Poly(styrene sulfonic acid)–grafted carbon nanotube as a stable protonic acid catalyst

Catalysis Communications 12 (2010) 217–221

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Short Communication

Poly(styrene sulfonic acid)–grafted carbon nanotube as a stable protonic acid catalyst Ke Liu a, Cuican Li a, Xiaohong Zhang b, Weiming Hua a, Dong Yang b,⁎, Jianhua Hu b, Yinghong Yue a,⁎, Zi Gao a a b

Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, PR China Department of Macromolecular Science, Fudan University, Shanghai 200433, PR China

a r t i c l e

i n f o

Article history: Received 15 July 2010 Received in revised form 2 September 2010 Accepted 4 September 2010 Available online 16 September 2010

a b s t r a c t Poly(styrene sulfonic acid)–grafted carbon nanotube was prepared and used as a solid acid catalyst in the alkylation of hydroquinone with tert-butanol. The material exhibits high activity as well as good stability as compared with other carbon-based solid acids. 73.3% of conversion and 53.7% of 2-TBHQ yield can be reached at 150 °C. © 2010 Elsevier B.V. All rights reserved.

Keywords: Carbon nanotubes Poly(styrene sulfonic acid) Alkylation of hydroquinone Stability

1. Introduction Acid-catalyzed reaction is one of the most important reactions for the production of chemicals. A significant number of acid-catalyzed reactions, such as Friedel–Crafts reaction, esterification, hydration and hydrolysis, are still carried out using liquid acids, like H2SO4 and HF, leading to a series of problems such as corrosion toward apparatus, environmental pollutions, and difficulties in separation of reactants and products. Replacing those unreusable and pollution-causing liquid acid catalysts by environmentally benign solid acid ones is the major trend. However, conventional solid acid catalysts have their own weakness, such as complexity of preparation, low catalytic activity, and poor stability [1–3]. Moreover, many solid acids lose their activities in water participating reactions because of easy chemisorption of water on their active sites. Therefore, water-tolerant solid acid catalysts with high efficiency are needed to be developed. Sulfonated carbon-based solid acid has attracted a lot of attentions as a new alternative material to liquid acid [4–6]. Such a material can be readily prepared by incomplete carbonization of sulfopolycyclic aromatic hydrocarbons [4] or sulfonation of incompletely carbonized organic compounds [5,6]. They have already exhibited high catalytic activity for various liquid-phase acid-catalyzed reactions, such as hydration of 2,3-dimethyl-2-butene, esterification of acetic acid and transesterification of triacetin with methanol [6,7]. However, the stability of this material is not satisfactory, especially in polar media, due to the leaching of polycyclic aromatic hydrocarbon-containing –

SO3H groups [7,8]. Furthermore, a large amount of concentrated sulfuric acid was employed in their preparation process, which is also harmful to the environment. Carbon nanotube (CNT) is of growing interest since its first synthesis by Iijima [9], because of its excellent physical and chemical properties, such as large surface area, high mechanical strength and stable structure [10]. However, carbon nanotube itself has little catalytic activity, thus modification with other functional groups is needed. Peng et al. had once synthesized a solid acid via directly sulfonating CNT [11], but the activity of obtained catalyst was not ideal as expected. Recently, a series of polymer-functionalized carbon nanotubes have been developed through in-situ radical polymerization in a poor solvent of polymer chains [12–14]. These materials have a lot of applications because of different organic groups on the CNT surface. Accordingly, sulfonic groups can be introduced onto the CNT surface by grafting poly-styrene sulfonic acid (PSA) using a similar method. In this work, PSA grafted CNT samples were prepared. Their textural, structural and acidic properties were characterized by N2 adsorption, SEM, FTIR, TGA and titration methods. The catalytic activity and stability of the samples towards alkylation of hydroquinone with tert-butanol were tested and compared with those of other sulfonated carbon-based solid acids. 2. Materials and methods 2.1. Materials

⁎ Corresponding authors. Tel.: + 86 2165642409; fax: + 86 2165641740. E-mail address: [email protected] (Y. Yue). 1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2010.09.010

The multi-walled carbon nanotubes (CNTs) with a purity of N99.5% (surface area 235 m2/g), prepared by the CVD method over an Fe/

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K. Liu et al. / Catalysis Communications 12 (2010) 217–221

Scheme 1. Synthesis of poly(styrene sulfonic acid)–grafted carbon nanotubes.

2.2. Catalyst preparation Poly(styrene sulfonic acid)–grafted CNT sample (PSA–CNT) was synthesized by in-situ radical polymerization following the procedures in the literature [12] except that methyl methacrylate was replaced by sodium styrene sulfonate. Typically, 2.0 g sodium styrene sulfonate and 50 mg CNTs were added into 100 ml deionized water. After 1 h treatment with low-energy ultrasonication, the solution was stirred vigorously for another 24 h. Then, 0.5 g persulfate was added as initiator. After 0.5 h of stirring, the solution was moved into a threeneck flask, heated to 70 °C for 3 h under protection of N2 flow, and subsequently kept for another 1 h at 85 °C. The obtained black precipitate was filtered, and dried at 100 °C overnight. The obtained sample, designated as PSS–CNT, was then ion-exchanged with a concentrated HCl solution (12 mol/l) overnight at 80 °C, filtrated through the PVDF membrane, washed by deionized water until no H+ was detected, and dried at 100 °C. The final product is designated as PSA–CNT. As a comparison material, directly sulfonated CNT, designated as SO3H–CNT, was prepared by adding 0.5 g CNT into 50 ml concentrated sulfuric acid and heated to 150 °C for 24 h. The suspension was filtrated, washed until no SO2− was detected, and dried at 100 °C. 4 Sulfonated carbon C–SO3H, was prepared according to the procedures in the literature [8]. 2.3. Characterization of catalysts The N2 adsorption/desorption isotherms were measured on a Micromeritics ASAP2000 instrument at a liquid N2 temperature. Specific surface areas of the samples were calculated from the adsorption isotherms by the BET method. Infrared spectra (FTIR) of the samples were recorded on a Cavatar-30 spectrometer. Thermal gravity analysis (TGA) was taken on a Rigaku Thermoflex thermal analysis system under a flowing N2 atmosphere at a heating rate of 10 °C/min. SEM studies were carried out with a Philips XL30 using an accelerating voltage of 15 kV. Sulfur content was measured by element analysis on a VARIO EL3 elemental analyzer. 2.4. Acidity measurement The density of surface acid sites was measured by a neutralization titration method [15]. Briefly, the sample was added into an aqueous solution of NaCl (in excess), and HCl formed due to the exchange of Na+ with proton on sulfonic groups was titrated by a standard solution of NaOH. The acidity was also measured by means of potentiometric titration [16,17]. The solid was suspended in acetonitrile, agitated for 3 h, and then titrated with 0.1 mol/l butylamine in acetonitrile. The electrode potential variation was recorded with a METTLER TOLEDO FE20 potentiometer.

2.5. Catalytic testing Alkylation of hydroquinone was carried out in a stainless steel autoclave with PTFE liner, using magnetic stirring. Typically, 0.5 g hydroquinone, 1.0 g tert-butanol and 0.2 g catalyst were added in the autoclave accompanied with 2 g xylene as solvent. The reaction lasts 4 h. The products were analyzed with a gas chromatograph equipped with a SE-54 capillary column (30 m × 0.25 mm × 0.3 μm) and a flame ionization detector. 3. Results and discussion 3.1. Structural characterization Poly(styrene sulfonic acid)–grafted carbon nanotubes (PSA–CNTs) were synthesized by in-situ radical polymerization of sodium styrene sulfonate, followed by ion exchanging with HCl solutions, as shown in Scheme 1. The PSS grafting densities of the prepared samples can be measured by weight loss from 280 to 700 °C on TG curves, which was illustrated in Fig. 1. It can be seen that the more sodium styrene sulfonate added during the preparation, the higher the grafting density obtained. However, taking into account the preparation efficiency and atom economy, the amount of sodium styrene sulfonate added was fixed as 2.0 g versus 50 mg CNT in the following study. Under this condition, the grafted PSS is about 7.7 wt.%. The SEM images of the CNT materials before and after polymer modification are shown in Fig. 2. Significant changes can be observed for the morphology of CNTs. The pristine CNT was piled disorganized and entwined without any order. But after grafting, PSS–CNT is showing an overall structure as sheets, and the arrangement is oriented towards a certain direction. The reason can be attributed to grafted polymers, which acted as bridges between CNTs, and limited the direction of arrangement of CNTs on some scale.

100

(a) 98

Ratio of Weight (%)

Al2O3 catalyst, were obtained from Chengdu Organic Chemical Co. Ltd. The inner diameter of CNTs is 3–10 nm. Sodium styrene sulfonate is purchased from Aladin Reagent Co. Ltd. and persulfate from Sinopharm Chemical Reagent Co. Ltd. All chemicals were used as purchased.

(b) (c) (d)

96

94

92

90 100

200

300

400

500

600

700

Temperature (oC) Fig. 1. TG curve of (a) pristine CNT and PSS–CNTs derived from the PSS:CNT ratio of (b) 10; (c) 20; and (d) 40.

K. Liu et al. / Catalysis Communications 12 (2010) 217–221

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Fig. 2. SEM images of (a) (c) pristine CNT and (b) (d) PSS–CNT.

Transmittance (%)

Fig. 3 illustrated the FTIR spectra of the materials before and after polymer modification, together with that of pure PSA for comparison. In contrast to the pristine CNT, on which no obvious absorbance was observed except for tiny peaks due to residual H2O, PSA–CNT displayed an IR spectrum with a broad band at 1500–1600 cm−1, characteristic of vibrations for benzene ring. The peaks at 540 cm−1, 650 cm−1, 1030 cm−1 and 1190 cm−1 confirmed the existence of a sulfonic group. In addition, the C–H stretch absorption peaks of methylene at 2850–3000 cm−1 and O–H stretch vibration of the sulfonic group (–SO3H) at ~ 3400 cm−1 are also detected on the IR spectrum of PSA–CNT. Peaks at 2300–2400 cm−1 are due to CO2 in the air, which has nothing to do with the grafted polymer. All above were

(a) (b)

(c)

consistent with the FTIR spectra of the pure PSA sample, indicating the successful grafting of PSA onto the surface of CNTs. The BET surface area of PSA–CNT is given in Table 1. The grafted sample maintains a large surface area as compared with other carbonbased catalysts, though a bit lower than the pristine CNT.

3.2. Catalytic activity 2-tert-butylhydroquinone (2-TBHQ) is an excellent antioxidant widely used in food industry, which is conventionally prepared by the alkylation of hydroquinone with tert-butanol or isobutylene [18–22]. The activity of PSA–CNT for alkylation of hydroquinone was tested and compared with those of other carbon-based catalysts, such as SO3H–CNT and C–SO3H. The results are listed in Table 1. During the reaction, 2,5-di-tert-butyl-hydroquinone (2,5DTBHQ), 2,5-di-tert-butyl benzoquinone (2,5-DTBBQ) and tertbutyl phenol ether (TBPE) were formed besides the aimed product 2-TBHQ. PSA–CNT exhibits high hydroquinone conversion as well as high 2-TBHQ yield, very close to that of C–SO3H, which is regarded as an excellent solid acid in liquid-phase reactions. Since the total acid sites on PSA–CNT are only 29% of those on C–SO3H, the TOF on PSA–

Table 1 Surface area and catalytic activity of carbon-based solid acids. Catalyst

3500

3000

2500

2000

1500

1000

500

Wavenumbers (cm-1) Fig. 3. Infrared spectra of (a) pristine CNT; (b) PSA–CNT; and (c) pure PSA.

Surface area Conversion Selectivity (%) (m2/g) (%) 22,5TBHQ DTBHQ

PSA–CNT 217 SO3H–CNT 194 C–SO3H 5

73.3 16.7 81.0

73.2 9.4 70.1

17.2 0.0 25.4

2,5DTBBQ

TBPE

4.2 0.6 1.0

5.6 90 3.5

Yield (%)

53.7 1.6 56.8

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K. Liu et al. / Catalysis Communications 12 (2010) 217–221

Table 2 Thermal stability of the PSA–CNT catalyst. Treating condition

Conversion (%)

Fresh 200 °C 300 °C 400 °C

73.3 65.7 25.7 3.3

Selectivity (%) 2-TBHQ

2,5-DTBHQ

2,5-DTBBQ

TPBE

73.2 77.1 47.4 39.9

17.2 10.9 17.0 6.9

4.2 5.4 10.1 12.7

5.6 6.6 25.5 40.6

Yield (%) 53.7 50.7 12.2 1.3

CNT is much higher. Meanwhile, PSA–CNT derivate from polymer grafting is far more active than SO3H–CNT prepared by direct sulfonation, not only in conversion but also in selectivity, which indicates that PSA–CNT possesses more acid sites with stronger acid strength, since the selectivity in this reaction is related to the acid strength of the catalyst. The lower acid strength causes the higher percentage of TBPE products. 3.3. Thermal stability The thermal stability of PSA–CNT catalyst was tested by calcinating the catalyst at different temperatures for 4 h. The results are summarized in Table 2. The activity of the catalyst remains almost unchanged after calcinating at 200 °C as compared with the fresh one. However, when the calcination temperature rises to 300 °C, a sharp decrease in activity is observed. Nearly no activity is detected while the calcination temperature reaches 400 °C. The results indicate that the temperature tolerance of PSA–CNT catalyst is between 200 °C and 300 °C, since decomposition of polymers on CNTs occurred at that temperature, which can be confirmed by the TG curve of PSA–CNT shown in Fig. 1. 3.4. Reusability of the catalyst The reusability of PSA–CNT catalyst was also tested. After reactions, the PSA–CNT was filtered off, washed with ethanol, dried in an oven at 100 °C and then reused in the reaction. The reaction data for the first three runs are listed in Table 3. A slow deactivation is observed in these cycle processes, the conversion of hydroquinone drops from 73.3% to 56.6%, while the selectivity of 2-TBHQ remains almost unchanged. However, comparing with the C–SO3H catalyst, whose hydroquinone conversion drops from 91 to 48% after three runs [8], the PSA–CNT catalyst is much more stable and reusable. Interestingly, after being stirred in diluted sulfuric acid at 60 °C, the catalytic activity of the used catalyst can be restored almost to the initial level.

number of the acid sites. On the other hand, these acid sites can be classified by their acid strength according to the following scale: E N 100 mV (very strong), 0 b E b 100 mV (strong) and −100 mV b E b 0 (weak) [16,17]. Fig. 4 shows the titration curves of the catalysts. The number of the acid sites was evaluated and listed in Table 4. The amount of the acid sites decreases markedly after the first run and then slowly afterwards. After 3 reaction runs, the total acid amount goes down from 0.343 to 0.212 mmol/g catalyst, which is in parallel with the results of acid–base titration (from 0.347 to 0.236 mmol/g). The decrease in the total acid amount was mainly caused by the reduction of the acid sites with strong acid strength (E N 100 mV), which indicates that the poison of the strong acid sites may account for the decrease of the activity of the PSA–CNT catalyst during the reactions, since the alkylation is a typical acid-catalyzed reaction. It was also proved by the fact that the activity can be restored by the recovered acidity (up to 90% of original one) after the regeneration. The probable reason for the loss of the acid sites may come from two aspects: leaching or poison of the active sites. Elemental analysis of the fresh and used catalysts shows that the weight content of S in the PSA–CNT catalyst decreases from 1.23% to 1.04% after three runs, indicating that the leaching of the SO3H group existed during the reaction. An observation by UV–vis shows that no leaching polymer peaks (~ 280 nm) were detected in the solution after the reaction, which means that the leaching of S comes from the desulfonation reaction of PSA, not from the leaching of PSA itself. However, this decrease (~15%) is much lower than the reduction of the acid amount, which is over 30%, showing that not all the catalytic activity reduction was caused by the leaching of the SO3H group. PSA–CNT catalyst retrieved after a cycle of reaction was filtered off, washed repeatedly in ethanol, and dried in an oven at 100 °C until the weight kept constant. A slight increase of weight (~5 wt.%) was observed, showing that a small amount of tar or polymeric compounds was deposited on the catalyst after the first run, which is probably another cause for the reduction in catalytic activity. A similar phenomenon was observed in the same reaction over MSUS(BEA) catalysts [20]. Since sulfonic acid is such a strong acid, the leaching of the sulfonic group into the liquid phase may lead the heterogeneous reaction to a homogenous one. To see whether this happens, an experiment was carried out as follows: the alkylation was carried out for the first 2 h catalyzed by PSA–CNT and then went on for the next 2 h without PSA– CNT. The results show that the conversion did not increase obviously without PSA–CNT in the solution containing leaching materials. So the alkylation catalyzed by the PSA–CNT catalyst is exactly a

3.5. Characterization of the reused catalyst To envision the reusability of PSA–CNT catalyst in the long term, the acidity of the catalyst after each reaction run was characterized by means of potentiometric titration with n-butylamine. It was suggested that the initial electrode potential indicated the maximum acid strength of the acid sites, and the value of n-butylamine consumed, where the plateau was reached, indicated the total

Table 3 Reusability of the PSA–CNT catalyst. Catalyst

1st run 2nd run 3rd run Regenerateda a

Conversion (%)

Selectivity (%) 2-TBHQ

2,5-DTBHQ

2,5-DTBBQ

TPBE

73.3 61.4 56.6 70.2

73.2 75.9 71.8 70.3

17.2 8.2 6.8 11.3

4.2 5.7 5.7 11.6

5.6 10.2 15.7 5.8

Washed in 30% diluted sulfuric acid at 60 °C.

Yield (%) 53.7 46.6 40.6 49.4 Fig. 4. Potentiometric titration curve of various PSA–CNT catalysts.

K. Liu et al. / Catalysis Communications 12 (2010) 217–221

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Table 4 Acidity of the PSA–CNT catalyst obtained by titration. Catalyst

Fresh catalyst After 1st run After 2nd run After 3rd run Regenerated a b c

Acid site (mmol/g)a E N 100 mV

0 b E b 100 mV

−100 mV b E b 0 mV

Total

0.191 0.063 0.049 0.046 0.173

0.023 0.025 0.032 0.026 0.023

0.129 0.131 0.148 0.140 0.111

0.343 0.219 0.229 0.212 0.307

Acid site (mmol/g)b Total 0.347 –c –c 0.236 –c

By potentiometric titration. By acid–base titration. Not measured.

heterogeneously catalyzed reaction and leaching sulfur content has a negligible catalytic activity. 4. Conclusions Poly(styrene sulfonic acid)–grafted carbon nanotubes (PSA–CNTs) were synthesized by in-situ radical polymerization of sodium styrene sulfonate and were used as a protonic acid catalyst in the alkylation of hydroquinone with tert-butanol. High activity as well as good stability can be achieved over this catalyst. A slight decrease of activity was observed during the recycling of the catalyst, which may be due to leaching of the sulfonic group and deposition of organic compounds. However, the stability of the catalyst is much higher than that of the sulfonated polyaromatic carbon catalyst. Moreover, the activity can be recovered mostly by treating the deactivated catalyst in diluted sulfuric acid at 60 °C. Acknowledgements This work was supported by the Chinese Major State Basic Research Development Program (2006CB806103), the National Natural Science Foundation of China (20633030, 20773027 and 20773028) and the Science and Technology Commission of Shanghai Municipality (08DZ2270500).

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