Borate Pathway to FAMEs at Near-Ambient Conditions from Used Oil

We present a straightforward, catalyst-free approach to the transesterification of vegetable oils, converting them into high-value biodiesel fatty acid methyl esters (FAMEs). Our method utilizes both commercial waste oils and virgin cooking oils, employing 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. Notably, our 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 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.