3.1 Industrial process for 2-ethylhexanol synthesis from fossil resources
The industrial process of synthesizing 2-ethylhexanol is production from fossil resources (Hafenstine et al., 2017b; Stewart et al., 2020). It involves three major steps: hydroformylation of propylene to butyraldehyde, self-condensation of butyraldehyde for 2-ethyl-2-hexenal, and hydrogenation of 2-ethyl-2-hexenal into 2-ethylhexanol (Fig. 6) (Zhang et al., 2014; Liang et al., 2015). During the first step, the Co/Rh based catalyst is commonly used, with the phosphine modified Rh catalysts mainly adopted in this step. The second step is often catalyzed by dilute aqueous solution of NaOH, in which butyraldehyde is quantitatively converted to 2-ethyl-2-hexenal. In the last step, hydrogenation can be carried out in the gas phase over either Ni or Cu catalysts. The overall process requires high capital cost due to high energy input, extensive use of alkali and treatment of organically contaminated caustic wastewater (Kelly et al., 2002; Li et al., 2021). To reduce the consumption of alkali during the condensation step, alternative catalysts have been designed to the condensation of butyraldehyde and production of 2-ethylhexanol. Liang et al. (2015) reported the use of Ni/Ce-Al2O3 for one-pot conversion of butyraldehyde to 2-ethylhexanol, and a maximum 2-ethylhexanol yield of 68.5% was obtained with 100% conversion. In their follow-up study, another Al2O3 based catalyst, Ni/La − Al2O3, was applied for the same reaction, and a maximum yield of 67.5% was obtained (Li et al., 2016b). Moore et al. (2017) reported a cascade condensation and hydrogenation reactions for 2-ethylhexanol production form butyraldehyde by Amberlyst 15, Pd/C and Ni/SiO2-Al2O3. First, butyraldehyde was converted to 2-ethylhexanal via the condensation and hydrogenation of the C = C bond by Amberlyst 15 and Pd/C in cyclohexane, followed by hydrogenation of aldehyde catalyzed by Ni/SiO2-Al2O3. Compared with the one-pot process, this cascade reactions showed a higher overall yield of 2-ethylhexanol of 95%, but the processes are more complicated. In terms of the source of the major feedstock (i.e., propylene), about 70% of propylene worldwide production comes from steam crackers, 28% from refinery fluid catalytic cracking units and 2% from specific on-purpose processes, such as propane dehydrogenation (Corma et al., 2005).
3.2 Lignocellulosic biomass as feedstock for producing 2-ethylhexanol precursors
In recent years, with the continuous consumption of non-renewable resources (coal, petroleum, etc.), the utilization of renewable biomass has become research highlights. Among various available biomasses, lignocellulose, such as agricultural wastes, forest residues and energy crops, is widely distributed and abundant (Yang et al., 2019; Andhalkar et al., 2022). Lignocellulose mainly comprises three components: cellulose, hemicellulose, and lignin (Liu et al., 2021; Andhalkar et al., 2022), as shown in Fig. 7. The depolymerizations and valorizations of these components determine the sustainable use of lignocellulosic biomass (Schutyser et al., 2018; Questell-Santiago et al., 2020). In petroleum refinery, crude oil was first refined into naphtha, gasoline, kerosene, gas oil and residue, among which the naphtha fraction was used for producing a few platform chemicals, such as ethylene, propylene, C4-olefines and aromatics (benzene, toluene, and xylene) (Cherubini, 2010). Then the petroleum derived platform chemicals can be converted into various chemicals and materials to meet different needs. To mimic the petroleum refinery, it was proposed that the biorefinery should first convert the lignocellulose into a limited number of platform chemicals, from which other commodity and bulk chemicals can be derived via different catalytic pathways to meet different utilization purposes (Li et al., 2016a; Li et al., 2018).
For producing 2-ethylhexanol from lignocellulosic biomass, the first step would be the conversion of it into precursors (platform chemicals). The main precursors that could be used for producing 2-ethylhexanol include ethanol, butanol, butyraldehyde, and syngas as discussed in following sections. In terms of producing syngas from lignocellulosic biomass, gasification is the dominant route. Depending on temperature, pressure and catalyst, the syngas compositions vary, but with CO and H2 usually as the major components (Nanda et al., 2014). CO and H2 can serve as starting materials for the synthesis of 2-ethylhexnal, which was discussed in later section. For producing other precursors, especially ethanol and butanol, the biological conversion is still the preferred pathway, which entails the conversion of carbohydrate fractions (i.e., cellulose and hemicellulose) in lignocellulosic biomass into platform sugars (e.g., glucose and xylose) first. However, due to the recalcitrant nature of lignocellulosic biomass, a pretreatment step is often required before it can be facilely converted into sugars via enzymatic hydrolysis. It is estimated that the pretreatment step can account for 20% of the total costs for cellulosic ethanol production, representing the single, most expensive processing step (Yang and Wyman, 2008). Besides, due to the low sugar concentration obtained from enzymatic hydrolysis of pretreated lignocellulosic biomass, a low product titer is often encountered in lignocellulosic biorefinery, which inevitably incurs a high cost for downstream processing (e.g, product recovery and purification) (Nguyen et al., 2016). Thus, development of cost-effective pretreatment that enables facile conversion of lignocellulosic biomass into fermentable sugars at high titer is critical for producing bio-based chemicals in an economically viable way. To this end, a variety of efficient pretreatment methods have been developed, which could potentially reduce the costs associated with precursors production and the 2-ethlyhexnanol production cost accordingly. Many excellent reviews are available to discuss the advantages and disadvantages of different pretreatment and their economic viability (Bhatia et al., 2020; Woiciechowski et al., 2020; Sidana and Yadav, 2022), and interested readers are referred to these reviews for more comprehensive understanding of pretreatment technology.
It is noteworthy that while traditional pretreatment, such as dilute acid pretreatment (DAP) and steam explosion, are still popular, several recently developed pretreatment technologies have the potential to replace these methods with lower costs and higher efficiency. Bao’s group modified the traditional DAP pretreatment by pre-soaking the biomass with dilute acid, followed by the introduction of steam onto the pre-soaked biomass (Zhang et al., 2011). This pretreatment could be conducted at extremely high solid loading (50%), but did not sacrifice the sugar yield during the enzymatic hydrolysis step as more than 80% yield was obtainable (Hou et al., 2018). Jin’s groups developed a pretreatment called densifying lignocellulosic biomass with chemicals (DLC), which utilizes pelletization to densify the biomass with simultaneous incorporation of either alkali or acids into the pellets (Shen et al., 2022; Yuan et al., 2022). This technology not only rendered lignocellulosic biomass with good digestibility without detoxification, but also enables the reduction of costs related to handling the biomass (e.g., transportation, storage, etc.) due to the much higher density of pelleted biomass. Hydrolysate with total sugar concentration up to 180 g/L could be obtained when the DLC pretreated corn stover was enzymatically hydrolyzed at 35% solid loading (Yuan et al., 2022). Chen et al. (2016) developed a deacetylation and mechanical refining (DMR) pretreatment in which lignocellulosic biomass was first subjected to deacetylation by dilute alkali solution, followed by mechanical refining to produce highly digestible biomass. Up to 230 g/L sugar hydrolysate could be obtained from the DMR pretreated corn stover at 28% solid loading. All these pretreatment methods were demonstrated to be scalable, and when the sugar hydrolysate derived from biomass pretreated by them was used for ethanol production, the minimum ethanol selling prices (MESP) was lower than that from DAP (Zhang et al., 2011; Chen et al., 2016; Yuan et al., 2022), suggesting their superiority in reducing production costs for producing sugar platforms. With further optimization of such pretreatment methods, the production costs for platform sugars would be expected to be lower, which will eventually reflect on the decreased production costs for their derived chemicals (i.e., 2-ethylhexanol here).
In addition to the advances in pretreatment, strides have also been made in development of more efficient enzyme cocktails for hydrolyzing cellulose, as well as development of more robust and efficient microbial chassis for converting biomass derived mixed sugars. For instance, lytic polysaccharide monooxygenases (LPMOs) are a group of mono-copper redox enzymes that is capable of disturbing the crystalline structure of the cellulose and facilitating cellulose hydrolysis. When LPMOs was coordinated with commercial cellulases, cellulose digestibility can be significantly improved (Müller et al., 2018). As a newly discovered enzyme, the properties of LPMO still could not meet the industrial needs for enzymatic saccharification processes (Forsberg et al., 2020). The advent of machine learning has spawned the generation of AlphaFold and CLEA, which are powerful tools for predicting enzyme structure and functions (Jumper et al., 2021; Yu et al., 2023). With the computer-aided understanding of enzymes, efficient design of LPMOs and other related enzymes with desired properties is highly possible, and would further promote the cellulose hydrolysis. Meanwhile, advanced genome editing tools, such as CRISPR, are available for us to reprogram microbial cell factories to meet different applications needs (Shanmugam et al., 2020). Thus, with the advances in pretreatment, enzyme engineering, microbial cell factories construction, production of precursors for synthesizing 2-ethylhexanol from lignocellulosic biomass is expected to reach more efficient and costly effective.
3.3 2-Ethylhexanol synthesis from butanol
Guerbet reaction is the condensation of two molecules of alcohol to form an alcohol with an increased number of carbon atoms (Kulkarni et al., 2018). In the case of 2-ethylhexanol synthesis by Guerbet reaction from butanol, two molecules of butanol are condensed to synthesize one 2-ethylhexanol (Miller and Bennett, 1961; Zhao et al., 2018). Renewable butanol can be produced from the canonical acetone-butanol-ethanol (ABE) fermentation with lignocellulosic biomass derived sugars as substrates. The sequence of Guerbet coupling of butanol has been highly debated, but is generally accepted to proceed through three distinct reaction steps. As shown in Fig. 8, during the reaction, butanol is first dehydrogenated into butyraldehyde with the concomitant production of H2, followed by self-condensation of butyraldehyde to yield 2-ethyl-2-hexenal. Finally, the 2-ethyl-2-hexenal is hydrogenated to produce 2-ethylhexanol.
As early as 1961, Miller and Bennett (1961) reported the synthesis of 2-ethylhexanol from butanol via Guerbet coupling during which activated copper and tripotassium phosphate were used as catalysts. This process required severe conditions and a high catalyst loading, while the selectivity to 2-ethylhexanol was rather low. Using hydroxyapatite comprising Mg, Ca, Sr and Ba as catalyst, Ozer et al. (2013) reported successful 2-ethylhexanol synthesis from butanol in gas phase, but the yield was still low (12%). To improve the selectivity and yield, more effective catalytic systems have been developed. A catalytic system based on copper chromite/BuONa for the coupling of butanol to 2-ethylhexanol was reported by Carlini et al. (2004) with a butanol conversion of 61.1% and a high selectivity of 100% obtained after reaction at 280 ℃ for 6 h. However, the byproduct of water formed during condensation can cause the hydrolysis of BuONa, and accordingly, deactivation and non-reusability of the catalyst. To prevent the catalysts deactivation caused by water, Han et al. (2021) integrated the 1,1-dibutoxybutane hydrolysis into Gubert coupling of butanol to improve 2-ethylhexanol production, with an overall 2-ethylhexanol yield of 51.7% achieved.
The above-mentioned catalytic systems require either pure butanol or gasified butanol as feeding substrates. However, the biological process for butanol production often features a low titer, and therefore, an aqueous butanol solution if no extensive separation and purification steps are used. Moreover, the catalysts used in above studies showed poor recyclability. Thus, the development of efficient, recyclable yet water-tolerant catalysts for Guerbet coupling of butanol would benefit the reduction of overall cost. Mu’s group reported the use of immobilized iridium catalyst for the condensation of aqueous butanol into 2-ethylhexanol, with the conversion and yield of 42% and 36% achieved, respectively (Xu et al., 2014). In their follow-up study, iridium nanoparticles supported on hierarchical porous N-doped carbon was used as an effective catalyst to catalyze the self-condensation of the butanol with the highest conversion of 49.8% and yield of 46.5% achieved (Liu et al., 2016a). Remarkably, both of the developed catalysts showed good reusability, and the recycled catalyst demonstrated similar catalytic performance as fresh catalyst for four consecutive runs. However, compared with the non-aqueous based catalytic processes, the aqueous ones showed lower 2-ethylhexanol yield due to the inhibitory effects of water.
One major limitation for the Guerbet coupling of butanol for 2-ethlyhexanol synthesis is the low conversion rate of butanol to butyraldehyde even at severe conditions, which leads to an overall low atom efficiency and high energy consumption. To tackle these issues, biocatalysts have been employed to reduce reaction severity and improve butyraldehyde selectivity and yield. In one study conducted by Hafenstine et al. (2017b), alcohol dehydrogenase was used to dehydrate butanol into butyraldehyde, which was further condensed into 2-ethyl-2-hexenal via the catalysis of β-alanine at room temperature. Although the reaction conditions are much milder than Guerbet coupling reaction, the overall yield from biocatalytic process is rather low due to the ineffective conversion of butanol to butyraldehyde. Recently, Stewart et al. (2020) modified this pathway by applying Gluconobacter oxidans as a whole cell biocatalyst to effectively oxidize butanol to butyraldehyde, which was then condensed into 2-ethyl-2-hexenal under the catalysis of lysine with an overall yield as high as 84% achieved. Thus, by carefully selecting the biocatalysts for the dehydration of butanol into butyraldehyde, effective production of 2-ethyl-2-hexenal, the precursor of 2-ethylhexanol, is feasible. Moreover, compared with Guerbet coupling, biocatalytic coupled chemocatalytic system is durable in aqueous butanol, and give higher yield with 70% yield of 2-ethylhexanol (Stewart et al., 2020). However, these studies used the homogenous catalysts for the condensation of butyraldehyde, incurring difficulties in catalysts recycling and reusing. In contrast, heterogenous catalysts, such as those based on silicas as support and amine containing silanes as catalysts, may be employed to catalyze the self-condensation of butyraldehyde while guaranteeing good reusability (Lauwaert et al., 2015). Of course, when these catalysts work with biocatalysts, their biocompatibility should be carefully examined to ensure that they exert no significant inhibitions on the biocatalysts.
Although conversion of butanol to 2-ethylhexanol is technically feasible, economical production of butanol from lignocellulosic biomass remains to be another barrier for 2-ethylhexanol production from butanol. Research efforts from both academia and industry have propelled the successful industrialization of butanol production from lignocellulosic biomass, but the biological production of butanol still face several challenges, including low titer, yield, productivity, and energy-intensive recovery (Green, 2011). In the classical ABE fermentation, the anaerobic bacterium Clostridium acetobutylicum is the key player for butanol production (Liao et al., 2015). Other clostridia, such as C. beijerinckii and C. saccharoperbutylacetonicum have also been explored for butanol production (Ezeji et al., 2007; Yao et al., 2017). The development of genome editing tools in recent years, on one hand, improved the performance of these solventogenic strains for butanol production. On the other hand, the nonconventional strains can also be repurposed for butanol production with some unique properties that traditional butanol producers do not have. For example, Zhang et al. (2018) recently reported harnessing the native CRISPR system of C. tyrobutyricum for genome editing, and successfully transformed this native butyrate producer into a hyperproducer of butanol, which can produce up to 26.2 g/L butanol under batch fermentation. To further improve the butanol titer and productivity, process development is also critical. To alleviate the inhibitory effect of butanol and improves its titer and productivity, different strategies have been developed to remove the butanol from the fermentation broth. The strategies include bi-phasic solvent extraction, air-stripping, pervaporation, and the integrated strategies (Cai et al., 2016; Rochón et al., 2017; Kushwaha et al., 2019). Nguyen et al. (2018) developed a process integrating distillation under low pressure and high cell density fermentation for production of butanol at higher titer and productivity, which reached 550 g/L and 14 g/L/h, respectively. Anbarasan et al. (2012) applied solvent extraction to improve butanol production from C. acetobutylicum, and integrated the extractive fermentation with chemical catalysis to continuously convert butanol into fuel precursors via C–C coupling reactions. Xin et al. (2018) found that the fed-batch culture could further increase butanol yield up to 33.90 g/L with in situ extraction using biodiesel. In addition to batch and fed-batch, studies have also reported the use of continuous fermentation for butanol production. High densities of bacteria can be accomplished by cell immobilization. Kihara et al. (2019) developed highly efficient continuous ABE production by cell recycling, resulting in ABE productivity of 5.32 g/L/h with high xylose consumption, which could promote the efficient utilization of lignocellulose. Furthermore, Chang et al. (2022) immobilized C. acetobutylicum ATCC55025 in a single-pass fibrous-bed bioreactor (FBB) for continuous production of butanol from glucose and butyrate at the highest productivity (16.8 g/L/h) ever achieved for biobutanol fermentation. Thus, the integration of metabolic engineering of butanol producers, fermentation process development, chemical catalysts development, as well as cell immobilization and cell recycling technology development may enable continuous production of 2-ethylhexanol without extensive product/intermediate separation, and would reduce the overall production costs accordingly.
Guerbet condensation of butanol to 2-ethylhexanolfeatures a simple reaction pathway and therefore a high atomic utilization and its feed n-butanol can be produced by bioprocesses (Serrano et al., 2018; Wang et al., 2020). As a whole, this route conforms to some of the requirements of the principles of green chemistry (Han et al., 2021). However, Guerbet coupling of butanol for 2-ethylhexanol production is neither cost-effective nor applied industrially at current stage because of undesirable reactions, high catalyst loading, and low yield (Ghaemi and Zerehsaz, 2018). Although butanol production from lignocellulosic biomass has been deployed at industrial scale, there are still several challenges limiting its economical production. Hence, the pathway from butanol to 2-ethylhexanol via Guerbet coupling is not economically feasible at current stage, and future efforts should be devoted to improving microbial butanol production and development of more efficient chemocatalysts for Cuerbet coupling of butanol. Combining extractive fermentation of butanol with Guerbet coupling may simplify the production process, and provide economic advantages for 2-ethylhexanol production from biogenic butanol. Integrating biocatalysts with chemocatalysts offers a new pathway for producing 2-ethylhexanol from butanol at milder conditions and higher yield, but this is not a mature process. In future studies, techno-economic analysis related to 2-ethlyhexanol production from biobased butanol have to be performed before we can determine if this pathway would be promising and economically feasible.
3.4 2-Ethylhexanol synthesis from ethanol
Ethanol is currently the dominant chemical produced from lignocellulosic biomass, and the Renewable Fuels Association of 2022 mandated the production of 15 billion gallons of bioethanol per year in U.S and is the world’s largest producer ( Renewable Fuels Association, 2022). Some recent studies have demonstrated that ethanol production from lignocellulosic biomass can reach up to 100 g/L with a yield of 75% using simultaneous saccharification and fermentation, a level comparable to corn ethanol production (Nguyen et al., 2017; Liu et al., 2018). Owing to the process development in recent years, the cellulosic ethanol yield is improved and its production cost was decreased accordingly. Liu and Bao (2017) conducted a comparison study to investigate the MESPs for corn ethanol and cellulosic ethanol, and found that the process based on dry dilute acid pretreatment and bio-detoxification gave a MESP of $1.79 for cellulosic ethanol, which was even lower than that corn ethanol ($2). Likewise, Yuan et al. (2022) and Chen et al. (2016) performed a techno-economic analysis for cellulosic ethanol production based on DLC and DMR pretreatment, respectively, and both found that the MESP for cellulosic ethanol was close to that of corn ethanol. Meanwhile, the inefficient utilization of xylose by Saccharomyces cerevisiae, which is one bottleneck for cellulosic ethanol production, has become gradually overcome. Recently, Chen et al. (2023) discovered a series of new xylose isomerase via big data mining and modified them via rational design. By introducing the xylulose isomerase into S. cerevisiase, an engineered strain able to efficiently utilize xylose was obtained, and can produce 94.8 g/L ethanol from pretreated corncob. Due to such advances, it could be envisioned that the cellulosic ethanol production would be further expedited. However, the ethanol blended with gasoline is unlikely to exceed the current 10% and the demand for gasoline is projected to decrease due to increased production of electric vehicles, suggesting a possible surplus of bioethanol in the near future (Moore et al., 2017). Given the improved bioethanol production capability, decreased demand for its, and its possible surplus, ethanol may serve as an abundant and relatively cheap starting material for 2-ethylhexanol production in the future. Direct 2-ethylhexanol production via Guerbet coupling of ethanol is often not practical since the selectivity to 2-ethylhexanol is pretty low. For example, Eagan et al. (2019) compared the 2-ethylhexanol production from Guerbet coupling of ethanol and butanol under the catalysis of calcium hydroxyapatite. Results from their study revealed that the yield of 2-ethylhexanol was only 4.6% from ethanol at a conversion of 89.9%, while it was 45.1% for butanol with a conversion of 74.2%. The lower yield mainly stems from the competing reactions for the formation of other alcohols, such as 2-ethyl-1-butanol, 1-hexanol, and butanol.
To reduce the side reactions and improve the overall yield of 2-ethylhexanol with ethanol as feedstock, one solution is to first dehydrogenate ethanol into more reactive acetaldehyde, which then undergoes poly aldol condensation and hydrogenation to yield 2-ethylhexanol (Poulikidou et al., 2019). Numerous studies have explored production of acetaldehyde from ethanol via either oxidative dehydrogenation or non-oxidative dehydrogenation by chemocatalysts or biocatalysts. Different from oxidative dehydrogenation which requires O2 and produces H2O as the by-product, the non-oxidative dehydrogenation requires no oxygen and produce H2 instead. In the context of 2-ethylhexanol production, the produced H2 may be used to hydrogenate the C = C in the condensed products of acetaldehyde, and save the cost correspondingly. Thus, non-oxidative dehydrogenation seems to be more compelling for 2-ethylhexanol production. Cu supported on oxides, such as Al2O3 and SiO2 are currently the dominant catalysts used for the dehydrogenation of ethanol to acetaldehyde via the non-oxidative pathway (Fujita et al., 2001; Wang et al., 2015; Shan et al., 2017). However, undesired reactions, such as aldolization and ketonization, occur along with the formation of acetaldehyde, leading to a low selectivity. Thus, modification of the Cu based catalysts is necessary to tune its selectivity. By supporting the Cu on exfoliated graphite oxide or nitrogen doped graphite oxide, the selectivity of acetaldehyde from ethanol dehydrogenation could reach up to 98%, and was further improved to 100% when water was added (Morales et al., 2016). Wang et al. (2018) also reported by supporting Cu on carbon-coated porous silica, the selectivity was improved from 76–93%. However, no matter which pathway is taken toward acetaldehyde production from ethanol, catalyst deactivation could not be avoided owing to sintering of metallic catalysts at the high dehydrogenation temperature (Shan et al., 2017). Thus, improving catalyst stability during catalytic reaction is critical. Surface modification of the metallic catalysts used for non-oxidation dehydrogenation of ethanol is an effective method to improve their stability. By incorporating a small amount of Ni into Cu catalyst could greatly improve the stability of Cu by preventing the sintering (Shan et al., 2017). Acetaldehyde condensation could yield crotonaldehyde, which can then undergo hydrogenation to produce butyraldehyde suitable for 2-ethylhexanol production.
In addition to the chemocatalysts used for dehydrogenation of ethanol to acetaldehyde, bio-catalysts have also been explored. Goodwin’s group reported the integration of biocatalyst and chemocatalyst for converting ethanol into 2-ethyl-2-hexenal in the fermentation broth (Hafenstine et al., 2017a). Catalyzed by alcohol dehydrogenase (ADH), ethanol was first oxidized into acetaldehyde, which then underwent condensation to form crotonaldehyde by an organocatalyst (β-alanine). The crotonaldehyde could be further hydrogenated by the in-situ generated H2 from the ethanol oxidation step under the catalysis of Pd nanoparticles, yielding butyraldehyde. After self-condensation catalyzed by β-alanine, the butyraldehyde is converted to 2-ethyl-2-hexenal. Although this proposed multi-catalytic systems could skip the costly step of separation and purification of intermediates, the major problem is the low yield (less than 10%) due to the ineffective conversion of alcohol to aldehyde.
3.5 2-Ethylhexanol synthesis from butyraldehyde
Compared with butanol, butyraldehyde has a much lower boiling point, which can be easily recovered with much less energy consumption. Moreover, butyraldehyde can fit well with the traditional processes for 2-ethylhexanol production with no further investment in process development, and can save the costs accordingly. Thus, if butyraldehyde can be directly produced from lignocellulosic biomass at a decent yield, the overall economics for 2-ethylhexanol production could be significantly improved.
As a matter of fact, several studies have reported the bioproduction of butyraldehyde via fermentation. A mutant strain of C. acetobutylicum with the alcohol dehydrogenase deactivated was capable of accumulating 1.6 g/L of butyraldehyde (Rogers and Palosaari, 1987). Ku et al. (2017) reported that by introducing butyraldehyde biosynthesis pathway and eliminating native alcohol dehydrogenase genes in Escherichia coli, the engineered strain can produce 0.16 g/L butyraldehyde. The butyraldehyde titer was further improved to 0.63 g/L when an overlayer organic solvent was applied to extract butyraldehyde from the fermentation broth, but was still lower than that of mutated C. acetobutylicum. Such difference may be ascribed to the better capability of C. acetobutylicum to draw the carbon flux toward butyraldehyde. Thus, producers of butanol or butyrate may be good host microorganism that can be engineered toward high titer butyraldehyde production. Although engineering of microorganisms for butyraldehyde production is possible, the high toxicity of butyraldehyde to microorganisms is a key barrier for its effective production. The toxicity of aldehyde on microorganisms has not been well understood, but it has been suggested that the damage of cell membrane and DNA by aldehydes are possible reasons (Kunjapur and Prather, 2015). Better understanding of the tolerance mechanisms of microorganisms to butyraldehyde will help the engineering of butyraldehyde producers highly tolerable to butyraldehyde. Moreover, process development for effective in-situ removal of butyraldehyde from fermentation medium as those adopted for butanol fermentation, such as solvent extraction, would also mitigate butyraldehyde inhibition on microorganisms and improve titer and productivity. Other than those traditional methods used for product removal, a more appealing way for removal of butyraldehyde and reducing its inhibition is in-situ conversion of butyraldehyde into 2-ethyl-2-hexenal (Fig. 9). Since 2-ethyl-2-hexenal is more hydrophobic than butyraldehyde, it has a higher partition coefficient than butyraldehyde in hydrophobic extraction solvents. By directly upgrading butyraldehyde into 2-ethyl-2-hexenal and extracting 2-ethyl-2-hexenal into hydrophobic solvents, inhibition of butyraldehyde to fermentation strains could be largely mitigated. Moreover, it would also skip the operation units for butyraldehyde separation and improve the overall economics possibly. It appeared that the key step in such processes is the design of catalysts that are effective in catalyzing butyraldehyde condensation and are compatible with butyraldehyde producers. Domaille et al. (2016) reported that by adding 0.4 M β-alanine to the butyraldehyde contained medium in the presence of E. coli, 2-ethly-2-hexenal could be produced at a yield of 70%, while the E. coli showed good viability. Albeit the good yield and biocompatibility offered by β-alanine, several challenges, such as economical catalyst recycling and reusing, remain to be overcome. Thus, in addition to the biocompatible chemocatalysts, other more efficient catalysts with higher conversion rates, such as enzymes capable of catalyzing the condensation reactions, should also be explored. One potential enzyme that can catalyze the condensation of butyraldehyde is the aldolase. Recently, Meng et al. (2021) reported the condensation of formaldehyde and acetaldehyde into 3-hydroxypropionaldehyde by a deoxyribose-5-phosphate aldolase (DERA). Nemr et al. (2018) also showed that the DERA can catalyze the condensation of two acetaldehyde into β-hydroxyaldehydes. As a matter of fact, aldolase have been explored to synthesize numerous aldol chemicals, as well as complex organic chemicals. However, there is a lack of studies concerning the identification of effective aldolases specific for butyraldehyde condensation. Metagenomic approaches can be leveraged in mining aldolases with high reactivity toward butyraldehyde condensation. Protein engineering serves as another powerful tool for improving properties of known aldolases and would provide new enzymes suitable for butyraldehyde condensation.
3.6 2-Ethlyhexanol synthesis from syngas
Lignocellulosic biomass gasification is a process in which the biomass react with air, oxygen, and/or steam to produce a gas product called syngas that contains CO, H2, CO2, CH4, and N2 in various proportions (Dagle et al., 2016). CO and H2, the major components in syngas, can be used for synthesis of propylene. Once the propylene is synthesized, it can be used as the feedstock for producing 2-ethylhexanol following the industrial process as introduced in Section 3.2, which includes its hydroformylation to butyraldehyde, self-condensation of butyraldehyde into 2-ethyl-2-hexenal, and hydrogenation of 2-ethyl-2-hexenal in 2-ethylhexanol. Fischer-Tropsch (FT) process is well-known for synthesis of hydrocarbon fuels from a CO–H2 mixture over iron- or cobalt-based catalysts, and is popular for propylene synthesis (Jun et al., 2004). However, the FT process used for producing propylene from syngas often experiences low selectivity, and the product distribution of FT is so complex that selective control of the reaction for producing target components is the bottleneck of this process (Zhai et al., 2016). Karim and Ahmed (2016) developed a process for converting syngas into propylene with a catalyst composed of Co, Mn, La, P and one alkali metal. This catalyzed process yielded a mixture of ethylene, propylene and aliphatic hydrocarbons, which underwent two more separation steps to recover propylene with a relatively high purity (Karim and Ahmed, 2016). Zhong et al. (2016) reported the use of cobalt carbide nanoprisms for producing propylene from syngas, with much improved propylene selectivity obtained due to the suppressed methane synthesis.
To improve the selectivity of syngas to propylene, an alternative pathway is the synthesis of methanol and dimethyl ether (DME) as intermediates, which are then converted into propylene (Fujiwara et al., 2019). In this pathway, syngas was first converted to methanol or DME, which then undergoes methanol to propylene (MTP) or DME to propylene (DTP) process for propylene synthesis (Lavrenov et al., 2015). Both of the MTP and DMP processes have been industrialized, and interested readers are referred to several recent excellent reviews covering this topic (Khanmohammadi et al., 2016; Ezhova et al., 2020).
In conclusion, to mimic industrial process for 2-ethylhexanol production, butanol, ethanol, butyraldehyde and syngas should be first produced from lignocellulosic biomass via biological, chemical or integrated pathways. Then, the obtained precusors could be upgraded into 2-ethylhexanol as illustrated in Fig. 10 and Table 2. Different from current industrials processes based on fossil resources as feedstocks, approaches based on lignocellulosic biomass have their own advantages, such as environmentally friendliness, reduced carbon emission, etc.
Table 2
The comparison between different 2-Ethylhexanol production pathways.
Intermediate compound | Resource | Major pathways | Characters | The conversion rate and yield | References |
Propylene | Fossil energy | (1) Hydroformylation of propylene to butyraldehyde; (2) self-condensation of butyraldehyde for 2-ethyl-2-hexenal; (3) hydrogenation of 2-ethyl-2-hexenal into 2-ethylhexanol | High conversion rate; but high energy input, extensive use of alkali and treatment of organically contaminated caustic wastewater, high cost, environmentally unfriendly | About 95%; 60–70% | (Liang et al., 2015; Li et al., 2016b; Moore et al., 2017; Poulikidou et al., 2019; Li et al., 2021) |
Butanol | Lignocellulosic biomass | (1) Dehydrogenated into butyraldehyde with the concomitant production of H2; (2) self-condensation of butyraldehyde to yield 2-ethyl-2-hexenal; (3) hydrogenation of 2-ethyl-2-hexenal to 2-ethylhexanol | A green route with a high atomic utilization, but high catalyst loading, and low yield | 84%; 45–70% | (Ghaemi and Zerehsaz, 2018; Stewart et al., 2020; Wang et al., 2020; Han et al., 2021) |
Ethanol | Lignocellulosic biomass | Method 1: (1) Guerbet condensation to C4+ alcohols; (2) dehydration to 2-ethylhexanol and other C8-C16 compounds; Method 2: (1) dehydrogenate ethanol into more reactive acetaldehyde; (2) undergoes poly aldol condensation and hydrogenation to 2-ethylhexanol | technical maturity and low cost of bioethanol from biomass, environmentally friendly; but more by-products, low catalytic selectivity | 70–90%; 70–80% | (Shan et al., 2017; Liu et al., 2018; Eagan et al., 2019; Poulikidou et al., 2019) |
Butyraldehyde | Lignocellulosic biomass | (1) self-condensation of butyraldehyde for 2-ethyl-2-hexenal; (2) hydrogenation of 2-ethyl-2-hexenal into 2-ethylhexanol | High conversion rate, environmentally friendly and low cost of bioethanol from biomass | About 95%; 70% | (Corma et al., 2005; Domaille et al., 2016; Li et al., 2016b) |
Syngas | Lignocellulosic biomass | (1) CO and H2 used for synthesis of propylene; (2) propylene to 2-ethylhexanol (the specific pathway is the same as above propylene pathway) | High conversion rate, environmentally friendly and low cost of bioethanol from biomass | About 95%; 60–70% | (Corma et al., 2005; Dagle et al., 2016; Li et al., 2016b; Ezhova et al., 2020) |
Note: The conversion rate and yield are based on the higher level of reported. |