Synthesis of a novel stabilized basic ionic liquid through immobilization on boehmite nanoparticles: A robust nanocatalyst for biodiesel production from soybean oil

Renewable Energy 138 (2019) 70e78

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Synthesis of a novel stabilized basic ionic liquid through immobilization on boehmite nanoparticles: A robust nanocatalyst for biodiesel production from soybean oil Shokoufe Hosseini a, G.R. Moradi a, *, Kiumars Bahrami b, c a b c

Catalyst Research Center, Faculty of Chemical and Petroleum Engineering, Razi University, Kermanshah, Iran Department of Organic Chemistry, Faculty of Chemistry, Razi University, Kermanshah, Iran Nanoscience and Nanotechnology Research Center (NNRC), Razi University, Kermanshah, Iran

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 March 2018 Received in revised form 26 December 2018 Accepted 12 January 2019 Available online 22 January 2019

A novel ionic liquid, chlorocholine hydroxide (CCH), was first synthesized and then stabilized on boehmite nanoparticles (BNPs-CCH). Variables such as weight percentage of catalyst, methanol to oil molar ratio and reaction time were evaluated to study the catalyst efficiency in the production of biodiesel from soybean oil and methanol. Results showed that the biodiesel yield maximum was obtained in 95.2% for optimal conditions of 11:1, 4.13 wt, 60
C and 4.4 h for the molar ratio of methanol to oil, weight percentage of catalyst, temperature and reaction time, respectively. In addition, the prepared nanocatalyst was applied 5 times under optimal conditions in order to evaluate the catalyst efficiency. According to the results it still indicated as high efficiency as in the production of biodiesel. Therefore, the strength and durability of the resultant nanocatalyst were confirmed in this research. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Biodiesel Ionic liquid Immobilization Nanocatalyst Nanoboehmite

1. Introduction In the early twentieth century, for the first time Rudolf Diesel referred oil to as a replacement for fossil fuel. So that, in the first two decades, oils only were used for certain uses and then completely replaced with petroleum [1,2]. Due to the occurrences of problems like increasing global energy demands, global climate change as a result of incremental greenhouse gas (GHG) emission, increase in fossil fuel prices and environmental concerns, the presence of a replacement for diesel fuel became essential [3e6]. Biodiesel is fatty acid (usually methyl and/or ethyl) alkyl esters derived from vegetable oil, animal fats and waste cooking oil through the catalytic transesterification reaction. It is worth noting this kind of fuel has been extensively considered as an appropriate alternative resource, owing to some valuable benefits such as, renewability, high combustion efficiency, sulfur-free, high cetane number (47), high oxygen content (10e12 wt %) and lower carbon dioxide (CO2) emission than that of the common petro-diesel [3,6e13].

* Corresponding author. E-mail address: [email protected] (G.R. Moradi). https://doi.org/10.1016/j.renene.2019.01.037 0960-1481/© 2019 Elsevier Ltd. All rights reserved.

Catalyst is known an important agent that accelerates the reaction and can lead to produce the desired product. Ionic liquids (ILs) are a new type of catalysts used as not only a co-solvent but also an extracting solvent in the reactions of biodiesel production. The ILs are organic salts consisting of various cations and onions, in which cations are organic, while anions can be organic or inorganic [14e16]. It has been reported the ILs compounds are available as
liquid at temperatures lower than 100 C. These catalysts found in acidic and alkaline types are used in various processes of biodiesel production. Some researchers have used the ionic liquid as catalyst in direct biodiesel production [17e20]. Newly, ionic liquids are extensively studied in the form of catalytic networks because of charming features like low melting temperature, negligible vapor pressure, low toxicity, high thermal stability and suitable solubility for most organic and inorganic compounds [21e23]. In general, ionic liquids have been investigated as an effective catalyst or solvent for biodiesel production [24,25]. However, using ionic liquids directly because of problems such as high viscosity, excess water consumption for product rinse, catalyst separation costs, etc. creates limitations [26e28]. To overcome the mentioned problems, it was reported that ionic liquids immobilization process was an appropriate method [29e31]. Supported ionic liquids (SILs) on magnetic nanoparticle

S. Hosseini et al. / Renewable Energy 138 (2019) 70e78

[32,33], metal oxide [34,35], molecular sieve [36], polystyrene [37], graphene oxide [38], poly divinylbenzene [39] and silica [40e42] or poly (ionic liquid)s (PILs) [43e45] all are examples of stabilization which inherits catalytic capabilities and eco-friendly features of homogeneous ionic liquids. Herein, in this study, an innovative basic ionic liquid was synthesized and immobilized on boehmite nanoparticles (BNPs) which is an environmentally friendly catalyst. Up to our knowledge, no studies have been conducted regarding the IL catalyst immobilized on the bohemite nanoparticles and its application in the biodiesel production. Above all, the most important characteristic of the catalyst is the convenience of the synthesis of ionic liquid and its base. Unlike many other ionic liquids and solid bases [46e49], for catalyst synthesis, there is no need of high temperature, specific gas, inert atmosphere or special equipment and there are no toxic waste materials in the production process. So, these features can be considered as the economic and significant benefits. The structure of the catalyst was characterized by FTIR, SEM and TEM. The effects of methanol to oil molar ratios, catalyst content and time on the efficiency of biodiesel production were investigated to get the optimum reaction conditions. Moreover, the reusability of the catalyst was assessed. 2. Materials and methods 2.1. Materials and reagents Methanol (>99.5%), normal hexane (95%), ethanol (>95.6%) and toluene were purchased from Merck Com. (2-Chloroethyl)-trimethyl ammonium chloride (98%), methyl laurate (methyl dodecanoate (>99.7%)) and Sodium hydroxide (>97.0%) were obtained from Sigma-Aldrich Com. Potassium hydroxide and aluminum nitrate nonahydrate (98.5%) were provided from Merck Com. Soybean oil (OLITALIA Fac. Italy, as oil source) was purchased from a local market. Distilled water was used throughout the experimental runs.

71

2.2. Boehmite preparation The synthesis method of boehmite nanoparticle was reported elsewhere [50]. Firstly, 6.49 g NaOH in 50 ml of distilled water and 20 g Al(NO3)3$9H2O in 30 ml distilled water were dissolved separately. Then, the former solution was added to the latter solution as dropwise under vigorous stirring. The resulting milky gel was subjected to be mixed by ultrasonic bath for 30 min at 25
C. The resultant nanoboehmite was filtered and washed by distilled water several times, eventually was dried in the oven at 225
C for 3 h. The nanoboehmite structure is given in Scheme 1. 2.3. Basic ionic liquid preparation To prepare the ionic liquid, according the reaction that is a single displacement reaction; reactants are reacted in a stoichiometric ratio. This reaction is presented in Scheme 2. Since the initial raw material for catalyst synthesis is organic, and KOH is soluble in ethanol, so for the reaction medium, ethanol is selected. The quantities of raw materials were dumped separately in ethanol and were dissolved by sonicated-assisted method for 20 min. Subsequently, the solutions were added to each other in two-necked flask, and then the resultant mixture was subjected to ultrasonic bath for 2 h. After reaction, white solid of KCl was removed by filtering. Ethanol was evaporated in 50
C for 8 h and remained discolored viscose liquid was basic-ionic liquid catalyst. 2.4. Immobilization of chlorocholine hydroxide on the boehmite nanoparticles For immobilization of chlorocholine hydroxide (CCH) on the BNPs, a 50 cc round bottom flask in fix paraffin bath on the heater stirrer equipped reflux system was used. In flask, 5 g CCH and 2.13 g nanoboehmite were added to 20 ml toluene, then kept at 100
C for 24 h to react to each other. After reaction, mixture was filtered and washed by ethanol, afterwards was dried in the oven at 50
C for 6 h. As shown in Scheme 3, negative ions of CCH, in two forms, have become stable with hydrogen bonding on hydroxyl groups of boehmite surface (in the scheme, only the combination of two

N

Cl O OH OH OH O

OH OH OH OH

Al-O OH OH

N

+

Cl

OH

Al-O O OH OH

OH OH

OH O H

BNPs-CCH Scheme 1. (BNPs) structure.

Scheme 3. Stabilization of (CCH) on (BNPs).

Cl N

Cl Cl

+

KOH

N

OH

CCH CCH = Chlorocholine Hydroxide Scheme 2. Synthesis of (CCH) from raw materials.

+

KCl

H

H

H

N

Cl

72

S. Hosseini et al. / Renewable Energy 138 (2019) 70e78

molecules is shown). The basicity of the CCH and final solid catalyst (BNPs-CCH) in methanol solution was determined by pH meter (Sartorius PB-10) and solid sample showed more alkaline property than ionic liquid.

percentage yield of produced biodiesel was calculated using equation below (1):

Yield ð%Þ ¼

2.5. Catalytic performance The desired amounts of methanol and soybean oil with molar ratios of 6:1 to 12:1 along with 2e5 wt % (by the weight of soybean oil) of catalyst were added to a flask equipped to a reflux condenser, and a magnetic stirring apparatus. According to previous work results, the optimum temperature was selected at 60
C [51] and the reaction time was in range of 2e5 h. After that, the reaction mixture was centrifuged for separation of catalyst from it. Then to remove glycerol and purify biodiesel, product was washed several times with 90
C distilled water, finally dried at 80
C for 2 h. 2.6. Characterization of produced biodiesel For biodiesel analysis, 1 g biodiesel was mixed with methyl laurate (almost 0.05 g, internal standard) in a tube, followed by the addition of n-hexane (1.5 ml). Fatty acid methyl ester (FAME) in samples was characterized via flame ionization detector (FID) by means of a HP 6890 gas chromatograph (GC). Capillary column was a BPX-70 with a length of 120 m, with a film thickness of 25 mm and internal diameter of 25 mm. Nitrogen was used as a carrier and an auxiliary gas was also used for the FID. One mL of sample was injected by a 6890 Agilent Series Injector. The input sample temperature to the injector was 50
C, which was increased to 230
C. Methyl laurate was added to biodiesel as a reference and samples were analyzed by GC which has been already described. The FAME

area of all FAME weight of reference  area of reference weight of biodiesel sample weight of producted biodiesel  100  weight of oil used (1)

2.7. Statistical design of experiments Central composite experimental design (CCD) to get the optimal condition for the yield of biodiesel was used. The effects of important variables such as methanol to oil molar ratio (X1), reaction time (X2) and weight percentage of the catalyst (X3) were investigated; while the response was biodiesel yield (Y). The range and coded level of the studied variables are listed in Table 1. Zero (0) value represents the center point for the variable while þ1 and 1 represent the upper and lower values studied for the three variables, also 2 and þ2 represent alpha values respectively [52,53]. A total of nineteen experimental runs were carried out as listed in Table 2. The full quadratic model of biodiesel yields (Eq. (2)) was established using the method of least squares. Y is referred to as the response (biodiesel yield) while b0 is the intercept coefficient (offset); b1, b2 and b3 are the linear terms, b11, b22 and b33 are the quadratic terms and b12, b13 and b23 are the interaction terms:

Y ¼ b0 þ b1 X1 þ b2 X2 þ b3 X3 þ b11 X 21 þ b22 X 22 þ b33 X 23 þ b12 X1 X2 þ b13 X1 X3 þ b23 X2 X3

(2)

Table 1 Coded variables with their actual values in optimization study.

2.8. Reusability of catalyst

Variables

Symbol

Ranges and Levels a (2)

1

0

þ1

þa (þ2)

Methanol to oil molar ratio Reaction Time (h) Catalyst Dosage (% Wt.)

X1 X2 X3

3 0.5 0.5

6 2 2

9 3.5 3.5

12 5 5

15 6.5 6.5

The experiment was studied under optimum conditions obtained from RSM study. After first reaction was done, separated catalyst without any rinsing, was used for next run in the same conditions. This process was done for 5 times.

Table 2 CCD matrix with response values. Standard Run

X1 Methanol to Oil Molar Ratio

Coded

Reaction Time (h)

Coded

Catalyst Dosage (% Wt.)

Coded

Biodiesel Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

6 12 6 12 6 12 6 12 3 15 9 9 9 9 9 9 9 9 9

1 þ1 1 þ1 1 þ1 1 þ1 2 þ2 0 0 0 0 0 0 0 0 0

2 2 5 5 2 2 5 5 3.5 3.5 0.5 6.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5

1 1 þ1 þ1 1 1 þ1 þ1 0 0 2 þ2 0 0 0 0 0 0 0

2 2 2 2 5 5 5 5 3.5 3.5 3.5 3.5 0.5 6.5 3.5 3.5 3.5 3.5 3.5

1 1 1 1 þ1 þ1 þ1 þ1 0 0 0 0 2 þ2 0 0 0 0 0

69.1 76.1 73.5 78.4 75.9 93.1 77.6 92.6 57.5 82.8 91.6 94.5 52.3 73.2 92.1 91.8 92.5 92.6 92.2

X2

X3

Y

S. Hosseini et al. / Renewable Energy 138 (2019) 70e78

3. Results and discussion 3.1. Catalyst characterization The most informative evidence, that confirmed the anchoring of CCH on the BNPs, was acquired by comparison of the FT-IR spectra of CCH and BNPs-CCH (Fig. 1). In spectra of BNPs and BNPs-CCH, several peaks at 475, 735 and 804 cm1 can be attributed to the characteristic absorption of AleO bonds [54]. In the BNPs-CCH spectrum, the stretching vibration of (CeCl) due to attachment of the CCH to the BNPs was observed at ~780 cm1 and also the (CeN) stretching band of CCH at ~985 cm1 appeared at ~990 cm1 in the BNPs-CCH spectrum. In addition, the BNPs-CCH spectrum shows strong and broad hydrogen-bonded OeH stretching bands centered between 2800 and 3700 cm1, which can be due to the formation of hydrogenic bonds between ionic liquid and solid surface. These observations clearly confirmed the formation of the BNPs-CCH catalyst. SEM and TEM pictures of BNPs are shown in Fig. 2. In both of figures, it is clear that a uniform and nanostructure is formed. SEM

73

and TEM images of the (BNPs-CCH) are given in Fig. 3. Using the relevant software, the average sizes of the particles were indicated. Obviously, the particle’s size has increased due to the stabilization of ionic liquid on the boehmite nanoparticles, and the adhesion between the catalyst particles has caused the creation of cavities, which shows themselves in the form of circles in the TEM image.

3.2. Statistical analysis in parametric study 3.2.1. Analysis of variance (ANOVA) Table 2 tabulates the CCD matrix with coded units together with the response values. An empirical model was tested with the ANOVA at confidence level of 95%. Based on the central composite design, the predicted model for biodiesel yield in this study is shown in Eq. (3):

Y ¼ þ92:24 þ 5:92X1 þ 0:85X2 þ 5:24X3  5:52X21 þ 0:20X 22  7:37X 23  0:55X1 X2 þ 2:53X1 X3  0:67X2 X3 (3)

4.5 CCH

Transmittance (%T)

4 3.5 C-Cl

3

O-H

sp3 C-H

C-N 985

BNPs

2.5 2 BNPs-CCH

1.5 1 C-Cl 780

0.5

O-H H-bonded

C-N 990

0 0

1000

2000

3000

4000

5000

Wavenumber (cm-1)

Fig. 4 illustrates the Parity plot, which shows the relationship between predicted values and observed (experimental) values for biodiesel yield in this study. In such experimental design, the R2 value in the Parity plot should be more than 0.75 to validate the empirical model [53]. The R2 value for predicted biodiesel yield is 0.9916; in which 99.2% of the variability in the data is accounted to the model. Thus, Eq. (3) is a valid model that expresses the effect of the factors on the response. Mathematical experimental model by analyzing of variance (ANOVA) with significant level of 0.05 was tested. The results of this test are presented in Table 3. ANOVA is used for significance level determination of second-order models. Statistical significance level of second order models is determined by F-value. When calculated F-value is greater than tabulated F-value, P-value will be much smaller, that is shows model statistical significance. Calculated F-value is defined by means squares regression (linear, squares and interaction terms) to mean square residual ratio [55]:

F  value ¼

Fig. 1. FT-IR of a) (BNPs), b) (CCH) and c) (BNPs-CCH).

MSregression MSresidual

Where:

180 nm

Fig. 2. a) SEM and b) TEM images of (BNPs).

(4)

74

S. Hosseini et al. / Renewable Energy 138 (2019) 70e78

250 nm

Fig. 3. a) SEM and b) TEM images of (BNPs-CCH).

95.00

MSregression ¼

MSresidual ¼

84.25

Predicted Values

SSregression DFregression

(5)

SSresidual DFresidual

(6)

Total degree of freedom is equal to total number of experiments minus one. Regression degree of freedom is equal to total number of terms minus one and residual degree of freedom is equal to total degree of freedom minus regression degree of freedom [39].

73.50

62.75

52.00 52.28

62.90

73.51

84.13

94.75

Observed Values Fig. 4. Parity plot for biodiesel yield.

3.2.2. Interaction effects of variables on biodiesel yield The interaction effects of the variables used in this study on the response are graphically shown in Fig. 5. Each three-dimensional surface plot represents the effects of two variables on the biodiesel yield as a response, while another variable is held constant. Fig. 5(a) demonstrates the effect of interaction X1X2 on the biodiesel yield (Y) at constant catalyst dosage (X3) of 3.5 %Wt. Based on the surface plot, the combined effect of the methanol to oil molar ratio and reaction time leads to the increment in biodiesel yield up to an optimum point. The biodiesel yield increases with methanol to oil molar ratio at higher reaction time. It is also clear that increase of reaction time has less effect on biodiesel yield. Fig. 5(b) illustrates the surface plot for the interaction X1X3 on

Table 3 Analysis of variance (ANOVA) for biodiesel yield model. Source

Sum of Squares (SS)

Degree of freedom (df)

Mean Square

F-Value

P-Value

Model X1: Methanol/Oil molar ratio X2: Reaction Time X3: Catalyst Dosage X1 X2 X1 X3 X2 X3 X21 X22 X23 Residual Lack of Fit Pure Error Cor Total R2: 0.9989 R2 adj: 0.9977

2904.87 560.51 11.73 439.95 2.31 51.51 3.78 722.40 0.96 1287.38 3.34 2.92 0.41 2908.21

9 1 1 1 1 1 1 1 1 1 9 5 4 18

322.76 560.51 11.73 439.95 2.31 51.51 3.78 722.40 0.96 1287.38 0.37 0.58 0.10

870.88 1512.36 31.65 1187.07 6.24 138.99 10.20 1949.19 2.60 3473.61

<0.0001 <0.0001 0.0003 <0.0001 0.0340 <0.0001 0.0109 <0.0001 0.1414 <0.0001

5.68

0.0587

significant

not significant

S. Hosseini et al. / Renewable Energy 138 (2019) 70e78

75

a) Biodiesel Yield (%)

95

85.5

76

66.5

57

5.00

12.00 4.25

10.50 3.50

9.00 2.75

7.50 2.00

6.00

b) 96

85

Biodiesel Yield (%) 74

63

52

5.00

12.00 4.25

10.50 3.50

9.00 2.75

Catalyst Dosage (wt.%)

7.50 2.00

6.00

c) 95

84.25

Biodiesel Yield (%) 73.5

62.75

52

5.00

5.00 4.25

4.25 3.50

3.50

Catalyst Dosage (wt.%)

2.75

2.75 2.00

2.00

Fig. 5. Response surface plot of biodiesel yield as function of (a) methanol to oil molar ratio and reaction time; (b) methanol to oil molar ratio and catalyst dosage, and (c) reaction time and catalyst dosage.

76

S. Hosseini et al. / Renewable Energy 138 (2019) 70e78

the biodiesel yield (Y) at constant X2 of 3.5 (h). It can be seen that both factors have the same effect on yield separately. Their increase, at first, increases the yield, and interaction of these two factors

approximately in the end of ranges has a positive effect on the biodiesel yield. The interaction X2X3 on biodiesel yield (Y) at constant X1 (methanol to oil molar ratio ¼ 9) is shown in Fig. 5(c). The interaction of catalyst dosage and reaction time is almost identical as the interaction reported in Fig. 5(a). It can be said that the interaction between these two factors, at the high reaction times and molar ratio of methanol to oil, has a favorable effect on the final yield. Therefore, all variables in this optimization model are significant on the biodiesel yield. 3.2.3. Optimization of biodiesel yield The predicted optimum conditions for the biodiesel production from the experimental data using statistical software were suggested as follows: (11:1) for methanol to oil molar ratio, 4.4 (h) for reaction time and 4.13 wt% for catalyst dosage. Transesterification of soybean oil was carried out for three times at optimized conditions, and the average value on biodiesel was calculated. The predicted and experimental biodiesel yields are reported as 95.56% and 95.2%, respectively, with 0.36% error. Thus, the optimum condition for the process is verified.

Fig. 6. Reusability of catalyst. (Reaction conditions: 60
C, 4.4 h, methanol: Soybean oil molar ratio of 11:1, catalyst dosage 4.13 wt%).

Table 4 The basicity of the basic catalysts. pH CCH (0.1 molL-1 in methanol) BNPs-CCH (0.1 molL-1 in methanol) BNPs-CCH (at the beginning of the first run) BNPs-CCH (at the end of the first run) BNPs-CCH (at the end of the fifth run)

12.10 12.98 11.62 11.23 10.41

3.3. Reusability of catalyst Fig. 6 shows the results of catalyst reusability tests. It is obvious that the yield of biodiesel decreases after the first run with fresh catalyst. The biodiesel yield remains 91.3% when the catalyst is used for the second time, and the catalyst still yields 83% after five successive runs. Reducing catalyst activity may be related to the loss of active sites during the reaction. For this purpose, the results of the catalyst basicity are given in Table 4. Obviously, over time and during reuse, the pH of the catalyst has decreased slightly. In Fig. 7, spectra related to fresh and five times used catalysts are shown. Evidently, the important bonds in the catalyst have been preserved. In Table 5, a comparison has been made between stabilized ionic liquid catalysts on their performance in biodiesel production. Evidently, BNPs-CCH is a promising heterogeneous basic catalyst for biodiesel production from soybean oil, along with satisfied performance, easy recovery and excellent stability under mild reaction conditions. 4. Conclusion

Fig. 7. The comparative FT-IR spectra of (BNPs-CCH) after 1 and 5 catalytic cycles.

In this work, (BNPs-CCH) as catalyst for synthesis of biodiesel from soybean oil was evaluated in terms of yield of biodiesel. The catalyst had the maximum efficiency when n(Methanol)/n(Soybean Oil) was 11:1, catalyst dosage was 4.13 wt% and the favorable reaction time was 4.4 h. The reusability test showed that catalyst had high potential for repeated use. The basicity of the (CCH) and (BNPs-CCH) in methanol solution was determined by pH meter (Sartorius PB-10). The (BNPs-CCH) possessed better basicity in

Table 5 Biodiesel synthesis over ionic liquid stabilized on solid base as the heterogeneous catalysts. Entry

Catalyst

Feedstock

Temp. (
C)

Catalyst

Methanol/Oil molar ratio

Time (h)

Biodiesel Yield (%)

Ref.

1 2 3 4 5 6 7 8

[Bmin]OH/MgeAleLa SO42/ZrO2eSiO2(Et)- [Ps-im] MBIAILs/MIL-101(Cr) Fe3O4@HKUST-1-ABILs SBA-15-pr-ILOH IL-Fe3O4@SiO2 FnmS-PIL BNPs-CCH

waste cooking oil soybean oil oleic acid soybean oil soybean oil soybean oil F. platanifolia L.f. oil soybean oil

65 150 67 65 65 160 85 60

3 wt% 5 wt% 11 wt% 1.2 wt% 7 wt% 0.6 g 4 wt% 4.13 wt%

12:1 18:1 10:1 30:1 20:1 12:1 9:1 11:1

6 3 4 3 8 6 5 4.4

98.7 99 91 92.3 95.4 90.1 92.8 95.2

[56] [57] [58] [59] [60] [61] [62] This Work

S. Hosseini et al. / Renewable Energy 138 (2019) 70e78

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