What are oxygenates?

Petrol is a complex mixture of refinery hydrocarbon streams, additives, and in many cases, oxygenated blending components.

The final composition must meet critical specifications and efficiency characteristics for optimal performance in a variety of engine and vehicle types and technologies.

Fuel composition and quality also have a direct impact on vehicular emissions, both evaporative and exhaust, which impact air quality, fuel efficiency, and greenhouse gas (GHG) emissions.

Oxygenates are organic compounds which contain one or more oxygen atoms. Oxygenates enhance octane and improve the combustion of other petrol components. Only two types of oxygenates are commonly used as petrol blending components: alcohols and ethers.

Oxygenates use in petrol goes back to the 1970s, when refiners sought to replace lead with less toxic octane boosters and increase petrol volumes. In fuel, the most commonly found oxygenates are: methanol (MeOH), ethanol (EtOH), methyl tertiary butyl ether (MTBE), ethyl tertiary butyl ether (ETBE), tertiary amyl methyl ether (TAME) and tert-amyl ethyl ether (TAEE).

	MTBE	ETBE	TAME	TAEE	DIPE
CAS Number	1634-04-04	637-92-3	0994-05-08	919-94-8	108-20-3
Molecular mass	88	102	102	116	102
Oxygen, wt.%	18.2	15.7	15.7	13.8	15.7
Solubility in water, g/L @25ºC	42	12	4.5	4.5	20
Boiling point, °C	55	72	86	102	68
RVP* blending, kPa	55	28	10	9	34
Density, kg/L	0.74	0.75	0.77	0.77	0.73
Stoichiometric air/fuel ratio	11.73	12.15	12.15	12.57	12.15
Net Energy Density, MJ/L	26	27	28	29	27
RON, blending	117	119	110	108	110
MON, blending	102	103	99	95	99

Physical and blending properties of alcohols (Source: Ullman Encyclopaedia, p.13)

	MeOH	EtOH	IPA	TBA	IBA	nBA
CAS Number	67-56-1	64-17-5	67-63-0	75-65-0	78-83-1	71-36-3
Molecular mass	32	46	60	74	74	74
Oxygen, wt.%	50.0	34.8	26.7	21.6	21.6	21.6
Boiling point, °C	65	78	82	83	108	117
Density, kg/L	0.8	0.79	0.79	0.78	0.8	0.81
Stoichiometric air/fuel ratio	6.46	8.98	10.33	11.16	11.16	11.16
Net Energy Density, MJ/L	16	21	25	27	27	27
RVP*, neat, kPa	31.7	17.3	12.4	11.7	4.1	2.8
RVP* blending, kPa	414	117.3	96.6	60.7	34.5	44.2
RON, blending	133	130	121	109	105	94
MON, blending	99	96	96	93	92	81

Physical and blending properties of alcohols (Source: Ullman Encyclopaedia, p.13)

Which are the most commonly used fuel ethers?

MTBE

Name: tert-butyl methyl ether, MTBE
EC Number: 216-653-1
CAS Number: 1634-04-4
Molecular formula: C5H12O

MTBE is a volatile, colourless liquid, with an odour similar to terpene. It has a boiling point of 55.2ºC and freezing point of -109ºC. MTBE is flammable and is soluble in other ethers, hydrocarbons, and alcohols. MTBE is only sparingly soluble in water (4.2 wt.%). MTBE is produced by acid-catalyzed condensation of methanol and isobutylene. MTBE is almost exclusively used to increase the octane and oxygen content of petrol.

The research octane number (RON) of MTBE is between 115-135, and the motor octane number (MON) between 98-110, depending on the base petrol blendstock. This range has been determined by a large number of experimental data obtained when formulating petrol.

The 2016 SAE paper ’Blending Octane Evaluation of Fuel Ethers: A Literature Review’ indicates that the RON level of MTBE is 117 and of MON is 102. Blending octane numbers of petrol are sensitive to the composition and octane numbers of the base petrol.

According to the MTBE Handbook, which takes into consideration some of the studies, a 15% v/v of MTBE represents a reasonable concentration of MTBE in petrol in terms of octane number increase, change in fuel stoichiometry, and commercial availability of MTBE. The high-octane properties of MTBE are particularly effective in upgrading low-octane unleaded petrol components such as naphtha and natural gasoline.

The boiling point of MTBE is low, which provides higher front-end octane numbers (FEON) to petrol. FEON is the octane number of a petrol fraction which boils below 100 degrees Celsius. This is an important element in cold-start conditions, when the low-boiling compoents of petrol get a chance to vaporize. MTBE is very effective in boosting the front-end octane and gives very high FEON numbers (135 RON). The FEON of MTBE is higher than that of butane, reformate, alkylate, and aromatics. FEON also increases engine efficiency during the low-speed acceleration stage.1

Among the properties that influence the performance of petrol are Reid Vapour Pressure (RVP) and distillation profile. In many countries, including the US and members of the European Union, the RVP is kept low to reduce evaporative VOC emissions which lead to ground-level ozone.

Adding MTBE to petrol may affect its RVP, depending on the RVP of the blendstock. For example, blending 15 wt.% MTBE in a 63 kPA blendstock decreases the RVP of the finished petrol by approximately 2 kPa (Figure 3). Hence, there is no need to remove butane with MTBE and, depending on the RVP limit of the finished petrol, additional butane can be added to the blend to reduce cost. This is one of several advantages of ethers over alcohols.
Figure 3 illustrates how ethanol and methanol both increase the volatility (RVP) of petrol. Adding ethanol to petrol causes an RVP increase ranging from 7 to 12 kPa depending on the RVP of the Blendstock for Oxygenate Blending (BOB). The RVP increase from 1.0 vol. % methanol is of the order of 15 kPa and increases to 20 kPa at 2.0 v/v methanol. This makes alcohols less useful than ethers for formulating low RVP petrol for ozone non-attainment areas. This also requires the refiners to remove additional butanes and pentanes when producing BOBs for alcohol blending, which increases cost. The RVP impacts of different oxygenates are compared in Figure.

MTBE boils in the same temperature range as other light refinery components. MTBE is soluble in any ratio with petrol. In contrast with alcohols, MTBE does not form azeotropes with other hydrocarbon components or depress the distillation curve (T50) of the finished petrol.

1 Handbook of MTBE and Other Gasoline Oxygenates. Halim Hamid Mohammed, Ashraf Ali March 11, 2004, CRC Press, p. 41.

ETBE

Name: Ethyl tert-Butyl Ether (ETBE)
EC Number: 211-309-7
EC Name: 2-ethoxy-2-methylpropane
CAS Number: 637-92-3
Molecular formula: C6H14O

Ethyl tertiary-butyl ether (ETBE) is a flammable liquid with a boiling point of 72ºC. ETBE is produced by the acid-catalyzed condensation of isobutylene and bio-ethanol. The blending vapour pressure of ETBE is 27.6 kPa, which is significantly lower than that of ethanol. The blending octane of ETBE is 119 RON and 103 MON. The effect of ETBE on the distillation characteristics is similar to MTBE. ETBE blends linearly with petrol and does not form azeotropes.

ETBE offers advantageous physical and chemical properties compared to ethanol for petrol blending including:

significantly lower blending volatility
no significant distortion of the distillation curve
better tolerance of wet distribution systems
lower octane sensitivity (RON-MON)
improved materials compatibility

ETBE offers advantageous physical and chemical properties compared to ethanol for petrol blending including:

significantly lower blending volatility
no significant distortion of the distillation curve
better tolerance of wet distribution systems
lower octane sensitivity (RON-MON)
improved materials compatibility

Blending ETBE with petrol for ethanol blending also imparts the following benefits:

Lower RVP, which leaves room for more light components,
Reduced water sensitivity,
Better compatibility with seals and gaskets,
Octane sensitivity (RON–MON) in line with finished petrol specifications requirements.

Zooming in on n-octane boosters: what are the health, environmental and mechanical risks?

Modern automotive fuels are composed of a blend of base fuel, various types of blending components, and additives. These additions serve to enhance fuel performance, particularly by preventing pre-ignition, also known as ‘engine knock’, during combustion in the engine. While numerous blending components and additives help raise the fuel’s octane number, and thus prevent engine knock, not all of them have equal effects on human health, the environment, and the durability of the engine.

In light of these concerns, Sustainable Fuels conducted a literature review to investigate the impact of nitrogen-based octane boosters. This review reveals that nitrogen-containing octane boosters are acutely toxic or harmful, often irritating to the eyes and/or skin or sensitizing, and cause damage to organs, often targeting the blood system. Their impact on the environment is critical as well, as the use of such octane boosters results in an additional release of nitrogen during fuel combustion, leading to higher-end levels of NOx, greenhouse gases, poorer air quality, and well-known toxicity to man and the environment.

Moreover, they are much less effective anti-knock agents than organometallic substances and must therefore be used at higher concentrations for satisfactory knock suppression. A number of studies indicate damaging effects on internal combustion vehicle engines resulting from their use, including pre-ignition, shortened induction period, and an increase in deposits (gum, tar, sludge). These factors collectively lead to reduced efficiency and potential engine failure.

The existing human health, environmental and technical data suggest that aniline-type nitrogen-containing octane boosters show lower performance than metal-based octane boosters, have an unfavorable human health and environment profile, and are detrimental to air quality and vehicle engines. Hence Sustainable Fuels recommends for existing and future nitrogen-containing octane boosters to only be allowed to be blended into fuels if they successfully pass a properly designed screening that considers both their efficacy and their safety for man and the environment.

Nitrogen-containing octane boosters are acutely toxic and harmful

Study shows ETBE is not likely to be a mutagenic carcinogen

Sustainable Fuels members are dedicated to the responsible production, use and promotion of the fuel ethers, which include (bio-) MTBE, (bio-) ETBE, (bio-) TAME and (bio-) TAEE. As fuel ether manufacturers we are committed to maximize the benefits for society whilst minimizing risks to human health and the environment. In parallel, we cooperate with regulators and civil society organizations to inform and educate about the societal benefits and potential risks of our products in a scientifically sound and unbiased manner.

Our product ETBE is globally used as a gasoline oxygenate, including in the EU, the United States and Japan. The main purpose of (co-) blending ETBE is to increase the octane level of the gasoline, enhance the fuel consumption efficiency and lower air pollution from tailpipe emissions. Inhalation of gasoline vapours is the primary potential route of human exposure to ETBE. As such, ETBE has been reported to cause liver adenomas (non-carcinogenic tumours) following inhalation in male rats in a 2-year study at the highest inhalation concentration of ETBE tested at 5000 ppm.

In our latest study, Sustainable Fuels have sought to understand if ETBE was causing the liver adenoma via a mutagenic mode of action.

Following detailed analysis of in vivo tests, Sustainable Fuels have found that ETBE does not appear to be a mutagenic driver for liver adenoma following inhalation exposure. This means that ETBE is not likely to be a mutagenic carcinogen.

While we stand ready and open to review, assess and investigate the impact of ETBE on human health on a continuous basis, the outcome of this latest study indicates that for general ETBE use cases, in particular for car refueling purposes, ETBE is a safe gasoline blend-stock. It does not only help improve the fuel’s combustion inside the engine, lower the CO2 and pollutant emissions values at tailpipe, but is, when correctly produced, handled and used, safe for human health.

ETBE does not appear to be a mutagenic driver