UC Santa Cruz chemists have developed a straightforward, catalyst-free approach to the transesterification of vegetable oils, converting them into high-value biodiesel fatty acid methyl esters (FAMEs). The method, described in a paper in Energy & Fuels, uses sodium tetramethoxyborate [NaB(OMe)₄] derived from the reaction of sodium borohydride (NaBH4) with methanol (MeOH).

NaB(OMe)₄ can also be produced using the inexpensive reagents trimethoxyborate [B(OMe)₃] and NaOMe in methanol and was also successful at producing FAMEs.

The process avoids the use of currently employed hydroxide bases, thereby averting the production of soapy diesel and eliminating the necessity for the removal of water from the reaction mixture and conversion of soapy impurities into FAMEs.

Furthermore, the byproduct glycerol-boron compounds form a solid and thus facilitate the isolation of the biodiesel by simple decantation.

Transesterification reactions were successful using cheap, abundant waste cooking oil from a major fast-food chain, as well as soybean oil, corn oil, olive oil, and animal fat, achieving an average yield of 85% for reactions performed at 80 °C for 1 h. Characterization through 1H and 11B NMR underscores the efficacy of the process and quality of the obtained biodiesel. Our method was extended to the synthesis of ethyl, propyl, and butyl fatty acid esters. This was achieved by introducing used cooking oil to NaB(OEt)₄, NaB(OPr)₄, and NaB(OBu)₄, formed by reactions of NaBH₄ with EtOH, PrOH, and BuOH, respectively.

—Lofgren et al.

In 2022, the US transportation sector alone used about 3 million barrels of diesel per day, accounting for about 75% of total consumption of the fuel in this country. That same year, diesel use accounted for about 10% of total energy-related CO₂ emissions in the United States, according to the US Energy Information Administration (EIA).

While some companies have turned towards electric vehicles to reduce their carbon footprint, the vast majority of fleets still run on diese. Because biodiesel production is difficult and energy intensive, adoption has been relatively slow. Of all the energy sources used by the US transportation sector in 2022, biofuels accounted for just 6%.

While individual consumers increasingly turn to solar and electric energy to power their homes and vehicles, America’s huge industrial sectors still rely on diesel fuel. Lead author Kevin Lofgren pointed out that the majority of the trucks, trains, and boats that ship goods around the world currently run on diesel engines and won’t be electrified any time soon.

Meanwhile, the researchers point out, biodiesel is a carbon-neutral fuel that is available today and approved to power these vehicles without the need for engine modifications.

According to the researchers, the method they discovered passes almost all industry standards for use as fuel in heavy machinery and transportation vehicles. The exception was water content, which was slightly higher than the acceptable value. The researchers expect that once this process is scaled up, the water content will be within acceptable levels.

Resources

• Borate Pathway to FAMEs at Near-Ambient Conditions from Used Oil; Kevin C. Lofgren, Sean Tierney-Bondy, Bakthan Singaram, and Scott R. J. Oliver Energy & Fuels 2024 38 (20), 19659-19667 doi: 10.1021/acs.energyfuels.4c03643