Mesoporous silica supported water-stable perfluorobutylsulfonylimide and its catalytic applications in esterification

Microporous and Mesoporous Materials 172 (2013) 51–60

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Mesoporous silica supported water-stable perfluorobutylsulfonylimide and its catalytic applications in esterification Qiu-Hong Yang a, Zhong-Hua Ma a,b,⇑, Jing-Zhong Ma a, Jin Nie b,⇑ a b

Department of Chemistry, College of Sciences, Huazhong Agricultural University, Wuhan 430070, PR China School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China

a r t i c l e

i n f o

Article history: Received 16 September 2012 Received in revised form 7 January 2013 Accepted 8 January 2013 Available online 16 January 2013 Keywords: Hybrid Brønsted acid Perfluorobutylsulfonylimide Water-stable catalysts Esterification

a b s t r a c t A new water-stable perfluorobutylsulfonylimide-functionalized acidic silica (PSFSI-MSMA15/SiO2) has been successfully prepared by copolymerization of 4-styrenesulfonyl(perfluorobutylsulfonyl)imide (1) with 3-(trimethoxysilyl)propyl methacrylate (MSMA) followed by condensation with tetraethyl orthosilicate under sol–gel conditions. N2 isotherm and TEM image showed that the material has high BET surface areas, large pore volumes, and abundant mesoporosity. Acid–base titration analysis gave the acid loading capacities of 0.66 mmol/g. The TG curves and XPS spectra exhibited good thermal stability (<210 °C) and hydrothermal stability (only ca. 5% H+ leakage observed) of the material. The material had acid-strength stronger than that of HZSM-5 determined by solid state 13C NMR spectrum combined with acetone-2-13C molecular probe technology. PSFSI-MSMA15/SiO2 performed effective in direct esterification of carboxylic acids and alcohols, recycling at least seven run without obvious loss of activity in high reaction temperature. The catalyst’s stability towards leaching and the effects of the composition and structure on the catalytic activity are discussed. Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction There is strong environmental as well as economic driver to use solid catalysts instead of liquid catalysts [1]. The reuse of the solid catalysts is governed by their deactivation, poisoning, and the extent of leaching in the reaction medium, for example, the H+ leakage of solid acid in water-containing system. Some reported solid acids can meet the requirements of both efficiency and stability [2–5]; nevertheless, the development of new water-stable solid acids is still the research focus in both academy and industry. In the design of solid acid, the effects of the roles of the acid sites and the framework structural factors on increasing the catalytic activity have been discussed by many literatures. Due to the powerful conjugative effect and electron-withdrawing properties of perfluoroalkylsulfonyl group (RfSO2–, Rf = fluorinated groups) significantly increasing the acidity [6,7], the perfluoroalkylsulfonyl-containing acid sites are considerable fascinating. For example, CF3SO3H is well-known super acid and (C4F9SO2)2NH holds the record as the strongest measured gas-phase superacid [6]. Bis[(per-

⇑ Corresponding authors. Address: Department of Chemistry, College of Sciences, Huazhong Agricultural University, Wuhan 430070, PR China. Tel.: +86 27 87288247; fax: +86 27 87282133 (Z.-H. Ma), tel.: +86 27 87543232; fax: +86 27 87543632 (J. Nie). E-mail addresses: [email protected] (Z.-H. Ma), [email protected]. edu.cn (J. Nie). 1387-1811/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2013.01.008

fluoroalkyl)sulfonyl]imide, (RfSO2)2NH, is thought particularly versatile and nice alternative of sulfonic acid group in many application fields including catalysis [5,8–12]. It can be prepared in a large variety, easily incorporated into polymeric systems, and used as a candidate of water-stable catalyst [5,12]. On the other hand, the framework structural factors, such as hydrophobic feature, specific surface area, pore size, would largely influence the catalytic activity and selectivity. Mesoporous silica has been widely used in catalysts due to their good thermal stability and adjustable mesopores (2–50 nm). However, the intrinsic hydrophilic properties of silica backbones must be reformed to inhibit catalyst deactivation in water-containing system [13,14]. Acid-catalyzed esterification of carboxylic acids with alcohols is a typical reaction in which the products and reactants are in equilibrium, and water impedes the equilibrium shift to the ester formation [1,3]. A stoichiometric amount of water as byproduct easily coadsorbs near the acid sites, resulting in their partial deactivation and/or decomposition [2]. Therefore, some heterogeneous sulfonic acid-catalyzed systems have proved efficient to address the issue [2,14–18], and several hydrophobic polymer-based perfluoroalkylsulfonimides with high catalytic activity have been also reported by us [5,10]. The hydrophobicity of the solid acids was favorable for achieving excellent catalytic performance. In the present work, we bonded a (perfluoroalkyl)sulfonyl]imidecontaining prepolymer, copoly[4-styrenesulfonyl (perfluorobutylsulfonyl)imide-3-(trimethoxysilyl) propyl methacrylate]

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Scheme 1. Synthesis route of SiO2-supported copoly[4-styrenesulfonyl(perfluoro-butylsulfonyl)imide-3-(trimethoxysilyl)propyl methacrylate] (PSFSI-MSMA15/SiO2).

(2) to the mesoporous silica to develop a new water-stable catalyst (noted as PSFSI-MSMA15/SiO2, Scheme 1). It was conceivable that incorporation of the organic propyloxycarbonyl propylene groups (–CH2CH2CH2OC(O)C(CH3)@CH2), which was introduced from 3(trimethoxysilyl)propyl methacrylate (MSMA), might provide hydrophobic features for the hydrophilic silica backbone while covalently attach the perfluoroalkylsulfonimide groups. PSFSIMSMA15/SiO2 was found to catalyze efficiently the condensation of carboxylic acids and alcohols in higher temperature and recycle stably without obvious activity loss. 2. Experimental 2.1. Materials All solvents (A.R.) for synthesis were commercially available in China and pretreated before used. Sodium-4-styrenesulfonate (industrial grade) was obtained from XZL Chemicals Co., Ltd. (China) and dried beforehand. NKC-9 resin, purchased from Nankai Chemical Plant, was soaked in ethanol, which was renewed every 5 h until it looked colorless. The resin was acidified with 5–10% of HCl and then scrubbed with water until no Cl ion could be detected (acid loading = 4.74 mmol/g). 2.2. Characterization of catalyst FT-IR and pyridine-FT-IR spectra were conducted by using an Avatar 330 Fourier Spectrometer with the KBr pellet technique. Prior to pyridine-FT-IR analysis, the dried samples were placed in a dry flask to absorb pyridine adequately under vacuum at 50 °C, and N2 flow was used to remove excessive absorbed pyridine. The solid-state 13C NMR experiments were performed at 9.4 T on a Varian Infinityplus-400 spectrometer using 7.5 mm probes under magic-angle spinning. The chemical shifts were referenced to hexamethylbenzene. GC–MS was recorded on SATURN2200 spectrometer equipped with a VF-5 column and electron impact source. TEM was performed with a FEI Tecnai G20. SEM picture of the sample surface was obtained by using Sirion TMP (FEI Company) fieldtransmitting scanning electron microscope equipment and the sample surface was decorated by a thin gold layer with a 682 de-

vice (Gatan Inc.) before imaging. Thermo gravimetric analysis (TGA) were carried out on a TG209 (Netzsch) instrument under air atmosphere from room temperature to 800 °C at a heating rate of 20 °C/min. X-ray photoelectron spectra (XPS) were measured on a VG Multilab2000 X-ray spectrometer equipped with a hemispherical electron analyzer and an Al anode X-ray exciting source (AlKa energy = 1486.6 eV). As common practice, the C1s peak at 284.6 eV is used as a reference for charge correction. The textural properties of the sample were measured using the BET procedure. Nitrogen adsorption–desorption isotherms were taken at 196 °C using a Zeton Altamira AMI-200 system. The surface area and pore volume/pore size distribution were calculated by the BET and BJH methods, respectively, and desorption branch was used. The sample was degassed at 150 °C for 6 h before measurement. 2.3. Synthesis of SiO2-supported copoly[4-styrenesulfonyl(perfluorobutylsulfonyl)imide-3-(trimethoxysilyl)propyl methacrylate] (PSFSIMSMA15/SiO2) The synthesis route of PSFSI-MSMA15/SiO2 is shown in Scheme 1. Sodium 4-styrenesulfonyl(perfluorobutylsulfonyl)imide (1) was prepared according to literature methods [19]. 2.3.1. Synthesis of copoly[sodium 4-styrenesulfonyl(perfluorobutylsulfonyl)imide-3-(trimethoxysilyl)propyl methacrylate] (3) The method used to synthesize the prepolymer 2 was based on the procedure reported by Wei and co-workers [20,21]. In a typical synthesis, 2 g of sodium salt monomer 1 (4.1 mmol) was dissolved in 3 mL methanol, mixed with 0.1796 g of MSMA (0.724 mmol). To the mixture, 1 mL of methanol solution containing 0.0224 g of AIBN was added at 70 °C under stirring. The copolymerization was performed for 3 h. As a result, a swellable solid could be obtained. Washed with methanol to remove the unconverted monomer and dried under vacuum, yellow prepolymer 2 was obtained (crude yield: 84%). The prepolymer 2 was sealed in a 30 mL Teflon-lined stainless steel autoclave and heated at 200 °C for 24 h to be dissolved. The insoluble residues were removed by filtration. The resulting solution was concentrated to about 10 mL and subsequently hydrolyzed at 40 °C for 45–50 min with 2 mL of 0.048 mol/L HCl. The solution of 3 was obtained for following use.

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2.3.2. Two-step sol–gel preparation of PSFSI-MSMA15/SiO2 An adapted method suggested by Harmer was adopted to prepare the solid acid [22]. A clear and homogenous sol solution was obtained by adding 800 lL of 0.048 mol/L HCl to 7.072 g of Si(OEt)4 (34 mmol). To the sol solution, the prepared solution 3 was rapidly added under vigorous stirring, and then a solution of 2.5 wt.% NH3
H2O dropwise. Within about 10 s, the solution gelled to form an elastic solid mass, which was slightly opaque. The solid gel was aged at 45 °C for 6 h in an oven, and then dried at 120 °C for 8 h. The product was ground into fine powders, passed through a 10-mesh screen, and then acidized with 50 mL of 6 mol/L HCl for 10 h. The residual HCl and the soluble materials were removed by water washing and further Soxhlet extraction for 24 h. Finally, the product was dried under vacuum at 120 °C for 8 h, yielding a hard material with a weight of 3.15 g compared to a theoretical weight of 4.11 g (assuming complete conversion to SiO2 (2.04 g)), which was designated as PSFSI-MSMA15/SiO2. The stability of PSFSI-MSMA15/SiO2 in boiling water was also tested to see if the perfluoroalkylsulfonimide groups tend to leach. The sample was boiled in distilled water for 24 h, and then filtered, dried, and characterized by TG and XPS (noted as refluxed PSFSIMSMA15/SiO2). The data were compared with that of the untreated sample. 2.4. Measurement of acid contents for PSFSI-MSMA/SiO2 The acid-exchange capacity was determined by titration with NaOH [23]. Typically, 0.3 g of solid was added to 50 mL of 1 mol/ L NaCl solution. The suspension was stirred at room temperature for 24 h until equilibrium was reached, and then filtrated. A precise amount of filtrate (20 mL) was subsequently titrated by dropwise addition of 0.0193 mol/L NaOH solution with phenolphthalein as indicator. The conclusive loading of the PSFSI-MSMA/SiO2 was determined by three parallel experiments. 2.5. Catalytic reactions Five parallel reactions were carried out in the presence of PSFSIMSMA/SiO2 at 120–125 °C, using 0.122 g of benzoic acid (1 mmol) and 0.148 g of butanol (2 mmol). These reactions were quenched in turn with 1 mL of acetone every other hour. Careful filtration was performed and the catalyst was washed thoroughly with acetone. The combined washings and the filtrate were transferred into a 10-mL volumetric flask, and a solution of the internal GC standard (200 lL of 0.1085 mol/L decane) in acetone was also added. The 10 mL of constant volume solution was analyzed by GC and GC– MS to calculate the yields and characterize molecular weight of the products, respectively (see supporting materials). The recovered catalyst was washed with acetone and dried at 100 °C under vacuum and reused for the next run. Another reaction was carried out under the optimized conditions to calculate the isolated yield. After dried with anhydrous magnesium sulfate, the solution was distillated under reduced pressure with a water pump to drive off excess butanol. Butyl benzoate was then obtained by reduced pressure distillation with oil pump (b. p. 148 °C, 11 mmHg). 3. Results and discussion 3.1. Heterogenization of perfluorobutylsulfonylimide The reported catalytic activity of perfluoroalkylsulfonylimide composite materials, which acted only in non-proton organic solvent system, excited our interest [9,19,24,25]. The purpose of this work is to provide a hybrid solid acid which is stable against rinsing in polar proton solvents (alcohol, water, etc.).

Table 1 Acid exchange capacity for various acid composites. Entry

1 2 3 4

Sample

PSFSI-VTES15/SiO2b PSFSI -VTES50/SiO2 PSFSI -MSMA15/SiO2 PSFSI -MSMA15/SiO2d

Acid loading/mmol/g Measured

Calculated

0.18 0.07 0.66 (0.62c) 0.59c

0.68 0.68 1.04 1.04

Loading ratio/% a

26 10 63 57

a

Calculated acid sites = mole number of PSFSI/(total mass of SiO2 and PSFSI). Reaction conditions: initiator AIBN (5 mol%, relative to sodium salt 1), stirred at 65–70 °C for 24 h. c The acid loading determined quantitatively by TGA method. d Refluxed in water for 24 h. b

Two silane species employed in the synthesis of precursor 2 include vinyl triethoxysilane (VTES) and 3-(trimethoxysilyl)propyl methacrylate (MSMA). Both of them are trialkoxysilyl- and vinylcontaining simultaneously, and thought as hydrolyzable and polymerizable silanes. The structure of VTES is less complex than that of MSMA, and the latter has a propyloxycarbonyl spacer group. When VTES was used to copolymerize with monomer 1, more than 5 mol% initiator (based on the concentration of monomer 1) and a 24-h reaction period were necessary to react smoothly. Only 0.18 mmol/g of acid loading was achieved if 15% mmol of VTES was cast (noted as PSFSI-VTES15/SiO2). The acid contents decreased with increasing amount of VTES, as shown in Table 1. That is, using more VTES was unhelpful. The reason for low acid contents of PSFSI-VTES15/SiO2 was related with the poor free radical polymerizability of VTES caused by dp-pp interactions between the Si atom and the vinyl group [26]. Contrarily, the Si atom within MSMA is away from the vinyl group, which will enhance the reactivity of the monomer due to the absence of dp-pp interactions [27]. For the conversion to prepolymer 2, 3 mol% initiator and a 3-h polymerization period were enough. Thus, the reactivity of MSMA was indeed different from that of VTES. As results, much stronger characteristic peaks and much more mass-loss were observed for PSFSI-MSMA15/SiO2, respectively in FT-IR spectra (Fig. 1(a, d)) and TG curves (Fig. 2(a, c)). If fully mixed with water and then filtrated, PSFSI-VTES15/SiO2 was expectably muddy, but PSFSI-MSMA15/SiO2 yet remained dry. Both the materials were composed of similar hydrophilic inorganic frameworks, and the hydrophilic acid groups must further enhance the hydrophilicity. The different observations on the hydrophobicity between PSFSI-MSMA15/SiO2 and PSFSI-VTES15/ SiO2 could be due to the presence of organic –COOCH2CH2CH2– and additional –CH3 groups of MSMA. It suggested that the two organic groups tuned hydrophilic-hydrophobic balance of the framework, thereby possibly influenced the catalytic activity. More important thing is that the obtained acid contents of PSFSIMSMA15/SiO2 would better meet the demands for catalysis, as described in the following items. 3.2. Acidic contents and acid strength of PSFSI-MSMA15/SiO2 The acid contents of PSFSI-MSMA15/SiO2 are listed in Table 1. Concentrated NaCl solution was used in ion exchange with the strong acid sites of hybrid material to obtain effective acid loading capacities considering the weak acidity of terminal silanols. The tested acid contents were 0.66 mmol/g. Compared to a calculated maximum value (1.04 mmol/g), 63% of PSFSI was combined into the hybrid material. The acid contents were sufficient to produce obvious catalytic effect. Solid-state NMR spectroscopy combined with molecular probe technology has proven to be a powerful tool for characterizing

1544

1353 1325

60

1353 1325

(a)

(a) 70

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Q.-H. Yang et al. / Microporous and Mesoporous Materials 172 (2013) 51–60

1448

54

1174 1138

50

1138

1174

(b)

Relative absorbance/%

(e) 40 (c) 30 (f)

(d) 20

10

0 (a) fresh PSFSI-MSMA15/SiO2 -10

(b) refluxed PSFSI-MSMA15/SiO2

(a) fresh PSFSI-MSMA15/SiO2

(c) recovered PSFSI-MSMA15/SiO2

(e) pyridine-PSFSI-MSMA15/SiO2

(d) fresh PSFSI-VTES15/SiO2

(f) pyridine-SiO2

-20 1600

1500

1400

1300

1200

1100 1600

1500

1400

1300

1200

-1

Wavenumbers/cm

Fig. 1. FT-IR spectra of: (a) fresh PSFSI-MSMA15/SiO2; (b) refluxed PSFSI-MSMA15/SiO2; (c) recovered PSFSI-MSMA15/SiO2; (d) fresh PSFSI-VTES15/SiO2; (e) pyridine-PSFSIMSMA15/SiO2 and (f) pyridine-SiO2.

105 100

o

212 C

95 90

Mass/%

85 80

(a)

75 70 65

(a) fresh PSFS-VTES15/SiO2

60

(b) refluxed PSFSI-MSMA15/SiO2

55

(c) fresh PSFSI-MSMA15/SiO2

50

(d) (b) (c)

(d) recovered PSFSI-MSMA15/SiO2 0

100

200

300

400

500

600

700

800

o

Temp. / C Fig. 2. TGA analysis of samples: (a) fresh PSFSI-VTES15/SiO2; (b) refluxed PSFSI-MSMA15/SiO2; (c) fresh PSFSI-MSMA15/SiO2 and (d) recovered PSFSI-MSMA15/SiO2.

surface acidity of solid acid catalysts, [28–30]. And acetone-2-13C is a reliable probe molecule for determining the relative acid strength of various solid acids. The stronger the Brønsted acidity, the stronger the hydrogen bonding between the carbonyl carbon of acetone and the acidic proton, and consequently the more downfield of 13C isotropic chemical shift of the carbonyl carbon. 13C CP/MAS spec-

trum of acetone-2-13C adsorbed on PSFSI-MSMA15/SiO2 is shown in Fig. 3, consisting of three groups of resonance peaks. The 231.0 and 219.1 ppm signals are caused by the carbonyl carbon of acetone interacting with Brønsted acid sites. That is, there are two types of acid sites. The 219.1 ppm signal is due to the relatively weak acid sites, which is thought as the peak of the physically

55

200

150

100

17.6 8.4

30.1

59.7

117.7

47.6

138.4 128.7

250

152.4

177.4

208.4

231.0

219.1

Q.-H. Yang et al. / Microporous and Mesoporous Materials 172 (2013) 51–60

50

0

ppm Fig. 3.

13

C CP/MAS spectrum of acetone-2-13C adsorbed on PSFSI-MSMA15/SiO2.

absorbed acetone-2-13C near the silanol groups [28]. And the 231.0 ppm signal is weak but clear, which corresponds to the low concentration of stronger acid sites. It is reasonally due to the peak of acetone-2-13C adsorbed on perfluorobutylsulfonylimide groups (SO2NHSO2C4F9). 208.4 ppm is ascribed to the free acetone. According to the results, the acid strength of PSFSI-MSMA15/SiO2 is stronger than that of organosulfonic acid-functionalized mesoporous silica MSU-SO3H and microporous zeolites HZSM-5, reported at 217.2 and 223 ppm respectively [28–31], but still weaker than that of 100% H2SO4 (244 ppm), a proverbial superacid. The decrease of the acid strength of PSFSI-MSMA15/SiO2 compared with that of (C4F9SO2)2NH is expected due to the chemical structural change of electron-withdrawing substituents in the acid functional group. In addition, the peaks between ca. 152.4–117.7 ppm can be ascribed to benzene rings and –C4F9 groups, and 177.4 ppm definitely to the polymerized carbonyl carbon of –COO(CH2)3– groups according to analyses by Ford [32]; the peaks between ca. 59.7– 8.4 ppm arise from carbon chain formed by polymerization of double bonds and –CH2CH2CH2–groups.

3.3. Characterization analysis The nitrogen adsorption–desorption isotherm of PSFSIMSMA15/SiO2 is of IV of IUPAC classification, indicating the

formation of mesoporosity (Fig. 4). And the isotherm shows a broad hysteresis loop at low relative pressure (0.45–0.95). The kind of loop is associated with porous materials that consist of agglomerate or packing of irregular and nonuniform spheres. Therefore, PSFSI-MSMA15/SiO2 has relatively broad pore size distributions. The dominant pores are in the mesopore range with a narrow peak at 3.8 nm and a second peak at 4.9–6.6 nm (inset), and a filling phenomenon due to microporosities are also observed in Fig. 4, which should be formed when the solid gel was aged and dried for gas emissions and backbone contraction. A high BET surface area (233 m2/g) and large pore volume (0.38 cm3/g) are observed. All the properties are comparable with those of reported acidic analog PSFSI/SBA-15 (5.3 nm, 178 m2/g, 0.19 cm3/g) [19]. PSFSI-MSMA15/SiO2 was identified with FT-IR and pyridine-FTIR. The FT-IR spectra from 1600–1100 cm 1 are shown in Fig. 1. It can be clearly seen that the characteristic absorption bands of the – SO2– linkage (tas 1325, ts 1174 cm 1) are in Fig. 1(a) [10,33]. Peaks at 1353 and 1138 cm 1 can be assigned to the asymmetric and symmetric C–F stretching vibration respectively [33]. The pyridine-FT-IR spectrum shows that PSFSI-MSMA15/SiO2 presents obvious Brønsted acid sites as well as Lewis acid sites (Fig. 1(e)). The intensive band at 1544 cm 1 and weaker one at 1448 cm 1 are assigned to Brønsted acid sites and Lewis acid sites, respectively. Another intensive band at 1492 cm 1 is attributed to a combination signal associated with both Brønsted acid and Lewis acid sites.

Volume/cm g

-1

150

3.83

0.015 0.010

4.89 6.55

200

Desorption, Dv/d

250

0.005 0.000 0

5 10 15 20 Pore diameter/nm

100

50 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Relative pressure, P/P0 Fig. 4. Nitrogen adsorption–desorption isotherms and the pore size distribution by BJH method from desorption branch (in inset) of PSFSI-MSMA15/SiO2.

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X-ray photoelectron spectroscopy analysis (XPS) and deconvolution results are shown in Fig. 5. The N1s (398.9 eV) and S2p (168.8 eV) binding energy is weak but clear. The curve-fitting of N1s was deconvoluted into two-type (399.0 (SO2–N), 401.6 (C@N, originated from AIBN)). The peak at 285.5 eV and vicinal weak peak at 292.2 eV of C1s signature the non-fluorinated hydrocarbonyl and –CF2CF2CF2CF3 groups, respectively. The curve-fitting of raw non-fluorinated hydrocarbonyl line was also deconvoluted into three-curve type (288.0(C@O), 285.9 (C–O), 284.6 (C–C, C–H)). Correspondently, F1s peak appears at 688.3 eV. The peak found at 154.0 eV is attributed to Si2s while the peak at 103.3 eV is owed to Si2p. The O1s binding energy is obviously found at 532.6 eV, and the curve-fitting was deconvoluted into six-curve type (533.8 (O–C@O⁄), 533.1 (O⁄–C = O), 532.8 (Si–O–Si), 532.0 (S@O), 530.5 (Si–O–C), 528.4 (maybe adsorbed oxygen)). The data show that PSFSI has been indeed supported. SEM and TEM images are shown in Fig. 6. The SEM image shows the non-uniform particles with a range of 3–10 lm in diameter. Some other small particles about 1 lm in diameter dispersing on the external surface can be observed. TEM image clearly shows

400000

that a large of disordered pores can be observed, and the integrity mesoporous structure is irregular. According to the given scale by the test system, the measured pore diameter range is between ca. 4.6–7.3 nm, which is in reasonable agreement with the results from the N2 adsorption–desorption isotherm. The highly dispersed porous structure is beneficial to enhancement of acid sites accessibility. The thermal stability of PSFSI-MSMA15/SiO2 was investigated by thermo-gravimetric analysis (TGA) and the results are shown in Fig. 2. Under 120 °C, the 5% weight loss in PSFSI-MSMA15/SiO2 is due to the loss of incorporated water of the support and no other significant mass-loss is observed (c). Compared with reported polymer-based analogs [5,10], the PSFSI-MSMA15/SiO2 achieves a significant improvement in thermal stability attributed to the more stable PSFSI (higher by 40 °C), and can be employed safely below 210 °C. Quantitative determinations of the organic content of PSFSIMSMA15/SiO2 are also performed by TGA [34]. PSFSI decomposes completely between 210–480 °C, and the corresponding weight loss of PSFSI-MSMA15/SiO2 within this range is taken as an esti-

Survey Al 300W PE 100eV 532.9

300000

688.8

200000 284.6 169.0 154.5

Counts/s

100000 103.8 0

400000

532.6

300000

688.3

200000 284.6

100000

168.8 154.0 103.3

refluxed PSFSI-MSMA15/SiO2 fresh PSFSI-MSMA15/SiO2

0 1200

1000

800

600

400

200

0

Binding Energy/eV 8800

20000 C1s Al 300W PE 25eV 285.2 18000 16000

8600

8200

284.6

12000

401.1

8000

292.4

8000 6000

7800

288.1

Counts/s

10000

Counts/s

399.1

8400

285.4

14000

N1s Al 300W PE 25eV 398.8

refluxed PSFSI-MSMA15/SiO2

4000 20000 18000

285.5 285.9 (C-O)

16000 14000 12000

292.2 (C-F)

10000

284.6 (C-C, C-H)

288.0 (C=O)

7600

refluxed PSFSI-MSMA15/SiO2

7400 9000

399 (SO2-N)

8800

398.9

8600

401.6 (C=N)

8400 8200

8000

8000

fresh PSFSI-MSMA15/SiO2

6000 294

292

290

288

286

284

Binding Energy/eV

fresh PSFSI-MSMA15/SiO2

7800 282

280

406

404

402

400

398

Binding energy/eV

Fig. 5. XPS binding energy peaks of fresh and refluxed PSFSI-MSMA15/SiO2.

396

394

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Q.-H. Yang et al. / Microporous and Mesoporous Materials 172 (2013) 51–60

11000 40000

F1s Al 300W PE 25eV

Si2p Al 300W PE 25eV

688.9

10000

103.9

9000

35000

8000 30000

7000 6000

25000

5000 4000

Counts/s

Counts/s

20000

refluxed PSFSI-MSMA /SiO2 15

45000 15000

688.3 40000 35000

3000

refluxed PSFSI-MSMA

15

/SiO 2

14000 103.3

12000 10000

30000

8000

25000

6000

20000

4000

fresh PSFSI-MSMA /SiO2 15

15000 696

694

692

690

688

fresh PSFSI-MSMA

2000

686

684

15

/SiO 2

112 110 108 106 104 102 100 98

682

96

Binding Energy/eV

Binding Energy/eV 8000 7500 7000

60000 O1s Al 300W PE 25eV

S2p Al 300W PE 25eV

532.9

169.3

50000

6500

40000

6000

30000

163.6

5500

Counts/s

Counts/s

10000

refluxed PSFSI-MSMA15/SiO2

4000 8000 7500

168.9

7000

refluxed PSFSI-MSMA15/SiO2

80000 70000

6500

50000

6000

40000

5000

30000

163.3 fresh PSFSI-MSMA15/SiO2

530.4528.9

0

532.8 (Si-O)

60000

5500

533.1

534

20000

5000 4500

533 532

20000

Binding Energy/eV

532 (S=O)

533.1 (O=C-O*)

528.4 (adsorbed oxygen)

530.5 (Si-O-C/H)

533.8 (O*=C-O)

10000

178 176 174 172 170 168 166 164 162 160

532.6

fresh PSFSI-MSMA15/SiO2 538

536

534

532

530

Binding energy/eV Fig. 5. (continued)

Fig. 6. SEM (left) and TEM (right) micrographs of PSFSI-MSMA15/SiO2.

528

526

58

Q.-H. Yang et al. / Microporous and Mesoporous Materials 172 (2013) 51–60

100

80

wt%

60

Yield(BuOH:PhCOOH=3:1, 10 mol%cat.) Conversion(BuOH:PhCOOH=3:1, 10 mol%cat.) Yield(BuOH:PhCOOH=2:1, 10 mol%cat.) Conversion(BuOH:PhCOOH=2:1, 10 mol%cat.) Yield(BuOH:PhCOOH=2:1, 5 mol%cat.) Conversion(BuOH:PhCOOH=2:1, 5 mol%cat.) Yield(BuOH:PhCOOH=3:1, no cat.)

40

20

cat.: PSFSI-MSMA15/SiO2 0 0

50

100

150

200

250

300

Time/min Fig. 7. Time courses of esterification of benzoic acid with 1-butanol under various conditions.

Table 2 Esterification of benzoic acid with 1-butanol under various reaction conditions.a

a b

Entry

Reaction conditions

Yield (%)

Conversion (%)

Selectivity (%)

1 2 3

BuOH:PhCOOH = 3:1, 3 h, 120 °C, 10 mol% catalyst BuOH:PhCOOH = 3:1, 8 h, 120 °C, no catalyst BuOH:PhCOOH = 2:1, 3 h, 120 °C, 10 mol% catalyst

4 5 6

BuOH:PhCOOH = 2:1, 4 h, 120 °C, 5 mol% catalyst BuOH:PhCOOH = 2:1, 3 h, 120 °C, 10 mol% NKC-9 BuOH:PhCOOH = 2:1, 8 h, 120 °C, SiO2

91 7 Run Run Run Run Run Run Run Run 82 73 10

>99 – 94 96 98 98 97 96 99 91 89 79 –

91 – 96 95 97 94 93 97 94 80 92 92 –

1 2 3 4 5 6 7 8

90 91 (90)b 95 92 90 93 93 73

Reaction conditions: PSFSI-MSMA15/SiO2 as catalyst unless noted otherwise (0.66 mmol/g, amount relative to benzoic acid), stirred at 120 °C for 3 h. Isolated yield.

mate of the total amount of acid contents [11,19]. The acid loading for fresh PSFSI-MSMA15/SiO2 is found to be 0.62 mmol/g (29% weight loss). So, the TGA data are in reasonable agreement with the acid–base titration results (0.66 mmol/g). The follow-up weight loss can be due to silanol condensation that gives off water.

SiO2 has the advantage of good stability in water. The elaborate description of chemical structure and stability analysis of PSFSIMSMA15/SiO2 were presented in other literature [11].

3.5. PSFSI-MSMA15/SiO2 catalyzed esterifications 3.4. Hydrothermal stability analysis We also tested the catalyst to see if perfluorobutylsulfonylimide groups tend to leach into the water. PSFSI-MSMA15/SiO2 was boiled in excessive distilled water for 24 h, and then the treated sample was further analyzed by TGA, FT-IR, and XPS. TGA curves of treated and fresh PSFSI-MSMA15/SiO2 showed tiny different weight-loss tendency and the acid contents were determined as 0.59 and 0.62 mmol/g, respectively (Table 1 and Fig. 2). These meant that the loss of acidic sites was ca. 0.03 mmol/g and 95% of them still retained. Additionally, the characteristic infrared absorption bands of the –SO2– (1325, 1174 cm 1) and C–F (1353, 1138 cm 1) appeared clearly for the treated sample (Fig. 1(b)). Correspondingly, the two XPS curves were pretty much the same (Fig. 5), and two weak peaks, 399.1 eV (N1s) and 169.0 eV (S2p), were still clear. All these results suggested that PSFSI-MSMA15/

Esterification was chosen as a model reaction to assess the catalytic activities of the PSFSI-MSMA15/SiO2 since this reaction with solid catalyst is relevant in both industry and academia [35]. Preliminary tests of the catalytic performance were carried out with benzoic acid and 1-butanol as the substrates. The reaction was monitored using gas chromatography (GC) and the results are shown in Fig. 7 and Table 2. Fig. 7 shows the profiles of the acid-catalyzed reaction of 1butanol with benzoic acid (3:1) at 120–125 °C when the loading of the catalyst was 10 mol%. The reaction reached its maximum 91% yield in 3 h and the benzoic acid was nearly completely converted (Table 2, Entry 1). When a 2:1 mixture (alcohol:acid) was esterified under the same conditions, the yield and conversion slightly decreased to 90% and 94%, respectively (Entry 3). Comprehensively considering the reactive efficiency and material cost, 2:1 (alcohol:acid) ingredient ratio was employed. Under the mole

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Q.-H. Yang et al. / Microporous and Mesoporous Materials 172 (2013) 51–60 Table 3 The results of esterification catalyzed by PSFSI-MSMA15/SiO2. Entry

Acid

Alcohol

MW of products

Conversion/%

1 2 3 4 5 6 7 8 9 10 11 12 13c

C6H5COOH C6H5COOH C6H5COOH C6H5COOH C6H5COOH C6H5COOH C6H5COOH m-CH3C6H4COOH m-CH3C6H4COOH m-CH3C6H4COOH m-CH3C6H4COOH m-CH3C6H4COOH C6H5CH2CH2COOH

CH3(CH2)3OH CH3(CH2)4OH CH3(CH2)5OH CH3(CH2)7OH (CH3)2CHCH2CH2OH C6H5CH2OH cyclo-C6H11OH CH3(CH2)3OH CH3(CH2)4OH CH3(CH2)5OH CH3(CH2)7OH (CH3)2CHCH2CH2OH C6H5CH2OH

178 192 206 234 192 212 204 192 206 220 248 206

93.9 94.2 98.0 92.1 96.7 87.9a 26.2 (29.8)b 92.3 88.7 90.1 91.4 91.6 87.5

a

Isolated yield. Conversion after reaction time 8 h in the brackets. Ref. [10]. Reaction conditions: catalyst PS-SO2NHSO2C4F9 (3.21 mmol/g, 0.10 equiv. relative to carboxylic acids), carboxylic acids (5 mmol), alcohols (15 mmol), stirred at 120 °C for 6 h. b

c

ratio, reducing the catalyst loading to 5 mol% led to only 72% yield and 76% conversion. The small increments of yield by 82% and conversion by 89% respectively were observed when the reaction was prolonged to 4 h (Entry 4), confirming that the mole ratio of 2:1 (alcohol:acid) and catalyst loading of 10 mol% were suitable for the reaction. Under the same conditions, NKC-9-catalyzed experimental results compared, which gave 73% yield and 79% conversion (Entry 5). Sulfonic acid resin NKC-9 can be viewed as a material with a hydrophobic polymer backbone, but the high-content sulfonic groups (4.74 mmol/g) would change the hydrophobicity of the resin [17,36]. Thus, strong acidic sulfonic groups could catalyze effectively while the by-product water would coadsorb near the acidity sites, resulting in fast equilibrium. Differently, in the case of PSFSIMSMA15/SiO2, there are much less strong acid sites (0.66 mmol/g) and the hydrophilicity is governed by the silanol groups, which show weak acidity and fail to catalyze the esterification. The superior catalytic activity of PSFSI-MSMA15/SiO2 suggested the inhibition effect of by-product water was overcome and the chemical equilibrium toward the ester formation was shifted. The result implied that the organic –COOCH2CH2CH2– and –CH3 groups in PSFSI-MSMA15/SiO2 catalyst balanced the intrinsic hydrophilicity, and the coadsorption of water was weakened. Furthermore, reducing disadvantage influences of water, which will cause the acid strength to level off and retard the catalytically acidic sites [3,37], the stronger acidity of PSFSI-MSMA15/SiO2 contributed to higher catalytic reactivity. In the control reaction, negligible yields were obtained without catalyst even when the reaction time was prolonged to 8 h (Entry 2). The results listed in Table 2 show that PSFSI-MSMA15/SiO2 was recoverable and reusable as expected even at high temperature (Entry 3). In the presence of fresh catalyst, benzoic acid with 2 equiv. of 1-butanol gave the corresponding product butyl benzoate in 90% yield. The catalyst can be easily recovered by simple filtration after reaction. The recovered catalysts were used with retention of high activity for 7 consecutive runs, and the average yield was 92%. But the eighth run gave a decreased yield in 73%, nevertheless retained conversion in 91%. In principle, the active site leaching would simultaneously affect both yield and conversion. To investigate whether it was just the acid sites leaching that caused the decreased yield, the ultimate recovered catalyst was characterized by FT-IR and TGA (noted as recovered PSFSI-MSMA15/SiO2). In the FT-IR spectrum (Fig. 1(c)), the characteristic bands of the –SO2– (tas 1325, ts 1174 cm 1) and C–F (tas 1353, ts 1138 cm 1) still appeared.

And the mass-loss trend of the TG curve was similar to that of the fresh catalyst (Fig. 2(c and d)). The acid contents determined quantitatively by TGA method corresponded very well with that of the fresh catalyst, 0.60 mmol/g versus 0.62 mmol/g (ca. 3% difference). Furthermore, a PSFSI-MSMA15/SiO2-catalyzed concentrated reaction liquor for an extended 8 h was also checked by FT-IR and no any C–F absorption peaks were observed between 1353 and 1138 cm 1 (see supporting materials), indicating no supported species leaked into the supernatant. The reason for the decreased yield remained to be elucidated, and could not exclude the effect of the micro-reaction experiment errors on the results after 7 consecutive runs. As well known, esterification reaction could be catalyzed by Brønsted or Lewis acids. To verify that what really matters is perfluoroalkylsulfonimide groups (SO2NHSO2C4F9) on PSFSI-MSMA15/ SiO2, pure SiO2-catalyzed controlled experiment was performed and the result is listed in Table 2. In the pyridine-FT-IR spectrum (Fig. 1), pure SiO2 presents weak signal at 1448 cm 1 and much weaker at 1492 cm 1, but nothing at 1544 cm 1, indicating itself only very weak Lewis acidity. So, it is believed that the Brønsted acidity of PSFSI-MSMA15/SiO2 originates primarily from SO2NHSO2C4F9 groups and Lewis acidity from SiO2 backbone, for only SO2NHSO2C4F9 groups were introduced into the material as acid sites. The SiO2-catalyzed reaction gave only 10% yield (Entry 6), almost as the result of the blank experiment (Entry 2). It proved that the weak Lewis acidity of SiO2 didn’t become involved in this catalytic process and the SO2NHSO2C4F9 groups performed. In order to further examine catalytic performances of PSFSIMSMA15/SiO2 for other carboxylic acids and alcohols, structurally diverse analogs were treated under the optimal conditions (10 mol% of catalyst, the mole ratio of 2:1 (alcohol:acid), 120– 125 °C, 3 h). The results are shown in Table 3. Primary alcohols reacted smoothly with 90% carboxylic acid conversion, but the reaction was relatively slow in the case of secondary alcohol, cyclo-C6H11OH (Entry 7). Maybe for the secondary alcohol, the hydrophobicity of the catalyst was not still enough to shift the chemical equilibrium toward the ester formation as for the primary alcohols. Compared with earlier work in which water-stable polymer-based sulfonamide (PS-SO2NHSO2C4F9) was used as catalyst (Entry 13), much less equilibration time for PSFSI-MSMA15/ SiO 2 -catalyzed esterification was needed (3 h versus 6 h). Regarding the pore volume and the amount of acid sites, the channels in PSFSI-MSMA15/SiO2 bearing concentrated acids, about average 1.74 mol/L acid sites in channels (amount of acid/pore volume), may have facilitated the esterification [38].

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4. Conclusions In summary, a water-stable perfluorobutylsulfonylimide-functionalized acidic silica catalyst was successfully synthesized by two-step sol–gel condensation method using copolymer of perfluorobutylsulfonylimide (PSFSI) and 3-(trimethoxysilyl)propyl methacrylate (MSMA), having acid contents of 0.66 mmol/g. The presence of organic –COOCH2CH2CH2– and additional –CH3 groups in the framework balanced the hydrophilicity and positively affected the catalytic activity of the hybrid materials. The solid acid was readily recyclable and eco-friendly in the esterification even at high temperature. It could be cycled seven runs without substantial decrease in activity, and acid site leaching was negligible. Acknowledgments We gratefully acknowledge the National Science Foundation of China (Nos. 21172083, 51173054) and Fundamental Research Funds for the Central Universities (No. 2011JC004). We also thank Dr. Jun Xu (Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences) for solid-state 13C NMR experiments and valuable analysis. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.micromeso.2013. 01.008. References [1] [2] [3] [4]

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