Observation and characterization of cellulose pyrolysis intermediates by 13C CPMAS NMR.: A new mechanistic model

The composition of char from heated Avicel cellulose was monitored as a function of heating time and temperature, using C-13 cross-polarization magic-angle spinning (CPMAS) NMR. Complex NMR line shapes observed in the carbohydrate region of the spectra are indicative of the presence of multiple carbohydrate forms. By successive spectral subtractions of the 300 degreesC pyrolysis char, the complex line shapes were separated into three distinct carbohydrate components that correspond to the crystalline cellulose starting material (SM), an intermediate cellulose (IC) that resembles a low degree-of-polymerization (low-DP) amorphous cellulose, and a disordered final carbohydrate (FC) that is characterized by a very broad 13C line width. Curve fitting was used to monitor the changes in the approximate abundance of these different carbohydrate forms relative to the aliphatic, aromatic, carboxyl, and ketone clusters of compounds of the char. The time evolution of the IC, together with its spectral line shapes, associate this component with the "active cellulose" intermediate that has long been postulated in many kinetic mechanisms for cellulose pyrolysis. After a heating period of 30 min, FC was the only remaining carbohydrate component. When subjected to prolonged heating, FC converted to aromatic carbons but not to aliphatic carbons, with little or no loss in char mass. This property distinguishes the FC as a char component that has not previously been recognized. Pyrolyses of cellulose with 1% K+ as KCl, and of pectin at 300 degreesC and cellulose at 350 degreesC were also performed. Evaluation of the combined data led to a new model for low-temperature cellulose pyrolysis. In this model, all char products are formed from IC, with FC being capable of forming aromatic carbon.