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FULL PAPER DOI: 10.1002/asia.201300131

Site-Selective Functionalization of Periodic Mesoporous Organosilica (PMO) with Macrocyclic Host for Specific and Reversible Recognition of Heavy Metal Gang Ye,*[a] Yuxiao Leng,[a, b] Feifei Bai,[a, b] Jichao Wei,[a] Jianchen Wang,[a] and Jing Chen*[a] Abstract: A novel kind of macrocyclichost-functionalized periodic mesoporous organosilica (PMO) with excellent and reversible recognition of PbII was developed. The macrocyclic host molecule cis-dicyclohexano[18]crown-6, with strong affinity to PbII, was carefully modified as a bridged precursor to build the PMO material. To break down the limit of the functionalization degree for PMOs incorporated with large-sized moieties, a site-selective post-functionalization method was proposed to further decorate the external

surface of the PMO material. The selective recognition ability of the upgraded PMO material towards PbII was remarkably enhanced without destroying the mesoporous ordering. Solidstate 13C and 29Si NMR spectroscopy, X-ray photoelectron spectroscopy (XPS), XRD, TEM, and nitrogen adsorption–desorption isotherm measureKeywords: host–guest systems · lead · macrocycles · mesoporous materials · molecular recognition

three research groups.[3] Then, great efforts were directed towards the development of this novel kind of organic–inorganic hybrid material.[4] PMOs offer a more promising opportunity for organic functionalities to be uniformly incorporated into the silica framework by using the bridged silsesquioxane [(R’O)3Si-R-SiACHTUNGRE(OR’)3] precursors, without compromising the mesoscopic characteristics of the materials.[5] Up to now, however, the bis-silylated precursors available for PMO construction, especially those with complex functionalities as bridge units, are very limited.[6] Most of them usually require subtle synthesis, which is, unfortunately, laborious and costly. The advance of PMOs relies on the further expansion of the precursor library. Furthermore, from a practical viewpoint, the enlargement of the number of specific functionalities anchored to the PMOs is often in demand, which is of great value in the realization of an effective detection or the binding of target molecules. But how to maintain the ordered mesoporous structure while enhancing the functionalization degree remains quite a challenge.[7] In recent years, host–guest chemistry has been linked with the research on ordered mesoporous organosilicas, which has paved the way for the development of new functional materials.[8] For this reason, highly selective sorbents and host materials for specific guests can be envisioned.[9] As we know, PbII is a common heavy-metal contaminant that has posed a considerable threat to human beings and other life forms. It has been reported that a macrocyclic host mole-

Introduction The advent of ordered mesoporous silicas has brought considerable vitality to the research on functional materials during the last two decades.[1] Owing to their tunable pore geometry, high surface area, and enormous possibilities for organic modification, these fascinating materials can potentially find applications in fields that range from catalysis, adsorption, and gas storage to controlled-release systems. Varieties of synthetic pathways including post-synthetic grafting and co-condensation, together with a rich palette of structure-directing agents (SDAs), have been developed to synthesize mesoporous silicas functionalized with organic moieties.[2] An exciting breakthrough in this area came in 1999 with the report of periodic mesoporous organosilicas (PMOs) by

[a] Dr. G. Ye, Y. Leng, F. Bai, J. Wei, Prof. J. Wang, Prof. J. Chen Institute of Nuclear and New Energy Technology (INET) Tsinghua University, Qinghua Yuan No.1 Haidian, Beijing (China) Fax: (+ 86) 10-62791740 E-mail: [email protected] [b] Y. Leng, F. Bai Faculty of Chemical Science and Engineering China University of Petroleum Changping, Beijing (China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201300131.

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ments were utilized for a full characterization of the structure, micromorphology, and surface properties. Reversible binding of PbII was realized in the binding–elution cycle experiments. The mechanism of the supramolecular interaction between the macrocyclic host and metal ion was discussed. The synthetic strategy can be considered a general way to optimize the properties of PMOs as binding materials for practical use while preserving the mesostructure.

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cule, cis-dicyclohexano[18]crown-6 (cis-DCH18C6), can specifically capture PbII by means of supramolecular interaction.[10] However, the marriage between this fascinating host molecule and the PMO materials has not yet happened. In our previous research, we succeeded in modifying the host molecule cis-DCH18C6 to be a bridged precursor for PMO preparation.[11] Herein, we report an original synthesis of a novel class of macrocyclic-host-functionalized PMO material that is specialized for PbII recognition. To break the limit of the functionalization degree for PMOs incorporated with large moieties, a site-selective post-functionalization scheme was proposed to optimize the binding ability of the PMO material for practical use.[12] A large number of macrocyclic host groups were anchored onto the external surface without damaging the mesoporous channels of the materials. We show that the selective binding of PbII in the presence of competing heavy-metal ions was greatly promoted relative to the intrinsic PMO counterpart.

Gang Ye, Jing Chen et al.

bridged precursor needs to be carefully controlled. The molar ratio of the reactant mixture was 0.05 HM/ 0.95 TMOS. Due to the bulky size and complex structure, more than 10 % of the macrocyclic host molecule in the cocondensation was found to clearly weaken the ordering of the PMO material. To serve the purpose of effective binding to the heavy metal, post-modification of the external surface was further performed to attach more macrocyclic host groups to the PMO material HM-PMO-G1. The modification was carried out in a site-selective manner.[14] During the synthesis, the mesoporous channels were still occupied by surfactant micelles, so the macrocyclic host groups were mainly grafted onto the external surface. After the reaction, the templates were removed by Soxhlet extraction, and an upgraded PMO material HM-PMO-G2 appended with macrocyclic host units on the outer surface was obtained. Solid-state 13C cross-polarization magic-angle spinning (CP-MAS) NMR spectroscopy was employed to identify the organic groups introduced to the mesoporous organosilicas HM-PMO-G1 and HM-PMO-G2 (Figure 2). The signals at d = 78.0, 70.3, 53.4, and 25.7 ppm correspond to the carbon atoms in the ether ring and cyclohexyl groups of the host moieties. The three carbon atoms in the spacers show signals at d = 48.7, 26.1, and 9.5 ppm. The results indicate that the host moieties were well preserved during the synthesis. Moreover, it can be seen that, in comparison, all the carbon signals evidently intensified for HM-PMO-G2, which implied that a large amount of the host moieties were grafted onto HM-PMO-G1 during the subsequent site-directed functionalization process.

Results and Discussion The macrocyclic host molecule cis-dicyclohexano[18]crown6 (cis-DCH18C6) has been well known for its excellent binding ability to lead ions in solution.[10] To incorporate the host molecules into the mesoporous organosilica, a series of elaborate chemical decorations are required. First, with dibenzo[18]crown-6 as the starting material, amino groups were introduced to the aromatic rings through typical nitration and reduction reactions.[13] Then the cis isomer was separated for the subsequent catalytic hydrogenation to transform the aromatic rings to cyclohexyl groups. The reaction conditions needed to be carefully controlled to avoid the breakage of the amino groups as well as the polyether ring. Finally, the amino-substituted cis-DCH18C6 was bis-silylated by means of a nucleophilic substitution reaction with 3chloropropyltrimethoxysilane to obtain a bridged precursor for the preparation of PMOs.[11b] The synthetic route of the mesoporous organosilica HMPMO-G2 functionalized with cis-DCH18C6 is illustrated in Figure 1. A PMO prototype tagged as HM-PMO-G1 was synthesized first by using the modified host molecule (HM) as a bridged precursor and tetramethyl orthosilicate (TMOS) as silica source for co-condensation under basic conditions (S + I system). It is noteworthy that, to obtain a well-ordered mesoporous structure, the content of the

Figure 2. 13C CP-MAS NMR spectra of HM-PMO-G1 and HM-PMO-G2.

Figure 1. Synthesis and site-selective modification of the periodic mesoporous organosilica (PMO) functionalized with macrocyclic host groups.

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Solid-state 29Si MAS NMR spectroscopy provides information about the silica matrix. Figure 3 shows the 29Si NMR spectra of the mesoporous organosilicas HM-PMO-Gn (n =

by XPS and elemental analysis (EA) were studied comparatively. The results are summarized in Table 1. As the escape depth of electrons is only a few nanometers, XPS emphasizTable 1. Elemental analysis and XPS survey of HM-PMO-Gn (n = 1 or 2). Samples C HM-PMO-G1 HM-PMO-G2

10.8 19.8

Elemental analysis [wt %] H N 2.71 4.07

0.46 1.42

N [wt %] by XPS 0.41 2.33

es the contribution from the outer layer of the material, whereas the EA method is concerned with the bulk chemical measurement.[14] Therefore, a comparative study can offer suggestive information about the location distribution of the host moieties. The analysis is based on the amount of N. For the HM-PMO-G1 sample, owing to the low molar ratio of the macrocyclic host molecules in the co-condensation system, it has a functionalization degree of 0.164 mmol g 1. Only a slight difference between the results of XPS and EA for this material was observed. However, after the site-selective modification in the second synthetic stage, it is noteworthy that the XPS survey gives a much higher N content (2.33 wt %) of HM-PMO-G2 than that obtained by EA (1.42 wt %). The result suggests that the sitedirected modification indeed took place in the expected manner. A large number of host moieties were anchored onto the external surface of the HM-PMO-G1 sample. It can be estimated that the functionalization degree for the external surface is about 0.343 mmol g 1. Actually, these host groups have a more convenient position that is easily accessible for the guest molecules in the environment. It is expected that the post-modification would afford the PMO material an enhanced binding ability. Powder X-ray diffraction (PXRD) was used to characterize the structural ordering of functionalized mesoporous organosilicas. Figure 5 shows the PXRD patterns of HMPMO-Gn (n = 1 or 2). It can be seen clearly that HM-PMOG1 has distinct diffraction signals that correspond to the

Figure 3. 29Si MAS NMR spectra of HM-PMO-G1 and HM-PMO-G2.

1 or 2). The three broad signals centered at d = 110.2 (Q4), 100.7 (Q3), and 65.2 ppm (T3) correspond to SiACHTUNGRE( O )4, MeOSiACHTUNGRE( O )3, and RSiACHTUNGRE( O )3 silicate species. In addition, Q2 resonance with low intensity can be observed at d = 91.9 ppm. The intensity of the T3 signal of HM-PMO-G2 is clearly enhanced relative to that of HM-PMO-G1. This is attributed to the RSiACHTUNGRE( O )3 groups in the bis-silylated host molecules introduced onto the external surface during the grafting process. X-ray photoelectron spectroscopy (XPS) was employed to investigate the composition of the mesoporous organosilicas. Figure 4 gives the survey spectra of the HM-PMO-Gn (n =

Figure 4. XPS spectra of the mesoporous organosilicas HM-PMO-G1 and HM-PMO-G2.

1 or 2) samples, which were calibrated by setting the main C 1s component due to C C/C H bonds at 284.8 eV. Signals of Si 2p, Si 2s, C 1s, N 1s, and O 1s were recorded. It can be seen that the evident N 1s signal emerges at around 400 eV, which corresponds to the N atom in the amino substituents of the macrocyclic host molecule. Meanwhile, the increase in the intensity of the C 1s signal can be clearly observed for HM-PMO-G2 due to the introduction of more organic constituents. To evaluate the functionalization degree of the cisDCH18C6 that contains PMO material, the results provided

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Figure 5. PXRD patterns of a) HM-PMO-G1 and b) HM-PMO-G2.

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(100), (110), and (200) reflections, which implies the sample has an ordered hexagonal pore arrangement (p6mm).[15] The well-formed mesoporous channels can be visually observed under transmission electron microscopy (TEM; Figure 6 a).


Gn (n = 1 or 2), it can be inferred that the first weight-loss period from 200 to 320 8C is related to the breakage of residual methoxy groups adjacent to silicon atoms. The second period up to 800 8C is due to the decomposition of the macrocyclic host groups. The difference in weight loss for the two samples reflects the different amount of the macrocyclic host introduced into the silica matrix, which is also consistent with the result of elemental analysis. Nitrogen adsorption–desorption measurements were carried out to evaluate the surface and pore properties of the mesoporous organosilicas. Type IV isotherms of the mesoporous organosilicas are shown in Figure 8. The PMO material HM-PMO-G1 has a Brunauer–Emmett–Teller (BET) surface area of 773.8 m2 g 1 and a pore volume of 0.253 cm3 g 1, whereas HM-PMO-G2 shows a less satisfactory result (467.3 m2 g 1; Table 2).

Figure 6. TEM images of a) HM-PMO-G1 and b) HM-PMO-G2.

During the subsequent functionalization by a site-selective method, as the pore channels of the as-synthesized sample were still taken up by the surfactant micelles, the macrocyclic host moieties, ideally, are grafted onto the external surface. As a result, the hexagonal pore arrangement was maintained in the product HM-PMO-G2, which was confirmed by the lower PXRD pattern in Figure 5 b (5-fold magnification). The TEM image in Figure 6 b also gives visual evidence. Meanwhile, it can be seen that the intensity of the Xray diffraction peak for HM-PMO-G2 shows a decrease relative to HM-PMO-G1. It reveals that, to some extent, the Figure 8. Nitrogen adsorption–desorption isotherms of HM-PMO-G1 and ordering degree of the sample became weakened during the HM-PMO-G2. subsequent decoration of the external surface. The thermal weight-loss curves of the mesoporous organosilicas HM-PMO-Gn (n = 1 or 2) meaTable 2. Surface and pore properties of HM-PMO-Gn (n = 1 or 2). sured by thermogravimetric analysis (TGA) are Samples BET surface area [m2 g 1] Pore size [nm] Pore volume [cm3 g 1] shown in Figure 7. It can be seen that when the 4.069 0.253 temperature is raised to 200 8C, only a slight weight HM-PMO-G1 773.8 HM-PMO-G2 467.3 4.271 0.133 loss (about 3 %) occurs due to the evaporation of the residual solvent. This means that the samples have a satisfactory thermal stability. Further inThe decrease in the specific surface area and pore volume crease in the temperature results in a two-stage weight loss. of HM-PMO-G2 is understandable. Although the postBased on the estimation of the weight change of HM-PMOmodification of HM-PMO-G1 was supposed to take place on the external surface in a site-directed way, it could not be exactly in that ideal situation for the real reaction system. It has been reported that, in some cases of selective functionalization, partial grafting reagents might enter the templatefilled channels to functionalize the inner pore systems.[16] Therefore, based on a similar consideration for our experiment, if some of the large macrocyclic host moieties obtained access to the pore openings and channels, it would cause a pore-blocking effect and result in a decrease in the surface properties. The Supporting Information shows the pore-size distribution of HM-PMO-G2. However, the narrow peak reveals that, from a statistical viewpoint, the uniformity of the pore size of HM-PMO-G2 was still well preserved. Figure 7. TGA curves of HM-PMO-G1 and HM-PMO-G2.

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The binding ability of the mesoporous organosilicas HMPMO-Gn (n = 1 or 2) towards heavy-metal ions was investigated. Typical heavy metals in industrial wastewater including CuII, ZnII, PbII, and CdII were employed to prepare the stock solution. The initial metal concentration for the batch experiment was 100 mg L 1, and the phase ratio was set to be 0.02 g per 1 mL. Figure 9 shows the removal ratio of each


porous organosilica was washed by deionized water (3 times) and dried for regeneration. The recycling effect is shown in Figure 10. For a better comparison, the removal rate in the first run was normalized to be 100 %. It revealed

Figure 10. Evaluation of the reusability of the mesoporous organosilica HM-PMO-G2 for PbII adsorption. [PbII] = 100 mg L 1, phase ratio = 0.02 g per 1 mL. Figure 9. Binding of heavy-metal ions by HM-PMO-G1 (left column) and HM-PMO-G2 (right column). [Metal] = 100 mg L 1, contact time = 60 min.

that the mesoporous organosilica HM-PMO-G2 has reversible binding ability to PbII in the cycle experiments, and the reuse efficiency is favorable. After 4 cycles, the binding ability of the material was largely preserved, and the removal rate only had an 8.6 % decrease. For this reason, mesoporous organosilica is believed to have the potential to be used as a separation material for selective removal of lead in industrial wastewater.

metal after the batch adsorption. Clearly, both the functionalized mesoporous organosilicas have specific recognition ability toward PbII in solution with coexisting interferences. This can be reasonably explained by the host–guest interaction between the lead ions and the host moieties.[10] The result also indicates that, after the chemical decoration and introduction to the mesoporous silicas, the macrocyclic host cis-DCH18C6 groups still retain excellent recognition toward PbII. Meanwhile, the HM-PMO-G2 material shows superior binding ability to PbII than HM-PMO-G1 does. After a contact time of 60 min, 95.1 % PbII was removed by HM-PMO-G2, whereas HM-PMO-G1 only had a removal rate of 40.1 %. The result suggests that, although the intrinsic PMO material HM-PMO-G1 has the advantage of better surface and pore properties, it is not the decisive factor for the material to have an excellent binding ability to the guest ions. Instead, as for HM-PMO-G2, the enhanced functionalization degree that took place on the external surface in particular, plays a dominant role during the binding process. The remarkable improvement should be attributed to the post-modification, which has upgraded the capability of the intrinsic PMO material for metal binding. As the macrocyclic host groups were covalently bonded to the silica matrix, the mesoporous material should be endowed with stability for repeated use, because the leakage of the host molecules, which usually happens for the separation materials prepared by physical methodology, can be avoided. In our study, binding–elution cycle experiments were carried out to evaluate the reusability of HM-PMOG2. After each binding process, the captured PbII was eluted by 0.1 mol L 1 ammonium oxalate solution. Then the meso-

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Mechanism Discussion Molecular recognition based on the host–guest interaction has been a topic of much concern for years.[8, 17] We developed in this study a new class of periodical mesoporous organosilica functionalized with macrocyclic host groups for PbII recognition. As far as we know, this is the first example of a PMO material specialized for PbII with excellent binding ability. To reach this goal, the challenge lies in the weighing of two factors that seem to be incompatible. On the one hand, a great number of the functionalities are expected to be anchored to the silica matrix. Therefore the mesoporous organosilica can serve as a selective separation material from a practical point of view. On the other hand, for the building of a PMO material, the amount of functionalities incorporated into the framework needs to be well controlled. For those with complex structure and bulky size, it is usually difficult to obtain a high degree of functionalization, or an ordered mesoporous structure cannot be guaranteed. To handle the situation, a site-directed modification method was employed in our research. During the postgrafting, more macrocyclic host groups were introduced onto the external surface of the PMO material, whereas the mesopore channels were protected by the template micelles. In this sense, it could be concluded that the site-directed

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strategy broke down the limitation on the amount of largesized functionalities to build PMO material, without obviously compromising the ordering of the mesoporous system. This strategy could be regarded as a general way to improve the performance of PMO materials. With regard to the mechanism for metal binding, the complexation between the macrocycle unit and PbII is believed to have the main contribution, as discussed in some of the previous reports.[10] It should be pointed out that, for the coordination complexes of macrocyclic hosts and cations, the hole-size fitting concept in traditional supramolecular chemistry has been incapable of giving a full explanation of the affinity between each other. It has been suggested that strain energy, solvation, and entropic factors should be also taken into account.[17a] One question is, in addition to the macrocycle unit, whether the NH groups in the host moieties played a role in the binding of PbII. Indeed, research efforts on amino-containing materials as sorbents to heavymetal ions have been reported before.[18] However, in our case, the NH groups acted as a linking point between the macrocyclic host and the alkyl chain spacer. Considering the lack of freedom and the steric hindrance of the adjacent macrocyclic group, it should be reasonably inferred that the contribution of the NH groups is not comparable to that of the macrocyclic hosts. All in all, the full mechanism of the binding effect of this macrocyclic-host-functionalized PMO material to PbII is still undetermined. More supporting evidence needs to be supplied. An in-depth theoretical explanation will rely on the calculation and modeling approaches.

ecule (1.45 g, 2 mmol) were premixed in THF (2 mL). The mixture was added quickly to the surfactant solution. After stirring at room temperature for 1 h, the mixture was heated to 90 8C under static conditions for 24 h aging. The as-synthesized HM-PMO-G1 was isolated by filtration and dried at 60 8C for 12 h. For a comparative study, the mesoporous product HM-PMO-G1 was obtained after the surfactant was removed by Soxhlet extraction in HCl/methanol for 3 days. Synthesis of HM-PMO-G2 As-synthesized HM-PMO-G1 (3.0 g) was added into anhydrous THF (200 mL) under a nitrogen atmosphere, followed by the addition of bis-silylated macrocyclic host molecule (4.35 g, 6 mmol), which was an excess amount to ensure a high functionalization degree. The reaction was performed under reflux at 80 8C for 5 h. Then the solid product was collected by filtration, and the unreacted host molecules were recovered from the filtrate. The final HM-PMO-G2 was obtained after the removal of the template by extraction as described for HM-PMO-G1. Heavy-Metal Binding The binding towards heavy metals including PbII, CuII, ZnII, and CdII was performed in a 25 8C thermostat. HM-PMO-Gn (n = 1 or 2; 0.1 g) was added into solution (5 mL) with an initial concentration of 100 mg L 1 for each metal. After vigorous agitating for 60 min, the aqueous phase was separated with a 0.45 mm micropore filter. The residual amount of the metals in the solution was measured by inductively coupled plasma atomic emission spectrometry (ICP-AES). The removal rate of the metal ions was calculated on the basis of material balance.

Acknowledgements This work is supported by the National Natural Science Foundation of China under project nos. 51103079 and 91226110.

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Conclusion In conclusion, a novel kind of periodical mesoporous organosilica (PMO) material, HM-PMO-G2, was designed and synthesized in this study. Host molecules cis-DCH18C6 were incorporated into the silica matrix, and, subsequently, grafted onto the external surface by following a site-directed method. Selective recognition of PbII in multicomponent heavy-metal solution was realized on the basis of the specific host–guest interaction. The reversible binding ability of the material was proven during the binding–elution cycle experiments. The new functional material might have the potential to be an excellent adsorbent for the decontamination of heavy-metal wastewater. Investigation on the more detailed complexation mechanism is still underway.

Experimental Section Synthesis of HM-PMO-G1 HM-PMO-G1 was synthesized by using cetyltrimethylammonium bromide (CTAB) as the structure-directing agent (SDA) under basic conditions (S + I system). First, a surfactant solution was prepared by adding CTAB (2.62 g, 7.2 mmol), NaOH (0.576 g, 14.4 mmol), and H2O (108 g, 6 mol) into a 500 mL flask followed by vigorous stirring. Tetramethoxysilane (TMOS; 5.78 g, 38 mmol) and the bis-silylated macrocyclic host mol-

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