Transformation of triglycerides to fatty acid methyl esters with hydrophilic sulfonated silica (SiO 2 SO 3 H) as catalyst and quaternary ammonium salts in toluene or DMSO

: Triglycerides of waste cooking oil reacted with methanol in refluxing toluene to yield mixtures of diglycerides, monoglycerides and fatty acid methyl esters (FAMEs) in the presence of 20% (w/w) catalyst/oil using the hydrophilic sulfonated silica (SiO 2 -SO 3 H) catalyst alone or with the addition of 10% (w/w) co-catalyst/oil [(Bu n4 N)](BF 4 ) or Aliquat 336]. The addition of the ammonium salts to the catalyst lead to a decrease in the amounts of diglycerides in the products, but the concentrations of monoglycerides increased. Mixtures of [(Bu n4 N)](BF 4 )/catalyst were superior to catalyst alone or Aliquat 336/catalyst for promoting the production of mixtures with high concentrations of FAMEs. The same experiments were repeated using DMSO as the solvent. The use of the more polar solvent resulted in excellent conversion of the triglycerides to FAME esters with all three-catalyst media. A simplified mechanism is presented to account for the experimental results. sulfonated silica SiO 2 -SO 3 H and the mixtures of SiO 2 -SO 3 H with Aliquat 336 and with [(Bu n4 N)](BF 4 ) were used successfully for the transesterification of waste cooking soy oil with methanol in refluxing toluene. Analysis of the quantities of triglycerides, diglycerides, monoglycerides and fatty acid methyl esters in the reaction’s products indicated different behaviors of the catalysts. Whereas the unmixed sulfonated silica was the fastest in promoting the transformation of monoglycerides to FAMEs, the mixed catalysts favored the transformation of diglycerides into monoglycerides. The salt (Bu 4 N)(BF 4 ) showed a high efficiency in producing mixtures with high percentages of monoglycerides and FAMEs, despite its difficulty in promoting the initial approach of the triglyceride to the sulfonated catalyst surface. When DMSO was used in place of toluene under the same reaction conditions, the catalytic systems tested were effective for the transformation of tri-, di- and monoacylglycerols in FAME and glycerol. The results obtained using the sulfonated silica, SiO 3 -SO 3 H, and the SiO 2 -SO 3 H/(Bu n4 N)](BF 4 ) in DMSO were within


Introduction
In the search for an environmentally friendly method for biodiesel synthesis by transesterification of triglycerides, several alternatives for the development of solid acid catalysts have been studied. In principle, solid catalytic mixtures with large pores through which bulky triglycerides can access as many acidic sites as possible were sought. These sites should be highly stable, and large pores should also allow esterification of free fatty acids that are also present in vegetable oils [1].
Recently, the propyl sulfonic acid-functionalized silica, SiO2-Pr-SO3H, was synthesized from commercial silica gel and 3-mercaptopropyltriethoxysilane, followed by the oxidation of SiO2-Pr-SH to SiO2-Pr-SO3H with H2O2. The functionalized silica was applied as an alternative to traditional sulfuric acid or sulfonic resins for catalyzing chemical transformations [2][3][4][5][6]. Although it was employed for the preparation of biodiesel by the methanolic esterification of free fatty acids (FFAs) present in vegetable oils [7], in simulated oils [8], and in beef tallow [9], there is no report of the use of sulfuric acid-functionalized silicas for the production of biodiesel from triacylglycerides under atmospheric pressure.
As part of an ongoing study of the use of SiO2-SO3H for clean synthesis [10][11][12][13], we report herein the direct preparation of fatty acid methyl esters (FAMEs) using methanol, waste cooking oil, and the hydrophilic SiO2-SO3H as a catalyst, alone or in combination with the quaternary ammonium salts Aliquat 336 or (Bu4N)(BF4) in refluxing toluene or in DMSO (Fig. 1).

Experimental
All the reactions were performed in air under atmospheric pressure and monitored by TLC with preprepared plates (Silica Gel 60 F 254 on aluminum).

Waste cooking oil and biodiesel analysis
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 4 January 2022 doi:10.20944/preprints202201.0013.v1 The official methods proposed by ISO 12966 were used to determine the compositional profile by gas chromatography with a flame ionization detector (GC-FID) (Shimadzu GC-2010). sample (0.5 mL injected) was prepared using heptane 99%. 1 H-and 13 C-NMR spectra were recorded on Bruker Avance 400 and Avance 500 spectrometers. These data are included in the supplementary material.

Raw materials and chemicals
Waste cooking oil was donated by the university restaurant. All the other reagents (analytical grade), including dry toluene, DMSO and methanol, were supplied by Vetec, São Paulo, Brazil.

Typical procedures
2.4.1. Reacting the triglycerides from waste cooking oil with methanol using the SiO2-SO3H catalyst.
Waste cooking oil (11.2520 g; 12.8856 mmol), methanol (22.50 mL, 17.8431 g, 556.90 mmol), toluene (244.0 mL) and the catalyst SiO2-SO3H (2.2504 g; 20% w/w of waste cooking oil) were mixed in a 500-mL round bottom flask equipped with a reflux condenser, and the mixture was refluxed for 72 h at 110 o C.
The mixture was cooled, and the solid catalyst was filtered. The methanol and toluene were evaporated separately on a rotary evaporator, purified by distillation and used in new reaction processes within this study. The organic layer was decanted into a separatory funnel, where the biofuel-containing upper phase was separated from the lower phase containing glycerol by decantation. The recovered glycerol was treated as previously described [11a]. The biofuel phase was dissolved in hexane (50 mL), extracted with 20 mL of a saturated solution of NaCl, dried with MgSO4 and concentrated.
The same procedure was repeated using DMSO as the solvent with heating at 110 o C for 72 h. After cooling, the solid catalyst was filtered, and the excess methanol and DMSO were removed on a rotary evaporator under high vacuum. The glycerol was separated from the oil with a separatory funnel. The biodiesel was diluted in hexane, and the mixture was treated as described above. The biodiesel was finally dried under high vacuum for 5 h.
The catalyst was transferred to a muffle furnace, heated for 2 h at 200 °C, cooled and stored in a desiccator before reuse. The glycerol was purified by adsorption using activated charcoal, and it was used in the synthesis of ketals [11].  The Aliquat was eluted from a silica column by hexane (30 mL), and the hexane was evaporated to recover the Aliquat. This procedure was repeated using DMSO as the solvent, and the recovery of the biodiesel was achieved as described above.

Results and Discussion
The compositional profile analysis of the commercial brand used in this work is described in Table 1. The main fatty acids in that oil were linolenic (C18:2) and oleic acids (C18:1); accordingly, the mean molecular weight of the fatty acids was determined to be 277.41 g.mol -1 , and the mean molecular mass of the triglycerides was 873.22 g.mol -1 . This profile was considered for calculating the molar ratio of waste cooking oil: methanol for the transesterification reaction.  ASTM D6584. The wt% composition of the mixture (FAMEs and glycerides) in the products is presented in Figure 1 and  Table   2). The choice of toluene as the solvent for the reactions and the use of quaternary ammonium salts as copromoters were indicated by the recently discovered efficiency of the mixture toluene, [(Bu n 4N)](BF4) and SiO2-SO3H in the esterification of fatty acids with solketal [13]. The highest yield (64.2%) of FAMEs was obtained using the combination of (Bu n 4N)](BF4) with SiO2-SO3H.   [10]. We also pointed out that the addition of a quaternary ammonium salt to that type of mixture could change the polarity of the toluene phase, stabilizing those cationic intermediates, and leading to very good yields of, for instance, linoleic acid solketal ester [13]. The mechanism of the formation of biodiesel (FAMEs) using the solid catalyst can be oversimplified by the assumption that the majority of the chemical transformations will occur at the surface of the catalyst. With this assumption in mind, the three stages for the complete conversion of TG into long-chain methyl esters should occur as  Once the diglyceride is produced with the release of the methyl ester, the alcohol functional group of the DG would have the necessary properties to anchor, or at least, to facilitate the approach of this new reagent to the active centers of the solid catalyst [10]. Therefore, we can assume with certain confidence that the velocity of the V2 (Fig. 1) process is much faster than V1, which means that, once formed, the DG molecules trapped at or near the active centers of the catalyst surface would rapidly (in the time frame of the overall reaction) be converted into monoglycerides with the release of the second methyl ester. Within this reasoning, step V3 (Fig. 1) would be almost as delicate as V1, also requiring the fine tuning of the reaction parameters time, temperature, solvent and co-catalyst. Because the DG molecule was anchored at Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 4 January 2022 doi:10.20944/preprints202201.0013.v1 the surface of the catalyst, the formation of the second alcohol functional group due to the transformation DGsto MGs should strengthen the forces binding that molecule to the highly hydrophobic catalyst surface. It is reasonable to assume, at this stage, that the formation of glycerol in the process V3 could possibly block the active center(s) where the actual process occurred because of the high affinity of the catalyst for that kind of polar, H-bond-prone molecule.
In this reaction scenario, the co-catalyst should play multiple tasks: lower the repulsion forces of the bulky TG molecules towards the catalyst surface to enhance the V1init process, stabilize the cationic intermediates from V1~V3, and lower the attraction of glycerol to the active sites after completion of the V3 step, effectively freeing those sites for the constructive continuation of the reactions V1-V2-V3 sequences. The three systems under study are capable of breaking triglycerides by transesterification reactions with methanol in refluxing toluene, but with slightly different behaviors, as is shown in the graph in Fig 2, constructed using the data from Table 2.
The samples obtained from the 72-h reactions catalyzed by SiO2-SO3H alone contained no TGs but had higher percentages at 13.9 % of DGs than the samples taken from the other reactions, a clear indication that the addition of quaternary ammonium salts are important to accelerate the transformation of DGsto MGs (step V2 in Fig 1). Interestingly, the reactions with the lone catalyst resulted in the lowest percentage of MGs (25.9 %), which could indicate that the ammonium salts are effective in stabilizing cationic species derived from protonated MGs, which would slow down their conversion to FAME and glycerol (step V3 in Fig. 1).
The appearance of 0.7% TG in the samples derived from the system including the (Bu n 4N)(BF4) seems to suggest that this co-catalyst slightly inhibits the V1 process of the binding of TG to an active center of the solid catalyst and the formation of the DGs, either due to the poor interaction of the ammonium salt with the bulky TGs, by the strong stabilization of a cation such as that in Fig. 3, thereby retarding the V1final step, or both. However, the action of the (Bu n 4N)(BF4) in these systems seems to be beneficial, for only 2.5% of DGs were found in the samples of those reactions ( In a recent study, DMSO was shown to be an efficient solvent for the SiO2-SO3H-catalyzed dehydration of fructose to 5-hydroxymethyl-2-furfural [14]. The interaction of DMSO with the catalyst led us to test the use of DMSO in transesterification reactions. Because of the inadequate results obtained using toluene with regard to the complete consumption of the di-and monoglycerides, the transesterification was performed using DMSO as the solvent in the second phase of the study. The remaining conditions for the reaction were the same as those used with toluene. The yields are presented in Table 3. With regard to the concentration of total glycerol, the SiO3-SO3H (0.08%) and SiO3-SO3H/(Bu4N)(BF4) (0.23%) catalytic systems were within the limits specified by ASTM D6584-17 (Max. 0.25%), which also demonstrates the high degree of conversion of the tri-, di-and monoacylglycerols. The concentration of total glycerol obtained with the SiO2-SO3H/Aliquat 336 catalytic system (1.62%) indicates that this system is less adequate for catalyzing the transesterification of waste cooking oil.

Conclusions
The the standards established by the ASTM, indicating that these catalytic systems are effective for the transformation of low quality cooking oils.