Estimation of oxygenates in gasoline by C-13 NMR spectroscopy

A direct and quick method for the estimation of alcohols (C-1-C-4) and ethers (C-5-C-6) has been developed by using C-13 NMR spectroscopy. The first method involves the calibration procedure, where the percentage integral areas of oxygenates in their characteristic alpha-carbon signals (50-75 ppm) versus concentration give a linear curve. The slope of the curve for all types of oxygenates has been found to be independent of the nature of gasoline (FCC, SR, hydrotreated, etc.) and also applicable in the case of samples containing mixture of alcohols and ethers. The second method is based on the estimation of group molecular weight (GMT) of blended gasoline containing oxygenates from their respective integral areas in the C-13 NMR spectra. Mo standards are required in the case of the group molecular weight procedure. Both the methods have given very accurate results for methanol (MeOH), ethanol (EtOH), propanol (PrOH; normal and iso), butanol (BuOH; normal and teptiary), methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), and tert-amyl methyl ether (TAME) present as a single or mixture of oxygenates in a blended gasoline samples. The pair of alcohols, such as EtOH, n-PrOH, and n-BuOH or t-BuOH, which are not possible to estimate by H-1 NMR, have been quantified by the C-13 NMR method. The optimum conditions for obtaining quantitative spectra have also been discussed.