Vulcanized Ethene-PMO: A New Strategy to Create Ultrastable

A periodic mesoporous organosilica (PMO) functionalized with sulfur units between the organic bridges has been successfully synthesized by a postsynth...
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Vulcanized Ethene-PMO: A New Strategy to Create Ultrastable Support Materials and Adsorbents María I. López,† Dolores Esquivel,‡ César Jiménez-Sanchidrián,† Pascal Van Der Voort,‡ and Francisco J. Romero-Salguero*,† †

Department of Organic Chemistry, Nanochemistry and Fine Chemistry Research Institute (IUIQFN), Faculty of Sciences, University of Córdoba, Campus of Rabanales, Marie Curie Building, Ctra. Nal. IV, km 396, 14014 Córdoba, Spain ‡ Department of Inorganic and Physical Chemistry, Center for Ordered Materials, Organometallics and Catalysis (COMOC), Ghent University, Krijgslaan 281, Building S3, 9000 Ghent, Belgium S Supporting Information *

ABSTRACT: A periodic mesoporous organosilica (PMO) functionalized with sulfur units between the organic bridges has been successfully synthesized by a postsynthetic cold vulcanization treatment performed on an ethenylene-bridged PMO material (ethene-PMO). The results of X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and nitrogen adsorption/desorption porosimetry show that the resultant materials have well-ordered hexagonal mesoporous structures with narrow pore size distributions. Moreover, 13C CP/ MAS NMR, 29Si MAS NMR, X-ray photoelectron spectroscopy (XPS), and elemental analysis prove the cross-linking between the organic moieties through additional sulfur bridges. Besides the incorporation of a new functionality to this PMO, the vulcanization treatment, reminiscent of that applied to rubber, improves the thermal and hydrothermal stabilities of the material. The vulcanized materials were employed as mercury adsorbents able to be reused without loss of their capacity in repetitive adsorption tests. or bridges with metal complexes).5 Among them, the ethenylene bridge has a special interest because it allows further functionalization by modification of the double bonds.10 A variety of organic reactions have been described to transform the double bonds located on the walls to other functionalities. The epoxidation of the CC bond in ethenylene-bridged PMOs (ethene-PMO) has resulted in an oxirane that can be subsequently modified by different ring-opening reactions to give PMOs functionalized with amine11 or sulfonic acids groups.12 Also, several dienes can react with the double bonds (dienophile) of ethene-PMOs through a Diels−Alder reaction.13−15 In these cases, the resulting pendant adducts were sulfonated. Furthermore, benzene rings were incorporated by a Friedel−Crafts reaction on an ethene-PMO, which were later subjected to sulfonation.16 All the above-mentioned PMOs were used for catalytic applications. One of the most common reactions to modify ethene-PMO is the bromination of the CC bonds.2,3,17 As the bromine atom is an excellent leaving group, it can be substituted by different nucleophiles giving rise to amines18 or thiol groups.19 These materials were used as adsorbents for arsenate and mercury adsorption, respectively.

1. INTRODUCTION Periodic mesoporous organosilicas (PMOs) were developed in 1999 by three different research groups,1−3 and since then these materials have attracted great scientific interest because they combine the rigidity of the inorganic structures and the flexibility and chemical versatility of the organic moieties.4,5 In contrast to other hybrid materials prepared from mesoporous silicas by grafting or co-condensation, the organic fragments in PMOs are abundantly and homogeneously distributed through their structure. Accordingly, they are more hydrophobic than their silica counterparts, which generally improves their performance in many applications.6 In comparison to other porous silica materials,7 they exhibit a higher thermal and mechanical stabilities.8 Furthermore, just like the mesoporous silicas, PMOs have high surface areas and narrow pore size distributions in the mesopore range.5 For these reasons, PMOs have been used in different fields, such as catalysis, adsorption, analytical chemistry, low-k materials, and biomedicine, among others.5,9 PMOs are synthesized by the simultaneous use of a soft template and a hydrolyzable organo bis-silane, (R′O)3−Si−R− Si−(OR′)3, where R is an organic functional linker and R′ usually represents a methyl or ethyl group. A multitude of bridging groups have been used, from the simplest (such as methylene, ethylene, ethenylene, or phenylene) to the most complex and advanced (like aromatic bridges, bridges containing nitrogen and sulfur, ionic bridges, chiral bridges, © 2014 American Chemical Society

Received: June 16, 2014 Revised: July 15, 2014 Published: July 16, 2014 17862

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2.5. Mercury(II) Adsorption. The Hg2+ adsorption capacities for ePMO-v1 and ePMO-v2 were measured using a batch adsorption process. Typically, the adsorbent (50 mg) was stirred at room temperature for 6 h in a Hg(NO3)2 aqueous solution containing 663 ppm of Hg2+ (50 mL). The pH of this solution was adjusted to 2 using HNO3 to avoid the precipitation of the metal ions during the adsorption experiments.25 The mixture was then filtered, and the amount of Hg2+ present in the adsorbent was measured by cold vapor atomic absorption spectrometry with a flow injection system (FICVAAS, PerkinElmer, FIMS 400). Prior to analysis, the solid sample was subjected to an acid microwave digestion under pressure (Anton Paar, Multiwave 3000). To regenerate the materials, the adsorbent (50 mg) was washed once with a mixed solution of 1 M HCl and 5% thiourea (10 mL) for 2 h at room temperature, and three times with water (20 mL), under stirring for 30 min at room temperature. Finally, the concentration of Hg2+ remaining in the adsorbent was determined by FI-CVAAS. Three adsorption/desorption cycles were performed. The adsorbent was evaluated after each cycle by XRD, elemental analysis, and XPS. 2.6. Characterization. XRD patterns were recorded on either a Siemens D-5000 or an ARL X̀ TRA (Thermo Scientific) powder diffractometer (Cu−Kα radiation). TEM micrographs were taken using a JEOL JEM 1400 instrument on samples supported on copper grids with carbon coating. Thermogravimetric analyzes were performed on a Setaram Setsys 12 electrobalance equipped with a Pt/Pt−Rh (10%) thermocouple. The sample (10− 15 mg) was introduced into a platinum sample holder and subsequently heated in an air flow (40 mL/ min) from room temperature to 1000 °C at a heating rate of 10 °C/min. The 13C CP/MAS and 29Si MAS NMR spectra were recorded at 100.61 and 79.49 MHz, respectively, on a Bruker Avance 400 WB dual channel spectrometer at room temperature. An overall of 1000 free induction decays were accumulated. The excitation pulse and recycle time for 13C CP/MAS NMR spectra were 6 μs and 2 s, respectively, and those for 29Si MAS NMR spectra 6 μs and 60 s. Chemical shifts were measured relative to tetramethylsilane standard. Before analysis, the samples were dried at 150 °C for 24 h. Deconvolution of the 29Si MAS NMR spectra was performed with PeakFit software. The content of silanol groups per silicon atom was calculated from the equation OH/Si = (2T1 + T2 + 2Q2 + Q3)/(T1 + T2 + T3 + Q2 + Q3 + Q4). XPS spectra were recorded with a SPECS Phoibos HAS 3500 150 MCD. Accurate binding energies (BE) have been determined with respect to the position of the C 1s peak at 284.8 eV. The peaks were decomposed using a least-squares fitting routine (Casa XPS software) with a Gauss/Lorenz ratio of 70/30 and after subtraction of a linear background. Elemental compositions were determined on an Eurovector EA 3000 analyzer. N2 isotherms were determined on a Micromeritics ASAP 2020 analyzer at −196 °C. The specific surface area of each solid was determined using the BET method over a relative pressure (P/P0) range of 0.06−0.20, and the pore size distribution was obtained by analysis of the adsorption branch of the isotherms using the Barrett−Joyner−Halenda (BJH) method. Prior to measurement, the samples were outgassed at 120 °C for 24 h. Particle size data were obtained in a Mastersizer S particle size analyzer by laser diffraction technology with a small volume

Despite the numerous procedures for modifying the ethenylene groups in PMOs, this is the first report on use of a vulcanization technique to enhance the stability of the material while simultaneously introducing sulfur-containing functional groups. Generally, the synthesis of thioether-bridged PMOs has been accomplished by using bridged organosilane precursors containing disulfide20,21 or tetrasulfide22,23 moieties, in some cases incorporating up to ca. 20 mol % of the Scontaining precursor in the synthesis mixture. Herein, we describe two procedures of cold vulcanization performed on an ethene-PMO (henceforth, ePMO) using sulfur monochloride (S2Cl2) as cross-linking agent. The resulting solids have been thoroughly characterized and finally essayed on the adsorption of mercury.

2. EXPERIMENTAL SECTION 2.1. Chemicals. The organosilica precursor, i.e., bis(triethoxysilyl)ethylene (95%, 80% trans-isomer), was purchased from ABCR. Surfactant Brij-76 (poly(oxyethylene(10)stearyl alcohol)) and sulfur monochloride (98%) were supplied by Aldrich. HCl (37%), HNO3 (65%), ethanol (96% v/v), dichloromethane (99.5%), and dry dichloromethane (99.9%) were provided by Panreac. Also, 1,2-dichlorobenzene (99%, Sigma-Aldrich), Hg(NO3)2·H2O (98+%, Sigma-Aldrich), and thiourea (≥99.0%, Fluka) were used. All these chemicals were used without further purification. 2.2. Synthesis of the Ethenylene-Bridged PMO (ePMO). The synthesis of this material was performed according to a previously reported procedure.24 In brief, Brij 76 (6 g) was added to a solution of 37% HCl (19.6 mL) in H2O (279 mL). The resulting mixture was stirred at 50 °C for 24 h. Later, bis(triethoxysilyl)ethylene was added dropwise. The Si:Brij-76:HCl:H2O molar ratio in the mixture was 1:0.10:2.85:186.1. After stirring for 24 h at the same temperature, the suspension was aged at 90 °C for 24 h under static conditions. Finally, the solid was filtered and dried in the air. The surfactant was removed by refluxing the as-synthesized material in an HCl solution (1 mL of 37% HCl in 50 mL of ethanol, per gram of solid) for 12 h. Subsequently, the solid was filtered and washed with ethanol. This process was repeated three times. The final material was then dried in a drying chamber at 100 °C under vacuum. 2.3. Cold Vulcanization Treatment. The cold vulcanization treatment was carried out according to the following procedure. One gram of ePMO material was heated overnight at 120 °C under vacuum and then added to a mixture of S2Cl2 (1.25 mL) and a solvent (50 mL). Depending on the solvent used, i.e., dichloromethane and 1,2-dichlorobenzene, two different materials, named as ePMO-v1 and ePMO-v2, respectively, were obtained. After the resulting suspensions were stirred for 24 h either under reflux or at 120 °C, respectively, materials ePMO-v1 and ePMO-v2 were recovered by filtration. Unreacted S2Cl2 was removed by fourfold refluxing each solid in dichloromethane for 1 h. 2.4. Hydrothermal Treatments. The corresponding material (0.3 g) was suspended in water (30 mL). The resulting suspension was stirred at room temperature for 8 h. Then, the solid was recovered by filtration and dried under vacuum at 120 °C. An analogous procedure was carried out under reflux. X-ray diffraction spectra were recorded before and after the hydrothermal treatments. 17863

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Figure 1. XRD patterns of the ePMO, ePMO-v1, and ePMO-v2 materials before and after the different hydrothermal treatments both at room temperature and at reflux.

Table 1. Physicochemical Properties of ePMO, ePMO-v1, and ePMO-v2 Materials

a

material

a0 (Å)a

BET surface area (m2 g−1)

micropore area (m2 g−1)b

pore volume (cm3 g−1)

pore diameter (Å)

wall thickness (Å)c

ePMO ePMO-v1 ePMO-v2

67 64 63

1122 803 749

105 38 2

0.92 0.64 0.68

33 32 32

34 32 31

Unit-cell dimension calculated from a0 = (2d100/√3). bDetermined by the t-plot method. cEstimated from (a0 − pore diameter).

Figure 2. Representative TEM images of ePMO, ePMO-v1, and ePMO-v2 materials, showing the pore system in the perpendicular direction to the pore axis.

the step at a relative pressure of 0.3−0.6, i.e., typical of mesoporous solids (Figure S1 in the Supporting Information). All materials exhibited a high surface area (Table 1), even though it decreased in both vulcanized PMOs, particularly in the ePMO-v2 material. The average pore diameter and the narrow pore size distribution confirmed the presence of pores in the mesopore range. In addition, pore volume dropped significantly in both vulcanized materials. Interestingly, the micropore area decreased drastically after vulcanization, particularly in ePMO-v2. Elemental analysis of vulcanized ethene-PMOs revealed a carbon to sulfur atomic ratio of 5.0 and 5.1 for ePMO-v1 and ePMO-v2, respectively. On the contrary, the same reaction carried out on other PMOs, i.e., ethylene- and phenylenebridged PMOs, gave rise to a negligible S content, thus indicating that S2Cl2 reacts selectively with the CC bonds but not with silanol groups. In fact, XPS analysis of ePMO-v1 and ePMO-v2 samples only exhibited one peak at approximately 164 eV, corresponding to a S 2p binding energy compatible with the presence of C−S and S−S bonds (Figure 3). Also, this

dispersion unit of Malvern Instruments. Prior to the measurements, the samples were dispersed in distilled water and sonicated for 10 min.

3. RESULTS AND DISCUSSION 3.1. Characterization of Vulcanized Materials. The XRD patterns of surfactant-free ePMO and the corresponding vulcanized materials, ePMO-v1 and ePMO-v2 (see Experimental Section for nomenclature), exhibited an intense lowangle (100) peak as well as broad and short second-order (110) and (200) peaks at higher incidence angles indicative of materials with 2D hexagonal (P6mm) mesostructures (Figure 1 and Table 1).24 These results indicated that the well-ordered two-dimensional hexagonal structure was preserved after the vulcanization process. Transmission electron microscopy (TEM) images of ePMO, ePMO-v1, and ePMO-v2 materials (Figure 2), which showed parallel fringes corresponding to side-on projections of the mesostructure, were consistent with the XRD results. Similarly, the N2 adsorption−desorption isotherms were of type IV with 17864

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Figure 3. XPS S 2p spectra of ePMO-v1 and ePMO-v2 materials before and after the adsorption of mercury (solid and dotted lines, respectively).

to the new carbon atoms formed in the cross-linking process by analogy with the reactions proposed for polymers (Chart 1).27,28 The basic structure coming from addition and crosslinking would correspond to 1. Peaks at around 25 and 35 ppm might be attributed to the new bridges in 1, i.e., CI and CII atoms, respectively. However, the existence of signals at ca. 41 (CIII) and 48 ppm (CVI) could only be explained by cleavage of Si−C bonds, yielding units of the type 2 and 3. Atoms CIV and CV would appear at ca. 35 and 25 ppm, respectively. In addition, upfield signals at ca. 11 and 20 ppm seemed to correspond to a bridge such as 4, whose carbon atoms CVII and CVIII would give signals at ca. 20 and 11 ppm, respectively. According to the intensity of the signals, material ePMO-v2 would exhibit a higher degree of functionalization and crosslinking than sample ePMO-v1. Those signals at around 138 and 132 ppm, which are more abundant in ePMO-v2 than in ePMO-v1, revealed the existence of pendant vinyl groups formed by cleavage of Si−C bonds.29 Also, a signal at ca. −3 ppm was present in ePMO-v2, which might be assigned to O3Si−CH3 units produced by cleavage of the bridges. 29Si MAS NMR spectra (Supporting Information Figure S2) of ePMO and vulcanized materials exhibited three signals centered at −64, −73, and −82 ppm, which can be ascribed to the T1 [CHCH−Si(OH)2(OSi)], T2 [CHCH−Si(OH)(OSi)2], and T3 [CHCH−Si(OSi)3] sites of the framework. No signals corresponding to SiO4 species (Qn sites with n = 1−4, i.e., a Si atom attached to four oxygen atoms) were detected in the region between −90 and −120 ppm for ePMO, thus precluding the cleavage of C−Si bonds during the synthesis of this material. However, in vulcanized materials signals also appeared at ca. −92, −102, and −111 ppm, which can be attributed to Q2 [(SiO)2Si(OH)2], Q3 [(SiO)3Si(OH)], and Q4

technique revealed the presence of chlorine in both samples, ePMO-v1 and ePMO-v2, amounting to 2.1 and 2.4 wt %, respectively. 13 C CP/MAS NMR experiments of vulcanized PMOs confirmed the functionalization of ethenylene bridges (Figure 4, Scheme 1). Although the spectra still exhibited an intense

Figure 4. 13C CP/MAS NMR spectra of ePMO-v1 and ePMO-v2 materials: *, spinning sidebands; †, surfactant; ‡, ethoxy groups.

signal at ca. 145 ppm, which corresponded to the sp2 carbons of the ethenylene bridges,13 new bands appeared below 50 ppm, clearly evidencing the addition reaction of S2Cl2 on the double bonds and concomitant formation of sp3 carbon atoms.26 Basically, the three shaded areas in Figure 4 could be attributed

Scheme 1. Pictorial Representation of Some Cross-Linking Reactions by Cold Vulcanization between S2Cl2 and the Ethenylene Bridges in ePMO

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Chart 1. Proposed Structures Formed by Reaction between the Ethenylene Bridges in ePMO and Sulfur Monochloridea

a

(S)x represents a monosulfide, disulfide, or polysulfide bridge.

[(SiO)4Si] units, respectively, thus corroborating the cleavage of C−Si bonds during the vulcanization treatment. The proportion of Qn sites in ePMO-v1 was below 10%, whereas it was 36.1% in ePMO-v2, in agreement with 13C NMR results. Interestingly, sulfur monochloride seemed to react preferentially with the double bonds more exposed to the surface of the pores, as deduced by the increased proportion of T3 sites from ePMO to ePMO-v1 and particularly to ePMO-v2. The content of silanol groups per silicon atom (OH/Si ratio) was 0.7 for the three materials. The cleavage of Si−C bonds in ethene-PMOs has been previously observed during thermal treatments through a process named metamorphosis, which consists of the transformation of bridging into terminal organic groups.29,30 Shirai et al.31 calculated the relative Si−C bond stability of various bridged organosilane precursors using molecular orbital theory calculations employing DFT methods. The predicted values for several aromatic bridges were consistent with the experimental results found during their polycondensation to organosilica hybrids. They proposed that the Si−C bond cleavage under acidic conditions proceeded in two steps: electrophilic attack to the carbon atom at the ipso-position and elimination of the Si(OCH2CH3)3 group through cleavage of the Si−C bond. During the vulcanization treatment, the Si−C bond cleavage in the ethenylene bridges might occur via a similar mechanism, but the reactive electrophile would be S2Cl2 rather than H+. 3.2. Thermal and Hydrothermal Stabilities of Vulcanized Materials. As depicted in Figure 5, vulcanized materials exhibited a similar thermal behavior to ethene-PMO. The three materials showed a first weight loss (below 110 °C) characterized by an endothermic peak due to the removal of physisorbed water. The second loss occurred in a wide temperature range between ca. 230 and 600 °C. This loss can be explained by the decomposition of organic matter, such as small amounts of remaining surfactant and particularly the decomposition of the organic bridges. All of them were associated with an exothermic peak, which was shifted to higher temperatures in the vulcanized materials, particularly in ePMOv2.8 Therefore, the vulcanization treatment increased the thermal stability of ePMO, as would be expected due to the incorporation of sulfur bridges. Moreover, the hydrothermal stability of the vulcanized materials was tested and compared with that of the parent ePMO. The latter material, synthesized using commercial bis(triethoxysilyl)ethylene as precursor, loses its mesostructure in water even at room temperature (Figure 1).8 On the contrary, vulcanized PMOs withstood the hydrothermal treatment at room temperature. In addition, after refluxing in water, ePMO-v2 preserved the quality of its mesostructure, exhibiting (110) and (200) reflections, whereas ePMO-v1 presented only the low-angle (100) peak. Clearly, the

Figure 5. Thermogravimetric curves (--- TGA), ( DTA), and (··· DTG) for the different materials.

vulcanization treatment improves the hydrothermal stability of the ePMO material. The increased thermal and hydrothermal stabilities of vulcanized ePMO proved that sulfur units were cross-linking its structure, so acting as additional bridges between the former ethenylene bridges. Also, the higher stability of ePMO-v2 than ePMO-v1 would involve a higher degree of cross-linking in the former. In fact, after the hydrothermal treatment under reflux the C to S atomic ratio (Figure 1) was lower for ePMO-v2 than for ePMO-v1. In the same sense, after the hydrothermal treatment, the structural contraction was higher for ePMO-v1 than for ePMO-v2. Particle size distribution curves (Figure 6) revealed that vulcanization led to a decrease in the particle size, probably due 17866

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practically constant throughout the successive cycles. The robustness of ePMO-v2 as Hg(II) adsorbent is highly remarkable. For comparison, two excellent mesoporous adsorbents with capacities of 1500 and 367 mg Hg2+ g−1 only preserved 30% (444 mg g−1) and 38% (138 mg g−1), respectively, of their original loading after only one recycle.33,34 Since the regeneration capacity is a key feature in the development of a good adsorbent, several desorption processes were tested to remove Hg2+ ions. The use of 12 M HCl34 resulted in a very low recovery of material. When the extractant solution consisted of either 2 M HCl18 or a 1.2 M KI solution, the residual amounts of Hg2+ in the adsorbent were 156 and 135 mg g−1, respectively. Nevertheless, the desorption of Hg2+ was almost quantitative with a solution of 1 M HCl and 5% thiourea.36 This last procedure was adopted for desorbing Hg2+ after each adsorption cycle. The amount of Hg2+ present in the solid after desorption was below 5 mg g−1 (Figure 7). Moreover, XRD analysis of ePMO-v2 (Figure 8) indicated that this material retained its hexagonal structure after each

Figure 6. Particle size distribution curves for ePMO (●), ePMO-v1 (■), and ePMO-v2 (▲).

to the shear forces during these treatments. Moreover, this implied that an interparticle cross-linking could be ruled out. 3.3. Mercury Adsorption. By virtue of the functionalization achieved through vulcanization by incorporation of sulfide bridges in the framework of ePMO, these materials were studied in the adsorption of mercury. Preliminary tests revealed that upon contacting both vulcanized materials, e-PMO-v1 and ePMO-v2, with an aqueous solution 663 ppm in Hg2+, their adsorption capacities determined by XPS analysis were 232 and 237 mg Hg2+ g−1, respectively. Accordingly, these materials were as efficient as other adsorbents reported in the literature under similar experimental conditions.32 For instance, an ordered mesoporous material with dipropyltetrasulfide bridges with 2.11 mmol S g−1 presented an adsorption capacity of 184 mg Hg2+ g−1,33 and a thiol-functionalized mesoporous ethylenebridged PMO with 2.12 mmol S g−1 showed a mercury uptake of 381 mg Hg2+ g−1.34 After adsorption of mercury in both ePMO-v1 and ePMOv2, the intensity of the S 2p peak decreased significantly, particularly in the former material (Figure 3). Also, the S 2p signal shifted to slightly lower binding energy due to the presence of newly formed Hg−S bonds upon complexation.35 As discussed previously, ePMO-v2 exhibited a higher degree of cross-linking and a higher stability than ePMO-v1. For that reason, the reusability of the former vulcanized material was tested. With this aim, ePMO-v2 was subjected to three complete adsorption−desorption cycles (Figure 7). The analysis of the amounts of Hg(II) in this material after each adsorption cycle revealed an uptake of 203, 207, and 205 mg g−1, respectively. Therefore, the adsorption capacity remained

Figure 8. XRD patterns of the adsorbent ePMO-v2 before each adsorption/desorption cycle.

adsorption/desorption cycle. A reduction of the unit cell dimension was observed after the first cycle, but subsequently it remained constant. Similarly, elemental analysis revealed that the C/S atomic ratio increased in the first cycle but became almost constant after successive cycles (Figure 8). In fact, it was 13.4 after the third adsorption cycle.

4. CONCLUSION For the first time, a cold vulcanization treatment, typically used for the modification of certain polymers, has been applied to a periodic mesoporous organosilica (PMO). Specifically, this procedure has been performed on an ethenylene-bridged PMO using sulfur monochloride as cross-linking agent. Thus, a significant amount of sulfur was incorporated into the initial material due to the formation of sulfide units between the former organic bridges. This functionalization mainly occurred on the CC bonds of the surface ethenylene bridges. While preserving its mesostructure, the vulcanized PMOs exhibited superior thermal and hydrothermal stabilities compared with the nonvulcanized material. This effect was more remarkable

Figure 7. Amount of adsorbed Hg2+ in ePMO-v2 material after each adsorption/desorption cycle. 17867

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stabilities of aged periodic mesoporous organosilicas. J. Phys. Chem. B 2003, 107 (46), 12628−12634. (8) Esquivel, D.; Jimenez-Sanchidrian, C.; Romero-Salguero, F. J. Comparison of the thermal and hydrothermal stabilities of ethylene, ethylidene, phenylene, and biphenylene bridged periodic mesoporous organosilicas. Mater. Lett. 2011, 65 (10), 1460−1462. (9) Esquivel, D.; Van Der Voort, P.; Romero-Salguero, F. J. Designing advanced functional periodic mesoporous organosilicas for biomedical applications. AIMS Mater. Sci. 2014, 1, 70−86. (10) Vercaemst, C.; Friedrich, H.; de Jongh, P. E.; Neimark, A. V.; Goderis, B.; Verpoort, F.; Van Der Voort, P. Periodic mesoporous organosilicas consisting of 3D hexagonally ordered interconnected globular pores. J. Phys. Chem. C 2009, 113 (14), 5556−5562. (11) Sasidharan, M.; Fujita, S.; Ohashi, M.; Goto, Y.; Nakashima, K.; Inagaki, S. Novel synthesis of bifunctional catalysts with different microenvironments. Chem. Commun. 2011, 47 (37), 10422−10424. (12) Sasidharan, M.; Bhaumik, A. Novel and mild synthetic strategy for the sulfonic acid functionalization in periodic mesoporous ethenylene−silica. ACS Appl. Mater. Interfaces 2013, 5 (7), 2618− 2625. (13) Esquivel, D.; De Canck, E.; Jimenez-Sanchidrian, C.; Van Der Voort, P.; Romero-Salguero, F. J. Formation and functionalization of surface Diels−Alder adducts on ethenylene-bridged periodic mesoporous organosilica. J. Mater. Chem. 2011, 21 (29), 10990−10998. (14) Nakajima, K.; Tomita, I.; Hara, M.; Hayashi, S.; Domen, K.; Kondo, J. N. A stable and highly active hybrid mesoporous solid acid catalyst. Adv. Mater. 2005, 17 (15), 1839−1842. (15) Nakajima, K.; Tomita, I.; Hara, M.; Hayashi, S.; Domen, K.; Kondo, J. N. Development of highly active SO3H-modified hybrid mesoporous catalyst. Catal. Today 2006, 116 (2), 151−156. (16) Dube, D.; Rat, M.; Beland, F.; Kaliaguine, S. Sulfonic acid functionalized periodic mesostructured organosilica as heterogeneous catalyst. Microporous Mesoporous Mater. 2008, 111 (1−3), 596−603. (17) Xia, Y. D.; Mokaya, R. To stir or not to stir: Formation of hierarchical superstructures of molecularly ordered ethylene-bridged periodic mesoporous organosilicas. J. Mater. Chem. 2006, 16 (4), 395− 400. (18) Nakai, K.; Oumi, Y.; Horie, H.; Sano, T.; Yoshitake, H. Bromine addition and successive amine substitution of mesoporous ethylenesilica: Reaction, characterizations, and arsenate adsorption. Microporous Mesoporous Mater. 2007, 100 (1−3), 328−339. (19) De Canck, E.; Lapeire, L.; De Clercq, J.; Verpoort, F.; Van der Voort, P. New ultrastable mesoporous adsorbent for the removal of mercury ions. Langmuir 2010, 26 (12), 10076−10083. (20) Grabicka, B. E.; Jaroniec, M. Microwave-assisted synthesis of periodic mesoporous organosilicas with ethane and disulfide groups. Microporous Mesoporous Mater. 2009, 119 (1−3), 144−149. (21) Hao, N.; Han, L.; Yang, Y. X.; Wang, H. T.; Webley, P. A.; Zhao, D. Y. A metal-ion-assisted assembly approach to synthesize disulfidebridged periodical mesoporous organosilicas with high sulfide contents and efficient adsorption. Appl. Surf. Sci. 2010, 256 (17), 5334−5342. (22) Kwon, O.; Park, S.; Seo, G. Exceptional performance of sulfonic acid-incorporated-MCM-41 mesoporous materials prepared using a silane containing polysulfide linkages in the acetylation of anisole. Chem. Commun. 2007, 40, 4113−4115. (23) Teng, M. M.; Wang, H. T.; Li, F. T.; Zhang, B. R. Thioetherfunctionalized mesoporous fiber membranes: Sol−gel combined electrospun fabrication and their applications for Hg2+ removal. J. Colloid Interface Sci. 2011, 355 (1), 23−28. (24) Burleigh, M. C.; Jayasundera, S.; Thomas, C. W.; Spector, M. S.; Markowitz, M. A.; Gaber, B. P. A versatile synthetic approach to periodic mesoporous organosilicas. Colloid Polym. Sci. 2004, 282 (7), 728−733. (25) Walcarius, A.; Delacote, C. Mercury(II) binding to thiolfunctionalized mesoporous silicas: Critical effect of pH and sorbent properties on capacity and selectivity. Anal. Chim. Acta 2005, 547 (1), 3−13. (26) ChemNMR Pro 11.0, C. U. CambridgeSoft Corporation.

for the material obtained by vulcanization at higher temperature due to its enhanced degree of cross-linking, even though a partial cleavage of Si−C bonds was observed. The vulcanized materials were used as mercury adsorbents. The material with the highest cross-linking degree was an exceptionally robust adsorbent showing the same mercury uptake in three consecutive adsorption−desorption cycles (ca. 205 mg g−1). This methodology could be extended to other materials provided that they have carbon−carbon double bonds in their structure. In such a way, the vulcanization procedure could both increase their stability and endow them with a new functionality.

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ASSOCIATED CONTENT

S Supporting Information *

N2 adsorption−desorption isotherms and pore size distributions of ePMO, ePMO-v1 and ePMO-v2 materials (Figure S1); 29 Si MAS NMR spectra of ePMO, ePMO-v1, and ePMO-v2 materials (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge funding of this research by the Spanish Ministry of Economy and Competitiveness (Project MAT2013-44463-R), Andalusian Regional Government (Project P10-FQM-6181 and FQM-346 group), and Feder Funds. The technical support and facilities from Córdoba and Málaga Universitieś SCAI are greatly appreciated. M.I.L. thanks the Spanish Ministry of Education, Culture, and Sports for a FPU teaching and research fellowship. D.E. is a postdoctoral researcher of the FWO-Vlaanderen (Fund Scientific ResearchFlanders), grant number 3E10813W.

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REFERENCES

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