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Carbon Nanotube-Based Solid Sulfonic Acids as Catalysts for Production of Fatty Acid Methyl Ester via Transesterification and Esterification Hang Liu, Jinzhu Chen, Limin Chen, Yisheng Xu, Xuhong Guo, and Dingye Fang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00156 • Publication Date (Web): 12 Apr 2016 Downloaded from http://pubs.acs.org on April 16, 2016

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ACS Sustainable Chemistry & Engineering

Carbon Nanotube-Based Solid Sulfonic Acids as Catalysts for Production of Fatty Acid Methyl Ester via Transesterification and Esterification

Hang Liu,†,‡ Jinzhu Chen,*,‡ Limin Chen,§ Yisheng Xu,*,† Xuhong Guo,† and Dingye Fang†

†

State-Key Laboratory of Chemical Engineering, East China University of Science and Technology.

130 Meilong Rd, Shanghai 200237, P.R. China ‡

Key Laboratory of Renewable Energy, Chinese Academy of Sciences. Guangzhou Institute of

Energy Conversion, Chinese Academy of Sciences. No.2 Nengyuan Rd, Wushan, Tianhe District, Guangzhou 510640, P.R. China §

College of Environment and Energy, South China University of Technology. 382 Zhonghuan Road

East, Guangzhou Higher Education Mega Centre, Panyu District, Guangzhou 510006, P.R. China *Corresponding author. E-mail address: [email protected] (J. Chen).

ABSTRACT: 1

ACS Paragon Plus Environment


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A series of polymer-carbon nanotubes composite materials (CNT-P-SO3H) were prepared by covalent grafting of multi-walled carbon nanotubes (CNT) with sulfonic acid-functionalized polymers

(P-SO3H)

including

poly(3-vinyl-1-sulfonic

acid

imidazolium

chloride)-grafted

multi-walled CNT (CNT-PVSAIC), poly(4-vinyl-1-sulfonic acid pyridinium chloride)-grafted multi-walled CNT (CNT-PVSAPC), and poly(4-styrenesulfonic acid)-grafted multi-walled CNT (CNT-PSSA). Such functionalization method provides a facile route to obtain various polyelectrolyte brushes on the surface of CNT in order to improve the dispersibility and modulate the acidity of CNT, to selectively introduce functional groups and densely create active sites over CNT for potential catalytic applications. Both CNT-PVSAIC and CNT-PVSAPC consist of cationic polyelectrolyte chains functionalized by sulfonic acid groups. Whereas, CNT-PSSA is composed of anionic polymer brushes grafted by sulfonic acid groups. The physicochemical properties of the CNT-P-SO3H were analyzed by BET, TGA, XRD, FT-IR, XPS, Raman and HRTEM techniques. The resulting CNT-P-SO3H materials exhibit excellent catalytic activity as CNT-based solid acids in liquid phase transesterification of triglycerides with methanol and esterification of oleic acid with methanol, which are typical model reactions for biodiesel production. The outstanding catalytic performance of CNT-P-SO3H catalysts is attributed to the combination of mesoporous structure together with well extended P-SO3H coating over the outer surface of the CNT, providing the formation of dense but uniform surface distribution of active sites.

KEYWORDS: biodiesel, carbon nanotubes, esterification, solid acid, transesterification

INTRODUCTION: 2


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Biodiesel, a renewable and biobased transportation fuel composed of alkyl esters of long-chain fatty acids, has attracted increasing attentions as an alternative to nonrenewable petroleum-diesel. 1-16 Biodiesel can be produced from vegetable oils, animal fat, and waste oils from food industry. Currently, the production of industrial biodiesel is mainly based on the transesterification (Scheme 1a) or esterification (Scheme 1b) reactions to obtain fatty acid methyl esters (FAME), which can be promoted by using either base or acid catalysis. 2-16 Although the base catalysts generally show a higher catalytic performance in these reactions, acid catalysts are more tolerant to free fatty acids and water present in oil feedstock. 2-16 Homogeneous acids (e.g., mineral acid and organic sulfonic acid) are highly active, but cannot be recovered and are closely associated with environmental, technical, and economic issues. In contrast, using of solid acids provides easy product separation and recycling of the catalysts. Moreover, solid acid catalysts are preferred because they can promote esterification of free fatty acids in parallel with transesterification of triglycerides present in oil feedstock without saponification. Therefore, solid acids enable a reduction of processing steps in biodiesel production, which meets the requirements of green and sustainable chemistry in the catalytic production of biodiesel. A variety of solid acid catalysts have already been examined for FAME synthesis via both transesterification of triglycerides and esterification of free fatty acid. 2-16 Compared with recently reported solid acid catalysts, such as sulfonated covalent organic frameworks (COF),17 sulfated mesoporous niobium oxide,

18

and Cs-exchanged heteropolyacid,

19

carbon-based solid acids are

among the most promising catalysts for the FAME productions. Firstly, carbon-based solid acids can be obtained from renewable resources conforming to the requirements of sustainable chemistry since carbon is one of the most abundant, cheap and readily available elements in our biosphere. 20,21 In 3



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addition, carbon-based materials generally show most versatile structures, which can benefit themselves in the field of renewable energy as well as environmental science. For instance, sulfonic acid group (SO3H)-bearing amorphous carbons, termed as “sugar catalysts”, were reported as promising and renewable acidic materials. These sugar catalysts possess acidic -SO3H groups on the catalyst surface and manifest hydrophobic feature, which can promote the diffusion of organic reagents.

16,22-25

However, the sugar catalysts were generally prepared by sulfonation of partially

carbonized organic molecules with concentrated sulfuric acid. In addition, the preparation of other commonly used acidic mesoporous carbon materials also involved sulfonation process with concentrated sulfuric acid as the reagent. Such kind of methods reduce the catalyst stability; on the other hand, leads to a random and indiscriminative formation of acidic sites on catalyst surface. In contrast, selective deposition of acidic -SO3H groups on carbon edges of delicate nanostructured materials with a large surface area can improve the accessibility of the bulky reactant molecules to acidic sites. 26 By using an improved synthetic control, a series of carbon nanotube (CNT)-, carbon nanofiber (CNF)-, and graphene-derived solid acids with delicate structure and tunable surface composition, texture, and acidic properties were recently developed as carbon-based acidic catalysts for various reactions. For example, poly(4-styenesulfonate acid)-grafted multi-walled CNT (CNT-PSSA) were investigated for fructose dehydration to biomass-based 5-hydroxymethylfurfural (HMF) esterification of lauric acid with methanol.

28

27

and

Hybrid materials of heteropoly acid and

nitrogen-functionalized CNT were reported for ethyl acetate hydrolysis, Beckmann rearrangement of cyclohexanone oxime, and Friedel–Crafts alkylation of toluene with 1-octene. 29 Polyaniline-sulfate deposited on CNT (CNT-PANI-S) was developed for methanolysis of triglycerides.30 CNF 4


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functionalized with benzenesulfonic acid groups was tested for transesterification of triolein with methanol26 and fructose conversion into HMF. 27 Sulfonated graphene and sulfonated graphene oxide were prepared for ethyl acetate hydrolysis,

31

transesterification of ethyl acetate with alcohols, 32

xylose dehydration to furfural, 33 transformation of carbohydrates to levulinic acid, 34 and conversion of HMF to biofuels. 35

Scheme 1. (a) Transesterification of triglyceride with methanol, (b) esterification of free fatty acid with methanol, and (c) proposed structures of CNT-PVSAIC, CNT-PVSAPC, CNT-PSSA, CNT-PVI, CNT-PVP and CNT-PSSNa (x indicates the degree of polymerization). 5



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CNTs are more interesting over other types of carbon materials, such as active carbon, graphene, and chitin derived carbon due to their excellent mechanical, physical, and chemical properties; therefore were chosen as the substrate for functionalization in this research. Actually, CNTs were suggested to combine almost all of the outstanding properties of the above carbon materials. In addition, CNTs were currently produced on an industrial scale and various methods were developed to modify CNTs for their usage in catalysis, materials science, environmental technology and energy research. Inspired by the above work, a series of polymer-CNT composite materials (CNT-P-SO3H) were prepared by covalent grafting of multi-walled CNT with sulfonic acid-functionalized polymers (P-SO3H) in this research. The investigated CNT-P-SO3H (Scheme 1c) includes

poly(3-vinyl-1-sulfonic

acid

imidazolium

chloride)-grafted

multi-walled

CNT

(CNT-PVSAIC), poly(4-vinyl-1-sulfonic acid pyridinium chloride)-grafted multi-walled CNT (CNT-PVSAPC), and CNT-PSSA (Scheme 1c). The obtained CNT-P-SO3H renders covalent grafting of various polyelectrolyte brushes on the surface of CNT with tunable dispersibility, acidity, functional groups and active sites and can be readily modified by using a variety of organic transformations. Moreover, all of the CNT-P-SO3H materials show almost unchanged graphitized surfaces of CNT moiety after covalent functionalization of pristine multi-walled CNT with P-SO3H. The resulting CNT-P-SO3H demonstrates catalytic performance as solid acids in liquid phase transesterification of triglycerides with methanol (Scheme 1a) and esterification of oleic acid with methanol (Scheme 1b).

EXPERIMENTAL SECTION:

6


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Each experiment for catalyst preparation and (trans)esterification reaction was duplicated for multiple times to ensure their repeatability. Preparation of CNT-PSSA, CNT-PVI and CNT-PVP CNT-PSSA, 27, 28 CNT-PVI36, 37 and CNT-PVP38, 39 were prepared according to literature methods (See the Supporting Information for details). Preparation of CNT-PVSAIC and CNT-PVSAPC In a typical procedure for CNT-PVSAIC preparation: a dried Schlenk flask was charged with a magnetic stirrer, CNT-PVI (50 mg), anhydrous 1,2-dichloroethane (250 mL), and chlorosulfonic acid (0.13 mL) under nitrogen atmosphere. After the reaction mixture was stirred for 2 h at room temperature, the flask was then placed in a thermostated oil bath at 65 °C for 24 h under nitrogen atmosphere. Finally, the remaining solid CNT-PVSAIC was separated by centrifugation, washed thoroughly with dry dichloromethane and anhydrous diethyl ether, and further treated under vacuum at 80 °C for 24 h. CNT-PVSAPC was prepared following the same synthetic procedure as for CNT-PVSAIC except that CNT-PVI was replaced by CNT-PVP. Transesterification of triglyceride with methanol In a typical procedure for transesterification of glyceryl triacetate with methanol: catalyst (20 mg), methanol (1.2 mL) and glyceryl triacetate (218 mg, 1.0 mmol) were separately added into an Ace Pressure Tube reactor as well. The mixture was heated at 60 °C for 10 h and the reaction was quenched by ice-water bath by putting the reactor into the ice-water bath to stop the reaction immediately. The resulting mixture was filtered to remove catalyst and decanted into a volumetric flask using methanol as diluents, then given amount of toluene (internal standard) was added. The quantification of the products was performed using GC calibrated for each compound with toluene as 7



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internal standard. GC-MS was used for identification of the products. Esterification of oleic acid with methanol In a typical procedure for esterification of oleic acid with methanol: Oleic acid (282 mg, 1.0 mmol), catalyst (20 mg) and methanol (1.2 mL) were separately added to the reactor. The mixture was heated at 60 °C for 10 h and the reaction was then quenched by ice-water bath by putting the reactor into the ice-water bath to stop the reaction immediately. The resulting mixture was filtered to remove catalyst, methanol solvent was removed under reduced pressure at ambient temperature. The mixture was then decanted into a volumetric flask using n-hexane as diluents and given amount of methyl heptadecanoate (internal standard) was added. The quantification of the products was performed using GC calibrated for each compound with methyl heptadecanoate as internal standard. Reusability of catalyst The reusability of CNT-PVSAIC was tested for transesterification of glyceryl tributyrate with methanol. Glyceryl tributyrate (302 mg, 1.0 mmol), methanol (1.2 mL), catalyst (15 mg) and toluene (30 mg) were used. Each time, the mixture was heated at 60 °C for 4 h and quenched by ice-water bath by putting the reactor into the ice-water bath to stop the reaction immediately. The reaction mixture was filtered; the mother liquor was analyzed by GC and GC-MS. While the insoluble catalyst was washed with anhydrous methanol, dried at 80 °C for 6 h under the vacuum, and reused directly as the catalyst for the next run under the same conditions (Figure S7, Supporting Information). Characterizations Surface area: The BET surface area measurements were performed with N2 adsorption-desorption isotherms at 77 K (SI-MP-10/PoreMaster 33, Quantachrome). After degassing the samples under 8


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vacuum at 120 °C for 24 h and then the isotherms were measured over the range of 10−6 < P/P0 < 0.1. FT-IR: Approximately 2 mg sample and 100 mg KBr were mixed in a quartz cell. The mixture was crush into power and were pressed (107 Pa) into self-supported discs (2 cm2 area). FT-IR analysis of the discs including sample were carried out using Bruker Tensor 27 spectrophotometer, equipped with a Data Station, at a spectral resolution of 1 cm−1 and accumulations of 128 scans. Spectra were recorded at room temperature under dinitrogen atmosphere. XRD: XRD patterns of the as-prepared samples were obtained by a Bruker Advance D8 diffractometer at 40 KV and 40 mA, using Ni-filtered Cu-Kα radiation with a scan speed of 0.3 sec/step, a step size of 0.02° in 2θ, and a 2θ range of 5–50. Approximately 15 mg sample was dehydrated under high vacuum at 100 °C before XRD analysis. TGA: Approximately 5–10 mg sample was used for thermogravimetric analysis. The sample was analyzed under a stream of dinitrogen using a TA Instrument Q600 SDT from room temperature to 600 °C with a scan rate of 10 °C/min. GC and GC-MS analysis: GC and GC-MS analysis were recorded on Agilent 6890 and ThermoQuest Trace 2000, respectively, equipped with a FID detector and a packed column (0.32 mm × 30 m KB-5 capillary column) with He as carrier gas. XPS analysis: the XPS spectra was performed with a Kratos Axis Ultra (DLD) photoelectron spectrometer operated at 15 kV and 10 mA at a pressure of about 5×10−9 torr using AlKα as the exciting source (1486.6 eV). C 1s photoelectron peak (BE = 284.6 eV) was used for the binding energy calibration. HRTEM analysis: the morphological analysis of CNT-PVSAIC, CNT-PVSAPC, and CNT-PSSA was carried out using transmission electron microscopy (TEM, JEM-2100HR). Samples for TEM studies were prepared by placing a drop of the suspension of CNT-PVSAIC, CNT-PVSAPC, and CNT-PSSA sample, respectively, in ethanol onto a copper grid coated by an ultrathin carbon film, followed by evaporating the solvent. Acid concentration: The total number 9



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concentrations of acid sites of the catalysts were determined by acid–base back potentiometric titration with hydrochloric acid according to literature methods. 40-42 A sodium hydroxide aqueous solution (0.1 M, 30 mL) was added to the catalyst (200 mg). Then the mixture was stirred for 60 min at room temperature under ultrasonic vibration and it was keeping stirring at room temperature for 12 hours. 20 mL of the supernatant solution was titrated with a hydrochloric acid (0.05 M) solution on Metrohm 877 Titrino plus instrument. Elemental analysis: elemental analysis of C, N, S and O for CNT-PVSAIC, CNT-PVSAPC, and CNT-PSSA was obtained from an Elementar Vario EL III. Raman spectra: Raman spectra were performed on LabRAM HR800-LS55 at ambient temperature and moisture-free conditions with an argon-ion laser at an excitation wavelength of 532 nm. The sample was loaded in an in situ cell and pre-treated in dry air flow at 100 °C for 1 h for dehydration.

RESULTS AND DISCUSSION: Composite materials of CNT-P-SO3H Scheme 1c shows the proposed structures of CNT-PVSAIC, CNT-PVSAPC, and CNT-PSSA. In this study, polyelectrolyte brush-modified solid acid CNT-PVSAIC was prepared by sulfonation of poly(1-vinylimidazole)-grafted multi-walled CNT (CNT-PVI, Scheme 1c) with chlorosulfonic acid. CNT-PVSAPC was prepared following the same synthetic procedure as for CNT-PVSAIC except that CNT-PVI was replaced by poly(4-vinylpyridine)-grafted multi-walled CNT (CNT-PVP, Scheme 1c). The grafting ratio of sulfonic acid group (-SO3H) in CNT-PVSAIC and CNT-PVSAPC were 76.8% and 39.7%, respectively, based on elemental analysis (Table S1, Supporting Information). The sulfonic acid grafting ratio was denoted as the molar ratio of -SO3H to imidazole units in CNT-PVSAIC and to pyridine units in CNT-PVSAPC, respectively (Scheme S1, Supporting

10


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Information). The precursors of CNT-PVI and CNT-PVP composites were synthesized by an in situ radical polymerization of 1-vinylimidazole and 4-vinylpyridine monomer, respectively, in the presence of multi-walled CNT. 36-39 CNT-PSSA was prepared by an in situ polymerization of sodium p-styrenesulfonate (SSNa) monomer in the presence of multi-walled CNT to give poly(sodium 4-styrenesulfonate)-grafted CNT (CNT-PSSNa, Scheme 1c), followed by acidification with hydrochloric acid. 46 According to the literatures, the CNT-polymer composites were fabricated by covalent immobilization of the corresponding polymer precursors on the surface of the CNT followed by propagation of the polymerization in the presence of corresponding monomers. 43-46 The formation of composites CNT-PVSAIC, CNT-PVSAPC, and CNT-PSSA were revealed by nitrogen physisorption analysis, thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (XRD), Raman spectra and high-resolution transmission electron microscopy (HRTEM). Table 1 and Table S1 (Supporting Information) show a summary of physical-chemical properties of the prepared CNT-P-SO3H.

Table 1. Properties of CNT-P-SO3H and pristine CNT Acid concentration Sample

[mmol g−1] [H+] b

[−SO3H] a

S content

TGA stability

Surface area c

Pore volume d

[wt%] a

[°C]

[m2 g−1]

[cm3 g−1]

Average pore diameter d [nm]

CNT-PVSAIC

1.46

1.38

4.69

up to 227

47.6

0.47

10.7

CNT-PVSAPC

1.02

1.14

3.28

up to 214

65.4

0.52

9.1

CNT-PSSA

0.71

0.65

2.27

up to 309

103.0

0.83

9.7

/

up to 600

155.1

1.00

14.6

CNT

a

/

Based on elemental analysis.

/

b

Acid concentration values were determined through acid–base back potentiometric titration.

Brunauer-Emmett-Teller (BET) surface area was evaluated using the BET method in the p/p0 range from 0.05 to 0.3.

d

c

Pore size

distribution curves were calculated using the adsorption branch of the isotherms and the Density Functional Theory (DFT) method, pore sizes were obtained from the peak positions of the distribution curves. 11



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Nitrogen adsorption and desorption isotherms and textural properties of precursor multi-walled CNT and CNT-P-SO3H were shown in Figure S1 (Supporting Information) and Table 1. The Brunauer-Emmett-Teller (BET) surface areas of various CNT-P-SO3H exhibit significant decreases when compared with that of the pristine multi-walled CNT. The decreased surface area of CNT-P-SO3H is presumably attributed to the formation of polymer coating covalently grafted on the porous surface of multi-walled CNT, leading to a partial pore blocking. In addition, the decrease of surface area after grafting polymer to multi-walled CNT is due at least in part to the increase in mass of the material by the weight percent of polymer, which can be calculated from the elemental analyses as discussed in the next paragraph. Of course, pore surface area/weight also is reduced by polymer filling pores between tubes in the aggregates. Moreover, all CNT-P-SO3H materials show dramatically lower mesopore volumes than that of initial sample CNT (Table 1). These distinct changes in the textural properties of CNT-P-SO3H further indicate successful modifications of CNT with P-SO3H. The thermal stabilities of the composite materials CNT-P-SO3H were investigated by thermal gravimetric-differential thermal analysis (TG-DTA, Figure S2, Supporting Information) under nitrogen atmosphere. Figure S2 shows the curves of weight loss versus temperature for CNT, CNT-PVSAIC, CNT-PVSAPC, and CNT-PSSA. As expected, CNT shows negligible weight loss up to 600 °C. While, the significant weight loss of CNT-P-SO3H composite, corresponding to polymer degradation, starts at about 227 °C for CNT-PVSAIC, 214 °C for CNT-PVSAPC, and 309 °C for CNT-PSSA, respectively. Evidently, CNT-PSSA has a higher thermostability than CNT-PVSAIC and CNT-PVSAPC. Moreover, the P-SO3H loading level of CNT-P-SO3H can be estimated from 12


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their respective TG results by comparing weight loss between CNT-P-SO3H and pristine CNT. The P-SO3H contents for CNT-PVSAIC, CNT-PVSAPC, and CNT-PSSA obtained from TG are 26.6 wt%, 24.5 wt%, and 8.2 wt%, respectively. Based on theoretical S contents of 15.22% in PVSAIC and 14.46% in PVSAPC, the quantification analysis of TG-DTA thus revealed that the S contents are about 4.05% in CNT-PVSAIC and 3.54% in CNT-PVSAPC, respectively, which is very close to the elemental analysis results as shown in Table 1. FT-IR spectra of pristine CNT and various CNT-P-SO3H were compared in Figure S3 (Supporting Information). The FT-IR spectrum of pristine CNT shows extremely low infrared absorption intensities. 27, 38 In the case of CNT-PVSAIC, the FT-IR spectrum shows the characteristic peak at 1066 cm−1 corresponding to the symmetric N−SO3 stretching vibration.

47, 48

The strong

absorptions at 1290 and 1178 cm−1 were assigned to the asymmetric and symmetric stretching of S−O vibrations in sulfonic acid group.

49, 50

For CNT-PVSAPC, the three characteristic peaks

observed at 1054, 1162 and 1235 cm−1 correspond to vibrational modes of N–SO3 bonds, symmetric and asymmetric stretching vibration modes of O–SO2 bonds, respectively. 51-53 Finally, the FT-IR spectrum of CNT-PSSA matches well with reported results, 27 showing two strong and characteristic peaks at 1593 and 1392 cm−1 assigned as the stretching modes of the phenyl group and sulfonic acid group, respectively. The above FT-IR analysis results thus verify the presence of polymeric P-SO3H segments in the as-prepared CNT-P-SO3H composite materials.

13



a

C(1s) O(1s)

N(1s)

Intensity

CNT-PVSAIC

Cl(2p) S(2p)

O(1s) N(1s)

CNT-PVSAPC

Cl(2p) S(2p)

O(1s)

CNT-PSSA

1000

800

600

S(2p)

400

200

0

Binding Energy / eV

b

S(2p) XPS

169.2

CNT-PVSAIC

168.9

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

CNT-PVSAPC

168.3 CNT-PSSA

174

172

170

168

166

164

Binding Energy / eV

Figure 1. (a) XPS scan survey and (b) S(2p) XPS spectra of CNT-PVSAIC, CNT-PVSAPC, and CNT-PSSA.

CNT-PVSAIC, CNT-PVSAPC, and CNT-PSSA were further evaluated by XPS. As shown in Figure 1a, peaks corresponding to oxygen, nitrogen, carbon, chlorine and sulfur are clearly observed in the survey scans for both CNT-PVSAIC and CNT-PVSAPC, confirming the presence of imidazolyl group in CNT-PVSAIC, pyridinyl group in CNT-PVSAPC, and sulfonic acid group in both CNT-PVSAIC and CNT-PVSAPC. The presence of chlorine peak indicates that Cl– ion acts as a counter ion of positively charged imidazolium ion in CNT-PVSAIC and pyridinium ion in 14


Page 14 of 40

CNT-PVSAPC, respectively (Scheme 1c). XPS result for CNT-PSSA shows the presence of oxygen, carbon and sulfur. The S(2p) XPS spectra of CNT-P-SO3H show one main peak corresponding to S(2p3/2), 54-59 indicating the presence of SO3H species in the CNT-P-SO3H samples (Figure 1b). A reduced binding energy in the S(2p) XPS spectra was observed in the order of CNT-PVSAIC > CNT-PVSAPC > CNT-PSSA (Figure 1b), suggesting an increase of electron density on sulfur atoms in the sequence of CNT-PVSAIC < CNT-PVSAPC < CNT-PSSA. Notably, a significantly high binding energy was observed for CNT-PVSAIC than that of CNT-PSSA, which can presumably be related to a strong electron-withdrawing effect of the positively charged imidazolium ion neighboring to the -SO3H group in CNT-PVSAIC (Scheme 1c). The above XPS spectra analysis thus further proves the proposed structures of CNT-PVSAIC, CNT-PVSAPC, and CNT-PSSA as depicted in Scheme 1c. The FT-IR and XPS analysis of CNT-P-SO3H samples thus indicate that -SO3H group covalently grafted to the composite materials of polymers and CNT rather than physisorbed on the composite materials.

002

100

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Intensity

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CNT CNT-PSSA CNT-PVSAPC CNT-PVSAIC 10

20

30

40

50

60

70

80

2θ / degree

Figure 2. Powder XRD patterns of CNT-PVSAIC, CNT-PVSAPC, CNT-PSSA, and CNT.

15



The powder XRD patterns of CNT-PVSAIC, CNT-PVSAPC, and CNT-PSSA exclusively show two characteristic peaks at 2θ values of 26° with high intensity and 43° with low intensity, matching with the characteristic peaks of the (002) and (100) packing of graphitic CNT, respectively (Figure 2). 60, 61

Notably, the intensity of characteristic diffraction peaks observed in CNT-P-SO3H decreases as

compared to the pristine CNT, which can be attributed to the change of the microstructure of carbon sheets in CNT moiety of CNT-P-SO3H samples. 62, 63

G D

1581

1346

CNT-PSSA

1581

1347

Intensity / a.u

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1583

CNT-PVSAPC

1572

CNT-PVSAIC

1349

1342

CNT

1000

1200

1400

1600

1800

2000

-1

Raman shift / cm

Figure 3. Raman spectra of CNT, CNT-PVSAIC, CNT-PVSAPC, and CNT-PSSA.

Raman spectroscopy was used to determine the variation of defect density in CNT moiety of composite CNT-P-SO3H before and after the polymerization process (Figure 3). For pristine CNT, the peak around 1342 cm–1 is associated with the vibrations of carbon atoms in the disordered graphite structure (D mode) corresponding to the defects in the curved graphite sheet, sp3 carbon, or other impurities (Figure 3).

63-69

Whereas, the peak near 1572 cm–1 is attributed to the graphite

structure of CNT (G mode). After covalent grafting with P-SO3H, the shifts of D and G bands of CNT moieties are negligible in CNT-P-SO3H if compared with the precursor CNT (Figure 3). Owing 16


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to the complex nature of the vibrational modes of disordered/amorphous and pristine graphene sheets, deconvolutions of the Raman spectra into individual components were further carried out using Gaussian and Lorentzian fitting procedures (Figure S4, Supporting Information) and the resulting deconvolution results are presented in Table S2 (Supporting Information). In the case of Gaussian fitting of CNT-PVSAIC sample, four component signals at 1350.9 cm–1 (D1, D), 1589.3 cm–1 (D2, G), 1546.7 cm–1 (D3), and 1348.8 cm–1 (D4) assigned to graphene egdes, graphene sheets, amorphous carbon and polyenes respectively, are clearly revealed in the Figure S4a (Supporting Information) and Table S2 (Supporting Information).

67-69

The intensity ratio of the D-band to

G-band (ID/IG) and the full width at half maximum (FWHM) was generally used to determine the relative extent of structural defects in CNT (Table S2, Supporting Information). 66 The ratios of ID/IG are 0.52 for CNT-PVSAIC, 0.51 for CNT-PVSAPC, and 0.70 for CNT-PSSA indicating almost unchanged graphitized surfaces of CNT moieties in CNT-P-SO3H. Moreover, the ratio of I/ITot (ITot = ID1 + ID2 + ID3 + ID4) further provides an accurate comparison for each component signals. Table S2 (Supporting Information) shows increased contents of amorphous carbon (ID3/ITot) and polyenes (ID4/ITot) as well as decreased contents of graphene egdes (ID1/ITot) and graphene sheets (ID2/ITot) for CNT-P-SO3H samples when compared with precursor CNT, suggesting successful functionalization of CNT with P-SO3H. The decline ratios of ID1/ITot were observed from CNT to CNT-PVSAIC, which can be related to the grafting level of P-SO3H to the carbons on graphene edges. This observation is in line with the P-SO3H loading level in CNT-P-SO3H as shown in TG analysis (Figure S2, Supporting Information). Typical HRTEM images for CNT-PVSAIC, CNT-PVSAPC, and CNT-PSSA are shown in Figure 4 and Figure S5 (Supporting Information). The morphology of these CNT-P-SO3H series 17



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closely resembles each other. The HRTEM images of these composite materials CNT-P-SO3H show a thin polymer coating around 3-5 nm with a rough and amorphous surface structure over CNT, verifying the successful functionalization of CNT with polymer (Figure 4a-c). In contrast, pristine CNT has a smooth and graphitized surface (Figure 4d). In addition, all of these CNT-P-SO3H composite materials do not lose quasi-one-dimensional structure and rougher surfaces of these CNT-P-SO3H materials were clearly observed in contrast to pristine CNT. Notably, in the obtained composite CNT-P-SO3H, P-SO3H localized mainly at the outer surface of CNT creating a layer encapsulating the supporting material CNT and forming a structure labeled “fiber in a jacket”. Therefore, during the preparation of CNT-P-SO3H precursors (CNT-PVI, CNT-PVP, and CNT-PSSNa), covalent grafting of CNT with corresponding monomers presumably took place mainly at the outer surface of the CNT. The data reported previously also revealed that coating of CNT with polyaniline occurred only at the outer surface of the CNT, and it was suggested that the polymerization of aniline inside the CNT was hindered by the restricted access of the reactants into the interior of the CNT.

30, 62, 70

Therefore, functionalization of CNT with polymer leads to an

extended P-SO3H coating over the outer surface of the CNT and creates an uniform distribution of dense active sites on the CNT surface. This type of surface structure is expected to facilitate the access of reactants and promote the migration of reagents away from the catalyst. Furthermore, the covalent grafting of polymer can well explain the strong reduction of CNT surface area and porosity. The Raman and HRTEM analysis of CNT-P-SO3H samples thus suggest that covalent grafting CNT with P-SO3H does not significantly destroy the delicate surface structure of pristine CNT.

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Figure 4. HRTEM images of (a) CNT-PVSAIC, (b) CNT-PVSAPC, (c) CNT-PSSA and (d) pristine CNT.

Since the catalytic performance of solid acid catalysts in transesterification and esterification is closely associated with their acidity, the amount of accessible acidic sites (acid concentration) were characterized by potentiometric acid–base back titration, whereas, the precise -SO3H loading levels were determined by elemental analysis. As described in Table 1, the pristine CNT is not acidic based on acid–base back potentiometric titration. In the cases of CNT-P-SO3H series, both the contribution of Brønsted acidity and the concentration of -SO3H in CNT-P-SO3H decrease in the order of CNT-PVSAIC > CNT-PVSAPC > CNT-PSSA, indicating that the acid concentrations of these CNT-P-SO3H parallels their -SO3H loading level. Such observations are in line with our previously reported results. 27, 71

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Transesterification and esterification

Table 2. Transesterification and esterification reactions over various CNT-P-SO3H catalysts a Entry

Catalyst

Rb

Time [h]

Conversion or Yield c [%]

1 2

CNT-PVI CNT-PVP CNT-PVSAIC CNT-PVSAPC CNT-PSSA CNT-PVSAIC CNT-PVSAPC CNT-PSSA CNT-PVSAIC CNT-PVSAPC CNT-PSSA CNT-PVSAIC CNT-PVSAPC

Me

10 10 10 10

12.3 15.1 >99.9 >99.9 90.4

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

CNT-PSSA CNT-PVSAIC CNT-PVSAPC CNT-PSSA CNT-PVSAIC CNT-PVSAPC CNT-PSSA AC-PVSAIC PVSAIC Amberlyst-15

Me Me Me Me Me Me Me n Pr n Pr n Pr n Pr n Pr n Pr C17H33 C17H33 C17H33 C17H33 C17H33 C17H33 n Pr n

Pr Pr

n

10 5 5 5 10 10 10 5 5 5 10 10 10 5 5 5 5 5 5

a

85.8 81.3 75.2 98.5 97.6 85.4 81.6 78.1 70.3 93.0 86.3 80.9 77.2 69.1 63.6 68.3 24.9 25.1

Reaction conditions: Catal. (20 mg), triglyceride or oleic acid (1.0 mmol), methanol (1.2 mL), 60 °C, (Entries 1–14, 21–23 for Scheme 1a and Entries 15–20 for Scheme 1b). b The R in the table refers to the substituent groups in Schemes 1a and b. c The conversions of glyceryl triacetate (Entries 1–8), glyceryl tributyrate (Entries 9–14, 21–23) and the yields of methyl oleate (Entries 15–20) were determined by GC analysis.

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Herein, the prepared CNT-P-SO3H samples were investigated as potential CNT-based solid acid catalysts. Their catalytic performance was systematically evaluated for transesterification of triglycerides with methanol and esterification of oleic acid with methanol, which are considered as typical model reactions in biodiesel production.

1-16

Initially, control experiments of glyceryl

triacetate methanolysis were carried out in the presence of CNT-PVI and CNT-PVP to evaluate the contributions from bare precursors before -SO3H functionalization. The conversions of glyceryl triacetate obtained in the blank runs were remarkably inferior, ranging from 12.3 to 15.1% (Table 2, Entries 1–2). The observed catalytic performances of the CNT-PVI and CNT-PVP are presumably related to Brønsted basicity of imidazolyl group in CNT-PVI and pyridinyl group in CNT-PVP, respectively. Table 2 shows all of the CNT-P-SO3H readily processed the transesterification of glyceryl triacetate and methanol (Scheme 1a) with outstanding conversions from > 99.9 to 90.4% at 60 °C (Entries 3–5), which is far beyond those achieved with regards to their corresponding unfunctionalized precursors such as CNT-PVI and CNT-PVP (Table 2, Entries 1–2). In addition, these solid acids CNT-P-SO3H also show excellent catalytic performance towards transesterification of glyceryl tributyrate and methanol with glyceryl tributyrate conversion ranging from 98.5 to 85.4% (Table 2, Entries 9–11). As expected, comparisons for the methanolysis of glyceryl triacetate and glyceryl tributyrate indicate that, at specified reaction conditions and given loading levels of catalysts, methanolysis of glyceryl triacetate (a short triglyceride molecule) reacts much faster than the reaction for glyceryl tributyrate consisting of long chain-triglyceride molecules. Such results are in agreement with the reported data. 30, 72 The catalytic activity of CNT-P-SO3H decreases in the order of CNT-PVSAIC > CNT-PVSAPC > CNT-PSSA in the methanolysis reactions of both 21



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glyceryl triacetate and glyceryl tributyrate (Table 2, Entries 6–8, 12–14). For comparison purpose, poly(3-vinyl-1-sulfonic acid imidazolium chloride)-grafted activated carbon (AC-PVSAIC) and polymer backbone poly(3-vinyl-1-sulfonic acid imidazolium chloride) (PVSAIC) alone were investigated for the methanolysis reactions of glyceryl tributyrate (Table 2, Entries 21–22). However, relatively low conversions of glyceryl tributyrate were observed if compared with CNT-PVSAIC under the investigated conditions, indicating excellent catalytic performance of CNT-polymer composite material. We further used commercially available and classical solid acid Amberlyst-15 as the catalyst for methanolysis of glyceryl tributyrate under the same conditions (Table 2, Entry 23). Amberlyst-15 exhibited low activity, although it was unexpected considering its stronger acidity. In addition to the transesterification of triglyceride with methanol, CNT-P-SO3H catalysts were also effective for esterification of C18 mono-unsaturated oleic acid and methanol with excellent yields of methyl oleate (Table 2, Entries 15–20, and Scheme 1b). It is well known, solid acid-promoted esterification reactions generally show low catalytic performance and suffer from slow reaction rates if compared with solid base-catalyzed reactions; therefore, a long process and high reaction temperature were usually observed in the acid-promoted reactions to reach full conversion. In our case, the CNT-P-SO3H is highly active for the esterification of oleic acid and methanol, which can be attributed to well extended P-SO3H coating with dense and uniform distribution of active sites -SO3H over the outer surface of the CNT. The surface distributions of active sites promote mass transfer; while, high -SO3H concentration of CNT-P-SO3H (Table 1) significantly enhance the catalytic activity according to the reported results. 2

22


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a

90 80 70 60 50 40

CNT-PVSAIC CNT-PVSAPC CNT-PSSA

30 20 10 0

50

100

150

200

250

300

350

Time / min

b

90

Glyceryl triacetate + MeOH

Glyceryl tributyrate + MeOH

o

-1

80

-1

TOF / molTG molSO3H h

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Glyceryl tributyrate Conversion / %

Page 23 of 40

70

o

30 C

30 C 72.1±0.2

o

60 C

o

60 C 65.2±0.2

60 50 40 30 20 10

39.8±0.2

35.0±0.2 28.6±0.1

27.0±0.1

23.5±0.1

20.6±0.1

14.1±0.1 9.2± 0.1

13.9± 0.1 9.8±0.1 C C A AI SS AP VS -P VS P T P T TCN CN CN

CN

T -P

SS

C

A

-P NT

VS

AP C

C

-P NT

VS

C AI

Figure 5. (a) The comparison of catalytic activities of CNT-P-SO3H based on an equimolar amount of -SO3H in the transesterification of glyceryl tributyrate and methanol. Reaction conditions: glyceryl tributyrate (302 mg, 1.0 mmol), catalyst (CNT-PVSAIC, 4.8 mg; CNT-PVSAPC, 6.9 mg; CNT-PSSA, 10.0 mg), methanol (1.2 mL), 60 °C. (b) The comparison of catalytic performance of CNT-P-SO3H based on TOFs for the transesterification reactions.

Previously, methanolysis of glyceryl triacetate was reported over various solid acids, and the glyceryl triacetate conversions of 50.2% and 31.4% over Amberlyst-15 and CNT-PANI-S catalysts were obtained after 3 hours at 55 °C. 30 Benzenesulfonic acid-functionalized CNF (d-CNF-w) and Amberlyst-15 were tested for transesterification of triolein and methanol with methyl oleate yields of 23



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72% and 22% after 4 hours at 120 °C. 26 Although it is impossible to perform a fair comparison of the catalytic activity of these carbon-based solid acids, because of the different reaction conditions employed in the literature studies. The above results can still demonstrate the superiority of CNT-PVSAIC catalyst in term of both excellent catalytic performance and mild reaction conditions required for the methanolysis reaction. In addition, catalytic performances of carbon-based solid acids for various reactions were systematically compared in Table S3 (Supporting Information), further demonstrating the superiority of our CNT-PVSAIC composites, presumably owing to its strong acidities, high surface areas and mesoporous structure. For instance, CNF-based solid acid d-CNF-w shows sulfonic acid site (-SO3H) density of 0.63 mmol g−1 and pore volume of 0.30 cm3 g−1, 26 which are evidently lower than our CNT-PVSAIC as shown in Table 2. In addition, sulfonated graphene oxide (S-RGO) was investigated as graphene-based solid acid catalyst for reaction of HMF with ethanol at 140 °C, producing 94% bioEs yield (Table S3, Supporting Information). 35 S-RGO has apparent BET surface area of 12 m2 g−1 and sulfur content around 1.08 mmol g−1, which are again lower than our CNT-PVSAIC (Table 2). The catalytic activities of CNT-PVSAIC, CNT-PVSAPC, and CNT-PSSA were further compared based on an equimolar amount of -SO3H site in CNT-P-SO3H. The catalytic activity of CNT-P-SO3H decreases with the following order of CNT-PVSAIC > CNT-PVSAPC > CNT-PSSA in terms of both glyceryl tributyrate conversion (Figure 5a) and methyl butanoate yield (Figure S6, Supporting Information) for the transesterification of glyceryl tributyrate and methanol. Figure 5b further shows quantitative insights into the catalytic performance of CNT-P-SO3H in terms of turnover frequency (TOF) for transesterification of triglyceride with methanol. Herein, the TOF for the transesterification was measured under a low triglyceride conversion level around 10−20%, given 24


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as the amount of consumed triglyceride (TG) per amount of sulfonic acid site (-SO3H) per hour for the investigated catalyst CNT-P-SO3H. Figure 5b indicates a decreased catalytic performance of CNT-P-SO3H with a sequence of CNT-PVSAIC > CNT-PVSAPC > CNT-PSSA under any specified reaction temperatures and triglyceride molecules. CNT-PVSAIC shows significantly high TOF values when compared with CNT-PVSAPC and CNT-PSSA indicating that the acid site (-SO3H) in CNT-PVSAIC proves to be most active among the investigated CNT-P-SO3H samples (Figure 5b). This results can be attributed to strong electron-withdrawing effect of positively charged imidazolium ion in CNT-PVSAIC. The electron-withdrawing effect of neighboring groups may lead to a decline of electron density on the sulfur atom in -SO3H group, which can be supported by S(2p) XPS analysis showing the highest binding energy of 169.2 eV for CNT-PVSAIC as plotted in Figure 1b. The decline of electron density of sulfur atom in -SO3H group will cause an increase of acid strength as well as catalytic performance of -SO3H group. Higher reaction temperature can significantly promote the catalytic performance of CNT-P-SO3H in the transesterification reactions and the TOF values for glyceryl triacetate methanolysis catalyzed by CNT-PVSAIC remarkably increased from 20.6 molTG molSO3H−1 h−1 at 30 °C to 72.1 molTG molSO3H−1 h−1 at 60 °C (Figure 5b).

9

Glyceryl tributyrate Conversion / %

Page 25 of 40

8

CNT-PVSAIC

7

slope = 1.49 2

R = 0.994

6 5

CNT-PVSAPC slope = 1.36

4

CNT-PSSA

2

R = 0.996

slope = 1.00 2

3

R = 0.996

2 1 1

2

3

4

5

6

7

Catalyst amount / mg

Figure 6. The effect of CNT-P-SO3H catalyst amount on glyceryl tributyrate conversion. Reaction 25



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condition: glyceryl tributyrate (302 mg, 1.0 mmol), specified catalyst amount, methanol (1.2 mL), temperature 60 °C, 1 h.

The effects of catalyst amount were examined using CNT-P-SO3H samples. Figure 6 shows the conversion of glyceryl tributyrate as a function of CNT-P-SO3H loading level. The conversions of transesterification reactions increase almost proportionally with the amounts of the CNT-P-SO3H catalysts within the range employed in this research. The observed linear plots further indicate that the CNT-P-SO3H catalysts were effectively distributed and active throughout the reactions, as previously suggested by Miller and co-workers on the kinetic model of triglyceride methanolysis using Amberlyst-15. 73 Figure 7a shows a typical transesterification reaction profile by using CNT-PVSAIC as the catalyst for methanolysis of glyceryl tributyrate. Glyceryl tributyrate quickly converted into methyl butyrate and diglyceride. The further transesterification of the diglyceride to monoglyceride was initiated after 128 min. Accordingly, the yield of diglyceride reached a maximum after 128 min. After that the yield of diglyceride declines with the presence of monoglyceride. The influence of the reaction temperature on CNT-PVSAIC-catalyzed transesterification reaction reveals that increasing the reaction temperature promotes the transesterification reaction (Figure 7b). The molar ratio of the triglyceride to methanol was further examined. Excess amount of methanol is generally required to shift the reversible equilibrium to the FAME side (Scheme 1a). In the current case, different glyceryl tributyrate to methanol molar ratios (1:60, 1:29, 1:5) were applied at a fixed reaction temperature of 50 °C as well as a fixed amount of CNT-PVSAIC (Figure 7c). Obviously, the transesterification system becomes more active as the concentration of methanol is increased.

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a

Glyceryl tributyrate Methyl butyrate Diglyceride Monoglyceride

Concentration / mmol mL

-1

0.20

0.15

0.10

0.05

0.00 0

100

200

300

400

500

600

Time / min

b 100 Glyceryl tributyrate Conversion / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

60

o

60 C o

30 C

40

20

0 0

100

200

300

400

500

600

400

500

600

Time / min

c Glyceryl tributyrate Conversion / %

Page 27 of 40

80

1:60 1:29 1:5

60

40

20

0 0

100

200

300

Time / min

Figure 7. (a) Product distribution, (b) influence of reaction temperature, and (c) influence of the ratio of glyceryl tributyrate to methanol on the transesterification of glyceryl tributyrate with methanol. Reaction conditions: (a) glyceryl tributyrate (302 mg, 1.0 mmol), CNT-PVSAIC (20 mg), methanol (1.2 mL), temperature 60 °C; (b) glyceryl tributyrate (302 mg, 1.0 mmol), CNT-PVSAIC (20 mg), methanol (1.2 mL), temperature 30 and 60 °C; (c) glyceryl tributyrate (302 mg, 1.0 mmol), CNT-PVSAIC (20 mg), methanol (0.2, 1.2, and 2.5 mL, respectively), temperature 50 °C.

27



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The reusability of CNT-PVSAIC catalyst was then investigated for transesterification of glyceryl tributyrate and methanol. A six-cycle experiment was performed at a low glyceryl tributyrate conversion level. As shown in Figure S7 (Supporting Information), the measured conversions of glyceryl tributyrate decreases from 52.4 to 40.9% after a six-cycle experiment, indicating that the catalyst CNT-PVSAIC loses partial activity during the recycling processes. Notably, treatment of the recovered catalyst with 0.1 M H2SO4 did not result in recovery of its catalytic activity. Our elemental analysis revealed that, in contrast to fresh CNT-PVSAIC, the recovered CNT-PVSAIC shows a reduced -SO3H concentration from 1.46 to 0.96 mmol g−1 (Table S1, Supporting Information). This observation presumably arises from the swollen of polymer layer and subsequent partial cleavage of covalently linked PVSAIC chains on CNT as shown in Scheme S2 (Supporting Information), resulting in a partial loss of catalytic active site (-SO3H) from CNT-PVSAIC during its recovery. 27, 46

CONCLUSIONS: In summary, composite materials (CNT-P-SO3H), obtained by covalent grafting of CNT with sulfonic acid-functionalized polymers (P-SO3H), show outstanding performance as CNT-based solid acids to transesterification and esterification reactions. The excellent properties of CNT-P-SO3H catalysts are attributed to a combination of mesoporous structure together with well extended P-SO3H coating over the outer surface of the CNT. Such structure provides the formations of uniform distributions of dense active sites on CNT surface. Besides, the surface distribution of active acidic sites is able to facilitate the access of reactants and promote the migration of reagents away from the catalyst. These features make CNT-P-SO3H composite materials interesting and ideal CNT-based solid acid catalysts for transesterification and esterification reactions.

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ASSOCIATED CONTENT: Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.xxxxxxx. Supplementary data include chemicals, synthetic procedures of CNT-PVI, CNT-PVP, and CNT-PSSA, illumination of −SO3H grafting ratio of various CNT-P-SO3H, mechanism for cleavage of polymeric chain during CNT-PVSAIC recycling experiment, properties of CNT-P-SO3H, Raman parameters of various CNT-P-SO3H, comparison of carbon-based solid acid catalysts for various reactions, nitrogen adsorption-desorption isotherms, TG-DTA analysis, FT-IR spectra, Gaussian and Lorentzian fitting procedures of Raman spectrum, HRTEM images, catalyst recycling, and typical GC analysis of transesterification (PDF).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel./Fax: (+86)20-3722-3380 (J. Chen). *E-mail: [email protected] (Y. Xu). Notes: The authors declare no competing financial interest.

ACKNOWLEDGEMENTS: We are grateful for the financial support from National Natural Science Foundation of China (21472189, 21172219, 21207039, 21306049), National Basic Research Program of China (973 Program, 2012CB215304), Science and Technology Planning Project of Guangdong Province, China (2015A010106010), the Open Project of State Key Laboratory of Chemical Engineering

29



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(SKL-ChE-14C01) and 111 Project Grant (B08021). We thank Qiumin Wu and Shenghong Dong for the assistance with the preparation of the AC-PVSAIC and PVSAIC.

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For Table of Contents Use Only

Carbon Nanotube-Based Solid Sulfonic Acids as Catalysts for Production of Fatty Acid Methyl Ester via Transesterification and Esterification

H. Liu, J. Chen,* L. Chen, Y. Xu,* X. Guo, and D. Fang

Composite materials of sulfonic acid-functionalized polymers and CNTs are efficient and recyclable CNT-based solid acids for transesterification and esterification.

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Carbon Nanotube-Based Solid Sulfonic Acids as ... - ACS Publications

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