Evaluation of Fourier Transform Near-Infrared for the Simultaneous Analysis of Light Alkene Mixtures

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Abstract
A Fourier transform near-infrared (FT-NIR) spectrometer incorporating a fiber-optic-coupled sample cell was used for simultaneous determination of ethylene and monoalkylated light alkenes in liquid mixtures at room temperature and pressure. Ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, and 1-decene were selected for the evaluation. Several additional alkylated alkenes (e.g., 1,1 dialkylated, cis, trans dialkylated, and trialkylated alkenes) were investigated to determine the effects of the molecular structure on this determination. The first overtone of the asymmetric =CH2 stretch of the monoalkylated alkenes was found to be unique for the light alkenes in the NIR region. This region of the spectrum was used for quantitative analysis using classical least-squares (CLS) regression. Ethylene/1-octene mixtures were studied as an example. The concentrations of these alkenes were determined with an error of less than 1% by weight.