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Synthesis and characterization of hybrid mesoporous materials of triblock copolymer and bridged silsesquioxane

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  • Save International Journal of M aterials and Chemistry 2013, 3(3A): 21-28 DOI: 10.5923/s.ijmc.201303.04 Synthesis and Characterization of Hybrid Mesoporous Materials Prepared with Triblock-Copolymer and Bridged Silsesquioxane Nina E. Velikova1,*, Yuliya E. Vueva2, Yordanka Y. Ivanova1, Yanko B. Dimitriev1, Isabel M. Miranda Salvado3, M. Helena F. V. Fernandes3 1Department of Silicate Technology, University of Chemical Technology and M etallurgy, Sofia, 1756, Bulgaria 2Department of M aterials Imperial College London Exhibition Road London, SW7 2AZ UK 3Ceramic and Glass Engineering Department CICECO, University of Aveiro, Aveiro, 3810-193, Portugal Abstract In this study functionalized SBA-15 mesoporous silica materials have been synthesized through a simp le co-condensation approach of bis-[3-(t rimethoxyosily l)propyl] amine (BTPA) and tetraethyl orthosilicate (TEOS) using xy lene like swelling agent, amphih ilic trib lock-copolymer (PEO-PPO-PEO) Pluronic P123 as structural direct ing agent under acidic condition, and inorganic salt has been used to improve mesostructure ordering and tailor framework porosity. The influence of a mount of BTPA on the structural properties of the resultant materia ls was investigated. The resultant materia ls have been characterized by powder X-ray d iffraction, FT-IR, 29 Si MAS NMR, 13C CP MAS NM R, SEM and elemental analysis. In order to e xa mine the potential of these materials as adsorbents for heavy meta ls Hg (II) adsorption experiments were also performed. The results from 29 Si MAS NM R and elemental analysis showed that with increasing the amount of BTPA in the synthesis mixture leads to increasing the incorporation of organic groups in the silica framewo rk. Adsorption of Hg (II) ions fro m aqueous solutions showed high adsorption capacities suggested that these materials have good potential to be used as adsorbents for Hg (II) ions in acid solutions. Keywords Silica-based Mesoporous Organic–Inorganic Hybrid Materials, A mine Functionalized Organosilicas, Surfactant Template Method 1. Introduction In the last 20 years have been developed organic-inorganic hybrid silica-based porous materials, because of their fascinating properties including a regular mesostructure along with high specific surface areas, thermal and mechanical stability, h ighly uniform pore distribution and tunable pore size, h igh adsorption capacity, as well as extraordinarily wide possibilities of functionalization[1–7]. Several researchers have reported various synthesis methodologies to prepare mesoporous oxide materials and proposed different mechanisms to exp lain the porous s tru ctu res [8]. Various factors, such as starting materials (e.g., alko xides, metal salts etc, surfactants as structure directing agent), reaction parameters (e.g. p H, temperature, solvent, co-solvent etc.) influence the format ion of porous structures and dictate the pore size, its distribution and ordering[8, 9]. * Corresponding author: (Nina E. Velikova) Published online at Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved The possibility of introduction of organic groups with different nature and their homogeneous distribution during the synthesis is a very important advantage connected with the sol-gel synthesis. The precursor BTPA (R`O)3Si–R–Si(OR`)3 is capable of undergoing cross-linking reactions and concurrently carry the organic functionality R a priori on board, so leading to materials in wh ich the density of the organic functional groups cannot be higher. These kinds of materia ls, where the functional group is bridged are called periodic mesoporous organosilicas (PMOs). The PMOs materials were firstly discovered 1999[10-12]. In some cases the ‘dilution’ of the reaction mixture with pure inorganic p recursors (TMOS / TEOS) is helpful, although at the (disproportionate) cost of the density of organic groups. The fact that the organic groups are covalently bonded and embedded in these bis-silylated co mpounds leads to mesostructured hybrids in which the organic groups are an integral part of the pore walls, meaning that the framework itself is modified. These materials are called ‘hybrids’ since the inorganic and organic parts are completely homogeneously mixed at the mo lecular scale within the whole samp le.[13] Integration of amine functional groups into the structure of mesoporous 22 Nina E. Velikova et al.: Synthesis and Characterization of Hybrid M esoporous M aterials Prepared with Triblock-Copolymer and Bridged Silsesquioxane organic-inorganic silica hybrids is of big interest due to the versatile applications of the resultant materials provided by the chemistry of the amine functional group, which include base-catalysis[14] coupling and immob ilizat ion of functional mo lecules and bio mo lecu les[15], drug delivery[16], adsorpt ion and sequestration of heavy metal ions[17, 18], etc. It has been suggested that preparing amine-functionalized materials with higher concentrations of functional groups will be important for enhancing their mo lecular accessibility in adsorbence and catalysis applications[19] In this paper we report on synthesis of mesoporous amine functionalized organosilicas prepared via a sol-gel process by using a co-condensation of a basic amine-bridged polysilsesquioxane bis-[3-(trimetho xyosilyl)p ropyl] amine (BTPA) and tetraethylortosilacate (TEOS) in the presence of nonionic surfactants. To estimate the influence of the amine-bridged polysilsesquio xane on materials structure we synthesized gels at different amounts BTPA. The samples were characterized by XRD, solid-state 29Si MAS NMR, 13C CP MAS NM R, FT-IR, SEM, e le mental analysis. In order to examine the potential of these materials as adsorbents for heavy metals, Hg (II) adsorption experiments were also performed. amine-functionalized mesoporous gels. Like swelling agent was used xylene (Aldrich). To improve mesostructure ordering and tailor framework porosity was used KCl (Aldrich). Ethanol (99.8%), d istilled water and Fuming hydrochloric acid (HCl, 36%, Pro mark Chemicals) was used for re mova l of surfactant. 2.2. Synthesis of Mesoporous Silica The gels were prepared through a one-step sol–gel process catalyzed by the –NH– groups of BTPA. In a typical synthesis (Scheme 1) 1.2g of Pluron ic P123 was dissolved in deionized water (10ml) and 52ml 1M HCl solution with stirring at 20oC. To this ho mogeneous solution xy lene was added, after one hour stirring at the same temperature TEOS was added and then the mixture was stirred for 1 hour at the same temperature. A fter homogenization of this mixtu re BTPA was added (drop-by-drop) under continuous stirring (Scheme 1). The sample co mposition is presented in Table 1. The solid products were collected by filtration, washed thoroughly with water, and air dried at 60℃. Th is material is referred to as the as-synthesized material. The as-synthesized materials are designated a. ABMO 1, A BMO 2, ABM O 3 and ABMO 4. 2. Experimental 2.1. Chemicals and Reagents Materials were synthesized using triblock copoly mer P123 PEO20PPO70PEO20, (Sig ma-A ldrich, Mn~5,800) as a structure directing agent. Tetraethyl orthosilicate (TEOS, MERCK) and bis[(3- trimetho xysilyl)propyl]amine (BTPA, Aldrich) were used in order to synthesize the 2.3. Surfactant Extracti on The surfactant was removed by soaking. 1.0 g of as-synthesized sample was soaking in solution of 150 ml ethanol and 1.7ml 36% HCl at 60 ℃ for 24 hours. The resulting solid was recovered by filtration, washed with ethanol, and dried in oven at 50 ℃ for 24 hours. This material is referred to as the surfactant extracted material. Sample ABMO 1 ABMO 2 ABMO 3 ABMO 4 Scheme 1. Sol-gel/surfactant template synthesis of hybrid porous materials Table 1. Composition of the samples P123 KCl [g] [g] 1.2 3.5 1.2 3.5 1.2 3.5 1.2 3.5 H2O 1M HCl Xylene [ml] [ml] [ml] 10 52 2.64 10 52 2.64 10 52 2.64 10 52 2.64 TEOS [ml] 2.64 2.64 2.64 2.64 BTPA [ml] 1.16 3.90 5.85 7.80 International Journal of M aterials and Chemistry 2013, 3(3A): 21-28 23 2.4. Characterization The surface of the resultant materials were determined by scanning electron microscopy (SEM) images was recorded on a Hitachi S-4100 scanning electron microscope with an acceleration voltage of 15 kV. Th is technique was used to observe the external mo rphologies of the as synthesized and surfactant extracted materials. Fourier transform infrared (FT-IR) measurements were performed on a MATTSON 7000 Spectro meter in the range 4000– 400 cm-1 and resolution 2 cm-1 The FT-IR spectra were recorded at room temperature using KBr pellets, 32 scans were signal averaged. FT-IR was used to confirm the removal of surfactants and the format ion of organosilica materials. 13C (100.61M Hz) cross-polarization mag ic angle spinning (CP MAS) and 29Si (79.49 MHz) MAS solid-state NMR experiments were recorded on a (9.4 T) Bru ker Avance 400 spectrometer. The e xperimental para meters for 13C CP MAS NMR e xpe riments: 9 kHz spin rate, 5 s pulse delay, for 29Si MAS NM R experiments: 5 kHz spin rate, 60s pulse delay. MAS NM R spectra were measured with 40 µs 1H 90º pulse, speed of rotation 50 kHz. 29Si solid-state NM R spectra were recorded at 79.49 MHz on a (9.4 T) Bru ker Avance 400 spectrometer 29Si mag ic angle spinning MAS NM R spectra were measured with 40 µs 1H 90º pulse, speed of rotation 50 kHz and a delay of 60 seconds. For X-ray phase analysis XRD (Rigaku/ New X-Ray Diffracto meter System "Geigetflex" D/Max- C Series), working with Cu-Kα radiation with a range of 0.4-5.0 (2θ), and scanning speed 0.01o 2θ/min. Elemental analyses for C, N and H were performed with a Truspec 630-200-200 elemental analyzer at combustion furnace temperature 1075℃. Hg(II). 3. Results and Discussions 3.1. Structural Properties The XRD patterns of the synthesized and polymer extracted materials with different amount BTPA are shown in Figure 1. A ll samples shows a peak at about 2θ = 1,16 o, which shows characteristics of ordered two-d imensional materials (2D) hexagonal structure[20-22]. Such "single diffraction peaks" have also been used for the mesoporous materials by earlier authors[22]. Widening of the peak and the absence of others can be exp lained by the radial arrangement of mesoporous channels in microsize part icles [23]. The absence of resolved higher-angle peaks indicates, however, that any structural order of the extracted hybrid materials does not extend over a long range, this could be to the faster gelation time of b is-[3-(triemetho xyosilyl) propyl] amine which provides inherently h ighly amo rphous materials as well as the strong interface of amine groups on the co-assembly process caused by the electrostatic and hydrogen bond interactions between the amine groups and the block co-poly mer and the silicate species[22]. Figure 1 show that the intensity of the peaks is the same fo r all samples, indicating that the rate of ordering of the resulting material is the same, i.e the amount of BTPA not affected. 2.5. Metal Adsorption Studies The adsorption properties of organic-inorganic hybrid materials towards Hg(II) ions were studied by means of the batch method. Experiments were carried out in acidic med ia (pH 2.5). Adsorption studies were performed using stoppered 50 ml Erlen meyer flasks containing about 0.1 g sample and 10 ml of Hg(II) solution. For this study, aqueous solutions with concentrations fro m 600 mg/ l to 2000 mg/l of Hg(II) ions were prepared. The mixtures were shaken at 25o C for 2 hours by an automatic shaker. On reaching equilibriu m, the adsorbent was collected by filtration through a Millipore filter (0.2 µm). The init ial and equilibriu m concentrations of the Hg(II) ions were determined by flame atomic absorption spectrometry on a Pye Un icam SP 192 (UK). All adsorption experiments were replicated and the average results were used in data analyses. The results shows that with increasing of the init ial concentration of HgII ions in the solution, the adsorption on the investigated materials decrease which is connected with saturation of the act ive p lace of the adsorbent. At concentration to 900 mg/ l the Hg(II) ions were retained practically fully on the investigated material. The adsorption capacities of ABM O 3 towards Hg(II) ions were determined at pH 2.5 using solutions with concentration of 900 mg/l Fi gure 1. XRD patterns of the samples after polymer extract ion The morphology of the hybrid gels was studied with SEM analysis. Figure 2 show SEM micrographs of the studied hybrid gels. All samples have nanoporous structure typical for material obtained sol-gel route. The gels are characterized by a smooth surface with an amo rphous structure composed of particles with a size around 0,2 μm. Which formed aggregates with size about 1-2 μm. The SEM images show that no significant differences in the morphology of the samples with increasing amount of BTPA. In order to characterize the structure of the resulting 24 Nina E. Velikova et al.: Synthesis and Characterization of Hybrid M esoporous M aterials Prepared with Triblock-Copolymer and Bridged Silsesquioxane materials was carried infrared spectroscopy. Figure 3 are presented IR spectra of synthesized gels. All samples are characterized by typical band relating to Si-O bonds, at 440-460 cm-1 bending of O-Si-O bond[24, 25] at 760-790 cm-1 and 1000-1100 is observed symmetric vibration band of Si-O-Si groups[26]. The appearance of the signal at 1415-1455 cm-1 is attributed to (Si-CH2) and (N-CH2) or (CCH2) of the precursor (BTPA) and surfactant (P123), respectively[27]. The signals at 2800-2890 cm-1 and 1469 cm-1 detect the presence of CH links contained in the precursor (BTPA) and surfactant (P123), which would mean that the surfactant was not removed completely by soaking. The appearance of the band at 2977-2979 cm-1 is characteristic for stretching of methylene groups[-CH2] building the BTPA molecules[28]. Another signal proving the inclusion of the bridging group in the structure of materia ls is the appearance of the signal at 1380 c m-1 related to the stretching vibration of n(CN) at C-N bonds[29], also the signal at 691 c m-1 typical of stretching of Si-CH2[30, 31] and this one at 1375 cm-1 at which is observed stretching of CN bond[32]. The signals at 1640 cm-1 and 3450 cm-1 can be associated with the presence of NH groups[29,33] or stretching vibration of OH g roups[25,34]. 29Si MAS NMR spectra of functionalized materials are presented on Figure 4. Clearly expressed six resonant peaks, three of which are at 110 ppm, 101 ppm and 92 ppm, respectively, as indicated Q4[Si (OSi) 4], Q3[(OH) Si (OSi)3] and Q2[(OH)2 Si (OSi)2] and another three about 66 pp m, 57 ppm and 50 pp m are designated as T3[(SiO)3SiC], T2[(SiO)2 (OH) SiC] and T1[(SiO) (OH )2SiC][35,36]. The appearance of the Tm peaks confirms that organic silane (BTPA) is included as part of the structure of the pore wall[37-39]. The ratio of the intensity of Tm and Qn peaks (Tm / Qn) increases with the increasing of the amount of BTPA, ind icating that the bridging organic part in the pore walls increases with increasing the amount of BTPA[38, 39]. These ratios are 0.428, 0.544, 0.582 and 0.673 respectively for ABMO 1, A BMO 2, A BMO 3 and ABMO 4 which are in good agreement with the results from elemental analysis, who shows that with increasing the amount of BTPA the content of nitrogen increase (Tab le 2). In addition, the appearance of Q3, Q2, T2 and T1 resonance peaks shows that the condensation between the both silicate precursors are not completed, this is confirmed by FT-IR spectra with appearance of signals around 920 cm-1 and about 940 cm-1 tipical for vibration of Si-OH bond[31, 32, 40]. Figure 2. SEM images of the samples International Journal of M aterials and Chemistry 2013, 3(3A): 21-28 25 Figure 3. FT-IR spectra of the samples Figure 4. 29Si MAS NMR spectra of the samples 26 Nina E. Velikova et al.: Synthesis and Characterization of Hybrid M esoporous M aterials Prepared with Triblock-Copolymer and Bridged Silsesquioxane Table 2. Result s from elemental analysis Sample ABMO 1 ABMO 2 ABMO 3 ABMO 4 Mass[mg] 1.019 1.204 1.352 1.087 %C 9.489 18.481 22.727 22.767 %H 2.3104 4.9853 5.5204 5.3839 %N 1.8239 3.8301 4.2733 4.3004 The results from 13C CP MAS NMR analysis of all samples shows resonans peaks from 10 to 50 pp m typical of sp3 carbon atoms which characterize organic bridging group of BTPA[41-46]. The three most intense peaks at around 10, 20 and 50 pp m correspond to the carbon atoms of the bridging group in the direction fro m left to right, as presented in Figure 5[22, 47]. The appearance of peaks in the range fro m 70 to 75 ppm indicate the presence of surfactant P123 i.e. not comp letely ext racted by soaking[22, 47-49], which is confirmed by FT-IR spectra in wh ich the signal appears at about 2900 - 3000 cm-1 wh ich are observed at stretching of CH bonds in CH3 groups[50,51]. capacity of the materials i.e the increasing the amount of BTPA leads to increasing of act ive places, in this case the active places are -NH- groups from the bridged group from BTPA precursor, in the result of this the adsorption efficiency of the materials increase. All materials are appropriate for Hg(II) removal form acidic aqueous solutions. The adsorption capacities were found to be 65.4 mg/g, 94.5 mg/g 114.7 mg/g and 128.7 mg/g for A BMO 1, ABM O 2, A BMO 3 and A BMO 4 respectively. The regeneration of the hybrid sorbents is from exceptionally importance. For the investigation of desorption of Hg (II) were used diluted mineral acids (0.1 М and 1 М HCl and 0.1 М, 1 М, 2 М HNO3) and EDTA (0.01 М and 0.1 М). We suggested that the cations, released fro m the acids will change the adsorbed Hg(II) ions or they will obtain complex with EDTA. In Table 4 are presented the data for the adsorption (in %) for d ifferent desorption agents. Table 4. Desorption of Hg(II) ions by hybrid material ABMO 3 Desorption agent 0.1 М HCl 0.1 M HNO3 1 M HCl 1 M HNO3 0.01 M EDTA 0.1 M EDT A Time[hour] 3 3 3 3 24 24 ABMO 3 1% 100% 6% 100% 10% 100% The obtained data shows that HCl is not appropriate for dsorption of Hg(II) ions, wile by using of 0.1M HNO3 and EDTA were achieved 100% desorption. The last one can be used for effective regeneration of hybrid sorbents. On the base of the results, probably the adsorption of Hg (II) ions on the material ABM O 3 is main ly physical adsorption. Figure 5. 13C CP MAS NMR spectra of sample ABMO 1 4. Conclusions 3.4. Metal Adsorption Studies The optimu m time to establish adsorption equilibriu m between investigated materials and Hg (II) ions of different concentrations was determined experimentally and was found to be within 20 minutes. The adsorption studies were performed ta king into account these preliminary e xperiments. We investigated the influence of amount of BTPA over the adsorption. In Tab le 3 are presented the results from adsorption of Hg (II) ions from the solution with different concentration on the investigated materials. Table 3. Influence of the BTPA amount on the materials adsorption Adsobent Ion ABMO 1 Hg ABMO 2 Hg ABMO 3 Hg ABMO 4 Hg C 0[mg/ l] 2000 2000 2000 2000 Amount adsorbed matter[mg/g] 65.4 94.5 114.7 128.7 Fro m these results we can conclude that the increasing of the amount of BTPA leads to increasing the adsorption Amine functionalized mesoporous organosilica materials have been synthesized with direct co-condensation of tetraethylortosilacate (TEOS) and bis-[3-(triemetho xyosilyl) propyl] amine (BTPA) under basic conditions catalyzed by the –NH– groups of BTPA. XRD results revealed that the materials have ordered structure but the absence of resolved higher angle p icks indicates that the order does not extend over a long rage. FT-IR, elemental analysis and 29Si MAS NMR data confirm that the materials are based on amine bridged polysilo xane and no degradation of the bridging groups occurs during the synthesis. FT-IR and 13C CP MAS NMR ana lyses indicate that part of the surfactant P123 is not fully dissolved in the extraction step and is still present in the materials. Adsorption of Hg (II) ions from aqueous solutions shows high adsorption capacities suggesting that these materials have a potential to be used as adsorbents for Hg (II) ions in acidic solutions. On the base of the obtained results, probably the adsorption of Hg (II) ions on the material ABMO 3 is main ly physical adsorption. By 0.1M HNO3 and EDTA were ach ieved 100% desorption of Hg(II) ions and regeneration of the hybrid material. International Journal of M aterials and Chemistry 2013, 3(3A): 21-28 27 ACKNOWLEDGEMENTS This paper has been produced with the financial assistance of the European Social Fund, project number BG051PO001-3.3.06-0014. The author is responsible for the content of this material, and under no circumstances can be considered as an official position of the European Union and the Ministry of Education and Science of Bulgaria. The authors kindly acknowledge of the Institute of General and Inorganic Chemistry at Bulgarian Academy of Sciences for the assistance at the metal adsorption studies of the synthesized hybrid materials. [12] T. Asefa, M . J. M acLachlan, N. Coombs, G. A. Ozin, Periodic mesoporous organosilicas with organic groups inside the channel walls, Nature, 402, 867–871, 1999. [13] Frank Hoffmann, M ichael Fröba, Vitalising porous inorganic silica networks with organic functions—PMOs and related hybrid materials, Chem. Soc. Rev., 40, 608–620, 2011 [14] X. Wang, Y. H. Tseng, J. C. C. Chan, S. Cheng, Catalytic applications of aminopropylated mesoporous silica prepared by a template-free route in flavanones synthesis, J. Catal., 233, 266, 2005. [15] C. Lei, Y. Shin, J. Liu, E. J. 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