4.1 Olefins

Given that ethanol is created by a straightforward dehydration process and that ethylene is the most widely manufactured petrochemical globally, ethylene is most likely the most obvious molecule to be obtained from ethanol. Nearly 80 % of the ethylene produced goes towards making polyethylene, ethylene oxide, ethylene dichloride, and ethylbenzene. Ethylene is utilised nearly exclusively as a chemical building block (Zimmermann and Walzl, 2009). Enzymatic solid acids catalyse the endothermic conversion of ethanol to ethylene. In the process, ethanol is either etherified to produce diethyl ether or directly dehydrated to produce ethylene. Since the ether can be later transformed into ethylene, it is regarded as a reactive intermediate. Diethyl ether is typically seen as a result of the reaction when it is conducted at temperatures between 150 and 300 °C, but at higher temperatures, the ether is easily changed to ethylene (Morschbacker, 2009). The principal secondary products generated from the reaction include acetic acid, ethyl acetate, acetaldehyde, acetone, methanol, methane, propane, propylene, butane, butenes, various hydrocarbons, carbon dioxide, carbon monoxide, and hydrogen. Ouyang et al., (2009) examined how bio-ethanol was catalytically converted to ethylene in a bioreactor using La-modified HZSM-5 catalysts. According to the stability test, selectivity and ethanol conversion over this catalyst could be sustained at levels over 98 % for over 950 h. High reactivity and stability lasting up to 830 h were also demonstrated by the regenerate catalyst. Shetsiri et al., (2019) examined ethylene synthesis from bioethanol in a sustainable manner using hierarchical ZSM-5 nanosheets. Two different models are employed to portray hierarchical ZSM-5 catalyst structures in order to compare reaction processes across various catalyst designs. These models include Brønsted acid sites situated on the external silanol surfaces, which contain Q3 species, and the interior surfaces of the bulk MFI framework, containing Q4 species, as depicted in Fig. 4A. A traditional model is also presnted in Fig. 4B. Mechanistic pathways that have been confirmed are illustrated in Fig. 4C. In the initial phase of the reaction, an ethanol molecule experiences dehydration over the Brønsted acid site of ZSM-5, leading to the creation of an ethoxide intermediate (I) through an ethyl carbenium transition state. The breakdown of this ethoxide intermediate into ethylene (I → II → III → IV) represents the first reaction pathway, while the conversion of a second ethanol molecule on the ethoxide surfaces to diethyl ether (DEE) (I → V → VI → VII → VIII) constitutes the second pathway (Chiang and Bhan, 2010), (Mortensen et al., 2011). Moreover, the Time on Stream (TOS) causes ethanol conversion to decrease from 89.3 % to 59.4 % even after several hours. However, as demonstrated in Fig. 4 (D, E), a significant conversion is still observed over the MFI-HieNS (37) catalyst (∼91.9). The MFI-HieNS (37) sample's activity has changed compared to the (MFI-CON(38)) catalyst, and there is a noticeable difference in the two samples' product selectivity. It is worth mentioning that ethylene demonstrates a significant level of selectivity, reaching nearly 90 %, in comparison to MFI-HieNS(37). Nevertheless, the dominant output of MFI-CON(38) is DEE, exhibiting a selectivity of 60.4 %, whereas the selectivity for ethylene stands at a mere 39.6 %.

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Fig. 4

Optimized structures in (A) and (B) represent the Brønsted acid sites at the external silanol surfaces of hierarchical ZSM-5 and the Brønsted acid sites at the interior surfaces of a standard MFI framework, respectively. Pathways for the proposed ethanol dehydration to ethylene and diethyl ether (DEE) are illustrated in (C). The time-on-stream (TOS) dependence of ethanol conversion is shown in (D), while (E) displays the selectivity and conversion of the products obtained after 15 h of TOS at a reaction temperature of 300 °C, a weight hourly space velocity (WHSV) of 10 h⁻¹, and a molar ratio of N₂ to EtOH of 2.5 (Shetsiri et.al., 2019).

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Using specialized catalysis, Iwamoto et al., (2015). investigated the conversion of propene from bio-ethanol. The study investigated the conversion of ethanol to propene using different catalysts: Ni ion-loaded silica MCM-41 (Ni-M41), Sc-modified In₂O₃ (Sc/In₂O₃), and a solid solution of Y₂O₃-CeO₂. The activity for propene synthesis followed the sequence Sc/In₂O₃ > Y₂O₃-CeO₂ > Ni-M41, but their stability during the process ranked as Y₂O₃-CeO₂ ∼ Sc/In₂O₃ > Ni-M41. The addition of water and hydrogen to the reactant stream significantly improved both the propene output and endurance of Sc/In₂O₃. The choice of catalyst had a profound impact on the reaction process. On Ni-M41, the dimerization of ethene played a crucial role, leading to a metathesis reaction between ethene and butenes, which was an essential step in propene synthesis. Conversely, on the other two oxide catalysts, the main pathway for ethanol conversion to propene involved the following steps: ethanol → acetaldehyde → acetone → propene. A study conducted by Zhang et al. (2017) investigated the conversion of bioethanol into propene using nano-HZSM-5 zeolite. Their research revealed that fluorinated HZSM-5 had rougher surfaces with etched crystals and defects compared to pure zeolite, which had a smoother surface. These irregular surfaces, fragile particles, and gaps between them were found to facilitate the formation of new mesopores, increasing the external surface area of the particles, as confirmed by N₂ adsorption–desorption experiments. XRD patterns (Fig. 5C) of HZ and HZ-F-20 before and after the reaction showed that the fresh catalysts had two peaks around 23°, which merged into a single peak after deactivation (Fig. 5D), indicating some damage to the zeolite HZSM-5 framework during the reaction. The active sites for converting ethanol to propene were identified as Brønsted acid sites (Bun et al., 1990). Various reaction pathways (Takamitsu et al., 2014), (Takahashi et al., 2013), (Inaba et al., 2006b) were proposed for the ethanol-to-propene (ETP) process, with ethene initially forming from ethanol and subsequently being converted in parallel to propene and butene. Due to the catalyst's large pore size, propene and butene had limited interaction time with the active acid sites, leading to their diffusion out of the channels. This effectively hindered the synthesis of aromatics, C5 + aliphatics, and paraffins (Xia et al., 2016). To assess the impact of time on stream (TOS) on the selectivity of C₂–C₄ olefins (propene, butylene, and ethylene) and aromatics during bioethanol-to-propene conversion, experiments were conducted and the results are shown in Fig. 5 (E, F, G, and H). The ethene selectivity increased gradually with TOS, while aromatics selectivity decreased as reaction time increased. Propene and butene selectivity exhibited similar fluctuations, initially rising, reaching a maximum, and then declining, as illustrated in Fig. 5 (F) and (G). This suggests that propene and butene are produced through both common intermediate and parallel pathways in HZSM-5 zeolites (Huangfu et al., 2016). Similar trends were observed in the fluorinated samples, but the initial propene selectivity was significantly higher, and catalytic stability improved compared to the parent HZ. Notably, the HZ-F-20 catalyst exhibited even greater advantages, reaching a maximum propene selectivity of 24.9 % within nine hours and extending the catalyst's lifetime (the duration it maintained a propene selectivity of at least 10 %) to 214 h, surpassing the original HZ catalyst's lifespan by more than five times. The long-term propene selectivity ranking was as follows: HZ-F-20 > HZ-F-15 > HZ-F-25 > HZ-F-10 > HZ-F-5 > HZ. In summary, the fluorine-modified HZSM-5 catalysts, particularly HZ-F-20, demonstrated significantly enhanced resistance to coke deposits, as evidenced by the catalytic test results (Fig. 5).

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Fig. 5

The HZ (A) and HZ-F-20 (B) TEM pictures. (D, C) The HZSM-5 samples' fresh and spent XRD patterns. E, F, G, and H HZ-S stands for 39 h, and HZ-F-20-S for 214 h. Time in the stream has an impact on the aromatic and C₂–C₄ olefin selectivity across various HZSM-5 zeolites. 500 °C, atmospheric pressure, ethanol/water = 9/1 vol/vol, and ethanol WHSV = 10 h^{− 1} are the reaction conditions (Iwamoto, 2015).

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By using the HZSM-5 zeolite to perform the ethanol-to-hydrocarbon reaction, Madeira et al. (2011b) were able to identify radical species and investigate their evolution through catalyst deactivation. Every coked sample was subjected to an infrared analysis in order to compare the primary vibration bands of the fresh zeolite with those linked to the carbon deposit region (Fig. 6A) and the OH region (Fig. 6B). At first, the researchers noticed that all of the coked samples in the area (Fig. 6A) had uniform coke molecule vibration bands. The strength of these bands is the only change that can be seen. The research indicates that vibration bands between 1500 and 1640 cm⁻¹ are frequently connected to aromatic rings. In contrast, vibration bands ranging from 1350 to 1470 cm⁻¹ are generally linked to the aromatics' alkyl branches; that is, they are connected to CH stretching δs(CH₂), CH stretching δs(CH₃), and/or δs([CH₃]₂C < ). This suggests that the carbon deposit formed in the framework is the result of alkylated aromatic molecules, which make up the compounds occluded in the framework. It was anticipated that the coke molecules would reduce the intensity of the silanol band (about 3740 cm⁻¹) and balance the acidic bands, which are bridged by OH groups, at about 3600 cm⁻¹. External cocaine is frequently linked to this band (MAGNOUX, 1987). The OH-bridging band (Fig. 6B) vanished from the original sample after just one hour of Time-On-Stream (TOS), suggesting a quick deactivation process. As the sample's time on stream (TOS) rises, the silanol band's intensity progressively diminishes, with the degree of this drop becoming increasingly noticeable. The yields of the two main products—ethylene and diethyl ether—obtained from the dehydration of ethanol are shown in Fig. 6C. Additionally, it displays the yield of C₃₊ hydrocarbon production as well as the full conversion of ethanol. For all four runs, the ethanol conversion was finished. Diethyl ether was identified at the end of the thirty-hour trial, albeit in negligible amounts. All of the ethanol was transformed entirely into C₃₊ hydrocarbons during the one-hour run. The majority of the higher hydrocarbons (C₃₊) are paraffinic and aromatic compounds, with five to eleven carbon atoms on average (Madeira et al., 2009b). Initially, the four coked samples as well as the fresh samples were analyzed using the EPR-Continuous Wave (EPR-CW) method. The resulting spectra are shown in Fig. 6D. With respect to the samples that were subjected to coke treatment, it is clear that every sample shows some sort of signal, indicating that this specific species has been present and formed since the reaction began. Each of these signals in the case of an organic radical species is uniquely defined by a Lorentzian line form. Depending on the particular sample, the line width fluctuates between 10 and 14 G, with the g factor at the center of these signals being 2.007. When examining the evolution of signal form and intensity with time of sample (TOS), an increase in signal intensity is seen from the 1-hour to the 16-hour TOS. This observation points to an increase in the concentration of spin. We appear to have reached a limit when comparing the 16- and 30-hour signals. Still, there is a small change in the signal shapes, giving the appearance of being slimmer. This suggests a shift in the species' characteristics. Fig. 6 displays the chromatograms for the soluble coke in the runs with a 1-hour and 30-hour TOS. The compounds found are clearly the same in both samples, although the peaks' intensities are different. The peaks that are seen in the first fifteen minutes of the one-hour run (Fig. 6E) are more intense than those that are seen in the thirty-hour run (Fig. 6F). In the 30-hour chromatogram, the peaks in the 23–30-minute period are more prominent than in the 1-hour chromatogram of the identical sample, where these peaks were not displayed. This suggests that the properties of TOS have changed, and there has been a rise in the amount that has accumulated on the catalyst. It also provides insights about the features of the current carbon deposit. These compounds are responsible for most of the highly alkylated monoaromatic compounds seen in the first 20 min of the chromatograms. It was found that these compounds were also found by online GC analysis over the course of our catalytic testing. Over the course of the next twenty to thirty minutes, more condensed aromatics that are still highly alkylated are added. These large, bulky molecules are most likely the main cause of the pore blockage, which prevents reactants from entering the channel (also known as micropore blockage) or products from being released.

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Fig. 6

In (A) and (B), the infrared spectra are presented for both the fresh and coked samples in the 1300–1700 cm⁻¹ and 3500–3800 cm⁻¹ regions. The reaction conditions for these tests were as follows: Temperature (T) = 623 K, Pressure (P) = 30 bar, N₂/EtOH = 4, and WHSV = 15 h⁻¹.(C) displays data on ethanol conversion and the production of ethylene, diethyl ether, and C₃₊ hydrocarbons (wt%) for the four tests conducted at different times on stream (TOS) of 1, 6, 16, and 30 h.(D) illustrates the relative EPR line width and the content of radical species (derived from spin concentration) for both the fresh and coked HZSM-5 (16) samples at TOS intervals of 1 h, 6 h, 16 h, and 30 h. Finally, (E) and (F) provide results from a GC–MS study of coke compounds recovered by CH₂Cl₂ from the coked samples of HZSM-5 (16) at TOS intervals of 1 h and 30 h, respectively, following solubilization by HF solution (Madeira et al., 2011b).

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4.2 Aldehydes and carboxylic acid

The production of either aldehydes or carboxylic acids can occur during the oxidation of ethanol, depending on the specific reaction conditions. Acetaldehyde is an essential industrial chemical that is used in the production of various chemicals. It can be selectively oxidized from ethanol under specific regulated conditions. Acetic acid is a widely utilized carboxylic acid in the production of plastics, textiles, and vinegar. It is produced through the additional oxidation of acetaldehyde. The adaptable dual-pathway process is crucial to the chemical industry due to its capability to generate significant intermediates for various applications (Jørgensen et al., 2007), (Redina et al., 2015). Tembe et al., (2009) conducted a study on the synthesis of acetic acid through the selective oxidation of ethanol using Au catalysts that were supported on various metal oxides. The capacity to oxidize ethanol was evaluated by employing catalysts containing 1 wt% of gold supported on TiO₂, Al₂O₃, and ZnO. The results indicate that Al₂O₃ exhibited the highest level of activity as the initial support for gold in the ethanol oxidation process, followed by ZnO and TiO₂.

In the study by Raynes and Taylor (2021), a catalyst consisting of zinc oxide-modified mordenite was employed for the dehydrogenation of (bio)ethanol to acetaldehyde. The research involved performing pXRD (powder X-ray diffraction) investigations on various ZnO/Na–MOR-(7) material loadings. All the samples successfully maintained the MOR framework type after the impregnation and calcination treatment, as indicated in Fig. 7A. Furthermore, pXRD reflections corresponding to ZnO were observed in the sample loaded with 10.0 wt% Zn, suggesting the presence of ZnO clusters in this material that were sufficiently large to produce a pXRD response (Ouyang et al., 2020). In Fig. 7B, solid-state Al NMR spectra of each ZnO/Na–MOR-(7) material confirmed that aluminum existed solely in tetrahedral framework positions (δAl ≈ 60 ppm), ruling out any impact of extra-framework alumina on catalysis (Nagy et al., 1984). SEM (scanning electron microscopy) imaging of the four catalyst variations did not reveal any changes in catalyst morphology or the presence of large ZnO clusters on the surface of the catalyst crystals (Fig. 7C). To further investigate the Zn distribution in the materials, a sample of each was embedded in resin, mechanically ground, polished with a diamond wheel, and then subjected to SEM-EDS (energy-dispersive X-ray spectroscopy) analysis. This preparation exposed the crystal interiors, allowing for the evaluation of element distribution within the zeolite crystals. Fig. 7E displays the selectivities to the main products for ZnO/Rb–MOR–(7), ZnO, and a physical mixture of ZnO and Rb–MOR–(7) at 0.2 h (A) and 4.0 h (B) TOS (total organic matter reaction time). After a 4-hour TOS, Fig. 7D shows that all catalysts achieved comparable acetaldehyde selectivities of over 80 %, with supported ZnO/Rb–MOR–(7) exhibiting the highest selectivity of 89 %.

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Fig. 7

(A) shows the pXRD (powder X-ray diffraction) patterns in the 2θ range of 5–55° for ZnO/Na–MOR-(7) materials containing different percentages of ZnO by weight (1.0, 3.5, 5.0, and 10 wt% ZnO). These samples were mounted on a glass pXRD slide during data acquisition. A reference diffractogram of ZnO (99.99 %, Sigma-Aldrich) was also acquired using the same analysis conditions as those used for the zeolite materials.(B) displays solid-state Al NMR spectra of the ZnO/Na–MOR-(7) materials with varying ZnO content (1.0, 3.5, 5.0, and 10 wt% ZnO by Zn).(C) presents conventional SEM (scanning electron microscopy) images of the ZnO/Na–MOR-(7) materials loaded with nominal percentages of 1.0, 3.5, 5.0, and 10 wt% ZnO by Zn.(D) and (E) provide information on the selectivities for major products at 4.0 h and 0.2 h TOS (total organic matter reaction time), respectively, for ZnO/Rb–MOR-(7) (marked with “×,” 300 mg), ZnO (marked with “▲,” 12.6 mg), and a physical mixture of ZnO and Rb–MOR-(7) (marked with “■,” 12.6 mg + 300 mg) at a reaction temperature of 400 °C for 4 h TOS (Raynes and Taylor, 2021).

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4.3 Ketones

Acetone finds extensive application as a solvent in medicine and polymer manufacturing, as well as serving as a feedstock for the synthesis of diacetone alcohol, methyl isobutyl ketone, isophorone, and cyanohydrins, which are precursors to methyl methacrylate (Chagas et al., 2018). Rodrigues et al., (2013) examined the one-pot acetone synthesis from ethanol, and Fig. 8 depicts the reaction process. The selectivity of various catalytic systems at isoconversion (∼80 %) is displayed in Fig. 8A. The primary byproducts of using ZrO₂ or CZA (Cu/ZnO/Al₂O₃) are acetaldehyde and ethylene, respectively. According to extensive literature research, ZrO₂ exhibits pairs of basic and acid Lewis sites (Zonetti et al., 2011a), which facilitate the conversion of ethanol to ethylene by dehydration. Conversely, ethanol is dehydrogenated on the Cu⁰ sites of CZA to produce acetaldehyde (Zonetti et al., 2011b). Almost reaching thermodynamic equilibrium, this reaction is at the experimental conditions used (Fig. 8A), known to create hydrogen. These two catalysts also produce trace amounts of carbon dioxide and acetone, as Fig. 8A illustrates. When the catalytic behaviour of the physical mixture of these oxides (CZA + ZrO₂ (1:2)) is compared with that of ZrO₂ and CZA, it is entirely different. The selectivity to acetone and carbon dioxide rises when ZrO₂ is added to CZA, whereas the selectivity to acetaldehyde falls. This time, under the same experimental conditions, Fig. 8B displays the distribution of the products as well as the conversion of ethanol utilising various catalytic systems. Low conversion from the CZA catalyst deviates significantly from thermodynamic equilibrium. The ethanol conversion increases when ZrO₂ is added to CZA (CZA + ZrO₂ (1:1), and the selectivity towards acetone and carbon dioxide likewise rises while it falls towards acetaldehyde. When ZrO₂ concentration in the physical mixture (CZA + ZrO₂ (1:2) is doubled, selectivity to acetone and carbon dioxide as well as ethanol conversion rise even more, but selectivity to acetaldehyde falls. Thus, the ZrO₂ surface may be the site of acetone production.

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Fig. 8

(A) The one-pot synthesis of acetone from ethanol and its reaction mechanism. (B) Distinct catalytic system selectivities during isoconversion. The temperature, weight of ZrO₂, weight of CZA, and composition of the gas mixture are N₂:H₂O:C₂H₅OH = 91:8:1 mol%, 400 °C, 25 mg, and 50 mg, respectively. CZA + ZrO₂ (1:2) is the physical mixture that uses these similar weights. To achieve the isoconversion, the flow rate was adjusted. (C) The different catalytic systems' selectivities and ethanol conversion. The gas mixture composition, temperature, weight, and flow rate of CZA are as follows: N₂:H₂O:C₂H₅OH = 91:8:1 mol%, 400 °C, and 25 mg, respectively; the flow rate is 70 mL min⁻¹. ZrO₂ weighs 25 mg and 50 mg, respectively, when it is used in the physical mixes CZA + ZrO₂ (1:1) and CZA + ZrO₂ (1:2) (Zonetti et al., 2011a and 2011b).

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The main disadvantage of Pt, Pd, and Ru based catalysts is their high cost, even if they show great activity in SRE (Song and Ozkan, 2010). As a result, bimetallic catalysts with non-noble metal additions and trace amounts of noble metal are being studied for SRE. Sliwa et al. assessed SRE for CuO/ZrO₂ catalysts doped with Mn, Ni, and Ga at 350 °C (Śliwa and Samson, 2021). By using the co-precipitation approach, the copper-based catalysts were created at a set (weight percentage) CuO/ZrO = 2.3 and a constant pH of 7. CuO and tetragonal ZrO₂ phases (t-ZrO₂) are detected for all catalysts. However, there are no discernible XRD peaks for MnO, Ga₂O₃, or NiO. The conversion of ethanol to all synthesized catalysts is high. Compared to Cu/Zr catalyst (XEtOH = 90 %), the inclusion of Ni and Ga results in an increase in ethanol conversion (XEtOH = 99 % and 98 %, respectively). Conversely, ethanol conversion shows a little reduction with Cu/Zr doping with Mn (XEtOH = 86 %). The observed variations in conversion are caused by variations in the synthesized catalysts' BET surface area. The BET surface area of the Cu/Zr catalyst increases with Ni and Ga doping, going from 35 m²/g_cat for Cu/Zr to 50 m²/g_cat and 51 m²/g_cat for Cu/Zr/Ni and Cu/Zr/Ga, respectively. Furthermore, the inclusion of Ga causes a notable 16 % decrease in selectivity to acetic acid. Gallium addition improves both BET surface area and copper dispersion. While adding Ga reduces the crystallization of the t-ZrO₂ phase and increases the thermal stability of a-ZrO₂, adding Mn promotes the crystallization of tetragonal ZrO₂.

2-Pentanone finds industrial applications primarily as a solvent, especially in the dewaxing process of high-boiling petroleum fractions like lubricating oils. It is also used in the manufacturing of pharmaceuticals, pesticides, nitrocellulose sprays, and synthetic resin coatings (He et al., 2005), (WEI et al., 2021). In their study, Subramaniam et al., (2020) explored the direct catalytic conversion of ethanol into C₅₊ ketones and investigated the role of a Pd–Zn alloy in catalytic activity and stability. They achieved highly selective ketone production (around 71 %) by directly impregnating 0.1 % Pd onto a ZnO-ZrO₂ catalyst. These ketones had a higher average carbon number (approximately 6) and minimal acetone selectivity (less than 10 %). A notable portion of the ketones belonged to the C₇₊ category, resulting from the sequential cross-condensation of ketones with aldehydes. The detailed product distribution from this experiment can be found in Fig. 9A inset. In order to gain a more comprehensive understanding of the reaction mechanism, the research collected data for the 0.1 wt% Pd–ZnO-ZrO₂ catalyst at lower conversion levels by varying the space velocity, as depicted in Fig. 9C. Fig. 9B illustrates the corresponding changes in product distribution, indicating that as the weight hourly space velocity (WHSV) increases, there are lower conversions and a significant shift in selectivity away from ketones towards esters and alcohols/aldehydes. Additionally, there is a reduction in the length of the product chains. These trends were also observed when the reaction temperature was lowered, suggesting that the in-situ formation of acetone was a slower secondary process. Fig. 9B provides information on the rates of formation of key compounds as a function of contact time. It's worth noting that lower contact times (<20 min) were not tested due to the formation of cyclic compounds resulting from unsaturated condensations. Importantly, the data shows that the total production of ketones is roughly equivalent in moles to the amount of CO₂ produced. This observation aligns with two possible pathways for intermediate production: either through a decarboxylation pathway or through water gas shift from CO generated via a decarbonylation pathway (Dagle et al., 2008).

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Fig. 9

A) the carbon yield distribution of major product groups with varying pd loading on zno-zro₂ at 340 °C, 0.21 MPa EtOH, 1.86 MPa N₂, and 0.15 h⁻¹ WHSV is depicted. The inset provides a closer look at the detailed product carbon selectivity derived from ethanol conversion over 0.1 % Pd-ZnO-ZrO₂ at 370 °C, 0.21 MPa EtOH, 1.86 MPa N₂, and 0.15 h⁻¹ WHSV. b) The molar yield of key products is shown as a function of contact time, which was conducted by varying the catalyst loading for ethanol conversion over 0.1 % Pd-ZnO-ZrO₂ at 370 °C, 0.21 MPa EtOH, 1.86 MPa N₂, and 0.15 h⁻¹ WHSV. The “*” denotes the sum of products, including all ketones such as acetone, 2-pentanone, and 3-penten-2-one, their respective alcohols like isopropanol and 2-pentanol, their respective dehydration products such as propene and trans-2-pentene, as well as isobutene from acetone self-condensation. c) A simplified reaction mechanism is provided for ethanol conversion to C₅₊ ketones, along with potential side reactions. Compounds in gray were not found in the product stream (Subramaniam et al., 2020), (Dagle et al., 2008).

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4.4 Alcohols

n-Butanol is a valuable chemical that finds use in the paint and coating, cosmetics, and pharmaceutical sectors as well as an extractant and solvent. In addition, it serves as a fuel by being mixed with petrol and utilised as a feedstock for the synthesis of other compounds, including butyl acrylate, methacrylate, acrylic acid, and acrylic esters (Ndaba et al., 2015), (Earley et al., 2015). An analysis of the one-pot liquid-phase catalytic conversion of ethanol to 1-butanol over aluminium oxide was conducted by Riittonen et al., (2012a). Among all the catalysts examined, the commercial HTC-500 (20.7 % Ni on alumina) had the highest level of selectivity (62 %). Additionally, a 20 % Ni solution made on alumina by the user had a modest level of selectivity (37 %) towards 1-butanol; nonetheless, the conversion was nearly four times higher than that of the HTC-500, yielding a sizable amount of acetaldehyde. XRD and TEM were used to characterize both nickel catalysts (Fig. 10A and B) (Riittonen et al., 2012b). It's interesting to note that the self-prepared catalyst has a much narrower particle size range and is devoid of aggregates, which are thought to be inert components. Fig. 10C depicts an example kinetic curve that displays the conversion of ethanol and the 1-butanol selectivity among liquid carbon products as a function of time.

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Fig. 10

TEM and XRD images of the commercial model catalyst 20.7 wt% Ni/Al₂O₃ HTC-500. The average particle size was found to be about twice as much as for the self-made Ni-catalyst and showed considerably broad size dispersion (Riittonen et al., 2012b).

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Sun et al., (2017) analyzed the efficient catalytic conversion of ethanol to 1-butanol via the Guerbet reaction over copper- and nickel-doped porous. The hydrogen neutral reaction sequence of the Guerbet coupling of ethanol to 1-butanol involves the following key steps: (a) dehydrogenation of ethanol to acetaldehyde, (b) aldol condensation of acetaldehyde including dehydration to afford the corresponding unsaturated C₄ products, and (c) hydrogenation to form saturated longer chain alcohols (Fig. 11A) (Gabriëls et al., 2015), (Kozlowski and Davis, 2013). In Fig. 11B, the study investigated the impact of varying reaction temperature (ranging from 180 to 320 °C) and changes in catalyst loading (from 0.05 to 0.2 g) on both conversion and yield values. As expected, the conversion consistently increased, starting at 2.3 % at 180 °C and reaching 56.5 % at 320 °C. Simultaneously, the yield of 1-butanol increased, peaking at 22.2 %, and the space–time yield achieved an impressive value of 704.6 g kgcat⁻¹h⁻¹ at 320 °C. However, it's important to note that the carbon balance decreased from 98.5 % to 75.0 % as the temperature increased. To delve deeper into the product formation profiles, experiments were carried out at 310 °C using 0.1 g of Cu10Ni10-PMO catalyst over a 24-hour period. Fig. 11C illustrates the results, where the conversion of ethanol exhibited a linear increase up to approximately 10 h, after which no significant change was observed. In contrast, the yield of 1-butanol reached a constant value of 21 % after 6 h. The initial increase in ethanol conversion during the first 10 h can be attributed to the competitive conversion of 1-butanol into higher alcohols, particularly between the 6th and 10th hour of the reaction (Riittonen et al., 2012c).

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Fig. 11

(A) Reaction network for Guerbet coupling of ethanol. (B) Influence of temperature and catalyst loading on ethanol conversion and distribution of products. Reaction conditions: Cu10Ni10-PMO (50–––200 mg), ethanol (3 mL), 6 h, decane (20 μL). (C) Product formation profile for the Guerbet reaction of ethanol to 1-butanol for 24 h. Reaction conditions: Cu10Ni10-PMO (100 mg), ethanol (3 mL), 310 °C, decane (20 μL) (Riittonen et al., 2012c).

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4.5 Lubricants

Carbon-carbon bond formation has a history in ethanol processing (as discussed earlier with the Lebedev and Guerbet reactions), new processes have emerged that combine ethanol with other small molecules derived from biomass. Notably, the Toste and Bell research groups have made significant contributions in this area, developing pathways to create novel bio-based lubricants and long-chain hydrocarbon fuels. They utilize a mixture of acetone, butanol, and ethanol, which is produced during acetone, butanol, and ethanol fermentation (Goulas and Toste, 2016). By employing reduced transition metals and K₃PO₄ as a base, it becomes possible to perform alkylation reactions with acetone, butanol, and ethanol. This alkylation process leads to a mixture of methyl ketones, which are subsequently doubly alkylated, resulting in mixed C₇-C₁₁ ketones, as illustrated in Fig. 12 (Anbarasan et al., 2012), (Sreekumar et al., 2015). These products, particularly when reactions are conducted at moderate space velocities and result in linear molecules, can be further processed through hydrodeoxygenation to produce bio-based alkanes suitable for use as fuels. To enhance the processing efficiency, it is preferable to use a solid base rather than K₃PO₄. Studies have shown that hydrotalcite-supported Pd and Cu catalysts can achieve the same reaction. Additionally, replacing butanol with i-propanol leads to improved product yields (Sreekumar et al., 2014). The use of Pd-Cu bimetallic nanoparticles supported on hydrotalcite is particularly advantageous because it mitigates decarbonylation reactions, which can reduce yields compared to monometallic Pd catalysts (Goulas et al., 2016). Furthermore, life cycle analyses of this process indicate a significant reduction (approximately 50 %-80 %) in greenhouse gas emissions when it is optimized for diesel production. By modifying the feedstock to include additional methyl ketones obtained from furans, further coupling reactions can occur, yielding higher molecular weight products (up to C₄₅) suitable for use as lubricants (Balakrishnan et al., 2015).

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Fig. 12

Routes for carbon–carbon coupling of ethanol, including the Lebedev process, the Guerbet reaction, and cross-coupling with the products of acetone-butanol-ethanol fermentation (Abdulrazzaq and Schwartz, 2019b).

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5.1 Hydrogen

Hydrogen has garnered substantial attention from both the scientific community and industries due to its potential as a clean-burning fuel. Presently, the primary method of hydrogen production involves methane steam reforming. However, alcohols, such as EtOH, have emerged as viable alternatives. They exhibit the advantage of facile decomposition in the availability of water at lessen temperatures (typically in the range of 250–600 ℃), offering the prospect of cleaner hydrogen generation (Jalowiecki-Duhamel et al., 2010), (BISWAS and KUNZRU, 2007b). Mondal et al., (2015a) conducted a study on the catalytic oxidative steam reforming of bio-ethanol for hydrogen production using a Rh-promoted Ni/CeO₂–ZrO₂ catalyst. TEM of the catalyst, both fresh and after 36 h of operation, are presented in Fig. 13 (A, B). It revealed that the size of NiO crystals fall within the range of 50–100 nm at the very early stage, consistent with XRD data. Notably, the TEM images of the catalyst after use show carbon deposits agglomerated on the catalyst surface (Bespalko et al., 2011). It's worth mentioning that there was no significant change in NiO crystallite size among both stages that represents a negligible sintering of the catalyst. However, the deactivation of catalysts mostly arises from the accumulation of carbon on the surface of the catalyst, hence diminishing the number of active sites available for the reaction. Fig. 13C presents the results of the investigation, showcasing the ethanol conversion, product selectivities, and hydrogen yield of two catalysts: a 30 %Ni/CeO₂–ZrO₂ catalyst and b 1 %Rh–30 %Ni/CeO₂–ZrO₂ catalyst. Notably, the incorporation of the noble metal Rh (1 wt%) to the 30 %Ni/CeO₂–ZrO₂ catalyst resulted in a significant increase in ethanol conversion and hydrogen yield (ranging from 3.5 to 4.6 mol/mol). The inclusion of this additional component resulted in an enhancement of the product, leading to an increase in the selectivity of hydrogen from 60 % to 71 %. Simultaneously, the selectivity towards carbon monoxide (CO) and methane (CH₄) decreased to roughly 4 % and 3 % respectively. The improved production of hydrogen and its increased selectivity can be due to the catalytic properties of rhodium (Rh), which facilitate the water gas shift reaction and methane steam reforming processes which inhibit the creation of unwanted by-products. Furthermore, the study revealed that with increase in temperature the conversion capacity also enhanced upto 99 % at 600 °C (Fig. 13D). At 500 to 600 °C, the production of H₂ also rises from 2.5 to 3.5 mol/mol in reactor. But beyond 600 °C, this decreased because of the decomposition reaction (Rxn-2). The highest hydrogen yield attained was roughly 3.5 mol of H₂ per mole of ethanol supplied, which is lower than the thermodynamic value of 5. The rise in temperature from 500 to 600 °C resulted in an increase in hydrogen selectivity from 63 % to 65 % and CO₂ selectivity from 18 % to 19 %. Conversely, a declining trend was noted in the selectivity of CH₄ (Mondal et al., 2015b).

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Fig. 13

Transmission Electron Microscopy (TEM) images (A) Represents the fresh 1 %Rh–30 %Ni/CeO₂–ZrO₂ catalyst (B) Represents the used 30 %Ni/CeO₂–ZrO₂ catalyst after a certain duration of operation. (C) The effects of introducing a noble metal at an operating temperature of 600 °C, atmospheric pressure (P = 1 atm), and a molar ratio of EtOH/H₂O/O₂ = 1:9:0.35. (D) The impact of temperature on ethanol conversion, product selectivity, and hydrogen (H₂) yield (Mondal et al., 2015b).

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Fang et al. (2014) used NiXMg₂AlOY ex-hydrotalcite catalysts to do a thorough investigation of hydrogen production from bioethanol. They looked at the catalysts' active sites, which are shown in Fig. 14 and comprise two cations in strong interaction—aluminum or nickel-magnesium. The standard expression for this active site is ^XNi–^YM, where x and y are the degrees of unsaturation of each cation. Earlier studies on Ni-based catalysts (Pirez et al., 2011) and a suggested active site for CeNi_XO_Y catalysts are the foundations of this approach. This provides an understanding of the possible process by which ethanol transforms on Ni_XMg₂AlO_Y compounds. In the context of the ^XNi–^YM ensemble, Fig. 14A illustrates the mechanism by which acetaldehyde and H₂ can be produced by the heterolytic abstraction of hydrogen from ethanol when the site (¹Ni–¹M) has less anionic vacancies. Every individual ^XNi–^YM ensemble is linked to a unique reaction, and the conversion of ethanol might result in a variety of products based on the active site's degree of unsaturation. For example, Fig. 14B demonstrates that H₂, CO, and CH₄ are formed at the ³Ni–¹M site. This model fits the observed synergistic effect in mixed oxides well, when several strongly interacting cations are present. It is worth mentioning that the presence of metallic Ni⁰ can also contribute to the carbon generation process from CH₄ (Ziebro et al., 2010) and/or CO (Yan and Liu, 2013). Nevertheless, blaming the activity on this species alone will not adequately explain the data obtained. Using in-situ X-ray diffraction, the characteristics of the Ni_XMg₂AlO_Y in the presence of H₂ at different temperatures were also investigated. The XRD profile of the Ni₃Mg₂AlO_Y catalyst, acquired during in-situ treatment in H₂, is shown in Fig. 14C. When comparing the diffraction patterns of the fresh catalyst to those at temperatures below 450 °C, no discernible changes were found. On the other hand, diffraction peaks at 2θ = 44.5 and 51.8°, which correspond to the (1 1 1) and (2 0 0) planes of metallic nickel, show that the metallic Ni⁰ phase began to form at 450 °C for 10 h. As the temperature rose to 620 °C, these peaks grew somewhat stronger, and at 76.2 °C, a new peak connected to the Ni (2 2 0) plane appeared. To prevent volume variation problems resulting from gas production, Fig. 14D shows the time history of the Single-Step Reforming (SRE) process under diluted circumstances (EtOH/H₂O/N₂ = 3/9/88). The Ni₃Mg₂AlO_Y catalyst fully converted ethanol at 450 °C and generated the anticipated SRE products, such as H₂ and CO₂, along with some CH₄. However, no CO production was seen. In these conditions, the H₂ yield was measured at 3.0 mol mol_EtOH⁻¹. For this reaction, a partial order to ethanol is indicated by the increase in conversion with decreasing ethanol partial pressure. The formation of CH₄ dropped to almost nothing as the reaction temperature rose to 650 °C, whereas the formation of CO rose to approximately 11 %. On a dry basis, the product distribution at 650 °C was 13 % CO₂, 76 % H₂, 11 % CO, and 0.2 % CH₄ (Mattos et al., 2012). One of the greatest findings published for inexpensive catalysts is the H₂ productivity of 5.0 mol mol_EtOH⁻¹on the Ni₃Mg₂AlO_Y catalyst at 650 °C. Importantly, at high temperatures, the water gas shift equilibrium limits reach the amount of 6.0 mol mol_EtOH⁻¹, and the obtained result exceeds previous reports (Lucrédio et al., 2010), where 4.5 mol mol_EtOH⁻¹ was reported using 150 mg of LaNiMgAl catalyst. and (with 200 mg of NiMg₄ZnAl catalyst at 700 °C) 4.9 mol mol_EtOH⁻¹ (Zeng et al., 2011).

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Fig. 14

Active site and mechanism modeling of Ni_XMg₂AlO_Y catalysts for H₂ production from transformation of bio-ethanol. Niⁿ⁺: Ni²⁺ or Ni^δ+; M^m+: Mg²⁺ or Al³⁺. (A) ¹Ni–¹M site, (B) ³Ni–¹M site. (C) In situ XRD patterns in H₂ for (A) Ni₁Mg₂AlO_Y and (B) Ni₃Mg₂AlO_Y mixed oxides. MgO (○), Ni–Mg–O (■), NiO (▽), Ni (▾). (D) Efficiency of the Ni₃Mg₂AlO_Y catalyst pre-treated in H₂ at 450 °C toward H₂ production. D) Ethanol conversion (♦), H₂ (♢), CO₂ (○), CO (●), CH₄ (▵) formation and H₂ yield (♢). EtOH/H₂O/N₂ = 3/9/88 (Fang et al., 2014).

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5.2 Hydrocarbon fuels

Fuels classified as hydrocarbons, such as petrol, diesel, and jet fuel, are essential energy sources for industry and transportation. They are produced by refining crude oil and are the main energy source used by automobiles and aeroplanes around the globe. Seeking greener and more sustainable substitutes for traditional hydrocarbon fuels is a major emphasis in the energy sector (Van der Borght et al., 2015b). The study conducted by Quintana et al. (2017) examined the potential of carbonate hydroxyapatite (CHAP) as a catalyst for the conversion of ethanol into hydrocarbon fuels (Lovón-Quintana et al., 2017c). Fig. 15 illustrates the ethanol reaction profiles over the CHAP catalyst with respect to time (h), reaction temperature (°C), and modified residence time W/F_Ethanol (g h mol⁻¹). To facilitate investigation, the reaction products were categorized into two primary groups: namely, lighter hydrocarbons including carbon chain lengths ranging from C₁ to C₃, and hydrocarbons containing carbon chain lengths spanning from C₄ to C₁₈ +. Fig. 15A illustrates that after around two hours of continuous reaction, ethanol conversion reached a steady state. Ethanol conversion was roughly 15 % at lower reaction temperatures (300 °C), but it rose with temperature and peaked at 500 °C (nearly 100 %) before falling to 41 % at 600 °C. Fig. 15B shows that from 300 to 500 °C, the production of C₁–C₃ stayed below 8 %. At temperatures more than 500 °C, on the other hand, the yield abruptly rose to 40 %. Fig. 15C shows that as reaction temperature rose, the yield of hydrocarbons higher than C₃ has been increased. This yield peaked at about 97 % at 500 °C and then dropped to 1 % at 600 °C. Significant quantities of dienes, aromatics (Fig. 15E), and ketones (Fig. 15F) were found between 400 and 500 °C. Reactions such as the Meerwein-Ponndorf-Verley (eq. (1) (Tsuchida et al., 2006), (Jones, 2014) and consecutive dehydration of crotyl alcohol to 1,3-butadiene (eq. (2) reaction and Lebedev's reaction (eq.3) can generate dienes, which can then produce butadiene (La-Salvia et al., 2015), (Makshina et al., 2012).

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Fig. 15

Ethanol reaction profiles over CHAP catalyst: (a) ethanol conversion as a function of time-on reaction; (b) product yield to lighter hydrocarbons with carbon chain lengths C₁–C₃ as a function of time-on reaction; (c) product yield to hydrocarbons with carbon chain lengths C₄–C₁₈₊ as a function of time-on reaction; (d–f) evolution of ethanol reaction rate (-r'_Ethanol) or product formation rate (r'_Product) as a function of reaction temperature at constant W/F_Ethanol of 40 g h mol⁻¹; and, (g–i) evolution of ethanol reaction rate or product formation rate as a function of modified residence time W/F_Ethanol at constant reaction temperature of 500 °C.

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At reaction temperatures near 500 °C, the CHAP catalyst showed greater activity for generating hydrocarbons C₄–C₁₈+, despite diffusional restrictions (Fig. 15D). At a fixed temperature of 500 °C, the effects of changed residence time (W/F_Ethanol ) on the reaction were investigated. There was a small increase observed in the production rate of product molecules per unit mass of catalyst and per unit time when the modified residence time was decreased (Fig. 15G), suggesting greater productivity of hydrocarbons C₄–C₁₈₊ with increasing ethanol flow (while keeping the catalyst mass constant). Nevertheless, neither the composition of reaction products nor the conversion of ethanol were considerably impacted by the change in modified residence time. All of the W/F_Ethanol values' curves (Fig. 15 F-I) were almost parallel and equally spaced. The range of 20 to 40 g h mol⁻¹produced the best yield (about 97 %) of C₄–C₁₈₊, with the EtOH productivity being most stable at 40 g h mol⁻¹. When W/F Ethanol was increased from 80 to 160 g h mol⁻¹, the selectivity of C₄–C₁₈₊ decends to almost 90 %. For the majority of hydrocarbon groups and ethanol conversion, the calculated reaction order (n) for the ethanol input was 1.0 ± 0.2. Aldehydes displayed a reaction order of roughly 1.3, but C₁–C₃, olefins, and dienes displayed a lesser order of 0.6 ± 0.1. All of the reactions that resulted in the synthesis of hydrocarbon groups, in summary, had rates that were directly correlated with the ethanol feed, with certain variances observed at various reaction temperatures and product compositions.

6.1 Process modeling and description

Advances in catalyst and process development have informed technoeconomic and life-cycle assessments for the chemical transformation of ethanol into fungible hydrocarbon fuel blendstocks (Hannon et al., 2020a). Historical endeavors in this domain predominantly centered on trilateral procedures encompassing dehydration, oligomerization, and hydrogenation. Nevertheless, the consolidated alcohol dehydration and oligomerization (CADO) technique is increasingly becoming prominent. The majority of technologies employed for catalytic conversion of ethanol to hydrocarbons generally encompass three essential stages prior to fractionation in order to comply with fuel standards. It involves the manufacturing of ethene the degradation of ethanol is utilized. Then, the process of oligomerization which involves the conversion of ethylene molecules into hydrocarbons with larger molecular weights. At last, hydrogenation is employed to achieve saturation of these oligomers, yielding a refined renewable fuel that is appropriate for incorporation into traditional fuel blends. Recent comprehensive research has examined the chemistries and processes involved in the conversion of ethanol into midrange distillate fuels. Typically, published designs necessitate elevated temperatures in the input reactor, exceeding 400 °C, together with pressures within the range of 30 to 40 atm, and the provision of hydrogen from an external source. Companies such as Lanza Tech are currently researching techniques based on this principle in partnership with Pacific Northwest National Laboratory (“Byogy renewables,” n.d.). The anticipated costs for converting ethanol to jet fuel using these three ways range from $3.38/GJ to $7.98/GJ ($0.11 to $0.26/L jet fuel), with comparable figures for diesel. When ethanol prices vary from $17.4/GJ to $52.1/GJ ($0.37 to $1.11/L ethanol) and reported yields are considered, the cost range for jet fuel produced by the three-step process is $25.5/GJ to $86.7/GJ ($0.83 to $2.84/L). In the United States, prices for petroleum-derived jet fuel have ranged from $7.9/GJ to $26.2/GJ ($0.26 to $0.80/L) over the last five years.

6.2 Cost estimation methodology

When examining the Capital Expenditure (CAPEX) and Operating Expenditure (OPEX) aspects of this conversion process, several key factors must be considered. CAPEX entails the initial investment in infrastructure, which includes reactors, catalysts, and facilities. These costs can be substantial, particularly when specialized equipment is required for catalytic processes (Becerra et al., 2017), (Gozan, 2020). Conversely, OPEX encompasses recurring operational costs, including energy, labor, maintenance, and raw materials such as the bioethanol feedstock (Becerra et al., 2018). The efficiency and stability of the catalyst play a crucial role in managing OPEX, as higher catalyst turnover and maintenance needs can significantly impact ongoing operational expenses. The TEA's outcome is vital for determining the overall economic feasibility of the process, taking into account factors like revenue generated from value-added chemicals and fuels, potential byproducts, and market conditions. It assesses the balance between CAPEX and OPEX to ensure cost-effectiveness and long-term sustainability (Cabrera Camacho et al., 2022). In an industrial setting where ethanol is produced and recovered through distillation, a beer column is employed to separate the ethanol-rich broth resulting from fermentation. This separation process involves the release of moist ethanol vapor from the top of the column, while water and any solid wastes are discharged from the bottom. In general, the moist vapor concentration is commonly seen to be roughly 40 wt% (wt%), which corresponds to a feed solution of about 6.4 wt% (8.1 vol/vol%). The aforementioned concentration exemplifies a procedure that yields second-generation ethanol. It is anticipated that the influence of cost estimates, which are dependent on the ethanol concentration exiting the beer column, on the consolidated alcohol dehydration and oligomerization (CADO) process will be negligible. This primarily affects capital costs (CAPEX). However, the sensitivity to cost estimates is expected to be somewhat greater for the production of anhydrous ethanol from 40 wt% ethanol vapor. In the preceding example, both operational expenditures (OPEX) and capital expenditures (CAPEX) are impacted, specifically in relation to the demand for distillation steam.

In this study, we conduct cost comparisons to evaluate the conversion of wet ethanol vapor into anhydrous ethanol or fungible blendstocks using the CADO method, considering several process configurations. Both instances are predicated on an annual ethanol manufacturing capacity of 61 million gallons per year (equivalent to 231 million liters per year), a common figure for small-scale ethanol facilities in the United States and Brazil, as posited by the National Renewable Energy Laboratory (NREL) in their Techno-Economic Analysis of cellulosic ethanol production. In the case of anhydrous ethanol, moisture-containing ethanol vapor traverses a rectifying column before being dehydrated via molecular sieve. During the CADO process, the gaseous form of ethanol is introduced into a catalytic reactor where it undergoes a chemical transformation, resulting in the formation of liquid hydrocarbons, light gases of ethylene, and water. The mixture is subsequently cooled to the surrounding temperature and introduced into a 3-phase separator. Within this separator, water is extracted from the lower section, less-dense liquid hydrocarbons are obtained from the middle section, and noncondensable gases are collected from the upper section. Approximately 73 % of the gas, by mass, is returned to the reactor for the purpose of converting it into liquid fuels. The remaining portion is combusted in order to decrease the reliance on natural gas for process heat (Hannon et al., 2020b).

6.3 Economic evaluation of bioethanol conversion

The estimation of the production cost of fuel-grade ethanol derived from moist ethanol vapor was conducted utilizing the 2011 NREL model (36). The expected prices for catalyst purchases, specifically for the process of transformation of wet ethanol into fungible blendstocks, were determined based on the NREL catalyst cost estimator (37). The current and future catalyst purchase prices were projected to be $70/kg and $30/kg, respectively, taking into account the current and anticipated metal loadings. The costs mentioned were confirmed by conversation with a producer specializing in commercial catalysts. The expected catalytic lifespan for experimental powder and pilot pellets catalysts was 6 months, however it was increased to 9 months for current commercial and future catalysts. This extension was calculated using half-lives and extrapolation from 200-hour aging studies. Longer-term aging studies might lend additional credibility to these statistics. A standard electricity rate of $0.05 per kilowatt-hour (kWh), which is commonly observed in the midwestern region of the United States, was utilized. The collective expenses associated with catalyst installation and electricity consumption constituted nearly 60 % of the overall operational expenditures. A supplementary fee of $1.85 per 1,000 L of water that has been treated was incorporated to account for the expenses associated with chemical wastewater treatment additives utilized in the elimination of dissolved hydrocarbons. At a unit cost of $2,000 per ton, catalyst disposal costs covered the cost of sending used catalyst back to the supplier. To be consistent with the NREL anhydrous ethanol model, additional expenses were incorporated, such as insurance, taxes, and maintenance (36). The labor demands associated with the transformation of fungible blendstocks into ethanol were presumed to be comparable to those required for ethanol dehydration, owing to the same intricacy of the unit activities entailed.

The NREL model, modified to 2019 USD, was utilized to estimate the Capital Expenditures (CAPEX) amounting to $19.7 million for the manufacture of anhydrous ethanol from wet ethanol vapor (36). The CAPEX numbers were annualized using a multiplication by 0.15, which represents the amortization over a 10-year duration at a 12 % interest rate. In accordance with the prescribed approach, the yearly cost of capital investment amounted to $0.031 per liter of anhydrous ethanol, with a lower heating value (LHV) of $1.46 per gigajoule. The production of fungible blendstocks from wet ethanol vapor is achieved through the utilization of the Consolidated Alcohol Dehydration and Oligomerization (CADO) process. This process involves several key equipment components, including two catalytic reactors that facilitate catalyst regeneration, a three-phase decanter, and four heat exchangers. Additionally, ancillary equipment such as pumps and load-out systems are employed to support the overall operation. The assumption was made that the requirements for storage tanks of fungible blendstock and water-products would be identical to those for ethanol storage. The aqueous solution containing relatively small amounts of dissolved hydrocarbons (usually ranging from 600 to 1,500 parts per million) that remained after the separation process was directed into the wastewater treatment system. The experimental setup consisted of two compressors: one was utilized to generate the required pressure of 0.417 MPa for the passage of wet ethanol through the reactor, while the other was employed for gas recycling purposes. It should be noted that the use of compressors may not be essential in scenarios where distillation proceeds under pressure. Given that a back pressure-controlled flash system might potentially be used in practical applications for the development of a new factory with environmental considerations, the decision to add a compressor into the design can be regarded cautious. As a result, the total installed CAPEX was expected to be around $15.3 million, which is comparable to $0.067/L of ethanol supply or $0.121/L of fungible blendstocks. For both current and future technology, CAPEX for processing via CADO was considered to remain the same. Using the identical 0.15 capital recovery factor as for anhydrous ethanol, the annualized CAPEX contribution for ethanol conversion to fungible blendstocks via CADO was $0.018/L ($0.56/GJ LHV) (Hannon et al., 2020b).