Organosulfonic acid-functionalized mesoporous composites based on natural rubber and hexagonal mesoporous silica

Materials Chemistry and Physics xxx (2014) 1e11

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Organosulfonic acid-functionalized mesoporous composites based on natural rubber and hexagonal mesoporous silica Sakdinun Nuntang a, Sirilux Poompradub a, Suchada Butnark b, Toshiyuki Yokoi c, Takashi Tatsumi c, Chawalit Ngamcharussrivichai a, d, * a

Fuels Research Center, Department of Chemical Technology, Faculty of Science, Chulalongkorn University, Patumwan, Bangkok 10330, Thailand PTT Research and Technology Institute, PTT Public Company Limited, Wangnoi, Ayutthaya 13170, Thailand Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan d Center of Excellence on Petrochemical and Materials Technology (PETROMAT), Chulalongkorn University, Patumwan, Bangkok 10330, Thailand b c

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Acidic NR/HMS-SO3H composites were prepared by in situ solegel process.
Propylsulfonic acid was functionalized onto HMS surface by direct cocondensation.
NR/HMS-SO3H exhibited a hexagonal mesostructure and high mesoporosity.
NR incorporated into the HMS structure improved the hydrophobicity of the composites.
NR/HMS-SO3H had a high esterification activity for lauric acid with ethanol.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 February 2014 Received in revised form 14 May 2014 Accepted 18 May 2014 Available online xxx

This study is the first report on synthesis, characterization and catalytic application of propylsulfonic acid-functionalized mesoporous composites based on natural rubber (NR) and hexagonal mesoporous silica (HMS). In comparison with propylsulfonic acid-functionalized HMS (HMS-SO3H), a series of NR/ HMS-SO3H composites were prepared via an in situ solegel process using tetrahydrofuran as the synthesis media. Tetraethylorthosilicate as the silica source, was simultaneously condensed with 3mercaptopropyltrimethoxysilane in a solution of NR followed by oxidation with hydrogen peroxide to achieve the mesoporous composites containing propylsulfonic acid groups. Fourier-transform infrared spectroscopy and 29Si MAS nuclear magnetic resonance spectroscopy results verified that the silica surfaces of the NR/HMS-SO3H composites were functionalized with propylsulfonic acid groups and covered with NR molecules. After the incorporation of NR and organo-functional group into HMS, the hexagonal mesostructure remained intact concomitantly with an increased framework wall thickness and unit cell size, as evidenced by the X-ray powder diffraction analysis. Scanning electron microscopy analysis indicated a high interparticle porosity of NR/HMS-SO3H composites. The textural properties of NR/HMS-SO3H were affected by the amount of MPTMS loading to a smaller extent than that of HMSSO3H. NR/HMS-SO3H exhibited higher hydrophobicity than HMS-SO3H, as revealed by H2O adsorption

Keywords: Composite materials Solegel growth Surface properties Nanostructures

* Corresponding author. Fuels Research Center, Department of Chemical Technology, Faculty of Science, Chulalongkorn University, Patumwan, Bangkok 10330, Thailand. Tel.: þ66 2218 7528; fax: þ66 2255 5831. E-mail addresses: [email protected], [email protected] (C. Ngamcharussrivichai). http://dx.doi.org/10.1016/j.matchemphys.2014.05.034 0254-0584/© 2014 Elsevier B.V. All rights reserved.

Please cite this article in press as: S. Nuntang, et al., Organosulfonic acid-functionalized mesoporous composites based on natural rubber and hexagonal mesoporous silica, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.05.034

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edesorption measurements. Moreover, the NR/HMS-SO3H catalysts possessed a superior specific activity to HMS-SO3H in the esterification of lauric acid with ethanol, resulting in a higher conversion level. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Esterification of carboxylic acids with alcohols, producing water as by-product, represents a well-known category of liquid-phase reactions of industrial interest due to the importance of organic ester products in the fuel, food, cosmetic and pharmaceutical industries. Currently, the synthesis of esters via esterification is achieved commercially using liquid acid catalysts, such as sulfuric acid, hydrochloric acid and para-toluene sulfonic acid (p-TsOH). These catalysts actively promote the complete conversion of carboxylic acids giving a high yield of the esters due to their high acidity and solubility in the reaction mixture. However, the use of homogeneous acid catalysts requires energy-inefficient processes for catalyst separation, product purification and treatment of the hazardous acid. The application of heterogeneous acid catalysts for esterification has received considerable attention since the process can be simplified using a column reactor packed with the solid catalysts [1e3]. More importantly, the heterogeneous catalysts themselves are recyclable, less toxic and non-corrosive. Various acidic materials have been investigated as heterogeneous catalysts for the esterification of simple carboxylic acids, such as sulfated zirconia [4], zeolites [5], heteropolyacids [6], acidic ion-exchange resins (Amberlyst series) [7e9]. The silica-supported Nafion® catalyst (SAC-13) has been studied as a catalyst in esterification reactions [10], due to its high acid strength from the presence of sulfonic acid groups and its higher thermodynamic stability than other acidic resins. Furthermore, SAC-13 exhibits highly accessible acid sites due to the porous silica matrix with adjustable pore openings at a nanometer scale that diminishes the likelihood of mass diffusion limitation, making it possible for the catalytic reaction of bulky reactants. SAC-13 contains perflurosulfonic acid groups with high hydrophilicity and large amount of surface silanol groups, which make it not suitable for water-sensitive reactions. Recently, it was demonstrated that the organiceinorganic composite as a mixture of graphite oxide and NaY zeolite exhibited superior activity to the pure zeolite catalysts in the dehydration of various alcohols to olefins [11]. Interest in the use of hexagonal mesoporous silicas (HMSs) functionalized with organo-sulfonic acid groups as esterification catalysts has increased due to their combination of an extremely high surface area and strength of sulfonic acid groups [12e15]. The incorporation of organo-functional groups on the mesoporous surface can be performed by either post grafting [16,17] or by direct co-condensation [18]. The functionalized materials attained by post-grafting exhibit a relatively well-ordered meso-structure, but are limited by the non-uniform distribution of the organic moieties due to organosilane precursors congregating on the pore mouth of the mesoporous channels and the external surface of mesoporous materials [19,20]. Co-condensation is a one-step process in which the hydrolysis and the condensation of tetraethoxysilane (TEOS) and mercapto-organosilane simultaneously occur around the micellar surfactant template in the presence of hydrogen peroxide (H2O2) as the oxidizing agent to convert the thiol groups in situ to the corresponding sulfonic acid [21]. Although this procedure provides a better control of the quantity and distribution of organofunctional groups in the mesoporous materials, residual silanol groups (^SieOH) on the surface contribute hydrophilicity and

promote the hydrolysis of esters via the adsorbed H2O. The amount of surface silanol groups in functionalized mesoporous silica has been decreased by the successive grafting of trialkylorganosilane [22,23], but the resulting materials possess low textural properties. In addition, use of organosilica precursor, such as 1,2-bis-(triethoxysilyl)ethane (BTSE), co-condensed with tetraethylorthosilicate (TEOS) as mixed silica sources can reduce an interaction of adsorbed H2O with the silica framework [24]. In this work, we report synthesis and characterization of acidic mesoporous composites, based on natural rubber (NR) and HMS formed via the in situ solegel technique and the simultaneous functionalization of HMS with organo-sulfonic acid groups (NR/ HMS-SO3H). The composites take advantage of both the high mesoporosity of HMS and the hydrophobicity of the NR improver. Their catalytic activities were tested in the esterification of lauric acid, as a representative long-chain carboxylic acid with ethanol to investigate an advantage of hydrophobicity of acidic mesoporous composites. The ethyl laurate attained can be used as biodiesel [25] and flavor ingredients [26]. Unique characteristics of the NR/HMSSO3H were compared with the propylsulfonic acid-functionalized HMS (HMS-SO3H) prepared under similar conditions. 2. Experimental 2.1. Preparation of mesoporous materials 2.1.1. Material and reagents The TEOS and 3-mercaptopropyltrimethoxysilane (MPTMS) (AR grade, >96%) were purchased from Tokyo Chemical Industry Co., Ltd. Dodecylamine (DDA) (AR grade, >96%), tetrahydrofuran (THF) (AR grade, 99.5%), H2O2 (AR grade, 30%), sulfuric acid (AR grade, 98%), absolute ethyl alcohol (AR grade, 99.5%) and lauric acid (AR grade, 99.9%) were purchased from Wako Pure Chemical Industries, Ltd. The NR (commercial grade) was supplied by the Thai Hua Chumporn Natural Rubber Co., Ltd. (Thailand). All of the material and reagents were used without further purification. 2.1.2. Synthesis of HMS Pure silica HMS was synthesized using DDA (a neutral primary amine) as the organic template, TEOS as the silica source and THF as the synthesis media at a gel molar composition of 0.10TEOS:0.04DDA:5.89H2O:0.37THF. The templating solution was prepared by dissolving DDA in a solution of THF and deionized water to which TEOS was added dropwise. This solution was rapidly stirred for 0.5 h at 40  C, and then aged for 18 h at ambient temperature in order to obtain the HMS. The resulting solid was recovered by filtration, thoroughly washed with deionized water, and dried at 100  C overnight. The dried HMS was then washed with 0.05 M sulfuric acid/ethanol solution at 70  C for 8 h to remove the organic template. 2.1.3. Synthesis of HMS-SO3H In a typical procedure, DDA was dissolved in a solution of THF and deionized water with stirring. Then, TEOS was slowly dropped into the solution and the synthesis mixture was stirred at 40  C for 0.5 h prior to adding MPTMS and H2O2 dropwise and stirring at 40  C for 1 h. The molar composition of synthesis mixture was 0.10TEOS:0.04DDA:5.89H2O:0.37THF:(n)MPTMS:(7n)H2O2, where

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3

n ¼ 0.01 or 0.02 mol. After ageing the resulting gel at ambient temperature for 18 h, the solid HMS-SO3H was harvested by filtration and air dried overnight. The organic template was removed by extraction as in Section 2.1.2. The HMS-SO3H catalyst was designated as HMS-SO3H (x) where x represents the MPTMS/ TEOS molar ratio in the synthesis mixture.

The acid level of the functionalized materials was determined by acid-base titration. Typically, 0.5 g of sample was equilibrated with a 20-mL solution of THF and ethanol (50% (v/v)) under stirring at room temperature for 18 h. The resulting mixture was then titrated with 0.02 M NaOH aqueous solution. The acidity was calculated from Eq. (1);

2.1.4. Synthesis of NR/HMS-SO3H and NR/HMS The acidic mesoporous composite based on NR and hexagonal mesoporous silica (NR/HMS-SO3H) was prepared via the in situ solegel method using THF as the synthesis media. Firstly, 1 g of NR sheet (1 mm thickness) was swollen in TEOS at room temperature for 16 h. The swollen NR sheet was then dissolved in THF to obtain a homogeneous solution. Subsequently, DDA was mixed with the NR solution, followed by the dropwise addition of TEOS with stirring. After 1 h, deionized water, MPTMS and H2O2 were sequentially added into the mixture with stirring and then the mixture was allowed to stand at 40  C for 1 h. The molar composition of synthesis mixture was 0.10TEOS:0.04DDA:5.89H2O:0.37THF:0.01NR:(n)MPTMS:(7n)H2O2, where n ¼ 0.01, 0.02 or 0.04 mol. The gel attained was aged at 40  C for 3 d after which it was precipitated in 100 mL of ethanol. The solid product was recovered by filtration, followed by drying under vacuum at 60  C for 2 h. Finally, the template occluded in the mesopores of the composite was removed as in Section 2.1.2. The composite was designated as NR/HMS-SO3H (x) where x represents the MPTMS/ TEOS molar ratio used in the synthesis. A composite of NR and pure silica HMS (NR/HMS) was synthesized by the same procedure and with a similar molar composition as the NR/HMS-SO3H (0.1), but without adding MPTMS and H2O2.

N¼

2.2. Characterization of HMS, NR/HMS and NR/HMS-SO3H The thermogravimetric/differential thermal analysis (TG/DTA) was carried out on a Rigaku Thermo Plus with a heating rate of 10  C min1 under a dry air flow (50 mL min1) to determine the silica content of the synthesized composites. The amount of sulfur incorporated in the mesoporous structure was evaluated using an Elementar Vario Micro Cube CHNS elemental analyzer. X-ray powder diffraction (XRD) patterns were obtained on a Rigaku Rint-Ultima III X-ray diffractometer employing Cu Ka radiation and an X-ray power of 40 kV and 40 mA. The measurement was started from 2q ¼ 0.5 e10 with a scanning step of 0.02 and a count time of 1 s. The repeating distance (a0) between pore centers of the hexagonal structure was calculated from p the ffiffiffi d-spacing of plane (100) (d100) using the formula; a0 ¼ 2d100 = 3 [27]. Fourier-transform infrared (FTIR) spectra were recorded on a JASCO FT/IR-4100 spectrometer equipped with a mercury cadmium telluride (MCT) detector. Each sample (10e20 mg) was pressed into a self-supported disk with a diameter of 20 mm and placed in a quartz cell attached to a closed-circulation system. The disk was pretreated by evacuation at 150  C for 1 h. The FTIR spectra were recorded at room temperature with total of 64 scans over 400e4000 cm1 at a resolution of 4 cm1. Solid-state 29Si magic angle spinning nuclear magnetic resonance (29Si MAS NMR) spectra were acquired on a JEOL-ECA400 NMR spectrometer at 79.4 MHz and a sample spinning frequency of 5 kHz to measure the relative concentration of silica species present in each material. The chemical shifts of the 29Si MAS NMR spectra are quoted in parts per million (ppm) with tetramethylsilane as the internal standard. The 29Si MAS NMR was analyzed by using a single-pulse method with a delay time of 60 s. The resolution of the 29Si MAS NMR spectra was sufficient for accurate peak assignments, and the relative peak area of each species was obtained by a curve-fitting analysis, using a series of Gaussian curves.



0:02  V 1000  W

 (1)

where N is the acidity (mmol g1), V is volume of NaOH solution consumed in the titration (mL) and W is sample weight (mg). The textural properties of the synthesis materials were analyzed by nitrogen (N2) adsorptionedesorption measurement at 196  C on a BEL Japan BELSORP-mini II instrument. All samples were pretreated at 150  C for 2 h and then measured for exact weight prior to the adsorption. The BrunauereEmmetteTeller (BET) method was employed to calculate the specific surface area (SBET) in the relative pressure (P/P0) range of 0.02e0.2 and the total pore volume (Vt) was obtained from the accumulative volume of N2 adsorbed at a P/P0 of about 0.990. The t-plot method was used to estimate the external surface area (Sext). The primary mesopore volume (Vp) was calculated from the slope of linear portion of the t-plot in the relative pressure range above which N2 was condensed inside the primary mesopores [28]. The calculation of pore size (Dp) was performed by using the BarretteJoynereHalenda (BJH) method using the adsorption branch data of N2 sorption isotherm. The material morphologies were studied by scanning electron microscopy (SEM). The FE-SEM images were recorded on a Hitachi SU9000 scanning electron microscope operating at 30 kV. The STEM mode was used to analyze transmission electron microscopy (TEM) images. The samples on copper grids were observed without any metal coating. All H2O adsorptionedesorption isotherms were measured by using a BEL Japan BELSORP-max instrument. Typically, a sample was pretreated at 150  C for 2 h before the sample weight was measured exactly. The experiment was conducted at 25  C. The monolayer adsorbed volume (Vm) of H2O was determined from analysis of adsorption data at a relative pressure below 0.2. 2.3. Esterification of lauric acid with ethanol The catalytic properties of the acidic mesoporous materials were evaluated for the esterification of lauric acid with ethanol. The reaction was performed in a 50-mL three-neck round-bottom flask equipped with a reflux system and a magnetic stirrer. In a typical reaction, 13.7 g of lauric acid and 6.3 g of ethanol (1: 2 M ratio of lauric acid: ethanol), were mixed in the flask. The reaction temperature was controlled using a silicone oil bath at 120  C. Subsequently, the respective pretreated catalyst (100  C, 2 h) was added into the reaction mixture at 1 wt.% (based on the weight of lauric acid used). A certain quantity of the reaction mixture was withdrawn at different time intervals up to 8 h of the reaction course, immediately diluted with pyridine and then subjected to sample preparation for composition analysis by gas chromatography (GC). A Shimadzu GC-2014 gas chromatograph equipped with a 30-m DB-5 capillary column and a flame ionization detector (FID) was used for the quantification of the residual lauric acid using ndecane (99%, TCI) as the internal standard. Prior to the analysis, Nmethyl-N-(trimethylsilyl)trifluoroacetamide (MSTFA) (>98.5%, Aldrich) was added into the liquid sample to convert the lauric acid to more volatile derivative, and then the sample volume was made up by adding n-heptane (99%, Aldrich) as solvent.

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acidified ethanol was effective since the nitrogen content was very low (<0.4 wt.%) in both the HMS-SO3H and NR/HMS-SO3H.

3. Results and discussion 3.1. Thermogravimetric (TG/DTA) analysis

3.2. XRD analysis The TG and DTA curves of the HMS-SO3H (0.1) and NR/HMSSO3H (0.1) composite, as representative samples of HMS-SO3H and NR/HMS-SO3H materials, respectively, are shown in Fig. 1. Both samples exhibited a three-stage weight loss. The first endothermic change occurred up to 110  C and was due to the loss of solvent and physisorbed water. The subsequent major weight losses consisted of two exothermic peaks at around 310 and 357  C were respectively attributed to the combustion of propylthiol groups and propylsulfonic acid groups functionalized on the HMS structure [29], indicating the presence of the thiol groups unconverted by the oxidation process. Margolese et al. reported that the complete oxidation of the mercaptopropyl group, introduced by the direct co-condensation, to the corresponding acid required a high molar ratio of H2O2:MPTMS [29]. It can be seen that NR/HMS-SO3H (0.1) exhibited higher weight loss of the propylthiol groups (~10 wt.%) than HMS-SO3H (0.1) (~6 wt.%). The result suggested that the NR molecules added in the synthesis mixture may hamper the oxidation process. Within this range, NR/HMS-SO3H (0.1) also gave a higher weight loss (~17 wt.%) than HMS-SO3H (0.1) (~12 wt.%), which likely reflects the additional thermal decomposition of NR in the NR/HMS-SO3H composite, since the thermal decomposition of pure NR occurred at 368  C [30]. Unfortunately, the exact NR content of NR/HMS-SO3H could not be determined due to overlapping the decomposition range of NR with that of propylsulfonic acid groups. It was supposed that the amount of NR in the NR/HMSSO3H materials would be close to that in NR/HMS (~10 wt.%) since all composites were synthesized by using the similar NR loading. In addition, the change of weight appearing at about 525  C was related to the condensation of surface silanol groups. The silica contents of HMS-SO3H and NR/HMS-SO3H prepared with different molar compositions, as estimated by the TGA technique, are summarized in Table 1. The amount of silica decreased with increasing MPTMS: TEOS molar ratios. HMS-SO3H possessed silica content in the range of 75.1e67.0%, while a smaller amount of silica was observed for NR/HMS-SO3H (65.6e58.1%). In addition, the elemental analysis revealed that the removal of DDA by

70

(A)

60 50

3.3. FTIR spectroscopy The presence of the NR or propylsulfonic acid groups in the HMS structure was confirmed using FTIR spectroscopy (Fig. 3). The stretching vibration of the siliceous framework (SieOeSi) of all

0

70

-10

60

(B)

-20

40

-40

DTA /

364 0C

-30 30 3020C 3490C

20

20 -50

10 0 -10 200

400 600 Temperature / °C

5240C

-50

10

-60

0

-70 800

-10

-40

TG/ wt%

30

TG/ wt%

5260C

-30

V

318 0C

0

0

-10

50

-20

40 DTA / V

Fig. 2 shows the XRD patterns of the pure silica HMS, HMSSO3H, NR/HMS and NR/HMS-SO3H composites after the extraction of template molecules. These materials exhibited one diffraction peak at 2q in the range of 1.0e3.0 , relating to characteristic (100) plane of the hexagonal mesoporous structure. The introduction of the organic species, either the propylsulfonic acid group or NR, into HMS decreased the structural arrangement. The effect of the MPTMS/TEOS molar ratio on the hexagonal mesostructure was similar for both the functionalized HMS and NR/HMS, although the loss of structure ordering was more obvious for NR/HMS-SO3H than for HMS-SO3H. An increased amount of MPTMS loading in the HMS-SO3H resulted in an increased d100 and unit cell parameter (Table 1), corresponding to the thicker wall of the silica framework. Comparing the structural data of NR/HMS in that of the pure silica HMS, the hexagonal unit cell and wall thickness were expanded with the presence of NR (Table 1). These results suggested that the NR molecules were incorporated in-between the mesoporous channels, since the mesopore size is too small to accommodate the bulky NR molecules [30]. Interestingly, the functionalization onto NR/HMS induced the unit cell contraction, while the thickness of pore wall was increased. This result indicated that adding organofunctional groups onto NR/HMS affected the arrangement of the hexagonal mesostructure more obviously than the functionalization of pure HMS. Similar to the case of HMS-SO3H, the unit cell parameter and the wall thickness of NR/HMS-SO3H were increased with increasing levels of MPTMS. It can be rationalized from these results that the mesoporous structure of both HMS-SO3H and NR/HMS-SO3H was functionalized with the propylsulfonic acid group.

-60

0

200

400 600 Temperature / °C

-70 800

Fig. 1. Weight loss and DTA curves of (A) HMS-SO3H (0.1) and (B) NR/HMS-SO3H (0.1) after removal of the organic template.

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Table 1 Physicochemical properties of mesoporous and acidic mesoporous materials synthesized under various conditions. Samplea

Silica contentb (%)

Sulfur contentc (mmol g1)

Acidityd (mmol Hþ g1)

SBETe (m2 g1)

Sextf (m2 g1)

Dpg (nm)

V th (cm3 g1)

Vpi (cm3 g1)

d100j (nm)

a0k (nm)

WTl (nm)

HMS HMS-SO3H (0.1) HMS-SO3H (0.2) NR/HMS NR/HMS-SO3H (0.1) NR/HMS-SO3H (0.2) NR/HMS-SO3H (0.4)

n.d. 75.1 67.0 78.5 65.6 61.7 58.1

n.d. 1.18 1.55 n.d. 0.94 1.59 2.22

n.d. 1.05 1.35 n.d. 0.83 1.18 1.61

852 759 673 570 560 543 524

548 494 274 268 399 288 151

3.15 2.60 2.54 3.19 2.33 2.33 2.08

2.57 1.72 1.00 1.96 2.28 1.80 1.18

0.73 0.53 0.43 0.42 0.35 0.34 0.29

4.3 4.5 4.7 4.9 4.4 4.5 4.4

5.0 5.2 5.4 5.7 4.8 5.2 5.1

1.8 2.6 2.8 2.5 2.7 2.8 3.0

n.d. ¼ not determined. a Number in parenthesis indicates the MPTMS/TEOS molar ratio. b Determined by thermogravimetry technique. c Determined by CHNS analyzer. d Determined by acid-base titration with NaOH aq. (0.02 M). e BET surface area. f External surface area from t-plot curves. g Pore diameter calculated using the BJH method. h Total pore volume. i Mesopore volume. j d100 from XRD analysis. k The repeat distance (a0) between the pore centers of the hexagonal structure was calculated from a0 ¼ 2d100/3½. l The framework wall thickness was determined by subtracting the BJH mesopore size from the repeat distance between pore centers.

mesoporous samples appeared between 1000 and 1300 cm1. The pure silica HMS exhibited a sharp band at 3740 cm1 and a broad band at around 3500 cm-1, which can be assigned to surface silanol groups as free and hydrogen-bonded species, respectively, [31]. After functionalization (HMS-SO3H), the bands related to the propylsulfonic acid groups were observed at 1360 cm1 (S]O stretching mode of sulfonic acid groups), 2850 and 2922 cm1 (CeH stretching of methylene groups) and around 3450 cm1 (OeH stretching vibration of the sulfonic acid groups) [32]. Simultaneously, the absorbance of the free silanol band was largely decreased. These results indicated that the silica surface was modified by incorporating the organosulfonic groups. On the other hand, NR/HMS gave various characteristic bands of the NR structure at 3010, 2960, 2920, 2848, 1655, 1440 and

1390 cm1 [33,34]. The band related to the free silanol group (3740 cm1) was decreased, while the band of hydrogen-bonded silanol species (3500 cm1) was increased. The results suggested that the NR molecules were bound onto the HMS surface. By comparing the spectrum of NR/HMS-SO3H (0.1) to that of NR/HMS, the bands representing eC^Ce of the NR backbone at 3010, 2960 and 1655 cm1 were decreased. This could be due to the high oxidizing power of H2O2 that can react with the eC^Ce of polyisoprene [35], but functional groups related to the possible oxidized products were not observed. The functionalization of propylsulfonic acid onto the surface of NR/HMS (NR/HMS-SO3H) was confirmed by the band at 1360 cm1 and by a further decrease in the free silanol group at 3740 cm-1. However, the band (~2580 cm1) related to the residual thiol groups was not observed

(A)

(B)

(a) HMS

(a) HMS

(b) HMS-SO3H (0.1)

Intensity / a.u.

Intensity / a.u.

(c) HMS-SO3H (0.2)

(b) NR/HMS (c) NR/HMS-SO3H (0.1) (d) NR/HMS-SO3H (0.2)

(e) NR/HMS-SO3H (0.4)

1

2

3

4

5 6 2-Theta/degree

7

8

9

10

1

2

3

4

5 6 7 2-Theta/degree

8

9

10

Fig. 2. XRD patterns of (A) pure silica and the functionalized HMSs and (B) primary and functionalized NR/HMS composites. All of the samples were extracted to remove the DDA template prior to the analysis.

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1440 1390 1243

2850

1360

(c)

3500

2922

(d)

1655

3010 2960 2920 2848

3500 3450

Absorbance/ a.u.

HMS composite gave a lower (Q2 þ Q3)/Q4 ratio than the pure silica HMS, indicating a smaller amount of free silanol groups on the HMS surface in the presence of NR. This result was consistent with the FTIR results (Fig. 3) that suggested the NR covered a part of the mesoporous silica surface. Increasing the MPTMS/TEOS molar ratio used in the preparation of HMS-SO3H and NR/HMS-SO3H caused a marked increase in the intensity of the Tm species as well as the STm/S(Tm þ Qn) ratio that corresponded with their sulfur content. Furthermore, the NR/HMS-SO3H composites gave a lower relative amount of Q2 and Q3 than the HMS-SO3H synthesized with a similar MPTMS/TEOS molar ratio. It can be concluded that the NR/ HMS-SO3H composites had a smaller content of free silanol groups than the HMS-SO3H materials.

1360

3460

6

3740

(b)

(a)

3.5. Sulfur content and acidity measurement 3900

3400

2900

2400

1900

1400

900

Wavenumber/ cm-1 Fig. 3. FTIR spectra of the (a) pure silica HMS, (b) HMS-SO3H (0.1), (c) NR/HMS and (d) NR/HMS-SO3H (0.1).

for both of HMS-SO3H and NR/HMS-SO3H, probably due to a small amount of propylthiol groups remaining in the HMS structure and a weak signal of the SeH stretching.

3.4. Solid state

29

Si MAS NMR studies

Fig. 4 shows 29Si MAS NMR spectra of the pure silica HMS and NR/HMS composites before and after the functionalization with propylsulfonic acid group. The three signals observed at 90, 101 and 109 ppm corresponded to the Q2 (Si(OSi)2(OH)2), Q3 (Si(OSi)3(OH)) and Q4 (Si(OSi)4) species, respectively, whilst the two signals at 56 and 66 ppm were attributed to T2 (RSi(OSi)2(OH)) and T3 (RSi(OSi)3 sites, respectively, where R is the organofunctional group. It was clearly seen that the functionalization decreased the intensity of Q2 and Q3 concomitantly with the generation of the Tm species. More precisely, the (Q2 þ Q3)/Q4 and STm/S(Tm þ Qn) ratios were determined from deconvolution of the 29Si MAS NMR spectra of these materials, and are summarized in Table 2. The primary NR/

The sulfur content and acidity of the HMS and NR/HMS functionalized with propylsulfonic acid group, as determined from the CHNS elemental analysis and acid-base titration, respectively, are summarized in Table 1. Increasing the MPTMS/TEOS molar ratio of the synthesis mixture resulted in an increased sulfur content and an enhanced acidity of the resulting materials. In all cases, the molar acidity level was less than the corresponding sulfur content due to the presence of the thiol groups unconverted by the oxidation with H2O2. This result was consistent with the results attained by TGA (Fig. 1). In addition, the acidity/sulfur content ratio decreased as the amount of added MPTMS increased. The conversion of sulfur to sulfonic acid for the HMS-SO3H and NR/HMS-SO3H series was in the range of 87e90% and 72e88%, respectively. The smaller sulfur conversion level observed for the NR/HMS-SO3H composites suggested that some mercaptopropyl groups were embedded in the NR layer, probably due to a hydrophobic attraction. 3.6. N2 adsorptionedesorption measurement The textural properties of the HMS-SO3H, NR/HMS and NR/ HMS-SO3H materials in comparison with those of pure silica HMS are summarized in Table 1, whilst the N2 adsorptionedesorption isotherms and BJH pore size distribution of HMS and NR/HMS with or without functionalization are shown in Figs. 5 and 6, respectively. The sorption isotherms of all materials exhibited type IV

(B)

(A)

Q

Q

Q Q

T T

T

Q

(c) NR/HMS-SO H(0.2)

Q

T

(c) HMS-SO H(0.2)

(b) HMS-SO H(0.1)

(b) NR/HMS-SO H(0.1)

(a) NR/HMS

(a) HMS

-50

-70

-90

-110

Chemical shift / ppm Fig. 4.

29

-130

-150

-50

-70

-90

-110

-130

-150

Chemical shift / ppm

Si MAS NMR spectra of the (A) pure silica and functionalized HMSs and (B) primary and functionalized NR/HMS composites.

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S. Nuntang et al. / Materials Chemistry and Physics xxx (2014) 1e11 Table 2 Relative intensities of the Tm and Qn groups (obtained from the deconvolution of the

29

7

Si MAS NMR) and the sulfur contents (obtained from CHNS analysis).

Sample

T (%)

T (%)

Q (%)

Q (%)

Q (%)

(Q2 þ Q3)/Q4 (%)

STm/S(Tm þ Qn) (%)

Sulfur content (mmol g1)

HMS HMS-SO3H (0.1) HMS-SO3H (0.2) NR/HMS NR/HMS-SO3H (0.1) NR/HMS-SO3H (0.2)

e 1.6 5.3 e 1.7 5.4

e 12.1 23.9 e 11.7 20.6

3.4 3.4 2.2 2.6 2.5 2.0

40.5 32.1 28.1 39.1 31.1 25.4

56.1 50.8 40.4 58.3 53.0 46.7

78 N/A N/A 72 N/A N/A

e 14 29 e 13 26

e 1.18 1.55 e 0.94 1.59

2

3

2

3

4

N/A ¼ not applicable.

ratios (Fig. 5B). Similar to the case of HMS-SO3H, the pore size distribution became broader and the mesopore diameter was decreased (Fig. 6B). However, their SBET and Vp were decreased to a much smaller extent than the HMS-SO3H series when the MPTMS/ TEOS molar ratio was increased from 0.1 to 0.2 (Table 1). Presumably a part of the organo-functional groups were not bound onto the internal surface of the primary mesopores. Some of the MPTMS molecules might have condensed with TEOS on the external surface leading to the organic moieties being stretched out to the NR layer covering the mesoporous channels due to the hydrophobic affinity of MPTMS and NR (Scheme 1). This explanation was supported by the severe decrease in the Sext when NR/HMS-SO3H was prepared with a MPTMS/TEOS molar ratio of 0.4 (Table 1).

isotherms (as defined by the IUPAC classification), with a hysteresis loop at a P/P0 ranging from 0.2 to 0.4, which are the characteristics of framework confined mesoporous materials. The large hysteresis loop observed at a high relative pressure (P/P0 > 0.8) for these materials could be due to N2 condensation inside the interparticle voids generated from their particle agglomerates. The isotherms of the HMS-SO3H materials possessed a lower N2 adsorbed volume and smaller hysteresis loop at P/P0 of 0.3e0.4 than that of HMS (Fig. 5A). A broader pore size distribution and smaller pore diameter were also observed for HMS-SO3H when compared to the pure silica HMS (Fig. 6A). The BET surface area (SBET), pore diameter (Dp), total pore volume (Vt) and mesopore volume (Vp) were all clearly decreased as the MPTMS loading level increased (Table 1). These results support that the organosilane was functionalized onto the primary HMS wall structure. The incorporation of the organo-functional group in the HMS structure also reduced the hysteresis loop at P/P0 > 0.8 (Fig. 5A) and decreased the external surface area (Sext) (Table 1), presumably because the particle size of the HMS-SO3H materials was larger than that of the size of pure silica HMS. Compared to the pure silica HMS, NR/HMS showed smaller hysteresis loops at a P/P0 of 0.3e0.4 and >0.8 (Fig. 5B). Although a lower SBET, Sext, Vt and Vp were observed for NR/HMS, the pore size (Dp) was not significantly affected by the presence of NR (Table 1). This observation emphasized the location of NR molecules that favorably covered the external surface of mesoporous channels, resulting in the thicker pore wall. The NR/HMS-SO3H series revealed a continuous reduction in the hysteresis loop at a P/P0 of 0.3e0.4 with increasing MPTMS/TEOS 2400

3.7. Electron microscopy The morphology of the HMS-SO3H (0.1) and NR/HMS-SO3H (0.1) composites is shown in representative SEM and STEM images in Fig. 7. The functionalized pure silica HMS exhibited large aggregates of nanosized silica particles (Fig. 7A), where the low interparticle porosity was in agreement with the N2 sorption isotherm of HMSSO3H (0.1) with a small hysteresis loop at P/P0 > 0.8 (Fig. 5A). The presence of NR as the composite with the HMS structure (NR/HMSSO3H (0.1)) yielded smaller particle aggregates and a better distribution of the particles (Fig. 7B), whilst the interparticle porosity was increased. The STEM images of the HMS-SO3H (0.1) and NR/ HMS-SO3H (0.1) (Fig. 7C and D) revealed that they possessed uniform framework-confined mesoporous channels of wormhole-like

(A)

(B)

4200

(e) NR/HMS-SO3H(0.4) 3600

(c) HMS-SO3H(0.2) 1800

(d) NR/HMS-SO3H(0.2)

(b) HMS-SO3H(0.1)

Va/cm3(STP) g-1

Va/cm3(STP) g-1

3000

1200

2400

(c) NR/HMS-SO3H(0.1)

1800

(b) NR/HMS

(a) HMS 1200

600

(a) HMS 600

0 0

0.1

0.2

0.3

0.4

0.5 p/p0

0.6

0.7

0.8

0.9

1

0 0

0.1

0.2

0.3

0.4

0.5 p/p0

0.6

0.7

0.8

0.9

1

Fig. 5. N2 adsorptionedesorption isotherms of the (A) pure silica and functionalized HMSs and (B) primary and functionalized NR/HMS composites.

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S. Nuntang et al. / Materials Chemistry and Physics xxx (2014) 1e11

(B)

(A)

(a) NR/HMS

dVp/ddp

dVp/ddp

(a) HMS

(b) HMS-SO3H(0.1)

(b) NR/HMS-SO3H(0.1) (c) NR/HMS-SO3H(0.2)

(c) HMS-SO3H(0.2)

(d) NR/HMS-SO3H(0.4)

0

2

4

dp/nm

6

8

10

0

2

4

dp/nm

6

8

10

Fig. 6. BJH pore size distribution of the (A) pure silica and functionalized HMSs and (B) primary and functionalized NR/HMS composites.

structures, similar to the conventional HMS reported in the literature [36]. 3.8. H2O adsorptionedesorption measurement The effects of the incorporation of propylsulfonic acid and/or NR on the hydrophobic properties of HMS-SO3H and NR/HMS-SO3H were evaluated by H2O sorption (Fig. 8). The isotherm of pure silica HMS was classified as type IV, where the large hysteresis loop and large volume of H2O adsorbed were related to its high SBET, Vp and Dp (Table 1). Taking into account its adsorption branch, HMS displayed a two-stage uptake of H2O, one at a medium relative pressure due to the adsorption of H2O in the primary mesopores, and the second at a relatively high pressure due to H2O filling into the interparticle voids. The H2O molecules were not totally desorbed from the HMS surface, as seen from the unclosed desorption branch end, indicating a relatively strong bonding of H2O on the silanol

groups. Irreversibly adsorbed H2O molecules have been reported previously to be consumed for breaking the siloxane bridges (SieOeSi) of the silicate framework [37]. The HMS-SO3H series exhibited a similar shaped isotherm to HMS (Fig. 8A), but the presence of the propylsulfonic acid in the mesoporous structure narrowed the hysteresis loop, indicating that H2O desorbed from the functionalized surface more easily. Moreover, the decreased amount of silanol groups in HMS-SO3H, as evidenced by the 29Si MAS NMR data (Fig. 4A), reduced the monolayer adsorption volume of H2O in the following order: HMS (82.3 cm3 g1) > HMS-SO3H (0.2) (70.3 cm3 g1) z HMS-SO3H (0.1) (67.3 cm3 g1). The NR/HMS and NR/HMS-SO3H composites also exhibited type IV shaped isotherms but the hysteresis loops were steeper than those for the pure silica HMS and the HMS-SO3H series prepared with a similar MPTMS/TEOS ratio. The decrease in the H2O affinity observed for the composites could be explained by the reduced

Scheme 1. Schematic representation of the formation of the hexagonal array of silicate micellar rods and NR in the presence of MPTMS.

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S. Nuntang et al. / Materials Chemistry and Physics xxx (2014) 1e11

9

Fig. 7. (A, B) SEM and (C, D) STEM images (400,000 magnification) of the (A, C) HMS-SO3H (0.1) and (B, D) NR/HMS-SO3H (0.1).

composites in following order: NR/HMS (52 cm3 g1) < NR/HMSSO3H (0.1) (58.7 cm3 g1) < NR/HMS-SO3H (0.2) 3 1 (60.8 cm g ) < NR/HMS-SO3H (0.4) (80.3 cm3 g1). This is due to the hydrophilicity of the acidic function in the propylsulfonic acid group. Overall, these results indicated that the NR/HMS-SO3H

level of exposed surface silanol groups, as seen from the FTIR (Fig. 3) and 29Si MAS NMR (Table 2) analyses. Although the NR/ HMS-SO3H series possessed a lower mesopore volume than the primary NR/HMS (Table 1), the monolayer adsorption volume of H2O increased with the amount of sulfonic acid contained in the

1800

1800

(A)

1400

1400

1200

1200 Va/cm3(STP) g-1

1600

Va/cm3(STP) g-1

1600

(c) HMS-SO3H(0.2)

1000

(d) NR/HMS-SO3H(0.4)

1000

800 (b) HMS-SO3H(0.1)

600

(B)

800

(c) NR/HMS-SO3H(0.2)

600

400

(b) NR/HMS-SO3H(0.1)

400

(a) HMS

200

200

0

(a) NR/HMS

0

0

0.1

0.2

0.3

0.4

0.5 p/p0

0.6

0.7

0.8

0.9

1

0

0.1

0.2

0.3

0.4

0.5 p/p0

0.6

0.7

0.8

0.9

1

Fig. 8. H2O adsorptionedesorption isotherms of the (A) pure silica and functionalized HMSs and (B) primary and functionalized NR/HMS composites.

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S. Nuntang et al. / Materials Chemistry and Physics xxx (2014) 1e11

Table 3 Esterification of lauric acid with ethanol over HMS-SO3H materials and NR/HMS-SO3H composites. Catalysts

Acidity (mmol g1)a

Initial rat e (mmol min1)b

Ethyl laurate yield (%)

TON (h1)c

Thermal (without catalyst) HMS-SO3H (0.1) HMS-SO3H (0.2) NR/HMS-SO3H (0.1) NR/HMS-SO3H (0.2) NR/HMS-SO3H (0.4)

e 1.05 1.35 0.83 1.18 1.61

0.02 0.17 0.20 0.24 0.28 0.30

29.2 51.0 61.8 67.7 73.8 69.7

e 71 64 129 102 83

Reaction conditions: stirring rate, 300 rpm; amount of catalyst, 1 wt.%; reaction temperature, 120  C; mole ratio (lauric acid to ethanol), 1:2; reaction time, 8 h. a Determined by acid-base titration with NaOH aq. (0.02 M). b Lauric acid conversion rate (mmol min1) determined over 30 min. c Turnover number, determined as the mmol of ethyl laurate yield/(acid density  weight of catalyst loading  time (0.5 h)).

composites possessed a higher hydrophobicity than the HMS-SO3H due to the incorporated NR molecules. 3.9. Esterification of lauric acid with ethanol Table 3 summarizes the results of the catalytic activity of the HMS-SO3H and NR/HMS-SO3H catalysts prepared with different MPTMS/TEOS ratios for the esterification of lauric acid with ethanol. All of the pure silica- and composite-based catalysts exhibited an increased initial rate of reaction with an increased acidity, due to the increased amount of active sites. Although the acidity of NR/HMS-SO3H composites was significantly lower than that of the HMS-SO3H prepared with a similar MPTMS loading level, NR/HMS-SO3H contributed a higher initial rate than HMSSO3H. The negative effect of the reduced acidity on the reaction rate over NR/HMS-SO3H might be compensated for by the enhanced diffusion of the reactants from the hydrophobic NR layer. Moreover, a higher ethyl laurate yield and TON were achieved over the NR/ HMS-SO3H catalysts. Since the esterification is in equilibrium with the hydrolysis of the ester product, the presence of NR in the composites may hamper the readsorption of H2O, as a by-product, onto the catalyst surface and so retard the reverse reaction. However, the ethyl laurate yield dropped to 69.7% when the esterification was catalyzed by NR/HMS-SO3H (0.4), which could be due to the increased affinity for H2O of this composite, as revealed by the H2O adsorptionedesorption (Fig. 8B). Therefore, the optimum molar ratio of MPTMS: TEOS in the preparation of NR/HMS-SO3H catalyst was in the range of 0.1e0.2. The reusability of NR/HMS-SO3H catalyst was preliminary evaluated under the same reaction conditions. The spent NR/HMSSO3H(0.2) was recovered from the reaction mixture by a filtration, followed by thoroughly washing with acetone, and drying at 100  C overnight. The catalyst could be repeatedly used in the esterification at least 4 times during which the ester yield was maintained around 70%. The yield loss (ca. 20 wt.%) was observed in the 5th repetition. It might be due to a strong adsorption of organic substances on the active sites as well as leaching of the organosulfonic groups. Further study is ongoing to reveal the cause of catalyst deactivation. 4. Conclusions A series of NR/HMS-SO3H composites with hexagonal mesostructure were successfully prepared as acidic composites using the in situ solegel process simultaneously with a one-pot functionalization of the HMS surface with a propylsulfonic acid group. However, the presence of NR molecules in the synthesis mixture lowered the oxidative conversion of the propylthiol group to the corresponding acid. The incorporation of NR and/or the organofunctional group into the HMS structure reduced the amount of free silanol groups and enhanced the hydrophobicity of the

materials. Increasing the amount of MPTMS increased the acidity of the obtained materials, but decreased the structural ordering and the textural properties. The detrimental effect of increasing the MPTMS/TEOS molar ratio on the Sext was more pronounced in the NR/HMS-SO3H composite than the HMS-SO3H one, suggesting that the functionalization occurred on the external surface of HMS to a certain degree in the presence of NR. A relatively high hydrophobicity of NR/HMS-SO3H was the key for the superior catalytic activity of HMS-SO3H in the esterification of lauric acid with ethanol. From the physicochemical and catalytic properties, a MPTMS: TEOS molar ratio of 0.1e0.2 was the optimum loading level of propylsulfonic acid group for the preparation of the NR/HMS-SO3H catalyst. Moreover, the NR/HMS-SO3H catalyst can be reused in the esterification at least 4 repetitions. Acknowledgments The authors gratefully acknowledge the financial support for this study from the Thailand Research Fund through the Royal Golden Jubilee Ph.D. Program (Grant no. PHD/0151/2551), the Center of Excellence on Petrochemical and Materials Technology (PETROMAT), Chulalongkorn University, and the PTT Public Co. Ltd. for partially support. The authors wish to thank Associate Professor Junko N. Kondo for suggestions, Mr. Hiroshi Yamazaki for the FTIR analysis and Mr. Ming Liu for the 29Si MAS NMR analysis. The authors also wish to express their thanks to Dr. Robert Douglas John Butcher (Publication Counseling Unit, Faculty of Science, Chulalongkorn University) for English language editing. References [1] L. Lianhua, L. Pengmei, L. Wen, W. Zhongming, Y. Zhenhong, Biomass Bioenergy 34 (2010) 496e499. [2] S.M. Son, H. Kimura, K. Kusakabe, Bioresour. Technol. 102 (2011) 2130e2132. [3] Q. Smejkala, J. Kolenab, J. Hanika, Chem. Eng. J. 154 (2009) 236e240. [4] B.M. Reddy, M.K. Patil, Chem. Rev. 109 (2009) 2185e2208. [5] A. Corma, Adv. Mater. 7 (1995) 137e144. [6] N. Mizuno, M. Misono, Chem. Rev. 98 (1998) 199e218. [7] K. Kun, R. Kunin, Polym. Sci. Part A 6 (1968) 2689e2701. [8] D.C. Kennedy, Ind. Eng. Chem. Prod. Dev. 12 (1973) 56e61. [9] M.A. Harmer, Q. Sun, A.J. Vega, W.E. Farneth, A. Heidekum, W.F. Hoelderich, Green Chem. 2 (2000) 7e14. [10] M.A. Harmer, W.E. Farneth, Q. Sun, J. Am. Chem. Soc. 118 (1996) 7708e7715. [11] A.D. Todd, C.W. Bielawski, Catal. Sci. Technol. 3 (2013) 135e139. [12] K. Nakajima, I. Tomita, M. Hara, S. Hayashi, K. Domen, J.N. Kondo, Catal. Today 116 (2006) 151e156. [13] I.K. Mbaraka, D.R. Radu, V.S.Y. Lin, B.H. Shanks, J. Catal. 219 (2003) 329e336. rez-Pariente, Microporous Meso[14] I. Díaz, F. Mohino, T. Blasco, E. Sastre, J. Pe porous Mater. 80 (2005) 33e42. [15] L. Sherry, J.A. Sullivan, Catal. Today 175 (2011) 471e476. [16] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548e552. [17] A. Stein, B.J. Melde, R.C. Schrodein, Adv. Mater. 12 (2000) 1403e1419. [18] W.M. Van Rhijn, D.E. De Vos, W.D. Bossaert, P.A. Jacobs, Chem. Commun. (1998) 317e318. [19] J.A. Melero, R. van Grieken, G. Morales, Chem. Rev. 106 (2006) 3790e3812.

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