Sustainable Bioethylene Production from Lignocellulosic Bioethanol: Performance of Zeolitic Catalysts and Mechanistic Insights

Abstract

Producing second-generation (2G) bioethylene through the dehydration of 2G bioethanol is a challenge, requiring the effective use of catalysts as an alternative to fossil-based ethylene production. This work evaluates the production of bioethylene from the catalytic dehydration of 2G bioethanol [from pine sawdust produced via a simultaneous saccharification and fermentation SSF process (53%)] using γ-Al2O3; ZSM-5, NH₄⁺Y, H-ZSM-5, and H-Y zeolite as catalysts. Yields of 94.6% (at 372 °C) and 85.5% (at 473 °C) of 2G bioethylene were obtained when using H-ZSM-5 and H-Y zeolite, respectively. These results demonstrate that the H-ZSM-5 zeolite showed the best performance for 2G bioethanol dehydration, producing high 2G bioethanol conversion and 2G bioethylene selectivity at a lower reaction temperature. Ethylene production from the catalytic dehydration of commercial (96%) and diluted (53%) ethanol was evaluated as a reference, along with the effects of the weight hourly space velocity (WHSV) and ethanol concentration. Varying the WHSV from 2.37 to 4.73 h⁻¹ at 312 °C and using commercial ethanol at 96%, produced similar ethanol conversion of 100% and ethylene yield of 100%. At 290 °C, with a WHSV of 2.37 h⁻¹ and 53% diluted commercial ethanol, H-ZSM-5 converted 76.83% of the ethanol and produced a 75.8% ethylene yield. A study based on density functional theory (DFT) has shown that diethyl ether is a key intermediate in the conversion mechanism on H-ZSM-5, proceeding through an ethoxide intermediate in the rate-determining step, with an apparent activation energy of 25.4 kcal mol⁻¹.

1. Introduction

Ethylene, also known as ethene (CH₂=CH₂), is the first member of the alkene family. It is a colorless gas with a boiling point of –103.7 °C and has low solubility in water and alcohol [1]. This gas is highly reactive and readily interacts with many other chemical compounds. For example, the addition of water to ethylene produces ethanol (ethyl alcohol). Its simple chemical structure and double bond make ethylene an excellent molecule for carrying out a variety of chemical reactions [2], for example, as a precursor of polyethylene, the most common plastic, having plenty of applications ranging from bags and pipes to all types of containers, bottles, and bazaar items [3].

Nowadays, many companies, such as Braskem, Dow Chemical, Sabic, LanzaTech, and TotalEnergies, produce bioethylene from first-generation bioethanol from crops like corn and sugarcane. Although these crops are renewable, their primary use is food production [4]. In contrast, bioethylene obtained from the dehydration of second-generation (2G) bioethanol derived from pine sawdust does not compete with food resources [5]. This method presents an excellent opportunity to reduce environmental impact compared to fossil resources, thanks to its favorable temperature efficiency and low investment costs [6].

This work on 2G bioethylene production focuses on the comprehensive utilization of residues from the wood industry. It utilizes readily available raw materials and established technologies for processing [7]. For example, a stirred reactor with recirculation for pretreatment using alkalis like sodium hydroxide to facilitate the delignification of the pulp, a Fed-Batch bioreactor for the fermentation process, which helps microorganisms grow better by adding small amounts of nutrients at precise moments during the process [8], and contribute to extending the lag phase in their growth curve, and a fixed-bed reactor widely used in ethanol dehydration due to its operational simplicity, high conversion efficiency, precise control of reaction conditions, and relatively low operational costs. In this framework, the production of bioethylene from pine sawdust involves discussing a 2G biorefinery that uses lignocellulosic biomass as a renewable resource for the production of biofuels [9], bioproducts, and high-value-added biomaterials, adapted to the circular economy, which seeks responsible consumption, the sustainable use of biodiversity, and the development of industrial innovation [10]. In this context, pine sawdust is an attractive option as a raw material for producing 2G bioethylene.

The northeast region of Argentina (NEA), particularly in the forested provinces of Corrientes and Misiones, has a greater availability of pine sawdust, resulting in large amounts of lignocellulosic waste. Pine sawdust can offer a valuable and beneficial solution, boosting local economies and bringing benefits on both social and economic levels. Obtaining 2G bioethylene from 2G bioethanol produced from pine sawdust is highly attractive due to its high availability and low cost, and it does not compete with food production. The process consists of the following stages: pretreatment, enzymatic hydrolysis, fermentation, purification, and dehydration. Previous works compared the fermentation strategies to find the highest production yield of 2G bioethanol, such as soda-ethanol pretreated pulps, a Cellic^® Ctec2 enzyme complex containing cellulases, beta-glucosidases, and hemicellulases for cellulose degradation to fermentable sugars, and Saccharomyces cerevisiae IMR 1181 (SC 1181), traditionally used in alcoholic fermentation to convert these sugars to bioethanol [9].

Regarding the dehydration step, the possibility of producing 2G bioethylene from the heterogeneous catalytic dehydration of 2G bioethanol is a challenge that requires the proper use of catalysts with good stability capable of achieving high bioethanol conversions and high bioethylene yields [7]. While alumina has proven to be effective for catalytic dehydration, it requires very high temperatures and can form coke during the reaction, which deactivates the catalyst [11]. On the other hand, zeolites, especially H-ZSM-5 and its modified versions with metals such as Cu, Zn, Mn, and La/Ce cations, are preferred due to their possibility of precisely control the quantity, the strength and nature of the acid sites, thus enhancing the process efficiency (Brönsted and Lewis acidic sites on the catalyst are crucial for the dehydration step) [12]. Zeolites are aluminosilicates in which the isomorphic substitution of silicon (Si₄⁺) by aluminum (Al₃⁺) produces a negative charge in the crystal lattice, which can be balanced by a proton, generating Brönsted acid-type protonic centers (H⁺) [13]. The generated H⁺ are created directly by the calcination of the zeolites at temperatures around 500 °C [7].

The catalytic dehydration step (Equation (1)) is an energy-absorbing reaction mechanism at temperatures between 300 and 500 °C [14]. Acidic catalysts are better for producing ethylene and diethyl ether during dehydration [15]. On the contrary, basic catalysts tend to favor dehydrogenation, resulting in acetaldehyde (Equation (2)). Employing acidic catalysts at low temperatures (around 300 °C) favors the reaction towards diethyl ether formation (Equation (3)), but at higher temperatures, ethylene becomes the main product [16]. Considering the reaction medium, the presence of water helps to stabilize the solid catalyst, control its acidity, and reduce secondary reactions such as ethylene oligomerization [17].

C₂H₅OH → C₂H₄ + H₂O

(1)

C₂H₅OH → C₂H₄O + H₂

(2)

C₂H₅OH → (C₂H₅)₂O + H₂O

(3)

The ethanol dehydration process for ethylene production follows mainly two routes: (a) Parallel reactions producing ethylene and diethyl ether simultaneously, and (b) Series reactions, where diethyl ether is formed first and then it is dehydrated to ethylene [18]. In this context and based on catalytic test results, it has been proposed that the ethanol conversion over zeolites may follow several pathways [19], including a hydrocarbon pool mechanism that produces larger hydrocarbons and water [20]. At low temperatures (<300 °C), the most widely accepted mechanism involves dehydration of ethanol to a diethyl ether intermediate, which then decomposes to ethylene. At higher temperatures, ethylene can also form directly via intramolecular dehydration [16]. Recently, Iadrat and co-workers [21] proposed two possible pathways using commercial and hierarchical FER and FAU zeolites: (i) Direct dehydration of bioethanol to ethylene, and (ii) conversion of bioethanol to diethyl ether (DEE) followed by decomposition of DEE to ethylene and ethanol. They concluded that the zeolite framework promotes different reaction pathways because its structure strongly influences the formation of specific intermediates. A theoretical study on H-ZSM-5 could therefore provide molecular-level insight into the reaction mechanism. Considering its different topology and pore structure compared to FER and FAU, such a study could offer a deeper understanding of the formation of intermediates, the transition states, and the energy barriers involved in each elementary pathway.

Previous studies by Cardozo et al. [22] on the design of an integrated biorefinery for bioethylene production from industrial pine sawdust in Argentina provide a detailed technical and economic analysis of all process stages: soda-ethanol pretreatment, simultaneous saccharification and fermentation (SSF), ethanol recovery, dehydration of ethanol to bioethylene, and final recovery of bioethylene.

As part of a comprehensive study on pine sawdust biorefinery covering different products and materials, this work evaluated the production of 2G bioethylene from the catalytic dehydration of 2G bioethanol using commercial catalysts. Commercial ethanol was used as a reference, analyzing its initial concentration, reaction temperature, and catalyst to achieve the highest ethanol conversion and ethylene final yield. The reaction mechanism on the zeolite surface was also discussed through DFT calculations.

2. Materials and Methods

2.1. Reagents

Commercial ethanol (96% concentration) from a sugar mill (Misiones, Argentina) was used as a reference. The 2G bioethanol (53% concentration) was obtained in a previous study [9]. The commercial catalysts γ-Al₂O₃, ammonium forms of zeolites ZSM-5 (SiO₂/Al₂O₃) 23 molar, and NH4⁺Y (SiO₂/Al₂O₃) 5.2 molar were provided by Sigma-Aldrich (Buenos Aires, Argentina).

2.2. Catalyst Preparation

The Ammonium forms of zeolites (ZSM-5 and NH₄⁺Y) were converted to proton forms H-ZSM-5 and H-Y by calcination treatment at 550 °C for 2 h with a heating ramp of 2 °C/min [7].

2.3. Catalyst Characterization NH₄⁺Y vs. H-Y and ZSM-5 vs. H-ZSM-5

The pyridine desorption technique was evaluated using FTIR (Thermo Scientific Nicolet IS10 spectrophotometer, Waltham, Massachusetts) to determine the active sites of the catalysts, including ammonium forms of zeolites (NH₄⁺Y and ZSM-5), and protonated zeolites (H-Y and H-ZSM-5). Subsequently, the samples were contacted with pyridine (3 Torr) at room temperature for 12 h. After this period, the probe molecule is desorbed for 1 h at 10-4 Torr at 250, 350, and 400 °C, with readings taken at each temperature. Acidic sites were identified by the peaks (around 1545 cm⁻¹ for Brönsted and 1455 cm⁻¹ for Lewis), and the area was used for quantification after subtracting the background signal.

Nitrogen sorptometry using Micromeritics ASAP 2020 equipment (Norcross, GA, USA) was utilized to determine the textural properties (surface area and pore geometry) of ammonium forms of zeolites and protonated zeolites through adsorption–desorption isotherms using the BET method [23]. Before adsorption, the samples were evacuated at 423 K and 10⁻⁵ Pa under vacuum for 8 h. Then, the adsorption–desorption isotherms were recorded at 77 K in a relative pressure range between 10⁻³ and 0.975. The total pore volume was measured according to the Gurvich rule at P/Pº = 0.975, and the surface area was estimated with the BET model.

2.4. Ethanol and 2G Bio-Ethanol Catalytic Dehydration Process

2G bioethanol, commercial ethanol, and commercial catalysts mentioned above were used for the dehydration process, varying the weight hourly space velocity (WHSV) between 2.37 and 9.47 h⁻¹. Protonated zeolites (H-ZSM-5 and H-Y) were also employed at the same catalyst dose. Temperatures from 200 °C to 450 °C were tested to determine which catalyst achieves the highest ethanol conversion and the optimal ethylene selectivity at the lowest possible temperature.

For the ethanol dehydration reaction, a conventional fixed-bed reactor consisting of a 6 mm external diameter quartz tube equipped with a heating oven (Estigia, La Plata, Argentina), a temperature controller (Ferca, CABA, Argentina), and a gas sampling system through a septum at its exit was used. Nitrogen was fed through on-off and flow-regulating valves (Swagelock, OH, USA)), bubbling into a glass reactor (Buchiglass, Uster, Switzerland)) containing ethanol at 25 °C. Its output was connected to the fixed-bed reactor through a 6 mm (external) diameter polyamide tube.

The ethanol was fed at a rate of 0.02 mL min⁻¹ using 15 mL min⁻¹ of N₂ as a carrier gas into the reactor containing between 0.1 and 0.4 g of powder catalyst (2.37–9.41 h⁻¹ WHSV).

Ethanol conversion (X_ETHANOL), ethylene selectivity (S_ETHYLENE), diethyl ether yield (Y_{DIETHYL ETHER}), and ethylene yield (Y_ETHYLENE) are defined by Equations (4)–(7):

$${\mathrm{X}}_{\mathrm{ETHANOL}}={\displaystyle \frac{\mathrm{M}\mathrm{e}0-\mathrm{M}\mathrm{e}}{\mathrm{M}\mathrm{e}0}}·100$$

(4)

$${\mathrm{S}}_{\mathrm{ETHYLENE}}={\displaystyle \frac{\mathrm{M}\mathrm{e}1}{\mathrm{M}\mathrm{e}0-\mathrm{M}\mathrm{e}}}·100$$

(5)

$${\mathrm{Y}}_{\mathrm{ETHYLENE}}={\displaystyle \frac{\mathrm{M}\mathrm{e}1}{\mathrm{M}\mathrm{e}0}}·100$$

(6)

$${\mathrm{Y}}_{\mathrm{DIETHYL} \mathrm{ETHER}}={\displaystyle \frac{\mathrm{M}\mathrm{e}2}{\mathrm{M}\mathrm{e}0}}·100$$

(7)

where Me, Me0, Me1, and Me2 correspond to the molar amounts of ethanol after (Me) and before the reaction (Me0), ethanol converted to ethylene (Me1), and diethyl ether after the reaction (Me2), respectively.

2.5. Reaction Product Characterization

The quantification of ethanol and the reaction products (ethylene and diethyl ether) was performed by gas chromatography using a Shimadzu GC-MS QP5050A equipped with a PE-Elite-Wax capillary column (30 m × 0.25 mm × 0.5 microns) and an FID detector.

2.6. Computational Methods

To consider the full topology of the zeolite cavity, the H-ZSM-5 catalyst was modeled using a 46T (T = Si and Al tetrahedral sites) cluster model, which incorporates both the straight and sinusoidal channels. The active site was positioned at the intersection of the channels. The terminal silicon atoms were saturated with hydrogen atoms positioned along the Si-O bond at a bond distance of 1.47 Å. Similar cluster models were employed in our previous studies [24,25,26,27,28].

The study utilized the two-layer ONIOM approach [29] specifically at the M06-2X/6-31+G(d):PM6 level of theory, within the Gaussian 16 software package [30] for all calculations. The ONIOM model was defined as 16T/46T, with the high-level layer consisting of the 16T region, including the active site and reactants within the cavity. The remaining atoms formed the low-level layer. During geometry optimizations, the reactants and the atoms in the high layer were relaxed, while the remaining atoms of the system were fixed at their crystallographically optimized position.

The stationary points were characterized by analyzing the Hessian matrix and the vibrational normal modes. Zero imaginary frequencies confirmed local minima, while one imaginary frequency identified transition states. Additionally, intrinsic reaction coordinate (IRC) calculations were performed in both the forward and reverse directions to confirm the connectivity between each transition state and its associated minima [31].

The initial energy (E_i), defined as the sum of the isolated zeolite (E_zeo) and ethanol (E_EtOH), is used as the reference point (set to zero) for calculating the energy changes along the reaction pathway.

The adsorption (E_ads) was calculated by Equation (8):

E_ads = E_{adsorbed EtOH} – (E_zeo + E_EtOH)

(8)

where E_{adsorbed EtOH} is the total energy of the optimized ethanol adsorbed complex, E_zeo the total energy of the zeolite, and E_EtOH the total energy of the isolated ethanol.

The relative energy changes (ΔEr) and intrinsic energy barriers (Ea_i) for each reaction step were determined using Equations (9) and (10). The apparent activation energy (E_app) for the transition states involved in the proposed mechanism was calculated relative to the initial energy (Ei).

ΔEr = E_FS – E_IS

(9)

Ea_i = E_TS – E_IS

(10)

In these equations, E_TS denotes the energy of the transition state (TS), E_IS refers to the energy of the preceding intermediate species, and E_FS represents the energy of the final state (product).

3. Results and Discussion

3.1. Catalyst Characterization by Pyridine Desorption Technique and Nitrogen Sorptometry (BET Method)

By analyzing the acid sites using the FTIR technique, we observed a higher amount of Brönsted sites compared to Lewis sites, both in the ammonium-form and protonated zeolites. The Brönsted acid sites play a vital role in the activation of bioethanol to hydrocarbons (particularly bioethylene) since they are considered the main active sites required for the dehydration reaction [32]. Additionally, the values obtained for both types of acid sites are similar between the two zeolite forms [23]. The N₂ adsorption isotherms of ammonium forms of zeolites and protonated zeolites corresponded to type I isotherms. The structural characterization of the catalysts used in this work was performed through pyridine desorption using the FTIR technique for the determination of acid sites (observed in

Table 1. Acid properties of commercial zeolite samples, before and after the calcination treatment.

Catalyst	Brönsted Acidity (mmol Py/g)			Lewis Acidity (mmol Py/g)
	250 °C	350 °C	400 °C	250 °C	350 °C	400 °C
NH4+Y	0.301	0.220	0.160	0.100	0.097	0.092
H-Y	0.258	0.016	0.108	0.101	0.080	0.068
ZSM-5	0.545	0.386	0.323	0.013	0.008	0.008
H-ZSM-5	0.321	0.272	0.231	0.049	0.050	0.044

), and nitrogen sorptometry (BET method) for the surface area, pore geometry determination, and the adsorption isotherms (see Supplementary Materials for details, Sections S1 and S2). Table 1 shows the details for the quantification of the Brönsted and Lewis acid sites expressed in units of mmol Py/g (millimoles of pyridine per gram of catalyst) at 250, 350, and 400 °C. Measurements for each assay were performed in duplicate.

The data presented in Table 1 show that there are no relevant differences in the acidic properties of the commercial zeolite catalysts and their protonated forms. The concentrations of both Brönsted and Lewis acid sites remain comparable across the different treatments, suggesting that the calcination procedure used to generate the protonated forms does not substantially alter the nature, strength, or quantity of acid sites within the zeolite framework.

According to Madeira et al. [32], the transformation of ethanol into hydrocarbon compounds requires Brönsted acid sites as the primary active sites in zeolite catalysts. The ethanol is directly dehydrated to form ethylene or DEE, depending on the strength of the Brönsted acid sites. Brönsted acid sites play a fundamental role. They are essential for facilitating relevant reactions such as dehydration, isomerization, and cracking. When there are more of these sites, the activity and selectivity of the catalyst improve significantly. In zeolites, the quantity and strength of Brönsted acid sites are directly related to the aluminum content in their structure. An increase in the aluminum content generates more acid sites, which raises the catalyst’s overall acidity [33].

3.2. Ethanol and 2G Bio-Ethanol Catalytic Dehydration Process

3.2.1. Temperature Effect on the Commercial Ethanol Dehydration Process, Used as Reference

Figure 1, Figure 2 and Figure 3 show the molar percentages of ethanol (96%), DEE, and ethylene with WHSV of 2.37 h⁻¹ in the dehydration mechanism, using a temperature range of 200 to 450 °C.

When analyzing the reaction temperatures in ethylene production, γ-Al₂O₃ used as a catalyst is quite effective, achieving an impressive yield of 98.46% molar ethylene at 390 °C (Figure 1). H-Y catalyst exhibits a similar pattern, requiring elevated temperatures (440 °C) to achieve 97.70% molar ethylene, even after calcination (Figure 3). However, when using ZSM-5 without calcination treatment, the yield is slightly lower at 97.05% molar, but at a higher temperature of 445 °C (Figure 2). In contrast, the H-ZSM-5 catalyst stands out for its efficiency, as it allows for the production of 96.62% molar ethylene at a significantly lower reaction temperature of just 290 °C, demonstrating that the calcination treatment favored ethanol dehydration using lower reaction temperatures and highlighting the advantage of H-ZSM-5 in terms of energy savings and effectiveness in the process.

According to Nguyen and Le Van Mao [34], between 250 and 270 °C, ethanol suffers an initial transformation into DEE, followed by subsequent dehydration into ethylene. Denise Fan et al. [35] comment that at 300 °C, the H-ZSM-5 can reach an ethanol conversion of 98% and an ethylene selectivity of 95%. Figure 1, Figure 2 and Figure 3, which illustrate ethylene production at temperatures below 300 °C, demonstrate that ethanol is dehydrated to the DEE intermediate and subsequently dehydrated to form ethylene through a series of reactions. It is to highlight that the formation of the DEE intermediate is detected as early as 200 °C in the H-ZSM-5 zeolite, which is a lower temperature than that reported by Nguyen et al. [34]. In contrast, DEE formation in the H-Y zeolite starts at 232 °C, while in γ-Al₂O₃ it occurs from 260 °C onward.

Comparing ethylene production at 276 °C using protonated zeolites (Figure 2 and Figure 3), significant differences can be observed between the H-Y and H-ZSM-5 catalysts.

With H-Y, the reaction primarily favors the formation of DEE, reaching 39.22% molar of DEE, while the formation of ethylene is very low, at only 1.22% molar. Additionally, ethanol conversion reaches only 58.85% molar. At the same temperature, the H-ZSM-5 catalyst performance is significantly better: the formation of DEE decreases to 14.02% molar, while ethylene formation increases considerably to 63.61% molar, accompanied by an ethanol conversion of 77.63%.

This trend becomes more pronounced at 286 °C. While H-Y continues to show high selectivity towards DEE (80.37%) and low selectivity towards ethylene (19.63%), with an ethanol conversion of 45.74%, the H-ZSM-5 catalyst achieves a significantly higher ethanol conversion of 92.85% and complete selectivity towards ethylene (100%), with no DEE detected as an intermediate product. These results demonstrate that the H-Y catalyst requires higher temperatures to favor ethylene formation, indicating a slower conversion process [22]. In industrial terms, this translates into higher energy consumption and, consequently, increased operating costs. In contrast, the use of H-ZSM-5 not only improves ethanol conversion but also allows the reaction to occur at lower temperatures, representing a more efficient and cost-effective option for ethylene production [22].

As said, the ethanol dehydration mechanism for ethylene generation is an endothermic reaction. So, the optimum reaction temperature range is high (between 300 and 500 °C). However, it is worth noting that using the H-ZSM-5 catalyst at a lower temperature of 290 °C achieves a molar ethylene yield of 96.62%. Maintaining the reaction temperature significantly contributes to energy costs in industrial applications. Lower temperatures favor DEE production but reduce ethylene yield. Figure 1, Figure 2 and Figure 3 show this decrease in ethylene yield at temperatures below 300 °C: 7.04% using ϒ-Al₂O₃ as a catalyst, 3.24% with ZSM-5, 22.5% with H-Y, and 8.7% with NH₄⁺Y. However, when using H-ZSM-5 as the catalyst, an ethylene yield of 96.62% is achieved, demonstrating that this material is the most efficient at temperatures below 300 °C. Results obtained using an H-ZSM-5 catalyst are comparable to those obtained by Moon Sanggil et al. [33] using H-ZSM-5 zeolites with SiO₂/Al₂O₃ ratios between 23 and 280 for the ethanol conversion at 250 °C and 5 h⁻¹ of WHSV. The H-ZSM-5 catalyst with (SiO₂/Al₂O₃) 23 molar produced the highest ethanol conversion and selectivity to ethylene compared to the H-ZSM-5 catalyst with a higher SiO₂/Al₂O₃ ratio. The increase in reactivity may result from a higher density of acid sites, derived from the increase in Al content in the zeolite structure. Bi et al. [36] reported that the H-ZSM-5 zeolite, with a SiO₂/Al₂O₃ ratio of 26, can achieve an impressive conversion of 98.6% and an ethylene selectivity of 99.2% at 240 °C. Moreover, its ability to catalyze the dehydration of ethanol to ethylene at lower temperatures (between 200 and 300 °C) makes it a highly valued material in the market, suggesting that it has the potential for further improving its efficiency.

In the ethanol dehydration, from the commercial catalysts (γ-Al₂O₃, ZSM-5, and NH4⁺Y) and their protonated forms (H-ZSM-5 and H-Y), the H-ZSM-5 catalyst exhibits greater conversion and selectivity in the process (96.62% ethanol conversion and 100% ethylene selectivity) at a reaction temperature of 290 °C, compared to the other catalysts.

3.2.2. Effects of Reagent Concentration and Weight Hourly Space Velocity (WHSV) in the 2G Bioethanol Dehydration Process

To evaluate the impact of reactant concentration and weight hourly space velocity (WHSV), commercial ethanol at 96% was used. Additionally, this commercial ethanol was also diluted to 53% to compare the results with those obtained from second-generation (2G) bioethanol.

Figure 4 and Figure 5 illustrate the influence of ethanol concentrations (96% and 53%) on ethanol conversion and ethylene yield, respectively, using the H-ZSM-5 as a catalyst with a WHSV of 2.37 to 9.47 h⁻¹.

For a 96% ethanol concentration in the dehydration process using H-ZSM-5, the ethanol conversion rates were similar at 2.37 and 4.73 h⁻¹, achieving 100% ethanol conversion and 100% ethylene yield at 312 °C. On the contrary, the ethanol conversion and ethylene yield notably decreased (62.6% and 44.6%, respectively, when the WHSV increased to 9.47 h⁻¹). So, the selected WHSV to maintain the ethylene yield was 4.73 h⁻¹.

For 53% ethanol, conversions of 82.08% and 58.87% and ethylene yields of 82.08% and 51.33% were obtained at 295 °C using flow rates of 2.37 and 4.73 h⁻¹, respectively. However, when operating near 320 °C, a 96.5% ethanol conversion and 91.9% ethylene yield are achieved.

Figure 6 shows bioethanol conversion and bioethylene yield using protonated zeolites (H-Y and H-ZSM-5). When comparing the performance of both catalysts at 360 °C, a significant difference can be observed: the H-ZSM-5 zeolite achieves a 91.76% ethylene yield, while the H-Y catalyst reaches only 61.86%. This difference highlights the superior efficiency of H-ZSM-5 in the dehydration of bioethanol, demonstrating its ability to promote the reaction even at lower temperatures.

Employing bioethanol (53%) with the H-ZSM-5 zeolite, a 94.6% bioethylene yield was obtained at 372 °C, while bioethylene yield was 85.5% when using the H-Y at 473 °C. Thus, in the bioethanol dehydration process, H-ZSM-5 zeolite demonstrated superior performance by achieving high bioethanol conversion and bioethylene yield at lower reaction temperatures. However, these results indicate that higher temperatures are required to eliminate the effect of water molecules present in the bioethanol produced from the fermentation of pretreated pine sawdust pulp. As previously mentioned, this bioethanol has a concentration of 53%, which could raise costs at a larger production scale.

When comparing the bioethylene yields obtained from 53% 2G bioethanol, 96% commercial ethanol, and 53% diluted ethanol at a temperature of 290 °C, using the H-ZSM-5 zeolite and a WHSV of 2.37 h⁻¹, yields of 75.46%, 76.8%, and 75.8% were obtained, respectively. However, the ethylene yield increases rapidly at 312 °C when using 96% commercial ethanol, reaching 100%. In contrast, when using diluted commercial ethanol at 53%, a temperature of 320 °C is required to achieve an ethylene yield of 94.5%. Conversely, achieving a bioethylene yield of approximately 92% requires a higher temperature of about 372 °C. Therefore, in the 2G bioethanol dehydration process, H-ZSM-5 zeolite demonstrated inferior performance compared to commercial ethanol and its diluted versions. Despite achieving high conversion and bioethylene yield, the process required higher reaction temperatures. This is attributed to the presence of impurities inherent to the biological production process of 2G bioethanol, which can inhibit the active sites of the catalyst, preventing complete dehydration at moderate temperatures.

3.3. DFT Study of the Reaction Mechanism for Bioethylene Formation Via Diethyl Ether Intermediate on H-ZSM-5 Zeolite

Understanding how typical reactants interact with active sites at the molecular level can provide alternative strategies for achieving the best combination of catalyst and reaction conditions. In this work, we explore the reaction mechanism proposed by Iadrat and co-workers [21] within H-ZSM-5, in which 2G bioethanol is converted to diethyl ether, and then decomposes. This mechanism is consistent with our experimental results, which identified DEE as an intermediate (Figure 2). The direct intramolecular dehydration of ethanol to ethylene is a possible pathway [37,38]. As direct ethylene formation did not occur, this direct pathway was not considered.

Figure 7 displays the most stable structures identified for the reaction mechanism involving the DDE intermediate on HZSM-5 Zeolite, while Scheme 1 shows the proposed mechanism based on these results. Figure 8 shows the corresponding energy profile of the overall mechanism. Table S2 (Supplementary Materials) lists the relative energy changes for each elementary step.

The mechanism begins with the adsorption of 2G bioethanol on the Brönsted acid site (BAS) of the zeolite (I), followed by the protonation of the alcohol through a transition state (II), where the Brönsted proton of the zeolite is transferred to the hydroxyl group of bioethanol. The subsequent dehydration of the alcohol, with loss of a water molecule (III), gives rise to an ethoxide intermediate bound to HZSM-5 (IV). The apparent activation energy (E_app1) of this initial step of the mechanism is 25.4 kcal mol⁻¹. Then, a second bioethanol molecule adsorbs onto the ethoxide intermediate and interacts weakly with it; the coadsorption energy of the coadsorbed complex (V) is −8.6 kcal mol⁻¹. Subsequently, the ethoxide is separated from the zeolite, and the reaction proceeds through a transition state (VI) in which an ethyl cation is formed and stabilized with both the zeolite and the second 2G bioethanol molecule. In this step, the C1 carbon atom of the ethyl cation is stabilized by the oxygen atom of the 2G bioethanol molecule (OM) positioned above it, while simultaneously interacting below with the basic oxygen (O_b) of the zeolite. The apparent activation energy for this step (E_app2) is 16.9 kcal·mol⁻¹. Subsequently, the protonated DEEH⁺ (VII) is formed and stabilized by interactions with the zeolite framework. A rearrangement of this intermediate, along with proton transfer to the zeolite, leads to the formation of the DEE intermediate (VIII). Then, through a third transition state (IX), the stretching and rupture of the C-O bond in DEE occur, and simultaneously, a hydrogen atom from the -CH₃ group is transferred to the zeolite (distance C···H = 1.23Å), leading to the formation of a 2G bioethanol molecule and an ethylene carbocation. The energy barrier is 18.1 kcal mol⁻¹ (E_app3). Finally, a bioethylene molecule is formed (X) upon the transfer of a hydrogen atom from the methyl group to the zeolite, thereby regenerating the BAS.

The results indicate that the rate-determining step in the process is the initial formation of the ethoxide intermediate, which is the first step in the overall mechanism. In this model, one molecule of 2G bioethanol is initially adsorbed onto the active site of the zeolite, and a second bioethanol molecule coadsorbs with the chemisorbed ethoxide intermediate, leading to the formation of the DDE intermediate in the subsequent step. The presence of ethoxide and DDE species in FER and FAU zeolites was confirmed by in situ DRIFTS spectroscopy by the 1180 and 1143 cm⁻¹ bands, and the 1640 cm⁻¹ band, indicating the production of water molecules during the formation of the ethoxide intermediate [20].

The formation of the C-O bond is evidenced by DFT calculation by a decrease in the C₁···O_Z distance, from 2.56 Å in TS1 (II) to 1.49 Å in the ethoxide intermediate (see Table S3 of Supplementary Materials). This bond length remains constant up to intermediate V, then elongates to 2.07 Å in the transition state TS2 (VI), and finally breaks, reaching a distance of 2.84 Å in the DEEH⁺, and 2.98 Å in DEE intermediate. The presence of DDE intermediate is observed up to 255 °C for H-ZSM-5, as shown in Figure 6 for 2G bioethanol (and up to 276 °C for ethanol 96%, Figure 3).

Recently, Ouayloul et al. [16] proposed an alternative pathway in which dimerized ethanol forms DEE at low temperatures when adsorbed on both Brønsted and Lewis sites. It then re-adsorbs on Lewis acid sites and decomposes into two ethylene molecules via dehydration. However, our theoretical study did not identify the dimeric ethanol species suggested by this mechanism.

Based on kinetic studies using ZSM-5 (Si/Al = 280) at temperatures between 200 and 300 °C, Becerra et al. [39] proposed a mechanism in which diluted ethanol adsorbs onto an active site (primarily Si–OH silanol Lewis species) involving the formation of an ethoxide intermediate and the release of water. They suggest that this ethoxide can then react either with another ethoxide or with another OH group from the zeolite to form ethylene. However, no DEE formation was identified in their studies, and the low activation energy calculated from a Langmuir–Hinshelwood kinetic model was attributed to the high Si/Al ratio of the ZSM-5 used.

4. Conclusions

In the production of 2G bioethylene from pine sawdust bioethanol, experiments were conducted using commercial ethanol as a reference, leading to the following conclusions.

The protonated H-ZSM-5 zeolite is the catalyst that achieved the highest ethanol conversion (96.6%) and ethylene selectivity (100%) at a lower temperature (290 °C), demonstrating that the calcination treatment favored the dehydration reaction. Using commercial ethanol (96%) with H-ZSM-5, complete ethanol conversion and 100% ethylene yield were achieved at 312 °C at WHSV values of 2.37 h⁻¹ and 4.73 h⁻¹.

An 82.2% ethanol conversion and 81.7% ethylene yield were obtained for ethanol dehydration (diluted commercial ethanol concentration at 53%) using WHSV of 2.37 h⁻¹ with H-ZSM-5 at 295 °C, whereas 58.9% ethanol conversion and 51.9% ethylene yield were achieved at the same temperature for a WHSV of 4.73 h⁻¹, demonstrating that with a lower concentration of reagent and higher WHSV higher temperatures are required to attain similar ethanol conversions.

H-ZSM-5 proved to be the most effective catalyst for 2G bioethanol dehydration, achieving high conversion and bioethylene yield at lower reaction temperatures. Using H-ZSM-5, a 94.6% bioethylene yield was obtained at 372 °C, compared with 85.5% at 473 °C using H-Y.

The bioethanol was distilled to 53% to assess the performance of a diluted system. The results indicate that greater dilution requires higher dehydration temperatures, highlighting the need for an economic-energy analysis to decide whether it is more cost-effective to distill the ethanol further or to dehydrate it at higher temperatures.

The DFT study provides detailed molecular-level insight into the mechanism of the ethanol conversion to ethylene on H-ZSM-5, which involves a diethyl ether intermediate, consistent with experimental observations. The proposed pathway reveals that the initial formation of the ethoxide intermediate through ethanol dehydration is the rate-determining step. Subsequent steps, including the formation of DEE and its decomposition into ethylene, are shown to be energetically feasible, with calculated energy barriers supporting the viability of the overall mechanism.