Under the framework of the global decarbonization trend, one of the most important green energy focus areas is the production of engine fuel components from renewable raw materials. Commonly recognized examples of successful implementation of such projects include, in particular, the production of first-generation bioethanol and biodiesel from fatty acid methyl ethers (FAMEs), chemicals that were extensively used in the USA, Brazil, and Europe in the early 20th century. On the other hand, bearing in mind the obvious drawbacks of these technologies (e.g., competition with food biomass, questionable cost effectiveness, and high specific carbon intensity), a growing body of research has been focused on finding new pathways for the transformation of renewable raw materials into biofuel components.

In this connection, a very promising approach is conversion of biomass to diols/polyols, primarily to glycerol, ethylene glycol, propylene glycol, and butanediols.

Of late, bioglycerol—a waste product from large-scale biodiesel production facilities—has served as a feedstock for hydrogenolysis into renewable propylene glycol [1]. Glycerol, ethylene glycol, and propylene glycol can also be produced by catalytic hydrogenolysis of carbohydrates in various designs that have been known for about 100 years [2]. 1,4-Butanediol and 2,3-butanediol can be successfully produced by fermentation of carbohydrate-containing raw materials [3, 4].

For polyols, one promising conversion route is the production of oxygenated fuel additives. Prior research has covered various types of chemicals as potential oxygenated additives for motor gasolines:

—cyclic ketals produced by condensation of polyols with ketones: glycerol acetone ketal [5–8], glycerol–glycerol butanone ketal, glycerol MIBK ketal, ethylene glycol acetone ketal [9], propylene glycol acetone ketal, and 2.3-butanediol butanone ketal [10];

—ethers produced by alkylation of polyols with alcohols or olefins: glycerol isopropyl ethers, glycerol sec-butyl ethers [11], glycerol tert-butyl ethers [12], propylene glycol monoisopropyl ether, and 2,3-butanediol mono-sec-butyl ether [10]; and

—mixed compounds: methyl, isopropyl, and tert-butyl ethers of solketal (cyclic ketal derived from glycerol and acetone) [9].

Among the above-listed ethers derived from diols and glycerol, di- and tri-tert-butyl ethers of glycerol (GTBEs) have exhibited the highest antiknock performance, expressed as a blending research octane number to blending motor octane number ratio (bRON/bMON) of about 135/117. On the other hand, the use of this additive faced the challenge of low volatility due to the high boiling point (213°C) [9]. The GTBE molecular structure suggests that the efficiency of this additive is attributable to the simultaneous presence of two groups commonly assumed to be “drivers” of high antiknock performance, namely the tert-butyl substituent and the free hydroxy group. Therefore, the task is to synthesize compounds that would combine the advantages of meeting the motor gasoline requirements with respect to boiling point and volatility as well as containing both tert-butyl and hydroxy groups. One possible solution to this task is preparation of mono-tert-butyl ethers derived from diols such as ethylene glycol, propylene glycol, or 2,3-butanediol, each representing a potentially renewable material. A number of tert-butyl monoethers based on renewable diols are presented below:

structure 1

These compounds are of interest as oxygenates added to gasoline blends for two reasons. First, available publications offer almost no description of the effects of vicinal (tert-butoxy)alkanols (including GTBEs and the above-listed compounds) on antiknock performance. This is despite the widespread use both of alcohols (e.g., methanol, ethanol, and C3–C4 alcohols) and tert-butyl ethers (e.g., methyl tert-butyl ether, ethyl tert-butyl ether, and methyl tert-amyl ether) as oxygenated gasoline additives. Second, glycol-derived and glycerol-derived ethers are known as hydrotropes, i.e. compounds that facilitate—due to their own amphiphilicity—the solubilization of lipophiles in aqueous media [13, 14]. This suggests that adding these compounds to gasoline could not only affect the antiknock performance, but also allow low-temperature phase stabilization (i.e. reduce the cloud point) of ethanol-containing gasolines. A similar effect has been outlined for a number of polyol derivatives using the case of cyclic ketals derived from ethylene glycol and glycerol [15].

The purpose of the present study was to identify the effects of adding PTBEs and di-GTBEs on the major physicochemical properties of motor gasoline. The additives were characterized, as individual chemicals, in terms of relevant physicochemical properties (e.g., density, boiling point, pour point, kinematic viscosity, and heat of combustion). A further goal was to identify the effects of the ethers (added in concentrations of 1 to 10 vol %) on the properties of base motor gasoline, such as density, fractional composition, vapor pressure, gum content, RON, and MON. Special attention was drawn to the effects of the tert-butyl ether additives on the antiknock performance and low-temperature phase stability of ethanol-blended base gasoline.

EXPERIMENTAL

Reagents. Propanediol-1,2 (high-purity grade), tert-butanol (AR grade), sulfuric acid (CP grade, 95.6%), and sodium hydroxide (high-purity grade) (all manufactured by Komponent-Reaktiv, Moscow, Russia) were used without additional treatment for the synthesis of diol tert-butyl ethers.

Components of base gasoline. By definition, base gasoline is a gasoline blend that closely approximates the composition of a commercial gasoline product prior to the introduction of additives.

To prepare base gasoline, the following components were used: straight-run gasoline (from TANECO Company, Russia); stabilized reformate (from Ryazan Oil Refining Company, Russia); light catalytic cracked gasoline (LCCG); heavy catalytic cracked gasoline (HCCG) (both from Gazpromneft Moscow Oil Refinery, Russia); alkylate; and isomerizate (both from LUKOIL Nizhny Novgorod Petroleum Organic Synthesis Company, Russia).

The base gasoline for testing ether additives and mixed ethanol+ether additives contained the following components (vol %): straight-run gasoline 4.5; reformate 46.5; LCCG 10.0; HCCG 6.0; alkylate 17.0; and isomerizate 16.0. Ethanol (CP grade, ChimMed, Moscow, Russia) was used to prepare ethanol-containing base gasoline. The base gasoline was prepared in two batches, differing in quality due to fluctuating concentrations of individual components.

Before using ethanol, the actual water content was evaluated from the density measured according to GOST 57037-2016.

Test methods for physicochemical properties of gasoline blends. The methods for characterization of the physicochemical properties of gasoline blends, with specification of relevant instrumentation, are summarized in Table 1.

Table 1. Test methods employed for the study
Full size table

When evaluating the bRON and bMON numbers, the octane number was assumed to vary additively depending on the volumetric concentration of the additive. The antiknock index was determined as the arithmetic mean of RON and MON.

Synthesis of Polyol tert-Butyl Ethers

Synthesis of PTBEs. Tert-butanol and propylene glycol, in a molar ratio of 1.25 : 1, were placed in a round-bottom flask equipped with a reflux condenser and a stirrer. Sulfuric acid (95.6%) was added in an amount of 5% by weight of diol. The mixture was stirred under heating to 55–60°C in a water bath for 36 h, then cooled at room temperature, after which the acid was neutralized with a three-fold molar excess of NaOH (as a 10% aqueous solution). The tert-butanol and the water resulting from the reaction were removed using a rotary evaporator (at a bath temperature of 40°C and a residual pressure of 2.7 kPa or 20 mmHg). The residue was distilled at atmospheric pressure using a laboratory packed column, where the desired distillate rich in the diol mono-tert-butyl ether was obtained at a boiling point of 152–154°C in 40–42% yield. The GC data showed 94.2% of the target propylene glycol mono-tert-butyl ethers (2-tert-butoxy propan-1-ol and 1-tert-butoxy propan-2-ol) in this distillate.

Synthesis of 1,3-di-GTBE. Glycerol 1,3-di-tert-butyl ether was synthesized from epichlorohydrin and tert-butanol according to the procedure described in our previous study [9].

The synthesized tert-butyl ether samples were analyzed by GC using a Crystal-Lux 4000M chromatograph equipped with a flame ionization detector and a 30 m×0.32 mm Supelcowax-10 column (with helium as a carrier gas).

RESULTS AND DISCUSSION

Properties and chemical composition of diol mono-tert-butyl ether samples. Bearing in mind the simple process design and high yields of the products, direct alkylation of glycols with tert-butanol was chosen as the optimal method for lab-scale synthesis of PTBE samples. Given the reaction stoichiometry of the catalytic synthesis, a tert-butanol excess of 1.25 with respect to diol was theoretically supposed to facilitate the selective formation of the monoether. In actuality, di-tert-butyl ether was formed as a byproduct: both the diether and free diol were detected as impurities in the monoether samples distilled at atmospheric pressure (Table 2). In the alkylation of propylene glycol, 1-monoether dominated among the isomers, with 92–93% selectivity.

Table 2. GC analysis of synthetic PTBE samples
Full size table

PTBEs appear as transparent liquids with a typical ether odor and a boiling point of 151–153°C (Table 3). As the ether samples were cooled, they showed no evidence of crystallization until the temperature reached minus 60°C. As had been expected when discussing the study objectives, the tested glycol ethers proved to be markedly “lighter” in terms of density, kinematic viscosity, and boiling point than the glycerol di-tert-butyl ethers extensively reported in previous publications [9, 12]. This gave reasonable grounds to assume that adding PTBE to motor gasoline would have a lesser effect on the gasoline volatility than in the case of GTBE. In particular, when 10 vol % of PTBE was added to the gasoline, the air-saturated vapor pressure (ASVP) decreased by 1.5 kPa, while identical addition of di-GTBE lowered this pressure by 4.8 kPa.

Table 3. Major physicochemical properties of synthetic samples of tert-butyl monoethers of renewable diols
Full size table

Effects of PTBE addition on gasoline quality. To investigate the relationship between PTBE addition and the physicochemical properties of motor gasoline, an oxygenate-free base gasoline blend was prepared. This base blend, obtained by compounding gasolines of different origins, had a composition similar to those of the most typical gasoline blends produced by Russian refineries. Increasing the PTBE concentration in the base gasoline predictably led to a growth in the density and oxygen content, though both parameters remained within the limits required by applicable regulations (725– 780 kg/m3 and no higher than 2.7 wt %, respectively, as per GOST 32513), even when the ether concentration reached as high as 10 vol % (Table 4). A similar relationship was observed for gum content, which remained below the regulatory limit (5 mg/100 mL). The PTBE effect on gasoline vapor pressure was negligible: the ASVP drop never exceeded 1.5 kPa (from 39.9 to 38.4 kPa for 10 vol % PTBE). This confirmed our assumption of the markedly lesser deterioration of volatility than in the cases previously described for solketal and GTBE [5, 9].

Table 4. Density, gum content, vapor pressure, and oxygen content in gasoline after PTBE addition
Full size table

Furthermore, as the PTBE concentration rose, the fractional composition of the gasoline predictably turned “heavier,” as indicated by the elevation of the initial, 50%, and end boiling points (Table 5). Nonetheless, given the relatively low intrinsic boiling point of PTBE, all the gasolines with ether addition of up to 10 vol % complied with the GOST regulations in terms of distillation analysis.

Table 5. Fractional composition of gasoline blend after PTBE addition
Full size table

The octane numbers of the resultant gasoline blend also increased with the PTBE concentration in base gasoline (Table 6). Compared to the homologous propylene glycol monoisopropyl ether (PIPE) described in [10], the PTBE exhibited a markedly higher efficiency, as evidenced by bRON/bMON of 101/98 and 120/111, respectively. According to previous reports, the octane-boosting effect was more pronounced when di-GTBE was added: the bRON/bMON amounted to 135/117, with the highest efficiency corresponding to the lowest additive concentrations (1.0–2.5 vol %) [9]. For PTBE, this effect was somewhat weaker. Overall, the octane improvement indicates that PTBE as an octane booster is comparable to MTBE and ethanol (bAKI = 108–112).

Table 6. Antiknock performance of gasoline blends after PTBE addition
Full size table

The previously reported comparative assessment of pure ethanol, pure solketal, and their 1 : 1 v/v mixture revealed no synergistic effect, either negative or positive. The data obtained in the present study for the ethanol and ethanol+PTBE additives (Table 7) did not allow for a definite conclusion. For example, adding 7.5 vol % of ethanol to the base gasoline increased RON/MON to 93.5/86.8. When the same amount of ethanol+PTBE (2 : 1 v/v) was added, the RON/MON rose to 95.4/87.0. Evidently, the mixed additive exhibited a noticeably higher efficiency than pure ethanol. On the other hand, adding 10.0 vol % of ethanol, ethanol+PTBE (3 : 1 v/v), and ethanol+PTBE (1 : 1 v/v) increased the RON/MON to 96.3/87.4, 96.1/87.3, and 95.9/87.2, respectively. Therefore, the introduction of 10 vol % of an oxygenated additive to the base gasoline did not create any non-additive effects. In this case, we only observe a higher molar efficiency of propylene glycol ether. Indeed, given that the molecular weight of PTBE is almost three-fold higher than that of ethanol, an equivalent octane boost can be achieved by introducing a smaller amount of the additive into gasoline.

Table 7. Physicochemical properties of gasoline blends after addition of ethanol vs. PTBE
Full size table

Furthermore, the experimental data confirmed the assumption that glycol tert-butyl ethers allow phase stabilization of ethanol-blended gasoline. For addition to gasoline, we used distilled ethanol additionally dried with 3A molecular sieves; this ethanol had a density of 807.4 kg/m3 (corresponding to 4.0 wt % of water). When 10 vol % of this ethanol was added, the gasoline had a cloud point of 15°C, and phase separation manifested itself even at positive temperatures. This can be explained by the relatively high weight content of water (measures are generally taken to reduce water content in ethanol before adding it to gasoline). The addition of identical amounts of the 3 : 1 and 1 : 1 ethanol+PTBE mixtures resulted in lowering the gasoline cloud point to minus 1°C and minus 50°C, respectively. Thus, the effect of PTBE is similar to that exerted by C3–C5 aliphatic alcohols and other (less common) stabilizers for ethanol-blended gasoline.

Effects of di-GTBE addition on gasoline quality. The effects of di-GTBE additives on the physicochemical properties of gasoline were detailed in [9, 12, 16]. We deemed it appropriate to revisit this subject within the scope of this study, taking into account two major considerations. First, our earlier data testify that di-GTBE ensures a markedly higher octane boosting efficiency when added in low concentrations (e.g., the MON increased by 1.5 points when 1.0–2.5 vol % was added) [9]. This pattern needed to be further verified. Second, to the best of our knowledge, the effects of di-GTBE on the quality of ethanol-blended gasoline had never been described.

The effects of the di-GTBE addition on the density, existent gum content, and vapor pressure of the gasoline (Table 8) proved to be similar to those for PTBE. The only exception was that increasing the di-GTBE concentration resulted in larger increments of density growth and vapor pressure reduction than in the PTBE case. This is associated with the higher density and boiling point of di-GTBE. Given a lower oxygen weight concentration in di-GTBE than in PTBE, the oxygen weight concentration in the gasoline blends also remained within the applicable GOST limits.

Table 8. Density, gum content, vapor pressure, and oxygen content in gasoline after di-GTBE addition
Full size table

The antiknock performance data for the gasoline blends with di-GTBE additives are presented in Table 9. The highest octane boosting efficiency was, again, observed at low concentrations. For example, the average bRON dropped from 128 to 121 when the additive’s concentration was raised from 1.0–3.0 vol % to 5.0–10.0 vol %. It is worth noting that there was almost no correlation between the calculated bMON and the di-GTBE concentration. At the same time, the absolute octane boosts were significantly lower than those observed in [9]: the bMON and bRON were as low as 104 and 124, respectively (versus 117 and 135). Thus, the data obtained in the present study confirm the previous assumption of the higher efficiency of di-GTBE in low concentrations involved in gasoline blending, whereas full consistency was not reached in terms of absolute efficiency.

Table 9. Antiknock performance of gasoline blends after di-GTBE addition
Full size table

Like PTBE, di-GTBE acted as a phase stabilizer for the ethanol-containing gasoline blends. Moreover, di-GTBE’s efficiency proved to be noticeably higher, probably due to its higher lipophilicity associated with the lower oxygen weight content: at identical concentrations of the additives, di-GTBE provided a 5°C larger cloud point depression than PTBE (Table 10).

Table 10. Physicochemical properties of gasoline blends after addition of ethanol vs. di-GTBE
Full size table

Furthermore, we investigated a synergistic effect between ethanol and di-GTBE manifested when the RON provided by the addition of ethanol+di-GTBE was higher than that for pure ethanol added in the same amount. This synergistic effect was observed in two out of the three cases. In particular, the addition of 7.5 vol % of ethanol increased the RON from 93.0 to 93.5, compared to 95.5 (a substantially higher number) provided by 7.5 vol % of ethanol+di-GTBE (2 : 1 v/v). The base blend with 10 vol % of ethanol exhibited RON 96.3. The same amount of ethanol+di-GTBE (3 : 1 v/v) increased the RON only to 96.0, thus revealing a somewhat lower efficiency of the binary additive. On the other hand, adding 10 vol % of ethanol+di-GTBE (1 : 1 v/v) resulted in RON 97.2, or 0.9 points higher than that in the 10 vol % ethanol case.

The applicable national standard (GOST 32513) limits the volumetric content of ethanol and other C5+ ethers in gasoline to no higher than 5 vol %. The data in Table 10 clearly show that the antiknock performance of base gasoline (RON 93.0) cannot be improved to the required level of ≥95 via adding ethanol alone. In contrast, the combined addition of ethanol and di-GTBE in the amount of 7.5 vol %, where ethanol accounted for 5 vol % and di-GTBE for 2.5 vol %, solved two problems simultaneously: (i) it boosted the octane number (without exceeding the oxygen content limit of 2.7 wt %); and (ii) ensured adequate phase stabilization of ethanol-blended fuel (obviously, decreasing the water content in ethanol shifts the cloud point towards negative temperatures). The same is true for the combined involvement of ethanol and PTBE (Table 8).

Finally, the addition of ethanol+di-GTBE was found to eliminate the negative effect of a high-boiling glycerol ether on gasoline volatility. In particular, the dry vapor pressure equivalent (DVPE) was 47.7 kPa for the gasoline with the addition of 7.5 vol % of ethanol, compared to 46.5 kPa when ethanol+di-GTBE (3 : 1 v/v) was added. Despite a certain elevation of the EBP in the latter case compared to the base gasoline (from 186 to 200°C), this EBP remained within the GOST limit (215°C). Thus, the combined use of tert-butyl ethers and ethanol in a volumetric ratio of 1 : 2, with their total concentration of 7.5 vol %, proved to be more efficient for improving gasoline quality than the addition of individual ethanol or individual ether in identical concentrations.

CONCLUSIONS

The paper describes the properties of propylene glycol mono-tert-butyl ether (PTBE) and glycerol di-tert-butyl ether (di-GTBE) as oxygenated additives for motor gasoline. The tert-butyl ethers were characterized, as individual chemicals, in terms of density, boiling point, pour point, heat of combustion, and kinematic viscosity. Both ethers were then investigated as additives (in concentrations of 1.0 to 10.0 vol %) to motor gasolines, including ethanol-based blends. The addition of these ether additives increased the density and average boiling point, and lowered the vapor pressure, of the tested gasoline blends; these effects were more pronounced for di-GTBE than for PTBE. Both ether additives enhanced the gasoline’s antiknock performance: the average bRON/bMON was 120/111 for PTBE and 124/104 for di-GTBE. When the ether additives were combined with ethanol, a phase stabilization effect was observed: increasing the ether content in the mixed additives markedly lowered the cloud point of the resultant ethanol-blended gasoline. It was shown that the addition of a 2 : 1 v/v ethanol+ether mixture provides the highest efficiency in enhancing the RON from 93.0 to the required level of ≥95.0, and that this target cannot be reached at all by the addition of ethanol alone.