The production of petrochemicals by processing of renewable vegetable resources attracts increasing researchers’ attention throughout the world [1–3]. This research field meets the principles of “green” chemistry and decarbonization of the economy, because the biomass grows at the expense of carbon dioxide utilization [4–6]. The recently started rearrangement of the global economy toward low-carbon development is accompanied by radical EU initiatives on market protection from “dirty” goods (by introducing a carbon tax) and by measures taken in China and other countries for tackling climate change. Bioisobutanol (isobutanol) is one of key compounds obtained by biomass processing. Its use as an additive to motor fuel and as a feedstock for petroleum chemistry can significantly contribute to the production of “green” hydrocarbons and other intermediates.

In this study, along with the IUPAC term 2-methyl-1-propanol and commonly used terms isobutyl alcohol and isobutanol, we used in some cases the term bioisobutanol. By so doing, we would like to emphasize that bioisobutanol was obtained specifically by biomass processing, by analogy with the term bioethanol introduced by GOST (State Standard) R 52808–2007: bioethanol is ethanol produced from biomass and/or biodegradable waste components and used as a biofuel. In cases when the isobutanol production procedure is not essential, we will use the traditional term isobutanol.

Studies by Prof. Arnold’s team (the United States) made a major contribution to the development of a process for producing bioisobutanol by fermentation of carbohydrates obtained by biomass processing [7–9]. The commercial production of bioisobutanol from carbohydrates was set up by Gevo and Butamax companies [10, 11]. Bioisobutanol is produced from carbohydrates that can be obtained from biomass of various origins. In turn, the produced bioisobutanol is considered by producers as a valuable feedstock for the synthesis of important petrochemicals: isobutene and butenes, p-xylene, hydrogen, etc. It can also serve as a motor fuel component capable of replacing ethanol.

PRODUCTION OF BIOISOBUTANOL BY FERMENTATION OF CARBOHYDRATES

The interest in using bioisobutanol as a feedstock for petroleum chemistry is largely due to recent advances in the field of biotechnology, which allowed the Gevo [12, 13] and Butamax [14, 15] companies to commercially implement processes for producing bioisobutanol from carbohydrates obtained by biomass processing. Studies in this field under the guidance of Prof. Arnold on the development of critically important enzymes for this purpose should be particularly noted [7–9, 16–19]. For these studies, Arnold was awarded the Nobel Prize in Chemistry in 2018. According to open-access data, about 0.24 kg of bioisobutanol is produced from 1 kg of corn in the bioisobutanol production process developed and implemented by Gevo [20]. This yield of bioisobutanol makes it competitive with the feedstock of petroleum origin and allows bioisobutanol to be considered as an alternative feedstock for producing various petrochemicals.

Synthesis of bioisobutanol from carbohydrates can be performed using such microorganisms as Saccharomyces cerevisiae, Corynebacterium glutamicum, Bacillus Subtilis, Escherichia coli, Clostridium cellulyticum [21–24], etc. In bioisobutanol synthesis using gene-modified Saccharomyces cerevisiae, the bioisobutanol yield was 6.6 [21] or 15 mg g–1 glucose [22]. When blocking competing reactions, the theoretical yield of bioisobutanol can reach 1 mol mol–1 glucose, or 0.41 g g–1 glucose. Atsumi et al. [25] studied the bioisobutanol production from glucose using gene-modified microorganisms Escherichia coli and Bacillus subtilis. Higher bioisobutanol yield, 0.35 g g–1 (0.86 mol mol–1), was reached with Bacillus subtilis. When using Corynebacterium glutamicum [24], the bioisobutanol yield was 0.77 mol mol–1 glucose. When using modified Bacillus subtilis strain under fermentation conditions with intermittent addition of carbohydrates to the solution, Li et al. [26] were able to obtain isobutanol in an amount of 5.5 g L–1 (0.36 mol mol–1 glucose).

Along with glucose, researchers in laboratory studies use as fermentation feedstock for isobutanol production also carbohydrates produced from various kinds of vegetable resources, including corn, wheat, sorghum, barley, sugarcane, etc. [27].

Jung et al. [28] produced isobutanol by fermentation of hydrolyzed bagasse using Enterobacter aerogenes with removed enzymes forming byproducts. The product was recovered using a pervaporation membrane. The productive capacity for isobutanol reached 0.32 g L–1 h–1.

Patel et al. [29] examined the possibility of producing isobutanol from cellulose using a combination of bacteria including Cellulosilyticum lentocellum, which decomposes cellulose to carbohydrates, E. Coli, which produces isobutanol, and the third bacterial species, Geobacter Metallireducens, which converts the by-products formed. The system preserved stability and allowed production of 7.7 g L–1 isobutanol.

The addition of NADH-dependent alcohol dehydrogenase isolated from Lactococcus lactis (AdhA) to Shimwellia blattae (p424IbPSO) led to a 19.3% increase in the isobutanol yield. The recombinant Shimwellia blattae strain (p424IbPSO, PIZPN TAB) containing transhydrogenase PntAB allowed production of 39.0% larger isobutanol amount compared to the initial strain; the productive capacity of 5.98 g L–1 was reached [30]. Both strains showed a considerably decreased yield of byproducts, lactic acid and ethanol.

Su et al. [31] used oligonucleotide-directed mutagenesis for developing a Saccharomyces cerevisiae strain with increased isobutanol tolerance. They obtained a strain exhibiting high viability in a medium containing 16 g L–1 isobutanol. The strain obtained allowed the isobutanol titer to be increased by 49.9% relative to the initial level.

Nitschel et al. [32, 33] used for isobutanol production modified Pseudomonas putida strains under anaerobic conditions. They obtained the modified P. putida strain KT2440 giving the isobutanol yield of 22 ± 2 mg per gram of glucose under anaerobic conditions [32]. Later this process was scaled for a 30-L bioreactor [33]. In the two-step bioprocess with the separated steps of bacterial growth and isobutanol production under microaerobic conditions, the isobutanol yield reached 60 mg per gram of glucose, and undesirable carbon loss in the form of 2-ketogluconic acid was prevented.

Yang et al. [34] described a process for producing isobutanol from oil palm empty fruit bunches using Escherichia coli JK209: 80.1 g of isobutanol can be obtained from 1 kg of the hydrolyzed feedstock.

Arnold et al. [7] were able to reach 100% bioisobutanol yield using gene-modified Escherichia coli. When organizing continuous recovery of the formed bioisobutanol from the fermentation mass under laboratory conditions, stable bioisobutanol yield of 0.29 g g–1 glucose is reached [35].

Thus, a number of microorganisms allow production of bioisobutanol from carbohydrates formed by biomass processing. However, for large-scale production of bioisobutanol from corn, Gevo company used gene-modified Saccharomyces cerevisiae [3]. This microorganism is extremely resistant to the action of fermentation products and can function at lower pH values. The gene-modified Saccharomyces cerevisiae patented by Gevo [12] allows production of the fermentation product containing up to 19 g L–1 isobutanol at the glucose concentration in the feed of 80 g L–1.

Figure 1 shows the route of metabolic transformations of carbohydrates into bioisobutanol under the action of modified S. Cerevisiae [3, 7]. The first step of the process, not shown in the scheme, is the transformation of carbohydrates into pyruvic acid. Then, pyruvic acid anions (pyruvates) under the action of acetolactate synthase transform into acetolactate ions. Under the action of the reduced form of nicotinamide adenine dinucleotide phosphate (NADP Н), acetolactate ions are reduced to 2,3-dihydroxyisovalerate ions, which are then dehydrated to 2-oxoisovalerate ions. Decarboxylation of the latter species yields isobutyraldehyde, which is reduced to isobutanol under the action of NADP Н. The valine formation is a possible side reaction.

Fig. 1.
figure 1

Scheme of isobutanol synthesis under the action of S. Cerevisiae (adapted from [7, 25]).

Full size image

The isobutanol obtained enzymatically from carbohydrates should be separated from the reaction mixture. Along with distillation, such processes as salting-out, adsorption, extraction, etc., are used for this purpose. Fu et al. [36] compared the extractive distillation and salting-out + distillation processes. The salting-out + distillation appeared to be cheaper. Claessens et al. [37] described a procedure for isobutanol recovery by adsorption on fully silica Beta zeolite. They state that this method shows promise for replacing distillation. However, the above-described methods have not yet found wide use.

Along with carbohydrates, methanol [38] and gas mixtures containing СО, Н2, and СО2 [39, 40] can also be used for bioisobutanol production using microorganisms. Ma et al. [38] used Methylorubrum extorquens for isobutanol production from methanol. The authors were able to increase the isobutanol titer by a factor of more than 20 compared to the initial Methylorubrum extorquens AM1 strain after overexpression of alsS gene coding acetolactate synthase and removal of ldhA gene coding lactate dehydrogenase. Replacement of the cellular framework by the strain resistant to isobutanol, isolated in the course of adaptive evolution of M. extorquens AM1, additionally increased the isobutanol production by a factor of 1.7, which led to the final titer of 19 mg L–1 in cultivation in a flask.

To produce isobutanol from gas mixtures containing СО, Н2, and СО2, Hermann et al. [39] used modified Clostridium ljungdahlii strain (C. ljungdahlii, CLJU). This strain allowed production of 0.02 g L–1 isobutanol. Additional blocking of the valine synthesis allowed the productive capacity for isobutanol to be increased by a factor of 6.5 to 0.13 g L–1. For producing isobutanol from gas mixtures containing СО, Н2, and СО2, Weitz et al. [40] used acetogenic bacteria Acetobacterium woodii and Clostridium ljungdahlii. The isobutanol yield was low, and additional introduction of ketoisovalerate was required for increasing it.

Along with procedures for producing isobutanol from carbohydrates using microorganisms, approaches to bioisobutanol production by the cell-free enzymatic method are being developed. Guterl et al. [41] described a cell-free bioreactor allowing production of bioisobutanol with a titer of 0.76 g L–1 from glucose; this value is considerably lower than that reached in the microbial procedure. Opgenorth et al. [42] used for isobutanol synthesis a molecular rheostat maintaining the preset ATP level in the molecular bioreactor. The cell-free system reached the maximal values of the productive capacity for isobutanol (1.3 g L–1 h–1) and isobutanol titer (24 g L–1) in two days; the yield was 91% of the theoretical level. This scheme was later improved by organizing continuous removal of the product, which allowed synthesis of isobutanol from glucose with the maximal productive capacity of 4 g L–1 h–1, titer of 275 g L–1, and yield of 95% in the course of 5 days [43].

The whole set of the above-given data demonstrates the possibility of using microbiological methods for preparing bioisobutanol from carbohydrates formed by processing vegetable raw materials on the commercial scale. To organize commercial production of bioisobutanol, Gevo company upgraded the plant for ethanol production from corn in Luverne (the United States) and successfully produced 50 thousand gallons (13 thousand liters) of bioisobutanol from corn. The company plans to expand the bioisobutanol production facilities. The key aspects of the Gevo technology are the use of gene-modified S. Cerevisiae [44] and continuous removal of the formed bioisobutanol from fermentation products [27, 45]. The Gevo technology can be termed a “hybrid” process combining a microbiological process and a chemical stripping process.

A specific feature of the bioisobutanol production by the Butamax company is the use of microorganisms with specially constructed DNA molecules introduced into them. The constructed DNA molecules code enzymes that accelerate each of the five reactions of bioethanol synthesis, shown in Fig. 1. This leads to an increase in the bioisobutanol formation rate and yield [46, 47]. The Butamax company also constructed a demo installation in Hull (the United Kingdom). In 2013, the Butamax company announced start of the upgrading of the ethanol production plant in Lamberton (Minnesota, the United States) for bioisobutanol production. To expand the production, the company plans to buy the ethanol production plant in Scandia (Kansas, the United States) [3].

The implemented processes for bioisobutanol production tend to expand owing to prospects for its further use and to trends toward decarbonization of the economy as a whole.

PRODUCTION OF ISOBUTANOL FROM OTHER ALCOHOLS OF BIOGENIC ORIGIN

The possibility of producing isobutanol from other low-molecular-mass alcohols that can be synthesized both by fermentation processes and from syngas produced by heat treatment of vegetable raw materials attracts researchers’ attention [48]. Syngas of biogenic origin can be converted to a mixture of С1–С3 alcohols at elevated pressure (>4.0 MPa) and temperature of 250–320°С using modified catalysts for Fischer–Tropsch process (based on Fe, Ni, Co, Mo, Rh) or for methanol synthesis (based on K, Zn, Cr, Cu) [48–51]. The products obtained contain also isobutanol in some cases, but in small amounts. To increase the isobutanol yield, the produced mixture of alcohols (methanol/ethanol/n-propanol) can be converted at higher temperatures to a product containing isobutanol, in particular, on methanol synthesis catalysts [48, 52].

Selective catalysts for producing isobutanol from lower alcohols by cross condensation of alcohols (Guerbet reaction) have been developed [52–64]. The occurring reactions allow synthesis of isobutanol in accordance with the scheme shown in Fig. 2.

Fig. 2.
figure 2

Scheme of Guerbet reaction (adapted from [52]).

Full size image

Carlini et al. [52], using copper chromite preliminarily reduced with hydrogen and sodium methylate as catalyst, were able to convert a mixture of methanol and n-propanol (molar ratio 6 : 1) to isobutanol, whose yield based on n-propanol reached 73–78% in 6 h of catalyst operation. The reaction was performed at 180–220°С in an atmosphere of nitrogen or hydrogen (3.0 MPa). The conversion of a mixture of methanol and ethanol (molar ratio 12.5 : 1) on the same catalyst yielded n-propanol and isobutanol [53]. The ethanol conversion in the 6-h run reached 61% at 98% selectivity with respect to isobutanol. Carlini et al. [53] also studied the conversion of a mixture of methanol, ethanol, and n-propanol (molar ratio 8 : 1 : 1). The ethanol conversion reached 71%; the products also contained ethanol, n-propanol, and isobutanol. The isobutanol content of the mixture obtained was 70 mol %. As noted in [53], the structure of isobutanol limits the possibility of its further transformations via Guerbet reaction.

The use of the Pd/C + MeONa catalyst in the conversion of the methanol/n-propanol mixture ensured the isobutanol yield higher than 90% (based on n-propanol) in 12 h [54]. Dissolution of palladium in the reaction mixture is noted.

Ueda et al. [55] in experiments on conversion of a methanol–ethanol mixture, catalyzed by magnesium oxide, found that, along with n-propanol, the reaction yielded isobutanol as the product of the reaction of the initially formed n-propanol with methanol.

To synthesize isobutanol from methanol and ethanol, Qiang Liu et al. [56] used a catalyst prepared by sodium borohydride reduction of iridium chloride IrCl3 preliminarily deposited onto carbon support functionalized with nitrogen-containing ligands. To a mixture of the catalyst and starting alcohols, NaOH was added, and the mixture was stirred in an autoclave for 16 h at 160°С. When using as an ethanol source the product of alcoholic fermentation of carbohydrates, purified to remove solid impurities, the ethanol conversion was 49%, and the selectivity with respect to isobutanol was 90%.

Wingad et al. [57] prepared isobutanol from a mixture of 10 mL of methanol and 1 mL of ethanol using catalysts based on ruthenium diphosphine complexes with base additives. On the best catalysts, the ethanol conversion in 2 h at 180°С reached 75% at 100% selectivity with respect to isobutanol.

The same authors have shown that the catalyst containing trans-[RuCl2(dppm)2] (dppm is 1,1-bis(diphenylphosphino)methane) is resistant to the action of water and is capable to transform a mixture containing methanol, ethanol, and water (simulating the alcoholic fermentation product) into isobutanol in 36% yield with 78% selectivity [58]. With beer used as a source of ethanol, the isobutanol yield was 29%.

On the whole, despite certain success, processes for isobutanol synthesis by the Guerbet reaction have not yet found practical use, which is probably associated with the properties of the catalysts used, containing base components. It should be noted that the Guerbet reaction using solely ethanol yields n-butanol [59–61].

FIELDS OF BIOISOBUTANOL APPLICATION

Bioisobutanol, i.e., isobutanol produced by fermentation of biomass processing products, should be considered as a kind of carbon-neutral competitive feedstock for preparing a number of petrochemicals. According to [62], bioisobutanol can be used in synthesis of approximately 40% of demanded chemicals (including butenes, toluene, and xylenes). Figure 3 shows a number of basic petrochemicals that can be produced from bioisobutanol using the existing industrial equipment [63].

Fig. 3.
figure 3

Petrochemical intermediates that can be produced from bioisobutanol (adapted from [63]).

Full size image

The Gevo company suggests using bioisobutanol that it produces as a high-octane component of motor fuels and as an intermediate for producing isobutene/butenes, p-xylene, jet fuel, and specialty chemicals. Utilization of bioisobutanol production waste as forage is suggested [64]. The Gevo company notes that an increase in the bioisobutanol yield makes its production and hence the production of various petrochemicals from it more profitable.

The bioisobutanol dehydration allows production of “green” isobutene, which is a feedstock for synthesizing such valuable products as tert-butyl ethers (fuel additives) [65], p-xylene [66], isooctane [67], polymers and rubbers [68], etc. The synthesis of many products shown in Fig. 3 occurs via formation of isobutene as an isobutanol transformation intermediate.

Isobutanol is also used in cross-condensation with acetone to obtain С7–С11 ketones, which, in turn, can be converted to alkanes/alkenes and amines as fuel additives [69].

Isobutanol is used in synthesis of heat- and acid-resistance lacquers [70] and in production of zinc O,O-diisobutyl dithiophosphate used as an additive to oils, consistent lubricants, and hydraulic oils resistant to wear and oxidation [71].

In addition, isobutanol is used in the catalytic process of supercritical biomass liquefaction. In [72], finely divided stems of Ferula orientalis L. were treated in a high-pressure reactor under supercritical conditions using various solvents. When using isobutanol as a solvent, the feed conversion was 50%, the bio-oil yield at 320°C was ~30%, and the yield of gaseous products (H2, CO, CO2) was 13%.

Wu et al. [73] studied the transformation of millet into liquid products using an isobutanol/water mixture as a solvent. On the Ni-HPMo/Fe3O4@Al-MCM-41 catalyst, the 84.7% feed conversion and the 55.0% yield of the liquid were reached.

The growing interest in the bioisobutanol production is largely due specifically to the possibility of using it as an effective additive to motor fuels, making them more environmentally friendly.

BIOISOBUTANOL AS AN ADDITIVE TO MOTOR FUEL

Among alcohols of biogenic origin, ethanol is the first in the production volume. Therefore, it is often used as an additive to motor fuels. However, the calorific value (CV) of isobutanol is higher than that of ethanol and closer to that of the motor gasoline. In addition, butanols can be admixed to gasoline in higher ratios than ethanol without affecting the fuel quality [74]. Butanols are less prone to phase separation with gasoline in the presence of water. This fact reduces the risk of corrosion of aluminum or polymer components of the fuel system and vessels of fuel-transporting vehicles [75–77]. The heat of vaporization and self-ignition point of butanol isomers are lower than those of ethanol, which favors better spraying and eliminates problems with cold startup and ignition of the gasoline–air mixture [75, 76]. The self-ignition point of isobutanol and n-butanol is 415 and 385°С, respectively [78]. Lower, compared to ethanol, polarity of butanols eliminates the problem of increased saturated vapor pressure, characteristic of mixtures of ethanol with motor gasoline. This reduces the evaporation loss in the course of filling and the tendency to cavitation and engine blocking with fuel vapor [79].

Isobutanol as a fuel additive has a number of advantages compared to n-butanol [80–82]. n-Butanol generates in the course of combustion mainly hydrogen radicals, whereas isobutanol mainly generates less reactive methyl radicals. Thus, n-butanol is characterized by shorter ignition time compared to isobutanol. In addition, n-butanol is characterized by higher flame propagation rate compared to isobutanol [83].

It is expected that the addition of isobutanol to a fuel will reduce the emissions of carbon and nitrogen oxides. To study how additions of isobutanol to gasoline influence the harmful emissions, Elfasakhany [84] used a one-cylinder SI engine (ignition system) operating in a wide rate range (2600–3400 rpm) at a fixed compression ratio of 7 : 1. According to [84], the emissions of CO and unburned hydrocarbons from pure gasoline were higher compared to mixed fuels at the engine rates lower than or equal to 2900 rpm. However, at higher rates, mixed fuels lead to higher emissions of CO and unburned hydrocarbons compared to the gasoline fuel. The CO2 emissions in the case of using isobutanol–gasoline mixtures were always lower (by up to 43%) compared to pure gasoline. On the other hand, the use of isobutanol–gasoline mixtures in an SI engine without additional tuning led to a decrease in the engine power throughout the rate range. However, the engine optimization for mixed kinds of fuel ensures high power and decreased emissions.

Karabektas and Hosoz [85] have found that addition of 10% isobutanol to diesel fuel reduces its knocking and the emissions of nitrogen and carbon oxides. Aakko-Saksa et al. [86] have shown that the addition of isobutanol to reduce the harmful emissions is more effective for indirect injection engines.

Thus, bioisobutanol should be considered as a promising environmentally friendly component of motor fuels based on the gasoline/isobutanol mixture.

PROMISING CATALYTIC PROCESSES FOR ISOBUTANOL TRANSFORMATION

Dehydration of bioisobutanol to isobutene is considered as one of the main routes of bioisobutanol processing [87]. Isobutene is a demanded petrochemical intermediate; it is used in production of rubbers, isooctane, and MTBE, in alkylation processes, etc. The importance of isobutene production by isobutanol processing was indicated by Dr. Axel Heitmann, Chairman of the Board of Management of LANXESS, one of world’s leaders in production of rubbers. As he said, the company as the world’s largest isobutene consumer considers it appropriate to produce isobutene from renewable resources as an alternative to traditional natural fuels. Various catalysts can be used for producing isobutene and its isomers from isobutanol; examples are given in Table 1.

Table 1. Catalysts and conditions of isobutanol transformation into butenes
Full size table

For example, as seen from Table 1 (no. 1), γ-alumina is a selective catalyst for isobutene synthesis from isobutanol [88]. Simulation of the bioisobutanol composition by using various additives did not affect the experimental results. On the other hand, γ-alumina subjected to heat and sulfuric acid treatments appeared to be considerably less selective (Table 1, nos. 2a, 2b) [89]. The use of other catalysts leads, as a rule, to the formation of a mixture of butenes and isobutene in various ratios. Zeolites of ferrierite type are more selective with respect to linear n-butenes (Table 1, nos. 8, 9) [94, 95].

The isobutanol transformations in the presence of catalysts based on zinc oxide with the addition of titanium and chromium oxides (Table 1, nos. 3a, 3b) are described in [90] by the occurrence of two parallel competing reactions: dehydration and dehydrogenation. It is assumed that strong Lewis acid sites (Ti+4 and Cr+3) interact with isobutanol (base) more efficiently. The use of unpromoted zinc oxide exhibiting moderate Lewis acidity leads to a decrease in the isobutanol conversion. Strong Lewis acidity of the catalyst with the addition of oxides favors predominant occurrence of dehydration, which leads to an increase in the isobutene yield. The competing dehydrogenation reaction occurring to a greater extent on unpromoted zinc oxide yields isobutanal. The authors believe that the zinc oxide surface contains a smaller amount of accessible Lewis acid sites compared to zinc oxide doped with Ti+4 and Cr+3 oxides. In addition, the zinc oxide surface contains an excess of oxygen anions. These facts in total lead to increased contribution of dehydrogenation, to isobutanal formation, and to decreased isobutene yield.

The formation of linear butenes on mixed titanium and silicon oxides (Table 1, nos. 4а, 4b) is attributed in [91] not to isomerization of the isobutene formed but to attainment of the thermodynamic equilibrium between the intermediately formed isobutyl carbenium ions and the subsequent elimination (Е1) to form linear butenes. Similar mechanism of the product formation was suggested in [92] for transformations of isobutanol on silicon oxide with the addition of titanium and tungsten oxides (Table 1, nos. 5а, 5b).

Du et al. [93] have shown that, when performing the isobutanol conversion on the 2% Ga2O3/SiO2 catalyst for a long time, the cis/trans-2-butene ratio increases; this trend is attributed to a decrease in the yield of trans-butene due to catalyst deactivation.

According to [96], the selectivity with respect to cis/trans-butenes formed by conversion of isobutanol to butenes on WO3/ZrO2 catalysts is independent of the catalyst acidity (WO3 content). The major product was isobutene (Table 1, no. 7).

Maury et al. [94, 95] assume that the isobutanol dehydration on ferrierite-type zeolite occurs with the formation of isobutyl carbenium ions on Brønsted acid sites (Fig. 4). The proton transfer leading to the formation of isobutene occurs without water but is inhibited in the presence of water, and also in the case of soft coke formation. The isobutyl carbenium ion can diffuse into the zeolite structure and react with accessible and sufficiently strong Brønsted acid sites, which leads to isomerization into linear carbocations via methyl shift.

Fig. 4.
figure 4

Assumed scheme of transformation of isobutanol into butenes on a zeolite of ferrierite type (adapted from [94]).

Full size image

These Brønsted acid sites (probably located at the entrance into zeolite pores) are not susceptible to the action of water and soft coke formed, which at increased contact time and long time of catalyst operation leads to high selectivity with respect to linear butenes. It is also assumed in [94] that the formation of water inhibits the proton shift in the isobutyl carbenium ion or deprotonation of acid sites favoring isobutene formation. However, the presence of water does not inhibit acid sites on which isomerization of isobutyl carbenium ions into linear carbocations occurs, which leads to the formation of linear butenes. That is, at high isobutanol conversion, the presence of water and of the formed coke favors inhibition of nonselective sites responsible for the proton shift, which leads to high selectivity of the formation of linear butenes.

Gunst et al. [97] studied the kinetics and mechanism of the transformation of isomeric butanols on HZSM-5 zeolite with the silica to alumina ratio of 50. As they showed, isobutanol can undergo dehydration already at 180°С, and isobutene is formed by direct dehydration of isobutanol and not via formation of diisobutyl ether as in the case of n-butanol.

The above data demonstrate the possibility of using isobutanol, including bioisobutanol, for selective synthesis of valuable petrochemicals, isobutene and isomeric butenes, on various oxide catalysts.

On the other hand, according to [98, 99], the isobutanol transformations on Ni0.5Zr2(PO4)3 and Na1–2xCuxZr2(PO4)3 involve competing reactions of isobutanol dehydration and dehydrogenation to form isobutene and isobutanal, respectively. Isobutanal can be selectively obtained on Bi4V2–2xCu2xO11–δ catalysts [100].

Bioisobutanol is also considered as a promising feedstock for producing valuable intermediates of basic organic synthesis: aromatic hydrocarbons, mainly those of the benzene–toluene–xylene fraction С6–С8 (BTX), and С2–С4 olefins. These products are formed via transformations of the initially formed butenes. Data on catalysts and conditions of isobutanol transformation, allowing synthesis of the above products, are given in Table 2.

Table 2. Catalysts and conditions of the isobutanol transformation into С2–С4 olefins and aromatic hydrocarbons
Full size table

Table 2 shows that zeolites of MFI structural type, promoted with zinc or gallium, are more effective catalysts for producing aromatic hydrocarbons (including BTX). For selective synthesis of С2–С4 olefins, it is preferable to use unpromoted zeolites of structural types MFI (ZSM-5 type) and FAU (USY type).

Van Mao and McLaughlin [101] have shown (Table 2, no. 1b) that, in transformation of isobutanol using catalysts based on HZSM-5 zeolites, higher pressure favors increased yield of arenes. Arenes are presumably formed from isobutene oligomerization products, and isobutene, in turn, is formed by isobutanol dehydration. According to the Le Chatelier principle, an increase in the pressure shifts the equilibrium toward formation of isobutylene oligomers. Introduction of 0.51% zinc promoter increased the yield of arenes insignificantly.

On the other hand, Yu et al. [102] believe that introduction of zinc into HZSM-5 zeolite favors an increase in the arenes yield due to suppression of the strong Brønsted acidity of the unpromoted zeolite, because the presence of strong Brønsted acid sites catalyzes cracking reactions yielding С3–С4 alkanes. Introduction of zinc ions also favors recombinant desorption of hydrogen atoms to form an Н2 molecule. This leads to suppression of the hydrogen transfer and to a decrease in the yield of С3–С4 alkanes and favors the dehydrogenation to form aromatic compounds. As can be seen (Table 2, no. 2d), promotion of HZSM-5 with 5.1% Zn considerably increases the yield of arenes. The Y and Beta zeolites, in contrast to HZSM-5, are more effective in the formation of С2–С4 olefins (Table 2, nos. 2а–2c).

Du et al. [93] attribute high content of ethylene and propylene in products of isobutanol transformations on HZSM-5 to the cracking of oligomers formed by oligomerization of isobutene as a primary product of isobutanol dehydration.

Introduction of gallium (Table 2, no. 3b) favors the dehydrogenation of the intermediates due to recombinant desorption of hydrogen atoms (as in the case with zinc), which leads to an increase in the yield of arenes.

Performing the isobutanol conversion with the additional feeding of carbon dioxide into the reactor [103] (Table 2, no. 4) also favors an increase in the yield of arenes. Presumably, the occurrence of the dehydrocyclization of butene oligomers to form aromatic hydrocarbons is favored by coupling of the hydrogen formed with carbon dioxide via reverse water-gas shift reaction.

As shown in [104], introduction of zinc and chromium into zeolite of HMFI structural type with the silica to alumina ratio of 40 increases both the total content of acid sites and the amount of weak and medium-strength acid sites. This favors an increase in the yield of arenes in isobutanol conversion (Table 2, no. 5). Unpromoted HMFI zeolite with the silica to alumina ratio of 136 produces predominantly С2–С4 olefins owing to prevalence of strong acid sites (Table 2, no. 5a).

The isobutanol transformations were also studied using as a catalyst НMFI/MCM-41 micro-mesoporous composite (Table 2, no. 6) synthesized by the bitemplate hydrothermal-microwave method [105, 106]. 100% isobutanol conversion was observed. The yield of liquid hydrocarbons was 61 wt %. The benzene content did not exceed 1 wt % at the total yields of arenes of 25 wt % and of iso- and cycloalkanes of 19 wt %.

As we showed in [107, 108], the НMFI/MCM-41 micro-mesoporous composite synthesized by the hydrothermal-microwave method and jointly promoted with zinc and chromium (Table 2, no. 7) can be considered as a promising catalyst for obtaining p-xylene from isobutanol. At 100% isobutanol conversion, the p-xylene yield was 7 wt %. The p-xylene content of the liquid hydrocarbons obtained reached 17 wt %, and the fraction of p-xylene in the isomer mixture reached 78%. Joint introduction of the zinc and chromium promoters influences the nature of Brønsted and Lewis acid sites of the catalyst, which probably favors high selectivity of the p-xylene formation. The p-xylene obtained is the most demanded xylene isomer and a valuable intermediate for producing terephthalic acid and poly(ethylene terephthalate) [109].

In [110], we reported the isobutanol conversion on the HMFI/SiC micro-mesoporous composite synthesized by the hydrothermal-microwave method directly in the protonic form. High total yield (84 wt %) of valuable petrochemicals, С2–С4 olefins and aromatic hydrocarbons, was reached. The BTX content of the arenes was 94%. In addition, we reached the highest, compared to the known catalysts for isobutanol conversion, productive capacity for propylene: 0.846 g gcat–1 h–1.

The use of isobutanol as a feedstock for syngas production by steam reforming or partial oxidation was studied in [111–115]. The development of such processes opens the route from bioisobutanol to “green” hydrogen and “green” products of Fischer–Tropsch synthesis.

The highest yield of hydrogen in partial oxidation of isobutanol on the Rh/Al2O3 catalyst reached 63% [114]. As noted in [112–114], high hydrogen yield was reached in steam reforming of isobutanol on Ni/Al2O3 catalysts. According to [112], the hydrogen yield in steam reforming of isobutanol on 25% Ni/Al2O3 catalyst was 84%, in agreement with the data of [113]. In [114], approximately 80% hydrogen yield was reached on the 4.3% Ni/Al2O3 catalyst.

Another process used for producing hydrogen from isobutanol is autothermal conversion in the presence of water and oxygen. In [116], the hydrogen yield on the 1% Rh/α-Al2O3 catalyst was 70% at 100% conversion of the alcohol.

In the series of catalysts containing Ru, Ni, Ce, and Zr on Al2O3 support, the best results in autothermal conversion of isobutanol were reached with the 0.3% Ru/10% Ni/3% Ce/Zr/Al2O3 catalyst, with which the coke formation was fully suppressed [117].

Tsodikov et al. [118] studied the isobutanol transformations on Pt/Al2O3 catalysts modified with the intermetallic compound TiFe0.95Zr0.03Mо0.02 or its hydride. At 350°С, pressure of 5.0 MPa, and isobutanol space velocity of 0.5 h–1 in an argon medium, the isobutanol conversion was 66–67%. The reaction yielded 6–12% gaseous products, 12–15% oxygen-containing products (mainly 2-methylpropanal and isobutyl butyrate), and liquid hydrocarbons of various compositions depending on the kind of the catalyst. On the hydride-containing catalyst, their yield was 17%, and they mainly consisted of dimethylhexanes (yield 11%), other isoalkanes (yield 5%), and xylenes (yield 1%). On the catalyst containing no hydride phase, the yield of liquid hydrocarbons (mainly olefins and dienes with traces of toluene) was 14%.

Thus, the catalytic transformations of isobutanol allow production of valuable petrochemicals: various butenes, aromatic hydrocarbons, С2–С4 olefins with high content of propylene and ethylene, and hydrogen.

SPECIFIC FEATURES OF ISOBUTANOL TRANSFORMATIONS ON ZEOLITE CATALYSTS

Table 2 shows that catalysts based on zeolites allow transformation of isobutanol into such valuable petrochemicals as С2–С4 olefins and aromatic hydrocarbons. Therefore, it is interesting to consider in more detail the published data on possible mechanisms of isobutanol transformations on zeolite catalysts.

Catalysts containing zeolites of MFI structural type (ZSM-5 type) show promise in transformation of isobutanol into aromatic hydrocarbons and С2–С4 olefins. The isobutanol transformations on zeolite catalysts involve a set of consecutive and parallel reactions. Their first step is dehydration of the alcohol to form isobutene, which is capable of further isomerization and oligomerization. The total acidity, ratio of Brønsted and Lewis acid sites, pore structure of the zeolite, and kind of promoting additives exert decisive influence on the course of subsequent transformations of butenes and on the composition of final products.

Du et al. [93] studied transformation of isobutanol into olefins and aromatic hydrocarbons on HZSM-5 zeolite and suggested the product formation scheme shown in Fig. 5.

Fig. 5.
figure 5

Scheme of isobutanol transformations on HZSM-5 zeolite (adapted from [93]).

Full size image

The isobutene and butenes formed by dehydration and isomerization dimerize to form С8 olefins, which can subsequently transform into С2–С5 and С6–С8 olefins by reversible cracking and oligomerization reactions. Du et al. [93] believe that direct cyclization and dehydrogenation of С6–С8 olefins occur to a minor extent and make a minimal contribution to the formation of aromatic compounds. They assume that aromatic hydrocarbons and low-molecular-mass С2–С5 alkanes are formed by hydrogen transfer reactions between С2–С5 and С6–С8 olefins.

In the same study, using the Fourier transform IR diffuse reflectance spectroscopy, the authors have found that the transformations of isobutanol on the gallium-promoted Ga-ZSM-5 catalyst involve the same reactions as on the HZSM-5 catalyst.

However, introduction of gallium favors an increase in the contribution of the dehydrocyclization of С6–С8 olefins to form aromatic hydrocarbons. Among aromatic compounds, С8 hydrocarbons (ethylbenzene and xylenes) prevail; they are formed by the reaction of two butene/isobutene molecules and dehydrocyclization of dimers. On the other hand, Du et al. [93] believe that benzene and toluene are formed by secondary processes such as cracking of С8 olefins and oligomerization of С2–С5 olefins. The authors note that introduction of gallium did not prevent the hydrogen transfer reactions, because the selectivity of formation of С2–С5 alkanes remained virtually the same.

Du et al. [93] also note that the yield of butenes on nonzeolitic catalysts 2% Ga–SiO2 and 4% Ga–SiO2 reached virtually 100% based on the isobutanol fed. That is, gallium oxide particles catalyze only the dehydration and isomerization of isobutanol but do not participate in the subsequent secondary transformations of isobutene and butenes. Therefore, Ga–SiO2 containing predominantly Lewis acid sites can be used as a selective catalyst for producing butenes. The occurrence of secondary processes yielding aromatic hydrocarbons is impossible without participation of Brønsted acid sites of HZSM-5 zeolite. Thus, the presence of a pair of Brønsted acid sites of zeolite and gallium ions (which partially substitute Brønsted acid sites) leads to a “synergistic effect” favoring the formation of arenes. Similar conclusion was made by other researchers who studied dehydrogenation of propane [119] and transformation of methanol into arenes [120].

Mo et al. [103] studied the isobutanol transformations on HZSM-5 zeolites, unpromoted and promoted with Ga or Zn, with the additional feeding of carbon dioxide into the reactor. They assume that carbon dioxide can react with hydrogen released in dehydrocyclization of olefins top form carbon monoxide and water. This, in turn, favors a shift of the equilibrium toward dehydrocyclization of olefins and formation of aromatic hydrocarbons (Fig. 6).

Fig. 6.
figure 6

Scheme of the shift of the equilibrium toward formation of aromatic hydrocarbons with the participation of carbon dioxide in isobutanol transformations on zeolites (adapted from [103]).

Full size image

The overall scheme of isobutanol transformations into aromatic hydrocarbons on promoted M/ZSM-5 zeolite, suggested in [103], is shown in Fig. 7.

Fig. 7.
figure 7

Assumed mechanism of isobutanol aromatization on M/ZSM-5 in the presence of carbon dioxide (adapted from [103]).

Full size image

Isobutylene is formed by dehydration of isobutanol and then dimerizes to form С8 isoolefins, which is followed by oligomerization and dehydrocyclization to form arenes. By the example of Ga–ZSM-5 catalyst, Mo et al. [103] proved that carbon dioxide activated on gallium particles couple hydrogen released in dehydrocyclization, which leads to an increase in the yield of aromatic hydrocarbons.

Published data on pathways of isobutanol transformations on zeolitic catalysts show that the isobutene formed by isobutanol dehydration undergoes oligomerization to form С8 and higher iso-olefins. However, the subsequent steps of the transformation of isobutene oligomers into aromatic hydrocarbons, according to published data, depend on the presence and kind of promoters. The presence of promoters favors the occurrence of dehydrocyclization processes, whereas on unpromoted zeolites arenes can be mainly formed by hydrogen transfer reactions. It should be noted that introduction of promoters (Zn, Ga) leads to the formation of additional Lewis acid sites, which, on the one hand, facilitates the alcohol dehydration and, on the other hand, can lead to a “synergistic effect” with Brønsted acid sites [93] on which oligomerization, cyclization, and dehydrocyclization reactions occur to form aromatic hydrocarbons. In addition, these promoters favor the dehydrogenation via recombinant desorption of hydrogen atoms.

CONCLUSIONS

The data presented show that the achievements of genetic engineering allowed commercial implementation of processes for bioisobutanol synthesis from products of processing of various vegetable raw materials. The existing bioethanol production facilities can be successfully converted to bioisobutanol production. As a result, bioisobutanol becomes a promising renewable feedstock for producing motor fuel components and petrochemical products. The use of bioisobutanol as an additive to motor fuel has a number of advantages compared to other bioalcohols.

Isobutylene and butenes are among key products of bioisobutanol processing. It is appropriate to use as a catalyst γ-alumina for the selective production of isobutylene and zeolite of ferrierite structural type for the production of linear butenes.

Catalytic systems based on zeolites of MFI structure allow production from bioisobutanol of С2–С4 olefins with high propylene and ethylene content and of aromatic hydrocarbons with high content of the benzene–toluene–xylene fraction, including p-xylene.

All these facts allow bioisobutanol to be considered as a competitive promising renewable feedstock, alternative to crude oil, for producing “green” hydrocarbons and a series of valuable petrochemicals. The use of bioisobutanol expands the resource base of petroleum chemistry and favors decarbonization of industry and reduction of the environmental impact by sustaining the balance between the carbon dioxide production and consumption.