Bromine addition and successive amine substitution of mesoporous ethylenesilica: Reaction, characterizations and arsenate adsorption

Microporous and Mesoporous Materials 100 (2007) 328–339 www.elsevier.com/locate/micromeso

Bromine addition and successive amine substitution of mesoporous ethylenesilica: Reaction, characterizations and arsenate adsorption Kojiro Nakai a, Yasunori Oumi b, Haruyuki Horie c, Tsuneji Sano b, Hideaki Yoshitake a

a,*

Division of Materials Science and Chemical Engineering, Graduate School of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan b Graduate School of Engineering, Hiroshima University, 1-3-2 Kagamiyama, Higashi-Hiroshima 739-8511, Japan c Tosoh Corporation, 4560 Kaisei-cho, Shunan 746-8501, Japan Received 20 September 2006; received in revised form 15 November 2006; accepted 15 November 2006

Abstract Bromination and subsequent ethylenediamine substitution of the C@C double bond in mesoporous ethylenesilica were carried out to explore the characteristics of this periodic mesoporous organosilica. The structures of the products (Br@PMO and EDA–Br@PMO, respectively) were analysed by IR, Br K-edge EXAFS and NMR spectroscopies, as well as X-ray diffraction and nitrogen adsorption. We showed (1) that the formulae of the two products that formed were [CHBrSiO1.5]0.45[CHSiO1.5]0.55 and [NH2CH2CH2NHCHSiO1.5]0.05 [CHBrSiO1.5]0.40[CHSiO1.5]0.55, respectively, (2) that the addition of Br2 at room temperature occurred on the C@C double bonds with disturbing the framework structure, (3) that IR absorption band of C@C bonds that reacted with Br2 is significantly different from that of inactive C@C bond, (4) that the length of the C–Br bond was considerably longer than in conventional alkyl bromides, and (5) that a large proportion of the m(C–Br) band remained at the same position in the IR absorption spectrum after the ethylenediamine (EDA) substitution, while a new m(C–Br) absorption also appeared. The mechanisms of these reactions are discussed at both the micro and mesoscopic levels. Arsenate adsorption on EDA–Br@PMO, in which the EDA is directly bound to the ‘‘surface’’ of the mesopores, was compared with adsorption on EDA–Pr–PMO, which was prepared by the direct synthesis of 3-chloropropyl-functionalized mesoporous ethanesilica followed by the substitution of Cl with EDA. The strength of the adsorption, as measured with the distribution coefficient, was greater for the former adsorbent than the latter. The origin of this difference was attributed to the distance between amino group and the surface.  2006 Elsevier Inc. All rights reserved. Keywords: Periodic mesoporous organosilica; Ethylenesilica; Bromination; Amine substitution; Arsenate adsorption

1. Introduction The functionalization of periodic mesoporous silicas by organic groups has been an important subject in developing new applications of mesoporous materials [1]; they have attracted the interest of chemists because they contain well-developed mesopores with a uniform pore structure. Post-synthetic grafting techniques are widely considered to be simple, easy and versatile and have been widely studied. However, it has been claimed that slow diffusion dur*

Corresponding author. Tel.: +81 45 339 4359; fax: +81 45 339 4378. E-mail address: [email protected] (H. Yoshitake).

1387-1811/$ - see front matter  2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.11.023

ing the grafting process can lead to non-uniform dispersion of the organic groups in the pores, and that pore-blockage is induced in some cases [1,2]. This difficulty can be avoided by the direct co-condensation of tetraalkoxysilane and organosilane with a structure-directing agent [1b,1e,1g,1h,3]. Nevertheless, the disadvantage of introducing partial inactivation of the functional groups has been pointed out as being an intrinsic problem for this co-condensation method [3–5]. Since this effect is believed to occur due to a strong interaction between the functional group and the silicon oxide/hydroxide in the synthetic gel, protection of the functional group by utilizing a stronger interaction with a surfactant molecule has been proposed [1h,6,7],

K. Nakai et al. / Microporous and Mesoporous Materials 100 (2007) 328–339

though the available combinations of functional groups and templates are quite limited. Periodic mesoporous organosilica (PMO) is another category of mesostructured material [8–10,14] that contains organic groups within a meso-framework, which act as bridges between silicon atoms. Such materials have been prepared by methods akin to mesoporous silicas, with a high degree of order and uniformity of the pores, by using a silsesquioxane (R 0 O)3SiRSi(OR 0 )3 as a precursor. Because of the use of a sole precursor, we ensure a uniform dispersion of the organic groups in the framework structure, a high loading of organic groups, and the avoidance of the pore narrowing/blockage that is often observed in the grafting of mesoporous silicas. Examples of silsesquioxanes that have been successfully incorporated into a framework include bis(triethoxysilyl)ethane [9–13], bis(triethoxysilyl)ethylene [14–16], bis(triethoxysilyl)acetylene [17], bis(triethoxysilyl)benzenes [15,18–22], 2,5-bis(triethoxysilyl)thiophene [17,23] and 1,4-bis(triethoxysilyl)ferrocene [17]. The variations in the organic groups mentioned above are not still large enough to match the applications in mesoporous, further functionalization has been carried out. Direct synthesis with 3-mercaptopropyltrimethyoxysilane for benzene-silica and ethane-silica has been employed for the preparation of sulfonic acid-functionalized PMOs [24]. A systematic study of the direct co-condensation of ethane-silica with various RSi(OR 0 )3 type silanes has also been carried out [25]. These direct co-condensation syntheses have clearly demonstrated that the ordered periodic structure is disrupted by the incorporation of silanes as well as the influence found in mesoporous silica. When using a vinylsilane in the direct synthesis of functionalized ethylene-silica, the disruptive effect seems larger than in the synthesis of the equivalent functionalized mesoporous silica [26]. The functionalization that is unique to PMO involves using the conversion of the organic group in PMO. It has been demonstrated that the methylene group in PMO can be converted to the Si–NH2–Si structure by ammonolysis [27]. In addition, bromination has been carried out in studies of the preparation of mesoporous ethylenesilica [14–16]. These studies show the possibility of designing particular sites for adsorption, catalysis, etc., by making use of various organic reactions. Since the framework of PMO exhibits both organic and inorganic characteristics, the relationship between the structure of the functional groups and the way in which they work should be carefully investigated. Since the structure of PMOs contains a functional group that is definitely larger than oxygen atom, they may show the structural characteristic between organic polymers and silica. If we compare data in the literature [22,28], a large difference can easily be observed in the conversion of the C@C double bond during the bromination of mesoporous ethylenesilicas, in spite of the similar pore sizes and the surface areas of these ethylenesilicas. This difference can be due to the effective exposure of C@C bond to bro-

329

mine, suggesting the need for study from the viewpoint of mesostructural and microstructural changes in the framework. The amino group-functionalization of mesoporous silica has aroused interest in applications of this solid in terms of coordination chemistry, catalysis and oxyanion adsorptions. However, this functional group interacts strongly with the surface, leading to structural disturbances such as lack of uniformity in grafting [27] and disruption of periodic order in direct co-condensation [4]. When PMO is employed as the mesoporous framework, it is possible to partially avoid these problems, since a considerable proportion of the silanol groups/siloxane bonds on the surface are substituted by a hydrophobic group. In this paper, we functionalize C@C double bonds in ethylenesilica with amino groups via the addition of bromine and subsequent substitution by ethylenediamine (EDA). In this functionalization process, the extent of reactions is particularly interesting, because of the intermediate characteristics of PMOs between organic polymer and inorganic solid. For this reason, we analyse the products by using a combination of elemental analysis, 13C NMR, EXAFS and IR spectroscopies, etc. This functionalization is compared in structural analyses and arsenate adsorption with EDA-modified ethanesilica that was prepared by the direct co-condensation of 3-chloropropylsilane and bis(triethoxysilyl)ethane and consecutive substitution of Cl by EDA and with an amino-functionalized mesoporous silica. 2. Experimental section 2.1. Materials Bis(triethoxysilyl)ethylene (BTESEY, (C2H5O)3SiCH@ CHSi(OC2H5)3), bis(triethoxysilyl)ethane (BTESE, (C2H5O)3SiCH2–CH2Si(OC2H5)3) and 3-chloropropyltrimethoxysilane (CPTMS, ClCH2CH2CH2Si(OCH3)3) were purchased from Gelest, Inc. Brij 56 (C16H33(OCH2CH2)nOH, n  10, Aldrich), Brij 76 (C18H37(OCH2CH2)nOH, n  10, Aldrich), ethylenediamine (EDA, Tokyo Kasei Kogyo Co., Ltd., reagent grade), bromine (Br2, reagent grade, Aldrich) were also commercially available. 2.2. Synthesis of mesoporous ethylenesilica Brij 76 was dispersed in 0.8 M hydrochloric acid by stirring and the mixture was then heated at 323 K for 12 h. BTESEY was then added to the solution before stirring for 12 h at the same temperature. The mixture was then transferred into a Teflon bottle and heated at 363 K for 24 h. The precipitate was collected by suction filtration and dried at room temperature. The molar ratio of the reactants was 0.11Brij 76:0.56BTESEY:222H2O:3.20HCl. The surfactant was extracted by the treatment with acidified ethanol (0.1 N HCl) twice at room temperature. This extracted solid is hereafter referred to as @PMO.

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2.3. Bromination of mesoporous ethylenesilica A surfactant-extracted mesoporous ethylenesilica, @PMO, was weighed in a sample tube after dehydration at 373 K for 12 h. This sample tube was installed in a sealed Teflon bottle with another sample tube containing bromine. The gaseous bromine diffused in the Teflon bottle reacts with the @PMO. After standing at room temperature for 12 h, the solid was heated at 373 K for 12 h to remove any physisorbed bromine molecules. The brominated mesoporous ethylenesilica was then weighed to calculate the Br content in the solid. The manipulation was carried out under Ar to avoid contact with the atmosphere. The brominated sample is represented as Br@PMO. A gaseous reaction is highly recommended when undertaking bromination in order to avoid contamination of the solvent, although the reaction is considerably rapid; under the present reaction conditions, 95% of the final conversion value was achieved within 2 min. It was difficult to control the amount of Br that reacted with the @PMO. 2.4. Substitution of Br with EDA in brominated mesoporous ethylenesilica The brominated mesoporous ethylenesilica was then heated in vacuum at 373 K for 12 h. The resulting solid was reacted with the equivalent molar EDA in toluene at 333 K for 12 h, followed by collection by suction filtration, washing with 0.1 N hydrochloric acid and ethanol and drying at 353 K. The solid formed after the substitution with EDA is designated as EDA–Br@PMO. 2.5. Direct synthesis of 3-chloropropyl-functionalized mesoporous ethanesilica

2000, Rigaku Corp.) using Cu Ka radiation at 40 kV and 20 mA. The nitrogen adsorption–desorption isotherms were recorded using a Nova 4200 (Quantachrome Inc.) after the samples were evacuated at 473 K for 3 h. The IR spectra were measured using a Perkin–Elmer System 2000 spectrometer in transmission mode. The powder was moulded into self-supporting disks, which were directly mounted in the sample holder of the spectrometer. 13CCP-MAS NMR spectra were recorded using a zirconia rotor with 7 mm diameter on a Varian VXP-400 at 100.706 MHz using a contact time of 5 ms. Tetramethylsilane was used as a chemical shift reference. The sample spinning frequency was varied from 4 to 5 kHz to verify the spinning side bands. Proton-decoupled 29Si-MAS NMR spectra were recorded on the same spectrometer at 79.4 MHz with a sample spinning frequency of 5 kHz. XAFS spectra were measured using Beamline 10B at the Photon Factory, High Energy Accelerator Research Organization, Tsukuba, Japan (Proposal #2005G196), with a ring energy of 2.5 GeV and a stored current of 300– 400 mA. A Si(3 1 1) channel-cut monochromator was used. Conventional transmission mode was employed, with detection by gas ion chambers. Br K-edge EXAFS spectra were acquired five times at room temperature, and the average v(k) was calculated from the extracted spectra. A standard automated data reduction procedure was used to extract the experimental EXAFS spectra using a REX 2000 (Rigaku Co.) program assembly. In order to determine the structural parameters, non-linear least square-fitting analysis was carried out for the inverse Fourier transform k3v(k) of the shells of the Fourier transform. The amplitudes and phase-shift functions were calculated using the FEFF 7.02 code. 2.8. Arsenate adsorptions

The above method for ethylenesilica was also followed for the preparation of 3-chloropropyl-functionalized mesoporous ethanesilica, except that we used mixture of BTESE, CPTMS and Brij 56 instead of BTESEY and Brij 76. The molar ratio of the reactants was 0.14Brij 56:0.22CPTMS:0.44BTESE:267H2O:1.92HCl. The solid product that was extracted is hereafter referred to as ClPr–PMO. If CPTMS was absent from the starting gel, mesoporous ethanesilica, which is denoted as –PMO, was obtained. 2.6. Reaction of chloropropyl-mesoporous ethanesilica with EDA In this reaction, we applied the same conditions and method as used for the EDA reaction with Br@PMO. The product is referred to as EDA–Pr–PMO.

The standard protocol in the adsorption experiments was as follows. The modified PMOs (50 mg) were placed in a container with an aqueous solution of KH2AsO4 (100 ml) and then the liquid was stirred. Although adsorption was completed within 5 min for most of the concentrations that we investigated, a maximum duration of 5 h was sometimes required when the concentration of the oxyanion was greater than 1700 ppm. The solution was filtered to remove solids and analysed by induced coupled plasma (ICP-mass) spectrometry. The initial pH of the arsenate solutions was between 6 and 7, while after the adsorptions this had changed to around pH 4. In this pH range, the dominant arsenate species in aqueous solution is H2 AsO 4. 3. Results and discussion

2.7. Characterizations

3.1. Chemical formulae and mesostructures of @PMO, Br@PMO and EDA–Br@PMO

The periodic structure of the organosilica frameworks was confirmed by powder X-ray diffraction (XRD, RINT

The chemical formula of @PMO is written as [CHSiO1.5] when the silanol group is ignored. The carbon weight

a b c 1

1.5

2

2.5

3

3.5

2theta/degree 800 700

a

600

V /ml(STP) g-1

percentage in [CHSiO1.5] (18.449%) agreed well with the experimental value of elemental analysis (18.1%) for @PMO, suggesting a practically complete removal of surfactant. The amount of bromine reacted with @PMO could be determined more precisely from the difference of the weights prior to and posterior to the bromination than by the elemental analysis using gas and ion chromatographies. The apparent measured formula of Br@PMO was [CHBrSiO1.5]0.45[CHSiO1.5]0.55; bromine was added to 45% of the C@C bonds in the solid @PMO. The elemental analysis by EDX demonstrated that the Br/Si atomic ratio was 0.49, when the hydrolysis product of 3-bromopropyltrimethoxysilane was employed as the standard material (see Supplementary data). The results for the amount of Br agree quite well and this value is almost concordant with the recent report on @PMO synthesized with Brij 76 (51%) [16]. The composition of EDA–Br@PMO is calculated with a C/N molar value (=10.5) as measured by elemental analysis: [NH3(Cl)CH2CH2NH2(Cl)CHSiO1.5]0.05[CHBrSiO1.5]0.40[CHSiO1.5]0.55. The composition of N (1.3 wt%) in the data of elemental analysis agrees with the calculated weight fraction of N (1.4 wt%) in the above formula. This formula implies that only 11% of Br–C bonds was converted. The surfactant was detected in @PMO by 13C NMR (vide infra). However, the influence on the above calculation is likely insignificant, because the elements directly measured were Br and N while Brij 76 is composed of C, H and O. The error could mainly appear in the weight of the samples, which is almost due to the framework of ethylenesilica (and Br in Br@PMO). The X-ray diffraction patterns and nitrogen adsorption isotherms of @PMO, Br@PMO and EDA–Br@PMO are shown in Fig. 1, and the mesostructural parameters calculated from these data are summarized in Table 1. The diffractions assigned to the (1 0 0), (1 1 0) and (2 0 0) indexes in @PMO, which are attributable to a 2D hexagonal structure [15], are almost unchanged after bromine addition. The nitrogen adsorption profiles of @PMO and Br@PMO are remarkably similar, except for the magnitude of the adsorption. The BET specific surface area had also decreased from 1080 to 749 after bromination, as shown in Table 1. Considering the changes in chemical formula and weight, from [CHSiO1.5] = 65 to [CHBrSiO1.5]0.45[CHSiO1.5]0.55 = 101, the specific surface area should be reduced to 64.3% by bromination, when structural changes such as swelling are negligible in the framework. The observed reduction in the BET surface area measurement was 69.4%, the pore sizes were almost unchanged (3.6 and 3.5 nm for @PMO and Br@PMO, respectively) and the change in value of d was also small. These agreements before and after the reaction imply that the density of the pore walls increased with no significant change in the mesoporous structure. In contrast, the diffraction peaks were diffused, the adsorption of nitrogen decreased considerably and the pore size narrowed after the reaction with EDA, implying an extensive modification of the mesostructure,

331

Intensity/a.u.

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500 400

b

300 200

c

100 0

0

0.5

1

P /P 0 Fig. 1. X-ray diffraction patterns and nitrogen adsorption isotherms for (a) @PMO, (b) Br@PMO and (c) EDA–Br@PMO.

even though the conversion due to substitution is much smaller than that due to Br2 addition. The post-modification of 3-halopropyl-functionalized MCM-41 and SBA-15 with EDA has been reported [29] with some degradation of the mesostructural order. The decrease in periodicity is larger in MCM-41 than in SBA15. Although we applied the same reaction procedure as described in Ref. [29], the peak change in XRD in this study is clearly larger than in the previous study on MCM-41 and SBA-15. The calculated wall thicknesses of @PMO and Br@PMO are 3.5 and 3.4 nm, respectively, as shown in Table 1. Considering the dimensions of a [O3SiCH@CHSiO3] unit, (approximately 0.3–0.6 nm), 10–20% of the total double bonds are exposed to the surface. Since the way that this unit is stacked in @PMO is unknown (but will most likely be random) this estimation is certainly rough. Nevertheless, the conversion by bromination is clearly larger

K. Nakai et al. / Microporous and Mesoporous Materials 100 (2007) 328–339

a

2RPa (nm)

VP (mm3 g1)

d100 (nm)

1080 749 646

3.6 3.5 1.8

1109 809 355

6.2 6.0 4.9

BJH pore size analysis.

than the surface exposure of double bonds. This disagreement suggests that bromine molecules could reach C@C bonds that are inaccessible for nitrogen molecules at 77 K. The X-ray diffractions for 2h = 10–85 are expanded in Fig. 2. These are typical patterns of amorphous solids. The peak around 19.8 was not changed significantly by the extraction of the surfactant. This unchanging pattern implies that the diffraction is mainly due to the atoms in the mesopore walls. In contrast, the peak shifted to 26.8 after bromination, while the diffraction around 19.8 was suppressed to almost disappear. This shift is attributed to an extensive reconstruction of the framework structure on a microscopic scale, and demonstrates that the addition of Br did not occur only to C@C double bonds on the top surface of PMO but also to those which are not accessible without a large distortion in the framework. The above results reveal that at room temperature, bromine molecules likely ‘‘thrust’’ through the ‘‘opening’’ in a network of „Si–O–Si„, „Si–C@C–Si„ and „Si–CBr– CBr–Si„ bonds, which are not permeable to nitrogen at 77 K. Bulk reactions between gases and inorganic solids have rarely been observed at room temperature. In the case of @PMO, the atoms are bound one-dimensionally at „Si–CH@CH–Si„ and „Si–CHBr–CHBr–Si„, and these parts can be ‘‘soft’’ enough to enlarge the distance from neighbouring chains by a conformational and vibrational changes caused by the thermal motion of small molecules that are sorbed at room temperature. However, the mesostructural framework of @PMO remains nearly com-

brominated

template-extracted

as-synthesized

0

20

40

60 80 2θ /degree

100

3.2. Spectroscopic characterization of chemical bonds in @PMO, Br@PMO and EDA–Br@PMO The infrared spectra of @PMO, Br@PMO and EDA– Br@PMO are plotted in Fig. 3. The infrared band that appears at 568 cm1 for Br@PMO is assigned to the Br– C stretching vibration. After reaction with EDA, the intensity of this peak declines, and a new peak appears at 538 cm1. In the mechanism of Br2 addition to a C@C double bond, Br+ bridges the C–C bond and then Br is attached from the other side of the Br+ (anti-addition) [30]. In the case of Br@PMO, a dibromo-form, „SiCHBr–CHBrSi„, is the only form that is produced according to this mechanism. Furthermore, the trans-conformation is allowed in this dibromo-chain, because the conformation is not variable due to the fixed siloxane bonds. These limitations on the reaction may define the surface species of bromine. A relatively large width of the band at 568 cm1 in spectrum (b) suggests that several chemical environments exist around the C–Br bonds and disturb the vibrational spectrum. On the other hand, no such pair site is necessary for the EDA substitution and, therefore, one Br in „SiCHBr–CHBrSi„ can rest after the substitution by EDA. The 538 cm1 band can therefore be assigned to the C–Br stretching vibration in the „SiCH(Br)–CH(EDA)Si„ species. Fig. 4 shows the 13C-MAS NMR of @PMO, Br@PMO and EDA–Br@PMO. The large single peak at 145.5 ppm

a

538 cm-1

@PMO Br@PMO EDA–Br@PMO

SBET (m2 g1)

plete, even after an extensive bulk reaction. This persistence of the mesostructure may be due to the „Si–O–Si„ structure where the atoms are three-dimensionally connected, as in most inorganic oxides. On the other hand, the nucleophilic substitution of Br by EDA is attributed to a ‘‘surface’’ reaction, since only 11% of the Br is substituted during the formation of EDA–Br@PMO, as discussed above. Although it is physically difficult to distinguish the surface and the bulk in an amorphous solid, these addition and substitution reactions, in a sense, exhibit the possibility of being labelled by chemical reactions.

568 cm-1

Table 1 Results of nitrogen adsorption of BTSEY derived PMOs

Absorbance/arb. unit

332

b c 0.1

120

Fig. 2. X-ray diffraction patterns for mesoporous ethylenesilica assynthesized, template extracted and brominated.

700

650

600

550 500 Wavenumber/cm-1

450

Fig. 3. IR spectra of (a) @PMO, (b) Br@PMO and (c) EDA–Br@PMO.

K. Nakai et al. / Microporous and Mesoporous Materials 100 (2007) 328–339

found in the spectrum of @PMO is attributed to the carbon in the C@C bond. After bromination, a broad peak centred at 32.8 ppm appeared, which is assigned to C–Br. A simultaneous decline in the peak at 145.5 ppm was also observed. Two peaks were found at 37.9 and 44.7 ppm in addition to that at 145.5 ppm in the spectrum of EDA– Br@PMO. The former peak is attributed to a C–Br bond remaining in the solid, and the latter is assigned to one (or both) of the carbons in EDA. The band at 32.8 ppm in Br@PMO has a large bandwidth, which is consistent with a broad C–Br band in the IR spectrum. It is clear that this band does not decrease uniformly after the substitution by EDA; the band for the C–Br bond appears at 37.9 ppm with a small band width. This decreasing profile after the EDA substitution suggests that the reactivity of C–Br bonds is closely related with the position of the resonance, though the details of the structures of these C–Br bonds have not been made clear by NMR.

surfactant

*

* a

*

*

b

*

*

c

250

200

150 100 50 Chemical shift /ppm

0

-50

Fig. 4. 13C-CP-MAS NMR of (a) @PMO, (b) Br@PMO and (c) EDA–Br@PMO. The spinning side-bands are marked with an asterisk (H).

1639

Absorbance

1575

a

1632

b

1900

1800

1700

1600

1500

1400

1300

-1

ν/cm

Absorbance

1664

1900

c

1575

1800

1700

333

1600

1500

1400

1300

-1

ν/cm

Fig. 5. IR spectra of (a) @PMO and (b) Br@PMO. The difference between (a) and (b) is also shown as (c) (=(a)  (b)).

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in the peak positions imply that the addition of Br2 does not occur uniformly, but rather that there are several C@C double bonds with different chemical natures and that bromine reacts selectively with the species exhibiting the absorption at ca. 1664 cm1 (the peak top of the differential spectrum) and 1575 cm1. It is interesting that a weakly-bound C@C bond (1575 cm1) was reactive, though the decrease in the main adsorption was due to the loss of a C@C bond that was more tightly-bound than

k 3χ (k )

We further investigated the IR absorption arising from the C@C double bond, in order to elucidate the difference in reactivity of the C@C double bonds in @PMO. Fig. 5 shows the IR spectra of @PMO and Br@PMO, along with their differential spectrum. The broad band due to the C@C double bond that appeared at ca. 1639 cm1 before the Br2 addition shifts to around 1632 cm1 after the reaction. A shoulder peak that had been observed at ca. 1575 cm1 decreased until it disappeared. These changes

0

2

4

6

8

10

12

14

|FT|

k / A-1

0

1

2

4

3

5

6

k 3χ (k )

r /A

3

5

9

7

11

13

k / A-1 Fig. 6. Br K-edge EXAFS spectrum of Br@PMO. (a) EXAFS oscillation k3v(k), (b) Fourier transform of the EXAFS and (c) curve-fitting of the inversely˚. 1A ˚ = 0.1 nm. Fourier transformed spectrum at r = 0.9–3.2 A

K. Nakai et al. / Microporous and Mesoporous Materials 100 (2007) 328–339

the average (ca. 1639 cm1). The reactivity of the latter species can be understood by considering the general tendency of electrophilic additions; a high electron density is preferable for the reaction rate, implying that tightly-bound double bonds react rapidly with bromine. The origin of the reactivity of the 1575 cm1-species is not clear; it is probably in tension, such as being positioned at the pore surface. The above results suggest that selectivity of Br2 addition in @PMO is closely related to the microstructure of the C@C double bond. We have not obtained any evidence showing that the double bonds far from the surface are inactive. Although an explanation by such a spatial factor may be attractive, the rapid termination of bromination, as described in the experimental section, prevented us from verifying this factor further. Fig. 6 shows the k3v(k) EXAFS oscillation, the radial distribution function and the result of curve-fitting analysis of the Br K-edge EXAFS of Br@PMO. The EXAFS oscil˚ was Fourier transformed. lation function in k = 3–13 A ˚ The first peak around 1.5 A is clearly assigned to the Br–C bond. However, the second largest peak around ˚ can be due to an unresolved sum of the contributions 2.8 A of the next nearest neighbours of C and Si scatterers. The curve-fitting results are summarized in Table 2. The coordination number for these three atoms is nearly unity, and is consistent with the structure of a brominated C@C double bond where each carbon is bound to a silicon atom. The appearance of the first shell as a strong peak reveals that a relatively uniform Br–C bond is formed during the bromination. According to the above discussion, the addition of bromine extends into the framework, where the substitution with amine does not occur. The appearance of the Br–C bond suggests that there is sufficient space around the C@C double bonds in @PMO for the Br2 addition reaction without resulting in considerable stereochemical inhibition in the product. In addition, the lack of a significant peak due to a Br–Br bond in the Fourier transform suggests that the large uptake of bromine in the bromination of PMO does not include the absorption of molecular bromine. (A tiny structure between the first and the second peaks in the Fourier transform was not able to be fitted with a Br–Br bond.) ˚ ) is significantly shorter The bond length of Br–C (1.83 A ˚ ). This length than the length in bromoalkanes (ca. 1.9 A depends on the bond order of the C–C chain; C–Br bound Table 2 Results of non-linear least-square fitting for the Br K-edge EXAFS spectrum of Br@PMO ˚) ˚) Scatterer r (A N DE0 (eV) r (A C C Si

1.829 ± 0.032 3.077 ± 0.046 3.095 ± 0.068

0.82 ± 0.28 1.00 ± 0.49 0.82 ± 0.64

15.4 ± 6.67 5.19 ± 6.1 3.73 ± 6.1

0.109 ± 0.36 0.089 ± 0.064 0.180 ± 0.069

r: bond distance, N: coordination number, DE0: shift of the energy ˚ = 0.1 nm. R fitting factor threshold, and r: Debye–Waller factor. 1 A was 4.5% and 5.3% for the first and the second shells, respectively. ˚ = 0.1 nm. 1A

335

to an alkynyl carbon is shorter than the bond to an alkenyl carbon, and that to the alkenyl group is shorter than to an alkyl carbon. The Br–C distance found in Br@PMO is ˚ [33]), suggesting close to that in bromobenzene (1.8674 A that the characteristics of Br in Br@PMO are similar to that of Br bound to an aryl C. Such discussions of the electronic state of the C@C bond in @PMO are supported by the similarity in position and intensity of the edge spectra, whose detailed analysis is now underway. We measured the 29Si NMR spectra and confirmed the absence of resonances due to Qn species, Si(OSi)n(OH)4n; no significant R–Si bond cleavage was found during the hydrothermal synthesis. The absence of a signal from a T1 species, R–SiOSi(OH)2, was also confirmed, though the intensity of T2 (d = 73.5 ppm; R–Si(OSi)2OH) is larger than that of T3 (d = 83.0 ppm; R–Si(OSi)3), suggesting that the condensation level is as low as the other mesoporous organosilicas from silsesquioxanes [10, 13a,16,21,31,32] synthesized under acidic conditions. The bromine addition reaction and the substitution by EDA are summarized in Fig. 7, indicating the different kinds of chemical bonds and their conversion. 3.3. Structural analysis of –PMO, ClPr–PMO and EDA–Pr–PMO We prepared an EDA-propyl group-functionalized mesoporous ethanesilica, EDA–Pr–PMO, for comparison with EDA–Br@PMO. Table 3 shows the results of elemental analysis and the parameters for the mesoporous structure. The lack of Cl–CH2– carbon in the NMR spectrum of EDA–Pr–PMO (vide infra) suggests that all of the chlorines are substituted by EDA in the reaction of EDA and ClPr–PMO. In the formula of EDA–Pr–PMO, [C5H15N2Cl2SiO1.5]x[H2CSiO1.5]1x, x is calculated to be 0.12 using the C/N molar ratio (=5.27). The validity of this calculation is confirmed by the percentage of carbon calculated in [C5H15N2Cl2SiO1.5]0.12[CH2SiO1.5]0.88, 20.8%, which agrees well with the experimental value in Table 3 of 20.5%. In reactions with a primary amine and a haloalkane, multiple substitutions may occur to form disubstituted secondary or tertiary amines. For an EDA– haloalkane reaction, up to 6 haloalkanes may theoretically be substituted by one EDA. However, if we assume that the reaction occurs with EDA:Cl = 1:2, x would become 0.2 and C/% = 24.7. This calculation does not agree with the experiment. The calculation with EDA:Cl = 1:3 (or less than 1:4) provides results that are less concordant with the experiment than that from EDA:Cl = 1:2. It can be concluded from these calculations that the formula of EDA–Pr–PMO is [NH3(Cl)CH2CH2NH2(Cl)CH2CH2CH2SiO1.5]0.12[CH2SiO1.5]0.88, and that the structure of the functional groups is EDA–propyl-(1-(2-aminoethyl)3-aminopropyl–). Furthermore, the weight percentage of carbon in ClPr–PMO (C/% = 20.0) agrees well with the formula [ClCH2CH2CH2SiO1.5]0.12[CH2SiO1.5]0.88 (C/ % = 20.2). The accordance of the ratio of the branch chain

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N Si consuming ν (C=C) = 1664 cm-1 1575 cm -1

Si

Br Si

Si

N consuming ν (C-Br) = 568 cm-1

Br

ν (C-Br) = 568 cm -1 r(C-Br) = 0.183 nm

ν(C=C) = 1639 cm-1 1575 cm -1

with

Si

Br

ν (C-Br) = 538 cm -1 568 cm -1 with

Si

Br Si ν (C=C) = 1632 cm

Si

Si

Si

Si

Br

Si

-1

ν (C-Br) = 568 cm -1

ν(C=C) = 1632 cm-1

conversion 11%

conversion 45%

Fig. 7. Transformation of chemical bonds and mesostructure in mesoporous ethylenesilica by Br2 addition and EDA substitution. The first reaction influences on the structure of pore wall, while the second reaction likely occurs near the pore surface.

Table 3 Results of elemental analysis and nitrogen adsorption of BTSE derived PMOs

–PMO ClPr–PMO EDA–Pr–PMO a

C (wt%)

H (wt%)

N (wt%)

SBET (m2 g1)

2RPa (nm)

VP (mm3 g1)

d (nm)

17.7 20.0 20.5

2.2 2.5 3.1

– – 3.89

1175 810 580

3.5 – –

1153 447 444

6.3 4.8 4.5

BJH pore size analysis.

in PMO between ClPr–PMO and EDA–Pr–PMO implies no decomposition of the propyl group during the EDA substitution. ClPr–PMO showed a significantly broad peak in XRD, and the jump around P/P0 = 0.5 in the nitrogen adsorption isotherm (which was observed in –PMO) was almost lost. The post-modification step by EDA substitution did not significantly modify the XRD patterns, though the specific surface area decreased considerably, as shown in Table 3. Since the specific mesopore volume of EDA–Pr–PMO (444 mm3 g1) is still larger than that in EDA–Br@PMO (355 mm3 g1), a porous structure with large pores still remains in EDA–Pr–PMO. The 13C NMR spectra in Fig. 8 shows resonance peaks attributed to the carbons in each organic structure. The peak at 4.4 ppm in the spectrum of –PMO is assigned to the carbon in ethanesilica. Those at 16.0 and 57.7 ppm are caused by the ethanol used for washing the solid. In the spectrum of ClPr–PMO, three peaks newly appeared at 9.9, 26.7 and 46.9 ppm in the spectrum for –PMO.

According to the literature [29,4], these peaks can be assigned to the a-, b- and c-carbons on the 3-chloropropyl group (Cl–CcH2–CbH2–CaH2–Si„). The peak at 46.9 ppm is asymmetric; this may include the signal from an unhydrolysed methoxy group in the silane. In addition to the signal at 4.4 ppm, peaks were found at 9.9(sh), 22.9, 40.2, 47.9(sh) and 51.0 ppm in the spectrum of EDA–Pr–PMO. These are attributed to the a-, b-, e-, d- and c-carbons in e d b a þ c the NHþ 3 C H2 C H2 NH2 C H2 –C H2 –C H2 –SiB structure. The peak at 46.9 ppm in the spectrum for ClPr–PMO disappeared, suggesting the consumption of chlorine by the substitution reaction. Together with the results of elemental analysis, 13C NMR of EDA–Pr–PMO reveals that the surface branch group has a uniform EDA–propyl structure. It has been pointed out that, in functionalized mesoporous silica prepared by direct synthesis, some of the functional groups can become inaccessible to aqueous ions from the exterior, or are ‘‘buried’’ in the pore walls. Whether or not they become inaccessible depends on the

K. Nakai et al. / Microporous and Mesoporous Materials 100 (2007) 328–339

337

1000000 100000

* γ

*

β

a α

*

10000

Kd

*

b

δ γ

1000 100

β

ε

α

c

EDA-Br=PMO

10

EDA-Pr-PMO

50

30

10

-10

1

Chemical shift/ppm

0

NH3+ ε

δ

Cl NH2

γ

β

α Si

0.001

As adsorption/mol

0.0015

g-1

Fig. 9. Distribution coefficient, Kd, in the adsorption of arsenate on EDA–Br@PMO and EDA–Pr–PMO. Adsorption conditions: 50 mg of adsorbent stirred for 5 h in 100 ml of an aqueous solution of KH2AsO4.

+

γ

β

0.0005

α Si

Fig. 8. 13C-CP-MAS NMR of (a) –PMO, (b) ClPr–PMO and (c) EDA– Pr–PMO. (*) indicates the peak due to ethanol.

interactions of the functional group and the active groups in hydrolysed silica, such as silanol [1h]. In the cocondensation of BTESE and CPTMS, most of the Cl is substituted in the reaction with EDA, implying that the 3-chloropropyl group is not occluded into the pore walls. In the synthesis using Brij 56, therefore, the interaction between chlorine atoms and silica during hydrolysis is not as large as that between chlorine and the surfactant–acid solution moiety. Nevertheless, in the 29Si NMR spectra, the intensity of the T2 peaks is larger than that of T3 in the spectra of – PMO and ClPr–PMO, suggesting a relatively low condensation level. 3.4. The adsorption of arsenate We carried out experiments involving the adsorption of arsenate on the above-mentioned EDA-functionalized mesoporous organosilicas, which have different lengths of alkyl chains attached to EDA group and hence different distances from the ‘‘surface’’. The distribution coefficient between the adsorbent and the solvent (=the amount of arsenate adsorbed on the surface divided by that in solution), indicating the strength of the adsorption, is plotted in Fig. 9. This parameter exceeded 2 · 105, which was our detection limit for As in solution by ICP-mass spectrometry, in the lowest adsorption region ð< initial½HAsO2 4  ¼ 5–6 ppmÞ: It dropped nearly vertically when the adsorption approached the saturation level. In between, a plateau appeared at ca. 1 · 102 on EDA–Pr–PMO, while Kd still

Table 4 Adsorption capacity of arsenate Adsorbent

EDA– Br@PMO

EDA–Pr– PMO

EDA–Pr– MCM-41a

Amino group [mmol g1] Capacity [mmol g1] As/N

0.87

2.8

2.8

0.33

1.2

0.61

0.38

0.42

0.22

a

[1-(2-Aminoethyl)-3-aminopropyl]trimethoxysilane–grafted MCM-41 [27].

decreases slowly in the region between 102 and 103 on EDA–Br@PMO. The capacity is larger on EDA–Pr– PMO than on EDA–Br@PMO, while the adsorption is stronger on EDA–Br@PMO than on EDA–Pr–PMO. Table 4 summarizes the adsorption capacity of these adsorbents. Although the capacity is smaller in EDA–Br@PMO than in EDA–Pr–PMO, the adsorption per amount of amino group is almost comparable (0.38 vs 0.42, respectively). A plateau also appeared around 1 · 102 in the Kd–As adsorption plot for EDA–Pr–MCM-41, prepared by the grafting of NH2CH2CH2NHCH2CH2CH2Si(OCH3)3 on MCM-41, and the As/N ratio was found to be 0.22 [27]. The characteristics in Kd is the similar to that found for EDA–Pr–PMO, while the capacity is smaller than in both of EDA-functionalized PMOs. On the other hand, the main body of the Kd curve is significantly larger on EDA–Br@PMO. This different behaviour can be explained as follows. The microstructure of the organic group on EDA–Pr–PMO is the same as the tether group of EDA– Pr–MCM-41 [27]. A significant structural difference between EDA–Pr–PMO and EDA–Pr–MCM41 is found in the surfaces of the pore walls; the former surfaces are composed of alkyl groups and siloxane bonds, while the latter have an amorphous silica surface. This implies that the propyl group is long enough to interact with the

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oxyanion independently of the chemical nature of the pore wall surface, even with a hydration sphere. On the other hand, one of the nitrogens is bound directly to the pore wall carbon on the surface of EDA–Br@PMO, and therefore the adsorption is much more sensitive to the chemical nature of the pore wall surface. It is likely that some of the amines could not interact with the hydrated arsenate, which decreases the capacity, and that the presence of the surface organic group could enhance the strength of binding of the adsorbate. 4. Conclusion Ethylenediamine(EDA)-functionalized mesoporous ethylenesilica (@PMO) was prepared by bromine addition and subsequent substitution by EDA, and arsenate adsorption was carried out on the structures that were formed. The adsorption results were compared with those on mesoporous ethanesilica prepared by the direct co-condensation of bis(triethoxysilyl)ethane (BTESE) and 3-chloropropyltrimethoxysilane (CPTMS). Bromine was added to 45% of the C@C double bonds in @PMO. In the diffraction data, the peaks from the mesostructural periodicity were almost unchanged, while the broad pattern due to the correlation of atomic position was considerably shifted, providing evidence for the disturbance of framework structure. On the other hand, the substitution by EDA occurred on 11% of the Br–C bonds, suggesting that this reaction would be limited on the surface. In the IR absorption spectra, the Br–C stretching vibration at 568 cm1 in Br@PMO was still observed in EDA–Br@PMO, where a new peak at 538 cm1 was also found. The latter peak was attributed to the Br–C bond in „Si-CH(Br)-CH(EDA)-Si„. We demonstrated that these IR observations were consistent with the conversion of Br2 addition and EDA substitution according to mechanisms in a homogeneous solution. In addition, they revealed that the C@C groups that reacted with Br2 was different from unreacted C@C bonds. The bond distance of Br–C (0.183 nm, revealed by EXAFS spectroscopy) was considerably shorter than that of most alkyl bromides. The As/N ratio at adsorption saturation and the distribution coefficient on EDA–Pr–PMO were close to those of EDA–Pr–MCM-41 (NH2CH2CH2NHCH2CH2CH2Si(OCH3)3–grafted MCM-41). On the other hand, the latter parameter was considerably larger in the adsorption on EDA–Br@PMO. The difference was explained by the distance of the EDA group from surfaces of the pore walls; the EDA group is directly bound to the ‘‘surface’’ in EDA–Br@PMO, while –CH2CH2CH2– binds EDA with the surfaces in EDA–Pr–PMO and EDA–Pr–MCM-41. Acknowledgement The authors thank Instrumental Analysis Centre, Yokohama National University for the elemental analyses.

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