Polyamine-functionalized mesoporous silicas: Preparation, structural analysis and oxyanion adsorption

Microporous and Mesoporous Materials 85 (2005) 183–194 www.elsevier.com/locate/micromeso

Polyamine-functionalized mesoporous silicas: Preparation, structural analysis and oxyanion adsorption Hideaki Yoshitake a

a,*

, Emi Koiso a, Haruyuki Horie b, Hiroyuki Yoshimura

b

Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan b Tosoh Corporation, 4560 Kaisei-cho, Shunan 746-8501, Japan Received 13 January 2005; received in revised form 7 April 2005; accepted 13 June 2005 Available online 8 August 2005

Abstract A series of polyamines, NH2(CH2CH2NH)n
1H (where n = 2–6, i.e. EDA-, DETA-, TETA-, TEPA- and PEHA), were attached to 3-chloropropyltrimethoxysilane (CPTMS) grafted MCM-41 (ClPrM41) and SBA-15 (ClPrS15) by the nucleophilic substitution of chlorine. The porous structure was maintained well after this synthesis. CHN elemental analysis implied that a polyamine molecule reacted with two chloropropyl chains on PrM41 while EDA, DETA and TETA (and higher amines) reacted with 1, 1.5 and 2 chloropropyl groups on ClPrS15. IR spectra revealed the presence of primary and secondary amino groups on EDA-, DETA- and on TETA-PrM41 and PrS15. Both bridge and bicrural structures are suggested for the major forms of the organic chains for polyamine-PrM41. On the other hand, the mode of binding shifted with the size of polyamine chain on PrS15. (We proposed linear structures for EDA, terminal bridge, linear and forked structures for DETA and bridged structures for TETA as surface species.) A clear even number effect (by which the amount of adsorption is enhanced in the cases of EDA, TETA and HEPA) was observed in an arsenate adsorption series on polyamine-PrM41, though the amount of adsorption decreased with the number of amino groups in the polyamine chain in polyamine-PrS15. Adsorbents prepared by the same procedure but with nonporous silica showed smaller As/N ratios in arsenate adsorption. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Mesoporous silica; Functionalization; Grafting; Post-modification; Arsenate adsorption

1. Introduction The use of surfactant molecules to form liquid crystals with a certain morphology for the synthesis of mesoporous silica (MPS) materials has constituted a major discovery in materials chemistry in the last decade [1–3]. These materials are characterized by their high surface areas, uniform and controllable pore sizes, and the periodic orders of their pore packing. They have invoked the attention of numerous scientists studying the functionalization of oxide surfaces in order to render them suitable for catalyses [4–7], adsorptions [8–13] and *

Corresponding author. Tel.: +81 453394359; fax: +81 453394378. E-mail address: [email protected] (H. Yoshitake).

1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.06.009

enzyme immobilizations [14,15]. Post-synthesis grafting of an organoalkoxysilane has often been used for these purposes. Although this method is simple and is applicable to most of the MPSs, the variety of silanes that are easily available is limited, in addition to difficulties in controlling the concentration of the organic groups and of their distribution in the mesopores. The number of functional groups on the surface of the pore walls is a critical factor in determining the capacities and the equilibrium constants for the adsorption of aqueous inorganic ions [11–13]. In addition, it has been claimed that the concentration of functional groups both on the surface and in the organic chain effects on the adsorption of oxyanions in a different manner [12]. Most of these problems will also be found in the synthesis and

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use of functionalized MPS prepared by the co-condensation [16,17] of a silane and tetraethyl orthosilicate. Considering the above problems, which can effect the use of organoalkoxysilanes for functionalization, it would be desirable to find a versatile reaction that occurs in a series of molecules with various numbers of amino groups on the surface of an MPS. The reactions of desirable functional molecules with the organic group pre-grafted on silica have been widely used in the preparation of organo-functionalized silicas. Brunel et al. have synthesized large amino groups in the mesopores via the reaction between 3-halopropylsilylated MPS with several amines [18]. We have proposed the fixation of a series of linear polyamines at chloropropyl group pre-grafted on MCM-41 [19]. j–OH þ ðMeOÞ3 SiRCl ! –O–SiðOMeÞ2 RCl ! j–O–SiðOHÞ2 RClmj–O–SiðOHÞ2 RCl þ NH2 ðCH2 CH2 NHÞn
1 H ! ðj–O–SiðOHÞ2 RÞm NHl ðCH2 CH2 NHs Þn where j– = MCM-41 surface, R = –(CH2)3– and l, m, n and s are non-negative integers. Although the removal of oxyanions such as arsenate and chromate has been an intriguing environmental issue, their oxygen-surrounded molecular structure obstructs selective adsorption from aqueous solutions that include a large concentration of anions such as sulfates, chlorides, phosphates etc. Our research group has proposed aminopropyl-functionalized MPSs as a successful adsorbent for arsenate, chromate and selenate. These have been prepared by post-synthesis grafting [12], direct synthesis co-condensation [17] and anionic templating with the help of the interaction between the cation-head in monoaminosilane and anionic surfactants [20]. The amine groups are exposed in samples prepared by the first and the last methods [17,20], while part of the functional groups are buried in the pore walls of solids prepared by co-condensation [17]. A comparison of these methods can offer the basis for an exploration of the effects of the uniformity of the distribution and the surface density of the amino groups in the mesopore. However, the number of amino groups in the grafted chain has only been investigated for mono-(3-aminopropyl, NH2CH2CH2CH2–), di-(1-(2-amino-ethyl)-3-aminopropyl, NH2CH2CH2NHCH2CH2CH2–) and triamino(propyldiethylenetriamine, NH2CH2CH2NHCH2CH2NHCH2CH2CH2–) functional groups because of the availability of silane reagents. One of the problems that remains is how to expand the number of amino groups in a grafted chain. The above two-step preparation is a direct method to prepare large polyamine chains on the surface. Another potential problem in preparing organo-functionalized (mesoporous) silicas is the difference in the dis-

tance between functional groups depending on the silica structure. In adsorption and catalysis, two or more sites can work simultaneously on a dense surface. However, density of functional groups cannot be evaluated by the number of organic groups per unit weight of solid, which is relatively easily obtained by elemental analysis. We have demonstrated that amino groups are inhomogeneously distributed in grafted MCM-41 by the comparison of adsorbing Co2+ and Fe3+ on amino-functionalized MCM-41 prepared by co-condensation and grafting [17]. In this study, the comparison between polyaminefunctionalized MCM-41 and SBA-15 reveals a clear difference in the microscopic structure of polyamine chains and the characteristics of arsenate adsorption.

2. Method 2.1. Chemicals Tetraethyl orthosilicate (TEOS, Tokyo Kasei Kogyo Co., Ltd., reagent grade) was used as a silica source. Tetramethylammonium hydroxide (TMAOH, 25% in water), dodecyltrimethylammonium chloride (DTMACl, >98%) and 3-chloropropyltrimethoxysilane (CPTMS, ClCH2CH2CH2Si(OCH3)3) were purchased from Tokyo Kasei Co. Ltd. A series of linear polyamines of the form NH2(CH2CH2NH)n
1H, i.e. ethylenediamine (EDA, n = 2), diethylenetriamine (DETA, n = 3), triethylenetetramine (TETA, n = 4), tetraethylenepentamine (TEPA, n = 5) and pentaethylenehexamine (PEHA, n = 6) were obtained from Tosoh Co. Reagent grade tetramethylammonium hydroxide (TMAOH, Aldrich) was used as-received. PluronicÒ P123 was offered by BASF Japan. FeCl3 Æ 6H2O was purchased from Aldrich (purity 98%). An aqueous solution of potassium arsenate (KH2AsO4, Pr.G., Wako Pure Chemical Industries Ltd.) was used for the arsenate adsorption experiment. This solution is toxic and should be handled wearing impermeable gloves and goggles in order to avoid contact with skin and eyes. 2.2. Synthesis of MCM-41 TEOS, DTMACl and TMAOH were mixed in water and the solution was stirred for 4 h at room temperature. The composition of the gel mixture was Si:DTMACl: TMAOH:H2O = 1:0.6:0.3:60. The mixture was kept at 373 K for 4 d. The white precipitate was filtered and dried at 393 K, followed by calcination at 903 K for 4 h. 2.3. Synthesis of SBA-15 A 4.0 g of PluronicÒ P123 was dissolved in 30 g of water and 120 g of 2 M HCl to which 8.50 g of TEOS was added. The mixture was stirred for 5 min and then

H. Yoshitake et al. / Microporous and Mesoporous Materials 85 (2005) 183–194

allowed to stand at 308 K for 20 h without stirring, followed by aging at 373 K for 1.5 d. The white precipitate was filtered and dried at 373 K, followed by calcination at 813 K for 7 h. 2.4. Grafting and functionalization of mesoporous silicas The MPSs were dehydrated at 423 K under vacuum to remove any water molecules adsorbed on the surface and then stirred vigorously in toluene containing 50 mmol g
1 of CPTMS. The solution was then heated to 383 K in dry nitrogen for 48 h. The powder was collected by filtration, washed with 2-propanol for 2 h and dried at 373 K overnight. The samples obtained from this process were denoted as ClPrM41 and ClPrS15. Ten grams of polyamine containing 1 g of ClPrM41 or ClPrS15 was stirred at 333 K overnight. The solid was filtered and washed with 0.01 N hydrochloric acid in order to remove the excess amines. The washing process was repeated and the resultant sample was denoted such as xxxA-PrM41 (or xxxA-PrS15), where xxxA is EDA, DETA, TETA, TEPA, or PEHA. The same syntheses were performed using Cab-o-sil M-7D (Cabot Corp. SBET = 200 m2 g
1) to form EDA-, DETA- and TETA-PrSiO2 (M-7D). 2.5. Characterization The periodic structure of the silica framework was confirmed by powder X-ray diffraction (XRD, XL Labo diffractometer, MAC Science Co., Ltd.) using Cu Ka radiation at 40 kV and 20 mA. The nitrogen adsorption–desorption isotherms were recorded using a BELSORP 28SA (BEL Japan Inc.) after the sample was evacuated at 473 K for 2 h. The IR spectra of the functionalized mesoporous silicas 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. Proton-decoupled 29Si MAS NMR spectra were recorded on a JEOL JNM-LA400WB 400 MHz spectrometer at 79.4 MHz with a sample spinning frequency of 5 kHz. 13C CP MAS NMR spectra were measured on the same spectrometer at 100 MHz using a contact time of 5 ms. A 7 mm zirconia rotor was used. 2.6. Ion adsorptions The standard procedure in the adsorption experiments was as follows. Fifty milligrams of modified MPSs were stirred for 10 h in 100 ml of an aqueous solution of FeCl3 or KH2AsO4. Although adsorption was completed within 5 min for most of the concentrations that we investigated, a maximum duration 5 h was required when the concentration of the oxyanion was

185

more than 1700 ppm. Therefore, the specific adsorptions were measured at 5 h. The solution was filtered to remove solids and analyzed by induced coupled plasma (ICP) spectrometry. The typical initial pH of the arsenate solutions was between 6 and 7, and after the adsorptions they had changed to around pH = 4. In this pH range, the dominant arsenate species in aqueous solution is H2 AsO
4. Some of the adsorption experiments were carried out using a commercial adsorbent with a similar structure without mesopores, 3-(ethylenediamino)propyl-functionalized silica gel (Aldrich, EDA-silica gel) in addition to the EDA-, DETA- and TETA-PrSiO2 (M-7D) that we prepared.

3. Results and discussion 3.1. Structure of polyamine-functionalized MCM-41 and SBA-15 Fig. 1 shows X-ray powder diffraction patterns of calcined (M41 or S15), CPTMS-grafted (PrM41 or PrS15) and ethylenediamine reacted (EDA-PrM41 or EDAPrS15) mesoporous silicas. The characteristic (1 0 0), (1 1 0), (2 0 0) and (2 1 0) peaks are clearly observed at 2h = 2.26°, 3.93°, 4.56° and 6.03° in the diffraction of M41. The (2 1 0) peak is not resolved from the background in the case of PrM41 and EDA-PrM41, in addition to the decline of the (1 0 0), (1 1 0) and (2 0 0) peaks. No significant shift in the peak position was observed due to the chemical treatment. On the other hand, the (1 0 0), (1 1 0), and (2 0 0) peaks are observed at 2h = 0.836°, 1.49° and 1.75° in the diffraction of S15. Although peaks for higher indices are not clearly observed in S15, the (1 0 0), (1 1 0), and (2 0 0) peaks are not declined in the case of PrS15 and EDA-PrS15 unlike PrM41 and EDA-PrM41. The peak shift was additionally negligible, i.e. 2h = 0.854°, 1.52°, and 1.77° for (1 0 0), (1 1 0), and (2 0 0) reflections, respectively, for both PrS15 and EDA-PrS15. These constant patterns imply that the framework structure of SBA-15 remains unchanged and is more stable than MCM-41 after grafting of CPTMS and EDA-functionalization. The nitrogen adsorption isotherms are shown in Fig. 2. The characteristic of mesoporous materials with narrow distributions of pore size becomes smeared in line with the synthesis steps of MCM-41. In the plots of SBA-15 materials, the jump around P/P0 = 0.75 decreased slightly in line with the synthesis steps while the position it occurred did not change significantly. The results of the calculation for the mesoporous structure are summarized in Table 1. A BET surface area of 1011 m2 g
1 and a mesopore volume of 1.07 cm3 g
1 were obtained for calcined MCM-41, which decreased to 880 m2 g
1 and 0.94 cm3 g
1, respectively, after

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H. Yoshitake et al. / Microporous and Mesoporous Materials 85 (2005) 183–194

EDA-PrM41

Intensity/a.u.

Intensity/a.u.

EDA-PrS15

ClPrM41

ClPrS15

M41 S15 0

2

4

6

8

10

0.5

1.5

2θ / deg

2.5

2θ / deg

EM-PrM41

ClPrM41

M41

Adsorbed amount [ml g-1]

Nitrogen adsorption [ml g-1]

Fig. 1. X-ray diffraction patterns of EDA-PrM41, ClPrM41, M41, EDA-PrS15, ClPrS15 and S15.

EDA-PrS15

ClPrS15

S15

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 P/P

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 P/P

Fig. 2. (Left) Adsorption–desorption isotherms for EDA-PrM41, ClPrM41 and M41. The offsets for EDA-PrM41 and ClPrM41 are 1000 and 500 ml g
1, respectively. (Right) Adsorption–desorption isotherms for EDA-Pr15, ClPrS15 and S15. The offsets for EDA-Pr15 and ClPrS15 are 1200 and 600 ml g
1, respectively.

grafting of chloropropylsilane. In addition, the pore size decreased from 2.9 to 2.6 nm. This kind of change often accompanies the grafting of silanes on MCM-41 [12,21– 23], and is usually attributed to the formation of an organic layer in the pores. Further decreases in the BET surface area, pore diameter and pore volume were clearly found with ethylenediamine-attached PrM41. A BET surface area of 640 m2 g
1 and a mesopore volume of 0.99 cm3 g
1 were obtained for calcined SBA-15, which decreased to 422 m2 g
1 and 0.68 cm3 g
1, respectively, after grafting of CPTMS, just as observed with MCM-41. The slight decrease of the pore size can be attributed to the formation of an organic layer in the pores. No further decreases in the BET surface area,

pore diameter and pore volume found after the EDAreaction implies different modes of surface interaction from that in MCM-41. 29 Si NMR spectra of MPSs and chloropropyl-functionalized MPSs are depicted in Fig. 3. The two peaks at
101.3 and
109.3 ppm commonly observed in the spectra are attributed to the Q3 (Si(OSi)3(OH)) and Q4 (Si(OSi)4) silicon atoms, respectively. After grafting on MCM-41, a new peak emerged at
49.6 ppm, which was assigned to the T1 (Si(OSi)R(OH)2) silicon. This mode of bonding is rather unique, since T2 (Si(OSi)2R (OH)) and/or T3 (Si(OSi)3R) are the dominant species in many of the 29Si NMR spectra of MPS after the grafting of silanes, as in the case of HMS (2Rp = 4 nm and

H. Yoshitake et al. / Microporous and Mesoporous Materials 85 (2005) 183–194 Table 1 BET surface area, pore diameter, mesopore volume and lattice d spacing of MCM-41, SBA-15 and the derivative mesoporous silicas

M41 ClPrM41 EDA-PrM41 S15 ClPrS15 EDA-PrS15

SBET (m2 g
1)

2Rpa (nm)

Vpa,b (cm3 g
1)

d100 (nm)

1011 880 716 640 422 419

2.9 2.6 2.2 8.2 7.1 7.6

1.07 0.94 0.72 0.99 0.68 0.70

3.9 4.0 3.9 10.6 10.3 10.3

Abbreviations: See text. a By the BJH method. b From adsorption branch.

SBET = 854–1406 m2 g
1) with 3-mercaptopropyltrimethoxysilane (MPTS) [9,10,24], SBA-15 (2Rp = 7.6 nm and SBET = 814 m2 g
1) with MPTS and 3-aminopropyltrimethoxysilane [25] and mesoporous silica (2Rp = 5.5 nm and SBET = 900 m2 g
1) with MPTS [8]. The spectral difference is explained by the small curvature of the mesopores, which can hinder the creation of a bond angle suitable for the formation of a siloxane bond with the organosilane. All of the pore sizes in the above references are more than 1 nm larger than those used in this study. In ClPrS15, the peaks are found at
49.1 and
57.0 ppm with the areas of 3.1% and 0.7%, respectively. These resonances are attributed to T1 and T2 silicons. The relative peak heights of Q4 and Q3 were P not changed significantly by functionalization. P T n/ (T n + Qn) (the ratio of R
Si to the total Si) of ClPrM41 and ClPrS15 was calculated to be 10.8% and 4.2%, respectively. The thick pore wall and the large population of T2 in SBA-15 likely reduce this ratio. The

T2 species is formed by reacting with two and three free silanols, respectively, while the formation of a T1 species needs only one Si–OH. The results of the elemental analysis of polyaminefunctionalized PrM41 are summarized in Table 2. The amount of nitrogen increased almost linearly with the number of amino groups in the polyamine molecule (the correlation coefficient R2 was 0.91 in the least square-fitting of the plot of N wt% vs. the number of N), implying a common mode for the fixation reaction at the chloropropyl chain on the surface of MCM-41. The molar ratio of C/N indicates the average stoichiometry of the reaction between the amine and the chloropropyl groups. When an amine molecule such as NH2(CH2CH2NH)n
1H (n = 2–6) is attached with m chloropropyl chains (m = 1, 2, 3, . . ., <3n), the respective C/N becomes (3m
2)n
1 + 2. The comparison in Table 2 clearly reveals that all the experimental C/N yields are close to the above calculation with m = 2. Several modes of chemical bond formation can be assumed for the polyamine molecules in the model with m = 2 as shown in Scheme 1. Even in the case of EDA-PrM41, two possible structures should be considered; the 1,2bridge ‘Si–(CH2)3N+H2CH2CH2N+H2(CH2)3–Sia and the bicrural (‘Si–(CH2)3)2N+HCH2CH2N+H3. DETAPrM41 can be further complicated into three possibilities, 1,3- and 1,2-bridges, ‘Si–(CH2)3N+H2CH2CH2N+H2CH2CH2N+H2(CH2)3–Sia and ‘Si– (CH2)3N+H2CH2CH2N+H((CH2)3Sia)CH2CH2N+H3, respectively, and 1,1- and 2,2-bicrurals (‘Si–(CH2)3)2N+HCH2CH2N+H2CH2CH2N+H3 and N+H3CH2CH2N+((CH2)3Sia)2CH2CH2N+H3, respectively. Various other species should be considered for the longer amines. The formation of bicrural species in

4

Q

Q

ClPrS15

3

ClPrM41

187

3

T

Q

Q

1

T T2

Q

4

3

2

1

T

2

Intensity

Intensity

Q

4

Q

Q

Q3

3

Q

S15

M41

Q

2

Q

-20

-40

-60

-80

-100

-120

Chemical shift / ppm

Fig. 3.

29

4

-140

-160

-20

-40

-60

-80

2

-100

-120

Chemical shift / ppm

Si MAS NMR spectra of ClPrM41, M41, ClPrS15 and S15.

-140

-160

188

H. Yoshitake et al. / Microporous and Mesoporous Materials 85 (2005) 183–194

Table 2 Results of elemental analysis of EDA-, DETA-, TETA-, TEPA- and PEHA-PrM41 Amine

C (wt%)

H (wt%)

N (wt%)

C/Na

N (mmol g
1)

m = 1b

m = 2b

m = 3b

ClPrM41 EDA-PrM41 DETA-PrM41 TETA-PrM41 TEPA-PrM41 PEHA-PrM41

5.3 6.7 5.1 6.4 7.6 9.4

1.3 2.7 2.2 3.0 4.5 4.6

– 1.8 1.9 2.5 2.8 3.9

– 4.3 3.1 2.9 3.1 2.8

– 1.26 1.38 1.81 1.97 2.75

2.50 2.33 2.25 2.20 2.17

4.00 3.33 3.00 2.80 2.67

5.50 4.33 3.75 3.40 3.16

a b

Molar ratio. C/N calculated for the reaction of one amine molecule having n amino groups with m chloropropyl groups.

EDA-PrM41 N N

N

N

Si

N

Si

1,2-bridge

N

Si

Si

N

N

Si

Si

Si

Si

1,1-bicrural

DETA-PrM41

N

N

N

Si

Si

1,3-bridge

N

N

N N

N

Si

Si

1,2-bridge

Si

N

N N

Si

N

Si

Si

1,1-bicrural 2,2-bicrural

TETA-PrM41 N

N

Si

N

N

Si

1,4-bridge

N

N

N N

Si

Si

1,3-bridge

Si

N

N N

N

N

N

N

Si

1,2-bridge

Si

N

Si

2,3-bridge

N

Si

Si

1,1-bicrural

N N

N

N N

N

N

Si

Si

2,2-bicrural

Scheme 1. Possible structures of EDA-, DETA- and TETA-PrM41.

EDA-PrM41 suggests the distance of chloropropyl groups is as large as ‘Si–(CH2)3N+(CH2)3–Sia. Table 3 shows the parallel data for polyamine-functionalized PrS15. Surprisingly, the amount of N does not considerably depend on the kind of polyamine (1.53 ± 0.28 mmol g
1). One explanation for the constant nitrogen amount is that each amino group occupies a certain exclusive volume or area. The comparison in Table 2 clearly reveals that the experimental C/N yield for EDA is close to the above calculation with m = 1 while that for TETA, TEPA, and PEHA are close to m = 2. The mode of attachment of DETA is just the average of m = 1 and m = 2. Thus, a linear form, ‘Si–(CH2)3N+H2CH2CH2N+H3 is reasonably assumed for the structure of the organic chain in EDAPrS15 while reactions occur at two amino groups of TETA, TEPA, and PEHA molecules. The absence of the 2:1 reaction (i.e. m = 2) in the EDA attachment

and the dominance of the reaction with m = 2 in the large (n P 4) amine attachments likely resulted from the distance of the chloropropyl groups on the surface of SBA-15. The separation of two chloropropyl groups is too large to make the chemical bonding such as ‘Si–(CH2)3N+H2CH2CH2N+H2(CH2)3–Sia in EDAPrS15 but will be enough small for a bridge such as ‘Si–(CH2)3N+H2CH2CH2N+HCH2CH2N+HCH2CH2N+H2(CH2)3–Sia in TETA-PrS15. This in turn suggests that the chain structure formed in the reaction with m = 2 is neither bicrural nor vicinal-bridge but can be terminal bridges (1,4-, 1,5-, and 1,6-bridges for TETA-, TEPA-, and PEHA-PrS15, respectively) and bridges-eloigne´s (such as 2,5-, 2,6- and 3,6-bridges). The possible structures are summarized in Scheme 2. The average surface density of the chloropropylgroup, which is calculated from the data of the elemental analysis and the nitrogen adsorption, is 2.1 nm
2 and 1.0 nm
2 for PrS15 and PrM41, respectively. The larger distance between organic groups in spite of the higher density in PrS15 is explained by the uniformity of grafting; i.e. these apparently contradictory results will be obtained by more homogeneous distribution of chloropropyl groups on ClPrS15 than on ClPrM41. Probably, the large pore size of SBA-15 allows the diffusion of CPTMS before reacting with silanols groups on the pore wall, while CPTMS is grafted near the pore mouths in MCM-41. We measured the infrared spectra to confirm the formation of these surface species. The region of the bending modes is shown in Fig. 4. The adsorption at Table 3 The result of elemental analysis of EDA-, DETA-, TETA-, TEPA- and PEHA-PrS15 Amine

C (wt%)

H (wt%)

N (wt%)

N (mmol g
1)

C/Na

ClPrS15 EDA-PrS15 DETA-PrS15 TETA-PrS15 TEPA-PrS15 PEHA-PrS15

5.2 5.5 4.4 4.4 5.5 5.4

1.2 1.7 1.5 2.2 1.5 2.0

– 2.4 1.8 1.8 2.3 2.3

– 1.68 1.28 1.25 1.67 1.67

– 2.8 2.8 3.0 2.7 2.7

a

Molar ratio.

H. Yoshitake et al. / Microporous and Mesoporous Materials 85 (2005) 183–194

N

N

N

N

N

N

N

Si

Si

Si

Si

DETA-PrS15 N N

N

N

Si

N

N N

N

Si

Si

N

Si

TETA-PrS15

N

N

N

N

N

Si

Si

N

N

N

Si

Si

Scheme 2. Model of the surface structure of EDA-, DETA- and TETA-PrS15.

1456 cm
1, which increased according to the number of anime groups in the polyamine molecule, is assigned to

MCM-41

0.3

1456

c Absorbance

1506

1596

1648 c Absorbance

SBA-15

0.2

1456

N

CH2 scissoring. In the spectra of polyamine-PrM41, the peaks at 1596 and 1506 cm
1 are attributed to asymmetric and symmetric bending of the primary amines ð–NHþ respectively, while that emerging at 3 Þ, 1648 cm
1 is assigned to the bending of secondary amines ð–NðRÞHþ 2 Þ. The band for the secondary amines was larger than that for the asymmetric bending of the primary amine in EDA-PrM41, while the latter absorption was larger than the former in the cases of DETA- and TETA-PrM41. In the spectra for polyamine-PrS15, the relative intensity of adsorption at 1456 cm
1, attributed to CH2 scissoring, increased with the number of anime groups in a polyamine molecule just as MCM-41. The observation of a peak at 1602 cm
1 (asymmetric bending of the primary amines,
1 –NHþ (bending of 3 Þ and a very weak one at 1656 cm þ secondary amines, –NðRÞH2 ) are consistent with the assignment of the chain structures by the result of elemental analysis. The rate of nucleophilic substitution of chlorine in RCl by an amine is dependent on the affinity of the lone pair on the nitrogen atom, and the trend is such that r(tertiary) > r(secondary) > r(primary). In the reaction of linear polyamines and a surface chloropropyl group, the reactivity of the terminal nitrogen is lower than the secondary nitrogen, suggesting that the first substitution occurs on the secondary amino groups in DETA and TETA to generate a forked form. The second substitution depends on the surface population of chloropropyl groups and stereochemical effects as well as on the electron densities of the nitrogen atoms. Assuming that the absorption coefficients are comparable between EDA-, DETA- and TETA-PrM41, then the population of

1656 1602

EDA-PrS15

189

b

b

a a

1880

1720

1560

1400

Wavenumber [cm-1]

1240

1800

1660

1520

1380

1240

Wavenumber [cm-1]

Fig. 4. (Left) Infrared absorption spectra of (a) EDA-PrM41, (b) DETA-PrM41 and (c) TETA-PrM41. (Right) Those of (a) EDA-PrS15, (b) DETAPr15 and (c) TETA-PrS15.

190

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secondary amines in EDA-PrM41 (1,2-bridge) is larger than that in DETA- and TETA-PrM41. The simplest explanation of these populations is the preferential formation of vicinal bridges (1,2-bridge in EDA and DETA, 1,2- and 2,3-bridges in TETA) because that in EDA produces no primary amino group but those in DETA and TETA produce primary amino groups equal to or more than the secondary amino groups. Although the formation of a vicinal bridge can explain the IR spectra, other kinds of reactions can occur in the polyamine reactions. The discovery of the bands from the primary amine in EDA-PrM41 demonstrates the formation of a bicrural species. This implies that this mode of reaction can also be applicable in the formation of DETA- and TETA-PrM41. The terminal bridges (1,2-, 1,3- and 1,4-bridges in EDA, DETA and TETA, respectively) produce no primary amino groups. Considering the clear bands for the primary amino groups, the contribution of this structure is not large in polyaminePrM41 and PrS15. In order to compare the length of the alkyl chains in the chloroalkyl-functionalized MCM-41, we grafted chloromethyltrimethoxysilane onto MCM-41 and then chemically treated it with EDA. The elemental analysis of the resultant material, EDA-MeM41, yields the population of amino groups and the C/N ratio shown in Table 4. Although the amino group population (1.21 mmol g
1) is very similar to that of EDA-PrM41 (1.26 mmol g
1), C/N = 1.6 implies a 1:1 reaction of EDA and the chloromethyl chain. Therefore, the structure of the organic chain in EDA-MeM41 is assigned to a linear diamine (‘Si–CH2N+H2CH2CH2N+H3). This simple reaction is clearly caused by the alkyl chain being too short to bind at more than one site on the EDA under a limited surface density of alkyl chains. Fig. 5 shows the 13C NMR spectra of ClPrM41 and EDA-PrM41. The peaks observed at 7.6, 26.2 coupled with 23.9, 46.1 and 49.4 ppm in the spectrum of ClPrM41 are assigned to a-, b-, c- and methoxy-carbons, respectively [20,26–29], using the notation ‘Si(OCH3)2CaH2CbH2CcH2Cl. The splitting of peaks that is often observed for b-carbon has been reported in 3-substituted propyl-functionalized silica [30], which probably results from the presence of conformational isomers [29]. Resonances at 8.3, 22.2, 39.8 and 50 ppm are observed in the spectrum of EDA-PrM41. The first three peaks are attributed to a-, b- and e-carbons and the last to unre-

α

EDA-PrM41 γ δ

β ε

*

80

*

60

40

20

0

Chemical Shift /ppm

ClPrM41

β

CH3Oγ

α

*

80

*

60

40

20

0

Chemical Shift /ppm Fig. 5. 13C CPMAS NMR spectra of EDA-PrM41 and ClPrM41. The chemical shifts are in ppm with respect to the TMS standard. The peaks at 17.5 and 57.9 ppm are due to physisorbed ethanol (marked with *). The peak around 65 ppm is due to carbon in the sample holder. See text for the assignment.

solved c- and d-carbons of the bicrural species (‘Si(OH)2CaH2CbH2CcH2)2N+HCdH2CeH2N+H3). Peak at 8.3, 22.2 and 50 ppm, respectively is also expected for the resonances of the a-, b- and (c- + d-) carbons in the bridge form (‘Si(OH)2CaH2CbH2CcH2N+H2CdH2CdH2N+H2CcH2CbH2CaH2Si(OH)2a). These observations and peak assignments are consistent with the results of elemental analysis and the infrared spectrum. The 13C NMR spectrum of ClPrS15 showed basically

Table 4 Results of elemental analysis of EDA-MeM41 C (wt%)

H (wt%)

N (wt%)

N (mmol g
1)

C/Na

m = 1b

m = 2b

m = 3b

m = 4b

2.29

1.26

1.70

1.21

1.6

1.5

2.0

2.5

3.0

a b

Molar ratio. C/N calculated for the reaction of an ethylenediamine molecule bound with m propyl-chains.

H. Yoshitake et al. / Microporous and Mesoporous Materials 85 (2005) 183–194

the same feature though the peaks were not clearly distinguished. 3.2. Ion adsorptions on polyamine-functionalized MCM-41 Fig. 6 shows an example of adsorption isotherms for arsenate. Saturation readily occurred at 1000 mg L
1, with 87 mg (g EDA-PrS15)
1 of adsorption. Most of the ion adsorption experiments showed such a readily adsorption. The adsorption capacities for ferric ions and arsenate on the polyamine-PrM41 are summarized in Table 5. The amount of ferric ion adsorbed increased monotonously with the number of amino groups in the polyamine chain. However, Fe/N is not constant and the maximum (0.29) was found in the case of DETAPrM41. On the other hand, arsenate adsorption is enhanced when the number of amino groups in a poly100 90

amine is even. This even number effect can be found more clearly in the series presented for the As/N ratio, which varies between 0.18 and 0.27, as shown in Fig. 7. The even number effect can appear when the specific coordination structure (possible at n = even) is stable. We confirmed that the adsorption capacity and As/N were independent of the chloride ion concentration and that As/N was reproduced within the error of ±0.02. Therefore, it was concluded that arsenate coordination is rather homogeneous and that the As/N ratio will reflect the stable coordination structure. The stability of chelate coordination by two –N+HxCH2CH2N+Hx– groups reasonably explains the even number effect and the As/N ratios of ca. 0.25. The maximum adsorption of arsenate on the polyamine-PrS15 is summarized in Table 6. The adsorption per unit weight adsorbent decreased with the increase of the number of amino groups in a polyamine chain. Because the amount of amino groups on polyaminePrS15 is nearly constant at 1.53 ± 0.28 mmol g
1 (Table 3), the decreasing behaviour is due to the decline of the activity of amino groups for binding arsenate anions. Considering the mode of reaction of a long polyamine, i.e. terminal-bridge and bridge-eloigne´, the attachment of the polyamine to the surface invokes a loss of the

70 60

0.4

50

0.35

40

0.3

30

0.25

polyamine-PrS15

As / N

As adsorption [mg/g-adsorbent]

80

191

20

0.2

polyamine-PrM41

0.15

10

0.1

0 0

500

1000

1500

2000

2500

3000

0.05

Equilibrium concentration of As[mg/L] 0 1

Fig. 6. Adsorption isotherm of arsenate on EDA-PrS15. Fifty milligrams of modified mesoporous silicas were stirred for 5 h in 100 ml of aqueous solution of KH2AsO4.

2

3

4

5

6

7

Number of N in Polyamine

Fig. 7. As/N at the saturation of arsenate adsorption on polyaminePrS15 (j) and polyamine-PrM41 (s). Table 5 Capacities of polyamine-functionalized PrM41 for Fe3+ and arsenate adsorptions Adsorbent

EDA-PrM41 DETA-PrM41 TETA-PrM41 TEPA-PrM41 PEHA-PrM41

Fe adsorption (mmol g
1)

Arsenate adsorption (mmol g
1)

0.28 0.40 0.42 0.44 0.54

0.30 0.26 0.49 0.36 0.52

Table 6 Arsenate adsorption capacities of polyamine-PrS15 Adsorbent

Adsorption (mmol g
1)

EDA-PrS15 DETA-PrS15 TETA-PrS15 TEPA-PrS15 PEHA-PrS15

0.62 0.42 0.28 0.32 0.26

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H. Yoshitake et al. / Microporous and Mesoporous Materials 85 (2005) 183–194

degree of freedom for conformational changes. The relatively rigid chain can inhibit the coordination of amino groups to an arsenate anion. The As/N is compared with that of polyaminePrM41 in Fig. 7. The As/N average stoichiometry decreases monotonously with the size of polyamine on PrS15 unlike the clear even number effect in polyamine-PrM41. The mode of attachment of a long polyamine on PrS15, i.e. terminal-bridge and bridge-eloigne´, results in loss of the degree of freedom for conformational change. This structural factor can influence the coordination strength of amino groups to an arsenate anion and increase the number of inactive amino groups. The amino-functionalized mesoporous silicas have shown orders of magnitude higher adsorption capacities for aqueous oxyanions than the other conventional adsorbents proposed in the literature [12,30,31]. However, the effect of mesopores has not yet been elucidated. We prepared several polyamine–functionalized aerosol silicas (Cab-o-sil M-7D, surface area = 200 m2 g
1) by the same procedure as described in the experimental section. Non-porous silica is a suitable substrate in a comparison of adsorption capacities to evaluate the effect of the mesoporous structure of MCM-41. In addition to this comparison, we also examined a commercial adsorbent (EDA-silica gel). The results are summarized in Table 7. The population of organic chains of functionalized Cab-o-sil is about 1/5 of that of polyamine-PrM41, while the density of nitrogen atoms is 40–50%. These values yield a different C/N ratio from that in polyamine-PrM41. C/N for polyamine-PrSiO2 (M-7D) is almost equal to n
1 + 2, where n is the number of amino groups in the polyamine molecule, implying a stoichiometry in the substitution reaction of (polyamine): (chloropropyl) = 1:1. The molecular structure in EDA-PrSiO2 (M-7D) is therefore in a linear form, ‘Si–CH2CH2CH2N+H2CH2CH2N+H3. Although the stoichiometry for DETA and TETA is the same, two possibilities should be considered for the chain structure, i.e. terminal amine-bound (linear) and inner amine-bound (forked). It is likely that this mode of reaction arises from the low surface density of surface Si–OH groups available in the grafting, leading to a large separation between the chloropropyl groups. The

amount of arsenate that can be adsorbed before saturation is lower for the adsorbent from the Cab-o-sil substrate than for that from MCM-41. This is partly because of the lower density of functional groups in the former. The value of As/N for EDA- and TETAPrSiO2 (M-7D) is 0.11, which is less than half for EDA- and TETA-PrM41. In contrast, As/N for DETA-functionalized silicas is almost the same. This comparison of As/N suggests that the even number effect in arsenate adsorption on polyamine-PrM41 is due to the structure of the organic chain. When the number of amino groups in a polyamine molecule is odd, the structure of polyamine-(propyl)2 is disadvantageous to the arsenate adsorption capacity, probably because it contains unreactive amino groups. Although the commercial functionalized-silica gel adsorbed a comparable amount of arsenate to EDA-PrSiO2 (M-7D), this is the result of a higher surface population of amino groups and a lower As/N ratio than in EDA-PrSiO2 (M-7D). 1-(2-Amino-ethyl)-3-aminopropyltrimethoxysilanegrafted MCM-41 (EDA-MCM-41) [12,13,30,31] is a linear EDA-Pr chain-functionalized form of MCM41. A significant difference is the fact that the EDA group is bound to two propyl chains in EDA-PrM41, while it is linearly connected to one propyl chain in EDA-MCM-41. Nevertheless, the calculated value of As/N are almost the same for the two solids. This result implies that the coordination structure of arsenate is not dependent on the mode of attachment of the EDA groups. Corriu et al. have recently reported the synthesis of cyclam-functionalized MSU using a reaction involving the substitution of the chlorine in a pre-grafted chloropropyl-group by a nitrogen atom on cyclam [32]. This procedure is easier than synthesizing cyclam-triethoxysilane and co-condensing with TEOS [33]. All of the chelating sites are exposed in the former method, while a part of them can be included in the wall or condensed separately from the mesoporous silica in the latter method. Several multi-step methods have been reported for synthesizing large organic groups on the MPSs [34– 37], as have been known as ship-in-bottle syntheses. The functionalization of MPSs using a reaction of the reactive group, such as a terminal chloroalkyl chain

Table 7 Results of elemental analysis and adsorption capacities of polyamine-substituted chloropropyl-functionalized non-mesoporous silicaa and commercial adsorbentb

EDA-PrSiO2 (M-7D) DETA-PrSiO2 (M-7D) TETA-PrSiO2 (M-7D) EDA-silica gelb a b

C (wt%)

H (wt%)

N (wt%)

N (mmol g
1)

C/N

As ads. (mmol g
1)

As/N

1.76 1.29 1.95 8.64

0.49 0.34 0.50 2.39

0.89 0.68 1.07 3.57

0.63 0.49 0.76 2.55

2.3 2.2 2.1 2.8

0.07 0.10 0.09 0.11

0.11 0.20 0.11 0.02

Cab-o-sil M-7D (SBET = 200 m2 g
1). 3-(Ethylenediamino)propyl-functionalized silica gel (Aldrich).

H. Yoshitake et al. / Microporous and Mesoporous Materials 85 (2005) 183–194

pre-grafted on the pore walls, will be a promising method for the preparation of custom-designed solids both at the meso- and microscopic levels.

4. Conclusion We prepared EDA-, DETA-, TETA-, TEPA- and PEHA-functionalized mesoporous silica by the reaction of pre-grafted CPTMS with polyamine. MCM-41 (2Rp = 2.9 nm) and SBA-15 (2Rp = 8.2 nm) were employed to compare the pore sizes. CPTMS was attached with one Si–OH on MCM-41 and this mode of attachment enhanced the population of organic chains. However, a polyamine molecule reacted with two chloropropyl chains on the pore wall. IR spectroscopy revealed that the functional groups included both primary and secondary amino groups. Vicinal-bridge and bicrural structures were proposed as the major forms of the surface organic chains on polyamine-functionalized MCM-41. When chloromethyltrimethoxysilane (CMTMS) was used instead of CPTMS, the EDA bound with one methyl group, yielding a linear diamine chain. The structure of the polyamine-functionalized SBA-15 (as analyzed by CHN elemental analysis and IR spectroscopy) was linear for EDA, bridged for TETA and larger polyamines and a mixture of linear, forked and bridged for DETA. The difference between chloropropyl silane pre-grafted on MCM-41 and SBA15 was discussed on the basis of a comparison of the structures of two series of polyamine on MCM-41 and SBA-15, together with silanes, CMTMS and CPTMS, on MCM-41. ClPrS15 had a higher density (2.1 chains/nm2) but showed a larger distance between the organic chains than ClPrM41 (1.0 chains/nm2), implying a higher inhomogeneity in the distribution of the chloropropyl chain on ClPrM41 than on ClPrS15. A clear even number effect, where the amount of adsorption is enhanced when the number of amines in the polyamine is even, was observed in a series of arsenate adsorptions on polyamine-PrM41, while Fe3+ adsorption increased with the number of amino groups in a polyamine molecule. In contrast, the amount of arsenate adsorption decreased with increasing size of the polyamine molecule in polyamine-PrS15. EDA-, DETA- and TETA-PrSiO2 (M-7D) showed considerably smaller As/N values for arsenate adsorption, implying that these mesoporous silica structure is essential for realizing a high adsorption capacity.

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