Synthesis of core-shell and hollow structured dual-mesoporous silica templated by alkoxysilyl-functionalized ionic liquids and CTAB

Materials Letters 211 (2018) 126–129

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Synthesis of core-shell and hollow structured dual-mesoporous silica templated by alkoxysilyl-functionalized ionic liquids and CTAB Huaying Gao, Yuming Zhou ⇑, Xiaoli Sheng, Shuo Zhao, Chao Zhang, Mingyu Zhang School of Chemistry and Chemical Engineering, Southeast University, Jiangsu Optoelectronic Functional Materials and Engineering Laboratory, Nanjing 211189, PR China

a r t i c l e

i n f o

Article history: Received 28 June 2017 Received in revised form 8 September 2017 Accepted 17 September 2017 Available online 27 September 2017 Keywords: Alkoxysilyl-functionalized ionic liquids Dual-mesoporous Structural Porous materials Core-shell silica

a b s t r a c t A simple and effective one-step method was proposed for the fabrication of dual-mesoporous silica, which was templated by alkoxysilyl-functionalized ionic liquid and cetyltrimethyl ammonium bromide (CTAB), with potential application in alkylation of o-xylene and styrene. Ionic liquid act as both template and silica source in this system. The influence of pH of solvent and concentration of alkoxysilylfunctionalized ionic liquid to the morphology of obtained silica was studied in detail. The results showed that the as-prepared silica materials with core-shell or hollow structure could be fabricated by adjusting the pH and ionic liquid concentration, which is strongly related to the unique features of alkoxysilylfunctionalized ionic liquid with organic silicon chain. The possible mechanism was proposed based on the interaction between ionic liquid and CTAB. This new method is also applicable to synthesize the other hierarchical porous silica materials by using organic silyl functional ionic liquid. Ó 2017 Published by Elsevier B.V.

1. Introduction Hierarchical porous materials have attracted much interest over the past few years, notably the mesoporous/macroporous, microporous/macroporous, microporous/mesoporous materials [1–3]. Besides, dual-mesoporous materials, owing to their high surface area, large pore volume and easily modifiable surface, have attracted considerable interest and broad application in adsorption, catalysis, and carriers [4,5]. To date, several different strategies have been developed for the synthesis of dual-mesoporous materials, such as the removal of framework atoms, dual templating with surfactants, and hard-templating methods [6–8]. Ionic liquids, as a novel green solvent, can form extended hydrogen bonds between molecules in the liquid state, which is the basis of molecular recognition and self-assembly processes [9]. Hence, ionic liquids can easily aggregate and produce micelles, then can be used as templates for the preparation of nanoporous materials [10]. Yang has synthesized dual-mesoporous silica using the ionic liquid [C16mim]3PMo12O40 (1-hexadecyl-3methylimidazolium phosphomolybdate) and the triblock copolymer P123 as co-templates [11]. In our previous work, we have successfully obtained the micro/mesoporous materials by templating of P123 and protic ionic liquid (triethylamine acetate) [12]. Furthermore, core-shell structured mesoporous silica spheres were ⇑ Corresponding author. Tel.: +86 25 52090617; fax: +86 2552090617. E-mail address: [email protected] (Y. Zhou). https://doi.org/10.1016/j.matlet.2017.09.067 0167-577X/Ó 2017 Published by Elsevier B.V.

successfully fabricated by utilizing cetyltrimethyl ammonium bromide (CTAB) and protic ionic liquid (butylamine acetate) as the co-templates in our previous work [13]. All of these could be an effective evidence to demonstrate ILs are the appropriate template to develop porous material. In this work, alkoxysilyl-functionalized ionic liquid with organic silicon chain, 1-methyl-3-[3-(trimethoxysilyl)propyl]imidazolium chloride (MTPI), was used as the template instead of the conventional ionic liquids. This kind of novel ionic liquid can be used as the source of silica besides the role of template in the system. With the condition that the two templates have hydrophobic chains with different sizes, MTPI and CTAB are excellent candidates to design dual-mesoporous silica materials through a one-step synthesis process. The influence of pH of solvent and MTPI concentration to the final structure was investigated. Based on the interaction between ionic liquid and CTAB, the possible mechanism was proposed.

2. Experimental Alkoxysilyl-functionalized ionic liquid was synthesized by ultrasonic/microwave synergistic method according to the previous work [14]. Dual-mesoporous silica materials were synthesized with CTAB and MTPI as co-templates in a solvothermal process. In a typical synthesis, CTAB (0.5 g) and appropriate amount of MTPI were completely dissolved in the mixture of water and ethanol (50 ml:30 ml), and its pH was adjusted by ammonia water, then

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Fig. 1. A:TEM images of the dual-mesoporous silica with different pH: a) pH = 9, b) pH = 10, c) pH = 11; SEM images of the dual-mesoporous silica with different pH: d) pH = 9, e) pH = 10, f) pH = 11; B: TEM images of the dual-mesoporous silica with different concentration of MTPI; SEM images of dual-mesoporous silica with different concentration of MTPI.

added TEOS. After vigorous stirring at room temperature for 24 h, the mixture was transferred into an autoclave for aging at 423 K for 3 days. Followed by filtration, the product was washed sequentially with water and ethanol, respectively. After drying under oven at 343 K overnight, the white as-synthesized solid powders were then calcined at 823 K in air for 6 h to obtain silica materials. The final products with different MTPI concentration were denoted as IL-X, in which X represents the mass of MTPI. 12Tungstophosphoric acid (HPW) catalysts incorporated into these dual-mesoporous silica were prepared by impregnation, and alkylation of o-xylene and styrene, as detective reaction to investigate the activity. The details were introduced in supporting information (Fig. S1 and Table S1). The morphologies and structures of final samples were characterized by obtaining transmission electron microscopy (TEM) images which were performed on FEI Tecnai G20 instrument and scanning electron microscopy (SEM) images which were recorded

on a JEOL JSM-5600L SEM Instrument with a working distance of 3–4 mm and an electron voltage of 3.0 kV. 3. Results and discussion Fig. 1A(a–c) present TEM images at different pH of precursor solvent, showing the morphology of silica from spherical to coreshell with the corresponding value of pH vary from 9 to 11. Fig. 1A(a) shows a well-defined silicon sphere with ordered porous structure, and there is a tendency from silica sphere to core-shell morphology as pH increased. Fig. 1A(b) is an important transition, changing from initial sphere to core-shell structure. Fig. 1A(d–f) present corresponding SEM images of the dual-mesoporous silica spheres, indicating the obtained silica materials consist of sphere morphology with hierarchical porous architectures. Overall, the change of pH caused negligible affection on the whole size of silica materials (ac. 300 nm).

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Fig. 2. XRD patterns (A), N2 adsorption-desorption isotherms and BJH. pore size distributions (inset) of IL-0.1 (B), Energy spectrum of IL-0.1 (C) TEM images of the dualmesoporous silica with different concentration of MTPI (D: only CTAB, E: only IL).

Fig. 3. Postulated mechanism of the dual-mesoporous silica induced at pH = 9 (a), pH = 10 (b), pH = 11 (c) (A), and with different MTPI concentration (B).

Fig. 1B shows the comparison of the results obtained with the dual-mesoporous structure, for different concentration of MIPI. CTAB as surfactant is composed by hydrophilic and hydrophobic, it is easy to produce sphere through self-assemble and then silica

deposited on its surface after hydrolysis. It can be seen that irregular morphology promoted by the increase of concentration of MIPI. Sample IL-0.1 has a clear core-shell structure different with sample IL-0.02 in hollow structure. Increasing the concentration

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of MTPI, sequentially, an irregular morphology of silica with disordered dual-mesoporous is obtained. SEM images of Fig. 1B are consistent with the result of TEM images. The XRD patterns, energy spectrum, N2 adsorption-desorption isotherm and the BJH pore size distribution of the sample IL-0.1 are shown in Fig. 3. Amorphous nature of silica network can be reflected on 2h = 15–30° of XRD pattern. From Fig. 3(B), the sample shows a type-IV curve with a wide capillary condensation step at p/p0 of 0.1–1.0, indicating the porous structure of the sample. The calculated BET surface area is 28.24 m2/g, and the calculated total pore volume is 0.07 cm3/g. Corresponding pore size distribution of the sample IL-0.1 is shown in the inset of Fig. 3(B). The distribution of elements was clearly shown on EDS (Fig. 3C), and C among the elements were due to carbon membrane of test. MTPI concentration makes great influence on morphology of silica, only in CTAB or IL as template, two kinds of extreme state, regular hollow sphere structure and irregular multiporous structure can be found (Fig. 2(D, E)). The probable formation mechanism of the dual-mesoporous structured silica materials, which is based on the interaction between MTPI and CTAB is tentatively elucidated (Fig. 3). MTPI comprises both a hydrophilic ionic headgroup and a hydrophobic organic chain, which may self-assemble into ordered structures like surfactants in an aqueous solution. In lower pH alkaline situation, CTAB can self-assemble form micelles to import mesopores, while most of MTPI in single molecule. Thus, hollow structure can be found. As pH increased, MTPI molecule can form micelles, moreover, the silica chain can act as source of silica to form ASiAOASiA structure. And core-shell structured silica was obtained in final (Fig. 1A). Especially, the mesopores in the shell and core are induced by CTAB micelles and MTPI micelles, respectively. In addition, MTPI may influence the formation of CTAB micelles under higher concentration, which leading a disordered structure in final (Fig. 1B). 4. Conclusion In conclusion, we have prepared alkoxysilyl-functionalized ionic liquid MTPI, which were confirmed to work as both template and source of silica, introducing mesoporous and establish ASiAOASiA structure with TEOS after hydrolysis. Dualmesoporous silica materials were obtained in one-step way, moreover, the structure of the final materials was systematically investigated by pH of precursor solution and ionic liquid concentration. Besides, the interaction between MTPI and CTAB could change along with the pH of solvent and MTPI concentration. Through this work, we introduced a new way to fabricate hollow and core-shell structure silica with dual-mesoporous in a facile and effective method, meanwhile, importing organic silyl functional ionic liquid to synthesize silica material. From a viewpoint of the chemical structure and application potential, HPW-coated dual-mesoporous silica showed certain catalytic performance in alkylation of o-xylene and styrene.

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Acknowledgments The authors are grateful to the financial supports of National Natural Science Foundation of China (Grant No. 21676056, 21376051 and 51673040), ‘‘Six Talents Pinnacle Program’’ of Jiangsu Province of China (JNHB-006), Qing Lan Project of Jiangsu Province (1107040167), Fund Project for Transformation of Scientific and Technological Achievements of Jiangsu Province of China (Grant No. BA2014100), The Fundamental Research Funds for the Central Universities (3207045421, 3207046302, 3207046409) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. (PAPD) (1107047002). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.matlet.2017.09.067. References [1] J.L. Blin et al., Hierarchically mesoporous/macroporous metal oxides templated from polyethylene oxide surfactant assemblies, Angew. Chem. Int. Ed. 42 (2003) 2872. [2] T. Sen, G.J.T. Tiddy, J.L. Cascic, M.W. Anderson, Macro-cellular silica foams: synthesis during the natural creaming process of an oil-in-water emulsion, Chem. Commun. 2182 (2003). [3] K. Nakanishi, Y. Kobayashi, T. Amatani, K. Hirao, T. Kodaira, Spontaneous formation of hierarchical macro-mesoporous ethane-silica monolith, Chem. Mater. 16 (2004) 3652. [4] J. Wei, Q. Yue, Z.K. Sun, Y.H. Deng, D.Y. Zhao, Synthesis of dual-mesoporous silica using non-ionic diblock copolymer and cationic surfactant as cotemplates, Angew. Chem. Int. Ed. 51 (2012) 6149. [5] Y. Deng, D. Qi, C. Deng, X. Zhang, D. Zhao, Superparamagnetic highmagnetization microspheres with an Fe3O4@SiO2 core and perpendicularly aligned mesoporous SiO2 shell for removal of microcystins, J. Am. Chem. Soc. 130 (2008) 28. [6] P. Kortunov et al., The role of mesopores in intracrystalline transport in USY zeolite: PFG NMR diffusion study on various length scales, J. Am. Chem. Soc. 127 (2005) 13055. [7] K. Kaviyarasu, E. Manikandan, Z.Y. Nuru, M. Maaza, Investigation on the structural properties of CeO 2 nanofibers via CTAB surfactant, Mater. Lett. 160 (2015) 61. [8] B. Sathyaseelan, E. Manikandan, K. Sivakumar, J. Kennedy, M. Maaza, Enhanced visible photoluminescent and structural properties of ZnO/KIT-6 nanoporous materials for white light emitting diode (w-LED) application, J. Alloys Comps. 651 (2015) 479. [9] M. Antonietti, D.B. Kuang, B. Smarsly, Z. Yong, Ionic liquids for the convenient synthesis of functional nanoparticles and other inorganic nanostructures, Angew. Chem. Int. Ed. 43 (2004) 4988. [10] Z. Ma, J.H. Yu, S. Dai, Preparation of Inorganic Materials Using Ionic Liquids, Adv. Mater. 22 (2) (2010) 261–285. [11] L. Yang et al., Fabrication of dual-mesoporous silica by triblock copolymers and metal-based ionic liquid: efficient and durable catalyst for oxidative desulfurization in fuel, RSC Adv. 5 (2015) 104316. [12] S. Zhao et al., Synthesis of micro/mesoporous silica material by dual-template method as a heterogeneous catalyst support for alkylation, RSC Adv. 5 (2015) 28124. [13] S. Zhao et al., One-step synthesis of core-shell structured mesoporous silica spheres templated by protic ionic liquid and CTAB, Mater. Lett. 178 (2016) 35. [14] X.Q. Fu et al., One-step synthesis of hierarchical aluminosilicates using alkoxyfunctionalized ionic liquid as a novel template, New J. Chem. 40 (2016) 6036.