Vacuum pyrolysis of cellulose: Fourier transform infrared characterization of solid residues, product distribution and correlations

Abstract

Vacuum pyrolysis of purified cellulose in constant rate of temperature increase experiments has been used, in conjunction with X-ray crystallography and FTIR of the solid residues, to probe the mechanism of conversion of cellulose into liquid products. The high yield of levoglucosan in the absence of cationic impurities, and the high yield of hydroxyacetaldehyde in the alternate pathway of impure or metal ion doped cellulose has been confirmed. The decomposition pathways correlate well with the yields of carbon oxides as has been found previously in different types of apparatus. The FTIR information shows that the residual solid is slowly transformed from a cellulose, i.e. pyranose ring structure, to a solid with little or no pyranose structure having considerable olefinic and carbonyl character at a conversion of about 50% of the solid to gaseous and liquid products. This occurs at about 325°C for the impure high DP cellulose and at about 300°C for the purified but lower DP cellulose. Increasing temperature continues to result in weight loss and increasing aromatization of the residual solid. The transition from a cellulosic to a dehydrated structure is much more clean cut in the case of the purified cellulose and the yield of levoglucosan (i.e. cyclised anhydro pyranose rings) is greater. An experiment conducted in the hydroxyacetaldehyde regime has demonstrated that the choice of pathways is dictated not only by the presence of the metal ions, but also is a function of the DP even at relatively high DPs.