Upgrading fast pyrolysis bio-oil quality by esterification and azeotropic water removal

Fast pyrolysis bio-oil has unfavorable properties that restrict its use in many applications. Among the main issues are high acidity, instability, and water and oxygen content, which give rise to corrosiveness, polymerization during storage, and a low heating value. Esterification and azeotropic water removal can improve all of these properties. In this work, low acidity biooils were produced from fast pyrolysis bio-oil via esterification with methanol or n-butanol. Esterification conversion was enhanced by azeotropic water removal prior to and/or during esterification. An additional hydrocarbon entrainer (n-heptane or petroleum ether) was required for efficient water removal. The product oils had total acid numbers ranging from 5 to 10 mg KOH/g and pH values from 4.0 to 5.6. The best results were obtained with 1:0.9:0.1 wt ratio of bio-oil, n-butanol, and n-heptane and p-toluenesulfonic acid (p-TSA) as catalyst. Removal of homogeneous catalyst (2 wt % p-toluenesulfonic acid (p-TSA)) was attempted by precipitation, centrifugation, and water washing, but only 41−82 wt % of the catalyst could be recovered from the product oil based on sulfur content. Solid acid catalysts were more efficient with methanol than n-butanol in dry conditions. An organic base (triethylamine) was tested for neutralizing the methanol esterified bio-oil’s residual acidity. Nitrogen content increased by 0.1−0.4 wt % when pH values of 6−8 were obtained.