February 11, 2013
Will biobutanol replace bioethanol? The use of bioethanol as a gasoline additive has advantages and disadvantages. On the positive side, ethanol is renewable and nontoxic; and it contains oxygen, which promotes cleaner automotive exhaust emissions. On the downside, ethanol has several operational problems: It has much less energy density than gasoline, which leads to lower fuel efficiency; it cannot be blended at levels much greater than 10%; and it cannot be blended into the gasoline until the end of the supply chain because of its miscibility in water.
Biobutanol is potentially a better alternative to ethanol. It has greater energy content than ethanol; it can be blended in gasoline at levels as high as 16%; it can be blended into gasoline in the refinery; and it can be transported through the existing fuel infrastructure. Like ethanol, it is renewable; but, unlike ethanol, the fermentation technology for converting sugars to biobutanol is difficult.
Alcohols can be dimerized by using the Guerbet reaction. Guerbet dimerizations are typically run with alcohols that have six or more carbon atoms to obtain dimers with twice as many carbon atoms. Short-chain alcohols such as ethanol are notoriously difficult to dimerize.
Y. Tanaka and M. Utsunomiya disclose a catalyst and operating conditions that allow the conversion of ethanol to n-butyl alcohol with good selectivity. In one example, RuCl2(PPh3)3 (0.239 mmol), Ph3P (0.716 mmol), t-BuOK (4.774 mmol), EtOH (6.307 g), and o-xylene solvent (5.525 g) are added to a stainless steel autoclave. Hydrogen gas (2.0 MPa) is introduced at 20 °C, and the reactor is heated to 180 °C for 2 h. The yield of n-BuOH is 21%, and the selectivity to n-BuOH is 93%. (Mitsubishi Chemical [Tokyo]. US Patent 8,318,990, Nov. 27, 2012; Jeffrey S. Plotkin)