Propyl-SO3H functionalized graphene oxide as multipurpose solid acid catalyst for biodiesel synthesis and acid-catalyzed esterification and acetalization reactions

Journal Pre-proof Propyl-SO3H functionalized graphene oxide as multipurpose solid acid catalyst for biodiesel synthesis and acid-catalyzed esterification and acetalization reactions Majid Masteri-Farahani, Mahdiyeh-Sadat Hosseini, Newsha Forouzeshfar PII:

S0960-1481(19)31801-4

DOI:

https://doi.org/10.1016/j.renene.2019.11.108

Reference:

RENE 12656

To appear in:

Renewable Energy

Received Date: 17 March 2019 Revised Date:

15 November 2019

Accepted Date: 18 November 2019

Please cite this article as: Masteri-Farahani M, Hosseini M-S, Forouzeshfar N, Propyl-SO3H functionalized graphene oxide as multipurpose solid acid catalyst for biodiesel synthesis and acid-catalyzed esterification and acetalization reactions, Renewable Energy (2019), doi: https:// doi.org/10.1016/j.renene.2019.11.108. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Author Contribution Statement Majid Masteri-Farahani: Supervision and corresponding author. Mahdiyeh-Sadat Hosseini: Investigation and writing. Newsha Forouzeshfar: Investigation and data collection.

Propyl-SO3H functionalized graphene oxide as multipurpose solid acid catalyst for biodiesel synthesis and acid-catalyzed esterification and acetalization reactions Majid Masteri-Farahani*, Mahdiyeh-Sadat Hosseini, Newsha Forouzeshfar Faculty of Chemistry, Kharazmi University, Tehran, Islamic Republic of Iran *Corresponding author: Email address: [email protected]; Tel/Fax: +98 21 86072706.

Propyl-SO3H functionalized graphene oxide as multipurpose solid acid catalyst for

1

biodiesel synthesis and acid-catalyzed esterification and acetalization reactions

2

Abstract

3

A graphene based acid catalyst, GO-PrSO3H, was prepared through a simple two-step process.

4

Surface modification with (3-mercaptopropyl) trimethoxysilane followed by oxidation of

5

sulfide groups led to the production of sulfonic acid sites on graphene oxide nanosheets. The

6

results of various physicochemical techniques approved the synthesis of desired catalyst. The

7

amount of acid sites was measured via the acid-base treatment with triethylamine which

8

exhibited 1.07 mmol/g H+ in the catalyst structure. Two kinds of acid-catalyzed reactions i.e.

9

esterification and acetalization were adopted to evaluate the catalytic performance of prepared

10

catalyst. More than 90% conversion was achieved for butyl acetate production in acetic acid

11

esterification with n-butanol. Moreover, methyl oleate as one of the main components of

12

biodiesel was produced with good yield over the prepared catalyst via oleic acid esterification

13

with methanol. The 1H-NMR technique was also conducted to characterize and determine the

14

amount of produced methyl oleate. Finally, the benzaldehyde acetalization with ethylene

15

glycol was performed which high conversion (92%) was obtained at 3 h. The catalyst

16

reusability for both esterification and acetalization reactions demonstrated the catalyst

17

stability after five reaction cycles.

18

Keywords: Solid acid catalyst; Graphene oxide; Sulfonic acid; Biodiesel; Esterification;

19

Acetalization.

20

1. Introduction

21

Esterification of carboxylic acids with alcohols has attracted much attention for industrial

22

manufacturing valuable chemicals such as solvents, fragrances, polymers, biodiesel, etc. in

23

recent years [1]. Because of the increase in energy consumption and considering the depletion

24

1

of non-renewable fossil fuels in future, biodiesel has been found to be a good alternative in

1

industry. Biodiesel is sustainable, renewable, clean, and sulfur-free fuel composed of fatty

2

acid monoalkyl esters, mainly produced via esterification of fatty acids or trans-esterification

3

of triglycerides available in edible or non-edible oils with short-chain primary alcohols using

4

acid or base catalysts [2-5]. However, it is important to esterificate free fatty acids such as

5

oleic acid, linoleic acid, palmitic acid etc. in incompatible feedstocks prior to use base

6

catalysts for transesterification reaction due to the soap formation. Hence, acid-catalysis is

7

more appropriate for biodiesel production [6,7].

8

On the other hand, acid-catalyzed acetal formation is one of the most important steps in

9

protecting carbonyl groups of aldehydes or ketones with various alcohols or diols in multistep

10

organic syntheses for manufacturing fine chemicals [8-10]. Typically, mineral acids such as

11

H2SO4, H3PO4, and p-toluene sulfonic acid are exploited as homogeneous catalysts to promote

12

the productivity of esterification and acetalization reactions [1,11]. Despite the high efficiency

13

of homogeneous catalytic processes, they suffer from some drawbacks such as difficult

14

separation and recycling of catalyst, severe corrosion, and large production of waste arising

15

from neutralization processes. In order to eliminate these problems for moving toward clean

16

processes, insoluble solid acid catalysts are considered as suitable alternatives in organic

17

transformations due to their facile separation, reusability, non-toxic property, and corrosion

18

elimination as well as waste decrement [12,13].

19

Scientific researches on designing catalysts with specific morphological and textural

20

properties has grown dramatically over the last few decades. The structural properties mainly

21

affect the catalytic activity which in turn is relevant to the number of active sites and their

22

availability for reactant molecules [14-17]. Solid carbon materials e.g. activated carbon,

23

carbon nanotube, fullerene, and graphene have been used as efficient supports to immobilize

24

catalytically active components which consequently leads to promote the selective formation

25

2

of desired products [18-22]. Graphene oxide (GO) containing a lot of oxygen functional

1

groups has been utilized to prepare graphene based acid catalysts. To date, the most utilized

2

reagents for sulfonation of the graphene nanosheets are sulfuric acid [23-30], fuming sulfuric

3

acid [31,32], chlorosulfonic acid [27, 33-38], and diazonium salt of sulfanilic acid [34, 39-52].

4

There are some reports on using graphene based solid acid catalysts in acid-catalyzed

5

reactions. For example, Zhang et al. [23] prepared GO supported dual sulfonic/carboxylic

6

acids to catalyze oleic acid esterification with methanol for biodiesel synthesis. In another

7

work conducted by Liu et al. [31], sulfated graphene was hydrothermally synthesized by using

8

fuming sulfuric acid and exploited as acid catalyst for acetic acid esterification with

9

cyclohexanol and 1-butanol, Pechmann reaction of resorcinol with ethyl acetoacetate and

10

hydration of propylene oxide. Moreover, the use of chlorosulfonic acid as sulfonation reagent

11

was reported by Wei et al. [36] to synthesis sulfonated graphene oxide as a solid acid catalyst

12

in hydrolysis of cellobiose, hydrolysis of isoflavone glycoside, and acetic acid esterification

13

with ethanol. Oger et al. [47] and Zhang et al. [48] prepared graphene supported aryl sulfonic

14

acid with using diazonium salt of sulfanilic acid for acid-catalyzed acetalization of carbonyl

15

compounds with glycerol and additive esterification of carboxylic acids with olefins,

16

respectively.

17

Despite the several use of graphene based acid catalysts in various organic reactions, the use

18

of immobilized propyl sulfonic acid on the surface of graphene oxide (GO-PrSO3H) is only

19

limited to catalyze the synthesis of star-shape phenolic compounds [53], and bisphenolic

20

antioxidants [27]. To best of our knowledge, there is no any report on utilizing GO-PrSO3H

21

acid catalyst in the esterification and acetalization reactions. In this research, we aim to

22

prepare and fully characterize propyl sulfonic acid functionalized graphene oxide and examine

23

its catalytic activity in acetic acid esterification with n-butanol, biodiesel synthesis through the

24

oleic acid esterification with methanol, and benzaldehyde acetalization with ethylene glycol.

25

3

This solid acid catalyst exhibited good catalytic performance compared with some other solid

1

acid catalysts reported earlier.

2

2. Experimental section

3

2.1. Synthesis of propyl sulfonic acid functionalized graphene oxide (GO-PrSO3H)

4

Graphene oxide (GO) was synthesized with modified hummers method [54]. Surface

5

silylation of GO was done by reacting excess amount of (3-mercaptopropyl) trimethoxysilane

6

(5 mmol, 1 g) with 1 g of ultrasonically dispersed GO in dry toluene (30 ml). The mixture was

7

refluxed for 24 h under N2 atmosphere. The obtained GO-PrSH was isolated by

8

centrifugation, soxhlet washed with chloroform, and dried in vacuum oven at 70 ºC overnight.

9

In order to convert the –SH groups to –SO3H acidic sites, the obtained GO-PrSH (1 g) was

10

dispersed in a mixture of double distilled water (50 ml) and methanol (50 ml) and then

11

hydrogen peroxide 30% (50 ml) was added as oxidizing reagent. After stirring at room

12

temperature for 16 h, the prepared GO-PrSO3H was filtered and washed thoroughly with

13

double distilled water to remove unreacted H2O2. Finally, the solid acid catalyst was acidified

14

with diluted H2SO4 (1 M) to protonate all of the sulfonate groups.

15

2.2. Measurement of acid sites

16

The GO-PrSO3H (0.05 g) was dispersed in ethanol (5 ml), followed by subsequent addition of

17

triethylamine (0.5 mmol). After stirring for 4 h at room temperature, the obtained solid was

18

separated by centrifugation, washed several times with cold ethanol, and dried in vacuum

19

oven at 70 ºC overnight. The amount of nitrogen related to adsorbed Et3N on sulfonic acid

20

sites was measured by CHN elemental analysis indicating the number of acid sites.

21

2.3. Catalytic activity of GO-PrSO3H

22

2.3.1. Esterification reactions

23 4

A mixture of acetic acid (30 mmol), n-butanol (45 mmol), and catalyst (0.1 g, dried in vacuum

1

oven at 70 ºC overnight) were transferred to a round bottomed flask and stirred at 120 ºC. In

2

the case of oleic acid esterification with methanol, 50 mmol of methanol was added to a

3

mixture of oleic acid (5 mmol) and catalyst (5 wt%). n-Decane (5 mmol) was used as internal

4

standard and the reaction was conducted at 80 ºC. After certain times, samples were

5

withdrawn from the mixture and diluted with toluene. Gas chromatography (GC) analysis was

6

conducted to determine the reaction progress. The nature and amount of produced biodiesel

7

from oleic acid esterification was also determined using 1H-NMR spectroscopy.

8

2.3.2. Acetalization reaction

9

A mixture of benzaldehyde (15 mmol), ethylene glycol (45 mmol), and cyclohexane (6.2 ml)

10

as water carrier reagent, were stirred at 90 ºC in a round bottomed flask. The reaction started

11

after addition of 0.05 g of activated catalyst to the reaction vessel. The sampling and analysis

12

was done similar to the esterification reactions.

13

3. Results and discussion

14

3.1. Preparation and characterization of GO-PrSO3H

15

Taking into account the fact that the surface of GO is rich in hydroxyl groups, it is known as a

16

suitable support for incorporating various active species via surface post modification. The

17

procedure used for preparing the GO based acid catalyst was shown in Fig. 1. At first, (3-

18

mercaptopropyl) trimethoxysilane was selected as silylating reagent containing thiol group.

19

The reaction was performed in dry toluene to prevent the possible side reaction of coupling

20

the silyl groups together by hydrolysis in the presence of water instead of interaction with GO

21

surface –OH groups. Finally, the hydrogen peroxide was used as oxidizing reagent to convert

22

–SH groups to sulfonic acid ones in mild reaction condition, followed by acidification with

23

sulfuric acid.

24 5

1

Fig. 1. Schematic preparation of the GO-PrSO3H.

2

FT-IR spectra of GO, GO-PrSH, and GO-PrSO3H samples were shown in Fig. 2. The advent

3

of five peaks at 3404, 1724, 1612, 1225, and 1056 cm-1 in the FT-IR spectrum of GO is due to

4

the oxygen functional groups i.e. O-H, C=O, C=C, C-OH, and C-O-C on the surface of

5

graphene oxide, respectively. After introducing 3-mercaptopropylsilyl groups on the surface

6

of GO, the C-H and Si-O-C stretching vibration peaks appeared respectively at 2923 and 1030

7

cm-1 in the GO-PrSH spectrum indicating the successful silylation process. The FT-IR

8

spectrum of GO-PrSO3H showed two additional peaks at 1157 and 1041 cm-1 respectively

9

related to S=O asymmetric and symmetric stretching vibrations which indicates the successful

10

formation of sulfonic acid sites on the surface of GO.

11

12

Fig. 2. The FT-IR spectra of GO, GO-PrSH, and GO-PrSO3H.

13

To evaluate the amount of acid sites in the prepared GO-PrSO3H, acid-base treatment between

14

the immobilized sulfonic acid and triethylamine (Et3N) was conducted. The results of CHN

15

6

elemental analysis revealed that approximately 1.07 mmol/g triethylamine interacted with

1

acidic sites indicating the exact number of H+ sites.

2

Fig. 3 displays the nitrogen adsorption-desorption isotherms and specific surface areas of GO

3

and the prepared GO-PrSO3H. The GO curve belongs to type IV isotherm and shows a

4

hysteresis loop due to the existence of mesoporosity created from the aggregation of GO

5

nanosheets whereas the GO-PrSO3H catalyst shows type II isotherm indicating its non-porous

6

structure [55,56]. The Brunauer–Emmett–Teller (BET) calculations revealed that the specific

7

surface area is decreased dramatically from 249 m2/g for GO to 9 m2/g for GO-PrSO3H. It can

8

be concluded that the grafting of propyl sulfonic acids on the surface of GO would lead to

9

fade mesoporosity and hence reduce the surface area.

10

11

Fig. 3. The N2 adsorption-desorption isotherms and BET surface areas of prepared GO and

12

GO-PrSO3H.

13

The XRD patterns of GO and GO-PrSO3H were depicted in Fig. 4. The oxidation of graphite

14

precursor to GO was confirmed by appearance of a peak at 2θ=12º and disappearance of

15

graphite characteristic peak at 2θ= 27º, indicating that oxygen moieties placed on the surface

16

of graphitic layers created the ordered graphene layers with the enlarged d-spacing of 7.3 Å

17

7

compared to graphite [33,36,55]. This well-arranged structure was disordered during the

1

sulfonic acid functionalization process followed by decrease in the intensity of GO peak in

2

GO-PrSO3H pattern.

3

4

Fig. 4. XRD patterns of GO and GO-PrSO3H.

5

To study the morphology of prepared GO-PrSO3H and distribution of its composing elements,

6

TEM, SEM, and EDX analyses were performed (Fig. 5). The sheet-like structure of the GO-

7

PrSO3H can be observed in Fig. 5a. The TEM image (inset of Fig. 5a) demonstrates the two

8

dimensional and crumpled transparent mono-layer of GO-PrSO3H nanosheets. The elemental

9

mapping of carbon, oxygen and sulfur indicates the uniform distribution of these elements in

10

the GO-PrSO3H (Figs. 5b-d).

11

8

1

Fig. 5. The SEM and TEM (inset) images of GO-PrSO3H (a), and corresponding elemental

2

mapping of carbon (b), silicon (c), sulfur (d), along with EDX spectrum (e).

3

Furthermore, the EDX spectrum completely shows the intended elements (Fig. 5e), verifying

4

the presence of propyl sulfonic acid groups in catalyst structure.

5

3.2. Catalytic activity of GO-PrSO3H

6

3.2.1. Esterification reactions

7

The solvent free acetic acid esterification with n-butanol was conducted as the first test

8

reaction to evaluate the catalytic behavior of prepared GO-PrSO3H. As shown in Fig. 6, the

9

9

highest acetic acid conversion (94%) was achieved at 4 h, while the blank test showed less

1

conversion (33%). This obviously demonstrates the efficiency of GO-PrSO3H catalyst in

2

promoting the rate of esterification reaction. The reaction continued up to 8 h, and no more

3

progress observed as the esterification is an equilibrium reaction, and the rate of backward and

4

forward reactions will be equal after a certain time (4 h here).

5

6

Fig. 6. Acetic acid esterification with n-butanol in the presence of GO-PrSO3H. Reaction

7

conditions: acetic acid = 30 mmol, n-butanol = 45 mmol, catalyst amount = 0.1 g, temperature

8

= 120 ºC.

9

Getting a good conversion in butyl acetate production motivated us to examine the catalytic

10

activity of GO-PrSO3H catalyst in the esterification of a fatty acid i.e. oleic acid with

11

methanol to produce biodiesel. In general, fatty acids are divided into saturated (e.g. palmitic

12

acid, stearic acid, etc.) and unsaturated (e.g. oleic acid, linoleic acid, etc.) long chain

13

carboxylic acids which most naturally found in animal fats and vegetable oils. However, high

14

concentration of oleic acid can be found in various vegetable oils such as sunflower, pecan,

15

macadamia, grape seed, peanut, sesame, sea buckthorn, and canola oils as shown in Fig. 7.

16

10

1

Fig. 7. The production of biodiesel from oleic acid found in vegetable oils.

2

Thereupon, the esterification reaction with an efficient acid catalyst without any substrate

3

pretreatment can lead to produce a green and environmentally friendly biodiesel from oleic

4

acid.

5

Typically, excess amount of alcohol is utilized in the fatty acid esterification to facilitate the

6

dispersion of solid acid catalyst in oily media and increase the collisions of substrates, leading

7

to more biodiesel production. Furthermore, like other esterification reactions, extra amount of

8

alcohol would prevent the backward reaction and more ester is produced over time. In this

9

study, the 1:10 molar ratio of oleic acid to methanol was selected and the reaction was

10

conducted at 80 ºC using 5 wt% of GO-PrSO3H catalyst. Table 1 and Fig. S1 show that in the

11

absence of catalyst, the maximum methyl oleate yield was about 3% at 8h, showing that the

12

reaction could not progress without acid catalyst. However, when the GO-PrSO3H catalyst is

13

added to the reaction mixture, the methyl oleate yield reached to 60% at 8 h indicating that the

14

presence of catalyst promotes the reaction rate.

15

16

17

18

19 11

Table 1. The results of the oleic acid esterification with methanol in the presence of GO-

1

PrSO3H.

2 COOCH3

COOH COOH

Entry

+ CH3OH

catalyst

COOCH3

Yield (%) a

Time (h)

2 (2)b

1

blank

2

GO-PrSO3H

40 (36)b

3

blank

3 (4)b

4

GO-PrSO3H

4

8 a

60 (57)b

The yield determined by means of GC.

b

Based on 1H-NMR

analysis. Reaction conditions: oleic acid = 5 mmol, methanol = 50 mmol, catalyst amount = 0.07 g, temperature = 80 ºC.

To prove the reliability of the results, this reaction was also followed by 1H-NMR technique.

3

Fig. 8 illustrates the sample preparation steps for 1H-NMR analysis. The reaction content was

4

centrifuged to separate the solid acid catalyst (Fig. 8a) and the supernatant containing the

5

mixture of methanol, n-decane, oleic acid, and produced methyl oleate was carefully poured

6

into a decantation funnel. In order to remove unreacted methanol, the mixture was washed

7

several times with hot distilled water. A biphasic solution was formed which the upper

8

solution contains the mixture of n-decane, oleic acid, and methyl oleate and the bottom

9

solution is the mixture of methanol and water (Fig. 8b). The latter phase was outpoured after

10

each washing and the upper clean mixture (Fig. 8c) was analyzed by 1H-NMR spectroscopy.

11

12

1

Fig. 8. The pictorial steps of the preparation and purification of biodiesel sample.

2

The methyl oleate yield was calculated based on following formula [41]:

3

The yield of methyl oleate (%) = (2ICH3/3ICH2)×100

4

Where ICH3 and ICH2 represent the integral of methyl oleate protons (3.63 ppm) and α

5

methylene protons attached to carbonyl groups (2.27 ppm), respectively.

6

Fig. 9 demonstrates the 1H-NMR spectra of pure oleic acid as well as the produced methyl

7

oleate in the presence and absence of catalyst. In all of these spectra, the oleic acid

8

characteristic peaks including olefinic and acyl protons can be observed, while the strong

9

singlet peak of methoxy protons is appeared only in methyl oleate produced in the presence of

10

GO-PrSO3H catalyst. In the absence of catalyst, the very low intensity of methoxy protons in

11

the product spectrum indicates the low methyl oleate yield. The calculated methyl oleate

12

yields based on this technique were given in Table 1. Similar to the data obtained from GC

13

analysis, there are considerable differences in methyl oleate yield with and without utilizing

14

catalyst. The flammability of samples was shown in Fig. 9d and the complete details of the

15

flammability process over time is shown in supporting video. Based on the obtained

16

observations, oleic acid extinguished quickly after a few seconds, showing its impermanent

17

flammability. The final product in the absence of catalyst showed the better flammability

18

compared to pure oleic acid owing to the existence of a little amount of methyl oleate (about

19

13

3%), while the product of the catalytic reaction exhibited high and permanent flammability

1

due to the formation of biodiesel (about 60%).

2

3

Fig. 9. The 1H-NMR spectra of pure oleic acid (a), the product formed in the absense (b) and

4

presence (c) of GO-PrSO3H catalyst. (d) The flammability of each sample.

5

The general process flow diagram and mass balance of biodiesel production are illustrated in

6

Fig. 10. The separation and purification steps are consistent with the procedures presented in

7

Fig. 8 discussed previously. Besides, the calculated mass balance shows that 1.59 kg of oleic

8

acid and 1.80 kg of methanol (oleic acid to methanol molar ration of 1:10) are required for

9

production of 1 kg of biodiesel in the presence of 79.39 g of freshly prepared solid acid

10

catalyst, considering the fact that the biodiesel yield is 60% at 8 h. At the end of reaction and

11

purification steps, the residual methanol (about 0.72 kg) can be recovered by distillation and

12

reused in another catalytic run.

13

14

1

Fig. 10. The process flow diagram along with mass balance of biodiesel production using the

2

prepared solid acid catalyst.

3

Table 2 illustrates the catalytic activities of earlier reported solid acid catalysts in comparison

4

with our prepared GO-PrSO3H in the esterification of acetic acid with n-butanol and oleic acid

5

with methanol. The solid acid catalysts show approximately equal or even much less catalytic

6

efficiency than prepared GO-PrSO3H catalyst with considering the whole reaction conditions.

7

This indicates that GO-PrSO3H catalyst is more useful in esterification reaction compared to

8

earlier ones.

9

15

Table 2. Comparison of the catalytic efficiency of reported acid catalysts with prepared GO-

1

PrSO3H catalyst in esterification of acetic acid with n-butanol and oleic acid with methanol.

2

No.

Acid catalysts

Reaction

RCOOH /

Catalyst

Temperature

Reaction

Yield

R'OH molar

amount

(˚C)

Time (h)

(%)

ratio

(wt%)

Ref.

GO-PrSO3H

1:1.5

5.5

120

4

94

This work

2

HSO3-MIL-101(Cr)

1:1

5

70

~6

~40

[57]

3

MIL-125s

1:1

6

90

7

75

[58]

4

SSA

1:2

20

130

2

84

[59]

5

RHANPSO3H

1:2

1.7

117

9

88

[60]

6

PSSF-mCNTs

1:4

2

120

10

55

[61]

7

PSSF-mCNTs-GO

8

HPW-NH2-SBA-15

Acetic acid + n-Butanol

1

1:4

2

120

10

70

[61]

1:1

4.9

90

17

96

[62]

1:1

4.9

90

17

96

[62]

3:1

8.1

150

12

88

[63]

2:1

23.8

100

6

93

[64]

9

HPW-SBA-15

10

20% TPA /AT-GMB

11

POM-IL

12

30%LPOM–HMS

3:1

10.8

115

5

88

[65]

13

CAF‐12/12

1.5:1

13.5

110

3.5

85

[66]

14

ZACF‐16/16/4

1.5:1

13.5

110

2

88

[66]

15

20 wt% WO3/SiC

2:1

2.3

120

6

88

[67]

16

WZr3

1:1

1.7

150

4

57

[68]

17

Al-MCM-41 (33)

2:1

5.4

200

9

88

[69]

18

GO-PrSO3H

1:10

5

80

8

60

This work

19

[BMIM]HSO4

1:2

10

80

4

~58

[70]

20

[EPy]HSO4

1:2

10

80

4

~15

[70]

21

[TEAm]HSO4

1:2

10

80

4

~12

[70]

22

[BHSO3MIM]HSO4

1:2

10

80

4

~72

[70]

23

[BMIM]ClO4

1:2

10

80

4

~7

[70]

[EPy]Br

1:2

10

80

4

~4

[70]

25

[TEAm]Cl

1:2

10

80

4

~4

[70]

26

mesoporous BEA

27

CD-1

28

CD-2

29

CD-3

30

TPA3/MCM-41

31 32 33

Oleic acid + Methanol

24

1:6

4

117

8

56

[71]

1:10

5

80

12

~4

[72]

1:10

5

80

12

~5

[72]

1:10

5

80

12

~92

[72]

1:10

10

60

8

~38

[73]

30% SiW11/MCM-41

1:10

n.d.

60

10

~32

[74]

CMK-3-873-SO3H

1:10

6.1

80

10

~58

[75]

Amorphous sugar

1:10

4.25

80

10

~53

[75]

1:10

17.7

80

10

~50

[75]

catalyst 34

Protonated-Nafion (NR50)

35

Zeolite CBV-720

1:10

28.33

80

10

~27

[75]

36

Glu-Fe3O4-SO3H

1:212

5

R.T.

12

72

[76]

MIL-125s = sulfated MIL-125, SSA = Silica Sulfuric Acid, RHANPSO3H = immobilized 7-amino-1-naphthalene sulfonic acid

16

on rice hush ash (RHA), PSSF-mCNTs-GO = poly(p-styrenesulfonic acid) grafted carbon nanotube/graphene oxide, HPW = TPA= H3PW12O40, AT-GMB= Acid activated Gujarat Mines Bentonite, POM = Polyoxometalate, IL = Ionic Liquid, LPOM = Lacunary Polyoxometalate, HMS = Hexagonal Mesoporous Silica, CAF = CeO2-Al2O3-Fe3O4, ZACF = ZrO2-Al2O3-CeO2Fe3O4, SiC = Silicon carbide, WZr = Zr-WOx/SiO2, BMIM = 1-Butyl-3-methylimidazolium, Epy = N-ethylpyridinium, TEAm = tetraethylammonium, BHSO3MIM = 1-sulfobutyl-3-methylimidazolium, CD = β-Cyclodextrin, CMK = The ordered mesoporous carbon, Glu = Glucose, n.d. = not determined, R.T. = Room temperature.

In addition to compare the catalysts in terms of their efficiencies, it is vital to mention their

1

environmental impact. In terms of biodiesel production and considering the environmental

2

impact, our prepared GO-PrSO3H was found to be harmless and benign in comparison with

3

TPA3/MCM-41 and 30% SiW11/MCM-41 catalysts owing to the presence of heavy tungsten

4

metal in these catalysts structure, and hence the probable leaching of these species would

5

results in the environmental pollution. Moreover, the separation process of ionic liquids such

6

as [BMIM]HSO4, [EPy]HSO4, [TEAm]HSO4, [BHSO3MIM]HSO4, [BMIM]ClO4, [EPy]Br,

7

and [TEAm]Cl from the reaction mixtures is more difficult than solid catalysts, so they can

8

cause a range of environmental issues. The other solid acid catalysts listed in Table 2 are

9

environmentally benign, exactly as the same as our prepared catalyst from the view point of

10

using non-toxic metal free substances and the separation process.

11

3.2.2. Acetalization reaction

12

To show the multifunctional behavior of prepared GO-PrSO3H catalyst in various acid-

13

catalyzed reactions, benzaldehyde acetalization with ethylene glycol was adopted as another

14

test reaction. The rate of this reaction would be enhanced if an acid catalyst is introduced to

15

the reaction mixture. Also, the use of water carrier reagent (cyclohexane in this study) to

16

entrap the produced water will cause further improvement in acetal production. Considering

17

these items, the acetalization reaction in presence of GO-PrSO3H was carried out and the

18

results are depicted in Fig. 11. As expected, without using the acid catalyst this reaction is

19

very slow and inefficient. Upon addition of the GO-PrSO3H catalyst, maximum conversion of

20

92% was obtained at 3h.

21

17

1

Fig. 11. The benzaldehyde acetalization with ethylene glycol in the presence of GO-PrSO3H.

2

Reaction conditions: benzaldehyde = 15 mmol, ethylene glycol = 45 mmol, catalyst amount =

3

0.05 g, temperature = 90 ºC.

4

The catalytic activities of earlier solid acid catalysts in benzaldehyde acetalization with

5

ethylene glycol are briefly listed in Table 3.

6

Table 3. Comparison of the catalytic efficiency of prepared GO-PrSO3H acid catalyst with

7

other reported solid acid catalysts in benzaldehyde acetalization with ethylene glycol.

8

Solid acid catalysts C6H5CHO + HOC2H4OH No.

C6H5CHOOC2H4 + H2O

Heterogeneous catalysts

Benzaldehyde / ethylene glycol molar ratio

Catalyst amount (wt%)

Reaction Temperature (˚C)

1

GO-PrSO3H

1:3

3

90

2

Fe3O4@C–SO3H

1:3

3

90

3

AIL-MFR

1:1.2

1.4

reflux

4

SO3H/NCF-600

1:5

1.9

90

5

SG-[(CH2)3SO3H-HIM]HSO4

1:1.8

8.2

110

6

HSiW-CS-Fe3O4

1:10

6

81

Reaction Time (h)

Efficiency (%)

Ref.

3

92

This work

2

88

[10]

2

85

[77] [78]

1

99

1.5

95

[9]

2

99

[79]

AIL-MFR = Acidic Ionic Liquid supported on Melamine-Formaldehyde Resin, NCF = Nitrogen-doped Carbon microfiber, SG = Silica Gel, HIM = Imidazolium, CS = Chitosan

Similar to the esterification reaction, the prepared GO-PrSO3H (row 1) shows comparable

9

catalytic activity toward acetal production in comparison with other supported sulfonic acid

10

(row 2-5) or heteropoly acid (row 6) catalysts. These observations imply that our solid acid

11

catalyst can be used in various organic reactions with good catalytic performance.

12

3.2.3. Catalyst reusability and life cycle

13

18

The life cycle of GO-PrSO3H solid acid catalyst (Fig. 12) begins with production of fresh

1

catalyst, following by thermal activation in vacuum oven at 70 ºC overnight to remove

2

probable solvents or moisture surrounding the active acidic sites. Then, the activated catalyst

3

involves in the esterification and acetalization reactions under pre-determined conditions. At

4

the end of each catalytic reaction, the catalyst is separated from the reaction media by

5

centrifugation and washed thoroughly with methanol and dried in vacuum oven overnight.

6

The activated recovered catalyst is reused in acetic acid esterification with n-butanol and

7

benzaldehyde acetalization with ethylene glycol under the same reaction conditions and the

8

obtained data were given in Fig. 13. As a consequence, the catalyst reusability shows no

9

considerable decrease in butyl acetate and 2-phenyl-1,3-dioxolane production after 5 cycles

10

due to the firmly bound sulfonic acid on the surface of GO. Therefore, the catalyst life cycle

11

would not contain any further recovery step for preparing the fresh catalyst due to the

12

comparable activity of reused catalyst to the fresh one.

13

14

Fig. 12. The proposed life cycle of GO-PrSO3H catalyst.

19

15

1

Fig. 13. The reusability of GO-PrSO3H acid catalyst for both acetic acid esterification with n-

2

butanol at 4h and benzaldehyde acetalization with ethylene glycol at 3h.

3

3.2.4. The reaction mechanisms

4

The mechanisms of esterification and acetalization reactions in the presence of prepared GO-

5

PrSO3H catalyst were shown in Fig. 14. As can be seen in esterification reaction, the carbonyl

6

group of carboxylic acid receives an acidic proton from the catalyst and a carbocation is

7

formed. The formed intermediate is receptive to nucleophilic attack of alcohol oxygen atom

8

which results in water removal from the structure and manufacturing the desired ester. The

9

hydrogen (H+) is returned to the catalyst to begin the catalytic cycle again.

10

Similar to esterification reaction, the acetalization process starts by absorption of an acidic

11

proton from catalyst by benzaldehyde carbonyl group to form carbocation on central carbon,

12

which makes it ready for subsequent nucleophilic attack of oxygen atom of ethylene glycol.

13

The proton exchange from attached oxygen of ethylene glycol to neighbor hydroxyl group

14

leads to water withdrawal along with oxonium formation. Finally, it is proposed that the

15

desired acetal is formed via the linkage of the other hydroxyl group of ethylene glycol to the

16

central carbon, and the migration of acidic proton to the catalyst structure keeps the catalytic

17

cycle alive [9].

18

20

Fig. 14. The mechanisms of esterification and acetalization reactions over GO-PrSO3H.

1 2

4. Conclusion

3

In summary, introducing propyl sulfonic acid groups as active acidic sites on the surface of

4

graphene oxide (GO) produced a graphene based acid catalyst (GO-PrSO3H) with high

5

surface acidity. Graphene oxide was selected as a proper solid support due to its high surface

6

area (249 m2/g) and bearing several hydroxyl functional groups appropriate for surface post

7

modification. The existance of covalently bound propyl-SO3H groups on the surface of GO

8

was corroborated by FT-IR spectroscopy and EDX analysis. To show the multipurpose

9

character of prepared graphene based acid catalyst, its catalytic behavior was investigated in

10

both esterification and acetalization reactions. As a consequence, this solid acid catalyst

11

showed excellent performance (94%) in acetic acid esterification with n-butanol. Biodiesel as

12

a clean fuel was produced with acceptable yield over GO-PrSO3H compared to the blank test

13

using oleic acid and methanol as substrates. Both GC and 1H-NMR techniques confirmed the

14

manufacture of methyl oleate. Also, the high catalytic efficiency of GO-PrSO3H acid catalyst

15

was observed in production of 2-phenyl-1,3-dioxolane at 3h via acetalization reaction.

16

21

Catalyst recyclability exhibited that this solid acid catalyst is able to catalyze both

1

esterification and acetalization reactions at least five times without loss of its activity.

2

Acknowledgements

3

The authors gratefully acknowledge financial support from the Iran National Science

4

Foundation (INSF) [Grant No. 96006456].

5

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Highlights •

GO-PrSO3H acid catalyst was prepared using sulfonic acid groups as active acid sites.

•

Esterification and acetalization reactions were performed in presence of GO-PrSO3H.

•

A clean fuel i.e. biodiesel was produced via oleic acid esterification with methanol.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: