TECHNICAL FIELD

This invention relates to hydrothermal production of alkanes.

BACKGROUND

Isooctane (2,2,4-trimethylpentane) is an important ingredient in hydrocarbon fuels that power cars, airplanes, and other machinery. Because of this, it is highly relevant to the transportation, manufacturing, and construction industries. One conventional method by which isooctane can be synthesized is a process called alkylation that uses large quantities of sulfuric acid or hydrofluoric acid to dimerize isobutane and isobutene. This method generates toxic waste that is costly to remediate. Another conventional way to synthesize isooctane is to dimerize isobutene to isooctene (2,2,4-trimethyl-2-pentene) with a supported acid catalyst, followed by hydrogenation with one of several metal hydrogenation catalysts (e.g., platinum or palladium). The supported acid catalysts generate costly waste, and the metal catalysts are rare materials and therefore expensive.

SUMMARY

Hydrothermal production of alkanes described herein involve water, Earth-abundant materials, heat, and a reaction vessel that can withstand moderate pressures. As such, these methods are low in cost and free of toxic waste.

Synthesizing an alkane includes heating a mixture including an alkene and water at or above the water vapor saturation pressure in the presence of a catalyst and one or both of hydrogen and a reductant, thereby hydrogenating the alkene to yield an alkane and water, and separating the alkane from the water to yield the alkane. The reductant includes a first metal and the catalyst includes a second metal.

Implementations of the general aspect can include one or more of the following features.

In some cases, the alkene is a second alkene and the mixture is a second mixture, a first alkene is combined with water to yield a first mixture, and the first mixture is heated above atmospheric pressure to yield the second mixture including the second alkene.

The general aspect can include dehydrating an alkyl alcohol having at least two carbons to yield the first alkene. Examples of suitable alcohols include butanol (e.g., tert-butanol or isobutanol). In one example, the first alkene is isobutene, the second alkene is isooctene, and the alkane is isooctane.

Heating the first mixture at or above the water vapor saturation pressure can include heating the first mixture to a temperature in a range of 250° C. to 350° C., a pressure in a range of 40 bar to 165 bar, or both. Heating the first mixture and the second mixture typically includes heating the first mixture in a first reactor and heating the second mixture in a second reactor. In some cases, the second mixture is transferred from the first reactor to the second reactor. In other cases, the first reactor and the second reactor are the same reactor, and no transfer occurs. Heating the second mixture typically includes heating to a temperature in a range 250° C. to 300° C.

The first metal oxidizes in water to yield one or more oxides of the first metal. In some cases, the first metal oxidizes in water to further yield molecular hydrogen. In certain cases, the first metal includes iron. The second metal remains substantially unoxidized. The second metal catalyzes the reduction of the second alkene with molecular hydrogen to yield the alkane. In one example, the second metal includes nickel.

Isolating the alkane from the water can be performed by mechanical separation (e.g., decanting, skimming, and the like).

The details of one or more embodiments of the subject matter of this disclosure are set forth in the accompanying drawings and the description. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims.

