In the first part of this study [1], we described the syntheses of tert-butyl esters of glycerol (GTBE) and propylene glycol (PTBE) and determined how these additives influence the physicochemical properties of automobile gasolines. As we found, the GTBE and PTBE additives to gasoline enhance its knock resistance: The calculated research and motor blending octane numbers (RONb/MONb) were 124/104 and 120/111, respectively. Such antiknock performance is comparable with that of such known oxygenate additives to gasolines as ethanol and methyl tert-butyl ether (MTBE). Furthermore, as we showed, inclusion of tert-butyl ethers into gasoline formulation jointly with ethanol (containing approximately 4–5 wt % water) allows the cloud point of gasolines to be considerably reduced. In this case, the ethers prevent the phase separation at low temperatures because of their amphiphilic properties. Therefore, compounds of this class have high potential, in particular, for use as hydrotropic solvents [2, 3].

This, second, part of the study deals with tert-butyl ethers of ethylene glycol (EG) and 2,3-butanediol (2,3-BD) as candidate automobile gasoline components. The technology of the saccharide hydrogenolysis to obtain glycols (diols) is well known and has precedents of commercial implementation [4]; the correction of the reaction conditions allows control of the selectivity with respect to products, including ethylene glycol [5]. 2,3-BD is notable in that it can be prepared by fermentation of carbohydrate-containing feedstock; it is important that relatively high concentrations of the products in the fermentation mass can be reached (2,3-BD titer 150 g/L) [6, 7], compared to acetone–butanol–ethanol (ABE) fermentation (the total titer of the products is 20– 35 g/L) [8, 9] and to production of ethanol by fermentation (40–60 g/L) [10, 11]. Thus, ethylene glycol (EG) and 2,3-BD can be considered as potentially renewable chemical feedstock whose production methods are being actively studied and improved.

EG and 2,3-BD derivatives (cyclic ketals and ethers) were characterized as additives to automobile gasolines in a number of previous studies [12–14]. However, there are no data in the literature on using mono-tert-butyl ethers of these diols as additives to gasolines. Therefore, as a continuation of the first part of our study, we determined the properties of mono-tert-butyl ethers of ethylene glycol (ETBE) and 2,3-butanediol (BTBE) (Fig. 1), including their effect as additives on the main physicochemical properties of gasolines, determining their quality.

Fig. 1.
figure 1

Mono-tert-butyl ethers of renewable diols.

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The goal of this study was to determine how the main physicochemical properties of automobile gasolines depend on the amounts of ETBE and BTBE additives.

These compounds were characterized as individual substances with the determination of a number of physicochemical properties (density, boiling point, crystallization onset point, kinematic viscosity, specific heat of combustion). We describe how additions of 1 to 10 vol % tert-butyl ethers influence the main properties of a base automobile gasoline (density, fractional composition, vVapor pressure (VP), gum content, RON, MON), including the case when the ethers are introduced in combination with ethanol.

EXPERIMENTAL

Chemicals. Ethylene glycol (analytically pure grade), tert-butanol (analytically pure grade), sulfuric acid (chemically pure grade), sodium hydroxide (pure grade) (all from Komponent-Reaktiv, Moscow, Russia), and 2,3-butanediol (98%, Sigma–Aldrich) were used for preparing tert-butyl ethers of diols without additional purification.

Components of base gasolines. For preparing a base gasoline, we used gasoline fractions whose origin and properties are indicated in Table 1. All the gasoline fractions were produced at Russian oil refineries. The base gasoline for testing the ether additives and ethanol + ether combined additives contained the following components (vol %): straight-run gasoline 4.0, reformate 45.0, catalytic cracker gasoline 25.0, alkylate 6.0, and isomerizate 20.0. For preparing ethanol-containing base gasoline, we used ethanol (chemically pure grade, Khimmed, Moscow, Russia).

Table 1. Main physicochemical properties of base gasoline components
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Before using ethanol, the actual water content was determined from the density measured according to GOST (State Standard) 57037-2016. The ethanol density at 20°С was 801.4 kg/m3, which corresponds to 4 wt % water content.

Methods for determining the physicochemical properties of gasoline mixtures. The analytical procedures for determining the physicochemical properties of gasoline mixtures and the equipment used are given in Table 2.

Table 2. Analytical procedures used in the study
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The research (RONb) and motor (MONb) blending octane numbers were calculated by formulas (1) and (2):

$${\rm{RONb}} = \;\;{{{\rm{RO}}{{\rm{N}}_{{\rm{blebd}}}} - {\rm{RO}}{{\rm{N}}_{{\rm{init}}{\rm{.}}\;{\rm{gas}}}}{C_{{\rm{vol}}{\rm{.}}\;{\rm{init}}{\rm{.}}\;{\rm{gas}}}}} \over {{C_{{\rm{vol}}{\rm{.}}\;{\rm{add}}}}}},$$
((1))
$${\rm{MONb}} = \;\;{{{\rm{MO}}{{\rm{N}}_{{\rm{blebd}}}} - {\rm{MO}}{{\rm{N}}_{{\rm{init}}{\rm{.}}\;{\rm{gas}}}}{C_{{\rm{vol}}{\rm{.}}\;{\rm{init}}{\rm{.}}\;{\rm{gas}}}}} \over {{C_{{\rm{vol}}{\rm{.}}\;{\rm{add}}}}}},$$
((2))

where RONblend is the research octane number of the blend, RONinit.gas is that of the initial gasoline, MONblend is the motor octane number of the blend, MONinit.gas is that of the initial gasoline, Cvol.init.gas is the volume concentration of the initial gasoline in the blend, and Cvol.add is the volume concentration of the additive in the blend.

The antiknock index (AKI) was calculated as the arithmetic mean of RON and MON.

Synthesis of tert-butyl ethers of EG and 2,3-BD. A round-bottomed flask equipped with a reflux condenser and a stirrer was charged with tert-butyl alcohol and glycol (EG or 2,3-BD) in 1.25 : 1 molar ratio. Sulfuric acid in an amount of 5 wt % based on diol was added. The mixture was stirred with heating to 55–60°C on a water bath for 36 h. Then, the mixture was cooled to room temperature, after which the acid was neutralized with a threefold molar excess of sodium hydroxide (in the form of 10% aqueous solution). The unchanged tert-butanol and water formed in the reaction were removed on a rotary evaporator (bath temperature 40°C, residual pressure 2.66 kPa, or 20 mmHg). The residue was fractionated at atmospheric pressure using a laboratory packed column. The target fraction enriched with the desired diol mono-tert-butyl ether was isolated (Тb = 152–154 and 162–163°C for ETBE and BTBE, respectively).

The samples obtained were analyzed with a Crystallux-4000М chromatograph equipped with a flame ionization detector (Supelcowax-10 capillary column, 30 m × 0.32 mm × 0.25 μm, carrier gas helium).

RESULTS AND DISCUSSION

Properties and Chemical Composition of Diol Mono-tert-butyl Ether Samples

Direct alkylation of alcohols with tert-butanol as a method for preparing ethers under laboratory conditions allows reaching high yields of target products and high selectivity of their formation. Therefore, it is widely used for preparing ethers of polyhydric alcohols [15–17]. For example, in synthesis of ethylene glycol tert-butyl ether (ETBE) under the chosen conditions, the target product yield reached 66% of the theoretical value.

The component composition of the synthesized ETBE and BTBE samples (according to the GLC data) is given in Table 3. The main reaction by-products are di-tert-butyl ethers of the corresponding diols and the unchanged diols. The purification method chosen does not ensure their complete removal from the target compounds.

Table 3. Component composition of the synthesized samples of ethylene glycol and 2,3-butanediol mono-tert-butyl ethers (according to GLC data)
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However, in the synthesis of BTBE by the same method the yield of the target product was very low and did not exceed 12–14% of the theoretical value. We failed to increase the yield by increasing the reaction time, taking tert-butyl alcohol (TBA) in a larger excess, and performing the synthesis with continuous heteroazeotropic distillation of water using a Dean–Stark trap. The butanediol transformations in side reactions (e.g., in dehydration to form methyl ethyl ketone) at the boiling point of TBA were not observed either. Most probably, the low yield of the desired ether is caused by steric hindrance arising in the course of the reaction of secondary OH groups of butanediol with TBA molecules. However, the scarcity of published data on the reactivity of 2,3-butanediol in reactions with tertiary alcohols does not allow us to go beyond mere assumptions.

The target mono-tert-butyl ethers (ETBE and BTBE) are colorless transparent liquids with characteristic ether odor and boiling points of 152–154 and 162–163°C, respectively (Table 4). No signs of ETBE crystallization were observed on cooling the samples down to –60°C. The ether viscosities (4.7 and 6.9 mm2/s at 20°C) were comparable to those obtained previously for a homolog, propylene glycol mono-tert-butyl ether (5.2 mm2/s at 20°С) [1].

Table 4. Main physicochemical properties of ETBE and BTBE samples
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Effect of Ethylene Glycol Mono-tert-butyl Ether (ETBE) Additions on the Quality Parameters of Automobile Gasolines

As a base gasoline blend (base gasoline) for studying the effect of ether additions on the operation properties, we chose a blend containing the main gasoline fractions commonly used in production of automobile gasolines at oil refineries. The RON/MON values of the base gasoline are 94.3/84.9 units. Therefore, the knock resistance of the base gasoline requires improvement to meet the requirements of GOST (State Standard) 32513-2013 to AI-95 gasoline. Because the base gasoline consists of hydrocarbon fractions, it contains no oxygen-containing or other octane boosters (alcohols, MTBE, methyl tert-amyl ether). With respect to the total sulfur content, the gasoline meets the requirements to K5 environmental class. The vapor pressure (VP) of the base gasoline is 63.3 kPa (DVPE), which meets the requirements of GOST (State Standard) (35–80 kPa in the summer period). The density at 15°C (754.2 kg/m3) is considerably lower than the upper limit prescribed by GOST (780 kg/m3).

The addition of ETBE to the base gasoline unambiguously influenced such parameters as the density, concentration of resins washed out with a solvent, and weight fraction of oxygen: They increased in proportion with the fraction of the additive (Table 5). The saturated vapor pressure showed no clear dependence on the content of the additive. Thus, ETBE additions exert no apparent negative effect on the gasoline volatility. All the gasoline blends containing 1 to 10 vol % ETBE met the requirements of GOST 32513-2013. For the blend with 10 vol % ETBE, the weight fraction of oxygen was at the upper limit of the admissible range.

Table 5. Values of the density, concentration of resins washed out with a solvent, saturated vapor pressure, and weight fraction of oxygen in gasolines with ETBE additions
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Because of relatively high boiling point of ETBE (Тb = 152–154°C), an increase in its content in the gasoline blend made the overall fractional composition of the fuel “heavier” (Table 6). The additive does not affect the amount of the distillation residue, which indicates that it is completely vaporized and forms no heavy products in the course of distillation.

Table 6. Fractional composition of gasoline blends with ETBE additions
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Along with the boiling end temperature and volume fraction of the residue in the flask, GOST 32513-2013 regulates the fractional composition of automobile gasolines via volume fractions of the vaporized gasoline at 70, 100, and 150°С (V70, V100, V150). All the three parameters of the base gasoline meet the requirements of the standard (Table 6). On adding ETBE, these parameters vary in proportion with the content of the additive. Namely, V70 and V100 decrease, whereas V150 increases. This is due to the relatively high TBE boiling point (152–154°C) and to an increase in the TBE fraction in the blend. The gasoline blends meet the GOST requirements throughout the interval of the concentrations of the additive, except V100 for the gasoline with 10 vol % ETBE, for which the volume fraction of gasoline vaporized up to 100°С (38.9 vol %) is below the minimum admissible level (40.0 vol %). Thus, when formulating gasoline blends with ETBE, it is necessary to take into account possible changes in the fractional composition and, if necessary, compensate them by adding larger amounts of low-boiling components (isomerizate, alkylate, light reformer naphtha).

The knock resistance of gasoline blends monotonically increased with an increase in the volume fraction of the added ETBE (Table 7). Addition of even 2.5 vol % ether was sufficient to reach the minimum admissible values of RON/MON prescribed by GOST 32513 (95.0/85.0). On the other hand, the addition of up to 10 vol % ETBE did not allow reaching the knock resistance prescribed for AI-98 gasoline (RON/MON = 98.0/88.0). The calculation based on the mean values of RONb/MONb shows that, to reach RON = 98.0, it is necessary to add to the base gasoline 17.0 vol % ETBE; the calculated value of MON will be 87.3 in this case. Thus, our results demonstrate limited possibility of increasing MON by adding ETBE. With respect to blending octane numbers RONb/MONb, ETBE as an octane booster is quite comparable with ethanol and MTBE. The traditional values indicated for ethanol are RON 109–110 and MON 90–92, and for MTBE, RON 117–118 and MON 101–102 [18, 19].

Table 7. Knock resistance characteristics of gasoline blends with ETBE additions
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On the other hand, our results confirm the initially formulated hypothesis that tert-butyl ethers of glycols act as phase stabilizers of ethanol-containing gasolines. Distilled ethanol additionally dried over 3A molecular sieves, used as an additive to gasoline, had a density of 807.4 kg/m3, which corresponds to 4.0 wt % water content. The gasoline with the addition of 10 vol % ethanol of such quality showed the first signs of phase separation already at positive temperatures (cloud point 15°C), which is due to relatively high weight fraction of water (as a rule, efforts are made to reduce the water content of ethanol before addition to gasoline). The gasoline with the addition of ethanol + ETBE, 3 : 1 v/v in the same amount had the 16°C lower cloud point: –1°C. With the addition of ethanol + ETBE, 1 : 1 v/v, the cloud point decreased to the level below –50°C. Thus, the action of ETBE is similar to that observed with traditionally used stabilizers of ethanol-containing gasolines, of which aliphatic alcohols С3–С5 are the most widely used.

One of the problems arising when including lower aliphatic alcohols into automobile gasoline formulations is heterogenization of gasoline blends in the presence of water; as a rule, it is most pronounced at lower temperatures. The main cause of this phenomenon is gradual saturation of gasoline with moisture due to hygroscopic properties of the alcoholic additive or direct contact of the fuel with water (e.g., occurring on the bottom of the reservoir). As a result, a phase enriched in water and alcohol is separated. Therefore, the fuel fed to an engine can have decreased knock resistance because of the loss of the oxygenate component. Ample attention was paid to this problem (for methanol and ethanol) [20–22].

We have found in the first part of our study [1] that additions of 1,2-propanediol mono-tert-butyl ether and glycerol di-tert-butyl ether can significantly reduce the tendency of an alcohol-containing gasoline to form a separate phase on contact with water and thus can exert a stabilizing effect.

On adding 5.0, 7.5, and 10.0 vol % ethanol (water content 4 wt %) into the base gasoline, we obtained blends with the cloud points of 5, 7, and 8°C (Table 8). On the other hand, the blends containing ETBE in the same amounts showed no signs of heterogenization down to –60°C. Partial replacement of the added ethanol by ETBE allowed the cloud point to be considerably decreased. In particular, replacement of one eighth/fourth part of alcohol by ETBE at the total volume fraction of the oxygenate additive of 10 vol % allowed the cloud point of the fuel to be decreased by 24/58°C. The similar trend was observed for blends with lower content of the oxygenate additive. No clear dependence of the octane-boosting performance on the composition of the binary oxygenate additive [alcohol + ETBE] was observed (Table 8). For example, ΔRON/ΔMON on adding 10 vol % ethanol to base gasoline was 2.8/1.0 units, on adding 10% ETBE it also was 2.8/1.0 units, and on adding 5% ETBE + 5% ethanol simultaneously it was 3.2/1.1 units. In combination with other data, these data demonstrate no apparent nonadditive effects on introducing binary additives of such composition into gasoline.

Table 8. Physicochemical properties of gasoline blends with additions of ethanol and ETBE
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Effect of 2,3-Butanediol Mono-tert-butyl Ether (BTBE) Additions on the Quality Parameters of Automobile Gasolines

The dependence of the properties of gasoline blends on the BTBE amount was, on the whole, similar to that observed with ETBE: An increase in the content of the additive led to an increase in the density and to changes in the parameters characterizing the fuel volatility. On the whole, the dependence of VP on the BTBE amount (Table 9) was considerably more pronounced. In particular, at the ETBE/BTBE content of 7.5 vol %, DVPE of gasoline was 63.0/56.9 kPa. Apparently, the main cause of differences in variation of the saturated vapor pressure is the difference in the boiling point and volatility of the ethers themselves: The boiling point of BTBE is 10°C higher than that of ETBE (162 and 152°C, respectively). Similar trend is observed in changes in the fractional composition parameters, although in this case the difference between the boiling temperatures for fuels with additions of ETBE/BTBE is small (up to 4°С) and is within the reproducibility interval of the GOST 2177 method. The antiknock performance of ETBE was higher than that of BTBE: The mean values of RONb/MONb for these compounds were 130/103 and 115/97, respectively.

Table 9. Physicochemical properties of gasoline blends with BTBE additions
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Similar difference in variation of the fuel properties on introducing BTBE is also observed when the low-temperature phase stability of ethanol-containing gasoline blends is concerned (Table 10). At similar compositions of the ether–alcohol additive, the cloud point depression was considerably larger for ETBE. For example, for the gasoline blend with 10.0 vol % binary additive [alcohol + ether, 3 : 1 v/v], the cloud point was –50°C for ETBE and –28°C for BTBE. The BTBE molecule is more lipophilic compared to ETBE because of larger amount of alkyl substituents; in this case, increased lipophilicity negatively affects the ability to dissolve water and ethanol at low temperatures.

Table 10. Changes in low-temperature properties of ethanol-containing gasolines on introducing BTBE
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CONCLUSIONS

Thus, we described the properties of mono-tert-butyl ethers of ethylene glycol and 2,3-butanediol and determined how the additions of these ethers (1–10 vol %) influence the properties of automobile gasoline. For both additives as pure substances we determined the density, boiling point, crystallization onset temperature, kinematic viscosity, and specific heat of combustion. Introduction of the ethers into gasoline was accompanied by an increase in its density and in the middle boiling point of gasoline and by a decrease in VP; BTBE exerted a stronger effect on the two latter parameters. ETBE, compared to BTBE, showed higher performance in enhancement of the knock resistance and reduction of the cloud point of the ethanol-containing gasoline. Introduction of these additives into ethanol-containing gasoline led to the phase stabilization, with the change in the cloud point depending on the amount and composition of the binary additive. For example, with 5.0% additive [alcohol + ETBE, 1 : 1 v/v], RON/MON increased to 95.8/85.7 (compared to 94.3/84.9 for the base gasoline), and the cloud point was below –60°C. Further increase in the concentration of the ether as a component of the binary additive led to an increase in RON and MON for the additive [alcohol + ETBE, 3 : 1 v/v] to 97.5 and 86.0, respectively.