Molecular modelling to investigate biomass pyrolysis chemistry

• Main Author: Gautam, Spandan
• Other Authors: Samir Hemant Mushrif
• Format: Theses and Dissertations
• Language: English
• Published: 2018
• Subjects: DRNTU::Engineering::Chemical engineering::Fuel
• Online Access: http://hdl.handle.net/10356/74221
• Institution: Nanyang Technological University

Depletion of non-renewable sources has led to efforts in the development of technologies to utilize lignocellulosic biomass as a potential feedstock. One of the promising techniques to convert biomass to fuels is fast pyrolysis. Cellulose, a major component of biomass, is made of the β-D glucose rings attached by a β-1,4 glyosidic bond (C-O-C). Cleavage of the glycosidic bond is the first and an important reaction step in the decomposition of cellulose during fast pyrolysis. Experimental studies have shown that cellulose fragmentation undergoes two distinct cleavages - high activation barrier, possibly associated mid/intra-chain cleavage and low activation barrier, possibly associated end-chain scission. However, fundamental understanding and mechanistic insights into the two distinct kinetic regimes is completely lacking. Hence, in this study, the mechanistic and kinetic differences in mid and end-chain glycosidic bond cleavages in a condensed phase environment using periodic boundary conditions is investigated by mimicking the condensed environment observed experimentally during cellulose fragmentation. The activation barriers for different end and mid-chain scission mechanisms were calculated using cellotetraose as model compound and are also compared with those for cellulose and cellobioise. The end-chain scission enthalpic activation barriers are calculated were found to be lower than the mid-chain scission barriers by ~18.7 kcal mol-1, ~6.5 kcal mol-1 and ~5.1 kcal mol-1 for transglycosylation, glycosylation and ring contraction mechanisms for cellotetraose, respectively. This is in excellent agreement with the reported experimental studies showing different kinetic parameters regimes for different scission mechanisms. The difference between the end and mid chain scissions was attributed to the inter and intra molecular interactions that govern the relative stabilisation of the transition state. These interactions were interpreted using Reduced Density Gradient (RDG) plots. Energy decomposition analysis shows that the intermolecular interaction stabilizes end-chain scission, while mid-chain cleavage transition state is stabilized by intramolecular interactions. The magnitude of intermolecular interactions for end-chain cleavage was observed to be higher than intra-molecular interactions of mid-chain cleavage, leading to a more stable end-chain fragmentation transition state and lower C-O-C bond scission barriers. This observation was consistent when compared with cellobiose and cellulose glycosidic bond breaking. To the best of our knowledge, this is the first study providing mechanistic insights into end and mid-chain glycosidic bond cleavage and the need to incorporate intermolecular interactions while simulating pyrolysis reaction mechanisms using DFT.