Supercritical CO2 as solvent for fatty acids esterification with ethanol catalyzed by Amberlyst-15

Journal Pre-proof Supercritical CO2 as solvent for fatty acids esterification with ethanol catalyzed by Amberlyst-15 Diego Trevisan Melfi, Kallynca Carvalho dos Santos, Luiz Pereira ´ Ramos, Marcos Lucio Corazza

PII:

S0896-8446(19)30738-7

DOI:

https://doi.org/10.1016/j.supflu.2019.104736

Reference:

SUPFLU 104736

To appear in:

The Journal of Supercritical Fluids

Received Date:

27 September 2019

Revised Date:

19 December 2019

Accepted Date:

19 December 2019

Please cite this article as: Melfi DT, dos Santos KC, Ramos LP, Corazza ML, Supercritical CO2 as solvent for fatty acids esterification with ethanol catalyzed by Amberlyst-15, The Journal of Supercritical Fluids (2019), doi: https://doi.org/10.1016/j.supflu.2019.104736

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Supercritical CO2 as solvent for fatty acids esterification with ethanol catalyzed by Amberlyst-15

Diego Trevisan Melfi1, Kallynca Carvalho dos Santos1, Luiz Pereira Ramos2, Marcos Lúcio Corazza1*

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Department of Chemical Engineering, Federal University of Paraná, CEP 81531-990,

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Curitiba, PR, Brazil.

Research Center in Applied Chemistry, Department of Chemistry, Federal University of

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Paraná, CEP 81531-990, Curitiba, PR, Brazil.

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*Corresponding author: Marcos Lúcio Corazza, PhD. E-mail: [email protected]; Phone:

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Graphical_Abstract

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55 (41) 3361-3587

Highlights 

Supercritical CO2 enhances the fatty acid esterification catalyzed by Amberlyst15. 1

   

Temperature and catalyst loading were the most significant variables for assisted scCO2 esterification. scCO2 provides higher initial reaction rates. CO2 interacts better with unsaturated acids. Esterification of different carboxylic acids and alcohols was boosted with scCO2.

ABSTRACT This work reports on the esterification of fatty acids with ethanol using Amberlyst-15 in

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a scCO2-assisted system. A factorial design was carried out for the following process variables: temperature (70 ºC to 110 ºC), ethanol to acid molar ratio (3:1 to 9:1) and catalyst to acid concentration (5 to 20 wt%), with a fixed amount of CO2 added to the

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reactor (28.62 g). The best reaction condition led to an 87% conversion at 110 °C, 9:1, and 20 wt% in 15 min of reaction time. An exploratory kinetic study illustrates the effect

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of catalyst amount and molar ratio in isothermal kinetic curves. Also, a catalyst reuse study was performed. Additionally, different carboxylic acids and methanol, instead of

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ethanol, were evaluated as reagents. At all proposed systems, the addition of scCO2

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increased the reaction conversion.

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Keywords: Esterification, supercritical CO2, ethanol, oleic acid, Amberlyst-15.

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1. Introduction

Biodiesel is traditionally produced through the transesterification of vegetable oils

with short-chain alcohols in the presence of a homogeneous alkaline catalyst [1,2]. High contents of free fatty acids (FFA) on the feedstock can lead to saponification simultaneously to transesterification, which is undesirable since the soap production consumes the alkaline catalyst, reducing its efficiency and bringing drawbacks in 2

downstream, mainly for separating esters from glycerol [3,4]. Thus, a preliminary esterification treatment before the transesterification reaction can be imposed on oils and fats with high acidity to avoid these issues and take advantage of the FFA content to enhance ester production and the biodiesel unit yield [2,5]. Conventionally, esterification reactions are carried out in the presence of homogeneous catalysts, such as sulfuric acid, due to high reaction yields reached in short reaction time [5,6]. However, this process presents disadvantages, such as toxicity,

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corrosion, large volumes of neutralization residues, and the need for downstream product purification steps [4,6]. Thus, several studies have been developed to find alternative

pathways to overcome these drawbacks and enable the use of cheaper feedstocks that

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have high acid values such as waste oils, crude oils, and animal fats [7].

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Heterogeneous catalysts are less corrosive and toxic, and more accessible to separate and reuse than homogenous catalysts. Besides, some of them can operate in mild

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conditions, making them an alternative for safe and economic processes [8–10]. Many different heterogeneous structures can catalyze both transesterification and esterification

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reactions: macroporous exchange resins, such as Pirolite, Relite and Amberlyst [11–13], carbon-supported materials [7], zeolites [14], lipases [15], zirconia [16], heteropolyacids

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immobilized on silica [17] and active clay minerals [18].

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Resins have attracted considerable attention because these porous polymeric solid catalysts contain a high number of active acid sites [19], and have considerable thermal and chemical strength [20]. Strongly acidic ion exchange resins, such as Amberlyst-15, have been proved to be effective catalysts for esterification and transesterification [13]. Son et al. [10] esterified oleic acid with methanol under Amberlyst-15 and obtained 84% of conversion at 120 °C in a continuous packed bed reactor. They pointed

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out that the phase partition inside the reactor was crucial to avoid poisoning of acid sites by water and to obtain high ester yields. However, since the residence time was not reported in their work, it is hard to make a fair comparison with batch studies. Hykkerud and Marchetti [20] and Shahid et al. [19] investigated the esterification of oleic acid with ethanol in the presence of Amberlyst-15. After 6 h of reaction at 75 ºC, they reached conversions of 53% and 42% using ethanol to oleic acid molar ratios of 6:1 and 5:1, and catalyst to fatty acid concentrations of 20 and 4 wt%. Shahid et al. [19] attributed the slow

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reaction rate to the difficulty that oleic acid has, due to its relatively long chain, to access the macroporous structure of Amberlyst-15 and find the catalytic-active sulfonic sites.

Thus, the use of Amberlyst-15 for fatty acid esterification with ethanol is still in

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need of improvement to acquire reasonable conversions in short reaction times. The main drawbacks of fatty acid esterification catalyzed by Amberlyst-15 seem to be related to

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catalyst deactivation due to the adsorption of water arisen from esterification and limited

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access of the reactants to the resin sites. In this context, the use of a solvent that improves mass transfer and promotes phase partition might enhance the catalytic performance of

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ion-exchange resins for the esterification of organic acids. Supercritical carbon dioxide (scCO2) has shown promising results as a facilitator

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for reactions with heterogeneous catalysts, improving the mass transfer, and leading to higher reaction yields [9,21,22]. By manipulating the temperature and pressure of

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operation, it is possible to tune the scCO2 density, increasing the system turbulence, and reducing external mass transfer limitations [21]. In the hydrogenation of levulinic acid to γ-valerolactone (identified as potential green fuel), the use of scCO2, besides improving the reaction yield, contributed to phase separation inside the reaction vessel. As presented by these authors, the downstream processing of the reaction product was simplified, lowering the operating cost for its purification, if needed [22]. Thus, applying scCO2 is 4

an attractive alternative to improve the catalytic esterification of fatty acids with ethanol, which, to the best of our knowledge, has not been reported in the literature so far. Hence, this work reports relevant assessments of a promising pathway that combines the advantages of scCO2 and a heterogeneous catalytic system (Amberlyst-15) to investigate the ethylic esterification of a commercial oleic acid preparation.

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2. Material and methods 2.1 Material

Amberlyst-15 (DOW Chemical Company) with a moisture content of 18 ± 1%

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was used as the esterification catalyst. Table 1 presents some typical properties of this macroporous ion-exchange resin [23]. The commercial oleic acid (named COA) was

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acquired from Alphatec (Paraná, Brazil). Stearic acid (95 wt%, CAS number 57-11-4),

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oleic acid (90 wt%, CAS number 112-80-1), lauric acid (≥ 98 wt%, CAS number 143-077) and levulinic acid (≥ 97 wt%, CAS number 123-76-2) were purchased from Sigma-

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Aldrich (São Paulo, Brazil). Methanol (≥ 99.8 wt%, CAS number 67-56-1) and ethanol (99.8 wt%, CAS number 64-17-5) were obtained from Neon (São Paulo, Brazil) and

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Êxodo Científica (São Paulo, Brazil). Carbon dioxide (> 99 wt% in the liquid phase, CAS

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number 124-38-9) was purchased from Air Liquide (Paraná, Brazil).

Commercial oleic acid was analyzed in terms of its fatty acid profile by gas

chromatography (GC-2010 Plus, Shimadzu), using a polar capillary column (SH-RtxWax, Shimadzu – 30 m x 0.32 mm; 0.25 μm) and detection by flame ionization (GCFID). Samples (1 μL) were injected at 240 °C in a split ratio of 1:10. The column program included one isothermal step at 100 ºC for 5 min, followed by one heating ramp at a rate

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of 4 ºC∙min-1 up to 240 °C and an isothermal holding of 5 min. Helium with a total flow of 32.5 cm³∙min-1 was used as the carrier gas. Samples were prepared according to the Ce2–66 AOCS method. Quantification was performed by area normalization, and the results were expressed as a percentage for each fatty acid present in the sample. COA was primarily composed of the following fatty acids: C18:1 (59.77 wt%), C18:2 (22.53 wt%), C16:0 (8.96 wt%), C16:1 (3.75 wt%) and C14:0 (1.94 wt%). The obtained chromatogram

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is given in the Supplementary Material.

2.2 Esterification reactions

Esterification was carried out in a 50 mL Parr benchtop stirred reactor (Parr Series

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4590, model 4597). All reactants, catalyst, and carbon dioxide were used as received, without any additional treatment. The molar mass of oleic acid (282.46 g∙mol-1) was used

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as the reference for COA molar ratio and acidity calculations.

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For all experiments, alcohol and the catalyst were loaded into the reaction vessel and let to stand for 15 min prior to the addition of the organic acid. Volumes of 5 mL and

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35 mL of the reactant mixture (alcohol + acid) were used for experiments in the absence and presence of CO2, respectively. Alcohol, catalyst, and acid were weighted and

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manually loaded into the reactor at room temperature, and the desired amount of CO2 was added to the system with a syringe pump operating at a constant temperature of 10 ºC

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(controlled with a thermostatic bath) and pressure of 150 bar. The stirring speed in the reactor was set to 500 rpm. When the reactor heating jacket was turned on, pressure and temperature ramps were recorded until the system reach the setpoint for the reaction temperature. This moment was considered as the initial time of the reaction. After the desired reaction time, the reactor was cooled, slowly depressurized at a rate around 10 bar∙min-1, and the content remaining in the reaction vessel was treated as a single sample. 6

Experiments were initially performed to evaluate the scCO2 effect as solvent for COA esterification with ethanol. These reactions were conducted at fixed reaction time (15 min), temperature (110 ºC), alcohol to acid molar ratio (3:1) and catalyst to acid concentration (5 wt%), varying the mass of scCO2 added to the system from 0 to 40 mL at 10 ºC and 150 bar (CO2 density of 954.18 kg∙m-3 [24]). After assessing the influence of scCO2, the effect of the process variables (temperature, molar ratio, and catalyst wt%) was investigated as described below.

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A 2³ factorial design was performed for the esterification of COA with ethanol,

keeping fixed the reaction time at 15 min, the addition of CO2 at 30 mL at 10 ºC and 150 bar (equivalent to 28.62 g for a CO2 density of 954.18 kg∙m-3 [24]). Temperature, ethanol

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to COA molar ratio and catalyst to acid wt% were used as factors, and the levels were defined with the following maximum and minimum values: 110 and 70 °C, molar ratio

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of 9:1 and 3:1, and 5 and 20 wt% of catalyst wt%. The central point in the factorial design

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(90 ºC, 6:1 and 12.5 wt%) was performed in triplicate, and the complete factorial design was carried out in duplicates. All experiments were randomly performed.

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Additional experiments were performed to investigate the COA esterification kinetics using Amberlyst-15 in scCO2 systems. The kinetic experiments were performed

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in the same conditions described above, with changes only in the reaction time. Lastly, further experiments were performed to evaluate the catalyst recovery and reuse and the

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catalytic performance of Amberlyst-15 using methanol instead of ethanol and different types of fatty and carboxylic acids instead of COA. For all experimental conditions, the acid conversion (Equation 1) was calculated based on initial and final acidity contents, which were measured based on the American Oil Chemist’s Society (AOCS) official method Ca-5a-40. Briefly, the acidity of 0.5 g

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duplicated samples diluted in ethanol was measured by titration with a standardized 0.05 mol∙L-1 sodium hydroxide solution.  Initial Acidity - Final Acidity  Conversion(%)    100 Initial Acidity  

(1)

The Statistica 7.0 software (Statsoft Inc., Tulsa, OK, USA) was used for the experimental design and data analysis, and all analyses were performed considering a

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confidence level of 95% (p ≤ 0.05).

3.1 Effect of supercritical CO2 as the solvent

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3. Results and Discussion

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In order to evaluate the effect of supercritical CO2 (scCO2) over the catalytic

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esterification of COA with ethanol, reactions were carried out varying the volume of CO2 added to the reactor, as already mentioned before. These volumes were from 0 to 40 mL, keeping the syringe pump at 10 oC and 150 bar (CO2 density 954.18 kg m-3 [24]). All

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these experiments were performed in duplicate at the same temperature (110 ºC), ethanol to COA molar ratio (3:1), and catalyst to COA concentration (5 wt%). The results are

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summarized in Table 2. At this point, it is worth to mention that a larger volume of the

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reagent mixture was used in reactions carried out without CO2 due to the mechanical configuration of the reactor, otherwise nor the stirrer nor the thermocouple would have been in contact with the reaction mixture.

Table 2 reveals that the CO2 addition led to an increase in the COA conversion from around 12% without CO2 to more than 70% with 6.6 wt% of CO2 to reagents mass

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ratio. These results demonstrate the positive influence that CO2 has on the catalytic efficiency of Amberlyst-15 for fatty acid esterification and confirm that the use of compressed fluids at supercritical conditions has a leveling effect on resin catalysts [25]. However, adding 40 mL of CO2 at the applied experimental conditions decreased the COA conversion, probably due to the excessive dilution of reactants and catalyst in scCO2. Such behavior is also certainly related to the phase behavior inside the reactor vessel [21]. Thus, a robust thermodynamic analysis, which will be the subject of further

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studies in our group, is essential for a better understanding and optimization of the proposed conversion process.

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3.2 Design of experiments

A 23 factorial design was performed to assess the effect of some process variables

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on the esterification of COA with ethanol using Amberlyst-15 and scCO2 as solvent.

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Table 3 presents the experimental conditions and the results for experiments that were carried out with the addition of 28.62 g of CO2 and 15 min of reaction time.

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Table 3 shows that the temperature and the catalyst loading had a positive effect on COA conversion [19,20]. The use of high ethanol to COA molar ratios, however,

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prejudiced the reaction in most cases, except when the highest temperature (110 ºC) and

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catalyst amount (20 wt%) were used. Shahid et al. [19] observed that up to a certain point the reaction rate is higher at lower ethanol to oleic acid ratios, while above this point, a higher molar ratio leads to higher reaction rates. This phenomenon, illustrated by an inversion on their kinetic curves, might have been the same observed in our experiments, and the reactions at lower temperatures and catalyst concentrations were not advanced enough after 15 min to be favored by high ethanol to COA molar ratios.

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This contradictory behavior can be explained by one or a combination of the following factors: higher ethanol loadings allowed higher swelling and availability of the active acid sites of the resin; residual water in ethanol may inhibit the catalyst activity more significantly at higher molar ratios; the catalyst dilution effect at higher molar ratios may lead to slower reaction rates; water formation (esterification byproduct) inside the catalyst may inhibit the catalyst activity and its exchange with ethanol may be favored when higher ethanol concentration is found in the reaction media.

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The replicate of the factorial design illustrates the experimental reproducibility, with a maximum standard deviation of 3.98 %, as presented in Table 3, and a small value of pure error acquired from the analysis of variance (ANOVA) summarized in Table 4.

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The ANOVA of the experimental data resulted in a high regression coefficient (99.51%)

with a linear model considering three-way interactions, which suggests good agreement

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between the data obtained experimentally and predicted by the model. The calculated lack

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of fit (F value) was lower than the tabulated F value for a confidence interval of 95% (p < 0.05), indicating that the proposed model does not suffer from lack of fit, and does not need to be adjusted to a quadratic model.

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Figure 1 depicts the parameters that most influence the esterification reaction of COA according to the adjusted model. In descending order of significance, temperature,

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catalyst amount, and the three-way interaction affected positively, molar ratio affected

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negatively, and associations with catalyst amount were marginally positive. Temperature to catalyst amount interaction had an insignificant effect on the response function. Even though temperature arguably plays a vital role in the studied reaction system,

it is worth pointing out that its effect is overestimated. Due to differences in the reactor heating ramps, as can be seen in the temperature and pressure profiles given in the

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Supplementary Material, experiments at higher temperatures had more reaction time compared to reactions that were carried out at lower temperatures.

3.3 Kinetic experiments Conditions of the factorial design at 110 ºC provided higher COA conversions and were elected to perform an exploratory kinetic study. The experimental kinetic curves are presented in Figure 2. Due to the mechanical configuration of the reactor, the reagents

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were loaded at room temperature, however, the reaction proceeded from the moment that the heating system was turned on until the moment that the setpoint temperature was

reached. Then, for the kinetic curves presented in this work, the zero time corresponds to

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the moment in which the reactor started to heat. Temperature and pressure profiles of

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each reaction run are given in the Supplementary Material.

Figure 2 shows that the catalyst amount plays an important role in the reaction

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rate, particularly at lower reaction times in the presence of large ethanol to COA molar ratios. Conversions around 90% were achieved after 30 min with an ethanol to COA

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molar ratio of 9:1 and 20 wt% catalyst to COA mass ratio. Although the catalyzed fatty

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acid esterification with methanol is known for its faster reaction rates [26], Park et al. [27] reached 90% conversion of oleic acid to methyl oleate only after 180 min at 80 ºC, 6:1 molar ratio and 20 wt% of Amberlyst-15. Santos et al. [28] carried out the auto-catalyzed esterification of oleic acid with ethanol using a 6:1 molar ratio in a continuous tubular reactor and also achieved conversions around 90% with short residence times of around 30 min. However, those authors used a much more severe temperature condition (280 ºC). Shahid et al. [19] reported an oleic acid conversion of 42% at 75 ºC using 4 wt% of 11

Amberlyst-15 and 5:1 of ethanol to oleic acid molar ratio. Compared to the kinetic curves presented in Figure 2, similar conversions were obtained for all conditions tested in this work in less than 10 min from the moment that the heating system was turned on. Figures 2(A) and (B) also illustrate that, as discussed in section 3.2, the initial reaction rates are higher at lower ethanol to COA molar ratios, and there is an inversion on this trend as the reaction proceeds. This result is particularly interesting from a process optimization perspective. It is likely that with less ethanol spent the same COA

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conversion could be reached in shorter residence times using a fed-batch system scheme, or in a continuous tubular reactor with side-feed of ethanol. Thus, a detailed kinetic evaluation and modeling of the process, which we intend to do as future work, is

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important to make the best use of this promising chemical pathway.

3.4 Catalyst reuse

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Catalyst reuse is an essential parameter for heterogeneous catalysis, especially for continuous system applications. A series of esterification reactions were performed

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considering the best condition of the experimental design (110 °C, 20 wt% of catalyst, 9:1 ethanol to COA molar ratio, 28.62 g of CO2, and 15 min of reaction). After each cycle,

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the supernatant was collected from the reactor vessel while the catalyst (without any

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further treatment, not even washing) remained inside for the next reaction cycle. Thus, since reused catalyst had impurities from the previous reactions, it was possible to evaluate the actual deactivation of the Amberlyst-15 catalyst. Figure 3 shows the conversion behavior during 6 reaction cycles. In the first three cycles, the conversion decreased from 86.34% to 73.68%, but in the other cycles, the catalyst was stable around the 70% conversion range. After 6 cycles, the conversion obtained with 20% of catalyst (73%) was still considerably higher than the conversion 12

obtained with 5% of the fresh catalyst at the same experimental conditions (56%). For comparative proposes, a run without Amberlyst-15 was performed at the same conditions, and the COA conversion found was insignificant (0.03%). Boz et al. [13] investigated the transesterification of waste cooking oil and after three reaction cycles (methanol to oil molar ratio of 9:1, 3 wt% Amberlyst-15 and 9 h of reaction time) a drop of only one point percent was found in reaction conversion. However, the catalyst had to be washed with different solvents and dried at 100 ºC prior

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to reuse. Hykkerud and Marchetti [20], who applied Amberlyst-15 for the esterification of oleic acid with ethanol, reported activity losses after a single use when the spent catalyst was washed only with ethanol and left it to dry at room temperature during 24 h.

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Unfortunately, these authors did not inform the impact of such reduced activity in the

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oleic acid conversion quantitatively.

3.5 Use of different reactants

In order to investigate the effect of using different reagents for the proposed

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process, both alcohol (methanol and ethanol) and carboxylic acid (levulinic acid, lauric acid, stearic acid, oleic acid, and COA) were varied. The reactions were carried out at the

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best condition of the experimental design (110 °C, 9:1 molar ratio, and 20 wt% for 15

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min) with 5 mL of reagent mixture with 28.62 g of CO2, or 35 mL of reagent mixture for solvent-free reactions. Figures 4 and 5 show that, regardless of the reagents, the use of CO2 as solvent

favored reaction performance. For the solvent-free system, methanol reached higher conversions in esterification compared to ethanol as already observed by others [26].

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However, by adding scCO2, the conversion with both reacting alcohols was similar (Figure 4), probably due to the proximity of the equilibrium conversion.

Figure 5 indicates that the use of scCO2 as solvent boosted the acid conversion for all evaluated reaction systems. The highest conversions in solvent-free and scCO2 conditions were 48.6% and 88.5%, respectively for levulinic acid. These values are higher

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than those obtained with fatty acids, probably due to the higher affinity between levulinic acid, CO2, and ethanol. By comparing the acquired conversions for lauric acid (C12:0)

and stearic acid (C18:0), the effect of polarity and chain length is pronounced. Due to its

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shorter molar mass, lauric acid (C12:0) seem to have an easier access to the resin active

sites compared to stearic acid (C18:0) in a solvent-free reaction condition, and such trend

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remains the same when scCO2 is added to the reactor vessel, although both conversions

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increased considerably.

The analysis of Figure 5 also reveals an interesting trend about the unsaturation effect. In both solvent-free and scCO2 systems, stearic acid (C18:0) presented lower

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conversions than oleic acid (C18:1), which in turn presented lower conversions than COA (60% for C18:1 and 23% for C18:2). It seems that, even in solvent-free condition and

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mainly in scCO2 media, the more unsaturated the fatty acid is, the easier is its access to

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the resin active sites, leading to higher conversions at the same reaction conditions. Future work concerning on a deeper investigation on the effect of chain length, unsaturation bonds and substituents would be a great contribution to better understand the observed trends.

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4. Conclusions In this work, the use of scCO2 as solvent for fatty acid esterification catalyzed by Amberlyst-15 was investigated. scCO2 helped mass transfer and increased COA conversion from 12.40% (without CO2) to 73.71% (with the addition of 28.62 g of CO2) after 15 min of reaction time. A design of experiments was performed, keeping fixed the reaction time at 15 min and the added mass of CO2 at 28.62 g and varying temperatures,

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the catalyst to COA concentrations, and ethanol to COA molar ratios. The highest COA conversion was 86.97% at 110 °C, 20 wt%, and 9:1 molar ratio. The ANOVA revealed

that temperature and catalyst concentration were the parameters that most influenced the

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reaction yield. Reaction kinetics showed that the reaction rate is strongly affected by the

amount of catalyst and that low molar ratios favor the initial reaction rate, while this factor

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was not pronounced at longer reaction times. The reusability of Amberlys-15 was

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verified, and a ten points percent drop was observed in the first three reuse cycles, remaining stable after this fall for three more reaction cycles. Finally, the use of different

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reagents suggested that the proposed process is suitable for a wide range of applications.

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Acknowledgments

The authors want to thank the Brazilian funding agencies (CNPq – grant numbers

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305393/2016-2, 309506/2017-4 and 435873/2018-0, Fundação Araucária – grant agreement 004/2019) for financial support and scholarships. This work was also financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001.

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[16] Y. Zhang, W.-T. Wong, K.-F. Yung, One-step production of biodiesel from rice bran oil catalyzed by chlorosulfonic acid modified zirconia via simultaneous esterification and transesterification, Bioresour. Technol. 147 (2013) 59–64. [17] R. Malhotra, A. Ali, 5-Na/ZnO doped mesoporous silica as reusable solid catalyst for biodiesel production via transesterification of virgin cottonseed oil, Renew. Energy. 133 (2019) 606–619. doi:10.1016/j.renene.2018.10.055.

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[18] L. Zatta, E.J. Paiva, M.L. Corazza, F. Wypych, L.P. Ramos, The Use of AcidActivated Montmorillonite as a Solid Catalyst for the Production of Fatty Acid Methyl Esters, Energy & Fuels. 28 (2014) 5834–5840.

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[19] A. Shahid, Y. Jamal, S. Jamal Khan, J. Ali Khan, B. Boulanger, Esterification

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Amberlyst 15, Arab J Sci Eng. 43 (2018) 5701–5709. doi:10.1007/s13369-017-

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[20] A. Hykkerud, J.M. Marchetti, Esterification of oleic acid with ethanol in the

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[21] L. Soh, C.-C. Chen, T.A. Kwan, J.B. Zimmerman, Role of CO2 in Mass Transfer, Reaction Kinetics, and Interphase Partitioning for the Transesterification of

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Triolein in an Expanded Methanol System with Heterogeneous Acid Catalyst, ACS

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[22] R.A. Bourne, J.G. Stevens, J. Ke, M. Poliakoff, Maximising opportunities in supercritical chemistry: the continuous conversion of levulinic acid to γvalerolactone in CO2, R. Soc. Chem. (2007) 4632–4634. doi:10.1039/b708754c. 18

[23] Z. Ziyang, K. Hidajat, A.K. Ray, Determination of adsorption and kinetic parameters for methyl tert-butyl ether synthesis from tert-butyl alcohol and methanol, J. Catal. 200 (2001) 209–221. doi:10.1006/jcat.2001.3180. [24] R. Span, W. Wagner, A new equation of state for carbon dioxide covering the fluid region from the triple-point temperature to 1100 K at pressures up to 800 MPa, J. Phys. Chem. Ref. Data. 25 (1996) 1509–1596. doi:10.1063/1.555991.

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[27] J.-Y. Park, D.-K. Kim, J.-S. Lee, Esterification of free fatty acids using watertolerable Amberlyst as a heterogeneous catalyst, Bioresour. Technol. 101 (2010)

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19

51.5942

(3) Temperature (ºC)

13.4779

(1) Catalyst (wt%)

8.2248

1*2*3

-5.7908

(2) Molar ratio

3.6912

1by2

2.8811

2by3

0.5039

1by3

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p=.05

Figure 1. Pareto chart obtained from the design of experiments presented in Table 3. 100

100

(B)

40

5% 20%

20 0

60 40

re

60

80

-p

80

Conversion (%)

Conversion (%)

(A)

5% 20%

20

0

30

60

90

120

150

0

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0

Time (min)

30

60

90

120

150

Time (min)

na

Figure 2. Experimental kinetic curves obtained for ethanolic esterification of COA catalyzed by Amberlyst-15 at 110 ºC, with 28.62 g of CO2 and ethanol to COA molar

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ratio of (A) 3:1 and (B) 9:1, comparing different catalyst amounts (5 and 20 wt%).

20

100 90 80

Conversion (%)

70 60 50 40 30 20 10 1

2

3

4

5

6

Cycle number

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0

Figure 3. Reusability of Amberlyst 15 in the esterification of COA with ethanol using

9:1. The error bars represent standard deviations.

90

re

100 Methanol Ethanol

lP

80

60 50

na

Conversion (%)

70

40

Jo

10

ur

30 20

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28.62 g of CO2 at 110 ºC, 20 wt% catalyst to COA and ethanol to COA molar ratio of

Solvent free

scCO2

Figure 4. Effect of the alcohol during the COA esterification at 110 ºC for 15 min using a molar ratio of 9:1 and 20 wt% Amberlyst-15 as catalyst. The error bars represent standard deviations of duplicate runs.

21

100 90 80

Levulinic acid Lauric acid Stearic acid Oleic acid COA

Conversion (%)

70 60 50 40 30 20 10 0 Solvent free

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scCO2

Figure 5. Effect of the fatty acid type during esterification with ethanol at 110 ºC for 15

min using a molar ratio of 9:1 and 20 wt% Amberlyst-15 as catalyst. The error bars

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ur

na

lP

re

-p

represent standard deviations of duplicate runs.

22

Table 1: Typical properties of Amberlyst-15 ion exchange resin [23]. Appearance

Surface area 50 m²∙g-1

Average pore diameter 240 Å

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Hard, spherical particles

Bulk Hydrogen density concentration 608 g/L 4.7 meq∙g-1

23

Table 2. Results acquired after 15 min of reaction by applying different volumes of CO2 at 110 ºC, ethanol to COA molar ratio of 3:1 and 5 wt% of catalyst to acid concentration. CO2 added (mL at 10 ºC, 150 bar)* 0

CO2 to reagents mass ratio -

Ethanol to Catalyst to COA COA molar (wt%) ratio 2.99  0.01 5.02  0.01

5

1.09  0.01

3.06  0.03

5.10  0.02

67.99  0.90

10

2.20  0.02

3.03  0.11

5.12  0.01

64.16  0.45

20

4.43  0.08

3.03  0.06

5.14  0.08

69.63  0.94

30

6.59  0.02

3.04  0.01

4.91  0.01

73.71  0.25

40

8.91  0.03

3.02  0.01

5.15  0.05

42.08  0.14

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12.40  0.44

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na

lP

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* CO2 density at this condition, 954.18 kg∙m-3 [24].

Conversion (%)

24

Table 3. Experimental conditions used in the 23 factorial design for the esterification of COA with ethanol using Amberlyst-15 as catalyst and scCO2 as solvent. Catalyst to COA (wt%)

Temperature (ºC)

Conversion (%)

3.04  0.03

70

6.24  3.24

4.91  0.00

3.04  0.00

110

5.28  0.00

8.96  0.00

70

5.08  0.03

8.98  0.05

110

12.32  0.15

5.94  0.08

90

5.34  0.06

12.30  0.15

Ethanol to COA molar ratio

5.90  0.14

12.50  0.04

6.03  0.01

20.19  0.02

3.02  0.02

19.93  0.08

90 90 70

110

-p

3.04  0.01

19.80  0.43

8.93  0.20

70

8.84  0.13

110

Jo

ur

na

lP

re

19.64  0.08

25

73.71  0.25

1.30  3.11 55.99  3.11 43.60  3.68 42.83  0.52

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Factorial design

44.69  3.98 27.19  2.80 76.19  2.55 11.40  1.46 86.97  1.91

Table 4. ANOVA table obtained from the factorial design reported in Table 3. SS

df

MS

F

p-value

(1) Catalyst

1038.61

1

1038.61

181.654

0.000000

(2) Molar ratio

191.73

1

191.73

33.533

0.000063

(3) Temperature

15219.85 1

15219.85 2661.960 0.000000

1 by 2

77.90

1

77.90

13.625

0.002715

1 by 3

1.45

1

1.45

0.254

0.622774

2 by 3

47.46

1

47.46

8.300

0.012867

1*2*3

386.77

1

386.77

67.647

0.000002

Lack of Fit

9.42

1

9.42

Pure Error

74.33

13 5.72

Total SS

17047.48 21

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Factor

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lP

re

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1.647

26

0.221806