(639r) Mechanistic Study of Guaiacol Fast Pyrolysis Using DFT Calculations, Microkinetic Model and Experiments

The use of fossil fuels raises severe environmental problems. The burning of fossil fuels produces around 10.7 billion tonnes (PBL NEAA 2016 report) of carbon dioxide (CO₂) per year [1]. Increasing concentrations of greenhouse gases causes an increase in the average temperature of the earth, which leads to global warming. Due to this reason, world is moving towards the utilization of renewable energy resources. Renewable energy technologies are needed for achieving a secure and sustainable energy, with high energy efficiency. In recent decades, the consumption of renewable feed stock such as lignocellulosic biomass has drawn an increasing attention to produce valuable chemicals and fuels. The primary components of lignocellulosic biomass include cellulose, hemi-cellulose and lignin. Lignin (15-30%) is the second abundant component in biomass and also a main by-product of the pulp and paper industries. The structure of lignin is complex and it is formed by polymerization of three phenyl propane monomers; viz. p-coumaryl, coniferyl and syringyl alcohols. Moreover, the structure of lignin is also complicated by the presence of different linkages such as β-O-4, α-O-4, 4-O-5, 5-5, β-β, β-1, and β-5. Among these the most abundant linkages of lignin is β-O-4 (40- 60%). Fast pyrolysis is a promising thermochemical conversion technique for the production of valuable phenolics and aromatic hydrocarbons from lignin. For this reason, the molecular mechanism of transformation of lignin is vital for understanding and improving the fast pyrolysis and catalytic fast pyrolysis processes. Lignin pyrolysis mechanism can be reasonably well understood through the lignin model compounds like phenolic monomers and dimers.

In this study, theoretical, kinetic and experimental analysis of fast pyrolysis of guaiacol, which is a model monomeric compound of lignin, was investigated. Fast pyrolysis experiments were carried out in micropyrolyzer and the evolved vapors were analyzed in gas chromatography-mass spectrometer. Experiments were conducted at different temperatures from 450 to 650 ^oC for high heating rate (> 1000 ^oC/s). The effect of temperature (450-650 ^oC) on the yield of phenolic compounds was studied. The main products from the pyrolysis of guaiacol were catechol, 2-hydroxybenzaldehyde, o-cresol, and phenol. The results suggested that conversion of guaiacol (27.7 to 100 %) increased with increasing temperature from 450 to 650^oC. The maximum yield of catechol (23.7 %) was observed at 600^oC. The highest phenol yield (10.8 %) was observed at 650^oC. The decomposition of the primary products seems to have occurred via dehydration, decarboxylation, demethylation, demethoxylation, dehydroxylation, decarbonylation and radical rearrangement reactions. Based on the experimental results, the reaction mechanism of fast pyrolysis of guaiacol was formulated. Thermodynamic and kinetic analysis of 19 reactions were theoretically investigated by using density functional theory methods at B3LYP /6-31++G (d, p) level. The kinetic parameters (Arrhenius activation energy and pre-exponential factor) of 19 reactions were theoretically calculated. The homolytic cleavage of O-CH₃ bond was the initial reaction in the reaction mechanism of pyrolysis of guaiacol, due to its low bond dissociation energy (52.9 kcal/mol). The effects of temperature (450-650^oC) and residence time (0-600 s) on product yields were evaluated by solving the rate equations of a detailed kinetic model. From the results, it can be concluded that the experimental and simulation trends are inline but the yields are different due to different operating conditions used and evaluation of the limited number of reactions. Even though the evaluation of limited number of reactions used in this study, the major products from the fast pyrolysis of guaiacol were captured.

References:

1. G. J. Jos. Olivier. Trends in global CO₂ emissions: 2016 report. PBL Netherlands environmental assessment agency. 2016, 2315.