Reactions of Mixture of Oxygenates Found in Pyrolysis Vapors: Deoxygenation of Hydroxyacetaldehyde and Guaiacol Catalyzed by HZSM-5

Pyrolysis is a promising thermochemical process to convert lignocellulosic biomass to renewable biofuel. Much research has been conducted on the catalytic upgrading of either vapors derived from whole biomass scale or on individual model oxygenates. However, not many studies investigated the upgrading and deoxygenation of a mixture of several oxygenates. In this study, we use a combination of techniques to probe the reactions of guaiacol and hydroxyacetaldehyde (HAA) on HZSM-5, their diffusion inside the zeolite catalyst pores, and the extractable products. The techniques we used included several NMR methods, gas chromatography, and thermogravimetric analysis coupled with Fourier transform infrared spectroscopy (TGA-FTIR). In nitrogen at 280 °C with HZSM-5 catalyst, HAA decomposes, cyclizes to form aromatics and phenolic compounds, as well as produces coke. Under the same conditions, guaiacol neither reacts nor forms coke. When the two molecules are present together at 280 °C, their reaction pathways are independent of each other. For HAA at 480 °C, the quantity of aromatics produced is much higher than at 280 °C. At 480 °C guaiacol forms mostly substituted phenolics, BTEX type molecules, and coke. When guaiacol and HAA are mixed together at 480 °C, the amount of coke formed is slightly higher while the aromatics produced in the form of toluene, naphthalene, and their substituted compounds are substantially higher than that can be predicted by simple summation of products in individual cases. After reactions with guaiacol alone or a mixture containing both guaiacol and HAA, the available micropore surface area decreased to zero, indicating plugging of the pores or blocking of pore entrance. However, in both cases the guaiacol and phenolics can be desorbed in an N2 atmosphere at a relatively low temperature range of 100–200 °C. Diffusion measurements indicate that size has a large effect on the pore diffusion coefficients of different oxygen molecules. After coke forms on the catalyst the diffusion coefficients of larger molecules such as guaiacol are affected more significantly than diffusion of small molecules such as water and methanol.