Experimental and computational insights into secondary decomposition chemistry in cellulose pyrolysis

Authors: KOSTETSKYY, PAVLO - Northwestern University; DENSON, MELBA - Washington State University; Terrell, Evan; GARCIA-PEREZ, MANUEL - Washington State University; BROADBELT, LINDA - Northwestern University

Submitted to: American Institute of Chemical Engineers Annual Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 7/14/2021
Publication Date: N/A
Citation: N/A

Interpretive Summary: Pyrolysis is a promising technology for producing clean fuels, chemicals, and materials from biomass (things like wood, grass, and agriculture plant wastes). A major challenge with pyrolysis, however, is that the chemical reactions involved can produce a very complicated mixture of products that is difficult to characterize. By improving our ability to understand the molecules that make up pyrolysis products, we will be able to more easily develop pyrolysis technologies to produce the fuels or chemicals that we want. In this research, we are studying the pyrolysis of cellulose, the primary building block of most biomass. Using some new laboratory techniques (specifically, high resolution mass spectrometry), we can analyze certain important pyrolysis products with much greater detail; however, there isn't a clear understanding of all the results that we get from this analysis. The goal of this work is to use our theoretical understanding of pyrolysis to study it with computational techniques (specifically, density functional theory) and compare the results we get in the laboratory to the results we get from the computer. The major focus of this presentation will be to thoroughly explain the theoretical basis of the computational analysis, highlight the major results from this analysis, and show directions for future work in this area.

Technical Abstract: 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 fate of biomass-derived pyrolysate and its composition have remained under debate, with varying compositions reported by a number of 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 events of pyrolysis, 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 produced during cellulose depolymerization 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.