While hydrocarbon fuels remain as the dominant energy resource for internal combustion engines, alcohols, especially methanol and ethanol, have also been used as fuels. Currently, the primary alcohol fuel is ethanol, which is commonly blended into gasoline in quantities of 5 to 10%. In fact, various fuels being produced today consist primarily of alcohols. For example, E-85 fuel contains 85% ethanol and 15% gasoline, and M-85 fuel has 85% methanol and 15% gasoline. While ethanol possesses excellent octane enhancement properties, there are several drawbacks to its use as a gasoline component, including: energy deficiencies (ethanol provides approximately 39% less energy than gasoline), high blending Reid Vapor Pressure (RVP) (at 10% of blending, the RVP=11 psi), and incompatibility with existing transportation facilities.
Historically, lead (Pb) was added to gasoline to increase its octane rating and thereby improve its antiknock properties. However, the use of lead in gasoline has now been eliminated in most countries for health and environmental reasons. In response to the need to increase octane ratings in the absence of lead, methyl-tertiary-butyl-ether (MTBE) was commercially introduced as an octane enhancing component of gasoline in the United States and other countries in the late 1970s. Legal restrictions on the minimum oxygen content of some gasolines—introduced in the 1990s as a means of reducing environmentally harmful exhaust emissions—encouraged a further increase in the concentration of MTBE in gasoline, which, by then, was being blended at up to 15% by volume. While MTBE is still widely used in the United Kingdom, its use has been in gradual decline in other regions of the world due to concerns about the harmful effects of MTBE itself. Specifically, its existence in groundwater has led to a decline in its use in countries such as the United States, where some states have actively legislated against its use. Today, in order to meet performance and legal requirements, the fuel industry in the United States is now replacing MTBE with fermented grain ethanol. However, producing the necessary quantities of grain ethanol to replace MTBE is problematic in specific regions, and the use of ethanol as a gasoline component has other drawbacks as discussed above.
Certain other alcohols (i.e., butanols), as well as butene oligomers (e.g., diisobutenes (DIBs)) can be used as combustible neat fuels, oxygenate fuel additives, or constituents in various types of fuels. The BTU content of butanols and diisobutenes is closer to the energy content of gasoline than either methanol or ethanol. Butanols have been thought of as second generation fuel components after ethanols. Specifically, 2-butanol and tert-butanol can be particularly advantageous fuel components, as they have blending octane sensitivities and energy densities comparable to those of MTBE and have been shown to have lower RVP at 15% concentrations relative to comparable ethanol blends. Similarly, DIB is a non-oxygenated fuel component with many advantages over other fuel additives. For instance, DIBs have higher RON, better anti-knock quality, and higher energy content compared with MTBE, as well as a lower RVP than MTBE, butanols and ethanol.
Butanols can be produced via the hydration of butenes, a process that typically utilizes an acid catalyst. While the production of butanols via hydration of butenes is a commercially important process, it is typically very costly. DIBs are produced via the oligomerization/dimerization of butenes, in particular isobutene. The dimerization of isobutene is also generally performed using acid catalysts, such as sulfuric acid and hydrogen fluoride; however, these catalysts tend to be highly corrosive in nature.
Both butanols and DIBs provide certain advantages over other existing fuel components. However, until now, there have not been any processes in place that are particularly effective for converting mixed olefins into alcohols—especially butenes into butanols—while also dimerizing part of the mixed olefins feed into oligomers such as DIBs without requiring the costly separation of either mixed butenes isomers in the feed or the mixed butanol isomers in the product. The combination of butanols and DIBs as a fuel additive would lead to enhanced RON, octane sensitivity, and energy density, as well as decreased RVP in gasoline.
Thus, there is a need for alternative gasoline oxygenates that possess comparable RON enhancement properties and a higher energy content than MTBE and ethanol, but that also eliminates the environmental and compatibility concerns of MTBE and ethanol. Additionally, there is a need for alternative fuel additives that lower the RVP of fuel in the absence of MTBE. Finally, there is a need for an alternative process that allows for both the hydration and oligomerization of mixed butenes to alcohols and oligomers, namely butanols and DIB, which can be used as octane enhancing components.