(495a) Condensed Phase Reactions of Polysaccharides during Fast Pyrolysis

The goal of this study is to investigate the early stages of polysaccharide deconstruction during fast pyrolysis. Fast pyrolysis of polysaccharides is typically studied by analyzing volatilized products, which precludes direct measurement of non-volatile intermediate products, namely anhydro-oligosaccharides. Focusing only on the volatile products biases theories against the role of large, intermediate products. We aim to overcome this analytic weakness.

We have employed two unique reactors that rapidly heat biomass and quench products, allowing us to truncate pyrolysis before its completion and recover solid products for chemical analysis. The smaller reactor, termed the Controlled Pyrolysis Duration (CPD)-Quench reactor, can control reactor residence time on the order of a few seconds. The larger reactor, a modified free fall reactor, has a fixed residence time of about one second but has the advantage of creating larger quantities of intermediate products for analysis. These methodologies allow us to examine condensed phase reactions during the early stages of polysaccharide pyrolysis.

We performed truncated pyrolysis on cellulose, the most abundant biomass polysaccharide, at three fast pyrolysis temperatures: 400, 450, and 500 °C. The CPD-Quench reactor, combined with gel filtration and high-performance liquid chromatography, shows that small anhydro-oligosaccharides form before substantial levoglucosan production – an indication that cellulose rapidly depolymerizes via cracking reactions before unzipping reactions produce significant levoglucosan. The signal from a flame ionization detector (FID) directly connected to a conventional micropyrolyzer correlated well with the time evolution of levoglucosan from the CPD-Quench reactor, suggesting that it could be used as a proxy for determining the rate of levoglucosan production in future cellulose depolymerization studies.

The larger quantities of intermediate products created in the free fall reactor make possible more detailed studies. The non-volatile products of truncated pyrolysis of cellulose, which are large, water-insoluble anhydro-oligosaccharides, were measured by size-exclusion chromatography and multi-angle light scattering (SEC-MALS)—the first known analysis of its kind—along with other analytical techniques.

The results from these experiments agree well with a simple population balance model of cellulose depolymerization. Using recently published rate parameters for cracking and unzipping of cellulose, the model predicts rapid depolymerization to small anhydro-oligosaccharides before significant levoglucosan is formed. The ultimate yield of levoglucosan from cellulose is strongly dependent upon both the yield of levoglucosan from the dimer cellobiosan as well as the relative rates of cracking and unzipping of anhydro-oligosaccharides. Deciding which of these mechanisms is most important in determining levoglucosan yields from cellulose requires further experimentation. Furthermore, there is evidence that the absolute rate of unzipping needs to be much higher than suggested in the literature to explain the experimentally observed evolution of levoglucosan during fast pyrolysis.