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Article

Silicotungstate- or Phosphotungstate-Catalyzed Glycerol Esterification with Acetic Acid: A Comparison of Zinc and Tin Salts

by
Marcio Jose da Silva
*,
Cesar Macedo Oliveira
,
Pedro Henrique da Silva Andrade
and
Neide Paloma Gonçalves Lopes
Chemistry Department, Federal University of Viçosa, Viçosa 36570-00, Minas Gerais, Brazil
*
Author to whom correspondence should be addressed.
Reactions 2025, 6(1), 19; https://doi.org/10.3390/reactions6010019
Submission received: 7 January 2025 / Revised: 4 February 2025 / Accepted: 17 February 2025 / Published: 7 March 2025
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Abstract

:
In this work, tin and zinc salts of silicotungstic and phosphotungstic acids were synthesized, characterized, and tested as catalysts for esterification reactions of glycerol with acetic acid (HOAc) to produce glycerol esters such as monoacetyl glycerol (MAG), which are used as additives in the pharmaceutical and food industries and in the manufacturing of explosives, or, in the case of di- or triacetyl glycerol (DAG and TAG), green bioadditives for diesel or gasoline. The activity of metal-exchanged salts (Zn, Sn) in H3PW12O40 and H4SiW12O40 heteropolyacids was evaluated in esterification reactions at room temperature. Among the catalysts tested, Sn2/3PW12O40 was the most active and selective toward the glycerol esters. The process’s selectivity can be controlled by changes to reaction conditions. The maximum selectivitiesy of DAG and TAG were 60% and 30%, respectively, using a 1:3 molar ratio of glycerol/HOAc and a Sn3/2PW12O40/673 K catalyst load of 0.4 mol%. Under these conditions, a glycerol conversion rate of 95% was observed and selectivity towards DAG and TAG was observed at 60% and 30%, respectively. The results were achieved after an 8 h reaction at a temperature of 333 K. The Sn3/2PW12O40/673 K catalyst demonstrated the highest efficiency, which was attributed to its higher degree of acidity.

Graphical Abstract

1. Introduction

Burning fossil fuels such as diesel and gasoline results in the emission of greenhouse effect gases, unburned hydrocarbon particulate materials, and acid rain gases (SOx, NOx) to the atmosphere, along with other environmental problems [1,2,3]. Consequently, the production of biofuels from renewable sources and biodiesels which generate fewer pollutants has increased significantly [4]. Besides biofuels, several fuel bioadditives can reduce these emissions [5]. These bioadditives can improve the physicochemical properties of fossil fuels, such as their octane and cetane numbers, cloud points, and calorific efficiency [6]. Unfortunately, despite efforts undertaken by economic and environmental authorities, no significant reduction in the consumption of these fossil fuels is predicted. Therefore, the search for a source of bioadditives that minimizes these impacts or biofuels which produce fewer pollutants should be intensified [7,8].
In this context, we examined glycerol, a coproduct of biodiesel generated at 10% weight. The production of biodiesel has led to an increase in the supply of glycerol, an attractive source of bioadditives such as acetals, ketals, ethers, or esters, which have been demonstrated to be very efficient when blended with gasoline or diesel [9,10,11].
In particular, glycerol esterification with acetic acid provides mono-, di-, or tri-esters that are of interest not only as fuel bioadditives but also as raw material for the pharmaceutical, food, polymer, and fine chemical industries [12,13,14,15]. Glyceryl esters are used in pharmaceutical applications as emulsifiers and stabilizers in oral medications, topical creams, and ointments. Additionally, glyceryl esters can be incorporated into products to enhance taste and mouthfeel. They are also used as food additives to extend shelf life and improve food quality [16].
Commonly, these reactions are catalyzed by soluble mineral acids such as sulfuric or hydrochloric acids [17]. Although inexpensive, these catalysts are corrosive and require neutralization [18]. Moreover, the product purification steps at the end of the process generate effluent and residues, making this an environmentally unfriendly route [19]. Alternatively, acid solid catalysts such as zeolite, clays, sulfonic resins, carbon materials, or acid-doped molecular sieves have been evaluated for glycerol esterification reactions with acetic acid [20,21,22]. Heterogeneous catalysts are mainly advantageous because they are easily recovered from the reaction medium and reused. However, the high polarity of the medium, besides the water generated as a byproduct, contributes to the leaching of the active phase and, consequently, the catalyst’s deactivation [23].
Keggin heteropolyacids arise as alternative catalysts because they have various attractive properties [24]. They are strong Bronsted acids and can be used as homogeneous or heterogeneous catalysts in acid-catalyzed or oxidation reactions [25,26,27,28,29,30]. These catalysts have been used in oxidation or acid-catalyzed reactions [31,32,33]. The modification in the structure of Keggin heteropolyacids generates soluble or insoluble salts in polar solvents [34]. Proton exchanges with metal cations improve the activity of these catalysts [35].
In this work, the protons of silicotungstic and phosphotungstic acids were exchanged with zinc(II) or tin(II) metal cations, generating heteropolysalts. They were evaluated as catalysts in esterification reactions of glycerol with acetic acid. These salts were characterized through infrared spectroscopy analyses, X-ray diffraction, and elemental analysis through MEV/EDS. Potentiometric titration curves with n-butylamine allowed us to evaluate the acidity of these catalysts. A comparison with pristine heteropolyacids (H3PW12O40, H4SiW12O40) was also performed. The impact of the main reaction parameters was assessed.

2. Materials and Methods

2.1. Chemicals

All the chemicals were used without any prior treatment. Acetic acid (99.9 wt.%) and glycerol (99.5 wt.%) were obtained from Sigma-Aldrich (Darmstadt, Germany). The hydrated heteropolyacids (H3PW12O40 n H2O and H4PSiW12O40 n H2O) were 99.9 wt.%. Synthesis precursors SnCl2 (99.6 wt.%) and ZnCl2 (99.5 wt.%) were obtained from Sigma-Aldrich.

2.2. Synthesis and Characterization of Tin- or Zinc-Exchanged Silicotungstic and Phosphotungstic Acid Salts

Tin or zinc phosphomolybdate or silicotungstate salts were prepared following an adaptation of a procedure described in the literature [36,37]. Typically, the catalyst was synthesized slowly by mixing 2 solutions; the first was 60 mL of an aqueous solution of heteropolyacid precursor heteropolyacid H3PW12O40, and the second was 40 mL of aqueous solution of ZnCl2 or SnCl2 (40 mL). The resulting solution was heated and stirred for 3 h at 333 K. After water evaporation, a white solid salt was left (Zn3/2PW12O40 or Sn3/2 PW12O40). After water vaporization at 373 K, the solid was dried in an oven at 393 K for 24 h. The same procedure was used to synthesize the salts of silicotungstic acid (Sn2SiW12O40 Zn2SiW12O40).

2.3. Catalyst Characterization

The infrared spectra were obtained in a Varian 660-IR spectrometer at a wavenumber range of 400 to 1300 cm−1. This is the fingerprint region where the main absorption bands of Keggin heteropolyanions are located. Powder diffraction patterns of silicotungstic and phosphomolybdic acids and their zinc or tin salts were analyzed by X-ray diffraction (XRD) spectroscopy in using an XRD system model D8-Discover Bruker using Ni filtered Cu-kα radiation (λ = 1.5418 Å), working at 40 kV and 40 mA, with a counting time of 1.0 s and a diffraction angle (2θ) ranging from 5 to 80 degrees.
Catalyst acidity strength was estimated by potentiometric titration, as described by Pizzio et al. [38]. The electrode potential variation was measured with a potentiometer (i.e., Bel, model W3B). Typically, 50 mg of salt or acid was dissolved in CH3CN and titrated with an n-butylamine solution in toluene (0.05 mol L−1).

2.4. Catalytic Runs

The catalytic tests were conducted in a three-necked glass flask with a capacity of 50 mL. An acetic acid solution containing a catalyst was magnetically stirred at a temperature of 333 K. The reaction commenced with the addition of glycerol and was monitored for 8 h. Toluene (0.10 mL) was used as the internal standard. Blank reactions were performed under the same conditions with each ketone and glycerol but without the catalyst.
The progress reaction was followed through GC analyses of periodically collected aliquots (Shimadzu 2010 equipment, FID, capillary column, Shimadzu, Kyoto, Japan). The conditions were 80 °C/ 3 min, 10 °C/min to 250 °C, and a 5 min hold time. The injector and FID were kept at 250 and 280 °C, respectively. The GC was fitted with a DB5 capillary column (0.25 μm × 0.25 mm × 30 m).
Analyses of mass spectrometry in a Shimadzu GC 2010 gas chromatograph coupled with MS-QP 5050A equipment (Tokyo, Japan) with a Carbowax capillary column (0.25 mm × 0.25 mm × 30 m) identified the main products. Helium was the carrier gas at 2 mL/min, and the GC injector and MS ion source temperatures were kept at 250 and 260 °C, respectively. The MS detector operated in the EI mode at 70 eV, with a scanning range of m/z 50–400. The products were co-injected in a GC chromatograph (Shimadzu 2010) with authentic previously synthesized samples [37]. The conversion was calculated using a calibration curve built with the glycerol and its esters to check the mass balance. The conversion was calculated using Equation (1):
% conversion = 100 × (Ai − Ar)/Ai
where Ai is the initial area of the GC peak of glycerol and Ar is the remaining area of the GC peak of glycerol in the aliquot analyzed.
The reaction selectivity was calculated through the relation between the GC peak of the product after its correction through the response factor (product/glycerol) (Ap) and the consumed area of the glycerol GC peak (Ai − Ar) (Equation (2)):
% selectivity = 100 × Ap/(Ai − Ar)

3. Results and Discussion

3.1. Catalyst Characterization

The fingerprint region of the infrared spectrum of Keggin anions shows absorption bands of vibrations involving the different oxygen atoms of heteropolyanions. The subscript of the oxygen atoms differentiates them concerning the place occupied in the heteropolyanion; Oa is the oxygen atom linked to the heteroatom (phosphorus or silicon); Ob is that belonging to the WO6 octahedral units sharing corners; Oc is that on an edge; and Od is a terminal oxygen atom bonded to the addenda atoms (tungsten) [39] (Figure 1). The octahedral units are depicted in blue, while the tetrahedral units are shown in red.
The infrared spectra of phosphotungstic acid and its zinc and tin salts are shown in Figure 2. The focus was to verify if the proton exchange led to a modification in the primary structure of phosphotungstate catalysts. The characteristic absorption bands involving the oxygen atoms of the Keggin anion of H3PW12O40 appeared at 1069, 971, 896, and 744 cm−1. The typical bands were assigned to the asymmetric stretching vibrations of bonds υas (P–Oa), υas (W–Od), υas (W–Ob–W), and υas (W–Oc–W) bonds, respectively [39,40]. It is important to highlight that, when the heteropolyanion W12O403− is decomposed, the band of absorption υas (P–Oa) undergoes a split, a modification that does not occur herein.
Comparing the infrared spectra of phosphotungstic acid and its tin and zinc salts, it is possible to note that most of the bands were seen at the same wavenumber. However, the absorption band attributed to the vibration of the υas (W–Ob–W) bond in the Zn3/2PW12O40 spectrum at 896 cm−1 showed a shoulder and slight shift toward the lower wavenumber if compared to H3PW12O40. These results agree with the literature [36,37,38].
The infrared spectra of silicotungstic acid and its zinc and tin salts are in Figure 3. The asymmetric stretching band (νas) is normally seen at 926 cm−1 but was absent. Conversely, the angular deformation (δ) of Si-O bonds was noticed at 510 cm−1. The vibrations of tungsten oxygen bonds resulted in bands at the following wavenumbers: 1004 cm−1 νas (W=Od), 908 cm−1 νas (W–Ob–W), and 742 cm−1 νas (W–Oc–W) [39]. Once more, most of the absorption bands of vibrations involving oxygen atoms of heteropolyanions of pristine heteropolyacid and their salts were noticed at the same wavenumbers. Nonetheless, the absorption band of the vibration υas (W–Ob–W) bond in the Sn2SiW12O40 spectrum at 908 cm−1 presented a small shoulder. All of the infrared spectra of silicotungstate and phosphotungstate catalysts showed an absorption band at a wavenumber greater than 1605 cm−1, which is attributed to the vibration of oxygen–hydrogen bonds in the cation dihydronium. When the protons are replaced with metal cations, this band can have its position or intensity modified. Herein, the intensity of mainly zinc salts increased. The infrared spectra showed that the primary unit of Keggin heteropolyacids with silicon or phosphorus as heteroatoms was preserved after the proton exchanges with zinc or tin metal cations [40,41,42,43].
While the infrared spectrum provides information about the primary structure of heteropolyacids or heteropolysalts (Keggin anion), the powder XRD patterns give details about the secondary structure, which is formed by the packing heteropolyanions simultaneously coordinated to the dihydronium cations or metal cations [40,41,42,43]. Therefore, the XRD patterns of Keggin heteropolyanions can be affected by the presence of water molecules and/or metal cations [44]. Figure 4a,b shows the diffractograms obtained through analyses of powder DRX.
The phosphotungstic anion has typical X-ray diffraction patterns at low angles, between 2θ 6 and 10° and 2θ 25 and 30° [43,44]. When the protons are exchanged with metal cations, it is expected that some peaks may be shifted and/or new diffraction lines may occur due to the possible contraction of the unitary cell [44]. According to Berry et al., the diffractogram of silicotungstic anion has a typical X-ray diffraction signal of a body-centered cubic secondary structure (2θ 9.5° and 26.0°) [44]. Conversely, the XRD patterns of phosphotungstic anions are of a cubic structure. The profile of the XRD obtained from tin and zinc salts was like that of the pristine acid, mainly for the phosphotungstate catalysts. However, the diffraction lines at low angles were slightly modified.
The elemental compositions of pristine heteropolyacids and their tin salts were determined through EDS/MEV analyses (Table 1). The same was performed for the zinc salts (omitted for simplification). All the results agree with the literature [45].
The potentiometric titration curves of the Keggin heteropolyacids and their salts are in Figure 5a,b. Although this technique does not allow us to distinguish the nature of acid sites (Lewis or Bronsted), it provides important information about their strength and amount. The Ei value led to classification of the acid sites as follows: Ei > 100 mV (very strong acid sites), 0 < Ei < 100 mV (strong acid sites) [38].
In general, lower volumes of n-butylamine were required to reach the equivalence point, which is given by the inflection points observed in the titration curves of tin and zinc heteropoly salts.

3.2. Catalytic Runs: A Comparison of Silicotungstates and Phosphotungstates of Zinc or Tin

Initially, the catalytic tests were carried out with metal heteropoly salts after reaching dryness at 373 K. Figure 6 presents the results of conversion and selectivity to glyceryl esters. Depending on the reaction conditions (catalyst load, temperature, molar ratio of HOAc to glycerol), the main product of glycerol acetylation can be diacetyl glycerol (DAG) or monoacetyl glycerol (MAG) (Scheme 1). Two possible MAGs or DAGs can be obtained, and it is always the less hampered hydroxy group or more reactive that undergoes the acetylation. Therefore, 1-MAG was formed due to the higher reactivity of the terminal hydroxyl group. On the other hand, 1,3-DAG is the main disubstituted product obtained. In the formation of 1,2-DAG, the acetyl group of the terminal hydroxyl hampers the formation of the vicinal product. Although these are reversible reactions, only one isomer was detected.
Figure 6 displays a typical chromatogram of glycerol esterification.
It is important to note that, in the absence of the catalyst at 333 K, only 1-MAG was obtained, but even with a HOAc excess (1:3) the conversion was lower than 10% after an 8 h reaction.
Tin and zinc phosphotungstates were more selective toward diacetyl glycerol, with similar glycerol conversions after an 8 h reaction. The same result was verified with silicotungstate catalysts.
The performances of metal silicotungstate and phosphotungstate catalysts and their pristine heteropolyacids were previously examined by our group [45]. However, no investigation was performed with the zinc catalyst. Particularly, the study about the impact of thermal treatment temperature on the Sn phosphotungstate revealed that, after treatment at 573 K, the catalyst became insoluble and had its catalytic activity increased (Figure 7) [45]. Herein, tin and zinc phosphotungstates and silicotungstates were treated at two temperatures (573 and 673 K) and evaluated in glycerol esterification reactions with acetic acid (Table 2).
Table 2 shows that, regardless of the catalyst or its thermal treatment temperature, after 8 h reaction and using 0.10 mol% of catalyst, TAG was always the minority product. Conversely, the most efficient catalyst led to a higher conversion and a greater selectivity toward the DAG. When treated at 573 K, tin phosphotungstate and tin silicotungstate were the most efficient catalysts. On the other hand, zinc phosphotungstate and zin silicotungstate achieved lower conversion and DAG selectivity.
Previously, Sn2+ heteropoly salts of phosphomolybdic and silicotungstic acids were used at the same temperature, glycerol:HOAc molar ratio, and catalyst load in glycerol esterification with HOAc. Although a higher conversion has been reached (70%) the selectivity was lower than those reached in Zn3/2PW12O40/673 K-catalyzed reactions. Moreover, those catalysts were not thermally treated and, therefore, they were soluble in acetic acid solution [45]. Transition metal salts of phosphotungstic acid were also evaluated in glycerol esterification with HOAc. The efficiency of metal phosphotungstate was Co2+ < Mn2+ ≅ Cu2 < Fe3+. Only FePW12O40 was more active than Sn3/2PW12O40/673 K evaluated herein, nonetheless, it was soluble in the reaction medium [37]. The selectivity toward the DAG reported in this paper was greater than those described in the literature [45].

3.3. The Effect of Main Reaction Parameters

The effect of molar ratio was assessed in the reactions with Sn3/2PW12O40/673 K or Sn3/2SiW12O40/673 K catalysts. The Sn3/2PW12O40/673 K-catalyzed glycerol esterification reaction with HOAc was performed using 1:3, 1:6, 1:9, and 1:12 molar ratios of glycerol to HOAc. Figure 8 displays the kinetic curves and the selectivity of products. Taking into account the conversion, a beneficial effect happened until 1:9 glycerol:HOAc. The conversion rate was 55% to 60%. With 1:12, there was a decline to 35%.
The consecutive character of reactions becomes noticeable when analyzing the selectivity of esters throughout the process. Regardless of the molar ratio, while the selectivity toward DAG gradually increased up to 80% after 8 h of reaction, a decrease in MAG and an increase in TAG selectivity occurred.
The impact of reactant stoichiometry was also evaluated on the Sn2SiW12O40/673 K-catalyzed reactions (Figure 9). Comparatively to the tin phosphotungstate catalysts, the tin silicotungstate was less effective, mainly when a higher HOAc load was used. Only in the reaction with a glycerol:HOAc proportion of 1:3 were the results comparable in terms of conversion or selectivity toward DAG. In the other reactions, a lower conversion was always achieved and MAG was the main product.
The superior performance of Sn3/2PW12O40 compared to Sn2SiW12O40 can be attributed to its higher strength of acidity (Figure 5).
Another important reaction variable is the temperature. Table 3 summarizes the main results obtained in reactions at different temperatures. An increase in temperature led to a higher conversion, which jumped from 25% to 60% when performed at 298 or 338 K, respectively. Once more, the consecutive character of reactions was evidenced by the variation in the selectivity of products. At lower temperatures, MAG is the main product. An increase in temperature led to a gradual increase in DAG selectivity with a consequent decline in MAG selectivity.
These data assure the endothermic character of this reaction. The same investigation was also performed with Sn2SiW12O40/673 K catalysts (Table 4). The behavior of Sn3/2PW12O40/673 K was also observed for the Sn2SiW12O40/673 K catalysts. An increase in reaction temperature led to higher conversions. The reaction selectivity was also impacted: the higher the temperature, the higher the DSG selectivity and concomitantly the lower the MAG selectivity.
The most efficient catalyst was selected to assess the effect of catalyst load. Increasing the load from 0.1 mol% to 0.2 mol%, both the conversion and reaction rate were significantly impacted (Figure 10). Within the first two hours of reaction, the maximum conversion (90%) was achieved, which stayed constant until the end of the reaction. The reactions using 0.2 to 0.4 mol% reached the same conversion after 3 h (90%).
Contrariwise, the reaction selectivity varied with a variation in catalyst load. Although DAG was always the main product, the selectivities of MAG and TAG were differently impacted. Indeed, in the reactions with a catalyst load equal to or higher than 0.3 mol%, the TAG became the secondary product while the MAG was the minority product.
The results in Figure 9 lead to the conclusion that the catalyst load controls the secondary product: MAG or TAG. The remarkable enhancement in selectivity for TAG when the catalyst load is increased is attributed to a greater amount of active sites in the solution when a greater catalyst load is used.
The reusability of Sn3/2PW12O40/673 K catalyst was assessed. At the end of the reaction, the solid catalyst was recovered from the solution through centrifugation, washed with propanol, and dried in an oven to 353 K. Afterward, the reactor was loaded, and the catalyst was added for a new run. This procedure was repeated three times. No significant decrease in conversion or selectivity to MAG or DAG was noticed.

4. Conclusions

Zinc or tin silicotungstates and phosphotungstates were successfully synthesized as demonstrated by the characterization data. The catalytic activity of these salts was evaluated in glycerol acetylation reactions with acetic acid. Although soluble when dried at 373 K, these catalysts became insoluble after thermal treatment at 573 or 673 K. Among the catalysts tested, the Sn3/2PW12O40 salt was the most active and selective toward the glycerol esters. This superior activity of tin phosphotungstate, if compared to the tin silicotungstate was attributed to it having the highest strength of acidity. A study of the effect of variation in reaction conditions showed that DAG and TAG can be the main products using the following reaction conditions: 0.4 mol% Sn3/2PW12O40 catalyst, 1:3 glycerol to HOAc proportion at 333 K. A higher proportion of HOAc to glycerol favors the formation of DAG, however, the TAG selectivity was not impacted. The consecutive character of esterification reactions (Gly to MAG, MAG to DAG, and DAG to TAG) was confirmed by monitoring the reaction selectivity throughout the reactions. The performance of phosphotungstate was superior to that of silicotungstate catalysts, regardless of the metal cation. Nonetheless, the tin catalysts were more efficient and selective toward the DAG or TAG esters.

Author Contributions

Conceptualization, M.J.d.S.; methodology, M.J.d.S., C.M.O. and N.P.G.L.; formal analysis, C.M.O.; investigation, C.M.O. and N.P.G.L.; resources, M.J.d.S. and P.H.d.S.A.; data curation, C.M.O. and P.H.d.S.A.; writing—original draft preparation, N.P.G.L. and M.J.d.S.; writing—review and editing, M.J.d.S. and N.P.G.L.; supervision, M.J.d.S.; project administration, M.J.d.S.; funding acquisition, M.J.d.S. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior—Brasil (CAPES)—Finance Code 001.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors are grateful for the financial support from CNPq and FAPEMIG (Brazil).

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Asghar, U.; Rafiq, S.; Anwar, A.; Iqbal, T.; Ahmed, A.; Jamil, F.; Khurram, M.S.; Akbar, M.M.; Farooq, A.; Shah, N.S.; et al. Review on the progress in emission control technologies for the abatement of CO2, SOx and NOx from fuel combustion. J. Environ. Chem. Eng. 2021, 9, 106064. [Google Scholar] [CrossRef]
  2. Zhao, H.T.; Wu, T.; He, J.; Kingman, S.; Shi, K.Q.; Shen, D.; Zhang, Y.Y. Simultaneous removal of SOx and NOx in flue gas at power stations over a Cu/Na-13X zeolite catalyst. Adv. Mater. Res. 2013, 650, 125–129. [Google Scholar] [CrossRef]
  3. Islam, A.; Teo, S.H.; Ng, C.H.; Taufiq-Yap, Y.H.; Choong, S.Y.T.; Awual, M.R. Progress in recent sustainable materials for greenhouse gas (NOx and SOx) emission mitigation. Prog. Mater. Sci. 2023, 132, 101033. [Google Scholar] [CrossRef]
  4. Anuar, M.R.; Abdullah, A.Z. Challenges in biodiesel industry with regards to feedstock, environmental, social and sustainability issues: A critical review. Renew. Sustain. Energy Rev. 2016, 58, 208–223. [Google Scholar] [CrossRef]
  5. da Silva, M.J.; Neide Paloma Gonçalves Lopes, N.P.G.; Rodrigues, A. A, Biodiesel Additives Synthesis Using Solid Heteropolyacid Catalysts. Energies 2023, 16, 1332. [Google Scholar] [CrossRef]
  6. Dodson, J.R.; Avellar, T.; Athayde, J.; Mota, C.J. Glycerol acetals with antioxidant properties. Pure Appl. Chem. 2014, 86, 905–911. [Google Scholar] [CrossRef]
  7. da Silva, M.J.; Ribeiro, C.A.J.; Rodrigues, A.A.; Silva, T.A. Fuel Bioadditives Synthesis from Furfural Glycerol Condensation over Vanadium-Substituted Cesium Phosphomolybdate Salts. Catal. Lett. 2024, 154, 3251–3263. [Google Scholar] [CrossRef]
  8. Luque, R.; Lovett, J.C.; Datta, B.; Clancy, J.; Campelo, J.M.; Romero, A.A. Biodiesel as feasible petrol fuel replacement: A multidisciplinary overview. Energy Environ. Sci. 2010, 3, 1706–1721. [Google Scholar] [CrossRef]
  9. Kirchhecker, S.; Dell’Acqua, A.; Angenvoort, A.; Spannenberg, A.; Ito, K.; Tin, S.; Taden, A.; de Vries, J.G. HMF–glycerol acetals as additives for the debonding of polyurethane adhesives. Green Chem. 2021, 23, 957–965. [Google Scholar] [CrossRef]
  10. Monteiro, M.R.; Kugelmeier, C.L.; Pinheiro, R.S.; Batalha, M.O.; da Silva César, A. Glycerol from biodiesel production: Technological paths for sustainability. Renew. Sustain. Energy Rev. 2018, 88, 109–122. [Google Scholar] [CrossRef]
  11. Tan, H.W.; Aziz, A.A.; Aroua, M.K. Glycerol production and its applications as a raw material: A review. Renew. Sustain. Energy Rev. 2013, 27, 118–127. [Google Scholar] [CrossRef]
  12. Cornejo, A.; Barrio, I.; Campoy, M.; Lázaro, J.; Navarrete, B. Oxygenated fuel additives from glycerol valorization. Main production pathways and effects on fuel properties and engine performance: A critical review. Renew. Sustain. Energy Rev. 2017, 79, 1400–1413. [Google Scholar] [CrossRef]
  13. Bohmer, N.; Roussiere, T.; Kuba, M.; Schunk, S.A. Valorisation of glycerol as renewable feedstock: Comparison of the exploration of chemical transformation methods aided by high throughput experimentation. Comb. Chem. High Throughput Screen. 2012, 15, 123–135. [Google Scholar] [CrossRef] [PubMed]
  14. Rodrigues, A.; Bordado, J.C.; Galhano dos Santos, R. Upgrading the glycerol from biodiesel production as a source of energy carriers and chemicals—A technological review for three chemical pathways. Energies 2017, 10, 1817. [Google Scholar] [CrossRef]
  15. Requena, S.O.; Montiel, C.; Máximo, F.; Gómez, M.; Murcia, M.D.; Bastida, J. Esters in the Food and Cosmetic Industries: An Overview of the Reactors Used in Their Biocatalytic Synthesis. Materials 2024, 17, 268–298. [Google Scholar] [CrossRef] [PubMed]
  16. Jia, M.; Jiang, L.; Niu, F.; Zhang, Y.; Sun, X. A novel and highly efficient esterification process using triphenylphosphine oxide with oxalyl chloride. R. Soc. Open Sci. 2023, 5, 171988. [Google Scholar] [CrossRef]
  17. Avhad, M.R.; Marchetti, J.M. Innovation in solid heterogeneous catalysis for the generation of economically viable and ecofriendly biodiesel: A review. Catal. Rev. 2016, 58, 157–208. [Google Scholar] [CrossRef]
  18. Lotero, E.; Liu, Y.; Lopez, D.E.; Suwannakarn, K.; Bruce, D.A.; Goodwin, J.G. Synthesis of biodiesel via acid catalysis. Ind. Eng. Chem. Res. 2005, 44, 5353–5363. [Google Scholar] [CrossRef]
  19. Corma, A.; Garcia, H. Lewis acids: From conventional homogeneous to green homogeneous and heterogeneous catalysis. Chem. Rev. 2003, 103, 4307–4366. [Google Scholar] [CrossRef]
  20. Guerrero-Ruíz, F.; Yara-Varon, E.; González, M.D.; Torres, M.; Salagre, P.; Canela-Garayoa, R.; Cesteros, Y. Use of biobased crude glycerol, obtained biocatalytically, to obtain biofuel additives by catalytic acetalization of furfural using SAPO catalysts. Fuel 2022, 319, 123803. [Google Scholar] [CrossRef]
  21. Samoilov, V.O.; Ramazanov, D.N.; Nekhaev, A.I.; Maximov, A.L.; Bagdasarov, L.N. Heterogeneous catalytic conversion of glycerol to oxygenated fuel additives. Fuel 2016, 172, 310–319. [Google Scholar] [CrossRef]
  22. Natalino, R.; Varejão, E.V.V.; Da Silva, M.J.; Cardoso, A.L.; Fernandes, S.A. p-Sulfonic acid calix [n] arenes: The most active and water tolerant organocatalysts in esterification reactions. Catal. Sci. Technol. 2014, 4, 1369–1375. [Google Scholar] [CrossRef]
  23. Trombettoni, V.; Lanari, D.; Prinsen, P.; Luque, R.; Marrocchi, A.; Vaccaro, L. Recent advances in sulfonated resin catalysts for efficient biodiesel and bio-derived additives production. Prog. Energy Comb. Sci. 2018, 65, 136–162. [Google Scholar] [CrossRef]
  24. Kim, I.; Kim, J.; Lee, D. A comparative study on catalytic properties of solid acid catalysts for glycerol acetylation at low temperatures. Appl. Catal. B Environ. 2014, 148, 295–303. [Google Scholar] [CrossRef]
  25. Batalha, D.C.; Ferreira, S.O.; da Silva, R.C.; da Silva, M.J. Cesium-Exchanged Lacunar Keggin Heteropolyacid Salts: Efficient Solid Catalysts for the Green Oxidation of Terpenic Alcohols with Hydrogen Peroxide. ChemistrySelect 2020, 5, 1976–1986. [Google Scholar] [CrossRef]
  26. Kozhevnikov, I. Sustainable heterogeneous acid catalysis by heteropoly acids. In Handbook of Green Chemistry; Wiley Online Library: Hoboken, NJ, USA, 2010; pp. 153–174. [Google Scholar] [CrossRef]
  27. Da Silva, M.J.; Liberto, N.A. Soluble and solid-supported Keggin heteropolyacids as catalysts in reactions for biodiesel production: Challenges and recent advances. Curr. Org. Chem. 2016, 20, 1263–1283. [Google Scholar] [CrossRef]
  28. Zhu, S.; Zhu, Y.; Gao, X.; Mo, T.; Zhu, Y.; Li, Y. Production of bioadditives from glycerol esterification over zirconia supported heteropolyacids. Bioresour. Technol. 2013, 130, 45–51. [Google Scholar] [CrossRef]
  29. Betiha, M.A.; Hassan, H.M.; El-Sharkawy, E.A.; Al-Sabagh, A.M.; Menoufy, M.F.; Abdelmoniem, H.M. A new approach to polymer-supported phosphotungstic acid: Application for glycerol acetylation using robust sustainable acidic heterogeneous–homogenous catalyst. Appl. Catal. B. 2016, 182, 15–25. [Google Scholar] [CrossRef]
  30. Da Silva, M.J.; Rodrigues, A.A. Metal silicotungstate salts as catalysts in furfural oxidation reactions with hydrogen peroxide. Mol. Catal. 2020, 493, 111104. [Google Scholar] [CrossRef]
  31. Timofeeva, M.N. Acid catalysis by heteropoly acids. Appl. Catal. A 2003, 256, 19–35. [Google Scholar] [CrossRef]
  32. Coronel, N.C.; da Silva, M.J.; Ferreira, S.O.; da Silva, R.C.; Natalino, R. K5PW11NiO39-catalyzed oxidation of benzyl alcohol with hydrogen peroxide. ChemistrySelect 2019, 4, 302–310. [Google Scholar] [CrossRef]
  33. Patel, A.; Singh, S. A green and sustainable approach for esterification of glycerol using 12 tungstophosphoric acid anchored to different supports: Kinetics and effect of support. Fuel 2014, 118, 358–364. [Google Scholar] [CrossRef]
  34. Coronel, N.C.; da Silva, M.J. Lacunar Keggin heteropolyacid salts: Soluble, solid and solid-supported catalysts. J. Clust. Sci. 2018, 29, 195–205. [Google Scholar] [CrossRef]
  35. Zhu, S.; Gao, X.; Dong, F.; Zhu, Y.; Zheng, H.; Li, Y. Design of a highly active silver exchanged phosphotungstic acid catalyst for glycerol esterification with acetic acid. J. Catal. 2013, 306, 155–163. [Google Scholar] [CrossRef]
  36. Castro, G.A.D.; Lopes, N.P.G.; Fernandes, S.A.; da Silva, M.J. Copper phosphotungstate-catalyzed microwave-assisted synthesis of 5-hydroxymethylfurfural in a biphasic system. Cellulose 2022, 29, 5529–5545. [Google Scholar] [CrossRef]
  37. Da Silva, M.J.; Liberto, N.A.; Leles, L.C.D.A.; Pereira, U.A. Fe4(SiW12O40)3-catalyzed glycerol acetylation: Synthesis of bioadditives by using highly active Lewis acid catalyst. J. Mol. Catal. A 2016, 422, 69–83. [Google Scholar] [CrossRef]
  38. Pizzio, L.R.; Blanco, M.N. A contribution to the physicochemical characterization of nonstoichiometric salts of tungstosilicic acid. Micropor. Mesopor. Mater. 2007, 103, 40–47. [Google Scholar] [CrossRef]
  39. Popa, A.; Sasca, V.; Bajuk-Bogdanović, D.; Holclajtner-Antunović, I. Synthesis, characterization and thermal stability of cobalt salts of Keggin-type heteropolyacids supported on mesoporous silica. J. Therm. Anal. Calorim. 2016, 126, 1567–1577. [Google Scholar] [CrossRef]
  40. Holclajtner-Antunović, I.; Mioč, U.B.; Todorović, M.; Jovanović, Z.; Davidović, M.; Bajuk-Bogdanović, D.; Laušević, Z. Characterization of potassium salts of 12-tungstophosphoric acid. Mater. Res. Bull. 2010, 45, 1679–1684. [Google Scholar] [CrossRef]
  41. Méndez, L.; Torviso, R.; Pizzio, L.; Blanco, M. 2-Methoxynaphthalene acylation using aluminum or copper salts of tungstophosphoric and tungstosilicic acids as catalysts. Catal. Today 2011, 173, 32–37. [Google Scholar] [CrossRef]
  42. Villabrille, P.; Romanelli, G.; Gassa, L.; Vazquez, P.; Caceres, C. Synthesis and characterization of Fe-and Cu-doped molybdovanadophosphoric acids and their application in catalytic oxidation. Appl. Catal. A 2007, 324, 69–76. [Google Scholar] [CrossRef]
  43. Borghese, S.; Louis, B.; Blanc, A.; Pale, P. Design of silver (I)-heteropolyacids: Toward the molecular control of reactivity in organic chemistry. Catal. Sci. Technol. 2011, 1, 981–998. [Google Scholar] [CrossRef]
  44. Berry, F.J.; Derrick, G.R.; Mortimer, M. Identification and characterization of stable phases of silicotungstic acid, H4SiW12O240∙nH2O. Polyhedron 2014, 68, 17–22. [Google Scholar] [CrossRef]
  45. Chaves, D.M.; Ferreira, S.O.; da Silva, R.C.; Natalino, R.; da Silva, M.J. Glycerol Esterification over Sn(II)-Exchanged Keggin Heteropoly Salt Catalysts: Effect of Thermal Treatment Temperature. Energy Fuels 2019, 33, 7705–7716. [Google Scholar] [CrossRef]
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Figure 1. Labels of oxygen atoms according to their localization in the Keggin anion.
Figure 1. Labels of oxygen atoms according to their localization in the Keggin anion.
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Figure 2. Infrared spectra of phosphotungstic acid and its tin and zinc salts.
Figure 2. Infrared spectra of phosphotungstic acid and its tin and zinc salts.
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Figure 3. Infrared spectra of silicotungstic acid and its tin and zinc salts.
Figure 3. Infrared spectra of silicotungstic acid and its tin and zinc salts.
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Figure 4. XRD patterns of phosphotungstic (a) and silicotungstic (b) acids and their zinc or tin salts.
Figure 4. XRD patterns of phosphotungstic (a) and silicotungstic (b) acids and their zinc or tin salts.
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Figure 5. Potentiometric titration curves of phosphotungstic (a) and silicotungstic acid (b) and their respective zinc or tin salts.
Figure 5. Potentiometric titration curves of phosphotungstic (a) and silicotungstic acid (b) and their respective zinc or tin salts.
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Scheme 1. Products of glycerol acetylation with HOAc.
Scheme 1. Products of glycerol acetylation with HOAc.
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Figure 6. Typical chromatogram of glycerol esterification (not optimized conditions).
Figure 6. Typical chromatogram of glycerol esterification (not optimized conditions).
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Figure 7. Conversion and selectivity of tin or zinc phosphotungstate-catalyzed (a) and tin or zinc silicotungstate-catalyzed (b) glycerol esterification reactions with acetic acid a. a Reaction conditions: glycerol (24.0 mmol), CH3COOH (72.0 mmol), catalyst load (0.10 mol%), 333 K.
Figure 7. Conversion and selectivity of tin or zinc phosphotungstate-catalyzed (a) and tin or zinc silicotungstate-catalyzed (b) glycerol esterification reactions with acetic acid a. a Reaction conditions: glycerol (24.0 mmol), CH3COOH (72.0 mmol), catalyst load (0.10 mol%), 333 K.
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Figure 8. Impact of glycerol:HOAc molar ratio on conversion and selectivity of Sn3/2PW12O40/673 K-catalyzed glycerol esterification reactions a. a Reaction conditions: glycerol (24.0 mmol); catalyst load (0.1 mol%); catalyst temperature (333 K); (a) 1:3; (b) 1:6; (c) 1:9; and (d) 1:12.
Figure 8. Impact of glycerol:HOAc molar ratio on conversion and selectivity of Sn3/2PW12O40/673 K-catalyzed glycerol esterification reactions a. a Reaction conditions: glycerol (24.0 mmol); catalyst load (0.1 mol%); catalyst temperature (333 K); (a) 1:3; (b) 1:6; (c) 1:9; and (d) 1:12.
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Figure 9. Effects of molar ratio of glycerol:HOAc on conversion and selectivity of Sn2SiW12O40/673 K-catalyzed glycerol esterification reactions a. a Reaction conditions: glycerol (24.0 mmol); catalyst load (0.1 mol%); catalyst temperature (333 K); (a) 1:3; (b) 1:6; (c) 1:9; and (d) 1:12.
Figure 9. Effects of molar ratio of glycerol:HOAc on conversion and selectivity of Sn2SiW12O40/673 K-catalyzed glycerol esterification reactions a. a Reaction conditions: glycerol (24.0 mmol); catalyst load (0.1 mol%); catalyst temperature (333 K); (a) 1:3; (b) 1:6; (c) 1:9; and (d) 1:12.
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Figure 10. Kinetic curves and ester selectivity of Sn3/2PW12O40/673 K-catalyzed glycerol esterification with HOAc a. a Reaction conditions: glycerol (24.0 mmol); HOAc:glycerol 3:1; temperature (333 K); catalyst load (a) 0.10; (b) 0.20; (c) 0.30; and (d) 0.40 mol%.
Figure 10. Kinetic curves and ester selectivity of Sn3/2PW12O40/673 K-catalyzed glycerol esterification with HOAc a. a Reaction conditions: glycerol (24.0 mmol); HOAc:glycerol 3:1; temperature (333 K); catalyst load (a) 0.10; (b) 0.20; (c) 0.30; and (d) 0.40 mol%.
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Table 1. Elemental composition of pristine heteropolyacids and their tin salts.
Table 1. Elemental composition of pristine heteropolyacids and their tin salts.
Exp.CatalystWeight (%)
SnX 1WO
1H4SiW12O40017722
2Sn2SiW12O40-773617023
3H3PW12O40017722
4Sn3/2PW12O40-773317323
1 X: Phosphorus or silicon atom.
Table 2. Conversion and selectivity of glycerol esterification reactions in the presence of phosphotungstates or silicotungstates of zinc or tin thermally treated at 573 K or 673 K a.
Table 2. Conversion and selectivity of glycerol esterification reactions in the presence of phosphotungstates or silicotungstates of zinc or tin thermally treated at 573 K or 673 K a.
EntryCatalyst/Thermal
Treatment Temperature		Conversion/%Percent/%
MAGDAGTAG
1Zn3/2PW12O40/573 K3445532
2Sn3/2PW12O40/573 K65157411
3Zn2SiW12O40/573 K2848557
4Sn2SiW12O40/573 K5818775
5Zn3/2PW12O40/673 K2275250
6Sn3/2PW12O40/673 K55157510
7Zn2SiW12O40/673 K159055
8Sn2SiW12O40/673 K5010855
a Reaction conditions: molar ratio glycerol: HOAc (1:3), catalyst load (0.1 mol%), time (8 h), temperature (333 K).
Table 3. Effect of reaction temperature on conversion and selectivity of Sn3/2PW12O40/673 K-catalyzed glycerol esterification reactions a.
Table 3. Effect of reaction temperature on conversion and selectivity of Sn3/2PW12O40/673 K-catalyzed glycerol esterification reactions a.
EntryReaction Temperature
(K)		Conversion/%Percent/%
MAGDAGTAG
1298 2588102
23083082153
33183574224
43285518775
53386075250
a Reaction conditions: molar ratio of glycerol:HOAc (1:3), catalyst load (0.1 mol%), time (8 h).
Table 4. Effect of reaction temperature on conversion and selectivity of Sn2SiW12O40/673 K-catalyzed glycerol esterification reactions a.
Table 4. Effect of reaction temperature on conversion and selectivity of Sn2SiW12O40/673 K-catalyzed glycerol esterification reactions a.
EntryReaction Temperature
(K)		Conversion/%Percent/%
MAGDAGTAG
1298 588120
23081588120
33182070300
43284525723
53385520755
a Reaction conditions: molar ratio of glycerol:HOAc (1:3), catalyst load (0.1 mol%), time (6 h).
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MDPI and ACS Style

da Silva, M.J.; Oliveira, C.M.; da Silva Andrade, P.H.; Lopes, N.P.G. Silicotungstate- or Phosphotungstate-Catalyzed Glycerol Esterification with Acetic Acid: A Comparison of Zinc and Tin Salts. Reactions 2025, 6, 19. https://doi.org/10.3390/reactions6010019

AMA Style

da Silva MJ, Oliveira CM, da Silva Andrade PH, Lopes NPG. Silicotungstate- or Phosphotungstate-Catalyzed Glycerol Esterification with Acetic Acid: A Comparison of Zinc and Tin Salts. Reactions. 2025; 6(1):19. https://doi.org/10.3390/reactions6010019

Chicago/Turabian Style

da Silva, Marcio Jose, Cesar Macedo Oliveira, Pedro Henrique da Silva Andrade, and Neide Paloma Gonçalves Lopes. 2025. "Silicotungstate- or Phosphotungstate-Catalyzed Glycerol Esterification with Acetic Acid: A Comparison of Zinc and Tin Salts" Reactions 6, no. 1: 19. https://doi.org/10.3390/reactions6010019

APA Style

da Silva, M. J., Oliveira, C. M., da Silva Andrade, P. H., & Lopes, N. P. G. (2025). Silicotungstate- or Phosphotungstate-Catalyzed Glycerol Esterification with Acetic Acid: A Comparison of Zinc and Tin Salts. Reactions, 6(1), 19. https://doi.org/10.3390/reactions6010019

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