Utilization of Glycerol by Ketalization Reactions with Acetone to Produce Solketal using Indion 225 Na as Catalyst

The increase of biodiesel production would cause an increase of glycerol as by-products. In the present study, utilization of glycerol by-products to form solketal using Indion 225 Na as a catalyst was investigated. The intrinsic kinetic data were taken in a batch reaction. The results showed that the temperatures and catalyst concentration have a significant influence on the conversion of glycerol. However, varying the acetone to glycerol ratio only had a marginal impact on glycerol conversion. A kinetic model based on Eley Rideal mechanism was developed to describe the reaction mechanism over the catalyst. The simulation results of glycerol conversion were compared to experimental data at temperatures from 308 K to 328 K. From the kinetic model, it was found that the pre-exponential factor of 2.59 min^-1, activation energy of 21.16 kJ/mol, acetone adsorption equilibrium of 0.62, and solketal desorption equilibrium of 0.03 were obtained. Comparison between the experimental and calculated glycerol concentrations showed that the model described the data well within the temperature range of 308 K to 328 K.

Acetone; Catalyst; Glycerol; Kinetic; Solketal

There has been growing interest in increasing the use of biodiesel as a renewable transportation fuel in Indonesia. According to the National Energy Plan (2017), Indonesia is expected to increase the portion of renewable energy from 23% of total energy consumption in 2025 to 31.2% in 2050. The increase use of renewable energy is an important step for Indonesia as it strives to decrease its greenhouse gas emissions. The increase of biodiesel production in the country will be accompanied by a corresponding increase of glycerol as a by-product of biodiesel production. Typically, glycerol yields as high as 10 wt% from transesterification reactions are obtained during biodiesel production. Therefore, there is an urgent need to utilize glycerol as a raw material to produce value-added chemicals along with the increase of biodiesel production.

        Many studies have reported the conversion of glycerol to produce various chemicals. Tan et al. (2013) and Kong et al. (2016) proposed glycerol conversion to 1,3-propanediol which plays an important role on production of polymers, cosmetic, food, lubricants and medicine. In addition, it can be converted to acrolein as an intermediate of acrylic acid production. Glycerol can also be converted to hydrogen and syngas as a platform for various chemicals production. A plasma electrolysis method to produce hydrogen was reported by Saksono et al. (2012). Regarding hydrogen production from glycerol, Slamet et al. (2015) used a TiO2 P25 photocatalyst modified with metals such as Platinum (Pt), Copper (Cu), and non–metal Nitrogen doping.

      In another study, Leong et al. (2016) reported the pyrolysis of crude glycerol using a microwave heating technique to produce pyrolyzed liquid, which can be used as fuels in combustion systems. Glycerol could also be converted to glycerol carbonate (Okoye and Hameed, 2016). Glycerol carbonate is known as a green organic solvent, which has a high boiling point. Ketalization of glycerol with acetone to produce an oxygenated compound such as solketal is an interesting question to consider, as solketal is known as a promising fuel additive (Reddy et al., 2011; Nanda et al., 2016).

Solid catalyst is generally preferred for solketal production as it improves the separation of catalyst. Specifically, zirconia and promoted zirconia catalysts have been used for the ketalization of glycerol (Reddy et al., 2011). Reddy et al. (2011) reported the promoted zirconia catalyst exhibited promising catalytic activity with the highest glycerol conversion of 98% using a sulphate zirconia catalyst at room temperature. da Silva et al. (2017) used SnF₂ as catalyst for the ketalization of glycerol with propanone to form solketal to obtain ca. 97% of glycerol conversion at ambient temperature. Nanda et al. (2014) reported on the use of ethanol as a solvent in glycerol acetylation using Amberlyst 35 as a catalyst, which produced a solketal yield as high as 74%. The mesoporous 5% Ni-1%Zr/AC catalyst has also been used for acetalization of glycerol without solvent, which yielded complete glycerol conversion (Khayoon and Hameed, 2013). Some catalysts, such as zeolite Beta, Amberlyst 15, and p-toluene sulfonic acid, have also been used for glycerol acetalization with acetone (da Silva et al., 2009). The results showed that the zeolite Beta with a high ratio of Si to Al attained a glycerol conversion of more than 95%. It was also found that the high ratio of Si to Al is beneficial as it caused the zeolite to block the entry of water into the pores.

Temperatures higher than 313 K are typically needed to obtain sufficient conversion to force water removal and to drive the forward reaction to produce solketal (Esteban et al., 2015). Vicente et al. (2010) investigated acetalization using crude glycerol with an SBA-15 catalyst and found high glycerol conversion. To drive the reaction equilibrium, they proposed a new method for water removal by refluxing the flask followed by water vaporization under vacuum pressure. Manjunathan et al. (2015) also produced solketal by reacting glycerol with acetone at an ambient temperature by using a modified beta catalyst. They obtained a glycerol conversion of 87.1% by using a catalyst loading of 7.5%. The pseudo homogeneous kinetics model for ketalization of glycerol using H-BEA as a catalyst was proposed by Rossa et al. (2017). At a temperature range of 313 K to 353 K, the activation energies for forward and reverse reactions were 44.77 kJ/mol and 41.40 kJ/mol, respectively. Nanda et al. (2014) investigated the kinetics of ketalization of glycerol at 293-323 K with Amberlyst 35 as a catalyst. They proposed a Langmuir-Hinshelwood kinetic model and obtained an activation energy of 55.6 kJ/mol. Overall, these studies show that the temperatures between 300-350 K are needed for glycerol conversion. As a result, kinetics studies with solid catalyst should be investigated within that range of temperatures.

The purpose of the present study was to develop a kinetics model of solketal synthesis from glycerol and acetone over an Indion 225 Na catalyst. Indion 225 Na is known to be an inexpensive ion exchanger compared to Amberlysts, and it is more accessible on the market. To achieve our proposed aim, glycerol conversion data were taken while varying reaction conditions such as temperature, catalyst concentration, stirrer speed, and acetone to glycerol ratios (Priadana, 2017; Fitriandini, 2017).

The acetalization of glycerol has been investigated in the present study by using the ion exchange resin Indion 225 Na. Temperature and catalyst loading significantly influenced the glycerol conversion rate. However, the molar ratio of acetone to glycerol had negligible effects on the solketal formation. The influence of stirrer speed was also negligible. As a result, it confirmed the absence of mass transfer resistance under high stirring rate. Thus, the reaction rate can be assumed to be measured under kinetic regime. A kinetic model based on the Eley-Rideal mechanism was developed to describe the reaction mechanism over the catalyst. The simulation results were compared to glycerol experimental data at temperatures ranging from 308 K to 328 K. From parameter calculations, we found a pre-exponential factor of 2.59 min^-1, an activation energy of 21.16 kJ/mol, an acetone adsorption equilibrium of 0.62, and solketal desorption equilibrium of 0.03. Comparison of glycerol concentrations between the simulated and experimental data showed that the model could describe the data well within the temperature range of 308-328 K. 

We would like to acknowledge the financial support received from the Ministry of Research, Technology and Higher Education of the Republic of Indonesia through the PDUPT Scheme, with contract number 124/UN1/DITLIT/DIT-LIT/LT/2018.

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