Radical-Facilitated Green Synthesis of Highly Ordered Mesoporous

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Radical-Facilitated Green Synthesis of Highly Ordered Mesoporous Silica Materials Guodong Feng, Jianyu Wang, Mercedes Boronat, Yi Li, Ji-Hu Su, Ju Huang, Yanhang Ma, and Jihong Yu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b00093 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018

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Journal of the American Chemical Society

Radical-Facilitated Green Synthesis of Highly Ordered Mesoporous Silica Materials Guodong Feng,†, ‡ ,※ Jianyu Wang,†, ‡ Mercedes Boronat,ǁ Yi Li,†, ⊥ Ji-Hu Su,§ Ju Huang¶, Yanhang Ma¶ and Jihong Yu*, †, ⊥ †

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, P.R. China. ǁ Instituto de Tecnologia Quimica, Universitat Politecnica de Valencia, Consejo Superior de Investigaciones Cientificas, Valencia, 46022, Spain. § CAS Key Laboratory of Microscale Magnetic Resonance and Department of Modern Physics, University of Science and Technology of China, 96 Jinzhai Road, Hefei, 230026, P.R. China. ⊥

International Center of Future Science, Jilin University, Changchun 130012, P.R. China. School of Physical Science and Technology, Shanghai Tech University, 393 Middle Huaxia Road, Pudong, Shanghai, 201210, P.R. China. ※ Key Laboratory of Advanced Molecular Engineering Materials, College of Chemistry and Chemical Engineering, Baoji University of Arts and Sciences, Baoji 721013, P. R. China. ¶

Supporting Information

ABSTRACT: In the hydrothermal synthesis of highly ordered

mesoporous silica material SBA-15, strong acid is typically required to catalyze the hydrolysis and condensation of silica species. Meanwhile, under strongly acidic conditions, the transition metal ions, e.g., iron ions, are difficult to incorporate into SBA-15 because of the facile dissociation of Fe-O-Si bonds. Here, we demonstrate an acid-free green synthetic strategy for the synthesis of highly ordered mesoporous SBA-15 and Fe-SBA-15 with the assistance of hydroxyl free radicals that are generated by physical or chemical methods. The prepared materials exhibit a large specific surface area compared to the counterparts prepared by conventional method under acidic conditions. Moreover, FeSBA-15 shows high metal loading efficiency as over 50%. Density functional theory calculations suggest that the hydroxyl free radicals exhibit higher catalytic activity than H+ ions for the hydrolysis of tetraethyl orthosilicate. This radical-facilitated synthesis approach overcomes the challenge to the direct synthesis of highly ordered SBA-15 and Fe-SBA-15 without adding any acid, providing a facile and environmentally friendly route for future large-scale production of ordered mesoporous materials.

With high surface areas and controllable pore architectures, ordered mesoporous materials have been employed in various fields, such as separation, catalysis, drug delivery, and nanosensors, etc.1 In general, mesoporous materials are synthesized under acidic or basic conditions, which is eco-unfriendly and not suitable for large-scale production.2 Several protocols have been developed for the synthesis of mesoporous materials under neutral conditions.3 However, the silica oligomers condense more rapidly than hydrolysis in neural media, producing significant amount of or-

ganic residues due to incomplete hydrolysis. The residual organic moieties weaken the interactions between hydrophilic block polymer and silica oligomers, giving rise to poorly organized mesocomposites. Mesoporous silica SBA-15 possesses a hexagonally ordered porous structure with controllable morphologies and texture properties.4 As other mesoporous materials, silica SBA-15 is typically synthesized under acidic conditions. Although several efforts have been made to synthesize SBA-15 under near neutral conditions by salt effect method5 or abrupt pH change,6 the synthetic procedures are too complicated for large-scale implementation. So far, facile and green synthesis of SBA-15 without adding any acid or additive remains challenging. The hydrolysis of tetraethyl orthosilicate (TEOS) can be promoted by the hydroxyl free radicals (•OH) without additional catalyst.7 Strikingly, our recent studies demonstrated that the presence of •OH radicals can remarkably accelerate the rate of SiO bond cleavage and depolymerization of the silica gel, and the rate of Si-O-Si bond remaking and polymerization of the silica species in the zeolite nucleation process.8 Hence, it is expected that the highly ordered mesoporous SBA-15 materials could be prepared in the absence of any acid by introducing •OH from electron pulse radiolysis, UV irradiation, Fenton reactions, chemical reactions, or high-voltage electrical discharge, etc.9 Here, we report a facile and green radical-facilitated synthetic method for highly ordered mesoporous SBA-15 without adding any acid. A modified multi-parallel reactor was used for the synthesis of SBA-15 allowing the introduction of •OH radicals under UV irradiations (Figure S1, ESI†). First, we investigated the synthesis of SBA-15 in the acid-free system of TEOS-P123-H2O under UV irradiation (3 W/m2) at 313 K. The low-angle XRD pattern of the prepared sample is shown in Figure 1a. The characteristic peaks of 100, 110 and 200 reflections for 2D-hexagonal mesoporous structures are observed.10 The TEM image (Figure 1b) also

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reveals the highly ordered mesoporous structure. The SEM image (Figure S2a, ESI†) shows that the sample is composed of aggregated particles with the average size of about 1 µm. The N2adsorption-desorption isotherms and pore size distributions of the calcined sample are shown in Figure 1c. The sample exhibits type-IV isotherm with an H1 type hysteresis loop, which is typical for 2D-hexagonal mesoporous structures with large pore sizes and narrowed size distributions.11 The pore size of the sample is 9.2 nm (Figure 1d), larger than that of the SBA-15 sample synthesized under conventional acidic conditions (Figure S3d, Table S1, ESI†). In contrast, the sample synthesized in the system of TEOSP123-H2O without UV irradiation exhibits disordered wormlike mesoporous structure (Figures 1e-h and S2b, Table S1, ESI†).

Figure 1. XRD patterns (left), HRTEM images (middle), nitrogen-adsorption-desorption isotherms and the pore size distributions (right) for the samples synthesized under UV irradiation (a-d), without addition of acid or radicals (e-h), adding Na2S2O8 (i-l) and adding Fenton reagents (m-p). UV conditions with different irradiances (1-15 W/m2) that might vary the •OH concentration were studied. XRD analysis reveals that lower or higher UV irradiances are not favorable for ordered SBA-15. This is because that the slower or faster hydrolysis-condensation of silica might affect the assembly of silicate oligomers and the template (Figure S4, ESI†). Meanwhile, we investigated the 29Si solid-state NMR for UV-irradiated precursors treated with different UV irradiances and the precursors without UV pretreatment in acid or acid-free system. The (Q3+Q4)/(Q2+Q3+Q4) area ratios of silica species in these samples vary with the synthetic conditions (Figure S5 and Table S3, ESI†). Notably, the precursor pretreated with UV irradiance of 3 W/m2 shows a higher (Q3+Q4)/(Q2+Q3+Q4) area ratio, comparable with that of the precursor in the acidic system, indicating that most of the silanols are existed in the polycondensation states. Furthermore, 29Si solid-state NMR analysis gives the Q3/Q4 ratio of the as-made SBA-15 via the radical route as 0.74, which is higher than that of the counterpart prepared via acidic route (0.64) (Figure S6 and Table S2, ESI†). The higher Q3/Q4 ratio suggests

the as-made SBA-15 sample via the radical route possesses the thinner silica walls than the counterpart prepared via acidic route.12 This agrees with the N2-adsorption results (Table S1, ESI†). Therefore, the as-made SBA-15 via the radical route exhibits a larger specific surface area than SBA-15 prepared under acidic conditions. To confirm that the hydroxyl radicals are introduced into the acid-free system of SBA-15 by UV irradiation, we conducted electron paramagnetic resonance (EPR) experiments to characterize the •OH radicals and the derived species. 5,5dimethylpyrroline-N-oxide (DMPO) as the spin-trapping agent of •OH was added into the initial reaction mixture, and the EPR signals were recorded in situ after the reaction mixture was irradiated for 60 s. For comparison, the initial reaction mixture without UV irradiation was also examined. As anticipated, the EPR signals (Figure 2a) of the DMPO-•OH adduct were observed after UV irradiation. The carbon-centered radicals (aN = 1.59 mT, aHβ = 2.27 mT) and oxidized DMPO radicals were also observed. Figure S7 shows the comparison of the experimental and the simulated EPR spectra of DMPO-•OH adducts, DMPO-carbon-centered radical adducts, and oxidized DMPO radicals, respectively. No clear signals were observed in the EPR spectrum of the initial reaction mixture without UV irradiation in the presence of DMPO (Figure 2b). These results confirm that the •OH radicals can be generated by the UV irradiation in the acid-free synthetic system.

Figure 2. EPR spectra of the initial reaction mixture containing DMPO (a) under UV irradiation; (b) without addition of acid or radicals; (c) with Na2S2O8; (d) with Fenton reagent. The EPR signals are marked as following: •OH (█); oxidized DMPO radicals (
); carbon-centered radicals (●). Besides introducing •OH through UV irradiation, sodium persulfate can be activated by heat, UV radiation, and metal activation to generate •OH with a reaction rate up to k = 6.5×107 M-1S-1 in a wide pH range.13 The SBA-15 was also synthesized in the reaction system of TEOS-P123-Na2S2O8-H2O with Na2S2O8 to introduce •OH. The characterizations of the as-prepared samples by low-angle XRD pattern (Figure 1i), TEM (Figure 1j), and N2 adsorption measurement (Figures 1k-l) indicate that the product possesses highly ordered mesoporous SBA-15 structure. Specifically, DMPO as the spin-trapping agent of hydroxyl radical was added to the initial reaction mixture of TEOS-P123-Na2S2O8-H2O. As shown in Figure 2c, the EPR signals arisen from the •OH captured by the DMPO, a characteristic 1:2:2:1 quartet pattern of the DMPO-•OH adduct, DMPO-carbon-centered radical adduct, and oxidized DMPO radical were observed. The SBA-15 was also synthesized in the acid-free system of TEOS-P123-H2O by adding the Fenton reagent. The Fenton reagent provides not only •OH radicals, but also iron for Fe-SBA-15. Due to the facile dissociation of Fe-O-Si bonds under strongly

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acidic hydrothermal conditions, iron is difficult to incorporate into SBA-15 directly.14 Figure 1m shows the low-angle XRD pattern of the mesoporous Fe-SBA-15 with a 2D-hexagonal structure. Since the ionic radius of Fe3+ is larger than that of Si4+, the unit cell is enlarged, and the peak of 100 reflection shifts to low angle15 (Figure S8, ESI†). From the SEM image, the mesoporous Fe-SBA-15 exhibits curved rope-like morphology (Figure S2d, ESI†). The N2-adsorption analysis (Figure 1o) indicates that the BET specific surface area of Fe-SBA-15 is 825 m2/g, which is similar to that of Fe-SBA-15 synthesized by two-step abrupt pH change method.16 Figure S9a shows the UV-Vis spectra of the samples. The absorption bands near 224 nm correspond to oxygen-to-metal charge transfers involving isolated tetrahedrally coordinated Fe2+ ions.16 The presence of a strong band near 320 nm indicates that the free iron oligomer or ferric oxide species might exist in the material. The absorption band at about 530 nm indicates the presence of extra-framework iron oligomer or aggregated iron oxide clusters in Fe-SBA-15.17 Moreover, the FTIR spectrum of Fe-SBA-15 exhibits a slight red-shift compared with that of SBA-15, indicating the incorporation of Fe into the silica framework and the formation of Fe-O-Si bonds (Figure S10, , ESI†).18 19 ICP measurement indicates that the Fe/Si molar ratios of the calcined Fe-SBA-15 and the initial reaction gel are 0.03 and 0.056, respectively (Table S1). The molar loading efficiency of Fe is about 50%, which is higher than that of previously reported FeSBA-15.16 Mössbauer spectra of the calcined Fe-SBA-15 acquired at 298 K (Figure S9b, Table S4, ESI†) indicate the presence of γFe2O3 and Fe3+ in the ratio of 2:1. Elemental mapping by EDS reveals that the Fe atoms are uniformly distributed within FeSBA-15 (Figure S11, ESI†). To identify the radicals in the synthetic system of Fe-SBA-15, the initial reaction mixture under Fenton conditions was characterized by EPR spectroscopy (Figure 2d, Figure S12, ESI†). The •OH (aN = aHβ = 1.50 mT), carbon-centered radicals (aN = 1.59 mT, aHβ = 2.27 mT), and oxidized DMPO radicals were observed. Figure S12 shows the comparison of the experimental and the simulated EPR spectra of DMPO-•OH adducts, DMPO-carboncentered radical adducts, and oxidized DMPO radicals, respectively. Theoretical calculations provide insights into the possible pathways in the hydrolysis of TEOS catalyzed by a •OH instead of H+. According to pathway I in Figure 3, the •OH interacts through its H atom with an O of TEOS, forming the weakly stable reactant R1. Then, in one step through TS1, the •OH binds to the Si atom while the •OCH2CH3 group in trans position detaches from Si, following an SN2-type mechanism. The Gibbs free energy barrier (Figure 4a) is similar to that found for the acidcatalyzed process (Figure S13, ESI†), and the reaction energy is 18.1 kcal/mol. The •OCH2CH3 radical generated in this process can react with water yielding ethanol and a new •OH. This is confirmed by the GC-MS experimental observation of ethanol (Figure S14).

Figure 3. Optimized geometry of the structures involved in the attack of •OH to TEOS generating the first hydrolyzed Si(OCH2CH3)3OH species. In pathway II, the •OH radical abstracts an H from an ethoxy group of TEOS without activation energy, generating water and a carbon-centered radical species (I2 in Figure 3), and releasing 23.5 kcal/mol. Next, I2 reacts with another •OH to form a closedshell I3 intermediate (see Figure 3) in a barrierless and exothermic step. I3 interacts with another •OH forming the weakly bound intermediate I4 that, by attack of •OH to the Si center forms a pentacoordinated intermediate I5, which decomposes easily into neutral Si(OCH2CH3)3OH and a CH3-CHOH-O• radical (Figure 3 and 4b). CH3-CHOH-O• can either decompose into ethanal and a new •OH or react with another •OH generating acetic acid and water. The experimental observation of ethanal and acetic acid (Figure S14, ESI†) confirms the viability of the above pathway. Alternatively, in pathway III radical •OH abstracts an H from I5, producing structure P2 in an extremely exothermic step (∆G = 139.1 kcal/mol, Figure 4c).

Figure 4. Gibbs free energy profiles for the •OH catalyzed hydrolysis of TEOS generating Si(OCH2CH3)3OH. (a) Pathway I generating ethanol. (b) Attack of •OH to I3 generating ethanal. (c) Attack of •OH to I3 generating acetic acid. Although the highest activation energy barriers in the acid catalyzed and radical pathways are similar (~15 kcal/mol), the reaction intermediates in the former process (Figure S13, ESI†) are less stable than initial reactants, and the driving force seems weaker. Hence, the radical mechanism is more efficient to hydrolyze TEOS, followed by acid media. This is consistent with our experimental results that the highly ordered materials can be easily synthesized through radical route comparable to the acid catalyzed route. In summary, an acid-free protocol for the synthesis of highly ordered mesoporous silica SBA-15 materials promoted by •OH mechanism has been developed. Compared with the conventional

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synthesis facilitated by strong acid, the radical route provides a simple, facile, and green method for the synthesis of highly ordered silica-based mesoporous materials. This environmentally friendly synthetic approach opens new perspectives for the future large-scale industrial production of highly ordered mesoporous materials.

ASSOCIATED CONTENT Supporting Information Experimental details, characterization data, SEM, TEM, EPR, 29Si NMR, EDS mapping, UV-Vis spectra, FTIR, Mössbauer spectra, GC-MS, nitrogen adsorption-desorption isotherms, and theoretical calculations, Figures S1-S14 and Tables S1-S4

AUTHOR INFORMATION Corresponding Author: Jihong Yu Email: *[email protected] Author Contributions ‡ These authors contribute equally to this work. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Key Research and Development Program of China (Grant No. 2016YFB0701100, 2013CB921800), the National Natural Science Foundation of China (21320102001, 21621001, 11227901) and the 111 Project (B17020). G. Feng acknowledges financial support from the China Postdoctoral Science Foundation (No. 2016M600228, 2017T1002 02).

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Journal of the American Chemical Society

Radical-Facilitated Green Synthesis of Highly Ordered Mesoporous Silica Materials Guodong Feng,†, ‡ ,※ Jianyu Wang,†, ‡ Mercedes Boronat,ǁ Yi Li,†, ⊥ Ji-Hu Su,§ Ju Huang¶, Yanhang Ma¶ and Jihong Yu*, †, ⊥ †

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, P.R. China. ǁ Instituto de Tecnologia Quimica, Universitat Politecnica de Valencia, Consejo Superior de Investigaciones Cientificas, Valencia, 46022, Spain. § CAS Key Laboratory of Microscale Magnetic Resonance and Department of Modern Physics, University of Science and Technology of China, 96 Jinzhai Road, Hefei, 230026, P.R. China. ⊥

International Center of Future Science, Jilin University, Changchun 130012, P.R. China. School of Physical Science and Technology, Shanghai Tech University, 393 Middle Huaxia Road, Pudong, Shanghai, 201210, P.R. China. ※ Key Laboratory of Advanced Molecular Engineering Materials, College of Chemistry and Chemical Engineering, Baoji University of Arts and Sciences, Baoji 721013, P. R. China. ¶

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Green synthesis of highly ordered SBA-15 via •OH radical route.

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