The intriguing case of organic impurities contained in synthetic methanol: a mass spectrometry based investigation

Abstract

The role of organic impurities in the methanol-to-olefin (MTO) industrial process catalyzed by zeolites is the subject of ongoing debate. We have found that methanol (HPLC and RPE grade) purchased from different chemical companies may contain organic impurities, whose ionization is the dominant process in the positive ion atmospheric pressure chemical ionization (APCI) spectrum of commercial CH(3)OH. Such impurities produce ions with elemental formulae C(n)H(2n+1)O(+) (n = 4, 5, 6); likewise, ionization of tetradeuterated methanol (CD(3)OD) leads to the corresponding fully deuterated series C(n)D(2n+1)O(+) (n = 4, 5, 6), an outcome which represents a clear evidence of their widespread diffusion. We suggest that their formation might be inherent to the chemical process whereby methanol is synthesized on an industrial scale. Mass spectrometry (MS) experiments, gas chromatography/mass spectrometry (GC/MS) analysis and nuclear magnetic resonance (NMR) measurements allowed us to establish that commercial methanol contains dimethyl acetals of simple alkyl ketones, such as propanone, butanone and pentanone. Ab initio calculations (DFT/B3LYP) proved useful to understanding the ionization mechanisms of such impurities.