| With continuing concern over fossil fuel shortage and severe environmental problems, more attention was being focused on exploitation of clean renewable energies. Bio-oil, a liquid product from the pyrolysis of biomass under conditions of rapid heating, is recognized as one of the most promising alternative fuels and receives increasing attention. The crude bio-oil is a complicated oxygen-containing mixture which generally contains a wide variety of alcohols, esters, ethers, aldehydes, ketones, acids, olefins, aromatics, phenols, carbohydrates, protein and derivatives> and as a result, crude bio-oil exhibits some undesired properties such as acidity, high viscosity,thermal instability, high water and oxygen content and low heating value compared with that of fossil fuels, which hinder the widespread use of bio-oil as a fuel. Therefore bio-oil needs to be upgraded to improve its quality for its practical application.Up till the present moment, gas chromatography (GC)/mass spectrometry (MS) is still a primary methodology for a full analysis of the organic components of bio-oil and the evaluation for bio-oil catalytic upgrading effect. GC/MS method, however, has obvious disadvantages for the analysis of the high-boiling fraction (HBF) of bio-oil which contain lots of high-molecular-weight or high-boiling-point compounds, and thus can not demonstrate complete composition and efficiency of upgrading process for bio-oil.Quantitative 31P-NMR spectroscopy was successfully used to measure the hydroxyl and carboxyl content in HBF of bio-oil before and after upgrading. The crude bio-oil was upgraded using Pt, Pd/Al2 (SiO3)3, Pt, Pd/C, Pt, Pd/MgO and HZSM-5 with different Si/Al ratios as catalysts in different reaction conditions. According to the P-NMR spectra, aliphatic OH moieties, carboxylic acids and phenolic OH (noncondensed and condensed phenolic structure) are main hydroxyl types in HBF of bio-oil. The noncondensed phenolic structure (guaiacyl and p-hydroxyphenyl phenolic hydroxyl) is derived from single-ring phenolic compounds such as guaiacol, phenol, eugenol, and vanillin. In comparison with the noncondensed phenolic hydroxyl, the condensed phenolic structure is almost non-existent in HBF of bio-oil. The experimental results showed that reaction condition has a great effect on efficiency of upgrading for HBF of bio-oil. Supercritical condition can promotes the conversion of aliphatic OH moieties and carboxylic acids, but has the disadvantage for noncondensed phenolic structure. On the contrary, noncondensed phenolic structure is converted more easily in convention condition. In addition, the acidity and alkalinity of catalysts are also an important factor on efficiency of upgrading for HBF, and both beneficial to esterification between aliphatic OH and carboxylic acids in HBF of bio-oil. The increasing of the acid strength of catalysts can promote the conversion of the aliphatic hydroxyl and carboxyl hydroxyl. In the upgrading process of HBF with the alkaline catalysts of Pt, Pd/MgO, aliphatic OH and carboxylic acids are significantly reduced due to esterification, however, the part of hydrolysis of the generated esters due to the alkali nucleophilic catalysis results in a residual amount of aliphatic OH and carboxylic acids after catalytic upgrading. During the super-critical upgrading process, carboxylic acids are almost eliminated using HZSM-5 catalysts with different Si/Al ratio, which is attributed to esterification with alcohols catalyzed by acidic HZSM-5 catalyst. The content of aliphatic OH moieties in the upgraded HBF decreases with the increasing of the acid strength of catalysts, which also controls the content of the non-condensed phenolic hydroxyl during the super-critical upgrading process. The acid strength of HZSM-5 catalysts could determine the catalytic conversion rate of noncondensation phenolic hydroxyl, and the generation rate of new phenolic compounds due to the catalytic cracking of aromatic oligomers in the meantime. Therefore, the content of noncondensation phenolic hydroxyl depends on the net conversion rate of phenolic compounds after catalytic upgrading.
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