Characterizing Biomass Fast Pyrolysis Oils by ¹³C NMR and Chemometric Analysis

Several biomass fast pyrolysis oils were characterized by ¹³C and DEPT (distortionless enhancement polarization transfer) NMR analysis to determine their chemical functional group compositions as related to their energy content. Pyrolysis oils were produced from a variety of feedstocks, including energy crops, woods, animal wastes, and oil seed presscakes, and were also compared with fossil fuels (gasoline and diesel). The ¹³C and DEPT NMR spectra of the biofuels and the two fossil fuel samples were measured, and each spectrum was divided into distinct regions according to the chemical shift ranges appropriate for various functional groups. The spectral intensities of each region were then quantified, and the carbon proton substitution numbers were determined. This approach provided helpful information on the chemical compositions of the oils, but it was found to oversimplify the information contained in the ¹³C NMR spectra, resulting in a substantial loss of important details. Hence, a chemometric method of analysis based on principle component analysis (PCA) was used to extract more of the useful information from the ¹³C NMR spectra. The intensities of the ¹³C NMR spectra from 15 pryolysis and two fossil fuel samples were binned in 2 ppm increments and subjected to unsupervised PCA. Comparison of the PCA scores plots with their respective loadings plots enabled a determination of the chemical shifts and, hence, the chemical functional groups that were most important in discriminating among the clusters. The clustering of the biomass samples based on PCA results were shown to track with their biological origin and their energy contents. Therefore, ¹³C NMR PCA analysis was shown to be a powerful and facile technique for classifying biomass fast pyrolysis oils