Characterization of various fast pyrolysis bio-oils by NMR spectroscopy

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

Submitted to: Energy and Fuels
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/1/2009
Publication Date: 4/15/2009
Citation: Mullen, C.A., Strahan, G.D., Boateng, A.A. 2009. Characterization of various fast pyrolysis bio-oils by NMR spectroscopy. Energy and Fuels. 23:2707-2718.

Interpretive Summary: Fast pyrolysis is a process that can convert biomass to a high density liquid called bio-oil. This is done by rapidly heating the biomass in the absence of air and then quickly cooling the vapors. Bio-oil can be sent on to be upgraded into liquid transportation fuels or used as a starting material for synthetic chemicals. For either of these uses knowledge of the chemical compounds that make up bio-oil is important. Because bio-oil is a very complex mixture, that can contain hundreds of different chemical compounds, this is difficult to do. The make-up of the bio-oil also differs depending on the identity of the biomass from which it is produced. Some methods have been able to identify some compounds in the bio-oil, but these techniques can only look at a portion of it or look at it in a qualitative manner. We used a technique called nuclear magnetic resonance (NMR) which can provide information on the types of compounds based on the chemical environment of carbon and hydrogen atoms that make up the bio-oil, and it can look at the entire bio-oil rather than just a portion. We have found that bio-oil produced from some feedstocks (like corn stover and switchgrass) contain more sugar like molecules and some (like wood-like biomass) contain more hydrocarbon-like molecules similar to those found in petroleum. This information will be important to bio-oil producers and to those refining bio-oil into fuels and chemicals.

Technical Abstract: NMR spectroscopy, including 1H, 13 C and DEPT spectra were used to characterize fast pyrolysis oil from numerous energy crops and other agricultural feedstocks. The bio-oils studied were produced from swithchgrass, alfalfa stems, corn stover, guayule (whole plant and latex extracted bagasse) and chicken litter. The 1H and 13C NMR spectra were integrated over spectra regions to quantify classes of carbon and hydrogen atoms in each bio-oil sample. DEPT spectra were used to quantify by protonation, the number of the carbon atoms in each of those classes to give further information of the types of molecules that are found in the bio-oil. The NMR spectra of the bio-oils varied greatly. The percentage of carbons and protons at the downfield regions of the NMR spectra tracked with the energy content of the bio-oil as well as the feedstock type but there was no such consistent trend for aromatic content. Degrees of branching in aliphatic portions of the bio-oil were inferred from percentages of CH1, CH2 and CH3 groups. Aromatic portions were found to be extremely complex, with substituted aromatic carbons outnumbering unsubstiuted aromatic carbons >2:1 in most cases.