Template LAAR 2021

Abstract-- Glycerol (Gly) is the major by-product of the biodiesel production (approximately 10 wt.%). There are different research focusing on developing glycerol utilization to produce value-added chemicals. One of them is the etherification with alcohols in the presence of acidic catalysts. In the present work, the catalytic behaviour of SiO₂-based system functionalized with heteropolyacids (HPA: H₃O₄₀PW₁₂.xH₂O) was studied in the etherification of Gly with benzyl alcohol (BA). The solids were impregnated with different solutions of a HPA (28, 35 and 40%) and characterized. Very good catalytic performance was achieved (94% conversion and 78% of selectivity to ME₁: 3-benzyloxy-1, 2-propanediol) after 360 min of reaction, at 120°C and Gly: BA= 3:1, with 10 wt.% of SiO₂-HPA₃₅. This system was compared with functionalized carbon-based catalysts (Cst-SO₃H and Ccs-SO₃H) and it was observed that the results were similar, conversion over 90% and selectivity between 70 -80% to some of the desired products (ME₁).

Keywords-- Heteropolyacid (HPA), carbon, sulfonic groups, heterogeneous catalysts.

I. INTRODUCTION

The increasing in the trend for green technology, such as the production of biodiesel, has indirectly generated a big raise of glycerol in the market. The great number of impurities as residual methanol, salts, and free fatty acids has limited its utilization. The research community has been proposing used both crude and refined glycerol, together with improved purification methods for crude glycerol, in order to deal with the offer exceeding demand. Besides, the studies have identified glycerol as a platform molecule for the synthesis of a lot of fine chemicals due to its three-hydroxyl groups (Ben et al., 2020; Shikhaliyev et al., 2021).

Glycerol is a molecule that combines important physical and chemical properties that make it a technically versatile product, compatible with many other substances and easy to handle to obtain high value products. Throughout these years, different reaction pathways such as hydrogenolysis, reduction, acetylation, dehydration, pyrolysis, oxidation, steam reforming, esterification and etherification have been utilized to convert glycerol to valuable chemicals that could find applications in various industrial fields (pharmaceutical, food, polymers, fuels, fertilizers, etc.) (Cespi et al., 2015; Dahdah et al., 2020; Kong et al., 2016; Faroppa et al., 2016).

The catalytic etherification of glycerol is an industrially interesting application. As a function of the alcohol used, selective glycerol etherification may lead to obtaining a wide range of products having biological applications, such as anti-inflammatory, antibacterial, antifungal, among others (Gu et al., 2008). Glycerol etherification with benzyl alcohol (BA) is an acid-catalysed reaction, resulting in a mixture of different products, including the by-product of BA self-condensation (dibenzyl ether, BE), being the desired products the mono-ethers (ME₁ and ME₂, see Fig. 1) (Kubota et al., 2014).

The glycerol etherification reaction can be carried out with homogeneous or heterogeneous catalysts. It has been studied with different heterogeneous acid catalysts because these systems offer the possibility for reusing and reduces environmental and operational problems (Gu et al., 2008; Da Silva et al., 2019; Jaworski et al., 2015; Chiosso et al., 2020; Chiosso et al., 2021).

On the other hand, heteropolyacids have been widely studied in recent decades. One of the reasons is related to them acidic character. Due to their strong Brönsted acidity, heteropolyacids (HPA) can be used instead of conventional acid catalysts, such as sulfuric acid. Furthermore, they can exhibit Lewis acidity depending on the constituent elements of heteropolycompounds. Among the advantages over the environmental impact, comprise of decreasing the amount of acid effluents compared to traditional technologies produce, and the easy recovery and reuse of the catalysts (Pizzio et al., 2003).

The HPA have some disadvantages as catalysts, such as low surface area (1–10 m² g^-1), low thermal stability and separation problems from reaction mixtures. To deal whit these disadvantages, zeolites, silica, activated carbon and polymers, have been used to immobilize them. Supported heteropolycompounds can be used as catalysts in gas-solid and liquid-solid reactions, and especially HPAs supported on SiO₂ have attracted interest as catalyst appropriate for reactions in polar media (Ferreira et

Figure 1: Products of the etherification reaction of glycerol with benzyl alcohol: 3-benzyloxy-1,2-propanediol (ME₁), 2-benzyloxy-1,3-propanediol (ME₂), 1,3-dibenzyloxy-2-propanol (DE₁), 1,2-dibenzyloxy-3-propanol (DE₂) and 1,2,3-tribenzyloxypropane (TE).

al., 2010). The HPA have been used as acid catalysts in reactions involving glycerol, such as dehydration (Atia et al., 2008) and esterification (Ferreira et al., 2009).

Based on these antecedents, in this work it is proposed to study catalytic behaviour (activity and selectivity) of HPA acid groups supported on SiO₂, in the etherification of Gly with BA. The results obtained will be compared to those obtained under the same reaction conditions with carbon functionalized catalysts (Cst-SO₃H and Ccs-SO₃H). The reactions will be performed in a solventless system in liquid phase.

II. METHODS

A. Preparation of the supports

-SiO₂

Commercial SiO₂was pre-treated to obtain catalysts with uniform particle size before use. A gel was formed with distilled water. Then, it was dried in an oven at 105°C during 24 h. Finally, it was ground and sieved to a particle size of 60 to 100 mesh.

-Carbon

Carbonaceous materials were prepared according to previous works. The solid called Cst was obtained by carbonization (in an atmosphere of N₂ in two stages) of anhydrous dextrose assisted with a template, according to the procedure reported in Chiosso et al., (2020), and the solid called Ccs was obtained following the methods published in Chiosso et al., (2021).

B. Material functionalization

-SiO₂ with heteropolyacids (HPA)

SiO₂ was functionalized by incipient wetness impregnation using an aqueous solution of hydrated tungstophosphoric acid (H₃O₄₀PW₁₂.xH₂O), called HPA. The acid concentration in the solution was adjusted to obtain different percentages (28, 35 or 40%) and it was added dropwise to the SiO₂ support until the "gel point" condition was completed. The catalyst was dried in oven at 105°C for 3 h to remove solvent. After, it was calcinated in two steps in a tubular furnace at a rate of 5°C min^-1 in atmosphere of N₂. In the first phase, it was heated from room temperature to 200°C and maintained in this temperature for one hour. Them, it was heated up to 250°C for another hours. Its materials were named SiO₂-HPAxx, where xx corresponds to percentage of HPA in the impregnation solution.

-Carbon with sulfonic groups (C-SO₃H)

Carbonous solids were functionalized in a glass reactor with concentrated sulfuric acid (H₂SO₄ 96%) using a ratio of 15 mL of acid per gram of carbon at 150°C under nitrogen flowing. Cst was sulfonated for 15 h while Ccs for 4 h. Afterwards, the acidic carbon were washed by distilled water until the presence of sulphate ions was not detected by reaction with Ba(NO₃)₂. Then these acidic carbons were oven-dried at 105°C during 24 h. The catalyst thus prepared were named Cst-SO₃H and Ccs-SO₃H.

C. Catalyst characterization

Density of acid sites of SiO₂-HPAxx catalysts were measured through potentiometric titration using n-butylamine solution 0.025 N at a flow rate of 0.05 mL min^-1 in a 794 Basic Titrino Metrohm equipment using a double junction electrode.

Acid densities of carbon materials were estimated by back-titration method. Approximately, 0.1 g of dried sample was added to 50 mL of basic solution depending on type of acid sites to be measured. This mixture was kept under stirring for 30 min at room temperature before titrating the remaining of basic solution by a standard solution of 0.01 M HCl. Total content of acidic groups was analysed by titration with 0.01 M NaOH, whereas -SO₃H and -COOH groups were estimated with 0.05 M NaHCO₃ solution.

The surface properties were obtained by N₂ sorption using a Micromeritics ASAP-2020 equipment at -196°C with samples degassed at 100°C during 11 h. The specific surface area was determined with BET method. The pore volumes were estimated by single point adsorption in the relative pressure at P/P0 ≥ 0.99.

The morphology of the catalysts was acquired by scanning electron microscopy images (SEM) using a Philips SEM 505 microscope operated at 25 eV. The main elements present in the samples after the functionalization methods were obtained using energy dispersive x-ray spectroscopy (EDX).

The surface chemistry was studied by Fourier transform infrared spectroscopy (FTIR) analysis on a Jasco 4200 instrument. The dried samples were mixed with KBr in 1:100 ratio and were scanned in the spectral range between 400 and 4000 cm^-1.

Figure 2: Curves of potentiometric titration.

Table 1. Results of the determination of acid sites and area BET.

Material	δ acid sites (mmol H⁺ g^-1)	Ei (mV)	S_BET (m²g^-1)	Vp (cm³g^-1)
SiO₂	--	46.1	257	1.13
SiO₂-HPA₂₈	--	232.2	238	0.99
SiO₂-HPA₃₅	0.07	520.8	198	0.82
SiO₂-HPA₄₀	0.05	462.6	192	0.79
Cst	0.4/ 0.25*	--	523	0.32
Cst-SO₃H	4.3/ 2.4*	--	34	0.03
Ccs-SO₃H	6.4/ 4.2*	--	2.6	0.001

^---: not determined,

^*acid sites determined with NaOH

^**acid sites determined with NaHCO₃.

Thermal stability of the catalysts was analysed in a thermobalance SDT 2960 Simultaneous DTA-TGA. The experiment was carried out under nitrogen flow at a rate of 100 mL min^-1 and using around 10 mg of sample. The temperature was studied between 30 to 700°C at a heating of 10°C min^-1.

D. Etherification of glycerol

The catalytic activity of the materials prepared was evaluated in the etherification of glycerol (Gly) reaction with benzyl alcohol (BA), under conditions optimized in previous work (Chiosso et al., 2020).

Experiments were performed in a glass batch reactor with magnetic stirring under N₂ atmosphere. They were carried out with a Gly: BA molar ratio of 3:1, at 120°C during 6 h and using a catalyst amount of 10 wt.% with respect to glycerol weight (catalyst/ Gly).

During of reaction time, micro samples were taken and analyzed by CG Varian CP-3800, provided with capillary column CP WAX 52 CB (30 m, 0.3 mm d.i.) and a flame ionization detector. The sample of reaction was mixed with butyl alcohol as internal standard and ethyl alcohol as solvent. The reaction compounds were identified by GC/ MS Shimadzu QP2010 Ultra with a capillary column SH-Rtx-5Sil MS (30 m, 0.25 mm d.i.).

III. RESULTS

The acid sites play an important role in the etherification of glycerol (Bozkurt et al., 2015). The results obtained for determination of acid site of synthesized materials are shown in Table 1. The potentiometric titration curves, with n-butylamine, obtained for SiO₂ and the SiO₂-HPAxx catalysts are presented in Fig. 2. The initial potential of electrode (Ei) indicates the maximum acid strength of the surface sites. The Ei suggest that the materials presented sites with very strong acid characteristics (Ei> 100 mV), according to the classification reported in literature (Osiglio et al., 2010; Ferreira et al., 2010).

For the carbon-based catalysts, a high density of total acid sites can be obtained after H₂SO₄ functionalization, resulting in a suitable method to increase the acidity of the synthesized materials (Sánchez et al., 2011). The relatively soft carbonization treatment used for the synthesis of Cst (200 and 450°C) also contributes to the acidity of the material, providing functional groups such as -COOH and -OH generated as products of the incomplete carbonization of the starting sugar (Boehm, 1994).

The N₂ adsorption/ desorption results of support, before and after its functionalization, also shown in Table 1. On both supports, SiO₂ and C can be seen that the functionalization method by impregnation (with HPA or H₂SO₄) generates a decrease in their surface area. These results agree with those published by other authors (Ferreira et al., 2010; Sánchez et al., 2011). It can also be seen that Na₂SiO₃ (used as "template" in Cst) plays a very important role in the surface characteristics of the support obtained (Han et al., 2003). The Ccs have a very low area (S_BET <10 m² g^{− 1}), but this is similar to the reported for materials synthesized with analogous methods (Mo et al., 2008).

Figure 4: FTIR spectra of SiO₂ and SiO₂-HPAxx.

Figure 5: TGA and DrTGA curves associated to mass loss of SiO₂-HPA₃₅ and SiO₂.

To analyse the morphological properties by SEM micrographs and EDX spectra, the materials SiO₂-HPA₃₅, Cst-SO₃H and Ccs-SO₃H were selected (Fig. 3). In the EDX spectrum of the HPA-containing material, the peak corresponding to P could be observed. This shows that the support has been successfully modified with H₃O₄₀PW₁₂ xH₂O. On the other hand, in the Cst–SO₃H and Ccs–SO₃H EDX measurements the presence of S was observed, confirming the existence of -SO₃H groups on both catalysts.

The FTIR spectra of SiO₂ and SiO₂-HPAxx are shown in Fig. 4. The solid IR spectrums were recorded using the KBr disk technique. For silica (SiO₂), the major peaks are broad antisymmetric Si–O–Si stretching from 1200 to 1000 cm^-1 and the symmetric Si–O–Si stretching near 800 cm^-1 (Shaterian et al., 2008). The systems SiO₂-HPAxx, have typical bands of the Keggin-anion structure [PW₁₂O₄₀]^3-: 1080 cm⁻¹ (P–O), 984 cm⁻¹ (W=O), 898 cm⁻¹ (W–O–W in corner shared octahedral) and at 812 cm⁻¹ (W–O–W in edge shared octahedral). However, some bands of HPA Keggin structures, are overlapped or partially overlapped with the bands of the silica matrix (Da Silva et al., 2019; Ferreira et al., 2010; Obali et al., 2008). The spectra also show, around 3450 cm^-1, a broad stretching absorption band of the -OH group.

The FTIR spectra of the systems Cst-SO₃H and Ccs-SO₃H have also been obtained, but were already published in previous works (Chiosso et al., 2020; Chiosso

Figure 6: Conversion of BA and selectivity obtained with different SiO₂-HPAxx catalysts. Reaction conditions: Gly: BA molar ratio 3:1, 10 wt.% of catalyst, 120ºC and 6 h reaction time.

et al., 2021) so they are not shown in this paper. In both spectra, characteristic bands of -SO₃H groups were observed. These bands confirm that the sulfonic groups were successfully introduced onto the carbonaceous materials. Furthermore, theses solids are rich in functional groups such as phenolic hydroxyl group, carbonyls and carboxylic, originated during the synthesis.

Figure 5 shows a thermogravimetric analysis of the SiO₂ support and a catalyst (SiO₂-HPA₃₅). In this figure, the mass loss (wt.%) as a function of temperature (TGA curve) and the first derivative of the TGA curve (DrTGA) for each sample, is observed.

The support has only one process of mass loss, representing less than 5%, which starts at low temperature (30ºC) and ends at approximately 120ºC. This process can be attributed to the loss of absorbed water. For the SiO₂-HPA₃₅ catalyst it is observed that the process of weight loss occurred gradually with an increase in the temperature. These phenomena resulted in two peaks below 200°C in the curves of DrTGA. The first is assigned to loss of adsorbed water (ca. < 120°C) and the second coordinated water (Da Silva et al., 2022). Obali and Dogu (2008) observed that decomposition process begins above 180°C, therefore, it can be stated that the system will not undergo thermal decomposition during the etherification reaction.

The results of the catalytic performance of the SiO₂-HPAxx materials are presented in Fig. 6. The influence of acidity can be observed, both on the conversion of BA and on the selectivity towards the Gly etherification products. The raised in the acidity, from 28 to 35% of HPA in the support impregnation solution, generated an increase in the alcohol conversion, reaching of 94% for SiO₂-HPA₃₅. However, when the percentage of HPA varied from 35 to 40%, there were neither significant change in the conversion of BA and not improvements in the distribution of products, was observed. Furthermore, by increasing the HPA amount the formation of the self-condensation product of alcohol (BE), was favoured, and it is seen an increment in the formation of other by-products that were identified as 2-benzyl-benzyl alcohol and 1, 2-diphenylethane (Chiosso et al., 2021). This can be explained,

Figure 7: Conversion of BA and selectivity obtained with SiO₂-HPA_35,Cst-SO₃H and Ccs-SO₃H. Reaction conditions: Gly:BA molar ratio 3:1, 10 wt.% of catalyst, 120ºC and 6 h reaction time.

considering the pKa values of Gly (14.15) and BA (15.40). The lower acidity of BA favours its protonation, even with low concentrations of H⁺, versus Gly. Accordingly, the protonation of BA is produced, and when it loses a water molecule a carbocation is formed, which is in turn attacked by a OH− of another BA molecule.

On the other hand, the three catalysts render the preferential formation of ME₁, due to the fact that the etherification of glycerol is sterically preferred in the primary −OH groups, because BA is a voluminous molecule (Gonçalves et al., 2015; Jaworski et al., 2015).

Further, the catalytic behaviour of the SiO₂-HPA₃₅ (which showed the best performance) was compared with Cst-SO₃H and Ccs-SO₃H catalysts, previously studied in our research group, under the same reaction conditions (Chiosso et al., 2020; Chiosso et al., 2021). In Fig. 7, it can be seen that the SiO₂-HPA₃₅ has a similar performance to that obtained with the carbonaceous systems, with conversions higher than 90% and selectivity between 70-80% to one of the searched products (ME₁). Therefore, it can be affirmed that, although the density of acid sites in the SiO₂-HPA₃₅ system (see Table 1) is much lower than of carbon-based catalysts, these sites are strongly acidic and effective for the etherification of Gly with BA.

Finally, if compared with some catalysts reported on glycerol etherification using benzyl alcohol, the results obtained in this work are better than those. Gu et al. (2008) obtained only 12% yield, with the C–SO₃H system, and Tekale et al. (2021) showed 35% of conversion of Gly with hexahydrate dodecatungstophosporic acid (DTP)/ Montmorillonite- K-10 (20% w/w) as catalyst. For the both systems, the main product was ME, with 89 and 70% of selectivity, respectively.

IV. CONCLUSIONS

The catalytic behaviour of the synthesized materials, SiO₂-HPAxx, was studied in the etherification of Gly with BA. The system SiO₂-HPA₃₅ was that showing the best catalytic performance. It was highly active (94%) and selective, at 120ºC and Gly: BA molar ratio 3:1. The high activity can be attributed to sites with very strong acid characteristics, which were similar to carbonaceous systems functionalized with sulfonic groups (Cst-SO₃H and CcsSO₃H). For every system, the main reaction product was 3-benzyloxy-1,2-propanediol (ME₁), and selectivity between 70 and 80% was obtained.

Therefore, the SiO₂-HPA₃₅ is interesting as catalyst for the glycerol etherification reactions, used instead of conventional acid catalysts (such as sulfuric acid) and a promising alternative for obtained value-added chemicals.

Acknowledgement

This work was supported by the Universidad Nacional de La Plata (Projects X757 and X903), ANPCyT (PICT 2016-0148 and PICT 2019-1962), UNNOBA (proyect SIB 2019, Exp.0596/2019) and CICPBA (proyect (PIT-AP-BA, Res. Nº428/ 2016) from Argentina. The authors are grateful to Pablo Fetsis, Laura Barbelli, and Dr. A. Sosa, CINDECA technicians, for BET, GC/ MS and FTIR analysis, respectively.

REFERENCES

Atia, H., Armbruster, U. and Martin, A. (2008) Dehydration of glycerol in gas phase using heteropolyacid catalysts as active compounds. J. Catal. 258, 71–82.

Ben, Z.Y., Samsudin, H. and Yhaya, M.F. (2020) Glyc-erol: Its properties, polymer synthesis, and applications in starch based films. Eur. Polym. J. 175 111377.

Boehm, H.P. (1994) Some aspects of the surface chemistry of carbon blacks and other carbons. Carbon. 32, 759-769.

Bozkurt, O.D., Tunç, F.M., Bağlar, N., Çelebi, S., Günbaş, I.D. and Uzun, A. (2015) Alternative fuel additives from glycerol by etherification with isobutene: Structure–performance relationships in solid catalysts. Fuel Process. Technol. 138, 780–804.

Cespi, D., Passarini, F., Mastragostino, G., Vassura, I., Larocca, S., Iaconi, A. Chieregato, A., Dubois, J.-L. and Cavani, F. (2015) Glycerol as feedstock in the synthesis of chemicals: a life cycle analysis for acrolein production. Green Chem. 17, 343–355.

Chiosso, M.E., Lick, I.D., Casella, M.L. and Merlo, A.B. (2020) Acid functionalized carbons as catalyst for glycerol etherifcation. Braz. J. Chem. Eng. 37, 129–137.

Chiosso, M.E., Casella, M.L. and Merlo, A.B. (2021) Synthesis and catalytic evaluation of acidic carbons in the etherification of glycerol obtained from biodiesel production. Catal. Today. 372, 107-114.

Dahdah, E., Estephane, J., Gennequin, C., Aboukaïs, A. and Aouad, S. (2020) Effect of La promotion on Ni/Mg-Al hydrotalcite derived catalysts for glycerol steam reforming. J. Environ. Chem. Eng. 8, 104228.

Da Silva, M.J., Julio, A.A., Ferreira, S.O., da Silva, R.C. and Chaves, D.M. (2019) Tin(II) phosphotungstate heteropoly salt: An efficient solid catalyst to synthesize bio additives ethers from glycerol. Fuel. 254, 115607.

Da Silva, M.J, Chaves, D.M., Ferreira, S.O, Da Silva, R.Ch., Filho, J.B.G., Oliveira Bruziquesi, C.G., and Al-Rabiah, A.A. (2022) Impacts of Sn(II) doping on the Keggin heteropolyacid-catalyzed etherification of glycerol with tert-butyl alcohol. Chem. Eng. Sci. 247, 116913.

Faroppa, M.L., Musci, J.J, Chiosso, M.E. Caggiano, C.G., Bideberripe, H.P., García Fierro, J.L., Siri, G.J. and Casella, M.L. (2016) Oxidation of glycerol with H₂O₂ on Pb‐promoted Pd/γ‐Al₂O₃ catalysts. Chin. J. Catal. 37, 1982–1990.

Ferreira, P., Fonseca, L.M., Ramos, A.M., Vital, J. and Castanheiro, J.E. (2009) Glycerol acetylation over dodecatungstophosphoric acid immobilized into a silica matrix as catalyst. Appl. Catal. B: Env. 91, 416–422.

Ferreira, P., Fonseca, I. M., Ramos, A. M., Vital, J. and Castanheiro, J. E. (2010) Valorisation of glycerol by condensation with acetone over silica-included heteropolyacids. Appl. Catal. B: Env. 98, 94–99.

Gonçalves, M., Castro, C.S., Oliveira, L.C.A and Car-valho, A. (2015). Green acid catalyst obtained from industrial wastes for glycerol etherification. Fuel Process. Technol. 138, 695–703.

Gu, Y., Azzouzi, A., Pouilloux, Y., Jérôme, F. and Barrault, J. (2008) Heterogeneously catalyzed etherification of glycerol: new pathways for transformation of glycerol to more valuable chemicals. Green Chem. 10, 164–167.

Han, S., Kim, M. and Hyeon, T. (2003) Direct fabrication of mesoporous carbons using in-situ polymerized silica gel networks as a template Carbon. 41, 1525-1532.

Jaworski, M.A., Vega, S., Siri, G.J., Casella, M.L., Romero Salvador, A. and Santos López, A. (2015) Glycerol etherifcation with benzyl alcohol over sulfated zirconia catalysts. Appl Catal: Gen. 505, 36–43.

Kong, P.S. Aroua, M.K. and Daud, W.M.A.W. (2016) Conversion of crude and pure glycerol into derivatives: A feasibility evaluation. Renew. Sustain. Energy Rev. 63, 533–555.

Kubota, M., Sakamoto, A., Komatsu, M., Maeno, K. and Masuyama (2014) Selective Preparation of Monobenzyl Glyceryl Ethers by the Condensation Reaction of Glycerol with Benzyl Alcohols in the Presence of Zeolite Catalysts. A. J. Oleo Sci. 63, 1057–1062.

Mo, X., Lopez, D.E., Suwannakarn, K., Liu, Y., Lotero, E., Goodwin Jr., J.G., and Lu, C. (2008) Activation and deactivation characteristics of sulfonated carbon catalysts. J. Catal. 254, 332–338.

Obali, Z. and Doǧu, T. (2008) Activated carbon–tungstophosphoric acid catalysts for the synthesis of tert-amyl ethyl ether (TAEE). Chem. Eng. J. 138, 548–555.

Osiglio, L., Romanelli, G. and Blanco, M. (2010) Alcohol acetylation with acetic acid using borated zirconia as catalyst. J. Mol. Catal. A-Chem. 316, 52–58.

Pizzio, L.R., Vázquez, P.G., Cáceres, C.V. and Blanco, M.N. (2003) Supported Keggin type heteropoly compounds for ecofriendly reactions. Appl. Catal. A: General. 256, 125–139.

Sánchez, J.A., Hernández, D.L., Moreno, J.A., Mondragón, F. and Fernández, J.J. (2011) Alternative carbon-based acid catalyst for selective esterification of glycerol to acetylglycerols. Appl. Catal. A-Gen. 405, 55– 60.

Shaterian, H.R., Ghashang, M. and Feyzi, M. (2008) Silica sulfuric acid as an efficient catalyst for the preparation of 2H-indazolo[2,1-b] phthalazine-triones Appl. Catal. A-Gen. 345, 128–133.

Shikhaliyev, K. Hameed, B.H. and Okoye, P.U. (2021) Utilization of biochars as sustainable catalysts for upgrading of glycerol from biodiesel production. J. Environ. Chem. Eng. 9, 104768.

Tekale, D.P., Yadav, G.D. and Dalai, A.K. (2021) Solvent-Free Benzylation of Glycerol by Benzyl Alcohol Using Heteropoly Acid Impregnated on K-10 Clay as Catalyst. Catalysts. 11, 1-16.

Received: September 5, 2022

Sent to Subject Editor: September 5, 2022

Accepted: October 3, 2022

Recommended by Subject Editor Laura Briand