Page | 5760

Catalytic production of biodiesel from esterification of lauric acid over a solid acid hybrid

Qiuyun Zhang 1,*

, Li Zhao 1, Taoli Deng

1, , Yutao Zhang

2, , Hu Li

3,4*

1 School of Chemistry and Chemical Engineering, Anshun University, Anshun, Guizhou, China 2 School of Resource and Environmental Engineering, Anshun University, Anshun, Guizhou, China 3 College of Engineering, Nanjing Agricultural University, Nanjing 210031, China 4 State-Local Joint Engineering Lab for Comprehensive Utilization of Biomass, Guizhou University, Guiyang 550025, China

*corresponding author e-mail addresses: qyzhang.asu@gmail.com | Scopus ID 57203621185

hli17@njau.edu.cn | Scopus ID 35933455300

ABSTRACT

In this study, an acidic heterogeneous hybrid (H4SiW/MIL-100(Fe)) was synthesized by incorporation of 12-silicotungstic acid (H4SiW)

into a porous metal-organic framework MIL-100(Fe) via a simple hydrothermal method. The hybrid was characterized by XRD, FT-IR,

SEM, and TGA, and those characterization analyses showed the H4SiW/MIL-100(Fe) hybrid was successfully synthesized and exhibited

good structural stability. The catalytic activity of H4SiW/MIL-100(Fe) was determined by esterification of lauric acid (LA) to produce

biodiesel. Optimum LA conversion of 80.3% was achieved over the H4SiW/MIL-100(Fe) hybrid under the reaction conditions of lauric

acid/methanol mole ratio of 1/12, 0.3 g catalyst dosage at 160°C for 3 h. After being repeatedly used for eleven cycles, the lauric acid

conversion of the catalyst could still maintain with >60%, which indicates potential cost-saving and affordable biodiesel synthesis

possibilities.

Keywords: 12-Silicotungstic acid; MIL-100(Fe); Immobilized; Lauric acid; Esterification; Biodiesel.

1. INTRODUCTION

In recent years, decreasing fossils fuels and increasing

greenhouse gas emissions led to global warming, which has

encouraged scientists to devote a search for alternative fuels that

can create less pollution and are more eco-friendly [1-3].

Biodiesel, as a promising eco-friendly alternative fuel, has high

potential to be a substitute for fossils fuels, and it can be produced

by either esterification of free fatty acids or transesterification of

triglycerides with short-chain alcohols in the presence of an acidic

catalyst [4]. In general, liquid acid (sulfuric acid, hydrochloric

acid, and phosphoric acid, etc.), which is relatively cheap, exhibits

high catalytic activity. However, it will lead to catalyst recycling

difficulties, corrosion of the equipment, and a large amount of

acidity wastewater [5]. To avoid the above problems, varieties of

solid acid catalysts can be used as more promising substitutes [6-

10].

Recently, due to its strong Brønsted acidity properties and

high catalytic activity, heteropolyacids with Keggin-type structure

have been widely studied for acid catalysis and catalytic synthesis

[11-13]. However, the heteropolyacids are unsuitable for

esterification due to their high solubility in a polar solvent, and

they also suffer from the drawback of the low specific surface area

[14]. Thus, great efforts have made to the development of

supported heteropolyacids, such as mesoporous H4SiW12O40-SiO2

[15], H3PW12O40/K10 [16], and (PW11)3/MCM-41 [17].

Currently, the heteropolyacids have been introduced to

metal-organic frameworks (MOFs), due to the extremely large

surface areas and appropriate porous pores of MOFs. Wee et al.

[18] reported that an original synthesis approach to prepare

Cu3(BTC)2 encapsulated Keggin HPW, which showed higher

catalytic activity in the esterification. Xie et al. [19] also

demonstrated that the encapsulation of Cs2.5H0.5PW12O40 in the

cages of UiO-66, and the catalyst demonstrate excellent catalytic

activities for the production of low-calorie structured lipids. Our

group has also studied the production of biodiesel over HPMo/Cu-

BTC and HSiW/UiO-66 hybrids, and the obtained results revealed

these hybrids exhibited good catalytic activity [20-21].

However, no work has been done so far for the production

of methyl laurate over a heterogeneous and stable acidic

H4SiW/MIL-100(Fe) hybrid catalyst in the literature.

In the present investigation, the encapsulation of H4SiW in

the cages of MIL-100(Fe) was developed and its catalytic ability

was tested for the esterification of lauric acid to methyl laurate,

biodiesel component. The characterization of the synthesized

hybrid was done using XRD, FTIR, SEM, and TGA. Further, the

stability of the catalyst had also been studied by reutilization tests.

2. MATERIALS AND METHODS

2.1. Materials.

Iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O, AR), lauric

acid (LA, AR), ethanol (AR), and methanol (AR) were obtained

from Sinopharm Chemical Reagent Co., Ltd. Silicotungstic acid

(H4SiW12O40·nH2O, H4SiW, AR), and 1,3,5-benzene tricarboxylic

acid (H3-BTC, AR) were obtained from Shanghai Aladdin

Industrial Inc. All the purchased chemicals were used without

further purification.

2.2. Synthesis of samples.

H4SiW/MIL-100(Fe) was synthesized by a hydrothermal

technique according to the following steps [21]: Fe(NO3)3·9H2O

(1.21 g) and H4SiW (1.0 g) were dissolved into 18 mL distilled

water, and H3-BTC (0.63 g) was then added into the above

mixture with stirring magnetically at room temperature for 1 h.

Volume 10, Issue 4, 2020, 5760 - 5764 ISSN 2069-5837

Open Access Journal Received: 10.03.2020 / Revised: 26.03.2020 / Accepted: 28.03.2020 / Published on-line: 01.04.2020

Original Research Article

Biointerface Research in Applied Chemistry www.BiointerfaceResearch.com

https://doi.org/10.33263/BRIAC104.760764

Catalytic production of biodiesel from esterification of lauric acid over a solid acid hybrid

Page | 5761

The mixture solution was put in 50 mL Teflon liner

autoclave, closed in an autoclave and heated in an oven for 6 h at

120 °C. After cooling, the resulting solid was centrifuged, and

washed with ethanol many times. Then, the solid was further

stirred for 3 h several times in a hot ethanol bath, and dried for 24

h in an oven at 120 °C. MIL-100(Fe) was also synthesized to

follow the same method in the absence of H4SiW.

2.3. Characterizations.

The X-ray powder diffraction (XRD) patterns of the

samples were recorded by a D8 ADVANCE (Germany) using

CuKĮ (1.5406 Å) radiation. Infrared spectra were recorded on a

PerkinElmer spectrum100 Fourier-transform infrared (FTIR)

spectrometer in the range of 400-4000 cm-1. The morphologies of

as-synthesized catalysts were observed by a Hitachi S-4800 field

emission scanning electron microscopy (SEM) with an

acceleration voltage of 2.0 kV. Thermogravimetric (TG) analysis

was carried out in a NETZSCH/STA 409 PC Luxx simultaneous

thermal analyzer.

2.4. Esterification experiment.

LA, methanol, and required amount of as-prepared

catalysts were loaded into the stainless steel autoclave.

Subsequently, the reaction system was heated to the specified

temperature. After reaction for the designated period, the catalysts

were centrifuged, and the liquid mixture was vaporized to remove

the excess methanol combined with the water produced. The

conversion of LA was calculated by titration according to the acid

value before and after the reaction, and the acid value was

calculated by ISO 660-2009 standard method.

3. RESULTS

3.1 Characterization.

The powder XRD patterns of the H4SiW, MIL-100(Fe),

and H4SiW/MIL-100(Fe) samples are presented in Figure 1. The

XRD pattern of the MIL-100(Fe) shows a broad diffraction peak

within 10° to 40° was attributed to a low level of crystallinity, and

the trend was the same as those reported in the literature [22].

Moreover, Keggin H4SiW anion diffraction peaks were

observed in a halo between 5° and 60° (2 theta) [23]. When the

synthesis was performed in the presence of H4SiW, a similar

phenomenon was also observed between MIL-100(Fe) and

H4SiW/MIL-100(Fe). More specifically, the XRD pattern of

H4SiW could not be detected from that of H4SiW/MIL-100(Fe),

this probably because of the H4SiW was uniformly distributed into

the MIL-100(Fe) framework. Thus, it can be seen that the

H4SiW/MIL-100(Fe) hybrid was successfully synthesized.

Figure 1. XRD patterns of H4SiW, MIL-100(Fe), and H4SiW/MIL-

100(Fe) catalysts.

The FT-IR spectra of the H4SiW, MIL-100(Fe), and

H4SiW/MIL-100(Fe) catalysts are shown in Figure 2. For the pure

H4SiW, The peaks at 804, 884, 927, and 980 cm-1 represent the

Keggin structure of H4SiW [24]. For MIL-100(Fe), the FTIR

spectrum observed in the region of 1636 and 1585 cm-1 is due to

the splitting of the coordinated -CO2- vibrations, FT-IR bands at

around 759 cm-1 and 710 cm-1 are due to the presence of C-CO2

bond. The FTIR peaks detected at 1454 cm-1 and 1383 cm-1

represent the existence of -COOFe metallic esters, and a similar

result was observed by other authors [25]. In strong contrast to

H4SiW and MIL-100(Fe) sample, the absorption peaks for

H4SiW/MIL-100(Fe) hybrids are also seen at 804, 884, 927, and

980 cm-1, which correspond to the W-Oe-W, W-Oc-W, Si-O,

W=O, respectively, suggesting that the pristine H4SiW were well

maintained in the materials synthesis process. These results

indicated that the pristine H4SiW molecules were introduced into

the MIL-100(Fe) matrix successfully.

Figure 2. FTIR spectra of H4SiW, MIL-100(Fe), H4SiW/MIL-100(Fe),

and H4SiW/MIL-100(Fe) after 11 reaction cycles.

SEM was recorded to observe the morphology of the

prepared hybrids; Hence, the morphology of the prepared

H4SiW/MIL-100(Fe) hybrids is studied using SEM and the results

are shown in Figure 3a-c.

The SEM image of MIL-100(Fe) shows many nearly-cube

and nearly-spherical particles with an average size of about 400-

500 nm can be found (Figure 3b). In accordance with previous

reports, similar morphologies have also been observed [26]. As

shown in Figure 3c, the SEM image of the H4SiW/MIL-100(Fe)

sample also demonstrates that a cube-like structure with the size of

about 300 nm, and the surface of these particles are relatively

smooth, suggesting that the H4SiW molecules are pretty uniformly

distributed on the MIL-100(Fe) crystals.

Meanwhile, no morphology difference was observed after

the introduction of the H4SiW species, which fits the XRD

Qiuyun Zhang, Li Zhao, Taoli Deng, Yutao Zhang, Hu Li

Page | 5762

analysis well, and it could be also seen that the frame structure of

MIL-100(Fe) was stable in the synthesis process.

Figure 3. SEM images of (a) H4SiW, (b) MIL-100(Fe), and (c)

H4SiW/MIL-100(Fe) catalysts.

Figure 4 shows the result of TGA analysis of H4SiW/MIL-

100(Fe) hybrid. In the temperature range of 40-300 °C, a 15%

weight loss was observed, which was due to the loss of water

molecules and solvent molecules solvent inner/outer the surfaces

of the catalyst. The second mass loss (20%) occurred in the range

of 300-700°C, which may be related to the decomposition of the

frameworks. Thus, the synthesized H4SiW/MIL-100(Fe) hybrid

shows greater stability up to 300 °C, and can be employed as a

solid acid catalyst for LA esterification.

Figure 4. TGA thermogram of H4SiW/MIL-100(Fe) hybrid.

3.2. Material performance.

Figure 5a shows the time effect on the esterification

reaction of biodiesel production from LA in the range of 0.5-5 h.

The reaction was performed at 160°C, 0.3 g of catalyst, and LA-

methanol mole ratio 1:12. In half an hour of reaction, the

H4SiW/MIL-100(Fe) hybrid was not effective because the LA

conversion was only 22.6%. With increasing reaction time in the

range of 0.5 to 3 h, LA conversion was also increased from 22.6%

to 80.3%, respectively. However, further increasing reaction time

resulted in the decrease of LA conversion, which may be probably

due to the addition of time causing the esterification reaction

reversion back by hydrolysis of methyl laurate to LA. Shuit et al.

[27] also reported a similar trend in their study. Hence, in the

present study, 3 h was set as the optimum reaction time.

Different LA to methanol molar ratios were used in

studying the influence of methanol content in the esterification of

LA, as presented in Figure 5b. The reaction was performed at

160°C, 0.3 g of catalyst after 3 h reaction. It was observed that the

conversion of LA increased gradually from 1:4 to 1:12. The

maximum LA conversion (80.3%) was obtained for the LA to

methanol ratio of 1:12. However, a significant change in LA

conversion was observed with the excess amount of methanol.

When the LA to methanol ratio was 1:16, the LA conversion was

decreased to 71.0%. This result may be due to the dissolution of

LA, intermediates and methyl laurate product, and the reduced

hybrid catalyst effect by filling its porous structure. Similar work

was performed by Sanger et al. [28] Thus, 1:12 LA to methanol

molar ratio was determined as the optimum for esterification of

LA.

The effect of reaction temperature from 100 to 180°C on

the conversion of LA was investigated and shown in Figure 5c.

The impact of reaction temperature was investigated between 100

and 180°C for 0.3 g of catalyst and LA-methanol mole ratio 1:12

after 3 h reaction. As the reaction temperature raising from 100 to

160°C resulted in an increase in LA conversion from 40.2% to

80.3%, respectively.

Figure. 5 Effect of (a) reaction time, (b) LA-methanol mole ratio, (c)

reaction temperature, and catalyst dosage (d) on LA conversion using

H4SiW/MIL-100(Fe) hybrids.

However, with a further increment of temperature from 160

to 180 °C, there was no major change in LA conversion,

indicating that the reaction had reached equilibrium [29].

Catalytic production of biodiesel from esterification of lauric acid over a solid acid hybrid

Page | 5763

Therefore, in this work, 160°C was the optimal reaction

temperature for the reaction.

The effect of H4SiW/MIL-100(Fe) catalyst weight on the

esterification reaction also was examined. The catalyst weight was

varied from 0.03 to 0.36 g by LA-methanol mole ratio 1:12 and

the reaction was carried out at a temperature of 160°C for 3 h. The

obtained results are shown in Figure 5d.

It was observed that the LA conversion increased as the

catalyst amount was increased. When the catalyst weight was 0.3

g, the LA conversion of 80.3% was obtained. However, it must be

noted that the excess amount of the catalyst can result in the

increased viscosity of the mixture and increased separation cost

[30]. 0.3 g is thus selected as the appropriate catalyst weight for

the present esterification reaction over H4SiW/MIL-100(Fe)

hybrids.

On the basis of the above experimental observation,

optimum conditions for esterification of LA were determined as

LA-methanol mole ratio of 1:12, 4 h for completion of the

reaction, temperature at 160°C, and catalyst weight of 0.3 g.

3.3. Stability and reusability.

An eleven-times recycling ability experiment was

performed to probe the reusability of H4SiW/MIL-100(Fe) hybrid.

The used catalyst was treated by centrifugation, washed two times

using methanol, and subsequently used for the next turn of methyl

laurate production. The results are presented in Figure 6. About

17% reduction was observed in the activity of the H4SiW/MIL-

100(Fe) catalyst compared to the fresh catalyst after eleven cycles,

and founded that the LA conversion maintained with >60%. It

may be the reason for the deposition of the LA and products onto

the surface of catalyst, and the loss of catalyst during each

separation process resulted in reduction of active sites. To

examine the structure of catalyst, FT-IR analyses of fresh and used

catalysts were investigated (Figure 2).

It was seen that a series of characteristic peaks were

observed in the range of 980-904 cm-1 and 1636-1383 cm-1 for the

fresh and eleventh cycle catalysts, respectively. These

characteristic peaks were assigned to the Keggin structure and

MIL-100(Fe) framework structure. It can be deduced that the

structure of the H4SiW/MIL-100(Fe) hybrid had essentially

retained after eleven reaction cycles.

Figure 6. H4SiW/MIL-100(Fe) hybrid reusability versus LA conversi on

of biodiesel synthesis. (Reaction conditions: LA-methanol molar ratio of

1:12, catalyst weight of 0.3 g, 160 °C, and 3 h).

4. CONCLUSIONS

In summary, a highly catalytically active H4SiW/MIL-

100(Fe) hybrid was prepared by wrapping H4SiW molecules onto

MIL-100(Fe) through a simple hydrothermal method. The

H4SiW/MIL-100(Fe) catalyst displayed excellent catalytic activity

for biodiesel production, which could be attributed to its strong

acidic sites and good stability. Meanwhile, H4SiW/MIL-100(Fe)

had a long lifetime and maintained activity at >60% LA

conversion even after being repeatedly used for 11 times, offering

the efficient encapsulated heteropolyacid catalyst for the

production of biodiesel.

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6. ACKNOWLEDGEMENTS

This research was supported by the Guizhou Science and Technology Foundation ([2020]1Y054), the Academician Workstation

of Guizhou Science and Technology Plan (Guizhou S&T Cooperation Platform Talents [2016]5602), the Science and Technology

Cooperation Project of Guizhou Province (LH [2017]7059), the Key Support Discipline in Agricultural Resources and Environment of

Anshun University, and the Creative Research Groups Support Program of Guizhou Education Department (KY [2017]049).

© 2020 by the authors. This article is an open access article distributed under the terms and conditions of the

Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).