ISSN 19950780, Nanotechnologies in Russia, 2010, Vol. 5, Nos. 11–12, pp. 808–816. © Pleiades Publishing, Ltd., 2010. Original Russian Text © E.V. Borodina, F. Roessner, S.I. Karpov, V.F. Selemenev, 2010, published in Rossiiskie nanotekhnologii, 2010, Vol. 5, Nos. 11–12.

ARTICLES

Synthesis and Characterization of Inorganic–Organic Composite Materials with AnionExchange Groups Based on Mesoporous Silicates E. V. Borodinaa, F. Roessnerb, S. I. Karpova, V. F. Selemeneva a

b

Voronezh State University, Universitetskaya pl. 1, Voronezh 394006 Industrial Chemistry 2, Carl von Ossietzky University, D26111, Oldenburg, Germany email: [email protected], frank.roessner@unioldenburg.de Received May 17, 2010

Abstract—Mesoporous materials functionalized with quaternary ammonium groups have been prepared by the surface modification of the MCM41 mesoporous material with NtrimethoxysilylpropylN,N,Ntri methylammonium chloride. The specific surface area, pore volume, and pore diameter were determined from nitrogen adsorption and desorption isotherms at 77 K. It was revealed that modification in toluene and water leads to a higher pore diameter. Based on this fact, it can be supposed that the nonhomogeneous filling of the inorganic matrix by the modification agent or partial changes in the hexagonal structure of the abovementioned samples takes place. Modification in methanol presumably proceeds without structural changes in the silicate backbone of the mesoporous material. IR spectra were used to control the modification. The structural changes during sample heating to 150–250°C have been shown by DRIFT. The TGA/DTA method revealed the ther mostability of composite materials. The loss of mass of the modified materials due to the destruction of the organic layer starts from 180°C, which corresponds to IR data. The degree of grafting is 13–51%. Keywords: functionalized mesoporous materials, MCM41, surface modification, quaternary ammonium DOI: 10.1134/S1995078010110091

INTRODUCTION Organic–inorganic hybrid materials attract parti cular attention because they combine the mechanical and thermal stability of the inorganic matrix with the high selectivity and reactivity of organic anion exchange resins. Organic–inorganic hybrid materials can be obtained by modifying an existing inorganic matrix with organosilanes [1–3] or, in one stage, using the sol–gel method [1, 4, 5]. Such materials are widely used in various fields of science and technology, such as chromatography [6], sorption processes [7], sensors [2, 3], and electrochemistry [1–4, 8, 9]. Particular interest in inorganic matrices is in regards to mesopo rous materials of MCM41 type, because they have an ordered structure. The MCM41 material consists of parallel tubes having a hexagonal mesopore structure with a size of about 40 Å [10]. In addition, they have a high surface area, making it possible to modify with organic functional groups [3, 8]. Classic synthetic ion exchangers are widely used in ionexchange chromatography [11–13]. However, their usage is limited to HPLC, which is associated with significant swelling [14] and a consequence of their limited use at high pressure [15]. Surfaceporous ion exchangers used in chromatography are character ized by low values of exchange capacity and thermal instability. Natural and modified inorganic materials

based on aluminosilicates are not widely used because of low chemical resistance at high temperatures and low exchange capacity and heterogeneity [15, 16]. Moreover, their use is limited to medium acidity; at a pH greater than 8, most of the materials are hydrolyt ically destructed. Currently, composite materials com bining the properties of inorganic matrices (silica gel or MCM41mesoporous material) and ionexchange properties [1, 4, 17], which overcome the disadvantages of synthetic polymeric ion exchangers, are becoming more and more popular. Obtaining organic–inorganic composite materials is the subject of several current studies [1–4]. The chemical modification of an inor ganic matrix with silanes with primary amino groups is described in the literature [1–4, 18–22]. The func tionalization of silica gels with quaternary amine groups is examined in [1, 4, 9]. However, amorphous silica gels are characterized by a disordered structure with a broad pore size distribution that does not make it possible to obtain materials with a strictly defined pore diameter. In the last decade there have been investigations on the synthesis of ion exchangers on the basis of ordered mesoporous materials by direct synthe sis [1–4, 8, 9] or by chemical modification [2, 3]. In this case, the preservation of the ordered hexagonal struc ture of the modified samples is one of the problems of modification. Combining the advantages of inorganic

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matrices with the selectivity and polarity of functional groups of a modifier affords the opportunity to obtain thermally and chemically stable organosilicate meso porous materials with ordered structures and narrow poresize distributions. Therefore, it is promising to synthesize materials which are applicable as selective sorbents and capable of operating at high tempera tures. Thermal stability of this material should be comparable or higher than that of polymeric ion exchange resins and also should have significant sorp tion capacity, selectivity, and hydrophilicity. The aim of this work is to synthesize composite materials with anion exchange properties by the chemical surface modification of mesoporous materi als with the preservation of the structure characteristic for them and with the further characterization of the properties of the samples. EXPERIMENTAL MCM41 (Sued Chemie Company, Germany) was used as an inorganic matrix, the basis for the synthesis of composite material. We used a 50% solution of chloride NtrimethoxysilylpropylN,N,Ntrimetil ammonium (TMT) in methanol (ABCR Company, Germany) as a modifier. The amount of the substance modifier in the reaction mixture per 1 g of inorganic matrices ranged from 0.0018 to 0.0036 mol [1, 9, 18]. The temperature ranged from 25°C to 60°C and the reaction time was 4–20 h [1, 17]. Next, the modified MCM41 was washed with 200 cm3 of the solvent and the samples were dried at 60°C for 24 h. To compare the influence that the reaction medium had on the properties of the materials, the reaction was carried out in water [17, 18], toluene [1, 4], and methanol media. Modified material below will be denoted as MWT modification in the water, MTT in toluene, and MMT in methanol, respectively. The determination of the surface area, volume, pore diameter, and pore size distribution was carried out on sorption–desorption isotherms of nitrogen at 77 K and in a relative pressure range of 10–5–0.99 on an Autosorb1 device (Quantochrom, United States). Before each measurement, the samples were degassed at 120°C for 8 h under vacuum. The method of BJH [23] was used to determine the poresize distribution. IR spectra of samples were recorded using the infrared spectrometer Bruker Equinox 55 with the Fourier transform diffuse reflectance technique (DRIFT). Each spectrum was taken in the wavenumber range 400–4000 cm–1 with a resolution of 4 cm–1. Samples were prepared by mixing the test sample with KBr (Merck, Germany) at a ratio of 1 : 4. Preliminary experiments showed that there is no ion exchange between the modified sample containing chloride ions and KBr at the applied conditions. To remove the adsorbed water, as well as to establish the structural changes of the sample, the Harricks cell device was used for in situ heating. We passed nitrogen through NANOTECHNOLOGIES IN RUSSIA

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the sample with a rate of 60 ml/min, making it possible to remove the products of desorption and decomposi tion upon heating from the addon. IR spectra were measured after keeping samples for 90 min at 25, 150 and 250°C. The thermal stability of modified MCM41 sam ples was determined on a TGA/SDTA 851e thermo analyzer (MettlerToledo GmbH, Germany). Ther mogravimetric measurements were made in an atmo sphere of nitrogen and oxygen. The maximum heating rate was 5°C per minute in the temperature range 25– 700°C. RESULTS AND DISCUSSION Grafted sorbents may differ as to the type of modi fier and its concentration. Surface coverage with an organic layer should be 10–60% and, in the best cases, up to 95% [15]. The selection of the modifier concen tration is explained by the necessity that the free OH groups participate in the surface modification. The amount of the substance modifier in the reaction mix ture ranged from 0.0018 to 0.0036 mol/g of inorganic matrix. The type of solvent affects the structure of modified samples. When selecting the conditions for modifying the MCM41, the mesoporous structure must preserve functionalized materials. The modifica tion of silica gel in aqueous [19] and MCM41 in tol uene [9] media with organosilanes with ammonium groups had been done before. As can be seen from the data on the adsorption and desorption of nitrogen, in the case of modification in the water and toluene (Fig. 1, curves 4, 5), regardless of the ratio of inorganic matrix and the modifier, it is likely that there are structural changes of mesoporous materials. Accordingly, the synthesis of composite materials also was carried out in the methanol. Isotherms of the sorption and desorption of nitro gen, measured for the original and modified samples of TMT, are characterized by the presence of two regions of hysteresis in the range of relative pressures 0.4–0.6 and 0.8–0.9, respectively (Fig. 1, curves 1, 2, 3). These curves refer to the type IV of isotherms accord ing to the IUPAC classification, which are typical for mesoporous materials. However, during the modifica tion of mesoporous material by TMT in toluene and aqueous environments, the change in the type of sorp tion isotherms is expressed in the disappearance of the hysteresis region in the pressure range 0.4–0.6 and a reduction in the amount of adsorbed N2 (Fig. 1, curves 4, 5). A change in the sorption isotherm type indicates a partial filling of pores with the modifying agent, which results in a significant reduction in the adsorption capacity of samples modified in toluene and water in relation to nitrogen. Data on the isotherms of adsorption and desorp tion of nitrogen demonstrate alterations of the surface area (SBET), volume (Vp), and average pore diameter

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Adsorbed volume, cm3/g

600

250 1

500

200

400 150

2 300 3

100

4

200 5

100 0

0.2

Adsorbed volume, cm3/g

300

700

50

0.4

0.6

0 1.0

0.8

P/P0 Fig. 1. Sorption isotherms of samples (1) MCM41, (2) MNM2, (3) MNM1, (4) MNW, and (5) MNT.

4

0.08

0.010

1

0.009 0.008

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0.007

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Dv(d), cm3/g/A°

Dv(d), cm3/g/A°

0.07

0.002

0.01 0 10

20

30

40

50 60 70 Pore diameter

80

90

0.001 0 100

Fig. 2. The distribution of pore volume by pore size (1) MCM41, (2) MNM2, (3) MNM1, (4) MNW, and (5) MNT

(Dh) of modified materials in comparison with the ini tial MCM41 sample (Table 1). Modification of the MCM41 was conducted under the same conditions except for the reaction medium (samples MNT, MNW, and MNM1). Modification in Table 1. Surface and bulk properties of the source and ami nated MCM41 in various solvents Sample

SBET, m2/g

Vp, cm3/g

D h, Å

MCM41 MNM2 MNT MNW MNM1

1330 919 113 265 807

1.165 0.736 0.267 0.383 0.706

35 33 94 58 35

water and toluene leads to a significant decrease in the surface area and pore volume and an increase in the average pore diameter. The most frequently used method for an analysis of mesoporous materials in constructing the distribution curves of the pore diam eter is the method of Barrett, Joyner, and Halenda (BJH) [23]. The calculated curves of pore size distri bution using data on the adsorption and desorption of nitrogen at 77 K are shown in Fig. 2. The parent meso porous MCM41 is characterized by a narrow pore size distribution (Fig. 2, curve 1). When conducting the synthesis in the toluene medium (as well as in water), the broadening of the pore size distribution takes place. On the one hand, the shift of maximum of the distribution toward smaller diameters (dpor ≤ 35 Å), under the condition of its broadening, indicates that modification leads to nonhomogeneous coverage,

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1419

3353 3354

3640

2892

2 3 1

3742

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2854

2764

4

1627 1633 1632

1476 1490

0.2

1419

0.3

3021

2956

1478

0.4

811

3274

5 4

5

3301

1488

Absorbance 0.5

3261

SYNTHESIS AND CHARACTERIZATION

3 1 ~ ~

0 1400

1500

1600

1700 3000 Wavenumber, cm−1

3500

Fig. 3. IR spectra of diffuse reflection for (1) MCM41, (2) MNM2, (3) MNM1, (4) MNW, and (5) MNT taken after keeping the samples in a flow of nitrogen at 25°C.

leading to the uneven coverage of the surface and the volume of the inorganic matrix modifier. On the other hand, the broadening of the pore size distribution (dpor ≥ 35 Å) indicates the formation of large mesopo res in the process of modification, which are obtained by the destruction of some walls of the hexagonal MCM41 structure. This is reflected in the increase in the average pore diameter (Table 1). To generate defin itive conclusions and evidence, studies using Xray structural analysis will be necessary. In the synthesis in methanol, a change in the bulk and surface character istics as compared with the original MCM41 is observed. However, in this case, the type of sorption isotherm and the position plot of capillary condensa tion remains unchanged (Fig. 1). Consequently, the specimen can be attributed to the orderedmesopo rous type. In this case, for these samples a narrow poresize distribution is retained, which is shifted to smaller pore diameters. Calculations show that the average pore diameter is Dh, assuming the existence of cylindrical pores [24] by the formula Dh = 4Vp/SBET, where Dh is average diameter of pores. For the samples synthesized in methanol, the aver age pore diameter does not differ from that for MCM 41 (32–35 Å). Obtaining the composite material in water and toluene leads to a significant increase in Dh (Table 1), which is presumably related to the partial destruction of the silicate base. Modification in water and toluene leads to a signif icant decrease in the surface area and pore volume and an increase in the average pore diameter. Along with a reduction in the surface and bulk characteristics and an increase in the average pore radius (Table 1), there is a change in the sorption isotherms, indicating a struc tural change of the hexagonal framework (Fig. 1). At this stage of work adsorption–desorption nitrogen NANOTECHNOLOGIES IN RUSSIA

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dataset, it is expected that only in the synthesis of the samples at different concentrations of modifier in methanol solution will not change the surface. To explain the structural changes, as well as to con firm the process of modification in this study, we used infrared spectroscopy (Fig. 3). The appearance of the bands 1475, 1490 cm–1 (δ of C–H groups in CH2 and CH3) [25] 2854, 2892, 2956, 3021 cm–1 (ν of C–H groups in CH2 and CH3) in the diffusereflectance IR spectra confirms the occur rence of modification of mesoporous MCM41 mate rial. The band at 1490 cm–1 can also be attributed to the vibrations of C–N bonds [26]. The broad absorp tion bands at 1630 and 3200–3400 cm–1 belong to the vibrations of adsorbed water associated with the sil anol groups by hydrogen bonds. The absorption band appearing at 3640 cm–1 can be assigned to the vibra tions of isolated pairs of adjacent OH groups con nected by hydrogen bonds (Scheme 1) [27]. There may also be an interaction between nitrogen atom of the quaternary aminogroup and the oxygen atom of silanol OH groups as shown in scheme 3 in the modified MCM41. Depending on the conditions of processing with chloride NtrimethoxysilylpropylN,N,N–trimetil ammonium, the intensity of the bands varies at 1490, 1478, and 3740 cm–1 varies. It should be noted that the

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H O

H O

Si O Si

Scheme 1. Formation of hydrogen bonds between OH groups on MCM41. 2010

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1394

0.1

2856 2895 2897 2957

2693 2730 2780

2472

3

2 3

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1679

1421

0.2

4 1475

1418

2

3736

0.3

4 5

3651

1476

2953 3011

0.4

3274

0.5

3257

1478

Adsorption

1 ~ ~

0 1400

1500

1600

1700 2500 3000 Wavenumber, cm1

3500

Fig. 4. IR spectra of diffuse reflection for (1) MCM41, (2) MNM2, (3) MNM1, (4) MNW, and (5) MNT at a temperature of in situ heating of 250°C.

reaction with the silanol groups takes place more intensively when the reaction happens in water and toluene. In this case, the band at 3740 cm–1 (ν OH in Si–OH) virtually disappears, as can be seen from the spectra 2, 4 and 5 (Fig. 3). The chemical modification does not involve all of the free OH groups, which is also confirmed in the ratio of the OH and amino groups calculated using the data of thermogravimetric analysis presented below. The registration of IR spectra in the mode of heat ing in situ made it possible to characterize the struc tural changes of the composites at elevated tempera tures. The spectrum and intensity of the absorption bands does not change when the temperature reaches 150°C. Keeping the samples at 250°C leads to changes in the infrared spectra (Fig. 4). The intensity of the bands at 3400–3500 cm–1, cor responding to the adsorbed water, is significantly reduced. Similar changes are observed in the field of deformation vibrations of OH groups at 1630 cm–1 (Fig. 4). Along with changes in the intensity of the absorption bands corresponding to adsorbed water, heating the samples to 250°C leads to the disappear ance of the band at 1490 cm–1, corresponding to the

Si

Cl− + N(CH3)3

O

O H

Si O

Si

Scheme 2. Interaction between the hydroxyl group and quaternary amine on the modified MCM41.

vibration of C–N, which can be attributed to the par tial destruction of C–N bonds. It is believed that this destruction is caused by deamination, i.e., the decom position of highbasic groups according to the reaction (Scheme 3) [18, 28]. IR spectra recorded at high temperatures evidence the thermal stability of the material, which is con firmed by the data of a thermogravimetric analysis. With an increase in temperature to 250°C, an increase in the intensity of the absorption bands at 3740 cm–1 takes place, indicating an increase in the number of silanol groups not involved in the formation of hydro gen bonds. With an increase in temperature, the destruction of hydrogen bonds occurs (Figs. 1, 2). Hydrogen bond breaking according to Scheme 1, as noted in [28], is accompanied by a decrease in the intensity of the band at 3640 cm–1 and increases in the intensity of the band at 3740 cm–1. The increase in the number of free silanol groups may also be due to the breaking of the C–N bond (Scheme 2). In this case, after the separation of N(CH3)3, the remaining graft aliphatic part can not participate in the interaction with the oxygen atom of the silanol group [1], which in turn can result in an increase in the number of free OH groups. Stepped temperature change during the in situ heating of the sample using infrared spectroscopy does not allow the continuous monitoring of the thermal stability of the composite, which can be implemented by TGA/DTA. Thermogravimetric curves taken in a nitrogen atmosphere are presented in Fig. 5. The first peak with a minimum at 55°C (Fig. 6) corresponds to the removal of the water, which is con firmed by infrared spectroscopy (Fig. 4). In the spectra taken at 250°C, a decrease in the intensity of the

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0

100

200

300

T, °C 400

500

813

600

700

1 0.002 2 −0.004 −0.006 −0.008

3

−0.010 −0.012 Fig. 5. TGA data obtained in a nitrogen atmosphere for samples of MCM41: (1) MNM2 and (2) MNM1.

0

50

100

150

200

−0.005

T, °C 250 300 1 2

dm/dT

−0.010

350

400

450

500

3

−0.015 −0.020 −0.025 −0.030

4

−0.035 Fig. 6. TGA data obtained in an oxygen atmosphere for samples of MCM41: (1) MCM41, (2) MNM2, (3) MNM1, and (4) MNT.

absorption bands takes place at 1630 and 3250–3350 cm–1, corresponding to the vibrations of the OH groups of water. A change in the intensity of the absorption bands corresponding to the vibrations of

the CH2, CH3, and N–H groups does not occur at a temperature of 150°C, but it is significant at 250°C (Fig. 4). On the TGA plot, the second peak with a minimum at a temperature of 250°C is presumably

R–(CH2)3–N+(CH3)3Cl–

R–(CH2)3–Cl + N(CH3)3

Scheme 3. Proposed mechanism of decomposition of ammonium groups; in this case, R = (SiO2)3–Si–.

Si O H

Si O

O

O

OMe MeO + O OMe Si Si O H OMe O Si O H Si O H

+

Si O –

N Cl

–3MeOH

Si

+

N Cl

–

O Si O O Si O H

Scheme 4. Proposed reaction mechanism for obtaining a highly basic anion exchanger on the basis of inorganic matrices. NANOTECHNOLOGIES IN RUSSIA

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Table 2. The results of a thermogravimetric analysis of MCM41 and MCM41 modified with Ntrimethoxysilylpropyl N,N,Ntrimethylammonium chloride Δmi /m0* 100% Sample

νmod, mmol/g

νmod/νOH, mol/mol OH

25–135°C

200–400°C

25–700°C

MCM41

3.41

1.02

4.43

–

–

MNM2

5.38

7.15

13.53

0.28 ± 0.03

0.043 ± 0.005

MNM1

11.06

13.81

24.87

0.50 ± 0.05

0.083 ± 0.011

MNT

10.04

27.76

37.80

1.08 ± 0.09

0.170 ± 0.008

* νmod/νOH is the amount of the modifier mole per 1 mole of OH groups.

associated with the removal of ammonium groups (Scheme 3). The third peak with a minimum at 450°C refers to the further decomposition of the grafted organic NtrimetoxyisilylpropylN,N,Ntrimethy lammonium chloride (Fig. 5). The figure shows that, for aminated specimens, the destruction of the grafted modifier begins at 180°C and ends at 550°C, which is corresponding to the data of DRIFT. Consequently, the data of thermogravimetric analysis confirms the above statement about the stability of the grafted func tional groups. Curves of the thermogravimetric analysis taken in an oxygen atmosphere (Fig. 6) make it possible to determine the stability of the modified material taking oxidation into account. The first peak with a minimum at 55°C (Fig. 6) corresponds to the removal of water from the sample. The second peak with a minimum at 250°C corre sponds to the complete decomposition of the organic modifier, which includes deamination and dealkyla tion. It should be noted that stability in the view of oxi dation (Fig.6) is comparable to the thermal stability of modified materials in an inert atmosphere (Fig. 5). 2

In [22] it was shown that, for 1 nm MCM41, there are 2.5–3 OH groups. Taking into account the surface area of MCM41 (1330 m2/g), the concentra tion of OH groups is 5.5–6.6 mmol/g [29]. The results of a thermogravimetric analysis for the samples, mod ified in methanol and toluene, allowed us to calculate the amount of modifier per 1 gram of sorbent (Qmod) and 1 mol OH groups (νmod/νOH) (Table 2). Accord ing to the given data, the modifier quantity per 1 g of sorbent depends on the type of solvent and the con centration of the modifier in solution. A higher degree of grafting modifier is achieved during the reaction in toluene than in methanol, which is confirmed by infrared spectroscopy for MNM1 and MNT samples. Moreover, increasing the concentration of the solution of the modifier in the MNM1 sample over MNM2 makes it possible to increase the amount of substance modifier in relation to the number of OH groups

(Table 2), which in turn leads to an increase in the degree of grafting of the organic functionalized layer. However, the large degree of grafting is probably accompanied by the partial destruction of the ordered mesoporous structure of the composite. In methanol, with a sufficient (although less than in toluene) degree of grafting, a modification presumably proceeds with preservation of the structure of the material. The degree of grafting on the OH groups is 0.043– 0.170 mole of modifier per 1 mole of OH groups of the original MCM41. In [27] it was shown that, when applying a modifier containing three methoxy groups, grafting occurs by three OH groups; therefore, a mod ification of the reaction involved 13–51% of the OH groups. Taking this into account, this way of reac tionsurface modification, which consists of a single stage, is shown in Fig. 4. Thus, in this paper the synthesis of composite materials in methanol achieved a satisfactory degree of organic coating layer [15], presumably while main taining the ordered structure of mesoporous materials. CONCLUSIONS Organic–inorganic hybrid materials have been synthesized by the chemical modification of a meso porous MCM41 with quaternary aminopropylsilane material. Structural changes of the ordered hexagonal structure of the samples in toluene and water media were revealed for sorption and desorption isotherms of the nitrogen, whereas this phenomenon does not occur in methanol. IR spectroscopy confirmed the modification of the samples and showed the structural changes of the composites with discrete changes of the temperature with in situ heating of the sample. With the continuous monitoring of the thermal stability of the composite by TGA/DTA, it was found that the materials are stable up to temperatures of T ≈ 170– 180°C, which coincides with the thermal stability of synthetic and anionexchanging resins and is consis tent with the data of IR spectroscopy. It was shown that the highest degree of grafting of the modifier hap

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pens during the reaction in toluene rather than in methanol. The amount of modifiers, per 1 mol of OH groups, is 0.043–0.170 mol, which corresponds to a degree of grafting of 13.0–51.0%. ACKNOWLEDGMENTS This work was supported by the Russian Ministry of Education and Science and the German Academic Exchange Service (DAAD) under the Mikhail Lomonosov 2008/2009 program. We thank our col league P. Adryan at the Department of Industrial Chemistry, Carl von Ossietzky University, for assis tance in investigating the sorption and desorption iso therm of nitrogen and infrared spectroscopy. We also thank M. Ahlers, an employee at the Department of Pure and Applied Chemistry, Carl von Ossiettski Uni versity, for carrying out the thermogravimetric analysis. This work was supported by the Federal Program “Research and Development on Priority Directions of ScientificTechnological Complex of Russia for 2007–2012,” Ministry of Education and Science of Russia (GK 02.552.11.7091).

9.

10.

11.

12.

13.

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