Synthesis of solketalacetin as a green fuel additive via ketalization of monoacetin with acetone using silica benzyl sulfonic acid as catalyst

Silica benzyl sulfonic acid (SBSA) was prepared as a catalyst for reacting monoacetin with acetone to synthesize solketalacetin as a green fuel additive. SBSA was synthesized using a commercially available silica gel. The catalytic activity of SBSA was superior to Amberlyst 36 and Purolite PD 206. Increasing acetone to monoacetin mole ratio increased solketalacetin yield with SBSA leading to the highest yield. Solketalacetin yield was reduced with temperature increase for all the catalysts. 


Introduction
Environmental pollution and global warming are among the most significant international concerns caused by widespread use of fossil fuels.To address these concerns, renewable energy carriers such as biofuel (e.g., biodiesel, bioethanol, etc.) have attracted a great deal of attention as alternatives to fossiloriented fuels (Goncalves et al., 2016).Among biofuels, biodiesel mainly produced via transesterification of vegetable oil triglycerides with methanol in the presence of an alkaline catalyst (e.g., NaOH), is largely utilized all around the word as mineral diesel replacement.Through this reaction, glycerol is also produced as a principal byproduct called crude glycerol (Kim et al., 2004;Gerpen et al., 2005).It is estimated that ~10 wt.% of the total biodiesel produced is crude glycerol (Johnson et al., 2009), mostly regarded as waste.This huge amount of glycerol could pose serious environmental threats if disposed of into the environment due to its highly organic nature.On the other hand, given its quantity, its efficient utilization with the aim of producing valueadded products is highly advisable.This could also enhance the economic viability of the biodiesel industry.In line with that, efforts have been put into converting crude glycerol into a wide range of products including fuel additives.
Some of glycerol derivatives such as glycerol ketals as well as acetals including diacetin, triacetin, and solketal have been used as oxygenated fuel additives so far (Zhou et al., 2008  .Among these, solketalacetin ((2,2-dimethyl-1,3-dioxolan-4yl)methyl acetate) is considered as a biodiesel additive leading to improved viscosity and flash point (Garcia et al., 2008).Solketalacetin is synthesized through the reaction of glycerol with acetone and acetic anhydride (Garcia et al., 2008).
Other acidic catalysts can also be used to catalyze the reaction between monoacetin and acetone.In line with that, numerous catalysts have been investigated for the acetalization and ketalization of glycerol to date.For instance, homogeneous catalysts including sulfuric acid, hydrofluoric acid, and paratoluene sulfonic acid were used for valorization of glycerol to ketals and acetals (Cablewski et al., 1994;Krief et al., 1998).However, homogeneous catalysts could cause corrosion problems and their application is associated with unfavorable environmental impacts.Moreover, the processes involved in separating these catalysts from the products are ineffective and costly.
On the contrary, using heterogeneous catalysts for the synthesis of fuel additives from glycerol, higher purity of final reaction products and easier recovery can be achieved.The main heterogeneous catalysts used for ketalization and acetalization reaction include niobium-zirconium oxide catalysts (Lauriol-Garbey et al., 2011), HZSM-5, HUSY, K-10 montmorillonite (Deutsch et al., 2007;Goncalves et al., 2008), heteropolyacids (Bhorodwaj et al., 2011), PMo-NaUSY (Ferreira et al., 2009), mesoporous silica (Melero et al., 2007), Nafion-H NR-50, H-BEA, Amberlyst-36 (Deutsch et al., 2007;Mota et al., 2010;Nanda et al., 2014), Amberlyst 15 (Zhou et al., 2013), Zeolite beta (Maksimov et al., 2011;Nanda et al., 2014), and MoO 3 /SiO 2 (Umbarkar et al., 2009).Another category of heterogeneous catalysts used to catalyze ketalization and acetalization reactions is cation-exchange resins which have been shown to have a higher activity as well as excellent selectivity for desired products in comparison with other heterogeneous catalysts (Klepacova et al., 2005;Goncalves et al., 2008).In our previous works, a method was developed for economical synthesis of solketalacetin in two stages using a type cation-exchange resin catalyst, namely Purolite PD 206 (Gorji et al., 2016 and2018).More specifically, in the first stage, monoacetin was synthesized through the reaction of glycerol with acetic acid.In the second stage, solketalacetin was obtained through the reaction of monoacetin with acetone in the presence of Purolite PD 206 as catalyst.In spite of the promising results obtained, the catalyst used suffer from a number of drawbacks including low reusability, leaching of the sulfonic acid group from the catalyst, and reduction of the catalyst activity vs. time (Gorji et al., 2016).It should also be mentioned that Purolite PD 206 is costly.
Therefore, the aim of this study was to develop a novel process for the ketalization of monoacetin with acetone to synthesize solketalacetin using a more stable and economically-viable catalyst.More specifically, silica benzyl sulfonic acid (SBSA) catalyst was synthesized from a commercially available silica gel which could also be used in a continuous system.Moreover, the performance of the synthesized SBSA catalyst was compared with those of the Amberlyst 36 and Purolite PD 206.

SBSA catalyst preparation
SBSA was prepared based on the published procedures with some modifications (Kumar et al., 2001;Najafi et al., 2015).First, 3.0 g of silica gel was kept in a solution of 0.1 mol HCl for 24 h under reflux condition at 90 o C. The resulted solid was filtered, washed, and dried under vacuum condition at 250 o C. Dried solid (2.0 g) was added to 10 mL benzyl alcohol dissolved in 20 mL toluene in the presence of sulfuric acid and refluxed with dean stark for 48h.After filtering, washing, and drying, the resulted solid was sulfonated with chlorosulfonic acid in chloroform under reflux condition for 4 h to produce SBSA catalyst.Finally, the SBSA catalyst was filtered, washed, and dried at 100 o C.

Monoacetin synthesis
Monoacetin was synthesized through the reaction of acetic acid and glycerol in a continuous flow system (Fig. 1).A plug flow reactor with 8 mm internal diameter and 3.2 cm length, containing 0.9 g of Amberlyst 36 as catalyst was placed in an oven equipped with a temperature controller (±1 °C).The pressure (±1 bar) was created via a backpressure regulator (model BP 1580-81, JASCO Co.).Feedstock solution was pumped into the reactor continuously using a high-performance liquid chromatography (HPLC) pump (model PU-980, JASCO Co.) (Fig. 2).Samples were collected in a cold trap and analyzed by GC-FID.The optimum conditions for the reaction of acetic acid and glycerol were obtained in our previous work, i.e., reaction temperature of 78 °C, acetic acid to glycerol mole ratio of 3.7:1, pressure of 1 bar, and the feed flow rate of 0.9 mL•min −1 (Gorji et al., 2018).Reaction of glycerol and acetic acid was conducted under these optimum conditions to obtain a mixture with the following mole percent; remaining glycerol (30%), monoacetin (62%), and diacetin (8%).The solution was then vacuum distilled to remove water and acetic acid from the mixture.

Sample analysis
Analysis of the samples in different experiments was conducted by a GC-FID (model SP-3420, Beifen-Ruili Analytical Instrument Co., Ltd., Beijing, China).An HP-5 capillary column with 30 m length, 0.25 mm i.d., and 0.25 µm film thickness was used and the carrier gas was argon.The GC injection port and the detector temperatures were set at 290 °C and 300 °C, respectively.The temperature programing used for the GC analysis was as follows: initial column temperature was set at 80°C for 3 min, then increased from 80 °C to 90 °C at the rate of 3 °C•min −1 , and from 90 °C to 280 °C at the rate of 35°C•min −1 .Finally, it was held at 280°C for 5 min.The quantification was performed by injecting an internal standard (2-ethyl-1-hexanol) and integrating the peak areas to establish the calibration curve.Methanol was used as solvent in sample preparations and then 0.4 µL of the sample was injected into the GC-FID.
The yield of the solketalacetin was calculated using Equation 1: (Eq.1)

Catalyst characterization
To prepare the SBSA catalyst, silica gel was refluxed in a solution of hydrochloric acid 0.1 M for 24 h.After drying, the remaining solid was refluxed in a solution of benzyl alcohol and toluene to react silica with benzyl alcohol in the presence of sulfuric acid and finally the sulfonation reaction was conducted with chlorosulfonic acid as shown in Figure 3. Preparation of the SBSA catalysts was monitored by FT-IR spectroscopy via the KBr dilution technique.Figures 4a, b, and C show the FT-IR spectra of the protonated silica gel in HCl, benzylated silica, and SBSA catalyst, respectively.The band observed at 1050 cm -1 was attributed to the stretching vibration of Si-O-Si groups.The bands appearing at 2850-3100 cm -1 reveals the C-H asymmetric and symmetric stretching of methylene group after benzylation of silica (Figs.4b and c).The broadband observed at 3000 -3500 (Fig. 4c) could be attributed to the acidic O-H groups after sulfonation.The sulfonic acid related bonds were observed at 650 and 1000-1200 cm -1 corresponding to the S=O stretching vibration of sulfonic groups and the O=S=O symmetric and asymmetric stretching modes, respctively (Miyatake et al., 1996).The acidity of the SBSA catalyst determined using an inverse acid-base titration stood at 5.8 mmol.g -1 acid content.

Catalyst activity
Solketalacetin formation via monoacetin ketalization with acetone (Fig. 7) was conducted in a continuous flow system.The reactor was filled with  1.0 g of each catalyst separately (i.e., SBSA, Amberlyst 36, and Purolite PD 206) and the feedstock solution was continuously pumped into the reactor.
The effect of acetone to monoacetin molar ratio on the yield of solketalacetin was investigated for each catalyst.The results of solketalacetin yield with different acetone to monoacetin molar ratio at 40 °C, 1 bar, and 0.4 mL.min −1 feed flow rate are summarized in Table 1.It is clear that by increasing the acetone to monoacetin molar ratio, the solketalacetin yield was increased.As shown in Table 1, the SBSA led to the highest solketalacetin yield which could be ascribed to the high acidity strength of SBSA.It has been reported that the rate of the ketalization reaction is correlated with the acidity of the catalyst used (Kumar et al., 2001;Nanda et al., 2014).The acidity of Amberlyst 36, Purolite PD 206, and SBSA catalyst was 5.6, 4.9, and 5.8 mmol.g -1 , respectively.The effect of temperature on the reaction of monoacetin with acetone was investigated at four different temperature values (i.e., 20, 40, 60, and 80 o C) in acetone to monoacetin molar ratio of 5, 1 bar, 0.4 mL.min −1 feed flow rate and 1.0 g of each catalyst used separately (Amberlyst 36, Purolite PD 206 and SBSA) (Fig. 8).At 20 °C, the maximum solketalacetin yield was observed with Amberlyst 36 but at 40 °C, the highest yield was observed with SBSA catalyst.In general, for all three catalysts, solketalacetin yield was reduced with increasing temperature, since ketalization is an exothermic reaction and higher reaction temperatures could lead to lower solketalacetin yields (Maksimov et al., 2011;Nanda et al., 2014).The catalysts stability was also determined by keeping the catalysts onstream for 24 h in acetone to monoacetin molar ratio of 5, 1.0 g catalyst, 40 °C reaction temperature, 1 bar, and 0.4 mL.min -1 feed flow rate.Figure 9 shows the activity of the catalysts vs. time, demonstrating that the catalysts were stable for at least 24 h.The reusability of the catalysts was examined by recycling the catalyst for four runs.Each catalyst was kept on-stream for 2 h in acetone to monoacetin molar ratio of 5, 1.0 g catalyst, 40 °C reaction temperature, 1 bar, and 0.4 mL.min -1 feed flow rate.After each run, the catalysts were regenerated by washing with methanol followed by drying in an oven at 70 °C for 5 h.Subsequently, the catalysts were reused under Please cite this article as: Ghaziaskar H.S., Gorji Y.M. Synthesis of solketalacetin as a green fuel additive via ketalization of monoacetin with acetone using silica benzyl sulfonic acid as catalyst.Biofuel Research Journal 17 (2018) 753-758.DOI: 10.18331/BRJ2018.5.1.3similar conditions to those of the first experiment.Figure 10 shows the results obtained on the reusability of the catalysts for solketalacetin synthesis.Accordingly, SBSA presented more favorable stability and catalyst activity for the solketalacetin synthesis.These results suggest that SBSA catalyst prepared from commercially available, low cost silica was an effective heterogeneous catalyst for the synthesis of solketalacetin as fuel additive.Moreover, it could be concludd the SBSA might also be a suitable catalyst for other chemical reactions requiring acidic catalysts.

Conclusions
The SBSA catalyst prepared from commercially available, low cost silica was found an effective catalyst for the synthesis of solketalacetin through the reaction of monoacetin with acetone.The catalytic activity and stability of SBSA was compared with those of Purolite PD 206 and Amberlyst 36 as two sulfonated catalysts.The increase in acetone to monoacetin molar ratio increased the solketalacetin yield for all used catalysts at 40 °C, with the SBSA leading to the highest solketalacetin yield.In terms of stability, all three catalysts showed only a slight decrease in their activities after being on-stream for 24 h.Moreover, the reusability tests revealed that the SBSA was more reliable compared with the other catalysts and was found stable after four consecutive runs.

Figure 5
shows the X-ray diffraction pattern of silica gel, benzyl silica, and SBSA catalyst revealing the amorphous structure of the silica (Shokrolahi et al., 2012; Sivasankaran et al., 2016).

Figure 6
Figure 6 presents the thermograms of thermogravimetric analysis (TGA) of silica gel in HCl, benzylated silica, and SBSA catalyst.The protonated silica gel showed a weight loss in the temperature range of 100-200 o C due to the elimination of surface-adsorbed water.While the benzylated silica showed a distinct weight loss at 350-450 o C due to loss of benzyl groups.The TGA of the SBSA catalyst showed two significant weight losses at 200-350 o C and 350 -450 o C due to the loss of sulfonic and benzyl groups, respectively (Guimaraes et al., 2009; Sasikala et al., 2014).The acidity of the SBSA catalyst determined using an inverse acid-base titration stood at 5.8 mmol.g -1 acid content.

Table 1 .
Effect of acetone to monoacetin molar ratio on solketalacetin yield with 1.0 g catalyst and at 40 °C reaction temperature, 1 bar, and 0.4 mL.min −1 feed flow rate.