Sodium Silicate Catalyst for Synthesis Monoacylglycerol and Diacylglycerol-Rich Structured Lipids: Product Characteristic and Glycerolysis–Interesterification Kinetics

Sodium silicate as heterogeneous base catalysts is more environmentally friendly and easily separated by filtration. The objective of this research was to evaluate the activated sodium silicate as catalyst for synthesis of monoacylglycerol (MAG) and diacylglycerol (DAG)-rich structured lipids (SLs) from a palm olein-stearin blend. Sodium silicate was activated and functional group was characterized. Reaction was performed using 5% catalyst (w/w) at various reaction temperature (70–120 °C) for 3 h in a batch stirred tank reactor. Physical properties of SLs, such as melting point, slip melting point, and hardness of SLs were determined. Reaction kinetics were also evaluated. The results show that Si−O bending was reduced and shifted to a Si−O−Na and Si−O−Si functional groups after sodium silicate activation. Temperature had a significant effect on SLs composition at higher than 90 °C. An increase in temperature produced more MAG, resulting in better product physical properties. The best reaction condition was at 110 °C. Rate constants and the Arrhenius equation were also obtained for each reaction step. In summary, the activated sodium silicate catalyzed glycerolysis-interesterification reaction, which produced MAG and DAG at temperature higher than 90 °C. Therefore, the physical properties of SLs were improved. Copyright © 2022 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).


Introduction
Structured lipids (SLs) are fats that have been chemically or enzymatically modified from their natural form to meet food and nutritional needs. The scope of SLs includes the manufacture of triacylglycerol (TAG), monoacylglycerol through the glycerolysis-interesterification reaction.
The purpose of this research was to evaluate the reaction kinetics and acyl glycerol profile of the glycerolysis-interesterification process using activated sodium silicate. In this research, MAG and DAG-rich structured lipids were synthesized from a palm stearin-olein blend using activated sodium silicate as the heterogeneous base catalyst. The reactions were modeled, and their rate constants were determined based on the reaction model. The effect of temperature on rate constants was predicted using the Arrhenius equation. Furthermore, the physical properties, such as melting profile and hardness of the product, and the interaction between the sodium silicate catalyst and SLs products were also evaluated.

Sodium Silicate Catalyst Activation
The sodium silicate was activated according to Perdana et al. [20]. Sodium hydroxide (NaOH) was weighed as much as 10 g, then dissolved in 10 mL of aquadest and stirred until homogeneous. The silica gel (7.5 g) was added gradually into the NaOH solution while stirring until a gel was formed. The gel was added to the porcelain crucible and heated in the furnace at 400 °C for 3 h. The activated gel was crushed with mortar. The result was sodium silicate powder, which could be directly used as a catalyst in the glycerolysis-interesterification reaction.

Glycerolysis-Interesterification Reaction at Various Temperature
The glycerolysis-interesterification reaction process was modified from Subroto et al. [5]. Palm stearin and palm olein ratio of 1:4 (w/w) were blended, then called oil. The oil was reacted with glycerol with a molar ratio of oil : glycerol = 1:2. Sodium silicate catalyst was added as much as 5% (w/w oil) and molecular sieve was added as much as 12% (w/w total reactant). The reaction was performed in a batch reactor at various temperatures (70, 80, 90,  100, 110, and 120 °C) and a stirring speed of  200 rpm for 3 h. Samples were taken at 0, 5,  10, 15, 30, 45, 60, 75, 90, 120, 150, and 180 min. The sample was then analyzed to determine TAG, MAG, and DAG concentrations. The properties of SLs products were also analyzed.

Characterization of Sodium Silicate Catalyst
Molecular structure and functional groups of sodium silicate catalyst were analyzed using Fourier Transfer Infrared (FTIR) Spectroscopy type 8201PC, Shimadzu Co., Japan, with a scanning range of 280-4000 cm −1 .
The total basicity of sodium silicate catalyst was analyzed using the Tanabe and Yamaguchi method [21,22]. About 20 mL of Benzene and 1 mL Bromthymol Blue indicator solution were added to a 100 mL Erlenmeyer flask. About 100 mg of 100-200 mesh sodium silicate catalyst was added to the solution. Sample was then titrated with 0.1 N Benzoic acid solutions until the green color disappeared.

Analysis of Acyl Glycerol Concentrations
The analysis of acyl glycerol was performed according to Subroto et al. [5]. The MAG, DAG, and TAG concentrations in each sample were analyzed using the Thin Layer Chromatography (TLC) scanner type Camag Automatic TLC Scanner III Dummy S/N (1.14.16) with Camag WinCATS software planar chromatography at wavelength 629 nm. Hexane, diethyl ether, and acetic acid (80:20:2, v/v/v) mixed solvent system was used as the mobile phase.

Reaction Kinetics Model
The glycerolysis-interesterification reaction was modeled as a combination of glycerolysis and interesterification reactions ( Figure 1). In this reaction, TAG reacts with glycerol to produce MAG and DAG, namely the glycerolysis process. It can also react with another type of TAG with a different fatty acid profile, namely interesterification. This reaction can increase the possibility of the formation of desired fat because the obtained results are in the form of a mixture of MAG, DAG, and TAG SLs, which all play a role in shaping the physical, chemical, and functional properties of the targeted fat [5].
In this model, 4 possible reactions occur at the same time (Figure 1), in which reaction (1) to (3) represent the glycerolysis process and reaction (4) represents the interesterification. In the glycerolysis process, equation (1) is the initial reaction, then continued with Equation (2) which DAG byproduct reacts with the excess glycerol. If the MAG production is excess, the reaction will continue to Equation (3) which MAG reacts with the raw material to produce DAG. In the interesterification, the acyl migration only occurs between TAG.
The following assumptions were made in order to generate the reaction kinetics model: (a) Equation (1) expressed as the initial reaction can be considered as an irreversible reaction because of the excess of glycerol and low product concentration. Thus, the reversible reaction (represented as k2) was neglected; (b) Since chemical catalyst leads to random glycerolysisinteresterification, the acyl migration on the glycerol backbone was challenging to identify because it varies widely over time. Thus, the acyl migration during the reaction was not analyzed, and the reaction kinetics determination was based on glycerolysis only; (c) A molecular sieve was used in this reaction. Thus, the forming of free fatty acid (FFA) during the reaction was neglected; (d) Catalyst was homogeneously dispersed in the system due to the mixing of reaction mixture at 200 rpm. Thus, mass transfer rate from the reactant to the catalyst can be neglected.
The following differential rate equations can then be used to describe the rate of change in concentration for each of the reaction components.
where, C is the concentration for each substance. Gly refers to glycerol. A computational code for the parameter estimation technique was written and implemented. The differential equations were solved with the ode15s subroutine, and the objective function was minimized with the lsqnonlin nonlinear data-fitting optimization subroutine. The relationship between temperature and reaction rate constant was shown by the Arrhenius Equation (9). (9) where, k is the reaction rate constant, A is the exponential factor, Ea is the activation energy, R is the ideal gas constant, and T is the reaction temperature. From the Arrhenius Equation (9) it can be linearized as in Equation (10), so that the value of the activation energy (Ea) and the exponential factor (A) can be obtained.
The obtained Arrhenius parameters were used to predict the MAG, DAG and TAG model at various temperature.

Analysis of Product Melting Point and Slip Melting Point
The slip melting point (SMP) analysis followed the AOCS Official Method Cc 3-25, and MP analysis followed the AOCS Official Method Cc 1-25 44, 45. Melted samples were inserted into a hematocrit tube about 1 cm height and then refrigerated overnight. Refrigerated samples were then heated gradually with the temperature increasing rate of about 0.5 °C/min until the samples flowed through the tube and changed into a clear liquid [23].

Analysis of Product Hardness
The product hardness was analyzed using the TA.XT Plus (Stable Micro Systems) type texture analyzer, with diameter probe 12.7 mm. Samples were prepared by melting the products at 80 °C for 15 min, then placed in a sample cup for about 1.5 cm in depth and con-tinued by placing the samples at room temperature until hardened and then keeping the samples in a refrigerator (around 10 °C) for approximately 18 h. Before measurement, samples were taken out from the refrigerator and thawed for about 1 h. Measurements were performed at room temperature [5].

Statistical Analysis
A one-way analysis of variance (ANOVA) was used to analyze the data. Tukey's test was used to identify the difference between samples. P-value of <0.05 were considered significant.

Characteristics of Sodium Silicate Catalyst
The silica powder and the activated sodium silicate catalyst were analyzed using Fourier Transform Infra-Red (FTIR) spectroscopy to determine the functional structure applied on the catalyst surface. The result is shown in Figure 2. The dominant functional group in silica powder was silicate ion, represented as a downward peak at a wavelength value ~1100 cm −1 (Figure 2(A)). An absorption band of Si−O bending also occurred at ~470 cm −1 [19,24]. After the silica powder was reacted and activated into an activated sodium silicate catalyst, the intensity of the silicate ion and Si−O bending was reduced, shifted into a Si−O−Na and Si−O−Si functional groups, shown by a sharp downward peak at ~1000 cm −1 and ~890 cm −1 , respectively (Figure 2(B)).
These results prove that the activation process improved the structure of sodium silicate. The Na + ions were organized in various patterns around the non-bridging oxygen after activation [25]. Consequently, the ion exchange process becomes easier and can lead to a higher catalytic activity. Besides, the formation of the Si−O−Si group during the activation process shows that the SiO4 4-tetrahedral shape changed from low polymerization conditions to high polymerization conditions. Changing the sodium silicate catalyst group to a more stable condition can provide a higher reaction conversion [19,25]. These transformations prove that the sodium silicate catalyst production and activation process was successful and can be used as a catalyst.
On the other hand, the total basicity of the activated sodium silicate catalyst was 9.19±0.08 mmol/g. This result was lower than the total basicity value reported by Guo et al. [19] at the same operating conditions. It is sug- gested that the difference in basicity value is due to the difference in the source of silica gel. Although it has a slight difference, the basicity value of the activated sodium silicate catalyst was still higher than other heterogeneous base catalysts [21,22,26]. A higher basicity value may result in a higher conversion rate [19].

Conversion of Acyl Glycerol at Various Reaction Temperature
Glycerolysis-Interesterification was conducted in a batch stirred tank reactor at various temperatures using the activated sodium silicate catalyst. The conversions of TAG, MAG, and DAG at each variation are shown in  Figure 3. Most of them implied that the TAG concentration decreased while the concentrations of MAG and DAG increased with the increasing time. This result proves that the glycerolysis-interesterification reaction using the activated sodium silicate catalyst produce a combination of MAG, DAG, and TAG in the products.
MAG and DAG concentrations were lower than TAG concentration. It shows that not all TAGs were converted to MAG and DAG. Also, that TAG was still the dominant component in the product. At 90 °C and above, both the decrease in TAG and the increase in MAG and FFA were optimum. The high temperature helps catalysts extract hydrogen from glycerol (E) (F) [28]. At 110 °C and 120 °C, the TAG, MAG, and DAG contents were at maximum value, which converted to 65%, 10%, and 25%, respectively. Temperature 110 °C was chosen to be the bestoperating condition because it requires lower energy, although it has a similar result with 120°C. It is suggested that the catalytic mechanism for glycerolysis-interesterification using the activated sodium silicate follows several steps [25,27]. The first step was the triglyceride enolate anion formation through alpha hydrogen abstraction by sodium silicate catalyst. The triglyceride enolate anion was then reacted with glycerol, leading to an ester interchange to form MAG and diglyceride anion. Finally, the proton was then transferred to diglyceride anion to form DAG in 2 possible ways: one is from the catalyst, and the other is from glycerol to produce more MAG. In addition, the triglyceride enolate anion was also able to react with MAG to produce DAG. This phenomenon may explain why the DAG value has a relatively stable value through time because it was always consumed and produced simultaneously during the reaction. The triglyceride enolate anion was also able to react with TAG as well to perform an interesterification mechanism. A beta-keto ester was formed during the process, which led to a TAG formation with a different fatty acid profile.
However, lower reaction temperature shows the opposite results. At the reaction temperature below 90 °C, the maximum reduction of TAG was around 80%, and the maximum rise of MAG was only about 3%. These results showed that low temperatures (especially 70°C and below) were insufficient to provide the appropriate MAG and DAG in products. Similar results were also reported by Zhong et al. [16] that 80°C was required and suitable to produce DAG by glycerolysis using KOH-MgO supported catalyst. Zhong et al. [16] also reported that CaO, MgO, and calcium hydroxide (Ca(OH)2) catalysts were not effective for glycerolysis at a temperature lower than 100 °C.

Composition of TAG, MAG, and DAG in Structured Lipid Products
The composition of TAG, MAG, and DAG in the end products at each reaction temperature is presented in Table 1. It could be seen at high temperature, the TAG value were significantly different than the value at low temperature (Table 1). Lower TAG value shows that some of TAG were converted to MAG or DAG as products. These results proved that a higher reaction temperature could convert more TAG into products than a lower reaction temperature.
The MAG value differed significantly in every reaction temperature. The MAG content at 120 and 110 °C had the highest value, while 80 °C had the lowest value. These results also proved that a higher reaction temperature could produce more MAG than a lower reaction temperature. No MAG content was produced at 70 °C. It proved that this temperature was insufficient to initiate the glycerolysisinteresterification reaction. The reaction using low temperature had been previously reported by Subroto et al. [5]. The TAG content could be decreased up to 60%, MAG and DAG content increased up to 12% and 28%, respectively, using immobilized C. antarctica lipase as the catalyst at 50 °C for 24 h. Although it is possible to produce the SLs using low temperatures, a longer reaction time was needed. Thus, a higher temperature is still preferable in MAG and DAG-rich SLs production.
The DAG value for each reaction was not significantly different ( Table 1). The value of DAG at the end of the reaction was also not much different from the initial value ( Figure  3). This shows that there is only a small increase in the DAG value at the end product. In glycerolysis reaction, DAG is an intermediate product that will react with glycerol to produce MAG ( Figure 1). However, the accumulation of MAG would react with the excess TAG to produce DAG, as the result, there is only a slight increase in the amount of DAG at the end of the reaction.

Physical Properties of Monoacylglycerol and Diacylglycerol-Rich Structured Lipids
The tested physical properties of the structured lipid products were SMP, MP, and product hardness. Results are shown in Table 2. Samples were compared with the previous data from Subroto et al. [5]. Compared to Palm Stearin-Palm Olein (PS-PO) blend, samples produced by glycerolysis-interesterification have significantly higher SMP and MP values ( Table  2). SLs products had 1.34-1.46 times higher and 1.36-1.52 times higher SMP and MP than the PS-PO blend, respectively.
It can also be seen that samples produced in higher reaction temperatures have significantly different SMP and MP values. The previous discussion from Figure 2 implied that higher reaction temperature leads to more MAG and DAG production. MAG and DAG have a higher MP than TAG, ranging from 55.5 °C to 77.0 °C depending on the type of fatty acid. The presence of MAG and DAG will contribute to the increase and variety of the melting profile of the structured lipid products, while the PS-PO blend did not contain any MAG [4,5].
Product hardness value can also be seen in Table 2. PS-PO blend showed a drastically lower hardness value than the product with glycerolysis-interesterification at any reaction temperature, ranging from 4.65 to 15.90-fold increase. Some samples from higher reaction temperatures (with significantly higher MAG and DAG content) also had a relatively higher hardness value. This issue happens because MAG and DAG are solid at room temperature, so the presence of MAG and DAG will increase the hardness of the structured lipid products [4].
At temperature 120 °C, MP and hardness value were significantly lower than the value at 110°C. These results could happen because random interesterification between TAG from PS and TAG from PO also occurred in this system. Depending on the iodine value, PS contains TAG with the structure of palmitateoleate-palmitate (p-o-p), palmitate-oleateoleate (p-o-o) and palmitate-palmitatepalmitate (p-p-p) fatty acids around 27.5-46.5%, 12.9-31.5% and 14.3-37.4%, respectively. Whereas the TAG structure of PO consists of p-o-p, p-o-o and p-p-p around 28.7-40.9%, 24.5-43.7% and 2.4-3.9%, accordingly [29][30][31]. As mentioned earlier, the fatty acid type could affect the melting properties of the TAG, MAG, or DAG [4]. The random interesterification at 120 °C may produce a certain fatty acid profile that has lower MP properties. Thus, the melting profile and hardness can fluctuate for each reaction temperature.

Reaction Rate Constant and Arrhenius Parameters
At a reaction temperature of 70 °C (  ating conditions. We conclude that this temperature was not enough to initiate the glycerolysis-interesterification process. Thus, the calculation of rate constants was only based on the higher temperature data (80-120 °C) using Equations (5) to (8). As we assumed that the reversible reaction in equation (1) is neglected, the calculation of k2 was also dismissed ( Table  3). The rate constants, k1, k3, and k5 showed the product formation in Equations (1), (2), and (3). While rate constants k4 and k6 showed the reverse reaction (reactant formation) for Equations (2) and (3), respectively. From data obtained in Table 3, it could be seen that equation (2) was mainly irreversible, while Equation (3) was reversible only at a lower temperature. These results probably correlated to the usage of higher temperatures for the experimental procedure.
The high temperature helps to improve the mass transfer between oil and glycerol. Since oil and glycerol are viscous fluids at room temperature and they are immiscible, increasing the reaction temperature can drop the fluid's viscosity and improve the reaction between oil and glycerol [32]. Moreover, high temperature increases the kinetic energy between molecules, thus the intermolecular movement will increase and lead to a faster diffusion [33].
Glycerolysis also follows endothermic reaction and usually occurs at high temperatures [34][35][36]. According to Le Chatelier's principle, temperature affects the reaction equilibrium, which the higher the temperature, the more the reaction leads to the formation of products [37]. Thus, the reaction towards reactant formation will be minimal, and at some point, may lead to an irreversible reaction.
Since equation (1) was the initial reaction, it was assumed that this reaction was irreversible. Thus the calculation was only based on the product formation. The reaction constants for the reversible reaction in equations (2) and (3) were calculated. For those reactions, the reaction to generate products was more remarkable than product degradation to form the reactants. It is supported by the values of k3 and k5, which were greater than that of k4 and k6. Thus, all reactions contribute to the MAG and DAG product formation. The most influential reaction on the product formation was the Equation (3). It is proved by the highest reaction rate constant value (k5) and the higher yield of DAG product than MAG at any reaction temperature. This issue is most likely happened due to the excess of TAG in the reactants and the accumulation of MAG over time, which shifts the reaction's equilibrium toward DAG formation, as reported by Moquin et al. [38].
The Arrhenius equation demonstrates the correlation between temperature and the reaction rate constant, as stated in equation (9). Since the value of k4 (Table 3) was mostly zero for reaction at temperature 90 to 120 °C, it can be concluded that reaction (2) is irreversible. Thus the Arrhenius equation for k4 was neglected. Therefore, the calculation of the Arrhenius constant and activation energy value was only based on k1, k3, k5, and k6 (Table 3).
From Table 3, it could be seen that the value of activation energy for k3, k5, and k6 was negative, while k1 was the only one that had a positive value. Since the activation energy value is less likely to be less than zero, it could be assumed that an intermediate complex could be formed during these reactions. Mozurkewich and Benson [39] explained that a tight transition state with low potential energy must be achieved to obtain a negative activation energy (Ea<0). This condition could be accomplished if the reaction undergoes a stable intermediate. The activation energy (Ea) is formed by the energy of transition state (ETS) subtracted with the energy of reactant (ER). At lower temperatures, the average energy of the transition state was lower than the average energy of the reactant, resulting in a negative value of activation energy. Nevertheless, in glycerolysisinteresterification, the activation energy for k1 is the most important since it was the initial reaction. Figure 4 shows the experimental data of glycerolysis-interesterification compared to the kinetic model from the differential equation and Arrhenius parameters substitution. The difference between the experimental data and kinetic models for reaction temperature 120, 110, 100, 90, and 80 °C were 1.0010 −2 , 1.0010 −2 , 2.0010 −2 , 3.0010 −2 , and 3.0010 −2 , respectively. The difference of the kinetic model between the differential equation and Arrhenius parameters substitute were 4.0010 −8 , 9.0010 −8 , 4.0010 −8 , 6.0010 −7 , and 2.5610 −6 , respectively.

Reaction Kinetics Model Evaluation
Kinetic models usually have a slight difference from the experimental data. Lionelli [40] explained that data are results of interactions between researchers and the world which are then processed as usable evidence to claim about a certain phenomenon, while models are conceptualized as representations of data. Data and models correspond to each other as they are a representation of the real condition and proposed theory, respectively, with data being closer to real condition phenomena and models being closer to theory. Thus, connecting the theory and real conditions using the relation between experimental data and models would likely create a missing gap.

Conclusion
Activation of sodium silicate resulted in reduction of Si−O bending. It shifted to a Si−O−Na and Si−O−Si functional groups, which were shown by a sharp downward peak at ~1000 cm −1 and ~890 cm −1 , respectively. MAG content was not produced at 70 °C. However, the temperature has a significant effect on SLs at higher than 90 °C. The best condition for this reaction was at 110 °C. TAG, MAG, and DAG were up to 65%, 10%, and 25%, respectively at 110 °C and above. SLs products had SMP and MP 1.34-1.46 times and 1.36-1.52 times higher than the PS-PO blend, respectively. It showed a drastic increase in the hardness value than PS-PO blend ranging from 4.65 to 15.90-fold. Furthermore, MP and hardness value at 120 °C were significantly lower than the value at 110 °C. It is because of random interesterification between TAG of PS and TAG of PO in this system. Besides, kinetic models implied that the reaction could be irreversible at a higher temperature. Equation (3) is the dominant reaction in glycerolysisinteresterification, which leads to a higher content of DAG than MAG in the end product. In summary, glycerolysis-interesterification of PS-PO blend for producing SLs using the activated sodium silicate as catalyst improved MP, SMP, and hardness of SLs due to an increase in MAG and DAG content at a higher temperature.