Eight kinds of paths of possible pyrolysis reactions were designed, and B3LYP/6-31＋＋G(d,p) methods in Gaussian 03 package was used for investigating the pyrolysis reaction mechanism of xylan monomer. The equilibrium geometries of the reactants, transition states, intermediate and products of every reaction path were optimized, and the transition states were checked using IRC method. The standard thermodynamic and kinetic parameters of every reaction path in 298～1098 K temperature ranges were calculated. The result shows that Path1 and Path2 are exothermic reaction and Path3 turn to exothermic reaction above 898 K and the other Paths are endothermic reaction. When the pyrolysis temperature is below 798 K the convert ratio of 2-furfuraldehyde (P₁) and water is the highest. Above 798 K, the main reaction path is Path4. From the kinetic analysis, we get three better path from all the paths of xylose molecule pyrolysis in different bond breaking-way. The rate-determining step of path2 is step5, whose activation energy is 236.9 kJ/mol. The rate-determining step of Path6 is step7, whose activation energy is 363.9 kJ/mol. The rate-determining step of Path7 or Path8 is step15, whose activation energy is 336.5 kJ/mol. According to results of thermodynamic or dynamic analysis, Path1 which produces 2-furfuraldehyde and water is given priority to supporting. Path6 which produces glycolaldehyde and methanal is given secondly supporting. Path7 or Path8 which produces glycolaldehyde, glycol and CO is given last supporting.

Key words: xylan monomer, density functional theory, pyrolysis mechanism