Synthesis of Biobased Polyester Polyol through Esterification of

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Kinetics, Catalysis, and Reaction Engineering

Synthesis of Bio-based Polyester Polyol through Esterification of Sorbitol with Azelaic Acid Catalysed by Tin (II) Oxide- A Kinetic Modeling Study Muhammad Ridzuan Kamaruzaman, Sim Yee Chin, Elaine Chiew Ling Pui, Haniif Prasetiawan, and Nurwadiah Azizan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02506 • Publication Date (Web): 21 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018

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Industrial & Engineering Chemistry Research

Synthesis of Bio-based Polyester Polyol through Esterification of Sorbitol with Azelaic Acid Catalysed by Tin (II) Oxide- A Kinetic Modeling Study

M.R. Kamaruzamana, S.Y. Chin*a,b, E. C. L. Puia, and H. Prasetiawanc, Nurwadiah Azizana a Faculty of Chemical and Natural Resources Engineering, Universiti Malaysia Pahang, LebuhrayaTunRazak, Pahang, Kuantan, 26300, Malaysia.

b Center of Excellence for Advanced Research in Fluid Flow, Universiti Malaysia Pahang, LebuhrayaTunRazak, Pahang, Kuantan, 26300,

Malaysia.

c Chemical Engineering Department, Universitas Negeri Semarang, Gd. E1 Kampus Sekaran Gunungpati, Semarang, 50229, Indonesia

*Email: [email protected]

Abstract A sustainable and renewable bio-based polyester polyol for the polyurethane production was synthesised through the esterification of azelaic acid and sorbitol catalysed by tin (II) oxide in a batch system. The studies on chemical equilibrium, reaction kinetics and important operating parameters were carried out. The temperature, molar ratio of sorbitol to azelaic acid and catalyst loading were varied in order to determine the best reaction conditions. The polyester polyol synthesised was tested for its fatty acid content through titration. The best operating condition found was at reaction temperature of 433K, sorbitol to azelaic acid molar ratio of 4:1 and catalyst loading of 2wt%, yielding 72% azelaic acid conversion after 6 hours of reaction. The presence of minute amount of sorbitan and isosorbide inferred the potential of sorbitol-based branched polyester formation with its backbone incorporated with these sorbitol anhydrides. The equilibrium study validated the endothermicity of the reaction. Meanwhile, the kinetic data well fitted to the Langmuir Hinshelwood Hougen Watson (LHHW) model with the activation energy of 14.43 kJ/mol.

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Industrial & Engineering Chemistry Research 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

Keywords Kinetic modeling, esterification, azelaic acid, sorbitol, polyurethane 1.0

Introduction Polyurethanes (PUs) are one of the most versatile groups of polymers suitable for the

application in the area of foams, elastomers, thermoplastics, adhesives, coatings, sealants and ﬁbers. Conventionally, PUs is produced by reacting petro-based polyester polyol with diisocyanate. Uncertainty in terms of price and availability of petroleum, in addition to global political and institutional tendencies toward the principles of sustainable development, has prompted the continuing effort to identify bio-based PUs. Sorbitol-based polyester is one of the potential polyester polyol for the production of bio-based PUs possessing comparable properties with the petro-based PUs. Sorbitol-based polyesters can be produced by direct, acid or base-catalysed esterification reaction of sorbitol (SL) with fatty acids at elevated temperature. It can also be produced through the basic catalysed transesterification of SL with triglycerides or fatty acid methyl esters. In recent years, most of the researches for the synthesis of sorbitol-based polyester was carried out using the route of acid catalysed esterification since the base catalysed reaction attributes to the unwanted highly coloured the products. Moreover, the anhydrization degree of SL increases in the presence of an


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acid catalyst, and therefore producing the mixtures with a high proportion of esters of sorbides. The isosorbide ester produced possesses rigid molecular structure which improves the mechanical properties of the PUs.1 Giacometti et al. prepared the surfactant through the esterification of SL with lauric acid (LA) using p-toluene sulfonic acid as a catalyst.2 The use of SL that underwent cyclisation prior the esterification reaction resulted higher LA conversion. A reaction temperature of 160°C was claimed to give desired rate of reaction and sorbitol lauric acid esters with better colour quality. Meanwhile, the same homogeneous catalyst was employed by Corma et al. for the synthesis of sorbitol oleic acid esters at the temperature of 135-200°C. In relative to the direct esterification of SL with oleic acid, a process involved the protection of sorbitol by ketalisation followed by esterification with oleic acid improved the quality in terms of hydroxyl value.3 Enzyme such as lipase in homogeneous phase has also been employed as a catalyst in the esterification of SL anhydrides (sorbitan and isosorbide) with oleic acid and decanoic acid.4-5 The advantages of the use heterogeneous catalytic system have prompted a comparative study of the esterification of oleic acid with sorbitol, isosorbide and lauryl alcohol using supported lipozyme as a catalyst.6 Despite its environmental benign and safe nature, the enzyme esterification of SL and its anhydrides with fatty acid has revealed low reaction rate that causing long reaction times. A long reaction time also required for the synthesis of sorbitol oleic acid esters through a two-step catalytic process (involving sorbitol ketalisation followed by esterification with oleic acid) using mordenite as catalyst.3 This time consuming reaction was ascribed to the relatively small number of acid sites in mordenite catalyst. Nonetheless, the shape selectivity of mordenite catalyst has led to the higher ratio of mono- to higher esters and hence the higher hydroxyl value of the final product in comparison with those obtained from the



homogeneously-catalysed process and other heterogeneously-catalysed process (heteropolyacids and other zeolite catalysts). To date, most of the studies on the sorbitol fatty acid esters synthesis used saturated mono-carboxylic fatty acid as the reactant. Very limited information about the heterogeneously catalysed esterification of SL with di-carboxylic fatty acid could be found in the open literature. Azelaic acid (AA) is industrially used in the manufacturing of pharmaceutical products, plasticizers, lubricants and hydraulic fluids. It belongs to a family of a naturally occurring saturated dicarboxylic fatty acid. It is a superior raw material in the formulation of high solids or solvent free systems ascribing to its excellent solubility in organic solvents and water.7 The polyesters made from AA was reported to have high-dimensional stability.8 In the present study, SL and AA were esterified to produce bio-based polyester polyol as the potential source for PUs production. The parametric and kinetic studies of this esterification reaction were carried out in a batch system. In spite of the superior activity of the germanium dioxide catalyst in the previous report,9 tin (II) oxide was selected as catalyst at present because it is more cost effective (cost of 20-fold lower) comparing to the germanium dioxide.10 2.0 Materials and Methods 2.1 Materials Azelaic acid (90%), potassium hydroxide (KOH) pellets and potassium hydrogen phthalate (KHP) were purchased from Merck. Sorbitol solution (70%) and ethanol (99.7%) were obtained from Sigma-Aldrich. Tin (II) oxide (99%) was supplied by Alfa-Chemistry possesses the surface area of 0.7m2/g, pore volume of 0.0022m3/g and pore size of 13nm.10 All chemicals were used without further purifications.


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2.2 Methods 2.2.1 Experimental procedure for the esterification reaction study The esterification reaction of SL and AA catalysed by tin (II) oxide was carried out in a 500ml three neck flask equipped with condenser. Temperature probe and controller were used to measure and maintain the reaction temperature. The SL and AA were heated separately. The water presented in the SL solution (70 %) was removed through vaporisation at 110°C. Subsequently, these two reactants were mixed to attain a reaction mixture with the initial volume of 200 ml before the catalyst was introduced. The samples were withdrawn every 30 minutes for the first hour and every 1 hour for the subsequent 6 hours. The total amount of sample withdrawn was less than 5% of the initial volume. The samples were immersed in an iced bath to cease the reaction. The important reaction parameters were varied during the reaction study. These parameters included molar ratio of SL to AA (MSL:AA) ranged from 1:1-6:1, reaction temperature from 403-433 K and catalyst loading from 1-8 wt%. The procedure for equilibrium study was identical to the esterification reaction study at different temperatures but the reaction time was prolonged until the conversion achieved equilibrium. The samples were taken every 5 minutes for the first 30 minutes. After the sampling at 1 hour, the samples were taken hourly for the subsequent 5 hours. The sampling was then done in every two hours until the equilibrium was achieved. The reaction was in equilibrium when the acid value was constant. The blank run (without catalyst) was carried out using a similar approach to evaluate the effect of the uncatalysed reaction.



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2.2.2 Analysis The acid value (AV) of the sample was determined by titration using KOH with phenolphthalein as indicator. The KOH solution was standardised using KHP. AV was calculated using Eq. 1.

AV = ( A × N × 56.1) / W

(1)

Where A is amount of 0.1M of KOH consumed for sample in ml, N is KOH concentration in mol/L and W is sample weight in g.

Consequently, the conversion of AA was calculated using Eq. 2.

X =

AV0 - AVt AV0

(2)

Where AV0 is the acid value of the sample when t=0 while AVt is the acid value of the sample at any time t.

The gas chromatography (GC) analysis was limited to certain compounds in the reaction mixture attributing to the unavailability of commercial GC standards. The reaction products (sorbitan and isosorbide) were determined by GC-FID using CP-TAP CB column with 25 m in length, 0.10 µm of film thickness, and 0.25 m of inner diameter. The oven temperature was ramped from 423 K to 493 K with the rate of 278 K/min. The split ratio was 20:1. The injector and detector temperature was set at 523 K and 653 K respectively.


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2.2.3 Kinetic Study The main esterification reaction of SL and AA is shown in Eq. 3. The possible side reaction are; 1. Dehydration of sorbitol (SL) to sorbitan (ST) as shown in Eq. 4; 2. Dehydration of ST to isosorbide (SB) as shown in Eq. 5; and 3. Esterification of ST and SB with azelaic acid as shown in Eq. 6 and Eq. 7. The gas chromatography analysis validated that only minute amount of ST and SB presented in the product. The side reactions can therefore be ignored in the reaction mixture at 160°C using tin (II) oxide as catalyst.

(3) + ↔ + (4) → + " (5) → + #$$ (6) + ↔ + " ' (7) + ↔ + #$$ $$

The reaction rate (rate of consumption of AA), rAA was determined by the differentiating the AA concentration-time profile. The reaction mechanism can be elucidated using different types of kinetic models (Pseudohomogeneous (PH) model, Eley Rideal (ER) model and Langmuir Hinshelwood Hougen Watson (LHHW) model) as shown in Eq. 8, 9 and 10 respectively.11



(

rAA = k f C AA C SL - C SA CW / K eq

(

)

)

rAA = k f C AAC SL - C SACW / K eq / (1 + K AAC AA + KW CW )

(

)

rAA = k f C AACSL - CSACW / K eq / (1 + K AAC AA + K SLCSL + K SACSA + KW CW )2

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(8) (9) (10)

Where kf is the forward rate constant, Keq is the equilibrium constant, C is the concentration, Ki is the adsorption constant for species i, the subscript of AA, SL, SA and W are azelaic acid, sorbitol, sorbitol azelate and water respectively.

The kinetic model of PH, ER or LHHW was then incorporated in to the ordinary differential equations (ODE) derived from the mass balance equation of batch reactor. ODE solver in MATLAB, ODE45 was used to solve these ODE. The FMINCON function in the optimisation toolbox of MATLAB was used to optimise the unknown kinetic parameters in the ODE while minimising the objective function (the difference between the experimental and predicted conversion of AA). The model was evaluated based on the mean absolute relative error percentage between calculated concentration and experimental concentration (MAE) as shown in Eq. 11.

)* =

,∑455 6474 84915/8

./01 2.3453 , ./01

:84915/

× 100

(11)

The activation energy (Ea) and pre-exponential (A) factor was obtained from Arrhenius equation as shown in Eq. 12. ln k f = - Ea / RT + ln A


(12)

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The Gibbs free energy, enthalpy ( ∆H ) and entropy ( ∆S ) were determined using Eq. 1315.

∆G o = - RT ln K eq ∆G o = ∆H - T ∆S o

ln K eq = -∆H o / RT + ∆S o / R

(13) (14) (15)

Where ∆G is Gibbs free energy, R is universal gas constant and T is temperature.

3.0 Results and Discussion 3.1 Effect of important operating parameters

3.1.1 Effect of temperature The effect of temperature was investigated by carrying out the esterification reaction in the temperature range between 403 K to 433 K, MSL:AA of 5:1, catalyst loading of 4 wt% and stirring rate of 600 rpm. The conversion of AA increased with temperature as depicted by the conversion-time profile in Figure 1. The average standard deviation of AA conversion were 1.57%, 2.66%, 1.12%, and 0.46% for the reaction temperature of 403 K, 413 K, 423 K and 433 K respectively. The corresponding standard deviation of the initial reaction rate is shown in Figure 2. It can be observed from Figure 2 that the initial reaction rate increased more than 20% with the increment of 10 °C in the reaction temperature. The temperature rise causes more frequent collision of the reactant which resulting more successful collision to break the bonds to form the sorbitol azelate (SA).12 The reaction temperature of 433 K was used for the subsequent studies.



90 80 Conversion (%)

70 60 50 40

403K 413K 423K 433K

30 20 10 0 0

5

10

15 20 Time (Hours)

25

30

Figure 1: Azelaic acid conversion-time profile for the equilibrium study at different reaction temperature with MSL:AA of 5:1, catalyst loading of 4 wt% and stirring rate of 600 rpm.

Initial reaction rate (mol/L.hr)

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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0.25 0.2 0.15 R² = 0.9882

0.1 0.05 0 400

405

410

415 420 425 Temperature (K)

430

435

Figure 2: Effect of reaction temperature on initial rate of reaction.


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3.1.2 Effect of Molar ratio The effect of molar ratio was studied to determine the best molar ratio for the esterification of SL with AA with other parameters remained constant. The usage of excess alcohol can shift the reaction equilibrium towards ester formation and shorten the time required to achieve equilibrium. The determination of best molar ratio is to prevent the increasing cost in recovery process when too much alcohol is used.12-14 Figures 3 and 4 show that the conversion and initial reaction rate were increased with the increase of MSL:AA from 1:1 to 4:1. The similar finding was found by Gulati et al.15 in their studies on the esterification of SL with stearic acid. Further increasing the molar ratio, the negative effect towards the conversion because the excess SL has blocked the active sites on the catalyst surface and subsequently prevent it from nucleophilic attack.16 Despite the identical conversion at 6th hour, the MSL:AA of 4:1 was chosen and used for subsequent studies due to its superior initial rate of reaction.


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60 1:1 3:1 4:1 5:1 6:1

50 40 30 20 10 0 0

2

4 Time (hours)

6

8

Figure 3: Azelaic acid conversion-time profile for the reaction at different MSL:AA with stirring rate of 600rpm, reaction temperature of 433 K and catalyst loading of 4 wt%.


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0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 0

2

4 Molar ratio (SL/AA)

6

8

Figure 4: Initial reaction rate for the reaction at different MSL:AA at stirring rate of 600rpm with reaction temperature of 433K and catalyst loading of 4wt%.

3.1.3 Effect of Catalyst Loading The catalyst loading was varied from 1wt% to 8wt% to study its effect towards esterification reaction. The other parameters were kept constant. Figure 5 shows that the increase of catalyst loading has led to a rise in conversion because of the increase of catalyst sites that promoting reaction.11,17,18 No significant change in conversion was observed when the catalyst loading was changed from 4wt% to 8wt%. The initial reaction rate for the reaction with the catalyst loadings of 4 wt% and 8 wt% were identical as shown in Figure 6. The positive effect of the increase in active site was offset by the adverse effect to the mass transfer when the catalyst loading of 8 wt% was introduced. The catalyst surface area that is in contact with reactants could be reduced with the excessive catalyst particle in the reaction mixture, hence introducing a higher mass transfer resistances. The catalyst loading of 4 wt% was the best condition since it


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provided sufficient active site to accommodate the reaction of the reactant molecules with negligible diffusion limitations.19,20


60 50 40

0wt% 1wt% 2wt% 4wt% 8wt%

30 20 10 0 0

1

2

3 4 Time (hours)

5

6

7

Figure 5: Azelaic acid conversion-time profile for the reaction at different catalyst loading of tin (II) oxide with stirring rate of 600 rpm, reaction temperature of 433 K and MSL:AA of 4:1.


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


0.6 0.5 0.4 0.3 0.2 0.1 0 0

2

4 6 Catalyst loading (wt%)

8

10

Figure 6: Initial reaction rate for the reaction at different catalyst loading with stirring rate of 600 rpm, reaction temperature of 433 K and MSL:AA of 4:1.



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3.4 Equilibrium Study The esterification process is limited by equilibrium conversion. The equilibrium constant, Keq was calculated experimentally according the Eq. 16. K eq = (C SA CW ) /(C AA C SL )

(16)

Figure 1 shows the conversion profile for the equilibrium study carried out at the reaction temperature ranged from 403-433 K, while keeping molar ratio (MSL:AA) at 5:1, catalyst loading at 4 wt% and stirring rate at 600rpm. The equilibrium conversion, Xe and the corresponding Keq are shown in Table 1. The increase in Keq and Xe with temperature implies that the esterification of SL with AA is endothermic. The current findings are in accordance to the study of Giacometti et al. on the esterification of SL with lauric acid catalysed by p-toluenesulfonic acid.2

Table 1: Equilibrium constant and equilibrium conversion at different temperature. Temperature Keq

Xe (%)

403

0.1295

52.4804

413

0.2374

62.4519

423

0.3462

68.5449

433

0.9239

82.4012

(K)

Based on the Van’t Hoff plot in Figure 7, the ∆Ho is 90706 J/mol while ∆So is 208 J/mol.K. The Gibbs free energy (∆Go) was 840 J/mol at 433K. The positive value of ∆So reflects


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that the amount of disorder or randomness in the system is increased as a consequence of the reaction.

-2.5 -2 ln (Keq)

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-1.5 y = -10910x + 24.963 R² = 0.9598

-1 -0.5 0 0.0023

0.00235

0.0024 1/T (1/K)

0.00245

0.0025

Figure 7: Van’t Hoff plot for the synthesis of sorbitol azelate at different reaction temperature.

3.5 Kinetic Modeling 3.5.1 Effect of Mass Transfer The equilibrium data was used to fit with the kinetic model. The equilibrium/kinetic studies were carried out under the operating conditions which were not limited by mass transfer effects. Mears criterion and Weisz Prater criterion in Eq. 17 and Eq. 18 respectively must be satisfied to ensure the negligence of the external and internal diffusion limitation.21

CM =

(-rAA,obs ) ρb Rc n < 0.15 kC C AAb


(17)


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Where rAA,obs is the rate of reaction, n is the reaction order, Rc is the catalyst particle radius, ρb is the bulk density of the catalyst, CAAb is the bulk concentration and kC is the mass transfer coefficient. kC in relation to the fluid and catalyst properties was estimated using the methods adopted from Geankoplis.22

CWP =

(-rAA,obs ) ρ b Rc2 Deff C AA

Synthesis of Biobased Polyester Polyol through Esterification of

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