Esterification of Glycerol with Acetic Acid using Nitrogen Based

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Kinetics, Catalysis, and Reaction Engineering

Esterification of Glycerol with Acetic Acid using Nitrogen Based Brønsted-Acidic Ionic Liquids John Keogh, Manishkumar S. Tiwari, and Haresh Manyar Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b01223 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 24, 2019

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Industrial & Engineering Chemistry Research

Esterification of Glycerol with Acetic Acid using Nitrogen Based Brønsted-Acidic Ionic Liquids John Keogh, Manishkumar S. Tiwari, and Haresh Manyar* Theoretical and Applied Catalysis Research Cluster, School of Chemistry and Chemical Engineering, Queen’s University Belfast, David-Keir Building, Stranmillis Road, Belfast, BT9 5AG, UK Email: [email protected], Phone: +442890976608, Fax: + 44 28 90 974687

KEY WORDS: Glycerol acetylation, Bio-fuels, Fuel additives, diacetin, triacetin, Brønsted-Acidic Ionic Liquids

Abstract: Glycerol esterification with acetic acid produces a mixture of mono-, di- and tri acetins, which are commercially important value-added products with wide range of industrial uses including their application as fuel-additives, thus contributing to environmental sustainability and economic viability of the bio-refinery concept. Glycerol esterification with acetic acid was studied using a range of nitrogen based Brønsted-acidic ionic liquids. Costeffective and easily synthesized Brønsted-acidic ionic liquids based on alkyl-pyrrolidone and alkyl-amine cations were synthesized and characterized using 1H NMR spectroscopy. The catalytic activity of Brønsted-acidic ionic liquids produced were investigated for the production of di- and triacetin from glycerol and acetic acid. Amongst all ionic liquids evaluated in this study, N-methyl-2-pyrrolidinium hydrogen sulfate [H-NMP][HSO4] was found to be the most active and cost-effective catalyst. The effect of significant reaction parameters on selectivity to

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the tri-substituted product, triacetin, was modelled using a Design of Experiment (DoE) approach with a response surface methodology involving a central composite design. The esterification process was optimised to maximise the production triacetin. Optimising the process this way naturally leads to lower levels of mono- and diacetin. Amongst the reaction parameters evaluated, temperature had the highest influence on product selectivity, followed by the glycerol to acetic acid molar ratio, and the model also showed dependence on the synergistic interaction between temperature and mole ratio. It is worth noting that agitation speed had minimal influence on the product selectivity. Under the optimized reaction conditions, >99% glycerol conversion was achieved with 42.3 % selectivity to triacetin, and a combined di- and triacetin selectivity of >95% within 1 hr.

1. Introduction

With reserves of conventionally used fossil fuels depleting, increasing energy demand and improved awareness towards global warming and climate change, significant importance has been placed on finding sustainable, environmentally friendly and economically viable alternative sources of fuels and chemicals. Biodiesel has emerged as a potential alternative to petroleum based fuels, as it is renewable, non-toxic and biodegradable 1. Typically biodiesel is produced through the transesterification of triglycerides, contained in vegetable oils, with methanol to produce fatty acid methyl esters (FAME) 2. A major problem of biodiesel is the production of a waste by-product, glycerol, which accounts for 10 wt% of the biodiesel production. It is important to find ways of valorising this waste product, not only to promote a circular economy but also to improve the economic viability of the biorefinery industry 3. In continuation to our interests in adding value to waste biomass resources 4-7, we here in report valorisation of glycerol


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through an esterification process with acetic acid to produce glycerol esters, as shown in Scheme 1. The reaction is a combination of stepwise reactions producing three ester products; mono-, di-, and triacetin, which have a wide range of applications

8-11.

In particular production of the fuel

additives di- and triacetins, can act as a double edged sword by dealing with waste glycerol whilst also improving the biodiesel fuel qualities.

HO

O

O

OH OH

-H2O OH

O

OH or HO

HO

O

OH O Monoacetin Isomers

Acetic Acid

Glycerol

O OH O

O

O

O

O

OH

O

-H2O O

O Triacetin

-H2O

OH

O HO

O

or

O O

O

O O

Diacetin Isomers

Scheme 1. Reaction scheme for the production of Acetins from the Esterification of Glycerol with Acetic Acid. Traditionally mineral acids have been used to catalyse the esterification, however these catalysts are hazardous to handle, corrosive and non-reusable12. Other acids have been investigated to overcome the drawbacks associated with mineral acids. The use of metal oxides was investigated by Hu et al 13. After 3.5 hours Sb2O5 gave good glycerol conversion of 96.8 % , however selectivity to triacetin was low at 12.6 %. The ability of ion-exchange resins, a class of solid acids, has been investigated thoroughly 14-16. Most recently Reinoso et al. investigated the use of Dowex Monosphere 650C. After 4 hours high glycerol conversion of 99.6 % was



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achieved with, selectivity to triacetin of 34 % and combined di- and triacetin selectivity of 88 %, at 120 ˚C using 4 wt % catalyst and a glycerol to acetic acid molar ratio of 9:1. Zeolites such as mordenite, HUSY and HZSM-5 have been investigated 17,18. Popova et al. reported high glycerol conversion of 93.5% and high selectivity to triacetin of 69.2 % using zirconium modified hierarchical mordenite. Other acids such as silica based solid acids and Heteropolyacids have also been investigated

11,19-21.

Sandesh et al. investigated the use of multiple solid acids for

glycerol esterification with acetic acid

22.

They found turnover frequencies for a cesium

phosphotungstate catalyst of 30.5 hr-1, zeolites to be 12-15 hr-1 and sulfated zirconia to be 9.2 hr1.

Whilst these acids offer good conversion and selectivity cost and reuse of these catalysts are

major issues. Ionic liquids have shown promising catalytic activities

23-24,

and provide an attractive

alternative to solid acids, with benefits such as good thermal stability, ease of handling and good recyclability 12. SO3H functionalised ionic liquids have been investigated by Li et al. et al

23.

25

and Liu

Liu et al. found [(HSO3-p)2im][HSO4] to be the most active catalyst, exhibiting 95%

conversion of glycerol and selectivity to di- and triacetin of 51.4 % and 5.5 % respectively after 30 mins at 100 ˚C. Huang et al. have reported the use of heteropolyacid-based ionic liquids consisting of pyridinium propyl sulfonate, tungstophosphoric acid and acetic acid achieving 85.9 % selectivity to triacetin after 4 hr at 105 ˚C with continuous water removal 26. Whilst these ionic liquids showed good activity and selectivity, multi-step synthesis methods and the use of expensive components limits their industrial use. In this work, simple to synthesize and cost effective protic ionic liquids were investigated in the esterification of glycerol with acetic acid, as shown in Scheme 2. The protic ionic liquids chosen in this study when compared to mineral acids such as sulfuric acid are environmentally


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cleaner, non-hazardous and reusable as well as economically viable for scale up due to cheap cost and have potential for subsequent commercialisation. A design of experiment approach was used to optimise the key reaction parameters and maximise the yield of triacetin produced.

HSO4O

N R1

H

R2

R1

HSO4-

N

H

R3

Scheme 2. Pyrrolidone and trialkylamine based ionic liquids with acidic protons shown in red. R1,2,3 = CH3, C2H5, C3H7, C4H9, C8H17.

2. Experimental

2.1 Materials and Methods

All chemicals were purchased from Alfa Aesar unless otherwise stated. N-ethyl-2-pyrrolidone, acetic acid and Amberlyst-15 hydrogen form wet were purchased from Sigma Aldrich. Sulfuric acid was purchased from Honeywell with bentonite clay provided by Oleon NV. All chemicals were used without further purification as commercially available, except amberlyst-15 which was dried in an oven overnight 1H NMR were recorded using a Bruker Avance 300 MHz instrument. Tetramethylsilane was used as an internal standard with DMSO-d6 as the solvent unless otherwise stated. 2.2 Catalyst Synthesis



N-methyl-2-pyrrolidinium hydrogen sulfate [H-NMP][HSO4] The [H-NMP][HSO4] ionic liquid was synthesized using the method reported in literature 27. Nmethyl-2-pyrrolidone (9.9 g, 0.1 mol) was added to a 100 ml round-bottomed flask immersed in an ice bath. Sulfuric acid 96 % solution (9.8 g, 0.1 mol) is added dropwise slowly under stirring. After the addition, the reaction is stirred for a further 4 hr at 40 °C to ensure the reaction had reached completion. The ionic liquid formed was washed with ethyl acetate (3 x 10 mL) to remove non-ionic residue and water was removed under reduced pressure. 1H NMR (DMSO-d6): δ (ppm) 1.85 (m, 2H), 2.25 (t, 2H), 2.66 (s, 3H), 3.31 (t, 2H), 6.96 (s, 1H). N-ethyl-2-pyrrolidinium hydrogen sulfate [H-NEP][HSO4] Synthesis similar to that of [H-NMP][HSO4]. 1H NMR (DMSO-d6): δ (ppm) 0.92 (t, 3H), 1.86 (m, 2H), 2.25 (t, 2H), 3.125 (q, 2H), 3.32 (t, 2H), 6.44 (s, 1H). The following alkylamine based ionic liquids were synthesized according to the literature 28. Tri-n-ethylammonium hydrogen sulfate [Et3NH][HSO4] For tri-n-ethylammonium hydrogen sulfate, triethylamine (10.1 g, 0.1 mol) was added to a 100 mL round-bottomed flask fitted with a condenser. Sulfuric acid 96 % solution (9.8 g, 0.1 mol) is added dropwise slowly under stirring at 60 °C. The reaction is then stirred at 70 °C for 2 hours to ensure the reaction had proceeded to completion. Water was then removed under reduced pressure. 1H NMR (DMSO-d6): δ (ppm) 0.91 (t, 9H), 2.819 (m, 6H), 8.74 (s, 1H). Tri-n-propylammonium hydrogen sulfate [Pr3NH][HSO4] – Synthesis was similar to that of tri-n-ethylammonium hydrogen sulfate, instead using tri-n-propylamine (14.3 g, 0.1 mol) during synthesis. 1H NMR (DMSO-d6): δ (ppm) 0.64 (t, 9H), 1.38 (m, 6H), 2.741 (m, 6H), 8.82 (s, 1H).


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Tri-n-octylammonium hydrogen sulfate [Oct3NH][HSO4] – Synthesis was similar to that of tri-n-ethylammonium hydrogen sulfate, instead using tri-n-octylamine (17.7 g, 0.05 mol) and sulfuric acid (4.6 g, 0.05 mol) during synthesis. 1H NMR (CDCl3): δ (ppm) 0.88 (t, 9H), 1.31 (d, 30H), 1.73 (s, 6H), 3.08 (t, 6H), 9.15 (s, 1H), 9.99 (s,1H). 2.3 Catalyst Characterisation The acid values of the ionic liquids were determined using an acid-base titration method, as reported by Ramli et al.29. Approximately 0.1 g of ionic liquid was dissolved in 10 ml of 2propanol. The mixture was then titrated with 0.1 M NaOH with phenolphthalein used as an indicator. The end-point of the titration was reached when the colourless solution turns pink. A blank titration was also performed. The following equation was used to calculate the acid value, which is defined as the amount of NaOH required to neutralise 1 g of ionic liquid. 𝐴𝑐𝑖𝑑 𝑉𝑎𝑙𝑢𝑒

()

𝑔 (𝐴 ― 𝐵)𝑥𝑀𝑥40 = 𝑔 𝑊

Equation 1 - Acid value of ionic liquids

The thermal stability of the ionic liquids was investigated using a Q5000 Thermogravimetric Analyzer (TA Instrument company). TGA were recorded using a ramp mode of 10 °C/min to 600 °C, with a nitrogen flow of 25 mL/min using platinum pans. Approximately 10-45 mg of sample was used for analysis. FT-IR spectra were recorded of neat samples using a Bruker Alpha FT-IR, with 24 scans acquired for each sample.

2.4 Typical esterification reaction of glycerol with acetic acid



The esterification of glycerol with acetic acid was performed in a 100 mL glass reactor, equipped with magnetic stirring and a condenser. The reactor assembly was kept in an isothermal oil bath at a known temperature and mechanically agitated at required speed of agitation. In a typical experiment, the reaction mixture consisted of a predetermined amount of glycerol, acetic acid and [H-NMP][HSO4]. Catalyst loading was determined in terms of molecular mol% of the limiting reagent, glycerol, unless otherwise stated. The reaction was carried out at required temperature for 1 hr. Samples of the reaction mixture were withdrawn periodically for analysis. For the initial catalyst screening and further optimisation of conditions using DoE, no method of water removal was employed. Analysis of the reaction was carried out using an Agilent 7820A GC equipped with a HP-5 capillary column (30 m x 0.32 mm x 0.25 µm) and a flame ionisation detector. GC analyses were carried out with an initial oven temperature of 140 °C with a 2 minute hold time, followed by an increase from 140 °C to 250 °C at a rate of 40 °C/min.

2.5 Design of Experiment (DoE) by Response Surface Model (RSM)

DoE was carried out with the aid of Stat-Ease Design Expert 11. It was determined that catalyst loading, temperature, stirrer speed, and molar ratio of glycerol to acetic acid were the important reaction parameters which would have an effect on glycerol conversion and selectivity to triacetin. Therefore a two-level four factor central composite design with axial points and central points was employed to optimize these factors.

3. Results and Discussion 3.1. Properties of the Ionic Liquids


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The acidic properties of the ionic liquids based on the acid-base titration method are stated in Table 1. The acidity values of the ionic liquids determined by the acid base titration follow the following order: [H-NMP][HSO4] > [H-NEP][HSO4] = [Et3NH][[HSO4] > [Pr3NH][[HSO4] > [Oct3NH][[HSO4].

Table 1. Acid Values of the Ionic Liquids. Ionic Liquid

Acid Value (g NaOH/ g IL)

[H-NMP][HSO4]

0.43

[H-NEP][HSO4]

0.40

[Et3NH][[HSO4]

0.40

[Pr3NH][[HSO4]

0.36

[Oct3NH][[HSO4]

0.26

FT-IR spectroscopy was used to characterise the structure of the ionic liquids, in particular the structure of the anionic species. Figure 1 shows the infrared spectra for the ionic liquid [HNMP][HSO4] in comparison with neat n-methyl-2-pyrrolidone and neat sulfuric acid.



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H2SO4 [H-NMP][HSO4]

NMP

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm )

Figure 1. FT-IR spectra for [H-NMP][HSO4], n-Methyl-2-pyrrolidone and sulfuric acid. The infrared spectra of [H-NMP][HSO4] features a S-OH vibration at 875 cm-1, a symmetric S=O at 1030.3 cm-1, a multiplet corresponding to vibrations of the {SO3} unit with the most intense band at 1135.4 cm-1, similar to that reported in literature by Matuszek et al.

30.

The

spectra also shows a C=O stretching peak corresponding to that present in the NMP at 1696.2 cm-1. Thermogravimetric analysis (TGA) was used to investigate the thermal stabilities of the ionic liquids tested, the results are shown in Figure 2.


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[H-NMP][HSO4]

100

[H-NEP][HSO4] [Et3NH][HSO4]

80

Percentage Weight (%)

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


[Pr3NH][HSO4] [Oct3NH][HSO4]

60 40 20 0 0

100

200

300

400

Temperature (°C)

500

600

Figure 2 - Thermogravimetric analysis (TGA) curves of the ionic liquids. From the TGA we can see that from ambient temperature to 100 °C there is an observable weight occurring. This is due to the loss of physically bound water which is present in the ionic liquids. From 100 to 200 °C there is a period of stability in the ionic liquids, however above 200 °C the ionic liquids start to degrade rapidly and lose mass. The TGA shows that the ionic liquids are stable for the temperature range that is required for the glycerol esterification reactions. 3.2. Comparison of the catalytic activity of different ionic liquids

A total of 5 ionic liquids were prepared, and each was tested in a reaction with glycerol and acetic acid in order to compare the catalytic activity. This was necessary to determine which ionic liquid not only gave high conversion but also good selectivity towards the desirable di- and triacetin products. Two cation structures based on n-alkyl-2-pyrroldione and trialkylamine precursors were investigated, to determine the effect that these would have on the reaction. These were chosen in order to promote the use of more cost effective ionic liquid catalysts. The



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reaction was also performed with conventional homogeneous and heterogeneous catalysts for comparison. The results for these experiments are shown in Table 2. Table 2. Catalytic activity of different ionic liquid catalysts compared with conventional homogeneous and heterogeneous catalysts in the esterification of glycerol with acetic acid. Glycerol

Acetin

Conversion

Selectivity (mol%)

TOF (hr-1)a

Ionic Liquid

(mol%)

Mono

Di

Tri

[H-NMP][HSO4]

99.0

20.8

60.3

18.9

98.1

[H-NEP][HSO4]

99.0

23.4

55.2

21.3

98.1

[Et3NH][HSO4]

97.3

39.5

55.4

5.1

96.4

[Pr3NH][HSO4]

98.1

40.7

52.9

6.4

97.2

[Oct3NH][HSO4]

97.9

28.4

60.1

11.5

97.0

Sulfuric Acid

99.4

12

63.3

24.7

98.9

Bentoniteb

50.9

76.3

21.3

2.4

-

Amberlyst-15b

90.4

48.8

43.0

8.1

-

Blank

47.4

100.0

0.0

0.0

N/A

Reaction conditions: glycerol 0.054 mol, acetic acid 0.326 mol, catalyst 2 mol%, agitation speed 800 RPM, temperature 100 °C, time 30 min. a – Turn over frequency (TOF) = moles of glycerol converted per mole of acid site per hour. b – Solid acids have been compared at 4.2 wt% in terms of glycerol. This is the same wt% loading as that of [H-NMP]HSO4. From the results shown in Table 2, it can be noted that ionic liquids based on the n-alkyl-2pyrrolidone cation are more active than those based on a trialkylamine cation. [H-NEP][HSO4] gives 99 % conversion and 21.3 % selectivity to triacetin, in comparison to [Oct3NH][HSO4]


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which gives comparable conversion at 97.9 % but only 11.5 % selectivity towards triacetin. It can also be noted that an increase in the alkyl chain length on the ammonium center results in an increased level of selectivity towards triacetin. [Et3NH][HSO4] yielding only 5.1 % triacetin after 30 minutes, in comparison to the 11.5 % selectivity exhibited by [Oct3NH][HSO4]. This may be attributed to an increase in the hydrophobic nature of the ionic liquid as a result of the increased alkyl chain length, repelling water away from the acidic center and preventing it from participating in the reverse reaction. From Table 2 it can be noted that neat sulfuric acid provides comparable activity as that of [H-NMP][HSO4], however as mentioned before sulfuric acid is hazardous to handle, corrosive and non-reusable

12.

Therefore use of sulfuric acid as a catalyst

should be avoided as it produces large amounts of waste and is not environmentally friendly. In comparison with bentonite clay, [H-NMP][HSO4] shows much higher activity. Bentonite achieves only moderate conversion of glycerol at 50.9 % conversion and low selectivity to triacetin at 2.4 %, after 30 minutes. In comparison with amberlyst-15, [H-NMP][HSO4] also exhibits higher activity with greater conversion and higher selectivity to triacetin. Amberlyst-15 was limited to 90.4 % conversion and 8.1 % triacetin selectivity, in comparison to the 18.9 % selectivity to triacetin achieved by [H-NMP][HSO4]. [H-NMP][HSO4] exhibits comparable catalytic activity to that of [H-NEP][HSO4], exhibiting high glycerol conversion and good selectivity towards triacetin. It can also be seen from the acid values that [H-NMP][HSO4] has the highest acid value out of all the ionic liquids tested. As a cation precursor n-methyl-2pyrrolidone is less expensive than n-ethyl-2-pyrrolidone. Since the cation precursor n-methyl-2pyrrolidone is less expensive than n-ethyl-2-pyrrolidone, we pursued all further optimisation studies using [H-NMP][HSO4] as the most suitable catalyst.



The reaction time profile using [H-NMP][HSO4] is shown in Figure 3. The sequential nature of the reaction can be seen from this graph. As glycerol is consumed, monoacetin forms first decreasing as diacetin forms. Following this diacetin is further converted and decreases in concentration as triacetin forms. Eventually a state of equilibrium is reached and no further conversion of the products takes place.

Conversion/ Selectivity (%)

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

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100

80

60

40

20

0 0

10

20

30

40

50

60

Time (minutes) Figure 3. Reaction composition-time profile for Glycerol esterification with acetic acid using [H-NMP][HSO4] ionic liquid as catalyst; () - % conversion of glycerol, () - % selectivity to monoacetin, () - % selectivity to diacetin, () - % selectivity to triacetin, Reaction conditions: Glycerol 0.054 mol, acetic acid 0.326 mol, catalyst 2 mol%, agitation speed 800 RPM, temperature 100 °C, time 30 min. In order to optimize the reaction conditions for the chosen catalyst, a Design of Experiment (DoE) approach was taken. Design of Experiment is a statistical and systematic methodology for investigating the effect of chosen parameters of then outcome of a given process. DoE has a number of advantages of the typically used one variable at a time (OVAT)


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approach. DoE takes into account interactions between variables, provides a global knowledge with a higher quality of information, and requires fewer experimental runs than with OVAT31.

3.3 Development of Response Surface Methodology model Response surface methodology (RSM) is used to model and analyze the effect of the chosen variables on the outcome of a reaction. A central composite design (CCD) is typically used in the optimization of the parameters

32.

The CCD is a full 2k factorial design involving 6 repeated

central points, and axial points. The following factors were chosen to be studied; (1) catalyst loading, (2) temperature, (3) stirring speed, and (4) glycerol to acetic acid molar ratio. The four factors were investigated at 5 levels (-α, -1, 0, +1, +α), as shown in Table 3. Low (-1) and high (+1) values were decided, with the software computing the central point values (0) and low and high axial point values. Table 3 - Actual values for the 5 level four-factor CCD matrix. Factor / unit

-α

-1

0

+1

+α

Catalyst loading / mol % glycerol

A

1

4

7

10

13

Temperature / °C

B

30

50

70

90

110

Stirrer speed / RPM

C

200

400

600

800

1000

Glycerol to acetic acid molar ratio

D

1

4

7

10

13

The 30 designated experiments were carried out in a randomized order to limit error. Table 4 shows the response from the CCD experimental runs plotted against values that were predicted by the model equation, as shown below. This equation is also used to generate the response surface plots which show the effect of the variables on the % triacetin yield. Table 4 - Experimental responses against predicted responses



Standar d

Coded Value A

B

C

D

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Actual triacetin

Predicted

yield / %

triacetin yield /

Residual

% 1

-1

-1

-1

-1

1.5

2.07

-0.5667

2

1

-1

-1

-1

6.2

8.83

-2.63

3

-1

1

-1

-1

16.3

14.8

1.5

4

1

1

-1

-1

17.8

18.83

-1.03

5

-1

-1

1

-1

3.6

2.91

0.6875

6

1

-1

1

-1

6.9

9.3

-2.4

7

-1

1

1

-1

17.2

13.57

3.63

8

1

1

1

-1

17

17.23

-0.2292

9

-1

-1

-1

1

3.2

4.15

-0.9458

10

1

-1

-1

1

8.4

12.03

-3.63

11

-1

1

-1

1

39.9

37.5

2.4

12

1

1

-1

1

40.8

42.66

-1.86

13

-1

-1

1

1

5.4

4.37

1.03

14

1

-1

1

1

9.2

11.88

-2.68

15

-1

1

1

1

37.1

35.65

1.45

16

1

1

1

1

41

40.43

0.5667

17

-2

0

0

0

5.5

10.69

-5.19

18

2

0

0

0

28.6

22.24

6.36

19

0

-2

0

0

0.6

-4.38

4.98

20

0

2

0

0

33.1

36.9

-3.8

21

0

0

-2

0

24.8

22

2.8

22

0

0

2

0

19

20.62

-1.62

23

0

0

0

-2

2.3

2.37

-0.0708

24

0

0

0

2

28.9

27.65

1.25

25

0

0

0

0

24.8

22.48

2.32

26

0

0

0

0

21.8

22.48

-0.6833

27

0

0

0

0

23.7

22.48

1.22

28

0

0

0

0

21.8

22.48

-0.6833


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29

0

0

0

0

25

22.48

2.52

30

0

0

0

0

17.8

22.48

-4.68

𝑇𝑟𝑖𝑎𝑐𝑒𝑡𝑖𝑛 𝑌𝑖𝑒𝑙𝑑 (%) = 22.48 + 2.89𝐴 + 10.32𝐵 ― 0.3458𝐶 + 6.32𝐷 ― 0.6813𝐴𝐵 ― 0.0937𝐴𝐶 + 0.2813𝐴𝐷 ― 0.5187𝐵𝐶 + 5.16𝐵𝐷 ― 0.1562𝐶𝐷 ― 1.51𝐴2 ― 1.56𝐵 2 ― 0.2927𝐶2 ― 1.87𝐷2 Equation 2 - Coded model equation.

The following diagnostic plots can be used to diagnose the model fit. Figure 4(a) shows the predicted response values and the experimental result values in the form of a scatter graph, with Figure 4(b) showing the normal probability plot. The normal probability plot of residuals indicates whether the residuals (deviation between the data and the fit) show a normal distribution. It can be seen from the graph that the distribution of residuals is normal as the plot follows a straight line. Outliers from the trend are not a cause for concern, as these are within the control limits.

Predicted vs. Actual

Design-Expert® Software Trial Version

50

Triacetin Color points by value of Triacetin: 41 0.6

(a)

(b)

Triacetin Color points by value of Triacetin: 41 0.6

40

Predicted

Normal Plot of Residuals

Design-Expert® Software Trial Version

30 20 10 0

99

Normal % Probability

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


90 70 50 20 5 1

-10 -10

0

10

20

Actual

30

40

50

-3

-2

-1

0

1

2

3

4

Externally Studentized Residuals



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Figure 4. (a) Experimental yield of triacetin against predicted model values (b) Normal probability plots of residuals. The yield of triacetin (%) can then be related to catalyst loading (A), temperature (B), stirrer speed (C), and glycerol to acetic acid molar ratio (D) using ANOVA for a quadratic model. The terms A, B, D, BD, B2 and D2 all had p-values of less than 0.05, indicating they are significant model terms, as shown in Table 5. Temperature was found to be the most significant variable (F value = 175.29), followed by the glycerol to acetic acid molar ratio (F value = 65.75), and finally the interaction between the two (F value = 29.17). It is worth noting that stirrer speed had a pvalue of 0.6636 indicating that it is not a significant model term. The ANOVA also provides data on the fit of the model, with the R2 value providing insight into the correlation between actual experimental values and predicted values. The R2 value was calculated by ANOVA to be 0.95 showing good correlation between actual and predicted values. The model is highly significant due to its low p-value and also high adjusted R2 value of 0.91.

Table 5 - Analysis of variance (ANOVA) for response surface quadratic model

Source

Sum of Squares

df

Mean Square

F-value

p-value

Model A-Cat Loading B-Temperature C-RPM D-Mole Ratio AB AC AD BC

4334.28 200.1 2556.47 2.87 958.87 7.43 0.1406 1.27 4.31

14 1 1 1 1 1 1 1 1

309.59 200.1 2556.47 2.87 958.87 7.43 0.1406 1.27 4.31

21.23 13.72 175.29 0.1968 65.75 0.5092 0.0096 0.0868 0.2952

< 0.0001 0.0021 < 0.0001 0.6636 < 0.0001 0.4865 0.9231 0.7723 0.5949


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BD CD A² B² C² D² Residual Lack of Fit Pure Error Cor Total

425.39 0.3906 62.14 66.34 2.35 95.68 218.76 182.71 36.05 4553.04

1 1 1 1 1 1 15 10 5 29

425.39 0.3906 62.14 66.34 2.35 95.68 14.58 18.27 7.21

29.17 0.0268 4.26 4.55 0.1611 6.56

< 0.0001 0.8722 0.0567 0.0499 0.6938 0.0217

2.53

0.1582

Figure 5a shows the effects of catalyst loading and reaction temperature on the percentage yield of triacetin, using a mole ratio of 10 and stirrer speed of 600 RPM. As temperature increases from 50 °C to 90 °C, percentage yield of triacetin increases. A similar trend can be noted with increasing the catalyst loading, however this is not as pronounced as the effect that temperature has. Figure 5b shows the effect of catalyst loading and speed of stirring on triacetin yield at 70 °C and a mole ratio of 10. It can be noted that stirrer speed shows little effect on the triacetin yield, with increases in RPM resulting in marginal decreases in the yield. Figure 5c shows the effects of catalyst loading and mole ratio on the triacetin yield at 70 °C and 600 RPM stirrer speed. Catalyst loading can be seen to increase the yield slightly as it increases. An increase in mole ratio results in an increase in triacetin yield. Lower mole ratios resulted in a shorter time period until equilibrium was reached preventing the reaction from proceeding further. Figure 5d shows the effects of temperature and stirrer speed on the percentage yield of triacetin, at 7 mol% catalyst loading and a mole ratio of 10. As stated before temperature has a marked effect on the yield of triacetin, with an increase in temperature leading to an increase in the yield. It can also be seen again that stirrer speed has little effect on the yield. Figure 5e shows the effect of mole ratio and temperature on the yield of triacetin, at 7 mol% catalyst loading and 600 RPM stirrer speed. It can be seen at lower temperatures that increasing the mole ratio of glycerol to acetic



acid has little effect on the triacetin yield. Similarly increasing temperature at a lower mole ratio only results in a small increase in the yield. An increase in temperature at higher mole ratios can be seen to promote a large increase in the triacetin yield to over 40 % at the conditions stated. The interaction between these two variables has a marked effect on the triacetin yield. Figure 5f shows the effect of stirrer speed and mole ratio on the triacetin yield, at 7 mol% catalyst loading and 70 °C. Again it can be seen that stirrer speed has little to no effect on the yield of triacetin.


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(a)

(b)

(c)

(d)

(e)

(f)



Figure 5. (a) 3D response surface plot for the effect of catalyst loading and temperature on triacetin yield; 600 RPM stirrer speed, mole ratio of 10; (b) 3D response surface plot for the effect of catalyst loading and stirrer speed on triacetin yield; 70 °C and mole ratio of 10; (c) 3D response surface plot for the effect of catalyst loading and mole ratio on triacetin yield; 70 °C and 600 RPM; (d) 3D response surface plot for the effect of temperature and stirrer speed on triacetin yield; 7 mol% catalyst loading, mole ratio of 10; (e) 3D response surface plot for the effect of temperature and mole ratio on triacetin yield; 7 mol% catalyst loading, 600 RPM stirrer speed; (f) 3D response surface plot for the effect of stirrer speed and mole ratio on triacetin yield; 7 mol% catalyst loading, 70 °C.

In order to optimise the conditions the model allows us to set goals for the parameters and outcome i.e. minimise or maximise the values, or to keep the values in the range set by the limits. By keeping the parameters within the limits set, and opting to maximise the yield of triacetin the optimum conditions were found, and shown in Table 6. Under these settings the maximum yield of triacetin was predicted to be 42.69 %. An experimental run was performed to evaluate the accuracy of the predicted results. Under the optimum conditions the following results were obtained; a glycerol conversion of 99.3 %, and selectivity to mono-, di- and tri- of 4.8 %, 52.9 % and 42.3 % respectively. This result shows not only high combined selectivity to di- and triacetin, but also shows the accuracy with which the model can predict results within the specified range.


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Table 6 - Optimum parameter settings Factor / unit

Optimum setting

Catalyst loading / mol %

9.57

glycerol Temperature / °C

90

Stirrer speed / RPM

400

Glycerol to acetic acid mole

10

ratio

3.4 Plausible reaction mechanism for glycerol esterification The plausible reaction mechanism of glycerol esterification with acetic acid is proposed to be analogous to earlier reports in literature, involving the activation of acetic acid carbonyl group 22. As shown in Scheme 3, the sequence starts with the activation of the acetic acid carbonyl group by [H-NMP][HSO4], followed by subsequent attach of carbonyl carbon by the oxygen of glycerol. The transfer of proton from the intermediate to adjacent hydroxyl group, then results in the loss of water molecule, producing monoacetin. Further reaction of monoacetin with acetic acid following similar reaction mechanism produces di and triacetins.



Scheme 3. Plausible reaction mechanism for glycerol esterification with acetic acid

3.5 The catalyst reusability The reusability of [H-NMP][HSO4] was evaluated in a series of recycling experiments. Upon completion of the reaction, excess acetic acid was distilled off under reduced pressure, followed


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by solvent extraction with diethyl ether. This resulted in the ionic liquid forming a separate layer at the bottom of the funnel. This procedure was repeated four times and the results are shown in Figure 6. The glycerol conversion remained constant over the 4 runs, however product distribution varies slightly. This may be attributed to partial loss of catalyst during the separation process. A 1H NMR of the recycled catalyst is included in the supporting information. Conversion/ Selectivity (%)

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


100 80 60 40 20 0 Fresh Glycerol Conversion

Recycle 1 Monoacetin

Recycle 2 Diacetin

Recycle 3 Triacetin

Figure 6. Effect of reusability of the [H-NMP][HSO4] catalyst. Reaction conditions: glycerol 0.054 mol, acetic acid 0.326 mol, catalyst 5 mol%, agitation speed 800 RPM, temperature 100 °C, time 30 min.

4. Conclusions

The esterification of glycerol with acetic acid was studied with nitrogen based Brønsted-acidic ionic liquids. The effect of the alkyl chain length of the ammonium cation was investigated, and it was discovered that a longer alkyl chain length led to increased yield of di- and tri- substituted products. The ionic liquids based upon pyrrolidone cations were found to be more active and again lead to increased yield in the di- and tri- substituted products. However, all of the ionic liquids tested gave greater than 97 % glycerol conversion using 2 mol% of catalyst at 100 °C after 30 minutes. [H-NMP][HSO4] was found to give high activity whilst remaining cost



effective. The esterification of glycerol was optimized by using DoE, amongst the reaction parameters, reaction temperature showed most significant influence on selectivity to di- and triacetins, followed by the glycerol to acetic acid molar ratio, and the model also showed dependence on the synergistic interaction between temperature and mole ratio. Optimum reaction conditions were 9.57 mol % catalyst loading, reaction temperature 90 °C, speed of agitation 400 RPM and glycerol to acetic acid mole ratio of 1:10. These conditions resulted in >99% conversion of glycerol, with >95% combined yield to di- and triacetins. The catalyst also exhibited good recyclability over 4 recycles, without any noticeable loss of activity.

Supporting Information 1H

NMR spectra for the prepared ionic liquids.

Acknowledgements The authors gratefully acknowledge the financial support from “The Bryden centre for advanced marine and bio-energy research” funded through the INTERREG VA Programme for PhD studentship to JK and the Invest Northern Ireland Competence Centre programme under the “Centre for Advanced Sustainable Energy” (CASE) project R3947CCE for postdoctoral funding for MT. The authors would also like to thank Prof. Tom Welton for his suggestion of the DoE approach.


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9. Sun Y.; Hu J.; An S.; Zhang Q.; Guo Y.; Song D.; Shang Q.; Selective esterification of glycerol with acetic acid or lauric acid over rod-like carbon-based sulfonic acid functionalized ionic liquids. Fuel, 2017 , 207, 136-145. 10. Huang M.; Han X.; Hung C.-T.; Lin J.-C.; Wu P.-H.; Wu J.-C.; Liu S.-B.; Heteropolyacid-based ionic liquids as efficient homogeneous catalysts for acetylation of glycerol. Journal of Catalysis, 2014, 320, 42-51. 11. Patel A.; Singh S.; A green and sustainable approach for esterification of glycerol using 12tungstophosphoric acid anchored to different supports: Kinetics and effect of support. Fuel, 2014, 118, 358-364. 12. Gupta P.; Paul S.; Solid acids: Green alternatives for acid catalysis. Catalysis Today, 2014, 236, 153170. 13. Hu W.; Zhang Y.; Huang Y.; Wang J.; Gao J.; Xu J.; Selective esterification of glycerol with acetic acid to diacetin using antimony pentoxide as reusable catalyst. Journal of Energy Chemistry, 2015, 24 (5), 632-636. 14. Dosuna-Rodríguez I.; Gaigneaux E. M.; Glycerol acetylation catalysed by ion exchange resins. Catalysis Today, 2012, 195(1), 14-21. 15. Kale S.; Umbarkar S. B.; Dongare M. K.; Eckelt R.; Armbruster U.; Martin A.; Selective formation of triacetin by glycerol acetylation using acidic ion-exchange resins as catalyst and toluene as an entrainer. Applied Catalysis A: General, 2015, 490, 10-16. 16. Reinoso D. M.; Tonetto G. M.; Bioadditives synthesis from selective glycerol esterification over acidic ion exchange resin as catalyst. Journal of Environmental Chemical Engineering, 2018, 6(2), 33993407.


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17. Popova M.; Lazarova H.; Kalvachev Y.; Todorova T.; Szegedi Á.; Shestakova

P.;Mali

G.;

Dasireddy V. D.B.C.; Likozar B.; Zr-modified hierarchical mordenite as heterogeneous catalyst for glycerol esterification. Catalysis Communications, 2017, 100, 10-14. 18. Gonçalves V. L. C.; Pinto B. P.; Silva J. C.; Mota C. J. A.; Acetylation of glycerol catalyzed by different solid acids. Catalysis Today, 2008, 133-135, 673-677. 19. Melero J. A.; van Grieken R.; Morales G.; Paniagua M.; Acidic mesoporous silica for the acetylation of glycerol: Synthesis of bioadditives to petrol fuel. Energy Fuels, 2007, 21(3), 1782-1791. 20. Trejda M.; Stawicka K.; Dubinska A.; Ziolek M.; Development of niobium containing acidic catalysts for glycerol esterification. Catalysis Today, 2012, 187(1), 129-134. 21. Zhu S.; Zhu Y.; Gao X.; Mo T.; Zhu Y.; Li Y.; Production of bioadditives from glycerol esterification over zirconia supported heteropolyacids. Bioresource Technology, 2013, 130, 45-51. 22. Sandesh S.; Manjunathan P.; Halgeri A. B.; Shanbhag G. V.; Glycerol acetins: Fuel additive synthesis by acetylation and esterification of glycerol using cesium phosphotungstate catalyst. RSC Adv., 2015, 5, 104354-104362. 23. Liu X.; Ma H.; Wu Y.; Wang C.; Yang M.;Yan P.; Welz-Biermann U.; Esterification of glycerol with acetic acid using double SO3H-functionalized ionic liquids as recoverable catalysts. Green Chem., 2011, 13(3), 697-701. 24. Ralphs K.; McCourt É.; Nockemann P.; Jacquemin J.; Manyar H.; Highly Selective Reduction of α, βUnsaturated Aldehydes and Ketones under Ambient Conditions using Tetraalkylphosphonium-based Ionic Liquids. ChemistrySelect, 2018, 3, 42, 11706-11711. 25. Li L.; Yu S.; Xie C.; Liu F.; Li H.; Synthesis of glycerol triacetate using functionalized ionic liquid as catalyst. J Chem Technol Biotechnol., 2009, 84(11), 1649-1652.



26. Huang M.-Y.; Han X.-X.; Hung C.-T.; Lin J.-C. Wu P.-H.; Wu J.-C.; Liu S.-B.; Heteropolyacidbased ionic liquids as efficient homogeneous catalysts for acetylation of glycerol. Journal of Catalysis, 2014, 320, 42-51. 27. Wagh K. V.; Bhanage B. M.; Greener approach for the synthesis of substituted alkenes by direct coupling of alcohols with styrenes using recyclable bronsted acidic [NMP][HSO4] ionic liquid. RSC Adv., 2014, 4(43), 22763-22767. 28. Wang C.; Guo L.; Li H.; Wang Y.; Weng J.; Wu L.; Preparation of simple ammonium ionic liquids and their application in the cracking of dialkoxypropanes. Green Chem., 2006, 8(7), 603-607. 29. Ramli NAS, Amin NAS. A new functionalized ionic liquid for efficient glucose conversion to 5hydroxymethyl furfural and levulinic acid. Journal of Molecular Catalysis A: Chemical. 2015, 407, 113121. 30. Matuszek K, Chrobok A, Coleman F, Seddon KR, Swadźba-Kwaśny M. Tailoring ionic liquid catalysts: Structure, acidity and catalytic activity of protonic ionic liquids based on anionic clusters, (HSO4)(H2SO4)x]− (x = 0, 1, or 2). Green Chem. 2014, 16 (7), 3463-3471. 31. Leardi R.; Experimental design in chemistry: A tutorial. Analytica Chimica Acta, 2009, 652 (1), 161172. 32. Chaudhary P.; Verma A.; Kumar S.; Gupta V. K.; Experimental design and optimization of castor oil transesterification process by response surface methodology. Biofuels, 2018, 9(1), 7-17.


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TOC/Graphical Abstract

O H

C

3

O C H3

O

O O

O

OH HO

3 CH

O

OH

+ H3 C

OH

Glycerol esterification to fuel-additives Synopsis The glycerol esterification using Brønsted-acidic ionic liquids was optimized by DoE to achieve >99% glycerol conversion and >95% di- and triacetin yield.


Esterification of Glycerol with Acetic Acid using Nitrogen Based

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