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Heterocyclic tri-urea isocyanurate bridged groups modified periodic mesoporous organosilica synthesized for Fe(III) adsorption
Journal of Solid State Chemistry 194 (2012) 392–399
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Heterocyclic tri-urea isocyanurate bridged groups modified periodic mesoporous organosilica synthesized for Fe(III) adsorption Vijay Kumar Rana a,c,d, M. Selvaraj b, Surendran Parambadath a, Sang-Wook Chu a, Sung Soo Park a, Satyendra Mishra c, Raj Pal Singh d, Chang-Sik Ha a,n a
Department of Polymer Science and Engineering, Pusan National University, Busan 609–735, South Korea Department of Chemical and Biomolecular Engineering, Pusan National University, Geumjeong-gu, Busan 609–735, South Korea c Department of Chemical Technology, North Maharashtra University Jalgaon–425001, India d Division of Polymer Science and Engineering, National Chemical Laboratory, Pune–411 008, India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 November 2011 Received in revised form 9 May 2012 Accepted 17 May 2012 Available online 1 June 2012
To achieve a high level of heavy metal adsorption, 1,10 ,100 -(1,3,5-triazine-2,4,6-triyl)tris(3-(3-(triethoxysilyl) propyl)urea) (TTPU) was synthesized as a novel melamine precursor and incorporated on the silica surface of periodic mesoporous organosilica (PMO). The melamine modified PMOs (MPMOs) were synthesized under acidic conditions using TTPU, tetraethylorthosilicate (TEOS) and Pluronic P123 as a template and the modified PMOs were characterized using the relevant instrumental techniques. The characteristic materials were used as adsorbents for the adsorption of Fe(III) ions. Fe(III) adsorption studies revealed MPMO-7.5 to be a good absorbent with higher adsorption efficiency than other MPMOs. & 2012 Elsevier Inc. All rights reserved.
Keywords: Periodic mesoporous organosilica (PMO) Melamine Fe(III) adsorption Adsorption efficiency
1. Introduction In 1999, periodic mesoporous organosilicas (PMOs) were synthesized using a range of bridged organic groups containing alkoxysilane ((R00 O)3Si-R0 -Si(OR00 )3) (R00 ¼-CH3 or -C2H5, R0 ¼methane, ethane, ethylene, acetylene, ferrocene, thiophene, and benzene) [1–3]. PMOs were reported to have organic and organo-metallic groups distributed uniformly over their silica surfaces with good physical properties, such as tunable chemical properties, chemical sensing and nano-material fabrication, and were used for the adsorption of heavy metal-ions and in controlled drug delivery systems [4–10]. Using the relevant organic moieties, the surface of PMOs can be modified to improve the adsorption capacity through surface hydrophobicity for the removal of heavy metal-ions. The thermal and mechanical properties of PMOs can also be improved by changing the nature of the organic groups. Many reports have examined the synthesis of PMOs using a range of organic spacerligand groups [11–17]. For example, Olkhovyk and Jaroniec incorporated benzene groups on the surface of PMOs [14]. PMOs synthesized using different organic functional groups containing organic moieties have been used for the adsorption of heavy metalions, such as Hg(II), Cu(II), Co(II), Eu(III), Gd(III) and Fe(III), and their
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adsorption efficiencies were also reported [18–28]. In particular, a recent review by Walcarius and Mercier on mesoporous organosilica adsorbents for the removal of organic and inorganic pollutants is highly tutorial and a good guideline on this subject[29]. In the present study, 1,10 ,100 -(1,3,5-triazine-2,4,6-triyl)tris(3(3-(triethoxysilyl)propyl)urea) (TTPU) with a high molecular weight (MW¼ 868.20) was prepared. Melamine was used as a key backbone molecule to synthesize the organosilica precursor because of its three amine-arm structure. The resulting precursor containing melamine core was expected to have a three-armed bridged structure. The core of this bridging group was an isocyanurate ring integrated with three triethoxysilyl groups through flexible propyl chains. Many amine groups with the urea group of the isocyanurate ring were expected to enhance the adsorption of metal ions (in this study, Fe(III)) due to the presence of a lone pair of electrons from the amine groups. The role of urea groups as metalbinding motifs is well known [28]. In addition, the three-armed bridged structure of the precursor might provide preferable spaces for the adsorption of metal ions owing to its molecular cavity structure. The organic bridging ligand group was incorporated successfully on the surface of PMOs by single-step method using TTPU and tetraethyl orthosilicate (TEOS) and poly(ethylene oxide)poly(propylene oxide)-poly(ethylene oxide) triblock copolymer (Pluronic P123) as a template under acidic conditions. These materials have also been used to obtain the organic bridged
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ligand group modified SBA-15. The obtained PMOs were characterized and used for the adsorption of Fe(III). 2. Experimental 2.1. Chemicals The organic bridging ligand group modified PMOs were synthesized using melamine (99%), 3-(triethoxysilyl) propylisocyanate (95%), tetraethoxysilane (TEOS, 99%), and poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer (Pluronic P123). All chemicals were used as received from Sigma–Aldrich Chemical Co. FeCl3.6H2O (Junsei Chemical Co., Ltd) was used as received for the adsorption studies. 2.2. Synthesis of TTPU For the synthesis of TTPU, melamine (1 mol) was reacted with 3-(triethoxysilyl) propyl isocynate (3 mol) in dimethyl formamide (DMF) or acetonitrile at 80 1C for 24 h. After cooling the reaction mixture, the excess DMF was removed from the mixture using a vacuum rotary evaporator. The mixture was washed several times with hexane and dried under vacuum for further 24 h at 60 1C. (Scheme 1). [(EtO)3Si(CH2)3NHCONH]3(N6C3H3). 1H NMR (d6 DMSO, 300 MHz): d 0.5 (t, 6H, -SiCH2), d 1.1 (t, 27H, –CH2CH3), d 1.5 (p, 6H, –CH2CH2CH2), d 3.1 (q, 6H, –CH2CH2NH), d 3.4 (q, 18H, –CH3CH2SiO), d 3.6 (t, 3H, –CH2NHCONH), d 6.05 (s, 3H, –NHCONH-heterocyclic ring). MS (m/z) calcd for 868.20 [Mþ 1], found 868.198. Yield 93%. 2.3. Synthesis of heterocyclic tri-urea bridged group incorporated PMOs (MPMOs) The MPMOs were synthesized using a previous published procedure [14]. In a typical experimental procedure, 1 g of Pluronic
Scheme 1. Preparation of TTPU organosilica precursor.
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P123 was dissolved in 34 mL of deionized water and 2.21 mL of 5.8 M-HCl with constant stirring for 3 h before adding the specified amounts of TTPU and TEOS. The molar composition of the synthesized mixture was as follows: (1 x) TEOS: x TTPU: 0.017 P123: 188 H2O: 5.8 HCl, where x denotes the number of moles of TTPU. The synthesis of PMO materials was performed using different percentages of TTPU expressed per silicon atom to one mole of the silica precursors, i.e., x¼2.5, 5, 7.5 and 10%. A given amount of TTPU was added to the mixture solution at 30 min prior to adding dropwise TEOS for completely dissolving TTPU in the mixtures. This mixture was stirred for 24 h before the hydrothermal treatment at 100 1C for 24 h. The resulting white precipitate was filtered, and the polymeric template was extracted with an ethanol/hydrochloric acid solution for 24 h at 60 1C. The extraction solution contained 100 mL of a 99% ethanol solution and 4 mL of 12 M HCl. The extraction method was repeated three times for all samples. (Scheme 2). 2.4. Adsorption of Fe(III) The MPMO materials for Fe(III) adsorption were tested using a published procedure [28]. In a typical experimental procedure, 10 mL of a 40 mM aqueous Fe(III) solution was added to 100 mg of the MPMOs. The reaction mixture was stirred for 24 h. Subsequently, the solid sample was recovered from the mixture, filtered off and washed thoroughly with water. After the adsorption experiments, slightly yellowish MPMO materials were recovered. After the Fe(III) adsorption experiments, the amount of Fe(III) adsorbed on MPMO was measured using an inductively coupled plasma optical emission spectrometer. 2.5. Characterization Small-angle X-ray scattering (SAXS) measurements were performed using a synchrotron X-ray source at the Pohang Accel˚ radiation with erator Laboratory (PAL, Korea): Co-Ka (l ¼1.608 A) an energy range of 4–16 keV. The 1H nuclear magnetic resonance (NMR, GEMINI-2000) spectrum of the precursor, 1,10 ,100 -(1,3,5triazine-2,4,6-triyl)tris(3-(3-triethoxysilyl)propyl)urea) (TTPU), was measured on a 300 MHz spectrometer using d6-DMSO as the solvent and tetramethylsilane (TMS) as the internal standard. To confirm the molecular weight of TTPU, the mass spectrum was taken on an Agilent 1100 LC/MSD SL model liquid chromatography-mass spectrometer. The nitrogen adsorption/desorption isotherms for the synthesized samples at 196 1C were measured using the Micromeritics ASAP 2010 surface area and pore size analyzer. All samples were outgassed at 100 1C for 12 h under vacuum. The specific surface area and pore size distribution of the samples was determined using the BET (Brunnauer–Emmett– Teller) method and BJH (Barrett–Joyner–Halenda) method with the adsorption branch of the isotherm plot, respectively. Scanning electron microscopy (SEM, Model 952888(8), Hitachi Ltd) images of the synthesized samples were obtained at an acceleration voltage of 20 kV. Transmission electron microscopy (TEM, JEOL 2021 F) was performed at an acceleration voltage of 200 kV. The PMOs/MPMOs samples pelletized with KBr were characterized by Fourier transform infrared (FT-IR, spectrum-GX) spectroscopy at ambient atmosphere. The 29Si and 13C cross-polarization (CP) magic angle spinning NMR (MAS NMR, Unity-Inova 400) spectra of the samples were obtained using 400 MHz spectrometer at room temperature with a 4 mm zirconia rotor spinning at 6 kHz. The ultraviolet (UV) absorption spectra of the samples were obtained using UV-visible spectrophotometer (U-2010, HITACHI Co.). Thermogravimetric analysis (TGA, TA instruments Q50) of the samples was performed under nitrogen at a heating rate of 10 1C/min. The amount of Fe(III) adsorbed on the MPMOs was
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Scheme 2. Schematic illustration of the preparation of MPMOs.
patterns, however, varied according to the percentage of TTPU with TEOS, as shown in Fig. 1 Three well-resolved peaks were observed in 2.5–7.5 wt% TTPU@MPMOs, but the intensity of two secondary 110 and 200 peaks noticeably decreased in 10 wt% of TTPU@MPMO. This result suggests that both TTPU and TEOS serve cooperatively in building the periodic MPMOs mesostructure up to a certain optimum TTPU/TEOS ratio (here, at 7.5 wt%), though the exact mechanism is not clear yet. It may be considered, however, that when the ratio of TTPU to TEOS is too high (here, at 10 wt%), the interaction between the reactive site of TTPU and TEOS increased and too many SiO2 are replaced with heterocyclic triurea bridging groups, resulting in the decrease of the order of the mesostructure. 3.2. SEM and TEM
Fig. 1. SAXS patterns of the as-synthesized MPMOs.
determined by ICP-optical emission spectroscopy (ICP-OES, JY ULTIMA@C HR, JOBIN YVON, France) in acquisition mode.
3. Results and discussion 3.1. SAXS Fig. 1 shows the SAXS patterns of the as-synthesized MPMOs with various TTPU concentrations (2.5–10%). The SAXS peaks assigned to the 100, 110, and 200 planes reflect a highly ordered two-dimensional hexagonal periodic mesostructure with a space group of p6mm lattice of the SBA-15 type. The intensity of the SAXS
The SEM images of MPMOs prepared with different percentages of TTPU to TEOS are shown in Fig. 2. Apparently, MPMO-2.5 had similar morphology as SBA-15, i.e., rod-like morphology with a bundle of rods of relatively uniform size of 1 mm length and 300 nm diameters, as shown in Fig. 2(a). On increasing the relative amount of TTPU to TEOS to 5–10 wt%, the morphology of MPMOs changed from rod-like one to hexagonal or spherical one.; i.e., MPMO-5 shows somewhat hexagonal morphology while MPMO-7.5 shows clearly hexagonal plate morphology of relatively uniform size ( 0.75 mm) (Fig. 2(b)–Fig. 2(d)). On the other hand, MPMO-10 has spherical morphology of large size (a few mm to ca. 10 mm) with the aggregated particles of smaller size, as shown in Fig. 2(e). The changes in the morphology of the MPMOs are associated with the fraction of TTPU incorporated in the framework wall and possibly various types of interactions, such as electrostatic attraction/repulsion, hydrogen bonding, and hydrophobic/hydrophilic interactions, between the organosilane and surfactant molecules at the micelle/water interface work together when producing the various particle shapes and sizes [30]. TEM images of MPMO-7.5 in Fig. 3 shows that
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Fig. 2. SEM images of the as-synthesized MPMOs; (a) MPMO-2.5, (b) MPMO-5, (c) low magnification image of MPMO-7.5, (d) high resolution image of MPMO-7.5, and (e) MPMO-10.
Fig. 3. TEM images of solvent-extracted MPMO-7.5 with (a) low and (b) high magnifications.
MPMO-7.5 has an excellent periodically well ordered hexagonal and honeycomb like arrangment with p6mm symmetry. Note that the TEM images show clear edge of a particle suggesting the uniformity (0.75 mm) of each partcle with hexagonal plate of MPMO-7.5.
3.3. N2 adsorption/desorption behaviour Fig. 4 shows the N2 adsorption/desorption of MPMOs revealing type IV isotherms with type H1 hysteresis loops. The hysteresis loops observed in the range of 0.5–0.8 P/P0 show the textural
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Fig. 4. N2 adsorption/desorption isotherms of solvent-extracted MPMOs (a) MPMO-2.5. (b) MPMO-5, (c) MPMO-7.5, and (d) MPMO-10.
Table 1 Textural properties of the MPMOs. Sample
Surface area (m2g 1)
Pore diameter ˚ (A)
Pore volume (m3 g 1)
Unit cell size ˚ (a0)a(A)
MPMO-2.5 MPMO-5 MPMO-7.5 MPMO-10
535 529 586 407
59.9 57.7 64.5 59.7
0.84 0.73 0.79 0.66
116.3 116.3 122.6 114.3
a Unit cell size (a0) was estimated from the SAXS data using the formula a0 ¼ 2 d100/O3.
Fig. 5. Pore size distribution of solvent-extracted MPMOs.
properties of the two-dimensional hexagonal periodic mesostructures for all the MPMOs, such as SBA-15 [31–33]. Fig. 5 shows the pore size distributions calculated using the BJH method. Table 1
summarizes the detailed textural properties of MPMOs along with the unit cell size that was estimated from the SAXS result in Fig. 1. One can see that MPMO-7.5 has a larger pore size and unit cell size than the other MPMOs. As already shown in the SAXS patterns in Fig. 1, such differences in the pore sizes and unit cell sizes depending on the types of MPMOs are believed to be related to the competitive reaction of TTPU and TEOS during the sol–gel reaction to result in silica networks of MPMOs, though the pore sizes and unit cell sizes of MPMOs were expected to be generally decreased due to the distribution of organic functional moieties
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on the interior surfaces of the mesopores. It was assumed above that both TTPU and TEOS serve cooperatively in building the periodic MPMOs mesostructure up to a certain optimum TTPU/ TEOS ratio (here at 7.5 wt%). From the results of SAXS, SEM, and nitrogen sorption behaviours, for convenience, we used the MPMO-7.5 for further detailed characterization and metal adsorption studies, unless otherwise specified, as a representative TTPU@MPMO, since the MPMO-7.5 showed the highest surface area and pore diameter as well as best morphology among all the TTPU@MPMO samples.
3.4. TGA and FT-IR results Fig. 6 shows typical TGA curves for the as-synthesized and solvent-extracted MPMO-7.5. The total weight loss of the assynthesized and solvent-extracted MPMO was 38% and 32%, respectively. In both cases, a small mass loss of adsorbed water occurred in the range, 40–120 1C. In the as-synthesized MPMO7.5, the weight loss between 200 and 450 1C was attributed to the loss ( 22 wt%) of the polymeric template and the organic moieties of TTPU on the inner surface of the silica walls. On the other hand, the weight loss ( 14 wt%) in the solvent-extracted MPMO-7.5 over the same temperature ranges were assigned to the organic moieties of TTPU on the inner surface of the silica walls and indicates nearly successful extraction of the polymeric template by a hydrochloric acid/ethanol solution. This result was confirmed further by FT-IR spectroscopy and 13C CP MAS NMR spectroscopy analyses. The FT-IR spectra revealed the presence of all the peaks of the TTPU group in the solvent-extracted MPMO samples (Fig. 7(a)). The vibration peaks at 2980, 2921, and 2850 cm 1 were assigned to the CH stretching vibrations of the –CH2CH2CH2– groups. The bands at 3458 and 1640 cm 1 were assigned to the –N–H and C¼O stretching vibrations of the amide bond of the urea group, respectively. The peak at 1472 cm 1 was assigned to the –N–H bending vibration of the same group [34] of TTPU on the silica framework. In particular, their intensities increased with increasing TTPU in the silica framework. The FT-IR spectra showed the presence of all peaks of the TTPU group in both solvent-extracted and as-synthesized MPMOs, whereas the peak at 1351 cm 1 was assigned to the CH3 bending vibration of P123 which appeared only in the as-synthesized MPMO, which confirms the polymeric
Fig. 6. Thermogravimetric analysis (TGA) curves of the as-synthesized and solvent-extracted MPMO-7.5.
Fig. 7. FT-IR spectra of the solvent-extracted MPMOs with (a) different TTPU contents and the compared FT-IR spectra of (b) as-synthesized and solventextracted MPMO-7.5.
template P123 (Fig. 7(b)) on the inner surface of the silica pore walls. To further confirm the template removal of MPMO, 13C CP MAS NMR analysis was carried out for both solvent-extracted and as-synthesized MPMOs. The peak at 70.4 ppm was corresponded to –CH2– carbons in poly(ethylene oxide) in the Pluronic 123, while three peaks at 23.0, 73.7 and 76.2 ppm confirmed the –CH3, –CH2–, –CH– carbons in poly(propylene oxide) in the Pluronic 123 [35]. These template peaks were almost disappeared after solvent-extraction (Fig. 8(a)). The FTIR and 13C CP MAS NMR spectra of MPMOs confirmed that the polymeric template was almost removed by the solvent-extraction method using the desired amounts of the hydrochloric acid/ ethanol solution. Fig. 8(a) presents the 13C CP MAS NMR spectrum of the representative as-synthesized and solvent-extracted MPMO-7.5. A weak peak at 9 ppm was assigned to aC carbons bonded directly to –Si in the –Si–aCH2–bCH2–cCH2–R fragment. In addition, two predominant peaks at 17.8 and 42.6 ppm confirmed the bC and cC carbons, respectively, where R is a reactive urea site of TTPU. The other two distinct peaks assigned to 156 and 165.5 ppm predict the dC and eC carbons from the C–N bond of the heterocyclic ring, and the –C ¼O bond of the urea groups of TTPU, respectively. This
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24 h stirring at room temperature, respectively. The mixture was filtered off without washing using a Syringe Filter. The filtrate solutions were 50 times diluted for UV/vis spectroscopy analysis (See Fig. S1). The result showed MPMO-7.5 exhibits highest adsorption behavior than other samples. The result may be also related to the most stable mesostructure with stable morphology and textural properties of the MPMO-7.5, as already shown in Table 1 and other SAXS, SEM and TEM results. That’s why MPMO7.5 was chosen for further adsorption experiments using ICP-OES analysis with kinetic analysis. MPMO-2.5 was also, however, chosen just for comparing the good adsorption capacity of MPMO-7.5 with another MPMO having different percentage of TTPU in the MPMO series. The kinetic study of the MPMO-7.5 filtrate solution after Fe(III) absorption with various treatment times at 273 nm on a UV-vis spectra showed that 24 h treatment in Fe(III) solution yielded least filtrate after Fe(III) adsorption (i.e., highest adsorption) for the MPMO-7.5 (see Fig. S2). This is why 24 h treatment condition for the Fe(III) adsorption was chosen in the present work. The Fe(III) adsorption capacity of the MPMO-7.5 and MPMO-2.5 adsorbents was measured by ICPOES. The results showed that the MPMO-2.5 (1 g) and MPMO-7.5 (1 g) adsorbed 39.2 and 57.2 mg Fe(III)-ions, respectively after treatment for 24 h. As expected, MPMO-7.5 showed better Fe(III) adsorption capacity than MPMO-2.5, meaning that a higher
Fig. 8. Solid-state (a) 13C CP MAS NMR spectra of as-synthesized and solventextracted MPMO-7.5 and (b) 29Si MAS NMR spectrum of solvent-extracted MPMO-7.5.
confirmed the homogeneous conformation of TTPU organosilica moieties in the MPMO frameworks. The 29Si MAS NMR spectrum of the solvent-extracted MPMO7.5 shows both T and Q sites, as expected (Fig. 8(b)). In the spectrum, two signals at 73.4 and 75.1 ppm confirmed the two Si environments of T2 [CSi(OH)(OSi)2] and T3 [CSi(OSi)3], respectively. The strong intensity of the peak due to the T3 site indicates the formation of highly condensed organosilica in the framework of the silica wall. The other three signals at 96.1, 105.0 and 111.1 ppm were assigned to the three Si environments peaks of Q2 (Si(OH)2(OSi)2), Q3 (SiOH(OSi)3) and Q4 (Si(OSi)4), respectively, indicating a high degree of TEOS crosslinking under the synthetic conditions employed. 3.5. Adsorption of Fe(III)-ions Adsorption experiments for Fe(III) were performed for the samples with different percentages of incorporated TTPU inside silica framework using UV/vis spectroscopy. The intensity of the UV/vis spectra of the MPMOs’ filtrate solution after Fe(III) absorption with various amounts of TTPU containing MPMOs at 273 nm was measured. 100 mg of MPMO-2.5, MPMO-5, MPMO-7.5 and MPMO-10 were added to 10 mL of 40 mM-Fe(III) solution with
Fig. 9. UV/vis spectra (a) and FT-IR spectra (b) of MPMO-2.5 and MPMO-7.5 with and without Fe(III) loading.
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percentage of multifunctional heterocyclic tri-urea isocyanurate bridged group (TTPU) incorporated on the well ordered PMOs increases the adsorption efficiency. To further confirm the results, UV/vis spectroscopy and FT-IR spectroscopy were also used to characterize the Fe(III) loaded MPMOs (Fig. 9). The UV/vis spectra of Fe(III) loaded into the MPMOs exhibited shoulders at ca. 250 nm with higher intensities due to the strong interaction between Fe(III) and TTPU (Fig. 9(a)). The Fe(III)-ion adsorption capacity of MPMO-2.5 and MPMO-7.5 was approximately 3 and 5 times higher, respectively, than the values reported in previous studies using other mesoporous silicas [28] due to the presence of abundant amine groups with the urea group of the isocyanurate ring. The urea groups play a role as metal-binding motifs and the three-armed bridged structure of the precursor provides preferable spaces of the adsorption of metal ions. In the FT-IR spectra of Fig. 9(b), the main characteristic peaks appeared at 960 cm 1 (Si–OH strechting), 1091 cm 1 (–Si–O–Si– stretching), and 1479 cm 1 (C–N bending) for both samples of MPMO-7.5 and MPMO-7.5/Fe. As discussed for the Fe(III) unloaded MPMO-7.5, the FT-IR bands at 1538 and 1646 cm 1 were assigned to the –N-H bending and C¼O stretching vibration of the amide bond of the urea groups, respectively. On the other hand, the FT-IR bands for the Fe(III) loaded MPMO-7.5 shifted slightly from 1538 to 1547 and 1646 to 1641 cm 1 for –N-H bending and C¼O stretching vibration of the amide bond of the urea groups, respectively, which confirms the chelating properties of functional groups. Besides, the shifting of peaks in the IR spectra of -NH and -C¼ O absorption peaks can be also attributed to hydrogen-bonding interactions between the functional groups and water molecules.An earlier detailed investigation of various cation-urea complexes, among them, in particular Fe(III)-urea, revealed that Fe(III) binds preferentially to the nitrogen atom and carbonyl oxygen in the urea group [28,36]. Therefore, a similar binding of Fe(III) may be also assumed for our MPMOs.
4. Conclusions A new organosilica precursor, TTPU, was synthesized successfully and characterized. TTPU was used as an organosilica functional agent in the preparation of MPMOs. SAXS and N2 adsorption confirmed that the MPMOs synthesized with different amounts of TTPU have well ordered mesoporous structures with a large surface area. SEM and TEM showed that the MPMOs have a rod-like morphology with a well ordered uniform pore structure. Based on the Fe(III) adsorption studies, MPMO-7.5 showed higher adsorption efficiency than the other MPMOs.
Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) through the Acceleration Research Program, Korea (MEST) (Acceleration Research Program (No. 2012-0000108); Pioneer Research Center Program (No. 2012-0000421/20120000422)), the Brain Korea 21 Project funded by Ministry of Education, Science and Technology, and a grant from the
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Fundamental R & D program for Core Technology of Materials funded by the Ministry of Knowledge Economy. M.Selvaraj greatly acknowledges the PNU research grant for 2 years.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jssc.2012.05.019.
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Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Heterocyclic tri-urea isocyanurate bridged groups modified periodic mesoporous organosilica synthesized for Fe(III) adsorption Vijay Kumar Rana a,c,d, M. Selvaraj b, Surendran Parambadath a, Sang-Wook Chu a, Sung Soo Park a, Satyendra Mishra c, Raj Pal Singh d, Chang-Sik Ha a,n a
Department of Polymer Science and Engineering, Pusan National University, Busan 609–735, South Korea Department of Chemical and Biomolecular Engineering, Pusan National University, Geumjeong-gu, Busan 609–735, South Korea c Department of Chemical Technology, North Maharashtra University Jalgaon–425001, India d Division of Polymer Science and Engineering, National Chemical Laboratory, Pune–411 008, India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 November 2011 Received in revised form 9 May 2012 Accepted 17 May 2012 Available online 1 June 2012
To achieve a high level of heavy metal adsorption, 1,10 ,100 -(1,3,5-triazine-2,4,6-triyl)tris(3-(3-(triethoxysilyl) propyl)urea) (TTPU) was synthesized as a novel melamine precursor and incorporated on the silica surface of periodic mesoporous organosilica (PMO). The melamine modified PMOs (MPMOs) were synthesized under acidic conditions using TTPU, tetraethylorthosilicate (TEOS) and Pluronic P123 as a template and the modified PMOs were characterized using the relevant instrumental techniques. The characteristic materials were used as adsorbents for the adsorption of Fe(III) ions. Fe(III) adsorption studies revealed MPMO-7.5 to be a good absorbent with higher adsorption efficiency than other MPMOs. & 2012 Elsevier Inc. All rights reserved.
Keywords: Periodic mesoporous organosilica (PMO) Melamine Fe(III) adsorption Adsorption efficiency
1. Introduction In 1999, periodic mesoporous organosilicas (PMOs) were synthesized using a range of bridged organic groups containing alkoxysilane ((R00 O)3Si-R0 -Si(OR00 )3) (R00 ¼-CH3 or -C2H5, R0 ¼methane, ethane, ethylene, acetylene, ferrocene, thiophene, and benzene) [1–3]. PMOs were reported to have organic and organo-metallic groups distributed uniformly over their silica surfaces with good physical properties, such as tunable chemical properties, chemical sensing and nano-material fabrication, and were used for the adsorption of heavy metal-ions and in controlled drug delivery systems [4–10]. Using the relevant organic moieties, the surface of PMOs can be modified to improve the adsorption capacity through surface hydrophobicity for the removal of heavy metal-ions. The thermal and mechanical properties of PMOs can also be improved by changing the nature of the organic groups. Many reports have examined the synthesis of PMOs using a range of organic spacerligand groups [11–17]. For example, Olkhovyk and Jaroniec incorporated benzene groups on the surface of PMOs [14]. PMOs synthesized using different organic functional groups containing organic moieties have been used for the adsorption of heavy metalions, such as Hg(II), Cu(II), Co(II), Eu(III), Gd(III) and Fe(III), and their
n
Corresponding author. Fax: þ82 51 514 4331. E-mail address: [email protected] (C.-S. Ha).
0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2012.05.019
adsorption efficiencies were also reported [18–28]. In particular, a recent review by Walcarius and Mercier on mesoporous organosilica adsorbents for the removal of organic and inorganic pollutants is highly tutorial and a good guideline on this subject[29]. In the present study, 1,10 ,100 -(1,3,5-triazine-2,4,6-triyl)tris(3(3-(triethoxysilyl)propyl)urea) (TTPU) with a high molecular weight (MW¼ 868.20) was prepared. Melamine was used as a key backbone molecule to synthesize the organosilica precursor because of its three amine-arm structure. The resulting precursor containing melamine core was expected to have a three-armed bridged structure. The core of this bridging group was an isocyanurate ring integrated with three triethoxysilyl groups through flexible propyl chains. Many amine groups with the urea group of the isocyanurate ring were expected to enhance the adsorption of metal ions (in this study, Fe(III)) due to the presence of a lone pair of electrons from the amine groups. The role of urea groups as metalbinding motifs is well known [28]. In addition, the three-armed bridged structure of the precursor might provide preferable spaces for the adsorption of metal ions owing to its molecular cavity structure. The organic bridging ligand group was incorporated successfully on the surface of PMOs by single-step method using TTPU and tetraethyl orthosilicate (TEOS) and poly(ethylene oxide)poly(propylene oxide)-poly(ethylene oxide) triblock copolymer (Pluronic P123) as a template under acidic conditions. These materials have also been used to obtain the organic bridged
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ligand group modified SBA-15. The obtained PMOs were characterized and used for the adsorption of Fe(III). 2. Experimental 2.1. Chemicals The organic bridging ligand group modified PMOs were synthesized using melamine (99%), 3-(triethoxysilyl) propylisocyanate (95%), tetraethoxysilane (TEOS, 99%), and poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer (Pluronic P123). All chemicals were used as received from Sigma–Aldrich Chemical Co. FeCl3.6H2O (Junsei Chemical Co., Ltd) was used as received for the adsorption studies. 2.2. Synthesis of TTPU For the synthesis of TTPU, melamine (1 mol) was reacted with 3-(triethoxysilyl) propyl isocynate (3 mol) in dimethyl formamide (DMF) or acetonitrile at 80 1C for 24 h. After cooling the reaction mixture, the excess DMF was removed from the mixture using a vacuum rotary evaporator. The mixture was washed several times with hexane and dried under vacuum for further 24 h at 60 1C. (Scheme 1). [(EtO)3Si(CH2)3NHCONH]3(N6C3H3). 1H NMR (d6 DMSO, 300 MHz): d 0.5 (t, 6H, -SiCH2), d 1.1 (t, 27H, –CH2CH3), d 1.5 (p, 6H, –CH2CH2CH2), d 3.1 (q, 6H, –CH2CH2NH), d 3.4 (q, 18H, –CH3CH2SiO), d 3.6 (t, 3H, –CH2NHCONH), d 6.05 (s, 3H, –NHCONH-heterocyclic ring). MS (m/z) calcd for 868.20 [Mþ 1], found 868.198. Yield 93%. 2.3. Synthesis of heterocyclic tri-urea bridged group incorporated PMOs (MPMOs) The MPMOs were synthesized using a previous published procedure [14]. In a typical experimental procedure, 1 g of Pluronic
Scheme 1. Preparation of TTPU organosilica precursor.
393
P123 was dissolved in 34 mL of deionized water and 2.21 mL of 5.8 M-HCl with constant stirring for 3 h before adding the specified amounts of TTPU and TEOS. The molar composition of the synthesized mixture was as follows: (1 x) TEOS: x TTPU: 0.017 P123: 188 H2O: 5.8 HCl, where x denotes the number of moles of TTPU. The synthesis of PMO materials was performed using different percentages of TTPU expressed per silicon atom to one mole of the silica precursors, i.e., x¼2.5, 5, 7.5 and 10%. A given amount of TTPU was added to the mixture solution at 30 min prior to adding dropwise TEOS for completely dissolving TTPU in the mixtures. This mixture was stirred for 24 h before the hydrothermal treatment at 100 1C for 24 h. The resulting white precipitate was filtered, and the polymeric template was extracted with an ethanol/hydrochloric acid solution for 24 h at 60 1C. The extraction solution contained 100 mL of a 99% ethanol solution and 4 mL of 12 M HCl. The extraction method was repeated three times for all samples. (Scheme 2). 2.4. Adsorption of Fe(III) The MPMO materials for Fe(III) adsorption were tested using a published procedure [28]. In a typical experimental procedure, 10 mL of a 40 mM aqueous Fe(III) solution was added to 100 mg of the MPMOs. The reaction mixture was stirred for 24 h. Subsequently, the solid sample was recovered from the mixture, filtered off and washed thoroughly with water. After the adsorption experiments, slightly yellowish MPMO materials were recovered. After the Fe(III) adsorption experiments, the amount of Fe(III) adsorbed on MPMO was measured using an inductively coupled plasma optical emission spectrometer. 2.5. Characterization Small-angle X-ray scattering (SAXS) measurements were performed using a synchrotron X-ray source at the Pohang Accel˚ radiation with erator Laboratory (PAL, Korea): Co-Ka (l ¼1.608 A) an energy range of 4–16 keV. The 1H nuclear magnetic resonance (NMR, GEMINI-2000) spectrum of the precursor, 1,10 ,100 -(1,3,5triazine-2,4,6-triyl)tris(3-(3-triethoxysilyl)propyl)urea) (TTPU), was measured on a 300 MHz spectrometer using d6-DMSO as the solvent and tetramethylsilane (TMS) as the internal standard. To confirm the molecular weight of TTPU, the mass spectrum was taken on an Agilent 1100 LC/MSD SL model liquid chromatography-mass spectrometer. The nitrogen adsorption/desorption isotherms for the synthesized samples at 196 1C were measured using the Micromeritics ASAP 2010 surface area and pore size analyzer. All samples were outgassed at 100 1C for 12 h under vacuum. The specific surface area and pore size distribution of the samples was determined using the BET (Brunnauer–Emmett– Teller) method and BJH (Barrett–Joyner–Halenda) method with the adsorption branch of the isotherm plot, respectively. Scanning electron microscopy (SEM, Model 952888(8), Hitachi Ltd) images of the synthesized samples were obtained at an acceleration voltage of 20 kV. Transmission electron microscopy (TEM, JEOL 2021 F) was performed at an acceleration voltage of 200 kV. The PMOs/MPMOs samples pelletized with KBr were characterized by Fourier transform infrared (FT-IR, spectrum-GX) spectroscopy at ambient atmosphere. The 29Si and 13C cross-polarization (CP) magic angle spinning NMR (MAS NMR, Unity-Inova 400) spectra of the samples were obtained using 400 MHz spectrometer at room temperature with a 4 mm zirconia rotor spinning at 6 kHz. The ultraviolet (UV) absorption spectra of the samples were obtained using UV-visible spectrophotometer (U-2010, HITACHI Co.). Thermogravimetric analysis (TGA, TA instruments Q50) of the samples was performed under nitrogen at a heating rate of 10 1C/min. The amount of Fe(III) adsorbed on the MPMOs was
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Scheme 2. Schematic illustration of the preparation of MPMOs.
patterns, however, varied according to the percentage of TTPU with TEOS, as shown in Fig. 1 Three well-resolved peaks were observed in 2.5–7.5 wt% TTPU@MPMOs, but the intensity of two secondary 110 and 200 peaks noticeably decreased in 10 wt% of TTPU@MPMO. This result suggests that both TTPU and TEOS serve cooperatively in building the periodic MPMOs mesostructure up to a certain optimum TTPU/TEOS ratio (here, at 7.5 wt%), though the exact mechanism is not clear yet. It may be considered, however, that when the ratio of TTPU to TEOS is too high (here, at 10 wt%), the interaction between the reactive site of TTPU and TEOS increased and too many SiO2 are replaced with heterocyclic triurea bridging groups, resulting in the decrease of the order of the mesostructure. 3.2. SEM and TEM
Fig. 1. SAXS patterns of the as-synthesized MPMOs.
determined by ICP-optical emission spectroscopy (ICP-OES, JY ULTIMA@C HR, JOBIN YVON, France) in acquisition mode.
3. Results and discussion 3.1. SAXS Fig. 1 shows the SAXS patterns of the as-synthesized MPMOs with various TTPU concentrations (2.5–10%). The SAXS peaks assigned to the 100, 110, and 200 planes reflect a highly ordered two-dimensional hexagonal periodic mesostructure with a space group of p6mm lattice of the SBA-15 type. The intensity of the SAXS
The SEM images of MPMOs prepared with different percentages of TTPU to TEOS are shown in Fig. 2. Apparently, MPMO-2.5 had similar morphology as SBA-15, i.e., rod-like morphology with a bundle of rods of relatively uniform size of 1 mm length and 300 nm diameters, as shown in Fig. 2(a). On increasing the relative amount of TTPU to TEOS to 5–10 wt%, the morphology of MPMOs changed from rod-like one to hexagonal or spherical one.; i.e., MPMO-5 shows somewhat hexagonal morphology while MPMO-7.5 shows clearly hexagonal plate morphology of relatively uniform size ( 0.75 mm) (Fig. 2(b)–Fig. 2(d)). On the other hand, MPMO-10 has spherical morphology of large size (a few mm to ca. 10 mm) with the aggregated particles of smaller size, as shown in Fig. 2(e). The changes in the morphology of the MPMOs are associated with the fraction of TTPU incorporated in the framework wall and possibly various types of interactions, such as electrostatic attraction/repulsion, hydrogen bonding, and hydrophobic/hydrophilic interactions, between the organosilane and surfactant molecules at the micelle/water interface work together when producing the various particle shapes and sizes [30]. TEM images of MPMO-7.5 in Fig. 3 shows that
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Fig. 2. SEM images of the as-synthesized MPMOs; (a) MPMO-2.5, (b) MPMO-5, (c) low magnification image of MPMO-7.5, (d) high resolution image of MPMO-7.5, and (e) MPMO-10.
Fig. 3. TEM images of solvent-extracted MPMO-7.5 with (a) low and (b) high magnifications.
MPMO-7.5 has an excellent periodically well ordered hexagonal and honeycomb like arrangment with p6mm symmetry. Note that the TEM images show clear edge of a particle suggesting the uniformity (0.75 mm) of each partcle with hexagonal plate of MPMO-7.5.
3.3. N2 adsorption/desorption behaviour Fig. 4 shows the N2 adsorption/desorption of MPMOs revealing type IV isotherms with type H1 hysteresis loops. The hysteresis loops observed in the range of 0.5–0.8 P/P0 show the textural
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Fig. 4. N2 adsorption/desorption isotherms of solvent-extracted MPMOs (a) MPMO-2.5. (b) MPMO-5, (c) MPMO-7.5, and (d) MPMO-10.
Table 1 Textural properties of the MPMOs. Sample
Surface area (m2g 1)
Pore diameter ˚ (A)
Pore volume (m3 g 1)
Unit cell size ˚ (a0)a(A)
MPMO-2.5 MPMO-5 MPMO-7.5 MPMO-10
535 529 586 407
59.9 57.7 64.5 59.7
0.84 0.73 0.79 0.66
116.3 116.3 122.6 114.3
a Unit cell size (a0) was estimated from the SAXS data using the formula a0 ¼ 2 d100/O3.
Fig. 5. Pore size distribution of solvent-extracted MPMOs.
properties of the two-dimensional hexagonal periodic mesostructures for all the MPMOs, such as SBA-15 [31–33]. Fig. 5 shows the pore size distributions calculated using the BJH method. Table 1
summarizes the detailed textural properties of MPMOs along with the unit cell size that was estimated from the SAXS result in Fig. 1. One can see that MPMO-7.5 has a larger pore size and unit cell size than the other MPMOs. As already shown in the SAXS patterns in Fig. 1, such differences in the pore sizes and unit cell sizes depending on the types of MPMOs are believed to be related to the competitive reaction of TTPU and TEOS during the sol–gel reaction to result in silica networks of MPMOs, though the pore sizes and unit cell sizes of MPMOs were expected to be generally decreased due to the distribution of organic functional moieties
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on the interior surfaces of the mesopores. It was assumed above that both TTPU and TEOS serve cooperatively in building the periodic MPMOs mesostructure up to a certain optimum TTPU/ TEOS ratio (here at 7.5 wt%). From the results of SAXS, SEM, and nitrogen sorption behaviours, for convenience, we used the MPMO-7.5 for further detailed characterization and metal adsorption studies, unless otherwise specified, as a representative TTPU@MPMO, since the MPMO-7.5 showed the highest surface area and pore diameter as well as best morphology among all the TTPU@MPMO samples.
3.4. TGA and FT-IR results Fig. 6 shows typical TGA curves for the as-synthesized and solvent-extracted MPMO-7.5. The total weight loss of the assynthesized and solvent-extracted MPMO was 38% and 32%, respectively. In both cases, a small mass loss of adsorbed water occurred in the range, 40–120 1C. In the as-synthesized MPMO7.5, the weight loss between 200 and 450 1C was attributed to the loss ( 22 wt%) of the polymeric template and the organic moieties of TTPU on the inner surface of the silica walls. On the other hand, the weight loss ( 14 wt%) in the solvent-extracted MPMO-7.5 over the same temperature ranges were assigned to the organic moieties of TTPU on the inner surface of the silica walls and indicates nearly successful extraction of the polymeric template by a hydrochloric acid/ethanol solution. This result was confirmed further by FT-IR spectroscopy and 13C CP MAS NMR spectroscopy analyses. The FT-IR spectra revealed the presence of all the peaks of the TTPU group in the solvent-extracted MPMO samples (Fig. 7(a)). The vibration peaks at 2980, 2921, and 2850 cm 1 were assigned to the CH stretching vibrations of the –CH2CH2CH2– groups. The bands at 3458 and 1640 cm 1 were assigned to the –N–H and C¼O stretching vibrations of the amide bond of the urea group, respectively. The peak at 1472 cm 1 was assigned to the –N–H bending vibration of the same group [34] of TTPU on the silica framework. In particular, their intensities increased with increasing TTPU in the silica framework. The FT-IR spectra showed the presence of all peaks of the TTPU group in both solvent-extracted and as-synthesized MPMOs, whereas the peak at 1351 cm 1 was assigned to the CH3 bending vibration of P123 which appeared only in the as-synthesized MPMO, which confirms the polymeric
Fig. 6. Thermogravimetric analysis (TGA) curves of the as-synthesized and solvent-extracted MPMO-7.5.
Fig. 7. FT-IR spectra of the solvent-extracted MPMOs with (a) different TTPU contents and the compared FT-IR spectra of (b) as-synthesized and solventextracted MPMO-7.5.
template P123 (Fig. 7(b)) on the inner surface of the silica pore walls. To further confirm the template removal of MPMO, 13C CP MAS NMR analysis was carried out for both solvent-extracted and as-synthesized MPMOs. The peak at 70.4 ppm was corresponded to –CH2– carbons in poly(ethylene oxide) in the Pluronic 123, while three peaks at 23.0, 73.7 and 76.2 ppm confirmed the –CH3, –CH2–, –CH– carbons in poly(propylene oxide) in the Pluronic 123 [35]. These template peaks were almost disappeared after solvent-extraction (Fig. 8(a)). The FTIR and 13C CP MAS NMR spectra of MPMOs confirmed that the polymeric template was almost removed by the solvent-extraction method using the desired amounts of the hydrochloric acid/ ethanol solution. Fig. 8(a) presents the 13C CP MAS NMR spectrum of the representative as-synthesized and solvent-extracted MPMO-7.5. A weak peak at 9 ppm was assigned to aC carbons bonded directly to –Si in the –Si–aCH2–bCH2–cCH2–R fragment. In addition, two predominant peaks at 17.8 and 42.6 ppm confirmed the bC and cC carbons, respectively, where R is a reactive urea site of TTPU. The other two distinct peaks assigned to 156 and 165.5 ppm predict the dC and eC carbons from the C–N bond of the heterocyclic ring, and the –C ¼O bond of the urea groups of TTPU, respectively. This
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24 h stirring at room temperature, respectively. The mixture was filtered off without washing using a Syringe Filter. The filtrate solutions were 50 times diluted for UV/vis spectroscopy analysis (See Fig. S1). The result showed MPMO-7.5 exhibits highest adsorption behavior than other samples. The result may be also related to the most stable mesostructure with stable morphology and textural properties of the MPMO-7.5, as already shown in Table 1 and other SAXS, SEM and TEM results. That’s why MPMO7.5 was chosen for further adsorption experiments using ICP-OES analysis with kinetic analysis. MPMO-2.5 was also, however, chosen just for comparing the good adsorption capacity of MPMO-7.5 with another MPMO having different percentage of TTPU in the MPMO series. The kinetic study of the MPMO-7.5 filtrate solution after Fe(III) absorption with various treatment times at 273 nm on a UV-vis spectra showed that 24 h treatment in Fe(III) solution yielded least filtrate after Fe(III) adsorption (i.e., highest adsorption) for the MPMO-7.5 (see Fig. S2). This is why 24 h treatment condition for the Fe(III) adsorption was chosen in the present work. The Fe(III) adsorption capacity of the MPMO-7.5 and MPMO-2.5 adsorbents was measured by ICPOES. The results showed that the MPMO-2.5 (1 g) and MPMO-7.5 (1 g) adsorbed 39.2 and 57.2 mg Fe(III)-ions, respectively after treatment for 24 h. As expected, MPMO-7.5 showed better Fe(III) adsorption capacity than MPMO-2.5, meaning that a higher
Fig. 8. Solid-state (a) 13C CP MAS NMR spectra of as-synthesized and solventextracted MPMO-7.5 and (b) 29Si MAS NMR spectrum of solvent-extracted MPMO-7.5.
confirmed the homogeneous conformation of TTPU organosilica moieties in the MPMO frameworks. The 29Si MAS NMR spectrum of the solvent-extracted MPMO7.5 shows both T and Q sites, as expected (Fig. 8(b)). In the spectrum, two signals at 73.4 and 75.1 ppm confirmed the two Si environments of T2 [CSi(OH)(OSi)2] and T3 [CSi(OSi)3], respectively. The strong intensity of the peak due to the T3 site indicates the formation of highly condensed organosilica in the framework of the silica wall. The other three signals at 96.1, 105.0 and 111.1 ppm were assigned to the three Si environments peaks of Q2 (Si(OH)2(OSi)2), Q3 (SiOH(OSi)3) and Q4 (Si(OSi)4), respectively, indicating a high degree of TEOS crosslinking under the synthetic conditions employed. 3.5. Adsorption of Fe(III)-ions Adsorption experiments for Fe(III) were performed for the samples with different percentages of incorporated TTPU inside silica framework using UV/vis spectroscopy. The intensity of the UV/vis spectra of the MPMOs’ filtrate solution after Fe(III) absorption with various amounts of TTPU containing MPMOs at 273 nm was measured. 100 mg of MPMO-2.5, MPMO-5, MPMO-7.5 and MPMO-10 were added to 10 mL of 40 mM-Fe(III) solution with
Fig. 9. UV/vis spectra (a) and FT-IR spectra (b) of MPMO-2.5 and MPMO-7.5 with and without Fe(III) loading.
V.K. Rana et al. / Journal of Solid State Chemistry 194 (2012) 392–399
percentage of multifunctional heterocyclic tri-urea isocyanurate bridged group (TTPU) incorporated on the well ordered PMOs increases the adsorption efficiency. To further confirm the results, UV/vis spectroscopy and FT-IR spectroscopy were also used to characterize the Fe(III) loaded MPMOs (Fig. 9). The UV/vis spectra of Fe(III) loaded into the MPMOs exhibited shoulders at ca. 250 nm with higher intensities due to the strong interaction between Fe(III) and TTPU (Fig. 9(a)). The Fe(III)-ion adsorption capacity of MPMO-2.5 and MPMO-7.5 was approximately 3 and 5 times higher, respectively, than the values reported in previous studies using other mesoporous silicas [28] due to the presence of abundant amine groups with the urea group of the isocyanurate ring. The urea groups play a role as metal-binding motifs and the three-armed bridged structure of the precursor provides preferable spaces of the adsorption of metal ions. In the FT-IR spectra of Fig. 9(b), the main characteristic peaks appeared at 960 cm 1 (Si–OH strechting), 1091 cm 1 (–Si–O–Si– stretching), and 1479 cm 1 (C–N bending) for both samples of MPMO-7.5 and MPMO-7.5/Fe. As discussed for the Fe(III) unloaded MPMO-7.5, the FT-IR bands at 1538 and 1646 cm 1 were assigned to the –N-H bending and C¼O stretching vibration of the amide bond of the urea groups, respectively. On the other hand, the FT-IR bands for the Fe(III) loaded MPMO-7.5 shifted slightly from 1538 to 1547 and 1646 to 1641 cm 1 for –N-H bending and C¼O stretching vibration of the amide bond of the urea groups, respectively, which confirms the chelating properties of functional groups. Besides, the shifting of peaks in the IR spectra of -NH and -C¼ O absorption peaks can be also attributed to hydrogen-bonding interactions between the functional groups and water molecules.An earlier detailed investigation of various cation-urea complexes, among them, in particular Fe(III)-urea, revealed that Fe(III) binds preferentially to the nitrogen atom and carbonyl oxygen in the urea group [28,36]. Therefore, a similar binding of Fe(III) may be also assumed for our MPMOs.
4. Conclusions A new organosilica precursor, TTPU, was synthesized successfully and characterized. TTPU was used as an organosilica functional agent in the preparation of MPMOs. SAXS and N2 adsorption confirmed that the MPMOs synthesized with different amounts of TTPU have well ordered mesoporous structures with a large surface area. SEM and TEM showed that the MPMOs have a rod-like morphology with a well ordered uniform pore structure. Based on the Fe(III) adsorption studies, MPMO-7.5 showed higher adsorption efficiency than the other MPMOs.
Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) through the Acceleration Research Program, Korea (MEST) (Acceleration Research Program (No. 2012-0000108); Pioneer Research Center Program (No. 2012-0000421/20120000422)), the Brain Korea 21 Project funded by Ministry of Education, Science and Technology, and a grant from the
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Fundamental R & D program for Core Technology of Materials funded by the Ministry of Knowledge Economy. M.Selvaraj greatly acknowledges the PNU research grant for 2 years.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jssc.2012.05.019.
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