorption of biomacromolecules such as proteins from solution onto solid surfaces is also of great scientific importance in many areas, such as biology, medicine, biotechnology, and food processing.12 Many porous materials were used for the protein adsorption,13,14 of which the mesoporous adsorbents have recently received particular attention for the adsorption of lysozyme that has well-understood structural characteristics.15 The adsorption of lysozyme was achieved on periodic mesoporous organosilicas (PMOs),3 MCM41,4 SBA-15,4 and nanoporous carbon molecular sieves16 in which the monolayer adsorption process has been observed. * To whom correspondence should be addressed. Tel.: +86 22 23509610. Fax: +86 22 23509610. E-mail: [email protected].

However, to the best of our knowledge, there are scarce reports about utilizing the hierarchical meso-/macroporous hybrid materials as adsorbents for heavy metal ions and proteins, though materials with hierarchical pores have enhanced properties compared with single-sized pore materials due to increased mass transport through the large pore channels of the material and maintenance of a specific surface area on the level of fine pore systems.17 By the combination of supermolecular assembly and macrotemplating, or by macroscopic phase separations, bimodal meso-/macroporous inorganic oxide-based materials could be prepared.17 A self-generation process has also been recently developed to the fabrication of hierarchical meso-/ macroporous metal oxides,18 phosphated metal oxides,19 and metal phosphates.20 Quite recently, nanostructured titaniaphosphonate hybrid materials with a porous hierarchy21 and meso-/macroporous titanium phosphonate materials22 have been successfully synthesized by this simple self-assembly strategy, which exhibited high adsorption capacity for heavy metal ions in the water. Herein, the synthesis of hierarchically meso-/macroporous aluminum phosphonate (AlPPh) hybrid materials is reported for the first time by a simple autoclaving method in the presence or absence of surfactant molecules. Because of the different natures of metal ions and biomacromolecules and the different interactions between the adsorbents and the adsorbates, their adsorption mechanisms on the surface of one special adsorbents should differ,4,7-9,16 and it will be significant to reveal them. Thus, the synthesized AlPPh materials were used for both the heavy metal ion adsorption and the lysozyme adsorption. Many factors that directly affect the adsorption behavior for metal ions and proteins, including the pore hierarchy, pore volume, pore diameter, electrostatic interactions, hydrophobic interactions, and

10.1021/jp903412m CCC: $40.75  2009 American Chemical Society Published on Web 06/15/2009

Meso-/Macroporous Aluminum Phosphonate Materials SCHEME 1: Structures of ATMP and BHMTPMPA

pH value of solution, were discussed, as well as the possible superiority of the hierarchically meso-/macroporous structure. 2. Experimental Section 2.1. Materials. Nonionic triblock copolymer F127 (EO106PO70EO106) was obtained from Nanjing Well Chemical Co., Ltd. Aluminum sec-butoxide (ASB) was obtained from Zhejiang Ultrafine Powders and Chemicals Co., Ltd. Amino tri(methylene phosphonic acid) (ATMP, Scheme 1) and bis(hexamethylenetriamine)-penta(methylenephosphonic acid) (BHMTPMPA, Scheme 1) were obtained from Henan Qingyuan Chemical Co. The lysozyme was obtained from Beijing Dingguo Biotechnology Co. Potassium phosphate, sodium carbonate, potassium chloride, and sodium hydroxide for buffer preparation were obtained from Tianjin Kermel Chemical Co. All chemicals were used as received without further purification. 2.2. Synthesis of Hierarchical Porous Aluminum Phosphonates. In a typical synthesis procedure, 2.99 g of ATMP or 6.85 g of BHMTPMPA was added into 100 mL of deionized water in the presence of 5 g of F127 under stirring, followed by dropwise addition of 2.46 g of ASB. After a further stirring of 24 h, the obtained mixture was sealed in one Teflon-lined autoclave and aged statically at 80 °C for 24 h. The product was filtered, washed with water, and dried at 60 °C. Removal of the surfactant was accomplished by Soxhlet-extraction with ethanol solution for 96 h, and the resultant sample was denoted as ATMP-F127 or BHMT-F127, respectively. As a comparison, surfactant-free synthesis of the sample was also performed by the same procedure, but in the absence of F127, and the obtained sample was denoted as ATMP-non or BHMT-non, respectively. 2.3. 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. N2 adsorption-desorption isotherms were recorded on a Quantachrome NOVA 2000e sorption analyzer at liquid nitrogen temperature (77 K). The samples were degassed at 150 °C overnight prior to the measurement. The surface area was obtained by the BrunauerEmmett-Teller (BET) method, and the pore size distribution was calculated from the adsorption branch of the isotherms by both the Barret-Joyner-Halenda (BJH) model and the nonlocal density functional theory (NLDFT) modeling method. Fourier transform infrared (FT-IR) spectra were measured on a Bruker VECTOR 22 spectrometer with KBr pellet technique, and the ranges of spectrograms were 4000-400 cm-1. X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max-2500 diffractometer with Cu KR radiation operated at 40 kV and 100

J. Phys. Chem. C, Vol. 113, No. 29, 2009 12855 mA. The chemical compositions of Al 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, 27Al, and 13C magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were recorded on a Varian Unity plus-400 spectrometer. Thermogravimetry (TG) and differential scanning calorimetry (DSC) were performed using a Netzsch STA409 instrument at a heating rate of 5°/min using R-Al2O3 as the reference. 2.4. Metal Ion Adsorption Test. Cu(II) ion adsorption test of the meso-macroporous hybrid materials was performed in batch mode. An aliquot of 0.01 g of the adsorbent was added into 50 mL of homoionic solution containing different concentrations (10, 20, 30, 40, 50 mg/L) of Cu(NO3)2. The mixture was stirring for 3 h, followed by centrifugation at 6000 rpm for 15 min. A total of 20 mL of obtained clear solution, 12 mL of ethanol, and 30 mL of dicyclohexanoneoxalyldihydrazone solution (0.4 g of dicyclohexanoneoxalyldihydrazone solved 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 volume of Cu(II) adsorbed was monitored by measuring the UV absorption at λmax ) 600 nm of the initial and final solutions. The results were also proved by atomic absorption spectroscopy (AAS) analysis. 2.5. Lysozyme Adsorption Test. The lysozyme adsorption test was carried out following the methodology described in the literature.4,16 A series of standard lysozyme solutions with different concentrations (20, 30, 60, 100, 150, 200 µmol/L) were prepared by dissolving different amounts of lysozyme in 25 mmol/L buffer solutions (pH 6.5 potassium phosphate buffer, pH 9.6 and 11 sodium bicarbonate buffer, and pH 12 potassium chloride buffer). In each adsorption experiment, 20 mg of the adsorbent samples was suspended in 4 g of the respective lysozyme solution and the fierce stirring was kept for 96 h to ensure the equilibrium reached. The amount of lysozyme adsorbed was monitored by measuring the UV absorption at λmax ) 281.5 nm of the initial and final solutions. Centrifugation prior to the analysis was necessary to avoid potential interference from suspended scattering particles in the UV-vis analysis. 3. Results and Discussion 3.1. Material Synthesis and Characterizations. The synthesis of hierarchical meso-/macroporous aluminum phosphonate was performed by adding aluminum sec-butoxide into an ATMP or BHMTPMPA solution in the presence or absence of surfactant F127, followed by autoclaving at 80 °C for 24 h. Removal of surfactant species was accomplished by extraction with ethanol solution at a relatively low temperature for the protection of the organophosphonate framework. The XRD patterns of all the synthesized samples show one wide diffraction peak in the 2θ range of 15-40° (Figure 1), indicating amorphous frameworks of aluminum phosphonates. Another diffraction around 2θ ) 5.5° appears in the patterns of BHMTnon and BHMT-F127, which is result from the organized array of alkylene -(CH2)6- chains of BHMTPMPA in the resultant materials.23,24 The nitrogen adsorption-desorption isotherms of the synthesized samples and their corresponding pore width distributions are shown in Figure 2, and their textural properties are listed in Table 1. All isotherms are of between type IV and type II, characteristic of mesoporous materials, according to the IUPAC classification. The nitrogen amount adsorbed rises very

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Figure 1. XRD patterns of the as-synthesized samples.

steeply at high relative pressure (P/P0 > 0.8), which suggests the presence of an appreciable amount of secondary porosity of very large pores (macropores).19,25 The isotherms of the hybrid samples exhibited type H3 hysteresis loops that do not level off at relative pressures close to the saturation vapor pressure, indicating these materials comprised of aggregates (loose assemblages) of particles forming narrow slitlike pores.25 Single narrow peaks centered at 4-5 nm were observed in the corresponding pore width distribution curves of the synthesized samples, determined by the NLDFT (nonlocal density functional theory) modeling method from the adsorption branch of the isotherms. This porosity should be as a result of the organized aggregation of aluminum phosphonate nanoparticles that was the hydrolysis product of aluminum alkoxide in an ATMP or BHMTPMPA solution. It is interesting to note that the surface area, pore volume, and the pore width of surfactant-assisted samples were all larger than those of nonsurfactant-assisted ones (Table 1), which indicates that the autoclaving synthesis with a surfactant could efficiently result in the improvement of porosity and textural property of the resultant aluminum phosphonate samples.19 The pore size distributions of these materials were also calculated by the BJH method (Figure S1 in the Supporting Information), though the BJH method somewhat underestimated

Ma et al. small mesopores, and the obtained BJH pore size values are listed in Table 1, too, for comparison. Figure 3 shows the representative SEM images of the hierarchical aluminum phosphonate materials synthesized with or without surfactant, revealing a uniform macroporous structure in all the samples using ATMP or BHMTPMPA as organophosphorus coupling molecules, synthesized in the presence or absence of surfactant. The macropores are in channel-like shape with a uniform pore diameter distribution of 500-2000 nm. The macrochannels are mainly of one-dimensional orientation, parallel to each other, perforative through almost the entire particle, which could be clearly seen from the side view of the samples (Figures 3b,f). This is consistent with those observed in meso-/macroporous boehmite AlOOH and γ-Al2O3 synthesized in the presence of a surfactant.26 Moreover, careful examination of these SEM micrographs (Figure 3a) reveals that the walls of the macropores are composed of small interconnected AlPPh granular particles. The fine particulate morphology indicates that the mesoporosity is probably partially due to the intraparticle porosity and partially due to the interparticle porosity.27 The well-defined macroporous structure and fine particulate morphology between the macrochannels were further confirmed by TEM observation. Figure 4 shows the TEM images of samples ATMP-non and BHMT-F127, being taken as representative. It is obviously revealed that the walls of macroporous network are composed of accessible wormhole-like mesopores formed by the aggregation of granular nanoparticles with more or less regular sizes of 20-40 nm. The genesis of such a hierarchical meso-/macroporous structure of aluminum phosphonates corresponds to a spontaneous self-assembly formation mechanism, as described previously,19,20 in which the surfactant molecule did not take a role in the formation of the macroporous structure, but influenced the textural properties and porosity of the resultant AlPPh materials. Figure 5 shows the FT-IR spectra of the as-synthesized samples. The strong broadband at 3400 cm-1 and the sharp band at 1640 cm-1 correspond to the surface-adsorbed water and hydroxyl groups.28 The sharp band at 1145 cm-1 is due to P-CH2Nd groups, while the P-O · · · Al stretching vibrations at 1050 cm-1 represent as a shoulder.29 The small bands at 1380

Figure 2. N2 adsorption-desorption isotherms and the corresponding pore width distribution curves of the synthesized samples, determined by DFT method: (a) ATMP-F127, (b) ATMP-non, (c) BHMT-F127, and (d) BHMT-non. The volume adsorbed was shifted by 300, 200, 100, and 0, and the pore volume was shifted by 0.036, 0.024, 0.036, and 0 for the curves of a-d, respectively.

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TABLE 1: Summary of the Physicochemical Properties of the As-Synthesized Samples chemical composition sample ATMP-non ATMP-F127 BHMT-non BHMT-F127 Ion-loaded Ly-loaded

SBET (m /g) DBJH (nm) DDFT (nm) Vpore (cm /g) C (%) He (%) Ne (%) Pf (%) Alf (%) a

77 154 60 128 118 24

2

b

1.8 2.3 1.8 1.9 1.5

c

4.3 5.2 4.1 5.0 4.9 2.4

d

3

0.38 0.44 0.36 0.42 0.41 0.09

e

7.62 7.52 10.23 10.02

4.32 4.11 5.03 5.03

2.31 2.28 2.10 2.08

19.68 19.66 7.78 7.55

11.42 11.20 4.52 4.43

R2 g

nmh (µmol/g)

Kdi (mL/g)

0.9796 0.9732 0.9821 0.9801

5.57 8.32 6.74 11.80

9675-1479 17281-2744 24069-3393 277485-6307

a

BET surface area calculated from the linear part of the BET plot. b Estimated using the adsorption branch of the isotherm by the BJH method. c Pore diameters calculated by the NLDFT method. d Single point total pore volume of pores at P/P0 ) 0.97. e Analyzed on a Vario-EL elemental analyzer. f Analyzed by inductively coupled plasma (ICP) emission spectroscopy. g Correlation coefficients for Langmuir model. h Monolayer adsorption capacities of adsorbents for lysozyme. i The distribution coefficient (Kd) of Cu(II) ion adsorption.

Figure 3. SEM images of ATMP-non (a,b), ATMP-F127 (c), BHMTnon (d), and BHMT-F127 (e,f).

Figure 4. TEM images of ATMP-non (a,b) and BHMT-F127 (c,d).

cm-1 and 1324 cm-1 could be attributed to the phosphoryl (PdO) frequency and the C-N stretching vibrations.30 The small band at 1437 cm-1 is due to the P-C stretching vibrations. In addition, one band at 1468 cm-1 and two bands at 2941 and 2868 cm-1, assigned to the C-H bending and stretching mode, were observed clearly in BHMT-non and BHMT-F127, while ATMP-non and ATMP-F127 presented no obvious band at 1468 cm-1 or only several weak bands of C-H stretching mode

Figure 5. FT-IR spectra of AlPPh samples: (a) ATMP-non; (b) ATMPF127; (c) BHMT-non; (d) BHMT-F127.

around 2887 cm-1, which is due to the presence of long alkyl chains (-[CH2]6-) of BHMTPMPA. The bands assigned to P-OH stretching vibrations were observed at 930 cm-1, which implies the existence of some dissociate phosphoryl oxygen not coordinated with the aluminum atoms. The 31P, 27Al and 13C MAS NMR spectra of ATMP-non and BHMT-non are shown in Figure 6, which are taken as being representative. The 31P MAS NMR spectra of ATMPnon and BHMT-non show broad signals around 5.1 and 8.0 ppm, respectively, which are in the area characteristic of phosphonates. The broadening of the resonance signal is due to the disordered or low-crystalline nature of solids. This signal has the similar chemical shifts found for PhP(OTi)3 units in molecular oxo phosphonato titanium clusters of phenylphosphonate-TiO2 hybrids31 and for diphosphonate groups (tP-CH2-Pt) in mesoprous aluminum phosphonates.32 27Al resonances at -24.6 and -15.6 ppm for ATMPnon and BHMT-non, respectively, were due to octahedral AlO6 sites,33,34 while no signals were found around 46 ppm (tetrahedral AlO4 sites). The broad 27Al signals observed indicate a wide distribution of sites typical of disordered solids and suggest the uniform distribution of P-OsAl and Al-OsAl bonds.35 13C MAS NMR spectra of the samples show major signals at 56.2 and 54.9 ppm for ATMP-non and BHMT-non, respectively, corresponding to the carbon atoms in nitrilomethylenephosphonate groups. Another sharp signal was detected at 25.9 ppm for BHMT-non, which was due to the carbon atoms in hexamethylene of BHMTPMPA. These suggest that no phase separation took place during the preparation of the hybrid samples, and ATMP and BHMTP-

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Figure 6.

13

Ma et al.

C, 31P, and 27Al MAS NMR spectra of ATMP-non and BHMT-non.

Figure 7. TG-DSC profiles of (a) ATMP-non and (b) ATMP-F127.

MPA groups were dispersed homogeneously within an aluminum phosphonate network.31 The thermal stability of the synthesized hybrids were determined by the thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC), shown in Figure 7. The TGA curves demonstrate initial weight loss of 23.8% (ATMPnon) and 19.3% (ATMP-F127) from room temperature to 250 °C, accompanied with a endothermal peak around 106 °C in the DSC curve, which may be assigned to the desorption of the

adsorbed and intercalated water. The weight losses of 13.8% (ATMP-non) and 12.4% (ATMP-F127) from 250 to 900 °C, accompanied with two exothermic peaks at around 320 and 780 °C, can be attributed to the decomposition of the organic species and the coke combustion. The TGA-DSC curves of the samples synthesized by using BHMTPMPA exhibit similarly to the sample ATMP-non amd ATMP-F127, indicating that the surfactant F127 could be removed completely by an ethanol extraction process. The ICP emission spectroscopy was employed to analyze chemical compositions of the resultant solids, and the results were listed in Table 1, with P/Al molar ratios approximate to 3:2. Combined with the conventional elemental analysis of C, H, and N, also listed in Table 1, all the samples synthesized with or without F127-assistance could be formulated as Al(C3H6O9NP3)0.50 · xH2O and Al(C17H34O15N3P5)0.30 · xH2O for the samples prepared using ATMP and BHMTPMPA, respectively, and alternative formulation can be expressed as Al(ATMP)0.50 · xH2O and Al(BHMTPMPA)0.30 · xH2O. As it can be seen, the carbon contents in BHMT-non and BHMT-F127 were obviously more than that in ATMP-non and ATMP-F127, which was introduced into the hybrid framework by the long alkyl chains (-[CH2]6-) in BHMTPMPA. 3.2. Heavy Metal Ion Adsorption. The synthesized meso-/ macroporous AlPPh materials contain organic functional groups in the framework, which could demonstrate some interactions with the heavy metal ions and thus their performances for heavy metal ion adsorption were studied, and the results of Cu(II) adsorption are summarized in Figure 8. Figure 8a shows the ability of as-synthesized AlPPh adsorbents to remove Cu(II) from homoionic solutions with different concentrations, giving the adsorption capacity sequence of BHMT-F127 > BHMTnon > ATMP-F127 > ATMP-non. When the concentration of Cu(II) was low (10-20 mg/L), most of the heavy metal ions in the solution could be removed, leading to the stable percentage of Cu2+ removed (BHMT-F127, 95.2-98.3%; BHMT-non, 80.3-83.4%;ATMP-F127,75.1-77.2%;ATMP-non,61.3-65.5%). When the Cu(II) concentration got higher (30-50 mg/L), the curves of the percentage of Cu2+ removed decreased dramati-

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Figure 8. Percentage Cu2+ removed (a) and log Kd (b) curves of the synthesized hybrid adsorbents.

cally. Notably, the tendency for different initial concentration of metal ion was similar for each sample, that is, the adsorption efficiencies increased with the decrease of the initial concentrations of metal ions, clearly demonstrating that the samples appeared to essentially tend to their high loading capacity in low metal ions concentration. The distribution coefficient (Kd) was determined using the following equation:1,5 Kd ) (ci - cf)Vsoln/(cfmads), 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 8b. The Kd values share the same sequence to that of adsorption capacity: BHMT-F127 > BHMT-non > ATMP-F127 > ATMP-non (Table 1). At low concentration of Cu(II) (10-20 mg/L), the Kd values of all the adsorbents were rather high (BHMT-F127, 277485 mg/L; BHMT-non, 24069 mg/L; ATMP-F127, 17281 mg/L; ATMPnon, 9675 mg/L); with the concentration of Cu(II) increasing to 50 mg/L, the Kd values decrease sharply to 6370 mL/g for BHMT-F127, 3393 mL/g for BHMT-non, 2744 mL/g for ATMP-F127, and 1479 mL/g for ATMP-non. This suggests that the adsorption is closely related to the concentration of the ions, and the complex organic motifs and the high surface areas could also contribute to the metal ion adsorption. Moreover, the Kd value for the synthesized adsorbents are comparable to those of Cu(II) adsorbents made up of functionalized mesoporous silica,36,37 indicating that the aluminum phosphonate adsorbents prove to be equally useful for removing metal ions such as Cu(II) from water. It is also supposed that the present synthesized hybrid AlPPh adsorbents could also be efficient in removing other kinds of heavy metal ions in the water, including Pb2+, Cd2+, and Hg2+. The adsorption isotherms of the AlPPh materials were shown in the Supporting Information (Figure S2), which do not quite fit the Langmuir adsorption model. The Cu2+ adsorption amounts increased with the equilibrium concentrations, giving the sequence of BHMT-F127 (140 mg/g) > BHMT-non (106 mg/g) > ATMP-F127 (94 mg/g) > ATMPnon (68 mg/g; when treated by 50 mg/L Cu2+ solution). Slight shifts were obtained of the P-C (+8 cm-1) and C-N (-10 cm-1) stretching vibrations on the FT-IR spectrum of the Cu2+ loaded AlPPh absorbents, which might be caused by the coordination bonds between Cu2+ and nitrogen in the hybrid framework. A broad shoulder appears ranging from 700 to 900 nm on the UV-vis spectrum of the Cu2+ loaded adsorbents, also related to the Cu complex on the surface of the materials.1,11

3.3. Adsorption of Lysozyme. The hydrophobic interaction adsorption of lysozyme was carried out to test the ability of the synthesized hybrids for the protein immobilizing. To find the optimal pH value for the adsorption of lysozyme, the hybrid adsorbents were tested at pH ) 6.5, 9.6, 11, and 12, respectively, and the sample BHMT-F127 was chosen as the representation here. Figure 9 shows the adsorption isotherm and monolayer adsorption capacity of lysozyme on BHMT-F127 at various pH values. These isotherms exhibit a sharp initial rise, suggesting a high affinity between lysozyme and the adsorbent surface, and finally the isotherms reach a plateau, as denoted type L (Langmuir) isotherm.16 The Langmuir model was employed as follows: ns ) Knmc/(1 + Kc), where K is the Langmuir constant, c is the lysozyme concentration, nm is the monolayer adsorption capacity, and ns is the amount of lysozyme adsorbed on the adsorbent. The monolayer adsorption capacity (nm) was calculated using the equation above. ns varies at different pH values: 11.80 µmol/g (pH ) 11) > 10.42 µmol/g (pH ) 12) > 9.23 µmol/g (pH ) 9.6) > 8.24 µmol/g (pH ) 6.5). The maximum adsorption was achieved at pH ) 11, which is the isoelectric point (pI, the pH value in solution at which the sum of the charges on the protein is zero) of lysozyme.4,16 The amount adsorbed increases from pH 6.5 to 11, but decreases when the pH is increased to 12. The same trend has been reported by Vinu et al.4 in the adsorption of lysozyme on MCM-41 and SBA-15, and detailed explanations have been made. In this work, the adsorption mechanism of lysozyme on AlPPh hybrid materials could be discussed by three ranges separated in Figure 8b: pH below and above the pI, pH close to pI. It is a well-known fact that the protein molecule is positively charged at a pH below pI and negatively charged at a pH above pI. With pH value increasing from 6.5 to 11, the net positive charges of the lysozyme molecule start to increase and so does the lateral repulsion between the lysozyme molecules.38 Thus, the protein molecules require more space and the monolayer capacity decreases. When the solution pH is increased from 11 to 12, the surface of the lysozyme molecule becomes negatively charged and enhances the electrostatic repulsion between protein molecules. Therefore, the monolayer capacity decreases in the same way. Consequently, when the solution pH values varies both below and above pI, Coulombic forces exhibited as the dominant driving force for the adsorption of protein onto the adsorbents, while hydrophobic interactions become more dominant than electrostatic interactions for the adsorption near pI, which was considered to be the main

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Figure 9. Adsorption isotherms (a) and monolayer adsorption capacity (b) of lysozyme on BHMT-F127 at various pH values. The solid lines in (a) represent a fit of the experimental data employing the Langmuir model.

Figure 10. Adsorption isotherms of samples for proteins at pH ) 11: (9) ATMP-non; (O) BHMT-non; (2) ATMP-F127; (0) BHMT-F127. The solid lines in this figure represent a fit of the experimental data employing the Langmuir model.

physicochemical principle underlying the adsorption near pI.39,40 Unlike the reported inorganic-framework zeolites4,16,39,40 used for adsorption of proteins, the present adsorbents made up of AlPPh have an inorganic-organic hybrid framework, which contains plenty of hydrophobic alkyl groups inside the framework. So the hydrophobic interaction between the organic groups inside the channel walls and the nonpolar side chains of the amino acids on the surface of lysozyme was greatly enhanced, leading to the increased monolayer adsorption capacity. Moreover, near the isoelectric point, the net charge of the protein is low and the Coulombic force between the protein molecules is minimal, which means a closer packing of the protein molecules is possible and the monolayer adsorption capacity increases. Furthermore, the adsorption of lysozyme on different samples was tested at pH ) 11, and the results are shown in Figure 10. The monolayer adsorption capacities and the correlation coefficients (R2), calculated by using the Langmuir equation, are shown in Table 1. The nm values of the F127-synthesized samples are larger than that of nonsurfactant-synthesized samples (8.32-11.80 µmol/g vs 5.57-6.74 µmol/g), which is consistent with the enlarged surface areas and pore sizes of the surfactant-assisted samples. Also, the samples BHMT-non and BHMT-F127 have larger monolayer adsorption capacities than

the ATMP-non and ATMP-F127. Because BHMTPMPA has extra long hydrophobic alkyl chains (-[CH2]6-) compared to ATMP, the AlPPh adsorbents synthesized with this specific phosphonates become more hydrophobic or organophilic in its adsorptive characteristics for lysozyme, making the higher adsorption capacity than the samples with similar surface areas but synthesized with organophosphonate of lower hydrophobic -CH2- groups. This phenomenon further demonstrates that the hydrophobic interaction is the dominant driving force at isoelectric point of proteins. Moreover, the monolayer capacities of the synthesized samples are comparable to those of sol-gel materials with similar surface areas,41,42 suggesting the AlPPh materials equally practical for immobilizing proteins. 3.4. Discussion. The spontaneous formation and the surfactant-assisted process are employed to obtain the hierarchical macro-/mesoporous structure of aluminum phosphonates. The genesis of this macro-/mesoporous hierarchy in the absence of the surfactant molecules corresponds to a spontaneous formation mechanism.10,21,22 The hydrolysis of aluminum sec-butoxide precursors in the phosphonic acid solution would result in the rapid formation of nanometer-sized Al-phosphonate particles, and simultaneously generating a lot of butanol molecules. Selfassmbly of Al-phosphonate particles and aggregations of Alphosphonates along with the microemulsions happens, together to produce the accessible mesopores. The hydrolysis reactions and polycondensation might produce microphase-separated domains of Al-phosphonate-based nanoparticles and water/ alcoholchannels,whicharetheinitiatorsofthemacrochannels.17,19,20 Thus, a hierarchical structure of uniform macrochannels with mesoporous walls could be generated. For the F127-assisted samples, F127 in solution could be adsorbed onto the surface of mesostructured F127/Al-phosphonate nanoparticles to form a bilayer structure at the interface, followed by the vesiculation of these bilayer structures and the formation of supermicelles by the coalescence of multiple micelles and the interaggregate interactions, finally leading to the production of an ordered array of macrochannels.19,20,26 It is interesting to note that the surface area, pore volume and the pore width of surfactant-assisted samples were all larger than those of nonsurfactant-assisted ones (Table 1), which indicates that the autoclaving synthesis with the surfactant could efficiently result in the improvement of porosity and textural property of the resultant aluminum phosphonate samples. Because of the different natures of two kinds of adsorbates, metal ions, and lysozyme, the adsorption behaviors of them are

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Figure 11. Simulated adsorption behaviors of the mesoporous adsorbents and hierarchically meso-/macroporous adsorbents. The adsorbents and adsorbates such as metal ions and proteins were painted white and dark blue respectively. The concentration of the adsorbates gets lower from material surface to the inner part of the pure mesoporous adsorbents, while the color changes from dark blue to light blue. The diffusion and the mass transfer inside the macrochannels were shown using yellow dash arrows, which benefit the ion/ protein solution’s impenetrating.

distinct. For the adsorption of heavy metal ions, the adsorption isotherms do not quite fit the Langmuir adsorption model (Figure S2 in the Supporting Information), the ions were adsorbed onto the hybrid adsorbents mainly by coordination to form the coordinate bonds, the adsorption could be indirectly observed from the UV-vis and FT-IR spectra reported in our previous work,11 showing some shifted peaks, and the adsorption of metal ions was usually tested in a near neutral system.5,10,11,21,22,43 For the adsorption of lysozyme, the isotherms of all the samples for lysozyme adsorption are denoted as type L (Langmuir) isotherm, indicating the monolayer adsorption process, the electrostatic interactions, lateral interaction, and hydrophobic interactions between lysozyme and the metal phosphonates are almost intermolecular, and the physicochemical properties of the protein-loaded adsorbents were greatly changed (Table 1), demonstrating the immobilizing of lysozyme. More importantly, the adsorption capacities for the two different adsorbates are determined by quite differed factors. Silica-based mesoporous organic-inorganic hybrid materials have recently been developed for removal of heavy metal ions from waste streams, while metal phosphonate adsorbents have been scarcely investigated. The observed high adsorption ability for heavy metal ions of inorganic-organic hybrid aluminum phosphonates is mainly caused by the bridged phosphonates containing ligands for binding metal ions. Thus, the organic motifs of the synthesized hybrid adsorbents are considered to be the dominant factor that determines the adsorption capacity. For the samples BHMT-F127 and BHMT-non, larger and more complex organic motifs (Scheme 1) allow the extensive infiltration of metal ions into the hybrid network and supply more coordination sites for the ions, which is the main reason that the specific adsorbents made up of BHMTPMPA exhibit superiority over the other adsorbents (BHMT-F127, BHMTnon > ATMP-F127, ATMP-non). Moreover, the enlarged surface area and pore volume may also make secondary contributions, which could be seen from the adsorption capacity sequences of samples made up of the same organophosphoric acid: BHMT-F127 (SBET ) 128 m2/g) > BHMT-non (SBET ) 60 m2/g) and ATMP-F127 (SBET ) 154 m2/g) > ATMP-non (SBET ) 77 m2/g). On the other hand, the adsorption of lysozyme exhibited quite differently. Lysozyme is a small globular protein with molecular

mass 14400 Da. It has a prolate spheroid shape with two characteristic cross sections: a side of dimensions of roughly 3.0 × 4.5 nm2 and an end of dimensions 3.0 × 3.0 nm2.44 The isoelectric point of lysozyme is around 11. In the range of physiological temperature, no detectable change in the structure was observed within the pH range from 1.5 to 12. The adsorption of lysozyme on the adsorbents is determined by several factors including electrostatic interactions, the hydrophobic interactions, and the lateral interaction, in which the hydrophobic interactions between inorganic-organic hybrid adsorbents, and the protein was considered to be more dominant near pI than other interactions.4,39,40 Hydrophobic interactions are a kind of van der Waals attraction, which is based on the reversible interactions of the hydrophobic groups in the pore wall of the adsorbents with the nonpolar regions on the biomolecule surface. Because of the monolayer adsorption of lysozyme, the large surface areas become the most important factor that could supply interaction spaces for the hydrophobic adsorbents and the biomolecule surface. Thus, the surfactantassisted samples with larger surface areas show higher adsorption ability: ATMP-F127 (SBET ) 154 m2/g) and BHMT-F127 (SBET ) 128 m2/g) > ATMP-non (SBET ) 77 m2/g) and BHMTnon (SBET ) 60 m2/g). With similar surface areas, samples with larger hydrophobic groups exhibit superiority: BHMT-F127 > ATMP-F127 and BHMT-non > ATMP-non. To compare the different states of the proteins and metal ions adsorbed in the pores of the adsorbents, N2 sorption technique was employed to analyze the adsorbent before and after metal ion and lysozyme adsorption. Figure S3 in the Supporting Information shows the N2 adsorption-desorption isotherms of BHMT-F127 before and after lysozyme (Ly-loaded) and metal ion (Ion-loaded) adsorption and corresponding pore width distribution curves. The surface area, pore width, and pore volume of Ly-loaded sample get a sharp fall compared with the unloaded BHMT-F127 (Table 1), which was caused by the filling of lysozyme molecules into the adsorbent pore system in such a monolayer adsorption way that the adsorption of nitrogen is hindered. It is believed that the functionalized and well-structured mesopores and the highly productive macropores have done great contribution to the biomacromolecule adsorption.13,14 On the contrary, the physicochemical properties of Ionloaded sample were hardly changed (Figure S3 and Table 1),

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because the complexed metal ions with the organic ligands in the pore wall occupied much less spaces than the biomacromolecules that it did not hinder the following adsorption of nitrogen. There have been many reports about the hierarchical porous materials as catalysts and adsorbents,21,25 in which the advantages of the macropores or macrochannels are demonstrated. In the present work, the reduced resistance to diffusion and improved mass transfer could be caused by macropores or macrochannels, as well as the establishment of an adsorption/ desorption equilibrium, benefiting the improvement of adsorption efficiency of the hybrid adsorbents. The comparison of the adsorption behaviors of the mesoporous adsorbents and hierarchically meso-/macroporous adsorbents are shown in Figure 11. The concentration of the adsorbates gets lower from material surface to the inner part of the pure mesoporous adsorbents due to the resistance caused by the disordered porous system, leading to less contact probability between adsorbents and adsorbates. However, the macrochannels supply direct access to penetrate the material, which benefit the ion or protein solution’s impenetrating into the inner mesopores. 4. Conclusions Hierarchical meso-/macroporous aluminum phosphonate hybrid materials were synthesized by a simple autoclaving method in the presence and absence of surfactant F127, which were used as adsorbents for heavy metal ion and lysozyme in aqueous solution. All the samples possess amorphous framework walls, inorganic-organic network and hierarchical porous structure. The large adsorption capacity of the heavy metal ions were caused by the coordination of the metal ions on the organic ligands inside the pore walls. The lysozyme adsorption results denoted type L (Langmuir) isotherm, and the hydrophobic interactions were considered to be dominant for the adsorption of proteins onto aluminum phosphonate materials. Thus, the synthesized hybrid AlPPh materials were confirmed to be multifunctional adsorbents for different kinds of adsorbates. Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 20473041 and 20673060), the National Basic Research Program of China (No. 2009CB623502), the Specialized Research Fund for the Doctoral Program of Higher Education (20070055014), the Natural Science Foundation of Tianjin (08JCZDJC21500), the ChineseBulgarian Scientific and Technological Cooperation Project, the MOE Supporting Program for New Century Excellent Talents (NCET-06-0215), and Nankai University. Supporting Information Available: N2 adsorption analysis and the adsorption isotherms for Cu(II) ions of the samples. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Dai, S.; Burleigh, M. C.; Ju, Y. H. J. Am. Chem. Soc. 2000, 122, 992. (2) Antochshuk, V.; Jaroniec, M. Chem. Commun. 2002, 258–259. (3) Qiao, S. Z.; Djojoputro, H.; Hu, Q. H.; Lu, G. Q. Prog. Solid State Chem. 2006, 34, 249–256.

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