On-line Study on the Catalytic Pyrolysis of Pine Wood over Li₂CO₃ Using Pyrolysis-Photoionization Mass Spectrometry

The catalytic effect of Li₂CO₃ on pine wood pyrolysis was studied by thermogravimetry (TG) and homemade pyrolysis-photoionization mass spectrometry (Py/PI-MS). The TG and derivative thermogravimetry (DTG) curves indicated that the pyrolysis temperature of pine wood was reduced and the yield of coke didn’t increase significantly in the presence of Li₂CO₃. The DTG curve of pure pine wood was characterized by a shoulder peak. Since the pyrolysis temperature of hemicellulose was lower than that of cellulose, the peak and shoulder peak were mainly attributed to the decomposition of cellulose and hemicellulose, respectively. The shoulder peak in the DTG curve of pine wood/Li₂CO₃ disappeared, indicating the pyrolysis temperature of cellulose was reduced obviously and the pyrolysis temperature of hemicellulose decreased slightly.
Py/PI-MS is an on-line analytical method which combines pyrolysis furnace with photoionization mass spectrometry. Thanks to the “soft” ionization method (10.6 eV), nearly few or no fragments are produced in the mass spectra. The experiment was carried out in two modes: (1) In temperature-fixed mode, the mass spectra of pine wood and pine wood/Li₂CO₃ pyrolyzates at different temperatures as well as the time-evolved profiles were obtained. (2) In temperature-programmed mode, the temperature-evolved profiles of pine wood and pine wood/Li₂CO₃ pyrolyzates were obtained. The results of Py/PI-MS showed that the three primary components of pine wood, i.e., cellulose, hemicellulose and lignin had different responses to the added catalyst, Li₂CO₃. The yield of pyrolyzates from cellulose and hemicellulose were decreased, while pyrolyzates yields from lignin increased significantly. From time-evolved profiles, the time to reach the maximum reaction rate of pyrolyzates were advanced in the presence of Li₂CO₃. From temperature-evolved profiles, the temperature to reach the maximum reaction rate of pyrolyzates were advanced due to added Li₂CO₃. The major pyrolysis products of hemicellulose had only one peak, which was consistent with their generation paths. The pyrolysis products of cellulose and lignin had two peaks, indicating that they had multiple different generation paths, which were also consistent with their generation paths. As mentioned above, Li₂CO₃ had a profound effect upon the pyrolysis of pine wood. It has been investigated by considering the effect of Li⁺ on the reaction energy barrier for the decomposition of pyrolyzates. The possible catalytic mechanism was proposed that the bond energy of C—O of pyrolyzates were reduced in the presence of Li⁺, thus promoting the decomposition of pyrolyzates. Pine wood pyrolyzates showed different responses to the catalyst, which were closely related to their different structures. This work provides the basis for the research of biomass catalytic pyrolysis mechanism and the potential of industrial application.