**1. Introduction**

Biodiesel is obtained predominantly by transesterification (chemically or enzymatic) of vegetable oils and animal fats and is known for its energy security awareness. It is composed of free fatty acid alkyl esters and has low toxicity and high biodegradability [1]. Biofuels are a clean energy source, whose combustion emits ≈ 35% fewer greenhouse gases compared with diesel fuel [1,2]. However due to technological limitations, biodiesel's cost is still higher than fossil diesel [1].

The transesterification of a variety of materials that contain fatty acids that include various vegetable and animal fats, vegetable oils, and edible oil processing residues such as soybean [3], sunflower [4], palm [5], rapeseed [6], canola [7], Jatropha, and cottonseed [8–10] generates a large amount of glycerol waste. For instance, for each 10 kg of biodiesel produced, 1 kg of glycerol is generated [11]. The increase of biodiesel production has, as a consequence, an increase of glycerol production. Solutions for this glycerol must be found [12–15] to ensure the economic feasibility of biodiesel. Yet this crude glycerol cannot be used in most of the traditional applications of glycerol such as food [16–19] and pharmaceutical industries [20–23], personal care products [24–26], anti-freezers [27,28], e-cigarette liquids [29–31], explosives [32] and many other processes as an intermediate compound [33–35] due to its poor quality. Additionally, the use of crude glycerol has been gaining ground as a component of heavier fuels and in the processes of obtaining acrylic acid [36]. A much less explored possible use of crude glycerol concerns the development of new solid materials generating high-value products. Coal is traditionally produced from waste, thus adding value to the residue [37,38]. However, the use of glycerol as a precursor in the preparation of carbon materials (carbon and activated carbon, Figure 1) is

**Citation:** Batista, M.; Carvalho, S.; Carvalho, R.; Pinto, M.L.; Pires, J. Waste-Glycerol as a Precursor for Carbon Materials: An Overview. *Compounds* **2022**, *2*, 222–236. https://doi.org/10.3390/ compounds2030018

Academic Editor: Juan C. Mejuto

Received: 23 June 2022 Accepted: 9 September 2022 Published: 16 September 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

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relatively new and unexplored in comparison with other precursors such as rapeseed [37], potato peel [38,39], sugarcane bagasse [40,41], waste coffee residues [42,43], waste rice husk [44,45], and waste corn [46,47], etc. The glycerol-based carbon materials are obtained in one step by in situ partial carbonization and sulfonation of glycerol with sulfuric acid. In turn, glycerol-based activated carbon materials are prepared in two steps: (i) partial carbonization and sulfonation of glycerol in the presence of sulfuric acid; (ii) chemical or thermal activation of the glycerol-based carbon material. The main advantages of the coal obtained from glycerol are the sulfonic acid groups on the material surface, which give them specific properties for diverse environmental applications such as catalyst, capacitor, and adsorbent materials. This review collected the existing data about the production and use of carbons from glycerol. Despite their potential, most of the research is not contemporary. We intended to show the potential and arouse the interest for this type of material, thus allowing the development of more effective glycerol-based carbons. The synthetic method of the glycerol-based carbons investigated is the same with few variations in the experimental conditions. More in-depth research into the synthesis may lead to carbons with improved properties for the desired applications. The use of these carbons in catalysis is the most explored application. Still, activated carbons have much potential as adsorbents of pollutants and capacitors, ye<sup>t</sup> they are unexplored, and new research is needed, as may be seen in this review. In this work, we intended to call the attention of scientists in the area by showing the potentialities of this by-product in developing advanced carbon materials and promote research in an area with grea<sup>t</sup> potential.

**Figure 1.** Synthesis of glycerol-based carbons and their main applications.

#### **2. Synthesis of Glycerol-Based Carbon Materials**

The synthesis of carbons from glycerol is a relatively new area concerning the use of glycerol. It mainly consists of the partial carbonization and sulfonation of glycerol with sulfuric acid. The main differences observed in the literature concerning the synthesis of carbons are variations in experimental conditions such as glycerol: sulfuric acid mass ratio, reaction temperature, and time. The paper of Devi et al. published in 2009 [48] was the first to synthesize carbon from glycerol. It used the one-pot reaction shown in Figure 1. The partial carbonization and sulfonation of glycerol were carried out with concentrated sulfuric acid (1:4 w/w) using soft experimental conditions. First, the glycerol and sulfuric acid mixture was heated to 180 ◦C. The mixture was kept at this temperature until foaming ceased. Then, the product was cooled to room temperature and washed with hot water under agitation until reaching neutral pH value. Two years later, the same authors synthesized other carbon using the same procedure but used pitch glycerol as a carbon source and at a different temperature (250 ◦C) [49]. The reaction yields were 50% and 40%, respectively, and no explanation for why a higher temperature had to be used for this last procedure was presented. The carbons were fully characterized by a grea<sup>t</sup> diversity of techniques (elemental analysis, X-ray photoelectron spectroscopy (XPS), X-ray Powder Diffraction (XRD), scanning electron microscopy (SEM), Fourier Transform InfraRed (FTIR), Magic-angle spinning (MAS) NMR 13C, Raman, potentiometric titrations, N2 isotherms, and thermogravimetry/differential thermal analysis (TG/DTA). The obtained carbons had a non-porous nature (<1 <sup>m</sup>2·g<sup>−</sup>1), a high density of sulfonic acid groups (–SO3H), and their catalytic capacity in the esterification of palmitic acid, tetrahydropyranylation, and dehydropyranylation was evaluated (See Section 3.1 for more details).

Mantovanic et al. [50] and Gonçalves et al. [51] prepared carbons by hydrothermal carbonization using a mixture of glycerol waste and sulfuric acid (different mass ratio) at 150 ◦C or 180 ◦C and using several reaction times (0.25–24 h). Interestingly, the authors successfully increased the number of sulfonic groups using sulfuric acid in a post-synthesis treatment [51]. The carbons also had a non-porous nature and a high numbers of acidic surface groups and were tested as catalysts in acetalization and etherification reactions (see Section 3.1 for more details).

The synthetic procedure described gives rise to non-porous carbons whose main application is in catalysis. For other applications, such as adsorption, the development of a porous structure is crucial. Typically, the synthesis of activated carbons from glycerol requires two steps, carbonization, followed by an activation step [52–54]. Some examples of those activated carbons obtained via chemical activation (KOH, ZnCl2, and H3PO4) [52,53] and thermal activation [55,56] may be found in the literature. Different activation agen<sup>t</sup> ratios and temperatures have been tested to vary the porosity of the obtained materials. For instance, the group of Ribeiro et al. [56], after obtaining the carbon using the already described procedure, carried out further calcination (120 ◦C, 400 ◦C, 600 ◦C—60 min in each temperature, plus 800 ◦C—240 min) under nitrogen flow. The obtained material showed high thermal stability, a basic character due to the decomposition of the sulphonic acid groups, and non-porous nature. This material was then thermally activated under an air atmosphere at different temperatures (150 ◦C, 200 ◦C, 300 ◦C, and 350 ◦C) for 1 h and generated porosity which increased with the temperature as shown in Figure 2. Additionally, the increase in temperature in the surface oxygen groups (lactones, phenols, and quinones) increases its acid character. Although rare, this work used activated carbons in the catalytic wet peroxide oxidation (CWPO) of 2-nitrophenol (See Section 3.1 for more details).

**Figure 2.** Influence of the activation temperature in the generation of porosity and development of microporosity.

Another way of obtaining porous carbons may be using a pore-forming agent. The work of Lee et al. [57] used a pure glycerol and crude waste glycerol as a carbon precursor for mesoporous carbon. It also explored glycerol as a pore-forming agen<sup>t</sup> for mesoporous silica. The mesoporous carbon was obtained by the carbonization of glycerol–silica nanoparticles at high temperatures (600 ◦C) under a nitrogen atmosphere. NaOH solution was used for the removal of the silica–nanoparticle framework. By simply changing the silica particle size in glycerol–silica nanocomposites or changing the silica particle size, it was possible to tailor the pore size and volume, surface area, and pore wall thickness of mesoporous carbon. Due to the presence of other components that may also act as a pore-forming agen<sup>t</sup> in the crude waste glycerol, its use in the synthesis leads to a multimodal pore size distribution of micropores smaller than 2 nm, small mesopores centered at 3.8 nm, and large mesopores above 10 nm.

To the best of our knowledge, the work published in 2016 by Álvarez-Torrellas et al. was the first to study the application of glycerol-based activated carbons as adsorbent materials [55]. The synthesis consisted in the partial carbonization of a glycerol– sulfuric acid mixture, followed by thermal activation. A glycerol and sulfuric acid mixture was heated to 180 ◦C for 20 min. The resulting material was calcinated in a tube furnace under a nitrogen flow (100 cm3·min−1) at different temperatures (120 ◦C, 400 ◦C and 600 ◦C) during 60 min and 800 ◦C during 240 min. Then the calcined material (GBCM) was thermally activated at different temperatures (200 ◦C, 300 ◦C and 350 ◦C, for 60 min) in a tube furnace under oxidative atmosphere (flow of 100 cm3·min−1). The textural properties were studied by N2 adsorption-desorption isotherms. The presence of the oxygenated groups was investigated by zeta potential and FTIR data. All obtained acid activated carbons (GBCM200, GBSM300 and GBCM350—where the subscript represents the activation temperature) presented high surface area and microporous structure developed. Their adsorption capacities were evaluated through flumequine and tetracycline (See Section 3.2 for more details).

Cui et al. [54] investigated glycerol as a liquid precursor for the preparation of activated carbon. The authors concluded that glycerol pyrolysis in the absence of acid generates no carbon material. This was justified by the evaporation of glycerol (boiling point of 290 ◦C) before it was carbonized. The description of different acids' roles and the absence of acid in carbon formation was reported. For this effect, glycerol was mixed with an acid (H2SO4, H3PO4, HCl, or CH3COOH) at volume ratios (10:1, 10:2 and 10:3 v/v). The solutions were added to a quartz boat and heated on the tube furnace in N2 atmosphere to 400 ◦C, 500 ◦C, 600 ◦C, 700 ◦C, or 800 ◦C for 1 h. The glycerol pyrolysis in the presence of HCl or CH3COOH did not produce carbon material. However, with H2SO4 or H3PO4 addition, glycerol pyrolysis generated carbon materials. According to the authors, the glycerol is dehydrated and polymerized when exposed to the presence of acids (H2SO4 or H3PO4) at moderate temperatures (<200 ◦C). Both acids induce dehydration of alcohol groups via protonation of the alcoholic oxygen (Figure 3).

**Figure 3.** General representation of the polymerization reaction.

In the case of HCl, it has a low boiling point (48 ◦C), and in the case of CH3COOH, because it is a weak acid, it cannot initiate the glycerol dehydration. In this context, carbon materials with various functional groups and porosities were prepared via sulfuric or phosphoric acid-mediated polymerization and carbonization followed by steam or CO2 activation. The porosity in the activated carbons reached surface areas up to 2470 <sup>m</sup>2·g<sup>−</sup><sup>1</sup> and pore volumes up to 1.44 cm3·g<sup>−</sup>1. The samples prepared with H3PO4 were consistently more mesoporous than samples prepared with H2SO4. The adsorption capacity of those materials was evaluated for the removal of gas phase volatile organic compounds (VOCs) and aqueous phase chromium Cr(VI) (See Section 3.2 for more details).

Naverkar et al. [58] prepared glycerol-based carbon by partial carbonization of glycerol using concentrated sulfuric acid (molar ratio 1:4) followed by thermal treatment. The sulfuric acid was added dropwise to glycerol (10 g) and stirred for 20 min at 180 ◦C. The carbonized material was further treated at 120 ◦C and 350 ◦C to obtain the samples GBC-120 and GBC-350. The carbon materials were characterized by XRD, FTIR, thermal analysis (TG/DTG/DTA), pHPZC measurements, SEM, and N2 adsorption-desorption at low temperature. The samples GBC-120 and GBC-350 presented BET surface areas of 21 <sup>m</sup>2·g<sup>−</sup><sup>1</sup> and 464 <sup>m</sup>2·g<sup>−</sup>1, respectively. They were studied for the adsorption of methylene blue (See Section 3.2 for more details).

Gonçalves et al. [53] also prepared glycerol-based activated carbon via two steps (polymerization + chemical activation). Firstly, the glycerol polymer was prepared by glycerol polymerization under reflux in the presence of sulfuric acid. The glycerol polymer was chemically activated with ZnCl2 or H3PO4. The authors also investigated several activated carbon synthesis conditions such as the type of activating agen<sup>t</sup> (ZnCl2 or H3PO4), the impregnation ratio (ZnCl2 (*X*Zn = 0.4 and 0.8) and H3PO4 (*X*P = 0.3 or 0.6) activating agen<sup>t</sup> mass/polymer mass), activation time, and temperature. They were evaluated as supercapacitor electrode and for the adsorption of organic contaminants (See Sections 3.2 and 3.3 for more details).

Glycerol-based magnetic carbon composites were synthesized by Medeiros et al. [59]. The carbon composites were prepared by mixing glycerol waste and iron(III) salt (heating to 380 ◦C, 600 ◦C, or 800 ◦C) for 3 h in a vertical reflux reactor. The textural properties of GFe3-380 (5 <sup>m</sup>2·g<sup>−</sup>1), GFe3-600 (140 <sup>m</sup>2·g<sup>−</sup>1), and GFe3-800 (136 <sup>m</sup>2·g<sup>−</sup>1) composites were evaluated by N2 adsorption–desorption isotherms. The carbon composites presented the following surface area 5 <sup>m</sup>2·g<sup>−</sup><sup>1</sup> (GFe3-380), 140 <sup>m</sup>2·g<sup>−</sup><sup>1</sup> (GFe3-600) and 136 <sup>m</sup>2·g<sup>−</sup><sup>1</sup> (GFe3-800). According to the pore size distribution for composites, the samples GFe3-600 and GFe3-800 contain both micropores and mesopores (essentially). The composites (GFe3-600 and GFe3-800) were tested as adsorbents of dyes (methylene blue and indigo carmine) (See Section 3.2 for more detail).

More recently, Batista et al. [52] prepared a series of glycerin-activated carbons from crude glycerin (82% glycerol) for application in the gas separation by adsorption processes. Glycerin-activated carbons were prepared via a two-step procedure involving carbonization followed by chemical activation with KOH. A mixture of industrial crude glycerin (82% glycerol) and concentrated sulfuric acid was prepared using a volume ratio of 1:0.5 (glycerol:H2SO4). The acid carbonization process was carried out in a Teflon lined Hydrothermal Autoclave at 180 ◦C for 6 h in an oven. The carbonized (glycerin-char) was washed with distilled water until the washing was neutral and dried (100 ◦C); The obtained solid (glycerin-char, crushed to fine powder of dimension < 0.297 mm) was mixed with an activating agen<sup>t</sup> (KOH) in distilled water, been stirring for 2 h (at ambient temperature) and when dried (100 ◦C). It was used two activation temperatures (700 ◦C and 800 ◦C) and weight ratios (1:1, 2:1 and 3:1, KOH:glycerin-char). The mixture (activating agent:glycerin-char) was activated in a horizontal furnace Thermolyne 21100 (under N2 flow 5 cm3·s<sup>−</sup><sup>1</sup> and 10 ◦C·min−1·h−1). The glycerin-activated carbons were washed with distilled water until the washing was neutral and dried at 100 ◦C. The prepared samples (G@700/1, G@700/2, G@700/3 and G@800/1, G@800/2 and G@800/3—the 700/800 corresponds to the activation temperature and the 1, 2 and 3 to the KOH:glycerin-char ratio) presented high surface areas (1166–2150 <sup>m</sup>2·g<sup>−</sup>1) and pore volumes between 0.63 and 1.03 cm3·g<sup>−</sup>1. These glycerin-activated carbons were evaluated as adsorbents for the adsorption separation of ethane and ethylene (See Section 3.2 for more details).

In another work, Batista et al. [60] modified a glycerin-activated carbon and zeolite type A surfaces with chitosan. The purpose of this work was different from the other works presented here. It was to evaluate the potential of those materials as H2S donors for therapeutic application. The activated carbon (Gta@600) was prepared by a combination of acid carbonization with H2SO4 followed by thermal activation (in a nitrogen flow rate = 5 cm3·min−<sup>1</sup> at 600 ◦C for 1 h). The modification of the material surface was obtained by adding chitosan dissolved in acetic acid solution (1 wt%). to a suspension of Gta@600. The chitosan-based carbon (Gta@600Chi) was characterized (FTIR, SEM, XDR, Elemental analysis and N2 adsorption–desorption isotherms). The adsorption capacity of H2S by Gta@600 and Gta@600Chi was performed to evaluate their use as H2S donors (See Section 3.2 for more information).

#### **3. Principal Uses of Carbons from Glycerol**
