Effective NH2-grafting on mesoporous SBA-15 surface for adsorption of heavy metal ions

Effective NH2-grafting on mesoporous SBA-15 surface for adsorption of heavy metal ions

Materials Letters 65 (2011) 1045–1047 Contents lists available at ScienceDirect Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i ...

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Materials Letters 65 (2011) 1045–1047

Contents lists available at ScienceDirect

Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t

Effective NH2-grafting on mesoporous SBA-15 surface for adsorption of heavy metal ions Yanling Zhao a,c, Qiang Gao b, Tao Tang a,c, Yao Xu a,⁎, Dong Wu a a b c

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, P. R. China Faculty of Material Science & Chemistry Engineering, China University of Geosciences, Wuhan 430074, P. R. China Graduate University of Chinese Academy of Sciences, Beijing 100049, P. R. China

a r t i c l e

i n f o

Article history: Received 10 August 2010 Accepted 23 December 2010 Available online 31 December 2010 Keywords: Surfaces Porosity Mesoporous materials Heavy metal ions removal

a b s t r a c t Utilizing the reactions between toluene diisocyanate (TDI), silanol, and ethylenediamine (EDA), NH2 groups were successfully grafted on mesoporous SBA-15 surface without any destroy to the mesostructure. TDI was employed as a ‘bridge’ molecule whose one NCO group was used to link SBA-15 surface silanols and the other one was left to link EDA. Such NH2-grafting is highly effective for a high loading amount of NH2 groups on SBA15 because the special stepwise grafting can avoid amino/silanol and amino/amino interactions. Subjected to remove toxic heavy metal ions in aqueous solution, the obtained NH2-SBA-15 showed very high adsorption rates 99.4%, 100%, 99.7%, 98.7% and 99.9% for Cu2+, Zn2+, Cr3+, Ni2+ and Cd2+, respectively, which should be attributed to the strong complexation reactions between metal ions and grafted NH2 groups. © 2010 Elsevier B.V. All rights reserved.

1. Introduction With attractive texture characteristics and excellent alkaline properties, amino-functionalized mesoporous silica received much attention [1,2]. Generally, functionalization with amino groups was realized through two independent methods, co-condensation method or grafting method. Unfortunately, no matter which method was used, mesoporous silica with high amino loading amount could hardly be obtained. In the case of the co-condensation method, the protonated amino groups under acidic condition would interfere the selfassembly of silica precursor and surfactant, and then resulted in the less ordered mesopores [3]. As for the grafting method, the amino groups early grafted on silica surface would hydrogen-bond seriously with surface silanols or with each other, thus preventing the subsequent amino-grafting on silica surface [4]. To overcome these shortcomings, enormous efforts have been taken with an emphasis on reducing amino/silanol and amino/amino interactions [3–8]. In the present work, we proposed a new method to synthesize amino-functionalized SBA-15 (see Fig. 1). Herein, toluene diisocyanate (TDI) was employed as a ‘bridge’ molecule whose one NCO group was used to link SBA-15 surface silanols and the other one was left to link ethylenediamine (EDA). Such method should have high link efficiency because TDI is a highly active molecule whose NCO groups can easily bond with OH or NH2 groups [9]. Meanwhile, the stepwise reaction can avoid the amino/silanol and amino/amino interactions. Thus, it is hopeful to synthesize amino-functionalized SBA-15 with a high loading

⁎ Corresponding author. Tel.: + 86 351 4049859; fax: + 86 351 4041153. E-mail address: [email protected] (Y. Xu). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.12.047

amount of amino groups. Moreover, the adsorption performance of the obtained material was investigated for removal of heavy metal ions. 2. Experimental The preparation of amino-functionalized SBA-15 involved three steps. First, pure siliceous SBA-15 was prepared according to the reported literature [10]. Second, 1.0 g of as-synthesized SBA-15 without template was added into a mixed solution containing 8.3 mL of TDI and 16.7 mL of toluene. After stirring the above mixture at 60 °C for 5 h, the product (denoted as T-SBA-15) was recovered and Soxhletextracted with CH2Cl2 for 48 h. At last, 1.0 g of dried T-SBA-15 was added into a mixed solution containing 20.0 mL of EDA and 20.0 mL of ether. After stirring the mixture at 20 °C for 24 h, the final product (denoted as NH2-SBA-15) was recovered, Soxhlet-extracted and dried. X-ray diffraction (XRD) patterns were recorded on a D8 Advance Bruker AXS diffractometer using Cu Kα radiation. N2 ad/desorption isotherms were measured at 77 K on a Micromeritics Tristar 3000 Sorptometer. Fourier transform infrared (FT-IR) spectra were obtained on a Nicolet Nexus 470 FT-IR analyzer. UV-Vis absorption spectra were measured on a Shimadzu 3150 UV-Vis spectrophotometer. 29Si MAS NMR experiments were performed on a Varian Infinityplus-300 spectrometer using 7.5 mm probe under magic-angle spinning. Thermogravimetric analyses (TGA) were performed on a Setaram TGA-92 thermogravimetric analyzer. In a typical adsorption experiment, 10 mL of a single metal solution was mixed with 100 mg of adsorbent at room temperature with agitation for 24 h. Metal ion concentrations, both in the initial and final solutions, were determined by inductively coupled plasma spectroscopy (ICP).

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Y. Zhao et al. / Materials Letters 65 (2011) 1045–1047

Fig. 1. Schematic illustration of our synthesis strategy of NH2-SBA-15.

3. Results and discussion XRD patterns of three SBA-15-type samples (see Supporting Information, Figure S1.) show an intense peak (100) and two wellresolved small peaks (110) and (200) in all the samples, indicating their highly-ordered hexagonal mesoporous structures [11]. N2 ad/ desorption isotherms and pore size distributions of samples (see Supporting Information, Figure S2.) reveal that all the samples possess type-IV isotherms and H1 hysteresis loops, further confirming the characteristics of mesoporous solids [11]. Also in Figure S2., the pore diameter displays this sequence: SBA-15 (7.7 nm) N T-SBA-15 (6.6 nm) N NH2-SBA-15 (5.3 nm). This is an expectative result, suggesting that SBA-15 surface has been stepwisely organic-functionalized. The detailed textural parameters of samples were collected in Table S1. (Supporting Information). The FT-IR spectra of SBA-15, T-SBA-15 and NH2-SBA-15 are shown in Fig. 2. The typical absorption band of NCO groups at 2280 cm− 1 [12] can be observed in the spectrum of T-SBA-15 sample, suggesting that TDI reacted with SBA-15 surface silanols. For NH2-SBA-15 sample, the absorption band at 2280 cm− 1 disappears completely, and two absorption bands of NH2 groups appear at 3186 cm− 1 and 3385 cm− 1 [13], which indicates that EDA has been one-end grafted on SBA-15 surface through TDI ‘bridge’. The UV-Vis absorption spectra of the three samples (see Supporting Information, Figure S3.) further confirms the above inference. Pure siliceous SBA-15 has no absorption in the wavelength range of 200 ~ 800 nm. In contrast, both T-SBA-15 and NH2-SBA-15 exhibit strong absorption below 300 nm, and the absorbance of T-SBA-15 is slightly higher than that of NH2-SBA-15. This phenomenon can be attributed to a big π-conjugation formed by the reaction of TDI and SBA-15 surface silanols, and the conjugation extent slightly decreased after bonding EDA with TDI, as shown in Fig. 1. Fig. 3 shows the 29Si MAS NMR spectra of SBA-15 and NH2-SBA-15, where Q4, Q3, Q3′ and Q2 respectively represent the silicon atom environments of (SiO)4Si*, (SiO)3Si*OH, (SiO)3Si*OC and (SiO)2Si*(OH)2. The relative peak areas in 29Si MAS NMR spectra were summarized

in Table S2. (Supporting Information). According to the literature [14], the transformation of Si-OH to Si-OC would make the Si chemical shift move toward lower field. Thus, the Q3 peak of SBA-15 is no longer present after NH2-grafting on SBA-15 surface, and the Q3′ peak appears at the chemical shift very close to Q4. Through data fitting in Table S2., it is found that both SBA-15 and NH2-SBA-15 have almost equal (SiO)2Si*(OH)2 contents. This might be explained by the reported conclusion that Si-OH of (SiO)2Si*(OH)2 was difficult to be organic-functionalized because of the strong hydrogen-bonding interaction between geminate Si-OH groups of (SiO)2Si*(OH)2 [15]. Also, the percent contents of surface Si-OH groups are found to be 57.8% for SBA-15 and only 13.2% for NH2-SBA-15, indicating that a high degree of NH2-grafting on SBA-15 surface (coverage of 77.2%) was realized. The TGA curves of SBA-15, T-SBA-15 and NH2-SBA-15 are shown in Figure S4. (Supporting Information). For all these samples, a slight weight loss is observed below 200 °C due to the residual water in sample. Both T-SBA-15 and NH2-SBA-15 exhibit considerable weight losses in the temperature range of 200 °C ~ 300 °C, which should be caused by the decomposition of organic components. And, the weight loss of NH2-SBA-15 is 19.9%, 1.32 times as that of T-SBA-15 (15.1%), which is in good agreement with the molecular weight ratio 1.33 of (TDI + EDA) to TDI. Thus, it can be concluded that TDI was connected to SBA-15 surface with one end, and then EDA was grafted via the other end of TDI. In addition, SBA-15 shows another slight weight loss in the temperature range of 200 °C ~ 600 °C owing to the further condensation between Si-OH groups. The amino-functionalized SBA-15 exhibited unique affinity for heavy metal ions. Table 1 shows the metal ion concentrations of simulated wastewater before and after adsorbed by SBA-15 and NH2SBA-15. It can be seen that NH2-SBA-15 adsorbent efficiently removed the heavy metal ions Cu2+, Zn2+, Cr3+, Ni2+, Cd2+, and the Zn2+ concentration was reduced to below the detection limit. Comparatively, no significant concentration changes for all metal ions are observed when pure siliceous SBA-15 was used to treat the

3'

Q

+

4

Q

Transmission (%)

SBA-15 T-SBA-15

Q2

-CH3

Q4

Q3

Q2

-NCO

NH2-SBA-15

NH2-SBA-15 -CH2-

SBA-15

-NH2

3000

2000

1000

-60

Wavenumber (cm-1) Fig. 2. FT-IR spectra of SBA-15, T-SBA-15 and NH2-SBA-15.

-90

-120

-150

ppm Fig. 3.

29

Si MAS NMR spectra of SBA-15 and NH2-SBA-15.

Y. Zhao et al. / Materials Letters 65 (2011) 1045–1047

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Table 1 Concentrations of heavy metal ions in wastewater before and after adsorption⁎. (unit: ppm). Solution

a

Cu2+

No treatment After adsorbed by SBA-15 After adsorbed by NH2-SBA-15 Adsorption rate on NH2-SBA-15

8.63 8.58 0.056 99.4%

b

Zn2+

c

10.02 10.10 — 100%

Cr3+

3.78 3.68 0.013 99.7%

d

Ni2+

7.92 7.88 0.100 98.7%

e

Cd2+

10.58 10.60 0.007 99.9%

⁎ Detection limit is (a) 0.002 ppm, (b) 0.005 ppm, (c) 0.004 ppm, (d) 0.005 ppm, and (e) 0.002 ppm.

wastewater under the same condition. Therefore, the concentration changes are indeed attributed to the complexation reactions between metal ions and grafted amino groups.

Appendix A. Supplementary data

4. Conclusions

References

An effective method was proposed to prepare surface NH2-grafted mesoporous silica, using TDI as the bridge molecule and EDA as the NH2 source. As expected, a high loading amount of amino groups on SBA-15 (coverage of 77.2%) was realized. More importantly, such method may be extended to graft other target groups on mesoporous silica surface. The obtained material could effectively remove toxic heavy metal ions in aqueous solution due to the strong complexation between metal ions and grafted amino groups.

Supplementary data to this article can be found online at doi:10.1016/j.matlet.2010.12.047.

[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

Acknowledgements The financial support from the National Natural Science Foundation of China (Grant No. 10835008) is acknowledged.

[13] [14] [15]

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