Xianglan Bai

Pyrolytic Sugars from Cellulosic Biomass

Depolymerization of cellulose offers the prospect of inexpensive sugars from biomass. Breaking the glycosidic bonds of cellulose to liberate glucose has usually been pursued by acid or enzymatic hydrolysis although a purely thermal depolymerization route to sugars is also possible. Fast pyrolysis of pure cellulose yields primarily the anhydrosugar levoglucosan (LG) whereas the presence of naturally occurring alkali and alkaline earth metals (AAEMs) in biomass strongly catalyzes ring-breaking reactions that favor formation of light oxygenates. Here, we show a method of significantly increasing the yield of sugars from biomass by purely thermal means through infusion of certain mineral acids (phosphoric and sulfuric acid) into the biomass to convert the AAEMs into thermally stable salts (particularly potassium sulfates and phosphates). These salts not only passivate AAEMs that normally catalyze fragmentation of pyranose rings, but also buffer the system at pH levels that favor glycosidic bond breakage. It appears that AAEM passivation contributes to 80 % of the enhancement in LG yield while the buffering effect of the acid salts contributes to the balance of the enhancement.

Quantitative Investigation of Free Radicals in Bio‐Oil and their Potential Role in Condensed‐Phase Polymerization

We report on the quantitative analysis of free radicals in bio-oils produced from pyrolysis of cellulose, organosolv lignin, and corn stover by EPR spectroscopy. Also, we investigated their potential role in condensed-phase polymerization. Bio-oils produced from lignin and cellulose show clear evidence of homolytic cleavage reactions during pyrolysis that produce free radicals. The concentration of free radicals in lignin bio-oil was 7.5×10(20)  spin g(-1), which was 375 and 138 times higher than free-radical concentrations in bio-oil from cellulose and corn stover. Pyrolytic lignin had the highest concentration in free radicals, which could be a combination of carbon-centered (benzyl radicals) and oxygen-centered (phenoxy radicals) organic species because they are delocalized in a π system. Free-radical concentrations did not change during accelerated aging tests despite increases in molecular weight of bio-oils, suggesting that free radicals in condensed bio-oils are stable.

The use of calcium hydroxide pretreatment to overcome agglomeration of technical lignin during fast pyrolysis

Technical lignin extracted from lignocellulosic biomass melts and agglomerates when heated, making its thermal processing problematic and impeding this pathway to low-cost renewable aromatics. We have developed a simple pretreatment of technical lignin with Ca(OH)2 that made possible the continuous pyrolysis of lignin in a fluidized bed reactor to produce a phenolic-rich bio-oil. The yield of bio-oil was approximately 38 wt% for pyrolysis in the temperature range of 450 to 600 °C. Average molecular weight of bio-oil was less than 288 Da indicating that the lignin was extensively depolymerized to phenolic monomers and dimers. Char from pyrolysis of pretreated lignin was fine powder in contrast to the large agglomerates of char produced from untreated lignin. The surface area of char from pyrolysis of pretreated lignin was relatively less than for char obtained from pyrolysis of whole biomass. The melting and agglomeration behavior of lignin appears to be due to the presence of phenolic hydroxyl, carboxylic acid and aldehyde groups in lignin. Pretreatment with Ca(OH)2 reduced these functional groups by forming hydroxylcalcium phenoxides, phenolic alcohols, and phenolic carboxylate salts that inhibited agglomeration.

Production of solubilized carbohydrate from cellulose using non-catalytic, supercritical depolymerization in polar aprotic solvents

We report yields of solubilized and depolymerized carbohydrate from solvent processing of cellulose as high as 94% without use of catalysts. Cellulose was converted using a variety of polar aprotic solvents at supercritical conditions, including 1,4-dioxane, ethyl acetate, tetrahydrofuran, methyl iso-butyl ketone, acetone, acetonitrile, and gamma-valerolactone. Maximum yield of solubilized products from cellulose, defined as both depolymerized carbohydrate and products of carbohydrate dehydration, was 72 to 98% at 350 °C for reaction times of 8–16 min. In all cases solvents were recovered with high efficiency. Levoglucosan was the most prevalent solubilized carbohydrate product with yields reaching 41% and 34% in acetonitrile and gamma-valerolactone, respectively. Levoglucosan yields increased with increasing polar solubility parameter, corresponding to decreasing activation energy for cellulose depolymerization.