Microwave-assisted preparation of coal-based heterogeneous acid catalyst and its catalytic performance in esterification

Journal of Cleaner Production 183 (2018) 67e76

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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro

Microwave-assisted preparation of coal-based heterogeneous acid catalyst and its catalytic performance in esterification Hewei Yu, Shengli Niu*, Tianrui Bai, Xincheng Tang, Chunmei Lu School of Energy and Power Engineering, Shandong University, Jinan, 250061, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 14 February 2018

Powder coal is carbonized under nitrogen atmosphere and then treated with concentrate sulfuric acid with the assistance of microwave radiation for heterogeneous acid catalyst synthesis and the capability in catalyzing esterification of oleic acid with methanol for biodiesel production is subsequently studied. The catalysts are characterized by N2 adsorption-desorption, ultimate analysis, X-ray diffraction, Raman spectra, attenuated total reflectance-Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and acid amount tests to obtain the physicochemical property. Microwave irradiation is a faster and simpler process than the conventional heating, and the sulfonation duration can be shortened to be 5 min. Under the carbonization temperature of 250
C for 30 min and sulfonation temperature of 75
C for 5 min, the synthesized catalyst gains the acid amount of 1.73 mmol g1 and esterification efficiency of 98.1% is achieved with the catalyst dosage mass percentage of 10 wt%, molar ratio of methanol to oleic acid of 12, esterification temperature of 65
C and esterification duration of 180 min, where the commercial Amberlyst-15 catalyst only presents the efficiency of 71.5% under the same condition. Though the catalytic capability is crippled during the recycling reusage, it can be easily regenerated with its mostly original catalytic activity. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Biodiesel Esterification Coal-based heterogeneous acid catalyst Microwave irradiation

1. Introduction The continuous consumption of fossil fuels is inevitable due to the promotion of economic development and improvement of life quality, and it causes the energy shortage and environmental pollution in turn (Lai et al., 2016). Biodiesel is regarded as a promising alternative to the fossil diesel because it is environmental benign, renewable and biodegradable. Besides, biodiesel possesses similar properties with the conventional petroleumbased diesel to guarantee its application directly or blended with the fossil fuel at an arbitrary ratio. Biodiesel is a mixture of monoalkyl esters of long-chain fatty acids, and produced from vegetable and animal oils/fats by transesterification and esterification with short-chain alcohols in the presence of acid or base catalysts or
jek et al., 2017; Huang et al., supercritical pressure condition (Ha 2015). The alkali-catalyzed transesterification is the common commercial process with high catalytic activity and short reaction duration (Alaba et al., 2016; Cheng et al., 2014), but it is restricted by

* Corresponding author. E-mail address: [email protected] (S. Niu). https://doi.org/10.1016/j.jclepro.2018.02.145 0959-6526/© 2018 Elsevier Ltd. All rights reserved.

the high cost of raw materials and production process. Acidic oils such as crude vegetable oils or waste cooking oils have been proposed as the potential resources due to the low cost and abundant availability (Konwar et al., 2014), where the base catalysts are not applicative as saponification generated by free fatty acids (FFAs). Compared with base catalysts, the acid ones are not sensitive to the raw materials quality and can convert FFAs into esters before the base catalyzed transesterification (Bala et al., 2017). Employment of the conventional mineral acid of concentrated H2SO4 or HCL for the acidated oils pretreatment is restricted by the strong corrosion, difficult recyclability and environmental unfriendly drawbacks (Xia et al., 2012). To qualify the esterification as a “green” process, heterogeneous acid catalysts, such as mesoporous silica based materials (Kocík et al., 2016), zeolite (Alaba et al., 2016), sulfonated mesoporous ZnO (Soltani et al., 2016) and carbon-based acid (Konwar et al., 2014), etc, gradually appear and they are easily separated from the liquid reactants through physical filtration. Among various heterogeneous acid catalysts, sulfonated carbonbased ones are reported to be promising in biodiesel production due to the advantages of chemical inertness, thermal stability and structural diversity, where incomplete carbonization to form aromatic hydrocarbon structure followed by sulfonation with

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Nomenclature YLC Yulin Coal N2 Nitrogen FFAs Free fatty acids HCL Hydrochloric acid H2SO4 Sulfuric acid NaCl Sodium chloride NaOH Sodium hydroxide KOH Potassium hydroxide ZnO Zinc oxide CT1(x)-ST2(y) C and S are for carbonization and sulfonation, T1, x and T2, y represent the temperature and duration of carbonization and sulfonation process, respectively z Catalyst dosage mass percentage, wt% g Molar ratio of methanol to oleic acid Te Esterification temperature,
C te Esterification duration, h h Esterification efficiency, % XRD X-ray diffraction FTIR Fourier transform infrared XPS X-ray photoelectron spectroscopy SBET Specific surface area, m2 g1

concentrated H2SO4 or fuming SO3 is frequently used. Besides, the carbon-bearing materials are relatively cheap and widely available, where biochar (Kastner et al., 2012), glycerol (Devi et al., 2009), rice husk (Li et al., 2014) and de-oiled canola meal (Rao et al., 2011), etc, have been reported. Coal is a polycyclic aromatic hydrocarbon polymer and mainly consists of carbonaceous organic compounds, where the aromatic rings are connected to oxygen-containing functional groups and alkyl side chains. Use of coal as the support material gains increasing attentions in recent years, besides the conventional applications of power generation, gasification and liquefaction, etc. To date, Babajide et al. (2010) utilized fly ash to prepare heterogeneous basic catalyst via wet impregnation under the potassium nitrate effect to catalyze transesterification. Zhong et al. (2014) synthesized a trichloroacetic acid modified coal tar pitch heterogeneous catalyst and studied the activity in catalyzing esterification for ethyl acetate production. However, synthesis of the heterogeneous acid catalyst from coal for biodiesel production is barely reported. Lengthy duration is required for sulfonation, which is 10 h for the carbonized seed powder (Dawodu et al., 2014), 10 h for the vegetable oil asphalt and 5 h for the sugar cane bagasse (Chin et al., 2012) in oil bath. In comparison with surface thermal conduction of the conventional method, microwave irradiation can convey the heat power straight to the reactants (Soltani et al., 2016). Moreover, polar molecules can selectively absorb microwave energy and nonpolar molecules are inert to the microwave dielectric loss, which dramatically accelerates reaction rate and greatly reduces reaction duration from hours to minutes (Zhang et al., 2012). In fact, microwave irradiation is especially appropriate for the sulfonation with concentrate H2SO4 and Maza et al. (2011) have successfully introduced microwave for the efficient and fast O-sulfonation of heparin oligosaccharide intermediates. However, the microwave assisted sulfonation for heterogeneous acid synthesis for biodiesel production has barely received attention in the literature. Given the above issues, a series of coal-based heterogeneous acid catalysts are synthesized with microwave assistance under the variant carbonization and sulfonation conditions to optimize the

catalytic capability in esterification of oleic acid with methanol. Meanwhile, the catalysts are characterized by N2 adsorption and desorption, ultimate analysis, X-ray diffraction (XRD), Raman spectra, attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photon spectroscopy (XPS) and acid amount tests to explain the catalytic performance. Then, influences of the catalyst dosage mass percentage, molar ratio of methanol to oleic acid, esterification temperature and duration on the catalytic capability are investigated. For comparison, commercial Amberlyst15 catalyst is mentioned. Finally, reused property of the coal-based heterogeneous acid is studied. The subject in this manuscript has been rarely reported in the previous studies and it hopes to supplement the fundamental insight in biodiesel production catalyzed by the carbon-based heterogeneous acid. 2. Materials and methods 2.1. Materials and equipment The coal powder is collected from Yulin, in Shaanxi province, China. The phosphoric acid (Sinopharm Chemical Reagent Co.,Ltd, Shanghai, China) with purity of 85% and sulfuric acid (Laiyang Fine Chemical Plant, Shandong, China) with purity of 98% are of analytical grade. Sulfonation and esterfication process are performed in a microwave synthesis reactor (MCR-3, lenk industrial development Co., Ltd, Shanghai, China), which is operated at atmospheric pressure with the frequency of 2450 MHz. 2.2. Catalyst synthesis Coal powder is sieved between 74 and 125 mm and dried at 105
C to remove moisture in advance. 15 g of dried coal powder is impregnated with 30 g of phosphoric acid (85%) and stirred at room temperature for 2 h. The slurry is dried at 105
C in oven overnight and carbonized under nitrogen atmosphere at the designed temperature (distributed from 250
C to 450
C and the first investigated parameter for the carbon-based heterogeneous acid synthesis) in a tubular reactor for the given duration (prolonged from 30 min to 240 min and the second investigated parameter for the carbon-based heterogeneous acid synthesis). Afterwards, the carbonized sample is washed repeatedly with the hot distilled water (exceeding 80
C) until the filtrate is neutral and then dried at 105
C till to mass constant. Then, about 5 g of the carbonized support and 75 mL of the concentrated sulfuric acid are consecutively poured into a 500 mL round-bottomed three-necked flask, which is placed in the microwave irradiation synthesis reactor, and heated at the designed temperature (distributed from 60
C to 135
C and the third investigated parameter for the carbon-based heterogeneous acid synthesis) for a given duration (prolonged from 1 min to 120 min and the fourth investigated parameter for the carbon-based heterogeneous acid synthesis) for sulfonation. Throughout the sulfonation, uniform mixture effect and constant temperature control are guaranteed by a magnetic stirrer and thermocouple. At termination, the suspension is cooled and washed with the hot distilled water thoroughly to be pH neutral. Then, it is dried at 105
C in oven. The obtained carbon-based heterogeneous acid catalyst is defined as CT1(x)-ST2(y), where C and S are for carbonization and sulfonation, T1, x and T2, y represent the temperature and duration of carbonization and sulfonation process, respectively. 2.3. Catalyst characterizations Microstructure parameters of the catalysts are measured via a TriStar II 3020 surface area and porosity analyzer (Micromeritics

H. Yu et al. / Journal of Cleaner Production 183 (2018) 67e76

Co., Ltd, USA) using nitrogen adsorption and desorption at 196
C. Prior to the measurement, the catalyst is degassed at 150
C for 1 h under vacuum. Then, the surface area is calculated from BrunauerEmmett-Teller (BET) method based on the adsorption data in the relative pressure (P/P0) range 0.05e0.3. Ultimate analysis of the catalysts are carried out using an elemental analyzer (Vario ELCUBE, Germany) with the accuracy of 0.3%, where the contents of carbon, hydrogen, nitrogen and sulfur are quantitatively determined and the oxygen content is calculated by difference. X-ray diffraction (XRD) patterns of the catalysts are obtained by an X-ray Diffraction instrument (Bruker Advanced D8, Germany) with Cu Ka radiation source at 40 kV and 100 mA. Intensity data is collected in the scanning range (2q) from 10
to 70
with a 0.02
step size and a scanning rate of 4
min1. Raman spectra of the catalysts are collected using a Raman single grating spectrograph (LabRAM HR Evolution, France). For excitation, a diode laser of 514 nm is focused through a 50 objective of the microscope and the spectra are acquired in the spectral range of 4000e100 cm1 with the acquisition time for each measurement of 10 s. Functional groups of the catalysts are detected using attenuated total reflectance-Fourier transform infrared spectroscopy (ATRFTIR) on a VERTEX 70 FTIR spectrometer (Bruker Co., Ltd, Germany). Each spectrum is collected in the wave number of 4500e600 cm1 with a resolution of 4 cm1. Surface species bonded to the catalyst surface are determine by X-ray photoelectron spectroscopy (XPS) on a Thermo escalab 250Xi (Thermo fisher, USA) instrument equipped with monochromatic A1 Ka radiation (1486.7 eV) with the binding energies range from 0 to 5000 eV. Acid-base neutralization titration method is introduced to determine acid amount of the catalyst. 0.5 g of catalyst is dispersed in 30 mL of the saturated NaCl solution (2 mol L1) and vibrated under the ultrasound effect for 30 min. After filtration, the filtration with phenolphthalein indicator is titrated via NaOH solution (0.02 mol L1) to pH 7. At last, acid amount of the catalyst is estimated by the NaOH volume consumed. The acidic strength of the catalyst is determined by Hammett indicator method, where indicators of Neutral red (pKa ¼ 6.8), Methyl red (pKa ¼ 4.8), Dimethyl yellow (pKa ¼ 3.3) and Crystal violet (pKa ¼ 0.8) are used. 0.1 g of dried catalyst is shaken in 2 mL methanol with Hammett indicator and left to equilibrate for 2 h. The changes of color are noted. Catalytic capability of the coal derived carbon-based heterogeneous acid derived is estimated in esterifying oleic acid with methanol with the microwave irradiation assistance and the experimental schematic is shown in Fig. 1. For each esterification, mixture of the oleic acid, methanol and carbon-based heterogeneous acid catalyst are heated in a 500 mL three-necked roundbottom flask by the microwave irradiation and magnetically stirred throughout the experiment. A thermal couple is used to feed back temperature of the esterification system to adjust the microwave irradiation power and a water-cooled condenser is fabricated to condense back the evaporated methanol. Parameters influencing the catalytic capability of the carbon-based heterogeneous acid in esterification involves the catalyst dosage mass percentage (2e12 wt%, based on the oleic acid mass), molar ratio of methanol to oleic acid (4e14), esterification temperature (45e70
C) and esterification duration (30e240 min) are investigated. Upon termination of each esterification, the heterogeneous acid catalyst is separated from the liquid products through vacuum filtration, after which the excess methanol and the produced water are removed by vacuum distillation. Finally, capability of carbon-based heterogeneous acid in catalyzing esterification of oleic acid with

69

1 2

3

4

5

Fig. 1. Schematic diagram of microwave irradiated sulfonation-esterfication unit (1microwave oven; 2- reflux condenser; 3- control panel; 4- thermal couple; 5-magnetic rotor).

methanol is labeled by the efficiency of acid value reduction (h), as shown in Eq. (1).

h¼

Ain  Aout  100% Ain

(1)

Where, Ain and Aout are the acid numbers of oleic acid and esterification product, respectively, mg KOH/g, and measured according to Eq. (2) of GB 5009.229e2016.

A¼

VKOH  cKOH  56:1 m

(2)

Where, A is acid number of oleic acid or its esterification product, mg KOH g1, VKOH is titration volume of KOH solution, mL, cKOH is normality of standard titration solution (0.1 mol L1), 56.1 is molar mass of KOH, g mol1, and m is weight of loaded esterification product, g.

3. Results and discussion 3.1. Catalyst characterizations Elemental analyses for coal of the partial carbonization product and its derived carbon-based heterogeneous acid catalyst are summarized in Table 1, where contents of both oxygen and sulfur are enormously increased and contents of carbon and hydrogen are conversely decreased. During sulfonation, sulfonic acid groups are immobilized onto the carbon material and abundant surface oxygen functional groups, such as carboxyl, lactone and phenolic hydroxyl, etc, are also generated by concentrated sulfuric acid oxidization, which result in the increment of O/C as well as O/H ratio in the synthesized carbon-based heterogeneous acid catalyst (Jiang et al., 2011; Li et al., 2013). The structural properties of various carbon materials are shown

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H. Yu et al. / Journal of Cleaner Production 183 (2018) 67e76

V. C400(120)-S90(120)

Table 1 Ultimate analyses of the carbon materials before and after sulfonation. Samples

Mass fraction wd/%

C250(30) C250(30)-S75(5)

C

H

O

N

S

71.81 61.60

4.20 3.87

19.85 24.59

0.81 0.77

0.21 5.35

O/C

O/H

0.28 0.40

4.73 6.35

G

in Fig. 2, where all the samples including the original coal, partial carbonization product and sulfonated heterogeneous acid exhibit the similar XRD patterns. A broad peak corresponding to the diffraction of C(002) observed at 2q of 20e30
is ascribed to the amorphous carbon and indicates both the carbon-based heterogeneous acid catalysts and the predecessor mainly consists of aromatic carbon sheets in considerably random fashion (Li et al., 2013). The weak diffraction peak at 2q of 35
e45
is attributed to the (100) axis of the graphite structure. The diffraction peak of C(002) gets more poignant and is shifted towards right slightly and the diffraction peak of C(100) becomes more distinct as carbonization temperature up to 400
C, suggesting further degree of carbonization, which may be responsible for the low sulfonic acid groups attached to carbon structure. Raman spectra of the partial carbonized coal and its derived heterogeneous acid catalyst are illustrated in Fig. 3, where two distinct bands are utilized for analyzing the graphitic structure. The band at around 1360 cm1 is typically ascribed to D mode associated with the presence of defects in the graphite layer (Dawodu et al., 2014), and the band at around 1600 cm1 for G mode is due to the vibration corresponding to the movement in opposite directions of two neighboring carbon atoms in single crystal graphite (Shu et al., 2010). Furthermore, the relative intensity ratio of the D/ G modes is nearly equal for partial carbonized coal and its heterogeneous acid catalyst to demonstrate the no significant difference in the average sizes of graphene of the two materials (Suganuma et al., 2010). ATR-FTIR spectra of the original coal, partially carbonized coal and derived heterogeneous acid catalyst are presented in Fig. 4. As for the original coal, bands range from 750 cm1 to 871 cm1 are ascribed to the out-of-plane bending of ring CeH bonds attached to aromatic and heteroaromatic compounds. Bands at 2920 cm1 and 2852 cm1 are for the aliphatic CeH symmetric and asymmetric

D

I

II 1000

1200

1400

1600

1800

2000

-1

Raman shift cm

Fig. 3. Raman spectra for carbonized coal (I) before and (II) after sulfonation.

750 871 816

I II 1700 1434 1583

III

1165 1026

2920 2852 3500

3000

2500

2000

1500

1000

-1

Wavenumber cm

Fig. 4. ATR-FTIR spectra of various carbon materials I. YLC, II. C250(30), III. C250(30)S75(5).

(002)

I II III (100)

IV V

10

20

30

40

50

60

70

2 Fig. 2. XRD patterns of various carbon materials I. YLC, II. C250(120), III. C250(120)S90(120), IV. C400(120), V. C400(120)-S90(120).

vibrations, respectively. Band at 1434 cm1 is for OeH bending vibration. Band around 1700 cm1 corresponding to C]O stretch in carboxylic acid reveals the presence of eCOOH groups and band at 1583 cm1 is assigned to C]C stretching in aromatic ring mode (Konwar et al., 2014). After the partially carbonized treatment, structure variations are observed, where the bands for the above CeH and OeH vibrations diminish. Further, vibration bands at 1165 cm1 and 1026 cm1 present in the sulfonated heterogeneous acid, which are identified as the O]S]O symmetric stretching and eSO3 stretching modes in eSO3H groups to confirm successful introduction of the eSO3H groups into the carbon materials (Suganuma et al., 2011). Besides, due to the partial oxidation effect generated by concentrated sulfuric acid, the peak intensity of C]O groups gets more intense in catalyst if compared with the partially carbonized product (Ezebor et al., 2014). X-ray photoelectron spectroscopy (XPS) measurement of the

H. Yu et al. / Journal of Cleaner Production 183 (2018) 67e76

catalyst is performed to determine surface species bonded to the carbon matrix and exhibited in Fig. 5. A common peak of the O 1s spectra (Fig. 5a) centered at around 532 eV is ascribed to oxygen atoms in SeO and SeOH and S 2p spectra (Fig. 5b) with a single peak at around 168 eV is typically attributed to sulfur in eSO3H groups. As for other peaks, such as the thiol group at 163 eV, is not detected in S 2p spectra to demonstrate association of sulfur atoms to eSO3H groups in the heterogeneous acid catalyst (Aacute et al., 2010). Furthermore, the distinctions of two samples embody in the peak intensity of O 1s and S 2p region. Meanwhile, C250(120)S90(120) presents higher sulfur and oxygen contents than C450(120)-S90(120), which indicates the significant influence of carbonization temperature on catalyst surface species and is accordance with the esterification efficiency in Fig. 6a and acid amount listed in Table 3 (discussed in section 3.2).

3.2. Synthesis conditions optimization To optimize the synthesis conditions, parameters of the carbonbased heterogeneous acid catalyzed esterification are set as the catalyst dosage mass percentage of 10 wt%, molar ratio of methanol to oleic acid of 10, esterification temperature of 65
C and esterification duration of 120 min. As presented in Fig. 6a, esterification efficiency is gradually decreased from 95.2% to 84.7% with carbonization temperature

(a)

I

increased from 250 to 400
C. Further increasing the temperature to 450
C, only 61.7% of esterification efficiency can be obtained. Analogously, the acid amount listed in Table 3 shows the unified variation tendency with the esterification efficiency due to the aggravated hardness of the carbon material with carbonization temperature impedes attachment of the sulfonic acid groups during sulfonation. However, the microstructure parameters of catalyst at various carbonization temperatures exhibit the opposite regularity. As can be seen in Table 2, surface areas ranging from negligible value to 5 m2 g1 are listed for the catalysts prepared at low carbonization temperatures of 250, 300 and 350
C, where the well-developed microstructure parameters are obtained as carbonization temperature is increased to 450
C. Interestingly, the C250(120)-S90(120) catalyst, which possesses the negligible specific surface area and the largest acid amount (1.70 mmol g1), exhibits the largest esterification efficiency of 95.2%, while the C450(120)-S90(120) catalyst, which possesses the largest specific surface area (407 m2 g1) and the lowest acid amount (0.47 mmol g1), exhibits the lowest esterification efficiency of 61.7%. The similar results were also reported by Fu et al. (2012) of the phosphoric acid-impregnated pulp fibers catalyst. Although the well-developed microstructure is beneficial for mass transfer during esterification, the acid amount may play a more important role in determining the catalytic activity of coal-based heterogeneous acid in this study.

I

(b)

O 1s

71

S 2p

II

II

542

540

538

536

534

532

530

528

526

174

172

170

Binding energy/eV

168

166

164

162

160

Binding energy/eV

C 1s

(c)

O 1s

I S 2p C 1s

O 1s

II S 2p 1200

1000

800

600

400

200

Binding energy/eV Fig. 5. XPS spectra of the heterogeneous acid catalyst. (a) Narrow XPS scan in S 2p region, (b) Narrow XPS scan in O 1s region, (c) XPS survey scan I. C250(120)-S90(120), II.C450(120)-S90(120).

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H. Yu et al. / Journal of Cleaner Production 183 (2018) 67e76

96.0

100

(b)

(a)

95

95.5

Esterification efficiency

Esterification efficiency

90 85 80 75 70 65

95.0

94.5

60 250

300

350

400

450

94.0

30

o

Carbonization temperature/ C

60

120

180

240

Carbonization duration/min 96.5

(c)

96

(d)

Esterification efficiency

Esterification efficiency

96.0

94

92

90

88

60

75

90

105

120

135

95.5

95.0

94.5

94.0

1

o

Sulfonation temperature/ C

5

15

45

90

120

Sulfonation duration/min

Fig. 6. Influence of the synthesis conditions on the catalytic capability. a. Carbonization temperature (carbonization duration of 120 min, sulfonation temperature of 90
C and sulfonation duration of 120 min), b. Carbonization duration (carbonization temperature of 250
C, sulfonation temperature of 90
C and sulfonation duration of 120 min), c. Sulfonation temperature (carbonization temperature of 250
C, carbonization duration of 30 min and sulfonation duration of 120 min), d. Sulfonation duration (carbonization temperature of 250
C, carbonization duration of 30 min and sulfonation temperature of 75
C).

Table 2 Microstructure parameters varied with carbonization temperature. Catalysts

SBET/m2 g1

Pore Volume/cm3 g1

C250(120)-S90(120) C300(120)-S90(120) C350(120)-S90(120) C400(120)-S90(120) C450(120)-S90(120)

e 1 5 115 407

e e e 0.06 0.22

Influence of carbonization duration on the catalytic capability and acid amount of carbon-based heterogeneous acid catalyst is

summarized in Fig. 6b and Table 3. With carbonization duration prolonging, the acid amount varies between 1.70 and 1.76 mmol g1. Additionally, all the coal-based heterogeneous acid catalysts prepared with disparate carbonization duration are demonstrated to be active for esterification of oleic and methanol, where esterification efficiency of higher than 95% can be obtained and no obvious decline is observed in Fig. 6b. Nevertheless, catalyst prepared with carbonization duration of 30 min possesses the highest acid amount and catalytic performance, which demonstrates carbonization duration of 30 min is sufficient for the carbon material formation to load the sulfonic acid groups. Hence, the

Table 3 Acid amount varied with preparation parameters. Catalysts

Acid amount/mmol g1

Catalysts

Acid amount/mmol g1

C250(120)-S90(120) C300(120)-S90(120) C350(120)-S90(120) C400(120)-S90(120) C450(120)-S90(120)

1.70 1.56 1.04 0.64 0.47

C250(30)-S90(120) C250(60)-S90(120) C250(120)-S90(120) C250(180)-S90(120) C250(240)-S90(120)

1.76 1.74 1.70 1.71 1.74

C250(30)-S60(120) C250(30)-S75(120) C250(30)-S90 (120) C250(30)-S105(120) C250(30)-S120(120) C250(30)-S135(120)

1.58 1.75 1.76 1.61 1.29 0.86

C250(30)-S75(1) C250(30)-S75(5) C250(30)-S75(15) C250(30)-S75(45) C250(30)-S75(90) C250(30)-S75(120)

1.59 1.73 1.72 1.74 1.73 1.74

H. Yu et al. / Journal of Cleaner Production 183 (2018) 67e76

carbonization procedure is rapid and energy efficient compared with glucose carbonized for 15 h (Takagaki et al., 2006) and peanut shell partially carbonized for 15 h (Zeng et al., 2014). Sulfonation with concentrated sulfuric acid is frequently used to graft acid active groups onto the carbon materials and influence of sulfonation temperature on catalytic capability is presented in Fig. 6c. Esterification efficiency is up to 95.8% from 94.9% when the sulfonation temperature is climbed from 60
C to 75
C and satisfactory value could be maintained till to sulfonation temperature of 105
C. Nonetheless, esterification efficiency is only 89.0% when sulfonation temperature is further increased to 135
C. Accordingly, the acid amount is slightly decreased to 0.86 mmol g1 from 1.75 mmol g1 at sulfonation temperature of 75
C due to the restricted incorporation of eSO3H groups under the harsh sulfonation condition. Sulfonation between concentrated sulfuric acid and aromatic carbon is a reversible thermal electrophilic substitution and certain duration is required for equilibrium. Compared with the traditional heating method for sulfonation (such as oil bath), sulfonation duration can be distinctly shortened under microwave irradiation. As shown in Table 3, acid amount of catalyst from 1.59 mmol g1 to 1.73 mmol g1 is obtained with sulfonation duration from 1 to 5 min and kept invariable if further prolonging sulfonation duration to 120 min, which is in good agreement with the esterification efficiency in Fig. 6d. This result demonstrates that microwave radiation is faster, simpler and more energetically efficient than the conventional heating method for carbon-based heterogeneous acid synthesis to produce biodiesel. Moreover, the Hammett indicator method can provide qualitative information about the acidity of the solid catalysts. The color of Dimethyl yellow is changed from yellow to red, but no color variation is observed when crystal violet is tested for the synthesized catalyst. Therefore, the acid strength lies in the range between pH 0.8 and 3.3, which demonstrates that the prepared catalyst extremely suitable for esterification with acidic characteristic. 3.3. Optimization study Generally, high catalyst dosage mass percentage means abundant active sites and contact probability between reagents and catalyst is subsequently intensified, where high esterification efficiency is obligatorily expected, especially for the poor mass transfer restricted by the undeveloped microstructure of the carbon-based heterogeneous acid catalyst (Man and Lee, 2011). It can be observed from Fig. 7a that the non-catalyzed esterification is very slow, while remarkable enhancement of esterification efficiency is observed from 71.9% at 2 wt% mass percentage to 95.7% at 10 wt% mass percentage ascribed to the intensive availability of catalytic sites (Sheikh et al., 2013). However, there is only marginal augment in esterification efficiency if the catalyst dosage mass percentage is further increased to 12 wt% due to the equilibrium limit. Similar results were reported by Zhao et al. with the peanut shell derived carbon-based heterogeneous acid catalyst (Zhao et al., 2010). Since esterification of oleic acid and methanol is reversible, excess amount of methanol is crucial to support the reaction moving forwards and gain high efficiency (Lokman et al., 2014). As shown in Fig. 7b, when the molar ratio is increased from 4 to 12, esterification efficiency is raised from 76.6% to 97.1% due to the consolidated forwards reaction by the excess methanol. However, further increasing the molar ratio to 14 makes no sense in raising esterification efficiency for the limited equilibrium of esterification and similar observation was also reported by Ezebor et al. (2014). Five esterification temperatures, which are distributed from 45
C to 70
C with an increment of 5
C, are selected to study the influence of reaction temperatures on catalytic capability of the

73

carbon-based heterogeneous acid in esterification. Increasing esterification temperature can boost esterification rate due to the reduced mass transfer limitation between reactant and catalyst (Caetano et al., 2008). As exhibited in Fig. 7c, an increment in reaction temperature from 45
C to 65
C distinctly heightens esterification efficiency from 80.1% to 97.3%. However, the esterification efficiency is markedly decreased to 84.1% at the reaction temperature of 70
C. Large numbers of bubbles are formed if esterification temperature is higher than the methanol boiling point (64.7
C under atmospheric pressure), where the poor contact between catalyst and reactants thereby inhibits the esterification (Zhang et al., 2012). Due to the mass transfer limitation caused by three immiscible phases of oleic acid, methanol and the carbon-based heterogeneous acid catalyst, lengthy esterification duration is normally required in previous studies to guarantee completion of esterification. With esterification deepening, products are accumulated with consumption of the reagents, where the reaction rate is thus weakened due to the reversible property of esterification. As illustrated in Fig. 7d, esterification duration evidently influences the esterification efficiency, especially in the initial 2 h when the reactants concentrations are at high level and esterification efficiency significantly climbed from 92.6% to 97.3% with the esterification duration prolonged from 30 min to 120 min. For the reaction duration of 180 min, esterification efficiency of 98.1% is obtained and then basically kept constant even further prolonging duration to 240 min. As demonstrated by the above catalytic results, the sulfonated coal-based heterogeneous acid is remarkably effective for the production of methyl oleate, where the topmost esterification efficiency of 98.1% is obtained under optimized condition of the catalyst dosage mass percentage of 10 wt%, molar ratio of methanol to oleic acid of 12, esterification temperature at 65
C and duration of 180 min. Catalyst synthesized at the optimal carbonization and sulfonation condition possesses the acid amount of 1.73 mmol g1 to demonstrate the effective utilization of concentrated sulfuric acid and validate the efficiency and economy of sulfonation process in comparison with many other carbon-based catalysts (Ezebor et al., 2014; Zhong et al., 2014). Although the negligible surface area is measured by N2 adsorption-desorption, the high densities of eSO3H and eCOOH hydrophilic groups (proved by FTIR and elemental analysis) facilitate the adsorption of a large amount of hydrophilic molecules such as methanol and favor the access of the reactants to the active sites, thus enhancing the catalytic performance. It is worth mentioning that more research work such as how to improve pore structure to further enhancing the mass transfer is in progress despite the high catalytic activity of this synthesized coal-based heterogeneous acid. Furthermore, a comparative study is performed with the widely used industrial heterogeneous acid catalyst of Amberlyst-15 under the unified esterification conditions of catalyst dosage mass percentage of 10 wt%, molar ratio of methanol to oleic acid of 12, esterification temperature of 65
C and esterification duration of 180 min. It is demonstrated that the carbon-based heterogeneous acid derived from coal exhibits much stronger capability in catalyzing esterification of oleic acid with methanol than Amberlyst-15, whose esterification efficiency is only 73.4% in comparison with 98.1% of the carbon-based one. Table 4 reveals a comparative analysis of esterification of FFAs with methanol catalyzed by the recently developed heterogeneous acid catalysts in the literature for biodiesel production, where the coal-based heterogeneous acid catalyst is of great application prospect and the appropriate operating parameters for methyl oleate production make it possible for comparing with other reported heterogeneous acid catalysts. Moreover, Table 5 shows the partial physicochemical indexes of

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H. Yu et al. / Journal of Cleaner Production 183 (2018) 67e76

100

(b) Esterification efficiency/%

95

90

85

80

75 4

6

8

10

12

14

Molar ratio of methanol to oleic acid 100

100

(c)

(d)

96

92

Esterification efficiency

Esterification efficiency

98

88

84

80

96

94

92

76

45

50

55

60

65

70

30

60

Esterification temperature/ oC

90

120

150

180

210

240

Reaction duration/min

Fig. 7. Optimization of the esterification parameters. a. Catalyst dosage mass percentage (molar ratio of methanol to oleic acid of 10, esterification temperature of 65
C and esterification duration of 120 min), b. Molar ratio of methanol to oleic acid (catalyst dosage mass percentage of 10 wt%, esterification temperature of 65
C and esterification duration of 120 min), c. Esterification temperature (catalyst dosage mass percentage of 10 wt%, molar ratio of methanol to oleic acid 12, esterification duration of 120 min), d. Esterification duration (catalyst dosage mass percentage of 10 wt%, molar ratio of methanol to oleic acid of 12, esterification temperature of 65
C).

Table 4 Comparison of esterification operating parameters and efficiency between different solid acid catalysts. Catalysts

FFAs

z/wt%

g

Te/
C

te/h

h/%

References

Sulfonated mesoporous ZnO De-oiled canola meal-derived Sugar cane bagasse Corn straw-derived Ordered mesoporous carbon-derived Coal-derived

palm fatty acid distillate oleic acid palm fatty acid distillate oleic acid oleic acid oleic acid

2 7.5 11.5 7 3.54 10

9 60 20 (weight ratio) 7 10 12

120 65 170 60 70 65

1.5 24 0.5 4 3 3

95.6 93.8 80 98 73.59 98.1

(Soltani et al., 2016) (Rao et al., 2011) (Chin et al., 2012) (Liu et al., 2013) (Liu et al., 2008) This study

density, water content, kinetic viscosity, carbon residue, ash content of the purified biodiesel, which meet the ASTM standard.

Table 5 Physicochemical indexes for the purified biodiesel. Index

3.4. Operational stability experiment of the catalyst To investigate the reusability, esterification of oleic acid with methanol catalyzed by the carbon-based heterogeneous acid derived from coal is performed under the above optimized condition. After each esterification, the heterogeneous acid catalyst is separated by suction filtration and then washed by ethanol to swill the oil substance. After dried at 105
C for 4 h in oven, the recycle heterogeneous acid is used in the next cycle esterification. Fig. 8

Density Kinetic viscosity Carbon residue Ash content Water content

Unit 3




g m ; 20 C mm2 s1; 40
C wt% wt% mg/kg

ASTM limits

Purified biodiesel

0.85e0.90 1.9e6.0 0.1 max 0.02 max 0.03 max

0.87 5.60 0.05 0.01 0.02

shows esterification efficiency catalyzed by the reused heterogeneous acid and the corresponding acid amount. Satisfying

H. Yu et al. / Journal of Cleaner Production 183 (2018) 67e76

reusability of the carbon-based heterogeneous acid is not expectedly performed and esterification efficiency is decreased by 21.3% for the third reused cycle, where the acid amount is accordingly decreased from 1.73 mmol g1 for the fresh heterogeneous acid catalyst to 0.55 mmol g1 of the spent catalyst after three cycles. It is revealed that the sulfonated groups attached to the carbon functionalities are leached out during esterification and deactivation of the other carbon-based heterogeneous acid catalyst has also been noticed in the previous studies (Chin et al., 2012; Rao et al., 2011). As it was suggested in the literature reported by Chin et al. (2012), the deactivation of catalyst was mainly due to the leaching of sulfonated groups into methanol phase. Moreover, to investigate the possibility of regeneration, the spent heterogeneous acid catalysts after the first and fourth cycles are sulfonated again for reactivation, where acid amount of 1.47 and 1.10 mmol g1 and esterification efficiency of 97.3% and 94.2% can be obtained, respectively. This clearly demonstrates that realization of catalyst regeneration by sulfuric acid treatment is also acceptable.

4. Conclusions A variety of coal-based heterogeneous acid catalysts capable of esterification of oleic acid and methanol are synthesized via incompletely carbonization followed by sulfonation method. Microwave irradiation is introduced to the sulfonation process which can greatly shorten the sulfonation duration to 5 min compared with other reported carbon-based heterogeneous acid, where the remaining synthesis parameters are optimized as carbonized at 250
C for 30 min and sulfonated at 75
C. Characterization analyses reveal that the resulting catalyst consists of amorphous carbon with 1.73 mmol g1 of acid amount. The catalyst exhibits good catalytic activity in esterification and 98.1% of conversion can be achieved at 65
C and 180 min of reaction duration, in the presence of 10 wt% of catalyst and methanol to oleic acid molar ratio of 12. Besides, the heterogeneous acid catalyst has significantly higher activity (e.g., 98.1%, 180 min) compared to the commercial Amberlyst-15 catalyst (e.g., 71.5%, 180 min). In the catalyst reused studies, the catalyst shows some loss in activity associated with the leaching out of active sites. However, the spent solid acid can be easily regenerated by sulfuric acid treatment and its original activity is mostly recovered.

1.8

Esterification efficiency 1.6 Acid amount

100

1.4

80

Acid amount/mmol g-1

Esterification efficiency/%

90

1.2 70

1.0 60

0.8 50

0.6 40

0.4 30 0.2 20 1

2

3

4

Recycling times Fig. 8. Reusability of the carbon-based heterogeneous acid. (catalyst dosage mass percentage of 10 wt%, molar ratio of methanol to oleic acid of 12, esterification temperature of 65
C and esterification duration of 180 min).

75

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