Characterization of biomass pyrolysis oils by diffusion ordered nmr spectroscopy

Authors: Mullen, Charles; Strahan, Gary; Boateng, Akwasi

Submitted to: ACS Sustainable Chemistry & Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/14/2019
Publication Date: 11/15/2019
Citation: Mullen, C.A., Strahan, G.D., Boateng, A.A. 2019. Characterization of biomass pyrolysis oils by diffusion ordered nmr spectroscopy. ACS Sustainable Chemistry & Engineering. 7:19951-19960. https://doi.org/10.1021/acssuschemeng.9b05520.
DOI: https://doi.org/10.1021/acssuschemeng.9b05520

Interpretive Summary: Biomass or non-food and feed plant material (e.g. crop residues, grasses, forest residues) is the most abundant renewable source of carbon available for the production of fuels and chemicals to replace some produced from fossil resources. It can be converted through a thermochemical conversion process known as fast pyrolysis, the heating of organic matter in the absence of oxygen, to a liquid called bio-oil. Bio-oil can be refined into gasoline and diesel fuels or used as a feedstock to produce commodity or specialty chemicals. However, bio-oils are highly complex mixtures which vary with biomass source and the particular process used to produce them making their complete analysis a challenge for analytical chemists. One of the challenges in analyzing bio-oil is that it is diverse both in the types of chemicals it contains and their size (molecular weight). In addition, these chemicals are difficult to separate because of the reactivity of the bio-oil. Therefore, we applied a technique called diffusion nuclear magnetic resonance (NMR) spectroscopy to a set of three bio-oils produced from switchgrass at three different temperatures. The objective of the study was to both learn more about the chemical nature of these bio-oils and also demonstrate the utility of this technique for bio-oils. This method produces an NMR spectrum, a common chemical analysis tool which can be interpreted to reveal what types of compounds are present, but also separates this signal to correspond to the molecular size. In this way a two dimensional plot is obtained. We were also able to quantify how much of the various types of chemical groups were contained within the various molecular weight ranges. This information will be valuable to those producing or utilizing pyrolysis bio-oils.

Technical Abstract: The complete chemical characterization of biomass pyrolysis oils is challenging because they are a complex mixture of numerous compounds containing many functional groups and have a large molecular weight range. Gas chromatography is a very effective method for detailing the chemical composition of the volatile portion of the bio-oil, but chemical characterization of higher molecular weight portions remains challenging. Advances have been made using NMR to get a broader picture of the chemical makeup of the bio-oils, but how the various functional groups or chemical classes are distributed over molecules remain difficult to determine without some prior tedious separation. Herein, diffusion ordered 1H NMR is applied to three biomass pyrolysis oils produced from switchgrass at temperatures of 500, 600 and 700 C. Diffusion NMR provides a plot of the 1H NMR spectra resolved based on molecular diffusivity which is generally directly proportional to the molecular weight of the compound. Furthermore, quantification of the signal intensity from the diffusion NMR experiments allows for comparison of how various functional groups are distributed over estimated molecular weight ranges. The diffusion NMR has provided a detailed comparison of these bio-oils. For example it has revealed that carbohydrates are present mostly as monosaccharides, disaccharides and trisaccharides. For aromatics, the 500 and 600 C samples consist of pyrolytic lignin, most of which is dimeric or trimeric phenolic units. In contrast, the 700 C sample consists mostly of polyaromatic hydrocarbons containing 2 or 3 rings, but PAHs with up to eight rings may be present. A detailed comparison of the three bio-oils based on the quantitative chemical information gained via diffusion NMR is presented in this manuscript.