Sustainable Synthesis of Pyridine Bases from Glycerol

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 zeolite-based 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.

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), β-picoline (3-methylpyridine), and γ-picoline (4-methylpyridine), whose structures vary 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].

Figure 1.

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

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].

E1

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].

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, 9, 10, 11, 12, 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, 15, 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 one-pot 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 H₂NNH₂·H₂SO₄, 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 ethyl-methylpyridines 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.

Figure 2.

Continuous-flow fixed-bed reactor for the single-step gas-phase aminocyclization between glycerol and ammonia.

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⁻¹, 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.

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⁻¹, 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⁻¹, 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⁻¹. 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.

Figure 3.

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

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].

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⁻¹; 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 Cu_4.6Pr_0.3/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⁻¹, 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, 15, 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 SO₄²⁻/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, 19, 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 h^−1, 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⁻¹ [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 β-picoline 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⁻¹; 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⁻¹, 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.

Catalyst	Feedstock composition^a	T(°C)^b	Space-velocity (h^−1)	X (%)^f	Y (%)^g	TOS (h)^h	Coke; (wt. %)^i	Ref.
HZSM-5	Pure gly, NH3/gly = 12/1	550	1^c	100	Total = 36; Py = 25.2; αP = 3.1; βP = 6.3; γP = 1.0	—	10	[9]
4.6%Cu/HZSM-5	20 wt% gly aq. soln., NH3/gly = 7/1	520	300^d	100	Total = 43; Py = 34.9; αP = 2.4; βP = 5.5	32	27.4	[10]
HZSM-5-At-acid	36 wt% gly aq. soln., NH3/gly = 4/1	425	0.60^e	100	Total = 29; Py = 15.7; αP = 1.9; βP = 10.0; γP = 1.2	11	N.R.	[11]
HZSM-5/11	20 wt% gly aq. soln., NH3/gly = 12/1	520	300^d	100	Total = 41; Py = 27.7; αP = 2.6; βP = 10.5	2	N.R.	[12]
Two-step process HZSM-5-At + ZnO/; HZSM-5-At-acid	20 wt% gly aq. soln., NH3/gly = 5/1	T1 = 330; T2 = 425	0.45^e	100	Total = 62; Py = 35.8; αP = 0.06; βP = 25.8	11	N.R.	[11]
Two-step process FeP/γ-Al2O3 Cu4.6Pr0.3/; HZSM-5	20 wt% gly aq. soln., NH3/gly = 7/1	T1 = 280; T2 = 420	300^d	100	Total = 60; Py = 39; βP = 20.2; 2,4-L = 1	10	N.R.	[13]
HF/ MgZSM-5	Molar ratio acr/H2O/NH3 = 1/2/2	425	500^d	100	Total = 59; Py = 26.6; βP = 30.4	12	N.R.	[18]
4.6%Cu-1%Ru/ HZSM-5	20 wt% acr. aq. soln., NH3/acr = 3.5/1	420	300^d	100	Total = 69; Py = 27; βP = 37; 2,4-L = 5.4	75	14	[19]
Y-45	Pure acr.,; molar ratio acr/NH3=; 1/2	360	4994^d	92.9	Total = 34; Py = 15; βP = 19.1	0.75	22.5	[20]
1%Zn/ HZSM-5	Molar ratio (acr. Diethyl acetal)/NH3/H2O = 1/4/1	450	0.85^e	100	Total = 61; Py = 26.9; βP = 34.3	11	8	[21]

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.

^a gly = glycerol, acr = acrolein, ^b T = reaction temperature, ^c WHSV = weight hourly space-velocity, ^d GHSV = gas hourly space-velocity, ^e LHSV = liquid hourly space-velocity, ^f X = feedstock conversion, ^g 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, ^h TOS = time on stream, ⁱ N.R. = not reported.

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 catalytic bed [9, 10, 11, 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.

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].

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].

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.

Acknowledgments

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.