Conversion of Glycerol into Aromatic Compounds

#### **Chapter 4**

## Sustainable Synthesis of Pyridine Bases from Glycerol

*Israel Pala-Rosas, José L. Contreras, José Salmones, Ricardo López-Medina, Beatriz Zeifert and Naomi N. González Hernández*

#### **Abstract**

Catalytic processes have been developed to obtain pyridine bases from glycerol, either by direct conversion or with acrolein as an intermediate. When producing acrolein as an intermediate, the reaction may proceed in a single reactor at temperatures above 400°C in co-feeding with ammonia. A system of two interconnected reactors can also be used: one reactor performs the catalytic dehydration of glycerol to acrolein, while in the second reactor acrolein reacts with ammonia to form pyridine bases. Both processes require the use of solid acid catalysts, for which ZSM-5 zeolitebased catalysts are the most studied. In the direct reaction between glycerol and ammonia, the most active catalysts were Cu/HZSM-5 and the composite zeolite HZSM-5/11. In the two-step systems, the dehydration of glycerol to acrolein over a HZSM-5 zeolite modified by alkali treatment or over a HZSM-22 zeolite modified by an alkali-acid treatment as catalysts in the first reactor, in combination with a Zn impregnated acid-treated-HZSM-5 zeolite have shown to be efficient catalyst pairs for the synthesis of pyridine bases from glycerol in two-step processes. When using acrolein or acrolein diacetals, the most active catalysts were a 4.6%Cu–1.0%Ru/ HZSM-5 zeolite in the presence of hydrogen, and a ZnO/HZSM-5-At-acid zeolite.

**Keywords:** pyridine, picolines, alkylpyridines, lutidines, glycerol, acrolein, zeolite catalyst, ammonia

#### **1. Introduction**

Pyridine bases are a family of aromatic heterocyclic compounds of commercial interest since they found applications as solvents in organic reactions, and as precursors of drugs, polymers, insecticides, herbicides, dyes, and adhesives, being pyridine, the series of picolines (α-, β-, and γ-picoline) and some lutidines the most important [1, 2].

The pyridine (azabenzene) structure is defined by a six-membered ring consisting of five carbon atoms and one nitrogen atom (**Figure 1**). It can be considered as an analog of benzene in which one CH group is replaced by a nitrogen atom [3]. On the other hand, the simplest alkyl-substituted pyridines are α-picoline (2-methylpyridine), βpicoline (3-methylpyridine), and γ-picoline (4-methylpyridine), whose structures vary

**Figure 1.** *Chemical structures of (a) pyridine, (b) α-picoline, (c) β-picoline, and (d) γ-picoline.*

according to the location where the methyl group is attached to the pyridine ring, either in position 2, 3, or 4 regarding the nitrogen atom. Physically, pyridine and picolines are considered to be dipolar, aprotic solvents, similar to dimethylformamide or dimethyl sulfoxide. They are colorless, flammable, irritating, toxic liquids with an unpleasant odor, and miscible in water and in most organic solvents [2, 3].

The chemical properties of pyridines are related to their structure, that is, ring aromaticity, presence of a basic ring nitrogen atom, π-deficient character of the ring, large permanent dipole moment, easy polarizability of the π-electrons, activation of functional groups attached to the ring, and presence of electron-deficient carbon atom centers at the α- and γ- positions. One or more of these factors can lead to different reactions of pyridine bases, namely electrophilic attack at nitrogen, electrophilic attack at carbon, nucleophilic attack at carbon or hydrogen, and free radical attack at carbon, besides varied substitution reactions of the carbon with N, O, S, halogen, or alkyl groups in the alkylpyridines [2].

The first industrial method for the production of pyridines was by their extraction from fossil sources such as coal tar, oil, and shale. However, it was only possible to obtain yields of less than 0.1% from a mixture of different pyridine bases and other organic compounds. In addition, the products obtained by this process had high sulfur contents, making it impossible to use them in pharmaceutical and agrochemical applications. So that, with a few exceptions, obtaining pyridine compounds by this method was expensive and had not been able to cover the industrial demand [3, 4].

Nowadays, the industrial synthesis of pyridine is based on the aminocyclization reaction (condensation/cyclization) between formaldehyde, acetaldehyde, and ammonia (NH3) using a ZSM-5 catalyst as shown in Eq. (1). This method generates a mixture of α-, β-, and γ-picoline as byproducts [1].

$$\text{2CH}\_3\text{CHO} + \text{H}\_2\text{C=O} + \text{NH}\_3 \xrightarrow[\text{}]{} \qquad \bigotimes\_{}^{\ast} \qquad \bigotimes\_{}^{\ast} \qquad \text{3-H}\_2\text{O} + \text{H}\_2\text{O} \qquad \text{(1)}$$

Also, the reaction between acrolein and ammonia has been used for the synthesis of β-picoline and pyridine by means of two parallel reactions, as shown in stoichiometric Eqs. (2a) and (2b). This process allows to modulate the products selectivity, avoiding the formation of α- and γ-picoline, being β-picoline the main reaction product [1].

*Sustainable Synthesis of Pyridine Bases from Glycerol DOI: http://dx.doi.org/10.5772/intechopen.111939*

Despite these synthesis methods, the high demand of pyridine bases has led to research on the use of different raw materials, such as aldehydes, ketones, and alcohols, from renewable sources, to improve the yield of a desired product [5, 6].

On the other hand, glycerol has gained importance as raw material in several catalytic processes, namely hydrogenolysis, dehydration, oxidation, and esterification, since it is industrially obtained as byproduct in the production of biodiesel from vegetable and algae oils [7]. Specifically, the aminocyclization reaction between glycerol and ammonia represents a potential alternative to the current industrial process for the synthesis of pyridine compounds, based on petroleum-derived aldehydes [8– 13]. In addition, the acrolein obtained by the catalytic dehydration of glycerol can also be used as feedstock for the production of pyridine bases [14–16].

In this context, this chapter presents the advances on the catalysts and reactor configurations employed for the synthesis of pyridine bases using glycerol and its derivatives, acrolein, and acrolein dialkyl acetals as raw materials.

#### **2. Synthesis of pyridine bases from glycerol in single-step processes**

The synthesis of pyridine compounds from glycerol, in batch and continuous onepot systems, has been reported, under pyrolysis or microwave heating conditions [8]. In the batch pyrolysis, pure glycerol and an ammonium salt, such as (NH4)2HPO4, NH4H2PO4, (NH4)2SO4, NH4Cl, NH4OAc, or H2NNH2H2SO4, were packed into a glass tubular reactor. The system was heated to a desired temperature and the reaction was carried out for about 1.5 h. The reason for using ammonium salts was to provide an acidic environment required for the conversion of glycerol to acrolein. Additionally, under thermal conditions, ammonium salts would decompose and release gaseous ammonia for its condensation and cyclization with the acrolein produced from glycerol. A mixture of pyridine and β-picoline was obtained and the highest product yield (36%) was reached when reacting glycerol with (NH4)2HPO4 at 450°C. However, glycerol also produced other volatile compounds, which polymerize with acrolein, resulting in tar formation and low product yields.

In the continuous system, an aqueous glycerol solution was fed to a tubular reactor, previously loaded with the ammonium salt and heated at 450°C. The best result (40% product yield) was obtained with (NH4)2HPO4 and a mixture of 1 g glycerol and 7.2 ml H2O. A mixture of pyridine, β-picoline, ethylpyridine, and ethylmethylpyridines was obtained, suggesting that during thermal degradation of glycerol, both acrolein and acetaldehyde were obtained as products. The low yield of pyridines was attributed to the uncontrolled formation and subsequent polymerization of acrolein at the reaction temperature [8].

For the microwave-assisted synthesis, glycerol and the ammonium salt were placed and closed into a glass vial, stirred, and subsequently irradiated by microwave energy to complete the reaction. The authors found that the addition of an organic acid, such as acetic acid, benzoic acid, or *p*-toluenesulfonic acid, improved the formation of pyridine bases [8].

On the other hand, the configuration of the continuous-flow fixed-bed reactor presented in **Figure 2** has been commonly used to evaluate the performance of solid catalysts for the gas-phase synthesis of pyridines from glycerol. The reactor, previously loaded with a certain amount of catalyst and heated at a required temperature, is fed at the top of the reactor with a gaseous stream composed of water, glycerol, and nitrogen (N2) as carrier gas. At the same time, a flow of preheated ammonia is introduced to react with the glycerol stream by effect of the catalyst. The stream at the reactor outlet contains the pyridine bases and byproducts.

The direct synthesis of pyridine bases from glycerol was performed in a continuous-flow fixed-bed reactor in presence of zeolite catalysts, using ammonia as carrier gas and as the reactive nitrogen source [9]. It was found that the optimal conditions were 550°C, a weight-hourly space-velocity (WHSV) of glycerol of 1 h<sup>1</sup> , a NH3/glycerol molar ratio of 12/1, and HZSM-5 zeolite (Si/Al = 25) as a catalyst. Total conversion of glycerol was reached with a total yield of pyridines around 35.6%. Pyridine was the main reaction product with a selectivity of 70.7%, while α-, β-, and γ-picoline exhibited selectivities of 8.6%, 17.8%, and 2.9%, respectively. Gaseous compounds, such as CH4, C2H4, C3H6, and CO, added a yield of 49.3%, and aromatics were produced at around 2.2% yield. After five reaction/regeneration cycles, a slight deactivation of the catalyst was observed.

*Sustainable Synthesis of Pyridine Bases from Glycerol DOI: http://dx.doi.org/10.5772/intechopen.111939*

The gas-phase aminocyclization between glycerol and ammonia in presence of a Cu/HZSM-5 catalyst has also been reported [10]. The reaction was performed in a fixed-bed reactor using a catalyst with 4.6% Cu supported on HZSM-5 zeolite with a ratio Si/Al = 38. The identified products were pyridine, α-picoline, β-picoline, acetonitrile, propionitrile, acetaldehyde, propylene, ethylene, and CO2. The best reaction conditions were 520°C, atmospheric pressure, a NH3/glycerol molar ratio of 7/1, and a gas-hourly space-velocity (GHSV) of 300 h<sup>1</sup> , reaching a total yield of pyridines around 42.8%, 34.9% of pyridine yield, 2.4% of α-picoline, and 5.5% of β-picoline yield, respectively.

The synthesis of pyridine bases from glycerol over a series of modified ZSM-5 zeolites in a continuous fixed-bed reactor was reported [11]. The catalysts tested were a series of metal oxide-impregnated ZSM-5 zeolite (ZnO, La2O3, and Fe2O3) , an alkali-treated zeolite (HZSM-5-At), and the alkali-acid treated ZSM-5 and ZSM-22 zeolites (HZSM-5-At-acid and HZSM-22-At-acid, respectively). The identified products in this process were pyridine, α-picoline, β-picoline, γ-picoline, small amounts of 3-ethylpyridine, 2-methyl-5-ethylpyridine, 3,5-dimethylpyridine, and benzene derivatives, as well as trace amounts of COx and C1 C2 hydrocarbons. At 425°C; LHSV = 0.60 h<sup>1</sup> , glycerol concentration of 36 wt%, molar ratio NH3/glycerol = 4/1, and time on stream (TOS) between 1 and 3 h, the HZSM-5-At-acid catalyst gave the highest total yield of pyridines (28.76%), with yields of pyridine, α-picoline, β-picoline, and γ-picoline around 15.67%, 1.90%, 10.02%, and 1.17%, respectively.

A composite zeolite HZSM-5/11 (SiO2/Al2O3 = 78) has been synthesized and employed as a catalyst in the reaction between an aqueous solution of 20 wt% glycerol and ammonia in a fixed-bed reactor [12]. The products obtained using this catalyst were pyridine, α-picoline, β-picoline, acetonitrile, propionitrile, acetaldehyde, C2H4, and C3H6. The analysis of the process variables on the synthesis of pyridines revealed that the optimal reaction conditions were a reaction temperature of 520°C, a molar ratio NH3/glycerol of 12/1, and a GHSV of 300 h<sup>1</sup> . At these conditions, glycerol reached total conversion and the total yield of pyridines was 40.8%, with selectivities of pyridine, α-picoline, and β-picoline around 27.7%, 2.6%, and 10.5%, respectively.

#### **3. Synthesis of pyridine bases from glycerol in two-step processes with acrolein as intermediate**

The conversion of glycerol to pyridine bases has also been carried out by first producing acrolein, and subsequently reacting it with ammonia [11, 13]. This process can be performed in a system with two coupled reactors. As shown in **Figure 3**, an aqueous solution of glycerol and a N2 flow are mixed, preheated/vaporized and fed to the first reactor, which was previously loaded with a solid acid catalyst. In this first stage of the process, the catalytic dehydration of glycerol to acrolein takes place at temperatures between 280°C and 350°C.

Subsequently, the stream of dehydration products is introduced to the second reactor, which is simultaneously fed with preheated ammonia to carry out the aminocyclization reaction between acrolein and ammonia to produce pyridine bases in presence of an acid catalyst at temperatures between 375°C and 475°C. Both reactors can be loaded with the same or different acid catalyst. According to literature, this process allows to improve the total yield of pyridines by performing separately the dehydration and aminocyclization reactions at adequate temperatures [11, 13].

**Figure 3.**

*Continuous flow reaction system for the two-step gas-phase conversion of glycerol and ammonia to pyridine bases. Adapted from ref. [11].*

Luo et al. [11] performed the synthesis of pyridines from glycerol in a two-step system comprised of a pair of reactors connected in series with different catalysts, denoted as a catalyst pair. The catalysts tested in the first reactor were the HZSM-5, HZSM-5-At, and HZSM-5-At-acid zeolites, while for the second reactor the HZSM-5-At-acid and the ZnO/HZSM-5-At-acid zeolites were evaluated. The reaction products identified in this process were pyridine, α-picoline, β-picoline, γ-picoline, 3 ethylpyridine, 2-methyl-5-ethylpyridine, 3,5-dimethylpyridine, benzene derivatives, trace amounts of COx, and C1 C2 hydrocarbons. The best results were obtained with the catalyst pair (HZSM-5-At + ZnO/HZSM-5-At-acid) at 330°C for the first reactor; 425°C for the second reactor; LHSV = 0.45 h<sup>1</sup> ; a glycerol concentration of 20 wt %; a molar ratio NH3/glycerol = 5/1; and TOS between 1 and 3 h, obtaining a total yield of pyridine bases around 62.25%, without the formation of γ-picoline.

Similarly, Zhang et al. [13] performed the conversion of glycerol to pyridine bases in a system of two series-connected reactors. The catalytic dehydration of a 20 wt.% glycerol aqueous solution to acrolein was accomplished at 280°C in the first reactor over an alumina (γ-Al2O3) catalyst modified with Fe and P. The output stream from the first reactor was fed to the second one, previously loaded with a Cu4.6Pr0.3/HZSM-5 catalyst, which reacted with ammonia to produce pyridine compounds. The identified products were pyridine, α-picoline, β-picoline, 2,4-lutidine, acetonitrile, propionitrile, ethylene, propylene, butylene, and CO2. At optimal reaction conditions, that is, 420°C in the second reactor, atmospheric pressure, GHSV = 300 h<sup>1</sup> , and NH3/ acrolein molar ratio of 7/1; glycerol achieved total conversion and the total yield of pyridines was 60.2%, while pyridine and β-picoline reached 39% and 20% yield, respectively. The impregnation of the zeolite with Cu and Pr resulted in the increase of the Lewis acidity and an improved dehydrogenation activity, enhancing the selectivity toward pyridine bases.

#### **4. Synthesis of pyridine bases from acrolein and acrolein derivatives**

Currently, the catalytic dehydration of glycerol, in the presence of a solid acid catalyst, is under research since it is considered a sustainable alternative method to the industrial process based on the partial oxidation of propylene for the synthesis of acrolein [14–16]. The use of acrolein for the synthesis of pyridine bases allows to modulate the products selectivity, enhancing the production of β-picoline and pyridine, without the formation of other pyridine compounds [3].

The synthesis of pyridine bases from acrolein in a batch process has been barely reported. The liquid-phase reaction of acrolein with ammonium acetate (CH3COONH4) over a SO4 <sup>2</sup>/ZrO2-FeZSM-5 catalyst was reported [17]. The process requires the addition of a C2-C6 carboxylic acid, preferably acetic acid (CH3COOH), as the reaction medium and solvent of acrolein. It was found that only β-picoline was generated, without the formation of any other pyridine compound. At the optimal conditions of 130°C as reaction temperature, a concentration of 14 wt% acrolein in the solution, a liquid flow of acrolein solution of 12 ml/h, and a catalyst usage of 0.7 g/g acrolein, the yield of β-picoline reached 60%. The presence of a carboxylic acid promoted the formation of β-picoline and retarded the polymerization of acrolein and its intermediate propylene imine.

Schematized in **Figure 4**, the gas-phase reaction between acrolein and ammonia in a continuous-flow fixed-bed reactor has also been reported [18–20]. Similarly, to the continuous single-step process described in Section 2, a preheated gaseous mixture of water, acrolein, and nitrogen is fed to the reactor previously loaded with a solid acid catalyst and heated to the reaction temperature. Simultaneously, a preheated flow of ammonia is introduced at the same end of the reactor as the acrolein stream, to come into contact with the catalytic bed and producing the pyridine bases.

#### **Figure 4.**

*Continuous-flow fixed-bed reactor for the gas-phase aminocyclization between acrolein and ammonia.*

This process has been studied with greater versatility than the direct reaction with glycerol, since the use of hydrogen as carrier gas has been explored, as well as the use of acrolein derivatives as raw material, specifically acrolein dialkyl acetals [21, 22].

The gas-phase reaction between acrolein and ammonia was studied by comparing the activity of a parent H-ZSM-5 catalyst and a series of zeolites modified with magnesium nitrate (Mg(NO3)2), hydrofluoric acid (HF), or both [18]. The reaction was performed at atmospheric pressure and 425°C in a fixed-bed reactor. When using the HF/MgZSM-5 catalyst, the total yield of pyridines achieved its maximum value (58.86%), with 30.38% being β-picoline and 26.59% being pyridine.

The use of hydrogen as a carrier gas in the synthesis of pyridine bases from acrolein and ammonia has been explored in presence of bimetallic copper-based ZSM-5 catalysts [19]. The identified products in this process were pyridine, α-picoline, β-picoline, 2,4 lutidine, acetonitrile, propionitrile, ethylene, propylene, butylene, and CO2. Among the tested catalysts, the 4.6%Cu–1.0%Ru/HZSM-5 zeolite produced the highest total yield of pyridines (69.4%) with pyridine and β-picoline yields around 27% and 37%, respectively, performing the reaction at 420°C, acrolein concentration of 20 wt.%, molar ratio NH3/acrolein = 3.5/1, GHSV = 300 h1, and H2 flow of 8.5 ml/min as optimal reaction conditions. The presence of Cu, Ru, and hydrogen enhanced the hydrogenation/dehydrogenation activity of the catalyst, promoting the conversion of acrolein to propionaldehyde, improving thus the formation of pyridine bases, notably of β-picoline. Additionally, the catalyst was stable with TOS, maintaining the total conversion of acrolein during 40 h, and decreasing gradually to 90.5% at 75 h of TOS.

The use of a catalyst other than ZSM-5 zeolite has been scarcely reported for the synthesis of pyridine bases from acrolein. Specifically, Y-type zeolites with different Si/Al composition has been reported as catalysts in the reaction between acrolein and ammonia at 360°C, pure acrolein, molar ratio NH3/acrolein = 2, and GHSV = 4994 h<sup>1</sup> [20]. The best catalytic performance was obtained with the catalyst with an atomic ratio Si/Al = 45. The acrolein conversion was around 93% while the pyridine and βpicoline yields were 15% and 19.1%, respectively. However, formaldehyde and acetaldehyde were also detected as reaction products and the catalysts were rapidly deactivated. It was found that the total acidity of the catalysts was the key factor for the conversion of acrolein and the type of acid sites influenced the products selectivity.

Acrolein dialkyl acetals have also been used as reagents for the synthesis of pyridines [21, 22]. When performing the gas-phase reaction between acrolein diethyl acetal and ammonia in a continuous fixed-bed reactor, the ZnO/HZSM-5 zeolite has shown superior catalytic performance than ZnO/HY and ZnO/α-Al2O3 catalysts. At 450°C; LHSV = 0.85 h<sup>1</sup> ; molar ratio of NH3/(acrolein diethyl acetal) = 4; TOS between 1 and 3 h; and 1 wt.% of Zn loading, the total yield of pyridines was 61.14%, with pyridine and β-picoline yields of 26.87% and 34.27%, respectively [21].

Similarly, the reaction between acrolein dimethyl acetal and ammonia was performed over a series of ZSM-5 zeolites treated with Mg(NO3)2, NH4F-HF, or both [22]. The reaction products were pyridine, α-picoline, β-picoline, and 3,5-dimethylpyridine. At 450°C, LHSV = 0.75 h<sup>1</sup> , molar ratio NH3/(acrolein dimethyl acetal) = 3.5/ 1, TOS = 1–3 h, and F/Mg-ZSM-5 catalyst with small particle size; the highest total yield of pyridines was 55.4%, with 14.5% for pyridine, 0.9% of α-picoline, 34.2% of β-picoline, and 5.8% yield of 3,5-dimethylpyridine. The catalytic activity of the catalyst was related to an adequate concentration of total acid sites and a ratio B/L < 1.

#### **5. The effect of catalyst acidic properties on the synthesis of pyridine bases from glycerol and acrolein**

The reaction conditions and catalytic performance of representative zeolite catalysts reported in the literature for the gas-phase synthesis of pyridine bases from glycerol, acrolein, and acrolein dialkyl acetals in fixed-bed reactors are presented in **Table 1**.

The main features that affect the catalytic performance of these catalysts in the synthesis of pyridine bases are their acidic properties, namely the type and strength of acid sites. In the single-step process, the reaction between glycerol and ammonia takes place through a complex network of simultaneous and subsequent reactions, among which the most relevant for the synthesis of pyridine bases are the catalytic dehydration of glycerol to acrolein, and its subsequent condensation/cyclization with ammonia to produce pyridine bases. Both reactions proceed at the same time in the single



*<sup>a</sup> gly = glycerol, acr = acrolein, <sup>b</sup> T = reaction temperature, <sup>c</sup> WHSV = weight hourly space-velocity, <sup>d</sup> GHSV = gas hourly space-velocity, <sup>e</sup> LHSV = liquid hourly space-velocity, <sup>f</sup> X = feedstock conversion, <sup>g</sup> Y = product yield,Total = total yield of pyridine bases, Py = yield of pyridine, αP = yield of α-picoline, βP = yield of β-picoline, γP = yield of γ-picoline, 2,4- L = yield of 2,4-lutidine, 3,5-DMP = yield of 3,5-dimethyl pyridine, <sup>h</sup> TOS = time on stream, <sup>i</sup> N.R. = not reported.*

#### **Table 1.**

*Reaction conditions and catalytic performance of zeolite catalysts during the synthesis of pyridine bases from glycerol, acrolein, and acrolein dialkyl acetals in continuous systems.*

catalytic bed [9–12]. In the two-stage systems, these reactions occurred independently in interconnected reactors [11, 13].

As presented in Eq. (3a), the production of acrolein from glycerol occurs primarily over the Brønsted acid sites (BAS) of the catalyst. However, other dehydration products are obtained, that is, the conversion of glycerol to acetol proceeds over Lewis acid sites (LAS) as in Eq. (3b), while formaldehyde and acetaldehyde can be produced from glycerol (Eq. 3c) or acetol, either over BAS or LAS. Depending on the type of catalyst and the reaction conditions, minor amounts of byproducts, such as aldehydes, ketones, carboxylic acids, and alcohols, in the range of C1–C3 may be obtained from the glycerol dehydration reaction [14, 15].

Furthermore, the reaction between acrolein and ammonia proceeds in acid medium, that is, a carboxylic acid in homogeneous reactions or over a solid acid catalyst in heterogeneous systems, mostly a ZSM-5 catalyst. Infrared spectroscopy and theoretical studies have demonstrated that acrolein can react with ammonia over LAS and BAS, producing propylene imine(prop-2-en-1-imine). This compound can undergo a Michael reaction over Brønsted or weak Lewis acid sites, condensing with another propylene imine, closing the ring structure, and producing β-picoline with the liberation of ammonia, as in Eq. (4a) [23, 24].

Additionally, as shown in Eqs. (4b) and (4c), the formation of pyridine takes place by a Diels-Alder reaction over strong Lewis sites, in which propylene imine condensates and cyclizes with acrolein or with another propylene imine, releasing CO or CH2NH, respectively [23, 24]. Similar reaction steps and imine intermediates have been reported in the reaction between formaldehyde, acetaldehyde, and ammonia obtaining pyridine, α-picoline, β-picoline, and γ-picoline as products [24, 25].

Experimental studies have revealed the importance of catalyst acidity on the synthesis of pyridine bases. The effect of the Si/Al molar ratio of HZSM-5 zeolites on the reaction between glycerol and ammonia has been reported [9]. The increase of the Si/ Al ratio from 25 to 80 decreased the total acidity from 580.6 to 92.4 μmol/g, resulting in the decrease of the total yield of pyridines with 26%, 22.85%, and 20.9% for Si/Al ratios of 25, 50, and 80, respectively. However, the pyridine selectivity increased from 67.3% to 69.3% and 72.3%, while the selectivity to β-picoline decreased from 21.4% to 17.9% and 16.1% in the same order.

The influence of acidity on the catalytic activity of a series of Cu/HZSM-5 zeolites, with Si/Al molar ratios of 25, 38, 50, 80, and 117, has been explored [10]. The total yield of pyridines increased from 40–43% with the change al Si/Al from 25 to 50. The further increase in the Si/Al ratio resulted in the decrease of the total yield of pyridines and pyridine. It was concluded that an appropriate proportion of BAS and LAS in the catalyst is a key factor for the synthesis of pyridine bases. However, the BAS/LAS ratio is not the only factor affecting the catalytic activity, but also the structure of HZSM-5 and the amount of Cu which enhanced the dehydrogenation/hydrogenation activity of the catalyst.

The acidity of a series of Mg- and HF-modified zeolites affected the gas-phase reaction between acrolein and ammonia [18]. The total yield of pyridines was 8.81%, 52.73%, and 58.86% for the MgZSM-5, HF/ZSM-5, and HF/MgZSM-5 catalysts, respectively. The MgZSM-5 zeolite presented a large quantity of acid sites and a low yield of pyridine bases, while the HF/ZSM-5 and HF/MgZSM-5 exhibited weaker and fewer acid sites and high yields of pyridine compounds. A certain concentration of

Brønsted acid sites and weak Lewis acid sites may promote the formation of β-picoline, while a high concentration of strong acid sites favored the synthesis of pyridine and polymers. It was concluded that the concentration and strength of acid sites promote the formation of pyridine bases, that is, proper amounts of BAS and weak LAS are necessary for the acrolein activation and pyridines formation, as well as the decrease in the formation of polymerization products (coke precursors).

A critical point of these processes is the catalyst deactivation by the deposition of carbonaceous compounds, which are formed by the polymerization of reaction products by the effect of the acid sites of the catalyst. Additionally, pyridine bases are strongly adsorbed on the acidic sites of the catalyst, being decomposed into carbon depositions [18, 20, 21]. In this sense, a proper amount and strength of acid sites can allow to control the polymerization reactions, and thus the catalyst deactivation.

In the direct reaction between glycerol and ammonia over molecular sieves, namely β-zeolite, MCM-41, and ZSM-5, the coke yield of the catalysts were 30.1%, 19.4%, and 13.7%, respectively, in agreement with the total acidity of the catalysts [9], as shown in **Figure 5**.

The comparison of HZSM-5 and ZnO/HZSM-5 catalysts in the reaction of acrolein diethyl acetal with ammonia, showed that both catalysts suffer a rapid deactivation resulting in the decrease of the total yield of pyridines. However, the ZnO/HZSM-5 zeolite, with a minor amount of total acid sites and a higher amount of weak acid sites than the HZSM-5 catalyst, exhibited the highest yield of pyridines even at longer TOS [21].

The characterization of the catalysts after reaction by 13C NMR, FTIR, and Raman spectroscopies has suggested that the coke formed on the zeolites (Y and ZSM-5) was constituted by aliphatic species with alkoxy groups, as well as large polyaromatic compounds, when using formaldehyde, acetaldehyde, acrolein, and acrolein diethyl acetal as reactants for the synthesis of pyridine bases [20, 26, 27].

#### **Figure 5.**

*Effect of the total acidity on the coke yield of molecular sieve catalysts in the reaction between glycerol and ammonia. Adapted from ref. [9].*

#### **6. Conclusions**

Pyridine bases can be obtained from glycerol and ammonia by an aminocyclization reaction, either in single-step or two-step processes. Glycerol derivatives, such as acrolein and acrolein dialkyl acetals, can also be used as raw materials for this reaction. The main process variables are the reaction temperature, the concentration of a reactant in water, the NH3/reactant molar ratio, and the space velocity in continuous fixed-bed reactors. When using glycerol as feedstock in the single-step process, the reactors operate usually at temperatures between 450°C and 550°C. The two-step process allows us to improve the total yield of pyridines by performing separately the dehydration and aminocyclization reactions at adequate temperatures, this is 280– 350°C and 375–450°C for the first and second reactor, respectively. Single-step processes with acrolein or acrolein dialkyl acetals require reaction temperatures between 420°C and 450°C. The catalysts for the revisited processes are based on ZSM-5 zeolite. The most active catalyst for the direct synthesis from glycerol is Cu/HZSM-5, while for the two-step process, the catalyst pair (HZSM-5-At + ZnO/HZSM-5-At-acid) exhibits higher activity and selectivity. When using acrolein, the most active catalyst is a 4.6%Cu-1%Ru/HZSM-5 zeolite with hydrogen as a carrier gas, while a 1%Zn/ HZSM-5 catalyst showed the highest yield of pyridines using acrolein diethyl acetal as raw material.

### **Acknowledgements**

The authors acknowledge the Instituto Politécnico Nacional and the Universidad Autónoma Metropolitana for their support to develop this investigation. I. Pala-Rosas thanks the support of the company Síntesis y Aplicaciones Industriales S.A.

### **Conflict of interest**

The authors declare no conflict of interest.

#### **Author details**

Israel Pala-Rosas<sup>1</sup> , José L. Contreras<sup>2</sup> \*, José Salmones<sup>1</sup> , Ricardo López-Medina<sup>2</sup> , Beatriz Zeifert<sup>1</sup> and Naomi N. González Hernández<sup>2</sup>

1 Higher School of Chemical Engineering and Extractive Industries, National Polytechnic Institute, Mexico City, Mexico

2 CBI-Energy, Autonomous Metropolitan University-Azcapotzalco, Mexico City, Mexico

\*Address all correspondence to: jlcl@azc.uam.mx

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Sustainable Synthesis of Pyridine Bases from Glycerol DOI: http://dx.doi.org/10.5772/intechopen.111939*

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[19] Zhang W, Duan S, Zhang Y. Enhanced selectivity in the conversion of acrolein to 3-picoline over bimetallic catalyst 4.6%Cu–1.0%Ru/HZSM-5 (38) with hydrogen as carrier gas. Reaction Kinetics, Mechanisms and Catalysis. 2019;**127**:391-411. DOI: 10.1007/ s11144-019-01558-0

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#### **Chapter 5**

## Catalytic Conversion of Glycerol to Bio-Based Aromatics

*Patrick U. Okoye, Estefania Duque-Brito, Diego R. Lobata-Peralta, Jude A. Okolie, Dulce M. Arias and Joseph P. Sebastian*

#### **Abstract**

Green application of biodiesel-derived glycerol will boost biodiesel production in terms of sustainability and economics. The glycerol to liquid fuels is a promising route that provides an additional energy source, which contributes significantly to energy transition besides biodiesel. This pathway could generate alkyl-aromatic hydrocarbons with a yield of ∼60%, oxygenates, and gases. MFI Zeolites (H-ZSM-5) catalysts are mainly used to propagate the aromatization pathway. This chapter presents the pathways, challenges, catalytic design, influences of catalyst acidity, metal addition, reaction condition, and catalysts deactivation on glycerol conversion to hydrocarbon fuels and aromatics. Studies revealed that time on stream, temperature, and weight hourly space velocity (range of 0.1–1 h−1) influences the benzene, toluene, and xylene BTX and benzene, toluene, ethylbenzene, and xylene BTEX yield. Acidity of the H-ZSM-5 could be tailored by metals, additives, and binders. Bronsted acidity promotes coke formation which results in reversible deactivation of the H-ZSM-5 catalyst. It is hoped that this study will promote intensified research on the use of glycerol for purposes of fuel generating and valuable products.

**Keywords:** BTX, BTEX, hydrodeoxygenation, hydrocarbon fuels, glycerol, biodiesel

#### **1. Introduction**

To achieve complete decarbonization, there is a need to transition from fossil fuels to renewable and sustainable alternative fuels. This is because fossil fuels are major sources of greenhouse gas emissions, which have contributed to rising earth temperatures and adverse climate change. Transportation sectors still rely heavily on fossil fuels and this trend must be discouraged to minimize the emission of noxious CO2 emissions [1, 2]. Liquid fuels generated from renewable energy sources, for use in heavy-duty vehicles are a promising option to reduce the impact of fossil fuels. Amongst the commercialized fuels from renewable energy, biodiesel has enjoyed wide acceptability because of lower CO2 and other greenhouse emissions, can directly be used in the current internal combustion engine without modifying the engine design, and requires facile production processes [3, 4]. More importantly, biodiesel can be obtained from natural sources including waste oil, second-generation nonedible seed oil, third-generation, microalgae and fungus, animal fats, etc. [5, 6].

The transesterification reaction of oils (triglycerides) and alcohol is a widely established route for the synthesis of biodiesel [7]. This route involves the reaction of a triglyceride and 3 mols of alcohol (preferably methanol) to produce 1 mol of fatty acid methyl ester (biodiesel) and glycerol as a byproduct. The glycerol produced in this process is about 10 g for every 100 g of biodiesel produced, which renders it abundant [8, 9]. The produced glycerol is cheap because of impurities of methanol, soap, and catalysts that require energy-intensive steps for purification [10, 11]. However, glycerol is a platform molecule with three hydroxyl groups which can be transformed into fuels and fine chemicals. Many reaction pathways such as acetylation [12], carboxylation [13], etherification [14], oxidation, dehydration, gasification, aromatization [15, 16], etc. have been adopted to valorize glycerol.

Conversion of glycerol to liquid hydrocarbon fuel is a very recent and notable research idea, which will boost the biodiesel process. This is because the hydrodeoxygenation and hydroisomerization of vegetable oils to green diesel is capital intensive. Hydrocarbon fuel is vital in energy transition because of its characteristics which include high density and ease of transportation. This liquid hydrocarbon contains mainly alkyl-aromatic hydrocarbons of cumene, xylene isomers, benzene, toluene, and traces of C9+ compounds [16–18].

The feed has a significant influence on the aromatic product distribution, coke formation, and the type of aromatic products obtained. When the compounds with a H/C ratio is less than 2, for instance, glycerol with 0.67, are used as feed, increasing coke formation on the catalyst is generally observed. Hence, dilution with water or alcohol is employed. The dilution of glycerol with a solvent that has a H/C ratio of 2 increases the H/C ratio and presents improved catalyst stability. This preferred solvent for dilution is methanol with an effective H/C ratio of 2 [19, 20]. The conversion of glycerol and methanol to gasoline has emerged as a promising route to valorize glycerol [21, 22]. Increasing the methanol/glycerol ratio from 10 to 40% favored the production of oxygenated compounds instead of aromatics. Also, when methanol/glycerol are used as feeds, the formation of trimethylbenzenes and xylenes is obvious. These compounds can be transformed into heavy C9 aromatics by dealkylation reaction to xylene and toluene (BTX). The use of higher alcohol like isopropanol and isobutanol for dilution results in the preferential generation of ethylmethylbenzenes, and ethylbenzene with xylenes and trimethylbenzenes (BTEX) from the alkylation of ethylene generated from the cracking of the alcohol or dehydration of ethanol [18].

Notably, benzene, toluene, and xylene (BTX) and/or benzene, toluene, ethylbenzene, and xylene (BTEX) have been the main focus of many researchers in this area of study because of the vast industrial applications of these aromatic compounds including their antiknocking characteristics [23]. In addition to the aromatic compounds, oxygenates such as propanal, hydroxyacetone, and propenal (acrolein) are the product of this process [24, 25]. Also, ethylene, methylene, and propylene gases could be derived from this process. The reaction is usually carried out in a fixed bed reactor at ~400°C under excess hydrogen, alcohols, and/or nitrogen gas flowrate at atmospheric pressure [18, 26, 27].

To promote this reaction, zeolites of ZSM-5 or protonated ZSM-5 have been widely used. However, the yield over these catalysts is generally low. To increase the catalytic activity, noble metals or transition metals have been functionalized on the ZSM-5 and used with remarkable performance [28, 29]. The noble metals of Pt, Pd, and Rh, promote the cleavage of C− O bonds of glycerol, which enhances the formation of hydrocarbons from polyols instead of H2 and CO2 gases [24, 30]. Another problem encountered

#### *Catalytic Conversion of Glycerol to Bio-Based Aromatics DOI: http://dx.doi.org/10.5772/intechopen.108148*

when ZSM-5 or H-ZSM-5 is used is rapid deactivation due to sintering, coking from reaction products, and attrition. This can be minimized by using a binder such as Al 2 O 3 [ 31 ]. The binder stops irreversible deactivation and prolongs the catalyst life. In all these, control of the reaction conditions such as the temperature, catalyst amount, and sometimes the atmosphere, greatly influence the yield of bio-based aromatics. The operating reaction conditions also determine the operating cost of the process.

 In this study, catalytic glycerol conversion to liquid hydrocarbon and bio-based aromatics are investigated with emphasis on the reaction mechanistic pathway and the influence of the reaction conditions such as temperature, time, and catalyst weight. The catalyst design, challenges, and deactivation mechanism are discussed. Prospects on bio-based aromatics are presented to reveal the knowledge gap and provide future guidelines for researchers and industries.

#### **2. Mechanism of glycerol conversion to biobased BTX and BTEX**

 The mechanism of glycerol to liquid hydrocarbon follows two main routes namely hydrodeoxygenation (HDO), followed by aromatization reaction. Hydrodeoxygenation is an established method of removing oxygen from biomass. Glycerol hydrodeoxygenation is usually carried out to synthesize 1,2-propanediol, which is an oxygenate compound used in pharmaceutical, tobacco, and cryogenic industries. This pathway occurs through simultaneous C-O bond cleavage and hydrogen addition [ 27 ]. Usually, hydrogen is added from an external source, however, recent studies have shown that *in situ* hydrogen generation from hydrogen donors such as methanol, formic acid, 2-butanol, ethanol, etc. could eliminate the need for external hydrogen [ 25 ]. The hydrodeoxygenation also follows two parts of hydration-hydrogenation and dehydrogenation-hydration-hydrogenation, which is the most common pathway. The dehydration happens on the acid catalyst sites, whereas the hydrogenation is catalyzed by the noble metals or Cu, Zn, Ni, Sn, etc. This is because these metals promote the aqueous phase reforming of glycerol to

#### **Figure 1.**

 *Reaction pathways for glycerol dehydration-hydrogenation and dehydrogenation-hydration-hydrogenation reaction to produce aromatics [ 17 ].* 

produce hydrogen that is consumed during the hydrogenation stage. **Figure 1** shows the liquid and gaseous steps for glycerol dehydration and dehydrogenation pathways.

Hydroxyacetone can be produced via dehydration or via another route that involves dehydration to obtain glyceraldehyde, followed by hydrogenation to hydroxyacetone [25, 27]. Also, there are other hydrocarbon mixtures of acetone, propenal, alcohols, ketone, paraffins, and olefins of ethylene and propylene. Both liquid and gaseous routes produce a hydrocarbon pool, which in the presence of a catalyst could be upgraded to benzene, toluene, and xylene (BTX) or benzene, toluene, ethylbenzene, and xylene (BTEX) and other oxygenate compounds [17]. Also, the hydrocarbon pool can be further cracked to light paraffin or olefins. Generally, the presence of strong acid sites results in the build up of heavy aromatics such as trimethylbenzene and tetra- methylbenzene, whereas the reduction in the strong catalytic sites that propagates cracking and cyclization reaction, inhibit the gas route. The liquid route however, is unaffected since the aldol condensation reaction requires weak acid sites [16, 17].

#### **3. Effect of HZSM-5 acidity, metals addition, and reaction conditions**

The unique three-dimensional cage-like crystalline structure and tunable acid properties of ZSM-5 render it a choice catalyst for many reactions including isomerization reaction, alkylation reaction, and aromatization reaction. Also, this catalyst possesses an appreciably high surface area and has been extensively used successfully to drive the reaction of methanol to aromatics. Studies on the glycerol to aromatics synthesis revealed that protonated ZSM-5 (H-ZSM-5) has been effective for BTX and BTEX production due to its acidity and shape-selective characteristics [32]. The H-ZSM-5 contains sinusoidal channels (0.51 × 0.5 *nm*) that are crossed with straight channels of the dimension (0.53 × 0.56 *nm*) with intersection channel of 0.9 nm size [33, 34]. However, the relatively large micropores of the catalysts limit the mass transport of large molecules and present a diffusion barrier, which ultimately results in undesired bulkier aromatics and cokes and eventual deactivation of the catalysts [35]. To correct this, hierarchical porous H-ZSM-5 catalysts with macro-meso-and micropores have been developed to ensure hitch-free diffusion of reactants. In the synthesis of hierarchical H-ZSM-5 catalyst, several factors should be considered (see **Figure 2**). These factors are the hydrophobicity of the catalyst since the reaction most times produce water, concentration, and strength of acid sites, presence of mesopores, and shape selectivity of the catalyst. Also, several metals have been added to stabilize the H-ZSM-5 and increase its acidity and efficiency [36]. This section presents the effect of acidity and the effects of metals used to functionalize H-ZSM-5 catalysts. Also, the influence of reaction conditions such as time on stream and reaction temperature was presented.

#### **3.1 Effect of catalyst acidity**

The acidity of the ZSM-5 and MFI (H-ZSM-5) is vital to promoting the aromatization or hydrodeoxygenation of glycerol. To determine the influence of the catalyst acidity, several analytical characterizations can be performed. A notable characterization technique employed is temperature desorption programming (TPD) using ammonia as a probe molecule [37]. Ammonia is used because it is basic and can easily interact with acid sites. The TPD can also be used to determine the number of active sites in mmol/g by deconvolution of the peaks using gamma distribution or the gaussian model [12]. The

#### *Catalytic Conversion of Glycerol to Bio-Based Aromatics DOI: http://dx.doi.org/10.5772/intechopen.108148*

 **Figure 2.**  *Considered factors in the application of hierarchical porous H-ZSM-5 [ 34 ].* 

TPD peaks could be classified as weak, moderate, and strong peaks. The weak peak can be appreciated from 150 to 220°C, the moderate acid sites are around 300–350°C and the strong sites are from 400°C and above [ 38 ]. For zeolites, only weak and strong acid sites are usually identified [ 39 ]. Although weak and moderate peaks contribute to the deoxygenation and aromatization, mainly the strong acidity favors a higher yield of aromatics.

 For instance, the yield of aromatics over catalyst of Zn/P/Si/ZSM-5 was lower than Zn/P/ZSM-5 catalyst by about 14.3% because the SiO 2 impregantion reduced the amount of the total acid sites by about 7.02% [ 38 ]. Also, the concentration of the surface acid sites is more vital than the total acid sites, because it is largely a surface dominated reaction. A similar study on atomic layer deposition (ALD) of Zn species on Sn/HZSM-5 zeolite revealed that above 20 cycles of ALD of Zn, precisely 40Zn, the interaction between the HZSM-5 and deposited Zn species were limited. Also, the strong acid sites reduced significantly, resulting in a reduction of BTX yield from close to C. 39% (for Sn/HZSM-5@20Zn) to C. 31% (for Sn/HZSM-5@40Zn) based on carbon yield [ 17 ]. Dealumination of HZSM-5 by steaming and acid leaching could produced different results. For instance, using 6 M nitric acid to remove extra-framework aluminum from the surface and channels of HZSM-5 and steaming to achieve the same purpose revealed significantly lower Brønsted acidity in steam treated HZSM-5. Whereas the concentration of the strong acid sites for the acid treated HZSM-5 remained almost unchanged. However, the weak acid sites decreased significantly, resulting in reduced total acidity. Likewise, BTX aromatic yield was higher for acid treated HZSM-5 (C. 28.1%) compared to the steam treated zeolite (C. 18.3%) [ 16 ].

 To evaluate the type and concentration of the acid site, Fourier transforms infrared (FTIR) of adsorbed pyridine are normally used [ 40 ]. The pyridine IR is usually conducted by heating a known mass of the catalyst at around 150°C for 3 h, followed by pyridine adsorption for 2 h and subsequent desorption of the pyridine at 200°C under vacuum [41–43].

From pyridine DRIFTS analysis, the types of acid sites normally encountered with ZSM-5 zeolites are Brønsted and Lewis acid sites. Due to high pressure and temperature conditions of the glycerol to liquid fuels and aromatics compounds, high coke selectivity and consequent deactivation often occur for this type of catalyst. Particularly, the strong acid sites, which are mainly Brønsted acids promote the undesired coke formation, because the coke deposits preferentially on these sites through different proton transfer steps that occur on the Brønsted acid sites [26]. It is important to mention that understanding of acidity types of the zeolite catalyst can be appreciated from the Si/Al ratio [44]. Tuning of the acid strength [41] and reduction in the acid density [45] are two notable strategies to reduce coke deposits. The tuning of the acid strength and concentration can be achieved by modifying the zeolite tetrahedral framework Al content of the H-ZSM-5 or exchanging the protons that compensate for the negative charge of the Al sites tetrahedral framework. The strategy adopted over the years to accomplish these are either post-synthesis modification (top-down) or in situ modification during synthesis (bottom-up) [26, 32]. For instance, the synthesis of H-ZSM-5 with different Si/Al ratios is a common bottom-up strategy to evaluate the effect of acidity. For the bottom-up strategy, the total acid sites increase when the Si/Al ratio decreases. Top-down strategies include dealumination by acid extraction or steam treatment and isomorphic substitution of Al or Si atom with Zr, Fe, Ga, In, etc. [21, 46–48], other metals, heteroatoms of phosphorus, sulfur, and alkali metals, and alkaline earth metals [16, 40, 43]. Although these substitution strategies, which focus on the framework tetrahedral sites have been successful, however, they often result in the damage of the framework or defects of the crystalline structure, which adversely impacts the reactants and products diffusion to and from the cage-like zeolite structure. Also, the accurate introduction of these metals could be very problematic and can lead to uncertainty in the pore dimension [26].

Evaluation of the H-ZSM-5 acidity after dealumination via washing with HNO3 and steaming revealed that the weak acid sites decreased, whereas the strong acid sites were unaffected. It is important to note that ZSM-5 zeolites have amorphous extra framework aluminum, which contributes significantly to the weak acid sites (although it could contribute slightly to the medium and strong acid sites with other silanols (Si-(OH)-Al) species), whereas the framework aluminum species are more related to the strong acid sites. Hence, the reduction in weak acid sites after dealumination can be attributed to the removal of the extra framework aluminum. The steaming treatment, however, resulted in a remarkable reduction of the acid concentration and strength, which shows the dealumination of the framework and extra-framework aluminum [16].

#### **3.2 Effect of metal addition**

Metal addition on H-ZSM-5 is bifunctional to provide stability as well as promote the dehydration/dehydrogenation reaction of glycerol to aromatics. A conducted study using only H-ZSM-5 reveal that predominantly oxygenates namely acrolein were obtained with about 11% of C6 −C9 compounds [30]. In this same study, the addition of metals (platinum and palladium), provided bifunctional properties, which promoted the C − O, C − H, O − H, and C − C bonds cleavages to achieve deoxygenation of the oxygenates [24]. Palladium however showed better performance in the conversion of the oxygenates to aromatics because of the higher H/metal ratio.

Notably, noble metals are preferred for fast kinetic activation of hydroxyl groups of glycerol and hydrogen dissociative adsorption [49]. Depending on the operating temperature they generally promote the hydrocarbon formation instead of the CO and H2 pathway.

Apart from noble metals, which are usually expensive and can impact the overall production cost of liquid hydrocarbon and biobased aromatics from glycerol, other metals have been used with appreciable performance. Binders such as Al2O3 have been used to prolong the H-ZSM-5 catalyst life. The mesopores of the Al2O3 provided a higher capacity to store coke deposits in amorphous form. Also, the binder promoted a high total BTX yield compared to H-ZSM-5 [31]. Another study investigated the incorporation of tin (Sn) species in ZSM-5 catalysts. The composite catalysts were treated hydrothermally using sodium hydroxide, followed by ion exchange with ammonium chloride. The H[Sn, Al] ZSM-5/0.3AT composite catalysts after ion exchange reaction showed a slight decrease in crystallinity, a decrease in total Bronsted acid sites with an increase in Lewis acid sites [36]. Also, the Sn incorporation replaced some medium and strong acid sites and new acid sites were formed. The addition of the Sn did not destroy the H-ZSM-5 morphology, and the composite catalyst showed intra-mesopores and micropores, which benefits mass transfer of reactants and products. Overall, the catalyst showed a higher carbon yield of BTX (C 32.1%) compared to H-ZSM-5 (C 17.8%). The H[Sn, Al] ZSM-5/0.3AT could sustain about 13 h of time on stream reaction (H-ZSM-5 sustained the reaction for only 3 h with severe deactivation due to coke deposits), which was attributed to the mesopores and tuned Lewis acid and Bronsted acid sites due to alkali post-treatment. Similar studies have demonstrated the effect of binary material of Sn, Zn incorporated on H-ZSM-5 with appreciable BTX yield and longer catalyst life [17]. The Zn has been reported to promote high BTX formation by suppressing H-transfer reactions and light paraffins [29].

#### **3.3 Effect of temperature**

Catalytic pyrolysis is employed to achieve glycerol to aromatics and liquid hydrocarbons conversion. The operating temperature is very vital because glycerol to aromatics is a consecutive reaction steps that involves dehydration, oligomerization, cyclization, aldol condensation, cracking, and dehydrogenation. Since these reaction pathways occur simultaneously and parallel, different product distributions could be obtained at different reaction temperatures. An effect of reaction temperature on glycerol to aromatics shows that at a low temperature of ~200°C, O − H and C − H bonds could be easily broken due to their lower activation energy. This activation could be prevented by a possible hydrogenation reaction that is propagated by high H2 content. An increase in temperature in the range of 300–400°C facilitated the bond cleavages of C −C and C − O species, which could happen simultaneously due to their similar activation energy [24]. However, breaking of the C −C bond at this temperature results in the formation of coke deposits, which could be graphitic or amorphous. Hence, the temperature should be tuned properly with the right amount of catalyst acidity to achieve the breaking of C − O bonds, while minimally avoiding the C −C bond. At 450°C, more of the liquid products are further cracked to gases, and syngas formation and methanation reactions are promoted [30]. Hence, the aromatics yield decreases, whereas the gaseous product increases as shown in **Figure 3** [30]. Most studies adopted 400°C as the optimum temperature to achieve the glycerol to aromatic conversion [18, 32, 50].

#### **Figure 3.**

 *Effect of pyrolysis temperature using Pd/H-ZSM-5 catalyst. Conditions: 1 atm, H 2 /glycerol molar ratio of 10, and 6 g cat·h/g glycerol (copyright obtained https://pubs.acs.org/doi/full/10.1021/acsenergylett.6b00421 ).* 

#### **3.4 Effect of time on stream**

 Time on stream for glycerol conversion to bio-based aromatics could be categorized into induction time, steady-state reaction time, and deactivation time [ 16 ]. Selectivity of the products is a clear function of the reaction time on stream; hence, it is important to optimize this factor. This has been generally established based on the yield of the aromatic intermediates in the cage structure of the H-ZSM-5 zeolite during aromatization. A study revealed that polymethyl benzenes and olefines, which is a very vital active hydrocarbon pools, were formed over the H-ZSM-5 cages as a product of the dehydration of the glycerol (oxycarbides) in the induction period [ 17 ]. This results in improved aromatic yields with a decrease in oxycarbides. After the induction period in a continuous reaction set-up, aromatization of glycerol proceeds via multiple steps of autocatalytic nonstop reaction of alkylation/dealkylation, oligomerization, and H-transfer. This stage usually has a relatively constant BTX and BTEX yield because the consumed hydrocarbon species are replenished. As the reaction time on stream is prolonged, the dealkylation step is suppressed with a corresponding decrease in lighter aromatic yield (toluene) and a buildup of heavier hydrocarbon species [ 38 ]. This was attributed to the coverage of the strong active sites by the heavier autocatalytic species, which suppresses the cracking and cyclization pathways propagated by the strong acid sites [ 51 ]. Hence, the gaseous route is mitigated allowing for the liquid synthesis. Further extension of the time on stream for continuous reaction set-up facilitates the conversion of the heavier intermediates (heavier hydrocarbon species) into carbon by a condensation reaction [ 52 ]. Eventually, the carbon formed blocks the micropores and the acid sites resulting in the catalyst deactivation. In the deactivation phase, the BTX and BTEX yields drop drastically, however, unconverted glycerol, acetol, acetaldehyde, and acrolein remained in the product [ 15 ]. **Table 1** shows the reported catalysts for glycerol conversion to aromatics and reaction conditions with selectivity towards BTX.

#### *Catalytic Conversion of Glycerol to Bio-Based Aromatics DOI: http://dx.doi.org/10.5772/intechopen.108148*


#### **Table 1.**

*Reported catalysts for glycerol conversion to aromatics.*

#### **4. Deactivation of catalysts**

Deactivation of catalyst is the loss of performance over time, and the difference in catalyst life depends on the contaminant and application. There are several mechanisms by which a catalyst can be deactivated including fouling, poisoning, sintering, leaching of active surface, and mechanical attrition. Thermal degradation deactivation could be found with catalysts used in petroleum cracking or polymers [54]. Poisoning is a deactivation mechanism whereby a contaminant is loosely or strongly adsorbed on the active sites of the catalyst, which prevents access to the reactant species. This type of deactivation could be reversible or irreversible. A notable poisoning mechanism is coking, which results in carbon deposits, which could mask the surface of the active sites or even block the catalyst pores (pore mouth-filling). Coke formation can be investigated by temperature gravimetric analysis and visual inspection to observe a color change, usually from off-white to black. In general, catalyst deactivation is inevitable, however, careful catalyst design and operation under mild conditions could prolong the catalyst life in any given application. The design of a suitable shape-selective catalyst with tunable acid properties could increase the peak BTX yield, and total productivity and prolong the catalyst life.

Aromatization reaction occurs through a series of consecutive reaction steps that involves aldol condensation, oligomerization, cracking, cyclization, dehydrogenation, dehydration, aromatization, etc. The strength and concentration of the acid sites influence the yield of BTX and BTEX; specifically, the amount of Bronsted and Lewis acids could propagate the deactivation of the catalysts. The Bronsted acid sites promote cracking, H-transfer, and oligomerization reaction and it is principally responsible for coke generation since it promotes the C −C bond breaking. A Survey of the literature shows that coke deposition is the main deactivation mechanism of the H-ZSM-5. The coking deactivation is considered reversible because the coke can be removed by oxidative treatment. The spent catalyst is subjected to high-temperature recalcination in the air to decompose the coke deposits. For instance, the stability analysis for H-ZSM-5/Al2O3 and H-ZSM-5 catalysts used in the synthesis of BTX showed that the Al2O3 binder had more coke accommodation capacity than only H-ZSM-5. This was attributed to the mesoporous nature of Al2O3. Besides, the coke formation on the H-ZSM-5/Al2O3 was amorphous, whereas that of H-ZSM-5 was graphitic [50, 55]. This implies that the decomposition temperature would vary widely and the graphitic coke obviously would result in irreversible deactivation [31].

There are other forms of deactivation of H-ZSM-5 related to the deformation of the structure, reduction of crystallinity, acid strength, and reduction of microporosity. As stated before, this type of structural defect can manifest during the top-down approach. A situation whereby the acidity is reduced by chemical treatment or steaming. In particular, the hydrophobicity and spatial constraints of some chemical agents such as CH3ONa and NaOH, in high concentrations could promote rapid desilication of the H-ZSM-5, which results in the damage of the catalyst microporous structure, diminished cage-like walls of the H-ZSM-5, and a drastic reduction in the crystallinity [52]. This situation presents a deformed H-ZSM-5 catalyst with low catalytic


#### **Table 2.**

*Reported H-ZSM-5-based catalysts, coke content, acidity, and number of reuse.*

*Catalytic Conversion of Glycerol to Bio-Based Aromatics DOI: http://dx.doi.org/10.5772/intechopen.108148*

performance towards the aromatization reaction of glycerol. Another study reported in situ deactivation due to the conditions of the pyrolysis and reaction intermediates. He et al. [56] in their study to produce bio-based aromatics from glycerol revealed that dealumination of H-ZSM-5 occurred, which severely affected the crystallinity and acidity of the catalyst. This dealumination was attributed to catalyst exposure to steam generated by glycerol dehydration and the framework interaction with intermediate oxygenates. These types of deactivation are irreversible and eventually result in the complete deactivation of the catalyst. **Table 2** shows the total acidity of some reported catalysts, total coke content, and catalyst lifetime.

#### **5. Prospects of catalytic glycerol to bio-aromatics**

The conversion of glycerol to liquid fuels and aromatics is a notable research effort towards the production of "green" drop-in fuels that can be used as aviation fuel or bio-based chemicals. The glycerol is subjected to pyrolysis and sometimes in situ hydrotreating to reduce the oxygenates under heterogeneous catalysts. Zeolites of MFI (ZSM-5) have been widely applied because of the shape-selective nature, tunable acidity, and structure of the catalyst that promotes dehydration and hydrodeoxygenation reaction to produce hydrocarbon pools. In particular, protonated ZSM-5 (H-ZSM-5) has been synthesized for this synthesis with different modifications to evolve hierarchical pores and moderate the Bronsted acidity. However, there are continuous improvements to this strategy to increase the yield of BTX and BTEX from glycerol pyrolysis.

Notable about this is the modification of the H-ZMS-5 with metals and steaming to dealuminate the catalyst or alkaline treatment. These techniques are deployed to stabilize the zeolite and achieve higher selectivity of BTX. Conducted studies have evidenced that most strategies used to achieve dealumination or functionalization with metals are invasive, i.e., the crystallinity is sometimes affected significantly [44]. Hence, it is pertinent to look for non-invasive methods, such as low-temperature plasma techniques and isomorphous substitution methods [46, 48]. These methods are capable of achieving grafting metals on the H-ZSM-5 with minimal damage to the crystallinity or the framework structure. Also, the synthesis of the zeolite with pore-templating agents to evolve hierarchical pores would benefit the mass transport of reactants and products from the catalyst active sites. Another aspect that needs further insights is the optimization of reaction conditions such as the temperature, time on stream, weight hourly space velocity (WHSV), and methanol/glycerol ratio. Optimization of these reaction influencing parameters will result in minimal operation cost and provide insights into the mechanistic pathways involved.

Wholistic study of the deactivation mechanism of the unmodified H-ZSM-5 based on time and spaced resolved analysis suggested that coke deposit increases with the time on stream. The longer time on stream, the more coke accumulation. The mechanism proposed for this scenario posits that initially, coke is formed at the microporous channel of the H-ZSM-5 catalyst, followed by accumulation on the external surface of the zeolite as the time on stream increases [15]. The primary cause of the coke formation is the strong acid sites of the H-ZSM-5 catalysts, which have been improved by the design of shape-selective catalysts [35], metals, additive addition [33], and dealumination [43]. It is important to search for more binders that are capable of accumulating coke in amorphous form as opposed to the graphitic form, which is energy-intensive to regenerate.

#### **6. Conclusions**

Conclusively, the aromatization of glycerol via pyrolysis methods could be promoted using MFI zeolites (HZSM-5 and ZSM-5) catalysts. The acid form of ZSM-5 (H-ZSM-5) possesses appropriate crystallinity and acidity to tailor the reaction to produce benzene, toluene, and xylene (BTX) and/or benzene, toluene, ethylbenzene, and xylene (BTEX) with some oxygenates. The crystalline cage-like structure, acidity, pore size, and channel of the H-ZSM-5 catalysts influence the product's yield and distribution. Also, the reaction conditions such as temperature, time on stream, weight hourly space velocity significantly influence the product distribution and the carbon yield (BTX and BTEX). Also, these factors impact the coke formation mechanism, especially the time on stream and reaction temperature. The addition of metals like Zn, noble metals, Sn, binders (Al2O3), and additives (heteroatoms) influence the H-ZSM-5 acid concentration, acid strength, and acid type. These chemical modifications of the H-ZSM-5 catalysts substitute the framework aluminum responsible for high Bronsted acidity, which promotes coke formation and stabilizes the catalysts. Deactivation caused by dealumination techniques and *in situ* reaction conditions destroys the crystalline cage-like zeolite framework and is irreversible. Whereas reversible coke deposition is removed by energy-intensive oxidation that could impact the porosity and crystalline structure of the catalyst.

Hence, it is recommended that further studies should be conducted on the synthesis of shape-selective and hierarchical porous H-ZSM-5 catalyst with moderate Bronsted acidity to minimize coke formation and promote mass transport. Non-invasive modification methods such as plasma techniques should be adopted to achieve metal and heteroatoms incorporation into the cage-like structure of the H-ZSM-5 without damage to the framework. An in-depth study on the technoeconomic analysis and life cycle analysis of the aromatization reaction will provide insights on the associated costs for comparison and the environmental impact of this process. Also, machine learning methods should be deployed to optimize the reaction conditions.

#### **Acknowledgements**

This work was financially supported by Dirección General de Asuntos del Personal Académico, Mexico under Programa de Apoyo a Proyectos de Investigación e Innovación Tecnológica (DGAPA-PAPIIT) Project Nos: IA102522, IG100217, and IA203320. PhD fellowship was granted to D.R. Lobato-Peralta by Consejo Nacional de Ciencia y Tecnologia, Mexico (CONACYT).

#### **Conflict of interest**

The authors declare no conflict of interest.

*Catalytic Conversion of Glycerol to Bio-Based Aromatics DOI: http://dx.doi.org/10.5772/intechopen.108148*

#### **Author details**

Patrick U. Okoye1 \*, Estefania Duque-Brito1 , Diego R. Lobata-Peralta1 , Jude A. Okolie2 , Dulce M. Arias1 and Joseph P. Sebastian1

1 Instituto de Energías Renovables, Universidad Nacional Autónoma de México, Temixco, Morelos, Mexico

2 Gallogly College of Engineering, University of Oklahoma, USA

\*Address all correspondence to: ugopaok@ier.unam.mx

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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