(34a) Experimental and Computational Insights into Secondary Decomposition Chemistry in Cellulose Pyrolysis

Pyrolysis of cellulose-based biomass for the production of renewable fuels and chemicals is an active area of research worldwide. A number of studies have focused on compositional characterization of pyrolysis reactor effluent, utilizing experimental and computational means. The composition of the biomass-derived pyrolysate has remained under debate, with varying compositions reported by different authors. It is well-known that feedstock identity, pretreatment method and process operating conditions have a strong effect on the observed product spectra. Additionally, it has been suggested that after the primary depolymerization reactions, the products undergo a series of secondary transformations such as dehydration, isomerization, fragmentation, repolymerization, and others. However, limited data are available on the composition of the pyrolysis oil itself, especially larger oligomeric species in the 400-700 Da range. Dehydration of the different molecules can result in a wide range of product species of varying size and degree of dehydration. In this work we report a combined experimental and computational study of elucidating cellulose pyrolysate composition, utilizing high-resolution Fourier transform-on cyclotron resonance mass spectrometry (FT-ICR MS) experiments and electronic structure calculations. Namely, the relative stabilities of different-size oligomers of glucose were evaluated using electronic structure calculations. A series of candidate molecules were compiled as likely constituents of the condensed pyrolysate phase, based on the calculated relative stabilities of monomer, dimer, trimer and tetramer states of glucose based carbohydrates. Each species was allowed to undergo multiple dehydration events, with the most stable structures selected. The findings in this work show that large oligomeric species can constitute a significant fraction of the pyrolysate and that dehydration of depolymerized cellulose oligomers is one of the main secondary chemical transformations these species undergo. A library of structures and corresponding stabilities was assembled toward elucidation of the mechanisms of cellulose pyrolysis and characterization of the pyrolysate composition.